Influence of Domain Size and Support Composition on the Reducibility of SiO and TiO Supported Tungsten Oxide Clusters.

Supported tungsten oxides are widely used in a variety of catalytic reactions. Depending on the support, the cluster size, oxidation state, reducibility and speciation of the tungsten oxides can widely differ. When promoted with a platinum group metal, the resulting spillover of hydrogen may facilitate the reduction of supported tungsten oxide species, depending on the support. High resolution scanning transmission electron microscopy imaging showed nanometer scale WO clusters were synthesized on SiO whereas highly dispersed species were formed on TiO. Results from H-temperature-programmed reduction showed the presence of Pd lowered the initial reduction temperature of SiO-supported WO species but interestingly did not affect that of TiO-supported WO . X-ray photoelectron and absorption spectroscopies showed the W atoms in SiO-supported WO species reduce from a +6 oxidation state to primarily +5 after thermal treatment in 5% H, while the fraction of W in the +5 oxidation state was relatively unaffected by reduction treatment of TiO-supported WO . The unusual behavior of TiO-supported WO was explained by quantum chemical calculations that reveal the lack of change in the oxidation state of W is attributed to charge delocalization on the surface atoms of the titania support, which does not occur on silica. Moreover, modeling results at <600 K in the presence of H suggest the formation of Brønsted acid sites, and the absence of Lewis acid sites, on larger aggregates of WO on silica and all cluster sizes on titania. These results provide experimental and theoretical insights into the nature of supported tungsten oxide clusters under conditions relevant to various catalytic reactions.