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Selective catalytic reduction of NOx with methane over indium supported on tungstated zirconia
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Catalysis Communications 8 (2007) 2243–2247 www.elsevier.com/locate/catcom
Selective catalytic reduction of NOx with methane over indium supported on tungstated zirconia Dong Yang a
a,b
, Junhua Li
a,*
, Mingfen Wen b, Chongli Song
b
Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China b Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 102201, China Received 2 February 2007; received in revised form 24 April 2007; accepted 25 April 2007 Available online 8 May 2007
Abstract Selective catalytic reduction (SCR) of NOx with methane was investigated over a series of non-noble metal (cobalt, manganese, nickel, tin, silver, indium) catalysts supported on tungstated zirconia (WZr). A great improvement of catalytic activity was found over the indium-loaded WZr catalysts upon the other WZr. The highest NOx conversion of 70% was achieved over an 1%In/WZr catalyst at 450 °C and 12,000 h1. InO+ was proposed to be the active site in NO reduction. H2O and SO2 significantly inhibited NO reduction + by competing with reactants to adsorb on InO+ site and forming inactive InðOHÞþ 2 and In(SO3) species. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Nitrogen oxides; Selective catalytic reduction; Methane; Indium; Tungstated zirconia
1. Introduction Nitrogen oxides, as a major pollutant of fossil fuel combustion, can cause many environmental problems, such as acid rain and photochemical smog. Selective catalytic reduction of nitrogen oxides by hydrocarbons (HC-SCR) is a promising technique to remove NOx in the present of excess oxygen for both stationary sources and mobile sources [1–3]. Methane, the main component of natural gas, has been attracted substantial attention as a reductant for the HC-SCR technique due to its low cost [4,5]. Many researchers have focused their investigations on zeolite based catalysts and on sulfated zirconia (SZr) based catalysts, such as Co–ZSM-5 [5,6], Pd–ZSM-5 [7,8], In– ZSM-5 [9,10], Pd/SZr [11], Co/SZr [12,13], Mn/SZr [14,15], and In/SZr [16]. These catalysts showed high activity under dry conditions, while some of catalysts also showed good activity under wet conditions. However, the stability of zeolite and SZr under hydrothermal condition is very poor [17,18], and this inherent drawback inhibits *
Corresponding author. Tel.: +86 10 62782030; fax: +86 10 62785687. E-mail address: [email protected] (J. Li).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.04.035
the practical application of these catalysts. Tungstate zirconia (WZr) is more stable than zeolite and SZr support. The most important is that the WZr possesses strong acid sites which is indispensable to the high activity and selectivity in CH4–SCR [19]. So WZr is a good choice for the substitution of zeolite and SZr. However, up to now, only Pd supported on WZr was reported to be used as a catalyst for the CH4–SCR reaction [18,20]. In this work, several non-noble metal (Co, Mn, Ni, Ag, Sn and In) loaded WZr catalysts were investigated for CH4–SCR reaction. Among these catalysts, only In/WZr revealed high NO reduction activity. The activity of In/ WZr was even higher than that of Pd/WZr. So the attention was mainly focused on the catalytic performance of In/WZr catalyst in this paper. 2. Experimental 2.1. Catalyst preparation The tungstated zirconia (WZr) or the sulfated zirconia (SZr) support was prepared by impregnating zirconium oxyhydroxide (AR grade, Shanghai Chemical Reagent
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D. Yang et al. / Catalysis Communications 8 (2007) 2243–2247
Work, China) with an aqueous solution of ammonium paratungstate (AR grade, Tianjin Juneng Chemical Factory, China) or ammonium sulfate (AR grade, Beijing Chemical Reagent Work, China), and the W and S contents in the final supports were 10 wt%. The supports were then dried at 120 °C overnight and calcined at 600 °C for 5 h. The zirconia support was obtained by directly calcinating zirconium oxyhydroxide at 600 °C for 5 h. Indium loading was accomplished by impregnation method using an aqueous solution of indium nitrate (AR grade, Sinopharm Chemical Reagent Factory, China). After dried at 110 °C overnight, it was then calcined in air at 650 °C for 5 h. The resulting catalysts are hereafter referred to as InaWZr, InaSZr or InaZr; where In refers to Indium, a refers to the weight content of indium in the catalyst, while WZr, SZr and Zr suggest that the support used in these catalysts are WZr, SZr, and