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Nanocrystalline La1 xKxFeO3 (x = 0 0.4) Oxides for Catalytic Removal of Soot from Practical Diesel Exhaust Emission Xiaoxiao Meng,† Fenglin He,† Xiangqian Shen,†,* Jun Xiang,† and Pan Wang‡ †
School of Material Science and Engineering, ‡School of Automotive and Traffic Engineering, Jiangsu University, Zhenjiang 212013, P.R.China ABSTRACT: The nanocrystalline, porous, perovskite La1 xKxFeO3 (x = 0 0.4) powders and La0.8K0.2FeO3-coated honeycomb ceramic device were prepared by the citrate-gel process and the citrate-gel assisted dip-coating method at a low calcination temperature of 600C for 6 h, respectively. All the perovskite powders can effectively catalyze the soot combustion and among them the La0.8K0.2FeO3 catalyst exhibits the highest catalytic activity for the soot combustion, with a lowest T50 (350 °C) owing to a high specific surface area and large pore sizes. This optimized La0.8K0.2FeO3 catalyst is proved by the bench test for the practical exhaust gas emission and the La0.8K0.2FeO3-coated honeycomb ceramic device has a effective capture of particulate matter from the exhaust emission at a low temperature and starts to efficiently catalyze oxidization reactions of the particulate matter at the temperature around 245 °C (T20), leads the smoke opacity near zero at the operational temperature of 400 °C.
1. INTRODUCTION The main pollutants for diesel engine exhaust emissions are soot and nitrogen oxides (NOx). Because of serious health and environmental concerns, there is increasingly a demand to develop technologies for controlling of emissions from diesel engines.1 5 Investigations proved that the catalytic route was effective for simultaneous removal of the soot and NOx.6 10 Among various catalysts, perovskite oxides have attracted a great attention because of their low cost, good catalytic activity, and high thermal stability.4 6,11,12 The perovskite oxides have a general formula of ABO3, where A designates a rare-earth or alkaline earth cation and B a transition metal cation.13 15 The most studied catalysts are the perovskites based on La at A-site and the elements at B-site are usually Co, Mn and Fe.16 19 As the catalytic combustion of soot by perovskite oxides is generally of heterogeneous reactions, the catalytic performance is mainly influenced by the surface-oxygen activity and contact state between the catalyst and soot.13,20 22 The surfaceoxygen activity in perovskite oxides is affected by ions substitutions and grain sizes, while the contact between the catalyst and soot is usually related to the specific surface area and pore structure of perovskite oxides.23 25 Teraoka et al.26 found out that La1 xKxMnO3 (x = 0.2 and 0.25) catalysts exhibited a high catalytic activity and selectivity, and were superior to La0.9K0.1CoO3. Wang et al.4 reported that the nanosized La1 xKxMnO3 had a very high catalyzing performance under a loose-contact condition, as the nanosized particles could contact well with the soot. In order to improve the contact between the catalyst and soot, three-dimensionally ordered macroporous (3DOM) perovskite oxides were prepared using a colloidal crystal of polymer spheres as templates.27,28 These 3DOM perovskite oxides showed a better catalytic combustion performance than the counterpart nanopowders. Although the reported perovskite oxides were attractive, compared to the noble metal based catalysts the catalytic activity for most of them was lower largely because of a small specific surface area,13,29 unsuitable pore sizes for the particulate r 2011 American Chemical Society
