No abstract

Embedded cluster models of the (001) terrace site have been used to determine adsorption structures and energetics for NO x (NO 2 , NO 3 ), SO x (SO 2 , SO 3 ), as well as for CO 2 on the entire sequence (MgO-BaO) of alkaline-earth-metal oxides. The calculations have been performed at the gradient-corrected density functional level. The oxidation of NO, NO 2 , SO 2 , and CO using surface peroxo species is also considered with the aim of contributing to the understanding of car emission control catalysts based on the NO x storage and reduction concept. For all substrates, the stability of the formed surface species is in the order NO 2 -≈ CO 3 2-< NO 3 -< SO 4 2-. Between substrates the stability of the adsorbates is determined by the substrate basicity, which in turn is regulated by the lattice parameter. The resulting order is MgO < CaO < SrO < BaO. Several different bonding modes and structures are found, which differ in terms of degree of charge transfer from the substrate and how the resulting charge deficiency in the substrate is distributed among the surface ions.

The decomposition of N 2 O through oxygen abstraction and surface peroxide ion formation is studied over the whole sequence, MgO-BaO, of alkaline-earth rock-salt metal oxides. The initial barrier to abstraction follows the expectations (MgO > CaO > SrO > BaO) of O 2chemistry. Good agreement with experiment is obtained for MgO and CaO, but for BaO the discrepancy is large. Consideration of both the Langmuir-Hinshelwood and the Elay-Rideal mechanisms suggests that for MgO the surface is regenerated already at low peroxide ion coverage through diffusion and recombination. For CaO, diffusion is hindered and recombination probably occurs only at higher peroxide ion coverage obtained through deposition of oxygen. SrO and BaO allow neither diffusion over the surface nor recombination of oxygen from two surface peroxide ions. Instead, the Elay-Rideal mechanism becomes active with activation energy at high coverage in good agreement with the experiment for BaO.