The vibrational spectroscopy of H2, N2, CO and NO adsorbed on the titanosilicate molecular sieve ETS-10

Abstract

Dihydrogen, dinitrogen, carbon monoxide and nitric oxide have been adsorbed, at nominally liquid nitrogen temperature, on Na⁺- and K⁺-exchanged ETS-10. IR spectroscopy shows formation of M⁺···(H₂), M⁺···(N₂)_n, M⁺···(CO)_n and M⁺···(NO) (n=1, 2, . . . ; M⁺=Na, K) adducts prevalently involving alkali-metal cations located in the 12-membered channels. These adducts give main IR absorption bands in the range 4050–4150 cm^-1 for H₂, 2331–2333 cm^-1 for N₂, 2148–2176 cm^-1 for CO, and 1820–1900 cm^-1 for NO, which are assigned to the fundamental stretching mode of the diatomic molecules polarized by the electric field created by the metal ions. On Na-exchanged samples, the Na⁺···(N₂) and Na⁺···(CO) species, formed at lowest dosage, evolve into Na⁺···(N₂)_n and Na⁺···(CO)_n (n=2, 3) species upon increasing the gas phase pressure. This reversible "‘solvation’' process is not observed for K-exchanged samples. For adsorbed CO, the high intensity of the IR spectra allowed us to observe and assign overtone bands and combination modes. This result does not find a comparable precedent for CO adsorbed on other zeolites. From bands corresponding to the combination of metal–carbon with carbon–oxygen fundamental stretching modes, the wavenumber values corresponding to metal–carbon stretching vibrations in Na⁺···CO and K⁺···CO were deduced: 122 and 107 cm^-1, respectively. Due to the strong tendency of NO towards dimerization, IR spectroscopy of adsorbed NO is complicated by the manifestations of cis and trans dimers interacting with alkali cations, and Na⁺···(NO) and K⁺···(NO) species could be distinctly observed only at the very initial stages of the adsorption process. Finally, as far as the H₂/ETS-10 system is concerned, the most relevant result is that the ortho and para forms are clearly detected in the Na⁺-ETS-10 sample.