Note: Descriptions are shown in the official language in which they were submitted.
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SILICIDE COMPOSITIONS CONTAINTLNG ALKALI METALS AND
METHODS OF MAKING THE SAME
Field of the Invention
[0002] The invention relates to alkali metal silicide compositions made
by the
interaction of alkali metals with silicon at temperatures below about 475 C.
The
compositions provide a stable source to reduce water, producing a source of
pure hydrogen
gas.
Background of the Invention
[0003] Alkali metals are very reactive in their metallic or neutral
state. Alkali metals
are very reactive toward air and moisture and may catch fire spontaneously
when exposed to
these agents. To avoid the inherent hazards associated with their activity,
the neutral metal
must often be stored in vacuo or under an inert liquid such as oil in order to
protect it from
contact with the atmosphere, which may result in oxidation or other reactions.
For example,
sodium metal is often stored in Nujol oil which must, to avoid unwanted
impurities, be
removed prior to use in chemical reactions. This places severe restrictions on
its shipment
and use.
100041 A number of compounds between alkali metals and silicon compounds
have
been prepared. For example, known compounds between sodium (Na) and silicon
(Si) exist
with stoichiometries that range from NaSi to Na(Si)6 (which is believed to be
Na8Si46) to
Na.Si136, with 1.5 <x < 24. (See Witte, J.; Schnering, H. G., "The Crystal
Structure of NaSi
and NaGe (in German)" Zeit Anorgan Allege Chemie 1964, 327, 260-273., Cros,
C.;
Pouchard, M.; Hagenmueller, P., "Two new Phases of the Silicon-Sodium System.
(in
French)" C. R Acad. Sc. Paris 1965, 260, 47644767., and He, J.; Klug, D. D.;
Uehara, K.;
Preston, K. F.; Ratcliffe, C. I.; Tse, J. S., "NMR and X-ray Spectroscopy of
Sodium-Silicon
Clathrates" J. Phys. Chem. B 2001, 105.). The known compounds are formed by
heating Na
with Si to high temperatures, always at or above 500 C, and in some r.ases
with removal of
Na vapor by condensation on a cold surface. (See He, J.; King, D. D.; Uehara,
K.; Preston,
CA 02842549 2014-01-28
K. F.; Ratcliffe, C. L; Tse, J. S., "NMR and X-ray Spectroscopy of Sodium-
Silicon
Clathrates" .I. Phys. Chem. B 2001, 105. and Mayeri, D.; Phillips, B. L.;
Augustine, M. P.;
Kauzlarich, S. M., "NMR Study of the Synthesis of Alkyl-Terminated Silicon
Nanoparticles
from the Reaction of SiC14 with the Zintl Salt, NaSi" Chem. Mater. 2001, 13,
765-770.).
Mayeri et al. react silicon in the presence of sodium to a temperature of
about 650 C to form
a sodium suicide. There have also been reports that a suicide of nominal
composition NaSi2
can be prepared by heating Na with quartz (Si02), although the evidence for
this composition
is slim. (See Novotny, H.; Schen., E., "A Ternary Compound in the System
Aluminum-
Silicon-Sodium (in German)" Metallforsch. 1947, 2, 76-80.).
[0005] It has often been assumed that NaSi is so reactive that it
must be considered to
be pyrophoric, or able to spontaneously ignite in the presence of air. It has
also been recently
characterized as "air and moisture sensitive." (See He, J.; Klug, D. D.;
Uehaza, K..; Preston,
, K. F.; Ratcliffe, C. I.; Tse, J. S., "NMR and X-ray Spectroscopy of
Sodium-Silicon
Clathrates" J. Phys. Chem. B 2001, 105.). This study showed, however, that the
clathrate
structure of Na8Si46 is non-reactive toward air and moisture. Id. However, it
is generally
agreed that the reaction of NaSi with water is rapid and "violent", such that
the heat of
reaction can ignite the hydrogen formed, just as occurs in the reaction of
alkali metals with
water. This places severe restrictions on storing and handling NaSi without
keeping it in
vacuo or under an inert atmosphere to avoid its inherent hazards.
