Том 53, № 10 (2017)

The dissipation theorem is applied to turbulent pipe flow. The eddy-viscosity profiles can be made to agree with some in the literature in the sense that the eddy viscosity starts at zero on the solid pipe wall, rises to a maximum, and declines again toward the center line. A relationship between the volumetric dissipation and the eddy viscosity is derived by means of an energy balance on a core of fluid of radius r. The question of what exponent to use on the radius in another governing equation is clarified, thereby giving better agreement with experimental data than other values tried. Negative values of the eddy viscosity can be obtained in some regions of the flow field, such as near the center line, and it is suggested that these can be eliminated by slight modification of the decay term. Better agreement with the shapes of friction-factor and mass-transfer curves could be achieved by further (empirical) modification of the stress dependence of parameters in the model.

Rotating ring disc electrode (RRDE) experiments are a classic tool for investigating kinetics of electrochemical reactions. Several standardized methods exist for extracting transport parameters and reaction rate constants using RRDE measurements. In this work, we compare some approximate solutions to the convective diffusion used popularly in the literature to a rigorous numerical solution of the Nernst–Planck equations coupled to the three dimensional flow problem. In light of these computational advancements, we explore design aspects of the RRDE that will help improve sensitivity of our parameter estimation procedure to experimental data. We use the oxygen reduction in acidic media involving three charge transfer reactions and a chemical reaction as an example, and identify ways to isolate reaction currents for the individual processes in order to accurately estimate the exchange current densities.

The electrochemical machining is aimed at the fabrication of parts of prescribed shape and dimensions by the anodic metal dissolution using tool-electrodes of various types. The task of the theory of electrochemical machining is to calculate the shape and dimensions of the workpiece depending on the shape of tool-electrode and operation conditions. In this work, a pseudotransient method of modeling, which is new for the steady-state electrochemical machining, is developed. In this method, first, the initial approximation of workpiece surface is prescribed; in the course of modeling, it shifts in the normal direction at a rate proportional to the residual of steady-state condition. The proposed method requires substantially lower computational cost than the non-steady-state method and can be used for the tool-electrodes of arbitrary shape.

According to the existing modified electrode theory, both radical-cations (polarons) and reduced quasi-particles of conducting polymer films include the same number of repeat units of polymer chains. As opposed to such supposition, any reduced repeat unit of polymer chains is assumed to be a separate quasiparticle in scope of the recently proposed approach to treating the conductance of polaron-containing films. To say nothing of the obtained new results of such approach, it is necessary to indicate that its applications have been restricted so far to the cases of either a homogeneous polaron population, which means the absence of any polaron transformations (for example, into bipolarons), or the presence of such transformations proceeding in equilibrium conditions. The main purpose of the presented communication is to derive charge carrier fluxes and material balance equations valid for non-equilibrium conditions and accounting for the above transformations and translocations of quasi-particles along polymer chains. Consecutive acts of any type of the indicated processes are treated as sequences of similar reactions proceeding in the film interior. This allows one to apply the Brönsted rule of linear correlation between activation energies of such reactions and their generalized heat effects, in order to express the reaction rate as a function of the local electric field. Using a lattice model of films formed by unramified polymer chains, non-stationary flux and material balance equations have been obtained. A comparison between the derived results and those conformed to the existing theory shows their significant difference. Some other inferences from the description carried out and possible prospects of its development are discussed.

10 MHz QCM quartz crystals, uncoated or coated with films of PMMA and/or PEO, were rapidly submerged into aqueous medium. The changes in these crystals’ resonance frequencies were recorded. In all cases after a few seconds crystal’s resonant frequency became constant at a value that depended upon whether the crystal was polished or etched and whether it was coated or uncoated with a polymer film. A blended polymer film composed of PMMA and PEO, when coated on a QCM crystal and then submerged in water showed a rapid frequency increase corresponding to the removal (leaching) of PEO from the film and creating a porous PMMA film whose pores were filled with water. The porosity of the leached film was determined.

The mechanism of action of photosensitizers used in the photodynamic tumor therapy is studied by using bilayer lipid membranes (BLM) which adequately simulate the plasma membranes of cells. The formation of singlet oxygen upon photoexcitation of photosensitizer (tetrasulfonated alumophthalocyanine) is revealed by the photosensitizer-induced decomposition of styryl dyes di-4-ANEPPS or RH-421 the adsorption of which on BLM gives rise to changes in the dipole potential at the membrane interface. This potential decreases upon illumination of the membrane covered with adsorbed phthalocyanine and dye molecules as a result of the dye oxidation by singlet oxygen and is recovered upon interruption of illumination as a result of adsorption of new dye molecules from the solution. The changes in the potential are observed both when the dye molecules are adsorbed on the same (cis) membrane side as the photosensitizer molecules and also when they are adsorbed on the opposite (trans) membrane side. When the dye molecules are adsorbed on the trans side, the kinetics of potential variations in the light and its subsequent recovery in the dark depends on the membrane size. This dependence is explained within the framework of a model which assumes that dye molecules can get into the lipid bilayer not only from the aqueous solution but also as a result of lateral diffusion from the bilayer-surrounding circular reservoir containing a lipid solution in decane. The quantitative description of experimental data in terms of this model allows the coefficient of lateral diffusion of di-4- ANEPPS molecules in the lipid bilayer to be assessed as 10^–7–10^–6 cm²/s. The oxidation rate of dye molecules on the cis side is shown to be lower, which is explained by inhibition of the formation of singlet oxygen by dye molecules.

We present a model Hamiltonian for electrochemical electron transfer, and use Green’s functions as the starting point for three different approaches to the calculation of rate constants: first order perturbation theory, which is equivalent to the Levich and Dogonadze theory, the calculation of adiabatic free energy surfaces, and propagation in time. We discuss the similarities and differences between these methods.

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.

A short review of the works where theoretical models for describing kinetics of catalytic X–H bonds breaking reactions (X = C, O, and H) over metal surfaces were developed on the basis of concepts of the Dogonadze–Kuznetsov–Levich quantum mechanical theory of chemical processes. Numerical values of the rate constants of these reactions over (111) surfaces of nickel, platinum and rhodium, which are considered as steps of a complex catalytic process of methane steam reforming (MSR) are calculated and compared with experimental data. These rate constants are used for simulations of microkinetic models of the MSR reactions on the catalysts. Effects of external parameters on the MSR rates and on isotope effects are described.