甲烷二氧化碳重整反应不仅可以将两种温室气体转化为更具有工业应用价值的合成气,而且反应产物中的H2/CO比也比较适宜合成气的深加工过程,兼具环境效益和经济效益,因此受到广泛的关注与研究.但是,阻碍该过程工业化的主要问题在于反应中Ni基催化剂非常容易积碳,从而导致催化剂失活.近年来,甲烷二氧化碳催化重整领域的研究主要集中在反应机理和催化剂设计,其中大多数的研究结果表明,Ni基催化剂的抗积碳性能取决于反应过程中积碳速率与消碳速率之间的平衡.CO2是该反应体系中唯一的氧源,因此Ni基催化剂的消碳能力在很大程度上取决于其对CO2裂解活化能力的强弱.早期的文献中一般认为,CO2的裂解活化与载体的Lewis碱性位点强弱相关,因此添加碱性氧化物助剂,比如MgO和CaO等,能够增强Ni基催化剂的碱性强度和CO2吸附性能,有利于催化剂表面碳物种的转化,从而增强催化剂的稳定性.已有文献报道,添加微量MgO助剂(< 1 wt%)尽管没有影响Ni基催化剂的碱性强度,但是能够明显增强Ni基催化剂的稳定性,但没有对此结果给出明确的解释.在非均相催化研究领域中,活性金属与助剂在催化剂表面的分散性,是研究其催化作用的重要前提.大部分甲烷二氧化碳催化重整研究工作中,助剂的引入通常采用浸渍法,但是这种制备方法并不能有效保证助剂的分散度.本研究工作利用了水滑石材料的"记忆效应",将0.42 wt% Mg2+引入到由Ni-Al水滑石前驱体焙烧后得到的Ni/Al2O3催化剂中.X射线能谱仪的结果表明,微量MgO助剂均匀分散在Ni/Al2O3催化剂表面上.经X射线衍射、CO2程序升温脱附和H2程序升温还原表征验证,添加微量的MgO助剂并没有对Ni晶粒尺寸、金属载体相互作用以及Al2O3载体表面碱性强度产生明显作用; 然而甲烷二氧化碳重整活性评价测试和反应后催化剂的O2程序升温氧化实验结果显示,微量MgO助剂能明显增强Ni/Al2O3催化剂的稳定性,并且有效地阻碍了石墨碳在催化剂表面的形成.表面脉冲吸附实验结果证实,微量MgO助剂促进了CO2在Ni颗粒表面的裂解活化,进而可以及时消除Ni金属表面由甲烷裂解产生的碳物种,防止其迁移、聚集和生成石墨碳.%The CO2reforming of CH4is studied over MgO-promoted Ni catalysts, which were supported on alumina prepared from hydrotalcite. This presents an improved stability compared with non-promoted catalysts. The introduction of the MgO promoter was achieved through the ''memory effect'' of the Ni-Al hydrotalcite structure, and ICP-MS confirmed that only 0.42 wt.% of Mg2+ ions were added into the Ni-Mg/Al catalyst. Although no differences in the Ni particle size and basicity strength were observed, the Ni-Mg/Al catalyst showed a higher catalytic stability than the Ni/Al catalyst. A series of surface reaction experiments were used and showed that the addition of a MgO promoter with low concentration can promote CO2dissociation to form active surface oxygen aris-ing from the formation of the Ni-MgO interface sites. Therefore, the carbon-resistance promotion by nature was suggested to contribute to an oxidative environment around Ni particles, which would increase the conversion of carbon residues from CH4cracking to yield CO on the Ni metal surface.
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