Effect of highly dispersed Co3O4 on the catalytic performance of LaCoO3 perovskite in the combustion of lean methane
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摘要: 本研究采用溶胶凝胶法，通过调变镧钴比合成了一种纳米新型钙钛矿类催化剂。利用物理吸附、ICP、XRD、H2-TPR、O2-TPD和XPS等技术对催化剂进行了表征，并对其在乏风甲烷氧化燃烧中的催化性能进行了研究。结果表明，高分散性的Co3O4纳米颗粒有利于甲烷的低温活化，且催化剂中镧钴钙钛矿体相可提供大量的晶格氧，促进高温下甲烷的催化燃烧速率和催化剂的高温稳定性。通过调变镧钴比例，可有效调变催化剂中Co3O4纳米颗粒的分散状态，进而实现催化剂低温活性和高温稳定性的有效统一。当La/Co比为0.9时，在空速为30000 mL/(gcat·h)的条件下，La0.9CoO3钙钛矿催化剂的甲烷起燃温度为382 ℃；稳定运行72 h后，甲烷转化率保持在95%以上。这些结果为今后开发低成本、高活性和高稳定性的甲烷燃烧催化剂提供了参考。Abstract: In this work, a series of nano LaCoO3 perovskite catalysts were effectively synthesized by a sol-gel method through modulating the La/Co molar ratio. These catalysts were characterized by ICP, XRD, N2 sorption, H2-TPR, O2-TPD, and XPS, and their catalytic performance in the lean methane combustion were then investigated. The results indicate that highly dispersed Co3O4 nanoparticles on the LaCoO3 perovskite catalysts are beneficial to the activation of CH4 at a low temperature, while the La-Co-perovskite bulk phase can provide a large amount of lattice oxygen, which can enhance the reaction rate of methane combustion and the catalytic stability at a high temperature. Through altering the La/Co molar ratio, the dispersion of Co3O4 nanoparticles in the La-Co-perovskite catalyst can be effectively modulated, to achieve the concurrence of low-temperature activity and high-temperature stability in the lean methane combustion. In particular, the La0.9CoO3 perovskite catalyst with a La/Co molar ratio of 0.9 exhibits excellent performance in lean methane combustion, with a light-off temperature of 382 ℃ at a space velocity of 30000 mL/(gcat·h), the light-off temperature of methane is 382 ℃, and the methane conversion rate is still maintained above 95% after 72 h of stable operation, indicating that the highly dispersed Co3O4 nanoparticles were beneficial to the low-temperature activation of CH4, and the lanthanum-cobalt-perovskite bulk phase in the catalyst could provide a large amount of lattice oxygen, which promotes the catalytic combustion rate of CH4 and the high-temperature stability of the catalyst under high-temperature conditions. By modulating the lanthanum-cobalt ratio, the dispersion state of Co3O4 nanoparticles in the catalyst can be effectively modulated, and then the effective unification of low-temperature activity and high-temperature stability of the catalyst can be achieved, which guides the future development of low-cost, high-activity and high-stability catalysts for methane catalytic combustion.
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Key words:
- catalytic combustion of lean methane /
- LaCoO3 /
- Co3O4 /
- high dispersion /
- lattice oxygen
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图 1 (a)镧钴系列催化剂的X射线衍射图，(b)36°−38.5°的局部X射线衍射谱图
Figure 1 (a)XRD patterns of LaxCoO3 catalysts；(b)XRD patterns of LaxCoO3 catalysts from 36° to 38.5°
图 3 (a)镧钴系列催化剂的Co 2p和(b) Co 2p3/2拟合谱图
Figure 3 Co 2p spectra (a) and Co 2p3/2 fitting spectra (b) of LaxCoO3 catalysts
图 7 (a)镧钴系列催化剂的甲烷氧化活性和 (b) La0.9CoO3在640 ℃下的稳定性测试
Figure 7 (a) Activity of various LaxCoO3 catalysts in methane oxidation; (b) Stability of various LaxCoO3 catalysts tested at 640 ℃ (0.2 g catalysts, CH4∶O2∶N2=1∶20∶79, GHSV=30000 mL/(gcat·h))
图 8 (a)甲烷浓度和(b)氧气浓度对La0.9CoO3催化甲烷燃烧的影响
