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Characterization of CeO2–WO3 catalysts prepared by different methods for selective catalytic reduction of NOx with NH3
Catalysis Communications 40 (2013) 145–148
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Characterization of CeO2–WO3 catalysts prepared by different methods for selective catalytic reduction of NOx with NH3 Caixia Liu, Liang Chen, Huazhen Chang, Lei Ma, Yue Peng, Hamidreza Arandiyan, Junhua Li ⁎ State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC), School of Environment, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 21 March 2013 Received in revised form 6 June 2013 Accepted 11 June 2013 Available online 25 June 2013 Keywords: Selective catalytic reduction (SCR) CeO2–WO3 Preparation methods NOx abatement
a b s t r a c t In this work, cerium–tungsten oxide catalysts were prepared by three methods: single step sol–gel (SG), impregnation (IM), and solid processing (SP). The catalysts were used for selective catalytic reduction (SCR) of NOx with ammonia over a wide temperature range. The results indicated that the catalysts prepared by the SP and IM methods exhibited better SCR activity than that prepared via the SG method in 175–500 °C. The excellent activity can be attributed to larger surface area, higher surface concentrations of Ce and Ce3+, enhanced NO oxidization ability, and greater number of surface acid sites. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxide (NO, NO2, and N2O) in exhaust from fossil fuel combustion is a major cause of photochemical smog, acid rain, and ozone depletion. Selective catalytic reduction of NOx with NH3 (NH3–SCR) is an effective and economical method to remove NOx. Currently, V2O5–WO3/TiO2 or V2O5–MoO3/TiO2 is the most widely used catalyst, but it has a relatively narrow temperature range of 300–400 °C [1,2]. Accordingly, many researchers have started to modify current catalysts and investigate novel catalysts to reduce vanadium loadings or to substitute the vanadium with other metal elements. Cerium oxide (CeO2) has received much attention for the SCR of NOx because of its prominence for NOx removal [3,4]. Many researchers have reported on CeO2 mixed with other oxides for SCR aided with NH3, such as Mn–Ce/TiO2 [5], CeO2–TiO2 [6], CexTi1−xO2 [7], CeO2–WO3/TiO2 [8], and CeO2/Al2O3 [9]. These compounds can store and release oxygen via the redox shift between Ce4 + and Ce3 + under oxidizing and reducing conditions, respectively [10]. Ceria can enhance the oxidization of NO to NO2, thereby increasing the SCR activity of NOx with NH3. In a previous study, our group developed a cerium–tungsten catalyst (CeO2–WO3) that showed high SCR activity, N2 selectivity, and SO2 durability over a wide temperature range (175–450 °C) [11,12]. Furthermore, the catalyst preparation method adopted is a critical factor dictating the interaction between the active components. Therefore, it is necessary to compare the activity and characterization of catalysts prepared by different methods.
⁎ Corresponding author. Tel.: +86 10 62771093. E-mail address: [email protected] (J. Li).
In this work, cerium–tungsten oxide catalysts were prepared by three methods: a single step sol–gel (SG), impregnation (IM), and solid processing (SP). The main purpose of this investigation is to determine the best method for preparing CeO2–WO3 catalyst with high NH3–SCR catalytic activity. 2. Experimental methods 2.1. Catalyst preparation 2.1.1. Single step sol–gel method Ammonium paratungstate was dissolved in an oxalic acid solution, and (NH4)2Ce(NO3)6 and citric acid were then added to the solution while stirring. The pH of this solution was adjusted to 1.0 using dilute nitric acid (0.1 mol/L). In the following steps, the sol was slowly vaporized at 60 °C to obtain a transparent colloidal solution, then dried at 120 °C for 10 h, calcined in air at 500 °C for 4 h. Finally, the catalyst was gained and signed CeO2–WO3(SG). 2.1.2. Impregnation method Ammonium paratungstate was dissolved in oxalic acid solution, to which CeO2 was added under stirring at 60 °C for 1 h, then ultrasonically treated for 2 h, dried overnight at 110 °C, and calcined in air at 500 °C for 4 h. Finally, the catalyst was gained and signed CeO2–WO3(IM). 2.1.3. Solid process method Cerium nitrate hexahydrate and ammonium paratungstate were first ground in an agate mortar, crushed for 3 h in a ball mill, and then calcined in air at 500 °C for 4 h, then the catalyst was gained and signed CeO2–WO3(SP).
