(521cy) Theoretical Insights into the Photocatalytic PFOA Degradation Mechanism over Boron Nitride

Perfluorooctanoic acid (PFOA) is a kind of pervasive, persistent, and harmful water contaminant that calls for effective degradation methods. We worked with our collaborators and reported that hexagonal boron nitride (hBN) is a promising photocatalyst for PFOA degradation under UVC illumination, with an activity ~2x higher than TiO₂ (see Environ. Sci. Technol. Lett. 2020, 7, 8, 613–619). By applying density functional theory (DFT), we investigated the favorability of the first photo-oxidation step of PFOA on hBN surface: C_nF_2n+1COO^- + h⁺ → C_nF_2n+1• + CO₂. The thermodynamic feasibility of the oxidation reaction was confirmed by the lower computed valence band position of hBN than the oxidation potential of deprotonated PFOA. We further computed the free energy and activation barrier of the surface reaction and found that the PFOA photo-oxidation is exothermic with a moderate barrier over illuminated hBN. The kinetic feasibility of the reaction was further confirmed by computing the electron transfer rate between the molecule and hBN surface using Marcus theory. To understand the excellent performance of the hBN with wide bandgap under UVC light, X-ray photoelectron spectroscopy (XPS) was applied to characterize the hBN surface defects. We identified nitrogen-boron substitutional defect (N_B), oxygen-at-nitrogen-site substitutions (O_N)_, as well as edge defects on the hBN material used in the experiment. The density of states analysis of the defective surface models suggests that the defects do not interfere with the oxidation reaction. Furthermore, N­_B defects introduce mid-gap states that enable the UVC light absorption and enhance charge carrier separation. Therefore, we propose that introducing more N_B defects is a promising strategy to enhance the photocatalytic degradation performance of hBN. Our work sets the theoretical basis for the photocatalyst design for PFOA removal in water treatment.