Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly alloy coating
Reexamination Certificate
2000-06-23
2002-04-30
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly alloy coating
C205S257000, C205S259000, C205S162000, C205S182000, C205S209000, C205S150000
Reexamination Certificate
active
06379524
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for preparing a composite membrane for separation of hydrogen gas and more particularly, to a method for preparing the composite membrane with better permeation and separation coefficient on hydrogen gas, prepared in such a manner that an alloy consisting of a palladium complex compound and transition metal is electroplated on a modified porous support under vacuum, thereby forming the thin plated-layer without pin-holes.
2. Description of the Related Art
Generally, it is advantageous to use a material with larger permeation and separation coefficients as a membrane for separation of a hydrogen gas. When the hydrogen gas is separated by a membrane derived from an organic polymer, the membrane manifests some poor physical properties such as its thermal stability, chemical resistance and durability. In particular, the selectivity of a membrane to a certain gas tends to be lowered as its gas permeability becomes larger, and thus a highly purified hydrogen cannot be obtained.
Meanwhile, a palladium-based separation membrane has a higher solubility and diffusion property to the hydrogen gas so that the selective separation of hydrogen gas may be available through its dense surface. In this context, some metals such as palladium have been mainly used so as to obtain a highly purified hydrogen from a mixing gas containing hydrogen.
However, the use of pure palladium membrane for the hydrogen separation is restricted, as hydrogen embrittlement caused by the transition between the &agr;- and &bgr;-phase occurs in membranes in contact with hydrogen at temperature below 300° C. and pressure below 2 HPA. Since the lattice constant of the &bgr;-phase is at least 3% larger than that of the &agr;-phase, the nucleation and growth of the &bgr;-phase cause strains in the metal and thereby the embrittlement of the material.
To comply with this matter, a method of using an alloy containing palladium and other metals has been suggested instead of use of pure palladium; With its proper prevention of any hydrogen saturation and interaction between a metal plated in the form of substitution and hydrogen atom, a palladium-alloy membrane may serve to prevent the regular arrangement of hydrogen atoms within the lattice and thus, any formation of a beta phase does not occur. The palladium-alloy membrane may further enhance the selectivity to hydrogen gas and durability, compared with a separation membrane derived from pure palladium. In particular, in order to improve the performance of membrane, a method of coating palladium thinly on a porous support has been under consideration and some studies on such composite-type separation membrane has been mainly conducted.
As disclosed in U.S. Pat. No. 2,773,561, the use of pure palladium or palladium-based alloy membrane with some metal in the absence of the composite-type separation membrane has some disadvantages in that a) this method is uneconomical due to an excess use of palladium in a membrane, and b) since the thickness of membrane should be in the range of 25-150 &mgr;m to endure the temperature and pressure, its larger thickness may lower the permeation rate.
In this respect, it is recommended that a palladium composite membrane containing a thin palladium alloy membrane on its support, be employed as a separation membrane for hydrogen gas instead of palladium metal membrane or thin palladium alloy membrane.
Examples of the existing materials used for a support of a palladium composite membrane includes glass (Chem. Lett., 10, 1687(1988)), inorganic materials such as porous ceramics (Ind. Eng. Chem. Res., 33, 616(1994)); J. Memb. Sci., 56,315(1991)) and stainless steel (J. Memb. Sci., 181(1993)). Whereas the weak strength of the glass and porous ceramics are susceptible to easy destruction and the sealing between a separation membrane and permeation cell may not be made available, a porous support derived from a stainless steel has several advantages such as a) lower material cost, b) little occurrence of corrosion and crack, c) easier processing, and d) higher mechanical strength for modulation.
In addition, examples of the plating methods designed to prepare a palladium composite membrane by means of coating palladium metal membrane and palladium alloy membrane on a porous support includes a) electroless plating (J. Memb. Sci., 77, 181(1993); Ind. Eng. Chem. Res., 32,3006(1993)), b) chemical vapor deposition (Ind. Eng. chem. Res. 33, 616(1994); J. Memb. Sci., 120,261(1996)) and c) sputtering (J. Memb. Sci., 94,299(1994): J. Memb. Sci. 104,251(1995)).
The electroless plating method comprising a variety of multi-pretreatment steps and several repeated activation steps at the surface of support has several disadvantages in that a) more prolonged treatment time is required, b) the thickness of film may be controllable due to continuous plate growth, and c) the excellent capacity of membrane may be hardly expected, since a membrane with a thickness of 10-20 &mgr;m should be prepared to form a pinhole-free layer.
The chemical deposition (CVD) method has also been used to coat thin films of thickness below 5 &mgr;m. Although the CVD method could prepare highly selective films, it required too high purity of constituents and strict conditions in view of an economical aspect. Also, since the CVD method cannot form an alloy, the method forms &bgr;-phase which induces hydrogen embrittlement.
The sputtering method has little separation effect of hydrogen gas via a palladium layer due to difficulty in forming defect-free film.
SUMMARY OF THE INVENTION
Therefore, an object of this invention is to provide a method for preparing a composite membrane for separation of hydrogen gas with improved permeability and separation capability to hydrogen gas, wherein a palladium alloy compound is electroplated on a porous support; instead of using palladium, a palladium complex compound is applied so as to lessen the difference of standard electropotential between palladium and transition metal used as alloy metal; the surface of porous support is modified to form a homogeneous, thin and pinhole-free layer; in particular, during electrodeposition, the thickness of an electrodeposited layer, so formed in the presence of vacuum, is thinly adjusted to further enhance the permeation and selectivity of hydrogen gas from a mixing gas containing hydrogen.
DETAILED DESCRIPTION OF THE INVENTION
This invention is characterized by a method for preparing composite membrane for separation of hydrogen gas using palladium, wherein an alloy containing palladium and transition metal is electroplated on a porous support.
This invention is explained in more detail as set forth hereunder.
This invention relates to a method for preparing a composite membrane where a palladium alloy complex is the form of homogenous and thin membrane is located, without any pin-hole formation, at one surface of porous support having possibly homogeneous micropores, to the extent that the diffusion of gas may not be affected. To this end, some major characteristics of this invention are that;
a palladium complex compound is used;
the surface of support is modified to thinly adjust an alloy layer without any pin-hole formation;
an electroplating method under vacuum state is applied.
According to this invention, an electroplating method is applied so as to form a palladium alloy membrane on the porous support. Under the conventional method, this method is not only simply and easy to adjust a thickness of the composite membrane, but also economical in that less time is required than the electroless plating and chemical vapor deposition.
In case of applying a general electroplating method designed to form a palladium alloy membrane, the amount of palladium from the composition of alloy is not easily adjusted due to big difference of standard electropotential between palladium and transition metal. For example, the standard electropotential of palladium and nickel used as a transition metal is 0.987V and −0.250V, resp
Lee Kew Ho
Lee Sang Hak
Nam Seung Eun
Korea Research Institute of Chemical Technology
Wong Edna
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