This dissertation employs the density functional theory (DFT) to investigate the oxidation of carbon monoxide (CO) on the W(111) surface and on the surface of Wn (n=10¡V15) nanoparticles. Since the properties of materials are significantly dependent on material size, we look into the influence of both the size and surface structure of tungsten catalysts on the CO oxidation process. The work contains two parts. Part 1: The adsorption and dissociation of O2 and CO on W(111) surface and Wn (n=10¡V15) nanoparticles. The chemical adsorption of O2 and CO on solid catalysts plays a very important role in heterogeneous catalysis for the CO oxidation reaction. The configurations, adsorption energies, vibration frequencies and electronic structures of adsorbates on W(111) and Wn (n=10¡V15) nanoparticles have been calculated to investigate their surface activity. The results indicate that adsorption of O2 and CO on Wn (n=10¡V15) nanoparticles are more stable compared to on the W(111) surface. The minimum energy pathways and transition states of chemical reaction processes on metal surfaces were also studied by the nudged elastic band (NEB) method. The dissociation barriers of O2 chemisorbed on Wn (n=10¡V15) nanoparticles are smaller those for the W(111) surface. Our results demonstrate that both the surface structure and size of metal significantly influence the adsorption and dissociation properties of adsorbates. Density functional theory-molecular dynamics (DFT-MD) simulation was also adapted to clarify the mechanism of O2 deposition on the W(111) surface. Observations of the variations of energy and bond lengths as a function of time show that the interaction between O2 and W atoms weakens the O¡VO bond, giving rise to the dissociation process. We conclude that the dissociation probability of an O2 molecule is affected by chemisorbed O2 coverage in the vicinity. Part 2: The mechanism of CO oxidation on W(111) and Wn nanoparticles. The oxidation of the CO molecule on transition metals usually follows two reaction pathways, either the Eley-Rideal (ER) mechanism or the Langmuir-Hinshelwood (LH) mechanism. In the ER mechanism, the CO molecule in the gas phase reacts directly with activated O2. The LH mechanism generally involves a few elementary steps, namely the co-adsorption of the O2 and CO molecules, O2 dissociation to form atomic oxygen, diffusion of atomic oxygen, and desorption of CO2. The oxidation of CO on a W10 nanoparticle surface and the W(111) surface are investigated by DFT calculations. Three pathways were studied in this dissertation: (i) CO + O2¡÷CO2 + O, (ii) CO + O2¡÷CO + O + O¡÷CO2 + O and (iii) CO + O¡÷CO2 via both LH and ER mechanisms. The calculated results show that CO oxidation on both the W10 nanoparticle and W(111) surfaces follow the ER rather than the LH mechanism. The CO oxidation on the W10 nanoparticle and W(111) surfaces occurs most easily via pathway (i) as compared to other two.

Ce travail s'inscrit dans les thématiques matériaux carbonés et matériaux pour l'énergie. Il s'agit d'une part de remplacer tout ou partie du platine, catalyseur de la pile à combustible basse température, par des matériaux moins coûteux et plus abondants ; et d'autre part d'optimiser l'accessibilité des sites catalytiques aux réactifs. WCl6 est imprégné en phase gazeuse en présence de dichlore sur un support dérivé du graphite expansé, avant d'être réduit/carburé afin d'obtenir des dérivés du tungstène (tungstène, carbures de tungstène). La taille et la nature des nanoparticules dépendent de la composition du gaz utilisé (H2 ou mélange H2/CH4) et de la température de réaction. En présence de H2, la réduction est complète à partir de 550°C. L'utilisation d'un mélange gazeux H2/CH4 (90/10, 85/15 ou 80/20) conduit à l'élaboration de nanoparticules de W2C et WC. L'augmentation de la température favorise le frittage des particules mais pas la carburation complète. Les tests en demi-pile effectués sur ces catalyseurs révèlent qu'ils sont peu performants. Des catalyseurs mixtes Pt-W2C sont obtenus en déposant successivement PtCl4 et WCl6. La réduction - carburation est réalisée à 900°C en présence d'un mélange H2/CH4 (85/15). Ces matériaux possèdent des performances électrocatalytiques bien supérieures à celles d'un catalyseur commercial pour un taux de platine équivalent. Ce travail concerne également la fonctionnalisation du support carboné par des groupements sulfonate. Le greffage de polystyrène sulfonate est efficace car il permet de diminuer la quantité de Nafion utilisée dans les électrodes et d'améliorer les performances catalytiques / This work is implicated in the development of carbon materials and of materials for energy applications. In the first part, we aim to totally or partially replace the platinum catalyst of the Proton Exchange Membrane Fuel Cell by less expensive and more abundant materials. In the second part, we optimize the catalytic sites accessibility for the reactants. WCl6 was impregnated on the Expanded Graphite derivated substrate by gaseous transport under chlorine pressure before being reduced and/or carburized in order to obtain tungsten derivatives (tungsten, tungsten carbides). The nature and size of the nanoparticles depend on the reducing or carburizing gas composition (H2 or H2/CH4 mixture) and the reaction temperature. Reduction is not complete until 500°C when using H2. W2C and WC mixtures are formed when using H2/CH4 mixtures (90/10, 85/15 or 80/20). The temperature increasing favours the sintering effect but not the complete carburization. All these obtained catalysts show low electrocatalytic performances when tested on a half-cell system. Platinum-tungsten hemicarbides catalysts were obtained by a successive deposition of PtCl4 and WCl6 while the reduction-carburization step was realized at 900°C under H2/CH4 mixture (85/15). These materials show higher electrocatalytic performances than a commercial platinum catalyst for an equivalent platinum ratio. The final part of this work concerns the carbon support functionnalization with sulfonate groups. Polystyrene sufonate grafting seems efficient for it helps to reduce the Nafion quantities used in the electrodes and to increase the electrocatalytic performances