Iron Nitrosyl Species in Fe-FER: A Complementary ... - ACS Publications

Apr 22, 2009 - A Fe/SiO2 and a Fe-FER sample containing iron oxide were also studied ... of NO on Fe-FER (containing no iron oxide species) gives diff...
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J. Phys. Chem. C 2009, 113, 8387–8393

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Iron Nitrosyl Species in Fe-FER: A Complementary Mo¨ssbauer and FTIR Spectroscopy Study Vanessa Blasin-Aube´,*,† Olivier Marie,† Jacques Saussey,† Anna Plesniar,† Marco Daturi,† Ninh Nguyen,‡ Christian Hamon,§ Mihail Mihaylov,| Elena Ivanova,| and Konstantin Hadjiivanov| Laboratoire Catalyse et Spectrochimie and Laboratoire de Crystallographie et Sciences des Mate´riaux, ENSICAEN, UniVersite´ de Caen, CNRS, 6 Bd Mare´chal Juin, 14050 Caen, France, IRMA, Parc Technologique de Soye, 9 rue de Galile´e, BP 64, 56274 Ploemeur, France, and Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria ReceiVed: January 23, 2009; ReVised Manuscript ReceiVed: March 23, 2009

FTIR spectroscopy was applied to the investigation of the nitrosyl complexes formed with the extra-framework iron species in Fe-ferrierite (Fe-FER). To clarify the loading effect on the nature of the species formed, six ferrierites with Fe loadings varying between 0 and 3.7 wt % were prepared via ionic exchange and investigated. A Fe/SiO2 and a Fe-FER sample containing iron oxide were also studied for comparison purposes. Adsorption of NO on Fe-FER (containing no iron oxide species) gives different nitrosyl species, and different iron sites can be evidenced depending on the iron concentration. The bands are assigned to mononitrosyl on iron whose oxidation state is determined to be +2 thanks to Mo¨ssbauer spectroscopy. In particular, one specific Fe2+ cation (typical of a highly loaded sample) appears to be easily converted to Fe3+ upon oxygen treatment, which makes these sites excellent candidates for catalytically active redox sites. 1. Introduction Transition-metal ions exchanged in pentasil containing “highsilica” zeolites (Si/Al g 6) such as ZSM-5 (MFI), ferrierite (FER), beta (BEA), and mordenite (MOR) structures exhibit a high and stable catalytic activity in a series of selective redox reactions of great importance. If these zeolites are exchanged with iron, they are particularly active catalysts for decomposition of N2O,1,2 oxidation of benzene to phenol using N2O,3,4 selective oxidation of methane,5 selective reduction of NOx (SCR-NOx) by hydrocarbons,6,7 and so forth. The type of metal, the metal-zeolite coordination, the location of the metal in the zeolite, and the zeolite topology are among the many factors thought to control catalytic activity. Any fundamental description of the catalytic efficiency of these materials should begin with understanding the nature of the metal active sites. Ferrierite has a two-dimensional channel system: a ten-ring (5.4 Å * 4.2 Å) and an eight-ring one (4.8 Å * 3.5 Å).8 According to the literature, in this zeolite matrix the isolated Fe cations can occupy four characteristic positions.9 Infrared spectroscopy of adsorbed probe molecules is known to be among the most useful methods to investigate the nature of surface centers. This method is based on the fact that the spectral features of the adsorbed molecules depend on oxidation and coordination states and on the degree of clustering of adsorption centers.10 Therefore, the principal methods that have been used to study the reactivity of zeolites are based on the adsorption of some typical probe molecules and their interaction with active sites. * To whom correspondence should be addressed. E-mail: [email protected]. † Laboratoire Catalyse et Spectrochimie, Universite´ de Caen. ‡ Laboratoire de Crystallographie et Sciences des Mate´riaux, Universite´ de Caen. § IRMA. | Bulgarian Academy of Sciences.

