Photochemical Reductions

The Fe -oxalate complex then adsorbs on goethite (step 2) where it exchanges an electron with a surface Fe atom to form Fe (step 3) and is itself reoxidized. 

Desorption of the reduced metal ion is the rate determining step and is assisted by protons and oxalate ions. The reoxidized surface complex also desorbs owing to its altered molecular structure and is thus available for further reaction. The reductive dissolution step is faster than the initial complexation process. Photochemical dissolution of hematite in acidic oxalate solution is faster when air is excluded from the system (by purging with N2) than when air is present (Taxiarchou et al. 1997). 

The Ea for the dissolution of hematite by mercapto carboxylic acids in acid media in the presence of UV radiation was lower (64 5 kj mol ) than that for dissolution in the absence of radiation (94 8 kJ mol ) (Waite et al. 1986). The reaction in both cases was considered to involve formation of an intermediate organic-Fe surface complex which decomposed as a result of intramolecular electron transfer to release Fe . UV irradiation enhanced the decomposition of the surface complex either through excitation of the ligand field states associated with the free electrons on the S atoms, or through high energy charge transfer states. 

Litter et al. (1991) found that the dissolution of maghemite was also considerably speeded up, once a dissolved Fe -oxalate or Fe -EDTA complex was reduced to an Fe complex by UV irradiation 1 = 254 nm). This system also showed an induction period which could be eliminated by addition of Fe (see Fig. 12.28). In a study concerned with dissolution of corrosion oxide, electrons from viologen radicals produced by y-radiation ( Co) were used to dissolve hematite and goethite (Mulvaney et al., 1988) it was observed that the Fe appearing in solution could only account for a fraction of the electrons consumed. The remainder was involved in conversion of the Fe oxide into magnetite. 

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In the laboratory, S2F2Q is prepared by the photochemical reduction of SF Cl in the presence of hydrogen (122). 

Selective Reduction. In aqueous solution, europium(III) [22541 -18-0] reduction to europium(II) [16910-54-6] is carried out by treatment with amalgams or zinc, or by continuous electrolytic reduction. Photochemical reduction has also been proposed. When reduced to the divalent state, europium exhibits chemical properties similar to the alkaline-earth elements and can be selectively precipitated as a sulfate, for example. This process is highly selective and allows production of high purity europium fromlow europium content solutions (see Calcium compounds Strontiumand strontium compounds). 

The benzopinacol obtained by photochemical reduction of benzophenone (p. 8) may be used directly without purification. 

Photochemical reduction. A deoxygenated aqueous solution of FMN (50 pM) containing 2mM EDTA is irradiated with a longwave UV light immediately before use (Nickerson and Strauss, 1960 Strauss and Nickerson, 1961). The reduction of FMN is accompanied by the formation of H202, which might be undesirable in some experiments. 

Aqueous plutonium photochemistry is briefly reviewed. Photochemical reactions of plutonium in several acid media have been indicated, and detailed information for such reactions has been reported for perchlorate systems. Photochemical reductions of Pu(VI) to Pu(V) and Pu(IV) to Pu(III) are discussed and are compared to the U(VI)/(V) and Ce(IV)/(III) systems respectively. The reversible photoshift in the Pu(IV) disproportionation reaction is highlighted, and the unique features of this reaction are stressed. The results for photoenhancement of Pu(IV) polymer degradation are presented and an explanation of the post-irradiation effect is offered. 

After observing the photochemical reduction of Pu(VI) and Pu(IV), it seems obvious that reaction (3) should be light-sensitive. However, it is not obvious how photons would affect the equilibrium concentrations of the plutonium species. The experimental results [3,4] are very interesting and are described below, but a complete explanation is yet to be developed. 

Keywords Catalysis Electrochemical reduction Hydroboration Hydrogenation Hydrosilylation Iron hydride complex Photochemical reduction... 

Two different types of zinc-porphyrins coordinated diiron complex act as catalysts for the photochemical reduction hydrogen evolution from water. In this system... 

Kuwabata S, Nishida K, Tsuda R, Inoue H, Yoneyama H (1994) Photochemical reduction of carbon dioxide to methanol using ZnS microcrystaUite as a photocatalyst in the presence of methanol dehydrogenase. J Electrochem Soc 141 1498-1503... 

They have also been used to bring about photochemical reduction of Hg + via Hg2 to Hg° (Troupis et al. 2005). 

Lehn and Ziessel166 have also developed systems for the photochemical reduction of C02. These systems are similar to those represented by Fig. 18. Visible-light irradiation of C02-saturated aqueous acetonitrile solutions containing Ru(bpy)2+ as a photosensitizer, cobalt(II) chloride as an electron acceptor, and triethyl-amine as a sacrificial electron donor gave carbon monoxide and... 

The effects of transition metals on the photochemical reduction of C02 to formaldehyde (0.1 %), formaldehyde to methanol (6-8%), and methanol to methane (ca. 10 5%) were examined172 in aqueous solutions, but the yields were very low as shown in parentheses for each reaction. 

Considerable progress has been made on C02 fixation in photochemical reduction. The use of Re complexes as photosensitizers gave the best results the reduction product was CO or HCOOH. The catalysts developed in this field are applicable to both the electrochemical and photoelectrochemical reduction of C02. Basic concepts developed in the gas phase reduction of C02 with H2 can also be used. Furthermore, electrochemical carboxyla-tion of organic molecules such as olefins, aromatic hydrocarbons, and alkyl halides in the presence of C02 is also an attractive research subject. Photoinduced and thermal insertion of C02 using organometallic complexes has also been extensively examined in recent years. 

One possible strategy in the development of low-overpotential methods for the electroreduction of C02 is to employ a catalyst in solution in the electrochemical cell, A few systems are known that employ homogeneous catalysts and these are based primarily on transition metal complexes. A particularly efficient catalyst is (Bipy)Re[CO]3Cl, where Bipy is 2,2 bipyridine, which was first reported as such by Hawecker et al. in 1983. In fact, this first report concerned the photochemical reduction of C02 to CO. However, they reasoned correctly that the complex should also be capable of catalysing the electrochemical reduction reaction. In 1984, the same authors reported that (Bipy)Re[C013CI catalysed the reduction of C02 to CO in DMF/water/ tetraalkylammonium chloride or perchlorate with an average current efficiency of >90% at —1.25 V vs. NHE (c. —1.5V vs. SCE). The product analysis was performed by gas chromatography and 13C nmr and showed no other products. 

An interesting alternative mechanism of activation is the photochemical reduction of Pt(IV) to Pt(II) (Fig. 3). In addition to photoreduction, photosubstitution and photoisomerization can also occur, making the photochemistry of Pt complexes difficult to predict and a careful analysis of the photoproducts imperative (21). We have been involved particularly in the development of photochemotherapeutic agents based on Pt(IV) and the study of their photodecomposition and (subsequent) interactions with... 

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