WO2008041969A2

WO2008041969A2 - Gas-selective permeable membrane system, and method of its production

Info

Publication number: WO2008041969A2
Application number: PCT/US2006/037935
Authority: WO
Inventors: Thomas Henry Vanderspurt; Rhonda R. Willigan; Kyle C. Cattanach; Zissis Dardas; Ying She; Sean C. Emerson; Caroline A. Newman
Original assignee: Utc Power Corporation
Priority date: 2006-09-28
Filing date: 2006-09-28
Publication date: 2008-04-10
Also published as: WO2008041969A3

Abstract

A membrane (10) permeable to a selected gas, e.g., H2, comprises a porous supporting structure (26, 28) having principally nano-sized pores (24) adjacent at least one surface, and plugs (22) of a material permeable substantially only to the selected gas, e.g., Pd or a Pd alloy, located in and sealing the nano-sized pores (24) against gas flow except for the selected gas. The plugs (22) are formed from coagulation of a colloidal solution of the selected material introduced to the nano-sized pores. The nano-sized pores (24) are sufficiently small to afford a strong support structure and minimize the size of the plugs (22), yet large enough to afford an effective diffusivity to the selected gas that does not inhibit its flow there through. The membrane (10) may be part of an integrated structure (12) also including a catalyst material (14), to facilitate the water gas shift reaction of an H2-containing reformate and the selective separation of H2 therefrom as a permeate. The method of making the membrane (10) with the selected-gas permeable plugs (22) in its nano-pores from a colloidal suspension is also included.

Description

Gas-Selective Permeable Membrane System, and Method of its Production

Technical Field

This invention relates to selective gas separation, and more particularly to palladium membranes for the separation of a selected gas from a gas stream. More particularly still, the invention relates to composite palladium membranes for hydrogen separation, and to such membranes incorporated as part of a larger system, and to the production of such composite palladium membranes.

Background Art

Gas separation and purification devices are used to selectively separate one or more target gasses from a mixture containing those and other gasses. One well known example is the use of certain membranes for the selective separation of hydrogen (H2) from a stream, flow, or region containing hydrogen in a mixture with other gasses. While the membranes for the selective separation of H₂ might generally be either polymers or metal, the polymer membranes are typically limited to use in low temperature environments. In circumstances where the membranes must be used in conjunction with high temperature processes, or processing, it becomes necessary to rely upon metal membranes .

In a typical example, the H₂ may be the product of a reformation and/or water gas shift reaction of a hydrocarbon fuel, and the H₂, following separation from other reformate or reaction gasses, may be used in a relatively pure form as a reducing fuel for the well- known electrochemical reaction in a fuel cell. The processes associated with the reformation and/or water gas shift reactions are at such elevated temperatures, as for example, reactor inlet temperatures of 700 ⁰C and 400 °C respectively, that H₂ separation, at or near those temperatures, requires the use of metal membranes. The metal perhaps best suiting these needs is palladium, which is selectively permeable to H₂, relative to other gasses likely to be present, and has high durability at these operating temperatures. ,

Composite palladium, or palladium alloy, membranes, consisting of a palladium layer deposited on a porous metal (PM) , oxidation resistant substrate, when integrated with the reformer or the water gas shift reactor, result in desirable H₂-perrαeation flux and offer significant advantages towards system size and cost reduction. Pd-Ag and Pd-Cu-based alloys are typically required for extended membrane stability in a sulfur-free or sulfur containing reformate, respectively, with the former being quite important for fuel cell power plants requiring a number of start up and shut down cycles. For a palladium alloy membrane to be produced by electroless plating (EP) or certain other techniques, high temperature thermal treatment, e. g., in the 550 ⁰C - 650 ⁰C temperature regime, in a controlled atmosphere is needed in the latter stages of the process. However, this thermal treatment will cause intermetallic diffusion of the porous metal substrate constituents into the Pd phase that is detrimental to H₂ permeance. An effective way to produce a Pd alloy membrane with the previously stated manufacturing processes is to provide the palladium membrane substrate with a thin ceramic layer that will serve as an intermetallic diffusion barrier. Examples of such techniques may be found in, for example, U. S. Patent 6,152,987 and U. S. published applications US 2004/0237779 and 2004/0244590 by Y. H. Ma, et al . In the instances cited above, this ceramic interlayer is grown thermally, either as an oxide from the metal support or as a separate phase like nitride from N₂ decomposition or carbide from a carbon-containing gas stream. The palladium membrane support is thermally treated in air, nitrogen or a carbon-containing gas at extreme temperatures and prolonged times to achieve this result.

Although Ma's method of electroless plating on an intermetallic diffusion barrier treated stainless steel is effective, it nonetheless requires many repetitions of both the tin treatment to activate the surface, and the actual electroless plating step. The membrane thickness necessary to seal the membrane completely against N₂ and CO leakage is governed by the size of the largest pores on the surface of the porous stainless steel (PSS) . This typically results in membranes over 10 microns (10,000 nm) in thickness. The resulting thickness of the Pd is representative of cost, both in terms of materials and also process complexity.

The transfer of hydrogen through a Pd membrane is generally considered to follow Sievert's law:

l^K* ^{' ! '} where Ji is the flux in moles per area per time, Q is the permeability of the membrane, 1 is the membrane thickness and pi is the partial pressure of the hydrogen on either side of the membrane. It will be noted that the concentration driving force under Sievert' s law is basedon a square^" root ^"dependency ^~(^"of^"the- partial -pressures) .- Nanoporous metal oxides like Y-stabilized zirconia membranes deposited on stainless steel or other suitable metallic substrates, can offer a smoother surface with much smaller pores, so that the formation of membranes 2 microns (2,000 nm) thick by electroless plating or other typical means can be considered. However, as the continuous Pd membrane gets thinner, its long term stability decreases as grain growth occurs and inter- granular leak paths develop. While the foregoing discussion of the prior art focuses on H₂-separation using Pd membranes, it will be understood that similar issues exist with respect to the separation of yet other gases using supported membranes of materials other than Pd.

