Ekaterina Mamontova

Isabelle Favier

Daniel Pla

Montserrat Gómez
email: gomez@chimie.ups-tlse.fr

Ekaterina Orcid

Mamontova

Organometallic interactions between metal nanoparticles and carbon-based molecules: A surface reactivity rationale

Keywords: Metal-based nanoparticles, C-C couplings, C-heteroatom couplings, Mechanistic insights, Surface reactivity

Metal-based nanoparticles (MNPs) have been widely used as catalysts in different types of chemical transformations, taking advantage of their intrinsic properties, such as geometry, specific surface area, sites (edges, corners, faces, kinks, steps…) and electronic configuration, which offer a wide-ranging variety of interactions with the reagents and products of catalytic processes. In addition, both the nature of the metal and the support (often MNPs are supported on solids or immobilized in liquid phases) play a crucial role in their catalytic behavior and plausible recycling. Nevertheless, research works reporting on the organometallic interactions at the metal surface are much more limited, often due to the difficulty of accessing to convenient experimental techniques. In this contribution, an overview from an organometallic point of view is presented regarding the interactions of organic compounds (reactants or products) at the surface of metal nanoparticles based on experimental data and calculations, helping to establish structure-reactivity correlations. Contents 1.

INTRODUCTION

 However, deep mechanistic studies have permitted to identify surface-like reactivity for different C-C or C-heteroatom couplings, exhibiting fine-tuning between the adsorption of reagents and products, permitting selective reactivity and avoiding catalyst poisoning. This review focuses on the reported works concerning C-C and C-heteroatom couplings catalyzed by MNPs, providing a special insight on metal surface-organic compounds interactions, based on experimental data including theoretical calculations, material characterization and control tests.

C-C HOMO-COUPLING REACTIONS

C-C bond formation transformations through metal-catalyzed homo-coupling reactions represent a useful and sustainable tool in organic synthesis. In this section, C(sp 2 )-C(sp 2 ) (Ullmann-type reactions catalyzed by Cu-, Co-, Pd-and Au-based nanoparticles) and C(sp)-C(sp) (Glaser couplings with metalbased nanoparticles from group 11) homo-couplings are reviewed, reporting mechanistic approaches in order to elucidate the role of the metal surface in the corresponding coupling. The multi-component Guerbet reaction is also discussed.

Ullmann and boronic acid couplings

 a myriad of methodologies with different transition metal catalysts has been developed to form C(sp 2 )-C(sp 2 ) and

C-N bond formation reactions 4.3.

A 3 couplings 4.4.

Oxyhalogenation reactions 5.

Fischer-Tropsch reaction 6.

Conclusions and perspectives Csp 2 -heteroatom bonds. These ways overcome the main limitations of the classical Ullmann coupling and in consequence permit to work under milder conditions, using inactivated substrates and exhibiting a better tolerance to functional groups. [START_REF] Monnier | and C-O Ullmann-Type Coupling Reactions[END_REF][START_REF] Sambiagio | Copper catalysed Ullmann type chemistry: from mechanistic aspects to modern development[END_REF][START_REF] Ribas | Cu(I)/Cu(III) catalytic cycle involved in Ullmann-type cross-coupling reactions[END_REF][START_REF] Beletskaya | Copper in cross-coupling reactions: The post-Ullmann chemistry[END_REF][START_REF] Hassan | Aryl-aryl bond formation one century after the discovery of the Ullmann reaction[END_REF] Sykes' group has studied the Ullmann coupling of bromobenzene on a Cu(111) surface by Scanning Tunnelling Microscopy (STM). [START_REF] Lewis | Atomic-scale insight into the formation, mobility and reaction of Ullmann coupling intermediates[END_REF] They demonstrated that the formation of biphenyl occurs at the metal surface through a mobile organometallic intermediate, containing two phenyl groups bound to a Cu atom that was extracted from the surface (Fig. 1). They could also conclude that the formation of this molecular intermediate at low temperature (160 K) is consistent with the rate limiting step for the studied reaction, being the biphenyl formation and not the Cu atom extraction from the metal surface or the formation of the organometallic intermediate, the step showing the highest energy barrier. The same group was also interested in the use of CoNPs grown on a Cu(111) surface for the homocoupling of bromobenzene, proving that Co and Cu catalyze the C-C coupling by different pathways. [START_REF] Lewis | Nanoscale insight into C-C coupling on cobalt nanoparticles[END_REF] Thus, authors proved that the in situ formed intermediates presented a different structure in relation to those coming from copper. STEM analyses (Scanning Transmission Electron Microscopy) evidenced that the bromobenzene dissociation at CoNPs surface happens at lower temperature (< 80 K) than that observed for Cu(111) surface (160 K). Furthermore, the phenyl groups are linked to the cobalt surface, without forming molecular organometallic species, what means that no cobalt atoms were removed from the nanoparticle surface in contrast to the copper reactivity (Fig. 2). After annealing the sample at 160 K, the formation of biphenyl was complete (for Cu(111), the temperature required was 350 K). Similar behavior was observed using benzylbromide as substrate.

Fig. 2 STEM images displaying the bromobenzene homo-coupling on cobalt nanoparticles: A and D images corresponds to bromobenzene deposited on the Co@Cu(111) surface; B and E images after annealing at 80 K (bromobenzene dissociates in bromo and phenyl species; C and F images corresponds to biphenyl formationon CoNPs (after annealing at 160 K); G shows a schematic representation of the Co/Cu(111) material: cobalt forms triangular nanoparticles on copper. Reproduced from [START_REF] Lewis | Nanoscale insight into C-C coupling on cobalt nanoparticles[END_REF] with permission from the Royal Society of Chemistry.

In the last decade, Au-and Pd-based nanoparticles have been efficiently applied to the aryl halide coupling taking advantage of their surface-like reactivity, tolerating both homogeneous (nanoparticles dispersed in a liquid phase) and heterogeneous (nanoparticles immobilized on solids) reaction conditions. [START_REF] Wang | ZIF-8-based vs. ZIF-8-derived Au and Pd nanoparticles as efficient catalysts for the Ullmann homocoupling reaction[END_REF] Concerning AuNPs supported on semi-conductor materials, Scaiano and coworkers proved the efficiency of AuNPs@TiO2 in the photoinduced C-C coupling of benzyl derivatives. [START_REF] Lanterna | Heterogeneous photocatalytic C-C coupling: mechanism of plasmon-mediated reductive dimerization of benzyl bromides by supported gold nanoparticles[END_REF] Control tests demonstrated that the reaction happens through a radical mechanism (radical intermediate trapped by TEMPO) rather than by a nucleophilic substitution reaction, requiring light (530 nm LED), a semi-conductor support and a sacrificial electron donor such as diisopropylethylamine; Au-free TiO2 gave low yield of the corresponding dimerization product (Fig. 3). Light-promoted coupling of aryl halides was carried out using AuNPs supported on the KNb3O8 semi-conductor which exhibits a bigger gap energy between valence and conduction bands than TiO2 (3.7 eV for KNb3O8 vs 3.0 eV for TiO2), showing a similar mechanism than that described by Scaiano's group. [START_REF] Crabbe | Light-activated Ullmann homocoupling of aryl halides catalyzed using gold nanoparticle-functionalized potassium niobium oxides[END_REF] Fig. 3 Top: Plausible mechanism for the AuNPs@TiO2 catalyzed photocatalytic reductive dimerization of 4nitrobenzylbromide. Bottom: Reduction of halo-benzyl derivatives by -aminoalkyl radical intermediates. Reproduced from [START_REF] Lanterna | Heterogeneous photocatalytic C-C coupling: mechanism of plasmon-mediated reductive dimerization of benzyl bromides by supported gold nanoparticles[END_REF] with permission from the Royal Society of Chemistry.

PdNPs on oxide-based supports have been efficient for aryl halide homo-couplings, some of them proving the synergic effect between the metal surface and the support. Thus, PdNPs immobilized on CeO2 porous nanorods, which show a large surface area, a considerable concentration of oxygen vacancies and a remarkable basicity, allowed the coupling of chlorobenzene, more difficult to be activated than bromo-and iodo-arenes, in water using glucose as reducing agent. [START_REF] Zhang | Insights into the effects of surface properties of oxides on the catalytic activity of Pd for C-C coupling reactions[END_REF] XPS (X-ray Photelectron Spectroscopy) and FT-IR (Fourier Transform Infrared spectroscopy) analyses evidenced that the strong basicity of the support with the high concentration of vacancies, induced a high electron density of surface palladium atoms and in consequence favored the oxidative addition of chloro-arenes at the metal surface, without studying the plausible leaching after this elementary step. Somsook and co-workers studied the synergy between Fe-doped La2O3 and PdNPs for a large scope of chloro-arenes, in ethanol under basic conditions. [START_REF] Chumkaeo | Synergistic Activities in the Ullmann Coupling of Chloroarenes at Ambient Temperature by Pd-Supported Calcined Ferrocenated La2O3[END_REF] An enhancement of the reactivity was observed using Fe-doped support in comparison to pristine La2O3. Based on the materials characterization and catalytic behavior, authors could prove that ethanol (acting also as reducing agent) was activated by La2O3 and chloro-arenes by the Pd surface, both processes showing a similar rate (Fig. 4). For PdNPs immobilized on amino-functionalized montmorillonite applied in the Ullmann coupling of iodobenzene, a palladium "release and catch" mechanism was inferred by TEM after several runs (observation of agglomerates), hot filtration test and ICP analyses (leaching of ca. 7% of the starting palladium loading). [START_REF] Varadwaj | Pd(0) Nanoparticles Supported Organofunctionalized Clay Driving C-C Coupling Reactions under Benign Conditions through a Pd(0)/Pd(II) Redox Interplay[END_REF] Hybrid materials constituted by Au-Pd bimetallic nanoparticles (BMNPs) supported on alumina, in the presence of TiO2 acting as photocatalyst, were applied in Ullmann coupling of aryl halides. [START_REF] Tyagi | Photocatalytic Ullmann coupling of aryl halides by a novel blended catalyst consisting of a TiO2 photocatalyst and an Al2O3 supported Pd-Au bimetallic catalyst[END_REF] XANES analysis of the AuPd BMNPs@Al2O3 revealed that both metal atoms are intimately close and an electron density transfer from Pd to Au atoms occurs. [START_REF] Tyagi | Novel blended catalysts consisting of a TiO2 photocatalyst and an Al2O3 supported Pd-Au bimetallic catalyst for direct dehydrogenative cross-coupling between arenes and tetrahydrofuran[END_REF] The formation of the by-product, benzene, increased linearly with the light intensity, while the formation of biphenyl was constant at high intensities, excluding a radical coupling mechanism. Monitoring and control studies permitted to conclude that the formation of biphenyl is initiated by the photo-generation of phenyl radicals which reacts with an iodobenezene molecule which has been activated at the metal surface (probably promoted by a plasmonic effect); thus, the homo-coupling follows a radical addition-elimination mechanism (Fig. 5). 48

Fig. 5 Plausible mechanism for the photocatalytic homo-coupling of iodobenzene using TiO2 blended with Au-Pd bimetallic nanoparticles supported on alumina. Reproduced from [START_REF] Tyagi | Photocatalytic Ullmann coupling of aryl halides by a novel blended catalyst consisting of a TiO2 photocatalyst and an Al2O3 supported Pd-Au bimetallic catalyst[END_REF] with permission from the Royal Society of Chemistry.

