Theor Chim Acta (1994) 87:293-305 Theoretica Chimica Acta © Springer-Verlag 1994 The bonding between NO and the NiO(100) surface* L. G. M. Pettersson Institute of Theoretical Physics, University of Stockholm, Vanadisv~igen 9, S-11346 Stockholm, Sweden Received December 7, 1992/Accepted April 5, 1993 Summary. The interaction between NO and different possible adsorption sites of the NiO(100) surface is studied. The Ni 2÷ cation gives a bonding to NO in reasonable agreement with experiment, but only if a crystal potential correspond- ing to less than completely ionic charges is assumed. The computed angle of 43 ° is also in good agreement with experiment. O 1- sites in both weak and strong crystal potentials also give a strong interaction with NO, 1.3 and 0.5 eV, respectively. In this case the angle is larger or around 70 °. The 0 2- anion and Ni 1+ sites do not give any significant bonding irrespective of assumed crystal potential and can be excluded as adsorption sites. The computed vibrational frequency for the adsorbed NO show shifts of + 50, - 85 and -200 cm -1 for adsorption on Ni 2+ in the weak potential, and 01- in strong and weak potential, respectively. Only one, downwards shifted, frequency has been observed in the experiment but the most likely candidate for the experimentally observed adsorption site with a binding of 0.5 eV, is still the Ni 2÷ in a weak potential. Nitrogen core level shifts are also computed and discussed and the fully screened core-hole state is obtained for a cluster model, NiF40 + NO, of Ni z + in NiO with an ionicity lower than the standard _ 2. Key words: NO - NiO(100) surface - Bonding 1 Introduction The interaction between NO and transition metal oxide surfaces is of great importance in the context of catalysts, e.g., for reducing the NOx content of exhaust gases. As a first step in such a reaction it may be assumed that a gas-phase NO molecule adsorbs at the surface before undergoing any reaction. However, in the case of NO the type of bonding and site at which the NO binds to the surface are largely unknown, both experimentally and theoretically. A particularly interesting case, where detailed experiments have been performed is the interaction between NO and the (100) surface of a cleaved NiO single crystal or a thin layer of NiO grown on nickel metal. For this system Kuhlenbeck et al. [1] have presented results * This work is dedicated to Prof. Inga Fischer-Hjalmars