Fe-containing ZSM-11 zeolites as active catalyst for SCR of NOx

Applied Catalysis A: General 266 (2004) 147–153

Fe-containing ZSM-11 zeolites as active catalyst for SCR of NOx Part II. XAFS characterization and its relationship with the catalytic properties Felix G. Requejo a , J.M. Ramallo-López a , Andrea R. Beltramone b , Liliana B. Pierella b , Oscar A. Anunziata b,∗ a

b

Dto. F´ısica, FCE, UNLP and IFILP (CONICET), La Plata, Argentina Grupo Fisicoqu´ımica de Nuevos Materiales, Centro de Investigación y Tecnolog´ıa Qu´ımica(CITeQ), Universidad Tecnologica Nacional, Facultad Cordoba, 5016 Cordoba, Argentina ; received in revised form 5 December 2003; accepted 21 December 2003 Available online 28 February 2004

Abstract This work describes the characterization by X-ray absorption spectroscopy (XAS) at Fe K-edge of the state of Fe in Fe-ZSM-11 zeolites prepared by novel sol–gel process used to decompose NOx to N2 by selective catalytic reduction (SCR), using iso-butane as reducer. Determining the Fe K-edge shift, the presence of Fen+ species in Fe-containing zeolite was determined and quantified. The symmetry and electronic characteristics of Fe in Fe-species are also discussed. The structural characteristics of the Fe species present in the catalysts are studied employing extended X-ray absorption fine structure spectroscopy. Fe2+ , Fe3+ and Fe2 O3 as isolated species were found as the active sites of the material for the SCR of NOx in concordance with Part I of this paper (Applied Catalysis, Part I, (2003)). © 2004 Elsevier B.V. All rights reserved. Keywords: Fe-zeolites; NO SCR; XAFS characterization

1. Introduction According to Part I of this work [1], Fe2+ , Fe3+ and Fe2 O3 in Fe-ZSM-11 zeolite are the species involved in the SCR of NOx reaction. Since active phases in catalysts are highly dispersed and diluted, the main advantage of X-ray absorption spectroscopy (XAS) at Fe K-edge over other techniques is that it provides direct information on charge density and the local environment of a specific absorbing atom without the requirement of long-range ordered structure and any special vacuum condition. The XANES spectroscopy incorporates the structure below the ionization potential (IP), as well as the structure that extends a few 10ths of eV above the IP. Spectra features below the IP are attributed to transitions of the excited electron to unoccupied molecular orbital. The region above the IP is dominated by multiple scattering effects of the outgoing electron wave. This part of the spectrum contains information on the geo∗

Corresponding author. Tel.: +54-3514690585; fax: +54-3514690585. E-mail address: [email protected] (O.A. Anunziata).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.12.032

metrical arrangement of atoms around the absorbing atom and on the electronic structure of this atom [2]. However, the XANES region of the spectrum cannot be described analytically in many cases, the available information has to be extracted by a “fingerprint” method, namely, by comparing the spectra of well-characterized reference compounds with the spectrum of the investigated compound. For the interpretation of the XANES spectra, some useful empirical rules are used. Those rules have been derived from a systematic study of well-known reference compounds. One important rule is the observation of the dependence of the energy of the edge on the valence of the investigated compound. Thus, as the oxidation state increases (and also by increasing the electronegativity of the neighboring atoms), higher energy of the shifted edge and pre-peak centroid position is obtained [3,4]. Another rule, is also obtained from the pre-edge region of XAS spectra. In addition to oxidation state, the intensity of the pre-peak associated with the 1s → 3d transition [5,6] have been shown to be sensitive to geometry of the iron atom. The total intensity of this transition has been shown

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Table 1 Edge shift (␦E), oxidation number (ON), relative proportion of Fe2+ (f) and absolute percentage in weight (w) present in each catalyst Catalyst

␦E

ON

f (%)

w (%)

