The influence of activation conditions on the performance of platinum/ruthenium methanol electro-oxidation catalysts surface enrichment phenomena

Starting from the electrocapillary equation for the ideally reversible electrode a relation has been derived between the parameters q^*-q^M+nFΛ₀ (q^M=electrode charge density, Λ₀=surface excess of oxidant) and the derivatives of the free energies of adsorption of the Ox and Red component with respect to potential. In this way a model is obtained for the explicit interpretation of the low frequency capacity C_LF and the high frequency capacity C_HF, occurring as parameters in the expression for the electrode admittance in the case of reactant and/or product adsorption. The experimental results obtained by Timmer et al. for the Pb(II) reduction in 1 M KCl appear to fit to this model if a linear isotherm with a potential-dependent adsorption coefficient is assumed. It is concluded that most probably the neutral PbCl₂ species is adsorbed.     

Study of redox behavior of Cd(II) and interaction of Cd(II) with proline in the aqueous medium using cyclic voltammetry

The redox behavior of Cd(II) and the interaction of Cd(II) with cyclic amino acid, proline, have been studied in 0.1 M KCl, 0.1 M NaClO₄ and acetate buffer of different pH. The CVs were recorded at glassy carbon electrode within the potential window 200 and ?1500 mV. The reference and counter electrode used were Ag/AgCl and Pt wire, respectively. The cyclic voltammograms show one pair of cathodic and anodic peaks for the Cd(II)/Cd(0) system indicating the involvement of two electron transfer processes. The peak potential shift and charge transfer rate constant (k_f) values strongly support the interaction between metal and ligand. The higher value of peak current ratio and peak potential separation (ΔE) indicate that the systems are quasireversible. The effect of supporting electrolyte and concentration of electro active species on the interaction were also studied.     

Confirming the Molecular Basis of the Solvent Extraction of Cadmium(II) Using 2-Pyridyl Oximes through a Synthetic Inorganic Chemistry Approach and a Proposal for More Efficient Extractants

The present work describes the reactions of CdI₂ with 2-pyridyl aldoxime (2paoH), 3-pyridyl aldoxime (3paoH), 4-pyridyl aldoxime (4paoH), 2-6-diacetylpyridine dioxime (dapdoH₂) and 2,6-pyridyl diamidoxime (LH₄). The primary goal was to contribute to understanding the molecular basis of the very good liquid extraction ability of 2-pyridyl ketoximes with long aliphatic chains towards toxic Cd(II) and the inability of their 4-pyridyl isomers for this extraction. Our systematic investigation provided access to coordination complexes [CdI₂(2paoH)₂] (1), {[CdI₂(3paoH)₂]}_n (2), {[CdI₂(4paoH)₂]}_n (3) and [CdI₂(dapdoH₂)] (4). The reaction of CdI₂ and LH₄ in EtOH resulted in a Cd(II)-involving reaction of the bis(amidoxime) and isolation of [CdI₂(L’H₂)] (5), where L’H₂ is the new ligand 2,6-bis(ethoxy)pyridine diimine. A mechanism of this transformation has been proposed. The structures of 1, 2, 3, 4·2EtOH and 5 were determined by single-crystal X-ray crystallography. The complexes have been characterized by FT-IR and FT-Raman spectra in the solid state and the data are discussed in terms of structural features. The stability of the complexes in DMSO was investigated by ¹H NMR spectroscopy. Our studies confirm that the excellent extraction ability of 2-pyridyl ketoximes is due to the chelating nature of the extractants leading to thermodynamically stable Cd(II) complexes. The monodentate coordination of 4-pyridyl ketoximes (as confirmed in our model complexes with 4paoH and 3paoH) seems to be responsible for their poor performance as extractants.     

Regression analysis of dependences of adsorption potential shifts on the charge in systems (Tl-Ga)/[N-methylformamide + mc KI + (1 − m)c KClO4], (Tl-Ga)/[N-methylformamide + mc KBr + (1 − m)c KClO4] and (Tl-Ga)/[N-methylformamide + mc KCl + (1 − m)c KClO4]

By the regression analysis of dependences of the adsorption potential shift (E _ads) on the electrode charge in systems (Tl-Ga)/[NMF + 0.1m M KI + 0.1(1 ? m) M KClO₄], (Tl-Ga)/[NMF + 0.1m M KBr + 0.1(1 ? m) M KClO₄], and (Tl-Ga)/[NMF + 0.1m M KCl + 0.1(1 ? m) M KClO₄] with the following m fractions of the surface-active anion: 0.05, 0.1, 0.2, 0.5, and 1, the adsorption parameters are calculated in terms of two models based on the Frumkin isotherm both considering the free adsorption energy as a quadratic function of the electrode charge, where one model takes into account the diffuse layer and the other ignores it. It is shown that for the studied electrode charges q ≤ 2 μC/cm², both models provide equal accuracy in calculating E _ads in the systems under study.     

