Porous metal and method of preparation thereof

Details Porous metal and method of preparation thereof Porous metal and method of preparation thereof

1999-12-08

2001-03-20

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

All metal or with adjacent metals

Porous

C075S722000, C075S732000

Reexamination Certificate

active

06203925

ABSTRACT:

This invention relates to porous metal-based materials, in particular porous metal-based materials having a substantially regular structure and uniform pore size, and to a method of preparing porous metal-based materials by reduction of a mixture containing a source of metal.
BACKGROUND OF THE INVENTION
Porous metals have found extensive applications in filtration, gas-flow control, catalysis, fuel cells, and batteries. Their open and interconnected microstructure allows facile diffusion of reactants and electrolytes to their relatively large surface areas. Conventionally, porous metals have been grouped into three categories on the basis of their microstructure, these categories being metal sponges or foams, supported aggregated colloidal metals, and micromachined metals. Metal sponges and supported aggregated colloidal metals represent the most important classes of porous metals and have been used for many years in industrial applications. Typical examples of these materials are platinum and palladium catalysts. Metal sponges are composed of an irregular framework of metal, 1 micrometer to 100 micrometers thick, surrounding interconnected pores with sizes in the range 0.5 micrometer to about 160 micrometers. Aggregated colloidal metals are composed of small primary particles of metal, typically spherical and of diameters in the range 2 nanometers to 1 micrometer, which are aggregated into large particles of irregular size and shape. These aggregated particles are often supported by non-metallic materials. The variability in pore sizes of metal sponges and aggregated colloidal metals is typically quite high. The sizes of pores in these materials span the top end of the mesoporous range, and the macroporous range. For the purpose of this application, the term mesoporous refers to pore sizes in the range approximately 13 to 200 Å, and macroporous refers to pore sizes greater than about 200 Å.
Conventional processes for preparing porous metals include powder sintering and electrochemical deposition. However, as described above these tend to produce metals with a variable pore size, generally in the macroporous range, which may not give a large specific surface area and the variable pore size does not permit its use for size-selective catalysis.
In the drive towards providing porous metals showing improved properties, for use in for example catalysts, batteries, fuel cells, electrochemical capacitors, quantum confinement effect devices, optical devices, sensors, electrosynthesis, electrocatalysis and filtration or chemical separation, to our knowledge no one has yet succeeded in developing an effective process for preparing at least mesoporous metal-based materials of regular structure and uniform pore size, with the attendant advantages in terms of properties which such metal-based materials might be expected to show.
Previously, we have shown that porous ceramic oxide monoliths can be condensed from lyotropic liquid crystalline phase media, whereby the liquid crystalline phase topology directs the synthesis of the material into a corresponding topology showing structural regularity and uniformity of pore size. However, it was not expected that this templating mechanism could be used to synthesise porous metal-based materials.
SUMMARY OF THE INVENTION
What we have found, surprisingly, is that porous metal-based materials can be prepared from homogeneous lyotropic liquid crystalline phases. Accordingly, the present invention provides a method of preparing a porous metal-based material having a substantially regular structure and uniform pore size which comprises reducing a mixture comprising: a source of metal; a solvent; and a structure-directing agent present in an amount sufficient to form a liquid crystalline phase in the mixture, to form a composite of metal-based material and organic matter, and optionally removing organic matter from the composite. Also, the invention provides a porous metal-based material having a substantially regular structure and substantially uniform pore size, wherein the pore size is in the mesoporous range.
DETAILED DESCRIPTION OF THE INVENTION
According to the method of the invention, a liquid crystalline mixture is formed and reduced. The mixture comprises a source material for the metal, dissolved in a solvent, and a sufficient amount of an organic structure-directing agent to provide an homogeneous lyotropic liquid crystalline phase for the mixture.
One or more source materials may be used in the mixture, for reduction to one or more metals. Thus, by appropriate selection of source material, the composition of the porous metal-based material can be controlled as desired. Suitable metals include for example the first, second and third row transition metals, in particular platinum, palladium, gold, silver, nickel, cobalt, copper, iron, lead, tin and indium, preferably platinum, palladium, gold and nickel. The metals may contain surface layers of, for example, oxides, sulphides or phosphides. Suitable source materials include hexachloroplatinic acid and ammonium tetrachloropalladate, preferably hexachloroplatinic acid.
The solvent is included in the mixture in order to dissolve the source material and to form a liquid crystalline phase in conjunction with the structure-directing agent, thereby to provide a medium for reduction to the porous metal. Generally, water will be used as the preferred solvent. However, in certain cases it may be desirable or necessary to carry out the reduction in a non-aqueous environment. In these circumstances a suitable organic solvent may be used, for example formamide or ethylene glycol.
The structure-directing agent is included in the mixture in order to impart an homogeneous lyotropic liquid crystalline phase to the mixture. The liquid crystalline phase is thought to function as a structure-directing medium or template for reduction to the porous metal. By controlling the nanostructure of the lyotropic liquid crystalline phase and reducing the mixture, porous metal-based material may be synthesised having a corresponding nanostructure. For example, porous metal-based materials formed from normal topology hexagonal phases will have a system of pores disposed on an hexagonal lattice, whereas porous metal-based materials formed from normal topology cubic phases will have a system of pores disposed in cubic topology. Similarly, porous metal-based materials having a lamellar nanostructure may be deposited from lamellar phases. Accordingly, by exploiting the rich lyotropic polymorphism exhibited by liquid crystalline phases, the method of the invention allows precise control over the structure of the porous metal-based materials and enables the synthesis of well-defined porous metal-based materials having a long range spatially and orientationally periodic distribution of uniformly sized pores.
Any suitable amphiphilic organic compound or compounds capable of forming an homogeneous lyotropic liquid crystalline phase may be used as structure-directing agent, either low molar mass or polymeric. These may include compounds sometimes referred to as organic directing agents. In order to provide the necessary homogeneous liquid crystalline phase, the amphiphilic compound will generally be used at an high concentration, typically at least about 10% by weight, preferably at least 20% by weight, and more preferably at least 30% by weight, based on the total weight of the solvent and amphiphilic compound.
Preferably, the structure-directing agent comprises a surface-active organic compound of the formula R
1
R
2
Q wherein R
1
and R
2
represent aryl or alkyl groups having from 6 to about 36 carbon atoms or combinations thereof, and Q represents a group selected from: (OC
2
H
4
)
n
OH, wherein n is an integer from about 2 to about 20; nitrogen bonded to at least two groups selected from alkyl having at least 4 carbon atoms, and aryl; and phosphorus or sulphur bonded to at least 4 oxygen atoms.
Other suitable compounds include organic surfactant compounds of the formula RQ wherein R represents a linear or branched alk

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