There thus remains a need to provide permeable- selective membranes that are effective, durable, and relatively economical to manufacture. There is particular need to provide an H₂-permeable membrane of Pd that is effective, durable, and relatively economical to manufacture .

Disclosure of Invention

The present invention takes advantage of the stability and strength of porous-metal supported, nano- porous oxide films, while providing very thin membranes, typically of metal or polymer, having high permeability to a selected gas, typically in the presence of other gasses. In a preferred example, the membrane is of Pd, or a Pd alloy, having selective permeability to H₂. The metal thickness of the Pd is determined principally by the pore size of the supporting substrate of nano-porous oxide film and/or porous-metal support member, in contrast with Pd membranes prepared by electroless plating or other means, like sputtering, of broad Pd distribution.

The present invention relies on plugging the nano- sized openings_^ in the pores of the porous supporting structure, typically the nano-porous oxide membrane, with similar-sized "plugs", so that each plug becomes a single crystallite Pd, or Pd alloy, attached by surface forces to the walls of the pore. Typically, the thickness of these plugs is on the order of the pore diameter. Thus, for instance, a stabilized nano-porous layer of zirconia with a pore size in the range of 80 nm to 100 nm, or 0.08 microns to 0.1 microns, is made gas tight by plugs of Pd, or Pd alloy, that are about 80 nm to 100 nm across and thick. In this way, the required quantity of Pd, or Pd alloy, is relatively small, the time required to form the Pd membrane is relatively reduced, and the membrane possesses good durability because of the lateral support afforded by the porous structure in which it is formed and resides. Indeed, preferably little or no Pd overlies the non-porous (i. e., solid) portions of the porous supporting structure because the requirement for an H₂- permeable membrane is met by the plugs of Pd or Pd alloy in the nano-pores of the supporting structure. This contributes to economies with respect to the amount of Pd material used and the process time for its formation.

Indeed, where the thickness of the membrane is quite small as determined by the pore size of the supporting substrate of nano-porous oxide film and/or porous-metal support member, the resistance through the Pd plug would not be significant compared to the external mass transfer limitation. Therefore, what remains is the effect of external mass transfer to the nanoporous oxide surface and the diffusion of gaseous species through the oxide pore structure to the proximate surface of the Pd plugs. This can be expressed as:

It ^■*^" IKMH

An important feature of this expression is that the concentration driving force has switched from a square root dependency based on Sievert's law to a linear dependency, which enhances the flux through the membrane. In this equation, D_eff is the effective diffusivity through the porous oxide layer, C₁ is the concentration (e. g., mol.πf³) of the hydrogen on either side of the membrane, and H is the partition coefficient between the concentration inside the membrane and the concentration in the gas mixture from which the H₂ is being separated. Thus, it is the effective diffusivity of the membrane combined with the ability of the Pd plugs to selectively separate H₂, that combine to provide the improved results of the invention.

The effective diffusivity in the pore structure can be represented by various models depending upon the pore size distribution. For example, a random pore model can be used where the micropore resistance to diffusion is dominant and the diffusivity is given by the expression:

1 ,

where D is the diffusivity in the pores, and ε is the porosity. In turn, D can be represented as:

where Di₂ is the bulk binary diffusion coefficient and D_k is the Knudsen diffusivity. The bulk binary gaseous diffusion coefficient can be calculated from empirical correlations. The Knudsen diffusion coefficient, which becomes important when the pore radius is less than the ^• mean free kinetic path length of the gaseous species, can be calculated from:

Djt/ (Cm²S-¹) » 4850rfι/ l '^{V M}

For a given operating pressure, porosity, and temperature, there is an optimum pore size below which Knudsen diffusivity becomes more dominant and reduces theoverall flux through the membrane . As an example, _a _ _ membrane, such as the one described hereinafter, operating at a constant 10 atm and 400 ⁰C with a porosity of 0.4 requires a minimum pore diameter of 40 nm to be free of Knudsen diffusion limitations. The effective diffusivity of such a membrane is approximately one hundred times greater than that of a microporous zeolite membrane, since the pore diameters are on the order of l nm, leading to significant diffusion limitations. In practice, the comparison above will not happen in a tubular arrangement, because the partial pressure of hydrogen will vary along the direction of flow as the hydrogen is removed. The membrane of the described embodiment of this invention would require pore diameters between 80 nm and 100 nm to be free of internal diffusion limitations. It has been further determined that increasing the pore diameter greater than 100 nm has no appreciable benefit from a mass transfer perspective.

Importantly, the process for forming the H₂-permeable membrane of Pd, or Pd alloy, which impregnates or plugs the pores of a porous supporting structure, relies upon the synthesis of a colloidal suspension of appropriately sized Pd, or Pd alloy, colloidal particles. Regulation of the size of the colloidal particles is effected at least partly by regulating the pH during the synthesis of the colloidal suspension. Further regulation of particle size is obtained by regulating the concentrations of the constituent solutions making up the colloidal suspension. That colloidal suspension of Pd or Pd alloy is applied to one side or the other of the porous supporting structure and caused to flow into and agglomerate and lodge within the pore openings. This is accomplished with at least the application of a pressure differential across the colloidal suspension and support membrane to cause it to enter and plug the pores of the support membrane. In addition to the application of a pressure differential, one or more optional supplemental processes may be employed to cause the colloidal suspension to flow into and deposit within the pore openings. For instance, the pores of the support membrane may be filled with a suitable liquid such as distilled water, and the pH of that liquid and the colloidal suspension are adjusted such that the surface of the porous material and the colloidal particles have opposite charges to facilitate entry of the colloidal suspension to the pores and resulting deposition on the walls of the pores to form

— 1 ~ the plugs which complete the membrane. Further alternatively or supplementally, the nano pores may be exposed to a solution containing an agent to coagulate and bind the colloidal particles to the walls of the nano pores. For Pd colloids prepared by a citrate method, a dilute solution of purified gelatin is effective, and then followed by the colloidal suspension. Still further alternatively or supplementally, the suspension may be treated with an agent to begin the agglomeration process immediately before passing it through the treated nano- porous layer. Effective agents are ammonium nitrate, ammonium acetate and the like. They destabilize the sol to begin the agglomeration process. Salts or other compounds containing possible poisons like S, P, etc. should be avoided.

