RU2487753C2 - Fuel additive containing cerium dioxide nanoparticles with altered structure - Google Patents

Abstract

FIELD: chemistry.SUBSTANCE: invention relates to a method of producing cerium dioxide nanoparticles and use thereof. Described is a method of producing cerium dioxide nanoparticles with a given lattice, which contain at least one transition metal (M), wherein: (a) an aqueous reaction mixture is prepared, said mixture containing a source of Ceions, a source of ions of one or more transition metals (M), a source of hydroxide ions, at least one nanoparticle stabiliser and an oxidant which oxidises the Ceion to a Ceion, at initial temperature ranging from about 20°C to about 95°C; (b) mechanical shearing of said mixture and passing said mixture through a perforated sieve to form a homogeneously distributed suspension of cerium hydroxide nanoparticles; and (c) creating temperature conditions that are effective for oxidation of the Ceion to a Ceion to form a product stream containing cerium dioxide nanoparticles, containing a transition metal CeMO, where x assumes a value from about 0.3 to about 0.8, said nanoparticles have a cubic fluorite structure, average hydrodynamic diameter ranging from about 1 nm to about 10 nm and geometric diameter from about 1 nm to about 4 nm, wherein the nanoparticle stabiliser is water-soluble and has a value Log Kranging from 1 to 14, where Kdenotes a constant for binding the nanoparticle stabiliser to a cerium ion in water and said temperature conditions that are effective for oxidation of the Ceion to a Ceion include temperature from about 50°C to about 100°C. Described is a method of preparing a homogeneous dispersion containing stabilised crystalline nanoparticles of cerium dioxide with a given lattice, containing a transition metal, CeMO, where M is at least one transition metal and x assumes a value from about 0.3 to about 0.8; (a) preparing an aqueous mixture which contains stabilised cerium dioxide nanoparticles containing a transition metal, CeMO, having a cubic fluorite structure, wherein, wherein said nanoparticles have average hydrodynamic diameter ranging from about 1 nm to about 10 nm and geometric diameter less than about 4 nm; (b) concentrating said aqueous mixture containing said stabilised cerium dioxide nanoparticles containing a transition metal, thereby forming an aqueous concentrate; (c) removing essentially all water from said aqueous concentrate to form an essentially water-free concentrate of stabilised cerium dioxide nanoparticles containing a transition metal; (d) adding an organic diluent to said essentially water-free concentrate to form an organic concentrate of said stabilised cerium dioxide nanoparticles containing a transition metal; and (e) merging said organic concentrate with a surfactant in the presence of a nonpolar medium to form said homogeneous dispersion containing stabilised crystalline nanoparticles of cerium dioxide containing a transition metal, CeMO, where M and x assume values given above. Described is a deposited coating for the catalytic converter of the exhaust system of an internal combustion engine, where said deposited coating is obtained using said homogeneous dispersion.EFFECT: catalyst that is efficient for a reduction or oxidation reaction is described, where said catalyst is obtained using said homogeneous dispersion.44 cl, 9 dwg, 2 tbl, 8 ex

Description

This application relates to: PCT / US 07/077545, METHOD FOR PRODUCING CERIUM DIOXIDE NANOPARTICLES and PCT / US 07/007535, FUEL ADDITION CONTAINING CERIUM DIOXIDE NANOPARTICLES, both of which were registered on September 4, 2007; which claim priority from: provisional application, serial number 60/824514, CERIUM-CONTAINING FUEL ADDITIVE, registered on September 5, 2006; provisional application, serial number 60/911159, REVERSE-MICELLAR FUEL ADDITIVE COMPOSITION, registered on April 11, 2007; and provisional application, serial number 60/938314, REVERSE-MICELLAR FUEL ADDITIVE COMPOSITION, registered May 16, 2007. The contents of all these applications are incorporated into this description by reference.

FIELD OF THE INVENTION

The present invention relates, in general, to cerium dioxide nanoparticles and, in particular, cerium dioxide, Ce _1-x M _x O ₂ nanoparticles containing one or more transition metals (M), and a method for producing such particles. These nanoparticles are suitable as components of fuel additive compositions, such as coated for catalytic converters or as a catalyst for the reduction / oxidation reaction.

BACKGROUND OF THE INVENTION

Shipping accounts for more than 5% of US GDP and contains more than 500,000 self-employed, private and government households, including property owners. They are a barometer of the US economy, representing approximately 70% of the tonnage carried by all types of domestic freight transport, including industrial and retail goods. This industry is carried out almost exclusively with the help of diesel engines (compression ignition engines), which are characterized by a higher torque achieved at low rpm and 25% greater thermodynamic efficiency compared to spark ignition engines (gasoline). As a result of the 2007 EPA, which mandates the reduction of nitrogen oxides (NO _x ) and diesel particles (DPM or soot), diesel vehicles must now be equipped with diesel oxidation catalysts (DOC) or some form of catalytic converter and burn diesel with very low sulfur content ULSD (<15 ppm S). These and other technologies, such as EGR (gas recirculation), must meet EPA emission standards. The ULSD requirement is the result of sulfur poisoning of precious metals in the DOC due to high sulfur levels. This decree has far-reaching consequences, since (on the road) diesel fuel in the USA is consumed in huge quantities, 650 M gallon / week, which is in second place only with respect to gasoline (1300 M gallon / week).

It is estimated that the costs due to these EPA regulations will add approximately $ 0.39 to the cost of one gallon of diesel. This value is divided into the following components: increased engine costs ($ 0.11 / gal), maintenance of particle traps ($ 0.05 / gal), fuel economy ($ 0.09 / gal), increased ULSD ($ 0.06 / gal) and lower energy content of ULSD fuel ($ 0.08 / gal).

It is clear that any technology that can reduce DPM and other emissions at the same time as increasing fuel economy (measured by increasing mpg) will be seen as a huge advantage for finance and the environment.

Diesel fuel additives, in particular additives that include inorganic metallic and metal oxide materials as opposed to organic materials, promise a reduction in DPM and improved fuel economy.

Kracklaurer, US patent No. 4389220, the contents of which are incorporated into this description by reference, describes a method for creating conditions for diesel engines in which a diesel engine runs on diesel fuel containing about 20-30 ppm dicyclopentadienyl iron for a period of time sufficient to remove carbon deposits from the combustion surfaces of the engine and to deposit a layer of iron oxide on the combustion surfaces, which is effective to prevent further buildup of carbon deposits. The diesel engine is then operated at a maintenance concentration of about 10-15 ppm dicyclopentadienyl iron or an equivalent amount of its derivative on a continuous basis. The maintenance concentration is effective in maintaining the catalytic layer of iron oxide on the combustion surfaces, but is insufficient to reduce the time delay in the engine. The added iron dicyclopentadienyl can lead to the formation of iron oxide on the surface of the engine cylinder (Fe ₂ O ₃ ), which reacts with carbon deposits (soot) to form Fe and CO ₂ , thereby removing deposits. However, this method can accelerate engine aging due to rust formation.

Valentine et al., U.S. Patent Application Publication No. 2003/0148235, the contents of which are incorporated herein by reference, describes specific bimetallic or trimetallic fuel combustion catalysts to increase fuel combustion efficiency. These catalysts reduce the contamination of heat-transfer surfaces with unburned carbon by limiting the amount of secondary additive ash, which can itself cause overloading of particle collectors or emissions of toxic ultrafine particles when they are used in the forms and quantities commonly used. By using fuel containing a fuel-soluble catalyst formed from platinum and at least one additional metal containing cerium and / or iron, the production of pollutants resulting from incomplete combustion is reduced. Ultra-low levels of non-toxic metal combustion catalysts can be used to improve heat recovery and reduce emissions of controlled pollutants. However, fuel additives of this type, in addition to using rare and expensive metals such as platinum, may require several months before the engine is “conditioned”. The expression "conditioned" means that all the advantages of this additive are not obtained until the engine has been working with this catalyst for a certain period of time. Initial conditioning may take 45 days, and potential benefits may not be obtained until 60-90 days. Additionally, free metal can be released from the exhaust system into the atmosphere, where it can then react with living organisms.

Cerium dioxide is widely used as a catalyst in converters to eliminate toxic exhaust gases and reduce particle emissions in diesel vehicles. In a catalytic converter, cerium dioxide can act as a chemically active component, releasing oxygen in the presence of reducing gases, and also removing oxygen by interacting with oxidizing particles.

Cerium dioxide can store and release oxygen by the reversible process represented by equation 1.

СеО ₂ ← → СеО _2-х + х / 2 О ₂ ( Lv. 1 )

This process is called oxygen storage capacity (OSC) by cerium oxide. Here, cerium oxide acts as an oxygen storage buffer (like a pH buffer), releasing oxygen in areas of space where oxygen concentration or pressure is low, and absorbing oxygen in areas of space where oxygen pressure is high. When x = 0.5, cerium oxide is effectively completely reduced to Ce ₂ O ₃ , and the maximum theoretical OSC is 1452 micromoles of O ₂ per gram of cerium oxide. The redox potential between Ce ³⁺ and Ce ⁴⁺ ions lies between 1.3 and 1.8 V and is highly dependent on the anionic groups present and the chemical environment (CERIUM: A Guide to its Role in Chemical Technology, 1992 by Molycorp. Inc , Library of Congress Catalog Card Number 92-93444)). This allows the described direct and reverse reactions to easily proceed in exhaust gases near the stoichiometric ratio of the required oxygen (15: 1). Cerium dioxide can provide oxygen for the oxidation of CO or hydrocarbons in an oxygen-depleted environment or, conversely, can absorb oxygen to lower levels of nitrogen oxides (NO _x ) in an oxygen-enriched environment. Similar catalytic activity can also occur when cerium dioxide is added as an additive to a fuel, for example diesel fuel or gasoline. However, for this effect to be useful, cerium dioxide must be particles of a sufficiently small size, i.e. nanoparticles (less than 100 nm) to remain suspended in the fuel due to Brownian motion and not to settle. In addition, since the catalytic effects depend on the surface area, the small particle size makes the nanocrystalline material more effective as a catalyst. The introduction of cerium dioxide into the fuel serves not only to act as a catalyst to reduce the toxic exhaust gases produced by the combustion of the fuel, for example, using a “water gas shift reaction”

СО + Н ₂ О → СО ₂ + Н ₂ ,

but also to facilitate the combustion of particles that accumulate in particle traps commonly used in diesel engines.

As already mentioned, cerium dioxide nanoparticles are particles having an average diameter of less than 100 nm. For the purposes of this description, unless otherwise stated, the diameter of the nanoparticles corresponds to their hydrodynamic diameter, which is a diameter determined using dynamic light scattering technology, and includes molecular adsorbates and the accompanying particle solvation shell. Alternatively, the geometric diameter of the particle can be calculated using transmission electron micrograph (TEM).

Vehicle on-board metering systems are known which distribute cerium dioxide into fuel before it enters the engine, but such systems are complex and require comprehensive electronic control to supply the appropriate amount of additive to the fuel. To avoid such complex on-board systems, cerium dioxide nanoparticles can also be added to the fuel at an earlier stage in order to achieve improved fuel efficiency. They can, for example, be introduced at a refinery, usually together with processing aids, such as, for example, cetane number improvers or lubricants, or added at a fuel distribution tank station.

Cerium dioxide nanoparticles can also be added to the fuel distribution center by injection into large (~ 100,000 gal) volumes of fuel, or in smaller warehouses of fuel companies, which will allow adjustment to specific individual requirements. In addition, cerium dioxide can be added at a gas station while fuel is being supplied to the vehicle, which will have the potential advantage of improving stabilization of particle distribution.

Cerium nanoparticles can form a ceramic layer on the cylinders of the engine and internal moving parts, thereby essentially transforming the engine into a catalytic device. Alternatively, they can be returned to the lubricating oil, where they accumulate. Their catalytic efficiency stems from the fact that they provide a source of oxygen atoms during combustion by proceeding with reduction according to equation (1); however, an induction period of several months is usually required before their mpg (mpg) benefits are achieved. This ultimately leads to better fuel combustion and reduces particulate emissions. In addition, when used as a fuel additive, these nanoparticles can provide improved engine performance by reducing engine friction. As an alternative input regimen, cerium dioxide nanoparticles can be added to the lubricating oil and act as a lubricant improving agent, reducing internal friction. It will also improve fuel efficiency.

