Adsorption of NO in Fe2+-Exchanged Ferrierite. A Density Functional

Simulated adsorption of NO shows that both ON− and NO− interactions with Fe2+ are at least metastable. Adsorption through the N atom, however, is ...
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J. Phys. Chem. C 2007, 111, 586-595

Adsorption of NO in Fe2+-Exchanged Ferrierite. A Density Functional Theory Study L. Benco,*,†,‡ T. Bucko,† R. Grybos,† J. Hafner,† Z. Sobalik,§ J. Dedecek,§ and J. Hrusak§ Institut fu¨r Materialphysik and Center for Computational Materials Science, UniVersita¨t Wien, Sensengasse 8, A-1090 Wien, Austria, Institute of Inorganic Chemistry, SloVak Academy of Sciences, DubraVska cesta 9, SK-84536 BratislaVa, SloVak Republic, and J. HeyroVsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejskoVa 3, CZ-18223 Prague 8, Czech Republic ReceiVed: August 30, 2006; In Final Form: NoVember 2, 2006

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 ∼2.5 times stronger. Two types of Fe-exchanged configurations were observed. Stable configurations with the cation located in a β site and exhibiting low adsorption energies of ∼180 kJ/mol were destabilized upon adsorption of NO. Less stable configurations, with the cation located in an R site and with higher adsorption energies of ∼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 σ and π bonding. σ bonding depletes the electron density in Fe dσ orbitals and leads to accumulation in the N pσ orbital. The π bonding causes an increase of the π-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 pσ density is accompanied by a depletion of the N pπ density oriented toward the O atom. On the contrary, the O atom exhibits a depletion of the σ-electron density and an increase of the π-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 π 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.

1. Introduction Iron-exchanged zeolites are active catalysts for the decomposition of N2O to N2 and O2 and are therefore potentially useful materials for the removal of N2O from industrial waste stream.1-12 The active sites are either bare Fe2+ cations or iron oxo species.13,14 The characterization of configurations with extraframework Fe2+ cation represents an extremely difficult task because of the low iron concentration in the active catalyst. NO is formed on the surface of Fe2+-exchanged zeolite as a reaction intermediate during the decomposition of N2O. The addition of extra NO accelerates the decomposition of nitrous oxide over the Fe zeolite. NO thus appears to be an important inorganic agent. Strong bonding of NO to Fe species provides * Corresponding author. E-mail: [email protected]. † Universita ¨ t Wien. ‡ Slovak Academy of Sciences. § Academy of Sciences of the Czech Republic.

peaks in IR spectra that are clearly detected even on samples with an Fe/Si ratio as low as 1/1000. NO is therefore extensively used as a sensitive probe molecule for iron species in zeolites.15-17 The properties of Fe2+ and Fe3+ cations occupying extraframework positions in zeolites have been the subject of several theoretical studies. Methane-to-methanol and benzeneto-phenol conversions have been analyzed by DFT calculations on small clusters.18 Decomposition of nitrous oxide over FeZSM-5 was investigated by Liu et al.19 The reactions of methane with [FeO2]+ and [OFeO]+ cations exchanged into ZSM-5 were investigated using DFT by Bell et al.14 Iron exchanged into ferrierite has been investigated by Broadbelt et al.20 and by van Santen et al.21 Broadbelt et al. determined the location of divalent metal cations, including Fe2+ cations, on the framework of ferrierite. They performed DFT calculations on small clusters extracted from the ferrierite framework, corresponding to R and β sites.22 Their calculated

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Adsorption of NO in Fe2+-Exchanged Ferrierite

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Figure 1. (a) Top view of the ferrierite structure showing the main channel (MC) and the side channel (SC). Dashed lines indicate the unit cell with Immm symmetry. (b) Positions of the tetrahedral T1-T4 sites and the three most populated positions of extraframework cations A, B, and C (ref 27).

infrared bands are consistent with the occupation of these sites by the metal cations. To keep the small cluster neutral, however, their model contains two Al atoms placed at a short distance. Such a model corresponds to a high-Al zeolite with a large Si/ Al ratio. Low-Al zeolite, requiring large and computationally demanding clusters, was not considered in their study. The full periodicity of the ferrierite framework was considered by van Santen et al.21 They investigated the properties of an Fe2+ cation occupying one specific site. The R site of ferrierite was chosen as the initial model because of its similarity with the R site in ZSM-5. They predicted the reaction path for benzene oxidation by N2O in reasonable agreement with previous cluster calculations.23 The activity of the cation depends on the local zeolite environment. Details of the local environment are difficult to investigate because experimental methods provide only a signal averaged over all local configurations in the zeolite sample. Using tools of solid-state chemistry and physics, we therefore performed an extensive study of local configurations of Fe2+ in ferrierite. For a divalent cation exchanged into an extraframework position of a zeolite, a crucial factor governing stability is the distance between Al atoms substituted into tetrahedral (T) sites of the framework. We therefore investigated models for both low-Al and high-Al ferrierite considering Al/Si substitutions in all irreducible T sites. Adsorption properties of representative configurations were tested via adsorption of NO. We evaluated bonding to Fe2+, adsorption energies, and finally, NO stretching frequencies depending on the local zeolite environment. 2. Structure and Computational Details The structure of ferrierite is displayed in Figure 1. The pore system of the ferrierite framework is classified as onedimensional.24 The main channel (MC), extending along the c lattice vector (cf. Figure 1a), is circumscribed by 10-membered rings (10MRs). The MC intersects smaller channels surrounded by 8MRs, extending along the b lattice vector (not displayed in Figure 1). Parallel to the MC stretches a narrower channel denoted as a side channel (SC, cf. Figure 1a). The experimental cell parameters of hydrated ferrierite are a ) 19.156 Å, b ) 14.127 Å, and c ) 7.489 Å.25 For dehydrated samples, slightly shorter cell parameters are observed. For K-exchanged dehydrated ferrierite, Cheetham et al.26 reported cell parameters of a ) 18.651 Å, b ) 14.173 Å, and c ) 7.404 Å. Positions of the four irreducible T sites T1-T4 are indicated in Figure 1b.

