Electrolytic reduction of Tc(VII) in nitric acid solution using glassy carbon electrode

Electroreduction of Tc(VII) was studied in nitric acid solution using glassy carbon electrode. The electroreduction was conducted at a constant potential –300 mV (vs. Ag/AgCl) with a potentiostat. It was found that the difference of the Tc concentration in the solutions before and after the electrolysis was negligibly small. This means that there were almost no TcO₂ or Tc deposited on the carbon fiber electrode during the electroreduction. Absorption spectra and distribution coefficients obtained by ion-exchange analysis indicated that Tc(VII) was reduced to Tc(IV).This work was financed by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) under the framework of The Development of Innovative Nuclear Technologies.     

Stripping voltammetric determination of traces of technetium with a glassy carbon electrode coated with tri-n-octylphosphine oxide

Technetium(VII) and Tc(IV) are concentrated from 3 M hydrochloric acid media by complexing with tri-n-octylphosphine oxide applied as a thin layer to a glassy carbon electrode. Differential-pulse cathodic stripping voltammetry from 0 V provides a stripping peak for Tc(VII) at ?350 mV (vs. Ag/AgCl). The detection limit after an enrichment time of 10 min is about 1.8×10^?8 M Tc(VII). Technetium(IV) produces a stripping peak near the Tc(VII) peak which can be used for rough estimates of the Tc(VII)/Tc(IV) ratio within limited ranges. Uranium(VI) in equimolar concentrations interferes.     

Catalytic activity of thiourea and its selected derivatives on electroreduction of In(III) in chlorates(VII)

It was found that thiourea, N-methylthiourea, N,N′-dimethylthiourea and N-allylthiourea accelerate the electroreduction process of In(III) ions in chlorates(VII). These substances are adsorbed on mercury from chlorates(VII). The relative surface excesses of thiourea and its derivatives increase with the increase of their concentrations and electrode charge. After adding thiourea, N-methylthiourea, N,N′-dimethylthiourea and N-allylthiourea to the solution an acceleration of the electroreduction process of In(III) ions occurs. This process depends on two factors: the adsorption of an accelerating substance on mercury and on the formation of complexes between a depolarizer and an accelerating substance on the electrode surface. The equilibrium of this complexing reaction determines the magnitude of the catalytic effect.     

Solvent extraction of Tc(VII) by the mixture of TBP and 2-nitrophenyl octyl ether

The extraction of Tc(VII) by the mixture of tri-n-butyl phosphate (TBP) and 2-nitrophenyl octyl ether (NPOE) has been studied. 0.2M NPOE-TBP can extract Tc(VII) effectively from 1M HNO₃ and 1M NaOH solutions with distribution ratios of 57.1 and 12.3, respectively. The distribution ratio of Tc(VII) decreases with increasing (>0.5M) HNO₃ concentration but increases with the increase of NaOH concentration. A pH 9 NaOH solution has proven to be suitable for Tc(VII) stripping. A simple extraction-stripping cycle can remove Tc(VII) from a sodium hydroxide solution. A more sophisticated extraction process is proposed to remove Tc(VII) from nitric acid solution because the co-extracted HNO3 prevents the direct stripping of Tc(VII) by NaOH solution of pH 9.     

Cobaltporphyrin‐adsorbed carbon black: highly efficient electrocatalysts for oxygen reduction

meso‐Substituted cobalt porphyrins adsorbed on carbon black were prepared as catalysts for the electroreduction of O₂. The catalyst, which was prepared by using a homogenizer in mixing cobalt tetraethylporphyrin and carbon black, gave rise to electroreduction of O₂ at a remarkably positive potential (E_p = 0.45 V versus saturated calomel electrode (SCE)) and showed a high selectively for the four‐electron reduction (n = 3.8). Electrochemical study and extended X‐ray absorption fine structure (EXAFS) analysis revealed that the adsorbed face‐to‐face dimeric aggregates of cobalt porphyrin molecules were highly efficient catalysts for electroreduction of O₂. Copyright © 2005 John Wiley & Sons, Ltd.     

