Electron transfer reaction and mechanism of oxidation of Inosine

Electrochemical oxidation of Inosine has been studied in the phosphate buffers of pH range 3.3-10.9 at pyrolytic graphite electrode. In the entire pH range a single well-defined oxidation peak (I_a) was observed, when the sweep was initiated in the positive direction. In the reverse sweep no cathodic peak was obtained. The peak potential of the oxidation peak was dependent on pH and shifted to less positive potential with increase in pH. The kinetics of the UV absorbing intermediate was followed spectrophotometrically and the decay occurred in a pseudo first order reaction having k values in the range 0.50-0.92 × 10⁻³ s⁻¹ in the entire pH range studied. The value of n was found to be 2.95 ± 0.3. The products of oxidation were silylated and characterized by using GC-Mass. Two tetramers having CC, CN, NN, CON and COOC linkages were identified. A plausible mechanism for the electrooxidation of Inosine has been suggested.     

Enhancement of ionic conductivity by mixing lithium borate with lithium aluminate

Several lithium borates (Salt A and Salt B) and a lithium aluminate (Salt C) with electron-withdrawing groups, OC₆F₅, OCOCF₃ or N(SO₂CF₃)₂, and oligoether chains (O(CH₂CH₂O)_nCH₃) directly bonded to the ate complex center, B or Al, were prepared. Lithium borate and lithium aluminate were mixed to get mix-salt electrolytes. Higher ionic conductivity was observed for the mix-salt than for the pure-salt. Conductivity as high as 1.1 × 10⁻⁴ S/cm at ambient temperature (25 °C) was achieved for the electrolyte in the optimized composition. The reason for such mixing effect on enhancement of ionic conductivity was discussed. Other electrochemical properties including electrochemical stability, compatibility with lithium anode and cyclic performance were also investigated for the mix-salt electrolytes.     

Two new siloxanic proton-conducting membranes: Part I. Synthesis and structural characterization

The development of stable polymer electrolytes having good proton conductivity, low cost and operating at medium temperatures represent a crucial step in the evolution of polymer electrolyte fuel cells. We describe two new siloxanic proton-conducting membranes that were synthesized through a two-stage protocol. In the first stage, a poly(methyl hydrosiloxane) precursor (P) bearing siloxane side chains with sulfonic acid groups was prepared. In the second step, the hydrolysis of pristine precursor or its derivative obtained by grafting siloxane chains on P yielded two types of membranes with the formulas {Si(CH₃)₃O[Si(CH₃)HO]_21.26[Si(CH₃)((CH₂)₃SO₃H)O]_1.8[Si(CH₃)((CH₂)₃Si(CH₃)₂O)O]₁₄Si(CH₃)₃}_n (A) and {Si(CH₃)₃O[Si(CH₃)HO]_21.26[Si(CH₃)((CH₂)₃SO₃H)O]_1.8[Si(CH₃)((CH₂)₃(Si(CH₃)₂O)_w)O]_v[Si(CH₃)((CH₂)₃Si(CH₃)₂O-)O]_14 − vSi(CH₃)₃}_n (B), with w = 20.31. Polymer membranes of A and B were prepared by means of a hot-pressing process at 80 °C and 10 t/cm². Scanning electron microscopy showed that A and B are rubbery materials with rough and transparent surfaces. Thermogravimetric investigations performed under air atmosphere disclosed that A and B are thermally stable up to at least 198 °C. DSC measurements yielded T_g(s) of −44 and −60 °C for A and B, respectively. The polymers exhibit ionic exchange capacities of 0.33 (A) and 0.15 meq/g (B). FT-IR and FT-Raman investigations revealed that the polymers consist of reticulated siloxane networks with pendant silicone chains having sulfonic acid groups.     

Estimation of standard reduction potentials of alkyl radicals involved in atom transfer radical polymerization

The redox properties of some alkyl radicals, which are important in atom transfer radical polymerization both as initiators and mimics of the propagating radical chains, have been investigated in CH₃CN by an indirect electrochemical method based on homogeneous redox catalysis involving alkyl halides (RX) and electrogenerated aromatic or heteroaromatic radical anions (D⁻). Dissociative electron transfer between RX and D⁻ yields an intermediate radical (R), which further reacts with D⁻ either by radical coupling or by electron transfer. Examination of the competition between these reactions, which depends on ED/D−°, allows determination of the standard reduction potential of R as well as the self-exchange reorganization energy λR/R⁻. The standard reduction potentials obtained for the radicals CH₂CN, CH₂CO₂Et and CH(CH₃)CO₂Me are −0.72 ± 0.06, −0.63 ± 0.07 and −0.66 ± 0.07 V vs. SCE, respectively. Quite high values of λR/R⁻ (from 122 to 164 kJ mol⁻¹) were found for all radicals, indicating that a significant change of structure accompanies electron transfer to R.     

