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Preparation and Characterization of the Multicomponent Mesoporous Mixed Oxide Catalysts La-Mn-Ce-O
ACTA PHYSICO-CHIMICA SINICA Volume 23, Issue 5, May 2007 Online English edition of the Chinese language journal
Cite this article as: Acta Phys. -Chim. Sin., 2007, 23(5): 641−646.
ARTICLE
Preparation and Characterization of the Multicomponent Mesoporous Mixed Oxide Catalysts La-Mn-Ce-O Yong Liu,
Ming Meng*,
Jinsong Yao,
Yuqing Zha
Department of Catalysis Science and Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China
Abstract:
A series of mesoporous multicomponent mixed oxide catalysts La-Mn-Ce-O with various ratios of
(nLa+nMn)/(nLa+nMn+nCe) were prepared by citric acid complexation-organic template decomposition (CAC-OTD) method. For comparison, the sample with the same composition was also prepared by conventional coprecipitation method. The results of N2 adsorption/desorption showed that the samples prepared by using CAC-OTD method possessed relatively large specific surface area and uniform pore diameter distribution (3.4−4.4 nm). The results of X-ray diffraction (XRD) identified the formation of La-Ce and Mn-Ce solid solution. Mn species were not detected by XRD. The results of X-ray photoelectron spectroscopy (XPS) showed that there was a strong electronic state interaction between Mn and Ce species, resulting in the formation of Mn 2p shake-up peak. Such interaction enhanced the transfer of oxygen species from Ce to Mn oxide and therefore increased the redox activity of the samples. The sample with the strongest shake-up peak showed the highest oxidation activity. The results of temperature programmed reduction (TPR) showed that the manganese species in the samples prepared by using CAC-OTD method were easier to be reduced, which was relevant to the oxidation activity of the catalysts. The results of activity evaluation showed that the light-off temperature of the LMC(0.5)-500 sample was about 50 ℃ lower than that of the sample prepared by using coprecipitation method. The samples prepared by using CAC-OTD method showed good thermal stability. Key Words:
Multicomponent mesoporous catalyst; Preparation; Citric acid complexation; Organic template; Structural
characterization
With the regulation for environmental protection becoming increasingly stringent, the requirement for three-way catalysts is becoming stricter. During the cold start of engines, a great deal of CO and hydrocarbons are present in the exhaust, most of which (approximately 50%−80%) are released into the air without effective purification because of the low temperature of the catalyst bed and the resulting low catalytic efficiency[1]. Therefore, it is necessary to explore the low-temperature oxidation catalysts with high performance. Although noble-metal catalysts possess good activity, it is very expensive. The transition metal oxide catalysts containing Co, Cr, Mn, and so forth are active for many catalytic reactions, such as the oxidation of CO and hydrocarbons, and the NOx decomposition and selective reduction[2−4].
Because manganese possesses the changeable structure of d electrons, it can easily change the oxidation state, enhance the redox recycle, and consequently give high oxidation activity. However, the manganese oxides prepared by using normal method and calcined at high temperature usually show low specific surface area and broad pore size distribution, which restrict the low-temperature oxidation activity. In recent years, the Si-containing mesoporous materials prepared by using the surfactant as template display large specific surface areas, narrow pore diameter, and uniform pore structure, which greatly enhance the transferring velocity of the reactant and the product. Therefore, these materials have been extensively used in many aspects, such as catalysis, adsorption, separation, and host-guest chemistry[5−9].
Received: October 8, 2006; Revised: December 11, 2006. * Corresponding author. Email: [email protected]; Tel: +8622-27892275 The project was supported by the National Hi-Tech Research and Development Program (“863” Program) of China (2006AA06Z348) and the National Natural Science Foundation of China (20676097). Copyright © 2007, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Yong Liu et al. / Acta Physico-Chimica Sinica, 2007, 23(5): 641−646
In the present study, based on the method of preparing mesoporous, Si-containing molecular sieves, the citric acid complexation (CAC) method for nanomaterials and the organic template decomposition (OTD) method for mesoporous materials are combined to prepare a series of multicomponent, mesoporous, Mn-containing mixed-oxide catalysts[10,11]. Several techniques were used for the structural characterization of La-Mn-Ce-O catalysts, and their catalytic properties were investigated using CO oxidation as model reaction.
1 1.1
Experimental Catalyst preparation
First, the La-Mn citrate complex precursors were prepared from lanthanum nitrate La(NO3)3·6H2O, manganese acetate Mn(CH3COO)2·4H2O, and citric acid. The cetyltrimethylammonium bromide (CTAB) was first dissolved in the distilled water and HCl solution with mass fraction of 10%. The La-Mn citrate complex precursor and cerium nitrate Ce(NO3)·6H2O were then simultaneously added to the above solution with vigorous stirring, resulting in a clear homogeneous solution. The solution was stirred for 2 h. Later, 2 mol·L−1 NaOH was quickly added to the solution with vigorous stirring. The pH was adjusted to 9.7±0.3. The resulting gel mixture was stirred for 6 h at room temperature, subsequently heated to adequate temperature, and maintained at this temperature for 24 h to increase the degree of condensation. Finally, the solid product was filtered, washed with ethanol, dried in air at 110 ℃, and calcined in air at 500 ℃ for 8 h. This catalyst is denoted as LMC(x)-y, where x and y represent the atomic ratio of (nLa+nMn)/(nLa+nMn+nCe) (nLa/nMn=1) and the calcination temperature (500 ℃), respectively. A series of mixed-oxide catalysts with different atomic ratios were prepared by changing the amount of cerium nitrate. For comparison, La-Mn-Ce-O mixed-oxide catalyst with a composition of x=0.5 and calcination temperature of 500 ℃ was prepared by using the conventional coprecipitation method, which is denoted as LMC(0.5)-500 CP.
Co Kα as radiation source (λ＝0.17902 nm), the tube voltage of 40 kV, and the tube electric current of 40 mA. The data from 2θ range of 20° to 110° were collected with the step size of 0.03°. XPS analysis was carried out in a PHI-1600 ESCA SYSTEM spectrometer using Mg Kα as X-ray source (EB=1653.6 eV) under a residual pressure of 5.0×10−8 Pa. The error of the binding energy was ±0.2 eV, and the contaminated carbon (C 1s, EB=284.6 eV) was used as a standard for binding energy calibration. H2-TPR measurement was carried out on a TPDRO 1100 SERIES apparatus supplied by Thermo-Finnigan. Each time, 30 mg of the sample was heated from room temperature to 900 ℃ at a rate of 10 ℃·min−1. A gas mixture of H2 and N2 (φ(H2)=5%) was used as reductant with a flow rate of 20 mL·min−1. The catalytic activities of the samples for CO oxidation were measured in a fixed-bed quartz tubular reactor (inner diameter was 8 mm) mounted in a tube furnace. The reactant was a gas mixture, which contained 1.3% CO and 4% O2 (volume fraction), balanced with pure N2. The gas hourly space velocity was 11250 h−1. The effluent gas from the reactor was analyzed by using a gas chromatograph (SP-3430, BFS, China) equipped with a TCD detector.