PdNPs stabilized by polyvinylpyrrolodine (PVP) were efficiently immobilized in glycerol and applied in homocoupling reactions of aryl halides. [START_REF] Favier | Palladium-mediated Radical Homocoupling Reactions: a Surface Catalytic Insight[END_REF] A deep mechanistic study, including reaction monitoring, electron paramagnetic resonance analyses, effect of radical scavengers and theoretical calculations, permitted to conclude that the reaction occurs through radical intermediates, requiring the synergy between neighboring palladium atoms which lead to the homocoupling product in a regioselective way (Fig. 6). 50

Fig. 6 Proposed mechanism for PdNPs-catalyzed reductive homo-coupling of aryl halide derivatives. Adapted from [START_REF] Favier | Palladium-mediated Radical Homocoupling Reactions: a Surface Catalytic Insight[END_REF] with permission from the Royal Society of Chemistry.

From a mechanistic point of view, the oxidative coupling of organoboron compounds has aroused great interest in the scientific community, mainly for Pd-and Au-catalyzed processes, using both homogeneous organometallic complexes and well-defined nanoparticles. [START_REF] Dhital | Oxidative Coupling of Organoboron Compounds[END_REF][START_REF] Poupart | Porous Gold Nanoparticle-Decorated Nanoreactors Prepared from Smartly Designed Functional Polystyrene-block-Poly(D,L-Lactide) Diblock Copolymers: Toward Efficient Systems for Catalytic Cascade Reaction Processes[END_REF][START_REF] Blanco | Palladium nanoparticles supported on graphene acid: a stable and eco-friendly bifunctional C-C homo-and cross-coupling catalyst[END_REF][START_REF] Prastaro | Homocoupling of arylboronic acids and potassium aryltrifluoroborates catalyzed by protein-stabilized palladium nanoparticles under air in water[END_REF] In this latter case, experimental and theoretical studies have been deeply carried out especially with AuNPs.

Corma and co-workers published the first report encompassing Au(III)-based supported ceria heterogeneous catalysts for preparing biaryl compounds through boronic acid homocoupling. [START_REF] Carrettin | Supported Gold Catalyzes the Homocoupling of Phenylboronic Acid with High Conversion and Selectivity[END_REF] In contrast to previous reports, the homocoupling takes place in the absence of both air and added base but requiring the presence of water, which generates hydroxyl groups on CeO2 surface, responsible of the formation of boric acid. These conditions are probably related to the small gold clusters formed. The formation of H2 was evidenced by Raman spectroscopy. XPS analyses and IR spectra using CO as a probe molecule, permitted to conclude on the presence of Ce(III) and the correlation between Au(III) and catalytic activity (no correlation found when using Au(I) or Au(0) catalytic materials), proving that Au(III) sites are the active centers.

It is important to highlight that for colloidal AuNPs stabilized by PVP (ca. 2 nm), molecular oxygen dissolved in water was found to be necessary to carry out the phenylboronic acid homocoupling. [START_REF] Tsunoyama | Colloidal Gold Nanoparticles as Catalyst for Carbon-Carbon Bond Formation: Application to Aerobic Homocoupling of Phenylboronic Acid in Water[END_REF] In this context, the O2 activation necessarily occurs at the metal surface. Based on the theoretical studies performed by Metiu and coworkers, 57 O2 preferentially binds to coordinative unsaturated Au atoms placed on corners, edges, steps or kinks, but not on faces. Later on, Corma's group studied the mechanism of activation of ArB(OH)2 considering Au38 nanoparticles (AuNPs ca. 1 nm) by DFT (Density Functional Theory) calculations, [START_REF] Boronat | Molecular approaches to catalysis[END_REF] taking into account that the activation of small molecules is not observed for AuNPs exhibiting diameters larger than ca. 4 nm. [START_REF] Turner | Selective oxidation with dioxygen by gold nanoparticle catalysts derived from 55atom clusters[END_REF][START_REF] Liu | Aerobic oxidation of cyclohexane catalyzed by size-controlled Au clusters on hydroxyapatite: size effect in the sub-2 nm regime[END_REF] Curiously, it was observed that when the AuNPs are partially oxidized (containing 2 oxygen atoms per Au38 NP), the energy barrier associated to the coupling of two aryl groups was lower in energy than in the non-oxidized Au38 NP (34.3 kcal/mol vs 4.7 kcal/mol). This DFT study proved that the crucial role of O2 in the homocoupling reaction is related to the coupling step and not to the activation of phenylboronic acid. 58

Glaser coupling

The interest for the chemistry of 1,3-diynes has significantly grown in the last years mainly due to their involvement as building blocks in the synthesis of different relevant compounds, 61 such as polymers, [START_REF] Liu | Acetylenic Polymers: Syntheses, Structures, and Functions[END_REF] natural products, [START_REF] Shun | Synthesis of naturally occurring polyynes[END_REF] and optoelectronic materials. [START_REF] Kivala | Conjugation and optoelectronic properties of acetylenic scaffolds and charge-transfer chromophores[END_REF] Usually, 1,3-diynes are obtained by Glaser oxidative coupling, involving both Cu(I) [START_REF] Glaser | Beiträge zur Kenntniss des Acetenylbenzols[END_REF] and Cu(II) salts 66 in the presence of ammonia or diamines such as tetramethylethylenediamine, under oxygen atmosphere. [START_REF] Akhtar | Transition metal catalyzed Glaser and Glaser-Hay coupling reactions: Scope, classical/green methodologies and synthetic applications[END_REF] In addition to homogeneous catalysts, heterogeneous catalytic materials have been also reported mainly concerning copper and palladium, [START_REF] Alonso | Heterogeneous Catalytic Homocoupling of Terminal Alkynes[END_REF] but also catalytic materials based on other transition metals such as Ni, [START_REF] Murugan | Green-Synthesized Nickel Nanoparticles on Reduced Graphene Oxide as an Active and Selective Catalyst for Suzuki and Glaser-Hay Coupling Reactions[END_REF] Ag, and Au. [START_REF] Boronat | Partially oxidized gold nanoparticles: A catalytic base-free system for the aerobic homocoupling of alkynes[END_REF][START_REF] Vilhanova | Subnanometer Gold Clusters on Amino-Functionalized Silica: An Efficient Catalyst for the Synthesis of 1,3-Diynes by Oxidative Alkyne Coupling[END_REF] From a mechanistic point of view, it seems difficult that the simultaneous activation of the terminal alkyne and O2 can take place on isolated copper cations. It is known that Cu-containing enzymes, binuclear metal structures are proposed to be the active sites for O2 activation; 72 also, in Cu-catalyzed azide-alkyne cycloadditions, it has been demonstrated the role of the multi-nuclear complexes and nanoparticles. [START_REF] Jin | Isolation of bis(copper) key intermediates in Cucatalyzed azide-alkyne "click reaction[END_REF][START_REF] Rodríguez-Rodríguez | Key Non-Metal Ingredients for Cu-catalyzed "Click" Reactions in Glycerol: Nanoparticles as Efficient Forwarders[END_REF] Corma and coworkers reported the synthesis of symmetric and unsymmetric 1,3-diynes by oxidative coupling of alkynes catalyzed by CuOx nanoparticles, both unsupported and supported on nano-TiO2. [START_REF] Liu | Facile Synthesis of Surface-Clean Monodispersed CuOx Nanoparticles and Their Catalytic Properties for Oxidative Coupling of Alkynes[END_REF] This work proved that small Cu-based nanoparticles are necessary (mean diameter ca. 2 nm), for both supported and monodispersed CuOx NPs. This work evidenced that copper clusters on titania (constituted by a few number of Cu atoms) were not appropriate for O2 activation in agreement with DFT calculations. However, for CuOx NPs of ca. 2 nm, O2 is transformed into peroxo species at the nanoparticle surface (based on in situ Raman and IR studies), being active intermediates for the alkyne coupling. Bigger CuOx NPs led to the formation of lattice oxygen species, exhibiting a lower activity for the oxidative coupling (Fig. 7). The formation of these oxygen-based intermediates agrees with TPR (Temperature Programmed Reduction) profiles. Therefore, the H2 consumption was very low for copper clusters compared with copper nanoparticles. Furthermore, when CuOx NPs of 2 nm were treated with phenylacetylene and 18 O2 under reaction conditions (90 °C, 1 h), neither H2 [START_REF] Mitsutaka | Chemical Vapor Deposition of Gold Nanoparticles on MCM-41 and Their Catalytic Activities for the Low-temperature Oxidation of CO and of H2[END_REF] O nor H2 [START_REF] Sivula | WO3-Fe2O3 Photoanodes for Water Splitting: A Host Scaffold, Guest Absorber Approach[END_REF] O was detected, meaning that the surface was reduced under catalytic conditions but it was not reoxidized, plausible due to the formation of peroxo intermediates; in contrast, for bigger CuOx NPs, both H2 [START_REF] Mitsutaka | Chemical Vapor Deposition of Gold Nanoparticles on MCM-41 and Their Catalytic Activities for the Low-temperature Oxidation of CO and of H2[END_REF] O and H2 18 O were detected, indicating a partial reduction and reoxidation by 18 O2 of the surface, in agreement with O2 dissociation at the surface transforming into a lattice oxygen species. Reprinted with permission from [START_REF] Liu | Facile Synthesis of Surface-Clean Monodispersed CuOx Nanoparticles and Their Catalytic Properties for Oxidative Coupling of Alkynes[END_REF] . Copyright 2016 American Chemical Society.

