In-situ DRIFTS and XANES identification of copper species in the ternary composite oxide catalysts CuMnCeO during CO preferential oxidation

Abstract

A series of CuMnCeO catalysts with 10% CuO (weight loading) and variable atomic ratios of Mn/Mn + Ce (0, 0.02, 0.05 or 0.10) were synthesized by co-precipitation and employed for CO preferential oxidation (CO PROX). After doping a small amount of manganese into ceria, the catalysts show improved catalytic performance as compared with the undoped one, especially the catalyst with the Mn/Mn + Ce atomic ratio of 0.05 (CMC-0.05) which displays the lowest temperatures for half CO conversion (T50 = 73 °C) and full CO conversion (T100 = 108 °C). In addition, it also exhibits the widest temperature window of full CO conversion (108–149 °C) and 100% selectivity of oxygen to CO2 below 117 °C. As revealed by XRD and UV-Raman, the presence of appropriate amount of Mn cannot only enhance the formation of Ce–Cu–Mn–O ternary oxide solid solution with fluorite structure, inhibiting the growth of CeO2 crystallite size, but also increase the oxygen vacancies. The results of H2-TPR and XPS indicate that the reducibility and amount of surface oxygen species of CMC-0.05 are also improved, both of which are beneficial to catalytic performance. In situ XANES results suggest that the bulk copper species still remain as Cu2+ ions below 200 °C even in H2-rich CO PROX atmosphere; however, the in-situ DRIFTS spectra indicate that during CO PROX below 120 °C the surface copper species are Cu+ ions which are regarded as the main active sites for CO PROX; above 120 °C Cu0 species appear, which enhance H2 oxidation, decreasing the oxygen to CO2 selectivity.