Preparation And Electrochemical Properties Of Palladium-based Noble Metal Nanomaterials

Precious metal nanomaterials are widely used in industrial catalysis,energy conversion and storage,biomedicine,sensors and other fields due to their special physical and chemical properties.With the development of heterogeneous catalysis,surface science and nanotechnology,we have gradually realized that the catalytic activity of precious metal nanomaterials is related to the element composition and atomic arrangement on the surface/near surface.First of all,starting from the structure design of precious metal nanomaterials,constructing anisotropic nanostructures can increase the specific surface area of precious metal nanocrystals,create more active sites,and the special structure can effectively prevent the catalysts from Ostwald ripening,dissolving and aggregating.From the component point of view,the introduction of other metals,metalloids or nonmetals into the noble metal nanomaterials can change the electronic environment around the active site and further optimize the reaction pathway,improve the intrinsic activity of the active site.The low abundance and high cost of precious metals further require us to increase the atomic utilization of precious metals.By introducing surface defects and controlling the position distribution of active sites on the catalyst surface,the atomic utilization of precious metals can be maximized.Based on the self-assembly behavior of surfactants,we accurately synthesized anisotropic Pd-based precious metal nanomaterials.By adjusting the synthesis parameters,the system can be expanded from single metal to ternary metals.By selecting different surfactants and adjusting the kinetic process of reduction and crystallization of the precursor,the surface structure and composition of the noble metal nanomaterials are rationally designed and constructed,and the anode reaction of the fuel cell is used as a probe to study the intrinsic activity of the catalysts.The main research work of this paper is as follows:(1)Using the self-assembly behavior of surfactant C₂₂TAC,an environmentally friendly synthesis strategy of bowl-shaped porous Pd nanospheres(Pd NBs)was developed.Using C₂₂TAC as the morphology control agent,H₂Pd Cl₄ as the precursor,ascorbic acid(AA)as the reducing agent,and reacting at room temperature for 30minutes,a Pd nanomaterial with asymmetric porous structure can be obtained.First,C₂₂TAC molecules self-assemble into vesicle micelles in aqueous solution,and electrostatic interaction/coordination with precursors.In the subsequent reduction process,Pd nanocrystals grow island-like on the surface of vesicle micelles to obtain three-dimensional porous skeleton.Due to the large number of surface defects and step atoms caused by the asymmetric and porous structure,the bowl-shaped porous Pd nanospheres exhibit excellent electrocatalytic performance for formic acid oxidation reaction,the mass activity(464A g^-1)is much higher than commercial Pd black(181A g^-1),and its initial oxidation potential is more negative than commercial Pd black catalyst.The synthesis method involves fewer reaction variables,and can be used to prepare asymmetric porous Pd nanomaterials on a large scale through a simple amplification synthesis system,which provides certain help for the industrial production of precious metal nanocrystals.(2)Designed a method to prepare ultrathin two-dimensional nanomaterials through lamellar micelle templates.Using H₂Pd Cl₄ and Ag NO₃ as precursors,C₂₂TAC as a dispersant and morphology regulator.Ultrathin two-dimensional Pd Ag nanomaterials(PdAgNSs)were obtained by simple AA reduction at room temperature.We further adjusted the reaction parameters and found that the kinds of surfactants and the appropriate reduction kinetic mechanism are the keys to obtaining PdAgNSs.PdAgNSs have an ultrathin and highly branched structure,which improves the atomic efficiency and specific surface area of precious metal.PdAgNSs have a high density of grain boundaries and edges,corners,steps,and kink atoms to increase reaction activity.In addition,nanochannels can accelerate the mass transfer of the reaction process.Benefiting from the above advantages,PdAgNSs has excellent activity and stability for electrocatalytic oxidation of formic acid.The mass activity of PdAgNSs is987 A g^-1,which is much higher than the commercial Pd black catalyst and other catalysts reported in the literature.This synthesis strategy provides an effective way for the synthesis of two-dimensional Pd-based nanomaterials.(3)A method for preparing PdAgCu nanodendrites(PdAgCu NDs)with highly branched structure using lamellar micelles as a confined reactor was developed.Using H₂Pd Cl₄,Ag NO₃ and Cu(NO₃)₂ as precursors,C₂₂TAC as morphology control agent,H₂O as solvent,and AA as reducing agent.PdAgCu NDs are composed of fractal structural units in a two-dimensional plane.There are abundant nanopores between the fractal structures.Good surface permeability can make the catalyst surface and internal atoms function as active sites.The interconnection of fractal structure provides a bridge for electron flows.In addition,the introduction of Ag and Cu to Pd nanomaterials can create more compositional advantages.PdAgCu NDs with dual advantages of structure and composition have good activity(3.5 A mg^-1)and stability for alkaline ethanol oxidation reaction.This synthesis strategy can further broaden the synthesis methods of multiple two-dimensional precious metal nanomaterials.