H_2分子在LaFeO_3表面吸附的第一性原理研究

First Principles Study on the Adsorption of H_2 Molecules on LaFeO_3 Surface

基于密度泛函理论的第一性原理方法,通过计算表面能确定La Fe O_3(010)表面为最稳定的吸附表面,研究了H_2分子在La Fe O_3(010)表面的吸附性质。La Fe O_3(010)表面存在La O和Fe O_2两种终止表面,但吸附主要发生在Fe O_2终止表面,由于La Fe O_3(010)表面弛豫的影响,使得凹凸不平的表面层增加了表面原子与H原子的接触面积,表面晶胞的纵向体积增加约2.5%,有利于H原子向晶体内扩散。研究发现,H_2分子在La Fe O_3(010)表面主要存在3种化学吸附方式:第一种吸附发生在O-O桥位,2个H原子分别吸附在2个O原子上,形成2个-OH基,这是最佳吸附位置,此时H原子与表面O原子的作用主要是H1s与O_2p轨道杂化作用的结果,H-O之间为典型的共价键。H_2分子的解离能垒为1.542 e V,说明表面需要一定的热条件,H_2分子才会发生解离吸附;第二种吸附发生在Fe-O桥位,1个H原子吸附在O原子上形成1个-OH基,另一个H原子吸附在Fe原子上形成金属键;第三种吸附发生在O顶位,2个H原子吸附在同一个O原子上,形成H_2O分子,此时H_2O分子与表面形成物理吸附,H_2O分子逃离表面后容易形成氧空位。此外,H_2分子在La Fe O_3(010)表面还可以发生物理吸附。

Based on the first principles calculations, the adsorption properties of H2 molecules on La Fe O3（010）surface are studied after the（010） surface was confirmed as the most stable surface. La Fe O3（010） surface consists of La O and Fe O2 terminated surfaces, but the adsorption mainly occurs on the Fe O2 terminated surface.Due to the surface relaxation, the contact area on the uneven surface lay between surface atoms and H atoms increased, and resulted into about 2.5% increase of the longitudinal volume of the unit cell surface which is beneficial to the H atoms diffusion within the crystal. The results indicate that, there are three kinds of chemical adsorption modes of H2 molecules on the surface of La Fe O3（010）： The best adsorption mode is that two H atoms are adsorbed to the two surface O atoms respectively, forming two-OH groups. At this position, the typical covalent bonds between H and surface O formed through the orbital hybridization of H1 s and O2 p. The energy barrier of H2 molecules dissociation is about 1.542 e V, indicating that the dissociative adsorption can be occurred only under certain thermal condition. The second mode is that one H atom adsorbed on the surface O atom,forming an-OH group, while the other H atom is adsorbed to the Fe atoms, forming a metal bond. The third mode is that two H atoms are adsorbed to the same surface O atom to form H2 O molecules which is physically adsorbed on the surface, but the surface oxygen vacancies can be easily formed after the H2 O molecules escaped from the surface. In addition, H2 molecules also can be physically adsorbed on La Fe O3（010） surface.