[0006] A major problem with the synthesis of NaSi materials has
been the need to
heat Na and Si in a closed system to prevent the condensation of Na at cold
sites.
Conventionally, for example, to prepare polycrystalline NaSi powder, excess Na
was heated
with Si in a molybdenum (Mo) tube that was welded shut and heated for three
days at 500 C.
(See Mayeri, D.; Phillips, B. L.; Augustine, M. P.; Kanzlarich, S. M., "NNER.
Study of the
Synthesis of Alkyl-Terminated Silicon Nanoparticles from the Reaction of SiC14
with the
Zintl Salt, NaSi" Chem. Mater. 2001, 13, 765-770.). In another study, a
stainless steel
container was used. (See He, J.; Klug, D. D.; Uehara, K.; Preston, K. F.;
Ratcliffe, C. I.; Tse,
=
J. S., "NMR and X-ray Spectroscopy of Sodium-Silicon Clathrates" J Phys. Chem.
B 2001,
105.).
2
=
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[0007] A need exists, therefore, to prepare an alkali metal
silicide composition
conveniently and inexpensively, so that it may be easily handled in air
without a significant
loss in its ability to reduce water. This reduction reaction would be able to
produce large
- amounts of hydrogen per unit mass of the solid.
Summary of the Invention
[0008) The invention relates generally to alkali metal silicide
compositions, methods
of making the alkali metal silicide compositions, and methods of using the
alkali metal
silicide compositions. Any alkali metal may be used in the practice of this
invention,
including Sodium (Na), Potassium (K), Rubidium (Rb), and Cesium (Cs).
[0009] Specifically, the invention relates to an alkali metal
silicide composition
comprising the product of mixing an alkali metal with powdered silicon in an
inert
atmosphere and heating the resulting mixture to a temperature below about 475
C, wherein
the alkali metal silicide composition does not react with dry 02. In this
embodiment, the
alkali metal silicide composition may be sodium "silicide, such as Na4Si4,
potassium silicide,
such as K4Si4, and the hie.
[0010] In addition, the invention relates to a sodium silicide
composition having a
powder X-ray diffraction pattern comprising at least three peaks with 2Theta
angles selected
from about 18.2, 28.5, 29.5, 33.7, 41.2, 47.4, and 56.2. Furthermore, the
invention relates to
a sodium silicide composition having a solid state 23Na Magic Angle Spinning
(MAS)
Nuclear Magnetic Resonance (NMR) spectra peak at about 18 ppm. In these
embodiments,
the sodium silicide may be Na4Si4, for example.
[0011] Moreover, the invention relates to a method of removing a
volatile or
flammable substance in a controlled manner, the volatile or flammable
substance being in the
presence of water, the method comprising the step of exposing the volatile or
flammable
substance to an alkali metal silicide composition, wherein the alkali metal
silicide
composition reacts exothermically with the water causing a controlled burn,
thereby
removing the volatile or flammable substance.
[0012] In addition, the invention relates to a method of removing
a volatile or
flammable substance in a controlled manner, the method comprising the steps of
exposing the
volatile or flammable substance to an alkali metal silicide composition, and
exposing the
alkali metal silicide composition to water, wherein the alkali metal silicide
composition reacts
exothermically with the water causing a controlled burn, thereby removing the
volatile or
flammable substance.
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[0013] F-urthermore, the invention relates to a method of making an
alkali metal
silicide composition comprising the steps of mixing an alkali metal with
powdered silicon in
an inert atmosphere and heating the resulting mixture up to a temperature
below about 475
C, wherein the alkali metal silicide composition does not react with dry 02.
[00141 In these embodiments, the alkali metal silicide composition may be
sodium
silicide, such as Na4Si4, potassium silicide, such as K4Si4, and the like.
Also, the exothermic
reaction between the alkali metal silicide composition and the water may
consume or clean
the volatile or flammable substance.
[0015] Moreover, the invention relates to a method for producing hydrogen
gas
comprising the step of contacting any of the alkali metal silicide
compositions described
herein with water.
