Density functional embedded cluster calculations on Lewis acid centers of the α-Al₂O₃(0001) surface: Adsorption of a CO probe

We have investigated the adsorption of CO on the cation-terminated (0001) surface of α-Al₂O₃ with a generalized-gradient density functional approach. We used cluster models that are consistently embedded in an elastic polarizable environment (EPE), employing a (classical) shell model scheme and bare pseudopotentials (Al^pp*) for cations at the cluster boundary. The newly implemented EPE method was found to perform well for describing structure and adsorption properties of Lewis acidic centers of the polar (0001) surface known for its strong relaxation with respect to the bulk terminated geometry. The calculated data are stable with respect to the size of the cluster models. For the embedded stoichiometric clusters [Al₄O₆]/Al₆^pp* and [Al₁₀O₁₅]/Al₁₃^pp* of different structure, the adsorption induced shift δν of the CO stretching vibration and the binding energy (BE) are 30-42 cm^-1 and 0.38-0.47 eV, respectively. The results for the frequency shifts are in good agreement with the value of 39 cm^-1 measured for CO/α-Al₂O₃(0001) at low coverages. Judged by the frequency shift and the adsorption energy, the interaction of CO molecules with three-coordinated Al³⁺ cations at the regular surface is very similar to that with Mg²⁺ corner sites of MgO polycrystallites, δν = 39 cm^-1, BE = 0.38 eV.