matter trap and their catalytic performances were estimated under the modeling conditions. Therefore, for the interests of industrial applications, the catalyst performance for the perovskite oxides needs a proof under practical conditions of diesel exhaust gas emission.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The nanocrystalline perovskite oxides of La1 xKxFeO3(x = 0 0.4) were synthesized by the citrate-gel process.15,30,31 The reagents used were analytic grade of La(NO3)3 3 6H2O, Fe(NO3)3 3 9H2O, K2CO3 and citric acid (CA). The molar ratio of citric acid to the total metal ions was 1:1. First, the required metal salts and citric acid were dissolved in deionized water and the solution pH 7.5 was adjusted with ammonia. After magnetically stirred for about 24 h at room temperature, the solution was transferred into a vacuum rotary evaporator to remove surplus water at 60 °C until a viscous liquid was obtained, which was used for the following powder formation and dip-coating as well. Then, the viscous liquid was dried in a vacuum oven at 90 °C for about 24 h to obtain a dried gel precursor, which was calcined at 600 °C for 6 h in air to form sponge powders. A cordierite honeycomb ceramic with diameter of 85 mm, length of 110 mm, 400 cells per square inch and specific surface area of 1.2 m2/g was dip-coated in the viscous liquid (with the viscosity of 1.47 mPa 3 s) for 30 min at room temperature. Then, the dip-coated honeycomb ceramic was dried at 90 °C for about 24 h in an oven and followed a heat treatment at 300 °C for 60 min. The coating and heat treatment procedure was repeated to load about 10 wt % of oxide catalyst onto the honeycomb Received: May 24, 2011 Accepted: August 23, 2011 Revised: August 10, 2011 Published: August 23, 2011 11037
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Table 1. Grain Sizes (D), Specific Surface Areas (SBET) and Tα of the La1 xKxFeO3 Catalysts D
SBET
Pav
T20
T50
T90
La1 xKxFeO3
(nm)
(m2/g)
(nm)
(°C)
(°C)
(°C)
LaFeO3 La0.95K0.05FeO3
28 26
14.7 18.2
5.77 6.40
281 277
392 381
460 453
La0.9K0.1FeO3
24
14.6
8.63
270
373
449
La0.8K0.2FeO3
22
16.9
11.59
242
350
409
La0.7K0.3FeO3
24
15.9
4.98
260
352
420
La0.6K0.4FeO3
23
15.2
5.92
265
353
418
Figure 1. Illustration of apparatus for catalyst performance evaluation in continuous flow of practical diesel exhaust gas emissions.
Figure 3. FT-IR spectra of perovskite oxides with various compositions obtained at 600 °C for 6 h. Figure 2. XRD patterns of perovskite oxides with various compositions obtained at 600 °C for 6 h.
ceramic. Finally, the loaded honeycomb ceramic was calcined at 600 °C for 6 h to form the perovskite oxide-coated honeycomb ceramic device with the coating thickness about 30 μm, which was used for the smoke opacity measurement under the practical diesel exhaust gases. The dip-coating process was described in detail in our previous work for the La Mn O-based catalysts.31 2.2. Catalyst Characterization. The structure was characterized by Fourier transform infrared spectroscopy (FT-IR) with a Nicolet 670 spectrometer, X-ray diffraction (XRD) with CuKα radiation (λ = 1.54 Å) and the XRD patterns were analyzed with the JADE (for windows) XRD pattern-processing software system. The morphology of powders and coatings was investigated using scanning electron microscopy (SEM, JSM-5600LV). The BET surface area was measured by the instrument of NOVA 2000e using N2 adsorption. 2.3. Catalyst Performance Analysis. The catalytic activity for soot combustion was examined by thermo-gravimetric (TG) analysis (SHIMADZU DTG-60H). Commercial amorphous carbon black (Printex U from Degussa) was used as the dummy diesel soot. The experimental procedure was similar to that described in the references.5,32,33The catalyst and dummy soot were mixed with a mass ratio of 9:1 in ethanol with ultrasonic agitation for 10 min to form a slurry. The slurry then was dried at 90 °C for 3 h and 10 mg of the mixture was analyzed by TG at a temperature range from room temperature to 700 °C with a heating rate of 5 °C/min under the air flow of 50 mL/min.
