Electrochemical Proton Transfer

Hydrogen evolution at metal electrodes is one of the most important electrochemical processes. The mechanisms of the overall reaction depend on the nature of the electrode and solution. However, all of them involve the transfer of proton from a donor molecule in the solution to the adsorbed state on the electrode surface as the first step. The mechanism of the elementary act of proton transfer from the hydroxonium ion to the adsorbed state on the metal surface is discussed in this section. 

An important observation on the proton behavior in chemical compounds is that it is a quantum particle. In particular, the frequency of its valence vibrations in molecules such as hydroxonium ion is on the order of Q 10 s [i.e., the energy of corresponding vibrational quantum TiQ. ( 0.3 eV) is much higher than the thermal 

The second important point is common for all charge-transfer reactions. The proton is a charged particle, and as such, interacts strongly with solvent polarization. The first model taking into account botfi tfiese points was proposed by Dogonadze et al. (1967). 

At least three components of the system change their state in the case of proton transfer reaction (1) electrons of the water molecule and the electrode providing the chemical bonding of the proton with a water molecule and the metal surface, (2) the proton itself, and (3) medium polarization. The characteristic times x, Xp, and x for 

Therefore, electrons adjust their state to any instant position of the proton and solvent polarization in both the initial (hydroxonium ion) and final (adsorbed hydrogen atom) states. The proton in the hydroxonium ion sees an average electron cloud but feels any instant configuration of solvent polarization. 

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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. 

Dogonadze and Kuznetsov showed for the first time the way to take into account the processes of transfer of heavy particles for reactions in liquids. This work was the basis for the first simplest quantum mechanical model of the electrochemical proton transfer process which was proposed by Dogonadze, Kuznetsov, and Levich in 1967 (see also Ref. 33). The expressions and conclusions of Dogonadze and Kuznetsov s work were used in a number of subsequent papers (also in some recent works, see, e.g.. Ref. 34). Therefore, it is necessary to consider this approach in more detail. 

Figure 10.13(a) shows the ammonia synthesis rates (mol H-s ) at constant voltage and inlet Ph2/Pn2 =0.82. Reaction rate increases from 6.9 x 10 mol H-s to 1.35 X 10 mol H s as increasing of reaction time. p = r/vo is defined as enhancement coefficient in this figure and its value is 2. A = Ar/(—I/F) is defined as inductive efficiency, reflecting the influence of electrochemical proton transfer to catalyst. This parameter is used to differentiate the inductive and non-inductive efficiency. The A is 0.6 in Fig. 10.13(a), indicating that 60% of proton transfers to... 

The ratio ARH/ARj (monoalkylation/dialkylation) should depend principally on the electrophilic capability of RX. Thus it has been shown that in the case of t-butyl halides (due to the chemical and electrochemical stability of t-butyl free radical) the yield of mono alkylation is often good. Naturally, aryl sulphones may also be employed in the role of RX-type compounds. Indeed, the t-butylation of pyrene can be performed when reduced cathodically in the presence of CgHjSOjBu-t. Other alkylation reactions are also possible with sulphones possessing an ArS02 moiety bound to a tertiary carbon. In contrast, coupling reactions via redox catalysis do not occur in a good yield with primary and secondary sulphones. This is probably due to the disappearance of the mediator anion radical due to proton transfer from the acidic sulphone. 

Of course, proton transfer can also occur between two reactants in the solution. As such, it is not an electrochemical reaction, unless it is combined with an electron exchange with the electrode. Such a combined electron-proton transfer can be represented by the scheme of squares shown in Fig. 2.8. Both electron and proton transfer... 

Grimminger J, Bartenschlager S, Schmickler W. 2005. A model for combined electron and proton transfer in electrochemical systems. Chem Phys Lett 416 316-320. 

Marcus, R. A., Similarities and differences between electron and proton transfers at electrodes and in solution, Theory of hydrogen evolution reaction, Proc. Electrochem. Soc., 80-3, 1 (1979). 

The first attempt to describe the dynamics of dissociative electron transfer started with the derivation from existing thermochemical data of the standard potential for the dissociative electron transfer reaction, rx r.+x-,12 14 with application of the Butler-Volmer law for electrochemical reactions12 and of the Marcus quadratic equation for a series of homogeneous reactions.1314 Application of the Marcus-Hush model to dissociative electron transfers had little basis in electron transfer theory (the same is true for applications to proton transfer or SN2 reactions). Thus, there was no real justification for the application of the Marcus equation and the contribution of bond breaking to the intrinsic barrier was not established. 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

More complicated reactions that combine competition between first- and second-order reactions with ECE-DISP processes are treated in detail in Section 6.2.8. The results of these theoretical treatments are used to analyze the mechanism of carbon dioxide reduction (Section 2.5.4) and the question of Fl-atom transfer vs. electron + proton transfer (Section 2.5.5). A treatment very similar to the latter case has also been used to treat the preparative-scale results in electrochemically triggered SrnI substitution reactions (Section 2.5.6). From this large range of treated reaction schemes and experimental illustrations, one may address with little adaptation any type of reaction scheme that associates electrode electron transfers and homogeneous reactions. 

Of hundreds of theoretically possible pathways, the list can be trimmed to four using linear sweep voltammetry (LSV) and chemical arguments [22]. The LSV method is an exceptionally powerful one for analyzing electrochemical processes [24-27]. From LSV studies, it was concluded that a single heterogeneous electron transfer precedes the rate-determining step, cyclization is first order in substrate, and that proton transfer occurs before or in the rate-determining step. The candidates include (a) e-c-P-d-p (radical anion closure). 

Because the reaction in a CL requires three-phase boundaries (or interfaces) among Nafion (for proton transfer), platinum (for catalysis), and carbon (for electron transfer), as well as reacfanf, an optimized CL structure should balance electrochemical activity, gas transport capability, and effective wafer management. These goals are achieved through modeling simulations and experimental investigations, as well as the interplay between modeling and experimental validation. 

The powerful biological machinery of energy conversion proceeds via redox reactions in aqueous media that involve electron and proton transfer between molecular entities. - Nature devised concerted sequences of these processes that generate electrochemical potential gradients across cell membranes and thereby enable the storage and the release of electrical energy. 

The cytochrome b(6)f complex mediates electron transfer between the PSI and PSII reaction centers by oxidizing hpophUic plastoquinol (PQH2) (see Figure 7.24) and reducing the enzymes plastocyanin or cytochrome Ce. The electronic connection also generates a transmembrane electrochemical proton gradient that can support adenosine triphosphate (ATP) synthesis instead of electron transport. 

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