Anodic Oxidation Peak Research Articles

“Deactivation of copper electrode” in electrochemical reduction of CO 2

Metallic Cu electrode can electrochemically reduce CO 2 to CH 4, C 2H 4 and alcohols with high yields as revealed by the present authors. Many workers reported that formation of CH 4 and C 2H 4 rapidly diminishes during electrolysis of CO 2 reduction. This paper shows that such deactivation of Cu electrode is reproduced with electrolyte solutions prepared from reagents used by these workers. Deactivated Cu electrodes recovered the electrocatalytic activity for CO 2 reduction by anodic polarization at −0.05 V versus she in agreement with the previous reports. Features of the deactivation depend greatly on the individual chemical reagents. Purification of the electrolyte solution by preelectrolysis with a Pt black electrode effectively prevents the deactivation of Cu electrode. Anode stripping voltammetry of Cu electrodes, which were deactivated during electrolysis of CO 2 reduction, showed anodic oxidation peaks at ca. −0.1 or −0.56 V versus she. The severer the deactivation of the Cu electrode was, the higher electric charge of the anodic peak was observed. It is presumed that some impurity heavy metal, originally contained in the electrolyte, is deposited on the Cu electrode during the CO 2 reduction, poisoning the electrocatalytic activity. On the basis of the potential of the anodic peaks, Fe 2+ and Zn 2+ are assumed to be the major contaminants, which cause the deactivation of the Cu electrode. Deliberate addition of Fe 2+ or Zn 2+ to the electrolyte solutions purified by preelectrolysis exactly reproduced the deactivation of a Cu electrode in CO 2 reduction. The amount of the deposited Fe or Zn on the electrode was below the monolayer coverage. Electrothermal atomic absorption spectrometry ( etaas) showed that Fe originally contained in the electrolyte solution is effectively removed by the preelectrolysis of the solution. Mechanistic difference is discussed between Fe and Zn in the deterioration of the electrocatalytic property of Cu electrode in the CO 2 reduction. The concentration of the impurity substances originally contained in the chemical reagents as Fe or Zn is estimated to be far below the standard of the impurity levels guaranteed by the manufacturers. Presence of trimethylamine in the electrolyte solution also severely poisons a Cu electrode in the CO 2 reduction. It was concluded that the deactivation of Cu electrode in CO 2 reduction is not caused by adsorption of the products or the intermediates produced in CO 2 reduction.

Voltammetric Behavior of Chlorophyll a at a Screen‐Printed Carbon Electrode and Its Potential Role as a Biomarker for Monitoring Fecal Contamination

Direct cyclic voltammetric determination of Chlorophyll a (Chl a) at a screen‐printed carbon electrode (SPCE) resulted in a single, irreversible anodic oxidation peak at E p = +400 mV vs. Ag/AgCl. Electrochemical investigations revealed that Chl a was adsorbing onto the SPCE surface. This adsorption phenomenon allowed the development of a method for Chl a determination, based on medium exchange followed by adsorptive stripping voltammetry (AdSV). The final protocol was optimised with respect to pH of accumulation/measurement buffer, accumulation time, and acetone content of accumulation buffer. Using this protocol, it was possible to determine Chl a in aqueous phosphate buffer over the concentration range 0.014–2.24 µM. This optimised protocol was applied to the determination of Chl a in faeces from dairy cows. The endogenous content of Chl a was calculated to be 15 mg g−1 of dried faecal matter. The mean recovery for the method was 104%, with an overall coefficient of variation of 18.4%. Based on these data, the detection limit for dried faecal matter was 300 µg in 10 mL of simulated teat wash water. This approach demonstrates that the electrochemical detection of Chl a is a potential means of monitoring faecal contamination in the dairy industry.

