Study on the Effect of Mn Structure and Valency on Water Oxidation Catalysis

Abstract

The development of a water oxidation catalyst has been a demanding challenge for the realization of overall water-splitting systems. The asymmetric geometry and flexible ligation of the biological Mn4CaO5 cluster are important properties for the function of photosystem II, and these properties can be applied to the design of a new inorganic water oxidation catalyst. In part I, we identified a new crystal structure, Mn3(PO4)2-3H2O, that precipitates spontaneously in aqueous solution at room temperature and demonstrated its superior catalytic performance at neutral pH. Computational analysis indicated that phosphate ligations in our crystal make Mn-O bonding longer and more distorted than in other Mn-based oxides. Such structural flexibility can stabilize Jahn-Teller distorted Mn(III) and thus facilitate Mn(II) oxidation, as monitored by electron paramagnetic resonance spectroscopy. Moreover, although intensive studies have explored the role of Mn element in water oxidation catalysis, it has been difficult to understand whether the catalytic capability originates mainly from either the Mn arrangement or the Mn valency. In part II, to decouple these two factors and to investigate the role of Mn valency on catalysis, we selected a new pyrophosphate-based Mn compound (Li2MnP2O7), which has not been utilized for water oxidation catalysis to date, as a model system. Due to the monophasic behavior of Li2MnP2O7 with the delithiation, the Mn valency of Li2-xMnP2O7 (x = 0.3, 0.5, 1) can be controlled with negligible change in crystal framework (e.g. volume change ~ 1 %). Interestingly, we observed that as the averaged oxidation state of Mn in Li2-xMnP2O7 increases from 2 to 3, the catalytic performance is enhanced in the series Li2MnP2O7 < Li1.7MnP2O7 < Li1.5MnP2O7 < LiMnP2O7. Moreover, Li2MnP2O7 itself exhibits superior catalytic performance compared with MnO or MnO2 because of the highly distorted Mn geometry in Li2MnP2O7. In summary, we selected Mn3(PO4)2-3H2O and Li2-xMnP2O7 compounds as artificial platforms for understanding the effect of Mn structure and valency on water oxidation catalysis, respectively. We think that this study presents valuable guidelines for developing an efficient Mn-based catalyst which can be comparable with the biological Mn4CaO5 cluster in photosystem II under neutral conditions with controlled Mn valency and atomic arrangement.