Characterization of Fe-Exchanged BEA Zeolite Under NH3 Selective


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Characterization of Fe-Exchanged BEA Zeolite under NH-SCR Conditions Jeongnam Kim, Andreas Jentys, Sarah M. Maier, and Johannes A. Lercher J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp309277n • Publication Date (Web): 07 Dec 2012 Downloaded from http://pubs.acs.org on December 8, 2012

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Characterization of Fe-Exchanged BEA Zeolite Under NH3-SCR Conditions Jeongnam Kim, Andreas Jentys, Sarah M. Maier and Johannes A. Lercher* Department of Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany

Abstract Structural changes to Fe3+ cationic species in Fe-exchanged zeolite BEA during the selective catalytic reduction (SCR) of NOx with NH3 were probed by UV-vis spectroscopy. The distribution between Fe2+ and Fe3+ species was characterized by IR spectroscopy of adsorbed CO. Upon heating to 723 K, some of the Fe cations formed Fe-O-Fe bonds that underwent reversible structural transformation under NH3-SCR conditions. The in situ formed Fe oxide clusters could be dissociated to isolated Fe cations at 423 K, while at higher temperatures O-bridged Fe clusters were again formed. The structure of the Fe cluster is related to the Al distribution in the zeolite probed by Co2+ ion exchange. We propose here that two Fe cations bound within one six-membered ring containing an Al pair form hydroxylated dimeric Fe-O-Fe in the zeolite. This was supported by a structure simulation of a binuclear [HO-Fe(III)-O-Fe(III)-OH] model. Keywords Fe-zeolite, UV-vis, CO, Al distribution, binuclear iron

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Introduction The NH3-SCR reaction (4NH3 + 4NO + O2 → 4N2 + 6H2O) has been developed addressing the demand for the abatement of NOx emission from the exhaust gas of diesel engines.1-3 Since the development of Fe-exchanged zeolites as highly active and stable catalysts for that reaction,4-6 the structure and activity of Fe in zeolites have been intensively investigated in attempts to obtain detailed structure-activity correlations. Despite numerous investigations, however, the structure of the active sites remains uncertain. In our view, the main difficulty in determining the active sites is related to various Fe species coexisting including isolated Fe cations, oligomeric FexOy species and Fe2O3 clusters.7-9 In addition, the oxidation state of Fe varies significantly depending on pretreatment and reaction conditions, making it even more difficult to understand the nature of active sites.10-14 Various spectroscopic techniques including UV-vis, electron spin resonance, and X-ray absorption spectroscopies (i.e., XANES and EXAFS) have been used to characterize the structure of Fe species in Feexchanged zeolites.1 Among them, UV-vis spectroscopy is the most suitable technique for estimating the fractions of different Fe species in zeolites.15-20 Specifically, the presence of isolated Fe cations in tetrahedral (Td) or octahedral (Oh) coordination, oligomeric FexOy clusters, and hematite-type Fe2O3 particles can be qualitatively determined by this technique. Quantitative determination, however, is not feasible due to unknown extinction coefficients, and because only Fe3+ species coordinated with oxygen ligands can be detected by UV-vis spectrsocopy. Nevertheless, UV-vis spectroscopy in combination with other spectroscopic techniques plays an important role in characterizing the structure of active Fe sites in zeolite catalysts.9,15-18,22 For example, in situ UV-vis measurements gave valuable information regarding structural changes of the Fe site as the NOx reduction proceeds,14,15 being a good indicator for the presence of active sites.16 An O-bridged binuclear Fe center has been often suggested as the active site in Fe-zeolites for NH3-SCR reactions.1,4,13 This proposed model is mainly based on spectroscopic data, and on the analogy to the binuclear Fe site in methane monooxygenase.23-25 On the other hand, some reports suggest that mononuclear Fe sites are more active than binuclear or oligonuclear Fe sites and that the latter are least 2 ACS Paragon Plus Environment

