In Situ/Operando IR and Theoretical Studies on the Mechanism of NH3–SCR of NO/NO2 over H–CHA Zeolites

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.

本研究借助原位/operando红外光谱（in situ/operando IR spectroscopy）与密度泛函理论（DFT）计算，揭示了NH3在H型CHA沸石（H–CHA zeolites）上选择性催化还原（SCR）NO/NO2混合气（即快速SCR（fast SCR））的反应机制。动力学结果表明，快速SCR的反应速率较H型CHA沸石上的标准SCR反应高出两个数量级。结合实验与计算结果，本研究明确了快速SCR的完整反应路径：该反应起始于NO2歧化为吸附态NO+与硝酸根（NO3–）物种的过程。吸附态NO+可与NH3反应生成N2与H2O；而硝酸根则在布伦斯特酸位点（Brønsted acid sites，以NH4+形式存在）上与NH3结合生成硝酸铵（NH4NO3）。在NO存在的条件下，硝酸铵可被NO还原为亚硝酸铵（NH4NO2），后者随后分解为N2与H2O；而在无NO的环境中，硝酸铵的热分解则会生成一氧化二氮（N2O）与H2O。本研究通过DFT计算得到了所有基元反应步骤的活化能与反应能，验证了实验提出的H型CHA沸石上快速SCR反应机制。在涉及NO2参与的SCR反应过程中，沸石的布伦斯特酸位点在关键基元反应中发挥了核心催化作用。