Mj. Calhorda et al., THEORETICAL-STUDIES OF ETHYLENE ADSORPTION AND OXIDATION ON CLEAN ANDOXYGEN-COVERED RHODIUM(111), Journal of molecular catalysis. A, Chemical, 97(3), 1995, pp. 157-171
The adsorption of ethylene on clean and oxygen-covered rhodium(111) wa
s studied using extended Huckel calculations with the tight-binding ap
proach. 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 si
gnificantly weakened, both by donation of electrons from the pi orbita
l 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 oxy
gen modifies the surface electronically, so that the three-fold site i
s preferred by ethylene and the on-top site becomes more stable than t
he two-fold. Steric effects are important for high oxygen coverage (th
eta approximate to 0.5) where close contacts between O and either C or
H atoms of the alkene lead to highly repulsive interactions. Steric e
ffects dominate on the Rh(111)-(2 x 1)-O surface. The C=C bond coordin
ates across a Rh-O bond to form an oxametallacycle structure which is
the lowest energy geometry found for this oxygen rich surface. This sp
ecies is proposed as a plausible intermediate along the path to ketone
production and rationalizes the experimental finding that oxygen-rich
conditions favor partial oxidation.