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Preparation of monolithic Pt–Pd bimetallic catalyst and its performance in catalytic combustion of benzene series
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ARTICLE IN PRESS Catalysis Today xxx (2013) xxx–xxx
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Preparation of monolithic Pt–Pd bimetallic catalyst and its performance in catalytic combustion of benzene series Lu Jiang, Na Yang, Jiqin Zhu ∗ , Chunyu Song State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 6 March 2013 Received in revised form 24 May 2013 Accepted 25 May 2013 Available online xxx Keywords: Pt–Pd bimetallic Cordierite Monolithic Catalytic combustion Benzene series
a b s t r a c t The monolithic Pt–Pd bimetallic catalysts supported on ␥-Al2 O3 using cordierite honeycomb ceramics as the first carrier are prepared by thermal adsorption method and characterized by XRD and SEM. The activities of the prepared catalysts are evaluated in a fixed bed reactor. It is determined that the suitable content of Pt–Pd is 0.1%, the molar ratio of additives Ce and Zr is 3:1 and their total content is 1%, the calcination temperature is lower than 500 ◦ C. The comparison between the prepared catalyst and the commercial catalyst are made. It is found the prepared catalyst shows higher activity and stability. © 2013 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Catalytic combustion is an effective method to destroy volatile organic compounds (VOC), which have some distinct advantages, such as cost saving, low reaction temperature, high removal rate and less formation of NOx [1,2]. The traditional pelleted catalyst results in high pressure drop in the reactor which is a main obstacle for VOC treatment system. Therefore, the structured catalysts based on honeycomb have attracting the interest of researchers for some significant advantages, such as low pressure drop, negligible scaling effect and good abrasion resistance [3–5]. The structured catalysts used for catalytic combustion are generally divided into three parts: the first carrier, the second washcoating and active components. Cordierite honeycomb ceramics is used as carrier. Its surface is coated with a cohesive, porous and stable washcoating. Then the additives and active metal component are adsorbed on the surface [6]. For catalytic combustion, the precious metals, such as Pt, Pd and Au, have excellent performance. Because the precious metals are expensive and scarce, the ideal catalyst should have high activity and stability with low content of precious metals. In this paper the monolithic Pt–Pd bimetallic catalysts supported on ␥-Al2 O3 using cordierite honeycomb ceramics as carrier are prepared and characterized. Their performance for catalytic combustion of toluene is investigated.
The cordierite honeycomb ceramics were pretreated with dilute nitric acid, and then the carriers were dried in the oven at 120 ◦ C for 2 h after cleaning by the ionized water. The transition sol solution was prepared using the mass ratio of deionized water:pseudo boehmite powder:polyethyleneglycol (4000) = 3:1:0.03. The pH of transition sol solution was adjusted from 3 to 4 by dilute nitric acid with magnetic stirring. The pretreated ceramics were immersed in the transition sol solution until saturated and then dried. The load of ␥-Al2 O3 washcoating amounted to 8–12% after repeating 3 or 4 times. Ce, Zr and active component were loaded using the hot adsorption method. At last the catalyst were dried and roasted. The activity of catalyst was tested in a fixed bed reactor and the bed temperature was automatically controlled by temperature controller with the fluctuation range of ±0.1 ◦ C. Volatile benzene series with different concentration were prepared by bubbling method. The gas concentration was determined using gas chromatography with the FID detector.
∗ Corresponding author. Tel.: +86 1064412054. E-mail address: [email protected] (Z. Jiqin).
3. Results and discussion 3.1. X-ray diffraction Fig. 1 shows the XRD spectra obtained for the catalysts based on the 0.1%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite and 0.2%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite. It can be seen that there are only the characteristic peaks of PdO and cordierite. The height and half-width of PdO diffraction
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peak for 0.1%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite is lower than those of 0.2%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite, which indicates active component of the former is highly dispersed. The diffraction peak of Pt is not found, which shows that it is also highly dispersed.
