Electrochemical Reactions Kinetics and Mechanism

For references on electrochemical reaction kinetics and mechanism, see, e.g., Newman and Thomas-Alvea, Electrochemical Systems, 3d ed., Wiley Interscience, 2004 Bard and Faulkner, Electrochemical Methods Fundamentals and Applications, 2d ed., Wiley, 2001 Bethune and Swendeman, Table of Electrode Potentials and Temperature Coefficients, Encyclopedia of Electrochemistry, Van Nostrand Reinhold, New York 1964, pp. 414-424 and Bethune and Swendeman, Standard Aqueous Electrode Potentials and Temperature Coefficients, C. A. Hampel Publisher, 1964. 

Simulation in Electrochemical Processes Reaction Kinetics and Mechanism on Metal Single Crystal Electrode Surfaces... 

The properties and applications of microelectrodes, as well as the broad field of electroanalysis, have been the subject of a number of reviews. Unwin reviewed the use of dynamic electrochemical methods to probe interfacial processes for a wide variety of techniques and applications including various flow-channel methods and scanning electrochemical microscopy (SEM), including issues relating to mass transport (1). Williams and Macpherson reviewed hydrodynamic modulation methods and their mass transport issues (2). Eklund et al. reviewed cyclic voltammetry, hydrodynamic voltammetry, and sono-voltammetry for assessment of electrode reaction kinetics and mechanisms with discussion of mass transport modelling issues (3). Here, we focus on applications ranging from measnrements in small volumes to electroanalysis in electrolyte free media that exploit the uniqne properties of microelectrodes. 

Electrochemical impedance spectroscopy (EIS) technique has been used for the experimental assessment of CO tolerance on different Pt-alloy catalysts and at different temperatures [187]. Hsing et al. [187] proposed that the critical potential at which pseudo-inductive behavior occurs could be used as a criterion for the evaluation of CO tolerance. A mathematical impedance model based on two state-variables (Pt-H and Pt-CO) was also developed to elucidate the reaction kinetics and mechanism of the H2/CO oxidation on a Pt/C catalyst [188]. In fact, this study has given better insight into explicitly understanding the impedance patterns and the quantitative assessment of the effect of applied potentials upon the oxidation reaction kinetics in a broad range of applied potentials. Nevertheless, with the consideration of only two adsorbed species, Pt-H and Pt-CO, the impedance model based on two state-variables was not able to explain the experimental observation... 

Reaction kinetics and mechanism of methanol oxidation Pt(110) NaHCOj. Na2C03, and NaOH solutions Pt was treated electrochemically to get the desired surface structure Oxidation proceeds with some poisoning species formation. Main reaction product is formate and CO. Potential range for OHa is 0.4 V< < 1.0 V Tripkovic et al. (1998a)... 

An excellent review covers the charge and discharge processes in detail (30) and ongoing research on lead—acid batteries may be found in two symposia proceedings (32,33). Detailed studies of the kinetics and mechanisms of lead —acid battery reactions are pubUshed continually (34). Although many questions concerning the exact nature of the reactions remain unanswered, the experimental data on the lead—acid cell are more complete than for most other electrochemical systems. 

Section 8 deals with reactions which occur at gas—solid and solid—solid interfaces, other than the degradation of solid polymers which has already been reviewed in Volume 14A. Reaction at the liquid—solid interface (and corrosion), involving electrochemical processes outside the coverage of this series, are not considered. With respect to chemical processes at gas-solid interfaces, it has been necessary to discuss surface structure and adsorption as a lead-in to the consideration of the kinetics and mechanism of catalytic reactions. 

Experimental studies in electrochemistry deal with the bulk properties of electrolytes (conductivity, etc.) equilibrium and nonequilibrium electrode potentials the structure, properties, and condition of interfaces between different phases (electrolytes and electronic conductors, other electrolytes, or insulators) and the namre, kinetics, and mechanism of electrochemical reactions. 

Electrocatalytic effects cannot be studied in depth without a detailed knowledge of the kinetics and mechanism of the electrochemical reaction being examined. In fact, diverse effects can be exerted on the reaction ... 

Great promise exists in the use of graphitic carbons in the electrochemical synthesis of hydrogen peroxide [reaction (15.21)] and in the electrochemical reduction of carbon dioxide to various organic products. Considering the diversity in structures and surface forms of carbonaceous materials, it is difficult to formulate generalizations as to the influence of their chemical and electron structure on the kinetics and mechanism of electrochemical reactions occurring at carbon electrodes. 

