Theoretical Design Of Palladium-based Catalysts For Acetylene Selective Hydrogenation Reactions

Ethylene（C₂H₄）is a measure of the level of the country’s petrochemical industry and occupies an important state in the national economy.The development of catalysts in acetylene（C₂H₂）selective hydrogenation is the most core technology in the whole ethylene refining process.Currently,Pd-based catalysts play a leading role in industrial applications because of their excellent activity and stability.However,Pd catalysts are expensive and have poor selectivity to ethylene,so they often need to be modified by adding co-catalysts.Furthermore,with the development of computational materials science,density functional theory（DFT）based simulation calculations can save a lot of manpower and material resources in the design and screening of new materials,and the catalytic reaction process can be more clearly understood by analyzing the electronic structure and catalytic properties,thus providing guideline for experiments.Herein,this thesis investigated the modified Pd-based catalysts based on DFT calculations,by using metallic elements Cu,Ag and Au and non-metallic elements B and C as modifying components.The adsorption properties of acetylene and ethylene,the mechanism of acetylene hydrogenation reaction and the selectivity of ethylene are investigated at molecular and electronic scales.The results show that:（1）Cu₁₃@Pd₄₂,Ag₁₃@Pd₄₂ and Au₁₃@Pd₄₂ core@shell catalysts were constructed by putting Cu,Ag or Au atoms in the core and sub-surface layers and Pd atoms in the shell layer.Then the effects on the catalytic properties of acetylene hydrogenation were investigated through metal-to-metal synergy.The type of inner and sub-surface metal elements can affect the activity and selectivity of C₂H₄ production.Cu₁₃@Pd₄₂,Ag₁₃@Pd₄₂ and Au₁₃@Pd₄₂ catalysts all exhibited higher ethylene selectivity compared to Pd₅₅ catalysts.In addition,the Cu₁₃@Pd₄₂ catalysts also exhibited higher activity(63.2 k J·mol^-1)due to the transfer of electrons from Cu atoms to Pd atoms.（2）The Cu₄₃Pd₁₂,Ag₄₃Pd₁₂ and Au₄₃Pd₁₂ crown-jewel single-atom catalysts were constructed in order to further reduce the amount of Pd by using Cu₅₅,Ag₅₅ and Au₅₅clusters as substrates and replacing the metal atoms with the lowest coordination number at the corner by Pd atoms.Then the effect on the catalytic performance of selective hydrogenation of acetylene was investigated.The results of the calculations showed that Au₄₃Pd₁₂ have no selectivity in catalyzing the conversion of acetylene to ethylene,due to the intermediate reaction of C₂H₃+H→CHCH₃ will occur;the adsorption energy of C₂H₂ on Ag₄₃Pd₁₂ is only 50.6 k J·mol^-1,which is lower than the activation energy of C₂H₂ hydrogenation reaction(66.4 k J·mol^-1).so Ag₄₃Pd₁₂is not suitable as a catalyst for industrial purification of ethylene.Moreover,the Cu₄₃Pd₁₂crown-jewel structure exhibited excellent catalytic performance and its activity and selectivity are higher than the Cu₁₃@Pd₄₂,Ag₁₃@Pd₄₂ and Au₁₃@Pd₄₂ core-shell structure catalysts,as well as the pure Pd₅₅ catalyst.（3）Non-metallic light elements with small atomic radius can enter the subsurface layer interstice of metal catalysts to influence the electronic structure and properties of the surface metal atoms.Therefore,the Pd_4LB_2L and Pd_4LC_2L catalysts were constructed by doping B and C atoms into the subsurface layer of Pd.The calculated results showed that the selectivity of both Pd_4LB_2L and Pd_4LC_2L is significantly enhanced compared to Pd（111）;the B element doping significantly weakens the adsorption strength of C₂H₄,and the catalytic selectivity for ethylene is up to 78.0 k J·mol^-1,which is superior to the corresponding core@shell and crown-jewel structure catalysts.