Comparison Of Adsorption And Reactions Of Pyrrole On Cu(100) And O/Cu (100)

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 C-2 and sigma(v) symmetries. The pi-interaction, involving the HOMO and HOMO-1 orbitals of pyrrole, results in the elongation of the C-2-C-3, C-3-C-4 and C-4-C-5 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 center dot center dot center dot O) occurs between the adsorbed O and pyrrole. The H-bonded pyrrole also has a flat-lying geometry and the bond lengths of C-2-C-3, C-3-C-4 and C-4-C-5 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-C)-C-2 and (N-C)-C-5 and longer C-2-C-3, C-3-C-4 and C-4-C-5 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 N-2, due to the effective N-C bond scission and relatively more surface oxygen available. Much enhanced H-2, additional HCN and (CN)(2), and termination of N-2 are observed at high coverages, representing the chemistry of pyrrolyl at a lower coverage of oxygen or without oxygen on Cu(100).