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Design and Preparation of Nanoporous Metals and Their Catalytic Performances

Author: XuCaiXia
Tutor: DingZuo
School: Shandong University
Course: Physical and chemical
Keywords: Go alloying Nanoporous Gold CO oxidation Nano pipe-like Nano - porous alloy Oxygen reduction Methanol Formic acid
CLC: TB383.1
Type: PhD thesis
Year: 2009
Downloads: 920
Quote: 4
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Abstract


This thesis is mainly to alloying metal material prepared by a series of nano-porous formation mechanism and characterization and applied research aspects are discussed, respectively, from the preparation of nano-porous materials include electrochemical alloying main content Nanoporous Gold and CO oxidation properties of the preparation of small aperture to alloying Redox combined preparation of nano pipeline like mesoporous platinum alloy nanostructures and their formation mechanism and electrocatalytic properties to alloying three yuan alloy preparation of nano-porous Pt / Au alloy and its electrocatalytic performance. The purpose of this paper is to develop a simple and effective way to preparation of nano-porous metal material such material vapor phase catalytic and electrocatalytic properties and explore, explore their prospects for application in fuel cells and industrial catalysis. Electrochemical go alloying Preparation small aperture Nanoporous Gold CO oxidation properties of two-electrode system, concentrated nitric acid as the electrolyte, platinum as the cathode, 25μm thick of Au / Ag alloy (42:58, wt.%) plate as the anode, the anode voltage (the voltage range of 0.6-1.0 V), etched at room temperature for 10-15 min to prepare a small aperture nanoporous gold a-NPG. a-NPG has a sponge-like structure of the three-dimensional bi-continuous, and the size of the hole wall is distributed evenly in about 6 nm. According to the mechanism of the formation of porous gold, the dissolution of silver in the surface roughening, surface smoothing a result of the diffusion of gold atoms are competing, this article improved to alloying of the gold and silver alloy, plus a certain anode voltage accelerate the dissolution of silver, gold diffusion is suppressed, shortened go alloyed time, the the prepared small size of the nano-porous gold. Based on the a-NPG this new non-supported gold catalyst system, a systematic study of the a-NPG series of the nature of the catalytic oxidation of CO, including the active species of the catalytic reaction, the reaction kinetics, and the reaction temperature, the size of the hole wall and of airspeed the catalytic activity. Nanoporous Gold for CO oxidation in the case of the absence of a metal oxide substrate shows a high catalytic activity. the metallic state in the a-NPG grant 2 , the active site of the catalytic reaction of CO and O For the influence of the concentration of the reactants on the reaction rate, the rate of the catalytic reaction to a concentration of CO by experimental observation greater impact, while the O a concentration of 2 only slightly affect the reaction rate, the experimental results show that CO adsorption of the surface of the catalyst in the a-NPG in the temperature range of discussion may be the reaction rate-controlling step, the reaction rate of the catalytic oxidation of CO on the a-NPG equation can be expressed as R = K [CO] 0.78 [O 2 ] 0.25 . Within the temperature range of 233-273 K, the reaction rate of the CO oxidation changes linearly as the temperature increases, at high temperatures, this trend is gradually slowed down. Arrhenius behavior analysis to estimate the catalytic oxidation of CO on a-NPG apparent activation energy of about 28.8 kJ / mol. Discussion of the impact of the size of nano-porous gold hole wall, the larger size of the a-NPG also shows a certain CO catalytic activity, showed a-NPG high activity was not the result of the quantum size effect may be associated with a-NPG bore wall smaller size present on the surface a large number of low-coordination gold atoms. Electrochemical dealloying prepared a-NPG catalyst preparation method is simple, easy to control the structure, high yield, the preparation method is the preparation of the porous metal material of the small size nano. Based on a series of studies conducted by a-NPG catalytic oxidation of CO by completely rule out the influence of substrate, conducted a series of studies on the cause of the catalytic activity of gold catalyst provides a new understanding and awareness. 2 to combined preparation of nano-pipe-like mesoporous platinum alloy structure and electrocatalytic properties of alloying with redox replacement wears simple research will go alloying Redox Combination prepared for a class of Pt group metals the nano-pipe-like mesoporous alloy material. Selectively etched in NaOH solution 50μm thick Cu 25 Al 75 (at.