Electrografting of the cyanomethyl radical onto carbon and metal surfaces

Nanometer layers are grafted on the surface of carbon or metallic electrodes by electrochemical reduction of iodo or bromoacetonitrile in acetonitrile. The structure of these layers, (carbon or metal)–[CH₂–CH(NH₂)–]_n, is determined by electrochemistry, ellipsometry and IRRAS. The bond between the surface and the organic layer is evidenced by ToF-SIMS. A mechanism is proposed to account for the formation of the layers: the grafting is assigned to the reaction of the cyanomethyl radical, CH₂CN, with the electrode surface and the latter is partly reduced to the cyanomethyl anion ⁻CH₂CN that attacks the first grafted –CH₂CN group, leading to the growth of the layer. It is also possible to produce the same radical by oxidation of the ⁻CH₂CN anion -obtained by deprotonation of acetonitrile-, but in this case only traces of grafting are detected on the electrode as the radical is trapped by the large excess of anions.     

New insights into the influence of activated carbon surface oxygen groups on H2O2 decomposition and oxidation of pre-adsorbed volatile organic compounds

In this study, the influence of the surface oxygen groups of activated carbons (ACs) on the decomposition of H₂O₂ and the consequent OH radicals generation is investigated. The oxidation of pre-adsorbed volatile organic compounds by H₂O₂ is also studied. Four ACs, with low percentage of inorganic matter (<0.2%), similar textural properties but differing in their surface oxygen content were evaluated. The surface oxygen groups of the ACs were characterised by using appropriate characterisation techniques (temperature programmed desorption and X-ray photoelectron spectroscopy). The kinetic curves of H₂O₂ decomposition were very similar for all the ACs. However, different profiles in the production of OH radicals were observed. OH radicals generation seemed to be promoted by low surface oxygen contents. Oxidation of two volatile organic compounds (VOCs) of different polarity, methyl ethyl ketone (MEK) and toluene, pre-adsorbed onto the ACs was finally investigated. H₂O₂ was used as oxidising agent. Both VOCs presented similar maximum oxidation rates, around 70%, in spite of their different hydrophobicity. Some evidences are provided supporting that oxidation of pre-adsorbed VOCs can take place in the inner pore structure of the ACs.     

Study on the oxidation of C4-phenolic-1,4-dihydropyridines and its reactivity towards superoxide radical anion in dimethylsulfoxide

Electrochemical characterization on glassy carbon electrode (GCE) and reactivity with superoxide radical anion in aprotic medium of three new synthesized C4-phenolic-1, 4-dihydropyridines is reported.Voltammetry, coulometry, controlled-potential electrolysis (CPE), UV-vis spectroscopy, ¹H NMR techniques were employed for the characterization of title compounds.The oxidation mechanism involves initially an oxidation process on the phenol moiety with the formation of the corresponding quinone followed by a second one affecting the dihydropyridine ring to give the pyridine derivative. Both processes appeared irreversible in character.Cyclic voltammetry was used to generate O₂⁻ by reduction on GCE of molecular oxygen in DMSO. The reactivity of DHPs towards O₂⁻ was directly measured by the anodic current decay of the radical in the presence of increasing concentration of tested 1,4-dihydropyridines and compared with the reaction of the reference antioxidant, Trolox. The linear correlations obtained between the anodic current of O₂⁻ and compound concentrations in the range between 0.01 mM and 1.00 mM allowed the determination of both the DHP antioxidant index (AI) and the concentrations needed to consume 50% of O₂⁻. Synthesized C4-phenolic 1,4-dihydropyridines exhibited significant scavenging capacity towards superoxide radical anion higher than Trolox and tested commercial 1,4-dihydropyridines.     

