CO and NO Adsorptions on Different Iron Sites of Fe-ZSM-5 Clusters

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CO and NO Adsorptions on Different Iron Sites of Fe-ZSM-5 Clusters: A Density Functional Theory Study Mehmet Ferdi Fellah* Department of Chemical Engineering, Middle East Technical University, Ankara, 06531 Turkey Department of Chemical Engineering, Yuzuncu Yil University, Van, 65080 Turkey ABSTRACT: Density functional theory (DFT) calculations were carried out in a study of CO and NO adsorptions on different iron sites of Fe-ZSM-5. The adsorption energies and distances were similar for CO adsorption on both small and large cluster calculations. The adsorption energies (ΔE) and enthalpies (ΔH) found for NO adsorption on [FeO]1þ-ZSM-5 and [Fe(OH)2]1þ-ZSM-5 clusters were in line with the previous theoretical values. Some additional energy was required for NO and CO adsorptions on [FeO]2þ-ZSM-5 (NO adsorption only), [FeOH]2þ-ZSM-5, and [Fe(OH)2]2þZSM-5 clusters because of their positive ΔG values. The scaled vibrational frequencies for adsorbed CO and NO molecules on Fe2þ-ZSM-5 cluster were computed as 1870 and 2149 cm-1, respectively. These values are in good agreement with the experimental values for the Fe2þ-CO and Fe2þ-NO bands (1882 and 2140 cm-1, respectively). Moreover, the calculated vibrational frequency (1897 cm-1) for NO adsorption on the [HO-FeOFe-OH]2þ-ZSM-5 cluster matches well with the experimental value (1892 cm-1).

1. INTRODUCTION Fe-exchanged ZSM-5 catalysts have good catalytic activities for a number of reactions. It has been experimentally reported1,2 that [Fe,Al]MFI gives good performance in the selective oxidation of benzene to phenol. The R-form of oxygen (extraframework oxygen) formed by decomposition of N2 O plays an important role in the direct oxidation of benzene on Fe-ZSM-5 that is confirmed by both experimental3-8 and theoretical9-13 literature. It should also be noted that Fe-ZSM-5 is an active catalyst for the stoichiometric decomposition of N2O to form R-oxygen, according to both experimental14-26 and theoretical25-34 reports. Benzene is selectively oxidized to phenol by interaction with the R-form of the surface oxygen produced on Fe-ZSM-5 zeolite by N2O decomposition.3-8,35 The following reaction is proposed for the decomposition of N2O to form R-oxygen7,8,36 N2 O þ ðÞR f ðOÞR þ N2

ð1Þ

FT-IR analysis of probe molecules is mostly used for the characterization of solid surfaces.37-41 Although CO is one of the most frequently used probe molecules,37 only a few IR studies have examined CO adsorption on Fe-ZSM-542,43 and on ironcontaining zeolites.44-50 However, NO is generally used to characterize iron sites on Fe-ZSM-5.1,42,51-70 Mihaylov et al.42,43 investigated CO and NO adsorptions on Fe-ZSM-5 catalyst and reported some IR frequencies for these adsorptions. The major challenge or debate in understanding the activity of Fe/ZSM-5 catalysts is about the nature of the active sites in the r 2010 American Chemical Society

ZSM-5 zeolite.71 Extraframework Fe species in the zeolite micropores can be present as mono-, bi-, or oligonuclear cationic species; neutral iron oxide species; or mixed oxide phases combining Fe and Al. Several experimental studies have utilized extended X-ray absorption fine structure, high-resolution X-ray absorption near-edge spectroscopy, Fourier transform infrared spectroscopy, electron spin resonance, electron paramagnetic resonance, X-ray diffraction, M€ossbauer spectroscopy, and UVvisible spectroscopy and have reported that iron sites of Fe-ZSM-5 are mononuclear sites,59,72-79 binuclear structures,78,80-84 and small iron oxide or oligomeric clusters.84-86 Many experimental studies23,25,35,36,59,87-89 have shown that Fe2þ sites are present in Fe-ZSM-5 catalyst and that they are the active sites for benzene oxidation to phenol by N2O. Volodin et al.79 studied the spin state of iron ions in Fe-ZSM-5 zeolites experimentally and reported that ferrous ions have integer electron spins (S = 2). They also reported that Fe-ZSM-5 catalyst contains an FeO complex where iron has a charge of 3þ. Based on an experimental90 XAFS study, it was reported that FeO complexes where Fe atoms are in the 3þ and 4þ oxidation states exist in Fe-ZSM-5 catalyst. Additionally, FeO complexes in solution in which iron has a charge of 4þ were observed using XAFS by Kemner et al.91 Several Fe-ZSM-5 clusters having different iron charges have been used in many theoretical studies. Yoshizawa et al.12,92 Received: August 10, 2010 Revised: December 3, 2010 Published: December 22, 2010 1940

dx.doi.org/10.1021/jp107534n | J. Phys. Chem. C 2011, 115, 1940–1951

The Journal of Physical Chemistry C

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Figure 1. Optimized geometries of (a) [Fe]1þ-ZSM-5, (b) [FeO]1þ-ZSM-5, (c) [FeOH]1þ-ZSM-5, (d) [Fe(OH)2]1þ-ZSM-5, (e) [Fe]2þ-ZSM-5, (f) [FeO]2þ-ZSM-5, (g) [FeOH]2þ-ZSM-5, (h) [Fe(OH)2]2þ-ZSM-5, (i) [FeOFe]2þ-ZSM-5, and (j) [HO-FeOFe-OH]2þ-ZSM-5 clusters.

