Note: Descriptions are shown in the official language in which they were submitted.
CA 02615054 2007-12-24
SYSTEM AND METHOD FOR GENERATING HYDROGEN GAS
This is a division of copending Canadian Application Serial No. 2,452,331,
based on PCT/US02/18805, filed June 14, 2002.
FIELD OF THE INVENTION
The invention relates to a method and arrangement for generating hydrogen gas
using a catalyst from a fuel such as borohydride. More particularly, the
invention
relates to a method and arrangement in which fuel is delivered to a catalyst
chamber by
means of internally generated differential pressure without requiring an
elaborate
electrically powered pumping system.
BACKGROUND OF THE INVENTION
Hydrogen is a "clean fuel" because it can be reacted with oxygen in hydrogen-
consuming devices, such as a fuel cell or combustion engine, to produce energy
and
water. Virtually no other reaction byproducts are produced in the exhaust. As
a result,
the use of hydrogen as a fuel effectively solves many environmental problems
associated with the use of petroleum based fuels. Safe and efficient storage
of hydrogen
gas is, therefore, essential for many applications that can use hydrogen. In,
particular,
minimizing volume, weight and complexity of the hydrogen storage systems are
important factors in mobile applications.
Several methods of storing hydrogen currently exist but are either inadequate
or
impractical for wide-spread consumer applications. For example, hydrogen can
be
stored in liquid form at very low temperatures. Cryogenic storage provides a
volume
density of 70 grams of hydrogen per liter, but is limited further by the
weight of tanks
required for storage which limits its use for consumer applications. In
addition, the
energy consumed in liquefying hydrogen gas is about 60% of the energy
available from
the resulting hydrogen. Finally, liquid hydrogen is not safe or practical for
most
consumer applications.
An alternative is to store hydrogen under high pressure in cylinders. However,
a
100 pound steel cylinder can only store about one pound of hydrogen at about
2200 psi,
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which translates into 1% by weight of hydrogen storage. More expensive
composite
cylinders with special compressors can store hydrogen at higher pressures of
about
4,500 psi to achieve a more favorable storage ratio of about 4% by weight.
Although
even higher pressures are possible, safety factors and the high amount of
energy
consumed in achieving such high pressures have compelled a search for
alternative
hydrogen storage technologies that are both safe and efficient.
An altemate hydrogen generation technology has been developed for producing
hydrogen on demand using a stabilized metal hydride solution and a hydrogen
generation catalyst system. Typically, such a system requires an elaborate
pumping
system and an electrical power supply for moving the metal hydride solution
into the
hydrogen generation catalyst system and for removal of spent reactant.
SUMMARY OF THE iNVENTION
Accordingly, it is an object of the invention to provide a hydrogen generation
system which does not require a pumping system which is operated by an
external
electrical power source.
It is another object of the invention to provide a hydrogen generation system
which requires no electrical power to operate.
It is a further object of the invention to provide a hydrogen generation
system
which can be used to augment of a hydrogen generation system containing an
electrically powered pump by reducing the differential pressure load on the
pump.
It is another object of the invention to provide a hydrogen power generation
system which can be used to augment military and small stationary hydrogen
generation
systems used as micropower sources.
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In accordance with one aspect of the present invention there is provided
apparatus
for use in a system for generating hydrogen and a spent material from a
reactant material,
said apparatus comprising a fuel container having first and second portions
separated by a
partitioning element, said first portion having an output port said second
portion having an
input port; a reactant material capable of generating hydrogen disposed within
said first
portion, and wherein said partitioning element is configured so as to move and
decrease
the volume of said first portion as said reactant material is outputted
through said output
port during operation of said system and said spent material is inputted
through said input
port to said second portion.
In accordance with another aspect of the present invention there is provided
an
arrangement for generating hydrogen gas comprising: (a) a catalyst chamber
comprising a
catalyst, (b) a fuel chamber configured to retain a reactant material under
pressure, said
reactant material capable of generating hydrogen gas when contacting said
catalyst, (c) a
spent fuel chamber connected to the catalyst chamber for receiving said
reactant material
after its contact with said catalyst and for receiving hydrogen gas generated
by the contact
of the reactant material with the catalyst.
In accordance with yet another aspect of the present invention there is
provided a
method of generating hydrogen gas comprising: providing a catalyst; providing
a fuel
chamber containing a reactant material under pressure, said reactant material
capable of
generating hydrogen upon contact with said catalyst; and bringing said
reactant material
and said catalyst into contact with one another using said pressure.
These and other objects of the invention are achieved in an exemplary
embodiment of the present invention by providing an arrangement for generating
hydrogen gas comprising a catalyst chamber including a catalyst, a fuel
chamber
comprising a reactant material capable of generating hydrogen gas when
contacting
the catalyst, a spent fuel chamber connected to the catalyst
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CA 02615054 2007-12-24
chamber for receiving spent reactant material and hydrogen gas, a conduit
between the
fuel chamber and spent fuel chamber which includes a check valve, and an
outlet
conduit connected to the check valve.
