Electrocatalysis by ad-atoms: Part III. Enhancement of the oxidation of carbon monoxide on platinum by ruthenium ad-atoms

The known vibrating interphase method when a.c. is measured fails while investigating very dilute solutions because of the ohmic drop of the potential in the solution. The proposed version of the method is free from this disadvantage. When a mercury electrode is vibrated an alternating voltage U is set up which depends upon the electrode potential E. The alternating voltage-potential curves U-E have a sharp minimum corresponding to the potential of uncharged surface (p.u.s.). Using the method U-E curves in HCl up to 1×10^?5M and in KCl in 5×10^?3M solutions were taken. Obtained p.u.s. data were extrapolated to the zero electrolyte concentration and the null point (n.p.) of mercury in water was determined. Calculation of the U-E curves in 0.01 M HCl at different vibration amplitudes was made. Calculated and measured data are in a good agreement, thus substantiating the principle of the method.     

Electrocatalysis by ad-atoms: Part XVII. Electronegativity effect on oxygen donation by oxygen-adsorbing ad-atoms and steric hindrance by alkyl groups in Shole control in aldehyde oxidation

The oxidation of propionaldehyde was examined on electrodes modified by Ru, Ag, Hg, Ge, Sn, As, Sb, Bi, S, Se and Te ad-atoms. The feature of the enhancement of propionaldehyde oxidation by ad-atoms was found to be quite similar to that of formaldehyde and acetaldehyde oxidation. The oxidation was found to be enhanced strikingly by oxygen donation by oxygen-adsorbing ad-atoms as well as by the inhibition of poison formation by S_hole control by non-oxygen-adsorbing ad-atoms, but to a lesser extent. In the enhancement by S_hole control, steric hindrance by alkyl groups is observed. In the enhancement by oxygen-adsorbing ad-atoms, the difficulty in oxygen donation from ad-atoms to aldehyde molecules depends on the size of the alkyl group: the smaller the electronegativity of the oxygen-adsorbing ad-atoms, the more difficult is this donation. The donation is discouraged by the orientation of aldehyde molecules caused by the induced negative charge of adsorbed oxygen atoms.     

Multi-electron oxidation of formaldehyde on a platinum electrode

The time evolution of a self-sustained potential oscillation under constant current oxidation of formaldehyde was observed on the rotating ring disk electrode assemble. Under the condition when the potential was controlled, the laser deflection voltammogram as well as the conventional cyclic voltammogram were measured. The mechanism of the multi-electron oxidationof formaldhyde on a platinum electode is discussed.     

Electrocatalysis by ad-atoms: Part VIII. Ag,Pb, Te and Tl ad-atoms for ethylene reduction

As already reported, ethylene reduction at a Pt electrode can be enhanced by ad-atoms of Cu and Hg. Another series of ad-atom species is reported, that is, Ag, Pb, Tl and Te, in which the enhancement order depends on the number of Pt sites occupied by an ad-atom. S_M. This effect originates from the difference between S_M and the number of Pt sites occupied by an ethylene molecule, which causes an increase in the number of Pt sites available for hydrogen adsorption. This increase results in the enhancement, in the order Ag>PbTl; Te which has the same S as ethylene has no enhancement effect.For the complete understanding of the function of the mixed surface in the electrocatalysis, a new concept, “catalytic domains”, parts of the mixed surface effective for the electrocatalysis, is introduced, on the assumption that the reaction rate is much higher than the rate of surface migration of the adsorbed species. Then the number of Pt sites which belong to the catalytic domain is calculated. Another concept, “reaction unit mesh”, is also introduced.     

Electrocatalysis by ad-atoms: Part IV. Enhancement of the oxidation of formic acid on Pt-Au(subs) and Au-Pt(subs) electrodes by bismuth ad-atoms

The roles of surface and bulk for electrocatalysis have been investigated. Bi ad-atoms enhance a platinum electrode to a great extent for HCOOH oxidation. In order to examine whether bulk platinum atoms are necessary for the electrocatalysis, monolayer or submonolayer amounts of platinum atoms were made to deposit on a gold electrode which is quite inactive for the oxidation. The deposition of a complete monolayer of platinum atoms on a gold electrode makes the electrode as active as a platinum electrode itself. Bi ad-atoms enhance this electrode to the same extent as they do a platinum electrode for the oxidation. Thus surface atoms, Pt and Bi atoms, having no bulk atoms on a quite inactive electrode work effectively for the electrocatalysis, the platinum atoms for the adsorption of the main reactant and the Bi atoms for blocking of the sites against the formation of poison. It is the adsorptive property of the surface that controls the electrocatalytic activity.     

The electrochemical behavior of ad-atoms and their effect on hydrogen evolution: Part II. Arsenic ad-atoms on platinum

The state of an arsenic layer electrodeposited at various potentials on platinum electrodes has been examined electrochemically. The relation between the state and the electrocatalytic activity for hydrogen evolution has been investigated. The number of vacant sites have been found to increase with lowering of the deposition potential. The activity has been found to depend not on the amount of deposited arsenic but on the number of vacant sites. The difference in the effects of arsenic and of copper on hydrogen evolution is pointed out. This is attributed to the difference in the affinity of the ad-atoms for hydrogen.     

