PNNL

Abstract

Rare-event simulations (climbing image nudge elastic band and metadynamics) were performed using density functional theory (DFT) and a hybrid exchange-correlation functional to elucidate and quantify the diffusion mechanisms of radiation-induced species (O-, H0, and H2) in boehmite (?-AlOOH) and gibbsite (?-Al(OH)3). The two Al (oxy-)hydroxide phases are known to have much different radiolytic activities, but the underlying mechanisms remain unknown. The DFT calculations revealed that O- diffusion occurred via proton-coupled hole transfer with high energy barriers in both phases. In contrast, energy barriers for H0 transfers were generally lower in boehmite than in gibbsite, suggesting a more facile diffusion in boehmite of H radicals to the surface, where H2 formation can take place. Another key difference was the ability of H0 and H2 to diffuse across the structural layers in gibbsite but not in boehmite. Therefore, while the formation of O and H radicals was energetically favored in gibbsite compared to boehmite, the DFT calculations indicated that the mechanisms of diffusion are responsible for the higher H2 yields measured for boehmite compared to gibbsite