Surface Science 412/413 (1998) 477–488 Surface complexes of nitric oxide: adsorption of anionic NO and N 2 O 2 on CaO Anders Snis, Itai Panas * Department of Inorganic Chemistry, Go ¨ teborg University, S-412 96 Go ¨teborg, Sweden Received 1 April 1998; accepted for publication 18 May 1998 Abstract Chemical reactions involving nitric oxide molecules and a calcium oxide surface are addressed by means of quantum chemistry. The regularised complete active space self-consistent field method (reg-CASSCF ) is employed to determine structures and vibrational frequency bands of the monomeric and dimeric surface species. Stabilities are computed by the CASPT2 method. Reactivity trends for the adsorbates are estimated for various surface sites, including bonding to the Ca and O surface atoms, to an oxygen vacancy and to two different edge sites. Implications of excess surface electrons to heterogeneous NO reduction processes at oxide catalysts are discussed. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Ab initio quantum chemical methods and calculations; Calcium oxide; Catalysis; Chemisorption; Nitrogen oxides; Single crystal surfaces; Surface defects; Vibrations of adsorbed molecules 1. Introduction concentration conditions, which introduce serious constraints on any useful catalyst material. The candidate catalysts are effectively reduced to noble The intriguing heterogeneous processes associ- metals and oxide materials. The former are attrac- ated with nitric oxide, NO, observed at metal as tive because surface oxidation becomes suppressed well as metal-oxide surfaces, is a continuous topic already at quite low temperatures, while the latter for research. The molecule, which is one of the are considered because of the high oxygen activity simplest and most stable radicals, is spontaneously natively present in these materials. Given the high formed in combustion processes at elevated tem- costs associated with noble metals, the present peratures. Being a major environmental hazard, it work has concentrated on seeking fundamental is of vital importance to remove NO from the understanding of general aspects in the catalytic exhaust gases. In order to find selective self-reduc- activities of oxides towards reduction of NO. tion catalysts, i.e. ones that allow for direct trans- Numerous studies have addressed the adsorp- formation of NO into N 2 and O 2 , it becomes tion and the catalytic reduction of nitric oxide. On necessary to learn more about the heterogeneous metal surfaces, NO is known to chemisorb both chemistry of these molecules. in molecular and dissociative forms. The former Most combustion processes occur at high oxygen mechanism involves the formation of NO dimers and a subsequent disproportionation step, * Corresponding author. Fax: +46 31 7722853; e-mail: itai@inoc.chalmers.se whereby N 2 O (g) +O (ads) are produced. This reaction 0039-6028/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S0039-6028(98)00469-5