Water-oxidation catalysis by synthetic manganese oxides--systematic variations of the calcium birnessite theme

Layered manganese oxides from the birnessite mineral family have been identified as promising heterogeneous compounds for water-oxidation catalysis (WOC), a key reaction for the conversion of renewable energy into storable fuels. High catalytic rates were especially observed for birnessites which co...

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Published in:Dalton transactions : an international journal of inorganic chemistry 2014-03, Vol.43 (11), p.4370-4379
Main Authors: Frey, Carolin E, Wiechen, Mathias, Kurz, Philipp
Format: Article
Language:English
Subjects:
Calcium
Calcium - chemistry
Catalysis
Catalysts
Heat treatment
Manganese Compounds - chemistry
Manganese oxides
Oxidation-Reduction
Oxides - chemical synthesis
Oxides - chemistry
Precipitates
Renewable energy
Tempering
Water - chemistry
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Summary:Layered manganese oxides from the birnessite mineral family have been identified as promising heterogeneous compounds for water-oxidation catalysis (WOC), a key reaction for the conversion of renewable energy into storable fuels. High catalytic rates were especially observed for birnessites which contain calcium as part of their structures. With the aim to systematically improve the catalytic performance of such oxide materials, we used a flexible synthetic route to prepare three series of calcium birnessites, where we varied the calcium concentrations, the ripening times of the original precipitates and the temperature of the heat treatment following the initial synthetic steps (tempering) during the preparation process. The products were carefully analysed by a number of analytical techniques and then probed for WOC activity using the Ce(4+)-system. We find that our set of twenty closely related manganese oxides shows large, but somewhat systematic alterations in catalytic rates, indicating the importance of synthesis parameters for maximum catalytic performance. The catalyst of the series for which the highest water-oxidation rate was found is a birnessite of medium calcium content (Ca : Mn ratio 0.2 : 1) that had been subjected to a tempering temperature of 400 °C. On the basis of the detailed analysis of the results, a WOC reaction scheme for birnessites is proposed to explain the observed trends in reactivity.
ISSN:1477-9226
1477-9234
DOI:10.1039/c3dt52604f