Oxidation of single-walled carbon nanotubes in dilute aqueous solutions by ozone as affected by ultrasound

A clean and simple wet chemical process using dilute aqueous ozone (O₃) solution with or without ultrasound (US) was used to functionalize single-walled carbon nanotubes (SWCNTs). Both O₃ and O₃/US treatments greatly increased the stability of SWCNTs in water. Results of X-ray photoelectron spectroscopy (XPS) showed that the surface oxygen to carbon atomic ratio increased by more than 600% after 72 h of O₃ treatment. Moreover, the effective particle size of SWCNTs was reduced from the initial 4400 to ∼300 and ∼150 nm, after 24 h of O₃ and O₃/US treatment, respectively. The zeta potential of treated SWCNTs decreased from 3.0 to −35.0 mV (at pH 4) after 2 h of treatment with both O₃ and O₃/US. Based on the XPS results, the oxidation pathway was proposed: at the onset of the oxidation reaction, the CC double bond was first converted to COH which was then oxidized to CO and OCOH concurrently. Oxidation reactions could be described well with first order expressions. Treatment time controlled the extent of surface oxidation and subsequently the stability and dispersion of SWCNTs in water.     

Spectroelectrochemical study of perchloroethylene reduction at copper electrodes in neutral aqueous medium

The potential-dependent chemical reaction of perchloroethylene (PCE) on copper in neutral noncomplexing aqueous media is explored by means of surface-enhanced Raman spectroscopy (SERS), linear sweep voltammetry and preparative electrolysis at controlled potential. Voltammetric peaks associated with copper oxide reduction in Na₂SO₄ solution in the presence and the absence of Cl⁻ are correlated with simultaneously acquired SER spectra. Perchloroethylene undergoes a dechlorination process at potentials at E ≤ −0.3 V vs. Ag/AgCl/KCl (3 M), as shown by the emergence of an intense CuCl stretching band at 290 cm⁻¹ and a CH stretching band together with the presence of Cl⁻ in the catholyte. In the potential region between 0 and −0.9 V vs. Ag/AgCl/KCl (3 M) a broad band assigned to CC structures is observed in the triple-bond region (∼1900 cm⁻¹, FWHM = 180 cm⁻¹). In addition, dichloroethylene (DCE) is detected (but not trichloroethylene (TCE)) in this potential region during preparative electrolysis. At potentials lower than −1 V vs. Ag/AgCl/KCl (3 M) carbon residues are the main product, detected on the copper surface by SERS (and confirmed by XPS), whereas in solution higher levels of dichloroethylene and trichloroethylene are detected with a DCE/TCE ratio below 1.     

Theoretical study of the structural effects of polymethylene amines on corrosion inhibition of iron in acid solutions

Quantum chemical calculations were performed on azacyclo C5 to C14 amines, open chain C6 to C14 amines and phenylazacyclo C5 to C14 amines. Inspection of the calculated parameters and corrosion inhibition efficiencies were made to observe any clear links, which might exist between the two. Possible correlations between experimental inhibition efficiencies and parameters such as dipole moment (μ), highest occupied (E_HOMO) and lowest unoccupied (E_LUMO) molecular orbitals and the differences between them, HOMO-LUMO gap (Δ), as well as some structural characteristics were investigated. The models of the inhibitors were optimized with the Modified Neglect of Diatomic Overlap (MNDO) method. The Quantitative Structure Activity Relationship (QSAR) approach has been used and a composite index of some quantum chemical parameters were constructed in order to characterize the inhibition performance of the tested molecules. The inhibition effect of polymethylene amines is closely related to orbital energies and/or energy gap and dipole moment.     

Electrochemical and spectral studies on the catalytic oxidation of nitric oxide and nitrite by high-valent manganese porphyrins at an ITO electrode

