Hydrogenase catalyst

Several other reports of studies on the [Ru(bpy)3] /MV scheme have appeared in which either platinum or hydrogenase catalysts were used. " Giro et al. managed to obtain hydrogen directly by using Ti " to quench the excited [Ru(bpy)3] + the reaction of Ti with protons is very rapid and competes favourably with the thermal back-reaction. The work of Miller and McLendon is particularly interesting. They used a series of [Ru(bpy)3] derivatives of the type (1) and (2) and were able to establish a linear relationship between electrode... 

The cationic complex [CpFe(CO)2(THF)]BF4 (23) can also catalyze the proton reduction from trichloroacetic acid by formation of Fe-hydride species and may be considered as a bioinspired model of hydrogenases Fe-H Complexes in Catalysis ) [44]. This catalyst shows a low overvoltage (350 mV) for H2 evolution, but it is inactivated by dimerization to [CpFe(CO)2l2-... 

The enzymes hydrogenase, nitrogenase, and formate dehydrogenase can be used to equilibrate reducing reagents with H2O/H2, N2NH3, and CO2/HCOOH, respectively.(53) In no case do the enzymes involve expensive noble metals as catalysts. 

The oxidation of hydrogen in fuel cells provides clean energy and water as the only byproduct. Application of hydrogenase for hydrogen electrode is able to improve the characteristics of the fuel cells. Thermostable hydrogenase from Thiocapsa roseopersicina is an appropriate catalyst for development of several systems for production and transformation of renewable energy based on molecular hydrogen. 

At present the hydrogenase is very expensive and commercially unavailable enzyme for large scale application. There are three approaches to design rather cheap hydrogenactivating catalyst ... 

In metal-containing biological catalysts it is the protein matrix surrounding the metal centres that provides the unique environment for the Fe and Ni atoms which allows hydrogenases to function properly, selectively and effectively. Therefore, a major goal of hydrogenase basic research is to understand the protein-metal interaction. 

Nature uses the transition-metal elements iron and nickel, rather than noble metals, and in their ionic form rather than the metals. As will be seen in this book, for the simplest chemical reaction, the metal-ion centres in hydrogenases are some of the most complex catalysts known. Their structures, which have just been elucidated, have proved to be an elegant and totally unexpected solution to the problem. The construction of these catalysts is in itself a molecular assembly line of extraordinary sophistication. 

In Chapter 2 we saw that hydrogenases of the three basic types are made by organisms that have existed over billions of years. In Chapter 6, the strnctnres of the proteins were laid ont in three dimensions. In Chapter 7 we saw that the metal centres of the protein could exist in particnlar chemical states. We can now begin to understand how the hydrogenases catalyse their reactions with snch extraordinary efficiency. Furthermore we ask, can similar catalysts be constrncted artificially ... 

With these possible applications in mind, we should review the significant characteristics of hydrogenase as a catalyst. Compared with most chemical catalysts, hydrogenases are large molecules. The protein has been selected by evolution, from an almost infinite number of possible structures. The whole protein is part of the machinery. Therefore even minor tampering with the protein, for example by site-directed mutagenesis, is likely to lead to unexpected changes in the properties of the enzyme. 

The mechanism of action, and organization of the catalytic sites, in hydrogenases are different from a solid catalyst such as platinum. For a start, the reaction of H2 with hydrogenase involves heterolytic cleavage into a hydron and a hydride. This contrasts with the reaction of H2 at the surface of a metal such as platinum, which is usually considered to involve the homolytic cleavage into two hydrogen atoms. Moreover in the enzyme, the catalyst is a cluster of metal ions (with oxidation states +2 or -h3) rather than the metal (oxidation state 0). 

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