Hydrated electron (e_aq⁻) generation from phenol/UV: Efficiency, influencing factors, and mechanism

A phenol/UV (253.7 nm) process to generate hydrated electron (e_aq⁻) was experimentally and theoretically studied in the present work, where monochloroacetic acid (MCAA) was selected as the probe of e_aq⁻. It was demonstrated that the e_aq⁻ generation efficiency was dependent on the phenol concentration and pH. To interpret the dependence, a mechanism for the generation of e_aq⁻ from phenol was proposed and confirmed by the quantum chemical calculations. Theoretically, phenol could eject e_aq⁻ and phenoxyl radical (C₆H₅[Formula presented]), followed by the addition of hydroxyl ion (OH⁻) to C₆H₅[Formula presented], and the simultaneous formation of phenol and p-hydroquinone was accomplished by hydrogen abstraction of the adduct with C₆H₅[Formula presented] as hydrogen acceptor (period I). The generated p-hydroquinone could also release e_aq⁻ with p-benzoquinone as the product (period II). Totally, one mole of phenol could generate four moles of e_aq⁻ via two periods, and two moles were generated in period I and two moles were in period II. Experimentally, e_aq⁻ could be ejected from phenol and phenolate, and the molar ratios of the species were determined by pH. Kinetically, the energy barriers of the electron release from phenol and phenolate were 63.7 kcal mol⁻¹ and 62.3 kcal mol⁻¹, respectively, which confirmed that the generation of e_aq⁻ from phenolate was much more efficient than that from phenol. These results may promote the development of novel e_aq⁻ reduction processes based on the phenolic compounds, since they are abundant in the environment.