For CO adsorption, there is, to our knowledge, no publication concerning Fe-FER. During its adsorption on zeolites, CO usually demonstrates multiple binding states, providing information about the oxidation and coordination state of the chargebalancing cations as well as their surface concentration.11 The adsorption of CO at low temperatures, studied by FTIR, has also been used for elucidating the location of cationic sites in zeolites.12 However, NO as a probe for iron ions also seems very efficient, and we will here focus on this probe molecule. Our choice is due to the fact that the nitrosyls of iron are relatively stable species and even at room temperature all accessible sites could be monitored by NO. Moreover, iron nitrosyls are characterized by extremely intense ν(NO) bands.13-16 Nitric oxide is a well-known probe molecule with a high affinity to Fex+ ions, leading to the formation of Fex+(NO)n nitrosyls (n ) 1, 2, 3). Most authors agree that nitrosyls are formed exclusively with the participation of Fe2+ ions: the interaction with Fe2+ causes a great perturbation of the dipole moment of the adsorbed molecule, resulting in a very high absorption coefficient of the N-O stretching in ferrous nitrosyls, whereas ferric nitrosyls are believed to be characterized by a low absorption coefficient.10,17,18 Studies concerning the adsorption of NO on Fe-containing zeolites deal essentially with Fe-ZSM-513,18-21 and Fe-silicalite.15,22,23 Wichterlova et al.20,24 published a series of articles about ferrierites and in particular about Fe-FER: they used the NO adsorption to study the local perturbation of some zeolite-specific framework vibrations. The authors proposed that, upon NO adsorption onto iron, the bonding between Fe2+ cations and zeolitic oxygen atoms was perturbed, thus leading to a shift of the concerted framework vibrations. They concluded from the observation of three distinct specific framework vibrations that among the four cationic positions proposed from XRD data9 only three distinct ones were significantly populated by iron.

10.1021/jp900699m CCC: $40.75  2009 American Chemical Society Published on Web 04/22/2009

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TABLE 1: Some Characteristic of the Samples Used in This Study sample

iron concn, wt %

iron precursor

parent sample

Fe/SiO2 Fe(M)-FER H-FER 0.96 Fe-FERa 1.1 Fe-FERa 2.5 Fe-FERa 2.7 Fe-FERa 3.7 Fe-FERa 1.5 57Fe-FERf

3.3 25 0 0.96 1.1 2.5 2.7 3.7 1.5

Fe(NO3)3 · 9H2O Mohr’s salt

SiO2 NH4FER NH4FER NH4FER NH4FER NH4FER NH4FER NH4FER NH4FER

a

ferrous ferrous ferrous ferrous ferrous ferrous

salta salta salta salta salta salta

Exact nature of the precursor is IRMA property.

