Insight into Acetylene Selective Hydrogenation on Transition Metal Catalysts

In Chapter 3, the influence of hydrogen coverage on the adsorption behavior of the reaction system was investigated. The results show that the increase of surface hydrogen coverage will greatly weaken the adsorption of surface species and change their surface adsorption configurations. In Chapter 4, we use a special molecular dynamics method to simulate the desorption process of ethylene under different coverage.

Two estimation methods for ethylene desorption barrier in some previous reports were selected to be compared with our results. A piecewise function model for ethylene desorption barriers were established. Meanwhile, the free adsorption energies of ethylene under different coverages were obtained by DFT total energy calculations and thermodynamic corrections.

The results indicate that the ethylene adsorption energy can be maintained around -0.33 eV at low coverage, and when the coverage exceeds 0.3 ML, it will rise rapidly with the increase of coverage. Subsequently, in Chapter 5, based on the conclusion of previous two chapters, a self-consistent coverage-dependent micro-kinetic model on the Pd (111) surface were built. The effects of different parameters on the selectivity and activity were discussed by the socalled sensitivity analysis method.

The selectivity of acetylene hydrogenation on the Pd(111) surface was proved to be mainly controlled by thermodynamic parameters, which may lead to a new understanding for this reaction system. We also found that the palladium surface would be reconstructed under high coverage situation. In this unique structure, the top-layer palladium atoms can be lifted up to 0.94 Å.

Then in Chapter 6, this uplifted surface structures which were caused by the surface reconstruction were investigated and the possible reasons were analyzed. The effect of the surface reconstruction, namely the thermodynamically stable structures and the ‘transient’ structures, on hydrogenation reactions is discussed: the ‘transient’ structures are more active for the acetylene hydrogenation. The understanding of the selective acetylene hydrogenation on different surfaces has long been focus in the field.

If an understanding can be obtained, it will be great help to screen suitable catalysts for this reaction. Thus, in Chapter 7, the acetylene selective hydrogenation on five different transition metal surfaces (Rh(111), Pd(111), Ir(111), Pt(111) and Ag(111)) were studied and the change of adsorption and reaction behaviors on these surfaces were explored. The results show that the adsorption on the silver surface is much weaker than that on other four metal surfaces and the weak adsorption on Ag(111) will lead to the changes of adsorption configurations of surface species.

In addition, a simple method to determine the system selectivity was proposed. The order of the selectivity obtained is: Ag(111) > Pd(111) > Ir(111) > Rh(111) = Pt(111), which is consistent with previous studies.