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selective towards the SCR reaction.1,17,20,22 This conclusion stems from a correlation, in which the turnover frequency decreases with increasing Fe content assuming statistical Al and Fe distribution in the zeolite. Since isolated Fe cations are mainly presented at very low Fe exchange levels (Fe/Al < 0.1), the mononuclear Fe seems to be the active site. However, at higher Fe loadings, the activity comparison of mononuclear and oligonuclear Fe sites determined from a simple statistical calculation would be inappropriate as, in most cases, the Al in zeolites are non-statistically distributed. This non-random distribution has been characterized by Wicherová et al. using Co2+ ions as probes.26-28 It has been found that the Al distribution in zeolite depends on synthesis conditions and Al content, and plays a decisive role in determining the SCR activity. It is, therefore, interesting to what extent the formation and location of active Fe species can be related to the Al distribution in zeolites. Recently we reported that Fe cationic species, mainly isolated, can be placed into BEA zeolites by wet-ion exchange up to the Fe/Al ratio of 0.3.29,30 A series of these FeBEA samples were thoroughly characterized using spectroscopy techniques. We determined oxidation states of Fe under reaction conditions by a combination of XANES and Mössbauer spectroscopy, revealing a reversible oxidation/reduction process of exchanged Fe2+/Fe3+ species.29 In related work, we concluded that the coordination of Fe as characterized by UV-vis, IR and in situ X-ray absorption fine-structure (XAFS) spectroscopies markedly depends on the reaction conditions.30 Under NH3-SCR conditions, the extraframework ion-exchanged Fe cations led to a temperature-dependent formation of bridging Fe-O-Fe dimers and finally to the insertion of Fe cations into zeolite T-atom positions while cooling to room temperature under reactant atmosphere. On the basis of those data, we proposed that active Fe-O-Fe dimers are formed in situ during the NH3-SCR reactions. The present work was undertaken to give a more detailed picture on the temperature-dependent formation of O-bridged Fe dimers. The Fe-exchanged BEA zeolite was pretreated at 723 K in the presence of N2 or O2 before the addition of reaction gases. Structural changes of Fe3+ species during the pretreatment and NH3-SCR reaction were monitored as a function of the temperature by UV-vis spectroscopy. The distribution between Fe2+ and Fe3+ species was characterized by IR spectroscopy of CO 3 ACS Paragon Plus Environment

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adsorbed at liquid N2 temperature. The Al distribution of the parent BEA sample was determined by ion exchange with Co2+, a method developed by Wichterlová et al.,31 and used for the structure simulation of binuclear Fe-O-Fe species in the zeolite framework.

Experimental Materials A FeBEA zeolite sample was used with a molar Fe/Al ratio of 0.3 (~ 1.0 wt% Fe), prepared by a wet-ion exchange method as described in ref.

29,30

. Briefly, zeolite HBEA (Si/Al = 18, Süd-Chemie AG)

was suspended in a 0.04 molar solution of FeSO4·7H2O (Fluka, 99 %) under N2 atmosphere. The pH was adjusted to 2 and the suspension was stirred at 353 K for 20 h. The FeBEA sample was collected by centrifugation, washed five times with distilled water, and freeze dried. The dried sample was held in N2 at 753 K for 2 h and then stored in air. For characterizing the Al distribution the HBEA zeolite sample was exchanged with Co2+ ions as described in ref

31

. The Co2+ ion exchange was carried out with a stirred 0.05 molar solution of

Co(NO3)2·6H2O (150 cm3 per g zeolite) at room temperature for 12 h, and repeated three times. After ionexchange the CoBEA sample was washed three times with distilled water and dried at 373 K. The chemical composition of the sample was determined by atomic absorption spectroscopy (AAS) using a Solaar M5 Dual Flame graphite furnace AAS from Thermo Fisher.

Characterization methods In situ UV-vis spectroscopy In situ UV-vis diffuse-reflectance (DR) measurements were performed with an Avantes Avaspec 2048 spectrometer. The FeBEA sample was used as powder and placed in a quartz reactor (6 mm inner diameter) with square optical-grade quartz windows. The reactor was placed horizontally in a lab-made heating chamber with an 8 mm diameter hole on top, through which a high-temperature optical fiber (Avantes FCR-7UV400-2ME-HTX UV-VIS reflection probe) could be vertically directed to the catalyst 4 ACS Paragon Plus Environment

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bed. The temperature was measured by a thermocouple located on the bottom of the catalyst bed. In a typical experiment, the UV-vis spectra were collected with an integration time of 104 ms, and 20 scans were accumulated. All DR UV-vis spectra are presented in the form of the Kubelka-Munk function defined as F(R) = (1-R)2/(2·R) with R = Rs/Rr, where Rs is the reflectance of the sample and Rr is the reflectance of HBEA used as reference. The intensity of the UV-vis spectra was slightly different despite the same FeBEA sample was used, which results from preparing individual samples for the experiments in N2 and O2, where the position of the UV-vis probe head slightly differed. Also the packing of the samples slightly affects the intensity of the resulting spectra. The spectra were deconvoluted using Gaussian bands with the Fityk 0.9.8 software. The FeBEA sample was treated at 723 K under two different conditions: in N2 or in 5 vol % O2 in N2 (flow 60 ml min-1). The UV-vis spectra were recorded at room temperature, 423, 523 and 623 K while heating with a rate of 10 K min-1 to 723 K. The spectrum at 723 K was taken after the sample was held at this temperature for 1 h. Subsequently, the sample was cooled to 423 K and a spectrum was recorded. In situ NH3-SCR experiments were carried out in a gas mixture of 2000 ppm NO, 2000 ppm NH3, 5 vol% O2 balanced in N2 with a total flow of 60 ml min-1. The temperature dependence of the NH3-SCR reaction was determined under steady-state conditions at 423, 523, 623 and 723 K. Afterward the sample was cooled to 423 K for a final measurement in order to validate the reversible nature of the Fe species. The distribution of Co2+ ions in the CoBEA sample was determined by UV-vis spectroscopy. Before the measurement, the CoBEA sample was treated at 773 K for 1 h in flowing He (30 ml min-1) to dehydrate the zeolite sample. The UV-vis spectrum of the dehydrated CoBEA sample was recorded at room temperature in flowing He. For quantitative analysis, reported absorption coefficients were used for the peaks α to δ: kα =11 ± 1 x 10-3, kβ =7 ± 0.7 x 10-3, kγ =5 ± 0.5 x 10-3, and kδ =1.3 ± 0.7 x 10-4 cm mmol g-1.31

