ZSM5 as a new superior catalyst for NO reduction with NH3

Applied Catalysis B: Environmental 73 (2007) 60–64 www.elsevier.com/locate/apcatb

Mn–Ce/ZSM5 as a new superior catalyst for NO reduction with NH3 Gabriela Carja a,*, Yoshikazu Kameshima b, Kiyoshi Okada b, Changalla D. Madhusoodana c b

a Department of Physical Chemistry, Faculty of Industrial Chemistry, Technical University of Iasi, 71 D. Mangeron, Iasi, Romania Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, Japan c Ceramic Technological Institute, BHEL, Bangalore 560012, India

Received 2 March 2006; received in revised form 2 June 2006; accepted 3 June 2006 Available online 21 July 2006

Abstract Mn–Ce/ZSM-5 catalyst prepared in an aqueous phase at 423 K exhibits a broad temperature window (517–823 K) for high NO conversions (75– 100%) in the process of selective catalytic reduction (SCR) by NH3 even in the presence of H2O and SO2. Both the zeolite matrix and the overexchanged amounts of manganese and cerium contribute to obtain a complex structure that owns microporous–mesoporous characteristics and specific surface properties. # 2006 Elsevier B.V. All rights reserved. Keywords: SCR; NO reduction; Manganese; Cerium; ZSM-5

1. Introduction Nowadays, nitrogen oxides emitted by stationary sources contribute up to nearly 48% to total (NO)x emissions therefore being an important source for air pollution (e.g. ozone depletion, photochemical smog, greenhouse effects and acid rain) [1]. The most effective technology to remove them is the selective catalytic reduction (SCR) of NO by ammonia (4NO + 4NH3 + O2 ! 4N2 + 6H2O). Although, many catalysts have been reported to be active for this reaction continuing efforts have been made in developing new catalysts more active within a broader temperature range [2,3]. Up to now, there have been two different approaches on the study of this catalytic process: one has focused on the low temperature range and the other on the high temperature range. For the low temperature range, catalysts containing transition metals have been investigated (e.g. MnOx/ Al2O3 [4], MnOx/NaY [5], CuO/TiO2 [6]). Qi and Yang recently reported Mn–Ce mixed oxides as a catalyst with superior activity for the process of NO reduction by NH3 at low temperature [7,8]. For the high temperature range, zeolites containing overexchanged amounts of different metals (Cu, Ni, Mn, Co and specially Fe) have shown high catalytic performances [9]. The combination between zeolite microstructure and over-exchanged

* Corresponding author. Tel.: +40 232 201231; fax: +40 232 201160. E-mail address: [email protected] (G. Carja). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.06.003

amounts [10] of different forms of metal cations and/or metal oxides is able to give rise to a complex structure that possesses special structural features and specific acid–base characteristics [11,12]. In particular, the textural and surface characteristics of these materials are able to play an important role in establishing their structure–catalytic activity relationship [13]. Considering together the information regarding the special performances of Mn–Ce mixed oxides for the low temperature range of the SCR process and the high activity for NO reduction, over the high temperature range, of zeolites with over-exchanged amounts of different cations, we prepared, by a specific method, ZSM-5 containing over-exchanged amounts of manganese and cerium. In this work, we report Mn–Ce/ZSM-5 as a new catalyst showing high performances, within a broader temperature window, for the process of SCR of NO by ammonia. The porous and the surface properties of the new structure formed by merging the zeolite matrix and overexchanged amounts of manganese and cerium have also been evaluated. 2. Experimental 2.1. Synthesis Manganese and cerium containing ZSM-5 samples (denoted as MnCeZ1 and MnCeZ2) were prepared in an aqueous phase by improving a previous reported method [14]. The zeolite