PdO
3.2. SEM images
b
a
15
20
25
30
35
40
45
50
2 (° ) Fig. 1. XRD patterns of 0.1%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite 0.2%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite (b).
(a)
and
SEM patterns of cordierite surface without and with washcoating have been presented in Fig. 2. As is illustrated in the SEM figures, the particles on the unloaded surface of the cordierite are rough and not suitable for the adsorption of the active metals. After washcoating pseudo boehmite, there is a gel-like layer on the surface, which can improve the cohesiveness and uniformity of surface. The results of SEM images of catalysts surface with different Pt–Pd contents have been presented in Fig. 3. As shown in the figures, for the catalyst with 0.1% Pt–Pd, the active component particles are uniform and small, which means it is high dispersed. While for the one with 0.5% Pt–Pd, the distribution of the active component tends to be uneven. What is more, the active adsorption bits are overlapped and the concentration of the impregnation solution becomes large. The SEM pictures of catalysts surface with different additives have been presented in Fig. 4. It can be seen that the different additives make a difference to the morphology of the surface. In the picture (A) where has 1%
Fig. 2. SEM images of cordierite surface without and with coating. (A) Cordierite without coating and (B) cordierite with coating.
Fig. 3. SEM images of 0.1%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite (A) and 0.5%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite (B).
Please cite this article in press as: J. Lu, et al., Preparation of monolithic Pt–Pd bimetallic catalyst and its performance in catalytic combustion of benzene series, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.05.026
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Fig. 4. SEM images of catalysts surface with different additives. (A) 0.1%Pt–Pd/1%Y/␥-Al2 O3 /cordierite and (B) 0.1%Pt–Pd/1%Y0.75 Zr0.25 /␥- Al2 O3 /cordierite.
yttrium oxide, there are many small pores on the surface, which can increase the surface area. What is more, the load area of the active component became larger, which makes it easier for adsorbing the active ingredients. In the picture (B), yttrium oxide and zirconia were added together to form solid solution according to the molar ratio Y:Zr = 3:1. As is shown in the picture, the addition of zirconia makes the pores bigger and deeper. 3.3. Acid treatment cordierite carrier The acid treatment of cordierite before coating can obtain a suitable surface area and many micro-pores are created [7–9]. The cordierite is placed into the acid solution with certain concentration and soaked for several hours at room temperature, then washed with deionized water and dried for 2 h at 120 ◦ C. At last, the weight loss rate can be measured and shown in Fig. 5. It can be seen from Fig. 5, with the acid treatment, the cordierite has a certain weight loss. At the beginning, the powders attached to the surface are dissolved in acid solution. Then Mg2+ and Al3+ ions in the cordierite dissolved out. Compared with oxalic acid, the nitric acid is mild and the strength and specific surface area of cordierite are feasible. 3.4. The precious metal loading For precious metal catalysts, the activity for catalytic oxidation reaction is Rh < Pd < Pt. While Pt and Pd present different activity on different reactants. For the oxidation of CO, CH4 and olefin, Pd
is superior of Pt [10,11]. For aromatic hydrocarbon, both of them are considerable [12–15]. The catalyst loaded with Pt–Pd bimetals has high catalytic activity and thermal stability and is the preferred catalyst for catalytic combustion of organics [16–18]. The main problem is as much as possible to reduce the content of Pt–Pd besides maintaining high catalytic activity. Under the condition of the toluene concentration of 5 g/m3 and the space velocity of 20,000 h−1 , the relationship of catalytic activity and the Pt–Pd loading are shown in Fig. 6. It can be seen from Fig. 6, the catalyst activity show increasing trend along with the increase of the precious metal content, especially when the content of precious metals increase from 0.1% to 0.2%, the improvement of activity is more obvious. The conversion temperature is reduced by 40 ◦ C when toluene conversion rate is 99%. While the precious metal content increases further, the activity of catalyst increases slowly. The reason may be that the concentration of precious metals became lager, the adsorption active sites overlapped, which reduce the active site in turn. Therefore, the proper precious metals content determined by this experiment is the 0.1%. 3.5. Ce and Zr loading In the preparation Pt–Pd catalyst, a variety of additive components can be used .The catalytic combustion can be influenced by the type of additives and the amount of loading. Ce and Zr are used as additives in common. To further determine the best load amount of Ce and Zr, a series of catalysts are prepared with the content of 100
40 nitric acid oxalic acid
80
Conversion/%
Mass loss/%
30
20
10
60 0.07% 0.1% 0.2% 0.3% 0.5% 0.4%
40 20
0
0
2
4
6
8
Time/h Fig. 5. The relationship between mass loss of carrier with the time of acid soaking.
