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The role of Brønsted acidity in the SCR of NO over Fe-MFI catalysts
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Microporous and Mesoporous Materials 111 (2008) 124–133 www.elsevier.com/locate/micromeso
The role of Brønsted acidity in the SCR of NO over Fe-MFI catalysts Michael Schwidder a, M. Santhosh Kumar b, Ursula Bentrup b, Javier Pe´rez-Ramı´rez c, Angelika Bru¨ckner b, Wolfgang Gru¨nert a,* b
a Laboratory of Industrial Chemistry, Ruhr University Bochum, P.O. Box 102148, D-44780 Bochum, Germany Leibniz Institute for Catalysis Rostock, Berlin Branch (former Institute of Applied Chemistry Berlin-Adlershof e.V.), Richard-Willsta¨tter-Str. 12, D-12489 Berlin, Germany c Laboratory for Heterogeneous Catalysis, Catalan Institution for Research and Advanced Studies (ICREA), Institute of Chemical Research of Catalonia (ICIQ), Av. Paı¨sos Catalans 16, 43007 Tarragona, Spain
Received 2 April 2007; received in revised form 8 July 2007; accepted 10 July 2007 Available online 19 July 2007
Abstract The selective catalytic reduction (SCR) of NO with isobutane and with NH3 was studied over Fe-MFI catalysts which differ strongly in Brønsted acidity but are similar in Fe content and structure of Fe sites, having shown similar activity in N2O decomposition in related work. The catalysts were prepared by exchange of Na-ZSM-5 (Si/Al ca. 14) with Fe2+ ions formed in situ by acidic dissolution of Fe powder and by steam extraction of framework iron from Fe-silicalite or from H-[Fe]-ZSM-5 (Si/Al ca. 30). The characterization of acidic properties by ammonia TPD and by IR of adsorbed pyridine at different temperatures revealed marked differences in acidity between exchanged and steam-activated samples, the latter being (almost) void of strong Brønsted sites. The structural similarity of the iron sites was confirmed by UV–Vis and EPR spectroscopic results. The weakly acidic samples were inferior both in isobutane-SCR and in ammonia-SCR. With isobutane, dramatic differences over the whole range of parameters studied imply a vital role of Brønsted acidity in the reaction mechanism (e.g. in isobutane activation). In NH3-SCR, large reaction rates were achieved with non-acidic catalysts as well, but a promoting effect of acidity was noted for catalysts that contain the iron in the most favorable site structure (oligomeric Fe oxo clusters). This suggests that an acid-catalyzed step (e.g. the decomposition of NH4NO2) may be rate-limiting at low temperatures. Ó 2007 Elsevier Inc. All rights reserved. Keywords: DeNOx; Fe-MFI; Isobutane; Ammonia; UV–Vis spectroscopy; EPR spectroscopy; Active sites; Acidity; FTIR of pyridine; NH3-TPD
1. Introduction Fe-ZSM-5 catalysts receive at present much attention due to their remarkable performance in reactions involving the activation of nitrogen oxides. In the attempts to elucidate the active Fe sites for the various reactions, the complexity of the Fe site structures coexisting in Fe-ZSM-5 of different preparations has led to many conflicting views. Thus, the selective reduction of NO by isobutane (isobutane-SCR) was suggested to be the catalyzed by binuclear
*
Corresponding author. Tel.: +49 234 322 2088; fax: +49 234 321 4115. E-mail address: [email protected] (W. Gru¨nert). 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.07.019
Fe oxo complexes formed upon washing and calcination of ZSM-5 loaded with iron by CVD of FeCl3 into the Hform [1,2]. Other authors considered only isolated Fe ions to support hydrocarbon-SCR over Fe-ZSM-5 [3]. Recently, some of us found that both isolated and oligomeric Fe oxo species can catalyze this reaction (and NH3-SCR as well) while oligomeric sites attack the reductant at higher temperatures limiting thus the selective temperature window (with isobutane more severely than with NH3) [4,5]. Both isolated and oligomeric Fe oxo species have been considered to be active in direct N2O decomposition [6], according to [7,8], however, with a clear preference for oligomeric species. The reduction of NO by CO or by hydrocarbons proceeds both on isolated and clustered sites [8,9].
M. Schwidder et al. / Microporous and Mesoporous Materials 111 (2008) 124–133
For several of these reactions, it is obvious that the zeolite Brønsted acidity is not a critical requirement. Thus, N2O decomposition and reduction by CO were both found to proceed swiftly over steamed Fe-silicalite which contains Al in the ppm range at maximum [7,8]. With the SCR of NO, there is less confidence about this question. While hydrocarbon-SCR is known to require Brønsted acidity over many catalytic systems (e.g. ZSM-5 modified with Pd, Ga, In and Ce [10–14]), the most plausible reaction mechanism proposed for isobutane-SCR over Fe-ZSM-5, which traces NH3-SCR as the final step of the hydrocarbon-SCR reaction sequence, does not explicitly refer to acid-catalyzed steps [15]. In NH3-SCR, acidity is generally considered to be favorable because acidic surfaces can adsorb NH3 in large quantities and thus provide a reservoir of the reductant in the vicinity of the reduction site. In the mechanism suggested by Topsoe et al. for the V2O5/TiO2 catalyst [16], an acidic V–OH group is also part of the active site, but there are also proposals that do not involve the activation of NH3 by a Brønsted center [17]. In an investigation of NH3-SCR over VO2+-exchanged ZSM-5, some of us came to the conclusion that the active site is a single VO2+ ion, which does not need the cooperation of Brønsted sites [18]. Very recently, Li et al. [19] identified an acid-catalyzed step which is probably involved in NH3-SCR: They found that the decomposition of the likely reaction intermediate NH4NO2 is accelerated by zeolite Brønsted sites. This reaction occurs already at rather low temperatures. Its potential role as rate-limiting step will depend on the rates of other steps in the reaction sequence. The assessment of the actual role of acidic sites in SCR reactions is not straightforward because a reliable conclusion can be only derived if the redox sites in the bifunctional catalysts used for the comparison are identical or close enough in structure to exclude their involvement in the catalytic effects observed. The most direct approaches, which involve poisoning of the acidic sites by bases or by back-exchange with alkali ions, may also affect the redox function of the catalyst. In former work, we obtained some indication that the relevance of Brønsted sites may be different for isobutane-SCR and NH3-SCR from the observation that a catalyst prepared via chemical vapor deposition (CVD) of FeCl3 onto a high-silica H-ZSM-5 (Si/Al 40) was active in the latter but very poor in the former reaction [19]. However, it was also found that this preparation route results in a multitude of Fe sites (from isolated Fe ions of different coordination via oligomeric sites of different nuclearity to large, disordered aggregates) and that the distribution of these species differs considerably between materials prepared from parent ZSM-5 samples of different Si/Al ratios [20]. Hence, the differences in activity may have originated from differences in the Fe site structure, which might be more relevant for isobutane-SCR than for NH3SCR. In the present paper, we compare Fe-ZSM-5 materials prepared via different routes, which are similar in their iron content and their catalytic activity in the direct N2O
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decomposition [21] but should, according to their Si/Al ratios and the preparation routes employed (exchange of Fe2+ ions into ZSM-5 vs. steam-activation of isomorphously substituted Fe-MFI) differ markedly in their acidity. UV–Vis and EPR spectroscopic data from these catalysts [5,9] show that among them are examples with close proximity in the structure of the Fe sites. In this paper, we describe the acidic properties of the catalysts on the basis of results from temperature-programmed desorption of ammonia and IR-spectroscopy of adsorbed pyridine and relate the drastic differences observed to those found in the catalytic behavior in the SCR reactions. 2. Experimental 2.1. Materials We will compare Fe-ZSM-5 catalysts made by exchange of Na-ZSM-5 (Chemiewerk Bad Ko¨stritz (Germany), Si/ Al 14) with Fe2+ ions produced in situ by acidic dissolution of iron powder (improved liquid ion exchange – ILIE, first proposed in Ref. [22]), with Fe-MFI (Fe-ZSM-5 and Fe-silicalite) in which iron initially located in the zeolite framework was extracted by high-temperature steaming. For the ion exchange, a slurry of Na-ZSM-5 and Fe powder was allowed to react with dilute HCl (initial pH – 1) under protective atmosphere for several days [5]. The catalysts, which contained 0.2–1.2 wt% Fe depending on the amount of Fe powder used in the synthesis, were then washed, dried and calcined in air at 873 K for 2 h. Along the manuscript, these samples are labeled as Fe-Z(m . n), where m . n is the Fe content. Details on the preparation of steam-activated Fe-MFI zeolites have been described elsewhere [7,23]. The isomorphously substituted zeolites, with Fe–Al–Si and Fe–Si frameworks, were calcined and activated in steam (300 mbar H2O in 30 ml (STP) min1 of N2) at ambient pressure and 873 K for 5 h, yielding sFe-ZSM-5 (Si/Al = 31 and 0.67 wt% Fe) and s-Fe-silicalite (Si/Al = 1 and 0.68 wt% Fe). For comparison, acidity and activity data for a catalyst made by the well-known CVD technique (CVD of FeCl3 onto H-ZSM-5 [1]) with a defective H-ZSM-5 of low Al content (Si/Al 40; Fe content – 2.6 wt%) will be given as well. The structural and catalytic properties of this material have been reported in Refs. [20,24] where it was labeled Fe-Z(B) and B(CVD, W1). The former label will be kept in the present paper. The information given here about the samples studied is summarized in Table 1. 2.2. Characterization The temperature-programmed desorption of ammonia was performed in two of the participating groups with somewhat different procedures. The parent H-ZSM-5 and Fe-Z(B) were measured with both protocols to allow a comparison of the results. Procedure A involved a Zeton/ Altamira AMI-1 instrument equipped with a Thermal
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Table 1 Overview over catalyst samples studied Catalyst
Si/Al
Preparation
Fe (wt%)
Fe-Z(0.3) Fe-Z(0.6) Fe-Z(1.2) s-Fe-silicalite s-Fe-ZSM-5 Fe-Z(B)
14 14 14 no Al 31 40a
Improved liquid ion exchange Improved liquid ion exchange Improved liquid ion exchange Steaming of Fe-silicalite Steaming of H-[Fe, Al]-ZSM-5 Gas-phas exchange of H-ZSM-5 with FeCl3
0.3 0.6 1.2 0.68 0.67 2.6
a Highly defective material, most Al extra-framework as shown by an IR study [24].
