Influence of lead and thallium underpotential adsorbates at silver single crystal surfaces on different redox reactions

The influence of underpotential Pb- and Tl-adsorbates at rotating disc silver single crystal surfaces (111), (100), and (110) on the kinetics of outer-sphere and inner-sphere redox reactions was studied in 0.5 M NaClO₄ and 0.5 M Na₂SO₄ solutions (2 < pH < 4) containing Fe³⁺, Ce⁴⁺ or NO ₃ ^– ions. While the reduction of Fe³⁺ and Ce⁴⁺ under limiting diffusion conditions was not affected by the metal adsorbates, a strong inhibition effect correlated to the degree of Pb- or Tl-adsorbate coverages was observed in the case of the reduction process in the presence of NO ₃ ^– ions. The results are interpreted in terms of a strong chemical interaction between the reactant or a reaction intermediate and the silver substrate, taking into account the previously proposed superlattice structures of the metal adsorbates, depending on the orientation of the silver single crystal surfaces and on the underpotential range.     

The electrocatalytic influence of underpotential lead adsorbates on the reduction of oxygen at glassy carbon electrodes in acid solution

The electrochemical reduction of oxygen and hydrogen peroxide has been investigated on a glassy carbon rotating disk electrode in 0.5 M HClO₄ at various concentrations of Pb²⁺. An overall 2-electron reduction of O₂ to H₂O₂ is found on glassy carbon which, in the presence of Pb²⁺, is positively catalyzed. The electrocatalytic effect can be correlated to an underpotential deposition (UPD) of lead at a limited number of active sites on the glassy carbon surface. The hydrogen peroxide reduction is nearly completely inhibited by the UPD of lead. The observed concentration dependence with respect to Pb²⁺ is in accordance with the Nernstian behaviour of the UPD process.     

The electrocatalytic influence of Pb and Tl adsorbates on the reduction of nitrobenzene and m-dinitrobenzene on platinum

The reduction of nitrobenzene and m-dinitrobenzene on platinum surfaces modified by Pb and Tl monolayers deposited at underpotentials was studied in aqueous acid solutions. It was found that Pb and Tl adsorbates markedly catalyze the reduction of the above substances. The enhancement of the overall reduction processed has been interpreted in terms of the change of the nitrogroup reduction mechanism from chemical mechanism on bare platinum to electron mechanism on Pt surfaces covered by Pb and Tl monolayers. The reaction paths of the reduction of nitrocompounds both on bare platinum and Pt surfaces modified by Pb and Tl adsorbates have been discussed on the basis of the diagnostic plots, which are known from the theory of the rotating disc-ring electrode.     

Reduction of dinitrobenzenes on mercury,bare silver and silver surfaces modified by underpotential Pb adsorbates in methanol

The electrochemical reduction of dinitrobenzenes on mercury, bare silver and silver surfaces modified by underpotential lead adsorbates was studied in methanolic solutions. The polarographic behaviour of the above substances in methanol presents remarkable similarities with that in aqueous buffered solutions. The addition of HClO₄ leads to the appearance of prewaves caused by local changes of the acidity during the reduction process due to the lack of buffer. On a bare silver substrate, dinitrobenzenes are reduced to phenylenediamines in acid solutions. The underpotential deposition of lead on a polycrystalline silver substrate caused a partial inhibition of the reduction of dinitrobenzenes in acid solutions. The reduction of the intermediate phenylenedihydroxylamines is completely inhibited when the silver substrate is covered by the underpotential lead adsorbate. In the presence of Cl^? ions a pseudo-catalytic effect was found which is interpreted in terms of a displacement of adsorbed Cl^? ions by the preferential adsorption of Pb in the underpotential region.     

Oxygen reduction electrocatalysis by underpotential deposited metal atoms at different single crystal faces of gold and silver

Electrocatalytic effects of underpotential lead and bismuth adsorbates on the cathodic reduction of O₂ and H₂O₂ in perchloric acid solution obtained on rotating disc Au and Ag single crystal surfaces of (111), (100) and (110) crystallographic orientation are discussed. On the clean Ag substrates the complete 4-electron reduction of O₂ to H₂O takes place whereas on Au the incomplete 2-electron reduction to H₂O₂ predominantly occurs, exhibiting pronounced structural effects. The underpotential deposition of Pb on Ag surfaces leads to a partial inhibition of the O₂ reduction whereas on Au (111) and (100) surfaces positive catalytic effects are observed. Measurements of H₂O₂ reduction indicate that an important part of the catalytisis on Ag and Au can be attributed to a decrease or increase of the rate of further reduction of H₂O₂ to H₂O. The results are discussed in terms of different adsorption models for the O₂ molecule.     

