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 (TC) 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: NiI, at lower temperature (Tm), assigned to a surface phase with similar stoichiometry to NiAl2O4, and the high Tm NiII, assigned to bulk-like NiAl2O4. On increasing TC, NiII area increases at the expense of NiI. Mo/Al2O3 catalysts showed spectra with two peaks which were almost insensitive to varying TC up to 660°C. The low Tm MoI peak was assigned to (partial) reduction of polymolybdates, and the high-Tm MoII to further reduction of polymolybdates plus that of 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 Tm of the MoI peak decreased on increasing Ni content between 0 and 5 wt% NiO. This effect was less apparent upon increasing TC, 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 MoI, and that the extent of MoI reduction was not affected by varying Ni concentration. In a similar way, Mo facilitated reduction of Ni, as reflected in the lower Tm of the NiI 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 NiI might be involved in the generation of active Ni-Mo-S phases.