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 synthesis of self-doped polyaniline and its use to the electrooxidation of ascorbic acid

Self-doped polyaniline (PAN) film on platinum electrode surface has been synthesized via electrochemical copolymerization of aniline with orthanilic acid (OAA). Fourier transform infrared, UV–Vis, and elemental analysis indicate the formation of the copolymer and that the copolymer has the structure of a head-to-tail coupling of aniline and OAA units. It was found that the internal doping of PAN with OAA can extend the electroactivity of PAN in neutral and even in alkaline media. The obtained self-doped PAN (PAN-OAA)-coated platinum electrode is shown to be a good surface for the electrooxidation of ascorbic acid (AA) in phosphate buffer solution of pH 7. The anode peak potential of AA shifts from 0.63 V at bare platinum electrode to 0.34 V at the PAN-OAA-modified platinum electrode with greatly enhanced current response. A linear calibration graph is obtained over the AA concentration range of 5–60 mM using cyclic voltammetry. Rotating disk electrode voltammetry and chronoamperometry have been employed to investigate the electrooxidation of AA. The PAN-OAA-modified platinum electrode shows good stability and reproducibility.     

The mechanism of formic acid electrooxidation on iron tetrasulfophthalocyanine-modified platinum electrode

The mechanism of formic acid electrooxidation on iron tetrasulfophthalocyanine (FeTSPc) modified Pt electrode was investigated with electrochemical methods. It was found that a “third-body” effect of FeTSPc on Pt electrode predominates during the electrooxidation process based on unusual electrochemical results. The modification leads formic acid electrooxidation to take place through a desired direct pathway, in which the mechanism is proposed to be the gradual dehydrogenation of formic acid and the reaction of formate with hydroxyl species.     

Generalized scheme of chemisorption,electrooxidation and electroreduction of simple organic compounds on platinum group metals

A generalized scheme of successive stages for chemisorption, electrooxidation and electroreduction of simple organic compounds from methane to CO₂ on platinum group metals has been developed on the basis of experimental data. The actual pathway of electrooxidation or electroreduction of an organic compound and the yield of the reaction products depend on the ratio between the rates of individual steps which, in their turn, are determined by various factors (potential, temperature, coverage of the surface with H_ads and OH_ads, with reacting and intermediate organic particles, as well as with foreign particles). The accumulation of chemisorbed compounds on the electrode surface depends on the relationship between the rates of their formation and subsequent transformation.     

Anodic Stripping Determination of Pt (IV) Based on the Anodic Oxidation of Cu from the Intermetallic Phase of Cu3Pt

It is shown that platinum can be determined by anodic stripping voltammetry at the peak of selective electrooxidation of copper from intermetallic phase with platinum of Cu₃Pt composition. The composition of intermetallic copper-platinum phase formed on the electrode during pre-electrolysis was calculated on the amount of potential displacement (ΔЕ) of copper electrooxidation.     

Methanol electrooxidation in alkaline solutions on platinum-based electrodes: Classical and dynamical approach

The electrochemical oxidation of methanol has been carefully studied due to its application in fuel cells. In this work electrooxidation of methanol was investigated on bare platinum electrode, the platinum electrode covered with Nafion and platinum supported on zeolite 13X. Along with classical electrochemical methods, attractor reconstruction was used to make rough distinction among possible reaction mechanisms on different forms of Pt. The obtained transient voltammogram records were used to calculate apparent rate constants for methanol oxidation limiting steps in transient period. All samples contributed to methanol oxidation by basically same reaction mechanism, but with significantly different apparent rate constants.     

Effect of perchlorate ion within the polypyrrole matrix on the electrooxidation of ethylene glycol in acidic media

The electrooxidation of ethylene glycol was investigated on platinum and perchlorate ions doped polypyroole film electrodes in acidic media. The presence of perchlorate ion was observed by XPS experiments. Optimum experimental conditions were determined. The electrooxidation kinetic of ethylene glycol was studied as a function of ethylene glycol concentration, potential scan limit and scan rate. Results suggest that the doping process has a strong effect on the oxidation reaction.     

Ring-disk electrode studies of the formation,growth and transformation of iodine films formed during the anodic oxidation of iodide on platinum

The kinetics and mechanism of the formation, growth and dissolution of iodine films on platinum during the electrooxidation of iodide have been investigated using rotating-disk and ring-disk techniques. The dissolution-precipitation mechanism of film formation and a linear growth law for film growth has been confirmed. The iodine film on platinum is believed to be made up of an ionically insulating barrier layer covered by a porous overlayer. The oxidation behavior of iodide and ferrous species shows that the iodine film is predominantly an iodide-ion conductor. Also, a film transformation, responsible for the transient features observed during iodide oxidation has been confirmed. This transformation changes the mechanism of iodide transport through the film from a partly “pore”-type to a Grotthus-type mechanism. The ring current maximum, which occurs at the same instant as the disk current minimum, reveals the mechanism of dissolution of iodine film during the transient period as involving complexation with iodide. Evidence for the mass-transport-controlled component of the potentiostatic transient response has been obtained from the sinusoidal hydrodynamic modulation response.     

