Reducibility of Ni-Mo/Al₂O₃ catalysts: A TPR study

Temperature-programmed reduction (TPR) has been employed to study the reducibility of Ni-Mo/γ-Al₂O₃ catalysts (0-5 wt% NiO; 12 wt% MoO₃) calcined at temperatures (T_C) between 400 and 800^°C. Catalysts were further characterized by XRD, BET area, and ammonia adsorption measurements. Ni/Al₂O₃ samples showed two broad TPR bands: Ni_I, at lower temperature (T_m), assigned to a surface phase with similar stoichiometry to NiAl₂O₄, and the high T_m Ni_II, assigned to bulk-like NiAl₂O₄. On increasing T_C, Ni_II area increases at the expense of Ni_I. Mo/Al₂O₃ catalysts showed spectra with two peaks which were almost insensitive to varying T_C up to 660^°C. The low T_m Mo_I peak was assigned to(partial) reduction of polymolybdates, and the high-T_m Mo_II to further reduction of polymolybdates plus thatof more refractory tetrahedrally coordinated and/or monomeric species. In Ni-Mo catalysts, XRD and NH₃ adsorption measurements, together with TPR data, indicate surface interaction between Ni and Mo, probably through the formation of Ni-Mo-O surface phases precursors of β-NiMoO₄. Ni facilitated the reduction of Mo, as the T_m of the Mo_I peak decreased on increasing Ni content between 0 and 5 wt% NiO. This effect was less apparent upon increasing T_C, indicating that it is related to the presence of Ni in or near the surface. However, quantitative TPR suggested that no appreciable amount of Ni was reduced simultaneously with Mo_I, and that the extent of Mo_I reduction was not affected by varying Ni concentration. In a similar way, Mo facilitated reduction of Ni, as reflected in the lower T_m of the Ni_I band in presence of Mo. Thus synergistic effects between Ni and Mo may be opperative not only in the sulfided state but also in the oxidic precursor of the catalysts. It is speculated that Ni ions responsible for Ni_I might be involved in the generation of active Ni-Mo-S phases.