4.  Results

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In this section, ex situ and in situ-UV/VIS-DR, EPR and FT-IR spectroscopic results obtained for the different catalysts are described in relation to their preparation procedure, as well as in dependence on their interaction with different reactants. Although catalytic tests have not been performed within this thesis, but in the laboratories of the cooperation partners Prof. Dr. W. Grünert and Dr. Javier Pérez-Ramírez, their main results are also shortly described in this section since their knowledge is essential for the discussion of structure-reactivity relationships (section 5.2)

4.1 Structure, distribution and redox behaviour of Fe species and surface acidity of the Fe-zeolites

First of all, results of the characterization of the zeolites by UV/VIS-DR, EPR and FT-IR spectroscopy are described in relation to their preparation procedure. These measurements were performed to investigate the effect of the genesis of the samples on the nature and distribution of iron species. For samples prepared by CVD, the influence of washing intensity, heating rate during calcination, Si/Al ratio of the zeolite matrix and use in the SCR reaction on the nature and distribution of iron species in Fe-ZSM-5 zeolites has been explicitly studied. Redox properties of iron species play a crucial role in the catalytic reactions, hence, redox kinetics of isolated Fe+3 ions and iron oxide clusters were studied by in situ UV/VIS-DRS. Surface acidity of the zeolites was analyzed by adsorption of pyridine using FT-IR spectroscopy.

4.1.1 UV/VIS-DRS studies

Assignment of UV/VIS-DRS signals

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A typical UV/VIS spectrum of an Fe-containing zeolite is shown in Fig. 3 (section 2.4.2). It is characterized by a broad absorption which is deconvoluted into subbands to facilitate the assignment of different iron species. In principle, two ligand-to-metal charge-transfer (CT) transitions, t1→ t2 and t1→ e, are to be expected for a Fe3+ ion [132]. For isolated Fe3+ ions they fall in the high energy range of the spectrum, usually below 300 nm, whereby their particular position depends on the number of ligands. Thus, CT bands of isomorphously incorporated tetrahedrally coordinated Fe3+ ions have been observed at 215 and 241 nm in Fe-silicalite [66] while a band at 278 nm is detected for isolated octahedral Fe3+ sites in Al2O3 [135]. Although a clear discrimination of CT bands of isolated Fe3+ ions in tetrahedral and higher coordination is not straightforward due to their similar wavelength range, these values suggest that CT bands of Fe3+ ions are red-shifted with increasing number of coordinating oxygen ligands. The same trend has been observed accordingly also for V5+ species [163]. Based on these considerations, subbands below 250 nm are assigned to isolated tetrahedral Fe3+ while those between 250 and 300 nm are attributed to isolated Fe3+ with a higher number of coordinating ligands.

CT bands between 300 and 400 nm are assigned to octahedral Fe3+ in small oligomeric Fe x O y clusters [66] while bands above 450 nm arise from larger Fe2O3 particles as can be seen, too, from the spectrum of the reference sample α-Fe2O3 (Fig. 6a). To facilitate band assignment, experimental spectra were deconvoluted into respective subbands (Fig. 3). For the deconvolution procedure, the lowest possible number of subbands has been used. Since the two CT transitions for the same Fe3+ species are experimentally not resolved, they have been fitted by one subband only for each type of Fe3+ species (tetrahedral and octahedral coordination). For deriving a quantitative estimate of the different Fe species coexisting in the zeolite samples, the percentage of the subbands with respect to the total area of the experimental spectrum has been multiplied by the overall Fe content (determined by ICP-OES) (Table 4.1). Being aware that the obtained values in Table 4.1 are an estimate only due to issues discussed below, they are nevertheless regarded to be helpful for a comparison of different samples.

Fig. 4.1. UV/VIS-DR spectra of reference samples (a) α-Fe2O3 and (b) γ-Fe2O3 recorded at 298 K.

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As seen from Fig. 4.1a, α-Fe2O3 does not contribute to the UV/VIS spectrum below 300 nm but γ-Fe2O3 (Fig. 4.1b), another reference sample, does contribute below 300 nm. This can cause some uncertainty when the area of deconvoluted subbands is used to derive the actual percentage of isolated sites in samples that contain also Fe x O y clusters because, depending on their structure, contributions of the particles in the wavelength region below 300 nm cannot be completely ruled out. In almost all particle containing samples, iron oxide particles are too small to be visible byXRD. However, EXAFS measurements of samples with higher Fe content (5.2 wt.%) point to the existence of oxide particles with an α-Fe2O3-like short range order [36]. Therefore, the existence of γ-Fe2O3 particles in the zeolites seems rather unlikely. However, to be on the safe side, the percentage of isolated and small oligonuclear Fe species in particle-containing samples (Table 4.1)has to be regarded as an upper limit.

In Fig. 2.3, light absorption above 300 nm occurs in a very broad range suggesting the superposition of CT bands for a variety of slightly different small oligonuclear Fe x O y clusters and larger Fe2O3 species. As mentioned above, for spectra deconvolution the lowest possible number of sub-bands in the range above 300 nm has been used that was needed to obtain a satisfactory fit of the experimental spectrum. This procedure is regarded to be acceptable, although a deconvolution of a broad and poorly structured experimental spectrum into sub-bands by mathematical means is always arbitrary to a certain extend. The subbands above 300 nm have to be understood in terms of reflecting a certain distribution of slightly different cluster geometries rather than representing a certain number of different individual cluster species.

In principle, d-d transitions of Fe3+ ions should also be considered between 350 and 550 nm. However, they are symmetry- and spin-forbidden and therefore, some orders of magnitude weaker than CT transitions. Hence, they are not visible and interpretation of the spectra is focused on the intense CT bands. Divalent iron does not contribute to the UV/VIS spectrum since its bands fall in the near-infrared range around 1000 nm [133]. However, Fe species in mixed valence iron oxides containing both Fe2+ andFe3+ give rise to intervalence charge transfer (IVCT) transitions in the visible and near infrared region due electron delocalization between Fe2+ andFe3+ ions.

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The UV/VIS-DRS results of differently prepared Fe-zeolites are presented below with respect to the preparation method.

Chemical Vapor Deposition (CVD)

In the following section, the influence of washing, calcination steps and of use in catalysis as well as defect density of the parent ZSM-5 matrix (Si/Al ratio) on the UV/VIS spectra is described. The percentage of the different Fe species has been derived by spectra deconvolution for each sample (Table 4.1).However, for the sake of clarity, deconvoluted subbands are only shown in selected figures.

Influence of washing intensity

It has been reported in the literature that thorough washing is crucial for the formation of highly dispersed Fe structures [36,56]. In this study, the influence of the washing step was assessed by performing the washing procedure of A(CVD) with a total amount of 1 or 10 l water per 5 g catalyst (A(CVD,W1) and A(CVD,W10), respectively) (Fig. 4.2a).

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Fig. 4.2. UV/VIS-DR spectra of hydrated Fe-ZSM-5 samples at 293 K. (a) A(CVD,W1) (experimental spectrum, thick solid line) and deconvoluted subbands (thin lines), assignments: --- isolated Fe+3, — small oligomeric Fe x O y moieties, …. Fe2O3-like clusters and A(CVD,W10) (experimental spectrum, broken line). (b) Experimental spectra of A(CVD,W1,C0.5) (solid line) and A(CVD,W10,C0.5) (broken line) after use in the SCR reaction. Samples were recalcined after catalysis in air at 823 K.

The UV/VIS spectrum of hydrated as-prepared A(CVD,W1) exhibits absorption mainly below 400 nm. The relative percentage of the deconvoluted subbands in Table 4.1 indicates that the majority of iron species is present as isolated ions and small oligomers. The band at ~500 nm suggests the presence of a certain amount of iron oxide particles which might have formed during the washing step.

In sample A(CVD,W10) light absorption above 400 nm is lower in comparison to sample A(CVD,W1) which indicates that intense washing diminishes the amount of large Fe3+ x O y clusters slightly (Table 4.1). This effect is still clearly seen even in the calcined samples A(CVD,W1,C0.5) and A(CVD,W10,C0.5) after catalysis, although the calcination step itself forces cluster formation. This influence will be separately discussed below. This is in good agreement with earlier studies which show less clusters in A(CVD,W10,C0.5) as compared to A(CVD,W1,C0.5) [36].

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Table 4.1. Percentage of the area of the sub-bands (I1 at λ< 300 nm, I2 at 300 < λ < 400 nm, and I3 at λ > 400 nm) derived by deconvolution of the UV/VIS-DRS spectra and corresponding Fe percentage derived from total Fe content determined by ICP [32,36,55,58,64,159,161].

Catalyst

I1a/ %

Fe1/wt.%

I2b/ %

Fe2/wt.%

I3c/ %

Fe3/wt.%

Total Fe Wt.%

A(CVD,W1)

47

2.53

45

2.43

8

0.43

5.4

A(CVD,W1,C0.5)

27

1.46

36

1.94

37

2.00

5.4

A(CVD,W1,C0.5) used

25

1.35

31

1.67

44

2.37

5.4

A(CVD,W1,C5)

27

1.46

35

1.89

38

2.05

5.4

A(CVD,W1,C5)used

22

1.19

30

1.62

48

2.59

5.4

A(CVD,W10)

46

2.30

47

2.35

7

0.35

5.0

A(CVD,W10,C0.5)used

27

1.35

36

1.80

36

1.8

5.0

A'(CVD,W1,C2)

44

2.19

45.2

2.25

10.8

0.54

5.0

B(CVD,W1)

28

0.73

45

1.17

27

0.70

2.6

B(CVD,W1,C5)

26

0.68

38

1.00

35

0.91

2.6

A(SSIE)5.2 uncalcined

32

1.66

37

1.90

31

1.62

5.2

A(SSIE)5.2

30

1.56

32

1.66

38

1.98

5.2

A(MR)0.5 uncalcined

91

0.45

9

0.05

-

-

0.5

A(MR)0.5

83

0.41

17

0.09

-

-

0.5

A(ILIE)0.2 uncalcined

100

0.2

-

-

-

-

0.2

A(ILIE)0.2

95

0.19

4.2

0.01

-

-

0.2

A(ILIE)0.3

95

0.28

5

0.01

-

-

0.3

A(ILIE)0.6

68.8

0.41

27.9

0.16

3.3

0.19

0.6

A(ILIE)0.7

60.7

0.42

35.7

0.25

3.5

0.02

0.7

A(ILIE)1.2 uncalcined

57.0

0.68

28

0.33

15

0.18

1.2

A(ILIE)1.2

40.8

0.48

41.7

0.5

17.5

0.21

1.2

Fe-ZSM-5(LIE)1.4

31.0

0.43

38.2

0.53

30.7

0.43

1.4

c-Fe-silicalite

100

0.68

-

-

-

-

0.68

ex-Fe-silicalite

93

0.63

7

0.04

-

-

0.68

ex-Fe-ZSM-5

44.3

0.29

41.5

0.27

14

0.09

0.67

c-Fe-beta

84

0.52

16

0.09

-

-

0.62

ex-Fe-beta

47

0.30

36

0.22

17

0.1

0.62

(Fe-SBA-I)0.95

89

0.88

11

0.11

-

-

0.95

a isolated Fe3+ in tetrahedral and higher coordination; b small oligomeric Fe x O y clusters; c large Fe2O3 particles

Influence of calcination

It has been reported that the calcination step is crucial for the iron constitution in the final catalyst [57,116]. Hence, the influence of the calcination on the distribution of iron species was assessed by comparing UV/VIS spectra of sample A(CVD,W1) before calcination and after calcination in air at 873 K using heating rates of 0.5 and 5 K/min for A(CVD,W1,C0.5) and A(CVD,W1,C5), respectively (Fig. 4.3a). It is clearly seen from the UV/VIS spectra (Fig. 4.3a) that light absorption above 400 nm being characteristic of extended Fe2O3-like clusters increased considerably after calcination indicating the formation of Fe2O3 clusters upon calcination. This effect is somewhat higher after calcination with 5 K/min than with 0.5 K/min (Fig. 4.3a and Table 4.1).

Fig. 4.3. UV/VIS-DR spectra of hydrated Fe-ZSM-5 samples recorded at 293 K. (a) A(CVD,W1) (thick solid line), A(CVD,W1,C0.5) (broken line) and A(CVD,W1,C5) (thin solid line). (b) B(CVD,W1) (broken line) and A(CVD,W1) (solid line); (c) B(CVD,W1,C5) (broken line) and A(CVD,W1,C5) (solid line). (d) A(CVD,W1,C0.5) before (thick line) and after (broken line) use in the SCR of NO, samples were recalcined after catalysis in air at 823 K.

Influence of the Al content and defect density of the parent ZSM-5 matrix

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The effect of Si/Al ratio on the distribution of iron species in the parent ZSM-5 support was studied by subliming FeCl3 into the ZSM-5 matrix B, which has a higherSi/Al ratio than matrix A (≈ 40 vs. ≈ 14).This, in turn, leads to a lower number of H+ sites available for exchange by Fe ions, and a large amount of internal silanol groups, most likely silanol nests [36]. As expected, the total Fe content in sample B(CVD,W1) is lower than in sample A(CVD,W1) (Table 3.1), but the Fe/Al ratio of 1.13 [36] shows that sample B(CVD,W1) is even more overexchanged than A(CVD,W1) (Fe/Al ≈ 0.9). The UV/VIS spectra of as-prepared samples B(CVD,W1) and A(CVD,W1) are compared in Fig. 8b. It can be seen that there is a slightly higher percentage of extended Fe2O3-like clusters in sample B(CVD,W1) than in sample A(CVD,W1) (Fig. 4.3b and Table 4.1).

Interestingly, this trend is not retained for the calcined samples. Calcination with a heating rate of 5 K/min leads to a slightly less pronounced formation of large Fe2O3-like aggregates in sample B(CVD,W1,C5) compared to A(CVD,W1,C5) as evident from UV/VIS spectra in Fig. 4.3c. This may be explained by the presence of silanol nests in sample B(CVD,W1,C5), which may serve as additional nuclei for aggregation as has been suggested earlier on the basis of FT-IR, Mössbauer and EXAFS data [36]. In well-structured HZSM-5 matrices Fe x O y clusters being formed upon calcination have been shown to migrate towards the external crystal surface where they can grow further in size [57]. In contrast, silanol nests present in highly defective matrices could keep the clusters inside the crystal, thus, preventing their further growth.

Influence of the SCR reaction

In Figure 4.3d, UV/VIS spectra of catalyst A(CVD,W1,C0.5) before and after use in the SCR of NO with isobutane (duration 1 working day) are compared in order to trace structural changes that might occur in a precalcined catalyst under reaction conditions. The light absorption above 400 nm reflecting extended Fe2O3-like clusters is only slightly more pronounced after catalysis. The contribution of the sub-bands above 400 nm to the total intensity increases from 37 % to 44 % after the catalytic run (Table 4.1). This is almost exclusively at the expense of the subbands between 300 and 400 nm while the contribution of the isolated sites remains constant. Hence, during catalysis, large oxide particles are formed rather from small oligomeric moieties than from isolated Fe sites [55].

Solid-State Ion Exchange (SSIE)

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The UV/VIS spectrum of sample A(SSIE)5.2 (uncalcined) is presented together with deconvoluted subbands in Fig. 4.4. From Fig. 4.4 and Table 4.1 it is evident that the sample contains a considerable amount of isolated Fe+3 ions (bands below 300) besides oligomeric Fe3+ x O y moieties (bands between 300 to 400 nm) and large Fe2O3 particles (bands above 400 nm). This is in line with earlier studies in which α-Fe2O3 has been identified in the sample by both EXAFS and XRD [36]. Calcination causes additional clustering as evidenced by the increasing area of the subbands above 400 nm and the corresponding Fe percentage in Table 4.1.

Fig. 4.4. UV/VIS-DR spectra and deconvoluted subbands of the hydrated uncalcined Fe-ZSM-5 zeolites recorded at 293 K, without pretreatment.

Mechanochemical Route (MR)

As discussed above, samples prepared by CVD and SSIE are highly heterogeneous with respect to the distribution of iron species which complicates assignment of catalytic activity in the SCR reaction to a particular iron species. Hence, it was essential to prepare a catalyst with preferably only isolated iron species. To this end Grünert et al. developed a so-called mechanochemical route to prepare highly dispersed iron species in the H-ZSM-5 matrix [32]. In the UV/VIS spectrum of the uncalcined sample A(MR)0.5 two intense bands appear at 228 and 290 nm (Fig. 4.4). Previous EXAFS measurements revealed that this sample is dominated by isolated Fe3+ sites with a mean coordination number between 4 and 6 suggesting that these Fe species are in both tetrahedral and higher coordination [32]. In agreement with these results the two strong bands in Fig. 4.4 are assigned to isolated Fe3+ sites in tetrahedral (228 nm) and higher coordination (290 nm). The band at 353 nm suggests the presence of a small amount of small oligomeric Fe3+ x O y clusters. These species were earlier not found by EXAFS spectroscopy, since, obviously, this method is less sensitive to photon scattering from higher shells. These results illustrate the benefits derived from UV/VIS-DRS. As observed in samples CVD and SSIE, clusters are also formed in A(MR)0.5 upon calcination at the expense of isolated Fe species as evidenced by a decrease in the area of subbands and the corresponding Fe content(Table 4.1). However, the superior effectivity of this preparation route in creating mainly isolated Fe3+ sites is clearly evident.

Liquid ion exchange

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The UV/VIS spectrum of Fe-ZSM-5(LIE)1.4 (Fig. 4.5) and the relative percentage of the deconvoluted subbands in Table 4.1 indicate that the majority of Fe3+ species is in the form of iron oxide clusters and iron oxide particles, besides about one third of the total iron content which is present as isolated Fe+3 ions. This is also in agreement with published HRTEM results [58], which indicated the presence of oxide particles with a size of ≈ 30 nm.

Fig. 4.5. UV/VIS-DR spectra and deconvoluted subbands of the hydrated Fe-ZSM-5 prepared by different liquid ion exchange procedures recorded at 293 K.

In contrast to conventional liquid ion exchange (LIE), improved liquid ion exchange (ILIE) produces samples with a high Fe dispersion as evidenced by the comparison of UV/VIS spectra of samples Fe-ZSM-5(LIE)1.4 and A(ILIE)1.2 with similar Fe content in Fig. 4.5 andTable 4.1. This is obviously due to the exchange of iron in the oxidation state +2. Therefore, this method was modified by Grünert and Schwidder for tuning the dispersion of Fe sites within a series of samples obtained after the same preparation route [64,164]. Deconvoluted UV/VIS spectra of calcined samples A(ILIE) are displayed in Fig. 4.5. The total spectral intensity as well as the relative intensity of the subbands strongly vary with the iron content as shown by Fig. 4.5 and Table 4.1. At low Fe content (A(ILIE)0.2 and A(ILIE)0.3) there is hardly any band above 400 nm indicating the absence of iron oxide particles. 95 % of the spectral intensity is accounted for by subbands below 300 nm (Table 4.1) indicating that the majority of Fe+3 species is well isolated. With increasing Fe content (A(ILIE)0.6 and A(ILIE)0.7), the percentage of Fe x O y clusters increases but still ca. 95 % of the spectral intensity is covered by subbands centered below 400 nm. This suggests that samples (A(ILIE)0.6 and A(ILIE)0.7) are dominated by isolated Fe+3 ions and small oligomers along with a negligible amount of large iron oxide particles (subband at ≈ 450 nm). Only the spectrum of A(ILIE)1.2 extends significantly to wavelengths above 400 nm indicating the presence of considerable amounts of large iron oxide particles.

