Synthesis and Catalytic Properties of Silver-based and Iron Oxide-based Nanocomposites

Author:Wang Wen Xia

Supervisor:xiao kai jun xia you nan

Database:Doctor

Degree Year:2019

Download:49

Pages:157

Size:14677K

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Metal-based nanocomposites have been widely explored for applications in many different fields including food industry,catalysis,biomedicine,energy and environmental-related field owning to their highly enriched physical and chemical properties.The catalytic properties of the nanocomposites highly depend on their size,shape,and/or twin structure.Over the past two decades,much effort has been directed toward the shape-controlled synthesis of nanocomposites for elucidating their structure-property relationship and optimizing their catalytic performance.In recent years,it is the main research direction to design the cost-efficient catalysts with enhanced catalytic activity and durability.In order to increase the utilization and catalytic activity/stability of the noble metal nanocrystals as well as enhance the dispersity and catalytic activity of the iron oxide nanoparticles,we develop a class of novel Ag-based icosahedral nanocrystals and iron oxide-based nanocomposites.We then systematically investigate their structure-property relationship.The contents of this thesis can be brodly devided into the following two aspects:(1)develop a robust route to the one-pot synthesis of Ag,Ag@PdnL and Ag@PtnL(n is the atomic layer of Pd/Pt)icosahedral nanocrystals with uniform and controllable sizes.PtAg icosahedral nanocages are also obtained by selectively etching the Ag.We then systematically investigate the growth mechanism of Ag icosahedral nanocrystals and the catalytic activity of Ag@PdnL,Ag@PtnL icosahedral nanocrystals and PtAg icosahedral nanocages toward the formic acid oxidation reaction.(2)sysntheis of Fe3O4/GO and Fe3O4/rGO/TiO2 nanocomposites and further investigated their catalytic properties by discoloration of RhB and TC-HCl in aqueous solution,respectively;The main contents and specific findings of this thesis are summarized as follows:(1)Facile Synthesis and Formation Mechanism of of Silver Icosahedral Nanocrystals with Uniform and Controllable Sizes.We report a robust route to the one-pot synthesis of Ag icosahedral nanocrystals with uniform and controllable sizes.The synthesis simply involves the reduction of CF3COOAg by OAm in ODCB at an elevated temperature under the protection of Ar.Our results indicate that multiply-twinned seeds are initially formed via homogeneous nucleation,followed by their growth into icosahedral nanocrystals with enlarged sizes.During the growth process,Ostwald ripening plays a key role in enabling the formation of nanocrystals with a narrow size distribution.In addition to the conventional approach involving the variation of reaction time,we are able to control the size of the icosahedral nanocrystals by adjusting the molar ratio of the Ag(I)precursor to the reducing agent,altering the reaction temperature,and/or introducing additional Ag(I)precursor.The Ag icosahedral nanocrystals embrace strong localized surface plasmon resonance,with a tunable peak position depending on the particle size.(2)Synthesis of Ag@Pd Icosahedral Nanocrystals as a Highly Efficient Electrocatalyst for Formic Acid Oxidation.We develop a facile protocol for the synthesis of Ag@Pd core-shell icosahedral nanocrystals as a cost-effective electrocatalyst.With a 12.4 nm Ag icosahedral seed,the Pd shell thicknesses can be readily tuned from 5.0 to 8.5 layer by varying the quantity of the Pd precursor and the reducing agent.Specifically,a pre-mixed solution containing 1,2-dichlorobenzene(ODCB),Pd(acac)2 and OAm was swiftly inject into the suspension of Ag icosahedral seeds in ODCB held at 180°C.Preliminary studies on their catalytic activity toward formic acid oxidation indicate that the Ag@PdnL icosahedral nanocrystals exhibit enhanced activity relative to that on the commercial Pd/C.Meanwhile,the synthetic control allows the catalytic properties of Ag@PdnL icosahedral nanocrystals to be optimized by the shell thickness.The Ag@Pd6.5L.5L icosahedral nanocrystals show the highest mass activity(10890 mA mg-1),1.9-fold enhancement than that of the commercial Pd/C(5744mA mg-1).Additionally,durability measurements indicate that the Ag@PdnL icosahedral nanocrystals are more stable than that of the commercial Pd/C.