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Standard and Fast Selective Catalytic Reduction of NO with NH on Zeolites Fe-BEA 3

Magdalena Jab#o#ska, Gerard Delahay, Krzysztof Krucza#a, Artur B#achowski, Karolina A. Tarach, Kamila Maria Brylewska, Carolina Petitto, and Kinga Góra-Marek J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05692 • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 7, 2016

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The Journal of Physical Chemistry

Standard and Fast Selective Catalytic Reduction of NO with NH3 on Zeolites Fe-BEA Magdalena Jabłońskaa, Gérard Delahayb, Krzysztof Kruczałaa, Artur Błachowskic, Karolina A. Taracha, Kamila Brylewskaa,d, Carolina Petittob, and Kinga Góra-Mareka,* a

Faculty of Chemistry, Jagiellonian University in Kraków, 3 Ingardena St., 30-060 Kraków, Poland,

phone: +48 12 663 2081, fax: +48 12 634 0515 b

Institut Charles Gerhardt de Montpellier, ICGM-MACS, 8 rue Ecole Normale, 34296 Montpellier

Cedex 5, France c

Institute of Physics, Pedagogical University, 2 Podchorążych St., 30-084 Kraków, Poland

d

Faculty of Materials Science and Ceramics, AGH University of Science and Technology in Kraków, 30

Mickiewicz Av., 30-059 Kraków, Poland *K. G.-M.: e-mail, [email protected]; phone, +48 12 663 20 81. Abstract Two Fe-containing BEA zeolites were prepared by ion exchange (IE; Fe-BEA) and post-synthesis (PS; Fe-BEA/DeAl) procedures. Similar Fe content (0.8-0.9 Fe wt.%) was evidenced for studied samples. Fe-BEA, prepared by ion exchange, contained essentially iron cations at the exchange sites of the zeolite. These iron cations are either isolated or bridged through an oxygen atom (Fe-oxo cations). However, in Fe-BEA/DeAl, a high portion of iron was introduced in the T-positions of zeolite lattice. The other part was either at an exchange positions or, deposited as oxide clusters on outer surface of zeolite grains. For Fe-BEA significantly higher acidity than for Fe-BEA/DeAl was evidenced by FT-IR studies with adsorption of NH3 and CO. The catalytic performance of Fe-BEA and Fe-BEA/DeAl was investigated in standard SCR (NO2/NO = 0) and fast SCR (NO2/NO = 0.85). Fe-BEA revealed high catalytic activity in both SCR NOx reactions. However, production of N2O was much more apparent over this catalyst than over Fe-BEA/DeAl. In line with EPR and IR studies the isolated or bridged through an oxygen atom extraframework iron oxo-sites in Fe-BEA were found to deliver higher catalytic activity than the iron oxo-sites in tetrahedral framework positions in Fe-BEA/DeAl.

1 ACS Paragon Plus Environment


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1. Introduction The selective catalytic reduction of NOx by ammonia is the most important and well-established process used to abate nitrogen oxides (NOx = NO + NO2) from stationary sources1-3 and diesel cars4-5. NOx are continuously reduced by NH3 on V2O5-TiO2 oxide with addition of either WO3 or MoO3 2,6. Due to narrow operating temperature window of the commercial catalysts, anatase to rutile phase transformation under reaction conditions, and vanadium pentoxide toxicity, there is a clear trend to replace V-based NH3-SCR catalysts with these zeolite-based7. A large number of H-form and metal exchanged zeolites were tested for NH3-SCR8-14. Among them iron modified zeolites, in particular FeBEA15-19 and Fe-ZSM-510,

20-30

, have been reported to be highly active catalysts for the process.

However, between different types of Fe species in Fe modified zeolites, only Fe3+ ions are the active sites for the NH3-SCR22,26,31. Depending on the zeolite type various Fe species can be formed, such as isolated and/or binuclear Fe ions at ion exchange positions, small oligonuclear FexOy clusters inside and/or outside the pores, and large Fe2O3 particles on the external surface21,25,32, which contribute to different catalytic behaviour18, 19. Schwidder et al.25 correlated the activity with the concentration of Fe sites determined by UV-vis spectroscopy. They found that not only mononuclear Fe ions are active for the SCR of NO by isobutene or by ammonia, but confirmed that also oligomers influence the overall activity. In the case of NH3-SCR, they concluded that oligomeric Fe-oxo species contribute to the activity with high efficiency. Considerable effort has been devoted to the understanding of the mechanism of the SCR of NO by ammonia over Fe modified zeolites. Long and Yang10,26, basing on the comprehensive FT-IR investigations, proposed that both ammonium ions as well as NO and NO2 adsorbed species play an important role in the SCR reaction on the iron-modified ZSM-5. Delahay et al.31 have reported that in the catalytic cycle the Fe(II) species are oxidized by O2 to Fe(III) oxo/hydroxo species and then consumed in the NOx intermediates production. Finally, NOx species react with ammonia to form water and nitrogen with concomitant reduction of Fe(III) to Fe(II). Summing up, there is an agreement that the Fe sites responsible for the activity in SCR and N2O decomposition are ionexchanged Fe-oxo species33. The oligomeric Fe species can play the role of active centres for NOx or N2O conversion only at high temperatures21,31. The high catalytic activity in SCR of NO by NH3 was obtained for the FeHBEA containing iron as pseudo-tetrahedral Fe(III) species18,19. In this work the Fe-exchanged BEA zeolites were prepared by (i) standard ion-exchange method with aqueous Fe(NO3)3 solution and (ii) post-synthesis procedure. A two-step post synthesis method consisted first of creating vacant T-sites by dealumination of a BEA zeolite with nitric acid and then impregnating the resulting highly siliceous BEA/DeAl zeolite with aqueous Fe(NO3)3 solution, used as Fe3+ ions precursor. Such procedure leads to the incorporation of iron into the framework of a BEA 2 ACS Paragon Plus Environment

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zeolite, i.e. into the previously created vacant T-sites. The resulting samples were characterized at both the macroscopic and molecular levels. The catalytic activity of zeolite catalysts in the standard and fast NH3-SCR together with an influence of the speciation of iron introduced into zeolites were discussed. Moreover, the role of acidity in NH3-SCR processes was investigated. 2. Experimental 2.1. Catalyst preparation The Fe-BEA zeolite was obtained by liquid ion exchange method. Parent ammonium form of zeolite NH4BEA (2 g) supplied by Zeolyst (CP814C) was dispersed in 200 ml Fe(NO3)3·9H2O solution (pH =3.5) containing 5⋅10-4 M of Fe3+ ions at 60 0C for 48 h. The resulting sample was filtrated, washed with distilled water and dried in air at RT for 48 h. Fe-BEA/DeAl zeolites were prepared by the post synthesis procedure reported earlier34,35. In the first step, the aluminium atoms were removed from the structure of BEA zeolite (Zeolyst, CP814C) by the treatment with 13 M HNO3 under stirring (5h, 60 0C). The resulting BEA/DeAl zeolite (2 g) was filtered and washed with distilled water, and next introduced to 200 ml aqueous solution of Fe(NO3)3·9H2O containing 5⋅10-4 M of Fe3+ ions (pH =3.5). The suspension was stirred at RT for 48 h, next temperature was increased to 70 0C until water was completely evaporated. The iron content, determined by ICP analysis, was equal 0.8 wt. % for Fe-BEA and 0.9 wt. % for Fe-BEA/DeAl. All obtained samples, both protonic and iron-forms, were calcined at 450 0C for 2 hours. 2.2. Catalyst characterization 2.2.1. Chemical composition, structural and textural data The Si, Al, and Fe concentrations in the studied materials were determined by the ICP OES spectroscopy with Optima 2100DV (PerkinElmer) instrument. The powder X-ray diffraction (XRD) measurements were carried out using X’Pert Pro Philips (PANalytical Cubix diffractometer), with CuKα radiation, λ=1.5406 Å and a graphite monochromator in the 2θ angle range of 5-40°. X-ray powder patterns were used for structural identification of the relative crystallinity value (%Cryst) for all the zeolites. The determination of the relative crystallinity value was based on the intensity of the peaks in the range between 100 to 600. The BET surface area and the pore volume of the samples were determined by N2 sorption at -196 °C using a 3Flex (Micromeritics) automated gas adsorption system. Prior to the analysis, the samples were degassed under vacuum at 250 °C for 24 h. The specific surface area (SBET) was determined using BET (Brunauer-Emmett-Teller) model according to Rouquerol recommendations36. The



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micropore volume and specific surface area of micropores were calculated using the Harkins-Jura model (t-plot analysis). 2.2.2. IR spectroscopic characterization Prior to FTIR studies, the materials were pressed into the form of self-supporting wafers (ca. 5-10 mg/cm2) and pre-treated in situ in an IR cell at 450 oC under vacuum conditions for 1 h. Spectra were recorded with a Bruker Tensor 27 spectrometer equipped with a MCT detector with the spectral resolution of 2 cm−1. All the spectra presented in this work were normalized to 10 mg of a sample. The NO (Linde Gas 99.5%), CO (Linde Gas Poland 99.5%) and NH3 (PRAXAIR, ≥99.8%) were used as adsorbates. Prior to adsorption nitric oxide (Linde Gas 99.5%), was purified by the freeze–pump– thaw technique. 2.2.3. XPS studies The X-ray photoelectron spectra (XPS) were measured on a Prevac photoelectron spectrometer equipped with a hemispherical VG SCIENTA R3000 analyser. The photoelectron spectra were measured using a monochromatized aluminium Al Kα source (E = 1486.6 eV,11 kV, 17 mA) and a lowenergy electron flood gun (FS40A-PS) to compensate the charge on the surface of nonconductive samples. The powder samples were pressed into indium foil and mounted on a dedicated holder then UHV evacuated. During the measurements, the base pressure in the analysis chamber was 5×10-9 mbar. The area of the sample analysis was approximately 3 mm2. The binding energy were charge-corrected to the carbon C 1s peak at 284.6 eV. Deconvolution of the Fe 2p3/2 peak of the catalyst was performed by fitting a Gaussian–Lorentzian (GL) function provided through the CasaXPS software. The Gaussian–Lorentzian ratio was fixed at 30, i.e. 70% Gaussian and 30% Lorentzian. 2.2.4. 57Fe Mössbauer studies Mössbauer transmission measurements were performed using the RENON MsAa-3 spectrometer equipped with the LND Kr-filled proportional detector and He-Ne laser based interferometer used to calibrate a velocity scale. A commercial 57Co(Rh) source kept at room temperature was applied for 14.41-keV resonant transition in

