An Indo Approach to H2 Chemisorption on the Fe (110) and (100) Faces

Abstract

One of the best known examples of surface catalyzed chemical reactions is ammonia synthesis for which Fe is an excellent catalyst. Hydrogen molecule adsorption on the Fe surface seems to be of special importance for the synthesis process. Hydrogen interaction with Fe surface has been studied both experimentally and theoretically: overlayers formation on Fe (110) plane and phase transitions among them were studied by means of low-energy electron diffraction (LEED) measurements and Monte Carlo simulations; EHMO calculations were done and indicated that activated N 2 might react on the Fe (111) surface with slightly activated H 2 chemisorbed in the flat-lying orientation or with 2H in bridging coordination. Recently, a pulsed-laser imaging atom probe study of NH 3 synthesis from N 2 and H 2 have found that ammonia molecule formed from the chemisorbed nitrogen and hydrogen atoms. They have also found that for increasing of the amount of NH 3 formed on the surface the gas H 2 –N 2 mixture has to contain more H 2 then is predicted from the stechiometric relations. In the study of the dynamics of the H 2 interaction with simple, two atomic Fe cluster, it has been found that H 2 molecule might dissociate even in low temperatures. There is an experimental evidence for H 2 dissociation; however, preceded by molecular absorption. As H 2 chemisorption is of importance for reaction with nitrogen, at least as a first step before the dissociation, chemisorption positions and orientation of the molecule relative to the surface are matter of interest. Local geometry of molecular hydrogen adsorption is the aim of the study in this chapter. This chapter discusses preliminary results of the semi-empirical, INDO type, calculations for H 2 adsorption on Fe clusters.