Photoinduced Hydrodefluorination Mechanisms of Perfluorooctanoic Acid by the SiC/Graphene Catalyst

Cleavage of the strong carbon-fluorine bonds is critical for elimination of perfluorooctanoic acid (PFOA) from the environment. In this work, we investigated the decomposition of PFOA with the SiC/graphene catalyst under UV light irradiation. The decomposition rate constant (k) with SiC/graphene was 0.096 h(-1), 2.2 times higher than that with commercial nano-TiO2. Surface fluorination on SiC/graphene was analyzed by X-ray photoelectron spectroscopy (XPS), revealing the conversions of Si-H bonds into Si-F bonds. A different route was found to generate the reactive Si-H bonds on SiC/graphene, substituting for silylium (R3Si+) to activate C-F bonds. During the activation process, photogenerated electrons on SiC transfer rapidly to perfluoroalkyl groups by the medium of graphene, further reducing the electron cloud density of C-F bonds to promote the activation. The hydrogen containing hydrodefluorination intermediates including (CF3(CF2)(2)CFH, CF3(CF2)(3)CH2, CF3(CF2)(4)CH2, and CF3(CF2)(4)CFHCOOH) were detected to verify the hydrodefluorination process. The photoinduced hydrodefluorination mechanisms of PFOA can be consequently inferred as follows: (1) fluorine atoms in perfluoroalkyl groups were replaced by hydrogen atoms due to the nucleophilic substitution reaction via the Si-H/C-F redistribution, and (2) generation of CH2 carbene from the hydrogen-containing perfluoroalkyl groups and the C-C bonds scission by the Photo-Kolbe decarboxylation reaction under UV light excitation. This photoinduced hydrodefluorination provides insight into the photocatalytic decomposition of perfluorocarboxylic acids (PFCAs) in an aqueous environment.