The following publications, all contents of which are incorporated herein by reference, describe fuel additives containing cerium oxide compounds.

Hawkins et al., US Patent No. 5,449,387, describes an oxide compound of cerium (IV) having the formula:

(H ₂ O) _p [CeO (A) ₂ (AN) _n ] _m

in which the radicals A, which are the same or different, represent each anion of an organic hydroxy acid AN having a pK _a greater than 1, p is an integer in the range from 0 to 5, n is a number in the range from 0 to 2 and m represents an integer in the range from 1 to 12. The organic hydroxy acid is preferably a carboxylic acid, more preferably a C ₂ -C ₂₀ monocarboxylic acid or a C ₄ -C ₁₂ dicarboxylic acid. These cerium-containing compounds can be used as catalysts for the combustion of hydrocarbon fuels.

Valentine et al., US patent No. 7063729, describes a low emission diesel fuel that includes a bimetallic fuel soluble catalyst with a platinum group metal and cerium, where cerium is provided as a fuel soluble hydroxyl oleate propionate complex.

Chopin et al., U.S. Patent No. 6,210,451, describes a petroleum-based fuel that includes a stable organic sol that contains cerium dioxide particles in the form of crystallite agglomerates (preferred size is 3-4 nm), an amphiphilic acid system containing at least one acid, in which the total number of carbons is at least 10, and an organic solvent medium. Adjustable particle size does not exceed 200 nm.

Birchem et al., US Patent No. 6,136,048 describes an activating additive for diesel engine fuel, which includes a sol containing particles of an oxygenate compound having d ₉₀ not greater than 20 nm, an amphiphilic acid system and a solvent. Particles of the metal oxygenate compound are prepared in solution by the reaction of a salt of a rare-earth element, such as a cerium salt, with a basic medium, followed by obtaining a precipitate formed by spraying or freeze-drying.

Lemaire et al., US patent No. 6093223, describes a method for producing aggregates of cerium oxide crystallites by burning hydrocarbon fuels in the presence of at least one cerium compound. This carbon black contains at least 0.1% of the mass of cerium oxide crystal aggregates, where the largest particle size is 50-10000 angstroms, crystallite size is 50-250 angstroms, and carbon black has a flash point of less than 400 ° C.

Hazarika et al., US Pat. No. 7,195,653 B2, describes a method for improving fuel efficiency and / or reducing fuel emissions from a fuel combustion apparatus, wherein the method comprises dispersing at least fine-grained lanthanide oxide, especially cerium dioxide, in a fuel of 1 to 10 ppm, in the form of a tablet, capsule, powder or liquid fuel additive, where the fine-grained lanthanum oxide is coated with a surfactant selected from the group consisting of alkyl carboxylic anhydrides and esters having HLB 7 or less. The preferred coating is dodecyl succinic anhydride.

Collier et al., U.S. Patent Application No. 2003/0182848, describes a diesel fuel composition that improves the performance of diesel particle traps and contains a combination of 1-25 ppm metal in the form of a metal salt additive and 100-500 ppm oil-soluble nitrogen-free ash-free detergent additives. This metal may be an alkali metal, alkaline earth metal, a metal of groups IVB, VIIB, VIIIB, IB, IIB or any of the rare earth metals having atomic numbers 57-71, especially cerium, or a mixture of any of these metals.

Collier et al., US Patent Application No. 2003/0221362, describes a diesel fuel composition for a diesel engine equipped with a particle trap, wherein the composition contains a hydrocarbon solvent and an oil soluble metal carboxylate or a metal complex derived from a carboxylic acid containing not more than 125 atoms carbon. This metal may be an alkali metal, alkaline earth metal, a metal of groups IVB, VIIB, VIIIB, IB, IIB or a rare earth metal, including cerium, or mixtures of any of these metals.

Caprotti et al., U.S. Patent Application No. 2004/0035045, describes a diesel fuel composition for a diesel engine equipped with a particle trap. This composition contains an oil-soluble or oil-dispersible metal salt of an acidic organic compound and a stoichiometric excess of metal. When added to the fuel, this composition provides 1-25 ppm of metal, which is selected from the group consisting of Ca, Fe, Mg, Sr, Ti, Zr, Mn, Zn and Ce.

Caprotti et al., US Patent Application No. 2005/0060929, describes a phase separation stabilized diesel fuel composition that contains a colloidal dispersed or dissolved metal catalyst compound and 5-1000 ppm stabilizer, which is an organic compound having a lipophilic hydrocarbon a chain attached to at least two polar groups, at least one of which is a carboxylic acid or a carboxylate group. This metal catalyst compound contains one or more organic or inorganic compounds or complexes of Ce, Fe, Ca, Mg, Sr, Na, Mn, Pt, or a mixture thereof.

Caprotti et al., US Patent Application No. 2006/0000140, describes a fuel additive composition that contains at least one colloidal metal compound or particles and a stabilizing component that is a condensation product of an aldehyde or ketone and a compound containing one or more aromatic fragments containing a hydroxyl substituent and another substituent selected from a hydrocarbon, —COOR or COR, where R is hydrogen or hydrocarbon. The colloidal metal compound preferably contains at least one metal oxide, preferred oxides are iron oxide, cerium dioxide or cerium doped iron oxide.

Scattergood, International Publication No. WO 2004/065529, describes a method for improving fuel efficiency for an internal combustion engine, which comprises adding cerium dioxide and / or doped cerium dioxide and possibly one or more fuel additives to the fuel.

Anderson et al., International Publication No. WO 2005/012465, describes a method for improving fuel efficiency for an internal combustion engine that contains a lubricating oil and gasoline, wherein the method comprises adding cerium dioxide and / or doped cerium dioxide to a lubricating oil or gasoline.

Cerium-containing nanoparticles can be obtained using a variety of technologies known in the art. Regardless of whether synthetic nanoparticles are made in a hydrophilic or hydrophobic environment, these particles usually require a stabilizer to prevent unwanted agglomeration. The following publications, all contents of which are incorporated herein by reference, describe some of these synthesis technologies.

Chane-Ching et al., US Pat. No. 6,272,269, describes a process for preparing storage-stable organic sols which comprises: reacting a basic reagent with an aqueous solution of a metal acid cation salt to form an aqueous colloidal dispersion containing an excess of hydroxyl ions; contacting this aqueous colloidal dispersion with an organic phase containing an organic liquid medium and organic acid; and separating the resulting mixture of the aqueous / organic phases into the aqueous phase and the resulting organic phase. Preferred metal cations are cerium and iron cations. Colloidal particles have hydrodynamic diameters in the range of 5-20 nanometers.

Chane-Ching et al., US patent No. 6649156, describes an organic sol containing particles of cerium dioxide, which are obtained using the process of thermal hydrolysis; organic liquid phase; and at least one amphiphilic compound selected from polyoxyethylene alkyl ethers of carboxylic acids, phosphates of polyoxyethylene alkyl ethers, dialkyl sulfosuccinates and quaternary ammonium compounds. The water content in these sols can be no more than 1%. The average crystallite size is approximately 5 nm, while agglomerates of particles from these crystallites have sizes from 200 to 10 nm.

Chane-Ching et al., US Pat. No. 7,008,965, describes an aqueous colloidal dispersion of a cerium compound and at least one other metal, where the dispersion has a conductivity of at most 5 mS / cm and a pH of 5 to 8.

Chane-Ching et al., U.S. Patent Application Publication No. 2004/0029978 (registered December 7, 2005), describes a surfactant formed from at least one nanoparticle that is amphiphilic and based on oxide, hydroxide and / or oxyhydroxide metal, with the surface of which organic chains are connected with hydrophobic properties. The metal is preferably selected from cerium, aluminum, titanium or silicon, and the alkyl chain contains 6-30 carbon atoms or a polyoxyethylene monoalkyl ether, the alkyl chain of which contains 8-30 carbon atoms, and the polyoxyethylene portion contains 1-10 hydroxyethylene groups. This particle is an isotopic or spherical particle having an average diameter of 2-40 nm.

Blanchard et al., U.S. Patent Application Publication No. 2006/0005465, describes an organic colloidal dispersion comprising: particles of at least one compound based on at least one rare earth element, at least one acid, and, according to at least one diluent, where at least 90% of the particles are single crystal. Example 1 describes the preparation of a colloidal solution of cerium dioxide from cerium acetate and an organic phase that includes an Isopar hydrocarbon mixture and isostearic acid. The obtained particles of cerium dioxide have a d _{50 of} 2.5 nm, the size of 80% of the particles was in the range of 1-4 nm.

Zhou et al., US patent No. 7025943, describes a method for producing crystals of cerium dioxide, which includes: mixing the first solution of a water-soluble salt of cerium with a second solution of alkali metal or ammonium hydroxide; shaking the resulting reagent solution under turbulent flow conditions, accompanied by passing gaseous oxygen through the solution; and the deposition of particles of cerium dioxide having a main particle size in the range of 3-100 nm. It was found that in example 1, the particle size is approximately 3-5 nm. There is no mention of a stabilizing agent, and it is assumed that the sols will eventually agglomerate and settle.

Sandford et al., WO 2008/002223 A2, describes an aqueous deposition technique that produces cerium dioxide directly without subsequent calcination. The Ce ³ cation is slowly oxidized to Ce ^{+ 4} with a nitrate ion, and a stable, non-agglomerating sol with a crystallite size of 11 nm (and approximately equal grain size) is obtained when acetic acid is used as a stabilizer. Interestingly, EDTA and citric acid produce grains with crystallite sizes of the order of several hundred nanometers.

Woodhead, James, L., U.S. Patent No. 4,231,893, describes the preparation of an aqueous dispersion of cerium oxide by acid treatment of Ce (OH) ₄ , which was obtained by peroxidizing Ce ³⁺ in a base. No size data were provided, and at the required pH of 1.5 to stabilize, the NO ₃ ^- anion is a likely stabilizer.

Noh et al., U.S. Patent Application Publication No. 2004/0241070, describes a method for producing a cerium dioxide single crystal nanopowder comprising: preparing cerium hydroxide by precipitation of a cerium salt in the presence of a mixture of solvents from an organic solvent and water, preferably in a ratio of about 0.1: 1 to about 5: 1 by weight; and the hydrothermal reaction of cerium hydroxide. The nanopowder has a particle size of approximately 30-300 nm.

Chan, US Patent Application Publication No. 2005/0031517, describes a method for producing cerium dioxide nanoparticles, which comprises: rapidly mixing an aqueous solution of cerium nitrate with aqueous hexamethylenetetramine, where the temperature is maintained at no higher than about 320 ° K when the nanoparticles are formed in the resulting mixture; and separating the formed nanoparticles. The mixing apparatus preferably comprises a mechanical stirrer and a centrifuge. In an illustrative example, it is reported that the obtained particles of cerium dioxide have a diameter of approximately 12 nm.

Ying et al., US Pat. Nos. 6,413,489 and 6,869,584, describe reverse-micellar synthesis of nanoparticles that are free from agglomeration and have a particle size of less than 100 nm and a surface area of at least 20 m ² / g. This method comprises introducing a ceramic precursor that contains barium alkoxide and aluminum alkoxide in the presence of an inverse emulsion.

A related publication, Ying et al., U.S. Patent Application Publication No. 2005/0152832, describes the synthesis, using reverse micellar technology in an emulsion having a water content of 1-40%, of nanoparticles that are free from agglomeration and have a particle size of less than 100 nm These nanoparticles are preferably metal oxide particles that can be used to oxidize hydrocarbons.

