EQCM Study of Iridium Anodic Oxidation in H2SO4 and KOH Solutions

In the present study anodic oxidation of iridium layer formed thermally on a gold‐sputtered quartz crystal electrode has been investigated by electrochemical quartz crystal microgravimetry (EQCM) in the solutions of 0.5 M H₂SO₄ and 0.1 M KOH. The emphasis here has been put on the microgravimetric behavior of iridium as a metal, because a few previous EQCM studies reported in literature have been devoted to iridium oxide films (IROFs). The objective pursued here has been to elucidate the nature of the main voltammetric peaks, which occur at different ranges of potential in the solutions investigated. It has been found that anodic oxidation of iridium electrode in 0.5 M H₂SO₄ and 0.1 M KOH solutions is accompanied by irregular fluctuations of the electrode mass at 0.4 V<E<0.8 V followed by regular increase in mass at 0.8 V<E<1.2 V. The cathodic process initially, at 1.2 V>E>0.9 V, proceeds without any or with slight increase in electrode mass, whereas at E<0.8 V a regular decrease in mass is observed. It has been found that mass to charge ratio characterizing the processes of interest is 2 to 3 g F^?1in acidic medium, whereas in the case of alkaline one it is 4 to 6 g F^?1. The main pair of peaks seen in the voltammograms of Ir electrode in alkaline medium at E<0.8 V is attributable to redox transition Ir(0)→Ir(III), whereas those observed in the case of acidic medium at E>0.8 V should be related to the redox process Ir(0)→Ir(IV) going via intermediate stage of Ir(III) formation. As a consequence of these redox transitions, the gel‐like surface layer consisting of Ir(III) or Ir(IV) hydrous oxides forms on the electrode surface.     

A polynuclear mixed-valent ruthenium oxide/cyanoruthenate composite that yields thin coatings on a glassy carbon electrode with high catalytic activity toward methanol oxidation

Polynuclear ruthenium oxide/cyanoruthenate films on carbon substrates were grown by cycling the potential between 0.5 and 1.0 V (vs SCE) for 5–90 min in fresh 2 mM RuCl₃·3 H₂O, 2 mM K₄Ru(CN)₆·3 H₂O, 0.5 M KCl solution at pH 2. During the positive scans, the cationic Ru(III,IV)-oxo polynuclear species interacted with the simultaneously formed anionic CN-bridged Ru dimers to yield sparingly soluble deposits on the electrode surfaces. Different thicknesses, typically corresponding to 2–50×10⁻¹⁰ mol cm⁻², could be obtained by varying the cycling times. The absorption spectra of the coatings obtained at SnO₂-covered glass electrodes were different from those known for RuO₂ films and exhibited two absorption bands at about 400 and 700 nm. IR spectroscopic measurements confirmed the presence of a CN group in the deposit, as well as showing significant aquation of the entire coating. Ru, O, N and K (from KCl) were detected by means of Auger electron spectroscopy. The surface electrochemistry and stability of the film were significantly enhanced in K⁺-containing supporting electrolytes. The system provides a durable catalytic surface which allows the voltammetric oxidation of methanol; in 0.5 M H₂SO₄+0.5 M K₂SO₄ electrolyte the resulting anodic peak is at 1.01 V (vs. SCE). Electrooxidation was not possible at bare carbon electrodes, at least before the onset of the electrolyte decomposition.     

