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

Abstract

Previous work has shown that Au supported on FePO 4 can be stable and active for CO oxidation and that oxygen from the FePO 4 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, FePO 4, and a non-reducible support, LaPO 4. H 2-TPR confirms that the Au/FePO 4 catalyst is highly reducible and that the reduction is strongly promoted by the Au, while neither LaPO 4 nor Au/LaPO 4 are reducible up to 500 °C. The nature of Au species was determined by CO adsorption. For Au/FePO 4, cationic Au is present after oxidative treatment, and metallic Au dominates after reductive treatment. The majority of the cationic Au observed on the FePO 4 support undergoes in situ reduction to metallic Au during rt CO adsorption. For Au/LaPO 4, 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/FePO 4, which accumulates during CO exposure, even after an oxidative pretreatment. Unexpectedly, CO interacts rapidly with Au/LaPO 4 to evolve CO 2 and form both adsorbed CO 2 and “carbonate-like” species, even though the LaPO 4 is non-reducible and Raman fails to find evidence for loss of structural oxygen. H 2 coevolves with CO 2 during CO-TPR of Au/LaPO 4 (but not for Au/FePO 4) leading to the conclusion that surface hydroxyl is the source of oxygen during CO exposure to Au/LaPO 4. Anionic Au is associated with the vacancies remaining after reaction of hydroxyl with CO.