Conductivity Detector (TCD). After mounting the previously calcined samples, they were treated in flowing He at 373 K, subsequently ammonia (3 vol% in He, 90 ml min1) was adsorbed at 373 K for 30 min. After physisorbed NH3 had been desorbed at the same temperature into flowing He for 90 min, the desorption ramp was started (5 K min1 between 373 and 873 K, with a 90 min isothermal period at the final temperature). The ammonia desorbed was trapped in dilute 0.05 N H2SO4 and subsequently titrated with 0.05 N NaOH. Procedure B was performed on a Micromeritics Autochem II 2920 equipped with a TCD as well. The calcined sample (35 mg) was pretreated at 723 K in He (30 ml min1) for 1 h. Afterwards, a mixture of 5 vol% NH3 in He (40 ml min1) was adsorbed at 473 K for 15 min. Subsequently a flow of He (30 ml min1) was passed through the reactor during 30 min to remove ammonia weakly adsorbed on the zeolite. This procedure was repeated three times. Desorption of NH3 was monitored in the range of 473–873 K at 10 K min1. The surface acidity of the Fe-zeolites was studied by FTIR spectroscopy of adsorbed pyridine. Spectra were recorded using a Bruker IFS 66 spectrometer equipped with a heatable and evacuable reaction cell with CaF2 windows, which is connected to gas-dosing and evacuation systems. The zeolite powder was pressed into self-supporting wafers with a diameter of 20 mm and a weight of 50 mg. Prior to pyridine adsorption, the samples were pretreated in flowing air at 673 K for 1 h followed by cooling to 373 K. Then, pyridine was adsorbed at this temperature for 1 h by bubbling the Ar flow through a pyridine-containing saturator. Physisorbed pyridine was evacuated during 5 min at 373 K and infrared spectra were recorded at different temperatures in the range of 373–673 K with 2 cm1 resolution and 100 scans. The relative concentration of Brønsted and Lewis acid sites was determined from the area of the absorption bands at 1545 cm1 and 1450 cm1, respectively. The UV–Vis and EPR data summarized here were previously published in [5,9] where experimental details can be found as well. The UV–Vis-spectra were measured with a Varian Cary 400 spectrometer equipped with a Harrick DRS accessory (Harrick), and deconvoluted into sub-
bands using the GRAMS/32 code (Galactic). The X-band EPR spectra were recorded on a Bruker ELEXSYS 500-10/12 spectrometer (microwave power = 6.3 mW, modulation frequency = 100 kHz, modulation amplitude = 0.5 mT). The magnetic field was measured in reference to the DPPH (2,2-diphenyl-1-picrylhydrazyl-hydrate) standard. 2.3. Catalytic tests The catalytic studies have been performed as described in [5]. In brief, the SCR reactions have been measured in catalytic microflow reactors using mixtures of 1000 ppm NO, 1000 ppm reductant (isobutane or NH3) and 2 vol% O2 in He, with a gas-hourly space velocity (GHSV) of 30,000 h1 and at temperatures between 823 and 523 K in the case of isobutane-SCR, and with a GHSV of 750,000 h1 and at temperatures between 873 and 423 K (temperatures decreasing during the measurement sequence in both cases) with the NH3 reductant. Prior to the runs, the catalysts were heated in flowing He at the highest reaction temperature for 30 min. The composition of the effluent gas was analyzed using calibrated mass spectrometry in the case of isobutane-SCR and a combination of photometric devices for detection of NO, NO2, and NH3 in the case of NH3-SCR (Binos 1000, Rosemount, for NO and NO2, Binos 1, Leybold, for NH3). In both cases, gas-chromatography was additionally used to analyze the quantity of nitrogen released to trace possible formation of NO2 or N2O in significant amounts by setting up a nitrogen balance. This balance typically rendered values between 95 % and 105 %. 3. Results 3.1. Acidic properties In Fig. 1, NH3 profiles recorded during the TPD of ammonia from H-ZSM-5 and different Fe-ZSM-5 samples are compared. The amounts of NH3 desorbed from the samples are given in Table 2. The comparison is somewhat complicated by the parallel application of two different experimental procedures (caused by a limited supply of some materials). With procedure A (adsorption at 373 K, Fig. 1a), two peaks arising from weakly and strongly acidic sites are seen in all runs while the first one was absent when procedure B (adsorption at 473 K, Fig. 1b) was applied. In the latter case, the desorption peaks were somewhat shifted towards lower temperatures, and the NH3 content in the effluent raised immediately after the start of the temperature ramp, which caused an asymmetry of the desorption peak of H-ZSM-5 and a second, earlier maximum with Fe-Z(B) (Fig. 1b). The ammonia desorbed at the start of the ramp arises obviously from sites that can be populated at 473 K, because after adsorption at 373 K, the first desorption maximum appears at T P 473 K. The downshift of the peak temperature is unexpected and its reason
M. Schwidder et al. / Microporous and Mesoporous Materials 111 (2008) 124–133 T,K 400
600
800
isothermal 1000
H-ZSM-5
TC1200D signal, a. u.
Fe-Z(0.6)
Fe-Z(B)
Fe-Z(1.2)
H-ZSM-5
Fe-Z(B)
s-Fe-ZSM-5 s-Fe-silicalite 400
600
800 T,K
Fig. 1. Ammonia TPD from H-ZSM-5 and from Fe-MFI of different preparation. (a) Fe-Z(0.6) and Fe-Z(1.2) prepared by improved liquid ion exchange, procedure A (see Section 2), (b) Fe-MFI by steam extraction of framework Fe from Fe-silicalite and H-[Fe]-ZSM-5 (procedure B). For comparison, the TPD profiles obtained with the parent H-ZSM-5 and with Fe-Z(B) using procedure A and B are given in panels a and b, respectively, as well.
Table 2 NH3 adsorption capacity of zeolites studied (see Fig. 1) Catalyst
H-ZSM-5 (Si/Al = 14) Fe-Z(0.6) Fe-Z(1.2) s-Fe-silicalite s-Fe-ZSM-5 Fe-Z(B) a
NH3 desorbed (mmol g1) after adsorption at 373 K (Aa, Fig. 1a)
473 K (Ba, Fig. 1b)
1.51 1.43 1.47 – – 0.32
0.75 – – 0.06 0.14 0.21
127
In the samples made via exchange (ILIE; Fig. 1a), the intensity of the high-temperature signal decreases markedly with increased Fe content as expected. At the same time, a shoulder emerges at the high-temperature side of the lowtemperature signal, presumably caused by a weak Lewis acidity of the Fe ions. The total amount of ammonia desorbed (Table 2) does not decrease significantly with increasing Fe content. Apparently, the loss of intensity at high-temperatures is outweighed by increases due to sites of lower strength. Fig. 1b and Table 2 confirm that steam extraction of framework Fe yields materials of inferior acidity. This applies in particular to s-Fe-silicalite where aluminium which could give rise to strongly acidic sites, is completely absent. s-Fe-ZSM-5 exceeds the latter somewhat in number and strength of sites. Fe-Z(B) is a material with very low acidity as well. The total ammonia amount desorbed from it is only slightly larger than in the case of s-Fe-ZSM-5, but it emerges mostly from weak sites. IR spectra of pyridine adsorbed on H-ZSM-5 and two Fe-ZSM-5 catalysts made from it via ILIE are given in Fig. 2. Spectra measured after desorption at 523 K are given to avoid the distortion of the results by the signal of hydrogen-bonded pyridine, which is observed in spectra measured at lower temperatures (Fig. 3, bands at 1445 and 1596 cm1 [25–27]). At 523 K, the well-known pyridine ring-mode bands indicative of Brønsted sites (PyH+ –1542 cm1, shoulder at 1635 cm1) and of Lewis sites (L-Py, 1450 cm1, 1610–1618 cm1) are well visible. As expected, the spectrum of H-ZSM-5 is dominated by the signal of PyH+ although some Lewis sites resulting from Al3+ are available as well. Upon introduction of iron, the center of the L-Py band shifts to lower wavenumbers, and the ratio between Brønsted and Lewis sites (PyH+/ L-Py) changes dramatically. With Fe-Z(0.6), this arises not so much from a disappearance of Brønsted acidity but predominantly from a strong enhancement of the L-Py band. With Fe-Z(1.2), the decrease in Brønsted acidity is more obvious while the intensity of the L-Py band, which is now broader due to a shoulder at 1442 cm1, decreases slightly.
See Section 2.
is unclear, therefore peak temperatures will not be discussed here. According to Table 2, the total NH3 amounts desorbed after procedures B and A are at a ratio of 1:2 with H-ZSM-5, and 1:1.5 with Fe-Z(B), the latter reflecting the larger relative weight of weakly bound ammonia on this material. Thus, apart from the above mentioned discrepancy in temperatures, the data from the two methods are well consistent.
Fig. 2. FTIR spectra of pyridine adsorbed at 523 K. Background spectra of the air pre-treated samples were subtracted.
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Fig. 3. FTIR spectra of pyridine adsorbed at different temperatures. Subtraction of background spectra was not applied.
