Photoexcited catalyst

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]. 

The authors have proposed a plausible mechanism for the trifluoromethylation reaction (Scheme 13.31). They assumed that a SET reduction ofCF3S02Cl224 by the photoexcited catalyst gives the oxidatively active catalyst and a CF3SO2CI radical anion 227, which rapidly collapses to generate the CF3 radical by the release of SO2 and chloride. The CFg radical then adds to a heteroarene to form the radical 228. Single-electron oxidation and subsequent deprotonation of the resulting cation 229 furnishes the trrfluoromethyl-substituted heteroarene products. 

It turned out that the dodecylsulfate surfactants Co(DS)i Ni(DS)2, Cu(DS)2 and Zn(DS)2 containing catalytically active counterions are extremely potent catalysts for the Diels-Alder reaction between 5.1 and 5.2 (see Scheme 5.1). The physical properties of these micelles have been described in the literature and a small number of catalytic studies have been reported. The influence of Cu(DS)2 micelles on the kinetics of quenching of a photoexcited species has been investigated. Interestingly, Kobayashi recently employed surfactants in scandium triflate catalysed aldol reactions". Robinson et al. have demonshuted that the interaction between metal ions and ligand at the surface of dodecylsulfate micelles can be extremely efficient. ... 

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])...

Chiral recognition of A-[Co(phen)3]3+ has been observed in a modified /3-cyclodextrin.772 Chiral discrimination has also been seen in photoinduced energy transfer from luminescent chiral lanthanoid complexes773 to [Co(phen)3]3+ and between photoexcited [Ru(bpy)3]2+ and [Co(phen)3]3+ co-adsorbed on smectite clays.774 The [Co(bpy)3]3+ ion has been incorporated into clays to generate ordered assemblies and also functional catalysts. When adsorbed onto hectorite, [Co(bpy)3]3+ catalyzes the reduction of nitrobenzene to aniline.775 The ability of [Co(phen)3]3+ to bind to DNA has been intensively studied, and discussion of this feature is deferred until Section 6.1.3.1.4. 

Photocatalysis uses semiconductor materials as catalysts. The photoexcitation of semiconductor particles generates electron-hole pairs due to the adsorption of 390 run or UV light of low wavelength (for Ti02). If the exciting energy employed comes from solar radiation, the process is called solar photocatalysis [21],... 

Figure 4.11 explains how the loaded metal can act as catalyst for both the reductive and oxidative reactions. The essential point is that the barrier height at the metal/semiconductor interface is changeable by the principle of Fig. 4 10 or other mechanisms. Thus, when the band bending is weak, photoexcited electrons mostly enter the metal and the metal acts as a catalyst for a reductive reaction (Fig. 4.11(A)). On the other hand, when the band bending is strong, the holes in the valence band mostly enter the metal and the metal acts as a catalyst for an oxidative reaction (Fig. 4.11(B)). The prevalence of one over the other depends on the magnitude of the band bending, i.e., on the relative rates of reaction of the electrons and holes at the metal-free semiconductor surface.

Our approach is vo establish eacn unit then comoine the units by utilizing molecular aggregates. Our progress towards this goal is described in Sections 19.3 (catalysts), 19.4 (photoexcited state electron transfer in a heterogeneous phase), and 19.5 (sensitization of Ti02 in water). 

Interfacial electron transfer across a solid-liquid junction can be driven by photoexcitation of doped semiconductors as single crystals, as polycrystalline masses, as powders, or as colloids. The band structure in semiconductors (281) makes them useful in photoelectrochemical cells. The principles involved in rendering such materials effective redox catalysts have been discussed extensively (282), and will be treated here only briefly. 

With the semiconductor oxidation catalyst, however, the surface becomes activated only upon photoexcitation. At low light intensities, the possibility that many holes are formed in the valence band is remote, so that the irradiated semiconductor powder becomes an effective one-electron oxidant. Now if the same chemistry ensues on the photochemically activated TiC>2 surface, then the reaction will proceed as in the bottom route of eqn 9. Thus, the carboxy radical is formed, producing an alkyl radical after loss of carbon dioxide. Since the semiconductor cannot continue the oxidation after the first step, the radical persists, eventually recapturing the conduction band electron, either directly or through the intervention of an intermediate relay, perhaps superoxide. The resulting anion would be rapidly protonated to product. 

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. 

A Mb reconstituted with cobalt(II) protoporphyrin IX was found to play a role as a catalyst for the photoinduced hydrogenation of acetylene when eosin is linked on the surface of the protein as shown in Fig. 24 (91). Irradiation of the eosin-linked cobalt Mb was found to produce a cobalt(I) species due to electron transfer from the photoexcited eosin moiety to cobalt(II) in the protein. In an aqueous solution, the cobalt(I) species is converted into cobalt(III)-hydride and the intermediate may react with acetylene in the presence of Na2edta [edta = ethylenediaminetetraacetic acid (ligand)] as a radical scavenger to... 

Catalyzed photolysis refers to catalysis of a photochemical reaction, for which there is a physical pathway for decay of the system back to its ground state (Figure 6.17). When the photocatalytic process occurs through photoexcitation of the catalyst, the physical decay may occur through recombination and/or thermal photoionization of the excited states, which ultimately leads to regeneration of the original state of the catalyst. Note that catalyzed photolysis is not catalytic in photons, contrary to photogenerated catalysis. 

Slower electron-hole recombination was observed in air, which was explained in terms of upward band bending near the surface because of adsorbed 02. In catalysts with high photocatalytic activity, the observed trapped carriers mainly exist near the particle surface and do not undergo rapid recombination after photoexcitation, as illustrated by Figure 16 (Anpo et al., 1991 Furube et al., 2001a). 

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