G. Carja et al. / Applied Catalysis B: Environmental 73 (2007) 60–64

material, NH4-ZSM-5 (Si/Al = 16.4, SBET = 400 m2 g1), supplied by Zeolyst International was used in its H+ form. One hundred millilitres of aqueous solution of manganese(II) acetylacetonate (0.025 M for MnCeZ1 and 0.04 M for MnCeZ2) was added during a 2.5 h period over 3 g of zeolite dispersed in 50 ml of water, at 423 K, under strong stirring in a refluxing flask. After the complete addition of the manganese salt, 100 ml of aqueous solution (0.01 M for MnCeZ1 and 0.02 M for MnCeZ2) of cerium nitrate was slowly added over a 2 h period in the synthesis medium. When addition of the cerium salt was completed, the temperature was slowly decreased to 353 K, the mixture was aged for 24 h and dried at 373 K overnight. The obtained materials were calcined at 723 K, for 5 h, under a stream of helium and oxygen. 2.2. SCR performance The catalytic experiments are performed at atmospheric pressure with a conventional fixed-bed flow reactor. The typical reaction conditions are as follows: 20 mg of sample (0.025 ml), 2000 ppm NO, 2000 ppm NH3 and 3% O2 in helium, balance He, 138.3 ml min1 total flow rate and a GHSV = 332,000 h1. The effluent composition was monitored continuously by sampling on line to a quadrupole mass spectrometer and nine masses characteristic of NO (30), NO2 (30, 46), N2O (28, 30, 44), NH3 (16, 17), H2O (17, 18), O2 (16, 32) followed.

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mounted onto double-sided sticky tape in an analysis chamber typically operating at 1.33  107 Pa. All binding energy (BE) values were determined referenced to the C 1s line (284.6 eV) of the carbon overlayer. The standard deviation of the peak position was within 0.1 eV. The surface (XPS) concentration of each element, expressed as surface atomic ratios, was calculated from the Mn 2p, Ce 3d, Al 2p and Si 2p peak areas and corresponding atomic sensitivity factors [15]. 2.3.4. N2 adsorption N2 adsorption isotherms were measured at 77 K on a Quantachrome Autosorb-1 automated gas adsorption system. Microcomputer processing controlled the analysis. Prior to the measurements, the samples were evacuated for 14 h under vacuum at 473 K. The standard Brunauer–Emmett–Teller method (BET) [16] was used to calculate the specific surface area (SBET) of the samples. Although, the applicability of BET method for highly microporous materials is questionable, the values of the surface area derived from the BET model in the adapted pressure range [0.01–0.1] were used only for comparative purpose. The micropore volume (mVp) and the mesoporous surface area (Amesop) were calculated according to the t-plot method [17]. 3. Results and discussion 3.1. XRD, XRF and XPS analysis

2.3. Characterization 2.3.1. XRF The chemical compositions of the synthesized samples were determined by X-ray fluorescence spectroscopy (Rigaku RIX2000 sequential X-ray fluorescence spectrometer). 2.3.2. XRD X-ray powder diffraction patterns were recorded on Shimadzu XRD 6100 diffractometer using monochromatic Cu Ka radiation (l = 0.154 nm), operating at 40 kV and 30 mA over a 2u range from 48 to 708. 2.3.3. XPS X-ray photoelectron spectroscopy spectra were recorded using a Perkin-Elmer Model5500-MT spectrometer equipped with Mg Ka radiation (1253.6 eV) operating at 15 kV and 20 mA. Microcomputer processing controlled the spectra acquisition and handling. Samples were analyzed as powders

The XRD patterns of the synthesized samples (not shown) are typical for well-crystallized ZSM-5 structure with high aluminum content. No diffraction peaks attributed either to metal and/or to metal oxide clusters were observed, suggesting that the formed manganese and cerium species are in the nanometer size range and well dispersed. The bulk and surface chemical compositions of the samples are listed in Table 1. For MnCeZ1 sample, the bulk Mn/Al and Ce/Al ratios are higher than the corresponded surface ratios, indicating that most of the manganese and cerium species are located inside of the zeolite structure. When the manganese and cerium content increases, in MnCeZ2 sample, the surface Ce/ Al ratio becomes higher than the corresponded bulk ratio suggesting that cerium species are enriched on the surface. The XPS binding energy values of the corresponding elements are shown in Table 2. For all samples, the BE values characteristic for Si 2p and Al 2p are equal to 102.99  0.1 and 74.42  0.1 eV, respectively. These values are close to those

Table 1 The bulk and surface chemical compositions of the MnCeZ samples Sample

MnCeZ1 MnCeZ2 a b

XRF bulk composition (atm%) Mn

Ce

Mn/Al

Ce/Al

Si

2.2 3.7

1.7 2.3

1.05 1.92

0.8 1.2

37.4 36.7

As determined by ICP analysis. For MNCeZ2 used catalyst, after 7 h on stream.

