Mechanistic stages of the SCR reaction – Insights into the trade-off between NO reduction and NH3 oxidation over CuSSZ-13 catalysts via isotopic 15NH3 and 18O2 TPSR and steady state studies supported by IR 2D COS and DFT modeling

Isotopic labeling investigations into SCR reaction were performed over CuSSZ-13 zeolites, obtained by one-pot synthesis or impregnation, and characterized by XRD, HR-SEM, TEM/EDX, EPR, and IR techniques. The course of SCR and internal ammonia oxidation (i-AMO) was revealed by isotopic TPSR and steady-state investigations using 15NH3 and 18O2 joined with IR Rapid Scan 2D-COS spectroscopy. The 14N15N and 15N2 profiles allowed for quantification of the relative contributions of the concurrent SCR and i-AMO reactions. Interpretation of the results was ascertained by DFT and first-principles thermodynamic modeling. Three temperature regions of SCR reactions, corresponding to NH3-rich, lean and dry conditions, were distinguished, and the main processes and intermediates involved in each stage were identified. For the conceptualization of SCR, a 2D diagram of the reactants and intermediates stoichiometry, oxidation state, and charge commensurability was constructed, and used for rationalization of the reaction mechanism.