Density functional model cluster study of adsorption of acetylene on magnesium oxide

Structural, energetic and vibrational properties of acetylene absorbed on terraces as well as edge and corner sites of MgO(0 0 1) have been investigated computationally using a gradient-corrected density functional method. The oxide substrate was represented by model clusters embedded in large arrays of point charges (PCs); positive PCs at the cluster borders were substituted by pseudopotentials of Mg2+ to reduce the artificial polarization of the nearby oxygen anions. From the calculations two types of adsorption complexes emerge: either a H atom of C2H2 interacts with an O2− anion of the substrate or the C–C triple bond interacts with a surface Mg2+ cation. However, for adsorption at the regular MgO(0 0 1) surface only the first case of H–O interaction with C2H2 perpendicular to the (0 0 1) plane is calculated to be stable, exhibiting very weak binding. Adsorption at edge and corner sites is notably stronger, with energies of 0.2–0.6 eV, favoring C–C interaction with Mg2+. In all cases, the infrared (IR) forbidden symmetric C–C and C–H stretching modes become activated due to adsorption. For all adsorption complexes considered, the stretching frequencies are calculated to be redshifted with respect to those of a free C2H2 molecule. The redshifts of the C–C and C–H vibrational frequencies computed for adsorption of acetylene molecules at the regular MgO(0 0 1) sites are closest to the IR frequency shifts measured at low coverage on polycrystalline MgO samples with extended (0 0 1) terraces.