Molecular Simulation Study Of Acetylene Selective Hydrogenation

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 TiO₂(101) support. When a single Pd atom adsorbed on the perfect anatase TiO₂(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 TiO₂(101) surface. The Pdn cluster on the anatase TiO₂(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 TiO₂(101) surface (Pd-TiO₂ 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-TiO₂ 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 TiO₂ and Al2O3 surface, we found that the adsorption energy of acetylene on the Pd3/TiO₂ surface was higher than that on the Pd3/Al2O3 surface. However, the adsorption energy of ethylene molecule on the Pd3/TiO₂ surface was lower than that on the Pd3/Al2O3 surface. The results demonstrated that the Pd3/TiO₂ catalyst has a better selectivity for acetylene molecule adssorption than Pd3/Al2O3 catalyst. The active sites for C2H2 on both Pd3/TiO₂ and Pd3/Al2O3 surface are three-coordinated hollow sites, but the active site for C2H4 is three-coordianted hollow site on the Pd3/TiO₂ surface and top site of two Pd atoms on the Pd3/Al2O3. The adsorption of C2H2 and C2H4 molecules on the Pd3/TiO₂ surface is competitive process but is non-competitive process on the Pd3/Al2O3 surface. These results implied that the Pd3/TiO₂ 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 TiO₂ 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/TiO₂ 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.