Loading Effects of CuO–CeO _{2– <i>y</i>} for CO Oxidation: A Combined Experimental and Theoretical Study

A series of CuO/CeO2 catalysts with different Cu contents (2 wt %, 1, 0.75, and 0.5 wt %) were prepared by the impregnation method and were characterized by various techniques such as X-ray diffraction, N2-adsorption/desorption, H2-TPR, Raman spectroscopy, and X-ray photoelectron spectroscopy. Moreover, their catalytic performance in CO oxidation was evaluated under both dry and wet conditions. In the absence of water, the T100 value decreased with Cu contents ranging from 0.5% to 0.75% and then leveled off from 0.75% to 2.0 wt %. However, a different variation trend emerged upon water addition. In the presence of water, the catalyst with 0.75–1.0 wt % Cu exhibited the highest catalytic activity, showing the lowest T100. These results indicated that excessive Cu was detrimental to CO oxidation under wet conditions but served as a spectator under dry conditions. To explore the possible mechanism, the DFT + U method was employed to investigate the adsorption of water molecules on different systems, including Cu-doped and undoped CeO2. Compared with CeO2 and CeO2–x, Cu-doped systems manifested a greater propensity for adsorbing water molecules, with the potential formation of two hydroxyl groups. It was found that the subsequent CO adsorption could interact with some of the surface lattice oxygens to form CO2, but the absolute value of formation energy was lower as compared to the pure Cu/CeO2(111) surface. The results could be used to account for the negative effect of water on the catalytic oxidation of CO. However, when CO is adsorbed onto the hydroxyl O atom, a formate-like species is formed. As expected, the formate-like species could be decomposed to generate CO2, thereby demonstrating a promoting effect of water, which was evidenced by additional catalytic testing at a high-temperature of 360 °C over the 0.75 CuCe catalyst. However, the water promoting effect was not found with the 2CuCe catalyst, demonstrating an obvious loading effect. This article provides foundational data that can be referenced for further research.