Influence of Domain Size and Support Composition on the Reducibility of SiO₂ and TiO₂ 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 WOx clusters were synthesized on SiO2 whereas highly dispersed species were formed on TiO2. Results from H2-temperature-programmed reduction showed the presence of Pd lowered the initial reduction temperature of SiO2-supported WOx species but interestingly did not affect that of TiO2-supported WOx. X-ray photoelectron and absorption spectroscopies showed the W atoms in SiO2-supported WOx species reduce from a +6 oxidation state to primarily +5 after thermal treatment in 5% H2, while the fraction of W in the +5 oxidation state was relatively unaffected by reduction treatment of TiO2-supported WOx. The unusual behavior of TiO2-supported WOx 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 H2 suggest the formation of Brønsted acid sites, and the absence of Lewis acid sites, on larger aggregates of WOx 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.