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 C₂ and σ_v 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).