%) alloy sheet prepared the nano porous copper (NPC) as a template and reducing agent. NPC shows a three-dimensional double continuous sponge-like morphology, and the size of the hole wall is distributed evenly around 50 nm, to alloying prepared NPC template, simple, effective, low cost and suitable for large scale preparation. replacement reaction by transmission electron microscopy (TEM), scanning electron microscopy and X-ray powder diffraction proved N-PC H 2 PtCl tubular nano-porous Pt / Cu alloy ( NM-Pt/Cu), wherein the size of the pipe diameter and the hole wall is about 60 and 10 nm, respectively. Hole wall by a diameter ~ 5 nm nanoparticles and a pore diameter of ~ 3 nm apertures. The solution reaction 4 K 2 PdCl produce a a similar nano pipeline like mesoporous structure, the size of the diameter of the pipe and the hole wall is about 50 and 6 nm. Substitution reaction process, the formation mechanism of such a pipe-like structure as follows: When NPC template with a platinum group metal precursor solution After mixing, the surface of the Cu precursor solution spontaneously substitution reaction, is reduced to produce a Pt or Pd atom is deposited onto The NPC's surface, as the reaction proceeds, more and more Cu atoms inside the structure from the NPC filtered off. Meanwhile, the the NPC structure inside the Cu gradually outward diffusion, deposited Pt or Pd atom solution with unreacted mutual diffusion of Cu atoms to form an alloy, while the exposure of Cu in solution gradually being replaced and alloying of Pt or Pd, and the product ultimately becomes Pt-rich Pt / Cu, or rich of Pd, Pd / Cu alloy of Cu due to the presence of Pt or Pd, of the large number of its surrounding atoms is passivation, leading to termination of the displacement reaction, and ultimately generate a pipe-like nano porous alloy structure. For the surface of the porous shell, may be Cu and the metal precursor solution more violent reaction or high surface energy gathered to form the nanoparticles resulting in the formation of a porous surface, so that the reaction process is to restore Pt or Pd atom. Small organic molecules electrocatalytic NM-Pt/Cu NM-Pd/Cu alloy electro-oxidation of methanol, formic acid oxidation and oxygen reduction, etc. with high activity and catalytic stability and strong anti-poisoning ability found in the catalytic performance tests . Such nanomaterials its excellent catalytic performance, simple and environmentally friendly preparation method, the unique structure of its application in industrial catalysis and fuel cell technology provides a significant advantage. Dealloying ternary alloy preparation nano-porous Pt / Au alloy and its electrocatalytic properties go alloying ternary alloy prepared a uniform structure of the platinum component nano-porous Pt / Au alloy, its ingredients are Pt 4 Au 1 Pt 1 Au 1 the Pt 1 Au 4 . Nano-porous Pt / Au alloy is dissolved by electrochemical etching of Pt / Au / Cu alloy of copper is obtained. SEM and TEM studies have shown that the selective removal of copper ternary alloy prepared a simple and effective three-dimensional bicontinuous nanoporous Pt / Au alloy. Similarly to the alloying conditions, with increasing gold content, the aperture hole wall roughening phenomena wherein Pt a 4 Au 1 , Pt the 1 Au 1 , Pt , Au 4 aperture with the size of the hole wall is 3,5,10 nm. XPS and XRD prove nano porous Pt 4 Au 1 with Pt 1 Au 1 has a very uniform alloy group stars and phase structure, while the Pt 1 Au 4 occurs a slight phase separation, resulting in a surface-enrichment gold. Such nano-porous structure in the lower gold content in the acidic solution of the Pt has played a role of the good structural stability. The electrocatalytic performance tests show that the different components of Pt / Au alloy electro-catalytic oxidation of methanol and formic acid showed significantly different performance. From the methanol catalytic look, Pt 4 Au 1 Pt 1 Au 1 has high catalytic activity of methanol , and Pt 1 Au 4 but shows the highest formic acid electrocatalytic properties due to the adoption of direct dehydrogenation catalytic mechanism. The experiments show that by refining alloy control the initial Pt / Au bimetallic ratio, can be different electro-catalytic activity of small organic molecules. Go alloying method of preparing the nano-porous alloy is a simple and effective method, a noble metal substantially no loss of the entire process, is adapted to large scale synthesis. Is particularly important, go alloying is an ideal \This method as an effective conventional method may be synthesized by other nanoporous alloy catalyst.

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