Polymer films on electrodes: Part III. Digital simulation model for cyclic voltammetry of electroactive polymer film and electrochemistry of poly(vinylferrocene) on platinum

The electrochemistry of electrodeposited poly(vinylferrocene) (PVF) and poly(vinylferrocene acrylonitrile) (PVFAN) films on platinum electrodes was studied in acetonitrile solutions using perchlorate or p-toluenesulfonate as the counter ion by cyclic voltammetry. A model is proposed for the cyclic voltammetric behavior of polymeric films on electrode surfaces. The model incorporates non-equivalent redox sites with interconversion between such sites, electron-transfer kinetics at substrate/film interface and diffusion within the film. Parameters are obtained which yield a good fit to the experimental results.     

Polymer films on electrodes: Part II. Film structure and mechanism of electron transfer with electrodeposited poly(vinylferrocene)

The structure and properties of electrodeposited poly(vinylferrocene) (PVF) films on platinum electrodes (PVF/Pt) were examined by electron microscopy, X-ray photoelectron spectroscopy, various electrochemical techniques and measurements of the film resistance. The data were consistent with a mechanism in which the polymer films are permeable to dis-solved reactants. A theoretical treatment of this situation for chronoamperometry is presented. The oxidation and reduction of a variety of dissolved reactants with redox potentials far removed from that of the PVF/PVF⁺ system at PVF/Pt occurred by diffusion of the electroactive species through the polymer film and subsequent reaction at the platinum surface.     

Ligand effects on the Pt(II) → Pt(0) reduction of dichlorobis(tertiary phosphine)platinum(II) complexes: Correlations between electrochemical data,spectroscopic data,and electronic ligand parameters

Cyclic voltammetry has been employed to study the diffusive, irreversible platinum(II) → platinum(0) reduction of three sets of structurally related complexes: cis-[PtCl₂P{p-C₆H₄X}₃)₂] (X = H, CH₃, Cl, F, OCH₃, N(CH₃)₂); cis-[PtCl₂(PPh₂R)₂] (R = CH₃, n-C₃H₇, n-C₅H₁₁, n-C₆H₁₃, n-C₁₂H₂₅) and cis-[PtCl₂(PR₃)₂] (R = CH₃, C₂H₅, CH₂ch₂CN). Relationships between the peak potentials for the Pt(II) → Pt(0) reduction and thermodynamic parameters which measure the electronic properties of the ligands are shown to exist for complexes of P{p-C₆H₄X}₃ ligands, implying a thermodynamic origin for the sensitivity of the peak potentials to structural change. Complexes of both P{p-C₆H₄X}₃ and PPh₂R ligands show correlations between peak potentials for reduction and the ³¹P{¹H} NMR spectroscopic parameter, ¹J(¹⁹⁵Pt, ³¹P). Correlations with values of δ(³¹P) exist in both cases, but a correlation with the coordination chemical shift, Δδ(³¹P), exists for complexes of PPh₂R, and not for complexes of P{C₆H₄X}₃. Complexes of PR₃ ligands show no correlation between the peak potentials measured for the Pt(II) → Pt(0) reduction and electronic or spectroscopic parameters, except possibly ¹J(¹⁹⁵Pt, ³¹P).     

Surface characterization of Pt electrodes using underpotential deposition of H and Cu: Part III. Surface improvement of the flame-annealed Pt(100) and Pt(111) electrodes via potential cycling

In Parts I and II of this series it was shown that the Pt(100) and Pt(111) surfaces pretreated by flame-annealing and quenching in aqueous electrolyte contain a high density of defects such as vacancies, Pt adatoms and clusters of Pt adatoms. In this paper we show that potential cycling including scans into the oxygen region in sulfuric or perchloric acid removes most of these sites and that a limited number of cycles yield hydrogen adsorption-desorption profiles (cyclic voltammograms) that compare favorably with those published by authors who established the structure using electron diffraction techniques. Some loss of longer-range surface order as a result of the potential cycling is indicated by an increase in the width at half-height of the monolayer copper stripping peaks. The possibility of surface improvement in the absence of surface oxidation and reduction is explored by potential cycling in hydrochloric acid.     