The use of a colloidal suspension to form nano- plugs in the nano-pores of a support material also extends to the usage with other plug-forming, selective gas permeability materials. Examples may include Nafion®, a sulfonated tetrafluoroethylene copolymer incorporating perflurovinyl ether groups terminated with sulfonate groups into a tetrafluoroethylene backbone, and XLPEO, a crosslinked poly,(ethylene oxide) .

Assuming a micro-porous metal support, as of stainless steel, having a nano-porous oxide membrane layer such as titania, hafnia, zirconia, stabilized zirconia where the stabilizing agents may include Y and the rare earths, or the like on one surface thereof, the colloidal suspension may be introduced to the nano-pores of the oxide membrane either indirectly through the larger pores of the stainless steel support or directly from the surface of the nano-porous oxide layer, depending on to which side of the structure the colloidal solution is applied. Ideally, for a nano-porous support medium having pore diameters preferably less than 200 nm (0.2 microns), and most preferably typically grouped near 80-100 run, the Pd or Pd alloy colloidal particles are preferably sized in the range of about 8 nm to 60 nrα to readily enter and quickly plug the nano pores of the support membrane. Colloidal suspensions with a range of particle sizes centered around about 0.4 to 0.6 of the typical pore size of the nano-porous layer and also containing some particles just smaller than the smallest pores in the nano-porous layer, are most preferred. As a further aspect of the invention, the H₂- permeable membrane is complimented by the inclusion of a water gas shift catalyst layer to form the H₂-permeable membrane system. This catalytic material may be a noble metal-loaded, nanophase, active-oxygen mixed metal oxide, typically cerium-containing, like cerium-zirconium, or doped cerium-zirconium oxide, where, the zirconia may be partially or wholly replaced with hafnia, and significant levels of titania may also be substituted. Alternatively, the principal oxide may be titaniawith significant levels of cerium and sufficient zirconium and hafnium to reduce lattice strain in both the fully oxidized or partially reduced working state. The dopants (<12% atomic percent on a metals basis) may be Nb, Ta, Mo or W, and promoters like rare earths, alkaline earths or thorium, Th, may also be present. The noble metals for the purpose of this invention include Ru, Rh, Pd, Re, Os, Ir, Pt and Au. The metallic components of the H₂-permeable membrane and the water gas shift catalyst layer should not be in direct contact because of the lack of performance caused by intermetallic diffusion. The Pd or Pd alloy plugs of the H₂-perrαeable membrane inside the pores of the nano-porous oxide layer are well positioned to avoid chemical interaction with the water gas shift catalyst. The water gas shift catalyst is, of course, positioned for catalytic contact with reformate or the like for the formation of H₂. The H₂-permeable membrane is adjacent to and forms part of a permeate region for the acceptance and collection of H₂ that permeates through the membrane. Though the H₂-permeable membrane and/or the H₂-permeable membrane system are typically tubular, they may assume other shapes such as planar, corrugated, or others that best suit the intended use.

The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.

Brief Description of Drawings

Fig. 1 is a functional, cross-sectional, diagrammatic view of a gas specific (H₂) -permeable membrane in accordance with the invention, and illustrated in the context of a tubular support member as part of a system that also supports a water gas shift reaction;

Fig.^' 2 is an enlarged view of the encircled portion of the H₂-permeable membrane of Fig 1; Fig 3 is graphical plot of the size distribution of nano-sized pores in an oxide layer forming part of the H₂- permeable membrane of the invention;

^■ Fig. 4 is a graphical plot of H₂ permeance and N₂ permeance vs time, for an H₂-permeable membrane in accordance with the invention

Fig. 5 is a photo-micrograph of a micro-porous stainless- steel support., with a. TiO^. na^'no-por.o.us_-coa±ing;_ . . and

Fig. 6 is a futher-magnified, photo-micrograph of the TiO₂ layer of Fig. 5, showing Pd plugs in the nanopores.

Best Mode for Carrying out the Invention

While the invention of forming gas-selective permeable membranes by depositing nano-plugs in the nanopores of a porous structure may be applicable to permeability of gases (including vapors) other than H₂ and suitable "plug" materials other than Pd, it is convenient to describe the invention in a preferred context of Pd plugs and selected H₂ gas permeability. Referring to Fig. 1, there is illustrated a functional, cross-sectional, diagrammatic view of an H₂-permeable membrane, generally designated 10, in accordance with the invention. The H₂- permeable membrane 10 is conveniently depicted in the context of a larger system 12 that also accommodates a water gas shift reaction, and in which a water gas shift catalyst 14 is, or may be, present as an integral part or adjunct of the H₂-permeable membrane 10. The H₂-permeable membrane system 12, in addition to the H₂-permeable membrane 10 and the water gas shift catalyst 14, is depicted in Fig. 1 as being tubular to define an enclosed water gas shift reaction region 16 therewithin. A further tubular outer wall 18, surrounding the tubular H₂ permeable membrane system 12 in radially-spaced relation, defines an annular chamber 20 for receiving H₂ permeate. H₂-rich reformate enters the water gas shift reaction region 16 where, in contact with the catalyst 14, it undergoes the water gas shift reaction to liberate additional H₂. That H₂ is then removed from the region in a known manner by selective permeation of only hydrogen atoms through the H₂-permeable membrane 10 to the permeate chamber 20.

