These De L'universite De Lyon Délivrée Par L'universite

Claude Bernard Lyon

Ecole Doctorale

M Pierre

Directeur De

M Christophe Copéret

M Christophe

Coperet M Stéphane Bellemin-Laponnaz

M Stéphane

Daniele M Sébastien

Donet M Nicolas

Mezailles Mme

Thieuleux M Chloé

Weiss François

Rapporteurs

M Stéphane Bellemin-Laponnaz

M Nicolas

Mezailles N° D'ordre Année

Pierre Laurent

Mézailles Mme

Mézailles Universite

Claude Bernard -Lyon

Secrétaire Général

M François-Noël

Gilly M Le Professeur

Hamda Ben

Hadid M Le Professeur

Philippe Lalle

M Le Professeur

Germain Gillet

M Alain Helleu

Composantes Sante

J Etienne

G Kirkorian

D Bourgeois

La Mme

C Professeure

Vinciguerra

Y Matillon

P Farge

François Bayard

Michel Jouve

Titi Krebs

Aurélie Vandeneynde

N° d'ordre Année 2012

1 aboutissant à l'ouverture de la plateforme « Nanochimie » dans les locaux

COMPOSANTES ET DEPARTEMENTS DE SCIENCES ET TECHNOLOGIE

one dimensional, two dimensional and three dimensional

Introduction

The synthesis of nanoparticles for catalysis applications has now a long history of research at an academic as well as at an industrial level but new synthesis procedures and better understanding of parameters that control the growth and the composition of metal nanoparticles are still of major interest today. Pt and Pd nanoparticles are among the best catalysts for numerous reactions from refining in the petroleum industry [START_REF] Curry | [END_REF] to fine chemistry 2 and biomedical applications. 3 In coherence with research programs currently pursued at the CEA Grenoble, we will mainly focus on two potential applications for such nanoparticles, car exhaust depollution and Polymer Electrolyte Membrane (PEM) fuel cells. The synthesis methods will then be described, emphasizing onto Metal Organic Chemical Vapor Deposition, Surface Organometallic chemistry (SOMC) and the colloidal approach.

Platinum and palladium based nanoparticles for catalysis 1.2.1. Car exhaust depollution

Pollution from car exhausts is a major concern and European programs (Euro 1 to Euro 6) are implemented every four years since 1993 with increase constraints concerning the nitrogen oxides (NO x ), carbon monoxide (CO) and hydrocarbons (HC) emissions thus pushing research and innovation towards more efficient catalysts.

Rhodium is the catalyst of choice for the NOx reduction 4 into N 2 , whereas Pt and Pd are used for the catalytic oxidation of CO and HC into CO 2 . This Rh/Pt/Pd system is referred as threeway catalyst (TWC) used in car's catalytic converters. Nanoparticles are supported onto a high surface area cordierite monolith with a honeycomb like structure and embedded into a washcoat with alumina as the principal component. Ceria is added to stabilize nanoparticles through the so-called Strong Metal Support Interaction (SMSI) 5 and to increase the thermal stability of the support. 6 It is moreover added to promote the low temperature water-gas shift reaction (WGSR) and to store oxygen under lean (fuel deficient) conditions. 7 The oxygen storage capacity comes from its unique property as it can be reversibly reduced from the Ce 4+ stable form to Ce 3+ , forming oxygen vacancies (bulk and surfacesee Equation 1). Upon thermal treatment under reducing atmosphere ceria thus forms non-stoichiometric oxides often described as CeO (2-x) with 0 ≤ x ≤ 0.5. This phenomenon and its influence over the catalytic properties of three-way catalysts has been widely studied. [8][9][10] This is of importance to keep an air to fuel ratio ( ) at a fixed value regardless the motor regime, rich ( < 14.6) or lean conditions ( > 14.6), this ratio allowing the best catalytic performances for both reduction and oxidation reactions (Figure 1). Nanoparticles have sizes between two and more than 20 nm particles (Figure 2). Moreover ceria was found to promote the redispersion of aggregated nanoparticles, this happened through atomic migration rather than particle coalescence/splitting. 12 Under oxidative conditions, oxygen absorb onto Pt particles, some mobile Pt oxide species migrates and are trapped onto ceria through the strong metal support interaction. Under reducing conditions, the Pt-O-Ce bonds are broken and new small Pt particles are formed.

Finally note that industrially, the catalysts are prepared by impregnation of cordierite monolith by sol-gel containing metal salts. This allows the formation of both the washcoat and metallic nanoparticles under subsequent thermal treatments; a major drawback of this method is that embedded nanoparticles are not all accessible for catalysis, generating poor noble-metal utilization yields.

Fuel-cell 1.2.2.1. Fuel-cell fundamentals

Fuel-cells convert chemical energy into electricity through redox reactions. The oxidation of the fuel at the anode leads to the formation of cation (positively charge) and electrons. A specifically designed electrolyte allows for the transfer of the cation but not of the electrons, these are driven through an electrical circuit and produce electricity. These electrons are recombined at the cathode with the cation and following a reduction reaction (most often with O 2 ) forms the byproduct of the fuel cell. The most common work-up is using H 2 as a fuel and atmospheric O 2 as the oxidant, the only by-product being water. Protons are produced at the anode and conducted to the cathode through a Polymer Electrolyte Membrane (Figure 3). This polymer is very often Nafion (produced by DuPont), a proton conductive polymer grafted with sulfonic acid functions. A schematic representation is presented in Figure 3.The reactions involved are the following:

At the anode (Hydrogen Oxydation Reaction -HOR): H 2 → 2 H + + 2 e - At the cathode (Oxygen Reduction Reaction -ORR):

½ O 2 + 2 H + + 2 e -→ H 2 O
Overall reaction:

H 2 + ½ O 2 → H 2 O
Note here that PEM fuel cell can use methanol too as a fuel (Direct Methanol Fuel Cell). This technology is actually under intense research [START_REF] Olah | Beyond Oil and Gas: The Methanol Economy[END_REF][START_REF] Dillon | [END_REF] as methanol would be safer and easier to handle than H 2 .

Figure 3. Schematic of a PEM fuel-cell H 2 /O 2 , reproduced from ref [START_REF] Services | L. Fuel cell handbook[END_REF] The theoretical potential of such system is 1.23 V vs. the Hydrogen Reference Electrode (HRE) but in practice this potential is limited by various factors such as gas diffusion, water transportation and electrical resistivity. The polarization curve (Figure 4) presents 3 main areas:

1) the kinetic is limited by the activation energy of the electrochemical reactions 2) the polarization curve is linear (U = R.I), this stems from the ohmic losses due the addition of various resistivity from the fuel cell set-up (electrolyte, interfaces)

3) diffusion limitations hinder the fuel cell efficiency Fuel cells are thus operated in the area 2, most often around 0.7 V.

Fuel cell catalysts

H 2 oxidation is fast while O 2 reduction is slow. The ORR is thus the limiting reaction of any fuel cell. Fuel cells are nowadays available for a wide range of stationary applications such emergency power systems to transportation application like public buses. The latter application suffers, among other issues, of the catalyst price. Indeed platinum is the best catalyst so far but its price tripled in the last 10 years from 13 to 38 €/g (with maximum up to 51 €/g); this is moreover a scarce resource. [START_REF] Sealy | [END_REF] Targets fixed by the American Department of Energy (DOE) are widely used for comparison purposes (Figure 5). Today the catalyst represents about 25% of the total price of the fuel-cell systems. The goal is thus to lower the content of platinum down to 0.15 mg.cm -2 (today's best performance have been achieved down to 0.4 mg.cm -2 ) with efficiency of 0.125 g.kW -1 in order to meet those targets (100 kW for a car at a 30 $.kW -1 price to be competitive against internal combustion motors -30 $.kW -1 as well). 17 Lowering the Pt-loading for the anode (H 2 feeding) has been achieved 18 down to 0.05 mg Pt /cm -2 due to the large activity of Pt toward the hydrogen oxidation reaction. Another issue at the anode is to avoid pollution of the catalyst, indeed CO is found in H 2 as a pollutant and is a strong poison for platinum catalyst. It strongly binds onto the Pt surface blocking the active sites access. Alloying Pt with other metals such as Ir gives satisfying results. 19 For the cathode (O 2 feeding), two strategies are pursued: i) the optimization of the electrode structure in order to increase the platinum utilization yield and limit the voltage loss and ii) the synthesis of more active Pt based alloys to improve the Pt mass activity (expressed in A.mg -1 Pt ). 20 Pt has been alloyed with many other metals such as Fe, Ni, Co 21 , Au 22 and Pd 23,24 for enhanced catalytic activities.

Moreover, the size of the nanoparticles is of importance as demonstrated by various studies, most agree on a size comprised between 2 and 3 nm for enhanced performances (Figure 6). 25,26 Finally, the shape is of prior interest as Pt facet do not all demonstrate the same catalytic properties towards the ORR. 27 Figure 6. a) ORR current densities and b) mass activities at 0.9 V vs. RHE as a function of the Pt nanoparticles size. O 2 saturated 0.1 M HClO 4 solution at 23°C, 1600 rpm and 50 mV.s -1 . Reproduced from ref 26 Note that replacing Pt by non-noble metal catalyst is a hot research field, for example hydrogenase like catalyst 28 (containing Fe and/or Ni) can be used for the HOR and nitrogen dopped carbon nanotubes 29 or heteroatomic polymers 30 have proved to be catalytically active for the ORR. But despite those effort Pt is still the best catalyst so far and will certainly remain essential on a short to medium time scale. [START_REF] Sealy | [END_REF] Efficient, versatile and easily scalable synthesis procedures are thus required.

Synthesis of supported nanoparticles

The synthesis of supported nanoparticles can be carried out using various methods, the most common [START_REF] Ertl | Editors Preparation of Solid Catalysts[END_REF] are i) salt impregnation by Incipient Wetness Impregnation (IWI), ii) ion exchange, iii) sol gel process with the addition of metal precursors, after various thermal treatments, nanoparticles are trapped into the matrix. Those methods are mature and are used for the preparation of large scale industrial catalysts. We will here focus on two methods that are less employed, Chemical Vapor Deposition (CVD) and the controlled decomposition of oxide grafted metallic complexes (also called surface organometallic chemistry -SOMC).

Metal Organic Chemical Vapor Deposition (MOCVD)

Metal Organic Chemical Vapor Deposition (MOCVD) is a technology initially dedicated to the deposition of metallic films onto 2D substrates. It is nowadays a mature process widely used in the microelectronic industry for film deposition onto wafers and for the treatment of large flat surfaces like windshields. This technique is indeed industrially very efficient as continuous processes are available. The deposition of many metals was described, e.g. Ni, [START_REF] Mond | [END_REF] Al, 33 W, 33 Au, 34 Ag, Cu 35 or even alloys such as Fe-Ni. 36 This technique has then been successfully applied to the synthesis of nanoparticles onto various substrates, whether being 2D or 3D; this research topic has been thoroughly reviewed. 37

MOCVD principles

The principle of MOCVD is to vaporize a metal precursor, usually an organometallic complex that will subsequently be decomposed onto the substrate and form either a metallic layer or nanoparticles depending on the process conditions. The vaporization is obtained under low pressure and specific temperature conditions. The decomposition of the metallic precursor can be induced by various methods as thermal, laser induced, plasma enhanced, electron beam induced decomposition. 38 A schematic representation of the process is presented Figure 7.

Figure 7. Schematic representation of the MOCVD process, reproduced from ref. 38 1) convection of the gaseous reagents 2) diffusion of the reagents towards the substrate

3) adsorption of the reagents onto the substrate 4) degradation of the complex and film/nanoparticles formation 5) desorption of the gas products of the reaction 6) diffusion of these products through the boundary layer 7) gas evacuation of the system

The deposition condition used can have a strong impact on the purity, the adhesion of the film/nanoparticles and their morphology. Among those conditions, the gas composition, the temperature, the nature of the substrate surface and the solvent are the most important.

Hydrogen can be used to help the reduction of the metallic precursor. For example Hierso et al. 39 synthesized Pt nanoparticles supported onto silica in a fluidized-bed MOCVD set-up, they

showed that the addition of H 2 decreased the decomposition temperature of (COD)Pt(Me) 2

(COD = 1,5-cyclooctadiene) from 240°C to as low as 90°C. This proceeded through the catalytic hydrogenation of COD into cyclooctane and methyl elimination yielding methane.

The substrate surface chemistry is of importance at it strongly influences the adsorption parameters, for example nucleation was found to be promoted by surface hydroxyl groups comparing the nanoparticle formation onto SiO 2 (high OH coverage), as prepared carbon nanotubes (low OH coverage) and acid treated nanotubes (high OH coverage -COOH functions). 40

Platinum organometallic complexes for MOCVD

The literature on platinum MOCVD demonstrates the relative maturity of this process;

recently mainly process conditions modification or set-up improvements are reported.

Applications in the preparation of electrodes for microelectronic, 41 highly resistant coatings 42 and catalyst 37 can be found. The variety of platinum complexes is fairly large and has been thoroughly reviewed by Thurier et al. 38 A good MOCVD precursor must present a sufficient volatility, an adequate stability window, a low production cost and lead to films of high purity with a fast deposition rate. Moreover, stability (against water and oxygen), toxicity and of course cost are important parameters that have to be taken into account.

Most of the platinum complexes (Figure 8) used are either from the β-diketonates family, Pt II diene complexes such as (COD)Pt(X) 2 or derivatives of the CpPtMe 3 complex (Cp = cyclopentadienyle). (COD)Pt(Me) 2 is widely used, despite being less volatile than β-diketonates or CpPtMe 3 , as it is commercially available, synthesized in high yields and relatively stable under air. Compared to impregnation methods widely used for the synthesis of supported nanoparticles, MOCVD present several advantages. Indeed this method avoids many steps as impregnation, washing, drying, calcination and activation that consume both time and energy. This method is thus fast and economical.

Surface Organometallic Chemistry (SOMC) 1.3.2.1. Principle

The goal of surface organometallic chemistry is the synthesis of efficient heterogeneous catalysts. The idea behind this chemistry is to bridge the gap between molecular chemistry/homogeneous catalysis and solid-state chemistry/heterogeneous catalysis. 43,44 By this approach, single-site heterogeneous catalysts are synthesized by reacting tailor made organometallic complexes with anchoring sites onto various substrates. Controlled grafting takes place onto silica surface silanols, their nature and density being controlled by thermal treatment under vacuum or an inert gas (dehydroxylation). As depicted in Figure 9, the grafted species can be further modified by various methods, allowing fine tuning of the metal coordination sphere. This chemistry led to the synthesis of catalysts designed for various reactions such as olefin polymerization, [45][46][START_REF] Copéret | In Fine chemical trough heterogeneous catalysis[END_REF] hydrogenation of olefins, [START_REF] Chupin | [END_REF] oxidation of olefins 49- 51 and olefin metathesis. [52][53][54][55][56] Surface species are mainly characterized by infrared spectroscopy, elemental analysis, X-Ray Photeoelectron Spectroscopy (XPS), Extended X-Ray Absorption Fine Structure (EXAFS) and solid-state nuclear magnetic resonance (NMR).

ab c

Figure 9. SOMC, schematic representation of the grafting and modification steps 44

Supports

Historically, SOM chemistry has been thoroughly studied using silica as a support as it presents a relatively simple surface chemistry and a high surface area (Aerosil -200 m 2 .g -1 ).

A deep understanding of the surface chemistry of the supports used for this chemistry is necessary. The nature and density of the surface anchoring site is a key point to generate well-defined surface species, the support being here considered as a ligand of the grafted metal.

Silica

The silica surface is composed of 3 types of silanol functionalities, isolated, vicinal and geminal as depicted Figure 10. The silanol surface density can be controlled by thermal treatment under vacuum, the so called dehydroxylation process originating from the condensation of silanols into siloxane brigdes (Figure 11). 57 The loss of surface area is limited up to 700°C, while above that temperature a significant drop is noticed due to silica nanoparticles sintering. Infra-red spectroscopy is the tool of choice to study silica surface composition, spectra of silica dehydroxylated at 200 and 700°C (SiO 2-200 and SiO 2-700 ) are presented in Figure 12.

OY O M R x-1 L n O M R' z L' m RY/
SiO 2-200 presents one sharp peak at 3747 cm -1 attributed to isolated silanols whereas the large absorption band centered around 3600 cm -1 was attributed to silanols in interaction which each other, i.e. vicinal and geminal silanols. On SiO 2-700 only isolated silanols are present as witnessed by the sharp peak at 3747 cm -1 . The three absorption bands at 1980, 1866 and 1633 cm -1 were attributed to deformation vibration of siloxane bridges. From its relative simplicity and well-defined surface chemistry, silica is a support particularly suitable to understand surface organometallic chemistry at a molecular level. arrangement, is surrounded by eight oxygen atoms.

We will here mainly focus on the surface composition of ceria as it is of major interest in order to further develop surface organometallic chemistry onto this oxide. The surface chemistry of ceria has been studied notably by the group of Jean-Claude Lavalley in the ninety's. IR spectroscopy is the tool of choice to study the surface composition of ceria. Three types of surface OH have been characterized, mono-coordinated μ 1 -OH (3710 cm -1 ), doubly bridging μ 2 -OH of two types (μ 2 -A 3650 and μ 2 -B 3634 cm -1 -Figure 14) and triply bridging μ 3 -OH (3500 cm -1 ). 59 The IR spectra of ceria calcined at 400°C and further reduced with H 2 at various temperature is presented Figure 13. The reduction of ceria by H 2 was limited to nonexistent at 200°C whereas at 400°C the reduction was emphasized by a strong modification of the surface OH composition. Only μ 2 -OH remained, of both A and B types.

Those two OH comes from the oxidation state of the ceria atom as depicted in Figure 14. The reduction by H 2 is concomitant to the formation of water. 60 Here spectra are recorded after evacuation at the same reduction temperature, and therefore water wass removed. The large band at 3450 cm -1 was thus attributed to the formation of surface hydroxyl groups resulting from the dissociation of H 2 . Ceria presents more IR bands bellow 1600 cm -1 attributed to carbonate species. 61,62 Figure 13. IR spectra of a ceria pellet 1) calcined under dry air at 400°C, 2) reduced by H 2 at 200°C, 3) at 400°C, reproduced from ref 59 .

Figure 14. The two types of μ 2 -OH onto ceria

The formation and modification of the surface OH species onto ceria has been widely studied but to our knowledge no effort have been done towards the rational control and the fine tuning of the surface OH composition. Moreover, to our knowledge, no surface organometallic chemistry onto ceria has been reported so far.

Nanoparticles synthesis

Ermakov et al. [START_REF] Ermakov | Catalysis by supported complexes[END_REF] proposed in the seventies to control the growth of nanoparticles through the careful decomposition of grafted surface species prepared via Surface Organometallic Chemistry (SOMC). This approach has been recently used by several groups working on the mechanism and the kinetic of formation of Ir nanoparticles supported on γ-Al 2 O 3 [START_REF] Mondloch | [END_REF] or the controlled formation of silica-supported Ru, 65 Pd 66 and Pt 67,68 nanoparticles. In the case of

μ 2 -B OH 3630cm -1 μ 2 -A OH 3660cm -1
Rh, 65 the reaction of Rh( 3 -C 3 H 5 ) 3 with silica, alumina and titania yielded ( EO)( EOE )Rh( 3 -C 3 H 5 ) 2 (E = Si, Al, Ti) along with the release of one mole of propylene (Figure 15). Then, reaction of ( EO)( EOE )Rh( 3 -C 3 H 5 ) 2 (E = Si, Al, Ti) surface species with H 2 led to metal particles. The mean sizes of the particles obtained are 1.5 nm on SiO 2 and TiO 2 supports, and 1.0 nm on Al 2 O 3 , which suggests that the oxide support plays a significant role on the final nanoparticle size. with the right ligands. 69 For instance, the reaction of an Au(I) bis(trimethylsilyl)-amido complex on silica partially dehydroxylated at 700 °C allowed to obtain a Au density of 0.5 Au I .nm -2 coordinated to (Me 3 Si) x NH 2-x and surrounded by trimethylsilyloxy surface species (Figure 16). This was crucial to generate 1.8 nm sized supported gold nanoparticles upon subsequent treatment under H 2 at 300 °C; such particles supported on passivated silica showed improved performances in PROX and liquid phase aerobic epoxidation of stilbene.

SOMC is thus a valuable tool for the synthesis of well-dispersed nanoparticles onto various supports but suffers from a lack of understanding of the parameters influencing the nanoparticle final size.

Colloidal route to the synthesis of nanoparticles

Besides synthesizing supported nanoparticles, a large research effort has been devoted to the synthesis of colloidal suspensions of nanoparticles. The main advantage here is the synthesis of nanoparticles with narrow size dispersion, a fine control of the shape/size and of the chemical composition.

The synthesis of colloidal nanoparticles demands three components, i) a metallic precursor that can either be metal salts or organometallic complexes, ii) a reductant such as an alcohol, NaBH 4 or H 2 and iii) a stabilizing agent such as polymers or dendrimers. Note that the solvent can be of importance, for example in the case of ionic liquid where it can act as a stabilizing agent. 70 Chaudret et al. have devoted their effort to the synthesis of colloidal nanoparticles from organometallic precursors, 71,72 they demonstrated the efficient nanoparticles synthesis under mild conditions. Ru(COD)(COT) (1,5-cyclooctadiene, and 1,3,5-cyclooctatriene) was used for the synthesis of Ru nanoparticles stabilized by alkylamines or polyvinylpyrrolidone (PVP) 73 and more recently by N-heterocyclic carbenes (NHC). 74 Those reactions were carried at room temperature in THF or pentane under 1-3 bars of H 2 and led to the synthesis of stable monodispersed nanoparticles from 1 to 3 nm in diameter. The synthesis of platinum and palladium nanoparticles were studied too from M(dba) 2 (dba = dibenzylideneacetone) complexes. Pd and Pt nanoparticles were stabilized by PVP 75 or nitrocellulose 76 and synthesized by reduction under H 2 . Those systems present the advantage of having stabilizing ligands weakly bonded to the nanoparticles allowing a full access to the surface for catalytic applications. Moreover bimetallic nanoparticles for example of Pd-Cu, 77 Pt-Ru 78 have been synthesized using similar procedures.

Sun et al. described the synthesis of Pt 79 and Pd 80 nanoparticles stabilized by oleylamine from M(acac) 2 complexes (acac = acetylacetone) using borane tributylamine as reductant.

Oleylamine, as stabilizing agent, presents the advantage of being easily washed out under acidic conditions after supporting nanoparticles and was moreover used as the solvent for the synthesis. The drawback of this method is its relative complexity as the temperature has a strong effect on the final nanoparticles size. Very recently, they describe the synthesis of Pt/Pd bimetallic nanoparticles using similar procedures. 23 This strategy was successfully implemented to Au 83 and Pt for the synthesis of hydrophobic 84 (stabilized by n-octylsilane) or hydrophilic 85 nanoparticles (stabilized by 3-chloropropylilane).

Both systems led to 1.8 ± 0.4 nm well dispersed nanoparticles. This modification of the surface chemistry led either to the integration of the nanoparticles in the walls or in the pores of a mesoporous silica matrix synthesized by sol-gel chemistry. Note that other stabilizing agents can be used such as tributyltin hydride ((n-Bu) 3 SnH) leading to the formation of stable Pt 3 Sn nanoparticles. 86 Figure 18. Ru nanoparticles stabilized by n-octylsilane, reproduced from ref 81 Finally, the use of a reductant can be avoided, several systems are able to act as both stabilizer and reductant, for example oleylamine has found to be able to both reduce and stabilize gold nanoparticles 87 or magnetite (Fe 3 O 4 ) nanoparticles 88 thus acting as a bi-functional stabilizer.

Research project

The synthesis of Pt and Pd based nanoparticles for catalysis can be done via in-situ and ex-situ methodologies. While in-situ methods such as dry impregnation are widely used, those methods demonstrated a lack of control of the nanoparticles size and shape. On another hand, ex-situ syntheses, as the colloidal approach, lead to well dispersed nanoparticles with a narrow size distribution and demonstrate a good control over both size and shape. The first question was thus: can we control the synthesis of supported Pt nanoparticles the way the colloidal approach does this? In the meantime, the goal of this project in partnership between the CEA Grenoble and the CNRS, was to explore different routes to the synthesis of nanoparticles with a stress on the industrial viability of the methods developed.