Brief Description of the Drawings
[0016] Figure 1 shows a Differential Scanning Calorimetry (DSC) pattern
illustrating
exothermic reactions between a mixture of Na and Si.
[0017] Figure 2 shows a powder X-ray difft ___________________________
action (XRD) pattern of a sodium silicide
composition of the invention and Na4Si4 prepared by conventional methods.
[0018] Figure 3 shows a powder X-ray difft ___________________________
action (XaD) pattern of a sodium silicide
composition of the invention.
[0019] Figure 4 shows a powder X-ray diffraction (al)) pattern of NaSi
prepared by
conventional methods.
[0020] Figure 5 shows a solid state 23Na MAS NMR spectra for a sodium
silicide
composition of the invention and a sodium silicide composition prepared by
conventional
methods.
[0021] Figure 6 shows a Differential Scanning Calorimetry (DSC) pattern
illustrating
exothermic reactions of the NaSi product of the invention.
Detailed Description of the Invention
[0022] As is shown in the attached Figures 1-6 and described herein, the
invention
relates to alkali metal silicide compositions comprising the product of mixing
an alkali metal
with silicon in an inert atmosphere and heating the resulting mixture to a
temperature below
about 475 C, wherein the alkali metal silicide composition does not react
with dry 01.
According to the processes described herein, the resulting composition can be
used as a
source of hydrogen by contacting the composition with water. While any alkali
metal,
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CA 02842549 2014-01-28
including sodium (Na), potassium (K), cesium (Cs), or rubidium (Rb) may be
used, it is
preferred that the alkali metal used in the alkali metal silicide composition
be either sodium
or potassium. In addition, any type of silicon, powdered silicon, or
crystalline powdered
silicon may be used, for example, powdered crystalline silicon (Alfa Aesar,
325 mesh). The
theoretical H2 yield from a sodium silicide composition of the invention is
approximately
= 0.098 kg H2/kg NaSi, more than double the Department of Energy (DOE) 2005
target for
hydrogen fuel sources and larger than the 2015 target of 0.081 kg H2/ kg fuel.
Accordingly,
sodium is the most preferred alkali metal, and sodium silicide is the most
preferred alkali
metal silicide composition.
[0023] Figure 1 illustrates a pair of exothermic reactions that
occur between Na and
Si in two temperature regions using a Differential Scanning Calodmetry (DSC)
display. The
DSC results were obtained using a Shimadzu DSC-50 instrument, and the
experiment was
conducted in a sealed copper vessel. During the experiment, a mixture of
stoichiometdc
amounts of sodium metal and silicon were combined and heated to about 550 C.
In general,
it is preferred that sodium metal and the silicon be mixed at a 1:1
stoichiometric ratio, or in a
mixture having a slight excess of silicon. At the conclusion of the
experiment, there was a
slight Na coating on the glass of the flask in which the reactions occurred.
In addition, it is
believed that about 80 % of the Na reacted with the silicon during the
experiment.
[0024] First run 101 shows the presence of a first exotherm
extending from
approximately 300 ¨ 450 C resulting in the release of about 1.0 ldlojoule of
heat per gram
(kJ/g) of Na used. The exotherm comes to a peak at about 420 C and returns to
the baseline
at about 472 C, at which point an endothenn begins to occur. The endotherm,
which extends
from about 472 ¨ 505 C has a trough at about 500 C, is believed to be
indicative of a
leeching of alkali metal from the material. A second exotherm extends from
about 505 ¨ 560
C, and results in the dissociation of NaSi to yield sodium metal in the amount
of about 0.25
kJ/g of Na used. The dissociation of Na is believed to be the product being
pyrophoric in
nature because of the presence of Na metal on the surface of the NaSi
material.
[0025] Thus, the DSC results of Fig. I clearly show that one or two
reactions occur to
form one or more of the suicides of sodium. After first run 101 was completed,
the resulting
material was re-heated a second time under the same conditions, with the
results shown as
second run 102. Second run 102 does not show a melting endotherm of Na at 98
C as
expected if sodium were released at approximately 550 C in the first
reaction, but does show
a complex series of further reactions. The lack of a significant melting
endotherm is
attributed to the reaction being slow in the DSC cup, so that the
decomposition reaction does
CA 02842549 2014-01-28
not have time to occur. Evidently though, the initial formation of NaSi is
followed by other
reactions of various complexities.
