Charles N. Reilley

Thin-layer electrochemistry: steady-state methods of studying rate processes

Abstract Potential applications are considered for twin-electrode thin-layer electrolysis. Determination of diffusion coefficients, electron-transfer rate constants, and concentration of electroactive species are possible using steady-state currents attainable with the twin-electrode technique. The technique is applied to the Fe(II)-Fe(III) and the quinone-hydroquinone systems.

A galvanic cell oxygen analyzer

Abstract A galvanic cell consisting of a silver-lead couple separated from the test sample by an oxygen permeable plastic membrane provides the basis of a simple oxygen analyzer. The current generated, while directly proportional to the amount of oxygen in the sample, also depends upon factors such as temperature, membrane thickness, kind and concentration of the supporting electrolyte, and the geometry of the cell. The analyzer has been used successfully with continuous recording devices and as a completely portable instrument with a readout on a microammeter for oxygen monitoring in natural waters and wastes. Oxygen content in non-aqueous systems and gaseous streams may be measured by the device.

Surface characterization of Pt electrodes using underpotential deposition of H and Cu: Part I. Pt(100)

Abstract This paper is the first in a series describing the in situ surface characterization of platinum electrodes using H and Cu deposited at underpotentials. The surface of a Pt(100) electrode pretreated by simple flame annealing and quenching in aqueous sulfuric acid is shown to contain a high concentration of defects such as vacancies and self-adsorbed Pt atoms. Adsorbed hydrogen is more strongly bound at these defects than on a uniform Pt(100) surface. Potential cycling in 1 M HCl produces a higher concentration of defects, while oxide formation and reduction in 0.5 M H 2 SO 4 has the opposite effect. The nature of (100)-like sites at a polycrystalline platinum electrode is also discussed.

Surface characterization of Pt electrodes using underpotential deposition of H and Cu: Part II. Pt(110) and Pt(111)

Abstract The surface characterization of Pt electrodes begun in Part I of this is extended to Pt(110) and Pt(111). Flame annealing and quenching in aqueous electrolyte yields the reconstructed Pt(110)-(1×2) surface, which gives two distinct hydrogen adsorption-desorption waves in dilute perchloric acid. The (1×2) structure is distinguished from other possible surface structures by comparison of the voltammetry with that of a vicinal (111) surface, namely Pt(331), and also by a detailed consideration of the effects of anions and cations on the hydrogen adsorption-desorption profile. The surface of a Pt(111) electrode pretreated by flame annealing and quenching in aqueous electrolyte is either highly defective or possesses a high degree of surface lattice strain, resulting in an unusually strong binding of adsorbed hydrogen. Our pretreatments also produce high binding energy hydrogen adsorption sites on (100), but not on (110) or any of the higher Miller index surfaces studied. The distinguishing feature is considered to be the low density of monoatomic steps at (111) and (100).

Thin-layer electrochemistry: use of twin working electrodes for the study of chemical kinetics

Abstract The steady-state currents attainable with twin electrode thin-layer electrochemistry offer a novel approach for the study of chemical kinetic processes. Expressions are presented- relating the chemical rate constant to the steady-state current, and the upper limit of pseudo-first-order rate constant measurable by the technique estimated to be 5·102 sec−1. A number of electrode materials suitable for use as twin working electrodes are listed, as well as the major limitations on the technique imposed by electrode design. Application of the technique to the study of bimolecular reactions between substances electrogenerated at opposing electrodes is discussed.

Hydrogen chemisorption and related anion effects on Pt(110) electrodes

Abstract The four primary states of hydrogen adsorption on polycrystalline platinum electrodes are assigned to sites characteristic of (100), (110), and (111) single crystal surfaces. The coupling between hydrogen adsorption and adsorbed sulfuric and hydrochloric acid anions is investigated. Time resolved staircase voltammetry is used to show the absence of a cathodic counterpart to the “third anodic peak”. The results of electrochemical relaxation measurements following small potential step perturbations are given for Pt(110) in sulfuric acid.

