Wang Wenji, Zhao Yi
College of Science, Northwest A&F University, Yangling, 712100, Shaanxi Province, P. R. China.
Phys Chem Chem Phys. 2015 Feb 28;17(8):5901-12. doi: 10.1039/c4cp05624h.
The dissociation and recombination rates of physisorbed H2, and the direct and steady state dissociation (i.e., the precursor mediated dissociation) rates of gas phase H2 on Ni(111), as well as the corresponding kinetic isotope effects, are calculated using the quantum instanton method, together with path integral Monte Carlo and adaptive umbrella sampling techniques. All these rates except the recombination one first decrease and then increase with the increasing temperature, and their minimum values appear at about 250, 300 and 250 K, respectively. These non-monotonic behaviors reveal that the quantum effect of H2 should be very remarkable at low temperatures. The steady state rates are smaller than the direct rates at low temperatures, however, they become larger than the direct ones at high temperatures, these two kinds of rates become equal at about 400 and 300 K on the rigid and quantum lattices, respectively. The quantum motion of the lattice can enhance the direct and steady state rates, and it increases the steady state rate much more than the direct one, for instance, the direct and steady state rates on the quantum lattice are 1.30 and 2.08 times larger than that on the rigid one at 300 K. The calculated kinetic isotope effects are much larger than 1, which reveals that H2 always has a larger rate than that of D2, and the direct process predicts much larger kinetic isotope effects than the steady state process at low temperatures. In addition, the kinetic isotope effects are not affected by the lattice motion.
采用量子瞬子方法，结合路径积分蒙特卡罗和自适应伞形采样技术，计算了物理吸附H₂的解离和重组速率、气相H₂在Ni(111)上的直接解离和稳态解离（即前驱体介导的解离）速率，以及相应的动力学同位素效应。除重组速率外，所有这些速率均随温度升高先降低后升高，其最小值分别出现在约250、300和250K处。这些非单调行为表明，H₂的量子效应在低温下应非常显著。稳态速率在低温下小于直接速率，但在高温下大于直接速率，在刚性晶格和量子晶格上，这两种速率分别在约400和300K时相等。晶格的量子运动会提高直接解离和稳态解离速率，且对稳态速率的提高幅度远大于直接解离速率，例如，在300K时，量子晶格上的直接解离和稳态解离速率分别比刚性晶格上的大1.30倍和2.08倍。计算得到的动力学同位素效应远大于1，这表明H₂的速率总是大于D₂的速率，且在低温下直接过程预测的动力学同位素效应比稳态过程大得多。此外，动力学同位素效应不受晶格运动的影响。
