Standard and Fast Selective Catalytic Reduction of NO with

jp6b05692 1..12Standard and Fast Selective Catalytic Reduction of NO with NH3 on Zeolites Fe-BEA Magdalena Jabonska,† Gerard Delahay,‡ Krzysztof Kruczaa,† Artur Bachowski,§ Karolina A. Tarach,†
Kamila Brylewska,†, Carolina Petitto,‡ and Kinga Gora-Marek*,†
†Faculty of Chemistry, Jagiellonian University in Krakow, 3 Ingardena St., 30-060 Krakow, Poland ‡Institut Charles Gerhardt de Montpellier, ICGM-MACS, 8 rue Ecole Normale, 34296 Montpellier Cedex 5, France §Institute of Physics, Pedagogical University, 2 Podchorzych St., 30-084 Krakow, Poland Faculty of Materials Science and Ceramics, AGH University of Science and Technology in Krakow, 30 Mickiewicz Av., 30-059 Krakow, Poland
*S Supporting Information
ABSTRACT: Two Fe-containing BEA zeolites were prepared by ion-exchange (IE; Fe-BEA) and postsynthesis (PS; Fe- BEA/DeAl) procedures. Similar Fe content (0.8 to 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). 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 the 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 selective catalytic reduction (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. 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 cars.4,5 NOx are continuously reduced by NH3 on V2O5−TiO2 oxide with the addition of either WO3
or MoO3. 2,6 Because of the 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-based.7 A large number of H-form and metal-exchanged zeolites were tested for NH3− SCR.8−14 Among them, iron-modified zeolites, in particular, Fe- BEA15−19 and Fe-ZSM-5,10,20−30 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−SCR. 22,26,31 Depending
on the zeolite type, various Fe species can be formed, such as isolated or binuclear Fe ions at ion-exchange positions, small oligonuclear FexOy clusters inside or outside the pores, and large Fe2O3 particles on the external surface,21,25,32 which contribute to different catalytic behavior.18,19 Schwidder et al.25
correlated the activity with the concentration of Fe sites determined by UV−vis spectroscopy. They not only found that mononuclear Fe ions are active for the selective catalytic reduction (SCR) of NO by isobutene or by ammonia but also confirmed that 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 Yang,10,26 based 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
Received: June 6, 2016 Revised: July 5, 2016 Published: July 6, 2016
Article
pubs.acs.org/JPCC
reduction of Fe(III) to Fe(II). In summary, there is an agreement that the Fe sites responsible for the activity in SCR and N2O decomposition are ion-exchanged Fe-oxo species.33
The oligomeric Fe species can play the role of active centers for NOx or N2O conversion only at high temperatures.21,31 The high catalytic activity in SCR of NO by NH3 was obtained for the FeHBEA containing iron as pseudotetrahedral Fe(III) species.18,19
In this work the Fe-exchanged BEA zeolites were prepared by (i) standard ion-exchange method with aqueous Fe(NO3)3 solution and (ii) postsynthesis procedure. A two-step postsynthesis 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 zeolite, that is, 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 SECTION 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 of Fe(NO3)3·9H2O solution (pH 3.5) containing 5 × 10−4 M of Fe3+ ions at 60 °C 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 previously reported.34,35 In the first step, the aluminum atoms were removed from the structure of BEA zeolite (Zeolyst, CP814C) by the treatment with 13 M HNO3 under stirring (5h, 60 °C). The resulting BEA/DeAl zeolite (2 g) was filtered and washed with distilled water and next introduced to 200 mL of 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 °C until water was completely evaporated. The iron content, determined by inductively coupled plasma (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 °C for 2 h. 2.2. Catalyst Characterization. 2.2.1. Chemical Compo-
sition, 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 of the zeolites. The determination of the relative crystallinity value was based on the intensity of the peaks in the range between 10 and 60°. The Brunauer−Emmett−Teller (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 model according to Rouquerol recommendations.36 The 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 Fourier transform infrared spectroscopy (FTIR) studies, the materials were pressed into the form of self-supporting wafers (ca. 5−10 mg/cm2) and pretreated in situ in an IR cell at 450 °C under vacuum conditions for 1 h. Spectra were recorded with a Bruker Tensor 27 spectrometer equipped with a mercury cadmium telluride (MCT) detector with the spectral resolution of 2 cm−1. All of 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 analyzer. The photoelectron spectra were measured using a mono- chromatized aluminum Al Kα source (E = 1486.6 eV,11 kV, 17 mA) and a low-energy 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 ultra-high vacuum (UHV) evacuated. During the measurements, the base pressure in the analysis chamber was 5 × 10−9 mbar. The area of the sample analysis was ∼3 mm2. The binding energy was 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, that is, 70% Gaussian and 30% Lorentzian.