160 170 180 190 200 210 220 230 240 250 o
Temperature/ C Fig. 6. Effect of Pt–Pd loading on catalytic activity.
Please cite this article in press as: J. Lu, et al., Preparation of monolithic Pt–Pd bimetallic catalyst and its performance in catalytic combustion of benzene series, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.05.026
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100
100 1.0% 1.5% 2.0% 2.5%
60
80
Conversion/%
Conversion/%
80
40 20
60 40
benzene toluene styrene
20
0 160 180 200 220 240 260 280 300 320 340
0 140
160
180
200
220
240
o
o
Temperature/ C
Temperature/ C Fig. 7. Effect of the Ce–Zr contents on the performance of catalysts.
Ce and Zr of 1.0%, 1.5%, 2.0% and 2.5% respectively (Ce:Zr = 3:1). Under the condition of the toluene concentration of 5 g/m3 and the space velocity of 20,000 h−1 , the catalytic activities have been investigated. The evaluation results are shown in Fig. 7. It can be seen from the results, when the content of Ce and Zr are 1.0%, the toluene conversion rate is the highest under the same temperature. 3.6. The calcination temperature The calcination temperature is another key factor that influences the catalytic activity. 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥Al2 O3 /cordierite catalysts are prepared with different calcination temperatures. Then the activities are evaluated with mass concentration of toluene of 5 g/m3 and space velocity of 20,000 h−1 . The results are shown in Fig. 8. It can be seen that with the increasing of calcination temperature, the combustion activity to toluene descends slightly. When the calcination temperature is lower than 500 ◦ C, the transformation of ␥-Al2 O3 to ␣-Al2 O3 does not take place and does not cause the change of particle size for the precious metals. 3.7. Catalytic combustion performance for aromatic hydrocarbons With 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥-Al2 O3 /cordierite catalyst, at space velocity of 20,000 h−1 , with the concentration of toluene, benzene, styrene was 5 g/m3 , 6 g/m3 and 5 g/m3 respectively. The catalytic combustion experimental results are shown in Fig. 9.
Fig. 9. Catalytic activity for different aromatic hydrocarbons.
The prepared catalysts have high activity for aromatic hydrocarbons. The conversion at the same temperature is styrene > toluene > benzene. The temperature when the conversion rate is above 95% for benzene, toluene and styrene are 220 ◦ C, 230 ◦ C and 210 ◦ C respectively. Using 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥-Al2 O3 /cordierite catalyst, at temperatures of 240 ◦ C and space velocity of 20,000 h−1 , the mass concentrations of toluene varied within the range of 1–10 g/m3 and the conversion are shown in Fig. 10. The conversion of toluene maintains >98% when the concentration is less than 5 g/m3 and fells slightly when the concentrations is >6 g/m3 . Even the concentration achieves 10 g/m3 , the conversion can reach 95%. With 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥-Al2 O3 /cordierite as the catalyst, at the temperature of 240 ◦ C and the concentration of toluene of 5 g/m3 , the space velocity varies from 5000 h−1 to 50,000 h−1 . The change of conversion is shown in Fig. 11. As can be seen in Fig. 11, when the space velocity is less than 20,000 h−1 , it has little effect on the conversion of toluene. When it is more than 20,000 h−1 , with the increasing of space velocity, the conversion of toluene is decrease. At the space velocity of 30,000 h−1 , the conversion can get 90%. Using 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥-Al2 O3 /cordierite catalyst, at the experimental condition in which the space velocity is 20,000 h−1 and the mass concentration of toluene is 6 g/m3 , the catalytic combustion experiments last for 1000 h. And the activity variations are shown in Fig. 12. It can be seen that the conversion of toluene remains at about 98% during 1000 h. Catalytic performance is very stable. It can be indicated that the prepared catalyst has excellent catalytic combustion stability for toluene.
