Spectroscopic Evidence and Density Functional Theory (DFT) Analysis of Low-Temperature Oxidation of Cu⁺ to Cu²⁺NO_x in Cu-CHA Catalysts: Implications for the SCR-NO_x Reaction Mechanism

Despite the intense investigation on the NH₃–SCR-NOx reaction mechanism catalyzed by small pore Cu-CHA zeolites, neither the rate-determining step of the process nor the exact nature of the active sites under reaction conditions are clearly established. In this work, in situ EPR and IR techniques combined with DFT calculations are applied to the study of the oxidation half-cycle of the NH₃–SCR-NOx reaction on Cu-SSZ-13 and Cu-SAPO-34 catalysts. EPR and IR spectroscopies unambiguously show that Cu⁺ is oxidized to Cu²⁺ at room temperature in the presence of the reaction mixture (NO, O₂, and NH₃) or NO and O₂, producing adsorbed NO₂, nitrites, and nitrates. Several pathways are proposed from DFT calculations to oxidize Cu⁺ cations placed in the plane of the 6R ring units of SSZ-13 and SAPO-34 to Cu²⁺, either by NO₂ alone or by a mixture of NO and O₂, with activation energy barriers lower than 70 kJ mol^–1. The results reported here demonstrate that a reaction mechanism invoking the formation of nitrate/nitrite intermediates on copper cations attached to the zeolite framework can be operational in the low-temperature region (T < 350 °C). Moreover, different intermediates, nitrites versus nitrates, are preferentially stabilized, depending on the catalyst composition, silicoaluminophosphate vs aluminosilicate.