Oscillatory reactions temperature oscillations

A chemical reaction can be designated as oscillatory, if repeated maxima and minima in the concentration of the intermediates can occur with respect to time (temporal oscillation) or space (spatial oscillation). A chemical system at constant temperature and pressure will approach equilibrium monotonically without overshooting and coming back. In such a chemical system the concentrations of intermediate must either pass through a single maximum or minimum rapidly to reach some steady state value during the course of reaction and oscillations about a final equilibrium state will not be observed. However, if mechanism is sufficiently complex and system is far from equilibrium, repeated maxima and minima in concentrations of intermediate can occur and chemical oscillations may become possible. 

For low values of the inlet gas temperatures (30°-80°C) oscillatory states within the particle have been observed. The periodic activity of the pellet has been observed both for the freely suspended and imbedded particles. While the temporal average temperature was 170°C, the temperature oscillations amounts to 60°C. The frequence of oscillations is very low the period is almost 1 hr. The temperature of the gas and solid oscillated in phase with the reaction rate. 

As early research on oscillatory reactions in heterogeneous catalysis began, little attention was given to the state of the catalyst surface. These first studies recorded the reaction rate by analysis of the product concentrations (see, e.g.. Refs. 3,81) or by measurement of catalyst temperatures 3,162). Later, however, attempts were also made to monitor the catalyst surface during the oscillations, first by measurement of the work function 81), and later by methods such as infrared (IR) spectroscopy 108) and low-energy electron diffraction (LEED) for HV oscillations 245). Table III lists methods employed to study oscillations. 

The methods discussed so fer are in principle applicable under all pressure and temperature conditions. Modern surface analysis tools used in the study of clean, well-defined surfeces, however, require high-vacuum conditions, thus limiting their application to reactions that oscillate under these conditions. Currently these include only the CO/O2, the CO/NO, and the NO/H2 reactions. Another possibility is the use of UHV methods on samples that have been introduced into a high-vacuum system after stopping the oscillatory reaction. This, however, violates the in situ measurement requirement. 

Rastogi and Rastogi (1980) investigated the oscillatory reaction in acetylacetone/ K.Br03/Mn(III)/H2S04 system and reported oscillations in Br and Mn(III)/Mn(II) and rate of temperature rise,... 

Rathousky and Hlavacek (1981) presented two mathematical models to illustrate the fact that the influence of adsorbed species on the rate of an isothermal catalytic reaction may lead to a complex dynamic pattern including multiplicity of steady states and oscillatory states. Multiple oscillations and horatian behavior can not be calculated from the models. Rathousky and Hlavacek (1982) studied CO oxidation on Pt/Al203 catalyst and observed changes in oscillations due to the variations in inlet temperature. For a narrow range they observed horatian behavior. Experiments show that interaction of two oscillatory processes cause horatian behavior. 

The component reactions of oscillatory reaction can be exothermic or endothermic. This aspect is expected to influence oscillatory characteristics. In this case of B-Z reaction involving malonic acid as substrate, it has been found that the rate of temperature rise oscillates rather than the temperature of the reaction mixture [19]. All the component reactions are found to be exothermic. Typical results are given in Fig. 9.7. Temperature begins to fall after IV2 hr. 

Detailed studies of the coadsorption of oxygen and carbon monoxide, hysteresis phenomena, and oscillatory reaction of CO oxidation on Pt(l 0 0) and Pd(l 1 0) single crystals, Pt- and Pd-tip surfaces have been carried out with the MB, FEM, TPR, XPS, and HREELS techniques. It has been found that the Pt(l 0 0) nanoplane under self-osciUation conditions passes reversibly from a catalytically inactive state (hex) into ahighly active state (1 x 1). The occurrence of kinetic oscillations over Pd nanosurfaces is associated with periodic formation and depletion of subsurface oxygen (Osub)- Transient kinetic experiments show that CO does not react chemically with subsurface oxygen to form CO2 below 300 K. It has been found that CO reacts with an atomic Oads/Osub state beginning at temperature 150 K. Analysis of Pd- and Pt-tip surfaces with a local resolution of 20 A shows the availability of a sharp boundary between the mobile COads and Oads fronts. The study of CO oxidation on Pt(l 0 0) and Pd(l 1 0) nanosurfaces by FEM has shown that the surface phase transition and oxygen penetration into the subsurface can lead to critical phenomena such as hysteresis, self-oscillations, and chemical waves. 

