Current oscillatory phenomena based on electrogenerated superoxide ion at the HMDE in dimethylsulfoxide

A study on the current oscillation based on the electrogenerated superoxide ion $({\mathrm{O}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-})$ at a hanging mercury drop electrode (HMDE) in dimethylsulfoxide solution containing tetra-n-alkylammonium perchlorate as the electrolyte was carried out. Cyclic voltammetric, potential step chronoamperometric, normal and reverse pulse voltammetric techniques were employed in this study. A bare glassy carbon electrode, a liquid hemispherical mercury (Hg) drop-coated gold (Au) electrode and a Hg film-coated Au electrode were also used as working electrodes to understand the mechanism of the current oscillation. The experimental conditions were optimized for a simple, regular and reproducible current oscillation. The specific adsorption phenomenon of iodide ion and the adsorption of the alkyl chain of the supporting electrolyte on the HMDE surface were also considered to clarify the experimental factors governing the current oscillation phenomenon. It has been concluded that the oscillation is mainly due to the promoted oxidation (dissolution) of the HMDE itself by the adsorption of ${\mathrm{O}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$, resulting in the formation–destruction of a passive film such as ${\mathrm{Hg}}_2({\mathrm{O}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}{)}_2$ on the electrode surface. A probable mechanism for the observed current oscillation is discussed.     

Characterization of iron hexachloroplatinate films and their electrocatalytic properties with NAD+, hemoglobin and O2

Iron hexachloroplatinate (FePtCl₆) films have been prepared by mixing Fe²⁺ and ${\mathrm{PtCl}}_6^{2-}$ ions in an aqueous KBr solution. The electrochemical quartz crystal microbalance (EQCM), rotating ring-disk electrode, UV–visible absorption spectroscopy, stopped-flow kinetic method and cyclic voltammetry were used to study the deposition and growth mechanism of the iron hexachloroplatinate films. The electrochemical and EQCM properties of the films indicated that a single redox process was confined to the immobilized iron hexachloroplatinate films. The deposition of an iron hexabromoplatinate film occurred when ${\mathrm{Pt}}^{\mathrm{IV}}{\mathrm{Cl}}_6^{2-}$ was electrochemically reduced to ${\mathrm{Pt}}^{\mathrm{II}}{\mathrm{Cl}}_6^{4-}$ and Fe³⁺ to Fe²⁺. In the aqueous KCl, pH 3.0, solution, ${\mathrm{Pt}}^{\mathrm{IV}}{\mathrm{Cl}}_6^{2-}$ was electrochemically reduced to ${\mathrm{Pt}}^{\mathrm{II}}{\mathrm{Cl}}_6^{4-}$ and the Fe²⁺ reacted with the ${\mathrm{Pt}}^{\mathrm{II}}{\mathrm{Cl}}_6^{4-}$ and ${\mathrm{Pt}}^{\mathrm{IV}}{\mathrm{Cl}}_6^{2-}$ species. The electrocatalytic reduction properties of NAD⁺ and hemoglobin were determined using the iron hexachloroplatinate films. The electrocatalytic and electrochemical reactions of NAD⁺ with an iron hexabromoplatinate film were investigated using the rotating ring-disk electrode method. The catalytic current increased with the increase of analyte concentration with slope of 1.6 μA/mM.     

Studying the binding of Cd2+ by ω-mercaptoalkanoic acid self assembled monolayers by cyclic voltammetry and scanning electrochemical microscopy (SECM)

Cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) have been used for studying the binding of protons and cadmium(II) ions by 8-mercaptooctanoic acid monolayer that was assembled on an Au surface. Binding of the cations by the monolayer affected the kinetics of electron transfer to negatively charged species, i.e., ${\mathrm{IrCl}}_6^{3-}$, in the solution. Increasing the pH of the solution causes the CV of ${\mathrm{IrCl}}_6^{3-}$ to become less reversible and the feedback current of the SECM to decrease as the distance between the microelectrode and the surface decreases. On the other hand, addition of Cd²⁺ to the solution at pH 6.8 increases the CV reversibility and the feedback current of the SECM as a result of neutralizing the negative charge of the monolayer by the metallic ions. A model based on the Frumkin correction was used to account for these findings and for extracting the heterogeneous association constant of Cd²⁺ by the monolayer.     

