Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FUEL ADDITIVE CONTAINING LATTICE ENGINEERED
CERIUM DIOXIDE NANOPARTICLES
[00011
Field of the Invention
100021 The present invention relates in general to cerium dioxide
nanoparticles and,
in particular, to cerium dioxide nanoparticles, Cei,M x02, containing one or
more
transition metals (M). and to a method for preparing such particles. These
nanoparticles
are useful as components of fuel additive compositions, as a wash coat for
catalytic
converters, or as a catalyst for a reduction/oxidation reaction
Background of the Invention
100031 The trucking industry accounts for more than 5% of the U.S. GDP and
is
comprised of more that 500,000 for¨hire, private and government fleets,
including owner
operators. It is a barometer of the US economy representing nearly 70% of the
tonnage
carried by all modes of domestic freight transportation, including
manufactured and retail
goods. This industry is powered almost exclusively by diesel engines
(compressive
ignition engines), which are characterized by high torque developed at low rpm
and 25%
greater thermodynamic efficiency compared to spark ignition (gasoline)
engines. As a
result of the 2007 EPA mandated emissions reductions in oxides of nitrogen
(N0x) and
diesel particulate matter (DPIVI or soot), diesel-powered vehicles are now
required to be
fitted with diesel oxidation catalysts (DOC) or some form of catalytic
converter and to
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burn ultra low sulfur diesel fuel, ULSD, (< 15 ppm S). These and other
technologies such
as EGR (emissions gas recirculation) are necessary to meet the EPA mandated
emissions
standards. The ULSD requirement is a consequence of sulfur poisoning of the
precious
metals on the DOC by high sulfur levels. This legislation has far ranging
consequences,
as (on road) diesel fuel in the US is consumed at a prodigious rate, 650M
gal/week, which
is second only to that of gasoline (1300 M gal/wk).
[0004] It is estimated that the imputed costs of the EPA mandates will add
approximately $0.39 to the cost of one gallon of diesel fuel. This is factored
into the
following components: increased engine costs ($0.11/gal), particle trap
maintenance
($0.05/gal), reduced fuel economy ($0.09/gal), increase in ULSD ($0.06/gal),
and lower
ULSD fuel energy content ($0.08/gal).
[0005] Clearly any technology that could provide a reduction in DPM and
other
emissions, simultaneously with an increase in fuel economy (as measured by an
increase
in miles-per-gallon) would be perceived as a tremendous financial and
environmental
benefit.
[0006] Diesel fuel additives, in particular, those that include to
inorganic metal and
metal oxide materials as opposed to organic materials, offer the promise of
reduced DPM
and improved fuel economy.
[0007] Kracklaurer, U.S. Patent No.4,389,220
describes a method of conditioning diesel engines in
which a diesel engine is operated on a diesel fuel containing from about 20-30
ppm of
dicyclopentadienyl iron for a period of time sufficient to eliminate carbon
deposits from
the combustion surfaces of the engine and to deposit a layer of iron oxide on
the
combustion surfaces, which layer is effective to prevent further buildup of
carbon
deposits. Subsequently, the diesel engine is operated on a maintenance
concentration of
from about 10-15 ppm of dicyclopentadienyl iron or an equivalent amount of a
derivative
thereof on a continuous basis. The maintenance concentration is effective to
maintain the
catalytic iron oxide layer on the combustion surfaces but insufficient to
decrease timing
delay in the engine. The added dicyclopentadienyl iron may produce iron oxide
on the
engine cylinder surface (Fe203), which reacts with carbon deposits (soot) to
form Fe and
CO2, thereby removing the deposits. However, this method may accelerate the
aging of
the engine by formation of rust.
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100081 Valentine, et at.. U.S. Patent Appl. Publ. No.2003/0148235
describe specific bimetallic or trimetallic fuel-
borne catalysts for increasing the fuel combustion efficiency. The catalysts
reduce
fouling of heat transfer surfaces by unburned carbon while limiting the amount
of
secondary additive ash, which may itself cause overloading of particulate
collector
devices or emissions of toxic ultra fine particles when used in forms and
quantities
typically employed. By utilizing a fuel containing a fuel-soluble catalyst
comprised of
platinum and at least one additional metal comprising cerium and/or iron,
production of
pollutants of the type generated by incomplete combustion is reduced. Ultra
low levels of
nontoxic metal combustion catalysts can be employed for improved heat recovery
and
lower emissions of regulated pollutants. However, fuel additives of this type,
in addition
to using the rare and expensive metals such as platinum, can require several
months
before the engine is "conditioned". By "conditioned" is meant that all the
benefits of the
additive are not obtained until the engine has been operated with the catalyst
for a period
of time. Initial conditioning may require 45 days and optimal benefits may not
be
obtained until 60-90 days. Additionally, free metal may be discharged from the
exhaust
system into the atmosphere, where it may subsequently react with living
organisms.
[0009] Cerium dioxide is widely used as a catalyst in converters for the
elimination of
toxic exhaust emission gases and the reduction in particulate emissions in
diesel powered
vehicles. Within the catalytic converter, the cerium dioxide can act as a
chemically active
component, acting to release oxygen in the presence of reductive gases, as
well as to
remove oxygen by interaction with oxidizing species.
[0010] Cerium dioxide may store and release oxygen by the reversible
process shown
in equation 1.
Ce02 + x/2 02 (eq. 1)
This process is referred to as the oxygen storage capability (OSC) of ceria.
Here ceria
acts as an oxygen storage buffer ( much like a pH buffer), releasing oxygen in
spatial
regions where the oxygen concentration or pressure is low and absorbing oxygen
in
spatial regions where the oxygen pressure is high. When x = 0.5, ceria is
effectively fully
reduced to Ce203, and the maximum theoretical OSC is 1452 micromoles of 02 per
gram
of ceria. The redox potential between the Ce3+ and Ce4+ ions lies between 1.3
and 1.8V
and is highly dependent upon the anionic groups present and the chemical
environment
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(CERIUM: A Guide to its Role in Chemical Technology, 1992 by Molycorp, Inc,
Library of Congress Catalog Card Number 92-93444)). This allows the described
forward
and backward reactions to easily occur in exhaust gases near the
stoichiometric ratio of
required oxygen (15:1). Cerium dioxide may provide oxygen for the oxidation of
CO or
hydrocarbons in an oxygen-starved environment, or conversely may absorb oxygen
for
reducing the levels of nitrogen oxides (N0x) in an oxygen-rich environment.
Similar
catalytic activity may also occur when cerium dioxide is added as an additive
to fuel, for
example, diesel or gasoline. However, for this effect to be useful, the cerium
dioxide
must be of a particle size small enough, i.e., nanoparticulate (less than 100
nm), to remain
suspended by Brownian motion in the fuel and not settle out. In addition, as
catalytic
effects depend on surface area, the small particle size renders the
nanocrystalline material
more effective as a catalyst. The incorporation of cerium dioxide in fuel
serves not only
to act as a catalyst to reduce toxic exhaust gases produced by fuel
combustion, for
example, by the "water gas shift reaction"
CO + H20 --> CO2 + H2 ,
but also to facilitate the burning off of particulates that accumulate in the
particulate traps
typically used with diesel engines.
[0011] As already noted, cerium dioxide nanoparticles are particles having
a mean
diameter of less than 100 nm. For the purposes of this disclosure, unless
otherwise stated,
the diameter of a nanoparticle refers to its hydrodynamic diameter, which is
the diameter
determined by dynamic light scattering technique and includes molecular
adsorbates and
the accompanying solvation shell of the particle. Alternatively, the geometric
particle
diameter can be estimated using transmission electron micrography (TEM).
[0012] Vehicle on-board dosing systems that dispense cerium dioxide into
the fuel
before it enters the engine are known, but such systems are complicated and
require
extensive electronic control to feed the appropriate amount of additive to the
fuel. To
avoid such complex on-board systems, cerium dioxide nanoparticles can also be
added to
fuel at an earlier stage to achieve improved fuel efficiency. They can, for
example, be
incorporated at the refinery, typically along with processing additives such
as, for
example, cetane improvers or lubricity agents, or added at a fuel distribution
tank farm.
[0013] Cerium dioxide nanoparticles can also be added at a fuel
distribution center by
rack injection into large (-100,000 gal) volumes of fuel, or at a smaller fuel
company
depot, which would allow customization according to specified individual
requirements.
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In addition, the cerium dioxide may be added at a filling station during
delivery of fuel to
a vehicle, which would have the potential advantage of improved stabilization
of the
particle dispersion.
[0014] Cerium nanoparticles may form a ceramic layer on the engine
cylinders and
internal moving parts, thereby essentially turning the engine into a catalytic
device.
Alternatively, they may be recycled in the lubrication oil where they
accumulate. Their
catalytic efficiency derives from the fact that they provide a source of
oxygen atoms
during combustion by undergoing reduction according to the equation (1);
however, an
induction period of several months is usually required before their mpg
benefit is
observed. This ultimately results in better fuel combustion and reduced levels
of
particulate material emissions. Additionally, when used as a fuel additive,
these
nanoparticles may provide improved engine performance by reducing engine
friction. As
an alternative mode of introduction, cerium dioxide nanoparticles can be added
to the
lube oil and act as a lubricity enhancing agent to reduce internal friction.
This will also
improve fuel efficiency.
[0015]
[0016] Hawkins et al., U.S. Patent No. 5,449,387, discloses a cerium (IV)
oxidic
compound having the formula:
[0017] (H20)p[Ce0(A)2(AH)nlm
[0018] in which the radicals A. which are the same or different, are each
an anion of
an organic oxvacid AH having a pIC greater than 1, p is an integer ranging
from 0 to 5, n
is a number ranging from 0 to 2, and m is an integer ranging from 1 to 12. The
organic
oxyacid is preferably a carboxylic acid, more preferably, a C2-C20
monocarboxylic acid or
a C4-C12 dicarboxylic acid. The cerium-containing compounds can be employed as
catalysts for the combustion of hydrocarbon fuels.
[0019] Valentine et al., U.S. Patent No. 7,063,729, discloses a low-
emissions diesel
fuel that includes a bimetallic, fuel-soluble platinum group metal and cerium
catalyst, the
cerium being provided as a fuel-soluble hydroxyl oleate propionate complex.
[0020] Chopin et al., U.S. Patent No. 6,210,451, discloses a petroleum-
based fuel that
includes a stable organic sol that comprises particles of cerium dioxide in
the form of
agglomerates of crystallites (preferred size 3-4 nm), an amphiphilic acid
system
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containing at least one acid whose total number of carbons is at least 10, and
an organic
diluent medium. The controlled particle size is no greater than 200nm.
[0021] Birchem et al., U.S. Patent No. 6,136,048, discloses an adjuvant for
diesel
engine fuels that includes a sol comprising particles of oxygenated compound
having a
d90 no greater than 20 nm, an amphiphilic acid system, and a diluent. The
oxygenated
metal compound particles are prepared from the reaction in solution of a rare
earth salt
such as a cerium salt with a basic medium, followed by recovery of the formed
precipitate
by atomization or freeze drying.
[0022] Lemaire et al., U.S. Patent No. 6,093,223, discloses a process for
producing
aggregates of ceric oxide crystallites by burning a hydrocarbon fuel in the
presence of at
least one cerium compound. The soot contains at least 0.1 wt.% of ceric oxide
crystallite
aggregates, the largest particle size being 50-10,000 angstroms, the
crystallite size being
50-250 angstroms, and the soot having an ignition temperature of less than 400
C.
[0023] Hazarika et al., U.S. 7,195,653 B2, discloses a method of improving
fuel
efficiency and/or reducing fuel emissions of a fuel burning apparatus, the
method
comprising dispersing at least one particulate lanthanide oxide, particularly
cerium
dioxide, in the fuel at 1 to 10 ppm, either as a tablet, a capsule a powder or
liquid fuel
additive wherein the particulate lanthanum oxide is coated with a surfactant
selected from
the group consisting of alkyl carboxylic anhydrides and esters having an HLB
of 7 or
less. The preferred coating is dodecyl succinic anhydride.
[0024] Collier et al., U.S. Patent Appl. Publ. No. 2003/0182848, discloses
a diesel
fuel composition that improves the performance of diesel fuel particulate
traps and
contains a combination of 1-25 ppm of metal in the form of a metal salt
additive and 100-
500 ppm of an oil-soluble nitrogen-containing ashless detergent additive. The
metal may
be an alkali metal, an alkaline earth metal, a metal of Group IVB, VIIB,
VIIIB, TB, IIB, or
any of the rare earth metals having atomic numbers 57-71, especially cerium,
or mixtures
of any of the foregoing metals.
[0025] Collier et al., U.S. Patent Appl. Publ. No. 2003/0221362, discloses
a fuel
additive composition for a diesel engine equipped with a particulate trap, the
composition
comprising a hydrocarbon solvent and an oil-soluble metal carboxylate or metal
complex
derived from a carboxylic acid containing not more than 125 carbon atoms. The
metal
may be an alkali metal, an alkaline earth metal, a metal of Group IVB, VIIB,
VIIIB, TB,
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JIB, or a rare earth metal, including cerium, or mixtures of any of the
foregoing metals.
[0026] Caprotti et al., U.S. Patent Appl. Publ. No. 2004/0035045, discloses
a fuel
additive composition for a diesel engine equipped with a particulate trap. The
composition comprises an oil-soluble or oil-dispersible metal salt of an
acidic organic
compound and a stoichiometric excess of metal. When added to the fuel, the
composition
provides 1-25 ppm of metal, which is selected from the group consisting of Ca,
Fe, Mg,
Sr, Ti, Zr, Mn, Zn, and Ce.
