An XPS study of dispersion and chemical state of MoO3 on Al2O3-TiO2 binary oxide support

Abstract

X-ray photoelectron spectroscopy (XPS) technique was employed to characterize Al2O3-TiO2 support and MoO3/Al2O3- TiO2 catalyst calcined at different temperatures from 773 to 1073 K. The Al2O3-TiO2 (1:1.3 mole ratio) binary oxide support was obtained by a coprecipitation procedure with in situ generated ammonium hydroxide. A nominal 12 wt.% MoO3 was impregnated over the calcined (773 K) support by a wet impregnation method. The initial characterization by X-ray powder diffraction, Fourier transform-infrared (FT-IR), and O2 chemisorption techniques revealed that the impregnated MoO3 is in a highly-dispersed state on the surface of the support. XPS electron binding energy (Eb) values indicate that the MoO3/Al2O3-TiO2 catalyst contains the mixed-oxide elements in the highest oxidation states, Ti(IV), Al(III), and Mo(VI), respectively. However, the core level Eb of Al 2p slightly increased with increase of calcination temperature, and this effect wasmore prominent in the case of molybdena-doped samples. A better resolved Mo 3d doublet was observed at all calcination temperatures. This was explained as due the coverage of alumina surface by titania, thereby lowering the interaction between molybdena and alumina. The XPS atomic ratios indicate that the Ti/Al ratio is sensitive to the calcination temperature. The Mo/Al ratio was found to be more than that of Mo/Ti ratio and decreased with increasing calcination temperature. A clear difference between the Al2O3 and the TiO2 surfaces, in terms of surface free energy, isoelectric point, and surface hydroxyl distribution was considered to be responsible for different distributions of molybdena over these supports.