To confirm the catalytic performance, the bench test was adopted and the measurement apparatus in this work is showed in Figure 1.31 The perovskite oxide coated honeycomb ceramic device was examined by the smoke opacity measurement under the practical diesel exhaust gas emission. An opacimeter (AVL Dismoke 4000) was used to record the smoke opacity of the exhaust gas. The diesel fuel (No. 0) was bought from Sinopec Co. of China and the engine worked with a revolution of 2000 r/min and a torque of 18.4 N 3 m.
3. RESULTS AND DISCUSSION 3.1. Perovskite Oxide Formation. The structure of La1 xKxFeO3 powders with various K ion substitutions calcined at 600 °C for 6 h was analyzed by the XRD technique and the XRD patterns are showed in Figure 2. It can be seen that the XRD patterns for all the powders are in a good agreement with the crystalline structure of perovskite LaFeO3 (JCPDS No. 371493), and no other crystalline phases are detected. Compared to the processes reported for the nanosized perovskite oxides,4,15,34 the calcination temperature of 600 °C in the present work is much lower. According to the full-width at half-maximum of the prominent crystal plane (121) as showed in Figure 2, the average grain size (D) can be estimated using the Scherrer’s equation and are represented in Table 1. From table 1, the La1 xKxFeO3 powders (x = 0 0.4) consist of nanosized grains about 22 28 nm. Figure 3 shows the FT-IR spectra of La1-xKxFeO3(x = 0 0.4). There are two strong absorption bands around 400 and 550 cm 1 in the IR spectra of all La1-xKxFeO3 samples. These two peaks are 11038
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Figure 4. SEM morphologies of different catalysts (a) LaFeO3, (b) La0.95K0.05FeO3, (c) La0.9K0.1FeO3, (d) La0.8K0.2FeO3, (e) La0.7K0.3FeO3, (f) La0.6K0.4FeO3.
attributed to Fe O bond’s bending and stretching vibrations in FeO6 octahedron, which indicates that the perovskite oxides are formed. With the substituted-K content increase from 0 to 0.4, the blue shift (559 to 577 cm 1) of the Fe O stretching vibration peak is observed, which means that the ion substitutions are accomplished and the covalent bond proportion in Fe O bonds is enhanced.35 While, it can be observed that with the ion substitution the two peaks become weaker and broader. This is caused by the charge imbalance arising from the replacement of La3+ ions by K+ ions. Consequently, oxygen vacancies will occur in the perovskite unit and the oxygen activity in the La1 xKxFeO3 catalysts can be improved.4,13 3.2. Catalyst Morphology and Specific Surface Area. The SEM morphologies of nanocrystalline La1 xKxFeO3 (x = 0 0.4) powders are showed in Figure 4. It can be seen that these powders are generally porous, composed of nanosized particles.
The pore structure and particle aggregation are in some degree influenced by the substituted-K content. For the La1 xKxFeO3 (x = 0, 0.05, 0.4) powders, there are almost no macropores observed and some particle agglomeration clearly takes place for the oxide with a high K content of 0.4 (Figure 4f). While, large pores about 200 500 nm are obvious for the powders as showed in Figure 4c, d, and e, corresponding to La0.9K0.1FeO3, La0.8K0.2FeO3 and La0.7K0.3FeO3. The pore structure and pore size in the La1 xKxFeO3 (x = 0.1 0.3) powders are similar to those in the 3DOM perovskite oxides which were prepared using a colloidal crystal of polymer spheres as templates, with pore sizes 280 320 nm.27,28 These pores of several hundreds of nanometers can act as nanofilters and will have a good capture for the particulate matter from the exhaust gas emission. Although the porous structure observed for the nanocrystalline La1 xKxFeO3 (x = 0 0.4) powders is generally caused by the decomposition of 11039
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Table 2. T50 Comparison of Perovskite Oxide Catalysts for Soot Combustion catalysts
T50 (°C)
preparation method
LaFeO3
550
combustion synthesis36
LaFeO3 LaFeO3
392 392
polymer template27 citrate-gel (present work)
La0.8K0.2FeO3
508
self-propagating high-temperature reaction18
La0.8K0.2FeO3
350
citrate-gel (present work)
Figure 5. Soot combustion for La0.9K0.1FeO3 and La0.8K0.2FeO3 by TG analysis.