57

Fe. Absorber thickness was amounted to 100 mg/cm2 for both

samples. Absorbers were kept at room temperature during spectra accumulation. 2.2.5. EPR studies Before EPR studies the samples were activated at 450 oC for 3 h under dynamic vacuum (10-4 Pa). The samples were heated with the heating rate 3 oC/min. Prior to adsorption nitric oxide (Linde Gas 99.5%), was purified by the freeze–pump–thaw technique, and the NO adsorption (5 Tr) was carried out at room temperature. After half an hour the samples were softly evacuated under dynamic vacuum (5x10-3 Pa) for 15 minutes. The EPR spectra were collected with Bruker ELEXSYS E500 4 ACS Paragon Plus Environment

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spectrometer equipped with the Xepr data system for spectra acquisition and manipulation, operating at X-band (9.5 GHz), modulation frequency 100 kHz, modulation amplitude 0.2 mT and with microwave power 2.0 mW. Measurements for each sample were carried out at room temperature and at -196 oC (liquid nitrogen). The g values of the iron centres were calculated by Xepr software, whereas a simulation procedure was used to determine the EPR parameters of NO containing species (EPRSIM32)37. 2.3. Catalytic tests The selective catalytic reduction of NOx (NO or NO/NO2) by NH3 was studied in a catalytic microflow reactor operating at atmospheric pressure. An aliquot of the catalyst (24 mg) was activated in situ at 450 °C for 1 h under a flow of O2/He (20/80, v/v) and then cooled to room temperature. The following compositions of the gas mixture for: (i) NH3-SCR of NO: [NO] = 0.1 vol.%, [NH3] = 0.1 vol.%, [H2O] = 3.5 vol.%, [O2] = 8.0 vol.%, (ii) NH3-SCR of NOx: [NO] = 0.0542 vol.%, [NO2] = 0.0458 vol.%, [NH3] = 0.1 vol.%, [H2O] = 3.5 vol.%, [O2] = 8.0 vol.%, diluted in pure helium was used. The weight hourly space velocity (WHSV) was about 250 l⋅g-1⋅h-1. The SCR was carried out on programmed temperature from 200 to 550 °C with the heating rate 6°C⋅min-1. In N2O decomposition the following conditions were applied: 75 mg of sample, [N2O] = 1000 ppm in He, total flowrate : 75 ml/min. The reactants and products were analysed by a quadruple mass spectrometer (Pfeiffer Omnistar) equipped with Channeltron and Faraday detectors (0-200 amu) following these characteristic masses: NO (30), N2 (14, 28), N2O (28, 30, 44), NH3 (15, 17, 18), O2 (16,32) and H2O (17, 18). 3. Results and discussion 3.1. Structural and textural characteristics Figure 1 presents X-ray diffraction patterns of protonic forms of the native BEA and the dealuminated BEA/DeAl as well as their Fe-analogues. The XRD patterns of all the samples are similar in their intensities and typical of BEA zeolite structure. Dealumination procedure did not perturb the crystallinity of BEA/DeAl zeolite (Table 1), there is no XRD evidence for extra lattice crystalline phase or long-range zeolite amorphization. The diffractograms recorded for Fe-zeolites showed that introduction of iron moieties into zeolite did not induce any significant changes in zeolitic structure. The absence of reflections originating from extraframework iron oxo and/or hydroxo species points to a good dispersion of iron moieties independently from the method of iron deposition. Tentatively, the crystalline domains, if they exist, might not provide the possible reflections due to low Fe concentration. Dealuminated BEA/DeAl zeolite and both Fe-analogues are characterized by BET specific surface area and micropore volume typical of the BEA zeolite structure, pointing to the preservation of textural properties of BEA upon aluminium extraction procedure and implementation



of iron moieties. The Fe-species deposition slightly reduced the micropore volume values suggesting the location of iron species inside zeolitic micropores. Table 1. The chemical composition obtained from ICP analysis. Relative crystallinity values (% Cryst) derived from XRD and textural parameters of native zeolites and their Fe-forms.

Sample name

Si/Al

Fe/Al

Fe ICP -1 [µ µmol·g ]

% Cryst

Stotal [m2·g-1]

Smeso [m2·g-1]

Vmicro [cm3·g-1]

BEA BEA/DeAl Fe-BEA Fe-BEA/DeAl

22 250 22 500

0.20 6.74

135 155

100 89 98 88

567 609 515 559

44 55 40 52

0.19 0.23 0.17 0.19

4000 2000

BEA 0

5

10

15

20

25

30

35

40

4000

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2000

Fe-BEA

0

5

10

15

20

25

30

35

40

4000

BEA/DeAl

2000 0

5

10

15

20

25

30

35

40

4000

Fe-BEA/DeAl

2000 0

5

10

15

20 25 o 2Θ /

30

35

40

Fig. 1. XRD patterns recorded at room temperature of parent and Fe-forms of zeolites BEA.

3.2. Quantification of acidic properties The Fe(III) ions in aqueous solutions are present in broad spectrum of complexes: from single-ion aqua-complexes [Fe(H2O)6]3+ via [Fe(H2O)5(OH)]2+ and [Fe(H2O)4(OH)2]+ to dimeric forms present as oxo-bridged [Fe2(µ-O)(H2O)8]4+ and di-oxo-bridged [Fe2(µ-O)2(H2O)8]4+ complexes. Thus coexistence of isolated Fe ions at various oxidation states, oxo- and hydroxo-complexes and iron oxide species in zeolites is widely reported. The treatment of zeolite in a vacuum involves auto-reduction of iron cations, that are present at exchange sites, according to the following equations: - iron oxo-cationic species FeEX/OX 6 ACS Paragon Plus Environment

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[OH-Fe-O-Fe-OH]2+→ 2 Fe2+ + O2 +H2O

(eq.1)

- iron isolated species FeEX/IS 2 [OH-Fe]2+→ 2 Fe2+ + 1/2 O2 + H2O

(eq.2)

Both processes are however significantly limited for previously calcined samples. For FexOy species, like small oxide clusters or large aggregate iron oxides, no reduction phenomenon is expected. Ammonia molecule is the probe widely used for quantification of both Brønsted and Lewis sites in solid acid catalysts38-40. Operating with the NH3‑SCR reactant molecule gives the possibility to refer the acidity measurement to the catalytic behaviour. Ligation of ammonia to acid centres leads to the appearance of the 1465-1450 cm-1 ammonium ion band and 1600-1625 cm-1 band assigned to NH3L (L=Lewis) acid sites adducts. The concentrations of both Brønsted (NH4+) and Lewis (NH3L) acid sites are calculated on the bases of the maximum intensities of the NH4+ and NH3L bands as well as the corresponding values of the absorption coefficients. The same methodology was employed in the present work with applying the previously determined values of the absorption coefficients39. It should be underlined that the ammonia sorption provides the overall picture of acidity but the concentration of Lewis sites cannot be accurately referred to a respective type of Fe-site. One of the reasons is the ligation of various number of ammonia molecules to iron sites of different types. Table. 2. Concentrations of Fe and Al determined by ICP analysis. The concentrations of Brønsted (B) and Lewis (L) acid sites determined in quantitative IR studies of ammonia sorption in investigated zeolites.

Sample name BEA Fe-BEA BEA/DeAl Fe-BEA/DeAl

Fe ICP [µ µmol·g-1] 135 155

Al ICP [µ µmol·g-1] 675 670 28 23

B [µ µmol·g-1] 400 165 45 102

L [µ µmol·g-1] 225 345 0 50

Table 2 gathers the concentrations of Brønsted and Lewis acid sites determined in IR quantitative measurements of ammonia sorption in both Fe-modified BEA zeolites and their H-forms. Introduction of iron cations in ion exchange positions in zeolite BEA was manifested by the significant reduction of the Brønsted sites amount, while the population of Lewis acidic centres was enhanced. At low Fe loadings, the formation of the highly dispersed isolated FeO+ species in extraframework positions was suggested, however some quantity of Fe2+ ions can be also expected (eq. 2). The species of both types act as Lewis sites. In Fe-BEA the concentration of Lewis sites newly generated by iron deposition is 120 μmol⋅g-1, in line with ICP analysis. It points to a high dispersion of iron(III) extraframework species. Extraframework cationic species are highly populated in zeolites of relatively low Si/Al (30)41-43. Therefore the quantity of FexOy particles is believed to be very low in comparison with



