2,2,6,6-Tetramethyl piperidine-1-oxyl (TEMPO)-mediated catalytic oxidation of benzyl alcohol in acetonitrile and ionic liquid 1-butyl-3-methyl-imidazolium hexafluorophosphate [BMIm][PF6]: Kinetic analysis

TEMPO (2,2,6,6-tetramethyl piperidine-1-oxyl) is electrochemically oxidized to a stable form of the cation (TEMPO⁺) in acetonitrile (CH₃CN) or 1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIm][PF₆]) media. Cyclic voltammograms were characterized by a well-defined one-electron reversible redox couple in both media at low scan rates. The reduced form of TEMPO⁺ is catalytically regenerated in a follow-up chemical reaction with benzyl alcohol (BA) in the presence of 2,6-lutidine. It was observed that in [BMIm][PF₆], the redox currents are largely suppressed compared to that in CH₃CN. The apparent heterogeneous electron-transfer rate constant () of the quasi-reversible redox reaction of TEMPO was determined at a Pt electrode and found to be 1.9 × 10⁻³ cm s⁻¹ and 4.5 × 10⁻² cm s⁻¹ in [BMIm][PF₆] and CH₃CN, respectively. With the aid of chronoamperometry (CA), the homogeneous rate constant for the catalytic oxidation of benzyl alcohol by TEMPO, in the presence of 2,6-lutidine in CH₃CN was estimated to be 5.53 × 10¹ M⁻¹ s⁻¹ which is approximately double, relative to the value of 2.91 × 10¹ M⁻¹ s⁻¹ determined in [BMIm][PF₆].     

Electrochemical behaviour of americium ions in LiCl-KCl eutectic melt

This work presents a study on the electrochemical properties of AmCl₃ in a molten LiCl-KCl eutectic, at a temperature range of 733-833 K. Transient electrochemical techniques, such as cyclic voltammetry and chronopotentiometry, on inert metallic tungsten working electrode have been used to investigate the reduction mechanism of Am³⁺ ions. The results show that Am³⁺ is reduced to Am metal by a two-step mechanism corresponding to the Am³⁺/Am²⁺ and Am²⁺/Am⁰ transitions. Formal standard potentials of Am³⁺/Am²⁺ ( versus Cl₂/Cl⁻ at 733 K) and Am²⁺/Am⁰ ( versus Cl₂/Cl⁻ at 733 K) redox couples as well as diffusion coefficients of Am³⁺ and Am²⁺ (2.4 × 10⁻⁵ and 1.15 × 10⁻⁵ cm² s⁻¹ at 733 K, respectively) have been calculated at three different temperatures. In the studied range of temperature, the DAm3+/DAm2+ ratio was found to be around 2. In addition, thermodynamic properties have been calculated for Am³⁺ () and Am²⁺ () and compared to thermodynamic reference data in order to estimate activity coefficients (Am³⁺ = 4.7 × 10⁻³ and Am²⁺ = 2.7 × 10⁻² at 733 K) in the molten LiCl-KCl eutectic.     

Electroreduction of peroxycitric acid coexisting with hydrogen peroxide in aqueous solution

The electrochemical reduction of peroxycitric acid (PCA) coexisting with citric acid and hydrogen peroxide (H₂O₂) in the equilibrium mixture was extensively studied at a gold electrode in acetate buffer solutions containing 0.1 M Na₂SO₄ (pH 2.0-6.0) using cyclic and hydrodynamic voltammetric, and hydrodynamic chronocoulometric measurements. The reduction of PCA was characterized to be an irreversible, diffusion-controlled process, and the cyclic voltammetric reduction peak potential () was found to be more positive by ca. 1.0 V than that of the coexisting H₂O₂, e.g., the values obtained at 0.1 V s⁻¹ for PCA and H₂O₂ were 0.35 and −0.35 V, respectively, vs. Ag|AgCl|KCl (sat.) at pH 3.3. The of PCA was found to depend on pH, i.e., at pH > 4.5, the plot of vs. pH gave the slope (−64 mV decade⁻¹) which is close to the theoretical value (−59 mV decade⁻¹) for an electrode process involving the equal number of electron and proton in the rate-determining step, while at pH < 4.5, the was almost independent of pH. The relevant electrochemical parameters, Tafel slope, number of electrons, formal potential (E⁰′), cathodic transfer coefficient and standard heterogeneous rate constant (k⁰′) for the reduction of PCA and the diffusion coefficient of PCA were determined to be ca. 100 mV decade⁻¹, 2, 1.53 V (at pH 2.6), 0.29, 1.2 × 10⁻¹² cm s⁻¹ and 0.29 × 10⁻⁵ cm² s⁻¹, respectively, and except for E⁰′, the obtained values were almost independent of the solution pH. The overall mechanism of the reduction of PCA was discussed.     

