Adsorption and electrooxidation of dimethyl ether on platinized platinum electrode in sulfuric acid solution

Adsorption and oxidation of dimethyl ether (DME) on the Pt/Pt electrode from 0.5 M H₂SO₄ is studied by measuring transients of current and potential, charging curves, and curves of electrooxidation in the adsorbed layer and also by cyclic voltammetry and steady-state polarization measurements. The DME adsorption is accompanied by dehydrogenation and destruction of its molecules to form a chemisorbed adsorbate that mainly consists of C1 species (HCO_ads and/or CO_ads) with (under certain conditions) a small amount of species that desorb at cathodic polarization. The adsorption and electrooxidation of DME are inhibited by adsorbed oxygen. The possible schemes of DME oxidation, where the reaction of DME chemisorption products with adsorbed oxygen-containing species is the limiting stage, are discussed.     

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.     

Adsorption of NO and CO and specific features of their interaction on the Pt(100) surface

This article presents an analytical review of the author’s results and the literature concerning the nature of species resulting from NO and CO adsorption on the unreconstructed (1 × 1) and reconstructed hexagonal (hex) Pt(100) surfaces, including specific features of the reactions between these species. At 300 K, both surfaces adsorb NO and CO mainly in their molecular states. When adsorbed on Pt(100)-1 × 1, the NO_ads and CO_ads molecules are uniformly distributed on the surface. Under the same conditions, the hexagonal surface undergoes adsorption-induced reconstruction with the formation of NO_ads/1 × 1 and CO_ads/1 × 1 islands, which are areas of the unreconstructed phase saturated with adsorbed molecules and surrounded with the adsorbate-free hex phase. In adsorption on structurally heterogeneous surfaces containing both hex and 1 × 1 areas, the 1 × 1 and hex phases are occupied in succession, the latter undergoing reconstruction into the 1 × 1 phase. The reaction between NO and CO on the unreconstructed surfaces occurs even at room temperature and results in the formation of N₂ and CO₂ in quantitative yield. On the hexagonal surface, a stable layer of adsorbed molecules as (NO_ads + CO_ads)/1 × 1 mixed islands forms under these conditions. Above 350 K, the reaction in the mixed islands is initiated by the desorption of small amounts of the initial compounds, and this is followed by rapid self-acceleration leading to a surface explosion yielding N₂, CO₂, and N₂O (minor product). These products show themselves as very narrow desorption peaks in the temperature-programmed reaction spectrum.     

Influence of structural defects on the electrocatalytic activity of platinum

Structural defects play major role in catalysis and electrocatalysis. Nanocrystalline (or nanostructured) materials composed of nanometer-sized crystallites joined via grain boundaries have been recognized for their specific structure and properties, differentiating them from single crystals, coarsely grained materials or nanometer-sized supported single-grained particles (Gleiter, Nanostruct Mater 1:1–19, 1992). In this paper, we use Pt electrodes, prepared by electrodeposition on glassy carbon and gold supports, as model nanocrystalline materials to explore the influence of grain boundaries and other structural defects on electrocatalysis of CO and methanol oxidation. We build on the recently established correlations between the nanostructure (lattice parameter, grain size, and microstrains) of electrodeposited Pt and the deposition potential (Plyasova et al., Electrochim. Acta 51:4447–4488, 2006) and use the latter to obtain materials with variable density of grain boundary regions. The activity of electrodeposited Pt in the oxidation of methanol and adsorbed CO exceeds greatly that for Pt(111), polycrystalline Pt, or single-grained Pt particles. It is proposed that active sites in nanostructured Pt are located at the emergence of grain boundaries at the surface. For methanol electrooxidation, the electrodes with optimal nanostructure exhibit relatively high rates of the “direct” oxidation pathway and of the oxidation of strongly adsorbed poisoning intermediate (CO_ads), but not-too-high methanol dehydrogenation rate constant. These electrodes exhibit an initial current increase during potentiostatic methanol oxidation explained by the CO_ads oxidation rate constant exceeding the methanol decomposition rate constant.

