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β catalyst for NOx reduction by NH3
Fuel Processing Technology 133 (2015) 220–226
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Ce doping effect on performance of the Fe/β catalyst for NOx reduction by NH3 Shui-Yan Jiang, Ren-Xian Zhou ⁎ Institute of Catalysis, Zhejiang University, Hangzhou 310028, PR China
a r t i c l e
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Article history: Received 4 October 2014 Received in revised form 5 December 2014 Accepted 3 February 2015 Available online 27 February 2015 Keywords: NH3-SCR Fe/β Ce doping Hydrothermal aging SO2 resistance
a b s t r a c t The Ce doping Fe/β catalysts were prepared by an easy incipient wetness impregnation method and characterized using various analytical techniques, including H2-TPR, XRD, UV–vis and XPS. The results demonstrated that the addition of cerium effectively improved the deNOx performance, hydrothermal stability and SO2 resistance of the Fe/β catalyst for the selective catalytic reduction (SCR) of NOx by NH3. The finely dispersed iron oxide nanoparticles on the surface of Ce–Fe/β catalysts and the compensative effect of cerium are dominating reasons for high catalytic performance. The Ce doping enhanced the hydrothermal stability of the Fe/β catalyst because it could control the growth of iron oxides and stabilize the dispersed iron species. Suppressing the deposition of sulfates by Ce addition made the Fe/β catalyst a great SO2 tolerance. It was speculated that the outstanding performance of Ce doping Fe/β catalysts made them attractive for practical application. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxides, such as NO and NO2, originated from mobile and stationary sources have been an important source of air pollution. Due to numerous environmental issues (e.g. acid rain, photochemical smog, ozone depletion and greenhouse effect) and serious damage to human health resulted from NOx, stringent regulations and great efforts have been made to limit NOx emissions [1–3]. Up to now one of the predominant technologies for eliminating NOx is regarded as the selective catalytic reduction with ammonia (NH3-SCR) [4,5]. The commercial V2O5–WO3(MoO3)/TiO2 catalyst has been applied widely and exhibits good catalytic performance [6]. However, some inevitable problems still remain, such as the toxicity of vanadia species, narrow activity temperature range and undesired oxidation of SO2 to SO3 [7–9]. Therefore, it is necessary to develop superior SCR catalysts. In recent years, zeolite-based catalysts promoted by various transition metals have attracted much more attention in diesel engines control [10,11]. Among them, Fe-exchanged zeolite catalysts have been found to be attractive candidates for SCR reaction because of their excellent SCR activity, high N2 selectivity and superior stability. M. Iwasaki et al. [12] investigated the effect of zeolite structure on the catalytic performance of fresh Fe/zeolite catalysts and ranked the following order: Fe/BEA N Fe/MFI N Fe/FER N Fe/LTL N Fe/MOR. C. He et al. [13] revealed that the 2.5 wt.% Fe/beta catalyst prepared by incipient wetness
⁎ Corresponding author. Tel.: +86 571 88273290. E-mail address: [email protected] (R.-X. Zhou).
http://dx.doi.org/10.1016/j.fuproc.2015.02.004 0378-3820/© 2015 Elsevier B.V. All rights reserved.
impregnation showed NO conversion over 97.0% in the temperature range of 300–500 °C at a high space velocity (1.9 × 105 h−1) and was stable up to a hydrothermal aging temperature of 750 °C. Besides, many other investigations, such as the reaction mechanism and influence of preparation method and iron content, were conducted as well [14,15]. But it can be seen that limited improvement of catalytic performance is achieved for single iron exchanged zeolite catalysts. Thus, the addition of extra metal ions as promoters may be a promising trial to further improve catalytic performance of Fe/zeolite catalysts. Cerium is a common rare earth metal and has been discussed vastly due to its particular properties. Especially, cerium which is called an oxygen reservoir can store and release oxygen via the redox shifted between Ce4 + and Ce3 + under oxidizing and reducing conditions, respectively [16,17]. A lot of cerium doping catalysts have been reported in many fields, such as Mn–Ce/ZSM-5 [18,19], CeO 2–MnO x [20], Cu–Ce/ZSM-5 [21,22] and Fe–Ce/TiO2 [23]. It is worth noting that the cerium modified Fe/zeolite catalysts used for NH 3-SCR reactions have been paid little attention. In addition, investigations on the deNOx performance and stability of Ce doping Fe/zeolite catalysts have not been systematically conducted. In this paper, to understand and elucidate Ce doping effect on deNOx performance of the Fe/β catalyst, the catalytic activity, hydrothermal stability and sulfur-resistance durability of Ce–Fe/β catalysts were systematically investigated. At the same time, the chemical state of iron species and the influence of cerium introduction on active iron species were clarified through a series of characterizations, which perhaps will be helpful to further develop more efficient NH3-SCR catalysts.
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2.2. Catalyst activity testing The activity measurement for NH3-SCR was conducted through a fixed-bed continuous flow reactor. An argon gas mixture containing 500 ppm NOx, 500 ppm NH3 and 5 vol.% O2 was introduced into the reaction tube at steady state. The concentrations of NH3, NOx (NO and NO2) and N2O in the inlet and outlet gas were measured by an IR spectrometer (Bruker EQUINOX 55) equipped with a 10 m gas cell. In the test, the total flow rate was fixed at 160 mL min−1, corresponding to a GHSV (gas hourly space velocity) of 48,000 h−1. The performance of the catalysts is shown in term of conversion defined as follows: NOx conversionð%Þ ¼
½NOx inlet−½NOx outlet ½NOx inlet
ð1Þ
The sulfur-resistance testing of selected catalysts was conducted using a continuous feed gas (500 ppm NH3, 500 ppm NOx, 5 vol.% O2 and 50 ppm or 100 ppm SO2, balance Ar) for 8 h at 280 °C. The conversion of NOx was measured at set intervals during the whole experimental process. 2.3. Characterization The powder X-ray diffraction (XRD) measurements have been conducted with a D/Max-2550pc system with Cu Kα (λ = 0.15406 nm) radiation in the 2θ ranges from 5° to 75°. For hydrogen temperature programmed reduction (H2-TPR) experiments, 100 mg of sample was put into a quartz reactor, pretreated in N2 (99.999%) stream for 30 min at 400 °C and then cooled down to 50 °C. The reduction process started with a ramp of 10 °C min−1 from 50 °C to 850 °C in a flow of 5 vol.% H2/Ar (40 mL min−1). UV–vis diffuse reflectance spectra (UV–vis DRS) were collected in air in the 200–800 nm wavelength range using the appropriate baseline correction on a UV-2401 PC (Shimadzu). Deconvolution of the UV–vis spectra into subbands was performed. X-ray photoelectro spectra (XPS) were measured on a Thermo ESCALAB 250 system using Al-Kα (1486.6 eV) X-ray source. The binding energy was calibrated by C1s peak of contaminant carbon (286.6 eV) as an internal standard. 3. Results and discussion 3.1. DeNOx performance 3.1.1. Catalytic activity of fresh catalysts The fresh Fe/β and Ce doping Fe/β catalysts were tested from 100 °C to 550 °C and the results of deNOx performance in NH3-SCR reactions are shown in Fig. 1 Based on the previous study, iron loading amount
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The Fe/β and Ce doping Fe/β catalysts were prepared by incipient wetness impregnation using an industrial H-β (SiO2/Al2O3 = 40) zeolite as the catalyst support. The Fe content of all the catalysts was fixed at 2.0 wt.% and the Ce concentration varied. It was performed by mixing H-β powders with the required amounts of Fe(NO3)3 and Ce(NO3)3 solution and stirred for 12 h at room temperature, then evaporated redundant water and dried for 2 h at 110 °C, finally calcined in air for 5 h at 500 °C. Finally, all the obtained powder samples were ground and then sieved to 40–60 mesh. The samples were denoted as 2%Fe/β, 0.5%Ce–2%Fe/β, 2%Ce–2%Fe/β and 5%Ce–2%Fe/β, respectively. To investigate the hydrothermal stability of catalysts, the selected samples were aged in a quartz tube reactor using compressed air containing 10 vol.% H2O at the flow rate of 25 mL min−1 for 24 h at 700 °C.