Electrochemistry of thorium in LiCl-KCl eutectic melts

This work presents a study of the electrochemical properties of Th chloride ions dissolved in a molten LiCl-KCl eutectic, in a temperature range of 693-823 K. Transient electrochemical techniques such as cyclic voltammetry, chronopotentiommetry and chronoamperometry have been used in order to investigate the reduction mechanism on a tungsten electrode and the diffusion coefficient of dissolved Th ions. All techniques showed that only one valence state was stable in the melt. The reduction into Th metal was found to occur according to a one-step mechanism, through a nucleation-controlled process which requires an overpotential of several 100 mV. At 723 K, the diffusion coefficient is D_Th(723 K) = 3.15 ± 0.15 × 10⁻⁵ cm² s⁻¹. EMF measurements indicated that, at 723 K, the standard apparent potential is (723 K) = −2.582 V versus Cl₂/Cl⁻, and the activity coefficient γThCl₄ (723 K) = 4.6 × 10⁻⁴ on the mole fraction scale (based on a pure liquid reference state).     

Electrochemical study of rat brain acetylcholinesterase inhibition by chlorofos: Kinetic aspects and analytical applications

An electrochemical sensor was applied for investigating the immobilized rat brain acetylcholinesterase inhibition by chlorofos. Two alternative routes were explored as response-generating reactions: (i) direct electrochemical oxidation of thiocholine produced upon acetylthiocholine enzymatic hydrolysis and (ii) reduction of the produced thiocholine with hexacyanoferrate (III), followed by hexacyanoferrate (II) electrochemical detection. The advantages of the direct way are simplicity and higher sensitivity compared to the indirect one, which however avoids the interferences because of the lower potential applied.Enzyme inhibition was identified as competitive, the increasing from 1.31 to 1.43 mmol L⁻¹ with chlorofos concentration in the range 0.2-1.0 mmol L⁻¹ and the maximal rate of the enzyme reaction remaining constant (I_max = 579.30 ± 5.71 μA) in the presence of chlorofos. The inhibition constant was calculated using the Dixon method (K_I = 10.07 mmol L⁻¹).The suppression of the acetylcholinesterase activity by the inhibitor, expressed as current decrease at a constant substrate concentration, was exploited for chlorofos quantification optimized by the design of experiments methodology. Optimal response was obtained for an acetylthiocholine concentration of 0.2 mmol L⁻¹, at 26 °C and pH 7.     

A third-generation hydrogen peroxide biosensor based on Horseradish Peroxidase (HRP) enzyme immobilized in a Nafion-Sonogel-Carbon composite

