Glassy Carbon Anode Research Articles

Identifying Mechanisms and Challenges for Electrochemical Oxidation of Cyclohexane to KA Oil

Electrochemical oxidation of cyclohexane to KA oil, a mixture of cyclohexanone and cyclohexanol, holds great promise for decarbonized chemical manufacturing based on the value of products and the thermodynamic equilibrium potential. However, fundamental understanding of this reaction is extremely limited. For example, even the number of electrons in this reaction has not yet been identified. In this work, we elucidate the mechanism of electrochemical cyclohexane oxidation to KA oil on fluorine-doped tin oxide (FTO), platinum, and glassy carbon anodes. Using three-electrode electroanalysis, isotopic labeling, and concentration studies, we show that electrochemical cyclohexane oxidation to KA oil is similar to its thermochemical analogue in that O2, not water, is the primary oxygen source. The reaction is initiated through the formation of cyclohexyl or hydroxyl radicals, depending on electrode and electrolyte composition. Additionally, crossover from undivided two-electrode cells is found to impact measurements such that cathodic reaction and reactor design may introduce potential artifacts to anodic activity and selectivity. These findings have significant implications for the technological viability of a theoretically promising electrosynthesis process.

Electrochemical Synthesis of Sulfonamides in Single‐Pass Flow

AbstractAn electrochemical multicomponent synthesis of sulfonamides at room temperature in single‐pass flow is presented. In contrast to batch‐type electrolysis, an undivided flow cell setup with a stainless‐steel cathode and either a boron‐doped diamond (BDD) anode or a glassy carbon anode can be employed. Simply by using SO2 stock solutions, less atom‐economic sulfur dioxide surrogates can be avoided. Moreover, no additional supporting electrolytes are required due to the in‐situ generation of amidosulfinates, which also serve as intermediate for this transformation. This protocol allows sulfonamides to be synthesized directly from non‐prefunctionalized electron‐rich arenes with amines and SO2. In total, 10 examples are demonstrated with isolated yields up to 92 %. The robust scalability of this electrosynthesis with an easy downstream processing was also proven.

Anodic Fluorination, Methoxylation, Acetoxylation, and Cyanation of Heteroatom Organic Compounds Using Boron‐Doped Diamond, GC, and Pt Electrodes

AbstractVarious anodic substitution reactions such as fluorination, methoxylation, acetoxylation, and cyanation of heteroatom compounds containing a sulfur or nitrogen atom were comparatively studied using boron‐doped diamond (BDD), Pt, and glassy carbon (GC) anodes. It was found that BDD anode is highly effective for these anodic substitution reactions similarly to Pt anode although both BDD and GC electrodes are carbon‐based materials.

Anodically‐Generated Alkyl Radicals Derived from Carboxylic Acids as Reactive Intermediates for Addition to Alkenes

AbstractElectrochemically driven C−C coupling has the potential to reduce the cost and environmental impact of some organic syntheses currently accomplished through thermochemical methods. Here, we use electrochemical oxidation of carboxylic acids as a source of reactive carbon‐centered radicals that enable radical addition to alkenes in the anode boundary layer. We demonstrate an optimization of reaction conditions to suppress the thermodynamically favored, but synthetically undesirable radical self‐coupling in favor of radical addition to styrene. In methanol solvent, 88 % selectivity and 72 % Faradaic efficiency for targeted functionalized benzenes are achieved. For low current densities, iridium anodes outperform platinum, gold, palladium, and glassy carbon anodes. With constant potential or constant current electrolyses, the deposition of organic by‐products on the catalyst surface leads to anode passivation. We show that periodic cathodic current pulses effectively regenerate the catalyst. Lastly, we confirm the role of free radicals in the reaction mechanism with a radical trap.

