3510
J. Phys. Chem. B 2001, 105, 3510-3517
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 3d10 singlet and 3d9s1 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 deNOx reactions differs depending on the type of zeolite structure and transition metal. For example, among cobaltexchanged 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 Cu2+ 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 eightmembered rings. A low coordination of Cu2+ 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). * Corresponding author. Fax: (+420 2) 858 2307. E-mail: petr.
[email protected].
The structure and coordination of copper ions in high-silica zeolites (ZSM-5 in particular) were theoretically studied previously (for example, see ref 6). Recently, we have performed an extensive computational study on the location and coordination of the isolated Cu+ and Cu2+ ions7,8 and interpretation of the photoluminescence spectra of Cu+ for various sites in ZSM-59 using a combined quantum mechanics/interatomic potential function method. In this contribution, we apply this method10 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+(3d94s1)-Cu+(3d10) transition energies are calculated at both singlet(3d10)- and triplet(3d94s1)-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-ζ basis sets are used for Cu, Si, Al, and H atoms, while for the O atoms a triple-ζ 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
10.1021/jp002679z CCC: $20.00 © 2001 American Chemical Society Published on Web 04/04/2001
Extraframework Cu+ Sites in Ferrierite
J. Phys. Chem. B, Vol. 105, No. 17, 2001 3511
carried out lattice energy minimizations using the shell model potential function alone. The lattice energies of selected structures were then minimized at the QM-Pot level. The size of the inner region (treated by DFT/B3LYP) varies depending on the type of Cu+ coordination. In this study we considered clusters of sizes ranging from three to nine TO4 units, AlSi2O2(OH)8 to AlSi8O11(OH)14. The binding energies of Cu+ ions to the ferrierite framework (with negative charge -1 due to Al substitution) are defined as the negative energies of the reaction
Cu+ + (FER)- f Cu/FER
(1)
Hence, in addition to lattice energy minimizations of the Cu/ ferrierite system, lattice energy minimizations of the negatively charged ferrierite framework have to be carried out. Thus, at the QM-Pot level, geometry optimizations of the negatively charged ferrierite framework with the same definition of the inner and outer regions as for the Cu/FER system were carried out. To make calculations feasible, we added a positive background charge and we corrected for it afterward (for details see ref 15). The relative binding energies for the individual Cu+ sites as defined above yield relative stabilities of different Cu+ sites only for those sites which have the Al atom in the same T-site. The energy minimization of the Cu(I)/FER system in the triplet state always started from the corresponding singletoptimized structure. Transition energies calculated at the singletand triplet-optimized geometries correspond to the vertical T1 r S0 excitation and S0 r T1 emission energies, respectively. The results for vertical excitation energies are the pure QM results of the clusters. The long-range contribution from the periodic zeolite structure cancels in our approach, because we employed the same parameters for the interaction of the zeolite framework with Cu+ in the 3d94s1 triplet excited state as in the 3d10 singlet ground state. Thus, the combined QM-Pot method was used in order to obtain reliable structures, while the transition energies were calculated with cluster models, the geometry of which were taken from QM-Pot calculations. The calculations have been carried out with the QM-Pot program,12 which makes use of the TURBODFT16 and GULP17 programs for DFT and shell model potential calculations, respectively. 3. Results 3.1. Distribution of the Framework Aluminum Atoms. In the ferrierite framework, five-membered ring building units are connected to form 10-membered ring channels running along the [001] direction (main channel) and intersected by eightmembered ring channels running parallel to the [010] direction (perpendicular channel).18 Two alternative space groups, Immm and Pmmm have been reported for the ferrierite structure. In the Pmmm space group, there are five nonequivalent T sites, while in the Immm symmetry the T4 and T5 sites are identical. These two structures differ in the T4-O-T5 angle, which is 180° and 170° for Pmmm and Immm structures, respectively. The energy difference between the Pmmm and Immm structures optimized using the empirical force field is about 0.1 kcal/mol.19 Considering the Immm structure, the numbers of T1, T2, T3, and T4 sites are in the ratio 1:2:2:4. The structure of ferrierite together with the T-position location and definition of individual copper sites is depicted in Figure 1. T positions can be classified with respect to their localization on the wall of the main or perpendicular channels. The T2 and T4 sites are located at the intersection of
Figure 1. Definition of the individual types of Cu+ sites and their location in the ferrierite framework. The 10-membered rings represent the main channel of ferrierite. T1, T2, T3, and T4 denote four distinguishable sites in the Immm symmetry. The M2, M5, and M7 denote the sites on the wall of the main channel, and the P5, P6, and P7 denote the sites on the wall of the perpendicular channel. The I2 and P2 sites are on the intersection of both channels.
