Ag Size/Structure-Dependent Effect on Low-Temperature Selective Catalytic Oxidation of NH₃ over Ag/MnO₂

Selective catalytic oxidation of ammonia (NH₃-SCO) into N₂ and H₂O is considered to be a promising technique to eliminate NH₃ pollution. Various Ag/MnO₂ catalysts were prepared and applied to low-temperature (especially <100 °C) NH₃-SCO. The Ag/MnO₂-X (X indicating the calcination temperature) catalyst had a variable Ag size and structure depending on the calcination temperature from 200 to 500 °C. The Ag/MnO₂-400 catalyst showed 10% NH₃ conversion at 35 °C, and the temperature was 90 °C for the complete removal of 50 ppm NH₃. More importantly, the N₂ selectivities of NH₃-SCO over Ag/MnO₂-400 at the temperatures ranging from 30 to 120 °C were higher than 96%. Turnover frequencies of NH₃ oxidation, N₂ selectivity, and byproducts depended on the Ag size/structure. Ag/MnO₂-400 with Ag particles of 2.4 nm diameter had the highest density of Ag particles over rod-shaped MnO₂ during the reaction, which would be favorable for the formation of adsorbed NO and NH₂NO intermediates to form N₂ and H₂O.