1566-7367/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.06.017
146
C. Liu et al. / Catalysis Communications 40 (2013) 145–148
In this work, all the prepared catalysts were crushed and sieved to 40–60 mesh prior to characterization. The weight ratio of CeO2/WO3 in the samples was 3:2. 2.2. Catalytic activity measurement The catalytic activity measurements were carried out in a fixed-bed quartz reactor (inner diameter 9 mm) using 0.5 g of catalyst. The feed gas mixture contained 500 ppm NO, 500 ppm NH3, and 3 vol.% O2 and balance in N2 with a total gas flow rate of 300 mL/min and GHSV of 47,000 h−1. The concentration of NOx, N2O, and NH3 in the inlet and outlet gas was measured using a Fourier transform infrared spectroscope (FT-IR, Gasmet Dx-4000) gas analyzer. The activity data were collected when the catalytic reaction substantially reached a steady-state condition for half an hour at each temperature. The performance of the catalysts is presented in terms of the conversion of NOx (X(NOx)) and selectivity of N2 (S(N2)) as defined by Eqs. (1) and (2). XðNOx Þ ¼
SðN2 Þ ¼
½NOx inlet −½NOx outlet 100% with ½NOx ¼ ½NO þ ½NO2 ð1Þ ½NOx inlet
½NOinlet þ ½NH3 inlet −½NO2 outlet −2½N2 Ooutlet 100%: ½NOinlet þ ½NH3 inlet
ð2Þ
2.3. Catalyst characterization X-ray diffraction (XRD) measurements were performed on a D/MAX-RB system with Cu-Kα radiation. BET-surface area was measured by N2 adsorption at 77 K using a Quantachrome Autosorb AS-1 System. Energy Dispersive Spectrometer (EDS) data were obtained with an Oxford INCA300 using 15 kV. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Mg-Kα radiation. The base pressure was approximately 3 × 10− 9 mbar. The binding energies were referenced to the C 1s line at 284.8 eV. The in situ Diffused Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) data were recorded with a Nicolet Nexus spectrometer equipped with a liquid-nitrogen-cooled MCT detector. Prior to each experiment, the samples were preheated in N2 at a flow rate of 100 mL/min and at 300 °C for 60 min and then cooled to room temperature. At room temperature, the background spectrum was recorded under N2 atmosphere. Then, 500 ppm NH3 was passed over the samples at 30 °C for 1 h. After purging with N2 at 30 °C for 60 min, the NH3-IR spectra were recorded at 30 °C. 3. Results and discussion 3.1. SCR catalytic activity Fig. 1(a) shows the conversion curves of NOx as a function of reaction temperature in the SCR process using CeO2–WO3 catalyst prepared by various methods. The figure reveals that the CeO2–WO3 catalysts prepared by the SP and IM methods yielded good catalytic activities over a broad temperature range (175–500 °C) but that, separately, CeO2 and WO3 presented slightly lower activity [11]. For the CeO2–WO3(SP) catalyst in particular, 70% NOx conversion was obtained at both 175 °C and 500 °C, and nearly 100% NOx conversion was achieved from 200 to 450 °C. By contrast, the CeO2–WO3(SG) catalyst showed its highest activities in the temperature range of 300–450 °C and had the lowest activities at low temperatures. Interestingly, the N2 selectivity of all these catalysts is extremely high in 175–500 °C (shown in Fig. 1(b)).
Fig. 1. NOx conversion (a) and N2 selectivity (b) profiles for the CeO2–WO3 catalysts prepared by different methods.
3.2. XRD and BET The XRD patterns of the catalysts prepared by different methods are shown in Fig. 2. The patterns clearly show that the diffraction peaks attributed to the cubic fluorite structure of CeO2 (PDF-ICSD 81-0792) and the Monoclinic WO3 structure (PDF-ICSD 83-0950) crystallites. Two typical crystallites were found in the samples prepared by SP and IM methods, but for the SG method only appears the cubic CeO2 crystallite and the diffraction peaks are broadened and slightly shifted to the direction with lower values compared to the pure cubic CeO2, which indicating that WO3 are mainly amorphous state or W and Ce formed some solution of Ce–W–O complex oxide in CeO2–WO3(SG). The surface areas of CeO2–WO3(SP), CeO2–WO3(IM) and CeO2– WO3(SG) catalysts are 49.1, 43.9 and 30.1, respectively (shown in Table 1). 3.3. EDS The weight concentrations of Ce, W, and O are also summarized in Table 1. Generally speaking, the weight concentrations of Ce, W and O in the catalysts are all close to the theoretical values. The weight of Ce is a little higher than theoretical value; the weights of W and O are a little lower than theoretical value. 3.4. XPS The atomic surface concentrations of Ce, W, and O are summarized in Table 1. As displayed in Table 1, the atomic concentrations of O on
C. Liu et al. / Catalysis Communications 40 (2013) 145–148
147
Fig. 2. XRD patterns of CeO2–WO3 catalysts prepared by different methods: (a) SG, (b) SP, and (c) SP methods.