The electroreduction of azides bound to nickel(II) ions in weak acidic media

Azide ions undergo reduction at the mercury electrode in slightly acidified media if they are coordinated to Ni(II) ions. Under such conditions, the electroreduction of Ni(II) to nickel amalgam is followed, at more negative potentials, by the parallel reduction of the ligand. The macro-scale electrolysis of slightly acidified solutions of $\mathrm{Ni}(\mathrm{II})''–''{\mathrm{N}}_3^{-}$ complexes showed the formation of ammonia and the evolution of bubbles of colorless gas. The electrode mechanism involves the direct electroreduction of ${\mathrm{N}}_3^{-}$ ions in the coordinated state according to the reaction scheme: ${\mathrm{N}}_3^{-}+4{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{N}}_2+{\mathrm{NH}}_4^{+}$ as the suggested main reaction pathway, accompanied by a side reduction of ${\mathrm{N}}_3^{-}$ by nickel amalgam. In terms of this mechanism, the average coordination number of the reacting $\mathrm{Ni}(\mathrm{II})''–''{\mathrm{N}}_3^{-}$ complexes could be deduced. A molecular mechanism of the electroreduction of the azide ion bound to Ni(II) central ion was suggested.     

Preparation and characterization of ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate mixed films and their electrocatalytic properties

Polynuclear mixed-valent films of ruthenium oxide/hexacyanoferrate $(\mathrm{RuO}/\mathrm{Fe}(\mathrm{CN}{)}_6^{3-})$ and ruthenium hexacyanoferrate (RuHCF) have been prepared using repetitive cyclic voltammetry. The deposition process and the films’ electrocatalytic properties in electrolytes containing various cations have been investigated. The cyclic voltammograms recorded the deposition of the mixed ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate films directly from the mixing of Ru³⁺ and $\mathrm{Fe}(\mathrm{CN}{)}_6^{3-}$ ions from the acidic aqueous solutions containing various anions. The electrochemical quartz crystal microbalance, cyclic voltammetry, and UV–Vis spectroscopy were used to study the growth mechanism of the mixed ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate films. The mixed ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate films exhibited four redox couples with formal potentials that demonstrated a proton effect in acidic aqueous solution and an anion effect. The electrochemical quartz crystal microbalance results indicated that the redox process was confined to the immobilized mixed ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate film. Electrocatalytic reactions of dopamine, epinephrine, norepinephrine, ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$, and ${\mathrm{SO}}_5^{2-}$ were carried out by the mixed ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate films. The electrocatalytic oxidation of ethyl alcohol, isopropyl alcohol, l-cysteine, and ascorbic acid by the mixed ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate films too was investigated.     

The complex formation of ions with a phospholipid monolayer adsorbed at an aqueous|1,2-dichloroethane interface

The complex formation of phosphatidylcholine adsorbed at a water|1,2-dichloroethane, W|DCE, interface with ions was evaluated based on the desorption behavior of lipids at the interface accompanied by an ion transfer and was confirmed by measuring voltammograms for the ion transfer facilitated by the complex formation with lipids. The desorption behavior of lipids was measured according to the Wilhelmy vertical plate method, the cell of which was improved to achieve polarization of the W|DCE interface. Voltammograms for the ion transfer at the W|DCE interface facilitated by the complex formation with lipids were recorded at a micro W|DCE interface in order to avoid the influence of convection caused by the desorption of lipids at the interface. Lipids adsorbed at the W|DCE interface showed complex formation with alkali metal ions, ${\mathrm{NH}}_4^{+}\text{,}({\mathrm{CH}}_3){\mathrm{NH}}_3^{+}\text{,}({\mathrm{CH}}_3{)}_2{\mathrm{NH}}_2^{+}\mathrm{or}({\mathrm{CH}}_3{)}_3{\mathrm{NH}}^{+}$, but not with $({\mathrm{CH}}_3{)}_4{\mathrm{N}}^{+}$ or anions such as ${\mathrm{SO}}_4^{2-}$, CH₃COO^?, Cl^?, Br^?, ${\mathrm{NO}}_3^{-}$, I^? or ${\mathrm{ClO}}_4^{-}$. Lipids also showed strong complex formation with a basic amino acid such as an arginine cation, which has two amino groups in the molecule. The strength of the complex formation with lipids was ${\mathrm{Arg}}^{+}>{\mathrm{Li}}^{+}\text{,}{\mathrm{Na}}^{+}>{\mathrm{K}}^{+}>{\mathrm{NH}}_4^{+}>{\mathrm{CH}}_3{\mathrm{NH}}_3^{+}>({\mathrm{CH}}_3{)}_2{\mathrm{NH}}_2^{+}>{\mathrm{Cs}}^{+}>({\mathrm{CH}}_3{)}_3{\mathrm{NH}}^{+}$.     

