Note: Descriptions are shown in the official language in which they were submitted.
000353 KI'/AL
CA 02354920 2001-08-10
1
A process for preparing a composite metal membrane,
the composite metal membrane prepared therewith and its use
Description
The invention provides a process for preparing a composite metal membrane on a
porous membrane support. Composite metal membranes of this type are used for
separating gas mixtures, in particular for separating hydrogen from a
reformate gas for
supplying fuel cells with the required fuel gas.
For this purpose, palladium or palladium alloy membranes on either porous or
non-
porous supports are normally used, such as compact palladium or palladium
alloy
membranes. Foils made of hydrogen-permeable metals, inter alia, are used as
non-
porous supports. The permeability of the membranes for hydrogen increases with
temperature. Typical operating temperatures are therefore between 300 and 600
°C.
T. S. Moss and R. C. Dye [Proc.-Natl. Hydrogen Assoc. Annu. U.S. Hydrogen
Meet.,
8th (1997), 357-365] and T. S. Moss, N. M. Peachey, R. C. Snow and R. C. Dye
[Int. J.
Hydrogen Energy 2~, (1998), 99-106 ISSN: 0360-3199] describe the preparation
and
use of a membrane which is obtained by applying Pd or PdAg by cathode
atomisation to
both faces of a foil of a metal from group SB. The thickness of the layers
applied to the
two faces may be varied so that an asymmetric component is produced (for
example:
0.1 pm Pd / 40pm V / 0.5 pm Pd). Permeation trials demonstrate twenty-fold
higher
hydrogen permeation as compared with self supported Pd membranes. Accordingly,
the
membrane described is suitable for use in a PEM fuel cell system instead of
the
traditional catalytic gas purification steps (water gas shift reaction and
preferential
oxidation of CO).
GB 1 292 025 describes the use of iron, vanadium, tantalum, nickel, niobium or
alloys
thereof as a non-porous support for a non-coherent, or porous, palladium
(alloy) layer.
The palladium layer is applied by a pressing, spraying or electrodeposition
process in a
thickness of about 0.6 mm to a support with a thickness of 12.7 mm. Then the
thickness
of the laminate produced in this way is reduced to 0.04 to 0.01 mm by rolling.
According to DE 197 38 513 C1, particularly thin hydrogen separation membranes
(thickness of layer less than 20 Vim) can be prepared by alternate
electrodeposition of
palladium and an alloy metal from group 1B or 8 of the periodic system of
elements to a
metallic support which is not specified in any more detail. To convert the
alternating
' 000353 KY/AL
CA 02354920 2001-08-10
2
layers into a homogeneous alloy, appropriate thermal treatment may follow the
electrodeposition process.
Either metallic or ceramic materials are suitable as porous supports for
palladium (alloy)
membranes. In accordance with JP 05078810 (WPIDS 1993-140642), palladium may
be
applied to a porous support by a plasma spray process for example.
According to Y. Lin, G. Lee and M. Rei [fatal. Today 44 (1998) 343-349 and
Int. J. of
Hydrogen Energy 25 (2000) 211-219] a defect-free palladium membrane (thickness
of
layer 20-25 pro) can be prepared on a tubular support made of porous stainless
steel
316L in a electroless plating process and integrated as a component in a steam
reforming reactor. At working temperatures of 300 to 400°C, a purified
reformate
containing 95 vol.% HZ is obtained. However, the optimum working temperature
is very
restricted because below 300°C the palladium membrane starts to become
brittle due to
the presence of hydrogen, whereas above 400 to 450°C the alloying
constituents in the
stainless steel support diffuse into the palladium layer and lead to
impairment of the
permeation properties.
Electroless plating processes are preferably used for coating ceramic
supports. Thus,
CVD coating of an asymmetric, porous ceramic with palladium is described by E.
Kikuchi (fatal. Today 56 (2000) 97-101] and this is used in a methane steam
reforming
reactor for separating hydrogen from the reformate. The minimum layer
thickness is
4.5 pro. If the layers are thinner, the gas-tightness of the layer can no
longer be
guaranteed. Apart from CVD coating with pure Pd, coating with palladium alloys
is also
possible, wherein the alloy with silver prevents embrittlement of the
palladium
membrane and increases the permeability to hydrogen.
