Study of grain size effect of lanthanum oxide catalyzed methane oxidation coupling reaction
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摘要: 采用水热合成法,制备了不同晶粒尺寸的La2O3催化剂。运用XRD、原位拉曼光谱、原位红外漫反射光谱、H2-TPR和O2-TPD等表征手段研究了不同晶粒尺寸La2O3催化剂的OCM反应性能和催化剂之间的构效关系。结果表明,La2O3催化剂的La−O键会随着温度的升高出现明显的伸长,从而影响La2O3对O2的吸附和动态储存。当晶粒尺寸增大至57.4 nm时,La2O3催化剂的储氧能力开始下降,同时伴随着表面氧物种,特别是超氧物种在催化剂表面富集,导致CH4和产物的过度氧化,降低OCM反应的选择性。晶粒尺寸为52.3 nm的L-La2O3催化剂在750 ℃时,表面氧物种含量适宜,储氧能力强。在CH4/O2为3,空速为1.6 × 105 mL/(g·h)的条件下表现出最佳的C2 + 烃选择性。Abstract: La2O3 catalysts with different grain sizes were prepared under hydrothermal condition. The structure activity relationship of La2O3 catalysts with different grain sizes were investigated by using in-situ XRD, Raman, FT-IR and H2-TPR, O2-TPD. The results showed that the La−O bond of the La2O3 catalyst showed a significant elongation with increasing temperature, which affected its adsorption and dynamic storage of O2. When increasing the grain size up to 57.4 nm, the oxygen storage capacity of the La2O3 catalyst started to decrease, accompanied by the enrichment of surface oxygen species, especially superoxide species, on the catalyst surface, which led to the over-oxidation of CH4 and products and reduced the C2 + hydrocarbons selectivity. The L-La2O3 catalyst with a grain size of 52.3 nm had a suitable content of surface oxygen species and a high oxygen storage capacity at 750 ℃. It exhibited the best C2 + hydrocarbon selectivity up to a CH4/O2 of 3 and a vacancy rate of 1.6 × 105 mL/(g·h).
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Key words:
- methane oxidation coupling /
- La2O3 /
- grain size /
- oxygen storage capacity
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表 1 催化剂的织构性质
Table 1 Texture properties of catalyst
Catalyst ABET/(m2·g−1)a vp/(cm3·g−1)b dp/nmc Crystallite size/nmd S-La2O3 9 0.07 29 42.5 M-La2O3 5 0.03 38 47.3 L-La2O3 3 0.02 41 52.3 XL-La2O3 4 0.02 40 57.4 a: BET surface area. b: BJH desorption cumulative volum of pores. c: BJH desorption average pore diameter (4V/A).
d: crystallite size was calculated by the Scherer formula based on the strongest diffraction (2θ=30°)表 2 不同温度下不同晶粒尺寸La2O3的拉曼峰偏移率
Table 2 Raman peak shift rates of La2O3 with different grain sizes at different temperatures
Catalyst Raman shift/cm−1 Drift rate/%a 20 ℃ 400 ℃ 550 ℃ 650 ℃ 750 ℃ 400 ℃ 550 ℃ 650 ℃ 750 ℃ S-La2O3 407 397 392 391 388 2.5 3.7 3.9 4.7 M-La2O3 407 396 392 391 388 2.7 3.7 3.9 4.7 L-La2O3 407 398 394 392 389 2.2 3.1 3.7 4.4 XL-La2O3 406 396 393 390 387 2.5 3.2 3.9 4.7 a: 400, 550, 650, 750 ℃ drift rate(%) = (20 ℃ Raman shift – 400, 550, 650, 750 ℃ Raman shift)100/20 ℃ Raman shift 表 3 不同晶粒尺寸La2O3在不同O2气预处理温度下吸附体相中近表层氧物种相对含量
Table 3 Relative content of oxygen species near surface oxygen species in bulk phase of La2O3 with different grain sizes at different O2 gas pretreatment temperatures
Catalyst Relative content of oxygen species in
adsorbate phase/(μmol·g−1)550 ℃ 650 ℃ 750 ℃ S-La2O3 24 36 47 M-La2O3 29 45 51 L-La2O3 36 59 69 XL-La2O3 33 53 63 -
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