The invention also provides in a further exemplary embodiment for a method
for generating hydrogen gas using such an arrangement in which pressure is
applied to
the fuel chamber causing reactant material to be transported to and contacted
with a
catalyst thereby generating hydrogen gas and spent reactant material.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an illustration of an arrangement for a hydrogen gas generation
system in accordance with the invention;
Figure 2 is a schematic illustration of the arrangement for a hydrogen gas
generation system in Figure 1;
Figure 3 is a schematic illustration for a hydrogen gas generation system in
accordance with the invention in which the fuel vessel includes a piston;
Figure 4 is a schematic illustration for a hydrogen gas generation system in
accordance with the invention in which the fuel vessel includes a flexible
bladder;
Figure 5 is a schematic illustration for a hydrogen gas generation system
including a fuel pump in accordance with the invention;
Figure 6 is a schematic illustration for a hydrogen gas generation system
including a fuel level sensor in accordance with the invention;
Figure 7 is an illustration of a volume exchange tank for use in the
invention;
Figures 8A, 8B and 8C are schematic illustrations of flexible bladder
arrangements for housing fuel, spent fuel or both respectively, in accordance
with the
invention; and
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Figures 9A is an illustration of a plurality of tanks arranged in parallel for
use in
the invention and Figure 9B is an illustration of a plurality of tanks which
can be
individually selected for use in accordance with the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides for an arrangement for generating hydrogen gas
which does not require an electrically powered pump to move catalyst or
reactant
components. Instead, the novel arrangement utilizes internally generated
differential
pressure to control transport of reactant to the hydrogen generation catalyst.
In an embodiment of the invention shown in Figure 1, the hydrogen gas
generation arrangement includes a borohydride fuel vessel 10 which is
connected to a
catalyst chamber 20 through a fuel inlet line 15. Upon reaction of the
borohydride fuel
with the catalyst, borate and hydrogen gas flow from the outlet of the
catalyst chamber
25 into the spent fuel chamber 30. By locating the catalyst remote from the
fuel
chamber, the fuel and spent fuel are separated from each other avoiding
constant
dilution of the fuel concentration.
The spent fuel chamber 30 is connected to the fuel vessel 10 by a conduit
having
a check valve 40 which allows hydrogen gas flow only in one direction. A
hydrogen
gas outlet line 50 conveys hydrogen gas out of the system. A delivery
pressure/flow
regulator 55 can be used to control hydrogen flowing to the outlet line 50. A
pressure
gauge 60 can be used to monitor the pressure in the system. A set pressure
regulator/relief valve 100 can be used to set and maintain the maximum
operating
pressure of the system by relieving pressure directly to the outlet line 50
via a bypass
line 110. As a safety precaution a manual pressure relief outlet line 70 and
manual
pressure relief safety valves 80 and 90 can release pressure in the event set
pressure
regulator/relief valve 100 is unable to set and maintain the pressure in the
system. A
fuel shut-off valve 120 can stop fuel from being transported to the catalyst
chamber.
The invention will be better understood in view of the following example which
is merely illustrative and is not meant to limit the scope of the invention.
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Example I
A schematic diagram of the arrangement in Figure I according to the invention
is shown in Figure 2. A fuel solution comprising 20 % NaBH4 by weight, 3 %
NaOH
by weight, and 77 % H20 by weight was poured into fuel vessel 10 such that
approximately half the vessel was filled with solution. An initial pressure
was supplied
to the system by introducing N2 gas through line 135 with regulator valve 55
in a closed
position. Note that the source of the initial pressure is not limited to N2
gas; for
example, it could alternatively be H2 gas arising from forcing a small amount
of fuel
over the catalyst. The pressure gauge 150 read 15 psi. The fuel shut-off valve
120 was
opened and as expected, no pressure drop was observed. To initiate hydrogen
generation, regulator valve 55 was opened about halfway. Gas began to flow out
of
outlet line 50 as evidenced by the appearance of bubbles in water beaker 180.
Very
soon after the bubbling began, fuel was observed moving through the fuel inlet
line 190
into the catalyst chamber 20 through check valve 210. The catalyst chamber 20
surface
temperature increased to approximately 60 C and borate and hydrogen gas flowed
into
spent fuel vessel 30.
The pressure observed at pressure gauge 150 was approximately 16 psi and was
relieved via set pressure regulator/relief valve 100 to the outlet line 50. As
the pressure
dropped to approximately 15 psi on gauge 150, more fuel flowed from fuel
vessel 10 to
catalyst chamber 20 due to the fact that the pressure PF in the fuel vessel 10
was greater
than the pressure Po in the spent fuel vessel 30. The pressure at gauge 150
rose again to
approximately 16 to 17 psi and the same cycle of pressure drop and pressure
rise due to
the transport of fuel and generation of hydrogen gas was repeated. The
generated H2
rate was measured as roughly 1.5 SLM (Standard Liters per Minute), or the
equivalent
of about 100 watts of equivalent fuel cell power, operating at a delivery of
approximately 15 psi.
The regulator 55 was adjusted to vary the flow rate and no detrimental effects
were observed. The outlet line 50 was blocked with a finger thereby stopping
gas flow.
CA 02615054 2007-12-24
Upon removal of the finger, flow generation immediately continued and no
detrimental
effects were observed.
The system was allowed to run until the fuel vessel 10 was virtually empty of
fuel. After the fuel was consumed the system was depressurized through outlet
line 50.
