U.S. patent application number 11/354489 was filed with the patent office on 2006-06-29 for method for preparation of nanometer cerium-based oxide particles.
Invention is credited to Harlan U. Anderson, Wayne Huebner, Xiao-Dong Zhou.
Application Number | 20060140837 11/354489 |
Document ID | / |
Family ID | 29418891 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060140837 |
Kind Code |
A1 |
Zhou; Xiao-Dong ; et
al. |
June 29, 2006 |
Method for preparation of nanometer cerium-based oxide
particles
Abstract
The invention comprises novel undoped and doped nanometer-scale
CeO.sub.2 particles as well as a novel semi-batch reactor method
for directly synthesizing the novel particles at room temperature.
The powders exhibited a surface area of approximately 170 m.sup.2/g
with a particle size of about 3-5 nm, and are formed of single
crystal particles that are of uniform size and shape. The
particles' surface area could be decreased down to 5 m.sup.2/g,
which corresponds to a particle size of 100 nm, by thermal
annealing at temperatures up to 1000.degree. C. Control over the
particle size, size distribution and state of agglomeration could
be achieved through variation of the mixing conditions such as the
feeding method, stirrer rate, amount of O2 gas that is bubbled
through the reactor, the temperature the reaction is carried out
at, as well as heating the final product at temperatures ranging
from 150.degree. to 1000.degree. C.
Inventors: |
Zhou; Xiao-Dong; (Rolla,
MO) ; Huebner; Wayne; (Rolla, MO) ; Anderson;
Harlan U.; (Rolla, MO) |
Correspondence
Address: |
Schultz & Associates, P.C.;One Lincoln Centre
Suite 1200
5400 LBJ Freeway
Dallas
TX
75240
US
|
Family ID: |
29418891 |
Appl. No.: |
11/354489 |
Filed: |
February 15, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10146824 |
May 15, 2002 |
7025943 |
|
|
11354489 |
Feb 15, 2006 |
|
|
|
Current U.S.
Class: |
423/263 ;
428/402 |
Current CPC
Class: |
B82Y 30/00 20130101;
C01P 2006/12 20130101; Y10T 428/2982 20150115; C01F 17/235
20200101; C01P 2002/72 20130101; C01P 2002/50 20130101; C01P
2004/04 20130101; C01P 2002/52 20130101; C01P 2004/64 20130101 |
Class at
Publication: |
423/263 ;
428/402 |
International
Class: |
B32B 5/16 20060101
B32B005/16; C01F 17/00 20060101 C01F017/00 |
Claims
1. A process for the production of cerium dioxide crystals
comprising: (a) providing a first solution of a water-soluble
cerium salt solution; (b) providing a second aqueous solution of an
alkali metal or ammonium hydroxide; (c) mixing said first and
second solutions together to form a reactant solution; (d)
agitating said reactant solution under turbulent flow conditions
while concomitantly passing gaseous oxygen through said reactant
solution; and (e) precipitating cerium dioxide particles having a
predominant particle size within the range of 3-100 nanometers.
2. The method of claim 1 wherein said cerium salt is cerous
nitrate.
3. The method of claim 1 wherein said second solution is an aqueous
solution of ammonium hydroxide.
4. The method of claim 3 wherein the ammonium hydroxide solution
ranges from 0.1 mol/l to 1.5 mol/l.
5. The process of claim 1 wherein the agitation of said reactor
solution is carried out within a reactor vessel to which one of
said first and second solutions is added followed by the addition
of the other said first and second solutions.
6. The method of claim 5 wherein the other first and second
solution is added to the reactor at a rate of from 0.5 ml/min to 10
ml/min.
7. The method of claim 5 wherein the first solution is added to the
reactor vessel followed by the addition of the second solution to
the reactor.
8. The method of claim 5 wherein the second solution is added to
the reactor followed by the addition of the first solution to the
reactor.
9. The method of claim 8 wherein said cerium salt is cerous
nitrate, said solution is an aqueous solution of ammonium
hydroxide, the first solution is added to the reactor at a rate
within the range of 0.5 ml/min to 10 ml/min, said agitating step is
accomplished by stirring the solution with an impeller at a rate
within the range of 100-5000 rpm during the mixing step, and
wherein the oxygen is passed through said reactant solution at a
rate from 1 ml/min to 500 ml/min.
10. The method of claim 1 wherein distilled water is added to the
reactor followed by the simultaneous addition of the first and
second solutions to the reactor.
11. The method of claim 1 wherein the agitating step is
accomplished by stirring the solution with an impeller at a rate
within the range of 100-5000 rpm during the mixing step.
12. The method of claim 1 wherein the oxygen is passed through said
reactant solution at a rate within the range of 1 ml/min to 500
ml/min.
13. The method of claim 1 that further comprises mixing a dopant
precursor solution comprised of the nitrate or acetate form of a
lanthanide series metal with the first and second solutions.
14. The method of claim 13 wherein the dopant precursor solution is
mixed with the first solution prior to mixing with the second
solution.
