Fuel additive containing lattice engineered cerium dioxide nanoparticles

Abstract

A process for making cerium dioxide nanoparticles containing at least one transition metal (M) utilizes a suspension of cerium hydroxide nanoparticles prepared by mechanical shearing of an aqueous mixture containing an oxidant in an amount effective to enable oxidation of cerous ion to ceric ion, thereby forming a product stream that contains transition metal-containing cerium dioxide nanoparticles, Ce.sub.1-xM.sub.xO.sub.2, wherein x has a value from about 0.3 to about 0.8. The 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 less than about 4 nm. The transition metal-containing crystalline cerium dioxide nanoparticles can be used to prepare a dispersion of the particles in a nonpolar medium.

Claims

1. A process for making lattice engineered crystalline cerium dioxide nanoparticles containing at least one metal (M), said process comprising: (a) providing an aqueous reaction mixture comprising a source of cerous ion, a source of one or more metal ions (M), a source of hydroxide ion, at least one water-soluble nanoparticle stabilizer, and an oxidant, wherein the metal ions (M) are ions of elements selected from the group consisting of rare earth metals and transition metals, and wherein said at least one water-soluble nanoparticle stabilizer comprises a compound selected from the group consisting of alkoxy substituted carboxylic acids, -hydroxyl carboxylic acids, -keto carboxylic acids, and polyacids, wherein said alkoxy substituted carboxylic acid comprises a compound of formula (Ia):
RO(CH.sub.2CH.sub.2O).sub.nCHR.sup.1CO.sub.2Y(Ia) wherein: R represents a methyl or an ethyl group; R.sup.1 represents a hydrogen or an alkyl group; Y represents a hydrogen or a counter ion; and n is an integer from 0 to 5; (b) shearing said mixture, thereby forming a homogeneously distributed suspension of cerium hydroxide nanoparticles; and (c) providing temperature conditions in the homogeneously distributed suspension of cerium hydroxide nanoparticles effective to enable oxidation of cerous ion to ceric ion, thereby forming a dispersion comprising metal-containing cerium dioxide nanoparticles, Ce.sub.1-xM.sub.xO.sub.2, wherein x has a value from about 0.02 to about 0.8, said nanoparticles having a crystalline cubic fluorite structure, a mean hydrodynamic diameter in the range of about 1 nm to about 10 nm, and wherein a size distribution of said nanoparticles has a coefficient of variation of less than 32%.

2. The process according to claim 1 wherein said mechanical shearing is effected in a colloid mill.

3. The process according to claim 1 wherein said temperature conditions effective to enable oxidation of cerous ion to ceric ion comprise a temperature of about 50 C. to about 100 C.

4. The process according to claim 3 wherein said temperature conditions effective to enable oxidation of cerous ion to ceric ion comprise a temperature of about 60 C. to about 90 C.

5. The process according to claim 1 wherein said sources of ions are introduced into said reaction mixture either concurrently or sequentially during said mechanical shearing.

6. The process according to claim 1 wherein said metal-containing cerium dioxide nanoparticles have a mean hydrodynamic diameter of about 8 nm or less.

7. The process according to claim 1, wherein the at least one water-soluble nanoparticle stabilizer comprises a compound selected from the group consisting of lactic acid, gluconic acid enantiomers, ethylenediaminetetraacetic acid, tartaric acid, citric acid, pyruvic acid, methoxy acetic acid, 2-(methoxy)ethoxy acetic acid, 2-[2-(2-methoxyethoxy)ethoxy] acetic acid, 2-hydroxybutanoic acid, and combinations thereof.

