On the structure sensitivity of ammonia synthesis on promoted and unpromoted iron

In recent years the techniques of surface science have been applied to the elucidation of the details of the mechanism of ammonia synthesis on Fe. In particular Ertl and co-workers have demonstrated the strongly anisotropic nature of nitrogen adsorption on well-defined Fe single-crystal surfaces. Thus the (111) plane is by far the most active for such dissociation, a property thought to be due to the presence of exposed seven-coordinated Fe atoms in this surface. The trend in the efficiency of nitrogen dissociation on these planes has been shown by Spencer et al. to parallel their activity for ammonia synthesis under kinetically limited conditions; thus the (111) plane shows 400 times the activity of the relatively smooth (110) surface. Ertl et al. have also investigated the effects of the presence of alkali metal on the nitrogen adsorption kinetics and have found that the main effect on the energetics is an increase in the heat of adsorption of molecular nitrogen, and a consequent reduction in the energy barrier to dissociation at the surfaces. They then postulate that the effect of promotion in industrial ammonia synthesis is to bring all planes to a similar activity (1b), with the (111) plane being least affected and the (110) being the most affected. The purpose of the present paper is to present data from the kinetic modeling of ammonia synthesis using the data derived by Ertl et al. and to examine whether the thesis described above regarding promoter action is correct.