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To elucidate whether the nature of the Fe species change during the calcination procedure, as-synthesized samples A(ILIE)0.2 and A(ILIE)1.2 were monitored by UV/VIS-DR spectroscopy during calcination as shown in Fig. 4.6. Interestingly, the whole spectral intensity increases upon calcination in comparison to the as-synthesized samples. This is due to the oxidation of divalent iron remaining in the sample after ILIE. After calcination the relative area of the subbands decreases below 300 nm and increases above 300 nm (Table 4.1) indicating the formation of iron oxide clusters at the expense of isolated Fe species. This is in line with the results of CVD, SSIE and MR samples where clustering was observed upon calcination, too.

Fig. 4.6. In situ UV/VIS-DR spectra of (a) A(ILIE)0.2 and (b) A(ILIE)1.2 recorded at 298 K before (thin solid line) and after (thick solid line) calcination in air at 823 K with a heating rate of 5 K/min.

Since iron species are participating as active redox sites in the SCR reaction, their ability to be reduced and reoxidized is of crucial importance. For investigating this property, redox kinetics of isolated Fe+3 ions and iron oxide clusters were studied by following the time dependence of the absorbance at 238, 290 and 350 nm in reducing and oxidizing. Ex situ UV/VIS-DR spectra of uncalcined A(MR)0.5, A(ILIE)0.2 and A(ILIE)0.3 are very similar (Fig. 4.4 and 4.5), revealing the presence of mainly isolated Fe+3 ions, while A(ILIE)1.2 contains, too, a certain amount of oligomeric iron-oxo-clusters (Fig. 4.5). Therefore, A(ILIE)0.3 and A(ILIE)1.2 were used as model samples to assess the redox properties of different isolated Fe+3 ions and iron oxide clusters separately.

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Isolated Fe+3 ions in tetrahedral and higher coordination give rise to two CT bands which are usually located below 300 nm. In the UV/VIS spectrum of sample A(ILIE)0.3 they fall around 225 nm and 285 nm respectively. This sample is ideal for redox studies of isolated Fe+3 ions, since only negligible contribution from agglomerated Fe x O y species is observed. The time dependence of reduction and reoxidation at 673 K was followed at the maximum of these two CT bands separately. As an example, this is shown for the band at 238 nmin Fig. 4.7.

Fig. 4.7. Time dependence of reduction and reoxidation of sample A(ILIE)0.3: (a) Absorbance at 238 nm as a function of time at 673 K in a flow of (1) 1 vol.% NH3/N2 for 2 h (gray line) and (2) reoxidation in air for 2 h (solid line); (b) corresponding UV/VIS-DR spectra at 673 K after oxidative pretreatment in air at 773 K for 1 h (thick solid line), after reduction (gray line) and after reoxidation (thin solid line).

Table 4.2. Rate constants derived for reduction and reoxidation of Fe species by UV/VIS-DRS.

↓75

At 238 nm, the experimental reduction and reoxidation curves can be fitted by assuming two pseudo-first order processes each, with different rate constants (Table 4.2). This suggests the involvement of at least two kinds of isolated Fe+3 species in the redox process which are contributing to this band. Taking account of the rate constants, one kind of iron species is easily reduced and reoxidized in the first few minutes and the other type is more slowly reduced but relatively easily reoxidized as shown in Table 4.2 (Fig. 4.7). The reason for the different reduction/reoxidation processes could be the presence of iron species at different pore positions in the zeolite matrix.

When comparing the results for the bands at 238 nm and 290 nm in sample A(ILIE)0.3 (Table 4.2), it is evident that both the bands show a similar behavior. However, the rate of reduction is somewhat higher for the latter representing octahedrally coordinated Fe3+. Differently, the rate of reoxidation is slower. This indicates that the isolated Fe+3 ions in octahedral and tetrahedral coordination possess different redox properties. Thus, it can be concluded that the former species are somewhat easily reduced and slowly reoxidized as compared to the latter species (Table 4.2).

Sample A(ILIE)1.2 contains the highest iron percentage among the samples prepared by ILIE. Moreover, ex situ UV/VIS-DRS results show that this sample contains a considerable amount of oligonuclear iron-oxo clusters. For analyzing the redox behavior of these species, the time dependence of absorbance was followed at 350 nm, in the maximum of the band being characteristic of oligomeric clusters (Fig. 4.8). In contrast to the CT bands of isolated Fe+3 ions at 238 and 290 nm for which rate constants of reduction and reoxidation are in the same order of magnitude, reoxidation of clusters reflected by the band at 350 nm is more than one order of magnitude faster than reduction (Fig. 4.8). Moreover, both reduction and reoxidation can each be fitted with one rate constant only (Fig. 4.8 and Table 4.2). This suggests that the properties of iron specieswithin clusters located in the channels may be more uniform than those of isolated sites.

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Fig. 4.8. Time dependence of reduction and reoxidation of sample A(ILIE)1.2:(a) Absorbance at 350 nm as a function of time at 673 K in a flow of (1) 1 vol.% NH3/N2 for 2 h (gray line) and (2) reoxidation in air for 2 h (solid line); (b) corresponding UV/VIS-DR spectra at 673 K after oxidative pretreatment in air at 773 K for 1 h (thick solid line), after reduction (gray line) and after reoxidation (thin solid line).

Interestingly, from the same experiment important additional information can be drawn on the redox properties of isolated Fe3+ ions in cluster containing samples. Fig. 4.8b shows that bands below 300 nm, which were assigned to isolated iron species, follow the same reduction and reoxidation trend as for cluster bands above 300 nm. This is in contrast to the reduction and reoxidation of the CT bands of isolated Fe+3 ions at 238 and 290 nm in the cluster-free sample A(ILIE)0.3. This indicates that the redox properties of isolated Fe3+ species change with increasing cluster content in the sample and possess similar redox properties to that of clusters.

Hydrothermal Synthesis

Different from sublimation and ion exchange procedures which use a crystalline zeolite matrix for incorporating Fe in extra framework positions, hydrothermal synthesis is based on the incorporation of Fe in framework positions during crystallization of the zeolite matrix followed by subsequent extraction by steaming. Using this preparation procedure, Fe-zeolites of different composition and/or pore structure have been prepared by Javier Pérez-Ramírez [58,159].

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Fig. 4.9. UV/VIS-DR spectra and deconvoluted subbands of the hydrated samples recorded at 298 K without pretreatment.

The spectra of calcined and steamed Fe-silicalite in Fig. 4.9 and the relative percentage of the deconvoluted subbands in Table 4.1 indicate that the majority of Fe3+ species in the sample is well isolated. The band at ≈ 350 nm after steaming suggests that a relatively small fraction of iron is present as small oligonuclear species. This could be due to the dislodgement of framework Fe3+ ions during calcination and/or a small fraction of iron which was not incorporated in the framework during the hydrothermal synthesis. No contribution >450 nm was observed, indicating the absence of large Fe2O3 particles in these samples.

In contrast to ex-Fe-silicalite, the percentage of isolated Fe3+ species is significantly lower in ex-Fe-ZSM-5, with extensive formation of Fe3+ x O y clusters and a small fraction of Fe2O3 particles (Fig. 4.9 and Table 4.1). The latter particles have been also detected by HRTEM in this sample with an average size of 1-2 nm [58].

↓78

As observed for calcined Fe-silicalite, calcined Fe-beta also contains mainly isolated Fe3+ species (Table 4.1 and Fig. 4.9). Similar to ex-Fe-ZSM-5, the percentage of isolated Fe3+ species is significantly lower in steamed Fe-beta with extensive formation of Fe3+ x O y clusters and a small fraction of Fe2O3 particles (Fig. 4.9 and Table 4.1) [159].

These results suggest that the framework composition of the zeolite plays an essential role in determining the iron speciation. Obviously, the presence of Al in the zeolite matrix partly prevents the incorporation of Fe in lattice positions since the latter are preferably occupied by Al. This might favour the formation of iron oxide clusters already during crystallization of the zeolite matrix [58].

Voltammetric characterization has shown that the extraction of framework iron in ex-Fe-silicalite is substantial after steam-treatment at 873 K, but not complete [155]. UV/VIS-DRS cannot discriminate between isolated Fe3+ ions in framework or extraframework positions just by the band position. Hence, the influence of Fe3+ reduction by H2 and reoxidation by air on the UV/VIS spectrum of the calcined and steamed samples has been used to further analyze the degree of iron extraction in the steam-activated catalyst (Fig. 4.10). After hydrothermal synthesis and calcination at 823 K, Fe-silicalite shows typical CT bands at 250 and 280 nm, originating from tetrahedral and higher coordination. These species which are predominant (ca. > 95%) in the sample are most probably incorporated in Si lattice positions (Fig. 4.9). They are not sensitive to a reductive treatment (20 vol.% H2 in N2 at 773 K for 1 h), as expected from the nature of framework Fe3+ ions, which are well shielded in the zeolite and thus resistant against reduction. In contrast, calcined Fe-beta shows a small decrease in band intensity upon reductive treatment (Fig. 4.10). This could be due to the presence of a small fraction of extraframework isolated Fe+3 ions in this sample. This is also evident from a small shoulder at 350 nm observed in calcined Fe-beta which indicates that a small fraction of iron was not incorporated in the framework during the hydrothermal synthesis or was already extracted during the calcination step.

↓79

Contrarily, the same reductive treatment over the steamed samples clearly shows that reduction of Fe3+ to Fe2+ causes a marked decrease of the Fe3+ CT band intensity. Additionally ex-Fe-beta with extensive clusters exhibits increase of light absorption in the range above 500 nm. This can be attributed to an intervalence charge transfer (IVCT) transition, which is induced by electron delocalization between Fe+2 and Fe+3 ions in partially reduced iron oxide species such as Fe3O4-like clusters [58,165,159]. However, reduced iron species are completely reoxidized in air flow (Fig. 4.10).

Fig. 4.10. UV/VIS-DR spectra of the calcined and steamed samples recorded at 293 K before reduction, after reduction in 20 vol.% H2 in N2 at 773 K for 1 h and after reoxidation in air at 773 K for 1 h.

Summarizing, the extraction of Fe3+ upon steaming at 873 K is significant in all samples. However, only in ex-Fe-silicalite the extraframework species remain well isolated while in Fe-beta and Fe-ZSM-5 iron extraction is connected with pronounced cluster formation.

Impregnation by incipient wetness

↓80

In the case of supported VOx species it has been shown that almost exclusively isolated, fully accessible VOx species can be deposited on the surface of mesoporous silica supports such as MCM-48 and SBA-15, due to their very high surface area [166]. Accordingly, the formation of highly dispersed FeOx moieties should be favoured on these supports, too. Hence, Fe was deposited on an Al-free SBA-15 support by impregnation with iron acetylacetonate (Fe3 +(acac)3). The corresponding UV/VIS spectra and the relative percentage of the deconvoluted subbands are shown in Fig. 4.11 and Table 4.1. These results indicate that in sample (FeSBA-I)0.95 the majority of Fe3+ species is well isolated. The band at ≈ 350 nm suggests that a relatively small fraction of iron is present as small oligonuclear species. No contribution >450 nm was observed, indicating the absence of large Fe2O3 particles [161].

Experimental UV/VIS spectra (thick solid lines) of hydrated and dehydrated (after pretreatment in air at 773 K for 1 h) samples in Fig. 16 show that in the latter a shoulder at ≈ 290 nm is completely missing which is clearly seen in the spectrum of the hydrated sample, which is assigned to isolated Fe3+ ions in environments with more coordinating ligands (5 or 6). This suggests that in the hydrated sample H2O molecules are coordinating to Fe sites that contribute to a band at 290 nm. Upon dehydration, H2O molecules are removed from the coordination sphere of Fe sites, which now contribute to a band below 250 nm, assigned to tetrahedrally coordinated isolated Fe3+ ions. This observation clearly supports the assignment of UV/VIS bands below 250 nm to tetrahedrally coordinated isolated Fe3+ ions and bands around 280 nm to higher coordination.

Fig. 4.11. UV/VIS-DR spectra of the calcined (Fe-SBA-I)0.95 sample recorded at 293 K: a) experimental spectrum of the hydrated sample (thick solid line) and deconvoluted subbands (thin lines). b) experimental spectrum after air treatment at 773 K for 1 h (thick solid line), after reduction in 20 vol.% H2 in N2 at 773 K for 1 h (gray line) and after reoxidation in air at 773 K for 1 h (broken line).

↓81

For comparative purposes, the calcined sample (Fe-SBA-I)0.95 with a similar distribution of iron species as compared to calcined and steamed Fe-silicalite and calcined Fe-beta was reduced by H2 and reoxidized by air at 773 K for 1 h respectively [161]. In view of the preparation method of the sample (Fe-SBA-I)0.95, Fe+3 ions are expected to be present at extraframework positions. Accordingly, the reductive treatment clearly shows that reduction of Fe3+ to Fe2+ causes a marked decrease of the Fe3+ CT band intensity, which is completely reversible on reoxidation in air flow (Fig. 4.11).

4.1.2 EPR studies

Ex situ EPR spectra were recorded for all samples at 293 and 77 K. Additionally, temperature dependent spectra were recorded for selected samples between 90 and 673 K while heating in air to obtain information on the magnetic behavior of iron species. In addition, some of the samples were treated in vacuum to gather information about the ability of the Fe species to change their coordination state. On the basis of these results an attempt was made to assign the EPR signals to particular iron species. This is a matter of ongoing discussion in the literature. The EPR results of different Fe-samples studied in this work are presented below in the same order as for the UV/VIS-DRS results, e.g., related to the preparation method.

Assignment of EPR signals

EPR spectra of almost all samples show typical signals at effective g values of 2, 4.3, and 6 (Fig. 4.12). Such signals were frequently detected in Fe-ZSM-5 [63,121-129] and also with Fe3+ ions in the other matrices. However, their assignment is by no means straightforward. The number and position of EPR transitions for Fe3+ ions observable in a powder spectrum depends sensitively on the local crystal field symmetry of these sites (reflected by the magnitude of the zero field splitting parameters D and E) and possible magnetic interactions between them. Signals at g' ≈ 4.3 and g' ≥ 6 arise from the | – ½ > ↔ < ½ | transition of isolated Fe3+ sites in strong rhombic (D >> hν, E / D = 1/3; g' ≈ 4.3) or axial distortion (D >> hν, E = 0, g' ≈ 6) when the zero-field splitting is large in comparison to the microwave energy hν [63,120]. This implies that an Fe3+ species giving rise to a line at g' ≈ 4.3 is more strongly distorted than an Fe3+ site represented by a signal at g' ≈ 6 due to the difference in the magnitude of E.

↓82

In zeolites, the line at g' ≈ 4.3 is frequently assigned to Fe3+ sites incorporated in tetrahedral framework positions while a line at g' ≈ 6 is attributed to isolated Fe3+ species in higher coordination [63,121-123,125-127]. However, it must be stressed, that just from the signal position alone it is not possible to conclude whether the respective Fe ions are octahedrally or tetrahedrally coordinated since the signal position is governed by the magnitude of D and E, i.e., the extent of distortion of the Fe coordination. This distortion can arise from both tetrahedral and octahedral coordination. To draw conclusions on the number of ligands associated with the g' ≈ 4.3 and 6 signals, additional aspects must be considered which are discussed in the following sections. Moreover, as will be shown below, an increase of the g' ≈ 4.3 signal at the expense of g' ≈ 6 and g' ≈ 2 lines can be induced by dehydration, implying that the loss of water ligands leads to a lower coordination number. Therefore, we assign the EPR signal at g' ≈ 4.3 to isolated tetrahedrally coordinated Fe3+ species and the line at g' ≈ 6 to isolated higher coordinated Fe3+ species in less distorted extraframework positions [55].

Signals at g' ≈ 2 can arise either from isolated Fe3+ in high symmetry (D, E ≈ 0) or from Fe x O y clusters in which magnetic interactions between the Fe3+ ions average out the zero field splitting. For isolated, highly symmetric Fe3+ species, the signal intensity should follow the Curie-Weiss law, i.e., I ~ 1/T. It appears that both types of signal can be observed in the spectra of studied samples.

Chemical Vapor Deposition (CVD)

EPR spectra of the samples A(CVD,W1) and B(CVD,W1) measured at 293 K and 77 K are shown in Fig.4.12.

↓83

Fig. 4.12. EPR spectra of hydrated Fe-ZSM-5 zeolites at 298 K (solid line) and 77 K (broken line).

Both samples show typical signals at g'-values of 2, 4.3, and 6 however, signal intensities for sample B(CVD,W1), in particular at g' ≈ 6, are slightly weaker. Signals at g' ≈ 4.3 and g' ≈ 6 arise from isolated Fe3+ sites in strong rhombic and axial distortion respectively. As expected for pure paramagnetic behavior according to the Curie-Weiss law, their intensities increase with decreasing temperature. In contrast, the intensity of the line at g' ≈ 2 does not markedly increase upon cooling. This suggests that the Fe3+ sites responsible for this signal are coupled by antiferromagnetic interactions within oxidic clusters, which reduce the number of unpaired spins contributing to the EPR signal. The presence of oxidic clusters in samples A(CVD,W1) and B(CVD,W1) has been confirmed, too, by UV/VIS-DRS results (Figs. 4.2 and 4.3).

Additional information about mutual interaction between iron species can be obtained from temperature dependent EPR measurements. It has been shown earlier that the temperature dependence of the signal intensity bears valuable information on the presence of magnetically coupled phases [167,168]. While well ordered crystalline α-Fe2O3 is antiferromagnetic below TN =960 K and, therefore, not EPR-active, it has been shown that nanoparticles of α-Fe2O3 (d ≈ 3 nm) do give rise to an EPR signal below TN due to incomplete compensation of the spin moments [167].

↓84

Therefore, series of temperature dependent spectra were recorded for selected samples in a wide temperature range. EPR spectra of sample A(CVD,W1,C0.5) are displayed in Fig. 4.13. It can be seen that the EPR signals at g' ≈ 6 and g' ≈ 4.3 decrease with increasing temperature as expected for paramagnetic behavior, whereby the intensity loss of the g' ≈ 4.3 signal is stronger suggesting shorter relaxation times in comparison to the Fe3+ species reflected by g' ≈ 6. At T ≥ 373 K those signals become narrower and better resolved. This is attributed to the loss of water molecules from the pores which are assumed to be located in the coordination sphere of the Fe3+ ions and give rise to a certain distribution of the zero field splitting parameters which enhances the line width.

Fig. 4.13. EPR spectra of hydrated sample A(CVD,W1,C0.5) during heating in air flow.

The broad signal at g' ≈ 2 does not show Curie-like behavior. In the high-temperature range it increases and narrows suddenly. This suggests that the samples contain antiferromagnetically coupled Fe x O y species with a Neel temperature of TN > 373 K. Above TN those species become paramagnetic and contribute to the EPR signal. The line narrowing is most probably due to effective spin-spin exchange interactions between neighboring Fe3+ species within the clusters. However, the g' ≈ 2 signal is anisotropic, this suggests that the signal is not due to a particular species with defined geometry but to a superposition of Fe x O y species with a certain size distribution.

Influence of washing intensity

↓85

In analogy to UV/VIS-DRS analysis (section 4.1.1, Fig. 4.2), the effect of washing intensity on the nature of iron species was also studied by EPR spectroscopy (Fig. 4.14). The temperature-dependent EPR measurements for used samples of A(CVD,W1,C0.5) and A(CVD,W10,C0.5) after catalysis are shown in Fig. 4.14a and b. In the spectra of A(CVD,W1,C0.5) after catalysis, an intense narrow signal appears at g' ≈ 2 above 373 K indicating the presence of extended Fe x O y clusters with antiferromagnetic coupling (Fig. 4.14a). In contrast, the increase of this line in A(CVD,W10,C0.5) after catalysis is much less pronounced. This suggests a weaker antiferromagnetic coupling which may be due to a smaller cluster size (Fig. 4.14b). These results are in agreement with UV/VIS-DRS results, which also showed that intense washing diminishes the amount of large iron oxide clusters slightly (Fig. 4.2b).

