Artificial photosynthesis: molecular systems for catalytic water oxidation.

Globally increasing energy demands and environmental concerns related to the use of fossil fuels have stimulated extensive research to identify new energy systems and economies that are sustainable, clean, low cost, and environmentally benign. Hydrogen generation from solar-driven water splitting is a promising strategy to store solar energy in chemical bonds. The subsequent combustion of hydrogen in fuel cells produces electric energy, and the only exhaust is water. These two reactions compose an ideal process to provide clean and sustainable energy. In such a process, a hydrogen evolution reaction (HER), an oxygen evolution reaction (OER) during water splitting, and an oxygen reduction reaction (ORR) as a fuel cell cathodic reaction are key steps that affect the efficiency of the overall energy conversion. Catalysts play key roles in this process by improving the kinetics of these reactions. Porphyrin-based and corrole-based systems are versatile and can efficiently catalyze the ORR, OER, and HER. Becau...

Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems

Molecular Catalysts for Water Oxidation

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal–oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal–oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and trans...

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

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

Cobalt porphyrins as homogeneous catalysts for water oxidation

The performance of a water-soluble cobalt porphyrin ([{meso-tetra(4-sulfonatophenyl)porphyrinato}cobalt(III)], CoTPPS) as a catalyst for the photoreduction of CO2 in fully aqueous media has been in...

Highly Efficient and Selective Photocatalytic CO2 Reduction to CO in Water by a Cobalt Porphyrin Molecular Catalyst

Enabling the production of solar fuels on a global scale through artificial photosynthesis requires the development of water oxidation catalysts with significantly improved stability. The stability of photosystems is often reduced owing to attack by singlet oxygen, which is produced during light harvesting. Here, we report photochemical water oxidation by CoFPS, a fluorinated Co-porphyrin designed to resist attack by singlet oxygen. CoFPS exhibits significantly improved stability relative to its non-fluorinated analogue, as shown by a large increase in turnover numbers. This increased stability results from resistance of CoFPS to attack by singlet oxygen, the formation of which was monitored in situ by using 9,10-diphenylanthracene as a chemical probe. Dynamic light scattering (DLS) confirms that CoFPS remains homogeneous, proving its stability during water oxidation catalysis.

Improving Singlet Oxygen Resistance during Photochemical Water Oxidation by Cobalt Porphyrin Catalysts

Iron tris(2-methylpyridyl)amine (TPA) and iron 1-(bis(2-methylpyridyl)amino)-2-methyl-2-propanoate (BPyA) salts are characterized as water oxidation catalysts (WOCs) using sodium periodate. Under the conditions used, these complexes serve as homogeneous WOCs as demonstrated via kinetic analysis and dynamic light scattering (DLS). The Fe(BPyA) salt serves as both a mononuclear and dinuclear catalyst, with the mononuclear form showing higher catalytic activity. Based on the H/D kinetic isotope effect and pH dependence, the rate determining step (RDS) in water oxidation (WO) by Fe(BPyA) is nucleophilic attack by water during O–O bond formation. In contrast, Fe(TPA) shows complex kinetic behavior due to the formation of multiple oxidation states of the complex in solution, each of which exhibits catalytic activity for WO. The RDS in WO by Fe(TPA) follows an equilibrium established between monomeric and dimeric forms of the catalyst. Under acidic conditions formation of the monomer is favored, which leads to an increase in the WO rate.

Mechanism of water oxidation by non-heme iron catalysts when driven with sodium periodate

Copper tetrasulfonatophthalocyanine (CuPcTS) is reported to serve as a catalyst for photochemical water oxidation via a radical coupling mechanism. Chloride greatly inhibits the water oxidation rate as a result of axial chloride binding to CuPcTS, preventing formation of the Cu oxyl or hydroxyl intermediate required for O-O bond formation.

Takashi Nakazono

Papers

Cobalt porphyrins as homogeneous catalysts for water oxidation

Highly Efficient and Selective Photocatalytic CO2 Reduction to CO in Water by a Cobalt Porphyrin Molecular Catalyst

Improving Singlet Oxygen Resistance during Photochemical Water Oxidation by Cobalt Porphyrin Catalysts

Mechanism of water oxidation by non-heme iron catalysts when driven with sodium periodate

Photochemical Water Oxidation Catalyzed by a Water‐Soluble Copper Phthalocyanine Complex