An infrared study on the chemisorption of tertiary phosphines on coinage and platinum group metal surfaces

The chemisorption of dimethylphenyl-, methyldiphenyl- and triphenylphosphine on evaporated gold, silver, copper, rhodium, iridium, palladium, platinum and nickel surfaces has been studied by means of infrared reflection–absorption spectroscopy (IRAS). Multilayers of physisorbed phosphine are formed on the surfaces of all metals studied except nickel after deposition from dilute toluene solution. The deposition rate varies for different metal surfaces and it is sometimes quite slow. The standard immersion time was 20 h in this study to secure that an equilibrium between the surface and the solution is reached. Several minutes of ultrasonic treatment are required to get rid of the physisorbed phosphine, leaving a very thin layer of chemisorbed phosphine on the metal surface. Most of the absorption bands in IRAS spectra of these thin layers show significant shifts, which are especially large for dimethylphenylphosphine. It is evident that the electron distribution in the entire phosphine molecules is changed and that the chemisorption to the coinage and platinum group metal surfaces is strong. Infrared spectra of coordination compounds of gold(I), silver(I) and copper(I) with dimethylphenyl-, methyldiphenyl- and triphenylphosphine and of the corresponding phosphine oxides have served as reference material for the chemisorbed phosphines. The spectra of the coordination compounds show similar shifts and intensity changes as the IRAS spectra of tertiary phosphines chemisorbed on the coinage and platinum group metals. This suggests that the studied phosphines are as strongly bound to the coinage and platinum group metal surfaces as to the monovalent coinage metal ions known to form very stable complexes with tertiary phosphines.     

Nanostructured platinum as an electrocatalyst for the electrooxidation of formic acid

Nanostructured platinum prepared by the chemical reduction of hexachloroplatinic acid dissolved in aqueous domains of the liquid crystalline phases of oligoethylene oxide surfactants, was examined as an electrocatalyst for the electrooxidation of formic acid. The electrocatalytic properties of the catalyst combining highly specific surface areas and a periodic mesoporous nanostructure were accessed in sulfuric acid solution containing 0.5 mol dm⁻³ formic acid using cyclic voltammetry (CV) and chronoamperometry. The electrocatalytic activity of the material at 60 °C, is characterised by a mass activity of 8.6 A g⁻¹ and a specific surface area activity of 26 μA cm⁻² at 0.376 V (vs. RHE). The resistance to CO poisoning was found to depend upon electrode potential. At hydrogen adsorption potentials, the material is easily poisoned, while the material shows high resistance to CO poisoning at potentials positive of the hydrogen region. These facts suggest that the decomposition of HCOOH on the mesoporous platinum is likely to proceed through a dual-path mechanism and the high surface area material is a potential electocatalyst towards the electrooxidation of small organic molecules.     

Kinetics and mechanism of “reduced CO2” electrooxidation on electrodispersed platinum in different acid solutions

The electrooxidation of “reduced CO₂” electroadsorbates on electrodispersed platinum electrodes has been investigated in 0.05 M HClO₄, 1 M HClO₄, 0.5 M H₂SO₄ and 1 M H₃PO₄ at 25° C through voltammetry and potential step techniques. The overall reaction comprises three distinguishable processes, namely a first order triggering process, and two second order surface processes. The latter are influenced remarkably by the solution composition (anions). The second order reaction mechanism involves two distinguishable “ reduced CO₂” electroadsorbates which react independently with the OH species formed from H₂O electrooxidation on the bare platinum sites as the reaction proceeds. An interaction term has to be included in the rate equations to account for the experimental results. The mechanistic interpretation accounts for the values of the number of electrons per site ranging from 1 to 2.     

Facile fabrication of nanoporous gold film electrodes

A novel one-step method has been developed for the fabrication of a three-dimensional (3D) nanoporous gold film (NPGF). The NPGF can be facilely made within 1 min from a pure gold substrate by applying a step potential just into the initial transition region of gold in an HCl medium. The pore formation and structural evolution have been revealed by scanning electron microscope, and the processes involve electrodissolution, disproportion, and deposition. The as-prepared 3D NPGF electrode has a large surface area and exhibits high catalytic activity in the electrooxidation of glucose. The NPGF electrode also shows excellent performance toward the electrooxidation of formic acid after being decorated with a tiny amount of Pt by electrodeposition.     

