WO2008002323A2

WO2008002323A2 - Production of cerium containing metal oxide grains

Info

Publication number: WO2008002323A2
Application number: PCT/US2006/048143
Authority: WO
Inventors: David Wallace Sandford; Thomas Nelson Blanton
Original assignee: Eastman Kodak Company
Priority date: 2005-12-23
Filing date: 2006-12-18
Publication date: 2008-01-03
Also published as: WO2008002323A3

Abstract

An aqueous precipitation process for the preparation of metal oxide particles, comprising adding a cerium +3 nitrate salt solution and a base together under turbulent mixing conditions in a precipitation reactor, where cerium +3 ions are oxidized to cerium +4 and precipitated metal oxide particles are directly obtained without need for a calcination step, wherein the improvement comprises adding acetic acid to the precipitation in an amount of from 1 to 40 molar percent, relative to the molar amount of cerium, and obtaining stable, substantially non-agglomerated nanometer size dispersed metal oxide particles.

Description

PRODUCTION OF CERIUM CONTAINING METAL OXTOE GRAINS

FIELD OF THE INVENTION

The present invention relates to the production of stable dispersions of nanometer size cerium containing metal oxide particles with uniform size and morphology produced by aqueous precipitation methods.

BACKGROUND OF THE INVENTION

There are many synthetic methods for the production of high surface area simple metal oxides. More common among them are: aqueous or organic co-precipitation, spray co-precipitation, chemical vapor deposition (Bai, Wei; Choy, K. L.; Stelzer, N. H. J.; Schoonman, J. Solid State Ionics (1999), 116(3,4), 225-228), plasmas (laser, microwave, radio frequency, electric arc; see for example US 6,669,823 Sarkas et al.) and hydrothermal (aqueous solutions at around 80° C) techniques. The precipitation techniques often involve the subsequent step of calcination, i.e. heating the material to very high temperatures, usually 500-900° C. Another viable synthetic route is the polyol-mediated preparation of nanoscale oxides in which a suitable metal precursor (acetate, halogenide, or alcoholate) is dissolved in high boiling point (246° C) diethylene glycol, DEG, and rapidly heated to between 180 and 240° C until a precipitate forms. The DEG acts as a chelating agent to stabilize the resultant particles and restrain their growth (Feldmann, C, Adv. Fund. Mater. (2003) 13 101-107).

Cerium dioxide (CeO₂, i.e. ceria) based metal oxide particulate materials demonstrate a broad spectrum of applications such as: catalysts for hydrocarbon fuel combustion, three-way catalysts for automotive exhaust gas treatment (catalytic converters), anode materials for solid oxide fuel cells, oxygen storage capacitors, fast ion conductors and ultra-violet radiation blockers. In addition ceria is widely used as a polishing agent for glass, quartz or silicon materials, which are used to make optical elements such as lenses, or hard disk data drives. Given the range of uses for ceria it is not surprising that there are a number of different methods used for its preparation. Cerium is a unique rare earth element for which the dioxide is the normal stable oxide phase; contrary to the other lanthanide rare earths for which Ln₂O₃ is the normal stoichiometry (the other exceptions being Pr₆On and Tb₄O₇). Ceria exhibits the cubic CaF₂ fluorite structure consisting of a network of Ce ions in a face centered cubic array leading to interior octahedral and tetrahedral holes, the latter of which are occupied by oxyanions. When some of these lattice sites are not occupied by the stoichiometrically required number of oxygen ions, oxygen vacancies are formed, resulting in CeO_2-X- This tendency to form oxygen vacancies gives rise to many of ceria' s useful properties, most notably its oxygen storage function (OSF) in which ceria either donates or withdraws oxygen from an environment in response to a shortage or excess of oxygen. The OSF of ceria can be succinctly summarized by the following chemical equation:

Equation 1: 2 CeO₂ <~> Ce₂O₃ + Vi O₂

The OSF enables ceria to act as an oxidation/reduction (redox) buffer, which is especially important in its catalytic applications.

For application as a fuel borne catalyst (e.g., as described in WO 03/040270, US 2003/0154646 and references therein) highly dispersed, small particles are critical for optimal performance and to avoid settling. In terms of particle size metrics, this corresponds to producing both a small primary crystallite size and a small grain size, along with a narrow grain size distribution, as described below.

In discussing ceria particle morphology and metrology it is important to clearly understand the definitions of some elementary and widely used terms. By primary crystallite size one refers to that size which is commonly determined by X-Ray Powder Diffraction (XRPD). A wider XRPD line width implies a smaller primary crystallite size. Quantitatively, the crystallite size (t) is calculated from the measured X-ray peak half width (B(radians)), the wavelength of the X-ray (λ), and the diffraction angle (θ) using the Scherrer equation

Equation 2. t = 0.9 λ/( B cos θ ) See B.D. Cullity, "Elements of X-Ray Diffraction" (1956) Addison- Wesley Publishing Company, Inc., Chapter 9.

A crystallite itself is typically composed of many unit cells, which is the most irreducible representation of the crystal structure. The primary crystallite size should not be confused with the final grain size. The final grain size is determined by how many of the crystallites agglomerate. Typically the grain size measurement is determined from dynamic light scattering experiments (provided by Honeywell Microtrac UPA- Ultrafine Particle Analyzer, or Zetasizer Nano ZS from Malvern Instruments). This also affords the size frequency distribution of the grains, the more uniform distributions being preferred. It is important to make the distinction that having a small primary crystallite size does not guarantee a small final grain size - this must be measured separately from the XRPD spectrum. However, a large primary crystallite size will preclude a small final grain size. Thus to fully characterize a particulate dispersion one would need a knowledge of final grain size (UPA or Zetasizer), size-frequency distribution (UPA or Zetasizer) and primary crystallite size (XRPD).

