THEORETICAL-STUDIES OF ETHYLENE ADSORPTION AND OXIDATION ON CLEAN AND OXYGEN-COVERED RHODIUM(111)

The adsorption of ethylene on clean and oxygen-covered rhodium(111) was studied using extended Huckel calculations with the tight-binding approach. Ethylene binds preferentially in a two-fold bridging site with the C=C and Rh-Rh bonds parallel on clean Rh(111). The C=C bond is significantly weakened, both by donation of electrons from the pi orbital to the surface and back-donation from the surface to the pi* orbital on the clean surface. The addition of a quarter of a monolayer of oxygen modifies the surface electronically, so that the three-fold site is preferred by ethylene and the on-top site becomes more stable than the two-fold. Steric effects are important for high oxygen coverage (theta approximate to 0.5) where close contacts between O and either C or H atoms of the alkene lead to highly repulsive interactions. Steric effects dominate on the Rh(111)-(2 x 1)-O surface. The C=C bond coordinates across a Rh-O bond to form an oxametallacycle structure which is the lowest energy geometry found for this oxygen rich surface. This species is proposed as a plausible intermediate along the path to ketone production and rationalizes the experimental finding that oxygen-rich conditions favor partial oxidation.