Lateral interactions in the desorption kinetics of weakly adsorbed species: unexpected differences in the desorption of C 4 alkenes and alkanes from Ag(110) due to oriented π-bonding of the alkenes

Abstract

Temperature programmed desorption (TPD), X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure (NEXAFS) techniques were used to compare the desorption kinetics of butanes and butenes. Even though these molecules are weakly adsorbed and of similar molecular weight, alkane desorption shows evidence of weak attractive intermolecular interactions, whereas alkenes show clear evidence for moderately strong repulsive interactions in desorption. This behavior is contrary to the weak attractive intermolecular interactions expected for both the butanes and the butenes based on theories of physical adsorption, since the second virial coefficients for both in the gas phase are negative below 550 K. Adsorption of the alkenes clearly introduces repulsive intermolecular forces that are not present in the gas phase. The origin of these intermolecular forces is revealed by NEXAFS measurements for the alkenes, which show a strong orientation of the double bond axis parallel to the surface, indicative of weak directional bonding between the surface and the alkene. It appears that this preferred orientation caused by the surface alters the intermolecular forces between colliding pairs of alkenes, leading to the repulsive interactions that produce the decrease in the activation energy for desorption with increasing alkene coverage. This effect does not occur for butane, whose weak intermolecular interactions in the adsorbed layer are consistent with the predictions for physical adsorption.