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Promotion effects of LaCoO3 formation on the catalytic performance of Co–La oxides for soot combustion in air
Catalysis Communications 51 (2014) 68–71
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Promotion effects of LaCoO3 formation on the catalytic performance of Co–La oxides for soot combustion in air Guchu Zou a, Mingxia Chen a,b, Wenfeng Shangguan a,b,⁎ a b
Research Center for Combustion and Environmental Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 19 February 2014 Received in revised form 19 March 2014 Accepted 20 March 2014 Available online 27 March 2014
a b s t r a c t Co1 − xLaxOy catalysts were synthesized by citric acid complex method and their catalytic activities for soot combustion in air were investigated in this study. The best performance was observed over Co0.93La0.07 oxide catalysts with T50 = 402 °C. Co4+ in the surface of LaCoO3 leads to the formation of more oxygen vacancies, thus contributing to the activities. © 2014 Elsevier B.V. All rights reserved.
Keywords: Co–La oxides Soot removal LaCoO3 Co4 + Oxygen vacancy
1. Introduction Diesel engines have experienced a rapid growth in market share mainly owing to their stronger power and better fuel economy than gasoline engines. Besides, they also produce less amount of carbon monoxide and unburned hydrocarbon in the exhaust. However, the diffusion combustion mode of diesel vehicle results in serious emission of nanoparticles (b50 nm) [1,2], which is hazardous to the environment and human health [3,4]. Therefore, many countries set strict limits on particulate matter (PM) emission in laws and regulations. Diesel particulate filter (DPF), an efficient way to trap soot, helps to meet the limits, but it needs to be regenerated when backpressure reaches alarm value. The conventional strategy for DPF regeneration is electrical heating, yet it is energy-consuming. A more economic route, continuous regeneration of DPF by means of coating catalysts, was developed to simplify the post-process system. The performance of DPF regeneration largely depends on the catalytic activity of catalysts; hence it is urgent to find a stable and active catalyst compatible with this route. Catalysts for soot abatement have been explored for decades [5]. As a typical spinel type catalyst, Co3O4 showed high activity in soot removal due to its strong redox ability [6,7]. A redox mechanism was proposed to explain the catalytic behavior of Co3O4 [8]. The perovskite-type oxides (ABO3), where site A generally stands for rare earth elements and site ⁎ Corresponding author at: Research Center for Combustion and Environmental Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China. Tel.: +86 21 34206020. E-mail address: [email protected] (W. Shangguan).
http://dx.doi.org/10.1016/j.catcom.2014.03.028 1566-7367/© 2014 Elsevier B.V. All rights reserved.
B is filled with transition metal elements, are promising catalysts for soot abatement owing to their high catalytic activity and stability. Among all the perovskite-type catalysts, LaCoO3 have been widely applied for their excellent catalytic performance for soot abatement. Two traditional strategies were reported to promote the activity over LaCoO3 catalysts. One is to replace cations at one or both of the A and B sites. This method leads to the generation of oxygen vacancies or the variation of valence state of metal ions at both sites [9], while the other is preparation into macroporous morphology. The structure can significantly improve the contact conditions between diesel soot and catalysts [10,11]. Although Co3O4 and LaCoO3 are common catalysts for soot removal, the combined effects of both the catalysts have not been reported yet. In this paper, Co1 − xLaxOy catalysts were prepared and their catalytic behaviors were investigated. Based on the experimental results, a possible determinant for the reaction was proposed.