At a jacket temperature of 305 K, the reactor model has an oscillatory response. The oscillations are characterized by apparent reaction run-away with a temperature spike. However, when the concentration drops to a low value, reactor then cools until the concentration builds, then the s another temperature rise. It is not unusual for chemid reactors to exhibit such widely different behaviors I different directional changes in the operating conditions. 

Undamped oscillations had been reported by Adlhoch et al. (160) for a Pt ribbon operated in the 10 5 torr range and by Schiith and Wicke (124) for a supported Pt catalyst working near atmospheric presure. An estimate for the former case yields temperature variations of the order of 10 K due to the exothermicity of the reaction in the latter case even periodic changes by 25 K were measured—quite obviously heat conductance is efficient enough to synchronize the oscillatory behavior of these systems. 

We have investigated the transitions among the types of oscillations which occur with the Belousov-Zhabotinskii reaction in a CSTR. There is a sequence of well-defined, reproducible oscillatory states with variations of the residence time [5]. Similar transitions can also occur with variation of some other parameter such as temperature or feed concentration. Most of the oscillations are periodic but chaotic behavior has been observed in three reproducible bands. The chaos is an irregular mixture of the periodic oscillations which bound it e.g., between periodic two peak oscillations and periodic three peak oscillations, chaotic behavior can occur which is an irregular mixture of two and three peaks. More recently Roux, Turner et. al. 

In this work the oxidative transformations of methane were studied with a catalyst system that combines an oxide and a metal component. The presence of both components gave rise to complex oscillation phenomena. The influence of pretreatment and reaction conditions over a wide range of parameters (temperature, total pressure, and oxygen concentration) on the oscillatory process was studied. The possible role of mass transfer and the balance of heat in the reactor were analyzed, and a model for the role of the components in the binary catalyst system is suggested. 

Oxidation of methane in the presence of such a binary oxide-metal catalyst proceeds in an oscillatory regime, and both temperature and concentration oscillations take place. Oscillations arise at the temperature at which the rate of reaction over the oxide component becomes noticeable ( 500°C). As temperature increases, the oscillation amplitude passes through a maximum. The oscillatory behavior disappears when complete conversion of oxygen is reached. In other words, the range of temperatures in which the oscillations are observed covers the range of oxygen conversions from 0 to 100%. 

Oscillations of physicochemical elements such as temperature, concentrations of chemical elements, etc. correspond to oscillatory solutions of the dynamic equations (differential equations) of the system under consideration. As discussed above, various reactions possess different types of these oscillatory mathematical solutions. The well known limit cycles were first observed and named as such by Poincare exactly a century ago. Although formulated by Poincare in general terms, the other mathematical solutions have recently been explored and named by mathematicians, e.g. attractors and exploded points consequently their application to chemical systems are more recent than the limit cycles. 

For the possible interpretation of the oscillatory behaviour the existenee of surface deposits of CxHy and CxHyOz type has been proved in TPO experiments measured for catalyst (II). Oscillation in the partial conversion regime is likely caused by the changes in the composition of the surface layer when the m-xylene concentration increases leading to excess heat formation, which in turn, gives rise to the overall temperature and the reaction rate increases. When the source for the excess heat is depleted (less deposit) the temperature is lowered and the rate decreases. After several over and undershoots, the system reaches a new stationary state. This self-sustained oscillation looks a general phenomena which is operates even if a methane and m-xylene mixture is introdused. 

CO, production rate versus temperature, simulated on a grid of 256 x 256 unit cells. Similar data obtained on larger grids do not differ significantly from the ones shown here. The reaction rate is given in molecules CO, produced per platinum atom. The average rates are drawn as well as the upper and lower limits of the rate in the oscillatory region. The amplitudes of the oscillations are the difference between the upper and lower limits. 

The observation of oscillations in heterogeneous catalytic reactions is an indication of the complexity of catalyst kinetics and makes considerable demands on the theories of the rates of surface processes. In experimental studies the observed fluctuations may be in catalyst temperature, surface species concentrations, or most commonly because of its accessibility, in the time variation of the concentrations of reactants and products in contact with the catalyst. It is now clear that spontaneous oscillations are primarily due to non-linearities associated with the rates of surface reactions as influenced by adsorbed reactants and products, and the large number of experimental studies of the last decade have stimulated a considerable amount of theoretical kinetic modelling to attempt to account for the wide range of oscillatory behaviour observed. 

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