gels during the calcination process with release of gases H2O, NOx, and CO2, the yielding mechanism for large pores needs a further systematic investigation. The specific surface area (SBET) and average pore size (Pav) measured by the BET method for the nanocrystalline La1 xKxFeO3 (x = 0 0.4) powders are presented in Table 1. The oxide powders as-prepared in this work have a high specific area up to 18.2 m2/g, which is much larger compared to 4.71 m2/g for the LaFeO3 powder obtained by Mihai et al.,29 and is similar to the specific surface areas for the La1 xKxFeO3 powders prepared by the citrate-gel process.4 When the substituted-K content at a low level (0 0.2), the Pav value shows a significant increase with the ion substitution, reaches a maximum at x = 0.2 and then reduces with a further increase of the K content up to 0.4. This is basically consistent with the SEM observations showed in Figure 4. It is interesting to note that the La0.8K0.2FeO3 powder is characterized with a relatively large SBET (16.9 m2/g) and a maximum Pav value (11.59 nm). 3.3. Catalytic Performance. The catalytic activity of the La1 xKxFeO3 (x = 0 0.4) catalysts for the dummy soot combustion is estimated by TG and it can be indicated by the temperature at which a given fraction of soot (Tα, α = 20, 50, 90 wt %) is oxidized. A lower value of Tα means a higher catalytic activity. Figure 5 shows typical TG analysis curves of soot combustion for La0.1K0.9FeO3 and La0.2K0.8FeO3, with the comparison of none-catalysts. From Figure 5, the soot combustion can be effectively catalyzed by the nanocrystalline La1 xKxFeO3 (x = 0.1 and 0.2) catalysts and the values of T20, T50, and T90 are far lower than that for the sample without catalysts. Furthermore, by comparing Tα the catalytic activity of La0.2K0.8FeO3 is significantly better than La0.1K0.9FeO3. To further investigate effects of the substituted-K content on the catalytic performance on La1 xKxFeO3 (x = 0 0.4) for soot combustion, T20, T50, and T90 for the nanocrystalline La1 xKxFeO3 catalysts with various K contents (x = 0 0.4) are presented in Table 1. With the substituted-K content increase from 0 to 0.2, the values of T20, T50, and T90 show an obvious reduction and then, with the K content further increase up to 0.4, they tend to increase. The optimized K content of 0.2 so that can be reasonably determined and the catalyst of La0.8K0.2FeO3 exhibits the best catalytic activity for the soot combustion, with T20, T50, and T90 being 242, 350, and 409 °C respectively. Similar optimum K contents were obtained by Teraoka et al for the La1 xKxMnO3
Figure 6. Smoke opacity for La0.8K0.2FeO3 at various exhaust gas emission temperatures.