This work has therefore been divided into three main chapters:

In chapter 2, we investigate the synthesis of new Pt complexes and their grafting onto silica and ceria leading to well-defined Pt II surface species. The further treatment, under H 2 at 300°C yielding nanoparticles, is described and the influence of the support and the Pt complex chemistry on the final nanoparticles size is discussed.

In chapter 3, we describe the preparation and characterization of Pt nanoparticles through the use of (COD)Pt(OSi(OtBu) 3 ) 2 as a precursor for Metal Organic Chemical Vapor Deposition and the further catalytic properties of such nanoparticles for the Oxygen Reduction Reaction.

In chapter 4, we focus on the preparation of colloidal dispersion of Pt and Pd nanoparticles using octylsilane acting as both the reducing agent and the stabilizer. The effect of the precursor chemistry is discussed for the synthesis of Pt nanoparticles. The further implementation of this methodology towards the synthesis of Pt/Pd bimetallic nanoparticles with various compositions is presented. The catalytic properties of those nanoparticles are again tested for the Oxygen Reduction Reaction.

In chapter 5, we conclude on this thesis work and briefly discuss the perspectives.

2.

From well-defined Pt II surface species to the controlled growth of oxide supported Pt nanoparticles

Introduction

Following work done in our group on gold nanoparticles supported onto silica 69 (as discussed in bibliography) we wanted to tune the structure of the grated precursor in order to control the final nanoparticles sizes and in view of generating Diesel Oxydation Catalysts (DOC). We thus decided to explore the formation of platinum nanoparticles through surface organometallic chemistry by grafting various species onto silica, used as a model surface, and ceria, of more interest as a support for DOC. The synthesis of new Pt complexes will be described followed by the grafting onto silica and finally ceria.

Synthesis and characterization of Pt II complexes

Symmetrical and dissymmetrical molecular complexes of general formula (COD)Pt(X)(Y)

were first prepared and characterized prior to investigate grafting. While the synthesis of the molecular complexes with (X = Y = Cl) 89 , (X = Y = Me) 90 , (X = Me Y = Cl) 91 , (X = Y = OSi(OtBu) 3 ) 92 and (X = Cl, Y = N(SiMe 3 ) 2 ) 93 were reported, the following Pt II organometallic complexes with complementary ligands sets were developed: (X = CH 3 , Y = OSi(OtBu) 3 ), (X = CH 3 , Y = N(Si(Me 3 ) 2 ) and (X = OSi(OtBu) 3 , Y = N(Si(Me 3 ) 2 ). Note however that the bisamido complex (X = Y = N(Si(Me 3 ) 2 ) as well as (COD)Pt(Cl)(OSi(OtBu) 3 ) could not be prepared so far.

Suitable crystals for single-crystal X-Ray diffraction were obtained for (COD)Pt(Me)(OSi(OtBu) 3 ) and (COD)Pt(N(SiMe 3 ) 2 )(OSi(OtBu) 3 ); selected bond distances are given in Table 1 and crystallographic data are available in Appendix 1. They both display the expected square planar geometry for Pt II d 8 molecular complexes (Figure 21). Comparing the two complexes, a shorter Pt-(C=C) distance (2.185 versus 2.268 Å) and a longer C=C bond length are observed for the alkene moiety trans to the amido vs. the methyl ligand (1.386 vs. 1.346 Å). This illustrates the greater -back donation in the amido complex, and it is consistent with the weaker -donating and the greater -donating ability of this ligand compared to methyl. The Pt-(OSi) bond lengths are 1.987 and 1.998 Å for the amido and the methyl complexes respectively; they are slightly longer than that observed in the bis-siloxy complex (1.982 Å) 92 and increases as the complementary ligands become more -donating (OSi(OtBu) 3 < N(SiMe 3 ) 2 < Me). Table 2 summarizes the characteristic signals in proton and carbon-13 NMR data (chemical shift and J P-X coupling) of all prepared molecular complexes (COD)Pt(X)(Y). As reported previously, chemical shifts and coupling constants of the olefinic protons of the COD ligand with 195 Pt 94 are characteristic spectroscopic signatures for such type of complexes, providing precious information about the nature of the X and Y ligands attached to Pt ( -and -donation); these data will be particularly useful to assign the structure of surface species (vide infra). 22) by comparison with (COD)Pt(Me)(OSi(OtBu) 3

(Table 1) and as recently disclosed as we were preparing this manuscript. 95 The peaks at 32 and 29 ppm are also consistent with the proposed structure as the two aliphatic carbon atoms of the COD ligand are no longer equivalent. Finally, the peak at 6 ppm is also a characteristic of the methyl ligand attached to Pt. On SiO The reaction of (COD)Pt(Me)(OSi(OtBu) 3 ) on SiO 2-200 is evidenced by the disappearance of the IR peak at 3747 cm -1 corresponding to isolated silanols (Figure S8). Note the presence of broad absorption bands centered around 3640 and 3413 cm -1 , corresponding to bonded silanol and adsorbed HOSi(OtBu) 3 , respectively. This is also accompanied with the appearance of (C-H) absorption bands associated with perhydrocarbyl ligands. The Pt loading is 6.8 wt% on SiO 2-200 (0.35 mmol.g-1 or 1 Pt.nm -2 , grafting yield = 40 %). Grafting also leads to the formation of 0.34 mmol of HOSiO(tBu) 3 per g of silica, which is close to 1 equiv. per grafted Pt. Note the absence of CH 4 formation upon grafting. In fact, carbon-13 CP-MAS NMR (Figure S9) shows peaks at 115 and 73 ppm for the olefinic carbons of the COD ligand, those peaks are respectively attributed to the olefinic carbons of the COD ligand in trans to the methyl ligand and to the surface siloxy. In addition, the peak at 27 ppm corresponds to the CH 2 of the COD, and the one at 30 ppm is attributed to absorbed HOSi(OC(CH 3 ) 3 ) 3 on the silica surface as demonstrated by a blank experiment (Appendix 8). Comparison with the carbon-13 CP-MAS NMR of (COD)Pt(Me) 2 @SiO 2-200 demonstrates the presence of very similar chemical shifts (118-78 ppm for the olefinic carbons of COD and 6 ppm for the methyl ligand). In addition, the similarity of the spectrum with the molecular equivalent (COD)Pt(Me)(OSi(OtBu) 3 ) (113.3-73.4 and 6.9 ppm) clearly shows that the structure of the surface complex is (COD)Pt(Me)(SiO≡). For SiO 2-700 , similar data are obtained; the only difference is the lower loading 4.1 wt% (0.20 mmol of Pt.g -1 or 0.6 Pt.nm -2 ) and the higher efficiency of grafting yield (79 %).

(COD)Pt(Me)(Cl)

In contrast, grafting of (COD)Pt(Me)(Cl) shows no consumption of the surface silanols and no appearance of typical (C-H) absorption bands related to COD or the methyl ligand by IR-DRIFT. Thus this molecular precursor is clearly unreactive towards surface silanols. For (COD)Pt(Me)(N(SiMe 3 ) 2 ), the reaction with SiO 2-200 is evidenced by the disappearance of the absorption peak at 3747 cm -1 corresponding to isolated silanols (Figure S12). The large absorption band centered around 3640 cm -1 corresponding to bonded silanols is still present, but in much lower intensity thus showing that bonded OH are also involved in the grafting process. The characteristics (C-H) absorption bands from COD (3013 cm -1 ), Me (2927-2845 cm -1 ) and SiMe 3 (2952-2890 cm -1 ) also appeared. High loading, which was implied by the removal of a large number of OH groups, is confirmed by elemental analysis, with 7.4 wt% Pt found (0.38 mmol of Pt.g -1 or 1.1 Pt.nm -2 , grafting yield = 44%). The carbon-13 CP-MAS NMR spectrum (Figure S13) of this solid exhibits peaks at 114, 72, 28, 5 and 0 ppm. The peak at 0 ppm can be assigned to OSiMe 3 , which results from the reaction of surface silanol groups with liberated (Me 3 Si) 3-x NH x . 69,96 Moreover, the peaks corresponding to the olefinic carbons of the COD ligand at 114 ppm and 72 ppm are attributed to the carbon atoms trans to the methyl and the surface siloxy ligands, respectively. This clearly points to a mono-grafting of (COD)Pt(Me)(N(SiMe 3 ) 2 ) via the selective cleavage of the Pt-N bond. It is noteworthy that on SiO 2-700 an additional peak of weaker intensity at 100 ppm is observed, which would imply the formation of (COD)Pt(N(SiMe 3 ) 2 )(OSi≡) as a minor species. In fact, 0.05 to 0.10 equiv. of methane per grafted platinum is detected in the gas phase upon grafting. Again on SiO 2-700 a lower loading is found, 3.8 wt% Pt (0.19 mmol of Pt.g -1 or 0.6 Pt.nm -2 ), highlighting the efficient reactivity of the surface silanols with this complex as 72 % of initial SiOH have reacted versus 44% for SiO 2-200 .

(COD)Pt(Cl)(N(SiMe 3 ) 2 )

The reaction of (COD)Pt(Cl)(N(SiMe 3 ) 2 ) towards silica was also investigated. In this case, a total disappearance of the isolated silanols according to IR spectroscopy was noticed (Figure S15); this was accompanied by the appearance of the characteristic (C-H) of COD (3022 and 3008 cm -1 ) and -SiMe 3 (2966 and 2934 cm -1 ). This translates into 4.2 wt% Pt loading (0.21 mmol of Pt.g -1 or 0.6 Pt.nm -2 , grafting yield = 25 %). The carbon-13 CP-MAS solid-state-NMR spectrum (Figure S16) displays 4 peaks at 104, 94, 31 and 0 ppm. The appearance of the olefinic protons of the COD ligand at 94 and 104 ppm is consistent with (COD)Pt(Cl)(OSi≡). This is also corroborated by the peak at 0 ppm attributed to passivated surface (≡SiOSiMe 3 ). Similar observations are obtained on SiO 2-700 : consumption of OH group upon grafting, a Pt loading of 3.1% (0.15 mmol of Pt.g -1 or 0.5 Pt.nm -2 , grafting yield = 59%) and a similar NMR spectrum. Finally, the reaction of (COD)Pt(OSi(OtBu) 3 )(N(SiMe 3 ) 2 ) on SiO 2-200 leads to a full consumption of isolated silanols as evidenced by the disappearance of the corresponding (OH) at 3747 cm -1 , while bonded silanols at 3640 cm -1 remained untouched (Figure S19). This is associated with the appearance of (C-H) of the three ligands. Note also, a broad absorption band centered around 3360 cm -1 , which is probably associated with the adsorption of NH functionalities or (tBuO) 3 SiOH. This leads to a Pt loading of 4.8 wt% (0.25 mmol of Pt.g -1 or 0.7 Pt.nm -2 , grafting yield = 29 %). Here again, the carbon-13 CP-MAS NMR spectrum (Figure S20) is very informative: two peaks at 105 and 95 (broad) for the olefinic COD, the former clearly attributed to a COD trans to N(SiMe 3 ) 2 , the latter being trans to the surface siloxy ligand. The two other peaks at 75 and 30 are typical of OtBu ligand, this at 28 ppm corresponds to the aliphatic carbons of COD, while the peaks at 4.5 and 0 ppm can be respectively attributed to Pt-N(SiMe 3 ) and ≡SiOSiMe 3 . Those data highlights the existence of two competing grafting modes whether involving the cleavage of the Pt-N or the Pt-O bond.

On SiO 2-700 similar data are obtained, the only differences are again the lower Pt loading of 3.7 wt% (0.18 mmol of Pt.g -1 or 0.6 Pt.nm -2 , grafting yield = 71%) and the presence of remaining isolated silanols interacting with adjacent ligands as evidenced by a peak at 3700 cm -1 in IR spectrum.

Overall, grafting occurs readily for most Pt II complexes independently of the silica used (SiO 2-200 and SiO 2-700 , Figure 23-A and-B) at the exception of (COD)Pt(Me) 2 for which Pt loading is much lower, e.g. 1.1 and 3.0 wt% on SiO 2-700 and SiO 2-200 , respectively;

(COD)Pt(Cl)(Me) being unreactive towards silanols in our hands. This confirms the poorer reactivity of Pt-C bonds towards surface silanols by comparison with Pt-O and Pt-N bonds (Table 3). The structures of surface species are readily assigned from the carbon-13 CP-MAS NMR spectra as the isotropic chemical shift of the olefinic moiety of the COD ligand display very specific chemical shifts as a function of its ligand in trans (Figure S25 and Table 4). B a) IR-DRIFT spectra of a) SiO 2-700 , b) (COD)Pt(Me) 2 @SiO 2-700 , c) (COD)Pt(OSi(OtBu) 3 ) 2 @SiO 2-700 , d) (COD)Pt(Me)(OSi(OtBu) 3 )@SiO 2-700 , e) (COD)Pt(Me)(N(SiMe 3 ) 2 )@SiO 2-700 , f) (COD)Pt(Cl)(N(SiMe 3 ) 2 @SiO 2- 700 , g) (COD)Pt(OSi(OtBu) 3 )(N(SiMe 3 ) 2 @SiO 2-700 99 This explains the selective grafting via cleavage of the M-X bond in dissymmetric complexes (COD)Pt(Me)(X) as well as the lower Pt loading upon grafting of (COD)Pt(Me) 2 .

Nanoparticles formation

To generate nanoparticles, the Pt II surface species were treated under H 2 either in static (150 mBar) or continuous flow conditions (1 bar, flow rate = 100 mL.min -1 ) at 300°C for 15 h following a heating ramp of 5 K.min -1 . In all cases, this treatment induces a dramatic change in coloration from white to black in agreement with the formation of supported Pt particles (Figure 24). This is accompanied with the recovery of surface silanols in all cases as evidenced by the reappearance of the sharp band at 3747 cm -1 associated with isolated silanols in the IR spectrum (Figure 25-A). This is in line with the cleavage of Pt-O bond and the formation of Pt nanoparticles.

(COD)Pt(Me) 2 , c) (COD)Pt(OSi(OtBu) 3 ) 2 , d) (COD)Pt(Me)(OSi(OtBu) 3 ), e) (COD)Pt(Me)(N(SiMe 3 ) 2 ), f) (COD)Pt(Cl)(N(SiMe 3 ) 2 ), g) (COD)Pt(OSi(OtBu) 3 )(N(SiMe 3 ) 2 ) h) HOSi(OtBu) 3 B. a') HOSi(OtBu) 3 @SiO 2-700 , b') (COD)Pt(OSi(OtBu) 3 ) 2 @SiO 2-700 , c') (COD)Pt(Me)(OSi(OtBu) 3 )@SiO 2-700 , d') (COD)Pt(OSi(OtBu) 3 )(N(SiMe 3 ) 2 )@SiO 2-700
Transmission electron microscopy (TEM) was performed on all samples and always confirmed the presence of Pt nanoparticles. Mean size and standard deviation after static H 2 treatments are given in Table 5. A representative picture of the obtained nanoparticles is given in Figure 26; all transmission electron micrographs can be found in appendix 2-7. We can first notice that most silica-supported nanoparticles synthesized from grafted Pt II species display a relatively small size from 2.1 to 3.3 nm and narrow distribution, the standard deviation ranging from 0.6 to 1.2 nm depending on the precursor; the exception being found for the chlorinated Pt complex (nanoparticles size from 5 to 40 nm). When comparing the silica supports, nanoparticles are systematically found larger on SiO 2-200 than on SiO 2-700 by factors ranging from 9 to 36%, which is to be related to the higher density of metal and the OH coverage. Additionally, in contrast to what was observed for Au, 69 surface passivation does not lead to smaller nanoparticles: in fact silica supported (COD)Pt(Me) 2 , (COD)Pt(Me)(OSi(OtBu) 3 ) and CODPt(Me)(N(SiMe 3 ) 2 ), which presents the same surface species, albeit with different Pt density (respectively 0.2, 0.6 and 0.6 Pt.nm -2 ) and SiMe 3 coverage, provide supported nanoparticles of similar sizes on SiO 2-700 i.e. 2.4, 2.0 and 3.0 nm, respectively. In fact, the most dramatic effect comes from the presence of a pendant Cl ligand. For instance, while grafted (COD)Pt(Cl)(N(SiMe 3 ) 2 ) associated with the surface species (COD)Pt(Cl)(OSi≡) yields large aggregates from 5 to 40 nm and 5 to 18 nm on SiO 2-200 and SiO 2-700 , respectively; grafted (COD)Pt(CH 3 )(N(SiMe 3 ) 2 ) associated with (COD)Pt(CH 3 )(OSi≡) surface species, provides well dispersed nanoparticles with a mean size of 3.3 nm on SiO 2-200 and 2.1 nm on SiO 2-700 (Table 6). Note that few larger aggregates are also found in this case. The formation of very large particle (> 20 nm) is prevented in part when flow conditions are used (Figure 27). Elemental analyses (Table 7) after H 2 treatment of (COD)Pt(Cl)(N(SiMe 3 ) 2 )@SiO 2-700 revealed the presence of chlorine, 0.7 (± 0.2) Cl/Pt ratio, for static H 2 treatment, whereas no chlorine is detected when a flow was used. This indicates that HCl formed under H 2 needs to be absent and at least eliminated to avoid the growth of nanoparticles.

Conclusion

Overall, cyclooctadienyl Pt II complexes of general formula (COD)Pt(X)(Y) are convenient precursors to prepare well-defined Pt II surface species and supported Pt nanoparticles with a small size, typically around 2 nm, and a narrow distribution; the smaller nanoparticles being obtained on highly dehydroxylated supports and lower Pt loading. For the grafting step, it is notable that the COD ligand is a good reporter of the coordination sphere around Pt, even

showing that a surface siloxy group is a slightly less strong donor ligand than the corresponding molecular tris(tert-butoxy) siloxy one. Overall, Pt II

Ceria surface organometallic chemistry

Ceria surface chemistry

Following this work on silica, we wanted to transpose it onto ceria. Ceria is indeed the support of choice for generating Diesel Oxydation Catalyst, mostly due to its O 2 storage properties. Before any grafting, the surface chemistry has to be controlled, Surface Organometallic Chemistry indeed supposes a good understanding of the surface oxide support in order to define at the molecular level the structure and reactivity of the grafted species.

Surface morphology

Ceria (Evonik Degussa VP60) is a crystalline material with a relatively low surface area (~ 60 m².g -1 ). In analogy with silica, ceria will be treated under vacuum at various temperatures in order to clean the surface from organic pollutants and to control the water coverage and surface OH (≡Ce-OH) density. Ceria was first calcined under air followed by dehydroxylation under secondary vacuum. The surface area was measured by N 2 adsorption and desorption at -196°C. A type II adsorption isotherm is recorded and linked to intergrain macroporosity, results are summarized in Table 8. Ceria presented a relatively good thermal stability with a loss of less than 10% of its surface area between 200 and 800°C.

IR-Study

The surface chemistry of ceria was studied by mean of IR spectroscopy. Ceria pellets were prepared and treated under various conditions into a set-up allowing in-situ analyses (Figure 28). On the spectrum of the as prepared pellet, bands at 3367 and 1637 cm -1 were associated with absorbed water (( (OH) and δ(OH)). Bands at 2963, 2937 and 2886 cm -1 corresponded to the (C-H) of organic pollution of the sample. The large absorption area between 1600 and 900 cm -1 was related to carbonate species 100 (δ(OH) and (C-O)) and δ(C-C) of hydrocarbons -pollution). After calcination, here at 450°C, the bands associated with the hydrocarbon pollution disappeared and the absorbed water was efficiently eliminated. Bands from (OH) at 3709, 3659 and 3630 cm -1 were found and associated respectively to monodentate OH (μ 1 )

and two types of bidentate OH (μ 2 -A and μ 2 -B -Figure 29). Only polydentate carbonate species 100 remained as demonstrated by the IR bands at 1460 and 1400 cm -1 . After dehydroxylation, only the μ 1 -OH band disappeared, all other bands, compared to the calcined sample, remained unchanged. Note that during the dehydroxylation process, a coloration change was noticed, the pellet turning from yellow to grey. This was indicative of an oxidation state modification from Ce(IV) to Ce(III).

-B OH ≈ 3630cm -1 μ2-A OH ≈ 3660cm -1
chromatography. A mean value of 3.6 ± 0.1 OH.nm -2 was measured, which is slightly higher than this found on silica dehydroxylated at 200°C, 2.5 OH.nm -2 .

OH was also titrated by integrating the OH bands by transmission IR spectroscopy. A ceria pellet was calcined at 200°C under air and further dehydroxylated under secondary vacuum at increasing temperature (Figure 30). From yellow after calcination, the pellet turned grey after dehydroxylation at 200°C. Between each temperature increase, the pellet was cooled down and a transmission infrared spectrum was recorded at room temperature. Results are presented in Figure 30. The peak at 2130 cm -1 was associated with electronic transition 101 of Ce 3+ or to CO arising from the decomposition of carbonate surface species 59 and the peak at 2339 cm -1 corresponded to residual gaseous CO 2 . As expected, the (OH) intensity was found to decrease with increasing dehydroxylation temperature. At 600°C, an OH and carbonates free surface was obtained. The total OH density was plotted versus the dehydroxylation temperature (Figure 31), an exponential type decay was observed as previously described for silica 57 and alumina. 102 The surface composition could be estimated by integrating each band and considering equal extinction parameters for μ 2 -A, μ 2 -B and bonded OH. The μ 2 -A/μ 2 -B ratio was found constant, equal to 0.8 (±0.1).

Attempts to rationalize the surface composition were made by reducing ceria under H 2 . Ceria reacts indeed with H 2 at 150°C to form water and reduced ceria as depicted below:

Ceria reduction A ceria pellet was first calcined under air at 200°C and dehydroxylated at 200°C for at least 14 hours (sample 1). After this first process, H 2 was added (150 mBar) and the pellet was heated at 150°C for at least 6 hours. The pellet was then again dehydroxylated at 200°C for 15 hours. After each step, an IR spectrum was recorded at room temperature. Four reduction/dehydroxylation cycles were performed and the surface chemistry was monitored by IR spectroscopy (Figure 32). The formation of water was confirmed by the large band at 3450 cm -1 and the appearance of a band of low intensity at 1630 cm -1 (not shown). Peaks corresponding to μ 2 -A and μ 2 -B species were integrated and plotted versus the reduction/dehydroxylation cycle numbers (Figure 33). The total OH content was plotted in black and correspond to the integration of the (OH) band from 3100 to 3710 cm -1 .

Figure 33. Integration of the bands under μ 2 -A, μ 2 -B and total integrated from 3700 cm -1 to 3100 cm -1 .

The total OH density remained constant after 5 cycles. Interestingly the reduction of the support was confirmed by the Ce-OH nature modification from μ 2 -A type to μ 2 -B type.

Overall the composition of the μ 2 -OH was switched from 37% of type A after the first cycle to less than 10% after 5 cycles. Note that at this low dehydroxylation temperature, most of the surface Ce-OH remained bonded (about 82 %).

Note here that surface organometallic chemistry on ceria will be made difficult by the high reactivity of ceria with O 2 and CO 2 . Indeed, the modification of the DRIFT spectra of dehydroxylated ceria showed the formation of carbonates species after storing it several weeks in a clean glove-box. Moreover, the dehydroxylation process on ceria powder did not

II)

Absorbance 3600 3700 3750 Wavenumber (cm -1 ) 3550 3650 give repeatable results. It was necessary to calcine ceria, and then reduce it under H 2 and finally dehydroxylate it to obtain repeatable spectra.

Platinum complexes grafting

The grafting of platinum species onto ceria was performed following the work done onto silica using similar procedures. Ceria calcined at 400°C under air, reduced under H 2 at 150°C and finally dehydroxylated at 400°C (CeO 2-400 ) was used for grafting, the hydroxyl coverage being estimated to be 1.1 OH.nm -2 , comparable to values found on SiO 2-700 (0.8 OH.nm -2 ).

