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

Structural, energetic and vibrational properties of acetylene absorbed on t erraces as well as edge and corner sites of MgO(0 0 1) have been investigat ed computationally using a gradient-corrected density functional method. Th e oxide substrate was represented by model clusters embedded in large array s of point charges (PCs); positive PCs at the cluster borders were substitu ted by pseudopotentials of Mg2+ to reduce the artificial polarization of th e nearby oxygen anions. From the calculations two types of adsorption compl exes emerge: either a H atom of C2H2 interacts with an O(2-)ion of the subs trate 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 sit es is notably stronger, with energies of 0.2-0.6 eV, favoring C-C interacti on with Mg2+. In all cases, the infrared (IR) forbidden symmetric C-C and C -W stretching modes become activated due to adsorption. For all adsorption complexes considered, the stretching frequencies are calculated to be redsh ifted 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 mol ecules at the regular MgO(0 0 1) sites are closest to the IR frequency shif ts measured at low coverage on polycrystalline MgO samples with extended (0 0 1) terraces. (C) 2001 Elsevier Science B.V. All rights reserved.