Pt－Mo／Al_2O_3和Pt－Co－Mo／Al_2O_3催化剂中Pt、Co助

Study of the Promoting Effects of Pt, Co on Pt-Mo/Al_2O_3 and Pt-Co-Mo/Al_2O_3 Catalysts

利用ＴＰＲ、Ｈ_２－ＴＰＤ技术，考察了氢气氛下的Ｐｔ－Ｍｏ／Ａｌ_２Ｏ_３和Ｐｔ－Ｃｏ－Ｍｏ／Ａｌ_２Ｏ_３催化体系中物种和电荷交换的现象和规律，揭示了Ｐｔ和Ｃｏ在表面Ｍｏ物种还原过程中助剂作用的本质．Ｐｔ－Ｍｏ／Ａｌ_２Ｏ_３的ＴＰＲ、Ｈ_２－ＴＰＤ结果证明，由于微量Ｐｔ参与了表面Ｍｏ物种还原时的物种和电子交换，有效地降低了其还原温度．Ｒｔ－Ｍｏ／Ａｌ_２Ｏ_３在氢还原过程中，氢和Ｍｏ容易形成载有活动氢的氢物种，并储存在催化剂表面，这种活动氢在Ａｒ中，甚至Ａｒ－Ｈ２混合气中可以释放出来。Ｐｔ－Ｃｏ－Ｍｏ／Ａｌ２Ｏ３的研究结果表明，Ｃｏ可以进一步促进Ｍｏ的还原．在样品预还原过程中，氢和Ｃｏ可以形成氢物种，其上的氢具有更强的可动性，很容易溢流到Ｍｏ物种的边上促进其还原．根据以上规律可以推测，加氢脱硫催化剂中Ｃｏ和贵金属的作用是使催化剂更容易形成低价的配位不饱和的钼中心，也就是ＨＤＳ活性中心．

The mechanism of promoting effects of Pt and Co on the reduction of octahedrally coordinated Mo6+ species on Pt-Mo/Al2O3 and Pt-Co-Mo/Al2O3 catalysts has been investigated using techniques of temperature programmed reduction and temperature programmed desorption of hydrogen. It was found that the existence of very slight amount of Pt greatly facilitated the reduction of surface Mo species on Pt-Mo/Al2O3and Pt-Co-Mo/Al2O3, brought about by the activation of charge and species exchange under hydrogen atmosphere by Pt. Thus, it was inferred implicitly that, in the course of H2pretreatment at comparatively low temperature, some kinds of molybdenum hydrogen species with mobile hydrogen were present on the surface of the catalysts, which could release its hydrogen in Ar, or even in Ar-H2(5% ) mixture. Furthermore, it was shown that Co accelerated the reduction of Mo species on Pt-Co-Mo/Al2O3by lowering its reduction temperature as much as 50 K, comparing with Pt-Mo/Al2O3. This was interpreted by that, the hydrogen in cobalt hydrogen species, formed during H2pretreatmet, had a feature of higher mobility, and could spill over onto Mo species easily to enhance its reducibility. The above mentioned principle implied that, owing to the promoting effects of Pt and Co, during HDS reaction it was more convenient for Pt-Mo/Al2O3and Pt-Co-Mo/Al2O3 catalysts to generate the surface hydrogen species with low valence state of Mo, i. e. the coordinated unsaturated sites, which was indispensable for HDS active centers.