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CO and soot oxidation over macroporous perovskite LaFeO3
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CO and soot oxidation over macroporous perovskite LaFeO3 Ping Xiao a , Linyun Zhong a , Junjiang Zhu a,∗ , Jingping Hong a , Jing Li b , Hailong Li a , Yujun Zhu b,∗ a Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, Wuhan, China b Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials, Heilongjiang University, Harbin 150080, PR China
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Article history: Received 12 October 2014 Received in revised form 9 December 2014 Accepted 7 January 2015 Available online xxx Keywords: LaFeO3 Macroporous Catalytic oxidation Carbon monoxide Soot
a b s t r a c t Bulk, supported and macroporous perovskite LaFeO3 are synthesized and investigated as catalysts for CO and soot oxidation. The structure and physicochemical properties of LaFeO3 are characterized by XRD patterns, N2 physisorption isotherms, TEM, XPS, O2 -TPD and H2 -TPR measurements. Pure LaFeO3 with desired textural structures are successfully prepared, and the porous sample exhibits the best activity to both reactions, pointing out the importance of fabrication of porous perovskite for the reactions. By correlating with the catalytic activities and the physicochemical properties, it is inferred that the best activity obtained from the porous LaFeO3 is attributed to its large surface area, rich active lattice oxygen and strong surface reducibility. A decrease in the activity of porous LaFeO3 is observed for both reactions when vapor is added, but the activity is sustainable even the vapor percentage increases up to 13.5%, and moreover, the activity can be recovered after vapor is removed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Exhaust emitted from internal combustion engine consuming fossil materials contains various substances that are harmful to the environment. The removal of these harmful substances is a huge task of today, especially with the increasing demands of environmental protection. Catalysis is an effective technology in solving environmental problems including exhaust. The crucial factor of this technology is to find a suitable catalyst, which can fit to the various situations of exhaust, such as the different types of substances, the variation in temperature at cold-start and normal operating conditions, and the co-existence of vapor in the exhaust. Among the catalysts investigated for exhaust removal, perovskite oxides receive great attentions. The low-cost, high thermal and hydrothermal stability, straightforward synthesis as well as considerable activity enable them as interesting catalysts for industrial application [1–5]. Also, they are attractive materials for scientific researches as they can be suitable model catalysts in correlating physicochemical properties with catalytic performances because of the structural features, of which the cation at either A- or B-site can be replaced by a foreign one without destroying
∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (J. Zhu), [email protected], [email protected] (Y. Zhu).
the matrix structure, and the content of oxygen vacancy and the oxidation state of B-site cation can be controlled as desired [6–9]. For perovskite oxide with ABO3 structure, the B-site cation is believed to be an active site of reactions and accounts for the structural stability [10,11]. It has been reported that LaCoO3 and LaMnO3 that exhibit higher activity for CO oxidation, methane combustion and soot oxidation are less stable at high temperatures. In contrast, LaFeO3 that shows lower activity to the reactions has better thermal stability, especially under the fluctuating redox conditions of automobile exhaust gases [11]. In view of the long-term use in industrial applications, LaFeO3 is more desired especially when the material is applied to high-temperature reactions. Surface area is a crucial issue needs to be considered when applying perovskite oxides for catalysis use, as they normally have low surface area because of the high temperatures and long calcination time used in the preparation process, which would limit their catalytic performances as catalysis belongs to a class of surface reaction. Furthermore, the surface area could also affect some physicochemical properties of a material such as the reducibility and the oxygen vacancy. Many strategies have been reported to increase the surface area of perovskite oxides by, for example, using organic complexes to homogenize and combust the metal precursors in the sol–gel or combustion methods [12–15], using high-surface-area carriers to support and disperse the materials [16–18], or using template or surfactant to prepare porous materials [19–21]. The latter is more desired especially when the material
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is used for solid soot oxidation [22–24], as its porous structure not only allows a good mass transfer of the reactant, but also exhibits more active sites on the surface accessible to the reactant. In this study, we prepared a porous LaFeO3 and investigated its catalytic performances for the oxidation of CO and soot powder, which are two components of exhaust emitted from mobile cars, and in order to illuminate its advantages in the reactions, bulk and supported LaFeO3 were synthesized and compared. Characterizations by XRD and TEM indicated that LaFeO3 with desired textural structures are successfully prepared. Catalytic results showed that the porous LaFeO3 exhibits high activity to CO and soot oxidation, owing to its large surface area, rich active lattice oxygen and strong reducibility, as evidenced by O2 -TPD, H2 -TPR and XPS measurements. To test its applicability in real conditions, effect of vapor on the catalytic performances of porous LaFeO3 is also investigated, showing a slight decrease in the activities when vapor is added to the reactant, but the activity is sustainable even the vapor percentage reaches up to 13.5 vol.%, and the activity can be recovered when the vapor is removed. 2. Experimental Porous LaFeO3 was prepared using PMMA colloidal crystals as template, which is synthesized according to the work reported by Li et al. [25]. The LaFeO3 /SBA-15 (loading: 55 wt%) was prepared by impregnation method as described in a previous work [17]. Bulk LaFeO3 was prepared similar as that for LaFeO3 /SBA-15, except without the addition of SBA-15. The catalysts were characterized by X-ray diffraction (XRD) patterns, transmission electron microscopy (TEM), N2 adsorption– desorption isotherm, X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2 -TPR) and oxygen temperature-programmed desorption (O2 -TPD). Both CO and soot oxidation were conducted on a continuous flow fixed-bed microreactor. For CO oxidation, the reactant 0.5% CO + 7.5% O2 + 92% Ar was passed through 0.1 g powder catalyst at a flow rate of 50 mL min−1 , and the activity was evaluated in terms of CO conversion. For soot oxidation, 5% O2 /Ar was passed through a mixture containing 0.1 g catalyst and 0.01 g soot (which were packed in a loose mode) at a flow rate of 50 mL min−1 and the temperature was raised from 150 to 650 ◦ C at a heating rate of 2 ◦ C min−1 . The activity was evaluated in terms of CO2 concentration determined in the effluent gases. In case the vapor was introduced, the reactant was passed through a water bubbler before entering the reactor, and the vapor percentage in the reactant was controlled by changing the temperature of the water bubbler. For the detailed procedures and parameters of catalyst preparation, characterization and activity tests, see the experimental section of supplementary information (SI). 3. Results and discussion 3.1. Characterizations Fig. 1 presents the wide-angle XRD patterns of the samples, showing that all possess the LaFeO3 perovskite structure (JCPDS 37-1493). In a previous work, we have shown that pure LaFeO3 phase can be synthesized in bulk LaFeO3 and LaFeO3 /SBA-15 [17]. Here, also for the porous LaFeO3 , no impurity such as La(OH)3 or Fe(OH)3 is observable in the XRD patterns, indicating that pure LaFeO3 is formed. The perovskite structure of samples is also confirmed by FT-IR spectra, which show a strong absorption band at 555 cm−1 assigned to the Fe O stretching vibration of octahedral FeO6 groups in the perovskites (see Fig. S1). The major difference in XRD patterns is the peak intensity. Bulk LaFeO3 has the strongest
Fig. 1. Wide-angle XRD patterns for bulk LaFeO3 , LaFeO3 /SBA-15 and porous LaFeO3 .