In accordance with the invention, the H₂-permeable membrane 10 and the larger membrane system 12 in which it is included are of novel construction and made in accordance with a novel process. The H₂-permeable membrane 10, broadly speaking, is comprised of H²-permeable "plugs" 22 of palladium (Pd) , or a Pd alloy, formed in substantially all of the nano pores 24 of a porous supporting structure. More specifically, the supporting structure is preferably comprised of a layer of micro- porous substrate 26, as of stainless steel or the like, having micro pores 27 and a thickness T_ss sufficient to safely contain the total differential pressure across the micro-porous pore, or tube, and an adjacent thin nano- porous layer 28 of ceramic or metal, with an oxide being preferred, having the nano pores 24 and a thickness T₀. The thickness, T₀, of this nano-porous layer 28 is between about 20 and 200 times the diameter of the average diameter of the nano-pores it contains. For instance, if the average diameter of pores 24 in nano-porous layer 28 is about 80 nm (0.08 micron), the layer 28 might typically be about 10,000 nm (10 micron) thick. The nano- porous layer 28 of metal or ceramic on the micro-porous substrate 26 is conveniently available, as specified, from a source such as Graver Technologies of Glasgow, Delaware, or Pall Corporation of East Hills, New York.

Where the thickness of the H₂-permeable plugs 22 of Pd is quite small as determined by the size of the nano pores 24 of the nano-porous oxide film and/or porous- metal support member, the resistance through the Pd plug 22 would not be significant compared to the external mass transfer limitation. Therefore, what remains is the effect of external mass transfer to the nanoporous oxide surface and the diffusion of gaseous species through the oxide pore structure to the proximate surface of the Pd plugs. This can be expressed as:

Ji — A (Ci -Cz) = ~_j — _.. . . _ Ji.T_755ff_

An important feature of this expression is that the concentration driving force has switched from a square root dependency based on Sieverf s law to a linear dependency, which enhances the flux through the membrane 10. In this equation, D_eff is the effective diffusivity through the porous oxide layer 28 and H is the partition coefficient between the concentration inside the membrane and the concentration in the gas mixture from which the H₂ is being separated. Thus, it is the effective diffusivity of the membrane combined with the ability of the Pd plugs 22 to selectively separate H₂, that combine to provide the improved results of the invention. The effective diffusivity in the pore structure 24 can be represented by various models depending upon the pore size distribution. For example, a random pore model, can be used where the micropore resistance to diffusion is dominant and the diffusivity is given by the expression:

_J_ !_ J_

where D_i2 is the bulk binary diffusion coefficient and D_k is the Knudsen diffusivity. The bulk binary gaseous diffusion coefficient can be calculated from empirical correlations. The Knudsen diffusion coefficient, which becomes important when the pore radius is less than the mean free kinetic path length of the gaseous species, can be calculated from:

For a given operating pressure, porosity, and temperature, there is an optimum pore size_below which Knudsen diffusivity becomes more dominant and reduces the overall flux through the membrane. As an example, a membrane, such as the one described hereinafter, operating at a constant 10 atm and 400 ⁰C with a porosity of 0.4 requires a minimum pore diameter of 40 nm to be free of Knudsen diffusion limitations. The effective diffusivity of such a membrane is approximately one hundred times greater than that of a microporous zeolite membrane. In practice, the comparison above will not happen in a tubular arrangement, because the partial pressure of hydrogen will vary along the direction of flow as the hydrogen is removed. The membrane of the described embodiment of this invention requires pore diameters between 80 nm and 100 nm to be free of internal diffusion limitations. It has been further determined that increasing the pore diameter greater than 100 nm has no appreciable benefit from a mass transfer perspective. Ideally, for a nano-porous support medium having pore diameters preferably less than 200 nm (0.2 microns), and most preferably typically grouped near 80-100 nm, the Pd or Pd alloy colloidal particles are preferably sized in the range of about 0.1 to 0.6 times this or about about 8 nm to 60 nm, to readily enter and quickly plug the nano pores of the support membrane. Colloidal suspensions with a range of particle sizes centered around about 0.4 to 0.6 of the typical pore size of the nano-porous layer and containing some particles just smaller than the smallest pores in the nano-porous layer are preferred, and result in the formation of Pd plugs 22.

The Pd plugs 22 typically reside in the nano pores 24 of the nano-porous layer 28, and are formed in a manner to reduce the quantity of Pd required while also achieving good strength, durability, and H₂-selective permeability of the membrane system 12. The thin nano- porous layer 28 may preferably be one or more of titanium dioxide, stabilized zirconium dioxide, zirconium dioxide containing elements^' like cerium, neodymium or first row transition that both stabilize crystal structure and prevent coke formation of the zirconia, hafnium dioxide, or an appropriate mixture of Zr-Ti, or Zr-Hf or Zr-Ti-Hf, with or without dopants, formed on a surface of the micro-porous substrate 26. Appropriately-sized Pd or Pd alloy colloidal particles, typically in the range of 8 nm to 60 nm, are synthesized and caused to flow into and block the pores of the membrane 10. The nano-porous oxide layer 28 preferably has a narrow pore size distribution, and this narrow pore size distribution is such that the pores are between about 10 times large and 2 times larger than the Pd or Pd alloy colloidal particles. In an example of a preferred configuration, the pore size distribution of the nano-porous oxide layer 28 is illustrated in Fig. 3, in which it is seen that a very large percentage of the pore sizes are between 80 and 90 nm (0.08 to o.09 microns), with by far the single largest _ portion being about 85 nm (0.085 microns) . This represents the so- called "mode" of the pore diameter distribution, and is also referred to herein as the "typical pore size" for convenience, though it is most accurately the most frequently occurring value of the pore diameter. In such instances, colloidal suspensions with a range of particle sizes centered around about 0.4 to 0.6 of the typical pore size of the nano-porous layer and containing some particles just smaller than the smallest pores in the nano-porous layer are preferred. It will be appreciated that a nano-porous material having a narrow pore size distribution of pores with a typical pore size mainly in the range of 40 nm to 200 nm may also be used, with concomitant adjustment in the size of the Pd or Pd alloy colloidal particles.

The Pd plugs 22 form in the nano-pores 24 as a single crystallite of Pd or Pd alloy from individual colloidal particles, attached by surface forces to the walls of the pore. Typically, the thickness of these plugs 22 is on the order of the pore diameter. Thus, for a nano-pore 24 having a diameter of 85 nm, the Pd plug 22 will of course also have a diameter of 85 nm and a thickness also on the order of 85 nm or somewhat more. Such reduced thickness of the Pd plugs 22 affords the formation of an H₂-permeable membrane 10 having high H₂ permeance with relatively little Pd or Pd alloy, and the associated support structure affords strength and durability. Care is taken to match as closely as possible the respective coefficients of thermal expansion (CTE) of the Pd or Pd alloy with the material of the nano-porous layer 28 of ceramic or metal, which in turn is matched closely with the CTE of the material of the micro-porous substrate 26.

