The Regulation Of Activity And Selectivity Via The Structure Modification Of Pd-based Catalysts In Acetylene Semi-hydrogenation

The trace amount of C₂H₂ in C₂H₄ will affect the irreversible deactivation of the C₂H₄ polymerization catalyst.Therefore,it is necessary to efficiently remove the trace amount of C₂H₂ in C₂H₄.At present,the main method for removing C₂H₂in industry is C₂H₂ semi-hydrogenation.Pd-based catalysts are widely used in C₂H₂ semi-hydrogenation due to their high C₂H₂ conversion rate,and the main types of catalysts include the single-atom catalysts,core-shell catalysts and supported catalysts.This study systematically investigated the regulation of the catalytic performance of C₂H₂ semi-hydrogenation via the adjustment of the microstructure modification on above three types of Pd-based catalysts;the results at the electron-molecular level reveals the effects of single-atom Pd coordination number and coordination environment,the existence of the shell Pd ensembles,the support type and the cluster sizes and other microstructure adjustment methods on the catalytic performance have been elucidated.The results are expected to provide structural clues for the rational design of high-performance catalysts in C₂H₂ semi-hydrogenation.The obtained main conclusions are as follows:（1）The effects of single-atom Pd coordination number and coordination environment in the Pd M（M=Cu,Ag,Au）catalysts on C₂H₄ selectivity and its formation activity in C₂H₂ semi-hydrogenation were studied,and the microscopic essence was clarified for the effects of single-atom Pd coordination number,coordination environment and coordination metal types on catalytic performance of C₂H₂ semi-hysrogenation.a)The single-atom Pd coordination number affects C₂H₄ selectivity and its formation activity.Pd Cu（111）with surface Pd coordination number of 9 easily leads to green oil to reduce C₂H₄ selectivity and deactivate the catalysts;Pd Cu（211）and Pd Cu（110）with the same surface Pd coordination number of 7presents different performance of C₂H₂ semi-hydrogenation due to the different surface Pd coordination environment.Interestingly,among four types of Pd Cu catalysts,Pd Cu（100）with surface Pd coordination number of 8 not only shows better C₂H₄ selectivity and its formation activity,but also effectively inhibits green oil formation.b)The single-atom Pd coordination metal types affect C₂H₄ selectivity and its formation activity.Interestingly,Pd Ag（100）and Pd Au（100）show better C₂H₄selectivity and its formation activity,but also effectively inhibits green oil formation,while Pd Cu（100）exhibits higher activity than Pd Ag（100）and Pd Au（100）for C₂H₂ semi-hydrogenation.c)The d-band center of single-atom Pd and its coordination metals affect C₂H₄ selectivity and its formation activity.The electronic properties analysis shows that there is a volcanic curve relationship between C₂H₄ selectivity and its formation activity and the d-band center.The Pd M catalyst with surface Pd coordination number of 8 has better catalytic performance for C₂H₂ semi-hydrogenation,which is attributed to the moderate surface d-band center.（2）The mechanism of C₂H₂ semi-hydrogenation on the Pd_shell@M_core（M=Cu,Ag,Au）alloy catalysts was studied,and the microscopic nature of shell Pd ensemble existence form affecting the catalytic performance was clarified.a)The shell Pd ensemble forms affect C₂H₄ selectivity and its formation activity.Among Pd@Cu catalysts,the Pd₇@Cu catalyst with both shell Pd dimer and trimer shows the best C₂H₄ selectivity and its formation activity.Meanwhile,Pd₇@Cu catalyst inhibits the green oil formation to deactivate the catalyst.b)The core metal types affect C₂H₄ selectivity and its formation activity.Compared with Pd _ML@Ag and Pd _ML@Au catalysts,the same interesting results have been verified on the Pd₇@Ag and Pd₇@Au catalysts.That is,both Pd₇@Ag and Pd₇@Au catalysts show the best C₂H₄ selectivity and its formation activity toward C₂H₂ semi-hydrogenation.c)There is a volcanic curve relationship of C₂H₄ selectivity and its formation activity with the d-band the center.Pd₇@Cu catalyst exhibits better C₂H₄selectivity and its formation activity,which is mainly attribited to its moderate d-band center.d)C₂H₄ selectivity and its formation activity mainly depend on the charge density difference and average Mulliken charge of surface Pd atoms.Pd₇@M（M=Cu,Ag,Au）catalysts with both shell Pd dimer and trimer exhibit high C₂H₄selectivity and its formation activity toward C₂H₂ semi-hydrogenation,which is attributed to the medium charge density difference and Mulliken charge.（3）The mechanism of C₂H₂ semi-hydrogenation on different types of carbon materials supported Cu and Pd cluster catalysts was studied,and the microscopic nature of the support types and cluster sizes affecting the catalytic performance of C₂H₂ semi-hydrogenation was clarified.a)The support types affect C₂H₄ selectivity and its formation activity.C₂H₄selectivity and its formation activity over the carbon materials supported Cu and Pd single-atom cluster catalysts are closely related to the support types.Among them,GDY support shows excellent catalytic performance.Pd₁/GDY catalyst shows high C₂H₄ selectivity and high activity toward C₂H₄ formation.Cu₁/GDY catalyst has poor C₂H₄ selectivity;However,it has relatively high activity of C₂H₄formation.b)The supported metal cluster sizes affect C₂H₄ selectivity and its formation activity.For GDY supported different sizes of Cu and Pd cluster catalysts,as the size of cluster increases,the activity generally shows a decreasing trend.However,C₂H₄ selectivity is less correlated with cluster sizes.In general,the activity of GDY supported Pd catalysts is better than that of GDY supported Cu catalysts.However,C₂H₄ selectivity of GDY supported Cu cluster catalysts is better than that of GDY supported Pd cluster catalysts.c)The average Mulliken charge of the metal atoms in the supported catalysts affects C₂H₄ selectivity and its formation activity.The electronic properties analysis shows that there is a volcanic curve relationship for C₂H₄ selectivity and its formation activity on the supported catalyst with the average Mulliken charge of the metal atoms.Pd₁/GDY catalyst has the best C₂H₄ selectivity and its formation activity,which is attributed to the moderate average Mulliken charge of its metal atom.Pd₁/GDY is a new type of supported catalyst among carbon materials supported single-atom catalysts that significantly improves the catalytic performance of C₂H₂ semi-hydrogenation.