LOW-TEMPERATURE ADSORPTION OF FORMALDEHYDE ON A PT(111) SURFACE - A THEORETICAL-STUDY

Several adsorption geometries of formaldehyde on a Pt(111) surface have been compared on the basis of an extended Huckel analysis. Two kinds of adsorbed molecules can exist; one perpendicular to the surface is bound through the oxygen atom (eta1 mode), and the other parallel to the surface is bound through the pi CO bond (eta2 mode). For each mode the number of metal atoms involved has been varied. The electronic description of the interaction between the organic ligand and the surface (simulated by a large cluster) leads to the distinction between stabilizing two-electron interactions and destabilizing four-electron ones. The latter play an important role, owing to the fact that platinum is a d10 metal with an almost filled d band. The more Pt atoms involved in the adsorption, the larger the stabilizing interactions, but also the larger the loss of bonding within the surface and the destabilizing interactions. The latter dominate so that the on-top site is preferred for the eta1 mode and the di-sigma site for the eta2 mode, which is the most stable.