Two coordination modes of CO in zeolites: A temperature-dependent equilibrium

CO interacts with exchangeable cations M+ (gray spheres in the picture) of zeolites to form M+CO and M+OC species (C: black; O: white) which are in a temperature-dependent equilibrium. For Na-ZSM-5 (M+=Na+) the difference in interaction energy amounts to 3.8 kJ mol-1, as determined by means of variable-temperature FT-IR spectroscopy.