Voltammetric behaviour, homogeneous charge transport dynamics and electrocatalytic properties of an Os 2+ functionalised pyrrole monomer

The Os 2+ functionalised pyrrole monomer, osmium-bis- N,N′-(2,2′-bipyridyl)- N-(pyridine-4-yl-methyl- (8-pyrrole-1-yl-octyl)-amine)chloride, 1, has been synthesised and characterised by spectroscopic (UV–Vis, 1H NMR, IR spectroscopy) techniques and cyclic voltammetry. Solution phase studies of 1 gave a redox couple associated with the Os 3+/2+ and an irreversible wave associated with the oxidation of the pyrrole moiety. Attempts to form stable polymeric films of 1 on vitreous carbon electrodes, by homopolymerisation, proved unsuccessful. However electroactive films of 1 were obtained by the electrooxidation of 1 through the Os 3+/2+ couple in low dielectric media such as toluene + acetonitrile mixtures. The efficiency of film deposition and properties depend upon the supporting electrolyte and the solvent employed. Films formed in mixtures of acetonitrile and low dielectric constant solvents (dichloromethane and toluene) are electrochemically active and exhibit a redox couple associated with the Os 3+/2+ system. The voltammetric behaviour of these solid-state films was investigated in both aqueous and non-aqueous solvents with a variety of supporting electrolytes. Upon redox switching between the Os 3+/2+ redox states the solid-state charge-transfer processes are coupled to the insertion/expulsion of anions from/to the solution phase. Scanning electron microscopy reveals that films formed in the presence of acetonitrile exhibit different morphology from films formed in acetonitrile-free solutions. Slow and fast linear sweep voltammograms have been employed to provide an absolute determination of the fixed site concentration as 1.5 M and an apparent diffusion coefficient of 5.2 × 10 −12 cm 2 s −1 in 0.3 M KCl. Electroactive films of 1 were then investigated for their electrocatalytic ability towards the oxidation of ascorbic acid in acidic medium. The anodic oxidation peak current was linearly dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range 0.1–2 × 10 −3 M of ascorbic acid with a correlation coefficient of 0.9993. The detection limit (3 σ) was found to be 5.5 × 10 −5 M.

In situ electrochemical atomic force microscopy of lead electrodes in sulfuric acid solution with or without lignin during anodic oxidation and cathodic reduction

Abstract The surface morphologies of lead sheets in sulfuric acid solution, with or without lignin, under temperature control during anodic oxidation and cathodic reduction are investigated by using in situ electrochemical atomic force microscopy combined with cyclic voltammetry. Whether lignosulfonate (lignin) is added or not, it is found at −20 °C that the precipitation of lead sulfate crystals occurs immediately after supersaturation of dissolved Pb 2+ ions at the anodic oxidation peak on the cyclic voltammogram. It is also observed that the density of lead sulfate crystals formed on the lead electrode after anodic oxidation decreases and the crystal size becomes larger when lignin is added. Furthermore, the addition of lignin delays dissolution of lead sulfate crystals during cathodic reduction at room temperature (RT).

Electrochemical oxidation of the chalcopyrite surface: an XPS and AFM study in solution at pH 4

The electrochemical oxidation of chalcopyrite (CuFeS 2) has been studied at pH 4 using voltammetry, coulometry, X-ray photoelectron spectroscopy (XPS) and both ex situ and in situ atomic force microscopy (AFM). Between 500 and 650 mV an anodic oxidation peak is observed, prior to the onset of the main decomposition reactions. Chalcopyrite electrodes in contact with electrolyte show some release of Cu into solution even without an applied potential. At 500 and 650 mV, the loss of Cu from the surface increases by a factor of 2 and 6, respectively. Oxidation at 500 mV results in the formation of a mixed oxide or hydroxide of iron, coincident with islands (<0.15 μm wide) of reaction products observed on the surface using AFM. The surface coverage of these islands increases with amount of charge passed. Oxidation at 650 mV shows similar processes have occurred, but with a greater island surface coverage and a more deeply altered surface. XPS depth profiling suggests iron oxide or hydroxide is now a major phase in the top ∼40 Å, with significant sulphate also formed. Observation of islands (alteration products) using in situ AFM under potential control shows that these features are not an artefact of the preparation methods.