The electrochemical self-assembly of the FAD modified zinc oxide films and their electrocatalytic properties

The flavin adenine dinucleotide (FAD) modified zinc oxide films have been prepared using repeated cyclic voltammetry to investigate both the deposition process and the films’ electrocatalytic properties. This paper describes the successful loading of electrochemically active molecules into ZnO by electrochemical method. The cyclic voltammograms recorded the direct deposition of the FAD/zinc oxide films over different scanning potential ranges from the mixed aqueous Zn²⁺ ions and FAD. In addition to the cyclic voltammetry, an electrochemical quartz crystal microbalance, UV–visible absorption spectroscopy, and the stopped-flow method were used to study the growth mechanism and their properties of the FAD/zinc oxide films. The results showed that the Zn²⁺ ions and FAD reacted by an electrochemical process to form a self-assembly FAD modified zinc oxide film. The FAD/zinc oxide films exhibited a single redox couple that included both the electron and proton transfer, with a formal potential that demonstrated a proton effect in acidic and basic solutions. The electrocatalytic reduction of NAD⁺ employing the FAD/zinc oxide films was investigated by cyclic voltammetry and UV–visible absorption spectroscopy methods. The electrocatalytic reduction of ${\mathrm{S}}_4{\mathrm{O}}_6^{2-}$, ${\mathrm{SO}}_5^{2-}\text{,}{\mathrm{S}}_2{\mathrm{O}}_8^{2-}\text{,}{\mathrm{ClO}}_3^{-}\text{,}{\mathrm{BrO}}_3^{-}\text{,}\mathrm{and}{\mathrm{IO}}_3^{-}$ ions using a FAD/zinc oxide film occurred in neutral aqueous solutions.     

Electrochemical investigation on hybrid viologen tetracyanoquinodimethanide LB films

Positively ionized monolayers of 1,1′-dioctadecyl-4,4′-bipyridilium were prepared on aqueous solutions of tetracyanoquinodimethane in a mixed valence state. The floating films were transferred onto ITO electrodes, yielding hybrid multilayers containing both viologen cations and $(\mathrm{TCNQ}{)}_2^{-\text{<mglyph src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ anions. The electrochemical properties of these modified electrodes were studied by cyclic voltammetry and showed reversible electrochemical processes with quite negative formal potentials of the viologen units. The Γ_c/Γ_a ratio was 1 and the electrochemical activity was preserved beyond the first layer. A comparison study of the hybrid viologen/TCNQ LB films with other films incorporating other counterions was carried out and the differences attributed to chemical complexation and a different arrangement induced by the counterion incorporated in the films.     

Determination of reaction rate between cathodically formed FeII-triethanolamine-complex and FeIII-hepta-d-gluconate complex by cyclic voltammetry

Electron transfer reactions between iron^II/III-complexes are of general interest for corrosion processes, biological processes and technical applications. The cathodic reduction of Fe^III-triethanolamine (TEA) complex to form the Fe^IITEA-form of the complex has been examined by cyclic voltammetry. In the presence of Fe^III-d-heptagluconate (HDGL), complex catalytic currents are observed due to homogenous chemical reaction between Fe^IITEA and Fe^III(HDGL)₂ which regenerates Fe^IIITEA. This method can be used to determine the rate constant of the redox reaction between Fe^IITEA and Fe^III(HDGL)₂ with $k_{\mathrm{f}}^{\prime }=88\pm 12{\mathrm{dm}}^3{\mathrm{mol}}^{-1}{\mathrm{s}}^{-1}$.     

