Adsorption properties of Fe-containing dealuminated BEA zeolites as revealed by FTIR spectroscopy

Microporous and Mesoporous Materials 131 (2010) 1–12

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Adsorption properties of Fe-containing dealuminated BEA zeolites as revealed by FTIR spectroscopy Konstantin Hadjiivanov a,*, Elena Ivanova a, Radoslav Kefirov a, Janusz Janas b, Anna Plesniar c, Stanislaw Dzwigaj d,e,*, Michel Che d,e,f a

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30-239 Kraków, Poland Laboratoire Catalyse et Spectrochimie, ENSICAEN, Université de Caen, CNRS, 6 Bd Maréchal Juin, 14050 Caen, France d UPMC Université Paris 6, UMR 7197, Laboratoire de Réactivité de Surface, 4 Place Jussieu, 75252 Paris Cedex 05, France e CNRS, UMR 7197, Laboratoire de Réactivité de Surface, 4 Place Jussieu, 75252 Paris Cedex 05, France f Institut Universitaire de France, France b c

a r t i c l e

i n f o

Article history: Received 1 September 2009 Received in revised form 22 November 2009 Accepted 25 November 2009 Available online 3 December 2009 Keywords: Adsorption BEA zeolite CO FTIR spectroscopy Iron NO

a b s t r a c t Adsorption properties of Fe-containing dealuminated BEA zeolites were investigated by FTIR spectroscopy of adsorbed CO and NO. Two Fe-containing SiBEA zeolite samples were prepared by a two-step post-synthesis method: creation of vacant T-atom sites (T = Si, Al) by dealumination of tetraethylammonium BEA zeolite with nitric acid followed by impregnation of the resulting SiBEA zeolite with an aqueous solution of Fe(NO3)3. The two samples differed in iron content (0.9 and 4.2 wt.%, for Fe0.9SiBEA and Fe4.2SiBEA, respectively). The parent SiBEA sample was characterized by IR bands at 3735 cm1 (isolated internal silanols), 3705 and 3515 cm1 (associated with hydroxyl nests at vacant T-atom sites). Upon the impregnation step, the bands at 3705 and 3515 cm1 practically disappeared, indicating consumption of the corresponding hydroxyls and incorporation of iron into the framework of SiBEA zeolite (also confirmed by XRD). In agreement with this, the IR spectra of the two samples revealed acidic bridging hydroxyls of a „Fe3+–O(H)–Si„ type characterized by a band at 3632 cm1 in higher concentration for Fe4.2SiBEA. The 3632 cm1 band shifted to 3352 cm1 after low-temperature CO adsorption (Dm  280 cm1) evidencing a high acidity of the bridging OH groups. Low-temperature CO adsorption experiments revealed the presence of mainly two families of Fen+ sites, evidenced by carbonyl bands at 2215 and 2186 cm1, respectively. The latter sites were in higher concentration for Fe4.2SiBEA. In addition, a minor fraction of iron sites were found to be able to form tricarbonyls (bands at 2155, 2123 and 2115 cm1). It was also deduced that the majority of iron introduced was in a Fe3+ state and the majority of these ions did not interact with probe molecules. Adsorption of NO leads to appearance of different mononitrosyls (1901, 1869 and 1842 cm1). With time and in the presence of NO, polynitrosyls (1920 and 1815 cm1) were also formed. Experiments on coadsorption of CO and NO reveal that the iron sites producing the 2215 cm1 carbonyls form nitrosyl species absorbing at 1901 cm1. It is suggested that highly electrophilic Fe3+ ions act as adsorption sites in this case. Treatment of the samples with CO at 673 K generated new Fe2+ sites monitored by CO at 2183, 2174 and 2166 cm1. NO adsorption revealed different mono-, di- and polynitrosyl species. A peculiarity in this case was that interconversion between poly- and dinitrosyl species was well observed. The amount of reduced iron was much higher for Fe4.2SiBEA than for Fe0.9SiBEA. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction Zeolites have found numerous applications in catalysis. Their H-forms are characterized by a strong Brønsted acidity and

* Corresponding authors. Tel.: +359 2 9793598; fax: +359 2 8705024 (K. Hadjiivanov), tel.: +33 1 44275291, fax: +33 1 44276033 (S. Dzwigaj). E-mail addresses: [email protected] (K. Hadjiivanov), stanislaw.dzwigaj@ upmc.fr (S. Dzwigaj). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.11.034

exchange of the acidic protons by metal cations gives rise to new acid/base and redox properties. However, to precisely control the catalytic properties of the final system, it is necessary to tune first the properties of zeolites. This has been achieved by various ways, one of them being isomorphous substitution of aluminum with an appropriate element (e.g. B, Fe, Ga) [1–3]. Recently, a series of iron-containing dealuminated BEA zeolites (FeSiBEA) has been prepared and characterized [4–6]. A two-step post-synthesis method has been used to introduce iron into BEA

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zeolite. The method consists of: (i) creation of vacant T-atom sites (T = Si, Al) by dealumination of BEA zeolite with nitric acid, and (ii) impregnation of the resulting SiBEA with aqueous Fe(NO3)3 solutions. It was found that iron was mainly in Fe3+ state and, for low metal contents, mainly in the form of isolated and homogeneously distributed tetrahedral sites indicating occupation of T-atom sites. For high iron content, the additional presence of extra-lattice FeOx oligomers and superparamagnetic Fe-oxyhydroxide was established. Janas et al. [6] have shown the effect of iron speciation on the catalytic properties of FeSiBEA zeolites. Thus, FeSiBEA with mainly framework tetrahedral Fe3+ species was found to be active in the selective catalytic reduction of NO by ethanol with selectivity toward N2 exceeding 90%. However, when additional octahedral Fe3+ species were present, the full oxidation of ethanol and NO by O2 became important, with formation of CO2 and NO2, respectively. Infrared spectroscopy is a valuable technique for investigating the surface properties of solids and brings detailed information on the type and acidity of the surface sites [7–12]. However, it seems that characterization of iron-containing samples with IR spectroscopy is somewhat restricted. There is a widespread opinion that Fe3+ sites are coordinatively saturated and thus not able to interact with probe molecules [7,8,13–23]. By contrast, FTIR seems to be suitable to characterize iron sites in a lower oxidation state. Note, however, that indirect information on Fe3+ sites, e.g. presence of Fe3+–OH or „Fe3+–O(H)–Si„ groups, can be obtained [1,17,29–37]. Irrespective of the fact that CO is one of the most informative and used IR probe molecules [8], only a few IR studies have been reported on CO adsorption on iron-containing porous materials [13–16,38–40]. Most authors find iron carbonyl bands at 2200– 2194 cm1 [13–16,39]. Zecchina et al. [38] studied CO adsorption on an ex-[Fe]ZSM-5 sample and observed bands at 2180, 2174 and 2157 cm1. All these bands were assigned to CO adsorbed on small iron oxide clusters. A band at unusual high frequency, 2215 cm1, was observed with Fe-silicalite and assigned to Fe3+– CO species formed with highly electrophilic extraframework iron cations [40]. Studies of NO adsorption on Fe-containing porous materials are numerous [13,17–29,40–56] and report different mono, di- and polynitrosyl species. To the best of our knowledge, there are only few IR studies where both CO and NO are used as probes to characterize the iron sites [13,17,40]. This work concerns the IR characterization of two FeSiBEA samples, with low (Fe0.9SiBEA) and high (Fe4.2SiBEA) iron content (0.9 and 4.2 wt.%, respectively), selected for their markedly different catalytic properties. We first analyze the background spectra before investigating the interaction of the zeolite with CO and NO. Coadsorption experiments are also reported. Finally, the samples are investigated after different redox treatments.