Mn, Ce loadinga (mmol gcat1) 0.76 0.94

XPS surface composition (atm%) Mn 2p

Ce 3d

Mn 2p/Al 2p

Ce 3d/Al 2p

Si 2p

0.67 2.27 0.81b

1.14 2.19 0.77

0.33 1.33 0.38

0.57 1.28 0.37

22.95 18.24 22.77

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G. Carja et al. / Applied Catalysis B: Environmental 73 (2007) 60–64

Table 2 The XPS data of MnCeZ samples Sample

BE (eV) Mn 2p

MnCeZ1 MnCeZ2

Ce 3d

2p 3/2

2p 1/2

3d 5/2

642.11 641.09

653.94 652.89

886.23 885.09

882.31 881.44

Al 2p

Si 2p

O 1s

74.42 74.37

103.01 102.99

532.09 532.18

529.84 529.57

previously reported for the tetrahedraly coordinated elements in zeolites framework [18]. The O 1s spectra shows two main features, one at 532.18  0.1 eV assigned to oxygen in a Si–O– Si environment [19,20] and the other around 529.6  0.2 eV assigned to oxygen in metal oxides [21]. The main Mn 2p3/2 component is observed at 642.11 eV for MnCeZ1 sample; the BE difference between this peak and the corresponding 2p1/2 peak equal to 11.7  0.1 eV indicates that most of the manganese species exist in a higher oxidation state in this sample. For MnCeZ2, the BE value of Mn 2p3/2 decreases to 641.09 eV, suggesting the lowering of the oxidation state of manganese when cerium species are enriched on the surface. Previously reported results demonstrate that mixed-valent manganese oxide species formed when manganese was loaded in the pores of MCM-48 [19]. BE values of Mn 2p3/2 between 640 and 645 eV were reported by Qi and Yang [22] for the manganese–cerium oxide catalysts; these values are higher than the corresponding binding energies characteristic to MnO, MnO2 and Mn2O3 therefore they suggest that strong interactions exist between manganese and cerium oxides. On the contrary a much narrow range, of Mn 2p3/2 BE values, is characteristic for our samples; this reveals the absence of strong interactions between manganese and cerium oxides dispersed on the surface of ZSM-5. It is reported that the Ce 3d XPS spectra owns six features of Ce4+ and four features of Ce3+ [23].

The patterns characteristic to Ce 3d XPS spectra (see Fig. 1) suggest a different contribution of Ce4+ and Ce3+ on the surface supporting the previously reported results for cerium exchanged ZSM-5 [24]. Moreover, the shift of BE values of Ce 3d5/2 XPS spectra from 886.23 and 882.31 eV (characteristic for MnCeZ1) to 885.09 and 881.44 eV, respectively (characteristic for MnCeZ2) (see Table 2) indicates that the XPS ratio of the redox couple Ce4+/Ce3+ is a function of the cerium–manganese content of the sample. For the used catalyst, the surface concentration of manganese and cerium strongly decreases. The variations of the surface concentrations of manganese and cerium species, after reaction, point to the direct involvement of the species present on the surface in the catalytic process.

Fig. 1. Ce 3d XPS spectra of (a) MnCeZ2 and (b) MnCeZ1.

Fig. 2. N2 adsorption-desorption isotherms of (*) MnCeZ1 and (!) MnCeZ2.