[0027] Caprotti et al., U.S. Patent Appl. Publ. No. 2005/0060929, discloses
a diesel
fuel composition stabilized against phase separation that contains a
colloidally dispersed
or solubilized metal catalyst compound and 5-1000 ppm of a stabilizer that is
an organic
compound having a lipophilic hydrocarbyl chain attached to at least two polar
groups, at
least one of which is a carboxylic acid or carboxylate group. The metal
catalyst
compound comprises one or more organic or inorganic compounds or complexes of
Ce,
Fe, Ca, Mg, Sr, Na, Mn, Pt, or mixtures thereof
[0028] Caprotti et al., U.S. Patent Appl. Publ. No. 2006/0000140, discloses
a fuel
additive composition that comprises at least one colloidal metal compound or
species and
a stabilizer component that is the condensation product of an aldehyde or
ketone and a
compound comprising one or more aromatic moieties containing a hydroxyl
substituent
and a further substituent chosen from among hydrocarbyl, -COOR, or -COR, R
being
hydrogen or hydrocarbyl. The colloidal metal compound preferably comprises at
least
one metal oxide, preferred oxides being iron oxide, cerium dioxide, or cerium-
doped iron
oxide.
[0029] Scattergood, International Publ. No. WO 2004/065529, discloses a
method for
improving the fuel efficiency of fuel for an internal combustion engine that
comprises
adding to the fuel cerium dioxide and/or doped cerium dioxide and, optionally,
one or
more fuel additives.
[0030] Anderson et al., International Publ. No. WO 2005/012465, discloses a
method
for improving the fuel efficiency of a fuel for an internal combustion engine
that
comprises lubricating oil and gasoline, the method comprising adding cerium
dioxide
and/or doped cerium dioxide to the lubricating oil or the gasoline.
[0031] Cerium-containing nanoparticles can be prepared by a variety of
techniques
known in the art. Regardless of whether the synthesized nanoparticles are made
in a
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hydrophilic or hydrophobic medium, the particles normally require a stabilizer
to prevent
undesirable agglomeration. The following publications
describe some of these synthetic techniques.
[00321 Chane-Ching et al., U.S. Patent No. 6,271,269, discloses a process
for
preparing storage-stable organic sols that comprises: reacting a base reactant
with an
aqueous solution of the salt of an acidic metal cation to form an aqueous
colloidal
dispersion containing excess hydroxyl ions; contacting the aqueous colloidal
dispersion
with an organic phase comprising an organic liquid medium and an organic acid;
and
separating the resulting aqueous/organic phase mixture into an aqueous phase
and a
product organic phase. Preferred metal cations are cerium and iron cations.
The colloidal
particulates have hvdrodynamic diameters in the range of 5-20 nanometers.
[00331 Chane-Ching, U.S. Patent No. 6,649,156, discloses an organic sol
containing
cerium dioxide particles that are made by a thermal hydrolysis process; an
organic liquid
phase; and at least one amphiphilic compounds chosen from polyoxyethylenated
alkyl
ethers of carboxylic acids, polyoxyethvlenated alkyl ether phosphates, dialkyl
sulfosuccinates. and quaternary ammonium compounds. The water content of the
sols
may not be more than 1%. The mean crystallite size is about 5 nm, while the
particle
agglomerates of these crystallites range in size from 200 to 10 nm.
[00341 Chane-Ching, U.S. Patent No. 7,008,965, discloses an aqueous
colloidal
dispersion of a compound of cerium and at least one other metal, the
dispersion having a
conductivity of at most 5 mS/cm and a pH between 5 and 8.
100351 Chane-Ching, U.S. Patent Appl. Publ. No. 2004/0029978 (abandoned
December 7, 2005), discloses a surfactant formed from at least one
nanoparticle that has
amphiphilic characteristics and is based on a metal oxide, hydroxide ancUor
oxyhydroxide, on the surface of which organic chains with hydrophobic
characteristics
are bonded. The metal is preferably selected from among cerium, aluminum,
titanium or
silicon, and the alkyl chain comprises 6-30 carbon atoms, or polyoxyethylene
monoalkyl
ethers of which the alkyl chain comprises 8-30 carbon atoms and the
polyovethylene part
comprises 1-10 oxvethylene groups. The particle is an isotopic or spherical
particle having
an average diameter of 2-40 nm.
100361 Blanchard et al., U.S. Patent Appl. Publ. No. 2006/0005465,
discloses an
organic colloidal dispersion comprising: particles of at least one compound
based on at
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least one rare earth, at least one acid, and at least one diluent, wherein at
least 90% of the
particles are monocrystalline. Example 1 describes the preparation of a cerium
dioxide
colloidal solution from cerium acetate and an organic phase that includes
Isopar
hydrocarbon mixture and isostearic acid. The resulting cerium dioxide
particles had a clso
of 2.5 nm, and the size of 80% of the particles was in the range of 1-4 nm.
[0037] Zhou et al., U.S. Patent No. 7,025,943, discloses a method for
producing
cerium dioxide crystals that comprises: mixing a first solution of a water-
soluble cerium
salt with a second solution of alkali metal or ammonium hydroxide; agitating
the resulting
reactant solution under turbulent flow conditions while concomitantly passing
gaseous
oxygen through the solution; and precipitating cerium dioxide particles having
a
dominant particle size within the range of 3-100 nm. In Example 1, the
particle size is
stated to be around 3-5 nm. No mention is made of a stabilizing agent and it
is anticipated
that the sols will eventually agglomerate and settle.
[0038] Sandford et al., WO 2008/002223 A2, describe an aqueous
precipitation
technique that produces cerium dioxide directly without subsequent
calcination. Cerous +3
cation is oxidized to ceric +4 slowly by nitrate ion, and a stable non-
agglomerated sol of
11 nm crystallite size (and approximately equal grain size) is obtained when
acetic acid is
used as a stabilizer. Interestingly, EDTA and citric acid produce grains with
crystallite
sizes on the order of several hundred nanometers.
[0039] Woodhead, James, L. U.S. Patent No. 4,231,893, teaches the
preparation of an
aqueous dispersion of ceria by the acid treatment of Ce(OH)4 which has been
obtained
from the peroxide treatment of Ce +3 in base. No sizing data are provided and
at the
required pH for stabilization, 1.5, the likely stabilizer is NO3- anion.
[0040] Noh et al., U.S. Patent Appl. Publ. No. 2004/0241070, discloses a
method for
preparing single crystalline cerium dioxide nanopowder comprising: preparing
cerium
hydroxide by precipitating a cerium salt in the presence of a solvent mixture
of organic
solvent and water, preferably in a ratio of about 0.1:1 to about 5:1 by
weight; and
hydrothermally reacting the cerium hydroxide. The nanopowder has a particle
size of
about 30-300 nm.
[0041] Chan, U.S. Patent Appl. Publ. No. 2005/0031517, discloses a method
for
preparing cerium dioxide nanoparticles that comprises: rapidly mixing an
aqueous
solution of cerium nitrate with aqueous hexamethylenetetramine, the
temperature being
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maintained at a temperature no higher than about 320 K while nanoparticles
form in the
resulting mixture; and separating the formed nanoparticles. The mixing
apparatus
preferably comprises a mechanical stirrer and a centrifuge. In the
illustrative example,
the prepared cerium dioxide particles are reported to have a diameter of about
12 nm.
[0042] Ying et al., U.S. Patent Nos. 6,413,489 and 6,869,584, disclose the
synthesis
by a reverse micelle technique of nanoparticles that are free of agglomeration
and have a
particle size of less than 100 nm and a surface area of at least 20 m2/gm. The
method
comprises introducing a ceramic precursor that includes barium alkoxide and
aluminum
alkoxide in the presence of a reverse emulsion.
[0043] A related publication, Ying et al., U.S. Patent Appl. Publ. No.
2005/0152832,
discloses the synthesis, by a reverse micelle technique within an emulsion
having a 1-
40% water content, of nanoparticles that are free of agglomeration and have a
particle
size of less than 100 nm. The nanoparticles are preferably metal oxide
particles, which
can be used to oxidize hydrocarbons.
[0044] Hanawa et al., U.S. Patent No. 5,938,837, discloses a method for
preparing
cerium dioxide particles, intended primarily for use as a polishing agent,
that comprises
mixing, with stirring, an aqueous solution of cerous nitrate with a base,
preferably
aqueous ammonia, in such a mixing ratio that the pH value of the mixture
ranges from 5
to 10, preferably 7 to 9, then rapidly heating the resulting mixture to a
temperature of 70-
100 C, and maturing the mixture of cerous nitrate with a base at that
temperature to form
the grains. The product grains are uniform in size and shape and have an
average particle
size of 10-80 nm, preferably 20-60 nm.
[0045] European Patent Application EP 0208580, published 14 January 1987,
inventor Chane-Ching, applicant Rhone Poulenc, discloses a cerium (IV)
compound
corresponding to the general formula
[0046] Ce(M)õ(OH)y(NO3)2
[0047] wherein M represents an alkali metal or quaternary ammonium radical,
x is
between 0.01 and 0.2, y is such that y = 4-z + x, and z is between 0.4 and
0.7. A process
for preparing a colloidal dispersion of the cerium (IV) compound produces
particles with
a hydrodynamic diameter between about 1 nm and about 60 nm, suitably between
about 1
nm and about 10 nm, and desirably between about 3 nm and 8 nm.
[0048] S. Sathyamurthy et al., Nano Technology 16, (2005), pp 1960-1964,
describes
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the reverse micellar synthesis of Ce02 from cerium nitrate, using sodium
hydroxide as the
precipitating agent and n-octane containing the surfactant
cetyltrimethylammonium
bromide (CTAB) and the cosurfactant 1-butanol as the oil phase. The resulting
polyhedral particles had an average size of 3.7 nm, and showed agglomeration
when
removed from their protective reversed micellar structure. Additionally, the
reaction
would be expected to proceed in low yield (for reactants A and B there are as
many AB
collisions resulting in product as AA and BB non productive collisions).
[0049] Seal et al.. Journal of Nano Particle Research, (2002), 4, pp 433-
448,
describes the preparation from cerium nitrate and ammonium hydroxide of
nanocrystalline ceria particles for a high-temperature oxidation-resistant
coating using an
aqueous microemulsion system containing AOT as the surfactant and toluene as
the oil
phase. The ceria nanoparticles formed in the upper oil phase of the reaction
mixture had a
particle size of 5 nm.
[0050] Seal et al., U.S. Patent No. 7,419,516
describes the use of rare earth metal oxide, preferably ceria,
nanoparticles as fuel additives for reducing soot. The particles, which are
prepared by a
reverse micelle process using toluene as the oil phase and AOT as the
surfactant, have
diameters in the range of about 2-7 nm, the mean being about 5 nm.
[0051] Pang et al., I Mater Chan., 12 (2002), pp 3699-3704, prepared A1203
nanoparticles by a water-in-oil microemulsion method, using an oil phase
containing
cyclohexane and the non-ionic surfactant Triton X-114, and an aqueous phase
containing
1.0 M AlC103. The resulting A1203 particles, which had a particle size of 5-15
nm,
appeared to be distinctly different from the hollow ball-shaped particles of
submicron size
made by a direct precipitation process.
[0052] Cuif et al. U.S. Patent No. 6,133,194
describes a process that comprises reacting a metal salt solution
containing cerium, zirconium, or a mixture thereof, a base, optionally an
oxidizing agent,
and an additive selected from the group consisting of anionic surfactants,
nonionic
surfactants. polyethylene glycols, carboxylic acids, and carboxylate salts,
thereby forming
a product. The product is subsequently calcined at temperatures greater than
500 C
(which would effectively carbonize the claimed surfactants).
[0053] It should be appreciated that, while many authors claim ceria
nanoparticles
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well below 5 nm, no X-ray or electron diffraction data have been presented to
unequivocally establish that the grains are indeed cubic Ce02 and not
hexagonal or cubic
Ce203. There is substantial doubt that cubic Ce02 is thermodynamically stable
at very
small grain sizes, and that the grains are, in fact, the reduced and more
stable hexagonal
Ce203 form. S. Tsunekawa, R, Sivamohan, S. Ito, A. Kasuya and T. Fukada in
Nanostructured Materials, vol 11, no. 1, pp 141-147 (1999)
[0054] "Structural Study on Monosize Ce02_x Nanoparticles" in particular
casts doubt
upon the existence of Ce02 at or below 1.5 nm.
[0055] Additional evidence for the existence of Cc 3+ (and by extension
Ce203) at
very small grain diameters comes from the work of Desphande et al. in Applied
Physics
Letters 87, 133113 (2005) "Size Dependency Variation in Lattice Parameter and
Valency
States in Nano Crystalline Cerium Oxide", who found a log linear relationship
between
the change in lattice constant,
[00561 Aa=a-a) (a0= 5.43 A in Ce02) and the crystal diameter, D, as
follows:
[0057] log Aa = -0.4763 log D -1.5029 (Eq. 2)
[0058] Thus, a grain diameter of 10 nm will experience a lattice strain or
change in
the lattice constant of 0.0103 A or 1.91%, whereas a 1 nil) diameter grain
will experience
a change of 0.031 A or 5.73 percent.
[0059] The extent to which Ce02 can act as a catalytic oxygen storage
material,
described by equation 1, is governed in part by the Ce02 particle size. At 20
nm particle
sizes and below, the lattice parameter increases dramatically with decreasing
crystallite
size (up to 0.45% at 6 nm, see for example Zhang, et al., Applied Physics
Letters, 80 1,
127 (2002)). The associated size-induced lattice strain is accompanied by an
increase in
surface oxygen vacancies that results in enhanced catalytic activity. This
inverse size-
dependent activity provides not only for more efficient fuel cells, but better
oxidative
properties when used in the combustion of petroleum fuels.
[0060] As described previously, various methods and apparatus have been
reported
for preparing cerium nanoparticles, including those described by Chane-Ching,
et al., U.S.
Patent No. 5,017.352: Hanawa, et al., U.S. Patent No. 5,938,837; Melard, et
al., U.S.
Patent No 4,786,325; Chopin, et al., U.S. Patent No. 5,712,218; Chan, U.S.
Patent Appl.