Electrochemically Generated Iodine Cations from a Glassy Carbon Electrode for Highly Selective Iodination of Anisole

The synthesis of aryl iodides from commercially available raw chemicals by simple, cheap and green strategies is of fundamental significance. Aryl iodides can undergo a series of homo-/cross-coupling reactions for the synthesis of important industrial chemicals and materials. Traditional methods require the electrophilic substitution on aromatic compounds by iodine or hypervalent iodine compounds, which suffers from the use of erosive halogens or hazardous oxidants. With the development of green chemistry in the field of electrochemical synthesis, anodic oxidation-derived I+ cations have been used for substitution reactions. However, the selectivity of the iodination by these electrochemical methods remains unsatisfactory. We believed that the anolyte is contaminated by trace platinum species from the working electrode. Herein, we report the generation of active I+ species from the anodic oxidation of I2 in acetonitrile using a glassy carbon electrode. With the presence of H+, electrolyte prepared with a glassy carbon anode can react with anisole to selectively form 4-iodoanisole with a yield as high as 97%. On contrast, the electrolytes prepared from Pt and graphite anodes finished the reaction with yields of 16% and 60% for 4-iodoanisole, respectively. This electrochemical method also applies to the iodination of toluene, benzonitrile and bromobenzene, delivering the target para-iodination products with 92%, 84%, and 73% yields, respectively. Thus, an atom-efficient and highly selective aryl iodination method was developed without the use of excessive oxidants.

Sustainable Electrochemical NO Capture and Storage System Based on the Reversible Fe2+/Fe3+-EDTA Redox Reaction

The removal of nitric oxide (NO), which is an aggregation agent for fine dust that causes air pollution, from exhaust gas has been considered an important treatment in the context of environmental conservation. Herein, we propose a sustainable electrochemical NO removal system based on the reversible Fe2+/Fe3+-ethylenediamine tetraacetic acid (EDTA) redox reaction, which enables continuous NO capture and storage at ambient temperature without the addition of any sacrificial agents. We have designed a flow-type reaction system in which the NO absorption and emission can be separately conducted in the individual reservoirs of the catholyte and anolyte with the continuous regeneration of Fe2+-EDTA by the electrochemical reduction in Fe3+-EDTA. A continuous flow reaction using a silver cathode and glassy carbon anode showed that the concentrations of Fe2+ and Fe3+-EDTA in the electrolyte were successfully maintained at a 1:1 ratio, which demonstrates that the proposed system can be applied for continuous NO capture and storage.

Hydrogen Peroxide Production from Oxygen and Water by Two-electrode Electrolytic Cell Using a Gold Nanoparticle-loaded Fluorine-doped Tin Oxide Cathode

Gold nanoparticles were deposited on fluorine-doped tin oxide (Au/FTO) with the mean particle size controlled by the deposition precipitation method. A two-electrode electrochemical cell with the structure of glassy carbon (anode) | 0.1 M NaClO4 | Nafion® | 0.1 M NaClO4 | Au/FTO with a mean Au particle size of 16.8 nm (cathode) stably and continuously produces H2O2 at applied voltage = 1.8 V with a rate of 1.08 × 102 µM h−1 and selectivity of 86.5% due to the high electrocatalytic activity of Au NPs for two-electron oxygen reduction reaction and the inactivity of FTO for H2O2 decomposition.

Improvement of the Performance in Membraneless Aqueous H2O2 Fuel Cells By Electroadsorption of Ad-Anions on the Surface of the Cathodes

