Multiple adsorption of NO on cobalt-exchanged chabazite, mordenite, and ferrierite zeolites: a periodic density functional theory study.

The adsorption of NO on Co(II)-exchanged chabazite (CHA), mordenite (MOR), and ferrierite (FER) has been investigated by periodic density functional theory calculations. The most stable configurations of Co(II) in alpha and beta sites of the zeolites with two framework Al/Si substitutions at short distances and Al-(Si)(n>1)-Al ordering are used for calculating the adsorption energy of NO molecules on Co(II) cations and at Al framework sites. The less stable configurations of alpha-Co(II)-MOR/FER show larger adsorption energies for one and two NO molecules. The bonding of one, two (and three) NO molecules to alpha/beta-Co(II) sites in CHA/MOR/FER induces a shortening of the N-O bond lengths because electron density is withdrawn from the antibonding orbital of the adsorbed NO molecule. The calculated nu(NO) stretching frequencies of mono- and dinitrosyl complexes at alpha/beta-Co(II)-MOR/FER are in good agreement with the experimental data. NO molecules adsorbed on alpha-Co(II)-MOR and on alpha-Co(II)-FER show similar NO stretching frequencies as nitrosyl complexes in Co(II)-MOR/-FER/-ZSM-5. Mononitrosyl complexes of alpha/beta-Co(II)-MOR/FER display nu(NO) frequencies blueshifted relative to the free NO, while in dinitrosyl complexes both the symmetric and asymmetric components are redshifted compared to the mononitrosyl frequency. The analysis of the vibrational spectra suggests that mononitrosyls are formed by adsorption at cation in both alpha and beta sites in MOR, FER, and ZSM-5, while dinitrosyl complexes exist only at alpha-type Lewis sites. This is important for the understanding of the reduction mechanism of NO to N(2). A larger adsorption capacity of alpha-Co(II)-FER compared to alpha-Co(II)-MOR is predicted.