Thermal and chromatographic behaviour of metal complexes. Part II. Complexes of platinum(II) with dipeptide ester

Experimental measurements of the rate of reduction of particles of Carol Lake and Kiruna ores have been made using pure hydrogen and pure carbon monoxide and mixtures of these two gases. The temperature range covered was 773–1143 K and throughout this range the reduction rate with hydrogen was greater than that with carbon monoxide. A retracting core model was found to best fit the experimental data even when granules of 9 × 10^?4 m diameter were used. Reduction with gas mixtures of hydrogen and carbon monoxide give rates intermediate between those of the pure gases.     

Obtaining thin films of “reactive polymers” on metal surfaces by electrochemical polymerization: Part II. Alcohol substituted polyphenylene oxide films

The selective electrochemical oxidation of the phenol function in the case of hydroxymethyl phenol derivatives (o^?, m^?, p-hydroxybenzyl alcohol) leads to “reactive polymer” films of polyphenylene oxide substituted by CH₂OH groups. The transformation of the hydroxyl function into an ester function by acetyl chloride indicates the reactivity of the CH₂OH group. As for the family of carbonylated polyphenylene oxide films, reactivity is limited to the superficial layers of film. Average film thickness is between 50–100 nm; however with the ferrocene-ferricinium system acting as a redox catalyst, it can reach about 300 nm. This catalytic mechanism intervenes only when the oxidation potential of the ferrocene-ferricinium couple is very similar to that of the phenol derivative.     

Hydridocyanoalkyl complexes of platinum(II). Insertion of olefins into the PtH bond

The preparation and spectroscopic properties are described of some platinum(II) complexes having a hydride ligand cis or trans to an sp³ carbon, viz. trans-PtH(YCN)(PPh₃)₂ and cis-PtH(YCN)(LL) with YCN = C₂H₄CN, n-C₃H₆CN, o-CH₂C₆H₄CN and LL = bis(diphenylphosphino)-ethene or -ethane. The complexes trans-PtH(YCN)(PPh₃)₂ can add a fifth ligand in solution; the resulting five-coordinate complex was observed by ³¹P NMR in the case of PtH(C₃H₆CN)(PPh₃)₃. Insertion of olefin (ethen, 1-cyanoethene, norbornadiene, allen) into the PtH bond of the trans-hydrido complexes occurs to give cis-dialkyl complexes, but the cis-hydrido complexes are unreactive. The mechanism of insertion is discussed in terms of the kinetics and the geometries of reactants and products.     

The acceleration of the hydrogen evolution reaction from acid solutions by sulfur compounds: Part II. Indium amalgam electrodes

Thiourea and its N-substituted derivatives accelerate the hydrogen evolution reaction at mercury, indium and indium amalgam electrodes. The differences in the effects observed with mercury and with indium amalgam electrodes can be accounted for in terms of differences in the adsorption of the organic compounds at the metal-solution interfaces.     

Hydrodynamic voltammetry at channel electrodes: Part II. Theory of first-order kinetic collection efficiencies

The theory of first-order kinetic collection efficiencies at the double channel electrode is developed for the following two schemes: (I) A±n₁e→B (at the generator electrode), B→^kP (in solution), B±n₂e→Y (at the detector electrode), (II) A±n₁e→B, B→^kA, B±n₂e→Y. The exact expressions for the kinetic collection efficiencies are obtained as ascending and asymptotic series with respect to the kinetic parameter. Further, approximate formulae in exponential forms are given, which hold within an error of about 2% for conventional electrode geometry. Finally, the validity of the approximate procedure, which has been used previously to obtain the kinetic collection efficiencies for fast homogeneous reactions, is discussed in comparison with the present theory.     