15. The method of claim 13 wherein the reactor is maintained at
room temperature.
16. The doped cerium oxide formed by the method of claim 13.
17. The method of claim 1 further comprising the step of vacuum
drying the synthesized powders.
18. The method of claim 1 further comprising the step of heating
the precipitate to result in the growth of the particles to a
desired size.
19. The method of claim 1 wherein each of the cerium dioxide
particles are a single crystal.
20. The cerium dioxide particles formed by using the method of
claim 1.
21. The cerium dioxide particles of claim 20 wherein the particle
size is further controlled within the range of 3 to 10
nanometers.
22. The cerium dioxide particles of claim 20 wherein the particle
size and shape are uniform.
23. A cerium dioxide composition comprising single crystal cerium
dioxide particles that are smaller than 100 nanometers in size.
24. The cerium dioxide composition of claim 23 wherein the
particles are smaller than 10 nanometers in size.
25. The cerium dioxide composition of claim 23 wherein the
particles are predominantly of a uniform size and shape.
26. A doped cerium dioxide composition comprising cerium dioxide
particles that are doped with a lanthanide series metal and have a
particle size less than 100 nanometers.
27. The doped cerium dioxide particles of claim 26 wherein the
particle size is less than 10 nanometers.
28. The doped cerium dioxide particles of claim 26 wherein the
particle size is within the range of 3-5 nanometers.
29. The doped cerium dioxide particles of claim 26 wherein each
particle is a single crystal.
30. The doped cerium dioxide particles of claim 26 wherein the
particles are predominantly of uniform shape and size.
31. A cerium dioxide composition comprising cerium dioxide
particles having a predominant particle size within the range of 3
to 100 nanometers produced by the process of: (a) mixing together a
first solution of a water-soluble cerium salt solution and a second
aqueous solution of an alkali metal or ammonium hydroxide to form a
reactant solution (b) agitating said reactant solution under
turbulent flow conditions while concomitantly passing gaseous
oxygen through said reactant solution; and (c) precipitating said
cerium dioxide particles having a predominant particle size within
the range of 3 to 100 nanometers.
32. The composition of claim 31 wherein said cerium dioxide
particles have a predominant particle size within the range of 3 to
10 nanometers.
33. The composition of claim 31 wherein said cerium dioxide
particles are predominantly of uniform particle size and shape.
34. A cerium dioxide composition comprising single crystal cerium
dioxide particles that are smaller than 100 nanometers in size.
35. The cerium dioxide composition of claim 34 wherein said
particles are smaller than 10 nanometers in size.
36. The cerium dioxide composition of claim 34 wherein said
particles are predominantly of a uniform size and shape.
37. A doped cerium dioxide composition comprising cerium dioxide
particles that are doped with a lanthanide series metal and have a
particle size less than 100 nanometers.
38. The doped cerium dioxide particles of claim 37 wherein said
cerium dioxide particles have a particle size of less than 10
nanometers.
39. The doped cerium dioxide particles of claim 37 wherein said
cerium dioxide particles have a particle size within the range of 3
to 5 nanometers.
40. The doped cerium dioxide particles of claim 37 wherein each
particle is a single crystal.
41. The doped cerium dioxide particles of claim 37 wherein said
particles are predominantly of uniform shape and size.
42. The doped cerium dioxide composition of claim 37 wherein said
lanthanide series metal is selected from the group consisting of
lanthanum, samarium and gadolinium.
43. The doped cerium dioxide composition of claim 42 wherein said
lanthanide series metal is samarium.
44. A cerium dioxide composition comprising doped cerium dioxide
particles having a predominant particle size within the range of 3
to 100 nanometers produced by the process of: (a) mixing together a
first solution of a water-soluble cerium salt solution and a second
aqueous solution of an alkali metal or ammonium hydroxide to form a
reactant solution; (b) mixing a dopant precursor solution comprised
of the nitrate or acetate form of a lanthanide series metal with at
least one of said first and second solutions and said reactant
solution; (c) agitating said reactant solution under turbulent flow
conditions while concomitantly passing gaseous oxygen through said
reactant solution; and (d) precipitating said doped cerium dioxide
particles having a predominant particle size within the range of 3
to 100 nanometers.
45. The cerium dioxide composition of claim 43 wherein said doped
cerium dioxide particles are produced by the process of mixing said
dopant precursor solution with said cerium salt solution.
46. The cerium dioxide composition of claim 44 wherein said
lanthanide series metal is selected from the group consisting of
lanthanum, samarium and gadolinium
47. The cerium dioxide composition of claim 45 wherein said
lanthanide series metal is samarium.
Description
FIELD OF THE INVENTION
[0001] This invention relates to novel undoped and doped
nanometer-scale metal oxide particles as well as a novel method for
directly synthesizing doped and undoped nanometer-scale CeO.sub.2
particles having a controlled particle size ranging from 3-100
nanometers.
