摘要
采用量子化学第一性原理计算对乙炔选择性加氢过程进行了比较系统的研究,本论文的研究不仅可以加深我们对乙炔选择性加氢催化过程的理解,为设计开发新型催化剂提供理论指导,而且对于其它催化材料的分子设计也有一定的借鉴意义。
首先,研究了活性组分Pd和锐钛矿型TiO_2载体之间的相互作用,发现当单个钯原子吸附在锐钛矿(101)完美表面时,最稳定的吸附位是两个相邻且配位数为二的氧原子构成的桥位。而当锐钛矿(101)缺陷表面存在氧缺位时,氧缺位则成为钯原子的最稳定吸附位。Pdn团簇在TiO_2(101)表面生长时,先呈平面结构生长,逐渐转变为三维立体结构。Pdn团簇在TiO_2表面的生长受两种作用力的驱动,成核初期Pd-TiO_2之间的作用是成核的主要驱动力;随着团簇的长大,Pd-Pd之间的作用逐渐成为了成核的主要驱动力。
其次,通过考察了不同载体(TiO_2和Al2O3)对乙炔选择性加氢过程的影响,发现乙炔分子在TiO_2负载的Pdn团簇上的吸附要强于其在Al2O3负载的Pd团簇上的吸附,而乙烯分子则正好相反。在Pd3/TiO_2表面吸附时,乙炔和乙烯分子所占据的稳定吸附位点相同,均为三配位空位,存在竞争吸附。在Pd3/Al2O3上吸附时,乙炔占据三配位空位,而乙烯吸附于两个Pd原子的顶位,两者不存在竞争。这些结果说明TiO_2负载Pd的催化剂有利于反应物乙炔分子的吸附和产物乙烯分子的脱附,与传统的Al2O3载体相比,更有利于乙烯选择性的提高。不同载体孔道中C2分子扩散性能接近,表明扩散对于加氢选择性没有太大影响。
最后,研究了第二金属Ag的引入对乙炔选择性加氢过程的影响,发现Pd-Ag在TiO_2表面形成合金团簇时,Ag原子倾向于吸附在Pd团簇之上,而非分散在载体表面。这样可以将Pd团簇分割成小的活性位点,从而起到抑制C2分子在表面的聚合反应,有利于催化剂选择性的提高。乙炔,乙烯分子在不同掺杂比例的负载型Pd-Ag合金团簇上的吸附结果显示,Pd-Ag合金团簇对C2H2, C2H4的吸附作用相比纯Pdn团簇有所减弱,但掺杂Ag后,对乙炔的吸附选择性增大,说明Ag的掺杂会使催化剂活性降低,但有利于乙烯选择性的提高。此外C4H6分子在Pd-Ag合金团簇表面的吸附能随着Ag原子掺杂比的提高逐渐减弱,说明Ag原子的添加有利于抑制绿油的沉积,延长催化剂寿命。
The focus of this dissertation is to investigate the acetylene selective hydrogenation using first principles based on slab calculations. The results from this study will be helpful to understand the acetylene hydrogenation mechanism and useful to design a new and highly efficient selective hydrogenation catalyst. Furthermore, our results will be beneficial to develop other new kinds of catalysts.
Firstly, we studied the interfacial interaction between Pd catalyst and anatase TiO_2(101) support. When a single Pd atom adsorbed on the perfect anatase TiO_2(101) surface, it prefers the bridge site formed by two neighbor 2-coordinated oxygen (2cO) atoms along the [010] direction, whereas Pd adatom prefers occupying the oxygen vacancy site when Pd was deposited on the defective anatase TiO_2(101) surface. The Pdn cluster on the anatase TiO_2(101) surface prefers to form plant-like conformation when the number of Pd atoms in the cluster is less than three, and then it turns to 3-dimensional structure. The growth of Pdn cluster was controlled by two kinds of forces. One is the interaction between Pd cluster and TiO_2(101) surface (Pd-TiO_2 interaction), and the other is the interaction between different Pd atoms in the adsorbed cluster (Pd-Pd interaction). At the beginning stage of cluster growth, the Pd-TiO_2 interaction is the main driving force, and then it will be replaced by the Pd-Pd interaction.
Secondly, we investigated the effects of different supports on the acetylene selective hydrogenation. Compared the adsorption of C2H2 and C2H4 molecules on the Pd3 cluster supported by TiO_2 and Al2O3 surface, we found that the adsorption energy of acetylene on the Pd3/TiO_2 surface was higher than that on the Pd3/Al2O3 surface. However, the adsorption energy of ethylene molecule on the Pd3/TiO_2 surface was lower than that on the Pd3/Al2O3 surface. The results demonstrated that the Pd3/TiO_2 catalyst has a better selectivity for acetylene molecule adssorption than Pd3/Al2O3 catalyst. The active sites for C2H2 on both Pd3/TiO_2 and Pd3/Al2O3 surface are three-coordinated hollow sites, but the active site for C2H4 is three-coordianted hollow site on the Pd3/TiO_2 surface and top site of two Pd atoms on the Pd3/Al2O3. The adsorption of C2H2 and C2H4 molecules on the Pd3/TiO_2 surface is competitive process but is non-competitive process on the Pd3/Al2O3 surface. These results implied that the Pd3/TiO_2 is beneficial to C2H2 (reactant) adsorption and C2H4 (product) desorption, which will improve the selectivity of C2H4.
Thirdly, the effects of Ag doping into Pd catalyst on the acetylene selective hydrogenation was studied. When adding Ag into the Pd catalyst, we found that the Ag atom preferred to deposite on the surface of Pd catalyst rather than the TiO_2 surface. The Ag doping can separate the Pd cluster into small areas, which can inhibit the copolymer reaction of C2H2 on the catalyst and enhance the selectivity of acetylene hydrogenation to ethylene. Investigating the adsorption of acetylene and ethylene molecules on the supported Pd-Ag alloy cluster, we found that the doping of Ag can not enhance the activity of the catalyst, but it is beneficial to enhance the selective adsorption of acetylene on the catalyst surface. Furthermore, the adsorption energy of C4H6 molecule on the Pd-Ag/TiO_2 is decreasing with the increase of Ag/Pd ratio, which means that the Ag doping can also inhibit the depositon of green oil molecules and prolong the usage of catalyst.
引文
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