Surface and Subsurface Oxygen on Platinum in a 0.5 M H2SO4 + 0.01 M CuSO4 Solution

The formation of a copper adatom layer on polycrystalline platinum in a copper sulfate solution is studied by cyclic voltammetry in different cycling ranges at 0.1 V s^–1. The copper adatom deposition kinetics is controlled by the following factors. The substrate's top layer structure during the oxygen exit onto the surface may be unstable at anodic limits E _a = 0.90–1.35 V. The concentration of copper oxides (active centers) may be higher at E _a = 0.8–0.95 V. The balance between different adsorption sites differs in different cycling conditions. Of importance is the number of complexes O_ss–Pt _n –SO₄ and O_ss–Pt _n –O_c, where O_ss is subsurface oxygen and O_c is chemisorbed oxygen.     

Surface and Subsurface Oxygen on Platinum in a 0.5 M H2SO4 Solution

The oxidation of polycrystalline platinum in 0.5 M H₂SO₄ is studied by cyclic voltammetry at potential scan rates of 5–500 mV s^–1 while varying the potential cycling range. The scheme, which is proposed for explaining the observed acceleration and deceleration of oxygen sorption at 0.75–1.0 V, accounts for the presence of oxygen in the subsurface layers of platinum (O_ss) and the formation of a barrier layer comprising complexes O_ss–Pt _n –SO₄. Cycling platinum secures certain steady-state contents of O_ss at 0.01–1.35 V. In an anodic scan, O_ss accumulates at E > 0.85 V (slow post-electrochemical stage) due to exchange of platinum and oxygen atom sites. In a cathodic scan, the desorption of most oxygen gives way to the adsorption of anions, which prevent residual O_ss from appearing on the surface. The residual O_ss disappears at E < 0.1 V after a sufficiently complete desorption of anions and the destruction of stable complexes O_ss–Pt _n –SO₄. Varying the potential cyclic limit leads, after a delay, to other steady-state O_ss contents.     

Surface and Subsurface Oxygen on Platinum in a Perchloric Acid Solution

The oxidation of polycrystalline platinum in perchloric acid is studied by cyclic voltammetry at a potential scan rate of 0.1 V s^–1 in various potential cycling ranges. The earlier model for the formation of a barrier layer of strong complexes consisting of subsurface oxygen O_ss, platinum atoms, and anions adsorbed on the latter is shown to correctly describe experimental results on the platinum oxidation in sulfuric and perchloric acids. The regularities in these acids are on the whole similar. A weaker adsorption of perchlorate anions as compared with bisulfate facilitates chemisorption of oxygen at 0.7–0.85 V and hinders exchange by sites Pt O at 0.85–1.35 V. A prolonged potential cycling with a cathodic limit of 0.27 V and low anodic limits leads to the accumulation of surface complexes O_ss–Pt _n –ClO₄, which hinder both the oxygen chemisorption and the exchange Pt O below 1 V. At more positive potentials, the complexes are destroyed and oxygen penetrates into subsurface platinum layers.     

Influence of the degree of surface oxidation of polycrystalline Rh electrodes on the underpotential deposition of Cu

The underpotential deposition of copper onto polycrystalline rhodium was studied as a function of the degree of oxidation of the electrode surface in acidic media using potentiodynamic techniques. Surface oxidation of the rhodium electrode was carried out using a triangular sweep potential between E _L (lower limit) and E _U (upper limit: 0.94≤E _U≤1.4 V). Cu electrodeposition was performed at the same time as the total or partial reduction of the oxidized species. The surface oxides produced at E _U≤1.09 V were completely reduced during Cu electrodeposition. In this case, the potentiodynamic I-E patterns for oxidative dissolution of Cu were characterized by three anodic peaks located at 0.41 V (peak I), 0.47 V (peak II) and 0.59 V (peak III) and the coverage degree by Cu, θ_Cu, was on the order of a monolayer. Surface oxides produced at E _U>1.09 V were partially reduced during the copper electrodeposition. In this case, the I-E profiles exhibited only two anodic peaks (II and III) and θ_Cu was <1. The Rh-oxygen species that remain on the electrode surface block the active sites of lower energy and modify the binding energy of strongly adsorbed Cu. Electronic Publication     

