摘要
采用XRD、EXAFS技术研究了不同Pd含量的Pd -Mo-K/Al2 O3 催化剂结构 ,并关联其合成低碳混合醇性能。结果表明 ,在氧化态Mo-K/Al2 O3 催化剂体系中添加Pd后 ,“K -Mo”物相晶粒变小 ,分散度提高 ,说明钯可能和钾钼物种发生了较强的相互作用。经硫化还原处理后 ,发生了氧硫交换 ,钼主要以MoS2 物种形式存在 ,其粒度随着Pd含量的增加而明显减小。尺寸的显著变化可能导致MoS2 与载体作用形式的改变 ,从而影响CO加氢催化反应的性能。在硫化态催化剂中 ,Pd的添加不仅能提高CO加氢合成醇的收率和选择性 ,而且有利于改善产物的分布。基于以上结果 ,认为“K -Mo”作用物种和Pd物种均为合成醇的催化活性组份 。
A series of palladium-modified Mo-K/Al 2O 3 catalyst samples was prepared by varying the palladium loading between 0 and 1.0% and maintaining molybdenum and potassium contents as constants. The structures of the samples were characterized by techniques of XRD, EXAFS and correlated to the catalytic properties of the samples for alcohol synthesis from synthesis gas. It is found that in the oxidic palladium-modified samples a strong interaction of the palladium modifier with the supported K-Mo-O species occurs. This interaction leads to a decrease in the size of the molybdenum species and stabilization of the cationic palladium species on the samples during sulfidation. Upon sulfidation, the sulfided molybdenum species in the palladium-free sample mainly present as large patches of MoS 2-like slabs with their basal sulfur planes interacting with the support surface. With the modification of palladium to the samples, the supported MoS 2-like species become highly dispersed as revealed by the decrease in the average size of the sulfided molybdenum species. The interaction of the palladium species with the molybdenum component may cause the basal planes of the MoS 2-like species to become oriented perpendicular to the support surface due to favorable bonding of the MoS 2 edge planes to the support through Mo-O-Al bonds. In comparison with the sulfided sample free of palladium, the properties of the palladium-modified samples for alcohol synthesis from synthesis gas are much improved. It most probably results from the synergic interaction of the palladium with the molybdenum species which gives rise to the appearance of the catalytically active sites.
基金
国家自然科学基金资助项目