A combined theoretical–experimental study on the acidity of WOx-ZrO2 systems

Abstract

This work provides a chemical approach to the relationship between structure and electronic behavior of the active surface of the WO_x-ZrO₂ system as a function of W loads. This study shows that the electronic hardness (η), the Lewis and Brønsted acidity are functions of the local coordination and of the polymerization degree of the WO_x domain. From theoretical calculations the observed behavior in the WO_x-ZrO₂ system is explained: the Brønsted acidity increases while the Lewis acidity decreases as the W centers go from tetrahedral to octahedral coordination and as the condensation degree of the WO_x domain increases. Our results also indicate that not all the Brønsted sites in the WO_x domains are equally acid, and that as the W load increases the most acid sites decrease in number due to the condensation process. This finding also means a decrease on the average acidity per H site. Additionally, our results suggest that for surface densities in the 4–7 W nm⁻² range, mainly dimeric-tungstate species are present. A maximum in Brønsted acidity was observed for a W surface density about 7 W nm⁻².