In addition to pure hydrogen separation membranes, membranes which are
provided
with a reactive layer in addition to the hydrogen separation layer (palladium)
are also
described for applications in fuel cell systems. Thus, the porous support for
a palladium
(alloy) membrane may be covered, for example on the face which is not coated
with Pd,
with a combustion catalyst. The heat released during combustion at the
reactive face is
then simultaneously used to maintain the operating temperature of the hydrogen
separation membrane (EP 0924162 A1 ). Such a component may then be integrated
in
the reforming process downstream of a reformer or incorporated directly in the
reformer
(EP 0924161 A1, EP 0924163 A1).
In addition, not only palladium membranes can be used for hydrogen separation
in the
fuel cell sector. EP 0945174 Al discloses a design for the use of universally
constructed
CA 02354920 2001-08-10
000353 KY/AL
3
layered membranes which may contain both fine-pore, separation-selective
plastics
and/or several ceramic layers and/or layers made of a separation-selective
metal
(preferably from groups 4B, SB or 8), wherein these layers are applied to a
porous
support (glass, ceramic, expanded metal, carbon or porous plastics).
The objective of developing metal membranes for the separation of hydrogen
from gas
mixtures is to obtain high rates of permeation for the hydrogen. For this
purpose, the
metal membrane must be designed to be as thin as possible while avoiding the
occurrence of leakiness in the form of holes. Such membranes can be processed
only in
a supported form. In order for the membrane support to have as little effect
as possible
on the permeation of hydrogen, it must have a high porosity. Thus there is the
difficulty,
in the case of known processes for preparing supported membranes, of
depositing a
defect-free membrane on a porous support. There are two problems involved
here. On
the one hand, the methods described for depositing for example palladium or a
palladium alloy can guarantee a relatively defect-free membrane layer only
above a
certain thickness of the layer. This minimum layer thickness is about 4 to 5
pm. On the
other hand, the coating techniques used for applying the membrane layer to the
porous
membrane support means that the average pore diameter of the membrane support
ought
not exceed a certain value because otherwise it would be impossible to apply
coherent
and defect-free coatings. The maximum pore sizes of known membrane support
materials, such as porous ceramics or porous metal supports, are therefore
less than
0.1 pm. This means that the resistance to flow of the gas through the pores
cannot be
reduced to a desirable extent.
WO 89/04556 describes an electrochemical process for preparing a pore-free
membrane
based on palladium supported by a pomus metal structure. In accordance with
the
process, a pore-free palladium(-silver) membrane on a porous, metallic support
is
produced by coating one face of a metal alloy foil (preferably brass) with
palladium or
palladium/silver (thickness of palladium layer: about 1 pm) using an
electrodeposition
process. The porosity of the support is produced later ,by dissolving the base
metal
components out of the brass foil. Dissolution is performed electrochemically,
wherein,
in a cyclic process, both metal support components are first taken into
solution but the
more base metal component is redeposited directly onto the palladium layer
(electrochemical recrystallisation). The less base metal component in the foil-
shaped
alloy thus goes virtually quantitatively into solution so that a porous metal
structure,
preferably a porous copper structure, remains as a support for the
palladium/silver
membrane.
CA 02354920 2001-08-10
000353 KY/AL
4
The process in accordance with WO 89/04556 has the disadvantage that the brass
foil
. used as support is virtually completely dissolved and has to be built up
again by
electrochemical recrystallisation. This means that the composite formed
between the
palladium layer and the support foil is destroyed. The mechanical strength of
the
recrystallised foil is low and its porosity is undefined.
The object of the present invention is to provide a simple and cost-effective
process for
the preparation of a composite metal membrane for separating hydrogen from gas
mixtures. Another object of the invention are composite metal membranes, the
membrane supports for which have a hitherto unrealisable, high, porosity
(average pore
sizes and pore volumes). A further object of the present invention are
composite metal
membranes in which the average pore size of the membrane support is greater
than the
thickness of the metal membranes.
These and other objects of the invention are achieved by a process for
preparing a
composite metal membrane which contains a thin metal membrane with a desired
thickness and a metallic membrane support with a porous structure, wherein the
metal
membrane and the membrane support consist of two different metals or metal
alloys.
The process is characterised in that a precursor of the metal membrane is
placed on a
non-porous precursor of the membrane support, the metal composite is produced
between the two precursors, the desired thickness of the metal membrane is
adjusted by
mechanically working the metal composite and then the porous structure for the
membrane support is produced.
Throughout this invention the terms "mechanical working", "metal working" and
"forming" are interchangeably used to denote chipless metal forming techniques
such as
rolling, pressing, flow moulding and deep-drawing.