Many of the conventional methods known in the art for preparing small particles or high surface area samples of metal oxides, employ precipitation or coprecipitation processes to prepare precursor materials, which are then converted to the oxides. Typical among these are aqueous acid-base neutralizations or ion exchange reactions of soluble metal nitrates, carbonates, and halides to form metal hydroxides. These metal or mixed metal hydroxides are converted to the oxides in a calcination step in which the material is heated to a range of 200 to 1000° C, or more typically 400 to 800° C for several hours.

Disadvantages of this process include 1) its expense in time and energy cost, 2) gaseous byproducts can be toxic or corrosive, 3) furnace environments can lead to contamination, 4) particles can agglomerate, which requires subsequent milling or grinding steps that often broaden the final grain size distribution. For these reasons, precipitation processes that produce metal oxides directly, without requiring calcination, may be preferred. Precipitation processes for the direct preparation of metal oxide particles without need for a calcination step are also known. Aqueous co- precipitation methods for the production of ceria are described, e.g., in US 5,389,352, US 5,938,837, and US2003/0215378. The basic precipitation process involves adding a cerium (+3) salt and a base such as ammonium hydroxide together under turbulent mixing conditions in a reactor in the presence of an oxidant, and converting the cerium (III) into a CeO₂ precipitate. In the absence of an intentionally added separate oxidant such as hydrogen peroxide, the nitrate ion of a cerous nitrate salt may perform the required oxidation function, although at higher temperatures than are typically required in the presence of added peroxide. The resultant solution can then be washed (e.g. ultrafiltration) to remove unwanted salts and materials from the ceria dispersion.

US 5,389,352 (Wang) discloses the precipitation of CeO₂ from an aqueous solution comprised of a water soluble trivalent cerium salt and an oxidizing agent wherein the solution is aged for a time not less than 4 hours, where no subsequent calcination or milling steps are required to produce materials for polishing applications. Redox reactions that result in the evolution of colorless gases (hydrogen and/or oxygen) are disclosed as part of the reaction mechanism. Examples employ reaction of cerous nitrate and ammonia either at elevated temperatures (above 100° C) and in a closed reaction container, or at room temperature and with the addition of hydrogen peroxide. High yields of crystalline CeO₂ are reported to be produced in elevated temperature examples, while no yield data is reported for the example at room temperature. An average crystallite size of 7 nm is indicated in the room temperature hydrogen peroxide example, but no final agglomerated grain size nor grain size distribution are reported.

US 5,938,837 (Hanawa) also discloses the precipitation OfCeO₂ from an aqueous solution based on reaction of cerous nitrate and ammonia. A pH range of between 5 and 10 (preferably between 7 and 9) along with a carefully timed temperature ramp up to 70-100° C within 10 minutes of initial mixing of reactants are employed, in combination with a maturation time for the reaction of between 0.2 to 20 hours, in order to achieve a direct aqueous precipitated cerium oxide with a desired single crystal grain size aim in the range 10 to 80 nm. Primary crystallite sizes determined from X-Ray Diffraction peak widths at half peak height (20 nm) are reported as being approximately consistent with particle (crystallite) sizes observed by Transmission Electron Microscopy (TEM), which TEM image was made after a deagglomeration step. No data is given with respect to actual grain size in solution prior to deagglomeration. Fig. 2 of US 5,938,837 itself reveals that the crystallites are still substantially agglomerated as clumps of crystallites even after a deagglomeration step. Therefore it is clear that the process is inadequate in regard to dispersibility of the crystallites.

US 2003/0215378 (Zhou et. al.) discloses another aqueous precipitation process for formation of cerium dioxide particles, with or without lanthanide metal (Zr, Y, Nb, Sm) dopants. A double jet addition (cerium nitrate vs. ammonium hydroxide in the range 0.1 to 1.5 mol/1) at a rate of 0.5 to 10 ml/min to a reactor with good impeller stirring (100-5000 rpm) followed by heating, affords single crystal particles which are claimed to be uniform in size and shape and smaller than 10 nm in size. Alternatively, the cerium nitrate salt may be added to a reactor already containing ammonia to produce what are claimed to be 3 nm size particles. Gaseous oxygen may be passed through the reactant solution. Particle size measurements were obtained from TEM samples prepared by ultrasonically dispersing the powders in ethanol, and confirmed to represent single crystal particle sizes by X-ray diffraction patterns, rather than aggregated precipitated grain size prior to ultrasonic dispersion. No data is given with respect to actual grain size in solution prior to ultrasonic dispersion. WO 00/48939 (Pickering) describes a precipitation process in which an oxidant is added to a solution of a metal capable of existing in two cationic oxidation states under conditions such that mixing of the solution and oxidant is substantially complete before precipitation of an oxide of the metal in its higher oxidation state occurs. A cerium nitrate solution maybe combined first with hydrogen peroxide and then with ammonium hydroxide, e.g., to undergo a homogeneous precipitation to produce a precipitate of the formula Ce(OH)₄. yOOHy. This washed and dried precipitate can suhsequently be hydrothermally treated in the temperature range 100 to 300° C to produce the metal oxide as a weakly agglomerated nanocrystalline powder. US 6,133,194 (Cuif et al.) suggests the use of anionic surfactants, non-ionic surfactants, polyethylene glycols, carboxylic acids, and carboxylate salts as additives in an aqueous precipitation or co-precipitation process involving cerium solutions, zirconium solutions, base and optionally an oxidizing agent, wherein the presence of the additive increases surface area measured after calcination of precipitated mixed hydroxides, such as (Ce₃Zr)(OH)-J. Two sets of time (2 and 6 hours) and temperature (500° C and 900° C, respectively) conditions are specified for the subsequent calcination step. For each specific additive class there is a preferred concentration range. While the presence of the additive during the reaction was shown to afford enhanced specific surface area for subsequently calcined materials, it is doubtful that the organic additive was still present after calcinations. There is no suggestion for use of any additives in a direct oxide precipitation process, such as those disclosed by Wang (US 5,389,352), Hanawa (US 5,938,837), and Zhou et. al. (US 2003/0215378)

