Catalytic combustion of soot particulates over La2-xKxNiMnO6 catalysts

Journal of Natural Gas Chemistry 20(2011)384–388

Catalytic combustion of soot particulates over La2−x KxNiMnO6 catalysts Wenjuan Shan∗ , Jiali Yang,

Lihua Yang, Na Ma

Institute of Chemistry for Functionalized Materials, College of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, Liaoning, China [ Manuscript received February 17, 2011; revised March 23, 2011 ]

Abstract Nanosized La2−x Kx NiMnO6 catalysts with ABO3 type perovskite-like structure were prepared by auto-combustion method using citric acid as a ligand to control particle size and morphology. The structures and properties of these perovskite-like oxides were investigated by X-ray powder diffraction (XRD) and temperature-programmed reduction (TPR). The catalytic activities for soot combustion were evaluated by temperatureprogrammed oxidation (TPO) with pure O2 and O2 /NOx as oxidant, respectively. In the La2−x Kx NiMnO6 catalysts, the partial substitution of K at A-site leads to an increase of the concentrations of high valence cation and oxygen vacancy, which enhance the catalytic activity for soot combustion. The optimal substitution amount of K was equal to x = 0.4 among these samples. Tp (peak temperature) in O2 -TPO profile was 420 ◦ C and Tp in O2 /NOx -TPO profile was 370 ◦ C over La1.6 K0.4 NiMnO6 catalyst for soot particulates combustion under loose contact conditions between catalyst and soot. Key words soot oxidation; La2−x Kx NiMnO6 ; perovskite-like; active oxygen

1. Introduction Diesel engine emissions, the main pollutants including soot and NOx , are known to be hazardous to the environment. Since the reduction of both soot and NOx emissions to regulated level cannot be accomplished by engine modifications alone, aftertreatment techniques for the simultaneous reduction of such emissions should be developed [1−3]. Since Yoshida et al. [4] proposed the catalytic conversion of soot into CO2 under an oxidizing atmosphere using catalytic soot traps. Since then, a large number of candidate oxidation catalysts and their catalytic mechanisms were summarized and discussed, including alkali metal or alkali earth metal catalysts, noble metal catalysts, transitional metal catalysts and compound catalysts [5,6]. Recently, several authors reported that perovskite and spinel oxides are active for simultaneous NOx -soot removal reaction [7−11]. According to the catalysis theory, the activities of ABO3 type perovskite catalyst mainly depend on B-site cation, while is influenced by A-site cation indirectly.

Teraoka et al. [12] found that La-K-Mn-O perovskite-type oxides are good candidate catalysts for diesel soot combustion, and they reported the relevant catalytic activities under tight contact conditions between the catalysts and soot particles. However, it is often a loose contact between the catalysts on the surface of filter and soot particles under practical conditions [13]. Li et al. [14] reported that the catalyst La0.9 K0.1 Co0.9 Fe0.1 O3−δ showed the highest activity for simultaneous NOx -soot removal, over which the maximal soot oxidation rate was achieved at only 362 ◦ C (Tm ), the NOx storage capacity reached 213 μmol·g−1, and the percentage for NOx reduction by soot was 12.5%. In the present study, the La2−xKx Niy Mn1−y O6 complex oxides were prepared by auto-combustion method, and the effects of the substituted amounts of A-site and B-site cations on the catalytic performances for soot particle combustion under loose contact conditions were investigated. The probable reasons of the activity enhancement induced by the Ksubstituted and Ni-substituted samples compared with unsubstituted sample were discussed.

∗

Corresponding author. Tel: +86-411-82156852; Fax: +86-411-82156858; E-mail: [email protected] This work was supported by the National Natural Science Foundation of China (No. 20603016) and the Liaoning Provincial Science & Technology Project of China (No. 20071074). Copyright©2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(10)60202-2

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2. Experimental 2.1. Catalyst preparation A series of La2−x Kx NiMnO6 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) complex oxides were prepared by auto-combustion method. The corresponding metal nitrates were used as starting materials to obtain an aqueous solution of La3+ , K+ , Ni2+ and Mn2+ with appropriate stoichiometry, respectively. 1.5 times molar amount of citric acid was added into the nitrate solutions and followed by heating and evaporating to dryness with vigorous stirring, going with burning or exploding. Finally, the precursor was calcined at 800 ◦ C for 4 h in static air.

structures [4,8]. All the characteristic diffraction peaks were belonged to perovskite structure and there was no miscellaneous peak can be detected. Compared with unsubstituted sample La2 NiMnO6 , there was no influence on the formation of perovskite structure with K-substitution. Table 1 shows that the average crystal particle sizes (D) of La2−x Kx NiMnO4 samples were between 19 nm and 25 nm. This revealed that La2−x Kx NiMnO4 samples prepared by auto-combustion method possessed nanometer particle sizes.

