The mechanism of N2O formation during the low-temperature selective catalytic reduction reaction (SCR) over Mn-Fe spinel was studied. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and transient reaction studies demonstrated that the Eley-Rideal mechanism (i.e., the reaction of adsorbed NH3 species with gaseous NO) and the Langmuir-Hinshelwood mechanism (i.e., the reaction of adsorbed NH3 species with adsorbed NOx species) both contributed to N2O formation. However, N2O selectivity of NO reduction over Mn-Fe spinel through the Langmuir-Hinshelwood mechanism was much less than that through the Eley-Rideal mechanism. The ratio of NO reduction over Mn-Fe spinel through the Langmuir-Hinshelwood mechanism remarkably increased; therefore, N2O selectivity of NO reduction over Mn-Fe spinel decreased with the decrease of the gas hourly space velocity (GHSV). As the gaseous NH3 concentration increased, N2O selectivity of NO reduction over Mn-Fe spinel increased because of the promotion of NO reduction through the Eley-Rideal mechanism. Meanwhile, N2O selectivity of NO reduction over Mn-Fe spinel decreased with the increase of the gaseous NO concentration because the formation of NH on Mn-Fe spinel was restrained. Therefore, N2O selectivity of NO reduction over Mn-Fe spinel was related to the GHSV and concentrations of reactants.

In this work, the novel relationships of N 2 selectivity of NO reduction over MnO x −CeO 2 with the gas hourly space velocity (i.e., GHSV) and the reactants' concentrations were discovered. Meanwhile, the mechanism of N 2 O formation during the low temperature selective catalytic reduction reaction (SCR) over MnO x −CeO 2 was studied using in situ DRIFTS study and the transient reaction study. N 2 O formation over MnO x −CeO 2 mainly resulted from the Eley−Rideal mechanism (i.e., the reaction between overactivated NH 3 and gaseous NO), and the Langmuir−Hinshelwood mechanism (i.e., the reaction between adsorbed NH 3 species and adsorbed NO x ) did not contribute to N 2 O formation. There was an excellent linear relationship of NO reduction and N 2 formation with gaseous NO concentration. Meanwhile, the reaction order of N 2 O formation with respect to gaseous NO concentration was nearly 1. However, the reaction orders of NO reduction, N 2 O formation, and N 2 formation over MnO x −CeO 2 with respect to gaseous NH 3 concentration were all higher than 0 due to the adsorption competition between NH 3 and NO+O 2 . Therefore, N 2 selectivity of NO reduction over MnO x −CeO 2 remarkably increased with the increase of gaseous NO concentration, and it slightly decreased with the increase of gaseous NH 3 concentration.

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