Nengjie
Feng
,
Yang
Wu
,
Jie
Meng
,
Chong
Chen
,
Lei
Wang
*,
Hui
Wan
and
Guofeng
Guan
*
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, PR China. E-mail: wanglei@njtech.edu.cn; guangf@njtech.edu.cn; Tel: +86-25-83587198
First published on 22nd October 2015
Three-dimensionally ordered macroporous (3DOM) La1−xCexFe1−yCoyO3 (x = 0–0.4, y = 0–0.6) perovskite catalysts were successfully prepared by colloidal crystal templating method and employed for soot combustion. The morphology, structure, and redox properties of the catalysts were characterized by XRD, FT-IR, SEM, BET, UV-Vis DRS, XPS, O2-TPD and H2-TPR techniques, and the catalytic activities for soot combustion were evaluated by a Temperature programmed oxidation (TPO) device using NO/O2 as oxidant. Co-substitution at the Fe-site of 3DOM LaFeO3 perovskites can effectively enhance the catalytic activity, however, the 3DOM structure is partly destroyed, which restricts the further improvement of the activity. The appropriate doping of Ce in the La-site can lower the solidification temperature of the precursors, and thus damage of the structure can be avoided. Additionally, the reducibility and surface adsorbed oxygen species of the catalyst are also improved as revealed by H2-TPR, O2-TPD and XPS. Whereas with an excess of Ce substitution, Co3O4 and CeO2 impurities will generate and grow as demonstrated by XRD and FT-IR. Among all the catalysts, 3DOM La0.7Ce0.3Fe0.4Co0.6O3 perovskite with a well-ordered macroporous structure possesses the highest activity for soot combustion.
In recent years, three-dimensionally ordered macroporous (3DOM) perovskites have been reported to show better activity for soot combustion, owing to the interpenetrating well-ordered macroporous structure.15–18 This structure allows the soot particles to enter the inner pores of catalysts without resistance, and thus providing more opportunities for soot particles to reach the active sites. So far, various kinds of 3DOM perovskites have been successfully prepared by the colloidal crystal templating method.14,15,19 Among them, 3DOM LaFeO3 perovskites are widely applied and easy to prepare for the low solidification temperature of Fe.16,20–22 Unfortunately, the activity of LaFeO3 for soot combustion was lower than that of LaCoO3 and LaMnO3.9 The introduction of Co into the 3DOM LaFeO3 can improve the activity obviously, while large amounts of Co will seriously destroy the 3DOM structure during calcination.23 Hence, it is a big challenge to prepare the Fe-based perovskites both with the well-ordered 3DOM structure and the high activity for soot combustion. In order to successfully prepare 3DOM structure oxides, the solidification temperature of metal precursors must be lower than the glass-transition temperature (Tg) of polymer templates.22 The doping of Co into the 3DOM LaFeO3 perovskites can increase the solidification temperature of metal precursor. For this reason, the polymer template will fuse and distort before the solidification of metal precursor and thus the well-ordered 3DOM structure cannot form. Ce is suitable to construct the 3DOM structure for its relatively low solidification temperature.22 And until now, different kinds of Ce-based catalysts have shown good activity for soot removal.24–27 The further substitution of Ce at La-site will lower the average solidification temperature of the precursor, so the metal precursor will solidify before the polymer templates melt and distort, and the destruction of 3DOM structure during calcination may be avoided. However, the substitution of A-site La by Ce in the 3DOM perovskites have rarely been discussed. The appropriate doping of Ce can avoid the destruction of 3DOM structure, meanwhile the redox properties and the catalytic activities of the catalysts will be further improved.28–30 Therefore, 3DOM LaFeO3 perovskites with simultaneous substitution of both A and B-site ions by Ce and Co with well-ordered macroporous structure and high activity for soot removal are attainable.
In this work, a series of Ce/Co simultaneously substituted 3DOM perovskite catalysts La1−xCexFe1−yCoyO3 (x = 0–0.4, y = 0–0.6) were prepared by colloidal crystal templating method. The XRD, FT-IR, SEM, BET, XPS, H2-TPR, O2-TPD and UV-Vis DRS techniques were performed to investigate the influences of Ce/Co-doping on morphology, structure, and redox ability of the catalysts. The catalytic performance for soot combustion was also discussed in detail.
