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

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 (CO)-C-12-(CO)-C-13 mixture reveals that the CO ligands in the di- and tri-carbonyls behave as independent oscillators. Low temperature N-15(2) adsorption can also be explained in terms of formation of Ca2+(N-15(2))(n)(n = 1-3) and Na+(N-15(2))(m) (m = 1-2) species. A theoretical model of the process is proposed and adsorption isotherms well describing the experimental results are derived.