Dihydrogen production

Savinov EN, Gruzdkov YA, Parmon VN (1989) Suspensions of semiconductors with microheterojunctions — A new type of highly efficient photocatalyst for dihydrogen production from solution of hydrogen sulfide and sulfide ions. Int J Hydrogen Energy 14 1-9... 

The reduction of protons is one of the most fundamental chemical redox reactions. Transition metal-catalyzed proton reduction was reviewed in 1992.6 The search for molecular electrocatalysts for this reaction is important for dihydrogen production, and also for the electrosynthesis of metal hydride complexes that are active intermediates in a number of electrocatalytic systems. 

Controlled one-electron reductions transform l,2,3,4-tetraphenyl-l,3-cyclopentadiene or 1,2,3, 4,5-pentaphenyl-l,3-cyclopentadiene into mixtures of the dihydrogenated products and the corresponding cyclopentadienyl anions (Famia et al. 1999). The anion-radicals initially formed are protonated by the substrates themselves. The latter are thermodynamically very strong acids because of their strong tendency to aromatization. As with the cyclopentadiene anion-radicals, they need two protons to give more or less stable cyclopentadienes. The following equations represent the initial one-electron electrode reduction of l,2,3,4,5-pentaphenyl-l,3-cyclopentadiene (CjHAtj) and explains the ratio and the nature of the products obtained at the expense of the further reactions in the electrolytic pool ... 

Figure 8.3 Influence of Ni and Fe on dihydrogen production during the acidogenesis phase in the anaerobic fermentation of FVC. The broken line represents the control.

An interesting comparison can be made with chemical techniques for glycerol treatment and dihydrogen production. Table 8.5 compares the performance of the above strain K4 with that of a catalytic conversion of aqueous glycerol [37]. 

Catalytic dihydrogen production form biomass is known as aqueous phase reforming (APR) [38]. It is interesting to note the absence of CO in the dihydrogen produced by the biosystem, which makes it immediately suitable for use in fuel-cells... 

The biotransformation of (K)-(+)-pulegone was also studied by a Japanese group [116]. The major bioconversion metabolite of this substrate with Botrytis allii was (-)-(1 / )-8-hydroxy-4-p-menthen-3-one. The secondary major product from this biotransformation was isolated and its structure established as piperitenone [117]. It is interesting to note that the same group also investigated the bioconversion of piperitone, the dihydrogenation product of piperitenone a strain of Rhizoctonia solani was found able to hydroxylate the substrate preferentially at the 6-position [118,119]. 

Figure 17 Dihydrogen production and dinitrogen binding in the reduction of [MoH2(T 2-02CMe)(dppe)2]+.

The experimental design is simple. A given sample of nitrogenase equipped with an ATP-generating system, Mg2+, and reductant is allowed to turn over without substrate (case I), and the dihydrogen production is monitored. The amount of dihydrogen produced is found to be equal (within experimental error) to the dithionite oxidized (50). Therefore, the electron balance equation is ... 

Fig. 3. Dihydrogen production in controlled pore glasses. The data are gathered from Refs. 24 and 25.

Perhaps this study on [Fe4S4(SPh)4]3- is closer to what happens during the reduction of dinitrogen at FeMo-cofactor in the enzyme. It is conceivable that the limiting stoichiometries of the various nitrogenases Equations (11), (18) and (19) merely reflect the ability of dinitrogen in each enzyme to divert the electron-flux away from dihydrogen production. 

As the Eqs. 2 and 3 indicate, electron transfer from the excited sensitizer (S ) to Co(lll) will be favored if the Co(III)/Co(ll) potential is less negative, whereas a more negative potential will favor H2 production. These two opposing influences are optimized in N3S3 donors with different apical substituents and they have been used for dihydrogen production from water at modest rates (61, 62). 

The close structural relation of germine with cevine is indicated by the following compounds which were obtained from the selenium dihydrogenation products 8-picoline, 2-ethyl-5-methylpyridine, cevanthrol, and cevanthridine. Finally, germine on oxidation with chromic-sulfuric acid afforded the same hexanetetracarboxylic acid, C10H14O8, that was similarly obtained from cevine (13). 

Combining relative energies and barriers displayed in Figs. 20, 22, and 24, as well as thermodynamic values listed in Table 8, the channel to loss of HD by dehydrogenating both methylene and ammonia is the most favored route for dihydrogen elimination in the reaction (1). The next one is the loss of H2 from NH3 activation. This accords approximately with the experimental characterization of dihydrogen products D2, HD, and H2 in a ratio of 70( 15) 100 80( 15) [17,18]. 

In this section we will first look at dihydrogen production in an acidic aqueous solution, whereby argon is continuously bubbled inside. Here, assuming unidirectional geometry, the steady states are described by the Nernst model due to convection, the composition remains homogeneous throughout the bulk of the electrolyte, i.e., in the area beyond the thickness layer <5. This composition is identical to the initial composition the pH is... 

Reaction of f-Bu2SiCl2 with lithium in THF also gave some hydrogenated cyclotetrasilanes (6) [102]. Bromination of the dihydrogenated product with NBS led to the dibromide which upon subsequent treatment with potassium gave the respective 1,3-dianionic compound [103]. 

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