Joseph E. Vitt

The importance of anodic discharge of H sub 2 O in anodic oxygen-transfer reactions

This paper discusses difference voltammetry at rotated disk electrodes which are applied to a study of several anodic O-transfer reactions that appear to occur concurrently with O{sub 2} evolution. This voltammetric technique was useful for extracting the rotation-dependent component of the total current from the large, virtually rotation-independent current for O{sub 2} evolution. Data for oxidation of I{sup {minus}} at Pt, Au, Pd, Ir, and glassy carbon electrodes show that the E{sub 1/2} for IO{sup {minus}}{sub 3} production is correlated with the overpotential for O{sub 2} evolution at these electrode materials. Data obtained at an Ir electrode for various reactions with widely varying E{sup o} values reveal uniform E{sub 1/2} values closely correlated with the potential for onset of O{sub 2} evolution in both alkaline and acidic solutions. The results support the conclusion that the anodic discharge of H{sub 2}O is a prerequisite of these anodic O-transfer mechanisms.

The Effect of Electrode Material on the Electrogenerated Chemiluminescence of Luminol

This paper reports on the oxidation of luminol and its concomitant electrogenerated chemiluminescence (ECL) which were studied at several electrode materials by voltammetry and chronoamperometry. The ECL intensity (I{sub ECL}) was inversely related to the activity of the electrodes. The lowest I{sub ECL}) was measured when luminol was oxidized to 3-aminophthalate (n {approx equal}4 eq mol{sup {minus}1}) at a nearly mass-transport limited rate at glassy carbon. The ECL kinetics were studied and the order of the reaction with respect to luminol was 3/2 at concentrations to ca. 1 mM when O{sub 2} was the coreactant. In the presence of H{sub 2}O{sub 2}, the ECL reaction was first order with respect to luminol. A reaction mechanism is proposed that is consistent with the inetic data and the inverse relationship between electrode activity and I{sub ECL}. The implications of these results are discussed with respect to imaging the spatial distribution of current density at electrode surfaces, including that of PbO{sub 2} films activated by adsorbed Bi(V). A value of 6.6 {times} 10{sup {minus}6} cm{sup 2} s{sup {minus}1} was determined for the diffusion coefficient of luminol in 0.1M NaOH.

Synthesis and characterization of Ru-Ti[sub 4]O[sub 7] microelectrode arrays

A synthesis is described for Ru microelectrode arrays within a conductive Ti[sub 4]O[sub 7] ceramic matrix. Data obtained by X-ray diffractometry and scanning electron microscopy are consistent with the existence of heterogeneous mixtures of Ru particles (ca. 0.8 [mu]m diam) within the Ti[sub 4]O[sub 7] matrices. No mixed metal oxides or other new compounds are detected. Rotated disk electrodes (RDEs) constructed from the Ru-Ti[sub 4]O[sub 7] materials are compared on the basis of their voltammetric response for the oxidations of I[sup [minus]] to I[sub 2] and IO[sup [minus]][sub 3] in 0.1M H[sub 2]SO[sub 4]. These results are rationalized on the basis of microelectrode arrays, i.e., elevated current densities and diminished sensitivity to changes in rotational velocity compared with solid Ru RDE. An RDE constructed from Ru-Ti[sub 4]O[sub 7] containing ca. 0.5% Ru by volume exhibits virtually zero dependence on rotational velocity for the oxidation of I[sup [minus]] to I[sub 2] and, therefore, this material functions as an ideal microelectrode array.

Current Oscillations during Iodide Oxidation at a Gold Rotating Disk Electrode

Oscillations in the electrode current were observed during the potentiostatic oxidation of iodide at a gold rotating disk electrode. In a 0.10 M KI solution, the oscillations were periodic, with a peak-to-peak amplitude equal to 70% of the peak current and a frequency of approximately 2 Hz. The oscillations continued for longer than 10 min, at which point the experiment was discontinued. Periodic oscillations in current were observed only for a narrow range of applied potentials, from 1.08 to 1.13 V vs. SCE in 0.10 M KI. At lower concentrations of KI, the oscillations were less periodic, had a smaller amplitude, and were sustained for shorter times (about 30-60 s for 20 mM KI). No oscillations were observed for concentrations less than about 5 mM. Maximum and minimum current values were measured from the oscillating currents observed at various rotation rates, and the resulting linear plots of inverse current vs. inverse square root of rotation rate gave n values of 1.28 and 0.24, respectively. Based on these results, a mechanism is discussed that involves the periodic formation and dissolution of a thick I 2 (s) film. The presence of this film decreases the rate of oxidation of iodide to iodine, thus yielding an apparent n value of less than one.

Electrocatalysis of Anodic Oxygen‐Transfer Reactions: Acetate‐Doped Lead Dioxide Electrodes in Sulfuric Acid Media

This paper reports that oxidations of dimethyl sulfoxide, toluene, and p-xylene at pure PbO{sub 2}-film electrodes were determined to be under extreme kinetic control with values of the apparent heterogeneous rate constant (k{sub app} {much lt} 1 {times} 10{sup {minus}1}. However, a significant increase in K{sub app} was observed at acetate-doped PbO{sub 2}-film electrodes (OAc-PbO{sub 2}) in 1M H{sub 2}SO{sub 4}. X-ray diffraction data indicated the OAc-PbO{sub 2} films retained the slightly distorted rutile structure of the {beta}-form of the pure PbO{sub 2}. Catalysis of the anodic discharge of H{sub 2}O to produce adsorbed hydroxyl radicals (OH{sub ads}) is concluded to account for the enhanced catalytic activity of the OAc-PbO{sub 2} films for the anodic oxygen transfer reactions in H{sub 2}SO{sub 4}.

Rotating ring-disk study of the oscillating electrochemical reaction of iodide at gold

Abstract The ring electrode of a Au–Au rotating ring-disk electrode was used to monitor the concentrations of I − and I 2 during oscillations in current that occur during I − oxidation at the disk electrode. With the ring electrode poised at a potential sufficient to oxidize I − , the anodic current at the ring decreased, while the anodic current at the disk increased, as was expected for a typical shielding experiment. However, when the ring was poised at a potential sufficient to reduce I 2 (or I 3 − ) in a collection experiment, the cathodic current also decreased simultaneously with the increase in anodic current at the disk. These results demonstrate convincingly that the oscillating current involves the periodic formation and dissolution of an I 2 film.

Observation of adsorbate coverage with electrogenerated chemiluminescence imaging

Abstract The spatial distribution of poly(phenylene oxide) on platinum electrodes was studied using the technique of electrogenerated chemiluminescence (ECL) imaging. The polymer was deposited on electrodes through the electrooxidation of phenol in aqueous solution. Electrochemical oxidation of phenol itself was effectively inhibited by the film, as was the electrogenerated chemiluminescen of luminol in alkaline peroxide. Using a sensitive microscopic imaging system, light from the ECL reaction was collected and used to show where film coverage occurred on the surface. At partial coverages of poly(phenylene oxide), some regions of the electrode were still able to support electron transfer and therefore the ECL images exhibited heterogeneity. Images showed that the nature and extent of coverage of poly(phenylene oxide) on the surface was a factor of phenol oxidation potential. The potential dependence of coverage was consistent with an isotherm that accounts for a high degree of attraction between adsorbed molecules, so that coverage increased abruptly over a relatively small potential range.