Oxygen reduction on electrodeposited silver catalysts in alkaline solution

In this study, silver was electrochemically deposited onto glassy carbon (GC) substrate using constant potential regime and tested for oxygen reduction reaction (ORR) in alkaline media. The surface morphology of Ag/GC electrodes was studied by scanning electron microscopy (SEM). It was established that after 10 s of deposition, a number of Ag nanoparticles with the size of 15 nm are produced that grow to about 45 nm after 300 s of electrodeposition. The ORR studies were conducted in 0.1 M KOH solution employing the rotating disk electrode (RDE) method. The Tafel slope at low current densities for electrodeposited silver is in the range from ?70 to ?80 mV. The RDE measurements showed that the electron transfer number (n) is 3.5 for smaller amounts of electrodeposited Ag, and it increases with increasing the loading of Ag on the GC surface. These n values suggest that the electroreduction of oxygen on Ag/GC electrodes proceeds mainly to water.     

Oxygen Electroreduction Catalysts Based on Polymer Complexes of Nickel with Schiff Bases

In energy-storage systems such as fuel cells and metal-air batteries, the main current-forming process is the reaction of oxygen electroreduction (OER). A simple method is proposed for synthesizing OER catalysts based on polymer complexes of transition metals (nickel, palladium) with Schiff bases prepared by electrochemical polymerization of starting monomers. The OER catalysts are prepared by thermolysis of polymers in inert atmosphere. Their properties are characterized by the methods of cyclic voltammetry with the use of a rotating disk electrode. The surface state (the catalyst film density, the size and composition of particles) is controlled by scanning electron microscopy with X-ray microanalysis. The electrode demonstrates the high catalytic activity in the oxygen electroreduction reaction in alkaline solutions (higher than 750 mA/mg of the initial polymer mass).     

Electrocatalysis of the Oxygen Reaction by Pyropolymers of N4 Complexes

Results of research into structural and electrocatalytic properties of metalloporphyrins and metallophthalocyanines pyrolyzed on carbon supports of various dispersion degree in the oxygen electroreduction reaction (OER) are analyzed. The pyrolysis products (pyropolymers) that form at T 800° in inert atmosphere contain centers Co(Fe)–N surrounded by carbon particles. The oxygen electroreduction reaction on pyropolymers in acid and alkali solutions is studied on a model gas-diffusion electrode and a rotating ring–disk electrode. The slopes of Tafel plots in an acid solution are 60 and 120 mV. On a disk electrode covered with a pyropolymer, the intermediate product of OER, hydrogen peroxide, is fixed on the ring electrode throughout the entire range of OER potentials. The activity of pyropolymers in the hydrogen peroxide electroreduction reaction in an acid solution is insignificant. In an acid environment, OER occurs via a parallel–successive mechanism with a slow stage of the attachment of the first electron. In alkali media, slopes of Tafel plots equal 40 and 120 mV at low and high polarizations, respectively. The amount of hydrogen peroxide fixed on the ring electrode corresponds to 2–5% of the disk electrode current. A pyropolymer is active in the hydrogen peroxide reduction. The slow stage in OER in an alkali environment is the attachment of the second electron at a low polarization and the attachment of the first electron, at a high polarization. In acid and alkali solutions a pyropolymer is methanol-tolerant.     

Glucose Biosensor Based on Multi‐Walled Carbon Nanotube Modified Glassy Carbon Electrode

An amperometric glucose biosensor based on multi‐walled carbon nanotube (MWCNT) modified glassy carbon electrode has been developed. MWCNT‐modified glassy carbon electrode was obtained by casting the electrode surface with multi‐walled carbon nanotube materials. Glucose oxidase was co‐immobilized on the MWCNT‐modified glassy carbon surface by electrochemical deposition of poly(o‐phenylenediamine) film. Enhanced catalytic electroreduction behavior of oxygen at MWCNT‐modified electrode surface was observed at a potential of ?0.40 V (vs. Ag|AgCl) in neutral medium. The steady‐state amperometric response to glucose was determined at a selected potential of ?0.30 V by means of the reduction of dissolved oxygen consumed by the enzymatic reaction. Common interferents such as ascorbic acid, 4‐acetamidophenol, and uric acid did not interfere in the glucose determination. The linear range for glucose determination extended to 2.0 mM and the detection limit was estimated to be about 0.03 mM.     