ZrO2, respectively. Co, Mn, Ni and Ag were introduced onto the WZr support by impregnation with the corresponding metal nitrate solution, while SnCl4 and PdCl2 were used as the source of Sn and Pd. 2.2. Catalytic activity measurement The activity measurements were carried out with a fixedbed quartz reactor (inner diameter 8 mm) using a 0.5 g catalyst of 60–80 mesh. The feed gas was a mixture contained 1000 ppm NO or NO2, 3000 ppm CH4, 10% O2, 0 or 100 ppm SO2, 0% or 10% H2O and N2 as the balance gas. The total flow rate of the feed gas was 100 mL/min, corresponding to a space velocity of about 12,000 h1. NO and NO2 concentration were analyzed with a chemiluminescence NO/NO2 analyzer (Thermal Environmental Instruments, model 42C), and CH4 concentration was detected by gas chromatograph (Shimadzu GC 17A). Catalyst activity was evaluated in terms of percent NOx and CH4 conversions, defined as: ðC in C out Þ 100; C in
radiation (1484.6 eV) and it was calibrated internally by carbon deposit C1s binding energy (BE) at 284.6 eV. 3. Results and discussion 3.1. Characterization of catalysts XRD patterns for ZrO2, In1Zr, WZr, and In1WZr catalysts are shown in Fig. 1. The XRD patterns of ZrO2 and In1Zr were that of monoclinic zirconia. While the XRD patterns of WZr and In1WZr showed mainly the tetragonal zirconia phase although small fraction of monoclinic phase was also detected for In1WZr catalyst. These results suggest that tungsten is helpful for the stabilization of tetragonal zirconia phase. In addition, none of these samples gave diffraction lines corresponding to In2O3. The XRD patterns of InWZr with indium contents various from 0.5% to 10% were also measured (not shown). They were all similar to that of the In1WZr catalyst, and no peaks of In2O3 were observed. These results indicate a high dispersion of In2O3 on the support surface. The BET surface area, pore volume and pore size of ZrO2, In1Zr, WZr, and In1WZr are summarized in Table 1. The surface area of WZr and In1WZr were much greater than that of ZrO2 and In1Zr. It was attributed to the stabilization effect of tungsten for the formation of tetragonal zirconia phase. The XPS results are given in Table 2 in terms of the binding energies of the In3d5/2 and W4f5/2 as well as of the atomic ratio of In/Zr and W/Zr in the sample surface. A binding energy of 444.7 eV for In3d5/2 was measured for the In1Zr catalyst, which was consistent with the literature values for the In2O3 bulk phase [21,22]. The binding energy of In3d5/2 for In1WZr was 0.5 eV higher than the value for the In2O3 bulk phase. Similar shift was detected over In/ ZSM-5 catalyst [22], and it was attributed to the formation of InO+ species. So similar InO+ species might be also formed in our In1WZr catalyst. The bind energy of W4f5/2 in WZr and In1WZr suggested that W was mainly in the state of 6+ [23]. The surface W/Zr atomic ratio of
where Cin and Cout are the NOx or CH4 concentration at the reactor inlet and outlet, respectively. The activity data were collected after the catalytic reactions were substantially reached to the steady-state conditions for half an hour at each temperature.
X-ray diffraction (XRD) patterns were determined using a Rigaku D/max-RB diffractometer. The analyses were performed with Cu Target (40 kV and 100 mA); a typical scan speed was 6°/min with a step of 0.002° in the range from 20° to 70°. BET surface area, pore size, and pore volume were measured by N2 adsorption–desorption method using a NOVA 3200e analyzer. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a PHI15300/ESCA system with Al Ka
Intensity (a.u.)
2.3. Catalyst characterization
In1WZr
WZr
In1Zr
ZrO 2
20
30
40 50 2 Theta (º)
60
70
Fig. 1. XRD patterns of ZrO2, In1Zr, WZr, and In1WZr catalyst.
D. Yang et al. / Catalysis Communications 8 (2007) 2243–2247 Table 1 Physical properties of ZrO2, In1Zr, WZr, and In1WZr
2245
50
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore diameter (nm)
ZrO2 In1Zr WZr In1WZr
27 24 78 76
0.13 0.12 0.25 0.19
5.3 8.3 1.7 1.1
40
NOx conversion (%)
Samples
Table 2 In3d5/2 and W4f5/2 binding energies, In/Zr and W/Zr atomic ratio of In1Zr, WZr, and In1WZr derived from XPS W4f5/2 (eV)
In/Zra
W/Zra
In1Zr WZr In1WZr
444.7 – 445.2
– 37.9 38.0
0.056 (0.011)b – 0.035 (0.012)b
– 0.071 (0.067)b 0.081 (0.067)b
a The atomic ratios were calculated from the area ratios of the XPS lines of In (3d), W (4f), and Zr (3d). Atomic sensitivity factors for these lines were those reported by Physical Electronics as empirically determined for the PHI15300/ESCA system. b The values in parenthesis are the expected ratios when elements are all homogeneously dispersed in the catalysts.