2. Experimental 2.1. Materials A tetraethylammonium BEA (TEABEA) zeolite (Si/Al = 11) provided by RIPP (China) was dealuminated by treatment with a 13 mol dm3 HNO3 solution for 4 h at 353 K under stirring, as described earlier [4–6,57–63]. The resulting dealuminated SiBEA zeolite (with Si/Al > 1300) was recovered by centrifugation, washed with distilled water and dried in air overnight at 353 K. In order to incorporate iron ions into vacant T-atom sites produced by dealumination, 2 g of SiBEA were stirred for 24 h at 298 K in 100 ml of an aqueous solution of Fe(NO3)39H2O. Two samples were prepared with solutions of different iron concentrations (0.4 and 1.8 mol dm3). The suspensions (pH 2.4–2.6) were stirred for 2 h

in air at 353 K until complete evaporation of water. The solids were then washed three times with distilled water and dried in air at 353 K overnight. The FexSiBEA samples hereafter referred to ‘‘as prepared” contained x = 0.9 and 4.2 wt.% of iron and are labelled Fe0.9SiBEA and Fe4.2SiBEA, respectively. Prior to adsorption, the samples were treated at 673 K first in oxygen (13.3 kPa, 1 h) and then under vacuum for 1 h: they are noted as ‘‘oxidized”. The samples noted as ‘‘reduced” were treated at 673 K first in CO (2.67 kPa, 1 h) and then under vacuum (1 h). 2.2. Techniques Chemical analysis of the samples was performed with inductively coupled plasma atom emission spectroscopy at the CNRS Centre of Chemical Analysis (Vernaison, France). Powder X-ray diffractograms (XRD) were recorded at ambient atmosphere on a Cary 5E spectrometer equipped with an integrator and a double monochromator. Specific surface area and adsorption isotherms of nitrogen at 77 K were measured on an ASP 2010 apparatus (Micromeretics). All samples were outgassed first at room temperature then at 623 K to a pressure <0.2 Pa. Microporous volumes and specific surface area were calculated using the Barrett, Joyner and Halenda (BJH) equation [64]. The DR UV–Vis spectra were taken with a Thermo Evolution 300 spectrometer equipped with a Praying Mantis diffuse reflectance accessory. FTIR spectra were recorded on a Nicolet Avatar 360 spectrometer accumulating 128 scans at a spectral resolution of 2 cm1. Selfsupporting pellets (ca. 10 mg cm2) were prepared from sample powders and treated directly in a purpose-made IR cell allowing measurements at ambient and low-temperatures. The cell was connected to a vacuum-adsorption apparatus allowing a residual pressure below 103 Pa. Before adsorption, CO (Linde AG, purity > 99.997%) was purified by passage through a liquid nitrogen trap and NO (Messer Griesheim, purity > 99.0%) by fraction distillation. 3. Results 3.1. XRD, BET and UV–Vis evidences for incorporation of iron into the zeolite framework The XRD patterns of the ‘‘as prepared” SiBEA, Fe0.9SiBEA and Fe4.2SiBEA samples are shown in Fig. 1. All patterns are similar and no diffraction peaks due to other crystalline phases are observed even for Fe4.2SiBEA. The d302 spacing related to the narrow main diffraction peak near 22.6° significantly increases from 3.912 Å for SiBEA (2h of 22.71°) to 3.939 (2h of 22.64°) and 3.941 Å (2h of 22.61°) for Fe0.9SiBEA and Fe4.2SiBEA, respectively (inset of Fig. 1), suggesting expansion of the matrix as reported earlier for VSiBEA zeolites [57,58,60] and incorporation of iron into the framework of BEA zeolite. Fig. 2 shows that SiBEA and Fe4.2SiBEA give very similar nitrogen adsorption isotherms, indicating that the incorporation of iron into SiBEA does not affect the crystallinity and does not lead to mesopores. Moreover, similar specific surface areas for SiBEA, Fe0.9SiBEA and Fe4.2SiBEA (range of 620–655 m2 g1), and the absence of extra framework crystalline compounds or long-range amorphization of the FexSiBEA zeolites indicate that iron is incorporated as well dispersed species. The DR UV–Vis spectra of the samples are presented in Fig. 3. The sample with low iron content, Fe0.9SiBEA, is characterized by two peaks at 222 and 245 nm which, according to data from the literature [44,53,65], can be assigned to isolated Fe3+ species. The

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22.61 - 207

c 22.64

b

- 490

- 222 - 245

b

- 350

23,0

- 267

22,5

c

- 28 8

b

a 200

10 15 20 25 30 35 40 45

400

500

600

Wavelength, nm Fig. 3. DR UV–Vis spectra of Fe0.9SiBEA (a) and Fe4.2SiBEA zeolites (b).

0.5

- 3673

Fig. 1. X-ray diffractograms of ‘‘as prepared” SiBEA (a), Fe0.9SiBEA (b) and Fe4.2SiBEA (c). The diffractograms are recorded at room temperature and ambient atmosphere.

d

3650 3600

3735 - 3520

Absorbance

e

0.1

d

b

- 3632

2θ,

300

0

- 3739

5

a

- 3632

Intensity, a.u.

a

Kubelka-Munk

22.71

c a 3705 -

b

a 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure, P/P0

3515 -

3735

b

3800

3600

3400

Wavenumber, cm

3200

-1

Fig. 2. Adsorption isotherms of nitrogen at 77 K on: SiBEA (a) and Fe4.2SiBEA (b). The samples are evacuated at room temperature and then at 623 K until a residual pressure below 0.2 Pa. Full symbols – adsorption; empty symbols – desorption. For convenience, the dataset for the SiBEA sample is shifted upwards along the Y-axes.

