Catalytic reactions heterogeneous

The temperature at which the maximum rate is found for exothermic reactions can be calculated from 

this temperature depends on the composition of the reaction mixture, Therefore, for exothermic equilibrium reactions the temperature should be lowered as more reactant is converted, to keep the rate high. Thus this type of reaction is often carried out in multiple stages (e.g. multiple reactors or multiple zones within a reactor), each with its own temperature, adapted to the reaction mixture composition. 

If chemical equilibrium is reached the reaction rate becomes zero and the reaction mixture composition must fulfil the equilibrium condition 

Where K is the equilibrium constant. Since R = 0 at equilibrium, K can be calculated as  

Heterogeneous catalytic reactions are reactions that take place on the surface of a solid catalyst. The following phenomena are significant in determining the microkinetics of these reactions  

In certain cases and together with the electrode reaction, in particular that of oxygen reduction at metal surfaces, a non-electrochemical regeneration mechanism operates which is heterogeneous in nature and involves the adsorbed product [165] obviously, it is very dependent on the electrode material and the available surface states. The reaction scheme is thus of the type 

For a first-order regeneration mechanism, the effect will be to shift the Levich-type plot at the limiting current upwards by a constant amount. At the RDE, the intercept is given by 

For a second-order regeneration mechanism, there is a more complex dependence on the mass transport however, this type of reaction is readily distinguishable from the homogeneous regeneration mechanism (Sect. 5.3). The range of second-order rate constants amenable to measurement at the RDE is 104 feh = 106 cm4 mol-1 s-1. 

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M. Boudart and G. Djega-Mariadassou, Kinetics of Heterogeneous Catalytic Reactions, Princeton University Press, Wnceton, NJ, 1984. 

The implementation of high-pressure reaction cells in conjunction with UFIV surface science techniques allowed the first tme in situ postmortem studies of a heterogeneous catalytic reaction. These cells penult exposure of a sample to ambient pressures without any significant contamination of the UFIV enviromnent. The first such cell was internal to the main vacuum chamber and consisted of a metal bellows attached to a reactor cup [34]- The cup could be translated using a hydraulic piston to envelop the sample, sealing it from... 

Boudart M and Djega-Mariadassou G 1984 Kinetics of Heterogeneous Catalytic Reactions (Prinoeton, NJ Prinoeton University Press)... 

Oxo Synthesis. Ad of the synthesis gas reactions discussed to this point are heterogeneous catalytic reactions. The oxo process (qv) is an example of an industriady important class of reactions cataly2ed by homogeneous metal complexes. In the oxo reaction, carbon monoxide and hydrogen add to an olefin to produce an aldehyde with one more carbon atom than the original olefin, eg, for propjiene ... 

For modern analysis, high sensitivity and selectivity of methods for the determination of these elements are important parameters. Catalymetric methods of analysis possess such characteristics. Using of heterogeneous catalytic reactions on the interface also penults to reduce determination time. 

The most common heterogeneous catalytic reaction is hydrogenation. Most laboratory hydrogenations are done on liquid or solid substrates and usually in solution with a slurried catalyst. Therefore the most common batch reactor is a stirred vessel, usually a stirred autoclave (see Figure 2.1.1 for a typical example). In this system a gaseous compound, like hydrogen, must react at elevated pressure to accelerate the process. 

In looking for the mechanism, many intermediates are assumed. Some of these are stable molecules in pure form but very active in reacting systems. Other intermediates are in very low concentration and can be identified only by special analytical methods, like mass spectrometry (the atomic species of hydrogen and halogens, for example). These are at times referred to as active centers. Others are in transition states that the reacting cheimicals form with atoms or radicals these rarely can be isolated. In heterogeneous catalytic reaction, the absorbed reactant can... 

Volume 15 Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.l. Golodets... 

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. 

It is not unusual for the full chemical potential of a reaction to be diminished by slower transport processes (i.e., to be transport limited). In fast liquid phase enzyme reactions, mechanical stirring rates can have a strong influence on the observed kinetics that may be limited by the rate of contacting of the reactants and enzymes. Most heterogeneous catalytic reactions take... 

Boudart, M., and Diega-Mariadassou, G. (1984). Kinetics of Heterogenous Catalytic Reactions. Princeton, NJ Princeton University Press. 

The development of methods for the kinetic measurement of heterogeneous catalytic reactions has enabled workers to obtain rate data of a great number of reactions [for a review, see (1, )]. The use of a statistical treatment of kinetic data and of computers [cf. (3-7) ] renders it possible to estimate objectively the suitability of kinetic models as well as to determine relatively accurate values of the constants of rate equations. Nevertheless, even these improvements allow the interpretation of kinetic results from the point of view of reaction mechanisms only within certain limits ... 

