Design And Preparation Of Nanoporous Metals And Their Catalytic Performances

This paper is focusing on designing and fabricating a novel class of nanoporous metallic materials and exploring their corresponding formation mechanism, catalytic and electrocatalytic properties.Investigations are based on several aspects mainly including:research on unsupported nanoporous gold(NPG) for CO oxidation,fabrication of multifunctional hierarchically hollow platinum-group bimetallic nanocomposites and their electrocatalytic activities, preparation of nanoporous Pt-based metals with controllable size and composition, and related characterization of their electrocatalytic performance.1.Preparation of unsupported NPG with small length scale and research on their CO performance.Typically,NPG sample with ligament size at～6 nm(a-NPG) was made by etching a piece of 25μm Ag/Au foil in a concentrated nitric acid for 10-15 min with an applied voltage of 0.6-1.0V.Platinum electrode was used as a cathode in a binary electrode system.Based on the dealloying principle,the evolution and formation of nanoporous structure is a competition between dissolution of Ag induced surface roughening and Au diffusion induced surface smoothing.By using the modified electrochemical dealloying method,the applied anode potential accelerates the dissolution of Ag and inhibits the diffusion of Au, resulting in the smaller length scale of NPG.Based on the unsupported NPG catalysts,we discussed the catalytic activity of NPG as a function of space velocity,gas concentration,and reaction temperature etc.It was found that a-NPG shows unique catalytic activity towards low-temperature CO oxidation in the absence of metal oxide support.In kinetic studies,we found that the reaction rate of CO oxidation on unsupported NPG depends significantly on CO concentration but only slightly on O₂ concentration. It is considered that the CO adsorption onto the NPG surface may play a decisive role in the reaction as the rate-limiting step.CO oxidation rate on NPG was expressed by the following equation:RCO=K[CO]^0.78[O₂]^0.25.With the increase in reaction temperature CO connversion rapidly increased between 235 and 273 K.At higher temperatures,the trend slowed,and CO reached near complete conversion.The analysis of Arrhenius behavior allows the extrapolation of the apparent activation energy to be 28.8 kJ/mol.Interestingly,we found that NPG with larger pore and ligament size also exhibited high CO catalytic activity.We think that the high catalytic activity of NPG originates form the high concentration of low-coordinated metallic Au atoms but not the quantum size effect.The electrochemical dealloying method to prepare NPG is simple and convenient with absolute yield and controllable structure,which enriches the preparation of small-sized nanoporous metallic materials.A series of research on NPG towards CO oxidation provides new insights into gold catalysis by completely excluding the effect of support.2.The preparation of nanotubular porous platinum-group bimetallic nanocomposites and research on their electrocatalytic performance.In this research,a general approach is employed to large-scale synthesis of a novel class of nanotubular mesoporous platinum-group-metals(PGMs)-based nanostructures based on a simple room-temperature dealloying and galvanic replacement reaction.Dealloying was used to synthesize nanoporous metals to act as sacrificial templates,and followed by reacting with H₂PtCl₆ and K₂PdCl₄ solution precursors.The resulted NM-Pt/Cu and NM-Pd/Cu structures were confirmed by transmission electron spectroscopy(TEM) and scanning electron spectroscopy(SEM).NM-Pt/Cu shows a three-dimensional(3D) bicontinuous nanotubular mesoporous structure with tube diameter around 80 nm and shell thickness about 10 nm.Interestingly,the shell surfaces are not smooth and seamless but rather have many small pores and grains around 3 nm.The NM-Pd/Cu structure also exhibits hierarchical hollow structure with the tube diameter around 60 nm and shell thickness about 6 nm.X-ray diffraction(XRD) analysis confirmed that the resulted NM-Pt/Cu and NM-Pd/Cu have the single-phase alloy structure.The possible formation mechanism of such the hierarchically hollow structure was explained as follows:as Pt(or Pd) atoms are reduced and deposited onto the NPC substrate,Cu atoms are progressively leached away.At the same time,these deposited precious metal surface atoms will interdiffuse with Cu to form an alloy.As the reaction goes on,more Cu will be depleted;and accordingly,the surface was gradually passivated due to the further incorporation of more precious metal atoms.Eventually,a PGM-based bimetallic nanotubular mesoporous structure forms,which also marks the completion of the displacement reaction.NM-Pt/Cu catalyst exhibits high catalytic activity and CO-tolerance toward methanol oxidation.Meanwhile,NM-Pd/Cu shows a unique activity towards oxygen reduction reaction.This synthesis strategy is facile and efficient to fabricate similar ultra-high surface area nanocomposites in large scale with hierarchically hollow structures.These nanostructures have obvious structural advantages in terms of unique catalytic properties,simple and clean processing, and saving precious metals,which suggest their great potential for use in heterogeneous catalysis and fuel cell technologies.3.Preparation of nanoporous Pt/Au alloys with controllable size and composition by dealloying ternary alloys and research on their electrocatalytic activity.A simple and general dealloying method was employed to fabricate nanoporous Au/Pt alloys with pre-determined Au/Pt molar ratios.Structural characterization by electron microscopes confirms that selective etching of Cu from Au/Pt/Cu alloy precursors results in the formation of 3-dimensional bicontinuous porous network structures with uniform pores and ligaments.The as-made Pt₄Au₁,Pt₁Au₁,and Pt₁Au₄ samples are all resulted in the similar 3D bicontinuous network with ligament size at 3,5,and 10 nm,respectively.A careful inspection shows that under the same dealloying conditions,nanoporous alloys with high Pt content generally result in smaller feature sizes.X-ray photoelectron spectroscopy and X-ray diffraction demonstrate that Pt₄Au₁ and Pt₁Au₁ nanoporous alloys adopt a relatively uniform bimetallic alloy structure across the entire length scale,while Pt₄Au₁ sample shows slight phase segregation,leading to an Au-rich surface layer.These nanostructures show distinct surface structure sensitive electrocatalytic properties. For methanol electrooxidation,nanoporous Pt₄Au₁ and Pt₁Au₁ showed an obviously improved performance in structure stability and electrocatalytic activity with higher specific activity and lower peak potentials,as compared with the commercial nanoparticle-based catalysts.In sharp contrast,nanoporous Pt₁Au₄ exhibited the best activity for formic acid oxidation amongst all samples,which was characterized by a preferential dehydrogenation reaction process at lower potential. The resulted nanoporous alloys typically represent a class of macroscopic nanostructured materials with a three-dimensional bicontinuous spongy morphology, which are expected to have evident structural advantages as compared with zero-dimensional nanomaterials.Dealloying method is extremely simple and flexible,which is appropriate for large-scale synthesis.In addition,this method is a chemical "green" method,which offers new opportunities for the preparation of nanoporous Pt-M bimetallic catalysts with high surface area and high activities.