The Adjustment Of Active Site Structure Of Cu Catalyst To Regulate The Catalytic Performance In Acetylene Selective Hydrogenation

The trace amounts C₂H₂ in the rich-C₂H₄ will cause the irreversible deactivation of C₂H₄ polymerization catalyst,so it is necessary to efficiently remove trace C₂H₂ in C₂H₄.At present,C₂H₂ selective hydrogenation is the commonly industrial method to remove trace C₂H₂ from C₂H₄.Nowadays,Cu catalyst has been widely used in C₂H₂ selective hydrogenation due to its high C₂H₄ selectivity,low cost,and the flexible structure adjustment.However,Cu catalyst exhibits poor C₂H₄ formation activity,and it is easy to form green oil that deactivate the catalyst.Thus,how to improve C₂H₄ activity on Cu catalyst and inhibit green oil formation remain the key problems to be solved urgently.In this dissertation,based on DFT calculations and microkinetic modeling,the influences of spatial scale of Cu active site,Cu active site types,generalized coordination number of Cu active sites and the subsurface adjustment of Cu active site on C₂H₄ selectivity,activity,and green oil formation in C₂H₂selective hydrogenation are systematically investigated.The results are expected to provide a solid theoretical basis and structural clues for the design and optimization of high-efficient Cu catalysts in the industry.The main conclusions are obtained as follows:1.C₂H₂ selective hydrogenation over the Cu and S/Cu catalysts have been investigated to elucidate the effect of spatial scale of Cu active sites on C₂H₄formation activity,selectivity and green oil formation;meanwhile,the microcosmic essence for the influence of spatial scale of Cu active sites in the Cu and S/Cu catalysts on the catalytic performance is clarified.Moreover,the types of Cu and S/Cu catalysts with excellent catalytic performance and the corresponding spatial scale of the most suitable active sites were obtained.（a）The spatial scale of Cu active sites affects C₂H₄ formation activity,selectivity,and green oil formation.Compared to Cu-4×4 catalyst,S/Cu-4×4catalyst with surface S coverage of 1/16 ML has little effect on the catalytic performance.Compared to Cu-3×3 catalyst,S/Cu-3×3 catalyst with surface S coverage of 1/9 ML showed better C₂H₄ formation activity,selectivity and the significant inhibition toward green oil formation.Cu-2×2 catalyst not only shows better C₂H₄ formation activity and selectivity,but also greatly inhibits green oil formation.For S/Cu-2×2 catalyst with surface S coverage of 1/4 ML,the electronic effect induced by high surface S coverage decreased C₂H₄formation activity,but increased C₂H₄ selectivity and inhibited green oil formation.Thus,both S/Cu-3×3 and Cu-2×2 catalysts are screened out to exhibit excellent catalytic performance,and the spatial scale of active sites is5.015 and 5.112?,respectively.（b）The d-band center of surface Cu atom and spatial scale of Cu active site affect C₂H₄ formation activity,selectivity,and green oil formation.The d-band center of surface Cu atom in S/Cu-3×3 catalyst is far away from Fermi energy level,which improves C₂H₄ formation activity.Meanwhile,S atom reduces the spatial scale of active sites and thus inhibits green oil formation.Due to its small spatial scale of active sites,Cu-2×2 catalyst is not conducive to the polymerization process but does not affect the hydrogenation process.Thus,both S/Cu-3×3 and Cu-2×2 catalysts not only show better C₂H₄ activity and selectivity,but also greatly inhibit green oil formation.2.C₂H₂ selective hydrogenation over the Cu catalysts with different Cu active site types have been investigated to elucidate the effect of Cu active site types and generalized coordination number on C₂H₄ formation activity,selectivity,and green oil formation;and the microcosmic essence for the influence of Cu active site types and generalized coordination number on the catalytic performance was further clarified based on the geometric and electronic structure properties.