Dr Sophie Hermans

Antonio Leyva

Professor Cyril Gomez

Dr Bruno Godard

Dr Katerina Chaudret

Dr. Soulantika

Dr Iann Gerber

Dr. Iker Del Rosal

Dr Regis Philippe

Dr Boris Guicheret

Anna Corrias

Professor Hervé Martinez

Dr Yann Tison

Mario Elias

Alejandro Yann

Procédés Catalytiques Lyon who carried out the experiments of myrcene hydrogenation and mass transfer studies.

Résumé

Ce travail de recherche se concentre sur la préparation de catalyseurs de site unique supportes sur matériaux carbonés et leur applicabilité aux réactions d'hydrogénation. Le chapitre 1 correspond à une revue complète des études expérimentales et théoriques visant à: i) préparer le SAC sur les matériaux carbonés, ii) comprendre les interactions métalsupport dans le SAC, et iii) étudier comment cela est lié aux performances catalytiques.

Dans le chapitre 2, nous avons pu comprendre la possibilité de produire différentes nanostructures Ru@fullerène qui peuvent être modulées nanométriquement en faisant varier les conditions lors de leur synthèse, solvant et rapport Ru/C60, à travers des nanoparticules (NP) bien définies à de très petits clusters et même des atomes simples (SA) avec une charge métallique élevée (~ 6-20% en poids). Nous avons appliqué ces nanostructures comme catalyseurs dans l'hydrogénation du nitrobenzène, constatant que Ru SA est moins efficace que Ru NP, performance qui a également été étudiée par des calculs DFT. Le chapitre 3 détaille un nouveau protocole simple pour la création de lacunes sur des supports carbonés. Nous proposons qu'il soit possible de créer des défauts/lacunes sur les supports par un processus contrôlé en deux étapes, qui comprend la subséquent création et décomposition de groupes oxygène sur la surface. La

General Introduction and objectives

Size reduction of the active phase in supported catalysts induces: i) the increase in the proportion of catalytically active surface species or in highly uncoordinated atoms, and ii) quantum size effect (for particle size close to the nanometer). This may result in enhanced catalytic performances (Figure 1). 1 Indeed, as the catalyst size is reduced, the surface atomic structure, electronic structure, and defects will change significantly.

Therefore, the catalytic activity or selectivity of the catalyst is often improved. The smallest size that can be reached for this type of object is of course the isolated atom deposited on a support. Discoveries of breakthroughs in the catalytic performance of supported isolated atom compared to traditional catalysts demonstrate the strong potential of these materials in various applications. Catalysts based on supported isolated atom have long existed in different forms (Figure 2). However, the possibility to visualize their presence in previously unknown systems has sparked enormous renewed interest in the past decade. 2 Metallic species with different size (nanoparticles (NP), clusters and single atoms (SA)) usually show different catalytic performances for various reactions, and many factors (particle size, shape, chemical composition, metal-support interaction, and metal-reactant interaction) can have significant influences on the catalytic properties. The electronic and geometric structures of isolated supported atoms, clusters, and nanoparticles have been recently reviewed by A. Corma et al. 3

Figure 2. Number of publications dealing with supported isolated atoms for catalytic applications.

The number of variables that can have an effect on the catalytic behavior is obviously quite high. Among them, A. Corma et al. highlight different important aspects linked to the origins of particle size effects in heterogeneous catalysis:

✓ The intrinsic electronic difference and binding capacities when going from SA to clusters and to NP.

✓ The dynamic characteristics of the catalytic process, which is directed by the metal-reactant interaction.

✓ The fact that these changes can be for the good or for the bad from a catalytic point of view. ✓ In the extreme case of supported SA, the real active site will be the "single metal atom", but the "ligand" atoms of the support will play an important role.

Concerning the last item, it is important to stress that, as in homogeneous catalysis, the direct environment of the SA will have a pivotal role on its catalytic performances.

Consequently, further advances in both the controlled synthesis and the characterization of supported SA catalysts are definitively required to move forward in the design. Another challenge for single atom catalysts (SAC) for industrial applications is to increase the active site density. In addition, the difficulty in experimentally verifying the structures of this type of catalysts has generated a growing reliance on computational methods to support hypotheses for the observed reactivity.

Single atoms undoubtedly have a different reactivity than metal NP conventionally used in catalysis, and today, the very large majority of catalytic studies addressing the subject are limited to a comparison of the reactivity of isolated atoms with that of metallic particles; even though these two types of species must coexist in many catalysts. Few studies have been reported up to day concerning a possible synergy between metal NP and SA, 456789 and in most of them very few efforts have been devoted to explain the origin of this synergy.

In that context, the objectives of this thesis were to explore new routes for the preparation of carbon-supported low and high loading SAC, and to investigate possible synergy between SA and NP for hydrogenation reactions. This thesis was carried out at the Laboratory of Coordination Chemistry (UPR 8241 CNRS) and involved active collaborations with the Laboratoire de Physique et Chimie des Nano-Objets (LPCNO -UMR 5215) in Toulouse and the Laboratoire de Génie des Procédés Catalytiques (LGPC -UMR 5285) in Lyon.

In chapter 1, we present a comprehensive bibliographic analysis of experimental and theoretical studies on SAC supported on carbon-based materials. Fundamental aspects dealing with the type of carbon materials used for SA deposition are discussed, together with the characterization of the resulting materials. Furthermore, an in-depth analysis was made on the use of carbon-supported SAC in a wide range of reactions, including thermal, photo-and electro-catalysis, trying to elucidate the metal-support interactions in SAC, and how it impacts the catalytic performances.

Chapter 2 describes an efficient method to produce high loading SAC. The main objective was to produce Ru fullerides consisting in a polymeric structure interspersing Ru atoms and fullerenes (C60) as monomers. We have also evaluated the possibility to introduce in a controlled way more metal, so as to produce novel architectures combining Ru clusters and C60. We studied the influence of solvent, temperature and Ru/C60 ratio on the morphology of the resulting products. These compounds were exhaustively characterized by WAXS, EXAFS, or XPS which, in combination with theoretical calculations such as DFT, allow the nature of the structures to be more precisely elucidated. In an innovative way, catalysis was used as a tool to discriminate the nature of the metallic species present in these structures.

In Chapter 3 a simple and reproducible method for preparing low loading SAC on carbon supports was proposed. First, we try to create and identify by magnetic measurements and EPR analyses the adsorption sites necessary to stabilize supported SA. Then, the dispersion of the active phase on the modified support was carried out. Trying to evaluate the scope of the method, different parameters were evaluated such as: solvent, impregnation time, precursor type, activation temperature, type of support and nature of the metal. Atomic resolution microscopy was used to characterize the resulting materials.

Finally, we attempted to develop a protocol for depositing both NP and SA on the carbon support, and for controlling the NP/SA ratio.

In chapter 4, we assessed the catalytic performances of the above-mentioned catalysts for the hydrogenation of different alkenes (terpenes such as myrcene, squalene and farnesene). Besides, exhaustive theoretical studies over a simpler model molecule were carried out in order to understand the catalytic hydrogenation and isomerization mechanisms. Factors such metal dispersion, concentration of oxygen surface groups and defects on the support were taken into account to evaluate the possibility of hydrogen spillover in these systems. We attempted to understand the cooperative effect between SA and NP, and how the control of the ratio between these two species allows optimizing the catalytic system design to produce ultra-active catalysts.

Chapter 5 provides a general insight of the performance of SAC for the thermocatalytic hydrogenation of carbon dioxide under methanation conditions. Six different metals were tested (Ru, Pt, Pd, Ni, Co, Cu), and a comparison of different catalysts was made, including samples containing only SA and other series with mixtures of SA and NP. Furthermore, SAC doping with alkali was investigated in order to evaluate its effect on the catalytic performances. Finally, prospects for further studies and obtaining optimized catalysts with better performances will be suggested. Heterogeneous catalysis on supported metals consists in multi-step surface reactions, in which most of the time, the underlying atoms of the active phase play no direct role in the reaction. The presence of SA with higher reactivity at the surface of this active phase has been proposed in 1925 by H.S. Taylor, in the case of Ni hydrogenation catalysts (Figure 1.1). 1 Somorjai and others have shown that surface defects such as steps and kinks on many metals may dramatically influence the reaction by 'poisoning' it or by acting as the catalytically active centers. 2 As the active phase is often made up of noble metals, expensive and for the most part identified as critical elements, a logical step in heterogeneous catalysis is to try to optimize their dispersion on the support. The maximum dispersion that can be achieved involves the use of isolated atoms on the support; however, their stability is usually not sufficient and therefore their more stable particle aggregation cannot be avoided, except if stabilization is accomplished by an environment not prone to their surface migration. From ref. [1].

In 1947, V. Haensel and co-workers at Universal Oil Products followed such a strategy for the development of a reforming Pt/Al2O3 catalyst. 4 In order to make such a process, involving inexpensive reagents and products and an active phase such as platinum, economically viable, it was imperative to minimize the amount of metal by maximizing its dispersion on the alumina support. The Pt content of this catalyst was 0.1 % w/w. The first commercial unit, at the Old Dutch Refinery in Muskegon Michigan, started to operate in November 1949. 5 This may be the first example of an industrial application of a bifunctional SAC, combining both isolated metal atoms and acidic functions. Seventy years later, the group of G. Hutchings and researchers from Johnson Matthey followed a similar strategy to replace the conventional Hg/C acetylene hydrochlorination catalyst by a Au/C catalyst. 6 They set as a target a Au loading < 0.25 % w/w, since loadings higher than this were considered not to be economically viable. In situ X-ray absorption fine structure spectroscopy experiments (EXAFS) have definitively shown that the active catalyst predominantly comprises Au(I) isolated cationic species. 7 In these two industrial catalysts, a strong metal-support interaction plays an essential role in preventing aggregation, creating stable and finely dispersed SAC. These two examples show that the concept of single atom catalysis, patented by an US company in 2010, 8 is attractive, particularly in the context of sustainable technologies that will make use of critical metals, which are expensive and of limited abundance. In this context, the use of SAC is particularly attractive because it limits the use of precious metals; but it is of course also necessary that it achieves optimum performance in terms of stability, activity and selectivity. Figure 1.2 shows that different types of heterogeneous catalysts can meet the definition of SAC: i) a supported single metal-atom catalyst containing only isolated single atoms embedded on a support, in this chapter we will call SAC this type of catalyst;

ii) site-isolated atoms incorporated in the framework of a material (i. e. in zeolite, metalorganic frameworks (MOF), or aluminum phosphate molecular sieves (ALPOs)); and iii) a supported homogeneous catalyst or site-isolated heterogeneous catalyst 9 that contains a coordination complex (that may contain one or several metallic atoms) anchored on a support. A generic term to name this set of catalysts was introduced by J.M. Thomas in 2005, "single site heterogeneous catalysts". 10 In this chapter, we will concentrate only on SAC dispersed on carbon materials. The readers interested by supported homogeneous catalysts on carbon materials are invited to refer to literature reviews. 11,12 The borderline between SAC and supported homogeneous catalysts is tenuous. Indeed, during catalysis, a SAC may become a supported homogeneous catalyst, due to chemisorption of reactants that will become ligands during the reaction; this phenomenon can be accompanied by a partial detachment of the metal from the support. [ 13 ]); and (c) a supported homogeneous catalyst, oxo-bridged zirconocenium coordination on a µ 3 -O alumina surface site. From ref. [14].

On the other side, Samantaray et al. have shown many years ago that a supported homogeneous catalyst can lead, after ligand removal, to SAC; 15 and the definition of single-site heterogeneous catalyst given by Zecchina et al. is the following: "The usual definition of single-site heterogeneous catalyst is a catalyst constituted by a metal atom, ion, or small cluster of atoms, held by surface ligands to a rigid framework". 16 It is thus interesting to notice that there is now more than forty years that the community of homogeneous catalysis and coordination chemistry, which strongly contribute to supported homogeneous catalyst, and heterogeneous catalysis, currently involved in SAC, try to meet; 17 and the day they do it, do not realize it! Up to now, supported homogeneous catalysts have been limited to few industrial applications, for example oligo-and polymerization or oxidation reactions. 18 This is mainly due to the fact that the synthetic approach used to prepare them was by molecular chemistry involving the use of coordination complexes, and the main goal, most of the times, is to maintain a coordination sphere around the metal as close as possible to the homogeneous catalyst. The result is that the linkage between the active phase and the support can be weak (often a single coordination, as shown on Figure 1.2c). SAC, on the other side can be very robust: Nie et al. have shown that a thermally stable catalyst, atomically dispersed Pt 2+ on CeO2, can become active for CO oxidation at 150 °C after steam treatment at 750 °C. 19 This is linked to the fact that chemists from solid state chemistry used a different synthetic approach, in which the catalyst precursor should be completely decomposed to create an as strong as possible interface with the support (multi-coordination, as shown in Figure 1.2a,b). We will see in this chapter that carbon materials can provide such an environment for SAC.

Since the beginning of the century, SAC have attracted a great deal of research interest, and many reviews have already been devoted to this subject. 20,21,22,23,24 The recent developments of characterization techniques, modelling tools, and the progresses in surface coordination chemistry have allowed significant advances in that field. It is plausible that SAC are operating in many catalytic reactions; however, in the past, the lack of powerful characterization tools such as aberration-corrected transmission electron microscopy, was detrimental to their identification. 25 Thus, relatively few studies dealing with SAC have been published in the last century. 26 They often rely on EXAFS studies;

for example Sachtler and co-workers have investigated in 1982 the structure of cobalt-porphyrin catalysts supported on active carbon, an active oxygen-reduction electrocatalysts. 27 Their works show that, after a heat treatment (700 °C under nitrogen), the carbon atoms originally present in the supported homogeneous catalyst have been displaced, although the CoN4 group is retained in the heat-treated material. It is also interesting to note that the first report on the visibility of SA by high-resolution scanning electron microscopy dated from 1970: it was about uranium atoms on a thin carbon film. 28 Carbon, due to its low atomic weight, is indeed an interesting element for visualizing metallic atoms by electron microscopy observations. Ten years later, direct observation of the behavior of heavy SA (U, Pt, Au, In, Cd, Ag, and Pd) on amorphous carbon substrates was reported. 29 In this study it was stated that "the motion of heavy atoms on amorphous carbon substrates is found to depend on the atom species and the local substrate structure". DFT calculations performed more than twenty five years later have

shown that transition metals (TM) such as V, Cr, Mn, Fe, Co, Mo and Ru have strong adsorption energy and strong migration barrier energy on graphene (G), whereas for Cu, Ag, and Au binding energies are very small and the diffusion barrier energies are also expected to be very small. 30 Electron microscopy as well as DFT calculations have also

shown that these mobile atoms can be trapped by mono-and di-vacancies in graphene. 31 Additionally, the first report, in 1991, on single atom detection by EELS was also about uranium atoms on carbon. 32 In 2015, the visualization of reacting PtSA deposited on carbon support in chemical reactions was reported. 33 All these progresses in surface science and modelling allow today an easier characterization of SA on carbon materials.

We have organized this chapter to have a complete overview of SAC on carbon materials.

We will first discuss the types of carbon materials for single atom deposition and the structure of the adsorption sites. Then, we will briefly present the main SAC characterization techniques, and present the preparation of these catalysts. Finally, we will discuss the performances of these catalysts.

Types of carbon materials for single atom deposition and nature of the adsorption sites

As stated in the introduction, the preparation of a SAC requires specific sites on the support that will act as traps for the metallic atoms. In that context, a perfect sp 2 carbon surface do not appear convenient to obtain stable SAC, even if for some metals on graphene, DFT calculations have shown strong adsorption energy, high diffusion barrier and significant charge transfer. 34 Most of the time however, the adsorption energy of metal adatoms on defect-free graphitic-like materials (0.2 to 1.55 eV) 35 are lower than the metal-metal bonding energies in supported NP (> 1.55 eV); 36 consequently, the formation of metal NP is favored over SAC. The presence of defects in the carbon material, such as vacancies will significantly increase the adsorption energy of metal adatoms. 37 The creation of defects is therefore a key step in the preparation of SAC. It can be achieved by physical or chemical methods, and will allow the anchoring of isolated atoms because of the potential existing around these centers. 38 The use of sp 2+δ materials presenting curved surface allows significantly higher binding energies, particularly with fullerenes, for which binding energy ranging between 1.25 and 3.1 eV have been reported for TM adatoms. 39 Consequently, the production of metal fullerides, in which single metallic atoms are bonded to C60 is possible, even if, in some cases, they constitute the kinetic product of the reaction. 40 Nitrogen or boron incorporation in carbon materials can efficiently enhance the binding energy of TM metal adatoms. 41 Two-dimensional porous carbon nitride sheet have attracted research interest because CN monolayers naturally come with regular pattern defect sites, which make it suitable for easy trapping of various atoms in a well-defined environment. 42 1.2.1. Graphitic carbon nitride and related structures Graphitic carbon nitride (g-C3N4) is a N-doped carbon material that exists both in 2D and 3D structures. It can be produced by polymerization of cyanamide, dicyandiamide or melamine precursors. The choice of the operating conditions of the reaction makes it possible to obtain, in a controlled manner, materials having different levels of condensation and, in fine, different properties. 43 According to this work, cyanamide-based precursors can polycondense into dicyandiamide, melamine, melam, dimelem, melon, melon sheet and g-C3N4. Polymeric g-C3N4 becomes unstable at above 600 °C. The first highly organized polymeric structure (melon) of composition C3N4 that is formed has pendant groups of amine type. Condensation continuing, less defective structures based on triazine or heptazine (2,5,8-triaminotri-s-triazine, or melem) elementary units are obtained. Although g-C3N4 is a promising material, its structure, which is believed to possess graphite like layered structure, is still under investigation. The two architectural units triazine and heptazine serve as a basis for obtaining allotropes of g-C3N4, which present different properties, particularly in terms of stability, because of the different environment of the nitrogen atom in these materials (Figure 1.3a-b). DFT calculations have shown that the polyheptazine-based material is 30 kJ.mol -1 more stable than the triazine one. 44 Therefore, it is recognized as the building block for graphitic carbon nitride in many recent works. 45 Structural information can be obtained through crystallographic methods;

XRD peaks at 2θ = 12.8° to 13.6° and the main peak at 26.8-27.6° are typical of this material. They correspond to the (100) and (002) planes, respectively; with intra-layer and inter-layer d-spacing of 0.680 and of 0.326 nm, respectively (Figure 1.3c). 46 Carbon and nitrogen atoms are sp 2 hybridized. The most reactive atoms in these materials are the nitrogen atoms connected to two carbon atoms. Efficient syntheses of: monolayer g-C3N4

2D nanosheet, 47 sub-microspheres of g-C3N4 (surface area of 89 m 2 g -1 ), 48 g-C3N4 mesoporous spheres (surface area of 550 m 2 g -1 ), 49 micro-rods (surface area of 15-60 m 2 g -1 ), 50 mesoporous g-C3N4 (surface area > 150 m 2 g -1 ), 51 have been reported, highlighting the versatility of this material.

Interestingly, Antonietti et al., have reported that in the case of metallic NP on g-C3N4 charge transfer occurs between the metal NP and the semiconductor, altering the electronic properties of both the metal and the semiconductor and in fine the catalytic performances. 52 Thus, the equilibration of the Fermi-levels makes in many cases the metal more electron rich and reductive, while the semiconductor become electron deficient. The possibility to tune the band gap of this material to extend visible light absorption through the introduction of point defects has been investigated. For example, through thermal heating, cyano groups as well as nitrogen vacancies could be introduced into the framework of g-C3N4. 53 Foreign element atoms in g-C3N4 can occupy either substitutional or interstitial sites. Eleven adsorption sites are possible in the heptazine-based structure:

i) five substitutional atomic sites labeled N'1, N'2, N'3, C'1 and C'2; ii) two interstitial sites I1-2 at the centers of the 6-membered ring and heptazine unit, respectively; and iii) four bonding sites marked as B1-4 bridging the N1-C1, C1-N2, N2-C2 and C2-N3 bonds (Figure 1.4). 54 The site I2 has the largest binding energy for metal atoms. Other forms of graphitic carbon nitride, although not so in-depth characterized, have been reported.

Graphene-like carbon nitride (g-CN) nanotubes have been produced from C3N3Cl3 and Na by a simple solvothermal method. 55 This material shows, as graphene, a honeycomblike structure but with six-membered carbon-nitrogen rings (see Figure 1.5a). 56 The size of the cavities is significantly different for triazine-based g-C3N4 (C6N6 cavity), heptazine-based g-C3N4 (C9N9 cavity) and g-CN (C12N6 cavity). This could have an effect on the adsorption of TM. The adsorption of 3d TM on g-CN (C12N6 cavity), g-C3N4 (C6N6 cavity) and g-C3N4 (C9N9 cavity) has been investigated by DFT. 57 It was found that 3d TM stand in the cavities of g-CN sheet while keeping the planar structure of the material (Figure 1.5b and 1.5c, for vanadium and iron atoms). Color code: nitrogen blue; carbon grey; vanadium violet; and iron orange. From ref. [57].

For C9N9 and C6N6 cavities, the planarity increases with the decrease in the metal's atomic number. 58 The calculated metal binding energies for selected 3d TM atoms to triazine-, heptazine-based g-C3N4 and g-CN are reported in Figure 1.6. From the energetics, it can be observed that in most cases, the binding energies are relatively strong, but some of them are smaller than the corresponding metal cohesive energies. For similar metal atoms, the corresponding binding energies to N4 type cavities were reported to be relatively higher (see next section, Figure 1.16b). 59 This can be attributed to the larger size of the N6 or N9 cavities, where the metal is bonding strongly to the closest nitrogen atoms, with remaining nitrogen atoms located somewhat far from it. Charge transfer from metal d-states to the π* states of the support have been evidenced

using Bader charge density. As a result of this charge transfer, the occupied anti-bonding states are found to be stabilized and shifted to lower energies. This result highlights the different behavior of SA related to metal NP, for which the charge transfer occurs from the support to the metal. 52 1.2.2. Graphene and carbon nanotubes These two materials can be composed of a single graphene layer or of few to multilayers.

Single layer graphene and single-walled carbon nanotubes (SWCNT) have a strong tendency to stack due to high surface energy, and constitute relatively expensive materials. Consequently, few layer graphene (FLG) and multi-walled carbon nanotubes (MWCNT) are generally preferred as supports for SAC synthesis. These two materials can be produced by catalytic chemical vapor deposition in high yield, and present relatively large surface area. 60 One considering adsorption on defect free pure sp 2 carbon materials (like FLG), the adsorption may basically occur on two types of surfaces: Even though this correspondence with organic chemistry species (carbene and carbyne) is not very rigorous, the carbon community generally accepts it. The basal plane surface of graphene or CNT is homogeneous and consists in sp 2 C atoms. Except for very small diameter CNT, the curvature of the tubes does not significantly affect the hybridization of these carbon atoms. On the contrary, the edges (prismatic surface) are very heterogeneous and may contain many surface groups, often oxygenated, and here also similarities with organic chemistry can be made: carbonyl, carboxylic acid, phenol… It is important to note that the reactivity of a surface group, even if it approaches that of organic species, is often different due to the presence of the surface, or to the porosity of the material. The ratio basal surface/prismatic surface is an important parameter when we consider adsorption; it can be estimated from NMR 61 or gas adsorption data. 62 The possible adsorption sites on graphene basal plane are top, T-(on top of a carbon atom); bridge, B-(bridging a C-C bond); and hollow, H6-(at the center of hexagons) sites (Figure 1.7b). 63 The nature and strength of the adsorption is correlated to carbon hybridization.

For small diameter CNT, strain is characterized by pyramidalization and misalignment of 61 the π-orbital. 64 Adsorption of an atom or a functional group onto the CNT external surface will release the strain. For classical MWCNT (external diameter around 10 nm) this phenomenon is negligible (very low pyramidalization angle and π-orbital misalignment), and the chemical reactivity approach that of planar graphite.

The convex outer surface of small diameter CNT is chemically more reactive than the concave (inner) surface, since pyramidalized carbon atom arrangement is optimum to form chemical bonds with reactants. Distortions of CNT will accentuate this phenomenon, suggesting that local reactivity can be significantly enhanced. Thus, tube diameter, but also CNT helicity, 65 another parameter that dictated the CNT electronic properties, are expected to affect the adsorption properties of carbon nanotubes. The CNT tips can be open or closed, according to chemical treatments. Although this surface is low compared to the total surface area of the material if CNT have not been cut, it is expected that it will react differently with single atoms, due the presence of reactive pentagons, and edges. Durgun and co-workers carried out a DFT study on the adsorption of 3d TM atoms onto zigzag and armchair SWCNT. 66 The H6 site was the more reactive for all 3d TM atoms, except nickel and copper atoms, which preferred the B adsorption site. Frapper and co-workers used DFT and molecular orbital analysis to investigate the trends in the adsorption of 3d TM atoms onto graphene and CNT surfaces (Figure 1.8). 34 It is obvious from this figure that the reactivity of the two surfaces for metal atom adsorption is very similar, despite the curvature of SWCNT. On both graphene and SWCNT it is the H6 sites the most reactive for metal adsorption. The metallic atoms bound mainly covalently, and as expected, the ionicity degrees varied with the electronegativity of the metal atom, with a charge transfer from the metal to the support. With the exception of Cr, Mn, and Cu, the other 3d TM atoms were chemisorbed with an η 6 hollow geometry (Eb = 1.09-1.74 eV). It is the strong hybridization between the dx2-y2 and dyz orbitals of the 3d-TM atoms and the pz orbitals of the carbon atoms that constitutes the main contribution to the covalent bonds. 67 Cr and Mn with half-filled atomic orbitals and Cu with its completely filled atomic orbital are only physisorbed on these surfaces. The adsorption binding energy increased as one moved in either direction away from the copper-manganese couple of the series. Sc, Ti, Fe, Co and nickel could be relatively stable as single-atoms on these carbon surfaces. For the other metals an easy surface diffusion and aggregation into particles can be anticipated. We can see however that, as expected, the metal adsorption binding energy on these defect-free surfaces are significantly lower than the ones calculated for graphitic carbon nitride. Systematic DFT studies performed for 3d, 4d, and 5d TM atom adsorption onto the graphene surface has confirmed that TM prefer to stand on the H6 sites when they are chemisorbed, but on B or T sites when they are physisorbed; which is the case of atoms with half-filled atomic orbitals (d 5 ) and filled atomic orbitals (d 10 ). 68 The trends for diffusion energy barriers follow the adsorption energy trends as shown on Figure 1.9. 69 Interestingly, in the case of platinum or aluminum atom adsorption on CNT, a significant curvature effect was reported. For Pt, 70 the adsorption energy was stronger on the outer wall than on the inner ones, and this adsorption energy increases when the tube diameter decreases. [69].

The contribution of the edge sites (A1, A2, Z1 and Z2, Figure 1.7b) to TM atom stabilization has been much less studied, although it is an important aspect of SAC stability. The reactivity of the edge sites of graphene and FLG towards 3d TM atom adsorption (Au, Cr, and Al) has been investigated. 71 The contribution of graphene sublayers to the total binding energy was predicted to be significant, although the edge binding energies were found to be substantially higher for all atoms in all cases (Figure 1.10a), and the defect-free regions of the lattice were completely free of single atoms (Figure 1.10b). The activation barriers of adatom migration have been calculated and were found to be small for all adatoms studied, implying high surface mobility. This suggests that these metal adatoms will migrate rapidly onto the graphene surface until they find a stable site, such as the A1-2 and particularly Z1-2 sites, for their binding to the support. The authors confirmed this hypothesis in the case of gold deposition on graphene.

Scanning transmission electron microscopy micrographs (Figure 1.10a) showed that Au atoms are located exclusively at edge sites, but also in some places contaminated by hydrocarbons, whereas the defect-free regions do not present any metal adatoms. It is thus obvious that the preparation of SAC with these elements on pristine graphene (or CNT) will be limited to very low loading (considering the basal plane/prismatic surface ratio), and that the SA will concentrate on edges. A similar tendency, i. e.; a preferred adsorption on edges than on pristine graphene, but also than on vacancies has been reported for iron. 31 Anomalous diffusion at graphene edges was reported both experimentally and theoretically. 72 It is finally worth mentioning that graphene edges in nanostructured carbon materials produced by chemical vapor deposition (often in the presence of dihydrogen either added in the gas phase or arising from hydrocarbon decomposition) should be H-terminated. In that case, the adsorption energy of the TM atom to a H-terminated site will decrease significantly. Thus, for an Fe atom, the binding energy to graphene was measured at 0.96 eV, and at 1.35 eV on a H-terminated zigzag edge site. 73 This latter value has to be compared to the binding energy of an iron atom to graphene zigzag edge site (5.77 eV). 72 In order to modify the chemical reactivity of graphene, defects or doping element can be introduced either during its synthesis or by post-treatment. In many materials as in graphene, crystalline defects constitute a very common type of defects. 74 Point defects such as Stone-Wales defect (SW), single vacancy (SV) and multiple vacancies, as well as 1D defects or grain boundaries (that could lead to nanolines of transition metals) 75 are the most common reactive sites (Figure 1.11 and 1.12). These 0D or 1D defects can strongly modify the local electronic structure of the sp 2 carbon materials, and thus significantly change their adsorption properties.

Stone-Wales defect (SW), (c) single vacancy (SV), (d) double vacancies (DV), (e) edge defect with pentagon ring at zigzag edge, octagon and fused pentagon carbon rings line defect with (f) odd number of octagon rings (GLD-558-01) and (g) even number of octagon rings (GLD-558-02), and pentagon-heptagon pairs line defects with (h) odd number of heptagon rings (GLD-57-01) and (i) even number of heptagon rings (GLD-57-02);

The larger grey and smaller white balls denote carbon and hydrogen atoms, respectively. From ref. [74].

Among the 0D defects, double vacancy (DV), produced for example by coalescence of two SV are suitable for metal adatom adsorptions. Indeed, adatom adsorption on these sites leads to thermodynamically more favorable situations. In addition, these 0D defects are stable up to relatively high temperatures. The DV (Figure 1.12b) can reorganize upon 73 (5-8-5); (e) DV(555-777); and (f) DV(5555-6-7777). From ref. [77].

Modelling studies based on DFT of TM atoms adsorbed on graphene vacancies have demonstrated that metal atoms show a high affinity to these adsorption sites compared to defect-free graphene surface. 78 The typical atomic configurations of TM-SV/DV complexes are shown in Figure 1.13, together with the example for an iron atom. 79 As the atomic radii of TM are larger than that of carbon, the metal adatoms are usually located above the graphene plane; this phenomenon is more pronounced on SV (Figure 1.13a) than on DV (Figure 1.13b). The binding energies for such complexes ranged between 2 and 8 eV, according to the systems, showing a strong bonding and pointing to the possible use of such species as SAC. The effect of vacancy on metal binding is, in fact, very similar to that observed for pyridine-like nitrogen doped graphene consisting in one carbon vacancy with three neighboring carbon atoms replaced by nitrogen. 80 (f,h) DFT optimized atomic models corresponding to (e,g). From ref. [79].

Binding energies of selected 3d TM adsorbed on defect-free graphene as well as on SV are shown on Figure 1.14. The binding energies of 3d TM adatoms placed on the H6 site of a defect-free graphene sheet ranged between 1 and 1.4 eV, which is much lower than the corresponding cohesive energies of the 3d TM atoms: this should favor clustering.

The much higher binding energies obtained in the case of a SV should allow SAC stabilization and limit the clustering on the surface. Finally, the adsorption of all of the elements of the periodic table up to an atomic number of 86 at the SV of a graphene sheet has also been analyzed by DFT. 81 For the large majority of the elements, strong binding energies (2-13 eV) with the SV site were found. Physisorptions were seen only and as expected for the elements of groups with filled atomic orbitals (d 10 and p 6 ) such as the noble gases. A correlation between the cohesive energies of the pure elements and their adsorption energies on the SV was observed. Indeed, these two values can be correlated to the tendency of these atoms to dissolve from the graphene SV. Beside these intrinsic structural defects, substitutional defects, such as heteroatoms introduced by doping are also attractive for TM immobilization on graphene or CNT surfaces.

For SAC preparation, nitrogen doping has been particularly studied due to the availability of a wide range of nitrogen-doped carbons, and to both Brønsted and Lewis basicity of these materials caused by nitrogen-doping. 82 As shown in Figure 1.15, it exists three different ways to bond a nitrogen atom in a graphene layer: pyridinic N, graphitic N, or pyrrolic N. Thermodynamically, the N pyrrolic doping is less stable compared to the doping of N graphitic and N pyridinic. The bonding orbitals in the graphitic N, and pyrrolic N groups have higher p orbital contributions than the pyridinic N groups. Twofold coordinated pyridinic N atoms can be present either in vacancies (N0 type) or at the edges (N1 type) of graphene layers. In such structures, the C-N bond length is between 1.35-1.38 Å, and it has been shown that these groups, and particularly the N0 type, show strong Brønsted and Lewis basicity. 83 N pyrrolic atoms are incorporated into 5-membered heterocyclic rings; the C-N bond length is between 1.42-1.44 Å. Three-fold coordinated graphitic nitrogen (or quaternary nitrogen) refers to N atom that substitute a carbon atom in the graphene layer. In these structures, the nitrogen has the lowest basicity. Thus, the N-doping contribute to the tuning of the local density of state around the Fermi level of the material. This may play a pivotal role in tuning the electronic and adsorption properties. In fact, DFT calculations have clearly demonstrate that CNT or FLG N-doping not only allows a strong interaction between metal adatoms and the support, but also impact the electronic structures of the immobilized atoms, which provide a promising approach for optimizing SAC. 85 The structural stability of 3d TM adatoms embedded in N-doped graphene with N graphitic, N pyridinic and N pyrrolic atoms, has been investigated by DFT calculations. 86,87 The TM adatoms forming a 4 N-centered structure (TM-N4V2, Figure 1.16a) shows the strongest binding, with binding energies of more than 7 eV (Figure 1.16b).

Graphene oxide

Graphene oxide (GO) is a complex material, and elucidating its structure is crucial to achieve control over its use as SA support. GO is a nonstoichiometric material with a lamellar structure, and its physico-chemical properties depend critically on synthesis procedures and post-synthesis treatments. GO is usually prepared by exfoliation of graphite with strong oxidants under acidic conditions. This highly exothermic (and potentially dangerous) reaction results in the formation of functional groups such as alcohols, epoxides, and carboxylic acids (Figure 1.17a). [91].

The oxidation level varied with the method of preparation, graphite precursor and reaction conditions. Different GO structural models have been proposed in the literature. 88 A common point of these models is the presence hydroxyl (-OH) and 1,2-ethers (epoxy) groups as basic functional groups on both sides of the GO sheet (Figure 1.17b). Even if the O/C ratio depends much on the oxidation processes, a C/O ratio of 1.5 fits well with many commercial products. 89 It has been found that the epoxide groups stay close to the -OH, but on the opposite sides of the graphene layer. 88 The reduction of GO is particularly studied since it can contribute to the restoration of graphene structure and properties. Different reduction procedures exist, resulting in different reduced GO (r-GO) presenting different properties, which in turn affect the final performance of r-GO. 90 3D molecular models of GO and r-GO and DFT calculations have shown that the removal of the -OH and -COOH groups by a reductive treatment restores a planar structure (Figure 1.17c). 91 There are relatively few theoretical studies dealing with adsorption of metals (or other elements) on GO or r-GO. 92 On GO supports the binding energy reported are higher than the one obtained on defect-free graphene. For example, for a single titanium atom, the binding energy are: 1.38 eV for Ti on graphene, 2.65 eV for Ti on a Stone Wales defect,

Fullerenes

In metallofullerenes the metallic atoms can be either trapped inside (endohedral metallofullerenes) 97 or bound outside (exohedral metallofullerenes) 98 the fullerene cage, without destruction of the cage. The introduction of the metal atom changes the electronic properties of C60 fullerene, with the work function of the material being higher than that of the pure C60. These metallofullerenes show a high chemical reactivity because of their much smaller HOMO-LUMO gaps. Stable substitutionally doped fullerenes or heterofullerenes constitute another type of compounds, in which one of the carbon atoms from the cage has been replaced by a doping atom. 99 These kinds of compounds can be seen as SAC, and have already found potential applications in catalysis. 100 In spite of a high symmetry, C60 fullerene presents different adsorption sites for single atom adsorption (Figure 1

Six different coordination modes have been reported:

• the η 1 hapticity with suspected σ bond, in which the metal atom is directly bounded above any carbon atom; • the η 2 hapticity; involving either two 6-membered rings (6,6) or one 6membered ring and one 5-membered ring (6,5), the η 2 (6,5) being not common; • the η 3 hapticity, in which the metal linked to three carbon atoms on either a 6-or 5-membered rings; • the η 4 hapticity, for which the metal atom linked to four carbon atoms;

• the η 5 hapticity, in which the metal is directly bounded to a 5-membered ring; and • the η 6 hapticity, in which the metal is directly bounded to a 6-membered ring.

Even if some examples of other hapticities exist, the great majority of exohedral metallofullerenes generally present η 2 , η 5 or η 6 hapticities. The fullerene family can be classified in the group of the non-planar conjugated systems. In C60, and due to the curvature, the hybridization at each carbon atom is not pure sp 2 but sp 2.3 . As hybridization depends on curvature, the hybridization in fullerene is not fixed but changes depending on the fullerene diameter, and thus on the number of carbon atoms in the molecule. Few data are available on the adsorption energies of TM on C60, and some of them differ largely for a single metal (see the case of Ni on Figure 1.20, which shows the adsorption energy of various TM on C60). 102 For Mn1@C60 η 6 hapticity leads to the most stable situation, whereas for CoSA@C60 both η 6 and η 5 hapticity coexist. For PdSA@C60 the η 2 hapticity was preferred. The structural and electronic properties of exohedral DFT was used to study the adsorption of 3d TM on C60 fullerene. 104 For Sc, Ti, Co, Ni, Cu and Zn the η 2 (6,6) hapticity lead to the more stable structure; while the η 6 hapticity was preferred for V, Cr, Mn, and Fe. The stronger adsorption energy was for Ni (5.11 eV), followed by Co (3.40 eV), Cr (3.15 eV). For the other elements, the order is: Ti (3.12 eV) > V (2.61 eV) > Cu (2.52 eV) > Mn (2.34 eV) > Sc (2.15 eV) > Fe (1.58 eV) > Zn (1.53 eV). In these structures, the positive charges on the transition metal atoms and the negative charges on the C atoms engaged in the TM-C bond ensure charge transfer from the TM to the C60 fullerene. For Ti, it was shown that if two atoms are present, they will prefer to cluster than to stay isolated on the C60 cage. 105 DFT was also used to study the adsorption of 3d TM (Ti, Cr, Fe, Ni, and Zn) on a N4 cavity doped C60 fullerene (C54N4). 106 All the adsorption energies are very large as compared to the corresponding TM metal cohesive energies so that the problem of TM aggregation is expected to be overcome, and the TM@C54N4 should result in stable SAC. The carbon bonds that connected aromatic C6 rings in graphdiyne are triple and single bonds that form an acetylene link. Thus, GY is made up of C6 rings and acetylene bridges, and GDY consists of C6 rings and butadiyne bridges. 107 The potential application of such material (particularly GDY) in catalysis has already been reported. 108 Their use as support for SAC could be of interest because of their structure presenting cavities and a welldefined environment that can host metal SA. Some theoretical works have reported the interaction of single TM atoms absorbed on GY or GDY. 109,110,111 Figures 1.21c,d display the most favorable adsorption site for six 3d TM (V, Cr, Mn, Fe, Co and Ni) on both GY and GDY. 112 In the case of iron, the adsorption energy was calculated to be -4.52 eV, whereas it was -5.26 eV in the case of Co and Ni decorated systems.113 These binding energies are significantly higher than the one obtained on graphene, and higher than the cohesive energies of the corresponding metals.

Material perspectives

It is important to note that most of the results presented in the previous sections are relying on DFT calculations. Standard functionals in DFT poorly capture dispersive interactions, which participate in long-range interactions and can influence the bonding character.

Additionally, many studies are dealing with single layer materials (for example graphene), and few studies are available for multi-layered materials. In the case of graphene, few-layer graphene and highly oriented pyrolytic graphite, some differences may exist in the adsorption energy values of TM. 114 It is thus important to take the given data as a guideline of TM adsorption. This overview of the different carbon (nano)materials that can be used for SA stabilization allows us to draw some conclusions concerning the rational choice of a carbon support for SA stabilization.

The use of defect free sp 2 carbon materials do not allow to stabilize SA sufficiently, since the TM-carbon interactions are weak compared to the cohesive energy of the bulk metal or of metal NP. The use of C60-fullerene, thanks to pyramidalization and π-orbital misalignment, allows reaching higher adsorption energy for single metal atoms. The presence of crystalline defects or heteroatoms in sp 2 carbon materials allows reaching high adsorption energy for metal adatoms. In order to have a more precise view of TM adsorption on carbon materials, we have concentrated our analysis on three elements presenting low, medium and high adsorption energy on defect free graphene: Au, Fe, and Pt, respectively. The Figure 1.22 shows the adsorption energy of these three elements on the different types of adsorption sites potentially present in these materials. This figure has been drawn from data collected in more than 50 scientific publications. In the case of defect-free graphene, the variations in adsorption energy observed for a given metal are linked to the use of different functionals or to the size of the models. The mean adsorption energy is 0.45 eV for gold, 1.75 eV for platinum, and 1.05 eV for iron. If we compare this value to the cohesive energy of the metal for a nanoparticle of 5 nm (dotted lines in the figures), it is obvious that SAC should not be obtained on this surface, particularly if we considered the low diffusion energy of these metals on defect-free graphene (0.028 eV for Au,115 0.14 eV for Pt,116 and 0.8 eV for Fe 35 ). The presence of defects or edges on the carbon surface allows to significantly increase the adsorption energy for single TM atoms.

Vacancies, either SV, DV or even triple vacancies (TV) are particularly reactive. Indeed, when one or multiple carbon atoms are removed from the graphene network, the created dangling bonds represent areas of greater chemical reactivity. For the three metals investigated, the presence of SV and/or DV is enough to stabilize SA, since the adsorption energy for single TM atoms is significantly higher than the cohesive energy117 of the TM in NP of 5 nm. However, the presence of such vacancies in the carbon materials is questionable. Indeed, they have most of the time been prepared in the presence of hydrogen, which should lead to H-terminated vacancies of much lower (if any) reactivity.118 A dissociation barrier of 1.1 eV has been reported for H2 on the SV of graphite. 119 A similar reactivity is expected to occur at graphene/graphite edges, since a dissociation barrier of 0.5 eV has been reported for H2 on armchair type edges of graphite,120 and the reaction on zigzag type edges is barrierless. 121 The atomic hydrogen adsorption energies are larger on the vacancy than on the edges. Production process of non-functionalized graphene edges have been reported, but they involved specific growth condition, such as sputtering in vacuum. 122 If produced, these highly reactive sites will allow the preparation of stable SAC. A competition between SA and hydrogen to saturate graphene vacancies can also occur during post-synthesis treatments, like reduction or during catalysis in the presence of H2. 123 If produced, these highly reactive sites are also able to easily react with small atmospheric molecules such as water or oxygen; in that case the vacancies (or edges) will be O-terminated. 124 Water and oxygen dissociate on SV with respective barriers of 1.6 and 0.2 eV, respectively. 119 The interaction of SA with O-terminated edges or vacancies could be relatively high, depending on the metal and on the type of oxygen functional groups created. Nevertheless, the force of the TM-O bond created is lower than the M-C bond in a vacancy. 125 As the incorporation of heteroelement such as oxygen, nitrogen, boron, 126 sulphur 127 or phosphorous 128 in the graphitic network can potentially improve single TM atom adsorption, the use of doped materials has been particularly studied; in particular with nitrogen containing materials such as g-C3N4. In the case of nitrogen doped carbon materials, different adsorption sites are available, which present significantly different affinity for SA. 129 This explain the differences in binding energy observed in Figure 1.22 for G-N samples. Similarly, oxidized carbon materials presented a large variety of surface oxygenated groups that will present different reactivity towards SA. The works of G. Hutchings and researchers from Johnson Matthey have shown that stable Au/C SAC can be prepared on activated carbons, a support that usually present sounds amount of oxygenated groups. 6 As one of the potential advantages of SAC is the possibility to create a well-defined chemical environment in a robust heterogeneous catalyst, the use of a support presenting different anchoring sites could be detrimental. That's why the use of carbon materials with a better-defined surface chemistry, as in g-C3N4 or graphdiyne is particularly attractive. For the other materials, we have to rely on the fact that the more reactive sites will be the ones that will stabilize SA preferentially. This poses the problem of the quantification of these sites. The use of high surface area support, with a high degree of functionalization should be preferred to reach high metal loadings in SAC.

