Synthesis Of Gold-Based Platinum Group Nanowire Catalysts And Study Of Their Relative Mechanism

Noble metal(e.g.,Au,Pd,and Pt)nanoparticles(NPs)have found widespread applications in the fields of catalysis,optics,biosensing,and electronics due to their unique physical-chemical properties.The catalytic properties of noble metal catalysts often vary dramatically with their size,morphology,and composition.By tuning the structure and morphology will make them own higher specific surface area and a large number of active sites,so as to improve their catalytic performance.For instance,one-dimensional(1D)noble metal nanowires(NWs)have shown excellent catalytic performance because of their high aspect ratios,anisotropy,high-density defects,which can increase the effective working area in the catalytic reaction process and improve the electron transport process.On the other hand,the large flexibility of NWs and their subsequent assembly ability make them especially important in the fabrication of flexible electrode materials with a high capacity and excellent conductivityIt's well known that both of pure Pd and Pt catalysts present excellent alcohol-catalysis activity in the field of direct alcohol fuel cells.However,due to their high cost,insufficient activity and poor stability,it is still urgent to explore an effective method for the preparation of the Pd-and Pt-based catalysts with low loadings and high activity.Alloys,core-shells,and multi-components have been widely used for improving the catalytic performance of Pd-or Pt-based catalysts via the addition of a second metal.Not only can the amount of Pd and Pt be reduced,but the synergistic effect and electronic effect among metals make them show better catalytic performance in comparison with their monometallic counterparts.For example,Pd-based catalysts always exhibit enhanced catalytic performance for ethanol under alkaline media,while Au has the excellent resistance to CO poisoning and other advantages.Therefore,the introduction of Au into Pd NW catalysts can not only reduce the consumption of Pd,but also effectively combine the advantages of Pd and Au to enhance the activity and stabilityPd and Pt not only have the similar physical-chemical properties,but also have their own characteristics.The electrocatalytic performance of Pd-based catalysts in alkaline fuel cells is generally better than that of Pt-based catalysts.While,Pt is the most effective catalyst to facilitate both hydrogen oxidation and oxygen reduction.Therefore,with the development of proton-exchange membrane fuel cells(PEMFCs),the design and synthesis of novel Pt-based catalysts are also a serious challenge.Undoubtedly,the introduction of Au can improve the catalytic stability of Pt-based catalysts.However,due to the limitation of surface energy between Au and Pt,Pt is usually deposited on the surface of the Au cores in the form of irregular particles,which is not conducive to improving the utilization of Pt and its catalytic activity.Since Pd both has good miscibility with Au and Pt,and the introduction of Pd can tailor the d band center of Pt,which is also the key factor to improve its catalytic performance.Therefore,it is of great significance to realize the effective preparation of ternary composite Pt-based NW catalysts via the combination of Au,Pd,and Pt.Moreover,most of the reported methods for the synthesis of composite noble metal NW catalysts generally present various shortcomings,such as low-yield,difficulty in removing surfactant and complex operation.Therefore,it is of great challenge and far-reaching significance to develop a simple and efficient method for the preparation of high-performance composite noble metal NW catalysts and apply it into the synthesis of other types of composite noble metal NW catalysts.Based on the current issues mentioned above,in the thesis,the synthesis and related reaction mechanism of electrocatalytic Au-Pt group(Pd,Pt)NWs are the two mainstreams.Firstly,the Au-Pd alloy NW networks(NWWN s)were synthesized via the antigalvanic reaction between Pd ions and in-situ Au NWs at room temperature without the aid of additional reducing agents,and its application in flexible fuel cells was further investigated.Next,the high catalytic activity of core-shell Au@PdPt alloy NWs with adjustable composition were successfully achieved by introducing a high quality Pd transition layer.Finally,in view of the special reaction phenomenon during the reductant-free preparation of AuPd and AuPt NWs,the