On the basis of these considerations, we used NO as a probe molecule to explore the coordination state, electrophilic properties, and accessibility of exchanged Fe2+ cations in the ferrierite zeolite and also made an attempt to gain further information on the distribution and reactivity of these Fe2+ cations. To clarify the loading effect on the iron distribution, a series with different Fe concentration was studied. Finally, as there is still a great debate regarding the assignment of the iron nitrosyl bands to Fe2+ or Fe3+ cation, we performed Mo¨ssbauer experiments to establish without any doubt the iron oxidation number and obtain complementary information on the iron distribution. Effectively, this technique is a choice technique to probe not only the iron oxidation state but also its coordination. However, Mo¨ssbauer is only sensitive to the 57Fe nucleus, whose isotopic natural abundance is quite low (2.1%). Our first attempts were unsuccessful: the spectra were too noisy to allow any reasonable fitting, and we then turned to prepare specially a “fresh” 57Fe-rich Fe-FER sample. 2. Experimental Section Materials. Nine samples were used for this study, and their notations as well as some characteristics are presented in Table 1. The Fe/SiO2 was prepared by incipient wetness impregnation using an aqueous solution of Fe(NO3)3 · 9H2O with a liquid/ solid ratio of 2.2 cm3 g-1 followed by calcination at 773 K for 3 h in air. The SiO2 used for the synthesis was AEROSIL 200. The parent zeolite sample was NH4-FER, kindly supplied by IRMA (Si/Al ) 8.8). H-FER was obtained from NH4-FER by in situ activation under a vacuum at 673 K. Fe(M)-FER sample was obtained from NH4-FER by 4-fold ion exchange with Mohr’s salt solution (0.1 M) for 4 h at room temperature. After each exchange, the solid was washed with a large amount of distilled water under magnetic agitation for 1 h, filtered, and dried under air at 353 K. Elemental analysis indicates a Fe/Al ratio ) 3.7 that corresponds to an iron loading of 25 wt %. The samples 0.96 Fe-FERa (0.96 wt % Fe), 1.1 Fe-FERa (1.1 wt % Fe), 2.5 Fe-FERa (2.5 wt % Fe), 2.7 Fe-FERa (2.7 wt % Fe), and 3.7 Fe-FERa (3.7 wt % Fe) were all prepared by ion exchange from ferrous solutions and kindly supplied by IRMA. They will further be called the “aged” samples since the ionic exchange steps were performed at least three years before the characterization step. The 57Fe-FER sample for Mo¨ssbauer experiment was prepared at the laboratory scale (90 mg) in a rigorously similar way but starting with a 57Fe-rich (99% purity) ferrous solution. Elemental analysis indicates for the 57Fe-FER sample a 1.5 wt % iron loading, which is sufficient to ensure reliable Mo¨ssbauer analysis and which leads to the denomination 1.5 57Fe-FERf, since it was characterized as freshly prepared.

Mo¨ssbauer Spectroscopy. The different iron sites in the structure were determined from Mo¨ssbauer spectroscopy. The 57 Fe powder Mo¨ssbauer resonance spectrum at room temperature was performed with a transmission geometry by use of a constant acceleration spectrometer and a γ-ray source from 57Co embedded in a rhodium matrix. The velocity scale was calibrated with an R-Fe foil at room temperature. The spectrum was further fitted with Lorentzian lines. The isomer shifts were referred to metallic R-Fe at 293 K. XRD. XRD analysis of powdered samples was carried out using a Philips PW 3710 diffractometer scanning at 0.2° min-1 (2θ range from 3 to 50°). FTIR. The IR experiments were carried out on a Nicolet Magna 550 FTIR spectrometer equipped with an MCT detector using an optical resolution of 2 cm-1. The powders were pressed into self-supported wafers of about 5 mg cm-2 and activated in situ (outgassing at 673 K for 1 h or treated in oxygen for 1 h at 673 K and then evacuated for 1 h at the same temperature) in a quartz cell equipped with KBr windows. The adsorption of NO was performed in two ways: (i) by introducing small doses of NO to the wafer heated at 423 K to favor diffusion and interaction with any possible iron site and (ii) at room temperature under equilibrium pressure of 50 Pa. In any case, the spectra were acquired at room temperature. For increasing small doses, the maximum amount introduced in the cell was around 0.2 mmol NO g-1 of catalyst, which corresponds to the NO quantity required for an interaction with all the iron sites without any noticeable NO equilibrium pressure (99.9) were supplied by Air Liquide. 3. Results and Discussion Assignment of the Iron Nitrosyl Bands. To obtain general information on the nature of the iron nitrosyl complexes in the different families of samples, we initially studied three ironcontaining catalysts (Fe/SiO2, Fe(M)-FER, and 2.5 Fe-FERa) by NO adsorption at 423 K. In the ν(OH) region (not represented here), distinct IR absorption bands are observed: at 3746 cm-1 for both Fe/SiO2 and Fe-FER and at 3603 cm-1 for Fe-FER only. They correspond to silanol groups and to Brønsted acid Si(OH)Al groups, respectively.25 According to the preparation procedure using ferric nitrate for iron loading, the Fe/SiO2 sample exhibits mainly iron oxide in the Fe2O3 form. The adsorption of NO at 423 K on Fe-containing catalysts gives rise to bands in the 1950-1700 cm-1 range (Figure 1A). Upon exposing Fe/SiO2 to NO, a single band of low intensity is observed at 1820 cm-1. The low intensity of this band is in agreement with the results reported in the literature: NO can hardly be absorbed on Fe2O3 (i.e., on Fe3+). Only a small amount of NO relative to Fe2+ cations at the surface of small Fe2O3 particles gives rise to this band at 1820 cm-1, which is assigned to Fe2+ mononitrosyls.26 Under the same conditions, two bands at 1876 and 1820 cm-1 with weak shoulders at 1893, 1842, and 1770 cm-1 were detected on the Fe(M)-FER sample. The band at 1820 cm-1 is identical to the band observed on Fe/ SiO2 and indicates the presence of Fe2+ mononitrosyls on Fe2O3. That is confirmed by the XRD pattern of Fe(M)-FER (Figure 1B), which shows the presence of Fe2O3 mixed with Fe-FER. During the ionic exchange, part of the iron precipitated to yield iron hydroxide oxide, which further converted to iron oxide upon

