N2 chemisorption on Ni(111). An infrared investigation under steady-state conditions

Abstract

The adsorption of N₂ on Ni(111) at 89–115 K under steady‐state (quasiequilibrium) conditions has been studied using Fourier‐transform infrared reflection absorption spectroscopy (FT‐IRAS) and low‐energy electron diffraction (LEED). At very low coverage, a single N 3/4 N absorption band is observed at 2218 cm⁻¹ which is assigned to a singleton N₂ adspecies. With increasing N₂ pressure, two other bands develop at 2212–2208 and 2204–2203 cm⁻¹, and a faint ( 7/8 × 7/8 )R30° LEED pattern is observed. At even higher pressure, a sharp ( 7/8 × 7/8 )R30° LEED pattern develops together with a relatively broad single band at 2194 cm⁻¹. The enthalpy of adsorption of N₂ on Ni(111) is estimated to be −8.4 kcal/mol at zero coverage from isothermal IRAS measurements. Slight attractive interactions between N₂ adspecies are detected. The origins of the vibrational features which are related to island formation will be discussed in detail.