A catalytic study of formic acid oxidation on preferentially oriented platinum electrodes

The oxidation of formic acid was examined by cyclic voltammetry and chronoamperometry in order to determine the rate of catalytic activity (reaction turnover) as a function of surface crystallography on preferentially oriented (electrochemically modified) platinum electrodes. The resulting turnover rates indicated a maximum fourfold current enhancement for an approximately 60% (111)-oriented surface versus a polycrystalline surface, suggesting that preferentially oriented electrodes are of potential practical significance.     

Voltammetry of UPD copper and formic acid as characterization of preferentially oriented polycrystalline platinum surfaces

In order to characterize the surface structure of electrochemically modified polycrystalline platinum, the voltammograms of underpotential deposited copper and of formic acid are studied. The operating conditions are chosen in such a way that a direct comparison to results on single crystal platinum electrodes is possible. It is shown that UPD copper in HF solution and formic acid oxidation are well suited to characterize the modified structure of platinum surfaces.     

Kinetics and mechanism of the oxygen electroreduction reaction on faceted platinum electrodes in trifluoromethanesulfonic acid solutions

The kinetics of the oxygen electroreduction reaction (OERR) were investigated on (1 1 1)-type and (1 0 0)-type faceted, and polycrystalline platinum electrodes in aqueous (0.05–1.0)m trifluoromethanesulfonic acid (TFMSA) using the rotating disc and ring-disc electrode techniques at 25°C. Reaction orders with respect to oxygen close to either 1/2 or 1 were found, depending on the TFMSA concentration and platinum surface morphology. At all TFMSA concentrations the formation of H₂O₂ was enhanced at (1 0 0)-type platinum surfaces. The difference in the electrocatalytic activity of platinum surfaces can be explained through data derived from the OERR formalism proposed by Damjanovicet al. The rate of the direct O₂ to H₂O electroreduction reaction increased steadily with the cathodic overvoltage irrespective of the platinum surface morphology, whereas a maximum H₂O₂ formation rate was found at about 0.5 V, depending on the TFMSA concentration. The H₂O₂ decomposition rate on (1 0 0)-type platinum electrode yielding H₂O approached zero within a certain potential range.     

Electroreduction of oxygen on gold-supported nanostructured palladium films in acid solutions

The electrochemical reduction of oxygen on thin Pd films with a nominal thickness of 0.25-10 nm on polycrystalline Au substrate (Pd/Au) was studied. The Pd films were prepared by electron beam evaporation and oxygen reduction was studied in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions using the rotating disk electrode (RDE) method. The surface morphology of Pd overlayers was examined by scanning tunnelling microscopy (STM). O₂ reduction predominantly proceeds through 4e⁻ pathway on all Pd/Au electrodes. The specific activity (SA) of oxygen reduction was lower in H₂SO₄ solution and decreased slightly with decreasing the Pd film thickness. In HClO₄, the SA was higher and not significantly dependent on the film thickness. The Tafel slope values close to −60 mV at low current densities and −120 mV at high current densities were found for all electrodes.     

Electroreduction of dimethyl maleate,dimethyl fumarate and α,β-dimethyl maleic acid on a platinum electrode in sulfuric acid aqueous solution

Electroreduction of ethylene derivatives was conducted on a smooth platinum electrode. Kinetic parameters and the study of the adsorption state of the test electrode lead to the conclusion that (i) the hydrogen atom and the organic reactant adsorb on different facets (or different kinds of sites) and (ii) the first addition of the hydrogen atom to the reactant at a boundary between the two facets (or the sites) is rate-determining. Difference in the rates among the ethylene derivatives is discussed in terms of the molecular properties calculated by CNDO/2 and EH methods.     

Impedance study of formic acid oxidation on platinum electrodes

The electrooxidation of formic acid on polycrystalline platinum electrodes in sulphuric acid is studied by a dynamic impedance method. Impedance spectra in the frequency range 0.7 Hz to 20 kHz, assembled from ac voltammograms, are interpreted in terms of a mechanism with three adsorbed species, although one of these may be free sites. The spectra show evidence for incipient oscillations in two potential regions, and observed oscillations were related to these spectra. The zeroes of the interfacial impedance were directly extracted from suitable equivalent circuits in distinct potential regions where oscillations may be triggered. The observation of only real zeroes of the interfacial impedance eliminates the possibility of pure potentiostatic oscillations arising from chemical reasons alone, and therefore potential is an essential variable.     