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Figure 1. (A) FTIR spectra of NO adsorbed at 423 K on Fe/SiO2, Fe(M)-FER, and 2.5 Fe-FERa. The spectra are background-corrected. (B) XRD patterns of Fe(M)-FER and 2.5 Fe-FERa.

Figure 2. (A) FTIR spectra of 0.96 Fe-FERa and H-FER activated (dashed line) and saturated after being exposed to NO (solid line). (B) FTIR spectrum of NO adsorbed at 423 K on 0.96 Fe-FERa. The spectrum is background-corrected.

calcination. The shoulders at 1842 and 1770 cm-1 can be assigned to Fe2+(NO)x species on Fe2+ truly isolated centers15,16,19,22,23 as already observed for zeolites with an important concentration of exchanged iron. The band at 1876 cm-1 and the shoulder at 1893 cm-1 are present on the spectra of the two Fe-ferrierites: they seem to be specific to the Fe cations compensating the negative charge hold by the ferrierite oxygen atoms and could correspond to the NO stretching frequencies of iron nitrosyl complexes formed with cations located in various crystallographic sites of the Fe-FER. To conclude, we emphasize that the Fe-FER catalysts provided by IRMA present no small particles of Fe2O3 (confirmed by XRD patterns) and contain predominantly iron ions in cationic sites. We will thus focus on these samples in the following study to establish the nature of iron species and their distribution. NO Adsorption on Fe-FER at 423 K. To prevent possible formation of polynitrosyl species and thus simplify the inter-

pretation of the spectra, NO adsorption was first performed with introduction at 423 K of small doses until “saturation” of the samples. Figure 2A depicts spectra of H-FER and 0.96 Fe-FERa after activation (outgassing under a vacuum at 673 K) and after NO adsorption. As observed from the zeolite transmission window (1000-800 cm-1), the introduction of iron in cationic exchange position leads to the appearance of concerted structural vibrations. The description of these framework vibrations was previously reported,20,24 but it appeared to us that their evolution was very complex and moisture sensitive so that we will not further consider them in this article. In the region of OH groups, the typical band characteristic of bridging OH groups of zeolite was found for both samples at 3603 cm-1 and a low intensity band at 3746 cm-1 reflected the presence of terminal SiOH groups.27,28 The addition of small NO doses on activated H-FER did not cause the appearance of any new band on the spectrum.

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Figure 3. (A) FTIR spectra of 1.1, 2.5, 2.7, and 3.7 Fe-FERa after saturation with NO (200 µmol NO/g). (B) FTIR spectra of NO adsorbed on 1.1, 2.5, 2.7, and 3.7 Fe-FERa. The spectra are background-corrected.

