Tantalum oxide-supported metal oxide (Re2O7, CrO3, MoO3, WO3, V2O5, and Nb2O5) catalysts:: synthesis, Raman characterization and chemically probed by methanol oxidation

The molecular structures and reactivity of tantalum oxide-supported metal oxide (V2O5, Nb2O5, CrO3, MoO3, WO3, and Re2O7) catalysts were determined by Raman spectroscopy and the methanol oxidation chemical probe reaction, respectively. The metal oxides form a two-dimensional surface metal oxide overlayer on the tantalum oxide support. Under ambient conditions, the hydrated surface metal oxide species possesses structures similar to the corresponding metal oxide ionic species present in acidic solutions (ReO4-, Cr2O72-, Mo8O264-, HW12O4210-, V10O286-, and Nb2O5 . nH(2)O). Under dehydrated conditions, only one surface ReOx species is present on the tantalum oxide support. For CrO3, MoO3, WO3, V2O5, and Nb2O5 supported on tantalum oxide, two dehydrated surface metal oxide species may be present: isolated and/or polymerized species. During methanol oxidation, the surface vanadium, chromium, and rhenium oxide sites on the tantalum oxide support behave as surface redox,sites, while the surface tungsten and niobium oxide sites on the tantalum oxide support are surface acidic sites. The surface molybdenum oxide sites on tantalum oxide possess both acidic and redox characteristics. No surface basic sites were found on any of the tantalum oxide-supported metal oxide catalysts. The relative redox activity and selectivity of the various tantalum oxide-supported metal oxides reflect the catalytic properties of the pure metal oxides. (C) 2003 Elsevier Science (USA). All rights reserved.