Circles denoted as A, B, and C in Figure 1b indicate possible positions of extraframework cations.27 The unit cell with Immm space group symmetry contains 108 atomic positions, 36 tetrahedral (T) sites, and 72 O sites. Our calculations were performed for the unit cell of dehydrated ferrierite26 using the Vienna ab initio Smulation Package (VASP).28-31 The spin-polarized Kohn-Sham equations of DFT were solved variationally,32 using the local exchange-correlation functional of Perdew and Zunger33 corrected for nonlocal effects according to Perdew et al.34 (PW91). Ultrasoft pseudopotentials35,36 and a plane-wave basis set were used as implemented in VASP.31 The atomic pseudopotentals were transformed to the all-electron potential using the Blo¨chl’s projector augmentedwave technique.37,38 The plane-wave basis set was expanded to a cutoff energy of 400 eV. Brillouin-zone sampling was restricted to the Γ point. Convergence was improved using a modest smearing of eigenvalues. During the relaxation of the atomic coordinates, no symmetry restrictions were applied. The harmonic stretching frequency of NO molecule adsorbed on the Fe2+ cation in different configurations was calculated using the finite-differences method.39 The correction for anharmonicity was evaluated with the program Anharm.40 3. Results and Discussion 3.1. Stability of Al/Si Substitutions. The properties of extraframework Fe2+ cations, which represent the strongest active sites of the zeolite, depend strongly on the stability of the whole configuration. The stability of the metal-exchanged zeolite is a complex function of the locations of Al/Si substitutions in the framework and of the locations of the extraframework Fe2+ cations. Moreover, with increasing Si/Al ratio, the concentration of Al atoms decreases to such an extent that each Fe2+ cation can interact with just a single Al atom in the framework. Compared to divalent cations interacting with two Al/Si substitutions, the stability of a cation placed next to a single Al atom is much lower. Because of their lower stability, such sites exhibit a higher reactivity.41 The Si/Al ratio therefore appears to be an important factor driving the reactivity of active sites in the zeolite. 3.1.1. Al/Si Substitutions. Different T sites of the zeolite framework exhibit different flexibilities and capacities to accommodate Al atoms. The positions of the four irreducible T sites in the ferrierite structure and the total energies of single Al/Si substitutions in the corresponding T sites are displayed in Figure 2. The reduced number of electrons, introduced by

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Figure 2. Energetics of Al/Si substitutions in different T sites (kJ/ mol). A fragment of the structure shows the location of four irreducible T sites in the 10-membered ring of the main channel (MC) and the 6-membered ring of the side channel (SC). Solid circles indicate four irreducible tetrahedral sites (T1-T4). Numbers in parentheses are energies of the Al/Si substitution of the T site relative to the most stable T2 site.

the substitution of Si by Al, is balanced by a homogeneous background charge density equivalent to a proton. This avoids the added complexity of different locations of the proton compensating for the charge of Al/Si substitution. The energies for Al/Si substitution in the four different T sites of ferrierite differ by up to 22 kJ/mol. The most stable T site connects the six-membered ring of the SC with the fivemembered ring forming the side wall of the MC. The capacity to accommodate an Al atom is slightly decreased for the T3 site (+13 kJ/mol) and the T4 site (+15 kJ/mol). The T1 site is the most disadvantaged site, with a penalty of +22 kJ/mol. Surprisingly, in mordenite, a similar site connecting the eight-, six-, and five-membered rings is the most advantaged T site for Al/Si substitution. This indicates that the properties of the T sites in zeolites not only are driven by the local symmetry of the site, but also depend on the topology of the framework. 3.1.2. Fe-Exchanged Ferrierite. The total energy of the Feexchanged zeolite depends on the number of efficient contacts formed between the Fe2+ cation and the O atoms of the framework surrounding the Al/Si substitution. Other important factors contributing to the stability of a particular configuration are the local flexibility of the framework supporting the formation of bonds between the extraframework cation and the O atoms of the framework and the stability of Al/Si framework substitutions. A divalent Fe2+ cation exchanged into a zeolite forms very stable configurations when connected to two Al/Si framework sites. The distances between pairs of Al/Si framework sites, however, are controlled by the Si/Al ratio. A strong binding of Fe2+ to two Al/Si substitutions occurs for Al-Al distances shorter than ∼7 Å when both Al atoms reside in the same 6MR. In ferrierite, with Al/Si substitutions distributed randomly over the framework, such configurations exist only for Si/Al ratios smaller than approximately 6. In materials with larger Si/Al ratios, the Fe2+ cation connects to only one Al/Si framework site. The total energies of Fe2+-exchanged ferrierite are displayed in Figure 3 for typical representative structures containing Al/ Si substitutions in different T sites. Geometric parameters are collected in Table 1. In configurations 1-4, the Fe2+ cation connects to two Al/Si substitutions, and in configurations 5-8, the cation interacts with a single Al/Si substitution. Configurations favoring strong bonding of the extraframework cation are in the 6MR with two Al/Si substitutions where the cation forms four Fe-O bonds. The most stable configurations are 1 and 2, with two Al atoms placed at a short distances ∼5.7 and ∼4.2 Å. In the former, Fe2+ occupies the β site, and the latter occupies

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Figure 3. Stable configurations for Fe2+ cations interacting with Al/ Si substitutions in different framework T sites. In configurations 1-4, the Fe2+ cation connects to two Al/Si substitutions, and in configurations 5-8, the Fe2+ cation binds to an isolated Al/Si substitution. The inset shows positions of adsorption sites (R and β site) in the six-membered rings of the main channel (MC) and in the side channel (SC). The solid circles represent the framework Al/Si substitution, and the empty circles represent the Fe2+ cation.