oxidation potentials of Au NPs with different sizes and morphology and their antigalvanic reaction(AGR)with palladium ions were explorated,which proved for the first time that the occurrence of AGR was closely related to the surface defects of NPs.This paper is divided into five chapters(Chapter 1 is the introduction),and the concrete research content is as follows:In Chapter 2,firstly,the Au NWs intermediates can be obtained by accurately controlling the reaction time of sodium citrate and chloroauric acid(HAuCl4)at room temperature.Secondly,the Au-Pd NWNs were obtained via the antigalvanic reaction between Pd ions and in-situ Au NWs,and their morphology and surface alloy structure were further characterized.Then,the Au95.1-Pd4.9 alloy NWNs with the best ethanol catalytic performance were obtained by carefully regulating their surface compositions.Finally,the Au95.1-Pd4.9 alloy NWNs were combined with perfluorosulfonic acid(Nafion)or carbon fiber cloth(CFC)to prepare the macroscopic and flexible electrode materials for fuel cells.In addition,highly stable Au95.i-Pd4.9 alloy NWNs hydrogels can also be prepared by combining the Au95.i-Pd4.9 alloy NWNs with agarose.Due to the high flexibility and strength of the Nafion-Au95.1-Pd4.9 alloy NWNs(NWN-Nafion)composite film,it can be readily transferred onto supporting solid substrates with different chemical natures and curvature.Moreover,the resulting NWN-Nafion composite film exhibits excellent electrocatalytic performance for ethanol oxidation in KOH alkaline medium.Their ECSA values(163.5 m2 gPd-1),mass activity(33.1 A mgPd-1),and specific activity(20.2 mA cm-2)are about 5.4-fold,165-fold,and 31-fold better than those(30.4 m2 gPd-1,0.20 A mgPd-1,0.66 mA cm-2)of commercial Pd/C catalysts,respectively.While,the Au95.i-Pd4.9 alloy NWNs@CFC(NWN-CFC)structure can be directly used as the flexible electrode of ethanol fuel cells,and its ECSA value can be up to 288.2 m2 gPd-1.At the same time,the cyclic voltammetry(CV)curves of the NWN-CFC were basically the same whether it is at bent,folded,rolled,or undeformed state,which further expanded the potential use of NWN-CFC as a flexible electrode for wearable devicesIn Chapter 3,firstly,by selecting appropriate type and amount of Pt precursor ions,the AuPt NWs with smooth surface were successfully prepared without the aid of additional reducing agents at room temperature,and their catalytic properties for methanol in alkaline and acidic mediums were further investigated.Then,the core-shell Au@PdPt alloy NWs with smooth surface were successfully achieved by introducing a high quality Pd transition layer and excessive ascorbic acid(AA).In addition,the Au69.8@Pd8.7Pt21.5 alloy NWs with optimal surface morphology and catalytic activity can be obtained by adjusting the addition amount of Pd and Pt precursors and the addition time of Pt precursor.Moreover,due to the high-density active sites on the NW surface and the electronic interaction among Au,Pd,and Pt,the resulting Au69.8@Pd8.7Pt21.5 alloy NWs exhibit excellent electrocatalytic performance(67.8 m2 gpt-1,781 mA mgpt-1,and 1.15 mA cm-2)for methanol oxidation in acidic medium,which are 1.9-fold,9.3-fold,and 5.0-fold better than those of Au3Pt NWs(36.6 m2 gPt-1,84 mA mgPt-1,0.66 mA cm-2),and 4.8-fold,6.5-fold,and 1.35-fold better than those of commercial Pt/C catalysts(14.1 m2 gPt-1,120 mA mgPt-1,0.85 mA cm-2),respectively.In chapter 4,firstly,the quasi-spherical Au NPs with sizes of 3,6,16 and 50 nm,as well as the multi-defects structures of 70 nm Au trisoctahedral(TOH),Au6-16 aerogels and 3 nm Au NWs were prepared.Then,the effect of surface ligand ions on the oxidation potential of Au NPs was investigated.On this basis,the oxidation potentials of Au NPs with different sizes and morphologies and their AGR with Pd ion were studied.Which confirmed that the oxidation potentials of quasi-spherical Au NPs were size-dependent,while that of multi-defects Au NPs is more due to the surface defects.More importantly,the discovery of this work further expands the current cognition of AGR,and provides a new design idea for the construction of multi?nanomaterials,which has important theoretical significance.Chapter 5 is a summary and forecast of the current research work.To sum up,in this thesis,multi-component AuPd,AuPt and Au@PdPt NWs were prepared and their reaction mechanism was further explored,which enriched the preparation methods of Au-based Pd,Pt catalysts and extended their applications in the flexible fuel cells.