WO2005012167A1 - Method of fabricating a hydrogen diffusion membrane - Google Patents

Method of fabricating a hydrogen diffusion membrane Download PDF

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Publication number
WO2005012167A1
WO2005012167A1 PCT/GB2004/003118 GB2004003118W WO2005012167A1 WO 2005012167 A1 WO2005012167 A1 WO 2005012167A1 GB 2004003118 W GB2004003118 W GB 2004003118W WO 2005012167 A1 WO2005012167 A1 WO 2005012167A1
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WO
WIPO (PCT)
Prior art keywords
support
palladium
coated
membrane
metal
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Application number
PCT/GB2004/003118
Other languages
French (fr)
Inventor
Hugh Gavin Charles Hamilton
Peter Albert Walker
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Johnson Matthey Public Limited Company
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Publication date
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Publication of WO2005012167A1 publication Critical patent/WO2005012167A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/02—Inorganic material
    • B01D71/022—Metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/10—Supported membranes; Membrane supports
    • B01D69/108—Inorganic support material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/22—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion
    • B01D53/228—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion characterised by specific membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0046—Inorganic membrane manufacture by slurry techniques, e.g. die or slip-casting
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0081—After-treatment of organic or inorganic membranes
    • B01D67/0088—Physical treatment with compounds, e.g. swelling, coating or impregnation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/10—Supported membranes; Membrane supports
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
    • C01B3/50—Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification
    • C01B3/501—Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification by diffusion
    • C01B3/503—Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification by diffusion characterised by the membrane
    • C01B3/505—Membranes containing palladium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2323/00—Details relating to membrane preparation
    • B01D2323/08—Specific temperatures applied
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/04—Characteristic thickness
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0405—Purification by membrane separation
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0465—Composition of the impurity
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0465—Composition of the impurity
    • C01B2203/047—Composition of the impurity the impurity being carbon monoxide
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0465—Composition of the impurity
    • C01B2203/0475—Composition of the impurity the impurity being carbon dioxide
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0465—Composition of the impurity
    • C01B2203/048—Composition of the impurity the impurity being an organic compound
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0465—Composition of the impurity
    • C01B2203/0495—Composition of the impurity the impurity being water

Definitions

  • the present invention relates to a method of fabricating a hydrogen diffusion membrane that is supported on a porous and gas permeable support.
  • the present invention relates to a method of fabricating supported palladium and palladium alloy layers that are suitable for use as hydrogen diffusion membranes.
  • Hydrogen is an important industrial gas and is used in a number of applications such as ammonia synthesis, methanol synthesis, chemical hydrogenation, metal manufacture, glass processing, fuel cells and semiconductor manufacture. For many applications, the hydrogen must be free of impurities, so techniques for hydrogen purification have been developed. Commercially available hydrogen typically contains impurities including carbon monoxide, carbon dioxide, oxygen, nitrogen, water and methane and it is often necessary to separate these impurities from the hydrogen.
  • One method of purifying hydrogen employs dense metallic layers as hydrogen diffusion membranes.
  • the metallic layers comprise metals that are selectively permeable to hydrogen such as tantalum, vanadium, niobium and palladium.
  • the metallic layer may be an alloy which can include other metals that, in their pure state, possess very limited hydrogen permeability.
  • the formed membranes are selectively permeable, and only hydrogen will pass through. The hydrogen is thus separated from the impurities.
  • Most commercial hydrogen diffusion membranes are palladium alloys, primarily for reasons of durability and achievable hydrogen permeation rate. Commonly used formulations are 77%Pd: 23%Ag and 60%Pd: 40% Cu. Many other binary and multicomponent alloy systems have been the subject of investigations into their hydrogen diffusion properties.
  • porous supports such as porous metallic, porous ceramic or porous glass supports.
  • Porous metallic supports such as those fabricated from stainless steel, are preferred because they are lower cost, have greater mechanical strength and durability, possess thermomechanical properties similar to those of the metallic diffusion membrane and can be more readily joined to other components in a gas purification apparatus.
  • US 6,152,987 discloses a method wherein palladium is deposited onto a porous stainless steel module by an electroless plating process.
  • the stainless steel Prior to the plating process the stainless steel is oxidised by heating in oxygen at 900°C for four hours.
  • the oxidised surface is surface activated by immersing the module in baths of SnCl 2 and PdCl 2 . The immersion is repeated five times, and the activated surface is dried. Following the surface activation the module is immersed in a plating solution. The plating procedure is repeated many times and the total plating time is between 18 and 25 hours.
  • WO 01/53005 discloses another electroless plating process wherein a porous ceramic support is subjected to a cleaning process, an activation process carried out by sequentially circulating acidic SnCl 2 and acidic PdCl 2 solutions inside the porous support, and a deposition process wherein a standard plating bath composition is circulated through the inner surface of the support whilst a gas pressure of l-5psig is maintained on the external surface of the support. The plating procedure is repeated many times to obtain the desired thickness.
  • US 6,379,524 and Nam et al J. Memb. Sci.
  • the known methods for fabricating supported hydrogen diffusion membranes are lengthy and involve a number of different steps. It is an object of the present invention to provide a new method of fabricating a hydrogen diffusion membrane on a support. Ideally, the method should have fewer steps and be simpler and quicker than known methods. In particular, the method should be suitable for forming membranes on stainless steel supports and should be suitable for forming palladium alloy membranes.
  • the present invention provides a method of fabricating a supported hydrogen diffusion membrane comprising the steps of: a) coating a support with a coating composition, thereby providing a coated support, b) firing the coated support at a temperature above 300°C, wherein the coating composition comprises one or more metal powders and a carrier liquid.
  • the carrier liquid which may also be termed the vehicle, must have a sufficient viscosity to hold the particles of the one or more metal powders in dispersion.
  • the method of the invention wherein metal powders in a carrier liquid are coated onto a support, is advantageous when compared to the method disclosed US 6,379,524 wherein nickel powder which is not dispersed in a liquid is coated onto a support.
  • the present invention avoids the safety problems associated with the use of fine, dusty particles and may provide smoother, more even coverage of metal onto the support.
  • the support is any suitable porous material known to the person skilled in the art, such as a metallic, ceramic or glass porous support.
  • the support must be strong enough to withstand typical hydrogen pressures within a hydrogen purification apparatus, and must be durable at temperatures of 400°C and higher.
  • the support may possess a uniform porosity throughout the bulk or possess a porosity that decreases from one side to another.