100261
Figs. 2 and 3 show powder X-ray diffraction (NRD) patterns of an alkali metal
=
silicide composition material of the invention. The powder X-ray diffraction
patterns were =
obtained using a Rigaku 200B X-ray diffractometer using a copper source. In
particular, Fig.
2 shows the powder X-ray diffraction pattern of Sample Ni-Si-4 (annealed at
about 400 C)
in a 0.7 mm diameter capillary tube. Fig. 3 shows a powder X-ray diffi ____
action pattern for an
additional sample.
[0027]
All of the peaks of Na4Si4 as calculated from the known crystal structure are
present in the experimental pattern. (See Witte, J.; Schnering, H. G., "The
Crystal Structure
of NaSi and NaGe (in German)" Zeit Anorgan Allege Chemie 1964, 327, 260-273.).
The top
pattern in Fig. 2, experimental pattern 201, and the pattern in Fig. 3
indicate the presence of
Na4Si4 or a closely related allotrope, in addition to the formation of a
different material, a
mixture of Na4Si4, unreacted silicon, and other unknown products that yield at
least seven
extra powder diffraction lines when compared to the bottom pattern in Fig. 2,
literature
pattern 202, and the pattern of Fig. 4, which are powder XRD patterns for
conventional NaSi.
Four of the new peaks at 2Theta angles of about 18.2, 28.5, 29.5 and 33.7 are
shown in
experimental pattern 201 Fig. 2, and six of the new peaks at 2Theta angles of
about 28.5,
29.5, 33.7, 41.2, 47.4, and 56.2 are shown in Fig. 3. The presence of these
additional peaks
*indicates that the sodium silicide of the invention is different from known
sodium silicides
found in the literature.
100281
Figure 5 shows a comparison of the solid state 23Na MAS NMR spectrum of
NaSi prepared by the methods of the invention and sodium silicide prepared by
the methods
given in the literature. (See He, J.; King, D. D.; Uehara, K.; Preston, K. F.;
Ratcliffe, C. I.;
Tse, J. S., "NMR and X-ray Spectroscopy of Sodium-Silicon Clathrates" ). Phys.
Chem. B
2001, 105.). The solid state 23Na MAS NMR spectrum was obtained using a Varian
WM.-
400S Spectrometer. As is evident in the figure, the shape and chemical shift
behavior are
very different from those reported by the literature. /d. In particular, the
NMR spectra
shown in Fig. 5 show that a peak 501 for the composition of the invention
occurs at
approximately 18 ppm, while the peak 502 for the sodium silicide material
taught by the
literature has a peak at approximately 52 ppm. Clearly, the environment of
most of the Na+ is
different in the two preparations. The normal chemical shift and symmetry of
the spectrum in
the composition of the invention suggests a more symmetric environment for Na+
than in the
Na4Si4 samples prepared elsewhere by a different method. This difference in
chemical shift
6
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definitively established the existence of a different composition than that
taught by the
literature. Furthermore, it should be noted that an earlier solid state 23Na
MAS NMR study
observed a very broad peak near the chemical shift reported in He and very
different from
that observed with the composition of the invention. (See Gryko, J.; McMillan,
P. F.;
Sankey, 0. F. "NTMR Studies of Na Atoms in Silicon Clathrate Compounds" Phys.
Rev. B.
= 1996, 54, 3037-3039, and He, J.; Klug, D. D.; Uehara, K.; Preston, K. F.;
Ratcliffe, C. I.; Tse,
J. S., "NMR and X-ray Spectroscopy of Sodium-Silicon Clathrates" .1. Phys.
Chem. B 2001,
105.). The single rather narrow NMR. peak observed during experimentation of
the
composition of the invention at the normal position of Na+ suggests that in
the samples of the
composition of the invention, the sodium ions are in a rather symmetrical
environment.
