Adsorption Mechanism of Acetylene Hydrogenation on the Pd (111) Surface

First-principles calculations are carried out to examine the adsorption of acetylene over the Pd(111)surface.A hydrogen adsorption system is initially investigated to confirm the reliability of the selected calculation method.Adsorption energies,Mulliken-populations,overlap populations,charge density,and projected density of states(PDOS)are then calculated in the optimised acetylene adsorption system.Results show that C_(2)H_(2) molecule tends to adsorb through the threefold parallel-bridge configuration that is computed to be the most stable.In this structure,the distance of the C-H bond is calculated to be 1.09 Å,and the C-C-H bond angle is 128°.The distance of the C-C bond in acetylene is 1.36 Å,increasing from 1.21 Å in the gas phase.Moreover,the C-C bond overlap population decreases from 1.98 to 1.38,revealing that the carbon configuration in C2H2 rehybridises from sp to sp^(2) and beyond.The obtained results are compared with available experimental studies on acetylene hydrogenation on single-metal surfaces.The PDOS study indicates that a carbonaceous layer may generate on the metal surface during acetylene adsorption.The carbonaceous layer can affect the adsorption and reaction of acetylene,thereby inactivating the metal surface.Our experiments also show that Pd exhibits high catalytic activity.