Preliminary study of quality control of99m Tc from99Mo generators

The separation of Tc(VII) from Mo(VI) by thin-layer and paper-chromatography is discussed. Some aspects concerning the formation and identification of lower oxidation states of Tc(VII) are also mentioned. Finally, a spot test is recommended for the determination of Mo(VI) and Al, which can be contaminants in the Tc(VII) solution eluted from the⁹⁹Mo column, filled with Al₂O₃.     

Favoring CO Intermediate Stabilization and Protonation by Crown Ether for CO2 Electromethanation in Acidic Media

The large-scale deployment of CO₂ electroreduction is hampered by deficient carbon utilization in neutral and alkaline electrolytes due to CO₂ loss into (bi)carbonates. Switching to acidic media mitigates carbonation, but suffers from low product selectivity because of hydrogen evolution. Here we report a crown ether decoration strategy on a Cu catalyst to enhance carbon utilization and selectivity of CO₂ methanation under acidic conditions. Macrocyclic 18-Crown-6 is found to enrich potassium cations near the Cu electrode surface, simultaneously enhancing the interfacial electric field to stabilize the *CO intermediate and accelerate water dissociation to boost *CO protonation. Remarkably, the mixture of 18-Crown-6 and Cu nanoparticles affords a CH₄ Faradaic efficiency of 51.2 % and a single pass carbon efficiency of 43.0 % toward CO₂ electroreduction in electrolyte with pH=2. This study provides a facile strategy to promote CH₄ selectivity and carbon utilization by modifying Cu catalysts with supramolecules.     

What’s that yellow powder? A nuclear forensic case study

A challenge to the incorporation of ⁹⁹Tc into a glass matrix is that ⁹⁹Tc is volatile at vitrification temperatures. Understanding how this volatilization occurs requires knowledge of the multiple chemistries which Tc may take during vitrification. This paper presents an overview of how the localized chemistry of ⁹⁹Tc has been determined in a series of ⁹⁹Tc-bearing glasses by chemometric analyses of X-ray absorption near edge spectra (XANES). Linear combination fitting and principal component analysis of the glasses’ XANES spectra suggested that the glasses contained 3–4 chemically distinct Tc environments. The identity of the detected chemistries were pertechnetates, (Tc(VII) as isolated oxyanions, or locally coordinated by Na or K), or isolated Tc(IV) species. The linear combination fitting distribution of local Tc sites agrees with reanalyzed Raman spectra, suggesting that as targeted KTcO₄ incorporation increases, a significant ion exchange takes place, and speciation in the glass changes to favor Tc(VII) formation, specifically NaTcO₄. Based on the statistical suggestion that not all Tc environments are accounted by the available standards, a new mechanism for the behavior of Tc during vitrification is proposed.     

Extraction of moderate oxidation state technetium species between 30 % tri-n-butyl phosphate and H2SO4/HNO3

The extraction of technetium species at oxidation state lower than +7 has been examined in sulfuric and sulfuric/nitric acid solutions using UV–Vis spectroscopy and optically transparent thin layer cell (RVC-OTTLE). Soluble Tc(III), TcO²⁺ and [Tc₂O₂]³⁺ species with absorption bands at 420–450, 400, and 502 nm, respectively, were detected as products of pertechnetates electroreduction. The distribution ratios of ⁹⁹Tc with lower than +VII oxidation state ionic species between 4 M H₂SO₄ and 30 % TBP/kerosene were found and are significantly lower than for TcO₄ ^? in the same solution.     