Properties of Adsorbed Oxygen Forms on a Defective Ag(111) Surface. DFT Analysis

A cluster model of an Ag12–3O (AS_V) adsorption center using layered silver oxide as a prototype is proposed. The model includes a cation vacancy V on the Ag(111) surface and oxide type subsurface oxygen atoms O_ox. Density functional theory (DFT) (B3LYP/LANL1MB approximation) is used to analyze the electronic structure of AS_V and oxygen adsorption on this center, AS_V+O AS–O. As shown by the calculations, the adsorbed oxygen is associated with the subsurface oxygen atoms O_ss to form structures similar to metal ozonides — Ag–O_ss–O_ep–O_ss–Ag–O_ox–Ag, containing electrophilic oxygen O_ep along with the oxide oxygen O_ox. The optical spectra of the AS_V and AS–O centers were calculated by the configuration interaction method with single excitations (CIS). For AS_V, the most intense absorption bands were obtained in the region 500-700 nm. Oxygen association is accompanied by a sharp decrease in spectrum intensity in the range 600-700 nm and an increase in the intensity of the peak at 500 nm. Vibration frequencies and (IR) intensities were determined for the AS_V and AS–O centers. The AS_V center exhibits a characteristic spectrum in the region 350-500 cm^–1, which corresponds to the frequency spectrum of the surface oxide Ag₂O. For associated oxygen forms (AS–O center), the calculations predict additional peaks around 980, 640 and 230 cm^–1. These peaks are due to the vibrations of the O_ss–O_ep–O_ss structural unit, stabilized at the cation vacancy.     

Electrooxidation of Organic Substances on Platinum Modified with Oxotungstate Adlayers

The behavior of a platinized platinum electrode (roughness factor 500) in 0.5 M H₂SO₄ containing Na₃PW₁₂O₄₀ (PW₁₂) is studied by cyclic voltammetry. It is established that the effect of the PW₁₂ additive on the adsorption of hydrogen and oxygen is similar to the influence exerted by specifically adsorbed anions: a predominant displacement of strongly-bound hydrogen and a heavy suppression of oxygen adsorption are observed. It is shown that the regularities of oxidation of methanol, ethylene glycol, and formic acid at platinized platinum undergoe substantial alteration in the presence of PW₁₂. In the region of potentials E _r = 0.6–0.8 V, where the limiting stage of these processes is the adsorption of organic molecules, the oxidation rates substantially decrease. The run of steady-state polarization curves in the region E _r = 0.40–0.55 V depends on the direction of potential change and the extreme value of E _r. With E _r varied from 0.6–0.8 V in the direction of lower values in the interval E _r < 0.45–0.50 V, the oxidation processes accelerate as compared with the rates in the absence of PW₁₂. The acceleration occurs if conditions are created for a preliminary adsorption of the tungstate.     

Synthesis of unsymmetrical compartmental oxime nickel(II) and copper(II) complexes: spectral, electrochemical and magnetic studies

