One step closer to the low-temperature CO oxidation over non-noble CuO/CeO2 nanocatalyst: The effect of CuO loading

Abstract

Concerning the catalytic field of CO oxidation for designing the cost-effective, stable, and highly active catalysts based on CuO/CeO2 nanocomposites, in this work, the urea-nitrate combustion method was used to synthesize a series of CuO/CeO2 catalysts with different CuO loadings (at%Cu) to test their catalytic activity in the CO oxidation, to find out the optimum at%Cu which allows the CuO/CeO2 composite to perform a higher activity in close to room-like temperature. The at%Cu of 0%, 2.5%, 5.0%, 7.5%, 10%, 20%, 30%, 40%, and 50% were set up to obtain a catalyst series. The resulting catalysts were characterized using EDX, PXRD, SEM, and other methods. It was shown that the CuO loading caused changes in the phase composition (crystalline CeO2, amorphous or crystalline CuO), the crystallite size (5–10 nm for CeO2; 20–33 nm for CuO), and the specific surface area (12.2–85.8 m2/g), as well as in the morphological and pore structure. These factors comprehensively influenced the catalytic performance of the samples. The initial temperature (ti) toward CO oxidation for the best catalyst obtained in this study belonged to the low-temperature region (ti < 50 °C). The temperature for 50% CO conversion (t50%) of the best catalysts was less than 100 °C that was much lower compared to pure CeO2 (t50% ≈ 312 °C). For CO oxidation, the active sites on the surface were correlated with the interactions between copper and cerium oxides; the improvement in catalytic activity is also related to the high concentration of oxygen vacancies or high oxygen storage capacity of the catalysts. Besides, the advantage in the specific surface area of the samples played as one of the decisive factors to the higher CO conversion. It was noted that the increase of CuO loading into the CuO/CeO2 catalysts exceeded 20% doesn’t lead to a further decrease in the temperature of CO oxidation. Thus, the optimum catalyst composition was determined to be the CuO/CeO2-20, and its properties were studied by the methods of Raman, XPS, TEM, H2-TPR, and EDS in detail. Results proved a strong synergistic effect – the coexistence of redox pairs Ce4+/Ce3+-Cu2+/Cu+, the formation of oxygen vacancies, as well as the presence of superficial lattice oxygen and adsorbed oxygen species leading to improve the catalytic activity of CuO/CeO2-20 catalyst. Moreover, the stability of the CuO/CeO2-20 catalyst was investigated and excellent results were obtained. We believe that the activity in CO oxidation of the CuO/CeO2-20 sample can even be enhanced by doping oxides of the other rare-earth and transition metals, which requires further study.