Probing adsorption sites for CO on ceria

Abstract

Ceria based catalysts show remarkable activity for CO conversion reactions such as CO oxidation and the water-gas shift reaction. The identification of adsorption sites on the catalyst surfaces is essential to understand the reaction mechanisms of these reactions, but the complexity of heterogeneous powder catalysts and the propensity of ceria to easily change oxidation states in the presence of small concentrations of either oxidizing or reducing agents make the process difficult. In this study, the adsorption of CO on CuO_x/Cu(111) and CeO_x/Cu(111) systems has been studied using infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. IR peaks for the adsorbed CO on O/Cu(111) with only chemisorbed oxygen, well-ordered Cu₂O/Cu(111) and disordered copper oxide [CuO_x/Cu(111)] were observed at 2070–2072, 2097–2098 and 2101–2111 cm⁻¹, respectively. On CeO_x/Cu(111) systems CO chemisorbs at 90 K only on Cu sites under ultra-high vacuum (UHV) conditions, whereas at elevated CO pressures and low temperatures adsorption of CO on Ce³⁺ is observed, with a corresponding IR peak at 2162 cm⁻¹. These experimental results are further supported by DFT calculations, and help to unequivocally distinguish the presence of Ce³⁺ cations on catalyst samples by using CO as a probe molecule.