Fenton chemistry at aqueous interfaces

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 111; no. 2; pp. 623 - 628
• Main Authors: Enami, Shinichi, Sakamoto, Yosuke, Colussi, Agustin J.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 14-01-2014; National Acad Sciences
• Subjects: Aerosols; Anions; Aqueous chemistry; Atmospheric chemistry; Hydrogen; Hydrogen Peroxide - chemistry; Ions; Iron; Iron - chemistry; Iron Compounds - chemical synthesis; Iron Compounds - chemistry; Mass spectra; Mass Spectrometry; Nanoparticles; Oxidation; Oxidation-Reduction; Ozone; Peroxides; Physical Sciences; Reactivity; Water - chemistry; advanced oxidation processes; nanoparticles; aerosols; metal ions; reactive oxygen species
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1314885111

In a fundamental process throughout nature, reduced iron unleashes the oxidative power of hydrogen peroxide into reactive intermediates. However, notwithstanding much work, the mechanism by which Fe ²⁺ catalyzes H ₂O ₂ oxidations and the identity of the participating intermediates remain controversial. Here we report the prompt formation of O=Fe ᴵⱽCl ₃⁻ and chloride-bridged di-iron O=Fe ᴵⱽ·Cl·Fe ᴵᴵCl ₄⁻ and O=Fe ᴵⱽ·Cl·Fe ᴵᴵᴵCl ₅⁻ ferryl species, in addition to Fe ᴵᴵᴵCl ₄⁻, on the surface of aqueous FeCl ₂ microjets exposed to gaseous H ₂O ₂ or O ₃ beams for <50 μs. The unambiguous identification of such species in situ via online electrospray mass spectrometry let us investigate their individual dependences on Fe ²⁺, H ₂O ₂, O ₃, and H ⁺ concentrations, and their responses to tert -butanol (an ·OH scavenger) and DMSO (an O-atom acceptor) cosolutes. We found that (i) mass spectra are not affected by excess tert -butanol, i.e., the detected species are primary products whose formation does not involve ·OH radicals, and (ii) the di-iron ferryls, but not O=Fe ᴵⱽCl ₃⁻, can be fully quenched by DMSO under present conditions. We infer that interfacial Fe(H ₂O) ₙ²⁺ ions react with H ₂O ₂ and O ₃ >10 ³ times faster than Fe(H ₂O) ₆²⁺ in bulk water via a process that favors inner-sphere two-electron O-atom over outer-sphere one-electron transfers. The higher reactivity of di-iron ferryls vs. O=Fe ᴵⱽCl ₃⁻ as O-atom donors implicates the electronic coupling of mixed-valence iron centers in the weakening of the Fe ᴵⱽ–O bond in poly-iron ferryl species.