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iron species present in the ion-exchanged positions. The opposite situation is observed for dealuminated BEA/DeAl zeolite. The extraction of aluminium atoms from the zeolitic structure remarkably reduced its ion-exchange ability (BEA vs. BEA/DeAl), however, after iron species implementation bigger amount of Brønsted acid sites were detected by ammonia (BEA/DeAl vs. FeBEA/DeAl). Thus Fe-BEA/DeAl prepared by two-step post synthesis method is believed to accommodate framework pseudo-tetrahedral Fe(III) species having significant impact to the global Brønsted acidity35,44. The formation of the bridging ≡Fe3+_O(H)-Si≡ hydroxyls was also verified in the IR spectra in the region of the O-H stretching vibrations (Fig. 1 SI). Leaching of BEA zeolite (spectrum a) by nitric acid solution leads to the extraction of framework aluminum and creation of vacant Tatom sites what is confirmed by the disappearing of the 3604 cm-1 band of the Si(OH)Al hydroxyls and development of a broad band of the silanol nests at 3520 cm-1 in the spectrum of BEA/DeAl zeolite (spectrum b). Upon impregnation of BEA/DeAl with aqueous solution of Fe(NO3)3, this band vanishes pointing to the involvement of Si–OH groups in silanol nests in the reaction with Fe3+ ions (spectrum c). The incorporation of Fe3+ cations into the framework of BEA/DeAl is directly evidenced by the appearance of an IR band at 3636 cm-1 attributed to the ≡Fe3+_O(H)-Si≡ acidic sites. The Fe species not involved into creation of protonic acidity in Fe-BEA/DeAl (50 μmol⋅g-1) bear the form of extraframework Lewis acid sites, i.e. oxides of Fe(II) and/or Fe(III). 3.3. Speciation of iron species 3.3.1. XPS investigations X-ray photoelectron spectroscopy is a versatile surface analysis tool widely used for qualitative evaluation of iron oxidation states. The iron 2p core levels are split into 2p3/2 and 2p1/2 doublets due to the spin-orbit coupling. The Fe2p3/2 and Fe2p1/2 peaks were found to be centered at ca. 712.7 and 725.9 eV (Fig. 2). Deconvolution of Fe 2p3/2 peaks revealed the presence of the band at 712.3 and 714.2 eV. Both components were attributed to the Fe(III) species45. It is worth noticing, that Fe 2p3/2 lines of the doublets on the high BE side of the Fe 2p photoelectron peaks (714.2 and 714.5 eV) exhibit BE higher than 712 eV. Many metal cations embedded in zeolite matrix were found to exhibit higher BE comparing with their BE in oxides45 due to their specific interactions with the zeolite framework. In our case the higher BE for Fe(III) might also reflect the occurrence of highly isolated species. Taking into account the information derived from XPS investigations and

57

Fe Mössbauer

spectroscopy (further discussed in Section 3.3.2), the Fe(III) species in pseudo-tetrahedral surroundings can be identified in Fe-BEA/DeAl as the main component of the Fe2p3/2 peak at 712.3 eV. For Fe-BEA zeolite the isolated Fe(III) species in the ion-exchanged positions are present as the only species. These data are in line with the results delivered by IR investigations where for Fe-


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BEA/DeAl the enhanced Brønsted acidity was attributed to Fe(III) ions grafted into framework Tpositons, i.e. the ≡Fe3+_O(H)-Si≡ acidic sites. Fe 2p3/2

Fe 2p1/2

712.3 714.2

counts / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe 2p1/2

Fe 2p3/2

720

715

710

FeBEA 714.5

712.5

FeBEA/DeAl 730 720 710 binding energy / eV

720

715

710

Fig. 2. XPS spectra of Fe-BEA and Fe-BEA/DeAl.

3.3.2. 57Fe Mössbauer and EPR studies The complex nature of Fe(III) species but also their high reactivity are significant limitations for their evaluation both in qualitative and quantitative manner. For instance, the unequivocal attribution of IR bands appearing upon sorption of NO and CO to Fe(III) sites still remains a major issue of debate. Thus the speciation of iron in zeolites studied was assessed in 57Fe Mössbauer and EPR studies. The

57

Fe Mössbauer spectra of zeolites were collected at room temperature and their fitted

parameters are shown in Figure 3 and listed in Table 3. For high-spin Fe complexes ligated to oxygencontaining ligands, i.e. those occurring in zeolites, the isomer shift values (IS) for Fe(III) are lower compared to those for Fe(II). The values of the isomer shift indicate that iron in studied samples occurs in the form of high spin trivalent ions. There are no traces of Fe(II). Value of isomer shift being higher than 0.3 mm/s indicates that in Fe-BEA sample all iron atoms are located in the environment being close to octahedral coordination symmetry and this value is typical for the nanoparticles of iron oxide-like species46,47. High octahedral coordination symmetry can be provoked by the ligation of water molecules to extraframework iron species. Contrarily, in Fe-BEA/DeAl sample about 30 at.% of iron atoms with IS = 0.18 mm/s is tetrahedrally coordinated. A correlation between line width and electric quadrupole splitting indicates that distributions of the quadrupole splitting occur in these materials48. Average value of quadrupole splitting is larger in Fe-BEA/DeAl sample in comparison with Fe-BEA. Thus this sample has more surface iron atoms interacting with the walls of the zeolite channel. This is an indication that iron species are more dispersed for the Fe-BEA/DeAl sample in comparison with Fe-BEA. We believed that



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these tetrahedral Fe(III) species are situated in vacant T-atom sites producing the bridging ≡Fe3+_O(H)-Si≡ hydroxyls in BEA matrix, in line with literature reports49. 100

100 99 98 97

3+

Fe-BEA

-4 -3 -2 -1 0 1 2 Velocity [mm/s] Fig. 3.

57

Fe (Td) 3+ Fe (Oh2) 3+ Fe (Oh3)

3+

Fe (Oh1) 3+ Fe (Oh2) 3+ Fe (Oh3)

3

4

99

Fe-BEA/DeAl

-4 -3 -2 -1 0 1 2 Velocity [mm/s]

3

4

Fe Mössbauer spectra recorded for Fe-BEA and Fe-BEA/DeAl after evacuation at 450 oC for 1 h. Spectra

recorded at room temperature.

Correspondingly, the information on nature of iron species in calcined samples was derived from EPR studies. Before EPR measurements the samples were treated under vacuum, thus the water molecules previously attached to iron species were effectively removed. Figure 4 presents the EPR spectra of Fe-BEA and Fe-BEA/DeAl registered at RT and -196 oC. The observed signals are due to Fe3+ ions surrounded by oxygen ligand, with high spin configuration and ground state 6S5/2 therefore the main features in EPR spectra are determined by the zero field splitting. In the all spectra the signal at geff ≈ 4.3 (denoted FeA) is present, and was assigned to oxygen surrounded Fe3+ ions in the strong rhombically distorted tetrahedral symmetry localized in framework and extraframework position50-56 or in small (nanometric) oxides species57. This signal arises from transition within the middle Kramers doublet. In the case of Fe-BEA two signals at geff ≈ 5.8 (FeB1) and ~ 6.3 (FeB2) are clearly visible particularly at spectra registered at -196 oC (Fig. 4B). Similar but not resolved line at geff ≈ 6 (FeB) can be identified in the EPR spectrum of Fe-BEA/DeAl. These signals are due to transition within the lowest Kramer doublet and can be qualified as easily accessible (vide infra) and therefore reactive penta- or hexacoordinated Fe3+ ions in extraframework positions exhibiting axially distorted higher than tetrahedral symmetry (possibly octahedral with strong tetrahedral distortion)23,25,55,57. The observed line is a perpendicular component of highly anisotropic spectrum with g⊥ ~ 6 (not resolved gx and gy) whereas parallel g component (g|| ≈ ge ≈ 2) is not visible, due to broadening of the line52,55,59,60. Apart from above described signals, the EPR spectrum of Fe-BEA/DeAl contains two additional lines in the region of g > 4.3, not present in the case of Fe-BEA. These signals at geff ≈ 5 (FeC) and geff≈ 8.3 (FeD) might be due to another type of distortion of the originally tetrahedral centre as suggested by Bordiga et al.60. The finding is in line with Mössbauer results which indicate presence of tetrahedrally coordinated Fe3+ in the Fe-BEA/DeAl zeolite. Additionally, in high magnetic field 10 ACS Paragon Plus Environment

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region, two main signals are visible, characterized by geff ≈ 2.3 (FeE) and giso ≈ 2.0 (FeF). The intensity of very broad signal at geff ~ 2.3, clearly visible in spectra registered at RT, decrease when experiments were performed at -196 oC, that means this line does not follow Curie’s law. The phenomenon can be rationalized by assuming mutual magnetic interaction of neighbouring Fe3+ ions and the signal can be assigned to FexOy particles on the external surface of zeolite53,55,56. The g factor of the last signal was determined by spectrum simulation by assuming sharp isotropic line with ΔB=0.5±0.1 mT and giso = 2.0025±0.0005. The line with g~2 is often attributed to isolated Fe(III) ions in high symmetrical octahedral (or tetrahedral) environment60-62. However the very small line width, leads to suggestion that this line is due to lattice defect (Si–O+–Al) generated by thermal treatment56,63. g = 4.3 g = 6.3

g = 2.0

a

b

g = 5.8

Fe-BEA

g∼6 g = 8.3

a

b g ∼5 0

g=2.3

150

300

Fe-BEA/DeAl 450

600

Magnetic Field / mT

Fig. 4. EPR spectra of Fe-BEA and Fe-BEA/DeAl registered at RT (a) and -196 oC(b).



