Fenton chemistry at aqueous interfaces.

Area of Science:

• Environmental Chemistry
• Inorganic Chemistry
• Chemical Kinetics

Background:

• Iron(II) catalyzes hydrogen peroxide (H2O2) oxidations, generating reactive intermediates crucial in natural processes.
• The precise mechanism and identity of these intermediates in iron-catalyzed H2O2 reactions remain subjects of scientific debate.

Purpose of the Study:

• To elucidate the mechanism of Fe(2+) catalysis in H2O2 oxidations.
• To identify the transient reactive intermediates formed during these reactions.
• To investigate the role of interfacial versus bulk water in iron-mediated oxidation processes.

Main Methods:

• Utilized aqueous FeCl2 microjets exposed to gaseous H2O2 or O3 beams.
• Employed in situ online electrospray mass spectrometry for rapid (<50 μs) identification of reaction species.
• Investigated reaction kinetics and intermediate behavior using varying concentrations of reactants and scavengers (tert-butanol, DMSO).

Main Results:

• Identified prompt formation of O=Fe(IV)Cl3(-), chloride-bridged di-iron ferryl species (O=Fe(IV)·Cl·Fe(II)Cl4(-) and O=Fe(IV)·Cl·Fe(III)Cl5(-)), and Fe(III)Cl4(-).
• Confirmed that detected species are primary products, not involving hydroxyl radicals (·OH), as tert-butanol had no effect.
• Demonstrated that di-iron ferryls are more readily quenched by DMSO (O-atom acceptor) than O=Fe(IV)Cl3(-), indicating different reactivity.

Conclusions:

• Interfacial Fe(H2O)n(2+) ions react >1000 times faster with H2O2 and O3 than bulk Fe(H2O)6(2+), favoring inner-sphere O-atom transfer.
• Di-iron ferryl species are more potent O-atom donors than O=Fe(IV)Cl3(-), attributed to electronic coupling in mixed-valence iron centers.
• The findings provide critical insights into the mechanism of iron-catalyzed oxidation reactions and the nature of reactive ferryl species.