N₂O Formation Pathways over Zeolite-Supported Cu and Fe Catalysts in NH₃-SCR

N₂O is a common byproduct in the NH₃-SCR reaction. We analyzed the N₂O formation pathways in NH₃-SCR over various catalysts (Cu-ZSM-5, Fe-ZSM-5, Cu-SAPO-34, Fe-SAPO-34, Cu-SSZ-13, and Fe-SSZ-13), aided by catalyst characterization using XRD, XPS, EDS mapping, and NH₃-TPD. The results showed that the NH₃ nonselective catalytic reduction was the major N₂O formation pathway for most of the Cu catalysts. The N₂O formation at lower temperatures (<300 °C) originated mainly from decomposition of NH₄NO₃. In addition, NH₃ nonselective oxidation was another reaction that formed N₂O especially at higher temperatures. The N₂O resulting from the Eley-Rideal mechanism was also favored at higher temperatures. The decomposition of NO to N₂O and O₂ also led to N₂O formation, although its contribution was minimal. The absence of N₂O yield over most Fe catalysts could be attributed to active N₂O decomposition and N₂O-SCR reactions. Moreover, varying O₂ and H₂O concentrations in the feed exerted strong influence on both N₂O formation and SCR activity. Decrease in O₂ level from 14% to 3% led to continual decline in N₂O formation but had no effect on SCR activity until reaching a threshold concentration of 2%. H₂O in lower concentrations (2-3%) facilitated N₂O formation and NO conversion due to increase in Brønsted acidity, while H₂O in higher concentrations (>5%) led to suppression of these reactions due to the coverage of active sites.