Computational Study of Extraframework Cu + Sites in Ferrierite: Structure, Coordination, and Photoluminescence Spectra Petr Nachtigall,* Marke ´ ta Davidova ´ , and Dana Nachtigallova ´ J. HeyroVsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejskoVa 3, 182 23 Prague 8, Czech Republic ReceiVed: July 26, 2000; In Final Form: February 1, 2001 The interaction of the Cu + ion in the lowest 3d 10 singlet and 3d 9 s 1 triplet states with the ferrierite matrix is studied computationally by means of a combined quantum mechanics/interatomic potential function technique. The excitation and emission energies found for the Cu/ferrierite and Cu/ZSM-5 systems are very similar. The 540 and 480 nm peaks in the photoluminescence emission spectra are assigned to the Cu + ion located on the channel intersection and on the walls of the main or perpendicular channels, respectively. The structure and coordination of individual binding sites of Cu + in ferrierite are very similar to those found in ZSM-5. Contrary to ZSM-5, the sites on the wall of the main and perpendicular channels of ferrierite are more stable than the sites located on the channel intersection. Therefore, the population of the sites on the channel intersection will be lower in ferrierite than in ZSM-5. It is suggested that the differences in populations of sites on the channel intersection found for ZSM-5 and ferrierite are responsible for the differences in catalytic activities of these systems. 1. Introduction The activity of transition-metal-exchanged high-silica zeolites containing pentasil rings (ZSM-5, ferrierite, or mordenite) for various deNO x reactions differs depending on the type of zeolite structure and transition metal. For example, among cobalt- exchanged zeolites, Co/ferrierite has been found to be more active for selective catalytic reduction (SCR) of NO with methane compared to Co/ZSM-5 and Co/mordenite. 1 Among copper-exchanged zeolites, the Cu/ZSM-5 system exhibits the highest activity in NO decomposition compared to mordenite or ferrierite. 2 For the Cu/ZSM-5 and Cu/mordenite systems, numerous experimental studies have been performed from which the coordination and siting of copper ions have been suggested. On the other hand, only few experimental data have been reported on the investigation of the siting of copper ions in ferrierite. 3 In particular, EXAFS experiments on Cu/ferrierite system are missing. Attfield et al. have studied the Cu 2+ coordination in ferrierite using synchrotron X-ray diffraction and ESR spectroscopy and found only one site in which copper is localized at the channel intersection of the 10- and eight- membered rings. A low coordination of Cu 2+ in this site has been suggested by these authors. Three dominant types of Cu sites have been suggested by Wichterlova et al. that are characterized by luminescence bands at 480, 510, and 540 nm. 4 Cu sites characterized by Cu + emission at 480 nm have been found for Cu/ferrierite with low Cu/Al and Si/Al ratios, while for high Cu/Al and Si/Al ratios Cu sites characterized by Cu + emission at 540 nm have been dominant. The emission band at 510 nm has been observed for a very high level of copper exchange. The qualitative interpretation of UV-vis spectra of Cu/zeolite systems has been known since the pioneering work of Klier and co-workers (see, for example, ref 5). The structure and coordination of copper ions in high-silica zeolites (ZSM-5 in particular) were theoretically studied previ- ously (for example, see ref 6). Recently, we have performed an extensive computational study on the location and coordination of the isolated Cu + and Cu 2+ ions 7,8 and interpretation of the photoluminescence spectra of Cu + for various sites in ZSM-5 9 using a combined quantum mechanics/interatomic potential function method. In this contribution, we apply this method 10 to study the interaction of the Cu + ion in the ferrierite framework with an Al atom in different T positions. For various Cu + sites, the Cu + (3d 9 4s 1 )-Cu + (3d 10 ) transition energies are calculated at both singlet(3d 10 )- and triplet(3d 9 4s 1 )-optimized geometries. The results obtained for the Cu + /ferrierite system are compared with those previously obtained for the Cu + /ZSM-5 system, and a possible explanation for the differences in catalytic activity is suggested. 2. Calculations The periodic structure of the zeolite is described by the combined quantum mechanics/interatomic potential function approach (QM-Pot). 10,11 Within this approach a finite inner part describing the Cu + ion and neighboring atoms is treated at the DFT level (employing the B3LYP functional) and embedded in the surrounding periodic zeolite framework (outer part), which is treated by the shell model ion pair potential. The details of this approach have been described in refs 7 and 9 and are only briefly reviewed here. The interaction parameters for Si, Al, O, and H atoms are from ref 12 and parameters for Cu + are from ref 7. Augmented double- basis sets are used for Cu, Si, Al, and H atoms, while for the O atoms a triple- basis set is used. 13 Polarization functions with exponents 0.35, 0.30, 1.2, and 0.8 are added for Si, Al, O, and H atoms, respectively. 14 Periodic boundary conditions are applied to a unit cell containing 72 T-sites (71 Si atoms and one Al atom) and 144 oxygen atoms. The lattice energy minimizations were performed in P1 symmetry. In localizing preferred Cu + sites, we first * Corresponding author. Fax: (+420 2) 858 2307. E-mail: petr. nachtigall@jh-inst.cas.cz. 3510 J. Phys. Chem. B 2001, 105, 3510-3517 10.1021/jp002679z CCC: $20.00 © 2001 American Chemical Society Published on Web 04/04/2001