Partial oxidation of soot to the high selectivity of CO under fluidized bed reactor

中国环境学会  2011年 06月22日

   

  Abstract

  Soot oxidation over a series of noble metals supported on SiO2 under fluidized bed reactor was studied compared to fixed bed reactor. Soot oxidation under fluidized bed reactor can be lowered 30-50℃ compared to fixed bed reactor. The option of the condition of fluidization soot oxidation was discussed and Ru/SiO2 shows the high activity with the lowest T10 when the gas flow is 150ml/min. Among the noble metal, Ru/SiO2 exhibits a big advantage over other catalysts under fluidized bed reactor.

 

  1.       Introduction

The emission of soot from the diesel engines can cause serious environmental problem, which can adsorb many kinds of organic chemicals as well as be the support of carcinogen. Hence, it is of important to remove soot from exhaust. Among various technical approaches for the control of diesel engine particulate emission, the direct and much valid way to reduce soot is to place a diesel particulate filter (DPF) in the exhaust stream, which needs to clean periodically because of the accumulation by combustion to prevent pressure increase build-up.

Many catalysts for soot oxidation have been reported, such as metal oxides [1-10], precious metals [11-17] and combined system [18-20]. However, most of experiment about simulated soot oxidation is carried out under fixed bed reactor, and the oxidation of soot under fluidized bed reactor which is more close to practical exhaust condition and has excellent heat and mass transfer characteristics has not been reported so far. The contact between soot and catalyst is an important parameter for the activity of the catalyst [2]. In practical condition of exhaust, they can not be intimate (‘tight’) contact but is loose. The contact between soot and catalysts will be improved under fluidized bed reactor which can evade the disadvantage of loose contact.

Generally, soot oxidized to CO2 completely is better than CO which can cause second pollution. Since some noble metal catalysts show high activities in the reduction of NO with CO [21-25], in our opinion, partial oxidation of soot to CO maybe benefit for the removal of NO.

In this work, we investigate the soot oxidation under fluidized bed reactor compared to fixed bed reactor, discuss the effect of gas flow rate on the catalytic activity, and carry out an exploratory research about the activities of a series of noble metals supported by SiO2.

 

  2.       Experimental

  2.1. Catalyst preparation

Ru, Rh, Pd, Ag, Ir, Pt and Au were prepared by iso-volumetric impregnation. The precursors used were RuCl3·nH2ORhCl3·nH2O, PdCl2, AgNO3, H2IrCl6·6H2O, H2PtCl6·6H2O and HAuCl4·4H2O (Platinum Group Metals China). The support used was SiO2 (Qingdao Haiyang Chemical Co., Ltd, 0.125-0.425mm, 450600m2/g). The support SiO2 were impregnated with an aqueous solution of the precursors, and left for one day at room temperature. They were then dried in air at 110 for 12h followed by calcination in air at 500 for 3h. The catalysts were screened to get granules of 0.150-0.250mm (i.e. 60-100 mesh) for measuring the catalytic reaction.

  2.2. Activity measurement

Catalytic activity was measured with temperature programmed reaction (TPR) technique. The experiments were carried out in a quartz microreactor (9mm of i.d.) that can be operated in either fixed- or fluidized-bed mode. The perforated plate and filter were added in the middle region of the microreactor. The reactor temperature was monitored with a thermocouple and controlled by PID-regulation system (Bachy, CKW-2200). The temperature was raised at a rate of 4·min-1 from 80 to 750. The concentrations of CO and CO2 were measured with an on-line as chromatograph (Shimadzu, Japan, GC-2014C), equipped with a flame ionization detector (FID).

Printex-U (Degussa AG; 100 m2/g) was used as model soot similar to literature [1], here, the 40-60 mesh was used for the activity measurement. The catalyst granules (0.2g) and model soot (0.02g) were carefully mixed and held on the perforated plate in the mircroreactor. The reaction gas which contains O24.4vol% diluted in Ar as the balance gas, was fed through the catalyst bed at a rate of 150ml/min. In the research about effect of gas flow on catalytic activity, the components of reaction gas were unchanged, but the total gas flow rate was changed.

