Electrocarboxylation of chloroacetonitrile by a Cobalt(I) complex of terpyridine

The electrocarboxylation of chloroacetonitrile (NC–CH₂–ClRCl) mediated by [Co^IIL₂]²⁺ (L = terpyridine) was investigated by cyclic voltammetry. Electrochemical studies under argon atmosphere showed that the monoelectronic reduction of [Co^IIL₂]²⁺ yielded a Cobalt(I) complex which after the loss of a terpyridine ligand reacted with chloroacetonitrile. The oxidative addition of chloroacetonitrile on [Co^IL]⁺ gave an alkylCobalt(III) complex [R–Co^IIIL]²⁺ which was reduced into an alkylCobalt(II) complex, highly unstable and decomposed into an alkyl anion and a Cobalt(II) complex. Under carbon dioxide atmosphere, Cobalt(I) complex was shown to be unreactive towards CO₂ but CO₂ insertion was observed in the alkylCobalt(III) complex [R–Co^IIIL] ²⁺ giving probably a CO₂ adduct [R–Co^IIIL(CO₂)]²⁺. This adduct presented a strong adsorption at the carbon electrode and was reduced at potential less cathodic than the one of alkylCobalt(III) complex. After reduction, the carboxylate RCO₂⁻ (NC–CH₂–CO₂⁻) was released and a catalytic bielectronic carboxylation of chloroacetonitrile took place. Controlled potential electrolyses confirmed the catalytic process and gave for cyanoacetic acid faradic yields up to 60% under low overpotential conditions.     

Electrocatalytic oxygen reduction and hydrogen evolution reactions on phthalocyanine modified electrodes: Electrochemical, in situ spectroelectrochemical, and in situ electrocolorimetric monitoring

This study describes electrochemical, in situ spectroelectrochemical, and in situ electrocolorimetric monitoring of the electrocatalytic reduction of molecular oxygen and hydronium ion on the phthalocyanine-modified electrodes. For this purpose, electrochemical and in situ spectroelectrochemical characterizations of the metallophthalocyanines (MPc) bearing tetrakis-[4-((4′-trifluoromethyl)phenoxy)phenoxy] groups were performed. While CoPc gives both metal-based and ring-based redox processes, H₂Pc, ZnPc and CuPc show only ring-based electron transfer processes. In situ electrocolorimetric method was applied to investigate the color of the electrogenerated anionic and cationic forms of the complexes. The presence of O₂ in the electrolyte system influences both oxygen reduction reaction and the electrochemical and spectral behaviors of the complexes, which indicate electrocatalytic activity of the complexes for the oxygen reduction reaction. Perchloric acid titrations monitored by voltammetry represent possible electrocatalytic activities of the complexes for hydrogen evolution reaction. CoPc and CuPc coated on a glassy carbon electrode decrease the overpotential of the working electrode for H⁺ reduction. The nature of the metal center changes the electrocatalytic activities for hydrogen evolution reaction in aqueous solution. Although CuPc has an inactive metal center, its electrocatalytic activity is recorded more than CoPc for H⁺ reduction in aqueous solution.     

Electrocatalytic carboxylation of 2-amino-5-bromopyridine with CO2 in ionic liquid 1-butyl-3-methyllimidazoliumtetrafluoborate to 6-aminonicotinic acid

A new electrochemical procedure for the electrocatalytic carboxylation of 2-amino-5-bromopyridine with CO₂ in ionic liquid, 1-butyl-3-methyllimidazolium tetrafluoborate (BMIMBF₄), to 6-aminonicotinic acid was investigated for the first time. The experiments were carried out in three electrodes undivided cell under mild conditions, and the use of volatile and toxic solvents and catalysts, as well as any other additional supporting electrolytes, was avoided. The electrochemical reduction behavior of 2-amino-5-bromopyridine in BMIMBF₄ had been studied by cyclic voltammetry with a reduction peak at −1.6 V (vs. Ag). 6-Aminonicotinic acid was obtained in 75% yield and 100% selectivity, under the optimized condition. Moreover, the ionic liquid was successfully recycled.     

Electrocarboxylation of benzyl chlorides at silver cathode at the preparative scale level

The electrocarboxylation of benzyl chlorides to the corresponding carboxylic acids performed at silver cathodes was investigated both theoretically and experimentally in order to find the influence of the operative parameters on the selectivity and on the Faradic efficiency of the process. Theoretical considerations were confirmed by the electrocarboxylation of 1-phenyl-1-chloroethane performed in undivided cells equipped with sacrificial anodes both in a bench-scale electrochemical batch reactor and in a continuous batch recirculation reaction system equipped with a parallel plate electrochemical cell. Selectivity and Faradic yields higher than 80% and 70%, respectively, were obtained by working under anhydrous conditions both under amperostatic and potentiostatic alimentation at appropriate values of either current density or applied potential.     