The position of the bands around 1600 cm1 can be considered as a measure of the Lewis acid strength of the surface sites [25]. Bands at higher wavenumbers (1618 cm1) result from strong Lewis sites whereas bands at lower wavenumbers (1610 cm1) indicate medium strong sites. Comparing the ratio of the bands at 1618/1610 cm1 it can be seen that the intensity of the 1618 cm1 band diminishes with increasing Fe content. This may indicate that stronger Al3+ Lewis sites are covered by FeOx species, while Fe3+ Lewis sites of intermediate strength will be created.
In Fig. 3, the intensity changes of the pyridine bands during thermal desorption can be seen. The rapid decay of the 1445 cm1 band on all three samples is obvious, which is mainly due to the desorption of hydrogen-bonded pyridine. Furthermore, pyridine adsorbed on Brønsted and Lewis sites is still observed at 673 K as indicated by the bands at 1545 cm1 and 1450 cm1, respectively. This also explains the broad high-temperature peaks in the NH3TPD profiles that obviously comprise the desorption of NH3 from both strong Brønsted as well as Lewis sites. The trend of band intensities with temperature is summarized in Fig. 4. With the Brønsted sites, the expected intermediate position of Fe-Z(0.6) between H-ZSM-5 and Fe-Z(1.2) is seen only after desorption at rather high-temperatures. The excess intensity at low temperature may indicate additional Brønsted sites of reduced strength (Fig. 4a). In Fig. 4b, the attention should be focused on the high-temperature region because at low temperatures the information about the Lewis sites is obscured by the band of hydrogen-bonded pyridine. It can be seen that in the Fe-exchanged samples the Lewis acidic sites are more abundant than those in H-ZSM-5. The strong decay in intensity even after desorption of H-bonded pyridine suggests a lower strength of the Fe-derived Lewis sites as indicated by the wavenumber shift discussed above. On the other hand, even after desorption at 673 K, Fe-Z(0.6) holds more Lewis-bound pyridine than H-ZSM-5. This may suggest that there is also a minority of strong Fe-derived Lewis sites. Upon increase of the Fe content to 1.2 wt%, the intensity of the L-Py signal decreases, obviously caused by the abundance of more clustered species in Fe-Z(1.2).
Fig. 4. Change of the FTIR band area for pyridium ions (PyH+) at 1545 cm1 (a) and for pyridine coordinated to Lewis-acid sites (L-Py) at 1445 cm1 (b) with desorption temperature.
M. Schwidder et al. / Microporous and Mesoporous Materials 111 (2008) 124–133
3.2. Structure of iron species UV–Vis and EPR spectra that provide relevant information about the structure of the Fe sites in selected catalysts are presented in Figs. 5 and 6 (see also [5,9]). The focus will be on the sample pairs Fe-Z(0.3)/s-Fe-silicalite and FeZ(0.3)/s-Fe-ZSM-5. In the UV–Vis spectra, all samples exhibit major bands below 300 nm. While the spectra of s-Fe-silicalite and of Fe-Z(0.3) are almost completely confined to this region, the remaining spectra extend to higher wavelengths. It has been reported earlier that signals below 300 nm arise from Fe3+ O charge-transfer bands of isolated ions in tetrahedral and octahedral coordination (220 nm and 285 nm, (t1 ! t2/t1 ! e transitions unresolved [20,28]). Signals between 300 and 400 nm may be assigned to oligomeric clusters, while bands at k > 400 nm indicate the presence of large Fe oxide aggregates [20]. By deconvoluting the spectra into sub-bands as shown in Fig. 5 (for numerical results see (see Table 3), it can be seen that in Fe-Z(0.3) and in s-Fe-silicalite more than 90% of the signal intensity arises from isolated Fe ions. In the two remaining samples, other species have considerable weight, and in general the degree of iron aggregation in s-Fe-ZSM-5 is between those in Fe-Z(0.6) and FeZ(1.2). From the viewpoint of UV–Vis spectroscopy, the same species occur in all catalysts. The most significant difference is a predominance of tetrahedrally coordinated isolated Fe species in s-Fe-silicalite, which is not found in its
1.0
counterpart with similar degree of clustering – Fe-Z(0.3). Among the remaining catalysts, there are just graduations in the degree of clustering, but the spectra are on the whole very alike. In Fig. 6a, EPR spectra of the Fe-ZSM-5 catalysts (in hydrated state) recorded at 77 K and 298 K are compared. In the spectra, several signals can be discerned, which have been discussed in our previous papers [9,20]. The signals at g 0 4.3 and g 0 6, which exhibit Curie-type behavior, are both attributed to isolated Fe3+ ions, in tetrahedral coordination (intra- or extra-framework location) [29–34] and with higher coordination numbers [35,36], respectively. In samples Fe-Z(0.3) and s-Fe-silicalite, for which UV–Vis data reveal the presence of only a negligible amount of clusters, there is another signal with Curie-type behavior at g 0 2, which is assigned to isolated Fe ions in a highly symmetric environment [5,9,20,31,37]. In s-Fe-ZSM-5, Fe-Z(0.6), and Fe-Z(1.2), which do contain clusters of different size and concentration, this signal is hardly seen since it is superimposed by a broader signal at g 0 2, which does not show Curie-type behavior and is attributed to clustered species with anti-ferromagnetic interactions between the Fe ions [5]. Above the Neel temperature, antiferromagnetic ordering collapses, the clustered Fe sites become paramagnetic and the EPR signal intensity increases. This is evident by comparing the relative EPR intensities of Fe-Z(0.3), Fe-Z(0.6) and Fe-Z(1.2) measured at 77 and 298 K (Fig. 6a) with those measured in air flow at
aggregated
mononuclear
oligomeric
1.0
aggregated oligomeric
Fe-Z(0.6) Normalized F(R)
Normalized F(R)
Fe-Z(0.3)
0.5
0.5
Fe-Z(1.2) Fe-Z(B)
0.0
0.0
1.0
1.0
s-Fe-ZSM-5 Normalized F(R)
Normalized F(R)
s-Fe-silicalite
0.5
0.0 200
300
400
500
Wavelength, nm
600
F(R)
mononuclear
129
700
0.5
0.0 200
300
400
500
600
700
Wavelength, nm
Fig. 5. UV–Vis spectra of Fe-MFI catalysts used in the present investigation, after calcination in air at 823 K (see also [5,9]).
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M. Schwidder et al. / Microporous and Mesoporous Materials 111 (2008) 124–133 g' = 4.3
g' = 4.3 g' = 6
g' = 6
g' = 2
g' = 2 Fe-Z(0.3)
Fe-Z(B)
Fe-Z(0.6) 77 K 298 K s-Fe-silicalite Fe-Z(1.2) s-Fe-ZSM-5
0
2000
4000
6000
0
2000
B, G
4000
6000
B, G Fe-Z(0.3) Fe-Z(0.6) Fe-Z(1.2)
g' = 4.3 g' = 6
0
2000
4000
6000
B, G
Fig. 6. EPR spectra of Fe-MFI catalysts used in the present investigation (see also [5,9,20]): (a) comparison of spectra recorded at room temperature and at 77 K, (Fe-Z(B) only at room temperature, not to scale, cf. [20]) and (b) spectra of samples prepared by improved liquid ion exchange measured at 623 K in air flow.
Table 3 Quantification of the UV–Vis spectra of Fe-ZSM-5 catalysts in Fig. 5 Catalyst
I1a (area%)
I2b (area%)
I3c (area%)
Fe-Z(0.3) Fe-Z(0.6) Fe-Z(1.2) s-Fe-silicalite s-Fe-ZSM-5 Fe-Z(B)
95.7 76.2 48.6 93.0 58.7 27
4.3 18.5 23.4 7.0 26.2 38
0 5.3 28.0 0 15.1 35
I1 – k < 300 nm, I2 – 300 < k < 400 nm, I3 – k > 400 nm. a Isolated Fe3+ in tetrahedral and higher coordination. b Oligomeric Fe3þ x Oy clusters. c Fe2O3 nanoparticles.
623 K (Fig. 6b). The temperature dependence of the g 0 2 signal in samples s-Fe-ZSM-5, Fe-Z(0.6), and Fe-Z(1.2) has to be differentiated from the broad g 0 2 signal in s-Fe-silicalite, which decreases in intensity at room temperature, i.e. exhibits Curie-type behavior. The latter broad feature arises probably from weak dipolar interactions between isolated iron sites in s-Fe-silicalite [9]. In summary, it can be stated in the light of the EPR results that practically the same iron species occur in all catalysts although some of them are almost void of FexOy clusters. Again, there are some special features with s-Fe-silicalite (the weak magnetic interactions between isolated sites, which indicates a relatively small average distance between very well distributed Fe sites), but beyond that qualitative differences between the Fe site structures cannot be discerned.
