H. Wroblowa

Electroreduction of oxygen: A new mechanistic criterion

Abstract A new diagnostic criterion was developed which allows a distinction to be made between the two main possible reaction paths in electroreduction of oxygen. This criterion was applied to the results obtained in the rotating ring—disc electrode system on gold in alkaline solutions, indicating the absence of the direct four-electron reduction.

The mechanism of the passivation of iron in neutral solutions: An ellipsometric and coulometric investigation

Ellipsometric and electrochemical transient techniques have been used to investigate the pre- and post-passive region on iron in neutral solutions. In particular, a new ellipsometric transient technique allows the examination of the dissolution and film formation on iron, even though this is undergoing roughening by etching. Stationary states of the metal surface were measured by classical ellipsometry. Measurement of transient states depend on the variation of light intensity, when the initial offset angle of analyser and polarizer are respectively at an angle of 25, 0 and 90°. Potentiostatic and galvanostatic coulometry and capacitance measurements were applied. Oxide-free surfaces with visible unidirectional scratches gave different values of Δ and ψ if the position of the sample was changed with respect to the oncoming light; apparent Δ and ψ were dependent also on the degree of polishing. If the metal was held at potentials during which it dissolved in the active region for more than ca 7 min and i > 10 μA/cm2, Δ and ψ changed with time and the original values were not re-attained on reduction; the double-layer capacitance was increased by 2–3 times by such treatment. A sufficient degree of polishing gave Δ and ψ for oxide-free iron in solution, which showed refractive index and adsorption coefficient the same as those determined by others in vacuum. The corresponding metal was taken as an oxide-free optical reference state. Coulometry showed 1–2 monolayers of a ferrous oxide at the peak of the current/potential curve. Far from this peak on the positive side, the galvanostatic transients during reduction are in two sections which correspond to a charge of 1 : 2 in the charge reduced. At less positive post-peak regions coulometry allowed evaluation of the ratio Fe2+ : Fe3+ in the film. Δ/potential and ψ/potential relations showed three regions; pre-peak, peak to 0 mV(she) and 0–900 mV(she). Δ/potential and ψ/potential relations measured at short (1 s) and long (300 s) times are consistent. Δ/time measurements down to 10−2 show changes of Δ from the beginning of anodic polarization. Ellipsometric measurements are affected by roughness except at an asymptotic degree of polishing. Metal dissolution may also effect roughening to an extent which completes ellipsometrically with effects due to oxide growth. A transformation of a ferrous to a ferric oxide begins at the i/V peak and is complete at + 500 mV to the peak. Such transformations are controlled by potential, and not by field. The film before the peak is a ferrous oxide phase, probably Fe(OH)2. Its growth kinetics indicate it is three dimensional, and formed by nucleation of adsorbed OH−. The ferric film is γ-Fe2O3. Film thickness l, refractive index n, and absorption coefficient, k, can be indicated within certain limits as functions of potential from ellipsometry. Application of coulometric thickness measurements to allow solution of the ellipsometric equations shows a growth of ferrous oxide to the peak, a region during which the oxide is transformed to a ferric oxide and then linear growth with potential to 40 A. In the pre-peak region, iron dissolves from oxide-free sites to solution. The current peak corresponds, essentially, to full coverage of the surface with Fe(OH)2. The fall of current after the peak could arise from the complete coverage. It is accompanied by the beginning of conversion to Fe2O3. In the passive region Fe2O3 growth continues.

Adsorption and kinetics at platinum electrodes in the presence of oxygen at zero net current

The rest potentials at Pt electrodes were determined in 1 N2SO4. Solutions saturated at 0°, 20u°, 50° And 80° with various partial pressures of oxygen ranging from 0.005 to 1 atm. Oxygen adsorption isotherms were obtained by cathodic reduction transient technique. The nature of the rest potentials is analyzed. The behavior of the Pt-O2-H2O system under open circuit conditions, appears to be best interpreted in terms of the following model. Oxide-free Pt electrodes immersed in an aqueous solution of H2SO4 acquire a mixed potential due to cathodic reduction of oxygen and oxidation of impurities as the anodic component. The value of this potential is simply dependent upon the oxygen pressure. The potential in turn determines the coverage of oxygen atoms, which arises from an equilibrium of the water discharge reaction with the Pt surface. According to this model, this reaction, and not the dissociative adsorption of oxygen, is the origin of oxygen coverage at the surface. The lack of reversibility of the O2 electrode would therefore result from the presence of impurities. Further analysis of the Behavior of noble metal electrodes In O2-saturated solutions is in progress.