Anodic Dissolution of Palladium in Sulfuric Acid: An Electrochemical Quartz Crystal Microbalance Study

Regularities of formation of a palladium oxide layer and its cathodic reduction in 0.5 M H₂SO₄ at 0.5–1.3 V (SHE) are studied by cyclic voltammetry, x-ray photoelectron spectroscopy, and electrochemical quartz crystal microbalance. A pure Pd plate and a 0.5-m-thick Pd coating on gold-sputtered quartz crystal is used for electrochemical and microgravimetric studies. It is shown that a Pd electrode dissolves electrochemically in 0.5 M H₂SO₄ when its potential is cycled between 0.5 and 1.3 V. In the case of 0.5-m-thick Pd coating on the gold substrate, the decrease in the electrode weight during one anodic–cathodic cycle is 1.0–1.5 g/cm². It is suggested that anodic process at 0.5–1.3 V (SHE) represents electrochemical oxidation of palladium, yielding a surface layer of poorly soluble Pd(OH)₂ and/or PdO phases, as expressed by the equation Pd + 2H₂O (Pd(OH)₂/PdO)_s + 2H⁺ + 2e. This surface layer, (Pd(OH)₂/PdO)_s, undergoes reduction during the cathodic process. About 5% of the total amount of ionized palladium dissolve in electrolyte.     

Thermal Activation of CH4 and H2 as Mediated by the Ruthenium Oxide Cluster Ions [RuOx]+ (x=1–3): On the Influence of Oxidation States

Thermal gas-phase reactions of the ruthenium-oxide clusters [RuO_x]⁺ (x=1–3) with methane and dihydrogen have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. For methane activation, as compared to the previously studied [RuO]⁺/CH₄ couple, the higher oxidized Ru systems give rise to completely different product distributions. [RuO₂]⁺ brings about the generations of [Ru,O,C,H₂]⁺/H₂O, [Ru,O,C]⁺/H₂/H₂O, and [Ru,O,H₂]⁺/CH₂O, whereas [RuO₃]⁺ exhibits a higher selectivity and efficiency in producing formaldehyde and syngas (CO+H₂). Regarding the reactions with H₂, as compared to CH₄, both [RuO]⁺ and [RuO₂]⁺ react similarly inefficiently with oxygen-atom transfer being the main reaction channel; in contrast, [RuO₃]⁺ is inert toward dihydrogen. Theoretical analysis reveals that the reduction of the metal center drives the overall oxidation of methane, whereas the back-bonding orbital interactions between the cluster ions and dihydrogen control the H−H bond activation. Furthermore, the reactivity patterns of [RuO_x]⁺ (x=1–3) with CH₄ and H₂ have been compared with the previously reported results of Group 8 analogues [OsO_x]⁺/CH₄/H₂ (x=1–3) and the [FeO]⁺/H₂ system. The electronic origins for their distinctly different reaction behaviors have been addressed.     

Iridium Anodic Oxidation to Ir(III) and Ir(IV) Hydrous Oxides

Iridium oxide films (IROFs) are known to have an enhanced or the so‐called super‐Nernstian (<59 mV/pH) pH‐sensitivity. The intention in the present study was to find out the reasons of such behavior and also to elucidate the nature of iridium anodic oxidation processes. The methods employed were combined cyclic voltammetry and chronopotentiometry. Iridium layers of 0.1 to 0.2 μm thickness, deposited thermally on titanium or gold‐plated titanium substrates, were used for investigations. IROFs on the surface of working electrodes were formed anodically by applying a constant potential in deaerated and oxygen‐containing solutions of 0.5 M H₂SO₄, 0.1 M KOH and 0.5 M H₃PO₄+KOH. Linear pH‐dependences of the stationary open‐circuit potential with the slopes close to 59 mV/pH were found for iridium electrode oxidized at 0.4 V–0.8 V (RHE) in deaerated and at 0.8 V–1.2 V (RHE) in O₂‐containing solutions. They were attributed to reversible Ir/Ir(OH)₃ and Ir/ IrO₂?nH₂O metal‐oxide electrodes, respectively. It has been suggested that the main current peaks seen in the voltammograms of iridium electrode in acid and alkaline solutions are of different nature. The difference between iridium electrode surface states in acid and alkaline solutions has been presumed to be the main reason of super‐Nernstian pH‐sensitivity of the IROFs. On the basis of the results obtained standard potential of Ir/Ir(OH)₃ electrode and the solubility product of Ir(OH)₃ have been evaluated: =0.78±0.02 V and K_sp=3.3×10^?64.     