Fig. 4. FTIR spectra (OH stretching region) of: parent SiBEA zeolite after evacuation at 673 K (a); ‘‘oxidized” (b) and ‘‘reduced” Fe0.9SiBEA zeolite (c); ‘‘oxidized” (d) and ‘‘reduced” Fe4.2SiBEA zeolite (e).

signal recorded with the Fe4.2SiBEA sample is much more intense, which is consistent with the higher iron content. In addition to isolated iron species (bands below 300 nm) there are additional peaks evidencing the existence of small oligonuclear clusters (350 nm) and Fe2O3 nanoparticles probably located at the external zeolite surface (490 nm) [65].

[57,58,61–64,66]. Three bands are observed at 3735, 3705 and 3515 cm1. The peak at 3735 cm1 is due to isolated internal silanol groups [66–68]. The peak at 3515 cm1 is assigned to hydrogen-bonded Si–OH groups located at vacant T-atom sites which form hydroxyl nests [57,58,61–64,66–68]. Originally, each nest includes four OH groups, but larger structures can also be formed. The corresponding terminal OH groups (experiencing H-bonding through the oxygen atom only) are characterized by the band at 3705 cm1 [66–68]. Introduction of iron markedly changes the spectra (Fig. 4, spectra b and d). The OH groups associated with vacant T-atom sites

3.2. Background IR spectra of oxidized and reduced samples The IR spectrum in the OH stretching region of the parent ‘‘oxidized” SiBEA (Fig. 4, spectrum a) was already discussed

3.3. Low-temperature CO adsorption on oxidized FexSiBEA: OH region CO is often used to simultaneously probe Lewis and Brønsted acidic sites. Interaction of CO with OH groups in zeolites is well documented [7–9,11]. Due to H-bonding, CO induces a broadening and a red shift of the OH bands. The higher the OH acidity, the larger is the shift of the OH modes and the higher the carbonyl stretching frequency [8]. However, the weak CO–OH interaction requires the experiments to be performed at low-temperature. Introduction of CO (100 Pa equilibrium pressure) onto oxidized Fe4.2SiBEA at 100 K leads to an erosion of the original hydroxyl bands and appearance of intense bands at 3655, 3450 and 3352 cm1 (Fig. 5, spectrum a). The intensity of the band at 3655 cm1 quickly decreases with the CO equilibrium pressure, while the band at 3739 cm1 is restored (Fig. 5, spectra b and c). The observed shift of ca. 85 cm1 indicates a weak acidity of the 3739 cm1 silanol groups on our sample, similar to that observed with the silanols of silica [8]. A shoulder located at 3595 cm1 also loses intensity very fast upon outgassing. However, the corresponding complexes are somewhat more stable than those giving the band at 3655 cm1. One could suppose that the band at 3595 cm1 is associated with that at 3673 cm1 registered before CO adsorption. However, analysis of the spectra shows that this is not the case, because the band

- 3352

b

d

3600 3400

- 3450

a

3352 -

- 3632

0.1

0.1

d e f

c g

3632 -

(bands at 3705 and 3515 cm1) are practically absent, indicating that the corresponding hydroxyls of SiBEA have reacted with the aqueous Fe(NO3)3 solution. The band due to silanol groups is shifted to 3739 cm1 with the Fe4.2SiBEA sample. A new band at 3632 cm1 is observed for the iron-containing samples (inset of Fig. 4) which is more intense for Fe4.2SiBEA. To the best of our knowledge, there is no IR study of BEA samples with aluminum substituted by iron. However, a series of studies of other types of zeolites, e.g. ZSM-5 [35–37], have shown that „Fe3+– O(H)–Si„ bridging hydroxyl groups absorb at higher frequencies than „Al3+–O(H)–Si„ (3625–3630 for H–[Fe]ZSM-5 vs. 3610– 3605 cm1 for H–[Al]ZSM-5). All authors unanimously agree that „Fe3+–O(H)–Si„ groups are slightly less acidic than their „Al3+–O(H)–Si„ analogues. On the basis of these data, we assign the band at 3632 cm1 observed with our FexSiBEA samples to „Fe3+–O(H)–Si„ hydroxyls. Additional proofs in favor of this assignment will be provided below. These results show that at least part of iron introduced in our samples occupies T-atom sites. The intensity of the band due to bridging hydroxyls is very often low for H–[Al]BEA samples. This is due to the fact that a large part of them are exchanged with aluminum, leading to a low concentration of OH groups [61–63]. Thus, the concentration of „Fe3+– O(H)–Si„ groups in our samples can not represent the number of iron ions in T-atom sites. Extraframework Fe–OH groups in H–[Fe]ZSM-5 were reported to exhibit a band at 3672 cm1 [36]. Such a band, registered here at 3673 cm1, although of a very low-intensity, is indeed present in the spectra of Fe4.2SiBEA (inset of Fig. 4), in line with its high iron concentration. Another interesting peculiarity of the spectrum of Fe0.9SiBEA is that the band corresponding to silanol groups (3735 cm1) is more intense than that of the parent zeolite, suggesting that part of these groups originally have been involved in H-interaction and have contributed to the bands at 3705 and 3515 cm1 in the parent SiBEA zeolite. Reduction of the samples hardly affects OH coverage. Only a small decrease in intensity of the silanol band at 3739 cm1 is observed with the Fe4.2SiBEA sample. The band at 3632 cm1 remains practically unaffected. Finally, we note that no traces of C–H bands were observed in the spectra indicating full decomposition of the template.

- 3655

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Absorbance

4

- 3739

3800

3600

3400

Wavenumber, cm

3200

-1

Fig. 5. FTIR difference spectra (OH stretching region) of ‘‘oxidized” Fe4.2SiBEA after adsorption of CO at 100 K: equilibrium CO pressure of 100 Pa (a) and development of the spectra during evacuation at 100 K (b–g).

at 3595 cm1 is also observed for Fe0.9SiBEA that does not exhibit the band at 3673 cm1. We thus conclude that some silanols have an enhanced acidity (Dm = 150 cm1). Indeed, similar conclusions were drawn earlier [69]. The disappearance of the intense band at 3655 cm1 upon the initial outgassing stages allows a more detailed analysis of the other bands in the region. It is seen that further outgassing gradually restores the band at 3632 cm1 (Fig. 5, spectra b–f), at the expenses of the broad bands at 3352 and 3450 cm1. The appearance of two shifted bands could indicate some heterogeneity of the Brønsted acidic sites. However, analysis of the spectra shows that, after a relatively fast loss of a component at 3450 cm1 (which in fact shows some heterogeneity of the hydroxyls), two bands at 3352 and 3450 cm1 change in parallel. Therefore, it seems more probable that both bands belong to the same complex. According to Onida et al. [70], the appearance of two separate bands is due to the coupling of the intermolecular transition with intramolecular modes. Taking into account the value of 3352 cm1 for calculations, it appears that the acidity of the hydroxyl groups observed at 3632 cm1 is very high, with corresponding Dm of ca. 280 cm1. For comparison, the corresponding shift of the OH band at 3610 cm1 observed with H–AlBEA was only slightly larger, namely Dm = 305 cm1 [61]. The CO-induced changes in the OH region of Fe0.9SiBEA (spectra not shown) are similar. In this case, however, the band at 3655 cm1 (arising from hydrogen-bonded silanols) is more intense, while the bands at 3450 and 3352 cm1 (associated with hydrogen-bonded acidic hydroxyls) appear with a reduced intensity.