3.3. DeNOx performance NO conversions in the SCR with isobutane and with NH3 are presented in Fig. 7 (data for exchanged samples cited from [5]). Fe-MFI catalysts prepared by extraction of Fe from framework positions, which are regarded as are very promising in N2O abatement reactions [38,39], are very poor in NO abatement. In isobutane-SCR where the Fe-ZSM-5 made by ILIE achieve peak conversions between 70% and 86%, s-Fe-silicalite converts <10% below 700 K, and achieves 20% conversion only above 800 K (Fig. 7a). The s-Fe-ZSM-5 catalyst exhibits an NO conversion maximum as does the vast majority of SCR catalysts, but the peak NO conversion is just above 20%. These catalysts behave similar as the Fe-Z(B) sample, which is given for comparison. As expected, the isobutane conversion trends differ strongly as well, with steeply increasing curves found for the exchanged catalysts but gradually increasing curves for the steamed Fe-MFI samples (Fig. 7c). While with the former the isobutane conversion is clearly lower than the NO conversion below the maximum, such high reductant utilization does not occur with the latter. In NH3-SCR, the relation between the two catalyst types is somewhat different (Fig. 7b). The steam-activated iron-containing zeolites are inferior here as well, but the activities of s-Fe-ZSM-5 and Fe-Z(0.3) are quite similar. s-Fe-silicalite is the poorest catalyst also for NH3-SCR, but while it is clearly inferior to s-Fe-ZSM-5 in isobu-
M. Schwidder et al. / Microporous and Mesoporous Materials 111 (2008) 124–133
a 100
b100 i-C4H10- SCR NO conversion, %
NH3- SCR
NO conversion, % 0 500
600
700
Fe-Z(0.3)
Fe-Z(0.6)
60 40 20
s-Fe-ZSM-5
s-Fe-silicalite
600
800
Fe-Z(1.2)
Fe-Z(B)
d 100
80
80 NH3 conversion
Isobutane conversion, %
80
0 400
800
c 100
60 40 20 0 500
131
60 40 20
600
T,K
700
800
0 400
600
800 T,K
Fig. 7. Catalytic behavior of FeMFI samples in the selective catalytic reduction of NO with isobutane (a, c – 1000 ppm NO, 1000 ppm isobutane, 2 vol% O2, 30,000 h1) and with NH3 (b, d – 1000 ppm NO, 1000 ppm NH3, 2 vol% O2, 750,000 h1).
tane-SCR, both catalysts give identical NO conversions with ammonia up to 700 K. A remarkable point with s-Fe-silicalite is its high NH3 oxidation activity. While the NO and the NH3 conversion curves are practically identical for all remaining catalysts, unselective oxidation of the ammonia becomes predominant over s-Fe-silicalite above 823 K. Another difference to isobutane-SCR is the relatively strong performance of the Fe-Z(B), which, although not being competitive with the better ILIE catalysts, achieved sizable NO conversions. 4. Discussion The relation between acidity and SCR activity will be discussed on the background of structural information about the Fe sites present that can be derived from Figs. 5 and 6 (for details see [5,9]). For the samples prepared via exchange with Fe2+ ions (ILIE), the UV–Vis spectra (Fig. 5, Table 1) show that the iron is largely isolated at low Fe content (>95% at 0.3 wt%). In Fe-Z(0.6), still 75% of the iron is present in isolated species, ca. 20% in oligomeric clusters, and only a minor amount has further aggregated into larger (disordered) particles. In Fe-Z(1.2), the ratio between the Fe atoms in these site types is approximately 50:25:25, i.e. the degree of aggregation is already significant here. The Fe speciation in Fe-Z(0.3) is similar to that in s-Fe-silicalite regarding the high degree of site isolation (Table 1). There are, however, differences in the coordination of iron, which is predominantly tetrahedral
in the latter and both tetrahedral and octahedral in the former (Fig. 5). The analogy in the Fe speciation is closer between Fe-Z(0.6) and s-Fe-ZSM-5, where similar sites are present in comparable amounts though with a stronger aggregation degree in the latter. The study of the acidity of Fe-MFI SCR catalysts has confirmed the expected trends, but has also revealed some interesting details. In the NH3-TPD profiles, the introduction of Fe species by ILIE has lead to a decrease of the high-temperature signal due to zeolite Brønsted sites, accompanied by the appearance of a shoulder around 523 K, which apparently arises from NH3 bound to Fe ions (Fig. 1a). It should be noted that the intensity loss of the high-temperature signal is significant already at 0.6 wt% Fe. As expected, the acidity of the samples prepared via steam extraction of framework Fe or by CVD of FeCl3 into a high-silica Fe-ZSM-5 is inferior (Fig. 1b). The acidic sites detected are mostly weak, however, a certain, very low amount of stronger acidic sites may be discerned in s-FeZSM-5 and in Fe-Z(B). The pyridine adsorption study (Figs. 2–4) confirms that the Fe ions have Lewis-acid character: upon introduction of 0.6 wt% Fe into H-ZSM-5, the L-Py signal is much enhanced and shifted from 1455 to 1450 cm1 (Fig. 2). Surprisingly, however, and in disagreement with the NH3 TPD profile, the intensity of the Brønsted band is almost the same in Fe-Z(0.6) as in H-ZSM-5 and decreases only at 1.2 wt% Fe. This trend is confirmed in the temperature dependence of the 1540 cm1 band (Fig. 4a), where the
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initially stronger intensity loss of Fe-Z(0.6) indicates the presence of a less acidic Brønsted site, which is absent (or not abundant) in Fe-Z(1.2). The assignment of this site is not straightforward. We attribute it tentatively to a Fe3+(OH) group on a site that is abundant in Fe-Z(0.6) but not in Fe-Z(1.2). According to the structural data cited here (Figs. 5 and 6, Table 1, for more details see [5,40]), FeZ(1.2) contains both isolated and oligomeric sites in larger (absolute) abundance than Fe-Z(0.6). It has been found, however, that some of the isolated sites (those characterized by g values of 4.3 and 6 in EPR) are rare in FeZ(1.2), probably due to their participation in the clustering that occurs at higher Fe content and higher temperatures [40] (Fig. 6b). OH groups associated with these Fe species might be candidates for the Brønsted sites of moderate acidity that give rise to the extra intensity in Fe-Z(0.6). Together with the structural data discussed above these results provide a good basis for the interpretation of the catalytic data. In isobutane-SCR (Fig. 7a and c), a key involvement of Brønsted sites is very likely. This is not so much obvious from the comparison between the non-acidic s-Fe-silicalite and the acidic Fe-Z(0.3) because the difference could also originate from the difference in the coordination of the predominantly isolated Fe sites. However, a large advantage of the acidic catalyst can be also seen in the comparison of s-Fe-ZSM-5 and Fe-Z(0.6) which contain Fe sites of similar structure. The poor performance of Fe-Z(B) supports the conclusion. It appears that the Brønsted sites are involved already in the activation of the isobutane because even the isobutane conversion is largely suppressed over the non-acidic samples. Given the important role of hydrocarbon deposits in isobutaneSCR over Fe-ZSM-5 [41], this is well in accordance with our earlier observation from an operando-DRIFTS study, according to which the interaction of isobutane-SCR feed with H-ZSM-5 leads to very similar hydrocarbon deposits as the interaction with Fe-ZSM-5 [42]. As these adsorbates are involved in the reduction of NOx activated on nearby Fe sites their absence should strongly impede the SCR reaction. A rate limiting role of acidity at a later stage (e.g. in NH4NO2 decomposition, which according to [15,19] is part of the reaction sequence) is less likely because even if isobutane-SCR proceeds via a concluding NH3SCR step, the rate of the latter is much larger than the observed rates of the former. With NH3 as the reductant, an influence of the Brønsted sites is less obvious (Fig. 7b and d). Still, the non-acidic samples s-Fe-silicalite and s-Fe-ZSM-5 exhibit the poorest performance. However, these two catalysts achieve identical NO conversions over quite a broad temperature range despite significant differences in acidity (Fig. 1b). Moreover, while the NO conversion levels off with s-Fe-silicalite, s-Fe-ZSM-5 performs quite similar as the strongly acidic Fe-Z(0.3). The difference might well be accounted for by a better accessibility of the Fe species in the latter sample. Finally, the remarkable performance of the non-acidic FeZ(B) in NH3-SCR (as opposed to isobutane-SCR, cf.
Fig. 7a) shows that the former reaction is possible also materials deficient in Brønsted sites. However, the very high activity of Fe-Z(0.6) and in particular of Fe-Z(1.2) is not fully compatible with this picture. It has been argued that clustered sites might be more effective in NH3-SCR than isolated sites as long as the dispersion remains high [5], but a large difference in activity is also seen between the strongly acidic Fe-Z(0.6) and the weakly acidic s-Fe-ZSM-5 (Figs. 7b and d), which has a similar Fe content and an only slightly more clustered Fe phase. This difference, which is illustrated by a marked decrease of the temperatures for 50 % conversion (827 K for s-Fe-ZSM-5, 653 K for Fe-Z(0.6)) is probably too large to be explained by increased local NH3 concentrations created by neighboring Brønsted sites, rather, it suggests that an additional reaction channel is opened by the presence of zeolite protons. This channel might be the acid-catalyzed decomposition of intermediate NH4NO2 as recently proposed by Li et al. [19]. It is known that this step proceeds also uncatalyzed, and it might be rapid enough at high temperatures to allow for large reaction rates. At low temperatures, the ammonium nitrite decomposition might become rate-limiting, and the catalytic effect of Brønsted sites on it would be felt in the total reaction rate. Therefore, while considerable NH3-SCR rates can be achieved with nonacidic samples as well, the development of top de-NOx performances may require the combination of oligomeric Fe oxo clusters with Brønsted sites. 5. Conclusions In the present study, Fe-MFI catalysts that have a similar structure of Fe sites (as revealed by UV–Vis and EPR spectroscopic data) and were found to exhibit comparable performance in N2O decomposition in companion work have been compared for their catalytic properties in the SCR of NO with isobutane and with NH3. The dramatic differences observed have been attributed to their strongly different acidity properties, which have been characterized by TPD of ammonia and by IR of adsorbed pyridine. Fe-ZSM-5 prepared by exchange of Na-ZSM-5 with Fe2+ ions formed in situ by acidic of Fe powder in acidic solution exhibits high acidity and high SCR activity. Fe-MFI prepared by steam extraction of framework iron from Fe-silicalite or from H-[Fe]-ZSM-5 is weakly acidic and poor in activity. The differences are dramatic for isobutane-SCR over the whole range of parameters studied, which indicates a vital role of Brønsted sites in this reaction. In NH3-SCR, large reaction rates can be achieved with non-acidic catalysts, but acidity definitely increases the activity provided the catalysts contain the iron in the most favorable structure (oligomeric Fe oxo clusters). This suggests that an acid-catalyzed step (e.g. the decomposition of NH4NO2) may become rate-limiting at low temperatures. The data show that the highest activities in NH3SCR may be achieved with catalysts containing both oligomeric Fe oxo clusters and Brønsted sites.