WZr was close to the expected values, suggesting that the surface and bulk morphology in WZr was symmetrical. For the In1WZr catalyst, the W/Zr ratio increased slightly beyond that of WZr, indicating the aggregation of tungsten on the catalyst surface which can be related to the high interaction of InO+ species with WZr support. The In/Zr ratio was found to be much higher than the expected values in both In1Zr and In1WZr, suggesting the aggregation of indium on the catalyst surfaces. The aggregation of In on the catalyst surface was more serious in In1Zr than In1WZr, perhaps attributed to the higher surface area of In1WZr than In1Zr as shown in Table 1. 3.2. CH4–SCR activity test The NOx conversion activity of WZr support and Co, Mn, Ni, Ag, Sn, In, Pd loaded WZr catalysts at 550 °C are compared in Fig. 2. NOx conversion over WZr support was lower than 10% at this temperature. The loading of Mn, Ni or Ag caused a pronounced decrease in the NOx conversion activity of WZr, and the loading of Co and Sn could slightly improve the activity of WZr. However, NOx conversion could be remarkably enhanced to 36% with loading indium on WZr, and the activity was even higher than that of the Pd/WZr catalyst. NOx and CH4 conversion activities over WZr, In1WZr, In1Zr, ZrO2, and In1SZr are shown in Fig. 3. The maximum NOx conversion over ZrO2 was 31% at 650 °C. Loading of indium on ZrO2 led to a pronounced decrease in the NOx conversion activity, but showed no influence on the activity for CH4 conversion. It suggested that In2O3 bulk phase over the In1Zr, as detected by XPS, was not active for either the reduction of NOx or the combustion of CH4. The WZr showed significantly
10
0 WZrC
oWZr MnWZrN iWZr AgWZrS nWZr InWZr PdWZr
Fig. 2. NOx conversion over various element loaded WZr catalyst at 550 °C. The content of Co, Mn, Ni, Ag, Sn and In were 2 wt%, and Pd was 0.1 wt%. Reaction conditions: 1000 ppm NO, 3000 ppm CH4, 10% O2, N2 as balance; GHSV = 12,000 h1.
a NOx conversion (%)
In3d5/2 (eV)
20
100 In1WZr WZr In1Zr ZrO2
80
In1SZr
60
40
20
0 250 300 350 400 450 500 550 600 650 700
Temperautre ( ºC)
b
100
CH4 conversion (%)
Samples
30
80 60 40 20 0 250 300 350 400 450 500 550 600 650 700
Temperautre ( ºC) Fig. 3. NOx and CH4 conversions over ZrO2, In1Zr, WZr, In1WZr, and In1SZr catalyst. (a) NOx conversion; (b) CH4 conversion. Reaction conditions: 1000 ppm NO, 3000 ppm CH4, 10% O2, N2 as balance; GHSV = 12,000 h1.
reduced activities for both NOx and CH4 conversion than that of the ZrO2. However, it can be readily be seen from Fig. 3 that the present of indium greatly enhanced the
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D. Yang et al. / Catalysis Communications 8 (2007) 2243–2247
NOx conversion of WZr. The highest NO conversion rate for In1WZr catalyst was 70% at 450 °C. The InO+ species, as discussed above, was attributed to the active site for NOx reduction and WZr might play the same role with ZSM-5, providing strong acid site for anchoring of InO+ species [24]. In addition, the entire CH4 conversion curve of In1WZr shifted toward much lower temperature than that of WZr, suggesting that the InO+ species was not only responsible for NOx reduction but also was an active site for CH4 oxidation. For comparative purpose, the activity of In1SZr for CH4–SCR was also tested and the result was shown in Fig. 3. Maximum NO conversion over the In1SZr was 46% and was appeared at 650 °C. Although the In1SZr showed higher NOx conversion activity than that of the In1WZr at temperature higher than 600 °C, the performance of the In1SZr at low temperature was more inferior compared to the In1WZr. This fact may be attributed to the weaker acid strength of the WZr compared to that of the SZr [23,25], since weaker acid strength can lead to a
a
100
weaker interaction between the InO+ species and the support. And thus the InO+ on the WZr would be more active than it was on the SZr. The effect of indium content on the NOx and CH4 conversion activity of the InWZr catalysts is shown in Fig. 4. The peak NOx conversions were all achieved at 450 °C and fell in the range of 57%–70% for the InWZr catalysts containing 0.5–10% indium, indicating a very little influence of indium loading on the activity of NOx reduction. Since the number of acidic sites on the WZr was limited, the ability of the WZr to anchor active InO+ sites was finite and In would aggregate to In2O3 at high loading of Indium. However, as shown above, In2O3 phase was not active for the conversion of both NOx and CH4. As a consequence, all of the InWZr catalysts showed similar NOx reduction activity. Comparing Fig. 4b with Fig. 3b, it is clear that CH4 conversion increased with increasing In loading up to a content of 1% and then it unchanged when In loading further increasing to 10%. This observation may suggest that the aggregation of In2O3 over WZr support began at the In loading of 1%. The effect of H2O and SO2 on the activity of the In1WZr catalyst was shown in Fig. 5. The presence of SO2 in the
In0.5WZr In1WZr In2WZr In5WZr In10WZr
60
40
20
a 100 SO 2
NOx conversion (%)
NO x conversion (%)
80
0 250 300 350 400 450 500 550 600 650 700
H2O+SO2
60 40 20 0 300
Temperature (ºC)
350
400
450
500
550
600
550
600
Temperaure(ºC)
100
b 80
CH4 conversion (%)
CH 4 conversion (%)
b
H2O
80
60 40 20 0 250 300 350 400 450 500 550 600 650 700
Temperature (ºC) Fig. 4. Effect of indium content on the NOx and CH4 conversion over InWZr catalysts. (a) NO conversion; (b) CH4 conversion. Reaction conditions: 1000 ppm NO, 3000 ppm CH4, 10% O2, N2 as balance; GHSV = 12,000 h1.