IR spectroscopy of adsorbed CO

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The IR spectra of the FeBEA sample were obtained on a Bruker VERTEX70 spectrometer equipped with a MCT detector. Self-supporting wafers (~ 10 mg/cm-2) were prepared and placed in a vacuum IR cell equipped with CaF2 windows. The sample wafer was treated at 723 K for 1 h in vacuum (p = 10-6 mbar) or in static synthetic air (25 kPa). In the latter treatment, the sample was first evacuated and then put in contact with static air. This procedure was repeated three times, while heating to 723 K. The air-treated sample was additionally evacuated at 723 K for 30 min prior to the adsorption measurement. After cooling to liquid N2 temperature, the sample was exposed to CO (99.97 vol %, used without further purification) with a partial pressure of 1.0 mbar. Subsequently, He (p ~ 15 mbar) was added to the sample cell to facilitate heat transfer under low-pressure conditions. The IR spectra were recorded at 4 cm-1 resolution, baseline corrected, and normalized to the intensities of the overtone and combination vibrations of zeolite BEA between 2095 and 1755 cm-1.

Structure simulation A periodic structure of BEA containing 2 Al atoms in a six-member ring close to the ß-type cationic site was used for the (3D periodic) DFT calculation. Two Fe atoms were placed near the Al atoms and the charges were balanced by OH groups. The geometry was optimized with respect to the energy using a double numerical basis set. After geometry optimization had been achieved, the final energy was calculated using the exchange correlation based on the Becke, Lee, Yang Parr method (BLYP). The program DMol3 was used for the calculations.32

Results UV-vis spectra during heat treatments The FeBEA sample was treated at 723 K either in flowing N2 or in the presence of O2 (5 vol % in N2) before the reaction. Changes in the distribution of Fe3+ species in the zeolite sample were probed by UV-vis spectroscopy as a function of the temperature. As shown in Fig. 1, the UV-vis spectra obtained at room temperature presented two broad absorption bands centered at 235 and 275 nm. According to the 6 ACS Paragon Plus Environment

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literature, these bands were assigned to the charge transfer (CT) bands from O ligands to isolated Fe3+ species (λ < 300 nm).14-22 The two CT bands arise from the t1 → t2 and t1 → e transitions, whereas their precise energies depends on the number of ligands.21,33 Typically, the transitions for Fe cations in Td symmetry are observed at 215 and 241 nm (below 250 nm), and for Fe in Oh symmetry are found in the ranges of 187−234 and 244−305 nm. It is thus most likely that the FeBEA prior to activation in N2 or the presence of O2 contained solely isolated Oh Fe3+ cations at ion-exchange positions. This result is consistent with ex situ UV-vis measurements already published.30 During heating to 423 K the UV-vis spectra of both samples became more intense below 250 nm, while the intensity of the CT band at 275 nm decreased. The first CT band at 215 nm was missing here due to a low transmittance of the optical fiber below 220 nm. This result is attributed to the formation of Fe3+ cations in Td coordination from octahedrally coordination. As suggested by Pérez-Ramírez et al., the formation of Td Fe species is likely caused by the release of water ligands during heating.21 Another important change during heating to 423 K was broadening of UV-vis spectra at 400 nm, indicating the formation of additional Fe species. These appear to be oligomeric FexOy clusters, as their CT transitions are between 300 and 400 nm.14-22 The formation of hematite-like Fe2O3 clusters, however, was small according to negligible absorption above 400 nm. Heating from 423 to 723 K resulted in clear changes in the UV-vis spectra of the samples treated in N2 or O2. During activation in N2 the intensity of all bands was gradually decreased with increasing temperature, but increased in the presence of O2. This indicates the reduction from Fe3+, to Fe2+ not visible in the UV-vis spectra, while in the presence of O2 Fe2+ species were oxidized to Fe3+. Deconvolution of the UV-vis spectra obtained at this stage was not possible, because of the appearance of an additional broad band above 400 nm assigned to inter-valence CT transitions between Fe2+ and Fe3+ species (λ > 500 nm), supporting a reduction process of Fe3+ in the zeolite sample.21 Despite the difficulty in deconvolution, it was tentatively concluded that the intensity of bands assigned to isolated Fe cations in Oh coordination and oligomeric FexOy species markedly decreased upon heating under N2, while the intensity of bands assigned to isolated Fe3+ cations remained unaffected. In the presence of O2, all CT bands except the one 7 ACS Paragon Plus Environment

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assigned to hematite clusters increased gradually with temperature. For details on the deconvolution see the Supporting Information.

