La compréhension des phénomènes de ségrégation superficielle induite par la présence de gaz est de première importance afin de modéliser "correctement" les propriétés catalytiques des catalyseurs bimétalliques. L'objectif principal de cette thèse a été de développer une méthodologie théorique capable de reproduire et de prédire le comportement de la ségrégation du Pd dans le système AuPd(100) en présence de CO. Cela a été réalisé grâce à la combinaison des calculs DFT et de la simulation Monte Carlo. Plus précisément, un modèle d’Ising basé sur un potentiel inter-atomique décrivant à la fois les interactions métal-métal, métal-gaz et gaz-gaz a été construit grâce à des calculs DFT. Ensuite, des simulations Monte Carlo ont été développées pour tracer les isothermes de ségrégation et pour obtenir des informations sur l'évolution de la concentration de Pd en surface et en volume en fonction du recouvrement en CO. Les résultats montrent une ségrégation inversée du Pd dès l’adsorption du gaz. Ainsi, la ségrégation de Pd induite par l'adsorption de CO a été simulée pour différentes températures et pression de CO. Les différents ordres chimiques de surface identifiés ont été analysés et leurs réactivités vis-à-vis de la réaction d’oxydation de CO ont été identifiées. / The understanding of surface segregation phenomena induced by the presence of gas is of prior importance to “correctly” model the catalytic properties of bimetallic catalysts. The main objective of this thesis is to develop a theoretical methodology able to reproduce and predict the segregation behavior in Au-Pd system exposed to CO gas. This is achieved thanks to the combination of Density functional calculations (DFT) and Monte Carlo (MC) simulations. Firstly a, DFT-based Ising model is considered to build inter-atomic potential that includes interactions between: (i) the metal atoms in the alloy; (ii) the metal atoms in the surface and the adsorbed molecules and (iii) the adsorbed molecules. Secondly, the Pd segregation isotherms and the evolution of the Pd surface concentration with the Pd bulk concentration as a function of the CO coverage are studied with Monte Carlo simulations. The results show a reversed segregation of Pd upon the adsorption of CO. Thus, adsorption-induced Pd segregation was analyzed through the calculation of segregation isotherms at different temperature and CO pressure. The different obtained chemical ordered phases are thoroughly analyzed and their reactivity toward CO oxidation reaction was investigated.

A molecular-level understanding of a heterogeneous catalytic reaction is the key goal of heterogeneous catalysis. A surface science approach enables the realization of this goal. However, the working conditions (ultrahigh vacuum (UHV) conditions) of traditional surface science techniques restrict the investigations of heterogeneous catalysis system under industrial working conditions (atmospheric pressures). Polarization Modulation Infrared Reflection-Absorption Spectroscopy (PM-IRAS) can be operated in both UHV and atmospheric pressure conditions with a wide temperature span while providing high resolution (4 cm-1 is used in this dissertation) spectra. In this dissertation, PM-IRAS has been employed as a major technique to: 1) obtain both electronic and chemical information of catalysts from UHV to elevated pressure conditions; 2) explore reaction mechanisms by in situ monitoring surface species with concurrent kinetic measurements. In this dissertation, NO adsorption and dissociation on Rh(111) have been studied. Our PM-IRAS spectra show a transition of NO adsorption on three-fold hollow sites to atop sites occurs at low temperatures (<275 K). NO dissociation is found to account for this transition. The results indicated the dissociation of NO occurs well below the temperature previously reported. Characterizations of highly catalytically active Au films have also been carried out. Electronic and chemical properties of (1 x 1)- and (1 x 3)-Au/TiOx/Mo(112) films are investigated by PM-IRAS using CO as a probe molecule. The Au overlayers are found to be electron-rich and to have significantly different electronic properties compared with bulk Au. The exceptionally high catalytic activity of the Au bilayer structure is related to its unique electronic properties. CO oxidation reactions on Rh, Pd, and Pt single crystals are explored from low CO pressures under steady-state conditions (less than 1 x 10-4 Torr) to high pressures (0.01-10 Torr) at various gaseous reactant compositions. Surface CO species are probed with in situ PM-IRAS to elucidate the surface phases under reaction conditions. These experimental results are used to correlate reaction kinetics and surface reactant species. It is evident that there is a continuum over the pressure range studied with respect to the reaction mechanism. The most active phase has been shown to be an oxygen-dominant surface. The formation of a subsurface oxygen layer is found to deactivate the reaction.