LOW-TEMPERATURE ADSORPTION OF ETHYLENE AND BUTADIENE ON PLATINUM AND PALLADIUM SURFACES - A THEORETICAL-STUDY OF THE DI-SIGMA/PI-COMPETITION

The different low temperature coordination modes of ethylene and butadiene on a platinum (111) face, (110) face and on a palladium (111) face are compared on the basis of extended Huckel calculations. The nature of the chemical interaction between the olefin and the surface is detailed and the electronic factors that govern the coordination mode of the hydrocarbon are underlined. The different surfaces are modelled by a 49 or a 44 atoms cluster. A correction is applied in the calculation in order to minimize the artefact introduced by this cluster representation of an extended surface. For the adsorption, the respective importance of two electrons interactions and four electrons repulsions is the key point for the determination of the preferred mode. The di-sigma coordination is more stable on platinum (111) but on the platinum (110) face the pi coordination yields the same adsorption energy than the di-sigma one. This is roughly the same result for the palladium (111) face. The pi mode is there favored by a decrease of the four electrons repulsions caused either by a smaller number of metal neighbours for the surface atom (Pt(110)) or by a reduced radial expansion of the metal orbitals (Pd(111)). This pi coordination is associated with a smaller hybridization of the ethylene molecule. The results are extended to the adsorption of butadiene and this allows a qualitative explanation of the better selectivity for butadiene partial hydrogenation on palladium compared with platinum.