WO 03/040270 (Wakefield) discloses mixed metal cerium oxide (Ce₁-XM_xO₂) used as a fuel additive, which has been doped with divalent or trivalent metals or metalloids that are transition metals or else a metal from group ILA, IIIB, VB or VIB of the Periodic Table (e.g. Rh, Cu, Ag, Au, Pd, Pt, Fe, Mn, Cr, Co, V). Rare earth oxide combinations of these cerium oxides are also claimed. The fuel additive composition is in a polar or non-polar organic solvent (aliphatic or aromatic hydrocarbon or aliphatic alcohol) and contains the above oxide coated with an organic acid (oleic acid), anhydride (dodecylsuccinic anhydride), or ester or a Lewis base additive. Other additives to the fuel composition can comprise: detergents, dehazers, anti-foamants, ignition improvers, anti-rust agents, deodorants, anti-oxidants, metal deactivators or lubricity agents. SUMMARY OF THE INVENTION

In accordance with one embodiment, the present invention is directed towards an aqueous precipitation process for the preparation of metal oxide particles, comprising adding a cerium +3 nitrate salt solution and a base together under turbulent mixing conditions in a precipitation reactor, where cerium +3 ions are oxidized to cerium +4 and precipitated metal oxide particles are directly obtained without need for a calcination step, wherein the improvement comprises adding acetic acid to the precipitation reactor in an amount of from 1 to 40 molar percent, relative to the molar amount of cerium, and obtaining stable, substantially non-agglomerated nanometer size dispersed metal oxide particles.

The present invention provides a facile and rapid method of production of stable dispersions of nanometer grain size cerium containing metal oxide particles with uniform morphology and size produced by aqueous precipitation methods well adapted to large-scale commercial production. The directly obtained precipitated nano-sized grains are stabilized against aggregation by an acetic acid additive, resulting in a minimally agglomerated and stable dispersion in aqueous media, avoiding the additional and potentially complicating steps of calcination (which may lead to agglomeration), milling or grinding.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the process of the present invention, a cerium +3 nitrate salt solution and a base are added together under turbulent mixing conditions in a precipitation reactor, where cerium +3 ions are oxidized to cerium +4 and precipitated metal oxide particles are directly obtained. Temperature and pH are maintained in the reactor so as to enable precipitated metal oxide particles to be directly obtained without need for a subsequent calcination step. In preferred processes, the cerium +3 nitrate salt solution and base are added together in the reactor and maintained at a temperature below the boiling point of water at ambient pressure to obtain the metal oxide particles. A specific preferred process employs the hexahydrate of cerous nitrate, with the pH of the reaction immediately following the complete addition of reactants being between 5 and 10. As described in US 5,938,837, any known base may be used in reactions with cerous nitrate to form precipitated cerium dioxide particles, but aqueous ammonia may be preferred over the use of hydroxides of alkali metals or alkaline earth metals if it is desired to avoid contamination of the final cerium dioxide with impurities such as the corresponding alkali metals or alkaline earth metals.

If an aqueous solution of cerous nitrate is maintained at relatively minor elevated temperatures (e.g., between 70° C and 100° C under ambient atmosphere) under either mildly acidic conditions or basic condition via the addition of ammonia, complexes of cerium hydroxynitrate Ce(OH)₂(Nθ₃)Η₂θ or (NH₄)₂Ce(NO₃)₅ -4HsO may be initially formed (depending upon whether cerous nitrate or ammonia is in excess). In either instance, as the intermediate complex material comprises Ce in the plus-three state, an additional oxidation step is required to obtain tetra-valent cerium and the resulting cerium dioxide particles. This oxidation step can be carried out by the nitrate anion accompanying the precursor cerium salt as specifically proposed below:

2NO₃ ^" + 4H⁺ + 2e^" -> N₂O₄ + 2H₂O

N₂O₄ ^■» 2 NO₂(g)

whereby the evolution of the brown gas, nitrogen dioxide (NO₂ ) is anticipated. While the use of a cerium nitrate salt solution provides nitrate ions which themselves function as oxidizing agents, additional oxidizing agents (e.g., hydrogen peroxide or gaseous oxygen) may also be added if desired to further facilitate the oxidation of the cerium +3 ions, and such added oxidizing agents may enable direct precipitation of metal oxide at lower temperatures (e.g., ambient room temperature). As described in US 5,389,352 and 5,938,837, reaction temperatures and maturation or aging times may be optimized to allow the oxidation reactions to go to completion. Once the tetra-valent cerium is formed, converted into Ceθ₂, and precipitated, the resulting precipitated particle dispersion may be subsequently subjected to the steps of filtration and drying. While previous cerium containing metal oxide particle precipitation processes have described obtaining nano-sized metal oxide primary crystallites, such previous processes typically also resulted in substantial aggregation of the precipitated primary crystallites such that the actual precipitated grain sizes were larger than may be desired for some applications. In accordance with the present invention, it has been found that performing the precipitation of cerium containing metal oxide particles in the reactor in the presence of acetic acid enables obtaining stable, substantially non-agglomerated nanometer size (e.g., less than 100 nm) dispersed metal oxide particles, of a size smaller than that obtained in the absence of the acetic acid. Acetic acid is added in an amount of from 1 to 40, more preferably 2 to 25 molar percent, relative to the molar amount of cerium, although other concentrations may also be preferred depending upon optimized precipitation reaction conditions. In various embodiments of the invention, the acetic acid may be added to the reactor before, during or after addition of the cerium nitrate salt solution and/or base to the precipitation reactor. In preferred embodiments, the acetic acid is present during the precipitation reaction.

