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

X-ray photoelectron spectroscopy (XPS) technique was employed to characteri ze Al2O3 TiO2 support and MoO3/Al2O3-TiO2 catalyst calcined at different te mperatures from 773 to 1073 K, The Al2O3-TiO2 (1:1.3 mole ratio) binary oxi de support was obtained by a coprecipitation procedure with in situ generat ed ammonium hydroxide. A nominal 12 wt.% MoO3 was impregnated over the calc ined (773 K) support by a wet impregnation method, The initial characteriza tion by X-ray powder diffraction, Fourier transform-infrared (FT-IR), and O -2 chemisorption techniques revealed that the impregnated MoO3 is in a high ly-dispersed state on the surface of the support. XPS electron binding ener gy (Eb) values indicate that the MoO3/Al2O3-TiO2 catalyst contains the mixe d-oxide elements in the highest oxidation states, Ti(IV), AI(III), and Mo(V I), respectively. However, the core level Eb Of Al 2p slightly increased wi th increase of calcination temperature, and this effect was more 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 co verage of alumina surface by titania, thereby lowering the interaction betw een molybdena and alumina, The XPS atomic ratios indicate that the Ti/Al ra tio 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 calcinati on temperature. A clear difference between the Al2O3 and the TiO2 surfaces, in terms of surface free energy, isoelectric point, and surface hydroxyl d istribution was considered to be responsible for different distributions of molybdena over these supports. (C) 2001 Elsevier Science B.V, All rights r eserved.