In Situ/Operando IR and Theoretical Studies on the Mechanism of NH<sub>3</sub>–SCR of NO/NO<sub>2</sub> over H–CHA Zeolites
Chong Liu, Grazia Malta, Hiroe Kubota, Kenichi Kon, Takashi Toyao, Zen Maeno, Ken‐ichi Shimizu
Abstract
The mechanism of selective catalytic reduction (SCR) of a NO/NO2 mixture by NH3 (fast SCR) over H–CHA zeolites was revealed by in situ/operando IR spectroscopy and DFT calculations. Kinetic results show that the rate of fast SCR is 2 orders of magnitude higher than that for standard SCR by H–CHA. Combined experimental and computational results show a complete mechanism of fast SCR initiated by the disproportionation of NO2 into adsorbed NO+ and NO3– species. NO+ reacts with NH3 to produce N2 and H2O, while NO3– reacts with NH3 on Brønsted acid sites (NH4+) to yield NH4NO3. In the presence of NO, NH4NO3 is reduced by NO to afford NH4NO2, which then decomposes into N2 and H2O, while thermal decomposition of NH4NO3 yields N2O and H2O in the absence of NO. The activation and reaction energies for all of the elementary steps are computed by DFT, which support the experimentally proposed mechanism of fast SCR over H–CHA. During the SCR process where NO2 participated, the Brønsted acid sites of the zeolite play an essential catalytic role in key elementary reactions.