Thus, according to the present invention, solid, pore-free metal foils are
initially used to
produce the composite metal membrane. For the gas separation membrane, a metal
foil
with a thickness between 50 and 100 pm is used as precursor. A foil of this
thickness
can be produced virtually pore-free in outstanding quality via a metal-
processing route.
This foil is placed on a thicker metal foil (or sheet) which later forms the
membrane
support. Thereafter the composite is formed between the two metal foils. This
is
preferably achieved by roll-bonding, explosive plating or diffusion welding.
The result
is a two-layered composite. Before bonding the metal foils, it is recommended
that the
contact areas be carefully cleaned and roughened in a known manner.
CA 02354920 2001-08-10
000353 KY/AL
When producing the metal composite by this process, a certain reduction in
thickness
takes place. After this, fizrther mechanical working procedures by means of
rolling,
pressing, flow moulding, deep-drawing or combinations of these forming
techniques
take place until the desired thickness of the metal membrane is achieved. The
measures
5 required for this such as, for example, thermal treatments between the
individual
forming steps, are known to a person skilled in the art of metals. The shape
of the final
composite metal membrane is not restricted to flat membranes. Rather, the
composite
metal membrane may be shaped to give various types of geometric structures
which
also have the advantage that their mechanical stability is substantially
better than that of
a flat membrane with the same wall thickness. The techniques which can be used
for
this are, for example, rolling, pressing, flow moulding or deep-drawing.
Mechanical
working the composite metal membrane to give thin tubules by means of a
drawing
process is mentioned in particular here.
The ratio of thicknesses between metal membrane and membrane support in the
final
composite metal membrane is preferably between 1:5 and 1:20 and corresponds to
the
ratio of the thicknesses of the initial foils before metal-processing has been
performed.
The metal-processing production of the metal membrane described has the
essential
advantage over known coating processes that a pore-free metal foil of high
quality can
be used initially and its freedom from pores can also be guaranteed after the
reforming
procedures.
Only after completing the reforming procedures is the porous structure of the
support
foil produced. The porous structure may be either a regular perforated
structure, which
can be produced, for example, by chemical, electrochemical or physical etching
processes, or else an open-pore structure with a statistical distribution of
pore sizes and
pore arrangements. The latter structure is preferably used. It can be produced
when the
precursor for the membrane support contains a two-phase or mufti-phase metal
alloy
and, after producing and reforming the metal composite, one or more alloy
phases are
electrochemically dissolved out of the membrane support.,
The membrane support preferably contains a eutectic alloy, wherein the porous
structure
is formed by electrochemical dissolution of the more base (more
ei~ectronegative) phase.
The eutectic alloy AgCu which contains an Ag-rich and a Cu-rich phase, for
example, is
especially suitable. The Cu-rich phase can be very easily dissolved out via an
electrochemical route. The Ag-rich phase then remains almost untouched.
Whereas the
membrane support in accordance with WO 89/04556 is completely dissolved and
then
CA 02354920 2001-08-10
000353 KY/AL
6
rebuilt, a rigid structure consisting of the Ag-rich alloy phase is retained
in accordance
with the present process, with corresponding positive effects on the stability
of the
membrane support.
Another advantage of the process according to the invention comprises the fact
that the
domain structure of the two-phase or mufti-phase metallic membrane support can
be
altered or adjusted within certain limits by choosing the alloy composition
and by
thermal treatment so that deliberate control of the porosity of the membrane
support is
possible. The pore diameter can be varied by the present process to a much
greater
extent than when using the traditional process. Thus it is also possible in
particular to
design the average pore diameter in the membrane support to be greater than
the
thickness of the metal membrane. Average pore diameters in the membrane
support
greater than 0.5 and less than 10 p,m are preferably striven for.
The copper content of the eutectic alloy is preferably between 20 and 80 wt.%,
with
respect to the total weight of alloy. Before dissolving the Cu-rich alloy
phase out of the
membrane support, the composite metal membrane is subjected to a thermal
treatment
at 400 to 750°C. On the one hand this reverses any structural changes
resulting from the
metal working process and on the other hand affects the structural
characteristics of the
membrane support, and thus its subsequent porosity in a desirable manner.
The proposed process is suitable for the preparation of supported metal
membranes
from a variety of materials. However, the metal membranes preferably contain
palladium or palladium alloys which have especially advantageous properties as
gas
separation membranes. Suitable palladium alloys are, for example, PdAg23,
PdCu40 or
a PdY alloy.
