Heterogeneous photocatalyst

Heterogeneous Photocatalysis. Heterogeneous photocatalysis is a technology based on the irradiation of a semiconductor (SC) photocatalyst, for example, titanium dioxide [13463-67-7] Ti02, zinc oxide [1314-13-2] ZnO, or cadmium sulfide [1306-23-6] CdS. Semiconductor materials have electrical conductivity properties between those of metals and insulators, and have narrow energy gaps (band gap) between the filled valence band and the conduction band (see Electronic materials Semiconductors). 

Recently was estimated an expected impact on the global chemistry of the atmosphere of the indirect heterogeneous photocatalytic reactions under the much more abundant near ultraviolet, visible and near infrared solar light [2]. As photocatalysts may serve atmospheric aerosols, i.e. ultrasmall solid particles that sometimes are embedded into liquid droplets. Aerosols are known to contain Ti02, Fc203, ZnO and other natural oxides, as well as metal sulfides of volcanic or antropogenic origin, that may serve as semiconductor photocatalysts (see Fig.5). Aerosols are known to be concentrated mainly in the air layers near the surface of the Earth, i.e. in the troposphere, rather than stratosphere. 

Interfaces between two different media provide a place for conversion of energy and materials. Heterogeneous catalysts and photocatalysts act in vapor or liquid environments. Selective conversion and transport of materials occurs at membranes of biological tissues in water. Electron transport across solid/solid interfaces determines the efficiency of dye-sensitized solar cells or organic electroluminescence devices. There is hence an increasing need to apply molecular science to buried interfaces. 

Saravanan P, Pakshirajan K, Saha P (2009) Degradation of phenol by Ti02-based heterogeneous photocatalysts in presence of sunlight. J Hydroenviron Res 3 45-50... 

Another specific and important aspect to consider is the possibility that an environmentally heterogeneous photocatalyst can lead to the undesirable formation of reaction intermediates which are more toxic than the starting reagents. For instance, the Ti02-based photodegradation of ethanol, a relatively innocuous air pollutant, occurs through its transformation into the more toxic acetaldehyde. Condensation reactions can also lead to the formation of traces of methyl formate, ethyl formate, or methyl acetate. Catalyst design is therefore important to increase the overall oxidation rate to ensure complete mineralization (formation of C02 and H20). 

Catalysis is known as the science of accelerating chemical transformations. In general, various starting materials are converted to more complex molecules with versatile applications. Traditionally, catalysts are divided into homogeneous and heterogeneous catalysts, biocatalysts (enzymes), photocatalysts, and electrocatalysts, which are mainly used... 

Sanjuan, A., Alvaro, M., Aguirre, G., Garcia, H. and Scaiano, J.C. (1998). Intrazeolite photochemistry. 21. 2,4,6-Triphenylpyrylium encapsulated inside zeolite Y supercages as heterogeneous photocatalyst for the generation of hydroxyl radical. J. Am. Chem. Soc. 120, 7351-7352... 

Frank SN, Bard AJ. 1977. Heterogeneous photocatalyst oxidation of cyanide ion in aqueous solutions at titanium dioxide powder. J Amer Chem Soc 99(l) 303-304. 

We study here the heterogeneous photocatalytic oxidation of neat toluene, using unloaded and iron-loaded Ti02 as photocatalysts. The influence on the... 

In summary, although the heterogeneous photocatalyt.ir oxidation of toluene has been previously investigated by several authors, our investigation, has attempted to further correlate experimental conditions ( such as structural as pects and concentration of photocatalysts, irradiation times, etc.) with chemi-... 

Sonophotocatalysis is photocatalysis with ultrasonic irradiation or the simultaneous irradiation of ultrasound and light with photocatalyst. Tnis method includes irradiation with alternating ultrasound and light. Ultrasound effects on heterogeneous photocatalytic reaction systems have been demonstrated by Mason,1 Sawada et al.,2) Kado et al.,3) and Suzuki et al.4) In these papers, not only acceleration of photocatalytic reactions but increase in product selectivity by ultrasonic irradiation has also been reported. It was postulated that ultrasound effects, such as surface cleaning, particle size reduction and increased mass transfer, were the result of the mechanical effects of ultrasound.1,5) Lindley reviewed these and other effects.5)... 

Heterogeneous photocatalytic reaction products and their production rates depend on the kind of photocatalysts. As noted above, each photocatalyst powder has different properties. In the case of the sonophotocatalytic reaction, products or their yields also depend on the kind of photocatalysts. The effect of surface area on product ratio was discussed in section (12.2.1C). The influence of surface area on product ratio was noted, but factors other than surface area must be introduced to explain the difference in product ratios. It is known that there are several crystal structures of Ti02. The major structures are anatase and rutile. 2... 

Photoexcitation of n-type semiconductors renders the surface highly activated toward electron transfer reactions. Capture of the photogenerated oxidizing equivalent (hole) by an adsorbed oxidizable organic molecule initiates a redox sequence which ultimately produces unique oxidation products. Furthermore, specific one electron routes can be observed on such irradiated surfaces. The irradiated semiconductor employed as a single crystalline electrode, as an amorphous powder, or as an optically transparent colloid, thus acts as both a reaction template and as a directed electron acceptor. Recent examples from our laboratory will be presented to illustrate the control of oxidative cleavage reactions which can be achieved with these heterogeneous photocatalysts. 

We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

Photocatalysts have been also successfully heterogenized in polymeric (catalyti-cally active) membranes. 

Novel photocatalytic membranes have been prepared by the heterogenization in PVDF membrane ofthe decatungstate (W10O324 ), a polyanionic metal-oxide cluster used as photocatalysts for oxidation reactions. 

Heterogeneous catalysis is a surface phenomenon, therefore the overall kinetic parameters are dependent on the real exposed catalyst surface area. In the supported systems only a part of the photocatalyst is accessible to light and to substrate. Besides, the immobilized catalyst suffers from the surface deactivation since the support could enhance the recombination of photogenerated electron-hole pairs and a limitation of oxygen diffusion in the deeper layers is observed. 

It is well known that heterogeneous photocatalysis can be successfully used to photodegrade or to transform a wide range of molecules in liquid-solid and in gas-solid systems. Nevertheless, the knowledge of fundamentals of photocatalysis is essential to understand the mechanistic aspects and to find the parameters that influence the process under investigation. Moreover, the development of new photocatalysts and their application in the various research fields is a mandatory task. 

The efficiency of such photocatalysts in influencing charge separation will depend sensitively on the dynamics of interfacial charge transfer, particularly as compared with the rate of electron-hole recombination. The presence of trap sites and dopants will profoundly influence the latter rates, and quantum yields attainable on such heterogeneous suspensions will clearly reflect such structural variants. A trapped carrier will possess a longer lifetime consistent with an enhanced ability to participate in charge transfer with a desired adsorbate. 

From the abovedescribed principle of heterogeneous photocatalysis, it follows that photocatalytic reaction rates depend upon the characteristics of the irradiation, the mass of the photocatalyst, and the concentration (or partial pressure) of the reactants. 

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