the R site. Configuration 2 is disadvantaged by 88 kJ/mol. This energy, however, was calculated for two Al atoms in neighboring T sites, i.e., violating the Loewenstein rule.42 Such a configuration results from the stacking of the flat unit cell of ferrierite in the c direction. Judging from our studies of Al/Si distribution in zeolites, we estimate that the violation of the Loewenstein rule introduces a penalty of ∼30 kJ/mol. Configuration 2 thus appears to be ∼58 kJ/mol less stable than structure 1 (cf. dashed line below configuration 2 in Figure 3). The local geometry of configurations 1-4 is displayed in Figure 4. In configuration 1, the Al/Si framework substitutions are in the T4 sites, and in 2, they are in the T3 sites. These two sites exhibit similar energetics for the Al/Si substitution (cf. Figure 2). The reason for the lower stability of the R site (configuration 2) is the lower local flexibility of the framework caused by the Al-Si-Al bridge connecting two Al sites on the back of the 6MR and fixing the diameter of the ring (cf. Figure 4f). The much larger Fe2+ cation therefore cannot fit into the ring and remains locked in a position above the plane of the ring with an O-Fe-O angle of ∼144° (cf. Figure 4b and 4f). In contrast, the lack of any cross-bridging in the β site allows an expansion of the 6MR, leading to an increase of the Al-Al distance to 5.65 Å in configuration 1, compared to 4.12 Å in 2. In the expanded 6MR of the β site, the Fe2+ cation is accommodated directly in the plane of the 6MR, with O-Fe-O angles close to 180°. In this position, the bonding of Fe with four O sites of the zeolite is more efficient. Side views of two configurations with the Fe2+ cation in the β and R sites are displayed in Figure 4e and f, respectively. In the less stable configuration 2, a slight elongation of the average Fe-O bond length to ∼2.02 Å is observed. NMR data indicate that a connection of the two Al atoms via an Al-Si-Si-Al chain is more frequent than a connection via an Al-Si-Al chain. This observation also agrees with the higher stability of configuration 1. Configurations 3 and 4 are considerably less stable than configuration 1. Two Al/Si substitutions in opposite corners of the 6MR lead to penalties +116 and +151 kJ/mol for 3 and 4, respectively. In both 3 and 4, a quasiplanar tetrahedral coordination of the Fe2+ cation is formed. In contrast to the more stable configurations 1 and 2, however, only two bonds bind to an O atom next to the Al site. Therefore, configurations 3 and 4 are less stable than configurations 1 and 2. The most efficient bonding leads to an Fe-O bond length of 2.00 Å or slightly

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TABLE 1: Total Energies and Geometric Parameters of Stable Fe-FER Structuresa configuration

1

2

3

4

5

6

7

8

type of site location(s) of Al Etot rAl-Al rFe-O

β T4, T4 0 5.650 1.996 1.998 2.009 2.010 2.003 176

R T3, T3 +88 (+58) 4.120 2.002 2. 003 2.041 2.047 2.023 144

β T2, T2 +116 6.670 1.965 1.980 2.094 2.124 2.041 172

R T1, T1 +151 6.847 2.015 2.018 2.130 2.136 2.075 142

β T4, T3 +148 10.03 1.959 1.975 2.045 2.078 2.014 175

β T2, T3 +204 7.08 1.931 2. 024 2.110 2.263 2.082 160

R T3, T1 +214 7.84 2.004 2.063 2.102 2.162 2. 083 138

γ T1, T3 +246 10.35 2.024 2.046 2.163 2.242 2.119 145

rFe-O (avg) O-Fe-O (avg) a

Energies in kJ/mol, distances in angstroms, and angles in degrees.

Figure 5. Geometries of configurations (a) 5, (b) 6, (c) 7, and (d) 8 with the extraframework Fe2+ cation connected to an isolated Al atom in different T sites. Configurations 5 and 6 accommodate the cation in the β site, and configuration 7 accommodates the cation in the R site. Configuration 8 forms upon Al/Si substitution in the T1 site with Fe2+ in the off-center position in the side channel (γ site, cf. ref 22). Figure 4. Geometries of the stable configurations (a) 1, (b) 2, (c) 3, and (d) 4 containing the Fe2+ cation in the 6MR with two Al/Si substitutions. Configurations 1 and 3 accommodate the cation in the β site of the side channel, and configurations 2 and 4 accommodate the cation in the R site of the main channel. Side views of configurations (e) 1 and 3 and (f) 2 and 4.

shorter (cf. bond distances of configuration 1 and 2 in Table 1). In configuration 3, only two such bonds are formed, and in configuration 4, all four Fe-O bonds are longer than 2.00 Å. Configuration 3 is 35 kJ/mol more stable than configuration 4. The reason is the lower energy of the Al/Si substitutions and the higher local flexibility of the β site. A single Al/Si substitution in the T2 site of configuration 3 is 22 kJ/mol more stable than a substitution in the T1 site of the configuration 4 (cf. Figure 2). The higher flexibility of the β site allows for more efficient Fe-O bonding when the Fe2+ cation resides in the 6MR. In configuration 3, the distance between the two Al atoms is 6.67 Å, whereas in configuration 4, the Al-Al distance is 6.85 Å. The reduced Al-Al distance leads to the formation of two shorter Fe-O bonds similar to those in the considerably more stable configurations 1 and 2 (cf. Table 1). The better local flexibility of the β site allows the accommodation of the Fe2+ cation almost in the plane of the four framework O atoms, whereas in the R site, the extraframework cation resides above the plane. The corresponding average O-Fe-O angles are 172° and 142° for 3 and 4, respectively (cf. Table 1). Configurations 5-8 are formed in ferrierite with large Si/Al ratios and also corresponding to large Al-Al distances. Because the Fe2+ cation connects to only one Al/Si substitution, configurations 5-8 are less stable than configurations 1-4. In analogy to configurations 1-4, Fe2+ is more stable in the β site than in the R site. Interesting is a comparison of the

