1 research outputs found
Molecular Cobalt Catalysts for O<sub>2</sub> Reduction: Low-Overpotential Production of H<sub>2</sub>O<sub>2</sub> and Comparison with Iron-Based Catalysts
A series of mononuclear pseudomacrocyclic
cobalt complexes have
been investigated as catalysts for O<sub>2</sub> reduction. Each of
these complexes, with Co<sup>III/II</sup> reduction potentials that
span nearly 400 mV, mediate highly selective two-electron reduction
of O<sub>2</sub> to H<sub>2</sub>O<sub>2</sub> (93–99%) using
decamethylferrocene (Fc*) as the reductant and acetic acid as the
proton source. Kinetic studies reveal that the rate exhibits a first-order
dependence on [Co] and [AcOH], but no dependence on [O<sub>2</sub>] or [Fc*]. A linear correlation is observed between logÂ(TOF) vs <i>E</i><sub>1/2</sub>(Co<sup>III/II</sup>) for the different cobalt
complexes (TOF = turnover frequency). The thermodynamic potential
for O<sub>2</sub> reduction to H<sub>2</sub>O<sub>2</sub> was estimated
by measuring the H<sup>+</sup>/H<sub>2</sub> open-circuit potential
under the reaction conditions. This value provides the basis for direct
assessment of the thermodynamic efficiency of the different catalysts
and shows that H<sub>2</sub>O<sub>2</sub> is formed with overpotentials
as low as 90 mV. These results are compared with a recently reported
series of Fe-porphyrin complexes, which catalyze four-electron reduction
of O<sub>2</sub> to H<sub>2</sub>O. The data show that the TOFs of
the Co complexes exhibit a shallower dependence on <i>E</i><sub>1/2</sub>(M<sup>III/II</sup>) than the Fe complexes. This behavior,
which underlies the low overpotential, is rationalized on the basis
of the catalytic rate law