2.2.4. 57Fe Mossbauer Studies. Mossbauer 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 57Fe. Absorber thickness was amounted to 100 mg/cm2 for both samples. Absorbers were kept at room temperature during spectra accumulation.
2.2.5. Electron Paramagnetic Resonance Studies. Before electron paramagnetic resonance (EPR) studies the samples were activated at 450 °C for 3 h under dynamic vacuum (10−4
Pa). The samples were heated with the heating rate 3 °C/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 (5 × 10−3 Pa) for 15 min. The EPR spectra were collected with Bruker ELEXSYS E500 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 °C (liquid nitrogen). The g values of the iron centers were calculated by Xepr software, whereas a simulation procedure was used to determine the EPR parameters of NO containing species (EPRSIM32).37
The Journal of Physical Chemistry C Article
DOI: 10.1021/acs.jpcc.6b05692 J. Phys. Chem. C 2016, 120, 16831−16842
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 % and (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 were used. The weight hourly space velocity (WHSV) was ∼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 flow rate: 75 mL/min. The reactants and products were analyzed 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 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 of extra lattice crystalline phase or long-range zeolite amorphization. The diffractograms recorded for Fe-zeolites showed that the introduction of iron moieties into zeolite did not induce any significant changes in
zeolitic structure. The absence of reflections originating from extraframework iron oxo or hydroxo species points to a good dispersion of iron moieties independent of 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 aluminum 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.
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)]
present as oxo-bridged [Fe2(μ-O)(H2O)8] 4+ and dioxo-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 autoreduction of iron cations, which are present at exchange sites, according to the following equations Iron oxo-cationic species FeEX/OX
− − − − → + ++ +[OH Fe O Fe OH] 2Fe O H O2 2 2 2 (1)
Iron-isolated species FeEX/IS
O H O2 2 2 2 (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 catalysts.38−40 Operating with the NH3−SCR reactant molecule gives the possibility to refer the acidity measurement to the catalytic behavior. Ligation of ammonia to acid centers 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 basis 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 coefficients.39 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 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 centers 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
Figure 1. XRD patterns recorded at room temperature of parent and Fe-forms of zeolites BEA.
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 (<20), while clustered iron oxides are dominant moieties in Al-poor zeolites (Si/Al > 30).41−43 Therefore, the quantity of FexOy particles is believed to be very low in comparison with iron species present in the ion-exchanged positions. The opposite situation is observed for dealuminated BEA/DeAl zeolite. The extraction of aluminum atoms from the zeolitic structure remarkably reduced its ion-exchange ability (BEA vs BEA/DeAl); however, after iron species implementation a greater number of Brønsted acid sites were detected by ammonia (BEA/DeAl vs Fe-BEA/DeAl). Thus, Fe-BEA/DeAl prepared by two-step post synthesis method is believed to accommodate framework pseudotetrahedral Fe(III) species, having significant impact on the global Brønsted acidity.35,44 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 (Figure 1 SI). Leaching of BEA zeolite (spectrum a) by nitric acid solution leads to the extraction of framework aluminum and creation of vacant T atom sites, which is confirmed by the disappearance of the 3604 cm−1 band of the Si(OH)Al hydroxyls and the 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 in creation of protonic acidity in Fe-BEA/DeAl (50 μmol·g−1) bear the form of extraframework Lewis acid sites, that is, oxides of Fe(II) or Fe(III). 3.3. Speciation of Iron Species. 3.3.1. XPS Investiga-
tions. 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 Fe 2p3/2 and Fe 2p1/2 peaks were found to be centered at ca. 712.7 and 725.9 eV (Figure 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) species.45 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 compared 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 57Fe Mossbauer spectroscopy (further discussed in Section 3.3.2), the Fe(III) species in pseudotetrahedral surroundings can be identified in Fe-BEA/DeAl as the main component of the Fe 2p3/2 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-BEA/DeAl the enhanced Brønsted acidity was attributed to Fe(III) ions grafted into framework T-positons, that is, the Fe3+−O(H)− Si acidic sites.