The electrocatalytic influence of underpotential Pb, Tl and Bi adsorbates on the electrochemical behaviour of benzofuroxan and o-benzoquinone dioxime on platinum

The electrochemical behaviour of benzofuroxan and o-benzoquinone dioxime on bare platinum and Pt surfaces covered by heavy metal monolayers deposited at underpotentials was studied in aqueous acid solutions. It was found that Tl, Pb and Bi adsorbates markedly catalyse the reduction of benzofuroxan. The enhancement of the overall reduction process has been interpreted in terms of the change of the reduction mechanism from chemical mechanism on bare platinum to direct electrochemical mechanism of Pt surfaces covered by the underpotential monolayers. The reduction waves of benzofuroxan and o-benzoquinone dioxime on Pt/M_ads surfaces were found to be kinetically controlled. The kinetic nature of the current was attributed to the dehydration reactions of the intermediate hydroxylamines formed in the course of the electrode reaction. It was also found that underpotential layers markedly improve the reversibility of the two-electron oxidation of o-benzoquinone dioxime. This catalytic effect has been interpreted in terms of the change of the reduction mechanism from an “inner sphere” mechanism involving adsorbed intermediates on pure platinum to an “outer sphere” one without complications from the adsorption of the reacting molecules on Pt/M_ads surfaces.     

The influence of adsorbing substances on the lead UPD onto (111) oriented silver single crystal surface—I

Effect of anions present in solution on the UPD of lead onto (111) oriented silver surface has been investigated. Linear sweep voltammetry was used. It was established that organic anions, such as acetate and citrate, shift the UPD phenomena towards the reversible potential of lead. Adsorption isotherms of lead indicate first order phase transition. Addition of molecular thiourea in solution, which is known to be strongly adsorbed, justified the assumption that the observed effects are due to the adsorbing properties of the anions.     

Effect of the surface roughness of conducting polypyrrole thin-film electrodes on the electrocatalytic reduction of nitrobenzene

Conducting polypyrrole (PPy) thin-film electrodes were prepared by the electropolymerization of pyrrole on gold-coated glass plates. Films of various roughnesses were obtained by the variation of the scan rates during electropolymerization. These thin films were modified by doping with 6mM of the dopant NiCl₂. The surface morphology of the films was studied by scanning electron microscopy and atomic force microscopy (AFM), which suggested films prepared with a high scan rate were rougher in nature than the films produced with a low scan rate. The electrocatalytic reduction of nitrobenzene was carried out with these electrodes with the cyclic voltammetry technique in acetonitrile containing 0.1M HClO₄ as a supporting electrolyte. The various results obtained show that the conducting PPy thin-film electrodes were catalytically active toward the electroreduction process. The modified PPy film electrodes doped with NiCl₂ were more active toward nitrobenzene electroreduction than the PPy film alone. The results indicate that the roughness of the films played a very important role in determining their catalytic activity. The PPy films that were more rough in nature were catalytically more active than the smooth films; this may have been due to the availability of more reactive sites in the case of rough films. The apparent diffusion coefficients of the PPy film electrodes were also calculated. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Electrochemical reduction of dichlorodifluoromethane on silver and lead electrodes

The electrochemical conversion of dichlorodifluoromethane (CFC-12) was studied on silver and lead electrodes. The main products detected were CHClF₂, CH₂F₂, CH₃F and CH₄. Cyclic voltammetry and constant potential electrolytic experiments in acetonitrile showed that on silver the reduction mechanism starts with the elimination of one chloride ion at –1.05 V. The current density and the product distribution strongly depend on the nature of the electrolyte used. Higher current densities were observed in methanol and acetonitrile, whereas in propylene carbonate (PC) the current density was found to be 13 times lower than that in acetonitrile. This difference was mainly attributed to the differing diffusion coefficient of CFC-12 in various solvents. A consecutive reaction mechanism was proposed to explain the experimental results.     