studied the reaction pathways and energetics for direct benzene oxidation to phenol on Fe-ZSM-5 zeolite represented as a [(SiH3)2AlO2(OH)2(FeO)] cluster. Benzene oxidation to phenol by N2O was also studied over (SiH3)4AlO4(FeO) and (SiH3)4AlO4(FeO2) clusters.9 Recently, computational studies investigated the direct benzene oxidation to phenol by N2O on Fe1þZSM-593 zeolite represented as a [Si4AlO4H12Fe] cluster, on Fe2þ-ZSM-571 zeolite represented as a [Si6Al2O9H14Fe] cluster, and on [FeO]1þ-ZSM-5 zeolite modeled as [Si4AlO4H12FeO].94 A [Si4AlO4H12Fe] cluster where Fe has a charge of 1þ was recently investigated for methane oxidation to methanol by N2O.95 A mononuclear Fe1þ in the Fe-ZSM-5 cluster has been investigated in many theoretical studies,9,10,29,92,93,96 whereas a mononuclear Fe2þ site in Fe-ZSM-5 has been used in two theoretical studies.13,70 Several clusters utilized in theoretical

calculations have contained an FeO site in which Fe has a charge of 3þ.9,12,29,92,94,96 ([FeOH]1þ)96 and ([Fe(OH)2]1þ)29 sites have also been used in some theoretical studies. Binuclear sites such as ([FeOFe]2þ)34,97-99 and ([Fe-μO-μOHFe]þ)100 where iron atoms have a charge of 2þ and ([HO-FeOFe-OH]2þ)91,92 in which Fe atoms exist in the 3þ oxidation state have also been studied theoretically for N2O decomposition and methane activation. Although there are some experimental and theoretical studies about the exact nature of the active iron site in Fe-ZSM-5 catalyst, it is still a challenging topic. The aim of this study was to analyze the adsorptions of CO and NO on different iron sites of Fe-ZSM-5 clusters where the formal valency of iron changes between 1þ and 4þ. DFT calculations using the B3LYP formalism with the 6-31G(d,p) 1941

dx.doi.org/10.1021/jp107534n |J. Phys. Chem. C 2011, 115, 1940–1951


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Table 1. Spin Multiplicity Numbers for Clusters and Systems Including Cluster and Adsorbing Molecules cluster type [Fe]1þ-ZSM-5 [FeO]1þ-ZSM-5 [FeOH]1þ-ZSM-5 [Fe(OH)2]1þ-ZSM-5 [Fe]2þ-ZSM-5 [FeO]2þ-ZSM-5 [FeOH]2þ-ZSM-5 [Fe(OH)2]2þ-ZSM-5 [FeOFe]2þ-ZSM-5 [HO-FeOFe-OH]2þZSM-5

Figure 2. Optimized geometry of a two-layer ONIOM cluster where Fe has a charge of 1þ. QM region represented as ball-and-bond view, and MM region represented as wireframe view.

basis set were used to obtain equilibrium adsorption geometries, relative adsorption energies, and vibrational frequencies.

2. SURFACE MODELS AND CALCULATION METHOD All calculations in this study were based on DFT101 as implemented in the Gaussian 03 suite of program.102 Becke's three-parameter hybrid method103,104 involving the Lee, Yang, and Parr correlation functional (B3LYP105) formalism was utilized to take into account the exchange and correlation. The 6-31G(d,p) basis set was used for all atoms including iron. Only small differences between cluster and periodic system calculations were observed in earlier works where Fe-ZSM-5 and Fe-ferrierite were used by Kachurovskaya et al.13,106 They studied the adsorption of complexes involving benzene, phenol, and the keto tautomer of phenol on iron-exchanged zeolite structures using cluster and periodic DFT calculations. Similar results such as atomic distances and adsorption energies were obtained using both types of calculation systems.13,106 Moreover, qualitative differences for chemical reactions on clusters have rarely been observed between clusters and periodic models of zeolites.107-109 In this study, ZSM-5 clusters that were cut from inside a ZSM5 channel were constructed using the Cartesian coordinates reported by Lermer et al.110 Two types of cluster were used to locate the iron sites on ZSM-5. First, one was a 5T ZSM-5 cluster

cluster

cluster-NO system

cluster-CO system

6, 69,10,12,29,32,92-96 6, 69,10,12,29,32,92-96 5, 596 6, 629,96 5, 513,71 5, 571 6 5 9, 934,97-99 9, 934,97,98