A new series of binuclear unsymmetrical compartmental oxime complexes (1–5) [M₂L] [M=Cu(II), Ni(II)] have been synthesized using mononuclear complex [ML] (L=1,4-bis[2-hydroxy-3-(formyl)-5-methylbenzyl]piperazine), hydroxylamine hydrochloride and triethylamine. In this system there are two different compartments, one has piperazinyl nitrogens and phenolic oxygens and the other compartment has two oxime nitrogens and phenolic oxygens as coordinating sites. The complexes were characterized by elemental and spectral analysis. Electrochemical studies of the complexes show two step single electron quasi-reversible redox processes at cathodic potential region. For copper complexes E¹ _pc=−0.18 to −0.62 and E² _pc=−1.18 to −1.25 V, for nickel complexes E¹ _pc=−0.40 to −0.63 and E² _pc=−1.08 to −1.10 V and reduction potentials are sensitive towards the chemical environment around the copper and nickel atoms. The nickel(II) complexes undergo two electrons oxidation. The first one electron oxidation is observed around +0.75 V and the second around +1.13 V. ESR Spectra of the binuclear copper(II) complexes [Cu₂L](ClO₄), [Cu₂L(Cl)], [Cu₂L(NO₃)] shows a broad signal at g=2.1 indicating the presence of coupling between the two copper centers. Copper(II) complexes show a magnetic moment value of μ_eff around 1.59 B.M at 298 K and variable temperature magnetic measurements show a −2J value of 172 cm⁻¹ indicating presence of antiferromagnetic exchange interaction between copper(II) centres.     

Initial Stages of Copper Electrocrystallization on a Glassy-Carbon Ring–Disk Electrode from Sulfate Electrolytes of Various Acidity: A Cyclic Voltammetry Study

Initial stages of copper electrocrystallization on glassy carbon from sulfuric acid electrolytes of pH 0.3 and 3.7 are studied by the cyclic voltammetry method on rotating and stationary ring–disk electrode. The rate of nucleation and growth of a metallic phase of copper in a 0.5 M Na₂SO₄ + 0.01 CuSO₄ (pH 3.7) solution is marginally higher than in a 0.5 M H₂SO₄ + 0.01 M CuSO₄ acid electrolyte (pH 0.3). Regularities governing the multistage discharge of copper ions, the formation of the new phase nuclei, and the deposit dissolution are analyzed. No copper adlayers form on glassy carbon at potentials more positive than the equilibrium potential of a reversible copper electrode, the copper nucleation occurs via the Volmer–Weber mechanism. The oxygen-containing surface groups of glassy carbon (quinone–hydroquinone, carbonyl, etc.) are probably active centers for the discharge of copper ions and the nucleation of the new phase. The results of the study are compared with the data on the kinetics of copper electrocrystallization on a platinum electrode.     

Initial stages of copper electrocrystallization on polycrystalline platinum and glassy carbon in the presence of acetonitrile

The kinetics of formation of copper adlayer, three-dimensional nucleation, and the deposit growth on polycrystalline platinum and glassy carbon in the 0.5 M H₂SO₄ + 10 mM CuSO₄ + (0–2) M acetonitrile (AcN) solutions at the cathodic overvoltages is studied using the methods of cyclic voltammetry and potentiostatic current transients on a ring-disc electrode. At [AcN] = 2 M, the process of formation of copper adatoms on platinum is significantly retarded. In the solutions with high contents of AcN, the processes of Cu⁺ ion production, the formation of their complexes with acetonitrile, hydrogen evolution, copper nucleation, and the deposit growth proceed in parallel. The contribution of any process to the overall current depends on the amount of adsorbed AcN at the surface of substrate and copper deposit and on the electrode potential. At [AcN] = 2 M, an increase in the cathodic overvoltage to 0.32 V leads to an abnormal increase in the current of Cu⁺ ion production on platinum, which is caused by insufficiently rapid formation of copper atoms in the reduction of Cu⁺(AcN) _x complexes.     

Migration Effects in Reversible System [Fe(CN)6]3–/4– on a Microelectrode

Migration factors Y are determined for an [Fe(CN)₆]^3–/4– reversible system at a platinum microelectrode 10 m in radius by measuring anodic and cathodic limiting currents at different concentrations of the redox forms in solution, from which Y _a and Y _c are calculated. The obtained results agree with theory. The experimental error for the Y _a/Y _c ratio is smaller than for each factor taken separately.     

