Preparation Of Supported Pd-based Catalysts By Spontaneous In-situ Reduction Method And The Study Of Dispersion Mechanism

As the foundation of the modern chemical industry,catalytic technology is widely used in petroleum refining,fine chemicals,environmental protection,energy development and other fields,which has played a crucial role in the development of the national economy.The catalyst is at the core of the catalytic technology,and the amount of the supported noble metal catalyst accounts for more than 50%of the total catalyst.The traditional preparation method of the supported metal catalyst is the impregnation method,which has the characteristics of simple production method and high production capacity,and is easy for large-scale industrial production.However,during the active metal loading process,the active metal components are not easily dispersed uniformly under the influence of solvation and clustering effects,and the active metal components are easy to agglomerate and inactivate during the subsequent reduction and heat treatment processes.In addition,it is difficult to form a strong interaction between the active noble metal component and the support after impregnation,leading to the migration or leaching of noble metals in the subsequent reaction process,which seriously affects the activity and service life of the catalyst.Therefore,research work aimed at further improving the performance of supported metal catalysts has always been a research hotspot in the field of catalysis.This paper takes the selective oxidation of alcohols and the selective hydrogenation of phenol as the target reactions.Based on the tunability of the chemical composition and structure of layered metal hydroxides/metal oxides,a series of reductive supports materials are designed and constructed.By matching the reduction potentials of the variable valence transition metal cations in reductive supports and the oxidized active noble metal ions,the controllable preparation of supported catalysts with highly dispersed active metal sites has been carried out,and the dispersion anchoring and reduction mechanism of active metal in the reduction sites of the support has been revealed.In the process of the preparation of supported noble metal catalysts by support spontaneous in situ reduction method,the anchoring and reduction of the active metals are carried out at the same time.Compared with the conventional impregnation method,the preparation process of the in situ reduction method is simplified,the economic cost is reduced,and the dispersion and stability of the active metals can be improved green and efficiently,which opens up a new idea for the green and efficient development of catalyst preparation technology.The selective oxidation of benzyl alcohol and the selective hydrogenation of phenol are used as probe reactions to explore the strengthening rules of the catalytic performance of the supported catalysts with highly dispersed active noble metal sites.(1)A series of LDHs support materials(CoAl-LDHs,CoNiFe-LDHs,CoCuAl-LDHs,FeAl-LDHs,CoFe-LDHs,CoAlCe-LDHs)with highly dispersed reduction sites are obtained through introducing variable reducing metal ions(Co2+,Fe2+,Ce3+,etc.)into LDHs support laminate by coprecipitation method.Utilizing the redox potential matching of transition metal ions and active metal cations in LDHs laminates and using them as a supports and reductive agent at the same time,the active metal cations are reduced by the support spontaneous in situ reduction method without additional reducing agents,and the reductive sites Co2+ loses electrons to form Co3+,and Pd2+ gains electrons to form Pd0.A series of supported Pd-based catalysts are prepared through the spontaneous redox reaction between the support and the metal precursor.Highly supported Pd-based catalysts are prepared by introducing cetyltrimethylammonium chloride(CTAC)modified in situ reduction method.The selective oxidation of benzyl alcohol is used as a probe reaction to investigate the difference in the reaction performance of Pd-based catalysts before and after modification.Through HAADF-STEM characterization,it is found that the active metal Pd on the catalyst surface obtained by the CTAC modification in situ method existed in the form of sub-nanocluster.XPS results show that the electronic interaction between CTAC,Co and Pd species strengthens the stable dispersion and immpbilization of Pd.The benzyl alcohol selective oxidation performance test show that the sub-nanocluster supported Pd-based catalyst exhibits relatively excellent catalytic activity,with a TOF as high as 11014 h-1.In addition,the good reusability and long-term stability prove that CTAC modified in situ reduction method can effectively inhibit the Oswald ripening of the active metal Pd particles.(2)The co-precipitation method is used to introduce the variable valence transition metal ions(Co2+,Ce3+,etc.)into the layered double hydroxides support phase structure,and the topological transformation of heat treatment is used to construct LDO support with the Co-Ce interface reductive sites and abundant oxygen vacancies(VO).The LDO is used as support and reducing agent to spontaneously reduce noble precursor for preparing a supported Pdbased catalyst.The phenol hydrogenation reaction is used as a probe reaction to investigate the difference in the reaction performance of the Pd-based catalyst before and after the topological transformation of the reductive support and before and after the addition of Ce species.HAADF-STEM and XPS showed that the atomically dispersed active metal Pd is distributed around the Co-Ce reduction sites on the catalyst surface.The catalyst shows good phenol hydrogenation performance under mild reaction conditions.The reaction kinetic show that the sub-nano electron-rich Pd site i is conducive to the activation of H2 molecules,and the abundant VO is conducive to the activation of phenol molecules,thus promoting the rapid hydrogenation of phenol.(3)The reductive Co3O4 support materials with different defect types(Co defect and O defect)is precisely constructed by the hydrothermal method.Utilizing the variable valence characteristics of Co element,and using it as a support and reducing agent at the same time,the Pd-based catalyst is prepared by support spontaneous reduction the oxidized Pd precursor by in-situ reduction technology.The phenol hydrogenation reaction is used as a probe reaction to investigate the difference in catalytic performance of Pd-based catalysts constructed with different defect types.Raman and PAS show that different types of vacancies(metal cobalt vacancies and oxygen vacancies)in the Co3O4 support material have different adsorption anchoring and reduction mechanisms for the metal precursors.Both metal cobalt vacancies and oxygen vacancies contribute to the anchoring of Pd species,and the metal cobalt vacancies have stronger adsorption to Pd species than oxygen vacancies.DFT theoretical calculations have also-verified this conclusion.The catalystof active noble metal Pd occupancy metal cobalt vacancy has a smaller average particle size(2 nm)than occupancy oxygen vacancy,which has excellent catalytic activity and cyclohexanol selectivity.