Oxygen reduction reaction on polycrystalline gold. Pathways of hydrogen peroxide transformation in the acidic medium

The mechanism of oxygen electroreduction on polycrystalline gold is studied in the acidic medium. Hydrogen peroxide is the main reaction product. However, two potential regions can be singled out in which the oxygen electroreduction reaction proceeds by different pathways. The first region is the potential interval close to the steady-state potential. Here, the oxygen electroreduction virtually completely produces peroxide. The second interval is the potential range of considerable cathodic polarization values. In this case, peroxide can be reduced to water. The low energy of hydrogen peroxide adsorption on gold determines the considerable overpotential of peroxide reduction. It is shown that on the gold electrode surface, the catalytic decomposition of peroxide occurs. The use of the method of electrochemical impedance spectroscopy allows the peculiarities of the oxygen reaction associated with hydrogen peroxide transformations to be revealed. In the acidic medium, the reactions of consecutive reduction of oxygen through the intermediate formation of hydrogen peroxide and the catalytic decomposition of the intermediate product are shown to proceed simultaneously. The ratio of rate constants of electrochemical stages depends on the potential. The chemical decomposition is observed both near the steady-state potential and in the cathodic region where considerable electrochemical reduction of peroxide occurs.     

Nanosize catalysts based on carbon materials promoted by cobalt tetra(<Emphasis Type="Italic">para</Emphasis>-methoxyphenyl) porphyrin pyropolymer for oxygen electroreduction

We present the results of electrochemical and structural investigations of several carbon materials: carbon blacks AD 100 and XC 72, ultradisperse diamond (UDD), multiwalled nanotubes (MWNT), various types of filament-like carbon materials (CFC series), and similar carbon materials promoted with cobalt tetra(para-methoxyphenyl) porphyrin (CoTMPhP) pyropolymer (PP). The electrochemical studies were performed at room temperature in 0.5 M H₂SO₄ by using a rotating disk electrode (RDE), a rotating ring-disk electrode (RRDE) (a thin layer of test material was applied onto the disk electrode), and a floating electrode. Structural characterization of initial and promoted carbon materials involved the determination of specific surface area by the BET method and by the polarization capacitance from cyclic voltammograms, and the particle morphology and dimensions by the transmission electron microscopy (TEM) method. The study of kinetics and mechanism of oxygen electroreduction on carbon materials promoted with CoTMPhP PP showed that the catalysts based on carbon materials of CFC and UDD series possess high specific activity in this reaction and high selectivity with respect to oxygen reduction to water. These catalysts are superior to the catalysts, in which carbon blacks AD 100 and XC 72 are used as the supports, in the specific activity.     

Restructuring and redispersion of silver on SiO2 under oxidizing/reducing atmospheres and its activity toward CO oxidation

The effects of oxygen-hydrogen pretreatments of nanosilver catalysts in cycle mode on the structure and particle size of silver particles, and subsequently the activity of the catalyst toward CO oxidation (or CO selective oxidation in the presence of H2), are reported in this paper. Ag/SiO2 catalyst with silver particle sizes of ca. 6 approximately 8 nm shows relatively high activity in the present reaction system. The adopting of a cycle of oxidation/reduction pretreatment has a marked influence on the activity of the catalyst. Oxygen pretreatment at 500 degrees C results in the formation of subsurface oxygen and activates the catalyst. As evidenced by in-situ XRD and TEM, the following H2 treatment at low temperatures (100 approximately 300 degrees C) causes surface faceting and redispersing of the silver particles without destroying the subsurface oxygen species. The subsequent in-situ FTIR and catalytic reaction results show that CO oxidation occurs at -75 degrees C and complete CO conversion can be obtained at 40 degrees C over such a nanosilver catalyst pretreated with oxygen at 500 degrees C followed by H2 at 100 degrees C. However, prolonged hydrogen treatment at high temperatures (>300 degrees C) after oxygen pretreatment at 500 degrees C induces the aggregation of silver particles and also depletes so much subsurface oxygen species that the pathway of CO oxidation by the subsurface oxygen species is inhibited. Meanwhile, the ability of the catalyst to adsorb reactants is greatly depressed, resulting in a 20 approximately 30% decrease in the activity toward CO oxidation. However, the activity of the catalyst pretreated with oxygen at 500 degrees C followed by hydrogen treatment at high temperatures (>300 degrees C) is still higher than that directly pretreated with H2. This kind of catalytic behavior of silver catalyst is associated with physical changes in the silver crystallites because of surface restructuring and crystallite redispersion during the course of oxygen-hydrogen pretreatment steps.     