Catalytic oxidation of nitric oxide and nitrite by water-soluble manganese(III) meso-tetrakis(N-methylpyridinium-4-yl) porphyrin (Mn(III)(4-TMPyP) was first studied at an indium-tin oxide (ITO) electrode in pH 7.4 phosphate buffer solutions. A stepwise oxidation of Mn(III)(4-TMPyP) through high-valent manganese porphyrin species has been observed by electrochemical and spectroelectrochemical (OTTLE) techniques. The formal potential of 0.63 V for the formation of OMn(IV)(4-TMPyP) has been estimated from OTTLE data. The product, oxoMn(IV) porphyrin, was relatively stable decaying slowly to Mn(III)(4-TMPyP) with a first-order rate constant of 3.7 × 10⁻³ s⁻¹. OMn(IV)(4-TMPyP) has been found to oxidize NO catalytically at potentials about 70 mV more negative than that previously reported for OFe(IV)(4-TMPyP) with good selectivity against nitrite. Nitrite was catalytically oxidized at potentials higher than 1.1 V presumably by OMn(V)(4-TMPyP). OMn(IV)(4-TMPyP) was observed as an intermediate species. Nitrate has been confirmed to be a final product of the electrolysis at 1.2 V, while at 0.8 V nitrite left unchanged, demonstrating that OMn(IV)(4-TMPyP) could not oxidize nitrite. A possible schemes of the catalytic oxidation of NO by OMn^IV(4-TMPyP) and NO₂⁻ by OMn(V)(4-TMPyP) have been proposed.     

Oxidization of carbon nanotubes through hydroxyl radical induced by pulsed O2 plasma and its application for O2 reduction in electro-Fenton

The oxidation of carbon nanotubes (CNTs) by hydroxyl radical produced by pulsed O₂ plasma in a gas-liquid hybrid discharge reactor was conducted with the goal of enhancing their solubility and improving the yield of H₂O₂ in electro-Fenton. Data from the characterization experiments showed that oxygen bearing groups (COH, COO, COOH, CO) were formed on the surface of CNTs. The possible mechanism indicated that introduction of oxygen bearing groups onto CNTs could be attributed to the attacks by hydroxyl radical. The oxidized CNTs were easily dispersed in ethanol. The H₂O₂ yield on the original CNTs was 102 mg/L at −0.85 V after 90 min; in contrast, H₂O₂ yield on CNTs-15 reached 146 mg/L under the same conditions, resulting from the enhancement of the accessibility of O₂ on CNTs. In the electro-Fenton, the removal of methyl orange on the original CNTs was around 40%, and it increased to 95% on CNTs-15.     

A DRIFTS study of the heterogeneous reaction of NO2 with carbonaceous materials at elevated temperature

Commercial carbon black, spark generator soot, Diesel soot from passenger car and high-purity graphite were used for the investigation of the reaction of carbonaceous materials with NO₂ applying diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The occurrence of infrared bands was analyzed as they were depending on the reaction temperature. The focus of interest was particularly on the conversion at an elevated temperature of 400 °C. The formation of oxidation products and the adsorption of NO₂ on the surface of the samples were observed. Infrared bands could be attributed to C(O)OR, RNO₂, and RONO as the main functionalities. The comparison of the results from the different samples revealed that different infrared signals appear when NO₂ is adsorbed either on aliphatic or graphitic domains of soot. However, the formation of characteristic bands for an acidic functional group did not occur. This supports the assumption, made in a prior temperature programmed desorption mass spectroscopy (TPD-MS) study, that this group is a transition state.     

Synthesis and high electrochemical activity of poly(aniline-co-2-amino-4-hydroxybenzenesulfonic acid)

Poly(aniline-co-2-amino-4-hydroxybenzenesulfonic acid) (PAAHB) was synthesized using chemical oxidative copolymerization of aniline and 2-amino-4-hydroxybenzenesulfonic acid (AHB) in the presence of an ionic liquid at 50 °C. The conductivity of the PAAHB copolymer synthesized at the optimum conditions is 0.47 S cm⁻¹ that is lower than that of polyaniline, but is slightly affected by water. The cyclic voltammograms demonstrate that the PAAHB copolymer has excellent redox activity from highly acidic solution to pH 12.0 in a wider potential range. This is attributed to the synergistic effect of the SO₃⁻ and OH functional groups in the copolymer chain and the ionic liquid incorporated into the PAAHB film. It is evident that the pH dependence of the redox activity and conductivity of the PAAHB copolymer prepared chemically is much better than that of polyaniline, and is further improved, compared to the PAAHB copolymer prepared electrochemically. The proton NMR spectrum of the PAAHB copolymer demonstrates that the SO₃⁻ group exists in the copolymer chain instead of the SO₃H group. The ESR spectra show that the ESR signal intensity is a function of the monomer concentration ratio of AHB to aniline in the mixture. The morphology of the PAAHB copolymer is also dependent on the monomer concentration ratio in the mixture.     

