Comparison of adsorption properties of adsorbates on Pt(111) and Pt(111)/γ-Al2O3 surface in the ethylene hydrogenation reaction:: MO-theory

Using an atom superposition and electron delocalization molecular orbital (ASED-MO) method, we have compared adsorption properties of adsorbates on the Pt(111) surface with the Pt(111)/gamma-Al2O3 surface in the ethylene hydrogenation reaction. In two-layer thick model systems, the calculated activation energy of the hydrogenation by the surface platinum hydride is equal to the energy by the hydride over supported platinum gamma-alumina. The transition structure on platinum is very close to the structure on the supported platinum/gamma-alumina surface. Hydrogenation by the surface hydride on platinum can take place easily because the activation energy is about 0.5 eV less than hydrogenation by ethylidene. On supported platinum/gamma-alumina the activation energy of the hydride mechanism is about 0.61 eV less than that of ethylidene mechanism. In one-layer thick model systems, the activation energy of hydrogenation by ethylidene is about 0.13 eV less than the activation energy of hydride reaction. The calculated activation energy by the hydride over the supported platinum gamma-alumina is 0.24 eV higher than the platinum surface.(6) We have found from this result that the catalytic properties of one-layer thick model systems have been influenced by the support but the two-layer thick model systems have not been influenced by the support.