UV-vis spectra under NH3-SCR conditions After the activation, FeBEA was contacted with the reaction gases consisting of 2000 ppm NO, 2000 ppm NH3, and 5 vol% O2 in N2 with a total flow of 60 ml min-1. The UV-vis spectra were recorded at the same temperatures studied before and also used for the EXAFS study published previously.29,30 In Fig. 2A the UV-vis spectrum of FeBEA pretreated in N2 and cooled to 423 K is included. After adding the reaction gases at 423 K the intensity increased and a shoulder at ~ 280 nm, assigned to Oh Fe3+ cations, was observed. These changes imply that the Fe species were significantly re-oxidized and the isolated Oh Fe3+ cations were restored in the presence of the reactant gasses at 423 K. At higher temperatures (up to 723 K), the shoulder at 280 nm disappeared and the overall intensity decreased slightly. Cooling to 423 K in the presence of reaction gases at the end of the experiment led to a nearly identical spectrum compared to that before heating, suggesting the complete reversibility of the interchanging Fe species. In the case of FeBEA pretreated in O2, there was only a small change in the overall intensity of the spectra in the region of Oh Fe3+ cations during the reaction at 423 K (Fig. 2B). This spectrum and its changes with the reaction temperature were quite similar to those of the sample pretreated in N2. This indicates that the pretreatment in N2 or in O2 has only a minor influence on structure of Fe species and its catalytic activity.29 The in situ UV-vis spectra obtained for the FeBEA sample pretreated in O2 were deconvoluted using peaks with a Gaussian shape as shown in Fig. 3. Clearly a fraction of isolated Oh Fe3+ cations with signature in the UV at ~ 280 nm was restored at the expense of oligomeric FexOy clusters (300 < λ < 400 nm). Heating to 523 K, however, led to the decrease of Oh Fe3+ cations, which most probably were

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converted into bridging Fe-O-Fe species confirming the results of the previous in situ EXAFS analysis, where the structure of Fe was investigated after activation and SCR reaction.30

IR spectra of the hydroxyl groups during pretreatment To get more insight into the nature of Fe in FeBEA, the OH stretching vibrations were followed by IR spectroscopy during heating from room temperature to 723 K in vacuum and in the presence of synthetic air (p = 25 kPa) (Fig. 4). At room temperature, the untreated FeBEA showed two well-resolved OH stretching bands. The band at 3744 - 3735 cm-1 was assigned to terminal Si-OH groups at external crystal surfaces, and the band at 3605 cm-1 to Brønsted acidic Si(OH)Al groups.34 The broad feature between 3400 and 3700 cm-1 is due to the presence of water on the sample and to H-bonded hydroxyl groups in hydroxyl clusters.34 Heating to 423 K led to the appearance of an additional weak band at 3686 cm-1, assigned to Fe3+-OH groups in FeBEA.8,35 This weak band disappeared almost completely upon heating to 723 K under vacuum, attributed to the reduction of the Fe3+-OH group to Fe2+ species under inert atmosphere, in agreement with UV-vis spectroscopy. On the other hand, in the presence of O2, the band corresponding to Fe3+-OH groups increased in intensity at higher temperatures, indicating the oxidation of Fe2+ species to Fe3+. Low-temperature CO adsorption after activation in vacuum or O2 was performed to characterize the distribution of Fe2+ and Fe3+ species. The normalized IR spectra of CO adsorbed on the sample activated in vacuum and in O2 are shown in Fig. 5. The dominant bands at 2190, 2175 and 2157 cm-1 are assigned to Fe2+-CO complexes, an acidic OH-CO adduct, and to CO interacting with Fe3+-OH groups.35 The weak bands around 2225 cm-1 and 2130 cm-1 are assigned to Al3+-CO species and physisorbed CO, respectively. The bands of the Fe2+ species were dominant in the sample treated in vacuum but not in O2. The fraction of Fe2+ species was 59 % (assuming the same extinction coefficients for CO adsorbed on Fe2+ and Fe3+ species) in the sample treated in vacuum, which is similar to our previous data obtained from a linear correlation between edge energy of XANES and oxidation state determined by Mössbauer spectroscopy.29 In the presence of O2, the Fe2+ species were oxidized to Fe3+, and only 28 % of Fe2+ 9 ACS Paragon Plus Environment

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species remained even after the treatment at 723 K. These Fe2+ species may be stabilized in the zeolite framework, while approximately 30 % of Fe species were easy reduced/oxidized and thus participate in the redox process during the heat treatment.

