Semiconductor powders

Furthennore, semiconductor powders can be employed for the catalytic generation of usefLil products such as Fl2 and O2 that can be used for the destmction of pollutants [98, 99 and 100]. 

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

Nonbiological methods for removal of trichloroethylene from water are also being studied. These include the use of a hollow fiber membrane contactor (Dr. A.K. Zander, Clarkson University), photocatalysis by solar or artificially irradiated semiconductor powders (Dr. G. Cooper, Photo-catalytics, Inc.), and micellar-enhanced ultrafiltration (Dr. B.L. Roberts, Surfactant Associates, Inc.). 

Fig. 5.14 Inoue et al. carried out a systematic study of the photocatalytic reduction of CO2 by different semiconductor powders in aqueous suspensions. Shown here is the energy correlation between semiconductor catalysts and redox couples in water, as presented in their paper. In principle, the solution species with more positive redox potential with respect to the conduction band level of the semiconductor is preferably reduced at the electrode. Photoexcited electrons in the more negative conduction band certainly have greater ability to reduce CO2 in the solution. (Reproduced from [240])...

One attractive approach to photochemical conversion and storage of solar energy is photofixation of carbon dioxide to C-1 organic compounds (formic acid, formaldehyde, methanol, and methane). Photoreduction of CO2 to formic acid and formaldehyde has been demonstrated by using n-type Bi2S3 and CdS semiconductor powders (particle size 300 00 mesh) as photoelectrocatalysts in emulsions... 

Frank SN, Bard AJ (1977) Heterogeneous photocatalytic oxidation of cyanide and sulfite in aqueous solutions at semiconductor powders. J Phys Chem 81 1484—1488... 

Benniston AC, Haniman A (2008) Artificial photosynthesis. Materials Today 11 26-34 Inoue T, Fujishima A, Konishi S, Honda K (1979) Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders. Nature 277 637-638 Halmann M (1978) Photoelectrochemical reduction of aqueous carbon dioxide on p-type gallium phosphide in liquid junction solar cells. Nature 275 115-116 Heminger JC, Carr R, Somorjai GA (1987) The photoassisted reaction of gaseous water and carbon dioxide adsorbed on the SrH03 (111) crystal face to form methane. Chem Phys Lett 57 100-104... 

Inoue, T., Fujishima, A., Konishi, S. and Honda, K. Photoelectrocatalytic Reduction of Carbon Dioxide in Aquesous Suspensions of Semiconductor Powders. Nature. 1979, 277 637. 

Photoassisted Reduction of Carbon Dioxide with Suspensions of Semiconductor Powders... 

Another disadvantage of using semiconductor powders is the difficulty in obtaining kinetic and thermodynamic data. 

The photoassisted reduction of C02 with suspended semiconductor powders gives, at present, very low energy efficiencies (at most, ca. 0.01% or less). The use of colloidal semiconductor particles is more efficient in some cases. 

Author has introduced the discovery of mechanolysis, a novel phenomenon of water splitting [9,10], which has been understood as a result of frictional electricity between the Teflon stirring rod and the Pyrex glass of the beaker, where pure water containing semiconductor powder is filled. Author [9] has pointed opt that the semiconductor must have the property of the hopping conductivity, and called tribolysis. There exists another type of mechanolysis, which may be due to the piezo electrolysis. This type is called piezolysis, but not discovered yet. 

Sayama, K., Arakawa, H. 1992b. Remarkable effect of Na CO, addition on photodecomposition of liquid water into H, and O, from suspension of semiconductor powder loaded with various metals. Chem Lett 2 253-256. 

Baba R, Nakabayashi S, Fujishima A, Honda K (1985) Investigation of the mechanism of hydrogen evolution during photocatalytic water decomposition on metal-loaded semiconductor powders J Phys Chem 89 1902-1905... 

Fig. 3. Redox Exchange on an Irradiated Metallized Semiconductor Powder... 

In a series of transition metal oxide semiconductor powders, photochemical activity in the decarboxylation of oxalic acid was controlled by surface properties and the presence of recombination centers, which in turn depended on the preparation method Similar effects have also been noted in the photodecarboxylation of pyruvic acid and formic acid... 

Photocatalytic reduction of C02 can be accomplished by suspending photosensitive semiconductor powders in aqueous solutions under irradiation, usually using UV light.129156 Photoreduction of C02, however, is in competition with H2 formation due to water decomposition, and leads to mixtures of reduced carbon products. Selectivity, therefore, is one of major problems of these processes. 

Water Photolysis by Metallized Semiconductor Powders 13.3.1 Gas-phase Water Photolysis by NaOH-coating... 

Besides the solution thickness on the catalyst and the concentration of deliquescent material, various other factors which may influence the yield of water photolysis over metallized semiconductor powders have been examined. 

This point may be located analytically by virtue of the fact that a semiconductor powder in a solution of adsorbing ions acts as a buffer for those ions everywhere but at the PZZP. Thus, potential drift or differential potentiometric titrations( 7,8 ) can be employed to determine the PZZP as illustrated in Figure 2 for CdS. Once the PZZP is determined in this fashion, a direct comparison of EA and is possible and has been done for a variety of semiconductors. (jO Figure 3 illustrates the vs. pH data for p-GaP and shows good agreement between the predicted at the PZZP from electronegativity calculations and the observed value.(9)... 

Other aromatic molecules also suffer oxygenation on irradiated semiconductor powders. For example, both ring and side chain oxidation products can be observed when toluene is allowed to contact excited Ti02> eq. 91 (289) ... 

The occurrence of cycloaddition (300) and cycloreversion (300,301) pathways mediated by excited semiconductor powders has also been reported, presumably by routes parallel to those discussed above for homogeneous electron-transfer cycloadditions. 

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105],... 

Heterogeneous semiconductor systems involve either suspensions or slurries of larger-sized semiconductor powders, or smaller colloids in solution. In principle, these semiconductor particles may act as tiny photoelectrolysis cells, similar to the photoelectrochemical systems discussed above. However, as many of the materials used for bulk electrodes are also described here in particulate form, both similar and new problems may arise, most notably irreproducibility in particle preparation, stability issues, and low C02 reduction rates. 

Among the earliest reports of semiconductor powders being used for the reduction of C02 was one made by Inoue et al. [106], who examined a wide range of semiconductors (W03, Ti02, ZnO, CdS, GaP, SiC, in 200-400 mesh), illuminated in aqueous solution. The study results showed that C02 could indeed be reduced to... 

A similar situation also is encountered in a miniaturized photoelectrochemical cell, i.e., on a metallized semiconductor powder, Figure 2. Here, the individual particle can be thought of as two electrochemical half cells which have eventually collapsed onto each other as the conductive wire connecting them became shorter and shorter. The oxidizing and reducing sites are thus found in close spatial proximity and the potential for subsequent chemical reaction between the initial oxidation and reduction products is excellent. In fact, so long as the respective rates of the oxidation and reduction half reactions differ appreciably, it may be unnecessary to metallize the semiconductor powder in order to... 

Although it is clear that photoinduced redox exchange can occur efficiently at the surface of an irradiated semiconductor powder, this redox chemistry will not find extensive use unless it provides access to new chemical transformations which are inaccessible with conventional reagents or to an improved selectivity in multifunctional molecules or in mixtures of reagents. 

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