3.3.2. 57Fe Mossbauer and EPR Studies. Not only the complex nature of Fe(III) species but also their high reactivity are significant limitations for their evaluation in both qualitative and quantitative manners. 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 Mossbauer and EPR studies. The 57Fe Mossbauer 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
Table 1. Chemical Composition Obtained from ICP Analysisa
sample name Si/Al Fe/Al FeICP (μmol·g−1) % cryst Stotal (m 2·g−1) Smeso (m
2·g−1) Vmicro (cm 3·g−1)
BEA 22 100 567 44 0.19 BEA/DeAl 250 89 609 55 0.23 Fe-BEA 22 0.20 135 98 515 40 0.17 Fe-BEA/DeAl 500 6.74 155 88 559 52 0.19
aRelative crystallinity values (% cryst) derived from XRD and textural parameters of native zeolites and their Fe forms.
Table 2. Concentrations of Fe and Al Determined by ICP Analysisa
sample name FeICP (μmol·g −1) AlICP (μmol·g−1) B (μmol·g−1) L (μmol·g−1)
BEA 675 400 225 Fe-BEA 135 670 165 345 BEA/DeAl 28 45 0 Fe-BEA/DeAl 155 23 102 50
aThe concentrations of Brønsted (B) and Lewis (L) acid sites determined in quantitative IR studies of ammonia sorption in investigated zeolites.
Figure 2. XPS spectra of Fe-BEA and Fe-BEA/DeAl.
High octahedral coordination symmetry can be provoked by the ligation of water molecules to extraframework iron species. Contrarily, in Fe-BEA/DeAl sample, ∼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 materials.48 The 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 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 reports.49
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, and 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 °C. 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 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 species.57
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 °C (Figure 4B). The 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), therefore, reactive penta- or hexacoordi- nated 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 line.52,55,58,59,60 Apart from the 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 center, as suggested by Bordiga et al.60 The finding is in line with Mossbauer results, which indicate the presence of tetrahedrally coordinated Fe3+ in the Fe-BEA/DeAl zeolite. Additionally, in high magnetic field 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, decreases when experiments were
Figure 3. 57Fe Mossbauer spectra recorded for Fe-BEA and Fe-BEA/ DeAl after evacuation at 450 °C for 1 h. Spectra recorded at room temperature.
Table 3. Hyperfine Parameters Derived from the Room-Temperature 57Fe Mossbauer Spectra for Studied Fe-Zeolitesa
57Fe Mossbauer data EPR data
sample name A [%] ± 5% IS (mm/s) QS (mm/s) Γ (mm/s) attribution of Fe species g factor attribution of Fe species
Fe-BEA bare Fe(II) not visible 22 0.36 0.58 0.22 [Fe(III)O]+, Fe(III)−μO2−Fe(III), Oh g⊥ ∼ 6 g ≈ 2 FeB 42 0.35 0.94 0.29 36 0.35 1.35 0.40
Fe-BEA/DeAl 30 0.18 0.5 0.6 Fe(III), Td geff ≈ 5 FeC 44 0.38 0.9 0.6 [Fe(III)O]+, Fe(III)−μO2−Fe(III), Oh g⊥ ∼ 6 g ≈ 2 FeB 26 0.39 1.9 1.1 Fe(III) oligomers, Oh geff ≈ 2.3 FeE
aThe symbol A denotes relative contribution of the particular sub-spectrum, 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 Γ stands for the absorber line width.
Figure 4. EPR spectra of Fe-BEA and Fe-BEA/DeAl registered at RT (a) and −196 °C(b).
performed at −196 °C; that means this line does not follow Curie’s law. The phenomenon can be rationalized by assuming mutual magnetic interaction of neighboring Fe3+ ions, and the signal can be assigned to FexOy particles on the external surface of zeolite.53,55,56 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) environment;60−62
however, the very small line width leads to the suggestion that this line is due to lattice defect (Si−O+−Al) generated by thermal treatment.56,63
3.3.3. IR Studies of Carbon Monoxide Sorption. As previously mentioned, the information on the amount of iron species derived from ammonia adsorption IR studies is not conclusive with 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 also be 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 postsynthesis procedure.35 Because of 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 (Figure 1, SI), which is consumed upon CO admission producing the 2173 cm−1 CO band.35 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 the 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 aluminum (EFAl) species, which were also found 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.60 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 autoreduction 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. The Fe(III) species were reported to be not detectable with
CO.35,44 Thus, only the concentrations of Fe2+ cations (Table 4) in the form of the exchangeable cations (Fe2+) and the oxo-
−
tetrahedral (Si/Al = 500 for Fe-BEA/DeAl) favor the hydrolysis process, finally resulting in oxide forms with different extents of dispersion. This process occurs at elevated temperatures, that is, 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
Figure 5. Maximum intensities of monocarbonyl (A) and mononitrosyl bands (B) formed in Fe-BEA and Fe-BEA/DeAl.