Specific features of the oxygen reaction on catalytic systems in acetonitrile-based electrolytes

The oxygen reaction is studied in acetonitrile solutions on various nanosystems: ХС72, 20Au/C, 20Pt/C, 15Ru/C, 20Pd/C, 20Pt10Ru/C, 20PdRu/C. It is shown that as regards their activity in the oxygen electroreduction reaction, the studied materials form the following series: Pd/C > PtRu/C > PdRu>Pt/C> Ru/C ≈ Au/C ≈ ХС72, whereas in the reaction of Li₂O₂ electrooxidation the activity series is different: Ru/C > PtRu/C > Pd/C > PdRu/C> ХС72 > Pt/C > Au/C. Assumptions are drawn on the nature of passivation for systems with the highest activity. The prospects of bimetallic catalysts (PtRu/C and PdRu/С) that combine the high activity in reactions of oxygen electroreduction and Li₂O₂ electrooxidation and also retain a considerable part of their activity on cycling are discussed. These results make it possible to judge on the possible applications of bimetallic nanosystems with bifunctional catalytic properties in lithiumoxygen fuel cells.     

The mechanism of tin-promoted electrochemical oxidation of organic substances on platinum

The promoting action of tin in the binary system platinum tin has been studied. A direct correlation of polarization and adsorption measurements has been carried out using an adsorption method of applying the promoter. The method of fast potentiodynamic pulses has been used to control the amount of tin and organic species on the surface. Studies have been made with smooth and platinized electrodes on the adsorption and oxidation of formic acid and methanol. A comparison of electro-oxidation rates at constant organic species coverage has made it possible to reveal the true catalytic effect as a result of introduction of tin. This effect has been found to be independent of both the nature of the substance under oxidation and the state of the electrode surface (smooth or rough). The promoting effect of tin is observed in the reactions where the limiting stage is represented by R+OH_ads; in the reactions where the limiting stage is represented by electron transfer or else the interaction R+H_ads, the promoting effect of tin does not show up, which fact points to the selectivity of its catalytic action.     

Electrocatalytic activity of platinum—polyaniline and palladium—polyaniline systems obtained by cycling the electrode potential

Steady-state current densities of electrooxidation of CH₃OH, HCOOH, and CO at the Pt-PAN-GC electrodes (where PAN and GC stand for polyaniline and glassy carbon, respectively) and those of electrooxidation of HCOOH at the Pd-PAN-GC electrodes are measured (per cm₂ of the true metal-catalyst surface area). The found higher activity of Pt and Pd particles incorporated in PAN, as compared with Pt/Pt and Pd/Pt, is attributed to interaction between metal-catalyst particles and the polymeric matrix. The activation effect is the most pronounced for the HCOOH electrooxidation at Pd-PAN-GC. The data concerning hydrogen evolution testify in favor of a decrease in exchange currents of this reaction upon going from Pt to Pt-PAN-GC electrodes     

Kinetics and mechanism of interaction between methanol and adsorbed oxygen on a smooth polycrystalline platinum electrode: Transients of the open-circuit potential

The method of transients of the open-circuit potential, combined with cathodic potentiodynamic pulses, is used for studying the methanol interaction with preliminarily adsorbed oxygen (O_ads) in 0.5 M H₂SO₄. It is established that, for the larger part of the time period required for a full reduction of a monolayer of O_ads in solutions of methanol on polycrystalline platinum, the process occurs at large coverages of the surface by O_ads (?_O). In the region of medium coverages, transients of the open-circuit potential are accurately described by the equation that corresponds to the mechanism of “conjugated reactions.” The mechanism of the methanol interaction with O_ads happens to be close to that for formic acid, which is explained by a dissociative character of these HCO compounds. Kinetic parameters characterizing the methanol reaction with O_ads in the region of large and medium coverages are determined and analyzed.     