Fig. 4.14. EPR spectra of hydrated samples of used A(CVD,W1,C0.5) (a) and A(CVD,W10,C0.5) (b) during heating in air flow. Samples were recalcined after catalysis in air at 823 K.

Influence of calcination

EPR spectra of sample A(CVD,W1) are shown in Fig. 4.15 before calcination as well as after calcination in air at 873 K for 2 h using heating rates of 5 and 0.5 K min-1. It is clearly seen that signals of isolated Fe3+ sites at g' ≈ 6 and g' ≈ 4.3 lose intensity upon calcination while the signal at g' ≈ 2 increases. This suggests that initially isolated Fe sites aggregate to form Fe x O y clusters. This effect seems to be slightly favored by higher heating rates as also observed by UV/VIS-DRS measurements (Fig. 4.3a and Table 4.1).

↓86

Fig. 4.15. EPR spectra of hydrated samples recorded at 293 K: before calcination A(CVD,W1) (thick solid line), after calcination at 873 K with a heating rate of 0.5 K/min A(CVD,W1,C0.5) (broken line) and after calcination at 873 K with a heating rate of 5 K/min A(CVD,W1,C5) (thin solid line).

Additional features of the calcination process are evident as shown by studying the effect of room-temperature evacuation and subsequent contact with ambient atmosphere for the uncalcined and the calcined samples A(CVD,W1) and A(CVD,W1,C0.5), where the latter had been rehydrated in ambient atmosphere between calcination and EPR experiment for an extended period of time (Fig. 4.16).

Fig. 4.16. Structural changes of the isolated Fe species in sample A(CVD,W1) during calcination; EPR spectra recorded at room temperature; initial hydrated state (thick solid line), after 2 h evacuation at 293 K (thin solid line), after 2 h calcination in air at 773 K (dotted line), and after reexposing to the ambient atmosphere (dashed line); initial states: (a) as-prepared sample A(CVD,W1); (b and c) calcined sample A(CVD,W1,C0.5) after long-term storage at the ambient atmosphere.

↓87

Fig. 4.16c shows the effect of thermal treatment in air at 773 K (instead of room-temperature evacuation) on the calcined and rehydrated sample A(CVD,W1,C0.5). It can be seen that evacuation of the uncalcined A(CVD,W1) material leads to complete disappearance of the signal at g' ≈ 6 while the one at g' ≈ 4.3 increases strongly (Fig. 4.16a). This shift is largely reversed by contact with ambient atmosphere. For the calcined but rehydrated sample A(CVD,W1,C0.5) the g' ≈ 6 signal remains upon room-temperature evacuation but has lost much intensity to that at g' ≈ 4.3 (Fig. 4.16b). When, instead of 2 h evacuation at room temperature, the latter sample is again thermally treated in air at 773 K, very different changes are induced (Fig. 4.16c): Now there is just a narrowing of the signals at g' ≈ 6 and g' ≈ 4.3 and the effect is completely reversible upon exposure to the ambient atmosphere.

An explanation of this behavior may set out from the assumption that [FeCl2]+ cations deposited on cation sites during the CVD step [31,56] [Eq. (4.1)] are hydrolyzed during the washing step. In this process, part of the iron species may acquire a distorted coordination sphere with a coordination number of 6 or 5 made up of water, charge-balancing OH groups and, possibly, oxygen of the zeolite wall, which is reflected in the signal at g' ≈ 6 [e. g., Eq. (4.2)].

H+ + FeCl3→ [FeCl2]+ + HCl (4.1)

↓88

[FeCl2]+ + n H2O + m Oz→ [Fe(H2O)n(Oz)m (OH)2]+ + 2 HCl (m + n = 3 or 4) (4.2)

Upon room-temperature evacuation, the water ligands may be reversibly removed, and the respective Fe species contribute to the signal at g' ≈ 4.3 which might arise from tetrahedral Fe species as frequently discussed in the literature (Fig. 4.16a). After calcination, however, if contact with the humid air is avoided, the coordination of most Fe ions is much different (Fig. 4.16c, dotted line). The difference is obviously caused by the more severe effect of the thermal treatment as opposed to room-temperature evacuation and consists presumably in a condensation of the charge-balancing OH groups on the Fe3+ ion. This process would lead to a cation with an extra-framework oxygen coordinated to 4 or 5 framework oxygens (Eq. (4.3), with a higher probability for p = 4 due to the steric conditions in the zeolite). This species is believed to give rise to the narrow signal at g' ≈ 6 after calcination (Fig. 4.16c), which is a typical feature of this type of mononuclear Fe sites in the dehydrated state. There is, however, also a minority species in tetrahedral coordination (g' ≈ 4.3).

[Fe(H2O)n(Oz)m (OH)2]+→ [(Oz)pFeO]+ (4.3)

↓89

(p = 4 or 5 for g' ≈ 6, and p = 3 for g' ≈ 4.3)

Upon contact with the humid ambient atmosphere, the outer coordination sphere of the Fe species is influenced by adsorbed water, which results in slightly different distortions at different lattice positions and causes the broadening of the signals, in particular that at g' ≈ 6, which can be seen in Fig. 21c. The reversal of the structural change upon dehydration (probably rehydration of the Fe=O units), however, appears to proceed much more slowly: This is suggested by a comparison of the spectra after 2h evacuation in Fig. 4.16a and 4.16b. In contrast to the initial sample in Fig. 4.16a, the one in Fig. 4.16b had been calcined but stored in ambient atmosphere for an extended period of time. The narrow g' ≈ 6 signal typical of the dehydrated structures is still left to an appreciable extent in the latter sample after 2 h evacuation (Fig. 4.16b), i.e. prolonged storage in the ambient atmosphere led to only incomplete rehydration of the dehydrated Fe site.

Influence of the Al content and defect density of the parent ZSM-5 matrix

Similar to the UV/VIS studies, the effect of Si/Al ratio on the nature of iron species in the catalyst was studied by EPR spectroscopy and the corresponding spectra are depicted in Fig. 4.17.

↓90

Fig. 4.17. EPR spectra recorded at 293 K: (a) as-prepared samples A(CVD,W1) (solid line) and B(CVD,W1) (broken line); (b) calcined samples A(CVD,W1,C5) (solid line) and B(CVD,W1,C5) (broken line).

The EPR signals for isolated Fe sites at low magnetic field are less intense for the uncalcined sample B(CVD,W1) than for A(CVD,W1) while the cluster signal at g' ≈ 2 is larger (Fig. 4.17a). A smaller percentage of isolated Fe3+ and slightly higher percentage of extended Fe2O3-like clusters in sample B(CVD,W1) is also evident from UV/VIS data (Fig. 4.3b, and Table 4.1).

In agreement with UV/VIS results (Fig. 4.3c), the opposite trend is observed for the calcined samples. Calcination with a heating rate of 5 K/min leads to a less pronounced formation of large Fe2O3-like aggregates in sample B(CVD,W1,C5) as evident from the signal g' ≈ 2 in EPR spectra in Fig. 4.17b.

Influence of the SCR reaction

↓91

Fig. 4.18 illustrates the effect of use in the catalytic reaction on a precalcined catalyst. The corresponding EPR spectra of A(CVD,W1,C0.5) before and after use in the SCR of NO with isobutane (duration 1 working day) are compared in Fig 4.18. In the EPR spectrum of used catalyst, the signal at g' ≈ 6 almost disappeared while the signal at g' ≈ 4.3 is virtually not effected. At the same time, the cluster signal at g' ≈ 2 grew enormously as compared to the fresh catalyst. These results do not agree with the corresponding UV/VIS results (Fig. 4.3d and Table 4.1)which indicate only a slight increase of the relative amount of iron oxide particles after use in catalysis. Obviously, the strong increase of the EPR signal at g' ≈ 2 after reaction is mainly due to intensified spin-spin exchange interactions between neighboring Fe sites which is probably caused by lattice ordering in larger Fe x O y clusters.

Fig. 4.18. EPR spectra of A(CVD,W1,C0.5) before (solid line) and after use in the isobutane-SCR of NO reaction (broken line) measured at 293 K.

The strong attenuation of the g' ≈ 6 line suggests that Fe species reflected by this signal, are modified in the catalytic reaction while those related to the g' ≈ 4.3 line remain unchanged. The UV/VIS data (Table 4.1), on the other hand, imply that this modification is not caused by cluster formation or reduction but just by a change in the Fe coordination geometry.

Solid-State Ion Exchange (SSIE)

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EPR spectra of uncalcined A(SSIE)5.2 sample at 293 K and 77 K (Fig. 4.19) show signals at effective g values of 2, 4.3 and 6. The spectra are similar to those of sample A(CVD,W1) which has similar Fe content (Fig. 4.12).

Fig. 4.19. EPR spectra of the uncalcined A(SSIE)5.2 sample (without pretreatment) at 293 K (solid line) and 77 K (broken line).

As observed from UV/VIS-DRS measurements, uncalcined A(SSIE)5.2 sample contains iron oxide clusters of different size along with a considerable amount of isolated iron ions (Fig. 4.4 and Table 4.1). While the isolated species are properly reflected by the signals at g' ≈ 6 and g' ≈ 4.3, the line at g' ≈ 2 is very weak, suggesting that the majority of the iron oxide clusters might be EPR silent at T ≤ 293 K due to antiferromagnetic interactions. Therefore, to gather additional information about mutual magnetic interactions between Fe3+ ions and subsequently the size of the clusters, temperature dependent EPR measurements were performed as shown in Fig. 4.20.

↓93

Fig. 4.20. EPR spectra of uncalcined A(SSIE)5.2 sample during heating in air flow.

The EPR spectra of sample A(SSIE)5.2 which were recorded in the low temperature range between 90-270 K show similar behavior to that of A(CVD,W1,C0.5) (Fig. 4.13). The signals at g' ≈ 6 and g' ≈ 4.3 decrease with rising temperature as expected for paramagnetic behavior.

Above 573 K a huge and very narrow signal appears at g' ≈ 2 which is much more intense than for sample A(CVD,W1,C0.5). This suggests that the A(SSIE)5.2 sample contains antiferromagnetically coupled Fe x O y species with a Neel temperature of TN > 573 K. The strong increase in intensity and line narrowing of g' ≈ 2 signal in A(SSIE)5.2 indicates the presence of larger and/or better ordered clusters than in the sample A(CVD,W1,C0.5). Such species were also earlier detected in the unwashed and uncalcined Fe-ZSM-5 (prepared by SSIE), by XRD, TPR and XAFS [36]. It is not unexpected that they remain also in the washed material.

Mechano chemical route (MR)

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EPR spectra of the uncalcined sample A(MR)0.5 at 293 K and 77 K (Fig. 4.21) show signals at effective g values of 2, 4.3 and 6.

Fig. 4.21. EPR spectra of the uncalcined A(MR)0.5 sample (without pretreatment) at 293 K (solid line) and 77 K (broken line).

The signals at g' ≈ 4.3 and g' ≈ 6 show similar behavior as observed for other samples, e.g., A(CVD,W1), A(CVD,W1,C0.5) and uncalcined A(SSIE)5.2. Interestingly, the signal at g' ≈ 2 increased upon cooling from room temperature to 77 K, as expected for paramagnetic Fe3+ species obeying the Curie-Weiss law. This behaviour, typical for isolated sites, relates to the high degree of dispersion of iron in this catalyst. This is in agreement with UV/VIS results that indicate the presence of the majority of Fe+3 species in isolated form (Fig. 4.4 and Table 4.1). However, in the EPR spectrum of uncalcined A(MR)0.5 at 77 K, a rather narrow and a very broad line are superimposed in the range of g' ≈ 2. The former signal loses intensity upon heating. Most probably, this line arises from isolated Fe3+ ions in high symmetry. The latter is not typical for ideally isolated sites.

↓95

To check for the presence of magnetic interactions between Fe3+ sites in oxidic moieties, temperature dependent EPR measurements were also performed analogous to samples A(CVD,W1,C0.5) and uncalcined A(SSIE)5.2 as shown in Fig. 4.22. With the exception of the narrow signal at g' ≈ 2, the shape and behavior of the EPR signals in the low temperature range between 90-270 K are rather similar for uncalcined A(MR)0.5, A(CVD,W1,C0.5) and A(SSIE)5.2. This was expected, since the samples might contain isolated sites of similar nature giving rise to similar EPR signals at g' ≈ 6 and g' ≈ 4.3. However, above 373 K a slight increase of the broad signal at g' ≈ 2 is observed, too, though the effect is much less pronounced than inA(CVD,W1,C0.5) and uncalcined A(SSIE)5.2 (Fig. 4.22). This suggests the presence of antiferromagnetically coupled Fe x O y species with a Neel temperature of TN > 373 K, although the lower intensity and the larger line width suggest that they are much less abundant and smaller than in samples uncalcined A(SSIE)5.2 and A(CVD,W1,C0.5). As mentioned above, these species were not detected by XAFS [32], illustrating the power of EPR spectroscopy for such investigations. Even the parent H-ZSM-5(A) which was measured for comparative purposes (Fig. 4.22b), and which contains only 0.05 % Fe shows a small g' ≈ 2 line with non-Curie behavior. This indicates that it is not free of small Fe x O y clusters. Moreover, signals at g' ≈ 4.3 and 6 were not detected in the parent H-ZSM-5(A). This shows clearly that the respective Fe3+ species are introduced by the various Fe loading procedures.

Fig. 4.22. EPR spectra of hydrated uncalcined samples A(MR)0.5 (a) and parent H-ZSM-5(A) (b) during heating in air flow.

Liquid Ion Exchange

In Fig. 4.23, ex situ EPR spectra recorded at 77 K and room temperature of calcined samples are presented. In the spectra, typical signals at effective g values of 2, 4.3, 6 and 9 can be distinguished. In addition, a narrow and a broad signal at g' ≈ 2 can be discerned. In samples A(ILIE)0.2 and A(ILIE)0.3 the g' ≈ 2 EPR signal increases markedly upon cooling to 77 K which is typical for pure paramagnetic behaviour according to the Curie-Weiss law. Therefore, this line is assigned to isolated Fe3+ ions in highly symmetric environment. This is also in agreement with UV/VIS-DRS results that indicate the presence of almost exclusively isolated Fe species in those samples (see Fig. 4.5 and Table 4.1). In ILIE samples with 0.6 and 0.7 % Fe, for which UV/VIS-DR spectra indicate the presence of a certain amount of oligomers and even some Fe2O3 nanoparticles (bands above 300 and 400 nm in Fig. 4.5) a broad line with a low field maximum around 2350 G at 298 K is superimposed on the narrow signal at g' ≈ 2 which might be due to Fe x O y clusters (Fig. 4.23). In sample A(ILIE)1.2 for which UV/VIS-DRS showed a pronounced trend to the formation of Fe2O3 particles (Fig. 4.5), the line width of this broad signal is even larger shifting the low field maximum to about 880 G while the narrow line, though well visible, does not increase with rising temperature which indicates a deviation from paramagnetic (isolated) nature (Fig. 4.23).

↓96

Fig. 4.23. EPR spectra of the hydrated samples at 293 K (solid line) and 77 K (broken line).

As mentioned earlier, the temperature dependence of the signal intensity bears valuable information on the presence of magnetically coupled phases. In view of this, temperature-dependent EPR measurements have been performed with samples A(ILIE)0.6 and A(ILIE)1.2 (Fig. 4.24). From Fig. 4.24 it can be seen that the signal at g' ≈ 2 of sample A(ILIE)0.6 narrows slightly and follows a Curie-like temperature dependence which is typical for both isolated Fe3+ ions but also for oxide nanoparticles in the superparamagnetic state. UV/VIS-DRS measurements (Fig. 4.5) suggest that this sample is dominated by isolated Fe3+ ions with a small contribution of oligomers. Due to their location inside the pore system, these oligomers are assumed to consist of only a few Fe atoms which is probably not enough to create superparamagnetic behaviour. Therefore, the g' ≈ 2 in sample A(ILIE)0.6 is regarded as arising from both highly symmetric isolated Fe3+ species as well as small oligomers with weak dipolar coupling.

Fig. 4.24. EPR spectra of hydrated samples during heating in air flow.

↓97

The EPR spectrum of sample A(ILIE)1.2 at 298 K is characterized by a very broad anisotropic line and two narrow features at g' ≈ 2 (Fig. 4.24). The broad line (marked with star) narrows and shifts to higher resonance field with increasing temperature. The same holds for one of the narrow lines (marked with an arrow). It has been shown that very small α-Fe2O3 particles of only a few nm size, in contrast to crystalline bulk α-Fe2O3, are EPR-active at low temperature and show ferrimagnetic behaviour [167]. In the EPR spectra, this is reflected by broad anisotropic signals, since the resulting magnetic moment vectors of the particles tend to align with the direction of the external magnetic field. With rising temperature, thermal fluctuations lead to the collapse of the ferrimagnetic order and the particles become superparamagnetic above the so-called blocking temperature. As a result, the EPR signal narrows and shifts towards g' ≈ 2 as the temperature increases [168]. Based on this, these two signals can be assigned to ferrimagnetic/superparamagnetic Fe2O3 particles of different size while the second narrow signal at g' ≈ 2 (marked with triangle) might be due to isolated and weakly interacting Fe3+ sites.

Interestingly, sample Fe-ZSM-5(LIE)1.4, which shows extensive clustering as evidenced by UV/VIS-DRS, does contain a small amount of highly symmetric isolated Fe3+ ions besides iron oxide clusters as evidenced by the temperature dependent g' ≈ 2 signal behaviour (Fig. 4.23). The g' ≈ 2 signal is a superposition of a narrow and a broad line. With decreasing temperature the narrow line increases in intensity (77 K spectrum in Fig. 4.23) as expected for paramagnetic isolated Fe3+ sites, while the broad line does not indicating the presence of clusters.

Hydrothermal Synthesis

Ex situ EPR spectra of ex-Fe-silicalite, ex-Fe-ZSM-5, c-Fe-beta and ex-Fe-beta were recorded at 77 K and room temperature as shown in Fig. 4.25. As observed for the other Fe-zeolites, too, samples show three signals at effective g values of 2, 4.3 and 6 and they can be assigned to similar species as for ILIE samples. The intensity of the line at g' ≈ 2 in the EPR spectra of the ex-Fe-ZSM-5 and ex-Fe-beta containing clusters, as evident from UV/VIS results (Fig. 4.9 and Table 4.1), at room temperature and 77 K is very similar, and does not follow the Curie-Weiss law as expected for paramagnetic species. This suggests that the Fe3+ sites responsible for this signal are coupled by antiferromagnetic interactions within oxidic clusters, which reduce the number of unpaired spins contributing to the EPR signal.

↓98

The g' ≈ 2 signal in ex-Fe-silicalite and c-Fe-beta increased upon cooling from 293 K to 77 K, as observed for A(MR)0.5 and ILIE samples with low Fe content, which is characteristic of paramagnetic isolated Fe3+ species. This indicates the high degree of iron dispersion in the catalysts. This is in excellent agreement with UV/VIS-DRS results which showed the presence of almost exclusively isolated Fe3+ ions besides a small fraction of oligomeric moieties (Fig. 4.9 and Table 4.1). Similar to A(MR)0.5 and ILIE samples with low Fe content, these samples, too, show a narrow and a very broad line in the range of g' ≈ 2 (see EPR spectra at 77 K).The latter line indicates the presence of weak dipolar interaction between the Fe3+ sites contributing to this signal. Considering these effects and similar to the assignment of g' ≈ 2 signal in A(MR)0.5 and ILIE samples, the signal at g' ≈ 2 in these samples is assigned, too, to highly symmetric Fe3+ species rather than isolated Fe sites that have weak dipolar interactions.

Fig. 4.25. EPR spectra of Fe-zeolites at 293 K (solid line) and 77 K (broken line).