A hydrogen peroxide sensor based on electrochemically roughened silver electrodes

Silver (Ag) electrodes were roughened by electrochemical oxidation-reduction cycles (ORC) in a KCl solution. The roughened Ag electrode exhibited a powerful electrocatalytic activity for the reduction of hydrogen peroxide (H₂O₂). Atomic force microscopy and electrochemical experiments confirmed that the electrocatalytic ability mainly resulted from the Ag nanoparticles produced in the process of ORC on the roughened Ag electrode. The electrochemical behaviors of the roughened Ag electrodes toward the reduction of H₂O₂ and the factors related to that reduction were investigated in detail.     

Electrocatalytic properties of NDGA and NDGA/FAD hybrid film modified electrodes for NADH/NAD redox reaction

The fabrication of monolayers composed of nordihydroguaiaretic acid (NDGA), and hybrid films composed of NDGA-flavin adenine dinucleotide (FAD) adsorbed films was performed in neutral aqueous solutions to produce electrochemically active thin films exhibiting one and two redox couples, respectively. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ growth of the NDGA and hybrid NDGA/FAD film monolayers. The NDGA modified film electrocatalytically oxidized NADH, ascorbic acid, dopamine, and N₂H₄ in neutral aqueous solutions. Well-separated voltammetric peaks were observed for dopamine and uric acid mixtures, and also for ascorbic acid and uric acid mixtures using the NDGA/GC modified electrode. When transferred to various aqueous buffered solutions, the two redox couples of the NDGA/FAD hybrid film and their formal potentials were observed to be pH-dependent. The electrocatalytic oxidation and reduction of NADH and NAD⁺ by a NDGA/FAD hybrid film in neutral aqueous solutions was carried out, and the electrocatalytic oxidation of NADH was performed using a NDGA/FAD hybrid film.     

Interaction between La and MP-11 in the physiological solution

In this paper, the interaction between La³⁺ and microperoxidase-11 (MP-11) in the imitated physiological solution was investigated with the electrochemical method, circular dichroism (CD) and ultraviolet-visible (UV-vis) absorption spectroscopy. It was found that the interaction ways between La³⁺ and MP-11 are different with increasing the molar ratio of La³⁺ and MP-11. When the molar ratio of La³⁺ and MP-11 is less than 2, La³⁺ mainly interacts with the metacetonic acid group of the heme group in the MP-11 molecules, causing the increase in the non-planarity of the porphyrin cycle in the heme group and the decrease in the content of the random coil conformation of MP-11. These structural changes would increase the exposure extent of the electrochemical active center of MP-11 and thus, La³⁺ can promote the electrochemical reaction of MP-11 and its electrocatalytic activity for the reduction of H₂O₂ at the glassy carbon (GC) electrode. However, when the molar ratio of La³⁺ and MP-11 is larger than 3, except binding to the carbonyl oxygen of the metacetonic acid group in the heme group, La³⁺ interacts also with the oxygen-containing groups of the amides in the polypeptide chains of the MP-11 molecules, leading to the increase in the contents of the random coil conformation in the peptide of the MP-11 molecule, comparing with that for the molar ratio of less than 2. However, the non-planarity of the porphyrin cycle in the heme group is similar to that for the molar ratio of less than 2. Perhaps, the increase in the contents of the random coil conformation in the peptide of the MP-11 molecule would partially shield the electrochemical active center of MP-11 and is not favorable to the electrochemical and electrocatalytic reaction of MP-11. Thus, the reversibility of the electrochemical reaction of MP-11 and its electrocatalytic activity for the reduction of H₂O₂ are lower than that for the molar ratio of less than 2.     

