The effects of potassium on ammonia synthesis over iron single-crystal surfaces

The effects of potassium on ammonia synthesis over model iron single-crystal catalysts of (111), (100), and (110) orientation have been studied under high-pressure reaction conditions (20 atm reactant pressure of nitrogen and hydrogen). Under these conditions, no more than 0.15 monolayers (ML) of potassium can be stabilized on the iron surfaces. The Fe(110) surface shows no activity for ammonia synthesis in this study with or without adsorbed potassium. The presence of potassium on the Fe(111) and Fe(100) surfaces increases the rate of ammonia synthesis markedly. At a low reaction conversion of 0.3% the rate over Fe(111) and Fe(100) is enhanced by a factor of two in the presence of potassium. The effect of potassium on Fe(111) and Fe(100) is enhanced as higher reaction conversions (i.e., increasing ammonia partial pressures) are achieved because potassium induces changes in the reaction orders for both ammonia and hydrogen. No change in the activation energy for the reaction is observed with potassium, suggesting that the reaction mechanism has not been altered. Temperature-programmed desorption shows that the adsorption energy of ammonia is significantly reduced when coadsorbed with potassium. A model is proposed in which the decrease of ammonia adsorption energy, induced by potassium, reduces the concentration of ammonia on the iron surface. This effect decreases the number of active sites blocked by the ammonia product, thereby increasing the rate of ammonia synthesis. The model also suggests that an additional effect of potassium is to increase the rate of nitrogen dissociative chemisorption by about 30% over Fe(111) and Fe(100) under ammonia synthesis conditions.