Iron doped lanthanum stannate pyrochlores (La2Sn2-xFexO7) for the simultaneous catalytic removal NOx and soot Zhongpeng Wang1,a, *, Zizi Wang2,b, Xiao Wang2,c, Qian Li2,d, Zonggang Mu2,e, Zhaoliang Zhang2,f, * 1 School of Resources and Environment, University of Jinan, 106 Jiwei Rd., Jinan 250022, China 2 College of Chemistry and Chemical Engineering, University of Jinan, 106 Jiwei Rd., Jinan 250022, China a
[email protected], b
[email protected], c
[email protected], d
[email protected], e
[email protected], f
[email protected], *corresponding authors Keywords: Simultaneous removal; NOx; Soot; Lanthanum stannate oxides; Pyrochlore; Iron doping. Abstract. A series of La2Sn2-xFexO7 (x=0, 0.2, 0.6, 1.0, 1.4, 1.8, 2.0) catalysts were prepared by a constant pH co-precipitation method, and their catalytic activity was investigated for simultaneous removal of NOx and soot. The incorporation of Fe strongly influences the crystal phase composition, surface area and redox properties of the catalysts. All the catalysts displayed soot oxidation activity with nearly 100% selectivity towards CO2. The doped solids exhibit higher activities than the undoped one, which may be related to the enhancement of reducibility derived from structure defects induced by doping. LSF0.6 may be a good catalyst with high soot oxidation activity (Ti = 334 ºC) and a high maximum productivity of N2 (PN2= 12.4%). Introduction Nitrogen oxides and soot particulate are the main pollutants in diesel engine emissions causing serious problems to global environment and human health. As the legislation limitation goes more stringent, it is necessary to develop after-treatment technologies to meet the more and more stringent emission regulations[1]. As a promising alternative, the simultaneous catalytic removal of NOx and soot particulates in a single trap was proposed by Yoshida et al.[2]. During the last few decades, various catalytic materials have been studied, with mixed metal oxides[3,4], spinels[5] and perovskites[6-8] being the outnumbering materials. This technology is ambitious, though there are some technical problems to be solved. The more exigent research subject is to develop new catalysts possessing low-temperature activity for both soot oxidation and NOx reduction. In recent years, pyrocholore-type oxides (general formular of A2B2O7) have drawn much attention for a wide variety of applications such as ionic/electronic conductors, hosts for fluorescence centers, high-temperature pigments, radioactive waste management and catalysts[9]. They have a structure composed of two different types of cation coordination polyhedra, in which the A-site positions typically occupied by larger cations are eight-fold coordinations while the B-site positions favored by smaller sized cations are six-fold coordination. The cations at the A site and the B site in the lattice can be replaced by the cations with different chemical valence or different oxidation-reduction property to synthesize the various kinds of pyrochlore compounds with different physical or chemical properties[10]. Especially, owing to their thermal stabilities and tunable structure, the catalytic properties of pyrochlore oxides have attracted much interest. They have been applied in various reactions such as oxidative coupling of methane[11], NO reduction[12], CO oxidation[13] and hydrocarbon combustion[14, 15]. Among them, rare earth stannate-based pyrochlores doped with transition metals exhibit excellent redox properties, which promote us to investigate their catalytic performance in simultaneous NOx-soot removal reaction. Advanced Materials Research Vols. 306-307 (2011) pp 1468-1472 Online available since 2011/Aug/16 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.306-307.1468 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 128.59.222.12, Columbia University Library, New York, USA-12/11/14,02:00:04) http://www.scientific.net http://www.ttp.net In the present study, we report the catalytic activities of Fe doped lanthanum stannate oxides for the simultaneous removal of NOx and soot. The physicochemical properties of the composite oxides prepared through the co-precipitation method are characterized by XRD, N2 adsorption-desorption, TPR and FT-IR techniques. The relationship among catalytic activity with material composition, surface area, and redox property was also discussed. Experimental Catalyst Preparation. A series of La2Sn2-xFexO7 (x=0, 0.2, 0.6, 1.0, 1.4, 1.8, 2.0) catalysts were prepared with a constant pH coprecipitation of a mixed salt solution (100 ml) and an aqueous ammonia solution (100 ml) following the reference[14]. The catalysts were calcined at 900 ºC for 5 h in air and designated as LSFx, while x represented the doped amounts of Iron. Catalyst Characterization. XRD was conducted with a BRUKER-AXS D8Adance X-Ray Diffractometer using Cu Kα radiation. N2 adsorption-desorption isotherms were measured on a Quanta Chrome NOVA1000 Sorptomatic apparatus. The BET specific surface area (SBET) was calculated by the standard BET method. Temperature-programmed reduction (TPR) analysis was carried out using a gas mixture of H2-N2 (5 % vol) and the gas flow (30 ml/min), sample weight (50 mg), and heating schedule (10 ºC/min). H2 consumption was measured using a thermal conductivity detector. Infrared spectra were recorded on a Bruker Tensor 27 spectrometer. Catalytic activity testing. In this paper, a commercially available carbon black was used as model soot[4]. The catalytic activity tests for soot combustion were carried out by a TPO technique in a fixed-bed flow reactor. Catalyst and soot (20:1 w/w) were well mixed, crushed and sieved to 20-40 mesh. The soot-catalyst mixture (0.33 g) was placed in an 8 mm U-shaped quartz reactor, and then pretreated in a helium flow at 300 ºC for 1 h in order to eliminate possible contaminants. After cooling down the mixture to 100 ºC and replacing the helium flow with the reaction gas flow (0.25 vol.% NO + 5 vol.% O2/He, flow rate 20 cm 3 /min), the TPO was started at a heating rate of 1.6 ºC/min. The outlet gas was analyzed with intervals of about 15 min using a TCD gas chromatograph (Shimazu GC-14B) with columns of molecular sieve 5A for CO2 and N2O and Porapak Q for separating N2, O2, NO and CO. From TPO results, two parameters were derived in order to evaluate the catalytic performance: one is the soot ignition temperature(Ti) estimated by extrapolating the steeply ascending portion of the CO2 formation curve to zero CO2 concentration, and another is the maximum productivity of N2 (PN2 ) calculated by 2[N2]out/[NO]in. Besides, the apparent activation energies (Ea) for the formation of CO2 were obtained using linear fitting of Arrhenius plots followed the reference[9].TN2 was the temperature at which the maximum productivity of N2 appeared. Results and Discussion XRD and BET Analysis. The XRD patterns of La2Sn2-xFexO7 samples are given in Figure 1. It can be seen that the gradual replacement of Sn with Fe leads to the gradual changes in the oxides phase composition. For the LSF0.0 and LSF0.2 samples, the typical diffraction patterns at 2θ around 29°, 33°, 48°, 57° and 60° were attributed to (222), (400), (440), (622) and (444) crystal planes of La2Sn2O7 with cubic pyrochlore structure conforming to the Fd-3m space group (JCPDS 13-0082). In contrast, for sample LSF2.0, a well-defined LaFeO3 phase (JCPDS 75-0541) together with a slight amount of La2O3 phase (JCPDS 73-1144) was detected. As for other tertiary oxides, with the increase of Fe content, the La2Sn2O7 phase tended to disappear, while the LaFeO3 phase became more evident. These results suggest that a small quantity of the doped Fe ions(x≤0.2) in the pyrochlore do not change the crystal purity and can be well distributed in the pyrochlore structure. It is well known[14] that the incorporation of functional dopants to A or B sites in the pyrochlore lattice leads to form defect structures and thus create more active sites to improve catalytic activity, however, doping often results in lower surface area. The BET specific surface areas (SBET) are Advanced Materials Research Vols. 306-307 1469 summarized in Table 1. Compared with the undoped LSF0.0 sample (12.6 m 2 ·g -1 ), the introduction of Fe decreases the surface area. There is a decrease trend in surface area with the increase of Fe content. LSF2.0 sample has the smallest surface area of 4.1 m 2 ·g -1 . Fig. 1 XRD patterns of. La2Sn2-xFexO7 samples Fig.2 TPR patterns of La2Sn2-xFexO7 catalysts (phases: •-pyrochlore, ▼-perovskite, ∇-La2O3) TPR Results. TPR was used to examine the redox properties of catalysts. Fig 2 gives TPR profiles for the synthesized pyrochlore oxides. LSF0.0 sample displays a low temperature peak between 450 ºC and 650 ºC and a high temperature peak above 650 ºC, which belongs to the reduction of Sn 4+ to Sn 2+ and Sn 2+ to Sn 0 , respectively. The replacement of Sn with Fe influences the reduction behavior, especially on the low temperature range, which is due to the interactions between Sn and transition metals. It can be seen that the low temperature peak shifts to the lower temperature range after the incorporation of Fe , that is, the doped samples are easier to be reduced [14]. According to the peak temperature of the low temperature reduction, the oxidation capacities of such solids changes as the sequence: LSF1.4>LSF0.6≈LSF1.0>LSF2.0>LSF0.2>LSF0.0. The improved oxygen mobility may be derived from the structure defects induced by doping, which can be helpful in oxidation reactions. FTIR Characterization. The metal ions in pyrochlores are situated in two different sub-lattices designated tetrahedral (A site) and octahedral (B site) according to the geometrical configuration of the neighboring oxygen. There are seven IR bands in the range of 750–50 cm -1 originating from vibration and bending of metal–oxygen bonds in the pyrochlore oxides [10]. The band at about 600 cm -1 is from the B–O stretching vibrations in the BO6 octahedron and the one around 400 cm -1 to the A–O stretching vibrations. Figure 3 presents the FTIR spectra of the catalysts in the range of 400-1800 cm -1 . It can be seen that ν(Sn–O) shifts