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Table 3. Hyperfine parameters derived from the room-temperature 57Fe Mössbauer spectra for studied Fe-zeolites. The symbol A denotes relative contribution of the particular subspectrum, IS stands for the isomer (total) shift of the particular sub-spectrum versus room temperature α-Fe, QS denotes the absolute value of the quadrupole splitting, while the symbol Г stands for the absorber line width. 57

Sample name

Fe-BEA

Fe-BEA/DeAl

A [%] ± 5%

IS [mm/s]

22 42 36 30 44 26

0.36 0.35 0.35 0.18 0.38 0.39

Fe Mössbauer data QS [mm/s] Г [mm/s] 0.58 0.94 1.35 0.5 0.9 1.9

0.22 0.29 0.40 0.6 0.6 1.1

g factor

EPR data Attribution of Fe species Not visible

[Fe(III)O] , Fe(III)–µO2–Fe(III), Oh

g⊥ ~ 6 g|| ≈ 2

FeB

Fe(III), Td + [Fe(III)O] , Fe(III)–µO2–Fe(III), Oh Fe(III) oligomers, Oh

geff ≈ 5 g⊥ ~ 6 g|| ≈ 2 geff ≈ 2.3

FeC FeB FeE

Attribution of Fe species bare Fe(II) +

12

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3.3.3. IR studies of carbon monoxide sorption As mentioned above, the information on the amount of iron species derived from ammonia adsorption IR studies is not conclusive with the regard to the nature of iron cations. The electron acceptor properties of redox sites are usually evaluated in CO and NO sorption experiments. Thus both probes were employed to investigate the speciation of iron in investigated zeolites. The spectra of CO adsorbed at -100 °C on studied zeolites are presented in Figure 5A. Hydrogen bonding of CO molecules to the ≡Al_O(H)-Si≡ acid groups led to the appearance of the 2177 cm−1 band. The band at 2173 cm-1 frequency can be also easily distinguished in the spectrum of CO adsorbed on Fe-BEA/DeAl zeolite. Usually this band is assigned to CO polarized by the bridging zeolite ≡Fe3+_O(H)-Si≡ hydroxyls for zeolites prepared by two step post synthesis procedure35. Due to the fact that the ≡Fe3+_O(H)-Si≡ groups are slightly less acidic than their ≡Al_O(H)-Si≡ analogues they are represented by the CO band of lower frequency. In the region of the O-H stretching vibrations the latter species are represented by the 3636 cm-1 band (Fig. 1 SI) which is consumed upon CO admission producing the 2173 cm-1 CO band35. The enhanced Brønsted acidity of the Fe-BEA/DeAl over BEA/DeAl (Section 3.2) also evidences the formation of the bridging acidic groups of the ≡Fe3+_O(H)-Si≡ type. Nevertheless, the participation of the ≡Al_O(H)-Si≡ groups in the 2173 cm-1 band cannot be excluded. The other bands originating from the interaction of probe with hydroxyls species are the 2163 and 2155 cm-1 bands typical of CO bonded to silanol groups. The rest of bands characterized by high frequencies were assigned to carbonyls formed by ligation of CO to electron acceptor Lewis sites. The 2199 cm-1 band was ascribed to carbonyls formed with extraframework aluminium (EFAl) species, which were found also in BEA as the result of its lowered thermal stability. The presence of electron acceptor Al-sites was confirmed in quantitative IR studies of ammonia sorption (Section 3.2). The bands at 2190 and 2183 cm-1 were qualified as the iron(II) carbonyls Fe2+(CO). The 2190 cm−1 band was unquestionably assigned to Fe2+(CO) monocarbonyls formed by the exchangeable isolated Fe2+ cations. The lower frequency band at 2183 cm−1 can be attributed to Fe2+ cations issued from iron in oxo-forms. Zecchina et al.50 has assigned the band at 2180 cm-1 to CO adsorption on small iron oxide clusters in H-[Fe]ZSM-5. As mentioned above, the treatment of the catalysts in situ at 450 °C under vacuum involves auto-reduction of iron oxo-cationic species FeEX/OX (eq.1) and iron isolated species FeEX/IS (eq.2) while small oxide clusters or large aggregate iron oxide FexOy are not supposed to be reduced. The mechanism of the SCR of NO by NH3 is believed to involve the Fe2+/Fe3+ redox cycle and the Fe(III)-oxo species are considered as the sites providing the catalytic activity in the reduction of NOx by ammonia. Thus the number of Fe2+ species detected with CO can be related to the easily reduced FeO+ moieties which can deliver high activity in SCR of NO by NH3.



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Page 14 of 30

The Fe(III) species were reported to be no detectable with CO35,44. Thus only the concentrations of Fe2+ cations (Table 4) in the form of the exchangeable cations (Fe2+) and the oxo-forms (Fe2+OXO) were calculated from the maximum intensities of the respective bands (Figure 3) and their absorption coefficients (2.52 cm/µmol)42. The presented spectra were subjected to deconvolution analysis to calculate the concentration of the Fe2+ ions existing in various forms. In both Fe-zeolites ca. 10 % of total Fe amount is detected as Fe(II) species. In Fe-BEA mainly isolated cation are present (Fe2+) while Fe-BEA/DeAl accommodates Lewis species represented by 2183 cm-1 band ascribed to extraframework Fe(II) oxide species, i.e. Fe2+OXO. The large distances between AlO4− tetrahedral (Si/Al=500 for Fe-BEA/DeAl) favour the hydrolysis process, finally resulting in oxide forms with different extents of dispersion. This process occurs at elevated temperatures, i.e. during thermal pretreatment of TMI-exchanged zeolites, and ensures the neutralization of AlO4− tetrahedra by protons while the majority of the TMIs are present in the oxide forms (eqs.1 and 2). However, the Fe2+ species concentration corresponds to the number of Fe(III) species being easily reduced in vacuum conditions. 2173

A

2183 A=0.01 2155

2190

Fe-BEA/DeAl

2183 2177 2163

2199

Fe-BEA 2220

2190

1863

A=0.01

2160

ν/cm-1

B

1837 1819

Fe-BEA/DeAl

1877

1846

Fe-BEA 1950

1875

1800

1725

ν/cm-1

Fig. 5. The maximum intensities of monocarbonyl (A) and mononitrosyl bands (B) formed in Fe-BEA and FeBEA/DeAl.


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Table 4. The concentration of the divalent Fe moieties derived from both ICP analysis (FeICP) and the quantitative IR studies of CO and NO sorption (Fe2+IR).

Sample name Fe-BEA Fe-BEA/DeAl

Fe ICP -1 [μmol g ] 135 155

Fe2+ 10 0

Fe2+IR [μmol g-1] CO Fe2+OXO ΣFe 5 15 10 10

NO 20 12

3.3.4. Interaction of iron species with nitrogen monoxide Sorption of nitrogen monoxide, i.e. NH3‑SCR reactant molecule, also provides valuable information on the status of iron species in zeolites and can allow referring the speciation of iron to the catalytic behaviour of studied materials. Unquestionable advantage of the use of NO as probe is the possibility of the IR quantitative analysis of the Fe2+ cations in zeolite matrix42. Similarly as for CO, the sorption of nitrogen monoxide delivered information on the amount of Fe2+ sites resulted from the treatment of zeolite in vacuum and reduction of Fe(III)-oxo centres with NO. It is a consensus that the mono- and dinitrosyls are formed solely on Fe2+ ions. Some authors44,51 however reported the Fe3+(NO) nitrosyl bands at 1885-1880 cm−1 frequencies. In our case consecutive sorption of NO doses led to the saturation of all Fe2+ cations with NO which was observed as the maximum intensities of Fe2+(NO) mononitrosyls of the 1885-1837 cm−1 bands (Fig. 5B). The IR spectra were recorded immediately upon the NO introduction in order to reduce the extent of the NO transformation into N2 and oxo-compounds of nitrogen in oxidation states +3 and +5. Since, several bands of mononitrosyl species were distinguished for Fe-BEA (1885, 1865 and 1846 cm-1) and FeBEA/DeAl (1863, 1837 and 1810 cm−1) (Fig. 5B), the Fe2+(NO) bands were subjected to deconvolution analysis. The Fe2+ sites concentration (Table 4) was calculated on the basis of the bands representative for maximum intensities of Fe2+(NO) mononitrosyls and their absorption coefficient (13.80 ± 0.06 cm/μmol)42. Both for Fe-BEA and Fe-BEA/DeAl the mononitrosyl Fe2+(NO) concentration was noticeably higher than the amount of Fe2+(CO) monocarbonyls adducts formed upon CO sorption. It suggests high reactivity of Fe(III)-oxo centres towards reduction with NO to Fe2+ cations. Similar conclusion can be derived when NO sorption was followed by EPR spectroscopy. After adsorption of NO in Fe-BEA, a new EPR signal with geff ≈ 4.0 (FeNO-A) appeared (Fig. 6). This signal is due to the interaction of the isolated Fe2+ ions with NO molecules52,64. The Fe2+-NO adduct can actually be considered as [Fe3+-NO-] complex with S=3/2 where a strong antiferromagnetic interaction resulted from an electron transfer from Fe2+ to NO. On the other hand, the signal from isolated Fe2+ cations species was not present in a EPR spectrum of Fe-BEA/DeAl, in line with CO sorption IR studies.



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g=2.3

g ~6

Page 16 of 30

g = 2.0

g ~6 g = 2.0

a

a

g = 4.0

b b Fe-BEA/DeAl

Fe-BEA 0

150

300

450

600

0

Magnetic Field / mT

150

300

450

600

Magnetic Field / mT

Fig. 6. EPR spectra of Fe-BEA and Fe-BEA/DeAl before (a) and after NO adsorption (b) registered at -196 oC.

Besides the FeNO-A signal, three lines assigned to FeNO-B sites are easily noticed in both samples (Figures 6, 7). The FeNO-B signal can be simulated with the following parameters: gxx = 2.094 ± 0.001, ∆B = 0.16 ± 0.01 mT; gyy = 2.058 ± 0.001, ∆B = 0.13 ± 0.01 mT; gzz = 2.015 ± 0.001, ∆B = 0.13 ± 0.01 mT (Fig. 7) and finally be assigned to defect iron site interacting with three NO molecules forming S=1/2 complex (Fe2+-(O/OH)-Fe2+)(NO3) ([Fe(NO)3]9 using the Enemark-Feltham notation65)66. For FeBEA/DeAl sample the signal is similar to that described above but the spectrum is weaker and lines are broader. The simulation points to a superposition of three signals denoted on the figure 7 as C1, C2 and C3. The signals C1 and C2 have roughly the same intensity. Since the C1 was simulated with g values similar to the ones used for simulation of the signal from FeNO-B it was also attributed to [Fe(NO)3] species despite the wider line. The second spectrum is characterized by gxx = 2.067 ± 0.001, ∆B = 0.27 ± 0.01 mT; gyy = 2.038 ± 0.001, ∆B = 0.24 ± 0.01 mT; gzz = 1.990 ± 0.001 ∆B = 0.25 ± 0.01 mT parameters and might be also attributed to (Fe2+-(O/OH)-Fe2+)(NO3) species characterized by slightly different symmetry of the centre. According to our best knowledge the presence of the signal characterized by such parameters has been not reported till now. This weak and ill-defined EPR signal and the absence of the signal at geff ≈ 4.0 indicate negligibly low concentration of Fe2+ ions in FeBEA/DeAl after NO adsorption. The last centre, C3, is identical to previously described FeF signal and is due to lattice defect (Si–O+–Al). Additionally, adsorption of nitric oxide resulted in disappearing of the signals around g ~ 6 in all the cases, pointing to coordinative unsaturation of these iron sites and indicate that these sites determine the catalytic activity.