Finally, it is important to note that in SAC the electronic metal-support interaction is of fundamental importance to establish correlations between the electronic properties of catalytic sites and catalytic activity. 130 In that context, the possibility to tune the electronic properties of carbon materials via doping is particularly appealing.

Single atom catalyst characterization

By definition, SAC only contain isolated single atoms dispersed on a support. Their characterization should ideally combine modeling studies and physicochemical/structural characterizations. From the modelling side, the reduction of the number of atoms is interesting from the practical point of view, since modelling will become less time consuming and simpler that with metallic nanoparticles. On the other side, the experimental confirmation of the existence of only isolated single metal atoms on a carbon support, and the determination of their chemical environment and spatial distribution is critical to develop SAC.

The most convincing and intuitive strategy is to directly visualize the SAC immobilized on supports. For this purpose, the use of scanning tunneling electron microscopy (STEM) 131 and aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) in combination with single-atom X-ray spectroscopy (atomic-resolution energy dispersive X-Ray spectroscopy (EDX) mapping) 132 provide a direct approach to precisely locate the isolated metal atoms in SAC. This can provide direct local structural information about the metallic species present on the support. Carbon materials are particularly interesting for these analyses since carbon is a light element, and the metal atoms are easily visualized. On the other side, carbon poorly scatters high energy electrons, and this limits the resolution obtained for carbon materials. Additionally, the electron beam may destroy the structure of the carbon support. A chemical analysis of individual metal atoms on high-surface-area supports is not easily accessible by EDX or electron energy loss spectroscopy (EELS). 133 Obtaining high resolution elemental mapping depends on the progress of the highly focused electron probe in a specific frame, on the collection of a significant number of characteristic Xrays over time, and on the absence of damage to the sample. However, EDX analyses can be used to address the nature of groups of individual atoms, as shown on Figure 1.23 that shows the STEM-HAADF image and the EDX analysis of a 15 %Co/CNT Fischer-Tropsch catalyst that has been doped with 0.1 % w/w of PtSA. A finer characterization to determine the environment close to the isolated atom, the number and type of bonds as well as the degree of oxidation of the metal atom can be performed through X-Ray absorption near-edge spectroscopy (XANES) and EXAFS in combination with modelling. 134 Such an approach has been followed for the characterization of RuSA@C60 SAC. 40 This technique however can be difficult to apply for some carbon materials that contain magnetic impurities from their synthesis. 140 13 C SS-NMR studies of carbon materials involves the use of cross polarization, to enhance the polarization of the unabundant 13 C nuclei through their interaction with 1 H nuclei, combined to magic angle spinning to limit peak broadening due to the chemical shift anisotropy. 141,142 Infrared spectroscopy represents one of the most important characterization technique in catalysis research that allows the direct monitoring of the interaction between probe molecules and the surfaces of the catalyst. 143 There also, the use of this technique with carbon materials is not trivial. 144 Indeed, spectra of carbon materials are difficult to obtain, mainly because of poor transmission, and uneven light scattering related to large particle size. 145 In principle, X-ray photoelectron spectroscopy (XPS) 146 analyses could provide valuable interaction on charge transfer in SAC. 147 These characterization methods, if performed in situ (EXAFS, IR)148 or under environment conditions (STEM) are also extremely useful to obtain insight into the nature of the catalysis with SAC.

Preparation of single atom catalysts

It is accepted that the application of all new materials is limited by the capability of their reproducible and large-scale production. In the case of SAC, the difficulties are obvious, and the inability to produce isolated atoms on carbon materials on a large scale is a problem yet. It is therefore essential to develop methods that allow the systematic preparation of heterogeneous catalysts containing isolated and well characterized active sites. This still remains a challenge, due to process low production efficiency (high cost precursors, high cost preparation devices, particle formation, etc.). As we previously mentioned, this section will be especially focused on the preparation of single catalytic active-sites over carbon materials. In order to have a broader view on the choice of catalytic supports (oxide, alloy or carbons), the reader is encouraged to refer to literature dealing with chemical methods for the preparation of SAC. 149 1.4.1. Low loading SAC (<1% w/w) Without any doubt, low loading metallic SA supported on carbon materials has far the advantage over the number of articles, owing almost 90 % of the total references to date.

There are plenty of methods to synthesize SAC, however all of them have a common characteristic: maximizing the metal-support interactions in order to generate single atoms as stable as possible, along with fighting against the natural tendency of the atoms to aggregate either during the synthesis processes or during the subsequent treatments.

From a general point of view, we have seen certain tendencies relative to the methods of insertion of SA into the carbon material.

Low loading catalysts have been typically prepared through "post-synthetic methods" i. e., the addition of the active phase is carried out on synthetized supports, and not during the synthesis of the support. As will be seen later, another approach is also possible, where the metallic atoms are integrated at the same time than the production of the carbon network. This latter method is preferred for obtaining higher metallic loadings (>1%). We will discuss below methods that have been regularly used to produce low loading carbon based-SAC. nanosheets150 and N-doped graphene nanosheets151 using the precursor (methylcyclopenta-dienyl) trimethylplatinum [MeCpPtMe3] and O2 as reactive gas.

After 50 cycles SA were obtain but also sub-nanometer clusters with a 1.52 wt% Pt. The line defects are considered to be either wrinkles or grain boundaries, points that act as nucleation sites for nanoparticle formation. These results suggest that a controlled number of cycles should be performed to obtain exclusively SA sites. Thus, after just one [Pd(hfac)2]-formalin ALD cycle on g-C3N4 (Pd1/g-C3N4), a Pd loading of 0.5 wt% is obtained. Aberration-corrected HAADF-STEM observations have shown that a high density of atomically dispersed Pd atoms was obtained on this support;154 neither PdNP nor subnanometer clusters were observed at low or high magnifications. The six-fold cavities on g-C3N4 are likely the anchoring sites for Pd atoms. When ten cycles are performed, 6.0 ± 1.2 nm PdNP are formed. In this case, the contribution of impurities, such as the oxygen-functional groups introduced during g-C3N4 synthesis can also promote the nucleation of these nanoparticles. Another approach is to use those functional groups to carefully control the formation of Pd species during ALD. 155 Graphene oxide powder was precisely heated to control the type and concentration of oxygenated surface functional groups. After annealing under helium at 1050 °C for 2 min, XPS analyses revealed that the dominant remnants groups on the graphene support were the phenolic groups. Next, Pd ALD was performed to reach a 0.25 wt% Pd loading, and to form exclusively randomly dispersed Pd atoms. If prolonged annealing is performed (10 min), a large decrease in the amount of phenolic groups is observed, so the Pd loading turned negligible (0.01 %). Additionally, when GO was annealed at lower temperatures (700 °C), the Pd loading became higher (1.7 %), and a considerable fraction of PdNP was produced. Comparable techniques, such as chemical or physical vapor deposition, cannot always provide such a level of uniformity, conformality and thickness control at the Angstrom level. It seems that ALD should be an efficient technique to achieve atomically dispersed catalysts. However, in some cases nucleation delay or island growth during the beginning of the film deposition are observed, inducing severe deviations from an ideal germination. Additionally, this method has some disadvantages such as: i) the lack of industrial reactors, ii) the relatively high price of the metal precursors as well as their reactivity/availability, iii) the high cost of the operation, and iv) the fact that traditionally, the process of ALD is very slow and this is known to be its major limitation.

Wet chemistry methods

In wet-chemistry also, since the precursor materials already contain single metal atom, the goal is to anchor those atoms onto the supports and to obtain a tough anchoring. A classical wet chemistry technique implies: i) introduction of the metal precursor on the support by different methods such as impregnation/ion-exchange, co-precipitation, or deposition-precipitation, ii) a drying step, and iii) a calcination and/or reduction step. It is important to stress that every step should be finely controlled to obtain reproducible procedures, and to avoid the production of metallic NP together with SA. An important factor to consider for this method as for the others is that carbon supports have a chemically inert surface in comparison with other supports such as oxides; so as already stated in the previous section, the odds to lead NP are much higher. Hence, a strong metalsupport interaction should be obtained in order to avoid aggregation of SA on the surface.

In practice, it is relatively easy to produce SAC by reducing the metal loading to an extremely low level, since the concentration of efficient anchoring sites should be extremely low in most of the carbon materials. Considering this principle, usually these wet chemistry methods are associated with a low metal loading to maintain high dispersion. Even with the simplest protocol it has been possible to prepare SAC. 156 In that case, the chosen support is firstly simply dispersed in deionized water together with an ionic metallic precursor. When the impregnation step is finished, the solvent is evaporated to proceed with a soft reduction under H2. Pd/CNF 157 was synthesized with a 0.04-0.5 wt% metal loading using this approach. The analyses indicate that the palladium was found to exist as SA just in the samples with concentration of 0. In order to increase the amount of supported metal, functionalization of carbon materials with different heteroatoms such as O, N, P or S can be performed. Thus, Fe/P-doped-gC3N4 have been synthetized using the same process as above mentioned but obtaining loadings up to 1 wt%. 167 Occasionally, techniques such as incipient wetness impregnation168 have also been used to generate atomically dispersed metallic species containing almost 1 wt% of active phase on functionalized carbons. Other examples of low loading SAC prepared by wet impregnation are given in Table 1.1. This approach can be considered part of the wet-impregnation classification because it comprises as a first step an impregnation but requires an extra step at high-temperature (>500 °C) or pyrolysis treatment to obtain the final catalyst. The objective of the high-temperature treatment is the anchoring of the metal and the introduction of the dopant (usually nitrogen) within the graphitic network. The doping is achieved by the incorporation of a nitrogen source as a solid such as urea169 or as a gas like NH3170 over the metal impregnated support during the annealing. For this preparation, GO171,172 has been recurrently selected as a support (see Figure 1.28 for a Co catalyst prepared on nitrogendoped GO).173 The high density of defects in GO and the large surface area of this negatively charged support, provide optimum characteristics for N-doping and uniform adsorption of a monolayer of metal cations for SA deposition. In that case, the density of the active sites could be maximized on the 2D surface for higher catalytic activity. Noble and non-noble single metal atoms have been embedded into porous carbon supports and stabilized by strong binding to nitrogen with this procedure. Liu and co-workers 174 studied the effect of N-doping on carbon black for the formation of platinum species. While the examination by STEM of multiple regions of the sample reveals that only individual Pt atoms were present in Pt1@N-CB; in the absence of nitrogen, PtNP were observed at the same Pt loading (0.4 wt %). It was shown that the nitrogen doping can enhance the interaction between the metallic atom and the carbon support, but also that it can promote metal-support electron transfer. Atomically dispersed Fe-Nx species anchored on nitrogen and sulfur co-doped carbon layers were obtained by coating CNT with 2,2bipyridine, potassium isothiocyanate, and an Fe precursor, followed by pyrolysis at 900 °C under N2, and acid leaching to dissolve undesired iron sulfide particles (Figure 1.29).175 TEM images of S,N-Fe/N/C-CNT show a 3D organization comprising a great number of carbon layers grown on entangled CNT. Notably, no NP can be found, showing a 0.8 % Fe atomic content, but in samples prepared without addition of SCN -salts (denoted N-Fe/N/C-CNT) few crystalline phases appear associated with Fe-based carbides.

Other methods

After impregnation, deposition of Ni and Fe over GDY was conducted by a facile electrochemical reduction method (Ni 0.28 wt%; Fe 0.68 wt%). 176 The presence of stable atoms over GDY surface was explained in terms of electronic interaction. GDY exists with strong electronegative sp-hybridized orbitals that, in the presence of a positively charged metal, allow the C(sp)-M(d) orbital overlapping. A three-step method based on pyrrole polymerization on oxidized CNT has been used to immobilize single Fe atoms and to produce a FeSA-N-PC/CNT SAC (Figure 1.30). 177 During this process and thanks to the spatial isolation effect of zinc, many pores were generated and a high concentration of Fe-Nx species was obtained instead of iron NP. Quantitative elemental analysis performed by XPS gives 0.3 at % of Fe, providing an outstanding activity, selectivity but also stability in the oxygen reduction reaction. Finally, an unexpected method of obtaining SA comprises the redispersion of noble metal nanoparticles (Pd, Pt, Au) deposed over ZIF-8 composites. Through in-situ environmental TEM was evidenced a dynamic process were sintering an atomization are competing at 900 ºC under inert atmosphere achieving 0.16, 0.51 and 0.18 wt% for Pd, Pt and Au, respectively. 178 Molecular dynamic simulations reveal the mechanism associated with the transformation of a metal NP into an array of stable SAs on a carbon surface. This study shows the pivotal role of defects in the carbon surface in trapping and stabilizing SA. 179 NP should perform Brownian-like random walk on the surface (above 1200 ºC) and when there are no defects, isolated atoms can collide and remain as NP. Contrarily, on a defective support free SA gets immobilized into the defect avoiding to merge back to NP.

From these findings, it appears that methods involving high temperature thermal posttreatments would be promising for the synthesis of low loading SAC. or direct approaches such as pyrolysis, template-sacrificial approach, and copolymerization. In these methods, the deposition of the active phase is carried out during the synthesis of the support, the final inclusion of the metal happens in the same process of formation of the graphitic network, which is generally achieved by pyrolysis of the carbon source. Although the manufacture of catalysts by this technique has been quite profitable for lab-scale preparation, it must be considered that the small control in the final metal loading and the fact that a large amount of metal is lost during the acid etching represent major disadvantages for upscaling. Additionally, the proportion of atoms on the surface and embedded in the carbon structure is difficult to control, and it is difficult to claim that all the atoms are located on the surface of the support, and therefore active in catalysis. PANI provided a rich nitrogen source to introduce the N-dopant into carbon matrix and to anchor an individual iron atom for the formation of a Fe-N4 site. In accordance with the data the pyrolyzation temperature is a fundamental parameter to control during the synthesis. The content of N and Fe is changed as well as the number of defects when temperature is increased from 700 to 800 and 900 ºC. The optimal results were found on FeSA/NCS-700 had 0.89 at % metal loading and highest total N content (8.54 at %) relating pyridinic N-species with Fe content. Based on DFT calculations the presence of these nitrogen groups can have synergistically enhanced the catalytic activity. A similar approach was used for the formation of atomically dispersed TM (Ni, Co, NiCo, CoFe, and NiPt) on N-doped CNT. 187 The N-doped CNT were produced through the catalytic decomposition of a dicyandiamide metal (Ni, Co) complex. wt%, which is the highest value reported to date. Based on XRD analyses, the authors proposed that the individual atoms are incorporated in the carbon layers rather than confined between the carbon layers.

1.4.2.2.

Template-sacrificial method.

By using a template, it is also possible to modulate the textural characteristics of the desired catalyst. The template-sacrificial approach involves the addition of a textural agent or "template" that confers greater surface and porosity, which is thereafter eliminated by an etching process using acids or bases. Figure 1. 33 190 shows three different template materials (SiO2 particles, ordered mesoporous silica SBA-15, and montmorillonite clay (MMT)) that have been used for the synthesis of CoSA@N-C and FeSA@N-C SAC. Vitamin B12 (VB12) and polyaniline-Fe (PANI-Fe) were selected as cobalt and iron precursors, respectively. The high nitrogen/carbon atomic ratios of these precursors allow, after pyrolysis, the production of N-doped carbon materials with high nitrogen content (9.3-9.5 at% N from XPS for the Co SAC). As shown on Figure 1.33, different microstructures were obtained according to the geometric characteristics of the original templates. For example, a lamellar structure was produced after pyrolysis with MMT. Although these samples contain a relatively high cobalt loading (1.3-1.4 at%), TEM observation did not show the presence of any metal NP after the etching step with HF that dissolves both the inorganic templates and the metal or metal oxide NP. There

are other examples of works using silica NP as template, from which worm-like porous materials were produced;204 but under appropriated conditions it was also possible to keep the spherical structure after template removal. 205,206 Iron oxide nanocubes self-assembled in cubic superlattices and stabilized by oleic acid ligands were also used as sacrificial template (Figure 1.34). In that process, the superlattices were obtained by a drying-mediated assembly of Fe3O4 nanocubes. After a calcination step at 500 °C under argon, the carbonized sample was treated 3-5 times with HCl until no iron was detected by ICP in the supernatant. In a final step, N-doping was realized by a heat treatment under ammonia at 900 °C. This latter treatment also contributes to the graphitization of the material, and to the partial reduction of Fe 3+ into Fe 2+ species. XANES analyses have shown that the coordination of iron was also affected by this treatment, from Fe-C coordination before NH3 treatment to Fe-N coordination after treatment. Although it is difficult to control the iron loading with this method, FeSA were systematically observed by STEM-HAADF (Figure 1.34)

for samples showing a 1-2 wt % iron loading.

Polymerization method

This method consists in a one-step reaction for network-doping during the g-C3N4 synthesis involving the use of a metallic precursor as a monomer. Silver tricyanomethanide with cyanamide (Figure 1.35) 194 were used as starting molecules to produce AgSA/g-C3N4 ranging from 1 to 10 wt% of silver. After the polymerization, carried out at 550 °C no Ag NP were observed. If silica nanosphere template was used during the synthesis, a reverse replication of the template was obtained after etching. This is an encouraging strategy to produce well-organized and dispersed SAC. Furthermore, the introduction of negative charges in the structure of carbon nitrides can be beneficial since it contributes to a change in the bandgap of this material. 195 Alternatively, thermal polymerization at 550 ºC of melamine-Cu complex and cyanuric acid produces porous Another report using this method indicates the use of inorganic metal salts (NiCl2•6H2O, FeCl3•6H2O) and dicyandiamide (DCDA) to produce the corresponding complexes for later growing Ni and Fe-doped g-C3N4 nano-patches around CNT skeletons, where isolated Ni and Fe atoms were embedded into the tri-s-triazine units of g-C3N4 (Fig. 1.36). 208 The metal contents in this sample were found to be 2.0 wt% Ni and 2.6 wt% Fe.

The reason to use CNT as a carbon-based skeleton is that: i) g-C3N4 can be immobilized in situ; ii) it benefits from charge transfer, iii) there is the suppression of the stacking between g-C3N4 layers, and iv) there is an improvement of the surface area. There are a couple of isolated cases, where other pathways are used to successfully achieve the deposition of high quantities of single atoms. Among them, we can find surface grafting. For example, the functionalization of the support with thiol groups, which are known for their high affinity for precious metals, allows the production of PtSA on thiolated CNT. 197,209 A fine control of the deposition rate by controlling temperature and pH has allowed the deposition of subnanometer patches of amorphous Co hydroxides on N-doped CNT. 196 Transition metal fullerides, in which each metallic atom acts as a binder between C60 fullerenes were produced by decomposition of coordination complexes of Co, Ru and Rh in the presence of C60. 210,211 This method is interesting because it is simple and allows obtaining high metal loading. High-energy ball milling was used to obtain in large quantity Fe-doped graphene nanosheets by reacting a nitrogencontaining iron precursor and graphene during 20 hours with steel balls. 294 The evaporation of niobium atoms from bulk phase by arc-discharge during the simultaneous decomposition of methane has also been used. 212 The formed black smoke that was deposited on the cool walls of the reactor was mainly composed of onion-like structures, containing 13.7 % w/w of Nb and 86.3 % of carbon. However, annular dark-field images unambiguously indicate that the single niobium atoms are prone to migrate from the carbon matrix and aggregated into niobium carbide clusters, reducing the dispersion.

Nanoporous graphene with nickel single-atoms was prepared by dealloying Ni30Mn70 alloy sheets with (NH4)2SO4 and further chemical vapor deposition using benzene for graphene growth. 213 The content of Ni (4-8 at. % by EDX) as dopants was controlled by the dissolution time of the alloy.

Catalysis with single atom catalysts on carbon-based materials

Without doubt, catalysis on isolated atoms has become one of the most promising areas for new discoveries in heterogeneous catalysis. 18,214 Indeed, in addition to saving metal, these isolated atoms often have a specific reactivity different from that obtained on metal surfaces. This can lead to an acceleration of the reaction rate and therefore to new reactivities, both in terms of activity and selectivity. In addition, as we saw in the previous section, their modes of preparation sometimes lead to entities anchored very firmly on the support, which gives them a great stability, which, too, can be exploited in catalysis. This last point distinguishes them particularly from supported homogeneous catalyst, which often lead to leaching, greatly limiting their industrial use. Finally, the fact that theoretical studies can be carried out easily should lead to a good understanding of these systems, and their optimization be simplified compared to supported catalysts based on metal particles. This set of advantages makes that SAC deposited on carbon materials have already been used in a large number of catalytic reactions in thermal-, electro-or photocatalysis. present an intrinsic activity close to that of Pt. 225 In that context, the combination of these nitrogen-sites with metal single atoms can boost the catalytic activity. Indeed, it has been shown that Co1-Nx/C or Fe1-Nx/C catalysts exhibit an electrochemical activity greater than the nitrogen-doped supports alone. 226 Theoretical evaluations validate those results confirming that the high electrochemical activity originates from the precise M-N coordination environment. 227 Even more, a report affirms that ORR does take place at a faster rate on a single atom than over NP. To prove that, a catalyst containing NP and SA, Co-NP/HNCS (HNCS=hollow N-doped carbon sphere) with 2.2 % w/w Co was evaluated in ORR in acidic 0.5 M H2SO4 media. 228 With the passage of time the catalytic activity E1/2 increased significantly by 32 mV due to the dissolution of CoNP in the acidic medium, suggesting that now, the exposed dispersed cobalt atoms (Figure 1.37) were responsible for the improved activity. After 5000 and 10000 cycles, the ORR activity of the catalyst remains constant, outlining the remarkable stability of this catalyst. The electrochemical performances of this system (E1/2 = 0.773 V vs reversible hydrogen electrode (RHE)), approaches the one of a commercial Pt/C catalyst (0.790 V).

Additionally, to have more evidence of the role of the SA the authors decide to use poisoning agents to block the metallic atom and thus limit their reactivity. Indeed, it is known that cyanate and thiocyanate anions coordinate strongly to metals, and consequently can passivate active sites. Following this principle, it has been shown that the introduction of CN -ions on FeSA/N-doped C or CoSA/N-doped C catalysts was detrimental to their performances in ORR, demonstrating the activity of these SAC. 193,220 Theoretical calculations have confirmed that FeN4 sites present in nitrogen-doped carbon materials can act as very efficient sites for oxygen adsorption and O=O bond cleavage, with performances reaching the ones of platinum. 229,230 Important achievements have also been obtained in terms of selectivity. Taking into account the mechanism of the reduction of oxygen, the reaction can proceed involving either four-electron reduction (4e -) from O2 to H2O (O2 + 4H + + 4e -→ 2H2O) or two-electron reduction (2e -) of O2 to H2O2 (O2 + 2H + + 2e -→ H2O2).231 A direct 4e -pathway is highly preferred in order to get high efficiency in fuel cell operations. ORR polarization curves at different rotation rates can be measured to determine whether the reaction occurs by a 4-or 2-electron pathway. By using the Koutecky-Levich equation, the electron-transfer number (η) can be calculated.

SAC have demonstrated an excellent selectivity, obtaining values of η > 3.5, pointing to the preferential path of 4e -and production of H2O. To validate this result, it is also possible to perform measurements to detect the generation of reactive intermediate species or hydrogen peroxide. 232 For SA supported on carbon, values below 5 % of conversion toward H2O2 were systematically reported. 233,234,235 In contrast, we can find examples of catalyst based on other carbon supports, particularly carbides, which are able to change the selectivity towards the formation of hydrogen peroxide, as for Pt/TiC 236 or Pt/S-doped-C. 185 The long-term stability of these systems, which is essential for commercial application of these catalysts, was often assessed during the amperometric measurements on ORR. SAC display a remarkable resistance and stability, and in the majority of the cases better than the benchmark Pt/C catalyst. The resistance of SAC to methanol crossover in a direct methanol fuel cell (DMFC) was also evidenced for Ru 222 and Fe 237 SAC. It is worth noting that when the mass activity is normalized by the metal loading, in 100 % of the cases the SAC show a better result in comparison with commercial Pt/C, which contains usually between 10 and 20 wt% of Pt, indicating that the SAC could notably increase the utilization of metal, provided higher metal loadings are achieved. 238 Besides the above-mentioned application SA bifunctional catalysts, capable of assist ORR and oxygen evolution reaction (OER), play a major role in the construction of zincair batteries. 239 Exploring efficient (and low cost) materials is of a paramount importance to promote the practical applications. To this effect, SAC over graphitic carbon-based porous materials have shown great potential due to their high specific surface area and good electroconductivity. Several publications have shown high catalytical performance of M-N-C sites for ORR and OER. For instance, Co-N-C, 240,241 Cu-N-C, 242 Mn-N-C 243 and Mo-N-C 244 deliver a charging-discharging gap of above 0.86 V, which is superior to that of a benchmark Pt-Ir/C catalyst (0.775 V) and 20 wt % Pt/C (0.83 V).

Hydrogen and oxygen evolution reactions

Hydrogen (HER) and oxygen (OER) evolution reactions are key reactions in various electrochemical systems. 245 For these reactions, as for ORR, it has been evidenced that the catalytic activity of SAC is highly correlated with the local environment of the metal center, namely its coordination number and electronegativity of the nearest neighbor atoms. 246 This result points the necessity to precisely control the synthesis of SAC. The HER is important in electrocatalysis as well as for the concept of a "hydrogen economy".

Indeed, hydrogen evolution from water electrolysis is one of the efficient ways to obtain clean hydrogen energy in the future. 247 Among the many electrocatalysts, multiple investigations 248 have demonstrated the effectiveness of isolated Pt sites, coordinated either directly by carbon atoms but mainly by nitrogen, as responsible for the catalytic performances, allowing reaching the best activity.

As example, platinum atoms can be immobilized on GDY by coordination with the alkynyl carbon atoms of the support (Figure 1.38). 249 These SAC show exceptionally high catalytic activity for HER, with a mass activity up to 26.9 higher than a state-of-the-art commercial Pt/C catalyst. This excellent behavior in HER was related to the optimal Gibbs free energy for hydrogen adsorption ∆𝐺 𝐻 * 𝑃𝑡 (0.092 eV) over an individual Pt active site anchored on GDY with a C2-Pt-Cl2 coordination environment (Figure 1.38), which is close to that on the surface of Pt metal (∆𝐺 𝐻 * 𝑃𝑡 ≈ -0.09 eV). A novel approach for stabilization of Pt atoms (0.53 % w/w) was carried out using the pyrolysis of Ptimpregnated mesostructured polydopamine particles to produce porous carbon matrix. [249].

Based on experiments and DFT calculation, it was proposed that the isolated Pt centers can activate the adjacent C/N atoms, making these non-metal atoms electrocatalytically active for HER. Another study proposed that due to the strong electron transfer from the Pt to the adjacent coordinated N, the unoccupied Pt 5d orbitals can interact with H 1s orbitals to form an hydride. 251 This allows not only to increase the number of chemically bonded H atoms, but also to decrease the adsorption energy of H2.

Other studies have proposed that atomic clusters (dimers, trimers, etc.) exist in many SAC, exhibiting superb electrocatalytic activity. Such atomic clusters deposited on defective graphene (Ptn-DG) 252 and N-doped graphene/g-C3N4 (Ptn-NG/C) 253 showed excellent HER performances. These catalyst with metal loading of 2 wt% show an activity 2.5 times higher than Pt/C at a overpotential of 50 mV (36.9 and 14.8 mA/cm 2 , respectively when current is normalized with the geometric surface area of the electrode) and 21 times higher than that of a 20 wt% Pt/C reference catalyst when normalization is made with the Pt loading.

In spite of these results, the high cost and scarcity of platinum largely limit its wide application. Therefore, it is significant to develop non-precious metal catalysts with high activity, good retention, low cost and environmental benignancy to replace Pt in both ORR and HER. In an effort to produce cheap, efficient and durable alternatives, mainly

Co, Ni and Fe SAC have been investigated. Fei et al. reported a low cost and direct approach for distributing cobalt atoms onto nitrogen-doped graphene (CoSA/NG) for HER. 173 This catalyst is robust and highly active in aqueous media with very low overpotentials (30 mV). Deep investigation and electrochemical testing confirm that the catalytically active centers are isolated cobalt atoms coordinated to nitrogen (MN4C4 species). These MN4C4 species were also predicted by DFT to be efficient catalysts for the OER with activities following the trend Ni > Co > Fe, which was confirmed by electrochemical measurements. 254 DFT calculations have also shown that single-atom transition metal (Pt, Pd, Co, Ni, Cu)

supported on g-C3N4 should be high-performance OER catalysts. 255 Finally, it is important to note that the overall water splitting reaction involves anodic OER and cathodic HER. Therefore, the ultimate goal for SAC would be to generate a unique bifunctional electrocatalyst capable to simultaneously promote both HER and OER.258,259,260 1.5.

Photocatalysis

Searching for highly efficient photocatalysts has become a strategic domain all over the world, due to their promising applications for energy conversion (particularly HER) and environment (photo-oxidation reactions). However, an important issue is that some of these catalysts are seriously mismatched with the solar spectrum. Their large band gap restricts their use to the UV-light. To face this this problem, g-C3N4 is particularly attractive, due to it moderate band gap (2.7 eV) that allows absorbing light up to 450 nm.

In spite of that, g-C3N4 suffers from a low quantum efficiency originated from its slow charge mobility and high recombination rate of photo-generated electron-hole pairs. The introduction of single-atom noble metals in the cavities of g-C3N4 provides an interesting strategy to modulate the electronic structure. Indeed, metal and non-metal atom doping are one of the most effective and convenient way to improve the separation rate of photogenerated electrons and holes. Au, 261 Fe 262 and Pt 263 single-atoms were embedded in g-C3N4 and used as a co-catalyst in photocatalytic HER. It was recently shown that N vacancies in g-C3N4 are efficient for the reactive metal-support interaction (RMSI). This allows not only improving the stability of supported PtSA thanks to strong Pt-C bonds, but also enhancing the performances of Pt SAC for capturing electron and evolving H2 thanks to electronic and geometric effects. 264 Pt SAC largely enhanced the hydrogen production with performances 9 times higher than that of PtNP, and 50 times higher than that of the bare support. Theoretical as well as experimental works have shown that Pt and Pd atoms trapped between g-C3N4 layers act as bridges that facilitate the charge carrier transfer between the layers (interlayer charge transfer), 265 exhibiting superior optical absorption. 266 In general, the participation of metallic impurities imposes additional binding functions, which lowers the band gap to enhance the absorption of visible light. 267 Figure 1.39 268 display the absorbance of the metal-containing samples in the visible region. In all cases the absorption of light was higher than that of pure g-C3N4.

Environmental remediation, through the degradation of organic pollutants such as dyes, hormones and medicines from the water have been studied over single atom photocatalyst. Co-N4/carbon quantum dots 269 was used for the degradation of organic dyes and Cu/C3N4 270 for degradation of norfloxacin antibiotic. Chen et al. prepared a set of different catalysts using Fe, Cu, Ti, Ni and Zn, getting the best results in the photooxidation of benzene to phenol with Fe/g-C3N4. 267 Another drawback of g-C3N4 is its low specific surface area, usually around 10 m 2 /g.271

To overcome this inconvenient, the same research group expanded the surface area of the catalyst by coating mesoporous silica (SBA-15) with nanometer thin Fe/g-C3N4 layers. 272 Table 1.3 shows the results obtained for phenol production from benzene oxidation using H2O2 and visible light. The TOF are markedly increased in the systems 0.2 % Fe/g-C3N4/SBA-15 compared with the system without SBA-15, either after exposition to radiation or not. This improvement is associated to the obtained high exposed structure of Fe/g-C3N4, and also to the homogeneous atomic Fe-coating achieved. It should be noted that Fe loadings higher than 0.1 % w/w lead to a decrease of the specific surface area, and consequently to lower reaction TOF. Finally, it can be seen that in the absence of g-C3N4, the Fe/SAB-15 SAC is practically inactive; this demonstrating the synergic effect between the metal and the carbon support. Among plenty of variables, the photooxidation activity can also be enhanced by the choice of the oxidants, such as H2O2, persulfate (PS) and peroxymonosulfate (PMS). The activation of the oxidant to produce highly reactive species occurs thanks to the electrons photogenerated on the catalyst surface. In this context, AgSA immobilized on a mesoporous g-C3N4 were produced and used as catalyst to degrade bisphenol A in the presence of PMS. 194 A positive effect of Ag content increment on the band gap energies was measured. The band gap decreases from 2.50 to 2.09 eV from 1 % to 10 % w/w Ag1/g-C3N4, respectively.

Table 1.3. Catalytic activities for phenol production from benzene oxidation by g-C3N4 based catalysts with (+) or without(-) visible light (λ > 420 nm).From ref. [272]

. This represents an improvement in the absorption of visible light. At the same time, the removal efficiency of bisphenol A increases from 88 % to 99.4 % with Ag contents from 1 % to 10 %. This improvement, directly associated with the increase of absorption in the visible region, is of course correlated to an increase in PMS activation, and thus to a greater concentration of reactive (SO4-) species generated due to the higher concentration of photogenerated electrons. Silver single-atoms serve as electron traps to capture those electrons from the of g-C3N4 conduction band, which results in a limited chargerecombination process, and further promote the photocatalytic efficiency of the AgSA/g-C3N4 SAC. AgSA/g-C3N4 SAC were also reported to be active for Naproxen photocatalytic degradation. 273 On the other hand, photocatalytic degradation of the Rhodamine B by H2O2 activated with FeSA/g-C3N4 SAC under visible-light (λ > 420 nm) irradiation has also been studied. 274 Although atomic Fe embedded in the g-C3N4 is capable to perform the oxidation of Rhodamine B by itself, the light absorption properties of g-C3N4 can be also manipulated due to the presence of non-metal doping (as phosphorous atoms), and therefore enhance even more the yields of the reaction. Figure A clear red shift of the absorption band can be observed for the P-CN sample, indicating a decrease of the band gap energy. Theoretical calculations have confirmed that the band gap energy dropped rapidly when nitrogen was gradually replaced by phosphorous to form C3N4-nPn compounds. 275 As a consequence, P-CN showed a higher activity than g-C3N4 for Rhodamine B photooxidation (Figure 1.40b). Additionally, the increase of the absorption intensity for iron loaded P-CN samples in visible light region, when increasing the Fe loading, points to a strong interaction between the support and the metal. For 0.5 %Fe/P-CN catalysts, the activities were further improved in relation to 0.5 %Fe/g-C3N4. This Fe doping, in addition to the cooperative effect of P-doping, which also improved the SBET, contribute to a further decrease of the band gap energy and to the limitation of the recombination of photo-generated electron-hole pairs. Another work reports loadings as high as 50 % w/w for Fe on g-C3N4, with the total absence of XRD peaks from iron species, such as Fe-Fe, Fe-O, Fe-Cl, and Fe-C. 274 These results suggest that iron species should be chemically coordinated to the support, most likely in the form of Fe-N bonds. These catalysts are able to activate H2O2 for the direct oxidation of benzene to phenol, and for the oxidative degradation of various organic dyes using H2O2 (e. g., Rhodamine B, methylene blue, methyl orange, and p-hydroxyazobenzene).

Catalyst SBET [m 2 /g] hv C6H6 conv. (%) TOF ×100 [a] [h -1 ] g-C3N4 - - 0 - g-C3N4 - + 0 - 10 % Fe/g-C3N4 8
Finally, DFT calculations were used to elucidate the role of AuSA in the enhancement of the photocatalytic activity of AuSA/GO. 276 It was found that the work function of the gold single atom anchored on GO is lower than the one of GO due to the charge transfer from Au to GO, suggesting an enhancement of surface activity. In this system, the gold atoms act as electron traps and play a mediating role in electron transfer to the GO conduction band. In addition, the photogenerated electron-hole pairs is easily separated, and the lower band gap of the AuSA/GO SAC compared to bare GO will facilitate the light absorption.

CO oxidation

The spectrum of study of CO oxidation has been dominated by metal oxides supports such as CeO2, 19 Al2O3, 277 and CuO,278 among others; in the field of carbon materials there Diffusion energy barriers are large enough to suppose that metal clustering issues will be absent, with the exception of Fe, for which an easy agglomeration of atoms over the graphene could be observed. Of course, the location of the single atom in the graphene layer (H6 site, N-doped site, SV or DV) will impact the course of the reaction, since strong electronic effects are anticipated, as shown in the first part of this review. Another way to predict clustering issues is the comparison between metal binding energy and its cohesive energy.

The binding energy of the atoms on graphene needs to be stronger than the cohesive energy. For instance, for Al-G (Table 1.4, entry 1) cohesive energy of Al-Al (3.39 eV)

is inferior to the binding Al-Cgraph (5.60 eV), in that way the dopant can disperse on graphene quite stably without formation of nanoparticles. The good predicted catalytic performances of this type of SAC can be correlated to the partially occupied d orbital of the metal that is localized close to the Fermi level, due to the metal-support interaction. % selectivity at 20 % conversion for all four molecules tested. In contrast, similar catalysis on PtNP/N-C induced the formation of multiple products, resulting from the cohydrogenation of the secondary functional group. Additionally, other groups have reported good activity for SAC in the hydrogenation of triple bonds. For the reduction of phenylacetylene, Cu atoms anchored over the surface-defective nanodiamond-graphene (Cu/ND@G) 306 and PdSA fully embedded in the inner walls of mesoporous nitrogendoped carbon foam nanospheres (Pd/MPNC) 307 show a higher activity compared with Cu1/nanodiamond, and their Pd NP counterparts, respectively. 308 Interestingly, comparing (Pd, Pt, Au) MNP@ZIF-8 composites with their SA counterparts, PdSA exhibit significantly better activity and selectivity than metal PdNP for semi hydrogenation acetylene. 178 According to DFT calculations, the performance of this SAC was rationalized considering that hydrogenation of ethylene to ethane on PdSA requires two barriers of 0.93 and 1.22 eV to be overcome (only 0.62 and 0.35 eV on PdNP).

Furthermore, in the case of PdSA-N4 site these barriers line above the ethylene level, which is not the case for PdNP. Also, coking resistance on PdSA is due to a geometric effect, since the multiple adjacent adsorption sites required for polymerization of acetylene or ethylene to form green oil (or coke) are not available, rendering fewer opportunities for coke formation than on an extended PdNP surface. Finally, HAADF-STEM images and EXAFS of Pd SAC showed the Pd remained atomically dispersed after catalysis and reducing treatment. Ni was also investigated for this reaction. . 315 This system also allows the selective production (> 99 %) of substituted anilines from functionalized nitroarenes. High performances of Co SAC on N-doped carbon supports were also reported by Zhou et al. 180 The CoSA/N-C SAC showed good performances in terms of activity, selectivity, and stability for the reduction under dihydrogen (3.5 bar) at 110 °C of various nitro compounds. Selectivity higher than 97 % were obtained at complete substrate conversion in water after 1.5 hours of reaction. This reaction can also be performed under milder conditions with nitrobenzene (40 °C, 1 bar H2) with an aniline yield of 98.7 % and a complete conversion. For comparison, a Pd/C catalyst under the same conditions allows also complete conversion but the selectivity to aniline was only 68 %. Pd SAC on r-GO were also reported to be extremely active for nitrophenol hydrogenation. 205 once again highlight the specific reactivity of SA compared to metallic NP. Moreover, this SAC was also active for the transfer hydrogenation reaction of nitrobenzene into aniline.

The performances of a single Pd atom catalyst on graphene were evaluated for the selective hydrogenation of 1,3-butadiene into butenes, and compared to those of different PdNP catalysts. 316 Under mild reaction condition (50 °C), the distribution of butenes (100 % selectivity) at 95 % conversion was 71 % 1-butene (the most desirable product),

22 % trans-2-butene and 6 % cis-2-butene. In order to largely reduce the contributions from the isolated Pd atoms, PdSA/graphene was annealed at 500 °C under Ar for 1 h to form PdNP with an average size of 5.5 nm. As expected, the new catalyst shows a pronounced decrease of the butane selectivity (≈ 75%) and a different product distribution with 40 % 1-butene, 40 % trans-2-butene and 14 % cis-2-butene. The author proposed that, contrarily to PdNP, on which butadiene adsorbed via the two double bonds (di-πadsorption), PdSA favors the adsorption of the substrate via the mono-π-adsorption mode, since the di-π-adsorption usually requires a large ensemble of Pd atoms on the surface.

The adsorption mode on SAC avoids complete hydrogenation of butadiene to butane and favors 1-butene production.

The performances of SAC were evaluated in the semi-hydrogenation of 2-methyl-3butyn-2-ol. 317 Pd SAC was produced on g-C3N4 supports presenting different crystallinity and different environment for the Pd atoms. The obtained results revealed an inverse correlation between the activity and the Pd oxidation degree. These findings further demonstrate the impact of tuning metal-support interaction in SAC by tailoring the electronic properties of the carbon support, where the least oxidized surface Pd 2+ species seems to be more active than the Pd 4+ species. and carbon matrix (12 mA cm -2 ), and also larger than those of most reported CO2 reduction electrocatalysts. Actives sites with a resulting porphyrin-like structure such as Zn-N4, 337 Fe-N4, 338 have been persistently identify as a responsible for good faradic efficiencies to CO up to 80%, while in the case of Co there are some inconsistences. All Co-N2, 339 Co-N4 340 and Co-N5 341 were reported to be the best coordination environment for the production of CO.

The possibility to use SA supported on graphene as CO2 electroreduction catalysts was investigated using DFT calculations. 342 Complete energy profiles allow to predict the formation of methanol on NiSA/G (limiting potential UL= -0.41 V), and particularly on PtSA/G (-0.27 V), and the production of methane on OsSA/G (-0.52 V) and Ru1/G (-0.52

V). For PtSA/G with the Pt atom located in a DV, the energy profile of the reaction is drastically different from on a Pt (211) surface, where energy barriers are much higher, and the product of the reaction is methane. Interestingly, it was shown that with SAC, the conventional linear relation between CO* binding energy and the binding energy of the CHO intermediate, which is well respected for metallic surfaces, strongly deviates from linearity (Figure 1.44).

Figure 1.44 Correlation between EB[*CO] and EB[*CHO] for transition metal (211) surfaces (black) vs. SAC (blue). The conventional scaling relation for M (211) is broken in the case of SAC. From ref. [342].