Determination of electrochemical kinetic parameters from the measurement of faradaic impedance during the growth of the dropping mercury electrode

The electrochemical kinetic parameters of the V(II)/V(III) couple in HBr solutions of different concentrations were determined from the measurement of faradaic impedance as a function of time during the growth of the dropping mercury electrode. The same method of analysis was applied to the study of the effect of uncharged surfactants on the electrode reaction of Cd(II) in 1 M NaNO₃ solutions. The rate constant of the vanadium system decreased with increasing concentration of HBr; this change of the rate constant was discussed in terms of the Frumkin double-layer effect. The relationship between the rate constant of Cd(II) and coverage of the surfactants was not linear, and followed the equation based upon Parsons' model of the blocking effect. The conditional rate constant of Cd(II) in the absence of surfactants was determined to be 0.6–1.1 cm s^?1 from the dependence of the rate constant on the coverage.     

Synthesis and spectroscopic,magnetic and cyclic voltammetric characterization of some metal complexes of methionine: [(C5H10NO2S)2MII]; MII = Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II)

Some metal complexes of DL–methionine were prepared in aqueous medium and characterized by different physico-chemical methods. Methionine forms 1:2 complexes with metal, M(II). The general empirical formula of the complexes is proposed as [(C₅H₁₀NO₂S)₂M^II]; where M^II = Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II). All the complexes are extremely stable in light and air and optically inactive. Magnetic susceptibility data of the complexes demonstrate that they are high spin paramagnetic complex except Zn(II), Cd(II) and Hg(II) complexes. The bonding pattern in the complexes are similar to each other as indicated by electronic absorption spectra and FTIR spectral analysis. The current potential data, peak separation (AE) and the peak current ratio (i_pa/i_pc) of the (Mn, Cu and Cd) complexes indicate that the charge transfer processes are irreversible, the systems are diffusion controlled and also adsorptive controlled. The charge transfer rate constant of metals in their complexes are less than those in their metal salts at identical experimental conditions due to the coordination of metal with methionine.     

Search for a model to describe the specific adsorption of anions A? in Ga/[N-Methylformamide + mc KCl + (1 ? m)c KClO4] Systems, where KA is KCl, KBr, or KI

The adsorption parameters for systems Ga/[NMF + 0.1m M KCl + 0.1(1 ? m) M KClO₄], Ga/[NMF + 0.1m M KBr + 0.1(1 ? m) M KClO₄], and Ga/[NMF + 0.1m M KI + 0.1(1 ? m) M KClO₄] are calculated by using the regression analysis of the adsorption potential shift vs. electrode charge dependences for the following molar fractions m of the surface-active anion: 0.05, 0.1, 0.2, 0.5, and 1 within the framework of two models. The models are based on the Frumkin isotherm with the free adsorption energy dependent on the electrode charge, of which one model takes into account the diffuse layer and the other ignores it. It is shown that for electrode charges q ?? 16 ??C/cm², both models provide equal accuracy; however, for higher q, preference should be given to the model that takes into account the contribution of the double layer diffuse part.     

Determination of electrochemical kinetic parameters by square-wave polarography: Polarographic reduction of Zn(II) in 1 M KCl,KBr, and KNCS solutions

A new method of determining electrochemical kinetic parameters by square-wave polarography was presented, in which the faradaic current at θ/2, θ being the half-period of superimposed square-wave voltage, was used for the analysis. The method gave the following kinetic parameters for the electrode reaction, Zn(II) + 2e(Hg), in aqueous solutions at 25° C: k_c^θ=0.0052 cm s^?1 and α_c=0.36 in 1 M KCl, k_c^θ=0.011 cm s^?1 and α_c=0.30 in 1 M KBr, and k_c^θ=0.020 cm s^?1 and α_c=0.52 in 1 M KNCS. Induced adsorption of Zn(II) on the dropping mercury electrode was suggested in solutions containing thiocyanate ions.     

