Wateroxidation catalysed by manganese compounds: from complexes to ‘biomimetic rocks’

Abstract

One of the most fundamental processes of the natural photosynthetic reaction sequence is the light-driven oxidation of water to molecular oxygen. In vivo, this reaction takes place in the large protein ensemble Photosystem II, where a μ-oxido-Mn(4)Ca- cluster, the oxygen-evolving-complex (OEC), has been identified as the catalytic site for the four-electron/four-proton redox reaction of water oxidation. This Perspective presents recent progress for three strategies which have been followed to prepare functional synthetic analogues of the OEC: (1) the synthesis of dinuclear manganese complexes designed to act as water-oxidation catalysts in homogeneous solution, (2) heterogeneous catalysts in the form of clay hybrids of such Mn(2)-complexes and (3) the preparation of manganese oxide particles of different compositions and morphologies. We discuss the key observations from the studies of such synthetic manganese systems in order to shed light upon the catalytic mechanism of natural water oxidation. Additionally, it is shown how research in this field has recently been motivated more and more by the prospect of finding efficient, robust and affordable catalysts for light-driven water oxidation, a key reaction of artificial photosynthesis. As manganese is an abundant and non-toxic element, manganese compounds are very promising candidates for the extraction of reduction equivalents from water. These electrons could consecutively be fed into the synthesis of "solar fuels" such as hydrogen or methanol.

AMnO3 (A = Sr, La, Ca, Y) Perovskite Oxides as Oxygen Reduction Electrocatalysts

“…3c) to 2+ [14,15,18]. The CV of CaMnO 3 is characterised by a broad reduction peak, ascribed to the reduction of + 4 to + 2 [14,43,44]. A similar response is observed from SrMnO 3 , although current magnitude is significantly weaker.…”

Metal-free organic sensitizers for use in water-splitting dye-sensitized photoelectrochemical cells

“…Much effort has been devoted to the development of earth-abundant water oxidation catalysts (11)(12)(13)(14), including a recently reported completely organic catalyst (15). Less attention has been paid to the development of earth-abundant sensitizers-an important problem for WS-DSPECs where absorber-to-catalyst ratios can exceed 1,000:1.…”

Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide

“…30,31 The current interest in water oxidation catalysts based on earth-abundant metals has stimulated researchers to explore in more detail the usage of Mnbased electrocatalysts for water oxidation. 5,17,20,32,33 The procedures for synthesis of the catalysts range from electrodeposition routes at constant potential (originally developed for Mn oxides designed for use in batteries) 20 through more intricate electrodeposition routes involving rotating electrodes and voltage-cycling protocols 17 to immobilization of pre-synthesized molecular or colloidal Mn-based catalysts on the electrode surface. 5,16,32 The goal of the present study is to pave the road for usage of electrodeposited Mn-based catalysts in water-splitting devices similar to the ''artificial leaf'' presented by Nocera and coworkers.…”