(607a) Effects of Domain Size and Support Composition on the Reactivity and Reducibility of Oxide-Supported Tungsten Oxide Clusters

Supported tungsten oxides (WO_x) have been studied as catalysts for a variety of reactions that involve H₂, such as the reduction of carboxylic acids. The active sites for carboxylic acid reduction are influenced by the presence of H₂ in the feed, which can affect the speciation of the WO_x clusters. Here, we used a combination of Density Functional Theory (DFT) calculations, global optimization methods, and ab initio thermodynamic modeling to evaluate the speciation and thermodynamic stability of titania and silica supported WO_x clusters.

We constructed molecular models for different WO_x domain sizes (monomers, dimers, trimers) supported on titania and silica, consistent with the small WO_x clusters we observed by STEM on our experimentally synthesized materials. To evaluate reactivity of the supported WO_x clusters with hydrogen, we globally optimized a library of structures with variable numbers of Brønsted acid sites and Lewis acid sites (from oxygen vacancies). Ab initio thermodynamic modeling (Figure 1) showed that WO_x speciation on TiO₂ forms acid sites at milder conditions than WO_x on silica. Consistent with this observation, H₂ TPR of the synthesized materials showed that catalysts on TiO₂ began consuming H₂ at ~600 K whereas catalysts on silica showed H₂ consumption at ~915 K. Bader charge analyses of WO_x on titania indicated that the oxidation state of W was not significantly affected by the removal of O atoms or addition of H atoms and remained close to +6. Tungsten retained its +6 state by restructuring upon oxygen-removal or delocalization of the electron for hydrogen-addition. Our results are corroborated by in-situ XPS of W, which revealed that the titania supported WO_x species did not change from an initial oxidation state of +6 during thermal treatments in H₂. However, in-situ XPS for silica supported WO_x showed that W is reduced, which is supported by Bader charge analysis.