Charge transfer between the metal and the support, as well as the lack of atomic ensemble are reasonable reasons that can explain these differences of reactivity. According to DFT calculations, it would also be possible to perform this reaction toward the selective formation of formic acid over embedded Ru, 343 Cu 344 and Ti 345 on graphene defects, pointing once again that tuning the coordination environment of single atoms is a fundamental aspect to take into account for the final application of the catalysts. In general, results are promising in CO2 reduction to C1 products. Production of C2+ compounds requires further exploration.

Finally, in an effort to mimic the process of natural photosynthesis, the aqueous CO2 the rate-limiting step. In the case of FeSA/G SAC it was shown by DFT that FeSA/G@SV has lower energy barrier than that on FeSA/G@DV for the cleavage of the first CH bond of methane. 362 Another group has investigated the transformation of methane to methanol not over one but two Pt atoms deposited on GO. 363 Apparently the presence of dimers allows the separate adsorption of CH4 on Pt(1) and O2 on Pt(2), which can favor the conversion. In a deeper insight of the behavior of SAC, the electro-oxidation of methanol has been used to analyze the influence of defects. 364 The calculations revealed that a SV allows stabilization of Pt, and that the PtSA/G@SV shows good tolerance towards CO, compared to PtSA/G or PtSA/G@SW. However, it was also shown that the C-H/O-H cleavage activation energy was also significantly increases on PtSA/G@SV, which should limit the catalytic activity. DFT calculations have also shown that the doping of CNT with nitrogen allows modulating the adsorption of several species involved in the methanol oxidation reaction, such as CH3OH, HCHO and HCOOH, which could be useful to modulate the catalytic performances. Besides, the direct dehydrogenation of propane to produce propylene has been investigated over PtSA supported on N-and B-doped graphene. 365 The use of a N-doped support allows to reach higher reactivity for C-H bond There also, the calculations have shown that it is the strong interaction between Au and a SV of graphene that tunes the energy level of Au d states, and finally the reactivity. In contrast, over TiSA/G@SV, C2H4 ethylene reacts directly with a peroxo ligand previously produced by the O2 adsorption, which lead to ethylene epoxidation. The computational results reveal that the highly electron deficient Ti centers significantly enhance the activation of the adsorbed O2. Without the defective graphene support, Ti atoms are not active for this reaction. SAC have been proposed to be very selective electrocatalysts for the electrochemical reduction of dinitrogen to produce NH3, in that sense that their use should limit very significantly the HER. 372 Various TM were investigated (Mo, Nb, V, Mn, Ti, Sc) both on graphene SV and in the MN4 environment of a N-doped graphene. It was proposed that the main reason of the selectivity was the formation on these SAC of very unstable MSA-H and MSA-NH2 species due to the absence of ensemble effect. The high potential activity of these catalysts originates from a charge transfer from the metal to the support (1.8 e -in the case of titanium) that makes Ti atoms very electrodeficient, which can favor nitrogen activation. Mo atoms supported on N-doped carbon support have also been predicted to be potential catalysts for this reaction. 373 1.

activation

Conclusion

The rapidly emerging field of single-atom catalysis aims at reducing precious metal loading in supported catalysts, and at tuning catalytic reactivity in numerous reactions.

As such, and provided there are stable, SAC present great potential for industrial catalysis in the future. The stabilization of SA on carbon supports is possible if there are sites allowing a strong interaction with the support. These sites may be vacancies, or heteroatoms introduced by doping. Some carbon supports such as g-C3N4 or graphyne and graphdiyne are particularly interesting because they allow a well-controlled environment of SA. Such an environment induces a strong stabilization of the SA, since multi-coordination occurs, as well as marked electronic effects that can significantly affect their reactivity. The use of carbon supports presents the advantage of being able to modulate these electronic interactions, for example by doping the support with specific heteroelements, such as nitrogen, oxygen, boron or sulfur. If some carbon supports have a relatively well-defined surface, which allows a fine control of the environment of the SA, it should be noted that the majority of carbon supports present a very heterogeneous surface. Substantial efforts are therefore necessary in the synthesis and characterization of these supports, to better control the type and amount of surface defects, in order to produce SAC with a control environment. With regard to the preparation of SAC on carbon supports, it appears quite simple to prepare catalysts with low metal loading on most supports using conventional methods of catalyst preparation. Methods that are more efficient are still needed to be developed to obtain SAC with high metal loadings (>1% w/w). Indeed, the methods used so far suffer either not to guarantee a localization of the SA on the surface of the support (atoms may be present in the bulk material); or to use washes with strong acids that are not compatible with the stated desire to reduce the amount of metal precursors in the preparation of these catalysts. Advanced characterization techniques, such as STEM, EXAFS, XANES, DRIFTS, and NMR, along with DFT studies allow most of the time verifying the atomic nature of the catalysts and preferred binding sites. However, unravelling the mechanisms of a SAC under reaction conditions requires a level of resolution that is yet to be demonstrated. The number of catalytic reactions for which SAC on carbon supports are used is rapidly expanding. The absence of atomic ensemble effects and the existence of electronic effects from the support that strongly modifies the electronic structure of a metal atom in SAC significantly affect the adsorption, and definitively induce a different reactivity compared to metallic nanoparticles. 374 Of course, depending on the reaction studied, these marked effects can be either positive or negative, and as for any catalyzed reactions, a reaction/catalyst adequacy must be found.

Chapter 2. High loading single atom catalysts

2. High loading single atom catalysts

Introduction

As reviewed in the previous chapter, extensive research is currently devoted to the reproducible synthesis of supported SA. 1,2,3,4 This is due to the specific physicochemical properties of these species compared to bulk metal but also to nanometric particles, 5 which open the route through applications in many fields such as quantum technologies, sensors, energy, environment, biology or catalysis. 6,7,8 In the last few years, various strategies for dispersing metal SA on supports have emerged. 9 Both of the most used method of preparation of high loading SAC (> 1%), pyrolysis of MOF and template-sacrificial approach, comprises sophisticated synthesis and hightemperature annealing steps, 10,11,12 producing porous carbons with embedded SA species, in which there is no control of the final metal loading, and the proportion of surface metallic atoms is hard to modulate, making very challenging to guarantee practical applications. On the other hand, taking advantage of solution chemistry, it is also possible to prepare atomically precise metal clusters, but mainly with few metals such as gold or silver. 13,14,15,16,17,18,19 The formation of these SA (or atomic clusters) necessarily requires stabilization to prevent aggregation, which would eradicate most of their desirable properties compared with bulk materials of identical composition. Additionally, for many applications these objects have to be integrated into a system, so the preparation of assemblies of subnanometric metal particles linked by strong covalent bonds could solve this problem. Very few strategies have been proposed to prepare such assemblies. One example is Pd-polymer micelles containing 0.7 nm Pd clusters, which have been prepared by ligand exchange from [Pd(PPh3)4]. 20 From these observations, it appears that the development of simple synthetic strategies to produce nanoarchitectures with SA and/or subnanometric particles, with significant metal loading, is particularly appealing and challenging.

Fullerene C60 is an interesting building block to stabilize single metal atoms or NP.

Indeed, the high degree of symmetry, the strong tendency toward polymerization of this molecule, and its coordinating geometries make it an ideal candidate for the construction of well-defined nanostructures. 21 In transition metal fullerides, depending on the amount of metal, the suggested structure could be polymeric with a chain-like -(M1@C60)narrangement of metal SA, or with two-or three-dimensional coordination. 22,23 Palladium, 24,25,26 and more recently ruthenium 27 spherical nanostructures containing SA have been produced. For Pd, it has been proposed that the synthesis of Pd fullerides can produce polymeric structures containing isolated Pd atoms with a stoichiometry between Pd1C60 and Pd4.9C60. 24,28 DFT calculations were used to predict the structure of different types of Pd fullerides. 22 The structures of one- However, a closer inspection of the literature data for the Pd-C60 system shows that these syntheses are sensitive to the experimental conditions and not selective. In the cases in which HRTEM was used to characterize the reaction products, metallic PdNP were also observed, even at low Pd/C60 ratio (Figure 2.2). 26,29 If a Pd3C60 fulleride has indeed been produced and characterized, 25,26 when a Pd/C60 ratio of 3 was used, it was not the only product of the reaction, and also significant amount of NP (5-20 nm) were observed. DFT calculations have suggested that both isolated atoms and weakly bonded metal aggregates may exist in equilibrium. 30 EXAFS analyses have allowed to propose the presence of Pd clusters in Pd3C60, 31 and XRD analyses have shown the presence of metallic Pd, starting from composition Pd2C60 and higher. 25 From these results, it seems that if the kinetic product of the reaction, the -(Pd1C60)-polymer, is rapidly formed, the incorporation of additional Pd in the structure is not straightforward.

We suspect that diffusional limitations should prevail, which may lead to Pd clusters or NP formation on the surface of the -(Pd1C60)-polymer.

From a previous report of our group, in which similar results were obtained in the case of ruthenium, 27 it appears difficult to produce fulleride metal-clusters, and this must be linked to the fact that once the kinetic product of the reaction is formed (the polymeric structures containing SA), the additional metal is deposited on the external surface of the spheres because it cannot diffuse in their porosity. Thus, if C60 fullerene-metal cluster complexes are well known for many years, 32 up to now, C60 metal-cluster fullerides are essentially hypothetical. 33 The work described in this chapter was conducted to try to overcome this difficulty in the case of Ru@C60 hetero-structures by using mixtures of solvents during the synthesis to favor the deposition of ruthenium in the porosity of the -(Ru1@C60)-n materials so as to obtain -(Run@C60)n-metal-cluster fullerides. Thus, we report the straightforward selfassembly synthesis of RuSA and Ru subnanometric particles with C60.

Results

Ru@C60 hetero-structure synthesis and TEM characterization

The Ru@C60 hetero-structures were synthesized by decomposing [Ru(COD)(COT)] in the presence of C60, varying the ratio Ru/C60, under H2 (3 bar) using dichloromethane (D), toluene (T) and methanol (M). The reaction conditions for each sample, the resulting metal loading, and the RuNP size are summarized in Table 2.1. From previous experiences, we observed well-defined spherical objects (30-40 nm, Figure 2.3a) when using pure CH2Cl2 for the synthesis (entry 1, Table 2.1, D-Ru1@C60). On the other hand, nanostructures of undefined shape were produced using pure toluene (entry 4, Table 2.1, T-Ru1@C60, Figure 2.3c). 27 This indicates the pronounced influence of the solvent on the shape of the structures produced. Nonetheless, regardless the solvent, no nanoparticles where detected in materials synthetized with a 1/1 Ru/C60 ratio. We proposed that these structures constitute the kinetic product of the reaction and contain only Ru single atoms Ru1@C60 (Figure 2.3b,d) connected by C60 with a  2 - 6 coordination as previously reported by our group for similar structures produced in dichloromethane. 27 We have attributed this morphology difference to the fact that C60 are much more soluble in toluene (2.80 mg/mL) than in CH2Cl2 (0.26 mg/mL). Indeed, it is known that selfassembly mainly based on specific amphiphilicity in a surrounding medium, may result in a selection of morphologies. 34 When we increased the Ru/C60 ratio to 2/1 in CH2Cl2 (entry 2, Table 2.1), the polymeric spheres are already covered by a shell of 1.2 nm size RuNP (Figure 2.4a,b), whose thickness increases with the increase of the Ru/C60 ratio. 27 Theoretically, based on thermodynamics arguments, the formation of Ru clusters and RuNP is favored over the increase of number of single atoms. Coordination energy between one Ru atom and two C60 is -84 kcal/mol, the cohesive energy per Ru atom in naked Ru13 is -96 kcal/mol or -90 kcal/mol in the -(Ru2@C60)n-model shown in Figure 2.5. Despite the fact that we cannot completely exclude the formation of complex 2D or 3D structures (such as in Figure 2.1) since they appear to be stable when increasing the metal/carbon ratio, 22 it seems that the electronic structures of various transition metals embedded in more structured systems remain very similar to the 1D case. Therefore, in our models we will retain only the wire configuration. 35 This result is consistent with the idea of "polymer swelling", where the solvent has the capability to penetrate through the cross-linked polymer network. In this case, -(D-Ru1@C60)n-polymer does not swell in CH2Cl2, so the additional Ru atoms on solution cannot be incorporated in the structure due to diffusional limitations, and nucleation of RuNP occurs on the surface of the nanospheres (Figure 2.4f). Using pure toluene as solvent -(T-Ru1@C60)-swelling favors the incorporation of Ru into the structure avoiding NP deposition as seen from HRTEM and STEM-HAADF images even for a 5/1 Ru/C60 ratio (entry 5, Table 2.1, and Figure 2.4c-e). Indeed, experimental evidence has shown the importance of polymer swelling by controlling the diffusion rate. 36 Interestingly, we noticed that the final Ru loading is lower in toluene than in dichloromethane at the same Ru/C60 ratio (Table 2.1). In order to increase the Ru loading we firstly increased the Ru/C60 ratio (entries 4-8, Table 2.1). By increasing the Ru/C60 ratio from 1/1 to 40/1, only a moderate increase in Ru loading was obtained, and a plateau is reached at 20 % w/w Ru. We also investigated the effect of the temperature (entries 7, 9 and 10, Table 2.1). An increase of the temperature of reaction from 25 to 100 °C for a Ru/C60 ratio of 20/1 does not allow increasing the Ru loading above 20% w/w. It is important to notice that, contrarily to the sample D-Ru@C60 2/1 (16.7% Ru w/w), 27 at 20 % w/w Ru loading, the sample produced in toluene does not contain any RuNP from HRTEM and STEM-HAADF observations (Figure 2.6). This result is again consistent with swelling of the T-Ru1@C60 structure, allowing incorporation of Ru atoms in the polymeric matrix. We evidenced toluene retention due to swelling in -(T-Ru1@C60)n-detected by TGA analyses (Figure 2.7a). It has already been reported that the Pd fulleride C60Pdn showed good adsorptivity toward toluene. 37 We performed DFT calculations including van der Waals interactions in LPCNO (Toulouse)

to measure the stabilization energy of toluene molecules on -( Ru1@C60)n-(Figure 2. 7b).

For one toluene molecule the stabilization energy was 12 kcal/mol, and it was 22 and 49 kcal.mol -1 for two and four toluene molecules, respectively. We discovered that the existence of the plateau is connected to the rapid formation of a stable complex [(1,5-cyclooctadiene)(toluene)Ru(0)] in solution by the reaction of the solvent with the ruthenium precursor. The formation of this product has been confirmed

by 1 H-NMR, and this complex was isolated from the yellow filtrates obtained at the end of the reactions (Figure 2.8). Trying to assess the reactivity of this molecule, separately, we synthesized this complex by mixing [Ru(COD)(COT)] and toluene in the presence of dihydrogen. 38 Using this compound as precursor in our synthesis, we confirmed that it hardly reacts in the presence of C60 under the reaction conditions. After 24 h reaction, a black solid is obtained in low yield (16%) that contains only 6% Ru w/w. In order to limit the formation of [Ru(COD)(toluene)] and to increase the Ru loading we investigated the use of solvent mixtures with toluene. Two solvents have been investigated, CH2Cl2 (entries 11-13, Table 2.1) and methanol (entries 14-16, Table 2.1), which can act as coordinating solvents to the Ru atoms to limit the formation of the stable Ru complex with toluene. 39,40 The use of toluene/CH2Cl2 mixtures (50/50) allows increasing the Ru loading (for a Ru/C60 ratio = 20) from 20 to 36%. The Ru reaches 34.7 wt.% for a toluene/methanol mixture (95/5), and 47.1 wt.% for a toluene/methanol mixture (50/50). The sample T95D5-Ru@C60 20/1 was analyzed by STEM-HAADF (Figure 2.9). The EDX profiles of this specific sample show a higher content of Ru on the nanostructure surface indicating that a core-shell structure starts to develop, with a Ru rich shell (Figure 2.9d). However, at this high Ru loading, the detection of RuNP was not straightforward.

The sample T95M5-Ru@C60 20/1 was also observed by STEM-HAADF and no Ru NP was clearly observed (Fig. The T50D50-Ru@C60 sample presented a similar metal loading as T95M5-Ru@C60 (Table 2.1, entries 13 and 14), and the formation of core-shell spherical particles is clearly observed (Figure 2.11a,b), on which the extra Ru atoms have the tendency to deposit on the external surface of the spheres (see Figure 2.11a for EDX mapping). There also, the structuration of Ru as metallic NP was not clear (Figure . 2.11b).

Finally, for the T50M50-Ru@C60 20/1 sample, no core shell structure was produced (Figure 2.11c-d), and once again the detection of metallic NP was not easy due to the extremely high Ru loading (47 % w/w, which corresponds to 6 Ru atoms for 1 C60). From these results, it appears that the use of solvent mixtures during the synthesis impacts the fate of the reaction. First, the use of toluene/CH2Cl2, and particularly of toluene/CH3OH mixtures allows to significantly increase the Ru loading, which reaches almost 50 % w/w in the T50M50-Ru@C60 sample. This could be attributed to a competition between the solvents for coordination to Ru, and to the generation of Ru intermediate species that are more easily decomposed than the stable [Ru(COD)(toluene)] complex. An easier coordination of methanol compared to dichloromethane is not surprising. Second, the choice of the second solvent (CH2Cl2 or CH3OH) has also an impact on Ru location. The use of CH2Cl2 does not allow depositing the extra Ru inside the hetero-structure and a shell of Ru species are deposited around the -(Ru1@C60)n-core. When CH3OH is used, the extra Ru atoms are incorporated inside the hetero-structure.

We propose that it is the swelling of the -(Ru1@C60)n-product that allows the incorporation of extra Ru atoms inside the hetero-structure. Methanol should act as a porogen (pore generating solvent) for the -(Ru1@C60)n-structure. In polymer chemistry, the porogen is the most influencing parameter to the surface area, porosity, and morphology of the polymer. 41 Finally, it is worth mentioning that if the presence of Ru subnanometric particles could be suspected in samples prepared using mixtures of solvents (up to 6 Ru atoms for 1 C60), the high metal loadings makes their identification by STEM-HAADF complex.

Raman, XPS, WAXS and EXAFS characterizations

Since fullerene C60 is a powerful electron acceptor, 42 we investigated by Raman spectroscopy the possibility of electron transfer in the materials synthesized in toluene. We observe a spectral shift as large as -16.2 cm -1 for the T-Ru@C60 20/1 sample, and a significant broadening with increasing the amount of Ru, caused by strong electronphonon interaction. Thus, a clear tendency can be observed where the displacement increases as the Ru content increases (Figure 2.12b). It is commonly accepted that the Ag(2) mode is downshifted by approximately 6 cm -1 transferred to C60 in alkali metal fulleride compounds. 44 The downshift of alkali metal fullerides depends on the number of metal atoms and each metal atom donates one electron to C60, because there are only 45, 1955-1958. ionic bonds between C60 and alkali atoms. For transition metal fullerides, the relationship between shift and composition is more complex since these compounds exhibit a large proportion of covalent bonding between metal and C60. However, a significant charge transfer is observed for our samples, suggesting the presence of electron deficient Ru species in the T-Ru@C60 series, as we had already shown for the D-Ru@C60 series. 27 The charge transfer from Ru to C60 was also evidenced by XPS (Figure 2.13), by comparing the binding energy of Ru3p3/2 (462.2 eV) with that of Ru 0 (461.2 eV). Raman 45 and XPS 46 signatures for charge transfer were already reported for Pd@C60 fullerides. DFT estimates of the charge transfer for various Run@C60 models (n = 1, 2, 4, 13) confirm this general trend, with values from 0.1 (Ru13@C60) to 0. Further analyses were performed in CEMES (Toulouse) by WAXS in order to shed some light on the possible presence of metallic Ru clusters in the Ru@C60 hetero-structures.

We first compared the T50D50-Ru@C60 and T50M50-Ru@C60 samples with the T-Ru@C60 sample at a similar Ru/C60 ratio of 20/1 (Fig. We also analyzed the samples prepared in pure toluene at different Ru/C60 ratio (Figure 2.13). As expected, for all ratios, it was impossible to detect a metallic Ru-Ru distance but a contribution in the 0.20-0.23 nm range could also systematically be observed. Finally, we compared the T95D5-Ru@C60 and T95M5-Ru@C60 samples with the T-Ru@C60 sample at a theoretical Ru/C60 ratio of 20/1 (Figure 2.16). Differences could be observed between the different samples but poorly defined RDFs and extreme complexity make any analysis irrelevant beyond the observation of a similar distance in the 0.20-0.23 nm range strongly pointing to Ru-C bonding. Despite the difficulty to apply WAXS to these complex systems, in which both SA and few atom clusters may be present, it was possible from this technique to detect metallic Ru in T50D50-Ru@C60.

This complexity is confirmed by the simulated PDF profiles (Figure 2.17 The sample prepared using toluene/methanol does show some differences, in that a hint of a second shell is detectable and the intensity of the peak is much lower than that of a sample produced using toluene with a comparable Ru/C60 ratio. There is an additional difference in the position of the maximum that is slightly shifted to shorter distances, as also evidenced by the shift in the oscillations in the k 2 χ(k). Data for all the samples were initially fitted with a single shell and a good fit was obtained with a Ru-C shell for all the samples. Ru-C coordination numbers are close to 8 for all samples (consistent with  2 - 6 coordination), with the main difference for the sample made using toluene/methanol being a larger Debye-Waller factor. For this sample, the fit did improve by adding a second Ru-Ru shell. In Fig. 2.18 the fitting results are shown and the fitting parameters are reported in Table 2.2. It should be pointed out that, due to the small weight of the second Ru-Ru shell in the sample T95M5-Ru@C60 and the strong correlation between coordination number and Debye-Waller factor, the values of the parameters reported in Table 2.2 can be considered as purely indicative. Notwithstanding, the fit improvement provides a suggestion that Ru clusters could be incorporated in the T95M5-Ru@C60 sample containing 34.7 % w/w of Ru. It is also worth mentioning that the EXAFS data reported for reduced Pt8 clusters deposited on carbon black do have common features with our results (Fig. 2.19). 51 The very small coordination number (CN = 2.7 ± 0.2) in the T95M5-Ru@C60 sample supports the formation of tiny clusters. For the sample prepared in pure toluene, the fact that only RuSA are produced with a welldefined environment ( 2 - 6 coordination) for Ru loading between 10 and 20% w/w is a remarkable result. Indeed, the challenges for SA catalysis for industrial applications are currently to achieve an accurate control over the local structure of metal SA and to increase the active site density. 52 Currently the maximum loadings obtained for SA type catalysts are around 10% w/w. 53,54,55 Since STEM-HAADF observations and WAXS and EXAFS analyses were not decisive concerning the presence or not of Ru subnanometric particles in all the samples obtained with a mixture of solvents, we decided to use catalysis to probe their presence. Indeed, in hydrogenation reactions, the electron deficient RuSA should be less efficient that RuNP/clusters for hydrogen dissociative chemisorption, which is often rate limiting. 56 The electro-deficiency in Ru SAC results in a lower d-band center that offers less electron back-donation to the antibonding orbital of H2 molecules, resulting in relatively high activation energy for H2 dissociation. 57,58 On the other side, it was reported that subnanometer Ru clusters can be very active hydrogenation catalysts. 59 It was also reported that among PdnC60 fullerides, active hydrogenation compounds are those with n > 3, 60 that is the samples suspected to contain PdNP. 30 We chose as first probe reaction the hydrogenation of nitrobenzene (NB) (Figure 2.20) to aniline (AN), since RuNP are known to be active for this reaction. 61,62,63,64 The reaction was studied at 30 bar H2 and 80 °C in ethanol with a Ru concentration of 0.025 mmol.

The activity of the catalysts expressed in moles of NB consumed per mole of Ru (determined by ICP) and per second, and being equivalent to an instantaneous Site Time Yield ("STY" in the following).

We independently checked that the C60 fullerene has no activity for this reaction under these experimental conditions. Table 2.3 shows the results obtained with the Ru@C60 samples prepared in different solvents and at different Ru/C60 ratio. The main products of the reaction were AN and cyclohexylamine (CA). Nphenylhydroxylamine (PHA), and N-ethylaniline (AN-Et) and N-ethylcyclohexylamine (CA-Et) both formed by N-alkylation from ethanol, were the only detected by-products.

All catalysts were found active for NB hydrogenation. The evolution of the NB conversion over the time is shown in Figure 2.21a for the Ru@C60 series prepared in toluene at different Ru/C60 ratio, and for the series prepared in toluene/methanol mixtures. For all the samples prepared in pure toluene (Ru/C60 ratio 1/1 to 20/1), for which only SA are present, the conversion of NB is low compared to the ones prepared in toluene/methanol mixtures. For these latter series, the activity increase is already observable for the T95M5-Ru@C60 sample, for which the presence of Ru clusters has been suggested from EXAFS analyses. The T50M50-Ru@C60 sample is by far the most active catalyst with a STY of 135.5 h -1 . Comparison between samples prepared in pure toluene and in solvent mixtures are shown in Fig. 2.21b. The T50D50-Ru@C60 sample, for which metallic Ru has been evidenced by WAXS is much less efficient (STY of 18.2 h -1 ) than the T50M50-Ru@C60 sample (STY of 135.5 h -1 ). This result should be related to the coreshell structure of the T50D50-Ru@C60 catalyst, for which the active ruthenium is limited to the external surface of the spheres ( Since for a RuSA, the 18-electron rule is satisfied upon the adsorption of 3 H2 molecules, the adsorption energy drops and no further H2 can be adsorbed. Interestingly, the Ru center coordination mode, changes from  2 - 6 to  2 - 2 with 3 partially activated H2

presenting elongated H-H bonds, around 0.9 Å (0.74 Å in H2). With larger clusters, more H2 can be adsorbed and dissociated, but adsorption energy values always reach a plateau (around -5kcal/mol) for clusters smaller than Ru13, experiencing a surface saturation effect. However, in the case of Ru13, for a value of 1.5H/per surface Ru, a ratio usually reported experimentally, 65,66 the energy per adsorbed H is still large and agrees well with previous reports. 67 Those results strongly suggest that very electro-deficient Ru SA and few atom aggregates are not performing for H2 activation compared to larger clusters.

We have also evaluated the catalytic activity of the most active T50M50-Ru@C60 catalyst for the hydrogenation of toluene to methylcyclohexane. Ruthenium has been widely used for this reaction since toluene is more strongly adsorbed on this metal than other metallic surfaces such Pd or Pt. 68,69 The results are depicted in Table 2.4. Although the results reported in this table show very contrasting catalytic activity for various Ru catalysts, a modest activity for the T50M50-Ru@C60 catalyst is measured compared to the best ruthenium catalysts. An explanation of this result could come from differences in the adsorption energy of toluene on the various catalysts. Indeed, when the size of a reactant molecule is comparable to that of a nanometer catalyst (such as toluene and the Ru clusters in T50M50-Ru@C60), the adsorption configuration of the molecule on the catalyst becomes different from that on their larger counterparts. Consequently, distinct catalytic performances can be observed for catalysts of different sizes. Thus, size-dependent adsorption of styrene was demonstrated on Pd clusters. 70 When the surface of the Pd cluster becomes too small (clusters between Pd7 and Pd16), the styrene molecule cannot fully bond with the cluster. Typical adsorption energy of toluene on Ru(0001) surface ranged from 48 to 54 kcal/mol. 71 DFT calculations should be performed in the future to determine the adsorption energy of toluene on a -(C60-Run-C60)n-structure to confirm this hypothesis.

Finally, the high TOF obtained in NB hydrogenation compared to previous studies, 72,73 incite us to evaluate the catalytic activity of the T50M50-Ru@C60 sample in the difficult hydrogenation of the tetra-substituted olefin 2,3-dimethyl-2-butene. This reaction can be performed with cationic iridium complexes under mild conditions (room temperature, 1-5 bar, TOF = 4000 h -1 ). 74,75,76 However, with some complexes irreversible deactivation process occurs, preventing reaction completion. 75 Colloidal metallic NP (Rh, Ru, Pt), although less active (TOF = 160-320 h -1 ) are also efficient catalysts for this reaction under relatively mild conditions (room temperature to 75°C, 1-10 bar of H2). 82,83 As far as supported catalysts are concerned (Rh, Co), much lower activities were reported (TOF = 2-10 h -1 ) under slightly harsher operating conditions (25 to 150°C, 5-40 bar). 84,85 At 50 °C and under 20 bar of H2, the reaction with the T50M50-Ru@C60 catalyst is completed within 2 h. The calculated STY at 20% conversion 470 h -1 is to the best of our knowledge the higher ever reported for supported metallic NP. For this reaction also, the catalysts prepared in pure toluene containing only SA are much less active (Fig. 2.21c), pointing their low hydrogenation activity. Finally, we have performed a series of calculations to propose the mechanism of tetra-substituted olefin 2,3-dimethyl-2-butene hydrogenation.

Interestingly due to the high steric hindrance induced by the small C60-C60 distance, the hydrogenation is not possible on RuSA. On Ru13H18, the hydrogenation reaction appears thermodynamically favorable, as it can be seen in Fig.

Conclusions

In conclusion, we have highlighted the crucial role of the solvent for the growth of subnanometric ruthenium NP in Ru@C60 hetero-structures. While RuSA are selectively obtained in pure toluene (Ru@C60 hetero-structures with very high Ru loading = 9-20% w/w), the use of a toluene/methanol solvent mixture makes it possible to increase the metal loading up to 50% w/w and to obtain ruthenium clusters uniformly distributed in the materials. These novel catalysts are extremely active for the hydrogenation reactions of nitrobenzene and tetra-substituted 2,3-dimethyl-2-butene. For this latter reaction a STY of 485 h -1 was calculated, which is the higher ever reported for supported metallic NP.

This work provides a general strategy for producing high loading and ultra-dispersed supported catalyst for enhanced catalytic performance.

Synthesis of Ru@C60 nanostructures

In a typical experiment the [Ru(COD)(COT)] complex was introduced in a Fisher-Porter bottle with a solution of fullerene C60 in the desired solvent. The resulting purple solution was stirred for 30 min at room temperature, after which the bottle was pressurized with 3 bar of H2. The solution, which turned black after few minutes of reaction, was kept under stirring overnight at room temperature. After this period, excess of H2 was eliminated and the volume of solvent was reduced under vacuum. Pentane was then added to precipitate the colloidal suspension. After filtration under argon with a cannula, the black solid was washed twice with pentane and filtrated again before drying under vacuum. The quantities of reactant used for each Ru/C60 ratio studied are detailed in Table 2.5.

Hydrogenation of nitrobenzene

Hydrogenation reactions were performed in Top Industry high pressure and temperature

Stainless steel autoclave with a controlling system. In a typical experiment, the autoclave was purged by three vacuum/argon cycles. The mixture of Ru@C60 catalysts (0.025 mmol Ru), dodecane (as internal standard, 1.1 mmol) and nitrobenzene (4.06 mmol) in 30 mL of ethanol was prepared in a glovebox, ultrasonicated for 5 min and then transferred into a high-pressure autoclave under argon atmosphere. The autoclave was heated to 80 °C and pressurized with 30 bar of H2; the stirring rate was fixed at 1000 rpm. Samples of the reaction mixture were taken periodically and then analysed by GC-FID. Quantitative analysis of reaction mixtures was performed via GC-MS using calibration solutions of commercially available products. The X-ray absorption spectra were recorded on the B18 beamline at the DIAMOND synchrotron (Oxfordshire, UK, Dr. Anna Corrias). Extended X-ray Absorption Fine Structure (EXAFS) spectra at the Ru (22117 eV) K-edge were collected at room temperature in transmission mode using a Si(311) monochromator. The monochromator energy scale was calibrated via a third ion chamber with a reference foil. The samples, in form of powder, were diluted with polyvinylpyrrolidone (PVP) in an appropriate concentration inside a glove box and pressed to form a pellet, which was then sealed in an aluminum pouch to avoid any oxidation. The data analysis was performed using the ATHENA and ARTEMIS software. 86 With ATHENA, the absorption edge, E0, is determined, and the absorption due to the isolated atom is subtracted, by fitting the preedge and post-edge regions to obtain χ(k). The software ARTEMIS is used to perform the fit of the EXAFS region to scattering models in R-space obtained by FEFF, validated on standard compounds. The number of fitted parameters was always lower than the number of independent points. Raman measurements were carried out with a Horiba XPLORA-MV2000 spectrometer in CEMES (Prof. Wolfgang Bacsa). For the measurements, an excitation wavelength of 532 nm and laser power of 0.084 mW was used. The samples were kept under vacuum and exposed to atmospheric air shortly before measurements.

The samples were analyzed by X-ray photoelectron spectroscopy (XPS) using a VG Escalab MKII spectrophotometer operating with a non-monochromatized Mg K source (1253.6 eV).

Chapter 3. Preparation of low loading carbon supported metal single atoms

3. Preparation of low loading carbon supported metal single atom

Introduction

As we saw in the previous chapter, effective SAC production relies on the ability to produce strong metal-support interactions. This leads us to meditate on the need to find susceptible sites to be used as traps to host metallic atoms. In this sense, when visualizing a perfect and smooth graphenic lattice (Fig. 1.11a), the obvious assumption would be that, except on the edges and if they are not decorated, there is not chance to anchor a metallic atom at least this lattice contains vacancies or defects within the stable hexagonal network of sp 2 carbons.

In general, defects in graphene can be categorized into two different groups: the first group, called intrinsic defects are those of non-sp 2 carbons in graphene. These defects are usually caused by the existence of non-hexagonal rings surrounded by hexagonal rings.

The second group consists in extrinsic defects where the crystalline order is perturbed with non-carbon atoms. The intrinsic defects of graphene can be divided into four categories: single vacancy defects, multiple vacancy defects, line defects and Stone-Wales defects (Fig. 1.11b-i). 1 We have shown in the bibliographic chapter that the stability of SA depends not only on the nature of the defects, but also of the metal (Fig.

1 .22). The general tendency being that carbon edges and single vacancy (SV), double vacancies (DV) and even triple vacancies (TV) are particularly efficient to trap metallic single atoms.

A SV is the simplest defect in graphene, which is formed by removing one atom from the lattice. According to DFT calculations, the energy needed to form a SV ranged between 7.2 and 8.58 eV, 2 values that are in good agreement with experimental measurements of 7.0 ± 0.5 eV. 3 This vacancy can diffuse through the network to be stabilized with estimated energies ranging between 0.87-1.70 eV, 4,5,6,7 suggesting that they could be observed even at 200 °C on CNT. 8 Tight-binding molecular dynamics simulations and first principles total energy calculations have shown that two SV can coalesce into a 5-8-5 DV (energy barrier for this process is 1.5-1.6 eV), which is further reconstructed at higher temperatures into a new defect structure, the 555-777 defect, by the Stone-Wales type transformation. 6 The energy barrier for the 5-8-5 to 555-777 transformation is high (5.2-5.7 eV) and therefore this state is considered to be inaccessible from thermal activation alone. Once formed, the immobile 5-8-5 DV will act as a nucleation site for further coalescence of additional mobile SV to form higher order multi-vacancy structures. DFT calculations have also shown that graphene layers with SV at the edge are energetically more stable than the layers with SV in the middle. 9 For graphite, it is suggested that the hexavacancy, is the most stable defect. This stability was explained by the dangling bond counting model, where the number of dangling bonds of the system decreased as the vacancy became more stable, 10 although the stability of the defect also depends on pentagons formed upon reconstruction of the system (Figure 3.1). 11 Other modeling studies have suggested that the thermally activated coalescence of SV will not result in 'holes' in the graphene structure, but in lines which will create 5-7 dislocation pairs and/or stacking faults (Figure 3.1c). 12 As mentioned before, the presence of vacancies is highly desired; dangling bonds around the vacancies can serve as bridges for the chemical connection or provide active sites for atom adsorption. 14 Even though some vacancies are created in the process of carbon material synthesis, important technologies have been developed to induce such defects later on. 15 During decades scientist have used high energy beams (ions or electrons) using sophisticated equipment such as scanning tunneling/atomic force microscopes to produce irradiation-induced vacancies. 16 These techniques possess high degree of control, being capable to remove even atom by atom to form monovacancy to multivacancy, by manipulating exposure time, size and energy of the focused beam (Figure 3.2). 17 Clearly, these modifications cannot be achieved with conventional macroscopic tools hence the limitations for mass production or scale-up are obvious. Adapted from ref. [19].

Despite the progress in the area, there is still a lack of efficient processes to scale up the creation of vacancies in the carbon matrix. In this context we decided to study the possibility of creating carbon vacancies in the surface of our materials by the decomposition of surface oxygen groups. In this chapter, we describe a simple procedure to incorporate defects in the graphitic network as well as characterization and their utilization as anchoring point to produce single atom catalysts.

Vacancies

We decided to resort to a common procedure such as functionalization through oxidation with nitric acid for subsequent defunctionalization in order to achieve a defective material. In a typical experiment, the 2 steps (functionalization and defunctionalization) were carried out as described in the experimental section and summarized in Figure 3.3.

C C C

C

Carbon support Oxidized carbon support Defective carbon support

Step II

Support activation

Step I Support oxidation

HNO 3 140ºC -2CO 2 , -H 2 Δ / Ar C C C C OH O O O H C C Figure 3.

Schematic representation of activation method on carbon materials.

Conceptually, step I involve the addition of different oxygen surface functional groups over the carbon support as seen in Figure 3.3. The oxidation of CNT with concentrated nitric acid leads to the formation of various functional groups, among them carbonyl, phenol or carboxylic groups, 20,21 which are mainly located on the edges but also in the frame of the aromatic planes. 22 Mapping at the nanometer scale by EELS/STEM analyses of the functional groups on oxidized multi-walled CNT, has shown that these groups are inhomogeneously distributed across the entire length of the nanotubes. 23 Step II attempts the removal of the previously formed oxygen-groups through thermal decomposition under an inert atmosphere. Each one of these groups decomposes at specific temperatures to form CO2 and/or CO (Table 3.2) and water. These decomposition temperatures are well-established, and can be determined by Temperature Programmed Decomposition (TPD) profiles. 24,25 The control of the presence of these surface groups in a given material could a priori be achieved by heating at a desired temperature to remove selectively specific groups. It is important to mention that the majority of these investigations primordially focus on the identification and quantification of the outgoing species, but do not provide further inquiries regarding the surface chemistry on the carbon. There are few reports dealing with the mechanism of thermal decomposition and the resulting species. In 1996, Menéndez et al. 27 proposed that thermal treatment under N2 atmosphere removes oxygen in the form of CO2 (predominantly at lower temperatures) and CO (predominantly at higher temperatures), leaving unsaturated carbon atoms (also called defective sites) at the edges, as shown in . Adapted from ref. [30].

A later publication in 2012 reports that the desorption of oxygen-bonded carbons generates large number of topological defects and vacancies in the graphene lattice. 31 The authors propose a five steps defunctionalization mechanism: a) the physically adsorbed and intercalated water is evaporated (physical water), b) the intermolecular dehydration takes place between neighboring carboxylic and/or hydroxyl (chemical water), c) the anhydrides decompose to evolve both CO2 and CO, d) the most stable carbonyls and quinones will decompose to evolve CO (Figure 3.5).

Figure 3.5. Proposed pathway for transformation and evolution of oxygen containing

functional groups during thermal treatment. From ref. [34].

Finally, Chernyak et al. 32 proposed a mechanism of thermal defunctionalization of multiwalled CNT proposing the formation of zwitterionic species after heterolytic cleavage of lactones and anhydrides, based on the mechanism of thermal decomposition in organic carboxylic acids. 33 From this analysis of the literature, it seems reasonable to propose that, after heat treatment the support will contain high density of reactive sites/defects (either radical or ionic species), which could be used for stabilization of metal single atoms. The fact that some active sites remain at room temperature, represents a great opportunity for the formation of new chemical bonds between carbon boundary atoms and atoms/molecules, as it has already been shown for NO molecule reactivity on freshly cleft graphite. 34 Shaffer and co-workers have used such an approach for the thermochemical organic functionalization of CNT. 35 In this work, the key step of the treatment is the activation of CNT at high temperatures (1000 °C) under inert atmosphere or in a vacuum, which results in the desorption of almost all oxygen containing functional groups at pre-existing defect sites and the generation of surface radicals. The reactive sites were used to initiate radical polymerisation of functional monomers leading to the grafting of oligomers (Figure 3.6). Such thermochemical activation was used for various organic functionalization of CNT,36 carbon black 37 or graphene. 38 Figure 3.6. Proposed mechanism for the thermochemical activation and grafting of CNT. From ref. [39].

Thermogravimetric analyses

To begin our study, it was essential to monitor oxidation degree and thermal stability of the carbon support (CNT). 39 To do so, we have performed thermogravimetric analyses (TGA) under nitrogen atmosphere on our supports. Figure 3.7 shows the TGA and differential TG (DTG) profiles of oxidized CNT after step I. As can be seen in Figure 3.7a, three weight loss areas located at 100 ºC, 200-300 ºC and 600-800 ºC are found in the DTG curve. The first signal corresponds to desorbed water and the second and third may be related with decomposition of carboxylic acids and phenols-lactones-carbonyl, respectively. From this data we decided to use 400 ºC as annealing temperature, which is in good agreement with previous reports in literature showing that below this point the labile oxygenated groups, weak and strong carboxylic acids, would have effectively decomposed. We also tested two other samples with different oxidation time (1, 3 and 5 h) for comparative purposes. After the activation process at 400 ºC during 1 hour (step II), the samples O1h-CNT, O3h-CNT and O5h-CNT have lost a percentage of mass of 3.4, 7.9 and 10.4%, respectively (Figure 3.7b). Although O5h-CNT was the material that contained the highest percentage of functional groups, and therefore potentially the largest number of active sites after defunctionalization, we decided to continue this study using the O3h-CNT support, since the process of washing and especially filtering required several day to be executed for the O5h-CNT sample, which could limit the progress of this investigation.

Temperature-programmed desorption analyses

Temperature-programmed desorption (TPD) was also used to evaluate the amount and types of oxygen-containing surface groups in these carbon materials. The key principle is to heat the material in an inert atmosphere and record spectra of outlet gases by mass spectrometry. 40 The amounts of CO and CO2 released during the thermal analysis were quantified at the end of each analysis.

In Figure 3.8 it can be observed the evolution of CO2, CO, H2O, CH4 and H2 (m/z =44, 28, 18, 16, 2, respectively) along the temperature range 25-1100 ºC for O3h-CNT and O3h-CNT400 samples. literature (Table 3.1), which correspond to desorption of carboxylic acid (250 ºC), carboxylic anhydrides (420 ºC) and lactone groups (610 ºC). After the subsequent thermal treatment at 400 °C, the carboxylic acid signal is practically inexistent; prove of the strong decarboxylation of the surface. The CO profile of O3h-CNT in Figure . 3.8b presents a weak peak at 200 ºC that can be ascribed to the decomposition of aldehyde groups, 41 or may be originated from the decomposition of oxygenated groups linked to nitrogen (from the HNO3 treatment), leading to CO evolution at temperatures below those usually observed. 42 The second peak, observed in the range of 380-530 ºC, can be associated to the decomposition of anhydride groups, known to produce both CO and CO2 during their decomposition. The larger peak at higher temperatures with maximum at 700 ºC, is expected for phenolic and carbonyl/quinone groups.