Table 4. Concentration of the Divalent Fe Moieties Derived from Both ICP Analysis (FeICP) and the Quantitative IR Studies of CO and NO Sorption (Fe2+IR)
Fe2+IR (μmol g−1)
CO
sample name FeICP (μmol g−1) Fe2+ Fe2+OXO ΣFe NO
Fe-BEA 135 10 5 15 20 Fe-BEA/DeAl 155 0 10 10 12
the number of Fe(III) species being easily reduced under vacuum conditions. 3.3.4. Interaction of Iron Species with Nitrogen Monoxide.
Sorption of nitrogen monoxide, that is, 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 behavior of studied materials. Unquestion- able advantage of the use of NO as probe is the possibility of the IR quantitative analysis of the Fe2+ cations in zeolite matrix.42 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 centers with NO. It is a consensus that the mono- and dinitrosyls are formed
solely on Fe2+ ions. Some authors,44,51 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 (Figure 5B). The IR spectra were recorded immediately upon the NO introduction to reduce the extent of the NO transformation into N2 and oxo-compounds of nitrogen in oxidation states +3 and +5. Because several bands of mononitrosyl species were distinguished for Fe-BEA (1885, 1865, and 1846 cm−1) and Fe-BEA/DeAl (1863, 1837, and 1810 cm−1) (Figure 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 For both 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 centers toward reduction with NO to Fe2+ cations. A similar conclusion can be derived when NO sorption is followed by EPR spectroscopy. After adsorption of NO in Fe-BEA, a new EPR signal with geff ≈ 4.0 (FeNOA) appeared (Figure 6). This signal is due to the interaction of the isolated Fe2+ ions with NO molecules.52,64 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 contrary, 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. Besides the FeNOA signal, three lines assigned to FeNOB sites
are easily noticed in both samples (Figures 6 and 7). The
FeNOB signal can be simulated with the following parameters, gxx = 2.094 ± 0.001, ΔB = 0.16 ± 0.01 mT; gyy = 2.058 ± 0.001, ΔB = 0.13 ± 0.01 mT; and gzz = 2.015 ± 0.001, ΔB = 0.13 ± 0.01 mT (Figure 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 Ene- mark−Feltham notation65).66 For Fe-BEA/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. Because the C1 was simulated with g values similar to the ones used for simulation of the signal from FeNOB, it was also attributed to [Fe(NO)3] species despite the wider line. The second spectrum is characterized by gxx = 2.067 ± 0.001, ΔB = 0.27 ± 0.01 mT; gyy = 2.038 ± 0.001, ΔB = 0.24 ± 0.01 mT; and gzz = 1.990 ± 0.001 ΔB = 0.25 ± 0.01 mT parameters and might also be attributed to (Fe2+−(O/OH)−Fe2+) (NO3) species characterized by slightly different symmetry of the center. According to our best knowledge the presence of the signal characterized by such parameters has been not reported until 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 Fe-BEA/DeAl after NO adsorption. The last center, C3, is identical to previously described FeF signal and is due to lattice defect (Si−O+−Al). Additionally, adsorption of nitric oxide resulted in disappearance of the signals around g ∼ 6 in all of the cases, pointing to coordinative unsaturation of these iron sites and indicating that these sites determine the catalytic activity. In IR studies besides the formation of the isolated Fe2+
cations, the transformation of nitrogen monoxide over catalysts also evidenced the production of nitrate species in both catalysts (Figure 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 (Figure 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
Figure 6. EPR spectra of Fe-BEA and Fe-BEA/DeAl before (a) and after NO adsorption (b) registered at −196 °C.
Figure 7. Experimental and simulated EPR spectra of Fe-BEA and Fe- BEA/DeAl after NO adsorption registered at −196 °C.
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. In summary, 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, a noticeable amount of Fe(III) is believed to be grafted in zeolite framework, while the nongrafted 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, that is, 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. 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
+ + → +4NO 4NH O 4N 6H O standard SCR3 2 2 2 (3)
Nitrogen is the desired product of this process, while also the formation of N2O is possible according to the reaction (eq 4)
+ + → +4NO 4NH 3O 4N O 6H O3 2 2 2 (4)
The results of the catalytic studies performed on Fe-BEA and Fe-BEA/DeAl zeolites and their protonic analogues are presented in Figure 9. The reference zeolites without iron were not catalytically active in the NO conversion up to 450 °C; at higher temperature NO conversion did not exceed 20% in both 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 a 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. The obtained samples have been also studied as catalysts for
the fast NH3−SCR of NOx (eq 5).
