The Effects Of Composition And Structure Of Cu-based And Pd-based Alloy Catalysts On The Catalytic Performance Of Acetylene Selective Hydrogenation

The trace amounts of C₂H₂ in C₂H₄ will reduce the activity of the catalyst and the properties of the polymer in the C₂H₄ polymerization reaction.Thus,it is necessary to remove trace C₂H₂ in the feedstock C₂H₄.C₂H₂ selective hydrogenation is the main method to remove trace C₂H₂.Cu-based and Pd-based catalysts are the most commonly used catalysts for C₂H₂ selective hydrogenation.Cu-based catalysts exhibit high C₂H₄ selectivity,but they are prone to the C-C polymerization to form green oil,resulting in the catalyst deactivation.Pd-based catalysts have high C₂H₂ conversion rates,but they tend to form hydrides and carbides,reducing the catalytic performance.Therefore,developing the catalysts with high activity,high selectivity and high stability has become a research focus in C₂H₂ selective hydrogenation.Using density functional theory calculation method,this paper systematically studies C₂H₂ selective hydrogenation on a series of Cu-based and Pd-based alloy catalysts,the effects of structural factors,including the second metal type,the composition ratio and surface structure on C₂H₄ formation activity and selectivity have been clarified.The research results are expected to provide ideas and theoretical guidance for the catalyst design of C₂H₂ selective hydrogenation.The main conclusions obtained are as follows:（1）C₂H₂ selective hydrogenation over the Cu_xM_y（M=Zn,Pt,Ni,Pd,Ag,Au）and Pd_xN_y（N=Zn,Ga,In,Sn,Cu,Ag,Au）intermetallic compounds have been investigated to elucidate the effects of second metal types and ratios on the catalytic performance of the reaction.a)The type of the second metal affects C₂H₄ formation activity and selectivity in the hydrogenation process.Among the Cu_xM_y IMCs,Cu₃Zn₁,Cu₁Pt₁,Cu₃Ni₁,Cu₃Pd₁ and Cu₃Ag₁ have higher C₂H₄ formation activity and selectivity than noble metal Pd,especially the Cu₃Pd₁.Among the Pd_xN_y IMCs,Pd₁Cu₃,Pd₁Au₁,Pd₃Au₁,Pd₁Ag₁,Pd₁Zn₁ and Pd₁In₁ have higher C₂H₄ formation activity and selectivity than Pd,especially the Pd₁Cu₃.Considering the activity and selectivity,the Cu₃Pd₁（or Pd₁Cu₃）with the surface isolated single-atom Pd active centers is the most conducive to the formation of gas phase C₂H₄.b)The ratio of the second metal affects C₂H₄ formation activity and selectivity of C₂H₂ hydrogenation process.Among the Cu_xM_y IMCs,Cu_xPd_y,Cu_xAg_y,Cu_xAu_y and Cu_xNi_y catalysts with a Cu/M ratio of 3/1 have high C₂H₄formation activity and selectivity.The Cu_xZn_y and Cu_xPt_y catalysts with Cu/M ratios of 3/1 and 1/1,respectively,have high C₂H₄ formation activity,while the Cu_xZn_y and Cu_xPt_y catalysts with Cu/M ratios of 1/1 and 3/1,respectively,have high C₂H₄ selectivity.Among the Pd_xN_y IMCs,Pd_xZn_y,Pd_xGa_y,Pd_xIn_y,Pd_xAg_yand Pd_xAu_y catalysts with a Pd/N ratio of 1/1 have high C₂H₄ formation activity and selectivity;Pd_xSn_y and Pd_xCu_y catalysts with Pd/N ratios of 3/1 and 1/3show high C₂H₄ formation activity and selectivity.c)The type and ratio of the second metal affect the formation of green oil and the dissociation ability of H₂ in the polymerization process.For the nine kinds of Cu₃Zn₁,Cu₁Pt₁,Cu₃Ni₁,Cu₃Pd₁,Cu₃Pd₁,Cu₃Ag₁,Pd₁Ag₁,Pd₁Ag₁ and Pd₁In₁ with better C₂H₄ formation activity and selectivity than Pd,green oil is easily produced on the Cu₁Pt₁,Cu₃Ni₁,Pd₁Au₁ and Pd₁Ag₁ in the polymerization process.However,Cu₃Zn₁,Cu₃Pd₁,Cu₃Ag₁,Pd₁Zn₁ and Pd₁In₁ can inhibit green oil formation and have high stability;further,among these five catalysts,H₂ is not easy to dissociate on the Cu₃Zn₁ and Cu₃Ag₁,while H₂ is easily dissociated on the Cu₃Pd₁,Pd₁Zn₁ and Pd₁In₁,which can provide sufficient H source to ensure the effective progress of C₂H₂ selective hydrogenation reaction.d)Considering C₂H₄ formation activity,selectivity,green oil formation,H₂dissociation ability and catalyst cost,Cu₃Pd₁（or Pd₁Cu₃）catalyst with the surface isolated single-atom Pd active sites is the best choice for the selective hydrogenation of C₂H₂ to C₂H₄.（2）The mechanism of C₂H₂ selective hydrogenation on a series of Pd-based alloy catalysts is studied,and the effect of the surface structure of Pd-based catalysts on the catalytic performance of the reaction is clarified.a)The surface structure affects H₂ dissociation activity.H₂ dissociation is easy on the Pd₁Cu₃ and Pd₁Ag₁ with uniform alloy structure;H₂ spontaneous dissociation occurred on twelve kinds of Pd-based alloy catalysts with core-shell and subsurface structures.Thus,above fourteen kinds of Pd-based alloy catalysts can provide sufficient hydrogen source for C₂H₂ hydrogenation reaction to ensure the effective progress of the reaction.b)The surface structure affects C₂H₄ formation activity and selectivity in the hydrogenation process.There is no C₂H₄ formation on the Pd_1L@Cu,Pd_3L@Pd₁Cu₃ and Pd_2L@Pd₁Ag₁ with core-shell structure.Considering the activity and selectivity,the core-shell structure Pd_1L@Pd₁Cu₃,Pd_1L@Ag,Pd_2L@Ag,Pd_3L@Ag and Pd_1L@Pd₁Ag₁ are not conducive to C₂H₄ formation,while the Pd_2L@Pd₁Cu₃,Pd₁Cu₃,Pd_1L-Cu_sub,Pd_3L@Pd₁Ag₁,Pd₁Ag₁ and Pd_1L-Ag_sub are conducive to C₂H₄ formation,especially Pd₁Cu₃,Pd_1L-Cu_sub,Pd₁Ag₁ and Pd_1L-Ag_sub.Electronic properties analysis shows that for Pd-Cu(Pd_2L@Pd₁Cu₃,Pd₁Cu₃ and Pd_1L-Cu_sub)and Pd-Ag(Pd_3L@Pd₁Ag₁,Pd₁Ag₁,and Pd_1L-Ag_sub)alloy catalysts favored C₂H₄ formation,the highest C₂H₄ formation activity is obtained when the distance between the d-band center of the outermost atoms and Fermi level is moderate.The more the average Bader charge of the outermost Pd atoms is,the higher the catalytic performance is.c)The surface structure affects the formation of green oil in the polymerization process.For the Pd₁Cu₃,Pd_1L-Cu_sub,Pd₁Ag₁ and Pd_1L-Ag_subwhich are more conducive to C₂H₄ formation,Pd_1L-Cu_sub,Pd₁Ag₁ and Pd_1L-Ag_subare easy to form green oil,while Pd₁Cu₃ can inhibit the formation of green oil,and has the higher catalytic stability.