A third-generation biosensor based on HRP and a Sonogel-Carbon electrode has been fabricated with the aim of monitoring hydrogen peroxide in aqueous media via a direct electron transfer process. The redox activity of native HRP, typical of thin-layer electrochemistry, was observed. The charge coefficient transfer, α, and the heterogeneous electron transfer rate constant, k_s, were calculated to be 0.51 ± 0.04 and 1.29 ± 0.04 s⁻¹, respectively. Topographic study by atomic force microscopy (AFM) shows that the enzyme may have been introduced inside the ionic cluster of the Nafion. The immobilized HRP exhibited excellent electrocatalytical response to the reduction of H₂O₂ and preserved its native state after the immobilization stage. Several important experimental variables were optimized. The resulting biosensor showed a linear response to H₂O₂ over a concentration range from 4 to 100 μM, with a sensitivity of 12.8 nA/μM cm⁻² and a detection limit of 1.6 μM, calculated as (3 S.D./sensitivity). The apparent Michaelis-Menten constant was calculated to be 0.295 ± 0.020 mM. The biosensor showed high sensitivity as well as good stability and reproducibility. The performance of the biosensor was evaluated with respect to four possible interferences.     

A theoretical study of electrical and electrochemical properties of dicyanomethylene derivatives of squaric acid

Theoretical studies on anions of 1,2-dihydroxycyclobuten-3,4-dione (squaric acid, H₂SQ) and the whole series of dicyanomethylene derivatives in gas-phase and in acetonitrile (AN) solution are carried out using density functional theory (DFT) and SCRF-DFT method at B3LYP theory level for the first time. Natural bond orbital (NBO) analyses indicate that π-electron delocalization in the series is stronger. Based on the most stable conformations, a linear correlation was observed between the highest occupied molecular orbital (HOMO) energies, or ionization potential (I_p) and the oxidation potentials measured by cyclic voltammetry, which supports experimental results that systematic substitution of the oxygen atoms in the C₄O₄²⁻ structure with C(CN)₂ groups causes a shift of both the oxidation potentials and towards more positive values, indicating that a consistent delocalization of the negative charge is exerted by these substituents and monoanions are more resistant against oxidation than their counterparts-dianions. The correlations were also observed between experimental K_sem and Gibbs free energy calculated in AN solution. The calculated gas-phase relative basicity of the six dianions follows the order SQ²⁻ > 4-CDCB²⁻ > 2,4-CDCB²⁻ > 3,4-CDCB²⁻ > 2,3,4-CDCB²⁻ > 1,2,3,4-CDCB²⁻ which support the experimental results that the dianions SQ²⁻ and 4-CDCB²⁻ can be protonated, in strongly acidic medium, to the corresponding monoanions, whereas no protonation occurs in the case of di-, tri- and tetra-substituted compounds.     

Investigation on diffusion behavior of Li in LiFePO4 by capacity intermittent titration technique (CITT)

Capacity intermittent titration technique (CITT) was used to investigate the chemical diffusion coefficient () of lithium-ion in LiFePO₄ cathode material. The values of at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 8.8 × 10⁻¹⁶ to 8.9 × 10⁻¹⁴ cm² s⁻¹ and from 1.2 × 10⁻¹⁶ to 8.5 × 10⁻¹⁴ cm² s⁻¹ in the voltage range from 3.2 to 4 V (vs. Li⁺/Li). The transfer coefficients of cathode (0.32-0.42) and anodic (0.26-0.3), and the standard rate constant (1.58 × 10⁻⁹ to 1.30 × 10⁻⁸ cm s⁻¹) were measured from the Tafel plots of LiFePO₄ in the equilibrium potential range from 3.06 to 3.45 V. From these kinetic parameters, the finite kinetics at interface was taken into account to revise the above values of . The revised values of at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 2.44 × 10⁻¹⁵ to 2.21 × 10⁻¹³ cm² s⁻¹ and from 5.81 × 10⁻¹⁶ to 3.22 × 10⁻¹³ cm² s⁻¹ in the voltage range from 3.2 to 4 V. Results show that the approximation of infinite charge-transfer kinetics leads to a spurious value of which is lower than the revised value, and the spurious extent depends on the galvano-charge current of CITT experiment.     