Cuif.et al. US6133194 describes precipitation of cerium and zirconium hydroxides, and subsequent calcining to obtain metal oxides, wherein a variety of additives, including carboxylic acids, are suggested for use during the formation of the oxides or precursors thereof in order to increase the surface area of the precipitated material after it is calcined. There is no teaching on the impact of any of such additives upon either the primary crystallite size, grain size or grain size distribution of aqueously precipitated material prior to calcining. We have investigated the impact of a variety of carboxylic acids and carboxylate salts upon the stability of aqueous dispersions of cerium containing oxides directly precipitated without the need for calcining (embodied in the grain size measurement derived from dynamic light scattering measurements of the resulting precipitates), and have discovered that acetic acid surprisingly provides reduced grain sizes in accordance with the direct metal oxide precipitation process of the invention, while other carboxylic acids suggested and actually employed in the hydroxide precipitation and calcinations process of Cuif et al were not effective in direct metal oxide precipitation processes.

In accordance with preferred embodiments, the use of acetic acid additive in the precipitation process of the present invention enables stable, substantially non-agglomerated nanometer size dispersed precipitated metal oxide particles to be obtained that have an average crystallite size of less than 20 nm (more preferably less than 15 nm) and an average aggregated grain size of less than 40 nm (more preferably less than 30 nm, and even more preferably less than 25 nm). Crystallite size refers to that size which is commonly determined by X-Ray Powder Diffraction (XRPD), while aggregated grain size refers to grain size measurement derived from dynamic light scattering analysis of the precipitated particles. Ultra- fine cerium containing metal oxide particles produced in accordance with the invention are also relatively uniform in both shape and size, that is to say the size frequency distribution is narrow, hi preferred embodiments of the invention, nanometer size dispersed precipitated metal oxide particles may be obtained with the desired very fine average grain sizes note above, as well with the substantial absence of significantly larger grains (e.g., with 99% of the particles having a size of less than 100 nm). Advantageously, the area weighted size frequency distribution of the grains may have less than a 25% coefficient of variation. Once formed in an aqueous precipitation process in accordance with the invention, the resulting ultra-fine cerium containing metal oxide particles may be conveniently re- dispersed in an organic solvent for use as a fuel additive similarly as described in WO 03/040270 and US 2003/0154646 referenced above. Turbulent mixing conditions may be obtained in the precipitation reactor by means of conventional stirrers and impellers such as disclosed, e.g., in Zhou et al. US 2003/0215378 referenced above. In a specific embodiment of the invention, the reactants are preferably contacted in a highly agitated zone of a precipitation reactor. Preferred mixing apparatus which may be used in accordance with such embodiment includes rotary agitators of the type which have been previously disclosed for use in the photographic silver halide emulsion art for precipitating silver halide particles by reaction of simultaneously introduced silver and halide salt solution feed streams. Such rotary agitators may include, e.g., turbines, marine propellers, discs, and other mixing impellers known in the art (see, e.g., U.S. 3,415,650; U.S.6,513,965, U.S. 6,422,736; U.S. 5,690,428, U.S. 5,334,359, U.S. 4,289,733; U.S. 5,096,690; U.S. 4,666,669, EP 1156875, WO-Ol 60511).

While the specific configurations of the rotary agitators which may be employed in preferred embodiments of the invention may vary significantly, they preferably will each employ at least one impeller having a surface and a diameter, which impeller is effective in creating a highly agitated zone in the vicinity of the agitator. The term "highly agitated zone" describes a zone in the close proximity of the agitator within which a significant fraction of the power provided for mixing is dissipated by the material flow. Typically it is contained within a distance of one impeller diameter from a rotary impeller surface. Introduction of a reactant feed stream into a precipitation reactor in close proximity to a rotary mixer, such that the feed stream is introduced into a relatively highly agitated zone created by the action of the rotary agitator provides for accomplishing meso-, micro-, and macro-mixing of the feed stream components to practically useful degrees. Depending on the processing fluid properties and the dynamic time scales of transfer or transformation processes associated with the particular materials employed, the rotary agitator preferably employed may be selected to optimize meso-, micro-, and macro-mixing to varying practically useful degrees.

Mixing apparatus that may be employed in one particular embodiment of the invention includes mixing devices of the type disclosed in Research Disclosure, Vol. 382, February 1996, Item 38213. In such apparatus, means are provided for introducing feed streams from a remote source by conduits that terminate close to an adjacent inlet zone of the mixing device (less than one impeller diameter from the surface of the mixer impeller). To facilitate mixing of multiple feed streams, they may be introduced in opposing direction in the vicinity of the inlet zone of the mixing device. The mixing device is vertically disposed in a reaction vessel, and attached to the end of a shaft driven at high speed hy a suitable means, such as a motor. The lower end of the rotating mixing device is spaced up from the bottom of the reaction vessel, but beneath the surface of the fluid contained within the vessel. Baffles, sufficient in number to inhibit horizontal rotation of the contents of the vessel, may be located around the mixing device. Such mixing devices are also schematically depicted in US Pat. Nos. 5,549,879 and 6,048,683.