3.4. Low-temperature CO adsorption on oxidized FexSiBEA: carbonyl region Fig. 6 shows the changes in the carbonyl region when CO is adsorbed on oxidized Fe4.2SiBEA. For convenience, the same set of spectra as that presented in Fig. 5 is given. Under CO equilibrium pressure of 100 Pa, six carbonyl bands are detected at 2215,

5

Absorbance

2200

- 2186

- 2215

f

g

g 2250

- 2174

- 2174

c d

2150

2100

Wavenumber, cm

2050

-1

2250

- 2155 2123 2115

2160

a

b a

- 2186

- 2215

2240

2160

- 2141 2135

2240

0.1

0.01

2200

- 2140 2134

g

-2215 - 2187

0.01

0.1

Absorbance

- 2157

- 2215 - 2186 - 2155 2123 2115

-2157

K. Hadjiivanov et al. / Microporous and Mesoporous Materials 131 (2010) 1–12

b d c e f 2150

2100

Wavenumber, cm

2050

-1

Fig. 6. FTIR spectra (carbonyl stretching region) of CO adsorbed at 100 K on ‘‘oxidized” Fe4.2SiBEA: equilibrium CO pressure of 100 Pa (a) and development of the spectra during evacuation at 100 K (b–g). The spectra are background corrected.

Fig. 7. FTIR spectra (carbonyl stretching region) of CO adsorbed at 100 K on ‘‘oxidized” Fe0.9SiBEA: equilibrium CO pressure of 100 Pa (a) and development of the spectra during evacuation at 100 K (b–f). The spectra are background corrected.

2186, 2174, 2157, 2141 and 2135 cm1 (Fig. 6, spectrum a). The bands at 2141 and 2135 cm1 are assigned to weakly bonded (physically adsorbed) CO [8,71] and disappear first upon outgassing (Fig. 6, spectra b and c). The next band to disappear is that at 2157 cm1. Its intensity correlates with that of the band at 3655 cm1 allowing assigning the 2157 cm1 band to CO bonded to silanol groups. Further outgassing provokes disappearance of the carbonyl band at 2174 cm1 (Fig. 6, spectra c–f), which is assigned to CO polarized by the acidic „Fe3+–O(H)–Si„ groups. Indeed, its intensity correlates with the decrease in intensity of the band at 3632 cm1. The wavenumber of this band is typical of CO interacting with bridging zeolite hydroxyls. Three bands of low-intensity at 2155, 2123 and 2115 cm1 are well resolved at relatively low CO coverages (inset of Fig. 6). These bands, as well as the bands at 2215 and 2186 cm1 do not correlate with any OH band and are thus assigned to iron carbonyls. Bands above 2200 cm1 are often observed after CO adsorption on zeolites and assigned to carbonyls formed with EFAL species [8]. In order to exclude the possibility the band at 2215 cm1 to originate from residual aluminum traces in our sample we have studied CO adsorption on the parent zeolite (details not reported). The results showed the absence of a band at 2215 cm1 and confirmed its assignment to iron carbonyls, as already proposed by Berlier et al. [40]. All bands assigned to iron carbonyls, although decreasing in intensity, cannot be removed by outgassing at 100 K. The three bands at 2155, 2123 and 2115 cm1 change in concert and are therefore tentatively assigned to one species, most probably polycarbonyls. They disappear upon outgassing at elevated temperatures (spectra not shown). The next band disappearing at temperatures >100 K is that at 2186 cm1 while the band at 2215 cm1 is the most resistant one. Fig. 7 gives the spectra of CO adsorbed at 100 K on Fe0.9SiBEA. The same bands as those detected with Fe4.2SiBEA are observed here but with different relative intensities. As expected, the band at 2174 cm1 (due to CO polarized by acidic hydroxyls) appears with reduced intensity, while that at 2157 cm1 due to CO bonded

to silanol groups is enhanced. From the iron carbonyl bands, that at 2215 cm1 is of almost the same intensity, while the other bands are less intense. Careful inspection of the band at 2215 cm1 observed with Fe0.9SiBEA shows that its intensity increases when CO coverage decreases (Fig. 8) suggesting that dicarbonyls are formed. Upon outgassing, they convert into monocarbonyls giving the band at 2215 cm1. The superimposition with other carbonyl bands does not allow determining the spectral parameters of the dicarbonyls. The same behavior of the 2215 cm1 band was observed for Fe4.2SiBEA (details not shown). In papers devoted to the adsorption of CO on iron-exchanged zeolites [13–16,38,39], it is generally agreed that Fe2+ ions in exchanged position form carbonyls observed in the 2200– 2190 cm1 region. This suggests that the band at 2187 cm1 is not associated with iron in exchanged position and we assign it to CO adsorbed on isolated Fe2+ species. The band at 2215 cm1 has an unusual high wavenumber for iron carbonyls. Note that the stretching frequency of adsorbed CO increases with the oxidation state of the metal [7,8]. This confirms the earlier assignment of the band to Fe3+–CO species [40]. Evidently, the respective iron ions are highly coordinatively insaturated and can attach more that one CO molecule. The three bands at 2155, 2123 and 2115 cm1, already tentatively assigned to polycarbonyl species, characterize particular iron sites. The low frequencies indicate some back p-bonding which is not typical of cation with oxidation state 2+ or higher. We suggest that these are tricarbonyls of Fe+ ions. In fact, monovalent Ni+ have been detected in matrices of ZSM-5 [72,73], FAU and SiO2 [74–77], as well as Co+ ions in ZSM-5 [73,78]. Ni+ ions form stable monocarbonyls [72–77]. Co+ ions form the so-called specified dicarbonyls (i.e., decomposing without formation of monocarbonyls) [73,78], characterized by a relatively low stability. The tricarbonyls of Fe+ observed here are less stable. This is most probably due to a restricted r-component of the Fe–CO bond. Note also that these polycarbonyls seem to be complex-specified, i.e. they are destructed without formation of monocarbonyl species.

K. Hadjiivanov et al. / Microporous and Mesoporous Materials 131 (2010) 1–12

0.002

0.01

- 1901

- 2215

6

b

- 1626

c

- 1815

- 1869 1842

e

1920

Absorbance

Absorbance

b c d

a

- 2080

a

a 2240

2220

2200

2200

2000

1800

1600 -1

-1

Wavenumber, cm

Wavenumber, cm 1

Fig. 8. FTIR difference spectra (2240–2190 cm region) of CO adsorbed at 100 K on ‘‘oxidized” Fe0.9SiBEA: equilibrium CO pressure of ca. 20 Pa (a) and development of the spectra during evacuation at 100 K (b–e). The spectra are background corrected.

