Hydrogen-reduction reaction, polarization

Fig. 3.16 Cathodic polarization of the hydrogen reduction reaction on iron showing the effect of pH. Curve for platinum shows influence of a metal with much higher exchange current density on the position of the hydrogen reduction curve. Source Ref 5...

Approximate polarization curves for the hydrogen-reduction reaction, H+ + e —> Y2 H2, were shown in Fig. 3.16 for pH values of 1, 3, and 5. These curves also are shown in Fig. 4.16, where the abscissa is in terms of current, I, rather than current density, i (Ref 8). 

The ruthenium-catalyzed reduction of polar bonds using H gas, rather than a sacrificial reductant such as isopropanol, is an atom-economical reaction that has been thoroughly explored. A key discovery by our laboratory was that neutral, stmctur-ally characterized metal-amido complexes such as 11 (Scheme 7) could cleave H heterolytically to yield the fran -dihydride complex 12, and that these are crucial intermediates during catalysis.R - Once the H-N-Ru-H moiety is in place, proton and hydride can then be transferred to the substrate. Having methyl groups instead of hydrogens on carbons alpha to the amido group (beta to the ruthenium) in this case and in the case of 1 (Scheme 4) was important to allow the isolation of an amido... 

The importance of diffusion polarization in corrosion results from the observation that in many situations, the current density of the reduction reaction is large enough to place it under diffusion control. Two important examples are the depletion of hydrogen ions in the solution adja-... 

Hydrogen evolution Irom the electroreduction of protons at different modified polymer electrodes was first investigated by Tourillon and Gamier, who studied the inclusion of bimetallic Ag-Pt particles into poly-3-methylthiophene(PMeT) and observed their electrocatalytic properties towards the proton reduction reaction [46], They demonstrated the positive effect of the Ag particles (from 15 /ig/cm ) on the reduction current due to an increase of the electrode conduction at low potentials, where PMeT is in its neutral undoped state, and put in evidence a minimum Pt loading (of about 10 tg/cm for a 170 nm thick film) for obtaining an enhanced catalytic activity compared to a platinized Ag-coated Au electrode without a polymeric film. A remarkable stability with time was observed under polarization at a constant potential ( — 0.4 V/SCE) without degradation of the modified electrode. 

The emf series of the standard half-ceU electrode potential on the hydrogen scale are given in Table 2.2. The reactions in this table are written as reduction reactions from left to right at T=25 °C. They have the same polarity as the reduction potential, which is measured experimentally. 

The second step in the procedure requires the working electrode to be anodicaUy polarized, yielding one of the dashed Hnes shown in Fig. 3.6. The electrode is then cathodicaUy polarized, and the other dashed Hne from Fig. 3.6 is obtained. The anodic polarization usuaUy results in the oxidation of the metal species, whUe the reaction resulting from cathodic polarization depends on the medium. In an aerated solution, the oxygen reduction reaction may be the prime cathodic reaction, while, in the case of deaerated aqueous solutions, hydrogen reduction could be the dominant reaction. In Fig. 3.6, the redox reaction is represented by a general reaction... 

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