Homogeneous semiconductor photocatalysis

In this chapter, an attempt has been made to address fundamental mechanistic and kinetic aspects of TiO2 photocatalysis of organophosphorus compounds. Comparisons between homogeneous (radiolysis) and heterogeneous (photocatalysis) hydroxyl-generating processes have helped to elucidate the reaction pathways and led to number of important mechanistic conclusions. From the various kinetic parameters, the overall rates and efficiencies for the degradation of organophosphorus compounds can be predicted and may find direct application in evaluation and implementation of semiconductor photocatalysis. 

Hydrazone cyclization and hydroalkylation [138-140] are rare examples of reactions conducted on a preparative scale, since the products were isolated in milligram amounts and not just identified in solution. As already mentioned in Section 6.2.5, photocorrosion of the semiconductor photocatalyst often prevents its use in preparative chemistry. This is very true also for colloidal semiconductors although the pseudo-homogeneous nature of their solutions allows one to conduct classical mechanistic investigations, until now they were too labile to be used in preparative chemistry [107, 141, 142]. In contrast to the above-mentioned reactions, in recent years we have isolated novel compounds on a gram-scale employing photostable zinc and cadmium sulfide powders as photocatalysts [97, 107, 143-145]. During this work we found also a new reaction type which was classified as semiconductor photocatalysis type B [45]. In contrast to type A reactions, where at least one oxidized and one reduced product is formed, type B reactions afford only one unique product, i.e., the semiconductor catalyzes a photoaddition reaction (see below). 

Reactions which formally can be classified as cycloadditions or retrocycloadditions have been observed in homogeneous solution with single electron oxidative initiation, so the observation of parallel activity on irradiated semiconductor suspensions is certainly reasonable. The first example of such a reaction involved the photocatalysis by ZnO or CdS of the ring-opening of a strained hydrocarbon which could also be opened in the dark by a single electron oxidant, ceric ammonium nitrate, Eq. (38)... 

Meissner, D., Sinn, C., Memming, R., Notten, P.H.L. and Kelly, J.J. 1986. On the nature of the inhibition of electron transfer at illuminated p-type semiconductor electrodes. In Homogenous and Heterogenous Photocatalysis" (Ed. E.Pelizzetti andN.Serpone). (D.Reidel, Dordrecht), pp. 317-333. 

Since photoexcitation induces significant enhancement of the reactivity of electron transfer, photochemical reactions via photoinduced electron transfer have been explored in homogeneous systems [43 52], On the other hand, the term photocatalysis has usually been used in heterogeneous systems involving photoinduced electron transfer across the gas-solid or liquid-solid interface [53-60], Photocatalysis has been extensively studied using a semiconductor particle as a photocatalyst [53-60], Photocatalysis is initiated by the absorption of a band gap photon... 

Although the lifetime of the reactive electron-hole pair is not known, the reasonable estimate of 10 -10 s leads to an electron transfer rate constant between 10 and 10 M s . In general, electron transfer reactions at the semiconductor-liquid interface are very fast [see Ref [33] and N. Serpone, E. Pellizetti (Eds.), Homogeneous and Heterogeneous Photocatalysis, Reidel, Dordrecht, 1986, p. 51]. 

The efficacy of semiconductor devices in photocatalysis is typically described by the quantum yield (j). While determining this parameter in homogeneous photochemistry presents no great difficulties, in heterogeneous media the challenges have been insurmountable to many workers for several years owing to the lack of an appropriate protocol. This was remedied by Serpone and Salinaro (1999), and appropriate experimental details were enumerated by Salinaro et al. (1999). Another useful parameter, the relative photonic efficiency 4ei (defined in Section 5.4.1) also provided a method by which work from many laboratories in environmental photochemistry could be calibrated when the more fundamental parameter 4> could not be accessed because of experimental limitations (Salinaro et al, 1999). 

Serpone N. and Pelizzetti E. (1986), Fundamental studies into the primary events in photocatalysis employing CdS and Ti02 semiconductors. Photoluminescence, laser flash photolysis, and pulse radiolysis , in Homogeneous and Heterogeneous Photocatalysis, Pelizzetti E. and Serpone N., eds., Reidel Publ. Co., Dordrecht, The Netherlands, pp. 51-90. 

Homogeneous doping of semiconductor particles with a small amount of metal ions such as Fe " and V " prolongs the electron-electron-hole separation and hence increases the photocatalytic efficiency [46,47]. However, doping of Ti02 with metal ions such as Cr + and Sb + creates electron acceptor and donor centers that accelerate the charge recombination—an undesirable result for photocatalysis [48-50]. 

Semiconductors excited by appropriate energy photons. Two weU-defined AOP s systems which have special interest, because natural solar light can be used, are heterogeneous photocatalysis with Ti02 and homogeneous photocatalysis by the photo-Fenton process. These processes are covered in detail in the following sections. 

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