Catalytic removal of diesel soot particulates over K and Mg substituted La1 − xKxCo1 − yMgyO3 perovskite oxides

Catalysis Communications 49 (2014) 15–19

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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom

Short Communication

Catalytic removal of diesel soot particulates over K and Mg substituted La1 − xKxCo1 − yMgyO3 perovskite oxides Shuqing Fang, Lei Wang, Zhichuan Sun, Nengjie Feng, Chen Shen, Peng Lin, Hui Wan ⁎, Guofeng Guan ⁎ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, PR China

a r t i c l e

i n f o

Article history: Received 7 December 2013 Received in revised form 27 January 2014 Accepted 28 January 2014 Available online 3 February 2014 Keywords: Soot combustion Perovskite Substitution K Mg

a b s t r a c t K and Mg substituted perovskite catalysts La1 − xKxCo1 − yMgyO3 (x = 0–0.4, y = 0–0.2) for soot combustion were prepared by citric acid complexation and characterized by XRD, FT-IR, SEM, TEM, EDS, H2-TPR, XPS and TG. Soot combustion was remarkably accelerated when K was introduced into LaCoO3. Then Mg was doped into the K substituted LaCoO3, soot combustion was further improved for the restrained growth of Co3O4 phase. K/Mg substitutions were responsible for enhancing activity of catalysts by improving reducibility as suggested by H2-TPR studies. Among all the catalysts, La0.6K0.4Co0.9Mg0.1O3 exhibited the highest activity. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Diesel engines have drawn public attention owing to their relatively high efficiency and low greenhouse gas emission. However, the soot particulate emissions from diesel exhaust have an impact on environmental protection and human health [1]. Catalytic route is effective for removal of the soot [2]. In the process of catalytic reaction, various kinds of catalysts have been used such as metal oxides [3,4], noble metals [5,6], spinel [7] and perovskite-type oxides [8]. Noble metal catalysts have been widely investigated in the field of diesel engine exhaust aftertreatment due to their high instinct catalytic activity. However, two major drawbacks (high cost and poor thermal durability) limite their widespread application. Recently, perovskite-type complex metal oxides have been extensively investigated as potential substitutes for noble metal catalysts in soot combustion because of their excellent catalytic performances and high thermal stability [9,10]. The perovskite oxides possess a general formula of ABO3, where A sites can be occupied by rare-earth metals or alkaline earth metals and B sites are usually chosen from transition metals [11]. If A and/or B site cations are substituted by other elements, structural distortion and valence change of A and/or B site cations will take place. The catalytic activity of the substituted perovskite-type complex metal oxides can be consequently improved. Wang et al. [12] found that the activity of Labased perovskites for soot oxidation followed the subsequent order: LaCoO3 N LaMnO3 N LaFeO3, suggesting that cobalt-based perovskites showed better soot oxidation ability compared with the other two perovskites. Recently, many studies focused on cobalt-based perovskite ⁎ Corresponding authors. Tel.: +86 25 83587198. E-mail addresses: [email protected] (H. Wan), [email protected] (G. Guan). 1566-7367/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2014.01.029

catalysts have been proposed [13]. It was reported that K/B′ (B′ = Fe, Ni, Cu) simultaneous substituted perovskite catalysts possessed higher activity for soot combustion than single substitution [14–16]. Perovskite catalysts doped with transition metals have been studied extensively for soot oxidation reactions. However, according to the best of our knowledge, little research has been focused on the incorporation of Mg into B site. In fact, low valence Mg doped into LaCoO3 can enhance the mobility of surface lattice oxygen, change the valence of Co and generate oxygen vacancies during the reaction. What's more, Mg can inhibit the sintering of the perovskite [13]. Therefore, K and Mg doped LaCoO3 perovskite for soot combustion is attainable. In this work, a series of La1 − xKxCo1 − yMgyO3 (x = 0–0.4, y = 0–0.2) perovskite catalysts were prepared and applied for diesel soot removal. Based on multiple characterizations, the influence of K/Mg substitution on the structures, reducibility and catalytic performance of catalysts for soot combustion was investigated in detail. 2. Experimental 2.1. Catalyst preparation The perovskite-type mixed oxides of La1 − xKxCo1 − yMgyO3 (x = 0–0.4, y = 0–0.2) were synthesized by the citric acid complexation method. The corresponding nitrate salts were dissolved in deionized water to obtain an aqueous solution of La3+, K+, Co2+ and Mg2+ with the expected stoichiometry. The molar ratio of citric acid to total metal ions was 1.2:1. The resulting solutions were evaporated to dryness at 75 °C with vigorous stirring. The clear solution gradually turned into sol and finally transformed into gel. Then, the wet gel was dried at 110 °C for 12 h. Afterwards, the resulting foamy solid was kept at 200