Performance of a membraneless aqueous H2O2 fuel cells were significantly improved by assembling prussian blue-modified glassy carbon anodes (GC/PB) and gold cathodes whose surfaces were enriched by H2PO4 - ad-anions (Au/H2PO4 -). In this way, glassy carbon anodes were modified by a potentiostatically-deposited prussian blue (PB = Fe4[Fe(CN)6)]3•xH2O) film, whereas H2PO4 - ad-anions were electroadsorbed on the polycrystalline gold cathodes by means of cyclic voltammetry. The as-prepared electrodes were inserted in H2O2 fuel cells filled by deoxygenated phosphates buffer solutions (PBS, pH 2) containing several initial H2O2 concentrations. A highest power performance of the electrode materials was found by comparing all the discharge curves (potential vs. current plots) obtained as a function of the initial H2O2 concentration fed to the cells. Effect of the gap between the electrodes over the fuel cells performance was previously explored and optimized. The tabulated results below, demonstrate that the performance achieved for the cells constructed in this work was significantly superior to the best performance reported for comparable H2O2 fuel cells. Thereafter, electrochemical impedance spectroscopy (EIS) and a Bisquert type electrical transmission line [2] (data not shown), were employed for better understanding how an optimized electron-transfer kinetics for the Au/H2PO4 - cathodes allowed improving to the fuel cells performance. Finally, our results demonstrated that, once the Au/H2PO4 - cathodes were inserted into the H2O2 fuel cells the fill factor and the maximal power density were improved with the aid of the establishment of a competition between the electrical and ionic conductivities occurring through the PB films at the Au/PB anodes. open-circuit potential (mV) maximal current density (mA/cm2) fill factor maximal power density (mW/cm2) This work 741±11 13.5±0.4 0.56±0.03 5.6±0.2 Ref. [1] 600 10 0.26 1.6

Selective and Scalable Electrosynthesis of 2H-2-(Aryl)-benzo[d]-1,2,3-triazoles and Their N-Oxides by Using Leaded Bronze Cathodes.

Electrosynthesis of 2H‐2‐(aryl)benzo[d]‐1,2,3‐triazoles and their N‐oxides from 2‐nitroazobenzene derivatives is reported. The electrolysis is conducted in a very simple undivided cell under constant current conditions with a leaded bronze cathode and a glassy carbon anode. The product distribution between 2H‐2‐(aryl)benzo[d]‐1,2,3‐triazoles and their N‐oxides can be guided by simply controlling the current density and the amount of the charge applied. The reaction tolerates several sensitive functional groups in reductive electrochemistry. The usefulness and the applicability of the synthetic method is demonstrated by a formal synthesis of an antiviral compound.

Rhenium behavior in the KF-KBF4-B2O3-KReO4 melt

The present paper is devoted to the study of the electrolyte temperature influence on the cathode process of rhenium reduction on the glassy carbon anode by the cyclic voltammetry method. Experimental dependencies, which characterize the influence of the KReO4 addition on the KF-KBF4-B2O3 system liquidus temperature, were built according to the obtained data on the thermal analysis. It was found that rhenium reduction at lower electrolyte temperatures requires higher values of cathode overpotential.

Electrochemical Synthesis of Azanucleosides

Recently, the nucleic acid medicine receiving a lot of attention because of various drug targets and few side effects compared with conventional low molecular medicine. The numerous nucleoside analogues have been reported. Especially, azanucleosides, in which the oxygen atom of furanose ring has been replaced with a nitrogen atom, show anticancer, antivirus, and antileukemia activity. Furthermore, azanucleosides exhibit tolerance towards nucleases when their incorporate into oligonucleotides. However, the traditional synthetic strategy requires multiple steps and harsh conditions, thereby limiting to obtain desired product with high yield. We developed mild condition and efficient synthesis of azanucleosides from prolinol derivative using by an electrochemical reaction in a 1.0 M lithium perchlorate-nitroethane and small amount of acetic acid mixture medium with a glassy carbon anode and platinum cathode. In this reaction, intermediate iminium cation was generated through anodic oxidation of N-α position. The nitroalkane solvent is suitable for cation-pool method because the cation intermediate species are stabilized at nearly room temperature. After 2.6 F of electricity, the nucleophiles were added into reaction medium and stirred overnight. We used each nucleobases (N6-benzoyl adenine: A(Bz), N4-benzoyl cytosine: C(Bz), N2-isobutyryl guanine: G(Ib)) as nucleophiles. The coupling with thymine (T) was carried out through silyl Hilbert-Johnson reaction from other prolinol derivative that has hydroxyl group in N-α position. It was also prepared by anodic oxidative C-H activation. As a result, desired deoxyribo-azanucleosides was obtained. However, the desired azanucleosides was generated as α and β anomer by equally ratio in this reaction. The natural nucleic acids are composed by only β-anomer nucleosides. In this reason, to regulate the anomeric selectivity is very important. Now, we are challenging the stereoselective synthesis of these deoxyribo-azanucleosides. Furthermore, we are applying our electrochemical method for the synthesis of ribo-azanucleosides.

