Mechanistic Insight into the Photo-Oxidation of Perfluorocarboxylic Acid over Boron Nitride

Hexagonal boron nitride (hBN) can photocatalytically oxidize and degrade perfluorocarboxylic acids (PFCA), a common member of the per/polyfluoroalkyl substance (PFAS) family of water contaminants. However, the reaction mechanism governing PFCA activation on hBN is not yet understood. Here, we apply electronic grand canonical density functional theory (GC-DFT) to assess the thermodynamic and kinetic favorability of PFCA photo- oxidative activation on hBN: CnF2n-1COO- + h(+) -> CnF2n+1 + CO2. The oxidation of all PFCA chains is exothermic under illumination with a moderate barrier. However, the longer-chain PFCAs are degraded more effectively because they adsorb on the surface more strongly as a result of increased van der Waals interactions with the hBN surface. The ability of hBN to act as a photocatalyst is unexpected because of its wide band gap. Therefore, we apply both theoretical and experimental analyses to examine possible defects on hBN that could account for its activity. We find that a nitrogen-boron substitutional defect (N-B), which generates a mid-gap state, can enhance UVC (ultraviolet C) absorption and PFCA oxidation. This work provides insight into the PFCA oxidation mechanism and reveals engineering strategies to design better photocatalysts for PFCA degradation.