Nanosize catalysts based on carbon materials promoted by cobalt tetra(<Emphasis Type="Italic">para</Emphasis>-methoxyphenyl) porphyrin pyropolymer for oxygen electroreduction

We present the results of electrochemical and structural investigations of several carbon materials: carbon blacks AD 100 and XC 72, ultradisperse diamond (UDD), multiwalled nanotubes (MWNT), various types of filament-like carbon materials (CFC series), and similar carbon materials promoted with cobalt tetra(para-methoxyphenyl) porphyrin (CoTMPhP) pyropolymer (PP). The electrochemical studies were performed at room temperature in 0.5 M H₂SO₄ by using a rotating disk electrode (RDE), a rotating ring-disk electrode (RRDE) (a thin layer of test material was applied onto the disk electrode), and a floating electrode. Structural characterization of initial and promoted carbon materials involved the determination of specific surface area by the BET method and by the polarization capacitance from cyclic voltammograms, and the particle morphology and dimensions by the transmission electron microscopy (TEM) method. The study of kinetics and mechanism of oxygen electroreduction on carbon materials promoted with CoTMPhP PP showed that the catalysts based on carbon materials of CFC and UDD series possess high specific activity in this reaction and high selectivity with respect to oxygen reduction to water. These catalysts are superior to the catalysts, in which carbon blacks AD 100 and XC 72 are used as the supports, in the specific activity.     

Adsorptivity of polyvinylpolypyrrolidone for selective separation of U(VI) from nitric acid media

Adsorptivity of polyvinylpolypyrrolidone (PVPP), a candidate resin with selectivity to U(VI) in HNO₃ media, to various metal ions was examined. It was found that PVPP has a strong adsorptivity to U(VI) in wide concentration range of HNO₃. The Scatchard plot analysis revealed that the adsorption of U(VI) by PVPP occurs at plural binding sites. The infrared spectroscopic analysis suggested that the strong binding site is due to the coordination of the carbonyl oxygen atom and the nitrogen atom in the pyrrolidone ring to UO₂ ²⁺. It was also found that fission product ions except Re(VII) as the simulant of Tc(VII) and Pd(II) are not adsorbed onto PVPP. The adsorptivities to Tc(VII) and Pd(II) species are weak, indicating that U(VI) can be separated from other metal ions by PVPP.     

Reductive immobilization and long-term remobilization of radioactive pertechnetate using bio-macromolecules stabilized zero valent iron nanoparticles

Reductive immobilization of radioactive pertechnetate (⁹⁹TcO₄^?) in simulated groundwater was studied by prepared carboxymethyl cellulose (CMC) and starch stabilized zero valent iron nanoparticles (nZVI), and long-term remobilization of reduced Tc was also evaluated under anoxic and oxic conditions. The stabilized nZVI can effectively reduce soluble ⁹⁹Tc(VII) to insoluble ⁹⁹Tc(IV), and they can be easily delivered into a contaminated groundwater zone and facilitate in situ remediation. In this study, CMC-stabilized nZVI showed higher reactivity than that using starch as the stabilizer. Batch experiments indicated that more than 99% of ⁹⁹Tc(VII) (C₀ = 12 mg/mL) was reduced and removed from groundwater by CMC-stabilized nZVI with a CMC content of 0.2% (w/w) at a broad pH of 5–8. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses further confirmed that ⁹⁹Tc(VII)O₄^? transformed into ⁹⁹Tc(IV)O₂ (s). The presence of bicarbonate exhibited insignificant effect on Tc immobilization, while humic acid (HA) inhibited reaction mainly due to retardation on electron transfer and formation of Tc(IV)-HA complexes. More interesting, the immobilized Tc(IV) remained insoluble even after 120 d under anoxic condition, while only ～21% was remobilized when exposed to air. Therefore, bio-macromolecules stabilized nZVI nanoparticles could be a viable alternative for in situ remediation of radioactive contamination in groundwater.     

Electroreduction of Oxygen and Electrooxidation of Methanol at Carbon and Single Wall Carbon Nanotube Supported Platinum Electrodes

The present research aimed at investigating the electrocatalytic properties and the electrochemical deposition of Pt nanoparticles on carbon powder, carbon nanotube and preparation of carbon and single wall carbon nanotube supported platinum electrodes. The Pt nanoparticles were synthesized by electroreduction of hexachloroplatinic acid in aqueous solution at ?200 mV. Electrocatalytic properties of the modified electrodes for oxygen reduction were investigated by cyclic voltammetry in O₂ saturated solution containing 0.1 M HClO₄. Methanol electrooxidation at the modified surfaces in 0.5 M HCLO₄ was studied by cyclic voltammetry. The corresponding results showed that the Pt/SWCNT/GC electrode exhibits more improved catalytical activity than the Pt/C/GC electrode.     