100 80 60 40 20 0 300
350
400
450
500
Temperaure(ºC) Fig. 5. Effect of H2O and SO2 on the NOx and CH4 conversion over In1WZr catalyst. (a) NO conversion; (b) CH4 conversion. Reaction conditions: 1000 ppm NO, 3000 ppm CH4, 10% O2, 100 ppm SO2, 10% H2O, N2 as balance; GHSV = 12,000 h1.
D. Yang et al. / Catalysis Communications 8 (2007) 2243–2247
reaction gas significantly inhibited the NOx reduction activity of the In1WZr. The maximum NOx conversion was also achieved at 450 °C but it was decreased from 70% to 33%. The inhibition effect of H2O on the catalytic activity of the In1WZr catalyst was more pronounced than that of SO2. The temperature window for reduction shifted by 50 °C to higher temperature when H2O was added to the reaction gas, while the maximum NOx conversion was decreased to 29%. H2O and SO2 could compete with reacþ tants for adsorbing on InO+ site and form InðOHÞ2 and In(SO3)+ species which were inactive for CH4–SCR [26]. The deactivation shown in NOx conversion reaction might be due to the formation of the two species. In the co-presence of H2O and SO2 in the reaction gas, the catalytic activity for NOx conversion was close to the case when only H2O was added. It means that H2O had a dominant influence on the catalytic activity. This fact might suggest that þ the formation of InðOHÞ2 species were easier than that + of In(SO3) species when H2O and SO2 co-present in the reaction gas. 4. Conclusions In/WZr catalyst exhibited high activities in SCR of NO with methane. Under a condition of 1000 ppm NO, 3000 ppm CH4, 10% O2, and GHSV of 12,000 h1, the maximum NOx conversion level of the In/WZr catalyst reached 70% and was achieved at 450 °C. The InO+ was attributed to be the active site for CH4–SCR, while In2O3 showed very little influence on the activity of the In/WZr catalyst. H2O and SO2 could significantly inhibit the NO reduction by adsorbing on InO+ and forming unactive + InðOHÞþ 2 and In(SO3) species. Further studies are in progress for enhancing the activity of the In/WZr catalysts by addition additives, such as Co, Mn, or Ce. Acknowledgements The work was financially supported by National Natural Science Fund of China (Grant No. 20677034 and
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20437010), and the National High-Tech Research and Development (863) Program of China (Grant No. 2006AA060301). References [1] M. Iwamoto, H. Yahiro, S. Shundo, Y. Yu-u, N. Mizuno, Appl. Catal. 69 (1991) 15. [2] M. Shelef, Chem. Rev. 95 (1995) 209. [3] K.A. Bethke, M.C. Kung, B. Yang, M. Shah, D. Alt, C. Li, H.H. Kung, Catal. Today 26 (1995) 169. [4] Y. Li, J.N. Armor, Appl. Catal. B 1 (1992) L31. [5] J.N. Armor, Catal. Today 26 (1995) 147. [6] M.C. Campa, S. De Rossi, G. Ferraris, V. Indovina, Appl. Catal. B 8 (1996) 315. [7] B.J. Adelman, W.M.H. Sachtler, Appl. Catal. B 14 (1997) 1. [8] A. Ali, W. Alvarez, C.J. Loughran, D.E. Resasco, Appl. Catal. B 14 (1997) 13. [9] X.J. Zhou, Z.S. Xu, T. Zhang, L.W. Lin, J. Mol. Catal. A 122 (1997) 125. [10] X.J. Zhou, T. Zhang, Z.S. Xu, L.W. Lin, Catal. Lett. 40 (1996) 35. [11] Y.H. Chin, A. Pisanu, L. Serventi, W.E. Alvarez, D.E. Resasco, Catal. Today 54 (1999) 419. [12] N. Li, A.Q. Wang, J.W. Tang, X.D. Wang, D.B. Liang, T. Zhang, Appl. Catal. B 43 (2003) 195. [13] L.F. Cordoba, W.M.H. Sachtler, C.M. de Correa, Appl. Catal. B 56 (2005) 269. [14] N. Li, A.Q. Wang, Z.M. Liu, X.D. Wang, M.Y. Zheng, Y.Q. Huang, T. Zhang, Appl. Catal. B 62 (2006) 292. [15] L. Ning, A.Q. Wang, X.D. Wang, M.Y. Zheng, R.H. Cheng, T. Zhang, Appl. Catal. B 48 (2004) 259. [16] W. Suprun, K. Schaedlich, H. Papp, Chem. Eng. Technol. 28 (2005) 199. [17] V. Houel, D. James, P. Millington, S. Pollington, S. Poulston, R. Rajaram, R. Torbati, J. Catal. 230 (2005) 150. [18] Y.H. Chin, W.E. Alvarez, D.E. Resasco, Catal. Today 62 (2000) 159. [19] C.J. Looughran, D.E. Resasco, Appl. Catal. B 7 (1995) 113. [20] Y.H. Chin, W.E. Alvarez, D.E. Resasco, Catal. Today 62 (2000) 291. [21] J.H. Li, J.M. Hao, X.Y. Cui, L.X. Fu, Catal. Lett. 103 (2005) 75. [22] T. Maunula, J. Ahola, H. Hamada, Appl. Catal. B 64 (2006) 13. [23] K. Oka, T. Nishiguchi, H. Kanai, K. Utani, S. Imamura, Appl. Catal. A 309 (2006) 187. [24] E. Kikuchi, M. Ogura, I. Terasaki, Y. Goto, J. Catal. 161 (1996) 465. [25] G.D. Yadav, J.J. Nair, Micropor. Mesopor. Mater. 33 (1999) 1. [26] E. Kikuchi, M. Ogura, N. Aratani, Y. Sugiura, S. Hiromoto, K. Yogo, Catal. Today 27 (1996) 35.