Mixing apparatus that may be employed in another embodiment of the invention includes mixers that facilitate separate control of feed stream dispersion (micromixing and mesomixing) and bulk circulation in the precipitation reactor (macromixing), such as descried in US Pat. No. 6,422,736. Such apparatus comprises a vertically oriented draft tube, a bottom impeller positioned in the draft tube, and a top impeller positioned in the draft tube above the first impeller and spaced therefrom a distance sufficient for independent operation. The bottom impeller is preferably a flat blade turbine (FBT) and is used to efficiently disperse the feed streams, which are added at the bottom of the draft tube. The top impeller is preferably a pitched blade turbine (PBT) and is used to circulate the bulk fluid through the draft tube in an upward direction providing a narrow circulation time distribution through the reaction zone. Appropriate baffling may be used. The two impellers are placed at a distance such that independent operation is obtained. This independent operation and the simplicity of its geometry are features that make this mixer well suited in the scale-up of precipitation processes. Such apparatus provides intense micromixing, that is, it provides very high power dissipation in the region of feed stream introduction. The invention is described primarily with respect to precipitation of cerium dioxide particles, however, it is also applicable to the precipitation of doped ceria wherein some fraction of the eerie ions is replaced by other metal ions. Possible dopants include, e.g., first row transition metals such as chromium, manganese, iron and cobalt, and lanthanide metals such as lanthanum, neodymium, samarium and gadolinium. The fraction of eerie ions replaced by the dopant can be as high as 50% but is typically no more than 25%. The presence of the dopant can usually be confirmed by a shift in the X-ray diffraction peaks for ceria indicating the substitution of an ion of different size than the eerie ion. For cases in which the dopant is of similar size as the eerie ion, other approaches need to be used to confirm the presence of the dopant. In some cases the dopant is of lesser charge than the eerie ion but this appears to be accommodated by altering the oxygen content of the ceria. For example, substitution of a 3+ ion for the eerie ion (4+) results in a product with the following formulation (wherein M is the 3+ dopant):

The invention is further illustrated by the following examples.

Example 1: Comparative, no stabilizer

A six-liter stainless steel sponge kettle was charged with 2L of distilled water and the temperature controlled at 80° C. A planar mixing device . previously described (Research Disclosure 38213, February 1996 ppl 11-114 "Mixer for Improved Control Over Reaction Environment") operating at 3000 rpm was used to ensure the homogeneity of the reactor contents. To this reactor 4OmL of a 28-30% solution OfNH₄OH was added. The resultant pH was 10.1. A peristaltic pump was used to deliver a solution containing 15Og of Ce(Nθ₃)₃-6H₂θ (0.34 moles) diluted to 25OmL with distilled water at a rate of 400mL/min. The measured pH was 6.3 and the solution was grayish/brown. The reaction was held at 80° C for 90min during which time there was gas evolution and the solution turned white. The final pH was 2.5. The final product was washed to a conductivity of <3mS and a portion was dried at ambient temperature. Powder X- ray diffraction confirmed the product was single-phase cerium dioxide. The mean crystallite size was 12nm (XRPD data) and the mean grain size was 44nm with a grain size frequency distribution ranging from 18nm to 243nm (UPA data; 1% of grains are less than 18nm, 99% of grains less than 243nm). Example 2: Comparative, potassium formate stabilizer

A six-liter sponge kettle was charged with 2L of distilled water and the temperature controlled at 80° C. The reactor contents were mixed as described in Example 1. To this reactor 42.5mL of a 28-30% solution OfNH₄OH and 3.7 grams (0.044 moles) of potassium formate were added (additive approximately 12.5 molar % relative to moles of cerium to be added). A peristaltic pump was used to deliver a solution containing 150g of CeQMOsVόH_∑O diluted to 25OmL with distilled water at a rate of 400mL/min. The reaction was held at 80° C for 90min during which time there was gas evolution and the solution turned white. The final pH was 2.88. The final product was washed to a conductivity of <3mS. The mean grain size from UPA after washing was 64nm with a grain size frequency distribution ranging from 36nm to 204nm (UPA data; 1% of grains are less than 36nm, 99% of grains less than 204nm).

Example 3: Comparative, sodium butyrate stabilizer

A six-liter sponge kettle was charged with 2L of distilled water and the temperature controlled at 80° C. The reactor contents were mixed as described in Example 1. To this reactor 42.5mL of a 28-30% solution OfNH₄OH and 4.8 grams (0.044 moles) of sodium butyrate were added. A peristaltic pump was used to deliver a solution containing 150g of Ce(NO₃)S-OH₂O diluted to 25OmL with distilled water at a rate of 400mL/min. The reaction was held at 80° C for 90min during which time there was gas evolution and a whitish precipitate was formed. However, the final product was foamy and malodorous, and particle size was accordingly not measured.

