Influence of Domain Size and Support Composition on\nthe Reducibility of SiO<sub>2</sub> and TiO<sub>2</sub> Supported\nTungsten Oxide Clusters

Supported tungsten\noxides are widely used in a variety of catalytic\nreactions. Depending on the support, the cluster size, oxidation state,\nreducibility and speciation of the tungsten oxides can widely differ.\nWhen promoted with a platinum group metal, the resulting spillover\nof hydrogen may facilitate the reduction of supported tungsten oxide\nspecies, depending on the support. High resolution scanning transmission\nelectron microscopy imaging showed nanometer scale WO<sub><i>x</i></sub> clusters were synthesized on SiO<sub>2</sub> whereas\nhighly dispersed species were formed on TiO<sub>2</sub>. Results from\nH<sub>2</sub>-temperature-programmed reduction showed the presence\nof Pd lowered the initial reduction temperature of SiO<sub>2</sub>-supported WO<sub><i>x</i></sub> species but interestingly\ndid not affect that of TiO<sub>2</sub>-supported WO<sub><i>x</i></sub>. X-ray photoelectron and absorption spectroscopies showed\nthe W atoms in SiO<sub>2</sub>-supported WO<sub><i>x</i></sub> species reduce from a +6 oxidation state to primarily +5 after\nthermal treatment in 5% H<sub>2</sub>, while the fraction of W in\nthe +5 oxidation state was relatively unaffected by reduction treatment\nof TiO<sub>2</sub>-supported WO<sub><i>x</i></sub>. The\nunusual behavior of TiO<sub>2</sub>-supported WO<sub><i>x</i></sub> was explained by quantum chemical calculations that reveal\nthe lack of change in the oxidation state of W is attributed to charge\ndelocalization on the surface atoms of the titania support, which\ndoes not occur on silica. Moreover, modeling results at <600 K\nin the presence of H<sub>2</sub> suggest the formation of Brønsted\nacid sites, and the absence of Lewis acid sites, on larger aggregates\nof WO<sub><i>x</i></sub> on silica and all cluster sizes\non titania. These results provide experimental and theoretical insights\ninto the nature of supported tungsten oxide clusters under conditions\nrelevant to various catalytic reactions.