The spent fuel collected in chamber 30 was examined. The spent fuel exhibited
a fairly
high conversion yield estimated as greater than 80%. Based on this experiment
it is
estimated than such an arrangement according to the invention could operate at
approximately 1.5 SLM for approximately 4.3 hours using 750 ml of 20 wt.%
NaBH4,
assuming a 100% conversion.
In another embodiment of the invention shown in Figure 3, the fuel vessel 10
can incorporate a piston 230 to retain the hydrogen gas pressure separate from
the fuel
liquid. This can allow the fuel vessel 10 to be oriented in any direction
without relying
upon the direction of gravity to direct the fuel downward out of the fuel
vessel 10. That
is, the piston 230 prevents the hydrogen gas providing the pressure PF from
traveling
through the fuel shut-off valve 120. In another embodiment of the invention
shown in
Figure 4, the fuel vessel 10 can incorporate a flexible bladder 250 to achieve
similar
directional independence of the fuel vessel 10.
In another embodiment of the invention shown in Figure 5, a fuel pump 270 can
be interposed between the fuel shut-off valve 120 and the catalyst chamber
inlet check
valve 210. The fuel pump 270 may require external power to operate, however,
the
system is enhanced due to the fact that the pump is required to pump only
against the
differential pressure (PB-PF) as opposed to a typical system in which the pump
would be
required to pump against the pressure PB. The reduced differential pressure
will reduce
the strain on the pump, may allow for delivery pressures higher than
achievable without
the differential pressure enhancement, and may make pump specification and
purchase
easier.
In another embodiment of the invention shown in Figure 6, a large reservoir of
fuel is stored at ambient pressure in a main fuel tank 300. A fuel level
sensor 340
senses the fuel level within the fuel vessel 10_ When the fuel level sensor
340 senses a
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CA 02615054 2007-12-24
predetermined "low" level, the fuel pump 320 tums on and pumps fuel through
the fuel
line 330 refilling the fuel vessel 10. When the fuel level sensor 340 senses
that the fuel
level has reached a predetermined "full" level, the fuel pump 320 turns off
ceasing
delivery of fuel via fuel line 330. Additionally, during this cycle, the spent
fuel drain
valve 350 can open allowing the hydrogen "pressure (PB) within the spent fuel
vessel 30
to push the spent fuel through the spent fuel drain line 370 into the ambient
pressure
main spent fuel tank 310. When the spent fuel level sensor 360 senses a
predetermined
"low" level of spent fuel, the spent fuel drain valve 350 can be closed. This
embodiment, while possibly requiring extemal power to run some of the
components
has the potential advantage of much larger fuel and spent fuel tanks that are
both at
ambient pressure. Essentially, this allows the system to be run for much
longer periods
of time (if not constantly) between refueling and/or draining spent fuel. Of
course, the
exact arrangement and sequence detailed in this example is simply illustrative
and is not
meant to limit the scope of the invention.
Figure 7 shows an altemative embodiment of the ambient pressure tanks 300
and 310 used in Figure 6. The two ambient pressure tanks 300 and 310 are
replaced by
a single "volume exchanging tank" comprising a fuel area 400, a spent fuel
area 410,
and a movable partition 420 (e.g., a piston). As fuel is consumed from the
fuel area 400
and spent fuel is returned to the spent fuel area 410, the movable partition
420 slides
such that space that originally occupied by fuel becomes occupied by spent
fuel. This
has the obvious advantage of reducing the overall volume needed to store both
fuel and
spent fuel. The movable partition 420 can be designed as a heat insulator.
Also, the
movable partition 420 can be eliminated if either the fuel, spent fuel, or
both materials
are contained within a flexible bladder 430 as in Figures 8A, 8B, and 8C.
Another embodiment of a volume exchanging tank is shown in Figures 9A and
9B. The system contains any number of discrete tanks that initially contain
fuel 500 or
are empty 520. When the fuel contained within a given fuel tank 500 is
consumed, the
tank is then to be used for storing spent fuel such as tank 510, and fuel is
consumed
from a different fuel tank 500. The system in Figure 9A shows how the tanks
can be
multiplexed in parallel via the fuel control valves 540 and spent fuel control
valves 530
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CA 02615054 2007-12-24
to direct the fuel into the fuel line 330 and direct spent fuel from the spent
fuel line 360,
respectively. Figure 9B shows an embodiment in which the tanks can be
selectively
filled in series via the control valves 550. The valves 530, 540, and 550 can
be manual
or actuated by other automatic means. Of course, the exact arrangement
detailed in this
example is simply illustrative and is not meant to limit the scope of the
invention. For
example, the system could be arranged in a cylindrical, spherical, or other
geometry.
The fuels used in the present invention include solutions of (i) a metal
hydride,
(ii) at least one stabilizing agent, and (iii) a solvent. The term "solution,"
as used
herein, includes a liquid in which all the components are dissolved and/or a
slurry in
which some of the components are dissolved and some of the components are
undissolved solids. The term "about," as used herein, means plus or minus 10%
of the
stated value.