stabilities of configurations 5 and 4. The stabilizing effect of two Al atoms located in opposite corners of the R site (configuration 4, Figure 4d) is smaller than the effect of a single Al in the T4 site of the β site (configuration 5, Figure 5a). A stability comparable to that of configuration 4, however, is achieved only for configuration 5 with a symmetrical location of Al in T4. In configuration 6 (Figure 5b) with a single Al atom located in the corner of the 6MR, the extraframework Fe2+ cation resides approximately in the center of the 6MR as in configuration 5. The stability of configuration 6, however, decreases by more than 50 kJ/mol compared to configuration 5 because the Fe2+ cation connects only to one O atom activated by a neighboring Al (Fe-O bond length 1.93 Å, cf. Table 1). Configuration 7 places the Fe2+ cation in an R site with one Al atom in position T3. This site is the least stable site, with the Fe2+ cation in the 6MR. The T1 site, located in the 8MR separating the MC and the SC, is the most disadvantaged T site for Al/Si substitution (cf. Figure 2). The Fe2+ cation connected to this Al site can reside in the MC in the 6MR or in the 5MR or moves through the 8MR into the SC (cf. Figure 5d). The most stable configuration forms in the SC, with the Fe2+ cation on the framework interacting with four O atoms of the 6MR. The most stable configuration 1 exhibits the shortest average Fe-O distance of ∼2.00 Å (Table 1). With decreasing stability of configurations 1-4, the average Fe-O distance smoothly increases to a value of ∼2.08 Å, and for configurations 5-8, it increases from ∼2.01 to ∼2.12 Å. The average Fe-O distance is therefore a good indicator of the stability of the configuration. Configurations 5-8 are realistic local structures in ferrierite with Si/Al ratios larger than 6. The compilation of extraframe-

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Benco et al.

Figure 7. Energy-level scheme for the interaction of the N pz orbital with the Fe dz2 orbital. Solid/dashed horizontal lines show the band positions of majority/minority states. Arrows indicate filling of energy levels with electrons. The small arrow represents half-filling of the band of spin-down Fe dz2 states. Figure 6. Spin-polarized partial density of states of the (a) N pz and (b) Fe dz2 orbitals responsible for the σ interaction of NO with Fe2+. Curves in the top/bottom panel show states before/after the adsorption of NO. The Fermi level is adjusted to 0 eV (dashed line).

work sites given by Mortier27 indicates that configurations with a position of the cation as in configurations 5-8 exist. The position of the cation in the center of the β site (5 and 6) corresponds to the A site according to Mortier’s notation.27 The center of the R site (7) corresponds to the B site, and the strongly off-center position in the side channel of configuration 8 corresponds to Mortiers’s C site. 3.2. Adsorption of NO. Adsorption of NO is simulated by adding a single NO molecule to the extraframework cation in Fe-exchanged ferrierites, including the eight typical configurations displayed in Figure 3. The analysis of the bonding is based on the evaluation of densities of states and differential electron densities. Adsorption energies were compared for configurations with different stability and for different locations of the extraframework Fe2+ cation. Finally, the NO stretching frequencies were calculated and compared with experimental data. 3.2.1. Bonding of NO to Fe2+. NO is a molecule able to form bonds through both the N and the O atoms. The energetics of N and O adsorption is discussed in section 3.2.2 below. A detailed analysis of the bonding was performed for adsorption of NO via the N atom in configuration 3 (cf. Figure 3 and Table 1). Figure 6 documents the effect of the Fe-to-N bonding on the σ-electron density of states (DOS). Displayed are the partial DOS of the N pz (Figure 6a) and the Fe dz2 orbitals (Figure 6b) before (top) and after (bottom) the adsorption of NO on Fe2+. In a free NO molecule, the N pz orbital represents the main component of the nonbonding 5σ orbital at approximately -7 eV. Upon adsorption, the N pz states are downshifted to approximately -8 eV, exhibiting the bonding character of the interaction (Figure 6a, bottom). No major differences are observed between spin-up and spin-down N pz states. The change of bonding on Fe2+ is documented by the partial DOS of the Fe dz2 orbital (Figure 6b). Before adsorption, the band of the majority states is completely filled, and the band of the minority states is half-filled (Figure 6b, top). Upon adsorption, the Fe dz2 states hybridize with N pz states, forming a bonding DOS at approximately -8 eV. The main part of the Fe dz2 majority states moves from about -2.5 to about -1.0 eV and

Figure 8. Spin-polarized partial density of states of the (a) Npx and (b) Fe dxz orbitals taking part in the π interaction. Curves in the top/ bottom panel show states before/after the adsorption. The Fermi level is adjusted to 0 eV.

remains occupied. The band of minority Fe dz2 states moves above the Fermi level to approximately +2.0 eV. The energy-level scheme for the interaction of the N pz orbital with the Fe dz2 orbital is displayed in Figure 7. The hybridization of the Fe dz2 states with the N pz states of the 5σ band of the NO molecule causes a stabilization of the N pz states and a destabilization of the Fe dz2 states. Both bands of majority and minority Fe dz2 states are pushed upward by ∼1.5-2.0 eV. The band of majority states remains occupied, but the band of minority states becomes completely depleted. The adsorption of the NO molecule on the Fe2+ cation thus induces a depletion of the minority spin dz2 density. The effect of adsorption on the π-electron density is documented in Figure 8 by the partial DOS of the N px and the Fe dxz states, and the energy-level scheme for the N px-Fe dxz interaction is displayed in Figure 9. In free NO (Figure 8a, top), π states constitute the 1π and 2π* spin-up molecular orbitals at approximately -8 eV and at the Fermi level, respectively.

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Figure 9. Energy-level scheme for the interaction of the N px orbital with the Fe dxz orbital.

