Elementary Steps in Ammonia Synthesis

Abstract

The net rate of a chemical reaction is determined by its mechanism, i.e., the sequence of elementary processes involved, and any attempt to gain real insight into the progress of a chemical transformation has therefore to start by studying the reaction intermediates and their individual kinetic, energetic, and structural properties on an atomic level. In heterogeneous catalysis this task is hampered in several ways: The conventional techniques used in investigations of homogeneous reactions are frequently only of limited applicability, and the catalyst surface is usually rather inhomogeneous (both structurally and chemically). Furthermore, its composition may differ considerably from that of the bulk. For these reasons information on the elementary steps of heterogeneously catalyzed reactions remained rather restricted for a long period of time. Emmett⁽¹⁾ concluded in a lecture on “Fifty years of progress in the study of the catalytic synthesis of ammonia” at a meeting held in 1974: “The experimental work of the past 50 years leads to conclusion that the rate-determining step in ammonia synthesis over iron catalysts is the chemisorption of nitrogen.... The question as to whether the nitrogen species involved on the surface is molecular or atomic is still not conclusively resolved, though, in the writer’s opinion, the direct participation of nitrogen in an atomic form seems more likely than that in a molecular form.”