Issue 7, 2016

Water oxidation catalysis – role of redox and structural dynamics in biological photosynthesis and inorganic manganese oxides

Abstract

Water oxidation is pivotal in biological photosynthesis, where it is catalyzed by a protein-bound metal complex with a Mn4Ca-oxide core; related synthetic catalysts may become key components in non-fossil fuel technologies. Going beyond characterization of the catalyst resting state, we compare redox and structural dynamics of three representative birnessite-type Mn(Ca) oxides (catalytically active versus inactive; with/without calcium) and the biological catalyst. In the synthetic oxides, Mn oxidation was induced by increasingly positive electrode potentials and monitored by electrochemical freeze-quench and novel time-resolved in situ experiments involving detection of X-ray absorption and UV-vis transients, complemented by electrochemical impedance spectroscopy. A minority fraction of Mn(III) ions present at catalytic potentials is found to be functionally crucial; calcium ions are inessential but tune redox properties. Redox-state changes of the water-oxidizing Mn oxide are similarly fast as observed in the biological catalyst (<10 ms), but 10–100 times slower in the catalytically inactive oxide. Surprisingly similar redox dynamics of biological catalyst and water-oxidizing Mn(Ca) oxides suggest that in both catalysts, rather than direct oxidation of bound water species, oxidation equivalents are accumulated before onset of the multi-electron O–O bond formation chemistry in Mn(III)–Mn(IV) oxidation steps coupled to changes in the oxo-bridging between metal ions. Aside from the ability of the bulk oxide to undergo Mn oxidation-state changes, we identify two further, likely interrelated prerequisites for catalytic activity of the synthetic oxides: (i) the presence of Mn(III) ions at catalytic potentials preventing formation of an inert all-Mn(IV) oxide and (ii) fast rates of redox-state changes approaching the millisecond time domain.

Graphical abstract: Water oxidation catalysis – role of redox and structural dynamics in biological photosynthesis and inorganic manganese oxides

Supplementary files

Article information

Article type
Paper
Submitted
26 Apr 2016
Accepted
09 Jun 2016
First published
10 Jun 2016

Energy Environ. Sci., 2016,9, 2433-2443

Water oxidation catalysis – role of redox and structural dynamics in biological photosynthesis and inorganic manganese oxides

I. Zaharieva, D. González-Flores, B. Asfari, C. Pasquini, M. R. Mohammadi, K. Klingan, I. Zizak, S. Loos, P. Chernev and H. Dau, Energy Environ. Sci., 2016, 9, 2433 DOI: 10.1039/C6EE01222A

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