Supplement to the theory of normal pulse voltammetry and its application to the kinetic study of methanol oxidation on a polycrystalline platinum electrode

The theory of normal pulse voltammetry (NPV) for complex multistep multielectron transfer processes on a plane electrode was advanced and applied to the completely irreversible process of methanol oxidation to formic acid in the potential range from 0.3 to 0.8 V versus Ag/AgCl. The kinetic parameters for this process, such as the standard rate constant (k0) and anodic transfer coefficient (alpha) for this irreversible heterogeneous electron transfer process at the electrode/solution interface and apparent diffusion coefficient (D(app)) for the homogeneous charge transfer process within liquid film near the electrode surface, were obtained with NPV theory from analyzing the dependence of current-potential curves upon the sampling times. The results showed that this process is truly a very slow, completely irreversible kinetic process, as k0 is in the order of 10(-9) cm/s for the rate-determining step. The values of k0 and D(app) decreased with the increase of methanol concentration, while alpha was independent of the concentration of methanol and its value was 0.35 +/- 0.05. Theoretical fitting is very consistent with the experimental data.     

Electrochemical determination of activation energies for methanol oxidation on polycrystalline platinum in acidic and alkaline electrolytes

The oxidation pathways of methanol (MeOH) have been the subject of intense research due to its possible application as a liquid fuel in polyelectrolyte membrane (PEM) fuel cells. The design of improved catalysts for MeOH oxidation requires a deep understanding of these complex oxidation pathways. This paper will provide a discussion of the literature concerning the extensive research carried out in acidic and alkaline electrolytes. It will highlight techniques that have proven useful in the determination of product ratios, analysis of surface poisoning, anion adsorption, and oxide formation processes, in addition to the effects of temperature on the MeOH oxidation pathways at bulk polycrystalline platinum (Pt(poly)) electrodes. This discussion will provide a framework with which to begin the analysis of activation energy (E(a)) values. This kinetic parameter may prove useful in characterizing the rate-limiting step of the MeOH oxidation at an electrode surface. This paper will present a procedure for the determination of E(a) values for MeOH oxidation at a Pt(poly) electrode in acidic and alkaline media. Values from 24-76 kJ mol(-1) in acidic media and from 36-86 kJ mol(-1) in alkaline media were calculated and found to be a function of applied potential and direction of the potential sweep in a voltammetric experiment. Factors that influence the magnitude of the calculated E(a) include surface poisoning from MeOH oxidation intermediates, anion adsorption from the electrolyte, pH effects, and oxide formation processes. These factors are all potential, and temperature, dependent and must clearly be addressed when citing E(a) values in the literature. Comparison of E(a) values must be between systems of comparable electrochemical environment and at the same potential. E(a) values obtained on bulk Pt(poly), compared with other catalysts, may give insight into the superiority of other Pt-based catalysts for MeOH oxidation and lead to the development of new catalysts which lower the E(a) barrier at a given potential, thus driving MeOH oxidation to completion.     

Electrochemical and FTIR spectroscopic studies of tyrosine oxidation at polycrystalline platinum surfaces in alkaline solutions

The electrochemical oxidation of tyrosine has been studied on polycrystalline platinum in sodium hydroxide using cyclic voltammetry and in situ Fourier transform infrared spectroscopy (FTIRS). According to infrared data, the first interaction of tyrosine through the carboxylate group on platinum is proposed. The results showed an onset for tyrosine oxidation at ca. 0.5 V with the formation of soluble formate ion as product. The sharp increase in the current vs potential slope from ca. 1.1 V obeys the detection of cyanate ions together with p-(methyl)-phenoxide ions. No adsorbed carbon monoxide as well as carbon dioxide was detected by this methodology in the whole studied potential range. These results contrast with those obtained in acid solution where two parallel reactions, one yielding the γ-lactone of hydroxy benzofurane and the other producing p-benzoquinone occurs for the oxidation of tyrosine on platinum. Dedicated to Prof. Dr. Teresa Iwasita on the occasion of her 65th birthday in recognition of her numerous contributions to interfacial electrochemistry.     

Electrochemical properties and electro-aggregation of silver carbonate sol on polycrystalline platinum electrode and its electrocatalytic activity towards glyphosate oxidation

This communication shows that colloidal suspensions of silver carbonate behave in a way which is new in voltammetric experience. Electrochemical signals typical of nucleation processes were observed during the electro-aggregation of silver carbonate on a platinum electrode. Modified electrodes, obtained by means of electro-aggregation of silver carbonate particles, showed catalytic activity towards the electrochemical oxidation of glyphosate, avoiding in this way typical passivation processes.     

