The bromine electrode Part II: reaction kinetics at polycrystalline Pt

The kinetics of bromide oxidation and Br₃^– reduction were studied at polycrystalline Pt electrodes in acidic media. The electrochemical behavior of equimolar Br₃^– and Br^– solutions was investigated, in a concentration range of the electroactive species between 0.1 and 1.0 M. E–log j plots did not exhibit linear segments, most probably because of extensive adsorption of bromine radicals. Further analysis supported the hypothesis of a Volmer–Tafel mechanism, with the chemical recombination step as rate determining. Electrosorption isotherms for Br radicals were found to be of the Frumkin type. The kinetics of Br₃^– reduction was controlled by the surface dissociation of the Br₂ molecules.     

Effects of Thiourea on Anodic Dissolution of Au and Surface Oxidation Behaviour in aq HClO4 Studied by Means of an EQCN

Anodic dissolution of Au is facilitated by the presence of thiourea owing to formation of strongly complexed Au ions. The present paper reports studies of this process using cyclic voltammetry (CV), chronopotentiometry and chronoamperometry, usefully complemented by nanogravimetry employing an electrochemical quartz-crystal nanobalance (EQCN). The molar masses per faraday for Au dissolution were determined from EQCN measurements, coupled with information derived from CV, chronopotentiometry and chronoamperometry, and clearly indicate that Au becomes dissolved over the potential range 0.45—0.65 V vs RHE via a 1e^– oxidation process in 0.5 M HClO₄ solution containing thiourea. The peak potential for Au dissolution in the presence of thiourea is about 600 mV less positive than that in the presence of Br^– or Cl^– (1.20 V vs RHE for Br^– and 1.39 V vs RHE for Cl^–). The linear relationship between anodic peak currents at about 0.630 V vs RHE and square-root of the sweep rate indicates that the Au dissolution process is diffusion-controlled. The anodic current efficiency for Au dissolution is 93%.     

Studies of Zn|ZnX2|polyaniline batteries. I. X.=Cl and Br

The cycling behaviour of polyaniline in chloride and bromide media is discussed. In both media some charge, equivalent to a charge storage capacity of 100 A h kg^–1, may be stored in the first oxidation step of polyniline. Further evidence is presented that the oxidation product is emeraldine rather than the fully oxidized imine. In chloride media, attempts to store more charge led to rapid irreversible changes in the polymer and degradation in battery characteristics. In bromide media, further charge may be stored via the Br^–/Br ₃ ^– couple and, although the Br₂ attacks the polymer slowly, the polymer retains the ability to cycle charge efficiently and gives a stable open-circuit voltage. The nature of the changes caused by Br₂ attack on the polyaniline is discussed. It is concluded that bromide is the better electrolyte, but self discharge by slow exchange of Br ₃ ^– in the polymer with Br^– in the surrounding electrolyte remains an unsolved problem.     

Movement of bromide as a tracer for nitrate in an Alfisol of the Indian Semi-Arid Tropics under rainfed condition

The variable responses of crops to added nitrogen (N) in Alfisols of the Indian semi-arid tropics are partly due to variable rainfall and partly due to variable losses of available-N. To measure the losses of N through leaching, which can be appreciable under some circumstances, a field experiment was conducted during the rainy season (June-September) of 1992, using bromide (Br) as a tracer for NO ₃ ^- . Bromide (as NaBr) was applied to bare fallow soil at a rate of 200 kg ha^–1 in microplots (2 m × 2 m) and its vertical movement was monitored periodically. Data on rainfall and Br^– distribution in the soil profile on different dates of soil sampling clearly indicated that the movement of Br^– was strongly dependent on rainfall. During the first month (15 June-15 July) after Br^– application, with scattered and light rainfall about 90% of the added Br^– remained in the soil profile (0.6 m). After continuous heavy rainfall in early August more than 90% Br^– had moved beyond 0.6 m depth. This indicates a very high risk of NO ₃ ^- leaching in this soil, and it is unavoidable without special measures to protect the applied N.     

Charge-discharge characteristics of polyvinylpyridinium hydrobromide perbromide-carbon paste electrodes

The work mainly focuses on the electrode characteristics of poly(4-vinylpyridine)-carbon paste composites in aqueous Br^– electrolytes. The reversible driving of the Br^–/Br ₃ ^– couple inside the polymer is initially much faster in KBr than in ZnBr₂. However, after repeated cycling, the paste electrodes are also conditioned to ZnBr₂, so that charging/discharging of bromine can be carried out with 1 A dm^–2 applied without causing excessive polarization. The possible use in secondary Zn/Br₂ batteries of a poly(4-vinylpyridine) solid matrix, which both complexes the halogen and collects the current, is a distinct possibility. This view is supported by the low self-discharge tested for poly(4-vinylpyridinium hydrobromide perbromide) which compares favourably with the previous state-of-the-art.Paper presented at the 7th Euchem Conference on Electrochemistry, Assisi, Italy, April 17–21 1989.     

