An XPS study of the dispersion of MoO 3 on TiO 2–ZrO 2, TiO 2–SiO 2, TiO 2–Al 2O 3, SiO 2–ZrO 2, and SiO 2–TiO 2–ZrO 2 mixed oxides

Abstract

X-ray photoelectron spectroscopy technique was employed to characterize TiO 2–ZrO 2, TiO 2–SiO 2, TiO 2–Al 2O 3, SiO 2–ZrO 2, and SiO 2–TiO 2–ZrO 2 mixed oxide supported MoO 3 catalysts. The investigated mixed oxide supports are obtained by a homogeneous coprecipitation method using urea as hydrolyzing agent. Molybdena (12 wt.%) was impregnated over these calcined (773 K) mixed oxide supports by a wet impregnation method from aqueous ammonium heptamolybdate solution. The XPS binding energy (BE) values of all the metals in the mixed oxide supports as well as Mo-containing catalysts are found to shift from the values of the individual metal component oxides. The shift in BE suggests that the Zr in TiO 2–ZrO 2 and Ti in TiO 2–Al 2O 3 acquire more negative charge after doping with MoO 3 on these supports. The observed BE shifts, due to variation in the lattice potential, are explained in terms of Kung’s model. The XPS atomic intensity ratio measurements show that the interaction between Mo and Al is strong and the dispersion of molybdena is more on Al 2O 3 portion of the TiO 2–Al 2O 3 mixed oxide. In the case of MoO 3/TiO 2–ZrO 2 and MoO 3/SiO 2–TiO 2–ZrO 2 samples, the Mo:Ti and Mo:Zr ratios show that the Ti 4+ and Zr 4+ both contribute equally in the dispersion of molybdenum on these corresponding mixed oxides. The FWHM values indicate the presence of different Mo(VI) species on TiO 2–Al 2O 3, and a homogeneous distribution on TiO 2–ZrO 2 and TiO 2–SiO 2 mixed oxide surfaces.