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Temperature (°C) Fig. 1. NOx (A) and NH3 (B) conversions over Fe/β and Ce doping Fe/β catalysts.
of 2 wt.% was chosen as the candidate for the preparation of Ce–Fe/β catalysts. After appropriate amount of cerium was added to Fe/β, a positive influence on catalytic activity was observed obviously, especially in region of the high temperature. As seen in Fig. 1A, for the 0.5%Ce–2%Fe/β catalyst, the NOx conversion greatly exceeds the Fe/β catalyst at high temperature (N 470 °C) with a little loss at low temperature, corresponding to operating temperature window being broadened. This trend continues with increasing cerium loading; the NOx conversion is enhanced considerably at high temperature and slightly decreases at low temperature. But the promoting effect of cerium at high temperature for the 5%Ce–2%Fe/β catalyst is less pronounced than that for the 2%Ce–2%Fe/β catalyst, which indicates that the excess Ce doping perhaps has a negative effect on high-temperature activity. Among all the catalysts prepared in this paper, the 2%Ce–2%Fe/β catalyst exhibits the best deNOx performance and shows above 90% NOx conversion within the range of 280–520 °C. It is noted that there is no byproduct (such as N2O) generation for all the catalysts, indicating the addition of cerium has not affected the N2 selectivity. In addition, ammonia conversion was also conducted during the reaction process. Fig. 1B shows that the trend of NH3 conversion is almost the same as that of NOx conversion. But all catalysts show relatively higher NH3 conversion than NOx elimination, especially at the temperature above 500 °C. That is due to the severe ammonia oxidation at high temperature, which competes with the process of SCR reaction and should be avoided as far as possible. In short, the results clearly demonstrate the Ce doping is beneficial to improve the SCR activity of the Fe/β catalyst. 3.1.2. Hydrothermal stability of catalysts To study the influence of hydrothermal aging on SCR activity, the 2%Fe/β and 2%Ce–2%Fe/β catalysts were treated at 700 °C for 24 h with flowing compressed air containing 10 vol.% H2O. The NOx conversions of 2%Fe/β and 2%Ce–2%Fe/β catalysts before and after aging treatment are displayed in Fig. 2. It can be seen that an obvious
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Temperature (°C) Fig. 2. Effect of hydrothermal aging on NOx conversion of 2%Fe/β and 2%Ce–2%Fe/β catalysts.
loss of NOx reduction activity occurs after hydrothermal treatment for both catalysts, which is probably the result of the migration and agglomeration of iron species [24]. Furthermore, the temperature windows of 90% conversion are 270–485 °C, 305–485 °C, 280–520 °C and 300–515 °C for fresh 2%Fe/β, aged 2%Fe/β, fresh 2%Ce–2%Fe/β and aged 2%Ce–2%Fe/β, respectively, which indicates that lowtemperature SCR is more sensitive to hydrothermal aging compared to the high-temperature SCR. Meanwhile, the shifts in temperature window of 90% NOx conversion for 2%Fe/β and 2%Ce–2%Fe/β are 35 °C and 25 °C before and after ageing, respectively. It can be seen that the deactivation of the 2%Ce–2%Fe/β sample is slighter than that of the 2%Fe/β catalyst, indicating that the 2%Ce–2%Fe/β catalyst is more resistant to hydrothermal ageing than the 2%Fe/β catalyst. This result demonstrates that the Ce doping further improves the hydrothermal stability of the Fe/β catalyst.
3.1.3. Sulfur-resistance testing of catalysts With regard to the actual use of lean SCR catalysts, sulfur poisoning is an important problem that should not be ignored. The effect of SO2 on the NOx conversion over 2%Fe/β and 2%Ce–2%Fe/β catalysts is displayed in Fig. 3. The samples were continuously exposed to feed gas containing 0 ppm, 50 ppm and 100 ppm SO2 at 280 °C for 10 h, respectively. It is
seen that the initial NOx conversion is about 97% and keeps steady within 10 h for both catalysts in absence of SO2. When adding SO2 to the feed gas, the deNOx performance of both catalysts decreases obviously. Particularly, the NOx conversion of the 2%Fe/β catalyst was down from 97% to 73.8% and 62.0% in 8 h for 50 ppm and 100 ppm SO2 poisoning, respectively. While the NOx conversion of the 2%Ce–2%Fe/β catalyst was accordingly down to 78.4% and 66.1% in both cases. It is quite clear that the 2%Ce–2%Fe/β catalyst exhibits less sensitivity to SO2 than the 2%Fe/β catalyst, which indicates that the Ce doping can improve the sulfur-resistance of the Fe/β catalyst. Furthermore, the SCR activity could not recover when SO2 was off, suggesting that the deactivation role of SO2 is not the competitive adsorption. The poisoning reason was expected as the formation of sulfates (SO2− 4 ) covering active sites [25,26]. According to the literatures [27,28], high temperature calcination for decomposing sulfates is presumably an effective way to achieve the whole or partial regeneration of catalysts. In order to investigate the feasibility of regeneration for the catalysts deactivated by SO2, the activity measurement was conducted over the 2%Ce–2%Fe/β sample after being exposed to 100 ppm SO2 at 280 °C for 8 h. It turned out that the low temperature activity of the sulfated sample decreased distinctly and the initial reaction temperature of achieving 100% NOx conversion increased by 10 °C, while the deNOx performance at high temperature was almost unaffected, which may be on account of sulfates decomposing at high temperature. It is speculated that hightemperature treatment may regenerate the catalysts deactivated by SO2, providing facilitation for practical application.
3.2. Characterization results of catalysts 3.2.1. XRD In order to probe the Ce doping effect and possible structural changes, the XRD patterns of Fe/β and Ce doping Fe/β catalysts before and after hydrothermal ageing treatment are presented in Fig. 4. The characteristic diffraction peaks for β zeolite can be observed over all fresh and aged catalysts, which indicates that zeolite crystal structures keep largely intact even under high-temperature hydrothermal conditions. Thus what can be concluded is that the loss of SCR activity is not the result of structural changes of β zeolite support. The diffraction lines corresponding to iron oxide crystallites (hematite, PDF 33-0664) were not detected among all fresh catalysts, which indicates that the Ce doping cannot apparently affect the dispersion state of iron oxide species. When the Ce loading is 0.5 wt.%, no
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Fig. 3. Effect of SO2 on deNOx performance of 2%Fe/β (A) and 2%Ce–2%Fe/β (B) catalysts.
Fig. 4. XRD patterns of fresh 2%Fe/β (a), fresh 0.5%Ce–2%Fe/β (b), fresh 2%Ce–2%Fe/β (c), fresh 5%Ce–2%Fe/β (d), aged 2%Ce–2%Fe/β (e) and aged 2%Fe/β (f) catalysts.