Synergistic effect in the electrocatalytic oxidation of methanol on platinum+palladium alloy electrodes

The electrocatalytic oxidation of methanol has been investigated on platinium+palladium alloy electrodes of different compositions in acid, neutral and alkaline aqueous solutions.The surface characteristics (composition and roughness factor) of the alloys and the stability of the electrodes in contact with different electrolytic solutions have been studied using cyclic voltammetry. In particular, a surface enrichment in platinum due to a preferential dissolution of palladium and an increase of the roughness factor with an increase of the palladium content has been shown.The electrocatalytic activity of different alloys for methanol oxidation has been characterized by exchange current densities obtained from extrapolation of Tafel lines of calculated equilibrium potential. The plot of these current densities vs. the surface composition leads to a synergistic effect, particularly important in alkaline medium. A reasonable explanation of this enhanced electroactivity at about 15 at.% in Pd is given on the basis of a decrease of electrode poisoning.     

Correction to: Catalytic oxidation of hydrogen on platinum

Journal of Thermal Analysis and Calorimetry - Due to a fault (oversight) of the authors, L’vov BV, Galwey AK, in Fig 1 of the article: ‘Catalytic oxidation of hydrogen on...     

Lateral modification and organization of mixed adlattices on polycrystalline platinum electrodes

Linear scan voltammetry of carbon monoxide on platinum was conducted with the electrode surface modified by coadsorbed organic and inorganic compounds (surface lateral modifiers). The current-potential peak of carbon monoxide electrooxidation moved in the positive direction in all cases studied. Two mechanistic components are proposed to account for our observation. The structural effects are related to the nucleation-growth mechanism of the oxidation which assigns the reaction zone to the perimeter of the growing islands of a surface oxidant. Electrooxidation of an ideally uniform and coherent CO adlattice should be unaffected by a modifier present outside of such islands. On the contrary, many small islands of CO, diluted by the two dimensional network of the surface modifiers, create a favorable situation for the lateral modification. The modifier's induced transitions between small and large islands are reflected in the shift of the CO oxidation peak potential.The second component relates to the redistribution of the modifier's electronic charge between the surface and the chemisorbed CO. A part of the charge is localized on the 5σ(CO)-5d(Pt) bond and increases the stability of surface attached CO. The electronic charge shifted towards the metal, can also screen the reacting CO/H₂O system against the growing electric field during the potential scan. Both effects are interpreted to increase the overpotential of the electrooxidation of CO and contribute to the gross effect observed experimentally.     

Optimisation of the plasma electrolytic oxidation process efficiency on aluminium

Plasma electrolytic oxidation (PEO) is a surface treatment technology enabling fabrication of adherent thick (50–150 µm) coatings on light metal alloys with significantly enhanced hardness, wear and corrosion resistance compared with other conventional treatments. The technology has the potential to play a significant role in the transport sector for replacement of steel with light‐weight materials of improved durability. A main limitation of PEO lies in its relatively high cost, associated with high energy consumption and low coating efficiency. The present work explores possible routes to improve the process efficiency. It is shown that a combination of conventional pre‐anodising with sequential PEO treatment reduces specific energy consumption up to five times because of an increase of the coating growth rate, up to 10 µm/min, compared with existing PEO processes. A further approach to improved coating efficiency involves PEO in electrolytes with suspended fine or nanoparticles, which results in the formation of thicker coatings in reduced time as a result of the incorporation of the particles from the electrolyte into the coating. Additionally, melting of the coating material during the micro‐arc discharge process leads to formation of stabilised high‐temperature phases, such as tetragonal and cubic zirconia, which provide significantly improved microhardness of the coating material and give a potential for thermal barrier applications. Copyright © 2009 John Wiley & Sons, Ltd.     

Insights into the enhanced tolerance to carbon monoxide on model tungsten trioxide-decorated polycrystalline platinum electrode

Polycrystalline platinum decorated by WO₃ nanoparticles (WO₃/Pt_pc) is used as a model electrode to gain insights into the enhanced tolerance to carbon monoxide (CO) observed on such composite materials. Bifunctional-type reactions involving WO₃ and Pt active sites are observed, such as hydrogen spill-over or the electrooxidation of CO molecules adsorbed on Pt sites neighboring the WO₃ nanoparticles. The resulting CO_ad-free Pt sites are active for the hydrogen oxidation reaction (HOR), thereby enhancing the HOR activity for WO₃/Pt_pc electrode relatively to bare Pt_pc in 300 ppm CO/H₂ saturated HClO₄ electrolyte. However, this bifunctional effect occurs exclusively for CO molecules weakly adsorbed on Pt, i.e. only for a small fraction of the CO_ad fully covering the Pt surface.     

Effect of traces of water vapor on the rate of ammonia decomposition on polycrystalline platinum

The influence of small (lower than 0.02) molar fractions of water on the rate of ammonia decomposition on platinum wires was studied for ammonia pressures varying between 6.7 and 93.3 kPa and temperatures between 800 and 1700 K. The presence of water strongly inhibited the rate of reaction in all experiments.

---

( 0,02) , 6,7 93,3 , 800 1700 K. .

     

Electrochemical oxidation of the nickel-EDTA complex on a platinum electrode

The electrochemical formation of a Ni(III)-EDTA (EDTA=ethylenediaminetetra-acetate) precipitate on a Pt electrode is observed. This precipitate can be reduced without dissolution. The electrochemical properties of this precipitate are analysed and compared with those of other tervalent nickel complexes.     

