Surface structures in ammonia synthesis

Abstract

Ammonia synthesis is one of the most structure sensitive catalytic reactions. Reaction studies using single crystals showed the open (111) and (211) crystal faces of iron and the$(11\bar 21)$ and$(11\bar 20)$ crystal faces of rhenium to be most active, while the close packed iron (110) and rhenium (0001) crystal faces were almost inactive. These studies suggest that seven (C₇) and eight (C₈) metal atom coordinated surface sites, which are available only on the active surfaces, are very active for the dissociation of dinitrogen, the limiting factor in the reaction rate under most experimental circumstances. In this paper, the experimental evidence for the existence of the C₇ sites in iron is reviewed. In addition, the role of potassium in creating a different active site which is less sensitive to the iron surface structure is discussed. Newly developed surface science techniques should permit investigations into the dissociation of dinitrogen at the C₇ sites and how the resulting chemisorbed nitrogen atoms are removed to allow for reaction turnover. Advances in LEED-surface crystallography, allowing detailed determination of relaxation in the clean metal surfaces and adsorbate induced restructuring of the metal surface, reopen the question of the real structure of the active sites in the presence of atomic nitrogen, or atomic nitrogen coadsorbed with potassium and oxygen. Investigation of the dynamics of surface restructuring involving the movements of both the substrate metal atoms and the chemisorbed atoms by surface diffusion becomes feasible by the availability of the high pressure/high temperature STM system built in our laboratory. Studies of the surface structures of the model iron catalysts under dynamic conditions, using 0.1 ms time resolution and atomic spatial resolution under reaction conditions are now possible.