Complex metal hydrides have been found to be useful in the hydrogen
generation systems of the present invention. These complex metal hydrides have
the
general chemical formula MBH4. M is an alkali metal selected from Group
1(fonnerly
Group IA) or Group 2 (formerly Group IIA) of the periodic table, examples of
which
include lithium, sodium, potassium, magnesium, or calcium. M may, in some
cases,
also be ammonium or organic groups. B is an element selected from group 13
(formerly Group IIIA) of the periodic table, examples of which include boron,
aluminum, and gallium. H is hydrogen. Examples of metal hydrides to be used in
accordance with the present invention include, but are not limited to, NaBHa,
LiBH4,
KBH4, Mg(BH.4)2, Ca(BH4)2, NI-I4BH4, (CH3)4NH4BH4, NaAlH4, LiAlH4, KAIH4,
NaGaH4, LiGaH4, KGaH4, and mixtures thereof. Without wanting to be limited by
any
one theory, it is believed that metal hydrides, especially borohydrides, are
most stable in
water at basic pH's, i.e., the metal hydrides do not readily decompose when in
contact
with water at high pH's. The following borohydrides are preferred: sodium
borohydride
(NaBH4), lithiuni borohydride (LiBH4), potassium borohydride (KBH4), ammonium
borohydride (NH4BH4), tetramethyl ammonium borohydride ((CH3)4NH4BH4),
quaternary borohydrides, and mixtures thereof.
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The metal hydride solutions of the present invention include at least one
stabilizing agent, since aqueous borohydride solutions slowly decompose unless
stabilized. A stabilizing agent, as used herein, is any component which
retards,
impedes, or prevents the reaction of metal hydride with water. Typically,
effective
stabilizing agents maintain metal hydride solutions at a room temperature (25
C) pH of
greater than about 7, preferably greater than about 11, and more preferably
greater than
about 13.
Useful stabilizing agents include the corresponding hydroxide of the cation
part
of the metal hydride salt. For example, if sodium borohydride is used as the
metal
hydride salt, the corresponding stabilizing agent would be sodium hydroxide.
Hydroxide concentrations in stabilized metal hydride solutions of the present
invention
are greater than about 0.1 molar, preferably greater than about 0.5 molar, and
more
preferably greater than about I molar or about 4% by weight. Typically, metal
hydride
solutions are stabilized by dissolving a hydroxide in water prior to adding
the
borohydride salt. Examples of useful hydroxide salts include, but are not
limited to,
sodium hydroxide, lithium hydroxide, potassium hydroxide, and mixtures
thereof.
Sodium hydroxide is preferred because of its high solubility in water of about
44% by
weight. Although other hydroxides are suitable, the solubility differences
between
various metal hydrides and various hydroxide salts must be taken into account
since
such solubility difference can be substantial. For example, adding too much
lithium
hydroxide to a concentrated solution of sodium borohydride would result in
precipitation of lithium borohydride.
Other non-hydroxide stabilizing agents include those that can raise the
overpotential of the metal hydride solution to produce hydrogen. These non-
hydroxide
stabilizing agents are preferably used in combination with hydroxide salts.
Nonlimiting
examples of non-hydroxide stabilizing agents include compounds containing the
softer
metals on the right side of the periodic chart. Nonlimiting examples of these
non-
hydroxide stabilizing agents include compounds containing lead, tin, cadmium,
zinc,
gallium, mercury, and combinations thereof. Compounds containing gallium and
zinc
are preferred, because these compounds are stable and soluble in the basic
medium. For
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CA 02615054 2007-12-24
example, zinc and gallium form soluble zincates and gallates, respectively,
which are
not readily reduced by borohydride.
Compounds containing some of the non-metals on the right side of the periodic
chart are also useful in stabilizing metal hydride solutions. Nonlimiting
examples of
these non-hydroxide stabilizing agents include compounds containing sulfur,
such as
sodium sulfide, thiourea, carbon disulfide, and mixtures thereof.
Preferably, the catalyst facilitates both aspects of the reaction of the metal
hydride and water: (i) the availability of a hydrogen site and (ii) the
ability to assist in
the hydrolysis mechanism, i.e., reaction with hydrogen atoms of water
molecules.
Metal hydride solutions are complex systems having multi-step reduction
mechanisms.
For example, borohydride has 4 hydrogens and an 8-electron reduction
mechanism.
Thus, once a single hydrogen atom is removed from a borohydride molecule, the
remaining moiety is unstable and will react with water to release the
remaining
hydrogen atoms. Catalysts that are useful according to the present invention
include,
but are not limited to, transition metals, transition metal borides, alloys of
these
materials, and mixtures thereof.
Transition metal catalysts useful in the catalyst systems of the present
invention are described in U.S. Patent No. 5,804,329, issued to
Amendola. Transition metal catalysts, as used herein, are catalysts containing
Group IB to Group VIIIB metals of the periodic table or compounds made from
these
metals. Representative examples of these metals include, but are not limited
to,
transition metals represented by the copper group, zinc group, scandium group,
titanium
group, vanadium group, chromium group, manganese group, iron group, cobalt
group,
and nickel group. Transition metal elements or compounds catalyze chemical
reaction
MBH4 + 2 H20 --> 4 H2 + MBO2) and aid in the hydrolysis of water by adsorbing
hydrogen on their surface in the form of atomic H, i.e., hydride H- or
protonic hydrogen
H+. Examples of useful transition metal elements and compounds include, but
are not
limited to, ruthenium, iron, cobalt, nickel, copper, manganese, rhodium,
rhenium,
platinum, palladium, chromium, silver, osmium, iridium, borides thereof,
alloys thereof,
CA 02615054 2007-12-24
and mixtures thereof. Ruthenium, cobalt and rhodium and mixtures thereof are
preferred.