Further support for this assignment was gathered by measuring the EPR spectrum of ex-Fe-silicalite after evacuation at ambient temperature for 2 h (Fig. 4.26). This treatment causes a dramatic decrease of the g' ≈ 2 line, together with the appearance of additional signals in the low-field range. This can be attributed to the removal of water molecules from the pore network of the zeolite adsorbed on Fe3+ species responsible of the g' ≈ 2 signal. Upon evacuation this leads to isolated Fe3+ sites in a lower symmetry (higher zero-field splitting) that give rise to additional signals in the low-field range. These spectral changes are slowly reversible when exposing the sample to ambient conditions. A very similar behaviour for the g' ≈ 2 signal upon evacuation and rehydration at room temperature was also observed by Berlier et al. [62] on iron silicalite (Si/Fe = 90) activated at 773-1073 K in vacuum. These authors assigned the g' ≈ 2 signal to isolated Fe3+ ions, which are solvated by water molecules in ambient atmosphere. Following their assignment, the result in Fig. 4.26 suggests that the majority of the Fe3+ species contributing to the g' ≈ 2 line in ex-Fe-silicalite are well isolated in extraframework positions, being accessible to gaseous molecules.

↓99

Fig. 4.26. EPR spectra of ex-Fe-silicalite recorded at 298 K before and after evacuation for 2 h and after re-exposure to ambient atmosphere for 1.5 and 16 h.

Impregnation by incipient wetness

In Fig. 4.27, ex situ EPR spectra of calcined (Fe-SBA-I)0.95 at 293 K and 77 K are depicted. The spectra not only show similar EPR signals at effective g values of 2, 4.3 and 6 but also similar behavior to that of uncalcined A(MR)0.5, A(ILIE)0.2, A(ILIE)0.3 and ex-Fe-silicalite, which show almost exclusively isolated Fe3+ ions besides a small fraction of oligomeric iron oxo clusters (see Figs. 4.21, 4.23 and 4.25 and Table 4.1). Hence, similar to these samples, the EPR signals in sample (Fe-SBA-I)0.95 can be attributed to differently distorted isolated Fe3+ ions. However, the broad line in the g' ≈ 2 range indicates the presence of weak dipolar interaction between the Fe3+ ions. These results are in line with UV/VIS-DRS which show a majority of iron in the sample as isolated iron species and a small fraction of iron association as oligomeric iron oxo species (Fig. 4.11 and Table 4.1).

Fig. 4.27. EPR spectra of (Fe-SBA-I)0.95 at 293 K (solid line) and 77 K (broken line).

4.2 Analysis of acidic properties of Fe-zeolites by FT-IR investigation of adsorbed pyridine

↓100

FT-IR spectroscopic measurements of adsorbed pyridine were carried out to elucidate the nature and relative amount of the acid sites in selected iron zeolites. The infrared spectra of the various samples (Figs. 4.28, 4.29 and 4.30) exhibit a band at 1545 cm-1 unequivocally assigned to Brønsted acid sites [148,159]. Bands around 1445 cm-1 arise from pyridine adsorbed on Lewis acid sites [148,159]. Bands around 1600 cm-1 are also due to pyridine adsorbed on Lewis sites and can be considered a measure of the strength of the Lewis sites [148]. Bands at 1581 and 1598-1596 cm-1 are attributed to weak and medium Lewis acid sites, respectively, while bands at 1612-1607 and 1623 cm-1 originate from strong and very strong Lewis acid sites, respectively [148]. However, the band at 1623 cm-1 is ambiguously discussed [148,150]. In zeolites this band is attributed to Brønsted acid sites [150], while the same band is ascribed to pyridine adsorbed on Al Lewis acid sites of γ-Al2O3 and amorphous Al2/SiO2. [148]. The latter assignment is supported by the fact that the FT-IR spectrum of the Al-free ex-Fe-silicalite (Si/Al ≈ ∞ and 0.68 wt.% Fe) after pyridine adsorption does not show a band at 1623 cm-1 (Fig. 4.30). According to infrared studies over iron supported on silica [149], the band at 1595-1598 cm-1 can be assigned to weakly adsorbed pyridine on Si Lewis acid sites in Fe-zeolites. The presence of different types of Lewis acid sites in Fe-zeolites can be inferred from the splitting of the bands at high wavenumbers (≈1600 cm-1), which is not the case in ex-Fe-silicalite, showing a single absorption band in this region (Fig. 4.30).

The ratio of Brønsted-to-Lewis acid sites can be estimated from the area of the bands at 1545 (pyridium ions, PyrH+) and 1445 cm-1 (pyridine coordinated to Lewis sites, PyrL), respectively, since the ratio of the extinction coefficients of these two absorption bands is ≈ 1 [169-172]. The integral intensities of Brønsted (1567-1515 cm-1) and Lewis (1467-1426 cm-1) acid sites of the samples are normalized on their surface area. As shown in Table 4.3, the ratio of Brønsted-to-Lewis acid sites depends on the zeolite type, Fe-content, preparation route and the corresponding treatment.

Attending to the intensity of the infrared bands at around ≈1600, 1545 and 1445 cm-1 in Fig. 4.28, samples A(CVD,W1,C0.5), A(CVD,W10,C0.5) and A(CVD,W1,C5) contain similar Brønsted and Lewis acidity (Table 4.3). The Brønsted acidity of the samples is similar to that of the parent H-ZSM-5(A) but the samples contain much higher amount of Lewis acidity. The latter can be attributed to the presence of iron oxide clusters in the samples A(CVD) which are not present in the parent H-ZSM-5(A) as evidenced by ex situ UV/VIS-DRS and EPR spectroscopic results (sections 4.1.1 and 4.1.2). These spectra show that A(CVD) samples consist of similar nature and distribution of iron species, however, with different size of clusters as shown by temperature dependent EPR spectra in Fig. 4.14 and as reported earlier in [36,55]. Therefore, it can be concluded that iron oxide clusters form additional Lewis sites on the sample. H-ZSM-5(A) shows additional bands at 1621 and 1596 cm-1 while in samples A(CVD) these two bands are extinguished and a new band at 1614 cm-1 arises. These observations indicate the change in the nature of the Lewis acidity of the A(CVD) samples as compared to the parent H-ZSM-5(A) which might be due to the presence of iron oxide clusters. Accordingly, a band in the same range (1612 cm-1) is observed for sample A(SSIE)5.2 which is dominated by clusters (Fig. 4.29).

↓101

Table 4.3. Areas of FT-IR bands normalized on the BET surface area of adsorbed pyridine associated with Brønsted and Lewis acidity and their corresponding ratio.

Sample

Band at 1545 cm -1
Brønsted sites

Band at 1445 cm -1
Lewis sites

Ratio of Brønsted /
Lewis acidity

H-ZSM-5(A)

3.4

1.8

188.8

A(CVD,W1,C0.5)

3.9

2.7

144

A(CVD,W10,C5)

3.1

2.9

106

A(CVD,W1,C5)

3.7

3.0

123

B(CVD,W1,C5)

0.8

1.58

50

A(SSIE)5.2

7.1

3.2

221

A(MR)0.5

5.0

1.8

277

A(ILIE)0.2

3.69

2.1

175.7

A(ILIE)1.2

2.78

3.14

87

ex-Fe-silicalite

0.15

0.9

16.6

c-Fe-beta

0.35

3.1

11.3

ex-Fe-beta

0.23

1.8

12.7

Fig. 4.28. Difference FT-IR spectra of pyridine adsorbed on Fe-ZSM-5 zeolites at 373 K, obtained by subtraction of the FT-IR spectrum of the bare zeolite after pretreatment at 673 K for 1 h.

The effect of Si/Al ratio (≈ 40 vs≈ 14) on the nature of iron species is significant as evidenced, too, by ex situ UV/VIS-DRS and EPR (Fig. 4.3 and 4.17) which show that higher Si/Al ratio (≈ 40, sample B(CVD,W1,C5)) favors cluster formation during CVD. However, after calcination the clusters in sample B(CVD,W1,C5) seem to be smaller than in samples A(CVD) with lower Si/Al ratio since they may be trapped in silanol nests which restrict their further growth. As expected, the FT-IR spectrum of adsorbed pyridine of sample B(CVD,W1,C5) shows low Brønsted acidity but contains a considerable amount of Lewis sites (Fig. 4.28 and Table 4.3). The low Brønsted acidity is obviously due to the higher Si/Al ratio and the observed Lewis acidity is due to the presence of intra zeolite iron oxide clusters and/or framework Al sites [36,55]. On the basis of these results it can be concluded that the higher Si/Al ratio (≈ 40) leads to lower acidity of the sample as compared to lower Si/Al ratio (≈ 14).

↓102

Interestingly, the sample prepared by solid state ion exchange, A(SSIE)5.2contains the highest density of Brønsted acid sites among the investigated Fe-zeolites including H-ZSM-5(A) besides a considerable amount of Lewis acid sites (Table 4.3 and Fig. 4.29). The acidity (Brønsted and Lewis) of the sample is almost double the amount as compared to the parent H-ZSM-5(A) (Table 4.3), while compared to the A(CVD) samples, A(SSIE)5.2 shows almost double the amount of Brønsted acidity but similar amount of Lewis acidity. This indicates that the SSIE preparation method creates additional Brønsted acid sites as compared to the CVD procedure. This conclusion is further supported by the similar Brønsted acidity in the parent H-ZSM-5(A) and A(CVD) samples (Table 4.3). Similar to A(CVD) samples, A(SSIE)5.2 shows an intense band at 1612 cm-1 while bands at 1621 and 1596 cm-1 are completely missing which are present in the parent H-ZSM-5(A) as shown in Fig. 4.28 and 4.29. This indicates the presence of similar nature of Lewis acid sites in A(SSIE)5.2 and A(CVD) samples. This is in good agreement with ex situ UV/VIS-DRS and EPR results which show similar nature and distribution of iron species, however, with different degrees of iron agglomeration. This indicates that the size of the clusters may not affect the Lewis acidity of the sample.

Fig. 4.29. Difference FT-IR spectra of pyridine adsorbed on Fe-ZSM-5 zeolites at 373 K, obtained by subtraction of the FT-IR spectrum of the bare zeolite after pretreatment at 673 K for 1 h.

The amount of Brønsted acid sites in the A(MR)0.5 (Si/Al ≈ 14 and 0.5 wt.% Fe) is increased as compared to the parent H-ZSM-5(A) and is between A(SSIE)5.2 and A(CVD) samples while the Lewis acidity of the sample is considerably lower than in the latter samples but is similar to that of parent H-ZSM-5(A) (Table 4.3 and Fig. 4.29). From the above observations it can be concluded that, similar to the SSIE preparation procedure,MR technique also creates additional Brønsted acid sites. However, at this point it cannot be conclusively stated how they are created by these two techniques. As shown by ex situ UV/VIS-DRS and EPR, the sample A(MR)0.5 contains almost exclusively isolated Fe3+ ions with a small fraction of oligomeric moieties (Table 4.1). Accordingly, additional Lewis acidity from iron oxide clusters cannot be expected for this sample as observed for cluster containing samples A(CVD) and A(SSIE)5.2. In agreement with these results sample A(MR)0.5 shows similar Lewis bands as the parent H-ZSM-5(A). This suggests that the nature of Lewis acidity of A(MR)0.5 is similar to that of H-ZSM-5(A) and mainly related to Al and Si sites.

↓103

Different from SSIE and MR techniques, ILIE does not create additional Brønsted acid sites as evidenced by samples A(ILIE)0.2 and A(ILIE)1.2 which show similar Brønsted acidity to that of H-ZSM-5(A) and A(CVD) samples. Lewis acidity of the A(ILIE)0.2 is similar to that of H-ZSM-5(A). This is in good agreement with ex situ UV/VIS-DRS and EPR results which show almost exclusively isolated Fe3+ ions in the A(ILIE)0.2. Accordingly, additional Lewis acidity is not expected for this sample. In contrast to this sample, A(ILIE)1.2 with extensive clustering as evidenced by ex situ UV/VIS-DRS and EPR results shows higher Lewis acidity as compared to H-ZSM-5(A), A(MR)0.5 and A(ILIE)0.2 and is similar to that of cluster containing A(CVD) and A(SSIE) samples. This further supports that iron oxide clusters provide additional Lewis acidity in the sample. Additional bands in the high wavenumber range at 1621 and 1596 cm-1 are observed over cluster free A(ILIE)0.2 similar to H-ZSM-5(A) and A(MR)0.5 but no band at 1612 cm-1 which was typically observed for cluster containing samples as in A(ILIE)1.2. In the latter sample the band at 1621 cm-1 is almost completely missing or may be present as a shoulder to 1612 cm-1 band but a band at 1596 cm-1 is observed.

Calcined c-Fe-beta contains weak Brønsted acidity but considerable Lewis acidity as evidenced by Fig. 4.30 and Table 4.3.

Fig. 4.30. Difference FT-IR spectra of pyridine adsorbed on Fe-ZSM-5 zeolites at 373 K. Difference spectra are obtained by subtraction of the FT-IR spectrum of the bare zeolite after pretreatment at 673 K for 1 h.

↓104

As compared to B(CVD,W1,C5) with similar Si/Al ratio, c-Fe-beta shows lower Brønsted acidity (Table 6). This can be attributed to the dealumination of Al3+ sites from framework positions upon calcination in the latter sample as reported in [159]. A decrease of Brønsted and Lewis acid sites is observed upon steaming of c-Fe-beta as evidenced by the reduced intensity of bands at 1546 and 1445 cm-1 in Fig. 4.30 and Table 4.3. Besides dealumination, the steam activation of calcined zeolites induces dislodgement of framework iron. This should in principle contribute to the acidity differences observed, despite the relatively low iron content compared to Al. This observation is in good agreement with ex situ UV/VIS-DRS and EPR results which show that c-Fe-beta contains the majority of iron as isolated Fe3+ ions probably in the framework positions while ex-Fe-beta shows extensive clustering. On the basis of these observations it can be concluded that the c-Fe-beta and ex-Fe-beta with Si/Al ≈ 36 possesses poor acidity even lower than that of the B(CVD,W1,C5) sample with higher Si/Al ≈ 40. Finally, as expected, ex-Fe-silicalite without Al in the framework exhibits poor Brønsted and Lewis acidity as evidenced by less intense FT-IR bands as compared to ex-Fe-beta in Fig. 4.30 and the corresponding areas of the bands in Table 4.3.

In summary, samples prepared using H-ZSM-5(A) support with Si/Al ≈ 14 show considerable amount of Brønsted and Lewis acidity. Interestingly, samples prepared by SSIE and MR techniques using this support show more Brønsted acidity than that of CVD and ILIE and even higher than H-ZSM-5(A). However, it is not yet clear how SSIE and MR techniques create additional Brønsted acidity. Sample prepared by CVD technique using H-ZSM-5(B) support with Si/Al ≈ 40 shows, as expected, poor Brønsted acidity as compared to samples prepared using H-ZSM-5(A). Interestingly, the Brønsted acidity of the sample is much higher than that of calcined and steamed Fe-beta samples which contain lower Si/Al ratio (≈ 36) than the H-ZSM-5(B). This is due to the dislodgement of framework Al and Fe even upon calcination and obviously after steam treatment in Fe-beta samples. Consequently, the reduced surface acidity is observed in the latter samples. However, in agreement, calcined and steamed Fe-beta samples show higher Lewis acidity than H-ZSM-5(B) due to lower Si/Al ratio.

4.3 Studies of the behaviour of Fe species in the presence of feed components by in situ techniques

In situ UV/VIS-DR, EPR and FT-IR spectroscopic studies were performed to identify active iron species and the adsorbed NOx species or reaction mechanism in the SCR of NO by NH3 and isobutane, the SCR of N2O by CO as well as N2O decomposition over Fe-zeolites, based on the propensity of the iron sites to undergo redox processes. To this end, the interaction of NH3, isobutane, NO, CO, N2O, CO+N2O and SCR reaction mixtures with the iron species in the catalysts has been investigated at 623 K. At this temperature, all catalysts have shown substantial activity in steady-state catalysis experiments (see section 4.4). Additionally, samples A(MR)0.5 and A(ILIE)0.3 were subsequently treated with NH3 and NO at 773 K to study the effect of temperature on the redox activity of different isolated Fe sites by EPR. Also, the interaction of NH3 and NO with the iron species in the catalysts was investigated at 293 K to study the influence of these probe molecules on the Fe sites and also to investigate the nature of adsorbed species. Hence, in this section results of EPR, UV/VIS-DR and FT-IR spectroscopic studies are described.

4.3.1 EPR, UV/VIS-DR and FT-IR spectroscopic studies during interaction of feed components in NH3-SCR of NO

Interaction of NH 3 with pre-oxidized samples A( MR)0.5 and A(CVD,W1,C0.5) at 293 K

↓105

Upon NH3 treatment at room temperature for 1 h, the EPR signal at g' ≈ 6 completely disappeared while the one at g' ≈ 2 slightly increased in intensity and the signal at g' ≈ 4.3 slightly decreased in A(MR)0.5 or is hardly affected in A(CVD, W1,C0.5) (Fig. 4.31).

Fig. 4.31. In situ EPR spectra of samples A(MR)0.5 (a) and A(CVD,W1,C0.5) (b) and in situ UV/VIS-DR spectra of A(MR)0.5 (c) recorded at 293 K after oxidative treatment at 773 K for 1 h (solid line) and after treatment in a flow of 1% NH3/He at 293 K for 1 h followed by flushing with He at 293 K for 15 min (dotted line).

In principle, this could be due to two reasons: i) reduction of Fe3+ to Fe2+ or ii) coordination of NH3 to these Fe+3 species which increases the local crystal field symmetry and shifts the signal towards g' ≈ 2. Considering the experimental conditions, i.e., room temperature and 1% NH3/He, the suppression of the EPR signal at g' ≈ 6 may not be due to the reduction of Fe+3 species but could be due to the coordination of NH3 to coordinatively unsaturated iron species that increases the site symmetry. This may be the reason for a slight increase in intensity at g' ≈ 2, which is more clearly seen for sample A(CVD,W1,C0.5) than for sample A(MR)0.5. This is due to the fact that in sample A(MR)0.5 the g' ≈ 6 signal is smaller than in sample A(CVD,W1,C0.5). Therefore, less NH3 ligated Fe3+ species contribute to the g' ≈ 2 signal in the former sample. Hence, this could explain the low intensity gain of g' ≈ 2 signal in sample A(MR)0.5 than in A(CVD,W1,C0.5). After NH3 treatment the samples were purged by pure He at 293 K temperature, which did not restore the original EPR spectra. This could be due to strong linking between Fe+3 sites and NH3 ligands. However, it is difficult to decide whether reduction of Fe3+ or coordination of NH3 is the reason for the spectral changes only from EPR. To clarify these results, UV/VIS-DR spectroscopy is a suitable technique since it provides information about simultaneous electronic and coordination changes.

↓106

In the corresponding UV/VIS-DRS spectra of sample A(MR)0.5, NH3 treatment causes a shift in the band position to a lower wavelength as observed in Fig. 4.31c. Interestingly, the band intensity is more or less similar which indicates that NH3 may considerably not reduce the iron species at room temperature but links to coordinatively unsaturated isolated iron species. Therefore, after NH3 treatment the band position slightly shifted to lower wavelength as compared to the oxidatively pretreated sample. This is expected since NH3 is a stronger field ligand than oxygen. Consequently, for electronic transitions higher energy is required. On the basis of these results it can be concluded that the disappearance of EPR signal at g' ≈ 6 is, probably, not due to the reduction of Fe sites but due to the change in the Fe site symmetry. However, reduction of a small amount of Fe species cannot be completely ruled out. The corresponding UV/VIS experiments have also been performed on sample A(CVD,W1,C0.5), but the effects are hardly visible since this sample is rather dominated by iron oxide clusters (not shown).