Kinetics of electrode processes on a bismuth electrode in acid solutions containing thiourea and formamidine disulphide

A study of the mechanism by which bismuth is electrodissolved in an aqueous solution of thiourea on the background of H₂SO₄ demonstrated that, in the thiourea concentration range 0.001 M < c < 0.5 M, a current oscillation is observed in cyclic voltammograms at E ≈ 0.4–0.3 V when the potential is swept from the anodic to the cathodic region. This oscillation is due to the loosening of the passivating film formed in the anodic process. It is shown that thiourea is not oxidized to formamidine disulphide at the bismuth electrode. thiourea and formamidine disulphide have mutually opposite effects on the height of the cathodic peak: the peak current falls with increasing thiourea concentration and grows with increasing formamidine disulphide concentration. According to the results of an X-ray fl uorescence analysis, sulfur is formed on the bismuth electrode upon its prolonged polarization of in a 0.5 M solution of thiourea. An explanation is provided for the experimental facts observed in the study.     

Electrochemical and physical characterization of Pt–Ru alloy catalyst deposited onto microporous–mesoporous carbon support derived from Mo2C at 600 °C

The electrochemical reduction of oxygen on binary Pt–Ru alloy deposited onto microporous–mesoporous carbon support was studied in 0.5 M H₂SO₄ solution using cyclic voltammetry, rotating disk electrode (RDE), and impedance method. The microporous–mesoporous carbon support C(Mo₂C) with specific surface area of 1,990 m²?g^?1 was prepared from Mo₂C at 600 °C using the chlorination method. Analysis of X-ray diffraction, photoelectron spectroscopy, and high-resolution transmission electron microscopy data confirms that the Pt–Ru alloy has been formed and the atomic fraction of Ru in the alloy was ～0.5. High cathodic oxygen reduction current densities (?160 A?m^?2 at 3,000 rev?min^?1) have been measured by the RDE method. The O₂ diffusion constant (1.9?±?0.3?×?10^?5?cm²?s^?1) and the number of electrons transferred per electroreduction of one O₂ molecule (～4), calculated from the Levich and Koutecky–Levich plots, are in agreement with literature data. Similarly to the Ru/RuO₂ system in H₂SO₄ aqueous solution, nearly capacitive behavior was observed from impedance data at very low ac frequencies, explained by slow electrical double-layer formation limited by the adsorption of reaction intermediates and products into microporous–mesoporous Pt–Ru–C(Mo₂C) catalyst. All results obtained for C(Mo₂C) and Pt–Ru–C(Mo₂C) electrodes have been compared with corresponding data for commercial carbon VULCAN® XC72 (C(Vulcan)) and Pt–Ru–C(Vulcan) electrodes processed and measured in the same experimental conditions. Higher activity for C(Mo₂C) and Pt–Ru–C(Mo₂C) has been demonstrated.     

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.     

High-performance supercapacitors of hydrous ruthenium oxide/mesoporous carbon composites

The amorphous hydrous ruthenium oxide/mesoporous carbon composites (denoted as RuO₂·xH₂O/MC), obtained by loading small amount of amorphous hydrous ruthenium oxide nanoparticles ranged from 0.9 to 5.4% by weight of Ru (denoted as RuO₂·xH₂O) on mesoporous carbon (MC), were investigated for the first time and were used for supercapacitors. Electrochemical measurements showed that RuO₂·xH₂O/MC composites not only have an enhanced specific capacitance but also retain the superior rate capability of MC. The RuO₂·xH₂O/MC composite with Ru loading of 3.6 wt% exhibited an increase of the specific capacitance of approximately 57% (from 115 to 181 F/g) at the scan rate of 25 mV s⁻¹ in 0.1 M H₂SO₄ aqueous electrolyte. The specific capacitance based on the mass of RuO₂ was estimated to be 1,527 F/g, by subtracting the contribution from MC in the composite. Cycle performance tests for RuO₂·xH₂O/MC composite (3.6 wt% Ru) showed that approximately 2.8% loss of the total capacitance was observed after 1,000 cycles.     