2. Experimental The catalysts were synthesized by citric acid complex method. Co(Ac)2, La(NO3)3 and citric acid (triple total amount of metal ions) were dissolved in distilled water. The solution was stirred in water bath at 80 °C till dryness. The viscous mixture experienced pre-calcination at 400 °C for 1 h followed by grinding and by final calcination at 700 °C for 4 h to remove the remaining citric acid. The catalysts were denoted as Co1 − xLax, where x (x = 0, 0.02, 0.04, 0.07, 0.17, 1) represented the molar proportion of lanthanum to total metal ions. The physical mixture
G. Zou et al. / Catalysis Communications 51 (2014) 68–71
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3. Results and discussion
Table 1 T50 value and BET surface data of Co1 − xLax catalysts (x = 0.02, 0.04, 0.07, 0.17). Catalysts
T50 (°C)
BET (m2/g)
Co3O4 Co0.98La0.02 Co0.96La0.04 Co0.93La0.07 Co0.93La0.07-pm Co0.83La0.17 LaCoO3
423 415 412 402 426 421 430
2.8 3.8 6.2 12.2 3.2 6.3 4.5
of Co0.93La0.07 was obtained by grinding LaCoO3 and Co3O4 prepared above at the same ratio. It was denoted as Co0.93La0.07-pm. The catalytic activities under air atmosphere were carried out on thermo gravimetric (TG) analysis using STA449F3 (NETZSCH, Germany). The tight mixture of typical model soot (Printex U) and catalyst was prepared at a mass ratio of 1:19 by grinding them together for 15 min. The mixture (20 ± 0.2 mg) was purged with air with a flow rate of 50 ml/min from 200 °C to 600 °C. The catalytic activity was evaluated by temperature named as T50, at which 50% of soot was consumed. The crystal structure of the materials was recorded by X-ray diffraction (Rigaku D/max-2200/PC Japan) with Cu ka (40 kV, 20 mA) at a scan speed of 6°/min. BET surface area measurements were performed on a Micromeritics BET surface area analyzer. X-ray photoelectron spectroscopy (XPS) was acquired with a Kratos Axis UltraDLD spectrometer (Kratos Analytical — a Shimadzu group company) using a monochromatic Al Kα source (1486.6 eV). All the binding energy (BE) values were calibrated by adopting BE value of contaminant carbon (C1s 284.6 eV) as a reference. Soot temperature programmed reduction (soot–TPR) was conducted on TG instrument mentioned above. The tight catalyst/soot mixture was heated under Ar flow at a flow rate of 50 ml/min from 100 °C to 950 °C. H2-TPR was performed on Micromeritics Chemisorb 2720 device. 0.1 g samples were pretreated under N2 at 300 °C for 30 min and cooled down to room temperature in the same atmosphere. Then the samples were exposed to 5% H2/N2 at a flow rate of 25 ml/min till the TCD signal was flat. Afterwards the tests were carried out under the same condition from room temperature to 600 °C at a heating rate of 10 °C per minute.
3.1. Catalytic activity The catalytic activities in air are listed in Table 1. It is shown that the T50 value of Co1 − xLax catalysts had an inverse relationship with the loading of lanthanum. The minimum value of T50 (402 °C) was achieved over Co0.93La0.07 catalysts. Further introduction of lanthanum didn't assist soot oxidation due to the formation of La2O3 with lower activity. Thus, Co0.93La0.07 had the best performance among all the samples, while activities of Co0.93La0.07-pm just lay between those of the Co3O4 and LaCoO3. 3.2. XRD and BET XRD patterns of all the catalysts are displayed in Fig. 1. Two separate phases, spinel type Co3O4 and perovskite type LaCoO3, were detected in the Co0.93La0.07 catalysts. The positive shifts of LaCoO3 peaks indicated a lattice shrink over Co0.93La0.07 samples. This phenomenon may result from substitution of La3+ for cation with smaller radius such as bivalent cobalt. Accordingly, no shifts were found over Co0.93La0.07-pm sample. All the diffraction peaks of Co0.93La0.07 were broadened, which revealed that crystal grain size of this sample was smaller than that of other catalysts. The results of XRD crystal phase and BET surface area are listed in Tables 2 and 1. The BET value of all the catalysts was rather low due to high temperature calcination. There was a negative correlation between the specific surface area and the amount of lanthanum components, with the exception of Co0.83La0.17 owing to a new phase (La2O3) introduction (Fig. S1). Although the minimum T50 value of Co0.93La0.07 corresponded to maximum BET value of 12.2 m2/g, the BET surface area may play a minor role in soot oxidation since the reaction occurred at the three phase boundary of soot, catalysts and reactant gas [12,13]. 3.3. Soot–TPR and H2-TPR Soot–TPR tests were applied to analyze the activity of surface oxygen and lattice oxygen. DTG curves are shown in Fig. 2. Two major species of
Intensity
(d)
(c) (b)
Intensity
20
21
22
23
24
25
2 Theta (degree)
d) c) b) a) 20
30
40
50
60
70
80
2 Theta (degree) Fig. 1. XRD patterns of (a) Co3O4, (b) Co0.93La0.07, (c) Co0.93La0.07-pm, (d) LaCoO3 and magnified XRD patterns of LaCoO3 (012).
