Surface, catalytic and magnetic properties of small iron particles II. Structure sensitivity of ammonia synthesis

On FeMgO catalysts with metallic iron particles between 1.5 and 30 nm, the ammonia synthesis at atmospheric pressure and between 570 and 680 K was found to be a structure sensitive reaction, with the turnover number of the small particles being an order of magnitude smaller than that for larger particles. The small particles were then treated with ammonia at 680 K to obtain a high virtual pressure of nitrogen with formation of an iron nitride. The latter was then decomposed in a hydrogen/nitrogen gas mixture. After this treatment, the iron particles possessed a higher turnover number which returned to its original lower value after prolonged treatment of the catalyst in pure hydrogen. The effects of treating the iron particles with ammonia, a hydrogen/nitrogen gas mixture, and pure hydrogen are explained by a nitrogen induced surface reconstruction which can be erased in hydrogen. Hydrogen reduced iron surfaces are rich in sites which are moderately active in ammonia synthesis. Upon exposure to synthesis gas, sites with higher catalytic activity appear on larger particles but hardly on small particles. The surface of the latter can, however, be reconstructed more readily by treatment with ammonia.