Sample A Sample B Sample C

−1.86 −1.05 0

2.38 2.65 3

62 35 0

0.62 1.05 0

to increase with decreasing coordination number for iron model complexes due to the loss of inversion symmetry at the iron site. We will discuss this point more extensively in Section 3. The importance of the extended X-ray absorption fine structure (EXAFS) technique in catalysis studies has long been acknowledged [7]. This powerful technique refers to the oscillatory structure in the X-ray absorption coefficient beyond 50 eV of threshold, where the photoelectron back-scattering responsible for this phenomenon is relatively weak. EXAFS analysis is no longer limited to first neighbors, and distance determinations are now often comparable in accuracy to those from X-ray diffraction measurements [8]. In a similar approach, Koningsberger and co-workers [9–11] have successfully applied XAS techniques to elucidate the nature of Fe sites in ZSM-5 zeolite, introduced by chemical vapor deposition of FeCl3 following the evolution at each step of the synthesis and during the reduction of NO. They found that the majority of iron is present as Fe oxo-hydroxo-binuclear complexes as has been also proposed by other authors [12]. In this work, our main objective is to characterize by XAFS the nature of Fen + species in Fe-containing zeolites prepared by sol–gel process, the Bronsted sites of the molecular sieve catalyst tuned with lower unoccupied molecular orbital (LUMO) of Fen + is probably the key of the efficient activity of this material for reduction of NOx to N2 with iso-butane in presence of oxygen with very low N2 O selectivity [1].

2. Experimental Catalysts preparation, catalytic activity, XRD, BET and FTIR results were described in Part I [1]. The samples under study, are denoted as samples A and B (1% and 3 wt.% of Fe, respectively, using FeSO4 ) and sample C (3 wt.% Fe using Fe(NO3 )3 ), the summary of their characteristics are shown in Table 1 of Part I of this work. 2.1. Fe K-edge X-ray absorption X-ray absorption spectra were measured using XAS beamline at the Laboratorio Nacional do Luz S´ıncrotron (LNLS), Campinas, SP, Brasil. Spectra were measured in transmission mode in air and room temperature. A Fe foil spectrum was used to calibrate the absolute energy scale and

␣-Fe2 O3 was measured as reference compound. Monochromators on the beamline was equipped with Si(1 1 1) crystals. The 0.3 mm vertical aperture of the beam definition slit in the hutch provided a resolution of about 2.5 eV at the Fe K absorption edge (7112 keV) for XANES experiments. Spectra were measured with an increment of 0.8 eV in the region from 7040 to 7200 keV and 2 eV in the region between 7200 and 7800 keV (EXAFS region). A complete description of the XAS beamline can be found elsewhere [13]. X-ray absorption data were analyzed using standards procedures [14]. To do that, the fine structure oscillations of each spectrum were isolated using the AUTOBK [15] program. The background function was approximated using a spline with knots evenly spaced in photoelectron wave number. The values of the spline at the knot energies were found by optimizing the Fourier transform of the isolated χ(k) spectrum, which was obtained from the following formula: χ(k) =

µ(E) − µ0 (E)

µ

(1)

where µ0 (E) is the background spline and µ is the edge step. The abscise was converted from energy to photoelectron wave number k by the relation:
k = η−1 2m(E − E0 ) (2) The obtained χ(k) was then Fourier transformed over a specified k range. The spline was adjusted so that this experimentally derived Fourier transform best matched the one obtained from theoretical calculation using the FEFF7 program [16]. As an ab initio calculation, FEFF uses a list of atomic coordinates in a cluster and physical information about the system, such as type of absorbing atom and excited core level for its calculation. In our case, the list of atomic coordinates was simplified using atoms [17] which generate the required coordinates starting from a crystallographic description of the ␣-Fe2 O3 system taken from the literature [18]. The data were then analyzed using the FEFFIT [19] program. In FEFFIT, the fitting model was expressed as a sum over all scattering geometries or simple paths j. Each individual path was described by an appropriate theoretical fitting standard from FEFF. The fitting standards were modified according to the EXAFS equation [20] χF (k) = Im

 Nj S 2 Fj (k) 0

j

2kR2j

ei(2kRj +Φj (k)) e−(2k

2σ2) j

(3)

