CO oxidation on phosphate-supported Au catalysts: Effect of support reducibility on surface reactions

Previous work has shown that Au supported on FePO4 can be stable and active for CO oxidation and that oxygen from the FePO4 can participate in the CO oxidation. In this paper, we have used gas transient DRIFTS-QMS, Raman, temperature-programmed reduction and CO oxidation activity measurements to compare adsorption and oxidation of CO on two comparably loaded Au catalysts supported on both a reducible phosphate support, FePO4, and a non-reducible support, LaPO4. H-2-TPR confirms that the Au/FePO4 catalyst is highly reducible and that the reduction is strongly promoted by the Au, while neither LaPO4 nor Au/LaPO4 are reducible up to 500 degrees C. The nature of Au species was determined by CO adsorption. For Au/FePO4, cationic Au is present after oxidative treatment, and metallic Au dominates after reductive treatment. The majority of the cationic Au observed on the FePO4 support undergoes in situ reduction to metallic Au during rt CO adsorption. For Au/LaPO4, no cationic Au is observed, but metallic Au is present after both oxidative and reductive treatment. In addition, metallic Au is accompanied by anionic Au, not seen on Au/FePO4, which accumulates during CO exposure, even after an oxidative pretreatment. Unexpectedly, CO interacts rapidly with Au/LaPO4 to evolve CO2 and form both adsorbed CO2 and carbonate-like species, even though the LaPO4 is non-reducible and Raman fails to find evidence for loss of structural oxygen. H-2 coevolves with CO2 during CO-TPR of Au/LaPO4 (but not for Au/FePO4) leading to the conclusion that surface hydroxyl is the source of oxygen during CO exposure to Au/LaPO4. Anionic Au is associated with the vacancies remaining after reaction of hydroxyl with CO. (C) 2010 Elsevier Inc. All rights reserved.