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gxx = 2.094 gyy = 2.058

gzz = 2.015

Fe-BEA

giso = 2.003

exp sim

Fe-BEA/DeAl gyy = 2.038

gzz = 1.990

exp sim

C-1 315

C-2

C-3 320

gxx = 2.067 325 330 Magnetic Field / mT

335

340

Fig.7. Experimental and simulated EPR spectra of Fe-BEA and Fe-BEA/DeAl after NO adsorption registered o

at -196 C.

In IR studies besides the formation of the isolated Fe2+ cations, the transformation of nitrogen monoxide over catalysts evidenced also the production of nitrate species in both catalysts (Fig. 8). With time, the bands typical of N2O (2224 cm-1) and nitrates (1620 and 1580 cm-1) started to develop more significantly for Fe-BEA material (Fig. 6). The NO+ ions are also observed as the product of the transformation of NO. Typically, nitrosonium ion in zeolites is detected at 2135 cm-1 35. Indeed, the band at the same frequency was detected for zeolite Fe-BEA. For Fe-BEA/DeAl a negligible intensity band of NO+ ion can be found at 2175 cm-1 pointing to the higher basicity of the oxygen bridging Fe and Si in ≡Fe3+_O(H)-Si≡ groups than their ≡Al_O(H)-Si≡ analogues. Summing up, the diverse population of framework Al provokes the differentiation in Fe dispersion in Fe-BEA and Fe-BEA/DeAl. In both zeolites the mononuclear Fe(III) species are present. In Fe-BEA such moieties are solely of the extraframework nature. In contrast, for Fe-BEA/DeAl noticeable amount of Fe(III) is believed to be grafted in zeolite framework while the non-grafted species can be considered as the extraframework sites. High reducibility of Fe(III) oxo-species was confirmed in EPR and IR studies of NO adsorption. As revealed by IR spectroscopy, the higher amounts of nitrate species were formed in Fe-BEA, i.e. zeolite accommodating extraframework mononuclear FeO+ entities as the major species. This Fe2+/Fe3+ redox cycle is crucial for the mechanism of the SCR of NO by NH3 .



+

2224

NO

1620

2175 2200 2100 2000

1675

1740

2135

1580

2224

Fe-BEA Fe-BEA/DeAl 2200

2000

1800

1600 ν/cm-1

Fig. 8. IR spectra (nitrosyl stretching region) of Fe-zeolites collected at RT after 30 min. contact time with adsorbed NO.

3.4. Catalytic performance The obtained zeolite materials were tested as catalysts for SCR of NO with ammonia into N2 and H2O according to the following main reaction (eq. 3)51: 4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O standard SCR

(eq. 3)

Nitrogen is desired product of this process, while also the formation of N2O is possible according to the reaction (eq. 4): 4 NO + 4 NH3 + 3 O2→ 4 N2O + 6 H2O 100

NO NH3

50

N2O

25

75 50 25 0

300

400

500 0

Temperature [ C]

Conversion [%]

Fe-BEA

75

100

Fe-BEA/DeAl

75

100 75

NO NH3

50

50

N2O

25 0 200

25

N2O production [ppm]

Conversion [%]

100

0 200

(eq. 4)

N 2O production [ppm]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

0 300 400 500 0 Temperature [ C]

Fig. 9. The results of activity tests for NH3-SCR of NO (24 mg of the catalyst, [NO] = 0.1 vol.%, [NH3] = 0.1 vol.%, [H2O] = 3.5 vol.%, [O2] = 8.0 vol.%, WHSV = 250 l⋅g-1⋅h-1) performed over Fe-catalysts. The dotted lines represent the conversion of NO for the native BEA and BEA/DeAl.

The results of the catalytic studies performed on Fe-BEA and Fe-BEA/DeAl zeolites and their protonic analogues are presented in Fig. 9. The reference zeolites without iron were not catalytically active in the NO conversion up to 450 oC, at higher temperature NO conversion did not exceed 20% in both


Page 19 of 30

cases. Deposition of iron(III) species in BEA and BEA/DeAl resulted in their activation in NH3-SCR process. The data clearly show the higher activity of Fe-BEA over Fe-BEA/DeAl. The activity of Fe-BEA zeolite is characterized by a complete NO conversion at 450°C. Only small amount of N2O was detected in the temperature range of 250-400°C and it was reduced in the higher temperature range. On the other side, lower catalytic activity was reported over Fe-BEA/DeAl with only 83% NO conversion at 550°C. The NH3 conversion curves were close to the NO conversion curves for all catalysts, providing the excellent selectivity as shown in Eq. 3.

Fe-BEA

75

NO NH3

50

NO2

75 50

NOx

25 0 200

N2 O

25 0

300

400

500 0

Temperature [ C]

100 Conversion [%]

100

100

75

75

Fe-BEA/DeAl 50

NO NH3 NO2

25 0 200

NOx N2O

300

400

50 25


Conversion [%]

100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

500 0

Temperature [ C]

Fig. 10. The results of activity tests for NH3-SCR of NO/NO2 (24 mg of the catalyst, [NO] = 0.0542 vol.%, [NO2] = 0.0458%, [NH3] = 0.1 vol.%, [H2O] = 3.5 vol.%, [O2] = 8.0 vol.%, WHSV = 250 l⋅g-1⋅h-1) performed over Fecatalysts.

The obtained samples have been also studied as catalysts for the fast NH3-SCR of NOx (eq. 5). NO + NO2 + 2 NH3→ 2 N2 + 3 H2O fast SCR

(eq. 5)

Figure 10 presents the conversions of NOx, NO, NO2, and NH3 and the formation of N2O in the fast SCR reaction with NO2/NO = 0.85 over iron-forms of zeolites BEA and BEA/DeAl. The presence of NO2 in the feed significantly increased the NOx conversions for tested samples. However, even in fast SCR, the Fe-BEA catalysts still showed higher NOx conversion than other system. The formation of N2O increased for both Fe-BEA and Fe-BEA/DeAl. The Fe-BEA gave the highest N2O production with a maximum concentration of about 22 ppm at around 341 °C (N2O/N2 selectivity = 2.3%/97.7%).



100 Conversion [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

75

Fe-BEA Fe-BEA/DeAl

50 25 0 350

400

450

500

550

o

Temperature [ C]

Fig. 11. The N2O conversion in N2O decomposition on Fe-BEA and Fe-BEA/DeAl (75 mg of the catalyst, [N2O] = 1000 ppm in He, total flowrate : 75 ml/min.).

The N2O decomposition catalytic studies were also completed (Fig. 11). The reaction started at about 350-370 oC and N2O conversion increased with reaction temperature. The BEA zeolites without iron species dispersed delivered poor catalytic activity: the N2O conversion was found to be below 8% in the whole temperature range. The Fe-BEA catalyst offered significantly higher catalytic activity in the process of N2O decomposition: above 60% of N2O conversion was achieved at 450 oC while at about 525 oC the N2O conversion reached above 95 %. Dealuminated Fe-BEA/DeAl catalyst was noticeably less active than Fe-BEA in N2O decomposition: the N2O conversion reached 40 % at temperature as high as 550 oC. 4. Discussion The tailoring of zeolitic catalyst activity requires to take into account several factors, including: (i) type of zeolite structure, (ii) speciation and amount of introduced iron species into zeolite structure, as well as (iii) zeolite acidity. Tailoring of the properties of the catalysts by the implementation of iron species of desired type and quantity is still open problem. In the literature, there is no general agreement on the method that is most suitable in order to obtain highly active and stable NH3-SCR zeolitic catalysts. What is more, the comparison of the results for different catalysts is problematic as they were obtained from studies performed in different laboratories under different conditions. Nevertheless, there is common agreement that the activity in the NH3-SCR of NO reaction is exclusively connected with the exchanged positively charged Fe species balanced by the zeolite negative framework, and not with any supported Fe61. The examples of iron modified zeolites for NH3-SCR of NO reported in the scientific literature are presented in earlier studies62. In this work an influence of speciation of iron introduced into BEA zeolites on their catalytic performance in both standard and fast NH3-SCR was examined. Iron was introduced into support using ion-exchange and post synthesis techniques. Spectroscopic studies revealed that the speciation of iron sites was strongly dependent on the catalyst preparation. The ion-exchange procedure 20 ACS Paragon Plus Environment