Preparation of Ag/HOPG model catalysts with a variable particle size and an in situ xps study of their catalytic properties in ethylene oxidation

The preparation of model silver catalysts supported on highly oriented pyrolytic graphite is described, and the effect of the Ag particle size on the catalytic ethylene oxidation into ethylene oxide, studied by in situ XPS and mass spectrometry, is considered. For a mean particle diameter of 8 nm, the adsorbed oxygen species characterized by an O 1s binding energy of 530.8 ± 0.2 eV (electrophilic oxygen) forms on the silver surface exposed to the ethylene-oxygen reaction mixture. Larger silver particles with a mean diameter of 40 nm additionally contain the adsorbed oxygen species characterized by an O 1s binding energy of 529.2 ± 0.2 eV (nucleophilic oxygen). The presence of both oxygen species on the surface of the larger particles ensures the formation of ethylene oxide, while the sample with the smaller silver particles is inactive in the epoxidation reaction. The O 1s signal at 530.8 eV is partly due to oxygen dissolved in the subsurface layers of silver.     

Assembly of two-dimensional ordered monolayers of nanoparticles by electrophoretic deposition

In this article the effect of field strength, temperature and square-wave pulse on the deposition structure of gold nanoparticles is investigated and 2D structures of silver and two kinds of rare-earth carbonate particles are synthesized by electrophoretic deposition (EPD). The results indicate that EPD is a general phenomenon that occurs on the electrode/sol interface and that the EPD method may be developed for the assembling of 2D structures of nanoparticles. On the other hand, the results also show that the composition and surface condition as well as the size distribution of the particles can affect the order of the particles in the monolayer. Received: 7 October 1999/Accepted: 25 January 2000     

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.     

Electroreduction of oxygen on copper-containing sulfide minerals

The state of the surface of and the oxygen electroreduction on the naturally occurring minerals bornite, chalcopyrite, and chalcosine are studied in borate buffer solutions using X-ray photoelectron spectroscopy, cyclic voltammetry, and the rotating disk electrode technique. The surface of the minerals in an oxygen-containing atmosphere is covered with compounds of copper and iron in the highest oxidation states, and the oxygen electroreduction occurs on these compounds. Electrocatalytic activity of sulfide minerals during cathodic polarization is presumably due to the participation in the oxygen electroreduction reaction of redox centers, i.e. ions of Fe(II) in bornite and chalcopyrite, and ions of Cu(I) bonded with the sulfide sulfur in each of the minerals.     

Influence of Pretreatment on the Interaction of Oxygen with Silver and the Catalytic Activity of Ag／SiO2 Catalysts for CO Selective Oxidation in H2

The interactions of oxygen with pre~reduced silver catalysts as well as their catalytic propertiesfor CO selective oxidation in H2 after oxygen pre-treatment are studied in this paper. It is found that the pretreatment exerts a strong influence on the activity and selectivity of the silver catalyst. A drop in activity and selectivity is observed after treating a pre-reduced catalyst with oxygen at low temperatures,whereas a converse result is obtained after an oxidizing treatment at high temperatures (T≥350℃). O2-TPD results show that surface oxygen species adsorbs on silver surface after the oxygen treatment at low temperatures. However, penetration of oxygen into the silver is enhanced by a high temperature treatment, meanwhile the surface oxygen species disappear. No other silver species except metallic silver are observed on all the catalysts by XRD, and the size of silver particle is not changed after the treatment with oxygen at low temperatures. The surface oxygen species formed by oxygen treatment can also be removed by hydrogen reduction. The strongly-adsorbed surface oxygen species prohibit the adsorption and diffusion of oxygen species in reaction gas on the surface of silver catalyst, causing the decrease in CO oxidation activity, in other words, it is important to obtain a clean silver surface for increasing the catalyst activity in CO removal from H2-rich feed gas. The differences in activity and selectivity due to the oxygen pretreatment at different temperatures axe discussed in terms of the changes in the surface/subsurface oxygen species of the silver particles.     