TABLE 1: Relative Energies (in kcal/mol) of Al Atom in Ferrierite for Different T Sites and Their Location Inside the Framework
a
T sitea
location
Erel
T2 T3 T1 T4 T5
channel intersection main channel perpendicular channel channel intersection channel intersection
0.0 1.2 1.3 9.1 9.3
Sites corresponding to labeling in the orthorhombic symmetry.
both channels, while the T1 and T3 sites are on the wall of the perpendicular and main channels, respectively. The energetical differences for the substitution of an Si atom by an Al atom calculated within the Pmmm space group with five nonequivalent T atoms are given in Table 1, together with their localization within the ferrierite framework. The difference between the most and least stable structures is 9.3 kcal/mol. The most preferred position for Si/Al substitution found is the T2 site. Structures with Si/Al substitution at T1 and T3 are only slightly higher in energy. Substitution at T4 and T5 yields structures which are 9.1 and 9.3 kcal/mol above the lowest energy structure, respectively. Because of negligible energetical differences for Si/Al substitution at the T4 and T5 sites and the similar localization of these two sites in ferrierite, only four T sites were considered in the present study. It should be noted that relative energies of structures with Al substituted into different T positions could change in the presence of water. Recently, Ricchiardi et al. found such an effect for Ti atom substitution into the ZSM-5 framework.20 3.2. Cu+ Siting and Coordination in the Electronic Ground State. For each Al position at least 10 lattice energy minimizations with different starting positions of the Cu+ ions were performed at the shell model potential level. For selected Cu+ positions the structures were reoptimized at the QM-Pot level. The Cu+ binding sites were classified according to their location in the ferrierite framework. The Cu+ sites located in the main channel, in the perpendicular channel, and on the intersection of two channels are denoted with letters “M”, “P”, and “I”, respectively. The number following the letter further specifies the type of binding site: “2” is used for sites where Cu+ is bonded only to two oxygen atoms of the AlO4
3512 J. Phys. Chem. B, Vol. 105, No. 17, 2001
Nachtigall et al.
TABLE 2: Site Types, Coordination Numbers, Cu-O Distances (in Å), and Binding Energies of Cu+ with Zeolite (in kcal/mol) as Obtained at the Shell Model Potential and QM-Pot Levels Al site site typea 1
2
3
4
T sites involvedb
shell model potential CN(O)c CN(OAl)d r(Cu-O)e Eb(Cu+)
P6
(1-2-2-1-2-2)
4
2
P6
(1-2-2-1-2-2)
4
2
P6
(1-2-2-1-2-2)
4
2
P5 M7 N
(1-2-4-4-3) (3-4-4-3-4-4)/1 (1-2-4-4-3)*
3 3 5
2 0 2
P6
(1-2-2-1-2-2)
5
2
P6
(1-2-2-1-2-2)
5
2
M5 I2 I2 N
(2-4-3-3-4) 2-2-4 2-2-4(1) (2-4-4-4-4)*
3 2 3 5
2 2 3 1
P5
(1-2-4-4-3)
5
2
P5
(1-2-4-4-3)
5
2
M7
(3-4-4-3-4-4)/1
4
2
P7 M2 M5 M2 M2
(1-2-4-4-4-3-3)/ 4 4-3-4 (2-4-3-3-4) 4-3-4 4-3-4
3 2 3 2 2
2 2 1 2 2
P7
(1-2-4-4-4-3-3)/4
4
2
P2 P7
2-4-4 (1-2-4-4-4-3-3)/4
2 4
2 2
P2 P7
2-4-3 (1-2-4-4-4-3-3)/4
2 4
2 2
N
(2-4-3-3-4)/4
5
3
N
(2-4-4-4-4)*
5
2
M5
(2-4-3-3-4)
5
2
2.06, 2.08, 2.21, 2.26 2.07, 2.08, 2.21, 2.25 2.05, 2.08, 2.20, 2.20 2.04, 2.21, 2.47 2.18, 2.22, 2.44 2.05, 2.12, 2.19, 2.23, 2.33 2.13, 2.21, 2.25, 2.42, 2.44 2.11, 2.24, 2.35, 2.36, 2.46 2.14, 2.32, 2.33 2.03, 2.04 2.03, 2.04, 2.47 2.01, 2.08, 2.09, 2.20, 2.30 2.06, 2.07, 2.13, 2.22, 2.26 2.06, 2.07, 2.10, 2.11, 2.43 2.14, 2.14, 2.34, 2.35 2.08, 2.15, 2.49 2.03, 2.03 2.01, 2.21, 2.24 2.01, 2.41 2.03, 2.42 2.13, 2.19, 2.31, 2.37 2.08, 2.08 2.12, 2.17, 2.33, 2.50 2.11, 2.19 2.04, 2.11, 2.21, 2.28 2.07, 2.16, 2.30, 2.42, 2.43 2.04, 2.11, 2.13, 2.15, 2.26 2.07, 2.13, 2.15, 2.17, 2.48
