Abstract
Mechanism of Fenton reaction, which is a most widely-used degradation test for organic materials using hydrogen peroxide (H_2O_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+} 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_2O_2 coordinating to the Fe^{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_2O_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.
Highlights
Mechanism of Fenton reaction, which is a most widely-used degradation test for organic materials using hydrogen peroxide (H2O2 ) and iron (Fe) cations, is revealed for the decomposition of hydrated Nafion membrane
The results show that the Fenton reaction proceeds with the barrier of about 30 kcal/mol for the ferrous ion hydration complex, while it significantly depends on the hydration number and requires the initial O–O bond extension of H 2O2 for the Fe+ hydration complex
The mechanism of Fenton r eaction[4], which is frequently used in the degradation test of organic materials, has been theoretically investigated for the decomposition of hydrated Nafion membrane
Summary
Mechanism of Fenton reaction, which is a most widely-used degradation test for organic materials using hydrogen peroxide (H2O2 ) and iron (Fe) cations, is revealed for the decomposition of hydrated Nafion membrane. Enami and coworkers performed mass spectroscopy experiment of aqueous microjet containing Fe(II) chloride (FeCl2 ) for selectively observing low-concentrated compounds produced at aqueous interfaces with very short time scale (< 5×10−5 s) and found that the H 2O2 decomposition proceeds not the OH radical formation but the FeO formation and it provides 1000–10,000 times larger reaction rate at the interface than that in aqueous solution[26] Since this reaction mechanism, proceeds inside the hydration complex without the OH radical formation, it does not make clear how the Fe cation hydration complex decomposes organic materials such as Nafion membrane. The author and coworkers suggested a new direct H 2O2-induced decomposition mechanism of Nafion m embrane[14] This mechanism is based on the hydration structure of Nafion membrane, which was revealed by the combination of experimental and theoretical studies[33,34]. The Fenton reaction mechanism of Fe cation hydration complex, to which H 2O2 molecule is coordinated, is revealed following our study on H 2 O2 decomposition[27]
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