An important facet of the invention is the use of a colloidal suspension, or sol, of the Pd or Pd alloy, for fluid introduction into and at least partly through the micro and/or nano pores of the porous supporting membrane structure of layers 26 and/or 28 to form the plugs 22 therein. The preparation of the Pd or Pd alloy colloidal sol may be accomplished by a wide variety of methods, as for example disclosed in a paper by John Turkevich and Gwan Kim in Science, 169, 1970, at pages 873-879.

One such method preferred herein for the preparation of a 7.5 nm sol is as follows: A Palladium Chloride, 9.3 x ICT⁴ M PdCl₂ stock solution, Solution A, is formed by dissolving 0.165 g of anhydrous PdCl₂ in 20 ml of 1 N HCl in a 1 liter volumetric flask and then diluting with double-distilled, dust-free water to bring the volume to the 1 liter mark. Then 100 ml of 9.3 x 10^~4 M PdCl₂ stock Solution A, or 9.3 x 10^~5 moles Pd, is mixed with 200 ml of dust-free 1 wt% sodium citrate solution (3.4 x 10^"2 M trisodium salt; 2-hydroxy-l, 2, 3-propanetricarboxylic acid, HOC(COONa) (CH₂COONa)₂), Solution B. This mixture is diluted to 500 mis and refluxed gently for about 6 hours. When the pH is 6.1, the resulting colloidal suspension contained 9.3 x 10^~5 moles of Pd/0.5 liter or 1.86 x 10^~4 moles Pd/liter as palladium particles typically have a quantity versus size distribution as follows: <3% 5nm; 15% ~6.3nm; 39% ~7.5nm; 28-8.7nm; 15% -lOnm; and -1% >110nm. To prepare a Pd sol in which the principal mode (i.e., greatest percentage) of the particle sizes is about 15 nm, the pH of the mixture above must be lowered slightly to the 5.1 to 5.8 range. This lowering of the pH is accomplished by adding acid, typically 1 N HCl dropwise with rapid stirring. Conversely, the size may be decreased by increasing the pH, as by the addition of NaOH. More generally, the principal mode of the resultant Pd sol varies with the pH of the synthesis solution before refluxing as follows: pH 5.1-5.8 ~ 15 nm; pH 6.1 ~ 7.5 nm; pH 6.4-6.9 ~ 50-120 nm with some 7.5 nm; and pH 8 ~ 50-120 nm free of any 7.5 nm. The preceding technique of pH control is used to initially establish a principal mode or size of the Pd nano-particles in the sol. Increasing the concentrations of Solutions A and B increases the particle size. One half the dilution gives ~ 11 nm particles, while twice the dilution gives ~ 5 nm particles. To grow the existing Pd nano-particles to the desired size, the starting 15 nm Pd sol is diluted 4 fold and the sodium citrate during dilution is adjusted so that after dilution it is ~ 2 x 10^"3 M. Equal volumes of 4.65 x 10^~4 M palladium chloride solution and 6.25 x 10^~3 hydroxylamine hydrochloride are added to the diluted sol from two dropping funnels at comparable rates with steady, smooth rapid stirring at ~25 ⁰C. Typical addition rates are 4 mis/minute per 100 mis of starting dilute Pd sol. It is desirable to protect the synthesis solution from air to minimize the catalytic oxidation of the citrate by the Pd. The average diameter of the particles in the final grown sol D_f compared to the diameter in the initial sol Di is given by: D_f = Di ( 1 + M_f/M_x)^1/3 , where Mi is the initial mass of Pd present in Pd growth nuclei and M_f is the mass of Pd added from the PdCl₂ solution. In general, doubling the initial size is the practical maximum, per growth step. For further growth, dilution and citrate concentration adjustment is recommended. Thus a sol of nominally 30 run Pd crystallites is obtained. Alternatively, the Pd can be alloyed with reducible metals such as Pt, Au, Ag, or Cu by adding the appropriate dilute metal complex solution to the Pd sol, and the appropriate reducing agent, for instance hydrogen gas, formaldehyde, hydrazine, hydroxylamine, sodium formate, etc. The alloying metal, like Pt or Au, may be added as a chloro complex salt in acid solution or the chloro complex acid like chloroplatinic acid for Pt or chloroauric acid for Au, etc. The alloy is formed by first growing a layer of the alloying metal on the surface of the Pd. This is made possible by the fact that if an extremely clean solution, free of suspended dust, etc, is used in a very clean, scratch-free container such as a new borosilicate glass flask, there are no nuclei for the dilute metal to crystallize on except the colloid. A new borosilicate glass flask can be prepared for colloidal formation by filling with just-prepared aqua regia, protecting from dust, and letting stand over night, rinsing thoroughly with dust-free, double- distilled water and then suspending the flask opening side down and directing a stream of fresh steam into the flask for at least an hour. This treatment sufficiently passivates the surface such that the metal from the added metal or metals deposits on the initial colloid, thus giving a colloid of larger bimetallic or multi-metallic particles.

The sol can be ion exchanged to replace sodium ions with ammonium ions etc. and the chloride ions with nitrate, acetate or hydroxyl ions, depending on the particular sol and how it is to be used. Sols that are to be ion-exchanged should be carefully protected from air though out their synthesis and subsequent handling. The vessels used should be scrupulously clean and dust free. Typically the ion exchange resin is added to a N₂ purged flask with a new Teflon stirring bar and the degassed sol is transferred to the flask. Removal of Na and Cl, for instance, greatly simplifies preparing a robust leak free membrane. It is also important that the nano-porous oxide substrate be defect free. Minor defects can be corrected by some variant of electroless plating before or after treatment with the sol, but it is greatly preferred that the oxide layer be tightly adhering to the porous metal support and be defect free.