The catalytic activity was evaluated by temperature of soot oxidation and selectivity to CO throughout a TPR run. The temperature of soot oxidation was estimated by the value of T10, T50, and T90, which were defined as the temperatures by which 10%, 50% and 90% of soot, respectively, were oxidized during the TPR run. The selectivity to carbon monoxide (SCO, %) was calculated by the ratio of CO to the sum of CO and CO2 in a TPR run, that is .

 

  3.       Result and Discussion

  3.1    Comparison between fluidized- and fixed-bed reactor

  Table 1 Activities of SiO2 and 1 wt. % Ru/SiO2 under fluidized and fixed bed reactor

  Catalysts and reactor

T10/℃

T50/℃

T90/℃

SCO/%

SiO2

Fluidized bed reactor

496

614

651

64.88

Fixed bed reactor

531

618

665

65.60

Ru/SiO2

Fluidized bed reactor

427

484

507

11.13

Fixed bed reactor

480

517

574

2.95

  Catalyst weight=0.2g, soot weight=0.02g, reactant conditions: 4.4 vol % O2+Ar, flow rate: 150ml/min

Table 1 shows the evaluated parameters of SiO2 and Ru/SiO2 in a TPR run with the gas flow rate is 150ml/min. Compared to soot oxidation over SiO2 under fixed bed reactor, T10 dropped about 30℃ under fluidized bed reactor, but there is no important variation in T50, T90 and Sco. Soot oxidation over 1 wt. % Ru/SiO2 under fluidized bed reactor, the T10 and T90 dropped about 60℃ more than that over SiO2, T50 also dropped about 30℃ in addition to the large change of Sco. It can conclude that soot oxidation under different reactor can express large effect on the T10, but catalyst plays an important role in the T50, T90 and Sco. Soot oxidation under fluidized bed reactor can make the reaction temperature window move to low temperature and improve the selectivity of CO. 

  3.2     The effect of gas flow on the fluidized bed reactor

Fig.1 shows the T10 and Sco in the soot oxidation with different gas flow from 75 to 200ml/min over 1 wt. % Ru/SiO2. From Fig.1, we can see that T10 goes up when the gas flow increase and reaches the highest when the gas flow is 100ml/min, then it goes down and touches the bottom when 150ml/min, at last it goes up again. This may be attributed to the change in the condition of fluidized bed reactor gradually. At the low gas flow and high gas flow rate, the condition of fluidized bed reactor can not reach the most suitable state for the oxidation of soot in the temperature programmed reaction, but it can reach an appropriate contact for the soot and catalyst when 150ml/min that is why the catalytic activity is better than other gas flow.

Fig.2 shows the activities of different noble metals supported on SiO2 with 150ml/min. From the figure above, we can see that Ru/SiO2 exhibits the advantage in the T10 which is the lowest among the catalysts, and about 70℃ lower than SiO2. The activities order is RuIrPdothers. Table 2 gives us the activities of different noble metals under fluidized and fixed bed reactor. It can conclude that Au/SiO2 and Ag/SiO2 can promote the reaction  but can not promote  which is not same as Pt/SiO2, Pd/SiO2, Ir/SiO2, Ru/SiO2 and Rh/SiO2 underin fixed and fluidized bed reactor. Soot oxidation under fluidized bed reactor can advance the soot oxidation to CO compared to fixed bed reactor, temperature is lowered and selectivity of CO is improved except for Ag/SiO2. Among the catalysts, selectivity of CO over Ru/SiO2 is improved obviously only less than Rh/SiO2, but T10 is about 100℃ less than Rh/SiO2. Hence, Ru/SiO2 possesses the best activity in the soot oxidation under fluidized bed reactor.