100
100 80 60
Conversion/%
Conversion/%
80
40 o
450 C o 500 C o 700 C
20 0
180
200
220
240
60 40 20
260
o
Temperatue/ C Fig. 8. Toluene conversion over 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥-Al2 O3 /cordierite catalysts with different calcination temperature .
0
0
2
4
6
8
10
Mass concertration g/m3 Fig. 10. Influence of concentration of toluene on catalyst performance.
Please cite this article in press as: J. Lu, et al., Preparation of monolithic Pt–Pd bimetallic catalyst and its performance in catalytic combustion of benzene series, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.05.026
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With the space velocity of 20,000 h−1 and the nonane content of 4 g/m3 , the prepared 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥-Al2 O3 /cordierite catalyst and a commercial catalyst are compared on the catalytic activity. The active components for both catalysts are 0.1%Pt–Pd. The results are shown in Fig. 13. As Fig. 13 shows, the light-off temperatures of commercial catalyst and the prepared catalyst for nonane are 160 ◦ C and 155 ◦ C respectively. The complete conversion temperature are 295 ◦ C and 280 ◦ C separately. As it can be seen from the curve, the rising rate of conversion for the prepared catalyst is faster than that of commercial catalyst.
100 90
Conversion/%
80 70 60 50 40 30 20
0
10000
20000
30000
40000
Space velocity/h
50000
-1
Fig. 11. Influence of space velocity on the conversion of toluene.
100 90 80
Conversion/%
5
70 60 50 40
4. Conclusions The Pt–Pd bimetallic monolithic catalyst supported on ␥-Al2 O3 using cordierite honeycomb ceramics as carrier is prepared by thermal adsorption method. The structure of the catalyst was characterized and the activity of the catalyst was evaluated. From the experiments it is determined that the suitable content of Pt–Pd is 0.1%, the molar ratio of additives Ce and Zr is 3:1 and their total content is 1%, the calcination temperature is lower than 500 ◦ C. The prepared catalyst has good catalytic combustion performance for benzene and other aromatic hydrocarbons. Acknowledgement
30
Project 21176010 supported by National Natural Science Foundation of China.
20 10 0
0
200
400
600
800
1000
References
Time/h Fig. 12. Stability test for the prepared catalyst.
100 Prepared catalyst Commercial catalyst
Conversion/%
80 60 40 20 0
160
180
200
220
240
260
280
300
o
Temperature/ C Fig. 13. The catalyst activity of nonane for the prepared catalyst and commercial catalyst.
