Dynamics of Proton Transfer on Boehmite (010) Surface in Anhydrous and Hydrous Environments

Boehmite (010) surface has a high density of ordered hydroxyl groups but no intrinsic hydrogen bonds. This represents a special case for which proton-transfer mechanism is unknown and different from those along a continuous hydrogen-bond network. In this study, probable pathways for proton transfer on boehmite (010) surface in both anhydrous and hydrous environments are explored, and energy barriers of these processes are calculated using the climbing-image nudged elastic band method based on density-functional theory. The results reveal that under anhydrous conditions, long-distance proton transfer is unlikely on clean (010) surface, but excess protons are mobile with a low proton-transfer energy barrier of 31.4 kJ mol(-1). Subsurface Zn dopants (for charge balance with excess protons) reduce the energy barrier to 21.1 kJ mol(-1). In contrast, water-mediated proton-transfer processes only need to overcome a low-energy barrier of 10.0 kJ mol(-1). The low-energy barriers suggest boehmite (010) surface can conduct protons, particularly in hydrous environments, making boehmite a promising filler material for proton exchange membranes.