Controllable Preparation And Electro-chemical Semi-hydrogenation Of Alkynes Of Palladium-based Catalysts

Olefin compounds are important raw materials for the synthesis of drugs,fine chemicals and polymer materials.Selective semi-hydrogenation of alkynes to alkenes is one of the important methods in industry.Hydrogen（H2）is used as the hydrogen source in the traditional thermochemical semi-hydrogenation（TSH）method,which has some safety problems.Besides,the over-hydrogenation,oligomerization of alkenes and the stability of TSH’s catalyst are the problems to be solved urgently.As a safe,efficient and green method,electrochemical semi-hydrogenation（ECSH）is favored by researchers,which has become the most potential alternative to TSH.In the ECSH process,the solvent containing proton（e.g.,water）is used as the hydrogen source that solve the safety problem of H2,and the electrocatalyst is attached to the conductive material that avoid the separation problem between the product and the catalyst,which ensures the stability of the catalyst.However,it is still the bottleneck to maintain high activity and selectivity at the same time in the current ECSH process.Palladium（Pd）is a perfect material for hydrogen storage and hydrogenation,which is widely used in hydrogenation research.Based on the above problems,this thesis starts with Pd-based materials.We designed and synthesized three different Pd-based catalysts by regulating Pd surface and interface structure and electronic structure,which are PdCu icosahedra with tensile strain,Pd NPs@MoS2 and the low-index Pd（hkl）planes,i.e.,Pd cubes,octahedrons and rhombic dodecahedrons（RDs）,respectively.At the same time,the performance of the different catalysts on ECSH was investigated,and the effect of interfacial water structure on ECSH in alkaline solution was discussed.The main contents of this thesis are as follows:1.PdCu icosahedral nanomaterials were designed as cathode catalysts for the ECSH of alkynes.PdCu icosahedral catalyst has double effects of surface tensile strain and alloy effect.The ECSH results showed that PdCu icosahedron showed excellent olefin selectivity in the potential range of–0.1 to–0.5V（vs.RHE）,and showed the highest activity at–0.3V,with its specific activity of 165.8μmol m^-2min^-1,which is 7.2 times of commercial Pd/C catalyst.Through the investigation of the mechanism,it is found that tensile strain enhances the formation of interfacial water structure,especially K·H2O and 2-HB·H2O.The results show that K·H2O can promote the splitting of H2O,thus promoting the ECSH performance of alkynes;2-HB·H2O can improve alkenes’’selectivity in ECSH.2.Single crystal catalysts with different Pd crystal planes were synthesized,and the effects of different Pd crystal planes on ECSH were investigated,and the reaction mechanism was analyzed from the perspective of interfacial water structure.Specifically,Pd rhombohedral dodecahedron,octahedron and cube with（110）,（111）and（100）plane were synthesized.Through the investigation of the electrocatalytic properties,the Pd（100）plane has the highest reaction activity and olefin selectivity.Under the same reaction conditions,the selectivity of MBE remained 95.0%when the conversion was 97.2%using Pd cube.In order to investigate the reaction mechanism,the water structure on different Pd crystal planes was characterized.It shows that Pd（100）plane has the strongest interfacial water regulation ability,which can promote the formation of K·H2O and 2-HB·H2O,thus promoting the performance of the electrocatalytic process and the selectivity of olefins.And the order of activity of low-index Pd（hkl）planes for ECSH is Pd（100）>Pd（111）>Pd（110）.3.An electrically driven MoS2-supported Pd nanoparticle catalyst was designed and the effects of single atoms（SAs）and nanoparticles（NPs）on ECSH were investigated.Based on the principle that sulfur can be used as the coordination element of metal elements on the surface of MoS2,Pd atoms are first anchored on the surface of MoS2 to form MoS2-supported Pd SAs catalyst（Pd SAs@MoS2）.Then Pd SAs@MoS2 tend to granulate and form（Pd NPs@MoS2）in alkaline solution under electric field.ECSH results show that,Pd NPs@MoS2 shows higher selectivity of olefins than Pd SAs@MoS2.It is proved that the single atoms tend to agglomerate in the electrochemical process,which is beneficial to improving the selectivity of olefins.