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.     

Peculiarities of copper underpotential deposition and nucleation on polycrystalline platinum in the presence of acetonitrile: Rotating ring-disk electrode

The initial stages of copper electrocrystallization on polycrystalline platinum in 0.5 M H₂SO₄ + 10 mM CuSO₄ + 0–200 mM acetonitrile (AcN) solutions are studied by the methods of cyclic voltammetry and potentiostatic current transients on a ring-disk electrode. Adsorbed AcN molecules accelerate both the underpotential deposition and the bulk deposition of copper due to the local electrostatic effects on the charged interface. With the increase in the additive concentration in solution, the contribution of the production of copper ions Cu⁺ is observed to increase due to the formation of Cu(AcN) _x ⁺ comlexes, particularly, for [AcN] ≥ 4 mM when the concentrations of acetonitrile and copper sulfate become comparable. In the presence of AcN, as well as in the copper sulfate supporting electrolyte, the adatomic layer is formed via the mechanism of the two-dimensional growth of Cu(1 × 1) phase islets.     

Initial stages of the copper electrocrystallization from a sulfuric acid electrolyte: Chronoamperometry at a platinum ring-disk electrode

Initial stages of the copper electrocrystallization on platinum from a sulfuric acid electrolyte are studied by measuring potentiostatic current transients (the chronoamperometry method) on a rotating and stationary ring-disk electrode. The number of active centers and the copper nucleation rate are shown to substantially depend on the electrochemical pretreatment of the electrode. The mechanism governing the formation of intermediate species (ions Cu⁺) during the nucleation of a new copper phase and the deposit dissolution is analyzed. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

Surface and Subsurface Oxygen on Platinum in a Copper Perchlorate Solution

The formation of an adatom layer on polycrystalline platinum and the three-dimensional nucleation of copper in a copper perchlorate solution are studied by cyclic voltammetry at 0.1 V s^–1 while varying potential ranges and by recording potentiostatic current transients. About 0.6 monolayers of copper adatoms are deposited when cycling with anodic limit E _a = 1.35 V, the process is slower than that in an acid sulfate solution. Decreasing E _a accelerates the process (nearly one monolayer forms for E _a = 0.80–0.95 V in a cathodic scan) due to an increased number of active centers (metastable copper oxides) and, probably, to a change in the platinum surface microstructure. Oxygen for copper oxides is presumably supplied by water molecules adsorbed on a monolayer of copper adsorbed atoms and by subsurface oxygen (O_ss), which appears on the platinum surface after the destruction of complexes O_ss–Pt _n –ClO₄. Both the copper nucleation and the deposit growth accelerate at higher concentrations of copper oxides, which form at low E _a. High cathodic overvoltages decrease the number of active crystallization centers due to reduction or removal of copper oxides.     

Copper Ion Catalyzed Electroreduction of Carbon Monoxide on Iridium Electrode

Substantial cathodic currents are observed on iridium electroplates in 0.5 M H₂SO₄ + (0.001–0.005 M) CuSO₄ + CO (sat) at potentials positive with respect to the Cu/Cu²⁺ equilibrium potential. These currents are shown to correspond to electroreduction of both CO and Cu²⁺ ions to Cu⁺, where the latter ions are formed in amounts indicating the formation of complexes. Among the products of CO reduction, methanol and formaldehyde are identified. A possible mechanism of CO reduction is discussed. It is assumed that both copper adatoms and Cu⁺ ions can play the role of intermediates in the mediator catalysis.__________Translated from Elektrokhimiya, Vol. 41, No. 7, 2005, pp. 804–809.Original Russian Text Copyright © 2005 by Podlovchenko, Gladysheva.     

Unusual electrochemical reduction of copper(II) to copper(I) in polyoxotungstates

The electrochemical behaviour of the copper-substituted Keggin-type and sandwich-type polyoxotungstate anions of the compounds α-[(C₄H₉)₄N]₄H[PW₁₁Cu^IIO₃₉] and α-B-[(C₄H₉)₄N]₇H₃[Cu^II₄(H₂O)₂(PW₉O₃₄)₂] was studied by cyclic voltammetry in acetonitrile. In both cases two copper 1-electron reduction waves were detected in the cathodic scan. The first one was due to the reduction of one Cu^II to Cu^I in the polyoxoanion and the second one to the consecutive reduction of the preformed Cu^I to Cu⁰, with the consequent deposition/adsorption of the ejected metal atom at the glassy carbon electrode surface. In the anodic scan, Cu⁰ was re-oxidised with regeneration of the initial copper(II) complexes, via a Cu^I intermediate. The observed two-step reduction of copper(II) to copper(0) and the formation of intermediate species containing copper(I) is here reported for the first time for copper substituted polyoxotungstates. The co-ordination of the acetonitrile molecules to the copper ions must play a role in the formation of the copper(I) species, which are not detected in aqueous solution.     