Impedance measurements of the underpotential deposition of Pb2+ and Tl+ on single crystal Ag surfaces

Investigations of metal ion adsorption in the underpotential region were carried out with the aid of the alternating voltage method. To this end the impedance behaviour of the system Ag (100)/Tl⁺, Ag(100)/Pb²⁺, Ag(110)/Tl⁺ and Ag(110)/Pb²⁺ at different potentials in the underpotential region in the frequency range of 100 mHz to 10 kHz was studied.Analysis of the electrode impedance shows that the adsorption of Pb²⁺ can be interpreted on the basis of a simple Dolin-Ershler model with accompanying diffusion. For Tl⁺ adsorption more complicated model is necessary with fast transfer at line inhomogeneities and slow transfer at homogeneous surface regions.     

Electrochemical reduction of 2,4,6-trinitro diphenylamine in buffered and unbuffered methanolic solutions

The polarographic reduction of 2,4,6-trinitro diphenylamine was found to give three waves of nearly equal height in all pH regions in buffered solutions and also in unbuffered methanol—water mixtures. The waves were shown to be diffusion-controlled and irreversible on the basis of the usual criteria. Controlled-potential electrolysis at the limiting region of the third wave gave n_app = 18 and thus it was shown that n_app = 6 for each of the waves. It was concluded that each of the nitrogroups in the compound is reduced to the corresponding amino group, the nitro groups ortho to the --- NH group in the parent compound being reduced first followed by the other. Cyclic voltammetric experiments in unbuffered solutions gave evidence for a new redox system which was attributed to amine-quinone diimine intermediate couple. It was inferred that each of the nitro groups in the compound is reduced to the amino function via this intermediate.     

Study on electroactive and electrocatalytic surfaces of single walled carbon nanotube-modified electrodes

An investigation of the electrocatalysis of single-walled carbon nanotubes modified electrodes has been performed in this work. Nanotube-modified electrodes present a surface area much higher than the bare glassy carbon surfaces as determined by capacitance measurements. Several redox probes were selected for checking the reactivity of specific sites at the carbon nanotube surface. The presence of carbon nanotubes on the electrode improves the kinetics for all the reactions studied compared with the bare glassy carbon electrode with variations of the heterogeneous electron transfer rate constant up to 5 orders of magnitude. The most important effects are observed for the benzoquinone/hydroquinone and ferrocene/ferricinium redox couples, which show a remarkable improvement of their electron transfer kinetics on SWCNT-modified electrodes, probably due to strong π–π interaction between the organic molecules and the walls of the carbon nanotubes. For many of the reactions studied, less than 1% of the nanotube-modified electrode surface is transferring charge to species in solution. This result suggests that only nanotube tips are active sites for the electron transfer in such cases. On the contrary, the electroactive surface for the reactions of ferrocene and quinone is higher indicating that the electron transfer is produced also from the nanotube walls.     

The kinetics of the anodic formation and reduction of phase silver sulfide films on silver in aqueous sulfide solutions

The kinetics of the potentiodynamic formation and reduction of silver sulfide films on a silver rotating disc electrode in aqueous sulfide solutions were investigated. Due to the inherently fast reaction between silver and the HS⁻ species, the physical processes of migration and diffusion of ions in solution were frequently rate limiting. Only when the rate of HS⁻ transport in solution was very high, did the rate of film growth become limited by the solid-state electromigration of silver ions through the silver sulfide film. By utilizing the low field approximation for field-assisted electromigration, the ionic conductivity of these films was found to be 5.9 × 10⁻⁴ S cm⁻¹. Silver sulfide film reduction occurred by the injection of electrons into the film to the film/electrolyte interface. All of the results have been supported by scanning electron microscope investigations.     

Building and characterization of catalysts on single crystal surfaces

The components of complex catalyst systems include transition metals, oxides, alkali metal or halogen additives and strongly chemisorbed sulfur or carbonaceous species. Modern surface science techniques identified the chemical and structural roles of these ingredients when used in combination with catalytic reaction rate studies. Using metal single crystal substrates, catalyst components have been deposited from the vapor phase and the complex catalyst system could be built this way. The Pt-Re-S, Rh-TiO₂, and Mo-Co-S systems are reviewed and the requirements for the proper surface structure and composition to obtain high reaction rates and selectivities are discussed.     