3.2. N2 adsorption The N2 adsorption isotherms are shown in Fig. 2. The isotherm characteristic of MnCeZ1 sample shows combined features characteristic to both types I and IV behavior (according to IUPAC classification) with high nitrogen uptakes at a low relative pressures (P/P0) and a remarkably enhanced uptake of nitrogen at a P/P0 value close to unity; this suggests the presence of both micro and mesoporosity in this sample. The t-plots results are given in Table 3. The contribution of the mesopores area to the total surface area equal to 28% coupled with the decreased values of the surface area value (SBET = 400 m2/g for the supplied zeolite) and micropore volume also confirms the development of the mesoporous properties. The combined micro-mesoporous

G. Carja et al. / Applied Catalysis B: Environmental 73 (2007) 60–64 Table 3 The porous properties of MnCeZ samples Sample

SBET (m2/g)

Vp (cm3/g)

mVp (cm3/g)

Amesop (m2/g)

MnCeZ1 MnCeZ2

350 270

0.19 0.17

0.127 0.105

98 94

structure of MnCeZ1 is in agreement to the results previously reported by Gervasini that also indicates the development of mesoporosity properties in Cu, Ni and Co over-exchanged ZSM5 [25]. For MnCeZ2, the isotherm features are enough close to that of MnCeZ1 though a lower adsorption capacity over the entire relative pressure range is observed in this case. This may be a consequence of the blocking micropores phenomena and the limited macro-mesoporosity when higher amounts of manganese and cerium are introduced in the ZSM-5 matrix. Moreover, the tplot results reveals the decrease of the micropore volume and an increased contribution of the mesopore area in the total surface area up to 35% that indicates less emphasized microporous features in MnCeZ2. Both the zeolite matrix (with regard to its dealumination and steaming process that could appear during the sample preparation in an aqueous phase at 423 K) and the overexchanged amounts of manganese and cerium introduced in the zeolite structure are able to contribute to the formation of a complex porous structure defined by specific micro-mesoporous features. 3.3. Catalytic activity The NO conversion as a function of temperature is shown in Fig. 3. Under the specified conditions, with a high GHSV (3.32  105 h1), both samples exhibit a similar trend of NO conversions though the sample characterized by higher manganese and cerium content exhibit better catalytic performances. The light-off temperature (50% NO conversion) is reached at ca. 523 K for MnCeZ1 while the value is shifted to

Fig. 3. NO conversion as a function of temperature, (5) MnCeZ2 and (*) MnCeZ1.

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488 K when manganese and cerium content increases in MnCeZ2. Ninety percent conversion is reached around 593 K for MnCeZ1 while the corresponding temperature value decreases to 548 K for MnCeZ2. Both samples reached the full conversion above 625 K. On the whole temperature range, the NO selectivity to N2 is only slightly below 100%. These results point out that the new designed catalyst shows a very good catalytic activity in the SCR NO by NH3 in a broader temperature range than the previous reported catalysts, type metals or metals oxides exchanged zeolites [13,11,26,27]; the combination between the over exchanged amounts of manganese and cerium and the zeolite matrix gives rise to the specific porous characteristic and redox properties that are able to act together in the NO reduction process. Moreover, the catalytic performances of the catalyst are a function of the cerium–manganese content of the sample. The resistance of the tested samples to deactivation by water vapor and SO2 is very important for industrial applications in the DeNOx processes. Moreover, it has been reported that the effect of water and SO2 on the catalytic activity of over-exchanged zeolites in the SCR process is a function of the nature of the exchanged metals [25]. Hence, we studied further the effects of H2O and SO2 on the NO conversion over MnCeZ2 at 548 K. The results are shown in Fig. 4. Before adding SO2 and water the SCR reaction was stabilized for 1.5 h at 548 K. Upon switching to a H2O, SO2 containing feed (2.0% H2O and 35 ppm SO2), a barely detectable decrease in the conversion is observed; after removing the water vapor and SO2 the catalyst activity is restored. When Mn–Ce mixed oxides were used as catalyst for the low-temperature SCR process a similar behavior regarding the effect of H2O and SO2 on the NO conversion was observed [7,8]. In conclusion, the new Mn–Ce/ZSM-5 catalyst exhibits high performance within a broad temperature window for the selective catalytic reduction of NO when ammonia is used as a reductant. NO conversion maintains above 75% in a broader temperature range (517–823 K) with a GHSV of 3.32  105 h1. The catalytic activity is stable even in the presence of water vapors and SO2. The facility of the catalyst preparation in an aqueous phase is cost effective and environmentally friendly, thus attractive for industrial application. Further studies including

Fig. 4. NO conversion as a function of time at 548 K on MnCeZ2 catalyst.

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