Dihydrogen evolution

Metal Hydride Complexes Electrogeneration and Electrocatalytic Dihydrogen Evolution 473... 

Polymeric films of [(//5-C s Me5)M(L)Cl]+complexes (M = Ir, Rh L = pyrrole-substituted bpy or phen) have been coated on an electrode by oxidative electropolymerization. The buildup of hydrido complexes in films is well known 27,28,30 the high electrocatalytic activity of these molecular electrode materials towards dihydrogen evolution in organic and aqueous electrolytes is also well known.25,31 For example, H2 is evolved at —0.55 V vs. SCE at a poly [(j75-C5Me5)-Rh(bpy)Cl]+ film in pH 1 aqueous solution.31... 

The stable hydride cluster Pd4(dppm)4(H)3]+ [dppm = bis(diphenylphosphino)methane] can be electrochemically generated from Pd2(dppm)2Cl2] at —1.35 V vs. SCE in DMF containing HC02H (Equations (21) and (22)). Subsequent addition of proton under the same reducing conditions induces dihydrogen evolution.43... 

V(OH)2 is a strong reducing agent and freshly prepared V(OH)2 reacts with water with dihydrogen evolution. In acidic solution, reduction of water may be induced by UV radiation.138 From solutions containing complexes with catechol there is evolution of dihydrogen with simultaneous oxidation of the metal to vanadium(III). The reaction is first order in vanadium(II) and autocatalytic (Scheme 7).145... 

Since the transition metal compounds proposed for the catalytic photocleavage of water (2, 3) appear to be inefficient, factors influencing the dihydrogen evolution from cis-dihydrido compounds deserve scrutiny. We know that dihydrogen is readily dissociated thermally from [RhH2(S)2(PPh3)3]+ (S = acetone) (9,10). The complex cannot be formed from water. RhH(PPh)3 lacks a sufficient nucleophilic character to form a cis-dihydro complex by adding water. The question then is how we could release H2 from Complex 2a. When heated to 90°C in dioxane, Complex 2a merely decomposes into an intractable oil irradiation (low-pressure Hg lamp) is also ineffective. 

Carbon monoxide inhibits the system to dinitrogen fixation but not to dihydrogen evolution. If dinitrogen fixation is undertaken in the presence of 2H2 then exchange occurs as shown in Eq. (85), but no 2H appears in solution. 

Among the numerous catalysts of dihydrogen evolution known at present (see e.g. the reviews [253-257]) the most promising for application in the vesicle systems... 

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. 

Fig. 8. The application of vesicles for photocatalytic water decomposition in sacrificial systems (a) — dihydrogen evolution in the vesicle cavity. Pt metal catalyst is anchored to the inner membrane // water interface (b) — dioxygen evolution in the bulk solution. Manganese oxide catalyst is anchored to the outer membrane // water interface of the vesicle...

In the vesicle suspension of Fig. 8 it was possible to isolate the centers for dihydrogen and dioxygen evolution and thus to avoid cross reactions of S+ and A- with the catalysts for H2 and 02 evolution, respectively. However, it turned out that 02 evolution gradually inhibits the H2 evolution, because oxygen evolved in the outer volume permeates across the membranes and destroys the apparatus for dihydrogen evolution located inside the vesicles. Note, that such a problem also arises for biological systems adapted to provide simultaneous evolution of H2 and Oz [275, 276],... 

Using the same nitrogenase preparation, dinitrogen is added to the reaction flask, and dihydrogen evolution and ammonia production are measured in the same reaction vessel. Under these circumstances (case 2), the electron balance Equation 4 obtains ... 

Deuterium does not affect dihydrogen evolution in either the presence or absence of dinitrogen. 

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