Publ. No.2005/0031517; and Zhou, etal.. U.S. Patent No.7,025,943,
However, current methods do not allow the
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economical, facile (i.e. non-calcined) and unambiguous preparation of cubic
Ce02
nanoparticles in high yield, in a short period of time at very high suspension
densities
(greater than 0.5 molal, i.e., 9 wt.% that are sufficiently small in size
(less than 5 nm in
mean geometric diameter), uniform in size frequency distribution (coefficient
of variation
[COV1 of less than 25%, where COV is the standard deviation divided by the
mean
diameter), and stable for many desirable applications. Additionally, it would
be very
desirable to produce particles that are crystalline, ie, a single crystal
rather than an
agglomeration of crystallites of various sizes such as are taught in the above
mentioned
art and technical literature.
[0061] Although substantially pure cerium dioxide nanoparticles are
beneficially
included in fuel additives, it may be of further benefit to use cerium dioxide
doped with
components that result in the formation of additional oxygen vacancies being
formed (Eq.
1). For this to occur, the dopant ion should be divalent or trivalent, i.e., a
divalent or
trivalent ion of an element that is a rare earth metal, a transition metal or
a metal of Group
IIA, IIIB, VB, or VIB of the Periodic Table. The requirement for crystal
charge neutrality
using these lower valence cations will drive Eq. 1 to the right, i.e., higher
extent of
oxygen vacancy formation. Metal dopant ions with smaller ionic radii than Ce
+4 (0.97 A
in an octahedral configuration) will also aid in oxygen vacancy formation
since this
process reduces two adjacent Ce+4 ions (one surface and one subsurface) to
Ce+3 whose
resultant larger ionic radius, 1.143 A, expands the lattice, thereby causing
lattice strain.
Thus substituting Zr+4 (ionic radius 0.84 A) or Cu+2 (ionic radius of six
coordinate
octahedral configuration is 0.73 A, four coordinate tetrahedral 0.57 A) will
relieve some
of this lattice strain. Additionally, Zr allows the formation of two adjacent
surface Ce+3
species (rather than one surface and one subsurface), which may be important
for very
small particles where approximately 50% of the ions are surface ions. One can
thus
appreciate that substitutional ion doping is preferred to interstitial ion
doping, where the
dopants occupy spaces between the normal lattice positions.
[0062] For the purposes of this discussion, we need to distinguish what is
meant by
doping as opposed to a lattice engineered crystal. In semiconductor physics,
the word
doping refers to n or p type impurities present in the parts-per-million
range. We use the
word doped crystal to refer to a crystal that has on or more metal dopant ions
present in
concentrations less than 2 mole percent (20,000 ppm). A lattice engineered
crystal, on the
other hand, can have one or more metal dopant ions present in the Ce02 crystal
at
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concentrations greater than 20,000 ppm up to 800,000 ppm (or 80% of the cerium
sub-
lattice). Thus a lattice engineered cerium dioxide crystal could have cerium
present as the
minor metal component.
100631 Doping of cerium dioxide with metal ions to improve ionic transport,
reaction
efficiency and other properties is described in, for example, "Doped Ceria as
a Solid
Oxide Electrolyte, H. L. Tuller and A. S. Nowick in J. Electrochem Soc., 1975,
122(2),
255; "Point Defect Analysis and Microstructural Effects in Pure and Donor
Doped
Ceria", M. R. DeGuire, et. al., Solid State Ionics, 1992, 52, 155; and
"Studies on
Cu/Ce02: A New NO Reduction Catalyst" Parthasarathi Bera, S.T. Aruna, K.C.
Patil,
and M.S. Hegde in Journal of Catalysis, 186, 36-44 (1999) and. The resultant
dopant
effects on the electronic and oxygen diffusion properties are described by
Trovarelli,
Catalysis by Ceria and Related Materials, Catalytic Science Series, World
Scientific
Publishing Co., 37-46 (2002) and references cited therein.
[0064] Trovarelli et al. in Catalysis Today, 43 (1998), 79-88, discuss the
preparation
of ceria-zirconia mixed oxides of fairly good compositional homogeneity using
a
surfactant-assisted approach. High specific surface areas, 230 m2/gm, are
obtained after
calcination of compositions at 723 K; however, sintering occurs at 1173 K as
the specific
surface area drops to 40 m2/gm (¨ 20 nm diameter).
[0065] Pulsed neutron diffraction techniques were used by E. Mamontov, et
al. I
Phys. Chem. B 2000, 104, 1110-1116 to study ceria and ceria-zirconia solid
solutions.
These studies established for the first time the correlation between the
concentration of
vacancy-interstitial oxygen defects and the oxygen storage capability. They
postulate that
the preservation of oxygen defects, which Zr aids, is necessary to ameliorate
the
degradation of OSC as a function of thermal aging. Zr02 was present at 30.5
mole %, and
the calcined particles had a diameter of approximately 40 nm, based upon BET
surface
area measurements.
[0066] Z. Yang et al. in Journal of Chemical Physics, (2006) 124 (22),
224704/(1-7)
calculated from first principles, using density functional theory, that an
oxygen vacancy is
most easily created close to a Zr center, and therefore these centers serve as
a nucleation
site for vacancy clustering. The released oxygen donates two electrons to Ce
+4 centers
neighboring the vacancy, resulting in two Ce +3 centers.
[0067] R. Wang et al. in 1 Chem. Phys. B, 2006, 110, 18278-18285 examined
the
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spatial distribution of Zr in Ce0.5 Zr 0.502 produced by a spray freezing
technique,
followed by calcination. They find that particle nanoscale heterogeneity, as
characterized
by Ce-rich cores and Zr-rich shells in particles in the 5.4 to 25 nm particle
size range, is
associated with more redox active materials. This finding implies that a
homogeneous
distribution of Zr and Ce results in decreased activity and is therefore not
preferred.
[0068] S. Bedrane et al. in Catalysis Today, 75, 1-4, 401-405 July 2002,
measured the
oxygen storage capability (Eq 1.) of 11 precious and noble metal (PM = Rh, Pt,
Rd, Ru,
and Ir) doped ceria (Ce02) and ceria-zirconia (Ce0.63Zr0.3702) compositions.
They
observe a leveling effect in which the Ce-Zr materials have an OSC that is
nearly
independent of PM concentration and is 2 to 4 times as great as the PM-loaded
Ce-only
materials.
[0069] H. Sparks et al. of Nanophase Technologies, Corp., using vapor phase
synthesis, manufactured ceria mixed with rare earth oxide nanomaterials (Mat.
Res. Soc.
Symp. Proc., Vol 788, 2004). They observe enhanced thermal stability of
nanocrystalline
particle size and an increase in OSC for the Zr-doped ceria (1:1); however
further
addition of La or Pr to the Zr composition, while better than ceria itself,
was poorer than
just the zirconium ceria combination. One can infer, from the reported
specific surface
areas, a particle size of 10 nm at 600 C, which increases to 40 nm at 1050 C.
[0070] The catalytic effects of Zr and Fe doped Ce02in the combustion of
diesel soot
were examined by Aneggi et al. in Catalysis Today,114, (2006), 40-47. They
reiterated the
fact that Zr enhances the thermal stability and OSC of pure ceria and found
that Fe203
gave better fresh results, but there was a net loss of activity after
calcination. A very
systematic level series in Zr and Zr with Fe was examined, including
crystallographic
data on these calcined particles that were approximately 21 nm. They
determined a
nanoparticle specific area threshold, 35 m2/gm (corresponding to a diameter of
less than
24 nm), in which the fresh versus aged activity was unchanged.
[0071] Copper-based catalytic systems have also received much attention. In
a very
thorough structural analysis of 3 and 5 atom percent Cu/Ce02, M. S. Hegde et
al., Chem.
Mater. 2002, 14, 3591-3601, demonstrated that Cu forms a distinct solid
solution of Ce 1-x
Cu x 02 with no discrete CuO phase. In these 50 nm agglomerated grains
produced by
combustion synthesis, the Cu is in the +2 state and is much more catalytically
active than
Cu in CuO. Furthermore, the oxygen ion vacancy is created around the Cu+2
cation.
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[0072] A. Martinex-Arias et al. in 1 Phys. Chem. B, 2005, 109, 19595-19603,
found
that the reduction of Cei_xCu,02 fluorite type nanoparticles (x= 0.05, 0.1,
and 0.2) was
reversible and that the oxidation state of Cu was higher than its normal
states (+1 or +2).
The dopant induced a large lattice strain in these ¨ 6 nm particles in the
oxide sub-lattice,
which favored the formation of oxygen vacancies. A reverse microemulsion
method
followed by calcination at 500C was used to prepare these materials.
[00731 Iron is another metal ion that has imbued Ce02nanoparticles with
enhanced
catalytic activity'. I. Melian-Cabrera et al. in Journal of Catalysis, 239,
2006, 340-346,
report enhanced activity (relative to the undoped materials) and optimal
catalytic
destruction of N20, an oxygen-limited reaction, with a 50/50 composition of
cerium and
iron oxide. The Fe-doped ceria is made by a co-precipitation method that
produces
particles in the 30 nm diameter range.
[0074] T. Campenon and colleagues in SAE special publication SP 2004, SP-
1860,
"Diesel Exhaust Emission Control" use iron doped ceria to control the ash
buildup in
diesel particulate filters.
[0075) R. Hu and colleagues in Shiyou Huagong (2006), 35(4), 319-323
examined
Fe-doped cerium dioxide made by a solid phase milling technique, followed by
calcination at various elevated temperatures. Iron doping improved the
catalytic activity
with respect to the combustion of methane while simultaneously decreasing
particle size.
[0076] Illustrative Examples 9 and 10 of U.S. Patent Appl. Publ. No.
2005/0152832
describe the preparation of, respectively, cerium-doped and cerium-coated
barium
hexaaluminate particles. Example 13 describes the oxidation of methane with
the cerium-
coated particles.
[0077] Talbot et al., U.S. Patent No. 6,752,979
describes a method of producing metal oxide particles
having nano-sized grains that consists of: mixing a solution containing one or
more metal
cations with a surfactant under conditions such that surfactant micelles are
formed within
the solution, thereby forming a micellar liquid; and heating the micellar
liquid to remove
the surfactant and form metal oxide particles having a disordered pore
structure. The
metal cations are selected from the group consisting of cations from Groups
1A, 2A, 3A,
4A, 5A, and 6A of the Periodic Table, transition metals, lanthanides,
actinides, and
mixtures thereof. Preparations of particles of cerium dioxide and mixed oxides
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containing cerium and one or more other metals are included in the
illustrative examples.
[0078] Illustrative example 9 of U.S. Patent Nos. 6,413,489 and 6,869,584
describes the inclusion of
cerium nitrate in the emulsion mixture to prepare cerium-doped barium
hexaaluminate
particles, which were collected by freeze drying and calcined under air to 500
C and
800C. The resulting particles had grain sizes of less than 5 nm and 7 nm at
500C and
800C, respectively. Illustrative example 10 describes the synthesis of cerium-
coated
barium hexaaluminate particles. Following calcination, the cerium-coated
particles had
grain sizes of less than 4 nm, 6.5 nm, and 16 nm at 500C, 800'C, and I100 C,
respectively.
[0079] Wakefield, U.S. Patent No. 7,169,196 B2
describes a fuel comprising cerium dioxide particles
that have been doped with a divalent or trivalent metal or metalloid that is a
rare earth
metal, a transition metal, or a metal of Group ha, IIIB, V13, or VIB of the
Periodic Table.
Copper is disclosed as a preferred dopant.
[0080] Oji Kuno in U.S. Patent No.7,384,888B2
describes a cerium-zirconium composite metal oxide
with a ceria core and zirconia shell haying improved high temperature
stability and stable
OSC. Ho ever. calcining at 700C is required for the preparation of the
material, which
shows a 10-20 percent improved catalytic activity with respect to hydrocarbon
and carbon
monoxide oxidation. No sizing data is provided to support the claim of 5-20 nm
particles,
no direct OSC measurements are quoted, and there is no analytical data to
support the
assertion of a core-shell geometry.
1008111 With regard tolOnm diameter or smaller nanoparticles, there are
multiple
concerns that cast doubt on the ability of metal ion dopants to be
incorporated in such
small particles. For example, an 8.1 nm particle will have less than 10% of
the Ce ions
on the surface. whereas a 2.7 nm particle (5 unit cells on an edge of each
0.54nm/unit
cell) will have 46.6% of the 500 Ce ions on the surface. Surface ions are
either 1/2 (for a
face) or 1/8 (corner) incorporated into the lattice; therefore, their binding
energies are
substantially reduced and their coordination requirements unfulfilled. The
difficulties
associated with the doping of (semiconductor) nanocrystals is discussed in
Science, 319,
March 28, 2008 by Norris et al. Characteristics such as the relative
solubility of the
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dopant in the crvstal vs solution, the diffusion of the dopant into the
lattice, its formation
energy, size and valence relative to the ions that are being replaced, kinetic
barriers such
as may be imposed by adsorbed surface stabilizers may all play a role in
determining the
extent, if am', to which a dopant metal ion may be incorporated into
nanocrystals of these
dimensions.
[0082] It is clear from the references just described, that, the majority
of the doping
work has occurred at relatively large particle size (20 nm or so) and was
carried out either
by calcining the initial cerium-metal dopant mixture, or by micellar synthesis-
--a process
that does not readily lend itself to large scale material production. In the
work describing
particles of a size less than 10 nm, the crystallographic form has not been
established nor
has conclusive evidence of incorporation been provided.
[0083] Thus there exists a need to readily incorporate a wide variety of
metal dopant
ions into the cerium sub-lattice of cubic Ce02 for very small nanoparticles
(less than
about 10 nm diameter) in a facile manner that does not require calcination
(500C or
greater) and to unequivocally demonstrate incorporation as opposed to the
production a
separately nucleated population of dopant metal oxide grains. As single
crystal particles
of ceria are unique_ so too would be a metal lattice engineered variant of
ceria.
Additionally_ it would be desirable to produce large commercially available
quantities of
these materials in an economical manner and in a relatively short period of
time.
[0084] A typical chemical reactor that might be used to prepare cerium
dioxide
includes a reaction chamber that includes a mixer (see, for example, Fig. 1 in
Zhou et al.
