Redox catalysts colloidal solutions

In the particular case of the carboxylates of the eosin family, a careful examination of electron transfer paths was made by K. Tokumaru and cowor-kers [158, 159, 160]. They found that eosin Y (EY ) was able to sensitize hydrogen evolution from water under visible light irradiation in a system made of an amine donor, methyl viologen (MV2+) mediator and redox catalyst such as colloidal platinum. In a water-ethanol solution of EY, in the presence of MV2+ and triethanolamine (TEOA), a maximum quantum yield of 0.3 was measured for the MV + radical cation formation. Kinetic measurements of the EY triplet quenching constant by MV2+ (kq = 3.109 M 1 s 1) and TEOA... 

Efficient Water Cleavage by Visible Light in Colloidal Solutions of Bifunctional Redox Catalysts... 

Around 1975, investigations of photoelectrochemical reactions at semiconductor electrodes were begun in many research groups, with respect to their application in solar energy conversion systems (for details see Chapter 11). In this context, various scientists have also studied the problem of catalysing redox reactions, for instance, in order to reduce surface recombination and corrosion processes. Mostly noble metals, such as Pt, Pd, Ru and Rh, or metal oxides (RUO2) have been deposited as possible catalysts on the semiconductor surface. This technique has been particularly applied in the case of suspensions or colloidal solutions of semiconductor particles [101]. However, it is rather difficult to prove a real catalytic property, because a deposition of a metal layer leads usually to the formation of a rectifying Schottky junction at the metal-semiconductor interface (compare with Chapter 2), as will be discussed below in more... 

Although photoelectrochemical systems are able to offer respectable conversion efficiencies, the refinement of other solution-based processes continues. Gratzel has reviewed photoredox processes, paying particular attention to the use of organized assemblies such as micelles and vesicles. He emphasizes the central role of efficient colloidal metal catalysts in these schemes and also describes the recent development of bifunctional redox catalysts that allow the combination of cycles for the generation of hydrogen and oxygen. 

For example Kurihara and Fendler [258] succeeded in forming colloid platinum particles, Ptin, inside the vesicle cavities. An analogous catalyst was proposed also by Maier and Shafirovich [164, 259-261]. The latter catalyst was prepared via sonification of the lipid in the solution of a platinum complex. During the formation of the vesicles platinum was reduced and the tiny particles of metal platinum were adsorbed onto the membranes. Electron microscopy has shown a size of 10-20 A for these particles. With the Ptin-catalyst the most suitable reductant proved to be a Rh(bpy)3+ complex generated photochemically in the inner cavity of the vesicle (see Fig. 8a). With this reductant the quantum yield for H2 evolution of 3% was achieved. Addition of the oxidant Fe(CN), in the bulk solution outside vesicles has practically no effect on the rate of dihydrogen evolution in the system. Note that the redox potential of the bulk solution remains positive during the H2 evolution in the vesicle inner cavities, i.e. the inner redox reaction does not depend on the redox potential of the environment. Thus redox processes in the inner cavities of the vesicles can proceed independently of the redox potential in the bulk solution. 

The kinetics found for the reactions at the solution/solid interface show some marked similarities with those at gas/solid [9, 49], gas/liquid, and liquid/liquid interfaces [268]. Whenever one of the phases is a liquid rather than a gas, mass transport is apt to become rate-controlling because of the smaller diffusion coefficients of species in liquids. Many of the catalysed redox reactions in Sect. 4 were indeed partly or wholly diffusion-controlled. These systems could be converted to surface-controlled ones simply by reducing the size of the catalysing material by using colloidal catalysts, for... 

These three techniques are employed along with others not mentioned here to investigate the catalytic nature of a reaction. It is difficult to obtain positive confirmation for one catalytic nature over another because of the ability of small amounts of homogeneous catalyst (concentrations below current detection methods) to catalyze reactions [11]. Leached atoms can readsorb rapidly to heterogeneous structures, either to a substrate or to the surface of the nanoparticles [17,18], In the following sections, we review some of the major results involving colloidal nanoparticles in solution-phase catalysis. The two reaction types that will be discussed in this chapter are redox reactions and carbon-carbon bond formation reactions. 

Dinitrogen has been photoreduced to ammonia in aqueous solutions containing colloidal transition metal catalysts (Nahor et al.). Kamogawa and Sato have observed redox photochromism in a crystalline l,r-diaryl-4,4 -bipyridinium salt. 

---