Preparation And Catalytic Performance Of Palladium Nanoparticles Modified By Quaternary Ammonium Salt Surfactants

The palladium nanomaterials possess an extremely important status in organic synthesis and petrochemical industry as catalysts due to their unique catalytic performance.It has been widely used in petroleum cracking,coupling reaction,catalytic oxidation,hydrogenation,and fuel cells,due to its high efficiency and selectivity.It has been investigated that the specific catalytic activity and selectivity of a palladium catalyst are strongly dependent on its crystal size,morphology and structure,etc.By changing these factors,it can essentially adjust the performances of palladium nano-crystals to adapt to various applications.The palladium nanoparticles?PdNPs?with various shapes and sizes were synthesized by a simple water-based route without seeds through kinetics control in the presence of different quaternary ammonium surfactants as capping agents.The effect of various preparation conditions such as the amount of surfactant,reaction time and temperature on the morphology,size and structure and catalytic performance of PdNPs were investigated.The XRD,TEM and UV-Vis were employed to analyze the as-prepared PdNPs catalysts.Meantime,the electro-catalytic performance of PdNPs catalysts were evaluated by the electro-oxidation of methanol reaction?MOR?in the alkaline medium.The PdNPs were synthesized in the presence of hexadecylpyridinium bromide?CTAB?using Na₂PdCl₄ as the precursor and L-ascorbic acid?AA?as the reduction agent.The optimum electro-catalytic performance was obtained when the amount of CTAB was n?CTAB?/n(Pd²⁺)=2:1,the reaction time was 1.5 h and the reaction temperature was 60?.The as-prepared PdNPs possess a cubic shape with various interstices inside and concave structures on the surface of the nanocubes with an average size of 20.5 nm and enclosed by high-index facets.The forward anodic peak current density of methanol oxidation was 12.19 mA cm^-2,which was much higher than the commercial Pd/C.The current density still remained 83.4%of the initial after1500 cycles,evidencing the long-term stability of PdNPs.The PdNPs were synthesized in the presence of hexadecylpyridinium bromide?HDPB?using Na₂PdCl₄ as the precursor and L-ascorbic acid?AA?as the reduction agent.The optimum electro-catalytic performance was obtained when the amount of HDPB was n?HDPB?/n(Pd²⁺)=0.6:1,the reaction time was 3 h,and the reaction temperature was 50?.The as-prepared PdNPs was in the shape of flocculus and it had a highly porous feature with an average size of 53.1 nm.The forward anodic peak current density of methanol oxidation was 19.39 mA cm^-2,which was much higher than the commercial Pd/C.The current density remained 40.7%of the initial after1500 cycles,evidencing the poor stability of PdNPs.The PdNPs were synthesized in the presence of 1-hexadecyl-3-methyl imidazolium bromide?HMIB?using Na₂PdCl₄ as the precursor and L-ascorbic acid?AA?as the reduction agent.The optimum electro-catalytic performance was obtained when the amount of HDPB was n?HMIB?/n(Pd²⁺)=0.2:1,the reaction time was 2 h,and the reaction temperature was 50?.The as-prepared PdNPs has a highly porous feature with an average size of 45.4 nm.The forward anodic peak current density of methanol oxidation was 13.11 mA cm^-2,which was much higher than the commercial Pd/C.The current density still remained 86.3%of the initial after 1500 cycles,evidencing the long-term stability of PdNPs.The results showed that the PdNPs synthesized using HDPB as capping agent exhibited the highest electro-catalytic activity,but the stability was relatively poor.On the whole,the PdNPs synthesized using HMIB as capping agent showed both better catalytic cativity and stability toward MOR.The different morphologies and structures of Pd nanoparticles obtained under three different kinds of surfactants ascribe to both of the space steric hindrance and the shielding effect of the head group.