Proton reduction

Kaesche considers that proton reduction may also play a role in polluted environments where the pH of the electrolyte is likely to be low. This would be particularly likely in the case of iron if the Schikorr mechanism, involving the presence of sulphuric acid, did in fact operate. However, Russian work" " has shown that oxygen depolarisation is many times more efficient in thin moisture films than in bulk solutions and therefore proton reduction may not be important in affecting corrosion rates. 

The cationic complex [CpFe(CO)2(THF)]BF4 (23) can also catalyze the proton reduction from trichloroacetic acid by formation of Fe-hydride species and may be considered as a bioinspired model of hydrogenases Fe-H Complexes in Catalysis ) [44]. This catalyst shows a low overvoltage (350 mV) for H2 evolution, but it is inactivated by dimerization to [CpFe(CO)2l2-... 

Sometime probably two billion years before humans became interested in efficient catalysts for four-electron, four-proton reduction of O2 to H2O, the so-called oxygen reduction reaction (ORR),... 

Modifiers Modification of the rhizosphere soil with (e.g., protons, reductants)... 

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. 

Only a limited number of true metal complex electrocatalysts have been proposed for proton reduction due to the difficulty inherent in the bielectronic nature of this reaction. It is obvious that the design of such electrocatalysts must take into account the lowering of the overpotential for proton reduction, the stability of the catalytic system, and the regeneration of the starting complex. 

It is well recognized that protonated phosphine complexes such as [M(dppe)(H)2]+ (dppe = 2-bis(diphenylphosphine)ethane), M = Co, Ir),39 [Fe(dppe)(L)H]+,40 or [Pt(PEt3)3H]+41 catalyze proton reduction at very negative potentials, 2 V vs. SCE. In contrast, the protonated [(,/s-CsI Is)CoIII P(OMe)3 2I I]1 complex is a catalyst for hydrogen production at —1.15 V vs. SCE at a Hg-pool cathode in pH 5 aqueous buffer.42 Dihydrogen is evolved from the reduced [(r/5-Cd fdCo1"-(P(OMe)3)2H]° form of the complex, which decays to H2 or reacts in a proton-hydride reaction. 

The second experimental system explored the reduction mechanism of another azo-dye, known as Sudan III (I-(4-pheuyIazophenyIazo)-2-naphthoI) [91]. Sudan III contains two azo groups rendering two successive two-electron, two-proton reduction steps at the mercury surface. Figure 2.68 shows a typical SW voltammetric response of Sudan III recorded in a borate buffer at pH 10.00. The first reduction step is chemically reversible, while the second one is irreversible. More importantly, the second reduction step proceeds at potential about 230 mV more negative than the first one, thus causing a well-separated voltammetric peak. The overall mechanism... 

Berberine is an alkaloid undergoing an irreversible four-electron and three-proton reduction to the electrochemically inactive compound canadine, which is also adsorbed on the mercury electrode surface. As predicted by the theory, the net peak current of berberine is a linear function of the frequency, whereas the peak current shifts linearly with log(/) with a slope of -45 mV. Based on the theoretically predicted value for the half-peak width, AE p/2 = (63.5 0.5) / c mV, the catho-... 

Electrocatalysis of proton reduction by metal complexes in solution has been widely studied [106-111] and confinement of molecular electrocatalysts for this process in polymeric films has also received some attention [111, 112]. This area has received much impetus from biochemical and structural studies of the iron-only... 

The potential level of the 02 evolving site of the photosynthesis (see Fig. 1) ranging around 0.82 V shows that a four-electron process occurs in it. The water oxidation site of the photosynthesis contains more than four Mn ions interacting with each other, thus leading to the four-electron reaction of water to give 02, Such a multielectron reaction leads to the generation of H2 from proton reduction as described later in chapter 4 on water photolysis. 

In the water photolysis system, the potential of Q should be lower than —0.41 V and that of C2 higher than 0.82 V. For the proton reduction, two electron process (Eq. (4), E0 = —0.41 V) is much more favorable than the stepwise reaction in which the first step (Eq. (5))... 

Many sulphides have been deposited using thioacetamide in acidic solutions (Chapter 6 describes most of these). For depositions using thioacetamide, as with thiosulphate, there are no detailed mechanistic studies. Both H2S formation and complex decomposition are possible in acid solutions, as discussed in Section 3.2.1.3. Deposition of CdS was accomplished using thioacetamide in acidic solution by exploiting electrolytic proton reduction to increase the pH locally at the cathode (substrate), and the mechanism was believed to be a surface-catalyzed decomposition of a Cd-thioacetamide complex [80]. 

It has been suggested that tricyclo[4.1.0.02 7]heptanc should have a low energy cwt/ -bonding orbital which readily accepts an electron to form an anion radical. The high electron density of the central bond then directs the subsequent protonation-reduction-protonation to give bicyclo[3.1.1]heptane.17... 

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