3 5, 596 4, 496 7, 796 4 4 3 2 8 8

4 4 5 6 5 5 4 3 9 9

containing five Si and four O atoms. One Al atom was placed in the T12 site of the framework surrounded by O and Si atoms in a 5T ZSM-5 cluster modeled as [(SiH3)4AlO4]-. [Fe]1þ, [FeO]1þ, [FeOH]1þ, and [Fe(OH)2]1þ sites in which the formal valences of iron are 1þ, 3þ, 2þ, and 3þ, respectively, were used to obtain neutral clusters. These clusters are represented in parts of a-d, respectively, of Figure 1. The second type of cluster was double 5T ring ZSM-5 clusters containing eight Si and nine O atoms. Two Al atoms were placed in different positions (T9 and T12 lattice positions) of each ring to obtain distant anionic sites in a double 5T ring ZSM-5 cluster modeled as [Si6Al2O9H14]2-. [Fe]2þ, [FeO]2þ, [FeOH]2þ, [Fe(OH)]2þ, [FeOFe]2þ, and [HO-FeOFe-OH]2þ sites in which the formal valences of iron are 2þ, 4þ, 3þ, 4þ, 2þ, and 3þ, respectively, were used to obtain neutral clusters. These clusters are shown in parts of e-j, respectively, of Figure 1. The dangling bonds of the terminal silicon atoms of all clusters were terminated with H atoms to obtain neutral clusters. All atoms of all of the clusters, except terminating H atoms, and the reactant and product molecules were kept relaxed. Terminating H atoms were kept fixed to orient in the Si-O direction of the next Si site. A larger cluster type containing one Al atom was constructed to investigate the effect of cluster size on adsorption. A two-layer ONIOM method was used to simulate ZSM-5 modeled by a Si83O149 cluster. In this method, 13 Si and 38 O atoms were in the QM region utilizing DFT, and the rest of the cluster (182 atoms in total) were in the MM region utilizing a universal force field (UFF). One Al atom was also placed in the T12 site of the framework surrounded by O and Si atoms in the ONIOM cluster, which is depicted in Figure 2. Four Si, four O, Al, and Fe atoms in the QM region were the only atoms kept relaxed as in the [(SiH3)4AlO4] type of clusters. The Fe atom has a charge of 1þ in the ONIOM cluster. The equilibrium geometry (EG) calculations were performed for determination of the relative adsorption energies. All energy values and energy differences in this study include zero-point energy (ZPE) corrections, which were obtained using frequency calculations at a temperature of 298 K because no experimental thermochemistry data are available for CO and NO adsorptions on Fe-ZSM-5 catalyst. The computed ÆS2æ values confirmed that the spin contamination was very small (after annihilation). Vibrational analysis was also performed to obtain vibrational frequencies, enthalpies, and Gibbs free energies for CO and NO adsorptions on Fe-ZSM-5 clusters. Mulliken population analysis111 was utilized to obtain Mulliken atomic charges and Mulliken atomic spin densities. Natural bond orbital (NBO)112 1942



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Table 2. Mulliken Atomic Charge and Spin Density Values and Electron Configurations of Iron Atoms in Clusters cluster type

Mulliken atomic charge

Mulliken atomic spin density

[Fe]1þ-ZSM-5

þ0.470

4.810

[core]4s1.073d6.004p0.09

1þ

þ1.003

4.123

[core]4s0.323d5.944p0.065p0.01

þ0.818

3.741

[core]4s0.423d6.084p0.045s0.01

þ1.185

4.238

[core]4s0.263d5.724p0.035p0.01

[Fe] -ZSM-5

þ0.794

3.818

[core]4s0.223d6.144p0.03

2þ

þ0.894

3.822

[core]4s0.273d5.914p0.034d0.015p0.01

þ1.018

4.219

[core]4s0.273d5.864p0.035p0.01

Fe1

þ1.111 þ0.831

4.219 3.812

[core]4s0.263d5.744p0.035p0.01 [core]4s0.303d6.154p0.03

Fe2

þ0.832

3.801

[core]4s0.313d6.154p0.03

Fe1

þ0.971

2.877

[core]4s0.313d5.964p0.025p0.01

Fe2

þ1.094

4.191

[core]4s0.263d5.754p0.035p0.01

[FeO] -ZSM-5 1þ

[FeOH] -ZSM-5 1þ

[Fe(OH)2] -ZSM-5 2þ

[FeO] -ZSM-5 2þ

[FeOH] -ZSM-5 2þ

[Fe(OH)2] -ZSM-5 [FeOFe]2þ-ZSM-5 2þ

[HO-FeOFe-OH] -ZSM-5

Figure 3. Optimized geometry of CO adsorption on a two-layer ONIOM cluster: (a) ONIOM representation (QM region represented as ball-and-bond view, and MM region represented as wireframe view). (b) QM region view (distance values in units of angstroms).

analysis was used to obtain electronic configurations of iron atoms. The convergence criteria involving gradients of maximum

electron configuration

Figure 4. Optimized geometries of (a) CO and (b) NO adsorptions on an [Fe]1þ-ZSM-5 cluster (distance values in units of angstroms).

force, root-mean-square (rms) force, maximum displacement, and rms displacement in Gaussian 03 software were 0.000450, 0.000300, 0.001800, and 0.001200, respectively. 1943



Figure 5. Optimized geometries of (a) CO and (b) NO adsorptions on an [FeO]1þ-ZSM-5 cluster (distance values in units of angstroms).

