Pergamon Solid State Communications, Vol. 90, No. 4, pp. 223-228, 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038- ! 098/94 $6.00+.00 0038-1098(94)E0154-4 MOLECULAR CHEMISORPTION OF OXYGEN ON Cu(100), Ni(100) AND Pt(111) Bo Hellsing and Shiwu Gao Department of Applied Physics, Chalmers University of Technology S-~12 96 GStebo~, Sweden (Received 15 January 1994,accepted for publication 18 February 1994 by B.Lundqvist) Abstract The electron structure of the oxygen molecule chemisorbed on metals is investigated using a semi-empirical embedded duster scheme. The influence of the metal d band for the interaction and the charge transfer is found to be qualitatively different for the Cu(100) surface compared to the transition metal surfaces Ni(100) and Pt(111). For Cu the metal s states are most important for the charge transfer from the surface to the x ° orbital of oxygen, while for Ni and Pt the metal d states play the dominant role. In a projected density analysis, it is demonstrated that the different location of the d-band for these substrates is crucial for the interaction between the oxygen x* orbital and the metal orbitals. In the case of O2/Pt(111), the lr'-d hybridization results in a ' significant fraction of unoccupied density of states, which is consistent with experimental observations. I. INTRODUCTION Dissociation of Oz is a fundamental step in many surface processes, such as corrosion, catalytic reactions and oxide growth. It is well known that the energy re- quired to dissociate the oxygen molecule is substantially decreased, in comparison with the gas phase value, when it chemically interacts with a metal surface. It is, how- ever, not clear which microscopic parameters determine the different behavior of the oxygen molecule when inter- acting with different metal surfaces. Theoretical studies of the O2-metal interaction is therefore important in or- der to reveal these parameters. To provide insight to this problem, it is necessary to calculate properties that can be related to experiments. The chemisorbed molecular state of oxygen has been observed at low temperatures on several metal sur- faces, e.g. Ag(ll0) 1, Cu(ll0) 2, Pd(lll) s, Pt(111) 4 and Cr(110) s. There are also indications from molecule sur- face scattering experiments that in dissociative sticking of oxygen on Pt(lll), this chemisorbed molecular state is in general an intermediate state e,r. Several theoretical studies of molecular chemisorp- tion on metals have been presenteda-12. These inves- tigations have given considerable insight to the under- standing of the dissociation mechanism in terms of e.g. changes in electron structure as the molecule approach a metal surface. For example, the results obtained from studies of O2 on a 24 Ag atom cluster11 and on a 30 Cu atom cluster 1~ indicate that the metal d-states play an importance role in the molecule-metal interaction . However, the existing controversy concerning the role of the d-electrons in the interaction of molecular oxygen and metals surfaces, motivates further investigations of this system. The present work is an extension of a previous work 12, with the aim to qualitatively describe the dif- ference between the cases when the oxygen molecule in- teracts with a noble metal and transition metal surface. For this study the systems O2/Cu(100) and O2/Ni(100) are chosen. Ni and Cu is chosen as they have the same bulk structure (fcc) and their lattice constants are simi- lar, 3.61/~ and 3.52 ~,. The main difference is the pres- ence of unoccupied d states for nickel. Our main results are as follows. We have found that in general, the oxygen 7r" resonance is located near the Fermi level. For 02 on transition metal substrates, the metal d-oxygen ~r ° hybridization is relatively strong, which yields unoccupied states (above the Fermi level). For noble metals the d band is well below the Fermi level and therefore they hybridize much less with the oxygen lr* state. The net result is that the charge trans- fer from the metal to the antibonding 7r* oxygen state is more efficient for noble metal substrates. The ex- perimentally observed wo-o vibrational frequency for molecularly chemisorbed oxygen indicates a substantial decrease in comparison with the gas phase value. This redshift is more pronounced in the case of noble metal substrates, which is consistent with our results. In the present paper we have also analyzed the sys- tem 02/P t (111), which has been studied experimentally in quite some detail 13-1s. By including also this system we are able to test the validity of our qualitative ideas of the chemical interaction between the oxygen molecule and metal surfaces and to make some comparisons with experiment. The semi-empirical method of calculation that we have used, limits ourselves to a qualitative and compar- ative study. Nevertheless, we believe that the essential physics of the O2-metal interaction is illuminated and 223