S. Fang et al. / Catalysis Communications 49 (2014) 15–19

°C to remove the organic ligands. At last, the precursor was heated to 400 °C and kept for 2 h, then calcined at 700 °C for 4 h. 2.2. Catalyst characterization X-ray diffraction patterns were recorded on a SmartLab-9 Japan automated power X-ray diffraction meter operating at 100 mA and 40 kV using Cu Kα (λ = 0.1541 nm) radiation. The data of 2θ from 20 to 80° were collected with a step scan of 0.02°. FT-IR spectra were collected on a Nicolet-6700 spectrometer with a resolution of 5 cm− 1 using anhydrous KBr as dispersing agent. Temperature-programmed reduction (TPR) was conducted on a BEL-CAT chemisorption analyzer equipped with a TCD detector. Each time, 30 mg sample was pretreated in argon stream at 200 °C for 2 h, and then cooled down to room temperature. 10% H2/Ar at a total flow rate 40 ml/min flowed over the sample from room temperature to 900 °C with heating rate of 10 °C/min. Scanning Electron Microscopy (SEM) was taken on a Hitachi S4800 field-emission SEM instrument operated at 5 kV. Transmission electron microscope (TEM) images and energy-dispersive spectroscopy (EDS) results were obtained with a JEOL instrument employing an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a PHI-5000 spectrometer using Al Kα (1486.6 eV) radiation as the excitation source. All binding energies were referenced to the C 1 s peak at 284.5 eV, Gaussian–Lorentzian and Shirley background was applied for peak analysis. A Perkin-Elmer Diamond TG/DTA instrument was used to obtain the weight loss and differential TG curves (DTG) of the samples. Each time, about 5 or 8 mg of the sample was heated at a constant rate. 2.3. Activity testing The catalytic activity for soot combustion under the tight contact mode was evaluated by TG/DTA technique. Printex-U from Degussa was used as a model substance of diesel soot. The soot was mixed with catalyst in a weight ratio of 1:9 by grinding in an agate mortar for 10 min to obtain a tight contact. These mixtures were heated from 100 to 700 °C at 10 °C/min heating rate in a flow of synthetic air (20% O2 and 80% N2, 50 ml/min). The temperature for maximal soot combustion rate (Tm) was used to evaluate the catalytic performance of the catalysts.

Table 1 The molar and weight percentage of the K and Mg doping concentration against each composition. La1 − xKxCo1 − yMgyO3

Weight percentageb

K

Mg

K

Mg

0 0.05 0.1 0.15 0.2 0.2 0.2 0.2

0 0 0 0 0 0.025 0.05 0.1

0 0.021 0.044 0.070 0.099 0.100 0.101 0.104

0 0 0 0 0 0.008 0.016 0.032

x x x x x x x x

= = = = = = = =

a

The molar ratio of K or Mg to total metal elements (La, K, Co, and Mg). The weight contents of K or Mg to total elements (La, K, Co, Mg, and O).

b

0, y = 0 0.1, y = 0 0.2, y = 0 0.3, y = 0 0.4, y = 0 0.4, y = 0.05 0.4, y = 0.1 0.4, y = 0.2