Hanawa et al., US patent No. 5938837, describes a method for producing particles of cerium dioxide, intended mainly for use as a polishing agent, which includes mixing, with stirring, an aqueous solution of cerium nitrate with a base, preferably aqueous ammonia, in this regard mixing, that the pH of the mixture lies in the range from 5 to 10, preferably from 7 to 9, then quickly heating the resulting mixture to a temperature of 70-100 ° C and maturing the cerium nitrate mixture with the base at this temperature to form grains. The resulting grains are uniform in size and shape and have an average particle size of 10-80 nm, preferably 20-60 nm.

European patent application EP 0208580, published January 14, 1987, inventor Chane-Ching, applicant Rhone Poulenc, describes a compound of cerium (IV) corresponding to the General formula

Ce (M) _x (OH) _y (NO ₃ ) ₂

where M is an alkali metal or Quaternary ammonium radical, x is from 0.01 to 0.2, y is such that y = 4-z + x, and z is from 0.4 to 0.7. A method for producing a colloidal dispersion of this cerium (IV) compound produces particles with a hydrodynamic diameter of from about 1 nm to about 60 nm, suitably from about 1 nm to about 10 nm, and preferably from about 3 nm to 8 nm.

S. Sanhyamurthy et al., Nano Technology 16, (2005), pp 1960-1964, describes reverse micellar synthesis of CeO ₂ from cerium nitrate using sodium hydroxide as a precipitating agent and n-octane containing cetyltrimethylammonium bromide surfactant (CTAB), and additional surfactant 1-butanol as the oil phase. The resulting polyhedral particles have an average size of 3.7 nm and exhibit agglomeration when removed from their protective inverse micellar structure. In addition, it is expected that this reaction will proceed with a low yield (for reagents A and B, there are many collisions AB leading to the product, and AA and BB unproductive collisions).

Seal et al., Journal of Nano Particle Research, (2002), 4, pp 433-448, describes the preparation of cerium nitride and ammonium hydroxide nanocrystalline particles of cerium oxide for a high temperature, oxidation-resistant coating using an aqueous microemulsion system containing AOT in as a surfactant; and toluene as an oil phase. Cerium oxide nanoparticles formed in the upper oil phase of the reaction mixture had a particle size of 5 nm.

Seal et al., US Pat. No. 7,419,516, the contents of which are incorporated herein by reference, describes the use of rare earth metal oxide nanoparticles, preferably cerium oxide, as fuel additives to reduce soot formation. Particles that were obtained using the reverse micellar method using toluene as the oil phase and AOT as a surfactant have diameters in the range of about 2-7 nm, an average diameter of about 5 nm.

Pang et al., J. Mater. Chem., 12 (2002), pp 3699-3704, prepared Al ₂ O ₃ nanoparticles using a water-in-oil microemulsion method using an oil phase containing cyclohexane and a non-ionic surfactant Triton X-114 and an aqueous phase, containing 1.0 M AlClO ₃ . The obtained Al ₂ O ₃ particles, which had a particle size of 5-15 nm, turned out to be completely different from the hollow spherical particles of submicron size obtained using the direct deposition method.

Cuif et al., US Patent No. 6,133,194, the contents of which are incorporated herein by reference, describes a process that involves reacting a metal salt solution containing cerium, zirconium or a mixture thereof, a base, optionally an oxidizing agent and an additive selected from the group consisting of anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids and carboxylate salts, to form a product. This product was then calcined at temperatures greater than 500 ° C (which would effectively carbonize the declared surfactants).

It should be borne in mind that although many authors claim cerium oxide nanoparticles are much lower than 5 nm, no X-ray or electron diffraction data are presented to clearly establish that the grains are indeed cubic CeO ₂ and not hexagonal or cubic Ce ₂ O ₃ . There is substantial doubt that cubic CeO ₂ is thermodynamically stable with a very small grain size, and that these grains, in fact, are a reduced and more stable, hexagonal form of Ce ₂ O ₃ . S. Tsunekawa, R, Sivamohan, S. Ito, A. Kasuya and T. Fukada in Nanostructured Materials, vol 11, no. 1, pp 141-147 (1999).

The Structural Study on Monosize CeO _2-x Nanoparticles, in particular, casts doubt on the existence of CeO ₂ at or below 1.5 nm.

Additional evidence for the existence of Ce ³⁺ (and, accordingly, Ce ₂ O ₃ ) for very small grain diameters comes from Dephandle et al., In Applied Physics Letters 87, 133113 (2005) "Size Dependency Variation in Lattice Parameter and Valency States in Nano Crystalline Cerium Oxide, "which discovered the following logarithmic linear relationship between the change in lattice constant,

Δa = aa ₀ (a ₀ = 5.43 Å in CeO ₂ ) and the crystal diameter D:

log Δa = -0.4763 log D -1.5029 (ur. 2)

Thus, a grain diameter of 10 nm will cause a lattice deformation or a change in the lattice constant of 0.0103 Å or 1.91%, while a grain diameter of 1 nm will cause a change of 0.031 Å or 5.73 percent.

The extent to which CeO ₂ can act as a catalytic oxygen storage material described by Equation 1 is determined in part by the particle size of CeO ₂ . At particle sizes of 20 nm or lower, the lattice parameter increases significantly with decreasing crystallite size (to 0.45% at 6 nm, see, for example, Zhang, et al., Applied Physics Letters, 80 1, 127 (2002)). The corresponding size-induced lattice deformation is accompanied by an increase in surface oxygen vacancies, which leads to increased catalytic activity. This inverse dependence of activity on size provides not only more efficient fuel cells, but also better oxidizing properties when used in the combustion of petroleum fuels.

As described above, various methods and devices for producing cerium nanoparticles have been reported, including those described by Chane-Ching, et al., US Pat. No. 5,017,352; Hanawa, et al, U.S. Patent No. 5,983,837; Melard, et al. US Pat. No. 4,786,325; Chopin, et al. US Pat. No. 5,712,218; Chan, US Patent Publication No. 2005/00331517; and Zhou, et al., US Pat. No. 7,025,943, the contents of which are incorporated herein by reference. However, modern methods do not provide the possibility of economical, easy (i.e., without calcination) and unambiguous production of cubic CeO ₂ nanoparticles with high yield, in a short period of time at very high suspension densities (more than 0.5 mol, i.e. 9 % mass), which are substantially small in size (less than 5 nm in geometric mean diameter), uniform in frequency distribution of size (coefficient of change [COV] less than 25%, where COV is the standard deviation divided by the average diameter), and stub lnymi for many desired applications. In addition, it would be very desirable to obtain particles that are crystalline, i.e. single-crystal, and not agglomeration of crystallites of various sizes, as described in the aforementioned prior art and technical literature.

Although nanoparticles of substantially pure cerium dioxide are advantageously included in fuel additives, it may be more beneficial to use cerium dioxide doped with components that lead to the formation of additional oxygen vacancies (level 1). For this to happen, the doping ion must be divalent or trivalent, i.e. divalent or trivalent ion of the element, which is a rare earth metal, transition metal or a metal of group IIA, IIIB, VB or VIB of the periodic table. The requirement that the crystal charge be neutral when using these cations with a lower valency will shift lv. 1 to the right, i.e. to a higher degree of formation of oxygen vacancies. Doping metal ions with an ionic radius smaller than Ce ⁺⁴ (0.97 Å in the octahedral configuration) will also contribute to the formation of oxygen vacancies, since this process reduces two neighboring Ce ⁺⁴ ions (one surface and one subsurface) in Ce ⁺³ with a large ionic radius of 1.143 Å, expands the lattice, thereby causing lattice deformation. Thus, substitution with Zr ⁺⁴ (ionic radius 0.84 Å) or Cu ⁺² (the ionic radius of the six-coordinate octahedral configuration is 0.73 Å, the four-coordinate tetrahedral 0.57 Å) will partially remove this lattice deformation. In addition, Zr allows the formation of two neighboring surface centers of Ce + ³ (rather than one surface and one subsurface), which may be important for very small particles, where approximately 50% of the ions are surface ions. Thus, it can be considered that doping with a substitutional ion is preferable to doping with an intercalating ion, when doping impurities occupy the space between the normal lattice positions.

For the purposes of this discussion, the authors must determine what is meant by doping as opposed to a crystal with a given (designed) lattice. In semiconductor physics, the word “doping” refers to n or p type impurities present in the ppm range. The authors use the term “doped crystal” to refer to a crystal that has doping ions of one or more metals present in concentrations of less than 2 mole percent (20,000 ppm). A crystal with a given lattice, on the other hand, can have doping ions of one or more metals present in a CeO ₂ crystal with concentrations greater than 20,000 ppm, up to 800,000 ppm (or 80% cerium sublattice). Thus, a cerium dioxide crystal with a predetermined lattice may contain cerium present as a smaller metal component.

Doping of cerium dioxide with metal ions to improve ion transport, reaction efficiency and other properties is described, for example, in Doped Ceria as a Solid Oxide Elecrolyte, HL Tuller and AS Nowick in J. Electrochem Soc., 1975, 122 (2), 255; "Point Defect Analysis and Microstructural Effects in Pure and Donor Doped Ceria", MR DeGuire, et al., Solid State Ionics, 1992, 52, 155; and "Studies on Cu / CeO ₂ : A New NO Reduction Catalyst" Parthasarathi Bera, ST Aruna, KC Patil, and MS Hegde in Journal of Catalysis, 186, 36-44 (1999). The effects of doping on electronic properties and oxygen diffusion are described by Trovarelli, Catalysis by Ceria and Related Materials, Catalytic Science Series, World Scientific Publishing Co. , 37-46 (2002) and references cited there.

Trovarelli et al. in Catalysis Today, 43 (1998), 79-88 discusses the preparation of cerium-zirconium mixed oxides with fairly good uniformity of composition using an approach involving surfactants. High specific surface areas, 230 m ² / mg, obtained after calcining the compositions at 723 ° K; however, sintering occurs at 1173 ° K, since the surface area drops to 40 m ² / mg (diameter ~ 20 nm).

Pulsed neutron diffraction techniques have been used by E. Mamontov, et al. J. Phys. Chem. B 2000, 104, 1110-1116 to study cerium and cerium-zirconium solid solutions. These studies first established a correlation between the concentration of vacancy-introduced oxygen defects and the ability to accumulate oxygen. The authors found that the preservation of oxygen defects, which Zr helps, is necessary to improve the decomposition of OSC as a function of thermal aging. ZrO _{2 was} present at 30.5 mol%, and the calcined particles had a diameter of approximately 40 nm based on BET measurements of the surface area.

Z. Yang et al. in Journal of Chemical Physics, (2006) 124 (22), 224704 / (1-7) calculated from first principles, using the density function theory, that an oxygen vacancy is most easily created close to the Zr center, and therefore these centers serve in as a nucleation center for the clustering of vacancies. The released oxygen gives two electrons centers Ce ⁺⁴ neighboring vacancy, leading to two centers Ce ^+3.

R. Wang et al. in J. Chem. Phys. B, 2006, 110, 18278-18285 investigated the spatial distribution of Zr in Ce _0.5 Zr _0.5 O ₂ obtained by spray freezing followed by calcination. The authors found that nanoscale particle heterogeneity, which is characterized by Ce-enriched nuclei and Zr-enriched shells in particles in the particle size range from 5.4 to 25 nm, is associated with more redox active materials. This discovery means that a uniform distribution of Zr and Ce leads to reduced activity and, therefore, is not preferred.

S. Bedrane et al. in Catalysis Today, 75, 1-4, 401-405 July 2002, the oxygen storage ability (level ₁ ) was measured in 11 compositions of cerium oxide (CeO ₂ ) and cerium zirconium oxide (Ce _0.63 Zr _0.37 O ₂ ) doped with valuable and noble metals (PM = Rh, Pt, Rd, Ru and Ir). The authors observed a leveling effect when Ce-Zr materials had OSC, which was almost independent of the concentration of PM and was 2-4 times greater than for PM-doped Ce materials without Zr.

