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):6705-13. doi: 10.1021/jp0566865.
The dissociative sticking coefficient for CH4 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. The experimental results are used to optimize the three parameters of a microcanonical unimolecular rate theory (MURT) model of the reactive system. The MURT calculations allow us to extract transition state properties from the data as well as to compare our data directly to other molecular beam and thermal equilibrium sticking measurements. We find a threshold energy for dissociation of E0 = 52.5 +/- 3.5 kJ mol(-1). Furthermore, 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 CH4 on Pt(111) for any combination of Ts and Tg even if the two are not equal to one another, indeed, the distribution of molecular energy need not even be thermal. Comparison of our results to those from recent thermal equilibrium catalysis studies on CH4 reforming over Pt nanoclusters ( approximately 2 nm diam) dispersed on oxide substrates indicates that the reactivity of Pt(111) exceeds that of the Pt nanocatalysts by several orders of magnitude.
采用泻流分子束和角度积分环境气体注入方法，测量了CH₄在Pt(111)上的解离 sticking 系数随气体温度（Tg）和表面温度（Ts）的变化。实验结果用于优化反应体系的微正则单分子速率理论（MURT）模型的三个参数。MURT 计算使我们能够从数据中提取过渡态性质，并将我们的数据直接与其他分子束和热平衡 sticking 测量结果进行比较。我们发现解离的阈值能量 E0 = 52.5 +/- 3.5 kJ mol⁻¹。此外，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 CH₄在 Pt(111)上对于 Ts 和 Tg 的任何组合的解离 sticking 系数，即使两者不相等，实际上，分子能量分布甚至不必是热的。将我们的结果与最近关于分散在氧化物衬底上的 Pt 纳米团簇（直径约 2 nm）上的 CH₄重整的热平衡催化研究结果进行比较表明，Pt(111)的反应活性比 Pt 纳米催化剂高出几个数量级。
