Surface Chemistry of 2-Iodoethanol on Pd(111): Orientation of Surface-Bound Alcohol Controls Selectivity

Temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) were used to investigate the thermal surface chemistry of 2-iodoethanol (ICH 2 CH 2 OH) on Pd(111). 2-Iodoethanol undergoes C–I scission upon adsorption at low temperatures, resulting in 2-hydroxyethyl (−CH 2 CH 2 OH) formation. At low coverage, this intermediate decomposes to form carbon monoxide and hydrogen during TPD. At higher coverage, hydroxyethyl reacts to produce ethanol, ethylene, water, 2-iodoethanol, and acetaldehyde, with nearly complete suppression of the decarbonylation pathway. DFT and HREELS results indicate that the coverage dependence of the reaction pathways is correlated with a change in the adsorption geometry of hydroxyethyl. At low coverage the C–O bond lies parallel to the surface, while at higher coverage it is shifted into a perpendicular position. Adsorbed iodine is observed to play a significant role in blocking sites and favoring the upright adsorption geometry. These results have important implications for the use of catalyst modifiers in controlling selectivity in reactions of functional alcohols.