For comparison, La1−xCexFe1−yCoyO3 catalysts without macroporous structure were prepared by the citric acid complexation method.8 Each time, the corresponding nitrate salts were dissolved in deionized water to obtain an aqueous solution of La3+, Ce3+, Co2+, and Fe3+ with the expected stoichiometry. The molar ratio of citric acid to total metal ions was 1.1:
1. The resulting solutions were evaporated to dryness at 80 °C with vigorous stirring. The clear solution gradually turned into sol and finally transformed into gel. At last, the wet gel was dried at 110 °C for 12 h and calcinated at 650 °C for 5 h.
For comparison, soot combustion experiment in the absence of NO and NO oxidation experiment were also carried out. NO oxidation experiment was carried out in a fixed-bed reactor. Each time, 180 mg catalyst and no soot was used for this reaction. The catalyst was placed into the quartz tube (i.d. = 14 mm) and heated from 200 to 650 °C (2 °C min−1) with reactant gas in a rate of 100 mL min−1. Reactant gas compositions were 5% O2 and 500 ppm NO with N2 balanced gas. The outlet NO2 concentration was analyzed by the infrared gas analyser (Infralyt 50).
The XRD patterns of 3DOM La1−xCexFe1−yCoyO3 perovskite catalysts are shown in Fig. 2(a), and the partial enlargement of XRD patterns ranging from 31.5° to 33.5° are shown in Fig. 2(b). All the catalysts prepared by colloidal crystal templating method possess ABO3 perovskite-type structure. It indicates that the calcination condition (650 °C for 5 h) is sufficient to format well-crystallized ABO3 type perovskite structure. For LaFe1−yCoyO3 catalysts, the perovskites exhibit only a single perovskite-type structure regardless of Co substitution level owing to the similar ionic radii of Co3+ and Fe3+ ions.32 With the increasing doping amount of Co, a slight shift to higher 2θ angles is also detected, which demonstrates that the Co has been incorporated into the lattice of perovskites successfully. After the addition of Ce, the intensity of diffraction peak decreases apparently, which implies that the substitution of Ce diminishes the crystallization degree of the catalysts.29 Meanwhile, the diffraction peaks shift to lower 2θ positions with the gradual increase of Ce content, suggesting the successful incorporation of Ce into the lattice of perovskite. In addition, two new characteristic peaks corresponding to CeO2 and Co3O4 appear at 28° and 37° when the doping amount of Ce is greater than 0.1. This is because the introduction of Ce into the perovskite may lead to the distortion of structure and the contraction of lattice, and as a result small amounts of CeO2 and Co3O4 are separated from perovskite.8,29 The crystallite sizes, which calculate by Scherrer equation using the XRD data of the most prominent line, are listed in Table 1. The size of LaFeO3 perovskite is 24.2 nm, and monotonously decreases to 22.9 nm with increasing Co substitution. When Ce is doped into the perovskite, the size of perovskite further reduces obviously. This is due to the dispersing effect of CeO2 and Co3O4 impurity phases separated from the perovskites.29
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Fig. 2 XRD patterns of 3DOM La1−xCexFe1−yCoyO3 catalysts: full patterns (a); enlarged reflections (b). |
Catalyst | 2θ (°) | D (nm) | BET (m2 g−1) |
---|---|---|---|
LaFeO3 | 32.18 | 24.2 | 21.7 |
LaFe0.8Co0.2O3 | 32.34 | 24.0 | 21.4 |
LaFe0.6Co0.4O3 | 32.52 | 23.8 | 19.8 |
LaFe0.4Co0.6O3 | 32.70 | 22.9 | 18.5 |
La0.9Ce0.1Fe0.4Co0.6O3 | 32.66 | 22.7 | 18.9 |
La0.8Ce0.2Fe0.4Co0.6O3 | 32.62 | 21.0 | 20.5 |
La0.7Ce0.3Fe0.4Co0.6O3 | 32.54 | 19.8 | 22.5 |
La0.6Ce0.4Fe0.4Co0.6O3 | 32.42 | 18.4 | 22.4 |
The specific surface areas of the catalysts are also listed in Table 1. It can be found that all these values are relatively high even after high temperature treatment, which are attributed to their well-ordered macroporous structures. The large surface area of 3DOM perovskite is favorable to achieving the large number of contact points between the catalyst and soot particulates. The specific surface area of 3DOM LaFeO3 is 21.7 m2 g−1, and a decreasing tendency of the values is observed along with the increase of Co substitution content. The main reason is that the addition of Co influences the ordered degree of the macroporous structures during the preparation.14,23 The fractured catalysts then accumulate together, leading to a decrease of the specific surface area. As shown in Fig. 1(e), LaFe0.4Co0.6O3 can hardly maintain the well-ordered macroporous structures with the lowest specific surface area of 18.5 m2 g−1. Interestingly, after the introduction of Ce, the corresponding values increase to some extent, with a maximum of 22.5 m2 g−1 at 30% Ce substitution amount. It means that the further introduction of Ce restrains the collapse of 3DOM structure.