The spin-down energy levels are located at about -7 and about +1 eV, and the exchange splitting is ∼1 eV compared to the spin-up states. Bonding to Fe2+ leads to the overlap of the N px orbital with the Fe dπ orbital. Both the 1π and 2π* bands of N px spin-up states are shifted to higher energies (Figure 8a, bottom). The 1π spin-up band moves to approximately -7 eV, and the 2π* band moves to approximately +2 eV. The position of the 1π spin-down band remains unchanged: it is centered at approximately -7.5 eV. The largest change within the NO molecule induced by the adsorption to the Fe2+ cation is observed for the 2π* spin-down molecular orbital. The band at approximately +1 eV splits into the bonding and antibonding component at about -1 and about +2 eV, respectively. The main Fe dxz band of majority states is centered at approximately -2.5 eV, and a flat band of states of more bonding character ranges from about -3 to about -5 eV (Figure 8b, top). In analogy with spin-up states of the NO molecule, no major change of spin-up energy states is observed upon adsorption. The Fe dxz band moves slightly upward to approximately -2 eV, and a low-intensity band is formed at approximately -7 eV. The Fe dxz band of minority states at approximately +1 eV splits into components at about -1 and about +2 eV. The oneto-one correspondence with the splitting of the N px spin-down states indicates the strong covalent character of the N pπ-Fe dπ interaction (cf. Figure 8a, bottom, and Figure 8b, bottom). The energy-level scheme, displayed in Figure 9, shows that adsorption destabilizes spin-up states of both the 1π and 2π* molecular orbitals of the NO molecule. The Fe dxz spin-up states split into the strongly bonding low-intensity band at approximately -7 eV and the main band slightly shifted from about -2.5 to about -2 eV. No change is observed for the spindown states of the 1π molecular orbital of NO. The spin-down states of both the 2π* orbital of the NO molecule and the Fe dxz orbital of the Fe2+ cation are empty, located at approximately +1 eV. Both the overlap of the N px orbital and the Fe dxz orbital and the close position of the energy levels lead to efficient bonding-antibonding splitting, with bonding N pπ-Fe dπ states at approximately -1 eV and antibonding states at approximately +2 eV. This bonding causes the transfer of the electron density within the N atom from spin-up to spin-down states and leads to a filling of the Fe dπ states. Figure 10 displays the change of the electron density distribution induced by the bonding of the NO molecule to the Fe2+ cation. Upon adsorption of NO, a complex polarization of both the adsorbed molecule and the transition metal cation

Figure 10. (a) Differential electron density showing the change of electron density induced by adsorption of NO on Fe2+. Dark/light gray areas indicate depletion/accumulation of the electron density. (b) Cut of the differential electron density. Dotted lines show N pz and Fe dxz orbitals.

occurs, but no change in the distribution of the electron density of the zeolite framework is observed. The newly formed Fe-N bond induces a change of both σand π-electron densities. Electron density is depleted from Fe dσ orbitals. The energy-level scheme for σ bonding (Figure 7) shows that only the spin-down electron density is depleted. The two interacting σ levels, N pz and Fe dz2, displayed in Figure 7, accommodate 3.5 electrons. The bonding states, formed by the N pz-Fe dz2 interaction, contain only 3 electrons. On the other hand, the interacting π levels, N px and Fe dxz, contain 3.5 electrons, and the π-bonding states accommodate 4 electrons (Figure 9). The balance of the filling of the energy levels thus indicates that adsorption of NO on Fe2+ causes a transfer of the electron density within the Fe2+ cation from σ states to π states. A pronounced increase of the π-electron density is observed between the Fe2+ cation and the N atom of the adsorbed molecule. The energy-level scheme of the π bonding (Figure 9) shows that new bonding states are formed from empty N px and empty Fe dxz spin-down states. These N px-Fe dxz bonding states are filled with one electron. The corresponding electron density is proportionally withdrawn from the 2π* molecular orbital of the NO molecule on one side and from the σ Fe dz2 orbital on the other side (Figure 7). On the N atom, a polarization of the N pπ-electron density is observed (Figure 10). A decrease of the N pπ density occurs on the side oriented toward the O atom, and a large increase of the N pπ density is oriented toward the Fe2+ cation. The energy-level scheme (Figure 9) shows that depleted states correspond to the 2π* spin-up electron density and the increase is due to the filling of the N px-Fe dxz bonding 2π* formed by the spin-down molecular orbital and the Fe dxz spin-down states.

