Catalytic oscillator

Bykov, V. I. and Yablonskii, G. S. (1981). Simplest model of catalytic oscillator. React. Kinet. Catal. Lett., 16, 377-81. 

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. 

Spatial and temporal patterns in catalytic oscillations (with D.G. Vlachos, F. Smith, and L.D. Schmidt). Physica A 188,302-321 (1992). 

Thus the mechanism formed by steps (l)-(4) can be called the simplest catalytic oscillator. [Detailed parametric analysis of model (35) was recently provided by Khibnik et al. [234]. The two-parametric plane (k2, k 4/k4) was divided into 23 regions which correspond to various types of phase portraits.] Its structure consists of the simplest catalytic trigger (8) and linear "buffer , step (4). The latter permits us to obtain in the three-dimensional phase space oscillations between two stable branches of the S-shaped kinetic characteristics z(q) for the adsorption mechanism (l)-(3). The reversible reaction (4) can be interpreted as a slow reversible poisoning (blocking) of... 

All the above mechanisms can be called the simplest catalytic oscillators. In all these mechanisms self-oscillations of the reaction rate are realized due to the combination of the fast system of steps (adsorption mechanism) leading to the sharp change in the number of unoccupied surface sites and of the "slow reversible step ensuring self-oscillations of their concentration. If the parameters of the "buffer step are sufficiently small compared with those of the main mechanism, all these oscillations will be typically relaxa-tional. 

Catalytic oscillations occur so commonly that the topic cannot be ignored, yet they are so complex that experimental characterization is seldom satisfactory and theoretical characterization is seldom complete. Therefore we have attempted to discuss experiments and their interpretations together so that readers can attempt to sort through alternate interpretations. However, in spite of many experiments and models, it appears that no oscillatory catalytic systems are as yet understood completely. 

The simulations discussed above are focused on the behavior of single catalytic oscillators at fixed reactant pressures. In the full-scale analysis of reactions on nm-supported particles, the reactant pressures should be calculated self-consistently with the reaction kinetics. At present, due to computational limitations, the self-consistent treatment can, however, be done only by using the MF equations (see, e.g., recent simulations [57] of oscillations in CO oxidation in a continuously stirred tank reactor). The MF approach does not, however, make it possible to scrutinize the reaction kinetics on the nm scale. Under such circumstances, the MC and MF treatments are complementary. In particular, the MC results may be employed in order to understand the limits of applicability of the MF approximation. 

Catalytic oscillations were detected for various catalysts using DTA techniques by Gallagher and Johnson (287). The rapid response and high sensitivity of DTA make it suitable for the study of these oscillations which may be more difficult to detect by other techniques. 

Self-organised, periodic phenomena have been found in several heterogeneous catal3dic systems [7], Such systems exist in a state far from thermodynamic equilibrium and exhibit at constant adjusted parameters of temperature, pressure and feed gas flow self-sustaining catalytic oscillations. 

A methanol-oxygen pre-treatment of the copper-catalysts at high temperatures is necessaiy to obtain catalytic oscillations in the copper/oxygen/methanol-system. For the pre-treatment the sample can be either treated with methanol-ojQrgen mixtures or alternating with oxygen and methanol at temperatures above 730 K. In a methanol-oxygen-helium flow (methanol to ojq gen ratio around 2 1, T > 730 K) the... 

At methanol-oxygen ratios from 1.6 to 1.3 (phase I, Fig. 3) the sample showed an oscillating oxidation state (see video pictures, at a methanol oxygen flow ratio of 1.2 (phase II, Fig. 3) the sample had been permanently oxidised while catalytic oscillations still sustained. Below a methanol-ojqrgen ratio of 1.1, no rate oscillations were observed. 

Although the Lorenz model is not a model of chemical kinetics, there is some similarity in both model types the right-hand side is of the polynomial type with first- and second-order terms. In this chapter, we will present results of the analysis of a nonlinear model—also with three variables— the catalytic oscillator model. 

Now the question is how to construct the simplest model of a chemical oscillator, in particular, a catalytic oscillator. It is quite easy to include an autocatalytic reaction in the adsorption mechanism, for example A+B—> 2 A. The presence of an autocatalytic reaction is a typical feature of the known Bmsselator and Oregonator models that have been studied since the 1970s. Autocatalytic processes can be compared with biological processes, in which species are able to give birth to similar species. Autocatalytic models resemble the famous Lotka-Volterra equations (Berryman, 1992 Valentinuzzi and Kohen, 2013), also known as the predator-prey or parasite-host equations. 

Seemingly, the description of catalytic oscillations at least requires a two-variable model with autocatalytic reactions or a three-variable model without autocatalytic reactions. Eigenberger (1978a,b) made a step toward the construction of a three-variable model by adding a so-called buffer step (4 in the following list) to an adsorption mechanism ... 

