Studies of the Di-iron(VI) Intermediate in Ferrate-Dependent Oxygen Evolution from Water

Reaction of ferrate(VI) with ABTS and self-decay of ferrate(VI): kinetics and mechanisms

Reactions of ferrate(VI) during water treatment generate perferryl(V) or ferryl(IV) as primary intermediates. To better understand the fate of perferryl(V) or ferryl(IV) during ferrate(VI) oxidation, this study investigates the kinetics, products, and mechanisms for the reaction of ferrate(VI) with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) and self-decay of ferrate(VI) in phosphate-buffered solutions. The oxidation of ABTS by ferrate(VI) via a one-electron transfer process produces ABTS(•+) and perferryl(V) (k = 1.2 × 10(6) M(-1) s(-1) at pH 7). The perferryl(V) mainly self-decays into H2O2 and Fe(III) in acidic solution while with increasing pH the reaction of perferryl(V) with H2O2 can compete with the perferryl(V) self-decay and produces Fe(III) and O2 as final products. The ferrate(VI) self-decay generates ferryl(IV) and H2O2 via a two-electron transfer with the initial step being rate-limiting (k = 26 M(-1) s(-1) at pH 7). Ferryl(IV) reacts with H2O2 generating Fe(II) and O2 and Fe(II) is oxidized by ferrate(VI) producing Fe(III) and perferryl(V) (k = ∼10(7) M(-1) s(-1)). Due to these facile transformations of reactive ferrate(VI), perferryl(V), and ferryl(IV) to the much less reactive Fe(III), H2O2, or O2, the observed oxidation capacity of ferrate(VI) is typically much lower than expected from theoretical considerations (i.e., three or four electron equivalents per ferrate(VI)). This should be considered for optimizing water treatment processes using ferrate(VI).

Electrocatalytic water oxidation by a monomeric amidate-ligated Fe(III)-aqua complex

The six-coordinate Fe(III)-aqua complex [Fe(III)(dpaq)(H2O)](2+) (1, dpaq is 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamido) is an electrocatalyst for water oxidation in propylene carbonate-water mixtures. An electrochemical kinetics study has revealed that water oxidation occurs by oxidation to Fe(V)(O)(2+) followed by a reaction first order in catalyst and added water, respectively, with ko = 0.035(4) M(-1) s(-1) by the single-site mechanism found previously for Ru and Ir water oxidation catalysts. Sustained water oxidation catalysis occurs at a high surface area electrode to give O2 through at least 29 turnovers over an 15 h electrolysis period with a 45% Faradaic yield and no observable decomposition of the catalyst.

Molecular water oxidation mechanisms followed by transition metals: state of the art

One clean alternative to fossil fuels would be to split water using sunlight. However, to achieve this goal, researchers still need to fully understand and control several key chemical reactions. One of them is the catalytic oxidation of water to molecular oxygen, which also occurs at the oxygen evolving center of photosystem II in green plants and algae. Despite its importance for biology and renewable energy, the mechanism of this reaction is not fully understood. Transition metal water oxidation catalysts in homogeneous media offer a superb platform for researchers to investigate and extract the crucial information to describe the different steps involved in this complex reaction accurately. The mechanistic information extracted at a molecular level allows researchers to understand both the factors that govern this reaction and the ones that derail the system to cause decomposition. As a result, rugged and efficient water oxidation catalysts with potential technological applications can be developed. In this Account, we discuss the current mechanistic understanding of the water oxidation reaction catalyzed by transition metals in the homogeneous phase, based on work developed in our laboratories and complemented by research from other groups. Rather than reviewing all of the catalysts described to date, we focus systematically on the several key elements and their rationale from molecules studied in homogeneous media. We organize these catalysts based on how the crucial oxygen-oxygen bond step takes place, whether via a water nucleophilic attack or via the interaction of two M-O units, rather than based on the nuclearity of the water oxidation catalysts. Furthermore we have used DFT methodology to characterize key intermediates and transition states. The combination of both theory and experiments has allowed us to get a complete view of the water oxidation cycle for the different catalysts studied. Finally, we also describe the various deactivation pathways for these catalysts.

A mononuclear copper electrocatalyst for both water reduction and oxidation

The oxidation and reduction of water is a key challenge in the production of chemical fuels from electricity.

Competitive oxygen-18 kinetic isotope effects expose O–O bond formation in water oxidation catalysis by monomeric and dimeric ruthenium complexes

Competitive ¹⁸O KIEs on water oxidation catalysis provide a probe of transition states for O–O bond formation.