70
G. Zou et al. / Catalysis Communications 51 (2014) 68–71
Table 2 Peak position and crystal sizes of Co3O4, LaCoO3, Co0.93La0.07 and Co0.93La0.07-pm calculated from XRD patterns. 2θ° (LaCoO3 (012))
D (nm)
2θ° (Co3O4 (311))
D (nm)
Co3O4 Co0.93La0.07 Co0.93La0.07-pm LaCoO3
/ 23.290 23.224 23.225
/ 26.3 44.7 43.3
36.880 36.890 36.884 /
44.5 22.6 43.9 /
active oxygen can be identified from DTG profiles. First being the surface − adsorbed oxygen including O− 2 and O (500–720 °C), while the other 2− being the lattice oxygen O (N720 °C) [14]. A promotion effect was obtained over Co0.93La0.07 compared with its physical mixture. The surface oxygen as well as the lattice oxygen was activated at lower temperature. It was noteworthy that the DTG peaks of Co0.93La0.07 corresponding to surface oxygen (564 °C) and lattice oxygen (793 °C) were close to Co3O4 (568 °C) and LaCoO3 (801 °C), respectively. Co0.93La0.07 seemed to combine the advantages of both materials. H2-TPR curves uncovering the redox ability of the catalysts were presented in Fig. 3. LaCoO3 was characterized by two groups of reduction peaks. The first group, occurring at 391 °C and 405 °C, was due to one-electron reduction of Co3+. LaCoO3 was initially reduced to lanthanum hydroxide and metallic cobalt. The other group, observed at 578 °C arose from reduction process of Co3+ to Co0 [15]. The broadened and asymmetric shapes for Co3O4 imply that there exists more than one stage for the reduction of trivalent cobalt to metallic cobalt. The hydrogen uptake was assigned to two consecutive reduction steps i.e., Co3+ to Co2+ and Co2+ to Co0 [16]. The profile of Co0.93La0.07 and Co0.93La0.07-pm was similar to that of Co3O4 below 550 °C because the characteristics of LaCoO3 were overlapped by a large proportion of Co3O4. However, the difference between Co0.93La0.07 and Co0.93La0.07-pm was still distinguishable. A minor peak at 362 °C indicated a stronger redox ability of Co0.93 La 0.07 than Co0.93La0.07-pm. 3.4. XPS XPS survey and its calculation results are demonstrated in Fig. 4 & Table 3. LaCoO3 was characterized by three peaks. The peak (OI) with a lower binding energy (~ 528.6 eV) was assigned to lattice oxygen. Two peaks (OIII, OIV) with higher binding energy represented absorbed oxygen. OIII (~530.8 eV) was attributed to adsorbed oxygen in oxygen vacancy [9]. In other words, OIII was the sign of chemisorbed oxygen. OIV (~ 532.7 eV) was ascribed to weakly bound surface oxygen [17].
400
578
Co0.93La0.07-pm Intensity
Catalysts
362 583
Co0.93La0.07
578 371 405
LaCoO3
356
Co3O4 100
200
300
400
500
600
Temperature(oC) Fig. 3. H2-TPR profiles of Co3O4, LaCoO3, Co0.93La0.07 and Co0.93La0.07-pm.
O II (~ 529.8 eV) responsible for lattice oxygen of Co3O 4 was seen over Co0.93 La 0.07 samples. In Co 2p 3/2 spectrum, shifts from 779.6 eV to 779.2 eV were also discovered in this study. It is known that the negative shifts of Co 2p 3/2 peaks were due to the existence of higher oxidation state of Co4+ [9,18]. This part of Co4+ is unlikely to originate from spinel Co3O4 since no low valence ions were introduced to the system to replace Co2+ or Co3+ in Co3O4. Co4+ possibly comes from the partial incorporation of Co2 + into A site of LaCoO3. The replacement of La 3 + by Co2 + leads to the oxidation of B-site ions (Co3 + to Co4 +), and the formation of oxygen vacancies so that the electrical neutrality of LaCoO3 can be maintained. Also, the proportion of oxygen adsorption slightly increased due to more oxygen vacancy in Co0.93La0.07 than that in Co0.93La0.07-pm. Judging by the results of activity tests above, it can be postulated that a synergistic effect may occur between Co3O4 and LaCoO3. The coexistence between the two oxides gives rise to a smaller size of crystal grain and the formation of Co4+. The latter can be one of the crucial factors for the promotion of the catalytic activity. In most cases, bivalent cobalt hardly replaces metal ions or rare earth ions at A site of ABO3 perovskite because it can easily occupy B site with higher oxidation state (Co3+). In present study, Co2+ that replaces A site in ABO3 perovskite was provided by Co3O4. Generally, Co3O4 is believed to undergo a redox mechanism in
LaCoO3 598 Co0.93La0.07-pm
801 895 Intensity
DTG (%/min)
Co3O4
568
Co0.93La0.07-pm
575
855
Co0.93La0.07
Co0.93La0.07 564 surface oxygen 200
400
600
793 LaCoO3
lattice oxygen 800
Temperature (oC) Fig. 2. DTG data of soot–TPR curves over Co3O4, LaCoO3, Co0.93La0.07 and Co0.93La0.07-pm.
