Formation of Ca 2+ (CO) 3 Complexes during Low-Temperature CO Adsorption on CaNaY Zeolite

Adsorption of CO on CaNaY at 85 K results in formation of three types of calcium carbonyl complexes. At high CO coverage (equilibrium CO pressures above ca. 100 Pa) Ca 2+ (CO) 3 species, characterized by an IR band at 2185 cm -1, are predominant. These species are decarbonylated stepwise and first converted into Ca 2+ (CO) 2 dicarbonyls at decreasing coverage during evacuation. The latter complexes lose one CO ligand at further evacuation and form linear Ca 2+ −CO species that are quite stable at 85 K. Adsorption of a 12 CO− 13 CO mixture reveals that the CO ligands in the di- and tricarbonyls behave as independent oscillators. The results obtained are explained by the existence of three coordinative vacancies of Ca 2+ cations in S II positions in Y zeolites.