Water oxidation to oxygen

Saladin F, Kamber I, Pfanner K, Calzaferri G. Photochemical water oxidation to oxygen at the solid/gas interface of AgCl on zeolite A. J Photochem Photobiol A Chem 1997 109 47-52. 

The reaction can take place at anode potentials largely lower than the thermodynamic potential of water oxidation to oxygen (1.23V/SHE under standard conditions). However, in order to achieve the dissociative activation of water (1.7), electrocatalytic electrodes are needed. In fact, the dissociative adsorption of water can be achieved only at electrodes (M) on which the bonding energy of M-OH and M-H exceeds the dissociation energy of water to H + HO. This is the case of Pt-Ru-based electrodes on which water activation can be achieved at 0.2-0.3 V/SHE. 

Brunschwig BS, Chou MH, Creutz C, Ghosh P, Sutin N (1983) Mechanisms of water oxidation to oxygen cobalt(IV) as an intermediate in the aquocobalt(II)-catalyzed reaction. J Am Chem Soc 105 4832-4833... 

Water oxidation to oxygen seems to be a many-electron process [81-82, 86, 87] if the reaction proceeds effectively with a high quantum yield. For this purpose it is necessary to abstract four electrons from two water molecules forming part of the reaction complex. This can be effected by a tetramer or two dimers of chlorophyll bound into a multi-center complex adsorbed at the interface. 

Figure 11 presents similar pH dependences of O2 yields with colloidal Co2+ salts as redox catalysts, reported recently hy Shafirovich et al. " Presxxmahly these redox catalysts assisted water oxidations to oxygen do not go through the same type of intermediates. 

The reason for the exponential increase in the electron transfer rate with increasing electrode potential at the ZnO/electrolyte interface must be further explored. A possible explanation is provided in a recent study on water photoelectrolysis which describes the mechanism of water oxidation to molecular oxygen as one of strong molecular interaction with nonisoenergetic electron transfer subject to irreversible thermodynamics.48 Under such conditions, the rate of electron transfer will depend on the thermodynamic force in the semiconductor/electrolyte interface to... 

This behaviour does not allow the catalytic processes of water oxidation to be detected on the time scale of cyclic voltammetry. However, during exhaustive electrolysis at the potential of the second process (Ew— +1.38 V), catalytic generation of gaseous oxygen is observed with restoration of the original complex.55... 

To compare quantitatively the current-voltage characteristic of an illuminated electrode, given by formula (31), with experimental data, Butler (1977) and Wilson (1977) measured the photocurrent, which arises in a cell with an n-type semiconductor photoanode ( 2, W03) when irradiated with monochromatic light at a frequency satisfying the condition ha>> Eg. In this case a light-stimulated electrochemical reaction of water oxidation with oxygen evolution... 

The ORR cannot be studied by feedback mode in neutral or acidic buffers. The tip-generated ORR product, that is, hydroxide ions, immediately react with aqueous protons to form water and do not have time to diffuse toward the substrate. A similar situation is encountered when the HOR is probed in neutral or alkaline media. Fernandez et al. [125] employed the TG/SC mode to study the ORR on the substrate in acidic solutions. The tip was placed close to the substrate and biased at a potential at which water was oxidized to oxygen. The substrate potential was fixed at a value corresponding to oxygen reduction to water. Under these conditions, the oxygen reduction rate at the substrate surface reflected its electrocatalytic activity. 

The invention of aerobic photosynthesis, the light-driven oxidation of water to oxygen, stands as one of the pivotal evolutionary innovations in the history of life on Earth. The process is carried out only at the oxygen-evolving complex (OEC) of PSII in plants and algae, as well as in cyanobacteria. Despite the biological uniqueness of water oxidation to 02, several of the core proteins of PSII have homologues in the so-called type I and type II anaerobic photosynthetic reaction... 

Thermodynamically, the electrochemical mineralization (EM) of any soluble organic compound in water should be achieved at low potentials, widely before the thermodynamic potential of water oxidation to molecular oxygen (1.23 V/SHE under standard conditions) as it is given by (1.1) ... 

The photogenerated electron-hole pairs, produced as a result of light absorption on the photoanode, are separated by an electric field within the semiconductor. On the one hand, the photogenerated holes migrate to the surface of the semiconductor where they oxidize water molecules to oxygen... 

The electrons that are provided by photosystem I are finally used to reduce CO2 to carbohydrates, while in photosystem II, water is oxidized to oxygen. Intense research over many decades has partially revealed the extremely complicated mechanism of natural photosynthesis. It follows that it is obviously rather difficult to imitate this in an artificial photosynthesis that is intended to convert and store solar energy in simple but energy-rich chemicals. Different approaches have been developed to solve this problem (i). It has been suggested to facilitate artificial photosynthesis by the assistance of redoxactive metal complexes in homogeneous systems. Generally, photoredox reactions of metal... 

Photosynthesis results in the reduction of the carbon atoms in carbon dioxide as glucose is formed. During this redox process, oxygen atoms in water are oxidized to oxygen gas. 

A newly discovered bacterium can reduce selenate ion, SeO iaq), to elemental selenium, Se(s), in reservoirs. This is significant because the soluble selenate ion is potentially toxic, but elemental selenium is insoluble and harmless. Assume that water is oxidized to oxygen as the selenate ion... 

The strong oxidizing holes produced by Cn ultimately cause the water oxidation to molecular oxygen. The performance of this process is a serious challenge to biological systems. If one supposes a univalent pathway, then the overall process of water oxidation can be subdivided into four individual electron transfer steps in the following way (different protonation reactions have been omitted for the sake of simplicity) ... 

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