2 2.1
Results and discussion N2 adsorption/desorption
The nitrogen adsorption/desorption isotherm of LMC(0.5)500 is shown in Fig.1, which shows a typical IV shape, with an evident hysteresis loop at p/p0=0.4−1.0 implying the presence of mesoporous structure in the sample. The BJH pore size distribution curves of LMC(x)-y are shown in Fig.2, from which it can be observed that the BJH pore size distribution is centered in the range of 3.4−4.4 nm, indicating that the multicomponent, mesoporous mixed-oxide catalysts with the uni-
1.2 Characterization of catalysts and evaluation of catalytic performance The measurement of the specific surface area (SBET) and the pore diameter distribution was done on a SORPTOMATIC 1990 SERIES apparatus (Thermo-Finnigan, Italy) by using the nitrogen adsorption/desorption method. The samples were pretreated in vacuum at 320 ℃ for 8 h before experiments. The specific surface area was determined from the linear part of the BET curve (p/p0=0.089−0.297). The pore diameter distribution was calculated from the desorption branch of N2 adsorption/desorption isotherms using the Barrett-JoynerHalenda (BJH) formula. XRD patterns were recorded on an X′pert Pro diffractometer (PANAlytical, Netherland) with a rotating anode using the
Fig.1 Nitrogen adsorption/desorption isotherms of the LMC(0.5)-500 sample LMC(0.5)-500 is La-Mn-Ce-O catalyst prepared by citric acid complexation-organic template decomposition (CAC-OTD) method, where the ratio of (nLa+nMn)/(nLa+nMn+nCe) is 0.5 and the calcination temperature is 500 ℃; nLa/nMn is 1.
Yong Liu et al. / Acta Physico-Chimica Sinica, 2007, 23(5): 641−646
form texture have been successfully prepared by using CAC-OTD method. Fig.2a and Fig.2b show the BJH pore diameter distributions of the LMC(0.5)-y and LMC(x)-500 samples, respectively. From Fig.2a, it can be observed that when the calcination temperature is increased from 400 ℃ to 600 ℃, the pore size distributions are slightly broadened, but the range for the most probable pore diameter varies slightly, and the mesoporous structures are kept in a good condition, indicating that the catalyst has good thermal stability. At the same calcination temperature of 500 ℃, the pore size distributions of LMC(0.3)-500 and LMC(0.7)-500 are broader than those of LMC(0.5)-500, and the symmetry of the former is not as good as that of the latter, suggesting that the pore size distribution is influenced to certain extent by the atomic ratio of the sample. The specific surface area and the pore structure data of the samples are summarized in Table 1. It can be observed that the tendency of specific surface areas of LMC(x)-500 samples to change is not the same as that of the atomic ratios, the sample LMC(0.5)-500 has the highest surface area. With the increase in the calcination temperature (400−800 ℃), the specific surface areas and the pore volumes of LMC(0.5)-y samples decrease gradually, but the sample calcined at 600 ℃ still has the surface area of 71.2 m2·g−1 and the pore diameter remains almost constant. Up to 800 ℃, the specific surface area of the sample decreases to 25.3 m2·g−1, implying the collapse of the mesoporous structure. However, the sample prepared by using coprecipitation method and calcined at 500 ℃ only shows a specific surface area of 33.2 m2·g−1 and a very small pore volume. In summary, the samples prepared by CAC-OTD method have high specific surface areas and good thermal sta-
Table 1 Surface area and pore texture data of LMC(x)-y catalysts Sample LMC(0.3)-500 LMC(0.5)-500 LMC(0.7)-500 LMC(0.5)-400 LMC(0.5)-600 LMC(0.5)-800 LMC(0.5)-500CP
SBET (m2·g−1) 87.2 95.3 82.1 103.5 71.2 25.3 33.2
Pore volume (cm3·g−1) 0.136 0.142 0.164 0.157 0.115 0.048 0.075
Pore diameter (nm) 3.74 3.80 3.86 3.68 3.87 − −
bility, and the pore size distributions are mainly influenced by the calcination temperature and atomic ratios of the samples. 2.2
XRD
XRD patterns of LMC(x)-y are shown in Fig.3. From Fig.3, it can be observed that for all the samples with different calcination temperatures and atomic ratios, the dominant diffraction peaks are the characteristic of CeO2 (cerianite, JCPDS #43-1002), with no detection of La2O3 or manganese oxide phases by XRD. It was reported that strong interaction could occur between Mn and Ce oxides, the manganese species with smaller radius can enter into the cerianite lattice easily, resulting in the formation of Mn-Ce solid solution, thus enhancing the dispersion of manganese species and making its detection difficult[12]. The diffraction peaks of CeO2 do not shift evidently because of the presence of same amount of manganese in these samples, so it is difficult to confirm the formation of Mn-Ce solid solution (Fig.3a). With the increase in manganese content, a progressive shift of the diffraction peaks of CeO2 to higher Bragg angles was observed, indicating that part of
Fig.2 BJH pore diameter distribution of the samples
Fig.3 XRD patterns of the samples
(a) LMC(0.5)-y; (b) LMC(x)-500
(a) LMC(0.5)-y and LMC(0.5)-500CP; (b) LMC(x)-500
Yong Liu et al. / Acta Physico-Chimica Sinica, 2007, 23(5): 641−646
manganese species has entered into the cerianite lattice, which has caused an expansion of its unit cell and a little decrease in the interplanar d spacing (Fig.3b). But it is worth noting that, in contrast to the normative diffraction peaks of CeO2 (2θ≈33.3°, 55.7°, and 66.5°), the diffraction peaks of CeO2 in all samples still shift to lower angles, which may have resulted from the entering of La3+ with larger radius (0.119 nm) into the crystal lattice of CeO2 (the radius of Ce4+ is 0.097 nm), forming the La-Ce oxide solid solution[13]. After La3+ enters into the crystal lattice of CeO2, because of the lower oxidation state of La3+ than that of Ce4+, some oxygen vacancies are generated to maintain the electron neutrality, which is beneficial to the promotion of oxygen mobility and redox properties of the samples. It is also reported that the introduction of smaller Mn3+ or Mn4+ cations into the crystal lattice of CeO2 can generate lattice defects throughout the matter[14,15], which in turn results in a remarkable increase in the oxygen mobility and diffusion in the lattice. During the reaction process, Ce could provide the active oxygen species to Mn oxide, thus enhancing the oxidation ability of Mn oxide. For La-Mn-Ce-O catalyst prepared by using coprecipitation method, the diffraction peaks of CeO2 can only be detected as well in the XRD patterns, but the peaks are weaker and broader, indicating the smaller crystal size of CeO2 in this sample than in others. However, the specific surface area and the pore volume of this sample are much lower than those of the samples prepared by the OTD method (see Table 1), because no good mesoporous pore structure is formed in the sample prepared by coprecipitation method. 2.3
XPS
Because the information on Mn species cannot be obtained from XRD, the oxidation state and surface atomic percentage of Mn species were characterized by XPS. The binding energy spectra of Mn 2p for the samples are shown in Fig.4. According to the standard binding energy value of Mn 2p3/2[16], it can be considered that the surface Mn species is close to Mn2O3, probably a small amount of β-MnO2 is present as well. It is also reported[12] that there exists more than one manganese species in Mn-Ce-O system, and the interaction between Mn and Ce enhances the transfer of electrons from Ce to Mn, changing the surface manganese species toward lower oxidation state and increasing the abundance of surface Ce4+ species.
