Mechanism of NH3–Selective Catalytic Reduction (SCR) of NO/NO2 (Fast SCR) over Cu-CHA Zeolites Studied by In Situ/Operando Infrared Spectroscopy and Density Functional Theory

In situ/operando infrared (IR) spectroscopy, kinetics, and density functional theory (DFT) calculations were combined to propose a comprehensive mechanistic model of the selective catalytic reduction (SCR) of the NO/NO2 mixture by NH3, the so-called fast SCR, over Cu-CHA zeolites. Steady-state kinetics for standard and fast SCR over H-CHA and Cu-CHA show that the promotional effect of NO2 on SCR is less significant for Cu-CHA than H-CHA, suggesting that the Brønsted acid site (BAS; H+OZ–) is important in fast SCR chemistry. In situ IR experiments show that NO2 disproportionates on Cu-CHA to NO+ on the cation-exchange site (NO+OZ–) of CHA and NO3– on CuII sites. Operando IR studies under transient or temperature-programmed surface reaction conditions indicate that the NO+OZ– intermediate is reduced by NH3 to yield N2 below 200 °C, while NO3– on the CuII site is rather stable under NH3 and is reduced by NO to afford NO+OZ–. BASs, rather than Cu sites, promote the side reactions via ammonium nitrate (NH4NO3). The dual-site (CuII and BAS sites) catalytic mechanism of fast SCR was verified by DFT calculations. First, two NO2 molecules are converted to a nitrous acid (HONO) intermediate and NO3– on the CuII site, which is then reduced by NO to afford HONO and regenerate the CuII site. HONO reacts with the BAS to afford NO+OZ–, which reacts with NH3 to produce N2 and H2O, via nitrosamide (NH2NO), along with the regeneration of the BAS.