Surface-Dependent Activation of Model α-Al₂ O₃ -Supported P-Doped Hydrotreating Catalysts Prepared by Spin Coating

Requirements for improved catalytic formulations is continuously driving research in hydrotreating (HDT) catalysis for biomass upgrading and heteroatom removal for cleaner fuels. The present work proposes a surface-science approach for the understanding of the genesis of the active (sulfide) phase in model P-doped MoS₂ hydrotreating catalysts supported on α-Al₂ O₃ single crystals. This approach allows one to obtain a surface-dependent insight by varying the crystal orientations of the support. Model phosphorus-doped catalysts are prepared via spin-coating of Mo-P precursor solutions onto four α-Al₂ O₃ crystal orientations, C(0001), A(11 $$ 0), M(10 $$ 0) and R(1 $$ 02) that exhibit different speciations of surface -OH. ³¹ P and ⁹⁵ Mo liquid-state NMR are used to give a comprehensive description of the Mo and P speciation of the phospho-molybdic precursor solution. The speciation of the deposition solution is then correlated with the genesis of the active MoS₂ phase. XPS quantification of the surface P/Mo ratio reveal a surface-dependent phosphate aggregation driven by the amount of free phosphates in solution. Phosphates aggregation decreases in the following order C(0001)≫M(10 $$ 0)>A(11 $$ 0), R(1 $$ 02). This evolution can be rationalized by an increasing strength of phosphate/surface interactions on the different α-Al₂ O₃ surface orientations from the C(0001) to the R(1 $$ 02) plane. Retardation of the sulfidation with temperature is observed for model catalysts with the highest phosphate dispersion on the surface (A(11 $$ 0), R(1 $$ 02)), suggesting that phosphorus strongly intervene in the genesis of the active phase through a close intimacy between phosphates and molybdates. The surface P/Mo ratio appears as a key descriptor to quantify this retarding effect. It is proposed that retardation of sulfidation is driven by two effects: i) a chemical inhibition through formation of hardly reducible mixed molybdo-phosphate structures and ii) a physical inhibition with phosphate clusters inhibiting the growth of MoS₂ . The surface-dependent phosphorus doping on model α-Al₂ O₃ supports can be used as a guide for the rational design of more efficient HDT catalysts on industrial γ-Al₂ O₃ carrier.