Co/SiO_2催化剂催化费托合成中CeO_2助剂的作用

Role of CeO_2 Promoter in Co/SiO_2 Catalyst for Fischer-Tropsch Synthesis

在Co/SiO2催化剂中加入少量CeO2助剂,能够提高其催化费托(FT)反应的活性和液态烃(C5+)的选择性,特别是可以生成优质的高附加值柴油及润滑油基础油(C12+),但助剂CeO2对Co/SiO2催化剂的作用目前仍存在争议.本文利用X射线衍射、程序升温还原、程序升温脱附、程序升温表面反应、X射线光电子能谱、扫描电镜和同位素脉冲反应结合H-D同位素交换等方法对催化剂进行了表征,并采用微反评价考察了催化剂的催化性能.表征结果表明,少量CeO2助剂的添加提高了活性组分Co在催化剂表面的分散度,降低了催化剂表面Co的晶粒度,使催化剂对CO和H2的吸附性能发生了改变.同位素脉冲反应结合H-D交换实验表明,CeO2的添加减弱了Co的金属性,提高了其吸附解离氢的能力和CO吸附量,增大了催化剂表面活性碳物种的浓度和聚合趋势,使FT合成的碳链增长几率增大至0.90,产物中液态烃选择性和重质烃含量明显增加,说明CeO2的加入有利于生成高附加值的柴油和润滑油基础油.

Fischer-Tropsch （FT） synthesis is gaining more and more interest because of the need ot high purity diesel fuels and lubricating oil. The cobalt-based catalyst is well known for its activity in the FT reaction. The addition of a small amount of ceria to the Co/SiO2 catalyst can increase the turnover rate and C5＋ selectivity, especially promote the formation of high hydrocarbons, but the role of ceria is still of controversy. In this study, the CeO2-promoted Co/SiO2 catalyst was prepared by the incipient wetness impregnation method and characterized by X-ray diffraction, temperature-programmed reaction, X-ray photoelectron spectroscopy, scanning electron microscopy, H-D isotopic exchange, and D2 pulse experiment. The results showed that adding a small amount of ceria improved the dispersion of cobalt, decreased the electron density of the cobalt particle, and enhanced the dissociation of CO on the catalyst surface. This might be due to the new catalytic active sites at which CO bonded to the Co atom with its carbon atom and to the adjacent partially reduced cerium oxide with its oxy gen atom. The D2 pulse in the FT reaction showed that the addition of ceria to Co/SiO2 increased the concentration of surface active carbon species （ -CH2- ） , the chain growth probability, and the selectivity for long chain hydrocarbons, which favored the formation of the high value diesel oil and lubricating oil.