Example 4: Comparative, malonic acid stabilizer

A six-liter sponge kettle was charged with 2L of distilled water and the temperature controlled at 80° C. The reactor contents were mixed as described in Example 1. To this reactor 42.5mL of a 28-30% solution OfNH₄OH and amounts of malonic acid ranging from lower to higher than 0.044 moles were added. Subsequently, in each case, a peristaltic pump was used to deliver a solution containing 150g of Ce(NC>3)₃-6H₂θ diluted to 25OmL with distilled water at a rate of 400mL/min. The reaction was held at 80° C for 90min during which time there was gas evolution. At the lowest amount of malonic acid, the final pH was 3.42 and the reaction product was tan in color. The final product was washed to a conductivity of <3mS. The mean grain size from UPA after washing was about 80nm with a grain size frequency distribution ranging from 43nm to 289nm (1% of grains are less than 43nm, 99% of grains less than 289nm). At higher levels of malonic acid, the reaction products were purple in color indicative of incomplete reaction.

Example 5: Comparative, malic acid stabilizer

A six-liter sponge kettle was charged with 2L of distilled water and the temperature controlled at 80° C. The reactor contents were mixed as described in Example 1. To this reactor 42.5mL of a 28-30% solution OfNH₄OH and amounts of malic acid ranging from lower to higher than 0.044 moles were added. Subsequently, in each case, a peristaltic pump was used to deliver a solution containing 15Og of Ce(NOa)₃-OHaO diluted to 25OmL with distilled water at a rate of 400mL/min. The reaction was held at 80° C for 90min during which time there was gas evolution. At the lowest amount of malic acid, the final pH was 4.1 and the reaction product was purple in color, indicative of incomplete reaction. The final product was washed to a conductivity of <3mS. The mean grain size from UPA after washing was about 75nm with a grain size frequency distribution ranging from 43 nm to 486nm (1% of grains are less than 43 nm, 99% of grains less than 486nm). At higher levels of malic acid, the reaction products were again purple in color.

Example 6: Comparative, EDTA stabilizer

A six-liter sponge kettle was charged with 2L of distilled water and the temperature controlled at 80° C. The reactor contents were mixed as described in Example 1. To this reactor 42.5mL of a 28-30% solution OfNH₄OH and amounts of ethylenediaminetetraacetic acid (EDTA) ranging from lower to higher than 0.044 moles were added. Subsequently, in each case, a peristaltic pump was used to deliver a solution containing 150g of Ce(NOs)S-OH₂O diluted to 25OmL with distilled water at a rate of 400mL/min. The reaction was held at 80° C for 90min during which time there was gas evolution. At the lowest amount of EDTA, the final pH was about 4.5 and the reaction product was purple in color, indicative of incomplete reaction. The final product was washed to a conductivity of <3mS. The mean grain size from UPA after washing was about 200nm with a grain size frequency distribution ranging from 36nm to 1635nm (1% of grains are less than 36nm, 99% of grains less than 1635nm). At higher levels of EDTA, the reaction products were again purple in color.

Example 7: Comparative, citric acid stabilizer

A six-liter sponge kettle was charged with 2L of distilled water and the temperature controlled at 80° C. The reactor contents were mixed as described in Example 1. To this reactor 42.5mL of a 28-30% solution OfNH₄OH and amounts of citric acid (a tricarboxylic acid, HO₂CCH₂C(OH)(CO₂H)CH₂CO₂H) ranging from lower to higher than 0.044 moles were added. Subsequently, in each case, a peristaltic pump was used to deliver a solution containing 15Og of Ce(NO₃)₃-6H₂O diluted to 25OmL with distilled water at a rate of 400mL/min. The reaction was held at 80° C for 90min during which time there was gas evolution. At the lowest amount of citric acid, the final pH was about 5 and the reaction product was tan in color. The mean grain size from UPA after washing was about greater than lOOOnm, with a grain size frequency distribution ranging from 102nm to 6500nm (1% of grains are less than 102nm, 99% of grains less than 6500nm). The reaction yield appeared to be low and XRD measurements showed the product to be somewhat amorphous.

Example 8: Comparative, lauric acid stabilizer

A six-liter sponge kettle was charged with 2L of distilled water and the temperature controlled at 80° C. The reactor contents were mixed as described in Example 1. To this reactor 42.5mL of a 28-30% solution OfNH₄OH and lauric acid were added. A peristaltic pump was used to deliver a solution containing 15Og of Ce(NO₃)₃-6H₂O diluted to 25OmL with distilled water at a rate of 400mL/min. A very thick gray product with a final pH of 6.4 was formed. Failure of the pH to decrease below 6.4 indicates the reaction failed to proceed very far toward completion. The final product was so viscous no attempt was made to measure grain size.

Example 9: Inventive, acetic acid stabilizer

A six-liter sponge kettle was charged with 2L of distilled water and the temperature controlled at 80° C. The reactor contents were mixed as described in Example 1. To this reactor 42.5mL of a 28-30% solution OfNH₄OH and 2.5mL (0.044 moles) of glacial acetic acid were added (acetic acid approximately 12.5 molar % relative to moles of cerium to be added). The resultant pH was 9.1. A peristaltic pump was used to deliver a solution containing 15Og of Ce(NOs)₃-OHaO diluted to 25OmL with distilled water at a rate of 400mL/min. The measured pH was 6.2 and the solution was grayish/brown. The reaction was held at 80° C for 90min during which time there was gas evolution and the solution turned white. The final pH was 3.0. The final product was washed to a conductivity of <3mS and a portion was dried at ambient temperature. Powder X-ray diffraction confirmed the product was single-phase cerium dioxide. The mean crystallite size was 1 lnm (XRPD data) and the mean grain size after washing was 14nm with a grain size frequency distribution ranging from 9nm to 36nm (UPA data; 1% of grains are less than 9nm, 99% of grains less than 36nm). B.E.T. surface area was determined to be 102 m²/g. High resolution TEM analysis was consistent with the presence of very small crystallites.