Relative stabilities of Ce(IV) and Ce(III) limiting carbonate complexes at 5-50 °C in Na aqueous solutions, an electrochemical study

The normal potential of the Ce(IV)/Ce(III) redox couple was determined by square wave voltammetry (SWV) at different temperatures in solutions with a constant ratio [CO₃²⁻]/[HCO₃⁻] ≈10 for high ionic strengths (3.29 mol dm⁻³ at 4.39 mol dm⁻³): varies from 259.5 to 198.0 mV/S.H.E. in the 15-50 °C range. Linear variations were found for versus (RT/F)ln(mCO₃²⁻), leading to the stoichiometry, Ce(CO₃)₆⁸⁻ for the Ce(IV) limiting complex. But the slopes of these linear variations were actually found in the range 1.8-1.9, not exactly 2. This was interpreted as dissociation of the Ce(IV) limiting complex following the reaction: Ce(CO₃)₅⁶⁻ + CO₃²⁻ → Ce(CO₃)₆⁸⁻ and as dissociation of the Ce(III) limiting complex following the reaction: Ce(CO₃)₃³⁻ + CO₃²⁻ → Ce(CO₃)₄⁵⁻; for which maximum possible values of log₁₀ K_IV,6 and log₁₀ K_III,4 were estimated via fitting in the 15-50 °C temperature range (log₁₀ K_IV,6 = 0.42 (0.97) and log₁₀ K_III,4 = 0.88 (7.00) at 15 °C (50 °C). The normal potential was found to decrease linearly with T, these variations correspond to , with T⁰ = 298.15 K and . The apparent diffusion coefficient of Ce(IV) was determined by direct current polarography (DCP), cyclic voltammetry (CV) and square wave voltammetry. It was found to depend on the ionic strength and to be proportional to T.     

Li-ion diffusion kinetics in LiMn2O4 thin films prepared by pulsed laser deposition

LiMn₂O₄ thin films were deposited on Au substrates by pulsed laser deposition (PLD). The Li-ion chemical diffusion coefficients of the films, , were measured by cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT), potentiostatic intermittent titration technique (PITT), and electrochemical impedance spectroscopy (EIS). It was found that the values by CV and PITT were in the order of 10⁻¹³ cm² s⁻¹, and those by EIS and GITT were in the range of 10⁻¹³ to 10⁻¹¹ and 10⁻¹⁴ to 10⁻¹¹ cm² s⁻¹, respectively. These data were compared with the previously reported values.     

Ionophores in polymeric membranes for selective ion recognition; impedance studies

Synthetic calix[4]arene-crown ionophores for selective Na⁺ (ionophore L1) and Cs⁺-ions (ionophore L2) recognition find application in ion-selective membrane electrodes (ISE) for analytical purpose. Selectivity coefficients for the electrodes with compounds L1 and L2 are  = −2.6 and  = −2.4, respectively. Electrodes of two different construction: all-solid-state (ASS) (with conducting polymer layer on glassy carbon or platinum as ion-to-electron transducer) and conventional ion-selective electrode (ISE) (with liquid electrolyte and Ag/AgCl) are presented and their properties and lifetime are being compared. Resistance of PVC membrane with ionophores L1 and L2 were within the range 0.15-1.4 MΩ depending on the type of the outer electrolyte and its concentration. Conductivity of the membranes was in the range 0.7 × 10⁻⁸ to 6 × 10⁻⁸ Ω⁻¹ cm⁻¹. Warburg coefficients σ were within 0.16 × 10⁴ to 12.7 × 10⁴ Ω s^−1/2, dielectric constant values ? were in a range 28-60 depending mainly on the type of plasticizer.     

Li-ion diffusion kinetics in LiFePO4 thin film prepared by radio frequency magnetron sputtering

LiFePO₄ thin films were prepared by radio frequency (RF) magnetron sputtering and were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and atomic force microscope (AFM). Li-ion chemical diffusion coefficients, , were measured by potentiostatic intermittent titration technique (PITT), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). The effects of Ag content, film thickness, and film orientation on the electrochemical performance and Li-ion chemical diffusion coefficients of the LiFePO₄ thin films were investigated. values were measured using the liquid electrolyte and the solid electrolyte, and the obtained values were discussed. The values by PITT and EIS were in the range of 10⁻¹⁴ to 10⁻¹² and 10⁻¹⁵ to 10⁻¹² cm² s⁻¹, respectively and that by CV was in the order of 10⁻¹⁴ cm² s⁻¹.     