Inactivation of Escherichia coli in Na2SO4 electrolyte using boron-doped diamond anode

Electrochemical disinfection in chloride-free electrolyte has attracted more and more attention due to advantages of no production of disinfection byproducts (DBPs), and boron-doped diamond (BDD) anode with several unique properties has shown great potential in this field. In this study, inactivation of Escherichia coli (E. coli) was investigated in Na₂SO₄ electrolyte using BDD anode. Firstly, disinfection tests were carried on at different current density. The inactivation rate of E. coli and also the concentration of hydroxyl radical (OH) increased with the current density, which indicated the major role of OH in the disinfection process. At 20 mA cm⁻² the energy consumption was the lowest to reach an equal inactivation. Moreover, it was found that inactivation rate of E. coli rose with the increasing Na₂SO₄ concentration and they were inactivated more faster in Na₂SO₄ than in NaH₂PO₄ or NaNO₃ electrolyte even in the presence of OH scavenger, which could be attributed to the oxidants produced in the electrolysis of SO₄²⁻, such as peroxodisulfate (S₂O₈²⁻). And the role of S₂O₈²⁻ was proved in the disinfection experiments. These results demonstrated that, besides hydroxyl radical and its consecutive products, oxidants produced in SO₄²⁻ electrolysis at BDD anode played a role in electrochemical disinfection in Na₂SO₄ electrolyte.     

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.     

Mechanism of heterogeneous catalytic ozonation of nitrobenzene in aqueous solution with modified ceramic honeycomb

The degradation efficiencies of nitrobenzene in aqueous solution were investigated by semi-continuous experiments in the processes of ozone alone, ozone/ceramic honeycomb (CH) and ozone/modified ceramic honeycomb (MCH). MCH with 1.0% Mn and 0.5% Cu had more pronounced catalytic ability than CH to accelerate the degradation of nitrobenzene, to increase the utilization efficiency of ozone, to improve the concentrations of hydrogen peroxide (H₂O₂) formation and hydroxyl radical (OH) initiation, and to enhance the removal efficiency of TOC. The modification process of CH with the metals enhanced the density of surface hydroxyl groups, which determines the initiation of OH from ozone decomposition and the generation of intermediate species on heterogeneous catalytic surface, yielding the acceleration of the degradation of nitrobenzene in aqueous solution. Possible reaction mechanism of ozone with heterogeneous catalytic surface in aqueous solution was proposed, and the formation mechanism of H₂O₂ and OH was also discussed according to the combined reactions in heterogeneous and homogeneous catalytic systems.     

Poly(methylene blue) doped silica nanocomposites with crosslinked cage structure: Electropolymerization, characterization and catalytic activity for reduction of dissolved oxygen

The poly(methylene blue) (PMB) modified glassy carbon electrode (GCE) exhibiting different electrochemical behavior was prepared via two methods, respectively. The PMB polymer, derived from the two-step electropolymerization, suffered structure conversion between Poly(leuco-MB) and Poly(MB) during cyclic voltammetric measurement and exhibited electrocatalytic activity for reduction of dissolved oxygen (DO). The monodispersed hollow methylene blue doped SiO₂ nanoparticles were synthesized in the W/O microemulsion. A new material, PMB doped SiO₂ nanocomposites, presenting monolayer sheets with crosslinked cage structure, were electrochemically polymerized on GCE surface. Compared with PMB film, the nanocomposite material provided a significantly improved sensitivity for reduction of DO and an excellent ability to resist interference from macromolecule contaminants. The detection of DO was performed using the nanocomposite material modified electrode. The calibration curve was linear over a DO concentration range of 0.112–5.78 mg L⁻¹ with a correlation coefficient of 0.998 and a detection limit (3σ) of 0.037 mg L⁻¹.     

Electrocatalytic degradation of aspen lignin over Pb/PbO2 electrode in alkali solution

A novel procedure about electrochemical catalysis degradation of aspen lignin with Pb/PbO₂ anode in the three-dimensional electrode (TDE) reactor was investigated. SEM, XRD and cyclic voltammogram tests were employed to study the surface morphology, composition and the electrochemical redox performance of the fabricated Pb/PbO₂ electrode. The lignin was cracked by OH and hydrogenated by [H] atom generated from alkaline water electrolysis, leading to the production of 4-methylanisole and other products. Raw material lignin concentration, current density, temperature and time were optimized. The pathway of electrocatalytic degradation and hydrogenation process of lignin in alkaline solution was also discussed.     

