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 T₅₀ 196.8 and 229.5 °C, and T₉₀ 239.2 and 292.1 °C. However, it was observed that while Co/CeO₂ displayed poor N₂ selectivity, Cu/CeO₂ demonstrated good N₂ 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. H₂-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.