Having thus described the synthesis of appropriately- sized Pd or Pd alloy colloidal particles, further consideration is given to their introduction to the nano pores 24 of the H₂-permeable membrane 10 to form the Pd plugs 22. Broadly speaking, and assuming membrane 10 comprises both a micro-porous substrate 26 and an adjacent thin nano-porous layer 28, the colloidal particles may be introduced to the nano pores 24 from either side or surface of the membrane 10, either directly without passage through the micro pores 27 or, preferably, first through the micro pores 27 and thence into the nano pores 24. Further still, to facilitate introduction of the colloidal sol into the nano pores directly, or through the micro pores into the nano pores, a pressure is conveniently applied to the sol in a known and suitable manner, such as by piston pump, gas pressure, or the like, to force it into the pores. That pressure is typically such as is required to cause the colloidal sol to flow into the micro and nano pores of the particular system, and may require a gradual increase as the nano pores 24 become increasingly plugged with the Pd or Pd alloy, until all of the nano pores are plugged. A coagulating solution is preferably applied to the nano pores 24 to facilitate the coagulation of the colloidal particles on the walls of the nano pores. Moreover, the pH of that coagulating solution, which typically pre- coats the walls of the nano pores 24, may be adjusted relative to the pH of the colloidal sol so as to have opposite charges and thus provide an attractive force to assist the migration and deposition of the colloidal particles within the nano pores.

Briefly by way of one example and starting with a thermally stable nano-porous metal oxide membrane layer 28 on a micro-porous metal support 26, the oxide is cleaned of adsorbed contaminants, as by chemical washing steps followed by double distilled water rinsing and heating in air and/or oxygen and/or steam, and then cooling in the same gas or another gas. The membrane, and particularly the nano-porous oxide layer 28, is saturated with a suitable liquid, which may be water or a low molecular weight oxygenate, etc., that also contains a coagulating or binder agent such as gelatin to aid in subsequently binding the colloidal particles to one another and to the walls of the nano pores 24. This binder agent may remain as a wash coat following removal of the solution that contains it. The colloidal suspension of Pd or Pd alloy is then introduced, in this example, directly to the nano-porous layer 28 following removal of the solution containing the binder agent. It is introduced in a manner, as in a closed system such as a tube or chamber or the like, where a pressure can be and is applied to urge the colloidal suspension into the nano pores 24. As an adjunct or supplement to the application of pressure to the colloidal suspension, the i pH of the liquid carrying the binder agent as well as the pH of the colloidal suspension may be adjusted such that the oxide surface and the colloidal particles have opposite charges and thus are attractive to facilitate the coagulation process. Under pressure, the colloidal suspension begins to flow through the nano pores 24 and the Pd particles deposit at the pore opening or just within the pore opening. The colloidal particles of Pd within a nano pore 24 collectively form a plug 22 which blocks the opening and the flow continues to unblocked openings until all the openings are blocked. When the flow ceases, assuming the pressure is sufficiently high to overcome the capillary resistance, the membrane 10 is rinsed with appropriate solutions or solvents, dried and heated. The rate of heating, the maximum temperature and the composition of the gas all require care and need to be tailored to the specific application. However, thoroughly dried membranes may be heated at about 1 °C/min in air to about 350 °C, held for 30 minutes and then cooled. Alternatively, at about 350⁰C when no significant additional carbon oxides are detected coming off of the membrane, the controlled atmosphere over /through the membrane tubes can be changed to an inert gas like N₂, Ar or He and, after a few minutes of purging or when the concentration of O₂ is below 0.5 vol. %, a reducing gas, preferably hydrogen, may be introduced. This should be done judiciously and the concentration increased stepwise to about 3% H₂ in inert gas. After at least 1 hour the concentration of hydrogen is decreased to "zero" (less than 0.01%). If it is the intention that the membrane be exposed to air, it should be passivated, i.e., the membrane is cooled to "room" or ambient temperature. At room temperature, oxygen in the form of air can be introduced, but this is preferably done slowly with the concentration of O₂ increasing from zero to 0.1% over the course of about 15 minutes, 0.1% to 0.5% over 30 minutes and 0.5% to 1% or another 30 minutes. The concentration of oxygen may then be increased to that of air over the course of an hour or longer as convenient. This treatment results in the passivation of a reduced metal surface. Alternatively, the above controlled atmosphere may change from oxidizing to reducing to inert, according to a schedule. The permeability of the membrane 10 to N₂ is then measured. If it is above 0.01m³/ (m² h atm^0-5) , for example, the membrane is either re-treated as above or is subjected to modified electroless plating designed to grow the Pd or Pd alloy particles within the pores. This latter-mentioned technique of supplementing with electroless plating is a hybrid approach.

The metallic Pd deposited from the colloidal sol acts as reduction sites for the reaction between a soluble Pd source and a reducing agent such as, but not limited to hydrazine, hydroxylamine hydrochlorides or citrate ion. Then after suitable washing and rinsing, the elevated temperature gas treatment is repeated until the membrane is judged sufficiently N2 gas tight.

In those instances where it may be deemed desirable to apply some limited supplemental electroless plating, that need may be determined in advance in the manner described above, and subsequent practice of the invention might begin with the limited electroless plating to plug or partly plug some of the nano pores, followed by the introduction of the colloidal solution to complete the major portion of the plugging.

By way of an alternative and perhaps preferred, example, the Pd or Pd alloy colloidal sol is introduced from the "backside" of the porous membrane 10 first via the micro pores 27 of the micro-porous stainless steel or similar substrate 26 and thence into the nano pores 24 of the oxide layer 28. The nano-porous oxide layer 28 as well as the micro-porous substrate 26 is suitably saturated with a liquid like distilled water, gelatin solution in water, or any solution that doesn't cause immediate colloidal coagulation but encourages adherence of the colloidal particles to the walls of the nano- pores, and placed in a concentrated solution of salts like ammonium nitrate or ammonium carbonate, etc. The high ionic strength of the solution on the nanoporous oxide side causes the Pd or Pd-alloy colloid to agglomerate and deposit on the walls of the nano pores 24, blocking them when the colloidal sol reaches the high ionic strength solution. As in the previous example, it is desirable if the pH of solution on the nano-porous oxide side is such that the charge on the oxide surface is opposite to the charge on the Pd or Pd alloy particles .

