The Effect Of The Modulation Of Strcture About Cu Catalysts On The Activity And Selectivity Of Acetylene Selective Hydrogenations

In industry,0.1-1%acetylene will be produced when ethylene is prepared by catalytic pyrolysis of hydrocarbons,and acetylene can cause irreversible deactivation of catalyst in the process of ethylene polymerization.As a result,it is necessary to remove trace acetylene from ethylene to reduce acetylene content to meet the requirements of ethylene polymerization.Selective hydrogenation of acetylene to ethylene is one of the important methods to remove acetylene.Noble Pd-based catalysts have good catalytic activity for acetylene hydrogenation,but the regulation of C₂H₄ selectivity and catalytic activity can not be achieved simultaneously.Recently,Cu-based bimetallic catalysts with non-noble metal Cu as active component have attracted wide attention due to the high C₂H₄ selectivity in acetylene selective hydrogenation at high temperature.However,the size of active component,the second metal doping,the composition and proportion of active component,as well as the metal-support interaction of Cu-based catalysts affect the catalytic performance of acetylene selective hydrogenation.Therefore,how to regulate the microstructure of Cu-based bimetallic catalysts to achieve high activity and selectivity of C₂H₄formation became the chemical basis of acetylene selective hydrogenation.In this paper,the mechanism of acetylene selective hydrogenation over Cu-based catalysts was systematically investigated using density functional theory calculations.The effects of the second metal doping mode,the size of active component and the metal-support interaction on the catalytic activity and selectivity of C₂H₄ formation in acetylene selective hydrogenation over Cu-based catalysts were clarified.The results provide a clear structural clue and theoretical guidance for the design of new and efficient Cu-based catalysts in the experiment.The main conclusions are obtained as follows:?1?The acetylene selective hydrogenation on a series of Pd-modified Cu nano-cluster catalysts have been investigated to elucidate the effects of promoter Pd,its doping mode and the cluster size on the catalytic activity and selectivity of C₂H₄ formation,and the target structure of Pd-modified Cu-based catalysts with higher selectivity and catalytic activity was obtained.a)The size of Pd-modified Cu-based catalyst affects C₂H₄ selectivity in acetylene selective hydrogenation.In comparison with the pure Cu cluster,promoter Pd cannot improve C₂H₄ selectivity and activity over the small-sized Cu₁₃ and the large-sized Cu₅₅ clusters.Only when Cu catalyst has a moderate size such as Cu₃₈ cluster,can significantly improve C₂H₄ selectivity and activity.Thus,Pd-modified Cu catalysts should be focused on the moderate size Cu catalysts,such as Cu₃₈ clusters,which can significantly improve the C₂H₄selectivity for acetylene selective hydrogenation.b)The doping mode of promoter Pd affects the selectivity and activity of C₂H₄ formation.For Pd-modified Cu₃₈ clusters,the individual Pd atom replacing the outer shell Cu atom cannot improve the C₂H₄ selectivity and activity,while the Pd ensemble composed of outer shell and its contiguous inner-layer Pd atoms can significantly enhance C₂H₄ selectivity and activity.c)For acetylene selective hydrogenation,controlling the size of Cu catalyst at a moderate size,followed by introducing Pd into Cu catalyst to form Pd ensemble composed of shell and its contiguous inner shell Pd atoms is an efficient way to improve the catalytic performance of C₂H₂ selective hydrogenation.?2?Based on above results,Pd ensemble composed of the surface and its contiguous subsurface Pd atoms in Cu catalysts significantly improve the C₂H₄selectivity and activity.Whether the metal Pd as the shell and other metals such as Cu as the core for core-shell catalysts can also exhibit better catalytic performance?Thus,acetylene selective hydrogenation on different compositions of M@Pd?M=Au,Ag and Cu?and M@Cu?M=Au,Ag and Pd?nanoclusters with different sizes have been investigated,which aim at identifying the effects of the composition and the nanocluster size on the selectivity and activity of C₂H₄formation.The corresponding structure and