Considering the specific surface area of the sample (220 m 2 /g measured by the BET method), surface density calculation of each group can be made to have an idea of the population of surface species, before and after step II. Also, considering the hexagonal unit cell of the graphitic planes containing two carbon atoms each 0.052nm 2 i.e., 38.5 C atoms per nm 2 , it was possible to estimate the number of functional groups per carbon atom (Table 3.2). As can be seen, after heating at 400 ºC during one hour the total density of groups was reduce by 30%, but only due to the selective elimination of the groups decomposing at low temperatures. Moreover, after heating the amount of lactones increased substantially, product of the intermolecular dehydration between neighboring carboxylic and hydroxyl groups, to generate thermally stable lactones (Figure 3.5). 31,32 The TPD profiles for water (m/z = 18), methane (m/z = 16) and hydrogen (m/z = 2) have also been recorded but not quantified. The profile of water (Figure 3.8c) shows an asymmetric peak with center at 100-200 ºC, which can be attributed to: i) the desorption of surface H2O, but also ii) the dehydration by combination of two adjacent carboxyl groups towards formation of carboxylic anhydrides (Figure 3.5) related to the shoulder at higher temperature (~300 ºC), 43 or the co-decomposition of a carboxylic group and a phenolic group giving rise to a lactone (Figure.

Raman analyses

Raman spectroscopy is a very valuable tool in the characterization of carbon materials since it is possible by this technique to discern even slight changes in the structure of the material. The Raman spectrum of O3h-CNT and O3h-CNT-400 (Figure 3.9) is characterized by the presence of two set of peaks. The first, the G band at 1582 cm -1 comes from the in-plane stretching of the C-C sp 2 bonds referring to honeycomb-like domains on graphitic materials. The other peaks that appear at 1345 cm -1 , 1614 cm -1 , 1692 cm -1 and 2938 cm -1 , are assigned to D, D′, 2D, D+D (D for defect or disorder) bands, respectively. Those are associated to out of plane vibrations from all non-regular portions.

The intensity of this peak is proportional to the amount of disorder randomly distributed, impurities or surface charges. 44 The ratio between the intensities of the D and G bands, ID/IG, provides a parameter that can be used for quantifying disorder. 45 We notice a slight increase of the intensity of the ID/IG after heat treatment, reflecting an increase of the amount of defects on O3h-CNT-400 compared to the unheated O3h-CNT. It should be noted that after the oxidation treatment, normally D and G signals suffer an upshift due to the "electron extraction" from the sp 2 structure. 46 In our case, all signals (Table 3.3) display a downshift after heating, which is a possible indicator of the recovery of electronic density.

O3h-CNT-400

It is interesting to note that the IG/I2D value has been related to the thickness of graphitic material. with a IG/I2D increase with the increase in the number of layers. Therefore, the reduction of this value after annealing could indicate an average reduction of the number of walls on the CNT (see Figure 3.10 for TEM analyses), although the exact quantification of these have been reserved for materials up to 8 layers, which here is not the case. 47 The intensity ratio between the D and D′ peak is able to probe the nature of the defects for moderate amount of disorder. 48 According to the obtained value (ID/ID′ = 4.14), it is more likely related to vacancy-like and boundary-like defects rather than to sp 3 hybridization defects. Furthermore, Casiraghi and co-workers noticed that defective graphene samples produced by ion-bombardment (vacancy-like defects) show a smaller ID/ID′ value than the sp 3 -type defective graphene (partially hydrogenated). 35 The decrease of the ID/ID′ value from 4.64 to 4.14 could thus be associated to a decrease of sp 3 -type defects and/or an increase of vacancy-like defects.

a) b)

An X-ray photoelectron spectroscopy (XPS) study was also performed in order to further investigate the surface chemistry of the carbon supports. The assignment of peaks was performed based on different reports found in the literature. 49,50 It has been possible to identify seven contributions in the C 1s profile (Figure 3.11a) as seen in Table 3.4. Interesting differences can be seen comparing both samples heated and not heated (Table 3.4). In spite of the evidences by TPD (performed under helium), where there can be observed a great loss of oxygen, the XPS C/O ratio did not change significantly. We relate this to the fact that due to the impossibility of carrying out the analyses under an inert atmosphere, the samples were exposed to air where they can re-adsorb oxygen to stabilize reactive surface species. 27 Indeed, vacancies in graphene are very reactive towards O2 and are quickly saturated by ether and carbonyl groups via a two-step barrierless mechanism. 51 The sp 2 signal at 284.1 eV increased after heating, confirming the recovery of aromatic structure; and consistently, the sp 3 signal at 285.0 eV was reduced. 52 More importantly, the peak associated to defects at 283.5 eV grows in the heated sample, as we aim with the proposed method. This signal could arise from C vacancies, pentagon and heptagon rings, the formation of fullerene like structures and also with functional groups attached to the graphitic lattice. 53,54,55 Another parameter of graphitic structure can be inferred from a shake-up (ShU) peak at the end of the C1s spectrum (291.2 eV). The ShU belongs to the π-electrons, which rapidly respond to the photo-excitation. The relative magnitude of this ShU signal can be correlated with the conjugation strength in the hexagonal lattice. 56 After heating, this contribution slightly decreased suggesting that despite the increase of the sp 2 character, the general conjugation of the material may be perturbed, more likely by the created defects. In the case of oxygen, Figure 3.11b, the C=O signal decreased in agreement with the TPD results, confirming the decrease in carboxylic groups as well as the permanence of -OH groups or the formation of ether groups evidenced by the relative increase of C-O signal. 57 The third component is probably related to the satellite structure of the spectrum or to adsorbed oxygen containing molecules such as water. 58

Detection of vacancy

The above characterization provides general insight about the graphitization degree and relative composition of our carbon support. Nevertheless, there is still need for a proper characterization of the chemical surface of the treated support. The detection of the defective species on carbons became a great challenge mainly due to their intrinsic heterogeneity. Several experimental and theoretical techniques have been used to detect different types of defects, and to understand how they locally change the material's properties. 59,60 For example, the direct observation of vacancies under the microscope has been successfully achieved for graphene 61 and SWCNT, 62 but the direct extrapolation of these studies on multi-layer materials, that is, graphite and MWCNT, becomes harder because the experimental signals became much more complex. For this reason, we have opted for the study of the magnetic properties of the material, which represents a powerful tool for characterization of electronic surface due to its extreme sensitivity to any magnetic "defect".

An important aspect that must be considered is that through traditional carbon nanomaterial production, such as chemical vapor deposition, a variety of ferromagnetic catalyst particles such Fe, FeNi or FeCo are involved, which usually end up encapsulated within the nanostructure even after purification protocols, what will influence the magnetic response of the samples. 63 In order to remove such impurities and obtain highly pure nanotubes we performed a high temperature annealing on the purified CNT sample, process that has been proven to remove all remaining metallic nanoparticles from the synthesis. 64 The treatment performed by Ana B. García (INCAR-CSIC, Oviedo, Spain) consisted in heating 5 g of purified MWCNT at 2800 ºC for 1 hour in argon flow using a Xerion-Graphite electrical furnace. Figure 3.12 shows the TEM images of the samples before and after annealing (CNT*). As can be seen on

Nuclear magnetic resonance analyses

Carbon-13 high-resolution solid-state nuclear magnetic resonance using magic angle spinning (MAS SS 13 CNMR) has been used as a primary method to characterize CNT.

Only a few groups have reported on the properties of carbon nanotubes using nuclear magnetic resonance. 65 The main difficulty concerns the large magnetic inhomogeneity induced by the catalyst used for the growth process, which prevents any magnetic measurements. Anyway, there is consensus that in CNT, the sp 2 signal is located around 100-125 ppm, according to the tube diameter D in nm ( = 18.3/D + 102.5). 66 The sample CNT* exhibits a poorly resolved peak that can be seen in Figure 3.13. This line shape has been related to a mixture of CNT of different types and diameters. 66 Additionally, a paramagnetic effect comes from the interaction between nuclei and unpaired electrons. As electrons have a magnetic dipole moment over 1000 times stronger than those of nuclear spins, it can dramatically alter the NMR signals nearby. 67 Although the quality of the spectrum is not adequate to accurately identify different species, some conjectures can be made based on the two peaks that can be distinguished at 183 and 159 65 ppm on O3h-CNT*-400 sample, which have been ascribed to carbon species at defects or edges. 68

Magnetization measurement analyses

An ideal graphene should be nonmagnetic itself due to π-symmetry electronic states formed by the unhybridized pz atomic orbital of sp 2 carbon atoms. Therefore, low magnetic susceptibility by the application of an external magnetic field is achieved on pure graphitic materials. Nonetheless, DFT studies have predicted that carbon atoms with unsaturated bonds play the role of magnetic impurities. For instance, reconstructed monoand trivacancies with unsaturated carbon bonds were predicted to contribute in magnetic signals. 69 Experimentally, it has been found that defects stated such as the carbon atom adsorbed on the bridge site, the single or multi-vacancies and the complex of carbon vacancies, can introduce a localized magnetic moment. 70,71,72 Ferromagnetism of graphene-based materials at room temperature has already been reported and saturation magnetization (MS) of 0.004-0.020 emu/g 73 were measured for rGO heat-treated at different temperatures. Figure 3.14a shows a typical ferromagnetic hysteresis loop associated to the magnetic catalysts (FeCo NP) used during the synthesis of the nanotubes. This points out that the acid treatments are not efficient to completely remove such impurities, and the encapsulated FeCo nanoparticles in the tips are hermetically sealed.

CNT* CNT

This result is in accordance with magnetic measurement reported in the literature (Table 3.5). The treatment at 2800 °C (CNT*) allows a complete removal of these impurities that correspond to 0.17 wt% (determined by ICP-OES analysis), resulting in nonmagnetic CNTs. Figure 3.14 highlights the drastic change in the magnetic properties on CNT*. The CNT free of magnetic impurities were oxidized 1, 3 and 5 hours in order to observe the influence of the oxygen surface groups on the magnetization (Figure 3.15a,b). The negative slope indicates a decrease in the value of the magnetic moments with increase in applied magnetic field, characteristic of diamagnetic material, showing that the oxygen functional groups do not induce paramagnetic response at room temperature. Nonetheless at 5 K a paramagnetic behavior was evidenced. A similar trend was noticed for graphene oxide quantum dots, 79 where the authors explain this contribution in terms of Pauli's and Curie's paramagnetism. In other words, this phenomenon can be present on materials with highly delocalized electrons in the space, and its magnetization would be directly proportional to the imposed magnetic field but inversely proportional to the temperature.

The additional energy per electron comes from the interaction between the electron spin and the magnetic field. Also, Tadyszak and co-workers, 80 observed a small paramagnetic signal at 5 K on graphene nanoflakes. Theoretical studies propose that the large π-conjugation structure of graphitic materials offers the required stability for unpaired electrons from radicals to generate net spin units. 81 Several reports have been published regarding magnetic signals on graphitic materials after high energy irradiations, relating those signals to the creation of different types of point defects with unpaired electrons such as vacancies or unsaturated edges. 82,83,71

Electron paramagnetic resonance analyses

Electron paramagnetic resonance (EPR) is also an effective method for studying the defect structure of nanostructured carbon materials. EPR signals are induced by itinerant electrons and unpaired electrons caused by dangling bonds. The energy differences studied in EPR spectroscopy are predominately due to the interaction of unpaired electrons in the sample with an imposed magnetic field. 84 The g-factor is the spectroscopic factor of free electron that can be calculated from equation (1), where h is the Plank constant, β is the Bohr magneton,  is the frequency of the radiation, and Bo the intensity of the magnetic field. 85 ( ) ≈ 2.00232

A free electron in vacuum has a g value ge= 2.00232. The experimental differences with this value give chemical information that lies in the interaction between the electron and the electronic structure of the molecule/material. For instance, in organic radicals, the g value is very close to ge ranging from 1.99 to 2.01. For transition metal complexes, the g value varies a lot because of the spin-orbit coupling and zero-field splitting. Usually it ranges from 1.4 to 3.0, depending on the geometry of the complex.

EPR spectrum of single-layer graphene obtained by the scotch-tape method has been reported, giving a Lorentzian line with g-value varying from 2.0040 at liquid helium temperature to 2.0045 at room temperature. The linewidth diminishes from 9 to ∼6.5 G with temperature from 4 to 300 K. 86 For the case of graphene the precise values of the components of the g tensor in graphite are: g┴, = 2.00314 and g║ = 2.05006. 87 Pure and perfect CNT are inactive to EPR, the experimental data often shows a very narrow signal with a Lorentzian line shape at g ≈ 2.002. 88 . As we have mentioned before, the magnetic properties of nano-sized graphenes and graphites are significantly perturbed by the presence of interruptions of the crystal structure. 89 It has been proposed that when localized defects in the sp 2 network or edge-structures are introduced, a broad line appears (g = 2.0035). Some researchers have deconvoluted this signal in different contributions.

For example, Ciric et al. 90 deconvolute the experimental response of graphene oxide sample into two components (Figure 3.17a), relating the broader one with the presence of defects. In the same way, Tadyszak et al. 91 analyzed anthracite powder, being capable to fit two Lorentzian lines in the spectrum (Figure 3.17b). The authors claim that the first constituent of the signal (representing the 80% of the total), can be related to the intrinsic conduction electrons of the carbon and the second correspond to the localized states due to lattice defects. Thus, this EPR response can be associated to the presence of unpaired electrons mainly found in fragments with an odd-number of electrons, or with unpaired electrons in structures with a peculiar topology, such the edges, dangling bonds and vacancies. The EPR measurements performed in this study are presented in Figure 3.18. O3h-CNT* does not show any signal at room temperature, but well resolved band was detected at -263 °C (10 K). A weak peak centered at 3383 G with a peak-to-peak linewidth of 3.13 G was detected. The spectrum was homogeneous, did not exhibit hyperfine coupling or distribution of the g value (2.003). 92 Magnetic field scanning over a broader field range did not reveal any additional resonances that would belong to frequently observed transition-metal impurities confirming the purity of the samples. O3h-CNT*-400 displayed a more intense signal with 13.9 G linewidth. Additionally, the g-factor was strongly anisotropic. When the magnetic field was parallel to the graphitic planes the gfactor had a minimum of g | | = 2.0002. A strong g shift was measured when the field is perpendicular to the planes (g┴ = 2.0033). It is worth mentioning that g values close to 2.0029 were predicted by Ishii et al. for dangling bonds in amorphous carbon. 93 Similar values were also obtained for point defects in electron irradiated CNT (2.0026), 94 or neutron-irradiated graphite (2.0023). 95 Our defective material presented a broader signal that can be deconvoluted in two Lorentzian lines with different linewidths. The narrow line (Figure 3.18, blue line) has been associated to interacting conduction electrons, while the broad one (Figure 3.18, red line) corresponds to spin-polarized localized electronic states. 90,91 It is important to mention that the sample was managed avoiding any contact with air so we can exclude the EPR line broadening being due to molecular oxygen.

With this combination of techniques, we can claim the effective creation of defects on the graphitic surface, although the specific nature of those defects needs further clarification.

At the same time, we propose that they can be used as trapping centers for metal atoms, thus leading to chemisorption with the formation of strong bonds. [Ru(COD)(COT)] (COD = 1,5 cyclooctadiene, COT = 1,3,5-cyclooctatriene) and the elimination of the remaining ligand. For details, see the experimental section at the end of this chapter. The full procedure is summarized on Figure 3.

consisting on heating

O3h-CNT during 1 hour at 400 °C. In a first set of experiments, a higher heat treatment temperature (1000 °C, to produce O3h-CNT-1000) was used to ensure the removal of a large fraction of oxygen functional groups trying to obtain maximal formation of reactive sites, such as described in ref. 35 The aimed Ru loading was 0.4 wt.% Ru.

C C C

C

Step III

Carbon support Oxidized carbon support

Single-atom sites on carbon

Step I

Support activation

Step I

Support oxidation

Solvent/ metallic precursor The most concise technique for confirming the successful insertion of metal atoms onto a support is high-resolution microscopy. Recent advances in microscopy allow for direct proving of the presence of single surface metal atoms in a sample. For this purpose, we resort to using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to characterize our samples.

In Figure 3.20 we can clearly identify plenty of bright dots on Ru/O3h-CNT-1000 thanks to the much higher atomic number of ruthenium compared to carbon atoms. It is worth to indicate the selective production of single atoms, since we were not able to visualize any cluster or RuNP throughout the entire microscopy grid. The visualization of the SA was easier on thinner tubes rather than on the thicker one. DFT calculations claim that Ru atoms can be trapped on defective graphene surfaces in the order Ru/SV > Ru/DV > Ru/SW. 107 The calculated binding energy of Ru on a 555-777 defective graphene (7.2 eV) is relatively high, therefore, the aggregation of these embedded metal atoms can be efficiently prevented. 108

Figure 3.20. HAADF-STEM micrograph and EDX analysis of a) 0.4%Ru/O3h-CNT-1000; and b) zoomed zone highlighted on red, magnification 3.6 times

To quantify the metal loading we performed inductively coupled plasma optical emission spectrometry (ICP-OES). The Ru/O3h-CNT-1000 sample prepared with a theoretical 0.4 wt.% Ru yielded a value of 0.12 wt%. This lower metal loading could be related with a low density of reactive sites. It can be explained by the phenomenon of rearrangement of carbon vacancies at high temperatures, conducting to the reconstruction of the graphitic structure. 109,110,111 Numerous studies have shown that with the increase of temperature of annealing, the structural ordering improves and a reduction in the overall defect concentration occurs. When lower temperatures are used, the resultant concentration of reactive defects should increase. We show in Table 3.6 the results obtained using O3h-CNT-T heat-treated at different temperatures. An optimal value was found at 400 ºC where the amount of ruthenium reached was 1.6 wt%. For this reason, all our further investigation will set 400 ºC as activation temperature for the supports. An evidence of saturation of the defect reactive sites is the change of color of the reaction media. In the cases of theoretical deposition 0.4 (0.4%Ru/O3h-CNT-400) and 1.0 wt% Ru (1.0%Ru/O3h-CNT-400), the pentane solution turned from yellow to colorless in a matter of minutes (Figure 3.21). In the case of higher metallic loading, the solution goes from intense yellow to pale yellow indicating no total deposition of the metal. Trying to get a general insight of the deposition process we have analyzed this supernatant by GC-MS. Before the analysis the liquid was filtrated with PTFE filter in order to remove any traces of solid. The analysis showed mostly the presence of COT in the solution (identification NIST library 97 % accuracy) and very low concentration of COD. This result suggests that under reaction conditions, COT is preferentially decoordinated from the complex. In this context, there are some examples in the literature dealing with ligand displacement in [Ru(COD)(COT)], 112,113 and it was generally accepted that COT ligand is more labile than COD. However, other studies report that reactions with alkyl phosphites, alkylphosphonites, and alkylphosphinites cause selective displacement of 1,5-COD.114 3.8). The peak at low temperature can be assigned to the decomposition of labile surface epoxy groups. 121 The formation of such groups has been investigated by theoretical calculations for Ru13/graphene catalysts. 119 The new epoxy surface functional groups arises from the reaction of the oxidized Ru cluster (due to air exposure of the samples) and the graphene surface (Figure 3.25). In STEM, homogeneous deposition was observed on Ru/O3h-CNT that may correspond to the [Ru(COD)(COT)] precursor anchored on the external surface of CNT by acetato ligands (Figure 3.26a,b). However, when the sample was heat-treated at 300 ºC 20/80 H2/Ar, RuNP of size diameter of 1.03 nm appeared over the entire surface of the support, unlike previous thermal tests, indicating that the anchoring over vacancies delivers higher stability than using surface -COOH groups. For metals such as gold, the calculated binding energy of AuSA to a defect site is 6-18 times (i.e., 3.02-9.56 eV) higher than that on a defect-free carbon surface (0.45 eV). In other words, it is evident that once Au atom is trapped on a defect site on the carbon surface, it is thermodynamically more stable. 122 To enlarge the scope of the deposited metals, we prepared samples 1.0%M/O3h-CNT-400 from six other organometallic precursors. The detail of each precursor and obtained metallic yield is presented on Table 3.9. The EDX signals have low intensity due to the high dispersion of the metal. It is worth mentioning that to facilitate the identification of Cu species, the interference signals that usually come from the grid were eliminated by dispersing the sample on a nickel grid.

Finally, we extend the experiments to other carbon supports and prepare SAC using a fibrous carbon material (FM), small carbon nanofibers (sCNF) and carbon black (CB, Vulcan XC-72). As mentioned before, the supports were prepared and purified in our laboratory following the protocol described in Ref. 123 , with the exception of CB, obtained from CABOT Corp. The samples present different metallic loadings, which cannot be related with any textural properties including BET surface area, pore volume, inter-defect distance (Ld) nor average stacking heights parallel to the fiber axis (Lc), which is a measure of surface reactivity (Table 3.10). Anyhow, a high density of SA was obtained, accompanied with few clusters Figure 3.28. In a recent report, researchers detect a synergic contribution from Ru nanoparticles in a system containing RuSA, but surprisingly, they did not seek for the role and influence of each species on the catalytic activity. 126 Additionally, some articles have been published

recently dealing with "two-component catalysts", highlighting the synergistic effect between atomically dispersed species and nanosized cluster/particles. 127,128,129 The

improved

c )

One technique to achieve two-component catalysts is the "top-down" approach where atoms can be released from nanoparticles by "re-dispersion" under specific reaction conditions. 132 For example, Pd-NPs@ZIF-8 nanocomposites were heated at 900 °C under an inert atmosphere for 3 h, after which the PdNP disappeared to give thermally stable PdSA. 133 Thus, great changes in the electron structures of metal clusters are created by giving more metallic availability. 134,135 However, sometimes the process occurs without a rational control, and the size of the resultant species cannot be tuned, which can be detrimental to catalyst performances. Thus, dramatic differences in rate and selectivity on active sites of differing Rh nuclearity have been observed. 136 Alternatively, the "bottomup" approach allows growing clusters from individual atoms by aggregation, creating nanomaterials in a clear and manageable manner, such as with atomic layer deposition. 137 The disadvantage of the latter method lies in its low practicality due to limitations in equipment and scale-up. That is why we present here a simple procedure that does not require sophisticated equipment to prepare catalysts containing simultaneously controlled amounts of single atoms and nanoclusters.

We discovered that by repeating the impregnation step we were able to produce a heterogeneous material composed of SA and NP/clusters. Furthermore, we realized that by controlling the number of impregnations cycles it was possible to tune the ratio of SA and clusters. More specifically, the following procedure has been followed:

Step III: Metal deposition. The solid obtained in step II was transferred to a Schlenk tube under argon avoiding contact with oxygen. Immediately, the solid is impregnated with 30 mL of pentane metal-containing solution transferred via canula. The concentration of the solution will correspond to the desire amount of metallic precursor to get a loading (e.g. 1.4 wt.% Pd, (bis(2-methylallyl)Pd)) divided X times (2, 3 or 4 parts), depending on the wished SA/NP ratio (smaller values of SA/NP ratio will require higher number of impregnations). The mixture was stirred one hour at room temperature under argon atmosphere. Afterwards, the solid is allowed to precipitate to remove the supernatant then washed 3 times with pure pentane and dried under vacuum overnight.

Step IV: Elimination of ligands. The previous solid was heated at 5 °C/min, up to 80 °C for one hour, at a flow rate of 80/20 Ar/H2. After, the sample is managed as step III. The number of times that step III is performed until to reach the given total metal loading will modify the SA/NP ratio. Finally, the samples were stored in air and room temperature.

For two, three and four cycles, the samples were labeled 0.70%x2 Pd/O3h-CNT-400, 0.46%x3 Pd/O3h-CNT-400 and 0.35% x4 Pd/O3h-CNT-400, respectively. The procedure is summarized in Figure 3.29. After each impregnation, the real metallic content was measured by ICP-OES. Table 3.11 display the values obtained after each cycle of precursor addition.

C C C

C

Metal deposition

Step III

Single-atom site + clusters

Step II Support activation

HNO 3 (conc.) 140ºC
Step I

Support oxidation

Step IV When analyzing the data, a linear correlation was obtained between nominal and actual metallic content (Figure 3.30). The variability of each cycle was low, indicated by the low deviation. These results suggest good efficiency and reproducibility of the method. We also characterized these samples by XPS, to prove the chemical and compositional properties of the resultant materials. The electronic state of Pd was studied. Making a comparison with the reference values of bulk Pd at core level of Pd 0 3d5/2 (335.10 eV), 138 we can see that 1.4%PdSA/O3h-CNT-400, presents a shift towards higher binding energies (335.58 eV), which has been broadly related to phenomena of charge transfer from the metal to the support, leaving electronically deficient PdSA. The ratio between these two peaks is 13/87 and 41/59 for 0.70%x2 Pd/O3h-CNT-400 and 0.35%x4 Pd/O3h-CNT-400 samples, respectively. These data, in association with the STEM-HAADF observations, confirm that the proportion of SA is much higher in the sample with two cycles than the one with four. The spectrum of O 1s (Figure 3.33) did not evolve considerably from the second impregnation onwards, which suggests that the subsequent insertion of Pd happens on the sites created during the first deposition (these acting as seeds), and does not occur on the remaining oxygen groups.

Conclusion

In summary, the preparation of SAC has been investigated through the reaction of a metallic precursor in the presence of a defective carbon support and the resulting materials have been characterized. We have evidenced the introduction of defects by a functionalization-defunctionalization process over a carbon support, thanks to magnetic techniques such EPR and SQUID. We have proved the importance of this process in the successful anchoring of stable metallic atoms. The temperature of activation as well as the choice of the precursor and solvent affect the final metallic loading. This method is reproducible and applicable to different metals (Ru, Pd, Pt, Ni, Co, Cu, and Ir were investigated) and supports (CNT, CNF, FM, and CB were used). Then, we have demonstrated in the case of Pd that it was possible to control the SA/NP ratio between 200 and 10 by repetition of impregnation cycles. Significant charge transfer from palladium to CNT was evidenced by XPS, which can be of importance when evaluating catalytic activity. This strategy allows engineering the minimal catalytic ensemble that we envision to produce in order to study a possible cooperative catalytic effect between SA and NP in Pd/C supported catalysts, which is the subject of the next chapter. Step II: Support activation. 1.0 g of OXh-CNT was placed in an alumina crucible and inserted into a quartz tube in a horizontal furnace. The support was heated under argon at 5 °C/min up to 400 °C during 1 hour (600 and 1000 °C were also used for comparison).

After the heating program the samples were allowed to cool down on argon and finally kept under an inert atmosphere to perform further experiments. The obtained product was labelled as OXh-CNT-400.

Step III: Metal deposition. The solid obtained at step II was transferred to a Schlenk tube under argon avoiding contact with oxygen. Separately, a desire amount of a solution containing the metallic precursor was prepared under inert atmosphere (for example: 0.4%; 12.5 mg of [Ru(COD)(COT)] was dissolved in 30 mL of degassed pentane).

Immediately, the yellow solution was mixed with the support under argon by transferring the liquid via cannula and let to react during one hour at room temperature under argon atmosphere. Afterwards, the solid was allowed to precipitate to remove the supernatant, then washed 3 times with pure pentane, and finally dried under vacuum overnight.

Step IV: Elimination of ligands. The prepared carbon material containing the metal was heated at 5 °C/min, up to 80 °C for one hour, at a flow rate of 80/20 Ar/H2. After this treatment the sample was stored in air.

Catalysts Characterization

The high-resolution analyses were conduct using a transmission-corrected field emission electron microscope, probe coupled to an energy-dispersive x-ray spectrometer (EDX)

and an energy loss spectrometer (EELS) JEOL JEM-ARM200F Cold FEG operating at 200 kV with a point resolution of > 1.9 Å.

Thermogravimetric analyses were performed in a Mettler Toledo TGA2 device with a heating rate of 5ºC/min and constant flow of N2 20 mL/min.

Temperature-programmed desorption analysis were obtained with a fully automated AMI-200 Catalyst Characterization Instrument (Altamira Instruments), equipped with a thermal conductivity detector and a quadrupole mass spectrometer (Dymaxion 200 amu, Ametek) and the sample was heated to 1100 °C using a constant flow rate of helium (25 cm 3 /min, STP).

The Raman measurements have been recorded with a Raman Horiba Jobin Yvon Labram HR 800 spectrometer in backscattering geometry using an optical objective 100 (NA 0.9).

The wavelength of the incident laser was 532 nm and it power was set to 1 mW.

XPS was performed using a Thermo Scientific K-alpha apparatus (hν (Al Kα) = 1486.6

eV); and the XPSPeak 4.1 software was used to perform curve fitting and to calculate the atomic concentrations.

NMR experiments were recorded on Bruker Avance 400 III HD spectrometer operating at magnetic fields of 9.4 T. Samples were packed into 2.5 mm zirconia rotors under argon inside a glove box. The rotors were spun at 10 kHz (r) at 295 K. 13 C Hahn-echo were synchronized with the spinning rate and performed with a recycle delay of 30 s. Chemical shifts were referenced to liquid TMS.

We have measured the magnetic properties of our samples with a superconducting quantum interference device-vibrating sample (SQUID-VSM) magnetometer MPMS5.

The magnetization loop was measured between -10 and 10 kOe at T = 300 K after cooling the sample from 27 °C (300 K) to -268 °C (5 K) under a constant applied magnetic field Hcool = 50 kOe.

EPR data were recorded using an Elexsys E 500 Bruker spectrometer, operating at a microwave frequency of approximately 9.5 GHz. All spectra were recorded using a microwave power of 10 mW across a sweep width of 1500 G (centered at 3100 G) with a modulation amplitude of 2 G. Experiments were carried out at 10 K using a liquid helium cryostat. The hydrogenation of alkenes is among the important technological reactions catalyzed by supported metals, finding application in the pharmaceutical, agrochemical, refining, and commodity chemical industries. As the catalyst active phase is often made of noble metals, expensive and for the most part identified as critical elements, a logical step is to optimize their dispersion on the support. 1 The highest dispersion that can be achieved corresponds to supported isolated metal atoms. Supported SA have coexisted with metal NP for a long time, 2 and may have played significant roles in many catalytic processes, but they have rarely been recognized as active sites. Today, aided by recent advances in synthesis methodologies, characterization techniques and computational modelling, single-atom catalysis has decidedly become the most active new frontier in heterogeneous catalysis. 3 Single atoms exhibit distinctive performances from the ones of NP for a wide variety of chemical reactions, and industrial applications employing them already exist. 4 The absence of atomic ensemble effects, and the charge transfer to or from the support, that strongly modifies the electronic structure of the metal atom in SA catalysts, significantly affect the adsorption, and definitively induce a different reactivity compared to metallic NP. 5 Current studies conducted on SA focus mainly on distinguishing them from NP and the possibility of synergy between these two species have been rarely explored. 6,7 Interestingly, it was recently proposed that a synergy may operate between PdSA and PdNP during ketone/aldehyde hydrogenation on Pd/TiO2 catalysts. 7 In this reaction, PdSA contributes for the C=O activation while PdNP allow the easy dissociation of H2 (Figure 4.1). From ref. [7].

Palladium on carbon is a very common catalyst for alkene hydrogenation. 8 Hydrogenations with PdSA have been already reported 3,9 but the metal-support interaction in such catalysts often results in an ionic/electron-deficient state of the metallic atoms that prevents an efficient H2 activation compared to metallic surfaces 7,9,10 and limits their application to specific selective hydrogenation reactions. 11,12 Although a direct H2 activation has been reported for Pd/Cu single-atom alloy, 11 on PdSA, H2 activation is performed thanks to support-assisted molecular hydrogen heterolytic dissociation. 13,14 Given that H2 dissociation is often the rate-limiting step in these reactions, efficient strategies toward engineering the minimal catalytic ensemble that can activate H2 would represent a substantial progress in pursuit of highly efficient Pd/C hydrogenation catalysts.

In this chapter, through a combined experimental-theoretical study (theoretical calculations performed in collaboration with the Laboratoire de Physique et Chimie des Nano-Objets (LPCNO)), we show that for alkene (myrcene, squalene, farnesene) hydrogenation (performed in collaboration with the Laboratoire de Génie des Procédés Catalytiques (LGPC) in Lyon a cooperative catalysis takes place between carbon-supported PdSA and PdNP. First, PdNP dissociate H2 easily, and subsequent hydrogen spillover provides Pd-H species on PdSA (H-PdSA). The H-PdSA are much more active than H-PdNP for the hydrogenation and isomerization of C=C double bonds. Thanks to the control of the SA/NP ratio on the support, which has been achieved in this work, the rational design of highly active catalysts becomes possible.

Hydrogenation and isomerization of alkenes

As a model substrate -myrcene, an acyclic polyunsaturated monoterpene with highly reactive 1,3-diene moiety, and an isolated tri-substituted C=C bond, was chosen (Figure 4.2). The hydrogenation of this important molecule has been studied over different homogeneous 15 or heterogeneous 16 catalysts. The complete hydrogenation to 2,6-dimethyloctane usually requires long reaction times (and 3 equivalents of H2), resulting in a low average Site Time Yield (STY). We recently reported 17 that a structure/activity correlation exists in Pd/C catalysts for myrcene hydrogenation, which integrates the metal dispersion, and the concentration of oxygen surface groups and defects on the support (see Figure 4.3). If the introduction of the dispersion values in this correlation and its impact on activity is not surprising, the influence of the percentage of oxygen and of defects on the carbon support was not clarified, and required further studies. The elucidation of the exact role of the percentage of oxygen and of defects on the carbon support on the catalytic activity was the main goal of my work. We decided to test Pd/CNT on this reaction considering the relatively high amounts of oxygen surface groups on CNT supports, 17 and compared it with two commercial Pd catalysts supported on alumina and activated carbon (see Table 4.1 section 4.3). The reaction was performed at 120 °C and under 20 bar. The pressure and temperature monitoring of the calibrated gas reserve provides the hydrogen consumption rate, and thus the apparent and instantaneous activity of the catalysts expressed in moles of H2 consumed per mole of Pd and per second, and being equivalent to an instantaneous Site Time Yield ("STY" in the following).

The instantaneous STY is determined by the local slope of the H2 reserve consumption curve as following, where nPd is the quantity of Pd determined by ICP:

𝑆𝑇𝑌 = ∆𝑛 𝐻 2 ∆𝑡 × 𝑛 𝑃𝑑 (𝑚𝑜𝑙 𝐻 2 𝑚𝑜𝑙 𝑃𝑑 𝑠 ⁄ ⁄ )
STY is qualified of apparent because (i) it addresses the overall consumption of H2 through multiple reactions and (ii) depending on the catalyst and the reaction progress, possible external and internal mass transfer resistances are likely. Indeed, the reactivity of the first C=C is very high (terminal and conjugated C=C) and the first moments of the reaction are physically limited.

Hopefully, this is not true for the whole range of reaction course. For robust and chemically consistent catalysts comparison, STY at 66% and 83% of reaction progress will be considered, i.e. when 2.0 and 2.5 equivalents of H2 per -myrcene are consumed respectively.

More details about the characterization of possible internal and external mass transfer limitations throughout the reaction progress are given in the experimental section 4.7.

The catalytic activity of Pd/CNT and Pd/C were similar and these two catalysts were significantly more active than Pd/Al2O3 (Figure 4. The major difference observed between these catalysts was the presence of PdSA on Pd/CNT and Pd/AC (Figure 4.7a-c). It is worth noting that the probable presence of atomically dispersed Pd species on reduced Pd/CNT catalysts has been recently discussed by De Jong and coworkers, 18 and that the presence of PdSA on commercial Pd/AC catalysts have been already reported. 2 A ratio SA/NP = 2 was measured from STEM-HAADF micrographs in the Pd/CNT catalyst (see Table 4.1 section 4.3). This catalyst (Figure 4.7c) will be named Pd2_SA/NP/CNT from now on. We independently checked that the SA/NP ratio of Pd2_SA/NP/CNT does not change significantly after catalysis. For this catalyst also, the mean PdNP size and SA/NP ratio (≈ 10 in this case) does not change significantly after catalysis (Figure 4.9). From these findings, we assumed that the presence of single atoms should be considered to rationalize the observed differences of reactivity. As discussed in the previous chapter, the existence of PdSA on carbon supports can be attributed to the presence of specific anchoring sites such as structural defects, which are necessary for their stabilization. 9,19 From the reaction profiles of Figures 4.3a,b, we can see that the reaction starts rapidly for all the samples, then for Pd/Al2O3 it slows down, while a high rate is maintained for the catalysts supported on carbon. We propose that this rate decrease for Pd/Al2O3 catalyst can be associated to the difficult hydrogenation of the internal double bonds of myrcene. 20 On the other hand, the stable activity of carbon-supported catalysts might be explained in terms of the possible involvement of acidic sites in the isomerization of the relatively crowded double bond to an easier to reduce one. The potential of acidic sites on the isomerization of alkenes has been early reported. 21 In our case, two types of acidic sites may coexist on the carbon-supported catalysts. do not establish the presence of significant amounts of carboxylic groups after Pd deposition, these groups acting as anchoring sites for the metal. 22 Thus, the presence of acidic sites that would assist the isomerization cannot explain the observed differences in reactivity. To gain further insight on the mechanistic pathway, DFT calculations were performed in LPCNO on PdSA and PdNP supported on graphene as model support.

Mechanistic insights from DFT calculations

To study the isomerization and hydrogenation reactions, 2-butene instead of myrcene was used as a model substrate (for simplifying computational calculations). Starting with the isomerization, we considered the trapping of a PdSA in a mono-vacancy, which is strongly exothermic, with an adsorption energy of roughly 100 kcal/mol. On PdSA, both, isomerization and the hydrogenation reactions begin with the exothermic π-coordination of 2-butene to Pd (by -24.6 kcal/mol) (1). From 1, the isomerization (Figure 4.13, black line) occurs via the formation of a π-allyl hydride intermediate as observed with homogeneous catalysts, 23 which requires to pass a high energy demanding transition state (36.4 kcal/mol) corresponding to the abstraction of one hydrogen atom from a methyl group. This leads to the formation of an unstable allyl intermediate (2), by +34.4 kcal/mol with respect to 1. Then, the H atom is transferred with almost no barrier with respect to 2 (2.8 kcal/mol) to the carbon atom in alpha position of the methyl group, leading to the endothermic formation of but-1-ene (+4.8 kcal/mol with respect to 1).

Figure 4.13. DFT mechanistic study of reaction mechanisms of isomerization of 2-butene mediated by PdSA and H-PdSA (in orange).

From 1, the hydrogenation of the but-2-ene (Figure 4.14, black line) occurs through the Horiuti-Polanyi mechanism: 24 (i) alkene and H2 coordination to Pd, (ii) H2 dissociation, (iii) hydrogen transfer to one of the alkene methine carbon with formation of a σ-bond between Pd and the second C-methine, and finally (iv) reductive elimination of the free alkane. In our case, the coordination of the 2-butene (1) and of a H2 molecule (4) to the PdSA is exothermic. These initial steps are followed by an endothermic (+21.6 kcal/mol with respect to 4) and kinetically demanding (activation barrier = +24.5 kcal/mol) H-H activation (5). This reaction exhibits a lower barrier than the isomerization reaction, by 9.9 kcal/mol. From 5, the reaction continues by the transfer of one of the hydrogen atoms to one of the methine carbon atoms. The energy barrier of this last step is only +8.1 kcal/mol with respect to 5 but +29.7 kcal/mol with respect to 4. This transfer induces the formation of a 2-butyl complex σ-coordinated to the Pd center ( 6), located at -11.7 kcal/mol with respect to the entrance channel. The subsequent reductive elimination occurs via an accessible transition state (+4.1 kcal/mol with respect to 6) and leads to the exothermic formation of butane (-42.1 kcal/mol with respect to the separated reactants).

In summary, the hydrogenation of the 2-butene into butane is preferred both from a kinetic and thermodynamic point of view over the isomerization to 1-butene. However, according to the high barriers, a poor catalytic activity is expected in both cases, as experimentally observed for hydrogenation using single Pd atoms. 25,26 For comparison, an activation energy of 18 kcal/mol has been reported for the hydrogenation of 2-butene on a Pd9/CNT catalyst. 27 In this context we also verified that the PdSA/CNT catalyst shows a very low activity for β-myrcene hydrogenation (see Figure 4.21 in the section 4.3 Control of the single atoms/nanoparticles ratio).

At this point, it is interesting to note that a completely different potential energy landscape is obtained if the catalyst is a hydrogenated SA or Pd hydride (H-PdSA, orange lines on Figures 4. 13 and 4.14). A plausible scenario for the formation of this active site is: (i) a nearly barrierless activation of H2 on PdNP (< 2kcal/mol on Pd13 supported on defective graphene monolayer), and (ii) hydrogen spillover. The latter phenomenon has been widely theoretically studied in the context of hydrogen storage and catalysis. 28,29,30 The entire spillover mechanism remains not fully understood, especially since some energy barriers remain high to be accessible at room temperature, but should correspond to a multi-step process after the dissociative adsorption of H2 on PdNP. The H-migration from the NP to the carbon support is supposedly the first step with large energy barrier to overcome and usually this step is not thermodynamically favorable for non-defective carbon supports. For instance a 60 kcal/mol energy barrier has been reported in the case of a Pt cluster on pristine graphene model, the final step being 35 kcal/mol above the initial one, 31 but when the graphene support is already hydrogenated the energy barrier is decreased and the thermodynamics become favorable. 32 However, it remains obscure how the support can be initially hydrogenated. Then H-diffusion on the carbon support occurs, with a relatively high energy barrier associated to a nearest-neighbor hopping process (> 20 kcal/mol on pristine carbon support) 19,30 It has also been proposed that the presence of oxygenated functional groups on the carbon support can facilitate the diffusion. 33 H-clustering could also play a role by disturbing the sp 2 network around the H-adsorption site, decreasing significantly the energy barrier. 34 As we can see on Figure 4.13 (orange line), the coordination of 2-butene over H-PdSA (1 H ) remains exothermic, -23.3 kcal/mol with respect to the separated reactants. However, in this case the isomerization reaction takes place through a so-called metal hydride additionelimination mechanism, strongly preferred for monohydrides. Thus, the isomerization proceeds via an initial hydrogen transfer to one of the -CH= carbons, followed by the dehydrogenation of the methyl group at α position with respect to the second methine carbon. This reaction is kinetically accessible with two low activation barriers (3.5 and 11.9 kcal/mol, respectively). However, even if the formation of the 2-butyl intermediate ( 2 H ) is exothermic with respect to 1 H by -6.0 kcal/mol, that of the 1-butene product (3 H ) remains endothermic by +2.8 kcal/mol. Concerning the hydrogenation (Figure 4.14, orange line), the coordination of H2 is followed by the transfer of the hydrogen atom to one of the methine carbons without H2 dissociation. The corresponding activation barrier (8.4 kcal/mol above 4 H ) corresponds to an easily kinetically accessible process. In the same way, the formation of the alkyl intermediate is found to be exergonic with respect to 4 H (-3.2 kcal/mol with respect to 4 H ). The last step corresponds to the heterolytic splitting of H2 leading to the regeneration of the H-PdSA and the butane product (7 H ).

The formation of butane is an exothermic process, by 25.2 kcal/mol with respect to 4 H (-42.4 kcal/mol with respect to the separated reactant). The corresponding activation barrier for this step is 12 kcal/mol. Therefore, this last step is the rate-determining step of an overall facile and exergonic reaction.

In order to confirm the key role of H-spillover in the cooperative catalysis between PdSA and PdNP, a Pd2_SA/NP/CNTHT catalyst (Figure 4.15) was prepared on CNTHT, from which the oxygen surface groups have been removed by thermal annealing at 1000 °C, 17 which should prevent Hspillover. Although this catalyst contains also a mixture of PdSA and PdNP (SA/NP ≈ 2), with mean particle size similar to the Pd2_SA/NP/CNT catalyst (Figure 4.15) it shows a significantly reduced activity for myrcene hydrogenation (Figure 4.16), which is quite similar to the one of the Pd/Al2O3 catalyst that contains only PdNP.

for details of calculations).

