Infrared spectroscopic study of N₂ chemisorption on rhodium surfaces

The adsorption of N₂ by alumina-supported Rh surfaces has been studied by high-sensitivity infrared transmission methods. For ¹⁴N₂ on Rh, a single N-N absorption band at 2257 cm^-1 is observed at high N₂ pressure (218 torr) and at temperatures below about 240 K. This chemisorbed N₂ species exhibits an adsorption energy of about 2.2 kcal mol^-1, based on infrared studies of the equilibrium situation. Physisorbed N₂ on Al₂O₃ was also detected below 200 K by its infrared spectrum. Its stretching frequency (2331 cm^-1) is identical with that of N₂(g), indicating a very weak electronic interaction. The adsorption energy for this physisorbed species is about 1.6 kcal mol^-1. This work is an example of the usefulness of high-sensitivity IR techniques, coupled with multiple-scan data acquisition methods, for studying weakly absorbing surface species in detail. The work also illustrates the utility of IR cells designed for working at cryogenic sample temperatures and capable of studying support and metal adsorption simultaneously.