An XPS study of the dispersion of MoO3 on TiO2–ZrO2, TiO2–SiO2, TiO2–Al2O3, SiO2–ZrO2, and SiO2–TiO2–ZrO2 mixed oxides

Abstract

X-ray photoelectron spectroscopy technique was employed to characterize TiO2–ZrO2, TiO2–SiO2, TiO2–Al2O3, SiO2– ZrO2, and SiO2–TiO2–ZrO2 mixed oxide supported MoO3 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 TiO2–ZrO2 and Ti in TiO2–Al2O3 acquire more negative charge after doping with MoO3 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 Al2O3 portion of the TiO2–Al2O3 mixed oxide. In the case of MoO3/TiO2–ZrO2 and MoO3/SiO2–TiO2–ZrO2 samples, the Mo:Ti and Mo:Zr ratios show that the Ti4C and Zr4C 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 TiO2–Al2O3, and a homogeneous distribution on TiO2–ZrO2 and TiO2–SiO2 mixed oxide surfaces.