Reductions, photocatalyzed

The photocatalyzed reduction of carbon dioxide at elevated pressure was also investigated. Porous glass beads were used to obtain efficient gas-liquid contact. With isopropanol as the solvent and 2-propyl formate as the reducing agent,the reaction products were carbon monoxide and hydrogen. The catalyst, chloro(tetraphenyl-porphinato)rhodium(III), was irradiated with visible light /21/. 

Tada, H., T. Ishida, A. Takao and S. Ito (2004). Drastic enhancement of Ti02-photocatalyzed reduction of nitrobenzene by loading Ag clusters. Langmuir, 20(19), 7898-7900. 

Reduction of carbon dioxide and its intermediates is catalyzed by illumination. Photocatalyzed reduction of CO2 to formaldehyde and methanol has been observed on semiconductor powders suspended in solution (25) as well as on GaAs electrodes (26). 

The chlorophyll - a destruction was inhibited by iron, hydro-quinone and ascorbate (Shimazaki et al , 1980) The reduced chlorophyll could be reoxidized by a number of oxidizing compounds. The net result was the photocatalyzed reduction of these compounds by AA. In many cases this reaction resulted in a net storage of chemical energy, e.g. riboflavin vas reduced by AA in an illuminated chlorophyll solution overcoming an adverse potential gradient of 0 2 volts ... 

The iron cycle shown in Fig. 10.14 illustrates some redox processes typically observed in soils, sediments and waters, especially at oxic-anoxic boundaries. The cycle includes the reductive dissolution of iron(lll) hydr)oxides by organic ligands, which may also be photocatalyzed in surface waters, and the oxidation of Fe(II) by oxygen, which is catalyzed by surfaces. The oxidation of Fe(II) to Fe(III)(hydr)-oxides is accompanied by the binding of reactive compounds (heavy metals, phosphate, or organic compounds) to the surface, and the reduction of the ferric (hydr) oxides is accompanied by the release of these substances into the water column. 

The reduction has also been carried out by adjusting the pH value with hydroxide ions. The reaction must now follow a different pathway the reduction is still photocatalyzed, but at pH > 13 a sharp decrease of the rate has been observed. The authors proposed the mechanism outlined below ... 

Type IT. When ED > ER and AG < 0, the reaction should be possible thermodynamically but may not occur because of a high energy of activation AG. The role of photoexcitation in this case is to provide an easier route for the reaction. These are called photocatalyzed reactions. The photoreduction of methylene blue by EDTA or other electron donors like stannous chloride fall in this category. The reaction is pH and concentration dependent and the reductant is consumed during the reaction. Dyes in the reduced state can act as very powerful reducing agents. 

Mineralization (meaning a complete oxidation) of numerous organic species cannot be regarded only as an oxidation process. In many cases oxidation must be preceded by reduction steps, eg photocatalyzed transformation of CC14 to C02 and Cl requires first reduction of carbon(IV) to lower oxidation states, followed by its reoxidation to C02 [30], In this context redox properties of an excited semiconductor play a crucial role. Photogenerated holes should support highly oxidative potential, but at the other surface sites an efficient reducer (electron) should also be available [64,65],... 

Reduction of C02 photocatalyzed by semiconducting materials may lead to formation of formaldehyde, formic acid, methanol, methane, and oxalate among other products [7], The earliest report on photocatalytic reduction of C02 with water at Ti02 was published by Inoue et al. [25], Almost at the same time the photoreduction of C02 to methane at SrTi03 was reported by Hemminger et al. [26], The same process can also be performed on AgCl/zeolites [27],... 

Y. L. Sandler Westinghouse Research Laboratories)-. In view of the magnitude of the optical gap in ZnO ( 3 e.v.), it seems very unlikely that illumination by means of an incandescent lamp as used in Professor Schwab s experiments (Lecture 24) would cause any appreciable electronic excitation from the valance band to the conduction band in a pure ZuO crystal. It seems more likely that the electrons come from impurity levels due to the presence of water. We have recently demonstrated that the reduction of silver ions in aqueous solution can be photocatalyzed in presence of pure Ti02 or Si02 by light of wavelengths not absorbed by these oxides when in a dry state. 

Figure 3.19, ZnS photocatalyzed dehydrodimerization of 2,5-DHF product formation rate in the absence (dotted line) and presence (solid line) of metal ions as function of the logarithm of exchange current density for water reduction at the correspionding metal electrode.

FIG. 12 Photocurrent transient responses associated with the heterogeneous quenching of the dimer ZnTPPS/ZnTMPyP by tetracyanoquinodimethane (TCNQ) at a water DCE interface. The redox couple FelCNlg /FelCN)/ was used as supersensitizer in the aqueous phase. The back electron transfer reaction, responsible for the photocurrent decay in the on-transient, is significantly quenched in the presence of the supersensitizer. According to the redox diagram in (b), the overall process at A 4>= —0.11 V corresponds to the reduction of TCNQ by the redox couple in the aqueous phase photocatalyzed by the porphyrin complex. Reprinted from Ref. 109 with permission from Elsevier Science. 

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