Fig.4 XPS spectra of Mn element in the samples (1) LMC(0.3)-500, (2) LMC(0.5)-500, (3) LMC(0.7)-500, (4) LMC(0.5)-400, (5) LMC(0.5)-600
It is worth noting that, for LMC(0.3)-500 and LMC(0.5)-500 samples, the evident shake-up satellites are present between the binding energy peaks of Mn 2p1/2 and Mn 2p3/2. The shake-up satellite is produced as follows: in the photoelectron emission process, because the electron vacancy sites are formed in the inner-shell layer, the electronic potential of the atomic center changes suddenly, causing the transition of valence electron to a higher binding energy state, and forming the shake-up of electron. The formation of shake-up is possibly correlated with the interaction between Mn atom and Ce atom, and the intensity of shake-up satellite is influenced by the proportion of Mn/Ce atomic ratio and the calcination temperatures of the samples. The shake-up satellite of LMC(0.5)500 is strongest, suggesting the remarkable interaction and the optimal synergy effect between Mn and Ce, accordingly showing the best oxidation activity (referring the activity evaluation results). The binding energy values and surface atomic percentages are listed in Table 2. From this table, as the atomic ratios increase from 0.3 to 0.7, the surface atomic percentage of Mn increases first and decreases later, this is not proportional to the bulk content of Mn. The surface atomic percentage of Mn in LMC(0.5)-500 is largest (6.5%), and the surface atomic ratio of La, Mn, and Ce is 1:1:1.6, similar to the raw material atomic ratio of 1:1:2. Furthermore, from Mn 2p binding energy peak in XPS spectra, it can be seen that the peak intensity of the sample with atomic ratio of 0.5 is strongest. For the samples with the same atomic ratio of 0.5 but calcined at dif-
Table 2 XPS results of LMC(x)-y samples Sample
EB/eV
Surface atomic percentage (%)
O 1s
Mn 2p3/2
La 3d5/2
Ce 3d5/2
O
Mn
La
LMC(0.3)-500
628.9
641.7
832.8
881.7
83.6
2.8
3.7
Ce 9.9
LMC(0.5)-500
628.9
641.2
833.6
881.7
76.8
6.5
6.5
10.2
LMC(0.7)-500
628.9
641.2
833.2
881.7
83.1
4.8
7.9
4.2
LMC(0.5)-400
628.9
641.2
833.6
881.7
86.3
4.1
4.8
4.8
LMC(0.5)-600
628.9
641.2
833.5
881.7
84.9
4.9
4.2
6.0
Yong Liu et al. / Acta Physico-Chimica Sinica, 2007, 23(5): 641−646
ferent temperatures of 400 ℃ and 600 ℃, the surface atomic percentages of Mn are only 4.1% and 4.9%, respectively. With the atomic ratios increasing from 0.3 to 0.7, the surface atomic percentage of Ce increases first, and decreases later. The reason for the former is the promoting effect of La to the dispersion of Ce species, whereas for the latter, it is the decrease in bulk Ce content. However, the surface atomic percentage of La increases with the increase in La content of the raw material. From XPS results, it can be observed that for these mesoporous mixed-oxide catalysts, the surface atomic percentages of La, Mn, and Ce are remarkably influenced by the calcination temperature of the samples and the atomic ratio between different elements in raw materials. 2.4
H2-TPR
The H2-TPR profiles for LMC(x)-y samples and pure MnO2, Mn2O3, and CeO2 are presented in Fig.5. From Fig.5, it can be observed that the profiles are featured by two main reduction peaks for MnO2 and Mn2O3, with a slight shoulder for MnO2 at the onset of reduction, similar to the reduction of pure manganese oxide reported by Kapteijn et al[17]. The reduction of Mn2O3 is consistent with the literature[18], undergoing the formation of the intermediate product Mn3O4. The TPR profile of pure CeO2 shows two reduction peaks at 501 ℃ and 746 ℃ similar to the reported results[19,20], and these two peaks are attributed to the reduction of surface and bulk lattice oxygen, respectively. Compared with pure oxide, the reduction peaks of the samples shift to lower temperature, and the main reduction peaks of the samples with different atomic ratios and calcined at the same temperature show great difference. At the temperature range from 200 ℃ to 500 ℃, the intensities and areas of the reduction peaks increase with the increase in Mn content, accordingly presuming that these peaks should mainly correspond to the reduction of manganese oxide species, and the reduction peaks of CeO2 are likely to be covered because of its much weaker intensity. The reduction of manganese oxides
reported by other researchers is also in this temperature region[14]. Compared to pure manganese oxides, the reduction peaks of the samples systematically shift to lower temperatures, indicating that, on the one hand, Mn oxides with higher dispersion and smaller crystal size are easily reduced; on the other hand, Ce addition can change the redox properties of Mn oxide and make it easier to be reduced by the interaction between the two oxides. For LMC(0.5)-500 sample, the reduction peaks shift to the lowest temperature (see Fig.5(7)), compared with all the samples with different atomic ratios and those prepared by using coprecipitation method, which suggests sufficient interaction between La and Ce, as well as between Mn and Ce in this sample. Such interaction improves the redox properties of Mn and Ce, so this sample is more reducible. It can also be observed from Fig.5 that there is a small reduction peak near 205 ℃, maybe due to a few of active oxygen species (O−2, O−, etc.) existing on the surface of the samples, or the presence of Mn4+ compensating the oxygen vacancy sites resulting from the interaction between Mn and Ce cation[21]. But this peak is not present in the sample prepared by using coprecipitation method, possibly because the interaction between Mn and Ce is so weak that the amount of active oxygen species is too small in this sample. It is worth noting that there is a strong reduction peak at 592 ℃ for LMC(0.7)-500 (see Fig.5(4)), whereas in other samples the peaks are very weak and broad. By analyzing the atomic percentages of Mn and Ce, it can be known that the Mn content is highest and Ce content is lowest in LMC(0.7)-500 sample, accompanied by the strongest peak intensity, whereas in LMC(0.3)-500 sample (Fig.5(5)), Mn content is lowest and Ce content is highest, accompanied by the weakest peak intensity. Hence, it is considered that the peak at this temperature mainly corresponds to the reduction in Mn species, possibly resulting from the reduction of perovskite LaMnO3 or perovskite-like LaMnO3 because of the higher reduction temperature (the atomic ratios of La/Mn are exactly 1 in all samples). Such species have not been detected by XRD because the calcination temperature is not high enough for the formation of well-defined crystal structure of the complex oxides. The TPR profiles of LMC(0.5)-y samples calcined at different temperature are similar to each other in the shape, but the peak temperatures change slightly. The peak temperature of the sample calcined at 500 ℃ is more than 20 ℃ lower than those of other samples, suggesting the easier activation of Mn–O bond and the stronger ability to participate in the redox recycle of Mn oxides formed at this calcination temperature. 2.5