8. A process for making lattice engineered crystalline cerium dioxide nanoparticles containing at least one metal (M), said process comprising: (a) providing an aqueous reaction mixture comprising a source of cerous ion, a source of one or more metal ions (M), a source of hydroxide ion, at least one water-soluble nanoparticle stabilizer, and an oxidant, wherein the metal ions (M) are ions of elements selected from the group consisting of rare earth metals and transition metals, and wherein said at least one water-soluble nanoparticle stabilizer comprises a compound selected from the group consisting of alkoxy substituted carboxylic acids, -hydroxyl carboxylic acids, -keto carboxylic acids, and polyacids, wherein said alkoxy substituted carboxylic acid comprises a compound of formula (Ia):
RO(CH.sub.2CH.sub.2O).sub.nCHR.sup.1CO.sub.2Y(Ia) wherein: R represents a methyl or an ethyl group; R.sup.1 represents a hydrogen or an alkyl group; Y represents a hydrogen or a counter ion; and n is an integer from 0 to 5; (b) shearing said mixture, thereby forming a homogeneously distributed suspension of cerium hydroxide nanoparticles; and (c) providing temperature conditions in the homogeneously distributed suspension of cerium hydroxide nanoparticles effective to enable oxidation of cerous ion to ceric ion, thereby forming a dispersion comprising metal-containing cerium dioxide nanoparticles, Ce.sub.1-xM.sub.xO.sub.2, wherein x has a value from about 0.02 to about 0.8, said nanoparticles having a crystalline cubic fluorite structure, a mean hydrodynamic diameter in the range of about 1 nm to about 10 nm, and a geometric diameter of less than 4 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A and 1B are, respectively, a TEM image and a particle size frequency analysis by TEM of CeO.sub.2 nanoparticles prepared by non-isothermal precipitation, as described in Example 1.

(2) FIG. 2 is an X-ray powder diffraction spectrum of cerium dioxide nanoparticles prepared as described in Example 1.

(3) FIG. 3A is a TEM image of 1.1 nm CeO.sub.2 nanoparticles prepared as described in Example 2; FIG. 3B is an electron diffraction pattern of these 1.1 nm particles; FIG. 3C is TABLE 1, containing calculated vs measured electron diffraction intensities for cubic and hexagonal CeO.sub.2 and Ce.sub.2O.sub.3 lattices.

(4) FIGS. 4A and 4B are, respectively, a TEM image and a particle size-frequency analysis by TEM of isothermally precipitated CeO.sub.2 nanoparticles, prepared by a triple jet process as described in Example 3.

(5) FIGS. 5A and 5B are, respectively, a TEM image and a particle size-frequency analysis by TEM of isothermally precipitated Cu-containing CeO.sub.2 nanoparticles, prepared as described in Example 4.

(6) FIGS. 6A and 6B are, respectively, a TEM image and a particle size-frequency analysis by TEM of isothermally precipitated Fe-containing CeO.sub.2 nanoparticles, prepared as described in Example 5.

(7) FIGS. 7A and 7B are, respectively, a TEM image and a particle size-frequency analysis by TEM of isothermally precipitated Zr-containing CeO.sub.2 nanoparticles, prepared as described in Example 6.

(8) FIGS. 8A and 8B are respectively, a TEM image and a particle size-frequency analysis by TEM of isothermally precipitated CeO.sub.2 nanoparticles containing Zr and Fe, prepared as described in Example 7. FIG. 8C are x-ray diffraction spectra of isothermally precipitated CeO.sub.2 nanoparticles and of isothermally precipitated CeO.sub.2 nanoparticles containing Zr and Fe, prepared as described in Example 7.

(9) FIG. 9 is a field emission gun TEM lattice image of CeO.sub.2 nanoparticles containing Zr and Fe, prepared as described in Example 7.

DETAILED DESCRIPTION OF THE INVENTION

(10) 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

(11) The present invention provides a process for making transition metal ion-containing cerium dioxide (CeO.sub.2) 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 eerie ion, thereby forming a product stream comprising transition metal-containing cerium dioxide nanoparticles, Ce.sub.1-xM.sub.xO.sub.2, 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 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.

(12) The present invention further provides for a continuous process for producing crystalline cerium dioxide CeO.sub.2 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.

(13) 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, 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.

(14) 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.

(15) In the presence of hydroxide ion, eerie 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.

(16) 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 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 1 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.

(17) 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.

(18) 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. Pat. Nos. 6,745,961 and 6,305,626, the disclosures of which are incorporated herein by reference.

(19) 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, 2-[2-(2-methoxyethoxy)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.

(20) 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, 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.

(21) 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.

(22) 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.

(23) 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. K.sub.BC represents the binding constant of the nanoparticle stabilizer to cerium ion in water. Log K.sub.BC for the nitrate ion is 1 and for hydroxide ion is 14. Most desirably, log K.sub.BC lies within this range, preferably in the middle of this range. Useful nanoparticle stabilizers include alkoxysubstituted carboxylic acids, -hydroxyl carboxylic acids, -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 -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 K.sub.BC such as EDTA with weak K.sub.BC stabilizers such as lactic acid are also useful at particular ratios. Large K.sub.BC stabilizers such as gluconic acid may be used at a low level, or with weak K.sub.BC stabilizers such as lactic acid.