BACKGROUND OF THE INVENTION
[0002] Cerium dioxide (CeO.sub.2) based materials have been studied
for use in various applications including 1) fast ion conductors;
2) oxygen storage capacitors; 3) catalysts; 4) UV blockers; and 5)
polishing materials. Pure and doped CeO.sub.2 exhibits the cubic
fluorite structure, similar to ZrO.sub.2. Doping CeO.sub.2 with
lanthanide series elements (e.g. Gd.sup.3+) results in the
formation of oxygen vacancies ([Gd.sup.3+]=2[Vo.sup.oo]), and a
high ionic conductivity, .sigma..sub.i In particular,
Ce.sub.0.9Sm.sub.0.1O.sub.1.95 exhibits a .sigma..sub.1=0.025
(.OMEGA.*cm).sup.-1 at 600.degree. C., which is more than five
times that of ZrO.sub.2 based materials. As such
Ce.sub.0.9Sm.sub.0.1O.sub.1.95 is an attractive choice for use as a
low temperature electrolyte and as an anode component in solid
oxide fuel cells (SOFC).
[0003] Ceria particles can also be used as catalysts, such as
three-way catalysts to purify exhaust gases, such as for
automobiles. This application requires a high oxygen storage
content (OSC). In order to improve the OSC, the ceria may be doped
with lanthanide elements. The use of high surface area,
nanocrystalline powder could benefit all of these applications.
[0004] Typically, processes for preparing nanocrystalline CeO.sub.2
involve simple oxidation of Ce metal clusters to form CeO.sub.2, or
solution processes that take advantage of the small solubility
product of Ce(OH).sub.3 (10.sup.-23). In addition, such processes
involve reaction temperatures of 100.degree. C. or higher. This
results in larger particle sizes and lower surface area of the
crystals. The particle size is inversely related to the specific
surface area ("SSA").
[0005] An example process is found in, U.S. Pat. No. 5,017,352
which discloses ceria having a SSA of at least 85.+-.5 m2/g. The
ceria particles are made from the hydrolization of cerium (IV)
nitrate solution in an acidic medium and followed by calcining the
washed and dried precipitate in the temperature range of
300.degree. to 600.degree. C. for a period of 30 minutes to ten
hours. This basic process can also be used to produce ceria having
a SSA of at least 130 m2/g as disclosed in U.S. Pat. No. 5,080,877.
The ceria is formed by reacting an aqueous solution of cerium (IV)
salt with an aqueous solution of sulfate ions to precipitate a
basic ceric sulfate, washing the precipitate with ammonia and then
calcined in a furnace at 400.degree. C. for 6 hours.
[0006] It is also possible to generate single crystal grains
ranging in size from 10 to 80 nm of cerium oxide that have a
uniform particle size and shape. This is disclosed in U.S. Pat. No.
5,938,837 as being accomplished by mixing cerous nitrate with a
base to keep the pH from 5 to 10 and then rapidly heating the
mixture to 70.degree. to 100.degree. C. and maintaining the mixture
at that temperature from about 30 minutes to 10 hours.
[0007] U.S. Pat. No. 4,786,325 discloses a method for the
production of a solid solution of cerium oxide and a lanthanide
series metal. This is achieved by combining a cerium salt, a basic
solution, and a lanthanide salt. The mixture is reacted at either
10-25.degree. C. or 40-95.degree. C., filtered, dried, and
calcinated at 600 to 1200.degree. C. for a period of time of 30
minutes to 10 hours. The particles are ground so that their mean
particle size is from 0.5 to 1.5 .mu.m and the resulting SSA is
from 2 to 10 m.sup.2/g.
[0008] U.S. Pat. No. 5,712,218 discloses a method for producing a
solid solution of cerium/zirconium mixed oxides that optionally can
include yttrium. The method involves mixing stoichiometric amounts
of soluble compounds of cerium, zirconium and optionally yttrium,
heating the mixture to at least 100.degree. C., and filtering out
the product. Optionally the product can be further calcinated at
between 200.degree. to 1000.degree. C. However, it is disclosed
that the calcinations process will reduce the surface area of the
solid solution. The SSA of the uncalcinated solid solution can
reach over 150 m.sup.2/g.
SUMMARY OF THE INVENTION
[0009] The present invention involves the use of a semi-batch
reactor process to synthesize metal oxide particles with
controllable particle size between 3 to 100 nm and with uniform
particle size and shape. The invention will be described in detail
with respect to the use of cerium, however the invention is
applicable to the use of iron, chromium, manganese, niobium,
copper, nickel, and titanium in place of or in combination with
cerium. The basic process involves mixing a cerium salt and an
alkali metal or ammonium hydroxide, which operates as a
precipitant, to form a precipitate, and then filtering and drying
the precipitate. The mixture is preferably constantly stirred at a
rate that ensures turbulent conditions to enhance the mixing.
