Synthesis And Catalytic Evaluation Of Pt-based And Pd-based Nano-alloys For Propane Dehydrogenation

Light alkane dehydrogenation is an important chemical reaction.It has been suggested that nano-alloys of small particle size exhibit an excellent catalytic performance in this reaction.Due to the limitation of previous scientific instruments,it is a great challenge to determine the precise structure of small nano-alloys and the structure-function relationship,especially when the particle size is smaller than 3 nm.Typically,alloys are classified into two categories depending on their structure:random substitutional solid solutions and intermetallic compounds;in this work,a series of nano-alloys were synthesized and synchrotron technologies were used to determine the precise structure of these alloys.Due to the surge of shale gas,propane dehydrogenation was employed as a model reaction to investigate the structure-function relationship for light alkane dehydrogenation.In this work,a series of 2-3 nm Pt and Pt-Sb nanoparticles were synthesized by a strong electrostatic adsorption and temperature programmed reduction method.In situ synchrotron XAFS,XRD and ACSTEM results suggest that the bimetallic nanoparticles possess a core-shell structure with a Pt core and a tetragonal Pt₁Sb₁ shell.The surface tetragonal Pt₁Sb₁ phase was not known in the bulk alloys and the structure formation is kinetic control.Propane dehydrogenation results suggest that the propylene selectivity of the Pt@Pt₁Sb₁ nanoparticles is about 99%,even under severe reaction conditions,and the TOF is 1.5 times higher than that of monometallic Pt catalyst.Pt-Sn bimetallic catalysts are the most widely investigated catalytic materials for light alkane dehydrogenation.It has been reported that there are several alloys coexisted in the Pt-Sb bimetallic system;in this case,it is a great challenge to investigate the structure-function relationship.To solve this problem,we synthesized Pt@Pt₁Sn₁nanoparticles of a core-shell structure,and the catalytic surface only exists one type of alloys(hexagonal Pt₁Sn₁ intermetallic compound).In addition,the surface structure of the nanoparticles was determined by EXAFS with a surface oxidized method.Catalytic evaluation results suggest that the propylene selectivity of the Pt@Pt₁Sn₁ nanoparticles is higher than 96%and the TOF is also higher than that of the monometallic Pt catalyst(1.8-4.3 times).Formation of intermetallic compounds is an efficient way to improve the catalytic performance of Pd-based catalysts for light alkane dehydrogenation.It is of great importance to investigate the geometric and electronic structure of Pd-based intermetallic compounds.In is study,a series of 2-3 nm Pd and Pd-Sb nanoparticles were synthesized.Pd K edge XAFS,Sb K edge XAFS,XRD and ACSTEM results suggest that a cubic Pd₁Sb₁ is formed on Pd nanoparticles with the incorporation with Sb.Pd L₃and L₂edge XANES results indicate that Pd 4d unfilled orbital states shift to a higher lever with the incorporation with Sb.Catalytic evaluation results suggest that the surface cubic Pd₁Sb₁intermetallic compound exhibits a higher propylene selectivity(>89%)than that of monometallic Pd catalyst,and the TOF is 2.8-8.5 times higher than that of Pd/Si O₂ catalyst.