Molecular catalysts for artificial photosynthesis: general discussion (original) (raw)
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Photosynthesis is considered to be one of the promising areas of cheap and environmentally friendly energy. Photosynthesis involves the process of water oxidation with the formation of molecular oxygen and hydrogen as byproducts. The aim of the present article is to review the energy (light) phase of photosynthesis based on the published X-ray studies of photosystems I and II (PS-I and PS-II). Using modern ideas about semiconductors and biological semiconductor structures, the mechanisms of H+, O2↑, e− generation from water are described. At the initial stage, PS II produces hydrogen peroxide from water as a result of the photoenzymatic reaction, which is oxidized in the active center of PS-II on the Mn4CaO5 cluster to form O2↑, H+, e−. Mn4+ is reduced to Mn2+ and then oxidized to Mn4+ with the transfer of reducing the equivalents of PS-I. The electrons formed are transported to PS-I (P 700), where the electrochemical reaction of water decomposition takes place in a two-electrode el...
Studies on Water Oxidation Catalysis and Mechanism toward Artificial Photosynthesis
2014
Water oxidation by cerium(IV) ammonium nitrate, CAN, with [Ir(Cp)(4,4’-R2-2,2’-bipyridine)(H2O)] (R = OH, OMe, Me or COOH) to evolve oxygen has been investigated together with the possible oxidation of the ligands by CAN. The apparent catalytic activity is highly dependent on the substituent R and the highest catalytic activity was obtained when R = OH. The apparent turnover frequency (TOF) of the catalytic water oxidation by CAN with [Ir(Cp){4,4’-(OH)2-2,2’bipyridine}(H2O)], which acts as a precatalyst, gradually increased during the reaction to reach the highest value among the Ir complexes. In the second run, the apparent TOF value was the highest from the beginning of the reaction. H NMR and dynamic light scattering measurements for solutions after the first run indicated formation of insoluble nanoparticles, which exhibited a much higher catalytic activity as compared with iridium oxide prepared by a conventional method. The 4,4’-R2-2,2’-bipyridine ligand was also efficiently o...
Oxidation of water in photosynthesis
Bioelectrochemistry and Bioenergetics, 1984
A model for the oxidation of water in chloroplasts is suggested. This model is based on certain physico-chemical principles and it accommodates the experimental results associated with the O,-evolving system. The model consists of two pools of heterogeneously bound manganese. Two manganese ions are bound to an enzyme system containing a water-splitting site and four manganese ions are bound to another protein system which connects the reaction centre of photosystem II to the water-splitting enzyme. The latter pool of manganese acts as an electron carrier and an electron trap. It is proposed that water is oxidised in the enzyme system to form H202 and 0; as transient species, and finally liberates 0,. This pathway is energetically feasible. The model explains the flash yield kinetics of O,-evolution and the action of various artificial electron donors and inhibitors of photosystem II. It gives a molecular picture and the quantum requirements of the redox reactions associated with the process of Os-evolution.
Journal of Molecular Catalysis A: Chemical, 2011
We report that a model complex of the Photosystem II-Water Oxidizing Complex (PSII-WOC) facilitates, exclusively, photocatalytic water oxidation from non-potable water sources like seawater, under suitable conditions. When the manganese cubane cluster [Mn 4 O 4 L 6 ] + , (L = (p-MeO-Ph) 2 PO 2 ), 1 + , is incorporated within a Nafion membrane deposited on an electrolytic anode that is poised at 1.00 V (vs. Ag/AgCl) and illuminated with light, catalysis of only water oxidation is observed in aqueous solutions of sodium chloride, including seawater. No chlorine formation can be detected. This effect is comparable to the ability of the PSII-WOC in marine and hypersaline organisms to catalyze, exclusively, water oxidation with chloride present within the WOC as an essential cofactor for activity. It stands in clear contrast to commercial water electrolyzers which generate chlorine gas at their anodes when filled with seawater. Investigations suggest that this effect originates largely in electrostatic repulsion of anionic chloride ions by the Nafion support. In this respect it also appears similar to the PSII-WOC, which harnesses a proteinaceous, proton-conducting environment with high cation affinity in its active site. Solar seawater electrolysis of this type offers a potentially unlimited source of clean hydrogen fuel for a future, pollutionfree economy.
REVIEW Artificial Photosynthesis Molecular Systems for Catalytic Water Oxidation
The Water Oxidation Catalyst − Catalytic Complexity at the Frontier. At a first glance, the oxidation of H 2 O seems to be a straightforward transformation, due to the structural simplicity of starting material and products. However, this task is definitely not easily accomplished, and the majority of redox processes in nature are single-or two-electron processes. The oxidation of H 2 O is an exception because it Scheme 2. Simplified Picture of the Photochemistry of [Ru(bpy) 3 ] 2+ Figure 5. Structural examples of artificial [FeFe]-hydrogenase (2) and cobalt diglyoxime complex (cobaloxime, 3).