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Issue 11, 2014
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Water-oxidation catalysis by synthetic manganese oxides – systematic variations of the calcium birnessite theme

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Abstract

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 Ce4+-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.

Graphical abstract: Water-oxidation catalysis by synthetic manganese oxides – systematic variations of the calcium birnessite theme

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Supplementary files

Article information


Submitted
20 Sep 2013
Accepted
01 Nov 2013
First published
01 Nov 2013

Dalton Trans., 2014,43, 4370-4379
Article type
Paper

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

C. E. Frey, M. Wiechen and P. Kurz, Dalton Trans., 2014, 43, 4370
DOI: 10.1039/C3DT52604F

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