peak intensity and LaFeO3 /SBA-15 has the weakest. By calculating the full-width at half-maxima (FWHM) of peak at 2 ≈ 32.2◦ , it is found that the value for bulk, supported and porous LaFeO3 increases from 0.219 to 0.448 and to 0.454. The crystal size, calculated based on the Scherrer equation, is accordingly decreased from 37.7 to 18.3 and to 18.0 nm (see Table 1). The weak peak intensity of LaFeO3 /SBA-15 could be additionally attributed to a diluted effect of SBA-15. Small-angle XRD patterns for LaFeO3 /SBA-15 show that the porous structure of SBA-15 is not destroyed after the formation of LaFeO3 , demonstrating that the LaFeO3 /SBA-15 composite is synthesized, as was previously observed [17]. As expected, the surface area of supported and porous LaFeO3 is improved relative to the bulk one (Table 1). The porous LaFeO3 exhibits surface area ca. five times bigger than the bulk one, suggesting that pores should be generated, which is also supported by the TEM image in Fig. 2A. For LaFeO3 /SBA-15, deposition of LaFeO3 does not destroy the porous structure of SBA-15 and the H1 hysteresis loop attributed to SBA-15 is present in the N2 physisorption isotherms (see Fig. S2) [17]. Although the value in Table 1 represents not the real surface area of LaFeO3 (it represents the surface area of LaFeO3 /SBA-15 composite), it can be imagined that the surface area should be larger than that of the bulk one, due to its smaller particle size, as demonstrated by Fig. 2B and C, which represent the TEM image of LaFeO3 /SBA-15 and bulk LaFeO3 , respectively. Fig. 3 shows the XPS spectra of La 3d, Fe 2p and O 1s for the bulk, supported and porous LaFeO3 . Both the La 3d3/2 and La 3d5/2 peaks are split and the degree of split increases from bulk LaFeO3 to LaFeO3 /SBA-15 (Fig. 3A). This split is caused by the transfer of electrons from O 2p to the unoccupied orbital of La 4f [26]. Thus, the increase in the peak split suggests that the influence of electrons on the unoccupied orbital of La 4f is stronger in LaFeO3 /SBA-15 than that in bulk LaFeO3 . The reason could be that the supported LaFeO3 has small particle size and the unoccupied orbital of La 4f is influenced not only by the electrons from O 2p of LaFeO3 , but also from that of SBA-15 (SiO2 ), which contains oxygen also. The peak split of La 3d for porous LaFeO3 is similar to that for the bulk one, but the peak position shifts to higher binding energy. The binding energy of Fe 2p shifts to higher position from the bulk to supported and further to porous LaFeO3 (Fig. 3B), suggesting the increase of Fe oxidation state or oxidizing capability, in order of porous LaFeO3 > LaFeO3 /SBA-15 > bulk LaFeO3 . To verify and explain the shift in the binding energy of La and Fe elements, the XPS spectra of oxygen atoms, which bond directly with La and Fe atoms and is the solely negative atom in LaFeO3 , are analyzed and deconvoluted by peak fitting method using
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Table 1 The crystalline size, BET surface area, molar ratio of OII /OIII in XPS spectra and H2 consumption in H2 -TPR measurements for bulk LaFeO3 , LaFeO3 /SBA-15 and porous LaFeO3 .a Sample
Crystal size (nm)b
SBET (m2 g−1 ) c
OII /OIII d
Bulk LaFeO3 LaFeO3 /SBA-15 Porous LaFeO3
37.3 18.3 18.0
4.3 161.6 20.6
4.83 – 2.575.57 f
a b c d e f
H2 consumption (mol)e T1
T2
T3
T4
19.8 13.9 58.1
40.7 29.4 73.1
66.1 69.1 134.5
– – 76.2
For the meaning of letters (OII , OIII , T1 , T2 , T3 , T4 ), refer text. Determined by the Scherrer equation from the XRD patterns. Determined by the BET method from the N2 physisorption isotherms. Data obtained from the O 1s XPS spectra. Data obtained from the H2 -TPR measurements. Data for the used sample.
the Gaussian functions (Fig. 3C). It should be noted that because SBA-15 also contains oxygen atoms that contribute to the O 1s spectra, the O 1s spectra of LaFeO3 /SBA-15 reflect not exactly the oxygen information for LaFeO3 , thus we did not make special effort on analyzing and discussing the oxygen properties of this sample. For the bulk and porous LaFeO3 , three peaks are fitted and for each peak the binding energy shifts to lower position from the bulk to the porous LaFeO3 , indicating that the oxygen in the latter is more negative. This is in accordance to the above results that the La and Fe elements of the porous LaFeO3 are more positive than these of the bulk one. Based on the classifications suggested in the previous work [27–30], the peak at 533 (535), 531(532) and 528(530) eV is attributed to oxygen originated from adsorbed water or hydroxyl group (OI ), chemically adsorbed oxygen (OII ) and lattice oxygen (OIII ), respectively. From the molar ratio of OII /OIII of the bulk and porous LaFeO3 (Table 1), it is seen that the former has higher OII /OIII than the latter, which is in accordance to previous results observed
for bulk and porous LaCoO3 [31,32], indicating that the percentage of lattice oxygen exposed on the surface of porous LaFeO3 is more than that of bulk LaFeO3 , or vice versa, the percentage of surface adsorbed oxygen on porous LaFeO3 is less than that of bulk LaFeO3 . It should, however, be noticed that the absolute amount of surface-adsorbed oxygen on porous LaFeO3 is more than that of bulk LaFeO3 , due to its larger surface area. Fig. 4 presents the O2 -TPD profiles of the samples. For O2 -TPD profile of perovskite oxides, four types of oxygen, can be classified based on the desorption temperature of the oxygen [33–37]. The first is physically adsorbed oxygen (O2 ), which stays physically on the surface and desorbs at temperature lower than 60 ◦ C; the second is ordinarily chemical adsorbed oxygen (O2 − ), which has weak interaction with the catalyst’s surface and desorbs at temperature of 60–300 ◦ C; the third is chemically adsorbed oxygen on the oxygen vacancies (O− ), which has strong interaction with the surface defect of catalyst and desorbs at temperature of 300–600 ◦ C; and
Fig. 2. TEM images for (A) porous LaFeO3 , (B) LaFeO3 /SBA-15 and (C) bulk LaFeO3 .
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Fig. 3. (A) La 3d, (B) Fe 2p and (C) O 1s XPS spectra for the bulkLaFeO3 , LaFeO3 /SBA-15 and porous LaFeO3 .
the last is lattice oxygen (O2− ), which is the unit composing the framework of perovskite oxide, and thus its desorption occurs at a relative high temperature, normally above 600 ◦ C. From the O2 -TPD profiles of the bulk, supported and porous LaFeO3 samples investigated here, it is however hard to differentiate the oxygen species desorbed from each sample based on the desorption temperature due to the very weak peak intensity observed in the profiles. Because of this, we thus did not make special effort on discussing the oxygen species desorbed from each sample. However, it is noticed that an intense and sharp desorption peak appears at temperatures of 600–800 ◦ C for the porous LaFeO3 , and no observable peak appears for the bulk LaFeO3 and LaFeO3 /SBA-15. This suggests that the porous LaFeO3 has more rich and active lattice oxygen than the bulk and supported LaFeO3 . This is crucial for reactions conducted at high temperature, where lattice oxygen could participate in the reaction.
Fig. 4. The O2 -TPD profiles for the bulk, supported and porous LaFeO3 .
To study the reducibility of samples, H2 -TPR measurements are conducted and the results are shown in Fig. 5. Bulk LaFeO3 shows an apparent reduction peak at 449 ◦ C, with two shoulder peaks at about 330 and 400 ◦ C. LaFeO3 /SBA-15 shows an apparent reduction peak at 436 ◦ C. Although definite shoulder peak is not observed, the slope at the left of the peak indicates that they should be existed. The shape of the reduction peak of porous LaFeO3 is complex. Besides a large shoulder peak at ca. 420 ◦ C, two split peaks at 481 and 533 ◦ C appear. It is known that there normally have four reduction steps for the reduction of LaFeO3 , corresponding to (1) the reduction of Fe4+ to Fe3+ ; (2) the reduction of surface Fe3+ to Fe2+ ; (3) the reduction of bulk Fe3+ to Fe2+ and (4) the reduction of Fe2+ to metallic iron [38–40]. Based on this classification, the reduction peaks are analyzed and deconvoluted, using the peak fitting function of the Origin software. Three peaks are fitted for bulk LaFeO3 and LaFeO3 /SBA-15, corresponding to the former three reduction steps, and are marked as T1 , T2 and T3 in the pictures. Four peaks, however, are fitted for the porous LaFeO3 , with the last one (T4 ) corresponding to the fourth reduction steps, namely the reduction of Fe2+ to metallic iron. This peak is fitted because its temperature (533 ◦ C) is far higher than that observed for the bulk LaFeO3 (449 ◦ C) and the LaFeO3 /SBA-15 (436 ◦ C). The large difference (ca. 100 ◦ C) in temperature indicates that this species is less reducible than that of bulk LaFeO3 and/or LaFeO3 /SBA-15. Consequently, a fourth peak is fitted for the porous LaFeO3 and is attributed to the reduction of Fe2+ to Fe◦ . The temperature at peak maximum and H2 consumption for each peak are marked in Fig. 5 and listed in Table 1. The porous LaFeO3 shows the largest H2 consumption for peaks T1 and T2 , suggesting that it has more Fe4+ /Fe3+ species on the surface, in accordance to the results obtained from XPS spectra. Even for the third reduction step, the porous LaFeO3 shows almost double H2 consumption than that for the bulk one, due to its rich active lattice oxygen as demonstrated in O2 -TPD measurements. The H2
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Fig. 5. H2 -TPR profiles for SBA-15 and the bulk, supported and porous LaFeO3 , and the corresponding peak fitting for each sample.