Molar percentagea

the Co3 O 4 phase, and the perovskite structural stability was thus enhanced. Fig. 2 showed the FT-IR spectra of the La1 − xKxCo1 − yMgyO3 catalysts. Two peaks at 560 and 596 cm−1 were attributed to two kinds of stretching vibration of Co\O bond in the CoO6 octahedron, and the band at 424 cm− 1 was assigned to Co\O bending vibration in the CoO6 octahedron [18]. These results proved that all the samples had ABO3 perovskite structure. The weak band at 667 cm−1 was assigned to Co3O4 phase, which was detected for K/Mg substituted catalysts, and the intensity gradually increased with the increase in the Ksubstituted amount. But with the introduction of Mg into perovskite, this band gradually declined, indicating the growth of Co3O4 was restrained. This conclusion was in good agreement with the XRD results. Fig. 3(a) displayed the XPS results of Co 2p in La1 − xKxCo1 − yMgyO3 catalysts. Two peaks corresponding to Co 2p1/2 and Co 2p3/2 were detected. The peak binding energy for Co 2p3/2 of LaCoO3 was similar to

a

La1-xKxCo1-yMgyO3 x=0.4, y=0.2 x=0.4, y=0.1 x=0.4, y=0.05

Intensity

16

x=0.4, y=0 x=0.3, y=0 x=0.2, y=0

3. Results and discussion

x=0.1, y=0 x=0, y=0

3.1. Catalysts characterization 20

30

40

50

60

70

80

2 Theta(degree)

b

La1-xKxCo1-yMgyO3

x=0.4, y=0.2 x=0.4, y=0.1

Intensity

Table 1 showed the molar and weight percentage of the K and Mg doping concentration against each composition. The XRD patterns of La1 − xKxCo1 − yMgyO3 perovskite catalysts were shown in Fig. 1(a). The main diffraction peaks of catalysts were in good agreement with the crystalline structure of perovskite LaCoO3 (JPCDS 48–0123), indicating that the perovskite structure was still well maintained after substitution. The partial enlarged detail of the XRD pattern ranging from 32° to 34° was shown in Fig. 1(b). A slight shift to lower 2θ angles was observed, which demonstrated that the K+ and Mg2+ ions were successfully incorporated into the lattice of perovskite structure. The weak peak detected at 2θ = 36.9° for La1 − xKxCo1 − yMgyO3 (x N 0.1) suggested the formation of trace amount of Co3O4. The intensity of Co3O4 diffraction peak was reinforced with increase in K substitution ratio, which indicated a gradual increase of the Co3O4 crystallite size. Because the ionic diameter of K+ (0.133 nm) was greater than that of La3+ (0.106 nm), substitution with K+ certainly caused structural distortion, leading to a separate Co3O4 phase [17]. However, the intensity of Co3O4 diffraction was weakened with the increase in Mg substitution ratio, because the doping of Mg 2 + (0.072 nm) into Co 3 + (0.065 nm) restrained the growth of

x=0.4, y=0.05 x=0.4, y=0 x=0.3, y=0 x=0.2, y=0 x=0.1, y=0 x=0, y=0

32.0

32.5

33.0

33.5

34.0

2 Theta(degree) Fig. 1. XRD patterns of La1 − xKxCo1 − yMgyO3 catalysts: full patterns (a); enlarged reflections (b).

S. Fang et al. / Catalysis Communications 49 (2014) 15–19 La1-xKxCo1-yMgyO4 667

x=0.4, y=0.2

596 560

424

x=0.4, y=0.1

Absorbance

x=0.4, y=0.05 x=0.4, y=0

x=0.3, y=0 x=0.2, y=0 x=0.1, y=0 x=0, y=0

1400

1200

1000

800

600

400

Wavenumber (cm-1) Fig. 2. FT-IR spectra of La1 − xKxCo1 − yMgyO3 catalysts.