Fig. 9. FTIR spectra of NO adsorbed at r.t. on ‘‘oxidized” Fe0.9SiBEA. Introduction of a small dose (ca. 50 lmol g1) of NO (a), development of the spectra with time (b) and under 10 Pa equilibrium pressure of NO (c). The spectra are background corrected.

3.5. NO adsorption on oxidized FexSiBEA

The band at 1901 cm1, formed independently of the other bands, is assigned to iron mononitrosyls. This wavenumber is very high for surface iron mononitrosyls. To the best of our knowledge, similar band has been reported so far only after NO adsorption on Fe-silicalite and assigned to Fe3+–NO adducts [40]. We agree with this assignment. Note that CO adsorption experiments revealed a carbonyl band with unusual high frequency that was also associated with Fe3+ sites. Some experiments with NO adsorption were performed with the parent zeolite in order to exclude any possibility for the band at 1901 cm1 to arise from residual aluminum in our sample. Although Al3+ ions do not form stable nitrosyl complexes, Venkov et al. [81] has proposed a band at 1938 cm1 to arise from Al3+(NO3)–NO species. The results (details not reported) showed that no bands around 1900 cm1 are formed on SiBEA after NO adsorption, which proves that the band at 1901 cm1 is due to iron nitrosyls. In the spectra registered after adsorption of NO on oxidized Fe4.2SiBEA (not shown), the band at 1901 cm1 is slightly less intense, while the most intense band is that at 1868 cm1.

Adsorption of a small amount of NO (ca. 50 lmol g1) on oxidized Fe0.9SiBEA generates a sharp band at 1901 cm1 and two bands of lower intensity at 1869 and 1842 cm1 (Fig. 9, spectrum a). With time, the band at 1869 cm1 increases in intensity, while the band at 1901 cm1 declines. New bands at 2080, 1920, 1815 and 1626 cm1 develop (Fig. 9, spectrum b). Simultaneously, the band at 3632 cm1 is eroded (spectra not shown). The increase of the amount of NO mainly leads to additional development of the bands at 2080 and 1626 cm1 (Fig. 9, spectrum c) and further erosion of the 3632 cm1 band. On the basis of literature data [13–17,19–23], we assign the bands at 1869 and 1842 cm1 to two kinds of Fe2+–NO species. The bands around 1920 and 1815 cm1, often detected with ironcontaining porous materials [18–21,23–25,40–43,47,55,56], are usually assigned to dinitrosyls. However, Spoto et al. [50] showed that these species contained more than two NO ligands and were thus identified to polynitrosyls. The band at 2080 cm1 can be assigned to NO+ [79]. It is well established that this species is produced after NOx adsorption on various zeolites with the participation of the zeolite acidic hydroxyls:

2Z—OH þ N2 O3 ! 2Z—O—½NOþ  þ H2 O

ð1Þ

In our case N2O3 is probably formed via oxidation of NO by Fe3+ species. However, direct oxidation of NO by Fe3+ ions is also possible:

Fe3þ þ NO ! Fe2þ þ NOþ

ð2Þ

Typically, nitrosonium ion in zeolites is detected at 2133 cm1 [79], a frequency also valid for [Al]BEA [61]. However, the frequency of NO+ is very sensitive to the environment [80]. It appears that the substitution of aluminum by iron in our sample strongly affects the frequency of NO+ in cationic positions: it is blue shifted by ca. 50 cm1 as compared to the Al-containing material. In addition, the appearance of NO+ suggests the presence of reactive surface species able to oxidize NO. The band at 1626 cm1 is attributed to the deformation modes of water produced by reaction (1) and/or to nitrate species.

3.6. Coadsorption of CO and NO on oxidized FexSiBEA In order to get more insight into the nature of the CO and NO adsorption sites, the coadsorption of CO and NO was investigated. Addition of NO to Fe0.9SiBEA with pre-adsorbed CO leads preferentially to erosion of the CO band at 2215 cm1 (seen as a negative band in the difference spectra) and development of the nitrosyl band at 1901 cm1 as well as small bands at 1868 and 1842 cm1 (Fig. 10, spectrum a). These results indicate that the 2215 cm1 carbonyls and the 1901 cm1 nitrosyls are formed with the same iron sites, most probably particular Fe3+ species. An increase of the amount of NO leads to practical disappearance of all carbonyl species and a more pronounced development of the other mono- and polynitrosyl bands (Fig. 10, spectrum b). Unfortunately, due to superimposition with the intense NO+ band at 2080 cm1 we are not able to analyze in details the behavior of the carbonyl bands below 2200 cm1.

7

- 2123 - 2115

0.1

0.05

- 2156

Absorbance

2200

2150

2100

2050

Wavenumber, cm

- 1868

Fig. 10. Changes in the FTIR spectra after addition of NO to pre-adsorbed CO (1 kPa equilibrium pressure) on ‘‘oxidized” Fe0.9SiBEA. Introduction of 1 (a) and 4 doses (ca. 25 lmol g1) of NO.

d

Absorbance

2250

- 2123 - 2115

b

-1

Wavenumber, cm

0.01

a

g h i j

- 2215

1800 -1

e f

2215 -

2000

2100

d

1842 -

a

2150

c

2183

- 1815

Absorbance

- 1901

f

b

2200

2155 -

2174

- 2080

0.01

- 1868

K. Hadjiivanov et al. / Microporous and Mesoporous Materials 131 (2010) 1–12

c

- 1901

b

Fig. 12. FTIR spectra (carbonyl stretching region) of CO adsorbed on ‘‘reduced” Fe4.2SiBEA: equilibrium CO pressure of 200 Pa, T = 100 K (a), development of the spectra during evacuation at 100 K (b–g) and at increasing temperatures (h–j). The spectra are background corrected.

to be underlined. First, the intensity of the negative band at 2215 cm1 is comparable to that of the positive band at 1901 cm1, indicating similar extinction coefficients of the carbonyl and nitrosyl species, respectively. Second, the nitrosyl bands develop even after occupation of the CO adsorption sites, i.e., the sites detected by NO on oxidized samples are more numerous that those detected by CO. These results can be rationalized assuming that some Fe3+ ions are reduced to Fe2+ in the presence of NO and the latter act as NO adsorption sites. Similar reduction behavior was recently reported with Fe3+ ions in Fe-FER [13,29]. This hypothesis is also supported by the fact that no similar phenomenon was observed with the reduced samples (see below). 3.7. Low-temperature CO adsorption on reduced FexSiBEA

2215 2186 -

a

2200

2000

1800 -1

Wavenumber, cm

Fig. 11. Changes in the FTIR spectra after addition of NO to pre-adsorbed CO (1 kPa equilibrium pressure) on ‘‘oxidized” Fe4.2SiBEA. Spectra (a–d) correspond to successive adsorption of small NO doses of ca. 25 lmol g1.