[1] W. Li, H. Gong, Journal of Physical Chemistry 26 (2010) 885–894. [2] A. Pérez-Cadenas, F. Kapteijn, J. Moulijn, Catalysis Today 44 (2006) 2463–2468. [3] B. Barbero, L. Costa-Almeida, O. Sanz, Chemical Engineering Journal 139 (2008) 430–435. [4] K. Barbara, W. Tylus, Catalysis Today 137 (2008) 324–328. [5] A. Machocki, M. Rotko, B. Stasinska, Catalysis Today 137 (2008) 312–317. [6] K. Yang, J. Choi, J. Chung, Catalysis Today 97 (2004) 159–165. [7] W. Wang, X. Liu, H. Zhang, Applied Science and Technology 34 (2007) 69–72. [8] B. Hai, L. Guo, Chinese Ceramic Industries 12 (2005) 1–4. [9] J. Hua, Q. Zheng, M. Wei, Molecular Catalysis 20 (2006) 550–555. [10] C. Ma, X. Li, M. Jin, Journal of Catalysis 28 (2007) 535–540. [11] K. Persson, K. Jansson, S. Jaras, Journal of Catalysis 28 (2007) 401–414. [12] K. Kim, S. Boo, H. Ahn, Journal of Industrial and Engineering Chemistry 15 (2009) 92–97. [13] K. Kim, H. Ahn, Applied Catalysis B: Environmental 91 (2009) 308–318. [14] D. Patricia, I. Virginia, M. Irene, Applied Catalysis A: General 392 (2011) 208–217. [15] S. Scirè, S. Minicò, C. Crisafulli, Applied Catalysis B: Environmental 40 (2003) 43–49. [16] M. Paulis, H. Peyrard, M. Montes, Journal of Catalysis 199 (2001) 30–40. [17] Z. Liu, J. Wang, J. Zhong, Journal of Hazardous materials 149 (2007) 742–746. [18] K. Persson, A. Ersson, S. Colussi, Applied Catalysis B: Environmental 66 (2006) 175–185.
Please cite this article in press as: J. Lu, et al., Preparation of monolithic Pt–Pd bimetallic catalyst and its performance in catalytic combustion of benzene series, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.05.026
ARTICLE IN PRESS Catalysis Today xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Preparation of monolithic Pt–Pd bimetallic catalyst and its performance in catalytic combustion of benzene series Lu Jiang, Na Yang, Jiqin Zhu ∗ , Chunyu Song State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 6 March 2013 Received in revised form 24 May 2013 Accepted 25 May 2013 Available online xxx Keywords: Pt–Pd bimetallic Cordierite Monolithic Catalytic combustion Benzene series
a b s t r a c t The monolithic Pt–Pd bimetallic catalysts supported on ␥-Al2 O3 using cordierite honeycomb ceramics as the first carrier are prepared by thermal adsorption method and characterized by XRD and SEM. The activities of the prepared catalysts are evaluated in a fixed bed reactor. It is determined that the suitable content of Pt–Pd is 0.1%, the molar ratio of additives Ce and Zr is 3:1 and their total content is 1%, the calcination temperature is lower than 500 ◦ C. The comparison between the prepared catalyst and the commercial catalyst are made. It is found the prepared catalyst shows higher activity and stability. © 2013 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Catalytic combustion is an effective method to destroy volatile organic compounds (VOC), which have some distinct advantages, such as cost saving, low reaction temperature, high removal rate and less formation of NOx [1,2]. The traditional pelleted catalyst results in high pressure drop in the reactor which is a main obstacle for VOC treatment system. Therefore, the structured catalysts based on honeycomb have attracting the interest of researchers for some significant advantages, such as low pressure drop, negligible scaling effect and good abrasion resistance [3–5]. The structured catalysts used for catalytic combustion are generally divided into three parts: the first carrier, the second washcoating and active components. Cordierite honeycomb ceramics is used as carrier. Its surface is coated with a cohesive, porous and stable washcoating. Then the additives and active metal component are adsorbed on the surface [6]. For catalytic combustion, the precious metals, such as Pt, Pd and Au, have excellent performance. Because the precious metals are expensive and scarce, the ideal catalyst should have high activity and stability with low content of precious metals. In this paper the monolithic Pt–Pd bimetallic catalysts supported on ␥-Al2 O3 using cordierite honeycomb ceramics as carrier are prepared and characterized. Their performance for catalytic combustion of toluene is investigated.