[0010] In carrying out the present invention a first solution of a
water-soluble cerium salt is mixed with a second solution of an
alkali metal or ammonium hydroxide are mixed together to form a
reactant solution. While the reactant solution is agitated under
turbulent flow conditions, oxygen is passed through the reactant
solution. Cerium dioxide particles having a predominant particle
size within the range of 3-100 nanometers are precipitated from the
reactant solution. In a preferred embodiment of the invention, the
second aqueous solution is an aqueous solution of ammonium
hydroxide with a concentration of ammonium hydroxide in water
within the range of 0.1 moles to 1.5 moles per liter. While
ammonium hydroxide is preferred, other alkali metal hydroxides,
such as sodium or potassium hydroxide, can be employed.
[0011] There are a number of variables involved in the mixing step
that can be controlled in order to synthesize ceria particles of
uniform shape at the desired particle size. First, the amount of
oxygen gas that is bubbled through the reactor as the reactants are
mixed will affect the particle size. Bubbling oxygen gas through
the reactor decreases the particle size of the ceria particles.
Using the oxygen gas allows the synthesis of ceria particles that
are as small as 3 nm as opposed to particles that are 12 nm when
oxygen is omitted. Second, adjusting the temperature at which the
reaction takes place will also affect the particle size. This
method will result in the synthesis of ceria particles of 15 nm at
20.degree. C. and 50 nm sized particles of ceria at 70.degree. C.
In addition, heating the produced ceria particles for one hour will
result in their coarsening to larger particle sizes depending on
the temperature being used.
[0012] Finally, the order with which the two reactants are mixed
will affect the pH value at which crystallization takes place. In
the case of adding the precipitate into the salt (PIS), the pH
starts out low, due to the slightly acidic nature of the cerium
salt. As a result, while the primary particle size is approximately
10 nm, the agglomerates are large and non-uniform in shape. On the
other hand, in the case of the addition of the salt into the
precipitate (SIP), the pH remains higher than 9 during the entire
reaction. This results in particle size approximately the same as
the primary particle size from the PIS process, however, there is
significantly less agglomeration and the particles were of uniform
size and shape due to homogenous nucleation.
[0013] Consequently, by using this process, it is possible to
synthesize ceria particles that have a uniform shape and size and
whose size is controllable within the range of 3 nm to 100 nm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a schematic representation of the reactor setup
used to carry out this method
[0015] FIG. 2 is a schematic of the scale of the mixing steps
[0016] FIG. 3 is a TEM image of the powders prepared at high
stirrer rate using SIP and bubbling oxygen through the reactor.
[0017] FIG. 4 is a graph of the pH evolution and the cerium ion
dissipation in the PIS process without using oxygen.
[0018] FIG. 5 is a graph of the pH evolution and the cerium ion
dissipation in the SIP process without using oxygen.
[0019] FIG. 6 is a TEM image of the powders prepared from the SIP
process without using oxygen.
[0020] FIG. 7 is an X-ray diffraction pattern of the particles from
the SIP process without using oxygen.
[0021] FIG. 8 is an X-ray diffraction pattern of the particles from
the SIP process with the use of oxygen.
[0022] FIG. 9 is a graph of the variation of CeO.sub.2 specific
surface area and particle size vs. annealing temperature.
[0023] FIG. 10 is a graph of the variation of CeO.sub.2 particle
size in nanometers vs. temperature.
[0024] FIG. 11 is a TEM image of the disordering structure of
nanometer CeO.sub.2 particles.
[0025] FIG. 12 is an X-ray diffraction pattern of Sm doped
CeO.sub.2, Sm.sub.2O.sub.3 and CeO.sub.2
DETAILED DESCRIPTION OF THE INVENTION
[0026] The liquid phase precipitation process of this method
includes three mechanisms: chemical reaction, nucleation, and
crystal growth. It was found that in most cases these three
mechanisms are fast, hence the mixing procedure has a large
influence on the product particle size and its distribution.
Therefore, control over the nucleation and growth mechanisms are
achieved by controlling the mixing conditions.
[0027] The prepared precursors for this method are: aqueous
solution of ammonium hydroxide (0.1 to 1.5*10.sup.-3 mol/g), cerium
salt solution, preferably cerium nitrate hexahydrate,
Ce(NO.sub.3).sub.3.6H.sub.2O, (GIF, 99.9%) solution (0.6 to
0.8*10.sup.-3 mol/g), and nitrate acetates of lanthanide series
metals as the dopant precursor. The use of excessive precipitant is
preferred so that the pH value is .apprxeq.9 after the reaction is
complete. The reaction can be carried out in a system as shown in
FIG. 1. The cerium salt and the ammonium hydroxide are fed into a
semi-batch reactor 8. This can be accomplished by placing the
ammonium hydroxide solution in reactor 8 and placing the cerium
salt in a precursor vessel 4 (SIP feeding). A Peristaltic pump 5 is
provided to pump the solution from the precursor vessel 4 into
reactor 8 at a fixed rate. Alternatively, the ammonium hydroxide
can be placed into precursor vessel 4 and the cerium salt placed in
reactor 8 (PIS feeding). Finally, there can be a second precursor
vessel (not shown) and peristaltic pump (not shown) and each
precursor can be separately fed into reactor 8 which would contain
distilled water. It is preferred that the solutions being fed into
the reactor by pump 5 is fed at a rate within the range of 0.5 to
10 ml/min. Any dopant precursor that is being used can either be
added to the cerium salt solution or fed into the reactor from a
separate precursor vessel by an additional peristaltic pump.
