Tuning The Structure Of Supproted Metal Catalysts To Improve The Catalytic Performance For Selective Hydrogenation

Selective hydrogenation is of siginificance for the industrial productions of bulk and fine chemicals,such as the production of ethylene and unsaturated alcohol,where the issue of selectivity needs to be addressed.Selective hydrogenation of acetylene is often employed to remove acetylene from ethylene industrially produced by the steam-cracking to avoid the poison of the polymerization catalysts downstream.Unsaturated alcohol,such as cinnamyl alcohol,is an important intermediate for the production of flavors,perfumes and pharmaceuticals.The industrial production of unsaturated alcohol,such as cinnamyl alcohol,mainly employs the selective hydrogenation of unsaturated aldehyde,i.e.,selectively hydrogenate C=O in unsaturated aldehyde.In this thesis,acetylene and cinnamldehyde were chosen as the cases to address the selectivity issue of the hydrogenation over supported metal catalysts.The combination of thermodynamics/kinetics analyses,DFT calculations and structural characterizations were employed to identify the dominant factors for the selective hydrogenation process over supported metal catalysts,based on which the structural properties of the catalysts were tailored toward the improved performance.The multi-faceted kinetics analysis and model calculations were first carried out to discriminate the active sites of Pd/CNT for acetylene hydrogenation,and then the electronic and geometric structures of the active sites were discussed.The subsurface strucuture of Pd catalysts were then modified by CNT support,and nitrogen doping of CNT was further employed to optimize the the electronic structure of Pd catalyst toward the enhanced ethylene selectivity.Subsequently,the modifications of surface and subsurface structures of Pd catalysts by In and their effects on the catalytic performance were elucidated,and then the compositions of Pd-In alloy catalysts were tuned to inverstigate the composition-dependent performance and effect of In on the kinetics behaviors.As a consecutive effort,the structure-performance of non-noble Ni-Ga catalysts were elucidated for acetylene hydrogenation to explore the potential of non-noble catalysts for acetylene hydrogenation.Finally,Pt was introduced into the subsurface of the Cu catalyst to improve its activity for the selective hydrogenation of C=O in cinnamaldehyde.The reaserch results are summarized as follows:(1)Active sites,electronic and subsurface structures.The kinetics analysis and model calculations show that the Pd(111)sites dominate the acetylene conversion and C4 formation,while the corner sites for ethan formation.The catalytic performance of the Pd catalyst depends on its electronic properties when Pd particle size is smaller than 3.1 nm.The presence of carbon in the lattice suppresses the formation of subsurface hydrogen and modifys Pd electronic properties,which weakens the adsorption strength of the Pd catalyst.The weakened adsorption of acetylene and ethylene enhances the activity and selectivity.The electronic properties of the Pd catalyst were further improved by the nitrogen doping of CNT.It was found that the graphitic nitrogen speices mainly contribute to the electronic interaction between N-CNT and Pd,which increases the Pd electron density and thus enhances ethylene selectivity,(2)Surface and subsurface structure of Pd catalysts modified by In.It was found that the modified surface and subsurface strucutures of Pd catalysts by In enhance the catalytic activity,selectivity and stability.The introduction of indium suppresses the formation of subsurface hydrogen,isolates the Pd ensemble sites and increases the Pd electron density,the later two of which weaken the adsorption of acetylene and ethylene on the Pd catalyst.The weaker adsorption of acetylene on Pd surface decreases the coverage of acetylene,and then provides more free active sites for the hydrogen activation,resulting in the higher hydrogenation activity.The weaker adsorption strength of the catalyst also promotes ethylene desorptioin and thus increases the ethylene selectivity.The enhanced stability of Pd-In catalysts is resulted from the inhibited formation of oligomer by indium.Moreover,the Pd-In catalysts show the compostion-dependent performance for acetylene hydrogenation due to the different extent of structural modification of Pd active sites by In.(3)Non-noble Ni-Ga alloy catalysts.Ni-Ga alloy nanoparticles derived from Ni/Ga/Mg/Al layer double hydroxides disperse well on the mixed oxides with being sized in ca.10 nm.The electron transfer from Ga to Ni increases the Ni electron density,and the introducation of Ga isolates Ni ensemble sites,both of which weaken the adsorption of acetylene and ethylene.Moreover,the barrier for the hydrogenation of ethylene is higher than the barrier for ethylene desorption.These factors contribute to the enhanced performance of Ni-Ga alloys for the selective hydrogenation of acetylene.Owing to the enhanced interaction between the metal nanoparticles and the mixed metal oxides as well as the anticoking ability,Ni-Ga catalysts exhibit an excellent long-term stability for the reaction.(4)Promotional roles of subsurface Pt.The Pt species in CuPt/SBA-15 catalysts have been shown to locate in the subsurface position of Cu when the Pt/Cu is smaller than 0.1.The CO-induced segregation of Pt toward the surface leads to the increase in the surface ratio of Pt,confirming the Pt enriched in the subsurface.The subsurface Pt promotes the activation of hydrogen on Cu surface,while remains the affinity of Cu surface to C=O,which contributes to the high activity and selectivity for the hydrogenation of unsaturated aldehyde to unsaturated alcohol.