Revealing the active Cu species in Cu/ZSM-5 catalysts for the selective catalytic oxidation of ammonia

Ammonia slip, a by-product of NH₃ processes, poses significant environmental and health risks, necessitating its removal via the selective catalytic oxidation of ammonia (NH₃-SCO). In this study, a series of Cu/ZSM-5 catalysts with varying Cu loadings was synthesized via ion-exchange, where the 2.1Cu/ZSM-5 catalyst achieved up to 99.5 % NH₃ conversion and 99.4 % N₂ selectivity at 400 °C. To investigate the active Cu sites in the NH₃-SCO reaction, the distribution of the Cu species, quantified using H₂-TPR and EPR analyses, was correlated with the reaction rate, revealing that isolated Cu²⁺ ions exhibited the strongest linear correlation. A controlled experiment involving the selective reduction of Cu²⁺ to Cu⁺ resulted in a significant decrease in the NH₃ conversion, confirming that Cu²⁺ ions are more active than Cu⁺ ions in the NH₃-SCO reaction. The turnover frequency (TOF) calculated based on the number of Cu²⁺ ions remained nearly constant, with a slight increase at higher Cu loadings, confirming the primary role of Cu²⁺ ions in the catalytic oxidation process and the minor contributions of other Cu species. Further NH₃-TPD and in situ DRIFTS analyses demonstrated that Cu ion-exchange provides higher NH₃ adsorption ability, potentially enhancing the activity of the catalysts for the NH₃-SCO. These findings offer valuable insights for developing more efficient Cu/zeolite catalysts for NH₃ removal in industrial applications.