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 ∼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 ∼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 ∼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 ∼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 ∼550 K.