Catalytic combustion of diesel soot over K2NiF4-type oxides La2-xKxCuO4

Catalytic combustion of diesel soot over K2NiF4-type oxides La2-xKxCuO4

JOURNAL OF RARE EARTHS, Vol. 26, No. 2, Apr. 2008, p. 254 Catalytic combustion of diesel soot over K2NiF4-type oxides La2-xKxCuO4 ZHU Ling (朱 玲)1, WA...

1MB Sizes 1 Downloads 17 Views

JOURNAL OF RARE EARTHS, Vol. 26, No. 2, Apr. 2008, p. 254

Catalytic combustion of diesel soot over K2NiF4-type oxides La2-xKxCuO4 ZHU Ling (朱 玲)1, WANG Xuezhong (王学中)2, LIANG Cunzhen (梁存珍)1 (1. Department of Environmental Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China; 2. Chinese Research Academy of Environmental Sciences, Beijing 100012, China) Received 20 August 2007; revised 10 December 2007

Abstract: Nanostructure K2NiF4 type oxides La2-xKxCuO4 complex oxides were prepared using the Sol-Gel method, characterized by X-Ray Diffraction (XRD), Fourier Transform Infrared (FT-IR), and Scanning Electron Microscopy (SEM). The catalytic activity for soot combustion was evaluated by the Temperature-Programmed Reaction (TPO) technique. The results demonstrated that the substitution quality of K+ for La3+ at the A-site would increase the catalytic activities of La2-xKxCuO4 for soot combustion greatly; the substitution quality affected the structure and catalytic activity obviously. The La1.8K0.2CuO4 complex oxides with tetrahedral structures had the best catalytic activity for soot combustion, and the ignition temperature of soot combustion was lowered from 490 to 320 °C. Keywords: K2NiF4-type complex oxides; soot; catalytic combustion; rare earths

The emissions of diesel engines are known to be hazardous pollutants for human health. One of the most dangerous components of diesel exhausts is particulate, which consists of agglomerates of small carbon particles with a number of different hydrocarbons and sulfates adsorbed on their surfaces[1-4]. A possible way to reduce particulate emission lies in filtering it with trap, and continuously burning it out, depending on the presence of a catalyst that promotes particulate combustion at a rather low temperature of exhaust emission (below 400 ˚C)[5]. Several authors have reported that perovskite and spinel oxides are active for soot removal. The substitution of alkali metal into the A-sites of perovskite and spinel oxides has been demonstrated to be quite effective for enhancing the activity[6-9]. However, as the other type of perovskite oxides, the K2NiF4-type complex oxides, have received only little attention, the correlative study about the K2NiF4-type complex oxides for soot combustion are relatively few[10]. In this study, the nanometric La2-xKxCuO4 perovskite-like oxides were prepared using the Sol-Gel method. Catalyst structures were characterized by XRD, FT-IR, and SEM techniques. The effluence of the substitution amount on the structure and the catalytic performances for soot combustion were also investigated.

1 Experimental

1.1 Catalyst preparation A series of La2-xKxCuO4 (x=0, 0.05, 0.1, 0.2, 0.25, 0.3, 0.4, 0.6, 0.8) K2NiF4-type complex oxides were prepared with the help of the Sol-Gel method. The corresponding metal nitrates were used as starting materials, for obtaining an aqueous solution of La3+, K+, and Cu2+ with appropriate stoichiometry, and a citric acid solution was chosen as a ligand. The solution was evaporated to get the precursor and finally the precursor was calcined at 800 ˚C for 4 h in static air. 1.2 Catalyst characterization The special surfaces of the samples were analyzed by N2 adsorption/desorption at liquid nitrogen temperature, using a Quantachrome NOVA-1200 gas absorption analyzer. The specific surface area was calculated with the BET equation. The X-ray diffraction (XRD) patterns of the samples were measured on a powder X-ray diffractometer (PANalytical XRD X’pert pro. MPD) using Cu Ka radiation (λ=0.1542 nm) in the 2θ range of 10˚–70˚ at a scanning rate of 4˚ per min. The tube voltage and current were set at 40 kV and 30 mA, respectively. The patterns were compared with JCPDS reference data for phase identification. FT-IR absorbance spectra were recorded on a Bruker Tensor 27 spectrometer in the wave number ranging between 2000 and 400 cm–1 after 128 scans at a resolution of

Foundation item: Project supported by Beijing Municipal Education Committee Program (KM200710017006) Corresponding author: ZHU Ling (E-mail: [email protected]; Tel.: +86-10-81294339)

ZHU L et al., Catalytic combustion of diesel soot over K2NiF4-type oxides La2-xKxCuO4

4 cm-1. The morphology of the catalysts was observed by SEM (S-4300N, Japan).

