RAIRS of small alkynes on Pd(100)

The adsorption of small alkynes on Pd(100) at 80 K has been studied using r eflection-absorption infrared spectroscopy (RAIRS). Ethyne is found to form a physisorbed multilayer at 80 K, which converts to a chemisorbed monolaye r when heated above 100 K. The ethyne is adsorbed with the C=C bond paralle l to the surface, through a di-sigma+di-pi interaction, and the H atoms til ted away from the surface. Propyne exhibits similar behaviour: at 80 K a ph ysisorbed multilayer forms which converts to a chemisorbed monolayer on hea ting to 100 K. Again adsorption is via the C=C bond with the H and CH3 grou ps tilted away from surface parallel. A disordered multilayer forms above t he chemisorbed layer. Small doses were found to chemisorb directly at 80 K. But-2-yne chemisorbed directly via the C=C bond at 80 K, and further addit ion of but-2-yne formed a disordered, physisorbed multilayer above the chem isorbed layer. The geometry of the chemisorbed species is discussed; ethyne is assigned conclusively to a configuration in which the HCCH plane is nor mal to the surface, but the spectra obtained for propyne and but-2-yne are consistent with either this or a configuration in which the HCCC/CCCC plane is tilted with respect to the surface normal.