The potentiodyanmic formation and reduction of a silver sulfide monolayer on a silver electrode in aqueous sulfide solutions

A potentiodynamic study of silver electrodes in aqueous sulfide solutions, carried out to form phase silver sulfide films, revealed that a monolayer of silver sulfice forms as a distinct and separate stage of film growth at an underpotential of about 0.12 V. The monolayer peak (and its cathodic counterpart) was also characterized by a linear relationship between peak current density and potential sweep rate and a constant charge density of about 0.2 mC/cm². The potentiodynamic E/i curves for the silver sulfide monolayer were simulated by computer on the basis of a mechanism of the initial adsorption of HS^? on the silver surface in a fast equilibrium step followed by a rate determining electron transfer step to form AgHS as a surface intermediate. The AgHS species then rapidly diffuses on the surface and joins a growing two-dimensional silver sulfide monolayer nucleus. Under the experimental conditons studied here, the formation and reduction of the silver sulfide monolayer was found to be of intermediate kinetic reversibility.     

Electrodeposition of silver particles from alkaline aqueous solutions and their electrocatalytic activity for the reduction of nitrate, bromate and chlorite ions

A pulsed electrochemical method was used for the preparation of silver deposit onto glassy carbon surfaces from an alkaline medium containing ammonia ligand. The morphology of the silver deposit, obtained under pulsed and cyclic conditions was examined and compared using the scanning electron microscopy (SEM) technique. From the experimental evidences, it can be inferred that pulsed method induces a favourable dendritic growth of silver particles, while deposited films obtained under cyclic voltammetry appear formed of globular particles with well regular and defined surface geometry. The electroanalytical performance of the deposited silver, towards the electroreduction of NO₃⁻, BrO₃⁻ and ClO₂⁻ ions, using cyclic voltammetry and chronoamperometry technique, was evaluated. Interesting analytical results, in terms of sensitivity and temporal reproducibility, in alkaline medium and under constant applied potentials (i.e., −1.45 and −0.6 V vs. SCE), were obtained.     

The underpotential deposition of bismuth and tellurium on cold rolled silver substrate by ECALE

Thin-layer electrochemical studies of the underpotential deposition (UPD) of Bi and Te on cold rolled silver substrate have been performed. Different approaches have been employed to investigate the influence of silver oxide film on Bi UPD. As a result, the precedent deposition of a little bismuth can effectively prevent silver from surface oxidation. The voltammetric analysis of underpotential shift demonstrates that the first Te UPD on Bi-covered Ag and Bi UPD on Te-covered Ag fit UPD dynamics mechanism. Thin film of bismuth telluride was formed using an automated flow deposition system, by alternately depositing Te and Bi. The electrochemical conditions necessary to form Bi₂Te₃ deposits of 50 cycles on cold rolled silver by ECALE are described here. X-ray diffraction indicated the deposits were Bi₂Te₃. EDX quantitative analysis gave the 2:3 stoichiometric ratio of Bi to Te, which is consistent with XRD result. Electron probe microanalysis of the deposits showed a worm-like network structure. The map of Te and Bi element indicated the distribution of both Te and Bi is homogeneous and locates the same sites, which is favorable to Te-Bi binary system. The composition analysis of structural expanded image also showed the approximately constant composition of Te:Bi ≈ 3:2 has taken place.     

Synthesis, characterization and electrocatalytic activity of silver nanorods towards the reduction of benzyl chloride

This paper describes a simple method of synthesizing silver nanorods using the polyol process, where propylene glycol serves both as a reducing agent and as a solvent in the presence of a capping reagent such as polyvinylpyrrolidone (PVP). The diameter and length of silver nanorods could be controlled by changing the AgNO₃/PVP ratio. The end-to-end assembly of the silver nanorods was found. The silver nanorods were characterized by using scanning electron microscopy, transmission electron microscopy, X-ray diffraction and absorption spectroscopy. The catalytic activity of a glassy carbon electrode with Ag nanorods exhibits extraordinary electrocatalytic activities towards the electro-reduction of benzyl chloride.