On the 0.96 Fe-FERa spectrum (Figure 2B), the presence of iron in the cationic sites of Fe-FER is reflected by an IR band at 1876 cm-1, characteristic of adsorbed NO on Fe ions. Bands around 1880 cm-1 have often been observed in the spectra of NO adsorbed on iron-containing zeolites, but their assignment seems to be ambiguous. A band close to 1876 cm-1 was previously assigned to mononitrosyl species associated with isolated Fe sites in either the 3+ state (Fe3+-NO)15,19,22,23,29-35 or the 2+ state (Fe2+-NO) state.14,18,20,24,36-41 Among others, Zecchina et al. assigned this band to Fe3+-NO in Fe-MCM-22,31,32 Fe-ZSM-5,19,32-35 and Fe-silicalite15,22,23,30 zeolites. In all these cases, the intensity of the band around 1880 cm-1 was relatively low. This could be explained by the very low absorption coefficient of NO onto Fe3+ species. However, we propose an alternative explanation: the band is due to NO connected to a small amount of Fe2+ species. In agreement with this, other researchers focusing on the most studied zeolite, FeZSM-5,14,18,36,37 assigned this band to Fe2+-NO. Concerning especially Fe-FER zeolites, all authors agree with the assignment of the 1876 cm-1 band to Fe2+-NO species. For Wichterlova et al.,20,24 this assignment is supported by the correlation between the intensity increase of the band at 1880 cm-1 of NO adsorbed on Fe2+ and a decrease in the intensity of the sum of perturbed skeletal bands at 915, 930, and 945 cm-1, associated with the coordination of the single Fe2+ ions to the framework oxygens exclusively. Recently, Benco et al.40,41 reported, after adsorption of NO on Fe-FER, a dominant band centered at 1876 cm-1, which they assigned to a mononitrosyl species on the Fe2+ cation in agreement with the NO stretching frequencies they calculated by DFT in a precedent article. In addition, Mo¨ssbauer studies revealed that after high-temperature vacuum treatments almost all iron in Fe-ZSM-5 catalysts is in the divalent state.42-46 The band at 1876 cm-1 that we observed with the 0.96 FeFERa is highly symmetric, which indicates only one type of Fe2+ species interacting with NO for this sample with rather low iron concentration. When the iron loading in Fe-ferrierites increases, the intensity of the band at 1876 cm-1 also increases and its shape becomes nonsymmetric with a new component of ν(NO), which appears

at higher wavenumbers (cf. Figure 3). This new component at 1893 cm-1 is included in the 1876 cm-1 broad band as a weak shoulder. Although well resolved in the second derivatives, it is hardly detected in the original spectra, thus making direct reliable quantitative analysis difficult. To better evidence the subcomponents of the ν(NO) massif, the spectrum of the NO adsorbed on the 0.96 Fe-FERa sample at saturation (one single type of Fe species) was subtracted from the spectra corresponding to NO saturated 1.1, 2.5, 2.7, and 3.7 Fe-FERa (cf. Figure 4.). According to the second derivative results, each spectrum was decomposed into two or three components (depending on the iron amount) with a very good fit. For 1.1 Fe-FERa, the new ν(NO) component is present at 1893 cm-1. This peak increases with the Fe loading. The main peak at 1876 cm-1 grows at the same time. For an Fe loading higher than 2.7 wt %, a third band at lower wavenumbers (1850 cm-1) is observed. NO and H2O coadsorption experiments (not reported here) have shown that this weak feature is due to mononitrosyl species perturbed by interaction with water molecules present in close contact with the NO coordination site and that this perturbation is only detectable for a high amount of mononitrosyls and thus only for a high iron loading. It has also been recently reported that water is evolved as a result of interaction of NO with Fe-OH groups.47 The most important result of these experiments is the indication that, upon increasing iron loading, a clear new ν(NO) component at 1893 cm-1 appears that we assign to a second mononitrosyl species formed on new Fe2+ cations forced to occupy a new crystallographic confined position by concentration effect. Considering our previous IR spectra interpretations, we chose to assign all our observed ν(NO) bands to nitrosyl formed upon interaction with Fe2+, rejecting the hypothesis of interaction with Fe3+. Effect of Pretreatment on NO Adsorption at 298 K. To complete our characterization of iron sites and particularly their redox behavior, we studied NO adsorption on Fe-FER after different pretreatments. We focused on the highly loaded 3.7 Fe-FERa sample to obtain high signal intensity in particular

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Figure 4. Decomposition of ν(NO) massif in the IR spectra of saturated Fe-FER samples (spectrum of NO adsorbed on 0.96 Fe-FERa was subtracted).