Erelf
146.7
0.0
144.4
2.3
142.2
4.5
129.7 128.0 109.1
CN(O)c CN(OAl)d
QM-Pot r(Cu-O)e
Eb(Cu+) Erelf
4
2
2.06, 2.12, 2.14, 2.44
159.1
0.0
17.0 18.7 37.6
2
1
1.96, 2.06
148.3
10.8
137.3
0.0
3
1
2.06, 2.04, 2.26
149.5
0.0
136.5
0.8
130.4 130.2 129.3 115.2
6.9 7.1 8.0 22.1
2 2 2
1 2 2
2.03, 2.04 2.05, 2.08 2.08, 2.08
145.5 145.5 144.8
4.0 4.0 4.7
106.8
30.5
106.1
31.2
138.3
0.0
3
2
2.05, 2.30, 2.10
152.0
0.0
136.5 133.9 131.6 130.0 126.1
1.8 4.4 6.7 8.3 12.2
3
1
1.99, 2.22, 2.29
145.2
6.8
140.3
1.7
2
1
1.96, 2.02
155.4
0.0
140.2 138.0
1.8 4.0
2
2
2.06, 2.08
151.0
4.4
129.9 125.6
12.1 16.4
121.2
20.8
117.1
24.9
111.7
30.3
a For definition, see Figure 1. b The particular T positions involved in the coordination with Cu+. Parentheses indicate that these T sites represent a ring, “/” denotes the bridging T atom, and “*” denotes that another ring is involved as well. c Number of oxygen atoms in coordination with Cu+ (atoms closer than 2.5 Å are considered). d Number of oxygen atoms of AlO4 tetrahedron in coordination with Cu+ (atoms closer than 2.5 Å are considered). e The numbers in italics correspond to the Cu-O bond distances between Cu and oxygen atoms not adjacent to Al. f The relative binding energies with respect to the energy lowest structure for particular Al position.
tetrahedron; “5” and “6” are used for sites on top of the fiveand six-membered rings, respectively; M7 denotes a site on top of the six-membered ring consisting of two pentasil rings; and P7 denotes seven-membered ring bridged by one SiO4 unit (see Figure 1). Three different types of the Cu+ binding sites were found: (i) the Cu+ ion interacts only with two oxygen atoms of the AlO4 tetrahedron (I2, P2, and M2); (ii) the Cu+ ion interacts with oxygen atoms of five-, six-, or seven-membered ring containing aluminum (M5, M7, P5, P6, and P7); and (iii) the Cu+ ion interacts with oxygen atoms belonging to more than one ring (N). The coordination numbers (CN), Cu-O bond distances, and binding energies for the structures with different T sites obtained at the shell model potential and QM-Pot levels are summarized in Table 2. The Cu+ ion is assumed to be coordinated to a
framework oxygen atom if the Cu-O bond distance is shorter than 2.5 Å.7 A good agreement between the relative binding energies of the Cu+ ion calculated at the shell model potential and QM-Pot levels is apparent. The relative stabilities of the individual Cu+ binding sites depend on the position of the Al atom. Al Atom in the T1 Position. In this case, the P6 site is the most stable structure. The Cu+ ion is coordinated to four oxygen atoms of the six-membered ring on the wall of the perpendicular channel with Cu-O bond distances of 2.06, 2.12, 2.14, and 2.44 Å. The Cu+ ion is only slightly above the plane of the sixmembered ring (see the top part of Figure 2). The P5 and M7 sites are over 10 kcal/mol higher in energy than the P6 site at both the QM-Pot and shell model potential levels. Al Atom in the T2 Position. The P6 site is the most stable
Extraframework Cu+ Sites in Ferrierite
Figure 2. The structure and coordination of the Cu+ ion at the P6(T1) site optimized at the singlet (upper part) and triplet (lower part) states. Clusters treated at the ab initio level (including link atoms) during the QM-Pot calculations together with Cu-O distances (in Å) are shown on the left. The location of the P6 site inside the ferrierite framework is shown on the right. Cu, circle; Al, black; Si, gray; O, dark gray; and cluster-terminating H, white.