consumption for peaks T1 and T2 of LaFeO3 /SBA-15 is larger than half (55%) of that of bulk LaFeO3 , which could be that, on the one hand, the former has smaller particle size and is more reducible, and on the other hand, some species (e.g., hydroxyl groups) from SBA15 are reduced. This is more crucial at high temperature, where the H2 consumption for peak T3 of LaFeO3 /SBA-15 is even comparable to that of bulk LaFeO3 . The temperature at peak maximum of T1 , T2 and T3 is similar for bulk LaFeO3 and LaFeO3 /SBA-15, both are in particle status and the absolute difference in H2 consumption is small (<10 mol). The temperature at peak maximum of T1 (334 ◦ C) and T2 (400 ◦ C) of the porous LaFeO3 is similar to that of bulk LaFeO3 and LaFeO3 /SBA-15. This seems strange as the porous LaFeO3 has stronger oxidability (see XPS spectra) and should be more reducible. However, considering the big difference in H2 consumption between them (about 40 mol, see Table 1), the similar reduction temperature observed here could be that the reduction temperature of porous LaFeO3 is delayed because of the more amount of H2 consumed. Large H2 consumption normally will delay the appearance of temperature at peak maximum as the reduction proceeds with reaction time, and this delaying effect is more pronounced if bigger difference in H2 consumption appears. Indeed, the temperature at peak maximum of T3 of the porous LaFeO3 (477 ◦ C) is even higher than that of bulk LaFeO3 (447 ◦ C) and LaFeO3 /SBA-15 (445 ◦ C) due to its large H2 consumption. This suggests that the reduction temperature at peak maximum should be carefully considered when used to estimate the reducibility of samples if the difference in H2 consumption is too large. 3.2. Catalytic performances 3.2.1. CO oxidation For CO oxidation, it is generally believed that the molecular oxygen should be dissociated into atomic oxygen before participating in the reaction [41,42]. Hence, a catalyst that can provide
sites (e.g., oxygen vacancies) and electrons for molecular oxygen adsorption and activation (e.g., O2 + 2e− = 2O− ) would facilitate the reaction. For perovskite oxides, it is known that the generation of oxygen vacancy accompanies the increase in oxidation state for the B-site cations and the enrichment of electrons, in form of, for example, Fe3+ − O − Fe3+ → Fe4+ −(e− ·) − Fe3+ + 1/2O2 . This implies that sample possessing more oxygen vacancies has stronger ability for oxygen adsorption and activation. Once the molecular oxygen is activated, it can react with adsorbed CO to yield CO2 and regenerate the active site (oxygen vacancy). As expected, the porous LaFeO3 that possesses the largest amount of oxygen vacancies and the strongest reducibility shows the lowest onset temperature and the highest catalytic efficiency for CO oxidation (Fig. 6A). Although it is hard to compare directly the amount of oxygen vacancies for the bulk and supported LaFeO3 due to the interference of SBA-15, it can be inferred that more oxygen vacancies should be generated in LaFeO3 /SBA-15, since the metals exhibit higher binding energies in the XPS spectra. In order to illuminate the advantage of using LaFeO3 /SBA-15 for CO oxidation at low temperature, the reaction rate of CO oxidation over the catalysts are calculated (Fig. 6B). Obviously, LaFeO3 /SBA-15 shows higher reaction rate for CO oxidation relative to bulk LaFeO3 at temperature above 200 ◦ C, due to the increased oxygen vacancies as well as possibly the support effect, which at least results in smaller particle size. To test whether the porous LaFeO3 could be applied to real exhaust environment, where vapor is present, the effect of vapor on the reaction is tested. The activity at each vapor percentage was measured for three times (or 45 min) to ensure that it reaches stable status. Fig. 6C presents the CO oxidation activities measured at different percentages of vapors at 340 ◦ C, showing that the activities are influenced when vapor is added. A decrease of 21% in the conversion is observed at 1.7 vol.% vapor percentage, but the activity is sustainable thereafter, even up to 13.5 vol.% vapor percentage. In particular, the activity can be recovered after vapor is removed.
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Fig. 6. (A) Catalytic activities and (B) the corresponding reaction rate of bulk, supported and porous LaFeO3 for CO oxidation; (C) effect of vapor on the activity of CO oxidation conducted on porous LaFeO3 at 340 ◦ C. Note: Reaction rate was calculated in terms of mole of CO converted per second and per mole of catalyst.
This indicates that although the presence of vapor influences the catalytic behavior of catalyst, it does not deteriorate the active site and the catalyst can endure up to 13.5 vol.% vapor in the reactant. 3.2.2. Soot oxidation Soot is one type of particulate matter (PM) in the exhaust. Soot with diameter below 2.5 m, known as PM 2.5, causes serious damage to the respiratory organs of human beings and is one of the major sources leading to smog formation. With the increase of automobiles, large amounts of soot are emitted to the air resulting in increasingly serious deterioration of our environment. Legislation on the emission of soot as well as other toxic matters has been issued in almost all the countries. Nevertheless, the removal of soot is still a big challenging task of today despite the achievements made over the years [43–48]. Other than gaseous CO, soot is a solid that cannot diffuse or mobilize freely to the catalyst’s surface or pores, which increases the difficulty of its catalytic oxidation removal. One effective way of improving the ability to soot oxidation is to increase the surface area of catalyst [23,24], so that the soot can be in contact with the catalyst as efficiently as possible and be oxidized. Fig. 7A shows the CO2 concentration yielded in the reaction as a function of reaction temperature over the catalysts. In accordance to that reported by Zhao et al. [23], the porous sample shows the best ability to soot oxidation because of its enhanced surface area and porous structure. Bulk LaFeO3 also exhibits good ability to soot oxidation and the activity is better than that of LaFeO3 /SBA-15. This could be that, on the one hand, the absolute active site is fewer in LaFeO3 /SBA-15 as only 0.055 g LaFeO3 (vs. 0.1 g in the bulk LaFeO3 ) is present, and on the other hand, the soot cannot be mobilized freely as CO, thus large amounts of soot stayed on the barren surface of SBA-15 (without LaFeO3 ) cannot be catalytically oxidized into CO2 . For comparison, the soot oxidation conducted over pure SBA15 and that in the blank experiment (without catalyst) is also tested, showing that SBA-15 can improve the reaction and soot can be selfcombusted at high temperature, but the major product is CO, not CO2 .