2.2. Catalyst characterization X-ray powder diffraction (XRD) patterns were recorded on a Siemens D-5000 power diffractometer using nickel˚ filtered Cu Kα radiation (1.5406 A). Temperature-programmed reduction (TPR) was conducted using a conventional apparatus equipped with a TCD detector. 25 mg sample was placed in a quartz tube (4.0 mm ID), then TPR was performed by heating the samples from 25 to 900 ◦ C at 10 ◦ C·min−1 in a 5% H2 -N2 mixture flowing at 40 mL·min−1. 2.3. Catalytic activity measurement The catalytic activity was evaluated by a home-made temperature-programmed oxidation (TPO) reaction apparatus. Before reaction, the soot and catalyst mixture, in a 1/4 weight ratio, was mixed carefully with a spatula for loose contact [12]. 100 mg catalyst-soot mixture diluted with 250 mg SiO2 was placed in a tubular quartz reactor (i.d. = 10 mm), and the reaction temperature varied from 300 ◦ C to 600 ◦ C at 1 ◦ C·min−1 during each TPO run. The inlet gas mixture was 50% air and 50% NO/N2 with a total flow rate of 60 mL·min−1. The catalytic stability was also evaluated by TPO reaction apparatus. The selectivity to CO2 (SCO2 ) was defined as SCO2 (%) =100 CCO2 /(CCO +CCO2 ). Where, CCO2 and CCO are the outlet concentrations of CO2 and CO, respectively. The catalyst, mixed with 20 mg soot, was performed several cycled reaction processes. The differences of catalytic activity among three cycles could show the stability of these catalysts. 3. Results 3.1. Structural and reduction properties of La2−x K x NiMnO6 catalysts The XRD patterns of all the samples gave several large diffraction peaks at 23.13o, 31.18o, 40.47o, 47.88o and 58.34o, as shown in Figure 1, which reveals that the complex oxides of La2−x Kx NiMnO6 possessed ABO3 type perovskite-like

Figure 1. X-ray diffraction patterns of La2−x Kx NiMnO6 catalysts Table 1. Average crystal sizes of La2−x Kx NiMnO6 catalysts calculated from XRD patterns Catalysts La2 NiMnO6 La1.9 K0.1 NiMnO6 La1.8 K0.2 NiMnO6 La1.7 K0.3 NiMnO6 La1.6 K0.4 NiMnO6 La1.5 K0.5 NiMnO6 La1.4 K0.6 NiMnO6

x Crystal surface 0 104 0.1 104 0.2 104 0.3 104 0.4 104 0.5 104 0.6 104

2θ (o ) β (radian) D (nm) 32.7561 0.0101 22.3 32.7161 0.0087 24.1 32.7028 0.0101 24.3 32.7694 0.0101 25.3 32.7428 0.0080 21.4 32.7961 0.0090 21.4 32.7428 0.0093 19.3

The intrinsic redox properties of catalysts play an key role in the activation of soot by oxygen. Temperature-programmed reduction (TPR) by H2 was used to measure these characteristics in the present work. During H2 -TPR process, not only the Mn+ ions with high valence were reduced to low valance ions or metal atoms by H2 , but also the oxygen ions were involved in the process due to the formation of H2 O molecules. Therefore, the reducibility of Mn+ and the mobility of lattice oxygen can be reflected by H2 -TPR measurement [15]. Figure 2 shows the H2 -TPR profiles of the series of La2−x Kx NiMnO4 perovskite-like catalysts. There were two kinds of reduction peaks at around 350–650 ◦ C (α) and 650–900 ◦ C (β), which can be attributed to the reduction of B-site cation in H2 -TPR for all the samples. A-site cation (La3+ ) partly replaced by K+ makes partial B-site cation with low valence change to high valence and oxygen vacancy increase to keep the charge balance [16]. The high valence cation leads to a high hydrogen