H. Sparks et al. from Nanophase Technologies, Corp., using synthesis from the gas phase, nanomaterials were obtained from cerium oxide mixed with rare earth oxide (Mat. Res. Soc. Sym. Symp. Proc., Vol 788, 2004). They observed increased thermal stability of nanocrystalline particles and an increase in OSC for Zr-doped cerium oxide (1: 1); however, the additional addition of La or Pr to the Zr composition, although better than cerium oxide itself, was worse than just the combination of zirconium and cerium oxide. From the presented specific surface areas, one can determine the particle size of 10 nm at 600 ° C, which increases to 40 nm at 1050 ° C.

The catalytic effects of CeO ₂ doped with Zr and Fe in the combustion of diesel soot were studied by Aneggi et al. in Catalysis Today 114, (2006), 40-47. They repeated the fact that Zr enhances the thermal stability and OSC of pure cerium oxide and found that Fe ₂ O ₃ gives better results when fresh, but loses activity after calcination. We studied a very systematic level series with Zr and Zr with Fe, including crystallographic data of these calcined particles, which were approximately 21 nm. The authors determined the threshold of the specific surface of nanoparticles, 35 m ² / g (corresponding to a diameter of less than 24 nm), at which the activity of the fresh sample relative to the aged was unchanged.

Copper-based catalyst systems have also attracted much attention. In a very thorough structural analysis of 3 and 5 atomic percent Cu / CeO ₂ , MS Hegde et al., Chem. Mater. 202, 14, 3591-3601, showed that Cu forms a special solid solution Ce _1-x Cu _x O ₂ without a discrete phase of CuO. In these grains, agglomerated up to 50 nm, obtained by combustion synthesis, Cu is in the +2 state and is much more catalytically active than Cu in CuO. In addition, an oxygen ion vacancy is created near the Cu ^{+ 2} cation.

A. Martinex-Arias et al. in J. Phys. Chem. B, 2005, 109, 19595-19603 found that the reduction of fluorite type Ce _1-x Cu _x O ₂ nanoparticles (x = 0.05, 0.1 and 0.2) was reversible, and that the oxidation state of Cu was higher, than her normal states (+1 or +2). The dopant caused a high lattice stress in these ~ 6 nm particles in the oxide sublattice, which promoted the formation of oxygen vacancies. A reverse microemulsion method followed by calcination at 500 ° C. was used to produce these materials.

Iron is another metal ion that gives CeO ₂ nanoparticles enhanced catalytic activity. I. Melian-Cabrera et al. in Journal of Catalysis, 239, 2006, 340-346 report increased activity (relative to undoped materials) and optimal catalytic decomposition of N ₂ O, limited by the reaction oxygen, for a 50/50 composition of cerium oxide and iron. Fe-doped cerium oxide is produced by coprecipitation, which leads to the formation of particles in the diameter range of 30 nm.

T. Campenon and colleagues in SAE special publication SP 2004, SP-1860, "Diesel Exhaust Emission Control" use iron-doped cerium oxide to control ash build-up in diesel particle filters.

R. Hu and colleagues in Shiyou Huagong (2006), 35 (4), 319-323 studied Fe-doped cerium dioxide obtained by solid phase grinding technology followed by annealing at various elevated temperatures. Doping with iron improved catalytic activity with respect to methane combustion while reducing particle size.

Illustrative examples 9 and 10 of the publication of US patent application No. 2005/0152832 describe the preparation, respectively, of cerium-doped and cerium-coated particles of barium hexaaluminate. Example 13 describes the oxidation of methane with cerium-coated particles.

Talbot et al., US Pat. No. 6,752,979, the contents of which are incorporated herein by reference, describes a method for producing metal oxide particles having nanosized grains, which consists of: mixing a solution containing one or more metal cations with a surfactant in such conditions that surfactant micelles are formed in solution, thereby forming a micellar fluid; and heating the micellar fluid to remove the surfactant and form metal oxide particles having a disordered porous structure. Metal cations are selected from the group consisting of cations from groups 1A, 2A, 3A, 4A, 5A and 6A of the periodic table, transition metals, lanthanides, actinides and mixtures thereof. The preparation of cerium dioxide particles and mixed oxides containing cerium and one or more other metals are included in illustrative examples.

Illustrative example 9 of US patent No. 6413489 and 6869584, the contents of which are incorporated into this description by reference, describes the inclusion of cerium nitrate in the emulsion mixture to obtain cerium-doped barium hexaaluminate particles, which were collected by freeze drying and calcined in air to 500 ° C and 800 ° C. The resulting particles had a grain size of less than 5 nm and 7 nm at 500 ° C and 800 ° C, respectively. Illustrative example 10 describes the synthesis of cerium-coated barium hexaaluminate particles. After calcination, the cerium-coated particles had a grain size of less than 4 nm, 6.5 nm and 16 nm at 500 ° C, 800 ° C and 1100 ° C, respectively.

Wakefield, US Patent No. 7,169,196 B2, the contents of which are incorporated herein by reference, describes a fuel containing cerium dioxide particles that have been doped with a divalent or trivalent metal or non-metal, which is a rare earth metal, a transition metal or a metal of group IIa, IIIB, VB or VIB of the periodic table. Copper is described as a preferred alloying agent.

Oji Kuno in US Pat. No. 7384888 B2, the contents of which are incorporated herein by reference, describes a cerium-zirconium mixed metal oxide with a cerium oxide core and a zirconia shell having improved temperature stability and a stable OSC. However, calcination at 700 ° C is required to obtain a material that exhibits 10-20 percent improved catalytic activity with respect to oxidation of hydrocarbon and carbon monoxide. No size data is provided to support the declaration of 5-20 nm particles, no direct OSC measurements are provided, and no analytical data are available to support the nuclear-shell geometry statement.

As for nanoparticles with a diameter of 10 nm or less, there is much that casts doubt on the ability of alloying additives of metal ions to penetrate into such small particles. For example, an 8.1 nm particle will have less than 10% cerium ions on the surface, while a 2.7 nm particle (5 unit cells at the edge with 0.54 nm / unit cell) will have 46.6% of 500 Ce ions on the surface. The surface ions are 1/2 (for the face) or 1/8 (angle) immersed in the lattice; consequently, their binding energies are substantially reduced and their coordination requirements are not satisfied. The difficulties associated with doping (semiconductor) nanocrystals are discussed in Science, 319, March 28, 2008 by Norris et al. Characteristics, such as the relative solubility of the dopant in the crystal relative to the solution, the diffusion of the dopant in the lattice, the energy of its formation, the size, valency relative to the ions that are replaced, the kinetic barriers that can be caused by adsorbed surface stabilizers, can all play a role in determining the degree of if it is, in which a doping metal ion can be introduced into nanocrystals of these sizes.

From the above links it is clear that most of the doping occurs at a relatively large particle size (20 nm or something) and is performed either by calcining the initial mixture with cerium dopant, or by micellar synthesis, a process that does not easily adapt to large-scale production of materials. In the work describing particles smaller than 10 nm in size, the crystallographic form was not established and there is not sufficient evidence of implementation.

Thus, there is a need for easy incorporation of a wide variety of alloying metal ions into the cerium sublattice of cubic CeO ₂ for very small nanoparticles (with a diameter of less than about 10 nm) in a light manner that does not require calcination (500C or more), and clearly demonstrate the incorporation as opposed to separately a nascent population of grains of alloyed metal oxide. Since the single crystal particles of cerium oxide are unique, it will be a variant of cerium oxide with a given metal lattice. In addition, it would be desirable to produce large, commercially available quantities of these materials in an economical manner and in a relatively short period of time.

A typical chemical reactor that can be used to produce cerium dioxide includes a reaction chamber that includes a stirrer (see, for example, FIG. 1 in Zhou et al. US Pat. No. 7,025,943, the contents of which are incorporated herein by reference). An agitator typically includes an axis and a propeller or turbine blades attached to the axis, and a motor that rotates the axis, so that the propeller rotates at high speed (from 1000 to 5000 rpm). The axis can move a turbine with flat blades for good meso mixing (micro level) and a turbine with inclined blades for macro mixing (pumping fluid through a reactor).

Such a device is described by Antoniades, US patent No. 6422736, the contents of which are incorporated into this description by reference. The described reactor is useful for fast reactions, such as the reactions represented by the equation below, where the AgCl product is a crystalline material having a diameter of the order of several hundred nanometers to several thousand nanometers.

AgNO ₃ + NaCl → AgCl + NaNO ₃

The cerium dioxide particles obtained using this type of agitation are often too large to be suitable for certain applications. It is highly desirable to have the smallest possible particles of cerium dioxide, since their catalytic ability, i.e. their ability to give oxygen to the combustion system (see equation 1) increases with decreasing particle size, especially for particles having an average diameter of less than 10 nm.

PCT / US 2007/077545, METHOD FOR PRODUCING CERIUM DIOXIDE NANOPARTICLES, registered on September 4, 2007, describes a mixing device that is capable of producing CeO ₂ nanoparticles _of up to 1.5 nm in high yield and very high suspension densities. This reactor has inlet ports for adding reagents, a propeller, an axis and a motor for mixing. The reaction mixture is contained in a reactor vessel. The addition of reagents, such as cerium nitrate, an oxidizing agent, and hydroxide ion, to this reservoir can lead to the formation of CeO ₂ nanoparticles, which are initially formed as very small nuclei. Stirring causes the nuclei to circulate; while the cores are continuously circulating in the reaction mixing mode, they grow (increase in diameter), as they include fresh reagents. Thus, after the initial stationary concentration of nuclei is formed, the population of these nuclei then grows into large particles in a continuous manner. If grain growth limiters are not used to complete the phase growth, this nucleation and growth process is undesirable if it is desired to limit the final particle size while maintaining a high particle density of the suspension.

An example of this nucleation and growth process in relation to the water deposition of CeO ₂ is in Zhang et al., J. Appl. Phys., 95, 4319 (2004) and Zhang et al., Applied Physics Letters, 80 127 (2002). Using cerium nitrate hexahydrate as a cerium source (very diluted at 0.0375 M) and 0.5 M hexamethylenetetramine as an ammonia precursor, 2.5-4.25 nm particles of cerium dioxide are formed in less than 50 minutes. These particles are then grown to 7.5 nm or more using a reaction time of the order of 250 minutes or 600 minutes, depending on the growth conditions. Limitations of particle size, concentration and reaction time exclude this method from consideration as an economically viable route to bulk commercial quantities of CeO ₂ nanoparticles.

I.H. Leubner, Current Opinion in Colloid and Interface Science, 5, 151-159 (2000), Journal of Dispersion Science and Technology, 22, 125-138 (2001) and ibid. 23, 577-590 (2002), and the references cited therein, provide theoretical work regarding the number of stable crystals formed at the molar rate of reagent addition, crystal solubility, and temperature. This model also takes into account the influence of diffusion, kinetically regulating the growth processes of Oswald's ripening agents and growth limiters / stabilizers on the number of crystals. High molar addition rates, low temperatures, low solubility and the presence of growth inhibitors all contribute to a large number of nuclei and, therefore, a smaller final grain or particle size.

In contrast to batch reactors, colloidal mills usually have flat-bladed turbines rotating at 10,000 rpm, whereby materials are forced through a sieve, the openings of which can vary in size from fractions of a millimeter to several millimeters. Usually there is no chemical reaction, but only a change in the size of the particles smashed by grinding. In certain cases, particle size and stability can be thermodynamically controlled by the presence of a surfactant. For example, Langer et al., In US Pat. No. 6,368,366 and US Pat. No. 6,363,237, the contents of which are incorporated herein by reference, describe an aqueous microemulsion in a hydrocarbon fuel composition obtained under strong shear conditions. However, the aqueous phase of the particles (discrete phase in the fuel composition) has a large size, of the order of 1000 nm.

