Do carbon nanotubes catalyse bromine/bromide redox chemistry?

The redox chemistries of both the bromide oxidation and bromine reduction reactions are studied at single multi-walled carbon nanotubes (MWCNTs) as a function of their electrical potential allowing inference of the electron transfer kinetics of the Br₂/Br⁻ redox couple, widely used in batteries. The nanotubes are shown to be mildly catalytic compared to a glassy carbon surface but much less as inferred from conventional voltammetry on porous ensembles of MWCNTs where the mixed transport regime masks the true catalytic response.

Schematic of a carbon nanotube impact in bromide solution.

The bromine–bromide redox couple plays an essential role in diverse energy storage devices including hydrogen–bromine, zinc–bromine, quinone–bromine, vanadium–bromide and bromide–polysulphide flow batteries.^1–5 The Br₂/Br⁻ redox couple is attractive as a cathode reaction due to its high standard potential, large solubility of both reagents, high power density and cost efficiency.⁶ The performance of such devices is generically limited by the thermodynamics and kinetics of the redox couple comprising the battery with fast (‘reversible’) electron transfer is essential. In many cases, including the Br₂/Br⁻ couple the electrode reaction involves more than one electron as given in the stoichiometric reaction:2Br⁻ − 2e⁻ ⇄ Br₂; E⁰ = 1.08 V vs. SHEwith, at high bromide concentrations, the possibility of the follow up chemical reaction⁷Br₂ + Br⁻ ⇄ Br₃⁻Since electrons are usually transferred sequentially this implies that the mechanism is multistep with any of the individual mechanistic steps in principle being rate limiting. For this reason catalysts are commonly required to enhance the electrode kinetics at otherwise favourable electrode materials. One type of catalyst which has seen wide usage, including for the Br₂/Br⁻ couple^8,9 are carbon nanotubes (CNTs) with suggested advantages which include high surface area and the inherent porosity of CNT composites.¹⁰ The deployment of CNTs as a porous composite presents a further level of complexity to the electrode reaction beyond its multistep character because of the ill-defined mass transport within the porous layer. In particular ascertaining the intrinsic electron transfer kinetics and hence the level of catalysis, if any, is essentially impossible since these are masked in the voltammetric response by diffusional mass transport effects.^11–14 Specifically the transport within the porous structure of CNT layers is dominated by thin-layer and other^15,16 effects which give the illusion of electrochemical reversibility. In order to unscramble possible electro-catalysis of the bromine/bromide couple a different approach is needed.In the following we study both the electro-oxidation of bromide (BOR) and the electro-reduction of bromine (BRR) at single MWCNTs via ‘nano-impact (aka ‘single entity’) electrochemistry’^17–20 in aqueous solution. In this approach a micro-wire electrode at a fixed potential is inserted in a suspension of CNTs in the solution of interest. From time to time a single CNT impacts the electrode, adopts the potential of the latter for the duration of the impact which in the case of CNTs can vary from 1–100 of seconds^21–23 and sustained catalytic currents flow if the oxidation/reduction of interest is faster at the nanotube in comparison with the micro-wire electrode. The catalytic currents are studied as a function of potential revealing the electron transfer kinetics. Fig. 1 shows the concept of the experiment.Open in a separate windowFig. 1Schematic representation of ‘nano-impact’ electrochemistry on a carbon micro wire electrode for the oxidation of aqueous bromide from which the kinetics of the BOR are inferred. Analogous experiments but showing negative impact currents allow the inference of the kinetics of the BRR.The BOR and BRR were studied first, however, voltammetrically at an unmodified glassy carbon (GC) electrode as shown in Fig. 2 (black line) using 5.0 mM solutions of either NaBr or Br₂ in 0.1 M HNO₃. The midpoint potential was 0.82 V versus the saturated calomel electrode (SCE) consistent with the literature values for the formal potential of the Br₂/Br⁻ couple.²⁴ The voltammograms were analysed to give transfer coefficients of 0.45 ± 0.01 and 0.33 ± 0.01 (ESI, Section 2^†) for the BOR and BRR respectively. Both processes were inferred to be diffusional and the diffusion coefficients D_Br⁻ and D_Br2 were calculated to be 2.05 (±0.04) × 10⁻⁵ cm² s⁻¹ and 1.50 (±0.04) × 10⁻⁵ cm² s⁻¹ (ESI, Section 3^†) using the Randles–Ševčík equation for an irreversible reaction the values are consistent with literature reports.²⁴ Then the electrodes were modified with 30 μg of MWCNTs consisting of ca. 125 monolayers (the calculation is given in the ESI, Section 9^†) of MWCNTs assuming that they are closely packed across the area of the GC electrode, and the resulting voltammograms are shown in Fig. 2 (red line). In comparison with the unmodified electrode, enhanced currents are seen for the Br₂/Br⁻ couple which partly reflects the enhanced capacitance of the interface reflecting in turn the large surface area of the deposited nanotubes (ca. 60–120 cm²). Larger signals are also seen indicating a thin layer contribution from the material occluded within the porous layer which also leads to the apparently quasi-reversible shape of the voltammograms obtained for both reactions. A log–log plot of peak current (I_p) vs. scan rate (ν) showed a gradient value of 0.68 (±0.01) and 0.66 (±0.03) for the BOR and BRR (ESI, Section 4^†) confirming a mixed mass transport regime^12,14 with a combination of semi-infinite diffusion and thin layer behaviour. The transition from the fully irreversible to the apparent quasi-reversible character is sometimes confused with electro-catalysis attributed to the CNTs rather than thin-layer diffusion. In order to ascertain the true catalytic response, single entity electrochemistry was measured to obtain the BOR and BRR responses at single CNTs.Open in a separate windowFig. 