Figure 5. FTIR spectra of 3.7 Fe-FERa sample after (a) outgassing under vacuum at 673 K and subsequent adsorption of NO (50 Pa at equilibrium), (b) oxidation under O2 at 673 K followed by evacuation at the same temperature and subsequent adsorption of NO (50 Pa at equilibrium). The spectra in the N-O region (right panel) are background-corrected.

for the band at 1893 cm-1. The results for NO adsorption (50 Pa equilibrium pressure, room temperature) on this sample pretreated under a vacuum at 673 K are presented in Figure 5, spectrum a. As previously observed, the band at 1876 cm-1 together with its shoulder at 1893 cm-1 are present, and we confidently assigned them to two kinds of Fe2+-NO species. The same experiment was performed after the sample was preoxidized under O2 at 673 K. In this case, we expected to increase the initial amount of Fe3+. Figure 5, spectrum b, effectively confirms that, in the case of an oxidative pretreatment, some Fe3+ species are formed: the new v(OH) band at 3674 cm-1 traduces the presence of Fe3+-OH groups.47,48 We also want to emphasize the fact that the appearance of these new Fe3+-OH groups was observed at the expense of Fe2+ from the sites monitored by NO at 1893 cm-1. Indeed, the band at

1893 cm-1 is more intense for the non-oxidized sample. In other words, the respective iron cations can transform from Fe3+-OH to coordinatively unsaturated Fe2+, whereas the other iron cations seem to have a fixed oxidation number as the intensity of the corresponding mononitrosyl band (1876 cm-1) does not change (or hardly changes) upon reducing or oxidative pretreatment. It was also observed that, in the presence of NO equilibrium pressure, the 1893 cm-1 mononitrosyls are converted into polynitrosyl species (details not reported). These observations evidence a high coordinative unsaturation of the respective Fe2+ sites. As we pointed out while describing the main literature data, there is still a great debate relative to the assignment of IR bands observed upon adsorption of the NO probe molecule over iron

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Figure 6. Mo¨ssbauer spectrum of “as-prepared” 1.5 57Fe-FERf recorded at 298 K and the corresponding optimized curve fitting with five components.

cations. It thus seemed to us fundamental to assess the cation oxidation degree by reliable independent techniques. 57 Fe-FER Study: Mo¨ssbauer and IR NO Adsorption Experiments. As already mentioned in the previous section, the Mo¨ssbauer spectroscopy is a very efficient tool to characterize the iron oxidation state and to give the quantitative distribution of the various sites. Unfortunately, there are very little data in the literature dealing with the characterization of iron-containing zeolites.42-46 Indeed, classically the studied zeolites loaded with transition metal do not contain an amount higher than 5 wt %, so taking into account that the technique is only sensitive to the 57Fe isotope whose natural abundance is around 2.1% the signal is always too weak and noisy to be exploited. We thus prepared the 1.5 57Fe-FERf sample using the procedure described in the Experimental Section to reach an 57Fe amount that allowed us to obtain a very nice Mo¨ssbauer spectrum after 34 h of acquisition. Figure 6 presents both the experimental Mo¨ssbauer spectrum and the curve fitting obtained using Mo¨ssbauer sub-bands characterizing five distinct iron species. We remind Mo¨ssbauer nonexperts that the generally accepted classification range49 for iron oxidation number depends on the isomer shift (IS): [0.1 < IS < 0.6] for Fe3+ and [0.7 < IS < 1.4] for Fe2+. Consequently, the most important result from this Mo¨ssbauer experiment is the evidence that iron in the Fe2+ oxidation state is the main species (∼90%) in our fresh sample, as expected from its preparation using ionic exchange starting from ferrous solution. This Fe2+ is mainly distributed on three distinct sites. Going in more detailed analysis, we can conclude that the most abundant Fe2+ site (50%, labeled I) is the one with the highest isomer shift (IS ) 1.34), which means it is the most coordinated species (at least when the experiment is made without any pretreatment, i.e., with the sample full of adsorbed water). Furthermore, the same site is characterized with the lower quadrupolar splitting (QS ) 1.49), which indicates the highest symmetry of the coordination sphere. The second more abundant Fe2+ site (18%, labeled II) is characterized by a lower coordination (IS decreases to 1.15) but a much more dissymmetric environment due to its quadrupolar splitting value increasing until 2.53. The third Fe2+ site is around the same abundance (16%, labeled III) but with even lower coordination (IS ) 1.02) and an intermediate site symmetry (QS ) 1.69). Finally, because of its high dissymmetry (QS ) 3.42) the very little abundant Fe2+ site (4%, labeled IV) can be associated with the iron cations located at the external surface of small Fe2O3 clusters as these last species can be

