Special features of methanol interaction with adsorbed oxygen at platinized platinum electrode: Transients of the open-circuit potential

Open-circuit potential transients are measured under the conditions of methanol interaction with the pre-adsorbed oxygen at platinized platinum electrode. The time necessary for complete removal of the adsorbed oxygen monolayer appeared being shorter by a factor of ～1.5 as compared with smooth polycrystalline platinum. The dependence of platinum surface coverage with adsorbed oxygen on the potential during its decay is found. It was shown that the reaction of methanol with the adsorbed oxygen is most slow at a high coverage (1–0.8). It is suggested that at these coverages, like the case of polycrystalline platinum, the adsorbed oxygen directly interacts with the methanol molecules from the solution. At moderate coverages (0.8–0.2), the reaction of the adsorbed oxygen with methanol at the platinized platinum is better described by the “conjugated reactions” mechanism. The specific rates of the methanol dissociative adsorption at the platinized platinum turned out to be close to those observed earlier for the polycrystalline platinum.     

High performance gold-supported platinum electrocatalyst for oxygen reduction

High performance gold-supported Pt electrocatalyst for the reduction of oxygen was prepared by replacing Cu adlayers, deposited potentiostatically on Au, with Pt at open-circuit potential in a 0.1 M HCl solution containing K₂PtCl₆. Auger Electron Spectroscopy and Atomic Force Microscopy reveal the surface modification. The kinetics of oxygen reduction on this platinum modified electrode was studied by the rotating-disc electrode technique. The activity of the electrode is lower than the activity of a smooth Pt electrode in the negative potential scan, but it is significantly higher in the positive scan.     

Tin underpotential deposition on platinum and its catalytic influence on the kinetics of molecular oxygen electroreduction

The formation and dissolution of tin ad-layers on polycrystalline platinum were analysed by cyclic voltammetry in aqueous 10^–4 M tin(II)/1 M sulfuric acid in the 0.05–0.70 V versus RHE range. At this concentration level it was possible to observe that platinum sites involving (110) planes are mainly related to tin underpotential deposition. In contrast to previous results, no irreversible adsorption was found in the course of the electrodeposition. Thermodynamic adsorption parameters were calculated from the potential dependence of tin surface coverage. Catalytic properties of this new surface were studied on the basis of oxygen electroreduction as a model. Kinetic runs were performed with rotating ring-disk electrodes on bare and tin-modified platinum surfaces. Molecular oxygen reduction on tin-modified platinum takes place through the production of both water and hydrogen peroxide. This interpretation was confirmed by calculating the reaction order with respect to oxygen. Electronic Publication     

Hydrogen peroxide electrochemistry on platinum: towards understanding the oxygen reduction reaction mechanism

Understanding the hydrogen peroxide electrochemistry on platinum can provide information about the oxygen reduction reaction mechanism, whether H(2)O(2) participates as an intermediate or not. The H(2)O(2) oxidation and reduction reaction on polycrystalline platinum is a diffusion-limited reaction in 0.1 M HClO(4). The applied potential determines the Pt surface state, which is then decisive for the direction of the reaction: when H(2)O(2) interacts with reduced surface sites it decomposes producing adsorbed OH species; when it interacts with oxidized Pt sites then H(2)O(2) is oxidized to O(2) by reducing the surface. Electronic structure calculations indicate that the activation energies of both processes are low at room temperature. The H(2)O(2) reduction and oxidation reactions can therefore be utilized for monitoring the potential-dependent oxidation of the platinum surface. In particular, the potential at which the hydrogen peroxide reduction and oxidation reactions are equally likely to occur reflects the intrinsic affinity of the platinum surface for oxygenated species. This potential can be experimentally determined as the crossing-point of linear potential sweeps in the positive direction for different rotation rates, hereby defined as the "ORR-corrected mixed potential" (c-MP).     

