Chemistry of N2O decomposition on active sites with different nature : Effect of high-temperature treatment of Fe/ZSM-5
Chemistry of N2O decomposition on active sites with different nature : Effect of high-temperature treatment of Fe/ZSM-5
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DOI：
10.1016/j.jcat.2005.12.013
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发表时间：
2006-02
影响因子：
7.3
通讯作者：
Keqiang Sun;Haian Xia;Ejm Emiel Hensen;V. Santen;Can Li
中科院分区：
文献类型：
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作者：
Keqiang Sun;Haian Xia;Ejm Emiel Hensen;V. Santen;Can Li
用稳态反应和暂态响应技术研究了固态离子交换法制备的Fe/ZSM-5催化剂823K在O2中焙烧和1173K在He中高温处理制备的催化剂上氧化亚氮的分解。高温处理使沸石微孔中铁物种的分布发生了深刻的变化，最显著的是，相当一部分寡核和阳离子铁物种转化为由骨架外Al(FeOAl物种)稳定的铁物种。在焙烧的Fe/ZSM-5催化剂上，一氧化二氮的分解速度比高温处理的慢得多。只有在后一种催化剂上才能观察到分子氧对N2O转化的抑制作用。暂态响应实验表明，焙烧后的Fe/ZSM-5分子氧和氮同时析出。通过吸附氧物种与气态一氧化二氮反应生成分子氮和氧的清除机制，提出了反应通过阳离子物种或低核氧化铁物种中孤立的Fe3+中心进行。相比之下，经过高温处理的催化剂的瞬变响应实验表明，与氮气生成相比，氧的脱附速度较慢，这表明复合氧脱附是速度限制的。这与分子氧抑制该速率的发现是一致的。结果表明，经过高温处理的催化剂中的亚铁离子在氧化亚氮分解反应中的活性远高于Fe3+中心。Fe2+物种很可能与高温处理后形成的铁铝混合物种有关。
Nitrous oxide decomposition on Fe/ZSM-5 catalysts, prepared by solid-state ion exchange followed by calcination in O2at 823 K or high-temperature treatment in He at 1173 K, was studied by steady-state reaction and transient-response techniques. High-temperature treatment induces profound changes in the distribution of iron species, most notably a substantial fraction of oligonuclear and cationic Fe species are converted to Fe species stabilized by extra-framework Al (FeOAl species) in zeolite micropores. Nitrous oxide decomposition is much slower over calcined Fe/ZSM-5 than over its high-temperature-treated counterpart. Only over the latter catalysts is inhibition of N2O conversion by molecular oxygen observed. Transient response experiments show that molecular oxygen and nitrogen evolve simultaneously for calcined Fe/ZSM-5. It is proposed that the reaction proceeds over isolated Fe3+sites in cationic species or oligonuclear Fe oxide species via a scavenging mechanism in which adsorbed oxygen species react with gaseous nitrous oxide to give molecular nitrogen and oxygen. In contrast, transient-response experiments for the high-temperature-treated catalyst show slow oxygen desorption compared with nitrogen formation, pointing to recombinative oxygen desorption being rate-limiting. This agrees with the finding that the rate is inhibited by molecular oxygen. It is proposed that ferrous ions in the high-temperature-treated catalyst show a much higher reactivity in nitrous oxide decomposition than Fe3+sites. The Fe2+species are most likely associated with the mixed iron and aluminium species formed upon high-temperature treatment.