The dependence of the kinetics of some simple outer-sphere electrode reactions on the nature of the electrode material

The kinetics of a number of simple inorganic electrode reactions that are known or expected to follow outer-sphere pathways have been examined at mercury, silver, platinum, and gold-aqueous interfaces in order to explore the effects of varying the electrode material on outer-sphere reactivity. The electroreduction kinetics of Co(III) ammine complexes exhibited only mild dependences on the nature of the electrode material which were compatible with the expected variations in double-layer effects. However, the electrooxidation of Cr²⁺ proceeded at strikingly higher overpotentials on the solid surfaces compared with mercury electrodes. Similar effects were also seen for the electrooxidation of V²⁺, Eu²⁺ and Ru²⁺ in the presence of Cr²⁺. Much larger rate constants were observed for these aquo reactions at solid surfaces in the absence of Cr²⁺, although Cr²⁺ had no influence on Co(NH₃)₆³⁺ electroreduction, or any reaction at mercury electrodes. It is speculated that the very large substrate effects upon the electrode kinetics of aquo couples arise from the influence of the inner-layer water structure on the reactant-solvent interactions experienced by these “structure-making” reactants at their plane of closest approach. The inhibiting influence of Cr²⁺ may be due to its ability to efficiently remove adsorbed catalytic contaminants by incorporation into a substitutionally inert Cr(III) electrooxidation product by means of a ligand-bridge mechanism.     

Change of the roginski “control band” in the case of surface diffusion of adsorbed particles

The effect of surface particle migration between different surface patches on the distribution of the catalytic activity of patches of a linearly inhomogenous surface has been studied. It has been shown that the diffusion can result in an equilibrium distribution of particles A_ads on the surface patches and, therefore, shift of the “control band” and significant change of the reaction rate.     

Electrocatalytic properties of Au(111)-Pd quasi-single-crystal film electrodes as probed by ATR-SEIRAS

Electrochemical and electrocatalytic properties of thin films Au(111-25 nm), which are quasi-single-crystal electrodes 25 nm thick made of gold with the (111) preferential orientation, and same electrodes modified with a monolayer (ML) of palladium are studied in 0.1 M solutions of HClO₄ and H₂SO₄ employing voltammetric techniques and surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS). Spectroscopic experiments demonstrate strong adsorption of electrolyte species (H₂O, OH_ads, anions) on the Pd surface. The weak and reversible adsorption of CO on Au(111-25 nm) does not change the interfacial-water structure. Adsorption of CO on the Pd-modified film results in an irreversibly adsorbed CO adlayer stabilized by co-adsorbed isolated water species. Various electrooxidation mechanisms are discussed. Electrochemical and spectroscopic investigations on the adsorption and electrooxidation of HCOOH on bare and 1 ML Pd-Au(111-25 nm) electrodes reveal that electrooxidation proceeds in both cases via a direct or dehydrogenation pathway. This mechanism involves the formation of formate as intermediate, which is detected by in situ ATR-SEIRAS. The reactivity on Pd-modified surfaces is higher than on bare gold. The specifically adsorbed anions (sulfate/bisulfate) and the oxide formation on the substrate surface lower the reactivity for CO and HCOOH on both surfaces. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1312–1329. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Inelastic electron scattering in the adsorbed system

The study revealed additional channels of inelastic electron scattering, which accompany the threshold excitation of the substrate Pt4d level — ionization of the valent states of adsorbed particles chemically bonded to the excited atom, and excitation of the surface plasmon vibrations. The conjugate excitation of this type shows up as a series of typical satellites in the spectra of disappearance potentials, which reflects the structure of valent states of adsorbed particles. Analysis of the satellite structure revealed the intermediate formation of NH _x,ads particles in the reaction NO_gas + H_ads on the surface of Pt(100) single crystal and, taking into account the earlier data, made it possible to formulate a general mechanism of selfoscillations in the NO + H₂ reaction on platinum metals. Mathematical modeling of reaction kinetics on the Pt(100) surface within the suggested mechanism demonstrated the presence of regular self-oscillations of the reaction rate at invariable values of the step constants.     