Anodic oxidation of isothiazolin-3-ones in aqueous medium by using boron-doped diamond electrode

Electrochemical degradation of biocide compound, isothiazolin-3-ones, was studied in aqueous medium, with Na2SO4 supporting electrolyte using boron-doped diamond (BDD) anode. The redox response of isothiazolin-3-ones at BDD was examined by cyclic voltammetric study. The degradation of isothiazolin-3-ones and its mineralization trend were monitored by UV–vis spectrophotometric method and total organic carbon (TOC) analyzer, respectively. The effect of operating parameters such as applied current density, biocide concentration, electrolyte pH and nature of supporting electrolytes (Na2SO4, NaNO3 and NaCl) on degradation rate was studied in detail. It was established that the hydroxyl radicals (OH) generated at BDD surface were responsible for the degradation and the mineralization of the biocide contaminant. The rate of degradation was almost independent of electrolyte pH but became faster as the applied current density increased and the biocide concentration decreased. The kinetic studies revealed that the biocide decay follows a pseudo-first-order rate. The apparent rate constant for the oxidation of isothiazolin-3-ones was determined to be 2.65×10−4s−1 at an applied current density of 25mAcm−2 in the presence of 0.1moldm−3 Na2SO4 at pH6.0. A poor mineralization efficiency was observed in the case of NaCl as supporting electrolyte which cause in-situ generation of chlorine based mediated oxidants such as Cl2 and OCl− which have negligible influence in mineralizing the isothiazolin-3-ones compared to peroxodisulfate (S2O82−) oxidants that formed in the case of Na2SO4. The oxidizing ability of the BDD anode was compared with those of Pt and glassy carbon anodes under similar experimental conditions.

Neutral Water Splitting Catalysis with a High FF Triple Junction Polymer Cell

We report a photovoltaics-electrochemical (PV-EC) assembly based on a compact and easily processable triple homojunction polymer cell with high fill factor (76%), optimized conversion efficiencies up to 8.7%, and enough potential for the energetically demanding water splitting reaction (Voc = 2.1 V). A platinum-free cathode made of abundant materials is coupled to a ruthenium oxide on glassy carbon anode (GC-RuO2) to perform the reaction at optimum potential (ΔE = 1.70–1.78 V, overpotential = 470–550 mV). The GC-RuO2 anode contains a single monolayer of catalyst corresponding to a superficial concentration (Γ) of 0.15 nmol cm—2 and is highly active at pH 7. The PV-EC cell achieves solar to hydrogen conversion efficiencies (STH) ranging from 5.6 to 6.0%. As a result of the solar cell’s high fill factor, the optimal photovoltaic response is found at 1.70 V, the minimum potential at which the electrodes used perform the water splitting reaction. This allows generating hydrogen at efficiencies that would be v...

Direct Imaging of the Electrochemical Deposition of Poly(3,4-ethylenedioxythiophene) by Transmission Electron Microscopy.

Conjugated polymers are electronically and ionically active organic materials of interest for use in a variety of devices. Electrochemical deposition is a convenient method for precisely fabricating conjugated polymer thin films, yet a detailed, quantitative understanding of nucleation and growth mechanisms has remained elusive. Here, we report direct imaging of the in situ electrochemical deposition of poly(3,4-ethylenedioxythiophene) (PEDOT) from an aqueous solution of EDOT monomer using Transmission Electron Microscopy with an electrochemical liquid flow cell. We found that PEDOT deposition began preferentially at the edge of the glassy carbon anodes at the beginning of the reaction. Fluctuating clusters of liquid-like oligomers were observed to form near the electrode surfaces. As the reaction continued, both the nucleation of new domains as well as the growth of pre-existing PEDOT deposits were observed, leading to systematic increases in film thickness and roughness.

