A. S. Kotkin

Mechanism of electroreduction of intermediates with and without a proton donor

Electrochemistry of various categories of intermediates was comparatively studied over wide range of electrode potentials, concentrations of proton donors (H3O+, NH4+), and temperatures. The suggested kinetic model considered two parallel pathways of electron transfer: either to an adsorbed intermediate (radical) or to the respective metastable complex with a proton donor. The electroreduction of all studied intermediates was found to obey the model. The formation of metastable complexes was shown to be facilitated for radicals having one or more functional groups. The particular reduction pathway is determined predominantly by the difference between overvoltages of electron transfer to a radical and to its metastable complex with a proton donor under the same experimental conditions (i.e. nature and concentration of a proton donor, electrode potential).

The elementary stages of electroreduction of alkyl(hetero)arylsulfones in water-organic media

Intermediates (IM) of methyl(2-pyridyl)sulfone (MPS) and tert-butyl(2-pyridyl)sulfone (TBPS) formed upon the transfer of the first electron are studied by methods of laser photoelectron emission (LPE). The capture constants of solvated electrons by the MPS and TBPS molecules were determined. The time-resolved voltammograms of each sulfone measured in water-organic mixtures (20–80% ethanol or 60% DMSO) are found to demonstrate a redox wave with half-wave potentials E1/2 = −1.34 and −1.37 V for MPS and TBPS, respectively. The dependence of rate constants for the one-electron IM reduction and oxidation on the potential is shown to obey the slow-discharge equation and the absolute magnitudes of rate constants are determined. The characteristic times of homogeneous transformations (decomposition, protonation) of MPS and TBPS radical anions do not exceed 3 × 10−7 s. The LPE data are compared with the results of preparative electrolysis and the mechanisms of both electrochemical and homogeneous reactions of IM are discussed.

Generation of phenylmethylketyl radicals in aqueous solutions and study of their transformation

Acetophenone and its intermediates formed upon the first electron transfer are studied by laser photoemission and traditional electrochemistry. It is shown that the intermediate reduction is affected by competition of the reactions of the formed radical-anions oxidation and their subsequent transformation to secondary products that are rapidly reduced at the electrode. From the comparison of the data obtained by the laser photoemission method and electrochemical measurements a conclusion was drawn that the product is a metastable complex (associate) of the radical-anion with water molecule; its formation rate constant is rather low (∼6 × 103 M−1 s−1). It was also concluded that bulk radical reactions dominate in aprotic media at moderate cathodic potentials; the acetophenone radical-anion is reduced at E ≤ −1.9 V (SCE). This conclusion agrees with the results of the acetophenone preparative electrolysis in DMFA, where marked yield of pinacon was observed at the potentials of limiting current of the 1st reduction wave, while the stage of 2nd electron transfer occurred at E ≤ −2.3 V (SCE).

Photoelectrochemical Behavior of Electrodes Containing One-Walled Carbon Nanotubes

Photoelectrochemistry of electrodes containing single-walled carbon nanotubes is studied for the first time ever. Photoemission of electrons from such electrodes illuminated by UV light is discovered. It is established that the photocurrent amplitude as a function of the imposed potential obeys the 5/2 law and that the work function into solution for nanotubes and a mercury electrode coincide to within 0.08 eV. Similarity of electrochemical behavior of intermediates on the nanocarbon and mercury electrodes is shown with the reduction of radicals ėCH2Cl as an example.

Effect of the EDL Structure on the Mechanism of Electroreduction of Adsorbed Intermediates

Effect of the Ψ1 potential on various kinetic modes of the β-hydroxyethyl radical reduction is studied using laser photoemission (LPE). A charged proton donor is found to make no impact on the electron transfer rate during the reduction of adsorbed radicals. Experimental results fit the model proposed earlier for the electroreduction of intermediates, which includes two parallel pathways for electron transfer—onto an adsorbed radical and onto a metastable complex radical–proton donor. Demonstrated is a complete analogy between time-resolved voltammograms for LPE-generated intermediates and classic polarograms.