  • the support is a metallic porous support, most preferably stainless steel.
  • the support may be of any geometry such as a flat sheet, but is preferably a tube.
  • a suitable support is a porous 316L stainless steel tube available from Mott Metallurgical Corporation (Conn., USA). Other suppliers of porous metal bodies include Pall Corporation (USA) and the Sinter Metals Division of GKN pic (USA).
  • the support may be coated on one side only, but may also be coated on both sides.
  • the method of the present invention provides a method of fabricating a palladium or palladium alloy membrane.
  • Preferred alloying metals include silver, gold and copper, preferably silver. In some formulations, more than one alloying metal may be used. The preferred amount of the alloying metal is well known to the skilled person, but generally will be less than 50wt%.
  • the thickness of the membrane is suitably less than lOO ⁇ m, preferably less than
  • the membrane It is desirable to make the membrane as thin as possible.
  • the rate of hydrogen permeation is proportional to the thickness of the membrane; decreasing the membrane thickness will result in higher hydrogen permeation rates, whilst at the same time reducing the amount of any expensive precious metal components.
  • the membrane must remain mechanically durable and capable of separating hydrogen with acceptable selectivity, suggesting that a minimum thickness is necessary to fulfil these criteria.
  • a thickness of approximately 1 -2 ⁇ m may form that lower limit.
  • the coating composition comprises one or more metal powders chosen from the metals which will make up the hydrogen diffusion membrane.
  • metal powders is used to describe all metal particles irrespective of aspect ratio.
  • the metal powders include one or more chosen from palladium, silver, copper and gold.
  • the coating composition comprises palladium and silver powders, palladium and copper powders, or palladium and gold powders, most preferably palladium and silver powders.
  • the coating composition comprises metal particles in the ratio that is optimum for operation of a palladium-based hydrogen diffusion membrane, eg 77%Pd: 23%Ag or 60%Pd: 40% Cu.
  • the average particle size of the metal powders is suitably in the range 0.001-50 ⁇ m, preferably 0.1-10 ⁇ m.
  • Each metal powder may have a uniform particle size distribution or may be present as a range of particle size distributions. There is no particularly preferred aspect ratio for the powders for the purpose of this invention.
  • the particles may, for example, be spherical, flake, columnar, or fibrous but are not limited to these forms.
  • the particle size distributions of the different metal powders are not required to be similar in the initial coating composition, but may in the course of further processing, be rendered to a similar state.
  • the amount of each metal powder in the coating composition is suitably 10-100wt%, preferably 10-90wt% as a percentage of the solid content of the composition.
  • the carrier liquid allows the uniform dispersion of the fine metal particles and provides a coating composition with an appropriate viscosity for the coating step, allowing smooth, even coating onto the support.
  • the carrier liquid is an organic solvent or an aqueous solvent and is neither adversely affected by nor adversely affects the other components of the coating composition.
  • the carrier liquid is an organic solvent.
  • Commonly used carrier liquids include terpineol, butylcarbitol, ethyl cellulose and butylcarbitol acetate.
  • the coating composition further comprises a binder. The function of the binder is to improve the interaction between the metal powders and the support and the interaction between the metal particles in the layer.
  • the binder is an inorganic binder and is suitably a frit material.
  • Frit materials are ceramic-type compositions that have been fused, quenched to form a glass and granulated. They are well known to those skilled in the art. They comprise a variety of oxides which may include SiO 2 , B 2 O 3 , Al 2 O 3 , Na 2 O, Li 2 O, ZrO 2 , ZnO, CaO, although other oxide materials are also commonly used. Suitable frits are described in US 4,835,038. A preferred frit material is a borosilicate material.
  • the inorganic binder material is suitably particulate and the average particle size of the inorganic binder is suitably in the range 0.001-50 ⁇ m, preferably 0.1-10 ⁇ m.
  • the particles may, for example, be spherical, flake, columnar or fibrous but are not limited to these forms.
  • the total amount of inorganic binder material in the coating composition is suitably 0.1-50wt%, preferably between 5-20wt% as a percentage of the solid content of the composition.
  • the coating composition comprises a binder which is an organic binder such as a polymeric binder.
  • the coating composition may comprise both an inorganic binder and an organic binder.
  • Organic binders can alter the viscosity and viscoelastic nature of the coating composition to allow improved coating onto the porous support. They can also provide green strength to the applied composition prior to the heat treatment to produce the final metal layer.
  • Suitable coating compositions comprising one or more metal powders, optionally one or more inorganic binders, optionally one or more organic binders and a carrier liquid can be purchased from several suppliers.
  • conductive pastes Such compositions are referred to as “conductive pastes”, “end termination pastes” or “cermet pastes” in the electronics industry and are used, although not exclusively, to form conductive tracks and/or electrodes.
  • Such formulations can be more generally known as “thick film pastes” but similar formulations can be called “thin film pastes”.
  • compositions may also be formulated for use outside the electronics industry, for example within the decorative/pigment industry where they can typically, but not exclusively, be known as "decorative metal pastes", “decorative precious metal pastes”, “functional pastes” or “functional metal pastes”.
  • Typical commercial paste formulations for the electronics industry may consist of a high (20,000-250,000mPa.s) viscosity dispersion of particles of a functional or decorative phase and a frit binder.
  • a high viscosity dispersion of particles of a functional or decorative phase and a frit binder.
  • the amount of solid material within the commercially available pastes is within the range of 10-90wt%, but more typically within the range 50-80wt%, based on the weight of the paste. However, for some processing applications, it is necessary to alter the proportion of solid material within the paste, typically to aid coating processing or to improve the appearance or thickness of the coated layer, and the amount of solid material in the final coating composition may be below 50wt%.
  • manufacturers of commercial pastes or compositions will supply solutions for dilution, formulated to be compatible with specific paste formulations. Suppliers of such functional pastes or compositions include Johnson Matthey pic (UK), E.I. Du Pont de Nemours and Company (USA) and Heraeus Inc.
  • the support is coated with the coating composition, preferably on one side only.
  • the means of coating the support may depend upon the geometry of the support.
  • a flat support may be coated by dip-coating, spray-coating or screen-printing.
  • the preferred tubular geometry may be coated by spray-coating and dip-coating, although the invention is not limited to these coating techniques, and may also include painting and "wash-coating".
  • the last technique may, for example, consist of holding a reservoir of the paste above the support, and releasing the reservoir such that the paste flows over the surface of the support, providing a uniform coating, either in the outside or on the inside of the support.
