Characterization of MoO3/TiO2-ZrO2 catalysts by XPS and other techniques

To explore thermal stability of TiO2-ZrO2 support, and dispersion and temperature stability of MoO3/TiO2-ZrO2 catalyst these systems were subjected to thermal treatments from 773 to 1073 K and were examined by X-ray photoelectron spectroscopy, X-ray diffraction, and FT-infrared techniques. The TiO2-ZrO2 mixed oxide support was obtained by a homogeneous coprecipitation method and MoO3 (12 wt.%) was impregnated over the 773 K calcined support by adopting a wet impregnation procedure. Characterization results suggest that the TiO2-ZrO2 when calcined at 773K is in X-ray amorphous state and gets converted into a crystalline ZrTiO4 compound beyond 873 K. The ZrTiO4 compound is thermally quite stable up to 1073 K calcination temperature in the absence of molybdena on its surface. In the case of MoO3/TiO2-ZrO2 catalyst, the molybdenum oxide is in highly dispersed state on the support surface when calcined at 773 K. However, above 773 K calcination it selectively interacts with ZrO2 portion of the TiO2-ZrO2 binary oxide and readily forms ZrMo2O8 compound by liberating TiO2. The ZrMo2O8 compound formation proceeds in a two-step process. In the first step, there is an incorporation of Mo6+ ions into the ZrO2 oxide matrix and then the crystal growth occurs. The liberated TiO2 appears in the form of both anatase and rutile phases with varying intensities. (C) 2000 Elsevier Science B.V. All rights reserved.