Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 140, Issue 34, Pages 10890-10899Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jacs.8b06394
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Funding
- Center for Molecular Electrocatalysis, an Energy Frontier Research Center - U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences
- Blue Waters Graduate Fellowship
- NSF [CHE-0741901]
- Paul and Margaret Bender Fund
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A soluble, bis-ketiminate-ligated Co complex [Co(N2O2)] was recently shown to catalyze selective reduction of O-2 to H2O2 with an overpotential as low as 90 mV. Here we report experimental and computational mechanistic studies of the Co(N2O2)-catalyzed O-2 reduction reaction (ORR) with decamethylferrocene (Fc*) as the reductant in the presence of AcOH in MeOH. Analysis of the Co/O-2 binding stoichiometry and kinetic studies support an O-2 reduction pathway involving a mononuclear cobalt species. The catalytic rate exhibits a first-order kinetic dependence on [Co(N2O2)] and [AcOH], but no dependence on [Fc*] or [O-2]. Differential pulse voltammetry and computational studies support Coin-hydroperoxide as the catalyst resting state and protonation of this species as the rate-limiting step of the catalytic reaction. These results contrast previous mechanisms proposed for other Co-catalyzed ORR. systems, which commonly feature rate-limiting protonation of a Co-III-superoxide adduct earlier in the catalytic cycle. Computational studies show that protonation is strongly favored at the proximal oxygen of the Co-III(OOH) species, accounting for the high selectivity for formation of hydrogen peroxide. Further analysis shows that a weak dependence of the ORR rate on the pK(a) values of the protonated Co-III(OOH) species across a series of Co(N2O2) catalysts provides a rationale for the unusually low overpotential observed for O-2 reduction to H2O2.
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