The hydrogen-electrode reaction in the isotopically mixed system—I. The relation between forward and backward unidirectional rates and evaluation of the stoichiometric number of the rate-determining step

A general consideration of the possible reaction routes of the hydrogen-electrode reaction leads to the classification of all the conceivable mechanisms into two groups according to the value of the stoichiometric number, ν, of the rate-determining step.

The previous treatment on the validity, in the isotopically mixed system, of the relation between the forward unidirectional rate of the over-all reaction, V_f, its backward rate, V_b, the overpotential, η, and ν; V_f/V_b = exp (− 2Fη/νRT), is re-investigated rigorously on the basis of general considerations. It is shown, in agreement with the previous conclusion, that this relation holds invariably irrespective of the isotope content and of the magnitude of the isotope effect, provided that V_f and V_b are replaced by the respective sums of contributions of both isotopes, and η by the potential of the test electrode referred to its rest potential in the same isotopically mixed system.

Based on the relation thus verified, a method is advanced that enables us to determine experimentally (a) ν by combining the rate of isotopic exchange with the polarization resistance of the electrode used as the catalyst, without neglecting the isotope effect, and (b) the magnitude of the isotope effect from similar data. For the hydrogen-electrode reaction on nickel in an alkaline solution, ν has been determined to be 1·0 and the deuterium separation factor in the hydrogen ionization reaction to be 1·5.     

The effect of additives on the practical electrolytic separation of hydrogen and deuterium. II. Separation factor at Hg in aqueous and non-aqueous media

Studies of the effects of additives on the electrolytic H/D separation factor,S _H/D, previously reported in Part I [2] for Pt and Fe electrodes, are extended to the case of Hg for which the mechanism of cathodic H₂ evolution is well established. The behaviour of urea and guanidine was investigated. In the case of urea, the separation factor at Hg is enhanced but decreases with increasing current density or cathodic potential. The dependence ofS _H/D on current density can be correlated with the diminishing surface excess of urea with increasing negative potential. Experiments with proton sources other than H₂O in liquid water were investigated by making measurements ofS _H/D from solutions of H₂O/HOD, CH₃COOH/CH₃COOD and CF₃COOH/CF₃COOD in CH₃CN with NaClO₄ as electrolyte. An interesting inverse H/D isotope effect arises with trifluoroacetic acid (TFA) as proton source; the effect is interpreted in terms of competitive reactions of H and D with TFA rather than unusual bonding and vibrational characteristics of the TFA activated complex. The results suggest that useful isotopic separations can be made by use of selective competitive reactions. A kinetic analysis of the inverse isotope effect is given.     

Studies on the mechanism of the hydrogen electrode reaction by means of deuterium tracer—II. The reaction mechanism

The exchange reaction between deuterium gas and light water was conducted with Ni, Rh, and Pt catalyst. Through analysis of isotopic composition of the gaseous hydrogen during the exchange reaction, the exchange rates of the individual steps, H₂ ? 2H(a) and H(a) + B ? H⁺B + e, of the hydrogen electrode reaction were determined at various hydrogen pressures and solution pH's, where H(a) is a hydrogen adatom and B = H₂O or OH^?. The rates of the steps were widely different among the metals studied but, throughout these metals, the hydrogen pressure dependence of the rate of the first step was with the power of 1·0–0·9, and that of the second step, 0·2–0·5. Both the rates were independent of solution pH. Generally, the former step is virtually rate-determining under low hydrogen pressures, whereas the latter takes over the role with increasing hydrogen pressure. Under ordinary pressures, the first step is virtually rate-determining on Pt but neither step is singly rate-determining on Rh and Ni.     

Change of mechanism of the hydrogen electrode reaction with overpotential—III. Potential dependence of the deuterium separation factor on Pt,Au and Ni cathodes

A relationship between the overall electrolytic deuterium separation factor of the hydrogen electrode reaction and the separation factors of the steps that constitute the reaction is formulated for the discharge-combination route, without assuming an overwhelming unique rate-determining step. This is utilized for the quantitative interpretation of sharp variation of the cathodic deuterium separation factor with overpotential observed on Pt and Au in aqueous H₂SO₄ and an insignificant variation on Ni in alkaline solutions. The variation on Pt and Au is interpreted in terms of change with cathodic overpotential of the mechanism from the rapid discharge—slow combination to the slow-discharge. The variation on Ni is expected to be significant only in the anodic region.     

