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 ... 

Fig. 7.6 Schematic diagram of core-shell nanorods of (a) InP-CdS (b) InP-ZnTe and use of nanorods in water photodecomposition. In (a), electrons are holes are localized in the shell and the core, respectively whereas, in (b), holes and electrons are localized in the shell and the core, respectively.

Third, to provide sufficient rate of water photodecomposition, the high specific rate of the transmembrane PET are needed. In this respect the suspensions of vesicles with a more developed membrane surface are preferable as compared to planar BLMS. 

Both reduced and Pt-modified powder samples were studied in distilled water and in aqueous solutions of HC1, H2SO4, HNO3 and NaOH. Water photodecomposition proceeds moderately in distilled water and in NaOH but is strongly suppressed in acidic aqueous media. The NaOH coating effect mimicks that found by other workers earlier (see Ref. 320 and text). 319... 

The [Ru(bpy)3] complexes are not the only sensitizers that can be used for cyclic water photodecomposition, indeed metal porphyrins, metal phthalo-cyanines, proflavin, and other compounds have been investigated as possible sensitizers. 

Several oxides (e.g. Ti02 [7], SrTiOs [8], n-SiG [24], p-GaP [25], Fe203 [26], etc.) and chalcogenides such as CdSe [23] have been used as anodes for water photodecomposition. n-Ti02, even today seems to be one of the most important materials, because it has been possible to extend its spectral response into visible portion of the solar spectrum through sensitization with organic dyes. A separate chapter is devoted... 

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. 

Metal complex catalysts allowed under mild conditions several reactions, which were previously unknown in chemistry, viz., reduction of molecular nitrogen to hydrazine and ammonia, alkane activation and oxidation, water photodecomposition, etc. 

CIO2 dissolves exothermically in water and the dark-green solutions, containing up to 8g/l, decompose only very slowly in the dark. At low temperatures crystalline clathrate hydrates, C102.nH20, separate (n 6-10). Illumination of neutral aqueous solutions initiates rapid photodecomposition to a mixture of chloric and hydrochloric acids ... 

There has been intense study of the complexes of bi- and polydentate ammines since the mid-1970s, driven by interest in the catalytic photodecomposition of water using the excited states of Ru(bipy)g+ (n = 2,3) and related systems (Figure 1.18) [5, 7, 8, 71]. 

Interfacial electron transfer in colloidal metal and semiconductor dispersions and photodecomposition of water. K. Kalyanasundaram, M. Gratzel and E. Pelizzelti. Coord. Chem. Rev., 1986, 69, 57 (338). 

An environmental protocol has been developed to assess the significance of newly discovered hazardous substances that might enter soil, water, and the food chain. Using established laboratory procedures and C-labeled 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD), gas chromatography, and mass spectrometry, we determined mobility of TCDD by soil TLC in five soils, rate and amount of plant uptake in oats and soybeans, photodecomposition rate and nature of the products, persistence in two soils at 1,10, and 100 ppm, and metabolism rate in soils. We found that TCDD is immobile in soils, not readily taken up by plants, subject to photodecomposition, persistent in soils, and slowly degraded in soils to polar metabolites. Subsequent studies revealed that the environmental contamination by TCDD is extremely small and not detectable in biological samples. 

Several facts have emerged from our studies with 2,7-DCDD and 2,3,7,8-TCDD. They are not biosynthesized by condensation of chloro-phenols in soils, and they are not photoproducts of 2,4-dichlorophenol. They do not leach into the soil profile and consequently pose no threat to groundwater, and they are not taken up by plants from minute residues likely to occur in soils. Photodecomposition is insignificant on dry soil surfaces but is probably important in water. Dichlorodibenzo-p-dioxin is lost by volatilization, but TCDD is probably involatile. These compounds are not translocated within the plant from foliar application, and they are degraded in the soil. 

Other environmental properties of interest are those that govern movement of chemicals, for these properties can influence not only the possibility of human exposure but also the lifetime and fate of the chemical. Clearly, if a nitrosamine is formed in, or introduced into, the soil and stays there, it presents little threat to man, and its lifetime will depend on the chemical or microbiological properties of the soil. If it should move to the surface and volatilize into the atmosphere, on the other hand, there will exist the possibility of human exposure via inhalation and also the possibility of vapor-phase photodecomposition. If a nitrosamine were to leach from soil into water, it could perhaps be consumed in drinking water alternatively, exposure of the aqueous solution to sunlight could provide another opportunity for photodecomposition. 

Wong AS, DG Crosby (1981) Photodecomposition of pentachlorophenol in water. J Agric Food Chem 29 125-130. 

Figure 1. Photodecomposition of water on Ru02/BaTi40g(, H2 O-Os), Ru02/Na2Ti60i3(H,H2 .O2) and Ru02/Ba4Tii303o(A,H2 A,02).

Sato, S. and White, J.M. (1980) Photodecomposition of water over Pt/ Ti02 catalysts. Chemical Physics Letters, 72 (1), 83-86. 

Water is involved in most of the photodecomposition reactions. Hence, nonaqueous electrolytes such as methanol, ethanol, N,N-d i methyl forma mide, acetonitrile, propylene carbonate, ethylene glycol, tetrahydrofuran, nitromethane, benzonitrile, and molten salts such as A1C13-butyl pyridium chloride are chosen. The efficiency of early cells prepared with nonaqueous solvents such as methanol and acetonitrile were low because of the high resistivity of the electrolyte, limited solubility of the redox species, and poor bulk and surface properties of the semiconductor. Recently, reasonably efficient and fairly stable cells have been prepared with nonaqueous electrolytes with a proper design of the electrolyte redox couple and by careful control of the material and surface properties [7], Results with single-crystal semiconductor electrodes can be obtained from table 2 in Ref. 15. Unfortunately, the efficiencies and stabilities achieved cannot justify the use of singlecrystal materials. Table 2 in Ref. 15 summarizes the results of liquid junction solar cells prepared with polycrystalline and thin-film semiconductors [15]. As can be seen the efficiencies are fair. Thin films provide several advantages over bulk materials. Despite these possibilities, the actual efficiencies of solid-state polycrystalline thin-film PV solar cells exceed those obtained with electrochemical PV cells [22,23]. 

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. 

Domen, K., Kudo, A., Tanaka, A., and Onishi, T., Overall photodecomposition of water on a layered niobate catalyst, Catal. Today, 8, 77,1990. 

Ohno, T. 1989. Spectrophotometric determination of total cyanide in surface waters following ultraviolet induced photodecomposition. Analyst 114 857-858. 

Paraquat is strongly adsorbed to soils and sediments and is biologically unavailable in that form however, it is not degraded significantly for many years, except in surface soils. In surface soils, paraquat loss through photodecomposition approaches 50% in 3 weeks. In freshwater ecosystems, loss from water column is rapid about 50% in 36 h and 100% in 4 weeks. In marine ecosystems, 50 to 70% loss of paraquat from seawater was usually recorded within 24 h. 

---