Improving the utilization of ruthenium oxide within thick carbon–ruthenium oxide composites by annealing and anodizing for electrochemical supercapacitors

The effects of annealing in air and anodizing on the capacitive behavior of carbon–ruthenium (denoted as C–Ru) composites fabricated by wet impregnation were investigated in 0.1 M H₂SO₄ by cyclic voltammetry (CV) and chronopotentiometry (CP). The utilization of Ru species within the thick composites (≈1000 μm) was greatly promoted by annealing in air at 240 °C for 8 h and anodizing in 0.1 M H₂SO₄ at 1.2 V for 1.5 h, due to the formation of Ru oxide and the transformation into a hydrous nature and the maximal specific capacity of Ru oxide (760 F g⁻¹ based on RuO₂) could be obtained. The crystalline information of the composites with annealing at different temperatures was obtained from X-ray diffraction (XRD) patterns. The morphology of C–Ru composites was examined by scanning electron microscope (SEM). The specific surface area and pore-size distribution of the composites with annealing and/or anodizing were analyzed by the BET method.     

Electrode Modified with Langmuir–Blodgett (LB) Film of Calixarenes for Preconcentration and Stripping Analysis of Hg(II)

A new type of current sensor, Langmuir–Blodgett (LB) film of calixarene on the surface of glassy carbon electrode (GCE) was prepared for determination of mercury by anodic stripping voltammetry (ASV). An anodic stripping peak was obtained at 0.15 V (vs. SCE) by scanning the potential from ?0.6 to +0.6 V. Compared with a bare GCE, the LB film coated electrode greatly improves the sensitivity of measuring mercury ion. The fabricated electrode in a 0.1 M H₂SO₄+0.01 M HCl solution shows a linear voltammetric response in the range of 0.07–40 μg L^?1 and detection limit of 0.04 μg L^?1 (ca. 2×10^?10 M). The high sensitivity, selectivity, and stability of this LB film modified electrode demonstrates its practical application for a simple, rapid and economical determination of Hg²⁺ in a water sample.     

Cyclic Voltammetric Behavior of Uranyl ion in Sulfuric Acid Solutions. Application to Some Nuclear Materials Characterization

Abstract

The U(VI) reduction at mercury electrode in sulfuric acid solutions was examined by cyclic voltammetry (C. V.). A diffusion coefficient, D, was (5.30 ± 0.08) × 10^?6 cm²/sec was obtained for the depolarizer at 25.0±0.2°C in 1 N K₂SO₄ (pH = 2). In 1 N K₂SO₄/1 N H₂,SO₄ systems the disproportionation of U(V) was found to occur with the constant rate of K_d/[H⁺] = 6.500 ± 1.000 M^?2 sec^?1.

In 1 M H₂SO₄ supporting electrolyte pure kinetic control was achieved over the range of scan rates and uranyl concentration (C) investigated, hence linear correlation between cathodic peak current and C (above 5x10^?6 M) was obtained. Strong complexing oxyanions, such as phosphate and pyrosulphate, do not interfere with the cathodic peak current. Rapid determination of O/U ratios in uranium oxides and of U in mixed U-Th materials were performed respectively in 1 M H₂SO₄/1.5 M H₃PO₄ and 1 M H₂SO₄/0.2 M K₂S₂0₄ supporting media, with a reproducibility of ± 1.3% standard deviation.     

Voltammetric Determination of Tin(II) and Iron(II) on Mechanically Renewed Graphite Electrode in Tin-Plating Electrolyte

The analytical properties of the cathodic peak of tin(II) reduction and the anodic peak of iron(II) oxidation on a graphite electrode were studied with the electrode surface mechanically renewed directly in a solution before applying a potential in each measurement. The influence of the organic components of the phenolsulfonic tin-plating electrolyte on the cathodic current of tin(II) reduction and anodic current of iron(II) oxidation was studied. A dc voltammetric method was proposed for determining tin(II) directly in the phenolsulfonic tin-plating electrolyte, and iron(II) after the electrolyte is diluted tenfold with a 0.5M H₂SO₄ supporting solution.     