Novel photocathodic characteristics of organic bilayer composed of a phthalocyanine and a perylene derivative in a water phase containing a redox molecule

An organic bilayer composed of metal-free phthalocyanine (H₂Pc, p-type semiconductor) and perylene derivative (PTCBI, n-type semiconductor) was found to involve novel characteristics of photoelectrode working in the water phase. When the photoelectrode characteristics were investigated when the PTCBI has contact with water containing the redox molecule (${\mathrm{Fe}}^{\mathrm{III}}(\mathrm{CN}{)}_6^{3-}$ (electron acceptor)), it was shown by voltammetry that the photocathodic current due to the ${\mathrm{Fe}}^{\mathrm{III}}(\mathrm{CN}{)}_6^{3-}$ reduction occurs at the present electrode, which is different from the ordinary characteristics at the n-type semiconductor/water interface of the Schttoky junction. Separate voltammetric studies showed that the photocathodic characteristics of the H₂Pc/PTCBI bilayer are consistent with those of the H₂Pc single layer, indicating that there are pin-holes in the PTCBI layer of the bilayer; that is, the H₂Pc has direct contact with ${\mathrm{Fe}}^{\mathrm{III}}(\mathrm{CN}{)}_6^{3-}$ dissolved in water. Thus, it is inferred that the photocathodic current occurs at the H₂Pc surface. However, the action spectrum for the photocathodic current indicated that a broad visible light absorption (400–750 nm) by only PTCBI can also induce photocurrent generation especially at wavelengths shorter than 500 nm where absorption of the H₂Pc is absent or relatively weak. This is supported by the previous knowledge that the PTCBI exciton alone can contribute to carrier generation through charge separation at the H₂Pc/PTCBI interface. This study showed the novel photocathodic characteristics at the organic solid/water interface coupled with electron conduction through the n-type semiconductor, in addition to the ordinary characteristics at the p-type semiconductor/water interface.     

Microwave enhanced electrochemistry: mass transport effects and steady state voltammetry in the sub-millisecond time domain

In situ microwave activation of electrochemical processes is possible by self-focusing of intense microwave radiation into a region close to the electrode|solution (electrolyte) interface of a microelectrode placed inside a microwave cavity. A systematic study of the microwave activation effects in electrochemical processes is reported for two redox systems, $\mathrm{Fe}(\mathrm{CN}{)}_6^{3-/4-}\mathrm{and}\mathrm{Ru}({\mathrm{NH}}_3{)}_6^{3+/2+}$, in aqueous KCl solution. Platinum microelectrodes of 100, 50, and 25 μm diameter are employed and at the 25 μm diameter electrode, extreme current enhancements of up to three orders of magnitude are detected. A typical Nernst diffusion layer thickness in aqueous solution of less than 100 nm can be achieved routinely and, consequently, high temperature steady state voltammetry is possible in the sub-millisecond time domain. Volatile reagents reduce the efficiency of this effect and therefore a steam bubble mechanism is proposed to explain the observations. Microwave effects on the rate of interfacial electron transfer are discussed.     

Adsorption of uracil on bismuth single crystal planes

Uracil adsorption at Bi(h k l) has been studied using impedance spectroscopy method. The limiting adsorption potential shift, Gibbs energy of adsorption $-\mathrm{\Delta }{G}_{\mathrm{A}}^{°}$ and interaction constant values a in the Frumkin isotherm have been calculated and compared with the corresponding data for Hg [V. Brabec, S.D. Christian, G. Dryhust, J. Electroanal. Chem. 85 (1977) 389; V. Brabec, S.D. Christian, G. Dryhust, Biophys. Chem. 7 (1978) 253]. It was found that the values of $|-\mathrm{\Delta }{G}_{\mathrm{A}}^{°}|$ increase in the order $\mathrm{Hg}<\mathrm{Bi}(111)<\mathrm{Bi}(01\overline{1})$, but the values of a are higher for Bi(h k l) compared with Hg [V. Brabec, S.D. Christian, G. Dryhust, J. Electroanal. Chem. 85 (1977) 389; V. Brabec, S.D. Christian, G. Dryhust, Biophys. Chem. 7 (1978) 253]. The values of the limiting adsorption potential shift are comparable for the Hg and Bi(h k l) electrodes.     

