Role of inhibition by anion adsorption in oscillatory phenomena produced in the course of electrocatalytic reduction of nitric acid at a platinized platinum electrode

Oscillatory phenomena occurring in the course of the reduction of nitric acid at a platinum electrode which were induced by the presence of chloride ions are presented. Both galvanostatic potential oscillations and potentiostatic current oscillations were observed. The influence of different parameters (concentration, current, potential) on the oscillatory behaviour was studied. By use of in situ radiotracer adsorption measurements it has been shown that the periodical changes of electrochemical parameters were coupled with those of chloride ion adsorption.Open-circuit potential oscillations were observed in a hydrogen atmosphere. This observation attests that the external electric circuit does not play a role in the generation of the galvanostatic oscillations.An attempt was made to give an explanation of the phenomena observed under galvanostatic and open-circuit conditions.     

Electrooxidation and adsorption of oligoelthylene glycols on platinized platinum electrode

The trends in adsorption and oxidation of oligoethylene glycols (OEG), namely, di-, tri-, and tetraethylene glycol, on a Pt/Pt electrode are studied. Using a combination of methods of open-circuit potential shifts at the adsorption of organic species and anodic voltammetric curves, it is established that the OEG adsorption on a Pt/Pt electrode is accompanied by hydrogenation, dehydrogenation, and partial destruction of molecules and also that the amount and composition of accumulated adsorbate depend on the initial adsorption potential and the polymer structure. The OEG oxidation on a platinum electrode is considerably hindered as compared with ethylene glycol. The reaction rate decreases with an increase in the OEG molecular mass and is largely limited by the adsorption of molecules on the electrode surface, which is evidenced by the weak dependence of the rate on the potential in the double layer region.     

On the electrochemical behaviour of pyruvic and lactic acids at a platinized platinum electrode in acid medium

The electrochemical oxidation of pyruvic and lactic acids at a platinized platinum electrode has been studied in acidic (M H₂SO₄) medium. The oxidation process takes place at potentials more positive than 900–1000 mV (on RHE scale). The final products are acetic acid and CO₂. Unambiguous polarization measurements could be carried out only in the case of preoxidized surfaces. For the interpretation of the phenomena observed, a mechanism involving reactions of organic species with electrochemically formed OH radicals is proposed.Studying the reduction of pyruvic acid, it has been stated that, similar to the behaviour of simple aliphatic ketones, the conversion of the

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group into -CH₂- occurs to a significant extent.     

Investigation of ruthenium deposition onto a platinized platinum electrode in sulfuric acid media

Ruthenium deposition onto platinized Pt electrode in 0.5 M H₂SO₄ solution is investigated. The I–V profiles of the Pt electrode covered with Ru depend on the potential of Ru deposition. This phenomenon is explained by the increasing degree of oxidation of the Ru layers deposited at higher potentials. Oxidation of Ru deposited at low potentials begins with comparatively slow processes. A mechanism for Ru deposition via ionization of hydrogen adsorbed on platinized Pt electrode is proposed.     

Indirect radiotracer study of the adsorption of organic compounds at platinized platinum electrodes

The possibility of the use of an indirect radiotracer method for the study of the adsorption of organic species has been discussed. It has been shown that by study of the adsorption of Cl-36 labelled chloride ions in the presence of organic species such as maleic, benzoic and m-nitrobenzoic acids there is a possibility of obtaining information concerning the adsorption of the latter compounds. Change in the adsorption behaviour of organic species connected with their electroreduction were detected and followed by indirect tracer measurements. In the light of the experimental results, some mechanistic aspects of the reduction of aromatic nitro compounds are discussed.     

Adsorption and electrooxidation of dimethyl ether on platinized platinum electrode in sulfuric acid solution

Adsorption and oxidation of dimethyl ether (DME) on the Pt/Pt electrode from 0.5 M H₂SO₄ is studied by measuring transients of current and potential, charging curves, and curves of electrooxidation in the adsorbed layer and also by cyclic voltammetry and steady-state polarization measurements. The DME adsorption is accompanied by dehydrogenation and destruction of its molecules to form a chemisorbed adsorbate that mainly consists of C1 species (HCO_ads and/or CO_ads) with (under certain conditions) a small amount of species that desorb at cathodic polarization. The adsorption and electrooxidation of DME are inhibited by adsorbed oxygen. The possible schemes of DME oxidation, where the reaction of DME chemisorption products with adsorbed oxygen-containing species is the limiting stage, are discussed.     