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(A) 5% Ce-2% Fe/ 2% Ce-2% Fe/ 0.5% Ce-2% Fe/
Absorbance
diffraction peaks of CeO2 are observed in Ce–Fe/β samples. These results show that iron and cerium species are well dispersed on the surface of β zeolite support as amorphous oxides, or aggregated in minicrystals invisible by XRD. Besides, for the 2%Ce–2%Fe/β and 5%Ce–2%Fe/β catalysts, diffraction peaks at 2θ = 28.7°, 47.9° and 56.8° can be identified, indicating crystallite CeO2 existing on the external surface of β zeolite [29,30]. Combining the catalytic activity results, it is deduced that excessive crystallite CeO2 may have some passive influence on high temperature activity. However, it is noted that typical peaks from crystalline α-Fe2O3 at 2θ = 35.6°, 49.5°, 54.1° and 62.5° appear for both aged samples, indicating the agglomeration and growth of dispersed iron species. This may decrease the deNOx activity of aged 2%Fe/β and 2%Ce–2%Fe/β catalysts [13,31]. Meanwhile, the particle size was estimated with Scherrer equation. It turned out that the sizes of iron oxides are 28.7 nm and 43.2 nm for aged 2%Ce–2%Fe/β and aged 2%Fe/β catalysts, respectively, which indicates that the Ce addition may ease the growth of iron oxides species during aging treatment. The better dispersion of iron species may improve the hydrothermal stability of the Ce doping Fe/β catalysts.
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Wavelength (nm) Fig. 6. UV–vis DRS spectra of different catalysts (A) and deconvoluted spectra of the 2%Ce– 2% Fe/β catalyst (B).
3.2.2. H2-TPR To understand the redox properties of the catalysts, the H2-TPR characterization was conducted and Fig. 5 displays the results. The spectra of fresh Ce containing samples were analyzed by the method of deconvolution. For 2%Fe/β catalyst, the H2 consumption signal (peak α) at about 415 °C is corresponding to the reduction of Fe3+ to Fe2 + in ion-exchanged positions [32,33], while the other peak (β) centered at around 540 °C is attributed to oligomeric iron oxo species or tiny Fe2O3 [34,35]. For Ce doping Fe/β catalysts, two deconvoluted peaks appear and with the augment of Ce loading, the temperature of peak α shifts to high temperature while the temperature of peak β shifts to low temperature. According to the literature [36], the consumption peak at about 540 °C obtained from CeO2 was attributed to the reduction of the surface-capping oxygen; thus the peak α is most likely related to the Fe3 + active species and the peak β is assigned to the simultaneous reduction of Fe and Ce species [37]. Definitively attributing H2 consumption peaks to different iron and cerium species is a tough task, but it is obvious that the reduction of Fe3+ species occurs at higher temperatures with increasing cerium loading. This may be the reason why the low-temperature SCR activity decreased slightly with the augment of cerium content. For aged 2%Ce–2%Fe/β and aged 2%Fe/β catalysts, a new peak (γ) centered at around 675 °C emerges and it is perhaps due to a further reduction of FeO to metallic Fe [33].
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3.2.3. UV–vis DRS Diffuse reflectance UV–vis spectroscopy was used to analyze nature and distribution of iron and cerium species as presented in Fig. 6. The absorption of ultraviolet region (b 450 nm) may jointly belong to O → Fe3+ ligand-to-metal charge-transfer (LMCT) transitions [38] and O2 − → Cen + [29,39]. The absorption of visible range is due to d–d transition of α-Fe2O3, a sole phenomenon to α-Fe2O3 [40]. For deconvoluted peaks in Fig. 6B, the three bands below 300 nm (I) are assigned to monomeric Fe3+ species in ion-exchanged positions and Ce3+ [41]. The bands between 300 nm and 400 nm (II) are related to oligomeric iron oxo-clusters and cerium oxides species whereas bands above 400 nm (III) are only corresponding to crystalline Fe2O3 particles located on the zeolite surface [42]. As shown in Fig. 6A, the spectral shapes for Ce–Fe/β catalysts vary slightly in comparison to the Fe/β sample, indicating the Ce doping effect on nature of iron species is very little. Furthermore, the distribution and relative amount of different species were quantified by calculation. According to the literatures [14,43], all iron species were SCR active and their NOx conversion rates showed different temperature dependencies: monomeric Fe3+ species were mainly responsible for SCR activity up to 300 °C and Fe2O3 particles contribution dominated at high temperature range. As seen in Table 1, compared with Fe/β catalyst, relative concentrations of monomeric Fe3 + and iron oxides species decrease and increase separately in Ce doping Fe/β catalysts. This may be the reason why the NOx conversion decreased slightly at low temperature and enhanced considerably at high temperature. Moreover, it is also found that the maximum amount of iron oxides species observed in the 2%Ce–2%Fe/β sample verifies its best high-temperature SCR activity. Besides, the ratio of Ce3+ to (Ce3+ + Ce4+) calculated according to the
540 Table 1 Relative concentration of different species measured by UV–vis spectroscopy.
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This indicates that the agglomeration of iron species occurred during hydrothermal treatment, which is confirmed by the XRD results. The results of H2-TPR indicate that the Ce doping significantly affects redox properties of the Fe/β catalyst, suggesting strong interactions between iron and cerium species.
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Temperature ( ) Fig. 5. H2-TPR profiles of fresh 2%Fe/β (a), fresh 0.5%Ce–2%Fe/β (b), fresh 2%Ce–2%Fe/β (c), fresh 5%Ce–2%Fe/β (d), aged 2%Ce–2%Fe/β (e) and aged 2%Fe/β (f) catalysts.
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Fig. 7. XPS results of 2%Fe/β and 2%Ce–2%Fe/β catalysts: deconvoluted Fe 2p3/2 spectra of fresh 2%Fe/β (A) and 2%Ce–2%Fe/β (B); S2p spectra of 2%Fe/β and 2%Ce–2%Fe/β catalysts after SO2 poisoning (C); surface Fe content of 2%Fe/β and 2%Ce–2%Fe/β catalysts before/after hydrothermal aging and 100 ppm SO2 poisoning (D).
