(15e) Theoretical Insight into Photo-Catalytic Pfas Degradation Mechanisms over Boron Nitride

Per- and polyfluoroalkyl substances (PFAS) are a pervasive contaminant in the environment that are energy-intensive to treat. PFAS removal through photocatalytic decomposition has emerged as an efficient strategy for eliminating PFAS because it can utilize light as a sustainable energy source and requires relatively mild reaction conditions. Working with collaborators, we recently reported that hexagonal boron nitride (h-BN) can efficiently degrade one of the most common PFAS molecules, perfluorooctanoic acid (PFOA), using UVC light [1]. Furthermore, h-BN outperforms state-of-the-art TiO₂ P25 catalysts under the same conditions. Scavenger experiments demonstrate that photo-generated holes are essential to the degradation process, yet the elementary steps of the PFOA oxidation mechanism are not clear. In this work, we apply density functional theory (DFT) in a grand canonical (GC) formalism to evaluate the thermodynamic and kinetic favorability of proposed photo-oxidative reaction steps in the PFOA degradation cycle over illuminated h-BN. We found that the computed valence band position of h-BN lies below the oxidation potential of deprotonated PFOA, which demonstrates that photo-generated holes on h-BN are sufficiently oxidizing to activate PFOA. Kinetic barriers computed with the GC-DFT formalism also demonstrate the kinetic favorability of the oxidation reaction. The ability of h-BN to degrade PFOA photo-catalytically has previously been unreported and is unexpected because its bandgap is too-large for UVC light absorption. Thus, in this work, we also investigated the role of h-BN defects and find that several stable substitutional defects can generate mid-gap states that promote the absorption of UVC light. While promoting UVC absorption, these defects do not affect the valence band position of h-BN and thus do not interfere with the oxidation of PFOA. Our work sets the theoretical basis for the design of better photo-catalysts for the disposal of PFOA from water.

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