Impact of M (M = Co, Cu, Fe, Zr) Doping on CeO2-Based Catalysts for Ammonia Selective Catalytic Oxidation at Low Temperatures

Selective catalytic conversion of ammonia to nitrogen is an effective method for reducing ammonia emissions from both stationary and mobile sources. In this study, CeO2-based catalysts (M/CeO2, M = Co, Cu, Fe, Zr) were synthesized using the sol-gel method and subsequently tested on a simulated gas experimental platform to assess their performance in NH3 selective catalytic oxidation (NH3-SCO). Results showed that Co/CeO2 and Cu/CeO2 catalysts exhibited high ammonia oxidation activity at respectively low temperatures, with T-50 196.8 and 229.5 degrees C, and T-90 239.2 and 292.1 degrees C. However, it was observed that while Co/CeO2 displayed poor N-2 selectivity, Cu/CeO2 demonstrated good N-2 selectivity. The superior catalytic performance of Cu/CeO2 and Co/CeO2 catalysts compared to Fe/CeO2 and Zr/CeO2 can be attributed to their distinct interactions with Ce. Subsequent characterization experiments were conducted to elucidate these interactions. BET and SEM analyses revealed that all M/CeO2 catalysts possessed a typical mesoporous structure. XRD and XPS results indicated that the primary phase of each catalyst was CeO2, and the incorporation of M transition metals did not alter the cubic fluorite structure. The interaction between the M metal and Ce varied, impacting the Ce3+ content on the catalyst surface, which in turn influenced oxygen species adsorption and ammonia oxidation activity. H-2-TPR and Raman spectroscopy analyses demonstrated that M metal incorporation shifted the CeO2 reduction peak, thereby altering reduction properties and affecting oxidation performance. In particular, the Co-metal composite shifted the reduction peak to a lower temperature, thereby enhancing the reduction properties and indirectly increasing oxidation activity.