size of deconvoluted Ce peaks increases with the augment of cerium content. This may affect the reduction of Fe3+ owning to the interaction between cerium and iron species. Combining H2-TPR results, more Ce3+ may reduce the reducibility of Fe3+ species. The UV–vis results suggest that Ce doping Fe/β catalysts are inclined to form more Fe2O3 species, which is beneficial to improve high-temperature SCR activity. 3.2.4. XPS The 2%Fe/β and 2%Ce–2%Fe/β catalysts were further investigated using XPS technique to understand the valence state of Fe species. Fig. 7A and B shows the results of deconvoluted Fe 2p3/2 peak for the 2%Fe/β and 2%Ce–2%Fe/β catalysts. The characteristic peak position of Fe 2p3/2 is at about 712 eV. Two distinct bands centered at around 711 and 713.8 eV are observed. These two peak positions are approximate to the range of Fe 2p3/2 binding energies of iron in FeO and Fe2O3 respectively [44,45], which indicates that both Fe2+ and Fe3+ exist on the catalysts. As seen in the XPS spectra, experimental data are shown as black dots and the fitted peaks are shown by solid black lines. The deconvoluted peak areas for Fe3+ and Fe2+ species are marked by filled space and blank space, respectively. The ratio between the peak areas for Fe3 + and the sum of Fe2 + and Fe3 + is 0.57 and 0.46 for 2% Fe/β and 2%Ce–2% Fe/β separately, indicating that the Ce addition decreases the relative percentage of Fe3+ species. According to the literatures [46, 47], Fe3+ sites may facilitate the reduction of NOx at low temperature. Thus, it may also be one of the reasons why the low-temperature activity of 2%Ce–2%Fe/β decreased slightly. To obtain the status of surface sulfur species, the XPS analysis was performed on the sulfated 2%Fe/β and 2%Ce–2%Fe/β catalysts. As
shown in Fig. 7C, the main peak of S2p spectrum is located at about 169.3 eV, corresponding to sulfate species [48]. It is obvious that the integration area of the peak for the sulfated 2%Fe/β sample is much more than that of the sulfated 2%Ce–2%Fe/β catalyst, indicating that more sulfates were produced on 2%Fe/β catalyst during SO2 deactivation process. That is, the presence of Ce can restrain the formation of sulfates on the surface of Ce–Fe/β catalysts, which may be the dominating reason for the improvement of SO2 tolerance over Ce doping Fe/β catalysts. Fig. 7D displays the surface Fe content measured by the XPS over 2%Fe/β and 2%Ce–2%Fe/β catalysts before/after hydrothermal ageing at 700 °C for 24 h and SO2 poisoning for 8 h. The surface iron content reduces upon the addition of Ce, which demonstrates that iron species may exist on the channel of zeolites in the form of nano oxides. This perhaps correlates to the decrease of low temperature activity and the increase of high temperature activity. From Fig. 7D, it clearly shows the increase of surface iron content for aged and sulfated catalysts. Generally, the augment of surface iron content for a fixed total amount of iron probably implies that iron species in ionexchanged positions or on the channels are migrated to external surface of zeolites. The formation of iron oxides for aged samples proved by the XRD results indicates the reduced dispersion of iron species after hydrothermal treatment. Upon aging, the surface Fe atom percent increases from 1.12 to 1.82 for the 2%Fe/β catalyst, while the surface Fe atom percent of the 2%Ce–2%Fe/β catalyst increases only from 0.87 to 1.27. It shows that the Ce addition can ease the migration of iron species. In other words, the presence of Ce is able to suppress the formation of bulk-type crystallite Fe2O3 and stabilize the
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iron species dispersion in the process of aging treatment, thus enhancing the hydrothermal stability of the Ce doping Fe/β catalyst. In addition, the surface Fe atom percent increases by 70.5% and 59.8% after SO 2 poisoning for the 2%Fe/β and 2%Ce–2%Fe/β catalysts, respectively. This indicates that the addition of cerium can also lessen formation of inactive iron sulfate species and then improve SO2 resistance, which is echoed by the result of S2p XPS spectra. 4. Conclusions In conclusion, the Ce doping has improved the catalytic performance of Fe/β catalyst for NH3-SCR. Cerium plays a key role in affecting the distribution and dispersion of iron active species and then affects the deNOx performance. Among all catalysts prepared in this paper, the 2%Ce–2%Fe/β catalyst obtained the widest activity temperature window, which exhibits the high NOx conversion (N90%) in a broad temperature range (280–520 °C). The addition of cerium could contribute to form more tiny iron crystallites and reduce the reducibility of Fe3 + species. The finely dispersed iron oxide nanoparticles on the surface of Ce–Fe/β catalysts were confirmed by XRD, UV–vis and XPS analyses and the compensation effect of cerium is responsible for their excellent catalytic performance. The NOx conversion of aged catalysts at low temperature reduced obviously on account of the migration and aggregation of Fe3+ active species. The Ce doping greatly enhanced the hydrothermal stability of the Fe/β catalyst because it could stabilize iron species dispersion and ease the growth of iron oxides species. In addition, the presence of Ce could effectively improve SO2 tolerance of the Fe/β catalyst by controlling the deactivating agents (sulfates) formation. Acknowledgments We gratefully acknowledge the financial supports from the Nature Science Foundation of China (No. 21177110) and the Zhejiang Leading Team of Science and Technology Innovation (No. 2009R50020). References [1] S. Brandenberger, O. Kröcher, A. Tissler, R. Althoff, The state of the art in selective catalytic reduction of NOx by ammonia using metal-exchanged zeolite catalysts, Catalysis Reviews 50 (2008) 492–531. [2] J. Li, H. Chang, L. Ma, J. Hao, R.T. Yang, Low-temperature selective catalytic reduction of NOx with NH3 over metal oxide and zeolite catalysts—a review, Catalysis Today 175 (2011) 147–156. [3] P. Granger, V.I. Parvulescu, Catalytic NOx abatement systems for mobile sources: from three-way to lean burn after-treatment technologies, Chemical Reviews 111 (2011) 3155–3207. [4] J. Li, R. Zhu, Y. Cheng, C.K. Lambert, R.T. Yang, Mechanism of propene poisoning on Fe–ZSM-5 for selective catalytic reduction of NOx with ammonia, Environmental Science & Technology 44 (2010) 1799–1805. [5] B. Guan, R. Zhan, H. Lin, Z. Huang, Review of state of the art technologies of selective catalytic reduction of NOx from diesel engine exhaust, Applied Thermal Engineering 66 (2014) 395–414. [6] P.G.W.A. Kompio, A. Brückner, F. Hipler, G. Auer, E. Löffler, W. Grünert, A new view on the relations between tungsten and vanadium in V2O5WO3/TiO2 catalysts for the selective reduction of NO with NH3, Journal of Catalysis 286 (2012) 237–247. [7] P. Balle, B. Geiger, S. Kureti, Selective catalytic reduction of NOx by NH3 on Fe/HBEA zeolite catalysts in oxygen-rich exhaust, Applied Catalysis B: Environmental 85 (2009) 109–119. [8] S. Djerad, M. Crocoll, S. Kureti, L. Tifouti, W. Weisweiler, Effect of oxygen concentration on the NOx reduction with ammonia over V2O5–WO3/TiO2 catalyst, Catalysis Today 113 (2006) 208–214. [9] J.P. Dunn, P.R. Koppula, H.G. Stenger, I.E. Wachs, Oxidation of sulfur dioxide to sulfur trioxide over supported vanadia catalysts, Applied Catalysis B: Environmental 19 (1998) 103–117. [10] V.I. Pârvulescu, P. Grange, B. Delmon, Catalytic removal of NO, Catalysis Today 46 (1998) 233–316. [11] J. Pérez-Ramírez, J.C. Groen, A. Brückner, M.S. Kumar, U. Bentrup, M.N. Debbagh, L.A. Villaescusa, Evolution of isomorphously substituted iron zeolites during activation: comparison of Fe-beta and Fe–ZSM-5, Journal of Catalysis 232 (2005) 318–334. [12] M. Iwasaki, K. Yamazaki, H. Shinjoh, NOx reduction performance of fresh and aged Fe-zeolites prepared by CVD: effects of zeolite structure and Si/Al2 ratio, Applied Catalysis B: Environmental 102 (2011) 302–309.