Interaction of NO with pre-oxidized and pre-reduced catalysts at 293 K studied by FT-IR spectroscopy

These experiments were performed to investigate the nature of the interaction of NO with Fe sites in Fe-ZSM-5 as well as the nature of adsorbed species on the pre-oxidized and pre-reduced surface of Fe-ZSM-5 zeolites.

Interaction of NO with pre-oxidized catalysts at 293 K

FT-IR spectra of samples A(MR)0.5, A(ILIE)0.3, A(ILIE)1.2, A(CVD,W1,C0.5) and pure support H-ZSM-5(A) after oxidative pretreatment and adsorption of 1% NO/He are shown in Fig. 4.32. The spectra obtained after 30 min adsorption of NO and before evacuation show bands at 2198, 2134, 1882, 1743, 1627, 1605 and 1574 cm-1. Similar bands have been observed previously upon adsorption of NO by several authors. An overview on their assignment is given in Table 2.2. Based on these studies, bands at 2198 and 2134 cm-1 are assigned to [NO+][N2O4] and NO+, respectively, while the band at 1743 cm-1 is ascribed to adsorbed N2O4 [86-89]. A band around 1880 cm-1 has always been observed for NO adsorbed on an iron site, although the nature of this Fe site is controversially discussed (Table 2.2). Taking into account that the catalysts in Fig. 4.32 have been pre-oxidized before NO adsorption, it is likely that the band at 1880 cm-1 arises from NO adsorbed at Fe3+ sites. However, N2O3 can also contribute to this band as it gives two bands at around 1880 and 1555 cm-1 [86]. Thus, the latter band probably also contribute to the nitrato band at 1575 cm-1. Interestingly, bands above 1700 cm-1 disappear completely upon evacuating the sample for 30 min (Fig. 4.33). This indicates that these bands arise from weakly adsorbed species only. In contrast, bands below 1700 cm-1 do not disappear completely upon evacuation, although differences in intensities are observed (Fig.4.33). However, it must be noted that the band at 1575 cm-1 over H-ZSM-5 completely vanished. This suggests the contribution of N2O3 to some extent to 1880 and 1575 cm-1 bands in the spectra of samples before evacuation (Fig. 4.32). This is in particular true for H-ZSM-5 since these bands disappeared completely after evacuation.

↓107

Fig. 4.32. In situ FT-IR spectra at 293 K after adsorption of 1% NO/He on oxidized surfaces and before evacuation; oxidative pretreatment in air at 673 K for 1 h followed by evacuation and cooled to 293 K.

Fig. 4.33. In situ FT-IR spectra at 293 K after adsorption of 1% NO/He on oxidized surfaces and after evacuation; oxidative pretreatment in air at 673 K for 1 h followed by evacuation and cooled to 293 K.

The band at 1605 cm-1 is no longer observed for the evacuated samples, suggesting that the respective species might be only weakly adsorbed. Chen et al. [86] assigned this band to weakly adsorbed NO2 since it is close to the asymmetric stretching frequency of gaseous NO2 (1610 cm-1). Bands at 1634, 1625 and 1570 cm-1 were assigned by the same authors [86,87] and Lobree et al. [88] to NO2/NO3 species whereby a more detailed description of their nature was not given. In contrast, Hadjiivanov et al. [89] and Davidov [153] assigned bands between 1650-1600 cm-1 to bridging bidentate nitrato and a band at 1575 cm-1 to chelating bidentate nitrato species. It is well known, that NO3 -and NO2 - anions give rise to bands at 1380 and 1260 cm-1, respectively [153]. For NO3 and NO2 species which are not purely ionic, an increase of the bond strength and, thus, a blue shift of the bands should be expected. This is in agreement with the results of Davidov [153], who reported bands of differently bound NO3 species to occur between 1480 and 1650 cm-1 while those of NO2 species were observed at lower wavenumbers between 1205 and 1520 cm-1. Based on these considerations, bands at 1633, 1620 and 1575 cm-1 in the spectra of evacuated samples (Fig. 4.33) are assigned to differently bound nitrato species. By comparing the band intensities in Fig. 4.33, it turns out that the relative intensity at 1575 cm-1 is higher on samples A(CVD,W1,C0.5) and A(ILIE)1.2 with the higher Fe content. This could suggest that the NO3 species reflected by the latter band are preferably adsorbed on Fe species. This is also supported when comparing the spectra of the different Fe-ZSM-5 samples with the one of H-ZSM-5. Band intensities over the bare H-ZSM-5 and samples with low Fe content do not differ much in intensity suggesting that bands associated with NO in the latter samples need not necessarily arise from NO adsorbed on Fe sites. Interestingly, H-ZSM-5 does not show a band at 1575 cm-1.

↓108

In summary, it can be stated that on pre-oxidized Fe-ZSM-5 samples weakly adsorbed [NO+][N2O4] (2198 cm-1), NO+ (2134 cm-1), Fe3+-NO (1882 cm-1), N2O4 species (1743 cm-1) and N2O3 species (1882 and 1555 cm-1) are formed that, however, can be easily removed by evacuation at 293 K. In addition, stable nitrato species reflected by bands between 1633 and 1575 cm-1 are formed, that persist evacuation, whereby the latter arises most likely exclusively from nitrate bound to Fe sites.

Interaction of NO with pre-reduced catalysts at 293 K

To learn more about the influence of the Fe valence state on the interaction with NO, the same experiments were performed with reductively pretreated samples. The adsorption of NO on samples A(MR)0.5, A(ILIE)0.2, A(ILIE)1.2, A(CVD,W1,C0.5) and H-ZSM-5(A) after reduction with NH3 was investigated at 293 K. The spectra obtained after adsorption of NO for 30 min and before evacuation are reported in Fig. 4.34. In contrast to the NO adsorption on oxidatively pretreated samples, the bands above 1635 cm-1 are only weakly present and the bands between 1635-1575 cm-1 decreased in intensity. In particular, sample A(CVD,W1,C0.5) with extensive clusters shows an intense band at 1465cm-1 which is completely missing in A(MR)0.5 and A(ILIE)0.2 (with almost exclusively isolated iron sites) and A(ILIE)1.2 (with relatively small clusters).

Fig. 4.34. In situ FT-IR spectra at 293 K after adsorption of 1% NO/He on pre-reduced surfaces and before evacuation; reductive pretreatment in 1% NH3/He at 673 K for 1 h followed by evacuation and cooled to 293 K.

↓109

Fig. 4.35. In situ FT-IR spectra at 293 K after adsorption of 1% NO/He on pre-reduced surfaces and after evacuation; reductive pretreatment in 1% NH3/He at 673 K for 1 h followed by evacuation and cooled to 293 K.

Furthermore, in sample A(CVD,W1,C0.5) the band at 1575 cm-1 and a shoulder at 1630 cm-1 completely disappeared and the one at 1620 cm-1 drastically decreased in intensity. Additionally, upon evacuation, the band at 1578 cm-1 drastically decreases in intensity in all samples and weak features above 1635 cm-1 completely disappear as shown in Fig. 4.35. These results suggest that Fe2+ species assumed to be formed upon pretreatment in 1% NH3/He do not favor neither the formation of weakly adsorbed species reflected by bands at 2198, 2134, 1882 and 1743 cm-1 nor the formation of nitrato species. This suggests that Fe3+ ions are essential for the activation of NO either by direct oxidation or by the intermediate formation of N2O4 (disproportionation). The band at 1465 cm-1 can be ascribed to the formation of nitrito species (-O-N=O) upon adsorption of NO on reduced iron sites, probably on reduced iron oxide clusters [153].

Interaction of NH 3 -SCR feed components with catalysts at elevated temperature

While FT-IR results described above provide information mainly on the nature of adsorbed species formed upon contact with NO, these experiments were performed to learn more about the behavior of the Fe sites in the presence of feed components. Samples are described in the order of increasing Fe x O y cluster formation, starting with samples that are dominated by isolated Fe sites.

A(MR)0.5

↓110

As shown by ex situ UV/VIS and EPR measurements (sections 4.1.1 and 4.1.2) this sample contains almost exclusively isolated Fe species in tetrahedral and higher oxygen coordination. Changing the flow at 623 K from air to NH3-SCR feed causes a substantial decrease of CT bands of isolated Fe3+ species in the UV/VIS-DR spectrum (Fig. 4.36a). This is due to the reduction of Fe+3 to Fe+2 ions. This suggests that the mean oxidation state of isolated iron ions under typical NH3-SCR conditions is probably slightly lower than +3. Interestingly, the reduced iron species were not completely reoxidized during 1 h air treatment (not shown), therefore, the sample was reoxidized in air at 773 K for 15 min and cooled to the reaction temperature of 623 K again. On subsequent switching of the flow from air to 0.1% NH3/He at 623 K the intensity of the two CT bands decreased much stronger than in the complete SCR feed (Fig. 4.36a). This decrease is even more pronounced for the band at 291 nm which is assigned to octahedrally coordinated Fe3+. This is in line with the iron redox kinetics (Table 4.2), which shows that octahedrally coordinated isolated Fe3+ ions are faster reduced than tetrahedrally coordinated ones. These reduced iron species are partly reoxidized by subsequent NO/He treatment.

In the corresponding EPR measurements, changing the flow from air to NH3-SCR feed causes a strong decrease of the signal at g' ≈ 6 and a less pronounced decrease of the line at g' ≈ 4.3 (Fig. 4.36b). Different from the g' ≈ 6 and g' ≈ 4.3 signals, the signal at g' ≈ 2, which is attributed to highly symmetric isolated iron species in this sample (see section 4.1.2), changes only very slightly. This suggests that Fe+3 species contributing to this signal are hardly reducible at this temperature or that the presence of redox cycle keeps the mean oxidation state of Fe at +3. To check this, the sample was subjected to subsequent 0.1% NH3 and 0.1% NO treatments at 773 K (Fig. 4.37).

Fig. 4.36. In situ UV/VIS-DRS (a) and in situ EPR (b) spectra of A(MR)0.5 at 623 K in different gas mixtures: air, NH3-SCR (0.1% NO, 0.1% NH3, 2% O2/He), 0.1% NH3/He and 0.1% NO/He. For UV/VIS measurements, samples were regenerated in airflow (20 ml/min) for 15 min at 773 K to restore the original spectra after NH3-SCR and cooled to 623 K. Spectra were measured after treatment of the samples in the respective mixtures at 623 K for 1 h. Catalyst pretreatment in air at 773 K for 1 h.

↓111

Fig. 4.37. In situ EPR spectra of A(MR)0.5 at 773 K in different gas mixtures: air (pretreatment), 0.1% NH3/He and 0.1% NO/He. Spectra were measured after treatment of the sample in the respective mixtures at 773 K for 1 h.

After 1 h NH3 treatment at 773 K, different from complete SCR feed treatment at 623 K, the intensity of the g' ≈ 2 and g' ≈ 4.3 signals decreased slightly while the one at g' ≈ 6 completely disappeared. Interestingly, subsequent NO treatment almost completely restored the original EPR spectrum. These results clearly demonstrate the different sensitivity of isolated Fe sites against reduction that decreases in the order g' ≈ 6 > g' ≈ 4.3 > g' ≈ 2. Accordingly, isolated Fe sites reflected by g' ≈ 6 which are most probably octahedrally coordinated (see section 4.1.2) are reduced already at lower temperatures compared to Fe sites reflected by g' ≈ 4.3 and g' ≈ 2. This is also suggested by the UV/VIS spectra in which the band at 291 nm being characteristic for octahedral Fe3+ decreases stronger already at 623 K than the one at 241 nm assigned to tetrahedral Fe3+ (Fig. 4.36a).

A(ILIE)0.2 and A(ILIE)0.3

Similar to sample A(MR)0.5 it has been shown that these two samples are dominated by isolated Fe3+ sites. Figure 4.38demonstrates thein situ UV/VIS-DRS and EPRspectra of samples A(ILIE)0.2 and A(ILIE)0.3 under different feed composition. UV/VIS-DRS spectra are characterized by isolated Fe+3 species with CT band maxima below 300 nm. The spectral intensity is markedly decreased under typical NH3-SCR conditions as compared to the UV/VIS-DR spectra of the samples in air (Fig. 4.38) due to a reduction of isolated Fe+3 ions. This behaviour is similar to that of A(MR)0.5 with similar nature and distribution of Fe species (Fig. 4.36). Interestingly, in agreement with Fe redox kinetics (Table 4.2), the intensity loss is more pronounced for the band at 290 nm representing Fe3+ in higher than fourfold coordination. As observed for A(MR)0.5, the reduced iron species were not completely reoxidized during subsequent 1 h air treatment at 623 K (not shown). Therefore, the samples were reoxidized at 773 K for 15 min and cooled to the reaction temperature of 623 K. On switching the flow from air to 0.1% NH3/He at 623 K the intensity of the whole spectrum decreased stronger than in the complete SCR feed indicating further reduction of Fe3+. However, the reduction of the bands at 240 and 290 nm follow the same trend as observed in the complete SCR feed and also as seen for A(MR)0.5, in agreement with the Fe redox kinetics (Fig. 4.7 and Table 4.2). The reduced iron species were partly reoxidized by subsequent NO/He treatment.

↓112

In the corresponding EPR spectra of A(ILIE)0.2 and A(ILIE)0.3, changing the flow from air to NH3-SCR feed causes a slight decrease of the signal at g' ≈ 4.3 and strong decrease of the line at g' ≈ 6, while the line at g' ≈ 2 is hardly influenced (Fig. 4.38). As for sample A(MR)0.5 this suggests that Fe3+ sites which are surrounded by more than 4 oxygen ligands and reflected by the signal at g' ≈ 6 are easier to reduce than tetrahedral Fe3+ represented by the line at g' ≈ 4.3. This is in agreement with the corresponding UV/VIS-DRS results which show under complete NH3-SCR feed slightly more reduction of octahedral Fe sites reflected by 290 nm band than tetrahedral Fe sites reflected by 240 nm band. Similar to sample A(MR)0.5 (Fig. 4.36) Fe3+ reduction is more pronounced in NH3/He flow and only partially reversible upon subsequent treatment with NO/He. In particular, the CT band at 290 nm does not return to its original intensity and this is also true for the EPR signal at g' ≈ 6. This is different when the experiment is performed at 773 K (Fig. 4.39).

Fig. 4.38. In situ UV/VIS-DRS and EPR spectra at 623 K in different gas mixtures, conditions as in Fig. 4.36. Bottom figures are enlarged EPR spectra.

↓113

Fig. 4.39. In situ EPR spectra of A(ILIE)0.3 at 773 K in different gas mixtures, conditions as in Fig. 4.37.

In this case the EPR spectrum is completely restored after NO/He treatment. Moreover, a small but significant change of the g' ≈ 2 EPR signal is also seen at 773 K, suggesting that Fe sites reflected by this line might become active at higher temperature compared to those reflected by the g' ≈ 6 and g' ≈ 4.3 signals. In analogy to the A(MR)0.5 catalyst, these results also indicate that the isolated Fe species are stable and do not aggregate upon reduction as observed by Kucherov et al. during reduction of Fe-ZSM-5 already at 523 K in 1% H2/He [119].

A(ILIE)0.7 and A(ILIE)1.2

In contrast to sample A(MR)0.5 and the two A(ILIE) samples with lower Fe content discussed above, samples A(ILIE)0.7 and A(ILIE)1.2 contain, besides isolated Fe, a certain amount of oligonuclear Fe x O y clusters. Sample A(ILIE)0.6 is very similar to sample A(ILIE)0.7 with respect to the nature and distribution of iron species as discussed in sections 4.1.1 and 4.1.2. Moreover, under typical SCR conditions this sample behaves like A(ILIE)0.7. Hence, the following discussion is focused on the latter sample only.

↓114

On switching the flow from air to NH3-SCR feed the intensity of the whole UV/VIS spectrum slightly decreased in A(ILIE)0.7 while in A(ILIE)1.2 the intensity of the band above 300 nm slightly decreases and the band below 300 nm hardly changes (Fig. 4.40). Interestingly, the reduction of isolated iron species in the samples is not as significant as observed for samples A(ILIE)0.2 and A(ILIE)0.3. This suggests that redox properties of isolated Fe3+ sites might change with increasing Fe x O y cluster content in the sample. It is well known from the literature [59] and has been demonstrated in section 4.1.1 and Fig. 8 that isolated Fe3+ can migrate out of their positions and be incorporated in Fe x O y clusters during calcination. It has also been shown that migration of isolated Fe3+ out of the pores to form clusters is particularly favoured for easily reducible sites [103,173]. Thus, it can be assumed that in samples with higher Fe content, preferentially those Fe3+ sites that are rather resistant against reduction might withstand the tendency to form clusters.

Fig. 4.40. In situ UV/VIS-DR spectra at 623 K in different gas mixtures, conditions as in Fig. 4.36.

When comparing the EPR spectra of sample A(ILIE)0.7 with those of samples A(ILIE)0.2 and A(ILIE)0.3 recorded under the same conditions, a rather similar behavior can be observed for the signals at low magnetic filed (Fig. 4.41). However, it must be noted that the total intensity of the lines at g' ≈ 6 and g' ≈ 4.3 in sample A(ILIE)0.7 is similar to that in samples A(ILIE)0.2 and A(ILIE)0.3 although the iron content is more than twice as high. This shows clearly that the relative percentage of (more easily reducible) isolated Fe3+ species represented by the signals at g' ≈ 6 and g' ≈ 4.3 decreases with rising iron content. This effect is even more pronounced for sample A(ILIE)1.2 in which those signals are hardly visible (Fig. 4.41). This result supports the conclusion derived from the UV/VIS-DRS measurements (Fig. 4.40) that, with increasing Fe content, only reduction resistant isolated Fe3+ sites might survive.

↓115

Considering the signal at g' ≈ 2, significant differences are observed for samples A(ILIE)0.7 and A(ILIE)1.2 upon treatment in 0.1% NH3/He in comparison to samples A(ILIE)0.2 and A(ILIE)0.3 (Fig. 4.38).These are most obvious for sample A(ILIE)1.2, in which the g' ≈ 2 signal is a superposition of at least two contributions, an intense broad line and a less intense narrower line. Possibly the two signals in the g' ≈ 2 range arise from oxidic clusters of different sizes.

Fig. 4.41. In situ UV/VIS-DR spectra at 623 K in different gas mixtures, conditions as in Fig. 4.36. Bottom figures are enlarged spectra.

Switching from air to NH3-SCR feed virtually does not change the EPR spectra, suggesting that cyclic reduction/reoxidation keeps the mean valence state of the oxidic cluster species unchanged at +3. This is in good agreement with the results of UV/VIS kinetic studies that revealed very fast reoxidation of Fe3+ x O y clusters (Fig. 4.8) and with in situ UV/VIS-DRS studies (Fig. 4.40). Changing the flow from NH3-SCR feed to NH3/He reduces the intensity of the narrow signal in the g' ≈ 2 range while the broad line increases. Increasing intensity upon partial reduction is characteristic of the formation of Fe3O4-like species with ferrimagnetic interaction [58,119]. The decrease of the narrow signal in sample A(ILIE)1.2 suggests that the oxidic moieties giving rise to this line might be smaller in comparison to those responsible for the broad line. Thus, reduction of Fe+3 to Fe+2 just reduces the number of EPR-active Fe+3 sites but does not give rise to ferrimagnetic order. In A(ILIE)0.7, the increase in intensity of the broad line is hardly seen indicating the presence of very few and/or small oligomeric Fe3+ x O y clusters in the sample which show less effective magnetic ordering. Isolated Fe3+ sites in sample A(ILIE)1.2 as reflected by the UV/VIS spectrum (Fig. 4.40) might contribute to the line at g' ≈ 2, too, but are hardly distinguished from the cluster signals. Switching from 0.1% NH3 to 0.1% NO/He flow restores the narrower subsignal even above its original value. This could be due to a change of the intrinsic magnetic interactions within the Fe3O4-like domains.

