Controllable Synthesis And Catalytic Properties Of PdH And PdNi Nanocrystals

Noble metal nanomaterials have attracted much attention due to their unique properties and excellent catalytic properties.As one of important noble metal elements,Pd has extremely broad application prospects in many fields including organic catalysis and fuel cells.However,due to the limited reserves and high cost of Pd,it is very important to improve the efficiency of atomic utilization and the catalytic activity.Recently,much attention has been drawn to improve catalytic activety of noble metal catalysts through electronic structure regulation,and one of an effective method is to cooperate the other element with noble metal.It was known that Pd has high solubility and permeability to hydrogen,it can absorb 900 times of gaseous hydrogen more than its own volume at room temperature and atmospher:ic pressure.The combination of Pd and H leads to the formation of PdHx compounds.The crystal lattice of Pd will undergo lattice expansion when H atoms diffuse into the Pd crystal lattice.The electronic structure will change acoordingly,and that could cause a great change of the catalytic property.By document survey,most of the synthesized PdHx compounds are unstable and the absorbed hydrogen is easy to overflow from Pd crystal lattice again.Therefore,it is necessary to develop a method to synthesis stable PdHX;compound.In addition,it is particularly important to systematically study the structure-catalytic relationship between the stable PdHx nanocatalyst with well-defined structure.Herein,our paper takes PdHx nanocrystals with different morphology and the phase transformation of PdNi binary alloy nanocrystals as research objects,and is devoted to exploring the synthesis method,formation mechanism and structure-activity relationship.The main scientific research progress is summarized as follows:1.Using n-butylamine as a hydrogen source,we prepared three types of PdHx nanocrystals exposed with three different low-index facets from Pd nanocrystals,the morphologies included cube with {100} facets,octahedra with {111} facets,and rhombic dodecahedra with {110} facets.The structure-activity relationships of the stable PdHO.43 nanocrystals were also investigated.All three PdHO.43 nanocrystals with different facets are stable and maintain the morphology of the precursor Pd nanocrystals very well.In heat treatment at 300 °C under argon,three types of PdHO.43 nanocrystals show different heat stability,the order is cube>octahedra>rhombic dedecahedra.The order is coincidently the same as the coopration of Pd nanocrystal with hydrogen,cube show the fastest speed in forming of PdH0.43 nanocrystals and the best thermal stability,followed by octahedra and rhombic dodecahedra.Finally,we tested the electrocatalytic properties of formic acid oxidation on PdH0.43 nanocrystals with three different facets.It was found that the formation of PdH0.43 structure can significantly reduce the overpotential of formic acid oxidation,and can also greatly increase the specific catalytic activity.2.PdHx nanosheets with adjustable H/Pd ratio was synthesized by tuning reaction time with oleylamine as a solvent and molybdenum carbonyl(Mo(CO)6)as a reducing agent.Using formic acid electrooxidation and ethanol electrooxidation as the probe reactions,the structure-activity relationships of PdHx nanosheets with different H contents were studied.It was found that the higher the H content,the lower the overpotential and the higher the oxidation current density.At the same time,during the surface treatment of PdH,nanosheets with n-butylamine,we found that Pd nanosheets can realize hydrogen storage at room temperature and form a stable[P-PdHx,,which provides us a new idea to produce PdHa-nanocatalyst with high efficiency and high catalytic activity.3.Metastable nanocrystals often exhibit different and even more excellent physical and chemical properties than stable nanocrystals due to its differences in atomic stacking.The hexagonal metastable phase(hcp)PdNi alloy nanocrystal is synthesized by using oleylamine as a solvent and formaldehyde as a reducing agent.During the experiment,we regulated reaction temperature,initial ratio of the precursors,the using amount of the reducing agent,and etc.We have found that formaldehyde is a key factor in the formation of hcp PdNi alloy nanoparticles,face-centered cubic phase(fcc)PdNi alloy nanoparticles formed in the presence of formaldehyde.The catalytic performances of two PdNi alloy nanocrystals with different phase structures(hcp and fcc)were studied towards ethanol electrocatalytic oxidation reaction under alkaline conditions.The results show that hcp PdNi has better catalytic activity than that of fcc PdNi.