Structure-reduction correlation of supported tungsten(VI)-oxo-species

Abstract

Influence of concentration on the solution chemistry of ammonium paratungstate (APT), before and after equilibration with alumina, titania or silica support particles, was examined by pH-metry and UV–vis spectroscopy. Then, tungsten(VI)-oxo-species were supported on each of the three oxides at various loading levels (2–20wt%-WO3) by wet-impregnation with APT-solutions, drying at 120°C, and subsequent calcination at 500°C for 10h. Bulk and surface structures of the materials thus obtained were characterized by X-ray powder diffractometry, UV–vis diffuse reflectance spectroscopy, temperature-programmed reduction profiles, and N2 sorptiometry. Various 2- (2D) and 3-dimensional (3D) structures were found to be assumed by the supported tungstates depending on (i) the kind of interactions prevailing at the support/solution interfaces, and (ii) the loading level. The hydrogen-reduction behavior and products were found to be sensitively dependent on the structure assumed by the supported tungstate species. The ease of reduction was found to be in accord with the following descending orders: (i) on a given support, 3D-strtuctured tungstate>2D-structured overlayers of polytungstate>2D-structured monolyers of polytungstate>2D-structured monolayers of monotungstate, and (ii) for a given structure assumed by supported tungstate on silica>titania>alumina.