Preparation And Catalytic Performance Of Pd-based Nanostructures

Palladium-based nanocrystals with particular shapes show superior physicochemical properties due to their high catalytic activity and selectivity for many catalytic reactions,and have been widely used as catalysts toward organic synthesis,fuel cells,electrochemical detection and hydrogen storage/sensors.High branched nanocatalysts always have a significantly enhanced catalytic activity and low cost owing to their large specific surface.However,till now,palladium-based nanocrystals with highly branched structure are still rare.In this work,we synthesized palladium-based nanomaterials with highly branched structure by a facile hydrothermal method.Then,we successively synthesized Pd@Pt core-shell tetrapods and Pd@Au core-shell nanotetrapods by using Pd tetrapods as hard templates.Finally,the prepared products are applied in various electrochemical fields such as the oxygen reduction reaction,formic acid oxidation reaction and reduction of 4-nitrophenol.The main researches are as follows:1.Three-dimensional highly branched palladium tetrapods(Pd-THBTs)have been constructed in the presence of polyvinylpyrrolidone(PVP)through the one-step hydrothermal reduction of ethylenediamine-tetramethylene phosphonate-palladium(II)(EDTMP-PdⅡ)by formaldehyde.The morphology and structure of the Pd-THBTs were fully characterized and growth mechanism was explored and discussed based on the experimental observation.The concave Pd tetrahedra grew into highly branched Pd tetrapods consisted of four nanothorn-like branches with tetrahedral dimension through interesting multi-generation nanocrystal overgrowth.The electrocatalytic activities of as-synthesized Pd-THBTs toward formic acid oxidation showed higher catalytic activity and stability for formic acid oxidation than commercial Pd black.For example,the ECSA-normalized specific peak current density of the Pd-THBTs(23.0 A mPd-2)is higher than the commercial Pd black(17.9 A mPd-2).After continuous 1000 potential cycling,the Pd-THBTs lost only 24.8%of the initial electrochemically active surface area(ECSA),while the commercial Pd black lost 70%of their initial ECSA.2.Pd@Pt core-shell tetrapods(CSNTPs)are prepared through a facile templates method.During the synthesis,Pd tetrapods act as hard templates to guide the growth of dendritic Pt shell.Various physical techniques confirm that the as-prepared Pd@Pt CSNTPs have core-shell structure and surface dendritic morphology.Meanwhile,the electrocatalytic activity and durability of Pd@Pt CSNTPs for the ORR in acidic media are systemically studied by various electrochemical techniques such as cyclic voltammetry,ORR polarization measurements,and accelerated durability test(ADT).After 8000 cycles ADT,Pd@Pt CSNTPs only lose 13%of the initial electrochemical surface area(ECS A),whereas the commercial Pt black lose 78%.Compared to Pt black,Pd@Pt CSNTPs show remarkably enhanced electrocatalytic activity and durability for the ORR owing to their specific structural characteristics and the synergistic effect between Pt shell and Pd core.3.The first Pd@Au core-shell nanotetrapods(Pd@Au CSNTPs)were synthesized through a facile templates method.Specifically,Pd nanotetrapods were utilized as the substrate for Au coating through chemically reducing HAuC14 with ascorbic acid(AA)in the presence of polyvinylpyrrolidone(PVP).The morphology,composition,and structure of Pd@Au CSNTPs were fully characterized by scanning and transmission electron microscopy,energy dispersive spectroscopy element mapping,X-ray powder diffraction,and X-ray photoelectron spectroscopy techniques,etc.Different from conventional spheric Au nanoparticles,the Pd@Au CSNTPs had very wide surface plasmon resonance(SPR)absorption band in the visible and near-infrared regions(500-1400 nm),showing especial SPR absorption features.Meanwhile,the Pd@Au CSNTPs exhibited remarkably enhanced catalytic activity for the hydrogenation reduction of nitro functional groups and C=N bond because of their specific structural characteristics.