Heyden Andreas, Peters Baron, Bell Alexis T, Keil Frerich J
Department of Chemical Engineering, Hamburg University of Technology, D-21073 Hamburg, Germany.
J Phys Chem B. 2005 Feb 10;109(5):1857-73. doi: 10.1021/jp040549a.
The reaction mechanism for nitrous oxide decomposition has been studied on hydrated and dehydrated mononuclear iron sites in Fe-ZSM-5 using density functional theory. In total, 46 different surface species with different spin states (spin multiplicity M(S) = 4 or 6) and 63 elementary reactions were considered. Heats of adsorption, activation barriers, reaction rates, and minimum energy pathways were determined. The approximate minimum energy pathways and transition states were calculated using the "growing string method" and a modified "dimer method". Spin surface crossing (e.g., O(2) desorption) was considered. The minimum potential energy structure on the seam of two potential energy surfaces was determined with a multiplier penalty function algorithm by Powell and approximate rates of spin surface crossings were calculated. It was found that nitrous oxide decomposition is first order with respect to nitrous oxide concentration and zero order with respect to oxygen concentration. Water impurities in the gas stream have a strong inhibiting effect. In the concentration range of 1-100 ppb, the presence of water vapor influences the surface composition and the apparent rate coefficient. This is especially relevant in the temperature range of 600-700 K where most experimental kinetic studies are performed. Apparent activation barriers determined over this temperature range vary from 28.4 (1 ppb H(2)O) to 54.8 kcal/mol (100 ppb H(2)O). These results give an explanation why different research groups and different catalyst pretreatments often result in very different activation barriers and preexponential factors. Altogether perfect agreement with experimental results could be achieved.
利用密度泛函理论，对Fe-ZSM-5中单核铁位点的水合和脱水状态下一氧化二氮分解的反应机理进行了研究。总共考虑了46种具有不同自旋态（自旋多重度M(S)=4或6）的不同表面物种和63个基元反应。确定了吸附热、活化能垒、反应速率和最小能量路径。使用“增长弦法”和改进的“二聚体法”计算了近似的最小能量路径和过渡态。考虑了自旋表面交叉（例如，O(2)解吸）。用Powell的乘子罚函数算法确定了两个势能面接缝处的最小势能结构，并计算了自旋表面交叉的近似速率。结果发现，一氧化二氮分解对一氧化二氮浓度为一级，对氧气浓度为零级。气流中的水杂质具有很强的抑制作用。在1-100 ppb的浓度范围内，水蒸气的存在会影响表面组成和表观速率系数。这在600-700 K的温度范围内尤为重要，因为大多数实验动力学研究都是在此温度范围内进行的。在此温度范围内测定的表观活化能垒从28.4（1 ppb H(2)O）到54.8 kcal/mol（100 ppb H(2)O）不等。这些结果解释了为什么不同的研究小组和不同的催化剂预处理常常导致非常不同的活化能垒和指前因子。总体而言，可以与实验结果实现完美吻合。