the surface of the prepared catalysts were evidently different, mainly as a result of the different preparation methods. To get a better understanding of the chemical state of Ce, and W on the catalyst surface, XPS spectra of Ce and W are presented in Fig. 3. Fig. 3 was deconvoluted by searching for the optimal combination with the correlation coefficients (R2) above 0.99 (PeakFit software package, Version 4.12). The peaks located at 882.2, 882.5, 888.3, 889.6, 898.2, 898.6, 901.2, 900.8, 907.2, 907.1, 916.8 and 916.4 represent the 3d104f1 state of Ce4+ ions [13,14]. As seen it, the intensities of feature peaks at 885.6, 903.1 and 903.0 belonged to Ce3+. Moreover, the Ce3+/(Ce3++Ce4+) ratio for CeO2–WO3 catalysts slightly decreased according to the sequence: SG b IM b SP. Similar peaks of binding energy at 35.7 and 37.8 eV are observed in the W4f region of all the samples, shown in Fig. 3(b). These two peaks located at W4f5/2 and 4f7/2 [15], can be well assigned to W6+. Seen from the two figures, no significant change in the peak position was observed among these samples, which indicated that the surface binding energies of Ce3d and W4f were not affected by preparation method. While the Ce valance state was different. In addition, the presence of Ce3+ may result in a charge imbalance, which would lead to oxygen vacancies and unsaturated chemical bonds. This situation could generate additional chemisorbed oxygen or weakly adsorbed oxygen species on the surface of the catalyst. That might affect the redox abilities, nitrogen oxides and ammonia adsorption capacity of catalysts. The results of H2-TPR, NO-TPD and NH3-TPD are showed in the supporting information (shown in Fig. S1, Fig. S2 and Fig. S3).
Fig. 3. XPS spectra of (a) Ce3d and (b) W4f over CeO2–WO3 catalysts.
attributed to the asymmetric and symmetric bending vibrations of the NH+ 4 species on the Brønsted acid sites [17]. The DRIFTS spectra show that the peak for the catalyst prepared by the SG method was very low, about 1220 cm−1, indicating that there were few acid sites on the surface of the CeO2–WO3(SG) catalyst. Some reported that WO3 could provide Brønsted acidic sites [18] and CeO2 might offer the Lewis acid sites [19]. Therefore it could be considered that the acidic sites were some related with the content of CeO2 and WO3 on the surface of catalysts.
3.5. NH3-adsorption Fig. 4 displays the DRIFTS spectra of the NH3 adsorption for different CeO2–WO3 catalysts at room temperature. The bands at 1157 and 1223 cm−1 can be assigned to the asymmetric and symmetric bending vibrations of the N\H bonds in NH3 coordinately linked to the Lewis acid sites [16]; the bands at 1443, 1454, and 1850–1640 cm−1 can be
Table 1 Surface atomic concentration of the various catalysts as determined by the XPS results (atomic%), EDS results (weight%) and BET (m2 g−1). XPS
SP IM SG
EDS 3+
O
Ce
W
Ce
74.12 75.96 77
19.84 12.76 11.08
6.04 11.28 11.92
28.0 18.6 16.5
/(Ce
3+
+Ce
4+
)
BET
O
Ce
W
SBET
19.02 17.82 18.45
57.18 52.75 53.2
23.8 29.4 28.4
49.1 43.9 30.1
Fig. 4. DRIFTS spectra of the CeO2–WO3 catalysts prepared by (a) SG, (b) IM and (c) SP.
