Comparison of adsorption and reactions of pyrrole on Cu(100) and O/Cu(100)

Abstract

Combined techniques of X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy and temperature-programmed reaction/desorption, with the assistance of density functional theory calculations, have been employed to investigate the adsorption, bonding structures and reactions of pyrrole (C4NH5) on Cu(100) and oxygen-precovered Cu(100) (O/Cu(100)). Pyrrole is reversibly adsorbed on Cu(100), with a nearly parallel orientation and preservation of the C2 and σv symmetries. The π-interaction, involving the HOMO and HOMO-1 orbitals of pyrrole, results in the elongation of the 2C–3C, 3C–4C and 4C–5C bonds of the adsorbed molecules. The activation energy of the N–H bond scission of pyrrole on Cu(100) is calculated to be 32.0 kcal ‧mol−1. On O/Cu(100), hydrogen bonding (N–H…O) occurs between the adsorbed O and pyrrole. The H-bonded pyrrole also has a flat-lying geometry and the bond lengths of 2C–3C, 3C–4C and 4C–5C are further increased. The activation energy of deprotonation of the H-bonded pyrrole is calculated to be 6.0 kcal ‧mol−1. This reaction process already occurs at 120 K to form parallel pyrrolyl (C4NH4) on O/Cu(100), which has shorter N–2C and N–5C and longer 2C–3C, 3C–4C and 4C–5C as compared to those of pyrrole on Cu(100). The pyrrolyl is stable below 350 K. At a low coverage of pyrrole on O/Cu(100), the main reaction products are H2O, CO, CO2 and N2, due to the effective N–C bond scission and relatively more surface oxygen available. Much enhanced H2, additional HCN and (CN)2, and termination of N2 are observed at high coverages, representing the chemistry of pyrrolyl at a lower coverage of oxygen or without oxygen on Cu(100).