In accordance with a specific example:

(Example 1) 100 ml of a 1.86 x 10^"3M Pd stock solution, (0.33Og of PdCl₂ in 20 ml of IM HCl and then diluting to 1 liter), was mixed with 100 ml of a 2.72 x 10^"1 M sodium citrate solution, (4.0Og of sodium citrate in 100 ml water) . Through the addition of IM NaOH, the pH of the reaction sol was adjusted until¹ it was approximately 8 - 8.5. The solution was then refluxed on an electric heating mantle for 2-4 hours until a light brown color was observed.

Next, a TiO₂ on Porous Stainless Steel Tube (PSST) from Graver^® with one end closed was used, where the TiO₂ layer was on the interior diameter of the PSST. A vacuum pump was attached to the closed end of the tube and the Pd colloidal sol was pulled through the membrane in order to fill the pores of the PSST and the TiO₂ nanoporous layer. The tube was then placed in a clean vessel with colloidal sol on one side and a 1OM ammonium nitrate and starch solution on the other. The system was allowed to set for 12-24h at 60 ⁰C. After being treated with the colloidal sol, the membrane was placed in furnace at 450 ⁰C for six hours at 4% hydrogen. It was then cooled in flowing, high purity N₂ to room temperature. To passivate the surface, the pure N₂ was gradually exchanged for N₂ with a few % O₂ at room temperature before exposing it to air.

The membrane tube was then transferred to a H₂ permeance test rig and flushed with N₂, then heated in N₂ to 350 C and the N₂ flow rate on the permeate side measured with a feed gas N₂ pressure of 29.4 psia, to detect pin holes and other imperfections (if any) in the membrane. Then the N₂ was replaced with H₂ at 29.4 psia and the initial H₂ flow through the membrane measured. It was then held under H₂ at 350 ⁰C and the H₂ flow then measured periodically. The H₂ was replaced with N₂ and the N₂ flow measured again. (Permeate side pressure ~14.7 psia. )

Fig. 4 illustrates that after a period of time the H₂ permeance increases more than the N₂ permeance, and then both stay constant. More specifically, the permeance of the membrane 10 to hydrogen (H₂) is depicted to rapidly increase to and remain at an acceptable level of about 8 m³/ (m²h atm⁰"⁵) , whereas the permeance to nitrogen (N₂) remains acceptably low at about 1 m³/ (m²h atm⁰'⁵) , although even lower values of < 0.1 m³/ (m²h atm⁰"⁵) are preferred and attainable.

Following the test described above, the sample was cooled under N₂ and sectioned for Scanning Electron Microscope examination. Figs. 5 and 6 show an elemental map of the membrane after three colloid/furnace treatments. Fig. 5 reveals the microporous stainless steel substrate containing the micron size pores, and the nanoporous TiO₂ oxide overlay containing the nano size pores. Fig. 6 is a further magnification of a portion of the TiO₂ oxide layer, showing the palladium particles deposited in nano pores of the ceramic layer of the membrane .

As a further aspect of the invention and returning to Fig. 1, the H₂-permeable membrane 10 is advantageously complimented by the inclusion of a water gas shift catalyst layer 14 to form the H₂-permeable membrane system 12» This catalytic material may be, but is not limited to, a noble metal-loaded, nanophase, active-oxygen mixed metal oxide, typically cerium-containing, like cerium- 5 zirconium, or doped cerium-zirconium oxide, where the zirconia may be partially or wholly replaced with hafnia, and significant levels of titania may also be substituted, and the dopants (≤12% atomic percent on a metals basis) may be Nb, Ta, Mo or W, and promoters like io rare earths, alkaline earths or thorium, Th, may also be present. The noble metals for the purpose of this invention include Ru, Rh, Pd, Re, Os, Ir, Pt and Au. The catalyst phase may be used alone or with an effective amount of a known binder such as alumina, silica, or the

15 like. As between the H₂-permeable membrane 10 and the water gas shift catalyst layer 14, the water gas shift catalyst is spaced from the Pd or Pd alloy plugs 22 of the H₂-permeable membrane to avoid the lack of performance that may be caused by intermetallic diffusion, and the

20 water gas shift catalyst is, of course, positioned for catalytic contact with reformate or the like for the formation of H₂. The Pd or Pd alloy plugs of the H₂- permeable membrane inside the pores of the nano-porous oxide layer are well positioned to avoid chemical

25 interaction with the water gas shift catalyst. The H₂- permeable membrane 10 is adjacent to and forms part of a permeate region 20 for the acceptance and collection of H₂ that permeates through the membrane 10. Whil-e- the^" embodiment pX±he-H₂-permeabϊe membrane system 12 depicted ^"30^~ in Fig. 1 chooses to place the water gas shift catalyst layer 14 on the interior surface of a tubular structure, it will be appreciated that the relative positions of the water gas shift catalyst layer and the Pd plugs 22 may be reversed. Indeed, the H₂-permeable membrane 10 and/or the

35 H₂-permeable membrane system 12 need not be tubular, but may assume other shapes such as planar, corrugated, or others that best suit the intended use.

Although the invention has been described and illustrated with respect to the exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made without departing from the spirit and scope of the invention.

Claims

Claims What is claimed is:

1 1.An H₂-permeable membrane (10), comprising: a porous

2 supporting structure (26, 28) having principally nano-

3 sized pores (24) adjacent at least one surface of the

4 porous supporting structure; and plugs (22) of Pd, or

5 an alloy of Pd, being located in and serving to β substantially seal the nano-sized pores (24) of the

7 porous supporting structure adjacent said at least one

8 surface against gas flow except for the permeation of

9 H₂ there through.

1 2. The H₂-permeable membrane (iθ) of claim 1 wherein the

2 nano-sized pores (24) have a relatively narrow size

3 distribution defining a typical pore size, and said Pd

4 or Pd alloy, plugs (22) are formed from coagulation of

5 a colloidal solution of the Pd or Pd alloy having been

6 introduced into said nano-sized pores.