This equation expresses the total calculated χF (k) for a particle central atom as a sum over all scattering geometries about that atom. The effective scattering amplitude Fj (k) and phase shifts Φj (k) for each path were calculated using FEFF. For each path j FEFFIT can modify the path distance Rj , the mean-square displacement (or Debye–Waller factor) σj2 around Rj , and the amplitude Nj S02 in some manner specified by the fitting model. In our case, an isotropic relaxation was assumed to calculate the Rj in the samples with higher Fe content. For the sample with 1% of Fe, the

F.G. Requejo et al. / Applied Catalysis A: General 266 (2004) 147–153

Typical values of R are a few percent, which means that our fit is limited by systematic errors in the model calculations. Quoted errors in our fitted variables include systematic contributions that are introduced from the measurements, theory, and analysis [19].

3. Results and discussion 3.1. Fe K-edge X-ray absorption near edge spectroscopy (XANES) The XANES pre-edge feature is an important tool for elucidating the local electronic structure and the local environment of transition metal ions. X-ray absorption spectroscopy (XAS) allows obtaining oxidation state, site symmetry, spin state, and crystal field splitting. To study Fe-species in microporous materials we used hard X-rays at the K edge. The pre-edge features at Fe K-edge are also sensitive to the electronic structure, even if they are less resolved than L-edges. Furthermore and in contrast with L2,3 -edges, the K pre-edge features are sensitive to the p–d hybridization that, for example, directly influences the physico-chemical properties of the materials [6,9–11]. The K pre-edge features are usually associated with electronic transitions from the 1s core orbital to the localized 3d orbitals (electric-quadrupole transitions) and to the 4p orbital mixed to the 3d (electric dipole transitions) [6,22–24]. They strongly depend on the local symmetry, which affects the degree of admixture between 4p and 3d orbitals. The

1

Normalized Absorption

distances were let to change independently to obtain a good fit, reducing the number of fitting parameters considerably. Nj was set to the probability of occurrence of the path and accounted for the average coordination number of the shell. The passive electron reduction factor S02 only depended on the absorbing atom and was determined from a reference compound (␣-Fe2 O3 ). FEFFIT also allows a shift in E0 , the energy reference for each path, which modifies the effective photoelectron wave number by the relation (2) and accounts for errors in the FEFF model. Only two parameters are fitted for the E0 values, as they can be considered to being dependent only on the type of atoms of the path, one for the Fe–O path and another for the Fe–Fe path. The same assumption is used for the Debye–Waller factors. Listed with our fit results are the XAFS reliability factor R and the reduced chi-squared χν2 , where ν is the number of degrees of freedom in the fit given by the number of independent points in the fit minus the number of free parameters. Each measurement contains about N = 29 independent points as given by the relation N = 1 + (2/π) R k, where R is the fitting range in R space and k is the data range in k space [21]. The number of fit parameters varies from 11 to 12. The XAFS reliability factor R is given by:  χ˜ data (Ri )| − |χ˜ model (Ri )|]2  R= i (4) ˜ data (Ri )|2 i |χ

149

23

Sample A Sample B Sample C Fe2O3

7.12 7.14 7.16 7.18 7.20

Energy [keV]

Fig. 1. XANES spectra with arbitrary offsets of Fe-ZSM-11 catalysts and Fe2 O3 .