The character of the oxide layers influences the kinetics and mechanism of the electrochemical reactions occurring on the platinum anode surface. The relation between the rate of oxygen evolution and oxide layer thickness is complex. In the region where the a-oxides exist, the reaction rate decreases with increasing oxide layer thickness. In the region where the P-oxides exist, the reaction rate depends little on oxide layer thickness or, according to some data, increases with increasing oxide layer thickness. 

Strong conformational changes may accompany electron transfer. This issue has been discussed in Section 1.5 and illustrated by an experimental example in Section 1.5.5, in the case where conformational change and electron transfer are concerted and the second electron transfer becomes easier than the first. Conformational changes do not necessarily cause the second electron transfer to be easier than the first. In all cases, their influence on the kinetics and mechanisms of electrochemical reactions should be analyzed. 

Selectivity in homogeneous reactions is treated in Refs. [2-6], while the selectivi-ties that are influenced in heterogeneous reactions by diffusion and adsorption are dealt with in Refs. [7, 8]. Kinetics and mechanisms of electrochemical reactions are covered in Chap. 1 of this volume and in Refs. [9-11]. 

In this chapter we discuss the electrochemical model of electroless deposition (Sections 8.2 and 8.3), kinetics and mechanism of partial reactions (Sections 8.4 and 8.5), activation of noncatalytic surfaces (Section 8.6), kinetics of electroless deposition (Section 8.7), the mechanism of electroless crystallization (Section 8.8), and unique properties of some deposits (Section 8.9). 

The fact that electrochemical processes are tied to electron transfer processes makes electrochemical methods generally less applicable for kinetics and mechanism studies than, for instance, spectroscopic methods. On the other hand, if the reaction under scrutiny involves a radical or radical-like species, electrochemical methods are invaluable tools that often provide a wealth of mechanistic detail. A major advantage of electrochemical methods for kinetics and mechanism studies is that intermediates (radical ions, radicals, etc.) may be formed and their chemical reactions studied at the same electrode in the same operation. 

The application of electrochemical methods for the study of the kinetics and mechanisms of reactions of electro chemically generated intermediates is intimately related to the thermodynamics and kinetics of the heterogeneous electron transfer process and to the mode of transport of material to and from the working electrode. For that reason, we review below some basics, including the relationship between potential and current (Section 6.5.1), the electrochemical double layer and the double layer charging current (Section 6.5.2), and the... 

It follows from Equation 6.12 that the current depends on the surface concentrations of O and R, i.e. on the potential of the working electrode, but the current is, for obvious reasons, also dependent on the transport of O and R to and from the electrode surface. It is intuitively understood that the transport of a substrate to the electrode surface, and of intermediates and products away from the electrode surface, has to be effective in order to achieve a high rate of conversion. In this sense, an electrochemical reaction is similar to any other chemical surface process. In a typical laboratory electrolysis cell, the necessary transport is accomplished by magnetic stirring. How exactly the fluid flow achieved by stirring and the diffusion in and out of the stationary layer close to the electrode surface may be described in mathematical terms is usually of no concern the mass transport just has to be effective. The situation is quite different when an electrochemical method is to be used for kinetics and mechanism studies. Kinetics and mechanism studies are, as a rule, based on the comparison of experimental results with theoretical predictions based on a given set of rate laws and, for this reason, it is of the utmost importance that the mass transport is well defined and calculable. Since the intention here is simply to introduce the different contributions to mass transport in electrochemistry, rather than to present a full mathematical account of the transport phenomena met in various electrochemical methods, we shall consider transport in only one dimension, the x-coordinate, normal to a planar electrode surface (see also Chapter 5). 

In studies of the kinetics and mechanisms of electrochemical reactions it is important to know, at least approximately, the oxidation or reduction potential of the substrate and... 

It may seem unnecessary to stress that it is important to know the product or the product composition of any electron transfer reaction before it is meaningful to discuss the kinetics and mechanism (see Chapter 2). Nevertheless, this simple rule is often violated. One reason probably is that it is not always a trivial task to isolate and identify the products from an electrochemical reaction using the usual arsenal of methods available to the organic chemist. A major problem is that often it is not an easy task to separate the product from the supporting electrolyte. In such cases, direct analysis of the product mixture without the need of a work-up, e.g. by LC-UV/vis-MS, is desirable. Analysis by this method of the product mixture resulting from the oxidation of 1,2,5-trimethylpyrrole is shown in Fig. 6.27. It is... 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

The foregoing has been concerned with the application of SERS to gain information on surface electronic coupling effects for simple adsorbed redox couples that are reversible in the electrochemical as well as chemical sense, that is, exhibit Nernstian potential-dependent responses on the electrochemical time scale. As noted in the Introduction, a major hoped-for application of SERS to electrochemical processes is to gain surface molecular information regarding the kinetics and mechanisms of multiple-step electrode reactions, including the identification of reactive surface intermediates. 

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