Zheng and coworkers have recently described the synthesis of hybrid copper-based nanoparticles supported on reduced graphene oxide (rGO), Cu/Cu2ONPs@rGO. [START_REF] Lu | Stabilized Cu/Cu2O nanoparticles on rGO as an efficient heterogeneous catalyst for Glaser homo-coupling[END_REF] Among the copper catalysts tested for the homocoupling of phenylacetylene including Cu(I) and Cu(II) salts, supported and unsupported Cu-based nanoparticles, the highest activity was obtained with Cu/Cu2ONPs@rGO, which was also efficient for the symmetrical coupling of different terminal alkynes (12 examples), being more active for aryl derivatives. Previous works of the same group using homogeneous catalysts proved the reduction of Cu(II) into Cu(I) by terminal alkynes (XANES, EXAFS and EPR studies) [START_REF] Zhang | Direct Observation of Reduction of Cu(II) to Cu(I) by Terminal Alkynes[END_REF] and the synergy between Cu(II) and Cu(I) evidenced by experimental (IR, EPR, XAS) and theoretical (DFT calculations) means. [START_REF] Bai | Cu(II)-Cu(I) Synergistic Cooperation to Lead the Alkyne C-H Activation[END_REF] Based on that, authors extrapolated the homogeneous reactivity to Cu/Cu2ONPs on rGO, conductor support exhibiting a high specific surface, and proposed the mechanism illustrated (Fig. 8). [START_REF] Lu | Stabilized Cu/Cu2O nanoparticles on rGO as an efficient heterogeneous catalyst for Glaser homo-coupling[END_REF] A first - interaction between terminal alkyne and rGO layers promotes the alkyne interaction with copper nanoparticles, which, in the presence of a base, leads to the activation of C-H bond and the corresponding formation of Cu-alkynyl intermediates; the synergy between close copper species favors the oxidative coupling and the reduction of copper nanoparticles, subsequently oxidized by molecular oxygen. Corma's group studied the mechanism of base-free symmetrical homo-coupling of terminal alkynes catalyzed by AuNPs and clusters, both unsupported and supported catalysts on ceria, with the aim of determining the nature of the gold active sites. [START_REF] Boronat | Partially oxidized gold nanoparticles: A catalytic base-free system for the aerobic homocoupling of alkynes[END_REF] DFT calculations showed that phenylacetylene can be adsorbed at Au(0), Au (+) and Au + sites; the basic oxygen atoms of the support permit its deprotonation (Fig. 9). Moreover, the alkynyl groups bonded to cationic gold atoms are more reactive than those linked to Au(0), and in consequence the activation barriers for the bimolecular coupling step are lower for AuNPs involving a cationic surface. Moreover, kinetic data indicated that the dissociation of molecular oxygen on AuNPs was the rate determining step, corroborated by the theoretical calculations.

Guerbet reaction

The tandem multi-step Guerbet reaction (involving dehydrogenation/aldol condensation/crotonisation/hydrogenation reactions) is an attracting process for the synthesis of branched monoalcohols, value added compounds for industrial production of plasticizers and detergents, including the synthesis of bioalcohols (Fig. 10); [START_REF] Zaccheria | The Role of Copper in the Upgrading of Bioalcohols[END_REF][START_REF] Aitchison | Homogeneous Ethanol to Butanol Catalysis-Guerbet Renewed[END_REF] further hydrodeoxygenation of the obtained branched alcohols makes it possible the production of branched hydrocarbons, components of aviation fuels. Although catalysts consisted of a basic metal oxide (such as MgO) have been widely studied, those composed of a transition metal supported on a basic solid, have received less attention, in particular from a mechanistic point of view. The presence of a transition metal mostly favors the dehydrogenation step of the starting alcohol, permitting to work under lower temperature compared to metal oxides and in consequence avoiding undesired by-products. [START_REF] Kozlowski | Heterogeneous Catalysts for the Guerbet Coupling of Alcohols[END_REF] Simakova and coworkers have studied the n-pentanol coupling with different platinum-group metals (Ru, Rh, Ir, Pd, Pt) supported on a variety of solids (C, Al2O3, TiO2, CeO2, ZrO2) doped with solid bases. [START_REF] Panchenko | Solid Base Assisted n-Pentanol Coupling over VIII Group Metals: Elucidation of the Guerbet Reaction Mechanism by DRIFTS[END_REF] These transition metals, in particular Pd and Pt, easily adsorb molecular and atomic hydrogen, promoting the first (alcohol dehydrogenation) and last step (crotonaldehyde hydrogenation). HRTEM (High Resolution Transmission Electron Microscopy) analyses of the as-prepared catalysts showed the presence of small and well-distributed MNPs (mean size 1.0-1.5 nm, except for Pd which exhibits an average particle size of 2.3-2.9 nm). For a given metal, the conversion and selectivity increased with the basic properties (proton affinity) of the support (TiO2 < ZrO2 < C < Al2O3). For catalysts containing carbon support, the conversion increased as follows: Ir < Ru < Pt < Pd, being PtNPs@Al2O3 doped with NaOH (named NaOH/Pt/-Al2O3) the most selective catalyst towards the formation of 2propylheptanol. Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectra of adsorbed ethanol on NaOH/Pt/-Al2O3 permitted to identify the formation of intermediate compounds produced in the Guerbet reaction. Therefore, at room temperature, the formation of acetaldehyde and hydroxyaldehyde could be inferred; after heating at 100 °C, new bands appeared corresponding to the formation of crotonaldehyde. These spectroscopic data corroborate the dehydrogenation/aldol condensation/hydrogenation steps according to a hydrogen borrowing reaction mechanism.

C-C CROSS-COUPLING REACTIONS

Since 1960's, C-C cross-coupling reactions, in particular applied in organic synthesis, are generally related to palladium-catalyzed transformations, recognized with the Nobel Prize in Chemistry 2010 conferred to R. F. Heck, E.-I. Negishi and A. Suzuki. In this section, we focus on mainly Pd-based nanoparticles (both mono-and bimetallic nano-catalysts) applied in C-C bond formation transformations, works including fundamental insights regarding surface reactivity based on experimental and in some cases also computational data.

Sonogashira coupling

Palladium-based catalysts in the presence or in the absence of Cu(I) salts (acting as cocatalyst and thus enhancing the reaction rate), have found a remarkable success in C(sp 2 )-C(sp) cross-couplings of aryl or alkenyl halides or triflates and terminal alkynes, [START_REF] Cassar | Synthesis of aryl-and vinyl-substituted acetylene derivatives by the use of nickel and palladium complexes[END_REF][START_REF] Dieck | Palladium catalyzed synthesis of aryl, heterocyclic, and vinylic acetylene derivatives[END_REF][START_REF] Sonogashira | Convenient synthesis of acetylenes. Catalytic substitutions of acetylenic hydrogen with bromo alkenes, iodo arenes, and bromopyridines[END_REF] leading to aryl-alkynes and conjugated enynes, intermediates particularly involved in the synthesis of drugs, natural products and organic materials. [START_REF] Chinchilla | The Sonogashira reaction: a booming methodology in synthetic organic chemistry[END_REF] Other than molecular catalysts, preformed Pd-Cu BMNPs have been employed in Sonogashira couplings, both BMNPs dispersed in liquid phases and immobilized on solid supports. Related to colloidal BMNPs, our group prepared Pd-Cu BMNPs by a co-reduction approach in glycerol under hydrogen atmosphere using different Pd/Cu ratios. Those obtained from an equimolar Pd/Cu ratio exhibited a cluster-in-cluster structure [evidenced by HAADF (High-Angle Annular Dark Field)-STEM and EDX (Energy Dispersive X-ray analysis) line scanning]. The as-prepared BMNPs were successfully applied in the synthesis of functionalized triazoles through one-pot tandem reaction, i.e. azide alkyne cyclisation coupling followed by Sonogashira reaction; this process displayed an excellent chemoselectivity due to the significant different rates of both processes (Cu-catalyzed triazole formation was faster than Pd-catalyzed C-C bond formation). [START_REF] Dang-Bao | Bimetallic Nanocatalysts in Glycerol for Applications in Controlled Synthesis. A Structure-Reactivity Relationship Study[END_REF] Domingos and coworkers synthesized core-shell Pd-Cu BMNPs in ethylene glycol/water by a polyol methodology. [START_REF] Olekszyszen | Coreshell PdCu bimetallic colloidal nanoparticles in Sonogashira cross-coupling reaction: mechanistic insights into the catalyst mode of action[END_REF] Based on kinetic profiles, poisoning experiments (Hg, CS2) and hot filtration tests, authors evidenced that both aryl halide and alkyne activation took place at the BMNP metal surface. However, HRTEM analysis after catalysis showed a significant size decrease of nanoparticles (from 3.8 to 2.8 nm, before and after catalysis respectively), pointing to a plausible metal leaching from the surface; this assumption was also supported by HRMS analysis which proved the presence of molecular copper acetylide species in the catalytic mixture. Therefore, after the oxidative addition to Pd(0) by the aryl halide and the formation of the alkynylcopper at the BMNP metal surface, a lixiviation of both palladium and copper species occurred before the transmetalation elemental step which occurred in solution (Fig. 11). These facts are in agreement with the Pd lixiviation after the oxidative addition on the metal surface proposed by Rothenberg and coworkers, for copper-free Pd-catalyzed Sonogashira coupling using palladium clusters as catalyst precursor. 91 Concerning Pd-Cu BMNPs immobilized on supports, Zhang, Gao and co-workers prepared well-defined Pd-Cu nanoalloys on montmorillonite, constituted of Pd(0) and Cu(I) (XPS analyses). [START_REF] Xu | Sonogashira Couplings on the Surface of Montmorillonite-Supported Pd/Cu Nanoalloys[END_REF] Catalytic materials containing Pd-Cu BMNPs of 10-11 nm (mean diameter) showed high activity and chemoselectivity in the coupling between aryl iodides and terminal alkynes, compatible with different types of functional groups. The synergistic effect between both metals was inferred by the dramatic decrease in activity when an equimolar amount of monometallic nanoparticles supported on montmorillonite was tested (from 97% to 26% yield for Pd-Cu nanoalloys and mixture of monometallic nanoparticles, respectively). Similar catalytic behavior was also observed for Pd-Cu nanoalloys anchored on graphene oxide. [START_REF] Sultana | Green synthesis of graphene oxide (GO)-anchored Pd/Cu bimetallic nanoparticles using Ocimum sanctum as bio-reductant: an efficient heterogeneous catalyst for the Sonogashira cross-coupling reaction[END_REF] Regarding gold-based catalysts, Lambert and co-workers demonstrated that supported zero-valent gold nanoparticles catalyzed the cross-coupling between iodobenzene and phenylacetylene, [START_REF] Beaumont | Identity of the Active Site in Gold Nanoparticle-Catalyzed Sonogashira Coupling of Phenylacetylene and Iodobenzene[END_REF] in contrast to the lack of activity shown by Au(I) and Au(III) supported materials. [START_REF] Goguet | Remarkable stability of ionic gold supported on sulfated lanthanum oxide[END_REF] It was proven that some leaching of ionic gold species into the liquid phase during the coupling happened, but this phase did not exhibit any catalytic activity, indicating that the Sonogashira reaction shows a surface-like reactivity. Furthermore, relative large AuNPs were more active than smaller ones pointing to the need that both molecules, iodobenzene and phenylacetylene, may be adsorbed on large facet planes; small AuNPs (<2 nm) favored the formation of biphenyl (homo-coupling process). [START_REF] Kyriakou | Sonogashira Coupling Catalyzed by Gold Nanoparticles: Does Homogeneous or Heterogeneous Catalysis Dominate?[END_REF] The same authors observed a similar trend using RhNPs supported on both -alumina and BaO. [START_REF] Kanuru | Evidence for heterogeneous Sonogashira coupling of phenylacetylene and iodobenzene catalyzed by well defined rhodium nanoparticles[END_REF] Regarding the nature of the gold sites, Corma and co-workers studied the Sonogashira coupling by both experimental (kinetic study) and theoretical (DFT calculations) means, using AuNPs supported on lanthanide oxides, such as La2O3 and CeO2 due to their higher selectivity towards the cross-coupling product than those immobilized on silica, alumina or titania, which favor the homocoupling of iodobenzene. [START_REF] Boronat | Making C-C Bonds with Gold: Identification of Selective Gold Sites for Homo-and Cross-Coupling Reactions between Iodobenzene and Alkynes[END_REF] This study evidenced that iodobenzene dissociates on Au(0) sites while phenyacetylene is activated on Au (+) ones, being the bimolecular surface elementary reaction the rate determining step. XPS analyses corroborated the presence of both neutral and ionic gold sites in the Au-based catalysts mainly favoring the cross-coupling reaction, in contrast to those promoting the formation of biphenyl (only Au(0) present in the nanoparticles).