Although CO2 is formed in each reaction, its concentration varies with the catalyst. To meet the carbon balance, the concentration of CO, which is a byproduct of the reaction, in the tail gas is also monitored (Fig. 7B). Large amounts of CO are produced in the blank experiment and that use SBA-15 as catalyst, and some are produced over the bulk and supported LaFeO3 . Negligible CO is detected when porous LaFeO3 is used as catalyst. These results indicate that soot can be burn off and oxidized into CO at high temperature by itself, but the further oxidation into CO2 depends on the properties of catalyst. Here, the porous LaFeO3 has the best ability to oxidize soot into CO2 relative to the other samples, which is probably attributed to its large surface area, rich active lattice oxygen as well as strong oxidizing ability, as discussed above. Table 2 lists the total amounts of CO + CO2 species produced in the reaction, showing that the carbon is approximately balanced. Blank experiment indicates that no black material in the reactor could be seen by the naked eye after reaction, confirming that soot is fully oxidized. From the selectivity to CO2 (SCO2 ), it is seen that porous LaFeO3 with large surface area exhibits high selectivity to CO2 , pointing out the importance of the contact area for total oxidation of solid soot. LaFeO3 /SBA-15 shows slightly lower CO2 selectivity than bulk LaFeO3 , which perhaps is due to its less (absolute) active sites and especially the fact that a certain amount of soot is stayed on the barren surface of SBA-15, thus cannot contact adequately with the active site and be oxidized into CO2 . The temperature for soot ignition (Tig , defined as 10% soot oxidation) and full oxidation (Tm , defined as the temperature at peak maximum of CO2 ) over the catalysts are also listed in Table 2, showing that the porous LaFeO3 gives the best catalytic performance, with Tig and Tm of 374 and 560 ◦ C, respectively. In comparison, the bulk LaFeO3 shows an increased Tig and Tm by 19 and 17 ◦ C, respectively. LaFeO3 /SBA-15 displays the worst catalytic performance, with Tig of 402 ◦ C and Tm of 578 ◦ C, the latter is similar to that of bulk LaFeO3 . Soot can be self-ignited at temperature of 410 ◦ C, and the temperature for the maximum yield of CO2 is 576 ◦ C, which is similar to that observed in the presence of bulk LaFeO3 . The low Tm observed in this case should be the reason that only a small
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Fig. 7. (A) CO2 and (B) CO concentration detected in the tail gas of soot oxidation reaction over different catalysts and, that under blank experiment (without catalyst); (C) effect of vapor on the catalytic performances of porous LaFeO3 for soot oxidation. Table 2 The Tig and Tm of soot oxidation, the amount of CO and CO2 produced, and the selectivity to CO2 detected over the investigated catalysts.
No catalyst SBA-15 LaFeO3 /SBA-15 Bulk LaFeO3 Porous LaFeO3 a b
Tig (◦ C)a
Tm (◦ C)b
CO (mol)
CO2 (mol)
CO + CO2 (mol)
SCO2 (%)
410 410 402 393 374
576 585 578 577 560
1.39E−04 1.12E−04 2.05E−05 1.63E−05 1.22E−06
9.08E−05 1.44E−04 1.84E−04 1.96E−04 2.21E−04
2.30E−04 2.56E−04 2.04E−04 2.12E−04 2.22E−04
39.5 56.3 89.9 92.3 99.5
The temperature for soot ignition. The temperature for soot full oxidation.
amount of CO2 is produced as the main product is CO. From the CO2 yield it is known that the presence of SBA-15 can improve the soot oxidation process (to CO2 ) although it has no contribution to the ignition temperature. Compared with previous literature,[49] the Tm observed in this study is higher, which could be that in previous work the reactant contains also NO, which can act as an oxidant and facilitate the reaction.[50] The effect of vapor on the catalytic performances of porous LaFeO3 for soot oxidation is also studied, to test if the catalyst could be used in simulated real engine exhaust conditions, as that for CO oxidation. Fig. 7C shows the CO2 yield obtained from the reaction at different vapor percentage as a function of reaction temperature. A slight increase in the temperature for the maximum CO2 formation is observed when vapor is added to the reaction, and the temperature remains unchanged irrespective of the further increase of vapor percentage (up to 13.5 vol.% in our tests). No CO is detected throughout the reaction, as that observed in the absence of vapor, indicating that the sample has well endurance to vapor during the reaction, and thus could be a potential candidate for industrial application in future. In justifying the stability of porous LaFeO3 , the used sample was subjected to XRD, BET and XPS measurements and compared with that of the fresh sample. XRD and BET results (see Figs. S2 and S3) indicate that the perovskite structure of porous LaFeO3 is well retained after the reaction, implying that the sample has good stability. XPS results (see Fig. S4) indicate that the binding energies of La 3d and Fe 2p shift to high position and that of O
1S shifts to low position, after the reaction. The reason could be that the surface oxygen is consumed during the reaction (by reacting with the soot), leaving more exposed positive metal ions and stronger bonded oxygen. That is, more oxygen vacancies are generated. Indeed, by comparing the molar ratio of OII to OIII (OII /OIII ; see Table 1), it is found that the ratio is increased after the reaction, from 2.6 to 5.6, justifying again that the surface oxygen is consumed and more oxygen vacancies are generated. 4. Conclusions Catalytic performances of bulk, supported and porous LaFeO3 for CO and soot oxidation have been investigated, showing that the porous LaFeO3 has the best ability to both reactions. TEM image and N2 physisorption isotherms confirm that the sample prepared using PMMA as template has porous structure and enhanced surface area, and thus increased contact area with the reactant. XPS, H2 -TPR and O2 -TPD results indicate that the porous LaFeO3 has more oxygen vacancy, stronger oxidizing ability and active lattice oxygen, explaining its well ability to CO and soot oxidation. The applicability of porous LaFeO3 to CO and soot oxidation in the presence of vapor is also investigated, showing a decrease in the conversion but has well endurance even up to 13.5% vapor in volume. The decreased activity can be recovered when the vapor is switched off, indicating that the active site is not destroyed and the porous LaFeO3 has strong resistance to vapor poisoning, potentiating its possible application for industrial removal of exhaust.
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Acknowledgements Financial support from the National Science Foundation of China (21203254), the Scientific Research Foundation for Returned Scholars, Ministry of Education of China (BZY11055), the Science and Technology Activities of Overseas Personnel Preferential Funding Project (No. BZY14038), the National Demonstration Center of Experimental Teaching on Ethnical Pharmacy, South-Central University for Nationalities, The Project-sponsored by SRF for ROCS, SEM (2013-1792), Program for New Century Excellent Talents in Heilongjiang Provincial University (1155-NCET-014), Postdoctoral Science-research Developmental Foundation of Heilongjiang Province of China (LBH-Q12022) and Innovative Research Team in Heilongjiang University (Hdtd2010-10) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015. 01.007. References [1] R.J.H. Voorhoeve, Advanced Materials in Catalysis, Academic Press, New York, 1977. [2] S. Hernández, G.A. Blengini, N. Russo, D. Fino, Ind. Eng. Chem. Res. 51 (2011) 7584–7589. [3] R. Zhang, W. Yang, N. Luo, P. Li, Z. Lei, B. Chen, Appl. Catal. B 146 (2014) 94–104. [4] Y. Wu, X. Ni, A. Beaurain, C. Dujardin, P. Granger, Appl. Catal. B 125 (2012) 149–157. [5] F.E. López-Suárez, A. Bueno-López, M.J. Illán-Gómez, A. Adamski, B. Ura, J. Trawczynski, Environ. Sci. Technol. 42 (2008) 7670–7675. [6] Z. Zhao, X.G. Yang, Y. Wu, Appl. Catal. B 8 (1996) 281–297. [7] Y. Wu, T. Yu, B.S. Dou, C.X. Wang, X.F. Xie, Z.L. Yu, S.R. Fan, Z.R. Fan, L.C. Wang, J. Catal. 120 (1989) 88–107. [8] J.J. Zhu, A. Thomas, Appl. Catal. B 92 (2009) 225–233. [9] M.A. Pena, J.L.G. Fierro, Chem. Rev. 101 (2001) 1981–2017. [10] H. Ziaei-Azad, A. Khodadadi, P. Esmaeilnejad-Ahranjani, Y. Mortazavi, Appl. Catal. B 102 (2011) 62–70. [11] I. Tan, H. Tanaka, M. Uenishi, K. Kaneko, S. Mitachi, J. Ceram. Soc. Jpn. 113 (2005) 71–76. [12] J. Zhu, Z. Zhao, D. Xiao, J. Li, X. Yang, Y. Wu, Catal. Commun. 7 (2006) 29–32. [13] H. Taguchi, S. Yamasaki, A. Itadani, M. Yosinaga, K. Hirota, Catal. Commun. 9 (2008) 1913–1915. [14] S. Specchia, C. Galletti, V. Specchia, Stud. Surf. Sci. Catal. 175 (2010) 59–67.