Elementary reaction rates of the hydrogen electrode reaction on nickel analyzed by means of deuterium tracer—II: The cathodic region

The rates of the constituent steps of the hydrogen electrode reaction on nickel were determined under cathodic polarization by means of deuterium tracer. The discharge—recombination reaction route was substantiated. Logarithm of the rate of the process 2H(a) → H₂ was dependent upon the overpotential component shared by the very step with the slope of 30 mV, or the rate was proportional to the square of activity of H(a), whereas that the process H₂ → 2H(a) was independent of it. Logarithm of the rate of formation of H(a) from the solution phase was dependent upon the total overpotential with the slope of ca 100 mV. Dependences of the mechanism and the step rates upon hydrogen pressure were discussed.     

Studies on the mechanism of the hydrogen electrode reaction by means of deuterium tracer—I. the reaction route

The reaction route of the hydrogen electrode reaction on Ni, Rh, Au, and Pt at the reversible potential was investigated through the exchange reaction between deuterium and light water. Various reaction routes involving hydrogen adatom H(a) and adsorbed hydrogen molecule-ion H⁺₂(a) as the reaction intermediates were considered. In order to single out the correct reaction route, these routes were examined, without assuming a priori the existence of a single rate-determining step, for satisfactory interpretation of the isotopic composition of the hydrogen gas during the exchange reaction. It is concluded that the reaction route consists of two consecutive steps,

and

, where B represents H₂O or OH^?.     

Studies on the mechanism of the hydrogen electrode reaction by means of deuterium tracer—III. The stoichiometric number

The stoichiometric number of the hydrogen electrode reaction on Rh, Ni, Pt, Au, Ag, and Ir was determined by means of deuterium tracer at the reversible potential. It is shown that, along with the change of the rate-determining step in the reaction route, H₂ ? 2H(a) followed by H(a) + B ? H⁺B + e, where B = H₂O or OH^?, the stoichiometric number v? experimentally observed should change by the relation, v? = (m + 2)/(m + 1), where m = v_d/2v_c and v_c and v_d are the exchange rates of these steps. The experimental v?vs m relation observed on these metals qualitatively satisfied this relation. Disagreement which existed were quantitatively explained by taking the isotope effect into account. The isotope effect of the individual steps, which were needed to interpret the disagreements, were found to be of reasonable magnitude.     

Change of mechanism of the hydrogen-electrode reaction with overpotential—II. hydrogen pressure equivalent to hydrogen overpotential and its relevance to the hydrogen embrittlement of metals

Electrochemical aspects of the hydrogen embrittlement are discussed in terms of a possible mechanism change of the hydrogen-electrode reaction occurring at the surface of the metal. Hydrogen pressure equivalent to hydrogen overpotential is formulated for the discharge-combination and the discharge-ion + atom-desorption routes in general forms, including the cases with comparable magnitudes of exchange cds of the constituent steps. The hydrogen pressure data observed by DeLuccia et al on iron in acidic solution are analysed. It is demonstrated that the data are well explained on the basis of the model that the hydrogen-electrode reaction on iron proceeds through the discharge-combination route with the exchange cd of the discharge step about one order of magnitude lower than that of the combination step.     

The behaviour of high polar organic solvents at platinum electrode — II. Adsorption and electrode reactions of acetonitrile

The adsorption of acetonitrile and reduction of adsorbed molecules at platinized platinum electrode were studied by potentiodynamic and radiometric techniques. During acetonitrile adsorption the reversibly and irreversibly adsorbed species are formed. One electron reduction process occurs in the potential range 0.45-0.15 V. Below 0.15 V the deeper reduction of adsorbed acetonitrile takes place — the final products are ammonia and ethane. The differences of smooth and platinized platinum electrodes properties are briefly discussed.     

