Theoretical Study Of Adsorption And Reaction Mechanism Of Acetylene Selective Hydrogenation On Pd(111)

The reaction of acetylene selective hydrogenation has important implications due to its important status in basic organic chemistry and chemical engineering industry. Both preparing acetylene, and removing acetylene impurity doped in ethylene, require in-depth research into the reaction mechanism of acetylene selective hydrogenation. In recent years, the catalysts used in acetylene selective hydrogenation, such as single metal, bimetal and nano catalyst, have been drawn much attention. However, there are little reports for the electron behavior in this adsorption system, and the activity changes of Pd catalysts after adsorption have been unclear so far.In the paper, the adsorptions of acetylene, ethylene and hydrogen on Pd(111) were completely investigated using first-principles method based on density functional theory (DFT), firstly. Secondly, the electron behavior in reactants and catalysts were discussed. Thirdly, we analyzed the changes in the activity of the catalysts surface after acetylene adsorption. Finally, the overall reaction energetics along the reaction path for acetylene selective hydrogenationand and the structures of transition-states and intermediates were calculated, and then the reaction mechanism was concluded.All the results show:(1) The structure is the most stable when H atom adsorbed at fcc site on Pd(111);(2) The3-fold parallel-bridge configuration, which C2H2molecule tends to adsorb, is the most stable, whose adsorption energy is-183.5kJ/mol. In this structure, the distance of C-H is1.09A and the C-C-H bond angle128°. The distance of C-C of acetylene is1.36A, increasing from1.21A in the gas phase, indicating that the C2H2molecule is activated;(3) The overlap population of C-C bond decreases from1.98to1.38. This variation is symbolic of a change in the bond order and bond character between the carbon atoms, and the C-C bond presents more double bond character;(4) The electron density around C atoms increases, while that of Pd atoms decreases, showing that electrons around Pd atoms are attracted by C atoms. Electrons transfer from catalyst surface to C2H2molecule, and the electrons between C and H atoms redistribute;(5) The results of d-band centre show that a carbonaceous layer may generate on the metal surface during the adsorption of acetylene. The carbonaceous layer can affect the adsorption and reaction of acetylene, which inactivates the metal surface;(6) The reaction mechanism of acetylene selective hydrogenation is:firstly, C2H2molecules adsorb on the Pd (111) surface, the adsorption energy is calculated to be-183.5kJ/mol. Secondly, the C2H2molecules and the H atoms co-adsorb on the metal surface, H atom reacts with neighboring Pd-C bond. The reaction involves the formation of a threecenter (Pd-C-H1) transition-state structure (TS1), whose energy is the highest in the first elementary reaction. Then, Hj-Pd bond break to form vinyl-adsorbed structure. The activation energy is+71kJ/mol and reaction is exothermic at31kJ/mol. Thirdly, vinyl reacts with neighboring Pd-C bond. The reaction involves the formation of a threecenter (Pd-C-H2) transition-state structure (TS2). Then, H2-Pd bond break to form ethylene-adsorbed structure. The activation energy is+80kJ/mol and reaction is exothermic at60kJ/mol. Finally, ethylene desorb from the metal, heat absorption is87.2kJ/mol.(7) At high coverage of gas molecules, the reaction rate of the second elementary reaction is faster than the first one. But acetylene hydrogenation becomes rate limiting when the rate reaches a certain extent. At low coverage of gas molecules, the reaction rate of the second elementary reaction is almost equal with the first one. These performance show that the coverage can significantly effect the reaction kinetics and reaction path.Present paper provides theoretical direction for the electron behavior in this adsorption system and the activity changes of Pd catalysts after adsorption.