WOx/TiO2 catalysts prepared by grafting of tungsten alkoxides: Morphological properties and catalytic behavior in the selective reduction of NO by NH3

Tungsten oxide on titania catalysts with loadings of 3.5-11 mu mol(W) m(BET)(-2) were prepared in a multiple grafting procedure using tungsten (VI)-oxo-methanolate and tungsten(v)-ethanolate as precursors. The efficiency of the deposition depended markedly on the combination of alkoxide precursor and solvent. Calcination at elevated temperature, 1023 K compared to 573 K normally used, resulted in the formation of additional Bronsted acid sites as indicated by TPD measurements. In line with this finding the activity for selective catalytic reduction of NO with NH3 increased by almost 100% upon calcination at 1023 K and nitrogen remained the only product observed. The structural changes induced upon tungsten deposition and calcination in dry O-2/Ar at different temperatures were followed using XRD and laser Raman spectroscopy. The formation of a stabilized polytungstate layer, indicated by a prominent Raman band at 985 cm(-1) was found to be favored by increasing WOx loading and calcination temperature. As a minor species, paracrystalline WO3 was found in samples with loadings higher than 9 mu mol(W) m(-2) after high temperature (1023 K) calcination in moist air. TPR investigations in combination with AAS indicated a stepwise reduction of W(VI), with W(IV) being stable up to ca. 1150 K. (C) 1996 Academic Press, Inc.