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The constant improvement of hydrotreating (HDT) catalysts, driven by industrial and environmental needs, requires a better understanding of the interactions between the oxide support (mostly alumina) and the MoS 2 active phase. Hence, this work addresses the supportdependent genesis of MoS 2 on four planar, single crystal -Al 2 O 3 surfaces with different crystal orientations (C (0001), R , M and A). In contrast to classical surface science techniques, which often rely on UHV-type deposition methods, the Mo is introduced by impregnation from an aqueous solution, in order to mimic the standard incipient wetness impregnation. Comparison between different preparation routes, impregnation vs. equilibrium adsorption (selective adsorption), is also considered. AFM, XAS, TEM and XPS show that the -Al 2 O 3 orientation has a clear impact on the strength of metal-support interactions at the oxide state with consequences on the sulfidation, size, stacking and orientation of MoS 2 slabs. Aggregation of molybdenum oxide particles is observed on the C (0001) plane suggesting weak metal-support interactions leading to high sulfidation degree with large slabs. Conversely, the presence of well-dispersed individual oxide particles on the R plane implies stronger metal-support interactions leading to a low sulfidation degree and shorter MoS 2 slabs. Both A and M facets, of similar crystallographic structure, display an intermediate behaviors in terms of sulfidation rate and MoS 2 size in line with intermediary metal-support interactions. Polarization-dependent Grazing-Incidence-EXAFS experiments as well as HR HAADF-STEM analysis allow us to demonstrate a surface-dependent orientation of MoS 2 slabs. A predominant basal bonding is suggested on the C (0001) plane in agreement with the existence of weak metal-support interactions. Conversely, a random orientation (edge and basal-bonding) is observed for the other planes. Generalization of these conclusions to industrial catalysts is proposed based on the comparison of the surface structure of the various model -Al 2 O 3 orientations used in this work and the predominantly exposed -Al 2 O 3 surfaces ((110), (100) and (111)).

A clear description of the morphology and location, with respect to the support, of metallic subnanometric particles remains a current strenuous experimental challenge in numerous catalytic applications such as naphtha reforming and biomass conversion. High-resolution HAADF-STEM coupled with in situ and tomographic analyses have been undertaken on a platinum (Pt) active phase supported on chlorinated alumina (γ-Al2O3) with 0.3 and 1 wt % Pt loadings, highlighting the formation of flat nanoparticles (NPs) of 0.9 nm diameter and Pt single atoms (SAs) in the reduced state. While SAs and weakly cohesive clusters are predominantly observed in the oxide state, with a coordination sphere of Pt composed of O and Cl as revealed by EXAFS, the ratio between SAs and Pt NPs in the reduced state is found to be about 2.8. This ratio is the same for the two metal loadings: both the total numbers of NPs and SAs increase at a higher metal loading. Electron tomography reveals that the vast majority of NPs are located on the edges or defects (steps, kinks) of the γ-alumina support crystallites. DFT calculations further highlight the optimized structures of NPs located at the γ-Al2O3 (110)–(100) edge and near-edge with a stability competing with NPs located either on the (110) or on the (100) γ-Al2O3 facet. A mathematical analysis of the segmented volumes shows that the average geodesic distances between NPs is linked to Pt loading: 9 nm for 1 wt % Pt and 16 nm for 0.3 wt % Pt. Evaluation of support tortuosity descriptors using the nanoparticle positions confirms a uniform distribution on the support. A square network geometric model compatible with the geodesic distances between NPs reveals that one to five NPs can be present at the same time on each alumina crystallite depending on Pt loading.

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Zinc oxide based materials are commonly used for the final desulfurization of synthesis gas in Fischer-Tropsch based XTL processes. Although the ZnO sulfidation reaction has been widely studied, little is known about the transformation at the crystal scale, its detailed mechanism and kinetics. A model ZnO material with well-determined characteristics (particle size and shape) has been synthesized to perform this study. Characterizations of sulfided samples (using XRD, TEM and electron diffraction) have shown the formation of oriented polycrystalline ZnS nanoparticles with a predominant hexagonal form (wurtzite phase). TEM observations also have evidenced an outward development of the ZnS phase, showing zinc and oxygen diffusion from the ZnO-ZnS internal interface to the surface of the ZnS particle. The kinetics of ZnO sulfidation by H(2)S has been investigated using isothermal and isobaric thermogravimetry. Kinetic tests have been performed that show that nucleation of ZnS is instantaneous compared to the growth process. A reaction mechanism composed of eight elementary steps has been proposed to account for these results, and various possible rate laws have been determined upon approximation of the rate-determining step. Thermogravimetry experiments performed in a wide range of H(2)S and H(2)O partial pressures have shown that the ZnO sulfidation reaction rate has a nonlinear variation with H(2)S partial pressure at the same time no significant influence of water vapor on reaction kinetics has been observed. From these observations, a mixed kinetics of external interface reaction with water desorption and oxygen diffusion has been determined to control the reaction kinetics and the proposed mechanism has been validated. However, the formation of voids at the ZnO-ZnS internal interface, characterized by TEM and electron tomography, strongly slows down the reaction rate. Therefore, the impact of the decreasing ZnO-ZnS internal interface on reaction kinetics has been taken into account in the reaction rate expression. In this way the void formation at the interface has been modeled considering a random nucleation followed by an isotropic growth of cavities. Very good agreement has been observed between both experimental and calculated rates after taking into account the decrease in the ZnO-ZnS internal interface.

The study of catalysts prepared by Pt deposition over mixtures of HUSY and HBEA zeolites revealed a selective deposition of Pt in HBEA sample. The characterization of nanoscale properties, as illustrated, was crucial to fully elucidate the structure of the bifunctional catalysts.

An efficient preparation method -based on the grafting and deposition of Ni and W molecular precursors onto partially dehydroxylated amorphous silica-alumina followed by a thermal treatment under H 2 S/H 2 -generates supported NiWS catalysts exhibiting enhanced activities in toluene hydrogenation, by comparison with conventional samples, prepared from metallic salts. A careful analysis of these materials by IR, XPS, TEM, HAADF-STEM, together with DFT calculations, reveals that the improved activity probably originates from the lower sulfidation temperature of W that improves distribution of Ni-W mixed sites at the edges of NiWS crystallites, thus providing an optimal compromise between intrinsic activity and surface concentration of active sites.

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