Electroreduction of cerium ions on silver electrode in halide melts at 973 K

The mechanism of electroreduction of cerium ions in equimolar KCl-NaCl melt is explored at 973 K. The effect of the anionic composition of the melt on the electroreduction of cerium ions is studied. It is shown that the electrodeposition of metal cerium from halide melts on a silver electrode is the primary electrochemical process that occurs at potentials more positive than those corresponding to the supporting-electrolyte decomposition. The electroreduction of chloride complexes of cerium on a silver electrode in the melt in both steady-and non-steady-state polarization modes at rates below V ≤ 0.5 V/s is limited by the diffusion delivery; at higher polarization rates, the charge-transfer stage predominates.     

氧在铁卟啉/聚吡咯膜修饰电极上的还原

用旋转环盘电极技术研究含中位四（ 对磺基苯基） 卟啉合铁（ＦｅＴＰＰＳ） 的聚吡咯膜覆盖的玻碳电极上的氧还原过程． 结果表明该修饰膜的存在降低了氧还原的过电位,还原产物中有Ｈ２Ｏ２ ,过程用异相氧化还原催化（ＥＣ） 机理解释． 与金或铂比较,碳是较好的电极基体材料,适合于产生破坏水中有机物所需的过氧化物． 在酸性介质中氧在该膜修饰电极上的还原速度比在中性介质中大,由ＫｏｕｔｅｃｋｙＬｅｖｉｃｈ 关系式求出修饰膜中催化剂和分子氧反应的表观速度常数值．     

Active Layer of an Oxygen Electrode Based on Nanocomposite Disperse Carbon Carrier + Laccase Material

Effect of the carbon material dispersivity on the efficiency of oxygen electroreduction by laccase immobilized on finely divided colloidal graphite (FCG) and carbon black AD-100 is studied. A highly active composite material based on FCG with laccase immobilized on it is proposed and investigated. This creates optimum conditions for direct bioelectrocatalysis by enzyme molecules. The specific oxygen reduction current calculated per enzyme molecule for nanocomposite FCG + laccase is five times that on an AD-100-based composite. Increasing the active-layer thickness, which is of importance for creating a gas-diffusion oxygen electrode, reduces specific activity of composite and only the activity of ultrathin layers is thickness-independent. This is explained by percolation restrictions on the electron transport, which reduce the number of catalytically active centers in the electrode's active layer that take part in reaction. The FCG particles are presumed to form agglomerates in the active layer. The size of the agglomerates is determined on the basis of computer-aided modeling of percolation processes and experimental data on the dependence of the specific capacitance of the active mass on the active-layer thickness. Hypotheses on the origin of percolation phenomena are put forth. One such hypothesis is that agglomerates of carbon particles are fractal clusters.     

The mechanism of electroreduction of nitrate ions on a hybrid electrode nanodispersed copper-MK-40 membrane

Based on a MK-40 sulfocation-exchange membrane, a hybride electrode material containing nanodispersed copper is prepared. The methods of scanning electron microscopy and X-ray diffraction (XRD) analysis reveal the formation of copper agglomerates measuring 250–470 nm and consisting of individual particles of 20–30 nm. The procedure of multistage chemical deposition of copper into the ion-exchange carrier makes it possible to obtain a continuous cluster of metal particles which determines the electron conducting properties of the resulting hybrid material. The electrochemical activity of the nanocomposite electrode is studied in the reaction of nitrate ion electroreduction. Nanodispersed copper deposited into the membrane is shown to intensify the electroreduction of nitrate ions by a factor of 1.5–2 as compared with a compact copper electrode. The electroreduction of nitrate ions on compact copper is shown to involve 6 electrons, whereas the electroreduction on the nanocomposite involves 8 electrons. The electroreduction products of nitrate ions are identified by the IR spectroscopy method.     