Al distribution of parent HBEA zeolite The spatial distribution of Al in the BEA framework was characterized by the method developed by Dedecek et al.,31 who have proposed to use Co2+ ions as probe molecules to determine the location of ion exchange sites in dehydrated BEA zeolites. The different Co(II) ion coordination is reflected in characteristic d–d transitions in the UV/Vis spectra. Since the Co ions exist exclusively as [Co(H2O)6]2+ complex during the ion exchange, two types of cationic sites in the zeolite can be balanced by these complexes. One is Al atoms paired in Al-O-(Si-O)1,2-Al sequences, and the other is single Al atoms in AlO-(Si-O)n>2-Al structures in a common channel and close proximity. After dehydration of the zeolite at 773 K, the distribution of Co2+ ions at individual cationic sites can be determined quantitatively by UV/Vis spectroscopy and related to the distribution of paired Al atoms in the zeolite. Three paired Al sites denoted as α (cationic site formed by an elongated six-member ring composed of twofold five-member rings), β (cationic site in an deformed six-member ring of the hexagonal cage), and γ (cationic site located inside the beta cage) have been characterized and their locations in BEA zeolites are shown in Fig. 6B. The Co2+ ion exchange of parent HBEA zeolite led to a Co2+/Al+ molar ratio of 0.24, implying that approximately half of Al sites were forming Al pairs in the zeolite. The UV-vis spectra of dehydrated CoBEA zeolite shown in Fig 6 contain contributions from Co ions at α-, β-, and γ-type positions. Following Dedecek et al.31 the α-type Co2+ ions are assigned to the single band at 690 nm. The β-type Co2+ ions are characterized by triple bands at 650, 610, and 570 nm, and the double bands at 520 and 510 cm-1 correspond to γ-type Co2+ ions. The result of the deconvolution presented here matches the literature, except that a band at 460 nm (corresponding to the β site) was not detected due to broad overlapping bands of δ-type Co-species at 470 and 320 nm. In contrast to α-, β-, and γ-type Co2+ ions characterized by d-d transitions, it has been suggested that the δ-type Co2+ species forms a bridging Co-O2-Co cobalt 10 ACS Paragon Plus Environment

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species showing a CT transition.27,28,31 Based on the absorption coefficients reported, the relative calculated concentrations of the Co2+ ions at the α-, β-, and γ-type and the δ-type Co-oxo species are shown in Fig. 6. Among the Co-species, the β-type Co2+ ions were predominant (75 %) in the HBEA zeolite. The β-type cationic site is located at an elongated six-member ring in the main channel of polymorphs A and B of zeolite BEA.31 The population of Co at the α-type cationic site was 15 %; for the γ-type site 3 % and the remaining Co found δ-type sites.

Structure simulation A structural model for an O-bridged Fe dimer, formed at high temperatures (> 573 K) at the βtype sites, was constructed by replacing two Si atoms with Al in the 6 MR of zeolite BEA. We assumed that the two Fe sites were closely bound at the same β site and thereby self-organized into dimers through dehydration upon heating. The dimers would be [HO-Fe(III)-O-Fe(III)-OH]2+ clusters, compatible with a binuclear model widely proposed in the literature.1,4,13,36-38 As shown in Fig. 7, the optimization of the structure resulted in a symmetric binuclear Fe cluster on top of six ring containing an Al atom pair. The Fe-Fe distance calculated was 3.20 Å, close to the distance (3.05 Å) determined by EXAFS.12,23,30 The distances between the Fe atoms and the O atoms from the zeolite were in a narrow range between 1.7 and 2.0 Å in Td coordination. This simulation agrees well with the preceding EXAFS analysis30 and supports the formation of binuclear Fe clusters at six-member ring containing two Al atoms.

Discussion In situ formation of O-bridged Fe dimers and their redox properties Characterization of the FeBEA sample (Fe/Al ~ 0.3) by UV-vis spectroscopy showed that Fe were presented mainly as isolated Fe3+ cations in an Oh environment after ion-exchange. The conclusion is based on the absence of absorption bands assignable to clustered FeOx. On the other hand, according to our previous Mössbauer data, significant concentrations of Fe2O3 particles (~ 24 % of all Fe) were present 11 ACS Paragon Plus Environment

