Study On The "Structure-Activity Relationship" Of Supported Platinum/Copper-based Catalysts In Heterogeneous Catalysis

The use of catalysts affects all aspects of our human society and undoubtedly lays an irreplaceable foundation for human progress and development.With the increase in energy demand,the green conversion of energy also puts forward higher requirements for scientific researchers.The research about more new and efficient catalysts is very important to improve energy efficiency and control environmental pollution.Studying the catalytic mechanism and the influencing factors of the catalytic reaction（support effect,pretreatment conditions,size effect,etc.）to reveal the"structure-activity relationship"of catalysts in the thermo-catalytic reaction,which is useful for providing important theoretical guidance for the development and design of more new high-performance catalysts（high activity,high stability,high selectivity）metal-supported catalysts.Conventional and simple synthesis methods were used to prepare and synthesize a series of metal-supported catalysts in this paper.By adjusting the metal loading,pretreatment condition,reaction condition,doping of auxiliary agent,etc.,the"structure-activity relationship"of catalysts in heterogeneous thermo-catalytic reactions was studied.The relevant research results of this paper are as follows:（1）The"structure-activity relationship"in the CO oxidation reaction of single atom Pt loaded on a small-sized Nb₂O₅ was studied.Sodium carbonate,ammonium bicarbonate,ammonium carbonate and urea were used as precipitating agents to precipitate niobium oxalate hydrate.The final experiment results proved that using urea as a precipitating agent can obtain high specific surface area,pure hexagonal phase nano-Nb₂O₅ powder(127 m²·g^-1,11 nm).The pure phase Nb₂O₅ was used to support Pt with different contents to obtain the Pt/Nb₂O₅ catalysts,in which the Pt single atoms were presented.After evaluating the performance of the catalyst,it was found that the catalyst after hydrogen pretreatment showed higher activity than that after oxygen pretreatment.After hydrogen pretreatment,some Pt single atoms combine to form very small clusters.The extremely small Pt clusters and low valence state are the reasons for the increase in CO oxidation reaction activity caused by hydrogen pretreatment.（2）The influence of Nb incorporation on Cu/SiO₂ catalyst and the"structure-activity relationship"in CO oxidation were studied.Loading 4 wt.%Nb and 3 wt.%Cu on SiO₂ to obtain a monometallic catalyst respectively,and simultaneously load Nb and Cu on SiO₂ to obtain a bimetallic catalyst.The catalyst of containing Nb and Cu exhibited higher activity than the catalyst of containing Cu only.H₂-TPR and XAFS show that the incorporation of Nb can interact with Cu,promote the reduction ability of Cu active species,increase the coordination number between Cu-Cu,and thereby improve the catalytic activity.（3）The“structure-activity relationship”study of the pretreatment conditions for the CO oxidation reaction of small-sized Cu/SiO₂ was explored.The deposition precipitation method was used to support Cu on silica,and then the catalyst was subjected to hydrothermal treatment and hydrogen（or oxygen）pretreatment for carbon monoxide oxidation.Activity tests showed that hydrothermal treatment can significantly improve the stability of the catalyst,and hydrogen pretreatment can greatly increase the catalytic activity compared to oxygen pretreatment（the catalyst was almost inactive after oxygen pretreatment）.In situ DRFTS,CO-TPR,H₂-TPR,XAFS and other characterization methods proved that hydrogen pretreatment can reduce Cu²⁺to a low valence state,and at the same time generate surface copper hydroxyl groups to provide active sites for the subsequent adsorption and activation of reactants,so hydrogen pretreatment treatment is the key step for catalyst activity;XAFS,TEM,N₂ adsorption and desorption and other characterization methods showed that hydrothermal pretreatment had no effect on the valence state of Cu,but it greatly changed the structure of the SiO₂ support and changed the coordination of Cu,which leaded to a more stable catalyst,provided more intermediate active sites for the catalyst,and produced more active copper hydroxyl groups.The surface hydroxyl groups of copper are essential for CO oxidation.（4）Preliminarily explored the supported copper-based catalysts for the epoxidation of propylene with oxygen to prepare propylene oxide.The effects of copper loading on the catalyst,pretreatment conditions,reaction atmosphere ratio composition,reaction temperature and other metal doping,and the use of different supports on the propylene epoxidation reaction were preliminary explored.Experiments show that the10 wt.%Cu copper catalyst was reduced under hydrogen at 300℃,and the reaction at250℃could get better activity.The conversion of propylene and the selectivity of propylene oxide could be optimized when the molar ratio of propylene to oxygen was1:1.When the conversion of propylene reaches 7%,the selectivity of propylene oxide is 5%.The simple doping of other metals（Nb,Mo,Bi,In,Li,Cs,Fe,Al,Sn,Sb,W）did not significantly improve the performance of the reaction or was not conducive to the reactivity.At the same time,the catalyst activity of 10 wt.%Cu loaded on different supports was screened.TS-1,S-1,SiC were not conducive to the selectivity of propylene oxide,while SiO₂ and Si₃N₄could obtain relatively stable propylene conversion and propylene oxide selectivity,which can be considered further research.In summary,this paper mainly studies the“structure-activity relationship”of Cu-based and Pt-based catalysts in the CO oxidation reaction,and initially explores the factors that affect the copper-based catalysts in the direct oxidation of propylene to propylene oxide.For the first time,a single-atom Pt catalyst was prepared on Nb₂O₅,and its“structure-activity relationship”in CO oxidation was studied.The structure and performance of Cu-based catalysts changed by hydrogen pretreatment were investigated,and the positive effect of Nb incorporation on Cu/SiO₂ for CO oxidation was confirmed.These research work will help us to expand and develop new catalysts,and at the same time help us understand the relationship between catalyst structure and catalytic activity.