An HREM study of the WO₃/TiO₂ monolayer catalyst system. Proposals for the overlayer structure

High resolution electron microscopy (HREM) has been used to characterise the WO3/TiO2 (anatase) catalyst system. Comparisons between pure samples of TiO2 in the uncalcined and calcined states with titania in the catalyst (loaded with 10 wt% WO3) indicate that the tungsten oxide overlayer preserves the surface roughness of the support (as observed in the pure uncalcined material). The calcination of pure anatase results in significant grain growth and surface smoothing. In the electron microscope, the tungsten oxide overlayer is revealed as 2D pseudo-hexagonal shaped clusters which appear to be epitaxially related to the support. After noting that the anatase support predominantly exposes {112}, {011}, {110} and (001) type facets we have combined this information with structural data on the overlayer derived from a previous EXAFS and XANES study by Hilbrig et al. that reported the presence of WO4 species and six-coordinate WO5 groups. By considering the arrangement of terminating oxygen atoms on each of the aforementioned anatase surfaces, we suggest ways in which the WOx species may be linked together to form the tungsten oxide overlayer. This approach has led us to conclude that, with the exception of the (001) surface, the overlayer may consist of WO4 dimers rather than chains of linked WO5 groups terminated by WO4 species