Co2O3 as reported in literature [19], indicating that Co3+ ions were the main valences of Co ions in LaCoO3. With increase in K substitution, the peak of Co 2p3/2 moved from 779.9 eV to 779.6 eV (La0.6K0.4CoO3), which indicated that some amounts of Co3+were changed into Co4+ [20,21]. The binding energy of Co 2p3/2 in La0.6K0.4Co0.9Mg0.1O3 (779.9 eV) increased compared to that of La0.6K0.4CoO3 (779.6 eV), suggesting that more Co4+ were formed after the replacement of Co3+ by low valence Mg [22]. The reducibility of the catalysts was characterized by H2-TPR technique, as shown in Fig. 3(b). Two major reduction stages were observed for all catalysts, the first one was ascribed to a partial reduction of the metallic cation Co3+,4+ into Co2+, while the second one was ascribed to the reduction of Co2+ into Co0. The first reduction step was a sign of high intrinsic oxygen reactivity which could be indicated as the critical factor in the soot oxidation state [23]. The two main reduction peaks of LaCoO3 appeared at about 505 °C and 650 °C. When K was introduced into LaCoO3 perovskite, the first reduction peak shifted to lower temperature. This phenomenon was attributed to the existence of Co4+ ions when La3+ ions were partially replaced by K+ ions, which was in good agreement with the XPS results. The Co4+ could be reduced to

Co 2p1/2

Co 2p3/2

La1-xKxCo1-yMgyO3

a

779.9 eV

779.6 eV

Intensity

x=0.4, y=0.1 x=0.4, y=0

779.8 eV

x=0.1, y=0

779.9 eV

x=0, y=0

800

795

790

785

780

775

Binding energy (eV)

b

La1-xKxCo1-yMgyO3

H2 consumption

455

x=0.4, y=0.2 x=0.4, y=0.1

358

423

x=0.4, y=0.05 416

x=0.4, y=0

416

x=0.3, y=0

479 505

x=0.1, y=0 x=0, y=0

100

200

300

400

Co2+ at a relatively low temperature [24]. Furthermore, with increase in K substitution ratio, the first reduction temperature was further lowered. It was mainly due to the increase in the amount of Co4+ ions. When 5% Mg was introduced into La0.6K0.4CoO3 perovskite, no big change for the first reduction peak was observed. However, after substituted by 10% Mg, the temperature of the first reduction peak was decreased significantly. Obviously, the optimal substitution amount of Mg was y = 0.1. According to the literature, the maximum substitution amount of Mg for Co was 10% [22], and the excess Mg would cover the surface of the perovskite instead of incorporating into the lattice of perovskite structure, which was responsible for the rise of the first reduction temperature. In summary, La0.6K0.4Co0.9Mg0.1O3 perovskite catalyst possessed the highest reducibility compared with the other as-prepared catalysts. The morphology of La1 − xKxCo1 − yMgyO3 perovskites was displayed in Fig. 4. SEM images of Fig. 4(a) and (d) showed that both LaCoO3 and Co3O4 were mainly consisted of spherical primary particles with a grain radius in the range of 100–200 nm. These primary particles condensed to form agglomerates. As seen in Fig. 4(b) and (c), after introduction of K and Mg into the catalysts, the sphere-like particles became larger and less uniform. Some fine grains adhering to the samples were observed in Fig. 4(b), a few of impurities appeared at high K content. In order to investigate the composition of these impurities in detail, TEM and EDS techniques were employed. As shown in Fig. 4(e) and (f), the large dark regions consisted of La, K, and Co (Cu peaks were due to the supporting copper grid) were ascribed to perovskite, while the light color spots on the edge composed solely of Co and O were attributed to cobalt oxides. According to the XRD and FT-IR results above, these cobalt oxides were ascribed to Co3O4. However, only small amounts of fine grains were formed on the surface of La0.6K0.4Co0.9Mg0.1O3 catalyst. This phenomenon could attributed to the incorporation of Mg into Co, and the growth of Co3O4 fine grains was thus inhibited. The average size of La0.6K0.4Co0.9Mg0.1O3 perovskite was about 220–250 nm, within 100–600 nm and meeting the requirements for soot particle trapper and soot oxidation [12], which was favorable to trap soot and provide the necessary solid–solid contact sites. 3.2. Catalytic soot combustion over La1 − xKxCo1 − yMgyO3 catalysts The performance of La1 − xKxCo1 − yMgyO3 and Co3O4 catalysts for soot combustion under tight contact mode was shown in Fig. 5. For comparison, the combustion of soot without catalyst was also presented. As shown in Fig. 5, the soot oxidation without catalysts started at 476 °C and the Tm was about 590 °C. LaCoO3 catalysts achieved high catalytic activity with the Tm decreased about 140 °C. After introduction of 10% potassium into LaCoO3, the Tm was decreased by 51 °C. With further increase in K amount, the Tm was accordingly declined. But the presence of excessive Co3O4 crystallites inhibited the further improvement of catalytic activity because Co3O4 owned the lowest catalytic activity compared with the La1 − xKxCo1 − yMgyO3 (x ≥ 0.1) series. Therefore, the activities of the La1 − xKxCo1 − yMgyO3 series were further improved with Mg loading by restraining the growth of the Co3O4 impurity in perovskites, and the optimal activity was obtained when the amount of Mg increased to 0.1. When 40% potassium and 10% magnesium was introduced into LaCoO3 simultaneously, the lowest Tm (359 °C) was observed. Based upon the above results, high catalytic activity of as-prepared catalysts was attributed to good structural stability, high reducibility and excellent oxygen species transferring ability [23,25]. 4. Conclusions