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CO and soot oxidation over macroporous perovskite LaFeO3 Ping Xiao a , Linyun Zhong a , Junjiang Zhu a,∗ , Jingping Hong a , Jing Li b , Hailong Li a , Yujun Zhu b,∗ a Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, Wuhan, China b Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials, Heilongjiang University, Harbin 150080, PR China
a r t i c l e
i n f o
Article history: Received 12 October 2014 Received in revised form 9 December 2014 Accepted 7 January 2015 Available online xxx Keywords: LaFeO3 Macroporous Catalytic oxidation Carbon monoxide Soot
a b s t r a c t Bulk, supported and macroporous perovskite LaFeO3 are synthesized and investigated as catalysts for CO and soot oxidation. The structure and physicochemical properties of LaFeO3 are characterized by XRD patterns, N2 physisorption isotherms, TEM, XPS, O2 -TPD and H2 -TPR measurements. Pure LaFeO3 with desired textural structures are successfully prepared, and the porous sample exhibits the best activity to both reactions, pointing out the importance of fabrication of porous perovskite for the reactions. By correlating with the catalytic activities and the physicochemical properties, it is inferred that the best activity obtained from the porous LaFeO3 is attributed to its large surface area, rich active lattice oxygen and strong surface reducibility. A decrease in the activity of porous LaFeO3 is observed for both reactions when vapor is added, but the activity is sustainable even the vapor percentage increases up to 13.5%, and moreover, the activity can be recovered after vapor is removed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Exhaust emitted from internal combustion engine consuming fossil materials contains various substances that are harmful to the environment. The removal of these harmful substances is a huge task of today, especially with the increasing demands of environmental protection. Catalysis is an effective technology in solving environmental problems including exhaust. The crucial factor of this technology is to find a suitable catalyst, which can fit to the various situations of exhaust, such as the different types of substances, the variation in temperature at cold-start and normal operating conditions, and the co-existence of vapor in the exhaust. Among the catalysts investigated for exhaust removal, perovskite oxides receive great attentions. The low-cost, high thermal and hydrothermal stability, straightforward synthesis as well as considerable activity enable them as interesting catalysts for industrial application [1–5]. Also, they are attractive materials for scientific researches as they can be suitable model catalysts in correlating physicochemical properties with catalytic performances because of the structural features, of which the cation at either A- or B-site can be replaced by a foreign one without destroying
∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (J. Zhu), [email protected], [email protected] (Y. Zhu).
the matrix structure, and the content of oxygen vacancy and the oxidation state of B-site cation can be controlled as desired [6–9]. For perovskite oxide with ABO3 structure, the B-site cation is believed to be an active site of reactions and accounts for the structural stability [10,11]. It has been reported that LaCoO3 and LaMnO3 that exhibit higher activity for CO oxidation, methane combustion and soot oxidation are less stable at high temperatures. In contrast, LaFeO3 that shows lower activity to the reactions has better thermal stability, especially under the fluctuating redox conditions of automobile exhaust gases [11]. In view of the long-term use in industrial applications, LaFeO3 is more desired especially when the material is applied to high-temperature reactions. Surface area is a crucial issue needs to be considered when applying perovskite oxides for catalysis use, as they normally have low surface area because of the high temperatures and long calcination time used in the preparation process, which would limit their catalytic performances as catalysis belongs to a class of surface reaction. Furthermore, the surface area could also affect some physicochemical properties of a material such as the reducibility and the oxygen vacancy. Many strategies have been reported to increase the surface area of perovskite oxides by, for example, using organic complexes to homogenize and combust the metal precursors in the sol–gel or combustion methods [12–15], using high-surface-area carriers to support and disperse the materials [16–18], or using template or surfactant to prepare porous materials [19–21]. The latter is more desired especially when the material
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is used for solid soot oxidation [22–24], as its porous structure not only allows a good mass transfer of the reactant, but also exhibits more active sites on the surface accessible to the reactant. In this study, we prepared a porous LaFeO3 and investigated its catalytic performances for the oxidation of CO and soot powder, which are two components of exhaust emitted from mobile cars, and in order to illuminate its advantages in the reactions, bulk and supported LaFeO3 were synthesized and compared. Characterizations by XRD and TEM indicated that LaFeO3 with desired textural structures are successfully prepared. Catalytic results showed that the porous LaFeO3 exhibits high activity to CO and soot oxidation, owing to its large surface area, rich active lattice oxygen and strong reducibility, as evidenced by O2 -TPD, H2 -TPR and XPS measurements. To test its applicability in real conditions, effect of vapor on the catalytic performances of porous LaFeO3 is also investigated, showing a slight decrease in the activities when vapor is added to the reactant, but the activity is sustainable even the vapor percentage reaches up to 13.5 vol.%, and the activity can be recovered when the vapor is removed. 2. Experimental Porous LaFeO3 was prepared using PMMA colloidal crystals as template, which is synthesized according to the work reported by Li et al. [25]. The LaFeO3 /SBA-15 (loading: 55 wt%) was prepared by impregnation method as described in a previous work [17]. Bulk LaFeO3 was prepared similar as that for LaFeO3 /SBA-15, except without the addition of SBA-15. The catalysts were characterized by X-ray diffraction (XRD) patterns, transmission electron microscopy (TEM), N2 adsorption– desorption isotherm, X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2 -TPR) and oxygen temperature-programmed desorption (O2 -TPD). Both CO and soot oxidation were conducted on a continuous flow fixed-bed microreactor. For CO oxidation, the reactant 0.5% CO + 7.5% O2 + 92% Ar was passed through 0.1 g powder catalyst at a flow rate of 50 mL min−1 , and the activity was evaluated in terms of CO conversion. For soot oxidation, 5% O2 /Ar was passed through a mixture containing 0.1 g catalyst and 0.01 g soot (which were packed in a loose mode) at a flow rate of 50 mL min−1 and the temperature was raised from 150 to 650 ◦ C at a heating rate of 2 ◦ C min−1 . The activity was evaluated in terms of CO2 concentration determined in the effluent gases. In case the vapor was introduced, the reactant was passed through a water bubbler before entering the reactor, and the vapor percentage in the reactant was controlled by changing the temperature of the water bubbler. For the detailed procedures and parameters of catalyst preparation, characterization and activity tests, see the experimental section of supplementary information (SI). 3. Results and discussion 3.1. Characterizations Fig. 1 presents the wide-angle XRD patterns of the samples, showing that all possess the LaFeO3 perovskite structure (JCPDS 37-1493). In a previous work, we have shown that pure LaFeO3 phase can be synthesized in bulk LaFeO3 and LaFeO3 /SBA-15 [17]. Here, also for the porous LaFeO3 , no impurity such as La(OH)3 or Fe(OH)3 is observable in the XRD patterns, indicating that pure LaFeO3 is formed. The perovskite structure of samples is also confirmed by FT-IR spectra, which show a strong absorption band at 555 cm−1 assigned to the Fe O stretching vibration of octahedral FeO6 groups in the perovskites (see Fig. S1). The major difference in XRD patterns is the peak intensity. Bulk LaFeO3 has the strongest
Fig. 1. Wide-angle XRD patterns for bulk LaFeO3 , LaFeO3 /SBA-15 and porous LaFeO3 .
