Photoredox system water

Figure 7.15 Photoredox system for the reduction of water to hydrogen based on...

Figure 7.15 shows a photoredox system capable of producing hydrogen from water. Photon absorption by Ru(bpy)2+ results in formation of Ru(bpy)3+, which undergoes oxidative quenching by methylviologen (MV2+). 

Figure 7.16 Photoredox system for the oxidation of water to oxygen based on oxidative quenching of excited Ru(bpy)2+ by the sacrificial Co(NH3)5C12+ Reprinted from C. Kutal, Photochemical Conversion and Storage of Solar Energy , Journal of Chemical Education, Volume 60 (10), 1983. American Chemical Society...

A photoredox system for the production of oxygen from water using Ru(bpy)3+ as a sensitiser is shown in Figure 7.16. This system involves oxidative quenching of excited Rufbpy) by the Co(III) complex Co(NH3)5C12+ ... 

The photoredox system Ru(bipy)2+/MV2+ (with EDTA or triethanolamine (TEOA) as the electron donor and Pt-catalysts) has been thoroughly investigated307-317 as a system leading to light-induced H2 evolution from water. Upon visible light excitation Ru(bipy)2+ reduces MV2+. 

Photopolymerizable coatings relief-image-forming systems, 6,125 Photoreactivity environmental effects, 1, 394 Photoredox properties bipyridyl metal complexes, 2, 90 Photoresist systems, 6,125 Photosensitive materials, 6, 113 Photosynthesis anoxygenic, 6, 589 magnesium and manganese, 6, 588 water decomposition models, 6, 498... 

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... 

Colloids and suspensions of semiconductors have also been utilised for the photoredox splitting of water. The principal advantage of a fine suspension is the large active surface area available. Reaction rates of H2 and O2 generation have been enhanced by loading the particles with small deposits of precious metals, as described by Memming (1988), but no practical system has been demonstrated. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

There Is now considerable evidence that a variety of photo-chemlcally Induced redox processes occur In natural water systems. At the present time however, little Is known about how these processes Influence redox equilibria of minor elements such as copper or other transition metals In the water column. A variety of elements exist In the upper water column In valence states which are unstable with respect to the O2/H2O couple. However, since little kinetic data Is available for probable photoredox processes Involving these elements, It Is Impossible to assess the Importance of photochemistry In relation to biologically mediated processes. 

From quantum yield measurements it has been shown that the photoactive species in these reactions are Ce dimers, and that the reduced Ce " ion deactivates the reactive states of these dimers. It is possible that these dimers act as two-electron acceptors in the redox reaction. The reduced Ce " ion undergoes a photoreaction in aqueous perchloric acid that results in the formation of hydrogen. It is plausible therefore to use aqueous cerium ion solutions for the photosplitting of water into hydrogen and oxygen. Since such a system requires both the Ce and Ce ions to be involved in the photoredox process, it is necessary that the experimental conditions can be controlled to independently effect each reaction. 

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