Fig.5 The TPR profiles of LMC(x)-y samples (1) Mn2O3, (2) MnO2, (3) CeO2, (4) LMC(0.7)-500, (5) LMC(0.3)-500, (6) LMC(0.5)-500CP, (7) LMC(0.5)-500, (8) LMC(0.5)-400, (9) LMC(0.5)-600, (10) LMC(0.5)-800
Evaluation of catalytic activity
The catalytic activities for CO oxidation of LMC(x)-y samples are shown in Fig.6. Comparing LMC(0.5)-500 with LMC(0.5)-500CP, it is found that the light-off temperature T50
Yong Liu et al. / Acta Physico-Chimica Sinica, 2007, 23(5): 641−646
(the reaction temperature for 50% conversion of CO) of the sample obtained by using OTD method is about 50 ℃ lower than that of the sample obtained by using coprecipitation method. The main reasons for the differences in the activities can be described as follows. First, the samples prepared using the OTD method have much larger specific surface areas (95.3 m2·g−1) than the samples prepared using the coprecipitation method (33.2 m2·g−1), thus the former has much higher dispersion of active components and more exposed active sites on the surface. Second, the efficiency for the utilization of inner surface area is promoted by the uniform mesoporous structure, which facilitates the diffusion and transport of the reactants and makes it easier for the contact between the reactants and the active surface sites. Finally, in the samples prepared by the OTD method, there exists very good synergism between different elements, and the stronger interaction between Mn-Ce makes Mn oxide reduce more easily, which gives rise to easier mobility of Mn–O bond (see TPR results), increasing the oxidation activities of the samples. From Fig.6, it can also be observed that the light-off temperature T50 (124 ℃) of LMC(0.5)-500 is approximately 17 ℃ and 24 ℃ lower than that of LMC(0.5)-400 and LMC(0.5)600, respectively. Although the sample calcined at 400 ℃ has the largest specific surface area (103.5 m2·g−1), its activity is not the best. Among LMC(x)-500 samples, the LMC(0.5)-500 also shows the highest activity, and the light-off temperature T50 (124 ℃) is approximately 36 ℃ and 42 ℃ lower than that of LMC(0.3)-500 and LMC(0.7)-500, respectively. Compared with La-Co-Ce-O catalysts reported in our previous study[11], although LMC(0.5)-500 has a relatively lower specific surface area, the light-off temperatures (T50 and T90) of LMC(0.5)-500 are even lower, and the oxidation activity is higher than that of La-Co-Ce-O catalysts with the same atomic ratios. On the basis of XRD, XPS, and TPR results, it can be concluded that the specific surface area is not the main factor that determines the catalytic activity of the samples, but the surface atomic percentages of La, Mn, and Ce, the mobility of Mn–O bond, and the synergism of La-Mn and Mn-Ce remarkably influence the catalytic activity. Obviously, the OTD method is superior to the coprecipitation method.
3
Conclusions
The mesoporous mixed-oxide catalysts La-Mn-Ce-O were successfully prepared by using CAC-OTD method, which showed very uniform mesoporous diameter distribution (3.4− 4.4 nm) and maintains the mesoporous structure very well after calcination at 600 ℃. This method was more advantageous than the traditional coprecipitation method, which could be suggested for the preparation of other mesoporous, nonsilica, multicomponent mixed-oxide catalysts. The structural characterization results of the samples showed that certain La-Ce and Mn-Ce solid solutions were formed, the manganese species had high dispersion, and the oxidation state of Mn was very close to that of Mn2O3. The samples showed novel catalytic activities for CO oxidation. The light-off temperature of the sample with (nLa+nMn)/(nLa+nMn+nCe) ratio of 0.5 and calcined at 500 ℃ was approximately 50 ℃ lower than that of the sample prepared by traditional coprecipitation method; it also possessed high thermal stability. However, the CO oxidation activities for samples were related not only to the specific surface areas and the surface atomic ratios of active components, more importantly, but also to the synergism between MnOx and CeO2 that functioned in storing and transferring oxygen species, and between MnOx and La2O3 that functioned in promoting the dispersion. The results of H2-TPR showed that the reducibility of active manganese oxides was directly related to the activity of the samples, and that the oxidation property of the catalysts was more strongly depended on the mobility of metal–oxygen bond (Mn–O).
References 1 Westerholm, R.; Christensen, A.; Rosén, Å. Atmospheric Environment, 1996, 30(20): 3529 2 Meng, M.; Zha, Y. Q.; Luo, J. Y.; Hu, T. D.; Xie, Y. N.; Liu, T.; Zhang, J. Appl. Catal. A, 2006, 301(2): 145 3 Hasen, K. K.; Skou, E. M.; Christensen, H.; Turek, T. J. Catal., 2001, 199(1): 132 4 Kaliaguine, S.; van Neste, A.; Szabo, V.; Gallot, J. E.; Bassir, M.; Muzychuk, R. Appl. Catal. A, 2001, 209(1−2): 345 5 Chen, Y. Y.; Han, X. W.; Bao, X. H. Chin. J. Catal., 2005, 26(5): 412 6 Kosslick, H.; Lishcke, G.; Landmesser, H.; Parlitz, B.; Storek, W.; Fricke, R. J. Catal., 1998, 176(1): 102 7 Zhang, Z. R.; Suo, J. S.; Zhang, X. M.; Li, S. B. Appl. Catal. A, 1999, 179(1−2): 11 8 Zhai, S. R.; Pu, M.; Gong, Y. J.; Zhang, Y.; Wu, D.; Sun, Y. H. Acta Phys. -Chim. Sin., 2002, 18(10): 911 9 Takahashi, R.; Sato, S.; Sodesawa, T.; Kawakita, M.; Ogura, K. J. Phys. Chem. B, 2000, 104(51): 12184 10 Zou, Z. Q.; Meng, M.; Luo, J. Y.; Zha, Y. Q.; Xie, Y. N.; Hu, T. D.; Liu, T. J. Mol. Catal. A, 2006, 249: 240 11 Luo, J. Y.; Meng, M.; Qian, Y.; Zha, Y. Q. Chin. J. Catal., 2006,
Fig.6 The CO conversion curves over LMC(x)-y samples
27(6): 471
Yong Liu et al. / Acta Physico-Chimica Sinica, 2007, 23(5): 641−646
12 Chen, H. Y.; Sayari, A.; Adnot, A.; Larachï, F. Appl. Catal. B, 2001, 32: 195 13 Ferrandon, M.; Ferrand, B.; Björnbom, E.; Klingstedt, F.; Neyestanaki, A. K.; Karhu, H.; Väyrynen, I. J. J. Catal., 2001, 202(2): 354 14 Terribile, D.; Trovarelli, A.; de Leitenburg, C.; Primavera, A.; Dolcetti, G. Catal. Today, 1999, 47(2): 133 15 Imamura, S.; Shono, M.; Okamoto, N.; Hamada, A.; Ishida, S. Appl. Catal. A, 1996, 142: 279 16 Wang, J. Q.; Wu, W. H.; Feng, D. M. The introduction to electron spectroscopy (XPS/XAES/UPS). Beijing, China: National De-
fense
Industry Press, 1992: 537
17 Kapteijn, F.; Singoredjo, L.; Andreini, A.; Moulijn, J. A. Appl. Catal., 1994, 3: 173 18 Trawczyński, J.; Bielak, B.; Miśta, W. Appl. Catal. B, 2005, 55(4): 277 19 Jobbágy, M.; Mariňo, F.; Schönbrod, B.; Baronetti, G.; Laborde, M. Chem. Mater., 2006, 18: 1945 20 Yao, H. C.; Yu Yao, Y. F. J. Catal., 1984, 86: 254 21 Ponce, S.; Peña, M. A.; Fierro, J. L. G. Appl. Catal. B, 2000, 24(3−4): 193
Cite this article as: Acta Phys. -Chim. Sin., 2007, 23(5): 641−646.