A novel biosensor based on electro-co-deposition of sodium alginate-Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>-graphene composite on the carbon ionic liquid electrode for the direct electrochemistry and electrocatalysis of myoglobin

A novel biosensor based on electro-co-deposition of myoglobin (Mb), sodium alginate (SA), Fe₃O₄-graphene (Fe₃O₄-GR) composite on the carbon ionic liquid electrode (CILE) was fabricated using Nafion as the film forming material to improve the stability of protein immobilized on the electrode surface, and the modified electrode was abbreviated as Nafion/Mb-SA-Fe₃O₄-GR/CILE. FT-IR and UV–vis absorption spectra suggested that Mb could retain its native structure after being immobilized in the SA-Fe₃O₄-GR composite film. The electrochemical behavior of the modified electrode was studied by cyclic voltammetry, and a pair of symmetric redox peaks appeared in the cyclic voltammograms, indicating that direct electron transfer of Mb was realized on the modified electrode, which was ascribed to the good electrocatalytic capability of Fe₃O₄-GR composite, the good biocompatibility of SA and the synergistic effects of SA and Fe₃O₄-GR composite. The electrochemical parameters of the electron transfer number (n), the charge transfer coefficient (α) and the electron transfer rate constant (k _s) were calculated as 0.982, 0.357 and 0.234 s^?1, respectively. The modified electrode exhibited good electrocatalytic ability to the reduction of trichloroacetic acid (TCA) with wide linear range from 1.4 to 119.4 mmol/L, low detection limit as 0.174 mmol/L (3σ), good stability and reproducibility.     

Photocatalytic mineralization of phenol catalyzed by pure and mixed phase hydrothermal titanium dioxide

The photocatalytic mineralization of phenol catalyzed by pure (anatase, rutile) and mixed phase hydrothermal TiO₂ was studied in aqueous solution employing different oxidative agents, H₂O₂ and O₂. In the case of H₂O₂, rutile particles, having large dimensions and high aspect ratio (size: 30–70 nm × 150–350 nm), display the highest catalytic activity due to their low tendency to recombine electrons and holes generated by UV irradiation. By using water dissolved gaseous O₂, the catalytic TiO₂ activity generally decreases and rutile displays the lowest efficacy. In fact, oxygen preferentially chemisorbs at the surface of the nanosized particles of anatase (5–15 nm) and acts as effective electron scavenger, inhibiting the electron-hole recombination. The number of electron and hole traps (Ti³⁺, O₂⁻ and O⁻) and the rate of formation of the short-lived hydroxyl radicals OH under UV irradiation, were evaluated by electron paramagnetic resonance (EPR). A correlation was suggested among the amount of the charge carrier centers, the rate of formation of OH radicals and the catalyst photoactivity. This confirms that the photocatalytic properties depend on the possibility that electrons and holes separately interact with the oxidative agents at the TiO₂ surface, inducing the formation of OH radicals.     

Electrochemical investigation of superoxide anion scavenging ability of 1,2,3-triketohydrindene hydrate in aprotic solvents

1,2,3-Triketohydrindene hydrate (NHy) shows well-defined redox electrochemistry in the formation of monoanionic radical (NHy⁻) and dianion (NHy²⁻) in nitrogen saturated aprotic solvents such as acetonitrile and dimethylsulfoxide. Cyclic voltammetry reveals that in an oxygen-saturated solution of DMSO, the oxidation peak of superoxide anion (O₂⁻) at −0.7 V versus Ag/AgCl wire electrode, decreases systematically with increasing NHy concentration. The similar behaviour is observed in the rotating disk voltammetry. On Pt disk, oxygen is reduced to O₂⁻ at a constant potential of −0.8 V and at Pt ring, O₂⁻ is oxidised to oxygen and the corresponding limiting current plateau in the ring voltammogram is decreased linearly as [NHy] is increased. In aqueous solutions, NHy is found to exhibit completely different redox chemistry due to its structural changes and hence showed no favourable redox potentials for efficient quenching of O₂⁻.     