The computational strategy employed in this study was as follows: Initially, the correct spin multiplicities (SMs) for the clusters were determined by single-point energy (SPE) calculations. The SM numbers were also found for the systems consisting of the cluster and adsorbing molecule, CO or NO. SPE values were calculated with different SM numbers for each cluster, and the SM number corresponding to the lowest SPE was accepted as the correct spin multiplicity. The cluster and the adsorbing molecule were then fully optimized geometrically by means of EG calculations. The adsorbing molecule was located over the active site of the cluster at a selected distance, and an EG calculation was performed to obtain the relative adsorption energy. The relative energy was defined as ΔE ¼ Esystem - ðEcluster þ Eadsorbate Þ where Esystem is the calculated energy of the given geometry containing the cluster and the adsorbing molecule at any distance; Ecluster is the energy of the cluster; and Eadsorbate is

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Figure 6. Optimized geometries of (a) CO and (b) NO adsorptions on an [FeOH]1þ-ZSM-5 cluster (distance values in units of angstroms).

the energy of the adsorbing molecule, namely, CO or NO in this case.

3. RESULTS 3.1. Optimization of Adsorbing Molecules and Clusters. The equilibrium geometries for CO and NO were obtained by taking the total charge as zero and the SM as a singlet and doublet, respectively. The optimized CO and NO molecules had distances of 1.138 and 1.159 Å, respectively. The vibrational frequencies of the C-O and N-O stretching modes were calculated to be 2208 and 1990 cm-1, respectively. The optimized geometries of the clusters shown in Figure 1 were obtained with neutral charge and the SM numbers calculated before. The SM numbers were also determined for the system including adsorbing molecule, CO or NO. The calculated SM numbers for all clusters are reported in Table 1, which also includes theoretical literature data. The Mulliken atomic charges, Mulliken atomic spin densities, and electron configurations of iron atoms for clusters are listed in Table 2. The Si-O distances 1944



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Figure 8. Optimized geometries of (a) CO and (b) NO adsorptions on an [Fe]2þ-ZSM-5 cluster (distance values in units of angstroms).

Figure 7. Optimized geometries of (a) CO and (b) NO adsorptions on an [Fe(OH)2]1þ-ZSM-5 cluster (distance values in units of angstroms).

of the clusters range from 1.55 to 1.68 Å. The corresponding experimental distances are between 1.55 and 1.65 Å.103 3.2. Test for Cluster Size Effect. CO adsorption was studied on a larger ZSM-5 cluster to investigate the effect of cluster size. A two-layer ONIOM method was used to simulate the larger ZSM-5 cluster modeled as the Si83O149 cluster shown in Figure 2. In this cluster, 13 Si and 38 O atoms are in the QM region utilizing DFT, and the rest of the cluster atoms are in the MM region utilizing a universal force field (UFF). The adsorption energy of CO was calculated to be 39.7 kcal/mol. The Fe-C and C-O distances were computed to be 1.811 and 1.142 Å, respectively. The stretching frequency of adsorbed CO was found to be 2157 cm-1. The optimized final equilibrium geometry of CO adsorption on the ONIOM cluster is presented in Figure 3. 3.3. CO and NO Adsorptions on Clusters. The adsorptions of CO and NO were first studied on an [Fe]1þ site. These reactions are highly exothermic reactions with relative adsorption energies of 43.8 and 67.9 kcal/mol, respectively. The Mulliken atomic charges and Mulliken atomic spin densities of iron atoms were calculated to be þ0.741, 3.312 and þ0.756, 3.266 for the

equilibrium geometries of CO and NO adsorption, respectively. The vibrational frequencies of the C-O and N-O stretching modes for adsorbed CO and NO were computed as 2074 and 1855 cm-1, respectively. The optimized geometries for CO and NO adsorptions on an [Fe]1þ site are shown in Figure 4. The adsorptions of CO and NO gave relative energy values of -17.3 and -18.7 kcal/mol, respectively, on an [FeO]1þ site. The Mulliken atomic charges and Mulliken atomic spin densities of iron atoms were computed to be þ0.912, 2.579 and þ1.019, 3.827 for equilibrium geometries of adsorbed CO and NO, respectively. The C-O and N-O stretching frequencies for adsorbed CO and NO molecules were calculated as 2180 and 1880 cm-1, respectively. Figure 5 shows the optimized geometries for CO and NO adsorptions on an [FeO]1þ site. The CO and NO adsorption reactions were found to have relative energies of -11.1 and -26.8 kcal/mol, respectively, if they occurred on an [FeOH]1þ site. The optimized final geometries for CO and NO adsorptions are depicted in Figure 6. The Mulliken atomic charges and Mulliken atomic spin densities of the iron atoms were calculated to be þ0.905, 3.762 and þ1.097, 3.862 for the equilibrium geometries of CO and NO adsorption, respectively. The vibrational frequencies of the C-O and N-O stretching modes for adsorbed CO and NO were obtained as 2177 and 1868 cm-1, respectively. The adsorptions of CO and NO were also studied on an [Fe(OH)2]1þ site. These reactions were found to be slightly exothermic reactions with relative 1945