The cordierite honeycomb ceramics were pretreated with dilute nitric acid, and then the carriers were dried in the oven at 120 ◦ C for 2 h after cleaning by the ionized water. The transition sol solution was prepared using the mass ratio of deionized water:pseudo boehmite powder:polyethyleneglycol (4000) = 3:1:0.03. The pH of transition sol solution was adjusted from 3 to 4 by dilute nitric acid with magnetic stirring. The pretreated ceramics were immersed in the transition sol solution until saturated and then dried. The load of ␥-Al2 O3 washcoating amounted to 8–12% after repeating 3 or 4 times. Ce, Zr and active component were loaded using the hot adsorption method. At last the catalyst were dried and roasted. The activity of catalyst was tested in a fixed bed reactor and the bed temperature was automatically controlled by temperature controller with the fluctuation range of ±0.1 ◦ C. Volatile benzene series with different concentration were prepared by bubbling method. The gas concentration was determined using gas chromatography with the FID detector.
∗ Corresponding author. Tel.: +86 1064412054. E-mail address: [email protected] (Z. Jiqin).
3. Results and discussion 3.1. X-ray diffraction Fig. 1 shows the XRD spectra obtained for the catalysts based on the 0.1%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite and 0.2%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite. It can be seen that there are only the characteristic peaks of PdO and cordierite. The height and half-width of PdO diffraction
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Please cite this article in press as: J. Lu, et al., Preparation of monolithic Pt–Pd bimetallic catalyst and its performance in catalytic combustion of benzene series, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.05.026
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peak for 0.1%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite is lower than those of 0.2%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite, which indicates active component of the former is highly dispersed. The diffraction peak of Pt is not found, which shows that it is also highly dispersed.
PdO
3.2. SEM images
b
a
15
20
25
30
35
40
45
50
2 (° ) Fig. 1. XRD patterns of 0.1%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite 0.2%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite (b).
(a)
and
SEM patterns of cordierite surface without and with washcoating have been presented in Fig. 2. As is illustrated in the SEM figures, the particles on the unloaded surface of the cordierite are rough and not suitable for the adsorption of the active metals. After washcoating pseudo boehmite, there is a gel-like layer on the surface, which can improve the cohesiveness and uniformity of surface. The results of SEM images of catalysts surface with different Pt–Pd contents have been presented in Fig. 3. As shown in the figures, for the catalyst with 0.1% Pt–Pd, the active component particles are uniform and small, which means it is high dispersed. While for the one with 0.5% Pt–Pd, the distribution of the active component tends to be uneven. What is more, the active adsorption bits are overlapped and the concentration of the impregnation solution becomes large. The SEM pictures of catalysts surface with different additives have been presented in Fig. 4. It can be seen that the different additives make a difference to the morphology of the surface. In the picture (A) where has 1%
Fig. 2. SEM images of cordierite surface without and with coating. (A) Cordierite without coating and (B) cordierite with coating.
Fig. 3. SEM images of 0.1%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite (A) and 0.5%Pt–Pd/1%Ce–Zr/␥-Al2 O3 /cordierite (B).
Please cite this article in press as: J. Lu, et al., Preparation of monolithic Pt–Pd bimetallic catalyst and its performance in catalytic combustion of benzene series, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.05.026
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Fig. 4. SEM images of catalysts surface with different additives. (A) 0.1%Pt–Pd/1%Y/␥-Al2 O3 /cordierite and (B) 0.1%Pt–Pd/1%Y0.75 Zr0.25 /␥- Al2 O3 /cordierite.