Molecular simulation of oxygen on supported platinum clusters

Molecular dynamics simulations are performed to study oxygen adsorption on platinum clusters supported on a graphite surface. The Sutton–Chen many-body potential is used for the Pt–Pt interaction, whereas a Steele potential was used to represent the carbon surface. The oxygen–oxygen intramolecular force is modeled by a harmonic oscillator model and other interactions are described by the Lennard–Jones potential. The results indicate an optimum loading of platinum for maximum specific adsorption of oxygen. Adsorption isotherms are constructed and the energies and orientation of adsorbed oxygen are reported. The relevance of this study to electrode processes is discussed.     

Surface oxidation and reduction of small platinum particles observed by in-situ X-ray diffraction

A new method is presented, in which the constructive Bragg interference intensity of small metallic clusters during the formation or decay of a surface compound is measured by an in situ X-ray technique. First results for a series of platinum catalysts supported on silica gel show intensity losses on the 111-peak of up to 40% under oxygen following a standard pretreatment. These losses are restored reversibly under hydrogen. The average thickness of the disrupted surface layer ranges between 0.9 to 4.0 Å depending on the treatment temperature and platinum crystallite size.     

Pt/Pd/C catalysts with ultra low platinum content for oxygen reduction reaction

The activity of Pt/Pd/C ETEK catalysts of the core-shell type with an ultralow content of platinum (0.5–15 μg cm^?2) based on a commercial palladium catalyst is shown to exceed the activity of commercial Pt/C ETEK catalysts in the oxygen reduction reaction. The activity sharply increases with the decrease in the platinum content down to values corresponding to monolayer and submonolayer of platinum on palladium. This dependence wasn’t observed for the same amounts of platinum deposited on the carbon support Vulcan XC-72. This makes it possible to conclude that the most probable factor responsible for the high catalytic activity of Pt/Pd/C ETEK is the effect of palladium on the electronic properties of platinum rather than the effect of structural modification of the platinum deposit induced by the decrease in the platinum amount deposited on a foreign metal or a carbon support.     

Mechanism of selective oxygen reduction on platinum by 2,2'-bipyridine in the presence of methanol

Mechanism of selective oxygen reduction on platinum by 2,2'-bipyridine in the presence of methanol has been investigated by in situ surface-enhanced infrared absorption spectroscopy. The addition of 2,2'-bipyridine caused the decrease of adsorbed water molecules and those existing near the surface of platinum. The formation of both CO and formate, the latter being the intermediate in the non-CO path for methanol oxidation, depressed in the presence of 2,2'-bipyridine, suggests that 2,2'-bipyridine hinders methanol oxidation via both non-CO and CO paths on platinum. The geometrical effect of 2,2'-bipyridine adsorbed onto platinum was also investigated by multisite Monte Carlo simulation. It is indicated that selective oxygen reduction is caused by the difference in the number of required adsorption sites between methanol and dioxygen molecules. The suppression of Pt oxide species by 2,2'-bipyridine is found to be another factor that enhances the oxygen reduction.     

Activation energies for oxygen reduction on platinum alloys: theory and experiment

A combined theoretical and experimental analysis of the electrode potential dependencies of activation energies is presented for the first step in oxygen reduction over platinum and platinum alloy catalysts in both polycrystalline and carbon supported form. Tafel data for several of the catalysts are used to predict potential-dependent activation energies for oxygen reduction over the 0.6-0.9 V range in strong and weak acid. Comparisons with the theoretical curve show good agreement above 0.8 V, suggesting a fairly constant preexponential factor. Arrhenius determinations of activation energies over the 0.7-0.9 V range yield little trend for weak acid, possibly because of the larger uncertainties in the Arrhenius fits, but the strong acid results have smaller uncertainties and for them the measured activation energies trend up with potential.     

