Investigation of hydrogenation reaction pathways on palladium(111) and surface modification by crown ethers

The pathways for the hydrogenation of adsorbed acetylene, vinylidene and ethylene were investigated using temperature-programmed desorption (TPD) and reflection-absorption spectroscopy. The chemistry of vinyl intermediate formed by the hydrogenation of both acetylene and vinylidene was investigated by adsorbing vinyl iodide on Pd(111). It was found that vinyl species hydrogenate more rapidly than adsorbed acetylene, indicating that the rate-limiting step in acetylene hydrogenation is the addition of the first hydrogen to acetylene to form a vinyl species. Experiments performed by grafting ethyl species onto the surface suggested that addition of the first hydrogen atom to adsorbed ethylene to form an ethyl species is the rate-limiting step in the hydrogenation reaction.; Crown ethers were investigated on a Pd(111) surface in order to synthesize self assembling systems using adsorbed crown ethers of various sizes to form surface rotaxanes, where adsorbate molecules penetrate the orifice of the crown ether ring to form an ordered system. Acetonitrile and methyl isocyanide could be used as axle molecules and investigation of their surface chemistry suggested that methyl isocyanide adsorbs on Pd(111) at 80K at low coverage with the isocyanide group parallel to the surface and it shifts to become perpendicular to the surface as the coverage increases. Acetonitrile is much less reactive and adsorbs in a linear geometry. The simplest crown ether, 1,4-dioxane adsorbs on Pd(111) at 80K to form an overlayer saturating at an exposure of ∼4L (1L = 1 × 10−6 Torrs) and desorbs with a peak centered at ∼268K in TPD. Reflection-absorption infrared spectroscopy of both normal and perdeuterated dioxane suggests that it converts from its usual chair conformation in the gas phase into a boat conformation in the monolayer, allowing the lone pairs on both oxygen atoms to interact with the surface. Analysis of the IR spectrum of 12-crown-4 suggested a similar bonding pattern on the Pd(111) surface where it adsorbs in a flat-lying geometry with C_4v symmetry with a saturation coverage of 0.073 ± .008 monolayers. 12-crown-4 thermally decomposes to leave predominantly CO and ethylidyne species on the surface where the desorbing CO removes three of the four crown ethers oxygen atoms, the fourth remaining on the surface. Small amounts of ethylene were also found to desorb along with a high molecular weight fragment. TPD and RAIRS investigation of 15-crown-5 and 18-crown-6 suggested similar surface interactions and thermal decomposition chemistry. Acetonitrile was adsorbed on crown ether covered surface and the reaction pathways were monitored using TPD and RAIRS technique and these results indicated the possibility of surface rotaxane formation on Pd(111).