Electrodeposition of gold from cyanide solutions on different n-GaAs crystal faces

The initial stages of the electrodeposition of gold from $\mathrm{Au}(\mathrm{CN}{)}_2^{?}$-solutions on n-GaAs were investigated by means of chronoamperometry and ex situ atomic force microscopy (AFM). Three different crystal orientations were examined, i.e. (1 1 1), $(\overline{1}\overline{1}\overline{1})$ and (1 0 0) n-GaAs. Analysis of the experimental current transients showed that the nucleation mechanism of Au on n-GaAs depends strongly on the surface orientation and the deposition potential, which was confirmed by AFM measurements. This can be explained by differences in chemical composition of the surface. Further, it was observed that the Au nuclei grow laterally, which results in a flat morphology.     

On the parameters affecting the characteristics of the “wired” glucose oxidase anode

We recently described a redox polymer with an $\mathrm{Os}(N,N^{\prime }\text{-}\mathrm{dialkylated}\mathrm{biimidazole}{)}_3^{2+/3+}$ redox center with a redox potential of ?195 mV vs. Ag|AgCl, 800 mV reducing relative to its 2,2′-bipyridine analog. The tethering of the centers to a poly(4-vinylpyridine) backbone through a 13 atom long spacer arm, provided for an apparent electron diffusion coefficient as large as 5 × 8.10^?6 cm² s^?1. The glucose electro-oxidation catalyst formed by “wiring” glucose oxidase with this polymer allowed the electro-oxidation of glucose already at ?360 mV vs. Ag|AgCl at pH 7.2, near the reversible potential of glucose oxidase, at a current density of 1.3 mA cm^?2 in the presence of 32 mM glucose concentration. Here we consider the parameters affecting the characteristics of this anode, compare the characteristics with those of an anode made with a short-tethered “wire” and describe the optimization of the composition of the “wired” glucose oxidase.     

Effect of niobium on the electronic properties of passive films on zirconium alloys

The effects of niobium on the structure and electronic properties of passive films formed on zirconium alloys in pH 8.5 buffer solution were examined by photo-electrochemical analysis. For Zr–xNb alloys (x = 0, 0.45, 1.5, 2.5 wt%), the photocurrent began to increase at an incident energy of 3.5–3.7 eV and exhibited the first peak at 4.3 eV and the second peak at 5.7 eV. From the (I_ph hν)^1/2 vs. hν plot, indirect band gap energies of $E_{\mathrm{g}}^1=3.01''–''3.47\mathrm{eV}\mathrm{and}{E}_{\mathrm{g}}^2=4.44''–''4.91\mathrm{eV}$ were obtained. With increasing Nb content, the relative photocurrent intensity of the first peak increased significantly. Compared with the photocurrent spectrum of the thermal oxide of Zr–2.5Nb, it was revealed that the first peak in the photocurrent spectrum for the passive film formed on Zr–Nb alloy was generated by two types of electron transitions; one caused by hydrous ZrO₂ and the other created by Nb. Two electron transition sources were overlapped over the same range of incident photon energy. In the photocurrent spectrum for passive film formed on Zr–2.5Nb alloy in which Nb is dissolved into the matrix by quenching, the relative photocurrent intensity of the first peak increased, implying that dissolved Nb acts as another electron transition source.     

Investigation of the electrochemical behavior of metallo-tetraazaporphyrin modified silver and pyrolytic graphite electrodes in aqueous nitrite solution

Five new complexes of the type [M(OBTTAP)] (where M = Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺; OBTTAP = 2,3,7,8,12,13,17,18-octakis(benzylthio)-5,10,15,20-tetraazaporphyrin(2?)) were used to prepare modified silver and pyrolytic graphite electrodes and their electrochemical behavior towards the reduction of aqueous nitrite solution was investigated in acidic, neutral and alkaline medium. Of these electrodes, [Co(OBTTAP)] and [Cu(OBTTAP)] modified electrodes showed affinity for ${\mathrm{NO}}_2^{-}$ ions and thereby exhibited significantly altered cathodic responses. The [Co(OBTTAP)]/Ag electrode in acidic, neutral and also in alkaline nitrite solutions, exhibited a new one electron irreversible reduction wave at E_p,c ～ ?0.2 V vs. Ag|AgCl. The [Cu(OBTTAP)]/Ag electrode in acidic and neutral nitrite solutions also behaved in a similar manner. However, in alkaline aqueous nitrite solution it showed two successive one electron reductions, the first reversible reduction at E_1/2 = ?0.26 V and the second irreversible reduction at E_p,c = ?0.73 V vs. Ag|AgCl. Controlled potential electrolysis of alkaline nitrite solutions using a [Cu(OBTTAP)]/Ag cathode led to isolation of a nitrosyl complex. The peak current, particularly with [Cu(OBTTAP)]/Ag microelectrode in alkaline medium, showed a linear response to $[{\mathrm{NO}}_2^{-}]$ and could be exploited in the electrochemical sensing and determination of ${\mathrm{NO}}_2^{-}$.     