interpretation of X-ray absorption spectra necessitates electronic structure calculations to obtain quantitative information from the spectra. Studies like those were already performed for Fe in the Fe-oxide systems [25–27]. In the next items, we present the different analyses of XANES results, which allow us to determine the nature of Fe-sites in our samples. 3.2. Chemical shift study for Fe oxidation state determinations Removing valence electron from an atom, the screening of core electrons provided by the valence electrons is reduced, and the core levels become more tightly bound. The edge and pre-peak chemical shift can be correlated with differences in oxidation state for an element [3,28]. The pre-edge centroid position for iron in Fe3+ is 1.4 ± 0.1 eV higher in energy than the corresponding for Fe2+ [28]. In a different way than other authors, who worked with higher energy resolution conditions than us (see [9–11]), and because of the indeterminations in background subtraction to isolate the pre-peak feature, we preferred to follow the energy shift at the edge than the pre-peak centroid position to determine the average oxidation state value for Fe ions like it has been done in other articles [29,30]. Fig. 1 shows the XANES spectra of each catalyst as well as ␣-Fe2 O3 spectrum used as experimental reference. The edge shift relative to the metallic reference, was determined by the integration of the spectra to the energy that leads to the same area for all samples, i.e. by integration of the empty electronic states below the Fermi level. This method proposed by Capehart [29] is independent of the fine structure, namely, the occurrence of certain peaks or shoulders in the raising edge, contrary to what happens with the simple determination of the inflection points. Using the data of Kataby et al. [30], average oxidation values for each sample are determined (Fig. 2 and Table 1). These averaged oxidation numbers can be understood as a linear combination of Fe2+ and Fe3+ ion species. Results show that Fe species are as Fe3+ in sample C, while there is a mixture of Fe2+ and Fe3+ ions in catalysts A and B. From

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Sample C

4.0

Fe2O3

3.5 Sample B

δE [eV]

3.0

Fe3O4 [22]

2.5 Fe amorph [22]

Sample A

2.0 1.5 1.0 0.5

FeO [22]

0.0 1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

Oxidation Number

Fig. 2. The energy shift of the iron K-edge as a function of the oxidation number of the compounds, analyzed according to Capehart [29] and using data from Kataby et al. [30].

these results, and supposing a linear superposition of Fe2+ and Fe3+ species present in each catalyst, we determined the weight percentage of Fe2+ present in each catalysts being of 0.6 and 1.0%, respectively (Table 1). This shows a first correlation between NOx conversion and the presence of Fe2+ in the catalysts. An intriguing aspect is the very low selectivity to N2 O observed for samples B and C after 400 ◦ C. Once again, the catalytic results showed in Part I [1] and the XANES data seem to indicate that, in the reaction pathway of the N2 O production at least a small amount of Fe3+ species are necessary to produce an adequate low selectivity to this non-desired reaction product. 3.3. Pre-edge feature studies for Fe site symmetry determinations In general, the small pre-edge peaks of the K-edge absorption spectra for transition metal compounds with partially filled d orbital have been assigned to the transition from 1s to nd orbitals even though it is dipole-forbidden transition. The transition has different intensities between tetrahedral and octahedral sites and thus, has been used to infer site symmetry in transition metal compounds [31,32]. Some molecular orbital calculation studies have reported that the pre-edge spectra of Fe compounds show a small absorption peak, assigned to the transition from 1s to nd, which might be related to the Fe 3d and 4p orbitals mixing and to the site symmetry [33,34]. They have shown that the pre-edge intensity generally increases with the departure from centro-symmetric coordination environment. Thus, the pre-edge absorption intensity increases in order of octahedral, five coordinate and tetrahedral sites. Complexes in noncentro-symmetric environments have more intense pre-edge features than centro-symmetric complexes [24,31,32,35]. This increase in intensity has been attributed to metal 4p mixing into the 3d orbital which provides some electric dipole allowed 1s → 4p character to the transition, and thus, adds to the intensity from the elec-