[0029] Figure 6 shows two DSC traces starting with the material of
the invention
comprising the product of the reaction of Na with an equimolar amount of
powdered Si,
heated to 400 C overnight. The first run 601 shows no melting endotherm of
sodium and no
significant exotherms up to 500 C, above which there is a significant
exothermic reaction,
probably the release of Na from Na4Si4 at about 550 C. This is confirmed by
the second run
602, which shows an endothenn due to the melting of Na (about half of that
initially used)
and a substantial exotherm starting at 300 C as a result of reaction between
the released Na
and the product formed by its release in the previous run.
[0030] To prevent the formation of the other materials at higher
temperatures, the
composition of the invention is created by heating an alkali metal and silicon
mixture to a
temperature below about 475 C, and most preferably, to a temperature of about
400 C,
which appears to be the optimal temperature for the formation of the Na4Si4
composition of
the invention. Compositions made at higher temperatures gave lower yields of
hydrogen
when reacted with water. In addition, the stability of the composition of the
invention in dry
air suggests that the procedures describe herein result in a "protected"
product. This
protection is likely due to the formation of a Si02-like coating on the
material.
[0031] Accordingly, the invention relates to a method of making an
alkali metal
suicide composition comprising the steps of mixing an alkali metal with
powdered silicon in
an inert atmosphere and heating the resulting mixture up to a temperature
below about 475
C, wherein the alkali metal suicide composition does not react with dry 02. In
this
embodiment, the step of heating may be staged heating occurring over an
extended period of
time period of hours comprising the steps of heating the resulting mixture up
to about 150 C,
heating the resulting mixture from about 150 C up to about 200 C, heating
the resulting
mixture from about 200 C up to about 250 C, heating the resulting mixture
from about 250
7
CA 02842549 2014-01-28
C up to about 300 C, and heating the resulting mixture from about 300 'V -up
to a
temperature below about 475 C. The step of heating the resulting mixture from
about 300 C
up to a temperature below about 475 C more preferably comprises heating the
resulting
mixture from about 300 C up to a temperature of about 390 - 425 C, and most
preferably ,
comprises heating the resulting mixture from about 300 C to a temperature of
about 400 C.
[0032] The invention also relates to a method of removing a volatile or
flammable
substance in a controlled manner, the volatile or flammable substance being in
the presence
of water, the method comprising exposing the volatile or flammable substance
to an alkali
metal silicide composition. In this embodiment, the alkali metal silicide
composition reacts
exothermically with the water causing a controlled burn, thereby removing the
volatile or
flammable substance. In addition, the invention relates to a method of
volatizing, driving of,
consuming, converting to a water-miscible species, or otherwise cleaning a
volatile or
flammable substance in a controlled manner, the method comprising the steps of
exposing the
volatile or flammable substance to an alkali metal silicide composition, and
exposing the
alkali metal silicide composition to water. In one embodiment, the alkali
metal silicide
composition reacts exothermically with the water causing a controlled burn,
thereby cleaning
the volatile or flammable substance. Moreover, the invention relates to a
method of
consuming a volatile or flammable substance in a controlled manner, the method
comprising
the steps of exposing the volatile or flammable substance to an alkali metal
silicide
composition, and exposing the alkali metal silicide composition to water. In
another
embodiment, the alkali metal silicide composition reacts exothermically with
the water
causing-a controlled burn, thereby consuming the volatile or flammable
substance. In each of
the above exemplary embodiments, it is preferred that the alkali metal
silicide composition be
sodium silicide. In addition, the applicability of the above-described methods
is most
apparent with respect to the cleaning of volatile and flammable substance
which cannot be
easily cleaned by conventional means, but instead require alternative means
for cleaning, for
example, chemical cleaning.
