Surface, catalytic and magnetic properties of small iron particles: III. Nitrogen induced surface reconstruction

Abstract

The turnover number for ammonia synthesis was found to be smaller on very small iron particles than on larger ones, but it could be enhanced by a pretreatment which seemed to bring about a surface reconstruction induced by nitrogen. The same pretreatment decreases the uptake of carbon monoxide by the catalyst and also the anisotropy energy barrier for relaxation of the metallic iron magnetization, as determined by Mössbauer spectroscopy. These new results can be explained by the creation upon pretreatment of the catalyst, of C 7 surface atoms on the small particles, where a C i atom is one with i nearest neighbors. As the concentration of C 7 atoms is expected to be smaller on very small iron particles than on larger ones, but is enhanced by nitrogen induced surface reconstruction, it is concluded that C 7 iron atoms are more active than others in ammonia synthesis.