N₂O Formation Mechanism during Low-Temperature NH₃-SCR over Cu-SSZ-13 Catalysts with Different Cu Loadings

Based on the in situ DRIFTS studies, the mechanism of formation and decomposition of NH4NO3 (decomposition to N2O) over Cu-SSZ-13 with different Cu loadings was investigated. Under standard SCR conditions, NH4NO2 can form over all catalysts, which can be more easily oxidized to NH4NO3 over higher Cu content catalysts because of their stronger oxidative ability resulting from the elevation of Cu(OH)+. Hence, a higher Cu content leads to more low-temperature N2O formation, while N2O barely occurred over Cu-0 and Cu-0.7 catalysts. When NO2 exists, NO3- forms from the disproportionation reaction of NO2 first, then reacts with NH3 to form NH4NO3, leading to N2O formation over all catalysts. Nevertheless, Cu2+ is active for the reaction between NH4NO3 and NO. The increase of Cu2+ allows more NH4NO3 consumption, and the self-inhibition of NH4NO3 alleviates, which allows NH4NO3 to decompose to N2O at lower temperatures, and N2O formation peak shifts toward lower temperatures.