In¯uence of potassium/oxygen layer on properties of iron surfaces W. Arabczyk, U. Narkiewicz, D. Moszyn Âski * Institute of Inorganic Chemical Technology, Technical University of Szczecin, Puøaskiego 10, 70-322, Szczecin, Poland Received 30 July 1998; received in revised form 27 January 1999; accepted 28 January 1999 Abstract In¯uence 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. Suf®cient 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, in¯uence of potassium on ammonia decomposition has been interpreted. Some other catalyst properties have been derived from the model. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Iron catalyst; Potassium; Desorption; Ammonia 1. Introduction The industrial process of ammonia synthesis from elements was developed more than 80 years ago. The catalyst used in this process (fused and reduces iron oxides with addition of potassium, calcium and alu- minium oxides) has remained almost unchanged since then [1]. Numerous studies carried out during these years helped to answer many questions but many of them are still unanswered. Some results and conclu- sions are still incomplete or controversial. The dissociative chemisorption of dinitrogen is known as a rate-limiting step in ammonia synthesis [2]. Dinitrogen adsorption [3] as well as ammonia synthesis [4] on the clean iron surfaces are structure- sensitive reactions, for example, dinitrogen dissocia- tion on Fe(1 1 1) is about 60 times faster than on Fe (1 0 0). Adsorbed potassium signi®cantly changes properties of the iron surface and in¯uences its activ- ity. After potassium adsorption the difference in activ- ity observed for potassium-free surfaces is eliminated [5]. The active phase of the iron catalyst is metallic iron covered with KO layer. Ratio of potassium to oxy- gen atoms was found to be 1:1 [6]. Strongin and Applied Catalysis A: General 182 (1999) 379±384 *Corresponding author. Fax: +48 91 433 0352; e-mail: mosiek@mailbox.tuniv.szczecin.pl 0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(99)00034-4