(24) 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. R.sup.1 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.sup.+ or K.sup.+. The stabilizer binds to the nanoparticles and prevents agglomeration of the particles and the subsequent formation of large clumps of particles.
RO(CH.sub.2CH.sub.2O).sub.nCHR.sup.1CO.sub.2Y(Ia)

(25) In another embodiment, the nanoparticle stabilizer is represented by formula (Ib), wherein each R.sup.2 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.sup.+ or K.sup.+, and p is 1 or 2.
XO.sub.2C(CR.sup.2).sub.pCO.sub.2Z(Ib)

(26) Useful nanoparticle stabilizers are also found among -hydroxysubstituted carboxylic acids such as lactic acid and among the polyhydroxysubstituted acids such as gluconic acid.

(27) 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 of oxides of sulfur after combustion.

(28) 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.

(29) 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. Pat. No. 6,897,270; Nickel, et al., U.S. Pat. No. 6,723,138; Campbell, et al., U.S. Pat. No. 6,627,720; Beck, U.S. Pat. No. 5,097,090; and Byrd, et al., U.S. Pat. No. 4,661,321; the disclosures of which are incorporated herein by reference.

(30) 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.

(31) 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.

(32) 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, 2-[2-(2-methoxyethoxy)ethoxy]acetic 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.

(33) 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.

(34) 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.

(35) The first and second reactant streams are combined to form a reaction stream. 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).

(36) 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.

(37) A schematic representation of a continuous reactor suitable for the practice of the invention is depicted in FIG. 3 of PCT/US2007/77545, METHOD OF PREPARING CERIUM DIOXIDE NANOPARTICLES, filed Sep. 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.

(38) 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 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.

(39) 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.

(40) 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.

(41) 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.

(42) The invention provides a method for formulating a homogeneous mixture that includes cerium dioxide (CeO.sub.2) 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.

(43) 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(NO.sub.3).sub.3(6H.sub.2O)+2NH.sub.4OH.fwdarw.Ce(OH).sub.2NO.sub.3+2NH.sub.4NO.sub.3+6H.sub.2O(3a)
2Ce(OH).sub.2NO.sub.3+H.sub.2O.sub.2.fwdarw.2CeO.sub.2+2HNO.sub.3+2H.sub.2O(3b)
Ce(NO.sub.3).sub.3(6H.sub.2O)+3NH.sub.4OH.fwdarw.Ce(OH).sub.3+3NH.sub.4NO.sub.3+6H.sub.2O(4a)
2Ce(OH).sub.3+H.sub.2O.sub.2.fwdarw.2CeO.sub.2+4H.sub.2O(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.

(44) In some cases, especially where ammonium hydroxide is not present in excess relative to the cerous ion, the species Ce(OH).sub.2(NO.sub.3) or (NH.sub.4).sub.2Ce(NO.sub.3).sub.5 may initially be present, subsequently undergoing oxidation to cerium dioxide.

(45) 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, -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 -hydroxy 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.

(46) 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.

(47) 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.

(48) 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.

(49) 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 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.

(50) 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).

(51) 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.

(52) Glycol ether surfactants that contain both an ether group and an alcohol group includes compounds of formula (Ic), in which R.sup.3 represents a substituted or unsubstituted alkyl group, and m is an integer of 1-8.
R.sup.3(OCH.sub.2CH.sub.2).sub.mOH(Ic)

(53) Other useful surfactants to effect the solvent shift include nonylphenyl ethoxylates having the formula, C.sub.9H.sub.19C.sub.6H.sub.4(OCH.sub.2CH.sub.2).sub.nOH, wherein n is 4-6.

(54) 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 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.

(55) 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.

(56) 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=I/E, 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 (5), 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.

(57) 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 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.

(58) The substantially homogeneous thermodynamic dispersion contains a minimal amount of water at a level of preferably no more than about 0.5 wt. %.

(59) 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.

(60) Desirably, the cerium dioxide nanoparticles have a primary crystallite size of about 2.5 nm0.5 nm and comprise one or at most two crystallites per particle edge length.