(COD)Pt(Me) 2 , (COD)Pt(Me)(N(SiMe) 3 ) 2 and (COD)Pt(Cl)(N(SiMe) 3 ) 2 were tested for grafting.

The efficient grafting of (COD)Pt(Me) 2 was monitored by IR-DRIFT (Figure 34). Similarly to silica, this complex poorly reacted with the OH functionality of the ceria surface, indeed only a partial consumption of surface hydroxyls was not found. Bonded hydroxyls did not react as the broad signal centered on 3350 cm -1 remained unchanged. The μ 2 type (OH) band was only partially consumed. The presence of the complex onto the surface was witnessed by the formation of absorption bands corresponding to (CH) of the Pt ligands below 3010 cm -1 .

(COD)Pt(Me) 2 , c) (COD)Pt(Me) 2 @CeO2-400, d) (COD)Pt(Me)(N(SiMe 3 ) 2 ), e) (COD)Pt(Me)(N(SiMe 3 ) 2 )@CeO 2-400 , f) (COD)Pt(Cl)(N(SiMe 3 ) 2 )@CeO 2-400 , g) (COD)Pt(Cl)(N(SiMe 3 ) 2 ).
For (COD)Pt(Me)(N(SiMe) 3 ) 2 (Figure 34 e) the spectrum showed the full consumption of the μ 2 -OH. Bonded surface OH seemed to react as no more broad band around 3350 cm -1 was found. Moreover, comparatively to CeO 2-400 and the pure (COD)Pt(Me)(N(SiMe 3 ) 2 ) spectra a new band at 1510 cm -1 was found and not attributed yet. Again, similar observations were made for (COD)Pt(Cl)(N(SiMe) 3 ) 2 (Figure 34 g). The consumption of μ 2 -OH and the concomitant appearance of (CH) vibration bands bellow 3010 cm -1 indicated the formation of surface species. Here, bonded surface OH remained untouched as witnessed by the large absorption band centered on 3350 cm -1 . For this latter complex carbon-13 CP-MAS solid-state NMR was used to characterize the surface species (Figure 35). The peaks at 92 and 74 ppm were attributed to the olefinic carbons of the COD onto Pt respectively in trans to the chlorine and to the surface (≡Ce-O-) ligands.

Nanoparticles formation

The formation of nanoparticles was tested in the same static conditions than those used for silica support, i.e. under 150 mbar of hydrogen at 300°C. The formation of nanoparticles was visually attested by the coloration change from light yellow to black. From IR-DRIFT spectra recorded after treatment (Appendix 12), the total elimination of the ligands was checked by the full disappearance of the (CH) vibration bands bellow 3010 cm -1 . The formation of platinum nanoparticles occurred by the cleavage of the Ce-O-Pt bonds as witnessed by the recovery of isolated μ 2 -OH. Note here that ceria itself was affected by H 2 at 300°C, as partial reduction and thus formation of water occurred. The samples being evacuated at 300°C no free water was found but the large OH band centered on 3350 cm -1 was associated to newly formed surface OH as already reported. 60 Nanoparticles were observed by TEM. For (COD)Pt(Me) 2 , nanoparticles were barely visible probably due to their very low size (0.8 nm nanoparticles were found -Appendix 13) and low Pt loading. For (COD)Pt(Me)(NSiMe) 3 ), nanoparticles of 1.2 nm were found with a narrow size dispersion of 0.2 nm. For (COD)Pt(Cl)(NSiMe) 3 ), nanoparticles were bigger with a diameter of 1.5 nm and a size dispersion of 0.3 nm. This is in strong contrast with what was observed onto silica where Cl, as a ligand, led to a dramatic increase of the nanoparticles size.

Here this size increase effect was probably limited by the influence of the substrate. Ceria is indeed known for giving rise to stronger metal/support interactions than silica 104 thus stabilizing nanoparticles even at high temperature under oxidizing conditions. 105 We hypothesized that this interaction counterbalanced the detrimental effect of Cl over the nanoparticles growth.

Conclusion

Through calcination and dehydroxylation procedure the ceria surface composition can be easily modified. The OH type and their concentration are respectively triggered by the oxidation state of ceria and the dehydroxylation temperature. Despite its relative complexity, grafting of isolated complexes onto ceria has been successfully achieved by using tools and methods previously developed for silica. The further treatment under H 2 and heating led to the formation of well dispersed nanoparticles. The platinum surface species chemistry, and particularly the presence of chlorine, proved to led to slightly larger nanoparticles, as observed for silica albeit with less dramatic effect. The strong metal support interaction encountered onto ceria is probably responsible for this limited growth.

Chapter conclusion

Surface organometallic chemistry was used to produce well dispersed Pt nanoparticles onto silica or ceria. Onto silica the surface OH concentration proved to be a key parameter to control the Pt loading. On both oxides the influence of the precursor chemistry, notably leading or not to the passivation of the surface was found to have little impact on the (COD)Pt(Cl)(N(SiMe 3 ) 2 ) 93 were prepared as described in the literature.

Infra-Red spectra were recorded on a Nicolet Magna 550 FT spectrometer using a custom infrared cell equipped with CaF 2 windows, allowing in-situ studies. Typically, 64 scans were accumulated for each spectrum (resolution 4 cm -1 ).

Elemental analyses were performed at "Mikroanalytisches Labor Pascher" in Germany.

Transmission Electron Micrographs (TEM) were obtained on a Philips CM120 Transmission

Electron Microscope at the "Centre Technologique des Microstructures Université Lyon 1" in France the acceleration voltage was set to 120 kV, and the grids were prepared by directly "dipping" the grid into the silica powder (this method avoided the use of a solvent and the potential leaching or aggregation of particles on the support). [START_REF] Curry | [END_REF] H magic angle spinning (MAS) and [START_REF] Olah | Beyond Oil and Gas: The Methanol Economy[END_REF] C cross polarization (CP)-MAS solid-state NMR spectra were recorded on a Brüker Avance 500 MHz spectrometer with a conventional double resonance 4 mm CP-MAS probe. The samples were introduced under Ar in a zirconia rotor, which was then tightly closed. In all experiments, the rotation frequency was set to ca. 10 kHz. Chemical shifts are given with respect to TMS as the external reference for both 1 H and 13 C NMR. [START_REF] Curry | [END_REF] H and [START_REF] Olah | Beyond Oil and Gas: The Methanol Economy[END_REF] C liquid-state NMR spectra were recorded on a Brüker Avance 300 MHz spectrometer. Chemical shifts are given with respect to the residual solvent peak chemical shift.

Platinum complexes preparation

Preparation of (COD)Pt(Me)(N(SiMe 3 ) 2 ).

To a suspension of (COD)Pt(Cl)(Me) (500 mg -1.4 mmol) in pentane (15 mL) was added a solution of LiN(SiMe 3 ) 2 (237 mg -1.4 mmol) in pentane (6 mL). The colorless reaction mixture turned light yellow, while LiCl precipitated. After 90 min at 25 °C, stirring was stopped allowing LiCl to settle, and the reaction mixture was filtered and evaporated to dryness to afford 521 mg of a targeted complex as a white to light yellow solid (77%).

Alternative preparation of (COD)Pt(Me)(N(SiMe 3 ) 2 ).

Complex 1 can also be prepared in 60 % yield using the above procedure from (COD)Pt(Cl)(NSiMe 3 ) 2 and MeLi.

Preparation of (COD)Pt(Me)(OSi(OtBu) 3 ).

To

Preparation of (COD)Pt(N(SiMe 3 ) 2 )(OSi(OtBu) 3 ).

To a mixture of (COD)Pt(Cl)(N(SiMe 3 ) 2 ) (200 mg -0.4 mmol) in benzene (5 mL) was added a solution of NaOSi(OtBu) 3 (115 mg -0.4 mmol) in benzene (10 mL). The reaction mixture was kept at reflux for 16 h, filtered and evaporated to dryness. The resulting solid was dissolved in pentane and crystallized at -30 °C yielding 150 mg (52%) of complex as yellow crystals suitable for X-Ray structure determination. [START_REF] Curry | [END_REF]

Grafting of (COD)PtMe 2 on CeO 2-400 : (COD)PtMe 2 @CeO 2-400 .

The experimental procedure described above was carried out in pentane using (COD)Pt(Me) 2

(1.2 equiv.; 72 mg; 0,22mmol) and CeO 2-400 (0.50 g, 0.18 mmol CeOH).

Grafting of (COD)Pt(OSi(OtBu) 3 ) 2 on SiO 2-700 : (COD)Pt(OSi(OtBu)

3 ) 2 @SiO 2-700 .

The experimental procedure described above was carried out in pentane using

Grafting of (COD)Pt(Me)(OSi(OtBu) 3 ) on SiO 2-200 : (COD)Pt(Me)(OSi(OtBu) 3 )@SiO 2-200 .

The experimental procedure described above was carried out in pentane using

Grafting of (COD)Pt(Me)(N(SiMe 3 ) 2 ) on CeO 2-400 : (COD)Pt(Me)(N(SiMe 3 )

2 )@CeO 2-400 .

The experimental procedure described above was carried out in pentane using (COD)Pt(Me)(N(SiMe) 3 ) (1.2 equiv.; 105 mg; 0,22mmol) and CeO 2-400 (0.50 g, 0.18 mmol CeOH).

Grafting of (COD)Pt(Cl)(N(SiMe 3 ) 2 ) on SiO 2-700 : (COD)Pt(Cl)(N(SiMe 3 ) 2 )@SiO 2-700 .

The experimental procedure described above was carried out in diethyl ether using The experimental procedure described above was carried out in pentane using (COD)Pt(Me) 2

(1.2 equiv.; 108 mg; 0,22mmol) and CeO 2-400 (0.50 g, 0.18 mmol CeOH).

Grafting of (COD)Pt(N(SiMe 3 ) 2 )(OSi(OtBu) 3 ) on SiO 2-700 :

(COD)Pt(N(SiMe 3 ) 2 )(OSi(OtBu) 3 ) @SiO 2-700 .

The experimental procedure described above was carried out in pentane using (COD)Pt(N(SiMe 3 ) 2 )(OSi(OtBu)

Grafting of (COD)Pt(N(SiMe 3 ) 2 )(OSi(OtBu) 3 ) on SiO 2-200 :

(COD)Pt(N(SiMe 3 ) 2 )(OSi(OtBu) 3 )@SiO 2-200 .

The experimental procedure described above was carried out in pentane using (COD)Pt((N(SiMe 3 ) 2 )(OSi(OtBu)

Grafting of (COD)Pt(Cl) 2

No attempt to graft (COD)Pt(Cl) 2 was done as no solvent was found to solubilize this complex. 1 H NMR , d1=1sec, 8 scans.

Appendix Appendix 1. Single Crystal X-Ray diffraction

13

C NMR, d1=1sec, 1875 scans.

Introduction

New Platinum precursors are required for nanoparticles deposition by Metal Organic Chemical Vapor Deposition. 38 One of the aims is to lower the deposition temperature for process, cost and method versatility issues. For example at temperature above 300°C, gas diffusion layers used in fuel-cells are partially calcined and thus damaged, a lower deposition temperature would broaden the number of possible substrates used in MOCVD and thus possible applications of this technology. For this purpose a recently described platinum complex with a low degradation temperature, 92 Platinum loadings from 200 to 650 μg.cm -2 were obtained. A trend, showing the linear increase of the platinum loading versus the deposition time, can be obtained (Figure 37). By MOCVD, the deposition rate depends on the wettability of the support by the metallic precursors. Here the deposition rate is estimated to 5 μgPt.cm -2 .min -1 for the two GDL supports, which both exhibit similar physical properties. From this deposition rate, a deposition yield, defined as the ratio between the amounts of platinum effectively deposited over the one introduced, can be calculated. Measurements of the mass of solution (and thus the volume) used at each run were attempted but due to set up constraints the measured values were smaller than the error. After numerous runs the injector tends to be stuck with early decomposed precursor. This made it difficult to reliably link the deposition time with the amount of precursor introduced. A test was performed with pure toluene; in the present conditions we found a flow rate of about 0.75 mL.min -1 . Based on those values and given the diameter of the deposition chamber (20 cm) the theoretical platinum deposition rate is 15.5 μgPt.cm -2 .min -1 . The deposition yield is thus 32% in agreement with yields generally obtained in MOCVD (15 to 30 %) 111 . Work is under progress to recover the non-reacted precursors by trapping the MOCVD exhaust gas in order to increase the overall yield of this deposition method and thus make it more economically viable.

Note that at very low loadings, values found by elemental analyses fall within the experimental error. This explains the inconsistent results found for 75 minutes deposition time. Platinum loadings are comparable on either E-TEK or 24BC, showing that there is no major influence of the GDL type.

On both types of GDL the ratio of silicium to platinum decreases with increasing deposition time as depicted Figure 38. On E-TEK the total amount of silica is about twice the one found on 24BC. Those high ratio (a theoretical ratio of 2 would have been expected) could come from the early decomposition of platinum at the injector level leading to an atmosphere enriched with -OSi(OtBu) 3 and the subsequent thermal decomposition of this ligand onto the GDLs.

Transmission Electron Microscopy (TEM)

In order to evaluate the particle morphology, TEM grids have been introduced into the deposition chamber and used as substrates for MOCVD. TEM analysis confirms the formation of platinum nanoparticles, a small increase in size from 3.7 to 4.5 nm being observed when increasing the deposition time. Relatively narrow size dispersions are measured, standard deviations being respectively 0.6 and 0.9 nm after 20 and 100 minutes of deposition. The small size increase is concomitant to an increase of the nanoparticles density onto the TEM grids. Indeed this density is increased from 216 nanoparticles per 100 nm² after 20 minutes of deposition to 274 nanoparticles per 100 nm 2 after 100 minutes of deposition. This indicates that two processes happened in the meantime, nucleation of new nanoparticles and limited growing of preexisting ones. Those nanoparticles densities associated with mean nanoparticle size allow us to calculate loadings; results compared to loadings obtained on GDLs are presented Table 10. The ratio between the loading after 100 and 20 minutes of deposition is quite similar in both cases (respectively 2.3 on TEM grids and 3.0 on GDL) but the absolute value is two orders of magnitude lower on TEM grids. This result is not surprising since GDLs are coated with a microporous carbon layer and thus develop a high surface area. The Z-contrast between the elements of the sample is outlined by TEM. In Figure 39 and Figure 40 particles of similar sizes but either black or light grey are observed, supporting the hypothesis of the formation of platinum particles along with silica particles.

Scanning Electron Microscopy (SEM)

After deposition and without further treatments GDLs are observed by SEM.

X-Ray Diffraction (XRD)

Silicon wafer ontroduced into the deposition chamber were used for those experiments. After deposition, wafers were analyzed by mean of X-Ray diffraction (XRD) to confirm the formation of crystalline platinum nanoparticles. Typical diffraction peaks (39.8, 46.2 and 67.2°) from platinum 112 are found, confirming the formation of crystalline platinum nanoparticles (see Figure 42). The peaks at 52.8, 54.4 and 55.8° are intrinsic to the silicon wafer support (see Appendix 15 for bare wafer XRD spectra). The platinum crystallites size can be evaluated using the Scherrer equation (see Appendix 16). For 20 and 100 minutes of deposition time, 3.3 (±1) nm and 2.3 (±1) nm crystallites are respectively found. Those values are slightly smaller those found on TEM grids (3.7 and 4.5 nm) and GDL (4 to 6 nm). This can either indicate a different deposition behavior on silica wafers leading to smaller nanoparticles or that nanoparticles on GDL are formed by the agglomeration of several crystallites. Unfortunately no SEM pictures of the deposited wafers are available to validate this hypothesis. Moreover, no silica crystalline phases were detected showing that the deposited silica was amorphous. For XPS analyses, Ti was chosen as a support for deposition by MOCVD as its orbital energy levels do not interfere with those of interest (Si, Pt). Titanium discs of high purity (99.99+%)

were used and analyzed for samples after 60 minutes of deposition. For comparison a second deposition was carried out in similar conditions using platinum norbornadiene dimethyl ((nbd)Pt(Me) 2 ) previously developed for MOCVD purposes. 113 The atomic concentrations are evaluated by integration of the XPS peaks using CasaXPS software. The results are summarized in Table 11. Note that the addition of each atomic percentage does not give 100% as nitrogen and sometimes fluorine (from the grease used to sealed samples under argon) are found as contaminants (total bellow 3 at. %). Note also that the Pt:Si ratio is equal to 2 as in the molecular precursor (COD)Pt(OSi(OtBu) 3 ) 2 . A Si:Pt ratio of 2 to 20 was found onto GDLs by elemental analyses (Figure 38). This probably arises from a bigger affinity of the liberated HOSi(OtBu) 3 with the GDL substrate compared to the titanium disc. In both cases the Ti support signal is not seen on the XPS spectra indicating the formation of a deposited layer (Pt, C contamination, SiO 2 ) with a thickness larger than the XPS probe depth ab (around 10 nm). The relative carbon contamination is higher when using the (COD)Pt(OSi(OtBu) 3 ) 2 compared to (nbd)Pt(Me) 2 , C:Pt ratio are respectively 10 and 1.6. The carbon contamination is higher than reported elsewhere but fine tuning of the deposition conditions is known to have a strong impact on it. 114 The Si2p spectra, with a binding energy of 103.2 eV, is typical of SiO 2 and confirmed the deposition of silicon dioxide (Figure 44). Those high energies are generally attributed to Pt IV and could be here related to a specific interaction between platinum and silica.

Electrocatalysis

Electrocatalysis evaluation of the catalyst was performed for the Oxygen Reduction Reaction (ORR) and Hydrogen Oxidation Reaction (HOR) in acid media in a half fuel-cell set-up, 116 please refer to experimental part for details. Main results are summarized in Table 12 and will be successively discussed. showing the same trend in the Pt surface area. The absolute value can differ from one method to another; the mean difference being in the 50 % range. This is probably due to the low loading of platinum on those samples and large errors on both the surface area measurements and elemental analyses.

CO stripping

A second method to measure the ESA is by the so called ''CO stripping'' method. 117 CO is indeed a strong poison for platinum and binds easily onto its surface. It is first adsorbed onto the surface and then oxidized into volatile CO 2 when applying a given tension to the half-cell set-up. For experimental details please refer to part 4. Results are presented and compared to ESA measured by the H 2 method in Figure 46. For the rest of the present study, Pt surface area will refer to the surface area measured by CO stripping. Indeed the surface area under the CO oxidation curve is much higher than the one observed for in the H upd zone leading to a smaller error and thus more accurate values.

CO stripping is a good way to evaluate the tolerance of the catalyst towards CO poisoning.

After one cyclic voltammetry under pure H 2 , the gas flow is switched to H 2 + 50 ppm CO.

The poisoning of the catalyst took only a few minutes. The reference, a PtCo catalyst, designed to be CO tolerant, shows no degradation of its properties in the same period of time (about 15 minutes). Typical behavior of pure Pt nanoparticles versus CO poisoning was recorded. A fast deactivation, within a few minutes range took place. Cyclic voltammograms The final j value is directly linked to the CO coverage on the particles, this value can be calculated as follow:

. Values of 99% are found for all samples confirming the total poisoning of the platinum electroactive surface area.

From the platinum loading and the platinum surface area, one can calculate the metal dispersion, defined as the ratio between surface platinum atoms and the total number of atoms per nanoparticles (Equation 2). Equation 2A : Avogadro's number 0.08 is the surface area per Pt atom (nm².atom -1 )

Dispersion is directly correlated to the particles size (see Appendix 17). Here very low dispersions are found while values between 20 and 30 % corresponding respectively to 6.6 The theoretical platinum ESA can be evaluated considering that a 100% of the platinum nanoparticles are accessible and a mean nanoparticle size of 5 nm. Results are summarized Table 13. Pt is defined as the catalytically active platinum over the total platinum content. Very small Pt, in the 1 to 2.4 % range, are obtained as compared to the reference where 32.6% of the platinum is catalytically active. This indicates that triple points, where gas, catalysts and electrons can meet, are not efficiently synthesized by MOCVD using the (COD)Pt(OSi(OtBu) 3 ) 2 precursor. This could be due to the formation of silica, an insulator, as it is the by-product of the decomposition of this very specific molecular precursor.

Hydrogen Oxidation Reaction

Hydrogen is very easily split onto Pt nanoparticles; low loadings (<50μg/cm²) of Pt meet both catalytic and economic constraints.

H 2 → 2 H + + 2 e - redox potential 0 V/SHE
The oxidation of H 2 is used here as a reporter of the membrane electrode assembly (MEA) quality. Current densities are arbitrary compared at 0.35 V/RHE under a pure H 2 flow -30 mL.min -1 (Figure 48). As expected, the current density is directly proportional to the electroactive surface and no influence of the GDL type is found.

Oxygen Reduction Reaction

For each sample, cyclic voltammograms (CV) were recorded under an O 2 flow (30mL.min -1 ) at a scanning rate of 5 mV.min -1 (Figure 49). To easily compare the performances of each GDL, the current values at 0.8 V/RHE are compared against the electroactive surface area (Figure 50). Those results have to be compared to a reference, here a PtCo catalyst (impregnated onto 24BC) produced by Tanaka -Japan with a loading of 540 μg.cm -2 . This sample shows much higher platinum electroactive surface, 108.9 Ptcm 2 .cm -2 compared to 1.8 to 4.8 Ptcm 2 .cm -2 for our samples. The current density (μA.cm-2) at 0.8V vs. RHE is 40 to 500 times higher than for MOCVD samples. If we compare the normalized currents versus the Pt surface area the performances of our particles are ca. 1 to 12 times lower than those of the PtCo reference sample (see Table 12).

Chapter conclusion

(COD)Pt(OSi(OtBu) 3 ) 2 proved to be an efficient precursor for the synthesis of platinum nanoparticles onto GDLs by DLI-MOCVD. Nanoparticles from 4 to 6 nm were obtained with narrow size dispersions. Platinum loadings from 100 to 600 μg.cm -2 , in the targeted range for The hypothesis c seems unlikely to occur as no core-shell structure was evidenced by microscopy. Hypotheses b, d and e cannot be rejected even if TEM microscopy leans towards the hypothesis b, as darker and brighter nanoparticles are found.

Despite the efficient synthesis of nanoparticles, (COD)Pt(OSi(OtBu) 3 ) 2 gives disappointing results in electrocatalysis. In the next chapter, a different approach will be tested using colloidal nanoparticles stabilized by silanes.

Note that the intimate Pt/SiO 2 structure could result is more thermally stable nanoparticles when deposited onto oxides and could have interest in car gas exhaust depollution as oxidation catalyst for hydrocarbons and CO removal. Effort will be pursued in this way using this Pt/SiO 2 precursor for deposition onto ceria/alumina honeycomb structures used in car exhausts.

X-Ray photoelectron spectrometry (XPS):

spectra were recorded at the CEA-Grenoble, LITEN, on the SSI-SProbe XPS spectrometer spectrometer with a Kα monochromated Al source. Spectra are recorded with a collection angle of 35° and a pass energy of 25 eV for high resolution spectra. The pressure in the analysis chamber was bellow 10 -9 Pa. Peaks are calibrated toward the C1s (adventitious contamination carbon) peak set at 284.8 eV. Synthetic components on Pt4f were analyzed with a Shirley background subtraction and the peaks shape were resolved with a combination of gaussians and lorentzians (30%).

The synthesis of (COD)Pt(OSi(OtBu)

3 ) 2 was carried out under a controlled atmosphere (Ar), using Schlenk and glove-box techniques for organometallic synthesis. For details please refer to chapter 2.

