Oscillatory chemical reaction catalysts

Two Catalysts in the Belousov-Zhabotinskii Oscillatory Chemical Reaction Dokl. Akad. 

Oscillatory chemical reactions always undergo a complex process and accompany a number of reacting molecules, which are indicated as reactants, products, or intermediates. An elementary reaction is occurred by the decrease in the concentration of reactants and increase in the concentration of products. Initial concentration of the intermediates of such reaction is considered low, which approaches almost pseudo-equilibrium state in middle at this moment speed of production is essentially equal to their rate of consumption. In contrast to this, an oscillatory reaction undergoes with the decrease in the concentrations of reactants and increase in the concentration of the products. But the concentrations of intermediates or catalysts species execute oscillations in far from equilibrium conditions [1]. An oscillatory chemical reaction is accompanied by some essential phenomenology called induction period, excitability, multistability, hysteresis, etc. [1, 4]. These characteristic phenomena could be useful to determine the mechanism and behavior of the oscillating reaction. 

The Belousov-Zhabotinsky (BZ) system is a methodically characterized chemical oscillation and provides an archetype scheme for smdy of wide ranges of patterning features in oscillatory chemical reactions [47-53]. This consists of bromination reaction initially and auto-oxidation of organic substrates is takes place in sequential processes by bromate ions. Overall, the reaction is catalyzed by redox catalysts in a concentrated water-acidic solution. 

If a chemical reaction is operated in a flow reactor under fixed external conditions (temperature, partial pressures, flow rate etc.), usually also a steady-state (i.e., time-independent) rate of reaction will result. Quite frequently, however, a different response may result The rate varies more or less periodically with time. Oscillatory kinetics have been reported for quite different types of reactions, such as with the famous Belousov-Zha-botinsky reaction in homogeneous solutions (/) or with a series of electrochemical reactions (2). In heterogeneous catalysis, phenomena of this type were observed for the first time about 20 years ago by Wicke and coworkers (3, 4) with the oxidation of carbon monoxide at supported platinum catalysts, and have since then been investigated quite extensively with various reactions and catalysts (5-7). Parallel to these experimental studies, a number of mathematical models were also developed these were intended to describe the kinetics of the underlying elementary processes and their solutions revealed indeed quite often oscillatory behavior. In view of the fact that these models usually consist of a set of coupled nonlinear differential equations, this result is, however, by no means surprising, as will become evident later, and in particular it cannot be considered as a proof for the assumed underlying reaction mechanism. 

The most extensively studied oscillatory chemical system is the Belousov-Zhabotinskii (BZ) reaction [1] which involves the oxidation of an organic susbstrate (for example malonic acid) by bromate in the presence of a metalion catalyst in sulfuric acid medium. 

Yatsmirski KB, Tikhonova LP, Kovalenko AS (1977) Possible use of vanadium(IV) and vanadium(V) ions as a catalyst for the oscillatory Belousov-Zhabotinskii chemical reaction. Theor Exp Khim 19 700 704... 

In chapter 12 we discussed a model for a surface-catalysed reaction which displayed multiple stationary states. By adding an extra variable, in the form of a catalyst poison which simply takes place in a reversible but competitive adsorption process, oscillatory behaviour is induced. Hudson and Rossler have used similar principles to suggest a route to designer chaos which might be applicable to families of chemical systems. They took a two-variable scheme which displays a Hopf bifurcation and, thus, a periodic (limit cycle) response. To this is added a third variable whose role is to switch the system between oscillatory and non-oscillatory phases. 

The starting point of a number of theoretical studies of packed catalytic reactors, where an exothermic reaction is carried out, is an analysis of heat and mass transfer in a single porous catalyst since such system is obviously more conductive to reasonable, analytical or numerical treatment. As can be expected the mutual interaction of transport effects and chemical kinetics may give rise to multiple steady states and oscillatory behavior as well. Research on multiplicity in catalysis has been strongly influenced by the classic paper by Weisz and Hicks (5) predicting occurrence of multiple steady states caused by intrapellet heat and mass intrusions alone. The literature abounds with theoretical analysis of various aspects of this phenomenon however, there is a dearth of reported experiments in this area. Later the possiblity of oscillatory activity has been reported (6). 

The wide range of reaction systems, catalysts, and reactors that exhibit oscillatory reaction rates reinforces the motivation for research in this field. Oscillations may be lurking in every heterogeneous catalytic system (one might speculate that every heterogeneously catalyzed reaction might show oscillations under the appropriate conditions), and it is crucial to know about this possibility when engineering a chemical process. 

Part II continues with a section on various approaches and transitions. Chapter 6 covers polymer networks and transitions from nano- to macroscale by Plavsic. The following chapter is on the atomic scale imaging of oscillation and chemical waves at catalytic surface reactions by Elokhin and Gorodetskii. Then next chapter relates the characterization of catalysts by means of an oscillatory reaction written by Kolar-Anic, Anic, and Cupic. Then Dugic, Rakovic, and Plavsic address polymer conformational stability and transitions based on a quantum decoherence theory approach. Chapter 10 of this section, by Jaric and Kuzmanovic, presents a perspective of the physics of interfaces from a standpoint of continuum physics. 

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