A(CVD,W1,C0.5)

↓116

As shown by ex situ studies (section 4.1.1) sample A(CVD,W1,C0.5) is characterized by pronounced formation of Fe x O y clusters. In the UV/VIS spectrum, a switch from air to NH3-SCR feed does not cause any significant changes (Fig. 4.42a). This suggests that cyclic reduction/reoxidation keeps the mean valence state of the iron species in steady state unchanged at +3. Accordingly, also the EPR spectrum under NH3-SCR feed does not change very much (Fig. 4.42b and d). It is interesting to compare the behavior of the EPR lines at g' ≈ 6 and g' ≈ 4.3 with those in the A(ILIE) samples discussed above. During treatment with the complete NH3-SCR mixture, the intensity of these lines decreases less and less as Fe x O y clusters become more and more dominant from A(ILIE)0.2 to A(ILIE)0.7. In sample A(CVD,W1,C0.5) with extended cluster formation, EPR signals at g' ≈ 6 and g' ≈ 4.3 remain almost constant during NH3-SCR (Fig. 4.42b).

Fig. 4.42. In situ UV/VIS-DRS (a) and in situ EPR (b, c and d) spectra of A(CVD,W1,C0.5) at 623 K in different gas mixtures, conditions as in Fig. 4.36.

Even upon switching from NH3-SCR to NH3/He flow the g' ≈ 6 signal does not completely disappear as it is observed for A(ILIE) samples without Fe x O y clusters (Fig. 4.42), and the line at g' ≈ 4.3 decreases only slightly. This is again a clear indication that isolated Fe sites in samples characterized by extensive cluster formation change their redox behavior as discussed above for samples A(ILIE)0.7 and A(ILIE)1.2. At the same time, the signal at g' ≈ 2 becomes broad, increases in intensity and shifts to a slightly lower magnetic field (Fig. 4.42c). Such behavior is typical for partially reduced iron oxide clusters with ferrimagnetic interaction [58,119]. Subsequent NO treatment shifts the signal back and increases the intensity markedly (Fig. 4.42c) and the signal at g' ≈ 6 is only partially restored (Fig. 4.42d), indicating the partial reoxidation of the reduced iron species. The increase in intensity of the g' ≈ 2 signal could be due to the change in intrinsic magnetic properties of the iron oxide clusters.

↓117

In summary, isolated Fe sites reflected by EPR signals at g' ≈ 6, g' ≈ 4.3 and g' ≈ 2 possess different redox properties and, hence, they behave differently under typical SCR conditions as revealed by in situ EPR experiments. Under typical NH3-SCR of NO, isolated Fe sites are partly reduced in cluster free samples (such as A(MR)0.5 and A(ILIE) with low Fe content) while in clustered samples (like A(ILIE)1.2 and A(CVD,W1,C0.5)) Fe sites are essentially in +3 oxidation state as evidenced by both in situ EPR and UV/VIS-DRS measurements.

4.3.2 EPR, UV/VIS-DRS and FT-IR spectroscopic studies during SCR of NO with isobutane

To elucidate differences in the behavior of the Fe species when isobutane is used as reducing agent instead of NH3, similar in situ investigations have been performed as described in section 4.3.1. The results are discussed below in the order of increasing cluster formation, starting with samples dominated by isolated Fe species.

A(MR)0.5

In the UV/VIS spectrum, switching flow from air to isobutane-SCR feed causes an increase of absorption in the whole UV/VIS spectral region (Fig. 4.43). This unexpected result could be due to the formation of N-containing organic deposits in the pores. Formation of N-containing organic species in the SCR of NO by hydrocarbons has been extensively discussed in the literature [87,90,174-178]. Based on in situ FT-IR results, the formation of several N-containing organic species such as alkyl nitrites, alkyl nitrates, alkyl nitriles, cyanates, isocyanates and alkyl radicals has been proposed. Most of these species are UV/VIS active and give bands below 400 nm hence, it is difficult to conclude which N-containing species is responsible for such behavior from UV/VIS only. Therefore, in situ FT-IR experiments have been performed for selected samples. These results will be discussed in the corresponding sections below. After treatment with the complete SCR feed, sample A(MR)0.5was calcined in air at 773 K for 15 min to restore the initial UV/VIS spectrum in order to continue with the further steps.

↓118

Fig. 4.43. In situ UV/VIS-DRS (a) and EPR spectra at 623 K in different gas mixtures (b and c): air, isobutane-SCR (0.1% NO, 0.1% isobutane, 2% O2/He), 0.1% isobutane/He and 0.1% NO/He. Spectra were measured after treatment of the samples in the respective mixtures at 623 K for 1 h. Catalyst pretreatment in air at 773 K for 1 h.

On changing the gas flow from air to isobutane/He at 623 K, the intensity of the bands below 300 nm slightly decreases and higher absorption above 400 nm can be observed. The former effect is due to the reduction of iron species and the latter effect might be due to the formation of carbonaceous deposits. The formation of carbonaceous deposits in the pores might proceed on acidic sites due to incomplete oxidation of isobutane. However, in contrast to isobutane/He treatment, a much stronger reduction of bands below 300 nm and no increase of absorbance above 400 nm were observed in the presence of NH3 (Fig. 4.36). This suggests that polymeric carbonaceous deposits probably also contribute to the UV/VIS spectrum below 300 nm. Hence, it is noteworthy that the real degree of reduction of isolated Fe sites below 300 nm can be underestimated by UV/VIS-DRS. After flushing with He for 15 min, the flow was changed to NO/He. The UV/VIS band intensity again increases below 400 nm but decreases above 500 nm. This suggests the formation of N-containing species upon reaction of NO with carbonaceous deposits formed in the previous isobutane/He treatment.

In the corresponding EPR spectrum, changing the flow from air to isobutane-SCR feed causes almost complete disappearance of the signal at g' ≈ 6 and a marked decrease in intensity at g' ≈ 4.3 (Fig. 4.43b inset). The observed changes of EPR signals at g' ≈ 6 and g' ≈ 4.3 are attributed to the reduction of Fe+3 to Fe+2 ions. Different from the low field signals, the signal at g' ≈ 2 practically does not change (Fig. 4.43b). This suggests that the Fe+3 species contributing to this signal are probably not easily reducible at 623 K as discussed in the NH3-SCR section for this sample (Fig. 4.36b). However, an additional sharp signal is superimposed in the g' ≈ 2 range (Fig. 4.43b) which was not observed in the EPR spectrum of this sample during NH3-SCR (Fig. 4.36b). This indicates the formation of some kind of carbon-containing radicals under isobutane-SCR. Changing the flow from isobutane-SCR feed to isobutane/He causes a further decrease of the EPR signal at g' ≈ 4.3 and the one at g' ≈ 2 decreases substantially. Note that upon NH3/He treatment only a very small intensity loss of the g' ≈ 2 line was observed at an even higher temperature (773 K, Fig. 4.37). These results clearly show that under isobutane/He feed isolated Fe sites are more strongly reduced than in NH3/He feed. This, however, is not detectable by UV/VIS-DRS due to the contribution of carbonaceous deposits as discussed above. These observations again demonstrate the benefits of using these two techniques for such studies and show that they are complimentary to a certain extent. Upon switching from isobutane/He to NO/He, the EPR signal at g' ≈ 4.3 slightly increases and the g' ≈ 6 remains completely missing while the one at g' ≈ 2 is enhanced even above its original intensity. The radical signal at g' ≈ 2 still retains similar intensity as observed in isobutane/He feed. So far, no conclusive explanation can be given for the increase of the g' ≈ 2 signal upon treatment with NO/He. Possibly, isolated Fe3+ species tend to agglomerate into larger clusters upon treatment with isobutane/He which revealed to be stronger reducing than the NH3/He mixture. Subsequent treatment with NO/He which causes partial reoxidation of those clusters could then give rise to a change in the intrinsic magnetic interaction as discussed similarly for the partially reduced sample A(ILIE)0.7 upon NO/He treatment (Fig. 4.41).

A(ILIE)0.2

↓119

As demonstrated by ex situ UV/VIS and EPR investigations the nature of the Fe species in sample A(ILIE)0.2 is almost the same as in A(MR)0.5, despite the fact that A(MR)0.5 contains slightly more oligomeric clusters than A(ILIE)0.2 (Table 4.1). Consequently, the behavior in the UV/VIS and EPR in situ experiments is also very similar (compare Fig. 4.43 and 4.44) and can be explained in the same way as for sample A(MR)0.5:

Again, the increasing UV/VIS intensity under isobutane-SCR feed suggests the formation of N-containing deposits being UV-active below 400 nm while the isobutane/He mixture alone causes Fe reduction (intensity loss below 400 nm) and carbon deposition (intensity gain above 400 nm). As observed for A(MR)0.5, the latter react with NO/He forming N-containing deposits (repeated intensity increase below 400 nm).

Also the EPR signals at g' ≈ 6 and g' ≈ 4.3 show virtually the same strong reduction as in the experiment with sample A(MR)0.5 which is even stronger than in the presence of NH3 indicating that isobutane is a stronger reducing agent (Fig. 4.38).

↓120

Furthermore, a narrow signal is superimposed on the Fe3+ line at g' ≈ 2 in isobutane-SCR which could be due to alkyl radicals as reported earlier in the SCR of NO by hydrocarbons [174]. However, it is more probable that carbon radicals in the carbon deposits contribute to this signal. This is supported by the fact that the radical signal intensity increases during isobutane/He treatment in which the amount of such deposits increases as evidenced by UV/VIS-DRS.

Fig. 4.44. In situ UV/VIS-DRS (left) EPR (right) spectra at 623 K in different gas mixtures: Conditions as in legend of Fig. 4.43.

In contrast to sample A(MR)0.5, the signal at g' ≈ 2 of isolated Fe3+ ions does virtually not change during SCR reaction and in isobutane/He feed it decreases a little less than in the case of A(MR)0.5 (Fig. 4.43). This suggests that the Fe+3 species contributing to this signal are hardly reducible at this temperature. Compared to A(ILIE)0.2 in which 95% of Fe is present as mononuclear Fe sites, in A(MR)0.5 only 83% of Fe is present as isolated Fe sites (Fig. 4.43 and Table 4.1), while 17% of Fe is present as small oligomers. Obviously, these clustered species are partially reduced upon isobutane/He treatment while the isolated Fe sites also contributing to the signal at g' ≈ 2 are harder to reduce.

↓121

To obtain more information about the nature of adsorbates detected already during the in situ-UV/VIS experiment described above, in situ FT-IR measurements were performed at 623 K under SCR conditions. Fig. 4.45 includes spectra measured after oxidative pre-treatment, after 1 h treatment in SCR feed followed by evacuation and after subsequent 1 h reoxidation at 673 K. For comparison, the same experiment was also performed with the bare H-ZSM-5(A).

It can be seen that the FT-IR spectra after SCR feed treatment are rather similar for H-ZSM-5(A) and A(ILIE)0.2. The strong band at 1630 cm-1 clearly indicates the formation of nitrate species [89,153] while bands below 1470 cm-1 point to the formation of adsorbed nitrite species [153]. However, it must be mentioned that bands in the latter range can also arise from deformation vibrations of adsorbed ammonium species [87,90]. In particular, for the ILIE samples this cannot be excluded, since an additional weak and broad band is observed at 3132 cm-1 (not shown) in the range for ν(N-H) vibrations, which is not visible for H-ZSM-5(A). Moreover, it is known that the intermediate formation of NH3 can occur during SCR of NO with hydrocarbons [90,179].

Fig. 4.45. In situ FT-IR spectra at 623 K after pretreatment (dotted line), isobutane-SCR for 1 h followed by evacuation (thick solid line) and after subsequent air treatment at 673 K for 60 min (broken line). Catalyst pretreatment in air at 673 K for 1 h and evacuation.

↓122

The band at 2240 cm-1 is certainly due to ν(C≡N) of nitrile, cyanate and/or isocyanate species which can only be formed upon reaction of NO with i-butane. Interestingly, this band is not observed over H-ZSM-5(A), suggesting that this reaction requires the presence of a sufficient amount of Fe species. Obviously, the very small Fe impurity of 0.05 % present in the commercial H-ZSM-5 is not enough to catalyse this process. In contrast, the formation of adsorbed nitrates and nitrites on zeolites can occur without participation of a transition metal site [180].

After subsequent treatment in air at 400 °C, the band at 2240 cm-1 almost disappeared due to the oxidative degradation of nitrile, cyanate and/or isocyanate species. The bands below 1700 cm-1 are markedly diminished in sample A(ILIE)0.2 but remain almost unchanged in H-ZSM-5(A). This suggests that in the former sample those bands might largely arise from oxidizable organic (alkyl) nitrates and nitrites while in H-ZSM-5(A) they may originate from inorganic species, the thermal degradation of which is probably not favoured at this temperature. Interestingly, a band at 1690 cm-1 in the range of ν(C=O) [181] is also observed over H-ZSM-5(A) which disappears upon reoxidation. This suggests that on the bare commercial H-ZSM-5(A) partial oxidation of the hydrocarbon is catalyzed to a certain extend by redox active impurities such as Fe.

A(ILIE)1.2

In contrast to samples A(MR)0.5 and A(ILIE)0.2, this sample contains a significant amount of Fe x O y clusters of different sizes. Switching the flow from air to isobutane-SCR feed does virtually not change the intensity of the UV/VIS spectrum (Fig. 4.46). This indicates that the average oxidation state of iron under typical isobutane-SCR conditions is +3 due to the presence of redox process. A similar behavior of this sample has been observed, too, under NH3-SCR feed (Fig. 4.40).

↓123

Fig. 4.46. In situ UV/VIS-DRS (left) EPR (right) spectra at 623 K in different gas mixtures, conditions as in Fig. 4.43.

In contrast to A(ILIE)0.2, no marked increase of absorbance is observed that could be attributed to the formation of N-containing species and/or carbon deposits. This might be due to the fact that such deposits are rapidly oxidized by the Fe x O y clusters being dominant in sample A(ILIE)1.2 and, thus, are easily removed from the surface. Changing feed flow from isobutane-SCR to isobutane/He does not form carbon deposits in the zeolites as observed for sample A(ILIE)0.2 but isobutane interacts with iron oxide clusters leading to a partial reduction of iron oxide clusters to Fe3O4-like clusters as evidenced by a slight decrease in intensity above 300 nm and a slight increase in the absorption in the visible region. The latter effect is associated with the lower wavelength tail of an intervalence charge transfer (IVCT) transition. Such a phenomenon is characteristic for mixed-valence iron oxides and has been observed in Fe3O4 nanoparticles [58,165], where the tail of the IVCT transition extends to the visible range and contributes to light absorption above 450 nm. Upon NO/He treatment the spectrum is partly restored, indicating that NO is able to reoxidize the clusters only partially.

In the EPR spectra (Fig. 4.46), on switching from air to isobutane-SCR feed, the decrease of the g' ≈ 6 and g' ≈ 4.3 signals is markedly less pronounced than for sample A(ILIE)0.2 (Fig. 4.44). This might be due to the fact that in sample A(ILIE)1.2, due to the influence of the higher Fe content and the dominating Fe x O y clusters, only more or less reduction-resistant isolated Fe3+ species persist. This agrees well with the respective in situ UV/VIS-DRS results (Fig. 4.46). The signal at g' ≈ 2 does not change indicating that the Fe ions remain essentially trivalent in time average due to the fast reoxidation of the clusters. This is in line with the results of UV/VIS kinetic measurements (section 4.1.1 and Fig. 4.8) which revealed that the reoxidation of clusters is more than one order of magnitude faster than their reduction.

↓124

Changing flow from isobutane-SCR feed to isobutane/He leads to the formation of a broad intense signal at g' ≈ 2.05 which indicates the formation of ferrimagnetic Fe3O4-like species. This behavior is similar to the one upon treatment with NH3/He, however the intensity increase is higher. This is probably due to a more pronounced reduction of the clusters by isobutane in comparison to NH3 and, consequently, a more pronounced formation of ferrimagnetic domains. By subsequent treatment in NO/He flow, the g' ≈ 2.05 signal narrows and shifts to g' ≈ 2 due to the partial reoxidation of reduced iron species as observed, too, in UV/VIS-DRS. However, the initial spectrum was not restored, indicating that just with NO alone a complete reoxidation of the ferrimagnetic clusters is not possible. Moreover, the signal at g' ≈ 2 reflects the superposition of two signals which probably represent ferrimagnetic clusters of different size.

The corresponding in situ FT-IR spectrum under isobutane-SCR conditions is shown in Fig. 4.45. Treatment of the sample for 1 h under SCR feed flow followed by evacuation gives rise to a band at 2240 cm-1 assigned to ν(C≡N) of nitrile, cyanate and/or isocyanate species, which was also observed for A(ILIE)0.2, however, with a markedly lower intensity. Again, this is a strong indication for the participation of Fe species in this process. Additionally, a series of bands at 1770, 1724 and 1690 cm-1 is observed which are typical for carbonyl stretching vibrations [181]. Carbonyl-like adsorbates can occur as intermediates in the oxidation of the hydrocarbon. Those bands are not visible on A(ILIE)0.2 which contains almost exclusively isolated Fe species. Therefore, it seems likely that the oxidative degradation of the hydrocarbon, which proceeds via intermediate formation of C=O moieties, is promoted by the presence of Fe x O y clusters and particles which are dominating species in A(ILIE)1.2.

A(CVD,W1,C0.5)

Similar to sample A(ILIE)1.2 (Fig. 4.46), a switch from air to isobutane-SCR feed does virtually not change the UV/VIS spectrum of A(CVD,W1,C0.5) (Fig. 4.47a). A similar behavior of the sample has been observed, too, under NH3-SCR feed (Fig. 4.42). This suggests the occurrence of redox cycle that keeps the mean valence state of the iron species unchanged at +3 in the sample.

↓125

Fig. 4.47. In situ UV/VIS-DRS (a) EPR (b, c and d) (c and d are enlarged spectra) spectra of A(CVD,W1,C0.5) at 623 K in different gas mixtures, conditions as in Fig. 4.43.

Subsequent change of gas flow from air to isobutane/He lowers the intensity of the band above 300 nm and increases absorption slightly in the visible region while the band below 300 nm hardly changes (Fig. 4.47a). The decrease in intensity above 300 nm and higher absorption above 400 nm is due to the partial reduction of iron oxide clusters to Fe3O4-like clusters as evidenced by the IVCT transition between neighboring Fe+2 and Fe+3 ions in partly reduced Fe3O4-like clusters [58,165]. Subsequent NO/He treatment causes the partial reoxidation of the reduced clusters. This behavior is similar to that of sample A(ILIE)1.2 (Fig. 4.46). However, the significant difference between these two samples is the pronounced formation of Fe3O4 like clusters in A(CVD,W1,C0.5) than in A(ILIE)1.2 as evidenced by the stronger light absorption in the visible range after isobutane treatment. This is due to the presence of larger iron oxide clusters in the former sample than the latter.

Switching from air to isobutane-SCR feed causes substantial decrease of the EPR signal at g' ≈ 6 while the signals at g' ≈ 4.3 and g' ≈ 2 practically do not change (Fig. 4.47c and d). These results are similar to NH3-SCR results of this sample and other cluster-containing samples such as A(ILIE)1.2 (Fig. 4.46). On changing the flow from SCR mixture to isobutane/He a broad singlet develops in the range of g' ≈ 2. This behavior is typical for partially reduced iron oxide clusters and formation of Fe3O4-like species with ferrimagnetic interaction [58] as discussed earlier. This is in good agreement with the corresponding UV/VIS results (Fig. 4.47a), which show IVCT in partially reduced Fe3O4 clusters. In agreement with the corresponding in situ UV/VIS, the intensity of the broad singlet around g' ≈ 2 is much higher in the sample as compared to the similar signal in A(ILIE)1.2 (Fig. 4.46). This confirms the presence of larger and higher amount of iron oxide clusters in the sample as also evidenced by ex situ results (section 4.1. and 4.1.2, Table 4.1). Subsequent NO treatment gives rise to an intense signal in the g' ≈ 2 range while the signal at g' ≈ 6 is weakly and at g' ≈ 4.3 is partially restored, indicating the partial reoxidation of the reduced iron species. As discussed for this sample and other clustered samples in NH3-SCR, the increase in the intensity of the g' ≈ 2 signal could be due to the change in intrinsic magnetic properties of the iron oxide clusters.

