Artificial photosynthesis involves several mechanisms of water oxidation. Generally, the operating mechanisms in the formation of oxygen-oxygen bonds are different. The first mechanism is the water nucleophilic attack (WNA) and the second mechanism is the interaction between two metal-oxygen (M-O) entities.

In WNA, a molecule of water from the solvent attacks an oxo group from the M-O species. Here, water acts as a nucleophile and the attack leads to the formation of metal hydroperoxide (M-O-O-H). Oxygen (O2) is subsequently released and the metal-oxygen entity is reduced to metal.

In the interaction of two M-O entities, which is often referred to as a reductive elimination or radical coupling, the two metal-oxygen species react together to form M-O-O-M entities. Further water oxidation of the M-O-O-M species releases oxygen (O2). This article discusses the mechanisms of water oxidation for different water oxidation catalysts used for artificial photosynthesis.

Chemically induced water oxidation

The mononuclear ruthenium catalyst has been described as one of the best catalysts that can achieve a turnover frequency (TOF) and turnover number (TON) of 300 s-1 and 50,000 respectively, using ceric ammonium nitrate (Ce(IV) (NH4)2Ce(NO3)6) as the chemical oxidant.

In an aqueous medium, a solvent water molecule aligns with a mononuclear ruthenium catalyst complex to generate a Ru-OH2 complex with concurrent misalignment of one of the pyridyls of the equatorial 2,2-bipyridine-6,60-dicarboxylate ([bdc]2-) ligand.

Researchers at the KTH Royal Institute of Technology and the Institute of Chemical Research of Catalonia (ICIQ) developed a molecular ruthenium catalyst [Ru(bda)(isoq)2] (H2bda 5 2,2′ -bipyridine-6,6′ -dicarboxylic acid; isoq 5 isoquinoline) for water oxidation. The researchers found the catalyst to speed up the water oxidation to a very high rate of reaction with a TOF greater than 300 s-1. The center of the ruthenium metal is capable of accommodating a seventh coordinated ligand when the oxidation state of Ru(IV) is achieved. This recovers the complete coordination of the pyridyl from the [bdc]2- ligand.

Researchers at the Arrhenius Laboratory in Stockholm University and the Biophysics Group at the University of Gothenburg developed a bioinspired manganese catalyst to photosynthesize water oxidation for artificial photosynthesis. The manganese complex has the capability to oxidize water to oxygen with the use of a one-electron donor type of oxidant, such as [Ru(bpy)3] 3+. The researchers claimed that the catalyst can yield a TOF of 0.027 s-1 and a TON of 25 under the conditions of a 7.2 pH (0.1 M phosphate buffer).

Iridium complexes have been previously developed to function as water oxidation catalysts. However, there are several arguments about whether the complexes serve as catalyst precursors to other elements that perform the catalytic job or as precursors to the nanoparticles of iridium oxide, which are active water oxidation catalysts.

Electrochemically induced water oxidation

Electrochemically induced water oxidation is an activation strategy, which utilizes a potentiostat as the source of electrons to be able to apply the most desirable potential as the desired pH. There are two activation strategies that can be followed in electrochemically induced water oxidation. First, the catalyst is held firmly onto the surface of the electrode, which is a forward step to integrating the water oxidation catalyst into a photoelectrochemical cell. This activation strategy can improve the reactivity through WOC stabilization after the heterogenization of the catalyst Second, the catalyst is kept in the homogeneous phase.

Several methods can be utilized in anchoring a homogenous catalyst onto a surface. These methods include chemisorption, which is the covalent bonding of the catalyst to the surface, encapsulation, which is the immobilization of the catalyst inside a matrix, electrostatic interaction, which involves exploiting the different charges between the water oxidation catalyst and the surface, and physisorption, taking advantage of Van der Waals forces.

Researchers at the University of North Carolina at Chapel Hill developed redox mediator-catalyst assemblies to study the catalytic and surface-electrocatalytic water oxidation for artificial photosynthesis. Their research shows that the mononuclear ruthenium catalyst has the best result when linked covalently to a [Ru(bpy)3]-type redox mediator following the heterogenization strategy. The researchers covalently linked the catalysts onto an Indium Tin Oxide (ITO) conducting glass via the interaction between the phosphonate moieties and the terminal –OH surface groups. They claimed that the catalyst performed better towards water oxidation with at least a TOF and TON of 0.6 s-1 and 28,000, respectively with the application of a 0.63 V overpotential at a pH of 1.

Different researchers have shown the capability of copper catalysts to oxidize water with a high TOF and high faradic efficiency. At the University of Washington, Seattle, researchers developed a soluble mononuclear copper-bipyridine water oxidation electrocatalyst. The researchers said that the catalyst can oxidize water with a faradic efficiency of 90% and TOF of 100 s-1 when an overpotential of 0.75 V is applied at pH 13.

Photochemically induced water oxidation

Using sunlight to oxidize water is an important point needed for mimicking natural photosynthesis. Therefore, the presence of a light-harvesting system is basic to the utilization of solar energy to oxidize the water oxidation catalyst. Also, a light-harvesting system helps to accumulate the oxidative equivalents required to release oxygen.

Sunlight absorption by a photosensitizer in photochemically induced water oxidation produces an excited state (P*), which has the capability to transfer electrons to the sacrificial electron acceptor (SEA) and produce the oxidized photosensitizer (P+). The oxidized photosensitizer oxidizes the water oxidation catalyst from a low-oxidation state to a high-oxidation state.

Then, the water oxidation catalyst oxidizes water to oxygen. One of the major issues with this method is the stability of the photosensitizer. The photosensitizer can be oxidized by a single oxygen molecule produced during the catalysis from the direct interaction between sunlight and the triplet oxygen.

A recent study recommended the building of chromophore-catalyst dyad molecules as a strategy for photochemically induced water oxidation. This idea comprises of linking two metal complexes covalently with each one playing a different role. One of the metal complexes acts as the water oxidation catalyst and the other functions as a light-harvesting antenna.

Future prospects

The current research on the mechanisms of water oxidation gives insights and direction for the development of more effective water oxidation catalysts. Future research can focus on developing tunable molecular catalysts with active sites and clear structures for water oxidation. Also, research should be conducted to understand the mechanism of water oxidation on heterogeneous catalysts.

Further, future research can focus on understanding the relationship between the structure and activity of water oxidation catalysts and their chemical dynamic states, and also on the mechanism of formation of O-O bond.