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

P. Moller, K. Wetzl, M. Eiswirth, G. Ertl. Kinetic oscillations in the catalytic CO oxidation Computer simulations. J Chem Phys 55 5328-5334, 1986. 

V. N. Kusovkov, O. Kortluke, W. von Niessen. Kinetic oscillations in the catalytic CO oxidation on Pt single crystal surfaces Theory and simulation. J Chem Phys 705 5571-5580, 1998. 

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. 

The oxidation of CO on Pt is one of the best studied catalytic systems. It proceeds via the reaction of chemisorbed CO and O. Despite its complexities, which include island formation, surface reconstruction and self-sustained oscillations, the reaction is a textbook example of a Langmuir-Hinshelwood mechanism the kinetics of which can be described qualitatively by a LHHW rate expression. This is shown in Figure 2.39 for the unpromoted Pt( 111) surface.112 For low Pco/po2 ratios the rate is first order in CO and negative order in 02, for high pco/po2 ratios the rate becomes negative order in CO and positive order in 02. Thus for low Pcc/po2 ratios the Pt(l 11) surface is covered predominantly by O, at high pco/po2 ratios the Pt surface is predominantly covered by CO. 

How relevant are these phenomena First, many oscillating reactions exist and play an important role in living matter. Biochemical oscillations and also the inorganic oscillatory Belousov-Zhabotinsky system are very complex reaction networks. Oscillating surface reactions though are much simpler and so offer convenient model systems to investigate the realm of non-equilibrium reactions on a fundamental level. Secondly, as mentioned above, the conditions under which nonlinear effects such as those caused by autocatalytic steps lead to uncontrollable situations, which should be avoided in practice. Hence, some knowledge about the subject is desired. Finally, the application of forced oscillations in some reactions may lead to better performance in favorable situations for example, when a catalytic system alternates between conditions where the catalyst deactivates due to carbon deposition and conditions where this deposit is reacted away. 

In these experiments, the catalytic activities were stable. That is, there was no evidence of deactivation. There was no evidence of oscillation either, unlike the case when the feed did not contain H2O, then oscillation was observed between about 723 to 773 K [15]... 

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

Recently there has been a growing emphasis on the use of transient methods to study the mechanism and kinetics of catalytic reactions (16, 17, 18). These transient studies gained new impetus with the introduction of computer-controlled catalytic converters for automobile emission control (19) in this large-scale catalytic process the composition of the feedstream is oscillated as a result of a feedback control scheme, and the frequency response characteristics of the catalyst appear to play an important role (20). Preliminary studies (e.g., 15) indicate that the transient response of these catalysts is dominated by the relaxation of surface events, and thus it is necessary to use fast-response, surface-sensitive techniques in order to understand the catalyst s behavior under transient conditions. 

According to analogous molecular mechanics analyses,38 this stereoselectivity mechanism would also operate for catalytic systems with oscillating stereocontrol, leading to atactic-isotactic stereoblock polymers,39,40 like those based on two unbridged 2-phenyl-indenyl ligands.40... 

Nonbonded energy interactions are able to rationalize not only the stereospecificities observed for different metallocene-based catalytic systems (isospecific, syndiospecific, hemi-isospecific, and with oscillating stereocontrol) but also the origin of particular stereodefects and their dependence on monomer concentration as well as stereostructures associated with regioirregular insertions. Nonbonded energy analysis also allowed to rationalize the dependence of regiospecificity on the type of stereospecificity of metallocene-based catalysts. 

Recently there has been an increasing interest in self-oscillatory phenomena and also in formation of spatio-temporal structure, accompanied by the rapid development of theory concerning dynamics of such systems under nonlinear, nonequilibrium conditions. The discovery of model chemical reactions to produce self-oscillations and spatio-temporal structures has accelerated the studies on nonlinear dynamics in chemistry. The Belousov-Zhabotinskii(B-Z) reaction is the most famous among such types of oscillatory chemical reactions, and has been studied most frequently during the past couple of decades [1,2]. The B-Z reaction has attracted much interest from scientists with various discipline, because in this reaction, the rhythmic change between oxidation and reduction states can be easily observed in a test tube. As the reproducibility of the amplitude, period and some other experimental measures is rather high under a found condition, the mechanism of the B-Z reaction has been almost fully understood until now. The most important step in the induction of oscillations is the existence of auto-catalytic process in the reaction network. 

The surface complementarity between the quantum activated complex and the catalytic surrounding media is the main idea of the present theory. The oscillating stereochemical control of the synthesis of thermoplastic elastomeric polypropylene recently reported by Coates and Waymouth [208] can be easily interpreted in terms of catalyst changing surface complementarity. Hill and Zhang have discovered a molecular catalyst that experiences a kinetic and thermodynamic drive for its own reassembly and repair under conditions of catalysis [209]. This is basically what an enzyme does when moving from the apo-structure towards the catalytically apt conformation. 

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