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allowed incorporating iron as cationic species either isolated or bridged through an oxygen atom (iron oxo cations) while the sample prepared by post-synthesis method accommodated a high proportion of iron substituted for aluminium in the zeolite lattice. In the latter case iron oxide clusters deposited on outer surface also were found. According to Schmidt et al.67 an Fe2+/Fe3+ redox cycle is involved in the mechanism of the SCR of NO by NH3, and the Fe(III)-oxo species were suggested to be the active sites in the reduction of NOx by ammonia. The comparison between Fe-BEA and other iron-modified zeolites, i.e. Fe-ZSM-5, Fe-MOR and Fe-ZSM-12 revealed its superior catalytic activity15,68,69. Boroń et al.18,19,69 studied widely Fe-BEA in the NH3-SCR of NO and found that the framework pseudo-tetrahedral ≡Al_O(H)-Si≡ and ≡Fe_O(H)-Si≡ sites have influenced the catalytic properties of the studied catalysts. The catalytic activity of our catalysts in both standard and fast SCR is believed to be also ruled by the concentration of Fe(III)-oxo species. The studies of NO adsorption confirmed the presence of highly reactive Fe(III)-oxo species being able to oxidise NO even at room temperature, as revealed by IR and EPR investigations. The NO oxidation was more efficient for zeolite Fe-BEA which provided extraframework iron oxo-cations as the only species. Zeolite with Fe(III) cations in the framework pseudo-tetrahedral positions was found to be less effective. This different reducibility of Fe(III) centres reflects their different structure. Wichterlová et al.43 have shown that mononuclear Fe(III)oxo species are more pronounced to be reduced to Fe(II) than dinuclear [(Fe(III)–µO2–Fe(III)]2+ complexes. The framework ≡Fe_O(H)-Si≡ sites were found to be the most resistant to the reduction. The high reactivity and thus low stability of FeO+ is supported by DFT calculations70. In our case the lability of oxygen in iron(III) oxo-complexes was determined in the decomposition of N2O. It has been nicely presented in the works of Wichterlova et al. (ref. 43 and references therein) that some information on the nature of the iron active site can be delivered by the comparison of the catalysts activity in both processes because both processes require the opposite Fe redox cycles. The N2O decomposition can be described in simplified terms as follows: N2O + * → N2 + O (eq. 6) 2 O* → O2 (eq. 7) The second step (eq.7) is the determining one therefore the rate of decomposition of N2O is function of the concentration of labile oxygens, thus reducibility of the catalysts. High activity of zeolite FeBEA in N2O decomposition points to accommodation of easily reducible Fe(III) moieties that can be transformed into Fe(II) species balanced by zeolitic framework. In line with the spectroscopic results (XPS, EPR, Mössbauer, and IR studies) the zeolite Fe-BEA/DeAl possesses the pseudo-tetrahedral Fe(III) in framework positions and extraframework FexOy species noticeably less prone to reduction, thus they did not deliver catalytic activity.



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Page 22 of 30

Indeed vital issue in standard and fast SCR is production of N2O (contributing to the greenhouse effect and destruction of the ozone layer). The formation of N2O in the fast SCR was already widely recognized over Fe-ZSM-571,72, but only few studies have investigated the SCR performance and the formation of N2O in fast SCR over Fe-BEA16. The formation of N2O is critical issue for commercial application of catalyst, however, it is worth noting that the diesel exhaust gases contain more than 90% of NO and less than 10% NO2. The main routes were suggested for nitrous oxide formation over Fe-modifies zeolites resulting from: (i) formation of surface species like ammonium nitrate (e.g. NH4NO3) and its subsequent decomposition (eq. 7)71,73: NH4NO3 → N2O + 2H2O

(eq.7)

(ii) reaction of NO2 with intermediate nitrites (eq. 8) (1) (2): NO2--(NH4+)2 + NO2 + 1/2 O2→ … → 2 N2O + 3H2O + 2H+

(eq.8)

and/or (iii) NO2-SCR reaction (eq. 9)71: 2NO2 + 2NH3→ N2 + N2O + 3H2O

(eq. 9)

As evidenced IR studies of ammonia adsorption both zeolites accommodate certain amount of Brønsted acid sites that provide NH4+ ions as active species. Also transformation of nitrogen monoxide over catalysts followed by IR spectroscopy evidenced the formation of nitrate species in significant amounts (Fig. 6). On the basis of this indications, we propose that NH4NO3 was formed and deposited on the catalysts surface, whereas at 250°C its decomposition occurred, resulting in the formation of N2O. In both standard and fast NH3-SCR, Fe-BEA revealed enhanced activity. A large difference was observed between Fe-BEA and Fe-BEA/DeAl in NH3-SCR of NO, while the selectivity towards N2 remained close to 100% for both catalysts. However, the catalytic tests showed the presence of N2O as by-product and the N2O profiles for both zeolites differ significantly. Over Fe-BEA, N2O appeared at around 250°C and with higher concentration than over Fe-BEA/DeAl. The concentration of N2O increases in the fast NH3-SCR, together with enhanced NO2 conversion than the NO conversion. Usually, the ability of the catalysts in N2O production is correlated with the acidic characteristic. Shi et al.74 have reported good activity in fast SCR and higher formation of N2O over hydrothermally aged catalyst, i.e. with lower concentration of Brønsted acid sites, than over fresh one. Contrarily, the Brønsted acid sites were suggested to accelerate decomposition of NH4NO275-77. The presence of Brønsted acid sites strongly accelerated the fast SCR over isolated Fe oxo-sites. Recent studies of Brandenberger et al.78,79 have shown that Brønsted acid sites are not required for high SCR activity but acidity might influence iron dispersion. Shwan et al.80 reported that the storage of ammonia in HBEA zeolite proceeded on two zeolite sites representing weak and strong Brønsted acid sites, while NO only could adsorb on Brønsted acid sites. Furthermore, the oxidation of NH3 and NO, and the NH3-SCR reaction are assumed to proceed with the participation of the Brønsted acid


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sites. Monomeric and dimeric iron cations as well as more clustered iron oxide particles were considered with regard to SCR activity. In low temperature SCR both NH3 and NO adsorb on monomeric iron species which is assumed to deliver the highest catalytic activity. Dimeric iron species were reported to provide the activity for high temperature SCR and NH3-oxidation. Iron particles, Fe2O3, are not active for NH3-SCR but for oxidation of NO. Other researchers confirmed that both Brønsted acid sites and added iron oxides are essential for high performance in NH3-SCR of NO over Fe-ZSM-581 as well as Fe-BEA15,69. Indeed, there is still significant controversy concerning the nature of acidity effect on the catalytic performance. The acid strength of ≡Al_O(H)-Si≡ and ≡Fe_O(H)-Si≡ sites evaluated in low temperature CO sorption studies followed by IR spectroscopy have evidenced significantly lower strength of the latter species67. Also in our studies Fe-BEA/DeAl showed a smaller downshift of the bridging hydroxyls band due to hydrogen bonding of CO molecule (ΔνOH…CO = 289 cm-1) for than Fe-BEA (ΔνOH…CO = 325 cm-1) – spectra not shown. A higher acidity of sample prepared by ion-exchange is due to the presence of ≡Al_O(H)Si≡ groups providing the protonic sites of significantly higher strength. Analysis of the catalytic tests revealed higher catalytic activity of Fe-BEA in both standard and fast SCR. Therefore, it could be concluded that vital population of highly acidic Al-originated sites (Al atoms in the zeolite structure) enhance the catalytic activity. Conclusions Fe-BEA and Fe-BEA/DeAl were found to be active catalysts in both standard and fast NH3-SCR in the presence of water vapour. The catalytic activity was strongly affected by iron speciation. EPR and IR studies revealed the presence of isolated or bridged species through an oxygen atom in zeolite FeBEA. These highly dispersed extraframework iron oxo-sites were found to more easily reduced, thus more catalytically efficient than the iron oxo-sites in tetrahedral framework positions in Fe-BEA/DeAl. Additionally, for such catalyst significantly higher acidity than for Fe-BEA/DeAl was evidenced by IR adsorption of NH3 and CO. The formation of N2O is a major drawback of the addition of NO2 into the feed to enhance the NOx conversion over both catalysts. Both high population and high strength of Brønsted enhanced the catalytic activity. ASSOCIATED CONTENT Supporting Information. IR spectra of studied samples in the region of O-H stretching vibrations revealing the extraction of framework aluminum and creation of vacant T-atom sites in dealuminated zeolite BEA/DeAl and the formation of the bridging ≡Fe3+_O(H)-Si≡ hydroxyls upon impregnation of BEA/DeAl with aqueous solution of Fe(NO3)3.



