P. E. Strizhak

Potential of chaotic chemical systems in nanotrace analysis based on the Belousov—Zhabotinskii reaction (BrO−3—malonic acid—ferroin). Determination of manganese(II)

A new method for the determination of trace amounts of manganese(II) in aqueous solutions by its affect on some types of the chaotic regimes in the Belousov-Zhabotinskii reaction has been proposed. The method is based on the high sensitivity of some types of chaotic regimes of the BZ reaction to small perturbations of initial conditions (e.g., low concentrations of some metal ions). The experimentally obtained detection limit for manganese is 3 pg/ml.

Synthesis, spectroscopic and redox behaviour of some copper(II) and copper(I) biomimetic complexes

Abstract Copper complexes with acyclic ligands have been synthesized with a chelate unit of the composition CuN x S y . In the electronic spectra of copper(II) complexes a highly intensive charge-transfer band n S→d x 2 − y 2 (Cu 2+ ) occurs within the range 620–720 nm. Corresponding copper(I) complexes were prepared by electrochemical reduction and characterized by electron spectroscopy. Values of the redox potential ( E 1/2 ) of the CuL 2+/+ pair as well as those of the rate constant of the electrode reaction ( k sh ) were obtained by cyclic voltammetry on a platinum electrode in acetonitrile. The behaviour of the complexes as new models of ‘blue’ copper proteins is discussed.

Oxidation of ascorbic acid by air oxygen catalyzed by copper(II) ions in batch and continuous flow stirred tank reactors: bistability, oscillations and stochastic resonance

We report various nonlinear phenomena observed in the oxidation of ascorbic acid (H2Asc) by molecular oxygen in the presence of copper(II) ions in a continuous flow stirred tank reactor (CSTR) as well as in a batch reactor (closed system). Transient irregular oscillations were observed in a batch reactor by varying the initial concentrations of H2Asc and copper(II) ions. The behavior of the system in a CSTR was studied by varying the flow rate (kf) and the inflow concentrations of H2Asc and copper(II) ions. At low inflow concentrations of copper(II) ions the system exhibits bistability and small amplitude regular oscillations. In a two-dimensional parametric plot with coordinates kf and [Cu2+]0 the regions of bistability and oscillations form a cross-shaped diagram if the inflow concentration of H2Asc is kept constant. For increasing values of kf oscillations emerge ia a Hopf bifurcation and disappear due to a homoclinic orbit. Simultaneously applied small amplitude periodic and stochastic perturbations imposed onto the flow rate close to the bistable region revealed the phenomenon of stochastic resonance, which is confirmed by an analysis of probability distribution functions, power spectra, and singular value decomposition.

Potential of the analyte pulse perturbation technique for the determination of polyphenols based on the Belousov-Zhabotinskii reaction

This paper introduces the use of the analyte pulse perturbation technique to the Belousov–Zhabotinskii reaction, developed in a continuous-flow stirred-tank reactor, in quantitative oscillating reaction-based determinations. This straightforward, expeditious approach was tested on the kinetic determination of various polyphenols (hydroquinone, catechol, resorcinol, pyrogallol and gallic acid) by their perturbation on the oscillating reaction. The proposed method relies on the relationship between changes in the oscillation amplitude of the chemical system and the analyte concentration, which is revealed for the first time in this work. The dynamic range is between 0.3–6.0 μmol and the method features good precision (RSD ca. 2%) and excellent throughput (60 samples h−1); the latter is a result of the expeditiousness with which regular oscillations are regained after each perturbation. The potential mechanism of action of polyphenols on the oscillating system is discussed.

Nanosize effects in heterogeneous catalysis

The principal nanosize effects observed in heterogeneous catalytic reactions are formulated on the basis of an analysis of published data. It is shown that strong effects are observed for nanoparticles with sizes in the range of 1-10 nm and weak effects in the range of 10-100 nm. It is shown that among the 19 effects discussed only three (the quantum-size effect, the charge effect, and the nanoparticle–liquid phase transition) can be used to explain the appearance of the catalytic activity of the nanoparticles and particularly of nano-sized gold.