Voltammetry at partially covered electrodes: Part II. Linear potential sweep and cyclic voltammetry

Effect of inhomogeneity of the electrode surface on the linear potential sweep and cyclic voltammograms is investigated theoretically and experimentally using model electrodes partially covered with photoresist layer. Good agreement between the theoretical and experimental results is obtained.     

Interfacial electrochemistry of cytochrome c at tin oxide,indium oxide,gold, and platinum electrodes

Cyclic voltammetry has been used to study the heterogeneous electron transfer kinetics of horse heart cytochrome c in pH 7 tris/cacodylate media at several electrode surfaces. Reversible voltammetric responses (formal heterogeneous electron transfer rate constant>10^?2 cm/s) were observed at bare gold electrodes and at tin-doped indium oxide semiconductor electrodes for certain experimental conditions. Quasireversible voltammetric responses were more typically observed at fluorine-doped tin oxide semiconductor electrodes, bare platinum electrodes, and at the indium oxide electrodes. Reaction rates at bare metal electrodes were strongly dependent on pretreatment procedures and experimental protocol. Reaction rates at metal oxide electrodes were strongly dependent on solution conditions, pretreatment procedures, and on the hydration state of the electrode surface. A general mechanistic scheme involving both interfacial electrostatic and chemical interactions is proposed for cytochrome c electrode reactions. The asymmetric distribution of surface charges on cytochrome c appears to play a dominant role in controlling electron transfer rates by its interaction with the electric field at the electrode surface. Electron transfer distances are also considered, and it is concluded that electron transfer between an electrode surface and the exposed heme edge of properly oriented cytochrome c molecules involves maximum distances of ca. 0.6–0.9 nm.     

Influence of sulfide and cyanide ions on the electrochemical behavior of the Fe(II)/Fe(Hg) system in thiocyanate solutions at mercury electrodes

The reduction and reoxidation processes of the Fe(II)/Fe(Hg) system in thiocyanate solutions at stationary mercury electrodes have been investigated by cyclic voltammetric, anodic stripping and controlled potential electrolysis methods. In 0.1 M NaSCN and 0.4 M NaClO₄ solution containing 1×10^?3M Fe(II), the voltammogram on the first cycle at. 0.05 V s^?1 gives two consecutive cathodic peaks near ?1.2 and ?1.39 V with a hysteresis on the reversal, and an anodic wave with two large peaks near ?0.58 and ?0.05 V and two small peaks near ?0.52 and ?0.43 V, respectively. The multicyclic voltammogram under the same conditions in the potential region between 0.00 and ?1.50 V gives a cathodic wave with a principal peak near ?1.02 V and two small peaks near ?0.02 and ?0.53 V, respectively, and an anodic wave with a principal peak near ?0.72 V, three small peaks near ?0.64, ?0.52 and ?0.40 V, and with a shoulder near ?0.05 V, respectively. The variation of the shape of the voltammogram on the second and subsequent runs is due to the formation of S^2? and CN^? during the process of electroreduction of Fe(II). A mechanism is proposed which involves an initial reduction of Fe(II)?SCN^? produced in an activation step at a mercury electrode, followed by the chemical redox reaction of a part of Fe(0)?SCN^? in the species giving FeS and CN^?, and takes into account the influence of FeS and CN^? on the further reduction and reoxidation of iron. Both FeS and CN^? stimulate further reduction, and reoxidation of iron. The hysteresis of the cathodic wave on the first cycle arises from the fact that Fe(II) is reduced more easily at the mercury electrode covered with FeS than at a pure mercury electrode.     

Voltammetry at partially covered electrodes: Part I. Chronopotentiometry and chronoamperometry at model electrodes

Equations for chronopotentiometry and chronoamperometry at partially covered electrodes have been derived using a model of hexagonal array of cylidrical spaces terminated, at the electrode surface, by concentric active and inactive regions. The boundary value problem was shown to be analogous to that for a charge transfer preceded by a chemical reaction. Experiments with the reduction of ferricyanide on gold model electrodes partially covered with photoresist layer showed excellent agreement with the theory. Application of the equations to estimation of coverage and size of active sites distributed on a electrode surface is discussed.     