[0033] The material of the invention may be used to clean any non-
miscible volatile
or flammable materials, including oils, fuels, etc. For example, the material
of the invention =
may be applied to an oil spill in a body of water. When the material of the
invention, such as
sodium silicide, contacts the surface of the water upon which the spill is
located, the material
reacts exothermically with the water causing a controlled bum. The ignition
may cause the
spill to ignite, thus combusting the spilled oil and cleaning the spill. This
use is particularly
advantageous because the amount of the material of the invention used is not
critical. After
8
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an initial amount of the material is applied to start the combustion,
additional amounts may
be applied as needed to further combust the spilled substance until the
desired amount is
removed. The material may be applied to such as spill in many ways, for
example, by
spraying or dropping out of an airplane in a manner similar to crop-dusting,
or from a
helicopter.
= [00341 In addition, the material of the invention may be placed
within a water soluble
storage container, for example, a water-soluble pouch, or be imbedded or
encapsulated in any
sort of containment material, for example, foams, polymers, porous materials,
and the like,
which can provide a time-released reaction with the water by delaying exposure
of the
material to the water. In this manner, the material may be delivered to a
spill in a body of
water, for example, an oil spill, by a boat, which could then leave the area
prior to the start of
combustion.
[0035] In addition, the material of the invention may be used to
clean volatile or
flammable substances in a dry environment. In this case, the material of the
invention may
be added to the volatile or flammable substance, for example, applied to the
surface of the
substance. Then, water may be introduced, for example, by spraying, to start
the reaction of
the material with the water and initiate combustion of the substance.
Similarly, the water
may be added to the volatile or flammable substance first, and then the
addition of the
material of the invention will initiate the combustion.
10036] The alkali metal silicide compositions of the invention vary
from free-flowing,
amorphous gay-black powders, in which the particles have a dull surface, to
hard particles
having a diameter of about 1-5 mm. The end product varies depending on the
method of
preparation. The ease of handling of the product, its low moisture absorption
from the air,
and its rapid reaction with water to produce hydrogen combine to make this
material a
convenient source of high yields of pure hydrogen.
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[0037] Examples:
[0038] In each of the following examples, evacuable Erlenmeyer flasks were
used to
prepare the samples of the alkali metal silicide material. The silicon was
inserted into the
flask, which was then attached to a vacuum line with an UltraTorr fitting and
a Kontes
vacuum valve. The flask was then heated with a flame under vacuum and inserted
into a
helium-filled glove box, where sodium pieces were added. After removal from
the box, the
flask was again evacuated and the sodium pieces were melted. When the pressure
stabilized
at about 3x10-5 torr, the tubing was sealed off above the flask and the flask
and its contents
were heated in a furnace overnight or longer at the desired reaction
temperature. Upon
completion of the reaction, the flask was again inserted into the glove box,
the tabulation was
broken open and the contents removed for storage and further study.
[0039] Example 1¨ Initial Experimentation
[0040] Although Na is known to react with borosilicate glass at
temperatures above
300 C, thereby causing darkening, powdered or solid products were prepared by
heating
equimolar mixtures of Na and crystalline powdered Si in evacuated 50 and 125
mL
Erlenmeyer flasks. Overnight heating at 500 C yielded gray-black powders with
some
crushable lumps. Three separate preparations made at this temperature yielded
products that
were found to release H2 in amounts equivalent to 0.042, 0.054, and 0.058 kg
H2 per kg of
sample. Accordingly, it was determined that the conditions of temperature and
composition
may not be optimal. The literature and the DSC results in Figs. 1 and 6
suggest that 500 C
might be too high a temperature. Accordingly, a sample was prepared by heating
the mixture
of sodium and silicon up to about 400 C. The resulting product gave a
hydrogen yield
equivalent to 0.072 kg 112 per kg of sample. This yield exceeds the target
yields of hydrogen
proposed by the DOE for both 2005 and 2010 and is nearly equal to the year
2015 target.
These results are very advantageous, especially in light of the simplicity of
the preparation
and the air stability of the product.