M. Schwidder et al. / Microporous and Mesoporous Materials 111 (2008) 124–133
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Microporous and Mesoporous Materials 111 (2008) 124–133 www.elsevier.com/locate/micromeso
The role of Brønsted acidity in the SCR of NO over Fe-MFI catalysts Michael Schwidder a, M. Santhosh Kumar b, Ursula Bentrup b, Javier Pe´rez-Ramı´rez c, Angelika Bru¨ckner b, Wolfgang Gru¨nert a,* b
a Laboratory of Industrial Chemistry, Ruhr University Bochum, P.O. Box 102148, D-44780 Bochum, Germany Leibniz Institute for Catalysis Rostock, Berlin Branch (former Institute of Applied Chemistry Berlin-Adlershof e.V.), Richard-Willsta¨tter-Str. 12, D-12489 Berlin, Germany c Laboratory for Heterogeneous Catalysis, Catalan Institution for Research and Advanced Studies (ICREA), Institute of Chemical Research of Catalonia (ICIQ), Av. Paı¨sos Catalans 16, 43007 Tarragona, Spain
Received 2 April 2007; received in revised form 8 July 2007; accepted 10 July 2007 Available online 19 July 2007
Abstract The selective catalytic reduction (SCR) of NO with isobutane and with NH3 was studied over Fe-MFI catalysts which differ strongly in Brønsted acidity but are similar in Fe content and structure of Fe sites, having shown similar activity in N2O decomposition in related work. The catalysts were prepared by exchange of Na-ZSM-5 (Si/Al ca. 14) with Fe2+ ions formed in situ by acidic dissolution of Fe powder and by steam extraction of framework iron from Fe-silicalite or from H-[Fe]-ZSM-5 (Si/Al ca. 30). The characterization of acidic properties by ammonia TPD and by IR of adsorbed pyridine at different temperatures revealed marked differences in acidity between exchanged and steam-activated samples, the latter being (almost) void of strong Brønsted sites. The structural similarity of the iron sites was confirmed by UV–Vis and EPR spectroscopic results. The weakly acidic samples were inferior both in isobutane-SCR and in ammonia-SCR. With isobutane, dramatic differences over the whole range of parameters studied imply a vital role of Brønsted acidity in the reaction mechanism (e.g. in isobutane activation). In NH3-SCR, large reaction rates were achieved with non-acidic catalysts as well, but a promoting effect of acidity was noted for catalysts that contain the iron in the most favorable site structure (oligomeric Fe oxo clusters). This suggests that an acid-catalyzed step (e.g. the decomposition of NH4NO2) may be rate-limiting at low temperatures. Ó 2007 Elsevier Inc. All rights reserved. Keywords: DeNOx; Fe-MFI; Isobutane; Ammonia; UV–Vis spectroscopy; EPR spectroscopy; Active sites; Acidity; FTIR of pyridine; NH3-TPD
1. Introduction Fe-ZSM-5 catalysts receive at present much attention due to their remarkable performance in reactions involving the activation of nitrogen oxides. In the attempts to elucidate the active Fe sites for the various reactions, the complexity of the Fe site structures coexisting in Fe-ZSM-5 of different preparations has led to many conflicting views. Thus, the selective reduction of NO by isobutane (isobutane-SCR) was suggested to be the catalyzed by binuclear
*
Corresponding author. Tel.: +49 234 322 2088; fax: +49 234 321 4115. E-mail address: [email protected] (W. Gru¨nert). 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.07.019
Fe oxo complexes formed upon washing and calcination of ZSM-5 loaded with iron by CVD of FeCl3 into the Hform [1,2]. Other authors considered only isolated Fe ions to support hydrocarbon-SCR over Fe-ZSM-5 [3]. Recently, some of us found that both isolated and oligomeric Fe oxo species can catalyze this reaction (and NH3-SCR as well) while oligomeric sites attack the reductant at higher temperatures limiting thus the selective temperature window (with isobutane more severely than with NH3) [4,5]. Both isolated and oligomeric Fe oxo species have been considered to be active in direct N2O decomposition [6], according to [7,8], however, with a clear preference for oligomeric species. The reduction of NO by CO or by hydrocarbons proceeds both on isolated and clustered sites [8,9].
M. Schwidder et al. / Microporous and Mesoporous Materials 111 (2008) 124–133
For several of these reactions, it is obvious that the zeolite Brønsted acidity is not a critical requirement. Thus, N2O decomposition and reduction by CO were both found to proceed swiftly over steamed Fe-silicalite which contains Al in the ppm range at maximum [7,8]. With the SCR of NO, there is less confidence about this question. While hydrocarbon-SCR is known to require Brønsted acidity over many catalytic systems (e.g. ZSM-5 modified with Pd, Ga, In and Ce [10–14]), the most plausible reaction mechanism proposed for isobutane-SCR over Fe-ZSM-5, which traces NH3-SCR as the final step of the hydrocarbon-SCR reaction sequence, does not explicitly refer to acid-catalyzed steps [15]. In NH3-SCR, acidity is generally considered to be favorable because acidic surfaces can adsorb NH3 in large quantities and thus provide a reservoir of the reductant in the vicinity of the reduction site. In the mechanism suggested by Topsoe et al. for the V2O5/TiO2 catalyst [16], an acidic V–OH group is also part of the active site, but there are also proposals that do not involve the activation of NH3 by a Brønsted center [17]. In an investigation of NH3-SCR over VO2+-exchanged ZSM-5, some of us came to the conclusion that the active site is a single VO2+ ion, which does not need the cooperation of Brønsted sites [18]. Very recently, Li et al. [19] identified an acid-catalyzed step which is probably involved in NH3-SCR: They found that the decomposition of the likely reaction intermediate NH4NO2 is accelerated by zeolite Brønsted sites. This reaction occurs already at rather low temperatures. Its potential role as rate-limiting step will depend on the rates of other steps in the reaction sequence. The assessment of the actual role of acidic sites in SCR reactions is not straightforward because a reliable conclusion can be only derived if the redox sites in the bifunctional catalysts used for the comparison are identical or close enough in structure to exclude their involvement in the catalytic effects observed. The most direct approaches, which involve poisoning of the acidic sites by bases or by back-exchange with alkali ions, may also affect the redox function of the catalyst. In former work, we obtained some indication that the relevance of Brønsted sites may be different for isobutane-SCR and NH3-SCR from the observation that a catalyst prepared via chemical vapor deposition (CVD) of FeCl3 onto a high-silica H-ZSM-5 (Si/Al 40) was active in the latter but very poor in the former reaction [19]. However, it was also found that this preparation route results in a multitude of Fe sites (from isolated Fe ions of different coordination via oligomeric sites of different nuclearity to large, disordered aggregates) and that the distribution of these species differs considerably between materials prepared from parent ZSM-5 samples of different Si/Al ratios [20]. Hence, the differences in activity may have originated from differences in the Fe site structure, which might be more relevant for isobutane-SCR than for NH3SCR. In the present paper, we compare Fe-ZSM-5 materials prepared via different routes, which are similar in their iron content and their catalytic activity in the direct N2O
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decomposition [21] but should, according to their Si/Al ratios and the preparation routes employed (exchange of Fe2+ ions into ZSM-5 vs. steam-activation of isomorphously substituted Fe-MFI) differ markedly in their acidity. UV–Vis and EPR spectroscopic data from these catalysts [5,9] show that among them are examples with close proximity in the structure of the Fe sites. In this paper, we describe the acidic properties of the catalysts on the basis of results from temperature-programmed desorption of ammonia and IR-spectroscopy of adsorbed pyridine and relate the drastic differences observed to those found in the catalytic behavior in the SCR reactions. 2. Experimental 2.1. Materials We will compare Fe-ZSM-5 catalysts made by exchange of Na-ZSM-5 (Chemiewerk Bad Ko¨stritz (Germany), Si/ Al 14) with Fe2+ ions produced in situ by acidic dissolution of iron powder (improved liquid ion exchange – ILIE, first proposed in Ref. [22]), with Fe-MFI (Fe-ZSM-5 and Fe-silicalite) in which iron initially located in the zeolite framework was extracted by high-temperature steaming. For the ion exchange, a slurry of Na-ZSM-5 and Fe powder was allowed to react with dilute HCl (initial pH – 1) under protective atmosphere for several days [5]. The catalysts, which contained 0.2–1.2 wt% Fe depending on the amount of Fe powder used in the synthesis, were then washed, dried and calcined in air at 873 K for 2 h. Along the manuscript, these samples are labeled as Fe-Z(m . n), where m . n is the Fe content. Details on the preparation of steam-activated Fe-MFI zeolites have been described elsewhere [7,23]. The isomorphously substituted zeolites, with Fe–Al–Si and Fe–Si frameworks, were calcined and activated in steam (300 mbar H2O in 30 ml (STP) min1 of N2) at ambient pressure and 873 K for 5 h, yielding sFe-ZSM-5 (Si/Al = 31 and 0.67 wt% Fe) and s-Fe-silicalite (Si/Al = 1 and 0.68 wt% Fe). For comparison, acidity and activity data for a catalyst made by the well-known CVD technique (CVD of FeCl3 onto H-ZSM-5 [1]) with a defective H-ZSM-5 of low Al content (Si/Al 40; Fe content – 2.6 wt%) will be given as well. The structural and catalytic properties of this material have been reported in Refs. [20,24] where it was labeled Fe-Z(B) and B(CVD, W1). The former label will be kept in the present paper. The information given here about the samples studied is summarized in Table 1. 2.2. Characterization The temperature-programmed desorption of ammonia was performed in two of the participating groups with somewhat different procedures. The parent H-ZSM-5 and Fe-Z(B) were measured with both protocols to allow a comparison of the results. Procedure A involved a Zeton/ Altamira AMI-1 instrument equipped with a Thermal
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Table 1 Overview over catalyst samples studied Catalyst
Si/Al
Preparation
Fe (wt%)
Fe-Z(0.3) Fe-Z(0.6) Fe-Z(1.2) s-Fe-silicalite s-Fe-ZSM-5 Fe-Z(B)
14 14 14 no Al 31 40a
Improved liquid ion exchange Improved liquid ion exchange Improved liquid ion exchange Steaming of Fe-silicalite Steaming of H-[Fe, Al]-ZSM-5 Gas-phas exchange of H-ZSM-5 with FeCl3
0.3 0.6 1.2 0.68 0.67 2.6
a Highly defective material, most Al extra-framework as shown by an IR study [24].