Electrochemical Processes at a Platinum Electrode in CuSO4 Solutions in the UPD Region and at Initial Electrocrystallization Stages: Effect of Polyethylene Glycol

Effect of polyethylene glycol (PEG; m.w. 3000) on the Cu_ad deposition at smooth platinum in 0.5 M H₂SO₄ + 0.1 M CuSO₄ solutions and on the initial copper electrocrystallization stages is studied. According to cyclic voltammetry, PEG barely affects the Cu_ad formation kinetics. Adding PEG hampers dissolution of deposited copper. As follows from chronoamperograms, PEG makes no impact on the nucleation character (progressing nucleation) but hampers the copper nucleation process.     

Effect of silver ions on codischarge of copper and silver at a platinum electrode at the copper underpotentials

Codischarge of copper and silver ions at potentials more positive than the equilibrium potential of the Cu⁺²/Cu system is studied by the voltammetry technique. At the Ag⁺ concentrationc below 10^-7 M, silver ions make no noticeable impact on the formation of a Cu_ad monolayer but decelerate that of supermonolayers of Cu at almost equilibrium potentials. Atc ≥ 4 × 10^-5 M, voltammograms indicate the concurrent adsorption of Cu_ad and Ag_ad and the nucleation and deposition of phase Ag     

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.     

Electrochemically driven reversible solid state metal exchange processes in polynuclear copper complexes

The electrochemical characteristics of polynuclear di-copper and tetra-copper complexes of an expanded "Robson-type" macrocyclic ligand are explored by solid state voltammetry in aqueous media. When adhered to a graphite electrode surface in the form of microcrystalline powders and immersed in aqueous buffer solution, these water-insoluble polynuclear copper complexes show well-defined voltammetric reduction and re-oxidation responses. The di-copper metal complexes [Cu₂(H₃L)(OH)][BF₄]₂ and the tetra-copper complexes [Cu₄(L)(OH)][NO₃]₃ with an O₄N₄ octadentate macrocyclic ligand L are shown to exhibit inter-related and proton concentration sensitive solid state voltammetric characteristics. At sufficiently negative potential, copper is extracted from the complexes to form a solid copper deposit and the neutral form of the insoluble free ligand. Upon re-oxidation of the copper deposit, Cu²⁺ undergoes facile re-insertion into the ligand sphere to re-form solid di- and tetra-copper complexes at the electrode surface. The reduction process occurs in two stages, with two Cu²⁺ cations being extracted in each step. The ability of the macrocyclic ligand to efficiently release and accumulate copper is demonstrated. Electronic Publication     

Effect of Temperature on Oxide Formation in Ligand-Deficient Cu|Cu(II), Ethylenediamine System

The influence of temperature on formation of oxide layers on copper electrode in solutions containing 0.01 M Cu(II), 0.005 M ethylenediamine, and 0.3 M K₂SO₄ as a supporting electrolyte at pH 5.3 is investigated. The rate of net process Cu + Cu²⁺ + H₂O Cu₂O + 2H⁺ proceeding under open-circuit conditions is supposedly controlled by interaction between copper electrode and Cu²⁺ aqua-ions. Well-defined voltammetric peak is observed at –0.75 V (SHE), the height of which may serve as a measure of Cu₂O formation rate. An activation energy and a formal rate constant of the process are found to equal 30 kJ mol^–1 and 0.17 s^–1.     

Investigation of the anodic decomposition of the solid electrolyte RbCu4Cl3I2

The process of electrochemical decomposition of the solid electrolyte RbCu₄Cl₃I₂ at a vitreous carbon electrode has been investigated. The anodic decomposition of the electrolyte occurs in two steps. At first, the oxidizing electrode reaction of Cu⁺ ions to Cu²⁺ ions takes place at potentials higher than 0.57 V and a layer of decomposition products is formed on the electrode surface, including the divalent copper compound RbCuCl₃. Then the oxidizing reaction of I^– ions occurs at potentials higher than approximately 0.67 V, with deposition of an iodine layer onto the electrode surface. The deposition rate of the layers of decomposition products is controlled by instantaneous nucleation and two-dimensional growth of the deposit. The total thickness of the passivating layer of decomposition products on the anode is equal to ca. 1 μm. Electronic Publication     

Contribution from the ion-exchange factor to the potential of a copper-containing electron-ion exchanger