Colloidal mills are not suitable for reducing the particle size of large particles of cerium dioxide, since these particles are too hard to crack in a reasonable amount of time. The preferred method for reducing large agglomerated particles of cerium dioxide from micron to nanometer size is to grind for several days in a ball mill in the presence of a stabilizing agent. This is a time-consuming, expensive method that invariably provides a wide distribution of particle sizes. Thus, there remains a need for an economical and easy method for the synthesis of large quantities, with a high density of the suspension, of very small nanometric particles of cerium dioxide having a uniform size distribution and containing ions of one or more transition metals, while maintaining the cubic fluorite structure of CeO ₂ .

Water precipitation may offer the usual route to cerium nanoparticles. However, in order to be applicable as a fuel combustion catalyst for fuels, cerium dioxide nanoparticles must exhibit stability in a non-polar environment, such as diesel fuel. Most stabilizers used to prevent agglomeration in the aquatic environment do not suit the stabilization task in a non-polar environment. When placed in a non-polar solvent, such particles tend to agglomerate immediately and therefore lose some, if not all, of their desired nanoscale properties. Thus, it would be desirable to form stable particles of cerium dioxide in an aqueous environment, maintain the same stabilizer on the surface of the particles, and then be able to transfer these particles to a non-polar solvent, where the particles will remain stable and form a homogeneous mixture or dispersion. In such a simplified and economical manner, the need to change the affinity of the surface stabilizer from polar to non-polar can be eliminated. Replacing stabilizers can include a complex displacement or separation reaction, tedious isolation and re-dispersion methods, such as, for example, precipitation and subsequent re-dispersion with a new stabilizer using a ball mill.

Thus, there remains a need for an efficient and economical method for synthesizing stable cerium dioxide nanoparticles containing transition metals in a polar, aqueous environment and then transferring these particles to a non-polar environment, where a stable homogeneous mixture is formed.

The use of cerium nanoparticles to provide a high temperature, oxidation resistant coating is described, for example, in "Synthesis Of Nano Crystalline Ceria Particles For High Temperature Oxidization Resistant Coating", S. Seal et al., Journal of Nanoparticle Research, 4, pp 433-438 ( 2002). The deposition of cerium dioxide on various surfaces was studied, including alloys of Ni, chromium and aluminum, and stainless steel, and on Ni, and surfaces coated with Ni-Cr alloy. It has been found that a cerium dioxide particle size of 10 nm or less is desired. The incorporation of cerium oxide particles subsequently inhibits the oxidation of the metal surface.

Rim, US patent No. 6892531, the contents of which are incorporated into this description by reference, describes a lubricating oil composition for a diesel engine that contains lubricating oil and 0.05-10% by weight of a catalytic additive containing cerium carboxylate.

As described above, the currently available fuel additives based on cerium oxide and doped cerium oxide improve the combustion of diesel fuel; however, further improvements are still needed. It would be desirable to obtain these fuel additives for diesel engines that provide additional improved fuel combustion due to the advantage of even smaller, below 5 nm, cubic CeO ₂ nanoparticles with higher specific surface areas. The increased oxygen storage ability provided by the inclusion of transition metals at these grain sizes is also very desirable. In addition, engine wear protection, reduced engine friction, and greater lubricity with simultaneously improved fuel efficiency would be extremely beneficial.

SUMMARY OF THE INVENTION

The present invention relates to a method for producing cerium dioxide nanoparticles with a predetermined lattice containing at least one transition metal (M), which contains: (a) obtaining an aqueous reaction mixture containing a source of cerium ions, a source of ions of one or more transition metals (M ), a source of hydroxide ions, at least one nanoparticle stabilizer and an oxidizing agent at an initial temperature in the range of from about 20 ° C to about 95 ° C; (b) the mechanical shift of the mixture and its passage through the perforated sieve, forming a suspension of cerium hydroxide nanoparticles; and (c) providing temperature conditions effective for oxidizing the Ce ³⁺ ion to the Ce ⁴⁺ ion to form a product stream containing cerium dioxide nanoparticles containing a transition metal, Ce _1-x M _x O ₂ . The cerium dioxide nanoparticles thus obtained have a cubic fluorite structure, an average hydrodynamic diameter in the range of from about 1 nm to about 10 nm, and a geometric diameter from about 1 nm to about 4 nm. The method can be performed both in batch and continuous mode.

The present invention further relates to a method for forming a homogeneous dispersion containing stabilized cerium dioxide nanoparticles containing a transition metal, Ce _1-x M _x O ₂ , which contains: (a) preparing an aqueous mixture that contains stabilized transition metal cerium nanoparticles, Ce _1-x M _x O ₂ having a cubic fluorite structure, an average hydrodynamic diameter in the range of from about 1 nm to about 10 nm, and a geometric diameter of from about 1 nm to about 4 nm; (b) concentrating an aqueous mixture that contains stabilized cerium dioxide nanoparticles containing a transition metal to form an aqueous concentrate; (c) removing substantially all of the water from the aqueous concentrate to form a substantially water-free concentrate of stabilized cerium dioxide nanoparticles containing a transition metal; (d) adding an organic diluent to a substantially water-free concentrate to form an organic concentrate of stabilized cerium dioxide nanoparticles containing a transition metal; and (d) combining the organic concentrate with a surfactant in the presence of a non-polar medium to form a homogeneous dispersion containing stabilized transition metal metal particles of cerium dioxide, Ce _1-x M _x O ₂ , where "x" has a value of from about 0, 3 to about 0.8, more preferably from about 0.40 to about 0.60.

Brief Description of the Drawings

Figa and 1B are respectively a TEM image and a frequency analysis of particle size using a TEM of CeO ₂ nanoparticles obtained by non-isothermal deposition, as described in example 1.

Figure 2 is a spectrum of x-ray powder diffraction of cerium dioxide nanoparticles obtained as described in example 1.

Figure 3A is a TEM image of a 1.1 nm CeO ₂ nanoparticle prepared as described in Example 2; 3B is an electron diffraction image of these 1.1 nm particles; Fig. 3C is a table 1 containing the intensities calculated from the measured electron diffraction for cubic and hexagonal CeO ₂ and Ce ₂ O ₃ gratings.

4A and 4B are respectively a TEM image and a frequency analysis of particle size using a TEM of isothermally deposited CeO ₂ nanoparticles obtained using the triple jet method as described in Example 3.

5A and 5B respectively represent a TEM image and a frequency analysis of particle size using a TEM of isothermally deposited Cu-containing CeO ₂ nanoparticles prepared as described in Example 4.

6A and 6B are respectively a TEM image and a frequency analysis of particle size using a TEM of isothermally deposited Fe-containing CeO ₂ nanoparticles prepared as described in Example 5.

7A and 7B are respectively a TEM image and a frequency analysis of particle size using a TEM of isothermally deposited Zr-containing CeO ₂ nanoparticles prepared as described in Example 6.

Figs. 8A and 8B are respectively a TEM image and a frequency analysis of particle size using a TEM of isothermally deposited CeO ₂ nanoparticles containing Zr and Fe obtained as described in Example 7. Fig. 8C is an X-ray diffraction spectrum of isothermally deposited CeO ₂ nanoparticles. containing Zr and Fe obtained as described in example 7.

Figure 9 is a TEM field emission image of a lattice of CeO ₂ nanoparticles containing Zr and Fe obtained as described in Example 7.

DETAILED DESCRIPTION OF THE INVENTION

In this application, the expression "transition metal" is understood as covering 40 chemical elements from 21 to 30, from 39 to 48, from 72 to 80, which are included in periods 4, 5, 6 of the periodic table, respectively.

The present invention provides a method for producing cerium dioxide (CeO ₂ ) nanoparticles containing transition metal ions, which comprises: (a) preparing an aqueous reaction mixture containing a source of cerium ions and one or more transition metal ions, a source of at least one hydroxide ion nanoparticle stabilizer and oxidizing agent; (b) the mechanical shift of the mixture and its passage through the perforated sieve, forming a suspension of cerium hydroxide nanoparticles; and (c) creating temperature conditions effective to oxidize the Ce ³⁺ ion to the Ce ⁴⁺ ion to form a product stream containing cerium dioxide nanoparticles containing a transition metal, Ce _1-x M _x O ₂ , which have a cubic fluorite structure , an average hydrodynamic diameter in the range of from about 1 nm to about 10 nm, and a geometric diameter from about 1 nm to about 4 nm. Crystalline cerium dioxide particles containing ions of one or more transition metals and having a monomodal size distribution and a monodisperse frequency distribution of size can be selectively obtained in this size range. Single-crystal particles contain from two unit cells per face for 1.1 nm particles to 5 unit cells per face for 2.7 nm particles, depending on the preparation conditions. Here, the word "crystalline" refers to particles that are not formed from numerous agglomerated crystallites of various sizes, but are a single crystal with well-defined sizes, given the number of constituent unit cells.

The present invention further provides a continuous process for the preparation of crystalline cerium dioxide CeO ₂ nanoparticles containing ions of one or more transition metals and having an average hydrodynamic diameter of from about 1 nm to about 10 nm, where the method comprises the step of combining Ce ³⁺ ions, one or more ions transition metals, an oxidizing agent, at least one nanoparticle stabilizer, and ion hydroxide in a continuous reactor.

The present invention also provides a method for producing cerium dioxide nanoparticles, which comprises the steps of: (a) providing an aqueous first reaction mixture comprising a source of Ce ³⁺ ions, ions of one or more transition metals and at least one nanoparticle stabilizer; (b) stirring the first reaction mixture while adding an oxidizing agent to obtain a second reaction mixture; (c) adding a source of hydroxide ions to the second reaction mixture when subjected to mechanical shear to form a third reaction mixture; and (a) heating the third reaction mixture to a temperature of from about 50 ° C to about 100 ° C to obtain crystalline cerium dioxide nanoparticles that contain ions of one or more transition metals and have a substantially unimodal and uniform frequency distribution in size.

The present invention also provides a method of forming a uniform mixture that contains the above crystalline cerium dioxide nanoparticles, at least one nanoparticle stabilizer, at least one surfactant, a mixture of glycol ethers and a non-polar medium. This method includes the steps of: (a) preparing an aqueous mixture that contains stabilized cerium dioxide nanoparticles obtained by closely associating the nanoparticle stabilizer with crystalline cerium dioxide nanoparticles; (b) concentrating the aqueous mixture comprising stabilized crystalline cerium dioxide nanoparticles to form an aqueous concentrate; and (c) removing substantially all of the water with a solvent moving glycol from the aqueous environment to the surrounding ether, combining the surfactant and optionally the joint surfactant with the displaced solvent concentrate in the presence of a non-polar medium to form homogeneous mixture.

In the presence of hydroxide ions, the Ce ⁴⁺ ion reacts to form cerium hydroxide, which when heated turns into crystalline cerium dioxide. The temperature in the reaction tank is maintained from about 50 ° C to about 100 ° C, more preferably about 65-95 ° C, most preferably about 85 ° C. Time and temperature can change places, higher temperatures usually reduce the time required to convert the hydroxide to oxide. After a period with these elevated temperatures, of the order of about 1 hour or less, and possibly about 0.5 hours, cerium hydroxide is converted to crystalline cerium dioxide, and the temperature of the reaction vessel is reduced to about 15-25 ° C. The crystalline cerium dioxide nanoparticles are then concentrated, and unreacted cerium and unnecessary by-products such as ammonium nitrate are removed most conveniently, for example, by diafiltration.

In one aspect of the present invention, a method for producing crystalline cerium dioxide nanoparticles containing one or more transition metal ions comprises: preparing an aqueous reaction mixture containing Ce ³⁺ ions, one or more transition metal ions, hydroxide ions, a stabilizer or combination of stabilizers and an oxidizing agent, carrying out the reaction at a temperature effective for generating small nuclei and achieving the subsequent oxidation of Ce ³⁺ ions to Ce ⁴⁺ ions and allowing the nuclei to grow and nanometric cerium dioxide. The reaction mixture is subjected to mechanical shear, preferably causing it to pass through a perforated sieve, forming a suspension of crystalline cerium dioxide nanoparticles having an average hydrodynamic diameter in the range of from about 1 nm to about 10 nm. Although the particle diameter can be adjusted in the range from 1.5 nm to 25 nm, preferably the crystalline cerium dioxide nanoparticles have an average hydrodynamic diameter of about 10 nm or less, more preferably about 8 nm or less, most preferably about 6 nm. It is desirable if the nanoparticles contain one or at most two primary crystallites per face of the particle, where each crystallite averages 2.5 nm (approximately 5 unit cells). Thus, the obtained nanoparticle size frequency is essentially monodisperse, i.e. has a coefficient of variation (COV) of less than 25%, where COV is defined as the standard deviation divided by the mean.