The opposite trend is observed with Fe4.2SiBEA (Fig. 11): the first carbonyl band to disappear is that at 2186 cm1 (a negative band in Fig. 11, spectra a and b) with formation of nitrosyls characterized by bands at 1868 and 1847 cm1. Then the band at 2215 cm1 disappears (Fig. 11, spectra b–d) while the nitrosyl band at 1901 cm1 develops. These results indicate that the 2215 cm1 carbonyl species are formed on the same iron sites involved in the formation of the 1901 cm1 nitrosyls. Consequently, the carbonyls absorbing at lower frequencies can be correlated with mononitrosyl bands below 1900 cm1. There are two points

In order to obtain information on the reducibility of iron species, the influence of a treatment with CO at 673 K was investigated. It has been reported that, in many cases, treatments with CO and with hydrogen lead to different species, while reductions by CO and by hydrocarbons lead to similar species [73]. Note that hydrocarbons are reactants (and possible reducers) in many catalytic reactions. After treatment at 673 K for 1 h first with CO (2.67 kPa) then under vacuum, the samples were investigated by CO and NO adsorption. It can be seen from Fig. 4 (spectra c and e) that the reduction does not affect the OH band at 3632 cm1. Therefore, the Fe3+ ions in T position are resistant to the reducing treatment applied. Adsorption of CO at 100 K on reduced Fe4.2SiBEA leads to an intense band at 2174 cm1, with shoulders at 2156, 2141, 2133, 2123 and 2115 cm1. A weak band at 2215 cm1 is also distinguished (Fig. 12, spectrum a). Simultaneously, the changes in the OH region already described (spectra not shown) are observed. However, when the initial OH spectrum is restored during outgassing, still intense bands at 2174, 2155, 2123 and 2115 cm1 are observed (Fig. 12, spectrum d). The band at 2215 cm1 remains unaffected. The contour of the main band at 2174 cm1 is complex and the second derivative (not shown) clearly reveals a second component at

K. Hadjiivanov et al. / Microporous and Mesoporous Materials 131 (2010) 1–12

- 2215 - 2184 - 2155 - 2115

8

2183

Absorbance

2166 - 2174

a

*

*

*

* *

c d

- 2215

0.05

2074 -

b

2250

a'' x 5

2100

- 2140

Absorbance

2200

e

- 2068

0.1

b'' x 2.5

- 2183 - 2174 - 2155 - 2147 - 2134 2124 - 2106 - 2089

- 2156

0.01

a

e

2200

2150

2100

2050

Fig. 13. FTIR spectra (carbonyl stretching region) of CO adsorbed at 100 K on reduced Fe0.9SiBEA: equilibrium CO pressure of 200 Pa (a) and development of the spectra during evacuation at 100 K (b–e). The spectra are background corrected.

2183 cm1. This band is clearly visible at lower CO coverages (Fig. 12, spectrum i). Because the bands cannot be associated with OH–CO interactions, the results suggest that the sample treatment with CO at 673 K leads to new types of Fe2+ sites of weak electrophilicity forming carbonyls detected at 2183 and 2174 cm1, respectively. In addition, the reduction by CO leads to a substantial increase in intensity of the bands assigned to polycarbonyls species. This indicates that they are formed with the participation of reduced iron sites, probably Fe+ cations. When CO is adsorbed on reduced Fe0.9SiBEA, the spectra do not strongly differ from those obtained with the oxidized sample (Fig. 13). Relatively intense bands at 2183, 2174 and 2166 cm1 are detected. The band at 2215 cm1 appears with a reduced intensity, suggesting that some of the corresponding iron ions have been reduced. In line with this observation, the 2215 cm1 band was not registered with a sample reduced with hydrogen at 673 K (details not reported). During outgassing, the stability of the bands decreases in the order: 2183, 2174 and 2166 cm1. The polycarbonyls are less stable than the 2166 cm1 species. The bands at 2215 and 2183 cm1 exhibit a comparable stability. 12

CO and

2150

2100

2050

2000

Wavenumber, cm

Wavenumber, cm

3.8. Low-temperature coadsorption of Fe4.2SiBEA

2200

-1

-1

13

CO on reduced

In order to obtain more information on the structure of the species supposed to be polycarbonyls we have studied coadsorption of 12 CO and 13CO on ‘‘reduced” Fe4.2SiBEA. We have chosen this sample because of the higher intensity of the carbonyl bands (in particular those associated with polycarbonyls) which facilitates the spectral analysis. It is well known that [82] if the species formed after 12CO adsorption are essentially monocarbonyls, the spectrum registered after coadsorption of 12CO and 13CO should be a simple sum of the spectra produced after adsorption of 12CO and 13CO, respectively. In contrast, policarbonyls should be characterized by additional bands resulting from mixed-ligand species, Fe(12CO)x(13CO)y. A typical spectrum registered after low-temperature 12 CO–13CO adsorption on reduced Fe4.2SiBEA sample is shown on Fig. 14 (spectrum a). Because of the superimposition of many

Fig. 14. FTIR spectrum of 12CO and 13CO isotopic mixture (12CO:13CO molar ratio  40:60) adsorbed on a ‘‘reduced” Fe4.2SiBEA sample (a). The isotopic mixture was adsorbed under 200 Pa equilibrium pressure at 100 K and then evacuated for 2 min at the same temperature; second derivative of the spectrum is denoted by a0 0 . For convenience the second derivative of a spectrum registered after 12CO adsorption at similar coverage is also shown (b0 0 ). The spectra are background corrected.

bands and for convenience, the second derivative of the spectrum, providing better spectral resolution, is also shown (Fig. 14, spectrum a00 ). The peaks at 2183, 2174 and 2155 cm1 were already observed in the experiments on 12CO adsorption (see Fig. 14, spectrum b00 ). Calculations show that the respective 13CO bands should appear at 2134, 2125 and 2107 cm1, respectively. Indeed, bands at these frequencies (±1 cm1) are registered in the spectra. The two bands at 2074 and 2068 cm1 are assigned to the 13CO counterparts of the masked 12CO bands at 2123 and 2115 cm1. However, there are two intense bands at 2147 and 2089 cm1 which cannot be assigned to monocarbonyls with a 12CO or 13CO ligand. Analysis of the second derivative of the spectra (Fig. 13, spectrum a00 ) indicates also the existence of at least three similar bands (marked with stars), at 2139, 2117 and 2082 cm1. Therefore, the experiments clearly show that polycarbonyl are formed after CO adsorption on our sample. Due to the superimposition of too many bands we are not able to determine exactly all spectral features of the polycarbonyls. However, the band at 2155 cm1 appears with a strongly reduced relative intensity which indicates it belongs to polycarbonyls. Now the question arises: How many CO ligands are involved in the polycarbonyl structure. Let us assume that the structures under considerations are dicarbonyls with a symmetric mode at 2155 cm1 and 12CO mode of the mixed Fe(12CO)(13CO) species at 2147 cm1. In such a case, calculations using approximate force field model [83] predicts antisymmetric modes of Fe(12CO)2 dicarbonyls at 2135 cm1 and 13CO mode of the mixed-ligand species at 2095 cm1 (note than the antisymmetric modes of dicarbonyls are expected to be more intense than the symmetric ones). However, analysis of the spectra registered after adsorption of 12CO indicates absence of a band at 2135 cm1. In addition the intense band registered after adsorption of isotopic mixture was at 2089 cm1 and not at 2095 cm1. Therefore the results indicate the existence of more than two CO ligands in the polycarbonylic structures. We tentatively assume formation of tricarbonyls.