[0028] Impeller 3 is provided to maintain turbulent conditions in
reactor 8. It is powered by motor 2 that preferably has a 0-15000
rpm range and is preferably operated in the 100-5000 rpm range.
Motor 2 is controlled by rate controller 1. The stirring rate
rapidly distributes the particles and prevents their concentration
from being localized at the region near the feed point. This
insures that micromixing is occurring as opposed to the slower
macromixing that would otherwise occur as a result of the reaction
only occurring at the surface of the drops of reactant. The scale
of mixing is schematically depicted in FIG. 2. An increase of the
impeller speed to generate turbulent conditions does not change the
primary particle size, but does significantly decrease the
agglomerate size. The onset of turbulent flow occurs when the
Reynolds number, R.sub.e, is .gtoreq.110.sup.4. The Reynolds number
is defined by R e = D 2 .times. N .times. .times. .rho. .mu. ,
##EQU1## where D is the motor's impeller diameter (m), N is the
impeller speed (rpm), .rho. is the liquid density (kg/m.sup.3) and
.mu. is the liquid viscosity (cp). Returning to FIG. 1, the rate
controller 1 is used to keep impeller 3 at the proper rpm range to
maintain turbulent conditions in reactor 8. Rate controller 1 also
automatically adjusts the power load to motor 2 in order to keep
impeller 3 at a constant rpm as the viscosity of the slurry in
reactor 8 changes.
[0029] FIG. 3 depicts a TEM micrograph of the resultant CeO.sub.2
particles when impeller 3 was set at 500 rpm, which corresponds to
a R.sub.e.apprxeq.1.310.sup.4. When the same method was used, with
the exception that impeller 3 was set at 100 rpm,
R.sub.e.apprxeq.2.610.sup.3, the primary particle size was the
same, however the agglomerate size was significantly increased.
[0030] The order the reactants are added also plays an important
role in the resulting powder. It appears that the nucleation and
growth of the Ce(OH).sub.3 occurs at the droplet:reactant
interface. The difference between whether the cerium salt is added
to the ammonium hydroxide (SIP feeding) or the ammonium hydroxide
is added to the cerium salt (PIS feeding) is the pH value at which
crystallization takes place.
[0031] FIG. 4 shows a graph with the pH value on the first ordinate
axis, time in minutes on the abscissa axis, and cerium hydroxide
concentration on the second ordinate axis for the PIS feeding
process. In PIS feeding, the pH value in the reactor is initially
very low (pH .apprxeq.3.8-4.3 for the cerium nitrate solution), and
increases rapidly with the addition of just a few drops of ammonium
hydroxide to a value of approximately 7.2. Further additions
resulted in a slight but steady increase in pH as the Ce.sup.+3
ions were consumed, with a sharp transition of pH when the reaction
was close to the end. FIG. 4 also shows the evolution of the
solubility product of [Ce.sup.+3][OH].sup.3 over the course of the
reaction. This value is less then the critical solubility constant
of Ce(OH).sub.3, which is .apprxeq.710.sup.-21. Under these
conditions, even though a nucleus may form at the drop:reactant
interface, it is in an unstable state because of the low pH value
of the bulk solution. This results in a redissolution process
called ripening. Consequently the particles synthesized are highly
agglomerated and non-uniform in shape.
[0032] As shown in FIG. 4, which shows PIS feeding with no oxygen
bubbling and the mixer set at 500 rpm, the reaction results in
interesting color changes to the slurry. The slurry was initially
purple in section a (low pH), transitioned to brown in section b
(intermediate pH), and then turned yellow in section c (high pH).
These color changes appear to relate to the valence state of the
Ce, with most likely purple corresponding to Ce.sup.+3, yellow
corresponding to Ce.sup.+4, and brown corresponding to a mix of
these two states.
[0033] FIG. 5 is a graph showing pH value on the first ordinate
axis, time in minutes on the abscissa axis, and cerium hydroxide
concentration on the second ordinate axis for the SIP feeding
process. During the SIP feeding process, the pH value always
remains higher than 9 (i.e. [OH.sup.-] higher than 10.sup.-5
mol/l). This is shown in FIG. 5 which shows the pH and
[Ce.sup.+3][OH.sup.-].sup.3 concentration changes as the reaction
progresses during the SIP feeding process, without any oxygen
bubbling and with the mixer set at 500 rpm. As also shown in FIG.
5, the slurry color changes immediately to brown upon the addition
of the cerium salt (section `a`) and then turns light yellow
(section `b`) over a period of only 1 minute.