Application of Novel Zn‐Ferrite Modified Glassy Carbon Paste Electrode as a Sensor for Determination of Cd(II) in Waste Water

This paper describes the preparation of a new sensor based on Zn‐ferrite modified glassy carbon paste electrode and its electrochemical application for the determination of trace Cd(II) ions in waste waters using differential pulse anodic stripping voltammetry (DPASV). Different Zn/Ni ferrite nanoparticles were synthesized and characterized using scanning electron microscopy (SEM) and X‐ray powder diffraction (XRPD). The prepared ferrite nanoparticles were used for the preparation of Zn‐ferrite‐modified glassy carbon paste electrode (ZnMGCPE) for determination of Cd(II) at nanomolar levels in waste water at pH 5. The different parameters such as conditions of preparation, Zn²⁺/Ni²⁺/Fe²⁺ ratio and electrochemical parameters, percentage of modifier, accumulation time, pH and accumulation potential were investigated. Besides, interference measurements were also evaluated under optimized parameters. The best voltammetric response was observed for ZnFe₂O₄ modifier, when the percentage of modifier was 3 %, accumulation time 9 min, pH of supporting electrolyte 5 and accumulation potential ?1.05 V. Thus prepared electrode displays excellent response to Cd(II) with a detection limit of 0.38 ppb, and selective detection toward Cd(II) was achieved.     

Polarographic analytical determination of Hg(II), Cu(II), Cd(II), As(III), Sb(III), Ti(IV) and U(VI) in the presence of Fe(III)

The analytical determination of Hg(II), Cu(II), Cd(II), As(III), Sb(III), Ti(IV) and U(VI) in the presence of Fe(III) and 1 M H₂SO₄ are investigated using the polarographic technique. The wave corresponding to the reduction of Fe(III) to Fe(II) was found to be completely suppressed by the addition of 1% pyrogallol. Thus, different mixtures of these elements, viz. Hg(II), Cu(II), Cd(II), As(III) and Fe(III)-mixture (A), Cu(II), Cd(II), Sb(III), As(III) and Fe(III)-mixture (B), and Cu(II), Cd(II), Ti(IV), U(VI) and Fe(III)-mixture (C), were quantitatively determined using 1% pyrogallol and 1 M H₂SO₄ as supporting electrolyte. The i₁/c results give excellent correlations in each case, as indicated from the results of leastsquares regression analysis.     

Polylysine supports electrofusion

A series of poly-l-lysine fractions (mean molecular masses M_m from 3970 to 132 300) at extremely low concentrations in the nanomolar range were combined with dielectrophoresis and rectangular pulsation of barley protoplasts in order to support electrofusion of membranes. Contrary to our results with dextran fractions, the poly-l-lysine fractions of high molecular masses M_m promote the synergistic effect of polymer-supported electrofusion yield more than two times. The M_m dependence of adsorption at the dropping mercury electrode is analogous to that of dextran; however, instead of a Langmuir(-Freundlich) isotherm a Frumkin type was found, because of intermolecular interaction in the adsorption layer. Therefore the adsorption mechanisms for the two biopolymers are contradictory in this respect. Again it was shown that such electrofusion experiments provide a sensitive test for two kinds of interactions between biopolymers and membranes. Moreover, the practical advantage for moderate electrofusion is the following: in presence of 10 μg ml⁻¹ (= 10^−3%) poly-l-lysine (M_m, 26300) the electrofusion yield (F_r=2.2) is the same as in polymer-free treatment with only one pulse, which must be 250 V cm⁻¹ higher. Finally, some possible mechanisms are proposed.     

Electrode kinetics of the metal complexes with aminopolycarboxylic acids: Part II. Cadmium-ethylenediamine-N,N,N′,N′-tetraacetate (EDTA) complexes

The d.c. polarographic current-potential curves of Cd(II)-EDTA complexes were examined in the pH range 0.5–10.0, to elucidate the mechanism of their electrode processes and to determine the relevant electrochemical kinetic parameters. It was shown that the first wave observed below pH 3 at ?0.58 to ?0.65 V vs. SCE is the reversible reduction wave of Cd(II) aquo-ion with kinetically-controlled limiting current, and the second wave observed above pH 1.5 at ?0.75 to ?1.21 V vs. SCE corresponds to the simultaneous irreversible reduction of four complex species, CdH₃L⁺, CdH₂L, CdHL^? and CdL^2?, where CdH_pL^(p?2)+ and L^4? denote the protonated complex species with p protons and the unprotonated EDTA ion, respectively. Analysis of the dependence of limiting current on the hydrogen ion concentration led to the conclusion that the preceding reaction determining the behaviour of limiting current is CdH₃L⁺?Cd²⁺+H₃L^? with k₃^d=6.3×10² s^?1 and k₃^f=3.3×10⁶ s^?1M^?1, where k₃^d and k₃^f are the dissociation and formation rate constants, respectively. On the other hand, from analysis of the dependence of half-wave potentials of the second wave on the hydrogen ion concentration, the kinetic parameters of the four complex species were evaluated, and are given in Table 1. Further, it was shown that the cathodic rate constants of these four charge transfer processes at some reference potential together with those of Cd(II)-HEDTA complexes fulfil the linear free energy relationship.     