592 J. Phys. Chem. C, Vol. 111, No. 2, 2007 Within the NO molecule, the changes of the σ- and π-electron density on the N atom and the O atom are of the opposite character. A slight expansion of the σ-electron density and a compression of the π-electron density on the N atom are accompanied by a compression of the σ-electron density and an expansion of the π-electron density on the O atom. The effect of this polarization of the electron density on the N-O bond length and the N-O stretching frequencies is discussed later. The energy-level scheme of the bonding of NO on the Fe2+ cation is qualitatively different from that developed by Blyholder for the interaction of CO and NO with a surface of the transition metal.43 According to the classical Blyholder scheme, the interaction of CO and NO with the metal surface induces σ donation of the electron density to the transition metal and π back-donation. A pronounced back-donation leads to the elongation of the bond length and to a decreased stretching frequency of the adsorbed molecule.44 For NO on the Fe2+ cation, no transfer of the electron density between the adsorbed molecule and the extraframework cation is observed. The bonding electron density is proportionally depleted from both interacting bodies and accumulated between the cation and the adsorbed molecule. The interaction between the partly occupied 2π* orbital of the NO molecule and the unoccupied dπ spindown states of the Fe2+ cation plays a dominant role. The overlap of these two orbitals leads to the formation of π-bonding states filled by 2π* electrons of the NO molecule and by electrons withdrawn from dσ states of the Fe2+ cation. The formation of the electron pair leads to a decrease of the magnetic moment of the NO-Fe2+ cluster compared to the magnetic moment of the Fe2+ cation (cf. below). Because of the formation of the bond between the molecule and the cation, the adsorbed molecule is depleted of electron density. Because of the antibonding character of the depleted electron density, the N-O bond becomes stronger upon adsorption. Adsorption thus leads to a shortening of the bond length and to an increase of the stretching frequency (cf. below). A similar bonding pattern is observed for adsorption on cations of nontransition metals.45 The CO molecule on the extraframework Al3+ cation exhibits a pronounced increase of the electron density between the cation and the adsorbed molecule. The bond is shortened and the stretching frequency is blue-shifted from 2140 cm-1 in the free CO to ∼2230 cm-1 in the adsorbed molecule.45 3.2.2. Adsorption Energies. NO can form adducts with the extraframework Fe2+ cation through both the N and the O atoms. The O bonding, however, is approximately 2.5 times weaker. For example, for configurations 3 and 6 (cf. Figure 3 and Table 1), the adsorption energies for the N/O adducts are 175/68 and 192/80 kJ/mol, respectively. A similar behavior is observed for adsorption of CO on active sites of zeolites. Adsorption energies for C bonding are ∼50% higher than those for O bonding. Because of the relatively small difference between the C and O bonding, both the C and O adducts are observed in spectra at low temperatures. Compared to CO, the NO molecule exhibits a much larger difference in adsorption energies for adsorption through different terminal atoms, and therefore, the NO molecule binds to the active sites of zeolites only through the N atom. Figure 11 shows the dependence of the adsorption energy on the total energy of a set of Fe2+-FER configurations, including the eight configurations compared in Figure 3 and Table 1. The energy of each particular configuration is given relative to the total energy of the most stable configuration 1 (cf. Figure 3 and Table 1). The calculated adsorption energies range between 175 and ∼260 kJ/mol. With decreasing stability of the Fe zeolite configuration, the adsorption energy of NO

Benco et al.

Figure 11. Adsorption energies of a single NO molecule adsorbed on an Fe2+ cation in FER, calculated for different configurations, vs the relative energy of each structure. Two solid lines are guides to the eye indicating two different types of active sites.

Figure 12. Total energy of Fe-FER before and after adsorption of a single NO molecule, relative to that of the most stable configuration 1 (cf. Figure 3 and Table 1). Structures below (above) the diagonal are stabilized (destabilized) upon adsorption of NO.

increases. Solid lines indicate the existence of two different types of active sites. Interestingly, these types are not distinguished according to the number of Al atoms interacting with the Fe2+ cation. Each group comprises configurations with both one Al/ Si substitution and two Al/Si substitutions. Evidently, the adsorption capacity of the Fe2+ cation is driven by the bonding of the transition metal to the zeolite framework. In configurations exhibiting lower adsorption energies, the extraframework Fe2+ cation resides directly in the plane of the framework O atoms of the side channel and forms four in-plane Fe-O bonds (Figure 4a,c). Strong bonding with the framework decreases the adsorption capacity of the Fe2+ cation. The adsorption energies of configurations with strongly stabilized Fe2+ vary between 175 and 190 kJ/mol. All configurations with Fe2+ in the R site exhibit higher adsorption energies ranging between 220 and ∼250 kJ/mol. To this group belong also the relatively stable configurations 2 and 4 with two Al sites in the same ring. A typical feature of all configurations with higher adsorption energies is the location of Fe2+ above the plane of the framework O atoms. In this position, the bonding capacity of the Fe2+ cation is not saturated by the interaction with the framework O atoms, and therefore, Fe2+ binds adsorbed molecules more strongly.

Adsorption of NO in Fe2+-Exchanged Ferrierite

Figure 13. Change of the total energy upon migration of the Fe2+ cation from the side channel (SC) into the eight-membered ring (8MR) between the SC and the MC. Migration of (a) the bare cation, (b) the cation with a single adsorbed NO molecule, and (c) the cation with two adsorbed NO molecules. Large (small) empty circles are Si (O) atoms. Large (small) solid circles are extraframework Fe2+ cations (framework Al/Si substitutions).

Configurations 5-8 are formed in zeolites with high Si/Al ratios where the Al sites are separated by large distances. A divalent Fe2+ cation can form configurations connecting a cation to just one isolated Al site, although such configurations are energetically less favorable structures (cf. Figure 3 and Table 1). A random distribution of the Al/Si substitution over all tetrahedral sites T1-T4 (cf. Figure 2) leads to the formation of configurations 5-8 in Fe-exchanged ferrierite. Although configurations 5 and 6 exhibit lower adsorption energies and configurations 7 and 8 higher adsorption energies, both types of Fe2+ sites are expected to exist in low-Al ferrierite. 3.2.3. Migration of Fe2+. The Fe2+-NO cluster formed upon adsorption of NO in Fe-exchanged zeolite is very stable at room temperature. Evacuation of samples does not lead to the removal of adsorbed NO molecules, and desorption occurs only at high temperatures. Two different patterns of bonding, demonstrated by adsorption energies in Figure 11, indicate that Fe2+-NO clusters can exhibit large differences in stabilities. Figure 12 displays the total energy of the intrazeolite Fe2+-NO cluster as a function of the total energy of the same Fe-exchanged zeolite for different configurations, including the eight typical configurations shown in Figure 3. The dependence is not smooth, but indicates the existence of two different types of intrazeolite Fe2+-NO clusters. For configurations above the diagonal, the adsorption of NO increases the total energy and decreases the stability. This occurs for configurations that have the Fe2+ cation in the β site of the side channel and exhibit lower adsorption energies (Figure 11). Bonding of the Fe2+ cation to the zeolite framework in the β site forms stable configurations for the bare cation, but is