peak intensity and LaFeO3 /SBA-15 has the weakest. By calculating the full-width at half-maxima (FWHM) of peak at 2 ≈ 32.2◦ , it is found that the value for bulk, supported and porous LaFeO3 increases from 0.219 to 0.448 and to 0.454. The crystal size, calculated based on the Scherrer equation, is accordingly decreased from 37.7 to 18.3 and to 18.0 nm (see Table 1). The weak peak intensity of LaFeO3 /SBA-15 could be additionally attributed to a diluted effect of SBA-15. Small-angle XRD patterns for LaFeO3 /SBA-15 show that the porous structure of SBA-15 is not destroyed after the formation of LaFeO3 , demonstrating that the LaFeO3 /SBA-15 composite is synthesized, as was previously observed [17]. As expected, the surface area of supported and porous LaFeO3 is improved relative to the bulk one (Table 1). The porous LaFeO3 exhibits surface area ca. five times bigger than the bulk one, suggesting that pores should be generated, which is also supported by the TEM image in Fig. 2A. For LaFeO3 /SBA-15, deposition of LaFeO3 does not destroy the porous structure of SBA-15 and the H1 hysteresis loop attributed to SBA-15 is present in the N2 physisorption isotherms (see Fig. S2) [17]. Although the value in Table 1 represents not the real surface area of LaFeO3 (it represents the surface area of LaFeO3 /SBA-15 composite), it can be imagined that the surface area should be larger than that of the bulk one, due to its smaller particle size, as demonstrated by Fig. 2B and C, which represent the TEM image of LaFeO3 /SBA-15 and bulk LaFeO3 , respectively. Fig. 3 shows the XPS spectra of La 3d, Fe 2p and O 1s for the bulk, supported and porous LaFeO3 . Both the La 3d3/2 and La 3d5/2 peaks are split and the degree of split increases from bulk LaFeO3 to LaFeO3 /SBA-15 (Fig. 3A). This split is caused by the transfer of electrons from O 2p to the unoccupied orbital of La 4f [26]. Thus, the increase in the peak split suggests that the influence of electrons on the unoccupied orbital of La 4f is stronger in LaFeO3 /SBA-15 than that in bulk LaFeO3 . The reason could be that the supported LaFeO3 has small particle size and the unoccupied orbital of La 4f is influenced not only by the electrons from O 2p of LaFeO3 , but also from that of SBA-15 (SiO2 ), which contains oxygen also. The peak split of La 3d for porous LaFeO3 is similar to that for the bulk one, but the peak position shifts to higher binding energy. The binding energy of Fe 2p shifts to higher position from the bulk to supported and further to porous LaFeO3 (Fig. 3B), suggesting the increase of Fe oxidation state or oxidizing capability, in order of porous LaFeO3 > LaFeO3 /SBA-15 > bulk LaFeO3 . To verify and explain the shift in the binding energy of La and Fe elements, the XPS spectra of oxygen atoms, which bond directly with La and Fe atoms and is the solely negative atom in LaFeO3 , are analyzed and deconvoluted by peak fitting method using
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Table 1 The crystalline size, BET surface area, molar ratio of OII /OIII in XPS spectra and H2 consumption in H2 -TPR measurements for bulk LaFeO3 , LaFeO3 /SBA-15 and porous LaFeO3 .a Sample
Crystal size (nm)b
SBET (m2 g−1 ) c
OII /OIII d
Bulk LaFeO3 LaFeO3 /SBA-15 Porous LaFeO3
37.3 18.3 18.0
4.3 161.6 20.6
4.83 – 2.575.57 f
a b c d e f
H2 consumption (mol)e T1
T2
T3
T4
19.8 13.9 58.1
40.7 29.4 73.1
66.1 69.1 134.5
– – 76.2
For the meaning of letters (OII , OIII , T1 , T2 , T3 , T4 ), refer text. Determined by the Scherrer equation from the XRD patterns. Determined by the BET method from the N2 physisorption isotherms. Data obtained from the O 1s XPS spectra. Data obtained from the H2 -TPR measurements. Data for the used sample.
the Gaussian functions (Fig. 3C). It should be noted that because SBA-15 also contains oxygen atoms that contribute to the O 1s spectra, the O 1s spectra of LaFeO3 /SBA-15 reflect not exactly the oxygen information for LaFeO3 , thus we did not make special effort on analyzing and discussing the oxygen properties of this sample. For the bulk and porous LaFeO3 , three peaks are fitted and for each peak the binding energy shifts to lower position from the bulk to the porous LaFeO3 , indicating that the oxygen in the latter is more negative. This is in accordance to the above results that the La and Fe elements of the porous LaFeO3 are more positive than these of the bulk one. Based on the classifications suggested in the previous work [27–30], the peak at 533 (535), 531(532) and 528(530) eV is attributed to oxygen originated from adsorbed water or hydroxyl group (OI ), chemically adsorbed oxygen (OII ) and lattice oxygen (OIII ), respectively. From the molar ratio of OII /OIII of the bulk and porous LaFeO3 (Table 1), it is seen that the former has higher OII /OIII than the latter, which is in accordance to previous results observed
for bulk and porous LaCoO3 [31,32], indicating that the percentage of lattice oxygen exposed on the surface of porous LaFeO3 is more than that of bulk LaFeO3 , or vice versa, the percentage of surface adsorbed oxygen on porous LaFeO3 is less than that of bulk LaFeO3 . It should, however, be noticed that the absolute amount of surface-adsorbed oxygen on porous LaFeO3 is more than that of bulk LaFeO3 , due to its larger surface area. Fig. 4 presents the O2 -TPD profiles of the samples. For O2 -TPD profile of perovskite oxides, four types of oxygen, can be classified based on the desorption temperature of the oxygen [33–37]. The first is physically adsorbed oxygen (O2 ), which stays physically on the surface and desorbs at temperature lower than 60 ◦ C; the second is ordinarily chemical adsorbed oxygen (O2 − ), which has weak interaction with the catalyst’s surface and desorbs at temperature of 60–300 ◦ C; the third is chemically adsorbed oxygen on the oxygen vacancies (O− ), which has strong interaction with the surface defect of catalyst and desorbs at temperature of 300–600 ◦ C; and
Fig. 2. TEM images for (A) porous LaFeO3 , (B) LaFeO3 /SBA-15 and (C) bulk LaFeO3 .
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Fig. 3. (A) La 3d, (B) Fe 2p and (C) O 1s XPS spectra for the bulkLaFeO3 , LaFeO3 /SBA-15 and porous LaFeO3 .
the last is lattice oxygen (O2− ), which is the unit composing the framework of perovskite oxide, and thus its desorption occurs at a relative high temperature, normally above 600 ◦ C. From the O2 -TPD profiles of the bulk, supported and porous LaFeO3 samples investigated here, it is however hard to differentiate the oxygen species desorbed from each sample based on the desorption temperature due to the very weak peak intensity observed in the profiles. Because of this, we thus did not make special effort on discussing the oxygen species desorbed from each sample. However, it is noticed that an intense and sharp desorption peak appears at temperatures of 600–800 ◦ C for the porous LaFeO3 , and no observable peak appears for the bulk LaFeO3 and LaFeO3 /SBA-15. This suggests that the porous LaFeO3 has more rich and active lattice oxygen than the bulk and supported LaFeO3 . This is crucial for reactions conducted at high temperature, where lattice oxygen could participate in the reaction.
Fig. 4. The O2 -TPD profiles for the bulk, supported and porous LaFeO3 .
To study the reducibility of samples, H2 -TPR measurements are conducted and the results are shown in Fig. 5. Bulk LaFeO3 shows an apparent reduction peak at 449 ◦ C, with two shoulder peaks at about 330 and 400 ◦ C. LaFeO3 /SBA-15 shows an apparent reduction peak at 436 ◦ C. Although definite shoulder peak is not observed, the slope at the left of the peak indicates that they should be existed. The shape of the reduction peak of porous LaFeO3 is complex. Besides a large shoulder peak at ca. 420 ◦ C, two split peaks at 481 and 533 ◦ C appear. It is known that there normally have four reduction steps for the reduction of LaFeO3 , corresponding to (1) the reduction of Fe4+ to Fe3+ ; (2) the reduction of surface Fe3+ to Fe2+ ; (3) the reduction of bulk Fe3+ to Fe2+ and (4) the reduction of Fe2+ to metallic iron [38–40]. Based on this classification, the reduction peaks are analyzed and deconvoluted, using the peak fitting function of the Origin software. Three peaks are fitted for bulk LaFeO3 and LaFeO3 /SBA-15, corresponding to the former three reduction steps, and are marked as T1 , T2 and T3 in the pictures. Four peaks, however, are fitted for the porous LaFeO3 , with the last one (T4 ) corresponding to the fourth reduction steps, namely the reduction of Fe2+ to metallic iron. This peak is fitted because its temperature (533 ◦ C) is far higher than that observed for the bulk LaFeO3 (449 ◦ C) and the LaFeO3 /SBA-15 (436 ◦ C). The large difference (ca. 100 ◦ C) in temperature indicates that this species is less reducible than that of bulk LaFeO3 and/or LaFeO3 /SBA-15. Consequently, a fourth peak is fitted for the porous LaFeO3 and is attributed to the reduction of Fe2+ to Fe◦ . The temperature at peak maximum and H2 consumption for each peak are marked in Fig. 5 and listed in Table 1. The porous LaFeO3 shows the largest H2 consumption for peaks T1 and T2 , suggesting that it has more Fe4+ /Fe3+ species on the surface, in accordance to the results obtained from XPS spectra. Even for the third reduction step, the porous LaFeO3 shows almost double H2 consumption than that for the bulk one, due to its rich active lattice oxygen as demonstrated in O2 -TPD measurements. The H2
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Fig. 5. H2 -TPR profiles for SBA-15 and the bulk, supported and porous LaFeO3 , and the corresponding peak fitting for each sample.
