Insights into the WOx Coverage-Dependent Location and Oxidation State of Noble Metals Supported on Tungstated Oxides: The Case of Rh/WOx-Ce0.62Zr0.38O2

Noble metals-promoted tungstated oxides have been shown to be profitable in a wide variety of catalytic reactions of environmental interest but to be detrimental in the hydrogenation of aromatics. The origin of the deleterious effect of tungstates on the hydrogenation performance of noble metals is still being debated. To provide further insights into this, the location and the oxidation state of Rh were investigated as a function of the W surface density (0-10 W/nm(2)) of Rh/WOx-Ce0.62Zr0.38O2 (Rh/W-CZ) catalysts after high-temperature reduction. For that purpose, a thorough characterization of the oxide phases was performed through N-2-sorption, X-ray diffraction, Raman spectroscopy, NO temperature-programmed desorption, and X-ray photoelectron spectroscopy (XPS), whereas the metallic phases were characterized by low-temperature H-2 chemisorption, XPS, N-2 Fourier transform infrared spectroscopy and benzene hydrogenation. It was found that Rh deposited on both tungstates and CZ, and did not sinter with increasing W surface densities. The observed linear decrease in the Rh hydrogenation performance of the WOx-promoted Ce0.62Zr0.38O2 below pseudo monolayer coverage of CZ (4.8 W/nm(2)cz) was assigned to a strong metal support interaction effect between the Rh particles and the nonreducible underlying WOx phase, resulting in the formation of electron-deficient Rh species (Rh delta+).