Catalysis Letters 7 (1990) 157-168 157 REGULARITIES OF SELECTIVITY AS A KEY FOR DISCRIMINATING FT-SURFACE REACTIONS AND FORMATION OF THE DYNAMIC SYSTEM Hans SCHULZ, Egon ERICH, Hartmut GORRE and Eric van STEEN Engler-Bunte-Institut, Universitiit Karlsruhe, 7500 Karlsruhe, Kaiserstrasse 12, Germany Fischer-Tropsch Synthesis, FT-CO-hydrogenation, FT-surface reactions, kinetic model Primary product compositions from FT-CO-hydrogenation have been obtained. A kinetic model has been used to calculate rate constants, rates and probabilities of the elemental surface reactions as a function of carbon number from product composition data. Formation of the catalytic system in the initial stage of an experiment is characterized by selectivity changes as related to elemental surface reactions. 1. Introduction The complexity of product composition in FT-CO-hydrogenation is regarded as a source of information about surface reactions involved and the nature of the catalyst under dynamic conditions. In order to obtain kinetic data on the elemental surface reactions within the mechanism of product formation, a kinetic scheme has to be modelled. The first quantitative model of chain growth by Herington [1] in 1946 respected the grossly exponential decline of moles of product compounds with increase of their carbon number. Anderson and Friedel in 1950 [2] extended the model to the formation of methyl-branched molecules. The model has been developed mathematically in more detail for quantitative description of product compositions by Wojciechowski et al. in 1983 [3,4]. Abstract kinetic parameters were introduced in order to reduce the complexity of mathematical treatment. Pichler, Schulz and Elstner in 1967 [5] inspected the Anderson-assumption of carbon number independent chain growth probability via capillary GC of various products and found appreciable deviations. A general kinetic model of surface reactions in FT-CO-hydrogenation using simply rate constants, rates and probabilities for individual elemental types of surface reactions as parameters was established by Schulz, Erich and Beck in 1987 [6-8]. This model is very flexibel and particularly suited to calculate individual kinetic constants for elemental surface reactions and thus to prove the validity of the simplified assumptions from detailed product composition data. It 9 J.C. Baltzer A.G., Scientific Publishing Company