Platinum electrodeposition mechanism during the reduction of platinum chloride complexes

The nature and conditions of the occurrence of chemical and electrochemical steps of the electroreduction of chloride complexes of Pt(II) and Pt(IV) during the electrodeposition of platinum in the potential region corresponding to the double-layer segment of the charging curve are discussed. The relationship between the platinum electrodeposition mechanism and properties of produced electrolytic deposits of platinized platinum, in particular, their true surface area and its accessibility to various species is considered. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

Hydrogen peroxide reduction on amalgamated platinum electrodes covered with a monolayer of stearic acid

The reduction of hydrogen peroxide and, for comparison, oxygen on an amalgamated platinum electrode covered with a monolayer of stearic acid is studied by methods of polarization curves and impedance spectroscopy. In contrast with the oxygen reduction, the reduction of dissolved hydrogen peroxide occurs predominantly on the monolayer surface, rather than inside it. This is explained by the difficulty of penetration of the polar molecule of hydrogen peroxide into hydrocarbon environment.     

Hydrogen peroxide reduction on single platinum nanoparticles

Understanding oxygen reduction, key to much of electrochemical energy transformation technology, crucially requires exploration of the role of hydrogen peroxide as a possible intermediate especially on catalysts such as Pt which can bring about the 4e reduction of O₂ to water. We reveal that at the single nanoparticle scale the direct platinum catalysed reduction of hydrogen peroxide is found – even at high overpotentials – not to be controlled by the rate mass-transport of the reagents to the interface but by a surface limited process. Further under alkaline (pH 12.3) and near mass-transport free conditions, the single nanoparticle hydrogen peroxide reduction rate goes through a maximum at potentials comparable to the surface deposition of hydrogen (H_upd) with the highest reaction rate occurring when the surface is partially covered in hydrogen.

At the single platinum nanoparticle scale the hydrogen peroxide reduction reaction is a surface limited process.     

The mechanism of oxygen catalytic reduction in the presence of platinum and silver nanoparticles

The reaction of oxygen reduction on the mercury electrode in the solution of the reversed micelles and in the presence of platinum and silver nanoparticles was studied. The data of inverse voltammetry show that in the presence of platinum nanoparticles the reaction can proceed via both two-electron and four-electron reaction mechanisms. In the case of silver nanoparticles it proceeds in accordance with the two-electron mechanism. Cumulative effect of catalytic action of platinum and silver nanoparticles on the molecular oxygen reduction was found.     

Electrochemical nanogravimetric studies of platinum in acid media

Electrochemical quartz crystal nanobalance (EQCN) is one of the most powerful tools to obtain information on the events occurring at the electrode surface. This method has been exploited to monitor the surface mass changes and hence to draw conclusions in respect of the formation and removal of adsorbed species and oxides as well as changes in the electrochemical double layer also in the case of platinum electrodes. However, the results that had been obtained so far are somewhat contradictory, and consequently diverse interpretations can be found in the literature. Therefore, it is worth to review the knowledge accumulated and to carry out systematic study in this respect. In this work smooth and platinized platinum electrodes in contact with acidic solutions were studied using EQCN technique. The effects of temperature, the nature of cations and anions, pH, concentrations, potential range were investigated on the electrochemical, and the simultaneously detected nanogravimetric responses. It is shown that in the underpotential deposition (upd) of hydrogen the adsorption/desorption of species from the solution phase is governed by the oxidative desorption/reductive adsorption of hydrogen; however, unambiguos conclusions cannot be drawn regarding the actual participation of anions and water molecules in the surface coverage. In the hydrogen evolution region a weak cation adsorption can be assumed and the potential of zero charge can be estimated. Cs⁺ cations affect the EQCN response in the hydrogen upd region. In some cases, e.g., in the case of upd of zinc the mass change can be explained by an induced anion adsorption. Two types of dissolution processes have been observed. A platinum loss was detected during the reduction of platinum oxide, the extent of which depends on the positive potential limit and the scan rate, and to a lesser extent on the temperature. The platinum dissolution during the electroreduction of oxide is related to the interfacial place exchange of the oxygen and platinum atoms in the oxide region. At elevated temperatures two competitive processes take place at high positive potentials: a dissolution of platinum and platinum oxide formation.     