Thermal desorption spectra of hydrogenated and water treated diamond powders

The thermal desorption mass spectra of hydrogenated and water treated diamond powders were measured in the range from room temperature to 1250°C. After preliminary outgassing up to 1150°C, samples were either hydrogenated under 1 Pa hydrogen at 500–1000°C or treated in water vapor at room temperature to 800°C. Hydrogenated diamond exhibited two desorption peaks due to H₂ at about 900 and 1050°C, while water treated one desorbed mainly H₂ and CO, and small amounts of H₂O and CO₂. Oxidation after hydrogenation and treatment with a mixture of hydrogen and oxygen showed some correlation between adsorption states of hydrogen and oxygen. The maximum amount of hydrogen desorbed (0.81 × 10¹⁵atom/cm²) was only one-third of the valu estimated as full surface coverage.     

Influence of the A-site cation in AMnO3+x and AFeO3+x (A = La,Pr, Nd and Gd) perovskite-type oxides on the catalytic activity for methane combustion

The effect of rare-earth ions (La, Pr, Nd and Gd) in AMnO_3+x and AFeO_3+x perovskites on the thermal behavior and on the catalytic activity for the deep oxidation of methane has been studied. AMnO_3+x perovskites showed after preparation an oxidative non-stoichiometry. Oxygen desorption analysis revealed for the four manganites different desorption steps occurring between 930 and 1370 K. Stoichiometry was reached after the first desorption step. Heating the samples at temperatures above 1300 K resulted in phase segregation to the simple oxides. AFeO_3+x perovskites were more stable towards thermal decomposition than the Mn-perovskites, showing no oxygen evolution up to 1400 K. The reducibility of these perovskites in hydrogen correlated inversely with the relative effective ionic radii of the trivalent rare-earth cations. Comparative catalytic studies were carried out in a fixed-bed microreactor at atmospheric pressure in the temperature range 600–1200 K. The activities at 770 K, expressed as reaction rates referred to the BET surface area, varied between 1.4 × 10^–7 and 2.9 × 10^–7 mol s^–1 m^–7 for the AMnO_3+x, and between 1.1 × 10^–7 and 1.6 × 10^–7 mols^–1m^–2for the AFeO_3+x perovskites.     

Electrode reaction of the Np3+/Np couple in LiCl–KCl eutectic melts

The electrochemical behaviour of the Np³⁺/Np couple in the LiCl–KCl eutectic salt was investigated by electromotive force measurements, cyclic voltammetry and chronopotentiometry in the temperature region between 723 and 823 K. The standard redox potential of the Np³⁺/Np couple vs Ag/AgCl (1.00 wt %) was measured and given by the equation, E _Np ³⁺ _/Np ^° = –2.0298 + 0.000706 T, where E is in V and T in K. The electrode reaction of the Np³⁺/Np couple was almost reversible under the conditions studied. The diffusion coefficient of Np³⁺, D _Np ³⁺, in the LiCl–KCl eutectic melts between 723 and 823 K was represented by the equation, D _Np ³⁺ = 2.22 × 10^–6 – 6.88 × 10^–9 T + 5.60 × 10^–12 T ² cm² s^–1. The adsorption and desorption peaks of Np at the Mo working electrode caused by underpotential deposition were also observed in the cyclic voltammograms, and the work function of Np was evaluated as 3.04 eV by peak analysis of the cyclic voltammograms.     

Thin films of Co3O4 and NiCo2O4 obtained by the method of chemical spray pyrolysis for electrocatalysis III. The electrocatalysis of oxygen evolution

The electrocatalytic properties of thin Co₃O₄ and NiCo₂O₄ films prepared on CdO-nonconductive glass by the method of chemical spray pyrolysis were investigated for oxygen evolution in varying KOH concentrations. The Tafel slopes were close to (2.3RT/F) V per decade for both oxides. The reaction order with respect to [OH^–] was found to be approximately 1.3. It was observed that Co₃O₄ is more active than NiCo₂O₄ towards oxygen evolution. A mechanism for oxygen evolution involving the electrochemical adsorption of OH^– as a fast step and the electrochemical desorption of OH forming an intermediate H₂O₂ as the rate-determining step has been suggested.     