Figure 9. Optimized geometries of (a) CO and (b) NO adsorptions on an [FeO]2þ-ZSM-5 cluster (distance values in units of angstroms).

adsorption energies of 2.5 and 1.9 kcal/mol, respectively. The C-O and N-O stretching frequencies for adsorbed CO and NO molecules were computed as 2243 and 1986 cm-1, respectively. The Mulliken atomic charges and Mulliken atomic spin densities of iron atoms were calculated to be þ1.175, 4.223 and þ1.173, 4.246 for the equilibrium geometries of CO and NO adsorption, respectively. Figure 7 presents the optimized geometries on an [Fe(OH)2]1þ site. The CO and NO molecules were found to be strongly adsorbed on an [Fe]2þ site. The relative adsorption energies were calculated to be -23.9 and -38.0 kcal/mol, respectively. The optimized geometries for CO and NO adsorptions on an [Fe]2þ site are shown in Figure 8. The Mulliken atomic charges and Mulliken atomic spin densities of iron atoms were computed to be þ0.756, 3.793 and þ0.972, 3.837 for the equilibrium geometries of CO and NO adsorption, respectively. The vibrational frequencies of the C-O and N-O stretching modes for adsorbed CO and NO molecules were determined as 2235 and 1945 cm-1, respectively. The adsorptions of CO and NO gave relative energies of -14.4 and -1.1 kcal/mol, respectively, on an [FeO]2þ site. The Mulliken atomic charges and Mulliken atomic spin densities of the iron atoms were calculated to be þ0.797, 2.616 and þ0.834, 2.946 for the equilibrium geometries of CO and NO adsorption, respectively. The C-O and N-O stretching frequencies for adsorbed CO and NO molecules were obtained as 2180 and 1904 cm-1, respectively. Figure 9 depicts the equilibrium geometries of CO and NO adsorption. The CO and NO adsorption reactions were found to have relative energies of -0.4 and þ2.3 kcal/mol, respectively, if they took

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Figure 10. Optimized geometries of (a) CO and (b) NO adsorptions on an [FeOH]2þ-ZSM-5 cluster (distance values in units of angstroms).

place on an [FeOH]2þ site. The optimized geometries for the CO and NO adsorptions are represented in Figure 10. The Mulliken atomic charges and Mulliken atomic spin densities of the iron atoms were calculated to be þ0.763, 3.748 and þ0.738, 2.076 for the equilibrium geometries of CO and NO adsorption, respectively. The vibrational frequencies of the C-O and N-O stretching modes for adsorbed CO and NO molecules were computed as 2193 and 1911 cm-1, respectively. The adsorptions of CO and NO were also investigated on an [Fe(OH)2]2þ site. These reactions have relative adsorption energy values of þ11.4 and þ9.4 kcal/mol, respectively. The C-O and N-O stretching frequencies for adsorbed CO and NO molecules were determined as 2237 and 1925 cm-1, respectively. The Mulliken atomic charges and Mulliken atomic spin densities of iron atoms were calculated to be þ0.924, 2.825 and þ0.986, 2.733 for the equilibrium geometries of CO and NO adsorption, respectively. Figure 11 represents the optimized geometries for adsorbed CO and NO on an [Fe(OH)2]2þ site. The adsorptions of CO and NO were also studied on dimeric iron sites. The relative adsorption energies were calculated to be -14.7 and -28.1 kcal/mol, respectively. The optimized geometries for CO and NO adsorptions on the [FeOFe]2þ site are shown in Figure 12. The Mulliken atomic charges and Mulliken atomic spin densities of the iron atoms (Fe1 and Fe2) were computed to be þ0.844, 3.827 and þ0.798, 3.755 for the equilibrium geometry of CO adsorption. The corresponding values for iron atoms for the equilibrium geometry of NO adsorption were þ0.874, 3.831 and þ1.011, 3.820, respectively. 1946



Figure 11. Optimized geometries of (a) CO and (b) NO adsorptions on an [Fe(OH)2]2þ-ZSM-5 cluster (distance values in units of angstroms).

The vibrational frequencies of the C-O and N-O stretching modes for adsorbed CO and NO molecules were obtained as 2185 and 1845 cm-1, respectively. The angles of the FeOFe site for the [FeOFe]-ZSM-5 cluster at the end of the CO and NO adsorption reactions were found to be 120.5 and 128.8, respectively. The angle of that site in the cluster before adsorption was calculated to be 117.7. The CO and NO adsorption reactions were found to have the same relative adsorption energy value (-7.3 kcal/mol) on the [HO-FeOFe-OH]2þ site. The optimized geometries for the CO and NO adsorptions are depicted in Figure 13. The Mulliken atomic charges and Mulliken atomic spin densities of the iron atoms (Fe1 and Fe2) were computed to be þ0.960, 2.870 and þ1.077, 4.177 for the equilibrium geometries of CO adsorption. The corresponding values for the iron atoms of the equilibrium geometry of NO adsorption were þ0.964, 2.865 and þ1.111, 4.127, respectively. The C-O and N-O stretching frequencies for adsorbed CO and NO molecules were determined as 2255 and 1973 cm-1, respectively. The angles of FeOFe for the [HO-FeOFe-OH]ZSM-5 cluster at the end of the CO and NO adsorption reactions were calculated as 157.9 and 146.7, respectively. The angle of that site in the cluster before adsorption was computed as 144.5.