[0041] The material obtained by heating to about 400 C produces both a
solution and
a black insoluble material, suggesting that the reaction of Na with Si is not
complete under
the stated conditions, as pure NaSi would be expected to produce products that
are
completely soluble in water according to a reaction such as:
2NaSi(s) + 5H20(Z) Na2Si205(aqueous) + 51121
[0042] Accordingly, it was determined that it might be possible to recover
the
unreacted Si and react it with more Na, thus increasing the ultimate yield to
nearly 100%
based on the Si used. To test this possibility, the residue was recovered,
approximately 0.5 g,
CA 02842549 2014-01-28
from the reaction of 1.0 grams of the second type of preparation described
above with water,
dried it, and reacted it with an equimolar =punt of sodium at about 400 C.
The resulting
material yielded a quantity of hydrogen equivalent to 0.023 kg H2 per kg of
sample. Thus,
the total yield from the original preparation was about 0.10 kg H2 per kg of
sample.
Recovery of the unreacted material from the initial preparation of Na + Si is
clearly possible.
= [0043] = The product of the reaction between Na and Si in
borosilicate glass at 400 C
is free of sodium. This is further evidenced by the lack of a peak due to the
presence of
metallic sodium shown in the solid state 23Na MAS NMR spectrum of Fig. 5. The
DSC of
the product of reaction of Na with Si at this temperature shown in Fig= 6
shows no melting
endotherm of Na. Instead, it only shows an exothermic peak at about 500 C.
The repeat run
clearly shows that this high temperature reaction produces sodium metal, in
agreement with
literature results. (Cros, C.; Pouchard, M.; Hagenmueller, P., "Two new Phases
of the
Silicon-Sodium System. (in French)" C. R. Acad. Sc. Paris 1965, 260, 4764-
4767., and He,
J.; King, D. D.; Uehara, IC; Preston, K. F.; Ratcliffe, C. I.; Tse, J. S.,
"NAIR and X-ray
Spectroscopy of Sodium-Silicon Clathrates" J. Phys. Chem. 13 2001, 105.).
[0044] Example 2 ¨ Stability in Air
[0045] The Na-Si material of the invention reacts immediately with
water to produce
hydrogen and release heat in the process. However, the material is completely
unreactive
toward dry oxygen over a period of at least one day. Unless the relative
humidity is high, the
powder can be weighed in air or transferred from one container to another. A
sample was
exposed to laboratory air in an aluminum weighing dish and only slowly reacted
with
moisture. After two hours, a small amount of liquid water was added, and the
black pieces
immediately evolved hydrogen. It is likely that the methods described herein
for preparation
of the composition of the invention result in an alkali metal suicide that is
protected by a
surface layer of silicon dioxide or some other composition. In any event, the
resulting
material is easy to handle in air, which results in the ability to produce
hydrogen on demand.
[0046] These results demonstrate that it is straightforward to
produce a stable
powdered or granular material that likely contains a suicide with the
stoichiometry NaSi,
together with an unknown amount of other substances (possibly glassy Si02 and
urireacted
silicon). The product, while stable in dry air and only slowly reactive in
moist air, produces
large yields of hydrogen when introduced into liquid water. The gaseous
product is pure
hydrogen, uncontaminated with anything except water vapor and small amounts of
silanes
such as SiRi. Thus, the material is an excellent source of hydrogen for use in
fuel cells.
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100471 Example 3 ¨ Reaction between sodium and silicon powder at 500 C.
[0048] Sample Na-Si-1 was prepared by introduction of 0.56 g of powdered
crystalline silicon (Alfa Aesar, 325 mesh) into an evacuable Erlenmeyer Flask.
After
outgassing under vacuum with a gas-oxygen torch (-- 300 C). the flask was put
into a .=
helium-filled glove box and 0.46 g. of sodium metal was added. After
evacuation and melting
of the sodium, the stem of the flask was sealed off under vacuum and the flask
and contents
were heated in a furnace for 2 hrs at 300 C, 22 hrs at 400 C and 48 hrs at
500 C. The Pyrex
flask became dark brown-red in the process and the product consisted of both
powder and
small hard lumps. In the glove box, 0.66 g of product was recovered. A 24.5 mg
sample
produced 0.517 millimoles (mmol) of hydrogen upon addition of water. This
corresponds to
0.0423 kg of H2 per kg of sample. If the hydrogen is produced only from sodium
silicide, the
amount corresponds to 43% Na4Si4.
