Henkelman Graeme, Arnaldsson Andri, Jónsson Hannes
Department of Chemistry and Biochemistry, The University of Texas at Austin, Texas 78712-0165, USA.
J Chem Phys. 2006 Jan 28;124(4):044706. doi: 10.1063/1.2161193.
The results of theoretical calculations of associative desorption of CH(4) and H(2) from the Ni(111) surface are presented. Both minimum-energy paths and classical dynamics trajectories were generated using density-functional theory to estimate the energy and atomic forces. In particular, the recombination of a subsurface H atom with adsorbed CH(3) (methyl) or H at the surface was studied. The calculations do not show any evidence for enhanced CH(4) formation as the H atom emerges from the subsurface site. In fact, there is no minimum-energy path for such a concerted process on the energy surface. Dynamical trajectories started at the transition state for the H-atom hop from subsurface to surface site also did not lead to direct formation of a methane molecule but rather led to the formation of a thermally excited H atom and CH(3) group bound to the surface. The formation (as well as rupture) of the H-H and C-H bonds only occurs on the exposed side of a surface Ni atom. The transition states are quite similar for the two molecules, except that in the case of the C-H bond, the underlying Ni atom rises out of the surface plane by 0.25 A. Classical dynamics trajectories started at the transition state for desorption of CH(4) show that 15% of the barrier energy, 0.8 eV, is taken up by Ni atom vibrations, while about 60% goes into translation and 20% into vibration of a desorbing CH(4) molecule. The most important vibrational modes, accounting for 90% of the vibrational energy, are the four high-frequency CH(4) stretches. By time reversibility of the classical trajectories, this means that translational energy is most effective for dissociative adsorption at low-energy characteristic of thermal excitations but energy in stretching modes is also important. Quantum-mechanical tunneling in CH(4) dissociative adsorption and associative desorption is estimated to be important below 200 K and is, therefore, not expected to play an important role under typical conditions. An unexpected mechanism for the rotation of the adsorbed methyl group was discovered and illustrated a strong three-center C-H-Ni contribution to the methyl-surface bonding.
本文给出了CH(4)和H(2)从Ni(111)表面缔合脱附的理论计算结果。利用密度泛函理论生成了最小能量路径和经典动力学轨迹，以估算能量和原子力。特别研究了表面下的H原子与表面吸附的CH(3)（甲基）或H的重组。计算结果并未显示出随着H原子从表面下位置出现而增强CH(4)形成的任何证据。事实上，在能量表面上不存在这样一个协同过程的最小能量路径。从表面下位置到表面位置的H原子跳跃的过渡态开始的动力学轨迹也没有导致甲烷分子的直接形成，而是导致了与表面结合的热激发H原子和CH(3)基团的形成。H-H键和C-H键的形成（以及断裂）仅发生在表面Ni原子的暴露侧。两种分子的过渡态非常相似，只是在C-H键的情况下，下面的Ni原子从表面平面上升起0.25 Å。从CH(4)脱附的过渡态开始的经典动力学轨迹表明，0.8 eV的势垒能量中，15%被Ni原子振动吸收，而约60%用于脱附的CH(4)分子的平动，20%用于振动。占振动能量90%的最重要振动模式是四个高频CH(4)伸缩振动。根据经典轨迹的时间可逆性，这意味着平动能在热激发的低能量特征下对解离吸附最有效，但伸缩模式的能量也很重要。据估计，CH(4)解离吸附和缔合脱附中的量子力学隧穿在200 K以下很重要，因此在典型条件下预计不会起重要作用。发现了一种吸附甲基旋转的意外机制，并说明了三中心C-H-Ni对甲基-表面键合的强烈贡献。
