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摘要
The properties of Fe-exchanged ferrierite were investigated by ab initio periodic DFT calculations. Stabilities of Al/Si substitutions were compared for all four irreducible tetrahedral (T) sites of the framework. For each T site, the most stable position of the extraframework Fe2+ cation is located in the six-membered ring, in agreement with experimental data. Depending on the location of the framework Al/Si substitutions, differences in the total energies of the Fe-exchanged configurations can be as high as 200 kJ/mol. Simulated adsorption of NO shows that both ON- and NO- interactions with Fe2+ are at least metastable. Adsorption through the N atom, however, is similar to 2.5 times stronger. Two types of Fe-exchanged configurations were observed. Stable configurations with the cation located in a P site and exhibiting low adsorption energies of similar to 180 kJ/mol were destabilized upon adsorption of NO. Less stable configurations, with the cation located in an a site and with higher adsorption energies of similar to 240 kJ/mol, were stabilized upon adsorption. A strong interaction of NO with Fe2+ can cause a migration of the extraframework cation to a new position in the zeolite framework. The interaction of NO with the Fe2+ cation combines both sigma and pi bonding. a bonding depletes the electron density in Fe d(sigma) orbitals and leads to accumulation in the N p(sigma) orbital. The pi bonding causes an increase of the g-electron density on both the N and Fe atoms. Adsorption induces extensive changes in the electron density distribution within the NO molecule. An expansion of the N p pi density is accompanied by a depletion of the N p pi density oriented toward the O atom. On the contrary, the O atom exhibits a depletion of the sigma-electron density and an increase of the pi-electron density. The complex polarization of the N-O bond leads to only a slight decrease of the bond length. Stretching frequencies calculated for configurations with different stabilities vary from 1866 to 1909 cm(-1). For several stable configurations, the calculated stretching frequency of 1876 cm(-1) is in good agreement with the maximum of the IR band, and for most configurations, the frequency is within the width of the experimental band. Too-high frequencies calculated for two Al atoms in the small rigid ring indicate that no such configurations exist in ferrierite structures. The bonding of NO to the Fe2+ cation is qualitatively different from the bonding on the surface of a transition metal characterized by the Blyholder scheme. The pi back-donation on the surface leads to a weakening of the N-O bond and a downshift of the stretching frequency. In contrast, in the bonding of NO to a transition metal cation, the electron density accumulates within the newly formed N-Fe bond, proportionally depleting both the molecule and the cation. The withdrawing of antibonding electron density leads to a strengthening of the intramolecular bond and to higher N-O stretching frequencies.
- 出版日期2007-1-18
