Multiple Adsorption of NO on Fe2+ Cations in the α- and β-Positions of Ferrierite: An Experimental and Density Functional Study

Abstract

Adsorption of NO on Fe-exchanged ferrierite is investigated by Fourier transform infrared (FTIR) spectroscopy and ab initio periodic density functional theory (DFT) calculations. The adsorption properties of single Fe species located in the β- and α-site representing the two most stable locations of the extraframework cation are probed. We consider a divalent Fe(II) cation with two framework Al/Si substitutions, three configurations with two Al atoms in the six-membered ring separated by the −Si−O−Si− chain, and one configuration with two Al atoms in different channels representing a low-Al zeolite. Upon adsorption of a single NO molecule, a tetragonal pyramid forms with four in-plane Fe−O bonds to the framework and one axial Fe−N bond. Upon adsorption of a second NO molecule on a Fe2+ cation in the β-2, α-1, and α-2 sites, a cis tetrahedral complex is formed. In the most stable configuration in the β-1 site, however, the planar Fe−O bonding is not destroyed but is completed to a trans octahedral complex. The tetrahedral complex contains an extraframework [Fe−(NO)2]+ (dinitrosyl) particle, carrying only a single positive charge. Because such a particle compensates only one Al/Si substitution, the resulting charge imbalance induces a high chemical reactivity of the framework Al sites. A third NO molecule therefore adsorbs on the activated oxygen atoms next to the Al site whose charge remains unbalanced, and it is oxidized to NO+. NO stretching frequencies of adsorption clusters formed in Fe−FER are similar to those recorded on Fe−ZSM-5 and Fe−silicatite. The calculated NO stretching frequencies agree well with experimental data and support the proposed assignment to mono-, di-, and trinitrosyl species. Before a trinitrosyl species is formed, one NO molecule adsorbs on the Al site and converts to NO+. The calculated stretching frequencies of the NO+ cation agree with the IR band at 2000−2100 cm-1.