Another characteristic of the process is the fact that the structure of the
boundary
surface of the metal membrane is provided by the surface structure of the
metal foil
used in the preparation and thus can be relatively smooth. Subsequent
production of
porosity in the membrane support affects the surface structure of the metal
membrane to
only an insubstantial extent. The final metal membrane. therefore has a very
uniform
thickness and is substantially smooth.
When preparing the gas separation membrane, the smallest possible membrane
thickness is striven for in order to endow the membrane with a high hydrogen
permeability. Gas separation membranes of palladium or palladium alloys with a
thickness of more than 20 p,m are of only little interest for separating
hydrogen from gas
mixtures due to the high cost of the noble metal and its low permeability.
Membranes
CA 02354920 2001-08-10
000353 KY/AL
7
with a thickness of less than 1 ~.m on the other hand are still very difficult
to obtain with
the proposed process and may have a number of defects. In addition, the
permeability to
undesired gases also increases at these small thicknesses. As a result of
these two
effects, the separating power of a membrane with a membrane thickness of less
than 1
~m drops to values which are no longer tolerable. Therefore, gas separation
membranes
with a thickness between 5 and 1 ~m are preferably prepared with the aid of
the process.
The porous, metallic membrane support is used to support the thin metal
membrane,
wherein the membrane support should impair the permeability of the laminated
membrane as little as possible, as compared with a freely suspended metal
membrane of
the same thickness. On the other hand, a certain minimum thickness of membrane
support is required in order to ensure the requisite mechanical stability of
the laminated
membrane. The thickness of the membrane support should therefore be less than
100 ~.m and should not be less than 20 Vim. Membrane support thicknesses
between 50
and 20 ~,m are preferably striven for.
The process described so far produces the composite metal membrane by metal
working
of a two-layered arrangement of a precursor of the metal membrane and a
precursor of
the membrane support. For certain material combinations of metal membrane and
membrane support, it may be expedient to also provide a temporary covering
membrane
for the metal membrane in order to improve the processability during the metal
working
process. A base metal alloy or a metal alloy which can be readily removed in a
chemical
way, without the metal membrane or the membrane support being attacked, is
chosen as
the material for the covering membrane. The covering membrane may be removed
before, at the same time as or after the production of porosity in the
membrane support.
Naturally, in this process variant, the specifications already mentioned with
regard to
the choice of materials for the metal membrane and the membrane support and
also for
their thicknesses in the final metal composite and for the porosity of the
membrane
support still apply.
In another process variant, a second membrane support is used instead of the
temporary
covering membrane. In this case the metal membrane is located between two
membrane
supports. After completing the forming process, the requisite porosity is
produced in
both membrane supports. Therefore, the second membrane support advantageously
consists of the same material as the first membrane support.
The process products of this process variant are thus symmetric, three-layered
composite metal membranes, wherein both faces of the gas separation membrane
are
CA 02354920 2001-08-10
000353 KY/AL
8
covered by porous metallic membrane supports. For certain material
combinations, this
process variant also has better processability during the metal working
process than is
the case when preparing the two-layered composite metal membrane. For this
process
variant, the specifications already mentioned with regard to the choice of
materials for
the metal membrane and the membrane support and also for their thicknesses in
the
final metal composite and for the porosity of the membrane support also still
apply.
The composite metal membranes prepared by the process according to the
invention are
preferably used for the separation of hydrogen from gas mixtures, in
particular from
reformate gas. The various process variants enable the preparation of
composite metal
membranes in which the membrane supports have a previously unrealisable, high,
porosity (average pore sizes and pore volumes). With thicknesses of gas
separation
membrane of 1 to 20, preferably 1 to 5 pm, the membrane supports) have an
average
pore size greater than 0.5 and less than 10 pm. Thus, it is possible for the
first time,
using the process described above, to produce composite metal membranes in
which the
average pore size in the membrane supports) is greater than the thickness of
the gas
separation membrane. These composite metal membranes therefore have
outstanding
hydrogen permeability.
The invention is explained in more detail by means of the following examples.
Example 1:
A foil of PdAg23 (dimensions: 30 x 0.07 x 500 mm) was placed between two foils
of
AgCu28 (dimensions: 30 x 1.0 x 500 mm). The contact areas were carefully
cleaned and
mechanically roughened beforehand. The three foils were welded together at a
front
face and then bonded to each other by metal-processing in a hot roll-bonding
procedure.