Onsite Deposition of Self-Repairing Biomimetic Nanostructured Ni Catalysts with Improved Electrocatalysis toward Glycerol Oxidation for H2 Production

In this study, a technology was developed for producing hydrogen via glycerol electrolysis. A homogeneous stock solution of a Ni catalyst was prepared, conserved and used for an alkali electrolyte with no precipitation due to the coordinative and protective effects of sodium tartrate and polyvinyl pyrrolidone (PVP). The deposition of the Ni(OH)2 catalyst and its electrocatalysis toward glycerol occurred nearly simultaneously. Tedious and laborious procedures for preparing common catalysts and immobilizing them on an electrode surface were not required. The Ni(OH)2 catalyst film self-repaired due to the presence of Ni in the alkali electrolyte. This technique also exhibited satisfactory electrocatalytic performance for hydrogen production via glycerol electrolysis with a high transformation efficiency and good stability. The Ni(OH)2 electrocatalyst electrolyzed glycerol at the glassy carbon anode to produce H2O and CO2, and this H2O was electrochemically reduced at the Pt cathodic electrode to release H2. The formation of bubbles and the presence of PVP provided the anodic electrode surface a brilliant iridescence similar to that of butterfly wings due to the distributed Ni(OH)2 nanostructures with large circular spaces and small nanoholes. Such systems using an abundance of the non-noble metal Ni have potential applications for solar energy and fuel cell development.

Preparation of Thylakoid/Polyaniline/Reduced Graphene Oxide/Glassy Carbon Integrated System and Photocurrent Enhancement

Solar energy is enormous, inexhaustible and environmentally friendly. Photosynthesis is a highly developed system that converts solar energy into chemical energy, taking place in thylakoid membrane of plants, algae and cyanobacteria. In the light reaction, solar energy is absorbed by light-harvesting complexes (LHC) containing antenna pigments. Absorbed solar energy is transferred to the reaction center, where the water photolysis into oxygen, protons and electrons occurs by oxygen evolving complex (OEC) attached to photosystem II (PSII). The generated electrons are transferred along the electron transfer pathway called Z-scheme and eventually stored in the form of NADPH. Since quantum yields of photosystem I (PSI) and photosystem II are very high, numerous attempts have been made to integrate these photosynthetic units (PSUs) into photoelectrochemical cells to directly convert light into electricity. Rather than using PSI or PSII, here we present our preliminary results on light energy conversion with thylakoid membrane that is installed on the electrode surface. Thylakoid membrane was isolated from spinach whose oxygen evolving activity was measured by a Clark-type oxygen sensor under the illumination. For the effective light energy conversion, we have developed an integrated system containing thylakoid multilayers on the glassy carbon (GC) electrode. First, 4-aminophenyl (4-AP)-modified GC surfaces were prepared. Isolated thylakoid was mixed with polyaniline (PANI) and graphene oxide (GO), and this composite material was applied onto the 4-AP-modified GC. Since GO has poor electric conductivity, we electrochemically reduced GO to highly conductive reduced graphene oxide (RGO) form. Thus prepared integrated system is denoted as Thyl/PANI/RGO/GC. This integrated system was proven effective in solar energy conversion. To achieve high photocurrent, we prepared the same system with different GO/chlorophyll ratios. After cycling potential to reduce GO to RGO, photocurrent was measured. The ratio of 3:1 (GO:chlorophyll) turned out to give highest photocurrent. The electron transfer pathways have been identified from the blocking experiments with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of QB site in PSII. Thylakoid suspension was treated with DCMU before immobilization and then applied to the electrode. Virtually no photocurrent enhancement was observed proving that the electron transfer was blocked. A complete solar cell composed of Thyl/PANI/RGO/GC anode and Pt/C air cathode was constructed. From the polarization curve, the maximum power density of 10.5 μW/cm2 with current density of 24.7 μA/cm2 under one sun illumination was obtained. The turnover frequency was calculated to be 5.8 water molecule oxidation per PSII unit per second, indicating that the immobilized thylakoid in the form of the multilayer films maintained its activity. This result shows that larger number of thylakoid units are needed for enhanced photocurrent and our method of integrating thylakoid using polyaniline and graphene oxide could be a useful way to this end.

