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
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
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DOI：
10.1021/acs.jpcc.1c06651
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发表时间：
2021-09-30
影响因子：
3.7
通讯作者：
Shimizu, Ken-ichi
中科院分区：
文献类型：
--
作者：
Liu, Chong;Malta, Grazia;Shimizu, Ken-ichi
结合原位/操作红外（IR）光谱、动力学和密度泛函理论（DFT）计算，提出了Cu-CHA沸石上NH3选择性催化还原（SCR）NO/NO2混合物的综合机理模型，即快速SCR. H-CHA和Cu-CHA上的标准和快速SCR的稳态动力学表明，NO2对SCR的促进作用对于Cu-CHA来说不如H-CHA显著，这表明布朗斯台德酸中心（BAS; H+OZ-）在快速SCR化学中是重要的。原位红外实验表明，NO2在Cu-CHA上发生了离子交换反应，生成CHA阳离子交换位（NO+OZ-）上的NO+和Cu-II位上的NO3-。在瞬态或程序升温表面反应条件下的Operando IR研究表明，NO+OZ-中间体在200 ℃以下被NH3还原产生N-2，而Cu-II位点上的NO3-在NH3下相当稳定，并被NO还原产生NO+OZ-。BAS而不是Cu位点通过硝酸铵（NH 4 NO3）促进副反应。通过DFT计算验证了快速SCR的双中心（Cu-Ⅱ和BAS中心）催化机理。首先，两个NO2分子被转化为亚硝酸（HONO）中间体和NO3-上的Cu-II网站，然后还原NO，以提供HONO和再生的Cu-II网站。HONO与BAS反应生成NO+OZ-，NO + OZ-与NH3反应生成N2-和H2O，沿着BAS的再生。
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 Bronsted 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 Cu-II sites. Operando IR studies under transient or temperature-programmed surface reaction conditions indicate that the NO+OZ- intermediate is reduced by NH3 to yield N-2 below 200 degrees C, while NO3- on the Cu-II 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 (Cu-II 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 Cu-II site, which is then reduced by NO to afford HONO and regenerate the Cu-II site. HONO reacts with the BAS to afford NO+OZ-, which reacts with NH3 to produce N-2 and H2O, via nitrosamide (NH2NO), along with the regeneration of the BAS.