Greeley Jeff, Mavrikakis Manos
Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
J Phys Chem B. 2005 Mar 3;109(8):3460-71. doi: 10.1021/jp046540q.
Periodic, self-consistent DFT-GGA calculations are used to study the thermochemical properties of both surface and subsurface atomic hydrogen on a variety of pure metals and near-surface alloys (NSAs). For surface hydrogen on pure metals, calculated site preferences, adsorption geometries, vibrational frequencies, and binding energies are reported and are found to be in good agreement with available experimental data. On NSAs, defined as alloys wherein a solute is present near the surface of a host metal in a composition different from the bulk composition, surface hydrogen generally binds more weakly than it binds to the pure-metal components composing the alloys. Some of the NSAs even possess the unusual property of binding hydrogen as weakly as the noble metals while, at the same time, dissociating H(2) much more easily. On both NSAs and pure metals, formation of surface hydrogen is generally exothermic with respect to H(2)(g). In contrast, formation of subsurface hydrogen is typically endothermic with respect to gas-phase H(2) (the only exception to this general statement is found for pure Pd). As with surface H, subsurface H typically binds more weakly to NSAs than to the corresponding pure-metal components of the alloys. The diffusion barrier for hydrogen from surface to subsurface sites, however, is usually lower on NSAs compared to the pure-metal components, suggesting that population of subsurface sites may occur more rapidly on NSAs.
采用周期性自洽密度泛函理论广义梯度近似（DFT-GGA）计算方法，研究了多种纯金属及近表面合金（NSA）上表面和次表面氢原子的热化学性质。对于纯金属表面的氢，报告了计算得到的位点偏好、吸附几何结构、振动频率和结合能，发现与现有实验数据吻合良好。在NSA（定义为溶质以不同于体相组成的成分存在于主体金属表面附近的合金）上，表面氢的结合通常比其与构成合金的纯金属组分的结合更弱。一些NSA甚至具有与贵金属一样弱地结合氢的不寻常性质，同时又更容易使H₂离解。在NSA和纯金属上，相对于H₂(g)，表面氢的形成通常是放热的。相比之下，相对于气相H₂，次表面氢的形成通常是吸热的（这一普遍规律的唯一例外是纯钯）。与表面氢一样，次表面氢与NSA的结合通常比与合金相应的纯金属组分的结合更弱。然而，与纯金属组分相比NSA上氢从表面到次表面位点的扩散势垒通常更低，这表明在NSA上次表面位点的填充可能更快发生。
