The deposition of Mo on γ-alumina by the equilibrium adsorption method starting from ammonium heptamolybdate has been studied. Spectroscopic results converge to indicate that a previously unrecognized species, i.e., the Anderson-type heteropolymolybdate [Al(OH)6Mo6O18]3-, plays a major role in this type of synthesis as it is quantitatively formed in the solution within a few hours, by reaction of the heptamolybdate with dissolved aluminic species. This results in a considerable increase of alumina solubility in conditions generally thought to be nonaggressive. Furthermore, this species is also present in the solid catalyst after deposition, although it is harder to observe than in the liquid phase. A parallel is drawn with a well-known idea from the field of geochemistry, i.e., ligand-promoted oxide dissolution. The relevance of this phenomenon in catalyst preparation is evaluated in realistic conditions corresponding to published studies and/or industrial procedures. It is concluded that strong metal−support interaction in the deposition stage by surface dissolution followed by reaction in the liquid phase is most likely to be an important phenomenon, not only for cationic metal precursors as previously known but also for anionic precursors such as molybdates.

The role of the oxide support on the structure of the MoS2 active phase (size, morphology, orientation, sulfidation ratio, etc.) remains an open question in hydrotreating catalysis and biomass processing with important industrial implications for the design of improved catalytic formulations. The present work builds on an aqueous-phase surface-science approach using four well-defined α-alumina single crystal surfaces (C (0001), A (112̅0), M (101̅0), and R (11̅02) planes) as surrogates for γ-alumina (the industrial support) in order to discriminate the specific role of individual support facets. The reactivity of the various surface orientations toward molybdenum adsorption is controlled by the speciation of surface hydroxyls that determines the surface charge at the oxide/water interface. The C (0001) plane is inert, and the R (11̅02) plane has a limited Mo adsorption capacity while the A (112̅0) and M (101̅0) surfaces are highly reactive. Sulfidation of model catalysts reveals the highest sulfidation degree for the A (112̅0) and M (101̅0) planes suggesting weak metal/support interactions. Conversely, a low sulfidation rate and shorter MoS2 slabs are found for the R (11̅02) plane implying stronger Mo-O-Al bonds. These limiting cases are reminiscent of type I/type II MoS2 nanostructures. Structural analogies between α- and γ- alumina surfaces allow us to bridge the material gap with real Al2O3-supported catalysts. Hence, it can be proposed that Mo distribution and sulfidation rate are heterogeneous and surface-dependent on industrial γ-Al2O3-supported high-surface-area catalysts. These results demonstrate that a proper control of the γ-alumina morphology is a strategic lever for a molecular-scale design of hydrotreating catalysts.