540
538
536
534
532
530
528
526
Binding Energy (eV) Fig. 4. XPS data for O1s spectra of Co0.93La0.07-pm, Co0.93La0.07 and LaCoO3.
G. Zou et al. / Catalysis Communications 51 (2014) 68–71
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Table 3 The percentages of different kinds of oxygen species and binding energies (BE) of Co 2p3/2 for Co0.93La0.07-pm, Co0.93La0.07 and LaCoO3 catalysts.
Co0.93La0.07-pm Co0.93La0.07 LaCoO3 a b
OI (%)
OII (%)
Surface lattice oxygena (%)
OIII (%)
OIV (%)
Surface adsorbed oxygenb (%)
Co 2p3/2 (eV)
9.66 5.37 38.92
34.44 37.27 0
44.1 42.64 38.92
50 51.27 46.33
5.9 6.09 14.75
55.9 57.36 61.08
779.5 779.2 779.6
The percentage of surface lattice oxygen equals to OI + OII. The percentage of surface adsorbed oxygen equals to OIII + OIV.
soot removal [8], while oxygen spillover mechanism for soot combustion over LaCoO3 catalysts is well-documented in literature [19]. The effects of both mechanisms are enhanced accompanied by the formation of Co4+. The enhancement of redox ability can be explained by the participation of Co4 + in the first reduction process in H2-TPR tests over Co0.93La0.07 samples. The enhancement of spillover mechanism can also be inferred from the analysis of soot–TPR tests. Surface oxygen was made of two parts. The first part expressed oxygen bound to Co3+, a conventional active site for Co3O4, and/or oxygen adsorbed at Co4+. The second part was assigned to oxygen adsorbed in vacancy. These species are unlikely to stay in one spot to wait for carbon feed. Prior to migration to soot to form a ‘three phase boundary’, adsorb oxygen is dissociated to active oxygen species accompanied with the diffusion of oxygen vacancy. Co4 + consequently favors the formation of more oxygen vacancies and accordingly exerts positive impact on oxygen spillover mechanism. 4. Conclusions A promotion role of LaCoO3 on the performance of CoxLa1 − xOy catalysts for soot abatement in air was discovered. Co4 + species resulting from partial replacement of La3+ in LaCoO3 by Co2+ stemming from Co3O4 lead to the generation of additional oxygen vacancies. These oxygen vacancies favor adsorption of oxygen and in turn assist soot combustion under air flow.
Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.03.028. References [1] H.J. Tobias, D.E. Beving, P.J. Ziemann, H. Sakurai, M. Zuk, P.H. McMurry, D. Zarling, R. Waytulonis, D.B. Kittelson, Environ. Sci. Technol. 35 (2001) 2233–2243. [2] J. Shi, R. Harrison, F. Brear, Sci. Total Environ. 235 (1999) 305–317. [3] A. Sydbom, A. Blomberg, S. Parnia, N. Stenfors, T. Sandström, S. Dahlén, Eur. Respir. J. 17 (2001) 733–746. [4] J. Kagawa, Toxicology 181–182 (2002) 349–353. [5] W. Shangguan, Y. Teraoka, S. Kagawa, Appl. Catal. B 8 (1996) 217–227. [6] Z. Wang, Z. Jiang, W. Shangguan, Catal. Commun. 8 (2007) 1659–1664. [7] M. Sun, L. Wang, B. Feng, Z. Zhang, G. Lu, Y. Guo, Catal. Today 175 (2011) 100–105. [8] G. Mul, F. Kapteijn, C. Doornkamp, J. Moulijn, J. Catal. 179 (1998) 258–266. [9] H. Wang, Z. Zhao, P. Liang, C. Xu, A. Duan, G. Jiang, J. Xu, J. Liu, Catal. Lett. 124 (2008) 91–99. [10] J. Xu, J. Liu, Z. Zhao, C. Xu, J. Zheng, A. Duan, G. Jiang, J. Catal. 282 (2011) 1–12. [11] G. Zhang, Z. Zhao, J. Liu, J. Xu, Y. Jing, A. Duan, G. Jiang, J. Rare Earths 27 (2009) 955–960. [12] B. Dernaika, D. Uner, Appl. Catal. B 40 (2003) 219–229. [13] W. Shangguan, Y. Teraoka, S. Kagawa, Appl. Catal. B 16 (1998) 149–154. [14] J. Ren, Y. Yu, F. Dai, M. Meng, J. Zhang, L. Zheng, T. Hu, Nanoscale 5 (2013) 12144–12149. [15] E.V. Makshina, S.V. Sirotin, V.V. Yushchenko, G.N. Mazo, M.W.E. Berg, K.V. Klements'ev, W. Grünert, B.V. Romanovskii, Kinet. Catal. 47 (2006) 49–53. [16] L.F. Liotta, M. Ousmane, G.D. Carlo, G. Pantaleo, G. Deganello, A. Boreave, A. GiroirFendler, Catal. Lett. 127 (2008) 270–276. [17] S.S. Shinde, K.Y. Rajpure, J. Alloys Compd. 509 (2011) 4603–4607. [18] Z. Li, M. Meng, Y. Zha, F. Dai, T. Hu, Y. Xie, J. Zhang, Appl. Catal. B 121–122 (2012) 65–74. [19] R. Zhang, N. Luo, B. Chen, S. Kaliaguine, Energy Fuels 24 (2010) 3719–3726