Mechanism of the effect of oxygen-modified carbon nanotubes on the kinetics of oxygen electroreduction on platinum

The relationship between the concentration of quinone groups in the electrode material containing carbon nanotubes (CNTs) and platinized carbon black and the efficiency of the use of the platinum surface in oxygen reduction was studied by cyclic voltammetry and rotating disc electrode methods. The effect of quinone groups on the oxygen coverage of the platinum surface and the density of the kinetic currents of molecular oxygen reduction on the platinum surface was investigated. A mechanism by which the oxygen-modified CNTs affect the kinetics of oxygen electroreduction on platinum was suggested.     

Changes in real surface area,crystallographic orientation and morphology of platinum electrodes caused by periodic potential treatments: phenomenological approach

The changes in surface structure of polyfaceted single crystal platinum spheres subjected to repetitive square-wave potential treatments in acid solution have been studied. The dependence of the prevailing type of change on symmetry, potential limits and frequency of the periodic potential has been determined. Results are discussed on the basis of different reactions occurring within definite potential windows and distinct rate controlling processes depending on the operating conditions. Accordingly, the surface restructuring involves a predominant change in either the real surface area, the crystallographic orientation or the entire surface morphology.     

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.     

Crystallographic rearrangement of platinum induced by square wave potentials

This paper provides experimental evidences of crystalline rearrangements on platinum surfaces by applying square wave potential perturbations. The phenomenon was followed by differential ex situ X-ray diffraction patterns and in situ cyclic voltammetry in sulfuric acid solutions. Various upper and lower potential limits were employed covering anion, hydrogen, and/or oxygen adsorption ranges. When the -0.05 to 1.50 V (vs. reversible hydrogen) potential region is covered an increase in the distribution of (200) planes is observed. However, when the 0.65 to 1.50 V region is used, (220), (311), and (420) planes developed. The development of a longitudinal propagation mode is responsible for this rearrangement. The new equilibrium position of the platinum surface atoms in the lattice was calculated from the minimization of the potential energy expression.     

Calculation of optimum thickness of active layer of oxygen and air cathodes of fuel cell with Nafion and platinum

It is shown that, for the electrodes of fuel cells with solid polymer electrolyte, the dependence of overall current on the active layer thickness contains an extremum. There is an optimum thickness of active layer, at which the overall current reaches its maximum possible value. The nature of this dependence is explained. The character of the distribution of electrochemical process intensity over the depth of active layer of cathode with solid polymer electrolyte is analyzed. The optimum thicknesses of active layers of oxygen and air cathodes of fuel cells with Nafion and platinum and the corresponding overall currents and contents of catalyst in the active layer are calculated. In the calculations, the temperature of fuel cell, the pressure in the cathode gas chamber, and the cathodic potential were varied. The optimization of active layer thickness of cathode with solid polymer electrolyte can reduce the platinum consumption, i.e. its amount per 1 kW of power produced in a membrane-electrode assembly.     

鲁米诺在铂电极上阳极电致化学发光的机理研究

研究了碱性鲁米诺溶液在多晶铂电极上的阳极电致化学发光(ECL)行为,观察到电极的预极化处理和溶解氧跟发光峰强度和峰形有直接关系。结合XPS谱图和Pt,Pt|S~a~d~s修饰电极的循环伏安特性,给出了鲁米诺阳极ECL两个发光通道的可能反应机理:(1)鲁米诺阴离子在表面有新鲜Pt原子的电极上氧化生成鲁米诺自由基,然后迅速与溶液中的氧反应形成0.22V(vs.Ag)处的发光肩峰;(2)电极表面的铂氧化物能加速原子态氧的发生过程,并增大0.60V(vs.Ag)附近ECL主峰的发光强度。