The electrochemical behavior of ad-atoms and their effect on hydrogen evolution: Part IV. Tin and lead ad-atoms on platinum

Hydrogen evolution on a platinum electrode decays against X_pt with the deposition of Sn ad-atoms and Pb ad-atoms in the same way as it decays with that of Ge ad-atoms, in which all of these ad-atoms occupy two platinum sites. In general the decay depends on the number of sites occupied by an atom of the ad-atom species.The potential ranges for oxygen adsorption by Sn ad-atoms and Pb ad-atoms are 0.45 to 1.24 V and 0.48 to 0.77 V, respectively, but the oxygen adsorbed by the latter ad-atoms is very small in amount.     

A further investigation of the catalysis by lead ad-atoms of formic acid oxidation at a platinum anode

Potential step experiments permit a more detailed investigation of the oxidation of formic acid in aqueous perchloric acid and at a platinum electrode covered by submonolayer amounts of lead. It is confirmed that lead atoms are very effective catalysts allowing high rates of oxidation over long periods of time. Indeed, at low concentrations of formic acid (<1 mM) the rate of oxidation can be determined by diffusion of formic acid in solution. At higher concentrations (10–500 mM); the current is kinetically controlled at short times, but later becomes diffusion controlled; it is suggested that the rate-determining step at short times is dissociative adsorption of formic acid at two platinum atoms adjacent to a lead ad-atom. The mechanism of catalysis is discussed.     

Oxidation of methanol on a platinum electrode in alkaline medium: Effect of metal ad-atoms on the electrocatalytic activity

A mechanistic investigation of the electrochemical oxidation of methanol was worked out in alkaline medium in order to interpret the effect of some foreign metal ad-atoms (Pb. Bi, Cd, Tl) on the electrocatalytic activity of platinum.The rate-determining step being a surface reaction between adsorbed hydroxyls and a methanol chemisorption intermediate, the positive effect met with Pb and Bi may be explained through the bifunctional theory of electrocatalysis, whereas the negative effect met with Cd and Tl may come from the inhibiting behaviour of these latter ad-atoms.     

Determination of formaldehyde by staircase voltammetry based on its electrocatalytic oxidation at a nickel electrode

In this paper, a novel method for detection of formaldehyde (HCHO), based on its electrocatalytic oxidation of HCHO at a nickel electrode, is reported. The mechanism of electrocatalytic oxidation and quantification of HCHO have been investigated by cyclic and staircase voltammetry, respectively. The electrocatalytic oxidation peak potential of HCHO is at about 475 mV vs. Ag/AgCl electrode; the peak current responds proportionally to concentrations of HCHO in alkaline solution. The linear range of detection is from 46.8 to 1640 μg/L (1.56 × 10⁻⁶ to 5.46 × 10⁻⁵ M) with a correlation coefficient of 0.996 and a detection limit of 23.4 μg/L (7.80 × 10⁻⁷ M). The relative standard deviation (RSD) is less than 6% (n = 5), and the recovery is in the range 98–106% for real samples. The result is consistent with that from the spectrophotometry. The text was submitted by the authors in English.     

Analysis of time-dependent kinetics for the oxidation of methanol on a platinum electrode and reaction mechanism

The reaction kinetics for the oxidation of methanol on a platinum electrode have been examined under precisely controlled conditions. The Tafel relations at constant surface coverages of the strongly adsorbed species show the existence of two potential regions where the predominant reaction path is different. The surface reaction of the strongly adsorbed species with OH(a) is rate determining at E > ca. 0.55 V, while the oxidative adsorption of methanol to form a reactive intermediate becomes the rate-determining step at E < ca. 0.55 V. In the latter potential region, the strongly adsorbed species is not oxidized so that its accumulation on the surface decreases the rate of the oxidative adsorption and thereby the total oxidation rate.     

Formic acid oxidation at platinized platinum electrodes: Part II. Effect of temperature on steady state oxidation process

A study has been made of the effect of temperature on the steady state oxidation of formic acid on platinized platinum electrodes in 0.5 M HCOOH+0.5 M H₂SO₄. The activation energy and entropy of activation for the anodic oxidation process decrease sharply with increase in electrode potential. The results provide additional evidence that the weakly adsorbed intermediate pathway proceeds through a rate-determining step involving the dissociation of weakly adsorbed formic acid molecule.     

