Mechanism for adsorption, dissociation and diffusion of hydrogen in hydrogen permeation barrier of α-Al2O3: A density functional theory study

Toward understanding physical interaction of hydrogen isotopes with α-Al2O3 barrier, adsorption, dissociation and diffusion of hydrogen in α-Al2O3(0001) slab have been investigated by density functional theory (DFT) and rate theory. H2 molecule, with parallel configuration, preferentially absorbs on a top Al atom site of first atomic layer on α-Al2O3(0001) surface, while H atom strongly bonds at a top O atom site of the second atomic layer, H atoms recombine into molecules on top Al atom sites of the third atomic layer. The barrier for H2 exothermic dissociation on surface is 0.79 eV. The potential energy pathways of H diffusion in α-Al2O3 are studied, predicting that H atom diffusion preferentially occurs via surface path rather than bulk path involving elementary reorientation and hopping steps. The surface-to-subsurface diffusion is significantly endothermic except for the surface and subsurface-to-bulk path. Mechanism, in well agreement with experimental result, of α-Al2O3 resisting hydrogen permeation has proposed.

► Favorable binding sites of H2 and H on clean α-Al2O3(0001) surface have identified.
► H2 dissociation on clean α-Al2O3(0001) surface is easier than H atoms recombination.
► H atom diffusion in α-Al2O3 occurs via surface path rather than bulk path.
► Bulk diffusion is possible but a high barrier of 1.58 eV must first be overcome.
► Mechanism of purity α-Al2O3 resisting hydrogen isotopic permeation has proposed.