This article will be devoted to analysis of some specific features of the kinetics of coupled heterogeneous catalytic reactions and to experimental results and conclusions derived from them, which were obtained by the present author and his coworkers. The general discussion of the kinetics of complicated reaction systems will be restricted to a brief characterization of fundamental approaches the survey of experimental works of other... 

III. Specific Features of the Kinetics of Coupled Heterogeneous Catalytic Reactions... 

A kinetic description of a heterogeneous catalytic reaction will in most cases be different when the reaction proceeds simultaneously with other reactions in a complex system, compared with the case where its kinetics was studied separately. The most important is the effect in the case where the reactions concerned take place on the same sites of the surface of a catalyst. Let us take, for example, the system of competitive reactions... 

In the case of coupled heterogeneous catalytic reactions the form of the concentration curves of analytically determined gaseous or liquid components in the course of the reaction strongly depends on the relation between the rates of adsorption-desorption steps and the rates of surface chemical reactions. This is associated with the fact that even in the case of the simplest consecutive or parallel catalytic reaction the elementary steps (adsorption, surface reaction, and desorption) always constitute a system of both consecutive and parallel processes. If the slowest, i.e. ratedetermining steps, are surface reactions of adsorbed compounds, the concentration curves of the compounds in bulk phase will be qualitatively of the same form as the curves typical for noncatalytic consecutive (cf. Fig. 3b) or parallel reactions. However, anomalies in the course of bulk concentration curves may occur if the rate of one or more steps of adsorption-desorption character becomes comparable or even significantly lower then the rates of surface reactions, i.e. when surface and bulk concentration are not in equilibrium. 

The example of consecutive, irreversible heterogeneous catalytic reaction of the type A —> B — C has been solved in a more general way by Thomas et al. (16). The authors considered scheme (III) with the listed values of the rate constants of surface reactions along with the constants of adsorption and desorption of the reactant A and of the product C. 

In this chapter we will discuss the results of the studies of the kinetics of some systems of consecutive, parallel or parallel-consecutive heterogeneous catalytic reactions performed in our laboratory. As the catalytic transformations of such types (and, in general, all the stoichiometrically not simple reactions) are frequently encountered in chemical practice, they were the subject of investigation from a variety of aspects. Many studies have not been aimed, however, at investigating the kinetics of these transformations at all, while a number of others present only the more or less accurately measured concentration-time or concentration-concentration curves, without any detailed analysis or quantitative kinetic interpretation. The major effort in the quantitative description of the kinetics of coupled catalytic reactions is associated with the pioneer work of Jungers and his school, based on their extensive experimental material 17-20, 87, 48, 59-61). At present, there are so many studies in the field of stoichiometrically not simple reactions that it is not possible, or even reasonable, to present their full account in this article. We will therefore mention only a limited number in order for the reader to obtain at least some brief information on the relevant literature. Some of these studies were already discussed in Section II from the point of view of the approach to kinetic analysis. Here we would like to present instead the types of reaction systems the kinetics of which were studied experimentally. 

The main results following from the investigation of stoichiometrically not simple heterogeneous catalytic reactions carried out in our laboratory can be summarized in this way ... 

The specific feature of polymerization as a catalytic reaction is that the composition and structure of the polymer molecule formed show traces of the mechanism of the processes proceeding in the coordination sphere of the transition metal ion to which a growing polymer chain is bound. It offers additional possibilities for studying the intimate mechanism of this heterogeneous catalytic reaction. 

Copper oxide, oxidation of CO over, 86 Coupled heterogeneous catalytic reactions, kinetics of, 1-49, see also Kinetics coupling through catalytic surface, 9-13 experimental studies, 22-49 apparatus and procedure, 25, 26 catalysts, 26-28... 

On the Theory of Heterogeneous Catalysis Jcro Horiuti and Takashi Nakamura Linear Correlations of Substrate Reactivity in Heterogeneous Catalytic Reactions M- Kraus... 

The behavior of kinetic systems with even a few interacting species can become very complex. L. Ber nek treats a few key principles and accompanies them with experimental observations in Kinetics of Coupled Heterogeneous Catalytic Reactions. In One-Component Catalysts for Polymerization of Olefins, Yu. Yermakov and V. Zakharov review results... 

The most complex type of gas-liquid-particle process is one in which gaseous components participate in a heterogeneous catalytic reaction, with the formation of gaseous products. The following elementary steps must occur in a process of this type ... 

There is a wide variety of solid electrolytes and, depending on their composition, these anionic, cationic or mixed conducting materials exhibit substantial ionic conductivity at temperatures between 25 and 1000°C. Within this very broad temperature range, which covers practically all heterogeneous catalytic reactions, solid electrolytes can be used to induce the NEMCA effect and thus activate heterogeneous catalytic reactions. As will become apparent throughout this book they behave, under the influence of the applied potential, as active catalyst supports by becoming reversible in situ promoter donors or poison acceptors for the catalytically active metal surface. 

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