Example 10: Inventive, same as Example 9 but the cerium nitrate is in the kettle and the base is pumped

The procedure from Example 9 was repeated except the NH₄OH solution was pumped into a kettle that contained the Ce(NOs)S-OH₂O solution. The mean crystallite size was 14nm and the mean grain size was 25nm with a grain size frequency distribution ranging from 15nm to 61nm (1% of grains are less than 15nm, 99% of grains less than 6 lnm).

Example 11: Inventive, same as Example 9 but the cerium nitrate and the base are added by double jet addition

The procedure from Example 9 was repeated except the NH₄OH solution and the Ce(Nθ3)₃-6H₂θ solution were added by double jet addition. The mean crystallite size was 14nm and the mean grain size was 25nm with a grain size frequency distribution ranging from 15nm to 61nm (1 % of grains are less than 15nm, 99% of grains less than 61nm).

Example 12: Inventive, Same as Example 9 with less (2/5x) acetic acid stabilizer The procedure from Example 9 was repeated but with ImL (0.017 moles) of glacial acetic acid. The mean crystallite size was lOntn and the mean grain size was 16nm with a grain size frequency distribution ranging from 1 lnm to 36nm (1% of grains are less than 1 lnm, 99% of grains less than 36nm).

Example 13: Comparative, Same as Example 9 with more (4x) acetic acid stabilizer

The procedure from Example 9 was repeated but with 1OmL (0.17 moles) of glacial acetic acid. The final pH was 5.8 and the solution stayed grayish/brown instead of turning white as in Example 9. Powder X-ray diffraction indicated the product was not single-phase cerium oxide. The mean crystallite size was 5nm and the mean grain size was lOOnm with a grain size frequency distribution ranging from 43nm to 243nm (1% of grains are less than 43nm, 99% of grains less than 243nm). Example 14: Inventive, Same as Example 9 with acetic acid stabilizer added after the 90^s hold

The procedure from Example 9 was repeated but with the glacial acetic acid added after the 90min hold, just prior to the washing step. The mean crystallite size was 12nm and the mean grain size was 35nm with a grain size frequency distribution ranging from 15nm to 243nm (1% of grains are less than 15nm, 99% of grains less than 243 nm).

Example 15: Inventive, Same as Example 9 with 2OmL NH₄OH The procedure from Example 9 was repeated but with 2OmL of a

28-30% solution OfNH₄OH. The final pH was 2.85 and the product yield was significantly less than in Example 9. The mean crystallite size was 9nm and the mean grain size was 13nm with a grain size frequency distribution ranging from 9nm to 26nm (1% of grains are less than 9nm, 99% of grains less than 26nm).

Example 16: Inventive, Same as Example 9 with 62.5mL NH4OH

The procedure from Example 9 was repeated but with 62.5mL of a 28-30% solution OfNH₄OH. The mean crystallite size was 12nm and the mean grain size was 18nm with a grain size frequency distribution ranging from 13nm to 43nm (1 % of grains are less than 13nm, 99% of grains less than 43 nm).

Example 17: Inventive, Same as Example 9 but with prop mixing

The procedure from Example 9 was repeated but with pitched blade turbine mixing. The mean crystallite size was 1 lnm and the mean grain size was 16nm with a grain size frequency distribution ranging from 1 lnm to 5 lnm (1% of grains are less than 1 lnm, 99% of grains less than 5 lnm).

Particle size results for the ceria materials precipitated in Examples 1-17 are summarized in Table 1 below. Table 1.

Grain Size

Crystallite Grain Freq. Dist.

Example Additive Feature Size Size Range Comment

1 None 12 nm 44 nm 18-243 nm Comparison

2 Potassium formate Ix level* - 64 nm 36-204 nm Comparison

3 Sodium butyrate Ix level - ** - Comparison

4 Malonic acid Lo/Hi levels - ~80 nm 43-289 nm Comparison

5 Malic acid Lo/Hi levels - ~75 nm 43-486 nm Comparison

6 EDTA Lo/Hi levels - 200 nm 36-1635 nm Comparison

7 Citric acid Lo/Hi levels - 1000+ nm 102-6500 nm Comparison

8 Laurie acid Lo/Hi levels - ♦♦* - Comparison

9 Acetic acid Ix level 11 nm 14 nm 9-36 nm Invention

Ce(NO₃)₃ in

10 Acetic acid kettle 14 nm 25 nm 15-61 nm Invention

11 Acetic acid Double Jet 14 nm 25 nm 15-61 nm Invention

12 Acetic acid 2/5x level 10 nm 16 nm l l-36 nm Invention

13 Acetic acid 4x level 5 nm 100 nm 43-243 nm Comparison

14 Acetic acid In Wash 12 nm 35 nm 15-243 nm Invention

15 Acetic acid LOwNH₄OH 9 nm 13 nm 9-26 nm Invention

16 Acetic acid High NU₁OH 12 nm 18 nm 13-43 nm Invention

17 Acetic acid Prop. Mixer 11 nm 16 nm 11-51 nm Invention

* 12.5 molar% relative to cerium

** Final product foamy and malodorous; grain size not measured

*** Reaction did not appear to go to completion; grain size not measured

Examples 9-12 and 14-17 employing acetic acid in accordance with the invention demonstrated a grain size reduction in comparison to Examples 1-8 employing no or other additives, as well as Example 13 employing acetic acid at approximately 50 molar% relative to cerium. Examples 9-12 and 15-17, wherein acetic acid was present during the precipitation reaction, demonstrated particularly advantageous results (average aggregated grain sizes of less than 30 nm, with 99% of the particles having a size of less than 100 nm). Example 18: Comparative, La doped ceria without a stabilizer