MOCVD

Direct Liquid Injection Metal Organic Chemical Vapor Deposition (DLI-MOCVD) was performed at the Laboratoire des Technologies de Surface (LTS) in CEA Grenoble on the "Murielle" equipment. The precursor was first dissolved (0.025 mol.L -1 ) in dried and degassed toluene and placed in a sealed vessel. The solution was pushed by N 2 . The carrier gas was an 80% N 2 /20% O 2 100 mL.min -1 flow. The substrate was heated to 270°C, above the decomposition temperature of the platinum precursor. The pressure into the deposition chamber was maintained at 1 torr and the wall temperature was set to 110°C. The injector was heated at 100°C and the injection frequency set to 2 Hz with an opening time of 2 ms. A scheme and a picture of the equipment are presented Figure 52 and Figure 53. For each deposition run a set of support was placed at the center of the substrate holder. A glass slide was used as a visual reporter of the efficient deposition as it turns grayish after deposition. A wafer (about 2×2 cm²) was used for further XRD characterization. A titanium disc (1 cm in diameter) of high purity (99.99+) from Goodfellow is used for XPS analysis, it was sonicated in acetone and air-dried prior to use. GDLs (8×8 cm²) of two types, 24BC (SGL group) and E-TEK (BASF) were used. TEM grids (carbon coated copper grids) were placed along with another supports to allow TEM characterization of the deposited nanoparticles. 5 runs were made with varying the deposition time from 20 to 100 minutes.

Sample preparation for half-cell tests

The catalysts deposed onto the gas diffusion layers (GDLs) were tested in fuel cell condition i.e. in the membrane electrode assembly referred as MEA. GDLs consist of a microporous mixture of carbon particles and polytretrafluoroethylene (PTFE) supported onto a macroporous carbon cloth. Platinum nanoparticles were deposited on the carbon microporous side. Two types of GDL were tested: 24BC produced by SGL and E-TEK from BASF. For half-cell set-up MEA composed of three layers were used (see Figure 54) and prepared as follow. To ensure a good contact (electrical pathway) between the catalyst nanoparticles and the Nafion membrane, Nafion was first deposited onto the GDLs. A solution of Nafion at 0.5 wt% in water/isopropanol (1:1) was prepared by mixing 1mL of Nafion 5 wt% in water, 4mL of deionized water (18.2 MΩ.cm) and 5mL of isopropanol. GDLs were impregnated using an aerograph with the appropriate amount of Nafion solution to ensure a Nafion loading of about 100 μg.cm -2 . After drying at 80°C for 10 min, GDLs and the Nafion membrane (NRE211 CS) were then hot casted together at 135°C for 2.5 min and then at 135°C under 40 kg.cm -2 for 2.5 more minutes following a previously developed method. Circular samples (ø=14 mm) were cut for testing.

Electroactive surface measurement

The total numbers of reactive surface sites on platinum were measured by two methods: absorption of H 2 or CO (CO stripping) at the catalyst surface followed by oxidation. Those two methods are based on the measurement of the charge needed to remove the absorbed monolayer (of H or CO). This is done by integrating the current in the anodic scan region (I>0). One H and one CO are absorbed per surface platinum atom. The electroactive surface area (ESA) can be obtained by using the two following equations: 0.210 mC.cm -2 being the charge needed to remove the H monolayer on 1 cm² of platinum (0.420 is the equivalent for CO removal). Q H and Q CO are measured by integrating the peaks corresponding to the H or CO monolayer removal, it is given in mC.

v is the scanning rate in mV.s -1 .

From this equation we understand that for a given ESA and thus a given Q x , ∫IdE increases with increasing scanning rate. N 2 was bubbled into the electrolyte for at least 20 minutes, and then the working electrode was flushed with N 2 (30 mL.min -1 ). Prior to measurements, the MEA is cycled from 0 to 1.1

V/RHE at a scanning rate of 20 mV.s -1 to ensure stabilization of the ESA. Cyclic voltammograms (CVs) were reproducible after 4 to 5 cycles. The ESA was measured on the last voltammogram recorded. Below is given an example for the H 2 surface area peak measurement (see Figure 57). Integration is done using EC-lab software.

CV under pure H 2 (Hydrogen Reduction Reaction) and CO stripping:

A first voltammogram was recorded under pure H 2 (30 mL.min -1 ) and then the gas flow was switched to H 2 + CO (50 ppm, 30 mL.min -1 ). CO is a strong poison for Pt and thus the current density will decrease. The faster the decrease the more rapidly the catalyst is poisoned. The poisoning of most sites on the catalyst surface took only a few minutes (<10 min). After switching for the H 2 /CO mix, CV between 0 to 0.65 V/RHE were recorded every 2.5 minutes.

After complete poisoning of the catalyst, typically 5 cycles, the gas at the working electrode was switched to nitrogen and a CV from 0 to 1.13 V/RHE was recorded. The absorbed CO was thus oxidized into CO 2 and the surface area under the CO oxidation peak was measured.

A second CV was drawn and used as the base line for the surface measurement (see Figure 58).

Introduction

The goal of this chapter is to describe the synthesis of octylsilane stabilized nanoparticles of platinum, palladium and of bimetallic Pt/Pd nanoparticles and the understanding of their synthesis pathway and final atomic composition. These syntheses were developed based on previous work performed on ruthenium, 82 platinum 84,85 and gold. 83 The catalytic properties will be tested in the Oxygen Reduction Reaction (ORR).

Platinum nanoparticles

From Pt 0 precursor

Synthesis using H 2 as an external reductant

The synthesis of Pt nanoparticles stabilized by octylsilane was previously published by our group 84,85 albeit with low insight into the nanoparticles growth mechanism. The starting Pt 0 complex was bis-dibenzylideneacetone platinum (Pt(dba) 2 ), an air stable compound easily synthesized. 120 1.8 ± 0.3 nm

The release of octane indicates the probable formation of Pt-Si intermediates, the cleavage of the Si-C bond and the potential incorporation of silicium atoms within the nanoparticles structure. Part of the alkyl chains would stay pending from the surface and thus stabilizing the nanoparticles. This hypothesized mechanism (Figure 60) implies the formation of Pt-Si intermediates that have not been isolated yet. This hypothesis is supported by kinetic observations where Pd (see below) was found to react faster than Pt. The faster reaction of palladium compared to platinum is in line with previous computational studies by Sakaki et al. 121 which showed lower calculated activation energies for the oxidative addition of SiH 4 over Pd(PPh 3 ) 2 than over Pt(PPh 3 ) 2 , this system being comparable to our RSiH 3 over M 0 reaction.

Figure 60. Platinum nanoparticles reaction scheme

Synthesis in the absence of H 2

The synthesis of platinum and palladium nanoparticles under H 2 proved to be very efficient but the explosiveness of H 2 makes it a potentially dangerous process. Moreover a solution of Pt(dba) 2 with one equiv. of octylsilane was left over for several hours under N 2 and a slight variation of the coloration, from deep purple to black was noticed. From this point the need for H 2 as an external reductant was questioned, and we decided to explore the synthesis in the absence of H 2 and at a higher temperature in order to speed up the nanoparticles formation process.

A Pt(dba) 2 solution was heated in THF at 60°C, neither coloration change nor formation of aggregates were observed after two hours demonstrating the good thermal stability of the precursors. When one equiv. of octylsilane was rapidly added (about 1 second), a dramatic coloration change from pink to dark black was noticed in about 1 min of time. TEM images of the nanoparticles after overnight reaction at 60°C are presented Figure 61. 1.6 nm nanoparticles were produced, this size being smaller than that obtained when the reaction proceeds at room temperature, while size distribution (σ = 0.3 nm) was not modified.

In-situ NMR monitoring of platinum nanoparticles formation

Despite previous work, the synthesis and structure of those nanoparticles are not yet fully understood. NMR analyses were performed in order to gain insights into the reactivity of this system. The synthesis of nanoparticles was monitored by NMR using a Young NMR tube.

The final Pt(dba) 2 concentration in the tube was 30 mmol.L -1 versus 1.5 mmol.L -1 for nanoparticles synthesis experiments. This higher concentration did not have any detrimental effect on the nanoparticles as proven by TEM images (see Appendix 18, well dispersed nanoparticles were produced). On the other hand, upon addition of 1 equiv. of octylsilane into the NMR tube, a fast coloration change was noticed demonstrating a faster kinetic. The formation of bubbles was also noticed within the first seconds after silane addition and before tightly closing the NMR tube. Spectra were recorded up to 14 h after starting the reaction (Figure 63). After addition of the silane, protons from the dba ligand between 7.2 and 7.8 ppm were not anymore visible. Strong and large peaks at 0.85 and 1.25 were attributed to both the silane The reactivity of octylsilane towards dibenzylideneacetone was tested without the presence of the Pt complex. When one equiv. of octylsilane and one equiv. of dibenzylideneacetone were mixed in THF-d 8 in an NMR tube, no reaction took place after overnight mixing at room temperature. Heating at 60°C for two hours did not induce either any reaction. Platinum thus proved to be necessary to induce the hydrogenation of dba into 1-5, dicyclohexylpentane. This implies the formation of H 2 at one point of the process, its splitting over the metallic nanoparticles and the further catalytic hydrogenation and reduction of dba. The reaction of silane with the metallic center yielding Pt-Si bonds and elimination of H 2 was hypothesized.

From Pt II precursors

The influence of the precursor used for such nanoparticles synthesis was studied; platinum II precursors were tested.

(COD)Pt(Me) 2

We first tested (COD)Pt(Me) 2 as a precursor; this product presents the advantage of being air stable and commercially available, making it a perfect candidate for nanoparticles synthesis. It

(COD)Pt(OSi(OtBu) 3 ) 2

Following the work performed in surface organometallic chemistry where (COD)Pt(OSi(OtBu) 3 ) 2 was found to be more reactive than (COD)Pt(Me) 2 , we decided to investigate the nanoparticles synthesis from this bisiloxy complex.

Synthesis and characterization

Upon addition of 0.3 or 1 equiv. of octylsilane at room temperature, the THF solution of (COD)Pt(OSi(OtBu) 3 ) 2 turned instantaneously from color-free to brown/black.

TEM imaging

TEM grids were prepared after 30 minutes and 15 h of stirring (Figure 65). Nanoparticles with a diameter of 1.2 nm and a narrow size dispersion (0.3 nm) were synthesized in a very reproducible manner. No influence of the number of equivalent of octylsilane between 0.3 and 1 was found; after 15 h of stirring, no aggregation or increase of the nanoparticles was observed. At room temperature, the total consumption of the platinum precursor was not achieved after 24 h according to NMR and longer reaction times or heating were necessary. The nanoparticles synthesis was thus carried out at room temperature (60 min) and pursued at 60°C for 15 h, NMR demonstrating the full consumption of the precursor under those conditions. No modification neither of the size nor of the dispersion was found (Figure 66). Those colloids are remarkably stable for months at room temperature, no aggregation or settling of nanoparticles was observed.

IR-monitoring of the reaction

The reaction was monitored by transmission infrared (IR) spectroscopy. After 15 h of reaction, the colloidal suspension was concentrated, one drop was placed between two KBr windows and the IR spectrum was recorded. After subtraction of the THF signal (Figure 67), the colloids spectra (d and e) showed the following peaks: the Si-H peak at 2150 cm -1 is no more visible indicating the full reaction of the Si-H. The C=C stretching vibration of COD at 1650 cm -1 as well as the C=C-H stretching band at 3007 cm -1 have also disappeared; these data being consistent with the hydrogenation of this ligand. The colloids spectra presented bending vibrations, δ(C-H) and δ(C-C) between 1100 and 1500 cm -1 , and stretching (CH)

vibrations between 2800 and 3000 cm -1 comparable to those of HOSi(OtBu) 3 and octylsilane (spectra c and f). Moreover the colloid spectra presented a broad peak at 3400 cm -1 similar to that of HOSi(OtBu) 3 and associated with (OH). Unfortunately, due to their extremely small particle size no separation of the nanoparticles from the reaction solution was possible by centrifugation. The IR spectrum of isolated nanoparticles was thus not recorded.

Overall, the IR spectra of the colloids suggested the formation of HOSi(OtBu) 3 as a byproduct and the probable hydrogenation of COD into cyclooctane. The full reaction of silane probably leading to ≡SiC 8 H 17 surface species was confirmed.

H 2 chemisorption

The nanoparticles size was also evaluated by H 2 chemisorption for comparison with TEM analyses. After synthesis (1 equiv. octylsilane, 60°C overnight), the colloids were impregnated (1 wt%) onto silica by the Incipient Wetness Impregnation method 124 (IWI).

Silica was previously calcined and dehydroxylated at 700°C under secondary vacuum. For IWI, a simply dried support is typically necessary (for silica a treatment at 120°C under primary vacuum would have been sufficient), but we used silica partially dehydroxylated at 700°C silica for ease of interpretation (this process ensure a simple IR-DRIFT spectrumisolated Si-OH, (OH) at 3740 cm -1 and Si-O-Si deformation bands bellow 2000 cm -1 ).

Samples were then dried under an argon flow and finally under primary vacuum at room temperature. The DRIFT spectrum of the as-prepared sample is presented in Figure 68 b. The broad peak at 3400 cm -1 ( OH) was attributed to absorbed HOSi(OtBu) 3 . To ensure a clean platinum surface ready for H 2 chemisorption and based on previous work, 84 supported nanoparticles were calcined under a dry air flow (40 mL.min -1 ) at 320°C for 15 h. IR-DRIFT showed the disappearance of multiple C-H stretching peaks (2800 to 3000 cm -1 -Figure 68 c), this process ensured the efficient removal of the pending alkyl chains. The remaining peaks at 2982 and 2931 cm -1 were attributed to tBu unit via the formation of HOSi(OtBu) 3 . Platinum was then reduced under a hydrogen flow (40 mL.min -1 ) at 500°C for 6 h. The IR spectrum is presented in Figure 68 d. Under such high temperature the remaining HOSi(OtBu) 3 was eliminated, no more (C-H) bands were found bellow 3000 cm -1 . The IWI process also ensured an efficient dispersion of the nanoparticles as no aggregate was found by TEM (Figure 69). Moreover, the subsequent thermal treatment under air and H 2 did not led to an increase of the nanoparticles size, demonstrating high stability of the nanoparticles under such conditions. Finally, H 2 chemisorption was performed on the reduced sample (Table 14). We consider 1.8 atom of hydrogen per surface platinum atom. 125 From the measured dispersion, the nanoparticle size was estimated considering a truncated cubic octahedron shape. Based on two samples, the dispersion was defined within ±8%, the nanoparticles size found was thus 1.6 ± 0.3 nm which is in excellent correlation with TEM analysis.

In-situ NMR monitoring of Pt nanoparticles formation

To gain more insights into the nanoparticles synthesis mechanism, liquid state NMR experiments were carried out. The reaction of one equiv. of octylsilane with one equiv. of (COD)Pt(OSi(OtBu) 3 ) 2 was carried out in a Young NMR tube in THF-d 8 . Upon addition of the silane, a quick coloration change occurred from color-free to brown; few bubbles were formed. NMR spectra from 2 minutes reaction to 110 h are presented in Figure 70. The peaks at 5.50/2.32 ppm were attributed to free cyclooctadiene (=C-H and -CH 2 -) and the one at 5.33 ppm to the silanol group of tris(tert-butoxy)silanol. The release of tris(tert-butoxy)silanol was further evidenced by the disappearance of the peak at 1.31 ppm associated with PtOSi(OC(CH 3 ) 3 ) 3 and the formation of a peak at 1.30 ppm attributed to the methyl signal of free tris(tert-butoxy)silanol (see Figure 71 and Appendix 20 d). The formation and further consumption of H 2 was evidenced by monitoring the peak at 4.55 ppm associated with dissolved H 2 . The consumption of H 2 was concomitant with the disappearance of the COD peaks, the formation of cyclooctene (5.51 and 1.51 ppm) and then its final hydrogenation into cyclooctane (1.54 ppm). This hydrogenation process underlined the formation of platinum nanoparticles as platinum surface is needed to split hydrogen and for the hydrogenation of cyclooctadiene.

The silane reactivity was evidenced by the disappearance of the SiH 3 triplet at 3.47 ppm. In the same time, the peak at 0.77 ppm (-CH 2 -Si) was shifted toward higher fields up to 0.68 ppm and the peak shape was broadened (Figure 72). Those observations are consistent with the formation of surface species as depicted in Figure 72. The formation of n-octane was evidenced by the appearance of a peak at 0.89 (CH 3 ) and 1.29 ppm (CH 2 ). The peak at 2.50 ppm observed after 110 h was associated to water from external contamination. Weak peaks at 4.63 and 4.72 ppm were formed; their attribution will be discussed later. Moreover, the diffusion coefficient (1920 μm 2 .s -1 ) is close to that of cyclooctane (1.54 ppm -1915 μm 2 .s -1 ).

The diffusion coefficients are directly related to the hydrodynamic radius of molecules (Equation 3). Here the nanoparticles size could be estimated from the diffusion coefficient of grafted octylsilane molecules. 126 Equation 3.

A hydrodynamic radius of 1.3 nm was calculated (Appendix 21) corresponding to nanoparticles with a 2.6 nm diameter. This is to be compared with TEM analysis where nanoparticles of 1.2 nm are found. By TEM the silane shell (≡SiC 8 H 17 ) cannot be seen (no contrast between the carbons of the silane chains and the carbon layer deposited onto the TEM grids) and only the platinum core is imaged. The total nanoparticle diameter would thus be theoretically of 3.2 nm considering straight octylchains (Appendix 22) covering the surface as depicted in Figure 74. Note that this was likely to lead to an overestimation of the nanoparticles size. NMR confirmed the formation of nanoparticles stabilized by octylsilane.

The oxidative addition of octylsilane and the further reductive elimination forming H 2 is hypothesized to explain the formation of the nanoparticles. proving the formation of surface species. After 9 h a steady state is reached, only a fraction (about a half) of the platinum complex had reacted and the liberated COD was not fully hydrogenated. Due to the particular reactivity induced by the methyl ligand, a second equiv.

of octylsilane was added and required to complete the reaction. After 12 h, full consumption of the Pt precursor was observed (Figure 76 c). The dramatic coloration change was monitored by mean of UV-Vis spectroscopy (Figure 78).

The absorption peak at 320 nm typical of Pd(dba) 2 faded after reaction indicating the total consumption of the precursor. The byproducts of the reaction were further analyzed. After filtration over alumina to trap the Pd nanoparticles the liquid phase was analyzed by gas chromatography coupled with mass spectrometry (GC-MS). 1-5,dicyclohexylpentane and n-octane were detected similarly to the results obtained for Pt(dba) 2 . The release of octane points out the formation of PdSi nanoparticles as already discussed for Pt.

Synthesis in the absence of H 2

As for the synthesis of platinum nanoparticles, the need for H 2 was questioned. A Pd(dba) 2 solution was heated in THF at 60°C, neither coloration change nor formation of aggregates were observed after two hours demonstrating the good thermal stability of the precursor.

Conclusion

In conclusion, palladium nanoparticles stabilized by octylsilane were successfully synthesized with or without H 2 following previous methods developed for platinum. They display a mean size of 1.7 nm and narrow size dispersion, the standard deviation being 0.4 nm. Based on those results we decided to test the possibility of expending this nanoparticles synthesis methodology towards the synthesis of bimetallic platinum/palladium nanoparticles.

Platinum/palladium bimetallic nanoparticles

Synthesis and characterization

The synthesis of bimetallic nanoparticles stabilized by octylsilane was attempted by simply mixing in THF both Pt 0 (dba) 2 and Pd 0 (dba) 2 precursors. They both display the same structure and have a similar reactivity towards octylsilane. The octylsilane to total metal content ratio was set to 1 as it previously demonstrated to lead to stable and well dispersed nanoparticles.

The reaction was carried out under 3 bars of hydrogen at room temperature to ensure complete reaction and hydrogenation of dba. Within 4 h a coloration change from purple to 1.7 ± 0.4 nm dark black was noticed. The reaction solution was further mixed at room temperature for 11 h (total reaction time of 15 h) to ensure the complete reaction of both precursors.

UV-Vis spectroscopy

UV-Vis spectroscopy was used to monitor the reaction. UV-Vis spectra of the pure precursors are very similar and present an absorption maximum at 320 nm associated with the dba ligand. The UV-Vis spectra of 3/1, 1/1 and 1/3 molar ratio of Pt(dba) 2 and Pd(dba) 2 are thus identical (Figure 80). After 15 h of reaction, an aliquot of the colloidal solution was analyzed, the intense peak due to the M(dba) 2 complexes at 320 nm was no more visible indicating a full consumption of both precursors. Again, GC-MS proved the formation of 1-5,dicyclohexylpentane and n-octane.

Transmission electron microscopy

After reaction, the colloidal solutions were analyzed by TEM (Figure 81). Nanoparticles from 1.4 to 1.8 nm are synthesized. For all compositions, similar size dispersion were found with a standard deviation from 0.3 to 0.5 nm (Table 15).

Infrared spectroscopy

IR analyses were carried out using silica supported samples before and after calcination. IR-DRIFT of the as-prepared and calcined samples were recorded after exposure to air (Figure 82). Typical signal of the silane aliphatic chain was observed ( (C-H) 2985, 2923 and 2855 cm -1 ) but the (Si-H) band at 2150 disappeared. Signals at 3064 and 3029 cm -1 , associated with (C=C-H) from residual dba aromatic rings or partially hydrogenated dba, were still observed. There was also a large band at 3400 cm -1 associated with water absorbed onto silica after exposure to air. After calcination, the (C-H) bands dramatically decreased and only bands with a very low intensity were found. This clearly indicated the efficient degradation of the aliphatic chains grafted onto the nanoparticles. Similar results were found for pure Pt or Pd as well as for bimetallic nanoparticles. Despite the loss of the organic ligands, the supported colloids were found to be stable as evidenced by TEM. Before and after calcination TEM pictures of the supported nanoparticles were recorded (Figure 83). Nanoparticles were evenly distributed onto the silica surface. After calcination neither aggregation nor increase of the nanoparticles size was noticed. The platinum, palladium and 1/1 Pt/Pd nanoparticles were studied by XPS (Figure 84) after deposition onto a titanium disc under argon. The XPS spectrum of Pt nanoparticles was already described 84 XP spectra were recorded after calcination under dry air at 320°C (Figure 85). Platinum presented then two main contributions attributed to Pt 0 and Pt 2+ (PtO) 115 respectively at 71.6/74.9 and 72.9/76.2 eV, the Pt-Si contribution, if still present, was no more visible. The same phenomenon was observed for palladium, the Pd 2+ being now the predominant species.

Interestingly, spectra of the bimetallic nanoparticles displayed both for palladium and platinum a third contribution at higher energies. As in the MOCVD, chapter this could be attributed to a specific interaction between the metals and silica. The formation of silica after calcination being supported by XPS too (Figure 86). The Si 2p signal switches indeed from 102.5 to 103. 4 EDX spectra were first recorded from Pt/Pd 1:1colloids directly deposited onto copper grids.

The EDX spectrum (Figure 88 a) demonstrated the presence of Pt and Pd in a 1/1 ratio along with silicium. During the EDX acquisition (about 1 minute), a drift of the sample occured, this phenomenon added to the high nanoparticles surface density did not allowed us to conclude on single nanoparticles composition. Moreover, those results were poorly reproducible due to a high carbon contamination resulting from the silane decomposition under the energetic electron beam of the HR-TEM microscope. Analyses were thus carried out on isolated nanoparticles supported onto silica and calcined. This ensured the stabilization of the nanoparticles and allowed for longer acquisition times. Those measures were repeated on 3/1 and 1/3 Pt/Pd samples, the calculated ratio came from the average of 5 to 10 measures, results are presented in Table 16. The platinum content is higher than expected for the 75/25 and 25/75 ratio. The standard deviation was ± 3 % but the error on the EDX measures was estimated to ca. 10 %. To confirm the presence of silicium into/onto the nanoparticles, an EDX spectrum was recorded on alumina supported sample after calcination (Figure 88 b). A Si signal was found at 1.75 keV confirming the Pt/Pd/Si composition of the nanoparticles but we were not able to record repeatable Si/M ratio.

. Loading on high surface area carbon

To study the intrinsic catalytic properties of nanoparticles, a high surface area carbon (Vulcan XC-72R) was used as support, [133][134][135] it presents a surface area of 242 m 2 .g -1 (Appendix 26).

Pure Pt, pure Pd and 1:3, 1:1 and 3:1 bimetallic nanoparticles were impregnated with a 50 wt% loading. The efficient dispersion of nanoparticles onto the high surface area carbon was checked by TEM (Figure 89). If dispersed in THF, the supported nanoparticles leach from the support, see Figure 89-a. In a water/isopropanol 80/20 solution, in which nanoparticles were less soluble, no leaching from the carbon was observed (Figure 89-b).