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in the same zeolite sample.29 This means that UV-vis spectroscopy is less sensitive than Mössbauer spectroscopy for the detection of aggregated Fe phases. The fraction of isolated Fe3+ species determined by Mössbauer spectroscopy was 24.5 %, and approximately a half of these were present as isolated Fe2+ cations. Thus, the Fe species characterized by CT bands in UV-vis spectra were a minor fraction of the Fe in the sample. Despite the limitation of UV-vis characterization, it is clear that isolated Fe in Td coordination, oligomeric FexOy, and hematite Fe2O3, can be easily formed upon heating to 723 K (Fig. 1). This change is attributed to the dehydration of hydroxyl and water ligands on extra-framework Fe3+ cations during the heating. These in situ formed Fe species were still present under NH3-SCR conditions (Fig. 2). The UV-vis data measured under ex situ conditions have been used by others to estimate the distribution of Fe species of different nuclearities and to correlate those with catalytic activities.15,17-20,22 This approach would be correct if the Fe species remained unchanged under reaction conditions. The present results show, however, that Fe species with different nuclearities are formed in situ and participate during the reaction (Figs. 1 and 2). Note that a feature, almost the same on an FeBEA sample with low Fe concentration (Fe/Al ~ 0.12), was observed under identical conditions (see Supporting Information). Therefore, we reject that Fe monomers represent a major active site for the NH3-SCR reaction, even if the Fe-sample had solely isolated Fe species after ion-exchange. The Fe in BEA zeolites can auto-reduce in N2 at high temperatures (> 523 K, Fig. 1). This reduction process has been observed previously and often associated with desorption of an O atom from between two Fe atoms.1,10,12,24,39 The reduction of Fe under inert atmosphere was characterized in this work using UV-vis spectroscopy by a gradual decrease of overall spectral intensity with increasing temperature. More importantly, among the CT bands detected, those assigned to Oh Fe (~ 280 nm) and oligomeric FexOy (300 < λ < 400 nm) decreased most markedly. The result might indicate that the Oh Fe is further transformed into other Fe species, while the FexOy clusters are reduced upon heating to 723 K in N2. This interpretation is in line with our previous XANES/EXAFS study revealing an average coordination change to Td Fe and formation of Fe-O-Fe bridges and their significant reduction during 12 ACS Paragon Plus Environment

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activation in He.30 We conclude that the in situ formed oligomeric Fe species play a major role in the selfreduction process of Fe in BEA zeolites. The Fe3+-OH groups in FeBEA zeolites are involved in redox processes. As characterized by IR spectroscopy (Fig. 4), the Fe3+-OH band (~ 3686 cm-1) appeared at 423 K but disappeared under selfreduction conditions, whereas the presence of O2 led to an intense Fe3+-OH band due to the oxidation with increasing temperature. From these results, we conclude that Fe in BEA zeolites is reduced to Fe2+ via dehydroxylation of Fe3+-OH groups, and oxidized vice versa. The low-temperature adsorption of CO characterized these Fe species in more detail (Fig. 5). The Fe2+ captured CO forming Fe2+-CO complexes, while the Fe3+ were mainly present as Fe3+-OH groups interacting with CO via hydrogen bonding.35 A clear difference in the distribution of Fe2+/Fe3+ species depending on the thermal treatment conditions supported a redox cycle between dehydroxylated Fe2+ species and Fe3+-OH groups. The degree of auto-reduction for the Fe was monitored in our previous study by a combination of XANES and Mössbauer spectroscopy.29 It showed that the fraction of Fe2+ cations increased from 51 % at room temperature to 62 % after activation at 723 K in He. The present study using IR spectroscopy of adsorbed CO confirms that result (Fig. 5) and also shows that activation in O2 led to a significant oxidation of the Fe as only 28 % of the Fe persisted as Fe2+. Under NH3-SCR conditions, bridging Fe-O-Fe species may be dissociated into isolated Oh cations (Fig. 2). This was observed at 423 K in either oxidizing or inert atmosphere. These Oh Fe were converted into O-bridged Fe species (Fig. 3) by increasing temperatures in the presence of the NH3-SCR reactants. The temperature-dependent formation of O-bridged Fe species at high temperatures was also confirmed in our EXAFS study.30 This strongly suggests that the Oh Fe3+ cations at ion-exchange positions are a key intermediate for the structural changes of Fe species in the zeolite catalyst during the NH3-SCR reaction. On the other hand, the formation of O-bridged Fe species and their stability under NH3-SCR conditions is highly dependent upon the reaction temperature, as clearly shown by the reversible change of spectra at 423 K after the NH3-SCR reaction. 13 ACS Paragon Plus Environment

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Fig. 8 summarizes the proposed structural changes of Fe3+ in FeBEA zeolites as a function of the temperature and the gas atmosphere. At room temperature Fe3+ cations are mainly bound to ion-exchange sites of the BEA zeolite. Since one Fe cation is located at one bridging hydroxyl group (Brønsted acid site),29,30 the excess charge of the Fe3+ cations had to be compensated by -OH groups. Heat treatment led to the formation of bridged [HO-Fe(III)-O-Fe(III)-OH]2+ species through dehydration, characterized by the appearance of an absorption band centered approximately on 320 nm in the UV-vis spectra. A band characteristic of isolated Fe3+ cations in Td coordination appeared simultaneously, and we conclude that the heat treatment can also transform Oh Fe3+ cations into isolated Td Fe without Fe-O-Fe bridges. Under NH3-SCR conditions, a redox cycle between the Fe2+ and Fe3+ species is concluded to exist utilizing NH3 as reducing and O2 as oxidizing agents. In the case of Fe dimers, the [HO-Fe(III)-O-Fe(III)OH]2+ species could be reduced to [Fe(II)-O-Fe(II)]2+ by removing the hydroxyl groups coordinated to the Fe3+ as indicated by IR spectroscopy. This redox cycle is the predominant process especially at high temperatures (> 573 K), owing to the temperature-dependent formation of bridged Fe-O-Fe. Conversely, at low temperature such redox cycle is less prominent because the Fe species cleave into isolated Oh sites at ion-exchange positions. We cannot rule out the contribution of isolated Fe sites to the NH3-SCR reaction, as those species are clearly observable in the UV-vis spectra. Cooling to room temperature after reaction led to incorporation of the Fe3+ cations into the zeolite framework, rather than return to the initial ion-exchange positions. The detailed analysis of the local structure of Fe by EXAFS on the catalysts after activation and cooling from SCR conditions to low temperature reported in our previous work30 indicated that Fe species can be eliminated from the lattice Tatom positions during activation at 723 K and can participate again in the formation of active bridging FeO-Fe species. This process allows a unique reversible transformation of the Fe species during subsequent NH3-SCR reactions.