Mechanical shear involves the movement of fluids along surfaces such as rotor surfaces, which results in shear stress. In particular, the laminar flow on the surface has zero velocity, and shear stress occurs between the zero velocity surface and a higher velocity flow from the given surface.

In one embodiment, the present invention uses a colloidal mill, which is commonly used to grind microemulsions or colloids, as a chemical reactor to produce cerium dioxide nanoparticles. Examples of useful colloidal mills include mills described by Korstvedt, US Pat. No. 6,745,961 and US Pat. No. 6,305,626, the contents of which are incorporated herein by reference.

Preferably, if the reagents include an aqueous solution of a source of Ce ³⁺ ions, for example, cerium (III) nitrate; an oxidizing agent 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 oxidizing agent, such as peroxide, is present, preferably in at least half the molar concentration of cerium ions. The concentration of hydroxide ions is preferably at least two, more preferably three times or even five times higher than the molar concentration of cerium ions.

Initially, the reaction chamber is maintained at a temperature low enough to generate cerium (III) hydroxide nuclei of small size, which can grow into nanometric crystalline particles of cerium dioxide after subsequent displacement to higher temperatures, leading to the conversion of Ce ³⁺ ions to the state of Ce ⁴ ions ⁺ . Initially, the temperature is suitably about 25 ° C. or less, although higher temperatures can be used without a substantial increase in particle size.

In one embodiment, the source of Ce ³⁺ ions, one or more transition metal ions, a nanoparticle stabilizer, and an oxidizing agent are in the reactor, and the ion hydroxide source, such as ammonium hydroxide, is added quickly with stirring, preferably over a period of about 10 minutes or less . Under certain conditions, such as adding a single stream of ammonium to metal ions, about 20 seconds or less are preferred, even more preferably about 15 seconds or less. In an alternative embodiment, a source of hydroxide ions and an oxidizing agent are in the reactor, and a source of Ce ³⁺ ions and one or more transition metal ions is added over a period of about 15 seconds to 20 minutes. In a third and preferred embodiment, the stabilizer is in the reaction tank, and cerium (III) nitrate with ions of one or more transition metals is simultaneously introduced into the reaction chamber with a separate stream of ammonium hydroxide, said hydroxide ions being in a molar stoichiometric ratio to Ce ³ ions ⁺ from about 2: 1 OH: Ce to about 5: 1 OH: Ce, with an optimal molar stoichiometric ratio of 2: 1, 3: 1 or even 5: 1 OH: Ce.

Ce ³⁺ ions react with an oxidizing agent in the presence of hydroxide ions, forming cerium hydroxide, which can be converted to crystalline cerium by heating. The temperature in the reaction tank is maintained from about 50 ° C to about 100 ° C, preferably from about 65-85 ° C, more preferably from about 70 ° C. The incorporation of certain transition metal ions, such as Zr and Cu, usually requires higher temperatures, approximately 85 ° C. After a period of time at these elevated temperatures, preferably about 1 hour or less, more preferably about 0.5 hours, the doped cerium hydroxide is substantially converted to crystalline cerium dioxide, and the temperature of the reaction vessel is reduced to about 15-25 ° C. Time and temperature variables can change places; higher temperatures usually require a shorter reaction time. The suspension of cerium dioxide nanoparticles is concentrated, and unreacted cerium and unnecessary by-products such as ammonium nitrate are removed, which can conveniently be accomplished by diafiltration.

A nanoparticle stabilizer is a critical component of the reaction mixture. Preferably, the stabilizer of the nanoparticles is water soluble and forms weak bonds with the cerium ion. K _BC denotes the binding constant of the stabilizer of nanoparticles with cerium ion in water. Log K _BC for the nitrate ion is 1 and for hydroxide ion is 14. Most preferably, if log K _BC lies in this range, preferably in the middle of this range. Suitable nanoparticle stabilizers include alkoxy substituted carboxylic acids, α-hydroxycarboxylic acids, α-ketocarboxylic acids, such as pyruvic acid, and small organic polyacids, such as tartaric acid and citric acid. Examples of alkoxylated carboxylic acids include di-, tri- and tetracarboxylic acids and combinations thereof, methoxyacetic acid, 2- (methoxy) ethoxyacetic acid and 2- [2- (2-methoxyethoxy) ethoxy] acetic acid (MEEA). Among α-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 high K _BC , such as EDTA, and stabilizers with low K _BC , such as lactic acid, are also applicable in special respects. High K _VS stabilizers such as gluconic acid can be used at low levels or with low K _VS stabilizers such as lactic acid.

In one desirable embodiment, the nanoparticle stabilizer comprises a compound of formula (Ia). In the formula (Ia), R is hydrogen, or a substituted or unsubstituted alkyl group, or an aromatic group, such as, for example, a methyl group, an ethyl group or a phenyl group. More preferably, R is a lower alkyl group, such as a methyl group. R ¹ is hydrogen or a substituted group, such as an alkyl group. In the formula (Ia), n is an integer of 0-5, preferably 2, and Y is H or a counterion, such as an alkali metal, for example, Na ⁺ or K ⁺ . The stabilizer binds to nanoparticles and prevents particle agglomeration and subsequent formation of large clusters of particles.

In another embodiment, the nanoparticle stabilizer is denoted by formula (Ib), where R 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 ⁺ or K ⁺ , and p is 1 or 2.

Suitable nanoparticle stabilizers are also found among α-hydroxy-substituted carboxylic acids, such as lactic acid, and among polyhydroxy-substituted acids, such as gluconic acid.

Preferably, the nanoparticle stabilizer does not include a sulfur element, since sulfur-containing materials may not be desirable for certain applications. For example, if cerium dioxide particles are included in the composition of the fuel additive, the use of a sulfur-containing stabilizer, such as AOT, can lead to undesirable emission of sulfur oxides after combustion.

The size of the obtained particles of cerium dioxide can be determined using dynamic light scattering, measurement technology to determine the hydrodynamic diameter of the particles. Hydrodynamic Diameter (cf. BJ 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", SH Chen and M. Kotlarchyk, World Scientific Publishing, Singapore, 1997), which is slightly larger than the geometric diameter of the particle, includes both the intrinsic particle size and the solvation shell surrounding the particle. When a ray of light passes through a colloidal dispersion, particles or droplets scatter part of the light in all directions. When the particles are very small compared to the wavelength of light, the intensity of the scattered light is uniform in all directions (Rayleigh scattering). If the light is coherent and monochromatic, such as a laser, for example, time-dependent fluctuations in the scattering intensity can be observed using a suitable detector such as a photomultiplier capable of operating in photon counting mode. These fluctuations come from the fact that the particles are small enough to undergo random thermal Brownian motion, and the distance between them therefore constantly changes. The amplifying and attenuating interference of light scattered by neighboring particles in the illuminated zone gives an increase in the intensity fluctuation in the plane of the detector, which, since it occurs due to the movement of particles, contains information about this movement. An analysis of the time dependence of intensity fluctuations can, therefore, give a particle diffusion coefficient, from which, according to the Stokes-Einstein equation and the known viscosity of the medium, it is possible to calculate the hydrodynamic radius or particle diameter.

In another aspect of the invention, a continuous process for the preparation of small crystalline cerium dioxide CeO ₂ nanoparticles containing transition metal ions, i.e., particles having an average diameter of less than about 10 nm, comprises combining Ce ³⁺ ions, one or more transition metal ions, an oxidizing agent, a nanoparticle stabilizer, or a combination of stabilizers, and ion hydroxide in a continuous reactor into which reactants and other ingredients are continuously introduced, and from which the product is continuously removed. Continuous methods are described, for example, in Ozawa, et al., US patent No. 6897270; Nickel, et al. US Pat. No. 6,723,138; Campbell, et al. U.S. Patent No. 6,677,720; Beck, US patent No. 5097090; and Byrd, et al., U.S. Patent No. 4,661,321; the contents of which are incorporated herein by reference.

A solvent, such as water, is often used in this method. The solvent dissolves the reagents, and the solvent stream can be adjusted to control the process. Advantageously, mixers can be used to agitate and mix reagents.

Any reactor that is capable of receiving a continuous stream of reactants and delivering a continuous stream of product can be used. These reactors may include continuous stirred tank reactors, reciprocating flow reactors, and the like. The reagents required to carry out the synthesis of nanoparticles are preferably loaded into the reactor in streams; those. they are preferably administered in the form of liquids or solutions. The reactants may be charged in separate streams, or certain reactants may be combined prior to loading the reactor.

Reagents are introduced into a reaction chamber equipped with a stirrer through one or more inlets. Typically, the reagents include an aqueous solution of a source of Ce ³⁺ ions, for example, cerium (III) nitrate, transition metal ions, such as, for example, iron nitrate or copper nitrate; an oxidizing agent 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 oxidizing agent, such as hydrogen peroxide, is preferably present in at least half the molar concentration of cerium ions. Alternatively, molecular oxygen may be bubbled through the mixture. The concentration of hydroxide ions is preferably two times higher than the molar concentration of cerium.

In one embodiment of the present invention, the method of forming small cerium dioxide nanoparticles comprises the step of forming a first aqueous reagent stream that includes Ce ³⁺ ions, for example, in the form of cerium (III) nitrate, one or more transition metal ions, and an oxidizing agent. Suitable oxidizing agents capable of oxidizing Ce (III) to Ce (IV) include, for example, hydrogen peroxide or molecular oxygen. Optionally, the first reagent stream also includes a nanoparticle stabilizer that binds to doped cerium nanoparticles to prevent particle agglomeration. Examples of suitable nanoparticle stabilizers have been mentioned above.

This method further includes the step of forming a second aqueous stream of reagents, which includes a source of hydroxide ions, for example, ammonium hydroxide or potassium hydroxide. Optionally, the second reagent stream further includes a stabilizer, examples of which are described previously. At least one of the first or second reagent streams, however, must contain a stabilizer or a combination of stabilizers.

The first and second reactant streams combine to form a reaction stream. Initially, the temperature of the reaction stream is kept low enough to form small cerium (III) hydroxide nuclei. Then, the temperature is increased, so that the oxidation of Ce (III) in Ce (IV) occurs in the presence of an oxidizing agent, and the hydroxide is converted into an oxide, forming a product stream, which includes crystalline cerium dioxide. The temperature for converting the hydroxide to 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 then increased to about 60-90 ° C. Isothermal precipitation at elevated temperatures, for example, 90 ° C, is an alternative way to obtain small nanoparticles, provided that the stage of growth can be inhibited by a suitable molecular adsorbate (growth restrictor).

Preferably, crystalline cerium dioxide nanoparticles with a predetermined lattice in the product stream are concentrated, for example, using diafiltration techniques using one or more semi-porous membranes. In one embodiment, the product stream comprises an aqueous suspension of crystalline cerium dioxide nanoparticles containing a transition metal that is reduced in conductivity to about 5 mS / cm or less using one or more semi-porous membranes.

A schematic representation of a continuous reactor suitable for carrying out the invention is shown in FIG. 3 in PCT / US 2007/77545, METHOD FOR PRODUCING CERIUM DIOXIDE NANOPARTICLES, registered on September 4, 2007. Reactor 40 includes a first reactant stream 41 containing aqueous cerium nitrate. An oxidizing agent, such as hydrogen peroxide, is added to the reactant stream through an inlet 42 and the reactants are mixed using a mixer 43a. A stabilizer is added to the resulting mixture through inlet 45, followed by stirring with a mixer 43b. The mixture from mixer 43b then enters mixer 43c, where it is combined with a second reagent stream containing ammonium hydroxide from inlet 44. The first and second reagent streams are mixed using mixer 43c to form a reaction stream that can undergo mechanical shear by passing it through the perforated sieve. In a further embodiment, the mixer 43c comprises a colloid mill reactor as previously described, which is equipped with inlet ports for receiving reagent 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 ° FROM.

