Heyden Andreas, Hansen Niels, Bell Alexis T, Keil Frerich J
Department of Chemical Engineering, Hamburg University of Technology, D-21073 Hamburg, Germany.
J Phys Chem B. 2006 Aug 31;110(34):17096-114. doi: 10.1021/jp062814t.
A number of experimental studies have shown recently that ppm-level additions of nitric oxide (NO) enhance the rate of nitrous oxide (N(2)O) decomposition catalyzed by Fe-ZSM-5 at low temperatures. In the present work, the NO-assisted N(2)O decomposition over mononuclear iron sites in Fe-ZSM-5 was studied on a molecular level using density functional theory (DFT) and transition-state theory. A reaction network consisting of over 100 elementary reactions was considered. The structure and energies of potential-energy minima were determined for all stable species, as were the structures and energies of all transition states. Reactions involving changes in spin potential-energy surfaces were also taken into account. In the absence of NO and at temperatures below 690 K, most active single iron sites (Z(-)FeO) are poisoned by small concentrations of water in the gas phase; however, in the presence of NO, these poisoned sites are converted into a novel active iron center (Z(-)FeOH). These latter sites are capable of promoting the dissociation of N(2)O into a surface oxygen atom and gas-phase N(2). The surface oxygen atom is removed by reaction with NO or nitrogen dioxide (NO(2)). N(2)O dissociation is the rate-limiting step in the reaction mechanism. At higher temperatures, water desorbs from inactive iron sites and the reaction mechanism for N(2)O decomposition becomes independent of NO, reverting to the reaction mechanism previously reported by Heyden et al. [J. Phys. Chem. B 2005, 109, 1857].
最近的一些实验研究表明，在百万分率水平添加一氧化氮（NO）可提高Fe-ZSM-5在低温下催化分解一氧化二氮（N₂O）的速率。在本工作中，使用密度泛函理论（DFT）和过渡态理论在分子水平上研究了Fe-ZSM-5中单核铁位点上NO辅助的N₂O分解。考虑了一个由100多个基元反应组成的反应网络。确定了所有稳定物种的势能极小值的结构和能量，以及所有过渡态的结构和能量。还考虑了涉及自旋势能面变化的反应。在没有NO且温度低于690 K时，大多数活性单铁位点（Z⁻[FeO]⁺）会被气相中低浓度的水毒化；然而，在有NO的情况下，这些被毒化的位点会转化为一种新型活性铁中心（Z⁻[FeOH]⁺）。这些后者的位点能够促进N₂O分解为表面氧原子和气相N₂。表面氧原子通过与NO或二氧化氮（NO₂）反应而被去除。N₂O分解是反应机理中的速率限制步骤。在较高温度下，水从无活性的铁位点解吸，N₂O分解的反应机理变得与NO无关，恢复到Heyden等人先前报道的反应机理[《物理化学杂志B》2005年，109卷，1857页]。
