NiO@SiO2 core-shell catalyst for low-temperature methanation of syngas in slurry reactor
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摘要: 采用改进的Stöber方法, 可控制备出具有不同形貌的NiO@SiO2核壳结构催化剂, 并在浆态床反应器(320 ℃) 上, 对其合成气低温甲烷化性能进行评价; 同时借助XRD、TEM、XPS和N2物理吸附等方法对反应前后催化剂的物化性质进行了表征。研究表明, 实验制备的催化剂形貌规整、粒径均匀, 且具有较好的热稳定性。在相同的制备条件下, 核颗粒粒径增大, 其SiO2壳层的厚度随之增加。在反应过程中, 部分催化剂的核壳结构遭到破坏并出现SiO2空壳, 是CO与壳层内的Ni作用生成易迁移的Ni羰基化物种(Ni (CO)x) 所致。催化剂的甲烷化活性随着核颗粒粒径的增加呈现下降趋势; 在不同的反应阶段, 催化剂的失活速率存在明显差异, 在反应的前20 h内, 催化剂出现快速失活, 20 h后失活缓慢, 但是催化剂的甲烷选择性都保持在80%左右。催化剂的失活, 一方面, 是因为反应过程中, Ni核颗粒发生了长大; 另一方面, 是由于壳层中3-5 nm的介孔的减少以及催化剂比表面积、孔容的下降。Abstract: A series of NiO@SiO2 core-shell catalysts were prepared using modified Stöber-method. Their catalytic performances in methanation of syngas were investigated in slurry reactor at 320 ℃. The catalysts before and after reaction were characterized by XRD, TEM, XPS, N2-physisorption, etc. It was found that the NiO@SiO2 core-shell samples had well-shape morphologies and relatively uniform scale. The catalyst test revealed that the methanation activity of these catalysts decreased dramatically with increase of core particle size. The three catalysts with distinct size of core and shell showed remarkably rapid deactivation in the initial period of 20 h and then deactivated slowly during the following reaction, while their CH4 selectivity maintained at about 80%. Void-shell was formed during the reaction probably because easy-migrated Ni (CO)x species were generated. Apparently, it was concluded that increase of core particle size, decrease of BET surface area and pore volume, and abatement of mesopores within 3-5 nm in the shell were responsible for the deactivation of these core-shell catalysts based on the characterization of catalysts.
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Key words:
- core-shell /
- nickel-based catalyst /
- slurry reactor /
- low-temperature methanation
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表 1 NiO颗粒粒径及反应前后NiO@SiO2的核壳尺寸
Table 1 Particle size of NiO, core size and shell thickness of NiO@SiO2
Sample Core d/nm Shell d/nm NiO-350 4.9 - NiO-350@SiO2 5.1 6.1 Used NiO-350@SiO2 8.1 5.0 NiO-400 8.1 - NiO-400@SiO2 8.6 11.2 Used NiO-400@SiO2 10.6 10.1 NiO-500 23.7 - NiO-500@SiO2 23.3 23.0 Used NiO-500@SiO2 27.8 20.6 表 2 反应前后催化剂的表面原子浓度及体相Ni含量
Table 2 Surface atom concentration and Ni content of catalyst before and after reaction
Sample Surface atomic concentrationsa/% Ni/Si
(atomic ratio)Nib/% Ni Si O NiO-350@SiO2 10.11 17.77 72.12 0.57 57.32 Used NiO-350@SiO2 9.85 17.12 73.03 0.58 55.92 NiO-400@SiO2 8.50 24.36 67.14 0.35 58.98 Used NiO-400@SiO2 5.09 27.73 67.18 0.18 61.21 NiO-500@SiO2 6.46 21.22 72.32 0.30 60.68 Used NiO-500@SiO2 4.61 28.38 67.01 0.16 63.14 Theoretical valuec 33.50 11.00 55.50 3.05 62.16 a: the surface atomic concentrations of catalysts were calculated by XPS; b: Ni bulk concentration of catalysts obtained from ICP c: based on the addition mass when catalyst is prepared 表 3 反应前后催化剂的N2吸附
Table 3 N2 sorption of NiO@SiO2 before and after reaction
Sample BET surface area A/(m2·g-1) Pore volume v/(cm3·g-1) Pore diameter d/nm NiO-350@SiO2 117.2 0.218 7.4 Used NiO-350@SiO2 52.7 0.147 11.2 NiO-400@SiO2 82.9 0.197 8.2 Used NiO-400@SiO2 43.5 0.101 9.3 NiO-500@SiO2 34.4 0.074 8.6 Used NiO-500@SiO2 23.7 0.073 12.3 -
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