255

greater than 0.6, there is a peak corresponding to KNO3, with cubic structure (01-081-0071), in the XRD spectra. The average crystal particle sizes (D) of the La2-xKxCuO4 samples are listed in Table 1. It is seen that the range of the samples is 40 to 60 nm, which reveals that the La2-xKxCuO4 catalysts have manometer sizes. The specific surface areas of the La2-xKxCuO4 samples are displayed in Table 1. The BET surface area is between 1 and 6 m2/g.

1.3 Catalytic activity measurement The model soot used was the Printex-U supplied by Degussa. Soot was carefully mixed with the La2-xKxCuO4 catalyst, in the weight ratio of 1/10 for a “loose” contact between the catalyst and the soot. The activities were evaluated in TPO tests. The TPO test was carried out on a steady-bed quartz reactor with a 110 mg test sample, in the temperature range of 30–700 ˚C, (at a heating rate of 10 ˚C/min), and in the 10%O2/N2 gas (flow rate of 500 ml/min). A nondispersive IR gas analyzer (TY-9800A) was used to monitor the concentration of CO2 continuously. The activity of the catalyst in TPO evaluation is represented by the ignition temperature (Ti) and the peak temperature (Tp). Ti and Tp are defined as the temperatures where the concentration of CO2 exceeds 20 μg/g at 5˚C interval and where the maximum CO2 is emitted.

2.2 FT-IR results FT-IR spectra of the La2-xKxCuO4 samples are presented in Fig.2. All the samples have a strong absorption band at 512 cm–1, which is the characteristic vibration of A-O(II)-B in the K2NiF4 type complex oxides to Ref.[12]. That is to say, all the samples are in perovkite-like structure. All the samples except x=0.2 have a stretching vibration absorption band at 685 cm−1, which belongs to the stretching vibration of B-O(I) in BO6. The strong absorption band (680 cm−1) of La2CuO4 indicates that it has an orthorhombic structure. When the K-substitution amount is 0.2, the disappearance of

2 Results and discussion 2.1 XRD results The XRD results of the La2-xKxCuO4 oxides are shown in Fig.1 and Table 1. All of them are in a single perovskite-like (A2BO4) type with orthorhombic or tetragonal structures. The structure changes from an orthorhombic to a tetrahedral structure[11], when La3+ is substituted by K+. All the samples possess A2BO4 perovskite-like type structures because the XRD patterns of all the samples give a large peak at 31.1º, which corresponds to the A2BO4 perovskite-like type structures (JCPDF: 880940). When the K-substitution amount is

Fig.1 XRD spectra for La2-xKxCuO4 samples

Table 1 Soot combustion activity over La-K-Cu-O series catalysts in TPO *

Samples

Structure

Ti/°C

Tp/°C

ΔT/°C

SBET/(m2/g)

L(311)/nm

Ea /(kJ/mol)

No catalyst

-

490

595

105

-

-

159.43

La2CuO4

La2CuO4 (O)

380

453

73

5.90

40.21

132.31

La1.95K0.05CuO4

La2CuO4(T)

350

403

53

2.41

49.56

122.82

La1.9K0.1CuO4

La2CuO4(T)

340

392

52

2.27

51.09

120.73

La1.8K0.2CuO4

La2CuO4(T)

320

387

67

2.13

56.36

119.78

La1.75K0.25CuO4

La2CuO4(T)

330

395

65

1.88

58.37

121.30

La1.7K0.3CuO4

La2CuO4(T+T**)

340

404

64

1.48

59.98

123.01

La1.6K0.4CuO4

La2CuO4(T+T**)

325

372

47

1.38

52.84

116.94

La1.4K0.6CuO4

La2CuO4(T+T**), KNO3(C)