Surface characterization of Pt electrodes using underpotential deposition of H and Cu: Part III. Surface improvement of the flame-annealed Pt(100) and Pt(111) electrodes via potential cycling

Abstract In Parts I and II of this series it was shown that the Pt(100) and Pt(111) surfaces pretreated by flame-annealing and quenching in aqueous electrolyte contain a high density of defects such as vacancies, Pt adatoms and clusters of Pt adatoms. In this paper we show that potential cycling including scans into the oxygen region in sulfuric or perchloric acid removes most of these sites and that a limited number of cycles yield hydrogen adsorption-desorption profiles (cyclic voltammograms) that compare favorably with those published by authors who established the structure using electron diffraction techniques. Some loss of longer-range surface order as a result of the potential cycling is indicated by an increase in the width at half-height of the monolayer copper stripping peaks. The possibility of surface improvement in the absence of surface oxidation and reduction is explored by potential cycling in hydrochloric acid.

Double potential step chronocoulometry. Part I. A reexamination of EC kinetic theory including the effects of electrode reactant and product adsorption

Summary Calculation of the theoretical double potential step chronocoulometry working curve (|Qb/Qf| vs. (kτ)1/2) for an irreversible pseudo first-order chemical reaction coupled to a heterogeneous charge transfer process (EC mechanism) by finite difference methods revealed an error in the previously published working curve for this situation. The analytical solution to the mass transport equations for the EC mechanism with double potential step boundary conditions is rederived. The working curve, calculated from this new analytical solution, was in close agreement with the finite difference results. In addition, a theoretical treatment is presented for double potential step chronocoulometry where the effect of electrode reactant and product adsorption on the charge ratio, |Qb/Qf|, both with and without EC kinetic complications, is incorporated. A procedure whereby experimentally determined charge ratios can be corrected for the effects of reactant and product adsorption has been developed.

Chronopotentiometry with programmed current: II. Response function additivity principles applied to current programming and multicomponent systems

Abstract The principle of additivity of response functions forms the basis of a new method for derivation of theoretical chronopotentiometric transition tune relations for programmed currents and multicomponent systems with diffusion-controlled transport. To use the method it is necessary to have available, for all types of single current impulses used, the characteristic current impulse response functions describing the diffusion gradients and surface depletions for single component systems. These are obtained through solution, of Ficks law. Response function additivity is utilized in multi-component systems by summing the n - and D -weighted surface depletions of all reactants; the resulting surface concentration response function is then at all times equal to the impulse current response function characteristic of the current impulse used for linear diffusion. In current programming , the n and D 12 -weighted surface depletion of the electroactive species is, by the response additivity principle for linear diffusion, equal to the summation of the response functions of all current impulses employed, each current impulse response function being written for the time of usage of the respective current impulse. Solution of Picks law to obtain the chronopotentiometric transition time relations for such cases is therefore unnecessary. Generalized relations are given for multi-component systems and current programming and for combinations of the two cases. The method of derivation is illustrated using a number of known cases. New equations are derived for the specific cases of (1) single component systems : step-reverse-ramp, step-reverse-square root of time, step-plus-negative ramp and ramp-reverse-ramp current reversal programs, step-wise reactions with any power of time current impulse, square root of time impulse program, staircase, square wave, and sawtooth periodic current programs (linear diffusion), and multi-step and step-reverse-step current programs (spherical and cylindrical diffusion), (2) multicomponent systems: step and square root of time current impulses (spherical and cylindrical diffusion).

Surface characterization of Pt electrodes using underpotential deposition of H and Cu: Part IV. Surface structural dependence of a non-equilibrium state of hydrogen adsorption

The cyclic voltammetric characterization of commercial platinum catalysts is discussed with regard to experimental variables such as nature and concentration of electrolyte and choice of sweep rate. Information available from electrochemical experiments such as surface area and distribution of hydrogen adsorption sites is obtained for BD Pt, a commercial platinum catalyst. The distribution of hydrogen adsorption sites at BD Pt is shown to be similar to that at smooth polycrystalline Pt. Also, underpotential deposition of copper is shown to be useful for surface area measurements under the proper conditions. A maximum copper coverage of 417 ..mu..C per real cm/sup 2/ is obtained at BD Pt.