size of core-shell catalysts with better catalytic performance for acetylene selective hydrogenation were obtained.a)The core composition of core-shell catalyst affects the catalytic activity and C₂H₄ selectivity.For M@Pd?M=Au,Ag,Cu?core-shell catalysts,the core M composition not only affect C₂H₄ selectivity but also affect the catalytic activity of C₂H₄ formation in acetylene selective hydrogenation,in which the metal Cu as the core for M@Pd core-shell catalyst present the excellent selectivity and activity of C₂H₄ formation than Au and Ag as the core.Thus,for M@Pd core-shell catalyst,the metal Cu as the core significantly improves the selectivity and activity of C₂H₄ formation.On the other hand,for M@Cu?M=Au,Ag,Pd?core-shell catalysts,the metal Pd as the core presents better C₂H₄selectivity and catalytic activity than the metals Au or Ag as the core.b)The shell composition of core-shell catalyst affects the selectivity and activity of C₂H₄ formation.Different sizes of Pd@Cu nanoclusters show poor C₂H₄ selectivity and activity than the corresponding sizes of Cu@Pd nanoclusters,which suggested that the metal Pd as the shell shows better C₂H₄selectivity and catalytic activity than the metal Cu as the shell.Thus,the core-shell catalysts with the metal Cu as the core and the Pd as the shell?Cu@Pd?are a suitable catalyst to fundamentally improve C₂H₄ selectivity and activity in C₂H₂ selective hydrogenation.c)The size of core-shell catalyst affects the selectivity and activity of C₂H₄formation.For Cu@Pd core-shell catalysts with different sizes,the sequence of the C₂H₄ selectivity and activity was:Cu@Pd₁₂<Cu₆@Pd₃₂<Cu₁₃@Pd₄₂.Thus,with the size increasing of Cu@Pd nanocluster,the selectivity and activity of C₂H₄ formation over Cu@Pd nanocluster increase,Cu₁₃@Pd₄₂ nanocluster exhibits the best activity of C₂H₄ formation.d)In the experimental preparation of core-shell catalyst used for acetylene selective hydrogenation,the combination of metal Cu as core and Pd as shell should be espencially considered,and keep the catalyst with a larger cluster size was beneficial to improve the C₂H₄ selectivity and activity for acetylene selective hydrogenation.?3?As mentioned above,the core-shell catalyst combined of Pd as the shell and Cu as the core showed higher C₂H₄ selectivity and activity.Thus,acetylene selective hydrogenation over supported Pd and Cu catalysts were studied to illustrate the effects of support type and metal-support interaction on the selectivity and activity of C₂H₄ formation.a)The type of supports affects the C₂H₄ selectivity and activity.For supported Pd catalysts,?-Al₂O₃ and MgO supports exhibits poor C₂H₄selectivity,while TiO₂ support shows high selectivity and activity of C₂H₄formation.For supported Cu catalyst,?-Al₂O₃ support cannot improve the C₂H₄selectivity in comparison with the unsupported Cu₃₈ cluster,whereas the oxygen defective MgO support effectively improve the C₂H₄ selectivity but reduced the catalytic activity.b)Supports properties?such as crystalline phase,oxygen defective?affect the catalytic activity and C₂H₄ selectivity.The anatase TiO₂ support exhibits higher C₂H₄ selectivity and catalytic activity than the rutile phase,furthermore,oxygen defects can significantly improve the C₂H₄ selectivity and catalytic activity;and for supported Cu catalyst,promoter Pd has improve the C₂H₄selectivity but reduced the catalytic activity of supported Cu catalyst..c)The metal-support interaction affects the selectivity and activity of C₂H₄formation.For supported Pd catalysts,the interaction between metal Pd and O-deficient anatase support was stronger than those of other types of supports,and the catalyst exhibits the highest C₂H₄ selectivity and activity.For supported Cu catalysts,the metal-support interaction between metal Cu and O-deficient MgO support is stronger than that between Cu and?-Al₂O₃ support,as a result,the catalyst exhibits high C₂H₄ selectivity and activity of formation.Moreover,the metal-support interaction between Pd and supports were much stronger than that of supported Cu catalysts,and the supported Pd catalysts shows better catalytic activity.Thus,stronger metal-support interaction was conducive to improve the selectivity and activity of C₂H₄ formation over the supported catalysts.