Characterization of Au-Rh/TiO₂ Bimetallic Nanocatalysts by CO and CH₃CN Adsorption: XPS, TEM and FTIR Measurements

On X-ray photoelectron spectra of the Au-Rh/TiO₂ catalysts the position of Au4f peak was practically unaffected by the presence of rhodium, the peak position of Rh3d, however, shifted to lower binding energy with the increase of gold content of the catalysts. Rh enrichment in the outer layers of the bimetallic crystallites was experienced. The bands due to Au⁰-CO, Rh⁰-CO and (Rh⁰)₂-CO were observed on the IR spectra of bimetallic samples, no signs for Rh⁺-(CO)₂ were detected on these catalysts. The band due to CH₃CN on Lewis acid centers shifted to lower wavenumbers with the increase of Rh content, which shows that the strength of Lewis acid sites weakens with the increase of Rh content of the catalysts. CH₃CN, on the other hand, dissociates producing CN_(a) species even at this temperature. From the shift to higher wavenumbers of the band due to CN_(a) the strengthening of the C—N bond with increasing Rh content has been established. The results were interpreted by electron donation from titania through gold to rhodium and by the higher particle size of bimetallic crystallites. The effects of catalyst composition, reaction temperature and composition of the reacting gas mixtures have been studied on the oxidation of CO in the presence of hydrogen (PROX process). The presence of both O₂ and H₂ reduced the surface concentration of CO adsorbed on metallic sites. Mass spectroscopic analysis of the gas phase showed that gaseous CO₂ formed in the highest amount in CO+O₂ mixture, the presence of H₂ suppressed the amount of CO₂ produced. This negative effect of hydrogen was the lowest on 1% Rh/TiO₂, the highest inhibition was observed on Au/TiO₂ systems.