Selective Catalytic Reduction of NO Over Fe-ZSM-5: Mechanistic Insights by Operando HERFD-XANES and Valence-to-Core X-ray Emission Spectroscopy

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• 作者：Alexey Boubnov ; Hudson W. P. Carvalho ; Dmitry E. Doronkin ; Tobias G眉nter ; Erik Gallo ; Andrew J. Atkins ; Christoph R. Jacob ; Jan-Dierk Grunwaldt
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：September 17, 2014
• 年：2014
• 卷：136
• 期：37
• 页码：13006-13015
• 全文大小：545K
• ISSN：1520-5126

文摘

An in-depth understanding of the active site requires advanced operando techniques and the preparation of defined catalysts. We elucidate here the mechanism of the selective catalytic reduction of NO by NH₃ (NH₃鈥揝CR) over a Fe-ZSM-5 zeolite catalyst. 1.3 wt % Fe-ZSM-5 with low nuclearity Fe sites was synthesized, tested in the SCR reaction and characterized by UV鈥搗is, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopy. Next, this defined Fe-zeolite catalyst was studied by complementary high-energy-resolution fluorescence-detected XANES (HERFD-XANES) and valence-to-core X-ray emission spectroscopy (V2C XES) under different model in situ and realistic working (operando) conditions identical to the catalyst test bench including the presence of water vapor. HERFD-XANES uncovered that the coordination (between 4 and 5), geometry (tetrahedral, partly 5-fold), and oxidation state of the Fe centers (reduced in NH₃, partly in SCR mixture, slight reduction in NO) strongly changed. V2C XES supported by DFT calculations provided important insight into the chemical nature of the species adsorbed on Fe sites. The unique combination of techniques applied under realistic reaction conditions and the corresponding catalytic data unraveled the adsorption of ammonia via oxygen on the iron site. The derived reaction model supports a mechanism where adsorbed NO_x reacts with ammonia coordinated to the Fe³⁺ site yielding Fe²⁺ whose reoxidation is slow.