2Cyclic voltammograms at pristine GC (black line) and 30 μg MWCNTs dropcast on GC (red line) at a scan rate of 0.05 V s⁻¹ (a) for the bromide oxidation reaction (BOR) in 5.0 mM NaBr in 0.1 M HNO₃, (b) for the bromine reduction reaction (BRR) in 5.0 mM bromine in 0.1 M HNO₃.For single entity measurements, a clean carbon wire (CWE, length 1 mm and diameter 7 μm) working electrode was used. Chronoamperograms were recorded at a constant applied potential of 0.2 V vs. SCE and 1.3 V vs. SCE for the BOR and BRR respectively (5.0 mM solutions). These values were selected in the light of Fig. 2 to provide a large overpotential for each reaction. Clear oxidative and reductive current steps were observed (Fig. 3). These were ascribed to the arrival of a MWCNT at the electrode surface and the resulting catalytic electron transfer for the duration of the impact. No steps were observed in the absence of MWCNTs (ESI, Fig. S4^†). The average residence time of the MWCNT was 1.2 (±0.5) seconds and the frequency of the collisions was 0.3 (±0.1) impacts per second. The average impact current for the BOR at 1.3 V vs. SCE was 2.8 (±0.2) nA (65 impacts) and for the BRR at 0.2 V vs. SCE it was 3.8 (±0.1) nA (70 impacts). The impact currents were assumed to be entirely faradaic since control experiments in 0.1 M HNO₃ solution in the presence of 100 μg of MWCNTs (in the absence of Br⁻ and Br₂) showed no obvious impacts as shown in ESI Section 10.^†Open in a separate windowFig. 3Chronoamperograms showing the impact step current (a) for the BOR in 5.0 mM NaBr in 0.1 M HNO₃ at 1.3 V vs. SCE, (b) for the BRR in 5.0 mM bromine in 0.1 M HNO₃ at 0.2 V vs. SCE.Further, impacts for both the BOR and BRR were observed at various potentials (ESI, Section 11^†) and analysed to obtain the average faradaic current at each potential. The average impact step current was plotted against the applied potential (Fig. 4). Two sigmoidal curves were obtained reflecting the current–potential response for either the bromide oxidation (BOR) or the bromine reduction (BRR). The curves reflect the average voltammograms (current–potential characteristics) for the Br₂/Br⁻ redox reaction at single carbon nanotubes. The shape of the two sigmoidal curves reflects the onset of electrolysis followed by a diffusion controlled plateau at high over-potentials.²⁵ Mass transport corrected Tafel analysis (Fig. 4; inset) showed the transfer coefficients β to be ca. 0.42 and α to be ca. 0.20 from the impacts for the BOR and BRR respectively (ESI, Section 6^†). The length distribution of the MWCNTs was calculated (ESI, Section 6^†) from the currents recorded at potentials corresponding to the plateau in Fig. 4 assuming that the reactions are (Fickian) diffusion controlled at the potentials used and by modelling the CNTs as cylindrical electrodes²¹ assuming a nanotube radius of 15 (±5) nm and the diffusion coefficients reported above. Chronoamperometry was also conducted for the BOR and BRR in the absence of MWCNTs at 1.3 V and 0.2 V vs. SCE respectively to confirm that no impact currents were contributed by the redox species in the electrolyte (ESI, Section 5^†). Alongside, chronoamperograms in 0.1 M HNO₃ and 100 μg show that the impact current was contributed only by the Br⁻ and Br₂ redox reaction and the results are shown in the ESI, Section 10.^†Open in a separate windowFig. 4Average step currents observed as a function of applied potential (a) for the BOR in 5.0 mM NaBr in 0.1 M HNO₃ at, (b) for the BRR in 5.0 mM Bromine in 0.1 M HNO₃; insets in both the cases show mass transport corrected Tafel analyses.The lengths were found to be 5.4 (±3.4) μm (BOR) and 5.9 (±1.3) μm (BRR) and are given in Fig. 5 (see ESI, Section 7^† for calculations). These values were compared with previously reported dark-field optical microscopy data and good agreement was observed with the literature value of 5.3 (±2.1) μm.²⁶ The observed consistency provides strong support for the choice of modelling the single entity voltammetry by analogy with that of a cylindrical electrode.Open in a separate windowFig. 5The length of MWCNTs calculated from the impact currents for the BOR (at 1.3 V vs. SCE) and BRR (at 0.2 V vs. SCE).It is evident that the single entity measurements allow a clear analysis of the catalytic behaviour of the carbon nanotubes by providing a well-defined diffusional regime conducive to the extraction of the electrode kinetics of both the bromide oxidation and the bromine reduction process. In contrast, electrodes were formed by ensembles of carbon nanotubes in the form of a porous layer where the mixed transport regime is not amenable to ready modelling and the dissection of thin-layer effects from the measured voltammetry. The electron transfer kinetics for both the BOR and BRR at single MWCNTs was then obtained via full simulation of the two single entity ‘voltammograms’ using the above measured diffusion coefficients and again treating the impacted MWCNT as a cylindrical electrode with uniform diffusional access and further assuming Butler–Volmer kinetics. For the BOR, one electron transfer was considered as given below,For the BRR the two electron transfer was modelled as,Br₂ + 2e⁻ → 2Br⁻The set of parameters used for the analysis are given in the ESI, Section 8.^† By using the transfer coefficients deduced from Fig. 4, the only unknown is the standard electrochemical rate constant k which is determined by fitting the impact voltammogram measured relative to a formal potential for the Br₂/Br⁻ couple of 0.82 V vs. SCE obtained from the voltammogram at pristine GC. Fig. 6 shows the fitting for the BOR and the BRR with rate constants k_BOR of 1.0 (±0.1) × 10⁻³ cm s⁻¹ and k_BRR of 5.0 (±0.1) × 10⁻⁴ cm s⁻¹ respectively. The transfer coefficients and rate constants obtained from impacts were compared to the voltammograms obtained at pristine GC for the BOR and BRR and are given in Open in a separate windowFig. 6DIGISIM simulated curves (black line) for average impact currents obtained at different potentials (red circles) (a) for the BOR with a rate constant (k_BOR) of 1.0 (±0.1) × 10⁻³ cm s⁻¹; (b) BRR with a k_BRR of 5.0 (±0.1) × 10⁻⁴ cm s⁻¹.Transfer coefficients and rate constants for the BOR in 5.0 mM NaBr in 0.1 M HNO₃ and the BRR in 5.0 mM bromine in 0.1 M HNO₃ obtained at the glassy carbon macroelectrode GC, and single MWCNT impact current