Electrocatalysis of metalloporphyrins: Part 12. Chemically modified electrodes with porphyrin-incorporated Nafion coatings

The electrocatalytic behaviour of cobalt(III) tetrakis(4-trimethylammoniumphenyl)porphyrin (CoTMAP) and the electroreduction mechanisms of dioxygen and dithiodipropionic acid (PSSP) at glassy carbon electrodes (GCEs) modified with Nafion coatings incorporating CoTMAP and Ru(NH₃)₆³⁺ have been studied. The experimental results indicate that the catalytic electroreduction of dioxygen using CoTMAP as a catalyst and Ru(NH ₃)₆³⁺ as a mediator could proceed at the Nafion coating—solution interface regardless of the state of the electrode surface. The catalytic electroreduction of dithiodipropionic acid could not proceed at the Nafion coating—solution interface but only at the GCE—solution interface, suggesting the importance of the adsorption of Co(I)TMAP on the GCE to the catalytic electroreduction of PSSP.     

Rotating ring-disk voltammetry: Diagnosis of catalytic activity of metallic copper catalysts toward CO<Subscript>2</Subscript> electroreduction

Using the rotating ring (platinum)—disk (glassy carbon) electrode methodology, electrocatalytic activity of the microstructured copper centers (imbedded within the polyvinylpyrrolidone polymer matrix and deposited onto the glassy carbon disk electrode) has been monitored during electroreduction of carbon dioxide both in acid (HClO₄) and neutral (KHCO₃) media as well as diagnosed (at Pt ring) with respect to formation of the electroactive products. Combination of the stripping-type and rotating ring-disk voltammetric approaches has led to the observation that, regardless the overlapping reduction phenomena, the reduction of carbon dioxide at copper catalyst is, indeed, operative and coexists with hydrogen evolution reaction. Using the fundamental concepts of surface electrochemistry and analytical voltammetry, the reaction products (thrown onto the platinum ring electrode) could be considered and identified as adsorbates (on Pt) under conditions of the stripping-type oxidation experiment. Judging from the potentials at which the stripping voltammetric peaks appear in neutral CO₂-saturated KHCO₃ (pH 6.8), formic acid or carbon monoxide seem to be the most likely reaction products or intermediates. The proposed methodology also permits correlation between the CO₂ electroreduction products and the potentials applied to the disk electrode. By performing the comparative stripping-type voltammetric experiments in acid medium (HClO₄ at pH 1) with the adsorbates of formic acid, ethanol and acetaldehyde (on Pt ring), it can be rationalized that, although C₂H₅OH or CH₃CHO are very likely CO₂-reduction electroactive products, formation of some HCOOH, CH₃OH or even CO cannot be excluded.     

Impact of the Pt catalyst on the oxygen electroreduction reaction kinetics on various carbon supports

Micro- and mesoporous carbide-derived carbons synthesized from molybdenum and tungsten carbides were used as porous supports for a platinum catalyst. Synthesized materials were compared with commercial Vulcan XC72R conducting furnace black. The scanning electron microscopy, X-ray diffraction, Raman spectroscopy, high-resolution transmission electron microscopy, and low-temperature N₂ adsorption methods were applied to characterize the structure of catalysts prepared. The kinetics of oxygen electroreduction in 0.5 M H₂SO₄ solution was studied using cyclic voltammetry and rotating disk electrode methods. The synthesized carbide-derived carbons exhibited high specific surface area and narrow pore size distribution. The platinum catalyst was deposited onto the surface of a carbon support in the form of nanoparticles or agglomerates of nanoparticles. Comparison of carbide-derived carbons and Vulcan XC72R as a support showed that the catalysts prepared using carbide-derived carbons are more active towards oxygen electroreduction. It was shown that the structure of the carbon support has a great influence on the activity of the catalyst towards oxygen electroreduction.     

