Effect of Ceria on Gold–Titania Catalysts for the Water–Gas Shift Reaction: Fundamental Studies for Au/CeOx/TiO2(110) and Au/CeOx/TiO2 Powders

Abstract

We have carried out a fundamental study of the WGS reaction on model Au/CeOx/TiO2(110) and powder Au/CeOx/TiO2 catalysts paying particular attention to the effect of ceria on the activity of the gold–titania systems. CeOx nanoparticles deposited on TiO2(110) act as anchoring sites for gold improving the dispersion of the admetal on the oxide support. When compared to a typical benchmark system like Cu(111), Au/CeOx/TiO2(110) catalysts exhibit TOFs, which are 10–30 times larger, and a substantial reduction in the apparent activation energy for the WGS, which decreases from 18 kcal/mol on Cu(111) to 7 kcal/mol on Au/CeOx/TiO2(110). Low concentrations of ceria (6 and 15 wt %) were deposited onto a titania powder support via a wetness impregnation process. 1 atom % gold was then deposited on the CeOx/TiO2 mixed-oxide supports via a deposition–precipitation (DP) method. The Au/CeOx/TiO2 powder catalysts were characterized with HRTEM and a combination of in situ time-resolved XRD and XAFS. The XRD measurements indicated that a main effect of ceria was to enhance the concentration of oxygen vacancies in the catalysts and, thus, help with the dissociation of water during the reaction. Results of in situ XAFS showed that the gold oxidation state in the Au/CeOx/TiO2 powder catalysts changed from ionic (Auδ+) to metallic (Au0) with the start of the WGS. The active phase for these powder catalysts contained gold particles with average sizes of ∼2 nm. This study shows that the phenomena observed in model Au/CeOx/TiO2(110) catalysts do provide useful concepts for the design and preparation of highly active and stable powder catalysts for the WGS reaction.