The mixture of 43c enters the reactor tube 45, which is located in the bath 46 with a constant temperature. which supports the pipe 45 at a temperature of approximately 60-90 ° C.

Crystalline cerium dioxide nanoparticles are formed in the reactor tube 45, which may include a spiral 50. The product stream then enters one or more diafiltration cells 47, where crystalline cerium dioxide nanoparticles are concentrated using one or more semi-porous membranes. One or more diafiltration cells can be connected in series to achieve product concentration in one pass, or the cells can be placed in parallel for very high volumetric productivity. Diafiltration cells can be arranged both sequentially and in parallel to achieve both high volumetric and fast productivity. Concentrated crystalline cerium dioxide nanoparticles exit the diafiltration cell through outlet port 49, and excess reagents and water are removed from the diafiltration cell 47 through outlet port 48. In an alternative embodiment, a stabilizer may be added to the second reactant stream through port 51 rather than to the first reactant stream through port 45.

In one embodiment of the invention, the product stream of concentrated crystalline cerium dioxide nanoparticles with a predetermined lattice exiting the diafiltration cell 47 is converted by changing the solvent to a substantially water-free environment of one or more glycol ethers. This can be done using dialysis bags or by passing aqueous nanoparticles through a diafiltration column with an organic diluent that preferably contains one or more glycol ethers. The organic diluent may further include alcohol. A suitable diluent comprises a mixture of diethylene glycol monomethyl ether and 1-methoxy-2-propanol.

The organic concentrate obtained by changing the solvent is combined with a surfactant, such as oleic acid, followed by combining with a stream that includes a non-polar solvent, such as kerosene or ultra-low sulfur diesel fuel, forming a homogeneous dispersion of crystalline cerium dioxide particles with a given lattice, which can be mixed with hydrocarbon fuels such as diesel.

Using a continuous method for producing crystalline cerium dioxide nanoparticles with a given lattice makes it possible to better control the production of particle nuclei and their growth compared to how it is done in batch reactors. The size of the nuclei can be adjusted using the initial concentration of the reactants, temperature, and the ratio of the stabilizer of the nanoparticles to the concentrations of the reactants. For small nuclei, low temperatures, less than about 20 ° C, and high ratios of nanoparticle stabilizer to reagent concentrations are favorable. In this way, very small nanoparticles having an average hydrodynamic diameter of less than about 10 nm, with geometric particle diameters of less than about 3 nm, can be obtained in an economical manner.

The present invention provides a method for producing a homogeneous mixture that includes cerium dioxide (CeO ₂ ) nanoparticles containing one or more transition metal ions, a nanoparticle stabilizer, a surfactant, glycol ethers and a non-polar solvent. Preferably, the nanoparticles have an average hydrodynamic diameter of less than about 10 nm, more preferably less than about 8 nm, most preferably about 6 nm with geometric particle diameters (determined using TEM) less than about 4 nm.

As described above, crystalline cerium dioxide nanoparticles with a given lattice can be obtained using various procedures. Typical synthesis routes use water as a solvent and result in an aqueous mixture of nanoparticles and one or more salts. For example, cerium dioxide particles can be obtained by reacting cerium (III) nitrate hydrate with hydroxide ions 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 by an oxidizing agent such as hydrogen peroxide, as shown in equation (3b). The stoichiometry of a similar ternary hydroxide is shown in equations (4a) and (4b).

Complexes formed with very high levels of base, for example 5: 1 OH: Ce, also lead to the formation of cerium oxide, albeit with much larger grain sizes, if there is no proper growth restriction.

In some cases, especially when ammonium hydroxide is not present in excess relative to the Ce ³⁺ ion, particles of Ce (OH) ₂ (NO ₃ ) or (NH ₄ ) ₂ Ce (NO ₃ ) ₅ may initially be present, subsequently being oxidized to cerium dioxide .

Crystalline cerium dioxide particles containing a transition metal are formed in an aqueous environment and combine with one or more nanoparticle stabilizers. Preferably, the cerium dioxide nanoparticles are either formed in the presence of stabilizer (s) or stabilizer (s) are added after their formation. Suitable nanoparticle stabilizers include alkoxy substituted carboxylic acids, α-hydroxycarboxylic acids such as pyruvic acid, and small organic polycarboxylic acids. Examples of alkoxy substituted carboxylic acids include methoxyacetic acid, 2- (methoxy) ethoxyacetic acid, and 2- [2- (2-methoxyethoxy) ethoxy] acetic acid (MEAA). Examples of α-hydroxycarboxylic acids; examples include lactic acid, gluconic acid and 2-hydroxybutanoic acid. Polycarboxylic acids include ethylenediaminetetraacetic acid (EDTA), tartaric acid and citric acid. In desired embodiments, the nanoparticle stabilizer comprises a compound of formula (Ia) or formula (Ib) as described above.

The reaction mixture includes, in addition to crystalline particles of cerium dioxide containing transition metals, one or more salts, for example, ammonium nitrate and unreacted cerium nitrate. The stabilizer particles can be separated from these materials and salts by washing with 18 MΩ of 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 environment. Washed stabilized cerium dioxide nanoparticles can be concentrated, if desired, using a semi-porous membrane, for example, to form an aqueous nanoparticle concentrate. Particles can be concentrated using another means, as well as, for example, by centrifugation.

The product stream is concentrated into a suspension having a density not exceeding approximately 35% (mass. Solids / mass, suspension) and a conductivity not exceeding approximately 5 mS / cm.

In one preferred embodiment, cerium dioxide crystalline particles containing transition metals are concentrated by diafiltration. Diafiltration technology uses ultrafiltration membranes, which can be used to completely remove, replace or reduce the concentration of salts in a mixture containing nanoparticles. This method selectively uses semi-permeable (semi-porous) membrane filters to separate the components of the reaction mixture based on their molecular size. Thus, a suitable ultrafiltration membrane will be sufficiently porous to retain most of the formed nanoparticles, allowing smaller molecules, such as salts and water, to pass through the membrane. In this way, the nanoparticles and the corresponding coupled stabilizers can be concentrated. Materials retained by the filter, including stabilized nanoparticles, are called a concentrate or retention product, effluent salts and unreacted materials are called filtrate.

Pressure can be applied to the mixture to accelerate the speed at which small molecules pass through the membrane (flow rate) and accelerate the concentration process. Other means of increasing the flow rate include the use of a large membrane having a high surface area and an increase in the pore size of the membrane, but without unacceptable loss of nanoparticles.

In one embodiment, the membrane is selected so that the average pore size of the membrane is approximately 30% or less, 20% or less, 10% or less, or even 5% or less than the average diameter of the nanoparticles. However, the pore diameter should be sufficient to allow the passage of water and salt molecules. For example, ammonium nitrate and unreacted cerium nitrate must be completely or partially removed from the reaction mixture. In one preferred embodiment, the average pore size of the membrane is small enough to hold particles with a diameter of 1.5 nm or more in concentrate. This corresponds to a protein size of approximately 3 kilodaltons.

Preferably, the concentrate includes stabilized nanoparticles and residual water. In one embodiment, the molar concentration of cerium dioxide nanoparticles is preferably more than about 0.5 mol, more preferably more than about 1.0 mol, even more preferably more than about 2.0 mol (about 35% solids in this dispersion).

Once the concentrate is obtained, most, if not all, of the water is removed by dialysis with glycol ethers. This is accomplished by placing the concentrate in a 2 kilodaltons dialysis bag with a mixture of diethylene glycol methyl ether and 1-methoxy-2-propanol, and allowing the water to transfer to the glycol ether medium, while the glycol ether medium replaces the water in the dispersion of nanoparticles. Several exchanges may be necessary (substituting glycol ether medium). Alternatively, the glycol ether mixture may be passed with aqueous crystalline particles of cerium dioxide containing transition metals through a diafiltration column, and a change of solvent is thus caused.

Glycol ether surfactants that contain both an ether group and an alcohol group include compounds of formula (Ic) in which R ³ is a substituted or unsubstituted alkyl group, am is an integer of 1-8.

Other suitable solvent-changing surfactants include nonylphenylethoxylates having the formula C ₉ H ₁₉ C ₆ H ₄ (OCH ₂ CH ₂ ) _n OH, where n is 4-6.

Once the crystalline particles of cerium dioxide containing transition metals are in an organic medium, still stabilized by the original stabilizer used in their preparation, but complexing with glycol ether, this mixture can be dispersed in a non-polar medium such as kerosene, which is compatible with most hydrocarbon fuels such as diesel and biodiesel. The surface of the particles is first functionalized with a surfactant, such as oleic acid, and possibly a joint surfactant, such as 1-hexanol, before being added to the hydrocarbon diluent. It is important to understand that this composition of the substance is not a reverse micellar water-in-oil emulsion, since very little water is present in it; rather, the positive charge on the surface of the cerium nanoparticles is complexed by the oxygen atoms of the ether and bound to the oppositely charged carboxylic acid. Carboxylic acid is present in a chemisorbed state and facilitates the miscibility of nanoparticles with a non-polar hydrocarbon diluent. Other surface functionalization materials, such as linoleic acid, stearic acid and palmitic acid, may be present instead of oleic acid. Typically, preferred materials are carboxylic acids with a carbon chain length of less than 20 carbon atoms, but more than 8 carbon atoms. Other suitable non-polar diluents include, for example, hydrocarbons containing from about 8 to 20 carbon atoms, for example, octane, nonane, decane or toluene, and hydrocarbon fuels such as gasoline, biodiesel and diesel.

For optimal miscibility and stability with non-polar hydrocarbons, it is desirable that very few electricity-conducting ions are present in the cerium dioxide concentrate. This situation can be achieved by concentrating the nanoparticles by diafiltration to a conductivity level of less than about 5 mS / cm, preferably up to about 3 mS / cm or less.

The resistivity is reverse conductivity, which is the ability of a material to conduct electric current. Conductivity tools can measure conductivity by including two plates that are located in the sample, applying a potential to the plates (usually a sinusoidal voltage) and measuring current. Conductivity (G), the inverse of resistivity (R), is determined from the values of voltage and current according to Ohm's law G = 1 / R = I / E, where I denotes the current in amperes and E denotes the voltage in volts. Since the charge on the ions in the solution facilitates the passage of electric current, the conductivity of the solution is proportional to the concentration of ions in it. The basic unit of conductivity is siemens (cm) or millisyms (mS). Since the cell geometry affects the conductivity, standardized measurements are expressed in units of specific conductivity (mS / cm) to compensate for changes in the size of the electrodes.

The present invention further relates to a method for producing a homogeneous mixture, which comprises cerium dioxide nanoparticles containing transition metals, at least one nanoparticle stabilizer, one or more medium replacing solvents, such as glycol ethers, at least one surfactant and a non-polar diluent or solvent. The first stage leads to the production of an aqueous mixture that contains stabilized cerium dioxide nanoparticles, where the nanoparticle stabilizer molecules are closely related to the nanoparticles. The second step involves the concentration of 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. The third step removes water bound to the nanoparticles using a non-ionic surfactant. The final step involves combining this concentrate with a substituted solvent with a non-polar solvent containing a surfactant, thereby forming a substantially homogeneous mixture, which is a thermodynamically stable, multicomponent, two-phase dispersion.

This essentially homogeneous thermodynamic dispersion contains the aforementioned nanoparticles in a concentration of at least about 2% by weight, more preferably at least about 9% by weight, and water at a maximum concentration of about 0.5% by weight.

The cerium dioxide nanoparticles containing transition metals have an average 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.

Preferably, the cerium dioxide nanoparticles have a primary crystallite size of approximately 2.5 nm ± 0.5 nm and contain one or at most two crystallites per particle face length.