↓126

In summary, in situ UV/VIS-DRS and EPR studies show a similar behaviour of the Fe sites under NH3- and isobutane-SCR, although isobutane seems to be stronger reducing than NH3. This is particularly evident by comparing the EPR spectra of sample A(CVD,W1,C0.5) in Fig. 4.42 and 4.47. While the g' ≈ 6 signal is still well visible under NH3-SCR feed, it has almost disappeared under isobutane-SCR feed. Besides, the more pronounced reducing power of isobutane-SCR conditions is also evident for Fe x O y clusters. While the respective EPR signal at g' ≈ 2 in cluster-containing samples like A(ILIE)1.2 and A(CVD,W1,C5) only slightly increases in NH3-SCR feed (Figs. 4.41 and 4.42), the strong increase under isobutane-SCR mixture in the same samples (Fig. 4.46 and 4.47) indicates a pronounced reduction of Fe2O3 to ferrimagnetic Fe3O4-like species.

In situ FT-IR results show that under isobutane-SCR conditions nitrate, nitrite and N-containing organic deposits such as nitrile, cyanate and/or isocyanate species are formed. Interestingly, the latter species are not formed over the bare H-ZSM-5(A) support while at low Fe content like in A(ILIE)0.2 they are much less pronounced. In contrast, these species are more pronounced over iron rich sample A(ILIE)1.2. This indicates the importance of Fe sites for the oxidation of NO and its subsequent reduction by isobutane via organic N-containing intermediates.

4.3.3 EPR and UV/VIS-DR spectroscopic studies during decomposition and SCR of N2O with CO

The abatement of N2O has been studied over Fe-MFI catalysts that are characterized by a similar distribution of differently structured Fe sites as observed for catalysts used in the SCR of NO. The only difference is that some of the Fe-MFI zeolites used for N2O abatement were prepared by other methods (section 3.1.1). Thus, ex-Fe-silicalite contains virtually only isolated Fe species as it is the case, too, for A(MR)0.5 and A(ILIE) with low Fe content, while the other three samples [A(ILIE)1.2, A'(CVD,W1,C2) and Fe-ZSM-5(LIE)1.4] contain, besides isolated Fe ions, a significant percentage of Fe x O y clusters and particles. In analogy to section 4.3 on SCR of NO, the behavior of the Fe catalysts in the abatement of N2O is described in the order of increasing Fe agglomeration, starting with ex-Fe-silicalite which contains isolated Fe sites only.

Steam-activated ex-Fe-silicalite

↓127

Upon changing the gas flow from air to CO/He, the UV/VIS spectrum does practically not change (Fig. 4.48). This suggests that isolated Fe3+ species in ex-Fe-silicalite, which are mainly tetrahedrally coordinated, are hardly sensitive to reduction by CO at this temperature. In principle, a comparable trend has also been observed upon NH3-treatment of samples with low Fe content such as A(MR)0.5 and A(ILIE). In those samples which, however, contain isolated Fe in both tetrahedral and octahedral coordination, the former species revealed to be more resistant against reduction (Fig. 4.36 and 4.37) than the latter.

Fig. 4.48. In situ UV-VIS-DRS (left) and EPR (right) spectra of ex-Fe-silicalite measured at 623 K upon subsequent treatment in different gas mixtures. Spectra were measured after treatment of the samples in the respective mixtures at 623 K for 1 h. Catalyst pretreatment in air at 773 K for 1 h.

The EPR spectrum of ex-Fe-silicalite changes significantly upon interaction with CO (Fig. 4.48). The signals at g' ≈ 7.15, 6.05, and 2, representative for differently distorted isolated iron sites, decrease in the presence of CO and a new broad signal at g' ≈ 3.01 appears. A signal at g' ≈ 4.3 in Fig. 53 is not observed at 623 K, probably due to short relaxation times at high temperature. Considering the UV/VIS results, the decrease of the EPR lines at g' of 7.15, 6.05, and 2 should not be due to reduction of Fe3+. Rather, it is probable that CO is chemisorbed on extraframework Fe3+ isolated ions, causing changes of the local symmetry and thus altering the position of the EPR signals. A similar shift of the Fe3+ signal upon changing ligands was observed for Fe-ZSM-5 zeolites prepared by impregnation of H-ZSM-5 with a FeCl3 solution [119]. In the as-prepared zeolites, a line at g' ≈ 3.65 was assigned to isolated (FeCl2)+ species. Upon calcination in air, this line disappeared due to the transformation of (FeCl2)+ into differently distorted FeO+ species, giving rise to EPR signals at g' ≈ 6.5, 5.6, and 4.27.

↓128

For ex-Fe-silicalite, the EPR changes are reversible upon switching from CO to N2O (compare EPR spectra in Fig. 4.48). The intensity of the band at 240 nm in the UV/VIS spectrum of ex-Fe-silicalite is slightly higher after contact with N2O, as compared to the initial spectrum of the sample in air (Fig. 4.48). This suggests that some Fe2+ ions could be present even after pretreatment in air, which can be oxidized by N2O but not by O2.

A(ILIE)0.3

The interaction of CO and N2O with A(ILIE)0.3 leads to important differences compared to ex-Fe-silicalite (Fig. 4.49). The UV/VIS spectrum of the sample increases in intensity upon switching from air to N2O/He. This might be due to the oxidation of Fe2+ ions by N2O, which remain even after pretreatment in air (Fig. 4.49).

Fig. 4.49. In situ UV-VIS-DRS (left) and EPR (right) spectra measured at 623 K upon subsequent treatment in different gas mixtures. Conditions as in legend of Fig. 4.48.

↓129

Subsequent treatment in CO/He decreases partly the intensity of the bands at 230 and 290 nm, which are assigned to isolated Fe sites in tetrahedral and octahedral coordination respectively. This effect is more pronounced for the latter band than the former (Fig. 4.49). The decrease of the band at 230 nm is surprising since this band was not reduced by CO at this temperature in ex-Fe-silicalite which is governed by mainly tetrahedrally coordinated isolated Fe sites. A possible explanation for the slight decrease of the band at 230 nm in A(ILIE)0.3 could be that the CT band of octahedral Fe sites at 290 nm extends below 250 nm and contributes to the overall intensity at 230 nm. Therefore, the reduction of the band at 290 nm could influence to a certain extent also the band at 230 nm. Another reason could be that tetrahedral Fe3+ sites in ex-Fe-silicalite and A(ILIE)0.3 are located in different pore positions. Note that the former sample was prepared by extraction of Fe from framework positions. These results suggest that octahedral isolated Fe sites are partly reduced by CO while tetrahedral isolated Fe sites are hardly sensitive against reduction by CO. This is highly likely, since tetrahedral isolated Fe can extend their coordination state rather than releasing an oxygen atom from the coordination sphere to undergo reduction. Differently, octahedral isolated Fe sites, which are coordinatively saturated, can give away an oxygen atom from the coordination sphere to undergo reduction rather than extend their coordination state. This could explain the different behavior of A(ILIE)0.3 and ex-Fe-silicalite. However, subsequent N2O treatment restores the UV/VIS spectrum similar to that of initial N2O spectrum.

In the corresponding EPR spectra of A(ILIE)0.3 (Fig. 4.49), switching from air to N2O causes a slight increase of the signals at g' ≈ 6 and 4.3. Taking into account of UV/VIS results, the observed changes can be attributed to oxidation of Fe2+ sites by N2O. Upon switching from N2O to CO, the signals at g' ≈ 2 and 4.3 are almost not affected. In contrast to these signals, signals at g' ≈ 6.4 and 5.6 decreased slightly in intensity indicating that isolated Fe sites with 5 or 6 coordinating ligands are partly reduced by CO. This is in line with the corresponding UV/VIS measurements which show that octahedral Fe sites are partly reduced by CO (Fig. 4.49).

These finding are different as compared to the results observed for ex-Fe-silicalite. The decrease of the EPR signals at g' ≈ 6.4 and 5.6 in A(ILIE)0.3 corresponds to octahedral isolated Fe3+ which are reduced by CO as concluded from in situ UV/VIS. Different from ex-Fe-silicalite, no pronounced signal at g' ≈ 3 was observed in A(ILIE)0.3 upon interaction with CO. Furthermore, EPR signals in the low-field range appear at slightly different g'-values in A(ILIE)0.3, suggesting that the distortion of the local symmetry of the isolated sites might differ slightly in comparison to ex-Fe-silicalite. Thus, the interaction of CO with such sites can lead to complexes of slightly different geometry in both samples, causing a shift of the EPR signal. This could also explain the absence of the g' ≈ 3 signal in A(ILIE)0.3. Finally, the reduced Fe sites are reoxidized upon switching from CO to N2O (Fig. 4.49).

A(ILIE)1.2

↓130

On switching the flow from air to N2O/He the UV/VIS bands below 300 nm slightly increased in intensity but bands above 300 nm did not change (Fig. 4.50). This suggests the oxidation of isolated Fe2+ sites as observed in A(ILIE)0.2. Subsequent treatment in CO/He decreases slightly the intensity of the bands between 250-300 nm assigned to octahedral Fe sites. Similar to ex-Fe-silicalite, the band at 250 nm, characteristic of tetrahedral isolated Fe3+ species, remains almost unaltered. This indicates that octahedral isolated Fe sites are partly reduced by CO but not tetrahedral isolated Fe3+ species. Differently, light absorption above 300 nm decreases strongly and the light absorption above 450 nm increases significantly. The former effect is due to the reduction of oligomeric clusters and consequently the latter effect is observed, due to the IVCT transition in partly reduced iron oxide clusters [58,165]. These results suggest a different behaviour of Fe3+ species upon interaction with carbon monoxide. Obviously, CO reduces oligonuclear Fe3+ x O y clusters at typical reaction temperatures. This differs from observations in the SCR of NO in which no marked reduction of Fe x O y clusters could be detected at this temperature (Figs. 4.40 and 4.46). It suggests that CO might be slightly more effective in reducing iron oxide clusters than NH3 and/or isobutane Switching back from CO to N2O reoxidizes the reduced Fe sites.

Fig. 4.50. In situ UV-VIS-DRS (left) and EPR (right) spectra of A(ILIE)0.2 measured at 623 K upon subsequent treatment in different gas mixtures. Conditions as in legend of Fig. 4.48.

In the corresponding EPR spectra of the sample, switching from air to N2O causes a slight increase of the signals at g' ≈ 6 and 4.3. This is due to the oxidation of divalent Fe sites as observed in the corresponding UV/VIS-DRS. CO treatment causes slight decrease of the signal at g' ≈ 6 while the one at g' ≈ 4.3 is almost not affected and the signal at g' ≈ 2 becomes slightly broader. The latter effect is due to the reduction of iron oxide clusters. These results are in line with the corresponding UV/VIS results, which show reduction of octahedral isolated Fe sites and oxidic clusters upon CO treatment. These reduced Fe sites are reoxidized by subsequent N2O treatment.

Steam-activated ex-Fe-ZSM-5

↓131

The subsequent interaction of CO and N2O with ex-Fe-ZSM-5 leads to similar results as in A(ILIE)1.2, which are associated with similar iron constitution. However, the UV/VIS spectrum of ex-Fe-ZSM-5 did not change upon switching from air to N2O/He indicating that the sample does not contain divalent Fe species unlike ILIE samples. But, treatment in CO/He decreases the intensity of the bands above 290 nm (Fig. 4.51). Similar to A(ILIE)1.2, the band around 250 nm is virtually not effected. These observations can be explained as for A(ILIE)1.2. Octahedral isolated Fe sites are slightly and clusters are stronger reduced by CO, whereas tetrahedral isolated Fe sites are not. Switching back from CO to N2O restores the band above 280 nm in ex-Fe-ZSM-5 and surprisingly, enhances the band around 250 nm above the maximum reached during the first N2O treatment. This result can be tentatively explained by assuming that the coordination of some of the Fe3+ species formed upon reoxidation by N2O is different from their initial state before reduction with CO. Phenomena like the partial dissolution of larger clusters and/or symmetry changes might lead to an increased absorbance around 250 nm. However, in any case, the changes are very small and the amount of Fe species involved in these processes is not significant.

Fig. 4.51. In situ UV-VIS-DRS (left) and EPR (right) spectra of ex-Fe-ZSM-5 measured at 623 K upon subsequent treatment in different gas mixtures. Conditions as in legend of Fig. 4.48.

In the corresponding EPR spectra of ex-Fe-ZSM-5 (Fig. 4.51), switching from air to N2O causes a slight increase of the signals at g' ≈ 6.4, 5.6 and 2.0. This can be associated to the change from a mixture containing a paramagnetic gas (O2) to a diamagnetic one (N2O), which causes line narrowing since the magnetic interaction between Fe3+ and O2 is suppressed. Interestingly, the signal at g' ≈ 4.3 is almost not influenced by this change, suggesting that the Fe3+ species contributing to this line are probably not accessible by gas-phase components.

↓132

The signals at g' ≈ 6.4, 5.6 and 2.0 in ex-Fe-ZSM-5 decrease upon switching from N2O to CO (Fig. 4.51). Based on the ex situ characterizations in section 4.1 (UV/VIS, and the temperature dependence in the EPR spectra), the signal at g' ≈ 2 in this sample is mainly assigned to oxidic iron clusters. Accordingly, its decrease upon CO treatment is associated to the reduction of clustered Fe3+ species. As observed in A(ILIE)1.2, the decrease of the EPR signals at g' ≈ 6.4 and 5.6 in ex-Fe-ZSM-5 is due to the reduction of octahedral isolated Fe3+ species as concluded from in situ UV/VIS. In agreement with the previous results and with the corresponding in situ UV/VIS results, tetrahedral isolated Fe sites are hardly effected by CO treatment as evidenced by a negligible change in the signal intensity at g' ≈ 4.3. Finally, the EPR signals at g' ≈ 6.4, 5.6 and 2 were restored upon switching from CO to N2O (Fig. 4.51).

Sublimed A'(CVD,W1,C2)

As shown in Fig. 4.52, the UV/VIS spectra of A'(CVD,W1,C2) in air and N2O were identical. Upon CO treatment, the intensity of the whole spectrum was substantially reduced and completely recovered by switching back to N2O. The intensity reduction in CO was expected for the bands above 280 nm, arising from octahedral isolated Fe sites and clustered iron species. However, the decrease of the band intensity below 250 nm (tetrahedral isolated Fe3+ species) was not anticipated, since these species were not reduced by CO in ex-Fe-silicalite and in other samples described above. It should be noted that the tail of the CT subbands arising from octahedral isolated Fe sites and Fe3+ x O y clusters in the sample extends into the low-wavelength range, contributing to the overall intensity below 300 nm. Thus, the marked reduction of the band above 280 nm influences the intensity of the signal at lower wavelengths. However, subsequent N2O treatment restores the original UV/VIS spectrum.

Fig. 4.52. In situ UV-VIS-DRS (left) and EPR (right) spectra of A'(CVD,W1,C2) measured at 623 K upon subsequent treatment in different gas mixtures. Conditions as in legend of Fig. 4.48.

↓133

The EPR spectra of A'(CVD,W1,C2) show a slight decrease of the signal at g' ≈ 2 upon switching from air to N2O, while the signals of isolated Fe3+ sites at low field remain practically unchanged (Fig. 4.52). Subsequent admission of CO leads to a very large broad singlet at g' ≈ 2.07 in the sample, which is typical of ferrimagnetic Fe3O4-like species that can only form by partial reduction of sufficiently large oxidic particles as present in this sample. Similar to the behavior observed for A(ILIE) samples and ex-Fe-ZSM-5, isolated octahedral Fe sites reflected by EPR signal at g' ≈ 6.4 are reduced.As shown in Fig. 4.52, switching back from CO to N2O narrows and shifts the g' ≈ 2 line to a higher magnetic field. Since the reduced Fe2+ sites are almost completely reoxidized by N2O, as concluded from UV/VIS-DRS, the change of the EPR signal should be related to changes of the intrinsic magnetic interactions within the ferrimagnetic clusters. In N 2 O, a very narrow signal at g'  2.003 is observed too. Such signal is typical for radical species rather than for Fe 3+ ions. Panov [100] attributed the unique performance of iron-containing ZSM-5 in the N2O-mediated oxidation of benzene to phenol to the formation of α-oxygen, the charge of which has been a matter of discussion. The ability of N2O to deposit paramagnetic O¯ species on partially reduced oxide surfaces is well known [182]. Such species originate narrow EPR signals near the free-electron g values, having an anisotropic g tensor with g⊥ > g|| [182,183]. However, when they become mobile, e.g., at high temperatures, g anisotropy averages out [183]. To the best of our knowledge, neutral oxygen atoms have never been detected by EPR, due to the short lifetime imposed by the their high reactivity [184]. In a recent work, Starokon et al. [185] have concluded that α-oxygen is an anion-radical species, designated as Oα¯. It has also been shown that O¯ species, when encaged in the micropores of CaO/Al2O3 crystals with zeolite-like structure can be rather stable [183].Based just on its g value, the narrow isotropic line in Fig. 4.52 cannot be conclusively assigned to O¯ species, since different radical species lead to similar signals in this region. However, considering that the signal appears only in the presence of N2O and/or after partial reduction of the sample with CO, i.e., under favorable conditions for formation of O¯ species, it does not seem unlikely that the signal arises from these radical species, being mobile within the zeolite pore network.

Finally, an in situ EPR experiment was performed by exposing A'(CVD,W1,C2) to different N2O-CO mixtures at 623 K (Fig. 4.53). A change from air to a reaction mixture containing of CO/N2O = 1 causes a slight decrease of the signals at g' ≈ 6.4 and g' ≈ 2, while the line at g' ≈ 4.3 remains unchanged. The N2O conversion in the experiment was 82%, in good agreement with the steady-state tests as reported in [58]. This result indicates that in the presence of equimolar amounts of N2O and CO, the average valence of active Fe species is +3 under steady-state conditions, with no formation of ferrimagnetic Fe3O4-like particles. This occurs when CO is added in excess (CO/N2O = 2), while the obtained N2O conversion is slightly increased by 5% (i.e., 87%).

Fig. 4.53.In situ EPR spectra of A'(CVD,W1,C2) in different gas mixtures: air at 773 K, 10 mbar N2O + 10 mbar CO in He (CO/N2O = 1) at 623 K, and 6.6 mbar N2O + 13.4 mbar CO in He (CO/N2O = 2) at T = 623 K and P = 1 bar. The N2O conversion obtained is shown in the figure.

Ion-exchanged Fe-ZSM-5(LIE)1.4

↓134

The in situ UV/VIS spectra of Fe-ZSM-5(LIE)1.4 shows similar behaviour to that of A(ILIE)1.2 (Fig. 4.54). As observed for sample A(ILIE)1.2, interaction with CO leads to an increase of light absorption above 450 nm. This is due to an IVCT transition by electron delocalization between Fe2+ and Fe3+ ions in mixed-valence iron oxides [58,165].

Fig. 4.54. In situ UV-VIS-DRS (left) and EPR (right) spectra of Fe-ZSM-5(LIE)1.4 measured at 623 K upon subsequent treatment in different gas mixtures. Conditions as in legend of Fig. 4.48.

