Recent progress in elucidating the mechanism of heterogeneous catalysis

Abstract

-The mechanism of the decomposition of ammonia on a tungsten surface and the Fischer-Tropsch synthesis over a Ru catalyst have been elucidated by following the full dynamic behavior of each of the adsorbed species under working conditions. In both cases the properties of the catalyst surfaces in their workingstate are remarkably different from those of the clean surfaces. The importance of such dynamic approach under working conditions has been emphasized for elucidating the mechanism of heterogeneous catalysis. INTRODUCT I ONThe mechanisms of reactions in heterogeneous catalysis have been studied in various manners by means of many kinds of instruments and techniques. Since in heterogeneous catalysis the reaction system consists of reactants and catalyst, one of the approaches to the study of mechanism has been the use of well-characterized catalysts. Thus, surfaces with known density of steps or kinks on the surface or with characteristic electronic structures have been investigated. Another approach has been the study of the characteristic behavior of reactants with time. Thus, the kinetics of the overall reaction have been examined under various reaction conditions by following the changes in the concentrations of reactants and products with time. The results have been interpreted on the basis of assumptions, most commonly, those of the Langmuir-Hinshelwood mechanism or the Eley-Rideal mechanism. Labelled compounds have also been employed to obtain deeper insights into catalytic reactions. Chemisorption of each of the reactants and products have also been studied.In all these approaches the catalyst is treated as existing in a "black-box" of which only the entrance and the exit are studied. Because the insides of the "black-box" are not looked at, that is, the catalyst is not examined in its working state, many controversial fundamental problems have remained unsolved.Under the reaction conditions, some of the reactants, products, intermediates or other species are usually adsorbed on the catalyst surface, and the properties of the catalyst surface sites are considerably influenced by the presence of such adsorbed species. The heat of adsorption and the electron accepting or donating properties or work function of the surface, for example, are generally affected by both the nature and amount of the adsorbed species. In some cases, such as catalysis by alloys, the composition of the catalyst at the surface is not only different from that of the bulk, but also altered by the chemisorption as the Auger techniques has revealed in many cases.The extent of adsorption on a catalyst surface during •the course of reaction cannot be estimated from separate adsorption measurements for each of the reactants and products. The nature and extent of adsorption in the working state is not only dependent upon the interactions among the surface species such as the formation of surface complexes, but also upon the position of the rate-determining step, because this largely determines the chemical potential of the react...

Understanding Heterogeneous Catalysis Through the Transient Method

“…The method has been advocated and used by Tamaru, who has recently reviewed his work since 1967 in this field (17). Dent and Kokes used the transient method in IR studies in 1970 (18).…”