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C. Liu et al. / Catalysis Communications 40 (2013) 145–148
Further, combined to XPS results, the presence of Ce3+ would lead to oxygen vacancies and unsaturated chemical bonds, which could generate additional chemisorbed oxygen or weakly adsorbed oxygen species on the surface of the catalyst. That would form much more acid sites. Therefore the higher surface concentrations of Ce and Ce3+ of CeO2–WO3(SP) and CeO2–WO3(IM) could bring larger amounts of acid sites. On the other hand, acid sites have been proved to be greatly beneficial for SCR reaction. So less acid sites over CeO2–WO3(SG) may result in less efficient for SCR activity.
4. Conclusion The results indicate that the catalysts prepared by the SP and IM methods have better SCR activities than those prepared by the SG method over a wide temperature range from 175 to 500 °C. On the basis of the obtained results, the excellent activity in SCR reaction can be attributed to the larger surface area, higher surface concentrations of Ce and Ce3+, enhanced NO oxidization ability, and greater number of surface acid sites, which are all factors that are dependent on the preparation process.
Acknowledgments This work was financially supported by the National Natural Science Fund of China (Grant No. 51078203) and the National High−Tech Research and Development (863) Program of China (Grant No. 2012AA062506).
Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.catcom.2013.06.017. References [1] G. Busca, L. Lietti, G. Ramis, F. Berti, Applied Catalysis B: Environmental 18 (1998) 1–36. [2] J.P. Dunn, P.R. Koppula, H.G. Stenger, I.E. Wachs, Applied Catalysis B: Environmental 19 (1998) 103–117. [3] G. Qi, R.T. Yang, The Journal of Physical Chemistry. B 108 (2004) 15738–15747. [4] R.Q. Long, R.T. Yang, Applied Catalysis B: Environmental 27 (2000) 87–95. [5] R.B. Jin, Y. Liu, Z.B. Wu, H.Q. Wang, T.T. Gu, Chemosphere 78 (2010) 1160–1166. [6] X. Gao, Y. Jiang, Y. Zhong, Z. Luo, K. Cen, Journal of Hazardous Materials 174 (2010) 734–739. [7] M.F. Luo, J. Chen, L.S. Chen, J.Q. Lu, Z. Feng, C. Li, Chemistry of Materials 13 (2001) 197–202. [8] L.A. Chen, J.H. Li, M.F. Ge, Environmental Science & Technology 44 (2010) 9590–9596. [9] Y.S. Shen, S.M. Zhu, T. Qiu, S.B. Shen, Catalysis Communications 11 (2009) 20–23. [10] B.M. Reddy, A. Khan, Y. Yamada, T. Kobayashi, S. Loridant, J.C. Volta, The Journal of Physical Chemistry. B 107 (2003) 5162–5167. [11] L. Chen, J.H. Li, W.J. Ablikim, J. Wang, H.Z. Chang, L. Ma, J.Y. Xu, M.F. Ge, Catalysis Letters 141 (2011) 1859–1864. [12] L. Chen, J.H. Li, M.F. Ge, W.J. Ablikim, J. Wang, Chinese Journal of Catalysis 32 (2011) 836–841. [13] F.B. Noronha, E.C. Fendley, R.R. Soares, W.E. Alvarez, D.E. Resasco, Chemical Engineering Journal 82 (2001) 21–31. [14] H. He, H. Dai, C.T. Au, Catalysis Today 90 (2004) 245–254. [15] G. Leftheriotis, S. Papaefthimiou, P. Yianoulis, A. Siokou, Thin Solid Films 384 (2001) 298–306. [16] F. Liu, et al., Applied Catalysis B: Environmental 93 (2009) 194–204. [17] S.D. Lin, A.C. Gluhoi, B.E. Nieuwenhuys, Catalysis Today 90 (2004) 3–14. [18] J.P. Chen, R.T. Yang, Applied Catalysis A: General 80 (1992) 135–148. [19] G. Qi, R.T. Yang, R. Chang, Applied Catalysis B: Environmental 51 (2004) 93–106.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Characterization of CeO2–WO3 catalysts prepared by different methods for selective catalytic reduction of NOx with NH3 Caixia Liu, Liang Chen, Huazhen Chang, Lei Ma, Yue Peng, Hamidreza Arandiyan, Junhua Li ⁎ State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC), School of Environment, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 21 March 2013 Received in revised form 6 June 2013 Accepted 11 June 2013 Available online 25 June 2013 Keywords: Selective catalytic reduction (SCR) CeO2–WO3 Preparation methods NOx abatement
