Heterogeneous Oscillations

We should emphasize that the studies and descriptions of homogeneous and heterogeneous oscillating catalytic reactions are based so far on the direct analysis of relevant kinetic schemes. It has been discovered that tra ditional kinetic modefing of some phenomena can only be satisfactory when particular mathematical approaches, such as the Monte Carlo method, are used. Tools of thermodynamics of nonequilibrium processes are not properly applied as yet. 

Fig. 1. Some examples of heterogeneous oscillations observed experimentally, (a) Sinusoidal time series for H2/O2 on a Pt wire (from Ref. 41). (b) Relaxation oscillations for CO/O2 on Pt/Al203 (from Ref. 100). (c) Oscillations after a single period doubling for CO/O2 on Pt(l 10) (from Ref. 231). (d) Model and experimental (inset) quasiperiodic oscillations for NO/CO on supported Pd (from Ref. 232). (e) Deterministic chaos produced by a period doubling sequence for CO/O2 on Pt(IlO) (from Ref. 231).

IIIC) Adamcikova, L., Sevcik, P. Heterogeneous Oscillation Reaction in Tartaric Acid —... 

Such reactions are called oscillating or periodic. Nowadays several dozens of homogeneous and heterogeneous oscillating reactions have been explored. Investigations of the kinetic models for these complex processes have allowed formulating a series of general conditions, which are required for the stable oscillations of the reaction rates and intermediate concentrations ... 

K. Fichthom, E. Gulari, R. Ziff". Self-sustained oscillations in a heterogeneous catalytic reaction A Monte Carlo simulation. Chem Eng Sci 44 1403-1411, 1989. 

Zhdanov VP. 2002. Monte Carlo simulations of oscillations, chaos and pattern formation in heterogeneous catalytic reactions. Surf Sci Rep 45 233-326. 

The majority of heterogeneous chemical and physical-chemical processes lead to formation of the intermediate particles - free atoms and radicals as well as electron- and oscillation-excited molecules. These particles are formed on the surface of solids. Their lifetime in the adsorbed state Ta is determined by the properties of the environment, adsorbed layer, and temperature. In many cases Ta of different particles essentially affects the rate and selectivity of heterogeneous and heterogeneous-homogeneous physical and chemical processes. Therefore, it is highly informative to detect active particles deposited on surface, determine their properties and their concentration on the surface of different catalysts and adsorbents. 

Growth of oscillations for heterogeneous catalysis model with poisoning A = amplitude, Tp = period... 

The specific models we will analyse in this section are an isothermal autocatalytic scheme due to Hudson and Rossler (1984), a non-isothermal CSTR in which two exothermic reactions are taking place, and, briefly, an extension of the model of chapter 2, in which autocatalysis and temperature effects contribute together. In the first of these, chaotic behaviour has been designed in much the same way that oscillations were obtained from multiplicity with the heterogeneous catalysis model of 12.5.2. In the second, the analysis is firmly based on the critical Floquet multiplier as described above, and complex periodic and aperiodic responses are observed about a unique (and unstable) stationary state. The third scheme has coexisting multiple stationary states and higher-order periodicities. 

Capsaskis, S. C. Kenney, C. N. 1986 Subharmonic response of a heterogeneous catalytic oscillator, the Cantabrator , to a periodic input. J. Phys. Chem. 90,4631-4637. 

Slin ko, M. G. Slin ko, M. M. 1978 Self-oscillations of heterogeneous catalytic reaction rates. Catal. Rev. Sci. Engng 17,119-153. 

Steady states and oscillations in homogeneous-heterogeneous reaction systems (with X. Song and L.D. Schmidt). Chem. Eng. Sci. 46,1203-1215 (1991). 

In the homogeneous systems at low temperatures, oscillation phenomena were also observed for this reaction system. Similar observations were made for the CO and 02 system. In order to explain the oscillation mechanism mathematically, a complicated kinetic scheme was devised which is capable of predicting the sustained oscillations [Yang (26, 27)]. Apparently a similar complicated kinetic mechanism governs the undamped oscillations in the heterogeneous system, however, a pertinent analysis has not been performed in the literature so far. 

Werner, H., Herein, D., Schulz, G. et al. (1997) Catalysis Letters, 49, 109-119. Slin ko, M.M. and Jaeger, N.I. (1994) Physicochemical Basis for the Appearance of Self-Sustained Oscillations in Heterogeneous Catalytic Systems, Elsevier. 

Self-oscillations have also been revealed for heterogeneous catalytic reactions. Hugo and Jakubith [7] and Wicke and co-workers [8] found self-oscillations for CO oxidation on platinum. In the period 1973-1975, M.G. Slinko and co-workers studied self-oscillations in hydrogen oxidation on nickel [9,10]. 

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