New insights into the oxidation pathways of apomorphine

A detailed study of the oxidative behaviour of apomorphine in aqueous media is reported. Resorting to the synthesis of apomorphine derivatives it was possible to identify all the anodic oxidation peaks of apomorphine, which are related to the oxidation of the catechol and tertiary amine groups. These findings were revealed to be important since they could lead to a better understanding of the biological interactions of apomorphine and gain insight into its metabolic pathways. During the voltammetric studies, it was also found that apomorphine forms a complex with borate through the catechol group leading to an increase of its oxidation potential. This property could be very useful with regard to the stabilization of apomorphine solutions since it could drastically reduce its autoxidation.

Influence of Topical Fluorides on corrosion behaviour of silver amalgam alloys

AbstractPotentiodynamic polarization and cyclic voltametry measurements were employed to study the effect of topical fluoride preparations on corrosion of silver amalgam alloy in synthetic saliva and aqueous fluoride solutions. Results support that small amounts of copper and indium alloyed to silver – tin amalgam enhance corrosion resistance considerably. Indiloy shifts both the open circuit potential and the anodic polarization curve to more positive potential values, this lead to a decreases in the current density. The weak activity of Indiloy as compared with Spheraloy is attributed to the fact that the amount of γ2 phase (tin–mercury) present in the former is much lesser than that present in the latter. Findings also show that fluoride solutions (NaF, SnF and SnF + HCI) enhance the dissolution rate of the investigasted alloys and the polarization curves illustrate the appearance of an active – passive transitions. This behaviour was discussed on the basis of complex formation. Cyclic voltammograms confirm the results obtained from the potentiodynamic polarization measurements and elucidate the anodic oxidation peaks of silver oxide and copper verifying the proposed mechanism.

Interfacial Electrochemistry of Pyrite Oxidation and Flotation.: I: Effect of Borate on Pyrite Surface Oxidation

Sodium tetraborate (Na2B4O7) has been widely used as an electrolyte and pH buffer in studying the interfacial electrochemistry of sulfide minerals in relation to sulfide mineral flotation. In all the previous studies published so far, borate was regarded as an inert electrolyte/pH buffer, and its reactions with the sulfide minerals were completely overlooked. In this first part of this series papers, the complicating effects of borate on the interfacial electrochemistry of pyrite have been studied. It has been demonstrated that borate is not an inert electrolyte/pH buffer. It strongly reacts with the surfaces of pyrite. In the borate solutions, the surface oxidation of pyrite is strongly enhanced. The first and rate-determining step of the reaction between borate and pyrite has been shown to be the following irreversible reaction:[formula]This reaction appears in the voltammogram as an anodic oxidation peak at potentials of more than 0.4 V lower than the commencement of pyrite oxidation in sodium perchlorate or nitrate electrolyte solutions. As the borate concentration increases, the peak current increases linearly, while the peak potential shifts positively at 240 mV per decade. On a rotating-disc electrode, the peak becomes a plateau. The limiting current density is a linear function of the square root of the rotation speed at relatively low rotation speeds. The Tafel slope, i.e.,dE/d{logI}, is close to 240 mV per decade and is independent of the rotation speed and borate concentration. The results indicate that charge transfer coefficient (α) is 0.25.

Anodic behaviour and passivation of a lead electrode in sodium carbonate solutions

Cyclic voltammograms of a lead electrode were obtained in Na2CO3 solution as a function of the starting potential, electrolyte concentration and voltage scanning rate. The shape of the voltammograms was found to depend on the starting potential as well as the sweep number. This is probably due to changes in the activation state of the electrode surface. The first anodic portion of the voltammograms is characterized by a shoulder and two peaks corresponding to the formation of PbCO3, PbO and PbO2, respectively. The cathodic portion shows the occurrence of two peaks corresponding to the reduction of PbO2 to PbO and PbO to Pb, successively, followed by the formation of PbH2. An increase in concentration of CO32−ions leads to a negative shift in the values of the peak potentials, Ep, accompanying the formation of PbO and PbO2. In addition, the current density for both the anodic oxidation peaks showed marked dependence on the concentration of the electrolyte. An increase in the scanning rate was observed to lead an increase in the size of the voltammograms. The current density of both the anodic peaks and the anodic passivation region were proportional to v1/2. Such behaviour is expected in a diffusion-controlled processes. In addition, the anodic peaks are shifted towards more positive values of potential, whereas the cathodic peaks are shifted in the negative direction, indicating irreversible formation of the passive film on the electrode surface.