9

K. Hadjiivanov et al. / Microporous and Mesoporous Materials 131 (2010) 1–12

b

2100

d

- 1920

Absorbance

1815

- 1613 - 1580

2200

a 2174 -

c a

2200

g

0.05

- 1768

- 2080

- 2245

0.2

- 1768

- 1841

g

- 1901

Absorbance

- 1868

0.02

- 1838

d

2000

1800

1600

2200

-1

2000

1800 -1

Wavenumber, cm

Wavenumber, cm

- 1841

Fig. 15. FTIR spectrum of NO (50 Pa equilibrium pressure) adsorbed on the ‘‘reduced” Fe0.9SiBEA sample (a), development of the spectrum with time (b) and after evacuation (c). The spectra are background corrected.

Fig. 17. Changes in the FTIR spectra after addition of NO to CO pre-adsorbed (1 kPa equilibrium pressure) on the ‘‘reduced” Fe4.2SiBEA sample. Addition of 1 dose (ca. 100 lmol g1) of NO (a) and development of the spectrum after 5 min (b); 2 doses of NO (c) and development of the spectrum after 5 min (d); 3 doses of NO (e) and development of the spectrum after 5 (f) and 10 min (g).

Fig. 16. Changes in the FTIR spectra after addition of NO to pre-adsorbed CO (1 kPa equilibrium pressure) on the ‘‘reduced” Fe0.9SiBEA sample: successive addition of 1 (a) and 2 doses (ca. 50 lmol g1) of NO (b).

The negative peak at 2224 cm1 also suggests some N2O in the gas phase. The nitrate band at 1613 cm1 masks the weak band at 1626 cm1 due to adsorbed water. The results indicate that NO disproportiation leading to N2O and nitrates takes place on the reduced sample. The coadsorption experiments with CO and NO on reduced Fe0.9SiBEA evidence a preferential displacement of CO from the 2174 cm1 sites and then from the 2215 cm1 sites (Fig. 16). Due to the relatively low NO equilibrium pressure, only dinitrosyls (1768 cm1) are detected with no polynitrosyls (1920 and 1815 cm1) formed. At the final stages, CO leaves the sites forming 2215 cm1 carbonyls while 1901 cm1 mononitrosyls are formed. The spectra recorded during NO adsorption on CO-precovered Fe4.2SiBEA are given on Fig. 17. A progressive decrease and ultimate disappearance of the complex carbonyl band at ca. 2175 cm1 is observed while mono-, di-, and polynitrosyl bands appear in the NO stretching region. Three facts can be noted: (i) the polynitrosyl bands appear more slowly (as expected from the equilibrium of poly- with dinitrosyls); (ii) the band at ca. 2215 cm1 is the last carbonyl band affected by NO adsorption (inset of Fig. 17) and (iii) the nitrosyl bands develop until carbonyl bands are eroded to finally disappear, showing that the CO and NO adsorption sites practically coincide. Note also that the overall intensity of the nitrosyl bands largely exceeds that of the negative carbonyl bands.

3.9. NO adsorption and CO–NO coadsorption on reduced FexSiBEA

4. Discussion

Spectra of NO adsorbed on reduced Fe0.9SIBEA are shown on Fig. 15. The species formed are similar to those observed with oxidized Fe0.9SiBEA. However, the relative intensity of the mononitrosyl (1868 and 1841 cm1) and polynitrosyl (1920 and 1815 cm1) bands is higher, while the band at 1901 cm1 is less intense. A weak component at 1768 cm1 is also detected and assigned to dinitrosyls in equilibrium with polynitrosyls [50]. In addition, bands at 1613 and 1580 cm1, assigned to nitrate species, and weak bands at 2245 cm1, due to adsorbed N2O are also formed.

4.1. Introduction of iron into vacant T-atom sites

0.02

- 1768

a

2215 -

- 2174

- 1901

Absorbance

b

2200

2000

1800 -1

Wavenumber, cm

Treatment of BEA zeolite by acid solutions leads to the extraction of framework aluminum and creation of vacant T-atom sites. In order to preserve the charge, the so-called hydroxyls nests are formed (Scheme 1). Ideally, each nest includes four silanol groups although larger structures can be formed. The appearance of such nests in SiBEA upon removal of Al atoms is revealed by the broad IR band at

10

K. Hadjiivanov et al. / Microporous and Mesoporous Materials 131 (2010) 1–12

Scheme 1.

Scheme 2.

3515 cm1 due to H-bonded Si–OH groups (Fig. 4, spectrum a). Upon impregnation of SiBEA zeolite with aqueous solution of Fe(NO3)3, this band practically disappears indicating consumption of the H-bonded Si–OH groups upon reaction with iron ions. It leads to the incorporation of iron into the framework of SiBEA (Scheme 2), as also evidenced by the progressive increase of the d302 spacing with Fe content (Fig. 1) and by the appearance of an IR band at 3632 cm1 attributed to the „Fe3+–O(H)–Si„ acidic sites. Note, however, that the increase in intensity of the band at 3735 cm1 in the spectrum of Fe0.9SiBEA, as compared to the parent SiBEA sample, indicates some deviations from this scheme and could be explained by the fact that not all hydroxyls from the nest have reacted with Fe3+ ions. In our case, the cation going into T-atom sites is Fe3+ (Scheme 2). As in the case of aluminum, the charge should be compensated by additional cation, e.g. proton. This is the reason of the existence of bridging „Fe3+–O(H)–Si„ hydroxyls. We have found a high acidity of these zeolitic hydroxyls, only slightly lower than that of their „Al3+–O(H)–Si„ analogues. An interesting observation is that iron affects the behavior of NO+ formed at cationic position by replacement of an acidic proton. Because the frequency of NO+ is correlated with the basicity of the oxygen to which it is bound [80], one can conclude that the oxygen bridging Fe and Si in FexSiBEA is more basic than that bridging Al and Si in [Al]BEA. The concentration of „Fe3+–O(H)–Si„ groups is higher for Fe4.2SiBEA than Fe0.9SiBEA. However, it was already noted that this cannot be a measure of the concentration of iron cations in T-atom sites, because part of the charge defects can be compensated by exchanged iron cations (see below). It should also be emphasized that iron ions in T-atom sites does not adsorb probe molecules [38] because they are coordinatively saturated. Our results demonstrate that iron in T-atom sites is stabilized in a trivalent state and cannot be reduced with CO even at 673 K. 4.2. Highly electrophilic iron cations The existence of strongly acidic hydroxyls in FexSiBEA is a premise to obtain iron ions in exchanged (cationic) position. There is a