It is also worth mentioning that the two catalysts Pd2_SA/NP/CNT and Pd2_SA/NP/CNTHT present a similar amount of Pd clusters (PdNP < 1 nm), so that the observed high activity measured for the Pd2_SA/NP/CNT catalyst cannot be only related to the presence of Pd clusters (Figure 4.15), even if it has been reported that noble metal clusters are often more active than nanoparticles. 35,36,37,38 The hydrogen spillover was experimentally probed for these two CNT-supported catalysts. As the spilled-over hydrogen migrates and readily reacts with the yellow WO3 oxide to form dark blue HxWO3 39,40,41 we used the WO3 tungsten oxide to diagnose the activation of H2 on these two catalysts. As shown in the photographs of Figure 4.17, the pure WO3 did not change color after hydrogen treatment. When mixed with the Pd2_SA/NP/CNTHT catalyst, no significant change in color after the hydrogen treatment was observed. While this Pd catalyst should activate hydrogen, the absence of surface oxygen groups on the support does not allow extended hydrogen spillover. In contrast, when WO3 was mixed with the Pd2_SA/NP/CNT catalyst this resulted in the darkest color of the tungsten species after hydrogen treatment at 20 °C for 3 min, demonstrating that hydrogen dissociation and extended spillover occurred on this catalyst. We infer that the significant spillover should be attributed to the presence of oxygenated surface groups on the carbon support. These latter experiments confirm that the presence of oxygen surface groups is necessary for the cooperative catalysis to proceed via H-spillover. The hydrogen spillover was also investigated by DFT in LPCNO to propose a complete pathway for the formation of H-PdSA species through the migration of H atoms initially adsorbed on H-saturated PdNP. As we can see on Figure 4.18, the overall reaction is kinetically accessible and thermodynamically favorable. Our starting geometry is a H-saturated PdNP deposited on a defective di-vacancy with an oxygen atom in its vicinity. From this starting configuration the first H migration from the PdNP to the closest C atom in front of the ether group is thermodynamically favorable (-38 kcal/mol), with a reasonable energy cost (activation barrier = 10.2 kcal/mol). Then a second H can easily migrate (the energy barrier is less than 2 kcal/mol) to the same carbon atom leading to the formation of a -CH2 group. Mind that the presence of an oxygen atom, in the vicinity of a stabilizing defect on the carbon support favors the thermodynamics, when it is not usually the case for pristine support. 42 Then we have evaluated the migration of one H from the -CH2 group to a neighbor C carbon atom, still involved in the di-vacancy ring. This step, with an energy barrier of 29.7 kcal/mol, is the ratedetermining step of the proposed profile. Then a second hopping process can proceed with a lower activation barrier (24.2 kcal/mol) to yield almost a single H atom adsorbed on the nondefective part of the carbon support. The adsorption of an extra H2 molecule on PdNP ensures the overall stability of the H-migration process. Then H-diffusion can occur with an energy barrier of around 24 kcal/mol, in line with previous works, or can be significantly decreased to less than 9 kcal/mol, when the H atom is surrounded by other H atoms, 6 in the present case.

Finally, the formation of the H-PdSA product take places through a barrierless H transfer to PdSA with a largely favorable stabilization energy of ca. -60.5 kcal/mol. Thus, the correlation that we have recently reported between the activity of Pd/C catalysts for -myrcene hydrogenation and their structure, 17 which integrates the dispersion of the metal, and the quantity of oxygen groups and defects on the support, arises from a cooperative catalysis between PdSA and PdNP that involves H-spillover. It is certainly necessary to have highly dispersed PdNP on the support to activate H2, but a high concentration of surface defects and oxygenated surface groups are also necessary on the carbon support, which favors the stabilization of PdSA and H-spillover, respectively. The resulting H-PdSA are highly active for alkene hydrogenation/isomerization.

Control of the single atom/nanoparticle ratio

From the above results, it becomes obvious that an optimized heterogeneous catalyst with an ultra-rational use of precious metals (often in short supply) could pass by an accurate control of the SA/NP ratio. With this in mind, we have tested samples with an engineered SA/NP ratio (for a similar Pd loading of 1.2 % w/w), in order to prove the influence of this parameter on the catalytic performance. Those samples were prepared with the previously presented protocol (Chapter 3, Section 3.5 Mixture single atoms and nanoparticles) and the samples have been relabeled as PdX_SA/NP/CNT, where X refers to the SA/NP ratio. Table 4.1 summarizes Pd loading, PdNP size, Pd dispersion, SA/NP ratio and atomic percentage of SA of all samples used in this study. Calculated over 20 NP from STEM-HAADF micrographs c)

Metal dispersion was evaluated from a universal mathematical relation between the mean relative size of metallic crystallites and their dispersion 1 . d) SA/NP ratio in number calculated over 500 elements from STEM-HAADF micrographs. e) Not determined

In detail, the quantification of the percentage of single atoms was calculated with:

𝐴𝑡𝑜𝑚𝑖𝑐 𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑆𝐴 = 𝑛 𝑃𝑑 𝑆𝐴 𝑛 𝑡𝑜𝑡𝑎𝑙 𝑎𝑡𝑜𝑚𝑠 𝑜𝑓 𝑃𝑑 (𝑆𝐴+𝑁𝑃) 𝑥100% ……….. (4.3)
Where the number of atoms on each spherical nanoparticle was calculated with: A good correlation was found between the results obtained with these two techniques (Figure 4.20, Table 4.2). These data, in association with the STEM-HAADF observations, confirm that the proportion of PdSA is much higher in the sample Pd200_SA/NP/CNT than in Pd10_SA/NP/CNT.

𝐴𝑡𝑜𝑚𝑠 𝑝𝑒𝑟 𝑠𝑝ℎ𝑒𝑟𝑖𝑐𝑎𝑙 𝑁𝑃 = 𝜋 𝐷 3 𝜌 𝑁 𝐴 6 𝑀 𝑤 ……………….…… (4.4)
Thus, this strategy allows engineering the minimal catalytic ensemble that will activate H2 for the cooperative catalysis to operate. Once again, it is interesting to note that for this sample series, for which the surface chemistry of the support is the same, it is not the sample that presents the higher proportion of palladium clusters that is the more active. Thus, the sample Pd60_SA/NP/CNT present a similar proportion of PdNP < 1 nm based on total atoms than the sample Pd10_SA/NP/CNT (Figure 4.19) but is significantly less active (Figure 4.21). Similarly, the sample Pd200_SA/NP/CNT, which presents a significant amount of clusters of different atomicity, is the less active of the series.

Finally, at a reaction progress of 2, there is a 44-fold increase of STY between the Pd10_SA/NP/CNT catalyst and Pd2_SA/NP/CNTHT (a catalyst containing NP with no possibility of hydrogen spillover, see cell solid foams 51,52,53 . Figure 24a presents the obtained evolution of the mean STY against the Turn Over Number (TON). As already mentioned, this mean STY cannot be compared to the instantaneous one followed in batch through H2 consumption. Here it is calculated from a mass balance regarding outlet flow composition between -myrcene, mono, di, and tri hydrogenated products. Depending on this distribution, the evolution of the corresponding reaction progress can be determined and is presented on Figure 4.24b. First, it is noticeable that a very high TON of 600 000 molH2/molPd is obtained, which is equivalent up to 100 recycling in batch mode with our typical conditions. Such a high TON is obtained because of the relatively low amount of Pd involved in the experiment (2 mg) together with a relatively long time on stream (> 35 hours).

Moreover, this indicates quite remarkable catalyst stability at all. The mean STY decreases quite regularly from 380 to 320 molH2/molPd/min at the final TON value of 600 000 molH2/molPd, which represent a loss of apparent activity of 15%. The corresponding evolution of the outflow composition indicates that the reaction progress decreases also with decreasing contributions of the second and third hydrogenation, while the first one remains constant. As in the batch experiments, this behavior for the first hydrogenation is explained by the gas-liquid external mass transfer limitation that hinder the intrinsic catalyst activity. For the slower second and third successive hydrogenations, the deactivation was logically more noticeable.

The coated catalyst was qualitatively examined by STEM-HAADF before and after this stability experiment (Figure 4.23). After the test the SA/NP ratio changes from ≈ 2 to ≈ 3 and the mean PdNP size increased from 2.5 to 3.2 nm. The slight deactivation could thus be structural but could also be explained by the strong adsorption of impurities from the natural starting material (β-myrcene). Although they could not be detected in the charge by GC because of their too low concentration, these impurities could continuously be absorbed on the catalyst, resulting in a gradual decrease in activity at high TON numbers. Studies are in due course to recover and characterize these adsorbed impurities.

Overall, the catalysts developed in this study appear resistant to sintering due to the stabilization of the SA by the functionalized carbon support and can undergo very long TON either in batch or continuous mode. Nonetheless in the chosen reaction, the catalysts appear sensible to a charge impurity and purification steps may be required to reach very high TON with a more stable activity over time. This fact was only detectable with the continuous mode operation because of its easier operability and ability to reach a very high TON. Moreover, in addition to efficient catalyst stability testing, the continuous mode operation appears very promising for the compact intensified and secure operation of this reaction.

Conclusion

In this chapter, we have reported that a cooperative catalysis operates between carbonsupported PdSA and PdNP for alkene hydrogenation and isomerization, which involved: i) dihydrogen activation on PdNP, ii) H-spillover on the support, and iii) the catalytic reaction on H-PSA species. This discovery provides a rational explanation for the structure/activity correlation that exists in Pd/C catalysts for myrcene hydrogenation. 17 While such a cooperative effect may occur in many Pd/C catalysts, it has never been recognized or exploited to improve catalytic performances in industrial catalysts. While this finding alone is important, major advances in heterogeneous catalysis can be achieved only if the SA/NP ratio can be controlled.

Indeed, the control of this ratio, a major achievement of this work, allows tuning the catalyst performances. For β-myrcene hydrogenation, STY variations of several orders of magnitude were measured as a function of the value of this ratio. Finally, other types of cooperative catalysis than the one reported herein can be envisioned, such as bifunctional catalysis with a single metal: metallic NP ensuring one function, and SA of the same element presenting an altered electronic density ensuring a second function. This has been recently proposed for the hydrogenation of aldehydes and ketones on Pd/TiO2 catalysts, 7 on which the electrodeficient PdSA contribute to the activation of the carbonyl group, while PdNP allow the dissociation of H2.

It is our hope that this work paves the way for new chemical reactivity involving a rational use of the active metallic phase. Pd/CNT (Pd2_SA/NP/CNT) catalyst was reduced in a horizontal tube oven under a nitrogen and hydrogen flow (20 vol% H2) at 300 °C for 2 h (25 °C to 300 °C at 10 °C/min). The same procedure was followed for the preparation of the 1.6% Pd/CNTHT (Pd2_SA/NP/CNTHT) catalyst by using the CNTHT support.

For PdSA synthesis (0.1% PdSA/CNT), the wet impregnation method was also used. The desired amount of palladium(II) nitrate dihydrate Pd(NO3)2•2H2O was added to an acetone solution (100 mL) containing 1 g of CNTox, so as to introduce 0.1 %w/w of metal phase. The solution was sonicated at room temperature for 6 h using a ultrasonic finger. The solution was then filtered and washed with acetone. The resulting solid was dried in an oven at 120 °C overnight.

Finally, the catalyst was reduced in a horizontal tube oven under a nitrogen and hydrogen flow (20 vol% H2) at 300 °C for 2 h (25 °C to 300 °C at 10 °C/min).

For PdX_SA/NP/CNT catalysts we use the procedure described in Chapter 3., Section 3.5 Mixture of single atoms and nanoparticles synthesis where X refers to SA/NP ratio, a recently patented process was used. 54 Finally, a 0.1%Pd1000_SA/NP/CNT catalyst with a SA/NP ratio > 1000 was also prepared by the wet impregnation method. The desired amount of palladium(II) nitrate dihydrate [Pd(NO3)2•2H2O] was added to an acetone solution (100 mL) containing 1 g of O-CNT, so as to introduce 0.1 %w/w of metal phase. The solution was sonicated at room temperature for 1 h.

Then, NaBH4 (0.1 g) was added at room temperature, and let to react for 1 h to complete catalyst reduction. The solution was then filtered and washed with water and then acetone. The resulting solid was dried in an oven at 120 °C overnight.

For comparison purpose in myrcene hydrogenation we also used a commercial 5%Pd/activated carbon catalyst and a 5%Pd/Al2O3 catalyst.

The palladium content in the products was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) performed at the LCC with a Thermo Scientific ICAP 6300 instrument. Elemental analyses of C, H were carried out on a PERKIN ELMER 2400 serie II elemental analyzer. TEM and HR-TEM analyses were performed at the "Centre de microcaracterisation Raimond Castaing, UMS 3623, Toulouse" by using a JEOL JEM 1011 CX-T electron microscope operating at 100 kV with a point resolution of 4.5 Å and a JEOL JEM 1400 electron microscope operating at 120 kV. The high-resolution analyses were conducted by using a JEOL JEM 2100F equipped with a field emission gun (FEG) operating at 200 kV with a point resolution of 2.3 Å and a JEOL JEM-ARM200F Cold FEG operating at transitions, as implemented by Henkelman et al., 63,64 using 3 intermediates images and force criteria of 0.05 eV/Å -1 to control the convergence. We have modeled the carbon support by a single graphene layer presenting a vacancy in (7x7) supercell, which can be either filled by a single Pd atom (PdSA-model) or served as an anchorage point for a fully hydrogenated Pd13H20 aggregate (PdNP-model). We recall that a sufficient vacuum height of 16 Å was used to limit spurious interaction between repeated images in the z-direction due to periodic boundary conditions. Calculations were carried out using the Gaussian09 package 65 at the DFT level by means of the hybrid density functional B3PW91. 66,67 For the Pd atom, the Stuttgart-Dresden pseudopotentials 68 were used in combination with their associated basis sets 69 augmented by a set of polarization functions (f-orbital polarization exponents of 1.472). 70 For the C and H atoms, a double-zeta 6-31G basis set 71,72,73,74,75,76,77

-myrcene hydrogenation

Myrcene catalytic hydrogenation has been carried out in a stirred tank reactor of 200 mL from Top Industrie. This reactor is equipped with 4 baffles and a gas inducing Rushton turbine having Hopefully, this is not true for the whole range of reaction course. For robust and chemically consistent catalysts comparison, STY at 66% and 83% of reaction progress are thus considered, i.e. when 2.0 and 2.5 equivalents of H2 per -myrcene are consumed respectively.

For hydrogenation and isomerization reactions, we have also used normalized time (time.cat), a standardized notation that is convenient and usual in catalysis to compare catalysts with different loadings of the active phase. The quantity of Pd was determined by ICP. For comparison with commercial catalysts (Figure 4.4), a time.cat* could be also used where nPd is replace by SPd, the quantity of surface Pd determined from metal dispersion.

To ensure a reproducible and precise initial time for the tests, heating of the reactor was first done with 1 bar of N2, then agitation was stopped and the N2 atmosphere was replaced by 20 bar of H2 and agitation turned on again, defining the initial time for the experiment. No sidereactions like Diels-Alder or oligomerization are encountered in the experimental domain investigated for this substrate. Thus, no selectivity issue has to be considered. The good matching between the evolution of H2 consumption profiles and of liquid substrate compositions has been checked (not shown here) and validates the experimental approach for reaction progress monitoring.

Squalene and -farnesene hydrogenation

The comparison between -myrcene, -farnesene and squalene hydrogenations has been carried out in the same experimental rig following the same procedure with identical experimental conditions excepting a slightly increased catalyst mass (280 mg instead of 200 mg). To illustrate the extension of the concept to other carbon supports, here FLG was chosen and compared to Al2O3. Squalene has been purchased from Sigma-Aldrich and -farnesene has been synthesized and purified accordingly to ref. 79.

4.7.4.3.

Catalyst stability testing under continuous reactor operation Catalyst stability has been investigated through a dedicated continuous reactor operation with coated metallic open-cell solid foams (Hollomet Foamet 800 µm). Stainless steel foam cylinders were precisely cut through EDM (Electrical Discharge Machining) to obtain cylindrical objects with a diameter of 4.3 mm and a length of 5 cm. 8 foam pieces were homogeneously coated with 12.5 mg of the Pd2_SA/NP/CNT catalyst (following the detailed procedure described elsewhere) 80 and inserted into a home-made millimeter-scale tubular reactor (40 cm long, 4.4 mm I. D.) to provide a reactor volume of 6 mL with a total catalyst amount of 100 mg. 60 mL of a solution of 1-octene (Sigma Aldrich, 99%) in heptane (1 M) was added in a 100 mL round bottom flask and then heat at reflux under H2 atmosphere. A sample was taken for ex-situ analysis and then catalyst was added in one portion. The mass of the catalyst was adjusted in order to have 3.75 mmol of Pd (0.4 mg, 60 ppm). Samples were taken as the function of time and were analyzed using GC-FID analysis.

The concentration of isomers of 1-octene and octane could be easily followed by GC as shown on

Tests to assess hydrogen spillover

To check for the H-spillover, we used WO3 to diagnose the activation of H2 in the various catalysts, because the spilled-over hydrogen migrates and readily reacts with the yellow WO3 to form dark blue HxWO3. 39 Samples made with 500 g of WO3 were mixed or not with 5 mg of catalyst and treated with H2 (100 mL/min) at 20 °C for 3 min. a) Estimated using correlations found in [81]. b) Thermodynamic calculations with PPR78 group contribution using PROSIM software for a flash equilibrium at constant P & T. c) Commonly taken equal to the graphite density. d) See for example [82,83] for such typical orders of magnitudes. e) Calculated from skeleton density and pore volume. f) See [84] for a discussion about mass transfer inside nanotubes bundles.

G-L external mass transfer

To evaluate the efficiency of the reactor set up to transfer hydrogen in the liquid phase, the mass transfer coefficient (kLaGL) of the reactor has been measured experimentally with a classical physical absorption technique ,85,86 following the H2 consumption in the regulated and calibrated 𝑘 𝐿 𝑎 𝐺𝐿 and 𝐶 𝐻2 * were estimated as 0.2 s -1 and 190 molH2/m 3 L respectively. The 𝑘 𝐿 𝑎 𝐺𝐿 appears typical of well-equipped laboratory stirred tank reactor (for H2). The experimental solubility is in agreement with the one calculated with the PPR78 thermodynamic model (187 mol/m 3 L).

Based on these mass transfer measurements, a theoretical maximum H2 consumption, 𝐹 𝐻2 𝑚𝑎𝑥,𝐺𝐿 (in molH2/s) can be defined for each experiment (4.8). This consumption can be linked to the corresponding theoretical apparent reaction rate, 𝑟 𝑝,𝑎𝑝𝑝 𝑚𝑎𝑥 (4.9), or to the corresponding theoretical First of all, it is noticeable that the 𝑆𝑇𝑌 𝑚𝑎𝑥 reached different values depending on the different amounts of Pd in each experiment. The 𝑓 𝑒𝑥,0.5 𝐺𝐿 , for most of the catalysts (except the Pd1000_SA/NP/CNT catalyst which contains a very low amount of metal) is revealed to be high indicating a strong G-L mass transfer impact on the STY0.5 measurements and the impossibility to discuss robustly these STY0.5. This is indicated in the main text of the article. Interestingly, the 2 more active catalysts present a higher STY0.5 compared to the theoretical STY max . This is a phenomenon already observed and explained in stirred tank reactors with very active fine slurry catalyst particles. 87,88 The approach detailed in ref. 87 is very close to an enhancement factor, E (-), for mass transfer G-L reactive absorption already well established. 89 In the present work, this situation is encountered. Thus, the first points at a reaction progress of 0.5 are used For reaction progresses of 2.0 and 2.5 molH2/molmyrcene, the impact of G-L mass transfer on the STY is drastically decreased as shown in the Table and a satisfactory comparison of the observed STY can be drawn on a chemical basis. The Pd10SA_NP/CNT, which is the most active one, is the only catalyst that may still present a possible partial G-L mass transfer hindrance of its intrinsic performance. This partial leveling is a signature of its very high intrinsic activity and does not change the conclusions made in this article. In a quantitative point of view, one can just expect an increase of the observed differences between this catalyst and the other ones.

L-S external mass transfer

In G-L-S slurry reactors, L-S external mass transfer is often less limiting than the G-L one. To verify this common assumption, the classical external mass transfer fraction, 𝑓 𝑒𝑥 𝐿𝑆 (defined in 4.13) has been used for reaction progresses of 0. L is the characteristic length of the catalyst (dp/6 for spherical particles) and 𝑘 𝑆 is the L-S mass transfer coefficient. It has been estimated using the correlation of Armenante and Kirwan. 90 The different results are presented in Table 4.5. At a reaction progress of 0.5 molH2/molmyrcene, only Pd/Al2O3 and Pd/C present a L-S mass transfer limitation. All the CNT catalysts appear free of external L-S mass transfer limitation.

For 𝜑 ′ > 1: 𝜂 𝑠𝑢 ≈ 1 𝜑 ′ and for 𝜑 ′ < 1: 𝜂 𝑠𝑢 ≈ 1 (4.16)

All the CNT catalysts at reaction progress 2.0 and 2.5 molH2/molmyrcene are free of internal limitations. This remains true for the reference Pd/Al2O3 and Pd/C at the reaction progress of 2.5 molH2/molmyrcene. For these catalysts, an intermediate regime with little mass transfer resistance may be present at a reaction progress of 2.0 molH2/molmyrcene because 1 < 𝝋′ < 3. This is due to the bigger size of the catalyst particles. Atmospheric CO2 is the main source of carbon for life on earth. It is produced by aerobic organisms and as a result of decay process of organic materials and fermentation. Also, since the industrial revolution, is released due to the use of fossil fuels and its concentration rise in the atmosphere has been related to global warming and ocean acidification. Nonetheless, during decades CO2 has been used, as refrigerant, food additive or even as a solvent. Even more, in recent years CO2 has been considered as a source of energy either by its direct reduction into hydrocarbons or by its use in the production of hydrogen. 1 Thus, CO2 could be no longer considered as an undesirable waste to be reused, but as a driving force for the sustainable future of the chemical industry. In that context, much attention has been paid to CO2 electro-, photoor thermocatalytic hydrogenation to various value-added products, such as CO, acetaldehyde, formic acid, methanol, methane, or lower olefins. 2 CO2 like water is well known to be very stable (ΔGf° = -396 kJ.mol -1 ). From the energetic point of view, the reduction of CO2 will require an amount of energy that depends on the downward steps of the carbon oxidation state, from +IV in CO2 to the value in the target product. Although most hydrogenation reactions are exothermic as shown in Table 5.1 (although no for reverse water gas shift -RWGS reaction), the low energetic level of CO2 leads to severe limitations of the thermodynamic equilibrium, and some of them are strongly endergonic as for formic acid or formaldehyde formation. 3 According to thermodynamic calculations, CO2 tends to convert to molecules with high carbon numbers at low temperature and high pressure. 3 However, these reactions are probably limited by kinetics. Thus, designing highly efficient catalysts is critical because CO2 hydrogenation involves a complex reaction network, which is restricted by thermodynamics and kinetics.

In heterogeneous catalysis, metal-support interactions (MSI), including electronic MSI, 4 can significantly influence the course of the reactions, making MSI modulation one of the few tools able to enhance catalytic performances in terms of activity, selectivity but also stability. 5 In general, the efficiency of MSI tuning rapidly decayed with increasing NP size, 6 and for a single metal and a given support high electronic MSI are expected for SA catalysts. 7 Unlike for NP, there is a large spatial variation in the bond and coordination environment of SA on the support surface due to support heterogeneities. This can significantly affect catalyst performances.8 The use of atomically dispersed metal for CO2 hydrogenation has recently been the subject of several studies (Table 5.2), and high selectivity towards a single product are most of the time reached on low loading catalysts (metal loading < 1 % w/w). It is worth to notice that thermal reduction of CO2 has been mainly studied over SA supported on reducible oxides, such TiO2, CeO2, In2O3 and ZnO2 and just few reports have been published on carbon-based SAC. The general trend observed is that CO is produced with isolated SA due to the fact that they bind CO weaker than metallic NP, and CH4 production occurs at NP surface. This means that SA and NP composed of the same metal on the same support can exhibit unique selectivity in a parallel reaction pathway: RWGSR preferentially occurs at SA, while methanation at NP.

However, a noticeable exception can be observed in the case of RuSA/CeO2 catalysts, 13,14 for which two different research groups reported totally different selectivity. a) Nitrogen-doped carbon. b) The catalyst is not stable and Ru NP are progressively formed leading to increased methane production. c) h-BN = porous hexagonal boron nitride. d) LDH = layered double hydroxide. e) A 1% Re/CeO2 containing mixture of clusters and SA produces selectively methanol. f) AP-POP = porous organic polymer with aminopyridine functionalities.

On RuSA/CeO2 catalysts prepared by Ru redispersion from Ru NP supported on CeO2, the Ru-O shell showed a coordination number of 5.5, and this catalyst selectively produced CO at 260 °C. 13 On RuSA/CeO2 catalysts prepared on nanowires containing significant amount of oxygen vacancies, the Ru-O shell showed a coordination number of 4.5, and this catalyst selectively produced CH4 at 260 °C. 14 These results highlight the importance of the coordination environment of SA and the presence of oxygen vacancies on reducible supports, which can actively participate to the reaction. 27 As far as activity is concerned, the study of Zhang et al.

who compared a RuSA/CeO2 catalyst with RuNP/CeO2 (Ru NP size = 1.2 and 4 nm) showed that these three catalyst are very selective for methane (SCH4 ≈ 100 %) and the activity order is RuNP-1.2nm/CeO2 > RuSA/CeO2 > RuNP-4nm/CeO2. Finally, the stability of SA under reaction conditions can also be an issue. This parameter was evaluated for NiSA/MgO, 10 PtSA/CeO2, 25 RuSA/CeO2, 13 RuSA/TiO2 13 and RuSA/Al2O3 12,13 catalysts. If nickel and platinum catalysts show good stability under CO2 hydrogenation conditions, ruthenium catalysts are prone to deactivation whatever the support used. It is therefore needed to tune binding strength between SA and supports because a suitable interaction is required to not only prevent SA agglomeration into NP, but also to perform high catalytic activity and selectivity. Typically, the fully occupied bonding orbitals correspond to the stability between the SA and the support, while the free electrons near Fermi level caused by the unsaturated coordination can be responsible for the catalytic activity. 28 In that context, the use of vacancy containing support should offer interesting perspectives. 29 In this chapter we tested in the CO2 reduction reaction a series of SA catalysts (Ru, Pt, Pd, Ni, Co and Cu) supported on carbon nanotubes prepared by stabilization of metal SA on vacancies in the surface of CNT. 27

Results and discussion

First, 1%MSA/CNTOX-400 (M=Pt, Ru, Pd, Ni, Co and Cu) were tested in the hydrogenation of carbon dioxide. The characterization of these catalysts is reported in Chapter 3 (See Section 3.4 Synthesis of supported single atom catalysts (SAC)), and the presence single metallic atoms dispersed all over the surface of CNT was evidenced. The CO2 hydrogenation was conducted under methanation conditions (CO2/H2 = 1/4) in single fixed-bed reactor configuration under 6.1 bar, at either 260, 300 or 340 °C, and with a GHSV of 16500 mL/h•g. Under such conditions, the CO2 conversion does not exceed 10 %, and we are thus comparing catalysts far away from the thermodynamic equilibrium. The apparent and instantaneous activity of the catalysts is expressed in moles of CO2 consumed per mole of metal and per hour, being equivalent to an instantaneous Site Time Yield ("STY" in the following). It is worth noting that two reactions, CO2 methanation and RWGS, run simultaneously under the present conditions, and the selectivity could depend strongly on the nature of the dispersed metallic phase, metal crystallite size, metallic loading, temperature or additives.

The main results of CO2 reduction with 1%MSA/CNTOX-400 catalysts are summarized on Figure 5.1 and Table 5 The reaction profiles show the evolution of STY with time on stream (ToS) at different temperatures. All samples were active in CO2 reduction and as expected, an increase of the STY with temperature was observed. The apparent activation energies ranged between 21 and 86 kJ/mol. Overall, catalytic activity was observed to be the greatest for platinum and the poorest for Cu. According to the activity values, metals can be ordered from more active to less active, in the following order Pt > Ru > Ni ~ Co > Pd > Cu. As we can see, the impact of the nature of the metal phase plays a role in the general catalytic performance. For all the metals, the main product was CO with a selectivity between 92-99% excluding for Ru, which produced methane in ~43%. Interestingly in this sample, as can be seen in figure 5.1b, the production of methane is favored with each increase in temperature, however the production to CO increase after a few hours of reaction. In terms of stability, the catalysts were analyzed before and after catalysis. HRTEM micrographs (Chapter 3, Figure 3.27) prove the existence of single atom species before reaction. After 30 h on stream at 6.1 bar of pressure, the formation of particles was evidenced for all the samples (Figure 5.2). This result is not surprising knowing that even under softer reductive conditions, clusters are formed after one hour at 1 bar of H2/N2 and 300 ºC for RhSA/TiO2 or 400 ºC for NiSA/hydroxyapatite. 41,42 The sintering of SA is a key factor to overcome for such applications, reason why it is critically important to seek for methods to improve their durability. The action of alkali promoters in heterogeneous catalysis has been broadly reviewed, 43,44,45 and it was shown that they can contribute to: i) increase the selectivity or specificity; ii) increase the activity; and more importantly iii) prolong the effective lifetime of the catalyst. 46 Their modes of action includes: i) intrinsic catalytic effect of the alkali; ii) creation of basic sites; iii) neutralization of acidic centers; iv) modification of the electronic properties of the catalyst surface by charge transfer from alkali metal to transition metal, which brings about a higher electron density on the transition metal; v) reduction of the volatility of the active component through compound formation; and vi) prevention of phase transformations. For CO2 methanation, various studies have shown that alkali addition to Ni or Ru catalysts supported on oxides contributes to an improvement of the catalytic performances. 47 This was attributed to the enhancement of the CO2 adsorption and activation thanks to surface basicity of the supports or to modification of the local electron density on the metal. In the case of SA catalysis, it has been shown that the addition of alkali to Pt 48 or Au 49 SA has a beneficial influence on their performances for the water-gas shift reaction thanks to SA stabilization through nearby Na-Ox moieties. Therefore, we decided to treat the prepared MSA/CNTox-400 with NaOH. With a simple aqueous impregnation, we introduce 2 equivalents of Na per mol of metal. We performed XPS analyses avoiding air exposure on sample RuSA-Na/CNTox-400 (Figure 5.3). We noticed that the addition of Na as promoter had a very positive impact on the STY, for all samples (Table 5. In the case of nickel, despites the STY increase upon Na addition, the stability of the catalyst is not optimal since after some hours on stream at 340 °C the STY of the NiSA-Na/CNTox-400 catalyst Ni catalysts. 47,54 This was not the case for the Co and Pd catalysts, for which a large increase of the activation energies was observed. Finally, in order to evaluate the cooperative effect between nanoparticles and single atoms, we have tested a series of 1%MSA+NP/CNTox-400 catalysts prepared using the method previously described (Chapter 3, Section 3.5 Mixture of single atoms and nanoparticles). As a brief reminder, the impregnation of the metal is done in two stages allowing the growth of clusters/nanoparticles but keeping the SA sites available. The results of STY and selectivity of 1%MSA+NP/CNT catalysts at 260 ºC, 300 ºC and 340 ºC are displayed in Table 5.5 and Figure 5.8 Comparing 1%MSA+NP/CNTox-400 versus 1%MSA/CNTox-400, there was no significant change in terms of activity. In general, the values of STY varied a few units but instead selectivity slightly decrease favoring the formation of CH4 in the case of Ru, Ni and Co. Indeed, this is not surprising since studies indicate that NP are more active for CH4 production, and that improvement of CH4 production are obtained with the increase of particle size. 55,56 Again, platinum presented the higher activity although after 10 hours at 300 ºC, the STY went from 55 to 50 molCO2/molPt.h. Contrarily, Ru was stable in all the range of temperature. From our previous studies we have proven the existence of a cooperative effect between SA and NP on liquid phase so it may proceed differently for this specific gas phase reaction. Still, the CO2 reduction mechanisms differ between systems, and the precise reaction pathways are still being investigated on both single atoms and nanoclusters.

Conclusion

We have reported a versatile and scalable approach to stabilize metallic single atoms on CNT.

Preliminary catalytic experiments have shown that these catalysts exhibit distinct performances for the hydrogenation of CO2 under methanation conditions. Pt and Ru catalysts were much more active than the ones prepared with other metals (Co, Ni, Pd, Cu). The sodium doping of the catalysts led to a significant increase in catalytic activity; however further studies are necessary to elucidate its specific role in these catalytic systems. Although the stability of the catalysts is reasonable (tested for 30 hours), it is still necessary to resort with stronger MSI, that can withstand the harsh operational conditions to avoid the formation of nanoparticles over the CNT. When nanoparticles/clusters were present along with SA, a different selectivity was evidenced. Surprisingly, the use of nickel, a metal well identified for its good methanation performances leads systematically to RWGS reaction and the selective production of CO. For the ruthenium catalysts, moderate CO selectivity was obtained, which can be increased by the 55 CH4 output flow = dry flow output x XCH4.

CO Yield (%) = % CO2 conversion -CH4 Yield (%)

𝐶𝐻 4 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 (%) = ( 𝐶𝐻 4 𝑦𝑖𝑒𝑙𝑑 (%) %𝐶𝑂 2 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 ) 𝑥100% 𝐶𝑂 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 (%) = ( 𝐶𝑂 𝑦𝑖𝑒𝑙𝑑 (%) %𝐶𝑂 2 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 ) 𝑥100%
We expressed the percentage of CO2 consumed by unit of time and by mole of metal, which corresponds to the STY

General conclusions and perspectives

In this thesis, we have focused on the preparation, characterization and evaluation of the catalytic properties in hydrogenation reactions of carbon-supported SA catalysts.

First, we have prepared catalysts with very high metal content (up to 20%) consisting on covalent assemblies of Ru and fullerenes. These Ru@C60 nano-architectures were synthesized by decomposition of [Ru(COD)(COT)] in the presence of C60 using an organometallic approach. The influence of the solvent and the Ru/C60 ratio on the nature of the metallic species were investigated. It was found that the use of pure toluene allows the exclusive production of RuSA-fullerene structures regardless of the Ru/C60 ratio (1/1, 5/1, 10/1, 20/1). As soon as small amounts of a second solvent (dichloromethane or methanol) is introduced, the morphology of the structure changes, with the suspicion of the formation of clusters in/on the polymeric structure. STEM-HAADF micrographs and EDX mapping suggested a core-shell distribution when a mixture T50%D50% was used in the synthesis. The use of methanol allows further metal deposition in the inner structure of the fulleride, achieving homogeneous metal distribution. We demonstrate that catalysis can be used as a method for identifying (or confirming the results from WAXS and EXAFS) the type of metal species in the final product.

For nitrobenzene hydrogenation, when SA were present, after 5 hours of reaction a maximum of 40% of nitrobenzene conversion was obtained, instead when clusters were suspected activity increases dramatically reaching 99% in 3 hours. This behavior was rationalized thanks to DFT calculations that show that electrodeficient RuSA (and few atom aggregates) are not efficient to activate H2, compared to larger clusters. Besides, for the hydrogenation of tetra-substituted 2,3dimethyl-2-butene, due the high steric hindrance induced by the small C60-C60 distance, the hydrogenation is not possible on RuSA. On large clusters, for instance Ru13H18, the hydrogenation reaction is thermodynamically favorable, and we experimentally evidence that the catalysts containing clusters are very efficient for this difficult hydrogenation reaction.

Second, we discovered a straightforward and efficient method to produce stable low loading SAC (~1%) on carbon materials. This method involves the creation of defects on the support to anchor metallic atoms. The entire process comprises four stages: i) creating carboxylic groups on the support by nitric acid oxidation (step I); ii) decomposing these groups under an inert atmosphere to produce carbon vacancies, CO2 and some water (step II); iii) depositing the metal on this defective support (step III); and iv) the removal of organic ligands possibly remaining on the surface of the catalysts. Factors such, as the time of oxidation and activation temperature were found to be of paramount importance to achieve a high density of defects. The defective CNT support was characterized by EPR and VSM techniques, indicating the presence of paramagnetic species related to spin-polarized localized electronic states. Although the exact nature (vacancies or edges) of the localized electronic states created by the thermochemical process is not known, these defects efficiently stabilized metallic RuSA. The deposition of the active phase was carried out by a simple impregnation of the defective support with a solution of the desired precursor. Besides Ru/CNT, this method was successfully used with six different metals (Pd, Pt, Co, Ni, Cu, Ir) and three other supports (CNF, FM, CB). A combination of organometallic precursors with labile ligands and non protic solvents yield higher metallic loading. Furthermore, we have proven that it was possible to control the SA/NP ratio by repetition of the impregnation step. We showed that for a given metal loading In chapter 4, through a combined experimental-theoretical study, we provide an explanation of the influence of metal dispersion and the concentration of oxygen surface groups and defects on the support of Pd/C catalysts for β-myrcene hydrogenation. Highly dispersed PdNP are necessary to activate H2. A high concentration of surface defects on the carbon support is necessary to stabilize PdSA. A high concentration of oxygenated surface groups is also necessary on the carbon support to allow hydrogen spillover. DFT calculations using 1-butene as a probe molecule, demonstrate that such combination allows a cooperative catalysis between PdNP and PdSA involving the formation of PdSA-H species, which are much more active than PdNP-H for hydrogenation but also isomerization. In addition, we have assessed the previously prepared Pd series on myrcene hydrogenation, and activity variations of several orders of magnitude were measured as a function of the value of SA/NP ratio. Surprisingly, the sample that presents the best performance (Pd10_SA/NP/CNTox-400) was not the one that contained the higher proportion of palladium clusters. This result indicates that a moderate number of clusters with an average diameter of 1.1 nm in combination with only 0.25% of the total Pd atoms as SA are the optimal conditions to carry out the hydrogenation of myrcene. The control of this ratio allows the development of a new generation of highly active catalysts integrating the ultra-rational use of precious metals.

Finally, in the last chapter we presented the preliminary results of the application of the prepared SAC on the hydrogenation of carbon dioxide. In agreement with the literature, SA species produce selectively CO as a product under methanation conditions. Pt and Ru were by far the most active metals at 340 ºC and 6.1bar of pressure. As anticipated, an increase in the temperature causes an increase in the activity due to the endothermic nature of the RWGS reaction. Unfortunately, after 30 hours on stream, a significant part of the atoms agglomerated, giving rise to the appearance of nanoparticles warning about the need to implement mechanisms to enhance the fixation of these species. Thus, the addition of 2 equivalents of Na per mol of metal, not only played a role on the stability (smaller particle size after catalysis) but also in terms of activity boosting the performance of all catalysts.

The work described in this doctoral thesis provides inspiration and perspectives for future research such as:

-The identification of the nature of the created defects stills need to be performed. In-situ measurements such time-resolved operando EPR spectroscopy would help to gain insight about these reactive species. Another approach could be the synthesis of molecular models (carbene molecular models) containing well-defined edge-like defects; and the study of their chemical reactivity towards selected organometallic compounds.

-Grafting other species different from transition metal atoms on the defective supports such coordination complexes in a manner to binding homogeneous catalysts or even ligands or other molecules for different applications than catalysis such gas sensors, biosensors or polymer functionalization could be envisaged.

-Enlarging the scope of the method to other supports and metals can be investigated.

Nitrogen-or sulfur-doped carbon supports represents interesting choices as starting materials for the SA deposition. There is a vast bibliography referring to the excellent stability of M-N-C moieties. S-doped carbons would also be interesting due to their affinity and strong interaction with precious metals, which could contribute to improve SA stabilization.

-The concept of cooperative catalysis between SA and NP can be extended to plenty other reactions involving hydrogen (or hydrogen spillover), such as nitrogen fixation, Fischer-Trosch, etc.

-Preliminary results obtained in the hydrogenation of CO2 encourage pursuing further investigation with platinum for example to perform CO2-FTS. In such a reaction, the role of the PtSA catalyst would be to transform CO2 to CO via the RWGS reaction but also to assist the reduction of a conventional Co-based FTS catalyst to improve its performances.

Chapter 6. Résumé de Thèse La présence de défauts dans le matériau carboné, tels que des lacunes, augmentera externe des sphères car il ne peut pas diffuser dans leur porosité se traduisant par une structure coeur-coquille. Ainsi, l'objectif des travaux décrits dans ce chapitre était de produire des hétérostructures Ru@C60 à forte charge de ruthénium en évitant la morphologie coeur-coquille en utilisant des mélanges de solvants lors de la synthèse pour favoriser le dépôt de ruthénium dans la porosité du matériaux -(Ru1@C60)-n de manière à obtenir des fullerides de NP métalliques - Des nanostructures de forme indéfinie ont été produites en utilisant du toluène pur (entrée 4, Tableau 6.1, T-Ru1@C60). 27 Cela indique l'influence prononcée du solvant sur la forme des structures produites (comparer avec des sphères bien définies produites dans du CH2Cl2). Step III

Carbon support Oxidized carbon support

Single-atom sites on carbon

Step II Support activation HNO 3 (conc.) 140ºC

Step I

Support oxidation

Solvent/ metallic precursor

Ar

Step IV Les résultats obtenus à partir de nanotubes de carbone (CNT) indiquent que les meilleures conditions pour créer des défauts sont à 400°C (D-CNT). En effet, à des températures plus élevées, un phénomène de réarrangement des lacunes a lieu, conduisant à la reconstruction de la structure graphitique. 37 De nombreuses études ont montré qu'avec l'augmentation de la température de recuit, l'ordre structurel s'améliore et une réduction de la concentration globale des défauts se produit. La formation et la décomposition des groupes oxygénés de surface ont été suivies par analyse TPD ; prouvant que le chauffage à 400°C est efficace pour éliminer les groupes acides carboxylique (Figure 6.6a). Dans le domaine de l'hydrogénations, des PdSA ont déjà été utilisés, 42 mais l'interaction métalsupport dans de tels catalyseurs entraîne souvent un état ionique/électron-déficient des atomes métalliques qui empêche une activation efficace du dihydrogène par rapport aux surfaces métalliques, 43 A partir des résultats ci-dessus, il devient évident qu'un catalyseur hétérogène optimisé avec une utilisation ultra-rationnelle de métaux précieux (souvent en pénurie) pourrait passer par un contrôle précis du rapport SA/NP. Dans cette optique, nous avons testé différents catalyseurs présentant des rapports SA/NP différents (pour une charge en Pd similaire de 1.4% p/p), afin de prouver l'influence de ce paramètre sur les performances catalytiques. Comme prévu, chaque échantillon ayant un rapport SA/NP différent présentait une activité catalytique différente, comme illustré sur Figure 6.13 . Plus important encore, nous avons trouvé que la valeur optimale pour l'hydrogénation du myrcène était de 10. 47 Dans ce contexte, une grande attention a été accordée à l'hydrogénation électro-, photo-ou thermo-catalytique du CO2 en divers produits à valeur ajoutée, tels que le CO, l'acétaldéhyde, l'acide formique, le méthanol, le méthane ou les oléfines inférieures. 48 Selon des calculs thermodynamiques, le CO2 a tendance à se convertir en molécules à grand nombre de carbone à basse température et haute pression. Cependant, ces réactions sont probablement limitées par la cinétique. Ainsi, la conception de catalyseurs hautement efficaces est essentielle car l'hydrogénation du CO2 implique un réseau de réactions complexes, qui est limité par la thermodynamique et la cinétique. Dans cette section, nous présentons les résultats de la réduction du CO2 sur une série de SAC (Ru, Pt, Pd, Ni, Co et Cu) supportés sur des nanotubes de carbone. 47

Ligand elimination Metal deposition

Defective carbon support

  caractérisation magnétique a reflété la présence d'espèce réactive avec un comportement paramagnétique soulevant d'éventuels radicaux en surface. Par la suite, nous avons montré qu'il était possible d'utiliser de tels défauts comme point d'ancrage pour stabiliser avec succès des atomes métalliques (Ru, Pd, Pt, Ni, Cu, Co, Ir) avec une charge allant jusqu'à ~ 1,6% en poids. Aussi, en améliorant cette méthode, nous sommes arrivés à être capables de contrôler le rapport SA/NP pour un chargement métallique donné allant de 10 à 200. Chapitre 4, à travers une étude expérimentale-théorique combinée, nous fournissons une explication de l'influence de la structure caractéristiques des catalyseurs Pd/C pour l'hydrogénation d'alcène. Des nanoparticules de Pd hautement dispersées (PdNP) sont nécessaires pour activer le dihydrogène. Une concentration élevée de défauts de surface sur le support de carbone est nécessaire pour stabiliser les atomes simples de Pd (PdSA), qui coexistent avec le PdNP sur les catalyseurs Pd / C. Une concentration élevée de groupes de surface oxygénés est également nécessaire sur le support de carbone pour permettre le débordement d'hydrogène. Nous démontrons qu'une telle combinaison permet à une catalyse coopérative d'opérer entre PdNP et PdSA qui impliquent la formation

Figure 1 .