[0034] Under the basic conditions during SIP feeding, the
solubility product of [Ce.sup.+3][OH.sup.-].sup.3 is much higher
than the solubility constant (K.sub.sp), meaning that the
supersaturation value, S = [ Ce + 3 ] .function. [ O .times.
.times. H - ] 3 K sp , ##EQU2## is very large. This establishes an
environment that favors homogenous nucleation. FIG. 6 is a TEM
image of particles made using the SIP feeding process, without any
oxygen bubbling and with the mixer set at 500 rpm. The result is
primary particles that are .apprxeq.10 nm and which are of a
uniform size and shape.
[0035] Returning to the system shown in FIG. 1, it is advantageous
to bubble oxygen gas through reactor 8 while carrying out the
reaction. This is accomplished by adding oxygen gas through a
stainless steel tube 6 and out a gas distributor 7 at a predefined
rate. It is preferred that the oxygen is bubbled through the
reactor within the range of 1-500 ml/min. In general, after
filtration, a powder cake appears brown due to the presence of
Ce(OH).sub.3 (purple) and CeO.sub.2 (light yellow). After aging
under ambient conditions, it transforms to a totally light yellow
powder (CeO.sub.2). Drying under a vacuum can accelerate this and
results in large amounts of water condensing on the container
walls. This appears to be caused by the reaction
2Ce(OH).sub.3+1/2O.sub.2.fwdarw.CeO.sub.2+3H.sub.2O. Therefore,
bubbling O.sub.2 during the mixing of the reactants can be applied
to speed up this conversion of Ce(OH).sub.3 to CeO.sub.2.
[0036] This is illustrated by experimental work in which ammonium
hydroxide was bubbled with oxygen for 1 minute and then the SIP
process was engaged. Adding droplets of the Ce(NO.sub.3).sub.3.6H2O
immediately turned the slurry purple and then over a period of
approximately 30 seconds it transitioned through a dark brown to a
light yellow color.
[0037] FIG. 7 is a graph of the XRD pattern from the SIP feeding
process with the intensity on the ordinate axis and 2.theta. on the
abscissa axis. The process was carried out at room temperature and
with stirring at 500 rpm's without any use of oxygen. It shows a
resulting particle size of 5 nm. FIG. 8 is another graph showing
the XRD pattern with the intensity on the ordinate axis and
2.theta. on the abscissa axis. The process used in FIG. 8 is the
identical process used in FIG. 7 except that oxygen was bubbled
through the solution during the reaction. The particle size in this
case is 3 nm and the particles are less agglomerated as shown in
FIG. 6, which is a TEM image of the resulting particles. However,
the powder shown in FIG. 7, while being more agglomerated than the
powder shown in FIG. 8, is only lightly agglomerated, and can be
easily re-dispersed in a solution.
[0038] It appears that bubbling the oxygen gas simply maintains the
equilibrium concentration of oxygen gas that is dissolved in the
solution. This is because the overall results indicate that the
nucleation step is the fastest, meaning that Ce(OH).sub.3 formation
is immediate and would not be impacted by the presence of an
O.sub.2 bubble. The oxidation reaction can either take place at the
surface of the O.sub.2 bubble or with dissolved O.sub.2. The
equilibrium concentration of oxygen in water-ammonium hydroxide
solutions ranges from 10 to 25 ppm. In a 500 ml reactor and a
typical batch size of approximately 10 grams of Ce(OH).sub.3, this
would not be sufficient fully oxidize all of the Ce(OH).sub.3 to
CeO.sub.2. The bubbling O.sub.2 would replenish the dissolved
O.sub.2 in the solution and allow this reaction to continue to
completion faster. In any case, the use of O.sub.2 bubbling during
the SIP process yields the finest and least agglomerated CeO.sub.2
powder.
[0039] In the system shown in FIG. 1, reactor 8 is maintained at a
constant temperature, preferably room temperature, through a
temperature controller 9. Varying the temperature that the reaction
is carried out at affects the particle size that is synthesized. In
experimental work carried out at 70.degree. C. the particles of
CeO.sub.2 were 50 nm. The same process carried out at 20.degree. C.
resulted in particles that were only 15 nm. As can be seen, the
smallest particle sizes occur around room temperature, so no
heating is needed in order to generate the smallest particle sizes.
However, the temperature of the reactor can be increased in order
to synthesize particles of CeO.sub.2 powder of a desired larger
size.
[0040] In addition, the particles synthesized with this process
will coarsen when heated. FIG. 9 is a graph showing the BET
specific surface area (m.sup.2/g) on the first ordinate axis,
temperature (.degree. C.) on the abscissa axis and particle size
(nm) on the second ordinate axis. It shows the SSA and the
corresponding particle size for annealing temperatures ranging from
150.degree. to 800.degree. C., all for a 1-hour soak time. FIG. 9
shows that the particle size increases slowly from 4 nm up to 10 nm
at 500.degree. C. and then begins to rapidly increase to reach 100
nm at 800.degree. C. This information can be plotted in an
Arrhenius manner as is shown in FIG. 10, which is a graph showing
the natural log of the particle size (nm) on the ordinate axis and
the inverse of the annealing temperature (.degree. C.) on the
abscissa axis, to show two distinct linear regions. The activation
energy in the low temperature range is 2.4 kJ/mol and in the high
temperature range the activation energy is 63.4 kJ/mol. Therefore,
it appears that there are two different mechanisms for crystal
growth at the different temperatures, which can be used to generate
ceria particles of the desired size.