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Ce doping effect on performance of the Fe/β catalyst for NOx reduction by NH3 Shui-Yan Jiang, Ren-Xian Zhou ⁎ Institute of Catalysis, Zhejiang University, Hangzhou 310028, PR China
a r t i c l e
i n f o
Article history: Received 4 October 2014 Received in revised form 5 December 2014 Accepted 3 February 2015 Available online 27 February 2015 Keywords: NH3-SCR Fe/β Ce doping Hydrothermal aging SO2 resistance
a b s t r a c t The Ce doping Fe/β catalysts were prepared by an easy incipient wetness impregnation method and characterized using various analytical techniques, including H2-TPR, XRD, UV–vis and XPS. The results demonstrated that the addition of cerium effectively improved the deNOx performance, hydrothermal stability and SO2 resistance of the Fe/β catalyst for the selective catalytic reduction (SCR) of NOx by NH3. The finely dispersed iron oxide nanoparticles on the surface of Ce–Fe/β catalysts and the compensative effect of cerium are dominating reasons for high catalytic performance. The Ce doping enhanced the hydrothermal stability of the Fe/β catalyst because it could control the growth of iron oxides and stabilize the dispersed iron species. Suppressing the deposition of sulfates by Ce addition made the Fe/β catalyst a great SO2 tolerance. It was speculated that the outstanding performance of Ce doping Fe/β catalysts made them attractive for practical application. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxides, such as NO and NO2, originated from mobile and stationary sources have been an important source of air pollution. Due to numerous environmental issues (e.g. acid rain, photochemical smog, ozone depletion and greenhouse effect) and serious damage to human health resulted from NOx, stringent regulations and great efforts have been made to limit NOx emissions [1–3]. Up to now one of the predominant technologies for eliminating NOx is regarded as the selective catalytic reduction with ammonia (NH3-SCR) [4,5]. The commercial V2O5–WO3(MoO3)/TiO2 catalyst has been applied widely and exhibits good catalytic performance [6]. However, some inevitable problems still remain, such as the toxicity of vanadia species, narrow activity temperature range and undesired oxidation of SO2 to SO3 [7–9]. Therefore, it is necessary to develop superior SCR catalysts. In recent years, zeolite-based catalysts promoted by various transition metals have attracted much more attention in diesel engines control [10,11]. Among them, Fe-exchanged zeolite catalysts have been found to be attractive candidates for SCR reaction because of their excellent SCR activity, high N2 selectivity and superior stability. M. Iwasaki et al. [12] investigated the effect of zeolite structure on the catalytic performance of fresh Fe/zeolite catalysts and ranked the following order: Fe/BEA N Fe/MFI N Fe/FER N Fe/LTL N Fe/MOR. C. He et al. [13] revealed that the 2.5 wt.% Fe/beta catalyst prepared by incipient wetness
⁎ Corresponding author. Tel.: +86 571 88273290. E-mail address: [email protected] (R.-X. Zhou).
http://dx.doi.org/10.1016/j.fuproc.2015.02.004 0378-3820/© 2015 Elsevier B.V. All rights reserved.
impregnation showed NO conversion over 97.0% in the temperature range of 300–500 °C at a high space velocity (1.9 × 105 h−1) and was stable up to a hydrothermal aging temperature of 750 °C. Besides, many other investigations, such as the reaction mechanism and influence of preparation method and iron content, were conducted as well [14,15]. But it can be seen that limited improvement of catalytic performance is achieved for single iron exchanged zeolite catalysts. Thus, the addition of extra metal ions as promoters may be a promising trial to further improve catalytic performance of Fe/zeolite catalysts. Cerium is a common rare earth metal and has been discussed vastly due to its particular properties. Especially, cerium which is called an oxygen reservoir can store and release oxygen via the redox shifted between Ce4 + and Ce3 + under oxidizing and reducing conditions, respectively [16,17]. A lot of cerium doping catalysts have been reported in many fields, such as Mn–Ce/ZSM-5 [18,19], CeO 2–MnO x [20], Cu–Ce/ZSM-5 [21,22] and Fe–Ce/TiO2 [23]. It is worth noting that the cerium modified Fe/zeolite catalysts used for NH 3-SCR reactions have been paid little attention. In addition, investigations on the deNOx performance and stability of Ce doping Fe/zeolite catalysts have not been systematically conducted. In this paper, to understand and elucidate Ce doping effect on deNOx performance of the Fe/β catalyst, the catalytic activity, hydrothermal stability and sulfur-resistance durability of Ce–Fe/β catalysts were systematically investigated. At the same time, the chemical state of iron species and the influence of cerium introduction on active iron species were clarified through a series of characterizations, which perhaps will be helpful to further develop more efficient NH3-SCR catalysts.
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2.2. Catalyst activity testing The activity measurement for NH3-SCR was conducted through a fixed-bed continuous flow reactor. An argon gas mixture containing 500 ppm NOx, 500 ppm NH3 and 5 vol.% O2 was introduced into the reaction tube at steady state. The concentrations of NH3, NOx (NO and NO2) and N2O in the inlet and outlet gas were measured by an IR spectrometer (Bruker EQUINOX 55) equipped with a 10 m gas cell. In the test, the total flow rate was fixed at 160 mL min−1, corresponding to a GHSV (gas hourly space velocity) of 48,000 h−1. The performance of the catalysts is shown in term of conversion defined as follows: NOx conversionð%Þ ¼
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The sulfur-resistance testing of selected catalysts was conducted using a continuous feed gas (500 ppm NH3, 500 ppm NOx, 5 vol.% O2 and 50 ppm or 100 ppm SO2, balance Ar) for 8 h at 280 °C. The conversion of NOx was measured at set intervals during the whole experimental process. 2.3. Characterization The powder X-ray diffraction (XRD) measurements have been conducted with a D/Max-2550pc system with Cu Kα (λ = 0.15406 nm) radiation in the 2θ ranges from 5° to 75°. For hydrogen temperature programmed reduction (H2-TPR) experiments, 100 mg of sample was put into a quartz reactor, pretreated in N2 (99.999%) stream for 30 min at 400 °C and then cooled down to 50 °C. The reduction process started with a ramp of 10 °C min−1 from 50 °C to 850 °C in a flow of 5 vol.% H2/Ar (40 mL min−1). UV–vis diffuse reflectance spectra (UV–vis DRS) were collected in air in the 200–800 nm wavelength range using the appropriate baseline correction on a UV-2401 PC (Shimadzu). Deconvolution of the UV–vis spectra into subbands was performed. X-ray photoelectro spectra (XPS) were measured on a Thermo ESCALAB 250 system using Al-Kα (1486.6 eV) X-ray source. The binding energy was calibrated by C1s peak of contaminant carbon (286.6 eV) as an internal standard. 3. Results and discussion 3.1. DeNOx performance 3.1.1. Catalytic activity of fresh catalysts The fresh Fe/β and Ce doping Fe/β catalysts were tested from 100 °C to 550 °C and the results of deNOx performance in NH3-SCR reactions are shown in Fig. 1 Based on the previous study, iron loading amount
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The Fe/β and Ce doping Fe/β catalysts were prepared by incipient wetness impregnation using an industrial H-β (SiO2/Al2O3 = 40) zeolite as the catalyst support. The Fe content of all the catalysts was fixed at 2.0 wt.% and the Ce concentration varied. It was performed by mixing H-β powders with the required amounts of Fe(NO3)3 and Ce(NO3)3 solution and stirred for 12 h at room temperature, then evaporated redundant water and dried for 2 h at 110 °C, finally calcined in air for 5 h at 500 °C. Finally, all the obtained powder samples were ground and then sieved to 40–60 mesh. The samples were denoted as 2%Fe/β, 0.5%Ce–2%Fe/β, 2%Ce–2%Fe/β and 5%Ce–2%Fe/β, respectively. To investigate the hydrothermal stability of catalysts, the selected samples were aged in a quartz tube reactor using compressed air containing 10 vol.% H2O at the flow rate of 25 mL min−1 for 24 h at 700 °C.