tric quadrupole mechanism. Since the electric quadrupolecoupled mechanism is so much weaker than the electric dipole-coupled mechanism, only a few percent of 4p mixing into the 3d orbitals can have a significant effect on the intensity of the 1s → 3d pre-edge feature. Fig. 1 shows the Fe pre-edge region in the experimental XANES spectra of the samples and references compounds. ␣-Fe2 O3 was taken as reference of Fe3+ ions octahedral (Oh) sites symmetry. A relative intense pre-peak (at around 7111 eV) is well assigned to Fe3+ ions with Td symmetry, which is not present in our samples. However, all spectra of catalysts shown in Fig. 1 exhibit a small and broad peak at around 7114 eV. The intensity of this peak is about the same in all samples. As was mentioned before, this pre-peak corresponds to transitions from 1s to 3d-like levels and is Laporte forbidden in systems with octahedral symmetry. Nevertheless, in real systems, were distortions from the perfect inversion center and a partial mixing of p and d levels occur, this pre-peak gain in intensity. On the contrary, under tetrahedral symmetry environments, without any inversion center and with strong p–d mixing, compounds are characterized by a strong peak in the pre-edge region due to the allowed transition from the ground state A1 to the final state T2 [28,36]. In our case, from both intensity and broadness of pre-peak at the samples spectra can be associated to Fe ions in distorted octahedral environment, i.e. Fe seem to be forming FeO6 octahedral units in the samples. The peak intensity is closely related to the symmetry around Fe atoms, and this peak become more intense as the symmetry is distorted from a regular octahedron [6,37]. Each pre-edge peak intensity of all the catalysts samples is a little larger than that of octahedral Fe3+ compounds such as ␣-Fe2 O3 . So, the local symmetry around Fe is octahedral and more distorted than that of ␣-Fe2 O3 . In particular, this is more evident for catalysts with presence of Fe2+ ions in samples A and B. Another remark related to the shape of the XANES spectra can be marked observing the main peaks denoted as 2 and 3 in Fig. 1. These peaks are attributed to the dipole-allowed transition from the 1s core level to the p␲ -polarized and the p␴ -polarized 4p orbital, respectively [38]. Then, it can be well noticed that the trend of the shape is the more Fe2+ ions content in the sample the lower the p␴ -polarized 4p orbital (and the higher p␲ -polarized 4p orbital). 3.4. Shape of spectra and iron species associated Fig. 3 shows the second derivative of XANES spectra of samples and ␣-Fe2 O3 reference. Different highlighted regions denoted as 1, 2 and 3 indicate portions of XANES spectra generally associated to different transitions. Region 1: assigned to 1s → 3d transition which could not be expected with the electric dipole matrix. It only appears with small absorption due to the electric quadrupole allowed transition (it is discussed in the previous section).

F.G. Requejo et al. / Applied Catalysis A: General 266 (2004) 147–153 2

1

151

3

k χ(k) [A. U.]

B

Sample B

2

C

Sample A

Sample C

nd

XANES 2 derivative [A. U.]

A

X

Fe2O3

c 0

d e 7.11

7.12

7.13

7.14

7.15

1

2 3 Distance [Å]

7.16

E [KeV] Fig. 3. Second derivatives of normalized Fe K-edge XANES spectra for X: ␣ Fe2 O3 ; (A) sample A; (B) sample B; (C) sample C.

The second derivatives peaks at this region are observed in similar positions. Region between 7115 and 7131 eV is associated to the dipole-allowed transition of 1s core electron to 4p energy level. Here, the transition and the core hole creation can produce two different final states. Region 2: between 7115 and 7120 eV, corresponds to the 1s → 4p main transition followed by the ligand metal charge transfer (shakedown process). Here, an increased effective nuclear charge with respect to the valence orbital cause putting down the orbital to lower energy levels, and then an electron from the O 2p orbital can be transferred to 3d orbital of the iron. In our case, sample C looks similar than the reference ␣-Fe2 O3 (spectra C and X). The other samples (spectra A and B), which possess Fe2+ species, exhibit a different behavior at the position of minimum b (Fig. 3) and also a maximum is growing on the right of region 2 when Fe2+ content is increasing and Fe3+ is decreasing in the catalysts. Region 3: between 7123 and 7131 eV. It is important to observe here, the region between 7128 and 7131 eV, which corresponds to the large absorption intensities in XANES spectra (Fig. 1). The large absorption peaks, corresponding to minima d and e, also corresponds to lines 1 and 2 denoted in Fig. 1 and discussed at the end of previous item. Moreover, in the samples, the second derivatives spectra show the same trend that we can observe for this region in Fig. 1, i.e. the similitude between catalysts and ␣-Fe2 O3 is in sample A < sample B ≪ sample C, which is coincident with the denoted Fe2+ content determined by fitting of the Fe K-edge shifting (Fig. 2). The differences in shape and position reported here for these minima (d and e) are similar to those reported by Grunes [3] (peaks denoted as C1 and C2, respectively in first derivatives spectra in that study) between Fe3+ and Fe2+ ions in Fe2 O3 and FeO compounds. This gives another evidence of the presence of Fe2+ and Fe3+ ions in the corresponding samples, as was obtained by XANES.

