A. I. Kokorin

Photodestruction of dichloroacetic acid catalyzed by nano-sized TiO2 particles

Abstract The photocatalytic activity of TiO2 nanoparticles of different size has been studied by following the photodegradation of dichloroacetic acid (DCA), and was tested by measuring the initial rate of the H+ and Cl− ion formation. The catalyst efficiency was found to be in correlation with the specific surface area of the semiconductor particles. The efficiency of the nanocrystalline TiO2 photocatalyst could be increased by doping the particles with copper ions. The structure and the partitioning of different surface copper complexes has been studied using ESR technique. The influence of the pH value of the solutions, as well as of the adsorbed copper ion content, on the efficiency of the DCA photodegradation is discussed in the paper.

Effect of electron and hole acceptors on the photoelectrochemical behaviour of nanocrystalline microporous TiO2 electrodes

Abstract A comparative study of the photoelectrochemical properties of nanocrystalline microporous, compact polycrystalline and single-crystal TiO 2 electrodes has been performed with an emphasis on the analysis of the short-wavelength region of the action spectra. Adding to the solution or immobilizing on the electrode surface some electron or hole scavengers has been shown to exert significantly more appreciable influence on the short-wavelength edge of photocurrent spectra of nanostructured electrodes as compared with those of polycrystalline compact or single-crystal electrodes. This influence is markedly stronger in basic solutions in comparison with acidic ones. The effects observed are explained by the change of limiting steps of the overall photoelectrochemical process in going from the conventional macroscopic to nanostructured electrodes.

Effect of solvent nature on spin exchange in rigid nitroxide biradicals

Intramolecular electron spin exchange as a function of temperature and solvent viscosity and polarity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two rigid nitroxide biradicals existing in one spatial conformation only. Temperature variations of the isotropic hyperfine splitting constanta and exchange integral value |J/a| were measured from EPR spectra and subsequently analyzed. The interaction of polar solvent molecules with >N-O fragments of nitroxide groups led to a slight decrease of the |J/a| value with the increase of temperatureT. In contrast, the interaction of polar solvent molecules with functional groups inside the bridge resulted in a noticeable increase of |J/a| vs.T. In the last case, a coverse relationship between the values of |J/a| and the hyperfine splitting constanta has been observed for solvents with different polarity.

Regularities of the spin exchange coupling through a bridge in nitroxide biradicals

The current state, achievements, problems and prospects of the intramolecular electron spin exchange interaction as a function of the nitroxide biradical composition and structure, type of the radical ring, temperature and the solvent nature are considered on the basis of the literature data and the results of our own experiments.

Electrodeposition of nanostructured diamond-like films by oxidation of lithium acetylide

Diamond-like carbon (DLC) films have been deposited by anodic oxidation of 4 M solution of lithium acetylide in dimethylsulfoxide on the surface of stainless steel or nickel electrode at room temperature and moderate anodic current densities (0.2–2.0 mA/cm2) in the range of electrode potentials 0.3–2.5 V (vs. sat. Ag|AgCl reference electrode). Electrodeposited DLC coatings represented complete and optically transparent films of a thickness 50–100 nm having dark island inclusions with a diameter 0.8–5.0 μm. The concentration and average size of these particles increased with the prolongation of deposition time. Micro-Raman spectra obtained by the focusing of laser beam onto these dark inclusions are characterized by a broad peak centered at 1500 cm−1 and weak peak at 1200 cm−1. With a defocused laser beam, there appear two well-distinguished peaks on the integrated Raman spectra – at 1530 and 1130 cm−1. Analysis of Raman spectra with the use of a Breit–Wigner–Fano lineshape and spectrum deconvolution indicates that the electrodeposited films consist of diamond-like nanostructured carbon with a high content (70–80%) of sp3 phase.

