Chemisorption of ethylene and acetylene on ultrathin Pd films on Mo(100)

Previous studies of the chemisorption properties of ultrathin (mono-, bi-, and trilayer) metal films have usually focused on CO adsorption. While the CO adsorption energy on these Pd films is often decreased, CO is one of the strongest π-acceptor ligands and probes mainly one aspect of the chemistry of these surfaces. We report here the first detailed study on ultrathin metal films of the chemisorption of two unsaturated hydrocarbons, C₂H₄ and C₂H₂, which initially bond to the surface as σ-donors and can rehybridize to form strong metal-carbon bonds. The adsorption and dehydrogenation of ethylene and acetylene on monolayer and ultrathin Pd films on Mo(100) were investigated using Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), and high-resolution electron energy loss spectroscopy (HREELS). The pseudomorphic monolayer of Pd on Mo(100) has greatly altered chemisorption properties for C₂H₄. Ethylene is weakly chemisorbed on the Pd monolayer, and the adsorbed species is much less rehybridized from sp² in the gas phase toward sp³ on this surface compared to C₂H₄ chemisorbed on the (100) surface of bulk Pd. In addition, a smaller fraction of the adsorbed ethylene dehydrogenates during TPD experiments on the Pd monolayer compared to thicker Pd films; i.e., the selectivity for reversible C₂H₄ adsorption is increased on the Pd monolayer. The weaker C₂H₄ chemisorption interaction of the Pd monolayer is consistent with previous CO adsorption studies, along with UV photoemission studies and electronic structure calculations showing a filled d-band. However, acetylene chemisorption is not affected like C₂H₄, and C₂H₂ is strongly rehybridized from sp in the gas phase toward sp³ on the Pd monolayer as it is on bulk Pd(100). We propose that the C₂H₂-Pd interaction is strong enough to rehybridize Pd in the monolayer back toward its normal bulk electronic structure (with a partially filled d-band) in order to have the favorable energy change from a large heat of adsorption for C₂H₂ and strong Pd-C bonding.