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Promotion effects of LaCoO3 formation on the catalytic performance of Co–La oxides for soot combustion in air Guchu Zou a, Mingxia Chen a,b, Wenfeng Shangguan a,b,⁎ a b
Research Center for Combustion and Environmental Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 19 February 2014 Received in revised form 19 March 2014 Accepted 20 March 2014 Available online 27 March 2014
a b s t r a c t Co1 − xLaxOy catalysts were synthesized by citric acid complex method and their catalytic activities for soot combustion in air were investigated in this study. The best performance was observed over Co0.93La0.07 oxide catalysts with T50 = 402 °C. Co4+ in the surface of LaCoO3 leads to the formation of more oxygen vacancies, thus contributing to the activities. © 2014 Elsevier B.V. All rights reserved.
Keywords: Co–La oxides Soot removal LaCoO3 Co4 + Oxygen vacancy
1. Introduction Diesel engines have experienced a rapid growth in market share mainly owing to their stronger power and better fuel economy than gasoline engines. Besides, they also produce less amount of carbon monoxide and unburned hydrocarbon in the exhaust. However, the diffusion combustion mode of diesel vehicle results in serious emission of nanoparticles (b50 nm) [1,2], which is hazardous to the environment and human health [3,4]. Therefore, many countries set strict limits on particulate matter (PM) emission in laws and regulations. Diesel particulate filter (DPF), an efficient way to trap soot, helps to meet the limits, but it needs to be regenerated when backpressure reaches alarm value. The conventional strategy for DPF regeneration is electrical heating, yet it is energy-consuming. A more economic route, continuous regeneration of DPF by means of coating catalysts, was developed to simplify the post-process system. The performance of DPF regeneration largely depends on the catalytic activity of catalysts; hence it is urgent to find a stable and active catalyst compatible with this route. Catalysts for soot abatement have been explored for decades [5]. As a typical spinel type catalyst, Co3O4 showed high activity in soot removal due to its strong redox ability [6,7]. A redox mechanism was proposed to explain the catalytic behavior of Co3O4 [8]. The perovskite-type oxides (ABO3), where site A generally stands for rare earth elements and site ⁎ Corresponding author at: Research Center for Combustion and Environmental Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China. Tel.: +86 21 34206020. E-mail address: [email protected] (W. Shangguan).
http://dx.doi.org/10.1016/j.catcom.2014.03.028 1566-7367/© 2014 Elsevier B.V. All rights reserved.
B is filled with transition metal elements, are promising catalysts for soot abatement owing to their high catalytic activity and stability. Among all the perovskite-type catalysts, LaCoO3 have been widely applied for their excellent catalytic performance for soot abatement. Two traditional strategies were reported to promote the activity over LaCoO3 catalysts. One is to replace cations at one or both of the A and B sites. This method leads to the generation of oxygen vacancies or the variation of valence state of metal ions at both sites [9], while the other is preparation into macroporous morphology. The structure can significantly improve the contact conditions between diesel soot and catalysts [10,11]. Although Co3O4 and LaCoO3 are common catalysts for soot removal, the combined effects of both the catalysts have not been reported yet. In this paper, Co1 − xLaxOy catalysts were prepared and their catalytic behaviors were investigated. Based on the experimental results, a possible determinant for the reaction was proposed.
