Transient Catalytic Studies

Abstract

The Importance of Transient Catalytic StudiesThe concept of using transient operations dates back to the late 1930s and the preliminary investigations conducted by Wagner and Hauffe [1,2]. Later, during the 1960s, Tamaru contributed substantially to this field in heterogeneous catalysis, in particular, by the use of the method of transient response [3] and by introducing the isotopic labeling technique [4]. Since then, transient methods have become standard tools for unraveling heterogeneously catalyzed reactions; for details, see the reviews in Refs. [5][6][7][8][9][10], monographs [11,12], and a special edition of Applied Catalysis [13]. The reasons for the wide application of transient methods are manifold. In chemical reactions, generally speaking, if the rate (rate equation, rate constant, and activation energy) of each of the elementary steps which comprise the overall reaction is known, the (micro)kinetics of the overall reaction may be understood. To take a simple example, in the reaction between hydrogen and bromine, the kinetics of the overall reaction follows a rather complicated rate law:However, if all the elementary reactions in the overall reaction are known, the kinetic behavior of the overall reaction may be explained on the basis of the rate constants, and the activation energies of the forward and backward reactions of the following elementary steps:In the case of such gas-phase reactions, the rate constants and activation energies of each of the elementary steps are independent of the concentration of reactants, reaction products, and reaction intermediates. This approximately also holds for reactions in solution.In the case of heterogeneously catalyzed reactions, the properties of the catalyst depend upon the concentrations of reactants, reaction intermediates, reaction products and, in some cases, spectator species that do not participate in the reaction path but which are chemisorbed on the catalyst surface. Furthermore, in many cases surface restructuring may occur along with the adsorption, and this results in marked changes in the reactivity of the surface. Consequently, even if the catalyst surface is kept in a well-defined state at the outset, as examined ex situ using a variety of spectroscopic techniques, it can be markedly different during the progress of reaction, and especially under industrially relevant reaction conditions. For example, in the case of oxide catalysts the extent of oxidation of the catalyst surface during the redox reaction is also influenced by the relative ratio of the rates of reduction and oxidation. This results in prominent changes in the properties of catalyst surfaces, depending upon the relative concentration of the reactants and the reaction temperature. In the case of acid catalysts, the acidity of the working catalyst surface (which should be associated with the catalytic activity) is that under the reaction conditions, and not that under the conditions far from those prevailing in the actual reaction.In the case of heterogeneous catalysis the rate co...

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