ARTICLE
Preparation and Characterization of the Multicomponent Mesoporous Mixed Oxide Catalysts La-Mn-Ce-O Yong Liu,
Ming Meng*,
Jinsong Yao,
Yuqing Zha
Department of Catalysis Science and Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China
Abstract:
A series of mesoporous multicomponent mixed oxide catalysts La-Mn-Ce-O with various ratios of
(nLa+nMn)/(nLa+nMn+nCe) were prepared by citric acid complexation-organic template decomposition (CAC-OTD) method. For comparison, the sample with the same composition was also prepared by conventional coprecipitation method. The results of N2 adsorption/desorption showed that the samples prepared by using CAC-OTD method possessed relatively large specific surface area and uniform pore diameter distribution (3.4−4.4 nm). The results of X-ray diffraction (XRD) identified the formation of La-Ce and Mn-Ce solid solution. Mn species were not detected by XRD. The results of X-ray photoelectron spectroscopy (XPS) showed that there was a strong electronic state interaction between Mn and Ce species, resulting in the formation of Mn 2p shake-up peak. Such interaction enhanced the transfer of oxygen species from Ce to Mn oxide and therefore increased the redox activity of the samples. The sample with the strongest shake-up peak showed the highest oxidation activity. The results of temperature programmed reduction (TPR) showed that the manganese species in the samples prepared by using CAC-OTD method were easier to be reduced, which was relevant to the oxidation activity of the catalysts. The results of activity evaluation showed that the light-off temperature of the LMC(0.5)-500 sample was about 50 ℃ lower than that of the sample prepared by using coprecipitation method. The samples prepared by using CAC-OTD method showed good thermal stability. Key Words:
Multicomponent mesoporous catalyst; Preparation; Citric acid complexation; Organic template; Structural
characterization
With the regulation for environmental protection becoming increasingly stringent, the requirement for three-way catalysts is becoming stricter. During the cold start of engines, a great deal of CO and hydrocarbons are present in the exhaust, most of which (approximately 50%−80%) are released into the air without effective purification because of the low temperature of the catalyst bed and the resulting low catalytic efficiency[1]. Therefore, it is necessary to explore the low-temperature oxidation catalysts with high performance. Although noble-metal catalysts possess good activity, it is very expensive. The transition metal oxide catalysts containing Co, Cr, Mn, and so forth are active for many catalytic reactions, such as the oxidation of CO and hydrocarbons, and the NOx decomposition and selective reduction[2−4].
Because manganese possesses the changeable structure of d electrons, it can easily change the oxidation state, enhance the redox recycle, and consequently give high oxidation activity. However, the manganese oxides prepared by using normal method and calcined at high temperature usually show low specific surface area and broad pore size distribution, which restrict the low-temperature oxidation activity. In recent years, the Si-containing mesoporous materials prepared by using the surfactant as template display large specific surface areas, narrow pore diameter, and uniform pore structure, which greatly enhance the transferring velocity of the reactant and the product. Therefore, these materials have been extensively used in many aspects, such as catalysis, adsorption, separation, and host-guest chemistry[5−9].
Received: October 8, 2006; Revised: December 11, 2006. * Corresponding author. Email: [email protected]; Tel: +8622-27892275 The project was supported by the National Hi-Tech Research and Development Program (“863” Program) of China (2006AA06Z348) and the National Natural Science Foundation of China (20676097). Copyright © 2007, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Yong Liu et al. / Acta Physico-Chimica Sinica, 2007, 23(5): 641−646
In the present study, based on the method of preparing mesoporous, Si-containing molecular sieves, the citric acid complexation (CAC) method for nanomaterials and the organic template decomposition (OTD) method for mesoporous materials are combined to prepare a series of multicomponent, mesoporous, Mn-containing mixed-oxide catalysts[10,11]. Several techniques were used for the structural characterization of La-Mn-Ce-O catalysts, and their catalytic properties were investigated using CO oxidation as model reaction.
1 1.1
Experimental Catalyst preparation
First, the La-Mn citrate complex precursors were prepared from lanthanum nitrate La(NO3)3·6H2O, manganese acetate Mn(CH3COO)2·4H2O, and citric acid. The cetyltrimethylammonium bromide (CTAB) was first dissolved in the distilled water and HCl solution with mass fraction of 10%. The La-Mn citrate complex precursor and cerium nitrate Ce(NO3)·6H2O were then simultaneously added to the above solution with vigorous stirring, resulting in a clear homogeneous solution. The solution was stirred for 2 h. Later, 2 mol·L−1 NaOH was quickly added to the solution with vigorous stirring. The pH was adjusted to 9.7±0.3. The resulting gel mixture was stirred for 6 h at room temperature, subsequently heated to adequate temperature, and maintained at this temperature for 24 h to increase the degree of condensation. Finally, the solid product was filtered, washed with ethanol, dried in air at 110 ℃, and calcined in air at 500 ℃ for 8 h. This catalyst is denoted as LMC(x)-y, where x and y represent the atomic ratio of (nLa+nMn)/(nLa+nMn+nCe) (nLa/nMn=1) and the calcination temperature (500 ℃), respectively. A series of mixed-oxide catalysts with different atomic ratios were prepared by changing the amount of cerium nitrate. For comparison, La-Mn-Ce-O mixed-oxide catalyst with a composition of x=0.5 and calcination temperature of 500 ℃ was prepared by using the conventional coprecipitation method, which is denoted as LMC(0.5)-500 CP.
Co Kα as radiation source (λ＝0.17902 nm), the tube voltage of 40 kV, and the tube electric current of 40 mA. The data from 2θ range of 20° to 110° were collected with the step size of 0.03°. XPS analysis was carried out in a PHI-1600 ESCA SYSTEM spectrometer using Mg Kα as X-ray source (EB=1653.6 eV) under a residual pressure of 5.0×10−8 Pa. The error of the binding energy was ±0.2 eV, and the contaminated carbon (C 1s, EB=284.6 eV) was used as a standard for binding energy calibration. H2-TPR measurement was carried out on a TPDRO 1100 SERIES apparatus supplied by Thermo-Finnigan. Each time, 30 mg of the sample was heated from room temperature to 900 ℃ at a rate of 10 ℃·min−1. A gas mixture of H2 and N2 (φ(H2)=5%) was used as reductant with a flow rate of 20 mL·min−1. The catalytic activities of the samples for CO oxidation were measured in a fixed-bed quartz tubular reactor (inner diameter was 8 mm) mounted in a tube furnace. The reactant was a gas mixture, which contained 1.3% CO and 4% O2 (volume fraction), balanced with pure N2. The gas hourly space velocity was 11250 h−1. The effluent gas from the reactor was analyzed by using a gas chromatograph (SP-3430, BFS, China) equipped with a TCD detector.
