Controlled Synthesis And Catalysis Properties Of Three Kinds Of Palladium Nanocrystals

Owing to the specific affinity with hydrogen,palladium(Pd)nanocrystals possess broad applications in the field of catalysis.Controlling the morphology of Pd nanocrystals is an efficient way to tune the catalytic performance.Because of the close relationship between morphology and the number of atoms on the crystal faces,edges and corners,it is of great influence for the manipulation of shapes of Pd nanocrystals on catalytic activity and selectivity.In order to explore the relationship between the morphology and properties of Pd nanocrystals,three kinds of Pd nanocrystals,named cube,octahedron and truncated cube,have been synthesized with characterizations via transmission electron microscopy(TEM)and high-resolution TEM(HRTEM).Furthermore,the electrocatalytic performance of the three were investigated.The liquid phase reduction method was applied to prepare Pd nanocubes with potassium tetrachloropalladium(K2PdCl4)as the precursor,polyvinylpyrrolidone(PVP)as the stabilizer,potassium bromide(KBr)as the capping agent and ascorbic acid(AA)as the reducing agent.Pd nanocubes in uniform shape and average size of 10.7 nm have been synthesized.The effects of reaction time,capping agent and reducing agent on the formation process of Pd nanocubes were explored in detail.The experimental results show that the capping agent plays a key role in the formation of Pd nanocubes.During the reaction,Br-would preferentially adsorb to the {100} facets,which could change the growth rate of the facets and promote the cubic shape of the crystal.The as-synthesized Pd nanocubes were used as the seed crystal for secondary growth,coupled with relevant research on the influence of the precursor,reducing agent,the reaction temperature and pH value of the reaction system.The results show that reducing agent has a significant effect on the crystal morphology of the secondary growth.When acetaldehyde was used as the reducing agent,Pd cubes as the seed,K2PdCl4 as the precursor,and PVP as the stabilizer,uniform Pd octahedron with a particle size of 27.3 nm can be obtained.Similarly,when glucose was used as the reducing agent,K2PdBr4 as a precursor,and other conditions unchanged,Pd truncated nanocubes with an average size of 16.5 nm can be obtained.In this work,the catalytic performance of those three kinds of Pd nanocrystals prepared above on the electrocatalytic oxidation of formic acid was investigated.It has been found that the activity of Pd nanocrystal is closely related to its morphology.The catalytic performance order of the three Pd nanocrystals decreases in the sequence of cube>truncated cube>octahedron.At the same time,their electrocatalytic stability was investigated by chronoamperometry.The results show that the stability of the cube is relatively the best.The catalytic activity of Pd nanocubes for hydrogenation of tetralin was also investigated.In a typic run the palladium nanocube was loaded on Al2O3,then the test was carried out under different conditions.The results show that when the reaction temperature is 280?,the selectivity from tetralin to trans-decalin is the best with the translcis ratio of 1.46.The morphology of the palladium nanocubes can remain unchanged within the experimental temperature range,which shows that the cubes have good stability.This result was also verified in the repeatability test,with 2 h as a test cycle,the morphplogy of the palladium nanocube didn't change until 5 cycles.