Role of anode material on the electrochemical oxidation of methyl orange

The anodic oxidation of methyl orange (MO, 5-(4-nitrophenylazo)salicylic acid) has been studied by cyclic voltammetry and bulk electrolysis, using a range of electrode materials such as Ti–Ru–Sn ternary oxide, lead dioxide and boron-doped diamond (BDD), glassy carbon (GC) and gold anodes. The results of voltammetries show that with all the electrode materials, in the potential region before oxygen evolution, the oxidation of MO involves simple electrode transfer that produces a polymeric film that deactivates the electrode surface, as confirmed by Fourier Transform Infrared Reflection-Absorption Spectroscopy (FTIRRAS) analysis. A very different behaviour was observed among the electrodes in the region of water decomposition. While BDD and PbO2 regained their initial activity by simple polarisation at 2.3 V vs. saturated calomel electrode (SCE) due to the production of high amount of hydroxyl radicals that destroy the polymeric film, TiRuSnO2, GC and gold cannot be completely reactivated, because they have a low overpotential for oxygen evolution, and this secondary reaction is favoured over polymer mineralization. The results of bulk electrolysis showed that after 3 h of polarisation at 10 mA cm−2, complete colour and chemical oxygen demand (COD) removal were obtained only with BDD anode. Using PbO2 MO was oxidised but a residual COD remains in the solution, while TiRuSnO2 permitted only a partial oxidation of MO.

Continuous treatments of estrogens through polymerization and regeneration of electrolytic cells

This study proposes a novel electrolytic method for simultaneous removal of trace estrogens and regeneration of electrolytic cells for long-term wastewater treatment. Continuous treatments of estrogens estrone (E1), 17β-estradiol (E2) and 17α-ethinyl estradiol (EE2) were theoretically and experimentally studied using an electrolytic reactor equipped with a multi-packed granular glassy carbon electrode reactor. Experimental results demonstrated that E1, E2 and EE2 were effectively removed through electro-polymerization on the granular glassy carbon (and Pt/Ti) anode counter. Polymer formed during continuous treatment was quickly decomposed and electrodes were regenerated completely by OH radicals produced through the reduction of ozone. Calculated overall energy consumptions were less than 10Wh/m3, demonstrating extremely low energy consumptions. In addition, a mathematical model developed based on the limiting mass transfer rate and post-regeneration could represent general trends in time series data observed in experiments.

Niobium Speciation in NaCl-KCl Based Melts: An Electrochemical and Spectroelectrochemical Study