Influence of temperature on the reduction kinetics of Bi(III) ion in the presence of cystine in chlorate (VII) solutions of decreased water activity

The results of the kinetic measurements of Bi(III) electroreduction on a mercury electrode in 1–8 mol dm^?3 chlorate (VII) solutions and in the presence of cystine demonstrate a dependence of the process on the temperature. The applied electrochemical techniques (DC polarography, cyclic and SWV voltammetry) allowed for the determination of the kinetic and thermodynamic parameters and their correlation with water activity. The catalytic activity of cystine was confirmed by the decrease in overall enthalpies of activation. The changes in the values of ΔH ^≠ and ΔS ⁰ for Bi(III) electroreduction in the presence of cystine with the increase of chlorate (VII) concentration showed that the mechanism is different in solutions with low water activity as compared to those with high water activity. Probably it is connected with a different structure of the activated complexes (Bi-Hg(SR)₂), mediating electron transfer.      

Voltammetric Determination of Metronidazole Using a Sensor Based on Electropolymerization of α‐Cyclodextrin over a Carbon Paste Electrode

An electrochemical sensor for metronidazole (MTZ) was built via the surface modification of a carbon paste electrode (CPE) by a film obtained through electropolymerization of α‐cyclodextrin (CPE_α‐CD). The CPE_α‐CD was characterized by cyclic voltammetry (CV) and atomic force microscopy (AFM), by both techniques was demonstrated that the polymer film is coating the electrode surface. The electroreduction behaviour of MTZ in HClO₄ media as a supporting electrolyte was studied by differential‐pulse voltammetric (DPV) technique. The DPV electrochemical process was observed to be diffusion controlled and irreversible. Under optimal conditions, the peak current was proportional to MTZ concentration in the range of 0.5 to 103.0 μM with a detection limit of 0.28±0.02 μM. The method was successfully applied to quantify of MTZ in pharmaceutical formulations. In addition, this proposed MTZ sensor exhibited good reproducibility, long‐term stability and fast current response.     

Strategy for Low Background‐Current Levels in the Electrochemical Biosensors Using Horse‐Radish Peroxidase Labels

This article describes an electrochemical strategy to achieve low background‐current levels in horse‐radish peroxidase (HRP)‐based electrochemical immunosensors. The strategy consists of (i) the use of an HRP substrate/product redox couple whose formal potential is high and (ii) the use of an electrode that shows moderate electrocatalytic activity for the redox couple. The strategy is proved by a model biosensor using a catechol/o‐benzoquinone redox couple and an indium tin oxide (ITO) electrode. The combined effect of high formal potential and moderate electrocatalytic activity allows o‐benzoquinone electroreduction with minimal catechol electrooxidation and H₂O₂ electroreduction. The detection limit for mouse‐IgG is 100 pg/mL.     

Anion sensitive voltammetry of fullerene C60 dissolved in 1,2-dichlorobenzene deposit in contact with aqueous electrolyte

Electroreduction of C₆₀ dissolved in hydrophobic solvent deposited on the electrode surface was studied. A microliter amount of C₆₀ and tetrahexylammonium perchlorate solution in 1,2-dichlorobenzene was deposited on basal plane pyrolytic graphite electrode and this electrode was immersed into an aqueous solution. The voltammetry shows three consecutive reduction–oxidation steps. The redox potential of first electroreduction step is sensitive on anion but not on cation present in the aqueous phase. This parameter also depends on electrolyte concentration in the aqueous and organic phase. It is proposed that electroreduction of C₆₀ is preceded by anion exchange and followed by anion expulsion to the aqueous phase. Similar anion effect on the redox potential is also observed for unsupported deposit indicating importance of initial partitioning of electrolyte into the organic phase.