Fenton reaction mechanism generating no OH radicals in Nafion membrane decomposition

AbstractMechanism of Fenton reaction, which is a most widely-used degradation test for organic materials using hydrogen peroxide (H$$_2$$ 2 O$$_2$$ 2 ) and iron (Fe) cations, is revealed for the decomposition of hydrated Nafion membrane. This reaction mechanism has been assumed to generate OH radicals. For a doubly-hydrated Nafion membrane model, Fenton reaction with divalent and monovalent Fe (Fe$$^{2+}$$ 2 + and Fe$$^+$$ + ) cation hydration complexes is explored for experimentally-supported hydration numbers using long-range correction for density functional theory. As a result, it is found that H$$_2$$ 2 O$$_2$$ 2 coordinating to the Fe$$^{2+}$$ 2 + hydration complexes first approaches Nafion side chains in high humidity, then leads to the C–S bond dissociation of the side chain to produce carbonic acid group and sulfonic acid ion. On the other hand, once electron transfer proceeds between iron ions, the O–O bond of the coordinating H$$_2$$ 2 O$$_2$$ 2 is extended, then the C–S bond is dissociated to produce trihydroxymethyl group and sulfur trioxide, which are rapidly transformed to carboxyl group and sulfonic acid ion in aquo. This mechanism is confirmed by the vibrational spectrum analysis of the decomposed product. Collective Nafion decomposition mechanisms also suggest that the decomposition reaction uses the recycle of generated Fe cation hydration complexes under acidic condition near membrane surface.