[0049] Example 4¨ Recovery of residue from the product of the invention
[0050] The second preparation of Na 4- Si (Sample Na-Si-2) corresponded
to 55%
Na4Si4 according to the yield of hydrogen. A 1.0 g sample of the product was
reacted with
water in a nitrogen-filled glove bag, with copious amounts of hydrogen
produced. The
reaction left a black residue that could not be recovered by centrifugation
because slow
evolution of gas continued to cause mixing. The continued evolution of
hydrogen would be
expected in this basic solution if the residue contained elemental silicon.
The product was
neutralized with HC1, washed by repeated centrifugation and dried. The
resulting black
powder (0.49 g) was again reacted with Na at 500 C and produced 0.021 kg of
112 per kg of
sample.
[0051] Example 5 ¨ Preparation of the highest ¨ yielding sample (Sample
Na-Si-4)
[0052] As shown in Fig. 1, it became apparent from the Differential
Scanning
Calorimetry (DSC) experiments on the heat evolved in the reaction of Na with
Si that there
were two exothermic processes occurring. It is believed that, after formation
of Na4Si4 at
around 400 C, further heating caused dissociation of the product with the
formation of Na
metal and other suicides. This result was somewhat surprising since pure
Na4Si4 is generally
prepared at or above 500 C. To test whether preparation at 400 C instead of
500 C
increases the yield of the product of the invention, a sample was prepared as
described in
Example 3, except that the flask and its contents were heated to 400 C
overnight. The =
resulting product had fewer lumps and gave a hydrogen yield that corresponded
to 73%
Na4Si4. The DSC of this sample shown in Fig. 6 confirmed the formation of
sodium upon
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CA 02842549 2014-01-28
heating to 560 C and showed that an exothemaic reaction resulted upon re-
heating, which is
believed to be the result of re-formation of sodium suicide.
[0053] In particular, Fig. 6 shows Differential Scanning Calorimetry
(DSC) traces of
the reaction of 4.9 mg of Na with 5.0 mg of Si. The mass of Na was determined
from the
measured heat of melting. It is likely that the exotherm with a peak at 400 C
is due to the
formation of Na4Si4 and that further heating causes a secondary reaction. As a
result, second
run 602 shows further reactions of the various products formed in first run
601. Second run
602 shows no melting endotherm of free sodium. This contrasts with the
behavior of a pre-
formed sample, annealed at 400 C which releases Na when heated to 550 C in
the DSC cup.
It is likely that the reaction to form Na4Si4 in the DSC cup is slow, so that
the second reaction
to release sodium does not have a chance to occur during the short time of the
DSC
experiment.
[0054] Example 6¨ Preparation of Potassium Silicide (KSi) Material
[0055] The KiSi material was produced by first mixing stoichiometric
amounts of
powdered (350 mesh) Si from Sigma-Aldrich with K metal chunks in an Erlenmeyer
flask,
equipped with a stem to attach to the vacuum line. This was done in a He-
filled glove box.
The He was pumped off and the mixture heated with a flame until the K melted
and the
system was outgassed to ¨ 10-5 Torr, at which point the Pyrex stem was sealed-
off with a
flame. The Erlenmeyer was then put into a muffle furnace and heated for about
two hours
each at 150 C, 200 C, 250 C, 300 C, and 350 C and then overnight at 400
C. The tube
was broken open in the glove box and the product was scraped off the walls.
The material
consisted of powder and chunks and the latter were ground up into a fine
powder. A sample
removed from the glove box was poured into an aluminum weighing dish and
exposed to
laboratory air. It reacted slowly, if at all, with no heating or flame. But
when a bit of powder
was dropped into a beaker of water it ignited immediately.
[0056] The yield of hydrogen upon reaction with water indicated only
about a 50
percent (50%) conversion to KSi, which indicates that a longer reaction time
or better
agitation during preparation is necessary. Accordingly, it is expected that
optimization of the
conditions of synthesis will lead to yields comparable to those achieved with
the sodium
silicide compositions of the invention.
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