STUDIES OF AMMONIA SYNTHESIS OVER IRON SINGLE CRYSTALS
Since 1913, ammonia has been produced in tonnage quantities by direct combination of nitrogen and hydrogen at high temperature and pressure. Most of it is used to make nitrogen-rich fertilizers, and the balance to manufacture explosives, of use in peace as in war. Ammonia synthesis occurs at the surface of iron catalysts. Because ammonia is a low-priced commodity chemical, the catalyst must be cheap and durable, and its activity as high as possible, so that temperature and pressure can be kept low to minimize the size and cost of huge industrial reactors. Factors responsible for the activity and durability of commercial iron catalysts have been elucidated in the 1980s by surface science studies conducted with model catalysts consisting of single crystals of iron approximately 1 cm in size.
The rate of ammonia synthesis at high temperature and pressure was measured over five of these large crystals of pure iron, cut in five different ways so as to expose five different facets. Two facets were found to be almost equally active, but more active in producing ammonia than the other three. It is logical to expect that the best commercial iron catalysts should expose one or both of the best facets. Is it so?
The answer seems to be yes, for a very simple reason. When the activity of a commercial iron catalyst is compared to that of single crystals under identical conditions, it is found that both values per iron atom exposed are about the same and equal to those found on the best two facets. Thus, it appears that the commercial catalyst has been optimized. How did this happen?
The answer to this second question is more subtle and related to the other desirable feature of a catalyst, namely, durability. Commercial iron catalysts consist of very small crystals of iron, a few millionths of a centimeter in size. As a result, the catalyst exhibits a very large surface area per unit volume, a most desirable characteristic to minimize reactor size.
However, tiny crystals of pure iron fuse under the harsh temperatures of ammonia synthesis, a phenomenon that leads to catalyst sintering and the death of the catalyst. Indeed, a commercial iron catalyst is not made of pure iron metal, but of iron to which certain oxides have been added. Because the added oxides are beneficial, they are called promoters. One of these promoters is aluminum oxide, alumina. Alumina by itself or on an iron surface is totally inactive in ammonia synthesis. Its role as a promoter has been ascribed in the past to its ability to prevent sintering of iron particles (i.e., to ensure catalyst durability).
Recent work with large single crystals does not refute this interpretation, but adds a significant dimension to the role of alumina as a promoter. Thus, surface science studies have revealed that the addition