The Preparation Of Several Binary Noble Metallic Micro-nano Materials And Their Applications In Catalysis

The optical, electronic/electrical, magnetic, and catalytic properties of nanomaterials are different from those of the corresponding bulk materials. Bimetallic nanomaterials possess special electronic structure, surface properties and chemical properties with respect to the monometallic material, and have broad applications in the energy source, chemical enginering, biology, medicine and other fields. So, to design and fabricate nanomaterials with special structures and functions has received wide attention of researchers. In this dissertation, several simple and convenient approaches have been developed to fabricate numbers of binary noble metallic nanomaterials. The catalytic properties of these nanomaterials have also been investigated, including electrocatalytic oxidation of small organic molecules, hydrogen production from decomposition of formic acid at room temperature and catalytic reduction of4-NP. The main contents are summarized as follows:1. Literature about the preparation bimetallic micro-nanostructured materials and their applications such as electrocatalytic oxidation of small organic molecules, catalytic reduction of4-NP and catalytic decomposition of formic acid at room temperature have been systematically reviewed.2. Metallic foams with hierachical porous structure could be prepared conveniently by electrodeposition utilizing hydrogen bubble dynamic templates. However, it was once considered that Au foams could not be prepared in this way for the overpotential of hydrogen evolution was low. We pioneered in fabrication of Au foams with hierachical porous structures from smooth Au in combination of treatment of square-wave potentials with hydrogen bubble dynamic templates, which demonstrated the possibility of electrodeposition of nobel metal foams. In this work, Pt-doped AuPt foam films with three-dimensional (3D) hierarchical pores consisting of interconnected dendrite walls were firstly fabricated by a strategy of cathodic codeposition utilizing the hydrogen bubble dynamic template based on the optimized electrodeposition conditions of gold foams. The AuPt foam films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Due to the special porous structure, the electronic property, and the assembly effect, the AuPt alloy foam films showed superior electrocatalytic activity toward the electrooxidation of formic acid in acidic solution, and the prepared3D porous AuPt alloy films also show high activity and long stability for the electrocatalytic oxidation of methanol, where synergistic effect plays an important role in addition to the electronic effect and assembly effect.3. PtPd alloy foam films comprised of nanodendrites have been further electrodeposited with the hydrogen bubble dynamic templates. The deposition conditions like potential, deposition time, H2SO4concentration, precursor concentrations were investigated in detail. We found that Pd played the predominant role in the formation of alloy foams and nanodendrites in comparison with respective electrodeposition of Pd and Pt. The porous films were characterized by scanning electron microscopy, energy dispersive X-ray and X-ray photoelectron spectroscopy. The nanodendritic PtPd porous films showed enhanced electrocatalytic activities toward the electrooxidation of methanol and ethanol in acidic solution.4. Hydrogen is a clean energy source in the future. Electricity can be obtained directly from the hydrogen-oxygen fuel cell. However, the storage and transfer of hydrogen gas are difficult. It is urgent to find a safe and convenient carrier for hydrogen storage. Formic acid has been considered as a promising in situ hydrogen source. So, considerable attention has been paid to hydrogen production from catalytic decomposition of formic acid at solid/liquid interface at room temperature. We found that3D porous thin films of AuPd and AgPd foams comprised of nanodendrites possess superior catalytic activity for the production of high-quality H2from formic acid decomposition at room temperature. The high catalytic activity is attributed to the presence of abundant active sites like steps, corners, kinks and edges in the nanodendrites, and to the electronic effect. Besides the high activity, there are some more advantages for the catalysts of nanodendrtic alloy foam films. For example, the foam films can be quickly electrodeposited in5min on a Ti substrate utilizing the template of hydrogen bubbles without needing organic additives and post-treatment. The hydrogen production is easily controllable, and we can get hydrogen and stop hydrogen production just by immersing the electrodeposited foam film into and pulling it out of the solution of HCOOH+HCOONa. The foam films can also be easily reactivated either by drying after water cleaning or by potential cycling in H2SO4solution.5. In consideration of the highly catalytic activity of AuPd and the AgPd porous structure with nanodendrite prepared by electrodeposition for formic acid decomposition at room temperature, we try to prepare agglomerated AgPd, Pd and AuPd porous catalysts containing nanodendrites and/or nanoparticles by chemical reduction with NaBH4, HCOONa or Mg powder as reductant without organic additives. We found that the agglomerated porous AgPd and Pd catalysts can efficiently decompose formic acid at room temperature to obtain hydrogen. Among these catalysts, the agglomerated porous AgPd catalyst prepared with the reductant of NaBH4showed the highest reactive activity for the decomposition of formic acid, and the catalytic activity of Ag1Pd3catalyst increased and reached a stable state during repeated performances. The high catalytic activity is attributed to the presence of abundant active sites like steps, corners, kinks and edges in the nanoparticles or nanodendrites, and to the electronic effect. Besides, the synthesis methods of these nanoporous catalysts are simple, fast and green, and the agglomerated catalysts can be easily separated from the reaction system.6. To prepare various micro/nano-materials through galvanic replacement reaction of a single metal precursor solution with a more active metal is a simple way, and this method is used widely. In this work, two simultaneous galvanic replacement reactions were explored to prepare a variety of bimetallic dendrites (AgCu, AgPd, AuPd, CuAu, CuPt and CuPd) within one-step, using commercial Mg powder as a sacrificial metal to reduce these bimetallic precursors in aqueous solution. We demonstrate in detail how morphologies and compositions of bimetallic dendrites were affected by these factors like potential differences between metal redox couples, molar ratios of precursors, ligands, reaction times and total precursor concentrations, taking dendritic AgCu as an example. In addition, monometallic Ag and Cu dendrites were also prepared for comparison. The application of these prepared dendrites was further explored as high performance catalysts for the reduction of4-nitrophenol in aqueous solution at room temperature. The following active order CuPd> CuPt> CuAu> Cu> Ag1Cu1> Ag3Cu1> AuPd> Ag1Cu3> AgPd> Ag was established for the prepared dendrites based on normalized kinetic parameters according to our proposed formula. In addition, weprepared colloids of AgPd, AuPd and Pd using Mg power as the reductant and polyvinylpyrrolidone (PVP) as the capping agent. These small colloidal nanoparticles showed very high catalytic activities toward the the reduction of4-nitrophenol in aqueous solution at room temperature.7. We pioneered in preparation of Pt hydrosols through facile dispersion of pure Pt wire with square wave potential (SWP) or alternating voltage (AV). In this work, Rh hydrosols have been prepared in a similar way by dispersion of pure Rh wire. Moreover, porous surface was also acquired. Both the Rh nanoparticles and the porous surface have high electrocatalytic activity toward the electroreduction of nitrate mainly due to their large active areas.