Determination of gases (NO, CO, Cl2) using mixed-mode regimes in the Belousov–Zhabotinskii oscillating chemical reaction

We show a principle possibility of the determination of gases concentrations (NO, CO, Cl(2)) by their affect on mixed-mode oscillations in the Belousov-Zhabotinskii (BZ) oscillating chemical reaction. These oscillations are characterized by single large amplitude oscillation followed by a number of small amplitude oscillations. The method of the determination is based on a construction of a calibration plot given by the dependence of the number of small amplitude oscillations on gas concentration.

Period adding and broken Farey tree sequence of bifurcations for mixed-mode oscillations and chaos in the simplest three-variable nonlinear system

A detailed study of the simplest three-variable model exhibiting mixed-mode oscillations and chaos is presented. We show that mixed-mode oscillations appear due to a sequence of bifurcations which is characterized by a combination of the Farey tree that is broken by chaotic windows and period adding. This scenario is supported by a family of one-dimensional return maps. The model also exhibits hysteresis between stable steady state and mixed modes.

Infinite period and Hopf bifurcations for the pH‐regulated oscillations in a semibatch reactor (H2O2–Cu2+–S2O2−3–NaOH system)

Dynamic behavior of the pH-regulated oscillations has been studied for the hydrogen peroxide oxidation of thiosulfate ions in the presence of trace amounts of copper(II) ions in a semibatch reactor. A solution of 0.08 M Na(2)S(2)O(3) and 0.112 M NaOH was flowed at 0.160 mL/min into 300 mL of solution containing the H(2)O(2) and Cu(2+) in a vessel. There exists a critical value of the H(2)O(2) or Cu(2+) concentrations below which the system does not oscillate. The oscillations appear due to an infinite period bifurcation at low initial concentrations of the H(2)O(2). The initial concentration of Cu(2+) may be considered as a bifurcation parameter in this case. Increase of the initial hydrogen peroxide concentration causes the pH-regulated oscillations through a nondegenerate supercritical Hopf bifurcation. The classification of bifurcations is based on the analysis of the behavior of oscillation amplitude and period at different initial concentrations of the H(2)O(2) and Cu(2+). Our results show a possibility to distinguish different scenarios for the appearance of transient oscillations in semibatch experiments. (c) 1996 American Institute of Physics.

Non-Fickian diffusion of methanol in mesoporous media: Geometrical restrictions or adsorption-induced?

The methanol mass transfer in the mesoporous silica and alumina/zeolite H-ZSM-5 grains has been studied. We demonstrate that the methanol diffusion is characterized as a time-fractional for both solids. Methanol transport occurs in the super-diffusive regime, which is faster comparing to the Fickian diffusion. We show that the fractional exponents defining the regime of transport are different for each porous grain. The difference between the values of the fractional exponents is associated with a difference in the energetic strength of the active sites of the surface of the media of different chemical nature as well as the geometrical restrictions of the porous media. Increasing by six-fold, the pore diameter leads to a 1.1 fold increase of the fractional exponent. Decreasing by three-fold, the methanol desorption energy results into the same increasing the fractional exponent. Our findings support that mainly the adsorption process, which is defined by the energetic disorder of the corresponding surface active sites, is likely to be the driving force of the abnormality of the mass transfer in the porous media. Therefore, the fractional exponent is a fundamental characteristic which is individual for each combination of the porous solid and diffusing species.

Catalytic properties of graphene material in the hydrogenation of ethylene

A graphene material was synthesized and investigated by atomic force microscopy, Raman spectroscopy, and X-ray diffraction analysis. It was shown that unlike activated carbon the obtained material exhibits catalytic activity in the hydrogenation of ethylene at temperatures in the range of 200-500 °C.