J. Phys. Chem. C, Vol. 111, No. 2, 2007 593

Figure 14. Dependence of the NO stretching frequency on the bond length of a single NO molecule adsorbed on Fe2+ in ferrierite. The solid circle and horizontal line indicate the frequency and the bond length of a free NO molecule and the maximum of the IR band observed for the monoadsorption of NO in Fe-FER and Fe-ZSM-5, respectively. Dashed lines represent the least-squares fits for three groups of configurations (A, B, C).

strongly destabilized upon adsorption of NO. Configurations with the Fe2+ cation in the R site and in small rings of the framework, such as 5MR and 6MR, are considerably less stable than configurations with the cation in the β site. In these less stable configurations, the cation resides above the plane of the framework O atoms. The cation therefore exhibits higher bonding capacity, leading to higher adsorption energies (Figure 10). Adsorption of NO on such configurations has a stabilizing effect, and therefore, the corresponding configurations are located below the diagonal in Figure 12. Destabilization of configurations with the Fe2+ cation in the β site can lead to a migration of the extraframework cation to a position where the cation is more stabilized. Figure 13 shows an example of such a migration. Configuration 6 (cf. Figure 3 and Table 1) contains the Al atom in the T2 site and the Fe2+ cation in the β site. Because of the location of the Al atom in the corner of the 6MR, configuration 6 is the least stable configuration with the Fe2+ cation in the β site. In the β site, the bare cation is quasisymmetrically coordinated by four O atoms in the plane of the side channel. In Figure 13, the interactions of the cation with O atoms of the framework are indicated by dashed lines. However, only one O atom, located next to the Al atom, has a strong stabilizing efect. After migration, the cation is asymmetrically located in the 8MR, interacting with three O atoms, where two are neighbors to the Al atom. Compared to the position in the 8MR, the bare Fe2+ cation in the β site is 21 kJ/mol more stable (Figure 13a). The Fe2+ cation with a single adsorbed NO molecule is displayed in Figure 13b. Upon adsorption, the configuration in the β site is destabilized (cf. configuration 6 in Figure 12). In contrast, adsorption of Fe2+ in the off-center position in the 8MR increases stability. The comparison of total energies shows that the combination of both interactions, cation-to-framework and cation-to-NO, leads to a higher stability of the Fe2+-NO cluster

594 J. Phys. Chem. C, Vol. 111, No. 2, 2007

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TABLE 2: Calculated Stretching Frequencies and Bond Lengths of NO Adsorbed on Fe-FERa configuration

free NO

1

2

3

4

5

6

7

8

type of site location of Al rNO fNO (harm) fNO (anharm)

-

β T4, T4 1.165 1908 1881

R T3, T3 1.169 1903 1876

β T2, T2 1.164 1910 1883

R T1, T1 1.161 1936 1909

β T4, T3 1.165 1902 1875

β T2, T3 1.166 1893 1866

R T3, T1 1.163 1904 1877

γ T1, T3 1.164 1900 1873

a

1.168 1903 1876

Frequencies in cm-1 and distances in angstroms.

located in the 8MR by 28 kJ/mol compared to the position in the β site. The energy gain of configuration 6 in the replaced position of the Fe2+-NO cluster is indicated in Figure 12 by the vertical arrow. The arrow shows that, upon migration, configuration 6 moves from the category of destabilized structures into the category of stabilized structures. Because the Fe2+ cation has the capacity to adsorb more than one NO molecule, the effect of cation migration can be more pronounced at increased partial pressure of NO. Figure 13c shows configurations with two adsorbed NO molecules. For Fe2+ in the β site (Figure 13c, left), the two NO molecules adsorb in axial positions. On the other hand, the cation in the 8MR (Figure 13c, right) binds the two NO molecules at Fe-N angles of ∼90°. Both effects, i.e., the migration of Fe2+ and the reconstruction of the coordination sphere, lead to a difference of 77 kJ/mol in favor of the configuration with Fe2+ in the 8MR. 3.2.4. Magnetic Moments. The typical location of an Fe2+ cation exchanged into the extraframework position of ferrierite is in the β site or in the R site. In the former, the cation forms four in-plane Fe-O bonds (Figure 4b), and in the latter, four Fe-O bonds keep the cation in the position above the plane of O atoms (Figure 4a). The (quasi-) planar arrangement allows both the high- and the low-spin configurations of the extraframework cation. Direct estimation of the spin state of the Fe2+ cation using ESR or magnetic susceptibility measurements is practically impossible. The former technique detects only Fe3+ in low-Al zeolites containing isolated noninteracting cations. Pure dehydrated Fe2+ zeolite can be prepared only at low Fe loadings. Thus, the framework negative charge is balanced by other positive charge species. This makes the estimation of the magnetic susceptibility of Fe2+ extremely difficult and inaccurate. UV/vis spectra are reported for both low- and high-spin configurations. The adsorption bands of the low-spin configuration of the extraframework Fe2+ cation are located in the visible region independent of the coordination of the transition metal cation. On the other side, the high-spin configuration exhibits adsorption only in the near-IR region, followed by charge-transfer bands in the UV region. Fe2+-exchanged zeolites in both hydrated and dehydrated states are white, exhibiting no bands in the visible region. This suggests that the configuration of Fe2+ ions coordinated to the zeolite framework is high-spin. Nevertheless, the coordination of extraframework cations in a zeolite is highly specific, and thus, the low-spin state of the Fe2+ cation cannot be excluded. The spin state of the Fe2+ cation in a zeolite matrix has not yet been studied. In the present work, the spin state was checked for all stable positions of the Fe2+ cation compared in Figure 3. Not only in typical locations in the β and R sites, but in all other configurations, the bonding of the cation to the zeolite framework leads to the high-spin configuration Fe d5v d1V, with a magnetic moment of 4 µB. Upon adsorption of NO, the pairing of the NO 1π* v electron with the Fe dσ V electron occurs (cf. Figures 7 and 9), leading to a final magnetic moment of 3 µB for the Fe2+-NO cluster in all intrazeolite configurations. 3.2.5. NO Stretching Frequencies. Figure 14 shows the dependence of the calculated NO stretching frequencies on the