Electropolymerized poly(2-vinylpyridine) coatings as ion-exchange polymer modified electrodes

The effects of pH and of the nature and concentration of the electrolyte on the electrochemical behaviour of the Fe(CN)^3–/4– ₆ charge-transfer reaction at a poly(2-vinylpyridine)-coated electrode formed by electropolymerization have been studied. Cyclic voltammetry during the Fe(CN)^3– ₆ incorporation process was combined with measurement of the saturated concentration of the Fe(CN)^3– ₆ confined in the films to investigate the electrochemical behaviour and the fundamental nature of the ion-exchange polymers. The poly(2-vinylpyridine) films formed by electropolymerization were found to have better properties (i.e., larger amount of Fe(CN)^3– ₆ can be incorporated at various pH values and films are more chemically stable under acidic conditions) as polymer-modified electrodes than those formed by solvent evaporation. Of the various anions studied, ClO^– ₄ was found to be distinct from the others (Cl^–, NO^– ₃, Br^– and SO^2– ₄). On the one hand, the polymer films exposed to ClO^– ₄ are more dense and rigid than those exposed to other anions and show relatively little electroactivity. On the other hand, when the films are exposed to increasing concentrations of Cl^–, the films become more swollen, thereby reducing the resistance within the film and enhancing the rate of charge-transfer from the outer film surface to the electrode surface.     

The temperature-programmed desorption of hydrogen from copper surfaces

The desorption kinetics of H₂ from a Cu/ZnO/Al₂O₃ catalyst for methanol synthesis were studied under atmospheric pressure in a microreactor set-up by performing temperature-programmed desorption (TPD) experiments after various pretreatments of the catalyst. Complete saturation with adsorbed atomic hydrogen was obtained by dosing highly purified H₂ for 1 h at 240 K and at a pressure of 15 bar. The TPD spectra showed symmetric H₂ peaks centered at around 300 K caused by associative desorption of H₂ from Cu metal surface sites. H₂ TPD experiments performed with different initial coverages resulted in peak maxima shifting to higher temperatures with lower initial coverages indicating that the desorption of H₂ from Cu is of second order. The microkinetic analysis of the TPD traces obtained with different heating rates yielded an activation energy of desorption of 78 kJ mol^–1 and a corresponding frequency factor of desorption of 3×10¹¹ s^–1> in good agreement with the kinetic parameters obtained with Cu(111) under UHV conditions.     

The activity and selectivity of oxygen atoms adsorbed on a Ag/α-Al2O3 catalyst in ethene epoxidation

The bonding of the oxygen species held on a Ag/-Al₂O₃ catalyst has been studied by temperature programmed desorption and their reactivity in ethene epoxidation by temperature programmed reduction using ethene as the reductant. The Ag/-Al₂O₃ catalyst was produced by the thermal decomposition of a Ag oxalate/-Al₂O₃ precursor. Oxygen desorbs from this Ag/-Al₂O₃ catalyst in two states, one (peak maximum temperature 520 K) having a desorption activation energy of 140 kJ mol^–1 – oxygen desorbing from Ag(111), and one (peak maximum temperature 573 K) having a desorption activation energy of 155 kJ mol^–1 – oxygen desorbing from a highly stepped or defected Ag surface. Temperature programmed reduction of the two oxygen states existing on the surface of the Ag/-Al₂O₃ catalyst using ethene as the reductant produced two peaks at 373 and 473 K in which ethene epoxide and CO₂ evolved coincidently. The peak at 373 K derives from the reduction of oxygen atoms adsorbed on Ag(111). The higher temperature peak (473 K) corresponds to the reduction of oxygen atoms adsorbed on highly stepped or defected Ag surface. The selectivity to ethene epoxide for the 373 K peak is ~ 57%, while that of the 473 K peak is 34%. The coincident evolution of ethene epoxide and CO₂ shows that the selective and unselective reaction pathways have a common surface intermediate – probably an oxametallacycle. The higher selectivity of the oxametallacycle formed by the bonding of ethene to the weaker Ag-O bond is considered to result from its having a lower activation energy to cyclisation than that produced by ethene bonding to the higher Ag-O bond.     

