Enhanced diffusion and permeation of hydrogen species on the partially carbon covered iron surfaces

Abstract Understanding hydrogen diffusion on and permeation into the species-covered metal surfaces is of great importance in the exploration of novel hydrogenation processes. By combining of density functional theory, ab initio thermodynamics and microkinetic simulations, we present the profound coverage effects on hydrogen diffusion on and permeation into the H/O/C-covered Fe(100) surfaces at different atomic coverages. Through comparing the calculated barriers of mostly 72 possible configurations, we found that the effect of hydrogen species adsorption is very weak, and the carbon species adsorption has very favorable effect in contrast to the oxygen species adsorption. Importantly, the strongly enhanced effects are respectively found within three partially carbon-covered configurations, i.e., H 1 C 1 -covered, O 1 C 1 -covered and C 1 C 1 -covered Fe(100) surfaces. Ab initio thermodynamics calculations provide the further evidences on the stabilities of these partially carbon-covered surfaces under different hydrogen pressures and temperatures. The phase of 0.5 ML surface hydrogen and 0.25 ML subsurface hydrogen is stably formed within the conditions of low surface temperatures and high hydrogen pressures. In addition, our microkinetic simulations accurately give the atomic coverages of surface and subsurface hydrogen species on these partially carbon-covered Fe(100) surfaces. The H 1 C 1 -covered Fe(100) surface is the most favourable due to the lowest dissociation barrier of molecular hydrogen.