ADSORPTION STUDY OF ETHYLENE DERIVATIVES ON PT(111) AT 95-K AS PROBEDBY NEXAFS

The interactions of unsaturated molecules with a metallic surface is o f great interest both from a theoretical point of view (charge transfe r mechanisms, surface reactivity, etc.) and from a practical point of view (catalysis, adhesion, corrosion, etc.). We have recently shown th at the substitution of one or two hydrogen atoms by CH3 groups on ehty lene (butene, propylene) has no effect on the interaction mode of the molecule on Pt(111): chemisorption with the formation of a di-sigma bo nd. We report in this paper some C K-edge NEXAFS experiments performed on substituted ethylene adsorbed on Pt(111) at 95 K where one hydroge n atom is replaced by a chemical function (acid, aldehyde). Acrylic ac id (CH2 = CH-COOH) is strongly chemisorbed on Pt(111) at 95 K (di-sigm a bond) whereas acrolein (CH2 = CH-COH) is pi-bonded. These results co mbined with XPS and UPS data reveal that the differences in the intera ctions come from differences in molecular orbitals conjugations which make possible (or impossible) an overlapping of the molecular orbitals with the metal d band.