Characterization of conducting copolymers of 2,2′‐bithiophene and pyrrole by cyclic voltammetry and UV/visible spectroscopy

AbstractSeveral polymers have been prepared by electropolymerization of mixtures of pyrrole (PY) and 2,2′‐bithiophene (BT). Cyclic voltammetry on prepared polymer films shows three anodic oxidation peaks, two of which match the oxidation potentials of homopolymeric PY and BT, and a third which is intermediate. UV/visible spectroscopy displays a unique spectrum for each of the reduced and oxidized forms of these polymers. Nernstian plots from UV/visible data exhibit three well‐defined redox couples in polymer films produced at 1.3 V. Overall, the data strongly support the formation of a copolymer, consisting of three distinct oxidizable units. Two of these can be attributed to short blocks of either PY or BT, and a third to random and alternate groupings of PY and BT. The polymers produced are electrically conductive, but the conductivity drops rapidly as BT units are introduced into a homopolymer of PY. © 1992 John Wiley &amp; Sons, Inc.

Electroreduction of Nifedipine

At pH > 6, the nitro group of nifedipine is reduced in a four-electron step to a hydroxylamino group. At pH < 6, the hydroxylamino derivative undergoes an acid-catalyzed dehydration, and the rate of this reaction governs the limiting current. Resulting quinonemethide is further reduced, and a six-electron reduction step results. Formation and reducibility of the quinonemethide is favored by conjugation with the 1,4-dihydropyridine ring. Further reduction of the hydroxylamino derivative is confirmed by the absence of the anodic oxidation peak on the reverse scan at pH < 6.5. At potentials more negative than that of the nitro group, the protonated form of the hydroxylamino derivative is reduced between pH 4 and 8 in direct current (dc) polarography and between pH 6 and 11 in cyclic voltammetry, the difference resulting from the difference in the measurement time window. The yields and fate of the hydroxylamino derivative obtained in controlled potential electrolysis with a hanging mercury drop electrode (HMDE) differ from those in dc polarography, indicating the presence of different competing reactions. For analyses, pH 2 and 11 are most suitable.

Characterization of NiCo 2O 4 electrodes for O 2 evolution : Part I. Electrochemical characterization of freshly prepared NiCo 2O 4 electrodes

In order to correlate the electrocatalytic activity and spinel structure, the NiCo 2O 4 catalyst has been characterized using cyclic voltammetric and rotating ring-disc electrodes. The surface morphology and composition of the freshly prepared NiCo 2O 4 layer are found to depend on the thermal treatment, particularly on the temperature: the increase in activity with decreasing temperature has been correlated with the increase in surface area and the change in surface composition. The voltammogram of a fresh NiCo 2O 4 electrode exhibits two anodic oxidation peaks, representing one-electron transfer surface redox reactions. The first anodic peak has been assigned to a Co 2+ 3+ transition, whereas the second oxidation peak can be attributed to a M 3+ 4+ transition (M = Co or Ni). Furthermore, it was shown that the highest oxidation state up to 1.55 V (vs. RHE) is 4+, and that the hydroxyl ion plays an important role in the electrochemical reactions prior to oxygen evolution. Three different models have been suggested for the conjugated electrochemical processes, before oxygen evolution takes place.

The Apparent Activation Energy for the Electrochemical Oxidation of HCOOH at a Platinum Electrode Chemically Modified by Bi Adatoms in 0.5M H 2 SO 4

The enhancement by Bi adatoms of the anodic peak for oxidation of at a Pt electrode in was studied as a function of temperature. The results are interpreted on the basis of an increased value of the pre‐exponential term in the Arrhenius rate equation, which is attributed to an increase in the frequency of the vibrational motion leading to desorption of the strongly adsorbed product of the organic oxidation. This increase is concluded to be the direct effect of the site‐to‐site surface migration of the freshly deposited Bi adatoms.