The characterization and bioelectrocatalytic properties of hemoglobin by direct electrochemistry of DDAB film modified electrodes

Direct electrochemistry of hemoglobin can be performed in acidic and basic aqueous solutions in the pH range 1-13, using stable, electrochemically active films deposited on a didodecyldimethylammonium bromide (DDAB) modified glassy carbon electrode. Films can also be produced on gold, platinum, and transparent semiconductor tin oxide electrodes. Hemoglobin/DDAB films exhibit one, two, and three redox couples when transferred to strong acidic, weak acidic and weak basic, and strong basic aqueous solutions, respectively. These redox couples, and their formal potentials, were found to be pH dependent. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ deposition of DDAB on gold disc electrodes and hemoglobin deposition on DDAB film modified electrodes. A hemoglobin/DDAB/GC modified electrode is electrocatalytically reduction active for oxygen and H₂O₂, and electrocatalytically oxidation active for S₂O₄²⁻ through the Fe(III)/Fe(II) redox couple. In the electrocatalytic reduction of S₄O₆²⁻, S₂O₄²⁻, and SO₃²⁻, and the dithio compounds of 2,2′-dithiosalicylic acid and 1,2-dithiolane-3-pentanoic acid, the electrocatalytic current develops from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in neutral and weakly basic aqueous solutions. Hemoglobin/DDAB/GC modified electrodes are electrocatalytically reduction active for trichloroacetic acid in strong acidic buffered aqueous solutions through the Fe(III)/Fe(II) redox couple. However, the electrocatalytic current developed from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in weak acidic and basic aqueous solutions. The electrocatalytic properties were investigated using the rotating ring-disk electrode method.     

Electrocatalytic reduction of dioxygen at a modified glassy carbon electrode based on Nafion-dispersed single-walled carbon nanotubes and cobalt–porphyrin with palladium nanoparticles in acidic media

Cathodic dioxygen (O₂) reduction was performed at a modified glassy carbon electrode (GCE) by single-walled carbon nanotubes (SWCNT)/Nafion^® (NF) film with cobalt (II) tetra (2-amino-phenyl) porphyrin (CoTAPP) and palladium (Pd) nanoparticles incorporated and employed as doping agents. Both the electrochemical behavior of SWCNT with a P(CoTAPP)–Pd nanoparticle matrix and the electrocatalytic reduction of O₂ were investigated using transmission electron microscopy (TEM), cyclic voltammetry (CV) and rotating ring-disk electrode (RRDE) techniques in 0.1 mol l⁻¹ H₂SO₄ aqueous solutions. The electrocatalytic reduction of O₂ at the SWCNT/NF/P(CoTAPP)–Pd composite film established a pathway of four-electron transfer reductions into H₂O. Hydrodynamic voltammetry revealed that the modified electrode was catalyzed effectively by the four-electron transferred reduction of dioxygen into H₂O with minimal generation of H₂O₂. The SWCNT/NF/P(CoTAPP)–Pd composite film showed a highly efficient electrocatalytic performance. P(CoTAPP)–Pd was an effective mediator for the reduction of dioxygen and was responsible for the enhanced catalytic activity.     

Electrocatalytic reduction of oxygen at glassy carbon electrode modified by polypyrrole/anthraquinones composite film in various pH media

The electrocatalytic reduction of dioxygen by one mono and four dihydroxy derivatives of 9,10-anthraquinone (AQ) incorporated in polypyrrole (PPy) matrix on glassy carbon electrode has been investigated. The electrochemical behaviour of the modified electrodes was examined in various pH media and both the formal potential of anthraquinones and reduction potential of dioxygen exhibited pH dependence. AQ and PPy composite film showed excellent electrocatalytic performance for the reduction of O₂ to H₂O₂. pH 6.0 was chosen as the most suitable medium to study the electrocatalysis by comparing the peak potential of oxygen reduction and enhancement in peak current for oxygen reduction. The diffusion coefficient values of AQ at the modified electrodes and the number of electrons involved in AQ reduction were evaluated by chronoamperometric and chronocoulometric techniques, respectively. In addition, hydrodynamic voltammetric studies showed the involvement of two electrons in O₂ reduction. The mass specific activity of AQ used, the diffusion coefficient of oxygen and the heterogeneous rate constants for the oxygen reduction at the surface of modified electrodes were also determined by rotating disk voltammetry.     

The electrocatalytic hydrogenation of soybean oil

Soybean oil has been hydrogenated electrocatalytically at a moderate temperature, without an external supply of pressurized H₂ gas. In the electrocatalytic reaction scheme, atomic hydrogen is produced on an active Raney nickel powder cathode surface by the electrochemical reduction of water molecules from the electrolytic solution. Adsorbed hydrogen then reacts with an oil’s triglycerides to form a hydrogenated product. Experiments were carried out at 70°C with a flow-through electrochemical reactor operating in a batch recycle mode. The reaction medium was a two-phase mixture of soybean oil in a water/t-butanol solvent containing tetraethylammoniump-toluenesulfonate as the supporting electrolyte. In all experiments the reaction was allowed to continue for sufficient time to synthesize a brush hydrogenation product. The effects of oil content, applied current, solvent composition, and supporting electrolyte concentration on the efficiency of hydrogen addition to the oil and on the chemical properties of the hydrogenated oil product were determined. The electrohydrogenated oil is characterized by a high stearic acid content and a low percentage of totaltrans isomers, as compared to that produced in a traditional hydrogenation process.     

