Mechanisms of Hydrogen Production

Maria L. Ghirardi, Pin Ching Maness, and Michael Seibert 

The light catalyzed oxidation of water by the PSII RC complex yields three products, O2, electrons, and protons (Fig. 1). Oxygen gas diffuses out of the cells while the protons accumulate in the lumenal space, since the latter cannot cross the lipophilic thylakoid membranes. During electron transfer from PSII to PSI, two electrons, coupled with two protons, are transported from the stromal side (outside) of the thylakoid membrane to the lumen (inside). This also contributes to the accumula tion of a proton gradient across the membrane. The proton gradient creates a proton 

Light absorbed by the organism s light-harvesting pigments (average 2.25 eV/photon) 45% 

Energy stored upon charge-separation (3.2 eV/2 photons) 32% 

Maximum energy that can be converted into H2 and 02 (1.23 eV/2 photons) 12-13% 

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Mechanism of Hydrogen Production Through NADH Pathway and Redox Potential... 

Figure 2 Mechanism of hydrogen production of NADH pathway.

The mechanism of hydrogen production at FeMoco is not well understood. 

Ca.ta.lysis, The mechanism of hydrogen abstraction from alcohols to form aldehydes (qv) over silver has been elucidated (11). Silver is the principal catalyst for the production of formaldehyde (qv), the U.S. production of which was 4 x 10 metric tons in 1993. The catalytic oxidation of... 

Entry from the aqueous phase The mechanism of electrochemical production of hydrogen on steel in aqueous solution has received much attention. It is accepted that the reaction occurs in two main stages. The hrst of these is the initial charge transfer step to produce an adsorbed hydrogen atom. In acid solution this involves the reduction of a hydrogen ion ... 

Stereochemistry and Mechanism of Hydrogenation of Naphthalenes on Transition Metal Catalysts and Conformational Analysis of the Products A. W. Weitkamp... 

Schwarz s model is a multiradical extension of the Ganguly-Magee model with some additional improvements, to be described later. Schwarz assumes that initially—that is, 10 11 s after the act of energy deposition in water—there appear five species, namely eh, H, OH, H30+, and H2. Their initial yields, indicated by superscript zero, are related by charge conservation and material balance. Thus, there are three independent initial yields, taken to be those of eh, H, and Hr The initial yield of H2 is identified with the unscavengable molecular hydrogen yield. No mechanism of its production is speculated, except that it is not formed by radical recombination. For the gaussian distribution of the radicals, two initial... 

Weitkamp, A.W., Stereochemistry and mechanism of hydrogenation of naphthalene on transition metal catalysts and conformational analysis of the products. Adv. Catal., 18,1-110 (1968). 

Although the latter product is a solvated mononuclear [Rh(MeOH)2(diphos)]+ cation, in the solid state it is isolated as a binuclear complex of formula [Rh2 (diphos)2](BF4)2, in which each rhodium center is bonded to two phosphorus atoms of a chelating bis(diphenylphosphino)ethane ligand, and to a phenyl ring of the bis(diphenylphosphino)ethane ligand of the other rhodium atom. This dimer reverts to a mononuclear species on redissolving. The mechanism of hydrogenation of the prochiral alkene methyl(Z)-a-acetamidocinnamate, studied in detail by Halpern [31], is depicted in Scheme 1.7. 

Yang, M., Nazhat, N. B., Jiang, X., Kelsey, S. M., Blake, D. R., Newland, A. C., and Morris, C. J., 1996, Adriamycin stimulates proliferation ofhuman lymphoblastic leukaemic cells via a mechanism of hydrogen peroxide (H2O2) production, Br. J. Hemeatol. 95 339-344. 

The formation of but-2-ene as an initial product raises some important questions regarding the mechanism of hydrogenation. In principle, two... 

Because this value has been found independently of a choice of detailed kinetic processes, the mechanism postulated by Calvert and Steacie for the photolysis of formaldehyde requires modification. If, for example, a reaction such as HCO - - CH20 —> H2 + CO + HCO is operative (58) then the activation energy measured for the rate of hydrogen production can no longer be simply associated with the activation energy of the reaction... 

However, the thermodynamics of reaction (61) in the absence of light are downhill by 0.43 V and hence the light is providing energy to overcome a kinetic barrier. If Eu1 is replaced by H2A, then the quantum efficiency of hydrogen production drops by nearly two orders of magnitude. Nevertheless, the net reaction (reaction 65) is thermodynamically uphill by 0.41 V at pH 4. The proposed reaction mechanism is now described by reactions (62), (63), (58), (59) and (64). The complexity of reduction reactions employing ascorbate are described elsewhere.170... 

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