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All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

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A binuclear manganese molecular complex [(OH2)(terpy)Mn(μ-O)2Mn(terpy)(OH2)](3+) (1) is the most prominent structural and functional model of the water-oxidizing Mn complex operating in plants and cyanobacteria. Supported on montmorillonite clay and using Ce(IV) as a chemical oxidant, 1 has been reported to be one of the best Mn-based molecular catalysts toward water oxidation. By X-ray absorption spectroscopy and kinetic analysis of the oxygen evolution reaction, we show that [(OH2)(terpy)Mn(μ-O)2Mn(terpy)(OH2)](3+) is transformed into layered type Mn-oxide particles which are the actual water oxidation catalyst.

Oxygen evolution catalysed by calcium manganese and manganese-only oxides was studied in (18)O-enriched water. Using membrane-inlet mass spectrometry, we monitored the formation of the different O(2) isotopologues (16)O(2), (16)O(18)O and (18)O(2) in such reactions simultaneously with good time resolution. From the analysis of the data, we conclude that entirely different pathways of dioxygen formation catalysis exist for reactions involving hydrogen peroxide (H(2)O(2)), hydrogen persulfate (HSO(5)(-)) or single-electron oxidants such as Ce(IV) and [Ru(III) (bipy)(3)](3+) . Like the studied oxide catalysts, the active sites of manganese catalase and the oxygen-evolving complex (OEC) of photosystem II (PSII) consist of μ-oxido manganese or μ-oxido calcium manganese sites. The studied processes show very similar (18)O-labelling behaviour to the natural enzymes and are therefore interesting model systems for in vivo oxygen formation by manganese metalloenzymes such as PSII.

A nano-size amorphous calcium-manganese oxide shows efficient water oxidation activity in the presence of cerium(IV) ammonium nitrate.

Previous measurements show that calcium manganese oxide nanoparticles are better water oxidation catalysts than binary manganese oxides (Mn 3 O 4 , Mn 2 O 3 , and MnO 2 ). The probable reasons for such enhancement involve a combination of factors: The calcium manganese oxide materials have a layered structure with considerable thermodynamic stability and a high surface area, their low surface energy suggests relatively loose binding of H 2 O on the internal and external surfaces, and they possess mixed-valent manganese with internal oxidation enthalpy independent of the Mn 3+ / Mn 4+ ratio and much smaller in magnitude than the Mn 2 O 3 -MnO 2 couple. These factors enhance catalytic ability by providing easy access for solutes and water to active sites and facile electron transfer between manganese in different oxidation states.nanomaterials | thermochemistry R esearch on calcium manganese oxides (CaMnOs) has been inspired by the water-oxidation centers in photosystem II (1-5), which is a manganese-calcium cluster Mn 4 CaO 5 (H 2 O) 4 supported by a protein environment. Because the capability for efficient water oxidation is unique to photosystem II among all biological photosystems (6, 7), these CaMnO materials that mimic the elemental composition, manganese oxidation state, and particle size of the photosynthetic water oxidation center are of special interest (8)(9)(10)(11)(12). Although a number of other metal compounds function as water-oxidizing catalysts (13), many contain rare and expensive metals like iridium and ruthenium; the advantage of manganese oxides (Mn-oxides) is that they are earth-abundant, inexpensive, and environmentally friendly (1)(2)(3)(4)(5)14).CaMnO phases have short-range order structure and lamellar morphology (12), which are hallmarks of phyllomanganates (classically known as hydrous Mn-oxides), and they possess a layered structure (15)(16)(17)(18)(19)(20). In this family of structures, manganese octahedra (and vacancies) form the layers, and charge-balancing cations (i.e., alkali and alkaline earth ions, protons) and water occupy the interlayer space.Our previous calorimetric studies of various oxide nanoparticles, including those of manganese, iron, and cobalt (21), show that different polymorphs and phases with different oxidation states have significantly different surface energies. For particle sizes below 100 nm, these differences affect the free energies of phase transitions and of oxidation-reduction reactions, shifting the latter by as much as several log(fO 2 ) units at a given temperature. This thermodynamic effect on redox equilibria is significant when one considers electrochemical potentials for water splitting or other catalytic reactions involving nanoparticles (14). In a chargecompensated layered structure, the Mn(III)/Mn(IV) equilibrium will be different from that of binary Mn-oxide phase assemblages, namely, Mn 3 O 4 (hausmannite), Mn 2 O 3 (bixbyite), and MnO 2 (pyrolusite). Thus, particle size, morphology, crystal structure, and mixed valence in the more complex ...