限域Ni/MCM-41催化抗积碳和金属烧结的甲烷干重整反应（英文）
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摘要
干重整反应为同时转化两种主要的温室气体甲烷和二氧化碳为合成气(CO和H2).发展干重整高温反应是转化工业废气(如焦炉煤气、煤制油尾气等)为合成气平台分子的有效手段.由于廉价的金属镍具有良好的甲烷解离能力,因此干重整反应中二氧化碳的解离很关键,可添加如MgO, BaO, CaO等碱土氧化物来加强二氧化碳的吸附,或添加具有氧空位的CeO_2, ZrO_2, La_2O_3的氧化物来捕集二氧化碳.双金属Ni Fe催化剂中, Fe通过将CO_2还原为CO和FeO来激活CO_2,然后FeO可通过氧化还原反应将解离的C*转化为CO和Fe,从而实现高温下活化CO2和表面C去除的完美结合.干重整反应面临高温下催化剂金属中心烧结和催化剂表面积碳严重的问题,而将活性金属粒子限域是一种有效阻止金属高温烧结的方法.本文利用乙醇诱导的毛细管作用力,发展了均匀负载Ni纳米粒子于MCM-41直型孔道结构内的简易方法.该限域结构催化剂的Ni金属负载量为10 wt%, X射线粉末衍射(XRD)测试显示无明显的Ni衍射峰,表明Ni颗粒高度分散,透射电子显微镜(TEM)表征结果表明Ni颗粒大小为2 nm左右, Ni颗粒主要分布在MCM-41的孔道内.程序升温还原(TPR)表明该限域结构催化剂具有较高的还原温度,说明NiO与硅氧化物之间有较强的相互作用.在反应条件下(700℃,常压,空速为45000 mL/g/h),催化剂具有高的甲烷转化率(72%,接近该温度下的平衡转化率), TOF达到667 mol CH4/molsurf.Ni/h.经过200 h反应后,甲烷转化率未见明显下降, H2/CO摩尔比维持在0.87左右.反应后TEM结果显示, Ni颗粒未见明显团聚(其平均粒径为3-4 nm左右),没有观察到Ni颗粒被碳包覆的现象.同时,反应后催化剂的拉曼光谱测试结果表明,催化剂上积碳为无定型碳,程序升温氧化(TPO)测试说明这种无定型碳更容易被气化,不会导致催化剂失活,热重分析(TGA)表明其平均积碳速率为0.26 mg/g/h.对比Ni纳米粒子负载于MCM-41外表面的催化剂,其甲烷初始转化率为65%,并且在反应开始后的12 h内快速失活.反应60 h后催化剂的XRD测试结果表明, Ni的衍射峰变强, Ni晶粒尺寸增大,并且出现了明显的石墨化碳的衍射峰.进一步TEM结果显示,平均Ni颗粒尺寸增大到16.7 nm,且催化剂表面布满积碳生成的碳纳米管,从高分辨TEM结果可以看出,大颗粒的Ni表面被多层石墨化碳覆盖.TPO测试结果显示,这种碳更难被气化, TGA分析得出平均积碳速率达到3.2 mg/g/h,是限域结构催化剂的12倍.这种石墨化碳阻断了金属中心和反应物分子间的接触,导致催化剂失活.
Development of dry reforming of methane and carbon dioxide is an effective route to convert industrial waste gases such as coke-oven gas and coal-to-oil gas into platform syngas. However, this process encounters severe problems of metal particle sintering and coke formation at high temperatures. In this work, we developed a new synthetic method for preparing confined Ni/MCM-41 catalysts, which impede the sintering of metal nanoparticles(NPs) and coke deposition at high temperatures, enabling them to be successfully applied to methane dry reforming. The method results in high activity and stability of the catalyst at 700 ℃ for 200 h. The Ni precursor is immersed in ethanol and impregnated into MCM-41 by the peculiar capillary action of hexagonal straight mesopores. By this method, 10 wt% Ni NPs(d = 2 nm) is equably confined to the mesoporous channels with strong metal-support interactions, as confirmed by HRTEM, TEM mapping, H2-TPR, and XRD measurements. Such a confined structure has a significant effect on the inhibition of metal NP agglomeration and carbon deposition during methane dry reforming, as evidenced by TEM, Raman, TGA, and TPO measurements of used Ni/MCM-41 catalysts. In contrast, unconfined Ni/MCM-41 catalysts, with Ni NPs located on the pore exteriors, are rapidly deactivated after 12 h due to the blocked contact between the active metal centers and the gas feedstock. Additionally, a fast increase in the Ni NP size and the formation of substantial carbon nanotubes on the unconfined catalyst surface are seen. This work offers a facile approach for the synthesis of anti-sintering, carbon-resistant confined Ni catalysts that can operate at high temperatures.
Development of dry reforming of methane and carbon dioxide is an effective route to convert industrial waste gases such as coke-oven gas and coal-to-oil gas into platform syngas. However, this process encounters severe problems of metal particle sintering and coke formation at high temperatures. In this work, we developed a new synthetic method for preparing confined Ni/MCM-41 catalysts, which impede the sintering of metal nanoparticles(NPs) and coke deposition at high temperatures, enabling them to be successfully applied to methane dry reforming. The method results in high activity and stability of the catalyst at 700 ℃ for 200 h. The Ni precursor is immersed in ethanol and impregnated into MCM-41 by the peculiar capillary action of hexagonal straight mesopores. By this method, 10 wt% Ni NPs(d = 2 nm) is equably confined to the mesoporous channels with strong metal-support interactions, as confirmed by HRTEM, TEM mapping, H2-TPR, and XRD measurements. Such a confined structure has a significant effect on the inhibition of metal NP agglomeration and carbon deposition during methane dry reforming, as evidenced by TEM, Raman, TGA, and TPO measurements of used Ni/MCM-41 catalysts. In contrast, unconfined Ni/MCM-41 catalysts, with Ni NPs located on the pore exteriors, are rapidly deactivated after 12 h due to the blocked contact between the active metal centers and the gas feedstock. Additionally, a fast increase in the Ni NP size and the formation of substantial carbon nanotubes on the unconfined catalyst surface are seen. This work offers a facile approach for the synthesis of anti-sintering, carbon-resistant confined Ni catalysts that can operate at high temperatures.
引文
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