Morphological effect of CeO2-MnOx catalyst on their catalytic performance in lean methane combustion
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摘要: 采用水热合成法制备了船形、扁球形及纳米片CeO2-MnOx复合氧化物。并运用低温N2吸脱附、XRD、SEM、TEM、H2-TPR、拉曼光谱、XPS等表征技术对不同形貌CeO2-MnOx复合氧化物的结构与其低浓度CH4催化燃烧反应性能之间的关系进行了关联。结果表明，CeO2-MnOx复合氧化物的形貌与其催化性能密切相关。其中，扁球形CeO2-MnOx复合氧化物的氧空位、Ce3+含量及表面吸附活性氧物种最多，其CH4催化燃烧反应活性最高，540℃时，可将CH4完全转化；其次是船形CeO2-MnOx复合氧化物催化剂，540℃时其CH4转化率为94.05%；与前两者相比，纳米片CeO2-MnOx复合氧化物催化剂的氧空位及表面吸附活性氧物种较少，活性较差，相同反应温度下，其CH4转化率仅为89.68%。Abstract: Ship-like, oblate spheroid and nano sheet CeO2-MnOx mixed oxides were prepared by hydrothermal method and characterized by N2 sorption, XRD, SEM, TEM, H2-TPR, Raman spectra and XPS. The relationship between the structure of CeO2-MnOx and their catalytic performance in lean methane combustion were then investigated. The results reveal that the catalytic performance of the mixed CeO2-MnOx oxide was greatly depended on its morphology. Among all of the samples, the mixed oblate spheroid CeO2-MnOx oxides with most oxygen vacancies, Ce3+ content and surface active oxygen exhibited the highest activity in lean methane combustion, with a complete CH4 conversion at 540℃. Following it, the mixed ship-like CeO2-MnOx oxides catalyst had 94.05% of the CH4 conversion at 540℃. On the contrast, the mixed nano sheet CeO2-MnOx oxides catalyst presented the lowest catalytic activity with only 89.68% CH4 conversion at the same reaction temprature owing to its lower oxygen vacancies and surface active oxygen.
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Key words:
- ceria /
- manganese oxide /
- lean methane /
- catalytic combustion /
- morphology
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图 1 CeO2及不同形貌CeO2-MNOx复合氧化物的XRD谱图
Figure 1 XRD patterns of CeO2 and the mixed CeO2-MnOx oxides with different morphology
a: CeO2; b: CeO2-MNOx(SP); c: CeO2-MNOx(OB); d: CeO2-MNOx(NS)
图 2 CeO2及不同形貌CeO2-MNOx复合氧化物的SEM照片
Figure 2 SEM images of CeO2 and the mixed CeO2-MnOx oxides with different morphology
(a), (b): CeO2; (c), (d): CeO2-MNOx(SP); (e), (f): CeO2-MNOx(OB); (g), (h): CeO2-MNOx(NS)
图 3 CeO2及不同形貌CeO2-MNOx复合氧化物的TEM照片
Figure 3 TEM images of CeO2 and the mixed CeO2-MnOx oxides with different morphology
(a): CeO2; (b): CeO2-MNOx(SP); (c): CeO2-MNOx(OB); (d): CeO2-MNOx(NS)
图 4 CeO2及不同形貌CeO2-MNOx复合氧化物的H2-TPR谱图
Figure 4 H2-TPR profiles of CeO2 and the mixed CeO2-MnOx oxides with different morphology
a: CeO2; b: CeO2-MNOx(SP); c: CeO2-MNOx(OB); d: CeO2-MNOx(NS)
图 5 CeO2及不同形貌CeO2-MNOx复合氧化物的拉曼光谱谱图
Figure 5 Raman spectra of CeO2 and the mixed CeO2-MNOx oxides with different morphology
