I. ANALYSIS OF ELECTROCHEMICAL MECHANISMS BY FINITE DIFFERENCE SIMULATION AND SIMPLEX FITTING OF DOUBLE POTENTIAL STEP CURRENT, CHARGE, AND ABSORBANCE RESPONSE RATIOS. II. THE APPLICATION OF SMALL POTENTIAL STEPS TO THE STUDY OF THE KINETICS OF NOBLE METAL SURFACE ELECTROCHEMISTRY.

摘要

Part I. An explicit finite difference digital simulation program used to calculate the double potential step chronoamperometric, chronocoulometric, and chronoabsorptometric response ratios for 22 two-parameter mechanisms is described. Distribution of errors in the data is discussed, along with some theoretical aspects of digital simulation. Computer analysis of double potential step experiments by Simplex fitting experimental results to the response ratio data set is described. The ability of double step methods to differentiate between mechanisms, and the probable error in kinetic parameters in fitting typical data to the response ratio data set, are demonstrated using the package.; Part II. Small potential step experiments were programmed on a minicomputer, the theoretical step response of several adsorption mechanisms worked out, and the step experiments were used in the study of hydrogen adsorption on platinum. Exponential decomposition was of central importance, and the topic is examined out of the context of any particular application. The theory and numerical implementation of the Gardner transform method of exponential decomposition is considered in detail. The programming of two small potential step experiments, cyclic staircase voltammetry (CSV) and squarewave voltammetry (SQWV), is described. The CSV program is flexible and easy to use. The SQWV program was designed specifically for a quantitative study of the kinetics of hydrogen adsorption on noble metals. The numerical problem of computing the complex admittance as a function of frequency from the squarewave response is discussed, and a program for accomplishing the transformation is documented. The relaxation kinetics of electrochemical surface reactions that can be modelled by a parallel combination of RC elements is discussed, and the step response for five mechanisms is worked out. Hydrogen adsorption on a platinum electrode in several electrolytes (dilute acids) was examined using the computerized step experiments. A new approach to the presentation, and a first step towards detailed mechanistic interpretation of the experiments, is discussed. The step response is transformed into an exponential decomposition spectrum (EXDS) where one axis is logarithm of relaxation rate, and the other axis is charge. The EXD spectra as a function of bias voltage produce a three dimensional EXDS voltammogram which is compared to theoretical EXDS voltammograms, and the parameters of the models obtained by a nonlinear least squares fitting procedure.