EQCM Study of Rhodium Anodic Dissolution in Sulfuric Acid

Rh anodic dissolution is studied in 0.5 M H₂SO₄ solution in the E range from 0.2 to 1.2 V (RHE) by means of EQCM, cyclic voltammetry, photometry, and XPS. Bright pure Rh electroplate 0.5 m thick on a gold sputtered quartz crystal electrode is used for electrochemical and microgravimetrical studies. It is found that the increase in Rh electrode weight during the anodic process is lesser than its decrease during the cathodic one. The difference is 120 ± 60 ng cm^–2. The electrode weight also decreases under open-circuit conditions, i.e. at E _{I = 0}. A linear relationship between the weight change and the charge exists for the anodic process. The presence of Rh(III) compounds in the solution and on the electrode surface is confirmed by a photometrical analysis and XPS measurements. It is assumed that the formation and reduction of Rh(OH)₃ phase on Rh electrode surface within E range investigated proceed according to equation Rh + 3H₂O [Rh(OH)₃]_s + 3H⁺ + 3e, where [Rh(OH)₃]_s is a surface layer of Rh(OH)₃ phase. is evaluated to be 0.6 V. Rh(OH)₃ partly dissolves in the electrolyte.     

Correction to: The effect of small silver inclusions on the palladium activity in formicacid oxidation reaction and corrosion stability

A PdAg deposit containing ~ 25 at.% Ag is obtained by the electrochemical codeposition from an aqueous solution of Pd and Ag sulfates (Au support, 0.5 M H₂SO₄). The deposit is characterized by means of various physical, physicochemical, and electrochemical methods. The PdAg deposit demonstrates the ~ 2 times higher specific activity (per the electrochemically active surface area (EASA) of Pd) in the formic acid oxidation reaction (FAOR) as compared with the individual Pd deposit prepared under the same conditions. The effect of silver additions on the palladium activity depends on many factors. The corrosion stability of PdAg is studied in 0.5 M H₂SO₄ solution based on the overall cyclic voltammograms (CVAs) and also on anodic and cathodic half-cycles in the region E = 0.3 − 1.25 V (vs. reversible hydrogen electrode (RHE)). The electrochemical estimates are compared with the results of direct analytical determination of dissolution products in solution after anodic polarization of deposits. The total amounts of Pd dissolved substantially increase with incorporation of Ag, which is associated, first of all, with the considerable increase in the EASA; at the same time, the specific dissolution of Pd also substantially increases. The possible factors determining the active dissolution of PdAg deposits are discussed; in particular, the specific mechanism of their dissolution via silver adatoms is proposed.

     

Direct Determination of Phosphite in Fertilizers by Flow‐Injection Amperometry

A sensor based on graphite electrode modified with palladium‐platinum‐palladium film is proposed for phosphite determination by flow‐injection amperometry. The modified electrode was prepared by a sequential cathodic deposition of Pd, Pt and Pd on a graphite electrode from 0.5% m/v PdCl₂+28% m/v NH₄OH and 2% m/v H₂PtCl₆+10% v/v H₂SO₄ solutions. After suitable conditioning, the electrode showed catalytic activity for phosphite oxidation when 0.15 V was applied. The proposed system handles approximately 50 samples per hour (0.01–0.05 mol L^?1 Na₂HPO₃; R²=0.9997), consuming ca. 70 μL of sample per determination. The limit of detection and amperometric sensibility were 5×10^?4 mol L^?1 and 1.5 mA L mol^?1, respectively. The proposed method was applied to analysis of fertilizer samples without pre‐treatment. Results are in agreement with those obtained by spectrophotometry and titrimetry at 95% confidence level and good recoveries (96–109%) of spiked samples were found. Relative standard deviation (n= 12) of a 0.01 mol L^?1 Na₂HPO₃ sample was 2%. The useful lifetime of modified electrode was around 220 determinations. For routine purposes it means that this electrode can be continuously used for 5 hours.     