2. Experimental The catalysts were synthesized by citric acid complex method. Co(Ac)2, La(NO3)3 and citric acid (triple total amount of metal ions) were dissolved in distilled water. The solution was stirred in water bath at 80 °C till dryness. The viscous mixture experienced pre-calcination at 400 °C for 1 h followed by grinding and by final calcination at 700 °C for 4 h to remove the remaining citric acid. The catalysts were denoted as Co1 − xLax, where x (x = 0, 0.02, 0.04, 0.07, 0.17, 1) represented the molar proportion of lanthanum to total metal ions. The physical mixture
G. Zou et al. / Catalysis Communications 51 (2014) 68–71
69
3. Results and discussion
Table 1 T50 value and BET surface data of Co1 − xLax catalysts (x = 0.02, 0.04, 0.07, 0.17). Catalysts
T50 (°C)
BET (m2/g)
Co3O4 Co0.98La0.02 Co0.96La0.04 Co0.93La0.07 Co0.93La0.07-pm Co0.83La0.17 LaCoO3
423 415 412 402 426 421 430
2.8 3.8 6.2 12.2 3.2 6.3 4.5
of Co0.93La0.07 was obtained by grinding LaCoO3 and Co3O4 prepared above at the same ratio. It was denoted as Co0.93La0.07-pm. The catalytic activities under air atmosphere were carried out on thermo gravimetric (TG) analysis using STA449F3 (NETZSCH, Germany). The tight mixture of typical model soot (Printex U) and catalyst was prepared at a mass ratio of 1:19 by grinding them together for 15 min. The mixture (20 ± 0.2 mg) was purged with air with a flow rate of 50 ml/min from 200 °C to 600 °C. The catalytic activity was evaluated by temperature named as T50, at which 50% of soot was consumed. The crystal structure of the materials was recorded by X-ray diffraction (Rigaku D/max-2200/PC Japan) with Cu ka (40 kV, 20 mA) at a scan speed of 6°/min. BET surface area measurements were performed on a Micromeritics BET surface area analyzer. X-ray photoelectron spectroscopy (XPS) was acquired with a Kratos Axis UltraDLD spectrometer (Kratos Analytical — a Shimadzu group company) using a monochromatic Al Kα source (1486.6 eV). All the binding energy (BE) values were calibrated by adopting BE value of contaminant carbon (C1s 284.6 eV) as a reference. Soot temperature programmed reduction (soot–TPR) was conducted on TG instrument mentioned above. The tight catalyst/soot mixture was heated under Ar flow at a flow rate of 50 ml/min from 100 °C to 950 °C. H2-TPR was performed on Micromeritics Chemisorb 2720 device. 0.1 g samples were pretreated under N2 at 300 °C for 30 min and cooled down to room temperature in the same atmosphere. Then the samples were exposed to 5% H2/N2 at a flow rate of 25 ml/min till the TCD signal was flat. Afterwards the tests were carried out under the same condition from room temperature to 600 °C at a heating rate of 10 °C per minute.
