Redox catalysts relay

Whilst [ Ru(bipy)3]2t itself is incapable of splitting water, its electron-transfer properties have been utilized for hydrogen production in a series of reactions involving cocatalysts (see equations 21 to 26). The first step involves electron transfer from the excited state complex to an electron relay (R), which in its reduced form is capable (in the presence of a suitable heterogeneous redox catalyst) of reducing protons to hydrogen. The [Ru(bipy)3]3+ which is formed is then capable of... 

Crucial to the success of reactions designed to produce hydrogen via intermolecular electron transfer reactions is the addition of an efficient redox catalyst which allows reduction of protons by the reduced form of the relay (e.g. MV+) formed by the initial photochemical electron transfer. 

Figure 9. Cyclic water cleavage by visible light in the presence of Ru(bipy)i as a sensitizer and bifunctional redox catalyst as an electron relay.

For these reactions to be exothermic, to take place in the dark, the (A/A ) and (D+/D) couples must satisfy the thermodynamic energy requirement, viz. E0 (A/A") < Eo (H+/ H2) and E0 (D+/D) > E0 (02/H20). The aim of all photoredox schemes is to photogener-ate such relays. As will be seen later in this section, it is possible to generate such relays in reasonably high quantum efficiency, but the efficiency of reactions (2.1) and (2.2) in the absence of redox catalysts, is often extremely low. 

Unfortunately, the number of systems where such a scheme has been shown to work efficiently are not many. This is mainly due to the gross inefficiency of the reduced relay A" to evolve H2, in reaction (2.10), in the absence of any redox catalysts. 

The primary redox products are then stabilized either through judicious choice of D/S/A relays or through Jhe use of multiphase systems. Generation of fuels 0 from S (or D ) and A (or S ) subsequently is achieved with the aid of redox catalysts. 

These are the first examples of "in vitro" systems which produce large amounts of by H reduction in water using visible light. The essential ingredients of systems of this type are a sensitizer (S), an electron relay (A) and a redox catalyst. Excitation of the sensitizer induces as electron-transfer ... 

The redox properties of Ru(bipy)5 " (ground state and excited state) have been taken advantage of Ru(bipy)3 is able to transfer an electron to a relay (MV, or a rhodium(III) complex or another electron acceptor) whose reduced form reacts with water to yield hydrogen the latter reaction might be accelerated by the presence of a heterogeneous redox catalyst. The ruthenium(II) complex is regenerated in the reaction between Ru(bipy) " and an electron donor D. This compound D is irreversibly converted to an oxidation product. An ideal system would, of course, use H2O as electron donor, with formation of O2. This remains to be done, but model systems for H2O oxidation have also been proposed [20] ... 

Figure 3.4.11 Synthetic molecular catalysts for hydrogen conversion. (A) [FeFe] hydrogenase model with functionalities for substrate binding (H2) and management of redox and proton equivalents (adapted from [162]). (B) Synthetic mononuclear Ni electrocatalyst with pending amines that function as proton relays proposed transition state for heterolytic H2 splitting and formation (adapted from [165]).

Actually, one of the most important applications of metal nanoparticles is in the field of catalysis. Catalysts should offer large specific area in order to accelerate the access of reactants to the active sites. Nanoparticles, such as those synthesized by radiolysis, are thus particularly efficient in a number of reactions. However, catalyzed reactions are controlled not only by the kinetics, but also by the thermodynamics. Thus, due to their redox properties, nanoparticles with small sizes and low polydispersities are able to play a role as intermediate electron relays in an overall electron transfer between a donor and an acceptor. 

The prototypical photochemical system for CO2 reduction contains a photosensitizer (or photocatalyst) to capture the photon energy, an electron relay catalyst (that might be the same species as the photosensitizer) to couple the photon energy to the chemical reduction, an oxidizable species to complete the redox cycle and CO2 as the substrate. Figure 1 shows a cartoon of the photochemical CO2 reduction system. An effective photocatalyst must absorb a significant part of the solar spectrum, have a long-lived excited state and promote the activation of small molecules. Both organic dyes and transition metal complexes have been used as photocatalysts for CO2 reduction. In this chapter, CO2 reduction systems mediated by cobalt and nickel macrocycles and rhenium complexes will be discussed. 

Figure 13. (a) Mechanism of catalytic electron transfer involving metal clusters as the relay. The thermodynamic conditions to be fulfilled are that the cluster redox potential be higher than the donor D potential and lower than the acceptor A potential. This implies that the size of the cluster is within the size range appropriate for an efficient redox potential. (b) Rough catalyst surface with variable local redox potentials. The catalytic efficiency is restricted to regions where the potential is between those of the donor and of the acceptor. On other sites, reduction of A or oxidation of D are predominant. 

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