The process of formation of the electrode potential of EI-21 electron-ion exchanger, composed of ultrafine copper particles and KU-23 sulfocationite, was studied. The potentials of a EI-21 powdery electrode with a platinum lead in copper(II) sulfate solutions of various concentrations (0.005–1.0 M) were measured using currentless-mode potentiometry. The potential of this electrode first shifted by 0.02–0.15 V in the negative direction with respect to a compact copper electrode, after which the shift eventually decreased to ?0.010 ± 0.003 V. It was demonstrated that the time evolution of the potential is determined by the interplay of electron and ion exchange. When EI-21 is placed onto a platinum lead, the role of the potential-determining reaction passes from Cu²⁺ + e^? ? Cu⁺ to Cu²⁺ + 2e^? ? Cu. At the same time, H⁺-Cu²⁺ ion exchange gives rise to a change in the ratio of the concentration of copper(II) ions in the internal and external solutions. The Donnan potential, which arises at the boundary between the electron-ion exchanger and the external solution, maintains a high concentration of copper(II) ions in the internal solution, a factor that facilitates the recrystallization of the particle distributed over the bulk of the exchanger. The process of recrystallization slows down with time to such an extent that the electrode potential stops changing, remaining at a level close to the equilibrium potential of the Cu²⁺/Cu pair. It was concluded that the internal stability of the system makes the potential of the EI-21 electrode sensitive to the dispersity of the metal component and the concentration of potential-determining metal ions in the external solution.     

Electrochemical formation of a cesium–tin alloy in an amide-type ionic liquid

Electrochemical formation of cesium–tin alloys in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing 0.5 M CsTFSA has been studied. Cathodic decomposition of BMPTFSA on a platinum electrode was suppressed by addition of CsTFSA, suggesting that Cs⁺ accumulated on the electrode surface and hindered the reduction of BMPTFSA. Multiple cathodic current peaks were observed on a tin electrode in the ionic liquid. Energy-dispersive X-ray analysis and X-ray diffraction results suggested formation of cesium–tin alloys after potentiostatic cathodic reduction on the tin electrode at room temperature.     

Cu(II) and Cu(I) complexes with 2-(3,5-diphenyl-1H-pyrazole-1-yl)-4,6-diphenylpyrimidine: Synthesis and structure. Catalytic activity of Cu(II) compounds in reaction of ethylene polymerization

The Cu(II) and Cu(I) complexes with 2-(3,5-diphenyl-1H-pyrazole-1-yl)-4,6-diphenylpyrimidine (L) of the composition CuLX₂ (X = Cl, Br) and CuL(MeCN)Br are synthesized. According to X-ray diffraction data, the complexes have molecular structures. The molecules L are coordinated to the copper atom in bidentate-cyclic mode, i.e., through the N² atom of pyrazole and N¹ atom of pyrimidine rings. The coordination polyhedron of the Cu²⁺ ion in CuLX₂ compounds is completed to a distorted tetrahedron with halide ions, that of the Cu⁺ ion in CuL(MeCN)Br compounds, with the bromide ion and the nitrogen atom of acetonitrile molecule. The CuLX₂ complexes (X = Cl, Br) in combination with cocatalysts (methylaluminoxane and triisobutylaluminium) exhibit catalytic activity in ethylene polymerization.     

Solvation of ions,part XXX. Thermodynamics of transfer of copper ions from water to solvent mixtures

Free energies and Walden products show that the Cu⁺ and Ag⁺ ions are specifically solvated by acetonitrile (AN), 2-hydroxycyanoethane (HAN), and by pyridine (Py) whereas Na⁺ is specifically solvated by water, in mixtures of these organics with water. The Cu²⁺ ion is specifically solvated by pyridine in pyridine-water mixtures, but by water in acetonitrile-water mixtures. Ion-solvent, coordinated solvent-bulk solvent, and solvent-solvent interactions produce large entropy losses in the order Cu⁺Ag⁺>Na⁺ for transfer of these ions from water to dilute acetonitrile-water. The metallurgically important oxidations of copper, silver, CuS and Cu₂ with CuSO₄ in water are strongly favored in an enthalpic and free energy sense by the addition of acetonitrile, but addition of acetonitrile also produces a large loss of entropy for the reactions.     

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.     

