Development of methods for determination of adsorption kinetics at metal electrodes

• Main Author: Moyana, Agata
• Other Authors: Baranski, Andrzej S.
• Format: Others
• Language: en
• Published: University of Saskatchewan 1996
• Subjects: chemistry; electrochemistry; surface chemistry; chemical kinetics
• Online Access: http://library.usask.ca/theses/available/etd-10212004-000613

Adsorption at metal electrodes is usually a very fast process and it plays a most important role in many areas of industry. The thermodynamics of the process are well known for many systems. However, there is currently no good method that allows a determination of very fast kinetics of adsorption to be made. Previously, many attempts at evaluation of kinetic parameters of adsorption were made, but in moat cases, due to the inadequacy of the experimental methods used, the parameters obtained were much lower than expected. This thesis aims at providing the means for determining the kinetics of adsorption at metal electrodes. The methods herein described are based on two different experimental techniques. These techniques are: (i) fast cyclic voltammetry (FCV, potential sweep rate up to 100000 V/s) and (ii) high frequency AC and FFT SW (Fast Fourier Transform Square-wave) voltammetry (frequency up to 50 MHz) at ultramicroelectrodes (5 or 6.25 ìm in radius). A theoretical description of the adsorption process for both kinds of experiments is presented. A simulation program was written to provide a better understanding of the process and to elucidate the development of methods for determining the kinetics of adsorption. Thermodynamic and kinetic descriptions of the process are based on the Frumkin adsorption isotherm. Both the equilibrium constant and the adsorption rate constant are treated as functions of potential and the electrode coverage. Comparison of results for different systems is presented as an analysis of the dependence of the adsorption rate constant on the equilibrium constant. FCV proved to be useful in the evaluation of kinetics of chemisorption (standard rate constant in the range of 10⁶ s$\sp{-1})$ but the results for adsorption of aliphatic alcohols were unreliable. High frequency AC methods allowed the determination of kinetics of physical adsorption. It was found that the activation energy of the adsorption process can be expressed as a linear combination of the electrical component of the standard free energy of adsorption (a major contribution) and the energy of lateral interactions (a minor contribution). At the zero charge potential the rate constant reaches the maximum value of $\rm(4.6\pm0.3)10\sp9\ s\sp{-1}.$