RAIRS of small alkynes on Pd(100)

The adsorption of small alkynes on Pd(100) at 80 K has been studied using reflection–absorption infrared spectroscopy (RAIRS). Ethyne is found to form a physisorbed multilayer at 80 K, which converts to a chemisorbed monolayer when heated above 100 K. The ethyne is adsorbed with the CC bond parallel to the surface, through a di-σ+di-π interaction, and the H atoms tilted away from the surface. Propyne exhibits similar behaviour: at 80 K a physisorbed multilayer forms which converts to a chemisorbed monolayer on heating to 100 K. Again adsorption is ia the CC bond with the H and CH3 groups tilted away from surface parallel. A disordered multilayer forms above the chemisorbed layer. Small doses were found to chemisorb directly at 80 K. But-2-yne chemisorbed directly ia the CC bond at 80 K, and further addition of but-2-yne formed a disordered, physisorbed multilayer above the chemisorbed layer. The geometry of the chemisorbed species is discussed; ethyne is assigned conclusively to a configuration in which the HCCH plane is normal 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.