4

5

Fig. 4. Fourier transform of the Fe K-edge EXAFS spectrum of samples A, B and C. The intensity of the Fourier transform of ␣-Fe2 O3 is shown for comparison purposes.

3.5. Fe K-edge extended X-ray absorption fine structure (EXAFS) EXAFS experiments were performed in order to determine the type and nature of Fe-species in the samples. Fig. 4 compares the intensities of the Fourier transforms of the EXAFS spectra. ␣-Fe2 O3 is also shown for comparison purposes. Sample C is quite similar pattern to ␣-Fe2 O3 . Sample A shows big differences in the first peak corresponding to Fe–O shells which now is well resolved in two different distances. Sample B shows an intermediate behavior. This trend is also obtainable from XANES results in previous section and comparable with the FTIR and XRD results showed in Part I [1]. This qualitative analysis is verified by the EXAFS fits shown in Fig. 5. Solid lines show the amplitude of the 1.5

2

7.10

k χ(k)

b a

A

1.2 0.9 0.6 0.3 0.0 1.2

B

0.9 0.6 0.3 0.0

C

1.2 0.9 0.6 0.3 0.0

0

2

4

6

Distance [Å]

Fig. 5. Fourier transform of k2 -weighed Fe K-edge EXAFS spectrum of samples A, B and C, respectively. Solid lines show the fitted EXAFS function obtained with parameters shown in Tables 2–4.

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F.G. Requejo et al. / Applied Catalysis A: General 266 (2004) 147–153

Table 2 Fe K-edge EXAFS results of sample C (C in Fig. 4) Atom type

N

O O Fe Fe Fe

2.7 2.0 0.8 2.4 1.6

χν2

(1) (1) (2) (2) (2)

D (Å)

σ 2 (Å2 )

E0 shift

1.92 2.08 2.89 2.96 3.35

0.002 0.002 0.003 0.003 0.003

−0.5 −0.5 2.0 2.0 2.0

(1) (1) (1) (1) (1)

(1) (1) (1) (1) (1)

(1) (1) (5) (5) (5)

= 6.5, R = 0.010.

Table 3 Fe K-edge EXAFS results of sample B (B in Fig. 4) Atom type

N

O O Fe Fe Fe

2.4 2.3 1.3 2.4 1.6

(1) (1) (3) (2) (2)

D (Å)

σ2

1.92 2.08 2.94 3.01 3.49

0.002 0.002 0.011 0.011 0.011

(1) (1) (2) (4) (4)

(Å2 ) (1) (1) (2) (2) (2)

E0 shift −2.0 −2.0 1.9 1.9 1.9

(3) (3) (5) (5) (5)

Catalyst

Sample A Sample B Sample C

fitted EXAFS function and their parameters are shown in Tables 2–4. The parameters for the different shells show that sample C has a ␣-Fe2 O3 -like structure with a small decrease in the coordination number of both Fe–O and Fe–Fe shells showing the dispersion of the oxide and some the presence of some Fe3+ specie as counter ion (Table 2). For sample B (Table 3), this diminution is more evident but the relation between the average coordination numbers of the first two shells of oxygen is not being conserved, showing a major contribution from the second shell. The most important changes are observed for sample A (Table 4). The distance between the first two shells is bigger leading to the two observed peaks in the Fourier transform. The relation between the average coordination numbers of these two shells is also bigger, appearing more oxygen atoms in the second shell. This behavior is consistent with XANES results. In effect, it was shown the Fe species appears as Fe2+ in samples A and B in 62 and 35%, respectively. The increase of oxygen neighbors at bigger distances is consistent with the formation of Fe2+ structures like FeO, which has six oxygen atoms as first neighbors at 2.15 Å [39]. In contrast, only Fe3+ cations are present in sample C and practically all of them are forming ␣-Fe2 O3 -like structures in this sample. Because of the coexistence of different phases, an structural characterization of the active species for the SCR of NO was not possible to obtain. Table 4 Fe K-edge EXAFS results of sample A (A in Fig. 4)