315

365

50

1.35

47.03

115.62

La1.2K0.8CuO4

La2CuO4(T+T**), KNO3(C)

325

382

57

1.26

44.98

118.84

* O: Orthorhombic structure; T: Tetrahedral structure; C: Cubic structure; ** Activation energy obtained from the streamlined Redhead analysis[13]; the pre-exponential factor was assumed as 1×109 min-1

256

JOURNAL OF RARE EARTHS, Vol. 26, No. 2, Apr. 2008

the absorption band may be attributed to the crystal structure’s transformation from orthorhombic to tetragonal, in which the symmetry of the Cu–O octahedron improves. However, at x≥0.4, because of the excess substitution of K+ for La3+, and the appearance of T* phase, the lattice distortion increases and the absorption band (680 cm−1) appears again. The γ3 vibration bands of KNO3 at 1382 cm−1 can be observed in all the spectra of the La2-xKxCuO4 sample, and the intensity of the vibration band increases with x increasing in La2-xKxCuO4 oxides. When the K-substitution amount is equal to and greater than 0.2, the γ2 vibration bands of KNO3 at 825 cm–1 are detected. Results obtained from the IR spectra support the results analyzed by the XRD patterns. 2.3 SEM results SEM images of the La2-xKxCuO4 samples are presented in Fig.3. The SEM image of La2CuO4 shows that the catalyst particles have an average particle size centered around 70 nm, with an intercross structure, and it has a much better dispersion than the K-substituted sample, La2-xKxCuO4. The size of La2-xKxCuO4 increases with an increase in x, as the radius of K+ is larger than La3+. The average particle size of La1.8K0.2CuO4 increases to 100 nm. Because of the lower melting point, the KNO3 conglomerated on the surface of La2-xKxCuO4 may cover the La2-xKxCuO4’s surface. Therefore, the figure of La1.2K0.8CuO4 cannot be distinguished clearly in the images.

combustion. Compared with the blank case, the temperature for soot combustion is lowered by 110 °C for La2CuO4 as catalyst. The soot combustion temperatures significantly reduce over all the samples, with K substitution for La, compared to the unsubstituted sample of La2CuO4. The catalyst shows the best performance for soot combustion when the K-substitution amount is 0.2, and the Ti decreases to 315 °C. On the contrary, with the continuous increase of x, the catalysts become inactive and the Ti and Tp rise. It is a complicated multiphase reaction for soot catalytic combustion. The redox properties of the catalysts, especially the oxidizing ability of the B site cation, in perovskite-like oxides governs the catalytic properties of soot oxidation. In case of the La2-xKxCuO4 catalyst, the cation at the A-site (La3+) is replaced by a larger radius cation K+. The structure parameters of La2-xKxCuO4 change and a defect occurs in the structure, which may enhance the activity of the lattice oxygen greatly. At the same time, the cation at the A-site (La3+)

2.4 Catalytic activity for soot combustion The TPO results as shown in Fig.4 and Table 1 are useful to investigate the catalytic activity of the catalysts for soot

Fig.2 FT-IR spectra of La2-xKxCuO4 samples

Fig.3 SEM images of La2-xKxCuO4 samples (a) La2CuO4; (b) La1.8K0.2CuO4; (c) La1.6K0.4CuO4; (d) La1.4K0.6CuO4; (e) La1.2K0.8CuO4

ZHU L et al., Catalytic combustion of diesel soot over K2NiF4-type oxides La2-xKxCuO4

Fig.4 TPO curves of soot combustion on La2-xKxCuO4 catalysts

is replaced by a lower valency cation K+, the positive charge reduced being balanced either by the formation of a higher oxidation state ion at the B-site (equation 1) or by the formation of oxygen vacancy (Vo) in the La2-xKxCuO4 (equation 2)[14]. Therefore the Cu3+, which has better catalytic oxidation activity than Cu2+, is produced. On the other hand, the concentration of oxygen vacancy (Vo) in La2-xKxCuO4 is also increased, which improves the adsorption and activation of oxygen on the catalyst surface. Therefore, it can improve the catalytic activities of the La2-xKxCuO4 catalysts. La2-xKxCu2+1-2xCu3+2xO4→La2CuO4+xLa3+|K+|″+2xCu2+|Cu3+| (1) La2-xKxλ(VO)Cu2+O4-λ→La2CuO4+xLa3+|K+|″+λ|O| (2)