3.1. Catalytic activity The catalytic activities in air are listed in Table 1. It is shown that the T50 value of Co1 − xLax catalysts had an inverse relationship with the loading of lanthanum. The minimum value of T50 (402 °C) was achieved over Co0.93La0.07 catalysts. Further introduction of lanthanum didn't assist soot oxidation due to the formation of La2O3 with lower activity. Thus, Co0.93La0.07 had the best performance among all the samples, while activities of Co0.93La0.07-pm just lay between those of the Co3O4 and LaCoO3. 3.2. XRD and BET XRD patterns of all the catalysts are displayed in Fig. 1. Two separate phases, spinel type Co3O4 and perovskite type LaCoO3, were detected in the Co0.93La0.07 catalysts. The positive shifts of LaCoO3 peaks indicated a lattice shrink over Co0.93La0.07 samples. This phenomenon may result from substitution of La3+ for cation with smaller radius such as bivalent cobalt. Accordingly, no shifts were found over Co0.93La0.07-pm sample. All the diffraction peaks of Co0.93La0.07 were broadened, which revealed that crystal grain size of this sample was smaller than that of other catalysts. The results of XRD crystal phase and BET surface area are listed in Tables 2 and 1. The BET value of all the catalysts was rather low due to high temperature calcination. There was a negative correlation between the specific surface area and the amount of lanthanum components, with the exception of Co0.83La0.17 owing to a new phase (La2O3) introduction (Fig. S1). Although the minimum T50 value of Co0.93La0.07 corresponded to maximum BET value of 12.2 m2/g, the BET surface area may play a minor role in soot oxidation since the reaction occurred at the three phase boundary of soot, catalysts and reactant gas [12,13]. 3.3. Soot–TPR and H2-TPR Soot–TPR tests were applied to analyze the activity of surface oxygen and lattice oxygen. DTG curves are shown in Fig. 2. Two major species of
Intensity
(d)
(c) (b)
Intensity
20
21
22
23
24
25
2 Theta (degree)
d) c) b) a) 20
30
40
50
60
70
80
2 Theta (degree) Fig. 1. XRD patterns of (a) Co3O4, (b) Co0.93La0.07, (c) Co0.93La0.07-pm, (d) LaCoO3 and magnified XRD patterns of LaCoO3 (012).
70
G. Zou et al. / Catalysis Communications 51 (2014) 68–71
Table 2 Peak position and crystal sizes of Co3O4, LaCoO3, Co0.93La0.07 and Co0.93La0.07-pm calculated from XRD patterns. 2θ° (LaCoO3 (012))
D (nm)
2θ° (Co3O4 (311))
D (nm)
Co3O4 Co0.93La0.07 Co0.93La0.07-pm LaCoO3
/ 23.290 23.224 23.225
/ 26.3 44.7 43.3
36.880 36.890 36.884 /
44.5 22.6 43.9 /
active oxygen can be identified from DTG profiles. First being the surface − adsorbed oxygen including O− 2 and O (500–720 °C), while the other 2− being the lattice oxygen O (N720 °C) [14]. A promotion effect was obtained over Co0.93La0.07 compared with its physical mixture. The surface oxygen as well as the lattice oxygen was activated at lower temperature. It was noteworthy that the DTG peaks of Co0.93La0.07 corresponding to surface oxygen (564 °C) and lattice oxygen (793 °C) were close to Co3O4 (568 °C) and LaCoO3 (801 °C), respectively. Co0.93La0.07 seemed to combine the advantages of both materials. H2-TPR curves uncovering the redox ability of the catalysts were presented in Fig. 3. LaCoO3 was characterized by two groups of reduction peaks. The first group, occurring at 391 °C and 405 °C, was due to one-electron reduction of Co3+. LaCoO3 was initially reduced to lanthanum hydroxide and metallic cobalt. The other group, observed at 578 °C arose from reduction process of Co3+ to Co0 [15]. The broadened and asymmetric shapes for Co3O4 imply that there exists more than one stage for the reduction of trivalent cobalt to metallic cobalt. The hydrogen uptake was assigned to two consecutive reduction steps i.e., Co3+ to Co2+ and Co2+ to Co0 [16]. The profile of Co0.93La0.07 and Co0.93La0.07-pm was similar to that of Co3O4 below 550 °C because the characteristics of LaCoO3 were overlapped by a large proportion of Co3O4. However, the difference between Co0.93La0.07 and Co0.93La0.07-pm was still distinguishable. A minor peak at 362 °C indicated a stronger redox ability of Co0.93 La 0.07 than Co0.93La0.07-pm. 3.4. XPS XPS survey and its calculation results are demonstrated in Fig. 4 & Table 3. LaCoO3 was characterized by three peaks. The peak (OI) with a lower binding energy (~ 528.6 eV) was assigned to lattice oxygen. Two peaks (OIII, OIV) with higher binding energy represented absorbed oxygen. OIII (~530.8 eV) was attributed to adsorbed oxygen in oxygen vacancy [9]. In other words, OIII was the sign of chemisorbed oxygen. OIV (~ 532.7 eV) was ascribed to weakly bound surface oxygen [17].
400
578
Co0.93La0.07-pm Intensity
Catalysts
362 583
Co0.93La0.07
578 371 405
LaCoO3
356
Co3O4 100
200
300
400
500
600
Temperature(oC) Fig. 3. H2-TPR profiles of Co3O4, LaCoO3, Co0.93La0.07 and Co0.93La0.07-pm.