This was observed neither for ex-Fe-ZSM-5 nor for A'(CVD,W1,C2) (Fig. 4.51 and 4.52) and could be attributed to the significantly larger Fe2O3 particles in Fe-ZSM-5(LIE)1.4, which can form typical Fe3O4-like species upon reduction of large Fe2O3 with a certain long range order. Furthermore, the mutual distribution of Fe3+ and Fe2+ species in A'(CVD,W1,C2) and Fe-ZSM-5(LIE)1.4 may differ which in turn can alter the exact position, intensity, and/or line width of the IVCT transition.

↓135

In the EPR spectra of Fe-ZSM-5(LIE)1.4, low filed signals for isolated Fe3+ species are hardly visible, in agreement with the high degree of clustering evidenced by this sample. Large iron oxide particles give rise to the broad EPR signal at g' ≈ 2 which does not change upon switching from air to N2O. In the presence of CO, the broad line accounting for ferrimagnetic Fe3O4-like particles develops. A similar EPR signal was observed in A'(CVD,W1,C2) (Fig. 4.52). However, in the latter sample, the intensity of this line is five times higher than in Fe-ZSM-5(LIE)1.4, although the total Fe content is only 3.5 times higher (see Table 3.1, in section 3.1.1). This is likely due to differences in the domain microstructure of the ferrimagnetic particles and not to a higher degree of Fe3+ reduction by CO in A'(CVD,W1,C2), since UV/VIS-DRS clearly shows a more pronounced reduction in Fe-ZSM-5(LIE)1.4 (Fig. 4.54). Upon contact with N2O, a narrow EPR signal around g' ≈ 2.003 arises in Fe-ZSM-5(LIE)1.4. This signal, attributed toO¯ species, is 17 times larger than in A'(CVD,W1,C2), despite the lower iron content in Fe-ZSM-5(LIE)1.4. These various observations further evidence the markedly different nature of this Fe-oxide phase in both catalysts.

4.4 Catalytic behaviour

Catalytic tests of the Fe-zeolites described in this section have not been performed within this thesis but in the laboratories of Prof. W. Grünert (SCR of NO), and Prof. Javier Pérez-Ramírez (direct decomposition and SCR of N2O). However, since knowledge of the catalytic behavior is essential for the discussion of structure-reactivity relationships (section 5.2), the results which have widely been published [36,55,58,64,161,162] are shortly mentioned blow.

4.4.1 Selective Catalytic Reduction (SCR) of NO with NH3and isobutane

In this section, the catalytic performance of selected Fe-containing samples is described that have been chosen to illustrate the influence of particular properties: 1) increasing Fe content and, thus, increasing degree of Fe site agglomeration (series of ILIE samples in comparison to A(CVD,W1,C5)). 2) Different Al content and, thus, different acidity of the MFI matrix (sample A(CVD,W1,C5), B(CVD,W1,C5) and ex-Fe-silicalite in comparison to A(ILIE) samples with low Fe content. 3) Different pore sizes (ex-Fe-silicalite and (Fe-SBA-I)0.95). Detailed results of the catalytic behavior of all samples studied in this thesis can be found elsewhere [36,55,64].

↓136

Fig. 4.55 shows the temperature dependence of the NO conversion in NH3-SCR over these Fe containing samples. NO conversions increase with increasing reaction temperature in all samples. For the series of ILIE samples it can be clearly seen that with increasing amount of isolated Fe sites from A(ILIE)0.2 to A(ILIE)0.3 (Table 4.1) activity increases gradually. Accordingly, sample A(MR)0.5 which is not included in Fig. 4.55 shows similar activity to that of A(ILIE)0.3. As the concentration of oligomeric sites increases with rising Fe content from A(ILIE)0.3 to A(ILIE)0.6 (Table 4.1), the NO conversion improves, too.Accordingly, A(ILIE)1.2 shows the highest activity within the ILIE series below 750 K while above this temperature NO conversion drops again. These results clearly demonstrate the contribution of both isolated and small oligomeric iron sites to the SCR reaction while the drop of NO conversion at high temperatures for sample A(ILIE)1.2 suggests that larger Fe x O y clusters could contribute to the non selective oxidation of the NH3 reductant.

Fig. 4.55. NO conversions in the selective catalytic reduction of NO with NH3 over different Fe-containing catalysts. Comparison of different preparation techniques, acidity and pore structures of the support. Feed composition: 1000 ppm NO, 1000 ppm NH3, 2% O2 in He at 750,000 h-1 [55,64].

Sample A(CVD,W1,C5) shows similar activity to that of A(ILIE)1.2 below 700 K but above this temperature activity drastically decreases which is much less pronounced over the latter. This is due to the oxidation of NH3 by iron oxide clusters. This is obviously much more pronounced over A(CVD,W1,C5) due to the presence of more and larger clusters than in A(ILIE)1.2 (Table 4.1). In agreement with this, sample A(SSIE)5.4 which contains similar Fe content but more or less the similar nature and distribution of Fe sites as that of A(CVD,W1,C5), exhibits similar catalytic activity (not shown).

↓137

To study the effect of acidity on the NH3-SCR of NO, the performance of B(CVD,W1,C5) and A(CVD,W1,C5) is compared (Fig. 4.55). These two samples were prepared by the same preparation method using different H-ZSM-5 supports with Si/Al ratio 40 and 14 respectively. Accordingly, B(CVD,W1,C5) shows poor acidity as compared to A(CVD,W1,C5) as evidenced by FT-IR studies of pyridine adsorption (Fig. 4.28). B(CVD,W1,C5) shows much less SCR activity than A(CVD,W1,C5) indicating that acidity may play an important role in the SCR. Furthermore, in spite of containing much higher Fe content and, consequently, higher amount of oligomeric clusters (Table 4.1), B(CVD,W1,C5) shows lower activity at all temperatures as compared to A(ILIE)0.6. This further supports that acidity, probably Brønsted acidity, is required for this reaction. Finally, despite high Fe content, Al free ex-Fe-silicalite shows very poor activity as compared to A(ILIE)0.2 and A(ILIE)0.3 with similar nature and distribution of Fe species (Table 4.1). This is, obviously, due to poor acidity. This is in excellent agreement with the above conclusion.

By comparing the catalytic performance of Al free ex-Fe-silicalite and (Fe-SBA-I)0.95, the influence of the framework structure on NH3-SCR reaction is studied (Fig. 4.55). Despite similar nature and distribution of Fe species in the samples (section 4.1), ex-Fe-silicalite shows poor activity whereas (Fe-SBA-I)0.95 completely failed. This indicates that the framework structure and/or pore geometry are crucial for determining the SCR activity. Thus, it was found that microporous materials like MFI structure is favorable for SCR reaction than mesoporous material like SBA-15.

Fig. 4.56 shows the temperature dependence of the NO conversions obtained in isobutane-SCR over the series of ILIE samples. First of all it must be noted that the ILIE samples are markedly less active with isobutane than with NH3 as a reactant since a much lower GHSV was required to reach similar NO conversions with isobutane. With increasing reaction temperature, the NO conversions increase in all samples and passes through a maximum. With increasing Fe content in the samples from A(ILIE)0.2 to A(ILIE)1.2 the NO conversion at the lowest reaction temperature (523 K) increases strongly from 10 to 40%. The NO conversion maximum decreases and shifts to lower temperatures, from 680 K (A(ILIE)0.2) to 620 K (A(ILIE)1.2).

↓138

A(ILIE)0.3 which contains almost exclusively isolated Fe sites as evidenced by ex situ EPR and UV/VIS-DRS results (Table 4.1), exhibits the best catalytic performance. This indicates that isolated Fe sites are playing a major role in the reaction. With increase in the concentration of oligomeric moieties in A(ILIE)0.6 (Table 4.1), the NO conversion drastically decreases at higher temperatures. This is mainly due to the unselective total oxidation of isobutane by oligomeric sites which are essentially in +3 oxidation state under typical SCR conditions as evidenced by in situ EPR and UV/VIS-DRS (section 4.3.2). This effect is even more pronounced for A(ILIE)1.2 which contains extensive clusters. It must be mentioned that NO conversion drops much more than in NH3-SCR as oxidic clusters gain influence in the samples. This indicates that isobutane is much more sensitive against total oxidation than NH3.

Fig. 4.56. NO conversions in the selective catalytic reduction of NO with isobutane over a series of ILIE samples. Comparison of different nature and distribution of Fe sites. Feed composition: 2000 ppm NO, 2000 ppm isobutane, 2% O2 in He at 42,000 h-1 [64].

Fig. 4.57 demonstrates the effect of the nature and distribution of Fe sites, framework composition and structure of the support on the isobutane-SCR activity. As compared to the performance of A(ILIE)1.2 in Fig. 4.56, the peak NO conversion shifts slightly to lower temperatures and NO conversion also increases slightly. This is due to the difference in the reaction conditions. Samples A(ILIE)1.2 and A(CVD,W1,C5) which contain extensive clusters show similar behavior on the whole. However, over A(CVD,W1,C5), in which clusters are more abundant than A(ILIE)1.2, NO conversion above 650 K decreases slightly more. With respect to the nature and distribution of Fe sites, samples A(CVD,W1,C5) and A(SSIE)5.4 are more or less similar. Accordingly, the latter sample shows similar activity as compared to the former (not shown).

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In contrast to these samples, B(CVD,W1,C5) which contains higher Si/Al ratio (40 vs 14) shows poor activity as compared to A(CVD,W1,C5). This can be attributed to the lack of acidity, mainly due to the poor Brønsted acidity as concluded in NH3-SCR. This is further supported by comparing the catalytic performance of A(ILIE)0.2 with Si/Al ≈ 14 (Fig. 4.56) and Al free ex-Fe-silicalite (Fig. 4.57). Accordingly, the former sample shows pronounced acidity while the latter shows poor acidity (Figs. 4.29, 4.30 and Table 4.3). However, both samples contain almost exclusively isolated iron sites (Table 4.1). In spite of containing only one third of the isolated Fe sites, which are believed to play a crucial role in the SCR reaction, A(ILIE)0.2 exhibits much higher activity than ex-Fe-silicalite. This confirms that Brønsted acidity is essential for this reaction.

Fig. 4.57. SCR of NO with isobutane over Fe containing catalysts. Comparison of different nature and distribution of Fe sites, framework composition and structure of the support. Feed composition: 1000 ppm NO, 1000 ppm isobutane, 2% O2 in He at 30,000 h-1 [55,64].

To investigate the effect of framework structures on the isobutane-SCR reaction, the catalytic performance of ex-Fe-silicalite and (Fe-SBA-I)0.95 is compared. Both samples do not contain Al in the framework but have very similar nature and distribution of iron species (Table 4.1). Interestingly, although ex-Fe-silicalite shows poor activity the activity of sample (Fe-SBA-I)0.95 is even worth showing almost negligible NO conversion in the whole temperature range. This clearly demonstrates that the intrinsic activity of MFI structure is an additional asset for this reaction.

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In summary, SCR activity either with NH3 or with isobutane increases with increasing Fe concentration at low temperature, suggesting that accessible Fe sites in isolated and oligomeric form play an active role. However, oligomers cause total oxidation of the reductant. This is much more severe even at lower temperatures for isobutane than for NH3. Therefore, the NO conversion drastically deteriorated above 600 K over cluster containing samples in isobutane SCR whereas this effect is much less pronounced in NH3-SCR. This suggests a more favorable role of small oligomeric clusters in NH3-SCR. Consequently, sample A(ILIE)1.2 shows superior activity at almost all temperatures as compared to the cluster-free sample A(ILIE)0.3. This is opposite in isobutane-SCR, where the cluster-free sample A(ILIE)0.3 exhibits the highest activity in a wide temperature window since clusters that can cause total oxidation of the reductant are missing.

The poor SCR activity of B(CVD,W1,C5) and ex-Fe-silicalite indicates that acidity is essential for these reactions. The SCR activity of sample (Fe-SBA-I)0.95 is almost negligible in both the reactions as compared to ex-Fe-silicalite, despite a very similar nature and distribution of iron species. From this it can be concluded that the intrinsic activity of MFI structure is favorable for these reactions.

4.4.2 Decomposition and Selective Catalytic Reduction (SCR) of N2O with CO

N2O conversion has been studied over ex-Fe-silicalite, ex-Fe-ZSM-5, A'(CVD,W1,C2) Fe-ZSM-5(LIE), A(ILIE)0.2, A(ILIE)1.2 and (Fe-SBA-I)0.95. These catalysts were prepared by different procedures and, hence, contain different nature and distribution of Fe species. Besides, some of the catalysts contain different amounts or no aluminum in the lattice, which gives rise to different acidic properties. Therefore, it allows us to study the effect of the nature and distribution of Fe species, acidic properties and framework structure on N2O decomposition and SCR of N2O with CO.

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Fig. 4.58 shows the N2O conversion vs. temperature resulting from steady-state experiments over ex-Fe-silicalite, ex-Fe-ZSM-5 and A'(CVD,W1,C2) in direct N2O decomposition.

Fig. 4.58. Conversions of N2O vs temperature in direct N2O decomposition at 9x105 g s mol-1. Partial N2O pressure was fixed at 1.5 mbar; balance He [58,96].

A'(CVD,W1,C2) shows the highest activity followed by ex-Fe-ZSM-5 and ex-Fe-silicalite. Furthermore, different activation energies were obtained for ex-Fe-ZSM-5 and ex-Fe-silicalite (≈ 140 and ≈ 155 kJ mol-1 respectively) as reported elsewhere [96]. It should be noted that despite similar Fe content, ex-Fe-silicalite contains almost isolated Fe sites while ex-Fe-ZSM-5 shows extensive clustering. Differently, A'(CVD,W1,C2) contains both isolated and oligomeric Fe sites almost equally (Table 4.1). As mentioned earlier (section 2.1.2), oxygen desorption is a rate limiting step in the direct N2O decomposition. Thus, for direct N2O decomposition oligomeric sites are preferred over isolated Fe sites due to easier oxygen recombination over Fe sites which are close together.

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Fig. 4.59 compares the activity of ex-Fe-silicalite and (Fe-SBA-I)0.95 in direct N2O decomposition. This is especially interesting, since the two catalysts contain Fe species of very similar structure as shown in section 4.1. Thus, for the first time, it is possible with these two samples to study the influence of the pore network explicitly. Ex-Fe-silicalite shows significant conversion in direct N2O decomposition at T > 650 K and the N2O conversion is complete at 800 K. The different behavior of this sample as compared to the one in Fig. 4.58 is due to the difference in the reaction condition. The conversion profile over (Fe-SBA-I)0.95 is similar but shifted by 200 K to higher temperatures indicating considerably lower activity than ex-Fe-silicalite [161].

Fig. 4.59. N2O conversion vs temperature in direct N2O decomposition at 3x105 g s mol-1. Partial N2O pressure was fixed at 1.5 mbar; balance He [161].

The SCR of N2O with CO was performed over the same samples used for direct N2O decomposition as well as over A(ILIE)0.2 and A(ILIE)1.2. The catalytic performance of ex-Fe-silicalite and ex-Fe-ZSM-5 in CO/N2O at a molar ratio of 0.6 is compared with A'(CVD,W1,C2) at molar feed ratios of 0.5 and 0.75 (see in the Fig. 4.60). In comparison to direct N2O decomposition (Fig. 4.58), reduction of N2O with CO starts at significantly lower temperatures over all samples. N2O conversion over A'(CVD,W1,C2) increases with increasing CO/N2O molar ratio from 0.5 to 0.75 indicating that the reaction is stoichiometric as shown by Eq. (4.4).

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N2O + CO → N2 + CO2 (4.4)

This is also suggested by the profile of the N2O conversion curves. N2O conversion over A'(CVD,W1,C2) sample rapidly increases in the temperature range of 500-600 K. Above 600 K CO becomes exhausted over this catalyst and hence, N2O conversion remains practically unchanged until 700 K. Above this temperature N2O conversion curves shift to that of the pure N2O decomposition.

Fig. 4.60. N2O conversion vs temperature at W/F(N2O) ≈ 9x105 g s mol-1 and P = 1 bar. Partial N2O pressure was fixed at 1.5 mbar and CO/N2O ratios at 0.5-0.75 were used, balance He [58,96].

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Despite the fact that both ex-Fe-silicalite and ex-Fe-ZSM-5 contain almost the same amount of iron, the former is markedly more active in the entire temperature range studied. While in ex-Fe-silicalite almost all Fe sites are well isolated, ex-Fe-ZSM-5 is characterized by a considerable amount of Fe x O y clusters which form at the expense of isolated Fe sites. Thus, the different behavior of the two samples shows clearly the superior catalytic performance of isolated Fe sites in the SCR of N2O. In principle, this is also confirmed by the behavior of sample A'(CVD,W1,C2). Although this sample contains roughly ten times more iron than ex-Fe-silicalite, it is only slightly more active in the low-temperature range, probably since most of the additional Fe is incorporated in oxidic clusters.

Fig. 4.61 compares the activity of different catalysts in the N2O reduction with CO at CO/N2O molar ratio of one. Both ex-Fe-silicalite and A(ILIE)0.2 contain almost exclusively isolated Fe sites. However, the occurrence of differently coordinated Fe sites (octahedral and tetrahedral) differs markedly in the samples. Thus, the former sample is dominated by tetrahedrally coordinated Fe sites besides a small amount of octahedral Fe sites, whereas the latter sample contains both types of Fe sites equally. However, despite lower Fe content, sample A(ILIE)0.2 shows slightly higher activity than ex-Fe-silicalite. This could be due to the differences in the occurrence of differently coordinated isolated Fe sites in the samples and/or presence of Al in the framework of A(ILIE)0.2. Interestingly, in spite of containing extensive clusters with relatively low amount of isolated Fe sites (Table 4.1), sample A(ILIE)1.2 shows higher activity at all temperatures as compared to ex-Fe-silicalite and (Fe-SBA-I)0.95. Therefore, the superior catalytic performance of the sample A(ILIE)1.2 should be related to the contribution of oligomeric sites along with isolated Fe sites.

Fig. 4.61. N2O conversion vs temperature over different catalysts in the N2O reduction with CO at CO/N2O=1. Conditions: W/Fo(N2O) = 3x105 g s mol-1 and P = 1 bar; balance He [161,162].

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Finally, despite of similar nature and distribution of Fe sites in ex-Fe-silicalite and (Fe-SBA-I)0.95, the latter sample shows considerably lower activity than the former. This indicates clearly that it is not the nature of the Fe sites but their confinement in the micropores of the MFI structure that accounts for the superior activity of ex-Fe-silicalite.

In summary, for direct decomposition of N2O, cluster-containing samples A'(CVD,W1,C2) and ex-Fe-ZSM-5 show higher performance than the cluster-free sample ex-Fe-silicalite. Thus, it can be concluded that oligomeric sites are preferred over isolated Fe sites for this reaction. In SCR of N2O with CO,A(ILIE)0.2 and ex-Fe-silicalite with almost exclusively isolated Fe sites show higher activity per mol of Fe than any other catalyst tested in this study. Therefore, it can be concluded that isolated Fe sites are essential for this reaction. However, contribution of oligomeric sites to the reaction cannot be completely ruled out as evidenced by the catalytic performance of sample A(ILIE)1.2. Contrarily, in spite of containing a slightly higher amount of isolated Fe sites (Table 4.1), (Fe-SBA-I)0.95 shows lower catalytic activity in both the direct decomposition as well as in the SCR of N2O in comparison to ex-Fe-silicalite. This observation is a clear evidence for the contribution of the intrinsic activity of MFI structure for these reactions than mesoporous SBA-15. Finally, from the catalytic behavior of A(ILIE)0.2 (Si/Al ≈ 14) and ex-Fe-silicalite (Si/Al ≈ ∞), which contain similar nature and distribution of Fe species, it can be concluded that surface acidity of the Fe-zeolites has negligible effect on these reactions.


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