Suzuki-Miyaura coupling

Initially described in 1979 and 1986, the Pd-catalyzed Suzuki-Miyaura cross-coupling pioneered in the formation of new C(sp 2 )-C(sp 2 ) and C(sp 2 )-C(sp 3 ) bonds, respectively, from aryl halides and boronic acid substrates. [START_REF] Miyaura | A New Stereospecific Cross-coupling by the Palladiumcatalyzed Reaction of 1-Alkenylboranes with 1-Alkenyl or 1-Alkynyl Halides[END_REF]100 Among all nanocatalyzed Suzuki-Miyaura cross-coupling reactions reported thus far in the literature, most of the works use either monometallic or bimetallic Pd catalyst systems. Thus, this review article will not only take advantage of the level of maturity reached in palladium nanoparticle catalysis to highlight surface mechanistic insights, but also detail the contributions of other transition metals from groups 8 to 10 enabling novel methodologies for the formation of C-C bonds. The key activation of aryl halides in Suzuki-Miyaura cross-coupling reactions has been classically described as an elementary oxidative addition of an aryl halide to a zero-valent Pd center taking place under a homogenous regime. However, nanocatalyst-enabled surface activation mechanisms of aryl halides substrates may differ from classical ones, as reported by Corma, García and co-workers for a catalytic system featuring smallsized nanoparticles that facilitated the oxidative addition via the formation of aryl Pd(II) intermediates. 101 In addition, substrate coordination on the metallic surface and steric effects can strongly tune the catalytic reactivity of preformed Pd NPs and favor oxidative addition as shown for 4bromoanisole. 102 Thus, the aim of this section is to highlight substrate activation pathways taking place on catalyst surfaces via key substrate interactions (Fig. 12). Seminal contributions by El-Sayed and co-workers in catalyzed Suzuki-Miyaura cross-couplings with PdNPs stabilized by PVP revealed that an excess of the stabilizer leads to enhanced stability in terms of size and shape due to Ostwald ripening inhibition, albeit on a catalytic activity decrease due to capping of the particle surface. 104 In addition, the boronic acid reagent acts as an stabilizing agent via binding through its boronate groups to the nanoparticle surface (as confirmed by FT-IR), 105 whereas iodobenzene and sodium acetate affected the stability of the nanoparticles by favoring Ostwald ripening via the leaching of molecular palladium species and their re-deposition for the reaction carried out at 100 °C in a MeOH-toluene solvent mixture (Fig. 13). Despite a weak interaction has been described between iodobenzene and the catalyst surface, it is worth to mention that the addition of biphenyl product to the reaction mixture resulted in low conversions via catalyst poisoning. Similar results were obtained by El-Sayed and co-workers for Pd nanocatalysts systems stabilized by polyamidoamine (PAMAM-OH) dendrimers. 106 Besides the size increase of spherical PdNPs catalysts, Ostwald ripening processes can be operative with other catalytic systems as observed for tetrahedral PtNPs systems that grow in size adopting a spherical shape under Suzuki-Miyaura cross coupling conditions. 105 The interplay of surface reactivity of Suzuki-Miyaura cross-coupling reaction of iodoanisole and phenylboronic acid with preformed PdNPs has been studied by Lee and co-workers via operando liquid-phase XAS monitoring (Fig. 14a-e). 103 Under their operando conditions (using KOMe as base at 60 °C in a MeCN-H2O solvent mixture), no metal leaching was observed, providing novel insights on the direct implication of surface density Pd defect sites in the Suzuki-Miyaura cross-coupling reaction catalyzed by PdNPs (Fig. 14f, size dependent TOF calculated taking into account all surface Pd atoms versus a constant TOF normalized to surface defect atoms). In addition, the kinetic profiles evidence a fast reaction without induction period followed by deactivation over time (Fig. 14g). Given the fact that classical substrate activation pathways via oxidative addition may be implicitly linked to the leaching of molecular species via the oxidation of the metal, finding alternative activation modes is crucial in the quest towards the development of novel catalyst systems with enhanced reactivity and selectivity outcomes. As an example, Kutubi, Kitagawa and Nagaoka reported in 2015 the preparation of a bimetallic catalyst system featuring alloyed Pd0.5Ru0.5 NPs stabilized with PVP (Fig. 15), exhibiting dual Lewis acidic/basic properties that facilitate the oxidative addition via the intrinsic charge density transfer from Pd towards Ru as evidenced by XPS characterization: +0.25 eV positive binding energy shift for Pd 3d5/2 (334.55 eV) together with a -0.80 eV negative binding energy shift for Ru 3p3/2 (460.60 eV) in comparison to analogous monometallic Pd and Ru NPs systems stabilized by PVP (334.30 and 461.4 eV, respectively). 107 The synergistic effects of alloyed PdRu NPs cooperatively participate in the activation of both Suzuki-Miyaura polar substrates via a rapid cleavage of haloarene C-X and boronic acid C-B bonds, yielding the formation of arylruthenium and arylpalladium species, respectively, in neighboring disposition that allow a reductive elimination to deliver the cross-coupled biphenyl product (Fig. 16). Furthermore, a series of analogous bimetallic Pd74M73 (M = Cu, Pt, Au, Rh, Ru) dendrimer encapsulated NPs were synthesized by Tang, Shi, Hu and co-workers and their catalytic performance towards the Suzuki-Miyaura cross coupling was assessed. Both Pd147 and Pd74Cu73 nanocatalyst systems furnished high yields (>79%) after short reaction times (45 min), and high TOFs (>8519 h -1 ). 108 Despite comparable energy barriers concerning the rate-determining step were determined for both monometallic Pd147 and bimetallic Pd74Cu73 dendrimer encapsulated NPs systems, substrate adsorption energies on the latter are slightly lower than those for monometallic ones. PdCu synergistic effects were assessed by DFT calculations, showing electron density differences that correlated nicely with a model encompassing the transmetalation of boronic acid at Cu sites on the catalyst surface, boosting the Suzuki coupling reaction. 108 Catalyst heterogenization on both inorganic oxides (SiO2, Al2O3, zeolites) or polymeric organic materials with special emphasis on entrapped nanoparticles in polymeric matrixes is an actively sought strategy as it represents a viable strategy towards catalyst enhancement via synergistic effects between metal NPs and the supports, as well as facilitated catalyst recovery and reuse. 109,110 Gupton, Khanna and co-workers have recently reported computational and experimental results shedding light on the nature of interfacial interactions between metal NPs and defect sites on a graphene surface. 111 The authors describe not only that the void sites on the surface of graphene provide enhanced stability against metal leaching through strong binding of the metal clusters, but also this support facilitates specific steps in the catalytic cycle, such as the oxidative addition via electron density transfer processes to the metal cluster, thus lowering the activation energy barrier of Suzuki cross-coupling reactions occurring at Pd@graphene catalyst surfaces (Fig. 17). 111 Furthermore, Blanco, Agnoli and coworkers have recently described a tunable methodology for the synthesis of size-controlled PdNPs (ranging from 1 to 9 nm) on graphene acid via impregnation of the support with Pd(OAc)2. 53 A similar activation mode via electron density donation to the metal sites has been described by Qu and co-workers for catalytic systems featuring PdNPs supported on porous CeO2 nanorods. The reported nanocatalysts composites can accelerate the initial oxidative addition step thanks to strong Pd-support interactions (Fig. 18), namely Pd sites enriched in electron density in combination with the strong basicity and a high number of oxygen vacancies in the CeO2 supports, as evidenced by a binding energy increase in core level Pd3d peaks in XPS (Fig. 18f). [START_REF] Zhang | Insights into the effects of surface properties of oxides on the catalytic activity of Pd for C-C coupling reactions[END_REF] The formation of reactive "ArPd(II)Br" species is proposed, but the authors do not comment whether the nature of the reactive intermediates is molecular or adsorbed on the catalyst surface. [START_REF] Zhang | Insights into the effects of surface properties of oxides on the catalytic activity of Pd for C-C coupling reactions[END_REF] Analogous synergistic effects have also been described by Ye and co-workers for highly active core-shell microspheres, bearing a magnetic Fe3O4 core, a layer of PdNPs and an outer shell of mesoporous CeO2. 112 Fig. 18 (a, b, c andd) Proposed mechanism for the Suzuki-Miyaura cross-coupling catalyzed by PdNPs@CeO2; (e) FT-IR spectra of a series of PdNPs@CeO2 catalysts; (f) XPS analysis of Pd nanocatalysts. Reproduced from [START_REF] Zhang | Insights into the effects of surface properties of oxides on the catalytic activity of Pd for C-C coupling reactions[END_REF] with permission from the Royal Society of Chemistry, license ID 1148288-1.

