(178c) Theoretical Insights into the Photocatalytic PFOA Degradation Mechanism over Boron Nitride

The water contaminant, perfluorooctanoic acid (PFOA), is pervasive, persistent, and harmful to human health. Effective methods for degrading PFOA to harmless substances are urgently needed. We were part of a team that reported the unexpected excellent photocatalytic performance of hexagonal boron nitride (hBN) for PFOA degradation under UVC illumination, with activity ~2x higher than TiO₂ (see Environ. Sci. Technol. Lett. 2020, 7, 8, 613–619). We applied density functional theory (DFT) to investigate the feasibility of the initial elementary steps of the PFOA degradation cycle over hBN: C_nF_2n+1COO^- + h⁺ → C_nF_2n+1• + CO₂. We found that the computed valence band position of hBN lies below the oxidation potential of deprotonated PFOA, which demonstrates the thermodynamic favorability of the oxidation reaction. We also calculated the free energy and activation barrier of the surface reaction and found that the oxidation reaction is exothermic with a moderate barrier. Marcus theory was applied to calculate the electron transfer rate explicitly, which further confirmed the feasibility of direct electron transfer between the PFOA anion and hBN. The excellent performance of hBN under UVC light is surprising because of its wide bandgap. X-ray photoelectron spectroscopy (XPS) was applied to characterize the hBN material and several surface defects were identified: nitrogen-boron substitutional defect (N_B), oxygen-at-nitrogen-site substitutions (O_N)_, as well as edge defects. By analyzing the density of states of the defective surface models, we found that the defects do not change the valence band position of hBN and thus do not interfere with the oxidation reaction. Furthermore, N­_B defects introduce mid-gap states that enable the UVC light absorption. Therefore, we propose that introducing more N_B defects can further enhance the photocatalytic degradation performance of hBN. Our work sets the theoretical basis for the design of better photo-catalysts for the disposal of PFOA from water.