426

x=0.2, y=0

17

500

600

700

Temperature (°C) Fig. 3. XPS and H2-TPR profiles of prepared catalysts.

800

A series of La1 − xKxCo1 − yMgyO3 (x = 0–0.4, y = 0–0.2) perovskite catalysts were prepared and employed for soot combustion. When La ions were partially replaced by K ions, the redox properties of catalysts were improved remarkably. Meanwhile, K substitution caused perovskite structural distortion slightly with generation of Co3O4 phase, inhibiting the further improvement of catalytic activity. However, B site substitution

18

S. Fang et al. / Catalysis Communications 49 (2014) 15–19

600

EDS

Co 500

Counts

400

300

La

Cu

200 La 100

Cu Co La K

0 0

Co K K La

Cu

La 5

120

Energy (keV)

10

15

EDS

Co

100

Counts

80 O 60 Cu 40

Cu C Co

20

Co

Cu

0 0

5

10

15

Energy (keV) Fig. 4. SEM images of samples: (a) LaCoO3; (b) La0.6K0.4CoO3; (c) La0.6K0.4Co0.9Mg0.1O3; (d) Co3O4. TEM images of La0.6K0.4CoO3 at different magnifications (e and f). EDS analysis of the corresponding marked areas.

by Mg was responsible for inhibition of the Co3O4 phase growth, which made for the enhanced structural stability of the perovskite. K and Mg simultaneous substitution improved the redox properties of the perovskites, ensuring a high catalytic activity for soot combustion. Among all the samples, La0.6K0.4Co0.9Mg0.1O3 exhibited the optimal catalytic performance with the lowest Tm at 359 °C under tight contact mode.

Acknowledgments The work presented above was supported by R&D Project for Environmental Protection of Jiangsu of China (no. 2012022), the Natural Science Foundation of Jiangsu Higher Education Institutions of China (grant no. 11KJB530007), the Foundation from State Key Laboratory of Materials-

S. Fang et al. / Catalysis Communications 49 (2014) 15–19

x=0.4, y=0.2

359

x=0.4, y=0.1

DTG/(%/min)

La1-xKxCo1-yMgyO3

387

x=0.4, y=0.05

372

x=0.4, y=0

370

x=0.3, y=0

371

x=0.2, y=0

385

x=0.1, y=0

397

x=0, y=0

448

Co3O4

442

uncatalyzed

100

200

590

300

400

500

600

700

Temperature (oC) Fig. 5. DTG profiles of soot combustion over prepared catalysts.

