Design Of Palladium-based Nanocrystal Surface/Interface Structure And Electrocatalytic Properties

With the increasing demand for energy in modern society,the rapid depletion of traditional energy resources and the corresponding environmental pollution problems are becoming increasingly serious.Therefore,it is urgent to develop clean and renewable energy sources.Electrocatalysis offers promising applications in the field of energy conversion due to its ability to make full use of renewable energy sources,mild reaction conditions,simple operation,and high energy conversion efficiency.Due to the similar lattice structure and lattice constants,palladium（Pd）is considered to be the most promising candidate to replace platinum（Pt）,but it still suffers from low activity and low catalytic efficiency.In order to solve the above problems of Pd-based materials,we can improve the catalytic performance by modulating the surface/interface properties of Pd,which can be done in the following ways:on the one hand,by modulating the morphology and particle size of nanomaterials or introducing porous structures to increase the number of active sites,and on the other hand,by introducing transition metal atoms to improve the catalytic behavior by exploiting the electronic interaction or synergistic effect between them and Pd.Therefore,this thesis is based on surface/interface modulation strategies（size,morphology,defects,component effects）to design and modulate a series of metal Pd catalysts to enhance catalyst performance and improve catalyst stability,with application to polyols oxidation and nitrogen reduction reactions.The main investigations are as follows:（1）The intrinsic activity of catalytic materials is enhanced by using precious metals in complex with organic functional molecules to modulate the morphology and size of nanomaterials.In this chapter,a functional coral-like Pd network nanostructured catalyst is successfully synthesized by a solvothermal method assisted by 1-hydroxyethyl-1,1-diphosphonic acid（HEDP）.HEDP acts as a surfactant and organic functional ligand during the formation process,which reduces the kinetic rate of the synthesis reaction.The slow reaction kinetics is essential for the formation of coral-like Pd network nanostructures with concave center and convex sides,providing more Pd active sites for electrocatalytic performance.The obtained coral-like Pd network nanostructures exhibite excellent catalytic activity for ethylene glycol oxidation reaction and glycerol oxidation reaction with a mass activity of 1785 m A mg^-1 and 1162 m A mg^-1,respectively,which is mainly attributed to the organic functional ligands and the unique morphological nanostructures.The theoretical calculations demonstrate that the P modification on the coral-like Pd network nanostructures modulates the d-band center of Pd,which shifts the d-band center of Pd upward,thus promoting the adsorption of polyhydroxy alcohols and the activation of OH_ads,and the conversion of-COH_x to CO₂.（2）The catalytic performance of the catalyst is enhanced by employing a defect engineering strategy to maximize the number of active sites activated by the catalyst.In this work,we synthesized defect-rich Pd Co Zn nanosheets（D-Pd Co Zn NSs）with porous structures by an alkali etching process,which have abundant defects and active sites that greatly improve the glycol oxidation performance.It was found that some of the Zn atoms were etched from the Pd Co Zn NSs,resulting in a large number of defects.The D-Pd Co Zn NSs exhibited excellent catalytic activity towards ethylene glycol with a mass activity of 9.5 A mg^-1,and the mass activity of the D-Pd Co Zn NSs remained well maintained with good stability after i-test for 7200 s.It was attributed mainly to defect engineering to optimize the electronic structure of Pd,thus improving the selectivity and persistence of the reaction pathway for CO₂ oxidation by ethylene glycol.（3）By introducing transition metal atoms into Pd（i.e.,changing the composition of the catalyst）in order to improve the utilization of precious metals and to enhance their electrocatalytic activity due to synergistic effects and modified electronic structures.A series of two-dimensional porous amorphous Pd Co M（M=Cu,Ag,Fe,Mo）nanosheets（NSs）were successfully synthesized by a general one-step solvothermal method,in which the prepared Pd Co Cu NSs possessed high electrical conductivity,ultrathin two-dimensional structure and large electrochemically active surface area,demonstrating strong electrocatalytic performance for nitrogen reduction reaction.The results revealed that the NH₃ yield of Pd Co Cu NSs in 0.1 M KOH electrolyte was 60.68μg h^-1 mg_cat^-1with a Faraday efficiency of 42.93%.Density functional theory further indicated that the introduction of transition metal Cu atoms into the amorphous Pd Co Cu NSs could not only regulate the electronic structure of Pd,but also promote the activation and hydrogenation of N species and inhibit the hydrogen precipitation reaction,thus improving the activity and selectivity of nitrogen reduction reaction.