Catalysis Communications 8 (2007) 2243–2247 www.elsevier.com/locate/catcom
Selective catalytic reduction of NOx with methane over indium supported on tungstated zirconia Dong Yang a
a,b
, Junhua Li
a,*
, Mingfen Wen b, Chongli Song
b
Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China b Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 102201, China Received 2 February 2007; received in revised form 24 April 2007; accepted 25 April 2007 Available online 8 May 2007
Abstract Selective catalytic reduction (SCR) of NOx with methane was investigated over a series of non-noble metal (cobalt, manganese, nickel, tin, silver, indium) catalysts supported on tungstated zirconia (WZr). A great improvement of catalytic activity was found over the indium-loaded WZr catalysts upon the other WZr. The highest NOx conversion of 70% was achieved over an 1%In/WZr catalyst at 450 °C and 12,000 h1. InO+ was proposed to be the active site in NO reduction. H2O and SO2 significantly inhibited NO reduction + by competing with reactants to adsorb on InO+ site and forming inactive InðOHÞþ 2 and In(SO3) species. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Nitrogen oxides; Selective catalytic reduction; Methane; Indium; Tungstated zirconia
1. Introduction Nitrogen oxides, as a major pollutant of fossil fuel combustion, can cause many environmental problems, such as acid rain and photochemical smog. Selective catalytic reduction of nitrogen oxides by hydrocarbons (HC-SCR) is a promising technique to remove NOx in the present of excess oxygen for both stationary sources and mobile sources [1–3]. Methane, the main component of natural gas, has been attracted substantial attention as a reductant for the HC-SCR technique due to its low cost [4,5]. Many researchers have focused their investigations on zeolite based catalysts and on sulfated zirconia (SZr) based catalysts, such as Co–ZSM-5 [5,6], Pd–ZSM-5 [7,8], In– ZSM-5 [9,10], Pd/SZr [11], Co/SZr [12,13], Mn/SZr [14,15], and In/SZr [16]. These catalysts showed high activity under dry conditions, while some of catalysts also showed good activity under wet conditions. However, the stability of zeolite and SZr under hydrothermal condition is very poor [17,18], and this inherent drawback inhibits *
Corresponding author. Tel.: +86 10 62782030; fax: +86 10 62785687. E-mail address: [email protected] (J. Li).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.04.035
the practical application of these catalysts. Tungstate zirconia (WZr) is more stable than zeolite and SZr support. The most important is that the WZr possesses strong acid sites which is indispensable to the high activity and selectivity in CH4–SCR [19]. So WZr is a good choice for the substitution of zeolite and SZr. However, up to now, only Pd supported on WZr was reported to be used as a catalyst for the CH4–SCR reaction [18,20]. In this work, several non-noble metal (Co, Mn, Ni, Ag, Sn and In) loaded WZr catalysts were investigated for CH4–SCR reaction. Among these catalysts, only In/WZr revealed high NO reduction activity. The activity of In/ WZr was even higher than that of Pd/WZr. So the attention was mainly focused on the catalytic performance of In/WZr catalyst in this paper. 2. Experimental 2.1. Catalyst preparation The tungstated zirconia (WZr) or the sulfated zirconia (SZr) support was prepared by impregnating zirconium oxyhydroxide (AR grade, Shanghai Chemical Reagent
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D. Yang et al. / Catalysis Communications 8 (2007) 2243–2247