Voltammetry at microcylinder electrodes: Part III. Chronopotentiometry

Expressions for chronopotentiometry at stationary microcylinder electrodes are presented. The transition time, τ, is expressed as a function of a single parameter λ ( = nFc^*_RD / ia, where a is the radius of the electrode, D is the diffusion coefficient, i is the current density, n is the number of transferred electrons, F is the Faraday constant and c^*_R is the bulk concentration. When the values of λ are small, the transition-time constant, i√τ / c^*_R, depends linearly on λ, with the intercept predicted from the Sand equation. Conversely, when values of λ increase, the transition time also increases exponentially. Therefore transition necessarily occurs no matter how small a current flows. An approximate equation for the transition time is presented, from which one can evaluate the diffusion coefficient. Equations for the potential-time curve and the quarter-wave potential are also obtained. The equations were tested experimentally using carbon fiber electrodes (a = 4.1 μ m) and platinum wire electrodes (a = 10-100μ m). The transition times obtained experimentally were in good agreement with those predicted theoretically for various values of the applied current, for several different radii of the electrodes.     

The electrochemical behaviour of cobalt in alkaline solutions part II. The potentiodynamic response of Co(OH)2 electrodes

The potentiodynamic behaviour of Co(OH)₂ hydroxide electrodes is studied in the potential range related to the appearance of CO(III) and CO(IV) species. The corresponding electrochemical reactions involve relatively fast proton transfer processes occurring at potentials close to those predicted from thermodynamics. Sandwich-type structures of the electrode/film/solution interface are assumed in the interpretation of the processes. They probably include configurational changes of reactants and products participating in the various electrochemical reactions.     

Hydrodynamic voltammetry at channel electrodes: Part VII. Current transients at double channel electrodes

Expressions for the transient current at the downstream electrode in response to galvanostatic electrolysis at the upstream electrode in the channel flow cell were derived by applying double Laplace transformation when the electrode reaction at the upstream electrode is kinetically controlled. The ratio of the transient current to the steady state current or the transient collection efficiency was calculated as a function of electrode geometry and θ

, where U_m is the mean flow velocity in the channel cell, D the diffusion coefficient of the electroactive species, b the half height of the channel, x₁ the length of the upstream electrode and t the time elapsed since the beginning of the galvanostatic electrolysis at the upstream electrode. Curves for the transient collection efficiency can be applied to evaluating the amount of adsorption at the upstream electrode when metal at the electrode is anodically dissolved in solution. Digital simulation was carried out. Transient curves, obtained analytically, were in good agreement with those evaluated from the digital simulation. In order to allow one to draw transient curves readily, we derived a simple approximate equation.     

Kinetics of the thermal dehydration of trans-fluoroaquobis(ethylenediamine)chromium(III) tetracyanometallate(II) [metal(II) = Ni(II), Pd(II) and Pt(II)]

The solid phase thermal deaquation—anation of trans-[CrF(H₂O)(en)₂][M(CN)₄] (M = Ni, Pd, Pt; en = ethylenediamine) has been investigated by means of non-isothermal DSC and isothermal and non-isothermal TG measurements. The physical model for these reactions (nucleation, growth, diffusion or intermediates) has been found by comparison of the isothermal and non-isothermal TG data for all the principal g(α) expressions (0.2?α?0.8) and by the shape of the isothermal curves. The values found for activation energy are low (～ 130 kJ mol^?1 for the Ni compound, ～ 140 kJ mol^?1 for the Pd compound, and ～ 100 kJ mol^?1 for the Pt compound). These data permit the assignment of the deaquation—anation mechanism of the S_N1 type involving a square-base pyramid activated complex and elimination of water as Frenkel defects.