Triphenylphosphine chalcogenides as efficient ligands for room temperature palladium(II)-catalyzed Suzuki-Miyaura reaction

A simple catalytic system based on PdCl₂ and triphenylphosphine chalcogenides (PPh₃X; X = O, S, Se) is found to be highly effective (up to 97% isolated yield) in the room temperature Suzuki-Miyaura reactions. Under the same experimental conditions, triphenylphosphine chalcogenides as ligands show superior activities compared to free triphenylphosphine.     

Thermophysical properties in medium temperature range of several thio and dithiocarbamates

The present study reports a DSC study of the thio-and dithiocarbamates: 3H-benzoxazole-2-thione (2-mercaptobenzoxazole), 3H-benzothiazole-2-thione (2-mercaptobenzothiazole), thiazolidine-2-thione (2-mercapto-2-thiazoline), oxazolidine-2-thione (2-mercapto-2-oxazoline) and tetrahydro-1,3-oxazine-2-thione (5,6-dihydro-4H-1,3-oxazine-2-thiol) in the temperature interval T=268 K and the melting temperatures. Temperatures, enthalpies and entropies of fusion are reported. No solid-solid phase transitions were observed for the compounds in the temperature interval studied. The heat capacity of the compounds as a function of temperature was measured.     

Unusual reaction of chloramine-T with araldoximes

Reaction of araldoximes with 4 equiv of chloramine-T in refluxing methanol produces N-(p-tolyl)-N-(p-tosyl)benzamides via addition of 2 equiv of chloramine-T to the intermediate nitrile oxide followed by extrusion of sulfur dioxide.     