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Figure 7. FTIR spectra of NO adsorbed on 1.5 57Fe-FERf sample after outgassing under vacuum at 673 K. Introduction of NO increasing small doses ca. (a) 4.2, (b) 8.4, (c) 16.7, (d) 25, (e) 33.4, (f) 41.7, (g) 62.6, (h) 83.5, (i) 104, and (j) 209 µmol · gcat-1 (“saturated”). For clarity, all spectra were corrected from the spectrum of 1.5 57Fe-FERf after activation.

detected from their isomer shift of 0.59 (Fe3+, labeled V) with a contribution of around 12%. This fresh 57Fe-enriched sample was rigorously prepared in the same way as the samples supplied from IRMA; however, it was characterized by Mo¨ssbauer “as-prepared” (in the hydrated form), whereas for IR experiments a thermal activation under a vacuum was performed. These differences in the pretreatment may prohibit any comparison relative to the iron oxidation number of the various samples. Nevertheless, previous work reports that when initially present in zeolite, the fraction of Fe(III) tends to decrease upon thermal treatment at 623 K in neutral atmosphere to yield Fe(II) species arising from Fe(III) autoreduction.46 These considerations thus consolidate us in our band assignments to nitrosyl species on Fe2+ cations; however, a potential “aging” effect had to be checked. Indeed, our prepared 1.5 57Fe-FERf sample was characterized fresh, whereas other samples supplied by IRMA were older: one could argue that oxidation of Fe2+ into the most stable Fe3+ oxidation state upon air contact might have occurred with time. For this purpose, NO adsorption was conducted on the 1.5 57Fe-FERf sample (same powder as the one used for the Mo¨ssbauer nondestructive experiment), and the corresponding spectra are given in Figure 7. A quick comparison between Figures 3 and 7 clearly indicates that qualitatively the spectra are identical: a main band at low wavenumber dominates the ν(NO) massif with a shoulder at higher wavenumber. The shift from 1876 to 1879 cm-1 and from 1893 to 1896 cm-1 when comparing natural iron samples with 57Felabeled iron sample may simply be explained by isotopic shift effect.50 Considering now the evolution of the ν(NO) massif with the NO adsorbed amount for one single sample, it appears that the high wavenumber shoulder is more evident for high value. Finally, to conclude this section, we emphasize that our ironcontaining FER samples, being fresh or aged, present qualitatively the same IR spectra after NO adsorption. The complementary use of Mo¨ssbauer spectroscopy applied on the fresh sample indicates that around 90% of iron is present on the +2 state, thus confirming our ν(NO) bands assignment to Fe(II) nitrosyls. Distribution of the Fe2+ Ions. The results of the use of NO as a probe molecule clearly show the existence of at least two families of accessible Fe2+ sites in Fe-FER and their concentration depends on the iron loading. The first type of the sites was observed with all samples and monitored by NO at 1876 cm-1. The second type of Fe2+ sites is observed with samples having

