Dynamic Formation of Brønsted Acid Sites over Supported WOx/Pt on SiO2 Inverse Catalysts─Spectroscopy, Probe Chemistry, and Calculations

Abstract

Platinum-tungsten oxides are among the most studied metal–metal oxide pair catalysts for C–O hydrogenolysis reactions. The Brønsted acid density and synergy between Pt and WOx, especially in the inverse structure, are critical to reactivity and selectivity. However, a clear molecular-level understanding of the formation and dynamics of Brønsted acid sites (BAS) is lacking. Here, using in situ spectroscopic characterizations (Raman and FTIR), chemical probing (CO chemisorption and pyridine titration), density functional theory (DFT) calculations, and a model reaction (tert-butanol dehydration), we demonstrate the structural evolution of WOx species and associated BAS dynamics in various environments. In situ Raman and DFT calculations show that below monolayer coverage, the WOx species stay as isolated monomers on the SiO2 support and W3Ox trimers on Pt. The W3Ox trimers on Pt are dynamic and 10× more active toward dehydration than the WOx species on the SiO2 support. H2 plays a complex role: at low temperatures (<473 K), it creates more BAS in the form of W3O7H on Pt by reversible hydrogen spillover, and at higher temperatures (>573 K), it partially reduces the W3Ox. We further show that the inverse configuration allows changes in the BAS density via catalyst pretreatments. This study provides a strategy for tuning Brønsted acid density and regenerating sites by pretreatment and catalyst composition.