Heterogeneous-homogeneous catalytic reaction models

Some concepts of the homogeneous-heterogeneous process of complex reactions have been obtained from a series of investigations by Polyakov and his school [141], Some results were reported in refs. 142 and 143. 

Under certain conditions, the catalyst surface can be a source for the formation of intermediates evolving into the gas phase. These intermediates can initiate chain reactions. 

Thus the logic of studies forces us to take into account the formation of homogeneous constituents in heterogeneous catalytic reactions and heterogeneous constituents in homogeneous processes. 

As far as the models accounting for these conceptions are concerned, their construction and investigation have just started. The development of these models is sure to be retarded by the absence of data on the detailed reaction mechanism and its parameters. The exception is ref. 147, where the authors construct an unsteady-state homogeneous-heterogeneous reaction model and analyze it with respect to the cyclohexane oxidation on zeolites. The study was aimed at the experimental interpretation of the self-oscillations found. The model constructed is in accordance with the law of mass action. 

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The potential importance of homogeneous catalytic reactions in synthesis gas transformations (i.e., hydrogenation of carbon monoxide) has been widely recognized in recent years. In the first place, such systems could provide structural and mechanistic models for the currently more important, but more difficult to study, heterogeneous catalysts. Secondly, product selectivity is generally more readily achievable with homogeneous catalysts, and this would be an obviously desirable feature in an efficient process converting synthesis gas to useful chemicals and fuels. 

Very often the rates of chemical transformations are affected by the rates of other processes, such as heat and mass transfer. The process should be treated as a part of kinetics. The gas/liquid mass transfer in multiphase heterogeneous and homogeneous catalytic reactions could be treated in a similar way. The mathematical framework for modelling diffusion inside solid catalyst particles of supported metal catalysts or immolisided enzymes does not differ that much, but proper care should be taken of the reaction kinetics. 

This complex and structurally related molecules served as a functional homogeneous model system for commercially used heterogeneous catalysts based on chromium (e.g. Cp2Cr on silica - Union Carbide catalyst). The kinetics of the polymerization have been studied to elucidate mechanistic features of the catalysis and in order to characterize the potential energy surface of the catalytic reaction. 

A metal cluster can be considered as a polynuclear compound which contains at least one metal-metal bond. A better definition of cluster catalysis is a reaction in which at least one site of the cluster molecule is mechanistically necessary. Theoretically, homogeneous clusters should be capable of multiple-site catalysis. Many heterogeneous catalytic reactions require multiple-site catalysis and for these reasons discrete molecular metal clusters are often proposed as models of metal surfaces in the processes of chemisorption and catalysis. The use of carbonyl clusters as catalysts for hydrogenation reactions has been the subject of a number of papers, an important question actually being whether the cluster itself is the species responsible for the hydrogenation. Often the cluster is recovered from the catalytic reaction, or is the only species spectroscopically observed under catalytic conditions. These data have been taken as evidence for cluster catalysis. 

This chapter begins, after this brief Introduction, by considering the different designs of HP IR cell, with particular emphasis on more recent developments. Applications of HP IR spectroscopy to mechanistic studies of catalytic reactions will then be discussed, illustrated by examples of both in situ catalytic investigations and model stoichiometric reactions. The chapter will concentrate on homogeneous catalytic processes. The reader is referred elsewhere for coverage of in situ IR spectroscopic methods in heterogeneous catalysis [1]. 

In the case of heterogeneous catalysis, a DCKM or microkinetic model must incorporate the added dimension of adsorbed chemical species as well as active versus non-active sites. To obtain the full predictive capability from reactant influent to product effluent, all possible reactions in the system, both heterogeneous and homogeneous, must be accounted for. To properly understand the catalytic reaction sequence, it is possible that seemingly unimportant intermediates on the surface may be what initiate gas phase reactions. To begin this elucidation, the surface chemical species and their properties must be known. 

It should be noted that the detailed modelling of heterogeneous catalytic reactions faces some specific difficulties. Compared with homogeneous systems, the limits of the field wherein the law of mass action analog (the surface-action law) can be correctly applied are less distinct. Still less reliable are the elementary step constants. Nevertheless, we believe that, despite the complexity of "real kinetics , the importance of studying the models fitting the law of mass action cannot be undervalued. These models describe the chemical components of a complex catalytic process properly and, on the other hand, they are a necessary step that can be treated as a first approximation. Our study is devoted to the analysis of just these models. 

In the majority of catalytic reactions discussed in this chapter it has been possible to rationalize the reaction mechanism on the basis of the spectroscopic or structural identification of reaction intermediates, kinetic studies, and model reactions. Most of the reactions involve steps already discussed in Chapter 21, such as oxidative addition, reductive elimination, and insertion reactions. One may note, however, that it is sometimes difficult to be sure that a reaction is indeed homogeneous and not catalyzed heterogeneously by a decomposition product, such as a metal colloid, or by the surface of the reaction vessel. Some tests have been devised, for example the addition of mercury would poison any catalysis by metallic platinum particles but would not affect platinum complexes in solution, and unsaturated polymers are hydrogenated only by homogeneous catalysts. 

This section briefly discusses some aspects of catalytic combustion mechanisms, i.e., surface reaction kinetics and heterogeneous-homogeneous reactions. Based on this discussion and the previous section, the extreme demands on combustion catalysts are presented. Finally, the role of mathematical modeling of this complex catalytic system is examined. 

I.. D. Pfefferle, Modeling heterogeneous-homogeneous reactions and transport coupling for catalytic combustion systems, Proc. 2nd Inl. Workshop Catalytic Combustion. Tokyo, 18-20 April, (H. Arai. ed.). Catalysis Society of Japan, Tokyo, 1994, p. 78. 

Pure diene-Fe(CO)3 complexes provide suitable model systems for basic catalytic studies. Many types of organometallic intermediates have been implicated to explain the mechanism of various catalytic reactions with transition metals (i, ii, 13, 21, 22). Little direct evidence on these intermediates has been obtained, however, especially in studies with heterogeneous catalysts. Our work with iron carbonyl complexes has afiForded a direct approach to studying active organometallic intermediates during the homogeneous reactions catalyzed by Fe(CO)5. 

One can assume that most (if not all) high-temperature gas-phase catalytic reactions proceeding at temperatures above 800 K are substantially heterogeneous-homogeneous (in the meaning defined above). Consequently, their adequate kinetic description and modeling require a development of corresponding approaches and procedures. 

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