Asymmetric synthesis of the highly methylated tryptophan portion of the hemiasterlin tripeptides

[reaction: see text] The asymmetric synthesis of the methylated tryptophan portion of hemiasterlin peptides is described. The key reactions are a SnCl4-mediated ring opening of epoxynitriles or epoxysulfones by N-methylindole followed by an asymmetric Strecker reaction. A second approach involving opening of glycidic esters by indoles is also described.     

Effect of mixed solvent media on the sorption and separation of uranium and plutonium on macroporous resins

Ion-exchange studies on uranium and plutonium using macroporous (MP) anion-exchange resins from an aqueous-organic solvent mixed media were carried out to develop a separation method. Out of the several water miscible organic solvents tried methanol and acetone were found to be best suited. Distribution data were obtained for U(VI) and Pu(IV) using three macroporous resins under various parameters. Based on these data, separation factors for Pu/U were calculated. Column experiments using Tulsion A-27(MP) were also carried out. The method has the advantage of loading plutonium from as low as 1M nitric acid in the presence of methanol or acetone and could be used satisfactorily for its recovery from solutions containing plutonium and uranium.     

Solvent extraction of Hf(IV) from mixed electrolyte solutions into di-2-ethylhexylphosphoric acid (HDEHP)

Extraction of hafnium(IV) from aqueous sulfuric and sulfuric-nitric acid solutions into di-2-ethylhexylphosphoric acid (HDEHP) in xylene has been investigated. The Hf(IV) species extracted from 5M sulfuric acid was found to be HfY₂(HY₂)₂ where Y and HY₂ represent the anions of monomeric HY and dimeric H₂Y₂ forms of HDEHP, respectively. In the presence of nitrate ion the species extracted are found to be Hf(NO₃)Y(HY₂)₂ and Hf(NO₃)₂)(HY₂)₂. But when the aqueous phase is 3.0M HNO₃+2.5M H₂SO₄ only one species, Hf(NO₃)₂(HY₂)₂ was extrated. No synergism was observed from 5M H₂SO₄ by HDEHP with the addition of thenoyltrifluoracetone (HTTA).     

Determination of aromatic primary amines at microg l(-1) level in environmental waters by gas chromatography-mass spectrometry involving N-allyl-n'-arylthiourea formation and their on-line pyrolysis to aryl isothiocyanates

Derivatization of aromatic primary amines to N-allyl-N'-arylthioureas by reaction with allyl isothiocyanate and GC-MS of the derivatives, when pyrolysis to aryl isothiocyanates occurs in the heated injector, has been used to determine aromatic amines in the range 0.5-50 microg l(-1) with a correlation coefficient, r, in the range 0.9902-0.9992. The limit of detection ranged 8 to 30 ng l(-1) when 60 ml of sample were preconcentrated, after derivatization, on a styrene-divinylbenzene copolymer sorbent. The pyrolytic cleavage of sym- and unsym-diaryl or alkyl-/arylthioureas has been rationalized. The chromatography of isothiocyanates is much superior to that of aryl amines and the specific mass fragmentation permits positive identification of amines. The method has been applied to spiked drinking water, groundwater and river water samples, when the recovery ranged from 84 to 109% with RSD of 5-9%, and to detect aromatic amines formed by reductive cleavage of azo dyes in effluents when the recovery of amine was in the range 81-95% with RSD 8-15%. The method is not applicable to nitroanilines.     

5-Bromouracil and 5-bromouracil–histidine complexes with metal(III) ions and their antitumour activity

Complexes of the [AILCl₂], [ML(OH)Cl] and [MLL(H₂O)Cl] type, where HL = 5-bromouracil; HL = histidine; M = Cr^III or Fe^III and M = Al^III, Cr^III or Fe^III were synthesized and characterized. The complexes are polymers with high temperature decomposition points and are insoluble in H₂O and common organic solvents. 5-Bromouracil is coordinated to the metal ion through the O atom of C₍₄₎=O and the N atom of N₍₁₎, while histidine coordinates through the O atom of —CO₂ ^– and the N atom of the —NH₂ groups. The _eff values, electronic spectral bands and e.s.r. spectra suggest a polymeric six-coordinate spin-free octahedral stereochemistry for the Cr^III and Fe^III complexes. The in vivo antitumour effect of 5-bromouracil and its complexes was examined on C₃H/He mice versus P815 murine mastocytoma. As is evident from their T/C values Cr^III and Fe^III complexes display significant and higher antitumour activity compared to 5-bromouracil while the Al^III complexes show lower activity. The in vitro results of the complexes on the same cells indicate that Cr^III and Fe^III complexes show higher inhibition on ³H-thymidine and ³H-uridine incorporation in DNA and RNA replication, respectively.