（a）The generalized coordination number of Cu active sites can be acted as a descriptor to characterize Cu active site type,which affects C₂H₄ formation activity,selectivity,and green oil formation.H₂ dissociation activity has an inverted volcanic-type relationship with the generalized coordination number of Cu active site,and with the increasing of Cu generalized coordination number,H₂ dissociation activity increases first and then decreases;among them,Cu_GCN4.8catalyst with defect site exhibits the highest H₂ dissociation activity.Meanwhile,Cu_GCN4.8 catalyst with defect site not only presents better C₂H₄ formation activity and selectivity,but also significantly inhibits green oil formation.（b）The generalized coordination number of Cu active sites affects the adsorption ability of the species,and further regulates C₂H₄ formation activity,selectivity,and green oil formation.C₂H₂ adsorption ability can be acted as a descriptor to evaluate C₂H₄ formation activity.For the catalysts that can inhibit green oil formation,C₂H₄ adsorption ability decreases with the increasing of the generalized coordination number;meanwhile,the activation free energy of C₂H₄hydrogenation to C₂H₅ firstly increases and then decreases,as a result,C₂H₄selectivity increases firstly and then decreases.Moreover,C₂H₃ adsorption ability increases firstly and then decreases,and it is up to the strongest on Cu_GCN4.8,which is not conducive to C₂H₃ coupling and thus inhibits green oil formation.On the other hand,C₂H₂ adsorption ability at Cu corner site decreases with the increasing of generalized coordination number.At the defect,step,and terrace sites,C₂H₂ adsorption ability increases firstly and then decreases with the increasing of generalized coordination number.Further,the change trend of C₂H₄ formation activity with the generalized coordination number is consistent with C₂H₂ adsorption ability.Thus,C₂H₂ adsorption ability is proposed to act as a descriptor to evaluate C₂H₄ formation activity.3.C₂H₂ selective hydrogenation over Cu catalysts with the subsurface adjustment have been investigated to elucidate the effect of Cu active site subsurface adjustment on C₂H₄ formation activity,selectivity,and green oil;meanwhile,based on the geometric and electronic effect,the microcosmic essence of the influences of the subsurface doped-heteroatom types and Cu surface coordination environment on the catalytic performance is clarified.（a）The types of subsurface heteroatom affect C₂H₄ formation activity,selectivity,and green oil formation.Over the heteroatom H,B,C,N and P doped-Cu（111）catalysts,B-Cu（111）,C-Cu（111）and N-Cu（111）catalysts show better C₂H₄ formation activity and selectivity,and significantly inhibit green oil formation.Over the heteroatom H,B,C,N,and P doped-Cu（211）catalysts,only P-Cu（211）could inhibit green oil formation and showed better C₂H₄ formation activity and selectivity.（b）Cu surface coordination environment affects C₂H₄ formation activity,selectivity,and green oil formation.The improvement of catalytic performance over the heteroatom doped-Cu（111）is generally better than that over the doped-Cu（211）,Thus,the subsurface heteroatom doping should focus on the high coordination Cu（111）rather than the low coordination Cu（211）.（c）The subsurface heteroatom doping could change Cu surface geometry and electronic structure and thereby affects C₂H₄ formation activity,selectivity,and green oil formation.In the view of geometrical structure,Cu subsurface heteroatom doping increases the Cu-Cu bond length,thus exposing highly active under-coordination Cu sites,increasing C₂H₄ formation activity.During the process of C₂H₄ hydrogenation,Cu active site bulged from the Cu surface,which is not conducive to C₂H₄ hydrogenation due to the steric hindrance,thus improves C₂H₄ selectivity.On the other hand,the presence of subsurface heteroatoms B,C,N and P in Cu（111）elongates the distance between two active sites to adsorb two C₂ species in the coupling reaction,which is not conducive to the coupling reaction and thus inhibits green oil formation.In the view of electronic structure,the subsurface heteroatom doping promotes the charge transfer between the heteroatom and Cu atom,thus improving its catalytic performance.C₂H₄ formation activity increases with the increasing of Bader charge of surface Cu atoms.Meanwhile,the d-band center of Cu atom above the heteroatom is far away from Fermi level,which is beneficial to improve C₂H₄formation activity.