U.S. Patent No, 7,025,943). A
mixer typically includes a shaft, and propeller or turbine blades attached to
the shaft, and
a motor that turns the shaft, such that the propeller is rotated at high speed
(1000 to 5000
rpm). The shaft can drive a flat blade turbine for good meso mixing (micro
scale) and a
pitched blade turbine for macro mixing (pumping fluid through out the
reactor).
[0085] Such a device is described in Antoniades, U.S. Patent No. 6,422.736,
The described reactor is useful
for fast reactions such as that shown by the equation below, wherein the
product, AgC1, is
a crystalline material having a diameter on the order of several hundred
nanometers up to
several thousand nanometers.
[0086] AgNO3 + NaC1 AgC1 + NaNa3
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[0087] Cerium dioxide particles prepared using this type of mixing are
often too large
to be useful for certain applications. It is highly desirable to have the
smallest cerium
dioxide particles possible as their catalytic propensity, i.e., their ability
to donate oxygen
to a combustion system (cf. equation 1), increases with decreasing particle
size, especially
for particles having a mean diameter of less than 10 nm.
[0088] PCT/US2007/077545, METHOD OF PREPARING CERIUM DIOXIDE
NANOPARTICLES, filed September, 4, 2007, describes a mixing device that is
capable
of producing Ce02 nanoparticles down to 1.5 nm, in high yield and in very high
suspension densities. The reactor includes inlet ports for adding reactants, a
propeller, a
shaft, and a motor for mixing. The reaction mixture is contained in a reactor
vessel.
Addition to the vessel of reactants such as cerium nitrate, an oxidant, and
hydroxide ion
can result in the formation of Ce02nanoparticles, which are initially formed
as very small
nuclei. Mixing causes the nuclei to circulate; as the nuclei continuously
circulate through
the reactive mixing regime, they grow (increase in diameter) as they
incorporate fresh
reactants. Thus, after an initial steady state concentration of nuclei is
formed, this nuclei
population is subsequently grown into larger particles in a continuous manner.
Unless
grain growth restrainers are employed to terminate the growth phase, this
nucleation and
growth process is not desirable if one wishes to limit the final size of the
particles while
still maintaining a high particle suspension density.
[0089] An example of this nucleation and growth process applied to the
aqueous
precipitation of Ce02 is the work of Zhang et al., J. Appl. Phys., 95, 4319
(2004) and
Zhang, et al., Applied Physics Letters, 80, 127 (2002). Using cerium nitrate
hexahydrate
as the cerium source (very dilute at 0.0375M) and 0.5 M hexamethylenetetramine
as the
ammonia precursor, 2.5 to 4.25 nm cerium dioxide particles are formed in times
less than
50 minutes. These particles are subsequently grown to 7.5 nm or greater using
reaction
times on the order of 250 minutes or 600 minutes, depending upon growth
conditions.
The limitations of particle size, concentration and reaction time would
exclude this
process from consideration as an economically viable route to bulk commercial
quantities
of Ce02 nanop articles.
[0090] I. H. Leubner, Current Opinion in Colloid and Interface Science, 5,
151-159
(2000), Journal of Dispersion Science and Technology, 22, 125-138 (2001) and
ibid. 23,
577-590 (2002), and references cited therein, provides a theoretical treatment
that relates
the number of stable crystals formed with molar addition rate of reactants,
solubility of
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the crystals, and temperature. The model also accounts for the effects of
diffusion,
kinetically controlled growth processes, Ostwald ripening agents, and growth
restrainers/stabilizers on crystal number. High molar addition rates, low
temperatures,
low solubility, and the presence of growth restrainers all favor large numbers
of nuclei
and consequently smaller final grain or particle size.
[0091] In contrast to batch reactors, colloid mills typically have flat
blade turbines ,
turning at 10,000 rpm, whereby the materials are forced through a screen whose
holes can
vary in size from fractions of a millimeter to several millimeters. Generally,
no chemical
reaction is occurring, but only a change in particle size brought about by
milling. In
certain cases, particle size and stability can be controlled thermodynamically
by the
presence of a surfactant. For example, Langer et al., in U.S. Patent No.
6,368,366 and
U.S. Patent No. 6,363,237
describe an aqueous microemulsion in a hydrocarbon fuel composition made under
high
shear conditions. However, the aqueous particle phase (the discontinuous phase
in the
fuel composition) has a large size, on the order of 1000 nm.
[0092] Colloid mills are not useful for reducing the particle size of large
cerium
dioxide particles because the particles are too hard to be sheared by the mill
in a
reasonable amount of time. The preferred method for reducing large
agglomerated
cerium dioxide particles from the micron size down into the nanometer size is
milling for
several days on a ball mill in the presence of a stabilizing agent. This is a
time
consuming. expensive process that invariably produces a wide distribution of
particle
sizes. Thus, there remains a need for an economical and facile method to
synthesize large
quantities, at high suspension densities, of very small nanometric particles
of cerium
dioxide having a uniform size distribution and incorporating one or more
transition metal
ions while still maintaining the Ce02 cubic fluoroite structure.
[0093] Aqueous precipitation may offer a convenient route to cerium
nanoparticles.
However, to be useful as a fuel-borne catalyst for fuels, cerium dioxide
nanoparticles
must exhibit stability in a nonpolar medium, for example, diesel fuel. Most
stabilizers
used to prevent agglomeration in an aqueous environment are ill suited to the
task of
stabilization in a nonpolar environment. When placed in a nonpolar solvent,
such
particles tend to immediately agglomerate and, consequently, lose some, if not
all, of their
desirable nanoparticulate properties. Thus, it would be desirable to form
stable cerium
dioxide particles in an aqueous environment, retain the same stabilizer on the
particle
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surface, and then be able to transfer these particles to a nonpolar solvent,
wherein the
particles would remain stable and form a homogeneous mixture or dispersion. In
this
simplified and economical manner, one could eliminate the necessity for
changing the
affinity of a surface stabilizer from polar to non-polar. Changing stabilizers
can involve a
difficult displacement reaction or separate, tedious isolation and re-
dispersal methods
such as, for example, precipitation and subsequent re-dispersal with the new
stabilizer
using ball milling.
[0094] Thus, there remains a need for an efficient and economical method to
synthesize stable transition metal-containing cerium dioxide nanoparticles in
a polar,
aqueous environment, and then transfer these particles to a non-polar
environment
wherein a stable homogeneous mixture is formed.
[0095] The use of cerium nanoparticles to provide a high temperature
oxidation
resistant coating has been reported, for example, in "Synthesis Of Nano
Crystalline Ceria
Particles For High Temperature Oxidization Resistant Coating," S. Seal et al.,
Journal of
Nanoparticle Research, 4, pp 433-438 (2002). The deposition of cerium dioxide
on
various surfaces has been investigated, including Ni, chromia and alumina
alloys, and
stainless steel and on Ni, and Ni-Cr coated alloy surfaces. It was found that
a cerium
dioxide particle size of10 nm or smaller is desirable. Ceria particle
incorporation
subsequently inhibits oxidation of the metal surface.
[0096] Rim, U.S. Patent No. 6,892,531
describes an engine lubricating oil composition for a diesel engine that
includes a lubricating oil and 0.05-10 wt.% of a catalyst additive comprising
cerium
carboxylate.
[0097] As described above, currently available cerium oxide- and doped
cerium
oxide-based fuel additives have improved fuel combustion of diesel engines;
however
further improvements are still needed. It would be desirable to formulate
these fuel
additives for diesel engines that provide further improved fuel combustion by
taking
advantage of even smaller, sub 5 nm nanoparticles of cubic Ce02 with higher
specific
surface areas. The increased oxygen storage capability enabled by the
inclusion of
transition metals at these grain sizes is also highly desirable. In addition,
protection of
engines from wear, reduced engine friction, and greater lubricity, with
simultaneously
improved fuel efficiency would be tremendously beneficial.
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Summary of the Invention
[0098] The present invention is directed to a process for making lattice
engineered
cerium dioxide nanoparticles containing at least one transition metal (M) that
comprises:
(a) providing an aqueous reaction mixture comprising a source of cerous ion, a
source of
one or more transition metal ions (M), a source of hydroxide ion, at least one
nanoparticle
stabilizer, and an oxidant at an initial temperature in the range of about 20
C to about
95 C; (b) mechanically shearing the mixture and causing it to pass through a
perforated
screen, thereby forming a suspension of cerium hydroxide nanoparticles; and
(c)
providing temperature conditions effective to enable oxidation of cerous ion
to ceric ion,
thereby forming a product stream comprising transition metal-containing cerium
dioxide
nanoparticles, Cei,M,(02. The cerium dioxide nanoparticles thus obtained have
a cubic
fluorite structure, a mean hydrodynamic diameter in the range of about 1 nm to
about 10
nm, and a geometric diameter of about 1 nm about 4 nm.
[0099] The present invention is further directed to a process for forming a
homogeneous dispersion containing stabilized transition metal-containing
cerium dioxide
nanoparticles, Cei,M,(02, that comprises: (a) providing an aqueous mixture
that includes
stabilized transition metal-containing cerium dioxide nanoparticles, CeiMO2,
having a
cubic fluorite structure, a mean hydrodynamic diameter in the range of about 1
nm to
about 10 nm, and a geometric diameter of about 1 nm to about 4 nm; (b)
concentrating the
aqueous mixture that includes the stabilized transition metal-containing
cerium dioxide
nanoparticles, thereby forming an aqueous concentrate; (c) removing
substantially all the
water from the aqueous concentrate, thereby forming a substantially water-free
concentrate of the stabilized transition metal-containing cerium dioxide
nanoparticles; (d)
adding an organic diluent to the substantially water-free concentrate, thereby
forming an
organic concentrate of the stabilized transition metal-containing cerium
dioxide
nanoparticles; and (d) combining the organic concentrate with a surfactant in
the presence
of a nonpolar medium, thereby forming a homogeneous dispersion containing
stabilized
transition metal-containing cerium dioxide nanoparticles, Cei,M02, wherein "x"
has a
value from about 0.3 to about 0.8
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Brief Description of the Drawings
1001001 FIGS. lA and 1B are, respectively, a TEM image and a particle
size
frequency analysis by TEM of Ce02nanoparticles prepared by non-isothermal
precipitation, as described in Example 1.
1001011 FIG 2 is an X-ray powder diffraction spectrum of cerium dioxide
nanoparticles prepared as described in Example I.
[00102] FIG 3A is a TEM image of 1.1 nm Ce02nanoparticles prepared as
described in Example 2. FIG 3B is an electron diffraction pattern of these 1.1
nm
particles.
[00103] FIGS. 4A and 4B are, respectively, a TEM image and a particle
size-
frequency analysis by TEM of isothermally precipitated Ce02 nanoparticles,
prepared by
a triple jet process as described in Example 3.
[00104] FIGS. 5A and 5B are, respectively, a TEM image and a particle
size-
frequency analysis by TEM of isothermally precipitated Cu-containing Ce02
nanoparticles, prepared as described in Example 4.
[00105] FIGS. 6A and 6B are, respectively, a TEM image and a particle size-
frequency analysis by TEM of isothermally precipitated Fe-containing Ce02
nanoparticles, prepared as described in Example 5.
[00106] FIGS. 7A and 7B are, respectively. a TEM image and a particle size-
frequency analysis by TEM of isothermally precipitated Zr-containing Ce02
nanoparticles, prepared as described in Example 6.
1001071 FIGS. 8A and 8B are respectively, a TEM image and a particle size-
frequency analysis by TEM of isothermally precipitated Ce02 nanoparticles
containing Zr
and Fe, prepared as described in Example 7. FIG 8C are x-ray diffraction
spectra of
isothermally precipitated Ce02 nanoparticles and of isothermally precipitated
Ce02
nanoparticles containing Zr and Fe, prepared as described in Example 7.
[001081 FIG 9 is a field emission gun TEM lattice image of Ce02
nanoparticles
containing Zr and Fe, prepared as described in Example 7.
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Detailed Description of the Invention
[00109] In this application, the term "transition metal" is understood to
encompass the
40 chemical elements 21 to 30, 39 to 48, 72 to 80, which are included in
Periods 4, 5, 6,
respectively, of the Periodic Table
[00110] The present invention provides a process for making transition metal
ion-
containing cerium dioxide (Ce02) nanoparticles that comprises: (a) providing
an aqueous
reaction mixture comprising a source of cerous ion and one or more transition
metal ions,
a source of hydroxide ion, at least one nanoparticle stabilizer, and an
oxidant; (b)
mechanically shearing the mixture and causing it to pass through a perforated
screen,
thereby forming a suspension of cerium hydroxide nanoparticles; and (c)
providing
temperature conditions effective to enable oxidation of cerous ion to ceric
ion, thereby
forming a product stream comprising transition metal-containing cerium dioxide
nanoparticles, Cei,Mõ02 , that have the cubic fluorite structure, with a mean
hydrodynamic diameter in the range of about 1 nm to about 10 nm and a
geometric
diameter of about 1 nm to about 4 nm. Crystalline, cerium dioxide particles
containing
one or more transition metal ions and having a monomodal size distribution and
a
monodisperse size frequency distribution can be selectively prepared within
this size
range. The single crystalline particles contain either two unit cells per edge
for 1.1 nm
particles up to 5 unit cells per edge for 2.7 nm particles particles depending
upon the
conditions of preparation. Here the word crystalline refers to particles that
are not
composed of multiple, agglomerated crystallites of various sizes but rather a
single crystal
of well defined dimensions dictated by the number of constituent unit cells.
[00111] The present invention further provides for a continuous process for
producing
crystalline cerium dioxide Ce02 nanoparticles containing one or more
transition metal
ions and having a mean hydrodynamic diameter of about 1 nm to about 10 nm,
wherein
the process comprises the step of combining cerous ion, one or more transition
metal ions,
an oxidant, at least one nanoparticle stabilizer, and hydroxide ion within a
continuous
reactor.
