Catalysts semiconductors

In most studies, heterogeneous photocatalysis refers to semiconductor photocatalysis or semiconductor-sensitized photoreactions, especially if there is no evidence of a marked loss in semiconductor photoactivity with extended use. It is meant here that the initial photoexcitation takes place in the semiconductor catalyst substrate and the photoexcited catalyst then interacts with the ground state adsorbate molecule [209]. 

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])...

Fig.S.The nature and general pathway of the photocatalytic action of a semiconductor catalyst particle. Eg is the band gap, E shows the direction of change of the energy for electrons...

Photocatalytic oxidation is a novel approach for the selective synthesis of aldehyde and acid from alcohol because the synthesis reaction can take place at mild conditions. These reactions are characterized by the transfer of light-induced charge carriers (i.e., photogenerated electron and hole pairs) to the electron donors and acceptors adsorbed on the semiconductor catalyst surface (1-4). Infrared (IR) spectroscopy is a useful technique for determining the dynamic behavior of adsorbed species and photogenerated electrons (5-7). 

Gondal MA, Seddiqi Z (2006) Laser-induced photo-catalytic removal of phenol using n-type WO3 semiconductor catalyst. Chem Phys Lett 417 124-127... 

Gondal MA, Sayeed MN, Seddiqi Z (2008) Laser enhanced photo-catalytic removal of phenol from water using p-type NiO semiconductor catalyst. J Hazard Mater 155 83-89... 

Although the role of rare earth ions on the surface of TiC>2 or close to them is important from the point of electron exchange, still more important is the number of f-electrons present in the valence shell of a particular rare earth. As in case of transition metal doped semiconductor catalysts, which produce n-type WO3 semiconductor [133] or p-type NiO semiconductor [134] catalysts and affect the overall kinetics of the reaction, the rare earth ions with just less than half filled (f5 6) shell produce p-type semiconductor catalysts and with slightly more than half filled electronic configuration (f8 10) would act as n-type of semiconductor catalyst. Since the half filled (f7) state is most stable, ions with f5 6 electrons would accept electrons from the surface of TiC>2 and get reduced and rare earth ions with f8-9 electrons would tend to lose electrons to go to stabler electronic configuration of f7. The tendency of rare earths with f1 3 electrons would be to lose electrons and thus behave as n-type of semiconductor catalyst to attain completely vacant f°- shell state [135]. The valence electrons of rare earths are rather embedded deep into their inner shells (n-2), hence not available easily for chemical reactions, but the cavitational energy of ultrasound activates them to participate in the chemical reactions, therefore some of the unknown oxidation states (as Dy+4) may also be seen [136,137]. 

Korsunovsky (65-68), Gross weiner (77), Stephens and co-workers (69) and also Markham and Laidler (70) point out that the catalytic activity of semiconductor catalysts in relation to the reaction of oxidation of water under illumination with light from the fundamental absorption band first increases with increasing radiation dose and then attains saturation at sufficiently high doses. 

Numerous data on the influence of adsorbed molecules on the photocatalytic activity of semiconductor catalysts in relation to the oxidation of water are evidence that acceptor molecules retard (65-68, 71-73, 77) and donor molecules speed up the reaction (65-68). 

Rate of Photocatalytic H2 and 02 Evolution over Various Kinds of Metal-Loaded Simple Semiconductor Catalysts Suspended in Both Aqueous Solution Containing Optimum Quantity of Na2C03 and Pure Water... 

Schwerzel, R.E. and Spahr, K.B., Hydrogen Production with Photoactive Semiconductor Catalysts Stabilized by Metallized Plasma Polymer Coatings, Final Report to U.S. Department of Energy, Brookhaven National Laboratory, Battelle, 1989. 

Photocatalytic degradation of environmental pollutants by solar energy is a very attractive technology for the remediation of contaminated water [253,323], In some variants of this process, solar UV radiation is absorbed by semiconductor catalyst particles suspended in water. TiOz photocatalytic particles are the most widely used for these applications. 

Sayama, K., Arakawa, H. 1994. Effect of NajCO, addition on photocatalytic decomposition of liquid water over various semiconductor catalysts. J Photochem Photobiol A Chem 77 243-247. 

Table fi.4 Photocatalytic decomposition of water over various semiconductor catalysts in NajCOi solution or pure water. 

Zou Z, Ye J, Arakawa H (2003) Photocatalytic water splitting into H2 and or O2 under UV and visible light irradiation with a semiconductor catalyst. Int J Hydrogen Energy 28 663-669... 

Lee JS (2006) Photocatalytic water splitting under visible light with particulate semiconductor catalysts. Catal Surv Asia 9 217-227... 

Ueno A, N. Kakuta N, Park KH, Einlayson ME, Bard AJ, Campion A, Fox MA, Webber SE, White JM (1985) Silica supported ZnS-CdS mixed semiconductor catalysts for photogeneration of hydrogen. J Phys Chem 89 3828-3833... 

The Matrix TiOa photocatalytic treatment system is a technology that destroys dissolved organic contaminants in water in a continuous-flow process at ambient temperature. The technology uses ultraviolet (UV) light and a titanium dioxide (TiOa) semiconductor catalyst to break hydroxide ions (OH ) and water (H2O) into hydroxyl radicals (OH ). The radicals oxidize the organic contaminants to form carbon dioxide, water, and halide ions (if the contaminant was halogenated). 

Table 14.3 Rate of photocatalytic H2 and 02 evolution over mixed oxide semiconductor catalysts suspended in both Na2C03 aqueous solution and pure water...

The low reproducibility of certain of the systems involving heterogeneous catalysts, described above, as well as the potential for charge separation using semiconductor catalysts has led to the use of heterogeneous catalysts supported on semiconducting materials, e.g. Ti02. 

Room Temperature Oxidations, Isotopic Exchanges, and Dehydrogenations over Illuminated Neat or Metal-Supporting Semiconductor Catalysts... 

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