Conductivity Detector (TCD). After mounting the previously calcined samples, they were treated in flowing He at 373 K, subsequently ammonia (3 vol% in He, 90 ml min1) was adsorbed at 373 K for 30 min. After physisorbed NH3 had been desorbed at the same temperature into flowing He for 90 min, the desorption ramp was started (5 K min1 between 373 and 873 K, with a 90 min isothermal period at the final temperature). The ammonia desorbed was trapped in dilute 0.05 N H2SO4 and subsequently titrated with 0.05 N NaOH. Procedure B was performed on a Micromeritics Autochem II 2920 equipped with a TCD as well. The calcined sample (35 mg) was pretreated at 723 K in He (30 ml min1) for 1 h. Afterwards, a mixture of 5 vol% NH3 in He (40 ml min1) was adsorbed at 473 K for 15 min. Subsequently a flow of He (30 ml min1) was passed through the reactor during 30 min to remove ammonia weakly adsorbed on the zeolite. This procedure was repeated three times. Desorption of NH3 was monitored in the range of 473–873 K at 10 K min1. The surface acidity of the Fe-zeolites was studied by FTIR spectroscopy of adsorbed pyridine. Spectra were recorded using a Bruker IFS 66 spectrometer equipped with a heatable and evacuable reaction cell with CaF2 windows, which is connected to gas-dosing and evacuation systems. The zeolite powder was pressed into self-supporting wafers with a diameter of 20 mm and a weight of 50 mg. Prior to pyridine adsorption, the samples were pretreated in flowing air at 673 K for 1 h followed by cooling to 373 K. Then, pyridine was adsorbed at this temperature for 1 h by bubbling the Ar flow through a pyridine-containing saturator. Physisorbed pyridine was evacuated during 5 min at 373 K and infrared spectra were recorded at different temperatures in the range of 373–673 K with 2 cm1 resolution and 100 scans. The relative concentration of Brønsted and Lewis acid sites was determined from the area of the absorption bands at 1545 cm1 and 1450 cm1, respectively. The UV–Vis and EPR data summarized here were previously published in [5,9] where experimental details can be found as well. The UV–Vis-spectra were measured with a Varian Cary 400 spectrometer equipped with a Harrick DRS accessory (Harrick), and deconvoluted into sub-
bands using the GRAMS/32 code (Galactic). The X-band EPR spectra were recorded on a Bruker ELEXSYS 500-10/12 spectrometer (microwave power = 6.3 mW, modulation frequency = 100 kHz, modulation amplitude = 0.5 mT). The magnetic field was measured in reference to the DPPH (2,2-diphenyl-1-picrylhydrazyl-hydrate) standard. 2.3. Catalytic tests The catalytic studies have been performed as described in [5]. In brief, the SCR reactions have been measured in catalytic microflow reactors using mixtures of 1000 ppm NO, 1000 ppm reductant (isobutane or NH3) and 2 vol% O2 in He, with a gas-hourly space velocity (GHSV) of 30,000 h1 and at temperatures between 823 and 523 K in the case of isobutane-SCR, and with a GHSV of 750,000 h1 and at temperatures between 873 and 423 K (temperatures decreasing during the measurement sequence in both cases) with the NH3 reductant. Prior to the runs, the catalysts were heated in flowing He at the highest reaction temperature for 30 min. The composition of the effluent gas was analyzed using calibrated mass spectrometry in the case of isobutane-SCR and a combination of photometric devices for detection of NO, NO2, and NH3 in the case of NH3-SCR (Binos 1000, Rosemount, for NO and NO2, Binos 1, Leybold, for NH3). In both cases, gas-chromatography was additionally used to analyze the quantity of nitrogen released to trace possible formation of NO2 or N2O in significant amounts by setting up a nitrogen balance. This balance typically rendered values between 95 % and 105 %. 3. Results 3.1. Acidic properties In Fig. 1, NH3 profiles recorded during the TPD of ammonia from H-ZSM-5 and different Fe-ZSM-5 samples are compared. The amounts of NH3 desorbed from the samples are given in Table 2. The comparison is somewhat complicated by the parallel application of two different experimental procedures (caused by a limited supply of some materials). With procedure A (adsorption at 373 K, Fig. 1a), two peaks arising from weakly and strongly acidic sites are seen in all runs while the first one was absent when procedure B (adsorption at 473 K, Fig. 1b) was applied. In the latter case, the desorption peaks were somewhat shifted towards lower temperatures, and the NH3 content in the effluent raised immediately after the start of the temperature ramp, which caused an asymmetry of the desorption peak of H-ZSM-5 and a second, earlier maximum with Fe-Z(B) (Fig. 1b). The ammonia desorbed at the start of the ramp arises obviously from sites that can be populated at 473 K, because after adsorption at 373 K, the first desorption maximum appears at T P 473 K. The downshift of the peak temperature is unexpected and its reason
M. Schwidder et al. / Microporous and Mesoporous Materials 111 (2008) 124–133 T,K 400
600
800
isothermal 1000
H-ZSM-5
TC1200D signal, a. u.
Fe-Z(0.6)
Fe-Z(B)
Fe-Z(1.2)
H-ZSM-5
Fe-Z(B)
s-Fe-ZSM-5 s-Fe-silicalite 400
600
800 T,K
Fig. 1. Ammonia TPD from H-ZSM-5 and from Fe-MFI of different preparation. (a) Fe-Z(0.6) and Fe-Z(1.2) prepared by improved liquid ion exchange, procedure A (see Section 2), (b) Fe-MFI by steam extraction of framework Fe from Fe-silicalite and H-[Fe]-ZSM-5 (procedure B). For comparison, the TPD profiles obtained with the parent H-ZSM-5 and with Fe-Z(B) using procedure A and B are given in panels a and b, respectively, as well.
Table 2 NH3 adsorption capacity of zeolites studied (see Fig. 1) Catalyst
H-ZSM-5 (Si/Al = 14) Fe-Z(0.6) Fe-Z(1.2) s-Fe-silicalite s-Fe-ZSM-5 Fe-Z(B) a
NH3 desorbed (mmol g1) after adsorption at 373 K (Aa, Fig. 1a)
473 K (Ba, Fig. 1b)
1.51 1.43 1.47 – – 0.32
0.75 – – 0.06 0.14 0.21
127
In the samples made via exchange (ILIE; Fig. 1a), the intensity of the high-temperature signal decreases markedly with increased Fe content as expected. At the same time, a shoulder emerges at the high-temperature side of the lowtemperature signal, presumably caused by a weak Lewis acidity of the Fe ions. The total amount of ammonia desorbed (Table 2) does not decrease significantly with increasing Fe content. Apparently, the loss of intensity at high-temperatures is outweighed by increases due to sites of lower strength. Fig. 1b and Table 2 confirm that steam extraction of framework Fe yields materials of inferior acidity. This applies in particular to s-Fe-silicalite where aluminium which could give rise to strongly acidic sites, is completely absent. s-Fe-ZSM-5 exceeds the latter somewhat in number and strength of sites. Fe-Z(B) is a material with very low acidity as well. The total ammonia amount desorbed from it is only slightly larger than in the case of s-Fe-ZSM-5, but it emerges mostly from weak sites. IR spectra of pyridine adsorbed on H-ZSM-5 and two Fe-ZSM-5 catalysts made from it via ILIE are given in Fig. 2. Spectra measured after desorption at 523 K are given to avoid the distortion of the results by the signal of hydrogen-bonded pyridine, which is observed in spectra measured at lower temperatures (Fig. 3, bands at 1445 and 1596 cm1 [25–27]). At 523 K, the well-known pyridine ring-mode bands indicative of Brønsted sites (PyH+ –1542 cm1, shoulder at 1635 cm1) and of Lewis sites (L-Py, 1450 cm1, 1610–1618 cm1) are well visible. As expected, the spectrum of H-ZSM-5 is dominated by the signal of PyH+ although some Lewis sites resulting from Al3+ are available as well. Upon introduction of iron, the center of the L-Py band shifts to lower wavenumbers, and the ratio between Brønsted and Lewis sites (PyH+/ L-Py) changes dramatically. With Fe-Z(0.6), this arises not so much from a disappearance of Brønsted acidity but predominantly from a strong enhancement of the L-Py band. With Fe-Z(1.2), the decrease in Brønsted acidity is more obvious while the intensity of the L-Py band, which is now broader due to a shoulder at 1442 cm1, decreases slightly.
See Section 2.
is unclear, therefore peak temperatures will not be discussed here. According to Table 2, the total NH3 amounts desorbed after procedures B and A are at a ratio of 1:2 with H-ZSM-5, and 1:1.5 with Fe-Z(B), the latter reflecting the larger relative weight of weakly bound ammonia on this material. Thus, apart from the above mentioned discrepancy in temperatures, the data from the two methods are well consistent.
Fig. 2. FTIR spectra of pyridine adsorbed at 523 K. Background spectra of the air pre-treated samples were subtracted.
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Fig. 3. FTIR spectra of pyridine adsorbed at different temperatures. Subtraction of background spectra was not applied.