[00112] The present invention also provides a process for making cerium
dioxide
nanoparticles that comprises the steps of: (a) providing an aqueous first
reaction mixture
comprising a source of cerous ion, one or more transition metal ions and at
least one
nanoparticle stabilizer; (b) stirring the first reaction mixture while adding
an oxidant,
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thereby producing a second reaction mixture; (c) adding a source of hydroxide
ion to the
second reaction mixture while subjecting it to mechanical shearing, thereby
forming a
third reaction mixture; and (d) heating the third reaction mixture to a
temperature between
about 50 C and about 100 C, thereby producing crystalline cerium dioxide
nanoparticles
that contain one or more transition metal ions and are substantially monomodal
and
uniform in size frequency distribution.
[00113] The present invention further provides a process for forming a
homogeneous
mixture that includes the aforementioned crystalline cerium dioxide
nanoparticles, at least
one nanoparticle stabilizer, at least one surfactant, a glycol ether mixture,
and a nonpolar
medium. The process comprises the steps of: (a) providing an aqueous mixture
that
includes stabilized crystalline cerium dioxide nanoparticles produced by close
association
of the nanoparticle stabilizer with the crystalline cerium dioxide
nanoparticles; (b)
concentrating the aqueous mixture including stabilized crystalline cerium
dioxide
nanoparticles to form an aqueous concentrate; and (c) removing substantially
all of the
water by solvent shifting from an aqueous environment to an glycol ether
environment,
combining the surfactant and optionally a co-surfactant with the solvent
shifted
concentrate in the presence of the nonpolar medium, thereby forming the
homogeneous
mixture.
[00114] In the presence of hydroxide ion, ceric ion reacts to form cerium
hydroxide,
which on heating is converted to crystalline cerium dioxide. The temperature
in the
reaction vessel is maintained between about 50 C and about 100 C, more
preferably
about 65-95 C, most preferably about 85 C. Time and temperature can be traded
off,
higher temperatures typically reducing the time required for conversion of the
hydroxide
to the oxide. After a period at these elevated temperatures, on the order of
about 1 hour or
less and suitably about 0.5 hour, the cerium hydroxide is converted to
crystalline cerium
dioxide, and the temperature of the reaction vessel is lowered to about 15-25
C.
Subsequently, the crystalline cerium dioxide nanoparticles are concentrated,
and the
unreacted cerium and waste by-products such as ammonium nitrate are removed,
most
conveniently, for example, by diafiltration.
[00115] In one aspect of the present invention, a method of making crystalline
cerium
dioxide nanoparticles containing one or more transition metal ions includes:
providing an
aqueous reaction mixture comprising cerous ion, one or more transition metal
ions,
hydroxide ion, a stabilizer or combination of stabilizers, and an oxidant, the
reaction
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being carried out at a temperature effective to generate small nuclei size and
to achieve
subsequent oxidation of cerous ion to ceric ion and enable the nuclei to be
grown into
nanometric cerium dioxide. The reaction mixture is subjected to mechanical
shearing,
preferably by causing it to pass through a perforated screen, thereby forming
a suspension
of crystalline cerium dioxide nanoparticles having a mean hydrodynamic
diameter in the
range of about I nm to about 10 nm. While the particle diameter can be
controlled within
the range of 1.5 nm to 25 nm, preferably the crystalline cerium dioxide
nanoparticles have
a mean hydrodynamic diameter of about 10 nm or less, more preferably about 8
nm or
less, most preferably, about 6 nm. Desirably, the nanoparticles comprise one
or at most
two primary crystallites per particle edge, each crystallite being on average
2.5 nm
(approximately 5 unit cells). Thus, the resulting nanoparticle size frequency
in
substantially monodisperse, i.e., having a coefficient of variation (COV) less
than 25%,
where the COV is defined as the standard deviation divided by the mean.
[00116] Mechanical shearing includes the motion of fluids upon surfaces
such as those
of a rotor, which results in the generation of shear stress. Particularly, the
laminar flux on
a surface has a zero velocity, and shear stress occurs between the zero-
velocity surface
and the higher-velocity flow away from the surface.
[00117] In one embodiment, the current invention employs a colloid mill,
which is
normally used for milling microemulsions or colloids, as a chemical reactor to
produce
cerium dioxide nanoparticles. Examples of useful colloid mills include those
described
by Korstvedt, U.S. Patent No. 6,745,961 and U.S. Patent No. 6,305,626,
[00118] Desirably, the reactants include an aqueous solution of a cerous
ion source, for
example, cerous nitrate; an oxidant such as hydrogen peroxide or molecular
oxygen; and
a stabilizer such as, for example, 242-(2-methoethoxy)ethoxy] acetic acid.
Typically,
a two-electron oxidant such as peroxide is present, preferably in at least one-
half the
molar concentration of the cerium ion. The hydroxide ion concentration is
preferably at
least twice, more preferably three times, or may even be five times the molar
cerium ion
concentration.
[00119] Initially, the reaction chamber is maintained at a temperature
sufficiently low
to generate small cerous hydroxide nuclei size, which can be grown into
nanometric
crystalline cerium dioxide particles after a subsequent shift to higher
temperatures,
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resulting in conversion of the cerous ion into the ceric ion state. Initially,
the temperature
is suitably about 25 C or less, although higher temperatures may be used
without a
significant increase in particle size.
[00120] In one embodiment, a source of cerous ion, one or more transition
metal ions,
a nanoparticle stabilizer, and an oxidant are placed in the reactor, and a
source of
hydroxide ion such as ammonium hydroxide is rapidly added with stirring,
preferably
over a time period of about 10 minutes or less. Under certain conditions such
as a single
jet addition of ammonia to metal ions, about 20 seconds or less is preferred,
even more
preferably about 15 seconds or less. In an alternative embodiment, a source of
hydroxide ion and an oxidant is placed in the reactor, and a source of cerous
ion and one
or more transition metal ions are added over a period of about 15 seconds up
to 20
minutes. In a third and preferred embodiment, the stabilizers are placed in
the reaction
vessel, and the cerous nitrate with one or more transition metal ions are
simultaneously
introduced into the reaction chamber with a separate jet of ammonium hydroxide
at the
optimum molar stoichiometric ratio of 2:1 , 3:1 or even 5:1 OH:Ce.
[00121] Cerous ion reacts with the oxidant in the presence of hydroxide ion to
form
cerium hydroxide, which can be converted by heating to crystalline cerium
dioxide. The
temperature in the reaction vessel is maintained between about 50 C and about
100 C,
preferably about 65-85 C, more preferably about 70 C. The incorporation of
certain
transition metal ions such as Zr and Cu typically require higher temperatures,
about 85 C.
After a period of time at these elevated temperatures, preferably about 1 hour
or less,
more preferably about 0.5 hour, the doped cerium hydroxide has been
substantially
converted to crystalline cerium dioxide, and the temperature of the reaction
vessel is
lowered to about 15-25 C. The time and temperature variables may be traded
off, higher
temperatures generally requiring shorter reaction times. The suspension of
cerium
dioxide nanoparticles is concentrated, and the unreacted cerium and waste by-
products
such as ammonium nitrate are removed, which may be conveniently accomplished
by
diafiltration.
[00122] The nanoparticle stabilizer is a critical component of the reaction
mixture.
Desirably, the nanoparticle stabilizer is water-soluble and forms weak bonds
with cerium
ion. KBC represents the binding constant of the nanoparticle stabilizer to
cerium ion in
water. Log KBC for the nitrate ion is 1 and for hydroxide ion is 14. Most
desirably, log
KBC lies within this range, preferably in the middle of this range. Useful
nanoparticle
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stabilizers include alkoxysubstituted carboxylic acids, a-hydroxyl carboxylic
acids, a-
keto carboxylic acids such as pyruvic acid, and small organic polyacids such
as tartaric
acid and citric acid. Examples of alkoxylated carboxylic acids include;
methoxy acetic
acid, 2-(methoxy)ethoxy acetic acid and 2-[2-(2-methoxyethoxy)ethoxy] acetic
acid
(MEEA). Among the a-hydroxycarboxylic acids, examples include lactic acid,
gluconic
acid and 2-hydroxybutanoic acid. Polyacids include ethylenediaminetetraacetic
acid
(EDTA), tartaric acid, and citric acid. Combinations of compounds with large
KBC such
as EDTA with weak KBC stabilizers such as lactic acid are also useful at
particular ratios.
Large KBC stabilizers such as gluconic acid may be used at a low level, or
with weak KBC
stabilizers such as lactic acid.
[00123] In one desirable embodiment, the nanoparticle stabilizer includes a
compound
of formula (Ia). In formula (Ia), R represents hydrogen, or a substituted or
unsubstituted
alkyl group or aromatic group such as, for example, a methyl group, an ethyl
group or a
phenyl group. More preferably, R represents a lower alkyl group such as a
methyl group.
Rl represents hydrogen or a substituent group such as an alkyl group. In
formula (Ia), n
represents an integer of 0-5, preferably 2, and Y represents H or a counterion
such as an
alkali metal, for example, Na+ or K . The stabilizer binds to the
nanoparticles and
prevents agglomeration of the particles and the subsequent formation of large
clumps of
particles.
RO(CH2CH20).CHR1CO2Y (Ia)
[00124] In another embodiment, the nanoparticle stabilizer is represented by
formula
(Ib), wherein each R2 independently represents a substituted or unsubstituted
alkyl group
or a substituted or unsubstituted aromatic group. X and Z independently
represent H or a
counterion such as Na + or IC', and p is 1 or 2.
X02C(CR2)pCO2Z (Ib)
[00125] Useful nanoparticle stabilizers are also found among a-
hydroxysubstituted
carboxylic acids such as lactic acid and among the polyhydroxysubstituted
acids such as
gluconic acid.
[00126] Preferably, the nanoparticle stabilizer does not include the element
sulfur,
since sulfur-containing materials may be undesirable for certain applications.
For
example, if the cerium dioxide particles are included in a fuel additive
composition, the
use of a sulfur-containing stabilizer such as AOT may result in the
undesirable emission
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of oxides of sulfur after combustion.
[00127] The size of the resulting cerium dioxide particles can be determined
by
dynamic light scattering, a measurement technique for determining the
hydrodynamic
diameter of the particles. The hydrodynamic diameter (cf. B. J. Berne and R.
Pecora,
"Dynamic Light Scattering: With Applications to Chemistry. Biology and
Physics", John
Wiley and Sons. NY 1976 and -Interactions of Photons and Neutrons with
Matter", S. H.
Chen and M. Kotlarchyk, World Scientific Publishing, Singapore, 1997), which
is slightly
larger than the geometric diameter of the particle, includes both the native
particle size
and the solvation shell surrounding the particle. When a beam of light passes
through a
colloidal dispersion, the particles or droplets scatter some of the light in
all directions.
When the particles are very small compared with the wavelength of the light,
the intensity
of the scattered light is uniform in all directions (Rayleigh scattering). If
the light is
coherent and monochromatic as, for example, from a laser, it is possible to
observe time-
dependent fluctuations in the scattered intensity, using a suitable detector
such as a
photomultiplier capable of operating in photon counting mode. These
fluctuations arise
from the fact that the particles are small enough to undergo random thermal
Brownian
motion, and the distance between them is therefore constantly varying.
Constructive and
destructive interference of light scattered by neighboring particles within
the illuminated
zone gives rise to the intensity fluctuation at the detector plane, which,
because it arises
from particle motion, contains information about this motion. Analysis of the
time
dependence of the intensity fluctuation can therefore yield the diffusion
coefficient of the
particles from which, via the Stokes Einstein equation and the known viscosity
of the
medium, the hydrodynamic radius or diameter of the particles can be
calculated.
1001281 In another aspect of the invention, a continuous process for
producing small,
transition metal ion-containing crystalline cerium dioxide nanoparticles, that
is, particles
having a mean diameter of less than about 10 nm, includes combining cerous
ion, one or
more transition metal ions, an oxidant, a nanoparticle stabilizer or
stabilizer combination,
and hydroxide ion within a continuous reactor, into which reactants and other
ingredients
are continuously introduced, and from which product is continuously removed.
Continuous processes are described, for example, in Ozawa, et al., U.S. Patent
No.
6,897,270; Nickel. et al., U.S. Patent No. 6,723,138; Campbell, et al., U.S.
Patent No.
6,627,720; Beck, U.S. Patent No. 5,097,090; and Byrd, et al., U.S. Patent No.
4,661,321.
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[00129] A solvent such as water is often employed in the process. The solvent
dissolves the reactants, and the flow of the solvent can be adjusted to
control the process.
Advantageously, mixers can be used to agitate and mix the reactants.
[00130] Any reactor that is capable of receiving a continuous flow of
reactants and
delivering a continuous flow of product can be employed. These reactors may
include
continuous-stirred-tank reactors, plug-flow reactors, and the like. The
reactants required
to carry out the nanoparticle synthesis are preferably charged to the reactor
in streams;
i.e., they are preferably introduced as liquids or solutions. The reactants
can be charged
in separate streams, or certain reactants can be combined before charging the
reactor.
[00131] Reactants are introduced into the reaction chamber provided with a
stirrer
through one or more inlets. Typically, the reactants include an aqueous
solution of a
cerous ion source, for example, cerous nitrate, a transition metal ion such
as, for example,
ferric nitrate or cupric nitrate; an oxidant such as hydrogen peroxide or
molecular oxygen,
including ambient air; and a stabilizer, such as, for example, 242-(2-
methoxyethoxy)ethoxylacetic acid. A two-electron oxidant such as hydrogen
peroxide is
present, preferably in at least one-half the molar concentration of the cerium
ion.
Alternatively, molecular oxygen can be bubbled through the mixture. The
hydroxide ion
concentration is preferably at least twice the molar cerium concentration.
[00132] In one embodiment of the present invention, a method of forming small
cerium dioxide nanoparticles includes the step of forming a first aqueous
reactant stream
that includes cerous ion, for example, as cerium (III) nitrate, one or more
transition metal
ions, and an oxidant. Suitable oxidants capable of oxidizing Ce(III) to Ce(IV)
include,
for example, hydrogen peroxide or molecular oxygen. Optionally, the first
reactant
stream also includes a nanoparticle stabilizer that binds to doped cerium
dioxide
nanoparticles, thereby preventing agglomeration of the particles. Examples of
useful
nanoparticle stabilizers were mentioned above.