Intramolecular spin exchange in flexible short-chain biradicals in solutions with various viscosity

Intramolecular electron spin exchange as a function of temperature and solvent viscosity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two short-chain flexible nitroxide biradicals. Certain thermodynamic parameters of the conformational rearrangements were calculated from the EPR spectra. The process of spin exchange in these biradicals dissolved in six different alcohols is compared with that in a nonpolar solvent (toluene) and with the thermodynamic characteristics of the solvents. No correlation is found on a macroscopic level (solvent viscosity) but on a microscopic level (solvent longitudinal relaxation times) a distinct correlation is seen. The original results are compared with literature data.

Mixed-valence copper complexes with the organic donors as catalysts for dichlorobutene isomerization

Abstract The formation of mixed-valence copper complexes with dialkyl sulfides or triphenylphosphine in the catalytic isomerization of dichlorobutenes was investigated kinetically and spectroscopically (UV–Vis, IR, EPR, NMR). These species were prepared both starting from Cu(I) and Cu(II) compounds. The complexes containing 3 or 4 copper ions in different oxidation states, substrate and donor molecules were found to play a key role in catalysis. Chlorolefins with allylic chlorine atom may stabilize mixed-valence complexes as bridging ligand between Cu(I) and Cu(II). The EPR spectra of these complexes provide the five-coordinated Cu(II) with a trigonal–bipiramydal geometry of coordination sphere. The simultaneous presence of Cu(I) and Cu(II) in polynuclear structure is proposed to favor the electron transfer step in the reaction mechanism.

Rotational and Translational Diffusion of Spin Probes in Room-Temperature Ionic Liquids

We have studied the rotational and translational diffusion of the spin probe 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPOL) in five imidazolium-based room-temperature ionic liquids (RTILs) and glycerol by means of X-band electron paramagnetic resonance (EPR) spectroscopy. Rotational correlation times and rate constants of intermolecular spin exchange have been determined by analysis of the EPR line shape at various temperatures and spin probe concentrations. The model of isotropic rotational diffusion cannot account for all spectral features of TEMPOL in all RTILs. In highly viscous RTILs, the rotational mobility of TEMPOL differs for different molecular axes. The translational diffusion coefficients have been calculated from spin exchange rate constants. To this end, line shape contributions stemming from Heisenberg exchange and from the electron-electron dipolar interaction have been separated based on their distinct temperature dependences. While the Debye-Stokes-Einstein law is found to apply for the rotational correlation times in all solvents studied, the dependence of the translational diffusion coefficients on the Stokes parameter T/η is nonlinear; i.e., deviations from the Stokes-Einstein law are observed. The effective activation energies of rotational diffusion are significantly larger than the corresponding values for translational motion. Effects of the identity of the RTIL cations and anions on the activation energies are discussed.

Formation of active catalysts in the system: chlorocuprates-CCl4-n-C10H22.

Transformations of anionic Cu(II) chlorocomplexes have been studied under conditions of catalytic exchange reactions between carbon tetrachloride and n-alkanes. It was shown that chlorocuprates are just precursors and are easily reduced to the genuine catalysts, that is, to the respective Cu(I) complexes. Both the composition and the geometric structure of the precursor (CuCl(4)(2-)) and, probably, the active site (CuCl(3)(2-)) have been investigated by several techniques (UV-vis spectroscopy, electron spin resonance (ESR), extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), and static magnetic measurements). The dependence of the metathesis velocity on the [Cl-]/[Cu] ratio was found to exhibit a maximum most likely corresponding to the highest content of trichlorocuprite CuCl(3)(2-).

The partial catalytic oxidation of toluene on vanadium and molybdenum oxides

The partial catalytic oxidation of toluene on pure and mixed vanadium and molybdenum oxides was studied over the temperature range 300–500°C. The main reaction products were maleic anhydride (MA), benzaldehyde (BA), and carbon oxides (COx) depending on the catalyst composition and reactor temperature. The samples containing more than 50% vanadium were characterized by conversion and selectivity close to those of pure vanadium oxide V2O5. Reaction temperature was found to influence the amount of products formed, primarily the amounts of MA and BA. The role played by the generation of the singlet molecular oxygen form in the samples and its influence on the selectivity of the reaction is considered.