Studies of N2O Adsorption and Decomposition on Fe–ZSM-5
Studies of N2O Adsorption and Decomposition on Fe–ZSM-5
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DOI：
10.1006/jcat.2002.3610
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发表时间：
2002-03
影响因子：
7.3
通讯作者：
B. Wood;J. Reimer;A. Bell
中科院分区：
文献类型：
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作者：
B. Wood;J. Reimer;A. Bell
利用红外光谱和程序升温反应研究了n2o与H-ZSM-5和Fe-ZSM-5的相互作用。采用固相交换法制备了Fe/Al比分别为0.17和0.33的Fe - zsm -5样品。结果表明，Fe-ZSM-5样品中的大部分铁以分离阳离子形式存在，与H +交换。在298 K时吸附在H-ZSM-5上的二氧化氮的红外光谱分别显示2226和1308 cm−1的波段与N - N和N - O键的振动有关。这些条带的位置相对于在气相中看到的条带表明，氮通过分子的氮端吸附。H-ZSM-5吸附二氧化氮的热估计为5千卡/摩尔。在Fe-ZSM-5中，由于氮氧与铁离子的相互作用，在2282和1344 cm−1处观察到额外的红外波段。在这里，相对于气相二氧化氮的振动特征的变化方向也表明分子通过其氮端进行吸附。二氧化氮在铁离子上的吸附热估计为16千卡/摩尔。氮氧对铁的吸附程度取决于铁的氧化态。在773 K条件下，用He或CO对样品进行预处理，其对n2的吸附程度高于用o2或n2对样品进行预处理。对Fe-ZSM-5样品进行了程序升温分解，结果表明，n2按化学计量量分解为n2和o2。经He预处理的样品，其N 2o分解活化能约为42 kcal/mol，反应速率系数的指前因子随Fe/Al比的增大而增大。这主要是由于Fe/Al比增大，fe3 +阳离子对fe2 +阳离子的自还原性增强。
Abstract The interactions of N 2 O with H–ZSM-5 and Fe–ZSM-5 have been investigated using infrared spectroscopy and temperature-programmed reaction. Fe–ZSM-5 samples with Fe/Al ratios of 0.17 and 0.33 were prepared by solid-state exchange. It was determined that most of the iron in the samples of Fe–ZSM-5 is in the form of isolated cations, which have exchanged with Bronsted acid H + in H–ZSM-5. The infrared spectrum of N 2 O adsorbed on H–ZSM-5 at 298 K exhibits bands at 2226 and 1308 cm −1 associated with vibrations of the N–N and N–O bonds, respectively. The positions of these bands relative to those seen in the gas phase suggest that N 2 O adsorbs through the nitrogen end of the molecule. The heat of N 2 O adsorption in H–ZSM-5 is estimated to be 5 kcal/mol. In the case of Fe–ZSM-5, additional infrared bands are observed at 2282 and 1344 cm −1 due to the interactions of N 2 O with the iron cations. Here, too, the directions of the shifts in the vibrational features relative to those for gas-phase N 2 O suggest that the molecule adsorbs through its nitrogen end. The heat of adsorption of N 2 O on the Fe sites is estimated to be 16 kcal/mol. The extent of N 2 O adsorption on Fe depends on the oxidation state of Fe. The degree of N 2 O adsorption is higher following pretreatment of the sample in He or CO at 773 K than following pretreatment in O 2 or N 2 O at the same temperature. Temperature-programmed decomposition of N 2 O was performed on the Fe–ZSM-5 samples and revealed that N 2 O decomposes stoichiometrically to N 2 and O 2 . A higher activity was observed when the catalysts were pretreated in He than when they were pretreated in N 2 O. For the He-pretreated samples, the activation energy for N 2 O decomposition was estimated to be 42 kcal/mol and the preexponential factor of the rate coefficient for this process was found to increase with Fe/Al ratio. This trend was attributed to the increasing autoreducibility of Fe 3+ cations to Fe 2+ cations with increasing Fe/Al ratio.