[00133] The method further includes a step of forming a second aqueous
reactant
stream that includes a hydroxide ion source, for example, ammonium hydroxide
or
potassium hydroxide. Optionally, the second reactant stream further includes a
stabilizer,
examples of which were described previously. At least one of the first or
second reactant
streams, however, must contain a stabilizer or stabilizer combination.
[00134] The first and second reactant streams are combined to form a reaction
stream.
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Initially, the temperature of the reaction stream is maintained sufficiently
low to form
small cerous hydroxide nuclei. Subsequently the temperature is raised so that
oxidation of
Ce(III) to Ce(IV) occurs in the presence of the oxidant, and the hydroxide is
converted to
the oxide, thereby producing a product stream that includes crystalline cerium
dioxide.
The temperature for conversion from the hydroxide to the oxide is preferably
in the range
of about 50-100 C, more preferably about 60-90 C. In one embodiment, the first
and
second reactant streams are combined at a temperature of about 10-20 C, and
the
temperature is subsequently increased to about 60-90 C. Isothermal
precipitation at an
elevated temperature, e.g., 90 C, is an alternative method for producing small
nanoparticles provided that the growth stage can be inhibited by a suitable
molecular
adsorbate (growth restrainer).
[00135] Desirably, the lattice engineered, crystalline cerium dioxide
nanoparticles in
the product stream are concentrated, for example, by diafiltration techniques
using one or
more semi-porous membranes. In one embodiment, the product stream includes an
aqueous suspension of transition metal-containing crystalline cerium dioxide
nanoparticles that is reduced to a conductivity of about 5 mS/cm or less by
one or more
semi-porous membranes.
[00136] A schematic representation of a continuous reactor suitable for the
practice of
the invention is depicted in FIG 3 of PCT/U52007/77545, METHOD OF PREPARING
CERIUM DIOXIDE NANOPARTICLES, filed September, 4, 2007. The reactor 40
includes a first reactant stream 41 containing aqueous cerium nitrate. An
oxidant such as
hydrogen peroxide is added to the reactant stream by means of inlet 42, and
the reactants
are mixed by mixer 43a. To the resulting mixture is added stabilizer via inlet
45,
followed by mixing by mixer 43b. The mixture from mixer 43b then enters mixer
43c,
where it is combined with a second reactant stream containing ammonium
hydroxide
from inlet 44. The first and second reactant streams are mixed using a mixer
43c to form
a reaction stream that may be subjected to mechanical shearing by passing it
through a
perforated screen. In a further embodiment, mixer 43c comprises a colloid mill
reactor,
as described previously, that is provided with inlet ports for receiving the
reactant streams
and an outlet port 45. In a further embodiment, the temperature of the mixer
43c is
maintained at a temperature in the range of about 10 C to about 25 C.
[00137] The mixture from 43c enters a reactor tube 45 that is contained in a
constant
temperature bath 46 that maintains tube 45 at a temperature of about 60-90 C.
Crystalline
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cerium dioxide nanoparticles are formed in the reactor tube 45, which may
include a coil
50. The product stream then enters one or more diafiltration units 47, wherein
the
crystalline cerium dioxide nanoparticles are concentrated using one or more
semi-porous
membranes. One or more diafiltration units may be connected in series to
achieve a
single pass concentration of product, or the units may placed in parallel for
very high
volumetric throughput. The diafiltration units may be disposed both in series
and parallel
to achieve both high volume and rapid throughput. Concentrated crystalline
cerium
dioxide nanoparticles exit the diafiltration unit via exit port 49, and excess
reactants and
water are removed from the diafiltration unit 47 via exit port 48. In an
alternative
embodiment, stabilizer may be added to the second reactant stream via port 51
rather than
to the first reactant stream via port 45.
[00138] In one embodiment of the invention, the product stream of concentrated
lattice
engineered, crystalline cerium dioxide nanoparticles exiting the diafiltration
unit 47 is
solvent shifted into a substantially water-free environment of one or more
glycol ethers.
This can be accomplished with dialysis bags or by running the aqueous
nanoparticles
though a diafiltration column with an organic diluent that preferably
comprises one or
more glycol ethers. The organic diluent may further include an alcohol. A
useful diluent
comprises a mixture of diethylene glycol monomethyl ether and 1-methoxy-2-
propanol.
[00139] The resulting solvent-shifted organic concentrate is combined with a
surfactant
such as oleic acid, followed by combination with a stream that includes a
nonpolar
solvent such as kerosene or ultra low sulfur diesel fuel, thereby forming a
homogeneous
dispersion of lattice engineered, crystalline cerium dioxide nanoparticles
that is miscible
with hydrocarbon fuels such as diesel.
[00140] The use of a continuous process for producing lattice engineered,
crystalline
cerium dioxide nanoparticles allows better control of the production of
particle nuclei and
their growth relative to that afforded by batch reactors. The nuclei size can
be controlled
by the initial reagent concentration, temperature, and the ratio of
nanoparticle stabilizer to
reagent concentrations. Small nuclei are favored by low temperatures, less
than about
20 C, and high ratios of nanoparticle stabilizer to reagent concentrations. In
this way,
very small nanoparticles having a mean hydrodynamic diameter of less than
about 10 nm,
with geometrical particle diameters less than about 3 nm, can be produced in
an
economical manner.
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[00141] The invention provides a method for formulating a homogeneous mixture
that
includes cerium dioxide (Ce02) nanoparticles containing one or more transition
metal
ions, a nanoparticle stabilizer, a surfactant, glycol ethers, and a nonpolar
solvent.
Preferably, the nanoparticles have a mean hydrodynamic diameter of less than
about 10
nm, more preferably less than about 8 nm, most preferably about 6 nm with
geometric
particle diameters (as determined by TEM) less than about 4 nm.
[00142] As described above, lattice engineered, crystalline cerium dioxide
nanoparticles can be prepared by various procedures. Typical synthetic routes
utilize
water as a solvent and yield an aqueous mixture of nanoparticles and one or
more salts.
For example, cerium dioxide particles can be prepared by reacting the hydrate
of cerium
(III) nitrate with hydroxide ion from, for example, aqueous ammonium
hydroxide,
thereby forming cerium (III) hydroxide, as shown in equation (3a). Cerium
hydroxide
can be oxidized to cerium (IV) dioxide with an oxidant such as hydrogen
peroxide, as
shown in equation (3b). The analogous tris hydroxide stoichiometry is shown in
equations (4a) and (4b).
Ce(NO3)3(6H20) + 2 NH40H ¨> Ce(OH)2NO3 + 2 NH4NO3 + 6H20 (3a)
2 Ce(OH)2NO3 + H202 ¨> 2 Ce02 + 2 HNO3 + 2 H20 (3b)
Ce(NO3)3(6 H20) + 3 NH4OH 4 Ce(OH)3 + 3 NH4NO3 + 6 H20 (4a)
2 Ce(OH)3 + H202 4 2 Ce02 + 4 H20 (4b)
Complexes formed with very high base levels, e.g. 5:1 OH:Ce, also provide a
route to
cerium oxide, albeit a much larger grain sizes if not properly growth-
restrained.
[00143] In some
cases, especially where ammonium hydroxide is not present in
excess relative to the cerous ion, the species Ce(OH)2(NO3) or (NH4)2Ce(NO3)5
may
initially be present, subsequently undergoing oxidation to cerium dioxide.
[00144] The transition metal containing, crystalline cerium dioxide particles
are
formed in an aqueous environment and combined with one or more nanoparticle
stabilizers. Desirably, the cerium dioxide nanoparticles are either formed in
the presence
of the stabilizer(s), or a stabilizer(s) is added shortly after their
formation. Useful
nanoparticle stabilizers include alkoxysubstituted carboxylic acids, a-
hydroxyl carboxylic
acids such as pyruvic acid, and small organic polycarboxylic acids. Examples
of
alkoxysubstituted carboxylic acids include methoxyacetic acid, 2-
(methoxy)ethoxy acetic
acid and 2-[2-(2-methoxyethoxy)ethoxy] acetic acid (MEEA). Examples of a-
hydroxy
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carboxylic acids include lactic acid, gluconic acid, and 2-hydroxybutanoic
acid.
Polycarboxylic acids include ethylenediaminetetraacetic acid (EDTA), tartaric
acid, and
citric acid. In desirable embodiments, the nanoparticle stabilizer includes a
compound of
formula (Ia) or formula (Ib), as described above.
[00145] The reaction mixture includes, in addition to transition metal
containing,
crystalline cerium dioxide particles, one or more salts, for example, ammonium
nitrate
and unreacted cerium nitrate. The stabilized particles can be separated from
these
materials and salts by washing with 18 Mohm water in an ultrafiltration or
diafiltration
apparatus. Low ionic strength (< 5 mS/cm) is highly desirable for particle
formation and
stabilization in a non-polar medium. The washed, stabilized cerium dioxide
nanoparticles
may be concentrated, if desired, using a semi-porous membrane, for example, to
form an
aqueous concentrate of the nanoparticles. The particles may be concentrated by
other
means as well, for example, by centrifugation.
[00146] In one preferred embodiment, the transition metal containing,
crystalline
cerium dioxide particles are concentrated by diafiltration. The diafiltration
technique
utilizes ultrafiltration membranes, which can be used to completely remove,
replace, or
lower the concentration of salts in the nanoparticle-containing mixture. The
process
selectively utilizes semi-permeable (semi-porous) membrane filters to separate
the
components of the reaction mixture on the basis of their molecular size. Thus,
a suitable
ultrafiltration membrane would be sufficiently porous so as to retain the
majority of the
formed nanoparticles, while allowing smaller molecules such as salts and water
to pass
through the membrane. In this way, the nanoparticles and the associated bound
stabilizer
can be concentrated. The materials retained by the filter, including the
stabilized
nanoparticles, are referred to as the concentrate or retentate, the discarded
salts and
unreacted materials as the filtrate.
[00147] Pressure may be applied to the mixture to accelerate the rate at which
small
molecules pass through the membrane (flow rate) and to speed the concentration
process.
Other means of increasing the flow rate include using a large membrane having
a high
surface area, and increasing the pore size of the membrane, but without an
unacceptable
loss of nanoparticles.
[00148] In one embodiment, the membrane is selected so that the average pore
size of
the membrane is about 30% or less, 20 % or less, 10% or less, or even 5% or
less than
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that of the mean diameter of the nanoparticles. However, the pore diameter
must be
sufficient to allow passage of water and salt molecules. For example, ammonium
nitrate
and unreacted cerium nitrate should be completely or partially removed from
the reaction
mixture. In one preferred embodiment, the average membrane pore size is
sufficiently
small to retain particles of 1.5 nm diameter or greater in the retentate. This
would
correspond to a protein size of approximately 3 kilodaltons.
[00149] Desirably, the concentrate includes stabilized nanoparticles and
residual water.
In one embodiment, the concentration of cerium dioxide nanoparticles is
preferably
greater than about 0.5 molal, more preferably greater than about 1.0 molal,
even more
preferably greater than about 2.0 molal (approximately 35% solids in a given
dispersion).
[00150] Once the concentrate is formed, most if not all of the water is
removed by
dialysis with glycol ethers. This is accomplished by placing the concentrate
in a 2
kilodalton dialysis bag with a mixture of diethylene glycol methyl ether and 1-
methoxy-2-
propanol, and letting the water exchange into the glycol ether medium while
the glycol
ether medium displaces the water in the nanoparticle dispersion. Several
exchanges may
be necessary (changes of glycol ether medium). Alternatively, the glycol ether
mixture
can be run with the aqueous transition metal containing, crystalline cerium
dioxide
particles through a diafiltration column and a solvent shift effected in this
manner.
[00151] Glycol ether surfactants that contain both an ether group and an
alcohol group
includes compounds of formula (Ic), in which R3 represents a substituted or
unsubstituted
alkyl group, and m is an integer of 1-8.
R3(OCH2CH2)11,0H (Ic)
[00152] Other useful surfactants to effect the solvent shift include
nonylphenyl
ethoxylates having the formula, C9H19C6H4(OCH2CH2)110H, wherein n is 4-6.
[00153] Once the transition metal containing, crystalline cerium dioxide
particles are
in an organic medium, still stabilized with the original stabilizer used in
their manufacture
but complexed by the glycol ether, the mixture can be dispersed into a non-
polar medium
such as kerosene, which is compatible with most hydrocarbon fuels such as
diesel and
biodiesel. The surface of the particle is first functionalized with a
surfactant such as oleic
acid and optionally a co-surfactant such as 1-hexanol before being added to
the
hydrocarbon diluent. It is important to realize that this composition of
matter is not a
reverse micelle water-in-oil emulsion, as there is very little water present;
rather, the
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positive charge on the surface of the cerium nanoparticle has been complexed
by the ether
oxygen atoms and bound to the oppositely charged carboxylic acid. The
carboxylic acid is
present in a chemisorbed state and facilitates the miscibility of the
nanoparticle with a
non-polar hydrocarbon diluent. Other surface functionalization materials such
as linoleic
acid, stearic acid, and palmitic acid may be used in place of oleic acid. In
general, the
preferred materials are carboxylic acids with carbon chain lengths less than
20 carbon
atoms but greater than 8 carbon atoms. Other suitable nonpolar diluents
include, for
example, hydrocarbons containing about 8 to 20 carbon atoms, for example,
octane,
nonane, decane and toluene, and hydrocarbon fuels such as gasoline, biodiesel,
and diesel
fuels.
[00154] For optimal miscibility and stability with non-polar hydrocarbons, it
is
desirable that very few ions be present in the cerium dioxide concentrate to
conduct
electricity. This situation can be achieved by concentrating the nanoparticles
through
diafiltration to a conductivity level of less than about 5 mS/cm, preferably
to about 3
mS/cm or less.
