还原条件对甲烷催化裂解催化剂活性影响探究

Effect of different pre-reduction situation on performance of the catalyst for decomposition of methane

用溶胶凝胶法制备了w(Ni)=50%的Ni/Al_2O_3催化剂,考察了不同还原条件对其催化裂解甲烷性能的影响。对新鲜的和使用后的催化剂进行了N_2物理吸附、XRD和H_2-TPR表征。结果表明:随着催化剂还原时间的延长,甲烷转化率先增加后降低,在还原90min时达最高,而氢气选择性则是逐渐增高;随着催化剂还原温度的提高,甲烷转化率呈现先增加后降低的趋势,氢气选择性则呈现逐渐增加的趋势,在800℃以上趋于100%。表征结果表明焙烧后的催化剂上存在3种不同形式的NiO,在还原过程中,自由态的NiO最易被还原,分散态的NiO次之,固定态的NiO最难被还原。将催化剂活性的降低归结于催化剂上大晶粒的镍物种在高温H_2氛围下发生的烧结现象。还原时间的延长有利于催化剂中NiO物种被还原,但是同时也发现,单纯通过延长还原时间无法达到完全还原催化剂中镍物种的目的。随着还原温度的提高,催化剂中的镍物种被完全还原,然而催化剂的烧结现象严重。因此,选择合适的还原条件将有利于催化剂表现出较高的活性。

A Ni/Al2O3catalyst（w（Ni）=50%） was prepared by the sol-gel method, and the effect of reduction conditions of the catalyst on its activity on methane catalytic cracking was investigated. The fresh and spent catalysts were characterized by N2 physical adsorption, XRD and H2-TPR. The results showed that, with the increase of reduction time, the conversion of methane increased firstly, and reached the highest at 90 min, then reduced a little, while the selectivity of hydrogen increased gradually; with the increase of reduction temperature, the conversion of methane increased firstly, then reduced, while the selectivity of hydrogen increased gradually, reaching nearly 100% at above 800℃. Characterization results indicated that there were three kinds of Ni O in the fresh catalyst, in which free Ni O was the easiest to be reduced, dispersed Ni O was a little harder to be reduced, and fixed Ni O was the most difficult to be reduced. The activity decrease of catalyst was attributed to the sintering of reduced large-grained NiO.Increasing reduction time was in favor of Ni O reducing, however, Ni O couldn＇t be completely reduced by increasing time only.Increasing reduction temperature could lead to a complete reduction of Ni O, but the sintering became serious. Selecting the appropriate reduction conditions would be beneficial to the catalyst to show higher activity.