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of 2 wt.% was chosen as the candidate for the preparation of Ce–Fe/β catalysts. After appropriate amount of cerium was added to Fe/β, a positive influence on catalytic activity was observed obviously, especially in region of the high temperature. As seen in Fig. 1A, for the 0.5%Ce–2%Fe/β catalyst, the NOx conversion greatly exceeds the Fe/β catalyst at high temperature (N 470 °C) with a little loss at low temperature, corresponding to operating temperature window being broadened. This trend continues with increasing cerium loading; the NOx conversion is enhanced considerably at high temperature and slightly decreases at low temperature. But the promoting effect of cerium at high temperature for the 5%Ce–2%Fe/β catalyst is less pronounced than that for the 2%Ce–2%Fe/β catalyst, which indicates that the excess Ce doping perhaps has a negative effect on high-temperature activity. Among all the catalysts prepared in this paper, the 2%Ce–2%Fe/β catalyst exhibits the best deNOx performance and shows above 90% NOx conversion within the range of 280–520 °C. It is noted that there is no byproduct (such as N2O) generation for all the catalysts, indicating the addition of cerium has not affected the N2 selectivity. In addition, ammonia conversion was also conducted during the reaction process. Fig. 1B shows that the trend of NH3 conversion is almost the same as that of NOx conversion. But all catalysts show relatively higher NH3 conversion than NOx elimination, especially at the temperature above 500 °C. That is due to the severe ammonia oxidation at high temperature, which competes with the process of SCR reaction and should be avoided as far as possible. In short, the results clearly demonstrate the Ce doping is beneficial to improve the SCR activity of the Fe/β catalyst. 3.1.2. Hydrothermal stability of catalysts To study the influence of hydrothermal aging on SCR activity, the 2%Fe/β and 2%Ce–2%Fe/β catalysts were treated at 700 °C for 24 h with flowing compressed air containing 10 vol.% H2O. The NOx conversions of 2%Fe/β and 2%Ce–2%Fe/β catalysts before and after aging treatment are displayed in Fig. 2. It can be seen that an obvious
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loss of NOx reduction activity occurs after hydrothermal treatment for both catalysts, which is probably the result of the migration and agglomeration of iron species [24]. Furthermore, the temperature windows of 90% conversion are 270–485 °C, 305–485 °C, 280–520 °C and 300–515 °C for fresh 2%Fe/β, aged 2%Fe/β, fresh 2%Ce–2%Fe/β and aged 2%Ce–2%Fe/β, respectively, which indicates that lowtemperature SCR is more sensitive to hydrothermal aging compared to the high-temperature SCR. Meanwhile, the shifts in temperature window of 90% NOx conversion for 2%Fe/β and 2%Ce–2%Fe/β are 35 °C and 25 °C before and after ageing, respectively. It can be seen that the deactivation of the 2%Ce–2%Fe/β sample is slighter than that of the 2%Fe/β catalyst, indicating that the 2%Ce–2%Fe/β catalyst is more resistant to hydrothermal ageing than the 2%Fe/β catalyst. This result demonstrates that the Ce doping further improves the hydrothermal stability of the Fe/β catalyst.
3.1.3. Sulfur-resistance testing of catalysts With regard to the actual use of lean SCR catalysts, sulfur poisoning is an important problem that should not be ignored. The effect of SO2 on the NOx conversion over 2%Fe/β and 2%Ce–2%Fe/β catalysts is displayed in Fig. 3. The samples were continuously exposed to feed gas containing 0 ppm, 50 ppm and 100 ppm SO2 at 280 °C for 10 h, respectively. It is
seen that the initial NOx conversion is about 97% and keeps steady within 10 h for both catalysts in absence of SO2. When adding SO2 to the feed gas, the deNOx performance of both catalysts decreases obviously. Particularly, the NOx conversion of the 2%Fe/β catalyst was down from 97% to 73.8% and 62.0% in 8 h for 50 ppm and 100 ppm SO2 poisoning, respectively. While the NOx conversion of the 2%Ce–2%Fe/β catalyst was accordingly down to 78.4% and 66.1% in both cases. It is quite clear that the 2%Ce–2%Fe/β catalyst exhibits less sensitivity to SO2 than the 2%Fe/β catalyst, which indicates that the Ce doping can improve the sulfur-resistance of the Fe/β catalyst. Furthermore, the SCR activity could not recover when SO2 was off, suggesting that the deactivation role of SO2 is not the competitive adsorption. The poisoning reason was expected as the formation of sulfates (SO2− 4 ) covering active sites [25,26]. According to the literatures [27,28], high temperature calcination for decomposing sulfates is presumably an effective way to achieve the whole or partial regeneration of catalysts. In order to investigate the feasibility of regeneration for the catalysts deactivated by SO2, the activity measurement was conducted over the 2%Ce–2%Fe/β sample after being exposed to 100 ppm SO2 at 280 °C for 8 h. It turned out that the low temperature activity of the sulfated sample decreased distinctly and the initial reaction temperature of achieving 100% NOx conversion increased by 10 °C, while the deNOx performance at high temperature was almost unaffected, which may be on account of sulfates decomposing at high temperature. It is speculated that hightemperature treatment may regenerate the catalysts deactivated by SO2, providing facilitation for practical application.
3.2. Characterization results of catalysts 3.2.1. XRD In order to probe the Ce doping effect and possible structural changes, the XRD patterns of Fe/β and Ce doping Fe/β catalysts before and after hydrothermal ageing treatment are presented in Fig. 4. The characteristic diffraction peaks for β zeolite can be observed over all fresh and aged catalysts, which indicates that zeolite crystal structures keep largely intact even under high-temperature hydrothermal conditions. Thus what can be concluded is that the loss of SCR activity is not the result of structural changes of β zeolite support. The diffraction lines corresponding to iron oxide crystallites (hematite, PDF 33-0664) were not detected among all fresh catalysts, which indicates that the Ce doping cannot apparently affect the dispersion state of iron oxide species. When the Ce loading is 0.5 wt.%, no
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Fig. 3. Effect of SO2 on deNOx performance of 2%Fe/β (A) and 2%Ce–2%Fe/β (B) catalysts.
Fig. 4. XRD patterns of fresh 2%Fe/β (a), fresh 0.5%Ce–2%Fe/β (b), fresh 2%Ce–2%Fe/β (c), fresh 5%Ce–2%Fe/β (d), aged 2%Ce–2%Fe/β (e) and aged 2%Fe/β (f) catalysts.
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(A) 5% Ce-2% Fe/ 2% Ce-2% Fe/ 0.5% Ce-2% Fe/
Absorbance
diffraction peaks of CeO2 are observed in Ce–Fe/β samples. These results show that iron and cerium species are well dispersed on the surface of β zeolite support as amorphous oxides, or aggregated in minicrystals invisible by XRD. Besides, for the 2%Ce–2%Fe/β and 5%Ce–2%Fe/β catalysts, diffraction peaks at 2θ = 28.7°, 47.9° and 56.8° can be identified, indicating crystallite CeO2 existing on the external surface of β zeolite [29,30]. Combining the catalytic activity results, it is deduced that excessive crystallite CeO2 may have some passive influence on high temperature activity. However, it is noted that typical peaks from crystalline α-Fe2O3 at 2θ = 35.6°, 49.5°, 54.1° and 62.5° appear for both aged samples, indicating the agglomeration and growth of dispersed iron species. This may decrease the deNOx activity of aged 2%Fe/β and 2%Ce–2%Fe/β catalysts [13,31]. Meanwhile, the particle size was estimated with Scherrer equation. It turned out that the sizes of iron oxides are 28.7 nm and 43.2 nm for aged 2%Ce–2%Fe/β and aged 2%Fe/β catalysts, respectively, which indicates that the Ce addition may ease the growth of iron oxides species during aging treatment. The better dispersion of iron species may improve the hydrothermal stability of the Ce doping Fe/β catalysts.
