Design And Synthesis Of Palladium Single Atom Catalyst For Selective Hydrogenation Study

In industrial applications,increasing the rate of chemical reactions conversion is usually accomplished with the help of catalysts.Metals usually play a core role in the catalyst,such as metal nanoparticles and metal complexes.Heterogeneous catalysts are widely used in the field of catalysis because of their advantages of easy separation and high repeated utilization rate.As a classically heterogeneous catalyst,the active metal sites of metal nanoparticles are usually located at the corners,edges and surfaces of crystals,showing different catalytic performance and low utilization rate of metal atoms.Compared with heterogeneous metal catalysts,the isolated metal center in the metal complex is completely exposed to the same position.Its active site is uniform.The target product obtained by reacting with reactant molecules in solution has high selectivity.However,the problems such as difficult recovery and easy inactivation of metal complexes in homogeneous system severely limit their industrial application.The problems posed by homogeneous and conventional heterogeneous catalysts have led to a search for ideal alternatives,which can make up for the above deficiencies by dispersing active metal sites on solid surfaces.Supported single atom catalyst has metal active sites containing only isolated metal atoms on the support surface.High dispersion of isolated metal atoms on the support surface can maximize the atomic efficiency of the metal,and it is easy to separate and recycle the supported single atom catalyst.Single atom catalyst and its catalysis have become the focus of research in recent years.However,when the particle size of metal reduces to a single atom,the surface free energy will significantly increase,promoting metal atoms to aggregate into small clusters or nanoparticles.Therefore,rational design and controlled synthesis of single atom catalysts are crucial significant for us to identify the properties of catalytic active sites and clarify the reaction mechanism.This paper focuses on Pd single atom catalysts,and the aim is to expand more simple and controllable methods for synthesis of single atom catalysts.Pd single atoms are anchored on different supports to regulate the local environment of Pd by partial ligand exchange strategy and coprecipitation method.The relationship between the structure and catalytic behavior in selective hydrogenation reaction is thoroughly studied.This paper mainly includes the following contents:（1）Single atom catalyst Pd₁/ZIF-8 was synthesized at ambient conditions via partial ligand exchange strategy,without harsh conditions and post treatment.In the synthesis process,Pd of Na₂Pd Cl₄ partially substituts Zn of ZIF-8 through partial ligand exchange triggered by Cl ions,resulting in single Pd atoms anchored on the ZIF-8 and dissociation of Zn into the solution.Pd of Pd₁/ZIF-8 remains part of the Cl ligands.Further,we have selected Zn complexes with a coordination environment similar to that of ZIF-8 to interact with Na₂Pd Cl₄.Studies at complexes level show that the phenomenon observed in Pd₁/ZIF-8 is a general reaction between Pd and Zn complexes with specific ligands.The atomic dispersion of Pd on ZIF-8 and the Pd–N and Pd–Cl bonds used to anchor Pd sites are verified by using the characterization techniques such as spherical aberration-corrected high angle ring dark field-scanning transmission electron microscopy and X-ray absorption fine structure spectroscopy.Pd₁/ZIF-8 combines the advantages of single site Pd and the robust porosity of ZIF-8.In the selective hydrogenation reaction of phenylacetylene for 3 h,the conversion is 100%and the selectivity of styrene is 93.6%,which has shown excellent selectivity of the target product.When the substrate was replaced by diphenylacetylene,the catalytic activity shows size selectivity.The conversion rate for 3 h is only 4.4%.（2）Single atom catalyst Pd₁/SnO_x was prepared by conventional coprecipitation method using SnCl₂ as metal precursor,anchoring Pd single atoms to Sn O_x via Pd–Sn bond.The catalyst exhibits strong metal–support interaction at low calcination temperature.We used X-ray absorption fine structure spectroscopy and other characterization techniques to confirm the monoatomic dispersion of Pd,and explored the evolution of Pd coordination environment after calcinating amorphous product obtained by coprecipitation method.Na₂Pd Cl₄ as the initial precursor,Pd only coordinates with Cl.After adding SnCl₂ ethanol solution Pd partially coordinates with Sn.This coordination environment is maintained after calcinating at low temperature.When the calcinating temperature exceeds 200°C,Cl gradually dissociates.Then the temperature continues to increase and O gradually coordinated with Pd.Ultimately Pd is completely coordinated with O.H₂ chemisorption results show that the Pd chemisorption capacity of Pd₁/Sn Ox obtained at low calcination temperature is significantly inhibited.The oxidation temperature increases,and the adsorption capacity gradually recovers.Pd₁/Sn O_x was successfully used in selective hydrogenation of 2-nitropropane at low calcination temperature.The selectivity of N-isopropyl hydroxylamine is significantly higher than Pd/C and Na2Pd Cl4/Si O2（Pd–Cl）.This catalytic behavior is attributed to the formation of Pd–Sn bond,and the recycles of hydrogenation reaction shows excellent stability.