[0041] FIG. 11 is a TEM lattice image of a collection of CeO.sub.2
primary particles after room temperature drying. It can be seen
that there are many crystal regions (supporting the XRD data) but
there is also a large fraction of the ensemble in disorder, perhaps
even amorphous. This state likely provides a large driving force
for diffusion and subsequent growth at higher temperatures. Lattice
diffusion typically has a lower activation energy then other
mechanisms so it is possible that simple atomic rearrangement and
ordering results in the slow crystal increase at lower
temperatures. At higher temperatures, boundary diffusion possibly
controls the particle size evolution because of the higher energy
associated with long range ordering and particle rearrangement.
Therefore, this data can be used to pick an annealing temperature
that will result in crystal growth to the desired size.
[0042] As disclosed above, many of the applications for CeO.sub.2
utilize the high ionic conductivity that can be achieved by
acceptor doping with lanthanide elements such as La.sup.3+,
Sm.sup.3+, and Gd.sup.3+. Of these, Sm.sup.3+ yields the highest
ionic conductivity. During the SIP process the supersaturation
values for Ce.sup.3+ ranges from 1.410.sup.13.about.1.410.sup.10
depending on how much the Ce.sup.3+ diffuses through the reactor
when it is added to the ammonium hydroxide. Using the K.sub.sp
values from Table I, the supersaturation value for Sm.sup.3+ is
5.410.sup.11. The theoretical and calculated values differ somewhat
in Table I most likely due to the assumption of equilibrium for the
calculated values. As a result of the supersaturation values,
during SIP feeding, it appears that Ce.sup.3+ and Sm.sup.3+
precipitate simultaneously. In addition, FIG. 12 is a graph showing
of a number of ERD patterns with the intensity on the ordinate axis
and 2.theta. on the abscissa axis. FIG. 12 shows the XRD patterns
of the as-synthesized (i.e. not thermally annealed)
Ce.sub.1-xSm.sub.xO.sub.2 (x=0.02, 0.05, 0.10, and 0.20) and
Sm.sub.2O.sub.3, along with CeO.sub.2 annealed at 800.degree. C.
for reference. Clear shifts in the diffraction peaks are evident as
greater amounts of [Sm.sup.3+] were added. This establishes that a
solid solution has formed. Similar results were achieved for
La.sup.3+ and Gd.sup.3+ doped CeO.sub.2. TABLE-US-00001 TABLE I
Experimental Calculated Element K.sub.sp K.sub.sp La 1.10E-19
5.01E-21 Ce 7.00E-21 1.26E-20 Sm 4.60E-23 3.16E-17 Gd 1.80E-23
2.51E-16
[0043] On the other hand, the supersaturation values for PIS
feeding (pH=7.3) are 1.1 for Ce.sup.3+ and 43.2 for Sm.sup.3+, for
[Ce.sup.3+]=1.0 mol/l and [Sm.sup.3+]=0.25 mol/l. These conditions
resulted in the successive precipitation of Ce.sup.3+ and Sm.sup.3+
hydroxides and consequently cation segregation in the dried powder.
However due to the fine particle size, it is believed that at
relatively low temperatures a solid solution would form.
[0044] The particle size and morphology were determined by
transmission electron microscopy (TEM, Philips EM420). Samples for
the TEM were prepared by ultrasonically dispersing the powders in
ethanol, and then droplets were placed on carbon-coated Cu grids.
Corresponding electron diffraction patterns (EDF) were used to
characterize the particle crystallinity, as well as X-ray
diffractometry (XRD; Scintag 2000). The specific surface area (SSA)
is inversely related to the particle size and is calculated by the
Brunauer-Emmett-Teller (BET) method. (Quantachrome; Nova 1000). The
particle .times. .times. size = 6 .rho. S .times. .times. S .times.
.times. A ##EQU3## where .rho. is the density of the powders
(g/cm.sup.3).
[0045] The theoretical density of CeO.sub.2 was calculated using
the lattice parameters calculated from the XRD pattern. X-ray line
broadening (20.degree..ltoreq.2.theta..gtoreq.100.degree.) was used
to calculate the x-ray coherence length, which corresponds to the
particle size after correcting for strain effects using the Lorentz
intensity breadth. The theoretical densities .rho..sub.th,
(kg/m.sup.3) of the lanthanide doped CeO.sub.2 compositions were
calculated by .rho. th = 4 n A .times. a 3 .function. [ M Ce
.function. ( 1 - x ) + M L .times. .times. n .times. x + M O
.function. ( 2 - 0.5 .times. x ) ] , ##EQU4## where M.sub.Ce,
M.sub.Ln and M.sub.O are the molecular weights of the sub-species
in kg/mole, n.sub.A is Avogadro's number (6.02310.sup.23/mole), and
`a` (meters) is the XRD lattice parameter. All lanthanide elements
were assumed to be in the 3+ valance state.