Study of the impedance of a platinum electrode in the system Br2/Br (HClO4, aq.)—II. Mechanism of the electrode reaction

The Randles circuit well represents impedance measurements carried out with activated Pt electrodes. This enables us to study the variation of j_o for redox reactions with concentration of the reactants, at constant potential, and also the variation of j_o with potential, keeping constant the concentration of one of the reactants. The results thus obtained indicate that the step Br₂ + e Br₂⁻ is rate-determining; it is followed or preceded by the rapid equilibria Br₂⁻ Br⁻ + Br 2Br Br₂. The mechanisms proposed hitherto for the electrochemical behaviour of the halogen/halide systems at inert electrodes are discussed, and it is reasoned that the ‘reversibility’ of these systems increases in the order Cl₂/Cl⁻ < Br₂/Br⁻ < I₂/I.     

The evaluation of the polarization resistance in a tubular electrode and its application to the hydrogen electrode reaction

An alternative method for the determination of the kinetic parameters involved in the elementary steps of the reaction mechanism of the hydrogen electrode reaction is proposed. It is based on the determination of the variation of the polarization resistance in a tubular platinum electrode with a laminar flow of electrolyte as a function of the activity of protons of the electrolyte solution. A theoretical expression that relates the experimental variables and the equilibrium polarization resistance is developed, which takes into account the current distribution along the electrode surface. The results are compared with others obtained previously, contributing to the verification of the kinetic mechanism through a completely different experimental procedure.     

The electrochemical reaction of sulphur-oxygen compounds—part II. Voltammetric investigation performed on platinum

The behaviour of the anions sulphide, thiosulphate, sulphite, dithionite and sulphate is examined on platinum using a triangular sweep technique. All sulphur anions, except for the sulphate anions, adsorb strongly to the platinum surface and prevent adsorbed hydrogen from being oxidized. The electrochemical behaviour of the anions on a platinum electrode may provide a method for analysing these compounds qualitatively and quantitatively.     

Electrochemical behaviour of the antimony electrode in sulphuric acid solutions—II. Formation and properties of the primary anodic layer

The formation of an anodic layer during the oxidation of antimony in sulphuric acid solutions as well as the structure and properties of the layer have been studied using linear sweep voltammetry, potentiostatic oxidation, ac impedance measurements, X-ray diffraction analysis and scanning electron microscopy. The electrochemical investigations as well as the shape of the impedance spectra during electrode passivation indicate that the surface layer probably has a complicated structure. Under the conditions of these experiments the passivating film is mainly amorphous, gel-like, exhibiting good ionic conductivity and a relatively high dissolution rate. Assuming that the anodic layer consists mainly of antimony hydroxide a reaction scheme for the growth and dissolution of this film is presented.     

The electrochemical reduction of O-nitrotoluene to O-tolidine—II. Voltammetry and reaction mechanism

O-tolidine obtained by reduction of o-nitrotoluene is used in the production of dyestuffs. Various steps in this reduction have been investigated by means of controlled-potential electrolysis and measuring voltammograms at rotating disc electrodes. Results of the first types of experiments are already published and those of the latter type are presented in the recent paper. Voltammograms are measured for the reduction of nitrotoluene, azoxytoluene and azotoluene with various electrode materials in alkaline ethanol—water solution and in alkaline aqueous solutions containing McKee salt and for the reduction of azoxytoluene and azotoluene in acidic ethanol—water solutions as well as in acidic aqueous solution containing McKee salt and acid. The reduction wave of nitrotoluene does not correspond to a single product reaction, whereas those of azoxytoluene and azotoluene correspond to a single-product process, respectively. It has been found likely that dimerization of the intermediate nitrosotoluene radical anion occurs on formation of azoxytoluene. This conclusion strongly deviates from the Haber scheme mostly proposed for the reduction mechanism of aromatic nitro-compounds and supports that of Fry.     

