Synthesis of three-dimensionally ordered macroporous LaFe0.7Co0.3O3 perovskites and their performance for chemical-looping steam reforming of methane
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摘要: 采用无皂乳液聚合法制备聚苯乙烯（PS）微球，通过自组装得到排列均匀有序的聚苯乙烯（PS）胶晶模板，然后经过浸渍和煅烧得到三维有序大孔（3DOM）钙钛矿型氧化物LaFe0.7Co0.3O3。通过扫描电镜、透射电镜和X射线衍射等手段对制备的3DOM钙钛矿型氧化物LaFe0.7Co0.3O3的物理化学性能进行表征。在固定床反应器上考察3DOM LaFe0.7Co0.3O3的甲烷化学链水蒸气重整性能。结果表明，聚苯乙烯（PS）微球粒径受苯乙烯单体使用量的影响，随着苯乙烯单体使用量的增加聚苯乙烯（PS）微球粒径呈增大的趋势；煅烧温度对三维有序大孔结构有显著影响，浸渍后模板在500℃煅烧下即能形成三维有序大孔结构比表面积达到19.820 m2/g，随着煅烧温度的升高三维有序大孔结构遭到部分破坏，在900℃煅烧下三维有序大孔结构遭到完全破坏。在氧载体与甲烷的反应前期，气体产物中CO2含量较高，是表面吸附氧将甲烷完全氧化所致，在表面吸附氧消耗完后体相晶格氧将甲烷部分氧化生成H2与CO。在水蒸气氧化阶段，水蒸气与还原态的氧载体发生反应生成氢气，产氢率为4.0-5.0 mmol/g。同时水蒸气氧化阶段气相产物中CO和CO2含量很低，说明3DOM LaFe0.7Co0.3O3具有优秀的抗积炭性能。Abstract: Three-dimensionally ordered macroporous (3DOM) LaFe0.7Co0.3O3 perovskite-type oxides were synthesized using a polystyrene (PS) colloidal crystal templating method. The obtained 3DOM LaFe0.7Co0.3O3 perovskites were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Brunauere-Emmette-Teller (BET) surface area. Its performance as oxygen carriers in chemical looping steam methane reforming (CL-SMR) to produce syngas (H2 + CO) and hydrogen were investigated in a fixed-bed reactor. The size of PS spheres obviously increases as the styrene addition increases. The calcination temperature is the major factor to affect the prepared 3DOM perovskite. SEM and TEM analysis show that the samples calcined at 500, 800 and 850℃ exhibit good 3DOM structures which collapse when the sample is calcined at 900℃. XRD results suggest that the obtained 3DOM LaFe0.7Co0.3O3 perovskites are pure crystalline. Two kinds of oxygen species, bulk lattice oxygen and surface adsorbed oxygen, are found to exist on the 3DOM LaFe0.7Co0.3O3 perovskites. The surface oxygen contributes to the complete oxidization of methane to CO2 and H2O in beginning of the reaction, while the bulk lattice oxygen tends towards partial methane oxidation to H2 and CO. In the methane conversion step, methane is partially oxidized into syngas at a H2/CO mol ratio close to 2:1 by the 3DOM- LaFe0.7Co0.3O3 in a wide range of the reactions, suggesting that the sample exhibits a good resistance to carbon deposition. In the steam oxidation step, the reduced perovskites are oxidized by steam to generate hydrogen with hydrogen productivity about 4 mmol/g oxygen carriers.
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Key words:
- 3DOM /
- perovskite /
- oxygen carrier /
- syngas /
- chemical-looping steam reforming of methane /
- hydrogen
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图 2 不同苯乙烯单体使用量及加入改性单体条件下合成聚苯乙烯(PS)微球的SEM照片
Figure 2 SEM pictures of PS spheres
(a): a dosage of 75 mL: styrene; (b): a dosage of 100 mL: styrene; (c): a dosage of 150 mL: styrene; (d): carboxyl modified polystyrene
图 3 不同煅烧温度下制备的3DOM LaFe0.7Co0.3O3氧载体的SEM和 TEM照片
Figure 3 SEM and TEM pictures of 3DOM LaFe0.7Co0.3O3
(a1，a2) : SEM of LFC-500; (b1，b2) : TEM of LFC-500; (c1，c2) : SEM of LFC-800; (d1，d2) : TEM of LFC-800;(e1，e2) : SEM of LFC-850; (f1，f2) : TEM of LFC-850; (g1，g2) : SEM of LFC-900; (h1，h2) : TEM of LFC-900
图 4 不同煅烧温度下制备的3DOM-LFC氧载体XRD谱图
Figure 4 XRD patterns of 3DOM-LFC with different calcination temperature
图 5 甲烷部分氧化反应气体产物组分随反应时间的变化
Figure 5 Gaseous products as a function of time in the methane conversion step
图 6 载氧体的反应性能
Figure 6 Catalytic performance of 3DOM-LFC
■: H2/CO (mol ratio); ●: CO selectivity; ▲: H2 selectivity; ▼: methane conversion
图 7 水蒸气氧化阶段中气体产物组分随反应时间的变化
Figure 7 Gaseous products as a function of time in the steam oxidation step
图 8 经过一次循环反应LFC-850的SEM和TEM照片
Figure 8 SEM and TEM pictures of LFC-850-1 cycle
(a1，a2) : SEM pictures of LFC-850-1cycle; (b1，b2) : TEM pictures of LFC-850-1cycle
表 1 不同煅烧温度下制备的3DOM-LFC氧载体的BET比表面积及晶粒粒径
Table 1 Specific surface area and crystallite size of 3DOM-LFC with different calcination temperatures
Oxygen carrier LFC-500 LFC-800 LFC-850 LFC-900 Specific surface area A/(m2·g-1) 19.820 17.305 10.192 4.485 Crystallite size d/nm 12.4 68.1 76.1 86.1 
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表 2 新鲜的LFC-850氧载体以及LFC-850经过循环反应后的BET比表面积及晶粒粒径
Table 2 Specific surface area and crystallite size of LFC-850-fresh and LFC-850 after cyclic redox
Oxygen carrier LFC-850-fresh LFC-850 after cyclic redox Specific surface area A/(m2·g-1) 10.192 8.581 Crystallite size d/nm 76.1 91.8
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