Catalytic Effects of Subsurface Carbon in the Chemisorption of Hydrogen on a Mg(0001) Surface: an Ab-initio Study

Ab initio density functional theory (DFT) calculations are performed to explore possible catalytic effects on
the dissociative chemisorption of hydrogen on a Mg(0001) surface when carbon is incorporated into Mg
materials. The computational results imply that a C atom located initially on a Mg(0001) surface can migrate
into the subsurface and occupy an fcc interstitial site, with charge transfer to the C atom from neighboring
Mg atoms. The effect of subsurface C on the dissociation of H2 on the Mg(0001) surface is found to be
relatively marginal:  a perfect sublayer of interstitial C is calculated to lower the barrier by 0.16 eV compared
with that on a pure Mg(0001) surface. Further calculations reveal, however, that sublayer C may have a
significant effect in enhancing the diffusion of atomic hydrogen into the sublayers through fcc channels. This
contributes new physical understanding toward rationalizing the experimentally observed improvement in
absorption kinetics of H2 when graphite or single walled carbon nanotubes (SWCNT) are introduced into the
Mg powder during ball milling.

本研究采用从头算密度泛函理论（ab initio density functional theory, DFT）计算，探究碳掺入镁基材料时，氢在Mg(0001)晶面的解离化学吸附过程所受的潜在催化效应。计算结果表明，初始吸附于Mg(0001)晶面的碳原子可迁移至次表层，并占据面心立方（face-centered cubic, fcc）间隙位，此时相邻镁原子会向该碳原子转移电荷。研究发现，次表层碳对H₂在Mg(0001)晶面的解离过程影响相对有限：相较于纯Mg(0001)晶面，完整的间隙碳次表层仅能将解离能垒降低0.16 eV。但后续计算结果显示，次表层碳可通过面心立方通道，显著促进原子氢向次表层的扩散过程。该研究为合理阐释实验中观察到的现象提供了新的物理认知：当在球磨过程中向镁粉中引入石墨或单壁碳纳米管（single walled carbon nanotubes, SWCNT）时，氢气的吸收动力学性能得到了提升。