Formation of binuclear Fe clusters in relation to the Al distribution

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Based on the EXAFS data, the O-bridged Fe species is most likely an Fe dimer.30 The UV-vis spectroscopy did not give any information regarding the nuclearity of the O-bridged Fe structure. The question remaining is how and where such binuclear Fe clusters form inside the zeolite structure. The Al distribution of the parent HBEA sample was characterized (Fig. 6), since it is inevitably related to the location of Fe cation species. As already suggested by Wichterlová et al.,26-28,31 the Al distribution in the BEA sample was not random; almost half of Al formed paired Al sites among which the ß-site was predominant (80 % among total Al pairs). The low incidence of a single Al site allowed us to simulate the structure of a binuclear Fe clusters in a specific cationic site, i.e., the ß-site, in the BEA zeolite. A quantum chemical calculation of Fe-oxide clusters revealed that this ß-site is a suitable position for [HO-Fe(III)-OFe(III)-OH]2+ dimers that can be formed in situ during the heating process (Fig. 7). This simulation indicates that the structure of binuclear Fe clusters can be related to the Al distribution of parent zeolite sample. Very recently, Dĕdeček et al. have reviewed the importance of the Al distribution and its control in zeolite catalysts containing transition metal ions (e.g., Cu, Co, and Fe).40 A general agreement is that at low metal concentration (exchange cation/Al < 0.15) the divalent cations are preferentially balanced by Al pairs in zeolites.27,40-42 The higher metal loading can lead to additional oxygen-bridged dimers at less proximate Al pairs. According to the present characterization results, the Fe ions in the studied sample were mainly bound as Fe-OH groups to a single Brønsted acid site rather than Fe2+ ions to the Al pairs. This supports a possibility for the formation of Fe dimers from the two Fe ions bound at the paired Al sites. On the other hand, the Fe species are also bound at isolated Al sites. This co-existence of both Fe species in Fe-exchanged zeolites again makes it difficult to discriminate the nuclearity of active Fe sites. An approach to answer this on-going debate would be a systematic control of Al pairs (i.e., concentration and distribution) in zeolites, as the structure of Fe is related to the Al distribution. This approach may be achieved by varying synthesis conditions, zeolite-structure-directing agents and Al concentrations.40

Conclusions 15 ACS Paragon Plus Environment

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The experiments show that the ion-exchanged Fe species initially presented as octahedrally coordinated cations can be readily transformed by heating into isolated Fe cations in Td symmetry, Fe-OFe dimers and Fe2O3 particles. Among them, the Fe-O-Fe dimers are most reducible and temperaturedependent. Their structural changes under NH3-SCR conditions were correlated to the NO conversion as a function of the reaction temperature. We attributed the low catalytic activity at low temperatures (e.g., 423 K) to the significant dissociation of binuclear Fe clusters into isolated Oh Fe3+ cations. A sharp activity increase between 523 and 623 K resulted from the temperature-dependent formation of active Fe dimers. The formation of the Fe dimers in the BEA zeolite is only possible at paired Al atoms. We proposed that the two Fe cations in close proximity within one six-member ring composed of Al-O-(Si-O)2-Al sequences are a main location for the formation of O-bridged binuclear Fe clusters. It would be interesting if the catalytic activity can be improved by control of the number and distribution of such Al paired sites. However, a catalytic role for isolated Fe sites cannot be absolutely rejected by the present investigation.

Acknowledgements This project was funded by the Bayerisches Staatsministerium für Wissenschaft, Forschung und Kunst. The authors would like to thank Martin Neukamm for AAS measurements, Monica Pop for assistance in in situ UV-vis measurements, and Stefanie Reiner for her contribution to low-temperature CO adsorption measurements.

Supporting Information Available Deconvolution of UV-vis spectra during the activation in O2 and NH3-SCR reaction and UV-vis spectra of FeBEA sample with a Fe/Al molar ratio of 0.12 are given. This information is available free of charge via the Internet at http://pubs.acs.org.