Co2 conversion and oxalate stability on alkali promoted metal surfaces: Sodium modified Al(100)

CO₂ conversion on alkali promoted metals in aprotic systems has been followed with surface sensitive spectroscopies. New results on sodium modified aluminum(100) are presented and compared with previous studies on magnesium [1], aluminum [2], and bulk alkali metals [3]. Electron energy loss spectra reveal two different states of CO₂ adsorption at 100 K and monolayer sodium coverage. Vibrational bands at 650 cm^–1 and 2325 cm^–1 correspond to weakly bound molecular CO₂ and a multitude of bands between 2300 cm^–1 and 460 cm^–1 to oxalate ions with low, possibly unidentate, coordination. Gentle annealing increases the coordination as apparent by vibrational shifts. This corresponds to oxalate to carbonate conversion, a process which is completed around room temperature. CO desorption was detected at 285 K and Auger measurements reveal a 13 C/O stoichiometry after high temperature annealing. We observe no release of CO₂ above 110 K but an additional weak state of CO desorption around 470 K. High temperature annealing causes decomposition of all intermediates and leaves the aluminum surface covered with an irreducible carbide and oxide overlayer. We suggest that CO₂ reduction and dimerization to C₂O₄ ^–2 is a common path to yield carbon deposition on all alkali promoted surfaces in hydrogen deficient systems. In contrast, oxalate decomposition is related to the specific chemistry of each substrate.     

Interaction of methane with surface of alumina studied by FT-IR spectroscopy

Methane adsorption on alumina was investigated by FT-IR spectroscopy at 173 K. Adsorbed methane gives four distinct IR bands at 3008, 3000, 2900 and 1305 cm^–1 which are attributed to v₁ (2900 cm^–1), v₃ (3008, 3000 cm^–1), and v₄ (1305 cm^–1) modes of methane respectively. The appearance of the v₁ mode indicates that the T_d symmetry of methane is distorted by the adsorption. The intensities of these bands increase significantly with outgassing temperatures of alumina, reach their maxima at an outgassing temperature near 773 K, and then decrease with further higher outgassing temperatures. Two hydroxyls with IR bands at 3750 and 3665 cm^–1 are perturbed evidently by the adsorbed CH₄ thereby resulting in two redshifted bands at 3707 and 3640 cm^–1. Coadsorbed CO slightly affects the adsorbed CH₄ indicating the very weak interaction between CH₄ and surface cations of alumina. It is proposed that the adsorbed CH₄ on alumina is formed mainly via the interaction of CH₄ with both surface hydroxyl and c.u.s. oxygen anion.     

Synthesis and dye separation performance of ferromagnetic hierarchical porous carbon

Ferromagnetic hierarchical porous carbon (FHPC) with nickel particles embedded in the hierarchical porous carbon skeleton was synthesized. The hierarchical macro–mesoporous skeleton was formed by dissolving a salt template of Na₂CO₃ and the ferromagnetic nickel particles were produced by in situ carbothermal reduction of nickel oxide. The saturation magnetization, remnant magnetization and coercive force of FHPC are 11.3 emu g⁻¹, 2.3 emu g⁻¹ and 55.7 Oe. The ferromagnetic property enables the magnetic separation of the FHPC from water. The surface chemical environments of the FHPC consist of different oxygen functional groups, like –OH, >COO and >CO groups, as well as a trace amount of aliphatic species of –CH₃ or -CH_2^- structures. Dye separation performance of the FHPC was investigated using methylene orange, and the adsorption capacity was 0.16 mg m⁻² with the adsorption kinetics constant of 2.2 m² mg⁻¹ min⁻¹, which is superior to that of magnetic carbon spheres.     

Electrooxidation of Cyanide on Cobalt Oxide Anodes

Oxidation of cyanide ions at a Ti/Co₃O₄ electrode in aqueous base solution has been investigated. The cyclic voltammetric curve for the oxidation of cyanide at Ti/Co₃O₄ shows a well formed wave prior to oxygen evolution at a potential where the spinel surface itself undergoes oxidation. Using a flow cell it is confirmed that the conversion of cyanide (CN^–) to cyanate (CNO^–) can be achieved galvanostatically with a reasonable current efficiency. As an example, at a current density of 100 A m^–2, CN^– concentration can be lowered from 10 to 0.2 mM with an electric energy consumption of about 18 kWh kg^–1 of CN^– oxidized and a global current efficiency of 28.5%. The oxide coating appears to be quite stable during repeated electrolyses.     

