Semiconductor catalysts dehydrogenations

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

Serpone N., Borgarello E., Pelizzetti E. and Barbeni M. (1985b), A visible light-induced dehydrogenation of alcohols. Improved efficiencies of hydrogen formation via coupling of two semiconductor catalysts , Chim. Ind. (Milano) 67, 318-324. 

Radha and Swamy (278) proposed a possible mechanism for the dehydrogenation of 2-propanol over La2MnM06 (M = Co, Ni, Cu). These authors found that admission of H2, together with the alcohol, does not have any influence on the reaction rate however, admission of acetone with 2-propanol decreases the reaction rate at all partial pressures. It can be inferred that H2 acts as a mere diluent whereas acetone has an inhibiting effect that may be due to its slow desorption. They also measured the conductivity changes of the catalyst in the presence of the reactants or products of the dehydrogenation. As a result of these studies it was concluded that the catalyst surface is covered predominantly with acetone under reaction conditions. Because acetone adsorbs by a donor-type mechanism, as shown by the decrease of the conductivity on its adsorption, its desorption involving electron transfer from the p-type semiconductor catalyst to the adsorbed species can be expected to be the slow process. 

The efficiency of semiconductor PCs in some reactions (such as dehydrogenation of organics, splitting of HjO and H2S, etc.) can be enhanced by depositing tiny islands of additional catalysts, which facilitate certain reactions stages that may not require illumination. For example, islands of Pt metal are deposited on the surface of the composite photocatalyst in Fig.6 with the aim to facilitate the step of H2 formation. 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

In this respect not much information can be obtained from investigations made to relate the semiconduction properties of the catalyst to its selectivity. Both the dehydrogenating ZnO and the dehydrating TiOa are w-type semiconductors, whereas the dehydrogenating MgO and the dehydrating Si02 are well-known insulators. 

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