Low temperature preparation of carbon-supported PdCo alloy electrocatalysts for methanol-tolerant oxygen reduction reaction

Carbon-supported PdCo alloy electrocatalysts of different Pd/Co atomic ratios were simply prepared in an aqueous solution at room temperature with NH₄F as a complexing agent and H₃BO₃ as a buffer, followed by NaBH₄ reduction. As-prepared PdCo bimetallic nanoparticles show a single-phase face-centered-cubic (fcc) disordered structure, and the mean particle size is found to decrease with increase in Co content. TEM images demonstrated that the as-prepared PdCo alloy nanoparticles are well dispersed on the surface of the carbon support with a small particle size and a relatively narrow particle size distribution. For example, the average particle size of a Pd₂Co₁/C catalyst is ca. 3.0 nm, which is much smaller than that of the PdCo/C bimetallic nanoparticles reported by others. An activity evaluation of the oxygen reduction reaction (ORR) on as-prepared PdCo/C catalysts with a rotating disk electrode (RDE) technique indicated that the maximum ORR mass activity was observed for a Pd:Co atomic ratio of 4:1, but the highest specific activity was found on a Pd:Co atomic ratio of 2:1. Kinetic analysis reveals that the ORR on PdCo/C catalysts follows a four-electron process leading to water. Moreover, the PdCo/C catalyst exhibited much higher methanol tolerance during the ORR than the Pt/C catalyst, assessing that it may function as a methanol-tolerant cathode catalyst in a direct methanol fuel cell (DMFC).     

Monitoring the real time growth of a self-assembled monolayer on a polymer electrolyte surface

The growth of a self-assembled monolayer (SAM) at the surface of a polymer electrolyte has been shown to inhibit the formation of the passivating layer that forms when the polymer is in contact with lithium metal. In this work, ac impedance spectroscopy was used to monitor the formation of SAM layers on polyethylene oxide (PEO) polymer electrolyte thin films as a function of time. To monitor SAM growth, thin PEO films were cast onto interdigitated electrodes. The electrodes were subsequently immersed in a saturated SAM solution and the film impedance was measured. SAM molecules with the general formula: H(CH₂)₃₂(CH₂CH₂O)_yH (y = 2, 10, 40) were used. Growth occurred due to interactions with the ethylene oxide portion of the SAM molecules with the PEO surface. To visualize SAM growth impedance data at a single frequency sensitive to changes at the solution interface was plotted verses time. At the point of immersion, a sharp increase in impedance was observed. With time, the rate at which the impedance increased slowed and ultimately leveled off presumably indicating the point at which a nearly complete monolayer had formed. SAM growth was verified using attenuated total reflectance infrared spectroscopy (ATR-IR).     

Covalent layer-by-layer type modification of electrodes using ferrocene derivatives and crosslinkers

A series of mono-substituted ferrocenes (Fecp₂R, R = CONHCH₂N(C₂H₄NH₂)₂ (4), R = CH₂NHCH₂N(C₂H₄NH₂)₂ (5)) and di-substituted ferrocenes (Fe(cpR)₂, R = COF (3)) have been prepared, with 4 and 5 representing new compounds. The ferrocenes were grown layer-by-layer on metal oxide electrodes and crosslinked. The underlying principle of synthesis is based on repetitive and fast amide forming reactions between trimesic acid chloride (2) or Fe(cpCOF)₂ (3) and tris(2-aminoethyl)amine (6), or one of the amine-substituted ferrocenes 4 or 5 on a 3-aminopropyl triethoxysilane (APS) (1) modified ITO, FTO or ATO electrode. Linear growth of the films electroactivity with the number of cascade steps is observed for up to 12 cascade steps yielding 1.1 × 10⁻⁸ ferrocene centers/cm² on a flat ITO electrode, whereas 3 cascade steps yield 2.2 × 10⁻⁷ ferrocene centers/cm² on a mesoporeous ATO electrode. In case of the ATO electrode, the inner walls of the mesopores exhibiting a very large surface are completely modified in the procedure. Beside this large coulometry fast electron transfer kinetics, high stability and low optical density in both oxidation states is observed.The electrodes were checked for their potential use as optically transparent counter electrodes in electrochromic devices.     

Diffusivity of anion vacancies in WO3 passive films

The WO₃ films were grown in 0.1 M HClO₄ aqueous solution, at different formation potentials (E_f) in the range of 2.0-7.0 V versus sce, on W electrode. The anion diffusion coefficient (DO) of WO₃ films was calculated from EIS spectra, following the surface charge approach (at high-field limit approximation), the Point Defect Model and the Mott-Shottky analysis. Among the parameters necessary to evaluate DO, the half-jump distance (a) is very relevant, given that a small variation in a has a great impact in the calculation of DO. In this work, it is proposed the half-jump distance (a) should be evaluated from spectroscopic data (available in the literature). The value of a (∼1.9 Å) is taken from lattice constants of a-WO₃ (amorphous-WO₃), with different values of N (coordination number), and the lattice constants of m-WO₃ (monoclinic-WO₃). The calculated value of DO was ∼3 × 10⁻¹⁷ cm²/s.