Preparation,Characterization Of Catalyst Pd-M@N（M=Co,Ni,Cu）and Study On Catalytic Performance Of H₂O₂ Synthesis

The direct synthesis of H₂O₂ by H₂ and O₂ under the action of a catalyst has the advantages of simple process,low production cost and environmental friendliness,and has been extensively studied in recent years.Currently,Pd-based catalysts are recognized as the best catalysts for direct synthesis of H₂O₂.However,conventional Pd-based catalysts have a larger Pd particle size and provide less active surface,resulting in lower H₂O₂ selectivity and yield.In order to reduce the particle size of the active component Pd nanoparticles and increase its active surface,the paper uses the non-noble metal M（Co,Ni,Cu,whose electronegativity is 1.9）,which is weakly electronegativity,as an auxiliary agent to add to the Pd（electrode The negative is 2.2）.On the one hand,based on the electron transfer theory,electrons on non-noble metals are transferred to Pd,changing the electronic structure of Pd and increasing the electron cloud density of the Pd surface,so that the Pd surface may exist not only in the form of Pd⁰,but also in the form of Pd²⁺,thereby enhancing Pd The degree of dispersion achieves the purpose of reducing Pd particle size and improving its active surface;on the other hand,the addition of non-precious gold can also reduce the cost of precious metal Pd.In addition,the previous research of the research group showed that the preparation of core-shell structure catalysts using Pd as the core and porous materials as the shell showed good stability,and solved the problem of migration and agglomeration of the active component Pd during the reaction of the conventional supported Pd-based catalyst.In view of this,the core-shell catalyst Pd-M@Nwas prepared by using the bimetallic Pd-M which is a non-noble metal as the core and the porous material（SiO₂,porous carbon）as the shell.The structure,morphology,active component crystal size and pore structure distribution of the catalyst were analyzed by TEM-EDS,XRD,pore structure analyzer,etc.,and the influence of non-noble metal species and ratio on the activity of H₂O₂ synthesis was discussed.The paper also discussed the effect of the addition amount of shell CTAB and the in-situ modification of Pd-Co bimetallic core on its synthesis of H₂O₂.The specific research contents are as follows:1.Preparation of bimetallic Pd-M as core and porous materials（SiO₂ and C）as shell core-shell structure catalyst Pd-M@N and its synthesis performance of H₂O₂.The results show that the addition of non-noble metal M reduces the Pd particle size of the active component.The particle size order of Pd on each catalyst with SiO₂ as the shell is:Pd-Co@SiO₂<Pd-Ni@SiO₂<Pd-Cu@SiO₂<Pd@SiO₂.The yield,selectivity and productivity of H₂O₂ synthesized by the catalyst are opposite to their Pd particle size.This is because the smaller the Pd particle size,the more active surfaces are provided,the more favorable the adsorption and activation of the reactant molecules,and the higher the catalyst activity.Similar conclusions were obtained for the preparation of catalyst Pd-M@HCS with C as the shell.2.The effect of m_Pd/m_Co ratio on the performance of catalyst Pd-Co@HCS to synthesize H₂O₂.The results show that with the increase of m_Pd/m_Co,the particle size of Pd first decreases and then increases.When the m_Pd/m_Co ratio is 4.4:1,the particle size of Pd nanoparticles is the smallest at 7.29 nm.The highest rate.After 5 cycles,the productivity of H₂O₂ synthesized by the catalyst decreased from 1996 mmol/gPd^-1·h^-1to 1826 mmol/gPd^-1·h^-1,and the productivity decreased by 8.5%,showing good stability.3.The effect of the CTAB addition of the Pd-Co@HCS catalyst shell and the in-situ modification of the core Pd-Co by Br on the performance of H₂O₂ synthesis The results show that the amount of CTAB added has no effect on the particle size of Pd crystals,but it obviously changes the structural parameters of the shell pores.As the amount of CTAB added increases,the pore volume of the shell micropores increases first and then decreases.When the amount of CTAB added is 4g,the pore volume of the shell micropores is the largest.The larger pore volume of the shell micropores is conducive to the adsorption of reactants and their enrichment on the surface of Pd,thereby improving the yield of H₂O₂ synthesized by the catalyst.In-situ modification of a small amount of Br reduces the size of Pd crystals in the catalyst Pd-Co@HCS.The appropriate amount of KBr added is 0.03g,and the size of Pd crystals is the smallest.After 5 cycles,the productivity of H₂O₂ synthesized by this catalyst decreased from 2123 mmol/gPd^-1·h^-1 to 2012 mmol/gPd^-1·h^-1,decreased of5.2%,showing better stability.