consumption for peaks T1 and T2 of LaFeO3 /SBA-15 is larger than half (55%) of that of bulk LaFeO3 , which could be that, on the one hand, the former has smaller particle size and is more reducible, and on the other hand, some species (e.g., hydroxyl groups) from SBA15 are reduced. This is more crucial at high temperature, where the H2 consumption for peak T3 of LaFeO3 /SBA-15 is even comparable to that of bulk LaFeO3 . The temperature at peak maximum of T1 , T2 and T3 is similar for bulk LaFeO3 and LaFeO3 /SBA-15, both are in particle status and the absolute difference in H2 consumption is small (<10 mol). The temperature at peak maximum of T1 (334 ◦ C) and T2 (400 ◦ C) of the porous LaFeO3 is similar to that of bulk LaFeO3 and LaFeO3 /SBA-15. This seems strange as the porous LaFeO3 has stronger oxidability (see XPS spectra) and should be more reducible. However, considering the big difference in H2 consumption between them (about 40 mol, see Table 1), the similar reduction temperature observed here could be that the reduction temperature of porous LaFeO3 is delayed because of the more amount of H2 consumed. Large H2 consumption normally will delay the appearance of temperature at peak maximum as the reduction proceeds with reaction time, and this delaying effect is more pronounced if bigger difference in H2 consumption appears. Indeed, the temperature at peak maximum of T3 of the porous LaFeO3 (477 ◦ C) is even higher than that of bulk LaFeO3 (447 ◦ C) and LaFeO3 /SBA-15 (445 ◦ C) due to its large H2 consumption. This suggests that the reduction temperature at peak maximum should be carefully considered when used to estimate the reducibility of samples if the difference in H2 consumption is too large. 3.2. Catalytic performances 3.2.1. CO oxidation For CO oxidation, it is generally believed that the molecular oxygen should be dissociated into atomic oxygen before participating in the reaction [41,42]. Hence, a catalyst that can provide
sites (e.g., oxygen vacancies) and electrons for molecular oxygen adsorption and activation (e.g., O2 + 2e− = 2O− ) would facilitate the reaction. For perovskite oxides, it is known that the generation of oxygen vacancy accompanies the increase in oxidation state for the B-site cations and the enrichment of electrons, in form of, for example, Fe3+ − O − Fe3+ → Fe4+ −(e− ·) − Fe3+ + 1/2O2 . This implies that sample possessing more oxygen vacancies has stronger ability for oxygen adsorption and activation. Once the molecular oxygen is activated, it can react with adsorbed CO to yield CO2 and regenerate the active site (oxygen vacancy). As expected, the porous LaFeO3 that possesses the largest amount of oxygen vacancies and the strongest reducibility shows the lowest onset temperature and the highest catalytic efficiency for CO oxidation (Fig. 6A). Although it is hard to compare directly the amount of oxygen vacancies for the bulk and supported LaFeO3 due to the interference of SBA-15, it can be inferred that more oxygen vacancies should be generated in LaFeO3 /SBA-15, since the metals exhibit higher binding energies in the XPS spectra. In order to illuminate the advantage of using LaFeO3 /SBA-15 for CO oxidation at low temperature, the reaction rate of CO oxidation over the catalysts are calculated (Fig. 6B). Obviously, LaFeO3 /SBA-15 shows higher reaction rate for CO oxidation relative to bulk LaFeO3 at temperature above 200 ◦ C, due to the increased oxygen vacancies as well as possibly the support effect, which at least results in smaller particle size. To test whether the porous LaFeO3 could be applied to real exhaust environment, where vapor is present, the effect of vapor on the reaction is tested. The activity at each vapor percentage was measured for three times (or 45 min) to ensure that it reaches stable status. Fig. 6C presents the CO oxidation activities measured at different percentages of vapors at 340 ◦ C, showing that the activities are influenced when vapor is added. A decrease of 21% in the conversion is observed at 1.7 vol.% vapor percentage, but the activity is sustainable thereafter, even up to 13.5 vol.% vapor percentage. In particular, the activity can be recovered after vapor is removed.
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Fig. 6. (A) Catalytic activities and (B) the corresponding reaction rate of bulk, supported and porous LaFeO3 for CO oxidation; (C) effect of vapor on the activity of CO oxidation conducted on porous LaFeO3 at 340 ◦ C. Note: Reaction rate was calculated in terms of mole of CO converted per second and per mole of catalyst.
This indicates that although the presence of vapor influences the catalytic behavior of catalyst, it does not deteriorate the active site and the catalyst can endure up to 13.5 vol.% vapor in the reactant. 3.2.2. Soot oxidation Soot is one type of particulate matter (PM) in the exhaust. Soot with diameter below 2.5 m, known as PM 2.5, causes serious damage to the respiratory organs of human beings and is one of the major sources leading to smog formation. With the increase of automobiles, large amounts of soot are emitted to the air resulting in increasingly serious deterioration of our environment. Legislation on the emission of soot as well as other toxic matters has been issued in almost all the countries. Nevertheless, the removal of soot is still a big challenging task of today despite the achievements made over the years [43–48]. Other than gaseous CO, soot is a solid that cannot diffuse or mobilize freely to the catalyst’s surface or pores, which increases the difficulty of its catalytic oxidation removal. One effective way of improving the ability to soot oxidation is to increase the surface area of catalyst [23,24], so that the soot can be in contact with the catalyst as efficiently as possible and be oxidized. Fig. 7A shows the CO2 concentration yielded in the reaction as a function of reaction temperature over the catalysts. In accordance to that reported by Zhao et al. [23], the porous sample shows the best ability to soot oxidation because of its enhanced surface area and porous structure. Bulk LaFeO3 also exhibits good ability to soot oxidation and the activity is better than that of LaFeO3 /SBA-15. This could be that, on the one hand, the absolute active site is fewer in LaFeO3 /SBA-15 as only 0.055 g LaFeO3 (vs. 0.1 g in the bulk LaFeO3 ) is present, and on the other hand, the soot cannot be mobilized freely as CO, thus large amounts of soot stayed on the barren surface of SBA-15 (without LaFeO3 ) cannot be catalytically oxidized into CO2 . For comparison, the soot oxidation conducted over pure SBA15 and that in the blank experiment (without catalyst) is also tested, showing that SBA-15 can improve the reaction and soot can be selfcombusted at high temperature, but the major product is CO, not CO2 .