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Acknowledgements This work was financed by Grant No. 2015/18/E/ST4/00191 from the National Science Centre, Poland. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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References 1. Forzatti, P., Present Status and Perspectives in De-NOx SCR Catalysis. Applied Catalysis A: General 2001, 222, 221-236. 2. Busca, G.; Lietti, L.; Ramis, G.; Berti, F., Chemical and Mechanistic Aspects of the Selective Catalytic Reduction of NOx by Ammonia over Oxide Catalysts: A Review. Applied Catalysis B: Environmental 1998, 18, 1-36. 3. Kapteijn, F.; Rodriguez-Mirasol, J.; Moulijn, J. A., Heterogeneous Catalytic Decomposition of Nitrous Oxide. Applied Catalysis B: Environmental 1996, 9, 25-64. 4. Enderle, C.; Vent, G.; Paule, M. BLUETEC Diesel Technology-Clean, Efficient and Powerful; 0148-7191; SAE Technical Paper: 2008. 5. Grossale, A.; Nova, I.; Tronconi, E., Study of a Fe–zeolite-based System as NH3-SCR Catalyst for Diesel Exhaust Aftertreatment. Catalysis Today 2008, 136, 18-27. 6. Bosch, H.; Janssen, F. Formation and Control of Nitrogen Oxides. Catalysis Today 1988, 2, 369-379. 7. Heck, R. M., Catalytic Abatement of Nitrogen Oxides – Stationary Applications. Catalysis Today 1999, 53, 519-523. 8. Byrne, J. W.; Chen, J. M.; Speronello, B. K., Selective Catalytic Reduction of NOx Using Zeolitic Catalysts for High Temperature Applications. Catalysis Today 1992, 13, 33-42. 9. Feng, X.; Keith Hall, W., FeZSM-5: A Durable SCR Catalyst for NOx Removal from Combustion Streams. Journal of Catalysis 1997, 166, 368-376. 10. Long, R. Q.; Yang, R. T., Catalytic Performance of Fe–ZSM-5 Catalysts for Selective Catalytic Reduction of Nitric Oxide by Ammonia. Journal of Catalysis 1999, 188, 332-339. 11. Sjövall, H.; Olsson, L.; Fridell, E.; Blint, R. J., Selective Catalytic Reduction of NOx with NH3 over Cu-ZSM-5 — The Effect of Changing the Gas Composition. Applied Catalysis B: Environmental 2006, 64, 180-188. 12. Huang, Y.; Cheng, Y.; Lambert, C., Deactivation of Cu/zeolite SCR Catalyst due to Reductive Hydrothermal Aging. SAE International Journal of Fuels and Lubricants 2008, 1, 466-470. 13. Delahay, G.; Mauvezin, M.; Coq, B.; Kieger, S., Selective Catalytic Reduction of Nitrous Oxide by Ammonia on Iron Zeolite Beta Catalysts in an Oxygen Rich Atmosphere: Effect of Iron Contents. Journal of Catalysis 2001, 202, 156-162. 14. Kieger, S.; Delahay, G.; Coq, B., Influence of Co-Cations in the Selective Catalytic Reduction of NO by NH3 over Copper Exchanged Faujasite Zeolites. Applied Catalysis B: Environmental 2000, 25, 19. 15. Frey, A. M.; Mert, S.; Due-Hansen, J.; Fehrmann, R.; Christensen, C. H., Fe-BEA Zeolite Catalysts for NH3-SCR of NOx. Catalysis Letters 2009, 130, 1-8. 16. Balle, P.; Geiger, B.; Kureti, S., Selective Catalytic Reduction of NOx by NH3 on Fe/HBEA Zeolite Catalysts in Oxygen-rich Exhaust. Applied Catalysis B: Environmental 2009, 85, 109-119. 17. Balle, P.; Geiger, B.; Klukowski, D.; Pignatelli, M.; Wohnrau, S.; Menzel, M.; Zirkwa, I.; Brunklaus, G.; Kureti, S., Study of the Selective Catalytic Reduction of NOx on an Efficient Fe/HBEA Zeolite Catalyst for Heavy Duty Diesel Engines. Applied Catalysis B: Environmental 2009, 91, 587-595. 18. Boroń, P.; Chmielarz, L.; Gurgul, J.; Łątka, K.; Gil, B.; Krafft, J.-M.; Dzwigaj, S., The Influence of the Preparation Procedures on the Catalytic Activity of Fe-BEA Zeolites in SCR of NO with Ammonia and N2O Decomposition. Catalysis Today 2014, 235, 210-225. 19. Boroń, P.; Chmielarz, L.; Gurgul, J.; Łątka, K.; Shishido, T.; Krafft, J.-M.; Dzwigaj, S., BEA Zeolite Modified with Iron as Effective Catalyst for N2O Decomposition and Selective Reduction of NO with Ammonia. Applied Catalysis B: Environmental 2013, 138–139, 434-445. 20. Krishna, K.; Seijger, G. B. F.; van den Bleek, C. M.; Makkee, M.; Mul, G.; Calis, H. P. A., Selective Catalytic Reduction of NO with NH3 over Fe-ZSM-5 Catalysts Prepared by Sublimation of FeCl3 at Different Temperatures. Catalysis Letters 2003, 86, 121-132.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

21. Iwasaki, M.; Yamazaki, K.; Banno, K.; Shinjoh, H., Characterization of Fe/ZSM-5 DeNOx Catalysts Prepared by Different Methods: Relationships Between Active Fe Sites and NH3-SCR Performance. Journal of Catalysis 2008, 260, 205-216. 22. Long, R. Q.; Yang, R. T., Superior Fe-ZSM-5 Catalyst for Selective Catalytic Reduction of Nitric Oxide by Ammonia. Journal of the American Chemical Society 1999, 121, 5595-5596. 23. Kumar, M. S.; Schwidder, M.; Grünert, W.; Brückner, A., On the Nature of Different Iron Sites and their Catalytic Role in Fe-ZSM-5 DeNOx catalysts: New Insights by a Combined EPR and UV/VIS Spectroscopic Approach. Journal of Catalysis 2004, 227, 384-397. 24. Schwidder, M.; Santhosh Kumar, M.; Bruckner, A.; Grunert, W., Active Sites for NO Reduction over Fe-ZSM-5 Catalysts. Chemical Communications 2005, 805-807. 25. Schwidder, M.; Kumar, M. S.; Klementiev, K.; Pohl, M. M.; Brückner, A.; Grünert, W., Selective Reduction of NO with Fe-ZSM-5 Catalysts of Low Fe Content: I. Relations Between Active Site Structure and Catalytic Performance. Journal of Catalysis 2005, 231, 314-330. 26. Long, R. Q.; Yang, R. T., Fe-ZSM-5 for Selective Catalytic Reduction of NO with NH3: A Comparative Study of Different Preparation Techniques. Catalysis Letters 2001, 74, 201-205. 27. Sun, Q.; Gao, Z.-X.; Chen, H.-Y.; Sachtler, W. M. H., Reduction of NOx with Ammonia over Fe/MFI: Reaction Mechanism Based on Isotopic Labeling. Journal of Catalysis 2001, 201, 89-99. 28. Qi, G.; Yang, R. T., Ultra-active Fe/ZSM-5 Catalyst for Selective Catalytic Reduction of Nitric Oxide with Ammonia. Applied Catalysis B: Environmental 2005, 60, 13-22. 29. Sun, Q.; Gao, Z.-X.; Wen, B.; Sachtler, W. M. H., Spectroscopic Evidence for a Nitrite Intermediate in the Catalytic Reduction of NOx with Ammonia on Fe/MFI. Catalysis Letters 2002, 78, 1-5. 30. Chen, H.-Y.; Sun, Q.; Wen, B.; Yeom, Y.-H.; Weitz, E.; Sachtler, W. M. H., Reduction over Zeolite-based Catalysts of Nitrogen Oxides in Emissions Containing Excess Oxygen: Unraveling the Reaction Mechanism. Catalysis Today 2004, 96, 1-10. 31. Delahay, G.; Valade, D.; Guzmán-Vargas, A.; Coq, B., Selective Catalytic Reduction of Nitric Oxide with Ammonia on Fe-ZSM-5 Catalysts Prepared by Different Methods. Applied Catalysis B: Environmental 2005, 55, 149-155. 32. Joyner, R.; Stockenhuber, M., Preparation, Characterization, and Performance of Fe−ZSM-5 Catalysts. The Journal of Physical Chemistry B 1999, 103, 5963-5976. 33. Li, J.; Li, S., A DFT Study toward Understanding the High Activity of Fe-Exchanged Zeolites for the “Fast” Selective Catalytic Reduction of Nitrogen Oxides with Ammonia. The Journal of Physical Chemistry C 2008, 112, 16938-16944. 34. Gurgul, J.; Łątka, K.; Hnat, I.; Rynkowski, J.; Dzwigaj, S., Identification of Iron Species in FeSiBEA by DR UV–vis, XPS and Mössbauer Spectroscopy: Influence of Fe Content. Microporous and Mesoporous Materials 2013, 168, 1-6. 35. Hadjiivanov, K.; Ivanova, E.; Kefirov, R.; Janas, J.; Plesniar, A.; Dzwigaj, S.; Che, M., Adsorption Properties of Fe-containing Dealuminated BEA Zeolites as Revealed by FTIR Spectroscopy. Microporous and Mesoporous Materials 2010, 131, 1-12. 36. Rouquerol, J.; Llewellyn, P.; Rouquerol, F. Is the BET Equation Applicable to Microporous Adsorbents? Studies in Surface Science and Catalysis 2007, 160, 49-56. 37. Spałek, T.; Pietrzyk, P.; Sojka, Z., Application of the Genetic Algorithm Joint with the Powell Method to Nonlinear Least-Squares Fitting of Powder EPR Spectra. Journal of Chemical Information and Modeling 2005, 45, 18-29. 38. Barzetti, T.; Selli, E.; Moscotti, D.; Forni, L., Pyridine and Ammonia as Probes for FTIR Analysis of Solid Acid Catalysts. Journal of the Chemical Society, Faraday Transactions 1996, 92, 1401-1407. 39. Van Oers, C. J.; Gora-Marek, K.; Sadowska, K.; Mertens, M.; Meynen, V.; Datka, J.; Cool, P., In Situ IR Spectroscopic Study to Reveal the Impact of the Synthesis Conditions of Zeolite Beta Nanoparticles on the Acidic Properties of the Resulting Zeolite. Chemical Engineering Journal 2014, 237, 372-379. 40. Datka, J.; Kawalek, A.; Gora-Marek, K., Acid Properties of NaKH-Ferrierites of Various Exchange Degrees Studied by IR Spectroscopy. Applied Catalysis a-General 2003, 243 (2), 293-299. 26 ACS Paragon Plus Environment

Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 47 48 49 50 51 52 53 54 55 56 57 58 59 60


