Precise Synthesis And Catalytic Performance Of Nanocarbon Supported Atomically Dispersed Metal Catalysts For Acetylene Hydrogenation

Ethylene,is an important raw material for the production of polyethylene.However,a trace amount of acetylene(-1%)is usually present in the ethylene feed produced from the petroleum industry,which would poison the catalysts used for ethylene polymerization and reduce the quality of polyethylene.Selective hydrogenating remnant acetylene in the raw olefin streams to ethylene while avoiding the over-hydrogenation to undesired ethane is an ideal solution to purify ethylene feed.Thus,selective hydrogenation of alkynes has great value both in basic research and industrial application.Pd nanoparticles are so far regarded as the most efficient catalyst for such catalytic reaction.However,pure Pd metal catalysts posses very poor selectivity for high acetylene conversion in the presence of ethylene.For this reason,pure Pd metal was modified with surface modifiers,such as lead acetate or quinoline(e.g.,those in Lindlar catalyst)to enhance the selectivity of acetylene hydrogenation.However,these catalysts were usually toxic,and large amounts of environmental pollutants(e.g.,Pb or sulfur containing compounds)could be produced as byproducts.The most commonly used industrial catalyst is based on supported Pd nanoparticles modified by Ag additives.Although the Pd-Ag catalyst prevents the usage of toxic promoters such as lead or sulfur,the extremely high cost of Pd and Ag leaves ample room for improving the cost-effectiveness in catalyst design.That is,the design of cost-effective,non-toxic and easily accesssible metal catalysts for selective hydrogenation of alkynes remains a challenge in both academia and industry.Aiming at the above challenges,this dissertation precisely fabricate nanocarbon supported atomically dispersed metal catalysts through easily formed metal-carbon bonds to fulfill highest atomic utilization,and examine their catalytic performance for acetylene hydrogenation.In combination with various characterization methods and theoretical calculations,this work further understand the structure-performace relationships of nanocarbon supported atomically dispersed metal catalysts for acetylene hydrogenation and,which paves the way for rational design of highly effective catalysts for selective hydrogenation of acetylene.The main contents are summarized as follows:(1)A new strategy to prepare Pd catalyst that consists of isolated Pd atoms anchoring onto the defective nanodiamond/graphene(ND@G)hybrid nanocarbon support(Pd1/ND@G)is reported.The Pd species in the Pd1/ND@G interact strongly with the support,becoming emdedded in the graphene defects through bonding with carbon.The lack of ?-H reservoir in the Pd1/ND@G can suppress further hydrogenation of ethylene to ethane,due to the control of reaction kinetics.In addition,the unique structure of the atomically dispersed Pd catalyst renders the easy desorption of the surface C₂H₄*species,which is the key for the excellent selectivity.In sumary,the suppression of further hydrogenation of ethylene to ethane is linked to the lack of unselective subsurface hydrogen and the presence of an energetic favored path for the aimed product,ethylene,desorption on the Pd1/ND@G.(2)A cheap atomically dispersed Cu catalyst supported on a defective ND@G(Cu1/ND@G)is fabricated to more effectively catalyze selective hydrogenation of acetylene,compared to the Cu-cluster catalyst supported over the same host(Cun/ND@G).Adequate experimental evidences show that isolated Cu atoms were anchored over the surface-defective ND@G support in Cu,/ND@G.DFT calculations reveal that the dissociation of H2 is the rate detemining step(RDS)for acetylene hydrogenation.The reaction barrier of RDS on Cu1 is much lower than that on Cun catalyst,which makes Cu1 catalyst exhibit much higher hydrogenation activity.In addition,Cu1 catalyst favors the desorption of C₂H₄*,which is the key for the excellent selectivity.In summary,the unique bonding structure and electronic property of Cu atoms on Cu1/ND@G facilitate the H2 activation and ethylene desorption,which clearly elucidates the importance of isolated Cu atoms in catalysts for high acetylene semihydrogenation performance.(3)A defective nitrogen-doped nanodiamond/graphene(ND@NG)support is prepared by doping nitrogen into the defect-rich graphene on the surface of ND@G.Two kinds of atomically dispersed Cu catalysts with different coordination structures are prepared by the electrostatic adsorption method:Cu1/ND@G,which features the Cu-3C coordination,and Cu1/ND@NG,which features the Cu-3N coordination.Cu1/ND@G catalyst exhibits more excellent catalytic activity for acetylene hydrogenation.Adequate experimental evidences show that isolated Cu atoms on ND@G support through Cu-3C bonding are more electron-rich than isolated Cu atoms on ND@NG support through Cu-3N bonding,which accelerates the RDS-dissociation of hydrogen and activiation of acetylene,and thus improve the acetylene hydrogenation catalytic activity.(4)Controlable preparation and catalytic performance of atomically dispersed bimetallic catalysts for acetylene hydrogenation are investigated.An atomically dispersed PdCu bimetallic catalyst is prepared by introducing a small amount of Pd metals as the second component into single-atom Cu catalyst through electrostatic adsorption associated with desorption precipitation method.The PdCu bimetallic catalyst shows the best catalytic activity than the single-atom Pd catalyst and the single-atom Cu catalyst,and maintains excellent ethylene selectivity.AC-HAADF-STEM and XAFS reveal that the Pd-Cu diatomic pairs are dispersed over PdCu bimetallic catalyst.As the new active sites,the Pd-Cu atomic pair favors to hydrogen dissociation and acetylene activiation by providing dual active sites.In addition,the effect of hydrogen spillover on the PdCu bimetallic catalyst also improves the hydrogenation activity of Cu atoms.