First-Principles Study of the Degradation of Perfluorooctanesulfonate and Perfluorobutanesulfonate on a Magnéli Phase Ti<sub>4</sub>O<sub>7</sub> Anode
Lei Li, Yaye Wang, Qingguo Huang
Abstract
The anodic degradation of perfluorooctanesulfonate (PFOS) and perfluorobutanesulfonate (PFBS) on a Magnéli phase titanium suboxide (Ti4O7) surface was investigated via density functional theory simulations. The adsorption energy on the Ti4O7 cluster surface decreases in the following order: OH• > C4F9SO3– (PFBS–) ∼ C8F17SO3– (PFOS–) > H2O. This suggests that OH• formed from water oxidation strongly adsorbs on the Ti4O7 cluster surface. C8F17SO3– and C4F9SO3– adsorbed on a model of an OH•-bound Ti4O7 cluster [Ti4O7(OH)] were examined computationally in response to different anodic potentials that were simulated by varying the number of added background charges. This indicates a gradual loss of one electron from C8F17SO3– and C4F9SO3– as the background charges increase on Ti4O7(OH). This is accompanied by elongation and eventual breakage of the C–S bond in PFOS and PFBS. The conversion from C8F17SO3– to C8F17SO3• by direct electron transfer involves the largest barrier energy (1.21 eV) in the process of PFOS degradation, while the breakage of the C–S bond in C4F9SO3• is the rate-limiting step of PFBS– degradation (1.49 eV). The first-principles computation here in combination with experimental measurements elucidated the elementary steps involved in the degradation of perfluoroalkyl acids (PFAAs) on a Ti4O7 anode and its dependence on the molecular structures of PFAAs.