The preparation of highly active catalytic materials with enhanced reusability properties has also been enabled by the immobilization of PdNPs on solids supports such as silicates (mesoporous SiO2, 113,114 mercaptopropyltrimethoxysilane-SiO2, 115 PEG-SiO2, 116 silicalite-1, 117 montmorillonite 47 ), ZnAl2O4 spinel, 118 ferrite, 119 magnetite, 120,121 chalcogels, 122 covalent organic frameworks, 123-125 metal organic frameworks (Eu-MOF, 126 MCM-41, 127 MOF-5, 128,129 MIL-101, 130 Cu-MOF, 131 ) Carbon-based materials (graphene, 111 graphene oxide, 132-134 graphene acid, [START_REF] Blanco | Palladium nanoparticles supported on graphene acid: a stable and eco-friendly bifunctional C-C homo-and cross-coupling catalyst[END_REF] Carbon nanotubes 135 ), polymeric matrixes [dendrimers, 108 poly(amido-amine) (G4-PAMAM), 106 2-aminobenzaldehyde modified chitosan nanofibers, 136 DIANION, 137 mesoporous polyaminophenols, 138,139 poly(ethylene glycol)-co-poly(Nisopropylacrylamide), 140 nylon rope 141 ], or composite materials of mixtures thereof. [142][143][144][145][146][147] Literature reports on surface-like Suzuki-Miyaura cross-couplings under aerobic conditions address the challenge of precluding boronic acid homo-coupling, a competing side reaction occurring in the presence of O2 via the formation of palladium(II) peroxo species 148 that can easily undergo two successive transmetallations with boronic acid substrates as aryl halide oxidative addition to Pd(0) is the rate determining step under classical molecular regimes. [149][150][151] Solvents exhibiting low solubility profiles for O2 or heterogeneized systems capable of precluding Pd oxidation under aerobic conditions might enable selective and sustainable Suzuki-Miyaura crosscouplings precluding homo-coupling reaction manifolds. The results of Ma and co-workers 120 , Wang and co-workers 130 , Ye and co-workers [START_REF] Li | Hierarchical nanospheres based on Pd nanoparticles dispersed on carbon coated magnetite cores with a mesoporous ceria shell: a highly integrated multifunctional catalyst[END_REF] provide nice examples of selective nanocatalysts systems operating in air, but further studies the operating catalytic reaction via precluding molecular homocoupling reaction manifolds merits further studies. Suzuki carbonylative coupling reaction is one of the most facile and effective routes for direct synthesis of biaryl ketones from carbon monoxide, aryl halides and arylboronic acids, with various functionalities. Recently, Hajra et al. reported the past decades developments in the carbonylative Suzuki-Miyaura carbonylative coupling reactions using modern methods based on transition-metal catalysts with special emphasis to Pd. 152 Also, Vessally and coworkers reviewed recent progress in the application of NPs for carbonylative Suzuki coupling. 153 Among different systems, the immobilization of Pd species on inorganic nanoparticles present interesting catalytic activities for the studied reaction. Thus, different works reported the preparation of functionalized nano-objects to stabilize Pd(II)complexes or PdNPs. By varying arylboronic acids and aryl iodides groups, the reaction mechanisms were proposed in literature for Fe3O4/L-Pd n catalysts (where L is stabilizing agent, as ligands or polymers, n = 0, 2). 154,155 Moreover, Ma and coworkers demonstrated that Fe3O4/L-Pd(0) (L = dopamine) system exhibits much inferior catalytic performance than Fe3O4/L-Pd(II). Such catalytic behavior could be ascribed to the fact that PdNPs are prone to agglomeration reaction. To prove this conjecture, TEM analyses for Fe3O4/L-Pd 0 and Fe3O4/L-Pd II after Suzuki carbonylative cross-coupling reaction have been performed. The agglomeration was obviously observed by TEM analyses of the catalyst after one reaction. After 5 cycles, very strong aggregation of PdNPs allowed to conclude that the nanosystem based on supported Pd II species is more stable than zero-valent PdNPs. The research maturity of Pd nanocatalysts reached thus far has fostered the research on selective Suzuki-Miyaura catalysts of other transition metals at the nanometric scale such as PtNPs, 105,156 , Au NPs 157-160 and Ru NPs 107,161 and bimetallic systems thereof 107,157,158,162 with light-tunable surface plasmon reactivity. 157 However the impact of abundant first-row transition metals as key enablers towards new sustainable chemical paradigms is foreseen in the years to come. Notably, catalyst nancomposites featuring Ni NPs 163 , NixOy NPs, 164 Co NPs 165 or Co(II) complexes, 166 , or bimetallic Co-Cu systems have recently been described. 167

Dehydrogenative cross-couplings

Dehydrogenative cross-coupling (DCC) offers a unique way of C-C or C-X (X = heteroatom) bond formation reactions. Numerous reviews have been published on such reactivity catalyzed by transition metal complexes, metal nanoparticles, and metal-free systems. 168,169 The application of nanocatalysts in cross-dehydrogenative coupling reactions has been recently reviewed by Vessally and co-workers. 170 A photocatalytic DCC reaction mechanism was proposed for hybrid materials combining noble metal NPs (e.g. Pt, Pd or Au-Pd bimetallic NPs) and semiconductor metal oxides (e.g. TiO2, CeO2). [START_REF] Tyagi | Novel blended catalysts consisting of a TiO2 photocatalyst and an Al2O3 supported Pd-Au bimetallic catalyst for direct dehydrogenative cross-coupling between arenes and tetrahydrofuran[END_REF]171,172 The proposed mechanism takes place as follows: (i) semiconductor metal oxide is excited by photoirradiation to generate the hole and electron; (ii) then transferred to metal NPs; (ii) hole would activate the organic molecules (e.g. C-H bond) to generate radical and a proton; (iii) the organic radicals combine to give a final product; (iv) the protons are reduced by the electrons migrated from semiconductor to MNPs to form H2. Yoshida et al. studied the DCC reaction between toluene and acetone catalyzed by MNPs@TiO2 (where M = Pd, Pt), carrying out different experiments with the aim of understanding the process, including EPR analyses (identification of radicals), kinetic experiments, and temperature-controlled photocatalytic tests. 171 Based on these observations, two catalytic mechanisms were proposed (Fig. 19) showing the importance of the metal NPs cocatalyst nature on the selectivity of the DCC products. Thus, the high activation energy for 1-(o-tolyl)propan-2-one determined from pseudo-Arrhenius plots of the temperature-controlled photocatalytic tests, suggests the involvement of a certain nonphotocatalytic process for PdNPs@TiO2 system. The authors proposed that PdNPs promote the DCC between toluene and acetone via radical addition-elimination mechanism. On the other hand, the weak activation energy determined for PtNPs@TiO2 indicates that PtNPs contribute as an electron receiver and promote the DCC reaction between the methyl group of toluene and acetone through a radical-radical coupling. Fig. 19 Proposed mechanisms for the photocatalytic DCC between toluene and acetone catalyzed by PdNPs@TiO2 (left) or PtNPs@TiO2 (right). Reprinted from 171 with permission from Springer Science Business Media, LLC, part of Springer Nature 2019. To avoid the possible alternation of the metal co-catalyst part on the TiO2 surface, a design of blended catalyst has been proposed by the same group, consisting in a physical mixture of a TiO2 photocatalyst and MNPs supported on Al2O3. [START_REF] Tyagi | Novel blended catalysts consisting of a TiO2 photocatalyst and an Al2O3 supported Pd-Au bimetallic catalyst for direct dehydrogenative cross-coupling between arenes and tetrahydrofuran[END_REF] The advantage of this system is the possibility to finely tune the catalytic properties of two catalysts to achieve high activity. Firstly, by examining the DCC of benzene and THF with different catalysts (i.e. Pd@TiO2, Pd@Al2O3, TiO2, TiO2 + Pd@Al2O3), the authors showed that PdNPs can work more efficiently if they are not directly supported on TiO2. However, the coexistence TiO2 component is essential to promote the DCC reaction. Thus, it was demonstrated that modification of one of the catalytic components, such as the insertion of small amount of Au to Pd@Al2O3 system, increased the blended catalyst activity and selectivity for the photocatalytic direct DCC between arenes and THF. The observed behavior was related to the electron deficient Pd species in Pd δ+ -Au δ-nanoparticles, suggested by Au LIII-edge XANES analyses, which can withdraw the electron density from an adsorbed arene to accelerate the reaction with a THF radical. This step is crucial for the proposed DCC reaction mechanism (Fig. 20).

Fig. 20

Proposed mechanism for the photocatalytic DCC between arenes and THF by the blended catalyst consisting of TiO2 photocatalyst and Pd-AuNPs@Al2O3. Reproduced from [START_REF] Tyagi | Novel blended catalysts consisting of a TiO2 photocatalyst and an Al2O3 supported Pd-Au bimetallic catalyst for direct dehydrogenative cross-coupling between arenes and tetrahydrofuran[END_REF] with permission from the Royal Society of Chemistry.