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3. Liu, Z.; Woo, S. I. Catal. Rev. 2006, 48, 43-89. 4. Chen, H.-Y.; Sachtler, W. M. H. Catal. Today 1998, 42, 73-83. 5. Ma, A.-Z.; Grünert, W. Chem. Commun. 1998, 71-72. 6. Long, R. Q.; Yang, R. T. J. Am. Chem. Soc. 1999, 121, 5595-5596. 7. Heinrich, F.; Schmidt, C.; Löffler, E.; Menzel, M.; Grünert, W. J. Catal. 2002, 212, 157-172. 8. Mauvezin, M.; Delahay, G.; Coq, B.; Kieger, S.; Jumas, J. C.; Olivier-Fourcade, J. Phys. Chem. B 2001, 105, 928-935. 9. Iwasaki, M.; Yamazaki, K.; Banno, K.; Shinjoh, H. J. Catal. 2008, 260, 205-216. 10. Lobree, L. J.; Hwang, I.-C.; Reimer, J. A.; Bell, A. T. J. Catal. 1999, 186, 242-253. 11. Long, R. Q.; Yang, R. T. J. Catal. 2000, 194, 80-90. 12. Battiston, A. A.; Bitter, J. H.; Heijboer, W. M.; de Groot, F. M. F.; Koningsberger, D. C. J. Catal. 2003, 215, 279-293. 13. Battiston, A. A.; Bitter, J. H.; Koningsberger, D. C. J. Catal. 2003, 218, 163-177. 14. Kumar, M. S.; Schwidder, M.; Grünert, W.; Bentrup, U.; Brückner, A. J. Catal. 2006, 239, 173-186. 15. Kumar, M. S.; Schwidder, M.; Grünert, W.; Brückner, A. J. Catal. 2004, 227, 384-397. 16. Pirngruber, G. D.; Roy, P. K.; Prins, R. Phys. Chem. Chem. Phys. 2006, 8, 3939-3950. 17. Schwidder, M.; Kumar, M. S.; Brückner, A.; Grünert, W. Chem. Commun. 2005, 805-807. 18. Høj, M.; Beier, M. J.; Grunwaldt, J.-D.; Dahl, S. Appl. Catal. B 2009, 93, 166-176. 19. Brandenberger, S.; Kröcher, O.; Tissler, A.; Althoff, R. Appl. Catal. A 2010, 373, 168-175. 20. Brandenberger, S.; Kröcher, O.; Tissler, A.; Althoff, R. Appl. Catal. B 2010, 95, 348-357. 21. Pérez-Ramírez, J.; Groen, J. C.; Brückner, A.; Kumar, M. S.; Bentrup, U.; Debbagh, M. N.; Villaescusa, L. A. J. Catal. 2005, 232, 318. 22. Schwidder, M.; Kumar, M. S.; Klementiev, K.; Pohl, M. M.; Brückner, A.; Grünert, W. J. Catal. 2005, 231, 314-330. 23. Marturano, P.; Drozdová, L.; Kogelbauer, A.; Prins, R. J. Catal. 2000, 192, 236-247.

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42. Penzien, J.; Abraham, A.; van Bokhoven, J. A.; Jentys, A.; Müller, T. E.; Sievers, C.; Lercher, J. A. J. Phys. Chem. B 2004, 108, 4116-4126.

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Figure captions Figure 1. UV-vis spectra of FeBEA zeolite during heating (A) in N2 and (B) in the presence of 5 vol % O2.

Figure 2. UV-vis spectra during NH3-SCR reaction (5 vol % O2, 2000 ppm NO, and 2000 ppm NH3) over FeBEA zeolites pretreated (A) in N2 and (B) in O2 atmosphere.

Figure 3. Deconvolution results of UV/vis spectra shown in Fig. 2B. Numbers in parenthesis indicate the wavelength of peak maxima after deconvolution. The areas were obtained by assuming the same absorption coefficients of different Fe species.

Figure 4. FT-IR spectra (OH stretching region) of FeBEA zeolite during heat treatment (A) under vacuum and (B) in the presence of synthetic air (25 kPa). The spectra were recorded while heating to 723 K.

Figure 5. FT-IR spectra (CO stretching region) of CO adsorbed on FeBEA zeolites at liquid nitrogen temperature after the heat treatment (black) under vacuum and (gray) in the presence of O2.

Figure 6. (A) UV-vis spectrum of the Co2+-exchanged BEA zeolite dehydrated at 773 K and its deconvolution result using Gaussian bands. The numbers in percentage indicate relative concentration of α, β, γ and δ site in the zeolite sample. (B) location of α, β, and γ-type Co2+ ions in the zeolite framework as reported in the literature.31

Figure 7. Structure simulation model of binuclear [HO-Fe(III)-O-Fe(III)-OH]2+ species bound at the βtype Al site of BEA zeolite.

Figure 8. Schematic representation for proposed structure changes of Fe3+ cationic species during the NH3-SCR reaction over FeBEA zeolite, based on the present UV-vis data. 20 ACS Paragon Plus Environment

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.

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Figure 7.

3.20 2.04

1.93 1.88 2.00

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Figure 8.

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Table of Contents (TOC)

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