N-O bond lengths. Data for a representative set of configurations are collected in Table 2. The frequencies were calculated according to the harmonic approximation using the finitedifferences method30 and were corrected for anharmonicity by a shift of 27 cm-1 evaluated according to Ugliengo.40 In Figure 14, the horizontal line indicates the maximum of the band observed for the stretching mode of NO adsorbed on both FeFER46 and Fe-ZSM-5.16 Gray area highlights the width of the band ranging from ∼1850 to ∼1900 cm-1. For different configurations, only a slight variation of the NO bond length between 1.161 and 1.169 Å is observed. Much more pronounced is the variation of stretching frequencies between 1866 cm-1 (configuration 6) and 1909 cm-1 (configuration 4). Figure 14 indicates the existence of at least three different types of configurations leading to a different dependence of the NO stretching frequency on the NO bond length. A very slight variation of the frequency by several wavenumbers is observed for group A comprising the most stable configurations 1-3. Adsorption on the least stable configurations with only one Al/ Si substitution leads to the group of frequencies designated as B. A common feature of all configurations in group C is two Al atoms in a small rigid ring (6MR, 5MR). Configurations A and B produce similar frequencies that reasonably correlate with the maximum of the experimental band and with the stretching frequency of a free NO molecule. The same frequency of 1876 cm-1 observed before and after adsorption means that the change of NO bonding upon adsorption (cf. Figure 10) retains the strength of the N-O bond. The frequencies of several configurations of series A and B coincide with the maximum of the experimental band. All remaining configurations of series A and B exhibit frequencies within the bandwidth. Because of the overlap of the interval of frequencies of the two series, IR spectra do not allow the adsorption sites with two Al/Si substitutions (A) to be distinguished from sites with one Al/Si substitution (B) connected to the extraframework Fe2+ cation. On the other hand, no NO stretching bands are observed at frequencies as high as those calculated for series C. This indicates that configurations containing two Al atoms in a small rigid ring and producing blue-shifted frequencies in the range 1905-1910 cm-1 are unlikely to exist. 4. Conclusions An extensive study of the energetics of Al/Si substitutions in the zeolite framework of ferrierite has been performed. Substitution in the T2 site is the most stable. The T3 and T4 sites are slightly disadvantaged (approximately +14 kJ/mol), and the T1 site is the least stable tetrahedral site (+22 kJ/mol). Comparison to mordenite shows that the energetics depend on the topology of the framework. For example, the T site connecting the 5MR, the 6MR, and the 8MR (T2 in mordenite and T1 in ferrierite) is the most stable in mordenite and the least stable in ferrierite. To compare the stabilities of Fe2+-exchanged ferrierites, two types of configurations are considered. Configurations with two Al atoms at a short distance (6) were examined. Calculated differences in stabilities measured up to 250 kJ/mol. The most stable position of the Fe2+ cation is in a 6MR (β or R site). Configurations with the extraframework Fe2+ cation connected to two Al sites are more stable. Such configurations, however, are rare in low-Al zeolites. Configurations with Fe2+ connected to just one Al site exhibit lower stability and higher reactivity. The Si/Al ratio therefore represents another important factor influencing the reactivity of zeolites. Simulated adsorption of NO on Fe2+ shows the existence of two types of Fe2+ cations. More stable are the cations in β sites, which exhibit lower adsorption energies of ∼180 kJ/mol. In contrast, the cations in R sites form less stable configurations, exhibiting higher adsorption energies of ∼240 kJ/mol. Upon adsorption of NO on less stable Fe2+ sites, more stable NOFe2+ configurations are formed. The increased stability can even lead to the migration of the extraframework cation to a different position on the inner surface of the zeolite. Bonding of NO on Fe2+ induces pronounced changes of the electron density on both the Fe2+ cations and the NO molecule. Adsorption induces the formation of N pπ-Fe dπ bonding states proportionally filled by electrons from both the N atom and the Fe2+ cation and leads to an increase of the π-electron density between Fe2+ and N. Extensive polarization of the electron density within the NO molecule leads to only a slight shortening of the interatomic distance. The NO stretching frequency calculated for stable configurations is similar to that of the free molecule and agrees well with the maximum of the IR band at 1876 cm-1. The stretching frequencies of other configurations are within the width of the experimental band. The frequencies of configurations with two Al atoms in a small rigid ring (5MR, R site) are too high compared to the band of observed frequencies. This indicates that no such configurations are present in ferrierite structures. The bonding of NO on the Fe2+ cation is qualitatively different from the bonding on the surface of a transition metal characterized by the Blyholder scheme. Bonding on the surface combines σ donation and π back-donation. The back-transferred electron density fills π-antibonding states of the NO or CO molecule. The intramolecular bond becomes weaker, exhibiting a larger interatomic distance and a downshift of the stretching frequency. In contrast, in the bonding of NO and CO to a transition metal cation, no transfer of the electron density occurs. The electron density accumulates within the newly formed bond, depleting proportionally both the molecule and the cation. Because of the antibonding character of the electron density withdrawn from the adsorbed molecule, adsorption leads to the strengthening of the intramolecular bond. The bond length becomes shorter, inducing higher stretching frequencies. Acknowledgment. This work was supported by the Austrian Science Fund under Project P17020-PHYS and by the Institut Franc¸ ais du Pe´trole. Computational resources were partly granted by the Computing Center of Vienna University (Schro¨dinger II). The work in Prag was supported by the Grant Agency of the Academy of Sciences of the Czech Republic under Project 1ET 400400413. References and Notes (1) Panov, G.-I.; Sobolev, V. I.; Kharitonov, A. S. J. Mol. Catal. 1990, 61, 85. 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