Platinum catalysed decomposition of hydrogen peroxide in aqueous-phase pulsed corona electrical discharge

Electrical discharges in water produced by a pulsed high voltage power supply generate chemically active species (OH, H₂, O₂, H₂O₂, HO₂ and O) that are capable of degrading various hazardous chemicals. Previous experimental studies showed that platinum high voltage electrodes in a pulsed corona electrical discharge lead to significantly higher pollutant removal in comparison to that with other electrode materials. In the present work it was observed that when nickel–chromium was used as a high voltage electrode, the pulsed corona electrical discharge in water produces hydrogen peroxide at a constant rate regardless of the initial pH of the solution. Replacement of the nickel–chromium electrode with a platinum high voltage electrode leads to the decomposition of hydrogen peroxide where the rate of decomposition increases with increasing pH. An Eley-Rideal mechanism describing heterogeneous catalytic hydrogen peroxide decomposition is proposed. It is assumed that the decomposition occurs on the surface of the platinum particles ejected from the platinum high voltage electrode. Combination of the experimental measurements and a mathematical model describing the platinum catalysed hydrogen decomposition suggests that the pH dependent hydrogen peroxide decomposition is caused by the adsorption of molecular hydrogen produced by the discharge and hydroxyl ions on the platinum surface. The influence of gases bubbled into the reactor (argon, oxygen and hydrogen) on the hydrogen peroxide decomposition was also tested by both experiments and the model. Finally, the model was utilized to predict molecular hydrogen and oxygen concentrations at three pH values when either nickel–chromium or platinum high voltage electrodes are used.     

Electrochemical fabrication of polyaniline/MnO2/graphite felt as free‐standing,flexible electrode for supercapacitors

Polyaniline/MnO₂/graphite felt (PMGF) composite, which can be used as a novel free‐standing, flexible electrode for supercapacitors, was fabricated via a facile electrochemical method. Polyaniline/graphite felt (PANI/GF) electrode was prepared by electropolymerization of PANI onto the GF. Subsequently, manganese dioxide (MnO₂) was electrodeposited on the surface of the PANI/GF electrode to prepare PMGF electrode. The microstructure and morphology of the as‐prepared samples were characterized by Fourier transform infrared spectra, X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. Specific surface area was examined using N₂ adsorption/desorption test. Cyclic voltammogram, chronopotentiometry techniques and electrochemical impedance spectroscopy were introduced to investigate the electrochemical performance of the composites. The PMGF electrode exhibited specific capacitance as high as about 630 F g⁻¹ at the current density of 0.5 A g⁻¹, which is much higher than that of PANI/MnO₂ composites reported previously. The high specific capacitance of PMGF may be attributed to the fact that the porous GF is a good conductive matrix for the dispersion of PANI/MnO₂ and it can facilitate easy access of electrolytes to the electrode, which results in enhancement of the electrochemical performance of the composite. Moreover, the specific capacitance of PMGF is much larger than that of MnO₂/GF (MGF), which may be ascribed to the participant of PANI, which contributes additional pseudocapacitance and electron transport path. POLYM. COMPOS., 34:819–824, 2013. © 2013 Society of Plastics Engineers     

New long-wavelength ethanolamino-substituted hypocrellin: photodynamic activity and toxicity to MGC803 cancer cell