to low frequency with the increase of Fe contents, indicating that the Sn–O bond strength weakens. The changes in Sn–O bond strength may influence oxygen mobility of the catalysts because the Sn–O bond is responsible for the release of lattice oxygens when enough energy is provided[16]. This can also be related to the TPR results where the reducibility of the samples was enhanced by doping. Catalytic Performance of the Simultaneous Soot-NOx Removal. The catalytic performances are compared by analyzing the TPO profiles. Fig. 4 shows the TPO results over LSF0.6 in the NO-O2-He atmosphere. Similar to the previous reports [9], the formations of CO2, N2 and N2O were observed within the same temperature range of 200-550 ºC evidencing the occurrence of the simultaneous NOx-soot removal reactions: the oxidation of soot by either NOx or O2, and the reduction of NOx by soot into N2 and N2O. The similar shapes of the formation curves for CO2, N2 and N2O indicate a closer temperature dependency and closer apparent activation energy. Catalytic activities in terms of Ti, PN2 and apparent activation energies are listed in Table 1. No CO was detected during the TPO process and the carbon mass balance was nearly 100 ± 5%, which suggested that the catalysts have an excellent selectivity to CO2 formation. A blank experiment was performed mixing the soot with SiO2, and the Ti was 520 ºC, while the soot was totally burnt at 790 ºC. Compared with the non-catalyzed soot combustion, the CO2 formation curves over catalysts shift to lower temperature and the catalyzed soot oxidation starts at 1470 Emerging Focus on Advanced Materials about 320~390 ºC. The doped catalysts exhibit higher soot oxidation activities than the undoped lanthanum stannate oxide, which may be related to the enhancement of reducibility [15] confirmed by TPR. The LSF1.4 sample shows the best activity with Ti = 320 ºC and Tp = 437 ºC. The value of Tp-Ti, which can represent combustion reaction velocity, decreases from 154 ºC for the blank sample to about 100~140 ºC for the catalyzed soot oxidation, indicating that catalyzed oxidation can speed up the soot combustion rate. It should be noted that the catalytic performance is almost independent on the surface area as listed in Table 1 because the catalyzed NOx-soot reactions supposedly takes place at the so-called ‘‘triple contact point’’ where the solid catalyst, the solid soot and the gaseous reactants (O2/NO) meet together. In addition, the apparent activation energies for catalyzed soot oxidation were also decreased from 150.4 kJ·mol -1 to about 70 kJ·mol -1 , thus the soot can be oxidized to CO2 at lower temperatures. Fig.3 Infrared spectra of La2Sn2-xFexO7 samples Fig.4 TPO results over LSF0.6 sample. Table 1 Catalytic activity of the simultaneous NOx-soot removal over La2Sn2-xFexO7 catalysts Catalyst Formulae SBET (m 2 ·g -1 ) Ti (ºC) Tp (ºC) Tp-Ti (ºC) Ea (kJ·mol -1 ) PN2 TN2 (ºC) Blank -- 120.6 520 674 154 158.4 8.0 661 LSF0.0 La2Sn2O7 12.6 390 530 140 63.3 8.4 530 LSF0.2 La2Sn1.8Fe0.2O7 9.7 364 492 128 70.7 8.5 482 LSF0.6 La2Sn1.4Fe0.6O7 9.5 334 449 115 67.2 12.4 434 LSF1.0 La2Sn1.0Fe1.0O7 9.9 337 450 113 74.8 9.8 458 LSF1.4 La2Sn0.6Fe1.4O7 7.0 320 437 117 69.9 9.6 410 LSF1.8 La2Sn0.2Fe1.8O7 4.4 356 471 115 65.4 8.0 482 LSF2.0 La2Fe2O7 4.1 347 452 105 69.5 8.3 458 As for NO removal, LSF0.6 shows the best activity with PN2 = 12.4% , which appeared at the temperature of the maximum soot combustion rate. TN2 values in Table 2 are near the maximum soot combustion rate for all catalysts, which suggests that NOx-soot reactions should exist. The conversion of NOx to N2 was relatively low, then the reduction of NOx by soot over pyrochlore catalysts in the presence of O2 was weak. In other words, the oxidation of soot by O2 was kinetically favored in comparison to NOx-soot reactions [5]. As a result, LSF0.6 may be a good catalyst with high soot oxidation activity (Ti = 334 ºC) and a high maximum productivity of N2 (PN2= 12.4% ). Conclusions A series of La2Sn2-xFexO7 catalysts for simultaneous NOx-soot removal were prepared by a constant pH co-precipitation, and only a small quantity of the doped Fe ions(x≤0.2) can be well distributed in the pyrochlore structure. All the catalysts display catalytic soot oxidation activity with nearly 100% selectivity towards CO2. The doped solids exhibit higher activities than the undoped one, which may be related to the enhancement of reducibility derived from structure defects induced by doping. As a result, LSF0.6 may be a good catalyst with high soot oxidation activity (Ti = 334 ºC) and a high maximum productivity of N2 (PN2= 12.4% ). 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Vol.10(2009), p. 1170. 1472 Emerging Focus on Advanced Materials Emerging Focus on Advanced Materials 10.4028/www.scientific.net/AMR.306-307 Iron Doped Lanthanum Stannate Pyrochlores (La2Sn2-XFeXO7) for the Simultaneous Catalytic Removal NOx and Soot 10.4028/www.scientific.net/AMR.306-307.1468 http://dx.doi.org/www.scientific.net/AMR.306-307 http://dx.doi.org/www.scientific.net/AMR.306-307.1468