Heterogeneous catalysis kinetic results

An interesting method, which also makes use of the concentration data of reaction components measured in the course of a complex reaction and which yields the values of relative rate constants, was worked out by Wei and Prater (28). It is an elegant procedure for solving the kinetics of systems with an arbitrary number of reversible first-order reactions the cases with some irreversible steps can be solved as well (28-30). Despite its sophisticated mathematical procedure, it does not require excessive experimental measurements. The use of this method in heterogeneous catalysis is restricted to the cases which can be transformed to a system of first-order reactions, e.g. when from the rate equations it is possible to factor out a function which is common to all the equations, so that first-order kinetics results. 

D.A. Rudd, L.A. Apuvicio, J.E. Bekoske and A.A. Trevino, The Microkinetics of Heterogeneous Catalysis (1993), American Chemical Society, Washington DC]. Ideally, as many parameters as can be determined by surface science studies of adsorption and of elementary steps, as well as results from computational studies, are used as the input in a kinetic model, so that fitting of parameters, as employed in Section 7.2, can be avoided. We shall use the synthesis of ammonia as a worked example [P. Stoltze and J.K. Norskov, Phys. Rev. Lett. 55 (1985) 2502 J. Catal. 110 (1988) Ij. 

The philosophy used to develop detailed chemical kinetic mechanisms for gas-phase reactions can, in principle, be extended to treat heterogeneous reactions, provided diffusion is also considered in the final analysis. Clearly, the problem in heterogeneous catalysis is considerably more complex because of the close proximity of a large number of atoms and their collective effect on reaction kinetics and mechanisms, and the inevitable variation of catalyst structure with time—for example, as a result of sintering and poisoning. 

Further evidence has been obtained to support the contention that the active catalysts are metal complexes dissolved in solution. With experiments reported in Table II, the kinetics of oxidation under standard conditions in the presence of various metal salts are compared with the rates of reaction when solid residues have been filtered from solution. The agreement between the rates in Cases 1 and 3 of Table II (where the amount of metal available is dictated by the solubility of metal complexes) shows that solid precipitates play little or no part in catalysis in all the systems studied. The amount of metal in solution has been measured in Cases 2 and 3 metal hydroxide complexes (Case 2) are not as soluble as metal-thiol complexes, and neither is as soluble as metal phthalocyanines (19). The results of experiments involving metal pyrophosphates are particularly interesting, in that it has previously been suggested that cobalt pyrophosphates act as heterogeneous catalysts. The result s in Table II show that this is not true in the present system. 

Chapter 2 describes the evolution in fundamental concepts of chemical kinetics (in particular, that of heterogeneous catalysis) and the "prehis-tory of the problem, i.e. the period before the construction of the formal kinetics apparatus. Data are presented concerning the ideal adsorbed layer model and the Horiuti-Temkin theory of steady-state reactions. In what follows (Chapter 3), an apparatus for the modern formal kinetics is represented. This is based on the qualitative theory of differential equations, linear algebra and graphs theory. Closed and open systems are discussed separately (as a rule, only for isothermal cases). We will draw the reader s attention to the two results of considerable importance. 

This book has been written by mathematicians and chemists, the collaborators of the Institutes of the Siberian Branch of the U.S.S.R. Academy of Sciences [The Institute of Catalysis (Novosibirsk), the Computing Centre (Krasnoyarsk) and the Tuva Complex Department (Kyzyl)]. It presents the results of 15 years activity of this Siberian team as reported in two earlier monographs (Kinetic Models of Catalytic Reactions, Nauka, Novosibirsk, 1983 and Kinetics for Model Reactions of Heterogeneous Catalysis, Nauka, Novosibirsk, 1984, both published in Russian). Unfortunately, these results are hardly known to English-speaking readers. 

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. 

In surface-controlled catalyses, the rate-determining step involves the reaction on the surface of an adsorbed reactant or of a derived species. The resultant kinetics may be more or less complicated depending upon the circumstances, and treatments of various cases have been given in reviews dealing with the heterogeneous catalysis of gas reactions [9, 31, 42]. Several... 

In essence, the procedures for reduction of complexity described in this book are applicable in principle to reactions on catalyst surfaces, provided the latter are uniform and no segregation of adsorbed species occurs. However, in view of the wealth of other complicating factors, the effort may well be beyond a point of diminishing returns unless a very simple rate equation results. While the strictly kinetic problems are largely analogous in both fields, the much greater complexity of the peripheral conditions in heterogeneous catalysis leaves less room for inclusion of finer reaction-kinetic detail. 

In continuous stirred-tank reactors (CSTRs), complex kinetics may give rise to multiple steady states even in isothermal operation, especially in heterogeneous catalysis. However, to unravel the causes may be difficult. Here, Feinberg s network theory can help [3]. It operates with a deficiency index that is a readily calculated zero or positive integer. The most useful result of the theory is ... 

In a series of papers, Suvorov et al. investigated heterogeneous catalysis of the cyclization of isolated aldehyde and ketone phenylhydrazones. y-Alumina was typically employed as catalyst in the vapor phase reaction at atmospheric pressure and at temperatures around 300 °C. A maximum yield of 60% was obtained from acetaldehyde phenylhydrazone as a result of thermal decomposition of the hydrazone [7] and the formation of benzene and aniline as by-products [8]. Kinetic studies indicated that the rate-determining step was desorption of product from the surface [9]. 

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