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 degrees 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.