Hydrogen water photodecomposition

Fujishima and Honda [16, 158] reported the photodecomposition of water using semiconductor photoelectrolysis cells (Figure 4.10). When the surface of the Ti02 electrode was irradiated with UV light, oxygen evolution was observed at the Ti02 electrode surface and hydrogen at the Pt black electrode. The overall water photodecomposition reaction ... 

Silica-supported molybdenum disulfide is a hydrogen evolution catalyst for water photodecomposition on semiconductors. This silica-supported M0S2 has an activity in acidic solution that is similar to that of dispersed platinum. The mechanism of the reaction involves electron transfer from the excited sensitizer molecule to the metal ion, followed by subsequent proton reduction by the reduced species. 

Sayama, K. and Arakawa, H., Significant effect of carbonate addition on stoichiometric photodecomposition of liquid water into hydrogen and oxygen from platinum-titanium (IV) oxide suspension,/. Chem. Soc., Chem. Commun., 150,1992. 

The degradation of mirex in water occurs primarily by photolysis. During the photodecomposition of mirex, the chlorine atoms are replaced by hydrogen atoms. The primary photoreduction product of... 

Fig. 7.1 Position of band edges and photodecomposition Fermi energies levels of various non-oxide semiconductors. E(e,d) represents decomposition energy level by electrons, while E(h,d) represents the decomposition energy level for holes vs normal hydrogen electrode (NHE). E(VB) denotes the valence band edge, E(CB) denotes the conduction band edge. E(H2/H20) denotes the reduction potential of water, and (H2O/O2) the oxidation potential of water, both with reference to NHE.

All photocurrents from a semiconductor, when measured near the limiting current region, have the type of appearance shown in Fig. 10.5. The events that lead to the production, e.g., of hydrogen from the photodecomposition of water on illumination of, say, p-type InP, are as follows ... 

The ionic pair of [Co(AMMEsar)] + cation with an anthracene carboxylate anion (A-Co(III)) was used as both a photosensitizer and an ETA in the photodecomposition of water to produce hydrogen [387]. The photoreduction of encapsulated cobalt(III) ion to cobalt(II) ion occurs on excitation of anthracene chromophore (v< 25 000 cm-i). The A-Co(III) complex shows almost no fluorescence (0<2x 10 ), whereas the A-Co(II) complex produces specific violet fluorescence (Fig. 66). The cobalt(II) complex is formed in the presence of EDTA on light irradiation of the A-Co(III) solution (v> 25 641 cm i). The visible band at 21 276 cm- disappeared, and violet fluorescence was observed. The quantum yield of cobalt(II) complex formation was... 

Stringer and Attrep compared hydrogen peroxide-sulphuric acid digestion and U.V. photodecomposition methods for the decomposition in water samples of triphenylarsine oxide, disodium methane-arsonate and DMAA to inorganic arsenic prior to reduction to arsine and determination by atomic absorption spectroscopy or by the AgDDC spec-trophotometric method . 

All these results concur to exclude the involvement of hydrogen peroxide as a coproduct of the photo-oxidation of water and intermediate of other photo-oxidation reactions at Ti02. The high rates of its photodecomposition at open circuit and photooxidation under anodic bias render unlikely the build-up of any significant concentration of H2O2 at the surface of irradiated titanium dioxide or in the nearby solution. 

The principles of redox catalysis applied successfully here to photodecomposition of water, can also be profitably employed to carry out efficiently other multi-electron reactions. Of special interest in the area of solar energy conversion are processes such as reduction of C02, N2 and NAD+ and oxidation of halide ions (Cl-, Br- or I-). With the latter, there exists the interesting possibility of complete decomposition of hydrogen halides with the reverse reaction used in a fuel cell to generate power. 

Because of their close relevance to the carbohydrate investigations now to be considered, considerable attention has been given in the foregoing Sections to the photodecomposition processes which occur in water and alcohols. At the present time, there is emerging a consistent pattern which indicates that, when a carbohydrate is irradiated in aqueous solution, one or more of the following processes may operate (a) photolysis of water by radiation of wavelength lower than 2000 A., and subsequent attack on the carbohydrate by the free radicals formed (6) direct photolysis of the carbohydrate (c) sensitized decomposition of water to produce radicals which may attack the carbohydrate and (d) photosensitized decomposition of the carbohydrate by a hydrogen atom or by an electron-transfer mechanism. 

The investigations of Bolland and Cooper and Wells, - discussed in Section IV, have been particularly valuable for indicating certain mechanisms for the photosensitized oxidation of cellulosic materials. The initial step is abstraction of a hydrogen atom by the excited dye, instead of any kind of electron-transfer process. Such a mechanism adequately explains the photo-oxidation which proceeds in the absence of moisture. When moisture is present, there is another process superimposed which also leads to photo-oxidation. Processes such as were described in relation to the sensitized photodecomposition of water (see Section III) may be intimately connected with the initiation step of the moisture-induced reaction. 

The photodecomposition of 4-(6-methoxy-2-naphthyl)butan-2-one (nabume-tone) in water probably involves the formation of the nabumetone radical cation. This leads to the formation of 6-methoxy-2-naphthalene carboxalde-hyde. Further study has examined the photodegradation of this ketone in -butanol where it was shown that a first-order degradation took place. An excited singlet state is involved, and the author proposes that both concentration and hydrogen bonding are important in this solvent. 

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