2 2.1
Results and discussion N2 adsorption/desorption
The nitrogen adsorption/desorption isotherm of LMC(0.5)500 is shown in Fig.1, which shows a typical IV shape, with an evident hysteresis loop at p/p0=0.4−1.0 implying the presence of mesoporous structure in the sample. The BJH pore size distribution curves of LMC(x)-y are shown in Fig.2, from which it can be observed that the BJH pore size distribution is centered in the range of 3.4−4.4 nm, indicating that the multicomponent, mesoporous mixed-oxide catalysts with the uni-
1.2 Characterization of catalysts and evaluation of catalytic performance The measurement of the specific surface area (SBET) and the pore diameter distribution was done on a SORPTOMATIC 1990 SERIES apparatus (Thermo-Finnigan, Italy) by using the nitrogen adsorption/desorption method. The samples were pretreated in vacuum at 320 ℃ for 8 h before experiments. The specific surface area was determined from the linear part of the BET curve (p/p0=0.089−0.297). The pore diameter distribution was calculated from the desorption branch of N2 adsorption/desorption isotherms using the Barrett-JoynerHalenda (BJH) formula. XRD patterns were recorded on an X′pert Pro diffractometer (PANAlytical, Netherland) with a rotating anode using the
Fig.1 Nitrogen adsorption/desorption isotherms of the LMC(0.5)-500 sample LMC(0.5)-500 is La-Mn-Ce-O catalyst prepared by citric acid complexation-organic template decomposition (CAC-OTD) method, where the ratio of (nLa+nMn)/(nLa+nMn+nCe) is 0.5 and the calcination temperature is 500 ℃; nLa/nMn is 1.
Yong Liu et al. / Acta Physico-Chimica Sinica, 2007, 23(5): 641−646
form texture have been successfully prepared by using CAC-OTD method. Fig.2a and Fig.2b show the BJH pore diameter distributions of the LMC(0.5)-y and LMC(x)-500 samples, respectively. From Fig.2a, it can be observed that when the calcination temperature is increased from 400 ℃ to 600 ℃, the pore size distributions are slightly broadened, but the range for the most probable pore diameter varies slightly, and the mesoporous structures are kept in a good condition, indicating that the catalyst has good thermal stability. At the same calcination temperature of 500 ℃, the pore size distributions of LMC(0.3)-500 and LMC(0.7)-500 are broader than those of LMC(0.5)-500, and the symmetry of the former is not as good as that of the latter, suggesting that the pore size distribution is influenced to certain extent by the atomic ratio of the sample. The specific surface area and the pore structure data of the samples are summarized in Table 1. It can be observed that the tendency of specific surface areas of LMC(x)-500 samples to change is not the same as that of the atomic ratios, the sample LMC(0.5)-500 has the highest surface area. With the increase in the calcination temperature (400−800 ℃), the specific surface areas and the pore volumes of LMC(0.5)-y samples decrease gradually, but the sample calcined at 600 ℃ still has the surface area of 71.2 m2·g−1 and the pore diameter remains almost constant. Up to 800 ℃, the specific surface area of the sample decreases to 25.3 m2·g−1, implying the collapse of the mesoporous structure. However, the sample prepared by using coprecipitation method and calcined at 500 ℃ only shows a specific surface area of 33.2 m2·g−1 and a very small pore volume. In summary, the samples prepared by CAC-OTD method have high specific surface areas and good thermal sta-
Table 1 Surface area and pore texture data of LMC(x)-y catalysts Sample LMC(0.3)-500 LMC(0.5)-500 LMC(0.7)-500 LMC(0.5)-400 LMC(0.5)-600 LMC(0.5)-800 LMC(0.5)-500CP
SBET (m2·g−1) 87.2 95.3 82.1 103.5 71.2 25.3 33.2
Pore volume (cm3·g−1) 0.136 0.142 0.164 0.157 0.115 0.048 0.075
Pore diameter (nm) 3.74 3.80 3.86 3.68 3.87 − −
bility, and the pore size distributions are mainly influenced by the calcination temperature and atomic ratios of the samples. 2.2
XRD
XRD patterns of LMC(x)-y are shown in Fig.3. From Fig.3, it can be observed that for all the samples with different calcination temperatures and atomic ratios, the dominant diffraction peaks are the characteristic of CeO2 (cerianite, JCPDS #43-1002), with no detection of La2O3 or manganese oxide phases by XRD. It was reported that strong interaction could occur between Mn and Ce oxides, the manganese species with smaller radius can enter into the cerianite lattice easily, resulting in the formation of Mn-Ce solid solution, thus enhancing the dispersion of manganese species and making its detection difficult[12]. The diffraction peaks of CeO2 do not shift evidently because of the presence of same amount of manganese in these samples, so it is difficult to confirm the formation of Mn-Ce solid solution (Fig.3a). With the increase in manganese content, a progressive shift of the diffraction peaks of CeO2 to higher Bragg angles was observed, indicating that part of
Fig.2 BJH pore diameter distribution of the samples
Fig.3 XRD patterns of the samples
(a) LMC(0.5)-y; (b) LMC(x)-500
(a) LMC(0.5)-y and LMC(0.5)-500CP; (b) LMC(x)-500
Yong Liu et al. / Acta Physico-Chimica Sinica, 2007, 23(5): 641−646
manganese species has entered into the cerianite lattice, which has caused an expansion of its unit cell and a little decrease in the interplanar d spacing (Fig.3b). But it is worth noting that, in contrast to the normative diffraction peaks of CeO2 (2θ≈33.3°, 55.7°, and 66.5°), the diffraction peaks of CeO2 in all samples still shift to lower angles, which may have resulted from the entering of La3+ with larger radius (0.119 nm) into the crystal lattice of CeO2 (the radius of Ce4+ is 0.097 nm), forming the La-Ce oxide solid solution[13]. After La3+ enters into the crystal lattice of CeO2, because of the lower oxidation state of La3+ than that of Ce4+, some oxygen vacancies are generated to maintain the electron neutrality, which is beneficial to the promotion of oxygen mobility and redox properties of the samples. It is also reported that the introduction of smaller Mn3+ or Mn4+ cations into the crystal lattice of CeO2 can generate lattice defects throughout the matter[14,15], which in turn results in a remarkable increase in the oxygen mobility and diffusion in the lattice. During the reaction process, Ce could provide the active oxygen species to Mn oxide, thus enhancing the oxidation ability of Mn oxide. For La-Mn-Ce-O catalyst prepared by using coprecipitation method, the diffraction peaks of CeO2 can only be detected as well in the XRD patterns, but the peaks are weaker and broader, indicating the smaller crystal size of CeO2 in this sample than in others. However, the specific surface area and the pore volume of this sample are much lower than those of the samples prepared by the OTD method (see Table 1), because no good mesoporous pore structure is formed in the sample prepared by coprecipitation method. 2.3
XPS
Because the information on Mn species cannot be obtained from XRD, the oxidation state and surface atomic percentage of Mn species were characterized by XPS. The binding energy spectra of Mn 2p for the samples are shown in Fig.4. According to the standard binding energy value of Mn 2p3/2[16], it can be considered that the surface Mn species is close to Mn2O3, probably a small amount of β-MnO2 is present as well. It is also reported[12] that there exists more than one manganese species in Mn-Ce-O system, and the interaction between Mn and Ce enhances the transfer of electrons from Ce to Mn, changing the surface manganese species toward lower oxidation state and increasing the abundance of surface Ce4+ species.