Figure 8 Effects of methane (a) and oxygen (b) concentrations on the catalytic combustion of methane over the La0.9CoO3 catalyst (CH4∶O2∶N2=x∶20∶(80−x) or CH4∶O2∶N2=1∶y∶(99−y), GHSV=30000 mL/(gcat·h)
表 1 镧钴系列催化剂的物性参数
Table 1 Physical properties of various LaxCoO3 catalysts
Sample La/Co molar ratio SBET/
（m2·g−1）vpore/
(cm3·g−1)dCo3O4-XRD
/nmLaCoO3 0.917 4.436 0.034 27.76 La0.95CoO3 0.906 4.311 0.121 27.59 La0.9CoO3 0.879 5.504 0.037 28.78 La0.8CoO3 0.781 5.466 0.066 21.05 Co3O4 − 4.276 0.055 79.68 notes: La/Co molar ratio wasdetermined by ICP; the specific surface area ( SBET ) and pore volume ( vpore) were derived from N2 sorption isotherms; the particle size ( dCo3O4 ) was calculated by Scherrer’s formula 
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表 2 镧钴系列催化剂的Co 2p3/2谱图拟合数据
Table 2 Co 2p3/2 spectra fitting results of various LaxCoO3 samples
Sample Co3 + Co2 + Co3 + /Co2 + BE/eV Fraction/% BE/eV Fraction/% LaCoO3 779.53 61 781.29 39 1.56 La0.95CoO3 779.76 60.16 781.47 39.84 1.51 La0.9CoO3 779.67 47.78 781.08 52.22 0.92 La0.8CoO3 779.61 43.71 780.99 56.29 0.78 Co3O4 779.52 45.06 781.02 54.94 0.82
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表 3 镧钴系列催化剂的O 1s谱图拟合数据
Table 3 Fitted results of O 1s XPS spectra of various LaxCoO3 samples
Sample Olatt Osurf Osurf/Olatt BE/eV Fraction/% BE/eV Fraction/% LaCoO3 528.59 36.92 530.98 63.08 1.71 La0.95CoO3 528.9 39.3 531.27 60.7 1.54 La0.9CoO3 528.85 48.1 531.3 51.9 1.08 La0.8CoO3 528.84 48.49 531.25 51.51 1.06 Co3O4 529.81 47.53 531.15 52.47 1.10
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表 4 镧钴系列催化剂的H2-TPR拟合数据
Table 4 Fitted H2-TPR results of various LaxCoO3 samples
Sample Peak 1
Co3 + →Co2 + （Co3O4）Peak 2
Co2 + →Co0（Co3O4）
Co3 + →Co2 + （LaCoO3）Peak 3
Co2 + →Co0（LaCoO3）Co3O4/LaCoO3 tred/℃ percentage/% tred/℃ percentage/% tred/℃ percentage/% LaCoO3 359.26 13.39 401.57 21.24 588.94 65.37 0.0149 La0.95CoO3 353.94 14.19 406.89 24.26 599.33 61.55 0.0623 La0.9CoO3 352 32.17 406.5 7.2 593.58 60.63 0.0747 La0.8CoO3 359.26 23.15 409.37 11.91 594.26 64.94 0.0199 Co3O4 350.56 22.52 447.7 77.48 − − − notes: tred refers to the temperature corresponding to the maximum value of the reduction peak and area percentage refers to the percentage of the reduction peak area to the total reduction peak area; According to the stoichiometric number, the relative content of Co3O4 and LaCoO3 is calculated by the reduction peak area in H2-TPR; that is, Co3O4/LaCoO3 = Speak-1/(0.5× Speak-3)
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表 5 甲烷催化燃烧代表性催化剂性能对比
Table 5 Comparison of representative catalysts in their performance in methane combustion
Number Catalyst Preparation
methodMethane initial
concentration/%GHSV/
(mL·gcat−1·h−1)Temp./℃ Stability test Ref. t10 t50 t90 temp. (℃)-
time (h)conv./% 1 La0.9CoO3 sol-gel method 1 30000 382 449 530 640−72 96 this work 2 Pd/Al2O3 impregnation 1 12000 298 346 401 370−20 68 [36] 3 BaMnAl11O19−d coprecipitation 0.5 18000 545 640 − − − [37] 4 Co3O4 coprecipitation 1 18000 285 373 455 360−30 80 [38] 5 LaCoO3 template method 3 30000 370 485 578 650−70 82 [11] 6 LaMnO3 template method 3 30000 405 480 570 650−70 98 [11] 7 MnOx/LaMnO3 acid etching method 2.5 30000 351 441 519 − − [39] 8 La0.8Sr0.2CoO3 acid etching method 1 44000 385 491 595 600−24 90 [40]
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