Fe2+ / uc

Fe3+ / uc

Fen+ / Al

Fe2+ / Al

Fe3+ / Al

0.639 1.083 –

0.392 2.018 3.095

0.193 0.581 0.580

0.119 0.203 –

0.073 0.378 0.580

FTIR, Lewis sites (Fe2+ )

0.031 0.041 –

Sample

NOx TOF (s−1 ) FTIRb , T = 400 ◦ C

XANESa T = 350 ◦ C

T = 400 ◦ C

T = 430 ◦ C

0.0149 0.0170

0.02137 0.01467

0.0235 0.0138

0.00224 0.00147

a

TOF of NOx according the data showed in Table 5. TOF of NOx from pyridine retained at 400 ◦ C and 10−4 Torr (Table 5). b

3.6. Integral reaction rate: TOF of NOx for the samples The analysis of NOx turn-over frequency (TOF) based on total Fe2+ species in samples A and B, calculated according to XANES and FTIR results (Table 5) are shown in Table 6. The TOF of NOx results obtained from XANES and FTIR are comparable. We must take account that the data obtained from XANES are due to all the Fe2+ species present in the samples, but the data used in the calculus of the TOF using FTIR are only from Fe2+ species which retain pyridine at 400 ◦ C (the reaction temperature chosen to calculate the TOF of NOx ). This is the reason of the different absolute value of TOF for the same samples. If we relate the TOF for samples B and A (XANES) XTOFB/A and (FTIR) FTOFB/A , we can see that they are very similar. Taking into account that the reaction formation rate (RFR, see Part I of this work) [1] of N2 O over sample A, is about six times higher than of samples B and C, between 270–400 ◦ C, and that it is very similar for samples B and C, we suggest that the type of Fe3+ species are very similar, but in the same way, they are different to the Fe3+ species in the sample A. This is the possible reason of the higher selectivity to N2 O of the sample A.

4. Conclusions

Atom type

N

D (Å)

σ 2 (Å2 )

E0 shift

O O Fe Fe O

1.5 (1) 2.5 (1) 1.95 (3) 5.6 (5) 2.6 (2)

1.87 2.05 2.96 3.39 3.49

0.0012 (4) 0.0012 (4) 0.011 (4) 0.011 (4) 0.0012 (4)

−1.0 −1.0 1.9 1.9 −1.0

(1) (1) (3) (4) (4)

XANES

Table 6 NOx turn-over frequency according to XANES results for Fe2+ , and FTIR results for Lewis sites (Fe2+ ) of the samples A and B

A B

χν2 = 5.7, R = 0.014.

χν2 = 6.4, R = 0.015.

Table 5 FTIR of pyridine (mmol/g) retained at 400 ◦ C before desorption at 10−4 Torr for 10 h for the samples A, B and C; and Fen+ species from XANES analysis

(3) (3) (5) (5) (3)

Fe-ZSM-11 has showed as active catalyst for the SCR of NOx . The state of iron in the three Fe-ZSM-11 samples prepared by sol–gel, was investigated by means of XANES and EXAFS. On the basis of XANES results, the quantity of Fe2+ and Fe3+ ions in the samples were determined. We could also distinguish the different electronic and geometric

F.G. Requejo et al. / Applied Catalysis A: General 266 (2004) 147–153

nature of each ion in the catalyst. EXAFS results were compatible with the quantity of Fe2+ and Fe3+ ions determined by XANES, showing the appearance of an oxygen shell at a short distance when Fe2+ ions were present. A fraction of Fe atoms appears as Fe2 O3 specially in the sample C. The NOx conversion appears as a function of Fe2+ species, but Fe3+ species show very low selectivity to N2 O.

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