M.W. Breiter

Voltammetric study of halide ion adsorption on platinum in perchloric acid solutions

Abstract Periodic current/potential curves were taken potentiostatically at 30 mV/sec on smooth platinum in 1 N HClO 4 with different additions of HCl, HBr, and HI, in the potential range between hydrogen evolution and deposition of oxygen or of the respective halide ion. The effect of increasing halide ion concentration, starting at 10 −6 M, on hydrogen adsorption and oxygen adsorption was studied systematically. Impedance measurements were made at 1000 c/s under the same conditions as the measurements of the current/potential curves. It is concluded from the dependence of the double layer capacitance upon the bulk concentration of halide ions in the double layer region that nearly a monolayer of adsorbed ions is present there for 0 c Cl − ≈ 10 −2 M, 0 c Br ≈ 10 −4 M, and 0 c HI ≈ 10 −5 M. The measurements at larger concentrations, especially for I − , show additional effects which are due to the deposition of halides and to the reverse reaction.

On the nature of reduced carbon dioxide

Carbon dioxide was allowed to interact at room temperature with different amounts of hydrogen chemisorbed on platinized platinum in sulphuric acid solutions. The reaction product, (CO2)r, was oxidized at constant current and constant rate of helium stirring. Analysis of the gas mixture by gas chromatography demonstrated that two electrons are required for the production of one CO2 molecule on the average. Similar experiments were carried out with carbon monoxide. The charging curves of (CO2)r and COads practically coincide in the potential region of the arrest of the charging curves and above, when measured with the same charging rate. It is suggested that (CO2)r is identical with carbon monoxide chemisorbed in the first layer on platinum.

Anodic oxidation of formic acid on platinum—I. Adsorption of formic acid, oxygen, and hydrogen in perchloric acid solutions

The coverage of bright platinum electrodes with formic acid molecules, oxygen atoms, and hydrogen atoms was determined as a function of potential and bulk concentration of formic acid in 1 N HClO4 during anodic and cathodic low-speed potential sweeps in the potential range between hydrogen and oxygen evolution. Anodic and cathodic charging curves were used. The techniques are discussed. The coverage with formic acid increases continuously below +0·4 V during the cathodic sweep and below +0·4 V during the anodic sweep. It decreases with increasing potential above 0·5 V and finally becomes negligibly small during the anodic sweep. The formation of the oxygen layer is slightly hindered in the presence of formic acid and starts at more anodic potentials than in 1 N HClO4. Hydrogen adsorption is decreased by about 75 per cent if a monolayer of formic acid molecules is present.

Comparative voltammetric study of methanol oxidation and adsorption on noble metal electrodes in perchloric acid solutions

Abstract Periodic current/potential curves were measured potentiostatically on smooth electrodes of platinum, iridium, rhodium, and gold at 30 mV/s in perchloric acid solutions with different additions of methanol (10−3 M to 1 M). Information on methanol adsorption was obtained from impedance measurements at 1000 c/s by volcammetry with superimposed a.c. voltage. The current/potential curves look similar on Pt and Pd on the one hand, and on Ir and Rh on the other hand. There is no noticeable methanol oxidation on gold. The peak current of the first wave during the anodic sweep decreases considerably in the order Pt > Pd > Rh > Ir. The capacitive component of the impedance shows methanol adsorption in the hydrogen and double layer region. The shape of the current/potential curves and the methanol adsorption are strongly influenced on platinum metals by oxygen layers which are formed or reduced electrochemically during the sweeps in different potential ranges.

Anodic oxidation of formic acid on platinum—II. Interpretation of potentiostatic current/potential curves. Reaction mechanism in perchloric acid solutions☆

Abstract The characteristic shape of periodic potentiostatic current/potential curves in 1 N HClO4 with different amounts of HCOOH is discussed on the basis of the previous adsorption measurements and of theoretical considerations. The experimental results can be satisfactorily interpreted by a generalized dehydration mechanism under consideration of the specific heterogeneity of the platinum surface. A concept is proposed which explains the appearance of three oxidation waves during the anodic sweep at bulk concentrations larger than 10−3 M and the appearance of only two waves at concentrations smaller than 10−3 M.

