Photocatalyst Photochemical reactions

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

The photocatalytic system is shown in Scheme 5, where BNAH is oxidized by the ZnP + moiety in the radical ion pair ZaP -Ceo (ki) produced upon photoirradiation of ZnP-Ceo, whereas HV " is reduced to HV by the Ceo" moiety of ZnP +-C6o ki). These individual electron-transfer processes compete, however, with the BET in the radical ion pair (/cbet)- This pathway was experimentally confirmed by photolysis of the ZnP-Ceo/BNAH/HV and ZnP-H2P-C6o/BNAH/HV + systems with visible light (433 nm) in deoxyge-nated PhCN [70], For instance. Fig. 4 depicts the steady-state photolysis in deoxy-genated PhCN, in which the HV absorption band (X ax = 402 and 615 nm) increases progressively with irradiation time. By contrast, no reaction occurs in the dark or in the absence of the photocatalyst (i.e., ZnP-Ceo or ZnP-H2P-C6o) under photoirradiation [70]. Once HV+ is generated in the photochemical reaction, it was found to be stable in deoxygenated PhCN. The stoichiometry of the reaction is established as given by Eq. (3), where BNAH acts as a two-electron donor to reduce two equivalents of HV [70] ... 

The decomposition of liquid water and the following reactions are the results of a typical chemical effect. In this case, however, overall water splitting does not occur because oxygen is not obtained but hydrogen and hydrogen peroxide are. On the other hand, it is impossible to decompose water by photochemical reaction under illumination with a xenon lamp. Although it is possible to decompose water by photocatalytic reaction using a desirable photocatalyst and photoirradiation, it is difficult to decompose in practice because of rapid backward reaction, the formation and accumulation of intermediates onto the surface of photocatalyst,10) and other reasons. 

Various efforts to apply photocatalysis to photoenergy conversion are described in Part III. Synthetic chemistry utilizing photocatalysis by semiconductors has been attracting attention as discussed in Chapters 11 and 12. The merits of the photocatalysts for synthetic chemistry are (a) multiple processes are possible, (b) catalysts can be separated easily and re-used, and (c) the reactions can proceed under ambient conditions, etc. (Chapter 11). In Chapter 12 photolysis and sonolysis are combined to obtain specific effects in addition to photochemical reactions. 

Alternatively, light is consumed and the reaction progress is possible only under continuous light absorption this option, called catalyzed photolysis, includes photoassisted generation of a reactive form of substrate or photocatalyst. In the former the process is called catalyzed photochemical reaction, whereas in the latter either catalyst activation may lead to formation of catalyst or photoinitiator, which initiates chemical transformations but is consumed within a reaction cycle, or the catalyst reacts with substrate in its excited state (photosensitization) in both cases... 

Photochemical Reactions of the Ligand in Re(I) Complexes Rhenium(I) Complexes as Highly Efficient Photocatalyst A. Mononuclear Rhenium(I) Complexes Reaction Mechanism Multicomponent Systems... 

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... 

According to the simple model of surface photochemical reactions illustrated earlier (eqs. 5.114-5.117), if reductive and oxidative processes, Le. interaction of reagent molecules with electrons and holes, respectively, take place only on the photocatalyst surface then the selectivity toward product formation by the reductive pathway can be expressed by eq. 5.136, and for product formation through the oxidative pathway by expression 5.137 (Emeline et al, 2003). 

Applying the solid plate models (Figs. 5.29 and 5.36) and the results obtained for the surface concentration of charge carriers, the behaviour of photocatalysts during photochemical reactions can be analysed. The model(s) indicates that the ratio is governed by various factors ... 

In order to estimate quantitatively the possible influence of heterogeneous photocatalysis on the composition of the atmosphere, one can compare the rates of the expected photochemical reactions of various atmospheric components over aerosol photocatalysts with the rates of the natural removal of these components from (or supply into) the atmosphere (see Table 1). 

Decarboxylation of silver carboxylates is a well known thermal process and is involved in the Hunsdiecker76 or Kolbe77 reactions. The Hunsdiecker reaction is the thermal decarboxylation of silver salts of acids and is used for the formation of bromoalkanes and related compounds, while the Kolbe process involves electrolysis of carboxylates as a route to decarboxylated radicals that can dimerize. Silver carboxylates are also photochemically reactive and the irradiation has been described as a facile process for the formation of alkyl radicals, as illustrated in equation 678. Later experimentation has shown that the irradiation of silver trifluoroacetate can serve as a route to trifluoromethyl radicals. This development uses irradiation of silver trifluoroacetate in the presence of titanium dioxide as a photocatalyst. The reaction follows the usual path with the formation of metallic silver and the formation of radicals. However, in this instance the formation of metallic... 

Furthermore, silica-based systems received attention as effective catalysts for several photochemical reactions, e.g olefin photo-isomerisation [12], olefin photo-oxidation [13-15], photomethatesis [16] and methane coupling [17]. Recently, it was also reported that silica mesoporous materials are more active photocatalysts than amorphous silica [18]. Among them, Mg/Si02 systems [13] were found to act as catalysts for the photo-oxidation of propene to propene oxide in the presence of molecular oxygen, which is an attractive path for the production of this industrially important chemical. 

Selective encapsulation of substrate(s), preorganization of substrates in the confined space of the NR, and an increase of local concentration within the NR can result in unusual regio- and stereoselectivity, reaction rate, and yield, and an alteration in the reaction pathway. It is worth nothing that NRs can also be used in photochemical reactions and development of photocatalysts. As an example, dendrimer-protected Tt02 nanoparticles were synthesized and their photocatalytic activity was compared to that of Ti02 nanoparticles in photodegradation of 2,4-dichlorophenoxyacetic acid. The results indicated the superior activity of dendritic photocatalysts, which was attributed to stabilization of nanoparticles and reinforcement of photocatalytic activity [43]. 

The challenging task of multielectron transfer reactions such as Oj and H reductions, which are efficiently performed by many metalloenzymes, was explored in seminal studies by Gray et oi in photochemical reactions of dirhodium(I) isocyanide complexes upon irradiation at 550 nm in aqueous HCl solutions, the Rh(I) isocyanide precursor RhjfCNlCHjljNC] forms and the dirhodium(II,II) salt [Rh2[CN(CH2)3NC] Cl2]Cl2 [151]. Further preeminent studies by Nocera et aL [40] provided access to a suite of dirhodium-based photocatalysts that enable production of and reduction in HX (X = halide) solutions, as well as valuable insight into the corresponding photocydes (Scheme 9.22), which are currently actively explored along with the reactivity and further development of judiciously chosen dirhodium units [152]. 

Due to the fact that the difference between a sensitised and a catalysed photoreaction is somewhat arbitrary, due to the different and complex mechanisms involved (static and dynamic sensitisation, interaction with a photoproduct, photoinduced reactions,...), the term, photocatalysis has therefore been defined as broad as possible without the specific implication of any special mechanism, and refers then to the action of a substance whose function is activated by the absorption of a photon. A photocatalyst can be described as one involved in the quantum yield expression for a photochemical reaction without its stoichiometric involvement or more precisely, it appears in the quantum yield expression for reaction from a particular excited state to a power greater than its coefficient in the stoichiometric equation [14]. 

---