Hydrogen generation by the reaction of H2O with Al2O3-based materials: a computational analysis

Abstract

The mechanisms for H₂O adsorption on γ-Al₂O₃(110) surface were investigated to illustrate the influence of oxide modifiers on the hydrogen generation reaction. Periodic density functional theory (DFT) calculations with the projected augmented wave (PAW) approach were carried out to study the adsorption of H₂O, OH, O and H species, as well as the reaction mechanisms of H₂O splitting and H₂ generation. Their corresponding structures and adsorption energies are also reported. The calculation results show that H₂O, OH, O and H are preferably bound at Al(I)-top, Al(III)-bridge, Al(I,II)-bridge and Al(III)-bridge sites with adsorption energies of −0.42, −5.01, −8.70 and −2.38 eV, respectively. The potential energy profiles for water splitting and hydrogen generation on the γ-Al₂O₃(110) surface are mapped out. We find that hydrogen generation on the surface occurs via two processes, namely, H₂O dehydrogenation and direct H₂ generation with an overall exothermicity of 2.24 eV. The nonexistence of intrinsic transition-state barriers and the high exothermicity for the reaction of H₂O_(g) + γ-Al₂O₃(110) → O_(ads)/γ-Al₂O₃(110) + H_2(g) result in rapid H₂ generation. The stepwise H₂ generation mechanism of adsorption on the γ-Al₂O₃(110) surface was also demonstrated using first-principles molecular dynamics simulations. In addition, the nature of the interaction between the adsorbate and the surface during the reaction was also analyzed by the local density of states and by Bader charge calculations.