Although CO2 is formed in each reaction, its concentration varies with the catalyst. To meet the carbon balance, the concentration of CO, which is a byproduct of the reaction, in the tail gas is also monitored (Fig. 7B). Large amounts of CO are produced in the blank experiment and that use SBA-15 as catalyst, and some are produced over the bulk and supported LaFeO3 . Negligible CO is detected when porous LaFeO3 is used as catalyst. These results indicate that soot can be burn off and oxidized into CO at high temperature by itself, but the further oxidation into CO2 depends on the properties of catalyst. Here, the porous LaFeO3 has the best ability to oxidize soot into CO2 relative to the other samples, which is probably attributed to its large surface area, rich active lattice oxygen as well as strong oxidizing ability, as discussed above. Table 2 lists the total amounts of CO + CO2 species produced in the reaction, showing that the carbon is approximately balanced. Blank experiment indicates that no black material in the reactor could be seen by the naked eye after reaction, confirming that soot is fully oxidized. From the selectivity to CO2 (SCO2 ), it is seen that porous LaFeO3 with large surface area exhibits high selectivity to CO2 , pointing out the importance of the contact area for total oxidation of solid soot. LaFeO3 /SBA-15 shows slightly lower CO2 selectivity than bulk LaFeO3 , which perhaps is due to its less (absolute) active sites and especially the fact that a certain amount of soot is stayed on the barren surface of SBA-15, thus cannot contact adequately with the active site and be oxidized into CO2 . The temperature for soot ignition (Tig , defined as 10% soot oxidation) and full oxidation (Tm , defined as the temperature at peak maximum of CO2 ) over the catalysts are also listed in Table 2, showing that the porous LaFeO3 gives the best catalytic performance, with Tig and Tm of 374 and 560 ◦ C, respectively. In comparison, the bulk LaFeO3 shows an increased Tig and Tm by 19 and 17 ◦ C, respectively. LaFeO3 /SBA-15 displays the worst catalytic performance, with Tig of 402 ◦ C and Tm of 578 ◦ C, the latter is similar to that of bulk LaFeO3 . Soot can be self-ignited at temperature of 410 ◦ C, and the temperature for the maximum yield of CO2 is 576 ◦ C, which is similar to that observed in the presence of bulk LaFeO3 . The low Tm observed in this case should be the reason that only a small
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Fig. 7. (A) CO2 and (B) CO concentration detected in the tail gas of soot oxidation reaction over different catalysts and, that under blank experiment (without catalyst); (C) effect of vapor on the catalytic performances of porous LaFeO3 for soot oxidation. Table 2 The Tig and Tm of soot oxidation, the amount of CO and CO2 produced, and the selectivity to CO2 detected over the investigated catalysts.
No catalyst SBA-15 LaFeO3 /SBA-15 Bulk LaFeO3 Porous LaFeO3 a b
Tig (◦ C)a
Tm (◦ C)b
CO (mol)
CO2 (mol)
CO + CO2 (mol)
SCO2 (%)
410 410 402 393 374
576 585 578 577 560
1.39E−04 1.12E−04 2.05E−05 1.63E−05 1.22E−06
9.08E−05 1.44E−04 1.84E−04 1.96E−04 2.21E−04
2.30E−04 2.56E−04 2.04E−04 2.12E−04 2.22E−04
39.5 56.3 89.9 92.3 99.5
The temperature for soot ignition. The temperature for soot full oxidation.
amount of CO2 is produced as the main product is CO. From the CO2 yield it is known that the presence of SBA-15 can improve the soot oxidation process (to CO2 ) although it has no contribution to the ignition temperature. Compared with previous literature,[49] the Tm observed in this study is higher, which could be that in previous work the reactant contains also NO, which can act as an oxidant and facilitate the reaction.[50] The effect of vapor on the catalytic performances of porous LaFeO3 for soot oxidation is also studied, to test if the catalyst could be used in simulated real engine exhaust conditions, as that for CO oxidation. Fig. 7C shows the CO2 yield obtained from the reaction at different vapor percentage as a function of reaction temperature. A slight increase in the temperature for the maximum CO2 formation is observed when vapor is added to the reaction, and the temperature remains unchanged irrespective of the further increase of vapor percentage (up to 13.5 vol.% in our tests). No CO is detected throughout the reaction, as that observed in the absence of vapor, indicating that the sample has well endurance to vapor during the reaction, and thus could be a potential candidate for industrial application in future. In justifying the stability of porous LaFeO3 , the used sample was subjected to XRD, BET and XPS measurements and compared with that of the fresh sample. XRD and BET results (see Figs. S2 and S3) indicate that the perovskite structure of porous LaFeO3 is well retained after the reaction, implying that the sample has good stability. XPS results (see Fig. S4) indicate that the binding energies of La 3d and Fe 2p shift to high position and that of O
1S shifts to low position, after the reaction. The reason could be that the surface oxygen is consumed during the reaction (by reacting with the soot), leaving more exposed positive metal ions and stronger bonded oxygen. That is, more oxygen vacancies are generated. Indeed, by comparing the molar ratio of OII to OIII (OII /OIII ; see Table 1), it is found that the ratio is increased after the reaction, from 2.6 to 5.6, justifying again that the surface oxygen is consumed and more oxygen vacancies are generated. 4. Conclusions Catalytic performances of bulk, supported and porous LaFeO3 for CO and soot oxidation have been investigated, showing that the porous LaFeO3 has the best ability to both reactions. TEM image and N2 physisorption isotherms confirm that the sample prepared using PMMA as template has porous structure and enhanced surface area, and thus increased contact area with the reactant. XPS, H2 -TPR and O2 -TPD results indicate that the porous LaFeO3 has more oxygen vacancy, stronger oxidizing ability and active lattice oxygen, explaining its well ability to CO and soot oxidation. The applicability of porous LaFeO3 to CO and soot oxidation in the presence of vapor is also investigated, showing a decrease in the conversion but has well endurance even up to 13.5% vapor in volume. The decreased activity can be recovered when the vapor is switched off, indicating that the active site is not destroyed and the porous LaFeO3 has strong resistance to vapor poisoning, potentiating its possible application for industrial removal of exhaust.
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Acknowledgements Financial support from the National Science Foundation of China (21203254), the Scientific Research Foundation for Returned Scholars, Ministry of Education of China (BZY11055), the Science and Technology Activities of Overseas Personnel Preferential Funding Project (No. BZY14038), the National Demonstration Center of Experimental Teaching on Ethnical Pharmacy, South-Central University for Nationalities, The Project-sponsored by SRF for ROCS, SEM (2013-1792), Program for New Century Excellent Talents in Heilongjiang Provincial University (1155-NCET-014), Postdoctoral Science-research Developmental Foundation of Heilongjiang Province of China (LBH-Q12022) and Innovative Research Team in Heilongjiang University (Hdtd2010-10) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015. 01.007. References [1] R.J.H. Voorhoeve, Advanced Materials in Catalysis, Academic Press, New York, 1977. [2] S. Hernández, G.A. Blengini, N. Russo, D. Fino, Ind. Eng. Chem. Res. 51 (2011) 7584–7589. [3] R. Zhang, W. Yang, N. Luo, P. Li, Z. Lei, B. Chen, Appl. Catal. B 146 (2014) 94–104. [4] Y. Wu, X. Ni, A. Beaurain, C. Dujardin, P. Granger, Appl. Catal. B 125 (2012) 149–157. [5] F.E. López-Suárez, A. Bueno-López, M.J. Illán-Gómez, A. Adamski, B. Ura, J. Trawczynski, Environ. Sci. Technol. 42 (2008) 7670–7675. [6] Z. Zhao, X.G. Yang, Y. Wu, Appl. Catal. B 8 (1996) 281–297. [7] Y. Wu, T. Yu, B.S. Dou, C.X. Wang, X.F. Xie, Z.L. Yu, S.R. Fan, Z.R. Fan, L.C. Wang, J. Catal. 120 (1989) 88–107. [8] J.J. Zhu, A. Thomas, Appl. Catal. B 92 (2009) 225–233. [9] M.A. Pena, J.L.G. Fierro, Chem. Rev. 101 (2001) 1981–2017. [10] H. Ziaei-Azad, A. Khodadadi, P. Esmaeilnejad-Ahranjani, Y. Mortazavi, Appl. Catal. B 102 (2011) 62–70. [11] I. Tan, H. Tanaka, M. Uenishi, K. Kaneko, S. Mitachi, J. Ceram. Soc. Jpn. 113 (2005) 71–76. [12] J. Zhu, Z. Zhao, D. Xiao, J. Li, X. Yang, Y. Wu, Catal. Commun. 7 (2006) 29–32. [13] H. Taguchi, S. Yamasaki, A. Itadani, M. Yosinaga, K. Hirota, Catal. Commun. 9 (2008) 1913–1915. [14] S. Specchia, C. Galletti, V. Specchia, Stud. Surf. Sci. Catal. 175 (2010) 59–67.
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Please cite this article in press as: P. Xiao, et al., CO and soot oxidation over macroporous perovskite LaFeO3 , Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.01.007