College of Chemistry & Pharmacy, Northwest A&F University, Yangling, 712100 Shaanxi Province, People's Republic of China.
State Key Laboratory for Physical Chemistry of Solid Surfaces and Fujian Provincial Key Lab of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China.
J Chem Phys. 2017 Jul 28;147(4):044703. doi: 10.1063/1.4995299.
Methane dissociation is a prototypical system for the study of surface reaction dynamics. The dissociation and recombination rates of CH through the Ni(111) surface are calculated by using the quantum instanton method with an analytical potential energy surface. The Ni(111) lattice is treated rigidly, classically, and quantum mechanically so as to reveal the effect of lattice motion. The results demonstrate that it is the lateral displacements rather than the upward and downward movements of the surface nickel atoms that affect the rates a lot. Compared with the rigid lattice, the classical relaxation of the lattice can increase the rates by lowering the free energy barriers. For instance, at 300 K, the dissociation and recombination rates with the classical lattice exceed the ones with the rigid lattice by 6 and 10 orders of magnitude, respectively. Compared with the classical lattice, the quantum delocalization rather than the zero-point energy of the Ni atoms further enhances the rates by widening the reaction path. For instance, the dissociation rate with the quantum lattice is about 10 times larger than that with the classical lattice at 300 K. On the rigid lattice, due to the zero-point energy difference between CH and CD, the kinetic isotope effects are larger than 1 for the dissociation process, while they are smaller than 1 for the recombination process. The increasing kinetic isotope effect with decreasing temperature demonstrates that the quantum tunneling effect is remarkable for the dissociation process.
甲烷的离解是表面反应动力学研究的典型体系。通过使用量子瞬时方法和分析势能面，计算了通过 Ni(111)表面的 CH 的离解和复合速率。Ni(111)晶格被刚性、经典和量子力学地处理，以揭示晶格运动的影响。结果表明，影响速率的主要是晶格的横向位移，而不是表面镍原子的上下运动。与刚性晶格相比，晶格的经典弛豫可以通过降低自由能垒来提高速率。例如，在 300 K 时，具有经典晶格的离解和复合速率比具有刚性晶格的速率分别高出 6 和 10 个数量级。与经典晶格相比，Ni 原子的量子离域而不是零点能进一步通过拓宽反应途径来提高速率。例如，在 300 K 时，具有量子晶格的离解速率大约比具有经典晶格的速率大 10 倍。在刚性晶格上，由于 CH 和 CD 之间的零点能差异，离解过程的动力学同位素效应大于 1，而复合过程的动力学同位素效应小于 1。随着温度的降低，动力学同位素效应的增加表明量子隧穿效应对于离解过程是显著的。