Cyclic voltamperometry

Cyclic voltammograms were recorded for each catalyst in de-aerated sulfuric acid at 0.5 mol.L -1 . The intensity was plotted versus the potential against the mercury/mercurous sulfate electrode. The potential of the Hg/HgSO 4 vs. the hydrogen reference electrode being 0.68 mV.

Catalysts were scanned from -0.67 mV (0.1 mV/RHE) to increasing voltage from 0.5 to 0.7 mV (Figure 90). Voltage was increased after two cycles. In the positive scanning direction, the first broad peak corresponds to hydrogen under potential desorption (H upd ) on the nanoparticles surface. The second peak corresponds to the formation of a hydroxide layer onto the surface and the oxidation of the metal. In the negative scanning direction (I < 0 mA) the first peak corresponded to reduction of the catalyst and the second at the minimum ab potential corresponded to the reduction of H + into H 2 . We compare on Figure 90 the platinum reference produced by Tanaka to octylsilane stabilized platinum nanoparticles. On cyclic voltammogram a) no modification of the H desorption peak and thus of the electroactive surface area (ESA) was noticed. For Pt/octylsilane, integration of the H upd surface area of each cycle showed no modification of the surface area too. The reduction of the platinum catalyst occured from 0.10 mV to 0.03 mV depending on the maximum potential reached during the cyclic voltamperometry (CV) experiments. 136 For Pt/octylsilane nanoparticles, this reduction occured at lower potential, from -0.10 to -0.33 mV. This is logically correlated to a stronger oxidation as seen between 0.4 and 0.7 mV in the positive scanning direction. All together those observations support the hypothesis of non-purely platinum nanoparticles and the probable incorporation of silicium inside its structure modifying its red/ox potential. No reference on Pt/Si was found but similar observations were found for alloyed nanoparticles, for example PdCo, 137 PtFe. 21 Figure 90. CV under N 2 of Pt/C with varying maximum potential: 0.5 -0.6 -0.7 and 0.5 mV/Hg. Arrows show the scanning direction.

CVs of colloidal nanoparticles under N 2 were compared to the reference (Figure 91).

Similarly to platinum and palladium nanoparticles, shifted single reduction peaks were observed for bimetallic nanoparticles witnessing the homogeneity of the surface. 138 Moreover, the Pt-H desorption peak presented a maximum potential that was a function of the nanoparticle composition as plotted in Figure 92. A linear increase in the potential was observed vs. the platinum content of the nanoparticles. This support the formation of alloyed nanoparticles against core/shell Pd/Pt nanoparticles as previously hypothesized from XPS results which was in good accordance with literature. 23,139 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 -0,2 The electroactive surface area (ESA) were measured by the integration of the H upd zone and reported in Table 17. The theoretical ESA was compared to the calculated one (see Chapter I) and the ratio ( M) gave the percentage of metal efficiently used for catalysis. Small ratios were found (0.1 to 28%) compared to the reference (56%); the nanoparticles being covered by octylsilane which then probably limited the surface accessibility to gases and hinder their catalytic properties.

-0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 -0,1 0,0 0,1 ). The mass current density could be calculated from the platinum loadings and plotted vs. the nanoparticles composition (Figure 93). Current densities from 0.12 to 1 A.g -1

Pt

were reported. Those values were 4 to 20 times lower than the one of the reference (3.9 A.g -1 Pt ). Of our catalyst, the 3:1 Pt/Pd nanoparticles was found to be 1.4 times more efficient for the ORR than pure platinum or 1:1 and 1:3 Pt/Pd ratios. Those results were comparable to results recently obtained by Lee et al. 24 where an enhancement of the mass current density of 2 was found for 3:1 Pt/Pd nanoparticles.

Chapter conclusion

The synthesis of platinum and palladium nanoparticles stabilized by octylsilane led to the formation of extremely small nanoparticles (bellow 2 nm) with narrow size dispersions (typically less than 0.4 nm). This methodology was successfully extended to Pt II precursors leading to even smaller nanoparticles (1.2 nm). The use of an external reductant (H 2 ) proved to be unnecessary making this nanoparticles synthesis method particularly simple, fast and safe. The formation of more complex systems, i.e. bimetallic nanoparticles was tested and successfully attained for Pt/Pd nanoparticles. For catalysis applications, these nanoparticles were further dispersed onto various supports: alumina, silica and high surface area carbon.

The removal of the alkyl chains is a key point for gas accessibility and is easily performed onto silica or alumina by calcination under air at 320°C. On high surface area carbon more work is necessary to achieved better catalytic performances in the Oxygen Reduction Reaction. For this reaction, 3/1 Pt/Pd nanoparticles proved to be more efficient than pure platinum.

The definite structure of octylsilane stabilized nanoparticles is not yet fully understood. From the release of n-octane during the synthesis, silicium atoms are believed to be embedded into the nanoparticles structure. After calcination, the formation of SiO 2 is proven by XPS. A core/ shell M/SiO 2 structure was expected but no evidence was found by microscopy. EXAFS results are currently under analyses and should lead to a better understanding of this system.

The straight forward synthesis of those nanoparticles and their particularly small size makes them very good candidates for catalytic applications and more work into that direction is encouraged.

Experimental

Preparation of palladium nanoparticles

Using dihydrogen as an external reductant

In a glove-box, 100 mg of Pd(dba) 2 (0.17 mmol) were introduced into a schlenk. Using schlenk lines, the complex was dissolved in freshly distilled THF and transferred into a batch reactor with thick glass walls. The THF volume was completed up to 116 mL. 0.58 mL of THF containing 25.1 mg of octylsilane (1 equiv. -0.17 mmol) were added. After partial evacuation, the reactor was pressurized under dihydrogen at 3 bars and stirred overnight at room temperature.

In the absence of dihydrogen

In a glove-box, 100 mg of Pd(dba) 2 (0.17 mmol) were introduced into a schlenk. Using schlenk lines, the complex is dissolved in 116 mL of freshly distilled THF. The solution was heated up to 60°C. After stabilization of the temperature 0.58 mL of THF containing 25.1 mg of octylsilane (1 equiv. -0.17 mmol) were rapidly added. The schlenk is quickly closed and stirred overnight at room temperature.

Using dihydrogen as an external reductant

In a glove-box, 100 mg (0.15 mmol) of Pt(dba) 2 were introduced into a schlenk. Using schlenk lines, the complex was dissolved in freshly distilled THF (over Na/benzophenone) and transferred into a batch reactor with thick glass walls. The THF volume was completed up to 100 mL. 0.50 mL of THF containing 21.7 mg of octylsilane (1 equiv. -0.15 mmol) were added. After partial evacuation, the reactor was pressurized under dihydrogen at 3 bars and stirred overnight at room temperature.

In the absence of dihydrogen

In a glove-box, 100 mg (0.15 mmol) of Pt(dba) 2 were introduced into a schlenk. Using schlenk lines, the complex was dissolved in 100 mL of freshly distilled THF. The solution was heated up to 60°C. After stabilization of the temperature 0.50 mL of THF containing 21.7 mg of octylsilane (1 equiv. -0.15 mmol) were rapidly added. The schlenk was quickly closed and stirred overnight at room temperature.

From Pd(dba) 2

Using dihydrogen as an external reductant

Similar procedure to Pt(dba) 2 was used with 86 mg (0.15 mmol) of Pd(dba) 2 , 100 mL THF and 0.5 mL of THF containing 21.7 mg of octylsilane (1 equiv. -0.15 mmol).

In the absence of dihydrogen

Similar procedure to Pt(dba) 2 was used with 86 mg (0.15 mmol) of Pd(dba) 2 , 100 mL THF and 0.5 mL of THF containing 21.7 mg of octylsilane (1 equiv. -0.15 mmol). (COD)Pt(Me)(OSi(OtBu) 3 )

A similar procedure as described above was used with 7 mg (1.2 μmol) (COD)Pt(Me)(OSi(OtBu) 3 ), 4 mL of THF-d8 and 1.7 mg (1 equiv. -1.2μmol) of octylsilane in 0.1 mL THF-d 8 .

After 12.5 h a second equiv. of octylsilane was added (1.7 mg in 0.1 mL THF-d 8 ) under argon.

DOSY

The 1H 2D NMR DOSY spectra were collected at 25°C on a Brüker spectrometer Avance III working at 500 MHz. Samples were dissolved in THF and classical 5 mm Young tubes were used. A modified Brüker dstebpgp3s sequence with π composite pulses and ZQ filter was used to improve phase stability in the spectra series. Short gradient duration pulses were obtained using a trapezoidal shape (p30=1 ms, falling and raising times of 50 s), and the diffusion delay was set at 100 ms. Data were processed with Topspin 2.3 and NmrNotebook 2.5 with the maximum number of iterations.

IR Transmission

A droplet of the concentrated colloidal suspension was placed into two KBr windows under air. IR spectra were recorded on a Nicolet Magna 550FT spectrometer. Typically 32 scans were accumulated for each spectrum (resolution 4 cm -1 ).

Diffuse Reflectance Infrared Fourier Transformed spectroscopy (DRIFT)

DRIFT spectra were recorded on a Nicolet Magna 550 FT spectrometer using a custom infrared cell equipped with CaF 2 windows, allowing in-situ studies. Typically, 64 scans were accumulated for each spectrum (resolution 4 cm -1 ).

Impregnation on silica and alumina

Nanoparticles were supported onto high surface area silica and alumina.

Silica dehydroxylated at 700 °C, referred as SiO 2-700 , was obtained as follow: silica (Aerosil Degussa, 200 m².g -1 ) was compacted with distilled water, dried at 110 °C for 2 days, calcined (500°C under air for 4 h) and partially dehydroxylated under secondary vacuum (10 -5 mBar) at 500°C for 12 h and then at 700°C for 4 h. The surface area is 185 ± 5 m².g -1 as measured by N 2 adsoprtion and desorption isotherms at -196°C.

-alumina from SASOL (SBa-200) was used. The support was first previously calcined at 500°C under a dry air flow (40 mL.min -1 ) for 12 h, evacuated for 12 h at 700°C under secondary vacuum. This ensures a stable (up to 700°C) and clean (water and organic free)

support. The surface area is 243 ± 2 m².g -1 as measured by N 2 adsorption and desorption isotherms at -196°C.

Nanoparticles were impregnated onto silica and alumina using the Incipient Wetness Impregnation (IWI) with a targeted final content of 1 wt%. The pore volume was measured by impregnating pure THF, and determined to be 1.45 mL.g -1 for silica and 1.90 mL.g -1 for alumina.

H 2 chemisorption

Chemisorption experiments were performed on a BELSORB-max from BEL JAPAN. Around 200 mg of silica supported nanoparticles (1wt%) previously calcined and reduced were introduced in the absorptions cells in a glove-box. The samples were then degassed at 623K for 3h under vacuum (10 -6 mbar). The chemisorption measurements were performed at 298K, the pressures at equilibrium being recorded when the pressure variation was below 0.02% per minute. Platinum particle size estimations were based on truncated octahedron geometry, assuming complete reduction, and an H/Pt adsorption stoichiometry factor of 1.8.

Conclusion and perspectives

In this PhD thesis, we have explored the preparation of supported Pt nanoparticles starting from well-defined organometallic compounds via Surface Organometallic Chemistry (SOMC), Metal Organic Chemical Vapor Deposition (MOCVD) and Colloidal chemistry in order to control the size, the distribution and the composition of the final systems. We also extended the colloidal approach to Pd and PdPt alloy to exploit the unique property of PtPd alloys for the Oxygen reduction Reaction.

First, we tuned the decomposition property of a Pt II complex and introduced (COD)Pt(OSi(OtBu) 3 ) 2 as efficient precursor to obtain small Pt nanoparticles via MOCVD.

While successful, the thus-obtained Pt nanoparticles turned out to be inefficient catalysts for fuel cell applications, because of the formation of silica from the molecular precursor, which probably helped with the control of particle size, but also led to a loss of contact between the Pt particle and the conducting support.

Second, we have also prepared a series of well-defined (COD)Pt II (X)(Y) complexes in view of generating supported Pt nanoparticles via controlling the Pt-density/loading. This was done via the grafting of those Pt complexes and the growth through the tuning of the X/Y ligands.

The study primarily focused on silica, but was also tested on ceria. On silica, it was shown that it was possible to exploit the controlled density of surface silanols, obtained through the dehydroxylation temperature, to control the Pt loading, the lower the OH density the smaller the Pt loading and the smaller the particle size (2-3 nm). We also showed that there was not a great impact of the nature of the X/Y ligand on the final Pt particle size unless a Cl-ligand was present (5-40 nm). This later effect could be partially circumvented when H 2 treatment was performed under flow conditions, which probably allowed HCl to be efficiently removed.

When N(SiMe 3 ) 2 was used as a ligand, the grafting and the formation of Pt particles was accompanied with the formation of ≡SiOSiMe 3 surface species, without a major impact on Pt particle size; this is in sharp contrast with what was found with gold. 69 Using the same approach, Pt nanoparticles were also obtained on ceria, but they display smaller sizes (below 1.5 nm); this is probably due to the so-called strong metal support interaction (SMSI). In fact, the effect of chlorine on the nanoparticle size was very limited on this support.

Third, we develop a colloidal approach, where nanoparticles are prepared at room temperature in the absence of dihydrogen from (COD)Pt II precursors through the use of octylsilane as a reducing agent and a stabilizer of the thus-formed Pt nanoparticles. This yielded Pt nanoparticles of very small sizes (1.2 nm), and this approach was extended to Pt 0 and Pd 0 dba precursors, where particle sizes of 1.8 and 1.7 nm were obtained, respectively. This was then possible to extend this approach to the formation of PtPd alloys of various composition.

Preliminary investigation by XPS of those nanoparticles show that PtSi x and PdSi x , are probably formed either as alloys or solid solutions, and EXAFS analysis is currently under way to identify the structure of these nanoparticles. These colloids were deposited onto high surface area carbon and then tested for the Oxygen Reduction Reaction, providing encouraging results, The Pt/Pd 3:1 alloy demonstrating an enhancement of the catalytic properties by a factor of 1.4 vs. pure Pt. More work is certainly necessary on these systems;

one key aspect to tackle being the clean removal of the organic ligands in ORR. Moreover, these nanoparticles were supported onto several supports (silica, alumina and ceria) and are currently tested for the preferential oxidation of CO.

Overall this work demonstrates the importance of understanding and controlling the chemistry at a molecular level, the metallic precursor having a strong influence for applications as different as surface organometallic chemistry, MOCVD or colloidal chemistry.

_______________________________________________________________ RESUME en français

Cette thèse a pour but la mise au point de méthode de synthèse de nanoparticles de métaux nobles pour la catalyse. Le projet s'est concentré sur la synthèse de nanoparticles composées de platine et de palladium. Trois voies de synthèses ont été explorées, i) par chimie organométallique de surface (COMS)), ii) par dépôt chimique en phase vapeur de composés organométalliques (MOCVD, Metal Organic Chemical Vapor Deposition et enfin iii) par voie colloïdale. La voie chimie organométallique de surface a permis la synthèse de nanoparticules supportées sur silice avec des tailles d'environ 2 nm. Le transfert de cette méthodologie vers l'oxyde de cérium a été effectué et permet d'obtenir des nanoparticules de moins de 1.5 nm de diamètre. Ces dernières recherches ont pour finalité l'obtention de catalyseurs d'oxydation supportés sur oxyde de cérium utilisé pour la dépollution automobile et ouvre la voie à la chimie organométallique de surface sur oxyde de cérium. Par MOCVD des nanoparticules de 4 à 6 nm ont été synthétisées sur couche de diffusion de gaz (GDL) pour des applications en pile à combustible. Des essais en électrocatalyse ont démontré la viabilité de la méthode de synthèse et l'activité catalytique des particules ainsi formées pour la réaction de réduction de l'oxygène (ORR), réaction clé des piles à combustible H 2 /O 2 .Enfin, pour cette même application, des nanoparticules de Pt, Pd et Pt/Pd dans différentes proportions ont été synthétisé par voie colloïdale. Ces nanoparticules, stabilisées par l'octylsilane ont une taille allant de 1.5 à 1.8 nm. Pour la platine, l'influence du précurseur organométallique et de la température ont été étudié, la synthèse sans apport de réducteur extérieur (H 2 ) et à partir de précurseur Pt II a permis l'obtention de nanoparticles stables de 1.2 nm. Leur utilisation pour l'ORR a démontré l'intérêt de former des alliages Pt/Pd, la composition 3:1 démontrant une amélioration d'un facteur 1.4 par rapport au Pt.

___________________________________________________________________________ TITRE en anglais

Molecular approach towards the design and the preparation of supported and non-supported Pt-and Pd-based nanoparticles ___________________________________________________________________________

RESUME en anglais

The goal of this thesis is to develop synthesis methods for platinum and palladium based nanoparticles. We mainly focused on three synthesis pathways, i) Surface Organometallic Chemistry (SOMC), ii) metal organic chemical vapor deposition (MOCVD) and iii) a colloidal approach. Surface organometallic chemistry allowed the synthesis of silica supported nanoparticles with sizes of about 2 nm. The transfer of this methodology to cerium oxide has been carried out and allowed obtaining nanoparticles of less than 1.5 nm in diameter. Those researches had for purpose the synthesis of oxidation catalysts supported on ceria used for automotive pollution and paves the way for the surface organometallic chemistry on this oxide. By MOCVD, Pt nanoparticles from 4 to 6 nm were synthesized onto gas diffusion layers (GDL) for applications in fuel cell catalysis. Electrocatalysis results demonstrated the viability of this synthesis method and the efficiency of these catalysts for the Oxygen Reduction Reaction (ORR). For the same application, nanoparticles of Pt, Pd and Pt/Pd in various proportions were synthesized by the colloidal approach. These nanoparticles stabilized by octylsilane had a size ranging from 1.5 to 1.8 nm. For platinum, the influence of the organometallic precursor and temperature have been studied and synthesis without adding external reducing (H 2 ) and from Pt II precursor allowed obtaining stable nanoparticles of 1.2 nm. Their use for the ORR has shown interest to form alloys, the Pt/Pd 3:1 composition demonstrating an improvement by a factor of 1.

Faculté

  Chimie Physique Electronique Institut Universitaire de Technologie de Lyon 1 Institut Universitaire de Formation des Maîtres Institut de Science Financière et d'Assurances Directeur : M. le Professeur F. De MARCHI Directeur : M. le Professeur F. FLEURY Directeur : Mme le Professeur H. PARROT Directeur : M. N. SIAUVE Directeur : M. le Professeur S. AKKOUCHE Directeur : M. le Professeur A. GOLDMAN Directeur : M. le Professeur H. BEN HADID Directeur : Mme S. FLECK Directeur : Mme la Professeure I. DANIEL Directeur : M. C. COLLIGNON Directeur : M. B. GUIDERDONI Directeur : M. P. FOURNIER Directeur : M. G. PIGNAULT Directeur : M. C. VITON Directeur : M. R. BERNARD Directeur : Mme la Professeure

2 CeO 2 =Figure 1 .

 21 Figure 1. Three-way catalyst performance determined by engine air to fuel ratio, reproduced from ref 11

Figure 2 .

 2 Figure 2. TEM, Diesel Oxidation Catalyst (DOC), Pt and Pd nanoparticles (in a 2:1 ratio), Renault reference catalyst, reproduced from CEA internal analysis report.

Figure 4 .

 4 Figure 4. Polarization curve of a typical PEMFC, reproduced from 15

1 2Figure 5 .

 15 Figure 5. Price targets set by the DOE, reproduced from ref 17

Figure 8 .

 8 Figure 8. a) β-diketonates, R = Me, C 5 H 11 , CF 3 , b) (CODPt((Me) 2 , c) R = H, Me, Et with increased volatility

Figure 10 .

 10 Figure 10. Silanols on silica, a) isolated, b) vicinal, d) geminal

Figure 11 .

 11 Figure 11. Silanol surface density vs. the dehydroxylation temperature, reproduced from ref58

Figure 12 .

 12 Figure 12. DRIFT spectra of silica dehydroxylated at 200 and 700°C

Figure 15 .

 15 Figure 15. Grafting of Rh onto oxide supports and nanoparticles formation under H 2

Figure 16 .

 16 Figure 16. Schematic representation of the grafting of gold I onto SiO 2-700 and the subsequent formation of nanoparticles under H2 at 300°C. Reproduced from ref.69

  Nanoparticles from 3.5 to 6 nm, depending on the final reduction temperature, were synthesized. The initial introduced Pt/Pd ratio proved to drive the final Pt/Pd ratio found into single nanoparticles. These nanoparticles demonstrated enhancement in the Methanol Oxydation Reaction for Direct Methanol Fuel Cells and composition dependent catalytic properties.

Figure 17 .

 17 Figure 17. a)TEM image of the 5 nmPd 67 Pt 33 NPs, b) High resolution HAADF-STEM image (left) and bright field image (right) of a single Pd 67 Pt 33 NP, c) HAADF-STEM image and the corresponding elemental map of several representative 5 nm Pd 67 Pt 33 NPs, d) Correlation between the amount of Pt(acac)2 added and the amount of Pt obtained in the final PdPt NP product. Reproduced from ref. 23

Figure 19 .

 19 Figure 19. Hydrophilic and hydrophobic Pt nanoparticles syntheses

Figure 20 .

 20 Figure 20. Synthesis of Fe 3 O 4 nanoparticles (7 to 10 nm) using oleylamine as both reductant and stabilizer

Figure 21 .

 21 Figure 21. ORTEP drawing of compound A) (COD)Pt(Me)(OSi(OtBu) 3 ) and (COD)Pt(OSi(OtBu) 3 )(N(SiMe 3 )).H atoms are omitted for clarity.

Figure 23 .

 23 Figure 23. A IR-DRIFT spectra of a) SiO 2-200 , b) (COD)Pt(Me) 2 @SiO 2-200 , c) (COD)Pt(OSi(OtBu) 3 ) 2 @SiO 2-200 , d) (COD)Pt(Me)(OSi(OtBu) 3 )@SiO 2-200 , e) (COD)Pt(Me)(N(SiMe 3 ) 2 )@SiO 2-200 , f) (COD)Pt(Cl)(N(SiMe 3 ) 2 @SiO 2-200 , g) (COD)Pt(OSi(OtBu) 3 )(N(SiMe 3 ) 2 @SiO 2-200

Figure 24 .PtFigure 25 -

 2425 Figure 24. Schematic representation of the nanoparticles formation from (COD)Pt(X)(N(SiMe 3 ) 2 )@SiO 2-700

Figure 26 .

 26 Figure 26 .TEM picture (COD)Pt(Me)(OSi(OtBu) 3 )@SiO 2-700 after H 2 static treatment

  -complexes with at least one O-and N-bound ligands are more reactive and preferred over those having Cl or only Cbound ligands; this allows the implantation of higher density of Pt at the silica surface and in-fine higher metal loading. Of the various ligands studied -(tBuO) 3 SiO, (Me 3 Si) 2 N, Me and Clchloride is the only one that dramatically influences and increases the mean size and the size distribution of Pt nanoparticles, probably through the formation of HCl, which helps Pt migration; but this can be partially controlled by using flow conditions in place of static ones.

Figure 28 .

 28 Figure 28. IR spectra of a ceria pellet a) as prepared, b) calcined at 450°C, c) dehydroxylated at 450°C.

Figure 29 .

 29 Figure 29. Schematic representation of the μ2 type of surface OH on ceria

Figure 30 .Figure 31 .

 3031 Figure 30. I) Transmission IR spectra of a ceria pellet calcined at 200°C under air and dehydroxylated for 5 to 12 hours at a) 200°C, b)300°C, c) 400°C, d) 450°C, e) 500°c and f) 600°C. II) Zoom 3750 to 3050 cm -1 region.

Figure 32 .

 32 Figure 32. I) Transmission IR spectra after reduction/dehydroxylation cycles. II) Only spectra after dehydroxylation are presented.

Figure 34 .