[00155] Resistivity is the reciprocal of conductivity, which is the ability
of a material
to conduct electric current. Conductivity instruments can measure conductivity
by
including two plates that are placed in the sample, applying a potential
across the plates
(normally a sine wave voltage), and measuring the current. Conductivity (G),
the inverse
of resistivity (R), is determined from the voltage and current values
according to Ohm's
law, G = 1/R = TIE, where I is the current in amps and E is the voltage in
volts. Since the
charge on ions in solution facilitates the conductance of electrical current,
the
conductivity of a solution is proportional to its ion concentration. The basic
unit of
conductivity is the siemens (S), or milli-Siemens (mS). Since cell geometry
affects
conductivity values, standardized measurements are expressed in specific
conductivity
units (mS/cm) to compensate for variations in electrode dimensions.
[00156] The present invention is further directed to a method for formulating
a
homogeneous mixture that includes transition metal-containing cerium dioxide
nanoparticles, at least one nanoparticle stabilizer, one or more solvent
shifted media such
as glycol ethers, at least one surfactant, and a nonpolar diluent or solvent.
A first step
provides an aqueous mixture that includes stabilized cerium dioxide
nanoparticles,
wherein molecules of the nanoparticle stabilizer are closely associated with
the
nanoparticles. A second step includes concentrating the stabilized crystalline
cerium
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dioxide nanoparticles while minimizing the ionic strength of the suspension to
form an
aqueous concentrate that is relatively free of anions and cations. A third
step removes the
water associated with the nanoparticles using a non-ionic surfactant. A final
step includes
combining this solvent shifted concentrate with a nonpolar solvent, containing
a
surfactant, thereby forming a substantially homogeneous mixture that is a
thermodynamically stable, multicomponent, bi-phasic dispersion.
[00157] The substantially homogeneous thermodynamic dispersion contains a
minimal
amount of water at a level of preferably no more than about 0.5 wt.%.
[00158] The transition metal-containing cerium dioxide nanoparticles have a
mean
hydrodynamic diameter of preferably less than about 10 nm, more preferably
less than
about 8 nm, most preferably about 6 nm, and a geometric diameter of about 4 nm
or less.
[00159] Desirably, the cerium dioxide nanoparticles have a primary crystallite
size of
about 2.5 nm + 0.5 nm and comprise one or at most two crystallites per
particle edge
length.
[00160] The aqueous mixture is advantageously formed in a colloid mill
reactor,
and the nanoparticle stabilizer may comprise an ionic surfactant, preferably a
compound
that includes a carboxylic acid group and an ether group. The nanoparticle
stabilizer may
comprise a surfactant of formula (Ia),
RO(CH2CH20).CHR1CO2Y (Ia)
wherein: R represents hydrogen or a substituted or unsubstituted alkyl group
or a
substituted or unsubstituted aromatic group; Rl represents hydrogen or an
alkyl group; Y
represents H or a counterion; and n is 0-5. Preferably, R represents a
substituted or
unsubstituted alkyl group, Rl represents hydrogen, Y represents hydrogen, and
n is 2.
[00161] Another suitable nanoparticle stabilizer comprises a dicarboxylate of
formula
(Ib),
X02C(CR2)pCO2Z (Ib)
wherein each R2 independently represents hydrogen, a substituted or
unsubstituted alkyl
group or a substituted or unsubstituted aromatic group; X and Z independently
represent
H or a counterion; and p is lor 2.
[00162] Other useful nanoparticle stabilizers are included in the group
consisting of
lactic acid, gluconic acid enantiomers, EDTA, tartaric acid, citric acid, and
combinations
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thereof
[00163] Concentrating the aqueous mixture is preferably carried out using
diafiltration,
which results in the reduction in conductivity of said concentrated aqueous
mixture to
about 5 mS/cm or less.
[00164] The surfactant used to shift the stabilized transition metal
containing,
crystalline cerium dioxide particles from an aqueous to a non-aqueous
environment may
advantageously comprise a nonionic surfactant, preferably a compound
comprising an
alcohol group and an ether group, in particular, a compound of formula (Ic),
R3(OCH2CH2)11,0H (Ic)
wherein R3 represents a substituted or unsubstituted alkyl group; and m is an
integer from
1 to 8.
[00165] The nonionic surfactant may also comprise a compound of formula (Id),
R3VOCH2CH2)11,0H (Id)
wherein R3 represents a substituted or unsubstituted alkyl group; 0 is an
aromatic group;
and m is an integer from 4 to 6.
[00166] The reaction mixture may further include a co-surfactant, preferably
an
alcohol.
[00167] Introduction of this solvent shifted concentrate is facilitated by
surfactants that
surface functionalize the nanoparticles. Preferred surfactants are carboxylic
acids such as
oleic acid, linoleic acid, stearic acid, and palmitic acid. In general, the
preferred materials
are carboxylic acids with carbon chain lengths less than 20 carbon atoms but
greater than
3 carbon atoms.
[00168] The nonpolar diluent included in the substantially homogeneous
dispersion is
advantageously selected from among hydrocarbons containing about 6-20 carbon
atoms,
for example, octane, decane, kerosene, toluene, naphtha, diesel fuel,
biodiesel, and
mixtures thereof When used as a fuel additive, one part of the homogeneous
dispersion
is with at least about 100 parts of the fuel.
[00169] In accordance with the invention, the transition metal is preferably
selected
from the group consisting of Fe, Mn, Cr, Ni, W, Co, V, Cu, Mo, Zr, Y and
combinations
thereof Preferred transition metals are Zr or Y, more preferably combined with
Fe.
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[00170] It may be beneficial to form a ceramic oxide coating on the interior
surfaces of
diesel engine cylinders in situ. The potential benefits of the coating include
added
protection of the engine from thermal stress; for example, Ce02 melts at 2600
C, whereas
cast iron, a common material used in the manufacture of diesel engines, melts
at about
1200-1450 C. Even 5 nm ceria particles have demonstrated the ability to
protect steel
from oxidation for 24 hours at 1000 C, so the phenomenon of size dependent
melting
would not be expected to lower the melting point of the cerium dioxide
nanoparticles of
the invention below the combustion temperatures encountered in the engine.
See, for
example, Patil et al., Journal of Nanoparticle Research , vol. 4, pp 433-438
(2002). An
engine so protected may be able to operate at higher temperatures and
compression ratios,
resulting in greater thermodynamic efficiency. A diesel engine having cylinder
walls
coated with cerium dioxide would be resistant to further oxidation (Ce02 being
already
fully oxidized), thereby preventing the engine from "rusting." This is
important because
certain additives used to reduce carbon emissions or improve fuel economy such
as, for
example, the oxygenates MTBE, ethanol and other cetane improvers such as
peroxides,
also increase corrosion when introduced into the combustion chamber, which may
result
in the formation of rust and degradation of the engine lifetime and
performance. The
coating should not be so thick as to impede the cooling of the engine walls by
the water
recirculation cooling system.
[00171] In one embodiment, the current invention provides transition metal-
containing,
crystalline, cerium dioxide nanoparticles having a mean hydrodynamic diameter
of less
than about 10 nm, preferably less than about 8 nm, more preferably 6 nm or
even less,
that are useful as a fuel additive for diesel engines. The surfaces of the
cerium dioxide
nanoparticles may be modified to facilitate their binding to an iron surface,
and desirably
would, when included in a fuel additive composition, rapidly form a ceramic
oxide
coating on the surface of diesel engine cylinders.
[00172] In one embodiment, a transition metal having a binding affinity for
iron is
incorporated onto the surface of the cerium dioxide nanoparticles. Examples of
iron
surfaces include those that exist in many internal parts of engines. Suitable
transition
metals include Mn, Fe, Ni, Cr, W, Co, V, Cu, Zr, and Y The transition metal
ion, which is
incorporated into the cerium dioxide nanoparticles by occupying a cerium ion
lattice site
in the crystal, may be introduced during the latter stages of the
precipitation of cerium
dioxide. The transition metal ion can be added in combination with cerous ion,
for
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example, in a single jet manner in which both cerous ion and transition metal
ion are
introduced together into a reactor containing ammonium hydroxide.
Alternatively, the
transition and cerous ions can be added together with the simultaneous
addition of
hydroxide ion. The transition metal-containing particles can also be formed in
a double
jet reaction of cerous ion with dissolved transition metal ion titrated
against an
ammonium hydroxide steam simultaneously introduced by a second jet.
Critically, it is
understood that sufficient nanoparticle stabilizer is present to prevent
agglomeration of
the nascent particles.
[00173] The surfactant/stabilizer combination may have the added benefit of
aiding in
the solvent shift process from the aqueous polar medium to the non-polar oil
medium. In
a combination of charged and uncharged surfactants, the charged surfactant
compound
plays a dominant role in the aqueous environment. However, as solvent shifting
occurs,
the charged compound is likely to be solubilized into the aqueous phase and
washed out,
and the uncharged compound becomes more important in stabilizing the reverse
micelle
emulsion.
[00174] Dicarboxylic acids and their derivatives, so called "gemini
carboxylates",
where the carboxylic groups are separated by at most two methylene groups, are
also
useful cerium dioxide nanoparticle stabilizers. Additionally, C2-C8 alkyl,
alkoxy and
polyalkoxy substituted dicarboxylic acids are advantageous stabilizers.
[00175] In accordance with the invention, nanoparticle stabilizer compounds
preferably comprise organic carboxylic acids such as, for example, 2-[2-(2-
methoxyethoxy)ethoxy]acetic acid (MEEA) and ethylenediaminetetraacetic acid
(EDTA),
lactic acid, gluconic acid, tartaric acid, citric acid, and mixtures thereof
[00176] Motor oil is used as a lubricant in various kinds of internal
combustion
engines in automobiles and other vehicles, boats, lawn mowers, trains,
airplanes, etc.
Engines contain contacting parts that move against each other at high speeds,
often for
prolonged periods of time. Such rubbing motion causes friction, forming a
temporary
weld, immobilizing the moving parts. Breaking this temporary weld absorbs
otherwise
useful power produced by the motor and converts the energy to useless heat.
Friction also
wears away the contacting surfaces of those parts, which may lead to increased
fuel
consumption and lower efficiency and degradation of the motor. In one aspect
of the
invention, a motor oil includes a lubricating oil, transition metal-
containing, crystalline,
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cerium dioxide nanoparticles, desirably having a mean diameter of less than
about 10 nm,
more preferably about 5 nm, and optionally a surface adsorbed stabilizing
agent.
[00177] Diesel lubricating oil is essentially free of water (preferably
less than 300
ppm) but may be desirably modified by the addition of a cerium dioxide
composition in
which the cerium dioxide has been solvent shifted from its aqueous environment
to that of
an organic or non-polar environment. The cerium dioxide compositions include
nanoparticles having a mean diameter of less than about 10 nm, more preferably
about 5
nm, as already described. A diesel engine operated with modified diesel fuel
and
modified lubricating oil provides greater efficiency and may, in particular,
provide
improved fuel mileage, reduced engine wear or reduced pollution, or a
combination of
these features.
[00178] Metal polishing, also termed buffing, is the process of smoothing
metals and
alloys and polishing to a bright, smooth mirror-like finish. Metal polishing
is often used
to enhance cars, motorbikes, antiques, etc. Many medical instruments are also
polished to
prevent contamination in irregularities in the metal surface. Polishing agents
are also
used to polish optical elements such as lenses and mirrors to a surface
smoothness within
a fraction of the wavelength of the light they are to manage. Smooth, round,
uniform
cerium dioxide particles of the present invention may be advantageously
employed as
polishing agents, and may further be used for planarization (rendering the
surface smooth
at the atomic level) of semiconductor substrates for subsequent processing of
integrated
circuits.
[00179] The invention is further illustrated by the following examples, which
are not
intended to limit the invention in any manner.
[00180] Example 1. Preparation of Cerium Dioxide Nanoparticles by Single-Jet
Addition.
[00181] To a 3 liter round bottom stainless steel reactor vessel was added
1.267 liters
of distilled water, followed by 100 ml of Ce(NO3)3=6H20 solution (600 gm/
liter
Ce(NO3)3=6H20). The solution was clear and had a pH of 4.2 at 20 C.
Subsequently, 30.5
gm of 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MEEA) was added to the vessel.
The
solution remained clear, and the pH was 2.8 at 20 C. A high sheer mixer, a
colloid mill
manufactured by Silverson Machines, Inc. that had been modified to enable
reactants to
be introduced directly into the mixer blades by way of a peristaltic tubing
pump, was
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lowered into the reactor vessel, the mixer head being positioned slightly
above the bottom
of the reactor vessel. The mixer was set to 5,000 rpm, and 8.0 gm of 30% H202
was
added to the reactor vessel. Then 16 ml of 28% - 30% NH4OH, diluted to 40 ml,
was
pumped into the reactor vessel by way of the mixer head in about 12 seconds.
The
initially clear solution turned an orange/brown in color. The high sheer mixer
was
removed, and the reactor vessel was moved to a temperature-controlled water
jacket,
where a mixer with an R-100 propeller was used to stir the solution at 450
rpm. The pH
was 3.9 at 25 C at 3 minutes after pumping the NH4OH into the reactor. The
temperature
of the reactor vessel was raised to 70 C over the next 25 minutes, at which
time the pH
was 3.9. The solution temperature was held at 70 C for 20 minutes, during
which time
the solution color changed from orange brown to a clear dark yellow. The pH
was 3.6 at
70 C. The temperature was lowered to 25 C over the next 25 minutes, at which
time the
pH was 4.2 at 25 C. Particle size analysis by dynamic light scattering
indicated a cerium
dioxide intensity weighted hydrodynamic diameter of 6 nm. The dispersion was
then
diafiltered to a conductivity of 3 mS/cm and concentrated, by a factor of
about 10, to a
nominal 1 Molar in Ce02 particles.
[00182] The cerium dioxide particles were collected, the excess solvent
evaporated off,
and the gravimetric yield, corrected for the weight of MEEA, was determined to
be
62.9%.