Fig.4 XPS spectra of Mn element in the samples (1) LMC(0.3)-500, (2) LMC(0.5)-500, (3) LMC(0.7)-500, (4) LMC(0.5)-400, (5) LMC(0.5)-600
It is worth noting that, for LMC(0.3)-500 and LMC(0.5)-500 samples, the evident shake-up satellites are present between the binding energy peaks of Mn 2p1/2 and Mn 2p3/2. The shake-up satellite is produced as follows: in the photoelectron emission process, because the electron vacancy sites are formed in the inner-shell layer, the electronic potential of the atomic center changes suddenly, causing the transition of valence electron to a higher binding energy state, and forming the shake-up of electron. The formation of shake-up is possibly correlated with the interaction between Mn atom and Ce atom, and the intensity of shake-up satellite is influenced by the proportion of Mn/Ce atomic ratio and the calcination temperatures of the samples. The shake-up satellite of LMC(0.5)500 is strongest, suggesting the remarkable interaction and the optimal synergy effect between Mn and Ce, accordingly showing the best oxidation activity (referring the activity evaluation results). The binding energy values and surface atomic percentages are listed in Table 2. From this table, as the atomic ratios increase from 0.3 to 0.7, the surface atomic percentage of Mn increases first and decreases later, this is not proportional to the bulk content of Mn. The surface atomic percentage of Mn in LMC(0.5)-500 is largest (6.5%), and the surface atomic ratio of La, Mn, and Ce is 1:1:1.6, similar to the raw material atomic ratio of 1:1:2. Furthermore, from Mn 2p binding energy peak in XPS spectra, it can be seen that the peak intensity of the sample with atomic ratio of 0.5 is strongest. For the samples with the same atomic ratio of 0.5 but calcined at dif-
Table 2 XPS results of LMC(x)-y samples Sample
EB/eV
Surface atomic percentage (%)
O 1s
Mn 2p3/2
La 3d5/2
Ce 3d5/2
O
Mn
La
LMC(0.3)-500
628.9
641.7
832.8
881.7
83.6
2.8
3.7
Ce 9.9
LMC(0.5)-500
628.9
641.2
833.6
881.7
76.8
6.5
6.5
10.2
LMC(0.7)-500
628.9
641.2
833.2
881.7
83.1
4.8
7.9
4.2
LMC(0.5)-400
628.9
641.2
833.6
881.7
86.3
4.1
4.8
4.8
LMC(0.5)-600
628.9
641.2
833.5
881.7
84.9
4.9
4.2
6.0
Yong Liu et al. / Acta Physico-Chimica Sinica, 2007, 23(5): 641−646
ferent temperatures of 400 ℃ and 600 ℃, the surface atomic percentages of Mn are only 4.1% and 4.9%, respectively. With the atomic ratios increasing from 0.3 to 0.7, the surface atomic percentage of Ce increases first, and decreases later. The reason for the former is the promoting effect of La to the dispersion of Ce species, whereas for the latter, it is the decrease in bulk Ce content. However, the surface atomic percentage of La increases with the increase in La content of the raw material. From XPS results, it can be observed that for these mesoporous mixed-oxide catalysts, the surface atomic percentages of La, Mn, and Ce are remarkably influenced by the calcination temperature of the samples and the atomic ratio between different elements in raw materials. 2.4
H2-TPR
The H2-TPR profiles for LMC(x)-y samples and pure MnO2, Mn2O3, and CeO2 are presented in Fig.5. From Fig.5, it can be observed that the profiles are featured by two main reduction peaks for MnO2 and Mn2O3, with a slight shoulder for MnO2 at the onset of reduction, similar to the reduction of pure manganese oxide reported by Kapteijn et al[17]. The reduction of Mn2O3 is consistent with the literature[18], undergoing the formation of the intermediate product Mn3O4. The TPR profile of pure CeO2 shows two reduction peaks at 501 ℃ and 746 ℃ similar to the reported results[19,20], and these two peaks are attributed to the reduction of surface and bulk lattice oxygen, respectively. Compared with pure oxide, the reduction peaks of the samples shift to lower temperature, and the main reduction peaks of the samples with different atomic ratios and calcined at the same temperature show great difference. At the temperature range from 200 ℃ to 500 ℃, the intensities and areas of the reduction peaks increase with the increase in Mn content, accordingly presuming that these peaks should mainly correspond to the reduction of manganese oxide species, and the reduction peaks of CeO2 are likely to be covered because of its much weaker intensity. The reduction of manganese oxides
reported by other researchers is also in this temperature region[14]. Compared to pure manganese oxides, the reduction peaks of the samples systematically shift to lower temperatures, indicating that, on the one hand, Mn oxides with higher dispersion and smaller crystal size are easily reduced; on the other hand, Ce addition can change the redox properties of Mn oxide and make it easier to be reduced by the interaction between the two oxides. For LMC(0.5)-500 sample, the reduction peaks shift to the lowest temperature (see Fig.5(7)), compared with all the samples with different atomic ratios and those prepared by using coprecipitation method, which suggests sufficient interaction between La and Ce, as well as between Mn and Ce in this sample. Such interaction improves the redox properties of Mn and Ce, so this sample is more reducible. It can also be observed from Fig.5 that there is a small reduction peak near 205 ℃, maybe due to a few of active oxygen species (O−2, O−, etc.) existing on the surface of the samples, or the presence of Mn4+ compensating the oxygen vacancy sites resulting from the interaction between Mn and Ce cation[21]. But this peak is not present in the sample prepared by using coprecipitation method, possibly because the interaction between Mn and Ce is so weak that the amount of active oxygen species is too small in this sample. It is worth noting that there is a strong reduction peak at 592 ℃ for LMC(0.7)-500 (see Fig.5(4)), whereas in other samples the peaks are very weak and broad. By analyzing the atomic percentages of Mn and Ce, it can be known that the Mn content is highest and Ce content is lowest in LMC(0.7)-500 sample, accompanied by the strongest peak intensity, whereas in LMC(0.3)-500 sample (Fig.5(5)), Mn content is lowest and Ce content is highest, accompanied by the weakest peak intensity. Hence, it is considered that the peak at this temperature mainly corresponds to the reduction in Mn species, possibly resulting from the reduction of perovskite LaMnO3 or perovskite-like LaMnO3 because of the higher reduction temperature (the atomic ratios of La/Mn are exactly 1 in all samples). Such species have not been detected by XRD because the calcination temperature is not high enough for the formation of well-defined crystal structure of the complex oxides. The TPR profiles of LMC(0.5)-y samples calcined at different temperature are similar to each other in the shape, but the peak temperatures change slightly. The peak temperature of the sample calcined at 500 ℃ is more than 20 ℃ lower than those of other samples, suggesting the easier activation of Mn–O bond and the stronger ability to participate in the redox recycle of Mn oxides formed at this calcination temperature. 2.5