  • Such an apparatus is typically used for coating ceramic or metallic monolith supports, used as catalyst supports for exhaust emission reduction catalysts for internal combustion engines.
  • Variations on the "washcoating” technique, for coating internal and external surfaces are known and appreciated by those familiar with the field. Such variations include application of an electrostatic field to attract the metal particles to the surface of the support.
  • the preferred means of coating for a tubular support is dip-coating, in which the support is held and dipped into a suitable volume of the coating composition, remaining within the composition for a period of time if required, and then extracted from the coating composition.
  • the rate of dipping and extraction depend upon the nature of the surface of the support.
  • a partial vacuum may be applied to the uncoated side of the support during and/or after the coating process.
  • pressure can be applied to the coated surface, using for example, air or gas pressure, or physical pressure such as a metallic or polymeric blade.
  • the support may be subjected to surface or bulk pre-treatments prior to the coating process, as a means of improving the quality of the coating and/or removing surface contaminants that may affect the quality of the coating and/or cause shortening of lifetime or other problems with the subsequently formed diffusion membrane.
  • the nature of the pre-treatment will depend upon the nature of the support being coated. For example, ceramic supports may be thermally oxidised to remove organic contaminants whilst metallic supports, such as stainless steel, may be degreased and cleaned using standard procedures, as outlined typically in ASTM A 380-99 and Annexes.
  • the support can be coated once, or more than once to increase the amount of deposited coating material and/or improve the quality of the coated layer.
  • the coated support is suitably dried.
  • the drying may be at ambient temperature or at elevated temperatures, in air or in an inert atmosphere, typically using recommended product-drying process specifications of 10 minutes at 150°C in a box oven or 8-10 minutes at a peak of 180°C in an infra-red drier.
  • the coated support is fired at a temperature above 300°C to improve the adhesion of the coated layer to the support.
  • Another function of the firing cycle is to remove any organic components that were in the coating composition; thus, at least part of the firing cycle may be spent in an oxidising atmosphere such as air.
  • part of the firing cycle may be carried out in a reducing atmosphere such as nitrogen or hydrogen-nitrogen gas mixes.
  • the firing step is carried out above 300°C, suitably between 600-1000°C, preferably between 750-850°C.
  • the temperature is ramped up from room temperature to the firing temperature at 30°C a minute, the temperature is maintained at the firing temperature for six minutes, and then the temperature is ramped down to room temperature.
  • the firing cycle can be a standard "bell-shaped" firing cycle or the rate of temperature increase and decrease can be different. The exact details of the firing cycle will be dependent upon the composition of the coating formulation and the recommendations of the commercial manufacturers and or suppliers.
  • the firing is suitably carried out in air, but in a preferred embodiment for stainless steel porous supports, a nitrogen rich, "reducing" gas is supplied to the furnace before the temperature reaches the maximum firing temperature.
  • a nitrogen rich, "reducing" gas is supplied to the furnace before the temperature reaches the maximum firing temperature.
  • One function of the presence of the reducing gas at elevated temperatures is to limit the degree of formation of metal oxide upon the surface of the stainless steel particles, preferably within the bulk of the stainless steel porous support. Formation of oxide on the surface of the steel particles results in an increase in the volume of those particles; this in turn causes a decrease in the pore volume of the body of the steel support and in turn can limit the gas permeation rate of the porous steel support. This can limit the effectiveness of the deposited metal diffusion membrane.
  • the coated layer formed on the surface of the support by the above described method may not be sufficiently continuous to function as a membrane. Such discontinuities in the coated layer can arise, for example, if incorrect firing procedures are used or if the surface of the porous support is excessively uneven.
  • the coating composition used may not be of the preferred formulation to produce a palladium-based separation membrane, for example 77%Pd: 23%Ag or 60%Pd: 40%Cu, but may, for example, be palladium deficient. Therefore, in a preferred embodiment the method of fabricating the hydrogen diffusion membrane comprises a further step which is after the firing step, (b), namely c) depositing metal onto the coated support. Depositing further metal onto the coated support will ensure a continuous metal layer that can function as a hydrogen diffusion membrane.
  • the metal is chosen from the metals that will make up the hydrogen diffusion membrane, so is suitably one or more metals chosen from palladium, silver, copper, gold.
  • palladium is deposited onto the coated support.
  • the deposition can be carried out using any deposition process known to the skilled person such as electroless plating, electroplating, chemical vapour deposition or sputtering, but is preferably an electroless plating process. Electroless plating processes are well known to the skilled person.
  • the coated support is suitably immersed in a bath that, in a preferred embodiment, comprises a palladium salt, for example palladium chloride, ammonia, ethylenediaminetetraaceticacid (EDTA) and hydrazine.
  • EDTA ethylenediaminetetraaceticacid
  • the deposition bath may be maintained at ambient temperature or at elevated temperature, but preferably at ambient temperature, and may be stirred to maintain bath homogeneity and assist the coating process.
  • the part to be plated may be held stationary or may be revolved within the bath to assist the coating process. Deposition of palladium may continue until the metal concentration in the bath is substantially reduced, for example over several hours. Alternatively, a sequence of baths may be used where the part is immersed for shorter periods of time and removed before substantial reduction of the metal concentration in the bath occurs.
  • deposition step There may be more than one deposition step, e.g. palladium could be deposited in a first deposition step and subsequently in a second deposition step. Alternatively, silver could be deposited in a subsequent deposition step.
  • a similar plating bath can be used for silver deposition, in which the palladium salt is replaced by a suitable silver salt, for example, silver nitrate.
  • the deposition step (c) includes an activation step, so that the step after the firing step (b) is c) activating the surface of the coated support with metal seed particles and depositing metal onto the activated, coated support.
  • Activating the surface can reduce the induction period at the start of the deposition process, increase the rate of the deposition process and increase the uniformity of the metal coating.
  • the metal seed particles are the same metal as the metal deposited onto the coated support.
  • the coated support can be immersed in an acidic SnCl 2 bath to sensitise the surface and subsequently immersed in an acidic PdCl 2 bath to seed the surface of the support with palladium nucleii. Rinsing with deionised water is carried out after each immersion. This sequence can be performed once or repeated several times to increase the number of palladium seed crystals formed on the surface of the coated support.
  • a dispersion of metal particles for example a metal colloid solution comprising metal nanoparticles, can be applied to the surface of the coated support. The deposited metal particles can stick to the surface of the coated support and act as seed crystals.