a b s t r a c t In this work, cerium–tungsten oxide catalysts were prepared by three methods: single step sol–gel (SG), impregnation (IM), and solid processing (SP). The catalysts were used for selective catalytic reduction (SCR) of NOx with ammonia over a wide temperature range. The results indicated that the catalysts prepared by the SP and IM methods exhibited better SCR activity than that prepared via the SG method in 175–500 °C. The excellent activity can be attributed to larger surface area, higher surface concentrations of Ce and Ce3+, enhanced NO oxidization ability, and greater number of surface acid sites. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxide (NO, NO2, and N2O) in exhaust from fossil fuel combustion is a major cause of photochemical smog, acid rain, and ozone depletion. Selective catalytic reduction of NOx with NH3 (NH3–SCR) is an effective and economical method to remove NOx. Currently, V2O5–WO3/TiO2 or V2O5–MoO3/TiO2 is the most widely used catalyst, but it has a relatively narrow temperature range of 300–400 °C [1,2]. Accordingly, many researchers have started to modify current catalysts and investigate novel catalysts to reduce vanadium loadings or to substitute the vanadium with other metal elements. Cerium oxide (CeO2) has received much attention for the SCR of NOx because of its prominence for NOx removal [3,4]. Many researchers have reported on CeO2 mixed with other oxides for SCR aided with NH3, such as Mn–Ce/TiO2 [5], CeO2–TiO2 [6], CexTi1−xO2 [7], CeO2–WO3/TiO2 [8], and CeO2/Al2O3 [9]. These compounds can store and release oxygen via the redox shift between Ce4 + and Ce3 + under oxidizing and reducing conditions, respectively [10]. Ceria can enhance the oxidization of NO to NO2, thereby increasing the SCR activity of NOx with NH3. In a previous study, our group developed a cerium–tungsten catalyst (CeO2–WO3) that showed high SCR activity, N2 selectivity, and SO2 durability over a wide temperature range (175–450 °C) [11,12]. Furthermore, the catalyst preparation method adopted is a critical factor dictating the interaction between the active components. Therefore, it is necessary to compare the activity and characterization of catalysts prepared by different methods.
⁎ Corresponding author. Tel.: +86 10 62771093. E-mail address: [email protected] (J. Li).
In this work, cerium–tungsten oxide catalysts were prepared by three methods: a single step sol–gel (SG), impregnation (IM), and solid processing (SP). The main purpose of this investigation is to determine the best method for preparing CeO2–WO3 catalyst with high NH3–SCR catalytic activity. 2. Experimental methods 2.1. Catalyst preparation 2.1.1. Single step sol–gel method Ammonium paratungstate was dissolved in an oxalic acid solution, and (NH4)2Ce(NO3)6 and citric acid were then added to the solution while stirring. The pH of this solution was adjusted to 1.0 using dilute nitric acid (0.1 mol/L). In the following steps, the sol was slowly vaporized at 60 °C to obtain a transparent colloidal solution, then dried at 120 °C for 10 h, calcined in air at 500 °C for 4 h. Finally, the catalyst was gained and signed CeO2–WO3(SG). 2.1.2. Impregnation method Ammonium paratungstate was dissolved in oxalic acid solution, to which CeO2 was added under stirring at 60 °C for 1 h, then ultrasonically treated for 2 h, dried overnight at 110 °C, and calcined in air at 500 °C for 4 h. Finally, the catalyst was gained and signed CeO2–WO3(IM). 2.1.3. Solid process method Cerium nitrate hexahydrate and ammonium paratungstate were first ground in an agate mortar, crushed for 3 h in a ball mill, and then calcined in air at 500 °C for 4 h, then the catalyst was gained and signed CeO2–WO3(SP).