The catalysts used in the catalyst systems of the present invention preferably
have high surface areas. High surface area, as used herein, means that the
catalyst
particles have small average particles sizes, i.e., have an average diameter
of less than
about 100 microns, preferably less than about 50 microns, and more preferably
less than
about 25 microns. The chemical reaction of borohydride and water in the
presence of
the catalyst follows zero order kinetics at all concentrations of borohydride
measured,
i.e., volume of hydrogen gas generated is linear with time. It is, therefore,
believed that
the reaction rate depends primarily on the surface area of the catalyst.
One method of obtaining catalyst particles with high surface areas is to use
catalysts with small average particle sizes. Although catalyst with small
average
particle sizes are preferred, small particles can be swept away by the liquid
metal
hydride solution if they are small enough to pass through the containment
system. Such
deficiencies can be avoided by forming large aggregates of the small catalyst
particles.
Large aggregate catalyst particles, as used herein, are masses or bodies
formed from any
small catalyst particles by well-known powder metallurgical methods, such as
sintering.
These metallurgical methods can also be used in making various convenient
shapes. It
is believed that these large aggregate catalyst particles maintain high
surface areas
because they are very porous. The catalyst particles are packed into a
catalyst chamber.
Altematively, the hydrogen generation catalysts can be formed into fine wires
or
a mesh of fine wires. These fine wires have a diameter of less than about 0.5
mm,
preferably less than about 0.2 mm, and more preferably less than about 20
microns.
In its simplest form, the catalyst chamber is a liquid and gas petmeable mesh
that traps or holds particulate catalysts, while allowing liquids and gases to
pass freely
through the containment system. In this embodiment the catalyst particles are
larger
than the spaces provided by the containment system. For example, metal hydride
solution can flow into the containment system to react with the catalyst,
while oxidized
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CA 02615054 2007-12-24
metal hydride, hydrogen gas, and unreacted metal hydride can easily pass out
of the
containment system.
Preferably, the catalyst particles can be encapsulated in a removable tube or
cylinder, wherein the ends of the cylinder are covered with the porous or mesh
material.
Porous or mesh material that are useful herein include ceramics, plastics,
polymers,
nonwovens, wovens, textiles, fabrics, carbons, carbon-fibers, ion exchange
resins,
metals, alloys, wires, meshes, and combinations thereof. Typically, the porous
or mesh
material is in the form of a sheet. Nonlimiting examples of porous or mesh
material
include nylon screens and stainless steel screens.
A contained high surface area catalyst can be obtained by binding or
entrapping
a transition metal catalyst onto and/or within a porous or nonporous substrate
by
chemical means. By porous is meant that the material is liquid and gas
permeable.
Generally, this process includes (i) dispersing a solution having a transition
metal ion
onto and/or within a substrate by contacting the solution with the substrate,
and (ii)
reducing the dispersed transition metal ions to the neutral valence state of
the transition
metal, i.e., metallic form. Without wanting to be limited by any one theory,
it is
believed that this unique process binds and/or entraps transition metal
catalyst at a
molecular level onto and/or within the substrate. These steps can also be
repeated to
obtain layers of transition metal molecules bound onto and/or entrapped within
the
substrate. High surface area for substrate bound catalysts, as used herein,
means that a
porous substrate has an effective surface area of greater than about 10 m2/g
or and a
nonporous substrate has an average diameter of less than about 50 microns.
Nonlimiting examples of porous substrates include ceramics and ionic exchange
resins.
Nonlimiting examples of nonporous substrates includes, metals, wires, metallic
meshes,
fibers and fibrous materials, such as ropes.
Transition metal ion, as used herein, means an anion, a cation, an anion
complex
or a cation complex of a transition metal that is described above. Transition
metal ions
can be obtained from dissolving salts of transition metals, which are readily
available
from commercial manufacturers, such as Alfa Aesar Company and Aldrich Chemical
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CA 02615054 2007-12-24
Company. The transition metal salts may be dissolved in any solvent, typically
water.
The reducing agent can be any material or compound that is capable of reducing
the
transition metal ion to its neutral valence state. Nonlimiting examples of
reducing
agents include hydrazine, hydrogen gas, glucose hydroxylamine, carbon
monoxide,
dithionite, sulfur dioxide, borohydride, alcohols and mixtures thereof.
Typically, most
transition metals that catalyze metal hydrides, such as borohydride, can also
be reduced
by the same metal hydrides. For example, borohydride is a suitable reducing
agent.