general rule that cations exchanged in zeolites are of low coordination, making them highly electrophilic with subsequent relatively strong r-bonds with CO and NO. This leads to a relatively high frequency of the adsorbed molecular probes. For instance, Cu+–CO complexes formed from oxide-supported Cu+ ions are observed at ca. 2130 cm1, while for Cu+ ions exchanged in ZSM-5, the Cu+–CO carbonyls absorb at 2158 cm1 and even dicarbonyls can be formed at room temperature [84]. Concerning Fe-[Al]BEA sample [17], one can see that indeed CO and NO adsorbed on exchanged Fe2+ cations are characterized by relatively high frequencies (2190 and 1876 cm1, respectively). Adsorption of CO and NO on FexSiBEA leads to formation of species with unusual high frequencies (2215 and 1901 cm1, respectively). Formation of dicarbonyls suggests adsorption on low coordinated and thus exchanged Fe ions. These unusual high frequencies can be explained by the high electrophilicity of iron ions. Because of the widespread opinion that Fe3+ sites can not adsorb probe molecules, one can expect the adsorption sites to be Fe2+ ions. Note that these ions should be bridged via oxygen to Fe3+ site(s) in T-atom sites (Scheme 2). One should expect a charge transfer from Fe3+ in T-atom sites to exchanged Fe2+ ions, leading to the unusual high electrophilicity of the exchanged cations. Two observations, however, impeach this interpretation. First, we have established that the concentration of highly electrophilic iron ions decreases upon reduction of the sample with CO, while the preservation of the intensity of the band due to bridging hydroxyls indicates that reduction does not affect Fe3+ ions in T-atom sites. Second, the extinction coefficients of CO and NO adsorbed on these ions are similar, while, on Fe2+ ions (reduced samples), the extinction coefficient of NO largely exceeds that of CO. These observations strongly suggest that the adsorption sites are exchanged Fe3+ ions, consistent with the high frequencies of adsorbed CO and NO. At this stage, it is not clear why these Fe3+ ions are active towards adsorption. One could speculate whether the effect of neighboring Fe3+ ions in T-atom sites or their polarizing effect on adsorbates is decisive for this behavior. 4.3. Properties of the other iron cations According to the UV–Vis spectra no oligonuclear iron species or iron oxide clusters exist in the Fe0.9SiBEA sample. Thus, the iron carbonyl bands below 2200 cm1 registered with this sample should be attributed to isolated Fe2+ species. Some reduced iron sites able to form polycarbonyl species (and supposed to be Fe+ cations) were found with both samples but their concentration was much higher with Fe4.2SiBEA (compare Figs. 12 and 13). Although these sites are more typical of reduced samples, traces were found on oxidized ones, probably as a result of the vacuum treatment applied. At present we are not able to determine the exact location of these sites, but it seems that they are connected with particular oligonuclear species (that are in concentration under the UV detection limit with the Fe0.9SiBEA sample). Earlier studies revealed that the increase of iron concentration slightly increased the concentration of iron in T-atom sites [6]. However, the majority of extra-deposited iron is in octahedral coordination. Evidently, these ions are not exchanged cations but probably form oligonuclear structures. As discussed earlier, iron ions on such species exhibit a relatively low electrophilicity, leading to relatively low frequencies for adsorbed CO and NO. These iron ions easily change oxidation state. Their properties, as deduced by the results of NO adsorption, are similar to those of iron ions in silicalite [50,51]. Catalytic measurements [6] indicate that these ions are active in the full oxidation of ethanol and NO.

K. Hadjiivanov et al. / Microporous and Mesoporous Materials 131 (2010) 1–12

5. Conclusions Upon removal of Al atoms from BEA zeolite to form the SiBEA, the resulting hydroxyl nests lead to a broad IR band at 3515 cm1 (due to H-bonded Si–OH groups). Upon impregnation of SiBEA with an aqueous solution of Fe(NO3)3 to form FexSiBEA, this band is practically not observed in the spectra of the FexSiBEA samples due to the reaction of H-bonded Si–OH groups with the iron precursor. This evidences the incorporation of iron into the SiBEA framework, as also shown by the increase of d302 with Fe content and by a new IR band at 3632 cm1 assigned to „Fe3+– O(H)–Si„ Brønsted acid sites. The band at 3632 cm1 shifts to 3352 cm1 (Dm  280 cm1) upon low-temperature CO adsorption. This indicates a high acidity of the bridging OH groups, only slightly lower than that of their „Al3+–O(H)–Si„ analogues. Part of iron in ‘‘oxidized” FexSiBEA corresponds to exchanged Fe3+ ions which exhibit unusual adsorption properties. With CO these sites produced carbonyls detected at 2215 cm1 that are able to convert into dicarbonyls at low-temperature. The nirtrosyl species formed with these sites absorb at 1901 cm1. The remaining Fe3+ sites are considered to be inactive towards adsorption of CO and NO. In addition, some isolated Fe2+ sites are detected on Fe0.9SiBEA by CO at 2186 cm1. Adsorption of NO leads to reduction of some Fe3+ sites and, as a result, two kinds of mononitrosyls are formed (1860 and 1842 cm1). With time, polynitrosyls (1920 and 1815 cm1) are also produced. Treatment of FexSiBEA with CO at 673 K (‘‘reduced” samples) generates Fe2+ ions detected by CO at 2183, 2175 and 2166 cm1. NO adsorption leads to different mono-, di- and polynitrosyls. Interconversion between poly- and dinitrosyl species is well evidenced. The amount of reducible iron is much higher for Fe4.2SiBEA than Fe0.9SiBEA. A minor fraction of iron ions was found to form tricarbonyls (bands at 2155, 2123 and 2115 cm1). They are more typical of Fe4.2SiBEA and they concentration is much higher with the reduced sample.

Acknowledgments

[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46]

R.K., E.I. and K.H. acknowledge financial support from the Bulgarian Scientific Foundation (Grants DO-02-82 and DO-02-184). S.D. gratefully acknowledges CNRS (France) for financing his research position. Special thanks are due to C. Clodic (Laboratoire de Réactivité de Surface, Paris) for performing the BET measurements.

[47] [48] [49] [50] [51] [52]

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