Unique CO-tolerance of Pt–WOx materials

Well-defined tungsten-oxide-supported platinum nanoparticles (Pt/WO_x) were elaborated by impregnation-reduction of a platinum salt onto commercial monoclinic WO₃. Field-emission gun scanning electron microscopy (FEG-SEM) and transmission electron microscopy (TEM) revealed that the Pt particles are well-distributed on the oxide support, present a narrow particle size distribution centered on ca. 2–3 nm and a low degree of agglomeration. Carbon black was added to ensure electronic percolation in the electrodes during the electrochemical measurements. CO_ads electrooxidation currents were monitored at potentials as low as 0.1 V vs. RHE on Pt/WO_x, demonstrating high CO-tolerance compared to carbon-supported Pt or PtRu catalysts.     

The effect of doping of the ultradispersed nickel powder by pyrocarbon on oxygen adsorption and Oads + CO reaction

Oxygen adsorption and chemisorption and the kinetics of interaction between adsorbed oxygen and CO on nickel ultradispersed powder (average size of particles, 20 nm) are studied. The ultradispersed nickel powder was dosed by the products of pyrolysis of chemisorbed ethylene (pyrocarbon) in the amount of 0.1–1.6 of a monolayer. The spectra of ferromagnetic resonance of the ultradispersed nickel powder before and after ethylene adsorption and pyrolysis and after adsorption and chemisorption of oxygen and its reduction by hydrogen are recorded. The magnetization of ultradispersed nickel powder increases upon dosing the surface with ethylene (C₂H_4ads) and pyrocarbon in the amount of 0.5 of a monolayer. Pyrocarbon inhibits the O_ads + CO reaction. The reaction orders with respect to O_ads and CO and the experimental activation energy change. The effects of small (less than a monolayer) and large (more than a monolayer) amounts of the modifying agent are different.     

Electroanalytical chemistry of metal complexes with S-containing ligands

Summary The main types of electrochemical reactions of complexes in organic and mixed solvents are considered. Fast electrochemical reactions (k _s=n×10^–2 cm · s^–1) were used to reduce the minimal concentrations (c _min) in a.c. voltammetry. The role of pre- and post-chemical reactions in the formation of the analytical signal in extraction voltammetry is elucidated. The principles of the influence of the special structure of the complexes on their redox properties are determined using complexes of the transition metals with phosphorus dithioacids and bidentate heterocyclic amines. The features of the electrooxidation and electroreduction of inert complexes of the platinum metals with heterocyclic dithioacids and organic sulphides in nonaqueous media are discussed. The products of the electrochemical reaction of these compounds in protic media catalyze the hydrogen evolution on the mercury electrode. Kinetic data of the chelate formation are used for developing highly selective methods for the determination of the platinum metals with c _min=2×10^–9–5×10^–8 mol/l.

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Elektrochemische Reaktionen von Komplexen mit S-haltigen Liganden

     

Mechanism of the reaction NO + H<Subscript>2</Subscript> on the Pt(100)-hex surface under conditions of the spatially nonuniform distribution of reacting species

The interaction of hydrogen with NO_ads/1 × 1 islands produced by NO adsorption on the reconstructed surface Pt(100)-hex was studied by high-resolution electron energy loss spectroscopy (HREELS) and the temperature-programmed reaction (TPR) method. The islands are areas of the unreconstructed surface Pt(100)-1 × 1 saturated with NO_ads molecules. The hexagonal phase around these islands adsorbs much more hydrogen near room temperature than does the clean Pt(100)-hex surface. It is assumed that hydrogen is adsorbed on the hexagonal surface areas that are adjacent to, and are modified by, the NO_ads/1 × 1 islands. The reaction of adsorbed hydrogen atoms with NO_ads takes place upon heating and has the character of so-called surface explosion. The TPR peaks of the products of this reaction—nitrogen and water—occur at T _des ～ 365–370 K, their full width at half-maximum being ～5–10 K. In the case of the NO_ads/1 × 1 islands preactivated by heating in vacuo above the NO desorption onset temperature (375–425 K), after the admission of hydrogen at 300 K, the reaction proceeds in an autocatalytic regime and the product formation rate increases monotonically at its initial stage. In the case of activation at 375 K, during the initial, slow stage of the reaction (induction period), hydrogen reacts with nitric oxide molecules bound to structure defects (NO_def). After activation at 425 K, the induction period is characterized by the formation and consumption of imido species (NH_ads). It is assumed that NH_ads formation involves N_ads atoms that have resulted from NO_ads dissociation on defects upon thermal activation. The induction period is followed by a rapid stage of the reaction, during which hydrogen reacts with NO_{1 × 1} molecules adsorbed on 1 × 1 areas, irrespective of the activation temperature. After the completion of the reaction, the areas of the unreconstructed phase 1 × 1 are saturated with adsorbed hydrogen. The formation of H_ads is accompanied by the formation of a small amount of amino species (NH_2ads).     