  • the activation and deposition step (c) is followed by a further activation and deposition step.
  • palladium seed crystals could be deposited, followed by electroless deposition of palladium, and then silver seed crystals could be deposited, followed by electroless deposition of silver.
  • the coating composition does not comprise metal powders in the ratio most favourable for use as a hydrogen diffusion membrane and further metal is deposited in a step (c), then it may be necessary to subject the fully-coated support to a process to promote alloying of the metals, so that the method comprises a further step: d) thermal treatment to homogenise the metals deposited in steps (a) and (c).
  • This thermal treatment could be performed, for example, during operation of the metal layer as a hydrogen diffusion membrane when the metals would intermix until a uniform layer composition had been reached.
  • operating temperatures for hydrogen separation processes using palladium-based membranes are within the range 350 - 450°C, temperatures at which homogenisation can occur.
  • the mixed- metal layer can be fully homogenised off-line in a separate process, by exposure to elevated temperatures, typically up to a maximum in the range 700-800°C.
  • this process can be carried out in an inert or noble gas atmosphere or a mildly reducing atmosphere.
  • the method of the present invention provides a quick and simple method for fabricating supported palladium or palladium alloy layers that may function as hydrogen diffusion membranes.
  • the coating composition comprises a frit material.
  • the frit material is generally found in an intermediate layer adjacent to the porous support and below the hydrogen diffusion layer. Therefore, in a further aspect the present invention provides a hydrogen diffusion membrane comprising a porous support and a metallic membrane layer, wherein there is an intermediate layer between the support and the membrane layer and the intermediate layer comprises fused glass that has been formed by heating a frit material.
  • the coating composition used in the examples was M3274B silver/palladium end termination paste, available from Johnson Matthey pic, UK.
  • the composition specification is: Ag:Pd ratio 3.5 : 1 Solids content 79% Viscosity 25,000cps +/- 3,000 cps Recommended thinners JM2
  • the paste comprises silver and palladium powders, a frit material and an organic solvent.
  • a layer of the M3274 PdAg paste was coated onto the external surface of a length of pre-cleaned, degreased 1" diameter porous stainless steel tube (Media Grade 0.2 - Mott Metallurgical, USA), using a dip-coating technique with vacuum applied to the internal surface of the tube during a portion of the dipping period.
  • the viscosity of the M3274 paste was reduced by addition of JM2 thinners. The solution was stirred from below in order to maintain particle suspension.
  • the coated piece was a uniform grey/silver colour after dipping and was fired in air to 780°C using a standard bell-shaped firing profile.
  • the adhesion and scratch resistance of the resulting coating was very high.
  • the measured nitrogen permeability, indicative of the nitrogen leak, was between 2-3m /m .h.atm .
  • a stainless steel tube was coated according to the method of Example 1 except that a modified firing process was employed.
  • the tube was coated by a dip-coating procedure including the application of a vacuum during a portion of the dipping process.
  • the tube was dried in air, then fired on a modified cycle to inhibit excessive oxidation of the underlying steel substrate.
  • the modified cycle involved heating the tube to 500°C in air and dwelling at that temperature for a period of time to ensure complete oxidation of the process organics.
  • the purge gas was then changed to nitrogen, and the part heated to 780°C under the standard procedure temperature ramp cycle.
  • the part was also cooled in nitrogen.
  • the measured nitrogen permeabilities of tubes coated and fired by this route were between 1-2.5 m 3 /m 2 .hr.atm 1 5 .
  • Porous stainless steel tubes coated according to Example 2 were activated by sequential dipping into fresh acidic solutions of SnCl 2 and PdCl 2j washing with deionised water after each step.
  • the tubes were air-dried at 70°C for one hour prior to immersion in a solution comprising palladium chloride, ethylenediamminetetraacetic acid (EDTA), ammonia, water and hydrazine, maintained at 25°C with moderate stirring.
  • the amount of palladium deposited was varied by varying the period spent in the deposition bath, with typically l-3 ⁇ m of palladium being the thickness achieved after 3 hours immersion.
  • Example 3 was repeated except that there was no activation step.
  • Porous stainless steel tubes coated according to Example 2 were immersed directly in an electroless plating composition comprising palladium chloride, ethylenediamminetetraacetic acid (EDTA), ammonia, water and hydrazine in concentrations typically used for electroless deposition.
  • the bath was maintained at 25°C, with moderate stirring.
  • Microscopic analysis shows that the deposited palladium had penetrated into the residual void space in the Pd/Ag coated layer, with the measured nitrogen permeabilities now reduced to between 0.08 - 0.15 m 3 /m 2 .hr.atm 1 5 .
  • Homogenisation of the system was achieved by holding the parts at 600°C in a 5%H 2 -N 2 atmosphere for 12 hours. The measured nitrogen permeabilities were of the same magnitude as those of the unhomogenised samples.

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Abstract

A method of fabricating a hydrogen diffusion membrane that is supported on a porous and gas permeable support is disclosed. The method comprises steps wherein a support is coated with a coating composition and is subsequently fired at a temperature above 300° C. The coating composition comprises one or more metal powders a carrier liquid, and optionally an inorganic or organic binder.

Description

METHOD OF FABRICATING A HYDROGEN DIFFUSION MEMBRANE
The present invention relates to a method of fabricating a hydrogen diffusion membrane that is supported on a porous and gas permeable support. In particular the present invention relates to a method of fabricating supported palladium and palladium alloy layers that are suitable for use as hydrogen diffusion membranes.
Hydrogen is an important industrial gas and is used in a number of applications such as ammonia synthesis, methanol synthesis, chemical hydrogenation, metal manufacture, glass processing, fuel cells and semiconductor manufacture. For many applications, the hydrogen must be free of impurities, so techniques for hydrogen purification have been developed. Commercially available hydrogen typically contains impurities including carbon monoxide, carbon dioxide, oxygen, nitrogen, water and methane and it is often necessary to separate these impurities from the hydrogen.
One method of purifying hydrogen employs dense metallic layers as hydrogen diffusion membranes. The metallic layers comprise metals that are selectively permeable to hydrogen such as tantalum, vanadium, niobium and palladium. The metallic layer may be an alloy which can include other metals that, in their pure state, possess very limited hydrogen permeability. The formed membranes are selectively permeable, and only hydrogen will pass through. The hydrogen is thus separated from the impurities. Most commercial hydrogen diffusion membranes are palladium alloys, primarily for reasons of durability and achievable hydrogen permeation rate. Commonly used formulations are 77%Pd: 23%Ag and 60%Pd: 40% Cu. Many other binary and multicomponent alloy systems have been the subject of investigations into their hydrogen diffusion properties.