Infrared studies of the effect of acetylene, hydrogen, and oxygen on CO adsorption on platinum hydrosols

Infrared spectra of CO-treated platinum hydrosols subsequently treated with acetylene, hydrogen, and oxygen reveal that v(CO)_ads decreases from 2070 cm⁻¹ with increasing gas-treatment time. This has been attributed to a reduction in the coverage of adsorbed CO. In Pt sol/CO/C₂H₂ systems, v(CO)_ads decreases to a limiting value of ca. 2060 cm⁻¹ after exposure to acetylene. In the Pt sol/CO/H₂ systems, v(CO)_ads decreases to ca. 2050 cm⁻¹ after exposure to hydrogen gas. The lower frequency in the Pt sol/CO/H₂ system has been attributed to CO adsorption on more active metal sites formed from the reduction of surface platinum oxides. Exposure of the CO-treated platinum hydrosols to O₂ gas was found to cause the eventual disappearance of the v(CO)_ads band in infrared spectra, which was attributed to oxidation of adsorbed CO to CO₂ by weakly bound surface layers of platinum oxides formed by the oxygen treatment.     

Behavior of Porous Titanium and Catalytically Active Electrodes Based on It in Acidic Chloride Solutions

The behavior of porous titanium and electrodes based on it, which are activated with Pt, Au, RuO₂, Co₃O₄, and MnO₂, in 20-% LiCl solution (pH –0.4 to –0.5) is studied. On porous titanium in the potential ranges 0.1 < E< 0.5 and 0.5 < E< 1.1 V (NHE), the formation of titanium hydrides and passive oxide layers, respectively, is observed; the processes decay with time. In the ranges E< 0.1 and E> 1.1 V, the dissolved oxygen reduction and chlorine evolution, respectively, are observed on porous titanium at high overpotentials. On porous titanium activated with thin-layer Pt, Au, and RuO₂coatings, the functional Evs. pH dependence, which is typical for these electrocatalysts, breaks down due to the conjugate reactions of titanium oxidation. On porous titanium activated with Co₃O₄and MnO₂, at pH below unity, chlorine evolution is observed; its rate is limited by the chlorine mass transfer into the bulk solution. Under a gas-diffusion control, the chlorine evolution rate is determined by the diffusion of absorbed hydrogen chloride. The conditions of application of porous titanium as the support for catalytically active electrodes of electrochemical sensors in acidic chloride solutions are considered.     

Formation of β- and β″-Al2O3 by the solid state reaction between NaAlO2 and γ-Al2O3

The solid state reaction of NaAlO₂ with γ-Al₂O₃ was investigated kinetically. Powdered compacts with various compositions (Al₂O₃/NaAlO₂ = 1–5) were fired at 700–1200°C for 1–768 hr. The amounts of the reaction product were determined by peak heights of X-ray diffraction patterns. β″-Al₂O₃ was formed predominantly from the sample with Al₂O₃/NaAlO₂ = 2. The firing time for the β″-Al₂O₃ formation was shortened as the firing temperature was raised, and the activation energy, E_a, for formation was about 130–135 kcal/mole. The sample of Al₂O₃/NaAlO₂ = 5 formed m-Al₂O₃ with the mullite structure and was observed to transform gradually to β-Al₂O₃. E_a for the m-Al₂O₃ formation and for the transition were about 55–60 and 40 kcal/mole, respectively, which resulted in E_a of about 95–100 kcal/mole for the β-Al₂O₃ formation. The mechanism of the m-Al₂O₃ formation is discussed briefly.     

Initial Stages of Copper Electrocrystallization on a Glassy-Carbon Ring–Disk Electrode from Sulfate Electrolytes of Various Acidity: Potentiostatic Current Transients

Initial stages of copper electrocrystallization on glassy carbon from sulfuric acid electrolytes of pH 0.3 and 3.7 are studied by the method of potentiostatic current transients on rotating and stationary ring–disk electrode. The rate of copper deposition in a 0.5 M Na₂SO₄ + 0.01 CuSO₄ (pH 3.7) solution is marginally higher than in a 0.5 M H₂SO₄ + 0.01 CuSO₄ acid electrolyte (pH 0.3) at the expense of adsorption of sulfate and hydroxide ions on the substrate surface and the copper crystals. Regularities governing the multistage discharge of copper ions, the formation of the new phase nuclei, and the deposit dissolution are analyzed. The results of the study are compared with the data on the kinetics of copper electrocrystallization on a platinum electrode. The acceleration of the copper deposition on glassy carbon in the acid solution of pH 0.3, as compared with platinum, is due to accelerated discharge of copper ions and increased number of univalent copper ions in the near-electrode layer of solution. The oxygen-containing surface groups of glassy carbon (quinone–hydroquinone, carbonyl, etc.) are probably active centers for the discharge of copper ions and three-dimensional nucleation.     