+ + → +NO NO 2NH 2N 3H O fast SCR2 3 2 2 (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 the 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 ∼341 °C (N2O/N2 selectivity = 2.3%/97.7%). The N2O decomposition catalytic studies were also
completed (Figure 11). The reaction started at about 350− 370 °C 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 °C, while at ∼525 °C 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 °C.
4. DISCUSSION
The tailoring of zeolitic catalyst activity requires us 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 an open problem. In the literature, there is no general agreement on the method that is most suitable 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
Figure 8. IR spectra (nitrosyl stretching region) of Fe-zeolites collected at RT after 30 min of contact time with adsorbed NO.
Figure 9. 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.
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 Fe.61 The examples of iron-modified zeolites for NH3−SCR of NO reported in the scientific literature are presented in previous studies.62
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 allowed incorporating iron as cationic species either isolated or bridged through an oxygen atom (iron oxo cations), while the sample prepared by postsynthesis method accommodated a high proportion of iron substituted for aluminum 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, that is, Fe-ZSM-5, Fe- MOR, and Fe-ZSM-12, revealed its superior catalytic activity.15,68,69 Boron et al.18,19,69 widely studied Fe-BEA in the NH3−SCR of NO and found that the framework pseudotetrahedral 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 also be 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 oxidize 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 pseudotetrahedral positions was found to be less effective. This different reducibility of Fe(III) centers reflects their different structure. Wichterlova 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+ are supported by density functional theory (DFT) calculations.70 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
+ * → +N O N O2 2 (6)
* →2O O2 (7)
The second step (eq 7) is the determining one; therefore, the rate of decomposition of N2O is a function of the concentration of labile oxygens and thus the reducibility of the catalysts. High activity of zeolite Fe-BEA 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, Mossbauer, and IR studies) the zeolite Fe-BEA/DeAl possesses the pseudotetrahedral Fe(III) in framework positions and extraframework FexOy species noticeably less prone to reduction, and thus they did not deliver catalytic activity. Indeed, a vital issue in standard and fast SCR is the
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-5,71,72
but only a few studies have investigated the SCR performance and the formation of N2O in fast SCR over Fe-BEA.16 The formation of N2O is a critical issue for commercial application of catalyst; however, it is worth noting that the diesel exhaust gases contain >90% of NO and >10% NO2. The main routes were suggested for nitrous oxide formation over Fe-modified zeolites resulting from (i) the formation of surface species like ammonium nitrate (e.g., NH4NO3) and its subsequent decomposition (eq 8)71,73
→ +NH NO N O 2H O4 3 2 2 (8)
Figure 10. 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 Fe catalysts.
Figure 11. N2O conversion in N2O decomposition on Fe-BEA and Fe- BEA/DeAl (75 mg of the catalyst, [N2O] = 1000 ppm in He, total flow rate: 75 mL/min).
(ii) the reaction of NO2 with intermediate nitrites (eq 9)
− + + → → + ++ +NO (NH ) NO 1 2
O ... 2N O 3H O 2H2 4 2 2 2 2 2
(9)
(iii) NO2−SCR reaction (eq 10)71
+ → + +2NO 2NH N N O 3H O2 3 2 2 2 (10)
As evidenced, IR studies of ammonia adsorption of both zeolites accommodate a 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 (Figure 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 toward N2 remained close to 100% for both catalysts; however, the catalytic tests showed the presence of N2O as byproduct, and the N2O profiles for both zeolites differ significantly. Over Fe-BEA, N2O appeared at ∼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, that is, with lower concentration of Brønsted acid sites than over fresh one. Contrarily, the Brønsted acid sites were suggested to accelerate decomposition of NH4NO2.
75−77 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 could adsorb only 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 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-BEA.15,69
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, has evidenced significantly lower strength of the latter species.67 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 are 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. The 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) enhances the catalytic activity.
5. 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 vapor. 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 Fe-BEA. These highly dispersed extraframework iron oxo-sites were found to be more easily reduced and 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 *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b05692.
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. (PDF)
AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +48 12 663 20 81. Fax: +48 12 634 0515. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was financed by Grant No. 2015/18/E/ST4/00191 from the National Science Centre, Poland.
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Standard and Fast Selective Catalytic Reduction of NO with