The effect of diffuse layer on adsorption potential shifts and differential capacitance in systems (In-Ga)/[N-methylformamide + mc KA + (1 - m)c KClO4], where KA is KCl, KBr and KI

By the regression analysis of dependences of the adsorption potential shift (E _ads) on the electrode charge in systems (In-Ga)/[NMF + 0.1m M KCl + 0.1(1 - m) M KClO₄, (In-Ga)/[NMF + 0.1m M KBr + 0.1(1 ? m) M KClO₄], and (In-Ga)/[NMF + 0.1m M KI + 0.1(1 - m) M KClO₄] with the following m fractions of the surface-active anion: 0.05, 0.1, 0.2, 0.5, and 1, the adsorption parameters are calculated in terms of two models both based on the Frumkin isotherm and considering the free adsorption energy as a function of the electrode charge, where one model takes into account the diffuse layer and the other ignores it. It is shown that for the studied electrode charges q ≤ 10 μC/cm², both models provide equal accuracy in calculating E _ads and the differential capacitance (C) in the systems under study. However, for determination of adsorption parameters, the regression analysis of E _ads vs. q curves has several advantages over the analogous analysis of C vs. q curves.     

Effect of metal nature on energy of specific adsorption of chloride ions from dimethyl formamide solutions

The specific adsorption of chloride ions on the renewable liquid (Cd–Ga) electrode from mixed [0.1_m М LiCl + 0.1(1–_m) М LiBF₄] solutions in dimethyl formamide (DMF) is studied with an ac bridge at the following fractions of surface-active anion m: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5 and 1. It is found that the data on the specific adsorption of Cl^– anion in the system can be quantitatively described by Frumkin’s isotherm. The free adsorption energy of Cl^– (ΔG _ads) is a quadratic function of electrode charge. The results are compared with the corresponding data for the Ga/DMF and (In–Ga)/DMF interfaces. It is shown that the adsorption energy of Cl–anions at the metal/DMF interface depends on the metal nature and increases in the series (In–Ga) ≈ (Cd–Ga) < Ga. The energy of metal–DMF chemisorption interaction, which hampers ion adsorption, increases in the same series. The analysis of the data uniquely indicates that the free energy of metal–Cl^– interaction (ΔG _M-CL-) increases in the series (In–Ga) ≈ (Cd–Ga) < Ga. Thus, in the series of electrodes studied, the variations in the energies of metal–Cl^– and metal–DMF specific interaction are correlated: the higher the energy of metal–DMF chemisorption interaction, the higher the energy of metal interaction with Cl^–.     

Kinetics of the Hg(II)/Hg electrode reaction in DMSO solutions of chloride ions

The electrode reaction Hg(II)/Hg in complex chloride solutions with dimethyl sulfoxide as solvent has been investigated at the equilibrium potential by the faradaic impedance method and a cyclic current-step method. The ionic strength was 1 M with ammonium perchlorate as supporting electrolyte, and the temperature was 25°C. Double-layer data have been determined by electrocapillary measurements. From the results of the kinetic measurements at ligand numbers ≤1.1 or ≥2.3 it is concluded that the overall charge transfer proceeds step-wise. The solvated Hg²⁺ and Hg₂²⁺ as well as the complexes HgCl_j^2?j and the dinuclear Hg₂Cl³⁺ contribute to the exchange current density. The rate constant of the step HgCl_j^2?j/ Hg(I) is found to increase with the number of Cl^? coordinated. This increase can be correlated to a decrease in solvation and a lengthening of the Hg?Cl distance. For 1.1 << 2.3, impedance measurements indicate a rate-controlling adsorption step. It is suggested that the uncharged HgCl₂ then forms an adsorbed network on the mercury surface.     

The admittance of an electrochemical cell with adsorption of electroactive species at the DME

Equations for the electrode admittance in, the case of adsorption of electroactive species have been derived, considering both a.c. and d.c. aspects of adsorption. It was shown that due to the d.c. aspects the function also contains a drop-time dependent term f(t_m): =c_OD_O^1/2+c_RD_R^1/2 +f(t_m)?s Under normal polarographic conditions the value of is constant, so c_O and c_R are drop-time dependent and consequently also the value of the Warburg coefficient σ, as well as the adsorption parameters C_LF and C_HF.     