Effect of CsF-concentration on electrolytic conductivity, viscosity and anodic reaction of nickel electrode in (CH3)3N-CsF–HF system at room temperature

In order to increase the formation ratio of perfluorotrimethylamine, (CF₃)₃N, to overall anode gas in electrolytic production using Ni anode, mixed melts of (CH₃)₃N·mHF + CsF·2.3HF were used as electrolytes at room temperature. The ionic conductivity of the mixed melts decreased with an increase in the CsF concentration, whereas the viscosity of the mixed melts increased with increasing the CsF concentration. AC impedance and XRD analysis revealed that the presence of CsNi₂F₆ in the oxidized layer formed on the Ni anode after electrolysis. The gas evolved at the Ni anode was composed of (CF₃)₃N, (CF₃)₂CHF₂N, CF₃(CHF₂)₂N, (CHF₂)₃N, CF₄, NF₃, CHF₃, C₂HF₅, and C₂F₆. The best ratio of (CF₃)₃N to the overall anode gas (52.11%) was obtained in the electrolyte of (CH₃)₃N·5.0HF + 50 wt% CsF·2.3HF mixed melt at 20 mA cm⁻².     

A system for accurate and fast measurement of the gas phase H/D ratio in electrolytic H/D separation factor experiments

Accurate and fast measurements of electrolytic hydrogen-deuterium separation factors are required to study short-term time effects and the effects of other variables, for example overpotential, before the cathode has suffered irreversible changes such as hydrogen embrittlement. A system for fast measurement of the hydrogen-deuterium ratio of the liberated hydrogen isotopes in argon as the sweep gas has been developed and is described in detail. Unique features include a modified Cartesian manostat and an automatic Toepler pump in the gas collection system, and an automatic mass spectrometer inlet system whose cycle time is about 4 min. Calibration procedures are outlined. The accuracy and reproducibility in the H/D ratio are ±1% and ±0.5% respectively, for the range 0.75 to 5.0 mol. % deuterium in mixtures containing 0 to 80 vol.% argon.     

Metal electrodes bonded on solid polymer electrolyte membranes (SPE)—II. The polarization resistance of Pt-Nafion electrode

The hydrogen and oxygen electrode reactions were examined on a Pt-SPE (Nafion) electrode with the condition that the platinum side was in contact with gaseous phase. The polarization curve shows an Ohmic relation determined mainly by the solution and membrane resistances. The removal of the solution resistance improves the above polarization characteristics to a large extent.     

A wall-jet electrode reactor and its application to the study of electrode reaction mechanisms Part III: Study of the mechanism of the a.c. electrolytic graining of aluminium in hydrochloric acid

The mechanism of the a.c. electrolytic graining of aluminium in hydrochloric acid is determined from the analysis of the potentiostatic transient behaviour of the system aluminium–electrolyte under anodic and cathodic polarization and comparison of experimentally determined transients with calculated values derived from a candidate mechanistic scheme. It has been established, that the oxidation of aluminium in the development of a distinct surface morphology occurs according to the Al3+ ions being dissolved from the surface and removed to the bulk of the solution, hence forming pits. Al(Cl)3 is a solid intermediate. The morphology developed, is determined by the excess of Cl– ions created at the electrode surface, with respect to the bulk concentration. The accumulation of Cl– ions is governed by the ratio between the rate constant for the formation of Al(Cl)3, set by the flux of charges forced across the electrode–solution interface per unit surface area taking part in the active dissolution of aluminium and the mass transport rate of the Cl– ions. The reduction of H+ ions in the cathodic half period of the applied alternating current is mass transport controlled. The concomitant rise in interfacial pH causes Al3+ ions formed in the preceding anodic half period, which are not yet removed from the electrode–solution interface, to precipitate as aluminium.     

Recoveries of deuterium from the separation of water–isotope mixture in batch-type thermal diffusion columns under unsteady-state operation

Heavy water (D₂O) is the most feasible moderator and coolant in nuclear-fission reactors while deuterium (D) will be the nuclear fuel for thermonuclear fusion in the distant future. The recoveries of deuterium in D₂O and HDO from the separation of water–isotope mixture (H₂O–HDO–D₂O) by thermal diffusion has been estimated. First, the degrees of separation for each component of water–isotope mixture in thermal diffusion columns were predicted, then the recoveries of deuterium in each component were estimated from the molecule weights. It was found that the maximum recovery of deuterium might be obtained when the feed concentration of D₂O is 0.22 mass fraction.