Performance of Zn-Al co-doped La2O3 catalysts in the oxidative coupling of methane
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摘要: 采用柠檬酸溶胶凝胶法制备了Zn掺杂和Zn-Al共掺杂的La2O3催化剂,运用原位表征技术研究了该催化剂在甲烷氧化偶联(OCM)反应中的构效关系。原位XRD表征结果发现,La2O3晶体在高温下沿c轴发生热膨胀。H2-TPR结果显示,La2O3基催化剂中含有两种类型的氧物种,即强结合氧和弱结合氧;XPS结果表明,强结合氧归属于为O−。Zn掺杂的La2O3催化剂在高温下形成更多的氧空位,能活化氧气产生更多的强结合氧,因而在OCM反应中表现出较好的催化性能。Al的共掺杂能促进Zn在La2O3中的分散,进一步增加强结合氧数量,提升OCM反应C2+烃的选择性。Abstract: Zn-doped and Zn-Al co-doped La2O3 catalysts were prepared by citric acid sol-gel method and characterized by a series of in situ technologies, to investigate the structure-activity relationship of La2O3-based catalysts in the oxidative coupling of methane (OCM). The in situ XRD results reveal a thermal expansion of the La2O3 crystal along the c-axis at high temperature. The H2-TPR results show two types of oxygen species on the La2O3-based catalysts, viz., the strong-binding oxygen species and weak-binding oxygen species; in addition, the XPS results indicate that the strong-binding oxygen species is probably attributed to anion radical O−. The doping with Zn can significantly increase the number of oxygen vacancies in the Zn-doped La2O3 catalysts, which can promote the activation of oxygen and generate more strong-binding oxygen species; as a result, the Zn-doped La2O3 catalyst shows better performance in OCM in comparison with the unmodified La2O3 catalyst. Moreover, the co-doping with Al can promote the dispersion of Zn in La2O3 and further raise the number of strong-binding oxygen species in the Zn-Al co-doped La2O3 catalysts, which is beneficial to enhance the selectivity to C2+ hydrocarbons in the OCM reaction
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Key words:
- Zn-Al co-doping /
- La2O3 /
- oxygen vacancies /
- crystal structure /
- oxidative coupling of methane (OCM)
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表 1 通过XRD计算的La2O3基催化剂晶格参数
Table 1 Crystalline phase properties of the La2O3-based catalysts measured by XRD
Catalyst Lattice parameter α = β /(°) γ /(°) a = b /Å c /Å L 3.9394 6.1395 90 120 LZ100 3.9383 6.1392 90 120 LZ50 3.9380 6.1359 90 120 LZ10 3.9368 6.1353 90 120 表 2 催化剂的比表面积和元素分析
Table 2 Elemental analysis and BET surface area of various La2O3-based catalysts
Catalyst La/Zn molar ratio SBET/(m2·g−1) Bulk, from ICP Surface, from XPS L − − 3.4 LZ100 104.0 107.7 2.6 LZ50 51.8 44.1 2.7 LZ10 10.1 7.7 2.6 1.0Al-LZ10 10.0 11.1 3.0 LA10 − − 5.0 表 3 La2O3基催化剂原位XRD精修结果
Table 3
In situ XRD refinement results of the La2O3-based catalysts Catalyst Lattice parameter Expansion rate 25 ℃ 750 ℃ a = b /Å c /Å a = b /Å c /Å a = b /% c /% L 3.9394 6.1395 3.9401 6.1470 0.02 0.12 LZ100 3.9383 6.1392 3.9389 6.1561 0.04 0.28 LZ50 3.9380 6.1359 3.9404 6.1589 0.06 0.38 LZ10 3.9368 6.1353 3.9377 6.1622 0.02 0.44 1.0Al-LZ10 3.9381 6.1382 3.9401 6.1582 0.05 0.33 表 4 催化剂上H2-TPR定量分析
Table 4 H2-TPR analysis results of various catalysts
Catalyst H2 consumption/(mmol·g−1) Oβ/Oα Oα Oβ Oβ + Oα L-100 ℃ 0.016 0.003 0.019 0.202 L-750 ℃ 0.064 0.064 0.128 0.992 LZ10-750 °C 0.065 0.086 0.151 1.322 LZ50-750 °C 0.057 0.111 0.168 1.947 LZ100-750 °C 0.056 0.116 0.172 2.052 1.0Al-LZ10-750 °C 0.066 0.150 0.216 2.268 LA10-750 °C 0.078 0.086 0.164 1.093 a:before H2-TPR, the catalyst samples were pretreated under 50% O2-50% N2 (40 mL·min−1) at 750 ℃ or at 100 ℃, as denoted after the catalyst label 表 5 样品的XPS O 1s拟合结果
Table 5 Contents of various oxygen species in the La2O3-based catalysts derived from O 1 s XPS spectra
Catalyst O2− O− $ {\rm{CO}}^{2-}_{3} $ $ {\rm{O}}^{-}_{2} $ O−/$ {\rm{O}}^{-}_{2} $ BE/eV x/% BE/eV x/% BE/eV x/% BE/eV x/% L 528.7 20.78 530.8 16.86 531.7 53.61 533.0 8.75 1.9 LZ10 528.8 25.31 530.8 20.63 531.7 44.33 533.0 9.73 2.1 LZ50 528.8 26.69 530.8 13.15 531.7 56.86 532.9 3.30 4.0 LZ100 528.8 27.56 530.8 12.31 531.7 57.29 532.7 2.84 4.3 1.0Al-LZ10 529.0 28.98 530.8 12.75 531.7 55.61 533.0 2.66 4.8 表 6 800 ℃下的OCM催化性能
Table 6 Performance of various La2O3-based catalysts in OCM at 800 °C
Catalyst CH4 Conversion /% O2 Conversion /% Selectivity /% C2H4 /C2H6 C2+ CO CO2 L 25.4 99.9 49.8 11.2 39.0 1.4 LZ10 25.2 99.9 51.3 11.0 37.7 1.3 LZ50 25.0 100.0 53.6 10.5 35.9 1.5 LZ100 26.0 99.9 53.8 10.4 35.8 1.5 1.0Al-LZ10 25.3 99.9 55.0 7.5 37.5 1.4 LA10 24.5 99.7 50.7 11.2 38.1 1.5 ZnO 12.0 80.1 29.5 8.7 61.8 0.5 -
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