Theoretical Study And Design Of Bimetallic Core-shell Heterogeneous Catalysts

Heterogeneous catalytic process plays an important role in the chemical industry and has a significant impact on social development.In order to achieve ideal catalytic performance,people have been committed to developing catalysts with high activity,high selectivity and long-term stability under reaction conditions for a long time.For different types of heterogeneous catalytic reactions,strategies of catalyst design such as size modification,component modification and fabrication of core-shell structure often exhibit different properties.Among them,bimetallic core-shell catalysts have unique advantages due to their large number of exposed active sites,wide range of regulation and clear structure-activity relationship,and they possibly have good catalytic performance in many reactions.In this dissertation,the limitations of size modification of monometallic catalysts in hydrogenation and oxidation reactions and the advantages of bimetallic core-shell catalysts are investigated by density functional theory(DFT)calculation.According to the features of different reactions and the modification mechanism of core-shell structure,bimetallic core-shell catalysts with high activity and selectivity are designed theoretically,some of which are confirmed by experimental results.Our work provides deep insights into the modification mechanism and rational design of bimetallic core-shell nanocatalysts in the heterogeneously catalytic reactions.1.For the experimentally observed particle size effect of Pt catalysts in the chemoselective hydrogenation of p-chloronitrobenzene,the calculation results reveal that the entanglement of size-dependent geometric and electronic effects is the reason for the activity-selectivity seesaw relationship.On this basis,we further theoretically constructed a Pt/Au bimetallic core-shell catalyst and predicted its high activity and selectivity due to enhanced adsorption and retention of high coordination sites.Au@Pt catalysts with core-shell architechture were then precisely synthesized using atomic layer deposition method and their high activity and selectivity in catalytic tests verified the rationality of theoretical prediction,breaking the limitation of particle size effect.2.In the semi-hydrogenation of acetylene,we first studied the particle size effect of monometallc Pd catalyst.The results show that although reducing the size of Pd can improve the dispersion,it will enhance the adsorption of intermediates such as acetylene and ethylene,leading to the reduced hydrogenation activity and ethylene selectivity.We further designed Pd/Ru bimetallic core-shell catalyst based on the features of hydrogenation reaction,and predicted that it could improve ethylene selectivity by weakening its adsorption through strain and ligand effect and could increase activity by weakening the adsorption of acetylene to a greater extent due to its larger structural unsaturation.Our theoretical prediction was then confirmed by the experimentally synthesized Ru@Pd core-shell nanocatalysts and their excellent catalytic performances of high activity and selectivity in acetylene semi-hydrogenation.3.For the preferential oxidation of CO in H2-rich atmosphere,the commonly used Pt group metals have the problem of CO blocking the active sites and limited activity at low temperature.We theoretically designed Cu/Rh bimetallic core-shell catalyst to inhibit CO poisoning by weakening CO adsorption and improve activity by promoting CO2 generation via associative mechanism.High selectivity and better structural stability than pure Cu were also demonstrated by calculation results.This dissertation mainly focuses on the modification mechanism of core-shell nanocatalysts and their advantages compared with other strategies of catalyst design.Based on the in-depth understanding of the structure-activity relationship at the microscopic level,the appropriate core and shell components are selected to construct the core-shell nanocatalyst based on the features of different reactions,so as to achieve the experimental results of high activity and high selectivity.It provides new insights into the rational design of high-performance heterogeneous catalysts.