O II (~ 529.8 eV) responsible for lattice oxygen of Co3O 4 was seen over Co0.93 La 0.07 samples. In Co 2p 3/2 spectrum, shifts from 779.6 eV to 779.2 eV were also discovered in this study. It is known that the negative shifts of Co 2p 3/2 peaks were due to the existence of higher oxidation state of Co4+ [9,18]. This part of Co4+ is unlikely to originate from spinel Co3O4 since no low valence ions were introduced to the system to replace Co2+ or Co3+ in Co3O4. Co4+ possibly comes from the partial incorporation of Co2 + into A site of LaCoO3. The replacement of La 3 + by Co2 + leads to the oxidation of B-site ions (Co3 + to Co4 +), and the formation of oxygen vacancies so that the electrical neutrality of LaCoO3 can be maintained. Also, the proportion of oxygen adsorption slightly increased due to more oxygen vacancy in Co0.93La0.07 than that in Co0.93La0.07-pm. Judging by the results of activity tests above, it can be postulated that a synergistic effect may occur between Co3O4 and LaCoO3. The coexistence between the two oxides gives rise to a smaller size of crystal grain and the formation of Co4+. The latter can be one of the crucial factors for the promotion of the catalytic activity. In most cases, bivalent cobalt hardly replaces metal ions or rare earth ions at A site of ABO3 perovskite because it can easily occupy B site with higher oxidation state (Co3+). In present study, Co2+ that replaces A site in ABO3 perovskite was provided by Co3O4. Generally, Co3O4 is believed to undergo a redox mechanism in
LaCoO3 598 Co0.93La0.07-pm
801 895 Intensity
DTG (%/min)
Co3O4
568
Co0.93La0.07-pm
575
855
Co0.93La0.07
Co0.93La0.07 564 surface oxygen 200
400
600
793 LaCoO3
lattice oxygen 800
Temperature (oC) Fig. 2. DTG data of soot–TPR curves over Co3O4, LaCoO3, Co0.93La0.07 and Co0.93La0.07-pm.
540
538
536
534
532
530
528
526
Binding Energy (eV) Fig. 4. XPS data for O1s spectra of Co0.93La0.07-pm, Co0.93La0.07 and LaCoO3.
G. Zou et al. / Catalysis Communications 51 (2014) 68–71
71
Table 3 The percentages of different kinds of oxygen species and binding energies (BE) of Co 2p3/2 for Co0.93La0.07-pm, Co0.93La0.07 and LaCoO3 catalysts.
Co0.93La0.07-pm Co0.93La0.07 LaCoO3 a b
OI (%)
OII (%)
Surface lattice oxygena (%)
OIII (%)
OIV (%)
Surface adsorbed oxygenb (%)
Co 2p3/2 (eV)
9.66 5.37 38.92
34.44 37.27 0
44.1 42.64 38.92
50 51.27 46.33
5.9 6.09 14.75
55.9 57.36 61.08
779.5 779.2 779.6
The percentage of surface lattice oxygen equals to OI + OII. The percentage of surface adsorbed oxygen equals to OIII + OIV.
soot removal [8], while oxygen spillover mechanism for soot combustion over LaCoO3 catalysts is well-documented in literature [19]. The effects of both mechanisms are enhanced accompanied by the formation of Co4+. The enhancement of redox ability can be explained by the participation of Co4 + in the first reduction process in H2-TPR tests over Co0.93La0.07 samples. The enhancement of spillover mechanism can also be inferred from the analysis of soot–TPR tests. Surface oxygen was made of two parts. The first part expressed oxygen bound to Co3+, a conventional active site for Co3O4, and/or oxygen adsorbed at Co4+. The second part was assigned to oxygen adsorbed in vacancy. These species are unlikely to stay in one spot to wait for carbon feed. Prior to migration to soot to form a ‘three phase boundary’, adsorb oxygen is dissociated to active oxygen species accompanied with the diffusion of oxygen vacancy. Co4 + consequently favors the formation of more oxygen vacancies and accordingly exerts positive impact on oxygen spillover mechanism. 4. Conclusions A promotion role of LaCoO3 on the performance of CoxLa1 − xOy catalysts for soot abatement in air was discovered. Co4 + species resulting from partial replacement of La3+ in LaCoO3 by Co2+ stemming from Co3O4 lead to the generation of additional oxygen vacancies. These oxygen vacancies favor adsorption of oxygen and in turn assist soot combustion under air flow.
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