Tunneling electrode reaction

Relations (18) and (19) are in good agreement with the experimental dependence (17). Such consideration of the activationless reactions is given elsewhere [31,33]. An example of the tunneling electrode reaction found in ref. 34 is the electrochemical desorption of hydrogen ... 

Examples of tunneling in physical phenomena occur in the spontaneous emission of an alpha particle by a nucleus, oxidation-reduction reactions, electrode reactions, and the umbrella inversion of the ammonia molecule. For these cases, the potential is not as simple as the one used here, but must be selected to approximate as closely as possible the actual potential. However, the basic qualitative results of the treatment here serve to explain the general concept of tunneling. 

In Chapter 7 general kinetics of electrode reactions is presented with kinetic parameters such as stoichiometric number, reaction order, and activation energy. In most cases the affinity of reactions is distributed in multiple steps rather than in a single particular rate step. Chapter 8 discusses the kinetics of electron transfer reactions across the electrode interfaces. Electron transfer proceeds through a quantum mechanical tunneling from an occupied electron level to a vacant electron level. Complexation and adsorption of redox particles influence the rate of electron transfer by shifting the electron level of redox particles. Chapter 9 discusses the kinetics of ion transfer reactions which are based upon activation processes of Boltzmann particles. 

Refs. [i] Conway BE (1999) Electrochemical processes involving H adsorbed at metal electrode surfaces. In Wieckowski A (ed) Interfacial electrochemistry, theory, experiment, and applications. Marcel Dekker, New York, pp 131-150 [ii] Climent V, Gomez R, Orts JM, Rodes A, AldazA, Feliu JM (1999) Electrochemistry, spectroscopy, and scanning tunneling microscopy images of small single-crystal electrodes. In Wieckowski A (ed) Interfacial electrochemistry, theory, experiment, and applications. MarcelDekker, New York, pp 463-475 [Hi] Calvo E] (1986) Fundamentals. The basics of electrode reactions. In Bamford CH, Compton RG (eds) Comprehensive chemical kinetics, vol. 26. Elsevier, Amsterdam, pp 1-78... 

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. 

Fig. 3.26a-e A scheme representing five stages of the SECM current-distance experiment, a The tip is positioned in the solution close to the Nafion coating on ITO. b The tip has penetrated partially into the film, and the oxidation of Os(bpy)3+ starts at the Pt tip, which was held at 0.8 V vs. SCE, where the electrode reaction is diffusion-controlled. The effective electrode (tip) surface grows with penetration, c The entire tip electrode is immersed in the film, but is still far from the ITO substrate that is biased at 0.2 V vs. SCE, where the reduction of the generated Os(bpy)3+ can take place, d The tip is sufficiently close to the substrate to observe positive SECM feedback, e The tip reaches the surface of ITO (the tunneling region) [15,387]. (Reproduced with the permission of the American Association for the Advancement of Science)... 

Fig. 24 - Hole generation by tunnelling mechanism in high electric field with consecutive electrode reactions...

Ye, T. He, Y. F. Borguet, E. 2006. Adsorption and electrochemical activity An in situ electrochemical scanning tunneling microscopy study of electrode reactions and potential-induced adsorption of porphyrins. J. Phys. Chem. B 110 6141-6147. 

The development of scanning probe microscopies and x-ray reflectivity (see Chapter VIII) has allowed molecular-level characterization of the structure of the electrode surface after electrochemical reactions [145]. In particular, the important role of adsorbates in determining the state of an electrode surface is illustrated by scanning tunneling microscopic (STM) images of gold (III) surfaces in the presence and absence of chloride ions [153]. Electrodeposition of one metal on another can also be measured via x-ray diffraction [154]. 

The probability matrix plays an important role in many processes in chemical physics. For chemical reactions, the probability of reaction is often limited by tunnelling tlnough a barrier, or by the fonnation of metastable states (resonances) in an intennediate well. Equivalently, the conductivity of a molecular wire is related to the probability of transmission of conduction electrons tlttough the junction region between the wire and the electrodes to which the wire is attached. 

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