To improve hydrophilicity and photoactivity of the new type of therapeutic agent, hypocrellin, a novel long-wavelength ethanolamino-substituted hypocrellin B (EAHB) was synthesized and its molecular structure was characterized by IR, NMR, MS, and UV–vis spectrometers, and EAHB had strong absorption at the phototherapeutic window (600–900 nm). Illumination of deoxygenated DMSO solution containing EAHB generated a strong electron paramagnetic resonance (EPR) signal, which was assigned to the semiquinone anion radical of EAHB (EAHB⁻). Absorption measurements displayed that the absorptive bands at 632 and 565 nm (shoulder) arose from the semiquinone anion radical (EAHB⁻) and the absorptive bands at 519 and 450 nm (shoulder) belonged to hydroquinone (EAHBH₂), which were formed via the decay of EAHB⁻ in water-contained solution. Superoxide anion radical (O₂⁻) was produced via electron transfer from EAHB⁻ (the precursor) to ground state oxygen. The presence of NADH, a bio-electron donor, significantly enhanced production of EAHB⁻ and O₂⁻. Singlet oxygen O₂ (¹Δ_g) could be produced via energy transfer from triplet EAHB to ground state oxygen molecules. The quantum yield of O₂ (¹Δ_g) and the relative quantum yield of O₂⁻ of EAHB were 0.15 and 0.76, respectively, with the parent compound hypocrellin B (HB) as the standard. It was inferred that Type I pathway was possibly a major photodynamic mechanism of EAHB. The study on photobiological action of EAHB on MGC803 cancer cells revealed that EAHB kept the same good phototoxic ability as HB but reduced 4 times cytotoxicity than HB, and also its photopotentiation factor increased 4-folds.     

A novel alkaline air electrode based on a combined use of cobalt hexadecafluoro-phthalocyanine and manganese oxide

Electrocatalytic reduction of O₂ with dual catalysts of cobalt 1, 2, 3, 4, 8, 9, 10, 11, 15, 16, 17, 18, 22, 23, 24, 25-hexadecafluoro-29 H, 31H-phthalocyanine (CoPcF₁₆) and MnOOH was studied in alkaline media with cyclic and rotating ring-disk electrode (RRDE) voltammetry. Cyclic voltammetric results show that CoPcF₁₆ possesses a good catalytic activity for redox-catalyzing an apparent two-electron reduction of O₂ with superoxide (O₂⁻) as an intermediate. The combined use of CoPcF₁₆ with MnOOH which shows a bifunctional catalytic activity toward the sequential disproportionations of the reduction intermediate and product, i.e. O₂⁻ and peroxide (HO₂⁻), eventually enables an apparent four-electron reduction of O₂ to be achieved at a positively-shifted potential in alkaline media. The possibility of utilizing the dual catalysts for the development of practical alkaline air electrodes was further explored by confining the catalysts in active carbon (AC) and carbon black (CB) matrices that are generally used as the substrate for constructing air electrodes. The RRDE voltammetric results suggest that an apparent four-electron reduction of O₂ reduction can be obtained at the as-prepared carbon-based air electrode at a potential close to that at the Pt-based air electrode, and that the as-prepared electrode shows a high tolerance against methanol and glucose crossover.     

An electrochemical study of H2O2 decomposition on single-phase γ-FeOOH films

The electrochemical reduction and oxidation kinetics of hydrogen peroxide on γ-FeOOH films chemically deposited on indium tin oxide substrates were studied over the pH range of 9.2-12.6 and the H₂O₂ concentration range of 10⁻⁴ to 10⁻² mol dm⁻³. The Tafel slopes for H₂O₂ reduction and oxidation obtained from polarization measurements are 106 ± 4 and 93 ± 15 mV dec⁻¹, respectively, independent of pH and the concentration of H₂O₂. Both the reduction and oxidation of H₂O₂ on γ-FeOOH have a first-order dependence on the concentration of molecular H₂O₂. However, for the pH dependence, the reduction has an inverse first-order dependence, whereas the oxidation has a first-order dependence, on the concentration of OH⁻. For both cases the electroactive species is the molecular H₂O₂, not its base form, HO₂⁻. Based on these observations, reaction kinetic mechanisms are proposed which involve adsorbed radical intermediates; HOOH⁻ and HO for the reduction, and HO₂H⁺, HO₂, and O₂⁻ for the oxidation. These intermediates are assumed to be in linear adsorption equilibria with OH⁻ and H⁺ in the bulk aqueous phase, respectively, giving the observed pH dependences. The rate-determining step is the reduction or oxidation of the adsorbed H₂O₂ to the corresponding intermediates, a reaction step which involves the use of Fe^III/Fe^II sites in the γ-FeOOH surface as an electron donor-acceptor relay. The rate constant for the H₂O₂ decomposition on γ-FeOOH determined from the oxidation and reduction of Tafel lines is very low, indicating that the γ-FeOOH surface is a very poor catalyst for H₂O₂ decomposition.     