Figure 3. The structure and coordination of the Cu+ ion at the P6(T1) and P6(T2) sites. Cu-O bond lengths obtained at the QM-Pot and shell model potential (in parentheses) levels are also depicted (in Å). Cu, circle; Al, black; Si, gray; O, dark gray; and cluster-terminating H, white.
site also with the Al atom in the T2 position. However, in this case the Cu+ ion is coordinated to only one oxygen atom of the AlO4 tetrahedron and two other framework oxygen atoms at the QM-Pot level. The P6 sites are compared in Figure 3 for the situations when the Al atom is in T1 or T2 position. M5 and I2 sites are about 4 kcal/mol less stable than the P6 site at
J. Phys. Chem. B, Vol. 105, No. 17, 2001 3513
Figure 4. The structure and coordination of the Cu+ ion at the M5(T2) site. Upper part: location of the M5 site inside the ferrierite framework. Lower part: cluster treated at the ab initio level (including link atoms) during the QM-Pot calculations. Cu-O bond lengths obtained at the QM-Pot and shell model potential (in parentheses) levels are also depicted (in Å). Cu, circle; Al, black; Si, gray; O, dark gray; and cluster-terminating H, white.
the QM-Pot level. The M5 site is depicted in Figure 4. The Cu+ ion is coordinated to only one oxygen atom of the AlO4 tetrahedron and one other oxygen atom of this ring. Al Atom in the T3 Position. In this case the most energystable structure is the M7 site, where the Cu+ ion is coordinated to two oxygen atoms of the AlO4 tetrahedron and one other oxygen of this six-membered ring. This site is very similar to the M7 site found for Cu+ in ZSM-5 (see Figure 4 in ref 7). The P7, M2, and M5 sites are within 7 kcal/mol of the M7 site. Al Atom in the T4 Position. The most stable is the P7 site, which is at the QM-Pot level about 4 kcal/mol more stable than the P2 site. At the P7 site the Cu+ ion is coordinated to one oxygen of the AlO4 tetrahedron and one other oxygen atom of the seven-membered ring (upper part of Figure 5). Since the T4 position is on the channel intersection, the P2 site could be denoted “I2”. However, to distinguish between this site and the I2 site when Al is in the T2 position (I2(T2)), we denote this site P2. This also reflects the fact that at this site the Cu+ ion is located more inside the perpendicular channel than on the channel intersection. In Table 2 the overall number of oxygen atoms in coordination with Cu+ (CN) and the number of oxygen atoms of the AlO4 tetrahedron in coordination with Cu+ (CN(OAl)) are reported. While the relative energies for individual sites are in reasonable agreement at the QM-Pot and interatomic potential function levels, the coordination numbers obtained at these two levels do not agree in several cases. Both CN and CN(OAl) obtained at the QM-Pot level are lower than those obtained at the interatomic potential function level. These changes can be explained by the topology of the rings in the ferrierite framework. During the QM-Pot optimization, the coordination of the Cu+ ion with those oxygen atoms that have their lone pairs oriented in a different direction than the Cu+ ion is located is lost. With the shell model potential level, structures with CN
3514 J. Phys. Chem. B, Vol. 105, No. 17, 2001
Figure 5. The structure and coordination of the Cu+ ion at the P7(T4) site optimized at the singlet (upper part) and triplet (lower part) states. Clusters treated at the ab initio level (including link atoms) during the QM-Pot calculations together with Cu-O distances (in Å) are shown on the left. The location of the P7 site inside the ferrierite framework is shown on the right. Cu, circle; Al, black; Si, gray; O, dark gray; and cluster-terminating H, white.
