Reviews oscillating reactions

Since the development of nonequilibrium thermodynamics in the late 1940s, initiated by the work of Prigogine (7), numerous reports have appeared dealing with the possibility of oscillations in reaction systems far from equilibrium. Initially the main focus of these studies was the Belouzov-Zhabotinskii liquid-phase reaction (2), but since the discovery of oscillating reactions in heterogeneous catalysis in the late 1960s (3-7), over 300 publications have described research in this field as well. This review focuses on this emerging and important area of research. 

Oscillating reactions In these coupled autocatalytic reactions, repeated undamped oscillations occur in concentrations of reactants. These reactions have been reviewed by Field. For example, in the cerium-catalyzed reaction of bromate with malonic acid, changes in concentration of bromide ion and in the Ce(rV)-Ce(III) ratio over several orders of magnitude occur repeatedly. These systems as yet are not well understood. For the bromate-malonic acid reaction a 10-step process for the mechanism has been proposed. ... 

Oscillations in the evolution of gas in the decomposition of NH4NO2 have been known for some time. Noyes reports that the results are consistent with the known mechanism of nitrosation of NH3 by N2O3. Nucleation of gas bubbles occurs when the system has approximately nineteen times the equilibrium concentration of dissolved N2. Reviews of oscillating reactions have been published. ... 

The Ce(IV)/Ce(III) (Br /BrOj ) system is one of several redox systems in which oscillating oxidation-reduction cycles can be observed. This particular system, with organic substrates like malonic acid (among others), is known as the Belousov-Zhabotinsky oscillator. This system has been studied for more than 15 years and has been the subject of several reviews and a number of symposia. The Ce--Br oscillating reaction system has been described in terms of a seven-step rate process (all reversible reactions), the Field-Koros-Noyes (FKN) mechanism (Field et al. 1972). 

In this section the whole field of exotic dynamics is considered this term includes not merely oscillating reactions but also oligo-oscillatory reactions, multiple steady states, spatial phenomena such as travelling reaction waves, and chaotic systems. All of these have common roots in autocatalytic processes. This area has continued to expand, and there is a case for treatment in future volumes by a specialist reviewer. An entry into the literature can be gained from a recent series of articles in a chemical education joumal, and in a festschrift issue in honor of Professor R. M. Noyes. Other useful sources are a volume of conference proceedings, and a volume of lecture preprints of a 1989 conference. The present summary is concerned with the chemical rather than the mathematical aspects of the topic. 

The catalytic oxidation of CO over platinum group metals is relatively simple and also important from the ecological viewpoint. In addition, this reaction exhibits a rich kinetic behavior, including regimes with sustained kinetic oscillations (for reviews, see [1-12]). Great interest in... 

Feinberg, M. (1981). Reaction network structure, multiple steady states, and sustained composition oscillations A review of some results. In Modelling of chemical reaction systems, eds K. H. Ebert, P. Deuflhard W. Jaeger, (Springer Series in Chemical Physics, Vol. 18), pp. 56-68. Springer Verlag, Berlin. 

It is worth asking whether perturbation methods might yield as much or more information about oscillating reactions, where it might be possible to probe not only constant or monotonically varying concentrations but also amplitude and phase relationships. Schneider (1985) reviewed a variety of model calculations and experiments on periodically perturbed chemical oscillators. The results, which show such features as entrainment, resonance, and chaos, are of considerable interest in the context of nonlinear dynamics, but shed little light on the question of mechanism. 

The development first of belief and then of interest on the part of chemists in oscillating reactions was spurred by two major developments, one theoretical, the other experimental. Studies in the field of nonequilibrium thermodynamics (Glansdorff and Prigogine, [1 ]. see Procaccia and Ross, [2 ] for a review) established that, sufficiently far from equilibrium, chemical oscillation was indeed consistent with the laws of thermodynamics. The accidental discovery in the Soviet Union (Belousov, [ 3 ] ) of a reaction which gave easily observable oscillations at room temperature evoked the interest of several chemists, first as an amusing lecture demonstration and then as a subject of serious research. It is interesting that the first homogeneous chemical oscillator (Bray, [4 ]), also discovered by serendipity almost 4o years before the Belousov-Zhabotinskii (bz) reaction, received little attention until after the BZ system had become a major focus of research. 

A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

Only a very few experimental studies have been made for detection of mnlti-plicities of steady states to check on theoretical predictions. The studies of multiplicities and of oscillations of concentrations have similar mathematical bases. Comprehensive reviews of these topics are by Schmitz (Adv. Chem. Sen, 148, 156, ACS [1975]), Razon and Schmitz (Chem. Eng. Sci., 42, 1,005-1,047 [1987]), Morbidelli, Vamia, and Aris (in Carberry and Varma, eds.. Chemical Reaction and Reacton Engineering, Dekker, 1987, pp. 975-1,054). 