(61) 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(CH.sub.2CH.sub.2O).sub.nCHR.sup.1CO.sub.2Y(Ia)
wherein: R represents hydrogen or a substituted or unsubstituted alkyl group or a substituted or unsubstituted aromatic group; R.sup.1 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, R.sup.1 represents hydrogen, Y represents hydrogen, and n is 2.

(62) Another suitable nanoparticle stabilizer comprises a dicarboxylate of formula (Ib),
XO.sub.2C(CR.sup.2).sub.pCO.sub.2Z(Ib)
wherein each R.sup.2 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 for 2.

(63) Other useful nanoparticle stabilizers are included in the group consisting of lactic acid, gluconic acid enantiomers, EDTA, tartaric acid, citric acid, and combinations thereof.

(64) 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.

(65) 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),
R.sup.3(OCH.sub.2CH.sub.2).sub.mOH(Ic)
wherein R.sup.3 represents a substituted or unsubstituted alkyl group; and m is an integer from 1 to 8.

(66) The nonionic surfactant may also comprise a compound of formula (Id),
R.sup.3(OCH.sub.2CH.sub.2).sub.mOH(Id)
wherein R.sup.3 represents a substituted or unsubstituted alkyl group; is an aromatic group; and m is an integer from 4 to 6.

(67) The reaction mixture may further include a co-surfactant, preferably an alcohol.

(68) 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.

(69) 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.

(70) 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.

(71) 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, CeO.sub.2 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 (CeO.sub.2 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.

(72) 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.

(73) 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 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.

(74) 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.

(75) 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, C.sub.2-C.sub.8 alkyl, alkoxy and polyalkoxy substituted dicarboxylic acids are advantageous stabilizers.

(76) In accordance with the invention, nanoparticle stabilizer compounds preferably comprise organic carboxylic acids such as, for example, 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MESA) and ethylenediaminetetraacetic acid (EDTA), lactic acid, gluconic acid, tartaric acid, citric acid, and mixtures thereof.

(77) 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, 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.

(78) 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.

(79) 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.

(80) The invention is further illustrated by the following examples, which are not intended to limit the invention in any manner.

Example 1. Preparation of Cerium Dioxide Nanoparticles by Single-Jet Addition

(81) To a 3 liter round bottom stainless steel reactor vessel was added 1.267 liters of distilled water, followed by 100 ml of Ce(NO.sub.3).sub.3.6H.sub.2O solution (600 gm/liter Ce(NO.sub.3).sub.3.6H.sub.2O). 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 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% H.sub.2O.sub.2 was added to the reactor vessel. Then 16 ml of 28%-30% NH.sub.4OH, 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 NH.sub.4OH 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 CeO.sub.2 particles.

(82) The cerium dioxide particles were collected, the excess solvent evaporated off, and the gravimetric yield, corrected for the weight of MESA, was determined to be 62.9%.

(83) 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.

(84) 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 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*lambda/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 2.50.5 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.

Example 2. Precipitation of 1.5 nm CeO2 Nanoparticles

(85) 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 nm. FIG. 3B represents the electron diffraction pattern of a representative sample of the precipitation. FIG. 3C contains Table I in which the intensities of the various diffractions rings {311}, {220}, {200} and {111} are analyzed within the framework of: cubic CeO2, cubic and hexagonal Ce.sub.2O.sub.3 and Ce(OH).sub.3. Clearly the percent deviations of analyzed ring intensity with crystal habit are minimal for the cubic fluorite structure of CeO.sub.2, thus establishing the existence of this polymorph down to this grain diameter.

Example 3. Preparation of CeO2 Nanoparticles by Isothermal Double-Jet Precipitation CeO2

(86) To a 3 liter round bottom stainless steel reactor vessel was added 1117 grams of distilled water. A standard Rv 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(NO.sub.3).sub.3.6H.sub.2O 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 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 CeO.sub.2 particles. Particle size-frequency analysis by transmission electron micrography (FIG. 4) revealed a mean particle size of 2.2 nm, with size frequency distribution having a coefficient of variation, COV, (one standard deviation divided by the mean diameter) of 23%. The calculated yield was 62.9%.

