Water-oxidation catalysis by manganese in a geochemical-like cycle

Abstract

Water oxidation in all oxygenic photosynthetic organisms is catalysed by the Mn₄CaO₄ cluster of Photosystem II. This cluster has inspired the development of synthetic manganese catalysts for solar energy production. A photoelectrochemical device, made by impregnating a synthetic tetranuclear-manganese cluster into a Nafion matrix, has been shown to achieve efficient water oxidation catalysis. We report here in situ X-ray absorption spectroscopy and transmission electron microscopy studies that demonstrate that this cluster dissociates into Mn(II) compounds in the Nafion, which are then reoxidized to form dispersed nanoparticles of a disordered Mn(III/IV)-oxide phase. Cycling between the photoreduced product and this mineral-like solid is responsible for the observed photochemical water-oxidation catalysis. The original manganese cluster serves only as a precursor to the catalytically active material. The behaviour of Mn in Nafion therefore parallels its broader biogeochemistry, which is also dominated by cycles of oxidation into solid Mn(III/IV) oxides followed by photoreduction to Mn²⁺.

Dye-sensitized PS- b -P2VP-templated nickel oxide films for photoelectrochemical applications

“…It, however, requires a fine regulation of photon, electron and proton management between both photocatalytic systems, which can be achieved through the grafting of the active components at the surface of transparent conductive electrode substrates [22]. Extending the n-type dye-sensitized solar cell (DSSC) technology, significant achievements in this direction have been reported recently regarding the preparation of molecular-based photoanodes for water oxidation [23][24][25][26][27][28][29]. Co-grafting of a water-oxidizing catalyst with a molecular dye on mesoscopic TiO 2 substrates yielded electrodes able to deliver up to 2 mA cm 22 photocurrent corresponding to O 2 evolution under visible irradiation [27].…”

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.…”

Preparation and Characterization of Catalysts for Clean Energy: A Challenge for X-rays and Electrons

“…From the EXAFS of the material, it could be shown that the short range connectivity of the material resembled that of the manganese oxide phase birnessite. [59] However, it was not known whether the EXAFS was from molecular-sized manganese clusters or an oxide material. To further examine whether manganese oxide particles had formed in the Nafion film, high-resolution TEM studies were undertaken on both the oxidized and reduced states of the Nafion films doped with either [Mn 4 O 4 L 6 ] þ and Mn 2+ from an acidified solution.…”

“…[59] In addition to discovering the origin of the catalytic material, XAS experiments were used to show that the origin of the photocatalysis was from photo-reduction of the birnessite-like phase to Mn 2þ , providing a biogeochemical-like cycle for the observed water oxidation mechanism. [59] In nature, such light sensitive manganese oxide materials are produced by bacteria, and then dissolve on irradiation forming Mn 2+ and oxygen. [60][61][62] …”