Bulk and Surface Structures of V2 O5 /ZrO2 Systems and Their Relevance for o -Xylene Oxidation

Abstract

Zirconia-supported vanadium oxide systems, with V2O5 loadings ranging from V/Zr ) 0.033-0.270 atom ratios (0.2-2 monolayers), were prepared by impregnation of a porous ZrO2 substrate with ammonium metavanadate solutions. The surface structures of air-calcined samples were elucidated by Raman and X-ray photoelectron spectroscopic techniques and thermal desorption methods. Some insight into how the bulk structures of these materials developed upon thermal treatment was derived from the X-ray diffraction patterns and temperature-programmed reduction. At low V content, surface-dispersed vanadium oxide species were formed, which produced the transformation of the ZrO2 support from the tetragonal phase into the monoclinic one. At higher vanadium oxide contents, a solid-state reaction between V2O5 and ZrO2 occurred, with subsequent formation of the ZrV2O7 phase, this being the major V-containing phase. The reactivity of these surface structures was examined by looking at their performance for the oxidation of o-xylene to phthalic anhydride. Activity tests indicated that surface vanadium oxide species were more active for the oxidation of o-xylene but had a lower selectivity to phthalic anhydride.