 1 Figure 1. Particle size effects in heterogeneous catalysis.

Chapter 1 .

 1 Single atom catalysts on carbon-based materials 1. Single atom catalysts on carbon-based materials 1.1. Introduction

3

 3

Figure 1 . 1 .

 11 Figure 1.1. Schematic representation of a portion of a Ni granule surface in cross-section. From ref. [1].

Figure 1 . 2 .

 12 Figure 1.2. Schematic illustrations of: a) a supported single Pd atom on carbon catalyst (SAC), the Pd atom is located in a carbon vacancy; (b) a site-isolated Al atom incorporated in the framework of a T site, zeolite ring, and zeolite channel system. (From ref.[ 13 ]); and (c) a supported homogeneous catalyst, oxo-bridged zirconocenium coordination on a µ 3 -O alumina surface site. From ref.[14].

Figure 1 .

 1 Figure 1.3. a) Triazine and b) polyheptazine-based g-C3N4 allotropes; and c) structural parameters (from XRD) of polyheptazine-based g-C3N4.

Figure 1 . 4 .

 14 Figure 1.4. Different adsorption sites on heptazine-based g-C3N4.

Figure 1 .

 1 Figure 1.5. a) Structure of graphene-like carbon nitride (g-CN); and b) top and side views of two types of TM@g-CN structures, taking V and Fe for examples, respectively. Color code: nitrogen blue; carbon grey; vanadium violet; and iron orange. From ref. [57].

  58 D. Ghosh, et al. Journal of Materials Chemistry C 2014, 2, 7943-7951.

Figure 1 . 6 .

 16 Figure 1.6. The binding energies (eV) of TM atoms adsorbed onto triazine-, heptazinebased g-C3N4, and g-CN.

  59 M.Yang, et al. Aip Advances 2015b, 5, 067136. 60 R. Philippe, et al. Chemical Vapor Deposition 2007, 13, 447-457.

  prismatic (edge), and basal plane surfaces (Figure1.7a). The graphene edges (if not Hterminated) are the most reactive sites of defect-free materials thanks to the presence of C atoms with unpaired electrons. Two types of sites exist: zigzag sites (carbene-like), and armchair sites (carbyne-like).

Figure 1 .

 1 Figure 1.7. a) Basal plane and prismatic surfaces on sp 2 carbon materials; and b) adsorption sites on sp 2 carbon materials: hollow H6, bridge B, top T, A1 and A2 high symmetry armchair sites, and Z1 and Z2 high symmetry zigzag sites.

Figure 1 .

 1 Figure 1.8. a) Binding energies (eV) of 3d TM atom adsorbed onto graphene, considering different possible adsorption sites; and b) binding energies (eV) corresponding to each 3d TM atom adsorbed onto graphene and SWCNT on the H6 site. From ref. [34].

  64 S. Banerjee, et al. Advanced Materials 2005, 17, 17-29. 65 O. Gulseren, et al. Physical Review Letters 2001, 87. 66 E. Durgun, et al. Physical Review B 2003, 67.

Figure 1 .

 1 Figure 1.9. a) Adsorption energy (Eads) of 3d, 4d, and 5d TM atoms on graphene, as well diffusion energies, Edif, both in eV. Results obtained with two different functionals: Perdewe-Burkee-Ernzerhof (PBE) functional (open circles and solid lines) and PBE-D2 dispersion correction (filled circles and dashed lines) DF results. b) Trends along the 3d (upper curve), 4d (middle curve), and 5d (bottom curve) series for the adsorption of TMadatoms on graphene on most stable sites plotted as calculated at PBE level using D2, D3, D3BJ, or TS methods to describe vdW forces. From ref.[69].

Figure 1 .

 1 Figure 1.10.a) The calculated binding energy for Au atoms adsorbed on pristine graphene (T, B, and H6 sites) and bound at monolayer edge sites (A1-2 and Z1-2 sites). The insets are the electron density images showing the difference in bonding character between adsorption and edge sites for Au. b) STEM HAADF images showing monolayergraphene sheet with bilayer and trilayer regions onto which a 5 Å layer of Au was evaporated. Individual Au atoms clearly bind preferentially to edge sites. From ref.[71].

Figure 1 .

 1 Figure 1.11. Perfect and defective graphene clusters. (a) Perfect graphene cluster, (b)Stone-Wales defect (SW), (c) single vacancy (SV), (d) double vacancies (DV), (e) edge defect with pentagon ring at zigzag edge, octagon and fused pentagon carbon rings line defect with (f) odd number of octagon rings (GLD-558-01) and (g) even number of octagon rings (GLD-558-02), and pentagon-heptagon pairs line defects with (h) odd number of heptagon rings (GLD-57-01) and (i) even number of heptagon rings (GLD-57-02);The larger grey and smaller white balls denote carbon and hydrogen atoms, respectively. From ref.[74].

  heating to produce three different configurations: i) a DV(5-8-5) that contains two pentagons and one octagon (Figure1.12d); ii) a DV(555-777) that contains three pentagons and three heptagons (Figure1.12e); and iii) a DV(5555-6-7777) that contains four pentagons, one hexagon and four heptagons (Figure1.12f). Among these configurations, the DV(555-777) is the most stable structure.76

Figure 1 . 12 .

 112 Figure 1.12. Schematic defects in graphene sheets: (a) single vacancy defect; (b) double vacancy defect; (c) Stone-Wales defect SW (55-77); (d) double vacancy DV(5-8-5); (e) DV(555-777); and (f) DV(5555-6-7777). From ref. [77].

Figure 1 . 13 .

 113 Figure 1.13. Typical atomic configurations of TM atoms adsorbed on SV and DV in a graphene sheet. Metal atom on a SV: Side view (a), top view (c). Note that the metal atom is above the surface, with an elevation h of up to 2 Å. Metal atom on a DV: Side view (b) and top view (d). The grey balls are metal atoms, the yellow balls carbon atoms. e)-h) Single Fe atoms in graphene vacancies. (e) Smoothed ACTEM images of single Fe atoms in (e) monovacancy and (g) divacancy sites shown in false colour. (f,h) DFT optimized atomic models corresponding to (e,g). From ref.[79].

Figure 1 . 14 .

 114 Figure 1.14. Binding energies of 3d TM atoms located: (i) at a H6 site of a graphene sheet (blue diamond); and (ii) on SV of a defected graphene sheet (red square). The cohesive energies of the respective 3d TM atoms are also shown (green triangle).

Figure 1 . 15 .

 115 Figure 1.15. Schematic representation of different types of N atoms (graphitic, pyridinic and pyrrolic N) in nitrogen-doped FLG and CNT. Reproduced with permission from ref. [84].

Figure 1 .

 1 Figure 1.16. (a) Optimized configurations and (b) formation energies for TM atoms embedded into different types of N-doped graphene. From ref. [59].

Figure 1 .

 1 Figure 1.17. (a) Representation of GO structure; (b) one of the most stable structure computed for GO and GO model side view along the armchair direction; and (c) 3D molecular models of GO and r-GO. From ref. [91].

Figure 1 .

 1 Figure 1.18. (a) Four possible sites depicted by the dashed lines for anchoring Fe atom on GO, where the Fe atom is bonded to the oxygen atoms; and (b) top (upper) and side (lower) views of the most stable Fe-GO structure. Color scheme: Fe, purple; C, grey; H, green; O, red. From ref. [95].

Figure 1 .

 1 Figure 1.19. Most common adsorption sites on C60 fullerenes.

  102 R. E. Estrada-Salas, et al. Journal of Molecular Structure-Theochem 2008, 869, 1-5.

  metallofullerenes with TM of group 9 and 10 have been investigated by DFT.103 Adsorption of metal atom on C60 results in elongated C-C bonds at the coordination. The electron-rich metals lower the electron affinity of C60, and the electron excess is mainly located on the carbon atoms engaged in the bonding with the metal.

Figure 1 . 20 .

 120 Figure 1.20. Adsorption energies for TM atoms adsorbed on C60.

1. 2 . 5 .

 25 Graphyne and graphdiyne The honeycomb structures of graphyne (GY) and graphdiyne (GDY), two 2D carbon allotropes of graphene, is shown in Figures 1.21a-b.

Figure 1 . 21 .

 121 Figure 1.21. Structure of: (a) graphyne and (b) graphdiyne; and c) and d) most favorable adsorption site for 3d TM on GY and GDY. From ref. [112].

Figure 1 . 22 .

 122 Figure 1.22. Adsorption energy of (a) gold; (b) platinum; and (c) iron on various sites of a graphene layer (black: defect free graphene surface; green: single vacancy; blue: double vacancy; pink: triple vacancy; red: graphene edges (zigzag type); grey: graphene edges (armchair type); yellow: Stone Wales defect; magenta: nitrogen doped graphene; orange: graphene oxide. The dotted line corresponds to the cohesive energy of the metal for a particle of 5 nm.

Figure 1 . 23 .

 123 Figure 1.23. Characterization of a 15 %Co-0.1 %Pt/CNT catalyst: (a) STEM-HAADF micrographs; and (b) EDX analysis.

Figure 1 .

 1 Figure 1.24 shows the STEM-HAADF, EXAFS, and DFT characterization of this SAC.EXAFS and DFT have shown that in this material, the RuSA show a η 2 -η 6 coordination to the C60. Other methods, such as solid-state magic-angle spinning NMR (SS-NMR),135 infrared spectroscopy with probe molecule such as carbon monoxide 136 and X-Ray photoelectron spectroscopy, 137 can also be used to obtain structural information of SAC.

Figure 1 . 24 .

 124 Figure 1.24. Characterization of a RuSA@C60 SAC: (a) STEM-HAADF micrographs of RuSA@C60; (b) magnitude of the Fourier Transform of k 2 -weighted Ru EXAFS; and (c) side view of an ideal 1D chain for Ru@C60 ratio in the η 2 -η 6 state. Ru atoms are in grey, carbon atoms in brown. From ref. [139]

Figure 1 .

 1 25 shows an overview of the complementary characterization methods (including DFT studies) required to assess structure and composition of SA on carbon materials.

Figure 1 . 25 .

 125 Figure 1.25. Overview of the complementary characterization methods and studies required to assess structure and composition of SAC on carbon materials.

Figure 1 .

 1 Figure 1.27 shows the evolution of Ru deposition on the triple boundary of a graphene particle with an increasing number of ALD cycles. 152

Figure 1 . 26 .

 126 Figure 1.26. Different steps of the ALD process for SAC preparation. From ref. [153].

Figure 1 . 27 .

 127 Figure 1.27. Evolution of Ru growth on graphene triple grain boundaries shown through magnified bright-field TEM images taken after: (a) 5, (b) 10, (c) 20, (d) 50, (e) 100, and (f) 200 ALD cycles, respectively. From ref. [152].

Figure 1 . 28 .

 128 Figure 1.28. Schematic illustration of the synthesis of Co@N-doped graphene catalyst (0.57 %Co w/w). From ref. [173].

Figure 1 . 29 .

 129 Figure 1.29. Synthetic process for S,N-Fe/N/C-CNT synthesis. From ref. [175].

Figure 1 . 30 .

 130 Figure 1.30. Preparation of a FeSA-N-PC/CNT SAC. From ref. [177].

Figure 1 . 31 .

 131 Figure 1.31. Formation of (a) CoNP@N-C (no zinc in the precursor); and (b) CoSA@N-C (presence of zinc in the precursor). From ref. [202].

Figure 1 .

 1 Figure 1.32. (a) Synthesis route of the Fe-SA/NCS catalysts. (b, c) HAADF-STEM and magnified images of Fe-SA/NCS. From ref. [203].

Figure 1 . 33 .

 133 Figure 1.33. Template synthesis of mesoporous C-N-Co (Fe) catalysts; i) impregnation; and ii) pyrolysis and etching: (a) interconnected vesicle-like frameworks; (b) well-defined linear array of mesoporous structures; and (c) nanosheet-like structures. From ref. [190].

Figure 1 . 34 .

 134 Figure 1.34. Schematic illustration of the fabrication of Fe-N-SCCFs. From ref. [193].

Figure 1 . 35 .

 135 Figure 1.35. Modification of a carbon nitride network with silver tricyanomethanide. From ref. [195].

Figure 1 . 36 .

 136 Figure 1.36. Schematic illustration of the fabrication of the rationally designed NiFe@g-C3N4/CNT. From ref. [208]. 1.4.2.4.Other methods.

Figure 1 .

 1 Figure 1.37. TEM images of CoNP/HNCS: (a) before reaction; (b) after reaction; and (c) HAADF-STEM image of CoNP/HNCS after CVs of 1000 cycles. From ref. [228].

Figure 1 . 38 .

 138 Figure 1.38. Schematic illustration for the synthesis of PtSA-GDY catalyst. From ref. [249].

  Cobalt on N-doped graphene (0.7 wt%) requires an ultra-low onset overpotential of approximately 210 mV on a glassy carbon electrode in both 1.0 M KOH and 0.1 M KOH solutions for OER.256 Although the most studied active sites for this reaction correspond to metal-nitrogen sites, it has been evidenced cases in which oxygen-coordinated metal promotes performance at OER. Atomically dispersed Ni-oxygen species on graphene-like carbon (Ni-O-G) show overpotential of 224 mV, two times lower than NiONP anchored on graphene (450 mV).257 XPS, EXAFS and DFT reveal that nickel has a valence state higher than +2 (+ 2.34) due to the coordination with six oxygen atoms in the configuration Ni-O4-(OH)2. These sites allow a lower energy barrier (the formation of O* is the rate-determining step) compared with NiO and even Ni/N-G SAC.

Figure 1 .

 1 Figure 1.39. (a) UV/vis spectra of g-C3N4 and M/g-C3N4 (M = Mn, Fe, Co, Ni, Cu); (b) PL spectra of M/g-C3N4 (M = Mn, Fe, Co, Ni, Cu) at 400 nm excitation at 298 K. From ref. [267].

1 .

 1 40a displays the semiconductor UV-Vis light absorption (originated by the charge transfer from the valence band populated by nitrogen 2p orbital to the conduction band formed by the carbon 2p orbital) of phosphorus doped carbon nitride (P-CN), which is compared to g-C3N4.167

Figure 1 .

 1 Figure 1.40. (a) UV-vis diffuse reflectance spectra of g-C3N4 based catalysts. (b) Rhodamine B degradation rate over g-C3N4 based catalysts under visible light. From ref. [167].

  is not major development. There are only few examples of the use of carbon-supported SAC in the CO oxidation reaction. 279 P. Zhou et al., 280 report the use of neighboring Pt and Ru (Pt-Ru) monomers on N-vacancy-rich g-C3N4 (Figure 1.41). The catalytic tests were carried out at atmospheric pressure with reactant feedstocks of 1 vol% CO and 20 vol% O2. At 150 ºC, PtRu/C3N4 shows a lowest conversion temperature (T100%) compared with samples having each metal independently, suggesting the synergistic promotion effect of Pt-Ru monomers. A stability test was carried out by continuous reaction during 50 hours showing 100% CO conversion all over the course of the essay.

Figure 1 . 41 .

 141 Figure 1.41. Schematic illustration of PtRu/C3N4 catalyst. From ref. [280]. In recent studies, theoreticians have taken CO oxidation as a prototype reaction, delivering interesting results about the possible utilization as effective catalysts of atomically dispersed species over different carbon matrices. Langmuir-Hinshelwood (LH), Eley-Rideal (ER) and termolecular Eley-Rideal (TER) mechanisms can operate in this reaction. As shown on Figure 1.42a, 281 in ER mechanism, gaseous CO directly reacts with preadsorbed O2, and CO2 is produced by the subsequent dissociation of the carbonate-like species.

Figure 1 . 42 .

 142 Figure 1.42. Schematic energy profiles of the (a) ER, (b) LH, (c) TER mechanisms, and (d) CO oxidation by atomic O along the minimum-energy pathway. All energies are given with respect to the reference energy (eV) of IS. From ref. [281].

  [a] Diffusion energy of the metal on the support. [b] Binding energy of the single atom to the support. [c] n.i. = no informed. [d] PVG = pyridinic vacancy in N-doped graphene.Additionally, the low adsorption energy of the product, and the large exothermicity of the reaction strongly contribute to an easy CO2 desorption and to the regeneration of the activated centers for subsequent CO oxidation. Considering the possibility to produce SAC on graphene and this literature on modelling studies, it is surprising that up today there is no abundant experimental reports on the use of SAC on carbon materials for this important reaction. 1.5.5. Hydrogenation reactions Selective hydrogenation is an important catalytic industrial process that until a few years ago only had thought possible to carry out on metallic NP. Nevertheless, it has been shown that SAC are capable of adsorbing the hydrogen molecule, and generate needed species to lead the hydrogenation of the reactants. For instance, PtSA/N-C (Figure 1.43) showed an excellent chemoselectivity in the semi-hydrogenation of terminal alkynes to alkenes (PtSA/N-C, with 0.43 wt% Pt loading). 305 At 10 bar of H2, 50 ºC (Figure 1.43a,b)/80 ºC (Figure 1.43c,d) and ratio Pt:substrate = 1:1200, SAC allows obtaining 99

Figure 1 . 43 .

 143 Figure 1.43.Catalytic performance of PtSA/N-C and PtNP/N-C. Reaction results for the hydrogenation of (a) 1-nitro-4-ethynylbenzene; (b) 1-ethynyl-4-vinylbenzene; (c) 1ethynyl-4-(phenylethynyl)benzene; and(d) 1-(dec-1-yn-1-yl)-3-ethynylbenzene on PtSA/N-C and PtNP/N-C. From ref. [305].

  than the B-doped graphene. The N-doped support renders the Pt center more electrodeficient (+0.45 from Bader charge analysis), while the B-doped support gives electrons to the metal (-0.85). The shift of the Pt d-band center, which impact the C-H activation step, explain the differences of activity, and a linear relation was established between C-H dissociation energy barrier, the complete reaction activation energy, and the charge on the PtSA. Additionally, the calculated activation energy on PtSA/N-G SAC is comparable or even lower than the calculated barriers on Pt(111) or Pt(211) surfaces, and Pt clusters, suggesting a higher activity of SA over NP. Ethylene epoxidation to produce ethylene oxide has been investigated on Au 366 and Ti 367 atoms dispersed on graphene. In the case of Au, the reaction would go through the formation of an intermediate species with a peroxametallacycle structure by the co-adsorption of C2H4 and O2. The dissociation of this species is the rate determining step (Ea = 0.84 eV) that produces ethylene oxide molecule and an adsorbed O atom. Further reaction of ethylene with the adsorbed O atom leads to the formation of a second ethylene oxide molecule.

3. 6

 6 %CoSA/Ndoped-C Yield ranging between 63 and 99 % according to the substrate 188 Suzuki coupling 0.6 wt% PdSA/C3N4 Conv = 63%, Selec = 90% TOF = 549h -1 , outperformed all of the investigated homogeneous catalysts 352 Reductive coupling of nitrobenzene CoSA/Ndoped-C CoSA/N-C (conv. 100% -S = 91.4%) > homogeneous Ir Pd catalyst (yield = 76%) 205 N2 reduction reaction 1.1% Fe-N-C The electrochemical N2RR was tested in N2-saturated 0.1 M KOH (30 mL in each cell compartment) at ambient T and P. Selectivity 100% to formation of ammonia Faradaic efficiency of 56.°C -8h -10 bar O2): conv. = 49 % -Saldehyde = 78 % Ethanol (80 °C-8h-10 bar O2): conv. =4 % -Saldehyde=98 % 192 Hydroalkoxylation of 3-buten-1-ol PtSA/CNT PtSA/CNT (conv. =70 %) > [Pt(COD)Cl2] (conv. = 10 %) >PtNP/CNT (conv.= ≪ 10 %) oxidation of bisphenol A via activation of peroxymonosulfate TOF=12.51 min -1 at room Tº, pH=6.0 371 368 M. F.

  (1-D), two-(2-D), and three-dimensional (3-D) polymers were examined. It was found that the most stable polymer is that represented by bonding via the [6,6] position of the C60 molecules with Pd in a distorted tetrahedral coordination (Figure 2.1).

Figure 2 . 1 .

 21 Figure 2.1. Optimized structures of the most stable configurations of the palladiumfullerene polymers: a) 1-D (C60Pd1); b) 2-D (C60Pd2); and c) 3-D (C60Pd3). The bonds of interest are displayed (in Å).

Figure 2 . 2

 22 Figure 2.2 TEM image of the Pd-C60 system prepared from 0.48 mM C60 and 0.73 mM Pd2(dba)3•CHCl3. Arrows indicate palladium metallic NP. From ref.[29]

Figure 2 .

 2 Figure 2.3.a) TEM of D-Ru@C60 1/1; b) STEM-HAADF of D-Ru@C60 1/1; c) TEM of T-Ru@C60 1/1; and d) STEM-HAADF of T-Ru@C60 1/1.

Figure 2 .

 2 Figure 2.4.a) STEM-HAADF of D-Ru@C60 2/1; b) TEM of D-Ru@C60 2/1; c) STEM-HAADF of T-Ru@C60 1/1; d) and e) TEM of T-Ru@C60 5/1; and f) scheme showing the incorporation of Ru on/in Ru1@C60 according to the nature of the solvent.

Figure 2 . 5 .

 25 Figure 2.5.Models of polymeric phases for Ru1-C60, Ru2-C60, Ru4-C60, Ru13-C60 building blocks used for the evaluation of charge transfer and the simulated Pair Distribution Function (PDF), and total charge loss for the corresponding Run cluster.

Figure 2 .

 2 Figure 2.6.a),b)TEM, d)STEM, c),d)STEM-HAADF micrographs and f)EDX mapping of the T-Ru@C60 20/1 sample.

Figure 2 .

 2 Figure 2.7. a) Mass loss of T-Ru@C60 10/1 swelled in pure toluene (24 h and then dried at room temperature during 3 days), TG curve measured under nitrogen flow 40 cm 3 /min; and b) Interaction of four toluene molecules with the -( Ru1@C60)n-structure.

Figure 2 .

 2 Figure 2.8. 1 H-NMR spectrum of [Ru(0) (1,5-cyclooctadiene)(toluene)] isolated from the yellow filtrates obtained at the end of the reaction of entry 7, Table 2.1.

Figure 2 .

 2 Figure 2.9. a) STEM; and b-d) STEM-HAADF micrographs of the T95D5-Ru@C60 20/1 sample.

  2.10), but no core-shell structure was formed. So, we can propose that for the samples prepared in toluene/methanol mixture, the extra Ru atoms are incorporated into the hetero-structure to provide more Ru atoms or subnanometric Ru clusters.

Figure 2 .

 2 Figure 2.10. a) and b) STEM; and c) STEM-HAADF micrographs of the T95M5-Ru@C60 20/1 sample.

Figure 2 .

 2 Figure 2.11. a), b) STEM-HAADF micrographs and EDX mapping of the T50D50-Ru@C60 sample; and c), d) STEM-HAADF micrographs and EDX mapping of the T50M50-Ru@C60 sample.

Figure 2 .

 2 Figure 2.12a shows Raman spectra (532 nm) of T-Ru@C60 at different Ru/C60 ratio. The spectral range is mainly focused on the pentagonal pinch mode Ag(2), because it has been proven that it is a reliable probe of metal fulleride polymer states. It is known that the energy of the Ag(2) mode (1460 cm -1 for pure C60) is sensitive to charge transfer in transition metal fullerides.43

Figure 2 .

 2 Figure 2.12. a) Raman spectra of (1) C60, (2) T-Ru@C60 1/1, (3) T-Ru@C60 5/1, (4) T-Ru@C60 10/1 and (5) T-Ru@C60 20/1; and b) Raman shift vs. Ru composition in C60 illustrates the broadening and down shift of Ag(2) peak with respect to increase in Ru content.

1 Figure 2 . 13 .

 1213 Figure 2.13.Ru3p XPS spectra of a) T-Ru@C60 1/1; and b) T-Ru@C60 20/1 samples.

  2.14). For T50D50-Ru@C60, a strong contribution can be observed at 0.267 nm in very good agreement with Ru-Ru metallic bond length (0.265 nm); confirming the presence of Ru clusters in this sample showing a core-shell structure (Figure2.11a). In the two other compounds, this distance could not be observed; however, they share a strong contribution close to 0.360 nm, likely also related to Ru-Ru distances but clearly non-bonding ones. A contribution in the 0.20-0.23 nm range is also observed for these two samples, which could be related to bonding of Ru with a light element (likely C, since Ru-C is close to 0.2 nm).

Figure 2 .

 2 Figure 2.14. a) WAXS Diffractograms of T95D5-Ru@C60, T95M5-Ru@C60 and T-Ru@C60 samples (Ru/C60 = 20/1); and b) related PDF.

Figure 2 .

 2 Figure 2.15. a) WAXS Diffractograms for T-Ru@C60 compounds at different Ru/C60 ratio; and b) related PDF.

Figure 2 .

 2 Figure 2.16. a)WAXS Diffractograms of T95D5-Ru@C60, T95M5-Ru@C60 and T-Ru@C60 samples (Ru/C60 = 20/1); and b) related PDF.

  ) derived from the DFT models previously used to evaluate the charge transfer, which are poorly comparable to the experimental profiles. It is however interesting to notice that no contribution at ca. 0.270 nm is observed for small clusters with a limited number of metallic Ru-Ru bonds on the simulated PDF profiles.

Figure 2 .

 2 Figure 2.17. Simulated PDF profiles derived from the models optimized at the DFT-PBE level of theory that are shown in Figure 2.3.DFT details regarding such simulations can be found in Ref.47,48,49,50.

Figure 2 .

 2 Figure 2.18. a) Fit of the EXAFS region; k 2 χ(k); and b) corresponding FTs for T-Ru@C60 1/1, T-Ru@C60 5/1, T-Ru@C60 10/1, T-Ru@C60 20/1, and T95M5-Ru@C60 20/1 using toluene/methanol (from bottom to top). Dots: experiment; Fit: full line.

Figure 2 .

 2 Figure 2.19. Fourier-transformed k3 -weighted Pt L3-edge EXAFS spectra of Pt8/CB.Inset dark-field STEM image. From ref.[51]

Figure 2 . 20 .

 220 Figure 2.20. Pathways for products and by products formed during NB hydrogenation to aniline.

Figure 2 .

 2 Figure 2.21. Time-conversion curve for a) NB hydrogenation with T-Ru@C60 and TM-Ru@C60 materials; and b) T-Ru@C60 20/1, T50M50-Ru@C60 and T50D50-Ru@C60; and c) for 2,3-dimethyl-2-butene hydrogenation with T-Ru@C60 and T50M50-Ru@C60.

Figure 2 .

 2 11a,b). In order to investigate the ability of various -(C60-Run-C60)n-hetero-structures to chemisorb H2 molecules, and thus get an idea on the activity of such metallic centers for further substrate hydrogenation, we have performed a series of calculations for naked Ru1, and Ru2, Ru4 and Ru13 clusters embedded in a 1D-C60 polymeric phase. Dissociative adsorption energies per H with respect to the H2 molecule number in interaction with a Run cluster are shown in Figure 2.22, as well as final geometry representations.

Figure 2 . 22 .

 222 Figure 2.22. Dissociative adsorption energy (in kcal/mol) per adsorbed H on various -(C60-Run-C60)n-hetero-structures.

  2.23, with a negative balance of 26 kcal/mol with respect to the separated reactants. The regeneration of the catalyst leading to the release of the 2,3-dymethylbutane molecule and the coordination of a H2 and 2,3-dimethyl-2-butene molecules is also favorable by -12 kcal/mol with respect to the final product (-38 kcal/mol with respect to the entrance channel). The first hydrogenation step leading to an alkyl intermediate is kinetically easily accessible with an activation barrier of 11 kcal/mol with respect to the -coordinated adduct.

Figure 2 .

 2 Figure 2.23 Possible reaction path of 2,3-dimethyl-2-butene hydrogenation.

2. 4 .

 4 Experimental 2.4.1. General methods All operations were carried out under argon atmosphere using standard Schlenk techniques or in an MBraun glovebox. Solvents were purified by standard methods or by an MBraun SPS-800 solvent purification system. [Ru(COD)(COT)] (COD= 1,5 cyclooctadiene, COT= 1,3,5-cyclooctatriene) was purchased from Nanomeps Toulouse, fullerene C60 from Sigma-Aldrich, Ar and H2 from Air Liquid. All these reactants were used as received. The ruthenium content was established by inductively coupled plasma optical emission spectroscopy (ICP-OES) with a Thermo Scientific ICAP 6300 instrument on samples dried 2 h at 200 °C.

2. 4 . 4 .

 44 Hydrogenation of 2,3-dimethyl-2-buteneIn a typical experiment, the Top Industry autoclave was purged by three vacuum/argon cycles. The mixture of Ru@C60 catalysts (5 mg), dodecane (as internal standard, 1.1 mmol) and 2.3-dimethyl-2-butene (10.0 mmol) in 30 mL of cyclohexane was prepared in a glovebox, ultra-sonicated for 5 min and then transferred into a high-pressure autoclave under argon atmosphere. The autoclave was heated to 50 °C and pressurized with 20 bar of H2; the stirring rate was fixed at 1000 rpm. Samples of the reaction mixture were taken periodically and then analysed by GC-MS. Quantitative analysis of reaction mixtures was performed via GC-FID using calibration solutions of commercially available products. 2.4.5. Catalyst characterization TEM analyses. TEM and HRTEM analyses were performed at the "Centre de microcaracterisation Raimond Castaing, UMS 3623, Toulouse". Low resolution TEM was performed by using a JEOL JEM 1011 CX-T electron microscope operating at 100 kV with a point resolution of 4.5 Å, and a JEOL JEM 1400 electron microscope operating at 120 kv. The high-resolution analyses were conduct using a JEOL JEM 2100F equipped with a Field Emission Gun (FEG) operating at 200 kV with a point resolution of 2.3 Å and a JEOL JEM-ARM200F Cold FEG operating at 200 kV with a point resolution of > 1.9 Å. The approximation of the particles mean size was made through a manual analysis of enlarged micrographs by measuring at least 200 particles on a given grid. Other TEM micrographs were acquired with a JEOL 2100F S/TEM microscope equipped with a FEG operating at 200 kV, a spherical aberration probe corrector and a GATAN Tridiem energy filter. The resolutions attained are 2 Å and 1.1 Å under parallel TEM mode and scanning STEM modes, respectively. For STEM-HAADF analyses the spot size was of 0.13 nm, a current density of 140 pA, the camera focal length was 10 cm, corresponding to inner and outer detection angle of the annular detector of about 60 mrad and 160 mrad. WAXS, EXAFS, Raman and XPS analyses. Wide Angle X-ray Scattering measurements were performed at CEMES (Dr. Pierre Lecante) on a diffractometer dedicated to Pair Distribution Function (PDF) analysis: graphite-monochromatized Molybdenum radiation (0.07169 nm), solid-state detection and low background setup.Samples were sealed in Lindemann glass capillaries (diameter 1.5mm) to avoid any Chapter 2. High loading single atom catalysts 123 oxidation after filling in a glove box. For all samples data were collected on an extended angular range (129 degrees in 2 theta) with counting times of typically 150s for each of the 457 data points, thus allowing for PDF analysis. Classic corrections (polarization and absorption in cylindrical geometry) were applied before reduction and Fourier transform.

Figure 3 . 1 .

 31 Figure 3.1. Reconstructions of the atomic network near vacancies in single-walled carbon nanotubes. Only front walls of the nanotubes are shown. From ref.[13]. As mentioned before, the presence of vacancies is highly desired; dangling bonds around

Figure 3 . 2 .

 32 Figure 3.2. First line: HRTEM images of proton-irradiated layer graphene. a) before irradiation, b) monovacancy after irradiation, c)555-777 rearranged divacancy. Adapted from ref. [ 18 ]. Second line: TEM images electron-beam irradiated CNT d) SWCNT, e), f) irradiated MWCNT with different beam size, g) model of electron-beam-irradiated CNT.Adapted from ref.[19].

Figure 3 . 4 .

 34 In conjunction with different characterizations and early theoretical work from Coulson, 28 the authors propose three types of defective sites, type I: isolated unpaired-electron (free-radical), type II: edge sites formed from the dangling carbon atoms by acquiring a triple-bond character; and type III: edge sites forming an inplane σ pair, with s 2 p 2 hybridization. Clearly, the resultant surface should be very (re)active due to the Lewis basicity, 29 especially from sites I and II. Eventually, this highly unstable structure would be stabilized by the adsorption of different atoms either O from the air or H from water.

Figure 3 . 4 .

 34 Figure 3.4. Proposed model for the stabilization of carbon surfaces upon heat treatment (∆) under a,b) N2, and b,c) H2. For simplicity and illustrative purposes, only two types of oxygen surface groups (pyrone and lactone) and three types of carbon active sites are shown. Type I, isolated unpaired-electron sites Ar-• (where Ar represents the aromatic (graphene) layer); type II, Ar-C≡C• (see text); and type III, divalent in-plane σ pairs, Ar -•* (where * represents a localized π electron). Adapted from ref. [30].

Figure 3 . 7 .

 37 Figure 3.7. TGA profiles of: a) O3h-CNT support; and b) CNT with different oxidation time. The analysis was carried out with the following program: ramp from 25 ºC to 100 ºC, isotherm at 100 ºC during 10 minutes, ramp from 100 ºC to 400 ºC, isotherm 400 ºC during 1 hour, finally a ramp from 400 ºC to 1000 ºC, all stages heated with rate of 5 ºC/min under 20 mL/min of N2.

Figure 3 .

 3 8a displays the three main CO2 desorption peaks with maximal temperature at around 250, 420 and 610 ºC. These results indicate the presence on the surface of chemical species with different thermal stability. We are aware that different carbon fractions could possibility present a co-decomposition but we identify the nature of the different species based on the most commonly accepted values in 40 I. V. Krasnikova, et al. In Carbon-Based Nanofillers and Their Rubber Nanocomposites, edited by S. Yaragalla, et al.: Elsevier (2019), pp. 75-137.

Figure 3 . 8 .

 38 Figure 3.8. TPD profiles: a) CO2; b) CO; c) H2O; d) CH4 and e) H2 obtained for O3h-CNT and O3h-CNT400.

  3.5). The methane signal (Figure. 3.8d) as well as the H2 signal (Figure. 3.8e) are relatively flat.

Figure 3 . 9 .

 39 Figure 3.9. Raman spectra of oxidized CNT and annealed oxidized CNT at 400 ºC. Lorentzian fitted D, G and D' bands of a) O3h-CNT and b) O3h-CNT-400.

Figure 3 .

 3 Figure 3.10 a,b) HRTEM micrographs of the MWCNT used in this study with a number of layers superior to 10.

Figure 3 . 11 .

 311 Figure 3.11. High-resolution a) C1s and b) O 1s XPS spectra of sample O3h-CNT-400: deconvoluted peaks using Shirley background.

Figure 3 .

 3 12a (purified CNT), catalyst particles remain indeed inside the tubes. Energy dispersive X-ray spectroscopy (Figure 3.12b) reveals that the composition of these particles corresponds to 44.62 wt.% of Fe and 55.38 wt.% of Co. A weight loss of 0.36 % was calculated by TGA after heating. The elimination of the catalyst at high temperatures is due to sublimation of FeCo nanoparticles that are swept from the oven by the vacuum pump. Most of the tips of the MWCNT were opened, and the catalyst particles embedded in the tips and the bodies of MWCNT were totally removed (Figure 3.12c,d). Afterwards CNT* was used as mentioned above in step I and step II to produce O3h-CNT* and O3h-CNT*-400, respectively.

Figure 3 .

 3 Figure 3.12. a) HRTEM micrographs of the as produced CNT highlighting a metallic nanoparticle, b) EDX analysis of the nanoparticle; and c,d) CNT annealed at 2800 ºC (CNT*).

Figure 3 . 13 C

 313 Figure 3.13. 13 C MAS SS-NMR of purified CNT*, O3h-CNT* and O3h-CNT*-400.

Figure 3 .

 3 Figure 3.14 a) Hysteresis loop of as prepared CNT at 5 and 300K b) Diamagnetic behavior of CNT after annealing the sample in argon atmosphere at 2800 °C.

Figure 3 . 15 . 73 O5hFigure 3 . 16 .

 31573316 Figure 3.15. Magnetization curves obtained for CNT*, oxidized CNT* with different time of oxidation, and oxidized CNT* with different time of oxidation heat-treated at 400 °C. a) CNT* and oxidized CNT* with different time of oxidation (300 K); b) CNT* and oxidized CNT* with different time of oxidation (5 K); c) CNT* and oxidized CNT* with different time of oxidation heat-treated at 400 °C (300K); and d) CNT* and oxidized CNT* with different time of oxidation heat-treated at 400 °C (5K).

Figure 3 .

 3 Figure 3.17. a) Two components GO EPR spectra recorded at 6 K, fitted by Gaussian and Lorentzian functions. The narrow Lorentzian line (ΔH = 3.4 G) is associated with conduction electrons, while the broad one (ΔH = 11 G) corresponds to defects. From ref. [102]. b) EPR powder spectrum of anthracite, recorded at 10 K. Deconvolution into two Lorentzian lines. From ref. [103].

Figure 3 .

 3 Figure 3.18. EPR spectra of O3h-CNT* and O3h-CNT*-400 measured at -263 °C (10K) and X-band at ~ 9.52 GHz.

Figure 3 . 19 .

 319 Figure 3.19. Procedure for the synthesis of single atom catalysts on graphitic materials.

Figure 3 . 21 .

 321 Figure 3.21. Change of color before and after deposition.

Figure 3 . 22 .Figure 3 .

 3223 Figure 3.22.Histograms and HAADF-STEM micrographs of Ru/O3h-CNT-400 prepared with Ru(NO)(NO3)2 in: a), b) ethanol and c), d) water.

Figure 3 .Figure 3 .

 33 Figure 3.23. TEM micrographs of sample 0.4%Ru/O3h-CNT400 a) fresh, b) reduced at 80 ºC, c) reduced at 150 ºC; and d) reduced at 300 ºC. Reduction with 80/20 Ar/H2 during one hour.

Figure 3 . 25 .

 325 Figure 3.25. Geometries of Ru13O30 + 1O atom on graphene. Top panel shows the initial state, in the middle lies the transition state geometry while the bottom panel shows the final state with the epoxy group. C atoms are in black, O in red, H in with white and Ru in red.

Figure 3 .

 3 Figure 3.26. HAADF-STEM micrographs sample 0.4%Ru/O3h-CNT without support activation a) and b) before reduction; and c) and d) after reduction at 300 °C.

Table 3 . 9 .

 39 Metal loading using different metal precursors determined by ICP-OES. followed was the same as for Ru. HAADF-STEM micrographs displayed in Figure3.27 show similar appearance as for Ru samples although the discernment of the Cu or Ni atoms was more difficult due to poor contrast because their lower Z ( 44 Ru >28 Ni and 29 Cu).

Figure 3 .

 3 Figure 3.27. HAADF-STEM micrographs of a) 1.0%Pd/O3h-CNT400; b) 1.0%Cu/O3h-CNT400; c) 1.0%Ni/O3h-CNT400; and d) 1.0%Pt/O3h-CNT400 together with respective EDX analyses.

Figure 3 .

 3 Figure 3.28. HAADF-STEM micrographs of: a) 2.0% Ru/O3h-FM-400; b) 1.5% Ru/O3h-sCNF-400; and c) 1.5% Ru/O3h-CB-400.

  performances have been explained in different ways. For instance, Tiwari et al., relate the positive effect of having both PtSA and Pt nanoclusters onto graphitic tubes for the hydrogen evolution reaction in acidic solutions. A synergistic conductivityenhancement effect between the PtSA and the Pt nanoclusters has been proposed. 130 On the other hand, He and co-workers indicate a metal-metal charge transfer from Ru particles to CoSA. 131

Figure 3 . 29 .

 329 Figure 3.29. Production of catalysts with controlled SA/NP ratio.

Figure 3 . 30 .

 330 Figure 3.30. Linear correlation between theoretical and real metallic loading after 2, 3, and 4 impregnation cycles.

Figure 3 .

 3 Figure3.31 shows HAADF-STEM images of 1.40% Pd/O3h-CNT-400 prepared by this process, for which the SA/NP ratio is controlled between 200/1 (one deposition cycle of 1.40% Pd) and 10/1 (four deposition cycles of 0.35% Pd). Furthermore, with the repetition of the process, the size of the clusters begins to grow due to the coalescence effect going from 0.50 nm to 1.1 nm.

Figure 3 .

 3 32shows a typical Pd 3d spectrum with characteristic doublet peaks corresponding to Pd 3d5/2 and Pd 3d3/2.

Figure 3 .

 3 Figure 3.31. HAADF-STEM micrographs mixtures of SA and NP at different ratio in a) 0.70% x2 Pd/O3h-CNT-400; b) 0.46% x3 Pd/O3h-CNT-400; and c) 0.35% x4 Pd/O3h-CNT-400 catalysts.

3. 7

 7 Experimental section 3.6.1 Catalysts preparation Step I: Support oxidation. 5.0 g of CNT prepared and purified as described in ref.123, were placed in a round bottom flask with 50 mL of HNO3 65% and refluxed at 140 °C for 1, 3 or 5 h. The acidic solution was filtered and the solid washed with distilled water. The solid was dried in an oven at 80 °C overnight to produce functionalized CNT O1h-CNT, O3h-CNT, and O5h-CNT.

Chapter 4 . 4 .

 44 Hydrogenation of alkenes with single atom catalysts Hydrogenation of alkenes with single atom catalysts 4.1. Introduction

Figure 4 . 1 .

 41 Figure 4.1. The PdSA+NP/TiO2 integrates both PdSA and PdNP sites on TiO2 surface, in which the PdSA sites undertake the activation task of C=O group while PdNP sites boost the dissociation of H2. The H atoms migrate to the C=O group bounded PdSA sites to complete hydrogenation.From ref.[7].

Figure 4 . 2 .

 42 Figure 4.2. β-Myrcene hydrogenation reaction.

Figure 4 . 3 .

