Selective oxidation of ammonia over copper-silver-based catalysts

A novel catalyst based on copper-silver was developed to solve the contradiction between the high conversion temperature of Cu-based catalyst and low N₂ selectivity of Ag-based catalyst during selective oxidation of ammonium gas. The Cu-Ag-based catalyst (Cu 5 wt.%-Ag 5 wt.%/Al₂O₃) displayed a relatively low complete conversion temperature (<320 °C) with a high N₂ selectivity (>95%). Increasing loading of Cu and Ag decreases N₂ selectivity. The low N₂ selectivity of Ag-based catalyst is possibly related to the formation of Ag₂O crystals. Improvement of N₂ selectivity of Ag-based catalyst was obtained by doping Cu to decrease crystallized Ag₂O phase. The temperature programmed reaction (TPR) data show that N₂O is the main byproduct of oxidation of ammonia at temperature lower than 200 °C. Two bands of nitrate species at 1541 and 1302 cm⁻¹ were observed on Ag 10 wt.%/Al₂O₃ at the temperature higher than 250 °C, which indicates the formation of NO_x during the selective catalytic oxidation of ammonia. No nitrate species was observed on Cu 10 wt.%/Al₂O₃ and Cu 5 wt.%-Ag 5 wt.%/Al₂O₃, while only one nitrate species (1543 cm⁻¹) existed on Cu 10 wt.%-Ag 10 wt.%/Al₂O₃. We proposed that mixing Ag with Cu inhibited the formation of NO_x during the selective catalytic oxidation of ammonia over Cu-Ag/Al₂O₃.