Kinetics of Zn/Zn(II) reactions in acidified solutions of potassium chloride

The title subject has been studied using stationary, single-pulse, and chronopotentiometric polarization measurements on the Zn/Zn(II) electrode and equilibrium measurements on the same and the Ag/AgCl electrode in 0.5–4 M chloride solutions at 25°C. The Zn/Zn(II) electrode reactions are found to occur in two consecutive charge-transfer steps with Zn(I) as intermediate. The ion-transfer step Zn/Zn(I) is too fast to exhibit its kinetics. The electron-transfer step Zn(I)/Zn(II) mostly occurs by the couple ZnCl₂(H₂O)_y/ZnCl₂(H₂O)_y, but species with one or no chloride ligand take over as the main electroactive ones at chloride (or salt) concentrations below 1 M. The value of y is not clearly revealed by the data. Some sluggishness in complex equilibration and some, double-layer effects are observed. A convenient scale for single-ion activities is described, used, and recommended.     

Cadmium(II) iodide molecular coordination compounds with 4-methylpyridine and 4-methylquinoline

The coordination compounds [CdI₂(4-MePy)₂] (I) and [CdI₂(4-MeQuin)₂] (II) where Quin is quinoline have been synthesized, and their structure has been solved. Crystals of complex I are monoclinic, space group C2/c, a = 13.353(1) Å, b = 16.653(1) Å, c = 14.380 (1) Å, β = 103.17(1)°, V = 3113.5(4) ų, ρ_calcd = 2.425 g/cm³, Z = 8. Crystals of complex II are monoclinic, space group P2₁/c, a = 10.647(1) Å, b = 25.264(1) Å, c = 8.610(1) Å, β = 113.73(1)°, V = 2120.1(3) ų, ρ_calcd = 2.044 g/cm³, Z = 4. Polymer [CdI₂(4-MePy)₂]_∞ chains running in the direction [001] are formed in the structure of complex I. Each of the two crystallographically nonequivalent Cd(1) and Cd(2) atoms are octahedrally surrounded by the four iodine and two nitrogen atoms of the 4-MePy ligand. The Cd(1)?Cd(2) distance in a chain is 4.33 Å. The structure of complex II is built of [CdI₂(4-MeQuin)₂] discrete neutral clusters. The two iodine and two nitrogen atoms of the 4-MeQuin ligand participate in the coordination of the Cd²⁺ ion. The cadmium coordination polyhedron is a distorted tetrahedron (Cd-I_avg, 2.72 Å; Cd-N_avg, 2.30 Å; angles N(I)CdN(I), 98.3–121.8°). The minimum and maximum values correspond to the ICdI angle and NCdN angle, respectively. Complex I is photoluminescent in the solid state at room temperature.     

The electrochemical Peltier effect observed with electrode reactions of Fe(II)/Fe(III) redox couples at a gold electrode

The electrochemical Peltier effect was studied at a gold electrode in solutions containing some Fe(II)/Fe(III) redox couples by measuring the local temperature change in the electrode/solution interphase under controlled-potential and controlled-current polarization. Relative values of the electrochemical Peltier coefficient for the cathodic process at equilibrium potential, which is denoted by (Π_c)_I=0, were determined by analyzing the observed temperature change as a function of current. The values of (Π_c)_I=0 were found to be positive for the Fe(H₂O)₆²⁺/Fe(H₂O)₆³⁺ systems in HClO₄ (1 M), HNO₃ (1 M), H₂SO₄ (0.5 M), and HCl (1 M), their magnitudes being very similar in the first three acid solutions, but smaller in the HCl solution. On the other hand, a negative value of (Π_c)_I=0 was obtained in the case of a Fe(CN)₆^4?/Fe(CN)₆^3? couple in a H₂SO₄ (0.5 M) solution. Such a difference in the Peltier coefficient is considered to be due to the difference in the ionic species of iron involved in the electrode reaction.     

A study of the gold/aqueous electrolyte interphase by modulated reflection spectroscopy

The admittance and differential reflectivity of an electrode of polycrystalline gold in contact with aqueous 0.02 M NaF, KClO₄, 0.01 M Na₂SO₄ and KCl have been measured in the range of potential where the electrode is ideally polarized (?0.7 V to 0.8 V NHE). In NaF and KClO₄ evidence was obtained for the compression of the adsorbed layer of water. The increase of the number of water molecules, with respect to the number in the least original state (at?4μC cm^?2), is 3% at +8 μC cm^?2 and at?17 μC cm^?2. At the potential of zero charge the water dipoles are oriented preferentially with the oxygen towards the metal. In KCl the optical measurements confirm the existence of two types of Au?Cl^? interaction and the covalent character of the bond at the most positive charges.