Proton-discharge step

In certain cases encountered experimentally, for example, for the HER at Ni or Ni-Mo alloys (75), the electrochemical barrier symmetry factor for the initial proton-discharge step [Eq. (4)] may be close to that for the electrochemical desorption step [Eq. (5)] then a limiting coverage ([Pg.42]

The course of the proton-discharge step in electrolytic H2 evolution was first considered in a semi-quantitative way by Gurney and Fowler(16). The interaction of the proton with water, as in... 

The chief methods by which these conclusions have been reached are summarised under electrode reaction mechanisms (q.v.). First, the T]-logj curve gives the value of the Tafel slope b, and, hence, the transfer coefficient a, since b = 2.303 RT/aF = 0.059/a (see activation overpotential). If the rate is determined by the proton discharge step, a will be simply the symmetry factor 3, and if this is approximately 0.5, b will have the value 0.118. This is what is found for the mercury group. 

This trend for a rate-determining proton discharge followed by a chemical desorption step at low q s and an electrochemical one at higher rj s seems to survive the change to alkaline solution shown here of course, the proton discharge occurs from water ... 

In electrochemical proton transfer, such as may occur as a primary step in the hydrogen evolution reaction (h.e.r.) or as a secondary, followup step in organic electrode reactions or O2 reduction, the possibility exists that nonclassical transfer of the H particle may occur by quantum-mechanical tunneling. In homogeneous proton transfer reactions, the consequences of this possibility were investigated quantitatively by Bernal and Fowler and Bell, while Bawn and Ogden examined the H/D kinetic isotope effect that would arise, albeit on the basis of a primitive model, in electrochemical proton discharge and transfer in the h.e.r. 

H atom may then enter the ground state of the system MH + HgO, i.e., the final state, or reform a proton by electron tunneling back to the metal. The discharge step may thus be represented by the equation... 

An important development in the 1930 s was the representation of the energetic course of electrochemical reactions, especially the step (I) of hydrated-proton discharge in the cathodic hydrogen... 

Reaction starts with proton discharge electrosorption (Volmer reaction, Eq. 1), and follows either or both electrodesorption step (Heyrovsky reaction, Eq. 2), and/or Hads recombination step (Tafel reaction, Eq. 3). If the steps are consecutive and one is the rate determining step (rds), theory predicts various values for Tafel slopes which are summarized in Table 1. 

Metal deposition is an example of a more general class of electrochemical reactions, ion transfer reactions. In these an ion, e.g. a proton or a chloride ion, is transferred from the solution to the electrode surface, where it is subsequently discharged. Many ion-transfer reactions involve two steps. The hydrogen-evolution reaction, for example, sometimes proceeds in the following way ... 

The disodium salt of diphenylacetylene dianion is stable in THE solution at -78°C. Methanol acts as a proton source toward the salt and causes the formation of a mixture of 1,2-diphenylethane with diphenylacetylene and small amounts of tran -stilbene (Chang and Johnson 1965, 1966). It seems logical that the reaction between (PhC=CPh) , 2Na and MeOH leads at first to PhCH=CHPh. The second step is supposed to consist of the further reduction of PhCH=CHPh at the expense of electrons from the nonreactedpartof the initial dianion. In principle, the electron transfer may proceed faster than the reaction of the initial dianion with protons. As a result, the diphenylacetylene dianion has to discharge into diphenylacetylene, whereas stilbene dianion has to form diphenylethane as follows ... 

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