 34 Figure 34. IR-DRIFT spectra of complexes grafted on CeO 2-400 a) CeO2-400, b) (COD)Pt(Me) 2 , c) (COD)Pt(Me) 2 @CeO2-400, d) (COD)Pt(Me)(N(SiMe 3 ) 2 ), e) (COD)Pt(Me)(N(SiMe 3 ) 2 )@CeO 2-400 , f) (COD)Pt(Cl)(N(SiMe 3 ) 2 )@CeO 2-400 , g) (COD)Pt(Cl)(N(SiMe 3 ) 2 ).

Figure 36 .

 36 Figure 36. TEM picture after static H2 treatment of a) (COD)Pt(Me)(N(SiMe 3 ) 2 )@CeO 2-400 , b) (COD)Pt(Cl)(N(SiMe 3 ) 2 )@CeO 2-400

Figure

  Figure S3. TEM

Figure

  Figure S6. TEM

13 C

 13 Figure S7.Liquid NMR of 2 (COD)Pt(Me)(OSi(OtBu) 3 ) in C 6 D 6 .

Figure S8 . 13 C 1 H

 S8131 Figure S8. IR-DRIFT

Figure S12 .

 S12 Figure S12. IR-DRIFT

13 C 13 C

 1313 Figure S13.Solid-state NMR of (COD)Pt(Me)(N(SiMe 3 ) 2 )@SiO 2-200

13 C 13 C

 1313 Figure S16.Solid-state NMR of (COD)Pt(Cl)(N(SiMe 3 ) 2 @SiO 2-200

Figure

  Figure S17. TEM

Figure S18 .

 S18 Figure S18. Liquid state NMR of 3 (COD)Pt(OSi(OtBu) 3 )(N(SiMe 3 ) 2 ) in C 6 D 6 .

Figure S19 . 13 C 13 C 2 s

 S1913132 Figure S19. IR-DRIFT

Figure 37 .

 37 Figure 37. Platinum loading vs. deposition time

Figure 38 .

 38 Figure 38. Evolution of the Si to Pt ratio versus deposition time

Figure 39 .Figure 40 .

 3940 Figure 39. TEM picture and size repartition diagram, 20 min deposition. 200 nanoparticles count: 3.7 nm (± 0.6)

Figure 41 .

 41 Figure 41. SEM picture (30 keV) and EDX spectrum, GDL 24BC, 100 min deposition.

Figure 42 .

 42 Figure 42. XRD spectrum of a wafer deposited with (COD)Pt(O(SiOtBu) 3 ) 2 , a) 20 minutes, b) 100 minutes

Figure 43 .

 43 Figure 43. XPS spectra of a) (COD)Pt(OSi(OtBu) 3 ) 2 and b) (nbd)Pt(Me) 2 after MOCVD on Ti/TiO 2 pellets.

  115

Figure 44 .

 44 Figure 44. Si 2p spectra of deposited (COD)Pt(OSi(OtBu) 3 ) 2

Figure 45 .

 45 Figure 45. Pt 4f spectra of MOCVD deposited a) (COD)Pt(OSi(OtBu) 3 ) 2 and b) (nbd)Pt(Me) 2 .

  between -0.01 and 0.24 V vs. RHE and the current density can be drawn versus time. For clarity, only current density values at 0.24 V versus times are plotted (Figure47).

Figure 47 .

 47 Figure 47. Current density vs. time under H 2 at 0.24 V/RHE from 0 to 200 seconds and then H 2 + CO (50ppm), a) 24BC, b) E-TEK

and 4 .

 4 1 nm particles are expected. The lower values indicate either very large particles (>50nm) or that only a fraction of the deposited particles are electroactive. The first hypothesis is rejected as SEM confirmed the formation of about 4 to 6 nm nanoparticles on GDLs and the absence of large platinum patches.

Figure 48 .

 48 Figure 48. Current density vs. electroactive platinum surface area. The blue dashed line is a help for the eyes.

Figure 49 .

 49 Figure 49. Cyclic voltammogram under O 2 , scanning rate 5 mV.min -1 , gas flow 30 mL.min -1 , RT, a) 24BC, b) E-TEK. Deposition time from 20 to 100 min.

Figure 50 .

 50 Figure 50. Current densities at 0.8 V/RHE

  8 V (μA/cm²) Pt surf ace area (cm²Pt/cm² of GDL) 8 V (μA/cm²) Pt surf ace area (cm²Pt/cm² of GDL) Etek economic viability of platinum as a fuel cell catalyst, 118 were achieved. The formation of Pt nanoparticles takes place concomitantly with the formation of silicon dioxide. The evaluation of the catalytic properties of such catalyst layer was made in the Oxygen Reduction Reaction in half-cell set-up. Despite the platinum loadings achieved and the small nanoparticles sizes, low catalytic properties were measured compared to those of the reference sample. In fact, XPS demonstrated an intimate interaction between silicon oxide and platinum. Since silicon dioxide is an insulating material, it likely hinders the electrical pathway from the nanoparticles to the conductive GDLs and thus lowers the electrocatalytic properties. This poisoning could result from several effects as depicted in Figure 51 a to e. a) Platinum and silicon dioxide nanoparticles are separately synthesized but in close relation. b) Silicon dioxide particles surrounding platinum could limit the electrical pathway to the GDL. c) A SiO 2 shell formed around the Pt nanoparticles could block the gas access to the catalytic sites. d) Silicon patches around the nanoparticles could lower the accessible surface area. e) Formation of a Pt-Si/SiO 2 solid solution.

Figure 51 .

 51 Figure 51. Scheme of the catalyst layer on GDL

  Elemental analyses were performed at "Mikroanalytisches Labor Pascher" in Germany.Transmission Electron Micrographs (TEM) were obtained on a Philips CM120 Transmission Electron Microscope at the "Centre Technologique des Microstructures Université Lyon 1" in France the acceleration voltage was set to 120 kV. Scanning electron microscopy (SEM) was performed at the CEA Grenoble at an acceleration voltage of 30 kV on the XX equipment. X-Ray diffraction analyses were performed on the LITEN, CEA Grenoble, Bruker D8 Advance diffractometer (40 kV & 30 mA) with CuKα radiation ( = 0.154 nm). The diffraction patterns were collected in the 2 angle range [20°-80°] at a scanning rate of 0.1°/min.

Figure 52 .

 52 Figure 52. DLI-MOCVD set-up 113

Figure 54 .

 54 Figure 54. Membrane electrode assembly used for half fuel-cell set-up

  Those tests were carried out at the Laboratoire Composants Piles à combustible, Electrolyse et Modélisation (LCPEM) of the CEA-Grenoble under the supervision of Dr. Nicolas Guillet.The characterization of our samples was performed in a half-cell set-up as described below (Figure55.). The standard electrolyte used was a H 2 SO 4 freshly prepared solution at 0.5 mol.L -1 and was deoxygenized by N 2 bubbling at least 20 minutes before testing. Three electrodes were used (working electrode, reference electrode and counter electrode). The counter electrode was made of platinum and the reference electrode was Hg/HgSO4 (E°= -0.698 mV vs. Reversible Hydrogen Electrode -RHE). The active part of the MEA has a geometric surface area of 0.5 cm². The feeding gas at the working electrode could be switch between N 2 , O 2 , H 2 or H 2 + 50ppm CO. Therefore we could evaluate the performances of the catalysts in the Oxygen Reduction Reaction (ORR), Hydrogen Oxidation Reaction (HOR) and against the CO poisoning.

Figure 55 .

 55 Figure 55. Half-cell set-up scheme

Figure 56 .

 56 Figure 56. Typical cyclic voltammogram under N 2 after CO adsorption on a Pt catalyst 117 1. H + + Pt surface + e -→ H-Pt(surface) 2. H-Pt surface → H + + Pt(surface) + e - 3. CO-Pt surface + H 2 O → CO 2 + 2H+ + 2e-+ Pt surface 4. Pt + xH2O → PtOx + 2xH + + 2xe -Metal oxidation 5. PtOx + 2xH + + 2xe -→ Pt + xH2O Metal reduction

Figure 57 .

 57 Figure 57. Cyclic voltammogram under N 2 at a 20 mV.s -1 scan. Sample 24BC 100 minutes.

Figure 58 . 3 . 4 . 4 . 2 . 14 . 8 Appendix 15 .

 58344214815 Figure 58. Cyclic voltammogram under N 2 after CO adsorption at a 5 mV.s -1 scan rate. Sample 24BC 100 minutes.3.4.4.2. Oxygen Reduction Reaction

  Upon addition of 1 equiv. of octylsilane, pressurization under 3 bars of H 2 and mixing, the coloration change from purple (color of Pt(dba) 2 ) to black indicating the formation of nanoparticles within 8 to 10 h. Nanoparticles of 1.8 nm in diameter were obtained with narrow size dispersions 0.3 nm (Figure 59).

Figure 59 .

 59 Figure 59. Pt nanoparticles synthesized from Pt(dba) 2 and stabilized by 1 equiv. octylsilane and the associated histogram with mean size and the associated standard deviation. 200 nanoparticles count.The byproducts of the reaction were further analyzed. After filtration over alumina to trap the Pt nanoparticles the liquid phase was analyzed by gas chromatography coupled with mass spectrometry (GC-MS). 1-5,dicyclohexylpentane and n-octane were detected.

Figure 61 .

 61 Figure 61. TEM pictures of Pt nanoparticles prepared at 60°C with one equivalent of octylsilane.

Figure 62 .

 62 Figure 62. a) Pt(dba) 2 , b) after reaction at 60°C, 15 h.

Figure 63 .

 63 Figure 63. Liquid state NMR of Pt nanoparticles formation in THF-d 8

  CH 3 and -(CH 2 ) 6 -see Appendix 20 a) and 1-5,dicyclohexylpentane. At 7.2 ppm, residual aromatic protons were attributed to non-hydrogenated dba. The broad shape of those peaks indicated the interaction of this residual dba with the nanoparticles surface. The expected H 2 signal at 4.55 ppm cannot be found indicating both its full consumption and its probable partial loss during the silane addition process. The peaks at 1.29 and 0.89 ppm were attributed to octane (see Appendix 20 b). Octane was found from the beginning of the reaction but its concentration evolves up to 14 h after the beginning of the reaction. No -SiH 3 signals was found (3.47 ppm) indicating the fast and total reaction of silane with the platinum complex. The -CH 2 -Si-signal (0.77 ppm) was not seen but could be overlapped by the large peak centered on 0.85 ppm and shifted/broadened due to the formation of Pt nanoparticles as already observed by other for Au122,123 or Ru73 and referred to as the "Knight shift" effect.

  was found to be unreactive towards octylsilane at room temperature after 3 days as no coloration change was observed. Addition of 3 bars of H 2 as an external reductant for 15 h did not led to any improvement of the reactivity. Finally, the heating at 60°C for 15 h yielded a brown solution and TEM microscopy revealed the formation of microsize aggregates (Figureinduceddegradation of the complex. The strong Pt-Me bonds are believed to hindered the reactivity of (COD)Pt(Me) 2 towards octylsilane.

Figure 64 .

 64 Figure 64. TEM image, (COD)Pt(Me) 2 + 1 equiv. octylsilane, 60°C, 15 hrs.

Figure 65 .

 65 Figure 65. TEM images, a) 0.3 equiv. octylsilane, RT, 30 min., b) 0.3 equiv. octylsilane, RT, 15hrs, c) 1 equiv. octylsilane, RT, 30 min., d) 1 equiv. octylsilane, RT, 15 hrs.

Figure 66 .

 66 Figure 66. TEM images, (COD)Pt(OSi(OtBu) 3 ) 2 , 1 equiv. of octylsilane a) 60 min at RT, b) 90 min at 60°C, c) 15 h at 60°C

Figure 67 .

 67 Figure 67. IR spectra of a) (COD)Pt(OSi(OtBu) 3 ) 2 , b) COD, c) HOSi(OtBu) 3 , d) 0.3 equiv. C 8 H 17 SiH 3 , e) 1 equiv. C 8 H 17 SiH 3 , f) C 8 H 17 SiH 3

Figure 68

 68 Figure 68. a) SiO 2-700 , b) after IWI, c) calcined ,d) reduced ,e) HOSi(OtBu)3 @ SiO 2-700 after calcination

Figure 69

 69 Figure 69. a) as prepared after IWI, b) calcined, c) reduced

  119

Figure 70 .

 70 Figure 70. NMR spectra, synthesis of Pt nanoparticles from (COD)Pt(OSi(OtBu) 3 ) 2 + 1 equiv. octylsilane at RT in THF-d 8 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm

Figure 72 .

 72 Figure 72. 1 to 0.5 ppm zoom-in

Figure 73 . 1 H

 731 Figure 73. 1 H-DOSY experiment of (COD)Pt(OSi(OtBu) 3 ) 2 + 1 equiv. octylsilane after complete reaction

Figure 74 . 4 . 2 . 2 . 3 .

 744223 Figure 74. Schematic representation of a platinum nanoparticle stabilized by octylsilane

Figure 75 .

 75 Figure 75. TEM images, colloids synthesized after 30 min. from 1 equiv. of octylsilane and a) (COD)Pt(Me)(OSi(OtBu) 3 ) and c) (COD)Pt(Cl)(Me).

Figure 76 . 1 H 1 .

 7611 Figure76.[START_REF] Curry | [END_REF] H NMR spectra at RT in THF-d 8 , synthesis of Pt nanoparticles from (COD)Pt(Me)(OSi(OtBu) 3 ) 2 A) 8 min after addition of 1 equiv. octylsilane, B) after 12.5 h, C) 12 h after addition of a second equiv. of octylsilane 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 ppm

Figure 77 .

 77 Figure 77. Palladium nanoparticles synthesized with one equiv. of octylsilane in THF

Figure 78 .

 78 Figure 78. UV-Vis spectrum of a) Pd(dba) 2 and b) Pd colloidal solution

  When one equiv. of octylsilane was rapidly added (about 1 second), a dramatic coloration change from pink to dark black was noticed in a few seconds. TEM images of the nanoparticles after overnight reaction at 60°C are presented in Figure79. No further NMR analyses were performed onto the Pd system. The formation process being probably the same as from Pt(dba) 2 as indicated by kinetic observations and by products analyses.

Figure 79 .

 79 Figure 79. TEM pictures of Pd nanoparticles prepared at 60°C

Figure 80 .

 80 Figure 80. a) Pd colloid, b) Pt(dba) 2 ; Pt(dba) 2 /Pd(dba) 2 molar ratio c) 3/1, d) 1/1, e) 3/1; f)Pd(dba) 2 . Peaks are normalized to 1 at maximum intensity.

Figure 81 .

 81 Figure 81. TEM images and corresponding size repartition diagram

Figure 82 .

 82 Figure 82. IR-DRIFT spectra after IWI at SiO 2-700 of a) Pt colloid, b) Pt calcined, c) Pt/Pd 1/1 colloid, d) Pt/Pd 1/1 calcined, e) Pd colloid, f) Pd calcined, g) pure octylsilane signal (absorbance)

Figure 83 . 4 . 4 . 2 .

 83442 Figure 83. TEM pictures of silica supported nanoparticles, as prepared and calcined

  and repeated for the purpose of this study. Pt-Pt and Pt-Si bonds were evidenced by peaks at 71.6/74.9 and 72.6/75.6 eV and confirmed the formation of Pt-Si bonds

Figure 86 .

 86 Figure 86. Si 2p XP spectra of Pt/Pd 1:1 nanoparticles before and after calcination

Figure 87 . 3 .

 873 Figure 87. HR-TEM picture of Pt/Pd 1:1 nanoparticles

Figure 88 .

 88 Figure 88. HAADF-STEM images and associated EDX spectra of Pt/Pd 1/1 nanoparticles, a) supported on TEM grid, b) supported on alumina and calcined

Figure 89 .

 89 Figure 89. TEM picture of Pd nanoparticles loaded onto Vulcan XC-72R a) dispersed in THF, b) dispersed in water/isopropanol 80/80.

Figure 91 .

 91 Figure 91. CV at 20 mV.s -1 scanning rate under N 2 .

Figure 92 .

 92 Figure 92. Potential of the Hupd peak as a function of the platinum content.

4. 7 . 2 . 3 . 2 4 . 7 . 2 . 4 . 3 ) 4 . 7 . 2 . 5 . 4 . 7 . 3 . 4 . 7 . 1 . 2

 72324724347254734712 From (COD)Pt(OSi(OtBu) 3 )In a glove-box, 125 mg (0.15 mmol) of Pt(dba) 2 were introduced into a schlenk. Using schlenk lines, the complex is dissolved in 100 mL of freshly distilled THF. After complete dissolution, 0.50 mL of THF containing 21.7 mg of octylsilane (1 equiv. -0.15 mmol) were rapidly added. The schlenk is quickly closed and stirred overnight at 60°C. From (COD)Pt(Me)(OSi(O t Bu) Similar procedure to (COD)Pt(OSi(OtBu) 3 ) 2 was used with 87 mg (0.15 mmol) of (COD)Pt(Me)(OSi(OtBu) 3 ) in 100 mL THF and 0.5 mL of THF containing 21.7 mg of octylsilane (1 equiv. -0.15 mmol). From (COD)Pt(Cl)(Me) Similar procedure to (COD)Pt(OSi(OtBu) 3 ) 2 was used with 53 mg (0.15 mmol) of (COD)Pt(Cl)(Me) in 100 mL THF and 0.5 mL of THF containing 21.7 mg of octylsilane (1 equiv. -0.15 mmol). Preparation of Bimetallic nanoparticles Pt/Pd 1:1 nanoparticlesIn a glove-box, 50 mg (0.075 mmol) of Pt(dba) 2 were introduced into a first schlenk and 43 mg (0.075 mmol) of Pd(dba) 2 were introduced in a second one. Using schlenk lines, the complexes were dissolved in freshly distilled THF and transferred successively into a batch reactor. The THF volume was completed up to 100 mL before 0.50 mL of THF containing 21.7 mg of octylsilane (0.15 mmol) were added. After partial evacuation, the reactor was pressurized under dihydrogen at 3 bars and stirred overnight at room temperature.Pt/Pd 3:1 nanoparticlesA similar procedure was used with 75 mg (0.11 mmol) of Pt(dba) 2 and 22 mg (0.04 mmol) of Pd(dba) 2 in 100 mL of THF. 0.50 mL of THF containing 21.7 mg of octylsilane (0.15 mmol) were added.Pt/Pd 1:3 nanoparticlesA similar procedure was used with 25 mg (0.04 mmol) of Pt(dba) 2 and 65 mg (0.11 mmol) of Pd(dba) 2 in 100 mL of THF. 0.50 mL of THF containing 21.7 mg of octylsilane (0.15 mmol) were added. UV-Vis spectroscopy UV-Vis spectra were recorded on a Perkin Elmer lambda 1050 spectrometer from 210 to 800 nm with 2 nm steps. Absorbance of the sample was measured against a reference of dry THF in a quartz cell and the samples were prepared by diluting 50 μL of colloidal solutions (initially at 1.5 mmol of metal per liter) into 3 mL of dry THF.In a glove box, 10 mg (1.2 μmol) of (COD)Pt(OSi(OtBu) 3 ) 2 were introduced into a Young NMR tube and stored into a schlenk. Using a schlenk line, 0.4 mL of THF-d 8 were introduced into the NMR tube. 1.7 mg (1 equiv. -1.2 μmol) of octylsilane in 0.1 mL THF-d 8 were added and the NMR tube was tightly closed. The tube was shaken a few seconds before being introduced into the NMR probe. 1 H-NMR spectra were recorded on a Brüker Avance 300 MHz spectrometer, 8 scans were recorded for each spectrum at regular intervals.

  125

4. 7 . 6 . 4 . 7 . 7 . Carbon supported nanoparticles preparation 4 . 7 . 7 . 1 . 4 . 7 . 7 . 3 . 4 . 7 . 8 . 2 Appendix 20 .Appendix 27 .

 764774771477347822027 X-Ray photoelectron spectroscopy (XPS)Spectra were recorded at the CEA-Grenoble, LITEN, on the SSI-SProbe XPS spectrometer with a Kα monochromated Al source. Samples are transferred from the glove-box to the spectrometer using a hermetic transfer box. Spectra are recorded with a collection angle of 35° and a pass energy of 25 eV for high resolution spectra. The pressure in the analysis chamber was bellow 10 -9 Pa. Peaks are calibrated toward the C1s (adventitious contamination carbon) peak set at 284.8 eV. Synthetic components on Pt4f were analyzed with a Shirley background subtraction and the peaks shape were resolved with a combination of gaussians and lorentzians (30%). Vulcan preparation Prior to use Vulcan (1 g) was cleaned in a 100 mL 5M HCl solution. After 60 seconds of sonication, the solution was stirred and heated up to 50°C for 18 h. The solution was centrifugated (6000 rpm, 10°C, 20 minutes) and the supernatant eliminated. 20 mL of water were added, the solution was sonicated for 60 seconds and again centrifugated, finally the supernatant was eliminated. This procedure was repeated 6 times until a constant pH (7) of the supernatant was reached. A final procedure was performed with ethanol and the Vulcan was finally dried at 80°C under ambient pressure. 4.7.7.2. Nanoparticles loading 22 mg of Vulcan were introduced into a 10 mL THF solution of 22 mg colloidal platinum nanoparticles previously prepared. This solution was sonicated for 15 minutes in order to load the nanoparticles. THF was then evaporated under an air flow. A sticky solid was obtained. This procedure was repeated for Pt, Pd and 3/1, 1/1, 1/3 Pt/Pd nanoparticles. Preparation of the rotating electrodes The carbon supported nanoparticles were dispersed by 5 minutes sonication in 22 mL of a water/isopropanol 80/20 solution, yielding a solution containing 1mg of metal per mL. Nafion (220 mg of a 5wt% Nafion solution in water -11 mg of Nafion) was added and again the solution was sonicated for 5 min. A 25 μL drop of the solution was deposited onto a glassy carbon rotating electrode (0.25 cm 2 ) and dried under ambient pressure at 80°C for about 10 minutes. Electrocatalysis Rotating disk electrode (RDE) measurements were conducted in an interchangeable RDE holder (Pine Instruments, USA). A Pt-foil counter-electrode and a Hg/HgSO 4 reference electrode (separated by an electrolyte bridge) were used in a standard three-compartment electrochemical cell. Tests are performed into freshly prepared sulfuric acid solution (0.5 mol.L -1 ). Voltammograms were recorded using the EC-Lab software. Cyclic voltamperometry under N Liquid state NMR in THF-d 8 (chemical shifts are referenced against the THF peak at 3.58 ppm): ORR polarization curve of pure and alloyed nanoparticles in H 2 SO 4 saturated with N 2 or O 2 and the difference O 2 -N 2 . Potential was sweep from 0.5 to 0 and back to 0.5 V at a 20 mV.s-1 scanning rate. The rotating electrode speed was set-

  V. MAUME-DESCHAMPS

	Abbreviations
	NMR	Nuclear Magnetic Resonance
	SS-NMR	Solid-State NMR
		chemical shift in NMR downfield from TMS, Hz or ppm
	TMS	tetramethylsilane, Si(CH3)4
	CP	Cross-Polarisation
	MAS	Magic Angle Spinning
	DOSY Hz n J A-B	Diffusion Ordered Spectroscopy Hertz, s -1 scalar coupling constants between nuclei A B separated by n bonds, Hz
	XRD	X-Ray Diffraction
	IR	infrared
	DRIFT	Direct Reflectance Infrared Fourrier Transform spectroscopy
	A-B)	deformation frequencies of A-B bond
	(A-B)	stretching frequencies of A-B bond
	TEM	Transmission Electron Microscopy
	XPS	X-ray Photoelectron Spectroscopy
	GC	Gas Chromatography
	MS	Mass Spectroscopy
	EXAFS	Extended X-ray Absorption Fine Structure
	HR-TEM	High-Resolution Transmission Electron Microscopy
	ESA	Electroactive Surface Area
	nm	nanometer
	Å	angström
	°C	relative temperature in Celsius degree: T = °C + 273.15
	MOCVD	Metal Organic Chemical Vapor Deposition
	SOMC	Surface OrganoMetallic Chemistry
	ORR	Oxygen Reduction Reaction
	HOR	Hydrogen Oxidation Reaction
	COD	1,5-cyclooctadiene
	dba	dibenzylideneacetone
	Me	methyl, CH 3
	tBu	tert-butyl
	equiv. eV	molecular equivalent (mol / mol) electron-volts, 1 eV = 1.60217646 × 10 -19 J
	h	hour
	min	minute
	s	second
	M	metal or molar concentration
	NP	nanoparticles
	RT	Room Temperature
	1D, 2D, 3D

1.3.2.2.2. Cerium oxide

  Cerium oxide, also called ceria, has a long history of academic research and industrial applications as it is a key component of three-way catalyst for car exhaust depollution as previously discussed. We are interested in developing SOMC onto this substrate for the design of Diesel Oxidation Catalysts. It has a general formula of CeO 2 and present a fluorite-like (CaF 2 ) cubic structure in which each cerium site, in a face-centered cubic (fcc)

Table 1 .