Commercially available, self-supporting palladium-based hydrogen separation membranes may have a thickness in the range 25-100μm, but more commonly between 50-70μm. The mechanical strength and durability of these membranes, typically tubular in form, is such that they are able to operate as hydrogen separation membranes with only limited requirement for physical support.
The successful development of durable palladium-based membranes having reduced wall thicknesses will result in membranes having higher hydrogen permeation rates whilst reducing the cost of the precious metal component. The mechanical properties of such membranes may require that they are supported on porous supports such as porous metallic, porous ceramic or porous glass supports. Porous metallic supports, such as those fabricated from stainless steel, are preferred because they are lower cost, have greater mechanical strength and durability, possess thermomechanical properties similar to those of the metallic diffusion membrane and can be more readily joined to other components in a gas purification apparatus.
Several methods are known in the art for depositing palladium and/or other metals, either sequentially or concurrently, onto porous supports. These typically include, but are not limited to, electroless plating, chemical vapour deposition and sputtering.
US 6,152,987 discloses a method wherein palladium is deposited onto a porous stainless steel module by an electroless plating process. Prior to the plating process the stainless steel is oxidised by heating in oxygen at 900°C for four hours. The oxidised surface is surface activated by immersing the module in baths of SnCl2 and PdCl2. The immersion is repeated five times, and the activated surface is dried. Following the surface activation the module is immersed in a plating solution. The plating procedure is repeated many times and the total plating time is between 18 and 25 hours.
WO 01/53005 discloses another electroless plating process wherein a porous ceramic support is subjected to a cleaning process, an activation process carried out by sequentially circulating acidic SnCl2 and acidic PdCl2 solutions inside the porous support, and a deposition process wherein a standard plating bath composition is circulated through the inner surface of the support whilst a gas pressure of l-5psig is maintained on the external surface of the support. The plating procedure is repeated many times to obtain the desired thickness. US 6,379,524 and Nam et al (J. Memb. Sci. 153, 1999, 163-173) disclose a process for manufacturing a supported Pd-Ni membrane wherein in a first step, fine nickel powder is dispersed on the surface of a stainless steel support and is then heated at 800°C for 5 hours. Subsequently the support is activated by treatment with sulphuric acid and immersed in a plating bath containing Pd and Ni salts.
The known methods for fabricating supported hydrogen diffusion membranes are lengthy and involve a number of different steps. It is an object of the present invention to provide a new method of fabricating a hydrogen diffusion membrane on a support. Ideally, the method should have fewer steps and be simpler and quicker than known methods. In particular, the method should be suitable for forming membranes on stainless steel supports and should be suitable for forming palladium alloy membranes.
Accordingly the present invention provides a method of fabricating a supported hydrogen diffusion membrane comprising the steps of: a) coating a support with a coating composition, thereby providing a coated support, b) firing the coated support at a temperature above 300°C, wherein the coating composition comprises one or more metal powders and a carrier liquid.
The carrier liquid, which may also be termed the vehicle, must have a sufficient viscosity to hold the particles of the one or more metal powders in dispersion. The method of the invention, wherein metal powders in a carrier liquid are coated onto a support, is advantageous when compared to the method disclosed US 6,379,524 wherein nickel powder which is not dispersed in a liquid is coated onto a support. The present invention avoids the safety problems associated with the use of fine, dusty particles and may provide smoother, more even coverage of metal onto the support. The support is any suitable porous material known to the person skilled in the art, such as a metallic, ceramic or glass porous support. The support must be strong enough to withstand typical hydrogen pressures within a hydrogen purification apparatus, and must be durable at temperatures of 400°C and higher. The support may possess a uniform porosity throughout the bulk or possess a porosity that decreases from one side to another. Preferably the support is a metallic porous support, most preferably stainless steel. The support may be of any geometry such as a flat sheet, but is preferably a tube. A suitable support is a porous 316L stainless steel tube available from Mott Metallurgical Corporation (Conn., USA). Other suppliers of porous metal bodies include Pall Corporation (USA) and the Sinter Metals Division of GKN pic (USA). The support may be coated on one side only, but may also be coated on both sides.
Suitably the method of the present invention provides a method of fabricating a palladium or palladium alloy membrane. Preferred alloying metals include silver, gold and copper, preferably silver. In some formulations, more than one alloying metal may be used. The preferred amount of the alloying metal is well known to the skilled person, but generally will be less than 50wt%. The thickness of the membrane is suitably less than lOOμm, preferably less than
50μm. It is desirable to make the membrane as thin as possible. The rate of hydrogen permeation is proportional to the thickness of the membrane; decreasing the membrane thickness will result in higher hydrogen permeation rates, whilst at the same time reducing the amount of any expensive precious metal components. However, the membrane must remain mechanically durable and capable of separating hydrogen with acceptable selectivity, suggesting that a minimum thickness is necessary to fulfil these criteria. A thickness of approximately 1 -2μm may form that lower limit.
The coating composition comprises one or more metal powders chosen from the metals which will make up the hydrogen diffusion membrane. The term "metal powders" is used to describe all metal particles irrespective of aspect ratio. Suitably the metal powders include one or more chosen from palladium, silver, copper and gold. Preferably the coating composition comprises palladium and silver powders, palladium and copper powders, or palladium and gold powders, most preferably palladium and silver powders. In an especially preferred embodiment, the coating composition comprises metal particles in the ratio that is optimum for operation of a palladium-based hydrogen diffusion membrane, eg 77%Pd: 23%Ag or 60%Pd: 40% Cu. The average particle size of the metal powders is suitably in the range 0.001-50μm, preferably 0.1-10μm. Each metal powder may have a uniform particle size distribution or may be present as a range of particle size distributions. There is no particularly preferred aspect ratio for the powders for the purpose of this invention. The particles may, for example, be spherical, flake, columnar, or fibrous but are not limited to these forms. The particle size distributions of the different metal powders are not required to be similar in the initial coating composition, but may in the course of further processing, be rendered to a similar state. The amount of each metal powder in the coating composition is suitably 10-100wt%, preferably 10-90wt% as a percentage of the solid content of the composition.
