Overall water splitting using powdered photocatalysts is a promising approach to large-scale solar hydrogen production. This Review details recent developments in particulate photocatalysts for overall water splitting based on one- and two-step photoexcitation systems.

Particulate photocatalysts for overall water splitting

Water oxidation is a key chemical transformation for the conversion of solar energy into chemical fuels Our review focuses on recent work on robust earth-abundant heterogeneous catalysts for the oxygen-evolving reaction (OER) We point out that improvements in the performance of OER catalysts will depend critically on the success of work aimed at understanding reaction barriers based on atomic-level mechanisms We highlight the challenge of obtaining acid-stable OER catalysts, with proposals for elements that could be employed to reach this goal We suggest that future advances in solar fuels science will be accelerated by the development of new methods for materials synthesis and characterization, along with in-depth investigations of redox mechanisms at catalytic surfaces

Earth-Abundant Heterogeneous Water Oxidation Catalysts

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn–Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25–90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn–Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn com...

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

Molecular catalysis plays an essential role in both natural and artificial photosynthesis (AP). However, the field of molecular catalysis for AP has gradually declined in recent years because of doubt about the long-term stability of molecular-catalyst-based devices. This review summarizes the development history of molecular-catalyst-based AP, including the fundamentals of AP, molecular catalysts for water oxidation, proton reduction and CO2 reduction, and molecular-catalyst-based AP devices, and it provides an analysis of the advantages, challenges, and stability of molecular catalysts. With this review, we aim to highlight the following points: (i) an investigation on molecular catalysis is one of the most promising ways to obtain atom-efficient catalysts with outstanding intrinsic activities; (ii) effective heterogenization of molecular catalysts is currently the primary challenge for the application of molecular catalysis in AP devices; (iii) development of molecular catalysts is a promising way to solve the problems of catalysis involved in practical solar fuel production. In molecular-catalysis-based AP, much has been attained, but more challenges remain with regard to long-term stability and heterogenization techniques.

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Artificial photosynthesis: opportunities and challenges of molecular catalysts

Photocatalysis promises a solution to challenges associated with the intermittent nature of sunlight which is considered as renewable and ultimate energy source to power activities on Earth. This review aims to provide a broad overview of the field. Insight into natural photosynthesis is discussed first, which provides a scientific basis for most efforts on photocatalysis. Afterwards, the details of four existing types of photocatalysis are presented, namely photosynthesis by plants, photosynthesis by microalgae, photocatalysis by suspension and photoelectrocatalysis. Detailed analyses of simple photocatalysts and integrated photocatalytic systems are followed to shed light on the different functionalities of different components in a working photocatalyst. Special attention is given to the roles played by surface and interface chemical phenomena. Lastly, perspectives on artificial photosynthesis are discussed briefly at the end.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/aenm.201700841

Photocatalysis: Basic Principles, Diverse Forms of Implementations and Emerging Scientific Opportunities

Photosynthetic splitting of water into oxygen by plants, algae, and cyanobacteria is catalyzed by the oxygen-evolving center (OEC). Synthetic mimics of the OEC, which is composed of an asymmetric manganese-calcium-oxygen cluster bound to protein groups, may promote insight into the structural and chemical determinants of biological water oxidation and lead to development of superior catalysts for artificial photosynthesis. We synthesized a Mn4Ca-cluster similar to the native OEC in both the metal-oxygen core and the binding protein groups. Like the native OEC, the synthetic cluster can undergo four redox transitions and shows two magnetic resonance signals assignable to redox and structural isomerism. Comparison with previously synthesized Mn3CaO4-cubane clusters suggests that the fourth Mn ion determines redox potentials and magnetic properties of the native OEC.

https://science.sciencemag.org/content/sci/348/6235/690.full.pdf

A synthetic Mn4Ca-cluster mimicking the oxygen-evolving center of photosynthesis

A synthetic model for the oxygen-evolving complex in Sr2+-containing photosystem II

Impact of obstructive sleep apnea on cancer risk: a systematic review and meta-analysis

The Cover Feature shows the synthesizing mechanism of the artificial Mn4Ca-cluster, a close mimic of the photosynthetic oxygen-evolving center (OEC). It is a great challenge and long-standing issue to reveal the synthesizing mechanism of the Mn4Ca-cluster in both natural and artificial photosynthesis. New results shed light on the synthesizing mechanism of the OEC and the rational design of artificial water-splitting catalysts. More information can be found in the Research Article by C. Chen, B. Xu, et al. 

Synthesizing Mechanism of the Mn
 4
 Ca Cluster Mimicking the Oxygen‐Evolving Center in Photosynthesis

The oxygen-evolving center (OEC) in photosynthesis is a unique Mn 4 CaO 5 -cluster that catalyzes the water-splitting reaction in nature. Understanding its catalytic mechanism for the O=O bond formation is of great challenge and long-standing issue, which is severely restricted by the lack of precise structure and mechanism mimics of this heterometallic-oxide cluster. Herein, we report two synthetic (Mn 3 XO 4 ) 2 O-clusters (X = Sr 2+ , La 3+ ) that closely mimic the heterometallic-oxide Mn 3 XO 4 cubane and three different types of μ-oxide bridges ( μ 2 -O 2 - , μ 3 -O 2 - , and μ 4 -O 2 - ) simultaneously as seen in the OEC. By resolving the crystal structures of both oxidized and reduced forms of the cluster, we have identified significant redox-induced structural changes that take place on the μ 2 -oxide bridge, rather than the μ 4 -oxide or μ 3 -oxide bridges. Our results provide chemical insights into understanding the reactivity of three different types of oxide bridges in the biological Mn 4 CaO 5 -cluster in PSII.

Changhui Chen

Papers

A synthetic Mn4Ca-cluster mimicking the oxygen-evolving center of photosynthesis

A synthetic model for the oxygen-evolving complex in Sr2+-containing photosystem II

Impact of obstructive sleep apnea on cancer risk: a systematic review and meta-analysis

Synthesizing Mechanism of the Mn 4 Ca Cluster Mimicking the Oxygen‐Evolving Center in Photosynthesis

Redox-Induced Structural Change in Artificial Heterometallic-Oxide Cluster Mimicking the Photosynthetic Oxygen-Evolving Center.