 43 Figure 4.3. Correlations between dispersion combined to non-basal plane surfaces and oxygen content of the catalyst (from XPS) and mean apparent Site Time Yield -STY (calculated at 90% of conversion) for Pd/CNT, Pd/FLG, Pd/FC, Pd/CNF and Pd/sCNF catalysts. Ref 17 (CNT: carbon nanotubes; FLG: few layer graphene; FC: fibrous carbon; CNF: carbon nanofibers; and sCNF: small diameter carbon nanofibers).

  4). Since the three catalysts present different PdNP size (Figure 4.6 a,c,e), in order to have a fair comparison with commercial catalysts, a STY* could be also used integrating the metal dispersion. A similar tendency was observed, and again higher STY* were obtained on the two carbon-supported catalysts (Figure 4.5).

Figure 4 . 4 .

 44 Figure 4.4. Catalytic results on Pd/Al2O3, Pd/C and Pd/CNT. Myrcene hydrogenation reaction. The reaction was carried out over supported Pd catalysts. PH2 = 20 bar -T = 120 °C -myrcene 1 M (80 mL) -solvent heptane -4 mg Pd a) Reaction progress as a function of time.cat. b) STY as a function of reaction progress.

Figure 4 . 5 .

 45 Figure 4.5. Catalytic results on Pd/Al2O3, Pd/C and Pd2_SA/NP/CNT as a function of surface Pd. -myrcene hydrogenation reaction. The reaction was carried out over supported Pd catalysts. PH2 = 20 bar -T = 120 °C -myrcene 1 M (80 mL) -solvent heptane -200 mg cat. a) Reaction progress as a function of time.cat*. b) STY* as a function of reaction progress.

Figure 4 . 6 .

 46 Figure 4.6. STEM-HAADF and TEM micrographs, and histograms of PdNP sizes of: a) fresh Pd/Al2O3; b) Pd/Al2O3 after β-myrcene hydrogenation; c) fresh Pd/C; d) Pd/C after β-myrcene hydrogenation; e) fresh Pd/CNT; and f) Pd/CNT after β-myrcene hydrogenation. Scale bar = 50 nm

Figure 4 . 7 .

 47 Figure 4.7. STEM-HAADF characterization. STEM-HAADF micrographs of: a) Pd/Al2O3; b) Pd/C; c) Pd2_SA/NP/CNT; d) PdSA/CNT; e) Pd2_SA/NP/CNTHT; and f) Pd10_SA/NPCNT catalysts. Scale bar = 10 nm.

Figure 4 . 8 .

 48 Figure 4.8. Comparison of the catalytic performances of Pd/Al2O3 and Pd/FLG for the total hydrogenation of myrcene, farnesene and squalene. Hydrogenation reactions carried out over supported Pd catalysts. PH2 = 20 bar -T = 120 °C -substrate 1 M (80 mL)solvent heptane -280 mg catalyst.

Figure 4 . 9 .

 49 Figure 4.9. STEM-HAADF micrographs, particle size distribution and PdSA/PdNP ratio for the Pd/FLG catalyst: a) and b) before catalysis; and c) and d) after myrcene hydrogenation.

  The first one corresponds to carboxylic acid groups (Bronsted acids) arising from the acid functionalization of the carbon material, which withstand the heat treatments carried out during catalyst preparation (see Chapter 3). The second type of acidic sites consists in the PdSA. DFT calculations performed in LPCNO on a PdSA/graphene single vacancy model show a charge transfer from Pd-4d to C-2p of -0.37 e-(Figure 4.10). Thus, a PdSA would act as Lewis acid due to its electronic deficiency.

Figure 4 . 10 .

 410 Figure 4.10. Isosurface plot of the electronic density differences for negative (in cyan) and positive (in yellow) isovalue of 0.002 e-/Å. The charge transfer from PdSA 4d orbitals to the defective (single vacancy in graphene) support is estimated to be 0.37 e-.

Figure 4 . 11 .

 411 Figure 4.11. Evolution of the composition of the solution during 1-octene hydrogenation as the function of time and catalysts PdSA/CNT, Pd/Al2O3 and Pd2_SA/NP/CNT. Reaction conditions: 1octene (1M) in heptane, Pd 0.4 mg (60 ppm), reflux (98 °C), H2 1 atm. In the case of the O-CNT catalyst no measurable activity was obtained.

Figure 4 .

 4 Figure 4.12. CO2 TPD profiles of the CNT support and the Pd2_SA/NP/CNT catalyst showing the disappearance of most of the surface carboxylic groups after catalyst preparation, and the appearance of a Pd-OOC-interface.

Figure 4 .

 4 Figure 4.14. DFT mechanistic study of reaction mechanisms of hydrogenation of 2-butene mediated by PdSA and H-PdSA (in orange).

Figure 4 .

 4 Figure 4.15. STEM-HAADF micrographs of Pd2_SA/NP/CNT and Pd2_SA/NP/CNTHT catalysts and related particle size distribution based on total particles and on total atoms (see section 4.3 for details of calculations).

Figure 4 .

 4 Figure 4.16. β-myrcene hydrogenation reactions carried out over Pd2_SA/NP/CNT and Pd2_SA/NP/CNTHT catalysts. PH2 = 20 bar -T = 120 °C -β -myrcene 1 M (80 mL)solvent heptane -200 mg cat.

Figure 4 .

 4 Figure 4.17. Photographs of samples made with 500 mg of WO3 mixed or not with 5 mg of catalyst before (left) and after (right) treatment with H2 (100 mL/min) at 20 °C for 3 min.

Figure 4 .

 4 Figure 4.18. DFT mechanistic study. Spillover mechanism corresponding to the migration of H adsorbed on PdNP leading to the formation of H-PdSA species.

Figure 4 . 19 .

 419 Figure 4.19. Mixtures of SA and NP at different ratio in 1.4% Pd/CNT catalysts.

Figure 4 . 20 .

 420 Figure 4.20. Correlation between the ratio Pd 0 /Pd δ+ determined by XPS and the fraction of SA in the same samples determined from STEM.

Figure 4 . 21 .

 421 Figure 4.21. Catalytic results for myrcene hydrogenation at different SA/NP ratio. a) Reaction progress as a function of time catalyst. b) STY as a function of reaction progress.

Figure 4 .

 4 16) and 195-fold increase with Pd1000_SA/NP/CNT (a catalyst containing almost exclusively SA). The evolution of STY with SA/NP ratio remains nearly the same for the reaction progress of 2.5(Figure 4.22). This fact further confirms the consistency of these results.

Figure 4 .

 4 Figure 4.22. STY at a reaction progress of 2 and 2.5 as a function of SA/NP ratio. Catalysts nomenclature is PdX_SA/NP/CNT where X refers to the SA/NP ratio.

Figure 4 .

 4 Figure 4.24 Catalyst stability testing in continuous mode using coated foam objects with Pd2_SA/NP/CNT catalyst (20 bar; 120 °C; 100 mg of catalyst; QL = 2 mL/min; QG = 2 NL/min, pure H2; Myrcene 1M in heptane). a) Mean STY as a function of Turn Over Number. b) Reaction progress versus Turn Over Number.

4. 7 .

 7 Experimental part 4.7.1. Palladium catalyst synthesis A wet impregnation method was used to prepare the palladium catalysts supported on carbon nanomaterials. The desired amount of palladium(II) nitrate dihydrate Pd(NO3)2•2H2O (Sigma-Aldrich) was added to acetone (100 mL) containing 1 g of carbon nanomaterial (O-CNT or O-FLG), so as to introduce 2 %w/w of metal phase. The solution was sonicated at room temperature for 1 h and magnetically stirred overnight. The solution was then filtered and washed with acetone. The resulting solid was dried in an oven at 120 °C overnight. Finally, the

6

  straight blades to provide an efficient G-L-S contacting. The catalytic tests were operated at 1200 rpm and 120 °C with 200 mg of catalyst powder (except for Pd/C and Pd/Al2O3 catalysts where 80 mg of catalyst has been used because of their Pd loading of 5 wt.%) under pure H2 atmosphere at a constant pressure of 20 bar. The liquid phase consists in 80 mL of a 1M myrcene (Sigma-Aldrich, 98% purity, corresponding to 3M of C=C double bond) solution in n-heptane (Sigma-Aldrich, 99%) containing a known trace of n-tetradecane as internal standard for final GC-FID analysis (DB17 column). In this configuration, the reactor is operated in batch mode for the liquid and solid phases and in semi-batch mode for the gas because hydrogen was constantly fed via a pressure regulator and supplied by a calibrated reserve. The pressure and temperature monitoring of this reserve provides the hydrogen consumption rate, and thus the apparent and instantaneous activity of the catalysts expressed in moles of H2 consumed per mole of Pd and per second, and being equivalent to an instantaneous Site Time Yield.The instantaneous STY is determined with the local slope of the H2 reserve consumption curve as following, where nPd is the quantity of Pd determined by ICP: of apparent because (i) it addresses the overall consumption of H2 through multiple reactions and (ii) depending on the catalyst and the reaction progress, possible external and internal mass transfer resistances are likely. Indeed, the reactivity of the first C=C is very high (terminal and conjugated C=C) and the first moments of the reaction are physically limited.

Figure 4 .

 4 25 shows a photo of the foam coated in the tubular reactor. The continuous experiments were performed at 120 °C and 20 bar applying a gas flowrate of 2 NL/min of pure H2 and a liquid flow rate of 2 mL/min of a 1M solution of myrcene in heptane corresponding to a liquid space velocity of 106.4 molmyrcene/molPd/min. Analysis by GC-FID (method and apparatus are identical to the one described in the previous β-myrcene hydrogenation section) at the reactor outlet allow to verify the mass balance and to determine the liquid flow composition. The corresponding mean Site Time Yield and Reaction progress are then calculated. It is noteworthy that this mean STY and the instantaneous STY used in the batch experiment are different and cannot be compared directly.

Figure 4 . 25 .

 425 Figure 4.25. Photo of the metallic foam coated in the tubular reactor for catalyst stability study under continuous reactor operation.

Figure 4 .

 4 Figure 4.26. H2-assisted isomerization of 1-octene.

Figure 4

 4

Figure 4 .Figure 4 .

 44 Figure 4.27 GC-FID of the reaction mixture.

4. 7 . 4 . 6 .

 746 Assessment of the impact of mass transfer for myrcene hydrogenationMass transfer limitations have been evaluated using common chemical engineering approaches regarding the limiting reagent H2. The physical properties of the different phases involved in the reaction are listed in the following Table4.3. Gas and liquid properties are given for 20 bar and 120 °C.

7 ) 3 L/m 3 R

 733 gas tank. Identical conditions to the experimental ones have been used but without any catalyst (1200 rpm, 80mL of 1M myrcene solution in heptane, 120 °C, constant PH2 = 20 bar). The H2 consumption profile of the reserve allowed adjusting and recovering 2 parameters of the system mass balance (eq. 4.5-4.7): the mass transfer coefficient 𝑘 𝐿 𝑎 𝐺𝐿 and the final saturation concentration of H2 in the solution (solubility) 𝐶 𝐻2 * .In these equations, 𝑑𝑛 𝐻2 𝐿 (𝑡) is the variation of the quantity of dissolved H2 in the liquid phase between t0 and t (mol); 𝑑𝑛 𝐻2 𝑅𝑒𝑠 (𝑡) is the corresponding variation of H2 in the calibrated reserve (mol); 𝑑𝑝 𝑟𝑒𝑠 (𝑡) is the corresponding pressure variation in the reserve (Pa); 𝑉 𝑟𝑒𝑠 is the volume of the reserve (m 3 ) ; 𝑇 𝑟𝑒𝑠 is the temperature of the reserve (K) ; 𝑉 𝐿 is the liquid volume inside the reactor (m 3 ) ; 𝜀 𝐿 is the liquid fraction inside the reactor (m ) ; 𝐶 𝐻2 * is the solubility of H2 in the liquid mixture (mol/m 3 ); 𝑘 𝐿 𝑎 𝐺𝐿 is the G-L mass transfer coefficient (s -1 ) with 𝑎 𝐺𝐿 the specific surface area for G-L mass transfer (

Chapter 5 .

 5 Hydrogenation of carbon dioxide over SA catalysts 5. Hydrogenation of carbon dioxide over SA catalysts 5.1. Introduction

  .

  3.

Figure 5 . 1 .

 51 Figure 5.1. STY and selectivity of 1%MSA/CNTox-400 at 260 ºC, 300 ºC and 340 ºC.

Figure 5 . 2 .

 52 Figure 5.2. TEM micrographs 1%MSA/CNTox-400 catalysts after catalysis. a) Pt, b) Ru, c) Ni and d) Cu.

Figure 5 .

 5 Figure 5.3. a) Ru 3p XPS spectra of RuSA/CNTox-400 b) Ru 3p XPS spectra of RuSA-Na/CNTox-400 c) Na 1s XPS spectra of RuSA-Na/CNTox-400.

  47 A. Petala, et al. Applied Catalysis B: Environmental 2018, 224, 919-927. 48 M. Yang, et al. Journal of the American Chemical Society 2015, 137, 3470-3473. 49 M. Yang, et al. Science 2014, 346, 1498. The signal of Na 1s was easily identified at 1071.8 eV, confirming the effective introduction of Na in the sample. Ru 3p binding energy were at 463.7 and 486.1 eV compared to 463.2 and 485.3 eV for RuSA/CNTox-400. The electronic state of Ru indicates an electron-deficient character after Na impregnation. This points to the absence of charge transfer from Na to Ru. This result is not surprising if we consider the work functions of Na, Ru and C, which should favor a charge transfer from Na to the carbon support. It is important to indicate that the further treatment with NaOH did not change either the metallic loading or the nature of the active phase. No nanoparticles were found after impregnation, ensuring the permanence of SA species on the support (Figure 5.4).

Figure 5 . 4 .

 54 Figure 5.4. TEM micrographs 1%MSA+Na/CNTox-400 catalysts beforer catalysis. a) Ni, b) Pd, c) Pt and d) Ru.

  4). Contrarily to Pt-Na/ZrO2 catalyst, 50 sodium doping on Pt catalyst increased the overall catalysts activity dramatically. Changes can be seen comparing PtSA/CNTox-400 and PtSA-Na/CNTox-400 where in average, STY increased 8 times regardless the temperature. The stability of the catalyst is also very good (Figure 5.5). 50 M. Martinelli, et al. International Journal of Hydrogen Energy 2020, 45, 18490-18501.

Figure 5 . 5 .

 55 Figure 5.5. STY and selectivity of 1%MSA-Na/CNTox-400 at 260 ºC, 300 ºC and 340 ºC.

Figure 5 .

 5 Figure 5.6c), pointing to a better stabilization for the Na-doped sample. For Pt, Ru and Ni catalysts, the small variations of activation energies between the promoted and unpromoted catalysts (Figure 5.7) imply that the presence of sodium does not change either the reaction mechanism or the rate limiting step, in accordance with published results on alkali promoted

Figure 5 . 6 .

 56 Figure 5.6. TEM micrograph of Ru, Pt and Ni after catalysis.

Figure 5 . 7 .

 57 Figure 5.7. Apparent activation measured for CO2 hydrogenation with the 1%MSA /CNTox-400, 1%MSA-Na/CNTox-400 and 1%MSA+NP/CNTox-400.catalyst series.

( 1 .

 1 4% in the case of Pd), it was possible to produce four different samples containing a SA/NP ratio of 200/1, 60/1, 40/1 and 10/1 by varying the number of impregnation cycles. Weight percentage values determined by ICP-OES after each impregnation, demonstrate the linearity and reproducibility of this approach. XPS data have shown a shift in the Pd 0 signal from Pd200_SA/NP/CNTox-400 to Pd10_SA/NP/CNTox-400, indicating a particle size-dependency pointing to a charge transfer between the metallic clusters and the support. The previous findings combined with STEM-HAADF imaging prove the evolution of the metallic dispersion along the series.

1 .

 1 Catalyseurs à base d'atomes métalliques isolés sur matériaux carbonéDepuis le début du siècle, les catalyseurs à base d'atomes métalliques isolés (SAC pour single atom catalysts) ont suscité un grand intérêt pour la recherche, et de nombreuses revues ont déjà été consacrées à ce sujet.1,2,3 Les développements récents des techniques de caractérisation, des outils de modélisation et les progrès de la chimie de coordination de surface ont permis des avancées significatives dans ce domaine. Il est plausible que les SAC opèrent dans de nombreuses réactions catalytiques; cependant, dans le passé, le manque d'outils de caractérisation puissants tels que la microscopie électronique à transmission avec correction d'aberration n'a pas permis leur identification.4 L'utilisation de SAC est particulièrement intéressante car elle maximise l'utilisation des métaux précieux; mais il faut bien entendu aussi qu'ils atteignent des performances optimales en termes de stabilité, d'activité et de sélectivité. La Figure6.1 montre que différents types de catalyseurs hétérogènes peuvent répondre à la définition de SAC: i) un catalyseur à un seul atome de métal supporté ne contenant que des atomes uniques isolés noyés sur un support, dans cette recherche nous appellerons SAC ce type de catalyseur; ii) des atomes isolés incorporés dans l'ossature d'un matériau (comme dans une zéolite, des MOF (Metal Organic Frameworks) ou des tamis moléculaires de phosphate d'aluminium (ALPO)); et iii) un catalyseur homogène supporté ou un catalyseur hétérogène à site isolé 5 contenant un complexe de coordination (pouvant contenir un ou plusieurs atomes métalliques) ancré sur un support.La préparation d'un SAC nécessite des sites spécifiques sur le support qui feront office de pièges pour les atomes métalliques. Dans le domaine des matériaux carbonés, une surface de carbone sp 2 parfaite ne semble pas adaptée pour obtenir un SAC stable. En effet, la plupart du temps, l'énergie d'adsorption des adatomes métalliques sur des matériaux de type graphitique sans défaut6 sont inférieures aux énergies de liaison métal-métal dans la nanoparticule (NP) supportée; 7 par conséquent, la formation de NP métallique est favorisée par rapport au SAC.

(

  Run@C60)n-. Ainsi, nous rapportons la synthèse d'auto-assemblage simple de particules subnanométriques de Ru et de RuSA avec le C60. Les hétéro-structures Ru@C60 ont été synthétisées en décomposant [Ru(COD)(COT)] en présence de C60, en faisant varier le rapport Ru/C60, sous H2 (3 bar) en utilisant du dichlorométhane (D), du toluène (T) et/ou du méthanol (M) comme solvants. Les conditions de réaction pour chaque échantillon, la charge métallique résultante et la taille des RuNP sont résumées dans Tableau 6.1. Tableau 6.1. Chargement et granulométrie moyenne des échantillons Ru@C60.

  partir d'analyses ICP. La valeur entre parenthèses est la charge de Ru visée compte tenu de la quantité initiale de précurseur métallique utilisée dans la synthèse. b) n. ré. = non détecté mode de pincement pentagonal Ag(2). On sait que l'énergie du mode Ag(2) (1460 cm -1 pour le C60 pur) est sensible au transfert de charge dans les fullerides de métaux de transition.29 En effet, un fort déplacement et un élargissement sont observés dans ce signal distinctif avec la teneur en ruthénium comme illustré dans Figure6.2. Des calculs DFT indiquent qu'un transfert de charge évolue de 0.62e/Ru sur RuSA à 0.11e/Ru dans le cas du cluster Ru13 vers les unités C60.

Figure 6 .

 6 Figure 6.3. a) Spectres Raman de (1) C60, (2) T-Ru@C60 1/1, (3) T-Ru@C60 5/1, (4) T-Ru@C60 10/1 et (5) T-Ru@C60 20/1; et b) le décalage Raman par rapport à la composition de Ru dans C60 illustre l'élargissement et le décalage vers le bas du pic Ag(2) par rapport à l'augmentation de la teneur en Ru.

Figure 6 .

 6 Figure 6.4. a) Diffractogrammes WAXS des échantillons T95D5-Ru@C60, T95M5-Ru@C60 et T-Ru@C60 (Ru/C60 = 20/1).

Figure 6 . 5 .

 65 Figure 6.5. Procédure de synthèse de catalyseurs mono-atomes sur matériaux carbonés.

Figure 6 .

 6 Figure 6.6. a) Profils TPD de désorption du CO2 des CNT oxydés et activés ; et b) signal RPE des CNT oxydés et activés. L'identification des sites défectueux après l'étape II a été réalisée par la combinaison d'analyses C 13 -NMR, SQUID et RPE. Les résultats indiquent la présence d'états électroniques

Figure 6 . 7 .

 67 Figure 6.7. Micrographies HAADF-STEM de : a) 1,0% Cu/D-CNT; b) 1,0% de Pt/D-CNT; c) 1,0% de Pd/D-CNT; d) 1,0% Ni/D-CNT ; et e) 1,0% Ru/D-CNT.

Figure 6 . 8 .

 68 Figure 6.8. Micrographies HAADF-STEM des mélanges de SA et de NP à des rapports différents dans : a)1.4%Pd/D-CNT ; b) 0.70% x2Pd/D-CNT; c) 0.46%x3Pd/D-CNT; et c) 0.35%x4Pd/D-CNT.

Figure 6 . 9 .

 69 Figure 6.9. Réaction d'hydrogénation du β-myrcène.

Figure 6 . 10 .

 610 Figure 6.10. Résultats catalytiques sur Pd/Al2O3, Pd/C et Pd/CNT. Réaction d'hydrogénation du myrcène. La réaction a été effectuée sur des catalyseurs Pd supportés. PH2 = 20 bar -T = 120 ° C -myrcène 1 M (80 mL) -solvant heptane -4 mg Pd a) Progression de la réaction en fonction du temps cat. b) STY en fonction de la progression de la réaction.

Figure 6 .

 6 Figure 6.11. Etude DFT des mécanismes de réaction de l'isomérisation du 2-butène sur PdSA (en noir) et H-PdSA (en orange).

Figure 6 .

 6 Figure 6.12. Etude DFT des mécanismes de réaction d'hydrogénation du 2-butène sur PdSA (en noir) et H-PdSA (en orange).

Figure 6 . 13 .

 613 Figure 6.13. Catalytic results for myrcene hydrogenation at different SA/NP ratio. a) Reaction progress as a function of time catalyst

Figure 6 .

 6 Figure 6.14. STY et sélectivité de 1% MSA/D-CNT à 260 ºC, 300 ºC et 340 ºC.

  V. A. Likholobov. In Catalysis by Unique Metal Ion Structures in Solid Matrices: From Science to Application, edited by G. Centi, et al. (2001), pp. 295-306.

62

J. P. Olivier, et al. Journal of Power Sources 2001, 97-8, 151-155. 63 K. Nakada, et al. In Graphene Simulation: InTech. Rijeka (2011), pp. 3-20.

  .19).101

97

H. Cong, et al. Coordination Chemistry Reviews 2013, 257, 2880-2898. 98 H. Shinohara Reports on Progress in Physics 2000, 63, 843-892. 99 C. G. Ding, et al. Journal of Chemical Physics 1999, 111, 8481-8485. 100 T. He, et al. Journal of Catalysis 2017, 354, 231-235. 101 D. Soto, et al. Molecules 2012, 17, 7151-7168.

Table 1 .

 1 1. List of low loading (<1 % w/w) SAC reported in the literature.

	2 wt% or less. Other

Table 1 . 2 .

 12 Selected examples of high loading (>1 % w/w) SAC reported in the literature.

	Entry	Sample	Preparation	Loading (wt%) Ref
		Co-N/C	pyrolysis of bimetallic MOF	1.5-4.3	174
		Ni-N/C	pyrolysis of MOF	1.5 [a]	182
		Fe-N/C	pyrolysis of MOF	1.76	183
		Co-N/C	pyrolysis of ZIF	0.34	184
		Pt-S/C	pyrolysis MOF	5.0	185
		Co-N/C	pyrolysis of MOF	3.5	186
		Ni and Co/N-CNT	pyrolysis of organometallic precursor	20.0	187
		Co-N/C	template-sacrificial approach	3.6	188
		Fe-N/C	template-sacrificial approach	2.9	189
	10	Co and Fe/N-C	template-sacrificial approach	1.3 -1.4 [b]	190
	11	Mo-N/C	template-sacrificial approach	1.32	191
	12	Ni and Fe-N/C	template-sacrificial approach	1.2	192
	13	Fe-N/C	template-sacrificial approach	1-2	193
	14	Ag/g-C3N4	copolymerization	1-10	194
	15	Ag/g-C3N4	copolymerization	1-10	195
	16	Co(OH)x-N-CNT	wet impregnation and annealing	17	196
	17	Pt/S-MWCNT	wet impregnation, NaBH4 reduction	20	197
	18	Fe-N/G	ball milling	1.5	198
	19	Ru/C60	decomposition of organic precursor	10.0	40
	20	Ni-porous graphene	wet impregnation and annealing	6.9	199
	21	Co-N-nanosheets	salt-template method	15.3	200

203 Y. Zhu, et al. ChemistrySelect 2020, 5, 1282-1287.

  250

245 L. Zhang, et al. Acs Catalysis 2017a, 7, 7855-7865. 246 H. Xu, et al. Nature Catalysis 2018b, 1, 339-348. 247 P. C. K. Vesborg, et al. Journal of Physical Chemistry Letters 2015, 6, 951-957. 248 C. Zhu, et al. Angewandte Chemie-International Edition 2017, 56, 13944-13960. 249 X.-P. Yin, et al. Angewandte Chemie International Edition 2018, 57, 9382-9386. 250 H. Zhang, et al. Science Advances 2018b, 4, eaao6657.

  68 eV. Other works have also reported prefer LH mechanism in the case of Cr/G,284 Fe/G, 285 Au/g-C3N4,286 Mn/G, 287 Co/G,288 Ni-N3/G, 289 Pt/penta-graphene 290 or Pd/G 291 . Contrarily to Pt, in Pd@CN (Figure1.42c) the TER mechanism is more preferable. It is worth noting that the termolecular Eley-Rideal mechanism was also proposed for Mn-N4/CNT 292 and Pd/G.293 In this mechanism, the dioxygen is activated in the presence of two CO molecules adsorbed on the Pd center, and the as-produced OOC-Pd-COO intermediate further dissociates to produce two carbon dioxide molecules synchronously. Au, Pd, Pt, Cu, Co, Mn, Ni and Al doped graphene are theoretically predicted to be potential catalysts for CO oxidation following a LH mechanism. Table1.4 enlists diffusion barriers, binding energies and energy barriers of the determinant step of the reaction. It is shown that the reaction could process readily with activation energy lower of 0.8 eV 294 . It is thus possible to anticipate a high catalytic activity with such SAC.

Table 1 .

 1 4. DFT calculated parameters for the reaction of oxidation of CO.

	Entry Metal Support	Diffusion barrier [eV]	Energy barrier [eV]	M-Cgraph Binding [eV] energy	Cohesive eV/atom energy	Eads [eV] CO	Eads [eV] O2	Eads [eV] CO2	Ref.
	1	Al	Graphene	2.98	0.32	5.60	3.39	0.83 1.57 0.16 295
	2	Au	Graphene	n.i. [c]	0.31	n.i.	3.81	1.53 1.34 0.13 296
	3	Fe	Graphene	0.66	0.57	n.i.	4.28	1.38 1.65 0.23 297
	4	Cu	Graphene	2.34	0.54	n.i.	3.49	1.71 2.67 0.49 298
	5	Pt	Graphene	5.62	0.58	7.23	5.84	0.86 1.49 0.03 299
	6	Pd	Graphene	1.55	0.57	5.63	3.89	1.07 1.13 0.36 300
	7	Ni	Graphene	3.41	0.58	7.57	4.44	1.25 1.65 0.33 301
	8	Mn	Graphene	5.91	0.41	6.81	2.92	1.70 1.96 0.20 302
	9	Co	PVG [d]	n.i.	0.86	n.i.	4.39	1.21 3.36 0.83 303
	10	Fe	PVG [d]	n.i.	0.13	7.77	4.28	1.19 0.95 0.12 304

  RuSA/N-C 322 and IrSA/S-C 323 catalysts. On CoSA/N-CNT catalyst, the authors proposed that the vast majority of CoSA resides inside of N-CNT being inaccessible to the reactant. After the liberation of the confined Co through laser irradiation in liquid, TOF (100 °C, 20 bar, substrate/Co = 400) goes from 13 to 128 h -1 proving the importance of the active site to be accessible. In contrast, the selectivity toward 1,2,3,4-C3N4 were studied.333,334 In these SAC, the Ni atoms have a NiN4 environment (XANES and DFT analyses), and the oxidation state of Ni is +1. A charge transfer from NiSA to the carbon 2π orbital of CO2 allows the formation of Ni-CO2 δ-species, which could reduce the activation energy needed for CO2 electroreduction. Among others, Ni-N4 335 results to be the more cited active site for CO2RR, although Rong and co-workers 336 indicate that the vacancies in the support are also of paramount importance. Ni-N3-V displays extremely high electrocatalytic activity for CO2 reduction, achieving a current density of 65 mA cm -2 at -0.9 V vs. RHE, with a slow scan rate of 1 mV s -1 . This value is much larger than those of Ni-N4 (19 mA cm -2 )

	CNT, 321 , tetrahydroquinoline (≈ 99%) is found to be independent of this treatment. The RuSA/N-C
	catalyst was more selective (99%) than RuNP deposited on carbon (79%), and the IrSA/S-
	C shown very high selectivity (> 99%) and activity with a TOF of 1200 h -1 (100 °C, 20
	bar, substrate/Ir = 10) to be compared to 1200 h -1 for a commercial Ir/C catalyst.
	1.5.6. CO2 reduction
	Catalytic CO2 reduction using SAC has been investigated in thermal-, 324 electro-325 and
	photo-catalysis.
	The valorization of biomass through the hydrogenation of model molecules such
	furfural 318 (IrSA/g-C3N4), benzaldehyde 319 (Co-Nx-C) or vanillin 320 (RuSA/g-C3N4) have
	been evaluated over SAC obtaining good results especially on selectivity. Finally,
	hydrogenation of quinoline into 1,2,3,4-tetrahydroquinoline was investigated on CoSA/N-

326

Only few examples of this reaction in gas phase has been reported using SA.

Kwak et al. 327

showed that PdSA/CNT was completely inactive for CO2 reduction. The addition of lanthanum oxide to the inactive PdSA/CNT system, allows to reach good conversion and very high selectivity towards CO, pointing to the importance of the support for this reaction, since it can contribute to CO2 activation. In this sense, SAC over oxides such as TiO2, 328,329 MgO 330 and CeO2 331 have been studied for CO2 hydrogenation.

For the electrochemical CO2 reduction reaction (CO2RR), carbon-supported SAC have attracted enormous attention. Indeed, the vast majority of studies associated to carbonbased single atoms are linked to electrochemical reactions.

332

Transition metal atoms Ncoordinated are the major active center for CO2 electroreduction due to the optimum binding energy with the intermediate species involved. EXAFS/XANES studies have allowed reaching some conclusions regarding the adsorption and activation of CO2 on this SAC. NiSA (2.8 to 4.6 % w/w) supported on g-

  reduction to CO over solar light was investigated on NiSA/G SAC. High FE over 90 % under significant currents up to 60 mA/mg have been obtained with this catalyst.346 1.5.7. Other reactions Other reactions as varied as acetylene dehydrogenation of N-heterocycles,347,348 hydrochlorination, 349 N-formylation of nitroarenes 350 coupling reaction, 351,352,353 α,βunsaturated aldehyde hydrogenolysis, 354 oxidation of ethylbenzene, 355 oxidation of alcohols, 356 cycloaddition of epoxides, 357 Fenton-like reaction 358 or N2 reduction 359 have been also catalyzed by carbon-supported SAC, demonstrating encouraging results, which could lead to further industrial applications. A summary of the most interesting results are summarized in Table1.5. Additionally, theoretical works have also highlighted the potential of SAC for other reactions. For example, the methane C-H bond activation, the first step in the mechanism of methane conversion into several desired products has been investigated.

360

The reaction has been studied over a set of single TM atoms such as Cr, Mn, Fe, Co and Cu adsorbed (H6 site adsorption) and embedded (substitution of one C by the TM) in graphene. According to the results, the activation energy for the C-H activation for Cr-, Mn-, Fe-, Co-, and Cu-methane systems are 1.67 eV, 1.94 eV, 1.08 eV, 1.32 eV and 1.72 eV, respectively. These results suggest that the C-H bond can be activated on Fe or Co systems. Additionally, cobalt showed the highest binding energies onto the support for the adsorbed and embedded cases, suggesting that a CoSA/G SAC could be an efficient catalyst for methane activation. Thus, employing first principles calculations, a two-step reaction mechanism was evidenced for direct CH4 oxidation to CH3OH over a CoSA/G SAC, with N2O as the O-donor molecule. 361 C-H activation was

Table 1 .

 1 5. Catalytic reactions performed on carbon-supported SAC.

	Reaction	Catalyst	Performances	Ref.
	Hydrogenative
	coupling of
	nitroarenes

Table 2 .

 2 1. Loading and mean particle size of the Ru@C60 samples.

	Entry Ru@C60 samples	Solvent	Temperature (°C)	Ru loading (%) a	Ru NP size (nm)
	1 D-Ru1@C60 1/1 27	CH2Cl2	25	10.6 (12.3)	n. d. b
	2	D-Ru@C60 2/1 27	CH2Cl2	25	16.7 (21.9)	1.2 ± 0.1
	3	D-Ru@C60 5/1 27	CH2Cl2	25	35.6 (41.2)	1.3 ± 0.1
	4 T-Ru1@C60 1/1 27	Toluene	25	9.1 (12.3)	n. d.
	5	T-Ru@C60 5/1	Toluene	25	11.0 (41.2)	n. d.
	6	T-Ru@C60 10/1	Toluene	25	16.6 (58.3)	n. d.
	7	T-Ru@C60 20/1	Toluene	25	20.0 (73.7)	n. d.
	8	T-Ru@C60 40/1	Toluene	25	19.2 (84.8)	n. d.
	9	T-Ru@C60 20/1	Toluene	50	19.6 (73.7)	n. d.
	10 T-Ru@C60 20/1	Toluene	100	20.5 (73.7)	n. d.
	11	T95D5-Ru@C60 20/1	Toluene (95%)/CH2Cl2 (5%)	25	24.4 (73.7)	n. d.
	12	T75D25-Ru@C60 20/1	Toluene (75%)/CH2Cl2 (25%)	25	35.8 (73.7)	n. d.
	13	T50D50-Ru@C60 20/1	Toluene (50%)/CH2Cl2 (50%)	25	36.0 (73.7)	suspecte d
	14	T95M5-Ru@C60 20/1	Toluene (95%)/MeOH (5%)	25	34.7 (73.7)	suspecte d
	15	T75M25-Ru@C60 20/1	Toluene (75%)/MeOH (25%)	25	41.6 (73.7)	suspecte d
	16	T50M50-Ru@C60 20/1	Toluene (50%)/MeOH (5 %)	25	47.1 (73.7)	suspecte d

a) From ICP analyses. The value between parentheses is the aimed Ru loading considering the initial amount of metallic precursor used. b) n. d. = not detected

Table 2 .

 2 2. Best fit parameters obtained by fitting the experimental EXAFS of samples.

	Coordination numbers (N), interatomic distances (R), Debye-Waller factors, ΔE0 and R-
	factors are shown.
	Sample		N	R(Å)	σ 2 (Å 2 )	ΔE0 R-factor
	T-Ru@C60 1/1	Ru-C	7.7±0.2 2.17 ± 0.02 0.006 ± 0.003 3±2 0.037
	T-Ru@C60 5/1	Ru-C	7.9±0.1 2.18 ± 0.01 0.005 ± 0.002 4±1 0.040
	T-Ru@C60 10/1	Ru-C	8.1±0.2 2.17 ± 0.02 0.005 ± 0.002 4±2 0.039
	T-Ru@C60 20/1	Ru-C	8.0±0.2 2.16 ± 0.02 0.005 ± 0.003 4±2 0.039
	T95M5-Ru@C60	Ru-C	8.0±0.2 2.16 ± 0.02 0.007 ± 0.002 3±2 0.013
	20/1	Ru-Ru 2.7±0.2 2.67 ± 0.03 0.015 ± 0.004	-	-
	2.2.3 Catalytic activity of the Ru@C60 nanostructures

Table 2 . 3

 23

		Nitro-group		Selectivity [%] a
	Ru/C60	STY (h -1 ) b Time (h) c	AN AN-Et	CA	CA-Et	PHA
	T-Ru@C60 1/1	14.5	2.3	82.4	8.5	7.1	2.0	-
	T-Ru@C60 5/1	15.0	2.2	85.3	4.6	-	-	10.1
	T-Ru@C60 10/1	13.4	2.5	78.4	6.4	5.5	3.6	2.9
	T-Ru@C60 20/1	12.5	3.0 d	86.7	5.4	3.1	-	4.9
	T95M5-Ru@C60 20/1	49.9	0.64 100.0	-	-	-	-
	T75M25-Ru@C60 20/1	107.36	0.34	83.5	-	-	2.9 13.6
	T50M50-Ru@C60 20/1	135.5	0.30	79.9	-	0.4	-	19.0
	T75D25-Ru@C60 20/1	12.6	2.5	75.9	14.3	-	-	8.8
	T50D50-Ru@C60 20/1	18.2	2.6	66.6	30.8	-	-	2.6
	Reaction conditions: 0.025 mmol Ru, 500 mg (4.06 mmol) nitrobenzene, 200mg (1.1 mmol) dodecane
	(internal standard), 30 bar H2, 80 °C, 30 mL EtOH.

. Results of hydrogenation of nitrobenzene in ethanol with different Ru@C60 catalysts. a determined by GC-FID using internal standard technique at ≈ 20% of conversion. b STY (molNBconverted/molRu.t) calculated at ≈ 20% of conversion. c Time to reach 20% of conversion. d Time to reach 16% of conversion, which is de maximal obtained with this sample.

Table 2 .

 2 4. Results toluene hydrogenation over ruthenium catalysts.

	Sample	Conditions	Time (hours)	Conversion (%)	TOF mol Metal*h) (mol conv./	Ref
	20/1 Ru@C60-T50M50	70 ºC, 30 bar H2, Tol/Ru ~ 900	2	>99	704 a)	b)
	Ru/CNF-P	100ºC, 30 bar H2 Tol/Ru ~ 11200	2.5	>99	35800	77
	Ru/C-silica	110ºC, 40 bar H2, Tol/Ru ~ 10000	1.02	>99	0.0098	78
	Ru/NHPC	100ºC, 14 bar H2, Tol/Ru ~ 12700	1	72	9160	79
	Ru/hectorite	50ºC, 50 bar H2, Tol/Ru ~ 3000	0.25	-	3550	80
	Ru/MgO	120ºC, 10 bar H2, Tol/Ru ~ 4000	1.5	100	1000	81

a) At 1h of reaction. b) This study

Table 2 .

 2 5. Reactant and solvent quantities of the prepared Ru@C60 samples.

	Ru@C60 samples	Solvent (mL) Fullere ne (mg)	Ru(COD) (COT) (mg)	Yield (mg)	Ru loading (%) a
	D-Ru1@C60 1/1 27	CH2Cl2 (300)	68.5	30	68.1	10.6
	D-Ru@C60 2/1 27	CH2Cl2 (400)	91.3	80	100	16.7
	D-Ru@C60 5/1 27	CH2Cl2 (400)	91.3	200	129	35.6
	T-Ru1@C60 1/1 27	Toluene (50)	91.3	62.5	116	9.1
	T-Ru@C60 5/1	Toluene (50)	114.2	250	136.8	11.0
	T-Ru@C60 10/1	Toluene (50)	57.2	250	88.9	16.6
	T-Ru@C60 20/1	Toluene (50)	28.6	250	58.3	20.0
	T-Ru@C60 40/1	Toluene (50)	14.3	250	18	19.2
	T-Ru@C60 20/1 (50 ºC)	Toluene (50)	22.9	200	31.7	19.6
	T-Ru@C60 20/1(100 ºC)	Toluene (50)	17.1	150	24.7	20.5
	T95D5-Ru@C60 20/1	Toluene (47.5)/ CH2Cl2 (2.5)	28.6	250	56.7	24.4
	T75D25-Ru@C60 20/1	Toluene (37.5)/ CH2Cl2 (12.5)	28.6	250	73.0	35.8
	T50D50-Ru@C60 20/1	Toluene (25)/ CH2Cl2 (25)	28.6	250	80.6	36.0
	T95M5-Ru@C60 20/1	Toluene (47.5)/ MeOH (2.5)	24.1	200	50.0	34.7
	T75M25-Ru@C60 20/1	Toluene (37.5)/ MeOH (12.5)	28.6	250	70.0	41.6
	T50M50-Ru@C60 20/1	Toluene (25)/ MeOH (25)	28.6	250	103.8	47.1

a) Determined by ICP-OES

Table 3 .

 3 1 Decomposition temperatures of different functional groups on carbon surfaces. From ref.[26].

	Group	Temperature of decomposition (ºC)	Pyrolysis gas
	Carboxylic acid (strong acids)	245	CO2
	Carboxylic acid (weak acids)	325	CO2
	Carboxylic anhydride	480-500	CO2 + CO
	Lactones	616-755	CO2
	Phenols	635	CO
	Carbonyls + quinones	700-950	CO

Table 3 .

 3 2 Quantification of different surface oxygen groups from TPD analysis.

		Percentage releasing	Desorbed amount	Oxygen group
		group (%)	(µmol/g)	surface density
	Group	% CO	% CO2	CO	CO2	Per nm 2	Per C
				O3h-CNT
	Carboxylic acid	-	43.5	-	419.7	1.15	0.030
	Lactone	-	8.3	-	79.9	0.22	0.006
	Carboxylic anhydride	22.6	48.2	597.1	465.5	1.63(CO) 1.27(CO2)	0.042 (CO) 0.033 (CO2)
	Aldehyde	6.9	-	181.7	-	0.5	0.013
	Phenol,
	carbonyl,	70.5	-	1858.2	-	5.09	0.132
	quinone
	Total			2637	965	8.23	0.214
				O3h-CNT-400
	Carboxylic acid	-	-	-	-	-	-
	Lactone	-	50.7	-	168.0	0.46	0.012
	Carboxylic anhydride	1.9	49.3	35.5	163.0	0.09 (CO) 0.45(CO2)	0.002 (CO) 0.012 (CO2)
	Aldehyde	-	-	-	-	-	-
	Phenol,
	carbonyl,	98.1	-	1766.5	-	4.84	0.13
	quinone
	Total			1801	331	5.80	0.037

Table 3 .

 3 3 Lorentzian fitted parameters of treated CNT from Raman analysis.

	Peak	Shift (cm -1 )	O3h-CNT Intensity (a.u.)	O3h-CNT-400 Shift (cm -1 ) Intensity (a.u.)
	D	1345.1	0.7797	1343.8	0.7910
	G	1582.6	0.5030	1580.5	0.4665
	D′	1614.2	0.1682	1612.7	0.1912
	2D	2692.1	0.1497	2691.9	0.16921
	ID/IG		1.54	1.68
	IG/I2D		3.33	2.78
	ID/ID′		4.64	4.14

46 H. Murphy, et al. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena 2006, 24, 715-720.

Table 3 .

 3 4.XPS C 1s and O 1s peak deconvolution assignments and relative content of species.

	Species	Binding Energy (eV) O3h-CNT (at%) O3h-CNT-400 (at%)
		C 1s
	vacancies	283.5	1.34	1.78
	C=C (sp 2 )	284.1	41.91	66.41
	C-C (sp 3 )	285.0	25.84	9.15
	C-OH, C-O	286.1	5.62	5.69
	C=O	287.3	8.13	3.73
	COOH, ester	289.1	6.87	4.86
	π-π* shake up	291.2	10.30	8.39
		O 1s
	C=O	531.3	59.63	53.61
	C-O	533.2	40.36	46.38
	Satellite (H2O)	536.0	0.01	<0.01
	C/O	94.3/5.7 (16.5)	94.8/5.2 (18.2)

Table 3 .