The FT-IR spectra of La1−xCexFe1−yCoyO3 catalysts are given in Fig. 3. There are two main characteristic bands in each samples. The absorption bands at 600 cm−1 are attributed to the Fe–O or Co–O stretching vibrations and the peaks around 400 cm−1 correspond to the O–Fe–O or O–Co–O deformation vibrations in the perovskites.23,28,33 The FT-IR results further demonstrate that the ABO3 perovskite structures are present and stable after substitution. When the content of Ce at A-site increases to 0.2, a new band phase at about 660 cm−1 related to the Co3O4 impurity is witnessed.34 Furthermore, with the increasing doping amount of Ce, there is a slight increase in the intensity of the corresponding band. These results are consistent with those of the XRD.
3DOM LaFeO3, LaFe0.4Co0.6O3 and La0.7Ce0.3Fe0.4Co0.6O3 are further characterized by using UV-Vis spectroscopy and the results are shown in Fig. 4. UV-Vis DRS is a significant characterization to investigate the geometry of the metal centres existing in the material. As shown in Fig. 4, all the three catalysts have ultraviolet absorption, which are ascribed to the electronic transitions from the valence band to conduction band owing to the octahedral coordination of oxygen around Fe in perovskite structure (O 2p → Fe 3d).35,36 However, the performance of visible light absorption is improved as a result of doping Co. It is mainly due to the presence of Co2+ in the tetrahedral and the Co3+ in the octahedral position, which proves that Co ions have been successfully incorporated into the perovskite structure.37,38 Since some absorption peaks of Ce are overlapped, the further introduction of Ce has little impact on the DRS spectra.
Fig. 5 shows the Ce 3d and Co 2p spectra of the 3DOM La1−xCexFe0.4Co0.6O3 (0 ≤ x ≤ 0.4) perovskite. As shown in Fig. 5(A), all the Ce-containing samples show two multiplets corresponding to the Ce 3d3/2 (u) and Ce 3d5/2 (v) levels. The binding energy peaks u′′′, u′′, u, v′′′, v′′ and v respectively located at about 916.0, 906.8, 900.1, 897.4, 888.3 and 881.7 eV are all belong to tetravalent Ce with different electron configuration states.39,40 With Ce content increasing, no obvious change, other than the increasing intensity of these binding energy peaks, is found in the spectra. It can be concluded that the main valence state of Ce in the 3DOM La1−xCexFe0.4Co0.6O3 (0 ≤ x ≤ 0.4) perovskite is tetravalence. It is mainly due to the better stability of Ce4+ than Ce3+ during high temperature calcination. As shown in Fig. 5(B), two peaks corresponding to Co 2p1/2 and Co 2p3/2 are detected in 3DOM La1−xCexFe0.4Co0.6O3 (0 ≤ x ≤ 0.4) catalysts. The peak binding energy of Co 2p3/2 for LaFe0.4Co0.6O3 is 779.6 eV, which is similar to that of Co2O3 reported in literature.41 It indicates that the main valence of Co ions in LaFe0.4Co0.6O3 is trivalent. With the increase of Ce substitution, slight shifts in the binding energies of Co 2p3/2 are found. The peak of Co 2p3/2 firstly moves from 779.6 eV to 780.3 eV (La0.8Ce0.2Fe0.4Co0.6O3), which means part of Co3+ have changed into Co2+ to maintain the electronic balance when Ce4+ ions are doped into the lattice of perovskite.42,43 When the Ce molar fraction is larger than 0.2, the peak shifts back to 779.9 eV (La0.6Ce0.4Fe0.4Co0.6O3) and the peak area decreases. It demonstrates that as the doping amount of Ce further increasing, the valence of Co slightly ascends.
