Reaction pathways of 2-iodoacetic acid on Cu(100): Coverage-dependent competition between C-I bond scission and COOH deprotonation and identification of surface intermediates

The chemistry of 2-iodoacetic acid on Cu(100) has been studied by a combination of reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reaction/desorption (TPR/D), and theoretical calculations based on density functional theory for the optimized intermediate structures. In the thermal decomposition of ICH ₂COOH on Cu(100) with a coverage less than a half monolayer, three surface intermediates, CH₂COO, CH₃COO, and CCOH, are generated and characterized spectroscopically. Based on their different thermal stabilities, the reaction pathways of ICH₂COOH on Cu(100) at temperatures higher than 230 K are established to be ICH₂COOH → CH₂COO + H + I, CH₂COO + H → CH₃COO, and CH₃COO → CCOH. Theoretical calculations suggest that the surface CH₂COO has the skeletal plane, with delocalized π electrons, approximately parallel to the surface. The calculated Mulliken charges agree with the detected binding energies for the two carbon atoms in CH₂COO on Cu(100). The CCOH derived from CH₃COO decomposition has a CC stretching frequency at 2025 cm^-1, reflecting its triple-bond character which is consistent with the calculated CCOH structure on Cu(100). Theoretically, CCOH at the bridge and hollow sites has a similar stability and is adsorbed with the molecular axis approximately perpendicular to the surface. The TPR/D study has shown the evolution of the products of H₂, CH₄, H₂O, CO, CO₂, CH₂CO, and CH₃COOH from CH₃COO decomposition between 500 and 600 K and the formation of H₂ and CO from CCOH between 600 and 700 K. However, at a coverage near one monolayer, the major species formed at 230 and 320 K are proposed to be ICH₂COO and CH₃COO. CH ₃COO becomes the only species present on the surface at 400 K. That is, there are two reaction pathways of ICH₂COOH → ICH ₂COO + H and ICH₂COO + H → CH₃COO + I (possibly via CH₂COO), which are different from those observed at lower coverages. Because the C-I bond dissociation of iodoethane on copper single crystal surfaces occurs at ∼120 K and that the deprotonation of CH₃COOH on Cu(100) occurs at ∼220 K, the preferential COOH dehydrogenation of monolayer ICH₂COOH is an interesting result, possibly due to electronic and/or steric effects.