Effect of cadmium and lead adatoms on the reduction kinetics of peroxodisulfate anions at platinized platinum in acid solutions

The reduction of peroxodisulfate anions on a rotating Pt/Pt disk electrode in solutions of sulfuric and perchloric acids with or without cadmium and lead salts is studied. At E _r > 0.2V (RHE) the reduction rate in HClO₄ exceeds that in H₂SO₄, but at E _r < 0.2V, the process in HClO₄ is strongly inhibited upon approaching E _r = 0. Lead adatoms catalyze the process, while cadmium adatoms inhibit it in H₂SO₄ and accelerate it at E _r < 0.2V in HClO₄. The results are interpreted by taking account of the specific adsorption of sulfuric-acid anions and their co-adsorption with cadmium cations, as well as the adsorption of peroxodisulfate anions on the Pt surface modified by lead and cadmium ions in perchlorate solutions.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 137–141.Original Russian Text Copyright © 2005 by Nikiforova, Petrii.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

Direct electrocatalytic reduction of hydrogen peroxide at a glassy carbon electrode modified with polypyrrole nanowires and platinum hollow nanospheres

A nonenzymatic sensor for hydrogen peroxide has been fabricated by dispersing platinum hollow nanospheres onto polypyrrole (PPy) nanowires to form a PPy-Pt hollow sphere nanocomposite on a glassy carbon electrode. The materials were characterized by transmission electron microscopy and scanning electron microscopy. The process and the sensor were characterized by electrochemical impedance spectroscopy, cyclic voltammetry, and chrono-amperometry and revealed that the electrode has a large electroactive surface area and small resistance to electron transfer. The linear range for the determination of hydrogen peroxide is from 3.5 µM to 9.9 mM, the detection limit is 1.2 µM (S/N?=?3), and the response time is 3 s. The electrode exhibited good stability and excellent repeatability.     

Concurrent oxygen reduction and methanol oxidation in sulfuric acid solutions on platinized platinum electrodes

Steady-state polarization curves are compared in solutions of 0.5 M H₂SO₄ + O₂ (saturated), 0.5 M H₂SO₄ + (0.005–0.1) M CH₃OH, and 0.5 M H₂SO₄ + (0.005–0.1) M CH₃OH + O₂ (saturated) on a Pt/Pt electrode. A considerable difference is found between the currents in mixed solutions and those expected based on the principle of additivity of currents in CH₃OH and O₂ individual solutions. The surface coverages with the CH₃OH and O₂ adsorption products are determined in the potential range of 0.2–0.9 V (RHE). Open-circuit potentials are measured in mixed solutions. The obtained results suggest that the direct heterogeneous interaction between methanol and oxygen occurs alongside with faradaic reactions. This is assumed to lead to a decrease in methanol electrooxidation currents at E ≥ 0.8 V and their increase at E ≤ 0.65 V.     

Mechanism of electroreduction of allyl alcohol at platinized platinum electrode in acidic aqueous solution.

The electroreduction of allyl alcohol to form propene at the platinized platinum electrode in acidic aqueous solution has been studied using CV plots, IR, ESR, and MS spectra, and a semiempricial MO method (MOPAC7/AM1, PM3). From the determinations of charge-transfer coefficients, reaction orders and apparent activation energy for the given reaction, the detection of the intermediates such as C(3)H(5)(+), C(3)H(5)(*), and C(3)H(5)(-) species, and PM3 calculations of charge distribution and frontier orbital energies of the reaction species C(3)H(5)OH and C(3)H(5)(+), the authors suggest that in acidic aqueous solution the production of propene via reductive splitting of the C-OH bond situated in the allyl position of allyl alcohol obeys a carbonium ion-carbanion mechanism.     

Radiotracer study of the adsorption of phosphoric acid on platinized platinum electrodes in the presence of different ions and oxalic acid

Radiotracer study of the adsorption of phosphoric acid on platinized platinum electrodes was carried out under different experimental conditions. The potential and concentration dependence of the adsorption was studied at low concentrations (5×10^?6–6×10^?3 mol dm^?3) in the presence of 1 M HClO₄ supporting electrolyte. A Freundlich type isotherm was found. The effect of different additives was studied. In the presence of Cl^? and HSO₄^? ions and oxalic acid the decrease of the adsorption of H₃PO₄ may be observed. On the other hand Cd²⁺ and Cu²⁺ ions induce the increase of the adsorption of H₃PO₄ in the potential range where their electrosorption takes place.     

Investigations of copper adsorption in hydrochloric acid media,via the ionization of hydrogen adsorbed on platinized platinum

Copper deposition in hydrochloric acid media, via the ionization of hydrogen adsorbed on platinized platinum, results in an adsorbed metal layer similar to that formed potentiostatically. However, the adsorbed hydrogen is replaced by adsorbed copper only to a certain extent, owing to the formation of Cu⁺ ions. This discrepancy depends on the chloride ion activity and the structure of the platinum black.