223
2% Fe/
(B) 2%Ce-2%Fe/
200
300
400
500
600
700
800
Wavelength (nm) Fig. 6. UV–vis DRS spectra of different catalysts (A) and deconvoluted spectra of the 2%Ce– 2% Fe/β catalyst (B).
3.2.2. H2-TPR To understand the redox properties of the catalysts, the H2-TPR characterization was conducted and Fig. 5 displays the results. The spectra of fresh Ce containing samples were analyzed by the method of deconvolution. For 2%Fe/β catalyst, the H2 consumption signal (peak α) at about 415 °C is corresponding to the reduction of Fe3+ to Fe2 + in ion-exchanged positions [32,33], while the other peak (β) centered at around 540 °C is attributed to oligomeric iron oxo species or tiny Fe2O3 [34,35]. For Ce doping Fe/β catalysts, two deconvoluted peaks appear and with the augment of Ce loading, the temperature of peak α shifts to high temperature while the temperature of peak β shifts to low temperature. According to the literature [36], the consumption peak at about 540 °C obtained from CeO2 was attributed to the reduction of the surface-capping oxygen; thus the peak α is most likely related to the Fe3 + active species and the peak β is assigned to the simultaneous reduction of Fe and Ce species [37]. Definitively attributing H2 consumption peaks to different iron and cerium species is a tough task, but it is obvious that the reduction of Fe3+ species occurs at higher temperatures with increasing cerium loading. This may be the reason why the low-temperature SCR activity decreased slightly with the augment of cerium content. For aged 2%Ce–2%Fe/β and aged 2%Fe/β catalysts, a new peak (γ) centered at around 675 °C emerges and it is perhaps due to a further reduction of FeO to metallic Fe [33].
675 690
f e
485
TCD signal
445
d
516
438
c 423
527
b 415
3.2.3. UV–vis DRS Diffuse reflectance UV–vis spectroscopy was used to analyze nature and distribution of iron and cerium species as presented in Fig. 6. The absorption of ultraviolet region (b 450 nm) may jointly belong to O → Fe3+ ligand-to-metal charge-transfer (LMCT) transitions [38] and O2 − → Cen + [29,39]. The absorption of visible range is due to d–d transition of α-Fe2O3, a sole phenomenon to α-Fe2O3 [40]. For deconvoluted peaks in Fig. 6B, the three bands below 300 nm (I) are assigned to monomeric Fe3+ species in ion-exchanged positions and Ce3+ [41]. The bands between 300 nm and 400 nm (II) are related to oligomeric iron oxo-clusters and cerium oxides species whereas bands above 400 nm (III) are only corresponding to crystalline Fe2O3 particles located on the zeolite surface [42]. As shown in Fig. 6A, the spectral shapes for Ce–Fe/β catalysts vary slightly in comparison to the Fe/β sample, indicating the Ce doping effect on nature of iron species is very little. Furthermore, the distribution and relative amount of different species were quantified by calculation. According to the literatures [14,43], all iron species were SCR active and their NOx conversion rates showed different temperature dependencies: monomeric Fe3+ species were mainly responsible for SCR activity up to 300 °C and Fe2O3 particles contribution dominated at high temperature range. As seen in Table 1, compared with Fe/β catalyst, relative concentrations of monomeric Fe3 + and iron oxides species decrease and increase separately in Ce doping Fe/β catalysts. This may be the reason why the NOx conversion decreased slightly at low temperature and enhanced considerably at high temperature. Moreover, it is also found that the maximum amount of iron oxides species observed in the 2%Ce–2%Fe/β sample verifies its best high-temperature SCR activity. Besides, the ratio of Ce3+ to (Ce3+ + Ce4+) calculated according to the
540 Table 1 Relative concentration of different species measured by UV–vis spectroscopy.
a 200
This indicates that the agglomeration of iron species occurred during hydrothermal treatment, which is confirmed by the XRD results. The results of H2-TPR indicate that the Ce doping significantly affects redox properties of the Fe/β catalyst, suggesting strong interactions between iron and cerium species.
250
300
350
400
450
500
550
600
650
700
750
800
Temperature ( ) Fig. 5. H2-TPR profiles of fresh 2%Fe/β (a), fresh 0.5%Ce–2%Fe/β (b), fresh 2%Ce–2%Fe/β (c), fresh 5%Ce–2%Fe/β (d), aged 2%Ce–2%Fe/β (e) and aged 2%Fe/β (f) catalysts.
Sample
Fe3+
FeOx cluster
Fe2O3
Ce3+ / (Ce3++Ce4+)
2%Fe/β 0.5%Ce–2%Fe/β 2%Ce–2%Fe/β 5%Ce–2%Fe/β
0.48 0.30 0.32 0.34
0.21 0.37 0.28 0.32
0.31 0.33 0.40 0.34
– 0.27 0.33 0.46
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(B)
(A) Fe3+
Fe3+
722
720
718
716
714
712
710
708
706
Intensity
Intensity
Fe2+
704
Fe2+
722
720
718
Binding energy (eV)
716
714
712
710
708
706
704
Binding energy (eV) 2.5
(C)
2% Fe/β 2% Ce-2% Fe/β
(D)
2.0
Fe Content ( At.% )
Intensity
2%Fe/β
2%Ce-2%Fe/β
1.5
1.0
0.5
173
172
171
170
169
168
167
166
165
Binding Energy (eV)
0.0
Fresh
Aged
SO2
Fig. 7. XPS results of 2%Fe/β and 2%Ce–2%Fe/β catalysts: deconvoluted Fe 2p3/2 spectra of fresh 2%Fe/β (A) and 2%Ce–2%Fe/β (B); S2p spectra of 2%Fe/β and 2%Ce–2%Fe/β catalysts after SO2 poisoning (C); surface Fe content of 2%Fe/β and 2%Ce–2%Fe/β catalysts before/after hydrothermal aging and 100 ppm SO2 poisoning (D).