4. DISCUSSION Table 1 displays the spin multiplicity (SM) numbers for all Fe-ZSM-5 clusters and the systems including CO and NO with available theoretical literature data. The SM numbers computed in this study for some clusters and their systems including a NO

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Figure 12. Optimized geometries of (a) CO and (b) NO adsorptions on an [FeOFe]2þ-ZSM-5 cluster (distance values in units of angstroms).

molecule are the same as the SM numbers found in the literature. Even though there are numerous theoretical references for the SMs of some clusters, there is only one study for the SM of the system including NO. The literature does not contain any study related to these clusters with a CO molecule. As mentioned before, only small differences between cluster and periodic system calculations were observed in earlier zeolite studies.13,106 It has been reported that similar atomic distances and adsorption energies are obtained using the two types of calculation systems. Nonetheless, the adsorption of CO was investigated on a larger cluster in this study to investigate the effect of cluster size. Similar adsorption energies for CO adsorption were obtained for both types of cluster calculations, which were found as -43.8 and -39.7 kcal/mol for small and large [Fe]1þ-ZSM-5 clusters, respectively. These values are in relatively good agreement with each other within the DFT error range. The adsorption geometries for the two types of calculations seem to be very similar (see Figures 3 and 4). Correspondingly, the calculated Fe-C and C-O distances in the equilibrium geometries for small and large clusters (1.778, 1.159 and 1.811, 1.142 Å, respectively) match reasonably well with each other. Consequently, small clusters can be used sufficiently to simulate Fe-ZSM-5 for CO and NO adsorptions. Tables 2-4 report the Mulliken atomic charges, spin densities, and electron configurations of iron atoms; theormochemistry data such as adsorption energy, enthalpy, and Gibbs free energy; and vibrational frequencies of C-O and N-O stretching modes for clusters and clusters including adsorbed CO and NO molecules. As can be seen from these tables, the iron atoms have high Mulliken spin densities, which indicates that the 1947



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unpaired electrons are localized on iron atoms. The Mulliken spin densities of iron atoms for clusters including adsorbed CO and NO molecules are higher than those of the nonadsorbed clusters. The small increase in spin density is also imitated in the Mulliken atomic charge values, which show a more positive

character on iron atoms in the clusters including adsorbed CO and NO in comparison to the nonadsorbed clusters. The FeOFe angles in the [HO-FeOFe-OH]- and [FeOFe]ZSM-5 clusters before the reactions were calculated to be 144.5 and 117.7, respectively. Sun et al.58 reported that the experimental Fe-O-Fe angle for a binuclear iron-oxo site of Fe-ZSM5 catalyst decreased from ∼150 to ∼130 when the catalyst was treated with a high temperature (hydroxylated versus dehydroxylated site). These angle values are in reasonable agreement with each other, as the values in this study were calculated at 298 K. The FeOFe angles for [FeOFe]- and [HO-FeOFe-OH]ZSM-5 clusters in this study increased after the adsorption reactions. An increase in the FeOFe angle after a reaction is a generally observed trend.34,99 The reason for this migt be that the iron atoms where the CO or NO molecules adsorb have a lower spin density after the adsorption reactions. When a NO molecule is adsorbed on Fe-ZSM-5 at room temperature, some bands related to dinotrosyl and trinitrosyl species64 or to mononitrosyls65 adsorbed in different iron positions in the ZSM-5 are detected. It has generally been reported that NO is adsorbed rather strongly on Fe2þ but only weakly or not at all on Fe3þ.41,64 In this study, the vibrational energies calculated and listed in Table 4 show that the NO molecule adsorbed strongly on Fe2þ but adsorbed weakly on Fe3þ. These vibrational energies were obtained on iron atoms having charges of 2þ and 3þ on [Fe]2þ, [FeOFe]2þ, [FeOH]1þ and [FeOH]2þ, [HO-FeOFe-OH]2þ, [FeO]1þ, [Fe(OH)2 ]1þ sites, respectively. The adsorption energy (-67.9 kcal/mol) of NO molecule on an [Fe]1þ-ZSM-5 cluster seems to be very high compared to general molecular adsorption energies. Although the adsorption or desorption energies in this system do not tend to be that high, based on experimental studies, it has been reported that NO molecules are adsorbed more strongly on iron sites with decreasing iron charges.41,64 The adsorption energies (ΔE) and enthalpies (ΔH) for NO adsorption on [FeO]1þ-ZSM-5 and [Fe(OH)2]1þ-ZSM-5 clusters in this study were calculated to be -18.7, -18.9 and -1.9, -0.4 kcal/mol, respectively. The corresponding theoretical values were reported as -14.7, -13.6 and -2.1,-0.9 kcal/mol, respectively, by Heyden et al.96 These values are in reasonable agreement with each other given that the energy values in this

Figure 13. Optimized geometries of (a) CO and (b) NO adsorptions on [HO-FeOFe-OH]2þ-ZSM-5 cluster (distance values in units of angstroms).