Concurrent oxygen reduction and methanol oxidation in sulfuric acid solutions on platinized platinum electrodes

Steady-state polarization curves are compared in solutions of 0.5 M H₂SO₄ + O₂ (saturated), 0.5 M H₂SO₄ + (0.005–0.1) M CH₃OH, and 0.5 M H₂SO₄ + (0.005–0.1) M CH₃OH + O₂ (saturated) on a Pt/Pt electrode. A considerable difference is found between the currents in mixed solutions and those expected based on the principle of additivity of currents in CH₃OH and O₂ individual solutions. The surface coverages with the CH₃OH and O₂ adsorption products are determined in the potential range of 0.2–0.9 V (RHE). Open-circuit potentials are measured in mixed solutions. The obtained results suggest that the direct heterogeneous interaction between methanol and oxygen occurs alongside with faradaic reactions. This is assumed to lead to a decrease in methanol electrooxidation currents at E ≥ 0.8 V and their increase at E ≤ 0.65 V.     

In situ study of electrochromic properties of self-assembled TiO2 nanotubes

Electrochromical properties of anodic self-assembled nanotubes were investigated. It was found that amorphous titania nanotubes were able to insert H⁺ ions in a highly reversible manner. Coloration of the TiO₂ nanotubes occurred at potentials below ?0.5 V vs. Ag/AgCl in 1M (NH₄)₂SO₄ aqueous solution. The proton insertion reaction probably leads to the formation of a Ti³⁺/Ti⁴⁺ solid solution in the amorphous titania electrode, as was shown by the analysis of the derivative curve. The nanotubular titania electrode shows reasonable color efficiency when compared with other electrochromic materials and it is a promising candidate for the fabrication of low-cost interdigitated electrochromic devices.     

Kinetics of anodic formation and cathodic reduction of MnO2 in the sulfate electrolyte solutions

The anodic formation of manganese dioxide is studied voltammetrically in a wide range of potential scan rate (V = 0.001–8 V/s). Using the diagnostic criteria of cronovoltammetric method, based on the original experimental data, the mechanism of electrooxidation of manganese ions in the acidic medium with subsequent reaction of disproportionation of the product of irreversible electrode reaction and hydrolysis yielding manganese dioxide is proposed. The kinetics of cathodic reduction of electrolytic manganese dioxide in the 0.5 M Na₂SO₄ solution is studied under the steady-state and non-steady-state potentiodynamic polarization conditions. From the experimental data, it is found that, in the acidic medium (pH 1–3), the mechanism of the electrode process changes depending on the cathodic potential scan rate: at the scan rate V < 0.5 V/s, MnOOH forms via one-electron transition leading, in its turn, to the partial deactivation of electrode surface with subsequent disproportionation of manganite. At the relatively high potential scan rates, manganite has no time to form, and the two-stage reduction via one-electron transitions at each stage is well pronounced. The parameters of the electrode processes are calculated.     

Variation of the capacity characteristics of a composite material consisting of acetylene black and polytetrafluoroethylene and of its wettability with an aqueous solution under the action of anodic current

The effect exerted by treatment with cyclic anodic current in 1 M H₂SO₄ in the interval 0.0–2.0 V on electrodes made of a porous (55 vol %) composite material consisting of A-437E acetylene black and polytetrafluoroethylene (60 wt %) was studied. The cyclic volt–ampere curves were recorded in 3 M KOH and 1 M H₂SO₄ to determine the double layer capacity. The anodic treatment leads to an increase in the volume of pores filled with the electrolyte and in the electrical capacity of the electrode due both to an increase in the area of the surface wetted with the electrolyte and to the pseudocapacity caused by oxidation of the carbon black surface.