Facile synthesis of WOx/ZrO2 catalysts using WO3·H2O precipitate as synthetic precursor of active tungsten species

Abstract

Zirconia-supported tungsten oxide (WOx/ZrO2) catalysts were successfully synthesized using a suspension containing amorphous hydrous zirconia precipitates [ZrOx(OH)4-2x·yH2O]n and tungstate monohydrate (WO3·H2O) precipitates. The procedure involved the dissolution of the WO3·H2O precipitate during the aging process with the release of oxyanion [WO4]2- species, interaction of this species with the surface of the [ZrOx(OH)4-2x·yH2O]n precipitate and, formation of active WOx species after thermal treatment. Non-bridging hydroxyl (OH−) groups present in the [ZrOx(OH)4-2x·yH2O]n precipitate act as an active agent for the WO3·H2O dissolution. N2 physisorption, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), temperature-programmed reduction using hydrogen (H2-TPR), temperature-programmed desorption of ammonia (NH3-TPD), Fourier-transform infrared (FTIR) spectroscopy of adsorbed pyridine, and Raman spectroscopy were used to elucidate the catalyst structure–performance relationship. The catalytic activity was evaluated for the oxidative desulfurization (ODS) of a model fuel containing dibenzothiophene (DBT). For a fixed WO3·H2O content, longer aging times improved the catalyst activity, reaching a maximum when WO3·H2O was completely dissolved. The increase in surface area and formation of more active Zr-WOx clusters and polytungstates are observed for the highest active catalysts. A synergetic effect between local Lewis and Brønsted acid sites seems to have contributed to the observed superior activity. The proposed strategy provides an efficient approach to produce active WOx/ZrO2 catalysts and may be applicable for designing other heterogeneous catalytic systems.