) 4 and 5 appear. However, most of the structures with CN ) 5 are more than 20 kcal/mol higher in energy compared to the most stable structure at a particular T position. If we consider only the structures optimized at both levels of theory, the average coordination number obtained at the shell model potential and QM-Pot levels is 3.3 and 2.5, respectively. 3.3. Cu+ Coordination in the Triplet State. The experimentally measured lifetimes of the triplet state are sufficiently long (on the order of microseconds)21 to ensure that in the electronically excited state the Cu+/FER system undergoes structure relaxation on the excited-state potential energy surface. Therefore, we studied the coordination changes upon excitation into the triplet state. The geometrical changes observed upon the excitation are summarized in Table 3. In the triplet state, the Cu+ ion moves away from the channel wall into the open space. Only small changes in the tripletoptimized structures are observed for the I2 and P2 sites. These changes are very similar to those observed for the I2 sites in Cu+/ZSM-5. The Cu-O bond distances shorten by at most 0.03 Å, and the coordination numbers remain unchanged. Also, for the P5 and M5(T2) sites the changes in geometry after excitation to the triplet state are relatively small. The coordination numbers do not change when going from the singlet to triplet state, and the bond distances between the Cu+ ion and the oxygen atoms of the AlO4 tetrahedron in coordination with Cu+ slightly shorten (by less than 0.05 Å). Although the Cu+ ion remains coordinated to non-AlO4 oxygen atoms, the Cu-O bond distances to the non-AlO4 oxygen atoms increase significantly, by 0.2 and 0.4 Å for the P5 and M5(T2) sites, respectively. In both cases, the Cu+ ion moves above the plane of the five-membered ring. These changes are more pronounced in case of the M5(T2) site. A somewhat different situation was found for the M5(T3) site, where the Cu+ ion loses its coordination to non-AlO4 oxygen atoms in the triplet state and it is coordinated to two AlO4 oxygen atoms.
Nachtigall et al. The M7 site becomes “I2-like” in the triplet state with very similar geometry changes as found for the Cu+/ZSM-5 system. The distances of the Cu+ ion to AlO4 oxygen atoms shorten by 0.02 and 0.17 Å and the distances to other oxygen atoms become larger by 0.8-2.0 Å. Although T3 site is not located on the channel intersection, the topology of the ferrierite framework does allow the Cu+ ion to move toward the open space on the channel intersection. In the P7 site, the Cu+ ion moves from the top of the seven-membered ring toward the channel intersection. During this change, Cu+ loses the coordination to the non-AlO4 oxygen atom (the change in Cu-O bond distance is 3.0 Å), while the CN(OAl) increases from 1 to 2. The resulting structure is “P2-like” (Figure 5). The changes observed for the P6 sites are similar to those found for the Z6 sites in the Cu+/ ZSM-5 system. The Cu+ ion remains coordinated to AlO4 oxygen atoms only and it is tilted away from the channel wall into the open space (Figure 2). 3.4. Vertical Excitation Energies. The vertical excitation energies calculated at the singlet-optimized structures for various Cu+ sites together with their coordination numbers are summarized in Table 4. When the Cu+ ion is located in the I2 or P2 site, the excitation energies are about 2.1 eV (590 nm). When the Cu+ ion is coordinated to the five-, six-, or seven-membered ring, the excitation energies are at least 0.9 eV larger than for the I2 and P2 sites. Cu+ in the P6 and P7 sites has the highest excitation energies (3.42-3.75 eV), while all excitations due to the P5, M5, and M7 sites fall into the range 3.01-3.16 eV. Thus, in the excitation scan of the photoluminescence spectra of