The carrier liquid allows the uniform dispersion of the fine metal particles and provides a coating composition with an appropriate viscosity for the coating step, allowing smooth, even coating onto the support. Suitably the carrier liquid is an organic solvent or an aqueous solvent and is neither adversely affected by nor adversely affects the other components of the coating composition. Preferably the carrier liquid is an organic solvent. Commonly used carrier liquids include terpineol, butylcarbitol, ethyl cellulose and butylcarbitol acetate. Suitably the coating composition further comprises a binder. The function of the binder is to improve the interaction between the metal powders and the support and the interaction between the metal particles in the layer. In one embodiment of the invention, the binder is an inorganic binder and is suitably a frit material. Frit materials are ceramic-type compositions that have been fused, quenched to form a glass and granulated. They are well known to those skilled in the art. They comprise a variety of oxides which may include SiO2, B2O3, Al2O3, Na2O, Li2O, ZrO2, ZnO, CaO, although other oxide materials are also commonly used. Suitable frits are described in US 4,835,038. A preferred frit material is a borosilicate material. The inorganic binder material is suitably particulate and the average particle size of the inorganic binder is suitably in the range 0.001-50μm, preferably 0.1-10μm. The particles may, for example, be spherical, flake, columnar or fibrous but are not limited to these forms. The total amount of inorganic binder material in the coating composition is suitably 0.1-50wt%, preferably between 5-20wt% as a percentage of the solid content of the composition. In an alternative embodiment of the invention, the coating composition comprises a binder which is an organic binder such as a polymeric binder. The coating composition may comprise both an inorganic binder and an organic binder. Organic binders can alter the viscosity and viscoelastic nature of the coating composition to allow improved coating onto the porous support. They can also provide green strength to the applied composition prior to the heat treatment to produce the final metal layer.
Suitable coating compositions comprising one or more metal powders, optionally one or more inorganic binders, optionally one or more organic binders and a carrier liquid can be purchased from several suppliers. Typically, such compositions are referred to as "conductive pastes", "end termination pastes" or "cermet pastes" in the electronics industry and are used, although not exclusively, to form conductive tracks and/or electrodes. Such formulations can be more generally known as "thick film pastes" but similar formulations can be called "thin film pastes". Such compositions may also be formulated for use outside the electronics industry, for example within the decorative/pigment industry where they can typically, but not exclusively, be known as "decorative metal pastes", "decorative precious metal pastes", "functional pastes" or "functional metal pastes". Typical commercial paste formulations for the electronics industry may consist of a high (20,000-250,000mPa.s) viscosity dispersion of particles of a functional or decorative phase and a frit binder. Often included, typically to ease or improve deposition processes and/or the final coated layer, are one or more polymeric binders, a carrier liquid or "vehicle", and modifiers such as surface wetting and flow-control aids. The amount of solid material within the commercially available pastes is within the range of 10-90wt%, but more typically within the range 50-80wt%, based on the weight of the paste. However, for some processing applications, it is necessary to alter the proportion of solid material within the paste, typically to aid coating processing or to improve the appearance or thickness of the coated layer, and the amount of solid material in the final coating composition may be below 50wt%. Typically, manufacturers of commercial pastes or compositions will supply solutions for dilution, formulated to be compatible with specific paste formulations. Suppliers of such functional pastes or compositions include Johnson Matthey pic (UK), E.I. Du Pont de Nemours and Company (USA) and Heraeus Inc.
The support is coated with the coating composition, preferably on one side only. The means of coating the support may depend upon the geometry of the support. For example, a flat support may be coated by dip-coating, spray-coating or screen-printing. The preferred tubular geometry may be coated by spray-coating and dip-coating, although the invention is not limited to these coating techniques, and may also include painting and "wash-coating". The last technique, may, for example, consist of holding a reservoir of the paste above the support, and releasing the reservoir such that the paste flows over the surface of the support, providing a uniform coating, either in the outside or on the inside of the support. Such an apparatus is typically used for coating ceramic or metallic monolith supports, used as catalyst supports for exhaust emission reduction catalysts for internal combustion engines. Variations on the "washcoating" technique, for coating internal and external surfaces, are known and appreciated by those familiar with the field. Such variations include application of an electrostatic field to attract the metal particles to the surface of the support.
The preferred means of coating for a tubular support is dip-coating, in which the support is held and dipped into a suitable volume of the coating composition, remaining within the composition for a period of time if required, and then extracted from the coating composition. The rate of dipping and extraction depend upon the nature of the surface of the support. In order to improve the quality of the coating, and/or increase the penetration of the metal particles into the support surface pores, a partial vacuum may be applied to the uncoated side of the support during and/or after the coating process. Alternatively, pressure can be applied to the coated surface, using for example, air or gas pressure, or physical pressure such as a metallic or polymeric blade.
The support may be subjected to surface or bulk pre-treatments prior to the coating process, as a means of improving the quality of the coating and/or removing surface contaminants that may affect the quality of the coating and/or cause shortening of lifetime or other problems with the subsequently formed diffusion membrane. The nature of the pre-treatment will depend upon the nature of the support being coated. For example, ceramic supports may be thermally oxidised to remove organic contaminants whilst metallic supports, such as stainless steel, may be degreased and cleaned using standard procedures, as outlined typically in ASTM A 380-99 and Annexes.
The support can be coated once, or more than once to increase the amount of deposited coating material and/or improve the quality of the coated layer. After the coating process, the coated support is suitably dried. The drying may be at ambient temperature or at elevated temperatures, in air or in an inert atmosphere, typically using recommended product-drying process specifications of 10 minutes at 150°C in a box oven or 8-10 minutes at a peak of 180°C in an infra-red drier. After suitable drying the coated support is fired at a temperature above 300°C to improve the adhesion of the coated layer to the support. Another function of the firing cycle is to remove any organic components that were in the coating composition; thus, at least part of the firing cycle may be spent in an oxidising atmosphere such as air. In an alternative embodiment, part of the firing cycle may be carried out in a reducing atmosphere such as nitrogen or hydrogen-nitrogen gas mixes. The firing step is carried out above 300°C, suitably between 600-1000°C, preferably between 750-850°C. In a preferred embodiment, the temperature is ramped up from room temperature to the firing temperature at 30°C a minute, the temperature is maintained at the firing temperature for six minutes, and then the temperature is ramped down to room temperature. The firing cycle can be a standard "bell-shaped" firing cycle or the rate of temperature increase and decrease can be different. The exact details of the firing cycle will be dependent upon the composition of the coating formulation and the recommendations of the commercial manufacturers and or suppliers. The firing is suitably carried out in air, but in a preferred embodiment for stainless steel porous supports, a nitrogen rich, "reducing" gas is supplied to the furnace before the temperature reaches the maximum firing temperature. One function of the presence of the reducing gas at elevated temperatures is to limit the degree of formation of metal oxide upon the surface of the stainless steel particles, preferably within the bulk of the stainless steel porous support. Formation of oxide on the surface of the steel particles results in an increase in the volume of those particles; this in turn causes a decrease in the pore volume of the body of the steel support and in turn can limit the gas permeation rate of the porous steel support. This can limit the effectiveness of the deposited metal diffusion membrane.