Fig.5 The TPR profiles of LMC(x)-y samples (1) Mn2O3, (2) MnO2, (3) CeO2, (4) LMC(0.7)-500, (5) LMC(0.3)-500, (6) LMC(0.5)-500CP, (7) LMC(0.5)-500, (8) LMC(0.5)-400, (9) LMC(0.5)-600, (10) LMC(0.5)-800
Evaluation of catalytic activity
The catalytic activities for CO oxidation of LMC(x)-y samples are shown in Fig.6. Comparing LMC(0.5)-500 with LMC(0.5)-500CP, it is found that the light-off temperature T50
Yong Liu et al. / Acta Physico-Chimica Sinica, 2007, 23(5): 641−646
(the reaction temperature for 50% conversion of CO) of the sample obtained by using OTD method is about 50 ℃ lower than that of the sample obtained by using coprecipitation method. The main reasons for the differences in the activities can be described as follows. First, the samples prepared using the OTD method have much larger specific surface areas (95.3 m2·g−1) than the samples prepared using the coprecipitation method (33.2 m2·g−1), thus the former has much higher dispersion of active components and more exposed active sites on the surface. Second, the efficiency for the utilization of inner surface area is promoted by the uniform mesoporous structure, which facilitates the diffusion and transport of the reactants and makes it easier for the contact between the reactants and the active surface sites. Finally, in the samples prepared by the OTD method, there exists very good synergism between different elements, and the stronger interaction between Mn-Ce makes Mn oxide reduce more easily, which gives rise to easier mobility of Mn–O bond (see TPR results), increasing the oxidation activities of the samples. From Fig.6, it can also be observed that the light-off temperature T50 (124 ℃) of LMC(0.5)-500 is approximately 17 ℃ and 24 ℃ lower than that of LMC(0.5)-400 and LMC(0.5)600, respectively. Although the sample calcined at 400 ℃ has the largest specific surface area (103.5 m2·g−1), its activity is not the best. Among LMC(x)-500 samples, the LMC(0.5)-500 also shows the highest activity, and the light-off temperature T50 (124 ℃) is approximately 36 ℃ and 42 ℃ lower than that of LMC(0.3)-500 and LMC(0.7)-500, respectively. Compared with La-Co-Ce-O catalysts reported in our previous study[11], although LMC(0.5)-500 has a relatively lower specific surface area, the light-off temperatures (T50 and T90) of LMC(0.5)-500 are even lower, and the oxidation activity is higher than that of La-Co-Ce-O catalysts with the same atomic ratios. On the basis of XRD, XPS, and TPR results, it can be concluded that the specific surface area is not the main factor that determines the catalytic activity of the samples, but the surface atomic percentages of La, Mn, and Ce, the mobility of Mn–O bond, and the synergism of La-Mn and Mn-Ce remarkably influence the catalytic activity. Obviously, the OTD method is superior to the coprecipitation method.
3
Conclusions
The mesoporous mixed-oxide catalysts La-Mn-Ce-O were successfully prepared by using CAC-OTD method, which showed very uniform mesoporous diameter distribution (3.4− 4.4 nm) and maintains the mesoporous structure very well after calcination at 600 ℃. This method was more advantageous than the traditional coprecipitation method, which could be suggested for the preparation of other mesoporous, nonsilica, multicomponent mixed-oxide catalysts. The structural characterization results of the samples showed that certain La-Ce and Mn-Ce solid solutions were formed, the manganese species had high dispersion, and the oxidation state of Mn was very close to that of Mn2O3. The samples showed novel catalytic activities for CO oxidation. The light-off temperature of the sample with (nLa+nMn)/(nLa+nMn+nCe) ratio of 0.5 and calcined at 500 ℃ was approximately 50 ℃ lower than that of the sample prepared by traditional coprecipitation method; it also possessed high thermal stability. However, the CO oxidation activities for samples were related not only to the specific surface areas and the surface atomic ratios of active components, more importantly, but also to the synergism between MnOx and CeO2 that functioned in storing and transferring oxygen species, and between MnOx and La2O3 that functioned in promoting the dispersion. The results of H2-TPR showed that the reducibility of active manganese oxides was directly related to the activity of the samples, and that the oxidation property of the catalysts was more strongly depended on the mobility of metal–oxygen bond (Mn–O).
References 1 Westerholm, R.; Christensen, A.; Rosén, Å. Atmospheric Environment, 1996, 30(20): 3529 2 Meng, M.; Zha, Y. Q.; Luo, J. Y.; Hu, T. D.; Xie, Y. N.; Liu, T.; Zhang, J. Appl. Catal. A, 2006, 301(2): 145 3 Hasen, K. K.; Skou, E. M.; Christensen, H.; Turek, T. J. Catal., 2001, 199(1): 132 4 Kaliaguine, S.; van Neste, A.; Szabo, V.; Gallot, J. E.; Bassir, M.; Muzychuk, R. Appl. Catal. A, 2001, 209(1−2): 345 5 Chen, Y. Y.; Han, X. W.; Bao, X. H. Chin. J. Catal., 2005, 26(5): 412 6 Kosslick, H.; Lishcke, G.; Landmesser, H.; Parlitz, B.; Storek, W.; Fricke, R. J. Catal., 1998, 176(1): 102 7 Zhang, Z. R.; Suo, J. S.; Zhang, X. M.; Li, S. B. Appl. Catal. A, 1999, 179(1−2): 11 8 Zhai, S. R.; Pu, M.; Gong, Y. J.; Zhang, Y.; Wu, D.; Sun, Y. H. Acta Phys. -Chim. Sin., 2002, 18(10): 911 9 Takahashi, R.; Sato, S.; Sodesawa, T.; Kawakita, M.; Ogura, K. J. Phys. Chem. B, 2000, 104(51): 12184 10 Zou, Z. Q.; Meng, M.; Luo, J. Y.; Zha, Y. Q.; Xie, Y. N.; Hu, T. D.; Liu, T. J. Mol. Catal. A, 2006, 249: 240 11 Luo, J. Y.; Meng, M.; Qian, Y.; Zha, Y. Q. Chin. J. Catal., 2006,
Fig.6 The CO conversion curves over LMC(x)-y samples
27(6): 471
Yong Liu et al. / Acta Physico-Chimica Sinica, 2007, 23(5): 641−646
12 Chen, H. Y.; Sayari, A.; Adnot, A.; Larachï, F. Appl. Catal. B, 2001, 32: 195 13 Ferrandon, M.; Ferrand, B.; Björnbom, E.; Klingstedt, F.; Neyestanaki, A. K.; Karhu, H.; Väyrynen, I. J. J. Catal., 2001, 202(2): 354 14 Terribile, D.; Trovarelli, A.; de Leitenburg, C.; Primavera, A.; Dolcetti, G. Catal. Today, 1999, 47(2): 133 15 Imamura, S.; Shono, M.; Okamoto, N.; Hamada, A.; Ishida, S. Appl. Catal. A, 1996, 142: 279 16 Wang, J. Q.; Wu, W. H.; Feng, D. M. The introduction to electron spectroscopy (XPS/XAES/UPS). Beijing, China: National De-
fense
Industry Press, 1992: 537
17 Kapteijn, F.; Singoredjo, L.; Andreini, A.; Moulijn, J. A. Appl. Catal., 1994, 3: 173 18 Trawczyński, J.; Bielak, B.; Miśta, W. Appl. Catal. B, 2005, 55(4): 277 19 Jobbágy, M.; Mariňo, F.; Schönbrod, B.; Baronetti, G.; Laborde, M. Chem. Mater., 2006, 18: 1945 20 Yao, H. C.; Yu Yao, Y. F. J. Catal., 1984, 86: 254 21 Ponce, S.; Peña, M. A.; Fierro, J. L. G. Appl. Catal. B, 2000, 24(3−4): 193