Mechanisms for adsorption, dissociation and diffusion of hydrogen in hydrogen permeation barrier of α-Al2O3: The role of crystal orientation

The mechanisms of adsorption of hydrogen on α-Al2O3(1-102) surface and of its diffusion in bulk are investigated, using first principles thermodynamics and kinetics, and compared with similar results obtained for the diffusion of hydrogen on α-Al2O3(0001) surface. Because of the different oxygen environments on both surfaces, the H binding energies on the (1-102) surface are 0.3–1.2 eV smaller than in the (0001) surface. The H2 binding energies on (1-102) and (0001) surfaces are resembled. We have identified four main mechanisms, leading to dissociation of H2, H migration on the surface, H diffusion into and inside the bulk. Equilibrium constant and activation barrier show that H2 dissociation is the most favorable process and significant diffusion of H into the bulk can occur more readily from the (1-102) surface compared to the (0001) surface. Based on the hydrogen interaction with α-Al2O3(1-102) surface, a mechanism of α-Al2O3 suppressing H-permeation is identified.