Anomalous effects of weak chemisorption on desorption kinetics of alkenes: The desorption of propylene and propane from Ag(110)

Abstract

Temperature programmed desorption (TPD), using x-ray photoelectron spectroscopy for absolute coverage determinations, was used to compare the desorption kinetics of weakly bound propane and propylene from Ag(110). The dependence of the activation energy on coverage was quantified by a linear relationship obtained from a leading edge analysis of the TPD curves. Whereas propane shows very weak attractive lateral interactions, propylene shows clear evidence for repulsive interactions in desorption. Weak attractive interactions are expected for both the propane and propylene based on theories of physical adsorption, since the second virial coefficients for both in the gas phase are negative below 550 K. Adsorption of propylene introduces repulsive intermolecular forces that are not present in the gas phase. We suggest that these repulsive forces originate in local interactions resulting from weak chemical bonding interactions between the surface and adsorbed propylene, which give rise to a preferred orientation of the double bond perpendicular to the surface. Fourier transform infrared spectroscopy results indicate that two adsorption states of propylene exist on the surface simultaneously. In the high coverage region the increase in repulsive interactions gives rise to a change in the dominant binding configuration as crowding increases.