Surface Science 402–404 (1998) 502–507 The effect of the real crystal structure of iron on the behaviour of surface contaminants W. Arabczyk, U. Narkiewicz * Technical University of Szczecin, Pulaskiego 10, 70-322, Szczecin, Poland Received 6 August 1997; accepted for publication 5 October 1997 Abstract The segregation kinetics studies of sulphur and carbon in iron and the adsorption kinetics of oxygen on iron have been discussed, taking into account the real structure of the monocrystalline sample. The deviation of the kinetics results from the t 1/2 law has been interpreted as the effect of the presence of the adsorption sites in the defects of the monocrystalline iron sample. The equilibrium between the adsorption states of sulphur, carbon and oxygen atoms in the defects and on the surface has been assumed. For sulphur and carbon atoms, the energetically more favourable adsorption sites are in the defects; however, for oxygen atoms the adsorption sites on the surface are more favourable. The free energy of the segregation of sulphur and carbon atoms from the defects to the surface has been assumed to be a linear function of the degree of filling of the adsorption sites in the defects. The calculations of the energies of iron clusters with a contaminant atom have been also considered. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Auger electron spectroscopy; Carbon; Iron; Oxygen; Sulphur; Surface defects; Surface segregation 1. Introduction The active sites for the adsorption of non-metal impurities could be localized on the surface, on the grain boundaries, but also in the defects opened The role of contaminants on the iron surface is important in many fields, such as metallurgy, to the surface. The effect of the presence of the active sites in the defects has been described for corrosion, adsorption, catalysis (iron is a compo- nent of many industrial catalysts, for example oxygen adsorption on iron [11,12] and for carbon segregation in iron [13]. ammonia synthesis, water gas conversion, the dehydrogenation of ethylbenzene to styrene or for In this paper, the possibility of the adsorption of oxygen, carbon and sulphur atoms in the defects Fischer–Tropsch synthesis). The segregation of non-metallic impurities in of a monocrystalline Fe(111) sample have been compared. iron and steel has been extensively studied by the group of Grabke [1–8], mainly as a grain boundary diffusion. In spite of the bulk and grain boundary diffusion, the contribution of pipe diffusion (fast 2. Adsorption model considering the real crystal diffusion paths) could be important, especially for structure sulphur in iron [9] and nickel [10]. In the real monocrystal sample the ideal arrange- * Corresponding author. un@mailbox.chemo.tuniv.szczecin.pl ment is disturbed by defects and a ‘‘mosaic struc- 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0039-6028(97)00955-2