1566-7367/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.06.017
146
C. Liu et al. / Catalysis Communications 40 (2013) 145–148
In this work, all the prepared catalysts were crushed and sieved to 40–60 mesh prior to characterization. The weight ratio of CeO2/WO3 in the samples was 3:2. 2.2. Catalytic activity measurement The catalytic activity measurements were carried out in a fixed-bed quartz reactor (inner diameter 9 mm) using 0.5 g of catalyst. The feed gas mixture contained 500 ppm NO, 500 ppm NH3, and 3 vol.% O2 and balance in N2 with a total gas flow rate of 300 mL/min and GHSV of 47,000 h−1. The concentration of NOx, N2O, and NH3 in the inlet and outlet gas was measured using a Fourier transform infrared spectroscope (FT-IR, Gasmet Dx-4000) gas analyzer. The activity data were collected when the catalytic reaction substantially reached a steady-state condition for half an hour at each temperature. The performance of the catalysts is presented in terms of the conversion of NOx (X(NOx)) and selectivity of N2 (S(N2)) as defined by Eqs. (1) and (2). XðNOx Þ ¼
SðN2 Þ ¼
½NOx inlet −½NOx outlet 100% with ½NOx ¼ ½NO þ ½NO2 ð1Þ ½NOx inlet
½NOinlet þ ½NH3 inlet −½NO2 outlet −2½N2 Ooutlet 100%: ½NOinlet þ ½NH3 inlet
ð2Þ
2.3. Catalyst characterization X-ray diffraction (XRD) measurements were performed on a D/MAX-RB system with Cu-Kα radiation. BET-surface area was measured by N2 adsorption at 77 K using a Quantachrome Autosorb AS-1 System. Energy Dispersive Spectrometer (EDS) data were obtained with an Oxford INCA300 using 15 kV. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Mg-Kα radiation. The base pressure was approximately 3 × 10− 9 mbar. The binding energies were referenced to the C 1s line at 284.8 eV. The in situ Diffused Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) data were recorded with a Nicolet Nexus spectrometer equipped with a liquid-nitrogen-cooled MCT detector. Prior to each experiment, the samples were preheated in N2 at a flow rate of 100 mL/min and at 300 °C for 60 min and then cooled to room temperature. At room temperature, the background spectrum was recorded under N2 atmosphere. Then, 500 ppm NH3 was passed over the samples at 30 °C for 1 h. After purging with N2 at 30 °C for 60 min, the NH3-IR spectra were recorded at 30 °C. 3. Results and discussion 3.1. SCR catalytic activity Fig. 1(a) shows the conversion curves of NOx as a function of reaction temperature in the SCR process using CeO2–WO3 catalyst prepared by various methods. The figure reveals that the CeO2–WO3 catalysts prepared by the SP and IM methods yielded good catalytic activities over a broad temperature range (175–500 °C) but that, separately, CeO2 and WO3 presented slightly lower activity [11]. For the CeO2–WO3(SP) catalyst in particular, 70% NOx conversion was obtained at both 175 °C and 500 °C, and nearly 100% NOx conversion was achieved from 200 to 450 °C. By contrast, the CeO2–WO3(SG) catalyst showed its highest activities in the temperature range of 300–450 °C and had the lowest activities at low temperatures. Interestingly, the N2 selectivity of all these catalysts is extremely high in 175–500 °C (shown in Fig. 1(b)).
Fig. 1. NOx conversion (a) and N2 selectivity (b) profiles for the CeO2–WO3 catalysts prepared by different methods.
3.2. XRD and BET The XRD patterns of the catalysts prepared by different methods are shown in Fig. 2. The patterns clearly show that the diffraction peaks attributed to the cubic fluorite structure of CeO2 (PDF-ICSD 81-0792) and the Monoclinic WO3 structure (PDF-ICSD 83-0950) crystallites. Two typical crystallites were found in the samples prepared by SP and IM methods, but for the SG method only appears the cubic CeO2 crystallite and the diffraction peaks are broadened and slightly shifted to the direction with lower values compared to the pure cubic CeO2, which indicating that WO3 are mainly amorphous state or W and Ce formed some solution of Ce–W–O complex oxide in CeO2–WO3(SG). The surface areas of CeO2–WO3(SP), CeO2–WO3(IM) and CeO2– WO3(SG) catalysts are 49.1, 43.9 and 30.1, respectively (shown in Table 1). 3.3. EDS The weight concentrations of Ce, W, and O are also summarized in Table 1. Generally speaking, the weight concentrations of Ce, W and O in the catalysts are all close to the theoretical values. The weight of Ce is a little higher than theoretical value; the weights of W and O are a little lower than theoretical value. 3.4. XPS The atomic surface concentrations of Ce, W, and O are summarized in Table 1. As displayed in Table 1, the atomic concentrations of O on
C. Liu et al. / Catalysis Communications 40 (2013) 145–148
147
Fig. 2. XRD patterns of CeO2–WO3 catalysts prepared by different methods: (a) SG, (b) SP, and (c) SP methods.
