Water reduction catalyst

Transition-metal catalyzed photochemical reactions for hydrogen generation from water have recently been investigated in detail. The reaction system is composed of three major components such as a photosensitizer (PS), a water reduction catalyst (WRC), and a sacrificial reagent (SR). Although noble-metal complexes as WRC have been used [214—230], examples for iron complexes are quite rare. It is well known that a hydride as well as a dihydrogen (or dihydride) complex plays important roles in this reaction. 

A cobalt complex 8 containing a redox active tetradentate bis-iminopyridine framework has been reported to support a water reduction catalyst, with activities of observed rate constant, of 10 M s derived from voltammetry measurements (Scheme 3) [16]. Ligand-centered reduction of the coordinated imine function has been proposed as the first electrocatalytic step followed by protonation. Notably, this compound was shown to operate even under basic conditions at pH 8 (buffer) to give 10 liter of H (mol catalyst" h" ) albeit with a modest Faradaic efficiency of only 60%. 

Photoinitiated electron collection in polyazine chromophores coupled to water reduction catalysts for solar H2 production 13CCR 1660. 

Fig. 4. (a) Ir turnover numbers (TON) (b) Rh turnover numbers and (c) kinetic and rate trace at 0.075 mM iridium photosensitizer (PS) and rhodium water reduction catalyst (WRC) concentration, from evaluating the effect of catalyst concentration on performance with [Ir(f- mppy)2(dtbbpy)](PF6) and [RhldtbbpylaKPFela in photosynthetic-H2 reactions (0.5-1.5 Atmol of PS and WRC in 10 mL of 0.6 M TEA in 80% THF-H2O, 460 mn, 500 mW, 22 h). Reprinted from Ref 60, copyright 2008, with kind permission from the American Chemical Society. 

It proves possible to anchor catalysts of H2 evolution to the outer and inner surface of the vesicle membrane. These catalysts are finely dispersed (10-20 A in diameter) metal Pt or Pd particles formed via reduction of appropriate salts in vesicle suspension (see [15, 16] and refs, therein). Among the viologen-type electron carriers a promising one is p-bis (1,2,5-triphenyl-4-pyridil)benzene which possesses reduction potential low enough for water reduction at neutral pH. Recently, using this mediator we succeeded in H2 evolution conjugated with PET... 

This latter point was stressed by some of us in a recent report studying NO storage and reduction on commercial LSR (lean storage-reduction) catalysts, in order to catch valuable information about the behaviour of typical NO storage materials in real application conditions. Nature, thermal stability and relative amounts of the surface species formed on a commercial catalyst upon NO and 02 adsorption in the presence and in the absence of water were analysed using a novel system consisting of a quartz infrared reactor. Operando IR plus MS measurements showed that carbonates present in the fresh catalyst are removed by replacement with barium nitrate species after the first nitration of the material. Nitrate species coordinated to different barium sites are the predominant surface species under dry and wet conditions. The difference in the species stabilities suggested that barium sites possess different basicity and, therefore, that they are able to stabilize nitrates at different temperatures. At temperatures below 523 K, nitrite species were observed. The presence of water at mild temperatures in the reactant flow makes unavailable for NO adsorption the alumina sites [181]. 

In the presence of 10% H20 but no C02, the same orange species accumulated in the solution and electrocatalysis was slow, with H2 being generated with a current efficiency of c. 85% (only a tiny amount of H2 was observed under the same conditions in the absence of the complex). In the presence of C02 the water reduction reaction was completely inhibited, showing that the orange species is less reactive towards water than COz and hence is a highly specific catalyst for the conversion of C02 to CO. 

Although there is evidence that quaternary ammonium salts are cleaved by sodium borohydride at high temperature [7], initial studies suggested that the quaternary ammonium borohydrides might have some synthetic value in their selectivity, e.g. aldehydes are reduced by an excess of the quaternary ammonium salts under homogeneous conditions in benzene at 25 °C, whereas ketones are recovered unchanged and are only partially reduced at 65 °C [2], The reduction of esters also requires the elevated temperature, whereas nitriles are not reduced even after prolonged reaction at 65 °C. Evidence that the two-phase (benzene water) reduction of octan-2-one by sodium borohydride was some 20-30 times faster in the presence of Aliquat, than in the absence of the catalyst [8], established the potential use of the mote lipophilic catalysts. 

With palladium catalysts aromatic chlorides are rather unreactive, however, nickel is able to catalyze the reactions of these substrates, too. The water-soluble catalyst was generated in situ from the easily available [NiCl2(DPPE)] and an excess of TPPTS by reduction with Zn in mixtures of 1,4-dioxane and water. Although it had to be used in relatively large quantities (10 mol %), the resulting compound catalysed the cross-coupling... 

Another approach to wards photocatalysis is to use dy as a sensitizer instead of a semiconductor as in photosynthesis. It is not the aim of this book to cover all the aspects of the sensitized photochemical conversion system, but typical sensitized systems for photocatalytic reactions of water are described in Chapter 18 The concept of a photochemical conversion system using a sensitizer and water oxidation/reduction catalysts is mentioned in Chapter 19, accompanied by a discussion on the sensitization of semiconductors. 

Figure 1. Basic features of sacrificial water reduction systems. (A) Homogeneous solution, with sensitizer, S, electron relay, R, sacrificial electron donor, D, and metal catalyst. (B) Catalyst-coated colloidal semiconductor dispersion, obviating the need for electron relay.

Actually, the reaction with alcohol may result in replacement of the —N2X group by —H or —OC2H5, and very often both types of products result. The most important factor in determining the course of the reaction is the nature of the substituents in the diazonium salt. The character of the products is also affected by the acid used for diazotiza-tion, the alcohol employed in the reduction, the presence of water and catalysts, and the reaction temperature. 

To our knowledge, at present there are no bilayer membrane systems which simultaneously satisfy all the requirements listed. Nevertheless, notable achievements have been made on the way towards their developments. Introducing appropriate catalysts in the vesicles it was possible to accomplish separately both water reduction to dihydrogen and its oxidation to dioxygen at the expense of irreversible consumption of sacrificial electron donors and acceptors, respectively. 

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