The coated layer formed on the surface of the support by the above described method may not be sufficiently continuous to function as a membrane. Such discontinuities in the coated layer can arise, for example, if incorrect firing procedures are used or if the surface of the porous support is excessively uneven. Secondly, the coating composition used may not be of the preferred formulation to produce a palladium-based separation membrane, for example 77%Pd: 23%Ag or 60%Pd: 40%Cu, but may, for example, be palladium deficient. Therefore, in a preferred embodiment the method of fabricating the hydrogen diffusion membrane comprises a further step which is after the firing step, (b), namely c) depositing metal onto the coated support. Depositing further metal onto the coated support will ensure a continuous metal layer that can function as a hydrogen diffusion membrane.
The metal is chosen from the metals that will make up the hydrogen diffusion membrane, so is suitably one or more metals chosen from palladium, silver, copper, gold. In a preferred embodiment, palladium is deposited onto the coated support. The deposition can be carried out using any deposition process known to the skilled person such as electroless plating, electroplating, chemical vapour deposition or sputtering, but is preferably an electroless plating process. Electroless plating processes are well known to the skilled person. The coated support is suitably immersed in a bath that, in a preferred embodiment, comprises a palladium salt, for example palladium chloride, ammonia, ethylenediaminetetraaceticacid (EDTA) and hydrazine. The deposition bath may be maintained at ambient temperature or at elevated temperature, but preferably at ambient temperature, and may be stirred to maintain bath homogeneity and assist the coating process. The part to be plated may be held stationary or may be revolved within the bath to assist the coating process. Deposition of palladium may continue until the metal concentration in the bath is substantially reduced, for example over several hours. Alternatively, a sequence of baths may be used where the part is immersed for shorter periods of time and removed before substantial reduction of the metal concentration in the bath occurs.
There may be more than one deposition step, e.g. palladium could be deposited in a first deposition step and subsequently in a second deposition step. Alternatively, silver could be deposited in a subsequent deposition step. A similar plating bath can be used for silver deposition, in which the palladium salt is replaced by a suitable silver salt, for example, silver nitrate.
In a particular embodiment, the deposition step (c) includes an activation step, so that the step after the firing step (b) is c) activating the surface of the coated support with metal seed particles and depositing metal onto the activated, coated support. Activating the surface can reduce the induction period at the start of the deposition process, increase the rate of the deposition process and increase the uniformity of the metal coating. Suitably the metal seed particles are the same metal as the metal deposited onto the coated support.
Activation methods are well known to the person skilled in the art. For example, the coated support can be immersed in an acidic SnCl2 bath to sensitise the surface and subsequently immersed in an acidic PdCl2 bath to seed the surface of the support with palladium nucleii. Rinsing with deionised water is carried out after each immersion. This sequence can be performed once or repeated several times to increase the number of palladium seed crystals formed on the surface of the coated support. Alternatively, a dispersion of metal particles, for example a metal colloid solution comprising metal nanoparticles, can be applied to the surface of the coated support. The deposited metal particles can stick to the surface of the coated support and act as seed crystals.
In a particular embodiment of the invention, the activation and deposition step (c) is followed by a further activation and deposition step. For example, palladium seed crystals could be deposited, followed by electroless deposition of palladium, and then silver seed crystals could be deposited, followed by electroless deposition of silver.
If the coating composition does not comprise metal powders in the ratio most favourable for use as a hydrogen diffusion membrane and further metal is deposited in a step (c), then it may be necessary to subject the fully-coated support to a process to promote alloying of the metals, so that the method comprises a further step: d) thermal treatment to homogenise the metals deposited in steps (a) and (c). This thermal treatment could be performed, for example, during operation of the metal layer as a hydrogen diffusion membrane when the metals would intermix until a uniform layer composition had been reached. Typically, operating temperatures for hydrogen separation processes using palladium-based membranes are within the range 350 - 450°C, temperatures at which homogenisation can occur. Alternatively, the mixed- metal layer can be fully homogenised off-line in a separate process, by exposure to elevated temperatures, typically up to a maximum in the range 700-800°C. In a preferred embodiment, this process can be carried out in an inert or noble gas atmosphere or a mildly reducing atmosphere.
The method of the present invention provides a quick and simple method for fabricating supported palladium or palladium alloy layers that may function as hydrogen diffusion membranes.
In a preferred embodiment of method of the present invention the coating composition comprises a frit material. After the firing step (b), the frit material is generally found in an intermediate layer adjacent to the porous support and below the hydrogen diffusion layer. Therefore, in a further aspect the present invention provides a hydrogen diffusion membrane comprising a porous support and a metallic membrane layer, wherein there is an intermediate layer between the support and the membrane layer and the intermediate layer comprises fused glass that has been formed by heating a frit material.
The invention will now be described by reference to examples that are illustrative and not limiting of the invention. Examples
The coating composition used in the examples was M3274B silver/palladium end termination paste, available from Johnson Matthey pic, UK. The composition specification is: Ag:Pd ratio 3.5 : 1 Solids content 79% Viscosity 25,000cps +/- 3,000 cps Recommended thinners JM2 The paste comprises silver and palladium powders, a frit material and an organic solvent.
EXAMPLE 1
A layer of the M3274 PdAg paste was coated onto the external surface of a length of pre-cleaned, degreased 1" diameter porous stainless steel tube (Media Grade 0.2 - Mott Metallurgical, USA), using a dip-coating technique with vacuum applied to the internal surface of the tube during a portion of the dipping period. The viscosity of the M3274 paste was reduced by addition of JM2 thinners. The solution was stirred from below in order to maintain particle suspension.
The coated piece was a uniform grey/silver colour after dipping and was fired in air to 780°C using a standard bell-shaped firing profile. The adhesion and scratch resistance of the resulting coating was very high. The measured nitrogen permeability, indicative of the nitrogen leak, was between 2-3m /m .h.atm .
