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

• Published in: Journal of physical chemistry. C Vol. 125; no. 40; pp. 21975 - 21987
• Main Authors: Liu, Chong, Malta, Grazia, Kubota, Hiroe, Toyao, Takashi, Maeno, Zen, Shimizu, Ken-ichi
• Format: Journal Article
• Language: English
• Published: American Chemical Society 14.10.2021
• Subjects: C: Chemical and Catalytic Reactivity at Interfaces
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.1c06651

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.