Mechanism of photocatalytic oxidation of guanine by BiOBr under UV irradiation

This article analyzes the mechanism details of guanine photocatalytic oxidation on the surface of BiOBr nanoparticles under UV irradiation. The reaction of O₂⁻ with radical species, generated from primary oxidation of guanine by hole, is proposed to be the dominant cause of guanine damage. The present work demonstrates that BiOBr photocatalysis provided good model to investigate the reaction of substrate radical with the in situ formed O₂⁻. It also implicates that the frequently-used biomarker for DNA damage 8-oxo-7,8-dihydro-2-deoxyguanosine (8-oxodG) may be ineffective in the situation that damage is caused by O₂⁻.     

Carbon dioxide as carbon source: Activation via electrogenerated O2 in ionic liquids

The activation of carbon dioxide has been obtained in O₂/CO₂ saturated ionic liquids, via electrochemically generated O₂⁻, at a less negative potential than the one of the direct cathodic reduction of CO₂. This electrochemical activation has been applied to the C–N bond formation from amines and carbon dioxide in the synthesis of organic carbamates. A competitive reaction between electrogenerated superoxide ion and imidazolium cations yielding 2-imidazolones has been pointed out. This procedure allows to avoid the utilization of volatile and toxic organic solvents, supporting electrolytes and catalysts.     

Oxidation pathways of malachite green by Fe-catalyzed electro-Fenton process

A very detailed scheme for the Fe³⁺-catalyzed electro-Fenton mineralization of malachite green as a model triarylmethane dye is presented. Bulk electrolyses of 250-mL aqueous solutions of 0.5 mM malachite green with 0.2 mM Fe³⁺ as catalyst have been carried out at room temperature and pH 3.0 under constant current in an undivided cell equipped with a graphite-felt cathode and a Pt anode to assess the performance of the electro-Fenton system. In situ electrogeneration of Fe²⁺ and H₂O₂ from quick cathodic reduction of Fe³⁺ and dissolved O₂ (from bubbled compressed air), respectively, allows the formation of the very oxidizing species hydroxyl radical (OH) in the medium from Fenton's reaction. A pseudo-first-order decay kinetics with an apparent rate constant of k_1,MG = 0.244 min⁻¹ was obtained for total destruction of malachite green by action of OH at 200 mA, requiring 22 min for total decoloration of the solution. In the same experimental conditions, overall mineralization was reached at 540 min. Up to 15 aromatic and short-chain carboxylic acid intermediates were identified along the treatment. The evolution of current efficiency was calculated from the chemical oxygen demand (COD) removal. Based on the time course of most of the by-products and the identification of inorganic ions released, some plausible mineralization pathways are proposed and thoroughly discussed. It has been found that the electro-Fenton degradation of malachite green proceeds via parallel pathways, all of them involving primary splitting of the triaryl structure initiated by attack of OH on the central carbon, thus yielding two different N-dimethylated benzophenones. Successive cleavage of the aromatic intermediates generates oxalic acid as the ultimate short-chain carboxylic acid, whereas N-demethylation of some of them produces formic acid as well. Oxalic acid and its Fe²⁺ complexes, as well as formic acid, can be slowly but totally mineralized by OH.