 1 Selected bond lengths and angles for (COD)Pt(X)(Y) compounds. [a]: this work.

	A	C1	Pt C2	C6	C5	B	C1 C2	Pt	C6 C5
		C7		O1		N		O1
	X	Y		Pt-(C=C trans X ) Pt-(C=C trans Y ) Pt1-X	Pt1-Y C=C trans X C=C trans Y Ref.
	Cl	Cl		2.164	2.177	2.315	2.309	1.376	1.388	[ 89 ]
	Me	Cl		2.280	2.14	2.085	2.316	1.347	1.383	[ 91 ]
	Me	Me		2.234	2.229	2.068	2.056	1.374	1.374	[ 90 ]
	OSi(OtBu) 3 OSi(OtBu) 3		2.129	2.129	1.982	1.982	1.331	1.331	[ 92 ]
	Me	OSi(OtBu) 3		2.268	2.11	2.042	1.987	1.346	1.413	[a]
	Cl	N(SiMe 3 ) 2		2.156	2.201	2.322	2.045	1.383	1.361	[a]
	OSi(OtBu) 3 N(SiMe 3 ) 2		2.131	2.185	1.998	2.054	1.390	1.386	[a]

Table 2 .

 2 NMR data of platinum compounds (COD)Pt(X)(Y) in C 6 D 6 , Pt-H coupling constants are given into brackets. δ HXC= is the chemical shift of the olefinic proton of the COD trans to the X ligand.

	1 H	13 C

.3. Grafting onto silica 2.3.1. Symmetrical complexes

	We first compared the reactivity of symmetrical Pt II complexes (COD)Pt(X)(Y) with X = Y =
	OSi(OtBu) 3 or Me using SiO 2-200 as support. The reactivity of (COD)Pt(Cl) 2 could not be
	evaluated in view of its poor solubility in most solvents. In the case of (COD)Pt(Me) 2, while
	the formation of CH 4 clearly indicate an grafting reaction, only a partial consumption of
	surface isolated silanols of SiO 2-200 was observed as evidenced by the presence of residual
	isolated silanols ( (OH) = 3747 cm -1 ) according to IR-DRIFT spectroscopy. This is
	accompanied with the concomitant appearance of the characteristic bands associated with the
	COD and methyl groups, albeit in a very low intensity (Figure S1 -Figure SX referred to
	figures available in the appendix section). This indicates a partial grafting of the Pt complex,
	and in fact the Pt content was only 3.0 wt% (0.16 mmol of Pt.g -1 or 0.5 Pt.nm -2 ), which
	correspond to a grafting on ca. 18% of the surface silanols, referred thereafter as grafting
	yield. The carbon-13 cross-polarization (CP) NMR spectrum (Figure S2) of
	(COD)Pt(Me) 2 @SiO 2-200 under magic angle spinning (MAS) exhibits peaks at 118, 78, 32, 29
	ppm and 6 ppm. The first two signals at low fields correspond to the alkene moiety of the
	coordinated COD, respectively trans to a Me and to a siloxy group, clearly indicating the
	formation of (COD)Pt(Me)(OSi≡) (Figure

.2.1. (COD)Pt(Me)(OSi(OtBu) 3 )

	2.3.2. Dissymmetrical complexes	2-700 , similar results are obtained, albeit with a
	lower Pt loading (1.1 %wt, 0.06 mmol of Pt.g -1 or 0.2 Pt.nm -2 ), but similar grafting yield (21
	%). In view of the low reactivity of the Pt-Me bond compared to the Pt-OSi bond towards
	protonolysis by surface silanols, we decided to investigate the grafting of dissymmetrical
	systems having a methyl and a more labile ligand (i.e. siloxy group) in order to hopefully
	generate higher grafting yield of Pt-Me surface species. We also investigated the
	corresponding	bis(trimethylsilyl)amido	and	chloro	complexes,	namely
	(COD)Pt(Me)(N(SiMe 3 ) 2 , (COD)Pt(Me)(Cl), as well as (COD)Pt(Cl)(N(SiMe 3 ) 2 ).
	2.3
		Figure 22. Surface Pt II species after grafting (COD)Pt(Me) 2
	While the grafting of (COD)Pt(OSi(OtBu) 3 ) 2 was already reported on ordered mesoporous
	silica (SBA type) materials, 92 here we discuss the grafting on non-porous silica partially
	dehydroxylated at 200°C and 700°C. IR-DRIFT spectroscopy (Figure S4) shows that the
	characteristic (OH) silanols vibration (3747 cm -1 ) disappeared whereas characteristic bands -
	(C-H) -corresponding to the COD ligand and the tBu appeared. Again stronger signals are
	found on SiO 2-200 than on SiO 2-700 in good correlation with the higher Pt loading, 3.9 wt.%
	(0.6 Pt.nm -2 , grafting yield = 23 %) versus 2.3 wt% (0.3 Pt.nm -2 , grafting yield = 44 %),
	respectively. Note also two additional broad (OH) peaks at 3700 cm -1 (weak) and 3360 cm -1
	(intense), which are attributed to surface SiOH interacting with adjacent ligands and adsorbed
	(tBuO) 3 SiOH -(O-H), respectively. The structure of the surface species was further assessed
	by carbon-13 CP-MAS solid-state NMR (Figure S5). The olefinic carbon atoms of COD
	appear as a broad signal at 93 ppm, which is close to that observed in the starting molecular
	complex (89.6 ppm). The slight downfield shift effect induced by the exchange of
	(tBuO) 3 SiO-by a surface siloxy ligand would indicate that the surface siloxy are somewhat
	less -donating. Moreover, signals at 72 and 30 ppm are observed and associated with the
	quaternary carbon and the methyl group of the (tBuO) 3 SiO-ligand; the signal at 28 ppm
	corresponds to the nearly equivalent CH 2 of the COD ligand. The surface species can thus be
	depicted as (COD)Pt(OSi(OtBu) 3 (OSiO≡).

Table 3 .

 3 The percentage of reacted silanols is calculated considering 0.86 mmol OH.g -1 on SiO 2-200 and 0.26 mmol OH.g -1 on SiO 2-700 .Overall, well-defined platinum (II) surface species have been effectively synthesized, by grafting on SiO 2-200 and SiO 2-700 and characterized at a molecular level. Complexes with O-or N-bound ligands (OSi(OtBu) 3 and N(SiMe 3 ) 2 ) have been proved to be more reactive towards

				SiO 2-200			SiO 2-700
			Pt (wt.%)	Pt/≡SiOH (A)	Pt.nm -2	Pt (wt.%)	Pt/≡SiOH (B)	Pt.nm -2
	(COD)Pt(Me) 2		3,00%	18.1%	0.5	1.1%	21.3%	0.2
	(COD)Pt(OSi(OtBu) 3 ) 2		3.9%	23.4%	0.6	2.3%	44.3%	0.3
	(COD)Pt(Me)(OSi(OtBu) 3 )		6.8%	40.3%	1,0	4.1%	78.6%	0.6
	(COD)Pt(Me)(N(SiMe 3 ) 2 )		7.4%	44.1%	1.1	3.8%	72.2%	0.6
	(COD)Pt(Cl)(N(SiMe 3 ) 2 )		4.2%	24.8%	0.6	3.1%	58.9%	0.5
	(COD)Pt(N(SiMe 3 ) 2 )(OSi(OtBu) 3 )	4.8%	28.8%	0.7	3.7%	70.5%	0.6
	Table 4. Carbon-13 solid-state NMR data for complexes grafted onto SiO 2-200
	X		Y	δ X C=	δ Y C=	δ ≡OSi C= δ Pt-CH 3
	Me		Me	118	-	78	6
	OSi(OtBu) 3 OSi(OtBu) 3	93	-	93	-
	Me	OSi(OtBu) 3	115	-	73	5
	Me	N(SiMe 3 ) 2	114	-	72	5
	Cl	N(SiMe 3 ) 2	104	-	94	-
	OSi(OtBu) 3	N(SiMe 3 ) 2	95	105	95	-
	It is noteworthy that grafting on both SiO 2-200 or SiO 2-700 leads to mono-grafted complexes,
	which is in sharp contrast to what is observed for the grafting of earlier transition-metal
	complexes (up to group 6), for which lower dehydroxylation temperature favors bis-grafted
	systems and higher dehydroxylation temperature mono-grafted species. 44,97,98 Thus, the high
	OH density on SiO 2-200 leads to a much higher Pt loading. Yet full loading (reaction of all
	surface silanols), which would lead to Pt loading of 17 wt.% (2.6 Pt.nm -2 ) on SiO 2-200 and
	5wt.% (0.8 Pt.nm -2 ) on SiO 2-700 , respectively, have never been reached. The theoretical
	maximum coverage has been calculated for all (COD)Pt(X)(OSi≡) species based on the
	projected surface area of these complexes (Appendix 10). Note that for (COD)Pt(Me)(OSi≡)
	and (COD)Pt(Cl)(OSi≡) the maximum surface coverage is calculated to be respectively 2.6
	and 2.8 Pt.nm -2 assuming mono-grafting on all surface silanols on SiO 2-200 (2.6 OH.nm -2 );
	grafting is thus limited by the reactivity of the molecular complexes or the complementary
	ligand (e.g. if passivation or adsorption occurs) and not by surface coverage. On the contrary,
	for the bis-siloxy complex and the amido-siloxy complex, the theoretical maximum surface
	coverage are between 1.0 and 1.3 Pt.nm -2 , thus no full consumption of surface silanols can be
	achieved and indeed on SiO 2-200 lower Pt density are found (0.6 and 0.7 Pt.nm -2 ). On SiO 2-700

protonolysis by surface silanols than C-bound ligands (Me); the O-and N-bound ligands have however similar reactivity.

Table 5 .

 5 Mean nanoparticles size, standard deviation and platinum loading.

		SiO 2-200			SiO 2-700
	mean	std. dev.	Pt	mean	std. dev.	Pt
	size (nm)	(nm)	wt.%	size (nm)	(nm)	wt.%

Table 6 .

 6 Particle size in nm and associated standard deviation from compounds grafted at SiO 2-700 .

static flow (COD)Pt(Me)(N(SiMe 3 ) 2 ) 2.1 (0.9) 1.7 (0.8) (COD)Pt(Cl)(N(SiMe 3 ) 2 ) 5 to 18 -6.3 (2.4) (COD)Pt(OSi(OtBu) 3 ) 2 2.0 (0.6) 1.8 (0.4)

Table 7 .

 7 Elemental analyses after grafting and H 2 treatments.

	7	0.4% 0.7

Pt C N Cl wt.% wt.% C/Pt wt.% N/Pt wt.% Cl/Pt (COD)Pt(Cl)(N(SiMe 3 ) 2 ) 3.1% 2.5% 13.3 0.4% 1.

Figure 27. Comparative TEM pictures of complexes grafted on SiO 2-700

Table 8 .

 8 Ceria BET surface area as a function of the calcination and dehydroxylation temperature. Error on the surface area is ± 3 m².g-1

	Calcination and dehydroxylation temperature (°C)	BET Surface area (m².g -1 )
	200	58
	400	57
	600	54
	800	52

  The large peak at 27 ppm was attributed to the CH 2 of the COD ligand. Similarly to what was observed for silica, the peak at 0 ppm was attributed to passivated surface (≡CeOSiMe 3 surface species). The peaks at 67 and 12 ppm were attributed to diethyl ether (grafting solvent) absorbed onto the surface as verified by a blank experiment (Appendix 11). The peak at 133 ppm was likely attributed to free COD that could result from a degradation of instable surface species or to carbonated species due to contamination.103 The carbon-13 CP-MAS solid-state NMR of the freshly CeO 2-400 presented no signal between -10 and 260 ppm.

						133		92	74	67	27	12	-0
	240	220	200	180	160	140	120	100	80	60	40	20	ppm
	Figure 35. 13 C CP/MAS solid state NMR spectra of (COD)Pt(Cl)(N(SiMe 3 ) 2 )@CeO 2 (d1 = 2 s, 30000 scans,
						lb=100Hz)

Overall, the Pt complexes seemed to graft on ceria similarly to what was observed on silica but the full characterization has not been done yet and requires further analyses (elemental analyses, solid-state NMR).

  On another hand Cl containing precursor led to a strong increase in the nanoparticles size through the probable formation of HCl. This phenomenon could be limited by treating the grafted samples under an H 2 flow as demonstrated for silica support. This first SOMC study onto ceria paves the way to further research on this support. For example, the study of the formation mechanism of nanoparticles from isolated species could be key to better understand and characterize the strong metal support interaction or the redispersion of agglomerated Pt nanoparticles as described recently.12

	(COD)Pt(Me) 2 , 90	(COD)Pt(Cl)(Me), 91	(COD)Pt(OSi(OtBu) 3 ) 2	92	and
	2.6. Experimental
	2.6.1. General procedures
	dehydroxylated at 400°C for 15 h.
	K 2 PtCl 4 99.9 % was purchased from Strem Inc. 1,5-cyclooctadiene 99+ % (COD), was
	purchased from Sigma-Aldrich, filtered over dried alumina and degassed prior to use.
	Tris(tert-butoxy)silanol 99.999 % (HOSi(OtBu) 3 ), lithium bis(trimethylsilyl)amide
	(LiN(SiMe 3 ) 2 ) and methyl lithium 1.6 M in Et 2 O (MeLi) were purchased from Sigma-Aldrich
	and used as received. Pentane, diethylether and dichloromethane were dried using an MBraun
	solvent purification system, contacted with MS 4 Å and degassed under vacuum. Dihydrogen
					2 , 89

1.2 ± 0.2 nm nanoparticles growth.

All experiments were carried out under a controlled atmosphere (Ar), using Schlenk and glove-box techniques for organometallic synthesis and silica surface modifications. Silica dehydroxylated at 700 °C, referred as SiO 2-700 , was obtained as follow: silica (Aerosil Degussa, 200 m².g -1 ) was compacted with distilled water, dried at 110 °C for 2 days, calcined (500°C under air for 4 hours) and partially dehydroxylated under secondary vacuum (10 -5 mBar) at 500°C for 12 h and then at 700°C for 4 h. Silica dehydroxylated at 200 °C, referred as SiO 2-200 , was dehydroxylated at 200°C under secondary vacuum for 15 hours, just after compacting. The density of surface silanols as determined by titration with methylmagnesium bromide was respectively 0.86 and 0.26 mmol.g -1 for SiO 2-200 and SiO 2-700 .

57,106

Ceria (Evonik Degussa VP 60) pellets were produced by finely grinding ceria using a mortar, 40 mg were placed in a mold (17 mm diameter) between two mica sheets and pressed using an hydraulic press applying a pressure of 1 t for 1 min. For grafting, ceria was first calcined under air at 400°C for 15 h, then reduced under static H 2 atmosphere at 150°C for 15 h and finally was purified over R3-11 BASF catalyst / MS 4 Å prior to use. Benzene was dried under Na/K/benzophenone and distilled prior to use. The molecular complexes (COD)Pt(Cl)

  15 H 33 NPtSi 2 37.6/6.9/2.9/40.8/11.7; found, 38.0/7.0/3.0/40.6/11.4. IR (cm -1 ): 3019, 3000,2955, 2924, 2888, 2843, 2795, 1493, 1478, 1433, 1397, 1339, 1312, 1251, 1214, 1180.

	1 H
	NMR (C 6 D 6 ; δ, ppm): 0.46 (s, 18H, -SiMe 3 ), 1.05 (t, 3H, Pt-CH 3 , J Pt-H = 80Hz), 1.47 (m, 4H,
	endo CH 2 ), 1.80 (m, 4H, exo CH 2 ), 3.79 (t, 2H, =CH, trans to N(SiMe 3 ) 2 , J Pt-H = 60 Hz), 5.18
	(t, 2H, =CH, trans to CH 3 , J Pt-H = 30 Hz). 13 C NMR (C 6 D 6 ; δ, ppm): 5.8 (t, CH 3 , Me, JPt-
	C=726.8 Hz), 7.0 (m, SiCH3), 28.1 (t, CH 2 , JPt-C=12.9 Hz), 30.8 (t, CH 2 , JPt-C=15.9 Hz),

84.0 (t, =CH, trans to N, JPt-C=161.3 Hz), 111.6 (s, =CH, trans to CH 3 , JPt-C=29.4 Hz). High-resolution mass spectrometry (m/z): [M+H] + 479.1876 measured (Δm = -0.9 ppm), 479.1872 calculated from C 15 H 33 NPtSi 2 . Elemental analysis: Pt/Si/N/C/H calculated for C

  (COD)Pt(Cl)(Me) (800 mg -2.3 mmol) suspended in pentane (10 mL) was added a solution of NaOSi(OtBu) 3 (648 mg -2.3 mmol) in pentane (10 mL). After 16 h at 25 °C, stirring was stopped, NaCl was filtered off, and the uncolored solution was concentrated at -

30°C. After removal of the supernatant, 835 mg (64%) of complex 2 was recovered as white crystalline solid.

[START_REF] Curry | [END_REF]

H NMR (C 6 D 6 ; δ, ppm): 1.16 (t, 3H, CH 3 , JPt-H=75Hz), 1.50 (m, 4H, endo CH 2 ), 1.62 (s, 27H, OCMe 3 ), 1.80 (m, 4H, exo CH 2 ), 3.57 (t, 2H, =CH, trans to OSi, JPt-H=66 Hz), 5.90 (t, 2H, =CH, trans to CH 3 , JPt-H=30Hz). 13 C NMR (C 6 D 6 ; δ, ppm): 6.9 (s, CH 3 , JPt-C=700 Hz), 27.6 (s, CH 2 ), 31.6 (s, CH 2 ), 32.2 (s, OCMe 3 ), 71.4 (s, OCMe 3 ), 73.4 (s, =CH, trans to OSi, JPt-C=227 Hz), 113.3 (s, =CH, trans to CH 3 , JPt-C=26.5 Hz). IR (cm -1 ): 3011, 2973, 2923, 2894, 2842, 1473, 1449, 1419, 1385, 1363, 1243, 1215, 1200, 1063.

Grafting procedures Grafting of (COD)PtMe 2 on SiO 2-700 : (COD)PtMe 2 @SiO 2-700 . Representative Procedure.

	2.6.3. Using a schlenk with double compartments separated with a glass filter, (COD)Pt(Me) 2 (1.2
	equiv.; 53 mg; 0,16 mmol) in pentane (5mL) was contacted with SiO 2-700 (0.50 g, 0.13 mmol
	SiOH) at 25°C. After 3 h, the solid is decanted, the supernatant was filtered, and the resulting
	solid was washed with pentane (4 × 5 mL) and dried under vacuum (10 -5 mBar) for 1 h to
	yield a white solid. Elemental Analysis (wt%): Pt/C 1.1/1.0. IR-DRIFT:
	3410/3057/3001/2945/2927/2877/2842/2801. Solid-state 13 C NMR (ppm): 118/78/33/30/6.
	Grafting of (
	71.8 (s, OCMe 3 ), 84.4 (s, =CH, trans to OSi(OtBu) 3 ), 100.8 (s, =CH, trans to N(SiMe 3 ) 2 ). IR
	(cm -1 ): 3056, 2963, 2935, 2895, 2846, 1474, 1456, 1434, 1387, 1363, 1247, 1198, 1065.

H NMR (C 6 D 6 ; δ, ppm): 0.54 (s, 18H, -SiMe 3 ), 1.31 (m, 4H, endo CH 2 ), 1.54 (s, 27H, OCMe 3 ), 1.94 (m, 4H, exo CH 2 ), 4.55 (t, 2H, =CH, trans to OSi, J Pt-H = 66 Hz), 6.01 (t, 2H, =CH, trans to N(SiMe 3 ) 2 , J Pt-H = 51 Hz). 13 C NMR (C 6 D 6 ; δ, ppm): 7.0 (s, SiMe 3 ), 29.2 (s, CH 2 ), 30.7 (s, CH 2 , COD), 32.3 (s, OCMe 3 ),

COD)PtMe 2 on SiO 2-200 : (COD)PtMe 2 @SiO 2-200 .

The experimental procedure described above was carried out in pentane using (COD)Pt(

Me

) 2 (1.2 equiv.; 173 mg; 0,52 mmol) and SiO 2-200 (0.50 g, 0.43 mmol SiOH). Elemental Analysis (wt%): Pt/C 3.0/2.1. IR-DRIFT: 3018/3005/2955/2930/2894/2648/2802. Solid-state 13 C NMR (ppm): 118/78/32/29/6.

Pt(OSi(OtBu) 3 ) 2 on SiO 2-200 : (COD)Pt(OSi(OtBu) 3 ) 2 @SiO 2-200 .

	SiOH).	Elemental	Analysis	(wt%):	Pt/C	4.1/4.8.	IR-DRIFT:
	3015/2977/2954/2932/2893/2888/2846/2800. Solid-state 13 C NMR (ppm): 115/73/30/27/4.
	The experimental procedure described above was carried out in pentane using
	(COD)Pt(OSi(OtBu) 3 ) 2 (1.2 equiv.; 172 mg; 0.21 mmol) and SiO 2-200 (0.20 g, 0.17 mmol
	SiOH).	Elemental	Analysis	(wt%):	Pt/C	3.9/5.3.	IR-DRIFT:
	3420/2975/2931/2905/2876/2850. Solid-state 13 C NMR (ppm): 93/72/30/27.

(COD)Pt(OSi(OtBu) 3 ) 2 (1.2 equiv.; 53 mg; 0.064 mmol) and SiO 2-700 (0.20 g, 0.053 mmol SiOH). Elemental Analysis (wt%): Pt/C 2.3/5.4. IR-DRIFT: 3370/2975/2933/2905/2876. Solid-state

[START_REF] Olah | Beyond Oil and Gas: The Methanol Economy[END_REF]

C NMR (ppm): 92/72/30/28.

Grafting of (COD)

Grafting of (COD)Pt(Me)(OSi(OtBu) 3 ) on SiO 2-700 : (COD)Pt(Me)(OSi(OtBu) 3 )@SiO 2-700 .

The experimental procedure described above was carried out in pentane using (COD)Pt(Me)(OSi(OtBu) 3 ) (1.2 equiv.; 37 mg; 0.064 mmol) and SiO 2-700 (0.20 g, 0.053 mmol

SiMe 3 ) 2 ) on SiO 2-700 : (COD)Pt(Me)(N(SiMe 3 ) 2 )@SiO 2-700 .

	(COD)Pt(Me)(OSi(OtBu) 3 ) (1.2 equiv.; 121 mg; 0.21 mmol) and SiO 2-200 (0.20 g, 0.17 mmol
	SiOH).	Elemental	Analysis	(wt%):	Pt/C	6.8/5.4.	IR-DRIFT:
	3013/2975/2951/2928/2890/2844/2801. Solid-state 13 C NMR (ppm): 115/73/30/27/5.
	Grafting of (COD)Pt(Me)(N(The experimental procedure described above was carried out in pentane using
	(COD)Pt(Me)(N(SiMe 3 ) 2 ) (1.2 equiv.; 30 mg; 0.064 mmol) and SiO 2-700 (0.20 g, 0.053 mmol
	SiOH).	Elemental	Analysis	(wt%):	Pt/C	3.8/3.0.	IR-DRIFT:
	3017/3002/2962/2957/2932/2895/2846/2802.	Solid-state	13 C	NMR	(ppm):
	117/101/76/32/29/7/0.
	SiOH).	Elemental	Analysis	(wt%):	Pt/C	7.4/4.9.	IR-DRIFT:
	3014/3000/2954/2927/2890/2844/2796. Solid-state 13 C NMR (ppm): 114/72/28/5/0.