Iron Nitrosyl Species in Fe-FER a higher iron loading, but in all cases their concentration is lower than the concentration of the first type of sites. These last sites can be easily oxidized forming Fe3+-OH species and/or possibly the so-called R-oxygen.47 Evacuation at 673 K leads to autoreduction of the Fe3+-OH species and appearance of Fe2+ ions. The latter form, with NO mononitrosyl species, is characterized by a band at 1893 cm-1. With time polynitrosyls are formed starting from these mononitrosyls (1893 cm-1), which is an activated process. These results indicate some rearrangement in the local structure of the active sites (i.e., “extraction” by NO of the Fe2+ ions from their original positions). However, these changes are fully reversible, which suggests that Fe2+ ions remain in the vicinity of their initial location. According to Mo¨ssbauer results obtained on a freshly prepared sample, a third Fe2+ site is detected; however, no direct spectroscopic evidence could be provided from NO adsorption. It seems likely that the main band at 1876 cm-1 could correspond to mononitrosyl species formed on two distinct iron sites. In such a case, the frequency of adsorbed NO would not be a sufficient criterion to distinguish between all kinds of cationic iron sites. Further experiments using CO as a probe molecule are under current study to give better IR evidence of these three distinct Fe2+ cations in FER zeolites. 4. Conclusions In this study, we report NO adsorption followed by infrared spectroscopy to characterize iron cations in Fe-ferrierite. Different iron sites forming mononitrosyl species are identified. The complementary use of Mo¨ssbauer spectroscopy enabled us to determine the iron oxidation state to be essentially +2, resolving therefore an interpretation conflict present in the specialized literature. For low Fe loading (by ion exchange), the main fraction of Fe2+ cations is suggested to be located in highly accessible positions of the ferrierite where ionic exchange takes place in the easiest way. When the Fe amount increases, a second site is detected: upon the ionic exchange process, the concentration effect would favor the occupation of less accessible positions (confined) or positions less favored from an energetic point of view. When an oxidative pretreatment is applied, only the iron cations in the confined positions lead to the formation of Fe3+-OH species. Moreover, NO appears to be able to form polynitrosyl species with these confined Fe2+ cations. It thus appears that both the oxidation and the coordination states of confined Fe2+ may change easily, which makes them excellent candidates for active redox sites. Acknowledgment. K.H., E.I., and M.M. acknowledge the financial support from the Bulgarian Scientific Foundation (Grant DO 02-184). References and Notes (1) Guzman-Vargas, A.; Delahay, G.; Coq, B. Appl. Catal., B 2003, 42, 369. (2) Heyden, A.; Peters, B.; Bell, A. T.; Keil, F. J. J. Phys. Chem. B 2005, 109, 1857. (3) Suzuki, E.; Nakashiro, K.; Ono, Y. Chem. Lett. 1988, 9, 953. (4) Panov, I.; Sobolev, V. I.; Dubkov, K. A.; Kharinotov, A. S. Stud. Surf. Sci. Catal. 1996, 101, 493. (5) Anderson, J. R.; Tsai, P. J. Chem. Soc., Chem. Commun. 1987, 193. (6) Feng, X.; Hall, W. K. J. Catal. 1997, 166, 368. (7) Chen, H.-Y.; Sachtler, W. M. H. Catal. Today 1998, 42, 73. (8) Vaughan, P. A. Acta Crystallogr. 1966, 21, 983. (9) Compilation of Extra Framework Sites in Zeolites; Mortier, W. J., Ed.; Butterworth Scientific: London, 1982. (10) Davydov, A. A. Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides; Wiley: New York, 1990. (11) Hadjiivanov, K. I.; Vayssilov, G. AdV. Catal. 2002, 47, 307.

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