Photoreduction, of metals

We have extended the seed-mediated technique for the synthesis of bimetallic nanoparticles, having core-shell type structure appending photoreduction of metal ions. It has been proved that the deposition of a less noble metal (M) as shell on a preformed nobler nanoparticle core (M ) seems to be very effective by UV activation. Using this seed-mediated method we were able to synthesize Aucore Agsheii particles. First for the preparation of gold seeds (S), TX-lOO (10 M) and HAuC (5.0 x 10 %) were taken in a quartz cuvette so that the final concentration of Au(III) ion remained 5.0 x 10 M. Then the... 

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. 

Photoreduction of CO2 to formate in metal sulfide colloids has been reported to provide a novel photosynthetic route for production of methanol as the end-product... 

Atiwi SM, Al-Jubori KE (1989) Photoreduction of CO2 by metal sulphide semiconductors. Sol Energy Mater 18 223-229... 

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). 

Van Dyke A reprographic process, based on the photoreduction of ferric ammonium oxalate to ferrous amonium oxalate, which in turn reduces silver nitrate to silver metal. 

It has been previously reported [21, 22] that metal colloids are formed by radiochemical reactions in water/alcohol solutions, in which the reduction of metal salts takes place by solvated electrons and free radicals produced under UV or y-ray irradiation. Ichikawa et al. have applied this photoreduction method to the surface-mediated reaction of metallic ions and succeeded in synthesizing metal/aUoy nanowires in the constrained cavities of mesoporous supports such as FSM-16 and MCM-41 [18-20, 23-25]. The adsorbed water and alcohol work not only as solvents in the nanoscale silica void space but also as a source of reducing species for metallic ions to metals under UV-vis and y-ray [11, 18, 19] irradiation. The results indicate the dense formation of Pt nanowires inside the charmels of mesoporous supports, such as FSM-16, which act as the templates. In fact, no any Pt wire is observed on the external surface of FSM-16 or amorphous silica surface. Short wires, 10 nm long, are also observed as a minor species in the samples in the initial stage of UV and y-ray irradiation. 

Much of the work on the photoreduction of carbon dioxide centres on the use of transition metal catalysts to produce formic acid and carbon monoxide. A large number of these catalysts are metalloporphyrins and phthalocyanines. These include cobalt porphyrins and iron porphyrins, in which the metal in the porphyrin is first of all photochemically reduced from M(ii) to M(o), the latter reacting rapidly with CO to produce formic acid and CO. ° Because the M(o) is oxidised in the process to M(ii) the process is catalytic with high percentage conversion rates. However, there is a problem with light energy conversion and the major issue of porphyrin stability. 

In the presence of alcohols, photoreduction occurs more easily. Ketones like acetone can act as a sensitizer in this case (9). Surfactants can be used as protective colloids instead of polymers (10). Irradiation with 60Co y-rays is more effective for the reduction of metal ions (11). 

Torigoe and Esumi proposed silver(I) bis(oxalato)palladate(II) as a precursor of Ag/Pd bimetallic nanoparticles stabilized by PVP (45). Photoreduction of the aqueous precursor in a quartz vessel gave Ag/Pd bimetallic nanoparticles at various concentrations. The particles deviate from spherical ones but are uniform. Each particle contains both metal elements, as confirmed by EDX measurement. The size can be changed with concentration of the precursor. The average composition of... 

Photoirradiation or heat treatment is also applied for the reduction of metal ions. In these cases, enough energy to cleave the metal-ligand bond is required for the reduction. Theoretically, the absorption band attributed to the charge transfer from ligand to metal must be excited by the photoirradiation. The coexistence of alcohol can assist the photoreduction. In this case, the corrdination of the alcohol to metal ions may decrease the reduction potential, or the ketyl radical produced may reduce the metal ions as shown in Scheme 9.1.7. 

Titania and silica glass thin films Au, Pt Photoreduction of HAuCl and K.2PtCl4 in ethanol-water in the presence of poly(N-vinyl-2-pyrrolidone) or poly(methyl vinyl ether led to metal particles (sizes depended on solvent composition the smallest, 2.8 nm in diameter, was obtained in 100% alcohol) which were mixed with Ti(i-OC3H7)4 and acetylacetone under N2. Subsequent to 30 minutes of stirring, exposure to moisture produced Ti02-embedded metal particles 74... 

Most of the reported photochemical reactions of lanthanide complexes involve some type of redox behavior.147 Photolysis (254—405 nm) of Eu2+ in acidic aqueous solution, for example, results in photooxidation of the metal and generation of H2 (equation 50).148 While the excited states responsible for this reaction nominally arise from 4/ -+ 5d transitions localized on the metal, strong mixing of the 5rf-orbital with ligand orbitals endows these states with appreciable CTTL character. Photoreduction of aquated Eu3+ can also be driven with UV ( 254 nm) light149 and forms the... 

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