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reactants. The soot conversion was calculated by integration of CO and CO2 concentrations over the recorded time intervals

consumption while the increased oxygen vacancy leads to a low hydrogen consumption in H2 -TPR process. Table 2 shows the results of H2 -TPR characterization over La2−xKx NiMnO6 catalysts. According to the results of Figure 2 and Table 2, it can be seen that when x = 0.4 and 0.6, the reduction peak temperatures of α and β peaks are lower than those of other catalysts in which x<0.4, and the H2 consumption is also higher than other catalysts. It indicates that La2−x Kx NiMnO6 samples with larger substitution amounts not only lead to larger concentration of surface oxygen species (O2− and O− ), but also strengthen the mobility of their lattice oxygen. 3.2. Catalytic activity for soot oxidation under O2 atmosphere In the practical case, the temperature of diesel exhausted is in the range of 200−400 ◦ C. But the soot combustion in air without catalysts showed that the oxidation of soot is at 550−600 ◦ C [17]. The soot is difficult to combust under common condition. Because of the experimental difficulty to supply the solid soot continuously to the reaction system, the reactions have been exclusively investigated by TPO technique, in which a mixture of catalyst and soot is heated in gaseous

Figure 2. H2 -TPR profiles of La2−x Kx NiMnO4 catalysts

Table 2. Results of H2 -TPR and CO2 selectivity during O2 -TPO over La2-x Kx NiMnO6 catalysts Catalysts

x

Tα (◦ C)

Tβ (◦ C)

Sα (a.u.)

Sβ (a.u.)

H2 consumption (mmol·g−1 )

SCO2 /%

La2 NiMnO6

0

509

821

1.39

2.39

0.061

97.2

La1.9 K0.1 NiMnO6

0.1

497

797

1.37

2.93

0.060

93.0

La1.8 K0.2 NiMnO6

0.2

465

787

1.43

1.89

0.062

93.2

La1.7 K0.3 NiMnO6

0.3

482

797

1.49

2.49

0.065

90.0

La1.6 K0.4 NiMnO6

0.4

437

769

1.75

2.75

0.076

100

La1.5 K0.5 NiMnO6

0.5

407

754

1.20

2.54

0.052

99.8

La1.4 K0.6 NiMnO6

0.6

440

796

2.24

1.92

0.097

99.9

which were always equal to 300 min because of each TPO running from 300 ◦ C to 600 ◦ C at 1 ◦ C·min−1 . Figure 3 shows the outlet CO2 concentration profiles during O2 -TPO over La2−x Kx NiMnO6 catalysts. The oxidation of soot started at 430 ◦ C and ended at 580 ◦ C over the unsubstituted sample La2 NiMnO6 . Compared with the catalyst without substitution, the temperatures decreased over Ksubstituted La2 NiMnO6 catalysts, but still not reached the actual requirements. In the La2−x Kx NiMnO6 catalysts, when the amounts of K-substitution was 0.2, the Tp (peak temperature) value of catalyst was lower than that of La2 NiMnO6 , but Ti (initial temperature) and Tf (finish temperature) were almost no change. When K-substituted value increased to 0.4, it can be seen that Ti decreased nearly 100 ◦ C, Tp reduced to 420 ◦ C. When K-substituted value was 0.6, it can be observed that Tp did not change much. The selectivity to CO2 (SCO2 ) is shown in Table 2. It can be observed that the selectivities to CO2 for soot combustion increased with the increasing Ksubstitution amount. So in the La2−x Kx NiMnO6 catalysts, the partial substitution of La with K at A-site greatly enhanced the catalytic activity for the removal of diesel soot. The optimal substitution amount of K (x) was equal to 0.4 among these

samples for the soot combustion under loose contact conditions between catalyst and soot.

Figure 3. Outlet CO2 concentration profiles during O2 -TPO over La2−x Kx NiMnO6 catalysts

Journal of Natural Gas Chemistry Vol. 20 No. 4 2011

3.3. Catalytic activity for soot oxidation under O2 -NOx atmosphere The catalytic activity for the soot combustion over La2−x Kx NiMnO6 catalysts in O2 -NOx was investigated by temperature-programmed oxidation (TPO). Figure 4 shows the outlet CO2 concentration profiles as a function of temperature. The oxidation of soot started at 330 ◦ C and ended at 540 ◦ C over the unsubstituted sample La2 NiMnO6 . Compared with the results of O2 -TPO, Tp decreased to 450 ◦ C. In the La2−x Kx NiMnO6 catalysts, when the amount of Ksubstitution was 0.2, the Tp value of the catalyst was lower than that of La2 NiMnO6 , but Ti and Tf were almost no change too. This result is corresponding to the result of O2 -TPO. When K-substitution value increased to 0.4, it can be seen that Ti decreased nearly 200 ◦ C, Tp reduced to 370 ◦ C. When Ksubstitution value was 0.6, it can be observed that Tp hardly changed compared with that of x=0.4. The Tp of each catalyst had decreased significantly. The catalytic activity was highest when K substitution value was 0.4, which proved that this series of catalysts have the potential of simultaneous removal of NOx and diesel soot.