 3 5. Comparison of the magnetic properties of different CNT.

	Sample	Tº (K)	(emu/g) Saturation of magnetization MS	(emu/g) Remanent magnetization MR	Hc (Oe) Coercivity field	Ref.
	O-CNT (FeCo)	5 300	0.14 0.12	0.07 0.05	1300 500	This work
	O-CNT (Fe)	5 77	1.89 2.57	--	1700 1950	74
	O-CNT (Fe)	4.5	0.22	0.03	1500	75
	CNT(Co)	130	0.17	0.07	600	76
	CNT (Fe)	4.2	8.5	2.9	1120	77
	CNT(FeCo) CNT-2500ºC	300 300	0.87 -	--	842 -	78

  96

93

N.

Ishii, et

al. Japanese Journal of Applied Physics 1981, 20, L673-L676. 94 F. Beuneu, et al. Physical Review B 1999, 59, 5945-5949. 95 K. A. Müller Physical Review 1961, 123, 1550-1552. 96 J. A. Rodríguez-Manzo, et al. ACS Nano 2010, 4, 3422-3428.

Table 3 .

 3 6. Ruthenium content varying temperature of activation determined by ICP-OES.

	Sample	Tº annealing (ºC)	Ru loading (wt. %)
	0.4%Ru/O3h-CNT-1000	1000	0.12
	2.0%Ru/O3h-CNT-1000	1000	0.31
	2.0%Ru/O3h-CNT-600	600	1.06
	2.0%Ru/O3h-CNT-400	400	1.60
	1.0%Ru/O3h-CNT-400	400	0.95
	0.4%Ru/O3h-CNT-400	400	0.37

Table 3 .

 3 7. Metal loading using different solvents determined by ICP-OES.

	Sample	Precursor	Solvent	wt. %
		[Ru (COD)(COT)] (0)
	2.0%Ru/O3h-CNT-400		Pentane	1.60
	2.0%Ru/O3h-CNT400		Ethanol	1.01
		Ru (NO)(NO3)3 (III)
	2.0%Ru/O3h-CNT400		Ethanol	0.38
	2.0%Ru/O3h-CNT400		Water	0.32

Table 3 .

 3 8. Quantification of temperature programmed desorption (TPD) profiles.

	Sample	Total CO2 (µmol/g)	Total CO (µmol/g)
		Supports
	O3h-CNT	965	2637
	O3h-CNT-400	331	1801
		Catalysts
	2.0%Ru/O3h-CNT	1892	4444
	1.6%Ru/O3h-CNT-400	1240	4699

Table 3 .

 3 10. Ruthenium content varying type of support determined by ICP-OES.

	Sample	Ru content (wt. %)	SBET (m 2 /g)	Pore Volume (cm 3 /g)	Ld	Lc
	2.0% Ru/ O3h-CNT-400	1.60	220	1.2	11.9 4.57
	2.0% Ru/ O3h-FM-400	1.41	215	0.4	10.1 6.71
	1.5%Ru/O3h-sCNF-400	1.33	296	1.0	11.4 4.64
	1.5%Ru/ O3h-CB-400	0.70	262 124	0.3	7.9	-

BET, Pore volume, Lc, Ld obtained from ref.

13

3.5 Mixture of single atoms and nanoparticles

For decades benchmark catalysts such as Pd/C, Ru/C, Pt/C have been used at the industrial level assuming they contain nanoparticles on carbon but, recent research has shown that even these commercial catalysts unconsciously contain supported single atoms.

125

These results make us wonder if the so-far "unseen" atoms, may play a role in the catalytic performances (see Chapter 4).

123

R. C. Contreras, et al. Journal of Catalysis 2019, 372, 226-244. 124 Y. Holade, et al. Physical Chemistry Chemical Physics 2014, 16, 25609-25620. 125 U. Petek, et al. Catalysts 2019, 9, 134.

Table 3 .

 3 11. Metallic loading of Pd/O-CNT samples with different number of impregnation cycles determined by ICP-OES.

		Metallic loading after nº	Ratio
	Sample	impregnation cycle (wt.%)	SA/NP a)
		1	2	3	4
	1.40%x1 Pd/O3h-CNT-400	1.33				200
	0.70%x2 Pd/O3h -CNT-400	0.74	1.43			65
	0.46%x3 Pd/O3h -CNT-400	0.62	0.95	1.37		45
	0.35%x4 Pd/O3h-CNT-400	0.38	0.68	0.93	1.29	15
	a) From HR-TEM, by manual count on Image J software. SA/NP ratio calculated considering the total number of atoms and
	particles counted in at least 20 images per sample.

Table 3 .

 3 12) and shifts towards lower binding energy (335.28 eV), which is in good agreement with bulk Pd 0 . 141 This upshift for SA species has been observed for other elements such as Pt.142

	139,140

Table 3 .

 3 12. XPS Pd 3d and O 1s peak deconvolution and relative content of species.

	Sample	Pd 0	Pd +	Atomic percentage (%) Pd + Pd 0 C=O	C-O	H2O
	1.40% x1 Pd/O3h-CNT-400	14.20	85.79	6.06	41.10 39.16 19.74
	0.70% x2 Pd/O3h-CNT-400	15.25	84.75	5.55	43.00 33.14 23.86
	0.46% x3 Pd/O3h-CNT-400	22.00	78.00	3.55	43.36 33.32 23.32
	0.35% x4 Pd/O3h-CNT-400	41.75	58.25	1.39	40.13 33.21 26.67

Table 4 .

 4 1. Pd loading, particle size, dispersion and SA/NP ratio.

		Catalyst	Pd loading (%)	NP size from TEM (nm) a)	Pd dispersion (%) c)	SA/NP ratio d)	At. % SA
		Pd/Al2O3	5	3.9 ± 1.1	31	n.d. e)	n.d. e)
		Pd/C	5	2.4 ± 1.0	46	n.d. e)	n.d. e)
		Pd/CNT
		also named	2	2.2 ± 1.1	49	2	n.d. e)
	Pd2_SA/NP/CNT
	Pd2_SA/NP/CNTHT	1.6	2.2 ± 1.1	49	2	1.5
		PdSA/CNT	0.1	-	-	no NP	n.d. e)
	Pd1000_SA/NP/CNT	0.1	1 ± 0.5 b)	-	1000	n.d. e)
	Pd200_SA/NP/CNT	1.2	0.5 ± 0.1	-	200	94
	Pd60_SA/NP/CNT	1.2	0.8 ± 0.3	-	60	48
	Pd40_SA/NP/CNT	1.2	0.9 ± 0.3	-	40	21
	Pd10_SA/NP/CNT	1.2	1.1 ± 0.6	-	10	14
		Pd/FLG	1.9	2.6 ± 1.6	43	10	n.d. e)
	a)	Calculated over 300 NP from STEM-HAADF micrographs
	b)

Table 4 .

 4 2. Fraction of SA in the same samples determined from STEM and ratio Pd 0 /Pd δ+ determined by XPS.

	Sample	STEM fraction of SA	XPS atomic percentage (%) Pd 0 Pd δ+ Pd δ+ /Pd 0
	Pd200_SA/NP/CNT	0.97	14.21	85.79	6.06
	Pd60_SA/NP/CNT	0.78	15.25	84.75	5.55
	Pd40_SA/NP/CNT	0.58	22.00	78.00	3.55
	Pd10_SA/NP/CNT	0.17	41.75	58.25	1.39

  4.7.3. DFT Gaussian calculations (LPCNO, Dr. Iann Gerber, Dr. Iker del Rosal and Prof. Romuald Poteau)

  augmented by a polarization function were used. The different intermediates and transition states were simulated with dispersion corrected functional. More precisely, dispersion corrections were treated with the D3 version of Catalytic experiments (LGPC, Dr. Boris Guicheret, Dr. Laurent Vanoye, Dr.Alain Favre-Réguillon, Prof. Claude de Bellefon and Dr. Régis Philippe)

	4.7.4. 4.7.4.1.
	Grimme's dispersion with Becke-Johnson damping. 78 The nature of the optimized stationary
	point, minimum or transition state, has been verified by means of analytical frequency
	calculations at 298.15 K and 1 atm. The geometry optimizations have been achieved without
	any geometrical constraints. IRC calculations were carried out to confirm the connectivity
	between reactant(s), transition state and product(s). Energy data in kcal/mol are reported in the
	gas phase.

Table 4 .

 4 3. Physical properties of the different phases at 120 °C and 20 bar.

	Phase	Name	Symbol	Value	Units
		Viscosity a	mL	1.71x10 -4	Pa.s
		Density	L	703	kg/m 3
	Liquid	Surface tension a	L	1x10 -2	N/m
		H2 diffusivity a	Dm,H2	1.99x10 -9	m 2 /s
		H2 solubility b	C*H2	187	mol/m 3 L
	Gas	Viscosity a Density	mG G	1.07x10 -5 1.22	Pa.s kg/m 3
		Skeleton density c	S	1900	kg/m 3 skeleton
		Pore volume d	Vpore	2.5	mL/g
		Dry particle density d	P	600	kg/m 3 particle
	CNT support	Wetted particle density Internal porosity e	p P	1080 70%	kg/m 3 particle m 3 void/m 3 particle
		Tortuosity f		1	-
		Mean Particle (agglomerate) diameter 4	dP	1-2	µm
		Skeleton density	S	3650	kg/m 3 skeleton
		Dry particle density	P	1460	kg/m 3 particle
	-Al2O3 support	Wetted particle density Internal porosity	p P	1880 60%	kg/m 3 particle m 3 void/m 3 particle
		Tortuosity		 3	-
		Mean Particle diameter	dP	25	µm
		Skeleton density c	S	1900	kg/m 3 skeleton
		Dry particle density	P	760	kg/m 3 particle
	Activated Carbon support	Wetted particle density Internal porosity	p P	1180 60%	kg/m 3 particle m 3 void/m 3 particle
		Tortuosity		 3	-
		Mean Particle diameter	dP	10	µm

Table 4 .

 4 4. Appraisal of G-L mass transfer limitations.

	Site Time Yield, 𝑆𝑇𝑌 𝑚𝑎𝑥 (4.10), both in full G-L mass transfer regime:
	𝐹 𝐻2 𝑚𝑎𝑥,𝐺𝐿 = 𝑘 𝐿 𝑎 𝐺𝐿 𝑉 𝑅 𝐶 𝐻2 *	(4.8)
	𝑟 𝑝,𝑎𝑝𝑝 𝑚𝑎𝑥,𝐺𝐿 =	𝐹 𝐻2 𝑚𝑎𝑥,𝐺𝐿 𝑉 𝑐𝑎𝑡𝑎	=	* 𝑘 𝐿 𝑎 𝐺𝐿 𝑉 𝑅 𝐶 𝐻2 𝑉 𝑐𝑎𝑡𝑎	----(4.9)
	𝑆𝑇𝑌 𝑚𝑎𝑥,𝐺𝐿 =	𝐹 𝐻2 𝑚𝑎𝑥,𝐺𝐿 𝑛 𝑃𝑑	=	* 𝑘 𝐿 𝑎 𝐺𝐿 𝑉 𝑅 𝐶 𝐻2 𝑛 𝑃𝑑	-----(4.10)
	Independently, following the instantaneous consumption curves of H2, experimental
	instantaneous apparent reaction rates or STY can be determined at any reaction progress. These
	latter can be compared to the corresponding theoretical ones to appreciate a fraction of external
	G-L mass transfer limitation, 𝑓 𝑒𝑥 𝐺𝐿 (4.11), on the measurements and is analogous of the L-S

  estimate an experimental E factor as shown in the 6 th column of the table. The values are equal to 1 when 𝒇 𝒆𝒙,𝟎.𝟓 𝑮𝑳 <100% and equal to the value of 𝒇 𝒆𝒙,𝟎.𝟓 𝑮𝑳 when >100%. The values obtained here for the two more active catalysts are consistent with those reported in ref. 87. For the other reaction progress, accordingly to these measurements, the 𝐹 𝐻2 𝑚𝑎𝑥,𝐺𝐿 (and STY max ) takes into

	account this factor accordingly to (4.12):
	𝐹 𝐻2 𝑚𝑎𝑥,𝐺𝐿 = 𝑬𝑘 𝐿 𝑎 𝐺𝐿 𝑉 𝑅 𝐶 𝐻2 *	(4.12)

87

Z. Junmei, et al. Chemical Engineering Journal 2006, 120, 149-156. 88 S. Karve, et al. Chemical Engineering Science 1990, 45, 587-594. 89 K. B. B. G. F. Froment. In Chemie Ingenieur Technik: John Wiley & Sons, . New York (1990).

to

Table 4 .

 4 5. Appraisal of L-S external mass transfer limitations.To evaluate a possible internal mass transfer limitation, the classical Weisz-Prater criterion 𝝋′ 89 has been evaluated. When the criterion is inferior to 1 a surface efficiency of 1 is stated. When

	Catalyst	𝑆𝑇𝑌 𝑚𝑎𝑥,𝐿𝑆 (molH2/ s/molPd)	𝑆𝑇𝑌 0.5 (molH2/ s/molPd)	𝑓 𝑒𝑥,0.5 𝐿𝑆 )	(-	𝑆𝑇𝑌 2.0 (molH2/ s/molPd)	𝑓 𝑒𝑥,2.0 𝐿𝑆 )	(-	𝑆𝑇𝑌 2.5 (molH2/s/ molPd)	𝐿𝑆 𝑓 𝑒𝑥,2.5 (-)
	Pd/Al2O3	137	109		79%		10.2	7.5%	4.6	3.4%
	Pd/C	541	168		31%		72	13.3%	26	4.8%
	Pd/CNT
	also named	31 486	144	0.5%	44	0.1%	22	0.07%
	Pd2_SA/NP/CNT
	Pd1000_SA/NP/CNT	642 309	184	0.03%	1.4	0.0002 %	0.7	0.0001%
	Pd200_SA/NP/CNT	48 294	104	0.2%	10.4	0.02%	4.5	0.01%
	Pd40_SA/NP/CNT	46 884	544	1.2%	104	0.2%	50	0.1%
	Pd10_SA/NP/CNT	49 791	672	1.4%	272	0.6%	106	0.2%
	At reaction progress of 2.0 and 2.5 mol/mol, no L-S mass transfer limitation remains at all for
	all the catalysts (except slightly for the Pd/C) and STY comparisons are consistent and
	chemically robust.
	4.7.4.9.	Internal mass transfer
	it is superior to 1, a surface efficiency is approximated as 1 ⁄ . The Table 4.6 presents the results 𝝋′
	and equations (4.14) to (4.16), the formula used for.
			𝜑 ′ =	𝑟 𝑝 𝐿 2 𝐷 𝑒𝑓𝑓 𝐶 𝐻2,𝑠𝑢				(4.14)
			𝐷 𝑒𝑓𝑓 =	𝛽 𝑃 𝜏	𝐷 𝑚,𝐻2

Table 5 .

 5 1. Thermodynamics of CO2 hydrogenation reactions.

	Reaction	H298 (kJ.mol -1 )	G298 (kJ.mol -1 )	 	
	CO2+ 4 H2  CH4 + 2 H2O	-164.8	-113.5	7.79 ×10 19
	CO2 + 3 H2  CH3OH + H2O	-49.5	3.5	2.45 ×10 -1
	CO2 + 3 H2  ½ CH3OCH3 + 3 /2 H2O	-61.3	-4.9	7.15
	½ CO2 + 3 H2  ½ CH2CH2 + 2 H2O	-64.0	-28.7	1.07 ×10 5
	CO2 + 3 H2  ½ CH3CH2OH + 3 /2 H2O	-86.7	-32.4	4.70 ×10 5
	CO2 + 2 H2  HCHO + H2O	35.7	55.9	1.63 ×10 -10
	2 CO2 + 4 H2  CH3COOH + 2 H2O	-64.8	-21.6	6.11 ×10 3
	CO2 + H2  CO + H2O	41.2	28.6	9.67 ×10 -6
	CO2 + H2  HCOOH	14.9	43.5	2.43 ×10 -8

Table 5 .

 5 2. Catalytic hydrogenation of CO2 on supported SA catalysts.

	Catalyst	Reaction conditions	Conversion (%)	Selectivity (%)	Ref.
	1.8% CoSA/SiO2	6 L•gcat -1 hr -1 ; 10 bar; CO:H2 = 1:4; 400 °C	7	95 (CO)	9
	10% NiSA/MgO	30 bar (72% H2, 18% CO2, 10% Ar for balance); 300 °C	< 3	100 (CO)
	0.1% NiSA/N-C a)	27 L•gcat -1 hr -1 ; 1 bar; CO:H2 = 1:4 (He as inert); 500 °C	14.5	96 (CO)
	0.1% RuSA/Al2O3 b)	1 bar (15% H2, 5% CO2, He for balance); 350 °C	3	84 (CO)
	0.5% RuSA/CeO2	1 bar; CO:H2 = 1:4 (N2 as inert); 260 °C	< 5	98 (CO)
	0.89% RuSA/CeO2	4.8 L gcat -1 hr -1 ; 1 bar; CO:H2 = 1:4 (He as inert); 260 °C	28	100 (CH4)
	0.01% Ru/FeOx	1.2 L•gcat -1 hr -1 ; 1 bar; CO:H2 = 1:1; 260 °C	4	100 (CO)
	0.6% RuSA/h-BN c)	GHSV = 18 000 h -1 ; 10 bar; CO:H2 = 1:4; 350 °C	29	93.5 (CH4)
	0.4% RuSA/LDH d)	1.0 M NaHCO3; 20 bar; CO:H2 = 1:1; 100 °C	-	100 (formate)
	0.5% RhSA/TiO2 (60% SA)	1 bar; CO:H2 = 1:4 (N2 as inert); 200 °C	-	35 (CO)
	0.2% RhSA/TiO2	1 bar; CO:H2 = 1:1 (He as inert); 200 °C		86 (CO)
	0.5% PdSA/Al2O3	1 bar (15% H2, 5% CO2, He for balance); 400 °C	18	67 (CO)
	1% PdSA/CNT	1 bar (15% H2, 5% CO2, He for balance); 350 °C	inactive	-
	0.1% PdSA/Fe3O4	60 L gcat -1 h -1 ; 1 bar; CO:H2 = 1:4; 300 °C	0.2	100 (ethanol)
	0.2% ReSA/CeO2 e)	in 1,4-dioxane; 10 bar CO2 -50 bar H2; 150 °C	-	95 (CO)
	0.7% IrSA/CeO2	10 bar (76% H2, 19% CO2, Ar for balance); 300 °C	2.9	>99 (CO)
	0.1% IrSA/TiO2	1 bar (45% H2, 45% CO2, He for balance); 350 °C	2	100 (CO)
	1.25% IrSA/AP-POP f)	1.0 mol/L aqueous triethylamine, 30 bar H2, 30 bar CO2, 120°C	-	100 (formate)
	0.05% PtSA/CeO2	48 L gcat -1 hr -1 ; 1 bar (62.5% vol. H2, 5% vol. CO2, N2 for balance); 340 °C	57	100 (CO)
	0.2% Pt/MoS2	in DMF; 32 bar; CO2:H2 = 1:3; 210 °C	-	93 (methanol)

  D. C. Upham, et al. Catalysis Science & Technology 2015, 5, 1783-1791. 28 Y. Wang, et al. Progress in Natural Science: Materials International 2019a, 29, 256-264.

29

G. S. Parkinson Catalysis Letters 2019, 149, 1137-1146.

Pt SA /CNT ox-400 Ru SA /CNT ox-400 Pd SA /CNT ox-400 Ni SA /CNT ox-400 Co SA /CNT ox-400 Cu SA /CNT ox-400 300ºC 260ºC 340ºC Selectivity CO (%) ToS (hours) Pt SA /CNT ox-400 Ru SA /CNT ox-400 Pd SA /CNT ox-400 Ni SA /CNT ox-400 Co SA /CNT ox-400 Cu SA /CNT ox-400

Table 5 .

 5 3. Catalytic performances of the produced catalysts on CO2 hydrogenation.

		Catalyst	ICP wt%	T (ºC)	CO2 conversion (%) a)	STY (molCO2/ molM.h) a)	CO sel. (%) a)	CH4 sel. (%) a)	Ea (kJ/ mol)
				260	0.92	29.81	99.31	0.68
	PtSA/CNTox-400	0.68	300	2.05	66.00	99.57	0.43	50.86
				340	4.14	133.35	99.72	0.28
				260	0.68	8.07	56.84	43.16
	RuSA/CNTox-400	0.95	300	2.5	30.00	54.8	45.18	86.69
				340	8.70	103.90	57.1	42.86
				260	0.34	4.20	98.95	1.05
	PdSA/CNTox-400	0.98	300	0.55	6.76	97.59	2.41	21.12
				340	0.81	9.89	97.11	8.88
				260	0.60	3.65	94.80	5.19
	NiSA/CNTox-400	1.09	300	1.96	11.87	95.04	4.96	75.04
				340	5.51	33.30	93.75	6.25
				260	0.19	0.99	93.50	6.50
	CoSA/CNTox-400	1.27	300	2.29	11.93	97.39	2.61	28.15
				340	6.74	35.09	96.16	3.84
				260	0.14	0.82	94.55	5.45
	CuSA/CNTox-400 1.21	300	0.18	1.08	92.75	7.25	76.17
				340	0.15	0.91	92.82	7.18
	a)	Average value

Table 5 .

 5 4. Catalytic performances of the 1%MSA-Na/CNT ox-400 catalysts on CO2 hydrogenation.

		Catalyst	T (ºC) CO2 conversion (%) a)	STY (molCO2/ molM.h) a)	CO sel. (%) a)	(%) a) CH4 sel.	Ea (kJ/mol)
			260	7.6	246.0	99.8	0.2
	PtSA-Na/CNTox-400	300	21.4	688.4	99.9	0.1	46.7
			340	30.1	970.5	99.9	0.1
			260	1.6	20.0	85.5	14.5
	RuSA-Na/CNTox-400	300	7.9	97.1	85.9	14.0	85.6
			340	20.7	252.3	80.3	19.7
			260	1.5	18.5	98.7	1.3
	PdSA-Na/CNTox-400	300	2.9	35.2	99.6	0.4	40.5
			340	5.5	66.5	99.7	0.3
			260	1.2	7.3	94.6	5.4
	NiSA-Na/CNTox-400	300	3.5	22.0	93.7	6.3	65.4
			340	8.1	50.7	95.5	4.5
			260	0.1	0.8	90.5	9.5
	CoSA-Na/CNTox-400	300	0.6	3.3	96.7	3.3	90.3
			340	2.4	12.6	98.2	1.8
			260	0.4	2.3	97.5	2.5
	CuSA-Na/CNTox-400	300	0.6	3.8	97.1	2.9	n.d. b)
			340	0.4	2.6	97.5	2.5
	a)	Average value
	b)	Could not be accurately determined due to low conversion obtained
	The RuSA-Na/CNTox-400 catalyst performed also very well in terms of STY compared to published
	performances of Ru/C catalysts containing metallic NP. 51 Thus, a 0.5%Ru/CNF catalyst (CNF
	= carbon nanofibers, RuNP = 1.6 nm) has a STY of 1080 molCO2/molRu.h at 400 °C52 compared
	to 247 molCO2/molRu•h for the 1%RuSA-Na/CNTox-400 catalyst at 340 °C, and a 3%Ru/CB catalyst
	(RuNP = 1.25 nm) has a STY of 7 molCO2/molRu•h at 240 °C, compared to 20 molCO2/molRu•h
	for the 1%RuSA-Na/CNTox-400 catalyst at 260 °C. Additionally, Ru selectivity to CO improved
	significantly from 56 % to 86 % after doping.

51 L. Roldán, et al. ChemCatChem 2015, 7, 1347-1356. 52 V. Jiménez, et al. Applied Catalysis A: General 2010, 390, 35-44.

Table 5 .

 5 5. Catalytic performances of the 1%MSA+NP/CNTox-400 catalysts on CO2 hydrogenation.

			ICP	T	CO2	STY	CO	CH4	Ea
		Catalyst	wt%	(ºC)	conversion (%) a)	(molCO2/m olM.h) a)	sel. (%) a)	sel. (%) a)	(kJ/m ol)
				260	1.09	27.80	98.30	1.70
	PtSA+NP/CNTox-400	0.86	300	2.06	52.55	99.82	0.18	50.86
				340	4.26	108.15	99.72	0.28
				260	0.74	8.48	46.93	53.07
	RuSA+NP/CNTox-400	0.99	300	2.76	31.64	53.66	46.34	83.11
				340	8.62	98.74	51.65	48.35
				260	0.39	4.99	98.42	1.58
	PdSA+NP/CNTox-400	0.92	300	0.50	6.53	97.76	2.24	21.12
				340	0.66	8.54	97.75	2.25
				260	0.66	4.33	91.93	8.07
	NiSA+NP/CNTox-400	1.01	300	2.24	14.64	89.73	10.27	71.03
				340	5.35	34.92	89.22	10.78
				260	1.59	8.48	87.62	12.38
	CoSA+NP/CNTox-400	1.24	300	2.56	13.63	93.01	6.99	30.59
				360	4.09	21.80	94.48	5.52
	a)	Average value

Reaction time (hour) Pt SA+NP /CNT ox-400 Ru SA+NP /CNT ox-400 Pd SA+NP /CNT ox-400 Ni SA+NP /CNT ox-400 Co SA+NP /CNT ox-400 260ºC 300ºC 340ºC Selectivity CO (%) Reaction time (hour) Pt SA+NP /CNT ox-400 Ru SA+NP /CNT ox-400 Pd SA+NP /CNT ox-400 Ni SA+NP /CNT ox-400 Co SA+NP /CNT ox-400

	Figure 5.8. STY and selectivity of 1%MSA+NP/CNT at 260 ºC, 300 ºC and 340 ºC.

  of alkali dopants. The different ability of single atoms and nanoclusters to dissociate hydrogen may also contribute to the selectivity differences. These results open perspectives for the design of single atom catalysts on carbon for the RWGS reaction, and on the possibility to run CO2-Fischer-Trosch synthesis (FTS)57,58 by combining a SA-based catalyst for the RWGS reaction that will transform CO2 into CO with a more conventional Co-based FTS catalysts to in fine produce carbon-neutral fuels. Encouraging first results have already been obtained in the group as part of Amel Ghogia's thesis, and these studies will continue as part of Canio Scarfiello's thesis in collaboration with the LPCNO and the school of Mines of Albi.

	5.4.2. Catalysts characterization
	TEM and HRTEM analyses were performed at the "Centre de microcaracterisation Raimond 𝑋 𝐴 = ( 𝐴𝑟𝑒𝑎 𝑜𝑓 𝐴 𝑠𝑖𝑔𝑛𝑎𝑙 𝐴𝑟𝑒𝑎 𝑜𝑓 𝑁 2 𝑠𝑖𝑔𝑛𝑎𝑙 1 𝑥𝑁 2 𝑓𝑙𝑜𝑤) 𝑥 𝑘 𝐴
	Castaing, UMS 3623, Toulouse". Low resolution TEM was performed by using a JEOL JEM
	1011 CX-T electron microscope operating at 100 kV with a point resolution of 4.5 Å, and a With a = CO2, CH4, H2, CO and k = response coefficient. The conversion rates of the reagents
	JEOL JEM 1400 electron microscope operating at 120 kv. The high-resolution analyses were were then calculated as follows:
	conduct using a JEOL JEM 2100F equipped with a Field Emission Gun (FEG) operating at 200
	kV with a point resolution of 2.3 Å and a JEOL JEM-ARM200F Cold FEG operating at 200 kV with a point resolution of > 1.9 Å. The approximation of the particles mean size was made 𝐶𝑂 2 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) = (1 -𝑋 𝐶𝑂2 𝑋 𝐶𝑂2 + 𝑋 𝐶𝐻4 + 𝑋 𝐶𝑂 ) 𝑥100%
	through a manual analysis of enlarged micrographs by measuring at least 200 particles on a 5.4. Experimental part 5.4.1. Catalysts preparation given grid. Other TEM micrographs were acquired with a JEOL 2100F S/TEM microscope equipped with a FEG operating at 200 kV, a spherical aberration probe corrector and a GATAN 𝐻 2 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) = (1 -𝐻 2 𝑂𝑢𝑡𝑝𝑢𝑡 𝑓𝑙𝑜𝑤 𝐻 2 𝐼𝑛𝑝𝑢𝑡 𝑓𝑙𝑜𝑤 ) 𝑥100%
	Tridiem energy filter. The resolutions attained are 2 Å and 1.1 Å under parallel TEM mode and With H2 output flow = dry flow output x XH2.
	scanning STEM modes, respectively.
	5.4.3. Catalytic tests 𝐷𝑟𝑦 𝑓𝑙𝑜𝑤 𝑜𝑢𝑡𝑝𝑢𝑡 = ( 𝑋 𝐶𝑂2 + 𝑋 𝐶𝐻4 + 𝑋 𝐶𝑂 𝐶𝑂 2 𝑖𝑛𝑝𝑢𝑡 𝑓𝑙𝑜𝑤	)
	The catalytic tests of CO2 hydrogenation were performed using a continuous flow stainless steel
	fixed bed reactor (height = 300 mm, e.d. = 9.52 mm, i.d. = 7.9 mm) under a total pressure of 6.1 bar. 200 mg of catalyst with a particle size in the 100-200 μm range were mixed with 1800 𝐶𝐻 4 𝑦𝑖𝑒𝑙𝑑 (%) = ( 𝐶𝐻 4 𝑜𝑢𝑡𝑝𝑢𝑡 𝑓𝑙𝑜𝑤 𝐶𝑂 2 𝑖𝑛𝑝𝑢𝑡 𝑓𝑙𝑜𝑤 ) 𝑥100%
	mg of SiC (Alfa Aesar). Before the catalytic test, the catalyst was reduced in situ at 400 °C for
	Preparation of 1% MSA+NP/CNTox-400: A desired amount of precursor to obtain 0.5wt.% is 4 h under a 1/4 mixture of N2/H2, at atmospheric pressure. Then, experiments were performed
	dissolved in 30 mL of pentane and mixed with 1g of previously activated CNTox-400 during one at a constant GHSV (Gas Hourly Space Velocity) of 16 500 mL/h.g. Catalytic tests were
	hour under an Ar atmosphere. Afterwards, the solid is precipitated, filtrated and washed three performed under a N2/H2/CO2 gas mixture of 1/4/1 at 260, 300 and 340 °C and 6.1 bar. The
	times to remove the possible non reacted precursor. This powder is dried one night under composition of the reactant/product mixture was analyzed using an on-line gas chromatograph
	vacuum, and is used to remake the impregnation process (0.5% MSA/CNTox-400). The totality of (500 Clarius) equipped with two TCD: one with argon as gas vector to quantify H2, CH4, and
	the previous product is heated at 80 ºC during one hour under a mixture of 20/80 H2/Ar flow, CO, and another with helium to quantify CO2. The GC is equipped with two Shincarbon
	and again after that time is mixed with 30 mL pentane solution of another 0.5wt.% of metallic columns (1/8, 2.0 mm, 80/100), and recorded the formation of methane and conversion of H2
	precursor for finally obtaining 1wt.% of metal. After one hour of reaction the solid is and CO2 every 8 min.
	precipitated, filtrated and washed three times to remove the possible non reacted precursor.
	Finally, the catalyst (0.5+0.5% MSA+NP/CNTox-400) was dried overnight and stored in air. The different response coefficients determined from the GC calibration allowed us to calculate
	the molar fractions (X) of the different molecules considered during the methanation reaction,
	as follows:

S. Navarro-Jaén, et al. Applied Surface Science 2019, 483, 750-761. 56 E. Marconi, et al. Catalysts 2019, 9, 375. addition Preparation of 1% MSA/CNTox-400: The catalysts were prepared by a 4-step synthesis method, comprising functionalization of the support, creation of defects, deposition of the metal and removal of organic ligand as described in Chapter 3. (for further information refer to Chapter 3, Section 3.4. Preparation of carbon-based SA). Preparation of 1% MSA-Na/CNTox-400: A desired amount of 1% MSA/CNTox-400 was treated with a proper volume of NaOH aqueous solution in order to obtain a ratio 1:2 Metal/Na. The mixture was sonicated during 15 min and magnetically stirred overnight at room temperature. The solid was filtrated using a PTFE membrane and washed until pH~6.5. The samples (MSA-Na/CNTox-400) were dried overnight at 80 ºC. 57 Y. H. Choi, et al. Applied Catalysis B: Environmental 2017, 202, 605-610. 58 W. Li, et al. RSC Advances 2018b, 8, 7651-7669.

  ce qui limite leur application à des réactions d'hydrogénation sélective

40

G. Malta, et al. Science 2017, 355, 1399. 41 J. N. Tiwari, et al. Advanced Energy Materials 2019, 9, 1900931. 42 C. Rivera-Cárcamo, et al. ChemCatChem 2018, 10, 5058-5091. 43 A. Granja-DelRío, et al. The Journal of Physical Chemistry C 2017, 121, 10843-10850.

  M. Aresta, et al. Chemical Reviews 2014, 114, 1709-1742. 48 L. Wang, et al. Chemical Society Reviews 2019b, 48, 5310-5349.Pd SA /Pd NP ratio L'hydrogénation du CO2 a été conduite dans des conditions de méthanation (CO2/H2 = 1/4) dans une configuration de réacteur à lit fixe sous 6.1 bar, à 260, 300 ou 340°C, et avec une GHSV de 16500 mL/h.g. Les principaux résultats de la réduction du CO2 avec des catalyseurs 1%MSA/D-CNT sont résumés sur le Tableau 6.4 et la Figure6.14, où les profils de réaction montrent l'évolution du STY avec le temps en service (ToS) à différentes températures.Tous les échantillons étaient actifs pour la réduction du CO2 et comme prévu, une augmentation du STY avec la température a été observée. Selon les valeurs d'activité, les métaux peuvent être classés du plus actif au moins actif, dans l'ordre suivant : Pt > Ru > Ni ~ Co > Pd > Cu. Comme on peut le voir, l'impact de la nature de la phase métallique joue un rôle sur les performances catalytiques générales. Pour tous les métaux, le produit principal était le CO avec une sélectivité comprise entre 92 et 99%, à l'exclusion du Ru qui produisait du méthane à ~ 43%. Ces résultats suggère qu'un catalyseur au Pd atomiquement ou presque atomiquement dispersé ne serait pas capable d'activer de manière homolytique H2 (pour former l'intermédiaire formiate conduisant in fine au méthane) ; au lieu de cela le PdSA était capable d'activer le dihydrogène de manière hétérolytique pour produire un intermédiaire carboxyle, expliquant la sélectivité envers le CO.49

	Tableau 6.4. Performances catalytiques des catalyseurs série 1% MSA/D-CNT sur
	l'hydrogénation au CO2.
	Catalyseurs PtSA/D-CNT RuSA/D-CNT	ICP wt% 0.68 0.95	T (ºC) 260 300 340 260 300	CO2 conversion (%) a) -myrcene conversion Pd /s) 0.92 2.05 (molH2/mol 4.14 0.68 STY at 66% 2.5	100 1000 10000	STY (molCO2/ molM.h) a) 29.81 66.00 133.35 8.07 30.00		CO sel. (%) a) 99.31 99.57 99.72 56.84 54.8	CH4 sel. (%) a) 0.68 0.43 0.28 43.16 45.18	Ea (kJ/ mol) 50.86 86.69
				340	8.70	10	1	103.90	10	57.1	100	42.86	1000
				260	0.34			4.20		98.95	1.05
	PdSA/D-CNT	0.98	300	0.55			6.76		97.59	2.41	21.12
				340	0.81			9.89		97.11	8.88
				260	0.60			3.65		94.80	5.19
	NiSA/D-CNT	1.09	300	1.96			11.87		95.04	4.96	75.04
				340	5.51			33.30		93.75	6.25
				260	0.19			0.99		93.50	6.50
	CoSA/D-CNT	1.27	300	2.29			11.93		97.39	2.61	28.15
				340	6.74			35.09		96.16	3.84
				260	0.14			0.82		94.55	5.45
	CuSA/D-CNT	1.21	300	0.18			1.08		92.75	7.25	76.17
				340	0.15			0.91		92.82	7.18
	a)	Valeur moyenne

  50,51 Le frittage de SA est un facteur clé à surmonter pour de telles applications, raison pour laquelle il est d'une importance cruciale de rechercher des méthodes pour améliorer leur durabilité. Tableau 6.5. Performances catalytiques des catalyseurs 1% MSA-Na/D-CNT pour l'hydrogénation du CO2.

	Catalyseur	T (ºC)	CO2 conversion (%) a)	STY molM.h) a) (molCO2/	(%) a) CO sel.	(%) a) CH4 sel.	Ea (kJ/mol)
		260	7.6	246.0	99.8	0.2
	PtSA-Na/D-CNT	300	21.4	688.4	99.9	0.1	46.7
		340	30.1	970.5	99.9	0.1
		260	1.6	20.0	85.5	14.5
	RuSA-Na/D-CNT	300	7.9	97.1	85.9	14.0	85.6
		340	20.7	252.3	80.3	19.7
		260	1.5	18.5	98.7	1.3
	PdSA-Na/D-CNT	300	2.9	35.2	99.6	0.4	40.5
		340	5.5	66.5	99.7	0.3
		260	1.2	7.3	94.6	5.4
	NiSA-Na/D-CNT	300	3.5	22.0	93.7	6.3	65.4
		340	8.1	50.7	95.5	4.5
		260	0.1	0.8	90.5	9.5
	CoSA-Na/D-CNT	300	0.6	3.3	96.7	3.3	90.3
		340	2.4	12.6	98.2	1.8
		260	0.4	2.3	97.5	2.5
	CuSA-Na/D-CNT	300	0.6	3.8	97.1	2.9	n.d. b)
		340	0.4	2.6	97.5	2.5

L'action de promoteurs alcalins en catalyse hétérogène a été largement étudiée,

52

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Acknowledgements

I would like to express my most sincere acknowledgments to my supervisor Professor

Synthesis of supported single atom catalysts (SAC)

The optimization of heterogenous catalytic systems has been of permanent interest in the last century, even more in the last decades, because theoretical and experimental results demonstrated that only surface active-site atoms are involved in the reaction mechanism.

Under this premise, in 2011 Zhang's group published an innovative study on the exceptional catalytic performance in CO oxidation over isolated Pt atoms supported on iron oxide. 97 There is consensus that this publication first coined the term "single atom catalysts" referring to heterogeneous catalysts with active phases comprised of a single metallic atom. Since then, hundreds of publications have focused on this topic. 98,99 In the particular case of carbon materials, significant efforts have been devoted in the development of carbon-based SAC due to their remarkable properties such as high specific surface area, tunable porosity, good electrical and mechanical conductivity, easy handling and low production cost, among others. These systems have been employed for promising sustainable energy and technology-related catalytic applications. 100,101,102 We thought that it could be possible to take advantage of the previously described defective support to introduce effectively metallic single atoms over the defective carbon support in order to produce carbon-based SAC.

We started this study with the preparation of ruthenium supported single atom catalysts on defective CNT (OXh-CNT-T). As discussed in the previous sections, this support should contain significant amounts of defects such as carbon vacancies. DFT studies of the spin coupling around a carbon atom vacancy in graphene have shown that the triplet state (spin-unpaired electrons) is slightly more stable (0.2 eV) than the singlet state (spinpaired electrons). 20,103,104 This singlet-triplet quasi-degeneracy shares similarities with the chemistry of carbenes in organic chemistry. 105,106 The two first steps (I and II described in the previous section) of the synthesis were followed by two subsequent steps (III and IV) that involve wet impregnation of D being the particle diameter measured by STEM, ρ the bulk metal density, NA the Avogadro number, and MW the molecular weight assuming that all the Pd is accessible. 43 The variation on this value along with SA/NP ratio gives an idea about how the palladium species evolve from one sample to another, and confirms that the method of preparation can effectively control single atom availability.

HRTEM images show that for the four 1.2% Pd/CNT samples prepared by this process (in grey on Table 4.1), the SA/NP ratio is controlled between 200/1 (a single deposition cycle of 1.2% Pd) to 10/1 (four deposition cycles of 0.3% Pd). The particle size distribution based on total particle number and on total atom number is given on Figure 4.19.

As transmission electron microscopy only informs us thanks to a very local analysis of the samples (poor statistic value), we also characterize these four samples by XPS, (Chapter 3, Section 3.5. Mixture of single atoms and nanoparticles) which constitutes a more general probe of the whole samples. The binding energy at 335.3 eV is in agreement with the value reported in the literature for metallic palladium. 44 For the sample Pd200_SA/NP/CNT, this binding energy is slightly upshifted presumably due to charge transfer between the very small particles present on this sample and the support or to particle size-dependent shift. 45,46 The binding energy at 337.3 eV can be attributed to surface oxides and/or to electron-deficient palladium species (Pd δ+ ) such as SA or small clusters in strong interaction with the support. 47,48,49 The Pd δ+ /Pd 0 ratio for the four samples are shown on Table 4.2 as well as the fraction of SA in the same samples determined from STEM. nitrogen flow did not significantly modify the particle size distribution and the SA/NP ratio of the catalyst (Figure 4.23a-f), pointing its good thermal stability. Such internals in milli-channels are ideal candidates to promote such powdered catalysts in a continuous reactor. The particularly confined hydrodynamics lead to interesting heat and mass transfer performances as well as a small pressure drop due to the wide-open structure of open-200 kV with a point resolution of > 1.9 Å. The particle size distribution was determined through a manual measurement of enlarged micrographs from different areas of the TEM grid (at least 300 particles). The size distribution reported as Pd atomic percentage (based on total atoms) in each size range were obtained by calculating the number of atoms (N) in each nanoparticle assuming a spherical shape using an equation described in Ref. 55. The SA/NP ratio (a number ratio) was measured from the STEM-HAADF analyses of at least 500 elements.

The samples were analyzed by X-ray photoelectron spectroscopy (XPS) using a VG Escalab MKII spectrophotometer operating with a non-monochromatized Mg K source (1253.6 eV). A SQuID magnetometer MPMS5 was used to investigate the magnetic properties. The M(H) loop was measured at T = 300 K after cooling the sample from 300 K to 2 K under a constant applied magnetic field Hcool = 50 kOe. The M(H) loops were measured between -50 and 50 kOe. EPR data were recorded using an Elexsys E 500 Bruker spectrometer, operating at a microwave frequency of approximately 9.5 GHz. All spectra were recorded using a microwave power of 20 mW across a sweep width of 150 mT (centered at 310 mT) with a modulation amplitude of 0.5 mT. Experiments were carried out at 110 K using a liquid nitrogen cryostat. Calculations using periodic boundary conditions were performed using the Vienna ab initio simulation package VASP, 56,57,58,59 which employs the full-potential projector augmented waves (PAW) framework. 60,61 Exchange-correlation effects were approximated using the PBE functional and applied in spin-polarized calculations. 62 A kinetic energy cutoff of 400 eV was found to be sufficient to achieve a total energy convergence within several meV, considering k-point sampling, i.e. with a (3x3x1) -centered grid in conjunction with a 0.05 eV-width Gaussian smearing. All atoms were fully relaxed in order to have forces smaller than 0.005 eV/Å -1 , and the Nudged Elastic Band method was used to estimate minimum energy paths of