Redox Catalysis in the H -Evolution Reaction from Water

Among the various acceptor relays that have been examined,methyl- viologen (MV ) has been found to he the most desirable one, due to its redox potential, high solubility in aqueous solution and the ease of reduction. The coupling of chloroplasts with MV relay and enzymes such as hytoogenase has been popular as a means of biophotolysis of water  

The fact that this latter process can he catalyzed hy Pt dispersions was discovered hy Green and Stickland in 193 Earlier we reviewed a nvimher of photochemical systems that have heen developed 

Pt content of the solutions of different particle size, namely 3.5 mg Pt/25 cc. This figure also shows two extra points with 100 X particle size but with low Pt levels 0.35 mg Pt/25 cc solution, the upper one, and 0.25 mg Pt/25 cc solution, the lower one. Quantum yield measurements indicate a stoichiometric relation between the viologen reduced in the photoprocess and the amount of hydrogen produced. One might object to the high Pt levels (lUO mg/ltr) required to obtain these high yields. Such concentrations are intolerable for practical systems. However, reduction in particle size to a radius of only 15 A leads to a one himdred fold increase in the activity of the catalyst. Hence, the same hydrogen output can now be achieved with a Pt concentration of only l.U mg/ltr. 

Catalyst Concentration on the Behaviour of MVt Absorption at 600 nm (curve a,b,c). In (d), the rate constants for the MVt decay are plotted versus Pt concentration of the Pt-PVA catalyst used (pH 5)  

U X 1011M 1s 1 indicating that this process is essentially diffusion controlled. The very high catalytic activity of these Pt particles enables one to catalyse other H2 producing reactions from reactive free radicals in competition to side reactions such as disproportionation. H2 production via proflavine , Pli2C0H 9 Ru(bpy ) illustrate this point. 

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