Influence of potassium oxygen layer on properties of iron surfaces

Influence of oxygen concentration in the iron bulk on desorption of potassium from the iron surface has been studied. Desorption of potassium from the iron surface, clean and precovered with oxygen, has been examined. It has been found that the higher the oxygen concentration in the bulk the higher the temperature required to decompose the potassium/oxygen layer existing on the iron surface. Sufficient oxygen concentration in the iron bulk makes potassium/oxygen layer to be stable on iron surface even at the temperature of ammonia synthesis. Diffusion of promoters from the surface of the iron catalyst onto a clean iron foil has also been studied. Only potassium tends to diffuse from the iron catalyst to the clean surface of iron. Considering the results of those experiments, model of the active surface of iron catalyst has been proposed. An adlayer consisting of equal amount of potassium and oxygen atoms is formed on the iron surface. The free sites for dinitrogen adsorption and ammonia formation are located under potassium layer in voids in the oxygen layer. Oxygen atoms bridge iron and potassium atoms and increase thermal stability of potassium on the iron surface. Based on that model, influence of potassium on ammonia decomposition has been interpreted. Some other catalyst properties have been derived from the model. (C) 1999 Elsevier Science B.V. All rights reserved.