P. Gray and S. K. Scott. Chemical Oscillations and Instabilities Nonlinear Chemical Kinetics. Oxford Clarendon Press, 1990. See also the Review Lecture by P. Gray. Instabilities and oscillations in chemical reactions in closed and open systems. Proc. Roy. Soc. Lond. A 415, 1-34 (1988). 

The classical problem of multiple solutions and undamped oscillations occurring in a continuous stirred-tank reactor, dealt with in the papers by Aris and Amundson (39), involved a single homogeneous exothermic reaction. Their theoretical analysis was extended in a number of subsequent theoretical papers (40, 41, 42). The present paragraph does not intend to report the theoretical work on multiplicity and oscillatory activity developed from analysis of governing equations, for a detailed review the reader is referred to the excellent text by Schmitz (3). To understand the problem of oscillations and multiplicity in a continuous stirred-tank reactor the necessary and sufficient conditions for existence of these phenomena will be presented. For a detailed development of these conditions the papers by Aris and Amundson (39) and others (40) should be consulted. 

The hydrated electron is characterized by its strong absorption at 720 nm (e = 1.9 x 104 dm3 mol-1 cm-1 (Hug 1981) the majority of the oscillator strength is derived from optical transitions from the equilibrated s state to the p-like excited state (cf. Kimura et al. 1994 Assel et al. 2000). The 720-nm absorption is used for the determination of its reaction rate constants by pulse radiolysis (for the dynamics of solvation see, e.g Silva et al. 1998 for its energetics see, e.g Zhan et al. 2003). IP only absorbs in the UV (Hug 1981), and rate constants have largely been determined by EPR (Neta et al. 1971 Neta and Schuler 1972 Mezyk and Bartels 1995) and competition techniques (for a compilation, see Buxton et al. 1988). In many aspects, H and eaq behave very similarly, which made their distinction and the identification of eaq" difficult (for early reviews, see Hart 1964 Eiben 1970 Hart and Anbar 1970), and final proof of the existence of the... 

An important realization that has been attained in recent years is that the processes of adsorption, desorption and rearrangement on the catalyst surface may themselves produce multiple reaction rates or oscillations. Wicke (35) and his colleagues found this some years ago and the reviews of Sheintuch and Schmitz (58) and Scheintuch (59) show how wide-spread the phenomenon is. It is dangerous to mention names when I am sure to omit many important ones but those of Yablonskii, Slin ko and their colleagues in Russia, Eigenberger and Hugo in Germany,... 

Self-sustained reaction rate oscillations have been shown to occur in many heterogeneous catalytic systems Cl—8]. By now, several comprehensive review papers have been published which deal with different aspects of the problem [3, 9, 10]. An impressive volume of theoretical work has also been accumulated [3, 9, ll], which tries to discover, understand, and model the underlying principles and causative factors behind the phenomenon of oscillations. Most of the people working in this area seem to believe that intrinsic surface processes and rates rather than the interaction between physical and chemical processes are responsible for this unexpected and interesting behavior. However, the majority of the available experimental literature (with a few exceptions [7, 13]) does not contain any surface data and information which could help us to critically test and further Improve the hypotheses and ideas set forth in the literature to explain this type of behavior. 

In a review article on oscillatory reactions (294), Sheintuch discusses the effect of introducing a heat balance for the catalyst rather than a mass balance for the reactor into the differential equation system for a surface reaction with oxidation/reduction cycles. Although the coverage equations alone can yield oscillatory behavior, as was the case for the models discussed in the previous section, Sheintuch s model is discussed in this section because introduction of the heat balance adds qualitatively new features. In this extended system complex, multiple peak behavior and quasiperiodicity was observed as shown in Fig. 8. Sheintuch also investigated the interaction of two oscillators. This work, however, will be treated in detail in Section V, were synchronization and chaos are discussed. 

Oscillations in the rate of CO2 production have been observed for many supported metal catalysts and single-crystal surfaces. Similar oscillations have been observed for most of the reactions discussed in this review. An example is shown in Fig. 10 for the NO + H2 reaction on Pt(lOO) (46). 

The coherent motion initiated by an excitation pulse can be monitored by variably delayed, ultrashort probe pulses. Since these pulses may also be shorter in duration than the vibrational period, individual cycles of vibrational oscillation can be time resolved and spectroscopy of vibrationally distorted species (and other unstable species) can be carried out. In the first part of this section, the mechanisms through which femtosecond pulses may initiate and probe coherent lattice and molecular vibrational motion are discussed and illustrated with selected experimental results. Next, experiments in the areas of liquid state molecular dynamics and chemical reaction dynamics are reviewed. These important areas can be addressed incisively by coherent spectroscopy on the time scale of individual molecular collisions or half-collisions. 

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