the Cu+/FER system we expect one narrow, lower energy peak, well-separated from the broad higher energy band. This higher energy band could possibly be separated into two or three peaks. Unlike Cu+/ZSM-5, there is no straight correlation between the total coordination number and the excitation energy for Cu+/ FER. This correlation holds only for the excitation energies of the sites with CN(OAl) ) 2. A different situation is observed for the Cu+ sites with CN(OAl) ) 1. In these structures the Cu+ ion is coordinated to one oxygen atom of AlO4 and one or two oxygen atoms on the opposite site of the ring. The singlettriplet splitting is largely determined by the repulsive interaction of the electron in the Cu(4s) orbital and the lone pairs on the surrounding oxygen atoms. To lower this repulsion, the Cu(4s) orbital must be directed into the open space. The NBO analysis carried out for the model system shows that this repulsion is lower for the pyramidal than for the planar coordination of the Cu+ ion. The vertical excitation energies found for structures with CN(OAl) ) 1 correlates well with the height of the Cu+ ion above the ring: the higher the Cu+ ion, the smaller the excitation energy. 3.5. Vertical Emission Energies. The vertical emission energies were calculated as the difference between the groundstate singlet and the lowest triplet states at the triplet-optimized geometries. In our recent study of the emission spectra of the Cu+/ZSM-5 system, we have shown that the relative emission energies depend on the number of oxygen atoms in the “second coordination shell” (2.5-3.25 Å). Due to the large size of the Cu+(4s) orbital, the repulsive interaction between the electron in the Cu+(4s) orbital and the electrons in the lone pairs of the oxygen atoms is important even for distant oxygen atoms, up to 3.25 Å. Table 5 shows the vertical emission energies calculated for several Cu+ sites, together with their coordination numbers (oxygen atoms within a distance of 2.5 Å) and the number of oxygen atoms within the distance 3.25 Å. The sites where in the triplet state the Cu+ ion is located on the channel
Extraframework Cu+ Sites in Ferrierite
J. Phys. Chem. B, Vol. 105, No. 17, 2001 3515
TABLE 3: Cu-O Distances (in Å) for Individual Types of Cu+ Sites for Triplet and Singlet Statesa site type
Al position
Cu-O(Al)b
Cu-Oc
I2 P2 P5 M5 M5 M7 P6 P7 P6
T2 T4 T1 T3 T2 T3 T1 T4 T2
2.02 (2.05), 2.08 (2.09) 2.04 (2.06), 2.06 (2.08) 1.97 (1.96), 3.14 (2.76) 2.10 (3.51), 2.11 (1.99) 1.99 (2.03), 3.30 (2.74) 2.03 (2.05), 2.03 (2.03) 2.07 (2.06), 2.07 (2.14) 2.03 (1.96), 2.04 (3.08) 2.01 (2.03), 3.38 (2.93)
3.28 (3.19), 3.48 (3.39), 4.02 (3.91), 4.20 (4.15) 3.45 (3.17), 3.52 (3.32), 4.02 (3.98), 4.23 (4.11) 2.28 (2.06), 3.65 (3.21), 4.11 (3.73) 6.10 (2.34), 6.18 (2.31) 2.44 (2.04), 2.85 (2.65), 3.25 (3.35), 3.83 (3.45) 4.05 (3.23), 4.07 (2.10), 4.27 (2.72), 5.59 (3.81) 2.85 (2.12), 2.97 (2.44), 3.74 (3.28), 3.80 (3.44) 3.40 (5.15), 3.54 (3.79), 5.03 (2.02), 4.37 (2.86) 2.57 (2.04), 2.88 (2.31), 3.45 (2.55), 4.06 (3.50)
a The bond lengths in parentheses are for singlet-optimized structures. b Cu-O distances for oxygen atoms of AlO4 tetrahedron. c Cu-O distances for oxygen atoms of non-AlO4 tetrahedra.
TABLE 4: The Vertical T1 r S0 Excitation Energies (in eV) a
site type
Al position
E(T1rS0)
CN
I2 P2 P5 M5 M5 M7 P6 P7 P6
T2 T4 T1 T3 T2 T3 T1 T4 T2
2.05 (605) 2.11 (588) 3.01 (412) 3.08 (403) 3.16 (392) 3.16 (392) 3.42 (363) 3.73 (332) 3.75 (331)
2(2) 2(2) 2(1) 3(1) 2(1) 3(2) 4(2) 2(1) 3(1)
a Number of oxygen atoms in coordination with Cu+. The number of O atoms of AlO4 tetrahedron in coordination with Cu+ is given in parentheses.
TABLE 5: The Vertical S0 r T1 Emission Energies (in eV) Calculated at the Triplet-Optimized Geometries site type
Al position
E(S0rT1)
CN(