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in few minutes, indicating that the catalyst kept high and stable activity even after two O2 -TPO cycles. Furthermore, the CO2 concentration did not decrease. The mixture was heated from 420 ◦ C to 600 ◦ C at a heating rate of 36 ◦ C·min−1 after three cycles, and there was a few soot without complete combustion at low temperature (at 420 ◦ C). This results reveals La1.6 K0.4 NiMnO6 catalyst can keep the high catalytic activity and good stability after several cycles.

Figure 5. Outlet CO2 concentration profiles during O2 -TPO cycles over La1.6 K0.4 NiMnO6 catalyst

4. Discussion

Figure 4. Outlet CO2 concentration profiles during O2 /NOx -TPO over La2−x Kx NiMnO6 catalysts

3.4. Catalyst stability at low temperature Catalyst stability was explored by successive TPO experiments using the same catalyst sample to mix with soot. Figure 5 shows three O2 -TPO cycles of soot oxidation over La1.6 K0.4 NiMnO6 catalyst at 420 ◦ C. In every TPO cycle, the CO2 concentration reached the top in short time, compared with the former cycle. After the first and second cycles, the unburned carbon generally became inert materials and covered on the surface of catalyst, which may cause the catalyst to lose activity. But in the third cycle, it can be observed that there was no change for CO2 concentration i.e., the CO2 concentration reached its highest value (higher than 8000 ppm)

The TPO results demonstrate that the La2−x Kx NiMnO6 perovskite-like complex oxides are good candidate catalysts for the catalytic removal of diesel soot particulates. The following three reasons can explain the activity enhancement for K-substituted samples compared with the unsubstituted sample (La2 NiMnO6 ). The first one is that A-site cations (La3+) are partly replaced by K+ and partial low valence cations at B-site change to high valence cations, which have better catalytic oxidation activity than low valence cations. The second one is the increase in the content of oxygen vacancy in La2−x Kx NiMnO6 . In Figure 2 and Table 2, it can be seen that when x = 0.4 and 0.6, La2−xKx NiMnO6 catalysts showed higher H2 consumption compared with other La2−xKx NiMnO6 samples. It reveals that the oxygen vacancies in La2−x Kx NiMnO6 catalysts increase with the change of the A-site cation valence [2]. The presence of a large amount of oxygen vacancies would enhance the ability to obtain active molecular oxygen on the surface of catalyst. Therefore, it can improve the catalytic activity for the removal of soot. The last one is the good contact between soot and nanoparticles of La2−xKx NiMnO6 perovskite-like oxides. The average crystal particle sizes (D) of La2−x Kx NiMnO4 samples were between 19 nm and 25 nm calculated by Scherrer equation using the (1 0 4) peak. The good contact between soot particle and catalyst surface is an indispensable condition for an active catalyst to

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play an important role in soot combustion. The primary particle size of soot is 25 nm, which is much larger than the pore diameter of common catalysts. Thus, soot particulates cannot enter their inter pores. It is envisaged that in the catalytic oxidation of soot only the outer surface of the catalysts plays an important role, as only the outer surface can provide the necessary solid-solid contact. Surface atoms of nanometric catalysts have extra and high surface free energies and they have good mobility. Thus, the contact between catalysts and soot is still very good even under loose contact conditions. 5. Conclusions The nanosized La2−x Kx NiMnO6 perovskite-like complex oxides have good catalytic performances on diesel soot particulates combustion under loose contact conditions. In the La2−x Kx NiMnO6 catalysts, the partial substitution of La with K at A-site enhances their catalytic activity which can be attributed to the production of high valence metal ions at B-site and nonstoichiometry of oxygen vacancies. The oxygen vacancy concentration has an important effect on the catalytic activity because the oxygen vacancy is beneficial to enhance the adsorption and activation of molecular oxygen. The optimal substitution amount of K is equal to x=0.4 among these samples. Acknowledgements We thank the important project of the National Natural Science Foundation of China (No. 20603016) and Liaoning Provincial Science & Technology Project of China (No. 20071074) for financial support of this research.

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