IR study of the interaction of hydroxyl groups of silica gel with Cr species of Phillips' type catalysts

Spectroscopic evidence for the interaction of hydroxyl groups and chromium ions was obtained using a catalyst prepared from chromyl chloride. A new OH peak, observed at 3705 cm^–1 after pumping away CO gas, is attributed to the direct interaction of OH with the low-valent chromium. This peak shifts to 3590 cm^–1 on contact with O₂ at room temperature and it is assigned to a hydroxyl interacting with the oxidized chromium. New assignments are also proposed for IR bands of CO presorbed on the catalyst. The peak due to CO at 2188 cm^–1 decreases as the OH intensity at 3705 cm^–1 increases, suggesting that the former peak arises from adsorption on Cr(II) species to which two oxygen atoms are attached.     

Conductivité électrique totale du systeme iodure d'argent-diiodure de diazonia-6,9-dispiro[5.2.5.3]heptadecane

Résumé La réaction entre l'homopipérazine, HN<(CH₂)₅>NH, et le dibromo-1,5-pentane, Br(CH₂)₅Br, conduit à la formation du dibromure de diazonia-6,9-dispiro[5.2.5.3]heptadécane de formule C₁₅H₃₀N ₂ ²⁺ ·2Br^–. Par permutation en milieu aqueux ou organique on obtient le sel iodé. Ce diiodure est associé à l'iodure d'argent dans des proportions variant entre 75 et 95% en équivalent de AgI. L'étude de la conductivité électrique totale de ce système en fonction de la température montre que le sel double renfermant 82,5% en équivalent de AgI présente, à 25°C, une conductivité électrique totale de 0,0083 ( cm)^–1.

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The reaction between homopiperazine, HN<(CH₂)₅>NH, and dibromo-1,5-pentane, Br(CH₂)₅Br, gives a diazonia-6,9-dispiro[5.2.5.3]heptadecane dibromide, C₁₅H₃₀N ₂ ²⁺ ·2Br^–. This salt is iodized by permutation in aqueous or organic solution. This diiodide is associated with silver iodide in proportions varying between 75 and 95% equivalent silver iodide. The study of the total electric conductivity shows that the double salt, containing 82.5% equivalent silver iodide, gives a conductivity of 0.0083 ( cm)^–1 at 25°C.

     

Evidence for strong metal-support interaction (SMSI) in Rh/TiO2 system

Rh(1 wt%)/TiO₂ samples were prepared by both incipient wetness and ion-exchange methods and were characterised by temperature programmed desorption (TPD), electron spin resonance (ESR), mass spectrometry (MS) and hydrogen chemisorption. The incipient wetness sample was found to be more favourable for the onset of SMSI state. The reduction of Ti⁴⁺ to Ti³⁺ during hydrogen spillover or due to the lattice oxygen (O^2–) deficiency seemed to be responsible for the SMSI state. A mechanistic pathway is proposed to explain the onset of SMSI behaviour.     

The behaviour of a trickle tower electrolytic cell for the production of cobalt(III) acetate from cobalt(II) acetate

The production of Co(III) acetate from Co(II) acetate using a bipolar trickle tower of graphite Raschig rings was investigated. Space time yields up to 18 kg m^–3 h^–1 were obtained, which showed no improvement over those achievable in a conventional plate and frame cell. A mathematical model of the system indicated that the electrode reactions occurred almost entirely at the opposing annular surfaces between consecutive layers of Raschig rings and that the unexpectedly low performance of the device was most probably due to the unfavourable mass transport conditions which existed in the intervening gaps.Nomenclature a annular cross sectional area of one Raschig ring (m²) - b _C kinetic exponential constant for reduction of Co(III) (V^–1) - b _A kinetic exponential constant for oxidation of Co(II) (V^–1) - b _H kinetic exponential constant for hydrogen evolution (V^–1) - b ₀ kinetic exponential constant for oxygen evolution (V^–1) - [Co(II)] concentration of Co(II) (mol m^–3) - [Co(III)] concentration of Co(III) (mol m^–3) - F Faraday constant (96 487 C mol^–1) - f fraction of total flow by-passing the annular gap between adjacent Raschig rings in a vertical row - I current per vertical column of rings (A) - k _C rate constant for reduction of Co(III) (A m mol^–1) - k _A rate constant for oxidation of Co(II) (A m mol^–1) - k _H rate constant for hydrogen evolution (A m^–2) - k _O rate constant for oxygen evolution (A m^–2) - k _L mass transfer coefficient (m s^–1) - Q flow rate per vertical row of Raschig rings (m³s^–1) - v volume of annular gap between adjacent Raschig rings in a vertical row (m³) - V superficial velocity of electrolyte (m s^–1) - _A anodic potential (V) - _C cathodic potential (V)