For this purpose, the foils were annealed in a tubular furnace at 600°C
for a period of 20
min under an inert gas (argon) and then rolled out on preheated roll faces
(200°C) with a
deformation aspect of 45% to form one composite foil.
Further metal working to give a composite metal membrane with a total
thickness of 0.1
mm was performed by conventional strip milling in the cold state with
deformation
aspects of about 15% and intermediate annealing at 600°C for 15 min in
a tubular
furnace under an inert gas after a total deformation aspect of about 70%.
After completing the rolling process, the composite metal foil was subjected
to thermal
treatment under an inert gas (argon) at 600°C for a period of 30 min
and cleaned by
cathodic degreasing. The Cu-rich phase in the AgCu28 alloy was then anodically
CA 02354920 2001-08-10
000353 KY/AL
9
dissolved out in a sulfuric acid electrolyte with 10% strength sulfuric acid
operated
potentiostatically at 40°C and with a constant bath voltage of 220 mV
over the course of
16 hours. This produced an open-pore structure in the membrane support foils.
Metallographic examination and images produced by a scanning electron
microscope
over the cross-section of the finally produced composite metal membrane showed
a
firmly adhering, dense PdAg membrane with a membrane thickness of 3 to S ~m
and,
on both faces, the porous AgCu support layers with open porosity and a pore
size of 1 to
2 p,m.
Example 2:
In this example, a gas separation membrane supported on one face was prepared.
The
advantage of this one-faced arrangement is the larger exposed access area on
the gas
supply face and the associated lower resistance to diffusion of the composite
membrane.
A foil of PdAg23 (dimensions 30 x 0.07 x 500 mm) was placed on a foil of
AgCu28
(dimensions 30 x 1.0 x 500 mm). The two foils were welded together at a front
face.
The contact areas of the foils had been cleaned and roughened beforehand, as
described
in example 1.
The metal laminate was produced by hot roll-bonding as in example 1. Further
processing was also performed as described in example 1.
Example 3:
In this example, a gas separation membrane supported on one face was also
prepared.
To facilitate the laminating process, the foil of PdAg23 (dimensions: 30 x
0.07 x 500
mm) was placed between two strips, one of which consisted of an AgCu28 alloy
and
subsequently formed the membrane support, whereas the second foil consisted of
copper. The copper foil was used only as a temporary support foil and was
completely
removed during the electrolytic treatment to form the pores in the membrane
support
foil.
Example 4:
In this example, a tubular composite metal membrane was produced.
A round plate of PdAg23 (diameter 60 mm; thickness 1 mm) was placed between a
lower round plate of AgCu28 (diameter 60 mm; thickness 12 mm) and an upper
round
CA 02354920 2001-08-10
000353 KY/AL
plate of copper (diameter 60 mm, thickness 8 mm). The contact areas had been
carefully
cleaned and mechanically roughened beforehand.
The round plates were inserted in a hydraulic press and pressed together with
a
compression force of 2000 kg/cm2 to produce the metal composite. This produced
a
5 reduction in thickness of about 10 %. Cylindrical pellets with a diameter of
12 mm were
cut out of the laminated plate produced in this way and moulded into tubular
blanks, the
walls of which consisted, from the inside to the outside, of a layer of
copper, a layer of
PdAg23 and a layer of AgCu28, in an inverted flow-moulding process using a
hydraulic
press. The tubular blanks were drawn out by conventional tube drawing, after
thermal
10 treatment at 600°C for 20 min under an inert gas and rapid cooling,
in several steps to
form tubes with an external diameter of 2 mm and a total wall thickness of 0.2
mm.
Between the individual drawing steps, appropriate intermediate annealing was
performed to provide sufficient softening for further forming.
The layer of copper found on the internal face of the tube wall was completely
removed
and the Cu-rich alloy phase was dissolved out of the AgCu alloy on the
external face by
electrochemical treatment so that thin-walled PdAg23 tubules with porous
support
structures of silver on the external faces were obtained.
Example 5:
Example 4 was repeated, but this time the round plate of copper used only for
temporary
support purposes was omitted so that the PdAg surface was present directly on
the
internal face of the tubes prior to the final electrochemical treatment. As a
result,
difficult dissolution of the internal coating, in particular in the case of
small tube
diameters, was not required.