a: CeO2; b: CeO2-MNOx(NS); c: CeO2-MNOx(OB); d: CeO2-MNOx(SP)
图 6 CeO2及不同形貌CeO2-MNOx复合氧化物的Ce 3d XPS谱图
Figure 6 Ce 3d XPS spectra of CeO2 and the mixed CeO2-MNOx oxides with different morphology
a: CeO2; b: CeO2-MNOx(SP); c: CeO2-MNOx(OB); d: CeO2-MNOx(NS)
图 7 不同形貌CeO2-MNOx复合氧化物的Mn 2p XPS谱图
Figure 7 Mn 2p XPS spectra of the mixed CeO2-MNOx oxides with different morphology
a: CeO2-MNOx(SP); b: CeO2-MNOx(OB); c: CeO2-MNOx(NS)
图 8 CeO2及不同形貌CeO2-MNOx复合氧化物的O 1s XPS谱图
Figure 8 O 1s XPS spectra of CeO2 and the mixed CeO2-MNOx oxides with different morphology
a: CeO2; b: CeO2-MNOx(NS); c: CeO2-MNOx(OB); d: CeO2-MNOx(SP)
图 9 CeO2及不同形貌CeO2-MNOx复合氧化物的CH4转化率与反应温度的关系
Figure 9 CH4 conversion of CeO2 and the mixed CeO2-MNOx oxides with different morphology
(1.0% CH4, 19.0% O2, balanced Ar and WHSV=30000 mL/(h·g))
图 10 520 ℃下CeO2-MNOx(SP)和CeO2-MNOx(OB)催化剂的CH4催化燃烧稳定性测试
Figure 10 Long-term stability test for CH4 catalytic combustion on CeO2-MNOx(SP) and CeO2-MNOx(OB) catalysts at 520 ℃ (1.0% CH4, 19.0% O2, balanced Ar and WHSV=30000 mL/(h·g))
表 1 CeO2及不同形貌CeO2-MNOx复合氧化物的化学组成、比表面积和XRD分析
Table 1 Chemical composition, surface area and XRD analysis results of CeO2 and the mixed CeO2-MNOx oxides with different morphology
Sample Ce/Mn (molar ratio) ABET/(m2·g-1) Cell parameter a/nm a Crystallite size d/nm b CeO2 - 93.4 0.5416 19.3 CeO2-MNOx(SP) 9.0 79.5 0.5411 22.7 CeO2-MNOx(OB) 8.9 88.6 0.5402 18.6 CeO2-MNOx(NS) 8.9 90.3 0.5408 17.1 a：calculated from the a value of the ceria (111) planes；b：calculated with the scherrer equation 
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表 2 CeO2及不同形貌CeO2-MNOx复合氧化物催化剂的H2-TPR定量分析
Table 2 H2-TPR quantitative analysis of CeO2 and the mixed CeO2-MNOx oxides with different morphology
Sample Peak position t/℃ H2 uptake /(μmol·g-1) Theoretical H2 uptake /(μmol·g-1)a CeO2 500 1820 2904 CeO2-MNOx(SP) 240, 310, 394, 452 242, 363, 798, 605 2619 CeO2-MNOx(OB) 246, 300, 404, 448 306, 353, 848, 746 2623 CeO2-MNOx(NS) 248, 306, 400, 441 255, 423, 804, 473 2628 a：theoretical H2 uptake was determined as the quantity of H2 required for the reduction of CeO2 and the mixed CeO2-MNOx oxides by assuming that CeO2 and MNOx (as MnO2) are stoichiometrically reduced to Ce2O3 and MnO, respectively
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表 3 CeO2及不同形貌CeO2-MNOx复合氧化物催化剂的XPS分析
Table 3 XPS analysis results of CeO2 and the mixed CeO2-MNOx oxides with different morphology
Sample Content w/% Ce3+ Mn4+ Mn3+ Mn2+ O″ CeO2 17.11 - - - 18.49 CeO2-MNOx(SP) 19.19 20.84 27.16 52.00 34.88 CeO2-MNOx(OB) 20.37 10.24 28.10 61.66 55.87 CeO2-MNOx(NS) 18.25 34.15 15.97 49.88 28.47
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