H2分子在LaFeO3表面吸附的第一性原理研究

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LaFeO₃(010)表面为最稳定的吸附表面,研究了H₂分子在LaFeO₃(010)表面的吸附性质。LaFeO₃(010)表面存在LaO和FeO₂两种终止表面,但吸附主要发生在FeO₂终止表面,由于LaFeO₃(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,H₂分子在LaFeO₃(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O2p轨道杂化作用的结果,H-O之间为典型的共价键。H₂分子的解离能垒为1.542 eV,说明表面需要一定的热条件,H₂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H₂O分子,此时H₂O分子与表面形成物理吸附,H₂O分子逃离表面后容易形成氧空位。此外,H₂分子在LaFeO₃(010)表面还可以发生物理吸附。

First Principles Study on the Adsorption of H2 Molecules on LaFeO3 Surface

Based on the first principles calculations, the adsorption properties of H₂ molecules on LaFeO₃(010) surface are studied after the (010) surface was confirmed as the most stable surface. LaFeO₃(010) surface consists of LaO and FeO₂ terminated surfaces, but the adsorption mainly occurs on the FeO₂ 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 LaFeO₃(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 H1s and O2p. The energy barrier of H₂ molecules dissociation is about 1.542 eV, 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 H₂O molecules which is physically adsorbed on the surface, but the surface oxygen vacancies can be easily formed after the H₂O molecules escaped from the surface. In addition, H₂ molecules also can be physically adsorbed on LaFeO₃(010) surface.