DeWitt Kristy M, Valadez Leticia, Abbott Heather L, Kolasinski Kurt W, Harrison Ian
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319, USA.
J Phys Chem B. 2006 Apr 6;110(13):6714-20. doi: 10.1021/jp055684h.
The dissociative sticking coefficient for C2H6 on Pt(111) has been measured as a function of both gas temperature (Tg) and surface temperature (Ts) using effusive molecular beam and angle-integrated ambient gas dosing methods. A microcanonical unimolecular rate theory (MURT) model of the reactive system is used to extract transition state properties from the data as well as to compare our data directly with supersonic molecular beam and thermal equilibrium sticking measurements. We report for the first time the threshold energy for dissociation, E0 = 26.5 +/- 3 kJ mol(-1). This value is only weakly dependent on the other two parameters of the model. A strong surface temperature dependence in the initial sticking coefficient is observed; however, the relatively weak dependence on gas temperature indicates some combination of the following (i) not all molecular excitations are contributing equally to the enhancement of sticking, (ii) that strong entropic effects in the dissociative transition state are leading to unusually high vibrational frequencies in the transition state, and (iii) energy transfer from gas-phase rovibrational modes to the surface is surprisingly efficient. In other words, it appears that vibrational mode-specific behavior and/or molecular rotations may play stronger roles in the dissociative adsorption of C2H6 than they do for CH4. The MURT with an optimized parameter set provides for a predictive understanding of the kinetics of this C-H bond activation reaction, that is, it allows us to predict the dissociative sticking coefficient of C2H6 on Pt(111) for any combination of Ts and Tg even if the two are not equal to one another.
使用泻流分子束和角度积分环境气体剂量法，测量了C2H6在Pt(111)上的解离粘附系数随气体温度（Tg）和表面温度（Ts）的变化。反应系统的微正则单分子速率理论（MURT）模型用于从数据中提取过渡态性质，并将我们的数据直接与超音速分子束和热平衡粘附测量结果进行比较。我们首次报告了解离的阈值能量，E0 = 26.5 +/- 3 kJ mol(-1)。该值仅微弱依赖于模型的其他两个参数。观察到初始粘附系数对表面温度有强烈依赖性；然而，对气体温度的相对较弱依赖性表明以下几种情况的某种组合：（i）并非所有分子激发对粘附增强的贡献都相同，（ii）解离过渡态中的强熵效应导致过渡态具有异常高的振动频率，以及（iii）从气相转动振动模式到表面的能量转移出奇地有效。换句话说，似乎振动模式特异性行为和/或分子旋转在C2H6的解离吸附中可能比在CH4中发挥更强的作用。具有优化参数集的MURT提供了对这种C-H键活化反应动力学的预测性理解，也就是说，它使我们能够预测C2H6在Pt(111)上对于Ts和Tg的任何组合的解离粘附系数，即使两者不相等。