catalysts,26 with the optimized K contents of 0.2 and 0.25. This can be explained by comprehensive consideration of effects of the surface-oxygen activity, specific surface area and pore size for the nanocrystalline La1 xKxFeO3 catalysts. Although a higher substituted-K content in La1 xKxFeO3 could result in more oxygen vacancies and a high surface-oxygen activity as analysis in the precious section 3.1, the specific surface area and pore size are decreased when the La1 xKxFeO3 with a very high K content (x = 0.3, 0.4) largely because of the particle agglomeration as observed in Figure 4f. On one hand, the increased surface-oxygen activity will benefit the soot combustion, on the other hand, the decreased specific surface area and pore size will deteriorate the contact between the soot and catalyst. The comparison of T50 values for the perovskite oxides reported in literatures and obtained in the present work is presented in Table 2. The T50 (392 °C) value for the LaFeO3 catalyst in this work is the same as that for the 3DOM LaFeO3 prepared by the colloidal crystal of polymer spheres as templates,27 and much lower than that for the catalyst obtained by the combustion synthesis method.36 For the La0.8K0.2FeO3 catalyst, compared to the catalyst prepared by the self-propagating high-temperature reaction process,18 the present T50 (350 °C) value is significantly lower. This catalytic performance improvement can be attributed to a higher specific surface area and larger pore sizes of the nanocrystalline La0.8K0.2FeO3 catalyst. The optimized nanocrystalline La0.8K0.2FeO3 catalyst was used for the bench test under the practical diesel exhaust emission and the measured smoke opacity for the catalyst-loaded honeycomb ceramic device and the bare honeycomb ceramic (nonecatalysts) at the commonly operational temperature range of 200 400 °C is shown in Figure 6. It can be seen that from Figure 6, the smoke opacity shows a monotonous reduction with 11040
dx.doi.org/10.1021/ie201112f |Ind. Eng. Chem. Res. 2011, 50, 11037–11042
Industrial & Engineering Chemistry Research the temperature, indicating the catalytic soot combustion is improved at a higher operational temperature, leading the smoke opacity around zero at 400 °C. Furthermore, even at a low temperature 200 °C, the exhaust emission has a very small smoke opacity about 1.6%, and it can be the result of the efficient capture of particulate matter from the exhaust emission by the porous La0.8K0.2FeO3 catalyst with large pores since the catalyst should have a very low catalytic activity at this temperature according to the above TG analysis (Figure 5). Then, the first significant reduction of the smoke opacity occurs around 245 °C, and this temperature basically corresponds the value of T20 (242 °C) for the catalyst (Table 1), which proves the soot combustion is effectively catalyzed by the La0.8K0.2FeO3 catalyst.
4. CONCLUSIONS (1) The nanocrystalline, porous, perovskite La1 xKxFeO3 (x = 0 0.4) powders and La0.8K0.2FeO3 coated honeycomb ceramic catalysts have been prepared by the citrate-gel process and the citrate-gel assisted dip-coating method at a low calcination temperature 600 °C for 6 h, respectively. (2) The specific surface area and pore size are influenced by the substituted-K content in La1 xKxFeO3 and the optimized chemical composition La0.8K0.2FeO3 is determined as the catalyst for the catalytic soot combustion, with a relatively large SBET (16.9 m2/g) and a maximum Pav value (11.59 nm). (3) All the La1 xKxFeO3 (x = 0 0.4) catalysts show a high catalytic activity for soot combustion. Among them, La0.8K0.2FeO3 is the best, with T20, T50, and T90 being 242, 350, and 409 °C respectively. (4) The bench test for the optimized La0.8K0.2FeO3 catalyst under the practical diesel exhaust emission proves that the La0.8K0.2FeO3-coated honeycomb ceramic is a promising device for the efficient removal of particulate matter from the diesel exhaust gas emissions at the operational temperature range of 200 400 °C. ’ AUTHOR INFORMATION Corresponding Author
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[email protected]. Tel./fax: +86-511-88791964.
’ ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant 50134020) and the Jiangsu Province’s Postgraduate Cultivation and Innovation Project (Grant CX10B-257Z). ’ REFERENCES (1) Martyn, V. T. Progress and future challenges in controlling automotive exhaust gas emissions. Appl. Catal., B 2007, 70, 2. (2) Ulrich, G. A.; Bernd, S. Engines and exhaust after treatment systems for future automotive applications. Solid State Ionics 2006, 177, 2291. (3) Sui, L. N.; Yu, L. Y. Diesel soot oxidation catalyzed by Co-Ba-K catalysts: Evaluation of the performance of the catalysts. Chem. Eng. J. 2008, 142, 327. (4) Wang, H.; Zhao, Z.; Xu, C. M.; Liu, J. Nanometric La1 xKxMnO3 perovskite-type oxides-highly active catalysts for the combustion of
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