A six-liter sponge kettle was charged with 2L of distilled water and the temperature controlled at 40° C. The reactor contents were mixed as described in Example 1. To this reactor 75.OmL of a 28-30% solution OfNH₄OH was added. A peristaltic pump was used to deliver a solution containing 127.5g of Ce(NO₃)3-6H₂O diluted to 25OmL with distilled water at a rate of 400mL/min. Subsequently, a peristaltic pump was used to deliver a solution containing 22.5g of La(NO₃)₃-6H₂O diluted to 10OmL with distilled water at a rate of 400mL/min. The solution was tan in color. The temperature was raised to 80° C and held for 90min during which time there was gas evolution and the solution turned white. The final product was washed to a conductivity of <3mS and a portion was dried at ambient temperature. Powder X-ray diffraction peaks were shifted to lower two-theta values compared to undoped CeO₂, consistent with the substitution of the larger La³⁺ ion for the Ce⁴⁺ ion. The mean grain size (UPA) was 130nm with a grain size frequency distribution ranging from 5 lnm to 289nm (1 % of grains are less than 5 lnm, 99% of grains less than 289nm).

Example 19: Inventive, La doped ceria with acetic acid stabilizer

A six-liter sponge kettle was charged with 2L of distilled water and the temperature controlled at 40° C. The reactor contents were mixed as described in Example 1. To this reactor 75.OmL of a 28-30% solution OfNH₄OH and 2.5mL of glacial acetic acid were added. The resultant pH was 10.1. A peristaltic pump was used to deliver a solution containing 127.5g of Ce(NO₃)₃-6H₂θ diluted to 25OmL with distilled water at a rate of 400mL/min. Subsequently, a peristaltic pump was used to deliver a solution containing 22.5g of La(NO₃)₃-6H₂O diluted to 10OmL with distilled water at a rate of 400mL/min. The measured pH was 8.3 and the solution was tan. The temperature was raised to 80° C and held for 90min during which time there was gas evolution and the solution turned white. The final pH was 5.4. The final product was washed to a conductivity of <3mS and a portion was dried at ambient temperature. Powder X-ray diffraction peaks were shifted to lower two-theta values compared to undoped CeO₂, consistent with the substitution of the larger La³⁺ ion for the Ce⁴⁺ ion. The mean crystallite size was lOnm and the mean grain size (UPA) was 22nm with a grain size frequency distribution ranging from 15nm to 43nm (1% of grains are less than 15ran, 99% of grains less than 43nm).

Particle size results for the lanthanum doped ceria materials precipitated in Examples 18-19 are summarized in Table 2 below.

Table 2.

Claims

CLAIMS:

1. In an aqueous precipitation process for the preparation of metal oxide particles, comprising adding a cerium +3 nitrate salt solution and a base together under turbulent mixing conditions in a precipitation reactor, where cerium +3 ions are oxidized to cerium +4 and precipitated metal oxide particles are directly obtained without need for a calcination step, wherein the improvement comprises adding acetic acid to the precipitation reactor in an amount of from 1 to 40 molar percent, relative to the molar amount of cerium, and obtaining stable- substantially non-agglomerated nanometer size dispersed metal oxide particles.

2. The process of claim 1, wherein the acetic acid is added to the reactor before the cerium nitrate salt solution.

3. The process of claim 1 , wherein the acetic acid is added to the reactor after the cerium nitrate salt solution.

4. The process of claim 1, wherein the acetic acid is added to the reactor with the cerium nitrate salt solution.

5. The process of claim 1, wherein the precipitated metal oxide particles have an average crystallite size of less than 20 nm and an average aggregated grain size of less than 40 nm.

6. The process of claim 5, wherein the precipitated metal oxide particles have an average crystallite size of less than 15 nm and an average aggregated grain size of less than 30 nm.

7. The process of claim 5, wherein the precipitated metal oxide particles have an average crystallite size of less than 15 nm and an average aggregated grain size of less than 25 nm.

8. The process of claim 5, wherein 99% of the precipitated metal oxide particles have a size of less than 100 nm.

9. The process of claim 1, wherein the cerium +3 nitrate salt solution and base are added together in the reactor and maintained at a temperature at or below the boiling point of water at ambient pressure to obtain the metal oxide particles.

10. The process of claim 9, wherein the reactor is maintained at a temperature of from 70- 10OC to obtain the metal oxide particles.

11. The process of claim 10, wherein the pH of the reaction immediately following the complete addition of reactants is between 5 and 10.

12. The process of claim 1 , wherein the base comprises ammonium hydroxide.

13. The process of claim 1, wherein intermediate complexes of either Ce(OH)₂(NO₃)H₂O or (NH₄^Ce(NO₃)S -4H₂O are formed during the course of the reaction.

14. The process of claim 1, wherein acetic acid is added to the precipitation reactor in an amount of from 2 to 25 molar percent, relative to the molar amount of cerium.

15. The process according to claim 1, further comprising washing the precipitated metal oxide particles by ultra filtration to remove unwanted byproducts.

16. Cerium containing metal oxide particles having an average crystallite size of less than 20nm and an average aggregated grain size of less than 40nm produced by the process of claim 1.

17. Cerium containing metal oxide particles according to claim 16, wherein 99% of the metal oxide particles have a size of less than 100 nm.

18. A fuel borne additive composition comprising a nanoparticulate dispersion of cerium containing metal oxide particles of claim 16.

19. A polishing agent composition comprising a nanoparticulate dispersion of cerium containing metal oxide particles of claim 16.