Reactions of nitrogen oxides (NO,N2O) with the surface of a copper-chromium oxide catalyst

Weight-space and IR spectroscopic methods have been used to investigate the reaction of NO and N₂O with the surface of a copper-chromium oxide catalyst in the form of copper chromite with a 20% excess of Cr₂O₃. Comparisons have been made between the relative reactivities of the different oxides (NO, N₂O, O₂) in oxidation of the Cu⁺ and Cu^o surface centers. The role of these centers in oxidation of CO by oxygen and by nitrogen oxides is discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 3, pp. 338–343, May–June, 1985.     

Space-time chaos in reactions of NADH with atmospheric oxygen in the presence of copper(II) complexes

In the oxidation of aqueous solutions of NADH by atmospheric oxygen in the presence of certain copper(II) compounds, an irregularly oscillating timewise change has been observed in the potential of a point platinum electrode (PPE), relative to either a silver chloride electrode or a second PPE located in the same solution. Phase portraits have been constructed on the basis of experimental data for all of the systems that have been investigated. It has been shown that the existence of chaotic oscillations, and also the form of the phase portrait of the systems, depend on the magnitude of the redox potential of the CuL²⁺/CuL⁺ pair. For compounds with E₀=0.60–0.64 V, the phase portrait has the form of a screw-type chaos, which then changes over to Lorentz-type chaos. Only for this group of complexes is chaotic behavior observed when the solution is stirred. For compounds with E₀=0.15–0.41 V, screw-type chaos is observed; and for compounds with E₀<0, no chaotic behavior has been detected.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 26, No. 2, pp. 175–184, March–April, 1990.     

Kinetics of underpotential deposition and nucleation of copper on the Pt(111) face in the presence of acetonitrile

The kinetics of underpotential deposition, three-dimensional nucleation, and growth of copper deposits at cathodic overpotentials on a Pt(111) electrode in solutions containing 0.5 M H₂SO₄, 10 mM CuSO₄, and 0–200 mM acetonitrile (AcN) is studied by the cyclic voltammetry, potentiostatic current transients, and scanning probe microscopy methods. At low volume concentrations of acetonitrile ([AcN] ≤ 4 mM), adsorbed acetonitrile molecules accelerate the formation of a co-adsorption lattice of copper adatoms with anions due to local electrostatic effects at the charged interface. At higher concentrations, the underpotential deposition process is hampered, but the desorption of copper adatoms occurs at potentials more positive than those at low acetonitrile concentrations. This effect is attributed to a stabilizing action of acetonitrile molecules situated on the layer of copper adatoms and, in part, on platinum. At [AcN] = 0.4–40 mM, adsorbed acetonitrile molecules accelerate the growth of the bulk copper deposit, but the nucleation stage is hindered. The dependence of the copper amount on the deposition potential at [AcN] = 40 mM exhibits a maximum at 0.15–0.17 V. This effect was previously observed in weakly acid solutions (pH 1.7–3.0) containing no acetonitrile. The maximum rate of the deposit growth corresponds to an optimum number of crystallites (which is not too great) and an optimum distance between the growing centers in conditions of mixed kinetics “diffusion + electron transfer.” A substantial number of complexes Cu(I)-AcN forms at high acetonitrile concentrations.     

