Propiolic Acid on Cu(100) and Oxygen-Precovered Cu(100): Multiple Adsorption States and Diversified Reaction Routes

Multiple adsorption geometries and diversified reaction pathways of the bifunctional propiolic acid (HC-C-COOH) on Cu(100) and oxygen-covered Cu(100) (O/Cu(100)) have been investigated, using the surface analytical techniques of X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, and temperature programmed reaction/desorption, with the assistance of density functional theory calculations. Upon adsorption on O/Cu(100) at 105 K, the deprotonation occurs promptly to form perpendicular HC-C-COO (propiolate) with the COO attaching to the surface. Decomposition of this intermediate results in the loss of CO and CO2 at similar to 230 K and formation of the plausible surface intermediates of CCCOOH, tilted HCCCOO, and CCH. The first two are derived from rearrangement or geometric transformation of the perpendicular HC-C-COO. CCH is the precursor for the C2H2 desorption at similar to 335 K. C2 (acetylide) and >C=C=O (ketenylidene) exist on the surface at higher temperatures and are responsible for the desorption of CO and CO2 at 580 K. On Cu(100), adsorption of propiolic acid at 105 K generates tilted HCCCOOH and HCCCOO, via the HCC group bonding to the surface. Perpendicular HC-C-COO is an additional species found at a higher coverage. Hydrogenation of the HCCCOO occurs, possibly involving the surface species of CHCHCOO and CH2CCOO. H2, H2O, CO, and CO2 evolve from decomposition of these carboxylates at similar to 315 K, also forming perpendicular CH2=CHCOO and >C=C=O. Adsorbed CH3CH2COO can be produced at a higher coverage. In addition, C2H2 and C2H4 desorb at similar to 360 K. Decomposition of the perpendicular CH2=CHCOO, >C=C=O, and CH3CH2COO is responsible for the products of H2, CO, CO2, and C6H6 found at similar to 550 K.