Research on electro-catalytic steam reforming of methane with modified Ni/γ-Al2O3 catalysts
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摘要: 提出了电催化作用下甲烷水蒸气催化重整的新工艺。基于工业常规Ni基催化剂,采用等体积浸渍法,以Ni为活性组分,γ-Al2O3为载体,MgO、CaO为助剂,制备了Ni/γ-Al2O3、Ni-MgO/γ-Al2O3和Ni-CaO/γ-Al2O3催化剂,考察了电流强度、重整温度、水蒸气与甲烷的物质的量比(水碳比,S/C)对不同催化剂的CH4转化率、H2产率、CO选择性和催化剂稳定性的影响。结果表明,电催化工艺有着良好的普适性,电流的引入能够提升CH4转化率、增加H2产率,尤其在低温下电流的促进作用显著。在三种催化剂中,Ni-CaO/γ-Al2O3催化效果最佳,在电流为4.5 A、S/C为3、重整温度为700℃时,CH4转化率就高达95%以上。稳定性测试表明,电流的通入还能显著提高催化剂的稳定性,延缓催化剂的积炭失活。通过对催化剂的分析表征,发现电流的通入提升了催化剂中NiO的还原程度,同时抑制了反应过程中NiCx向石墨炭的转化,从而可延缓催化剂因积炭覆盖反应活性位点而造成的失活。Abstract: A novel electro-catalytic technique for catalytic methane steam reforming was developed. Based on the conventional industrial Ni-based catalyst, aseries of catalysts, including Ni/γ-Al2O3, Ni-MgO/γ-Al2O3 and Ni-CaO/γ-Al2O3, were prepared using incipient wetness impregnation method with Ni as the active component, γ-Al2O3 as the carrier, and MgO or CaO as the promoter. Experiments were performed to investigate the effects of electric current intensity, reforming temperature, and molar ratio of water vapor to methane (water/carbon ratio, S/C) on CH4 conversion, H2 yield, CO selectivity and catalyst stability. The results indicated that the electro-catalytic technique had good adaptability, and the introduction of electric current could improve the CH4 conversion and increase the H2 yield. Such effects were more intensive at lower reforming temperatures. Among the three catalysts, Ni-CaO/γ-Al2O3 catalyst exhibited the best catalytic efficiency, with the CH4 conversion over 95% under conditions of 4.5 A, S/C of 3, and 700℃. Stability tests of the catalysts showed that the electric current could improve the stability of catalysts and delay the deactivation caused by coke deposition. The characterization results proved that the presence of electric current enhanced the reduction degree of NiO in the catalyst and inhibited NiCx conversion to graphite carbon, resulting the delay of catalyst deactivation caused by carbon deposition over reactive sites.
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表 1 稳定性测试过程中催化效率的下降
Table 1 The decline in catalytic efficiency during stability test
Catalyst t /℃ I /A Decline in efficiency/% 10Ni/γ-Al2O3 700 0 9.6 700 4.5 5.6 10Ni-3MgO/γ-Al2O3 700 0 7.1 700 4.5 3.3 10Ni-3CaO/γ-Al2O3 650 0 4.1 650 4.5 1.6 表 2 稳定性测试后催化剂的含碳量
Table 2 Carbon contents of the catalysts after stability tests
Catalyst I /A C w/% Decrease /% 10Ni/γ-Al2O3 0 0.89 20.2 4.5 0.71 10Ni-3MgO/γ-Al2O3 0 0.39 15.4 4.5 0.33 10Ni-3CaO/γ-Al2O3 0 0.52 28.8 4.5 0.37 表 3 催化剂的微观结构性质
Table 3 Microstructure properties of the catalysts
Catalyst Condition BET surface
A/(m2·g-1)Pore volume
v/(cm3·g-1)Average pore
diameter d/nmNi0
particle size *d/nm10Ni/γ-Al2O3 fresh catalyst 118.9 0.64 21.6 (NiO) 0 A reformed 104.6 0.57 21.9 18.6 4.5A reformed 106.1 0.56 21.1 13.5 10Ni-3MgO/γ-Al2O3 fresh catalyst 114.3 0.60 21.0 (NiO) 0 A reformed 105.2 0.56 21.3 11.5 4.5 A reformed 107.8 0.53 19.9 8.9 10Ni-3CaO/ γ-Al2O3 fresh catalyst 107.8 0.60 22.1 (NiO) 0 A reformed 98.2 0.52 21.4 12.4 4.5 A reformed 103.0 0.53 20.7 11.0 *: the average particle size of Ni is calculated using Scherrer formula based on the XRD diffraction peaks 表 4 Ni0所占百分比(Ni0/(Ni0+Ni2+))
Table 4 Percentage of Ni0 (Ni0/(Ni0+Ni2+))
Catalyst I (0 A) /% I (4.5 A) /% 10Ni/γ-Al2O3 23.8 24.0 10Ni-3MgO/γ-Al2O3 17.3 35.8 10Ni-3CaO/γ-Al2O3 25.1 29.0 表 5 NiCx和石墨炭所占百分比
Table 5 Percentages of NiCx and graphite carbon
Catalyst NiCx /% Graphite carbon /% I (0A) I (4.5A) I (0A) I (4.5A) 10Ni/
γ-Al2O314.3 20.8 69.1 62.2 10Ni-3MgO/
γ-Al2O358.4 65.7 27.6 22.4 10Ni-3CaO/
γ-Al2O347.9 54.0 40.0 35.2 -
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