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