Alonso's group studied CuNPs on zeolite Y to catalyze DCC of tertiary amines with terminal alkynes in the presence of tert-butylhydroperoxide (TBHP) as oxidant to yield propargylamines. 173 The authors showed that the mechanism involves radical species and the couple Cu(I)/Cu(II) (Fig. 21 Top). To demonstrate this, several experiments were carried out, one of them using a radical scavenger that inhibited the propargylamine formation (especially with TEMPO). Thus, the formation of acetone from TBHP was observed by adding 2,4-dinitrophenylhydrazine to obtain the corresponding hydrazone under the reaction conditions (70 °C, 5 h, solvent-free) even without catalyst. Concerning the catalytic material, the authors performed XPS analysis for CuNPs after catalysis Cu(2p3/2) peaks (Fig. 21 Bottom). CuNPs showed 4 peaks corresponding to Cu2O, CuO and two other satellite peaks corresponding to Cu 2+ species. After catalysis, Cu(II) species disappeared and the sample was mainly constituted of Cu2O. After oxidative treatment with TBHP, the starting CuNPs were regenerated, showing that the Cu(II) species play a crucial role in DCC. Based on these results, the following mechanism was proposed: (i) the Cu(I) phenylacetylenide complex is formed from the Cu2ONPS and phenylacetylene; (ii) in parallel, the decomposition of TBHP occurs, leading to terbutoxyl and hydroxyl radicals, and further to methyl radicals and acetone; (iii) the obtained molecules oxidize Cu2ONPs into CuONPs and then reduced with the concomitant oxidation the amine in its radical cation; (iv) H-transfer between the methyl radical and the radical cation amine give an iminium; (v) when iminium is added to the copper phenylacetylenide complex, generation of the propargylamine is observed. In a similar way, a radical process for DCC reaction between tetrahydroisoquinolines and nitroalkanes was also proposed for Cu-oxide based NPs supported on TiO2 (Fig. 22). 174 Radical traps and catalyst poisons were used to study the plausible DCC inhibition. XPS studies were performed to demonstrate the strong interaction of CuNPs with the nitro group of nitroalkanes, presumably, promoted by CuONPs. The involvement of metal oxide on the MNPs surface was observed for other catalytic systems. Thus, Loh et coworkers showed that IrO2 species on the 0.1 wt% Ir@Mg4AlO catalyst promote the kinetically important step in the DCC of indole and methanol. 175 XRD measurements of the obtained material showed neither IrO2 and not metallic iridium diffraction pattern in 0.1 wt % Ir@Mg4AlO. Similarly, XPS studies showed oxidized Ir species detected despite very weak binding-energy peak due to low Ir content. Then, to investigate the catalytic mechanism, time-dependent product evolution for indolemethanol coupling reaction was studied by GC. Different separate experiments showed that the condensations of indole with paraformaldehyde gave the 3,3′-bis(indolyl)methane (3,3′-BIM), suggesting the formaldehyde could be the intermediates in indole-methanol coupling to 3,3′-BIM transformation. It should be note that the selectivity for 3,3′-BIM produced by indole and formaldehyde by adding paraformaldehyde gradually was much higher than by adding paraformaldehyde at one time, indicating the in situ formed formaldehyde on 0.1 wt % Ir@Mg4AlO with low concentration is one of the crucial factors to allow the reaction proceeding in a selective manner. Moreover, isotope labeling study demonstrated the expected deuterium incorporation at the carbon of bridging methylene group, but with low reaction rate. This observation indicates that the dissociation of a C-H and/or O-H in methanol in the first step catalyzed by 0.1 wt % Ir@Mg4AlO is involved as a kinetically important step (Fig. 23). Characterization and a systematic study of the influence of RuNPs with different oxidation on their catalytic activity in DCC reaction between tetrahydroisoquinolines and nucleophiles (e.g. indole or nitromethane) via single-electron transfer were performed by Yan and coworkers (Fig. 24). 176 EPR analyses of RuNPs with different content of Ru(III) species were carried out. Samples with a higher content of Ru(III) present the higher catalytic activity due to the generation of superoxo-or peroxolike species (*O2 •-). The content of Ru(III), as well as the oxidation states of Ru and RuO2 NPs have been investigated by XAFS and XPS. The obtained data showed that Ru(0) was present in RuNPs centers, and in the interface, the oxidized Ru triggered the DCC reaction. Based on these data, a heterogeneous catalytic mechanism was proposed, as well as volcano shaped correlation between the oxidation state of the NPs surface and their catalytic activity was established.

Fig. 24 DCC mechanism between tetrahydroisoquinolines and nucleophiles catalyzed by RuNPs. Reproduced from 176 with permission from the Royal Society of Chemistry.

C-heteroatom cross-couplings

Carbon-oxygen and carbon-nitrogen bonds are present in a wide variety of compounds, particularly in biologically active molecules. [177][178][179] These organic compounds are often synthesized through efficient and low-cost catalytic coupling reactions by the formation of C-X bonds. 180,181 The reactivity comprehension permits the design of selective catalysts and thus leads to sustainable processes. 182

C-O bond formation reactions

Friend's group studied the synthesis of esters by vapor-phase of methanol with primary alcohols (such as ethanol and n-butanol) catalyzed by unsupported AuNPs (mean diameter <2 nm). 183 The esterification occurred through activation of alcohols by adsorbed oxygen atoms present on AuNPs by a mechanism involving alkoxy and hemiacetal intermediates. Au(111) surface was oxidized with ozone prior to the reaction with the corresponding alcohols. The slower rate of -C-H dissociation for methoxy group than that for ethoxy or butoxy groups led to a high selectivity for cross-couplings, efficiently competing with homo-couplings. The mechanism of the coupling reaction of ethanol with methanol was proposed based on different experimental data (Fig. 25): product distribution analysis, isotopic labelling studies showing that the -hydride elimination occurs from the ethoxy (and not from methoxy), high resolution electron energy loss (HREEL) analyses showing the vibrational mode of methoxy-d0 and ethoxy-d5 onto O/Au(111) surface, and temperature programmed heating studies. A similar mechanism was proposed for the coupling of aldehydes with methanol. 184 This mechanism was also studied for the oxidative cross-coupling between methanol and unsaturated alcohols (such as methallyl alcohol or allyl alcohol) on oxygen-activated Au(110) surface to give the corresponding methacrylate derivatives. 185 Authors evidenced that the presence of C=C bond increases the binding efficiency and the methyl group triggers an additional van der Waals interaction at the metal surface. N-doped CoNPs (2.55 nm) supported on active carbon were applied to cross-couplings between substituted benzaldehydes and methanol under oxygen flow. 186 CoNPs were prepared by pyrolysis under N2, from Co(NO3)2 adsorbed on active carbon with a melamine-formaldehyde resin, followed by H2SO4 treatment. XPS analyses evidenced a Co-N-pyridine interaction based on the positive shift of the N1s binding energy in relation to the metal-free support (398.1 eV vs 398.9 eV, respectively). Therefore, authors suggested that the activation of oxygen was promoted by the Co-N coordination sites; the pyridinic nitrogen atoms act as base promoting the deprotonation of methanol. Kinetic studies showed that the presence of hindered substituents on the aldehyde in ortho-position affects the reaction rate more than electronic effects, proving that the nucleophilic attack of the methoxy group to the carbonyl is the rate limiting step. Shimizu and coworkers reported the synthesis of amides through hydration of nitriles, catalyzed by PdNPs@carbon. 187 The catalytic material (PdNPs exhibiting a mean size of 6.5 nm) was prepared by exposure of commercial Pd/C under H2 at 500 °C followed by cooling to room temperature under He atmosphere. PXRD and EXAFS analyses showed that PdNPs were constituted of Pd(0). However, XPS analysis revealed also the presence of Pd(II) species at the surface of PdNPs. This as-prepared catalytic material was used for the hydration of nitriles in water. The presence of adsorbed oxygen atoms (acting as Bronsted base for the dissociation of water via hydrogen bond) and Pd(0) sites (acting as Lewis acid activating the nitrile moiety) led to a high catalytic activity (Fig. 26). The carbon-based support also showed a better adsorption ability in comparison to inorganic oxides (such as alumina or zircona) of methylacetates due to its higher hydrophobic nature. The kinetic isotope effect (KIE) of the acetonitrile hydration in H2O and D2O was relatively low (KIE = 1.2), indicating that the dissociation of water has an influence on the reaction rate but is not the determining step. The Hammet parameter was determined for para-substituted benzonitriles; the positive slope value indicated a negative charged transition state for the rate determining step corresponding to the nucleophilic addition of OH  species to the carbon of the nitrile group. DTF studies corroborated these experimental data. Ullmann-type couplings between haloarenes and phenol derivatives catalyzed by bimetallic spinelnano-materials, in particular M-based ferrites (MFe2O4, where M = Cu, 188 Ni, 189,190 ,Co 190 ), were highly active and selective towards the formation of aromatic ethers, being recyclable catalytic systems. Control tests evidenced that the coupling process proceeded through oxidative addition of the haloarene facilitated by the high surface of NPs and their easier electron transfer, discarding a nucleophilic substitution; furthermore, these couplings were not promoted by radical intermediates. A synergetic effect between M and iron in these ferrites seems likely by electronic transfer between M(II) and Fe(III). Decarboxylation processes of fatty acids to give long-chain alkenes or alkanes exhibit a great interest to substitute non-renewable fossil fuel resources. 191,192 Both experimental and theoretical studies on carboxylic acids indicate that the decarboxylation occurs through an initial activation and dehydrogenation of C-H bonds in 2 and 3 positions (related to COOH function) before producing the C-O or C-C bond scission. In particular, AuNPs have proven their efficiency in many oxidation reactions. [193][194][195] From a mechanistic point of view, Yates and coworkers studied the oxidative dehydrogenation of acetic, propionic and butyric acids promoted by AuNPs (ca. 3 nm ) supported on TiO2 to form Au2C=C=O ketenylidene species (Fig. 27). 196 In-situ FT-IR analysis confirmed that carboxylates were adsorbed on TiO2 surface after deprotonation of the COOH function. Then O2 dissociates at the dual Au-Ti(IV) sites giving O ad atoms on Au surface, which act as Lewis base for the further C-H bond activation. FT-IR analysis also proved the formation of Au2C=C=O. Experiments run under 18 O2 (no Au2C=C= 18 O species was observed) or under vacuum (no formation of Au2C=C=O in the absence of oxygen) indicate that gold ketenylidene species are formed firstly by activation of the C-H bonds followed by the C-O bond activation. The absence of kinetic isotopic effects with deuterated propionic and butyric acids reveals that the C-H bond cleavage is not the rate determining step and that the dehydrogenation step occurs before the C-O (rate determining step) and the C-C bond cleavage.