Nonlimiting examples of suitable substrates include ceramics, plastics,
polymers, glass, fibers, ropes, nonwovens, wovens, textiles, fabrics, the many
forms of
carbon and carbon-fibers, ion exchange resins, metals, alloys, wires, meshes,
and
combinations thereof. Nonlimiting examples of ceramic substrates with various
pore
sizes include metal oxides, zeolites, perovskites, phosphates, metal wires,
metal meshes,
and mixtures thereof. Specific examples of suitable substrates include, but
are not
limited to zirconium oxides, titanium oxides, magnesium oxides, calcium
oxides,
zeolites, cationic exchange resins, anionic exchange resins, fibrous
materials,
nonwovens, wovens, aramid fibers such as NOMEXO and KEVLAR ,
polytetrafluoroethylene (PTFE), and combinations thereof. Since metal hydride
solutions can have a high pH, substrates that do not dissolve or react with
caustics are
preferred. Also preferred are porous substrates with effective surface areas
of greater
than about 50 m'/g or nonporous substrates with an average diameter of less
than about
50 microns.
When the substrate is in the form of beads, it is preferable to have the beads
in a
containment system, as described above, wherein the average diaineter of the
beads is
greater than the spaces of the containment system. Furthermore, if the
substrate has a
surface treatment, such treatments can be removed by appropriate methods, such
as by
boiling or applying a solvent. For example, substrates treated with wax can be
boiled.
Altematively, the wax can be removed by using acetone. Similarly, the starch
on
textiles can be removed by boiling in water.
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The substrates, except for the ion exchange resins described below, can be
treated with the catalyst in the following manner. The substrate is first
soaked in a
solution containing the transition metal salt, e.g., ruthenium trichloride.
Solutions
having concentrations close to saturation are preferred. This step disperses
the
transition metal salt into and/or onto the substrate. The treated substrate is
then dried,
typically with heat. Optionally, the treated substrate can be filtered before
being dried.
Note that the treated substrate is not rinsed. It is believed that the drying
step promotes
absorption of the transition metal ions onto and/or within the substrate by
removing the
solvent. The dry, treated substrate is then subjected to a solution containing
a reducing
agent, such as sodium borohydride, at a concentration sufficient to provide
complete
reduction, e.g., 5% by weight of sodium borohydride. Although this step can be
conducted at room temperature, it is preferred to reduce the absorbed
transition metal
ions at an elevated temperature, e.g., greater than about 30 C, to increase
the reduction
rate. It is believed that the reduction step converts transition metal ions
into its neutral
valence state, i.e., the metallic state. After rinsing with water, the
substrate is ready for
use as a catalyst in the reaction of the metal hydride and water to produce
hydrogen gas.
The method can be repeated to obtain a desired loading of transition metal
onto and/or
within the substrate.
This method to obtain a contained high surface area catalyst can also be
adapted
to utilize chemical vapor deposition technology (CVD) by forming a transition
metal
complex that can be evaporated, i.e., boiled or sublimed, in a vacuum. The
transition
metal complex includes a transition metal ion, as described above, and a
chemical vapor
deposition complexing compound. Since the substrate is cold, the transition
metal
complex will recondense onto the substrate. Any suitable substrate, as
described above,
can be used. Any suitable chemical vapor deposition complexing compound that
is
known in the art can also be used. Nonlimiting examples of metal complexes
useful for
chemical vapor deposition are metal diketonates such as Ru(acac) or Co(acac)3
and
metal alkoxides such as Ti(OiPr)4 (acac = acetylacetonate; OiPr =
isopropoxide). The
transition metal complex that is deposited on the substrate can then be
reduced by any
of the above described reducing agents.
14
CA 02615054 2007-12-24
Alternatively, this method can be adapted to utilize electroplating
techniques,
i.e., electroplating a conductive substrate in a solution having a transition
metal ion.
Useful transition metal ions are described above. The transition metal can be
electroplated onto a conductive substrate, such as nickel or stainless steel
fine wire,
screens comprising such fine wires, or metallic sheets. Typically, such fine
wires can
liave an average diameter of less than about 20 microns, preferably less than
about 10
microns, and more preferably less than about 2 microns.
In one preferred mode of electrochemical plating, a rough coating is obtained
instead of the typical smooth or "bright coatings." Without wanting to be
limited by
any one theory, it is believed that these rough coatings have a high surface
area. These
rough coatings are often black in color, and are typically referred to in the
art of
electrochemical plating by the element name followed by the word "black,"
e.g.,
platinum black or ruthenium black. Most of the transition metals described
above can
be coated as " transition metal blacks." The exact conditions may vary between
the
elements, but the common parameter is application of a varying voltage during
the
plating process. "Varying voltage" means that the voltage is changed,
alternated,
stepped up, or stepped down in a cyclic or noncyclic manner. For example, a DC
voltage can be turned on or off over time. Alternatively, the current can be
periodically
reversed, or the voltage may be switched from a lower to higher voltage and
then back
to the lower voltage. It is also common to superimpose an AC signal onto a DC
source.