the surface of the prepared catalysts were evidently different, mainly as a result of the different preparation methods. To get a better understanding of the chemical state of Ce, and W on the catalyst surface, XPS spectra of Ce and W are presented in Fig. 3. Fig. 3 was deconvoluted by searching for the optimal combination with the correlation coefficients (R2) above 0.99 (PeakFit software package, Version 4.12). The peaks located at 882.2, 882.5, 888.3, 889.6, 898.2, 898.6, 901.2, 900.8, 907.2, 907.1, 916.8 and 916.4 represent the 3d104f1 state of Ce4+ ions [13,14]. As seen it, the intensities of feature peaks at 885.6, 903.1 and 903.0 belonged to Ce3+. Moreover, the Ce3+/(Ce3++Ce4+) ratio for CeO2–WO3 catalysts slightly decreased according to the sequence: SG b IM b SP. Similar peaks of binding energy at 35.7 and 37.8 eV are observed in the W4f region of all the samples, shown in Fig. 3(b). These two peaks located at W4f5/2 and 4f7/2 [15], can be well assigned to W6+. Seen from the two figures, no significant change in the peak position was observed among these samples, which indicated that the surface binding energies of Ce3d and W4f were not affected by preparation method. While the Ce valance state was different. In addition, the presence of Ce3+ may result in a charge imbalance, which would lead to oxygen vacancies and unsaturated chemical bonds. This situation could generate additional chemisorbed oxygen or weakly adsorbed oxygen species on the surface of the catalyst. That might affect the redox abilities, nitrogen oxides and ammonia adsorption capacity of catalysts. The results of H2-TPR, NO-TPD and NH3-TPD are showed in the supporting information (shown in Fig. S1, Fig. S2 and Fig. S3).
Fig. 3. XPS spectra of (a) Ce3d and (b) W4f over CeO2–WO3 catalysts.
attributed to the asymmetric and symmetric bending vibrations of the NH+ 4 species on the Brønsted acid sites [17]. The DRIFTS spectra show that the peak for the catalyst prepared by the SG method was very low, about 1220 cm−1, indicating that there were few acid sites on the surface of the CeO2–WO3(SG) catalyst. Some reported that WO3 could provide Brønsted acidic sites [18] and CeO2 might offer the Lewis acid sites [19]. Therefore it could be considered that the acidic sites were some related with the content of CeO2 and WO3 on the surface of catalysts.
3.5. NH3-adsorption Fig. 4 displays the DRIFTS spectra of the NH3 adsorption for different CeO2–WO3 catalysts at room temperature. The bands at 1157 and 1223 cm−1 can be assigned to the asymmetric and symmetric bending vibrations of the N\H bonds in NH3 coordinately linked to the Lewis acid sites [16]; the bands at 1443, 1454, and 1850–1640 cm−1 can be
Table 1 Surface atomic concentration of the various catalysts as determined by the XPS results (atomic%), EDS results (weight%) and BET (m2 g−1). XPS
SP IM SG
EDS 3+
O
Ce
W
Ce
74.12 75.96 77
19.84 12.76 11.08
6.04 11.28 11.92
28.0 18.6 16.5
/(Ce
3+
+Ce
4+
)
BET
O
Ce
W
SBET
19.02 17.82 18.45
57.18 52.75 53.2
23.8 29.4 28.4
49.1 43.9 30.1
Fig. 4. DRIFTS spectra of the CeO2–WO3 catalysts prepared by (a) SG, (b) IM and (c) SP.
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Further, combined to XPS results, the presence of Ce3+ would lead to oxygen vacancies and unsaturated chemical bonds, which could generate additional chemisorbed oxygen or weakly adsorbed oxygen species on the surface of the catalyst. That would form much more acid sites. Therefore the higher surface concentrations of Ce and Ce3+ of CeO2–WO3(SP) and CeO2–WO3(IM) could bring larger amounts of acid sites. On the other hand, acid sites have been proved to be greatly beneficial for SCR reaction. So less acid sites over CeO2–WO3(SG) may result in less efficient for SCR activity.
4. Conclusion The results indicate that the catalysts prepared by the SP and IM methods have better SCR activities than those prepared by the SG method over a wide temperature range from 175 to 500 °C. On the basis of the obtained results, the excellent activity in SCR reaction can be attributed to the larger surface area, higher surface concentrations of Ce and Ce3+, enhanced NO oxidization ability, and greater number of surface acid sites, which are all factors that are dependent on the preparation process.
Acknowledgments This work was financially supported by the National Natural Science Fund of China (Grant No. 51078203) and the National High−Tech Research and Development (863) Program of China (Grant No. 2012AA062506).
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