(532cx) Theoretical Methods for Assessing the Feasibility of PFOA Oxidation on Photo-Catalyst Surfaces

Removing the pervasive, persistent and harmful water contaminant, perfluorooctanoic acid (PFOA) efficiently is a concerning topic in water treatment. Working with collaborators, we found that hexagonal boron nitride (hBN) is an excellent photocatalyst for PFOA degradation under UVC illumination, which outcompetes TiO₂ under the same conditions(see Environ. Sci. Technol. Lett. 2020, 7, 8, 613–619). We applied density functional theory (DFT) to examine the feasibility of the initial oxidation step of PFOA degradation cycle over hBN: C_nF_2n+1COO^- + h⁺ → C_nF_2n+1• + CO₂. We computed the redox potential of the PFOA oxidation, and found the redox potential is less positive than the valence band maximum (VBM) of hBN, which suggests that the photo-generated holes can oxidize PFOA thermodynamically. We also calculated the explicit reaction free energy and activation barrier of the surface reaction as a function of surface charge density in a grand canonical (GC) formalism and found the reaction is exothermic with a surmountable barrier. Also, Marcus theory was applied to calculate the electron transfer rate. This further implies the kinetic feasibility of the electron transfer between the anion and the hBN surface. The excellent photocatalytic performance of hBN under UVC illumination is unexpected because of its wide bandgap. By analyzing the density of states of the defective surface models, we found the nitrogen-boron substitutional defect (N_B) on hBN surface can generate mid-gap states that promote the UVC light absorption. Furthermore, the presence of N_B does not change the VBM of hBN and thus does not affect the feasibility of oxidation reaction. Therefore, we propose that introducing more N_B defects can further enhance the performance of hBN on PFOA degradation.