Molecular/electronic structure–surface acidity relationships of model-supported tungsten oxide catalysts

Abstract

A series of model-supported WO 3 catalysts were synthesized on preformed Al 2O 3, Nb 2O 5, TiO 2, and ZrO 2 supports by impregnation of aqueous ammonium metatungstate, (NH 4) 10W 12O 41⋅5H 2O. The molecular and electronic structures of the supported tungsten oxide phases were determined with in situ Raman and UV–vis spectroscopy, respectively. The supported tungsten oxide structures are the same on all oxide supports as a function of tungsten oxide surface density (W/nm 2). Below monolayer coverage (<5 W/nm 2), both monotungstate and polytungstate surface WO x species are present under dehydrated conditions and the polytungstate/monotungstate ratio increases with increasing surface coverage. Above monolayer coverage (>5 W/nm 2), crystalline WO 3 nanoparticles are present on top of the surface WO x monolayer. Above ∼10 W/nm 2, bulk-like WO 3 crystallites become dominant. The number of catalytic active sites and surface chemistry of the supported tungsten oxide phases were chemically probed with CH 3OH dehydration to CH 3OCH 3. The specific oxide support was found to significantly affect the relative catalytic acidity of the surface WO x species (Al 2O 3 ≫ TiO 2 > Nb 2O 5 > ZrO 2) to that of the supported WO 3 nanoparticles. Consequently, no general relationship exists between the molecular/electronic structures or domain size and the specific catalytic acidity of the supported tungsten oxide phases present in the model-supported WO 3 catalysts.