Potentiostatic voltammetry at spherical electrodes and microelectrodes in the presence of product

An explicit analytical solution for the complete anodic–cathodic I–E–t response of a reversible electrode process in presence/absence of amalgamation under potentiostatic conditions is presented. To obtain this solution we have taken into account the electrode curvature, using in the case of amalgamation, Koutecký’s approximation (i.e. the finite electrode volume has not been considered). Explicit expressions for the concentration profiles and the surface concentrations have been also deduced. All the results are applicable to electrodes of any radius including ultramicroelectrodes when both species are soluble in the electrolytic solution for any value of the ratio D_O/D_R. When amalgamation takes place, analytical results were compared with the numerical ones deduced by using the rigorous condition of null net flux in the electrode centre, pointing out that Koutecký’s approximation remains valid even for electrode radius and time values at which the diffusion layer reaches the electrode centre. For high electrode sphericities the I–t curves present a cross-linking for applied potential values higher than the equilibrium potential which is the cause of the appearance of a “peak” near to the anodic limit of the I–E curve.     

Voltammetry of immobilized cytochrome c on novel binary self-assembled monolayers of thioctic acid and thioctic amide modified gold electrodes

Horse heart cytochrome c (cyt c) was adsorbed on the binary self-assembled monolayers (SAMs) composed of thioctic acid (T-COOH) and thioctic amide (T-NH₂) at gold electrodes via electrostatic interaction. The cyt c adsorbed on the modified gold electrode exhibited well-defined reversible electrochemical behavior in 10 mM phosphate buffer solution (PBS, pH 7.0). The surface concentration (Γ) of electroactive species, cyt c, on the binary SAMs was higher than that in single-component SAMs of T-COOH, and reached a maximum value of 9.2 × 10⁻¹² mol cm⁻² when the ratio of T-COOH to T-NH₂ in adsorption solution was of 3:2, and the formal potential (E^0′=(E_pa+E_pc)/2) of cyt c was −0.032 V (vs. Ag|AgCl (3 M NaCl)) in a 10 mM PBS. The interaction between cyt c and the binary SAMs made the E^0′ shift negatively when compared with that of cyt c in solution (+0.258 V vs. NHE, i.e., +0.058 V vs. Ag|AgCl (3 M NaCl)). The fractional coverage of bound cyt c was a 0.64 theoretical monolayer. The standard electron transfer rate constant of cyt c immobilized on the binary SAMs was also higher than that on single-component SAMs of T-COOH, and the maximum value of 15.8 ± 0.6 s⁻¹ was obtained when the ratio of T-COOH to T-NH₂ in adsorption solution was at 3:2. The results suggest that the electrode modified with the binary SAMs functions better than the electrode modified with single-component SAMs of T-COOH.     

Direct electrochemistry of myoglobin in a room temperature ionic liquid aqueous solution and its catalysis to H<Subscript>2</Subscript>O<Subscript>2</Subscript>

Using 1-ethyl-2-methylimidazolium trifluoroacetate (EMImTfa) as the supporting electrolyte, a couple of well-defined and reversible redox peaks of Myb could be observed at the basal plane graphite (BPG) electrode through direct electron transfer between the protein and the BPG electrode, whose anodic and cathodic peak potentials were at −0.098 V and −0.144 V vs. Ag | AgCl, respectively. Both anodic and cathodic peak currents increased linearly with the potential scan rates. Compared with the supporting electrolyte of phosphate buffer solution, EMImTfa played an important role for the direct electron transfer between Myb and the BPG electrode. Further investigation suggested that Myb was adsorbed tightly on the surface of the BPG electrode in the presence of EMImTfa to form a stable, approximate monolayer Myb film. Myb adsorbed on the BPG electrode surface could retain its biological activity and showed a remarkable electrocatalytic activity for the reduction of H₂O₂ in an EMImTfa aqueous solution. Based on these, a third-generation biosensor could be constructed to directly detect the concentration of H₂O₂ in EMImTfa aqueous solution with a limit of detection of 3.24 × 10⁻⁸ M. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 3, pp. 363–368. The text was submitted by the authors in English.