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

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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.

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