Photochemical Water Decomposition

Electrolysis, and thermochemical and photochemical decomposition of water followed by purification through diffusion methods are expensive processes to produce hydrogen. 

Colorless or white cubic crystals or granules becomes yellowish when exposed to bright light due to photochemical decomposition liberating traces of free iodine density 3.13 g/cm melts at 681°C vaporizes at 1,330°C highly soluble in water, 140 g/lOOmL at 20°C aqueous solution readily dissolves iodine sparingly soluble in ethanol (about 2 g/lOOmL at 25°C) and acetone slightly soluble in ether and ammonia. 

The photochemical decomposition of chlorine water, bromine water, and iodine water.—In 1785, C. L. Berthollet20 noticed that chlorine water is gradually decomposed by exposure to light, forming aq. hydrochloric acid and oxygen. He said ... 

The published research on the photochemical decomposition of di-azonium salts suggests that the two processes, a heterocyclic and a homolytic process, analogous to those of the thermal decomposition may occur. Various workers 36 187 have reported that phenols are formed when diazonium salts are photolyzed in water and aryl ethers result when an alcohol replaces water as the solvent. Homer and Stohr122 report that a process analogous to reductive deamination occurs in preference to ether formation results in alcohols. The importance of free radical intermediates in the photodecomposition, based on magnetic susceptibility measurements, has been stressed.25 Lee and his co-workers171 have recently suggested that in ethanol the photodecomposition of a diazonium salt occurs via a radical intermediate while in water an ionic process predominates. Thus, photodecomposition of a nitrobenzene diazonium chloride in water yielded both a nitrophenol and a chloronitrobenzene in ethanol, on the other hand, the major product of photolysis was the reduction product, nitrobenzene. 

Scheme 2 Proposed mechanism for the photochemical decomposition of water catalyzed by [PtH(PEt3)3]+ ...

INTERMOLECULAR ELECTRON TRANSFER REACTIONS IN THE PHOTOCHEMICAL DECOMPOSITION OF WATER... 

More success in the photochemical decomposition of water using visible light has been achieved by attempting to mimic the photosynthetic processes by electron transfer, via a series of catalysts, from the excited state of a dye molecule to a proton. This is accompanied by oxidation of hydroxide ion by the oxidized form of the dye. 

Sturini, M., Fasani, E., Prandi, C., Casaschi, A., and Albini, A., Titanium dioxide-photocatalysed decomposition of some thiocarbamates in water, /. Photochem. Photobiol. A Chem., 101, 251, 1996. 

Figure 9. Schematic of biogeochemical processes producing and consuming DMS in marine environments. DMSP in certain phytoplankton and marsh grasses may be slowly metabolized and released diretfly. Disruption of DMSP-containing cells, either by herbivory or by microbial decomposition, results in DMSP release into solution and enhanced production of DMS. DMS may be oxidized microbially to DMSO, and perhaps DMSO photochemical decomposition of DMS in surface waters may also occur. Residual DMS may escape into the atmosphere, where it undergoes further photochemical degradation.

X 109 and 8. 9 x 109 s 1 for Ndm and Ybm, respectively. Both the above-described in-termolecular mechanism, as well as an intramolecular pathway in the ternary complex with aad which forms in solution, are responsible for the observation of NIR luminescence in these systems. Addition of water to the toluene solutions quenches the NIR luminescence, while it enhances the visible CL emission of the corresponding solution of Eum and Tbm (Voloshin et al., 2000c). Neodymium and ytterbium tris(benzoyltrifluoroacetonates) display the same CL as tta complexes, although for Ybm its intensity is about 2.5 times lower than for the tta chelate. On the other hand, almost no CL is detected for acetylacetonate complexes (Voloshin et al., 2000a). Thermal or photochemical decomposition of aad also triggers CL from [Pr(dpm)3] and Pr(fod)3], both in the visible (from the 3Pi, 3Po, and 1D2 levels) and in the NIR at 850 nm ( Do -> 3F2 transition) and 1100 nm ( D2 3F4 transition). The excited... 

De Ruiter et al. [4] observed that photochemical decomposition by ultraviolet irradiation of dansyl derivatives of chlorinated phenolic compounds in methanol-water mixtures led to the formation of highly fluorescent dansyl-OH and dansyl-OH3 species. The optimal irradiation time was 5.5s. This reaction was utilised in a post-column photochemical reactor in the high performance liquid chromatography determination of highly chlorinated phenols in river water. The method calibration curve (for dansylated pentachlorophenol) was linear over three orders of magnitude. 

The illumination source was a xenon illuminator (Cermax, ILC Technology, Sunnyvale, Cal.) operated at 180-320 W and situated 34-38 cm from the photolysis cell. The spectrum of the output of this lamp closely simulated the solar spectrum over the 300-700 nm wavelength region. To avoid thermal decomposition of samples undergoing illumination, the near-infrared component of the lamp output was attenuated by inserting a 20-cm water filter between the lamp and a sample. A portion of each sample of adsorbed PAH was stored in the dark, as a control, to ensure that no detectable non-photochemical decomposition of the PAH occurred during the duration of illumination. 

Samanidou, V., Fytianos, K., Pfister, G., Bahadir, M. (1988) Photochemical decomposition of waters of Northern Greece. Sci. Total Environ. 76, 85-92. 

H.H. Tonnesen, and A.-L. Grislingaas, Photochemical stability of biologically active compounds II Photochemical decomposition of mefloquine in water. Int. J. Pharm. 60 157-162, 1990. 

Hori H,Takano Y, Koike K, Kutsuna S, Einaga H, IbusukiT. Photochemical decomposition of pentafluoropropionic acid to fluoride ions with a water-soluble heteropolyacid photocatalyst. Appl Catalysis B Environ 2003 46 333-40. 

The facile changes of oxidation state exhibited by rhodium complex catalysts pointed the way to the employment of rhodium complexes in the photochemical decomposition of water."... 

The experiments with N2 cryostats are usually run at temperatmes between 100 and 170K the desire for the lowest possible temperatme is balanced by the necessity to avoid icing. Such temperatmes are almost always sufficient to prevent both the chemical (or photochemical) decomposition of the sample and the solvent loss. The choice between rapid (flash) and slow cooling depends on the natme of the sample. When the thermal expansion coefficient is likely to be high, as is usually the case with conformationally flexible molecules, slow cooling is preferable if the crystal stability permits it. On the other hand, flash-cooling is the only technique possible for protein crystals, because slow cooling will allow the crystallization water to form ice crystals,... 

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