Electrocatalytic carboxylation of aromatic ketones with carbon dioxide in ionic liquid 1-butyl-3-methylimidazoliumtetrafluoborate to α-hydroxy-carboxylic acid methyl ester

A new electrochemical procedure for the electrocatalytic carboxylation of aromatic ketones with carbon dioxide in ionic liquid, 1-butyl-3-methylimidazolium tetrafluoborate (BMIMBF₄), to α-hydroxycarboxylic acid methyl ester was investigated for the first time. The electrochemical behavior of acetophenone in BMIMBF₄ was studied by cyclic voltammetry with a reduction peak at −1.9 V (vs. Ag). The electrolyses experiments were carried out in an undivided cell under mild conditions without any toxic solvents, catalysts and supporting electrolytes, followed by addition of an alkylating agent, affording the α-hydroxycarboxylic acid methyl ester in a moderate yield (62%). The results showed that the yields were strongly affected by various factors: temperature, current density, charge passed, electrode material and substrate concentration. Moreover, the ionic liquid was successfully recycled for this reaction.     

Structural characterization and electrocatalytic properties of Au30Zr70 amorphous alloy obtained by rapid quenching

The present work describes the characteristics of Au₃₀Zr₇₀ amorphous alloy as a precursor of a catalyst for hydrogen evolution both in acid and in basic environments and for oxidation of methanol and formaldehyde in alkaline solutions. Amorphous Au–Zr ribbons were prepared by melt spinning methods and were characterized by X-ray diffraction, optical and scanning electron microscopy. In order to obtain an active electrocatalyst, the surface of the amorphous ribbons was treated by immersion in 1 M HF solution. The results of electrocatalytic tests were compared to those obtained with untreated ribbons and also with electrodes made in polycrystalline pure gold. Moreover, some untreated samples were aged in air for 30 days before the electrochemical measurements, in order to investigate the effect of surface oxidation on the electrochemical behaviour of the alloys. The HF treatment yielded a porous structure rich in nanocrystalline gold particles which had better electrocatalytic activity than untreated ribbons or polycrystalline gold electrodes. Ageing in air produced a duplex phase structure, comprising of ZrO₂ scales and nanocrystalline gold which had also improved electrocatalytic activity.     

Nickel-decorated MoS2/MXene nanosheets composites for electrocatalytic oxidation of methanol

Nickel incorporated on MoS₂/MXene composites (NiMoS₂/MXene) via a wet impregnation method is used as an anode electrode material for methanol electro-oxidation. X-ray diffraction, X-ray photoelectron spectra, and scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy techniques were used for the confirmation of MoS₂/MXene formation. Textural properties of catalysts were obtained in N₂ adsorption-desorption analysis. Electrochemical measurements in 0.1 M KOH demonstrated the better electrocatalytic activity of NiMoS₂/MXene catalysts. The NiMoS₂/MXene system exhibited enhanced electrocatalytic activity for methanol oxidation due to low onset potential offered by Ni, high tolerance toward CO poisoning by MoS₂, and high conductivity and high mechanical stability of MXene. NiMoS₂-3/MXene catalysts exhibited high current density, electrochemical active surface area, long-term stability, and low Rct value. Based on the electrochemical results, NiMoS₂/MXene catalysts is a highly electroactive anode material. Hence can be utilized in fuel cell applications such as Direct Methanol Fuel Cell (DMFC).     

Electrochemical reduction of carbon dioxide at ruthenium dioxide deposited on boron-doped diamond

Study of carbon dioxide reduction at RuO₂-coated diamond electrodes showed that conductive metallic oxides are promising electrocatalysts for this process and allow higher reduction products to be obtained. The main reduction products obtained in acidic and neutral media were formic acid and methanol, with product efficiencies as high as 40 and 7.7%, respectively. It was observed that conductive diamond is a very well suited material for studying the electrocatalytic properties of conductive metallic oxides, because its use resulted in a negligible substrate effect. This feature will greatly enhance the ability to understand the relationships between the intrinsic electrochemical behavior and the electrocatalytic behavior, in view of developing new effective electrocatalysts. The use of conductive diamond could also allow better utilization of the electrocatalyst by avoiding the need for thick films.     