Dissolution and passivation of vertical porous zinc electrodes in alkaline solution

Abstract The anodic dissolution and passivation of vertical porous zinc electrodes that were prepared by pressing zinc powder or amalgamated zinc powder into small disks were studied in 6 M KOH + 0.25 M ZnO by measuring potentiostatic current/potential curves at a rate of 1 mV/s and by taking photomicrographs of the electrodes at the beginning of the runs and after repeated cycling. Comparison of the current/potential curves of porous and smooth electrodes demonstrates that only a small fraction of the interior of the different porous electrodes participates in the electrochemical processes. The lower part (about 10%) of vertical electrodes of smooth zinc or pressed zinc powder are much less reactive than the rest of the electrodes. Repeated cycling leads to a finer porosity of electrodes made of zinc powder. Amalgamation (5%–10%) appears to reduce the latter two effects.

Voltammetric study of the reduction of molecular oxygen on bright platinum in perchloric acid solution

Abstract The cathodic reduction of molecular oxygen was studied on bright platinum under voltammetric conditions in 1 N HClO4. Periodic current-potential curves were measured at 30 mV/s, and the ohmic and capacitive components of the impedance platinum/solution were determined under the same condition at 1000 c/s in the potential range between hydrogen and oxygen evolution. It follows from the impedance measurements in the presence and absence of O2 reduction (oxygen or helium saturation of the electrolyte) that the oxygen coverage or the hydrogen coverage at a given potential are the same in the presence and absence of O2 reduction. This eliminates reduction mechanisms in which the formation of adsorbed oxygen atoms as intermediates or the participation of adsorbed hydrogen atoms are postulated. A mechanism compatible with the experimental results is discussed. Oxygen coverage inhibits O2 reduction. Adsorption of chloride ions has a similar effect.

Voltammetric study of the inhibition of methanol oxidation on platinum by chloride ion in acid solution

Abstract A study of the inhibition of methanol oxidation on smooth platinum in perchloric acid solutions by chloride ion was carried out by means of periodic potentiostatic current/potential curves and impedance measurements at 1000 c/s under voltammetric conditions. The concentration of chloride was gradually increased from 10 −6 M to 10 −2 M. The rate of methanol oxidation at the same potential decreases with increasing chloride-ion concentration and is already rather small at 10 −2 M. The impedance measurements demonstrate that the inhibition is due to a blocking of surface sites by chloride-ion adsorption in the respective potential ranges. Chloride-ion adsorption is predominant over the adsorption of methanol molecules.

Galvanostatic studies of passivity and breakdown of passivity of titanium in hydrochloric acid solutions

Abstract The formation of oxide layers was followed at 10 mA on titanium rods in hydrochloric acid solutions by measuring the potential and the electrode impedance at 10,000 Hz as a function of time. The anodic current is largely used for oxide formation until a maximum is passed in the potential/time curves. The thickness of the oxide film does not increase significantly with time after this maximum and is about the same at HCl concentrations between 0.1 M and 10 M. The oxide film begins to break down preferentially at wedges and grooves in the potential range between 11.5 to 14.5 V. The dissolution of titanium under complex formation occurs at these areas. The decrease of electrode potential with increasing current above a certain value results from an increase of the number of dissolution areas.

Voltammetric study of the anodic activation of platinum

Abstract The gradual transition of platinum electrodes from a state of small reactivity to one of large reactivity by the repetitive formation and reduction of the oxygen layer has been investigated by cyclic voltammetry. Changes of the surface state were followed by simultaneous recording of the current/potential curves and of the ohmic and capacitive component of the impedance of the platinum/solution interface at 10000 c/s. The results demonstrate that the activation is mainly due to the oxidation of a layer of organic impurities sitting originally on the surface. The blocking of the surface is slowly removed.