Fig. 6 shows the O 1s XPS spectra of La1−xCexFe1−yCoyO3 catalysts. The O 1s spectra have been divided into three components using the curve-fitting approach to get a relative content of different oxygen species. The detailed results are listed in Table 2. It can be concluded that the peaks at 533.1–533.4 eV are attributed to surface adsorbed molecular water, and the peaks at 513.1–531.4 eV are ascribed to the surface adsorbed oxygen species (Oads). Another low BE peak at about 519.0 eV is due to the lattice oxygen species (Olat).43 For many oxidation reactions, the adsorbed oxygen species are very active at low temperatures and the area ratio of Oads/Olat is a sign to evaluate the oxidative performances of catalysts.44 From Table 2, it can be found that the addition of Co in the B-site increases the Oads/Olat ratio, and the ratio increases with Co-substituted amounts ascending. This ratio further rises when La partly substituted by Ce, and it reaches the highest point when 30% Ce is doped. Obviously, the catalyst La0.7Ce0.3Fe0.4Co0.6O3 possesses the largest amount of surface adsorbed oxygen species, which can accelerate the reaction during soot combustion.
Catalyst | H2O | Oads | Olat | Oads/Olat | |||
---|---|---|---|---|---|---|---|
BE (eV) | Area | BE (eV) | Area | BE (eV) | Area | ||
LaFeO3 | 533.3 | 392.4 | 531.2 | 7247.6 | 528.9 | 9244.7 | 0.78 |
LaFe0.8Co0.2O3 | 533.4 | 491.1 | 531.2 | 7397.9 | 529.1 | 8982.7 | 0.82 |
LaFe0.6Co0.4O3 | 533.3 | 392.4 | 531.3 | 7559.4 | 528.9 | 8877.3 | 0.85 |
LaFe0.4Co0.6O3 | 533.3 | 713.9 | 531.3 | 8105.7 | 528.9 | 9093.3 | 0.89 |
La0.9Ce0.1Fe0.4Co0.6O3 | 533.1 | 854.6 | 531.1 | 7876.1 | 528.9 | 9386.8 | 0.84 |
La0.8Ce0.2Fe0.4Co0.6O3 | 533.3 | 638.4 | 531.2 | 8426.6 | 529.0 | 9404.8 | 0.90 |
La0.7Ce0.3Fe0.4Co0.6O3 | 533.3 | 747.7 | 531.4 | 8477.5 | 529.0 | 9039.8 | 0.94 |
La0.6Ce0.4Fe0.4Co0.6O3 | 533.1 | 759.8 | 531.2 | 6107.7 | 528.7 | 9594.4 | 0.64 |
O2-TPD measurements were carried out to investigate the adsorption and activation of oxygen on the catalysts. The O2-TPD curves of 3DOM LaFeO3, LaFe0.4Co0.6O3 and La0.7Ce0.3Fe0.4Co0.6O3 are shown in Fig. 7. There are three main oxygen desorption peaks for each sample, <150 °C, 150–550 °C, and >500 °C, which are ascribed to the physical adsorption oxygen species, surface active oxygen species and lattice oxygen species, respectively.16,45 The surface active oxygen species are very important for soot combustion because the reaction often takes place at this temperature range. After Co is introduced, the peak area between 150–550 °C increases. With the further doping of Ce in A-site, a further increase in peak area is observed. At this point, both the substitution of Ce and Co can increase the amount of surface active oxygen species, and thus the catalytic activities for soot combustion can be improved. The results are consistent with those of O 1s XPS results.
