Abbott Heather L, Harrison Ian
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319, USA.
J Phys Chem B. 2005 May 26;109(20):10371-80. doi: 10.1021/jp0505375.
A 3-parameter local hot spot model of gas-surface reactivity is employed to analyze and predict dissociative sticking coefficients for CH(4) incident on Ir(111) under varied nonequilibrium and equilibrium conditions. One Ir surface oscillator and the molecular vibrations, rotations, and translational energy directed along the surface normal are treated as active degrees of freedom in the 14 dimensional microcanonical kinetics. The threshold energy for CH(4) dissociative chemisorption on Ir(111) derived from modeling molecular beam experiments is E(0) = 39 kJ/mol. Over more than 4 orders of magnitude of variation in sticking, the average relative discrepancy between the beam and theoretically derived sticking coefficients is 88%. The experimentally observed enhancement in dissociative sticking as beam translational energies decrease below approximately 10 kJ/mol is consistent with a parallel dynamical trapping/energy transfer channel that likely fails to completely thermalize the molecules to the surface temperature. This trapping-mediated sticking, indicative of specific energy transfer pathways from the surface under nonequilibrium conditions, should be a minor contributor to the overall dissociative sticking at thermal equilibrium. Surprisingly, the CH(4) dissociative sticking coefficient predicted for Ir(111) surfaces at thermal equilibrium, based on the molecular beam experiments, is roughly 4 orders of magnitude higher than recent measurements on supported nanoscale Ir catalysts at 1 bar pressure, which suggests that substantial improvements in catalyst turnover rates may be possible.
采用一种三参数的气-表面反应性局部热点模型，来分析和预测在不同非平衡和平衡条件下，CH₄入射到Ir(111)上的解离 sticking 系数。在14维微正则动力学中，一个Ir表面振子以及沿表面法线方向的分子振动、转动和平动能量被视为活性自由度。通过对分子束实验进行建模得出，CH₄在Ir(111)上解离化学吸附的阈值能量为E(0)=39 kJ/mol。在sticking系数超过4个数量级的变化范围内，束流与理论推导的sticking系数之间的平均相对差异为88%。当束流转动能低于约10 kJ/mol时，实验观察到的解离sticking增强现象，与一个平行的动力学俘获/能量转移通道相一致，该通道可能无法使分子完全热化至表面温度。这种俘获介导的sticking，表明在非平衡条件下从表面有特定的能量转移途径，在热平衡时，它对整体解离sticking的贡献应该较小。令人惊讶的是，基于分子束实验预测的热平衡下Ir(111)表面的CH₄解离sticking系数，比最近在1 bar压力下对负载型纳米级Ir催化剂的测量值高约4个数量级，这表明催化剂周转率可能有大幅提高。
