FTIR study of low-temperature CO and 15N2adsorption on a CaNaY zeolite: formation of site-specified Ca2+(CO)3 and Ca2+(15N2)3 complexes

Abstract

Adsorption of CO at room temperature on a CaNaY zeolite results in formation of several species: Ca²⁺(CO) (2198 cm⁻¹); Ca²⁺(CO)₂ (2188 cm⁻¹), Ca²⁺(OC) (2094 cm⁻¹), Na⁺(CO) (2172 cm⁻¹) and Na⁺(OC) (2119 cm⁻¹). At 85 K no O-bonded CO is observable but, in addition to the C-bonded complexes found at room temperature, two additional species have been detected: Ca²⁺(CO)₃ (2185 cm⁻¹) and Na⁺(CO)₂ (2167 cm⁻¹). At high CO coverages (equilibrium CO pressures above ca. 100 Pa), the Ca²⁺(CO)₃ and Na⁺(CO)₂ species are predominant. The Ca²⁺(CO)₃ complexes are decarbonylated stepwise and first converted into Ca²⁺(CO)₂ and then into Ca²⁺(CO) species with decreasing coverage. The latter are quite stable at 85 K. The Na⁺(CO)₂ species stepwise lose the two CO ligands. Adsorption of a ¹²CO–¹³CO mixture reveals that the CO ligands in the di- and tri-carbonyls behave as independent oscillators. Low temperature ¹⁵N₂ adsorption can also be explained in terms of formation of Ca²⁺(¹⁵N₂)_n (n = 1–3) and Na⁺(¹⁵N₂)_m (m = 1–2) species. A theoretical model of the process is proposed and adsorption isotherms well describing the experimental results are derived.