  Table 2 Activities of different noble metals under fluidized and fixed bed reactor

  Catalysts

Fixed bed reactor

Fluidized bed reactor

T10/℃

SCO/%

T10/℃

SCO/%

Pt/SiO2

545

0.53

526

3.38

Pd/SiO2

487

5.53

476

7.18

Ir/SiO2

493

3.22

465

11.47

Ru/SiO2

480

2.95

427

11.13

Au/SiO2

508

33.65

512

46.16

Rh/SiO2

514

0.37

520

25.33

Ag/SiO2

518

51.74

506

45.30

SiO2

531

65.60

496

64.88

  Catalyst weight=0.2g, soot weight=0.02g, reactant conditions: 4.4 vol % O2+Ar, flow rate:150ml/min

  4.       Conclusion

  Temperature of soot oxidation can be lowered and selectivity of CO can be improved under fluidized bed reactor compared to fixed bed reactor. Gas flow plays an important role in the soot oxidation under fluidized bed reactor for the different condition of fluidization. In the range of 75-200ml/min, Ru/SiO2 exhibits highest activity at low temperature when the gas flow is 150ml/min. Soot oxidation need the lowest temperature over Ru/SiO2 compared to other catalysts. Partial oxidation of soot can be improved over Ru/ SiO2 evidently only less than Rh/SiO2, but its T10 is about 100℃ lower than Rh/SiO2. Hence, Ru/SiO2 shows big advantage over other noble catalysts in the soot oxidation.

  Acknowledgments

 This work was supported by the Double-Hundred Plan in Shenzhen.

  References

  1.       John P.A.Neeft, Michiel Makkee, Jacob A.Moulijn. Catalysts for the oxidation of soot from diesel exhaust gases. I An exploratory study. Applied Catalysis B: Environmental. 1996, 8:57~78.

  2.       John P.A.Neeft, Olaf P. van Pruissen, Michiel Makkee, Jacob A. Moulijn. Catalysts for the oxidation of soot from diesel exhaust gases. II. Contact between soot and catalyst under practical conditions. Applied Catalysis B: Environmental. 1997, 12:21~31.

  3.       Giovanni Neri, Lucio Bonaccorsi, Andrea Donato, Candida Milone, Maria Grazia Musolino, Anna Maria Visco. Catalytic combustion of diesel soot over metal oxide catalysts. Applied Catalysis B: Environmental. 1997, 11:217~231.

  4.       Shetian Liu, Akira Obuchi, Junko Uchisawa, Tetsuya Nanba, Satoshi Kushiyama. An exploratory study of diesel soot oxidation with NO2 and O2 on supported metal oxide catalysts. Applied Catalysis B: Environmental. 2002, 37:309~319.

  5.       Isabela C.L.Leocadio, Christianne V. Minana, Silvana Braun, Martin Schmal. Effect of experimental conditions on the parameters used for evaluating the performance of the catalyst Mo/Al2O3 in diesel soot combustion. Applied Catalysis B: Environmental. 2008, 84:843~849.

  6.       Eleonora Aneggi, Carla de Leitenburg, Giuliano Dolcetti, Alessandro Trovarelli. Diesel soot combustion activity of ceria promoted with alkali metals. Catalysis Today. 2008, 136:3-10.

  7.       Benjaram M. Reddy, Komateedi N. Rao. Copper promoted ceria-zirconia based bimetallic catalysts for low temperature soot oxidation. Catalysis Communications. 2009,10:1350-1353.

  8.       Jian Liu, Zhen Zhao, Jie Lan, Chunming Xu, Aijun Duan,Guiyua Jiang, Xinping wang, and Hong He. Catalytic combustion of soot over the highly active (La0.9K0.1CoO3)x/nmCeO2 catalysts. Journal of Physcial Chemistry. 2009, 113:17114-17123

  9.       Y. Teraoka, K. Nakano, S. Kagawa, W. F. Shangguan. Simultaneous removal of nitrogen oxides and diesel soot particulates catalyzed by perovskite-type oxides. Applied Catalysis B: Environmental. 1995, 5:L181~L185.

  10.   W. F. Shangguan, Y. Teraoka, S. Kagawa. Promotion effect of potassium on the catalytic property of CuFe2O4 for the simultaneous removal of NOx and diesel soot particulate. Applied Catalysis B: Environmental. 1998, 16:149~154.

  11.   Junko Oi Uchisawa, Akira Obuchi, Zhen Zhao, Satoshi Kushiyama. Carbon oxidation with platinum supported catalysts. Applied Catalysis B: Environmental. 1998, 18:L183~L187.