Plasma-Assisted Electrochemical Exfoliation of Graphite in the Pulsed Mode

The exfoliation of commercially produced GR-280 graphite into few-layered graphene (FLG) structures was accomplished by applying repetitive pulses with an amplitude of ±(150–200) V to graphite electrodes immersed in a 0.3 M Na2SO4 aqueous solution. In comparison with the commonly used electrochemical exfoliation methods [1, 2], a distinguishing feature of this work was the application of a specially developed high-current generator, which formed high-voltage pulses with a pulse rise time of ~0.5 μs on a capacitance–resistance load (the graphite electrodes of an electrochemical cell). In our case, the high rate of the growth of cathodic polarization on an electrode with a smaller surface area at current densities higher than 20 A/cm2 in terms of apparent graphite surface area led to the explosive boiling of solution in the nearelectrode region and the formation of a vapor–gas shell around the electrode accompanied by intense light and acoustic generation. In this case, intense electrode destruction was observed, which manifested itself in the turbidity of solution near its surface. It can be hypothesized that the pulsed gas generation accompanied by the generation of hydraulic shock waves leads to the mechanical loosening of the graphite surface similarly to ultrasonic treatment. Moreover, the wedging action of bubbles formed in the interplanar space between the basal planes of graphite can be an additional factor that increases the efficiency of surface dispersion. Electrolytic plasma is not formed upon the subsequent anodic polarization [3], and the electrode surface is attacked by the active intermediates of water decomposition, primarily, by hydroxyl radicals. The latter process leads to the decoration of graphite with oxygen-containing functional groups. The electrochemical functionalization of carbon nanospecies, which was described in detail in the world literature, occurs by an analogous mechanism. In particular, data on the surface modification of single-walled carbon nanotubes and highly oriented carbon nanowalls by the application of anodic potentials were reported by Komarova and coauthors [4, 5]. An opaque suspension of FLG structures was formed in the electrochemical cell after only ~10 min at a pulse duration of 20–50 ms and a pulse ratio of 2–3. The aqueous electrolyte dispersion obtained was converted into a stable suspension of graphene structures in water with a concentration of ~2–15 mg/mL without electrolyte traces after several stages of decantation, centrifugation, and short-term ultrasonication. The FLG structures were characterized using a wide range of currently available techniques: optical and scanning electron microscopy (SEM); UV–VIS–NIR, IR, and Raman spectroscopy; and thermogravimetry. Figure 1a shows the SEM image of the FLG suspension sediment of graphene-like particles with uneven edges and a wide lateral size distribution (the half-width was estimated at 100–600 nm). Figure 1b shows the Raman spectra of an aqueous dispersion of FLG structures obtained at an excitation length of 976 nm. In accordance with published data [6], the shape of the spectrum corresponds to moderately defective few-layered graphene structures: the G/D line intensity ratio is ~1.7, and the wide 2D line is shifted toward smaller wave numbers with respect to that of graphite. The IR spectra given in Fig. 1c for FLG powders display the most intense absorption bands at ~3430– 3450 cm−1, which can be attributed to the vibrations of C–OH bonds in hydroxyl and carboxyl groups, and an absorption band at 1720 cm−1 corresponds to C=O vibrations in carboxyl groups. In addition, the FLG samples contained CH2 fragments or methylene groups, since there are absorption bands at 2923 and 2855 cm−1, and CH3 groups (2963 cm−1) [7]. The above absorption bands are also characteristic of graphite; however, they are expressed much more weakly. Figure 1d shows the UV–VIS–NIR spectrum of the aqueous suspension of FLG; it exhibits features typical of FLG: a maximum at λ ≈ 270 nm and considerable absorption in the visible region [2]. According to the elemental and thermogravimetric analysis data, the volumetric oxygen content of FLG powders was ~8.3 at %, which indicates a sufficiently high degree of their functionalization.

Investigation of electrochemical behavior of secondary products of capture of OH radicals by dimethyl sulfoxide molecules using laser photoemission

Electrode reactions of intermediates formed during capture of OH radicals by dimethyl sulfoxide (DMSO) molecules were studied using laser photoemission in aqueous buffer solutions in the pH range from acidic to basic. The results were compared with characteristics of one-electron reduction of methyl radicals generated via photoemission from methyl halides CH3X (X = Cl, I). From these experiments, it was concluded that intermediates in these systems were identical since the primary product of capture of OH radicals by DMSO molecules, i.e., adduct (CH3)2SO. (OH), was spontaneously decomposed to form .CH3 with a time as low as <2 × 10−5 s. Some anomalies were found on time-resolved voltammograms of intermediates in the pH range from weakly basic to weakly acidic and at illumination times of an electrode with UV light Tm ≤ 90–300 ms. These features were presumably caused by rather slow formation of organomercury intermediates as interaction products of the components of the system DMSO—OH radical—mercury electrode.

Electrochemical Behavior of Electrodes Containing Single-Walled Nanotubes

The differential capacitance and voltammograms of electrodes that contain single-walled carbon nanotubes are measured in aqueous electrolytes. The discovered dependence of the capacitance on the measurement frequency is attributed to structural features of nanomaterials used. Electrochemical characteristics of nanotube electrodes are close to those of glassy carbon electrodes, with the difference that the discharge current in the former is substantially higher at cathodic potentials in the presence of N2O. This effect is presumably caused by an autoelectron emission of electrons from nanotubes into electrolyte.