Work, China) with an aqueous solution of ammonium paratungstate (AR grade, Tianjin Juneng Chemical Factory, China) or ammonium sulfate (AR grade, Beijing Chemical Reagent Work, China), and the W and S contents in the final supports were 10 wt%. The supports were then dried at 120 °C overnight and calcined at 600 °C for 5 h. The zirconia support was obtained by directly calcinating zirconium oxyhydroxide at 600 °C for 5 h. Indium loading was accomplished by impregnation method using an aqueous solution of indium nitrate (AR grade, Sinopharm Chemical Reagent Factory, China). After dried at 110 °C overnight, it was then calcined in air at 650 °C for 5 h. The resulting catalysts are hereafter referred to as InaWZr, InaSZr or InaZr; where In refers to Indium, a refers to the weight content of indium in the catalyst, while WZr, SZr and Zr suggest that the support used in these catalysts are WZr, SZr, and ZrO2, respectively. Co, Mn, Ni and Ag were introduced onto the WZr support by impregnation with the corresponding metal nitrate solution, while SnCl4 and PdCl2 were used as the source of Sn and Pd. 2.2. Catalytic activity measurement The activity measurements were carried out with a fixedbed quartz reactor (inner diameter 8 mm) using a 0.5 g catalyst of 60–80 mesh. The feed gas was a mixture contained 1000 ppm NO or NO2, 3000 ppm CH4, 10% O2, 0 or 100 ppm SO2, 0% or 10% H2O and N2 as the balance gas. The total flow rate of the feed gas was 100 mL/min, corresponding to a space velocity of about 12,000 h1. NO and NO2 concentration were analyzed with a chemiluminescence NO/NO2 analyzer (Thermal Environmental Instruments, model 42C), and CH4 concentration was detected by gas chromatograph (Shimadzu GC 17A). Catalyst activity was evaluated in terms of percent NOx and CH4 conversions, defined as: ðC in C out Þ 100; C in
radiation (1484.6 eV) and it was calibrated internally by carbon deposit C1s binding energy (BE) at 284.6 eV. 3. Results and discussion 3.1. Characterization of catalysts XRD patterns for ZrO2, In1Zr, WZr, and In1WZr catalysts are shown in Fig. 1. The XRD patterns of ZrO2 and In1Zr were that of monoclinic zirconia. While the XRD patterns of WZr and In1WZr showed mainly the tetragonal zirconia phase although small fraction of monoclinic phase was also detected for In1WZr catalyst. These results suggest that tungsten is helpful for the stabilization of tetragonal zirconia phase. In addition, none of these samples gave diffraction lines corresponding to In2O3. The XRD patterns of InWZr with indium contents various from 0.5% to 10% were also measured (not shown). They were all similar to that of the In1WZr catalyst, and no peaks of In2O3 were observed. These results indicate a high dispersion of In2O3 on the support surface. The BET surface area, pore volume and pore size of ZrO2, In1Zr, WZr, and In1WZr are summarized in Table 1. The surface area of WZr and In1WZr were much greater than that of ZrO2 and In1Zr. It was attributed to the stabilization effect of tungsten for the formation of tetragonal zirconia phase. The XPS results are given in Table 2 in terms of the binding energies of the In3d5/2 and W4f5/2 as well as of the atomic ratio of In/Zr and W/Zr in the sample surface. A binding energy of 444.7 eV for In3d5/2 was measured for the In1Zr catalyst, which was consistent with the literature values for the In2O3 bulk phase [21,22]. The binding energy of In3d5/2 for In1WZr was 0.5 eV higher than the value for the In2O3 bulk phase. Similar shift was detected over In/ ZSM-5 catalyst [22], and it was attributed to the formation of InO+ species. So similar InO+ species might be also formed in our In1WZr catalyst. The bind energy of W4f5/2 in WZr and In1WZr suggested that W was mainly in the state of 6+ [23]. The surface W/Zr atomic ratio of
where Cin and Cout are the NOx or CH4 concentration at the reactor inlet and outlet, respectively. The activity data were collected after the catalytic reactions were substantially reached to the steady-state conditions for half an hour at each temperature.
X-ray diffraction (XRD) patterns were determined using a Rigaku D/max-RB diffractometer. The analyses were performed with Cu Target (40 kV and 100 mA); a typical scan speed was 6°/min with a step of 0.002° in the range from 20° to 70°. BET surface area, pore size, and pore volume were measured by N2 adsorption–desorption method using a NOVA 3200e analyzer. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a PHI15300/ESCA system with Al Ka
Intensity (a.u.)
