No abstract

The interaction energy and equilibrium distance for adsorption of CO on a perfect MgO(OO1) surface has been calculated, using a cluster approach for representing the surface. The cluster size has been varied from one single Mg"+ ion up to Mgr405. Ab initio model potentials (AIMP) were used to embed the explicitly described cluster while the rest of the crystal was taken to be point charges. The AIMP model potentials are shown to be a necessary and reliable improvement, compared with using only point charges as a model for the surrounding crystal. It is found that the electrostatic attraction and Pauli repulsion almost cancel each other and the small binding energy obtained is ascribed to dispersive forces. The interaction energy is calculated to be 0.07-0.09 eV (depending on which approach to basis set superposition errors carrections is taken); this is rather low in comparison with the experimental value of 0.3-0.4 eV.

Atomistic modeling of reconstructions and relaxation of the nonpolar (101 h0) and (112 h0) ZnO surfaces has been performed to obtain more realistic surface structures for embedded cluster quantum chemical calculations of the reactivity of ZnO toward H 2 dissociation. Only small differential geometrical effects are found for the cation and anion top-layer relaxations. Both surfaces are found to be unreactive toward H 2 dissociation. For the polar, Zn-terminated (0001) surface several different reconstructions were studied; the lowest surface energy was obtained for a 4×4 reconstruction containing single-step plateaus and valleys. The highest exothermicity was found for H 2 dissociation over a corner site, involving a low-coordination oxygen, on the plateau. The edge sites on the plateau also lead to exothermic reactions, while the regular sites between plateaus are unreactive. The determining factor for the exothermicity is the coordination of the anion, but the ionization potential at the anion site also shows a correlation with the exothermicity. No cooperative effects were found. The vibrational frequencies were computed in qualitative agreement with experiment for the O-H vibration, while the strong experimental Zn-H peak at 1710 cm -1 has not been reproduced. The quantum chemically computed chargeson the ions are found to vary with coordination and position; this could have an effect on the atomistic modeling which presently does not include coordination-dependent terms in the potentials. Using an unembedded, gas-phase ZnO unit as a model leads to unrealistically high reaction energies; this is also the case if reconstruction of the (0001) surface is neglected. This demonstrates the need to develop realistic embedding schemes.

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.