Surface metal modifiers for methanol electrooxidation on platinum; silver and mercury

The catalytic activity of platinum surfaces towards methanol electrooxidation can be modified by the deposition of a second metal using different methodologies. There is little information about the catalytic performance of polycrystalline platinum modified by silver and mercury adatoms using spontaneous and electrochemical deposition methods. Cyclic voltammetrics have been performed to compare the current vs potential profiles of modified platinum surfaces in acid solution at room temperature. The inhibition of the hydrogen adatom voltammetric profile by foreign metal adatoms on platinum was used to calculate the degree of surface coverage by the metal. Poisoning effects were checked by anodic stripping experiments of methanol residues on the modified platinum surfaces at adsorption potentials in the hydrogen electrosorption region using a micro flux cell. Methanol solution oxidation was also evaluated at slow scan rates of up to 0.8 vs reversible hydrogen electrode (RHE) on the platinum-modified surfaces. The comparison between the amounts of carbon-monoxide-type residues and the solution oxidation of methanol was analysed to check for their utility as catalytic surfaces for direct methanol fuel cells. Dedicated to Professor Dr. Algirdas Vaskelis on the occasion of his 70th birthday.     

Hydrogen adsorption and methanol electrooxidation on platinum films between 293 and 163 K

Hydrogen adsorption on plantinum polycrystalline films in methanolic medium has been studied between 293 and 163 K by cyclic voltammetry. At the same time, the electrooxidation of the solvent was followed down to the solidification point of the electrolyte. The oxidation reactions of the residues coming from methanol adsorption on platinum are thermally activated phenomena which are controlled by diffusion at very low temperature, whereas the mechanism of hydrogen adsorption does not seem to be thermally activated. The adsorption of hydrogen on Pt in methanolic medium is not all “blocked” below T = 243 K. Moreover, at low temperature, it seems to be stronger than that at room temperature.     

Electrooxidation of methanol on upd-Ru and upd-Sn modified Pt electrodes

The electrochemical oxidation of methanol has been investigated on underpotentially deposited-ruthenium-modified platinum electrode (upd-Ru/Pt) and on underpotentially deposited-tin-modified platinum electrode (upd-Sn/Pt). The submonolayers of upd-Ru and upd-Sn on a Pt electrode increased the rate of methanol electrooxidation several times as large as that on a pure Pt electrode. The best performance for methanol electrooxidation was obtained on a ternary platinum based catalyst modified by upd-Ru and upd-Sn simultaneously. The influence of the submonolayers of upd-Ru adatoms and upd-Sn adatoms on the oxidation of methanol in acid has been investigated. The effect of Ru on methanol electrooxidation lies on the distribution of Ru adatoms on a Pt surface. It has been shown that as long as the amount of upd-Ru deposits were controlled in a proper range, upd-Ru deposits would enhance the methanol oxidation obtained on a Pt electrode at whichever deposition potential the upd-Ru deposits were obtained. The effects of tin are sensible to the potential range. The enhancement effect of upd-Sn adatoms for the oxidation of methanol will disappear as the electrode potential is beyond a certain value. It is speculated that there exists a synergetic effect on the Pt electrode as adatoms Ru and Sn participate simultaneously in the methanol oxidation.     

Characterization and methanol electrooxidation studies of Pt(111)/Os surfaces prepared by spontaneous deposition

