Characteristics of Catalytic Reactions

During the reaction, the catalyst remains unchanged in mass and chemical composition at the end of reaction. However, the physical properties of the catalyst may change. 

A very small quantity of the catalyst can produce an appreciable effect on the speed of reaction, e.g. an amount of 10 6-lCT8 mol dm 3 of platinum group metal ions is sufficient to catalyse various redox reactions. 

A catalyst cannot start a reaction but can only effect, i.e. decrease or increase the rate of reaction. Since the catalyst is reproduced at the end of reaction, it does not contribute any energy to the system. The free energy change thus will be same in presence or in absence of the catalyst. The catalyst works as an agent to find out an alternative path for the reaction. 

The catalyst does not effect the final state of equilibrium. The equilibrium 

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The chemistry of the stratospheric ozone will be sketched with a very broad brush in order to illustrate some of the characteristics of catalytic reactions. A model for the formation of ozone in the atmosphere was proposed by Chapman and may be represented by the following "oxygen only" mechanism (other aspects of... 

Problem 1. Define catalyst and catalysis. Mention the types and classification of catalysis. Discuss the characteristics of catalytic reactions. 

This retardation of reaction rate with increase in reactant partial pressure is characteristic of catalytic reactions controlled by a surface reaction mechanism. Langmuir-Hinshelwood surface reaction rate mechanisms for single and dual site mechanisms are respectively ... 

Another characteristic of catalytic reactions is that it cannot in general be assumed, once reaction is established at a certain rate under given conditions, that the rate will remain constant with the passage of chronological time. Catalysts normally lose some or all of their specific activity for a desired chemical transformation with time of utilization. This effect, normally referred to as deactivation, can come from a number of different sources and is often very important in the analysis and/or design of catalytic processes and reactors. 

So far we have considered the effect of pore structure on the general characteristics of catalytic reaction rates. We have seen that pore structure can strongly affect the activity, the temperature coefficient. 

Fewer EXAFS works have been devoted to the study of catalytic systems under reaction conditions due, as already said, to inherent limitations of the technique at high, working temperatures characteristic of catalytic reactions. Among these, a majority include studies of Cu/ZnO (Cu/Si02) in methanol synthesis.The solid state physics of the active copper phase in methanol synthesis is a rather intriguing problem which has not achieved consensus concerning the oxidation state and the hosting of the Cu phase characteristics. The EXAFS works mentioned above elucidated mainly the importance of the metallic state in the reaction. Similarly, the metallic state has been shown to be of importance in the water gas shift reaction (WGS) in Cu, Au, and bimetallic Pd-Cu systems supported on ceria. The importance of ceria vacancies on the activation of water and of the metal (and more precisely, of the metal at support boundaries) for CO activation appear as key elements for this reaction. The bimetallic Pd-Cu work analyses the modulation of Pd behaviour by effect of the alloy with the base... 

Moreover, Fig. 2.2 points out further statistics data on palladium membranes applied in the field of membrane reactors (MRs), devices combining the separation properties of the membranes with the typical characteristics of catalytic reaction steps in only one unit. In particular, this figure reports the number of publications on palladium-based membranes reactors with respect to the total number of publications in the membrane reactors area. 

Returning to a rather general characteristic of catalytic reaction mechanisms, consider the following schematic reaction... 

The Car-Parrinello quantum molecular dynamics technique, introduced by Car and Parrinello in 1985 [1], has been applied to a variety of problems, mainly in physics. The apparent efficiency of the technique, and the fact that it combines a description at the quantum mechanical level with explicit molecular dynamics, suggests that this technique might be ideally suited to study chemical reactions. The bond breaking and formation phenomena characteristic of chemical reactions require a quantum mechanical description, and these phenomena inherently involve molecular dynamics. In 1994 it was shown for the first time that this technique may indeed be applied efficiently to the study of, in that particular application catalytic, chemical reactions [2]. We will discuss the results from this and related studies we have performed. 

On his return to Princeton after the war, Hugh Taylor organized catalytic research at the Frick Chemical Laboratory. He applied high vacuum technique, liquid air cryoscopy to the study of adsorptive characteristics of catalysts, correlating rates of catalytic reactions and rates of adsorption. He introduced the concept of activated adsorption and defended it against all comers. ... 

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. 

Since the time constants of catalytic reactions and the sorption uptake of molecules of various types on crystalline MS, e.g. zeolites, alumlnophosphates and others, are within comparable ranges, the diffusion coefficient represents one of the important rate characteristics of both catalytic and sorptive... 

The Langmuir-Hinshelwood treatment of the kinetics of surface catalyzed reactions affords a useful representation of some of the characteristics of catalytic hydrogenation. It is a limiting form of more exact equations which recognize that, even though the elementary steps are reversible, few if any will be at equilibrium (ref. 15). Not surprisingly, alternative assumptions regarding the relative rates of the forward and reverse elementary reactions can lead to approximate equations of the same form. 

Non-uniformity of catalytic sites A characteristic of a catalytic surface is that its sites may differ in their thermodynamic and kinetic properties. In the kinetic description of catalytic reactions on non-uniform surfaces, a parameter a is frequently used to connect changes in the activation energy of activated adsorption with the enthalpy of the adsorption... 

Since the classification is essentially based on rates of catalytic reactions relative to rates of diffusion of redox carriers, there are oxidation reactions that are intermediate between the two limiting cases. We note that neither the molecular size nor the polarity of reactant molecules is the principal characteristic determining the type of catalysis. Although oxide ions migrate rapidly in the bulk, bulk type II catalysis is not observed for oxidation catalyzed by Bi-Mo oxides. In this case the rate-limiting step is a surface reaction. 

Attention should be paid to the reactivity of 1 and 11, which react with nucleophiles. It is well known that Grignard reagents react with electrophiles forming Mg(II), whereas Pd complexes generate Pd(0) after reacting with nucleophiles. Grignard reactions cannot be carried out catalytically, because it is difficult to reduce Mg(II) to Mg(0) in situ. However, formation of Pd(0) by the reaction of nucleophiles shows the possibility of catalytic reactions. This is the most important characteristic of Pd chemistry. Ni compounds react with both electrophiles (stoichiometric) and nucleophiles (catalytic), depending on the substrates. 

It is surprising that complicated dynamic behaviour proved to be characteristic of the simplest and quite ordinary kinetic models of catalytic reactions, namely of the Langmuir Hinshelwood adsorption mechanism. We are possibly at the initial stage of interpreting the kinetics of complex reactions and the "Sturm und Drang period has not yet been completed. 

Pseudo-steadiness is one of the most essential characteristics for catalytic reactions. In what follows this concept is specified more rigorously on the... 

It is the necessity to interpret critical effects observed in experiment that is a stimulus for the elaboration of a totality of various models accounting for various steps of complex catalytic processes. So far research workers have not come to a unified viewpoint about the factors causing critical effects, but most of them ascribe the complex dynamic behaviour of reactions by the kinetic peculiarities of their mechanism. In principle, a "complete model of catalytic reactions can be suggested that would include the following principal characteristics (1) a detailed reaction mechanism a hypothesis about an ideal adsorbed layer (2) biographical inhomogeneity of the cat-... 

Although there are different types of catalytic reactions, the following features or characteristics are common to most of them. These features are often referred to as the criteria of catalysis. 

We distinguish integral and differential characteristics of catalytic properties. One of the integral characteristics is the extent of reaction Generally, any chemical reaction, whether overall or an elementary step, can be represented by a stoichiometric equation ... 

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