Oriented Chemical Engineering, Nanjing Tech University (ZK201305) and the Practice and Innovation Training Project for College Students of Jiangsu Province (no. 201310291004Z).

References [1] B. Giechaskiel, B. Alfoeldy, Y. Drossinos, J. Aerosol Sci. 40 (2009) 639–651. [2] L. Li, X. Shen, P. Wang, X. Meng, F. Song, Appl. Surf. Sci. 257 (2011) 9519–9524. [3] D. Weng, J. Li, X. Wu, F. Lin, Catal. Commun. 9 (2008) 1898–1901.

19

[4] P.A. Kumar, M.D. Tanwar, N. Russo, R. Pirone, D. Fino, Catal. Today 184 (2012) 279–287. [5] B. Azambre, S. Collura, P. Darcy, J.M. Trichard, P. Da Costa, A. García-García, A. Bueno-López, Fuel Process. Technol. 92 (2011) 363–371. [6] Y. Wei, J. Liu, Z. Zhao, C. Xu, A. Duan, G. Jiang, Appl. Catal. A Gen. 453 (2013) 250–261. [7] M. Zawadzki, W. Walerczyk, F.E. López-Suárez, M.J. Illán-Gómez, A. Bueno-López, Catal. Commun. 12 (2011) 1238–1241. [8] G. Pecchi, B. Cabrera, E.J. Delgado, X. García, R. Jimenez, Appl. Catal. A Gen. 453 (2013) 341–348. [9] S. Li, R. Kato, Q. Wang, T. Yamanaka, T. Takeguchi, W. Ueda, Appl. Catal. B Environ. 93 (2010) 383–386. [10] P. Doggali, S. Kusaba, Y. Teraoka, P. Chankapure, S. Rayalu, N. Labhsetwar, Catal. Commun. 11 (2010) 665–669. [11] D.W.J.R.J.H. Voorhoeve Jr., J.P. Remeika, P.K. Gallagher, Science 195 (1977) 827–833. [12] H. Wang, J. Liu, Z. Zhao, Y. Wei, C. Xu, Catal. Today 184 (2012) 288–300. [13] J. Zhang, X. Weng, Z. Wu, Y. Liu, H. Wang, Appl. Catal. B Environ. 126 (2012) 231–238. [14] Z. Li, M. Meng, F. Dai, T. Hu, Y. Xie, J. Zhang, Fuel 93 (2012) 606–610. [15] Z. Li, M. Meng, Q. Li, Y. Xie, T. Hu, J. Zhang, Chem. Eng. J. 164 (2010) 98–105. [16] Z. Li, M. Meng, Y. Zha, F. Dai, T. Hu, Y. Xie, J. Zhang, Appl. Catal. B Environ. 121–122 (2012) 65–74. [17] F. Bin, C. Song, G. Lv, J. Song, C. Gong, Q. Huang, Ind. Eng. Chem. Res. 50 (2011) 6660–6667. [18] N.A. Merino, B.P. Barbero, P. Grange, L.E. Cadus, J. Catal. 231 (2005) 232–244. [19] R. Tan, Y. Zhu, Appl. Catal. B Environ. 58 (2005) 61–68. [20] Y. Wen, C. Zhang, H. He, Y. Yu, Y. Teraoka, Catal. Today 126 (2007) 400–405. [21] Y. Wang, J. Ren, Y. Wang, F. Zhang, X. Liu, Y. Guo, G. Lu, J. Phys. Chem. C 112 (2008) 15293–15298. [22] S. Sun, L. Yang, G. Pang, S. Feng, Appl. Catal. A Gen. 401 (2011) 199–203. [23] N. Russo, S. Furfori, D. Fino, G. Saracco, V. Specchia, Appl. Catal. B Environ. 83 (2008) 85–95. [24] J. Xu, J. Liu, Z. Zhao, C. Xu, J. Zheng, A. Duan, G. Jiang, J. Catal. 282 (2011) 1–12. [25] J. Liu, Z. Zhao, C. Xu, A. Duan, Appl. Catal. B Environ. 78 (2008) 61–72.