In still another example, this method to obtain a contained high surface area
catalyst can also be adapted to utilize sputter deposition technology, e.g,,
physical vapor
deposition, which is well known to those skilled in the art of surface coating
technology. In sputter deposition, atoms of a metal surface are vaporized by
the
physical ejection of particles from a surface induced by momentum transfer
from an
energetic bombarding species, such as an ion or a high-energy neutral atom,
preferably
from one of the inert noble gases. The target atoms evaporate into the vacuum
chamber
and then condense on the substrate to form a thin film. Typically, the
hydrogen
generation catalyst substrate is mounted in a sputtering chamber, with one
side facing
up or down toward a metal electrode target (examples include, but are not
limited to Ni,
CA 02615054 2007-12-24
Pt, Ru, Os, Ag or alloys of these metals). After evacuating the chamber, an
inert gas,
such as argon, is used to backfill the chamber to a pressure from about 10 to
about 50
millitorr (from about 1.3 to about 6.7 Pa). The sputtering process is
initiated by
applying a high voltage between the target and the chamber wall. The
sputtering
process is continued for an amount of time (typically a few minutes but
ranging from
less than a minute up to a few hours) according to the desired thickness of
catalyst
loading on the substrate. Upon completion of sputtering, air is readmitted to
the
chamber to remove the coated substrate.
While most of these substrates simply absorb the solution of transition metal
salts, ion exchange resins offer some surprising and interesting
characteristics. Ion
exchange resins are porous polymeric materials having active groups at the end
of the
polymer chains. Typically, polymers used in ion exchange resins include, but
are not
limited to, polystyrene, epoxy amines, epoxy polyamines, phenolics, and
acrylics. Ion
exchange resins are classified into anionic exchange resins and cationic
exchange
resins. These resins are commercially available as beads, typically having
particle sizes
from about 20 mesh to about 100 mesh. The resins are also available as sheets
and can
be fabricated into any shape desired.
Anionic exchange resins attract anions because the active groups at the ends
of
the polymers have positive charges. Nonlimiting examples of positively charged
active
groups include a quatemary ammonium, tertiary amine, trimethyl benzyl
ammonium,
and/or dimethyl ethanol benzyl ammonium. Commercial anionic exchange resins
are
typically supplied in the Cl' or OH- fom1, i.e., easily replaceable chloride
ions or
hydroxide ions are bound to the active groups having positive charges.
Commercially
available anionic exchange resins include, but are not limited to, A-26, A-36,
IRA-400
and IRA-900, manufactured by Rohm & Haas, Inc., located in Philadelphia,
Pennsylvania; Dowex 1, Dowex 2, Dowex 21 K, Dowex 550A, Dowex MSA-1, and
Dowex MSA-2, manufactured by Dow Corporation; Duolite A-101 D, Duolite A-102
D, and Duolite A-30 B; and lonac A-540, lonac A-550, and lonac A-300.
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CA 02615054 2007-12-24
Cationic exchange resins attract cations because the active groups at the ends
of
the polymers have negative charges. Nonlimiting examples of negatively charged
active groups include sulfonic acid, carboxylic acid, phosphonic acid, and/or
aliphatic
acid. Commercial cationic exchange resins are typically supplied in the Na+ or
H+
form, i.e., easily replaceable sodium or hydrogen ions are bound to the active
groups
having negative charges_ Commercially available cationic exchange resins
include, but
are not limited to, Nafion resins, manufactured by Dupont Corp., located in
Wilmington, Delaware; IRA-120 and Amberlyst 15 manufactured by Rolun & Haas,
Inc., located in Philadelphia, Pennsylvania; Dowex 22, Dowex 50, Dowex 88,
Dowex
MPC-1, and Dowex HCR-W2 and Dowex CCR-1, manufactured by Dow Corporation;
Duolite C-3, Duolite ES-63, and Duolite ES-80; and lonac 240.
Anionic exchange resin beads are treated with the catalyst in the following
manner. A transition metal salt is dissolved in an acid having the
corresponding anion
that can form an anionic complex of the transition metal. For example,
ruthenium
trichloride can be dissolved in hydrochloric acid to form chlororuthenic acid,
wherein
the ruthenium is contained in an anionic complex, i.e., [RuCl6]-3. Typically,
the anionic
,
complex of a transition nietal is characterized by the chemical formula
[My'+X61(' 6)
wherein M is a transition metal, y is the valence of the transition metal, and
X is an
anion with a single negative charge. The concentration of the transition metal
solution
can be varied accordingly, but a concentration close to saturation is
preferred. The
acidic solution containing the anionic transition metal complex can then be
exchanged
onto the anionic exchange resin beads by contacting the anionic exchange resin
beads
with the anionic transition metal solution. Typically, this is done either by
soaking the
beads in the solution or dropwise adding the solution onto the beads. Without
wanting
to be limited by any one theory, it is believed that the anion associated with
the active
group of the resin is exchanged with the anionic transition metal complex.
Exchange,
as used herein, means that the ion associated with the active groups of the
ion exchange
resin, e.g., the chloride, is substituted with the ion of the transition
metal. As a result, a
very strong chemical (ionic) bond is formed between the anionic transition
metal
complex and the active group of the ion exchange resin at each active group
site.
