Electrochemical characterization of oxygen reduction by FeII[ethylenediaminetetraacetate]
By Laine, Derek F.; McAllister, Simon D. & Cheng, I. Francis
Published in Journal of Electroanalytical Chemistry
2007
Abstract
The kinetics of the reduction of O2 by FeIIEDTA is evaluated using electrochemical methods and shows a promising approach to the study of oxygen activation by metal complexes. Beneficial to this method is the use of stable FeIIIEDTA, which can be electro-reduced to generate air sensitive FeIIEDTA. The FeII/IIIEDTA oxidation states act as an electron mediator between the electrode and O2(aq). At a sweep rate of 5 mV/s, cyclic voltammeteric waves indicate an EC’ type mechanism. The kinetics of O2 reduction by FeIIEDTA is evident by the EC’ current and is dependent on pH with a maximum at pH 3. This rate quickly decreases below pH 3 and above 8 and is nearly constant between pH 4–8. This behavior is strongly correlated with the FeIIEDTA species distribution diagram indicating that the protonated FeIIEDTA–H complex is the optimal species for O2 reduction. Applying computer simulation to representative cyclic voltammograms provides rate constants for the reaction steps involved in O2 reduction by FeIIEDTA. The mechanistic steps evaluated consist of (1) the binding of O2 to FeIIEDTA, (2) the reduction of bound O2 by FeIIEDTA to produce O 2 -, and (3) further reduction of O 2 - by FeIIEDTA to produce H2O2. We report a rate constant of 1.07 X 104 M-1 s-1 for step (1), 6.23 X 109 s-1 for step (2), and 1.00 X 104 M-1 s-1 for step (3) at pH 3. The data is consistent to literature values obtained by stopped-flow techniques.