(3)Synthesis of PtAg Icosahedral Nanocages as a Highly Efficient Electrocatalyst for Formic Acid Oxidation.Designing a hollow structure of Pt catalyst offers an effective route to improve the electrocatalytic performance and maximize the utilization efficiency of precious Pt.we demonstrate a facile seed-mediated method to synthesize Ag@Pt core-shell icosahedral nanocrystals and then PtAg icosahedal nanocages by wet chemical etching.The synthesis of Ag@PtnLL core-shell icosahedral nanocrystals involved the reduction of Pt(acac)2 by oleylamine(OAm)in the presence of Ag icosahedral nanocrystals at an elevated temperature.The galvanic replacement between Pt and Ag was blocked by passivating the surface of Ag icosahedral nanocrystals with the corver layer of OAm,as well as by increasing the reducing power of OAm at 180°C when deposition of Pt.We could finely regulate the thickness of the Pt shell by varying the amount of Pt(acac)2 added into the solution of Ag icosahedra.The PtAg icosahedral nanocages were then obtained by selective etching of Ag.Due to the porous architecture and synergistic ligand and geometric effect between Pt and Ag,the PtAg icosahedral nanocages exhibit a 6.52-fold mass activity enhancement for the formic acid oxidation reaction relative to the commerical Pt/C in acid media,as well as a higher resistance to CO poisoning.(4)Electrostatic self-assembly of Fe3O4/GO nanocomposites and their application as an efficient Fenton-like catalyst for degradation of rhodamine B.The Fe3O4 nanoparticles were successfully modified with the 3-aminopropyltrimethoxy-silane and homogeneously deposited onto the surface of GO.They were used as Fenton-like catalyst to degrade Rhodamine B and displayed a high catalytic activity,demonstrating the synergistic effect between the superior adsorption properties of GO and the excellent catalytic activity of Fe3O4/H2O2 system.The effect of concentrations of RhB solutions,loading of catalyst,initial pH of pollutant solutions and temperature on the adsorption process were systematically studied.Besides,the possible catalytic mechanism and degradation pathway for RhB molecules by Fe3O4/GOnanocomposites and H2O2 was proposed based on the liquid chromatography-mass spectrometry(LC-MS)analysis.The result reveals that the·OH radicals should be the main actives species during catalytic degradation of RhB by the Fe3O4/GO/H2O2 system.In addition,the catalyst is reusable and shows efficiency up to 5 cycles.(5)Graphene oxide supported titanium dioxide&ferroferric oxide hybrid,a magnetically separable photocatalyst with enhanced photocatalytic activity for tetracycline hydrochloride degradation.Fe3O4/rGO/TiO2 nanocomposites were synthesized by depositing of TiO2 on the surface of the Fe3O4/rGO nanocomposite via a hydrothermal method.The asprepared Fe3O4/rGO/TiO2 photocatalyst exhibited an enhanced photocatalytic activity for the degradation of tetracycline hydrochloride(TC-HCl)over a wide pH range from3.0 to 11.0.At optimal conditions,a 92.6%degradation rate of TC-HCl was achieved within330 min.The enhanced photocatalytic activity could be ascribed to the synergistic effect of the Photo-Fenton reaction and electron transportation of graphene.The products of TC-HCl degradation were identied by LC-MS and the possible pathway and mechanism of photocatalysis of the TC-HCl was proposed.Four kinds of probe compounds were used to study the main active radicals.The result reveals that the e-radicals and·O2 radicals should be the two main actives species during photocatalytic degradation of TC-HCl by the Fe3O4/rGO/TiO2nanocomposites.In addition,the catalyst is reusable and shows efficiency up to 5 cycles.