The position of the bands around 1600 cm1 can be considered as a measure of the Lewis acid strength of the surface sites [25]. Bands at higher wavenumbers (1618 cm1) result from strong Lewis sites whereas bands at lower wavenumbers (1610 cm1) indicate medium strong sites. Comparing the ratio of the bands at 1618/1610 cm1 it can be seen that the intensity of the 1618 cm1 band diminishes with increasing Fe content. This may indicate that stronger Al3+ Lewis sites are covered by FeOx species, while Fe3+ Lewis sites of intermediate strength will be created.
In Fig. 3, the intensity changes of the pyridine bands during thermal desorption can be seen. The rapid decay of the 1445 cm1 band on all three samples is obvious, which is mainly due to the desorption of hydrogen-bonded pyridine. Furthermore, pyridine adsorbed on Brønsted and Lewis sites is still observed at 673 K as indicated by the bands at 1545 cm1 and 1450 cm1, respectively. This also explains the broad high-temperature peaks in the NH3TPD profiles that obviously comprise the desorption of NH3 from both strong Brønsted as well as Lewis sites. The trend of band intensities with temperature is summarized in Fig. 4. With the Brønsted sites, the expected intermediate position of Fe-Z(0.6) between H-ZSM-5 and Fe-Z(1.2) is seen only after desorption at rather high-temperatures. The excess intensity at low temperature may indicate additional Brønsted sites of reduced strength (Fig. 4a). In Fig. 4b, the attention should be focused on the high-temperature region because at low temperatures the information about the Lewis sites is obscured by the band of hydrogen-bonded pyridine. It can be seen that in the Fe-exchanged samples the Lewis acidic sites are more abundant than those in H-ZSM-5. The strong decay in intensity even after desorption of H-bonded pyridine suggests a lower strength of the Fe-derived Lewis sites as indicated by the wavenumber shift discussed above. On the other hand, even after desorption at 673 K, Fe-Z(0.6) holds more Lewis-bound pyridine than H-ZSM-5. This may suggest that there is also a minority of strong Fe-derived Lewis sites. Upon increase of the Fe content to 1.2 wt%, the intensity of the L-Py signal decreases, obviously caused by the abundance of more clustered species in Fe-Z(1.2).
Fig. 4. Change of the FTIR band area for pyridium ions (PyH+) at 1545 cm1 (a) and for pyridine coordinated to Lewis-acid sites (L-Py) at 1445 cm1 (b) with desorption temperature.
M. Schwidder et al. / Microporous and Mesoporous Materials 111 (2008) 124–133
3.2. Structure of iron species UV–Vis and EPR spectra that provide relevant information about the structure of the Fe sites in selected catalysts are presented in Figs. 5 and 6 (see also [5,9]). The focus will be on the sample pairs Fe-Z(0.3)/s-Fe-silicalite and FeZ(0.3)/s-Fe-ZSM-5. In the UV–Vis spectra, all samples exhibit major bands below 300 nm. While the spectra of s-Fe-silicalite and of Fe-Z(0.3) are almost completely confined to this region, the remaining spectra extend to higher wavelengths. It has been reported earlier that signals below 300 nm arise from Fe3+ O charge-transfer bands of isolated ions in tetrahedral and octahedral coordination (220 nm and 285 nm, (t1 ! t2/t1 ! e transitions unresolved [20,28]). Signals between 300 and 400 nm may be assigned to oligomeric clusters, while bands at k > 400 nm indicate the presence of large Fe oxide aggregates [20]. By deconvoluting the spectra into sub-bands as shown in Fig. 5 (for numerical results see (see Table 3), it can be seen that in Fe-Z(0.3) and in s-Fe-silicalite more than 90% of the signal intensity arises from isolated Fe ions. In the two remaining samples, other species have considerable weight, and in general the degree of iron aggregation in s-Fe-ZSM-5 is between those in Fe-Z(0.6) and FeZ(1.2). From the viewpoint of UV–Vis spectroscopy, the same species occur in all catalysts. The most significant difference is a predominance of tetrahedrally coordinated isolated Fe species in s-Fe-silicalite, which is not found in its
1.0
counterpart with similar degree of clustering – Fe-Z(0.3). Among the remaining catalysts, there are just graduations in the degree of clustering, but the spectra are on the whole very alike. In Fig. 6a, EPR spectra of the Fe-ZSM-5 catalysts (in hydrated state) recorded at 77 K and 298 K are compared. In the spectra, several signals can be discerned, which have been discussed in our previous papers [9,20]. The signals at g 0 4.3 and g 0 6, which exhibit Curie-type behavior, are both attributed to isolated Fe3+ ions, in tetrahedral coordination (intra- or extra-framework location) [29–34] and with higher coordination numbers [35,36], respectively. In samples Fe-Z(0.3) and s-Fe-silicalite, for which UV–Vis data reveal the presence of only a negligible amount of clusters, there is another signal with Curie-type behavior at g 0 2, which is assigned to isolated Fe ions in a highly symmetric environment [5,9,20,31,37]. In s-Fe-ZSM-5, Fe-Z(0.6), and Fe-Z(1.2), which do contain clusters of different size and concentration, this signal is hardly seen since it is superimposed by a broader signal at g 0 2, which does not show Curie-type behavior and is attributed to clustered species with anti-ferromagnetic interactions between the Fe ions [5]. Above the Neel temperature, antiferromagnetic ordering collapses, the clustered Fe sites become paramagnetic and the EPR signal intensity increases. This is evident by comparing the relative EPR intensities of Fe-Z(0.3), Fe-Z(0.6) and Fe-Z(1.2) measured at 77 and 298 K (Fig. 6a) with those measured in air flow at
aggregated
mononuclear
oligomeric
1.0
aggregated oligomeric
Fe-Z(0.6) Normalized F(R)
Normalized F(R)
Fe-Z(0.3)
0.5
0.5
Fe-Z(1.2) Fe-Z(B)
0.0
0.0
1.0
1.0
s-Fe-ZSM-5 Normalized F(R)
Normalized F(R)
s-Fe-silicalite
0.5
0.0 200
300
400
500
Wavelength, nm
600
F(R)
mononuclear
129
700
0.5
0.0 200
300
400
500
600
700
Wavelength, nm
Fig. 5. UV–Vis spectra of Fe-MFI catalysts used in the present investigation, after calcination in air at 823 K (see also [5,9]).
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g' = 4.3 g' = 6
g' = 6
g' = 2
g' = 2 Fe-Z(0.3)
Fe-Z(B)
Fe-Z(0.6) 77 K 298 K s-Fe-silicalite Fe-Z(1.2) s-Fe-ZSM-5
0
2000
4000
6000
0
2000
B, G
4000
6000
B, G Fe-Z(0.3) Fe-Z(0.6) Fe-Z(1.2)
g' = 4.3 g' = 6
0
2000
4000
6000
B, G
Fig. 6. EPR spectra of Fe-MFI catalysts used in the present investigation (see also [5,9,20]): (a) comparison of spectra recorded at room temperature and at 77 K, (Fe-Z(B) only at room temperature, not to scale, cf. [20]) and (b) spectra of samples prepared by improved liquid ion exchange measured at 623 K in air flow.
Table 3 Quantification of the UV–Vis spectra of Fe-ZSM-5 catalysts in Fig. 5 Catalyst
I1a (area%)
I2b (area%)
I3c (area%)
Fe-Z(0.3) Fe-Z(0.6) Fe-Z(1.2) s-Fe-silicalite s-Fe-ZSM-5 Fe-Z(B)
95.7 76.2 48.6 93.0 58.7 27
4.3 18.5 23.4 7.0 26.2 38
0 5.3 28.0 0 15.1 35
I1 – k < 300 nm, I2 – 300 < k < 400 nm, I3 – k > 400 nm. a Isolated Fe3+ in tetrahedral and higher coordination. b Oligomeric Fe3þ x Oy clusters. c Fe2O3 nanoparticles.
623 K (Fig. 6b). The temperature dependence of the g 0 2 signal in samples s-Fe-ZSM-5, Fe-Z(0.6), and Fe-Z(1.2) has to be differentiated from the broad g 0 2 signal in s-Fe-silicalite, which decreases in intensity at room temperature, i.e. exhibits Curie-type behavior. The latter broad feature arises probably from weak dipolar interactions between isolated iron sites in s-Fe-silicalite [9]. In summary, it can be stated in the light of the EPR results that practically the same iron species occur in all catalysts although some of them are almost void of FexOy clusters. Again, there are some special features with s-Fe-silicalite (the weak magnetic interactions between isolated sites, which indicates a relatively small average distance between very well distributed Fe sites), but beyond that qualitative differences between the Fe site structures cannot be discerned.
