Tantalum oxide-supported metal oxide (Re 2O 7, CrO 3, MoO 3, WO 3, V 2O 5, and Nb 2O 5) catalysts: synthesis, Raman characterization and chemically probed by methanol oxidation

Abstract

The molecular structures and reactivity of tantalum oxide-supported metal oxide (V 2O 5, Nb 2O 5, CrO 3, MoO 3, WO 3, and Re 2O 7) 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 (ReO 4 −, Cr 2O 7 2−, Mo 8O 26 4−, HW 12O 42 10−, V 10O 28 6−, and Nb 2O 5· nH 2O). Under dehydrated conditions, only one surface ReO x species is present on the tantalum oxide support. For CrO 3, MoO 3, WO 3, V 2O 5, and Nb 2O 5 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.