Investigation of thin-film α-and β-Ni(OH)2 electrodes in alkaline solutions

Thin films of α - and β -Ni(OH) 2 were investigated on Pt, C and Ni substrates. The growth of α -Ni(OH) 2 during cathodic deposition from Ni(NO 3 ) 2 electrolyte was followed ellipsometrically; a transparent film is formed with refractive index n − ki =1.41±0.03−0 i (λ=546.1 nm). For β -Ni(OH) 2 , prepared by conversion of α -Ni(OH) 2 in strong alkaline solution, the refractive index was found to be 1.46±0.03−0 i . In the cyclic voltammograms of α - and β -Ni(OH) 2 the anodic oxidation peak for β -Ni(OH) 2 is about 70 mV higher than for α -Ni(OH) 2 . The characteristic peaks of both α - and β -Ni(OH) 2 are observed at film electrodes that were deposited at very low cd. For γ-NiOOH a refractive index 1.54−0.39 i is obtained (if no change in thickness during oxidation is assumed), or 1.74−0.51 i if a thickness change on the basis of the ratio of the densities is assumed. The optical transients during oxidation- and reduction-cycling indicate a different behaviour of α - and β -Ni(OH) 2 .

Investigation of thin-film α- and β-Ni(OH) 2 electrodes in alkaline solutions

Thin films of α- and β-Ni(OH) 2 were investigated on Pt, C and Ni substrates. The growth of α-Ni(OH) 2 during cathodic deposition from Ni(NO 3) 2 electrolyte was followed ellipsometrically; a transparent film is formed with refractive index n− ki=1.41±0.03−0 i (λ=546.1 nm). For β-Ni(OH) 2, prepared by conversion of α-Ni(OH) 2 in strong alkaline solution, the refractive index was found to be 1.46±0.03−0 i. In the cyclic voltammograms of α- and β-Ni(OH) 2 the anodic oxidation peak for β-Ni(OH) 2 is about 70 mV higher than for α-Ni(OH) 2. The characteristic peaks of both α- and β-Ni(OH) 2 are observed at film electrodes that were deposited at very low cd. For γ-NiOOH a refractive index 1.54−0.39 i is obtained (if no change in thickness during oxidation is assumed), or 1.74−0.51 i if a thickness change on the basis of the ratio of the densities is assumed. The optical transients during oxidation- and reduction-cycling indicate a different behaviour of α- and β-Ni(OH) 2.

The electrochemical behaviour of dithiocarbamato complexes of palladium(II) in DMSO at the mercury electrode

The reduction of palladium(II) dithiocarbamates at the mercury electrode in DMSO has been investigated using a number of electrochemical techniques. All the palladium dithiocarbamates gave similar polarograms with only one main reduction wave. Limiting currents varied linearly with ν h in the concentration range 0.20–1.0 m M and the electrode reaction was first order with respect to the depolarizer. Half-wave potentials were independent of ligand concentration over the range 2.0 m M to 0.128 M. Log-plot analyses gave linear plots with slopes indicative of a one-electron, quasi-reversible reduction. On examination by cyclic voltammetry, most of the complexes exhibited voltammograms consisting of seven peaks, four cathodic reduction peaks and three anodic oxidation peaks. In agreement with the results of polarography, cyclic voltammetry and chronoamperometry showed that, for the main reduction wave, electron transfer is essentially quasi-reversible: however, there was evidence for increasing departure from diffusion control and for the presence of an ECE-type reaction. The chemical reaction involves a dissociation to produce a palladium species more easily reduced than the palladium dithiocarbamate. For the five complexes investigated, rate constants for the chemical step showed a time dependence at electrolysis time above 3 s, consistent with an ECE mechanism. Exhaustive reduction of palladium dithiocarbamate at the mercury pool electrode gave non-integral n values (2> n>1), also in agreement with an ECE mechanism.