Hole scavenger

The photolytic reduction of N2 at TiO -suspensions was at first reported by Schrauzer et al. Small amounts of NH3 and N2H4 were obtained as products. The highest activity was found with anatase containing 20-30 % rutile. Very low yields were also obtained with p-GaP electrodes under illumination It is much easier to produce NH3 from NO -solutions at CdS- and Ti02-particles using S -ions as hole scavengers . Efficiencies are not reported yet. Recently the formation of NH3 from NO was observed at p-GaAs electrodes under illumination. In this case NH3-formation was only found in the presence of transition metal ions or their complex with EDTA. 

Kaneco, S., Shimizu, Y., Ohta, K., and Mizuno, T. (1998) Photocatalytic reduction ofhigh pressure carbon dioxide using Ti02 powders with a positive hole scavenger. Journal of Photochemistry and Photobiology A Chemistry, 115 (3), 223-226. 

It is possible that colloidal photochemistry will provide a new approach to prebiotic syntheses. The work described previously on redox reactions at colloidal ZnS semiconductor particles has been carried on successfully by S. T. Martin and co-workers, who studied reduction of CO2 to formate under UV irradiation in the aqueous phase. ZnS acts as a photocatalyst in the presence of a sulphur hole scavenger oxidation of formate to CO2 occurs in the absence of a hole scavenger. The quantum efficiency for the formate synthesis is 10% at pH 6.3 acetate and propionate were also formed. The authors assume that the primeval ocean contained semiconducting particles, at the surface of which photochemical syntheses could take place (Zhang et al 2007). 

Lee SG, Lee SW, Lee HI (2001) Photocatalytic production of hydrogen from aqueous solution containing ChT as a hole scavengers.Appl Catal A Gen 207 173-181... 

Fig. 3 Results for the photocatalytic reduction of nitrate in the presence of 0.04 M HCOOH as hole scavenger over titania and Ag/Ti02 catalysts prepared by impregnation (IMP) or photodeposition (Photo) and comparing use of Degussa P25 (49 m g ) with Hombikat UV 100 (250 m g ) as supports. ...

The rate constants of the forward class (2) reactions are much slower than those of the class (1) reactions, although some of electron donors have comparably low adiabatic IPgS. These rate constants do not correlate with the free energies of hole scavenging... 

Figure 15 Top Photographic latent image formation in undoped (left) and formate-doped and gold-sulfide sensitized AgBr crystals with the hole-scavenging step (center). Secondary reduction step by formyl radical (right). Bottom Sensitometry curves for gold-sulfide sensitized emulsions, undoped or formate-doped, and developed after 5 or 20 min (texp = 10 sec, development with aminophenol and ascorbic acid). The same absorbance is observed for a number of photons absorbed 5 or 10 times less, respectively, than in the undoped emulsion. (From Ref. 200.)...

Since the concentration of C02 in water is rather low, increasing pressure may enhance the performance of photoreduction. Studies using Ti02 in water showed that hydrocarbons such as methane and ethylene, which were not produced at ambient pressure, were obtained under high pressure.161 166 Methane was formed as the main reduction product when the reduction was performed in isopropyl alcohol, a positive hole scavenger.167... 

TiOz coated with potassium ferrocyanide proved to be an effective catalyst for the reduction of C02 to formic acid and formaldehyde.169 A very stable and reproducible catalytic system was prepared by immobilizing Ni2+ and Ru2+ complexes into Nation membrane, which was used for the selective reduction of C02 to formic acid.170 Formic acid was again formed when Zn and Co phthalocyanines were adsorbed onto a Nation membrane on irradiation with visible light in acidic aqueous solution containing triethanolamine as a hole scavenger. Cobalt comns (B i2) acting as homogeneous catalysts in acetonitrile-methanol solutions induced the formation of formic acid and CO.172... 

Colloidal particles of CdS were examined by Eggins et al. [131]. In C02-saturated aqueous solutions containing tetramethyl ammonium chloride, illumination with an arc-lamp produced formic acid and formaldehyde along with glyoxylic acid and acetic acid. Either hydroquinone or Na2S03 was used as the hole scavenger to increase yields. When CdSe colloids were examined for C02 photoreduction, those with a particle size <50 A were seen to produce formic acid under illumination in aqueous solution, whilst larger particles exhibited no photoreduction [132],... 

---