Grafting of (COD)Pt(Me)(N(SiMe 3 ) 2 ) on SiO 2-200 : (COD)Pt(Me)(N(SiMe 3 ) 2 )@SiO 2-200 .

The experimental procedure described above was carried out in pentane using (COD)Pt(Me)(N(SiMe 3 ) 2 ) (1.2 equiv.; 98 mg; 0.21 mmol) and SiO 2-200 (0.20 g, 0.17 mmol

SiMe 3 ) 2 )@SiO 2-200 .

  The experimental procedure described above was carried out using (COD)Pt(Cl)(N(SiMe 3 ) 2 ) (1.2 equiv.; 258 mg; 0,52 mmol) and SiO 2-200 (0.5g, 0.43 mmol SiOH). Elemental Analysis (wt%): Pt/C 4.2/2.8. IR-DRIFT: 3023/3009/2962/2942/2934/2900/2853. Solid-state 13 C NMR (ppm): 104/94/31/0.

	Grafting of (
	(COD)Pt(Cl)(N(SiMe 3 ) 2 ) (1.2 equiv.; 103 mg; 0,16 mmol) and SiO 2-700 (0.50 g, 0.13 mmol
	SiOH).	Elemental	Analysis	(wt%):	Pt/C	3.1/2.5.	IR-DRIFT:

3022/3008/2964/2934/2902/2888. Solid-state

[START_REF] Olah | Beyond Oil and Gas: The Methanol Economy[END_REF]

C NMR (ppm): 106/88/60/35/32/0.

Grafting of (COD)Pt(Cl)(N(SiMe 3 ) 2 ) on SiO 2-200 : (COD)Pt(Cl)(N(

COD)Pt(Cl)(N(SiMe 3 ) 2 ) on CeO 2-400 : (COD)Pt(Cl)(N(SiMe 3 ) 2 )@CeO 2-400 .

Grafting of (COD)Pt(Cl)(Me) on SiO 2-200 : (COD)Pt(Cl)(Me)@SiO 2-200 .

  The experimental procedure described above was carried out in dichloromethane using (COD)Pt(Cl)(Me) (1.2 equiv.; 73 mg; 0.21 mmol) and SiO 2-200 (0.20 g, 0.17 mmol SiOH). No signals were found neither by IR-DRIFT nor by solid-state[START_REF] Olah | Beyond Oil and Gas: The Methanol Economy[END_REF] C NMR.

3 ) (1.2 equiv.; 151 mg; 0.21 mmol) and SiO 2-200 (0.20 g, 0.17 mmol SiOH). Elemental Analysis (wt%): Pt/C 4.8/4.8. IR-DRIFT: 2977/2958/2933/2900/2852. Solid-state 13 C NMR (ppm): 105/95/75/30/28/5/0.

Table S -1.

 S

	Figure S2.
	Solid-state NMR of (COD)Pt(Me) 2 @SiO2 -200
			(COD)Pt(Me)(OSi(OtBu) 3 )	(COD)Pt(OSi(OtBu) 3 )(N(SiMe) 3 )
			(2)	(3)
	Formula		C 21 H 42 PtO 4 Si	C 26 H 57 NPtO 4 Si 3
	Formula weight (g.mol -1 )	581.74	727.09
	Crystal size (mm)		0.38x0.21x0.13	0.63x0.53x0.45
	Color, morphology		coloreless	light yellow
	(g.cm -3 )		1.563	1.452
	Lattice type, crystal system	Monoclinic	Triclinic
	Space group		P2 1 /n	P -1
	Z		2	2
	Cell constant
	a (Å) C CP/MAS solid state NMR spectra of (COD)Pt(Me) 2 @SiO 2-200 (d1 = 6 s, 10484 scans, 9.465(1) 10.5195(5) 13
		b (Å)	8.6144(9) lb=100Hz)	11.5936(6)
	Solid-	c (Å)	30.436(3)	14.5167(8)
		(°)	90	72.425(5)
		(°)	95.187(9)	80.632(4)
		(°)	90	89.822(4)
		V (Å 3 )	2471.4(4)	1663.3(2)
	F 000		1168	744
	μ (Mo Kα) (mm -1 )		5.75	4.36
	θ (°)		3.6-29.5	3.5-29.5
	Scan type		scans	scans
	Temperature (K)		150	150
	Measured reflections		22806	43516
	Refinement		full least-square matrix on F 2	full least-square matrix on F 2
	Independent relections		6069	8476
	No. Parameters		244	317
	R[F 2 >2 (F 2 )]/ wR 2		0.047/ 0.149	0.031/ 0.066
	GoF		0.99	1
	Max peak in final diff. map (e-Å -3 )	2.09	1.86
	Min peak in final diff. map (e-Å-3)	-3.57	-1.76

state NMR of (COD)Pt(Me) 2 @SiO 2-700

	13 C CP/MAS solid state NMR spectra of (COD)Pt(Me) 2 @SiO 2-700 (d1 = 4 s, 33702 scans,
	lb=100Hz)
	Solid-state

NMR of (COD)Pt(Me) 2 mixed in the solid state with SiO 2-700

	113	100	98	97	74	32	29	27	9	6	3
	118			78		32	29		6
	140 130 120 110 100 90 80 70 60 50 40 30 20 10		ppm
	13 C CP/MAS solid state NMR spectra of (COD)Pt(Me) 2 + SiO 2-700 (d1 = 2 s, 3433 scans,
	140 130 120 110 100 90 80 70 60 50 40 30 20 10 lb=100Hz)		ppm
	118			78		33		30			6
	140 130 120 110 100 90 80 70 60 50 40 30 20 10		ppm

  NMR (C 6 D 6 ; δ, ppm): 6.9 (s, CH 3 , JPt-C=700 Hz), 27.6 (s, CH 2 ), 31.6 (s, CH 2 ), 32.2 (s, OCMe 3 ), 71.4 (s, OCMe 3 ), 73.4 (s, =CH, trans to OSi, JPt-C=227 Hz), 113.3 (s, =CH, trans to CH 3 , JPt-C=26.5 Hz).

	13 C
	5.95	5.90	5.85	3.68	3.57	3.46	1.80	1.62	1.50	1.29	1.16	1.04
											*
													*
		2.0			2.1			4.4	27.9	3.4
			113.32	73.43	71.45		32.27	31.61	27.66		6.96
	140 130 120 110 100 90 80 70 60 50 40 30 20 10	ppm

C

  NMR (C6D6; δ, ppm): 5.8 (t, CH 3 , Me, JPt-C=726.8 Hz), 6.8 (m, SiCH3), 28.1 (t, CH 2 , JPt-C=12.9 Hz), 30.8 (t, CH 2 , JPt-C=15.9 Hz), 83.9 (t, =CH, trans to N, JPt-C=161.3 Hz), 111.6 (s, =CH, trans to CH 3 , JPt-C=29.4 Hz).

	13
					5.23	5.18	5.13			3.89	3.79	3.69				1.80	1.47	1.18	1.05	0.92	0.46
	7.0	6.5		6.0	5.5		5.0		4.5	4.0		3.5	3.0	2.5	2.0			1.5	1.0	0.5	ppm
						2.0				2.0				4.4		4.6	2.9	17.0
	127.89	111.60	111.41	111.21			84.86	83.79	82.72						30.72	30.62	30.52	28.02	27.93	27.85	10.45	7.24	7.02	6.88	6.74	6.52
		120	110	100		90		80	70	60	50	40			30		20	10	ppm

  C NMR (C6D6; δ, ppm): 6.8 (s, OCMe 3 ), 7.0 (s, SiMe 3 ), 29.2 (s, CH 2 ), 30.7 (s, CH 2 , COD), 32.3 (s, OCMe 3 ), 71.8 (s, OCMe 3 ), 84.4 (s, =CH, trans to OSi), 100.8 (s, =CH, trans to N).

	6.10	6.01	5.92	4.64	4.54	4.43	1.94	1.54	1.31	0.54
	1 H NMR (C6D6; δ, ppm): 0.54 (s, 18H, -SiMe 3 ), 1.31 (m, 4H, endo CH 2 ), 1.54 (s, 27H, 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm 18.5 4.8 27.8 4.3 1.9 2.0 140 130 120 110 100 90 80 70 60 50 40 30 20 10 ppm OCMe 7.0 6.83 7.00 28.63 29.17 30.73 30.90 31.69 32.28 71.81 84.40 100.81 128.07

3 ), 1.94 (m, 4H, exo CH 2 ), 4.55 (t, 2H, =CH, trans to OSi, JPt-H=66 Hz), 6.01 (t, 2H, =CH, trans to N, JPt-H=51Hz).

[START_REF] Olah | Beyond Oil and Gas: The Methanol Economy[END_REF]

Table 9 .

 9 (COD)Pt(OSi(OtBu) 3 ) 2 , was tested. Indeed, during thermogravimetric experiments under an N 2 flow and a heating ramp of 10°C/min Tilley et al.observed the decomposition of the product yielding pure Pt (ceramic yield 22.7 wt% vs 23.5 wt% theoretically), the loss of 1.8 equivalent of HOSi(OtBu) 3 beeing shown byPlatinum was then deposited starting from the thus-prepared Pt II precursor onto gas diffusion layers (GDLs) used as fuel-cell catalyst supports.107 Five deposition times were tested; experimental details can be found in part 4. The MOCVD process 108,109 was carried out on Elemental analyses, results in wt% are given with a ± 0.01 wt% error. This corresponds to 100 μg.cm -2 for 24BC and 180 μg.cm -2 for E-TEK.

	two types of GDLs: 24BC and E-TEK, note that 24BC (104.1 g.m -2 ) has a paper-like
	morphology whereas E-TEK (178.8 g.m -2 ) has a fabric-like structure; they are both coated on
	one	side	with	microporous	carbon.	GDLs	are	preimpregnated	with

1 H NMR experiments. The onset decomposition temperature is about 150°C and total decomposition is achieved around 230°C. This low decomposition temperature makes this precursor a good candidate for MOCVD as lower temperature will induce broader uses on fragile supports and lower costs.

3.2. Results and discussion

3.2.1. Precursor characterization

(COD)Pt(OSi(OtBu) 3 ) 2 was synthesized according to the procedure developed by Tilley et al.

92

from (COD)Pt(Cl) 2 and NaOSi(OtBu) 3 ) 2 with high yields (>80 %) and purity (for NMR see

Appendix 14)

. polytetrafluoroethylene (PTFE). This polymer has two functions: binding together the carbon fibers and giving hydrophobic properties to the GDL to avoid flooding at the cathode; 110 those two GDLs being widely used for research purposes and current applications of PEM fuel cells.

Table 10 .

 10

			Pt loading (μg.cm -2 )
	deposition time	nanoparticle density (NP.100 cm -2 )	calculated (on TEM grids)	by ICP (on GDL)
	20	216	1.2	208
	100	274	2.8	625

Table 11

 11

	. Atomic percentage measured by XPS
		(COD)Pt(OSi(OtBu) 3 ) 2 (nbd)Pt(Me) 2
	O	40.5 %	46.0%
	C	44.1%	31.8 %
	Pt	4.4 %	20.0 %
	Si	9.0 %	-

Table 12

 12

	. Main electrocatalysis results
	deposition time (min)	Pt (μg/cm²)	Pt ESA CO stripping (cm²Pt/cm²)	j at 35mV under H 2 (mA/cm²)	j at 0.8V under O 2 (μA/cm²)	A/gPt at 0.8V	μA/cm²Pt at 0.8V	dispersion
		20	208	2.2	35.0	258.0	1.2	115.2	0.2 %
		45	208	2.5	50.0	348.0	1.7	140.3	0.8 %
	24 BC	60 75	416 208	2.9 2.8	39.9 87.5	216.0 421.0	0.5 2,0	75.0 152.5	0.4 % 0.8 %
		100	625	4.8	140.0	258.0	0.4	54.2	0.5 %
		20	179	2.6	23.3	70.0	0.4	26.9	0.8 %
		45	179	1.8	35.0	42.0	0.2	22.8	0.4 %
	E-TEK	60 75	358 358	2.5 2.4	21.9 70.0	30.6 68.3	0.1 0.2	12.3 28.9	0.1 % 0.1 %
		100	536	3.4	116.7	93.0	0.2	27.4	0.4 %
	REF PtCo Tanaka		540.0	108.9	155.6	17150.0	31.8	157.5	8.2 %
	3.2

.3.1. Electroactive surface area 3.2.3.1.1. Hydrogen adsorption

  The electroactive surface area (ESA -Figure46) has been determined by measuring the surface area under the cyclic voltammogram in the anodic part in the H-Pt surface removal range (or H underpotential desorbtion H upd -see experimental part for details). Figure 46. Comparison of the measured Pt surface area by the H 2 or the CO adsorption method on both types of GDLs. The error on the platinum surface area is ± 0.3 cm 2 Pt .cm -2 .

The platinum surface area increases with increasing platinum loading. This indicates the nucleation of more nanoparticles instead of the growing of pre-existing nanoparticles as previously confirmed by microscopy. CO and H 2 adsorption methods correlate well together,

Table 13 .

 13

				Pt surface area (cm²Pt/cm²)
		deposition time	Pt loading (ICP) μg/cm²	ESA (CO stripping)	calculated for 5 nm nanoparticles	νPt
		20	208.2	2.2	128.6	1.7%
	24 BC	45 60 75	208.2 416.4 208.2	2.5 2.9 2.8	128.6 257.2 128.6	1.9% 1.1% 2.2%
		100	624.6	4.8	385.8	1.2%
		20	178.8	2.6	110.4	2.4%
	E-TEK	45 60 75	178.8 357.6 357.6	1.8 2.5 2.4	110.4 220.9 220.9	1.6% 1.1% 1.1%
		100	536.4	3.4	331.3	1,0%
	REF
	PtCo		540	108.9	333.5	32.6%
	Tanaka

Table 14

 14

	. H 2 chemisorption
	Pt loading	1%
	n Pt mmol.g -1	0.051
	nH per surface Pt	1.8
	Q max (mmol H 2 .g -1 )	0.039
	dispersion	68%
	particle size based on cubic octahedron	1.54

Table 15 .

 15 Monometallic and bimetallic nanoparticles size and standard deviation

	composition Pt/Pd	size (nm)	standard deviation (nm)
	1/0	1.8	0.3
	3/1	1.4	0.5
	1/1	1.6	0.3
	1/3	1.8	0.5
	0/1	1.7	0.3
	The nanoparticles size did not vary very much with increasing the palladium content with a
	lowest value of 1.4 nm for ca. 3/1 Pt/Pd ratio.
	TEM highlights the Z contrast, the formation of separated Pt and Pd nanoparticles should thus
	be visible (platinum nanoparticles being darker than palladium). A mix of separately prepared
	Pt and Pd nanoparticles was observed by TEM (Appendix 24). Unfortunately no strong Z
	contrast was evidenced. One cannot conclude by low resolution TEM analysis whether
	monometallic platinum and palladium nanoparticles or bimetallic Pt/Pd nanoparticles were
	synthesized.

  arising from the formation of the octylsilane shell or the incorporation of Si into the nanoparticles structure. As prepared Pd nanoparticles presented 2 broad 3d level signals that can be deconvoluted, similarly to Pt, into two pairs of peaks at 336.2/341.6 eV and 337.6/342.9 eV, these peaks were respectively attributed to Pd 0 and Pd 2+ .115 The Pd 2+ signal could be interpreted as Pd-Si. The Pt 4f signal of the bimetallic nanoparticles with a 1/1 ratio was very similar to the one of the Pt monometallic which presented peaks at 71.8/75.2 and 72.8/76.1 eV attributed again to Pt-Pt and Pt-Si bonds. Surprisingly, the Pd 3d signal of the bimetallic nanoparticles presented only the Pd 0 contribution at 336.2/341.6 eV. This could be interpreted as resulting from a core shell structure, the core being Pd enriched but a Pd-Si contribution arising from the incorporation of Si into the nanoparticles structure would still have been expected. The formation of core shell Pt/Pd nanoparticles with an enriched Pd core was previously described by others 128 and was coherent with a faster reaction of the palladium precursor. This, anyhow, confirms the formation of bimetallic nanoparticles instead of separated palladium and platinum nanoparticles.

					1.8 ± 0.3 nm			1.8 ± 0.3 nm
					20 nm				20 nm
					1.8 ± 0.3 nm			1.8 ± 0.4 nm
				Pt 4f	Pd				Pd 4d
	Pt						Pd
					20 nm				20 nm
					1.7 ± 0.4 nm			1.8 ± 0.4 nm
	80	78	76	74	72	70	348	344	340	336	332
			Binding Energy (eV)				Binding Energy (eV)
	PtPd 1:1					PtPd 1:1
	80	78	76	74	72	70	348	344	340	336	332
			Binding Energy (eV)				Binding Energy (eV)
	Figure 84. Pt 4f and Pd 3d level XP spectra of Pt, Pd and Pt/Pd 1:1 nanoparticles stabilized by 1 equiv.
						octylsilane

whether

  eV, respectively attributed to partial Si oxides (and thus Pt-Si or Pd-Si) and

	SiO 2 115 .
				Pt 4f					Pd 4d
	Pt calcined				Pd calcined
	80	78	76	74	72	70	348	344	340	336	332
			Binding Energy (eV)				Binding Energy (eV)
	PtPd 1:1					PtPd 1:1
	calcined					calcined
	80	78	76	74	72	70	348	344	340	336	332
		Binding Energy (eV)				Binding Energy (eV)
	Figure 85. Pt 4f and Pd 3d level XP spectra of Pt, Pd and Pt/Pd 1:1 nanoparticles stabilized by 1 equiv.
						octylsilane and calcined

Table 16 .

 16 Pt/Pd ratio measured by EDX on nanoparticles impregnated on alumina and calcined

	Pt/Pd theoretical	Pt/Pd
	composition	measured
	75/25	85/15
	50/50	51/49
	25/75	37/63

Table 17 .

 17 Electroanalyses methods confirmed the formation of Pt/Pd alloys with varying composition and demonstrated to be valuable tools for the analysis of colloidal nanoparticles. The catalytic properties of these nanoparticles were tested in the Oxygen Reduction Reaction. Lower activities compared to the platinum reference were found. The surface octylsilane chains are believed to hinder the gases accessibility to the catalytic sites or could act as an insulating material between the carbon support and the nanoparticles. The elimination of those aliphatic chains is thus probably necessary to increase the electroactive surface area. The simple calcination process under air cannot be used because Vulcan has a low decomposition temperature (100 to 200°C) in the presence of platinum.140 A previously developed process where UV irradiation is used could be tested.141 Despite this limitation, ORR results demonstrate higher catalytic properties for the Pt/Pd 3:1 nanoparticles. The Pt/Pd alloy nanoparticles synthesis method developed is fast and reproducible. It moreover leads, to our knowledge, to the smallest Pt/Pd nanoparticles reported so far.

			ESA (cm²/cm²)
	sample	NP size (nm)	measured (H upd )	calculated	M	j (A.g -1	Pt )
	Pt ref	2	75	133	56%	3,9
	Pt	1,6	14	163	9%	0,7
	Pt/Pd 3:1	1,4	51	183	28%	1,0
	Pt/Pd 1:1	1,6	5	163	3%	0,1
	Pt/Pd 1:3	1,8	0,2	148	0,1%	0,2

Figure 93. Current density per electrode surface area at 0.2 V/Hg vs. the catalyst composition.

4.7.2. Preparation of monometallic nanoparticles 4.7.2.1. From Pt(dba) 2

  Chimie Organométallique de Surface, platine, paladium, allliages, particules, silice, cérine.

	4
	compared to pure Pt.
	___________________________________________________________________________
	DISCIPLINE Chimie
	__________________________________________________________________________
	MOTS-CLES

___________________________________________________________________________ INTITULE ET ADRESSE DE L'U.F.R. OU DU LABORATOIRE : C2P2, UMR 5265, CNRS-Université Lyon 1-ESCPE Lyon, ESCPE Lyon, 43 Bd du 11 Novembre 1918 F-69616 Villeurbanne, France.

Remerciements

Appendix

Nanoparticles formation

Preparation of supported Pt nanoparticles from oxides supported Pt precursors under static H 2 atmosphere.

Representative procedure. In a glove box, 80 mg of the silica supported Pt precursor was transferred in a 200-mL glass reactor. After evacuation of the reactor (10 -5 mBar), H 2 was added (150 mBar), and the system was kept at 300°C for 15 h following a heating ramp of 5 K.min -1 from 25 °C. After cooling down to 25 °C, the reactor was evacuated (10 -5 mBar), and the solid (dark grey to black) was transferred and stored in a glove-box.

Preparation of silica-supported Pt nanoparticles from silica supported Pt precursors under a continuous flow of H 2 .

Representative procedure. To a reactor equipped with a glass frit, 80 mg of the silica supported Pt precursor were placed on the frit in the glove-box. After connection to the H 2 line and purging the tubing, the reactor was opened, put under H 2 flow (ca. 100 mL.min -1 ), and heated up to 300°C with a heating ramp of 1 to 5 K.min -1 . After 15 h, the gas flow was switched off, the reactor was cooled down to room temperature and evacuated. The resulting solid (dark grey to black) was transferred and stored in a glove-box. For the methyl-siloxy complexes grafting, IR-DRIFT reveals the presence of tBu units on the surface, to address the question whether the silanol is absorbed or grafted on the surface, SiO 2- 700 was contacted with 1.2 equivalents of tris(tert-butoxy)silanol in pentane for 3 hours followed by multiple washings and secondary vacuum drying, mimicking the full process of a

Figure S25.

Appendix 10. Maximum surface coverage for (COD)Pt(R)(OSi≡) species

The maximum radius of the grafted molecule is determined from DRX data of (COD)Pt(R)(OSi(OtBu) 3 ) when available. For (COD)Pt(Cl)(OSi≡) the measure is derived from the (COD)Pt(CH 3 )(OSi(OtBu) 3 ) complex, considering the carbon atom of the methyl ligand as a chlorine atom. We consider the molecules as discs in dense packing, with a surface density of 0.9069. The red arrows are pointing to Pt nanoparticles.

nm

Platinum Nanoparticles deposited from (COD)Pt(OSi(OtBu) 3 ) 2 on GDLs by DLI-MOCVD and application in fuel-cell catalysis

Based on the measure of the full width at half maximum (FWHM) of the diffractions peaks, the crystallites size can be evaluated by the Scherrer formula:

With K the shape factor fixed to 0.9 for truncated cubo-octahedron nanoparticles, the wavelength of the X-Ray source (1.5406 Å), the FWHM in radian and the Bragg angle in radian. FWHM are measured using the OriginPro 8.1 Software and a Gaussian type fitting of the peaks. The error is estimated to be in the nanometer range on the final crystallites value.

Appendix 17. Correlation between the nanoparticles size and dispersion. 119

Calculations are based on the truncated cubo-octahedron geometry typically observed for platinum crystalline nanoparticles.

Colloidal nanoparticles stabilized by octylsilane, from monometallic to bimetallic systems

The electrolyte solution was degassed under N 2 bubbling at least 20 minutes before use. The electrode rotating frequency was set to 100 rpm. Successive cycling were performed from -0.67 to 0.5, 0.6 and 0.7 mV vs. the Hg/HgSO 4 reference electrode at a scanning rate of 20 mV.s -1 . rate. For each maximum, two cycles were recorded. The maximum potential was then again set to 0.5 mV and two cycles are recorded. This procedure ensured the stabilization of the electroactive surface area and repeatable measurements.

Cyclic voltamperometry under O 2

The electrolyte solution was saturated with oxygen by bubbling at least 20 minutes before use.

The electrode rotating frequency was set to 100 rpm. CVs were recorded from 0.5 to -0.67 mV at a scanning rate of 20 mV.s -1 . The Oxygen Reduction current were compared at 0.