Table 3. Mulliken Atomic Charge and Spin Density Values and Electron Configurations of Iron Atoms and Theormochemistry Data and Vibrational Frequencies of C-O Stretching for CO Adsorption cluster

Mulliken

Mulliken atomic

electron

type

atomic charge

spin density

configuration

ΔH

ΔG

frequency of C-O

[Fe]1þ-ZSM-5

þ0.741

3.312

[core]4s0.413d6.344p0.03

-43.8

-43.7

-34.6

1994

[FeO]1þ-ZSM-5

þ0.912

2.579

[core]4s0.323d6.194p0.03

-17.3

-18.8

-5.7

2096

[FeOH]1þ-ZSM-5 [Fe(OH)2]1þ-ZSM-5

þ0.905 þ1.175

3.762 4.223

[core]4s0.303d6.134p0.03 [core]4s0.293d5.724p0.035p0.01

-11.1 -2.5

-10.8 -0.6

-2.5 þ2.7

2093 2156

[Fe]2þ-ZSM-5

þ0.756

3.793

[core]4s0.303d6.064p0.02

-23.9

-24.1

-14.9

2149

2þ

[FeO] -ZSM-5

þ0.797

2.616

[core]4s0.313d6.204p0.02

-14.4

-15.3

-3.5

2096

[FeOH]2þ-ZSM-5

þ0.763

3.748

[core]4s0.283d6.304p0.015p0.01

-0.4

-1.4

þ9.8

2108

[Fe(OH)2]2þ-ZSM-5

þ0.924

2.826

[core]4s0.303d5.984p0.025p0.01

þ11.4

þ10.9

þ21.9

2150

Fe1

þ0.844

3.827

[core]4s0.303d6.154p0.03

-14.7

-15.7

-4.1

2100

Fe2

þ0.798

3.755

[core]4s0.313d6.114p0.03

[HO-FeOFe-OH] -ZSM-5 Fe1 Fe2

þ0.960 þ1.077

2.870 4.177

[core]4s0.313d5.974p0.025p0.01 [core]4s0.313d5.784p0.034d0.015p0.01

-7.3

-7.4

þ2.2

2168

2þ

[FeOFe] -ZSM-5 2þ

a

ΔE

(kcal/mol) (kcal/mol) (kcal/mol) stretchinga (cm-1)

Frequency values scaled by 0.9613 to reproduce experimental fundamentals. 1948



ARTICLE

Table 4. Mulliken Atomic Charge and Spin Density Values and Electron Configurations of Iron Atoms and Theormochemistry Data and Vibrational Frequencies of N-O Stretching for NO Adsorption cluster type [Fe]1þ-ZSM-5 [FeO]1þ-ZSM-5 [FeOH]1þ-ZSM-5 [Fe(OH)2]1þ-ZSM-5 [Fe]2þ-ZSM-5 [FeO]2þ-ZSM-5 [FeOH]2þ-ZSM-5 [Fe(OH)2]2þ-ZSM-5 [FeOFe]2þ-ZSM-5

Fe1 Fe2 [HO-FeOFe-OH] -ZSM-5 Fe1 Fe2 2þ

a

Mulliken Mulliken atomic atomic charge spin density þ0.756 þ1.019 þ1.097 þ1.173 þ0.972 þ0.834 þ0.738 þ0.986 þ0.874 þ1.011 þ0.964 þ1.111

3.266 3.827 3.861 4.246 3.837 2.945 2.076 2.733 3.831 3.820 2.865 4.127

ΔE ΔH ΔG frequency of N-O (kcal/mol) (kcal/mol) (kcal/mol) stretchinga (cm-1)

electron configuration [core]4s0.483d6.184p0.035s0.01 [core]4s0.303d5.974p0.044d0.015p0.01 [core]4s0.323d6.134p0.045s0.01 [core]4s0.263d5.724p0.045p0.01 [core]4s0.273d5.954p0.024d0.015p0.01 [core]4s0.283d6.154p0.03 [core]4s0.273d6.214p0.025s0.014d0.015p0.01 [core]4s0.273d6.024p0.035p0.01 [core]4s0.273d6.144p0.03 [core]4s0.273d5.974p0.034d0.01 [core]4s0.313d5.964p0.025p0.01 [core]4s0.293d5.764p0.025p0.01