Analysis of the chemical diffusion coefficient of lithium ions in Li3V2(PO4)3 cathode material

The chemical diffusion coefficients of lithium ions (DLi⁺) in Li₃V₂(PO₄)₃ between 3.0 and 4.8 V are systematically determined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT). The DLi⁺ values are found to be dependent on the voltage state of charge and discharge. Based on the results from all the three techniques, the true diffusion coefficients measured in single-phase region are in the range of 10⁻⁹ to 10⁻¹⁰ cm² s⁻¹. Its apparent diffusion coefficients measured in two-phase regions by CV and GITT range from 10⁻¹⁰ to 10⁻¹¹ cm² s⁻¹ and 10⁻⁸ to 10⁻¹³ cm² s⁻¹, respectively, depending on the potentials. By the GITT, the DLi⁺ varies non-linearly in a “W” shape with the charge-discharge voltage, which is ascribed to the strong interactions of Li⁺ with surrounding ions. Finally, the chemical diffusion coefficients of lithium ions measured by CV, EIS and GITT are compared to each other.     

Anomalous IR optical properties of aggregates of Pd nanoparticles induced through electrochemical cyclic voltammetry

IR optical properties of Pd nanoparticles with different size and aggregation state were studied in the current paper. The dispersed Pd nanoparticles () stabilized with poly(N-vinylpyrrolidone) (PVP) were synthesized by the seeding growth method, in which the seeds were formed step by step through reducing H₂PdCl₄ with ethanol. The dispersed Pd nanoparticles of much large size () were grown from the by keeping the colloid of undisturbed for 150 days at room temperature around 20 °C. The aggregates of () were prepared through an agglomeration process induced during a potential cyclic scanning between −0.25 V and 1.25 V for 20 min at a scan rate of 50 mV s⁻¹. Scanning electron microscope (SEM) patterns confirmed such aggregation of . Fourier transform infrared (FTIR) spectroscopy together with CO adsorption as probe reaction was employed in studies of IR optical properties of the prepared Pd nanoparticles. The results demonstrated that CO adsorbed on films substrated on CaF₂ IR window or glassy carbon (GC) electrode yielded two strong IR absorption bands around 1970 cm⁻¹ and 1910 cm⁻¹, which were assigned to IR absorption of CO bonded on asymmetric and symmetric bridge sites, respectively. Similar IR bands were observed in spectra of CO adsorbed on films, except the IR bands were much weak, whereas CO adsorbed on film produced an IR absorption band near 1906 cm⁻¹, and an anomalous IR absorption band whose direction has been completely inverted around 1956 cm⁻¹. The direction inversion of the IR band of CO bonded to asymmetric bridge sites on was ascribed to the interaction between Pd nanoparticles inside the aggregates. Based on FTIR spectroscopic and cyclic voltammetric results, the aggregation mechanism of Pd nanoparticles from to has been suggested that the agglomeration of Pd nanoparticles was driven by the alteration of electric field across electrode-electrolyte interface, when the PVP stabilizer was stripped via oxidation during cyclic voltammetry.     

Evaluation of electrical energy per order (EEO) with kinetic modeling on the removal of Malachite Green by US/UV/H2O2 process

The kinetics of decolorization of Malachite Green (MG) as a model cationic dye from textile industry, by US/UV/H₂O₂ process, was investigated with nonlinear regression analysis. The experimental results indicated that the decolorization kinetics of MG in this process fit well by pseudo-first order kinetics. With nonlinear regression analysis a model was developed for pseudo-first order constant (k_ap) as a function of operational parameters such as initial concentrations of H₂O₂ (25-600 mg l^− 1) and MG (1.82-9.87 mg l^− 1), temperature (294-307 K) and power density (0.049-0.16 W ml^− 1) as following:

     

Mechanism of formation and electronic structure of semiconducting ZnSb nanoclusters electrodeposited from an ionic liquid

Electrocrystallization of Sb and the compound semiconductor ZnSb has been investigated by in situ SPM methods at the electrified ionic liquid/Au(1 1 1) interface at an elevated temperature of 50 °C for the first time employing the ionic liquid ZnCl₂-[C₄mim]⁺Cl⁻ (45:55). Prior to the underpotential deposition (UPD) process of Sb, ZnCl₃⁻ anions adsorb on the gold surface at the open-circuit potential (OCP). An ordered region - showing the characteristic of a Moiré-like pattern - coexists with a disordered region indicative of an interfacial phase transition. When the potential is reduced to −0.40 V versus Pt/Pt(II), 2D electrocrystallization of Sb starts showing a typical structure of the first monolayer. Further decreasing the potential to −0.5 V a second layer of Sb islands occurs. Stepping the potential from the UPD region to −0.60 V, the OPD of Sb sets in showing randomly dispersed clusters of homogeneous size. Near the ZnSb deposition potential, at ∼−0.95 V, a nearly homogeneous distribution of clusters of spherical shape with diameters up to 15 nm is found. Their corresponding STS curves exhibit an obvious semiconducting behaviour with a gap-energy of ∼0.6 ± 0.2 eV. Experiments at deposition conditions on the Sb-rich or Zn-rich side relative to the ZnSb deposition potential show an obvious doping effect - in the case of Zn excess - which is revealed by the corresponding normalized conductance (NC) spectra.     

Evolution of the local structure and electrochemical properties of spinel LiNixMn2−xO4 (0 ≤ x ≤ 0.5)

A series of Ni substituted spinel LiNi_xMn_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized to study the evolution of the local structure and their electrochemical properties. X-ray diffraction showed a few Ni cations moved to the 8a sites in heavily substituted LiNi_xMn_2−xO₄ (x ≥ 0.3). X-ray photoelectron spectroscopy confirmed Ni²⁺ cations were partially oxidized to Ni³⁺. The local structures of LiNi_xMn_2−xO₄ were studied by analyzing the and A_1g Raman bands. The most compact [Mn(Ni)O₆] octahedron with the highest bond energy of Mn(Ni)O was found for LiNi_0.2Mn_1.8O₄, which showed a Mn(Ni)O average bond length of 1.790 Å, and a force constant of 2.966 N cm⁻¹. Electrolyte decomposition during the electrochemical charging processes increased with Ni substitution. The discharge capacities at the 4.1 and 4.7 V plateaus obeyed the linear relationships with respect to the Ni substitution with the slopes of −1.9 and +1.9, which were smaller than the theoretical values of −2 and +2, respectively. The smaller slopes could be attributed to the electrochemical hysteresis and the presence of Ni³⁺ in the materials.     

Oxygen reduction mechanism on copper in a 0.5 M H2SO4

The mechanism of the oxygen reduction reaction (ORR) in a naturally aerated stagnant 0.5 M H₂SO₄ was studied using electrochemical methods. The cathodic polarization curve showed three different regions; electrochemical impedance spectroscopy (EIS) measurement was used accordingly. The EIS data were analyzed, and the mechanism for the ORR was proposed consequently. The three regions include a limiting current density region with the main transfer of 4e⁻ controlled by diffusion (−0.50 V < E < −0.40 V), a combined kinetic-diffusion region (−0.40 V < E < −0.20 V) with an additional 2e⁻ transfer due to the adsorption of the anions, and a hump phenomenon region (−0.20 V < E < −0.05 V), in which the chemical redox between the anodic intermediate and the cathodic intermediate , together with the electrochemical reaction, synergistically results in the acceleration of the ORR. Therefore, a coupled electrochemical/chemical process (the EC mechanism) in the hump phenomenon region was proposed, and a good agreement was found between the experimental and fitted results. The EC mechanism was confirmed by the deaerated experiments.