Dihydrogen evolution reaction

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... 

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. 

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 ... 

A reaction system consisting of a Fe(II) salt, BSA, Ne2S, and 2-mer-captoethanol afforded a dark brown solution whose absorption spectrum ( max 420 nm) had the appearance of a [4Fe-4S] + cluster (96). Formation of additional BSA-bound Fe-S species cannot be discounted, however. The albumin-cluster protein showed weak hydrogenase activity in the form of dihydrogen evolution in the presence of reduced methyl viologen (96). The protein was not further characterized. 

Some aqueous reactions form a gas as a product. These reactions, as we learned in the opening section of this chapter, are called gas evolution reactions. Some gas evolution reactions form a gaseous product directly when the cation of one reactant reacts with the anion of the other. For example, when sulfuric acid reacts with lithium sulfide, dihydrogen sulfide gas is formed. 

The carbon anode showed an overpotential reaction, adsorbing hydrogen and preventing gas evolution until -0.65 V versus NHE. The proton absorption could block dihydrogen evolution until it became more thermodynamically feasible [83,86]. The reversible Mn02 oxidation reactions at the cathode show oxygen overpotential to 1.4 V versus NHE, which allows the device s potential window to be extended. 

AIL Schlenk fiask, which is attached to a pressure-release valve, is flushed with nitrogen and charged with dicyclopentadiene (400 mL). Freshly cut sodium metal (10.00 g, 0.435 mol) is added. The mixture is heated for 6 h at 160°C. The solution initially turns blue (around 35°C), but then slowly discolors. At 150°C, a white solid begins to precipitate. When the alkali metal is quantitatively consumed, the dihydrogen evolution stops. To ensure a quantitative conversion, the heating is continued for another 30 min after the dihydrogen evolution ends. The reaction mixture is cooled, and the white residue is collected by filtration, washed with n-pentane (3x50mL), and dried in vacuum. The urueacted dicyclopentadiene (260 mL) can be reused later for the same reaction. Yield 38 g (99%). ... 

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