[2010年01月19日]

N2OFe/ZSM-5上分解研究取得进展

Schematic description of the promotional effect of NO on direct N2O decomposition.    

Fe/ZSM-5NOx选择催化性还原、N2O催化分解、CH4的低温选择氧化、一步法选择氧化苯制苯酚等反应显示出独特的催化性能。研究Fe/ZSM-5的活性中心结构及其催化性质具有重要的学术意义和应用前景。

近来,我们研究组采用原位光谱方法特别是原位Raman光谱在研究Fe/ZSM-5催化剂的活性中心和合成机理取得了一系列进展(J. Catal. 254 (2008) 383-396; J. Phys. Chem. C 112 (2008) 9001-9005; J. Phys. Chem. C 112 (2008) 16036-16041; Chem. Eur. J. 15 (2009) 3268-3276Chem. Commun. 2009in pressetc.) ,深入认识了Fe/ZSM-5催化剂的活性中心本质和合成机理。

研究发现Fe/ZSM-5 NO能促进N2O分解的奇特性能,完全不同于在贵金属催化剂上NO能够抑制N2O的分解。然而,Fe/ZSM-5NO促进N2O分解的机理以及其活性中心的认识仍然不清楚。这是由于Fe/ZSM-5催化剂上铁物种分布特别复杂(孤立的铁物种、双原子铁物种及低聚的铁物种等共存)。如何区分这些不同的铁物种以及如何实现在原位条件下检测这些活性铁物种的变化成为本工作的难点。即使表征铁物种比较强有力的工具,如穆斯鲍尔谱(Mössbauer),由于其灵敏度低和难以实现原位表征而很难得到活性铁物种的结构信息。然而,利用共振拉曼光谱可以选择性地得到不同铁物种的结构信息。

在本研究中,我们结合瞬变应答方法、原位红外和拉曼光谱方法来研究Fe/ZSM-5NO促进N2O分解的机理和活性中心。结果表明,743 cm-1拉曼谱峰的出现或消失是和铁活性中心直接相关的。通过原位拉曼光谱首次得到了N2O分解活性中心是一种双原子铁活性中心的光谱证据。基于以上的实验结果,我们提出了活性中心模型及NO促进N2O分解的反应机理(图1)。文章发表在近期出版的Journal of Catalysis上。

Article

The promotional effect of NO on N2O decomposition over the bi-nuclear Fe sites in Fe/ZSM-5

Haian Xia, Keqiang Sun, Zhimin Liu, Zhaochi Feng, Pinliang Ying, Can Li*

 JCatal, In Press, Available online 8 January 2010

   Abstract

   Combining transient response method, steady-state kinetics, in-situ IR, and in-situ Raman spectroscopic techniques, the effect of NO on N2O decomposition over Fe/ZSM-5 catalyst was investigated in detail, which reveals that the active sites are bi-nuclear Fe sites. The presence of NO increases the desorption rate of O2 and lowers the apparent activation energy of N2O decomposition. Trace amount of water vapor can lead to the deactivation of the Fe/ZSM-5 catalyst due to the oxidation of the active Fe2+ sites during N2O decomposition. On the other hand, NO treatment causes the regeneration of the inactive Fe sites by releasing O2 at relatively low temperature. For the first time, it was found that a Raman band at 743 cm-1, assigned to the bi-nuclear Fe sites, changes accordingly with the deactivation and regeneration of the Fe/ZSM-5 catalyst. The in-situ IR spectroscopic study reveals that NO treatment of the deactivated catalyst can remove the hydroxyl groups bound to the inactive Fe3+ species. These results demonstrate that H2O and NO play important roles in the structural transformation between the hydroxylated bi-nuclear Fe3+ sites and the dehydroxylated bi-nuclear Fe2+ sites in Fe/ZSM-5. Our conclusion is that the positive effect of NO on N2O decomposition is due to the dehydroxylation of the hydroxylated bi-nuclear Fe sites facilitated by NO, as well as the enhancement of the O2 desorption rate regulated by NO.

 

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