C-N bond formation reactions

Che and coworkers applied AuNPs (ca. 14.5 nm) supported on graphite in the oxidation of different organic compounds under oxygen, in particular the formation of N-(benzylidene)benzylamines benzylamines from benzylamines. 197 In contrast to Ru-based catalysts which preferentially lead to the formation of nitriles, 198 AuNPs@graphite gave selectively N-(benzylidene)benzylamines. Based on several studies, such as radical trap tests, Hammet plot and kinetics experiments, authors proposed a mechanism including a hydrogen-transfer process from the benzylamine to the metal and an oxidation step involving metal hydrides at the AuNPs surface (Fig. 28). Thus, the first step corresponds to the amine coordination at Au + surface sites. The participation of positively-charged intermediates was inferred from the negative slope of the Hammett plot (parameter) for the oxidation of para-substituted benzylamines. Then, C-H bond cleavage takes place at the benzylic position, being the rate determining step deduced from kinetic isotopic effects. A hydride shift occurs at the AuNPs surface and then O2 acts as an H acceptor giving H2O2 (not detected due to the high temperature of the reaction, 110 °C). This role of H-acceptor of oxygen was confirmed by the control reaction (oxidation of dibenzylamine to N-(benzylidene)benzylamine in presence of H2O2 under Ar atmosphere, but exhibiting a lower yield). The presence 2,6-di-tert-butyl-4-methylphenol as radical scavenger did not perturb the reaction.

Beller and coworkers described the synthesis of sulfonamides catalyzed by RuNPs supported on magnetic Fe3O4, through an eco-friendly approach consisting of direct coupling between unfunctionalized sulfonamides and alcohols. 199 A dehydrogenation-condensation-hydrogenation tandem process took place. Kinetic isotopic effect was observed for the competitive reactions of benzylic alcohol-d7 and benzylic alcohol with p-toluensulfonamide, indicating that the alcohol dehydrogenation was the rate determining step (kH/kD = 2.86 for the benzyl alcohol dehydrogenation and kH/kD = 0.74 for the hydrogenation of N-benzylidene-p-toluenesulfonamide) (Scheme 1).

Scheme 1 dehydrogenation-condensation-hydrogenation tandem process catalyzed by RuNPs supported on magnetic Fe3O4.

Garcia and coworkers reported iron and cobalt based nanocatalysts, supported on a graphitic carbon matrix, including FeCo alloys (sizes in the range 10.9 -26.5 nm), to be applied in C-N oxidative coupling of aromatic N-H and amide compounds. 200 FeNPs@C (mainly constituted of Fe(0) (XRD analysis) with the presence of Fe(II) and Fe(III) at the metallic surface proven by XPS) appeared as the most active and recyclable system, for which some mechanistic insights were carried out, mainly focused on the characterization of the organic intermediates. These studies showed that the amide radical is the key intermediate of the process, while FeNPs are the responsible to form the first radicals by TBHP degradation (Fig. 29). Fig. 29. C-N oxidative coupling of aromatic N-H and amide compounds catalyzed by FeNPs@C.

A 3 coupling

Three components tandem reaction of Aldehyde, Alkyne and Amine, commonly called A 3 coupling, has received a lot of attention in recent years. [201][202][203] Indeed, it leads to the synthesis of propargylamines and their derivatives, useful precursors in the development of different organic substrates, natural products, and drugs. 204 Over the last decades, different catalytic systems have been investigated for convenient preparation of propargylamines under green conditions. Thus, homogeneous catalytic systems based on transitionmetal salts have been reported, presenting high reactivity and selectivity. 201 In the heterogeneous catalyzed methodology, a large number of A 3 couplings catalyzed by MNPs (e.g. Cu, Au, Ag, Ni, Fe, Co and Zn) have been reported, however the research of their catalytic mechanism remains limited. 205,206 Generally, mechanisms based on the formation of well-known π complexes between metals and alkynes (R-C≡C-M) are proposed, which then react with the iminium ion formed by condensantion between the aldehyde and the amine, resulting in the production of the corresponding propargylamine and regeneration of the metal catalyst. However, when CuNPs synthesized in glycerol were used, the mechanism appeared to be different, as evidenced by spectroscopic means (Fig. 30). 207 Actually, FT-IR studies proved a weak  bonding interaction between phenylacetylene and the metal surface [broad band showing a   -30 cm -1 in relation to free phenylacetylene: 2108 (free Ph-C≡C-Ph) vs 2070 (capping Ph-C≡C-Ph) cm -1 ; for phenylethynylcopper(I), the C≡C stretching appeared as a narrow absorption band at 1927 cm -1 ), in accordance with that observed for RuNPs. 208 Moreover, UV-vis analyses showed that the surface plasmon resonance absorption band of CuNPs was not affected by the presence of phenylacetylene (578 nm), being larger most likely due to the capping effect triggerend by the absoprption of reagents at the metal surfaced; after addition of morpholine, an extra absorption band appeared, attributed to a M-L charge transfer transition. This behavior is in clear contrast with that observed with phenylethynylcopper(I) where two absorption bands were observed due to a M-L charge transfer band and an intramolecular  * ligand band; in the presence of morpholine, free phenylacetylene was released and Cu(II) species were formed. These spectroscopic data point out a surface-like reactivity, proving the absence of molecular copper species.

Oxyhalogenation reactions

Catalytic methane oxyhalogenation is the process involving the C-H bond activation of methane with hydrogen halide and oxygen, leading to the generation of hydroalkanes and water. It permits the methane functionalization with HX recovery. 209,210 This methodology appears particularly interesting for the selective transformation of methane and light hydrocarbons (C2-C4), which represents an important current challenge. Ideally, the catalyst must simultaneously activate the three reagents, precluding alkyl halide oxidation and further halogenation. Pérez-Ramirez and co-workers have recently reported the methane oxychlorination and oxybromination catalyzed by noble nanoparticles supported on SiO2 (mean diameter of MNPs in the range of 6-18 nm), such as Ru-, Pt-, Ir-, Rh-and Pdbased NPs, being RuO2 NPs the most active catalytic system (Fig. 31). 211 Authors evidenced the transformation of the metal oxide nanoparticles into metal silicide, metal, and halogenated phases during the reaction (PXRD and Raman analyses), proving the high reactivity shown by the catalyst surfaces. Operando photoelectron photoion coincidence spectroscopy (PEPICO) analyses carried out over Pd-based NPs@SiO2, proved that the catalyst promotes the formation of radical halide species, which are the responsible of the methane activation in the gas phase; activation at the catalyst surface cannot be ruled out.

Fischer-Tropsch reaction

Fischer-Tropsch reaction (FT) is a process which converts syngas, a mixture of H2 and CO, into hydrocarbons mainly for fuel applications but also for the synthesis of valuable chemicals. [212][213][214] Fe, Co, Ni and Ru based NPs are commonly applied for this transformation.

Regarding FT mechanism, it is accepted that the process follows a step-wise carbon chain growth, after an initial activation of both H2 and CO at the metal surface of the nanocatalyst. Numerous research groups studied the chain growth processes. In the scope of this review, the interaction between reactants and MNPs surface is highlighted. The reaction mechanism for FT synthesis with iron catalysts has been reported by B. Devis. 215 One of the difficulties to study this mechanism is related to the ability of metals to transform into their corresponding oxides or carbides. The role of carbides and their performance in FT is still not totally clear and represent a goal of current studies. 216,217 In 2016, Chaudret and coworkers proposed an elegant approach using labelled [START_REF] Burda | Chemistry and Properties of Nanocrystals of Different Shapes[END_REF] CO to monitor the reaction by gas-phase NMR spectroscopy and mass spectrometry. 218 It was demonstrated that iron carbide NPs can be labeled with [START_REF] Burda | Chemistry and Properties of Nanocrystals of Different Shapes[END_REF] C to study their role in the CO hydrogenation. Heating of the as-prepared nanomaterial at 250 °C under H2 led to the carbon desorption and exclusive formation of methane. When the reaction was performed at 210 °C, a mixture of unlabeled hydrocarbons was observed, suggesting the absence of exchange between carbide C atoms at the metal surface and CO in the gas phase. Contrarily, the presence of [START_REF] Burda | Chemistry and Properties of Nanocrystals of Different Shapes[END_REF] CH4 was demonstrated at 250 °C, confirming that FT process goes through a surface carbide mechanism (Fig. 32). The same group also performed synchrotron-based high-resolution XPS to determine both the nature and concentration of the CxHy surface intermediates on fcc-CoNPs surface. 221 The experiments showed that CO promotes the transformation of C2Hx adsorbates into ethylidyne and its further dimerization (Fig. 34) Martínez and coworkers investigated the influence of Ru concentration in bimetallic CoRuNPs@TiO2 (10 wt% of Co and 0.1-1.2 wt% of Ru) on FT synthesis. 223 The FT activity in terms of metal-time-yield and initial turnover frequency was analyzed for both Ru-free and Ru-doped catalytic materials. It was observed that both catalytic parameters were maximized for catalysts with 0.1-0.2 wt% Ru. To understand this behavior, in situ characterizations were performed. FT-IR experiments under flowing syngas at 220 °C and 1 bar showed the CoNPs surface reconstruction, creating more defected Co(0) with the highest Ru content (1.2 wt % of Ru). Unfortunately, the lack of surface reconstruction was observed for Ru free Co@TiO2 system, for which linear CO-Co 0 IR vibration band stays unchanged at 220 °C. Thus, the lower amount of Co(0) sites in Co@TiO2 system could be at the origin of its lower FT activity in comparison to CoRu@TiO2. Furthermore, in situ XPS analysis revealed that the CoNPs surface remained zero-valent during FT reaction when it was catalyzed by CoRu@TiO2 with low Ru loading (0.1-0.2 wt%), while oxide-like cobalt nanoparticles (CoOX) were detected for samples with Ru content higher than 0.2 wt%. Such effect could arise from a partial electron charge transfer from Co(0) to the more electronegative Ru(0), favoring the adsorption of O atoms formed upon CO dissociation on positively charged Co δ+ at high Ru content (Fig. 35). In the absence of Ru, Auger spectrum of Co@TiO2 showed the shoulder at 779.5 eV which was attributed to the formation of cobalt carbide species during the FT reaction. Based on these experimental results, authors concluded that the FT activity for CoRu@TiO2 catalytic materials is limited by the formation of CoCX (in the absence of Ru promoter) and the presence of CoOX (at Ru loading above 0.2 wt%) (Fig. 36).

