Nano-sized manganese oxides as biomimetic catalysts for water oxidation in artificial photosynthesis: a review

Abstract

There has been a tremendous surge in research on the synthesis of various metal compounds aimed at simulating the water-oxidizing complex (WOC) of photosystem II (PSII). This is crucial because the water oxidation half reaction is overwhelmingly rate-limiting and needs high over-voltage (approx. 1 V), which results in low conversion efficiencies when working at current densities required for hydrogen production via water splitting. Particular attention has been given to the manganese compounds not only because manganese has been used by nature to oxidize water but also because manganese is cheap and environmentally friendly. The manganese-calcium cluster in PSII has a dimension of about approximately 0.5 nm. Thus, nano-sized manganese compounds might be good structural and functional models for the cluster. As in the nanometre-size of the synthetic models, most of the active sites are at the surface, these compounds could be more efficient catalysts than micrometre (or bigger) particles. In this paper, we focus on nano-sized manganese oxides as functional and structural models of the WOC of PSII for hydrogen production via water splitting and review nano-sized manganese oxides used in water oxidation by some research groups.

Scheme 1.

During water oxidation, in addition to oxygen evolution, electrons and protons are also produced, which could be used in synthesizing fuel and useful compounds. (Online version in colour.)

Figure 1.

The whole structure of the CaMn₄O₅(H₂O)₄ cluster resembles a distorted chair, with the asymmetric cubane. There is only a small fraction of the residues that come in direct contact with the manganese–calcium cluster. In other words, the structure could be considered as a nano-sized manganese–calcium oxide in protein environments. The image was made with visual molecular dynamics (VMD) and is owned by the Theoretical and Computational Biophysics Group, NIH Resource for Macromolecular Modelling and Bioinformatics, at the Beckman Institute, University of Illinois at Urbana-Champaign. (a) The original data for figure 1 are from Kamiya & Shen [18] (PDB: 3ARC). (b) CaMn₄O₅(H₂O)₄ cluster and the surrounding amino acids. (Online version in colour.)

Scheme 2.

A comparison of the reactions involved in water oxidation in the presence of (a) [Ru(II)(bpy)₃]³⁺ and (b) chlorophyll. The biological water oxidation process involves three basic steps. First, trapping of light energy by chlorophyll pigments and rapid energy transfer to the reaction centre Chl, resulting in its oxidation to Chl⁺. Second, rapid electron donation to Chl⁺, through an oxidizable protein side-chain tyrosine 161 in D1 peptide. Third, oxidation of water to molecular oxygen within the WOC. (Online version in colour.)

Figure 2.

(a) SEM micrographs of particles after the aggregation of colloidal particles. (b) Visible spectrum of soluble form of nano-sized colloidal manganese(IV) oxide. An image of soluble form of nano-sized colloidal manganese(IV) oxide in a tube. (Online version in colour.)

Figure 3.

TEM images of (a) nano-sized layered aluminium–manganese, (b) layered manganese–calcium and (c) layered zinc–manganese oxides.

Scheme 3.

The proposed mechanism of water oxidation in the reaction of manganese oxides and Ce(IV) or [Ru(bpy)₃]³⁺ as an oxidant. Proposed role(s) of calcium, aluminium or zinc could be facilitating high oxidation states and/or the formation a moderate structure for water oxidation at manganese oxides. Oxidized manganese ions are shown in pink. (Online version in colour.)

Figure 4.

(a) SEM and (b) TEM images of manganese oxide prepared by decomposition of manganese nitrate solution at 100°C for 24 h.

Figure 5.

TEM images of the manganese oxide prepared in the presence of (a,b) BSA, (c,d) manganese oxide–BSA film and (e,f) HRTEM images of manganese oxide–BSA film. (g) A three-dimensional image of bovine serum albumin molecule. Imidazole groups as proposal sites for interaction with manganese ions are shown in yellow. (Online version in colour.)