Catalytic performance of CuCoCe supported on nitrogen-doped carbon nanotubes for the synthesis of higher alcohols from syngas
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摘要: 以三聚氰胺为氮源，控制其与碳纳米管混合比例，经过高温焙烧得到不同氮含量的氮掺杂碳纳米管（xN-CNTs）载体；通过浸渍法制备xN-CNTs担载的CuCoCe催化剂，研究了氮掺杂对其催化合成气制低碳醇性能的影响。采用X射线衍射（XRD）、N2吸附-脱附、H2程序升温还原（H2-TPR）、NH3程序升温脱附（NH3-TPD）和X射线光电子能谱（XPS）等表征手段，分析催化剂结构特性，关联了构效关系。结果表明，氮的掺杂量会影响催化剂活性组分Cu的存在状态及分散情况，减少可还原Co物种的数量，降低催化剂表面酸强度及酸量，使得长链烃类的生成受到抑制，总醇选择性明显提高。分析认为，掺杂在碳管上N的形态分布及掺杂量是影响上述因素的关键。Abstract: A series of nitrogen-doped carbon nanotubes (xN-CNTs) were obtained by treating the mixture of melamine and carbon nanotubes at high temperature; the CuCoCe catalysts supported on xN-CNTs were then prepared by impregnation method and used in the synthesis of higher alcohols from syngas. The CuCoCe/xN-CNTs catalysts were characterized by XRD, N2 sorption, H2-TPR, NH3-TPD and XPS and the effect of nitrogen content in xN-CNTs on the catalytic performance of CuCoCe/xN-CNTs in the higher alcohols synthesis was investigated. The results show that the content of nitrogen in xN-CNTs has a significant influence on the existence and dispersion of Cu on the CuCoCe/xN-CNTs catalysts; the presence of nitrogen can reduce the number of reducible Co species and lower the acid strength and amount on the catalyst surface, which helps to suppress the long-chain hydrocarbons formation and improve total alcohol selectivity. It is proposed that the morphological distribution and doping amount of nitrogen on the carbon tubes may play a crucial role in enhancing the catalytic performance of CuCoCe/xN-CNTs in the higher alcohols synthesis.
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Key words:
- nitrogen doping /
- carbon nanotubes /
- syngas /
- higher alcohols /
- CuCoCe
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图 1 不同氮含量催化剂反应前的XPS谱图
Figure 1 A survey of XPS spectra of fresh CuCoCe/xN-CNTs catalysts with different N contents
图 2 不同催化剂反应前后的XRD谱图
Figure 2 XRD patterns of different CuCoCe/xN-CNTs catalysts
(a): before reaction; (b): after reaction
图 3 不同催化剂反应前的H2-TPR谱图
Figure 3 H2-TPR profiles of various fresh CuCoCe/xN-CNTs catalysts before reaction
图 4 催化剂反应前的NH3-TPD谱图
Figure 4 NH3-TPD profiles of various CuCoCe/xN-CNTs catalysts before reaction
图 5 不同催化剂反应前的XPS谱图
Figure 5 XPS spectra of various CuCoCe/xN-CNTs catalysts before reaction
(a): Cu 2p; (b): Co 2p; (c): Ce 3d; (d): N 1s
表 1 不同氮含量载体的元素分析
Table 1 Elemental analysis results of the xN-CNTs supports with different nitrogen contents
Support Melamine/CNTs
ratio by weightw/% N C H O CNTs 0/3 0.09 95.19 0.69 4.03 0.1N-CNTs 0.1/3 1.31 96.39 0.71 1.59 0.3N-CNTs 0.3/3 2.28 95.35 0.58 1.79 0.6N-CNTs 0.6/3 3.08 94.61 0.60 1.71 
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表 2 载体及对应催化剂的织构性质
Table 2 Textural properties of the xN-CNTs supports and corresponding CuCoCe/xN-CNTs catalysts
Support A/(m2·g-1) d /nm vp/(cm3·g-1) Catalyst A/(m2·g-1) d /nm vp/(cm3·g-1) CNTs 199.3 20.3 0.84 Cat-0 153.6 13.2 0.46 0.1N-CNTs 195.6 15.4 0.61 Cat-1 160.4 10.5 0.35 0.3N-CNTs 186.5 14.5 0.53 Cat-2 164.7 10.6 0.35 0.6N-CNTs 177.0 16.8 0.51 Cat-3 160.5 10.3 0.36 notes: A: specific surface area; d: average pore diameter; vp: pore volume
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表 3 不同催化剂反应前后Cu物种平均晶粒粒径
Table 3 Average crystallite size of Cu species in different catalysts before and after reaction
Catalyst Before reaction After reaction Cu2O crystal size d/nm Cu0 crystal size d/nm Cat-0 14.4 22.9 Cat-1 18.8 32.3 Cat-2 17.0 30.1 Cat-3 10.3 (CuO) 31.8
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表 4 H2-TPR耗氢量
Table 4 Hydrogen consumption of various CuCoCe/xN-CNTs catalysts determined from H2-TPR
Catalyst Reduction peak
t/℃H2 consumption
/(mmol·gcat-1)Cu2+/Cu+
(mol ratio)Cat-0 181 229 472 2.12 0.96 1.05 1.10 Cat-1 175 252 481 1.71 1.42 0.55 0.60 Cat-2 191 256 503 1.99 1.06 0.49 0.94 Cat-3 195 260 504 2.59 0.84 0.50 1.54
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表 5 催化剂的酸中心分布
Table 5 Distribution of acid sites on the catalysts
Catalyst Desorption peak t/℃ Peak area weak acid middle acid strong acid weak acid middle acid strong acid Cat-0 150 217/283 394 140 86 31 Cat-1 135 195/280 381 88 104.6 16 Cat-2 133 192/258 384 68 89 12 Cat-3 130 188/256 381 67 74 7
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表 6 XPS分析中各物种的结合能
Table 6 Binding energy data of different Cu and Co species on CuCoCe/xN-CNTs from XPS analysis
Catalyst Binding energy E/eV pyridinic N pyrrolic N graphitic N Cu 2p3/2 (Cu+) Cu 2p3/2(Cu2+) Co 2p3/2(Co2+) Cat-0 - - - 932.4 934.3 779.8 Cat-1 398.2 400.2 403.4 932.6 934.5 780.4 Cat-2 398.5 400.4 403.3 932.6 934.5 780.2 Cat-3 398.7 400.8 403.7 932.5 934.6 780.1
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表 7 不同催化剂的催化性能
Table 7 Performance of various CuCoCe/xN-CNTs catalysts in the synthesis of higher alcohols from syngas
Catalyst CO conversion
x/%Selectivity s/C-mol% C2+OH/ROH
/C-mol%STYROH
/(mg·g-1·h-1)CH4 C2-5 ROH CO2 Cat-0 17.9 23.2 46.9 18.3 11.6 81.3 102.0 Cat-1 12.5 30.7 33.5 23.5 12.3 70.4 96.9 Cat-2 10.7 32.1 36.1 20.7 11.1 68.4 73.5 Cat-3 12.0 29.9 38.3 20.3 11.5 72.2 79.3 notes: CH4 and C2-5, hydrocarbons; ROH, total alcohols; reaction conditions: 300℃, 3.0MPa, V(H2)/V(CO) = 2, feed flow rate of 150mL/min; the data reported are the average data in the 48h reaction process
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