1 3. The H₂-permeable membrane (10) of claim 2 wherein the

2 colloidal solution is formed of Pd or Pd alloy

3 ^■ particles, the typical size of the particles in the

4 colloid being in the range of one-half to one-tenth the

5 typical pore size.

l 4. The H₂-permeable membrane (10) of claim 3 wherein the

² _^ _ typical pore size of the nano-sized pores (24) of the

3 porous supporting structure are in the range of 80 to

4 100 nm.

1 5. The H₂-permeable membrane (10) of claim 1 wherein the

2 porous supporting structure comprises a layer of nano-

3 porous oxide (28) on a porous metal substrate (26) , the

4 layer of nano-porous oxide forming the at least one

5 surface of the porous supporting structure in which the

6 Pd or Pd alloy plugs (22) are formed.

6. The H∑-permeable membrane (10) of claim 5 wherein the porous metal substrate (26) is formed of stainless steel having pores (27) of greater diameter than the nano-pores (24) of the oxide layer (28).

7. The H2-permeable membrane (10) of claim 1 wherein the plugs (22) of Pd or Pd alloy in the nano-sized pores (24) of the porous supporting structure comprise at least a majority of the total mass of Pd or Pd alloy at and adjacent to said at least one surface of the porous supporting structure.

8. The H₂-permeable membrane (10) of claim 7 wherein the plugs (22) of Pd or Pd alloy in the nano-sized pores (24) of the porous supporting structure comprise substantially the entirety of the total mass of Pd or Pd alloy at and adjacent to said at least one surface of the porous supporting structure.

9. An integrated structure (12) to facilitate the water gas shift reaction of an H₂-containing reformate and the selective separation of H₂ therefrom as a permeate, comprising: a supporting barrier (26) of porous metal having 1^st and 2^nd opposite surfaces; a porous layer of supported catalyst material (14) supported by the supporting barrier of porous metal and disposed substantially on the 1^st surface thereof for contact with the H₂-containing reformate in a water gas shift reaction region (16) to facilitate a water gas shift reaction; a thin nano-porous membrane layer (28) of oxide supported by the supporting barrier (26) of porous metal and disposed substantially on the 2^nd surface thereof, the nano-porous membrane layer having principally nano-sized pores (24) therethrough; and plugs (22) of Pd or Pd alloy located in and serving to seal the nano-sized pores (24) of the nano-porous membrane layer (28) against gas flow except for the permeation of H₂ therethrough to a permeate region (20) , thereby to selectively separate H₂ from the water gas shift reaction region (16) .

10. The structure (12) of claim 9 wherein the plugs (22) of Pd or Pd alloy in the nano-sized pores (24) of the nano-porous membrane (28) comprise at least a majority of the total mass of Pd or Pd alloy at and adjacent to said thin nano-porous membrane layer.

11. The method of making an H₂-permeable membrane, comprising the steps of: selecting a porous supporting structure (26, 28) having principally nano-sized pores (24) adjacent at least one surface of the porous supporting structure; preparing a colloidal suspension of Pd, or Pd alloy, colloidal particles, said Pd or Pd alloy particles being smaller than the nano-sized pores in the porous supporting structure; relatively immersing at least the nano-sized pores of the porous supporting structure in the colloidal suspension of Pd or Pd alloy, colloidal particles; and causing the colloidal suspension of Pd or Pd alloy, colloidal particles to flow into the nano-sized pores of the porous supporting structure and agglomerate to form plugs of the Pd, or Pd alloy, in the nano-sized pores to substantially seal them against gas flow except for the permeation of H₂ there through.

12. The method of claim 11 wherein the agglomeration of the colloidal suspension of Pd or Pd alloy colloidal particles in the nano-sized pores of the porous supporting structure comprises the step of coagulating the colloidal suspension within the nano-sized pores.

13. The method of claim 11 wherein the step of causing the colloidal suspension of Pd or Pd alloy colloidal particles to flow into the nano-sized pores of the porous supporting structure comprises the step of applying a pressure to the suspension and directed to cause it to flow into the nano-sized pores.

14. The method of claim 13 wherein the nano-sized pores are defined by respective pore walls and further comprising the step of bathing the walls of the nano- sized pores with a coagulating solution to facilitate coagulation of the colloidal particles thereon.

15. The method of claim 14 comprising the further step of adjusting the pH of the coagulating solution relative to the pH of the colloidal suspension so as to have opposite charges such that resulting attractive forces facilitate coagulation of the colloidal particles.

16. The method of claim 11 wherein the nano-sized pores have typical pore diameters less than about 200 nm and the Pd or Pd alloy colloidal particles in the colloidal suspension are sized in the range of about 0.1 to 0.6 times that typical diameter.

17. The method of claim 16 wherein the typical pore diameters of the nano-sized pores are grouped at about 80 nm to 100 nm, and the Pd or Pd alloy colloidal particles in the colloidal suspension are sized in the range of about 8 nm to 60 nm.

18. The method of claim 11 wherein the step of preparing a colloidal suspension of Pd, or Pd alloy, colloidal particles includes the step of regulating thepH of the colloidal suspension to thereby regulate the sizes of the colloidal particles.

19. A membrane (10) permeable to a selected gas, comprising: a porous supporting structure (26, 28) having principally nano-sized pores (24) adjacent at least one surface of the porous supporting structure; and plugs (22) of a material permeable substantially only to the selected gas, being located in and serving to substantially seal the nano-sized pores (24) of the porous supporting structure adjacent said at least one surface against gas flow except for the permeation of the selected gas there through.

20. The membrane (10) of claim 19 wherein the membrane is to operate under certain operating conditions, the porous supporting structure (26, 28) has an effective diffusivity to the selected gas and is selected such that the nano-sized pores (24) have a relatively narrow size distribution defining a typical pore size, and said typical pore size is selected to be as small as possible without limiting the effective diffusivity of the selected gas under said operating conditions.