Studies of the mechanism of the anodic oxidation of ethylene in acid and alkaline media

Abstract The ethylene oxidation reaction on smooth and platinized platinum has been studied at 80°C in solutions of H 2 SO 4 + K 2 SO 4 and NaOH + K 2 SO 4 of constant ionic strength = 1.5. Reaction rates were measured as a function of potential, pH and partial pressure of ethylene. Coulombic efficiency of the reaction was determined by measurements of CO 2 production in acidic solutions, and of C 2 H 4 consumption in alkaline solutions. The following parameters have been found: Coulombic efficiency is 100 ± 1% in acidic and 90 ± 5% in alkaline solutions, i 0 = 10 −8 A cm −2 on platinized and 10 −10 A cm −2 on smooth Pt respectively. Activation energy A = 17 ± 2 kcals. At higher overpotentials diffusion limiting current was obtained. At V = 0.9 (R.H.E.), the current drops to negligible values due to the oxide formation. Coverage of the electrode with the ethylenic radical at 1 atm has been evaluated by two independent methods as θ E = 0.55 ± 0.2. The reaction mechanism in the Tafel potential range was interpreted in terms of water discharge as the rate-determining step over the complete pHrangeinvestigated. In the potential range higher than that of the Tafel region the rate control is shifted to mass transport of ethylene. At P E = 1 atm, the Tafel potential range is limited by oxide formation before diffusion control sets in. A possible explanation of the anomalous pH effect in reactions concerning water discharge has been given.

Adsorption of organic compounds at the metal/solution interface: Thiourea on gold electrodes☆

Abstract The adsorption of thiourea on gold has been investigated as a function of potential and concentration, using a radiotracer method. The behaviour of the system is interpreted in terms of a simple molecular model. The distribution of potential in the double layer and the dominant energetic factors governing adsorption, viz., chemical binding energy, field-dipole interaction, distortion of the double layer and lateral interaction have been evaluated and are discussed.

The mechanism of the electrochemical oxidation of oxalic acid

Abstract Oxalic acid oxidation on platinized platinum has been studied at 80°C in solutions of H 2 SO 4 , Na 2 SO 4 and NaOH. Reaction rates were measured as a function of potential, pH and concentration of oxalic acid. Coulombic efficiency was determined by measurements of CO 2 production. Empirically i = kc H 2 C 2 O 4 0·35 c H+ −0·55 exp ( FV / RT ). Coulombic efficiency of oxidation to CO 2 is ca 100%. No oxidation occurs in alkaline solutions. The results are interpreted in terms of a mechanism having the following characteristics. Undissociated acid is the reacting species. Oxidation occurs under Temkin conditions. Both electrons are supplied by the organic species. The second charge transfer is the rate-determining step.

Flow-through electrodes: II. The I3/−/I− redox couple

Summary o (i) Voltammetric characteristics of the I 3 / − /I − flow-through system on porous graphite have been obtained in neutral and acid electrolytes. (ii) The mechanism of the I 3 / − /I − redox reaction on graphite has been determined. It corresponds to that found by Vetter on platinum. (iii) The standard apparent exchange current density of the I 3 / − +2ē→3I − reaction is of the order of 2 Acm −2 (geom. area) both in neutral and acid electrolytes. (iv) Good agreement was found between experimental data and the numerically computed curves based on parameters obtained using diagnostic criteria previously derived. This confirms the validity of the proposed methods of mechanism determinations using flow-through electrodes.

Anodic oxidation of ethylene on noble metals and alloys. Parametric and isotopic examination of mechanisms

The anodic oxidation of ethylene has been studied on smooth noble metals and certain binary alloys, in aqueous solutions at 80°C, in the steady state. Reaction orders are derived from a parametric analysis and several types of mechanism are suggested and shown to occur. Studies in isotopic substitution serve to confirm these mechanisms. An experimentally derived “volcano” relationship between the heat of sublimation of the electrocatalytic substrate, and the rate of ethylene oxidation on it, is interpreted in terms of variable rates and variable rate-determining steps.

Experimental and theoretical examination of methods of obtaining double-layer parameters

Abstract Capacity and electrocapillary measurements were carried out in identical HCl solutions (0.01–3 N). By integrations and differentiations, both sets of data were used to obtain values of the interfacial tension, charge density, ionic surface excesses and capacity. Surface excesses from capacity data were evaluated by thermodynamic and non-thermodynamic methods. There is agreement between the data derived from the two sets of experimental results, except for two instances: the surface excesses derived from capacity data by thermodynamic methods deviate at low concentrations, those derived by non-thermodynamic methods deviate at high concentrations, from the surface excesses derived from interfacial tension data. This latter deviation is attributed to a failure of diffuse-layer theory to apply under the present conditions, while the former arises from more fundamental differences between the measuring systems. A mechanism is proposed which accounts for the differences in results obtained at low concentrations. It is shown that these differences are not present when the evaluation is carried out by a new method based on the use of the charge density as electrical variable.

Flow-through electrodes: I. Diagnostic mechanistic criteria

Summary Diagnostic criteria allowing the determination of the electrode-kinetic equation for electrode reactions using flow-through systems have been derived. They are listed in Table 1 together with the conditions under which the measurements should be carried out.