Example 4. Copper-Containing CeO2 Nanoparticles Ce0.9Cu0.1O1.95

(87) 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 Ce(NO.sub.3).sub.3.2.5H.sub.2O. 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 CeO.sub.2 lattice but instead existed as a separate Cu.sub.2O.sub.3 population.

Example 5. Iron-Containing CeO2 Nanoparticles Ce0.9Fe0.1O1.95 (CeO-255)

(88) 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(NO.sub.3).sub.3.9H.sub.2O. 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%.

Example 6. Zirconium-Containing CeO2 Nanoparticles Ce0.9Zr0.15O2 (CeO 257)

(89) 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(NO.sub.3).sub.2.6H.sub.2O. 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 Ce.sub.0.82Zr.sub.0.18O.sub.1.91, which given the relative insolubility of ZrO.sub.2 to CeO.sub.2, would account for the enhanced Zr content (18% vs 15%).

Example 7a. Zirconium- and Iron-Containing CeO2 Nanoparticles Ce0.9Zr0.15Fe0.1O1.95 (CeO-270)

(90) 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(NO.sub.3).sub.3.9H.sub.2O and 12.76 grams of ZrO(NO.sub.3).sub.2.6H.sub.2O. 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 ZrO.sub.2 and Fe.sub.2O.sub.3 grain populations. The calculated yield was 78%. Inductively coupled plasma atomic emission spectroscopy revealed a stoichiometry of Ce.sub.0.69Fe.sub.0.14Zr.sub.0.17O.sub.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 CeO.sub.2. The lack of a peak (denoted by an arrow) at 32 deg two theta means that there is no free ZrO.sub.2, i.e., it is all incorporated into the cerium lattice. Also, the lack of peaks at 50 and 52 degrees two theta indicate no separate population of Fe.sub.2O.sub.3 (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(n/2d=sin ) which is consistent with the smaller ionic radii of Fe.sup.3+ (0.78 A) and Zr.sup.4+ (0.84 A) relative to the Ce.sup.4+ (0.97 A) which it is replacing. Thus, we conclude that the transition metals are incorporated into the CeO.sub.2 lattice and do not represent a separate population of neat ZrO.sub.2 or Fe.sub.2O.sub.3 nanoparticles. The unimodal size-frequency distribution also supports this conclusion.

Examples 7b-f Zirconium- and Iron Containing CeO2 Nanoparticles Varying Systematically in the Amount of Iron (15%, 20%, 25%, 30%) at 15% Zirconium and 20% Iron at 20% Zirconium

(91) 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.

(92) 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.

(93) 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.

(94) The accompanying TABLE 2 contains the Oxygen Storage Capacity (1 sigma reproducibility in parenthesis) and the fast (k1) 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% H.sub.2. 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-dependent in the range of about 2 nm-20 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 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.

(95) TABLE-US-00001 TABLE 2 Comparison of OSC for Cerium Dioxide Nanoparticle Variations OSC Reduction Reduction (moles/g) Rate constant Rate constant (Std. Dev. k1 10.sup.3 k2 10.sup.3 Sample moles/g) (/min) (std dev.) (/min) (std dev.) Sigma Aldrich 296 (1.65) CeO.sub.2 (20 m) CeO.sub.2 (2 nm) 349 CeFe.sub.0.10O.sub.2 470 (1% surf) CeZr.sub.0.15O.sub.2 592 (3) CeZr.sub.0.15Fe.sub.0.10O.sub.2 1122 (3) 3.1 (0.4) 0.9 (0.15) CeZr.sub.0.15Fe.sub.0.15O.sub.2 1359 (33) 5.9 (0.04) 2.0 (0.2) CeZr.sub.0.15Fe.sub.0.20O.sub.2 1653 (6) 3.4 (0.4) 1.1 (0.3) CeZr.sub.0.15Fe.sub.0.25O.sub.2 2013 (1) 3.1 (0.4) 1.1 (0.2) CeZr.sub.0.15Fe.sub.0.30O.sub.2 2370 (4) 2.6 (0.1) 1.0 (0.1) CeZr.sub.0.20Fe.sub.0.20O.sub.2 1661 (7) 4.9 (1.3) 1.2 (0.2) CeZr.sub.0.20Fe.sub.0.20O.sub.2 1636 (1) 9.5 (0.6) 3.9 (0.2) citric acid

(96) 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.