Ana Ivanović-Šašić

Mixed-mode oscillations and chaos in return maps of an oscillatory chemical reaction

The return maps, as an element of mathematical phenomenology appropriate for general examinations of complex dynamic states of the oscillatory systems were used to detect and explain the evolution of mixed-mode oscillations and chaos in a six-dimensional nonlinear reaction system of the Bray–Liebhafsky (BL) reaction, a well-studied nonlinear chemical reaction system that exhibits complex dynamic behavior. For this purpose principally different Poincaré sections were applied and different transition scenarios between periodic and aperiodic states were examined by numerical simulations. It is shown that emergence of new periodic patterns can be detected by return maps already within chaotic windows. Besides, we also show that the higher dimensionality of manifold gives the impression of having several layers of manifolds.

Methanol electrooxidation in alkaline solutions on platinum-based electrodes: Classical and dynamical approach

The electrochemical oxidation of methanol has been carefully studied due to its application in fuel cells. In this work electrooxidation of methanol was investigated on bare platinum electrode, the platinum electrode covered with Nafion and platinum supported on zeolite 13X. Along with classical electrochemical methods, attractor reconstruction was used to make rough distinction among possible reaction mechanisms on different forms of Pt. The obtained transient voltammogram records were used to calculate apparent rate constants for methanol oxidation limiting steps in transient period. All samples contributed to methanol oxidation by basically same reaction mechanism, but with significantly different apparent rate constants.

Intermittent Chaos in the Bray–Liebhafsky Oscillator. Dependence of Dynamic States on the Iodate Concentration

Chaotic dynamic states with intermittent oscillations were generated in a Bray–Liebhafsky (BL) oscillatory reaction in an isothermal open reactor i.e., in the continuously-fed well-stirred tank reactor (CSTR) when the inflow concentration of potassium iodate was the control parameter. They are found between periodic oscillations obtained when [KIO3]0 < 3.00 × 10–2 M and stable steady states when [KIO3]0 > 4.10 × 10–2 M. It was shown that the most chaotic states obtained experimentally somewhere in the middle of this region are in high correlation with results obtained by means of largest Lyapunov exponents and phenomenological analysis based on the quantitative characteristics of intermittent oscillations.

Return map analysis of the highly nonlinear Bray–Liebhafsky reaction model

By numerically simulated Bray–Liebhafsky (BL) reaction under a continuously fed well stirred tank reactor (CSTR) conditions, we discussed the attractors and Poincaré 1D maps with respect to flow rate as the control parameter. The new technique of the return maps from transient trajectories over the slow manifold is developed and applied in order to explore its multilayered structure related to dynamical states (periodic and aperiodic -chaotic oscillating modes) of the system. Kinetic relations underlying the slow manifold structure are briefly discussed.

Oscillators: Phenomenological mappings and analogies: First part: Mathematical analogy and chains

New analytical and numerical results of dynamics for both linear and nonlinear system with two degrees of freedom are presented. For a mechanical chain system with two degrees of freedom, oscillations are investigated analytically and numerically with corresponding comparing between free and forced oscillatory dynamics of linear and nonlinear system. Also, energy analysis and transient energy between the mass particles in the system are discussed. Using Mihailo Petrovics theory of the mathematical phenomenology elements, phenomenological mappings in vibrations, signals, resonances and dynamical absorptions in models with two degrees of freedom - the abstractions of a different real system dynamics are identified and presented. Mathematical description of a chain mechanical system with two mass particles coupled by linear and nonlinear elastic springs and with two degrees of freedom is given. By analysis of corresponding solutions for free and forced vibrations, series of related two-frequency regimes and resonant states, as well as dynamical absorption states, are identified. Besides, by mathematical analogy and phenomenological mappings, the analysis of series of dynamics of other two degrees of freedom models dynamics (torsional system, double pendulum system, double electrical circuit) is performed.

Bifurcation analysis: a tool for determining model parameters of the considered process

In this paper, we intend to show the importance of the bifurcation analysis in understanding of an oscillatory process. Hence, we use the bifurcation diagram of the Bray–Liebhafsky reaction performed in continuous well-stirred tank reactor under controlled temperature variations for the determination of the activation energies as well as rate constants of particular steps appearing in the kinetic model of oscillatory reaction mechanism. This approach has led us to the development of general procedure for treatment of experimentally obtained data and extracting kinetic parameters from them, which was very important considering that some rate constants of the already proposed model could not be determined experimentally and have to be fitted (or guessed). Also, the proposed approach has the potential to inspire the refinement of already proposed models and the development of a new one that will be able to reproduce experimentally obtained system’s dynamical features more successfully. In particular, the dynamic states of the Bray–Liebhafsky oscillatory reaction have been analyzed experimentally and numerically using already proposed model together with qualitative and quantitative analysis of bifurcation diagrams in both cases.

Modified Clays in Environmental Protection

Bentonite is cheap, natural raw material, whose usage is not detrimental to the environment. Bentonite clay is primarily composed of of smectite minerals, usually montmorillonite. Bentonite can be easily modified. Some of these modifications include pillaring and organomodification. Pillared clays (PILC) with incorporated mixed Al–Fe oxide pillars were used as Fenton-like catalysts in the catalytic oxidation of organic water pollutants in the presence of H2O2. The intercalation of quaternary alkyl ammonium cations resulted in the obtainment of organobentonites as adsorbents for the simultaneous removal of organic and inorganic pollutants. These sorbents are useful in the purification of multicomponent dye/toxic metal solutions and real waste waters. Organobentonites and PILCs were used as electrode materials in electroanalytic sensors and biosensors in environmental protection. Bentonite modification led to the synthesis of adsorbents for the removal of organic and toxic and radioactive inorganic pollutants, catalysts for the oxidation of water pollutants, sensors and biosensors, electrocatalysts.

Thermodynamics of the adsorption of different dyes onto bentonite modified with hexadecyltrimethylammonium cation

Removal of two different dyes: Acid Orange 10 (AO10) and Reactive Black 5 (RB5) from their aqueous solutions using organobentonite as adsorbent was investigated. The experiments were carried out at different temperatures (298, 313, 323, and 333 K) in order to obtain thermodynamic parameters for adsorbate/adsorbent system i.e., activation energy, Gibbs free energy, enthalpy and entropy. The results of thermodynamic studies indicated that the adsorption of both dyes onto organobentonite is an endothermic process, while the values for activation energies (76 kJ mol−1 for AO10 and 51 kJ mol−1 for RB5) indicated that chemisorption occurred.