Fenton chemistry at aqueous interfaces

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 Fe2+ catalyzes H2O2 oxidations and the identity of the participating intermediates remain controversial. Here we report the prompt formation of O=(FeCl3-)-Cl-IV and chloride-bridged di-iron O=Fe-IV center dot Cl center dot(FeCl4-)-Cl-II and O=Fe-IV center dot Cl center dot(FeCl5-)-Cl-III ferryl species, in addition to (FeCl4-)-Cl-III, on the surface of aqueous FeCl2 microjets exposed to gaseous H2O2 or O-3 beams for <50 mu s. The unambiguous identification of such species in situ via online electrospray mass spectrometry let us investigate their individual dependences on Fe2+, H2O2, O-3, and H+ concentrations, and their responses to tert-butanol (an center dot 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 center dot OH radicals, and (ii) the di-iron ferryls, but not O=(FeCl3-)-Cl-IV, can be fully quenched by DMSO under present conditions. We infer that interfacial Fe (H2O)(n)(2+) ions react with H2O2 and O-3 > 10(3) times faster than Fe(H2O)(6)(2+) 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=(FeCl3-)-Cl-IV 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.