The reducibility of the catalysts were characterized by H2-TPR technique, as displays in Fig. 8. For pure LaFeO3 perovskite, only one reduction stage appears at about 414 °C, which can be ascribed to the reduction of Fe4+ to Fe3+, while the Fe3+ cannot be further reduced under the experimental conditions.46 The reduction temperature, as a sign of intrinsic oxygen reactivity, is the critical factor in the soot oxidation state. The lower reduction temperature indicates the higher catalytic activity for soot oxidation. As the Co-substitution increases, the reduction peak moves towards the lower temperature accordingly. This is due to the increased content of Co3+ in the perovskites.47 When A-site La is further replaced by Ce, two new peaks between 310 °C and 550 °C emerge. According to the results of XRD and FT-IR, the lower reduction peak (310–370 °C) is attributed to the Co3O4 particles dispersed on the surface of the perovskite.48 While the higher reduction peak (430–550 °C) is assigned to the outermost layers reduction of Ce4+ to Ce3+ in the CeO2.49 Meanwhile, the special oxygen storage ability of Ce doped into the perovskite may make the reduction peak remarkably shift to the lower temperature in perovskite.30 When the content of Ce at A-site reaches up to 0.3, the reduction temperature is the lowest, which demonstrates that the La0.7Ce0.3Fe0.4Co0.6O3 has the best reducibility among all the as-prepared catalysts. However, when excessive amount of Ce is added, all the reduction peaks slightly shift to the higher temperature owing to the increasing amount of impurity phases on the surface of perovskite.
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Fig. 9 The conversion rate of soot over 3DOM La1−xCexFe1−yCoyO3 (0 ≤ x ≤ 0.4, 0 ≤ y ≤ 0.6) catalysts. |
Catalyst | T 10 (°C) | T 50 (°C) | T 90 (°C) | (%) |
---|---|---|---|---|
Soot (without catalyst) | 500 | 590 | 653 | 63.2 |
LaFeO3 (particle) | 440 | 512 | 574 | 95.2 |
LaFeO3 (3DOM) | 405 | 478 | 543 | 97.2 |
LaFe0.8Co0.2O3 (particle) | 419 | 494 | 560 | 96.2 |
LaFe0.8Co0.2O3 (3DOM) | 389 | 460 | 525 | 96.3 |
LaFe0.6Co0.4O3 (particle) | 398 | 475 | 541 | 95.8 |
LaFe0.6Co0.4O3 (3DOM) | 370 | 444 | 508 | 96.9 |
LaFe0.4Co0.6O3 (particle) | 379 | 459 | 522 | 96.1 |
LaFe0.4Co0.6O3 (3DOM) | 360 | 438 | 499 | 96.1 |
La0.9Ce0.1Fe0.4Co0.6O3 (particle) | 376 | 448 | 514 | 95.1 |
La0.9Ce0.1Fe0.4Co0.6O3 (3DOM) | 351 | 422 | 485 | 97.3 |
La0.8Ce0.2Fe0.4Co0.6O3 (particle) | 372 | 442 | 500 | 96.3 |
La0.8Ce0.2Fe0.4Co0.6O3 (3DOM) | 345 | 414 | 477 | 97.0 |
La0.7Ce0.3Fe0.4Co0.6O3 (particle) | 369 | 437 | 495 | 95.2 |
La0.7Ce0.3Fe0.4Co0.6O3 (3DOM) | 339 | 410 | 473 | 97.2 |
La0.6Ce0.4Fe0.4Co0.6O3 (particle) | 404 | 478 | 540 | 95.8 |
La0.6Ce0.4Fe0.4Co0.6O3 (3DOM) | 377 | 446 | 511 | 96.9 |
CeO2 (3DOM) | 380 | 463 | 529 | 96.8 |
Co3O4 (3DOM) | 374 | 451 | 516 | 97.0 |
For 3DOM LaFeO3 catalyst, the T50 of soot decreases to 478 °C and the combustion temperature further declines monotonously along with the increase of Co-substitution. For soot combustion, the activity of LaFeO3 is much lower than that of LaCoO3, and thus substitution of Co for Fe can improve the activity of the perovskites. When the doping amount of Co increases to 0.6, the promotion effect on catalytic activity trends to flat. The above characterization results show that the substitution of Co destroys the well-ordered macropore structure. The fractured macropore skeletons pile up on the surface of the catalysts, just like the nanoparticles prepared by the citric acid-ligated combustion method.51 The nanoparticle catalysts show relatively low activities than the corresponding 3DOM ones.14,52 As a result, the destruction of 3DOM structure caused by Co-substitution partly inhibits the further improvement of the catalytic activity.
