Active Sites in Fe/MFI Catalysts for NOx Reduction and Oscillating N2O Decomposition
Active Sites in Fe/MFI Catalysts for NOx Reduction and Oscillating N2O Decomposition
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DOI：
10.1006/jcat.2000.3034
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发表时间：
2000-12
影响因子：
7.3
通讯作者：
El-Mekki El-Malki-El-Mekki-El-Malki-146598724;V. Santen;W. Sachtler
中科院分区：
文献类型：
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作者：
El-Mekki El-Malki-El-Mekki-El-Malki-146598724;V. Santen;W. Sachtler
通过将 FeCl3 升华到 H-MFI 上或通过固态离子交换制备的 Fe/MFI 催化剂通过 XRD、FTIR 和 ESR 光谱进行了表征。已鉴定出连接的 Fe3+ 离子，例如扭曲四面体配位中的 (Fe(OH)2)+ 或 (Fe=O)+、多核氧阳离子，例如 (HO-Fe-O-Fe-OH)2+ 和 Fe2O3 氧化物颗粒。它们的相对丰度取决于硅/铝比和铁含量。通过 FTIR 谱带识别的硝基和硝酸盐基团是在将高 Fe 负载量的 Fe/MFI 暴露于 N2O 时形成的。这些吸附络合物在 200°C 时会被苯还原，但在室温下不会。它们与 CO 的还原反应发生在比桥接氧更低的温度下。与在 500°C、100 kPa 的 O2 中煅烧相比，在 320°C 和 0.5 kPa 下暴露于 N2O 中可将更多的活性氧泵入 Fe/MFI。热分解在相同温度下释放NO和O2。在 Fe/MFI 上，稀释进料流中 N2O 催化分解为 N2+12O2 的速率是 N2O 的一级。对于低 Fe 负载量的样品，速率常数非常低，但对于高 Fe 负载量，速率常数会跳至更高的值，这表明根据最近的 EXAFS 结果，多核 Fe 团簇的重要作用。在 H2O 蒸汽存在下，过度交换的 Fe/MFI（Fe/Al=1；Al/Si=1/14）观察到稳定的等温振荡，表明存在两种不同的催化活性状态。使用干进料或在低负载的 Fe/MFI 上未观察到振荡。
Fe/MFI catalysts prepared either by sublimation of FeCl3 onto H-MFI or by solid state ion exchange were characterized by XRD, FTIR, and ESR spectroscopy. Ligated Fe3+ ions, such as (Fe(OH)2)+ or (Fe=O)+ in distorted tetrahedral coordination, multinuclear oxocations, such as (HO-Fe-O-Fe-OH)2+, and Fe2O3 oxide particles have been identified. Their relative abundances depend on the Si/Al ratio and the Fe loading. Nitro and nitrate groups, identified by their FTIR bands, are formed upon exposing Fe/MFI with high Fe loading to N2O. These adsorption complexes are reduced with benzene at 200°C but not at room temperature. Their reduction with CO occurs at a lower temperature than that of the bridging oxygen. Exposure to N2O at 320°C and 0.5 kPa pumps 30% more active oxygen into Fe/MFI than calcination in O2 of 100 kPa at 500°C. Thermal decomposition releases NO and O2 at the same temperature. The catalytic decomposition rate of N2O to N2+12O2 in diluted feed streams over Fe/MFI is first order in N2O. The rate constant is very low for samples with low Fe loading, but jumps to a much higher value for high Fe loadings, indicating the important role of multinuclear Fe clusters in accordance with recent EXAFS results. Stable isothermal oscillations are observed with overexchanged Fe/MFI (Fe/Al=1; Al/Si=1/14), in the presence of H2O vapor, showing that two different states of catalytic activity exist. No oscillation is observed with a dry feed or over Fe/MFI with low loading.