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由于Ni基催化剂具有高活性和低成本的优势，因此被广泛应用于甲烷化反应中。但是，Ni基催化剂的缺点主要有高温下Ni纳米颗粒的团聚、易形成积碳和低温活性差（250 °C）。因此，提高Ni基催化剂在合成气甲烷化中的低温催化性能是一项十分重要，但具有挑战性的工作。因为在低温和高浓度CO的条件下容易形成羰基镍，导致催化剂出现低温失活现象。鉴于此，本文通过原位还原水滑石得到了镶嵌在AlOx基底上的Ni基催化剂并用于合成气甲烷化反应，探究了水滑石的制备过程和不同助剂对催化剂甲烷化性能的影响。本论文的主要研究内容如下：（1）通过在镍铝水滑石的合成过程中添加不同的铵盐，然后将镍铝水滑石不经煅烧直接还原得到Ni催化剂并应用于合成气甲烷化反应，探究不同铵盐对水滑石结构和其催化性能的影响。通过XRD、SEM、H2-TPR和TEM等表征结果发现添加氟化铵合成的水滑石结晶度较差，纳米片的尺寸大且纳米片彼此相互堆叠在一起，相互作用较强，因此所需的还原温度较高，得到的催化剂中Ni颗粒尺寸较大。而添加硝酸铵和氯化铵制备的催化剂中Ni颗粒尺寸较小，其在合成气甲烷化反应中具有高的低温催化性能。（2）通过在镍铝水滑石的合成过程中改变溶剂的组成，然后将镍铝水滑石不经煅烧直接还原得到Ni催化剂并应用于合成气甲烷化反应，探究不同溶剂对水滑石结构和其催化性能的影响。结合XRD和H2-TPR结果发现，以乙醇为溶剂制备的镍铝水滑石层间距较大，层间阴离子和层板间的静电作用较弱，因此在400 °C时就可以被完全还原，制得的催化剂中Ni颗粒分散度低且H2吸附量小，其在合成气甲烷化反应中低温催化性能较差。而以去离子水为溶剂制备的镍铝水滑石层间含有较多的碳酸根离子，层间阴离子和层板间的静电作用较强，所需的还原温度较高，制得的催化剂中Ni颗粒分散度高且H2吸附量大，具有较高的低温甲烷化活性。（3）通过在镍铝水滑石的合成过程中添加不同的助剂（Mn、Zr和V），分别制备了镍锰铝、镍锆铝和镍钒铝三元水滑石，经过H2还原后得到了Mn、Zr和V助剂修饰的Ni催化剂。XRD、H2-TPD、XPS和TEM等结果表明掺杂的助剂进入了水滑石的层状结构中，对水滑石的形貌和结晶度基本没有影响。另外，添加助剂能有效提高催化剂中Ni颗粒的分散度、活性氧空位的数量和H2吸附量，从而显著提高催化剂的低温催化性能、抗积碳性能和高温稳定性。其中，掺杂助剂Zr对催化剂中Ni颗粒的分散度和H2吸附量的提高最明显，因此其具有最高的低温催化性能。;Ni-based catalysts are widely used in methanation reactions due to their high activity and low cost. However, the disadvantages of the Ni-based catalysts are mainly the agglomeration of Ni nanoparticles at high temperature, easy formation of carbon deposits, and poor low-temperature activity (250 °C). Therefore, it is important and challenging to improve the low temperature catalytic performance of Ni-based catalysts in syngas methanation. Nickel carbonyl is easily formed under low temperature and high concentration of CO, thus the catalyst is deactivated at a low temperature. In view of this, the Ni-based catalysts embedded in AlOx substrate were obtained by in-situ reduction of hydrotalcites and used in the syngas methanation reaction. The preparation process of hydrotalcites and the influence of different additives on the methanation performance of the catalysts were investigated. The main research contents of this thesis are as follows:By adding different ammonium salts in the synthesis process of NiAl-hydrotalcites, and then directly reduction the NiAl-hydrotalcites without calcination to obtain Ni-based catalysts and applied it to methanation reaction of syngas. The effect of different ammonium salts on the structure and catalytic performance of the hydrotalcites were investigated. The results of XRD, SEM, H2-TPR and TEM showed that the hydrotalcites synthesized by the addition of ammonium fluoride had poor crystallinity, the size of the nanosheets was large and the nanosheets were stacked on each other, and the interaction was strong, so the required reduction temperature was higher, and the size of Ni particles in the obtained catalyst was larger. The catalysts prepared by adding ammonium nitrate and ammonium chloride had a small Ni particle size, and high low-temperature catalytic performance in the syngas methanation reaction.By changing the composition of the solvent during synthesis of the NiAl-hydrotalcites, and then directly reducing the NiAl-hydrotalcites without calcination to obtain the Ni-based catalysts and applied it to the methanation reaction of the syngas. The effect of different solvents on the structure and catalytic performance of the hydrotalcites were investigated. Combined with XRD and H2-TPR results, it was found that the NiAl-hydrotalcites layer prepared by used ethanol as a solvent has a large spacing, and the electrostatic interaction between the interlayer anions and the interlayer is weak, so it can be completely reduced at 400 °C. In the catalyst, the Ni particles have low dispersion and the H2 adsorption amount is small, and with poor low-temperature catalytic performance in the syngas methanation reaction. The NiAl-hydrotalcites prepared by using deionized water as a solvent contained more carbonate ions, so the required reduction temperature is higher, due to the stronger electrostatic interaction between the interlayer anions and the interlayer. The prepared catalyst possessed high dispersion of Ni particles, H2 adsorption amount, and high low-temperature methanation activity.NiMnAl-LDHs, NiZrAl-LDHs and NiVAl-LDHs were successfully prepared by adding Mn, Zr and V in the synthesis process of NiAl-LDHs. The Ni-based catalysts modified with Mn, Zr and V promoters were obtained from in-situ LDHs. XRD, H2-TPD, XPS and TEM results show that the doped additives have entered the layered structure of hydrotalcites, and have no effect on the morphology and crystallinity of hydrotalcites. In addition, the addition of additives can effectively improve the dispersion of Ni particles, the number of active oxygen vacancies and the amount of H2 adsorption, thereby significantly improving the low-temperature catalytic performance,carbon deposition resistance and high-temperature stability of the catalysts. Among them, the doping Zr has the most obvious improvement on the dispersion of Ni particles and the adsorption amount of H2 in the catalyst, so it has the highest low-temperature catalytic performance.
|陆彬. 水滑石衍生的镍基催化剂及其甲烷化性能研究[D]. 中国科学院大学,2019.|
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