3 Conclusion The nanometric-structure La2-xKxCuO4 perovskite-like complex oxides had good catalytic performances for purifying soot from diesel exhaust, under loose contact conditions. In the case of the La2-xKxCuO4 catalyst, the partial substitution of K+ into A-site La3+ improved the catalytic activities, and the combustion temperature of soot decreased with increasing x values. The combustion temperature for soot combustion on La2-xKxCuO4 catalysts, with K substitution amount between 0.05 and 1, was between 315 and 380 °C. The optimal substitution amount of potassium x was equal to 0.2 among these samples.

References: [1] John P A Neeft, Michiel Makkee, Jacob A Moulijn. Diesel particulate emission control. Fuel Processing Technology, 1996, 47: 1.

257

[2] Acres G J K, Harrison B. The development of catalysts for emission control from motor vehicles:early research at Johnson Matthey. Topics in Catalysis,2004, 28(1-4): 3. [3] Barry A A L Van Setten, Michiel Makkee, Jacob A Moulijn. Science and technology of catalytic diesel particulate filters. Catalysis Reviews, 2001, 43(4): 489. [4] Sua D S, Jentoft R E, Müller J O, Rothe D, Jacob E, Simpson C D, Tomovic Z, Müllen K, Messerer A, Pöschl U, Niessner R, Schlögl R. Microstructure and oxidation behaviour of Euro IV diesel engine soot: a comparative study with synthetic model soot substances. Catalysis Today,2004, 90: 127. [5] Hu Hui, Wang Shuxia, Zhang Xiaoling, Zhao Quanzhong, Li Jin. Study on simultaneous catalytic reduction of sulfur dioxide and nitric oxide on rare earth mixed compounds. Journal of Rare Earths, 2006, 24(6): 695. [6] C van Gulijk, Heiszwolf J J, Makkee M. Selection and development of a reactor for diesel particulate filtration. Chemical Engineering Science,2001, 56: 1705. [7] Debora Fino,Nunzio Russo,Emanuele Cauda,Guido Saracco, Vito Specchia. La-Li-Cr perovskite catalysts for diesel particulate combustion. Catalysis Today, 2006, 114: 31. [8] Teraoka Y, Nakano K, Kagawa S, Shuangguan W F. Simultaneous removal of nitrogen oxides and diesel soot particulates catalyzed by perovskite-type oxides. Applied Catalysis B: Environmental, 1995, 5: L181. [9] Zhu Junwu, Sun Xiaojie, Wang Yanping, Wang Xin, Yang Xujie, Lu Lude. Solution-phase synthesis and characterization of perovskite LaCoO3 nanocrystals via a Co-precipitation route. Journal of Rare Earths, 2007, 25(5): 601. [10] Migaku Kobayashi,Ryoko Katsuraya,Tsubasa Nara, Yusuke Tomita, Hiromi Nakano, Naoki Kamegashira. Phase behavior and crystal structure of perovskite-type rare earth complex oxides. Journal of Rare Earths, 2006, 24(6): 668. [11] Shangguan W F, Teraoka Y, Kagawa S. Simultaneous catalytic removal of NOx and diesel soot particulates over ternary AB2O4 spinal-type oxide. Applied Catalysis B : Environmental,1996, 8: 217. [12] Zhu Junjiang, Zhao Zhen, Xiao Dehai, Li Jing, Yang Xiangguang, Wu Yue. Characterization and catalytic activity in NO decomposition of La2-xSrxCuO4 (0≤x≤1) compounds with T* phase structure. Materials Chemistry and Physics, 2005, 94: 57. [13] Dernaika B, Uner D. A simplified approach to determine the activation energies of uncatalyzed and catalyzed combustion of soot. Applied Catalysis B: Environmental,2003, 40: 219. [14] Zhu Junjiang, Zhao Zhen, Xiao Dehai. Effect of valence of copper in La2-xThxCuO4 on NO decomposition reaction. Catalysis Communications,2006, 7: 29.