3.3. DeNOx performance NO conversions in the SCR with isobutane and with NH3 are presented in Fig. 7 (data for exchanged samples cited from [5]). Fe-MFI catalysts prepared by extraction of Fe from framework positions, which are regarded as are very promising in N2O abatement reactions [38,39], are very poor in NO abatement. In isobutane-SCR where the Fe-ZSM-5 made by ILIE achieve peak conversions between 70% and 86%, s-Fe-silicalite converts <10% below 700 K, and achieves 20% conversion only above 800 K (Fig. 7a). The s-Fe-ZSM-5 catalyst exhibits an NO conversion maximum as does the vast majority of SCR catalysts, but the peak NO conversion is just above 20%. These catalysts behave similar as the Fe-Z(B) sample, which is given for comparison. As expected, the isobutane conversion trends differ strongly as well, with steeply increasing curves found for the exchanged catalysts but gradually increasing curves for the steamed Fe-MFI samples (Fig. 7c). While with the former the isobutane conversion is clearly lower than the NO conversion below the maximum, such high reductant utilization does not occur with the latter. In NH3-SCR, the relation between the two catalyst types is somewhat different (Fig. 7b). The steam-activated iron-containing zeolites are inferior here as well, but the activities of s-Fe-ZSM-5 and Fe-Z(0.3) are quite similar. s-Fe-silicalite is the poorest catalyst also for NH3-SCR, but while it is clearly inferior to s-Fe-ZSM-5 in isobu-
M. Schwidder et al. / Microporous and Mesoporous Materials 111 (2008) 124–133
a 100
b100 i-C4H10- SCR NO conversion, %
NH3- SCR
NO conversion, % 0 500
600
700
Fe-Z(0.3)
Fe-Z(0.6)
60 40 20
s-Fe-ZSM-5
s-Fe-silicalite
600
800
Fe-Z(1.2)
Fe-Z(B)
d 100
80
80 NH3 conversion
Isobutane conversion, %
80
0 400
800
c 100
60 40 20 0 500
131
60 40 20
600
T,K
700
800
0 400
600
800 T,K
Fig. 7. Catalytic behavior of FeMFI samples in the selective catalytic reduction of NO with isobutane (a, c – 1000 ppm NO, 1000 ppm isobutane, 2 vol% O2, 30,000 h1) and with NH3 (b, d – 1000 ppm NO, 1000 ppm NH3, 2 vol% O2, 750,000 h1).
tane-SCR, both catalysts give identical NO conversions with ammonia up to 700 K. A remarkable point with s-Fe-silicalite is its high NH3 oxidation activity. While the NO and the NH3 conversion curves are practically identical for all remaining catalysts, unselective oxidation of the ammonia becomes predominant over s-Fe-silicalite above 823 K. Another difference to isobutane-SCR is the relatively strong performance of the Fe-Z(B), which, although not being competitive with the better ILIE catalysts, achieved sizable NO conversions. 4. Discussion The relation between acidity and SCR activity will be discussed on the background of structural information about the Fe sites present that can be derived from Figs. 5 and 6 (for details see [5,9]). For the samples prepared via exchange with Fe2+ ions (ILIE), the UV–Vis spectra (Fig. 5, Table 1) show that the iron is largely isolated at low Fe content (>95% at 0.3 wt%). In Fe-Z(0.6), still 75% of the iron is present in isolated species, ca. 20% in oligomeric clusters, and only a minor amount has further aggregated into larger (disordered) particles. In Fe-Z(1.2), the ratio between the Fe atoms in these site types is approximately 50:25:25, i.e. the degree of aggregation is already significant here. The Fe speciation in Fe-Z(0.3) is similar to that in s-Fe-silicalite regarding the high degree of site isolation (Table 1). There are, however, differences in the coordination of iron, which is predominantly tetrahedral
in the latter and both tetrahedral and octahedral in the former (Fig. 5). The analogy in the Fe speciation is closer between Fe-Z(0.6) and s-Fe-ZSM-5, where similar sites are present in comparable amounts though with a stronger aggregation degree in the latter. The study of the acidity of Fe-MFI SCR catalysts has confirmed the expected trends, but has also revealed some interesting details. In the NH3-TPD profiles, the introduction of Fe species by ILIE has lead to a decrease of the high-temperature signal due to zeolite Brønsted sites, accompanied by the appearance of a shoulder around 523 K, which apparently arises from NH3 bound to Fe ions (Fig. 1a). It should be noted that the intensity loss of the high-temperature signal is significant already at 0.6 wt% Fe. As expected, the acidity of the samples prepared via steam extraction of framework Fe or by CVD of FeCl3 into a high-silica Fe-ZSM-5 is inferior (Fig. 1b). The acidic sites detected are mostly weak, however, a certain, very low amount of stronger acidic sites may be discerned in s-FeZSM-5 and in Fe-Z(B). The pyridine adsorption study (Figs. 2–4) confirms that the Fe ions have Lewis-acid character: upon introduction of 0.6 wt% Fe into H-ZSM-5, the L-Py signal is much enhanced and shifted from 1455 to 1450 cm1 (Fig. 2). Surprisingly, however, and in disagreement with the NH3 TPD profile, the intensity of the Brønsted band is almost the same in Fe-Z(0.6) as in H-ZSM-5 and decreases only at 1.2 wt% Fe. This trend is confirmed in the temperature dependence of the 1540 cm1 band (Fig. 4a), where the
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initially stronger intensity loss of Fe-Z(0.6) indicates the presence of a less acidic Brønsted site, which is absent (or not abundant) in Fe-Z(1.2). The assignment of this site is not straightforward. We attribute it tentatively to a Fe3+(OH) group on a site that is abundant in Fe-Z(0.6) but not in Fe-Z(1.2). According to the structural data cited here (Figs. 5 and 6, Table 1, for more details see [5,40]), FeZ(1.2) contains both isolated and oligomeric sites in larger (absolute) abundance than Fe-Z(0.6). It has been found, however, that some of the isolated sites (those characterized by g values of 4.3 and 6 in EPR) are rare in FeZ(1.2), probably due to their participation in the clustering that occurs at higher Fe content and higher temperatures [40] (Fig. 6b). OH groups associated with these Fe species might be candidates for the Brønsted sites of moderate acidity that give rise to the extra intensity in Fe-Z(0.6). Together with the structural data discussed above these results provide a good basis for the interpretation of the catalytic data. In isobutane-SCR (Fig. 7a and c), a key involvement of Brønsted sites is very likely. This is not so much obvious from the comparison between the non-acidic s-Fe-silicalite and the acidic Fe-Z(0.3) because the difference could also originate from the difference in the coordination of the predominantly isolated Fe sites. However, a large advantage of the acidic catalyst can be also seen in the comparison of s-Fe-ZSM-5 and Fe-Z(0.6) which contain Fe sites of similar structure. The poor performance of Fe-Z(B) supports the conclusion. It appears that the Brønsted sites are involved already in the activation of the isobutane because even the isobutane conversion is largely suppressed over the non-acidic samples. Given the important role of hydrocarbon deposits in isobutaneSCR over Fe-ZSM-5 [41], this is well in accordance with our earlier observation from an operando-DRIFTS study, according to which the interaction of isobutane-SCR feed with H-ZSM-5 leads to very similar hydrocarbon deposits as the interaction with Fe-ZSM-5 [42]. As these adsorbates are involved in the reduction of NOx activated on nearby Fe sites their absence should strongly impede the SCR reaction. A rate limiting role of acidity at a later stage (e.g. in NH4NO2 decomposition, which according to [15,19] is part of the reaction sequence) is less likely because even if isobutane-SCR proceeds via a concluding NH3SCR step, the rate of the latter is much larger than the observed rates of the former. With NH3 as the reductant, an influence of the Brønsted sites is less obvious (Fig. 7b and d). Still, the non-acidic samples s-Fe-silicalite and s-Fe-ZSM-5 exhibit the poorest performance. However, these two catalysts achieve identical NO conversions over quite a broad temperature range despite significant differences in acidity (Fig. 1b). Moreover, while the NO conversion levels off with s-Fe-silicalite, s-Fe-ZSM-5 performs quite similar as the strongly acidic Fe-Z(0.3). The difference might well be accounted for by a better accessibility of the Fe species in the latter sample. Finally, the remarkable performance of the non-acidic FeZ(B) in NH3-SCR (as opposed to isobutane-SCR, cf.
Fig. 7a) shows that the former reaction is possible also materials deficient in Brønsted sites. However, the very high activity of Fe-Z(0.6) and in particular of Fe-Z(1.2) is not fully compatible with this picture. It has been argued that clustered sites might be more effective in NH3-SCR than isolated sites as long as the dispersion remains high [5], but a large difference in activity is also seen between the strongly acidic Fe-Z(0.6) and the weakly acidic s-Fe-ZSM-5 (Figs. 7b and d), which has a similar Fe content and an only slightly more clustered Fe phase. This difference, which is illustrated by a marked decrease of the temperatures for 50 % conversion (827 K for s-Fe-ZSM-5, 653 K for Fe-Z(0.6)) is probably too large to be explained by increased local NH3 concentrations created by neighboring Brønsted sites, rather, it suggests that an additional reaction channel is opened by the presence of zeolite protons. This channel might be the acid-catalyzed decomposition of intermediate NH4NO2 as recently proposed by Li et al. [19]. It is known that this step proceeds also uncatalyzed, and it might be rapid enough at high temperatures to allow for large reaction rates. At low temperatures, the ammonium nitrite decomposition might become rate-limiting, and the catalytic effect of Brønsted sites on it would be felt in the total reaction rate. Therefore, while considerable NH3-SCR rates can be achieved with nonacidic samples as well, the development of top de-NOx performances may require the combination of oligomeric Fe oxo clusters with Brønsted sites. 5. Conclusions In the present study, Fe-MFI catalysts that have a similar structure of Fe sites (as revealed by UV–Vis and EPR spectroscopic data) and were found to exhibit comparable performance in N2O decomposition in companion work have been compared for their catalytic properties in the SCR of NO with isobutane and with NH3. The dramatic differences observed have been attributed to their strongly different acidity properties, which have been characterized by TPD of ammonia and by IR of adsorbed pyridine. Fe-ZSM-5 prepared by exchange of Na-ZSM-5 with Fe2+ ions formed in situ by acidic of Fe powder in acidic solution exhibits high acidity and high SCR activity. Fe-MFI prepared by steam extraction of framework iron from Fe-silicalite or from H-[Fe]-ZSM-5 is weakly acidic and poor in activity. The differences are dramatic for isobutane-SCR over the whole range of parameters studied, which indicates a vital role of Brønsted sites in this reaction. In NH3-SCR, large reaction rates can be achieved with non-acidic catalysts, but acidity definitely increases the activity provided the catalysts contain the iron in the most favorable structure (oligomeric Fe oxo clusters). This suggests that an acid-catalyzed step (e.g. the decomposition of NH4NO2) may become rate-limiting at low temperatures. The data show that the highest activities in NH3SCR may be achieved with catalysts containing both oligomeric Fe oxo clusters and Brønsted sites.
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