IDENTIFICATION OF ETHYLENE-DERIVED SPECIES ON AL2O3-SUPPORTED RH, IR, PD, AND PT CATALYSTS BY INFRARED-SPECTROSCOPY

We have used infrared spectroscopy to study ethylene adsorption on alumina-supported Rh, Ir, Pd, and Pt at and below room temperature. The adsorption of ethylene at room temperature gives a set of infrared bands indicating the presence of both pi-bonded ethylene and ethylidyne on the surface of each metal. At lower temperatures di-sigma-bonded ethylene is formed which converts to ethylidyne as the temperature is raised. We have used the C-C stretch and the CH2 scissors mode of pi-bonded ethylene as a measure of the bond strength between the molecule and the metal surface and find that the bond stength follows the sequence Rh > Pd, Ir > Pt. We have attempted to explain the difference in the bond strengths among these metals by using simple qualitative arguments.