[00183] A transmission electron microscope (TEM) was used to analyze the
cerium
dioxide particles. A 9-microliter solution (0.26M) was dried onto a grid and
imaged to
produce the image shown in FIG. 1. The particles show no signs of
agglomeration, even
in this dried-down state. In solution, the particles would be expected to show
even less
propensity to agglomerate. The size frequency distribution of the cerium
dioxide particles
(plotted in FIG 1), determined by transmission electron micrography (TEM),
yields a
geometric diameter of about 2.6 nm. Additionally, the size distribution is
substantially
monomodal, i.e., only one maximum, and uniform, 19% COV, with most of the
particles
falling in the range 2 nm to 4 nm.
[00184] FIG. 2 shows an X-ray powder diffraction pattern 70 of a sample of the
dried
cerium dioxide nanoparticles, together with a reference spectrum 71 of cerium
dioxide
that was provided by the NIST (National Institute of Standards and Technology)
library.
The line positions in the sample spectrum match those of the standard
spectrum. The two
theta peak widths were very wide in the sample spectrum, which is consistent
with a very
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small primary crystallite size and particle size. From the X-ray data (Cu K
alpha line at
about 8047 ev) and the Scherrer formula (d = 0.9*Iambda/delta*cos(theta),
where lambda
is the x-ray wavelength, delta the full width half maximum, and theta the
scattering angle
corresponding to the x ray peak), the primary crystallite size was calculated
to be 2.5
0.5 nm (95 % confidence of 5 replicas). Since the particle itself is the size
of this
crystallite, there is only one crystal per particle, therefore we refer to
this composition as
crystalline cerium dioxide to distinguish it from all previous art in which
the
nanoparticles are comprised of agglomerates of crystallites of various sizes.
1001851 Example 2. Precipitation of 1.5 nm Ce02Nanoparticles.
This precipitation follows Example 1, except that the stabilizer combination
of EDTA and
lactic acid in the ratio 20:80 and at a level of 76.4 gm EDTA disodium salt
and 74.0 gm
of 85% lactic acid is used instead of the MEEA stabilizer. FIG. 3A is a high
magnification TEM indicating a grain size substantially smaller than 5 nm and
estimated
to be 1.1 +/- 0.3 rim. FIG. 313 represents the electron diffraction pattern of
a
representative sample of the precipitation. In Table 1, the intensities of the
various
diffractions rings {311}, {220}, {200} and {111} are analyzed
within the framework of: cubic Ce02, cubic and hexagonal Ce203 and Ce(OH)3.
Clearly
the percent deviations of analyzed ring intensity with crystal habit are
minimal for the
cubic fluorite structure of Ce02, thus establishing the existence of this poly-
morph down
to this grain diameter.
1001861 Example 3. Preparation of Ce02Nartoparticles by Isothermal Double-Jet
Precipitation Ce02
[00187] To a 3 liter round bottom stainless steel reactor vessel was added
1117 grams
of distilled water. A standard RN- 100 propeller was lowered into the reactor
vessel, and
the mixer head was positioned slightly above the bottom of the reactor vessel.
The mixer
was set to 700 rpm, and the reactor was brought to a temperature of about 70
C. Then
59.8 grams (98%) of methoxyacetic acid were added to the reactor. A double jet
precipitation was conducted over a period of five minutes by pumping a 250 ml
solution
containing 120.0 grams of Ce(NO3)3.6H20 into the reactor concurrently with a
solution
containing 69.5 grams (28-30%) of ammonium hydroxide. A distilled water chase
into the
reactor cleared the reactant lines of residual materials. Then 10.2 grams of
50% non-
stabilized hydrogen peroxide was added to the reactor and its contents over a
period of 40
43
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,
TABLE 1
d, Ang Cubic Percent Cubic Percent Hexagonal
Percent
expt. E.D. Ce02 Difference Diffraction Ce203 Difference
Diffraction Ce203 Difference
3.2066 3.1234 2.67 111 4.539 29.35 211 3.37 4.85
2.8795 2.7056 6.43 200 2.623 9.78 411 3.03 4.97
1.9655 1.9134 2.72 220 2.374 17.21 332 2.945 33.26
1.6694 1.6318 2.31 311 2.184 23.56 510 2.254 25.93
1.3807 1.5622 11.61 222 1.806 23.55 611 1.945 29.01
1.2365 1.3531 8.61 400 1.717 27.98 541 1.733 28.65
1.1134 1.2415 10.31 331 1.685 33.92
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seconds. Initially, the reaction mixture was an opaque dark orange brownish
liquid in the
pH range 6 to 7. The reaction mixture was heated for an additional 60 minutes,
during
which time the pH dropped to 4.25 (consistent with the release of hydronium
ion via
reactions (3a) and (3b) and the mixture became clear yellow orange color. The
reaction
was cooled to 20 C and diafiltered to a conductivity of 3 mS/cm to remove
excess water
and unreacted materials. This resulted in concentrating the dispersion by a
factor of about
10, or nominally 1 Molar in Ce02 particles. Particle size-frequency analysis
by
transmission electron micrography (FIG 4) revealed a mean particle size of
2.2nm, with
size frequency distribution haying a coefficient of variation, COV, (one
standard
deviation divided by the mean diameter) of 23%. The calculated yield was
62.9%.
[00188] Example 4. Copper-Containing Ce02Nanoparticles Ce 0.9Cu 0.10 1.95
The conditions of example 3 were repeated, except that the cerium nitrate
solution
contained 108.0 grams of cerium nitrate hexahydrate, and 6.42 grams of
Cu(NO3)3.2.5
H20. These metal salts were dissolved separately and then combined to form a
250 ml
solution. The reaction proceeded as described in Example 3 except that the
hydrogen
peroxide was added over a period of 40 seconds after the cerium and ammonia
had been
added. Particle size-frequency analysis by transmission electron micrography
(FIG.5)
revealed a mean particle size of 2.5 nm, with size frequency distribution
having a
coefficient of variation, COV, (one standard deviation divided by the mean
diameter) of
25%. Note the absence of a bi-modal distribution; a secondary peak would be an
indication that the Cu was not incorporated into the Ce02 lattice but instead
existed as a
separate Cu203 population.
[00189] Example 5. Iron-Containing Ce02Nanoparticles Ce 0.9Fe 9.10 1.95
(Ce0-255)
[001901 The conditions of Example 4 were repeated, except that the metal salts
solution contained 108.0 grams of cerium nitrate hexahydrate, and 11.16 grams
of
Fe(NO3)3=9 H20. These metal salts were dissolved separately and then combined
to form
a 250 ml solution. The reaction proceeded as described in Example 4. A TEM of
the
precipitated particles (FIG 6A) and particle size-frequency analysis by
transmission
electron micrography (FIG. 6B) revealed a mean particle size of 2.2 +/-0.7 nm,
with size
frequency distribution having a coefficient of variation, COV, (one standard
deviation
divided by the mean diameter) of 32%. The calculated yield was 55.1%.
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[00191] Example 6. Zirconium-Containing Ce02Nanoparticles Ce 0.9Zr 0.150 2
(Ce0
257)
[00192] The conditions of Example 4 were repeated except that the metal salts
solution
contained 101.89 grams of cerium nitrate hexahydrate, and 9.57 grams of
ZrO(NO3)2=6
H20. These metal salts were dissolved separately and then combined to form a
250 ml
solution. The reaction proceeded as described in Example 4, except that the
temperature
of the reaction was carried out at 85 C. Particle size-frequency analysis by
transmission
electron micrography (FIG. 7A) revealed a mean particle size of 2.4 +/- 0.7
nm, with size
frequency distribution having a coefficient of variation, COV, (one standard
deviation
divided by the mean diameter) of 29%. Inductively coupled plasma atomic
emission
spectroscopy revealed a stoichiometry of Ce0.82Zro.1801.91, which given the
relative
insolubility of Zr02 to Ce02, would account for the enhanced Zr content (18%
vs 15%).
[00193] Example 7a. Zirconium- and Iron-Containing Ce02Nanoparticles Ce0.9
Zro.i5Fe 0.101.05 (Ce0-270)
[00194] The conditions of Example 4 were repeated, except that the metal salts
solution contained 84.0 grams of cerium nitrate hexahydrate, 11.16 grams of
Fe(NO3)39
H20 and 12.76 grams of ZrO(NO3)2=6 H20. These metal salts were dissolved
separately
and then combined to form a 250 ml solution. The reaction proceeded as
described in
Example 4, except that the temperature of the reaction was carried out at 85
C, and the
hydrogen peroxide solution (50%) was elevated to 20.4 gm and added over a
period of ten
minutes. Particle TEM (FIG 8A) and particle size-frequency analysis by
transmission
electron micrography (FIG. 8B) revealed a mean particle size of 2.2 +/-0.6 nm,
with size
frequency distribution having a coefficient of variation, COV, (one standard
deviation
divided by the mean diameter) of 27%. Again, a monodisperse, unimodal
distribution
supports the idea of co-incorporation as opposed to separately renucleated
Zr02 and
Fe203 grain populations. The calculated yield was 78%. Inductively coupled
plasma
atomic emission spectroscopy revealed a stoichiometry of Ce 0.60 Fe 0.14 Zr
0.170 0.915.
Again, the relatively more concentrated Fe and Zr with respect to the nominal
amounts
reflects the greater insolubility of their hydroxide precursors relative to
that of cerium
hydroxide. Also in FIG. 8C is an x-ray powder diffraction pattern of this
sample (top
curve) compared to the transition metal free Ce02. The lack of a peak (denoted
by an
arrow) at 32 deg two theta means that there is no free Zr02, i.e., it is all
incorporated into
the cerium lattice. Also, the lack of peaks at 50 and 52 degrees two theta
indicate no
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separate population of Fe203 ( ie incorporation of Fe into the cerium
lattice). Note the
shift to larger 2 theta at large two theta scattering angle, which indicates a
distortion or
contraction of the lattice- ( nk/ 2d = sin 0) which is consistent with the
smaller ionic radii
of Fe 3+ (0.78A) and Zr 4+ (0.84A) relative to the Ce 4+ (0.97A) which it is
replacing.
Thus, we conclude that the transition metals are incorporated into the Ce02
lattice and do
not represent a separate population of neat Zr02 or Fe203nanoparticles. The
unimodal
size-frequency distribution also supports this conclusion.
[00195] Examples 7b-f Zirconium-and Iron Containing Ce02Nanoparticles varying
systematically in the amount of iron (15%, 20%, 25%, 30%) at 15% zirconium and
20%
iron at 20% zirconium.
[00196] The conditions of Example 7a were followed; however the amount of iron
or
zirconium was adjusted to give the nominal stoichiometries indicated, using
the
appropriate metal containing salt solution while the overall cerium nitrate
hexahydrate
was reduced to accommodate the increased concentration of the iron or
zirconium
transition metal.
[00197] FIG. 9 is a Field Emission Gun TEM lattice image of the particles made
in
Example 1. Two of the particles are circled for clarity. Note the small number
of lattice
planes that define a single crystal having a diameter of less than 5 nm.
Aqueous sols of various materials were heated for 30 minutes in a muffle
furnace at
1000 C. These thoroughly dried samples were measured for OSC and the kinetics
at
which they reached their maximum OSC using thermogravimetric techniques, as
described by Sarkas et al., "Nanocrystalline Mixed Metal Oxides-Novel Oxygen
Storage
Materials," Mat. Res. Soc. Symp. Proc. Vol. 788, L4.8.1 (2004). Typically, one
observes
a very fast initial reduction rate in nitrogen gas containing 5% hydrogen,
followed by a
second slower rate.
The accompanying TABLE 2 contains the Oxygen Storage Capacity (1 sigma
reproducibility in parenthesis) and the fast (kl) and slow (k2) rate constants
(1 standard
deviation in parenthesis) for reduction of various lattice engineered ceria
nanoparticles
(all 2 nm except the Sigma Aldrich control) in a nitrogen gas at 700 C
containing 5% H2.
These values have been cross-checked against a second TGA instrument (average
2.6%
difference), against gas flow differences (average 1% deviation) and replicate
sample
preparation at 1000 C for 30 minutes (average 1.54% deviation). From the
entries in
TABLE 2, we see that the OSC of cerium dioxide particles does not appear to be
size-
46
CA 02747547 2011-06-17
WO 2010/071641
PCT/US2008/087133
dependent in the range of about 2 nm-20 p.m. This may be a consequence of
sintering to
larger particles. Note that OSC increases approximately by 50% with the
addition of
zirconium and is accompanied by a 10 x rate increase. Furthermore, the
addition of iron
to the Zr-containing material affords nearly three times the OSC at a 10-fold
rate
compared to cerium dioxide particles containing no transition metal ions.
These values
are more than triple the values in the cited reference. The beneficial effect
of citric acid on
the reduction rate constant seems to suggest that the stabilizer may have an
effect on the
particle surface area or morphology even after it has been pyrolyzed.
TABLE 2¨ Comparison of OSC for Cerium Dioxide Nanoparticle Variations
Sample OSC (p moles/g) Reduction Rate constant Reduction Rate
constant
(Std. Dev. pmoles/g) k1 x 10^3 (/min) (std dev.) k2 x10^3 (/min)
(std dev.)
Sigma Aldrich 296 (1.65)
Ce02 (20 pm)
Ce02 (2 nm) 349
CeFe0.1002 470(1% surf)
CeZr0.1502 592(3)
CeZr015Fe0.1002 1122 (3) 3.1 (0.4) 0.9 (0.15)
CeZr015Fe0.1502 1359 (33) 5.9 (0.04) 2.0 (0.2)
CeZr0.15Fe0.2002 1653(6) 3.4 (0.4) 1.1 (0.3)
CeZr0.15Fe0.2502 2013 ( 1) 3.1 (0.4) 1.1 (0.2)
CeZr0.15Fe0.3002 2370 (4) 2.6 (0.1) 1.0 (0.1)
CeZr0.20Fe0.2002 1661 (7) 4.9 ( 1.3) 1.2 (0.2)
CeZr0.20Fe0.2002
citric acid 1636 (1) 9.5 (0.6) 3.9 (0.2)
[00198] While the invention has been described by reference to various
specific
embodiments, it should be understood that numerous changes may be made within
the
spirit and scope of the inventive concepts described. Accordingly, it is
intended that the
invention not be limited to the described embodiments, but will have full
scope defined
by the language of the following claims.
47
