CaO/MgO modified perovskite type oxides for chemical-looping steam reforming of methane
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摘要: 甲烷化学链蒸汽重整(Chemical-looping steam methane reforming,CL-SMR)是基于化学链燃烧的概念而提出的一种新颖的技术。在重整反应器中,甲烷与载氧体中的晶格氧发生部分氧化反应生成合成气(H2/CO物质的量比为2.0),还原后的载氧体进入到水蒸气反应器中,与水蒸气反应恢复晶格氧的同时生成H2。以钙钛矿型氧化物LaFeO3为载氧体用于甲烷化学链蒸气重整过程,同时通过碱金属CaO和MgO对LaFeO3进行负载,以增大载氧体的比表面积、热稳定性和抗积炭能力。通过X射线衍射(XRD)、H2程序升温还原(H2-TPR)、BET比表面积分析(BET)和X光电子能谱(XPS)对载氧体进行表征。结果表明,三种载氧体均表现出较高的反应活性和合成气选择性,循环后仍能保持钙钛矿的结构。从反应性能、选择性和抗积炭能力等方面综合考虑,LaFeO3-CaO的效果最好,五次循环后具有很好的再生性。Abstract: Chemical-looping steam methane reforming (CL-SMR) is a novel method proposed on the base of chemical looping combustion (CLC) technology. In the CL-SMR scheme, methane is partially oxidized to syngas (H2/CO(molar ratio)=2.0) by the lattice oxygen in reformer reactor in the absence of gaseous oxidant, and then the reduced oxygen carrier is oxidized by steam to produce hydrogen in steam reactor. The use of perovskite type oxide LaFeO3 as an oxygen carrier in CL-SMR was studied. While the basicity of CaO/MgO modified oxygen carriers, LaFeO3-CaO and LaFeO3-MgO, were also synthesized aiming to increase specific surface area, thermostability, and resistance to coke formation. The synthesized oxides were characterized by X-ray diffraction (XRD), H2-temperature-programmed reduction (H2-TPR), Brunauer-Emmett-Teller (BET) surface area and X-ray photoelectron spectroscopy (XPS). Three oxygen carriers exhibited high active and selective for syngas production from methane, and maintained perovskite type over cyclic redox operations. The LF-CaO sample is the best candidate for the CL-SMR of the three samples judging from the reactivity, selectivity, and resistance to carbon formation. It showed good regenerability during 5 redox reactions.
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Key words:
- chemical-looping /
- methane reforming /
- perovskite /
- CaO/MgO modified /
- redox
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Figure 2 XRD patterns of oxygen carriers of (a) fresh (b) regenerated
●: orthorhombic phase; □: La2O3; ◆: CaO; ◇: MgO
Figure 9 Catalytic performance of LF-CaO in five successive recycles
(a)methane reduction step (b) steam oxidation step ■: CH4 conversion; ●: CO selectivity; ▲: H2 selectivity
Table 1 Position and area percentage of the major reduction peaks for the three samples
Sample Peak position t/℃ Fractional peak area /% LF 515 3.3 702 96.7 LF-CaO 403 41.8 545 8.3 640 49.9 LF-MgO 435 2.9 560 45.4 780 51.7 
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Table 2 Specific surface area of perovskites
Oxygen carriers LF LF-CaO LF-MgO Specific surface area A/(m2·g-1) 3.5 20.3 21.7 Average pore diameter d/nm 28.4 3.7 11.2
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Table 3 Surface elemental composition and relative proportion for the samples measured by XPS
Oxygen carrier Surface compositions /% La Fe Ca Mg OI OII OIII LF 20.2 7.7 - - 27.3 17.1 27.7 LF-CaO 13.7 5.5 7.8 - 24.8 26 22.2 LF-MgO 15.4 5.4 - 9.6 15.6 27.2 26.7
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