Electrochemically created roughened lead plate for electrochemical reduction of aqueous CO2

Surface-roughened Pb electrodes were prepared through a facile oxidation–reduction cycle. Compared with their smooth surface counterparts, the electrodes exhibited significantly higher activity, selectivity, and energy utilization in the electrocatalytic reduction of CO₂ to HCOOH using water under ambient temperature and pressure. Furthermore, the modified electrodes maintained high activities after operating repeatedly for five batches. The enhanced performance of these electrodes is attributed to the enlarged active surface area and increased number of reactive species associated with the three-dimensional structure of the surface. Both the hydrogenation mechanism and the hydrogencarbonate mechanism were affected during the electrochemical CO₂ reduction.     

Visualization of electrocatalytic activity of microstructured metal hexacyanoferrates by means of redox competition mode of scanning electrochemical microscopy (RC-SECM)

Transition metal hexacyanoferrates are versatile inorganic compounds widely employed for the assembling of sensors and biosensors in a variety of different electroanaytical applications. A modified version of the recently introduced redox competition mode of scanning electrochemical microscopy (RC-SECM) was exploited to visualize the local electrocatalytic activity of microstructured Prussian blue (PB) films towards the reduction of H₂O₂ with improved lateral resolution. The PB films were electrochemically deposited in a spot on glassy carbon surfaces using a droplet cell. The influence of the potential applied to the PB modified surface on the current at the SECM tip was evaluated when both the sample and the SECM tip were competing for H₂O₂ in solution. Thus, high local electrocatalytic activity is indicated by low currents at the SECM tip. The same strategy was successfully employed for the characterization of the performances of a biosensor employing the enzyme glucose oxidase (GOx) immobilized within a polymer hydrogel matrix on the top of PB-modified glassy carbon electrodes.     

Catalysis of the reduction of phenyldinitro-methane and phenylnitromethane on Pt by underpotential monolayers

The reduction of phenyldinitromethane and phenylnitromethane on platinum surfaces covered by heavy metal monolayers deposited at underpotentials was studied in aqueous acid solutions by means of voltammetric and electrolysis techniques. It was found that Tl, Pb and Bi adsorbates markedly catalyse the reduction of phenyldinitromethane. The enhancement of the overall reduction processes has been interpreted in terms of the change of the nitrogroup reduction mechanism from chemical mechanism on bare platinum to direct electrochemical mechanism on Pt surfaces covered by the underpotential monolayers. The electrocatalytic activity of the Pt/M_ads surfaces for both phenyldinitromethane and phenylnitromethane reduction was found to be more pronounced compared to that of mercury. In the case of phenyldinitromethane differences in the electrocatalytic selectivity of these modified electrodes were also observed.     

Characterization of a stirred tank electrochemical cell for water disinfection processes

Laboratory experiments were performed to characterize the behaviour of an electrochemical cell equipped with boron-doped diamond anodes and to verify its effectiveness in water disinfection. The hydrodynamic regime was determined when the cell worked either in batch or in continuous mode. Galvanostatic electrolyses of aqueous 1 mM Na₂SO₄ solutions were performed to investigate on the oxidant production in different experimental conditions. The same solutions contaminated by E. coli, enterococci and coliforms were used as test media to verify the effectiveness of the system in the disinfection process. Experimental results indicated that the major inactivation mechanism of bacteria in the electrochemical cell is a disinfection by electrochemically generated oxidants, however a cooperative effect of superficial reaction has to be taken into account. The great capability of BDD anode to produce reactive oxygen species (ROS) and other oxidizing species during the electrolysis allows to establish a chlorine-free disinfection process.     

Highly dispersed carbon-supported Pd nanoparticles catalyst synthesized by novel precipitation–reduction method for formic acid electrooxidation

The highly dispersed and ultrafine carbon-supported Pd nanoparticles (Pd/C) catalyst is synthesized by using an improved precipitation–reduction method, which involves in Pd^II → PdO·H₂O → Pd⁰ reaction path. In the method, palladium oxide hydrate (PdO·H₂O) nanoparticles (NPs) with high dispersion is obtained easily by adjusting solution pH in the presence of 1,4-butylenediphosphonic acid (H₂O₃P-(CH₂)₄-PO₃H₂, BDPA). After NaBH₄ reduction, the resulting Pd/C catalyst possesses high dispersion and small particle size. As a result, the electrochemical measurements indicate that the resulting Pd/C catalyst exhibits significantly high electrochemical active surface area and high electrocatalytic performance for formic acid electrooxidation compared with that prepared by general NaBH₄ reduction method.