  12.   Junko Oi Uchisawa, Akira Obuchi, Atsushi Ogata, Ryuji Enomoto, Satoshi Kushiyama. Effect of feed gas composition on the rate of carbon oxidation with Pt/SiO2 and the oxidation mechanism. Applied Catalysis B: Environmental. 1999, 21:9~17.

  13.   Junko Oi-Uchisawa, Akira Obuchi, Shudong Wang, Tetsuya Nanba, Akihiko Ohi. Catalytic performance of Pt/MOx loaded over SiC-DPF for soot oxidation. Applied Catalysis B: Environmental. 2003, 43:117~129.

  14.   Junko Oi-Uchisawa, Shudong Wang, Tetsuya Nanba, Akihiko Ohi, Akira Obuchi. Improvement of Pt catalyst for the soot oxidation using mixed oxide as a support. Applied Catalysis B: Environmental. 2003, 44:207~215.

  15.   Mitsuru Hosoya, Masatoshi Shimoda. The application of diesel oxidation catalysts to heavy duty diesel engines in Japan. Applied Catalysis B: Environmental. 1996, 10:83~97.

  16.   Robert J. Farrauto, Kenneth E. Voss. Monolithic diesel oxidation catalysts. Applied Catalysis B: Environmental. 1996, 10:29~51.

  17.   H. J. Stein. Diesel oxidation catalysts for commercial vehicle engines: strategies on their application for controlling particulate emissions. Applied Catalysis B: Environmental. 1996, 10:69~82.

  18.   Shetian Liu, Akira Obuchi, Junko Oi-Uchisawa, Tetsuya Nanba, Satoshi Kushiyama. Synergistic catalysis of carbon black oxidation by Pt with MoO3 or V2O5. Applied Catalysis B: Environmental. 2001, 30:259~265.

  19.   S. J. Jelles, B.A.A.L. van Setten, M. Makkee, J.A. Moulijn. Molten salts as promising catalysts for oxidation of diesel soot: importance of experimental conditions in testing procedures. Applied Catalysis B: Environmental. 1999, 21:35~49.

  20.   I. Atribak, A. Bueno-Lopez, A. Garcia-Garcia, P. Navarro, D.Frias, M. Montes. Catalytic activity for soot combustion of birnessite and cryptomelane. Applied Catalysis B: Environmental. 2010, 93:267~273.

  21.   Masaru Ogura, Aya Kawamura, Masahiko Matsukata, and Eiichi Kikuchi. Catalytic activity of Ir for NO-CO reaction in the presence of SO2 and excess oxygen. Chemistry Letters. 2000, 29:146-147.

  22.   Aiqin Wang, Lei Ma, Yu Cong, Tao Zhang, Dongbai Liang. Unique properties of Ir/ZSM-5 catalyst for NO reduction with CO in the presence of excess oxygen. Applied Catalysis B: Environmental. 2003, 40:319~329.

  23.   Aiqin Wang, Dongbai Liang, Changhai Xu, Xiaoying Sun, Tao Zhang. Catalytic reduction of NO over in situ synthesized Ir/ZSM-5 monoliths. Applied Catalysis B: Environmental. 2001, 32:205~212.

  24.   Masaaki Haneda, Tomohiro Yoshinari, Kazuhito Sato, Yoshiaki Kintaichi and Hideaki Hamada. Ir/SiO2 as a highly active catalyst for the selective reduction of NO with CO in the presence of O2 and SO2. Chemcial Communications. 2003,22:2814-2815.

  25.   Masaaki Haneda, Pusparatu, Yoshiaki Kintaichi, Isao Nakamura, Motoi Sasaki, Tadahiro Fujitani, Hideaki Hamada. Promotional effect of SO2 on the activity of Ir/SiO2 for NO reduction with CO under oxygen-rich conditions. Journal of Catalysis. 2005, 229:197-205.

  


  
*
Corresponding author. Tel./fax: +86 755 26033472-602

 E-mail address: Ouyangfh@hit.edu.cn (F. Ouyang)

  

 
污染防治与管理更多>>
循环经济与绿色产业发展 更多>>
低碳经济与可持续发展更多>>
中国面临的主要环境问题及对策 更多>>