Lengths of cleaned Mott porous stainless steel tube were subjected to the same heat treatment without the PdAg coating formulation; the measured nitrogen permeabilities were found to be between 9-17 m3/m2.hr.atm1'5. The porous stainless steel tube is typically found to have nitrogen permeabilities between 67-77 m3/m2.hr.atm1,5 in the as-received state, suggesting that excessive oxidation of the underlying steel was occurring during the firing in air and causing blockage of the substrate pore structure. Cross-sectional analysis by electron microscopy showed the coated layer to follow the surface contours of the steel substrate very well, with a non-metallic frit layer lying between the Pd/Ag layer and the underlying steel surface. The coated Pd/Ag layer showed a moderate degree of porosity, responsible for the measured nitrogen permeability.
EXAMPLE 2
A stainless steel tube was coated according to the method of Example 1 except that a modified firing process was employed. The tube was coated by a dip-coating procedure including the application of a vacuum during a portion of the dipping process.
The tube was dried in air, then fired on a modified cycle to inhibit excessive oxidation of the underlying steel substrate. The modified cycle involved heating the tube to 500°C in air and dwelling at that temperature for a period of time to ensure complete oxidation of the process organics. The purge gas was then changed to nitrogen, and the part heated to 780°C under the standard procedure temperature ramp cycle. The part was also cooled in nitrogen. The measured nitrogen permeabilities of tubes coated and fired by this route were between 1-2.5 m3/m2.hr.atm1 5.
The benefit of this processing modification is to maintain a high gas permeability, measured as 38-40 m /m .hr.atm through the underlying metallic support. This would minimise the hydrogen dwell time on the downstream side of the membrane and increase the rate of flow through the membrane.
EXAMPLE 3
Porous stainless steel tubes coated according to Example 2 were activated by sequential dipping into fresh acidic solutions of SnCl2 and PdCl2j washing with deionised water after each step. The tubes were air-dried at 70°C for one hour prior to immersion in a solution comprising palladium chloride, ethylenediamminetetraacetic acid (EDTA), ammonia, water and hydrazine, maintained at 25°C with moderate stirring. The amount of palladium deposited was varied by varying the period spent in the deposition bath, with typically l-3μm of palladium being the thickness achieved after 3 hours immersion.
Microscopic analysis shows that the deposited palladium had successfully penetrated into the residual void space in the Pd/Ag coated layer, with the measured nitrogen permeabilities now reduced to between 0.07 - 0.17 m3/m2.hr.atm1'5. Homogenisation of the system was achieved by holding the tube at 600°C in a 5%H2-N2 atmosphere for 12 hours. The measured nitrogen permeabilities were of the same magnitude as those of the unhomogenised samples.
EXAMPLE 4
Example 3 was repeated except that there was no activation step. Porous stainless steel tubes coated according to Example 2 were immersed directly in an electroless plating composition comprising palladium chloride, ethylenediamminetetraacetic acid (EDTA), ammonia, water and hydrazine in concentrations typically used for electroless deposition. The bath was maintained at 25°C, with moderate stirring. Microscopic analysis shows that the deposited palladium had penetrated into the residual void space in the Pd/Ag coated layer, with the measured nitrogen permeabilities now reduced to between 0.08 - 0.15 m3/m2.hr.atm1 5. Homogenisation of the system was achieved by holding the parts at 600°C in a 5%H2-N2 atmosphere for 12 hours. The measured nitrogen permeabilities were of the same magnitude as those of the unhomogenised samples.

Claims

1. A method of fabricating a supported hydrogen diffusion membrane comprising the steps of: a) coating a support with a coating composition, thereby providing a coated support, b) firing the coated support at a temperature above 300°C, wherein the coating composition comprises one or more metal powders and a carrier liquid.
2. A method according to claim 1, wherein the coating composition further comprises a binder.
3. A method according to claim 2, wherein the binder is an inorganic binder.
4. A method according to claim 3, wherein the inorganic binder is a frit material.
5. A method according to claim 2, wherein the binder is an organic binder.
6. A method according to any preceding claim, wherein the support is a metallic, ceramic or glass porous support.
7. A method according to claim 6, wherein the support is a stainless steel support.
8. A method according to any preceding claim, wherein the hydrogen diffusion membrane is a palladium or palladium alloy membrane.
9. A method according to claim 8, wherein the hydrogen diffusion membrane is a palladium alloy membrane comprising one or more metals chosen from silver, gold, copper and platinum.
10. A method according to claim 9, wherein the hydrogen diffusion membrane is a palladium-silver alloy membrane.
11. A method according to any one of claims 8- 10, wherein the one or more metal powders in the coating composition are chosen from palladium, silver, copper, gold and platinum.
12. A method according to claim 11, wherein the coating composition comprises palladium and silver powders.
13. A method according to any preceding claim, wherein the carrier liquid is an organic solvent.
14. A method according to any preceding claim, wherein in step (a) the support is dip coated.
15. A method according to any preceding claim, wherein in step (b) the coated support is fired at a temperature between 600-1000°C.
16. A method according to any preceding claim, comprising a further step which is after step (b), c) depositing metal onto the coated support.
17. A method according to any one of claims 1 to 15, comprising a further step which is after step (b), c) activating the surface of the coated support with metal seed particles and depositing metal onto the coated support.
18. A method according to claim 16 or claim 17, wherein in step (c) metal is deposited onto the coated support using an electroless plating process.
19. A method according to any one of claims 16 to 18, comprising a further step d) thermal treatment to homogenise the metals deposited in steps (a) and (c).
20. A hydrogen diffusion membrane comprising a porous support and a metallic membrane layer, wherein there is an intermediate layer between the support and the membrane layer and the intermediate layer comprises fused glass that has been formed by heating a frit material.
PCT/GB2004/003118 2003-07-31 2004-07-19 Method of fabricating a hydrogen diffusion membrane WO2005012167A1 (en)

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US3428476A (en) * 1965-06-22 1969-02-18 Engelhard Min & Chem Method for producing hydrogen diffusion cells
EP0783919A1 (en) * 1996-01-11 1997-07-16 Bend Research, Inc. Composite hydrogen separation element and module

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US3413777A (en) * 1965-06-22 1968-12-03 Engelhard Min & Chem Hydrogen diffusion and method for producing same
US3428476A (en) * 1965-06-22 1969-02-18 Engelhard Min & Chem Method for producing hydrogen diffusion cells
EP0783919A1 (en) * 1996-01-11 1997-07-16 Bend Research, Inc. Composite hydrogen separation element and module

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