Conclusions and perspectives

This review focused on coupling reactions catalyzed by preformed (un)supported metal-based nanoparticles, including mechanistic studies in particular those that highlight substrate-metal interactions at the surface of the nanocatalysts. These transformations often work under complex conditions, involving several reagents, solvents and/or additives, making mechanistic studies difficult both at experimental and computational level. Design of appropriate experiments together with analysis by cutting-edge techniques permit to evidence specific metal-reagent interactions at the surface of nanoparticles. The surface activation pathways via synergistic effects between nanoparticles and supports, taking into account reaction kinetics are key to the development of orthogonal transformations. Thus far, Glaser-Hay heterocouplings for the preparation of unsymmetrical 1,3-diynes has been achieved with nanocatalyst composites of small Cu/Cu2O NPs on covalent organic frameworks. 224 In addition, the exploitation of novel activation modes by means of surface plasmon resonance effects, magnetic properties, and light harvesting capabilities, among others, brings forth new opportunities for tailormade catalytic nanomaterials. From a mechanistic point of view, the advances on characterization tools, including operando techniques in order to allow reaction monitoring, signify a major challenge in this field.

Fig. 1

 1 Fig. 1 STEM images showing the progression of the Ullmann coupling on Cu(111) (imaging at 5K): a) Bromobenzene on Cu(111) annealed to 80 K; insets corresponding to high-resolution images of the clusters (3 and 5 molecules); b) clusters of the Ullmann organometallic intermediates and bromo atoms at the copper surface after annealing at 160 K; c) formation of biphenyl after annealing at 350 K; insets corresponding to highresolution images of the organometallic Ph-Cu-Ph intermediate and biphenyl product. Reproduced from 40 with permission from the Royal Society of Chemistry.

Fig. 4

 4 Fig. 4 Plausible mechanism of the formation of aryl methyl ketone and biphenyl catalyzed by PdNPs@Fe-doped La2O3. Reproduced from 46 with permission of John Wiley and Sons and Copyright Clearance Center (license number: 5147050704036) 2018.

Fig. 7 a

 7 Fig. 7 a) Schematic representation of the transformation of Cu-based species; b-d) Proposed mechanisms depending on the size of the catalytic material: b) CuOx/TiO2-0.25%; c) CuOx/TiO2-2.5%; d) CuOx/TiO2-10%.Reprinted with permission from[START_REF] Liu | Facile Synthesis of Surface-Clean Monodispersed CuOx Nanoparticles and Their Catalytic Properties for Oxidative Coupling of Alkynes[END_REF] . Copyright 2016 American Chemical Society.

Fig. 8

 8 Fig. 8 Plausible mechanism for Glaser homo-coupling catalyzed by Cu/Cu2O NPs@rGO. Reproduced from 76 with permission of Elsevier and Copyright Clearance Center (license number: 5147061417629) 2019.

Fig. 9

 9 Fig. 9 Interaction of phenylacetylenyl radical with Au(0) and cationic gold (both Au + and Au + ) sites on isolated AuNPs. Relationship between calculated energies (data in brackets, kcal/mol) and atomic charge on gold atoms. Metal-carbon distances in Å. Reproduced from 70 with permission of Elsevier and Copyright Clearance Center (license number: 5147070651959) 2014.

82

 82

Fig. 10

 10 Fig. 10 Schematic representation of Guerbet reaction. Reprinted with permission from 81 . Copyright 2016 American Chemical Society.

Fig. 11

 11 Fig. 11 Plausible mechanism for Sonogashira cross-coupling catalyzed by PdCu bimetallic nanoparticles. Reproduced from 90 with permission from the Royal Society of Chemistry.

Fig. 12 .

 12 Fig. 12. Plausible reaction manifolds operating in PdNPs catalyzed Suzuki-Miyaura cross-coupling reactions. Reproduced from 103 with permission from the Royal Society of Chemistry, license ID 1149708-1.

Fig. 13

 13 Fig. 13 TEM images and statistical distributions of a, b) preformed PdNPs stabilized with PVP; c, d) after refluxing in sodium acetate; e, f) after refluxing in the presence of sodium acetate and phenylboronic acid; g, h) after refluxing in the presence of NaOAc and iodobenzene. Adapted with permission from 104 . Copyright 2003 American Chemical Society.

e

  Fig. 14 (a, b, c and d) TEM micrographs of PdNPs stabilized by PVP of the following sizes: 1.8, 2.6, 3.1 and 4.0 nm, respectively, with a high resolution micrograph representative of the surface geometry of a 1.8 nm; (e) Operando Pd K-edge XAS fits for Suzuki-Miyaura reaction monitoring of iodoanisole and phenylboronic acid with 1.8 nm PdNPs stabilized by PVP. Inset represents normalized, time-dependent EXAFS spectra with real/simulated radial distribution functions for different Pd cuboctahedra; (f) Plot of TOFs versus the total (•) or surface atom densities with defects (○) of the largest nanoparticle evidencing the structure-sensitive Suzuki-Miyaura coupling reaction of iodoanisole and phenyboronic acid over PdNPs; (g) Reaction profiles for Suzuki-Miyaura cross-coupling of iodoanisole and phenylboronic acid catalyzed by the as-prepared PdNPs systems. Reproduced from 103 with permission from the Royal Society of Chemistry, license ID 1149708-1.

Fig. 15

 15 Fig. 15 HAADF-STEM image of bimetallic Pd0.5Ru0.5 NPs stabilized with PVP after three runs of Suzuki-Miyaura coupling reaction and TEM-EDS analyses of the area framed with a red line. a) HAADF-STEM image; b-d) EDS mapping for Pd (green), Ru (red) and its overlay showing the co-localization of both metals in yellow; e) linescan profile along the arrow in (a) Pd (green) and Ru (red). Reproduced from 107 with permission of John Wiley and Sons and Copyright Clearance Center (license number: 5153740025431) 2015.

Fig. 16

 16 Fig. 16 Plausible mechanism for the Suzuki-Miyaura coupling catalyzed by a bimetallic Pd0.5Ru0.5 NPs system with PVP as stabilizing agent. Reproduced from 107 with permission of John Wiley and Sons and Copyright Clearance Center (license number: 5153740025431) 2015.

Fig. 17

 17 Fig.17 Reaction pathway for oxidative addition of bromobenzoic acid on a Free Pd13 cluster, and supported Pdn clusters on defected graphene, n=4, 13, and 14. Reproduced from 111 with permission from the Royal Society of Chemistry, license ID 1148395-1.

Fig. 21 (

 21 Fig. 21 (Top) DCC reaction mechanism between N,N-dimethylaniline and phenylacetylene catalyzed by CuNPs@ZY in the presence of TBHP. (Bottom) XPS spectra of CuNPs@ZY at the Cu 2p3/2 level at the end of the DCC (a) and after reoxidation (b). Reproduced from 173 . Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 22

 22 Fig. 22 DCC reaction mechanism between tetrahydroisoquinolines and nitroalkanes catalyzed by CuxONPs@TiO2 (x = 1, 2) Reproduced from 174 . Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 23

 23 Fig. 23 Schematic representation of DCC reaction mechanism between indole and methanol catalyzed by 0.1 wt % Ir@Mg4AlO. Reprinted with permission from 175 . Copyright 2019 American Chemical Society.

Fig. 25

 25 Fig. 25 Mechanism of Allyl Alcohol Coupling with Methanol on oxygen-activated Au(110) surface. Reprinted with permission from 185 . Copyright 2016 American Chemical Society

Fig. 26

 26 Fig. 26 Reaction mechanism hydration of acetonitrile (computed relative energies in kJ mol -1 ) on the Pd(111) surface covered with the surface oxygen atom. Reprinted with permission from 187 . Copyright 2012 American Chemical Society

Fig. 27

 27 Fig. 27 Partial oxidation of acetic, propionic, and butyric carboxylic acids on Au/TiO2 catalysts. Reprinted with permission from 196 . Copyright 2014 American Chemical Society

Fig. 28

 28 Fig. 28 Mechanism of Aerobic Oxidation of Benzylic Amines to Imines involving AuNPs@graphite as catalyst. Reprinted from 197 . Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 30 (

 30 Fig. 30 (Top) Synthesis of propargylamine catalyzed by CuNPs in glycerol. (Bottom) FT-IR (a) and UV-vis (b) spectra corresponding to the experiments of CuNPs, phenylacetylene and morpholine in glycerol. Reproduced from 207 . Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 31

 31 Fig. 31 SiO2-supported catalysts structure-performance relationships for methane oxychlorination (MOC left) and oxybromination (MOB right). The left scale indicates catalyst reactivity: low (light gray), medium (gray), high (black) for methane oxyhalogenation at 753 K. The right scale indicates the total selectivity to halomethanes at 753 K and at ca. 20% methane conversion, corresponding to the mark on diamond map. The lines arising from the symbols point to the phases identified by X-ray diffraction and Raman analysis which vary from metal halides (top), to metallic (center), metal silicides (left and right), and metal oxides (bottom). Reprinted with permission from 211 . Copyright 2019 American Chemical Society

Fig. 32

 32 Fig. 32 Reactions investigated for iron carbide NPs. Reproduced from 218 . Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 33 FT

 33 Fig.33FT-IR spectra of an adsorbed acetylene + 2 H layer on CoNPs at 250 K in vacuum (blue) and in the presence of 1 × 10 -6 mbar CO (red). Reprinted from 221 with permission from Elsevier.

Fig. 34 (

 34 Fig. 34 (a) High resolution XPS spectra after heating ethylene-saturated Co(0001)NPs under vacuum conditions at the indicated temperatures; (b) Heat map of C1s XPS spectra carried out during the heating of ethylenecovered CoNPs under vacuum conditions; (c) Heat map of C1s XPS spectra carried out during the heating of C2H2ad/2Had-covered surface in the presence of CO (1×10 -7 mbar); (d) High resolution XPS spectra after heating C2H2ad/2Had in CO at the indicated temperatures. Note: for b and c, h = 380 eV at 7 K for each spectrum; blue means low intensity; red, high intensity (heating rate of 0.2K s -1 ). Reprinted from 222 with permission from Springer Nature.

Fig. 35

 35 Fig. 35 Schematic representation of the formation of Co-O species on positively charged surface Co atoms in CoRu@TiO2 with Ru content was higher than 0.2 wt%. Reprinted with permission from 223 . Copyright 2020 American Chemical Society.

Fig. 36

 36 Fig. 36 Schematic representation of the influence of Ru content on the nature of surface cobalt species in Co-Ru@TiO2 under FT conditions, showing the formation of Co-C in the absence of Ru and of Co-O at high Ru concentration >0.2 wt %. Reprinted with permission from 223 . Copyright 2020 American Chemical Society.

f g

Acknowledgments References

Acknowledgments

The Centre National de la Recherche Scientifique (CNRS) and the Université Toulouse 3 -Paul Sabatier are gratefully acknowledged for their financial support. Authors thank to POCTEFA Interreg program for funding this work (EFA308/19 TRIPyr).