Molten chlorides can be used for niobium electrowinning and electrorefining. Understanding niobium electrochemical properties and speciation in chloride melts is essential for designing or optimizing the industrial electrochemical process. Apart from the technological value, the information on the kinetics of the electrode reactions can also be used for verifying theoretical predictions concerning the nature of the heterophase electrochemical reactions.In the present study a variety of electrochemical techniques (polarization curves measurements (PCM), chronoamperometry (CA), chronopotentiometry (CP), linear (LVA), cyclic (CVA) and square-wave voltammetry (SWVA), impedance spectroscopy) were applied for studying electrode processes involving niobium species in NaCl-KCl-based melts. In addition the most valuable and reliable information about niobium speciation was obtained by combining electrochemical and spectroscopic (measuring electronic absorption spectra, EAS) methods.The results of spectroelectrochemical experiments showed that the major product of niobium anodic dissolution in chloride melts at current densities up to 40 mA/cm2 were niobium(III) species, NbCl6 3-. In the potentiostatic regime of anodic dissolution of niobium metal niobium(III) complex ions were formed at potentials below -1.15 V vs.chlorine reference electrode. The polarization curves characterizing the anodic dissolution of niobium consist of three or more waves. The first wave is associated with reaching limiting current density of niobium ionization according to the three-electron scheme. Further increase of applied current leads to the oxidation of Nb(III) to Nb(IV) ions. These conclusions were confirmed by determined values of potentials of niobium or indifferent electrodes in the equilibrium with the melts containing Nb(III) and Nb(IV) ions.When studying anodic dissolution of niobium we found that at high current densities there is sudden increase of the ohmic part of voltage drop sometime accompanied by increase of the anode potential. As a result stationary and non-stationary polarization dependencies exhibit unstable behavior. This is explained by the salt passivation, SP. The product of the SP of niobium is unstable NbCl4, which decomposes to NbCl3.13 and volatile NbCl5.The results of electrochemical experiments (PCM, CVA, LVA, CA, CP and SWVA) showed that soluble niobium(V) species were formed during Nb(IV) electrooxidation and represented the highest oxidation state of niobium in NaCl-KCl melts. We noticed that peak potential value (both cathodic and anodic) on CV changes with scan rate, whereas the ip/ν1/2 value remains constant. The formation of Nb(V) species was confirmed by the disappearance of absorption bands in the visible part of the spectrum due to Nb(V) species formation during elecrooxidation of niobium-containing melts at potentials up to -0.5 V. The quasireversibility of Nb4+↔Nb5+reaction was explained by the instability of niobium(V) species.Niobium(IV) ions were formed when potentiostatic electrolysis of NbCl3-NaCl-KCl melts (prepared by anodic dissolution of niobium metal) was performed at -(1.0÷0.8) V vs.chlorine reference electrode on a glassy-carbon anode, Fig. 1.The electroreduction of niobium on a tungsten electrode in NbCl3-NaCl-KCl melts at potentials below -1.45 V led to the formation of metallic niobium (Fig. 2), no evidence of any intermediate product formation was obtained.Linear voltammetry measurements showed that intermediate products can be formed during elecroreduction of NaCl-KCl-NbCln (n=3.8-4.1) melts. Most likely the electroreduction of Nb(IV) results in the formation of niobium(III) ions. The shape of cyclic voltammograms recorded in niobium containing melts depended on the scan rate. Similar sets of cyclic voltammograms were obtained at different temperatures and niobium concentrations. SWVA measurements performed on different electrodes also confirmed that there is more than one step of electroreduction. The obtained results were explained by overlapping of the following electrode reactions: Nb(IV) → Nb(III), Nb(IV) → Nb(0) and Nb(III) → Nb(0).

Anodic properties of silafluorenes

Three 1,3-diphenyl ring-substituted silafluorenes (I–III) and one ring-unsubstituted derivative, 1,1-diphenylsilafluorene (IV) were characterized electrochemically by cyclic voltammetry measurements in dichloromethane-acetonitrile (1:4, v/v)-lithium perchlorate at both Pt and glassy carbon (GC) anodes. Compounds I–III show at least two irreversible oxidation waves, the first being at a lower potential (around 1.4V) and the second around 1.75V whereas IV exhibits only one irreversible oxidation wave at 1.85V (vs. Ag/AgCl). Electrolysis at a C (graphite) anode under controlled potential conditions led to the expulsion of the silicon moiety in all cases. Introducing phenyl groups on the silafluorene ring at the 1 and 3 positions in I–III afforded 1,3,5-triphenylbenzene (1) as the sole aromatic hydrocarbon product. However, ‘naked’ compound IV showed less selectivity, producing a mixture of products involving biphenyl, substituted biphenyls (and additional unidentified aromatic hydrocarbon). The silicon fragments formed mostly linear and cyclic siloxanes on both C and Pt anodes. However at Pt the reaction was less selective than at C and produced in addition “intermediate” products containing a single C–Si bond (out of the two in the parent silafluorene ring) due to only one C–Si bond cleavage.