The physical and catalytic properties of silica supported platinum or rhodium model catalysts are studied under both ultra high vacuum (UHV) and elevated pressure reaction conditions (>1torr). Platinum or rhodium nanoparticles are vapor deposited onto a SiO2/Mo(112) surface and characterized using various surface analytical methods. CO chemisorption is utilized as a surface probe to estimate the concentration of various sites on the nanoparticles through thermal desorption spectroscopy (TDS) and infrared reflection absorption spectroscopy (IRAS) along with microscopy techniques to estimate particle size. The results are compared with hard sphere models of face centered cubic metals described as truncated cubo-octahedron. Results demonstrate the excellent agreement between chemisorption and hard sphere models in estimating the concentration of undercoordinated atoms on the nanoparticle surface. Surfaces are then subjected to high pressure reaction conditions to test the efficacy of utilizing the rate of a chemical reaction to obtain structural information about the surface. The surfaces are translated in-situ to a high pressure reaction cell where both structure insensitive and sensitive reactions are performed. Structure insensitive reactions (e.g. CO oxidation) allow a method to calculate the total active area on a per atom basis for silica supported platinum and rhodium model catalysts under reaction conditions. While structure sensitive reactions allow an estimate of the types of reaction sites, such as step sites (≤C7) under reaction conditions (e.g. n-heptane dehydrocyclization). High pressure structure sensitive reactions (e.g. ethylene hydroformylation) are also shown to drastically alter the morphology of the surface by dispersing nanoparticles leading to inhibition of catalytic pathways. Moreover, the relationships between high index single crystals, oxide supported nanoparticles, and high surface area technical catalysts are established. Overall, the results demonstrate the utility of model catalysts in understanding the structure-activity relationships in heterogeneous catalytic reactions and the usefulness of high pressure reactions as an analytical probe of surface morphology.

L'objectif de ce travail était d'étudier de manière approfondie l'effet de l'indium dans des catalyseurs trimétalliques de reformage catalytique à base de platine, étain et indium. Des catalyseurs ont ainsi été synthétisés selon différents protocoles de préparation et de multiples caractérisations (spectroscopies Mössbauer d'étain, XANES, EXAFS, IR-CO, MEBT, chimisportion du CO, TPR et TPD) ont été mises en oeuvre afin d'élucider l'impact de l'indium sur la nature des phases métalliques présentes au sein du catalyseur. Il a ainsi été montré que la nature des sites métalliques dépend de la méthode d'introduction de l'indium. Quand l'indium est précipité avec la source d'alumine des alliages de type PtxSn sont obtenus alors que lorsque les métaux sont imprégnés sur la surface de l'alumine des espèces subsitutionnelles Pt-Sn sont formées. Il a aussi été montré que l'augmentation de la teneur en indium entraine une augmentation de la concentration atomique d'étain dans les alliage PtxSn et un remplacement de l'étain par l'indium dans les espèces substitutionnelles. Nous avons également réussi à préparer des catalyseurs avec des quantités élevées d'alliage Pt3Sn sur la base des connaissances acquises sur l'impact de l'indium et en déposant l'étain par une réaction organométallique contrôlée de surface. D'un point de vue catalytique, les tests de reformage du n-heptane mettent en évidence l'impact positif de l'indium puisque les catalyseurs à base de Pt-Sn-In sont moins sélectifs pour les réactions parasites d'hydrogénolyse et d'hydrocraquage et plus sélectifs pour l'isomérisation que les catalyseurs à base de Pt-Sn. / This work consists of a detailed study on the effect of indium in alumina supported trimetallic PtSnIn-based naphtha reforming catalysts. These catalysts were reproduced using different preparation protocols and the indium effect was investigated using 119Sn Mössbauer spectroscopy, XANES, EXAFS spectroscopies, IR-CO, STEM, CO chemisorption, TPR and TPD. It appears that the nature of the metallic active centres depends on the method with which indium was introduced. When co-precipitating the indium precursor with the Al source, PtxSn alloys were formed, whereas when metals were impregnated on the surface, substitutional Pt-Sn alloys were observed. Increasing the In content in the frst type of catalysts leads to an increase in the Sn concentration in PtxSn alloys, whereas a gradual replacement of Sn by indium in susbstitutinal alloys is observed in the second type of catalysts. These results allowed preparing catalysts with high Pt3Sn alloy contents using the effect of indium in catalysts prepared by Sn organometallic controlled surface reactions (CSR). From a catalytic point of view, n-heptane reforming tests show that trimetallic PtSnIn-based catalysts are less selective to hydrogenolysis and hydrocracking reactions, and highly selective to isomerisation, compared to the bimetallic PtSn-based catalyst.