Coadsorption of carbon monoxide and copper atoms on rhodium electroplates

The partial removal of copper atoms from a preliminarily formed Cu_ad layer and the adsorption of Cu²⁺ cations in the presence of a CO_ads monolayer were demonstrated for Rh/Pt electrodes by means of transients of current and potentiodynamic and analytical measurements. The electrooxidation of the mixed CO_ads + Cu_ad layer in the course of anodic potential scans is shown to be accompanied by simultaneous removal of both species in a wide potentials range.Translated from Elektrokhimiya, Vol. 41, No. 3, 2005, pp. 367–370.Original Russian Text Copyright © 2005 by Gladysheva, Podlovchenko.     

XPS, SIMS and SNMS applied to a combined analysis of aerosol particles from a region of considerable air pollution in the upper Rhine valley

 The water acceptance of aerosol particle surfaces is a key factor for atmospheric hygiene as it initiates gravitational settling by water up-take. To examine the concurrent influences on the surface of real airborne particles concerning the deposition of hydrophobic organic material, six particle sampling campaigns were performed in polluted outdoor-air under different air conditions. The particles were examined with SNMS, SIMS, and XPS with special view of the chemical inventory of the surface region. The total elemental inventory obtained with SNMS shows carbon compounds in all particle size classes. Soot seems to govern the submicron particles while the coarse fraction contains soil dust or fly ash. Depth-resolved analysis shows organic carbon compounds to be surface-enriched and to dominate the composition of the topmost molecular layers of the particles independent of the collection time and particle sizes. However, chlorides and ammonium sulfate were also found at the surface which will always reduce the hydrophobicity of the surface caused by organic compounds. No correlation was discovered between the ozone or NO₂ concentration of the air and the type and quantity of the organic surface components. Received: 1 November 1996/Revised: 17 January 1997/Accepted: 22 January 1997     

Cleavage of Σ-bonds by electroreduction: The EED mechanism

The data on the cleavage of Σ-bonds by electroreduction, found in the literature and obtained by the authors, are analyzed. A new EE_D mechanism is proposed. It is shown, using reactions of reduction of various organic compounds (halogenorganic compounds, ethers, organic phosphates, nitroamines), that this is a rather common reaction path. The EE_D processes are intermediate between processes E_DE and ECE. If the reorganization of reacting species in the first electron transfer is minimal, either E or ECE process is realized; with maximum reorganization, the E_DE process; and at intermediate values, the EE_D process.     

Corrosion of Copper in Presence of Acetic Acid Derivatives

The anodic corrosion of copper in presence of acetic acid derivatives were determined by measuring the limiting current. It is found that the rate of corrosion increased by decreasing H₃PO₄ concentration and electrode height. The experimental results showed that the inhibition efficiency increased with increasing concentration of the investigated compounds at a fixed temperature, but decreases with increasing temperature. Values of activation energy indicate that the reaction is diffusion controlled. The isotherm Langmuir, Temkin, and Flory Huggins are applied. The values of free energy of adsorption (ΔG_ads) obtained indicate the spontaneous adsorption of the inhibitor. The overall mass transfer correlations under the present conditions have been obtained using dimensional analysis method. The results agreed with the previous studies of mass transfer to rotating cylinder in turbulent flow.At the end of the corrosion process the morphology of the specimens after experiment is monitored using scanning electron microscope (SEM). SEM examination of the copper surface revealed that these compounds inhibited copper from corrosion by adsorption on its surface to form protective film. The presence of these organic compounds adsorbed on the electrode surface was confirmed by SEM investigations.