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Tungsten oxide supported on titania is an important material for the selective catalytic reduction of nitrogen oxides NO{sub x}. A structure analysis by means of X-ray absorption spectroscopy (XANES and EXAFS) is reported for materials prepared by spreading of WO{sub 3} 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 L{sub 1} and W L{sub 3} edges indicated that tungsten is hexavalent and anchored to the surface as WO{sub 5} and WO{sub 4} 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{double bond}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 WO{sub 5} units. The chains are assumed to be terminated by WO{sub 4} units, the WO{sub 5}/WO{sub 4} ratio thus increasing with chain length or island size, which obviously increases with loading. |
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