size of deconvoluted Ce peaks increases with the augment of cerium content. This may affect the reduction of Fe3+ owning to the interaction between cerium and iron species. Combining H2-TPR results, more Ce3+ may reduce the reducibility of Fe3+ species. The UV–vis results suggest that Ce doping Fe/β catalysts are inclined to form more Fe2O3 species, which is beneficial to improve high-temperature SCR activity. 3.2.4. XPS The 2%Fe/β and 2%Ce–2%Fe/β catalysts were further investigated using XPS technique to understand the valence state of Fe species. Fig. 7A and B shows the results of deconvoluted Fe 2p3/2 peak for the 2%Fe/β and 2%Ce–2%Fe/β catalysts. The characteristic peak position of Fe 2p3/2 is at about 712 eV. Two distinct bands centered at around 711 and 713.8 eV are observed. These two peak positions are approximate to the range of Fe 2p3/2 binding energies of iron in FeO and Fe2O3 respectively [44,45], which indicates that both Fe2+ and Fe3+ exist on the catalysts. As seen in the XPS spectra, experimental data are shown as black dots and the fitted peaks are shown by solid black lines. The deconvoluted peak areas for Fe3+ and Fe2+ species are marked by filled space and blank space, respectively. The ratio between the peak areas for Fe3 + and the sum of Fe2 + and Fe3 + is 0.57 and 0.46 for 2% Fe/β and 2%Ce–2% Fe/β separately, indicating that the Ce addition decreases the relative percentage of Fe3+ species. According to the literatures [46, 47], Fe3+ sites may facilitate the reduction of NOx at low temperature. Thus, it may also be one of the reasons why the low-temperature activity of 2%Ce–2%Fe/β decreased slightly. To obtain the status of surface sulfur species, the XPS analysis was performed on the sulfated 2%Fe/β and 2%Ce–2%Fe/β catalysts. As
shown in Fig. 7C, the main peak of S2p spectrum is located at about 169.3 eV, corresponding to sulfate species [48]. It is obvious that the integration area of the peak for the sulfated 2%Fe/β sample is much more than that of the sulfated 2%Ce–2%Fe/β catalyst, indicating that more sulfates were produced on 2%Fe/β catalyst during SO2 deactivation process. That is, the presence of Ce can restrain the formation of sulfates on the surface of Ce–Fe/β catalysts, which may be the dominating reason for the improvement of SO2 tolerance over Ce doping Fe/β catalysts. Fig. 7D displays the surface Fe content measured by the XPS over 2%Fe/β and 2%Ce–2%Fe/β catalysts before/after hydrothermal ageing at 700 °C for 24 h and SO2 poisoning for 8 h. The surface iron content reduces upon the addition of Ce, which demonstrates that iron species may exist on the channel of zeolites in the form of nano oxides. This perhaps correlates to the decrease of low temperature activity and the increase of high temperature activity. From Fig. 7D, it clearly shows the increase of surface iron content for aged and sulfated catalysts. Generally, the augment of surface iron content for a fixed total amount of iron probably implies that iron species in ionexchanged positions or on the channels are migrated to external surface of zeolites. The formation of iron oxides for aged samples proved by the XRD results indicates the reduced dispersion of iron species after hydrothermal treatment. Upon aging, the surface Fe atom percent increases from 1.12 to 1.82 for the 2%Fe/β catalyst, while the surface Fe atom percent of the 2%Ce–2%Fe/β catalyst increases only from 0.87 to 1.27. It shows that the Ce addition can ease the migration of iron species. In other words, the presence of Ce is able to suppress the formation of bulk-type crystallite Fe2O3 and stabilize the
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iron species dispersion in the process of aging treatment, thus enhancing the hydrothermal stability of the Ce doping Fe/β catalyst. In addition, the surface Fe atom percent increases by 70.5% and 59.8% after SO 2 poisoning for the 2%Fe/β and 2%Ce–2%Fe/β catalysts, respectively. This indicates that the addition of cerium can also lessen formation of inactive iron sulfate species and then improve SO2 resistance, which is echoed by the result of S2p XPS spectra. 4. Conclusions In conclusion, the Ce doping has improved the catalytic performance of Fe/β catalyst for NH3-SCR. Cerium plays a key role in affecting the distribution and dispersion of iron active species and then affects the deNOx performance. Among all catalysts prepared in this paper, the 2%Ce–2%Fe/β catalyst obtained the widest activity temperature window, which exhibits the high NOx conversion (N90%) in a broad temperature range (280–520 °C). The addition of cerium could contribute to form more tiny iron crystallites and reduce the reducibility of Fe3 + species. The finely dispersed iron oxide nanoparticles on the surface of Ce–Fe/β catalysts were confirmed by XRD, UV–vis and XPS analyses and the compensation effect of cerium is responsible for their excellent catalytic performance. The NOx conversion of aged catalysts at low temperature reduced obviously on account of the migration and aggregation of Fe3+ active species. The Ce doping greatly enhanced the hydrothermal stability of the Fe/β catalyst because it could stabilize iron species dispersion and ease the growth of iron oxides species. In addition, the presence of Ce could effectively improve SO2 tolerance of the Fe/β catalyst by controlling the deactivating agents (sulfates) formation. Acknowledgments We gratefully acknowledge the financial supports from the Nature Science Foundation of China (No. 21177110) and the Zhejiang Leading Team of Science and Technology Innovation (No. 2009R50020). References [1] S. Brandenberger, O. Kröcher, A. Tissler, R. Althoff, The state of the art in selective catalytic reduction of NOx by ammonia using metal-exchanged zeolite catalysts, Catalysis Reviews 50 (2008) 492–531. [2] J. Li, H. Chang, L. Ma, J. Hao, R.T. Yang, Low-temperature selective catalytic reduction of NOx with NH3 over metal oxide and zeolite catalysts—a review, Catalysis Today 175 (2011) 147–156. [3] P. Granger, V.I. Parvulescu, Catalytic NOx abatement systems for mobile sources: from three-way to lean burn after-treatment technologies, Chemical Reviews 111 (2011) 3155–3207. [4] J. Li, R. Zhu, Y. Cheng, C.K. Lambert, R.T. Yang, Mechanism of propene poisoning on Fe–ZSM-5 for selective catalytic reduction of NOx with ammonia, Environmental Science & Technology 44 (2010) 1799–1805. [5] B. Guan, R. Zhan, H. Lin, Z. Huang, Review of state of the art technologies of selective catalytic reduction of NOx from diesel engine exhaust, Applied Thermal Engineering 66 (2014) 395–414. [6] P.G.W.A. Kompio, A. Brückner, F. Hipler, G. Auer, E. Löffler, W. Grünert, A new view on the relations between tungsten and vanadium in V2O5WO3/TiO2 catalysts for the selective reduction of NO with NH3, Journal of Catalysis 286 (2012) 237–247. [7] P. Balle, B. Geiger, S. Kureti, Selective catalytic reduction of NOx by NH3 on Fe/HBEA zeolite catalysts in oxygen-rich exhaust, Applied Catalysis B: Environmental 85 (2009) 109–119. [8] S. Djerad, M. Crocoll, S. Kureti, L. Tifouti, W. Weisweiler, Effect of oxygen concentration on the NOx reduction with ammonia over V2O5–WO3/TiO2 catalyst, Catalysis Today 113 (2006) 208–214. [9] J.P. Dunn, P.R. Koppula, H.G. Stenger, I.E. Wachs, Oxidation of sulfur dioxide to sulfur trioxide over supported vanadia catalysts, Applied Catalysis B: Environmental 19 (1998) 103–117. [10] V.I. Pârvulescu, P. Grange, B. Delmon, Catalytic removal of NO, Catalysis Today 46 (1998) 233–316. [11] J. Pérez-Ramírez, J.C. Groen, A. Brückner, M.S. Kumar, U. Bentrup, M.N. Debbagh, L.A. Villaescusa, Evolution of isomorphously substituted iron zeolites during activation: comparison of Fe-beta and Fe–ZSM-5, Journal of Catalysis 232 (2005) 318–334. [12] M. Iwasaki, K. Yamazaki, H. Shinjoh, NOx reduction performance of fresh and aged Fe-zeolites prepared by CVD: effects of zeolite structure and Si/Al2 ratio, Applied Catalysis B: Environmental 102 (2011) 302–309.
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