Selective transfer of Ag+ at the water|1,2-dichloroethane interface facilitated by complex formation with a calixarene derivative

The facilitated transfer of silver, Ag⁺, has been studied at the Interface between Two Immiscible Electrolyte Solutions (ITIES). The transfer was achieved with the assistance of a calixarene-based silver ionophore. An investigation of the mechanistic details of the transfer was conducted using cyclic voltammetry at both micro and macro liquid|liquid interfaces. The mechanism was found to follow a Transfer by Interfacial Complexation (TIC)/Transfer by Organic phase Complexation (TOC) mechanism. The complex stoichiometry was found to shift from 1:1 to 1:2, metal:ligand, with increasing ionophore concentration. The logarithms of the complex association constants, $\log {\beta }_1^{\mathrm{o}}\mathrm{and}\log {\beta }_2^{\mathrm{o}}$, were estimated at 12.4 and 14.5, respectively. The charge transfer current was also found to be limited by diffusion of the transferring species and was unchanged by the presence of a range of interferents. The system thus shows promise for selective analytical applications.     

Theoretical and experimental study of the catalytic hydrogen evolution reaction in the presence of an adsorbed catalyst by means of square-wave voltammetry

Surface catalytic electrode mechanism based on the hydrogen evolution reaction is analyzed both theoretically and experimentally under conditions of square-wave voltammetry (SWV). The electrode mechanism involves preceding chemical reaction in which the adsorbed catalyst (Cat_(ads)) is undergoing protonation at the electrode surface, i.e., ${\mathrm{Cat}}_{(\mathrm{ads})}+{\mathrm{H}}_{(\mathrm{aq})}^{+}?{\mathrm{CatH}}_{(\mathrm{ads})}^{+}$. The protonated form of the catalyst $({\mathrm{CatH}}_{(\mathrm{ads})}^{+})$ is irreversibly reduced yielding the initial form of the catalyst and atomic hydrogen, i.e., ${\mathrm{CatH}}_{(\mathrm{ads})}^{+}+\mathrm{e}\to {\mathrm{Cat}}_{(\mathrm{ads})}+{\mathrm{H}}_{(\mathrm{aq})}$. The concentration of protons is assumed to be constant in the course of the voltammetric experiment due to the buffered solution, whereas the current is controlled by the variation of the surface concentration of both unprotonated and protonated forms of the catalyst. The overall voltammetric behavior of the system is a specific combination of a simple surface irreversible electrode reaction, surface catalytic reaction and surface reaction preceded by a chemical reaction (CE mechanism). The effect of the thermodynamics and kinetics of the preceding chemical reaction, as well as the kinetics of the irreversible electrode reaction, to the SW voltammetric response is examined in detail. The overall catalytic effect is predominantly controlled by the ratio of the protonation rate constant and the time window of the voltammetric experiment that is represented by the frequency of the potential modulation. Theoretical predictions are illustrated by experiments with the hydrogen evolution reaction at the hanging mercury drop electrode catalyzed by adsorbed famotidine.     

Kinetic studies of reduction of nitrate ions at Sn-modified Pt electrodes using a quartz crystal microbalance

An electrochemical quartz crystal microbalance (EQCM) was used to determine the surface coverage of adsorbed nitrate during the reduction of nitrate on Sn-modified Pt electrodes. The rate equation was derived as a function of the surface coverage of nitrate, the concentration of hydronium ion and the electrode potential for an electrode with a tin coverage of ca. 0.35, which showed the highest activity of the electrodes reported so far. N₂ (30%) and ${\mathrm{NH}}_4^{+}(62\%)$ were produced. From the Tafel slope at a constant surface coverage (?0.115 V decade^?1) and the reaction orders (1.1 and 0.94 with respect to the surface coverage of the adsorbed nitrate and the concentration of hydronium ion, respectively), it is concluded that the rate-determining step is the reaction of the adsorbed nitrate with the hydronium ion and an electron.