17
CA 02615054 2007-12-24
Upon exposure to a reducing agent, such as sodium borohydride, the anionic
transition metal conlplex is reduced at the exchange site to its neutral
valence state, i.e.,
the metallic state. The result is a distribution of transition metal catalyst
molecules in
andlor on the resin. The process may be repeated to obtain higher metal
content if
desired, because the reduction step restores the anion at the positively
charged active
groups of the exchange resin. It is believed that the restored anion
associated with the
active group is either the anion that had been formerly associated with the
transition
metal, e.g., chloride from the [RuCl6]'3, or the reducing agent. After rinsing
with water,
the treated anionic exchange resin beads are ready for use as a catalyst in
the reaction of
the metal hydride and water to produce hydrogen gas.
Catalyst treatment of cationic exchange resin beads require a slightly
different
procedure, because the affinity of the cation transition metal complexes for
the cationic
exchange resins is much weaker than the affinity of anion transition metal
complexes
for the anionic exchange resins. Despite this additional complication,
cationic exchange
resins are particularly useful because they can typically withstand harsher
environments, especially higher temperatures.
Although transition metals are formally written in their cationic valence
state,
e.g. Ru+3, transition metals form anionic complexes in the presence of common
complexing ions, such as chloride. Such anionic transition metal complexes
would
have little or no attraction for a cation exchange resin bead having
negatively charged
active groups. This can be avoided by using transition metal salts having non-
complexing anions. Non-complexing anions, as used herein, refers to ions that
are
typically very large and contain a central atom that is fully coordinated,
thereby leaving
little activity for further complexing with the transition metal. Nonlimiting
examples of
non-complexing anions of this type include perchlorate (C104'),
hexafluorophosphate
(PF6-), and tetrafluoroborate (BF4 ), and mixtures thereof. Transition metal
salts having
non-complexing anions can be obtained via a precipitation reaction with a
transition
metal salt and an equimolar amount of a compound having a non-complexing
anion.
The compound having the non-complexing anion is chosen so that the anion from
the
transition metal salt precipitates out with the cation associated with the non-
complexing
18
CA 02615054 2007-12-24
anion. For example, a solution of ruthenium trichloride can be reacted with an
equimolar amount of silver perchlorate solution. The chloride will precipitate
out of
solution as silver chloride and leave ruthenium perchiorate in solution. Since
perchlorate ions can not complex like chloride ions, only the ruthenium will
be hydrated
in the cationic form, i.e., [Ru-xH2 OJ3+, wherein x refers to the number of
water
molecules. It is believed that the hydrated ruthenium typically has a chemical
formula
[Ru-6H2O]+3
The pH of the solution containing both transition metal ion and non-complexing
ion should be adjusted to as close to 7 as possible without precipitation of
ruthenium as
a hydrated oxide, before contacting the cationic exchange resin beads.
Preferably, the
solution containing the transition metal ion and the non-complexing ion has a
pH of
greater than or equal to about 2, more preferably greater than or equal to
about 4, most
preferably greater than or equal to about 7. This pH adjustment prevents
hydrogen
cations, H+, from competing for cationic sites, i.e., associate with the
negatively
charged active groups, of the cationic exchange resin. For example, if a I
Molar
solution of ruthenium is used and the pH is 2, ruthenium ions will outnumber
hydrogen
ions by a factor of 100. Although the ratio of ruthenium ions to hydrogen ions
at pH 2
is sufficient, the ratio would be even better at pH's closer to 7. Without
wanting to be
limited by any one theory, it is believed that upon contacting the cationic
exchange
resin beads with the transition metal salt solutions, the positively charged
transition
metal ions exchange with the positive ions initially associated with the
negatively
charged active groups of the cationic exchange resin.
To insure high displacements of the transition metal ions without using
excessive quantities of transition metal salt solutions, the exchange can be
performed by
contacting the cationic exchange beads with transition metal salt solutions in
a tube or
column. This method can also be used to treat the previously-described anionic
exchange resins. The tube or column is usually mounted vertically and filled
with
cationic exchange beads. The solution containing transition metal ions and non-
complexing ions is allowed to pass through the column of beads. Typically,
more dilute
solutions are used first and then progressively more concentrated solutions
can be used
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CA 02615054 2007-12-24
thereafter, thereby allowing the use of the concentrated solutions from the
end of prior
batches at the beginning of subsequent batches. Large quantities of catalyst
treated
cationic resin beads can be produced by utilizing a continuous counter-current
system
that allows virtually complete utilization of ruthenium and complete
saturation of the
beads. A continuous counter-current system means contacting the more dilute
ruthenium solution with the less treated beads and the more concentrated
ruthenium
solution with the more treated beads. After exchanging the transition metals
onto
and/or into the beads, the cationic exchange resins are rinsed with deionized
water and
then reacted with a solution containing a reducing agent, such as sodium
borohydride,
to reduce the rutheniunl to its neutral valence state. Higher transition metal
content can
be obtained by repeating the exchange and/or reduction steps, because the
reduction
step restores cations at the negatively charged active groups of the exchange
resin. It is
believed that the restored cation associated with the active group is provided
by the
reducing agent, i.e., sodium from the sodium borohydride. After rinsing with
water, the
treated cationic exchange resin beads are ready for use as a catalyst in the
reaction of
the metal hydride and water to produce hydrogen gas.
In a preferred embodiment the catalyst chamber is a wound spiral of tubing
including catalyst such that fuel enters the tubing at the center of the
spiral and flows
through the spiral wound tubing.