41. Gora-Marek, K.; Gil, B.; Datka, J., Quantitative IR Studies of the Concentration of Co2+ and Co3+ Sites in Zeolites CoZSM-5 and CoFER. Applied Catalysis a-General 2009, 353, 117-122. 42. Gora-Marek, K.; Brylewska, K.; Tarach, K. A.; Choi, M., Quantitative Aspects of the Identification of Fe(II) Moieties in ZSM-5 Zeolites with Various Pore Hierarchies. Dalton Transactions 2015, 44, 8031-8040. 43. Sazama, P.; Wichterlová, B.; Tábor, E.; Šťastný, P.; Sathu, N. K.; Sobalík, Z.; Dědeček, J.; Sklenák, Š.; Klein, P.; Vondrová, A., Tailoring of the Structure of Fe-cationic Species in Fe-ZSM-5 by Distribution of Al Atoms in the Framework for N2O Decomposition and NH3-SCR-NOx. Journal of Catalysis 2014, 312, 123-138. 44. Kefirov, R.; Ivanova, E.; Hadjiivanov, K.; Dzwigaj, S.; Che, M., FTIR Characterization of Fe3+–OH Groups in Fe–H–BEA Zeolite: Interaction with CO and NO. Catalysis Letters 2008, 125 (3), 209-214. 45. Hawn, D. D.; DeKoven, B. M., Deconvolution as a Correction for Photoelectron Inelastic Energy Losses in the Core Level XPS Spectra of Iron Oxides. Surface and Interface Analysis 1987, 10, 63-74. 46. Garten, R. L.; Delgass, W. N.; Boudart, M., A Mössbauer Spectroscopic Study of the Reversible Oxidation of Ferrous Ions in Y Zeolite. Journal of Catalysis 1970, 18, 90-107. 47. Kuzmann, E.; Nagy, S.; Vertes, A. IUPAC Recommendations and Technical Reports-Critical Review of Analytical Applications of Mossbauer spectroscopy Illustrated by Mineralogical and Geological Examples. Pure and Applied Chemistry 2003, 75, 801-858. 48. Burns, R. G., Mineral Mössbauer Spectroscopy: Correlations between Chemical Shift and Quadrupole Splitting Parameters. Hyperfine Interactions 1994, 91, 739-745. 49. Hajjar, R.; Millot, Y.; Man, P. P.; Che, M.; Dzwigaj, S., Two Kinds of Framework Al Sites Studied in BEA Zeolite by X-ray Diffraction, Fourier Transform Infrared Spectroscopy, NMR Techniques, and V Probe. The Journal of Physical Chemistry C 2008, 112, 20167-20175. 50. Guglielminotti, E., Spectroscopic Characterization of the Fe/ZrO2 System: 1. CO Adsorption. The Journal of Physical Chemistry 1994, 98, 4884-4891. 51. Madia, G.; Koebel, M.; Elsener, M.; Wokaun, A., Side Reactions in the Selective Catalytic Reduction of NOx with Various NO2 Fractions. Industrial & Engineering Chemistry Research 2002, 41, 4008-4015. 52. Volodin, A. M.; Dubkov, K. A.; Lund, A., Direct ESR Detection of S=3/2 States for Nitrosyl Iron Complexes in FeZSM-5 Zeolites. Chemical Physics Letters 2001, 333, 41-44. 53. Chen, H. Y.; El-Malki, E.-M.; Wang, X.; van Santen, R. A.; Sachtler, W. M. H., Identification of Active Sites and Adsorption Complexes in Fe/MFI Catalysts for NOx Reduction. Journal of Molecular Catalysis A: Chemical 2000, 162, 159-174. 54. Dzwigaj, S.; Stievano, L.; Wagner, F. E.; Che, M., Effect of Preparation and Metal Content on the Introduction of Fe in BEA Zeolite, Studied by DR UV–Vis, EPR and Mössbauer Spectroscopy. Journal of Physics and Chemistry of Solids 2007, 68, 1885-1891. 55. Zhilinskaya, E. A.; Delahay, G.; Mauvezin, M.; Coq, B.; Aboukaïs, A., EPR Investigation of FeExchanged Beta-Zeolites. Langmuir 2003, 19, 3596-3602. 56. Gil, B.; Adamski, A., Complementary Use of IR and EPR Spectroscopies for Characterization of Iron Species in Thermally Treated MFI-type Zeolites. Microporous and Mesoporous Materials 2010, 127, 82-89. 57. Kim, M.-Y.; Lee, K. W.; Park, J.-H.; Shin, C.-H.; Lee, J.; Seo, G., Catalytic Decomposition of Nitrous Oxide over Fe-BEA Zeolites: Essential Components of Iron Active Sites. Korean Journal of Chemical Engineering 2010, 27, 76-82. 58. Volodin, A. M.; Zhidomirov, G. M.; Dubkov, K. A.; Hensen, E. J. M.; van Santen, R. A., Spin Design of Iron Complexes on Fe-ZSM-5 Zeolites. Catalysis Today 2005, 110, 247-254. 59. Schreurs, J. W. H., Low Field Hyperfine Structure in the EPR Spectra of Mn2+ Containing Glasses. The Journal of Chemical Physics 1978, 69, 2151-2156. 60. Bordiga, S.; Buzzoni, R.; Geobaldo, F.; Lamberti, C.; Giamello, E.; Zecchina, A.; Leofanti, G.; Petrini, G.; Tozzola, G.; Vlaic, G., Structure and Reactivity of Framework and Extraframework Iron in 27 ACS Paragon Plus Environment


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Page 28 of 30

Fe-Silicalite as Investigated by Spectroscopic and Physicochemical Methods. Journal of Catalysis 1996, 158, 486-501. 61. Fejes, P.; Nagy, J. B.; Lázár, K.; Halász, J., Heat-treatment of Isomorphously Substituted ZSM-5 (MFI) Zeolites: An ESR and Mössbauer Spectroscopy and Kinetic Study. Applied Catalysis A: General 2000, 190, 117-135. 62. Kucherov, A. V.; Shelef, M., Quantitative Determination of Isolated Fe3+ Cations in FeHZSM-5 Catalysts by ESR. Journal of Catalysis 2000, 195, 106-112. 63. Shih, S., Chemical Reactions of Alkenes and Alkynes with Solid-state Defects on ZSM-5. Journal of Catalysis 1983, 79, 390-395. 64. Fisicaro, P.; Giamello, E.; Berlier, G.; Lamberti, C., Paramagnetic Nitrosyliron Adducts in Pentasilic Zeolites: an EPR Study. Research on Chemical Intermediates 2003, 29, 805-816. 65. Enemark, J. H.; Feltham, R. D., Principles of Structure, Bonding, and Reactivity for Metal Nitrosyl Complexes. Coordination Chemistry Reviews 1974, 13, 339-406. 66. Malykhin, E. S.; Volodin, M. A.; Zhidomirov, M. G., Spin States of Iron-Nitrosyl Adsorption Complexes Formed in Fe-ZSM5 Zeolites. Applied Magnetic Resonance 2008, 33, 153-166. 67. Schmidt, R.; Amiridis, M. D.; Dumesic, J. A.; Zelewski, L. M.; Millman, W. S., In Situ Moessbauer Spectroscopy Studies of Fe-Y Zeolites for the Selective Catalytic Reduction of Nitric Oxide by Ammonia. The Journal of Physical Chemistry 1992, 96, 8142-8149. 68. Pieterse, J. A. Z.; Booneveld, S.; van den Brink, R. W., Evaluation of Fe-zeolite Catalysts Prepared by Different Methods for the Decomposition of N2O. Applied Catalysis B: Environmental 2004, 51, 215-228. 69. Boroń, P.; Chmielarz, L.; Gurgul, J.; Łątka, K.; Gil, B.; Marszałek, B.; Dzwigaj, S., Influence of Iron State and Acidity of Zeolites on the Catalytic Activity of FeHBEA, FeHZSM-5 and FeHMOR in SCR of NO with NH3 and N2O Decomposition. Microporous and Mesoporous Materials 2015, 203, 73-85. 70. Li, G.; Pidko, E. A.; van Santen, R. A.; Li, C.; Hensen, E. J. M., Stability of Extraframework IronContaining Complexes in ZSM-5 Zeolite. The Journal of Physical Chemistry C 2013, 117, 413-426. 71. Iwasaki, M.; Shinjoh, H., A Comparative Study of “Standard”, “Fast” and “NO2” SCR Reactions over Fe/zeolite Catalyst. Applied Catalysis A: General 2010, 390, 71-77. 72. Devadas, M.; Kröcher, O.; Elsener, M.; Wokaun, A.; Söger, N.; Pfeifer, M.; Demel, Y.; Mussmann, L., Influence of NO2 on the Selective Catalytic Reduction of NO with Ammonia over FeZSM5. Applied Catalysis B: Environmental 2006, 67, 187-196. 73. Kamasamudram, K.; Henry, C.; Currier, N.; Yezerets, A., N2O formation and mitigation in diesel aftertreatment systems. SAE International Journal of Engines 2012, 5, 688-698. 74. Shi, X.; Liu, F.; Xie, L.; Shan, W.; He, H., NH3-SCR Performance of Fresh and Hydrothermally Aged Fe-ZSM-5 in Standard and Fast Selective Catalytic Reduction Reactions. Environmental Science & Technology 2013, 47, 3293-3298. 75. Li, M.; Yeom, Y.; Weitz, E.; Sachtler, W. M. H., An Acid Catalyzed Step in the Catalytic Reduction of NOx to N2. Catalysis Letters 2006, 112, 129-132. 76. Schwidder, M.; Santhosh Kumar, M.; Bentrup, U.; Pérez-Ramírez, J.; Brückner, A.; Grünert, W., The Role of Brønsted Acidity in the SCR of NO over Fe-MFI Catalysts. Microporous and Mesoporous Materials 2008, 111, 124-133. 77. Long, R. Q.; Yang, R. T., Reaction Mechanism of Selective Catalytic Reduction of NO with NH3 over Fe-ZSM-5 Catalyst. Journal of Catalysis 2002, 207, 224-231. 78. Brandenberger, S.; Kröcher, O.; Wokaun, A.; Tissler, A.; Althoff, R., The Role of Brønsted Acidity in the Selective Catalytic Reduction of NO with Ammonia over Fe-ZSM-5. Journal of Catalysis 2009, 268, 297-306. 79. Brandenberger, S.; Kröcher, O.; Casapu, M.; Tissler, A.; Althoff, R., Hydrothermal Deactivation of Fe-ZSM-5 Catalysts for the Selective Catalytic Reduction of NO with NH3. Applied Catalysis B: Environmental 2011, 101, 649-659. 80. Shwan, S.; Jansson, J.; Korsgren, J.; Olsson, L.; Skoglundh, M., Kinetic Modeling of H-BEA and Fe-BEA as NH3-SCR Catalysts—Effect of Hydrothermal Treatment. Catalysis Today 2012, 197, 24-37. 28 ACS Paragon Plus Environment

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81. Chen, X.; Li, W.; Schwank, J. W., Reactivity of NH3 over (Fe)/H-ZSM-5 Zeolite: Studies of Temperature-Programmed and Steady-State Reactions. Catalysis Today 2011, 175, 2-11.



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on Zeolites Fe-BEA - ACS Publications - American Chemical Society

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