2.3. Catalyst characterization
In1WZr
WZr
In1Zr
ZrO 2
20
30
40 50 2 Theta (º)
60
70
Fig. 1. XRD patterns of ZrO2, In1Zr, WZr, and In1WZr catalyst.
D. Yang et al. / Catalysis Communications 8 (2007) 2243–2247 Table 1 Physical properties of ZrO2, In1Zr, WZr, and In1WZr
2245
50
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore diameter (nm)
ZrO2 In1Zr WZr In1WZr
27 24 78 76
0.13 0.12 0.25 0.19
5.3 8.3 1.7 1.1
40
NOx conversion (%)
Samples
Table 2 In3d5/2 and W4f5/2 binding energies, In/Zr and W/Zr atomic ratio of In1Zr, WZr, and In1WZr derived from XPS W4f5/2 (eV)
In/Zra
W/Zra
In1Zr WZr In1WZr
444.7 – 445.2
– 37.9 38.0
0.056 (0.011)b – 0.035 (0.012)b
– 0.071 (0.067)b 0.081 (0.067)b
a The atomic ratios were calculated from the area ratios of the XPS lines of In (3d), W (4f), and Zr (3d). Atomic sensitivity factors for these lines were those reported by Physical Electronics as empirically determined for the PHI15300/ESCA system. b The values in parenthesis are the expected ratios when elements are all homogeneously dispersed in the catalysts.
WZr was close to the expected values, suggesting that the surface and bulk morphology in WZr was symmetrical. For the In1WZr catalyst, the W/Zr ratio increased slightly beyond that of WZr, indicating the aggregation of tungsten on the catalyst surface which can be related to the high interaction of InO+ species with WZr support. The In/Zr ratio was found to be much higher than the expected values in both In1Zr and In1WZr, suggesting the aggregation of indium on the catalyst surfaces. The aggregation of In on the catalyst surface was more serious in In1Zr than In1WZr, perhaps attributed to the higher surface area of In1WZr than In1Zr as shown in Table 1. 3.2. CH4–SCR activity test The NOx conversion activity of WZr support and Co, Mn, Ni, Ag, Sn, In, Pd loaded WZr catalysts at 550 °C are compared in Fig. 2. NOx conversion over WZr support was lower than 10% at this temperature. The loading of Mn, Ni or Ag caused a pronounced decrease in the NOx conversion activity of WZr, and the loading of Co and Sn could slightly improve the activity of WZr. However, NOx conversion could be remarkably enhanced to 36% with loading indium on WZr, and the activity was even higher than that of the Pd/WZr catalyst. NOx and CH4 conversion activities over WZr, In1WZr, In1Zr, ZrO2, and In1SZr are shown in Fig. 3. The maximum NOx conversion over ZrO2 was 31% at 650 °C. Loading of indium on ZrO2 led to a pronounced decrease in the NOx conversion activity, but showed no influence on the activity for CH4 conversion. It suggested that In2O3 bulk phase over the In1Zr, as detected by XPS, was not active for either the reduction of NOx or the combustion of CH4. The WZr showed significantly
10
0 WZrC
oWZr MnWZrN iWZr AgWZrS nWZr InWZr PdWZr
Fig. 2. NOx conversion over various element loaded WZr catalyst at 550 °C. The content of Co, Mn, Ni, Ag, Sn and In were 2 wt%, and Pd was 0.1 wt%. Reaction conditions: 1000 ppm NO, 3000 ppm CH4, 10% O2, N2 as balance; GHSV = 12,000 h1.
a NOx conversion (%)
In3d5/2 (eV)
20
100 In1WZr WZr In1Zr ZrO2
80
In1SZr
60
40
20
0 250 300 350 400 450 500 550 600 650 700
Temperautre ( ºC)
b
100
CH4 conversion (%)
Samples
30
80 60 40 20 0 250 300 350 400 450 500 550 600 650 700
Temperautre ( ºC) Fig. 3. NOx and CH4 conversions over ZrO2, In1Zr, WZr, In1WZr, and In1SZr catalyst. (a) NOx conversion; (b) CH4 conversion. Reaction conditions: 1000 ppm NO, 3000 ppm CH4, 10% O2, N2 as balance; GHSV = 12,000 h1.
