AN HREM STUDY OF THE WO3 TIO2 MONOLAYER CATALYST SYSTEM - PROPOSALS FOR THE OVERLAYER STRUCTURE/

High resolution electron microscopy (HREM) has been used to characteri se the WO3/TiO2 (anatase) catalyst system. Comparisons between pure sa mples of TiO2 in the uncalcined and calcined states with titania in th e catalyst (loaded with 10 wt% WO3) indicate that the tungsten oxide o verlayer preserves the surface roughness of the support (as observed i n the pure uncalcined material). The calcination of pure anatase resul ts in significant grain growth and surface smoothing. In the electron microscope, the tungsten oxide overlayer is revealed as 2D pseudo-hexa gonal shaped clusters which appear to be epitaxially related to the su pport. After noting that the anatase support predominantly exposes {11 2}, {011}, {110} and {001} type facets we have combined this informati on with structural data on the overlayer derived from a previous EXAFS and XANES study by Hilbrig et al. that reported the presence of WO4 s pecies and six-coordinate WO5 groups. By considering the arrangement o f terminating oxygen atoms on each of the aforementioned anatase surfa ces, we suggest ways in which the WOx species may be linked together t o form the tungsten oxide overlayer. This approach has led us to concl ude that, with the exception of the (001) surface, the overlayer may c onsist of WO4 dimers rather than chains of linked WO5 groups terminate d by WO4 species.