Catalytic activity of the Pt(111)/Os surface toward methanol electrooxidation was optimized by exploring a wide range of Os coverage. Various methods of surface analyses were used, including electroanalytical, STM, and XPS methods. The Pt(111) surface was decorated with nanosized Os islands by spontaneous deposition, and the Os coverage was controlled by changing the exposure time to the Os-containing electrolyte. The structure of Os deposits on Pt(111) was characterized and quantified by in situ STM and stripping voltammetry. We found that the optimal Os surface coverage of Pt(111) for methanol electrooxidation was 0.7 +/- 0.1 ML, close to 1.0 +/- 0.1 Os packing density. Apparently, the high osmium coverage Pt(111)/Os surface provides more of the necessary oxygen-containing species (e.g., Os-OH) for effective methanol electrooxidation than the Pt(111)/Os surfaces with lower Os coverage (vs e.g., Ru-OH). Supporting evidence for this conjecture comes from the CO electrooxidation data, which show that the onset potential for CO stripping is lowered from 0.53 to 0.45 V when the Os coverage is increased from 0.2 to 0.7 ML. However, the activity of Pt(111)/Os for methanol electrooxidation decreases when the Os coverage is higher than 0.7 +/- 0.1 ML, indicating that Pt sites uncovered by Os are necessary for sustaining significant methanol oxidation rates. Furthermore, osmium is inactive for methanol electrooxidation when the platinum substrate is absent: Os deposits on Au(111), a bulk Os ingot, and thick films of electrodeposited Os on Pt(111), all compare poorly to Pt(111)/Os. We conclude that a bifunctional mechanism applies to the methanol electrooxidation similarly to Pt(111)/Ru, although with fewer available Pt sites. Finally, the potential window for methanol electrooxidation on Pt(111)/Os was observed to shift positively versus Pt(111)/Ru. Because of the difference in the Os and Ru oxophilicity under electrochemical conditions, the Os deposit provides fewer oxygen-containing species, at least below 0.5 V vs RHE. Both higher coverage of Os than Ru and the higher potentials are required to provide a sufficient number of active oxygen-containing species for the effective removal of the site-blocking CO from the catalyst surface when the methanol electrooxidation process occurs.     

The effect of the system polydispersity on voltammograms of nanoparticles electrooxidation

The paper proposes a mathematical model describing electrooxidation of a polydisperse system of metal nanoparticles from the surface of an indifferent macro-electrode. It is shown that the degree of dispersion of a nanoparticle ensemble affects the shape of oxidation voltammograms. When the degree of dispersion rises and the average size of nanoparticles becomes smaller, the range of oxidation potentials increases. The results of the experimental study of electrooxidation of gold nanoparticles with different degrees of dispersion are given. The particles were localized on the surface of graphite screen-printed electrodes. A good agreement between the parameters of the experimental and calculated voltammograms confirms the correctness of the proposed model.     

Kinetics and mechanism of nitrate and nitrite electroreduction on Pt(100) electrodes modified by copper adatoms

Kinetics and mechanism of nitrate and nitrite reduction on Pt(100) electrode modified by Cu adatoms have been studied in solutions of sulfuric and perchloric acids by means of cyclic voltammetry and in situ IR-spectroscopy. It has been shown that the surface redox process with participation of ammonia or hydroxylamine at 0.5–0.9 V occurs only on the Cu-free platinum. The causes of this effect could be low adsorption energy of nitrate reduction products on copper or changes in the composition of the products (ammonia for Pt(100) and N₂O for Pt(100)+Cu). Nitrate reduction on Pt(100)+Cu electrode is much faster in the perchloric acid solution (by several orders of magnitude) as compared with unmodified platinum as a result of induced adsorption of nitrate anions in the presence of partly charged Cu atoms. In the solutions of sulfuric acid the rate of nitrate reduction is considerably lower as copper adatoms facilitate adsorption of sulfate anions, which block the adsorption sites for the nitrate.     

Pt nanoparticles/graphene paste electrode for sodium borohydride electrooxidation

The graphene paste electrode (GPE), modified with platinum nanoparticles (NPt), is fabricated and characterized by electrochemical impedance spectroscopy and cyclic voltammetry. Borohydride electrooxidation and hydrogen evolution reaction (HER) are investigated by cyclic voltammetry at surface of the fabricated electrode. Linear sweep voltammetry of NPt/GPE and NPt/carbon paste electrode in different concentrations of borohydride was studied. Results demonstrate good conductivity and electrocatalytic activity of NPt/GPE toward borohydride electrooxidation and HER. The morphology of platinum nanoparticles is studied by atomic force microscopy.     

Dimethoxymethane (DMM) electrooxidation on carbon-supported Pt-based nanosized catalysts for PEMFC

Previous studies on dimethoxymethane (DMM: CH₃OCH₂OCH₃) on platinum poly- and single crystals allowed us to propose a general mechanism of DMM electrooxidation. At the time, making electrodes for proton exchange membrane fuel cells (PEMFC) with nanoparticles (based on Pt) was encouraged. It is well known that the improvement of Pt activity for electrocatalysis is possible by modifying platinum with other metals able to increase the kinetics of specific steps of the reaction (activation of water for example). Nanosized PtM/C electrocatalysts have been synthesized by the Bönneman method and characterized for DMM electrooxidation. Voltammetry, in situ IRTF spectroscopy and fuel cell tests were carried out to better understand DMM oxidation reaction. Voltammetry and fuel cell tests showed that PtRuMo and PtRu are the most active catalysts at high potential, whereas PtSn and PtMo have a best activity at low potentials. In situ IR experiments allowed the observation of CO_ads and CO₂ bands.