乙腈-水相1,4-二氯苯在铂电极上的电氧化行为

为有效去除化工废水中的1,4-二氯苯,研究了Pt电极上乙腈-水混合溶剂中1,4-二氯苯电氧化机理。计时电流法研究表明,氧化过程中既有电极表面失电子的直接电化学氧化,又有基于活性自由基的间接氧化。通过SEM表征和Pt电极表面观察,证明Pt电极在2.3 V(vs. SCE)的电位下氧化p-DCB过程中,表面存在黄色有机聚合物膜,致使体系的导电性变差。降解途径推断表明,反应首先经历了脱氯和对氯酚、对苯醌等芳香类中间体的生成;接着开环生成小分子有机酸;最后小分子羧酸进一步氧化成CO₂。     

On the reduction of oxygen at platinum—oxygen alloy diaphragm electrodes

The steady-state reduction of oxygen in O₂-saturated 2 N H₂SO₄ solution was obtained at constant current on the front side of a Pt foil diaphragm before and after strong anodization of the back side. Strong anodization converts the Pt to a PtO alloy. In all cases, the overvoltage for O₂ reduction was less at the PtO alloy than at the Pt cathode. This enhanced catalytic activity of Pt containing dissolved oxygen is interpreted in terms of the band approximation model of metals. The dissolved oxygen in such high concentrations generates a 2p-band in the alloy which interacts with the 5d- and 6s-bands of the Pt. Electrons spill into the 5d-band causing a raising of the Fermi level of the metal with respect to the energy distribution of adsorbed species on the electrode surface. Consequently, it requires less cathodic polarization to transfer electrons from the metal to the species adsorbed on the metal surface by quantum mechanical tunnelling or by passing over the potential energy barrier.     

The oxygen film on platinum electrodes and the electro-oxidation of acetate ions

The recent literature is reviewed and a consistent mechanistic and mathematical model is presented which can account for the varied phenomena observed experimentally during both steady-state and unsteady-state electrolysis of aqueous acetate solutions at Pt electrodes. In particular the following competing reactions are considered:

1.

complete oxidation of acetate ions to carbon dioxide,     

New electrodes for oxygen evolution in acidic solution

Conventional electrowinning of metals such as zine and copper from 1–1.5 kmol m^–3 H₂SO₄ electrolytes involves anodic oxygen evolution at Pb alloy/PbO₂ anodes operating at 200–800 A m^–2. The oxygen overpotential, estimated to be about 0.6 V, constitutes a significant proportion of the cell voltage (typically 2.5 V for Cu and 3.3 V for Zn). The objective of this work was to lower the anode overpotential and so decrease the process specific energy requirements, by devising new anode materials based on: (1) PTFE-bonded PbO₂ catalysts, supported on Pb–Ag alloys; (2) modification of the pore structure of porous electrodes to maximize the utilization of the available surface without the use of PTFE bonding. Both methods are shown to produce significant benefits in lowering oxygen overpotentials, though the latter technique has been tested only in alkaline electrolytes, as yet. However, with the former approach, substrate oxidation through the porous catalyst layer has been found to cause catalyst shedding after>60 h at 1 kA m^–2, though careful pre-oxidation of the anode substrate appears to extend the anode life.Paper presented at the 2nd International Symposium on Electrolytic Bubbles organized jointly by the Electrochemical Technology Group of the Society of Chemical Industry and the Electrochemistry Group of the Royal Society of Chemistry and held at Imperial College, London, 31st May and 1st June 1988.     

Voltammetric response of iron hydroxide layers precipitated on platinum electrodes in alkaline solution

Voltammograms of thin and thick iron hydroxide layers chemically precipitated on platinum were obtained. The influence of the precipitation conditions of iron hydroxide was studied. Different types of combined potential/time perturbation programmes were employed to correlate the anodic and cathodic voltammetric peaks and to detect possible ageing-like effects. The electrochemical response of the iron hydroxide layer depends considerably on the layer thickness and precipitation conditions. The voltammetric behaviour of the precipitated iron hydroxide layer is interpreted in terms of the complex structure involving an inner poorly hydrated layer and an outer porous layer recently proposed from in situ ellipsometry data.     

Characterization of platinum electrodes in H2-saturated sulphuric acid solution under steady-state potentiostatic conditions

The character of platinum-bead and -(111) electrodes at anodic steady-state potentials was determined by analysing with cathodic galvanostatic pulses for reducible, Pt-associated species. There was a considerable amount of potential overlap in the reduction of adsorbed and dermasorbed O. The θ_so^2?4- dependence on potential and the effects of adsorbed SO^2?₄ on H and O sorption were shown. SO^?₄ adsorption passivated the surface for H₂ oxidation below 1·0 V and stabilized the associated oxygen species at higher potentials. Pre-saturation of Pt with dermasorbed oxygen caused a hysteresis effect requiring potentiostating at or below 0·85 V for its removal. H₂ has a strong effect on the steady-state character below 1·0 V, but has essentially no effect for the transient formation of Pt—O, Pt-bead and -(111) electrodes showed important differences.     

Investigation of oxygen reduction on activated carbon electrodes in alkaline solution

Oxygen reduction has been studied on gas diffusion electrodes made from various kinds of activated carbon materials. The potentiodynamic method was used to study the electrochemical behavior of the reduction reaction, while nitrogen gas adsorption and powder X-ray diffraction were used to determine the pore size distribution and the crystal structure of the carbon material, respectively. The relationship between the catalytic activity of activated carbon and the surface content of edge orientation was discussed. The specific catalytic activity of activated carbon is determined by the percentage of the edge orientation on the surface. The higher the content of the edge plane, the lower the oxygen reduction overpotential. The reduction overpotential is also related to the effective surface area, which determines the local current density.     

