Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319, USA.
J Chem Phys. 2013 Dec 7;139(21):214707. doi: 10.1063/1.4837697.
A dynamically biased (d-) precursor mediated microcanonical trapping (PMMT) model of the activated dissociative chemisorption of methane on Pt(111) is applied to a wide range of dissociative sticking experiments, and, by detailed balance, to the methane product state distributions from the thermal associative desorption of adsorbed hydrogen with coadsorbed methyl radicals. Tunneling pathways were incorporated into the d-PMMT model to better replicate the translational energy distribution of the desorbing methane product from the laser induced thermal reaction of coadsorbed hydrogen and methyl radicals occurring near T(s) = 395 K. Although tunneling is predicted to be inconsequential to the thermal dissociative chemisorption of CH4 on Pt(111) at the high temperatures of catalytic interest, once the temperature drops to 395 K the tunneling fraction of the reactive thermal flux reaches 15%, and as temperatures drop below 275 K the tunneling fraction exceeds 50%. The d-PMMT model parameters of {E0 = 58.9 kJ/mol, s = 2, η(v) = 0.40} describe the apparent threshold energy for CH4/Pt(111) dissociative chemisorption, the number of surface oscillators involved in the precursor complex, and the efficacy of molecular vibrational energy to promote reaction, relative to translational energy directed along the surface normal. Molecular translations parallel to the surface and rotations are treated as spectator degrees of freedom. Transition state vibrational frequencies are derived from generalized gradient approximation-density functional theory electronic structure calculations. The d-PMMT model replicates the diverse range of experimental data available with good fidelity, including some new effusive molecular beam and ambient gas dissociative sticking measurements. Nevertheless, there are some indications that closer agreement between theory and experiments could be achieved if a surface efficacy less than one was introduced into the modeling as an additional dynamical constraint.
一种动态偏置（d-）前体介导微正则捕获（PMMT）模型被应用于甲烷在 Pt(111)上的活化离解化学吸附的广泛离解 sticking 实验中，并通过详细平衡应用于吸附氢与共吸附甲基自由基热缔合解吸的甲烷产物状态分布。隧道途径被纳入 d-PMMT 模型中，以更好地复制在 T(s) = 395 K 附近发生的共吸附氢和甲基自由基激光诱导热反应中离解出的甲烷产物的平移能分布。尽管隧道对于在催化感兴趣的高温下 CH4 在 Pt(111)上的热离解化学吸附不重要，但一旦温度降至 395 K，反应热通量的隧道分数达到 15%，而当温度降至 275 K 以下时，隧道分数超过 50%。d-PMMT 模型参数{E0 = 58.9 kJ/mol, s = 2, η(v) = 0.40}描述了 CH4/Pt(111)离解化学吸附的表观阈值能、参与前体复合物的表面振荡器数以及分子振动能相对于沿表面法线的平移能促进反应的效率。平行于表面的分子平移和旋转被视为旁观自由度。过渡态振动频率是从广义梯度近似-密度泛函理论电子结构计算中得出的。d-PMMT 模型以良好的保真度复制了广泛的实验数据，包括一些新的扩散分子束和环境气体离解 sticking 测量。然而，有一些迹象表明，如果在建模中引入小于一的表面效率作为附加动力学约束，理论与实验之间可以更紧密地一致。