yttrium oxide, there are many small pores on the surface, which can increase the surface area. What is more, the load area of the active component became larger, which makes it easier for adsorbing the active ingredients. In the picture (B), yttrium oxide and zirconia were added together to form solid solution according to the molar ratio Y:Zr = 3:1. As is shown in the picture, the addition of zirconia makes the pores bigger and deeper. 3.3. Acid treatment cordierite carrier The acid treatment of cordierite before coating can obtain a suitable surface area and many micro-pores are created [7–9]. The cordierite is placed into the acid solution with certain concentration and soaked for several hours at room temperature, then washed with deionized water and dried for 2 h at 120 ◦ C. At last, the weight loss rate can be measured and shown in Fig. 5. It can be seen from Fig. 5, with the acid treatment, the cordierite has a certain weight loss. At the beginning, the powders attached to the surface are dissolved in acid solution. Then Mg2+ and Al3+ ions in the cordierite dissolved out. Compared with oxalic acid, the nitric acid is mild and the strength and specific surface area of cordierite are feasible. 3.4. The precious metal loading For precious metal catalysts, the activity for catalytic oxidation reaction is Rh < Pd < Pt. While Pt and Pd present different activity on different reactants. For the oxidation of CO, CH4 and olefin, Pd
is superior of Pt [10,11]. For aromatic hydrocarbon, both of them are considerable [12–15]. The catalyst loaded with Pt–Pd bimetals has high catalytic activity and thermal stability and is the preferred catalyst for catalytic combustion of organics [16–18]. The main problem is as much as possible to reduce the content of Pt–Pd besides maintaining high catalytic activity. Under the condition of the toluene concentration of 5 g/m3 and the space velocity of 20,000 h−1 , the relationship of catalytic activity and the Pt–Pd loading are shown in Fig. 6. It can be seen from Fig. 6, the catalyst activity show increasing trend along with the increase of the precious metal content, especially when the content of precious metals increase from 0.1% to 0.2%, the improvement of activity is more obvious. The conversion temperature is reduced by 40 ◦ C when toluene conversion rate is 99%. While the precious metal content increases further, the activity of catalyst increases slowly. The reason may be that the concentration of precious metals became lager, the adsorption active sites overlapped, which reduce the active site in turn. Therefore, the proper precious metals content determined by this experiment is the 0.1%. 3.5. Ce and Zr loading In the preparation Pt–Pd catalyst, a variety of additive components can be used .The catalytic combustion can be influenced by the type of additives and the amount of loading. Ce and Zr are used as additives in common. To further determine the best load amount of Ce and Zr, a series of catalysts are prepared with the content of 100
40 nitric acid oxalic acid
80
Conversion/%
Mass loss/%
30
20
10
60 0.07% 0.1% 0.2% 0.3% 0.5% 0.4%
40 20
0
0
2
4
6
8
Time/h Fig. 5. The relationship between mass loss of carrier with the time of acid soaking.
160 170 180 190 200 210 220 230 240 250 o
Temperature/ C Fig. 6. Effect of Pt–Pd loading on catalytic activity.
Please cite this article in press as: J. Lu, et al., Preparation of monolithic Pt–Pd bimetallic catalyst and its performance in catalytic combustion of benzene series, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.05.026
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benzene toluene styrene
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Ce and Zr of 1.0%, 1.5%, 2.0% and 2.5% respectively (Ce:Zr = 3:1). Under the condition of the toluene concentration of 5 g/m3 and the space velocity of 20,000 h−1 , the catalytic activities have been investigated. The evaluation results are shown in Fig. 7. It can be seen from the results, when the content of Ce and Zr are 1.0%, the toluene conversion rate is the highest under the same temperature. 3.6. The calcination temperature The calcination temperature is another key factor that influences the catalytic activity. 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥Al2 O3 /cordierite catalysts are prepared with different calcination temperatures. Then the activities are evaluated with mass concentration of toluene of 5 g/m3 and space velocity of 20,000 h−1 . The results are shown in Fig. 8. It can be seen that with the increasing of calcination temperature, the combustion activity to toluene descends slightly. When the calcination temperature is lower than 500 ◦ C, the transformation of ␥-Al2 O3 to ␣-Al2 O3 does not take place and does not cause the change of particle size for the precious metals. 3.7. Catalytic combustion performance for aromatic hydrocarbons With 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥-Al2 O3 /cordierite catalyst, at space velocity of 20,000 h−1 , with the concentration of toluene, benzene, styrene was 5 g/m3 , 6 g/m3 and 5 g/m3 respectively. The catalytic combustion experimental results are shown in Fig. 9.