Electroenzymatic reactions with oxygen on laccase-modified electrodes in anhydrous (pure) organic solvent

The electroenzymatic reactions of Trametes hirsuta laccase in the pure organic solvent dimethyl sulfoxide (DMSO) have been investigated within the framework for potential use as a catalytic reaction scheme for oxygen reduction. The bioelectrochemical characteristics of laccase were investigated in two different ways: (i) by studying the electroreduction of oxygen in anhydrous DMSO via a direct electron transfer mechanism without proton donors and (ii) by doing the same experiments in the presence of laccase substrates, which display in pure organic solvents both the properties of electron donors as well as the properties of weak acids. The results obtained with laccase in anhydrous DMSO were compared with those obtained previously in aqueous buffer. It was shown that in the absence of proton donors under oxygenated conditions, formation of superoxide anion radicals is prevented at bare glassy carbon and graphite electrodes with adsorbed laccase. The influence of the time for drying the laccase solution at the electrode surface on the electroreduction of oxygen was studied. Investigating the electroenzymatic oxidation reaction of catechol and hydroquinone in DMSO reveals the formation of various intermediates of the substrates with different electrochemical activity under oxygenated conditions. The influence of the content of aqueous buffer in the organic solvent on the electrochemical behaviour of hydroquinone/1,4-benzoquinone couple was also studied.     

Surface State of a Silver Catalyst for Ethylene Glycol Oxidation

The surface state of electrolytic silver before and after treatment with a reaction mixture in the course of ethylene glycol oxidation to glyoxal was studied using X-ray photoelectron spectroscopy and scanning electron microscopy. It was found that electrophilic forms of adsorbed oxygen, which participate in the selective conversion of ethylene glycol, were formed on the surface of electrolytic silver crystals under exposure to oxygen under conditions similar to catalytic process conditions. The treatment of the catalyst with a reaction mixture resulted in the formation of filamentous carbon deposition products. A mechanism of formation of carbon-containing products was proposed.     

Electrodeposited noble metal particles in polyelectrolyte multilayer matrix as electrocatalyst for oxygen reduction studied using SECM

Taking the advantage of the stability and penetrability of polyelectrolyte films formed by layer-by-layer (LbL) deposition, noble metal particles of Pd and Pt were fabricated in a preformed polyeletrolyte multilayer film by galvanic deposition. The metal deposition occurred as metal particles and they were tested for their properties as electrocatalyst for oxygen reduction. Atomic force microscopy (AFM) was used to characterize the morphology of the particle films. The noble metal particles were investigated by cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) with respect to oxygen reduction. The results show that the electrocatalytic properties of the Pd particle film can be adjusted by the electrodeposition time. The hydrogen peroxide formed as an intermediate during electroreduction of dioxygen was conveniently measured in the SECM using the substrate-generation/tip-collection mode. The relevance of the main reduction pathways could be extracted from fitting the current transients to an analytical model.     

电泳沉积制备氧化石墨烯-银电极及其氧还原催化性能

通过在1-甲基-2-吡咯烷酮（NMP）中超声剥离氧化石墨制备出稳定的氧化石墨烯（GO）分散液,添加AgNO₃使氧化石墨烯吸附Ag+而带正电荷。采用电泳沉积法使GO沉积到阴极的玻璃碳电极上,Ag+被电化学还原为单质银,均匀的分散在GO片层当中。通过AFM、SEM、Raman、XRD及元素面扫分析对制备电极的形貌、结构进行表征。在碱性环境中进行氧还原测试,结果表明GO+Ag电极的氧还原起始电位较玻璃碳电极最大正移228mV,还原电流密度最大为7.564mA·cm^-2,是玻璃碳电极的3.4倍。通过不同转速下的线性扫描曲线绘制Koutechy-Levich图,计算氧还原反应的电子转移数为3.3。