Calcium and strontium phosphates coatings on titanium formed by the plasma electrolytic oxidation

Oxide-phosphate coatings were formed in electrolytes with Ca(II) and Sr(II) polyphosphate complexes. The data on an effect of a ratio of metal polyphosphate complexes in the electrolyte, treatment time on an elemental and phase compositions, thickness and morphology of coatings were obtained. The approach is promising for application of the coatings containing calcium phosphate with additives of phosphates of other metals to titanium implants.     

Initial stages of copper electrocrystallization on polycrystalline platinum and glassy carbon in the presence of acetonitrile

The kinetics of formation of copper adlayer, three-dimensional nucleation, and the deposit growth on polycrystalline platinum and glassy carbon in the 0.5 M H₂SO₄ + 10 mM CuSO₄ + (0–2) M acetonitrile (AcN) solutions at the cathodic overvoltages is studied using the methods of cyclic voltammetry and potentiostatic current transients on a ring-disc electrode. At [AcN] = 2 M, the process of formation of copper adatoms on platinum is significantly retarded. In the solutions with high contents of AcN, the processes of Cu⁺ ion production, the formation of their complexes with acetonitrile, hydrogen evolution, copper nucleation, and the deposit growth proceed in parallel. The contribution of any process to the overall current depends on the amount of adsorbed AcN at the surface of substrate and copper deposit and on the electrode potential. At [AcN] = 2 M, an increase in the cathodic overvoltage to 0.32 V leads to an abnormal increase in the current of Cu⁺ ion production on platinum, which is caused by insufficiently rapid formation of copper atoms in the reduction of Cu⁺(AcN) _x complexes.     

Bulk CO oxidation on platinum electrodes vicinal to the Pt(111) surface

Bulk CO oxidation has been studied on platinum stepped surfaces belonging to the series Pt(S)[n(111) × (111)], using a hanging meniscus rotating disk electrode (HMRDE) configuration. The general shape of the voltammograms is not significantly affected by the presence of the steps. However, the curves shift towards negative values as the step density increases. Thus, in the positive-going scan, a linear relationship is observed for the dependence of the potential for the ignition peak vs the step density for surfaces with terraces wider than five atoms, shorter terraces deviate from this behavior. In the negative-going scan, a similar situation is observed for the potential where the current drops to zero. In this case, Pt(111) electrode also deviates from the expected behavior because of the formation of the ordered bisulfate adlayer on the electrode. The anion readsorption process is also observed by recording the HRMDE voltammograms at a high scan rate. All these results have been analyzed in light of a common mechanism, discussing the possible role of the steps in the stability and reactivity of the CO adlayer. In memoriam of Francisco C. Nart, an excellent scientist, colleague, and friend.     

Influence of tin on the oxidation of methanol on a platinum electrode

The influence of adsorbed tin and tin(IV) ions on the oxidation of methanol chemisorbed species as well as the methanol from the bulk of the solution was studied on a Pt electrode by voltammetric and radiometric methods. It was found that tin is not adsorbed as an ad-atom but rather as a divalent ion. Enhancement of the electrocatalytic oxidation of chemisorbed species derived from methanol was observed only in the potential range from 0.4 to 0.8 V. The influence of some factors on the electrocatalytic properties of platinum is discussed.     

Size effects on the electrochemical oxidation of oxalic acid on nanocrystalline platinum

The electrocatalytic activity of platinised platinum (Pt Pt) electrodes in the electrooxidation of oxalic acid was found to be dependent on the degree of ageing. Pt Pt electrodes prepared by electrodeposition were aged by cycling the potential with an upper positive potential limit corresponding to Pt surface oxidation. This procedure results in surface reconstruction with an increase of mean particle size. The changes of surface area and roughness of Pt Pt during ageing have been discussed in terms of sintering processes for supported catalysts or ceramic materials. An increase of mean particle size is accompanied by a decrease in oxygen adsorption, e.g. through changes in the surface concentration of defects on the particle surface. Two possible mechanisms for the electrooxidation of oxalic acid involving either an oxygen adsorbate species (CE mechanism) or direct electrode transfer can be distinguished. Changes of oxidation rate are related to changes of oxygen coverage with ageing.     

Influence of moisture on transient oxidation and reduction of alumina-supported platinum

The transient surface behavior on oxidation and reduction of a partially oxidized alumina-supported platinum catalyst containing residual water have been observed by AC electrical conductance (G) measurements.     

Anodic oxidation of cyanide and cyanate ions on a platinum electrode

The oxidation processes of cyanide and cyanate ions on a Pt electrode in aqueous and methanol solutions were studied by infrared spectroscopy. In aqueous solution, the cyanide ion was oxidized to cyanate and successively to carbon dioxide. The reaction proceeded on an oxidized platinum surface. In methanol solution, HNCO is the main product during anodization.