After the introduction of Ce, the activity of the catalyst is enhanced with the decrease of soot combustion temperature. Based upon above characterization results, the improvement of the activity for soot combustion can be assigned to the following two aspects. Firstly, the introduction of Ce can avoid the collapse of 3DOM structure. The interpenetrating well-ordered macroporous structure permits soot particles to go through the structure with less diffusion resistance and the available active sites of the catalysts will be increased remarkably. The maintained 3DOM structure can improve the activity of catalysts from the structure advantage. Secondly, as revealed by O 1s XPS and O2-TPD results, Ce-doped samples possess more surface adsorbed oxygen species which facilitate the transferring of oxygen species during soot combustion. Meanwhile, the introduction of Ce also improves the reducibility of the catalyst, which accelerates the redox circulation during soot combustion. The improvement of intrinsic property of the catalyst also enhances the catalytic activity. However, the excess addition of Ce decreases the activity due to the formation of CeO2 and Co3O4 impurities as the 3DOM CeO2 and Co3O4 exhibit lower catalytic activities compared with the 3DOM La1−xCexFe0.4Co0.6O3 (0 ≥ x ≥ 0.4) catalysts. Among all the catalysts, the highest activity is obtained when the doping amount of Ce is 0.3 and the corresponding T10, T50, T90, and are 330, 410, 473 °C, and 97.2%, respectively.
The catalytic activities of 3DOM La0.7Ce0.3Fe0.4Co0.6O3 for soot combustion in the absence of NO were also evaluated by TPO method and the results are shown in Fig. 11. As shown in Fig. 11, the catalytic performance of 3DOM La0.7Ce0.3Fe0.4Co0.6O3 in the absence of NO is lower than that in the presence of NO, with the T10, T50 and T90 of soot increasing from 339, 410, and 473 °C to 378, 457, and 520 °C, respectively. In order to study the effect of NO in the reaction, NO oxidation experiment was also carried out. Fig. 12 shows the NO2 concentration profile during NO oxidation over 3DOM La0.7Ce0.3Fe0.4Co0.6O3 catalyst. For comparison, the profile without catalyst is also included. As shown in Fig. 12, NO cannot convert to NO2 without the help of catalyst as the outlet NO2 concentration is low. After 3DOM La0.7Ce0.3Fe0.4Co0.6O3 catalyst is added, the outlet NO2 concentration increases remarkably at first, and then decreases with the temperature increasing. The corresponding temperature of maximum NO2 concentration is about 380 °C, which is accorded with the soot combustion temperature. NO2, as a strong oxidant, can oxidize soot much more effective than O2.53 It can be concluded that the soot is oxidized by NO2 converted from NO, rather than directly oxidized by O2 during soot combustion.
Table 4 lists the relevant results reported in literature. As seen in Table 4, many researches focus on perovskite-type catalysts on soot combustion. Among them, the perovskite-type catalysts with noble metal (Entry 1 and 2)16,54 or alkali metal (Entry 3 and 4)14,32 show the higher activities for soot combustion than other catalysts. The component of reactant gas and the contact between soot and catalyst also affect the soot combustion temperature. Considering the factors above, the catalysts in this work have relatively high catalytic activities compared with the similar component of catalyst (Entry 5–7).28,55,56
Entry | Catalyst | Reactant gas | Contact | T m or T50 or Tp (°C) | References |
---|---|---|---|---|---|
1 | La0.7Ag0.3MnO3 | 2000 ppm NO + 5% O2 | Loose | 401 | 54 |
2 | Au0.04/LaFeO3 (3DOM) | 2000 ppm NO + 5% O2 | Loose | 368 | 16 |
3 | La0.9K0.1CoO3 (3DOM) | 2000 ppm NO + 5% O2 | Loose | 378 | 14 |
4 | La0.6K0.4CoO3 | 800 ppm NO + 10% O2 | Loose | 382 | 32 |
5 | La0.8Ce0.2Mn0.7Co0.3O3 | Air | Tight | 424 | 28 |
6 | La0.7Ce0.4Fe0.5Co0.5O3 | 10% O2 | Loose | 433 | 56 |
7 | LaCe0.2CoO3 | 10% O2 | Tight | 435 | 55 |
8 | La0.7Ce0.3Fe0.4Co0.6O3 (3DOM) | 500 ppm NO + 5% O2 | Loose | 410 | This work |
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