Fig. 9. Catalytic activity for different aromatic hydrocarbons.
The prepared catalysts have high activity for aromatic hydrocarbons. The conversion at the same temperature is styrene > toluene > benzene. The temperature when the conversion rate is above 95% for benzene, toluene and styrene are 220 ◦ C, 230 ◦ C and 210 ◦ C respectively. Using 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥-Al2 O3 /cordierite catalyst, at temperatures of 240 ◦ C and space velocity of 20,000 h−1 , the mass concentrations of toluene varied within the range of 1–10 g/m3 and the conversion are shown in Fig. 10. The conversion of toluene maintains >98% when the concentration is less than 5 g/m3 and fells slightly when the concentrations is >6 g/m3 . Even the concentration achieves 10 g/m3 , the conversion can reach 95%. With 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥-Al2 O3 /cordierite as the catalyst, at the temperature of 240 ◦ C and the concentration of toluene of 5 g/m3 , the space velocity varies from 5000 h−1 to 50,000 h−1 . The change of conversion is shown in Fig. 11. As can be seen in Fig. 11, when the space velocity is less than 20,000 h−1 , it has little effect on the conversion of toluene. When it is more than 20,000 h−1 , with the increasing of space velocity, the conversion of toluene is decrease. At the space velocity of 30,000 h−1 , the conversion can get 90%. Using 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥-Al2 O3 /cordierite catalyst, at the experimental condition in which the space velocity is 20,000 h−1 and the mass concentration of toluene is 6 g/m3 , the catalytic combustion experiments last for 1000 h. And the activity variations are shown in Fig. 12. It can be seen that the conversion of toluene remains at about 98% during 1000 h. Catalytic performance is very stable. It can be indicated that the prepared catalyst has excellent catalytic combustion stability for toluene.
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Mass concertration g/m3 Fig. 10. Influence of concentration of toluene on catalyst performance.
Please cite this article in press as: J. Lu, et al., Preparation of monolithic Pt–Pd bimetallic catalyst and its performance in catalytic combustion of benzene series, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.05.026
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With the space velocity of 20,000 h−1 and the nonane content of 4 g/m3 , the prepared 0.1%Pt–Pd/1%Ce0.75 Zr0.25 /␥-Al2 O3 /cordierite catalyst and a commercial catalyst are compared on the catalytic activity. The active components for both catalysts are 0.1%Pt–Pd. The results are shown in Fig. 13. As Fig. 13 shows, the light-off temperatures of commercial catalyst and the prepared catalyst for nonane are 160 ◦ C and 155 ◦ C respectively. The complete conversion temperature are 295 ◦ C and 280 ◦ C separately. As it can be seen from the curve, the rising rate of conversion for the prepared catalyst is faster than that of commercial catalyst.
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4. Conclusions The Pt–Pd bimetallic monolithic catalyst supported on ␥-Al2 O3 using cordierite honeycomb ceramics as carrier is prepared by thermal adsorption method. The structure of the catalyst was characterized and the activity of the catalyst was evaluated. From the experiments it is determined that the suitable content of Pt–Pd is 0.1%, the molar ratio of additives Ce and Zr is 3:1 and their total content is 1%, the calcination temperature is lower than 500 ◦ C. The prepared catalyst has good catalytic combustion performance for benzene and other aromatic hydrocarbons. Acknowledgement
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Project 21176010 supported by National Natural Science Foundation of China.
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References
Time/h Fig. 12. Stability test for the prepared catalyst.
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Please cite this article in press as: J. Lu, et al., Preparation of monolithic Pt–Pd bimetallic catalyst and its performance in catalytic combustion of benzene series, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.05.026