-67.9 -18.7 -26.8 -1.9 -38.0 -1.0 þ2.3 þ9.1 -28.1

-68.2 -18.9 -27.7 -0.4 -37.8 -2.8 þ1.7 þ8.7 -28.8

-58.7 -9.1 -15.2 þ1.4 -28.9 þ11.1 þ12.7 þ20.8 -17.2

1783 1807 1796 1909 1870 1830 1837 1851 1774

-7.2

-7.3

þ3.0

1897

Frequency values scaled by 0.9613 to reproduce experimental fundamentals.113

study were calculated at a temperature of 298 K whereas the corresponding theoretical values were computed in the temperature range of 600-800 K. Furthermore, some additional energy is required for NO adsorption on [FeO]2þ-ZSM-5, [FeOH]2þZSM-5, and [Fe(OH)2]2þ-ZSM-5 clusters because their ΔG values are relatively more positive. Similarly, some energy is also required for CO adsorption on [FeOH]2þ-ZSM-5 and [Fe(OH)2]2þ-ZSM-5 clusters. The vibrational frequency for NO adsorption on an [Fe]2þZSM-5 cluster in this study was computed to be 1870 cm-1, using a scaling factor of 0.9613.113 The equivalent IR frequency for the Fe2þ-NO mode was reported as 1882 cm-1 at low temperature.42 The band near 1880 cm-1 is generally assigned to mononitrosyl coordinating to Fe2þ ions located in five-membered rings of Fe-ZSM-5 catalyst.51,58,63,64,66-68 The band at 1875 cm-1 was reported for Fe2þAlOx particles formed on the external surface of ZSM-5 zeolite.66,69 These values are in good agreement with each other. The calculated vibrational frequency value (1897 cm-1 with application of a scaling factor) for NO adsorption on the [HO-FeOFe-OH]2þ-ZSM-5 cluster matches well with the experimental value (1892 cm-1)58 reported for mononitrosyl on the binuclear iron-oxo site of Fe-ZSM-5 catalyst. The band around 1765 cm-1 was assigned to a low-spin FeINOþ complex in several experimental studies.65,70 The calculated vibrational frequency value (1783 cm-1) for an adsorbed NO molecule on an [Fe]1þ site in this study is relatively near this experimental band. The band at 1830 cm-1 was obtained for NO adsorption on an [FeO]2þ site in this study. In the same way, Lezcano et al.70 reported one experimental value of 1838 cm-1 for FeOx particles. Some IR bands in the range between 2200 and 1580 cm-1 have been covered for NO adsorption on Fe-ZSM-5 catalyst.63 Mihaylov et al.42 also reported some vibrational bands such as 2135, 1896, 1843, and 1766 cm-1 for NO adsorption on Fe-ZSM-5 catalyst at low temperature. FT-IR spectra including several bands at 1917, 1890, 1850, 1808, and 1766 cm-1 were recorded in an experimental study by Berlier et al.69 These bands can be assigned to different iron sites such as sites used in this study because some frequency values for NO adsorption in this study are in this band range. Moreover, they also reported that the vibrational IR frequency for Fe2þ-CO mode is 2140 cm-1 under low-temperature adsorption conditions. The corresponding frequency value for CO adsorption on an [Fe]2þ-ZSM-5 cluster in this study was calculated to be 2149 cm-1 with application of a scaling factor of 0.9613.113 These values are in good

agreement with each other. Berlier et al.69 also monitored Fe-CO bands in the range of 2200-2194 cm-1. Some of these bands can also be assigned to other iron sites used in this study because some frequency values for CO adsorption in this study are in this band range.

5. CONCLUSIONS The elementary reactions of the adsorptions of CO and NO on different iron sites of Fe-ZSM-5 clusters have been studied using DFT. The distances and adsorption energies for CO adsorption were similarly obtained on both small and large clusters. The adsorption energies (ΔE) and enthalpies (ΔH) for NO adsorption on [FeO]1þ-ZSM-5 and [Fe(OH)2]1þZSM-5 clusters are in reasonable agreement with the corresponding theoretical values. Some additional energy is required for NO and CO adsorptions on [FeO]2þ-ZSM-5 (for NO adsorption only), [FeOH]2þ-ZSM-5, and [Fe(OH)2]2þ-ZSM-5 clusters because their ΔG values are relatively more positive values. The scaled vibrational frequencies for adsorbed CO and NO on Fe2þ-ZSM-5 clusters (1870 and 2149 cm-1) are line with experimental frequency values for Fe2þ-CO and Fe2þ-NO bands (1882, 1875 and 2140 cm-1). Additionally, the calculated vibrational frequency (1897 cm-1) for NO adsorption on a [HO-FeOFe-OH]2þ-ZSM-5 cluster matches well with the experimental value (1892 cm-1). ’ AUTHOR INFORMATION Corresponding Author

*E-mail: mff[email protected].

’ ACKNOWLEDGMENT The author thanks Prof. Dr. Isik Onal for support with the Gaussian 03 package. This research was supported in : :: software part by TUB ITAK through TR-Grid : :Project. :: e-Infrastructure TR-Grid systems are hosted by TUB ITAK ULAKB IM and METU. Visit http://www.grid.org.tr for more information. The author also thanks Dr. Mehmet Zahmakiran for editing of the text. ’ REFERENCES (1) Hensen, E. J. M.; Zhu, Q.; Janssen, R. A. J.; Magusin, P. C. M. M.; Kooyman, P. J.; van Santen, R. A. J. Catal. 2005, 233, 123–135. 1949


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CO and NO Adsorptions on Different Iron Sites of Fe-ZSM-5 Clusters

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