X-RAY ABSORPTION-SPECTROSCOPY STUDY OF THE TITANIA-SUPPORTED AND ALUMINA-SUPPORTED TUNGSTEN-OXIDE SYSTEM

Tungsten oxide supported on titania is an important material for the selective catalytic reduction of nitrogen oxides NO(x). A structure analysis by means of X-ray absorption spectroscopy (XANES and EXAFS) is reported for materials prepared by spreading of WO3 in physical mixtures with the support oxide as well as by impregnation from aqueous solution. A comparison is also made with alumina-supported materials. Analysis of the XANES region at the W L1 and W L3 edges indicated that tungsten is hexavalent and anchored to the surface as WO5 and WO4 units, the relative proportion of which increases with loading. When water is absorbed, pseudooctahedrally coordinated species are formed in both cases. The analysis of the EXAFS provides additional support for the existence of these structures, which contain oxo groups W = O and W-O-W bridges. A tentative structure model is proposed, in which islands of surface tungstate species are formed by branched chains of WO5 units. The chains are assumed to be terminated by WO4 units, the WO5/WO4 ratio thus increasing with chain length or island size, which obviously increases with loading.