[0046] The crystal grain size was determined by powder x-ray
diffraction, analyzing the pattern by simulation based upon the
Gaussian and Lorentz distribution after correcting for the strain
effect. The equation, which was used, is shown as: .beta. total =
.beta. XRCL + .beta. Strain = 0.9 .times. .lamda. t .times. .times.
cos .times. .times. .theta. + 4 .times. ( .DELTA. d ) d .times. tan
.times. .times. .theta. . ##EQU5## A plot of .beta..sub.total(cos
.theta.) vs. sin .theta. has the intersection of 0.9.lamda./t,
where .lamda. is the wavelength of generated x-ray and t is the
sample x-ray coherence length, i.e. the crystal grain size. This
was compared to the particle size calculated above to ensure that
each particle was a single grain crystal.
[0047] In order to further illustrate the present invention and the
advantages thereof, the following specific examples are given, it
being understood that same are intended only as illustrative and in
no way limiting:
EXAMPLE 1
[0048] Ammonium hydroxide aqueous solution with a concentration of
1.510.sup.-3 mol/g was placed in a semi-batch tank reactor. A
0.510.sup.-3 mol/g solution of cerium nitrate aqueous solution was
the fed into the reactor (SIP feeding). There was a 20% excess of
the ammonium hydroxide solution. The feeding rate was controlled by
a peristaltic pump supplied by Fisher. The ammonium hydroxide
solution was constantly stirred at a rate of 300 rpm with the power
load of the stirrer being automatically adjusted with the changing
viscosity of the slurry in the reactor. The reactor temperature was
set at room temperature. Oxygen was bubbled into the reactor at a
rate of 20 l/min as controlled by a gas flow-meter. The slurry was
vacuum filtered and then vacuum dried at room temperature. The SSA
data were found to be about 150 m.sup.2/g and the TEM microscopy
photos showed that the particle size is around 3-5 nm. This was
confirmed to be the same size as a single crystal from the x-ray
diffraction pattern.
EXAMPLE 2
[0049] The same setup as in example 1 is used. This time PIS
feeding was used with ammonium hydroxide aqueous solution used as
the feeding precursor and cerium nitrate solution in the reactor.
The feeding rate was controlled between 0.5 ml/min to 8 ml/min. At
a reactor temperature of 70.degree. C. the average synthesized
particle size was 50 nm and at a reactor temperature was of
20.degree. C. the average particle size was 15 nm.
EXAMPLE 3
[0050] PIS feeding was carried out as in example 1 at room
temperature, a feeding rate of 5 ml/min and a stirrer rate of 1000
rpm. When oxygen was bubbled through the reactant mixture the
smallest particle size obtained was 4 nm. Without the use of oxygen
the smallest particle size obtained was 12 nm.
EXAMPLE 4
[0051] The method used in Example 1 was repeated using double
feeding, which is where ammonium hydroxide aqueous solution and
cerium nitrate solution are both used as feeding solutions into a
reactor that contains distilled water. The feeding rate was kept in
the range of 1 ml/min to 8 ml/min. The temperature was 25.degree.
C. and the mixture was stirred to establish turbulent conditions.
The average particle size is 10 nm. Oxygen was not used in this
example.
EXAMPLE 5
[0052] Solid solutions were observed using the above method with
the Lanthanide element in a nitrate or acetate compound that was
dissolved in water to form an aqueous solution, which was used as
the dopant precursor.
[0053] a. Niobium-citric acid aqueous solution was used as the
precursor in the double feeding method to form niobium and cerium
mixed compounds. These compounds were transferred to solid solution
after being sintered.
[0054] b. Yttrium nitrate or acetate aqueous solution was used as
the lanthanum dopant precursor and mixed with the cerium nitrate
solution. This mixed solution was used as the feeding solution in
SIP feeding. A solid solution resulted from the reaction.
[0055] c. Zirconia hydroxy acetate aqueous solution or the acetate
aqueous solution was used as the dopant precursor and mixed with
the cerium nitrate aqueous solution. This mixed solution was used
as the feeding solution in SIP feeding. A solid solution resulted
from the reaction.
[0056] d. Double feeding of the doped element precursors from a, b,
and c were used as a separate feeding solution in double feeding
method. The solution in the reactor was distilled water. The
reaction resulted in the formation of a solid solution in each of
the cases.
[0057] Having described specific embodiments of the invention, it
is understood that modifications thereof may be suggested by those
skilled in the art, and it is intended to cover all such
modifications as filed within the scope of the appended claims.
* * * * *