Supported metal membrane, a process for its preparation and use

Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Inorganic carbon containing

Reexamination Certificate

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C502S439000, C502S527120, C502S527150, C502S527240, C428S312800, C428S319100, C428S116000, C428S613000, C428S553000, C095S055000, C095S056000, C096S004000, C096S011000

Reexamination Certificate

active

06649559

ABSTRACT:

DESCRIPTION
The invention provides a supported metal membrane which contains a metal membrane on a porous membrane support, as well as a process for its preparation and its use. Supported metal membranes of this type are used for separating gas mixtures, in particular for the separation of hydrogen from a reformate gas for supplying fuel cells with the required fuel gas.
For this purpose, palladium or palladium alloy membranes on either porous or non-porous supports are normally used, such as compact palladium or palladium alloy membranes. Foils made of hydrogen-permeable metals, inter alia, are used as non-porous supports. The permeability of the membranes for hydrogen increases with temperature. Typical operating temperatures are therefore between 300 and 600° C.
T. S. Moss and R. C. Dye [Proc.-Natl. Hydrogen Assoc. Annu. U.S. Hydrogen Meet., 8th (1997), 357-365] and T. S. Moss, N. M. Peachey, R. C. Snow and R. C. Dye [Int. J. Hydrogen Energy 23(2), (1998), 99-106 ISSN: 0360-3199] describe the preparation and use of a membrane which is obtained by applying Pd or PdAg by cathode sputtering (atomization) to both faces of a foil of a metal from group 5B. The thickness of the layers applied to the two faces may be varied so that an asymmetric component is produced (for example: 0.1 &mgr;m Pd/40 &mgr;m V/0.5 &mgr;m Pd). Permeation trials demonstrate twenty-fold higher hydrogen permeation as compared with self-supported Pd membranes. Accordingly, the membrane described is suitable for use in a PEM fuel cell system instead of the traditional catalytic gas purification steps (water gas shift reaction and preferential oxidation of CO).
GB 1 292 025 describes the use of iron, vanadium, tantalum, nickel, niobium or alloys thereof as a non-porous support for a non-coherent, or porous, palladium (alloy) layer. The palladium layer is applied by a pressing, spraying or electrodeposition process in a thickness of about 0.6 mm to a support with a thickness of 12.7 mm. Then the thickness of the laminate produced in this way is reduced to 0.04 to 0.01 mm by rolling.
According to DE 197 38 513 C1, particularly thin hydrogen separation membranes (thickness of layer less than 20 &mgr;m) can be prepared by alternate electrodeposition of palladium and an alloy metal from group 8 or 1B of the periodic system of elements to a metallic support which is not specified in any more detail. To convert the alternating layers into a homogeneous alloy, appropriate thermal treatment may follow the electrodeposition process.
Either metallic or ceramic materials are suitable as porous supports for palladium (alloy) membranes. In accordance with JP 05078810 (WPIDS 1993-140642), palladium may be applied to a porous support by a plasma spray process for example.
According to Y. Lin, G. Lee and M. Rei [Catal. Today 4.4 (1998) 343-349 and Int. J. of Hydrogen Energy 25 (2000) 211-219] a defect-free palladium membrane (thickness of layer 20-25 &mgr;m) can be prepared on a tubular support made of porous stainless steel 316L in a electroless plating process and integrated as a component in a steam reforming reactor. At working temperatures of 300 to 400° C., a purified reformate containing 95 vol. % H
2
is obtained. However, the optimum working temperature is very restricted because below 300° C. the palladium membrane starts to become brittle due to the presence of hydrogen, whereas above 400 to 450° C. the alloying constituents in the stainless steel support diffuse into the palladium layer and lead to impairment of the permeation properties.
Electroless plating processes are preferably used for coating ceramic supports. Thus, CVD coating of an asymmetric, porous ceramic with palladium is described by E. Kikuchi [Catal. Today 56 (2000) 97-101] and this is used in a methane steam reforming reactor for separating hydrogen from the reformate. The minimum layer thickness is 4.5 &mgr;m. If the layer is thinner, the gas-tightness of the layer can no longer be guaranteed. Apart from CVD coating with pure Pd, coating with palladium alloys is also possible, wherein the alloy with silver prevents embrittlement of the palladium membrane and increases the permeability to hydrogen.
In addition to pure hydrogen separation membranes, membranes which are provided with a reactive layer in addition to the hydrogen separation layer (palladium) are also described for applications in fuel cell systems. Thus, the porous support for a palladium (alloy) membrane may be covered, for example on the face which is not coated with Pd, with a combustion catalyst. The heat released during combustion at the reactive face is then simultaneously used to maintain the operating temperature of the hydrogen separation membrane (EP 0924162 A1). Such a component may then be integrated in the reforming process downstream of a reformer or incorporated directly in the reformer (EP 0924161 A1, EP 0924163 A1).
In addition, not only palladium membranes can be used for hydrogen separation in the fuel cell sector. EP 0945174 A1 discloses a design for the use of universally constructed layered membranes which may contain both fine-pore, separation-selective plastics and/or several ceramic layers and/or layers made of a separation-selective metal (preferably from groups 4B, 5B or 8), wherein these layers are applied to a porous support (glass, ceramic, expanded metal, carbon or porous plastics).
The objective of developing metal membranes for the separation of hydrogen from gas mixtures is to obtain high rates of permeation for the hydrogen. For this purpose, the metal membrane must be designed to be as thin as possible while avoiding the occurrence of leakiness in the form of holes. Such membranes can be processed only in a supported form. In order for the membrane support to have as little effect as possible on the permeation of hydrogen, it must have a high porosity. Thus there is the difficulty, in the case of known processes for preparing supported membranes, of depositing a defect-free membrane on a porous support. There are two problems involved here. On the one hand, the methods described for depositing for example palladium or a palladium alloy can guarantee a relatively defect-free membrane layer only above a certain thickness of layer. This minimum thickness is about 4 to 5 &mgr;m. On the other hand, the coating techniques used for applying the membrane layer to the porous membrane support means that the average pore diameter of the membrane support ought not exceed a certain value because otherwise it would be impossible to apply coherent and defect-free coatings. The pore sizes of known membrane support materials, such as porous ceramics or porous metal supports, are therefore less than 0.1 &mgr;m. This means that the resistance to flow of the gas through the pores cannot be reduced to a desirable extent.
WO 89/04556 describes an electrochemical process for preparing a pore-free membrane based on palladium supported by a porous metal structure. In accordance with the process, a pore-free palladium(-silver) membrane on a porous, metallic support is produced by coating one face of a metal alloy foil (preferably brass) with palladium or palladium/silver (thickness of palladium layer: about 1 &mgr;m) using an electrodeposition process. The porosity of the support is produced later by dissolving the base components out of the brass foil. Dissolution is performed electrochemically, wherein, in a cyclic process, both components are first taken into solution but the more base component is redeposited directly onto the palladium layer (electrochemical recrystallisation). The less base component in the foil-shaped alloy thus goes virtually quantitatively into solution so that a porous metal structure, preferably a porous copper structure, remains as a support for the palladium/silver membrane.
The process in accordance with WO 89/04556 has the disadvantage that the brass foil used as support is virtually completely dissolved and has to be built up again by electrochemical recrystallisation. This means that the composite or

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