Isotopic and Chemical Titration of Acid Sites in Tungsten Oxide Domains Supported on Zirconia

Abstract

The acid and surface properties of WOx−ZrO2 solid acid catalysts were examined as a function of WOx surface density. Surface density strongly influences the size and electronic properties of WOx domains and the rate of acid-catalyzed reactions on these materials. The adsorption and isotopic exchange of acid and redox probe molecules (NH3, pyridine, H2, and O2) were used to measure the density and type of active sites in the reducing environments required to achieve high isomerization reaction rates. In this manner, the role of H2 and of surface density on the formation of Bronsted or Lewis acid sites was determined by combining infrared spectroscopy and desorption measurements using NH3 titrants. At submonolayer WOx coverages, the density of Bronsted acid sites increased with increasing WOx surface density up to ∼0.20 NH3/W atom, indicating their presence at the interface between ZrO2 and polytunsgtate domains. Above monolayer WOx coverages, Bronsted acid site densities (per W atom) decreased markedly because of the formation of WO3 clusters. The addition of H2 during NH3 adsorption or desorption increased only slightly the ratio of Bronsted to Lewis acid sites and did not change the total adsorption uptake. H2 chemisorption uptakes at typical reaction temperature (523 K) showed that the number of H atoms adsorbed on WOx−ZrO2 was negligible at low WOx surface densities (<4 W/nm2). Hydrogen uptakes increased as reducible polytungstate domains formed with increasing surface density, reaching a maximum value of 0.063 H/W at 7.35 W/nm2. H2 uptakes decreased at higher surface densities because of reduction processes that lead to oxygen removal from WOx domains, as shown by O2 chemisorption uptakes on these samples. Oxygen-deficient WO3-x species are unable to stabilize adsorbed hydrogen by delocalizing electron density to form Hδ+. The density of these minority Hδ+ Bronsted acid sites formed during exposure to H2, and the rate of o-xylene isomerization on WOx−ZrO2, shows a similar dependence on WOx surface density. These findings suggest that temporary acid sites formed by H2 dissociation and adsorption on basic oxygens in WOx domains lead to very active Bronsted acid sites, which catalyze o-xylene isomerization with turnover rates much higher than on H-ZSM5. Isotopic exchange of surface OH groups with D2, after H2 pretreatments that lead to chemisorbed hydrogen, confirmed the low density of O−H species formed from H2 on WOx−ZrO2 (0.017 H atoms/W atom at 7.8 W/nm2).