A molecular-level understanding of heterogeneous catalyst preparation (deposition of a dissolved metallic salt on a metal oxide support) is a prerequisite for precise control of the final properties of these catalysts. Nevertheless, most studies of adsorption reactions are performed on high surface area oxides with a number of crystallographically different surfaces with ill-defined sorption sites. A fundamental description of sorption mechanisms is thus not possible in most cases. One way to mitigate this problem is to simplify the sorption system by using oriented single crystals that have a limited number of well-defined surface sites.

α-Al₂O₃ single crystals wafers with two crystallographic orientations (0001) et (1-102) have been used as model supports for the characterization of hydrotreating Ni promoted Mo/Al₂O₃ catalysts. Deposition of Mo^VI and Ni^II is carried out by immersing the wafers in the impregnation solution before washing and drying with a flow of N₂. Ni adsorption is carried out in the presence of a chelating ligand (ethylenediamine) with variable en/Ni ratio (1 to 3).

The samples are characterized by XPS, AFM and Grazing-Incidence EXAFS (GI-EXAFS). The latter technique is well suited for the study of surface systems due to an enhance surface sensitivity allowing to work at the low surface coverages imposed by low surface area single crystals.

Macroscopic (XPS, AFM) and molecular-scale characterization techniques (GI-EXAFS) show that the adsorption of Mo^VI is site-specific with no adsorption on the (0001) face since this orientation exposes only neutral surface hydroxyls. Conversely, Mo^VI is grafted on the most basic sites of the (1-102) face. Ni^II adsorption leads to the precipitation of a nickel hydroxide phase despite the presence of chelating ligands. These experiments reveal that ethylenediamine has no influence on the adsorption mode of Ni^II while it is claimed that the addition of this ligand improves the HDS performance of NiMo catalysts.

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