A selective electrolytic sensor for nitrite based on a modified platinum electrode

The electrolytic sensor described is based on the oxidation of nitrite at a platinum electrode modified with chemisorbed iodine and coated with a thin layer of quaternized poly(4-vinylpyridine), qPVP. The sealed sensor uses an anion-exchange membrane to separate Donnan transport of nitrite across the membrane and controlled potential electrolysis at the Pt/qPVP indicator electrode. The sensor has a linear response to nitrate concentration in aqueous samples over the range 4 × 10^?6?2 × 10^?3 M nitrite. The detection limit is 2 × 10^?6 M nitrite. The sensor is free of interference by nitrate, dissolved oxygen, cations, and many neutral species. Anions that are electroactive at 0.7 V vs. Ag/ AgCl would interfere, but they are uncommon in most samples. Initial tests with lake water samples suggest that this sensor is unaffected by this matrix. The system was also evaluated for monitoring nitrite levels in spiked meat extracts.     

Studies on analytical methods by amperometric titration using a rotating platinum electrode: Part 47. Successive Determination of L-Cysteine and L-Cystine by Amperometric Titration

The successive determination of {spl}-cysteine and {spl}-cystine (2–5 μmol of each) in mixtures is achieved by amperometric titration with standard potassium iodate solution using a rotating platinum wire electrode at + 0.6 V vs. SCE. {spl}-Cysteine is titrated in acidic bromide-containing solution; sodium chloride (15 g) is then added and the titration is continued to determine L-cystine.     

Heterogeneous selective oxidation of formaldehyde on oxide catalysts:In situ FTIR study of formaldehyde surface species on a V-Ti-O catalyst and oxygen effect

The interaction of formaldehyde with a highly selective V-Ti-O catalyst for the oxidation of formaldehyde to formic acid is studied by Fourier-transform infrared (FTIR) spectroscopy at 70–200‡C. In a flow of formaldehyde/oxygen mixture and in a mixture without oxygen at optimal temperatures for formic acid formation (100–140‡C), methoxy groups and other oxygenates are formed in small amounts. These are two bidentate formates and covalently bound monodentate formate. The fact that similar oxygenates are observed independently of the presence of oxygen in the reaction mixture suggests the participation of the catalyst oxygen in their formation. Oxygen accelerates the desorption of bidentate formates. Bidentate formates of one type decompose in a flow of air at 100–150‡C, and bidentate formates of the other type decompose at 170–200‡C.     

Kinetics and mechanism of glucose electrooxidation on different electrode-catalysts: Part I. Adsorption and oxidation on platinum

The processes of adsorption and electrooxidation of glucose on a smooth platinum electrode have been investigated in a wide range of pH values. It is found that glucose adsorption are platinum is accompanied by dehydrogenation of adsorbed molecules. The θ_R vs. E_r dependence represents a bell-shaped curve with unequal sides and with a maximum at E_r = 0.2 V at 0 < pH < 12 or at E_r = 0.4 when pH > 12. The kinetics of adsorption is described by the Roginsky-Zel'dovich equation, and the dependence of the steady-state coverage on the glucose bulk concentrations by the Temkin isotherm.It is shown that in the case of glucose adsorption on platinum Q^dehyd._H > Q_H, i.e. when glucose is brought into contact with a platinum electrode, the catalytic decomposition of glucose molecules occurs in addition to the formation of strongly chemisorbed particles. The transient current at E_r < 1.0 V is a current due to the ionization of hydrogen formed during adsorption with dehydrogenation of glucose and its catalytic decomposition. The glucose electrooxidation rate under steady-state conditions at E_r < 0.7 V is determined by the interaction of the chemisorbed carbon-containing particle with OH_ads. The slow step of glucose electrooxidation in the potential range 1.0 < E_r < 1.5 V is the interaction of glucose molecules from the solution bulk with the surface platinum oxide, the latter undergoing a quick electrochemical regeneration thereafter.The basic regularities and mechanism of glucose electrooxidation on platinum are shown to be analogous to those obtained earlier for such elementary organic fuels as formaldehyde and formic acid.     

Electrochemical reactions with protonations at equilibrium: Part X. The kinetics of the p-benzoquinone/hydroquinone couple on a platinum electrode

The kinetics of the p-benzoquinone/hydroquinone Q/QH₂ couple on a platinum electrode are analysed on the basis of the theory presented earlier (E. Laviron, J. Electroanal. Chem., 146 (1983) 15) for the nine-member square scheme when the protonations are assumed to be at equilibrium, using experimental data from the literature. The square scheme is of the NN type. The Tafel plots and the variations of the experimental apparent rate constants between pH 0 and 7 are in good agreement with the theoretical predictions. The heterogeneous rate constants found for the elemental electrochemical steps are as follow: Q Q^?, k_h3=1/6×10^?3 cm s^?1; QH^.QH^?, k_h5=0.11 cm s^?1; QH⁺QH^., k_h2?160 cm s^?1; k_h4 for the reaction QH₂^+.QH₂ is in the range 0.5–4 cm s^?1. Between pH 0 and 7, the reaction sequence during the reduction is, for the most part, successively H⁺e^?H⁺e^?, e^?H⁺H⁺e^?, and e^?H⁺e^?H⁺ (reverse sequence during the oxidation).