Electroreduction of peroxycitric acid coexisting with hydrogen peroxide in aqueous solution

The electrochemical reduction of peroxycitric acid (PCA) coexisting with citric acid and hydrogen peroxide (H₂O₂) in the equilibrium mixture was extensively studied at a gold electrode in acetate buffer solutions containing 0.1 M Na₂SO₄ (pH 2.0-6.0) using cyclic and hydrodynamic voltammetric, and hydrodynamic chronocoulometric measurements. The reduction of PCA was characterized to be an irreversible, diffusion-controlled process, and the cyclic voltammetric reduction peak potential () was found to be more positive by ca. 1.0 V than that of the coexisting H₂O₂, e.g., the values obtained at 0.1 V s⁻¹ for PCA and H₂O₂ were 0.35 and −0.35 V, respectively, vs. Ag|AgCl|KCl (sat.) at pH 3.3. The of PCA was found to depend on pH, i.e., at pH > 4.5, the plot of vs. pH gave the slope (−64 mV decade⁻¹) which is close to the theoretical value (−59 mV decade⁻¹) for an electrode process involving the equal number of electron and proton in the rate-determining step, while at pH < 4.5, the was almost independent of pH. The relevant electrochemical parameters, Tafel slope, number of electrons, formal potential (E⁰′), cathodic transfer coefficient and standard heterogeneous rate constant (k⁰′) for the reduction of PCA and the diffusion coefficient of PCA were determined to be ca. 100 mV decade⁻¹, 2, 1.53 V (at pH 2.6), 0.29, 1.2 × 10⁻¹² cm s⁻¹ and 0.29 × 10⁻⁵ cm² s⁻¹, respectively, and except for E⁰′, the obtained values were almost independent of the solution pH. The overall mechanism of the reduction of PCA was discussed.     

Electrode kinetics of oxygen reduction on platinum in trifluoromethanesulphonic acid

The kinetics of oxygen reduction at Pt in trifluoromethanesulphonic acid (TFMSA) (0.05–6.0 M) and in 1.0 M TFMSA with addition of small concentrations of phosphoric acid (0.003–0.1 M) was investigated using therrde technique. In TFMSA, the oxygen reduction current on the oxide-covered Pt was found to be smaller than that on the oxide-free Pt surface. This result is consistent with the greater amonut of hydrogen peroxide produced on the oxide-covered Pt. A reaction order of one-half with respect to the oxygen concentration for the oxygen reduction reaction was obtained from the ring-disc data. Addition of increasing amounts of phosphoric acid to TFMSA resulted in a progressive decrease in the oxygen reduction current and in an increase of the reaction order with respect to oxygen. On the basis of these experimental results the reaction mechanism proposed for the oxygen reduction of Pt in TFMSA is the fast dissociative adsorption of oxygen, followed by the slow electron transfer step.     

Adsorption of acetic acid on platinum,gold and rhodium electrodes

The adsorption of acetic acid has been studied on platinum, gold and rhodium electrodes from aqueous electrolytes. On the platinum electrode, the evidences have been presented that in the double layer region of the electrode potentials: (i) the undissociated acetic acid molecule is adsorbed, (ii) the adsorption process is reversible and (iii) the adsorption occurs in the second ad-layer of the interfacial region, ie on the top of the chemisorbed water molecules. In the hydrogen region, the slow (reductive) chemisorption of this compound has been observed.The similarity in the character of the adsorption of acetic acid in the double layer region of the three electrodes investigated have been shown.     

Mechanisms for the evolution and ionization of oxygen at platinum electrodes

By considering published data on the kinetics of the oxygen electrode at a platinum surface it is suggested that the electrode process follows the same path in both acid and alkaline solution. This path is identified, and attention is drawn to certain restrictions on the use of diagnostic criteria.     

Electrocatalysis of oxygen reduction by quinones adsorbed on highly oriented pyrolytic graphite electrodes

The reduction of oxygen in alkaline solution has been studied on highly oriented pyrolytic graphite (HOPG) electrodes modified with various quinones using a rotating disk electrode (RDE). The electrode surface was modified by adsorption of quinones from a 0.1 M KOH solution. The oxygen reduction activity of these electrodes was considerably higher than that for unmodified HOPG and characteristic current maxima for oxygen reduction was observed. All quinones studied catalysed the two-electron reduction of oxygen to hydrogen peroxide. The peak potentials for oxygen reduction were in good correlation with the redox potentials of the quinones that were found from the cyclic voltammograms in oxygen-free solutions. The results obtained give further evidence that oxygen reduction is catalysed by the semiquinone radical and that the redox potential of the quinone is the most important factor determining its electrocatalytic activity for oxygen reduction.