reduced activities for both NOx and CH4 conversion than that of the ZrO2. However, it can be readily be seen from Fig. 3 that the present of indium greatly enhanced the
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D. Yang et al. / Catalysis Communications 8 (2007) 2243–2247
NOx conversion of WZr. The highest NO conversion rate for In1WZr catalyst was 70% at 450 °C. The InO+ species, as discussed above, was attributed to the active site for NOx reduction and WZr might play the same role with ZSM-5, providing strong acid site for anchoring of InO+ species [24]. In addition, the entire CH4 conversion curve of In1WZr shifted toward much lower temperature than that of WZr, suggesting that the InO+ species was not only responsible for NOx reduction but also was an active site for CH4 oxidation. For comparative purpose, the activity of In1SZr for CH4–SCR was also tested and the result was shown in Fig. 3. Maximum NO conversion over the In1SZr was 46% and was appeared at 650 °C. Although the In1SZr showed higher NOx conversion activity than that of the In1WZr at temperature higher than 600 °C, the performance of the In1SZr at low temperature was more inferior compared to the In1WZr. This fact may be attributed to the weaker acid strength of the WZr compared to that of the SZr [23,25], since weaker acid strength can lead to a
a
100
weaker interaction between the InO+ species and the support. And thus the InO+ on the WZr would be more active than it was on the SZr. The effect of indium content on the NOx and CH4 conversion activity of the InWZr catalysts is shown in Fig. 4. The peak NOx conversions were all achieved at 450 °C and fell in the range of 57%–70% for the InWZr catalysts containing 0.5–10% indium, indicating a very little influence of indium loading on the activity of NOx reduction. Since the number of acidic sites on the WZr was limited, the ability of the WZr to anchor active InO+ sites was finite and In would aggregate to In2O3 at high loading of Indium. However, as shown above, In2O3 phase was not active for the conversion of both NOx and CH4. As a consequence, all of the InWZr catalysts showed similar NOx reduction activity. Comparing Fig. 4b with Fig. 3b, it is clear that CH4 conversion increased with increasing In loading up to a content of 1% and then it unchanged when In loading further increasing to 10%. This observation may suggest that the aggregation of In2O3 over WZr support began at the In loading of 1%. The effect of H2O and SO2 on the activity of the In1WZr catalyst was shown in Fig. 5. The presence of SO2 in the
In0.5WZr In1WZr In2WZr In5WZr In10WZr
60
40
20
a 100 SO 2
NOx conversion (%)
NO x conversion (%)
80
0 250 300 350 400 450 500 550 600 650 700
H2O+SO2
60 40 20 0 300
Temperature (ºC)
350
400
450
500
550
600
550
600
Temperaure(ºC)
100
b 80
CH4 conversion (%)
CH 4 conversion (%)
b
H2O
80
60 40 20 0 250 300 350 400 450 500 550 600 650 700
Temperature (ºC) Fig. 4. Effect of indium content on the NOx and CH4 conversion over InWZr catalysts. (a) NO conversion; (b) CH4 conversion. Reaction conditions: 1000 ppm NO, 3000 ppm CH4, 10% O2, N2 as balance; GHSV = 12,000 h1.
100 80 60 40 20 0 300
350
400
450
500
Temperaure(ºC) Fig. 5. Effect of H2O and SO2 on the NOx and CH4 conversion over In1WZr catalyst. (a) NO conversion; (b) CH4 conversion. Reaction conditions: 1000 ppm NO, 3000 ppm CH4, 10% O2, 100 ppm SO2, 10% H2O, N2 as balance; GHSV = 12,000 h1.
D. Yang et al. / Catalysis Communications 8 (2007) 2243–2247
reaction gas significantly inhibited the NOx reduction activity of the In1WZr. The maximum NOx conversion was also achieved at 450 °C but it was decreased from 70% to 33%. The inhibition effect of H2O on the catalytic activity of the In1WZr catalyst was more pronounced than that of SO2. The temperature window for reduction shifted by 50 °C to higher temperature when H2O was added to the reaction gas, while the maximum NOx conversion was decreased to 29%. H2O and SO2 could compete with reacþ tants for adsorbing on InO+ site and form InðOHÞ2 and In(SO3)+ species which were inactive for CH4–SCR [26]. The deactivation shown in NOx conversion reaction might be due to the formation of the two species. In the co-presence of H2O and SO2 in the reaction gas, the catalytic activity for NOx conversion was close to the case when only H2O was added. It means that H2O had a dominant influence on the catalytic activity. This fact might suggest that þ the formation of InðOHÞ2 species were easier than that + of In(SO3) species when H2O and SO2 co-present in the reaction gas. 4. Conclusions In/WZr catalyst exhibited high activities in SCR of NO with methane. Under a condition of 1000 ppm NO, 3000 ppm CH4, 10% O2, and GHSV of 12,000 h1, the maximum NOx conversion level of the In/WZr catalyst reached 70% and was achieved at 450 °C. The InO+ was attributed to be the active site for CH4–SCR, while In2O3 showed very little influence on the activity of the In/WZr catalyst. H2O and SO2 could significantly inhibit the NO reduction by adsorbing on InO+ and forming unactive + InðOHÞþ 2 and In(SO3) species. Further studies are in progress for enhancing the activity of the In/WZr catalysts by addition additives, such as Co, Mn, or Ce. Acknowledgements The work was financially supported by National Natural Science Fund of China (Grant No. 20677034 and
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