Metal oxides photoreduction

Photo-oxidation reactions, 32 118 Photoreduction, metal oxides, 31 123 Phthalic acid, esterification, 17 340 Phthalocyanines EDA complexes of, 20 328-330 catalytic activity for hydrogen exchange reaction, 20 329,330 electronic configuration of, 20 330 organometallic complexes, 30 276-277 Phyllosilicates, see Layer lattice silicates, catalysts... 

Organic compounds have the potential to abiotically reduce Fe(III) and Mn(IV) (Luther et al., 1992 Stone, 1987). Phenols and a variety of other aromatic compounds reduce Fe(III) rapidly at acidic pH, but slowly at circum neutral pH (LaKind and Stone, 1989). Humics can reduce Fe(III) effectively at circumneutral pH and they are abundant in soils and sediments. Because humics and other organic compounds often serve as electron shuttles between metal-reducing bacteria and metal oxides (Lovley et al., 1996a), it may be difficult to separate microbial and nonmicrobial sources of electrons. Finally, aerobic photoreduction of Fe(III) has been observed in freshwater and marine environments (Barbeau et al., 2001 Emmenegger et al., 2001), but it is unknown to what degree this process... 

At constant temperature the activity of the photoimmobilized catalysts decreases gradually. Temperature rise from 300 to 360 K leads to a new increase in the rate of partial oxidation production. Time decrease in the activity of these catalysts at constant temperature is described by the equation W=A Int, which is typical for processes with a variable number of active sites. Decrease in the activity of Me-Ph catalysts after attaining the low-temperature maximum can be ascribed to reoxidation of photoreducted metal ions... 

Two systems will be discussed. The first is the system of metal oxides in which photoreduction of metal oxides is applied to controlling the valency of metal ions. The second is the system of metal carbonyls which shows the photoinduction of its catalytic activity by the irradiation of metal carbonyls adsorbed on metal oxide supports. 

Prussian blue is a mixed Fe(II)/Fe(III) complex polymeric species in which Fe(II) is octahedrally coordinated by C, and Fe(lll) is octahedrally coordinated by N, to give a structure containing Fe(II)-C-N-Fe(lll)-N-C-Fe(II)-linkages, in which the colour originates from electron transfer between the two metal oxidation states. It was discovered in 1704, and used in blueprints and also in the cyanotype photographic process developed by Herschel (see Chap. 11). The cyanotype process is made possible by the photochemical reduction of Fe(III) citrate (or oxalate) to Fe(II), which reacts with ferricyanide present in the coating formulation to give Prussian blue. A similar photoreduction of Fe(lII) oxalate to Fe(ll) is used in the ferrioxalate actinometer (see Chap. 14). 

Photo-oxidation or reduction is often found if the complex is irradiated in the charge-transfer bands (see above) photo-oxidation of the metal occurring if the transition is M - -L. Thus the photochemical generation from Ir(IV)Cl6 of a species active in forcing filaments of E. coli may well involve the photoreduction of Ir(IV) to Ir(III) since the intense bands in the visible spectrum of Ir(IV)Clcharge-transfer bands. A report has appeared of the photo-aquation of IrCl -(43). 

The continuous flow method is still necessary when one must use probe methods which respond only relatively slowly to concentration changes. These include pH, Oj-sensitive electrodes, metal-ion selective electrodes,thermistors and thermocouples, " epr and nmr detection. Resonance Raman and absorption spectra have been recorded in a flowing sample a few seconds after mixing horseradish peroxidase and oxidants. In this way spectra of transients (eompounds I and II) can be recorded, and the effext of any photoreduction by the laser minimized. ... 

Metal acetylacetonates quench triplet species generated by flash photolysis of aromatic ketones and hydrocarbons.330-333 More recently, these reactions have been studied from a synthetic standpoint. Triplet state benzophenone sensitizes photoreduction of Cu(MeCOCHCOMe)2 by alcohols to give black, presumably polymeric, [Cu(MeCOCHCOMe)] . This reacts with Lewis bases to provide complexes of the type CuL2(MeCOCHCOMe) (L = bipyridyl/2, ethylenediamine/2, carbon monoxide, Ph3P). Disubstituted alkynes yield Cu(C2 R2 XMeCOCHCOMe) but terminal alkynes form CuQR acetylides.334 The bipyridyl complex of copper(I) acetylacetonate catalyzes the reduction of oxygen to water and the oxidation of primary and secondary alcohols to aldehydes and ketones.335... 

The deposition of silver in tissues is the result of the precipitation of insoluble silver salts, such as silver chloride and silver phosphate. These insoluble silver salts appear to be transformed into soluble silver sulfide albuminates, to bind to or form complexes with amino or carboxyl groups in RNA, DNA, and proteins, or to be reduced to metallic silver by ascorbic acid or catecholamines (Danscher 1981). The blue or gray discoloration of skin exposed to ultraviolet light in humans with argyria may be caused by the photoreduction of silver chloride to metallic silver. The metallic silver is then oxidized by tissue and bound as black silver sulfide (Danscher 1981). Bucklet et al. (1965) identified silver particles deposited in the dermis of a woman with localized argyria as being composed of silver sulfide. 

Primary photoreactions leading to net oxidation or reduction reactions of coordination compounds are well known and are often the result of decay paths accessible only from CT states. A number of coordination compounds yield photoelectron production in solution, the Ru(2,2 -bipyridine)3+ ion has been shown to be an electron donor from its electronically excited state, and photoreduction of several metal complexes has been studied in detail. Discussion of these three areas should reveal most of the important principles associated with photoredox and CT state chemistry. 

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