Chemical kinetics for generalized two-step reaction schemes

Abstract

Chemical reaction kinetics for steady-state catalytic reactions on heterogeneous catalysts are generally governed by a limited number of elementary steps and the nature of these steps is controlled by the degree of rate control and the surface coverage of reaction intermediates. The generalized two-step reaction scheme, first proposed by Michel Boudart, is used in this paper to drive analytical expressions for the surface coverages by the Most Abundant Reactive Intermediate (MARI); the surface coverage by the Most Abundant Surface Intermediate (MASI); the fraction of the free surface; the degree of rate control for each kinetically significant step; and the overall reaction rate. The two most ubiquitous reaction mechanisms in heterogeneous catalysis, i.e., the associative reaction mechanism and the dissociative reaction mechanism are studied. The applicability of the derived expressions is demonstrated by two case studies: acetone hydrogenation for the associative case and ammonia synthesis for the dissociative case. Scaling relations and BEP correlation are incorporated in the generalized two-step reaction scheme to determine the surface properties of the optimal catalyst. Acetone hydrogenation is used as a case study to demonstrate the applicability of the derived expressions for optimal catalyst. Finally, the application of generalized two-step reaction schemes in understanding the response of a catalyst surface to transient reaction conditions is discussed. Transient reaction kinetics of acetone hydrogenation is used as a case study to demonstrate the applicability of the generalized two-step reaction approach.