(581a) Photo-Catalytic Treatment of Perfluoroalkyl Substances (PFAS) over Hexagonal Boron Nitride

Perfluoroalkyl substances (PFAS) are a pervasive contaminant in the environment that are energy-intensive to treat through traditional absorption and separation strategies. Direct degradation of PFAS through catalytic treatment has emerged as a viable alternative. Working with collaborators, we reported that two-dimensional (2D) hexagonal boron nitride (hBN) unexpectedly exhibits high catalytic activity for the photo-catalytic degradation of perfluorooctanoic acid (PFOA). Furthermore, h-BN outperforms three-dimensional (3D) TiO₂ P25 catalysts under the same conditions. In this work, we demonstrate that the 2D properties of hBN are responsible for its enhanced photo-catalytic activity for PFAS degradation. We apply density functional theory (DFT) to investigate the thermodynamics and kinetics of PFAS adsorption and degradation over hBN and TiO₂. We found that the valence band maximum (VBM) position of both hBN and TiO₂ lie below the oxidation potential of PFOA, which demonstrates that both catalysts can photo-oxidize PFOA. The VBM of TiO₂lies below that of hBN suggesting that TiO₂ should more readily oxidize PFOA based on the thermodynamic favorability of the reaction. However, we find that PFOA oxidation is enhanced over the 2D basal plane of hBN, which is hydrophobic in nature and interacts strongly with the fluorinated tail group of PFOA. Conversely, on TiO₂ the surface is covered with a dissociated water later that readily scavenges photo-generated holes and interferes with PFOA oxidation. Water dissociates on the TiO₂ to saturate under-coordinated metal and oxygen sites that are generated when the 3D TiO₂ bulk is cleaved to form a surface. Since hBN is a 2D material, no under-coordinated sites exist on the surface for dissociative water adsorption. As such, it is the 2D nature of hBN that is responsible for its high hydrophobicity, which in turn is responsible for its high photo-catalytic activity toward PFAS.

[1] Environ. Sci. Technol. Lett. 2020, 7, 8, 613–619