A Surface Science and Catalytic Study of the Effects of Aluminum Oxide and Potassium on the Ammonia Synthesis Over Iron Single-Crystal Surfaces

The status of present-day ammonia synthesis is the product of research spanning almost a century. Much of the effort has been directed toward elucidating the mechanism of the promoter effects of potassium and aluminum oxide on the rate of ammonia synthesis. Out of this work has evolved a multitude of techniques, concepts, and ideas which have profoundly affected catalytic chemistry. Many reviews have been written on this subject,(1–3) but it becomes evident from them that an understanding at the molecular level of the promoters is still lacking. A principal reason for this deficiency is that the bulk of this early work could only use indirect methods to study the catalyst. For example, a large amount of kinetic data relating the gas phase ammonia concentration to the surface concentration of promoters has been obtained.(4,5) This type of information is important for optimizing the concentration of promoters but it fails to reveal the effects at the atomic level of the promoters within the working catalyst. With the advent of combined surface-science/high-pressure systems, high-pressure reaction data (>1 atmosphere) can now be correlated to the structure of the catalyst surface at the atomic level, which is determined in the ultrahigh vacuum environment (<10−8 torr). The combination of surface science and high-pressure catalysis provides powerful tools in the study of the reactivity and structure of surfaces. This chapter will be devoted to describing how surface science work, combined with high-pressure data, has elucidated the structure sensitivity and the role of potassium and aluminum oxide in ammonia synthesis. The structure sensitivity of ammonia synthesis will be presented first, since it serves as necessary background when explaining potassium and aluminum oxide promotion in ammonia synthesis.