Study On The Controlled Synthesis Of Palladium Nanoparticles And Their Catalytic Activity

The properties of metal nanomaterials are strongly dependent on their size, morphology,composition and structure. As one of the noble metals, palladium has also attracted greatinterests because of its extraordinary properties. It can be widely used as the primary catalystand its applications in catalysis are related to its remarkable capacity for hydrogen absorption.To improve its catalytical activity, a lot of efforts have also been devoted in the past decade tothe fabrication of palladium nanostructures with monodispersive sizes and well-definedmorphologies. Up to now, however, Pd nanoparticles with various morphologies have beenobtained such as spheres, cubes, tetrahedra, octahedra, icosahedra, tetrahexahedra andnanorods, nanowires, nanosheets, and so on, while monomorphological metallic nanocrystalswith well-defined multipods or highly branched structures have still been rarely reported. Inthis paper, the shape-controlled syntheses of concave tetrahedral and branchedmultipod-shaped Pd nanocrystals as well as their electrocatalytic activities were investigated.By oil-bath heating, uniform, dispersive and well-defined concave tetrahedral Pdnanocrystals with a mean size of about55.6nm were readily synthesized with Pd（acac）₂as aprecursor, PVP as a stabilizer, DMF as a solvent and CO as a reducing agent at100oC for3h.The as-prepared Pd nanocrystals were characterized by UV-vis absorption spectroscopy,transmission electron microscopy （TEM）, X-ray diffraction （XRD） and X-ray photoelectronspectrometry （XPS）. The as-prepared concave tetrahedral Pd nanocrystal consists of {111}and {110} facets, in which the cut-out pyramid holds {110} facets and the outer planes hold{111} facets. The selective adsorption of CO on {110} facets in the synthesis process was thekey to the formation of concave tetrahedral Pd nanocrystals. The adsorption of CO on thefaces of Pd nanocrystals can be well controlled by adjusting the CO flow and the temperature,so that CO was adsorbed selectively on {110} facets and their growth was inhibited. As aresult, concave tetrahedral Pd nanocrystals were formed. CO stripping voltammetrymeasurements confirmed that not only the presence of {110} facet but also the preferentialadsorption of CO on {110} facet in the forming process of concave tetrahedral Pd.Well-defined and dispersive tetrapod-shaped Pd nanocrystals with a mean size of about37.2nm were successfully synthesized with Pd（acac）₂as a precursor, PVP as a stabilizer, mixed DMF/water as a solvent and CO as a reducing agent in the presence of an appropriateamount of sodium acetate at100oC for3h by oil-bath heating. The products werecharacterized by UV-vis absorption spectroscopy, TEM, XRD and XPS. The Pd tetrapodconsists of {311} planes. The effects of the reaction parameters on the formation oftetrapod-shaped Pd nanocrystals were investigated. The results show that the presence ofcarboxylate anions and the CO flow rate are the key to the growth of tetrapod-shaped Pdnanocrystals. The adsorption of carboxylate anions on Pd seeds resulted in a change of thecoverage of CO on the faces of palladium. Due to the co-adsorption of CO and carboxylateanions on Pd planes, the confined-growth contributes to the formation of tetrapod-shaped Pd.Uniform, dispersive and well-defined tetrapod-thorn Pd nanocrystals, which hold jointsof branch with a mean length of85.7nm and a cross width of22.3nm were synthesizedsuccessfully with a high yield by using Pd（acac）₂as a precursor, PVP as a stabilizer, mixedDMF/water as a solvent and CO as a reducing agent in the presence of an appropriate amountof SDS at100oC for3h by oil-bath heating. The products were characterized by UV-visabsorption spectroscopy, TEM, XRD and XPS. Pd atoms addition at the four apex anglesgenerated due to the co-adsorption of CO, SDS and PVP on the faces of Pd seeds, so that thecrystals gorw along the apex angles. The co-adsorption results in a confined growth in severalcycles and improves the formation of tetrapod-thorn Pd nanocrystals.The catalysis activities of the as-prepared branched tetrapod Pd nanocrystals were studiedby electrocatalytic oxidation of formic acid. The results show that the tetrapod-thorn Pdnanocrystals, on which the maximum current density was measured to be12.52mA·cm-2,demonstrated the highest electrocatalytic activity which is3times greater than those ofcommercial Pd black, and the activity of the tetrapod-shaped Pd nanocrystals was2.35timesgreater than those of commercial Pd black. The order of electrocatalytic activities is asfollows: tetrapod-thorn Pd> tetrapod-shaped Pd> small nanoparticles> Pd black, indicatingthat the catalytic activity is dependent on the crystalline structure.