Photocatalyst selectivity

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. 

Shiraishi, Y. and Hirai, T. (2008) Selective organic transformations on titanium oxide-based photocatalysts. Journal of Photochemistry and Photobiology C Photochemistry Reviews, 9 (4), 157-170. 

Matos,J., Garcia,A., Cordero,T., Chovelon, J.M., and Ferronato, C. (2009) Eco-friendly Ti02-AC photocatalyst for the selective photooxidation of 4-chlorophenol. Catalysis Letters, 130 (3 1), 568-574. 

Ohtani, B., Iwai, K., Kominami, H., Matsuura, T., Kera, Y., and Nishimoto, S. (1995) Titanium(IV) oxide photocatalyst of ultra-high activity for selective N-cyclization of an amino acid in aqueous suspensions. Chemical Physics Letters, 242 (3), 315-319. 

Metal ions or their oxides were combined with Ti02 nanomaterials in order to improve the catalytic activity and selectivity of the Ti02 photocatalysts. Ag-doped Ti02 was used to degrade gaseous sulfur compounds [158], Ag nanoparticle-doped Ti02... 

Sanjuan, A., Alvaro, M., Corma, A. and Garcia, H. (1999). An organic sensitizer within Ti-zeolites as photocatalyst for the selective oxidation of olefins using oxygen and water as reagents. Chem. Commun. 1641-1642... 

In classical kinetic theory the activity of a catalyst is explained by the reduction in the energy barrier of the intermediate, formed on the surface of the catalyst. The rate constant of the formation of that complex is written as k = k0 cxp(-AG/RT). Photocatalysts can also be used in order to selectively promote one of many possible parallel reactions. One example of photocatalysis is the photochemical synthesis in which a semiconductor surface mediates the photoinduced electron transfer. The surface of the semiconductor is restored to the initial state, provided it resists decomposition. Nanoparticles have been successfully used as photocatalysts, and the selectivity of these reactions can be further influenced by the applied electrical potential. Absorption chemistry and the current flow play an important role as well. The kinetics of photocatalysis are dominated by the Langmuir-Hinshelwood adsorption curve [4], where the surface coverage PHY = KC/( 1 + PC) (K is the adsorption coefficient and C the initial reactant concentration). Diffusion and mass transfer to and from the photocatalyst are important and are influenced by the substrate surface preparation. 

The selective oxidation and, more generally, the activation of the C-H bond in alkanes is a topic of continuous interest. Most methods are based on the use of strong electrophiles, but photocatalytic methods offer an interesting alternative in view of the mild conditions, which may increase selectivity. These include electron or hydrogen transfer to excited organic sensitizers, such as aryl nitriles or ketones, to metal complexes or POMs. The use of a solid photocatalyst, such as the suspension of a metal oxide, is an attractive possibility in view of the simplified work up. Oxidation of the... 

Zhang, N., et ah, Assembly of CdS nanoparticles on the two-dimensional graphene scaffold as visible-tight-driven photocatalyst for selective organic transformation under ambient conditions. The Journal of Physical Chemistry C, 2011.115(47) p. 23501-23511. 

The use of nanocarbon-semiconductor hybrid materials thus offers a great potential to the design and development of novel/improved photocatalysts and photoanodes, but it is necessary to have a detailed understanding of the many factors which determine the overall properties. This chapter will analyze these aspects presenting a concise analysis of the topic with selected relevant developments in the field, mainly in the last few years. 

In considering photoactivity on metal oxide and metal chalcogenide semiconductor surfaces, we must be aware that multiple sites for adsorption are accessible. On titanium dioxide, for example, there exist acidic, basic, and surface defect sites for adsorption. Adsorption isotherms will differ at each site, so that selective activation on a particular material may indeed depend on photocatalyst preparation, since this may in turn Influence the relative fraction of each type of adsorption site. The number of basic sites can be determined by titration but the total number of acidic sites is difficult to establish because of competitive water adsorption. A rough ratio of acidic to basic binding sites on several commercially available titania samples has been shown by combined surface ir and chemical titration methods to be about 2.4, with a combined acid/base site concentration of about 0.5 mmol/g . 

A few studies did address chemical selectivity in these oxidations. Pattenden and coworkers, for example, showed that primary alcohols could be selectively oxidized to the corresponding aldehydes, without appreciable overoxidation, when the platinized TiO2 photocatalyst was suspended in benzene, Eq. (11). Poor yields were obtained... 

In this section, new methods for the stereospecific epoxidation of 2-hexene and the selective oxidation of naphthalene using Ti02 as photocatalyst under UV illumination are introduced. Both these reactions under similar conditions have been examined by several authors, but the conditions for the selective epoxidation and selective oxidation was optimized only recently. For the epoxidation of olefins, only molecular oxygen is needed for the reactant. It is emphasized that the epoxidation reaction proceeded stereospecifically in high yield. In the case of... 

Selective Oxidation of Naphthalene by Molecular Oxygen and Water Using Ti02 Photocatalysts... 

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)... 

Concluding this section, when very fine particles were dispersed in the reactant solution, i.e., the number of particles and the surface area increased, the reactivity of the sonophotocatalytic reaction decreased and the product ratio became lower. In general, for photocatalytic reactions, the finer the photocatalyst, the better for the reaction. However, for sonophotocatalytic reactions it was found that the finer the particles such as Ti02-B in the reactant solution, the worse the product ratio. Since it is impractical to obtain and use a photocatalyst of very large particle size to increase the activity limitlessly, a suitable particle size must be selected to obtain high performance in the sonophotocatalytic reaction. 

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. 

The redox potentials of the valence and conduction bands for different semiconductors varies between +4.0 and —1.5 volts vs. the normal hydrogen electrode (NHE), respectively. Therefore, by careful selection of the photocatalyst a wide range of molecules can be converted via these processes [3]. 

Membrane processes, indeed, thanks to their selective properties, allow not only to recovery and to reuse the photocatalyst but also to enhance the residence time of the substrates to be degraded or to obtain a selective separation of the products. 

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