Copper(II) ion sensor based on electropolymerized undoped conducting polymers

A solid state copper(II) ion sensor is reported based on the application of electropolymerized undoped (neutral) polycarbazole (PCz) and polyindole (PIn) modified electrodes. The new sensor shows high selectivity to Cu²⁺ ions with a detection limit of 10^–5 M. PCz and PIn are formed respectively by the anodic oxidation of 50 mM carbazole and 5 mM indole monomers in dichloromethane containing 0.1 M tetrabutylammonium perchlorate on a platinum electrode using a single compartment cell. Potentiostatic polymerization of both the monomers are carried out at 1.3 V and 1.0 V vs. Ag/AgCl, respectively. Perchlorate ions were electrochemically removed from the polymer films by applying – 0.2 V vs. Ag/AgCl. Polymer-coated electrodes are incubated in 1 M KCl solution for 8 h followed by incubation in distilled water for 2 h before using as a metal ion sensor. The undoped PCz and PIn electrodes were found to be highly selective and sensitive for Cu²⁺ ions with little selectivity for Pb²⁺ and negligible response towards Ag⁺, Hg²⁺, Cu⁺, Ni²⁺, Co²⁺, Fe²⁺, Fe³⁺ or Zn²⁺. Potentiometric responses for Cu²⁺ ions are recorded for both the sensor electrodes together with a double-junction Ag/AgCl reference electrode. Calibration curves for Cu²⁺ are reported for both ion sensors. The polymer-modified electrodes were found to be stable for several weeks. Electronic Publication     

Cathodic processes in copper(II) solutions containing gluconic acid

The kinetics of cathodic processes proceeding in the acidic 0.01 M Cu(II) solutions containing gluconic acid and 0.5 M Na₂SO₄ as the supporting electrolyte is studied. According to the spectrophotometric data, in the moderately acidic solutions, a monoligand complex of CuL⁺ predominantly forms. Its concentration stability constant is 10^2.2 M⁻¹. In the cathodic voltammograms, a well-defined plateau of the limiting current is observed. The height of the plateau obeys the Levich equation. The effective diffusion coefficient decreases from 4.2 × 10⁻⁶ to 2.5 × 10⁻⁶ cm²/s with increasing complexation degree of the system. An analysis of normalized Tafel plots showed that the exchange current density of Cu²⁺ + e → Cu⁺ process decreases with increasing concentration of ligand or with increasing pH value. Thereby, the cathodic chargetransfer coefficient remains constant (0.33 ± 0.02). A comparison of the kinetic data with the results of deposit surface examination points to significant surface activity of the ligand. The gluconate chemisorption can be accompanied by the incorporation of the fragments, which were formed as a result of its destruction, into the electrodeposits.     

Investigation of electrochemical reactions in the solid state cell Cu/RbCu4Cl3I2/C

Using experimental potential values for a vitreous carbon electrode in contact with the RbCu₄Cl₃I₂ solid electrolyte, the concentration of Cu²⁺ ions in the electrolyte was determined. At 0.5 V, the concentration of Cu²⁺ was 1.25×10¹⁸ cm^–3. The estimated values of the Cu²⁺ ion concentration in RbCu₄Cl₃I₂ (0.8%) and the potential of the vitreous carbon electrode after electrochemical decomposition of RbCu₄Cl₃I₂ (0.606 V) correspond to experimental values of 2% and 0.58 V, respectively. This demonstrates the adequacy of the model describing the electrode potential of Cu²⁺ as a function of the concentration in RbCu₄Cl₃I₂. When the C/RbCu₄Cl₃I₂ interface was polarized, the diffusion coefficient of Cu²⁺ was 1.5×10^–8 cm² s^–1. Investigations of the interface between the copper electrode and RbCu₄Cl₃I₂ were carried out by galvanostatic and potentiostatic methods. A 1-μm layer of cuprous oxide, Cu₂O, was discovered on the interface of the copper electrode with RbCu₄Cl₃I₂. This layer blocks the course of the electrochemical reaction Cu⁰–e^–⇌Cu⁺ with participation of copper metal. The copper electrode behaves as an inert redox electrode at low overvoltages. In this case, at the Cu₂O/RbCu₄Cl₃I₂ interface an electrochemical reaction with Cu²⁺ ion participation, Cu⁺–e^–⇌Cu²⁺, takes place. The results suggest that the reaction rate is limited by slowing the Cu²⁺ diffusion in RbCu₄Cl₃I₂. The initial Cu²⁺ ion concentration in the electrolyte near this interface is about 1.4×10¹⁷ cm^–3. The exchange current density is about (4±2)×10^–6 A cm^–2. At potentials ϕ>8–10 mV, an electrical breakdown of the Cu₂O layer takes place, allowing copper metal to ionize to Cu⁺. We suggest that at 10 mV<ϕ<100 mV the rate of this reaction is limited by the formation and growth of copper nuclei and at ϕ>120 mV the reaction rate is limited by charge transfer. Electronic Publication