The aqueous mixture is advantageously formed in a colloid mill reactor, and the nanoparticle stabilizer may contain an ionic surfactant, preferably a compound that contains a carboxylic acid group and an ether group. The nanoparticle stabilizer may contain a surfactant with the formula (Ia),

where: R is hydrogen, or a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aromatic group; R ¹ represents hydrogen or an alkyl group; Y is H or a counterion; and n is 0-5. Preferably, R is a substituted or unsubstituted alkyl group, R ¹ is hydrogen, Y is hydrogen, and n is 2.

Another suitable nanoparticle stabilizer contains a dicarboxylate of formula (Ib),

where each R ² independently represents hydrogen, a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aromatic group; X and Z are independently H or a counterion; and p is 1 or 2.

Other suitable nanoparticle stabilizers are included in the group consisting of lactic acid, gluconic acid, EDTA, tartaric acid, citric acid, and combinations thereof.

The concentration of the aqueous mixture is preferably carried out using diafiltration, which leads to a decrease in the conductivity of said concentrated aqueous mixture to about 5 mS / cm or less.

The surfactant used to transfer stabilized crystalline cerium dioxide particles containing a transition metal from an aqueous environment to a non-aqueous may preferably comprise a non-ionic surfactant, preferably a compound containing an alcohol group and an ether group, in particular a compound of the formula (Ic)

where R ³ denotes a substituted or unsubstituted alkyl group; and m is an integer from 1 to 8.

The nonionic surfactant may also contain a compound of formula (Id),

where R ³ denotes a substituted or unsubstituted alkyl group; F denotes an aromatic group; and m is an integer from 4 to 6.

The reaction mixture may further include a joint surfactant, preferably alcohol.

The introduction of this solvent-moved concentrate is facilitated by surfactants that functionalize the surface of the nanoparticles. Preferred surfactants are carboxylic acids, such as oleic acid, linoleic acid, stearic acid and palmitic acid. Typically, preferred materials are carboxylic acids with a carbon chain length of less than 20 carbon atoms, but more than 3 carbon atoms.

The non-polar diluent included in the substantially homogeneous dispersion is advantageously selected from 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 a homogeneous dispersion is taken with at least about 100 parts of the fuel.

According to this 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 and Y, more preferably combined with Fe.

Using a homogeneous dispersion containing stabilized crystalline cerium dioxide nanoparticles containing a transition metal / according to the present invention, as defined above, it is possible to form a coating for a catalytic converter of an exhaust system of an internal combustion engine.

Thus, it may be advantageous to form a ceramic oxide coating on the outer surfaces of the cylinders of an In situ diesel engine. Potential benefits of this coating include additional thermal protection for the engine; for example, CeO ₂ melts at 2600 ° C, while cast iron, a common material used in diesel engines, melts at about 1200-1450 ° C. Even 5 nm particles of cerium oxide showed the ability to protect steel from oxidation for 24 hours at 1000 ° C; therefore, it is not expected that phenomena depending on the melting size will lower the melting point of the cerium dioxide nanoparticles of this invention below the combustion temperatures found in the engine. See, for example, Patil et al., Journal of Nanoparticle Research, vol 4, pp 433-438 (2002). An engine protected in this way 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 will be resistant to further oxidation (CeO _{2 is} 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, MTBE oxygenates, ethanol and other cetane number improvers, such as peroxides, also increase corrosion when introduced into the combustion chamber, which can lead to rust and engine life and performance degradation. This coating should not be so thick as to interfere with the cooling of the engine walls by means of a cooling system with water recirculation.

In one embodiment, the present invention provides crystalline cerium dioxide nanoparticles containing transition metals having an average hydrodynamic diameter of less than about 10 nm, preferably less than about 8 nm, more preferably 6 nm or even less, which are useful as a fuel additive for diesel engines . The surfaces of cerium dioxide nanoparticles can be modified to facilitate their attachment to the iron surface, and preferably, when incorporated into the fuel additive composition, quickly form a ceramic oxide coating on the surface of the cylinders of a diesel engine.

In one embodiment, a transition metal having an affinity for binding to iron is embedded in the surface of cerium dioxide nanoparticles. Examples of iron surfaces include surfaces that exist in many internal parts of engines. Suitable transition metals include Mn, Fe, Ni, Cr, W, Co, V, Cu, Zr, and Y. A transition metal ion, which is incorporated into cerium dioxide nanoparticles, taking the place of the cerium ion in the lattice in the crystal, can be introduced during later cerium dioxide precipitation steps. The transition metal ion can be added in combination with a Ce ³⁺ ion, for example, as a single jet in which both Ce ³⁺ ion and a transition metal ion are introduced together into a reactor containing ammonium hydroxide. Alternatively, Ce ³⁺ and transition metal ions can be added together while the hydroxide ion is added. Particles containing transition metals can also be formed in reaction with two jets of Ce ³⁺ ion with dissolved transition metal ions titrated with respect to a stream of ammonium hydroxide introduced simultaneously by a second jet. It is important to understand that a sufficient amount of nanoparticle stabilizer is present to prevent agglomeration of the resulting particles.

A surfactant / stabilizer combination may have the added benefit of assisting in the process of replacing a solvent from an aqueous polar medium to a non-polar oily medium. In a combination of a charged and an uncharged surfactant, a charged surfactant plays a dominant role in the aqueous environment. However, when solvent replacement occurs, the charged compound is likely to dissolve in the aqueous phase and wash out, and the uncharged compound becomes more important in stabilizing the emulsion with reverse micelles.

Dicarboxylic acids and their derivatives, the so-called "double carboxylates", where the carboxyl groups are separated by at most two methylene groups, are also suitable stabilizers of cerium dioxide nanoparticles. In addition, C ₂ -C ₈ alkyl, alkoxy and polyalkoxy substituted dicarboxylic acids are preferred stabilizers.

According to the invention, the nanoparticle stabilizers preferably contain organic carboxylic acids, such as, for example, 2- [2- (2-methoxyethoxy) ethoxy] acetic acid (MEAA) and ethylenediaminetetraacetic acid (EDTA), lactic acid, gluconic acid, tartaric acid, citric acid and mixtures thereof.

Motor oil is used as lubricating oil in various types of internal combustion engines in cars and other vehicles, ships, lawn mowers, trains, airplanes, etc. Engines contain contacting parts that move relative to each other at high speeds, often over extended periods of time. This movement causes friction, forming a temporary connection, stopping the moving parts. The rupture of this temporary connection absorbs the useful power produced by the motor, and turns the energy into useless heat. Friction also wears out the contact surfaces of these parts, which can lead to increased fuel consumption and reduced efficiency and motor parameters. In one aspect of the invention, motor oil comprises a lubricating oil, crystalline cerium dioxide nanoparticles containing transition metals, preferably having an average diameter of less than about 10 nm, more preferably about 5 nm, and possibly a surface-stabilizing agent.

Diesel lubricating oils are substantially free of water (preferably less than 300 ppm), but can be modified in a desirable manner by adding a cerium dioxide composition in which cerium dioxide has been changed solvent from its aqueous environment to an organic or non-polar environment. Cerium dioxide compositions include nanoparticles having an average diameter of less than about 10 nm, more preferably about 5 nm, as already described. A diesel engine operating with a modified diesel fuel and a modified lubricating oil provides greater efficiency and may, in particular, provide improved fuel consumption per mile, reduced engine abrasion or reduced pollution, or a combination of these features.

Metal polishing, also called grinding, is the process of smoothing metals and alloys and polishing to a bright, smooth, mirror state. Metal polishing is often used to improve automobiles, motorcycles, antiques, etc. Many medical instruments are also polished to prevent clogging in irregularities of the metal surface. Polishing agents are also used to polish optical elements, such as lenses and mirrors, to a smooth surface of the order of a fraction of the wavelength of light with which they work. The smooth, rounded, homogeneous particles of cerium dioxide of the present invention can mainly be used as polishing agents, and can be additionally used for planarization (execution of surface smoothness at the atomic level) of semiconductor substrates for the subsequent production of integrated circuits. Also, a homogeneous dispersion containing stabilized crystalline cerium dioxide nanoparticles containing a transition metal formed according to the present invention can be used to obtain a catalyst effective for the reduction reaction or oxidation reaction, that is, as catalysts similar to how they are used in the prior art, about which it was discussed in the description on p.5, the first paragraph (red line).

The invention is further illustrated by the following examples, which are not intended to limit the invention in any way.

Example 1. Obtaining cerium dioxide nanoparticles by adding a single jet.

1.267 liters of distilled water was added to a 3-liter round-bottom stainless steel reaction vessel, followed by 100 ml of Ce (NO ₃ ) ₃ · 6H ₂ O solution (600 gm / liter Ce (NO ₃ ) ₃ · 6Н ₂ О). The solution was clear and had a pH of 4.2 at 20 ° C. Then 30.5 gm of 2- [2- (2-methoxyethoxy) ethoxy] acetic acid (MEED) was added to the vessel. The solution remained clear and the pH was 2.8 at 20 ° C. A high speed mixer, a colloidal mill manufactured by Silverson Machines, Inc., which was modified to allow direct reagents to be introduced into the mixer blades by means of a peristaltic tube pump, was lowered into the reaction vessel, the mixer being positioned slightly above the bottom of the reaction vessel. The mixer was set at 5000 rpm, and 8 gm of 30% H ₂ O _{2 was} added to the reaction vessel. Then 16 ml of 28% -30% NH ₄ OH diluted to 40 ml was pumped into the reaction vessel by means of a mixer head in approximately 12 seconds. The initially clear solution turned orange-brown in color. The high speed mixer was removed and the reaction vessel was transferred to a temperature controlled water jacket, where an R-100 mixer was used to stir the solution at 450 rpm. The pH was 3.9 at 25 ° C. at 3 minutes after the NH ₄ OH was pumped into the reactor. The temperature of the reaction vessel was raised to 70 ° C. over the next 25 minutes, during which the pH was 3.9. The temperature of the solution was maintained at 70 ° C for 20 minutes, during which time the color of the solution changed from orange-brown to transparent dark yellow. The pH was 3.6 at 70 ° C. The temperature was lowered to 25 ° C. over the next 25 minutes when the pH was 4.2 at 25 ° C. Particle size analysis using dynamic light scattering showed a hydrodynamic diameter of suspended cerium dioxide of 6 nm. The dispersion was then diafiltered to a conductivity of 3 mS / cm and concentrated, approximately 10 times, to nominally 1 M CeO ₂ particles.

Cerium dioxide particles were collected, the excess solvent was evaporated and the mass yield of 62.9%, corrected for the mass of MEEA, was determined.

A transmission electron microscope (TEM) was used to analyze cerium dioxide particles. A 9 microliter solution (0.26 M) was dried on a grate and the image shown in FIG. 1 was obtained. The particles showed no signs of agglomeration, even in this dried state. Particles were expected to show an even lower tendency to agglomerate in solution. The frequency distribution of particle size of cerium dioxide (delayed in figure 1), determined using transmission electron microscopy (TEM), gave a geometric diameter of approximately 2.6 nm. In addition, the size distribution was essentially monomodal, i.e. only one maximum, and uniform, 19% COV, with most particles falling in the range from 2 nm to 4 nm.

Figure 2 shows an X-ray diffraction pattern of a 70 sample of dried cerium dioxide nanoparticles together with a comparison spectrum 71 of cerium dioxide, which was provided by the NIST library (National Institute of Standards and Technology). The positions of the lines in the spectrum of the sample correspond to the lines of the standard spectrum. The two theta peak widths were very wide in the spectrum of the sample, which corresponds to the small size of the primary crystallites and the particle size. From the x-ray data (CuK alpha line at approximately 8047 eV) and Scherer's formula (d = 0.9 * lambda / delta * cos (theta), where lambda indicates the x-ray wavelength, delta indicates the width at half maximum, and theta indicates the scattering angle corresponding to this x-ray peak), the size of the primary crystallites was calculated 2.5 ± 0.5 nm (95% confidence in 5 repetitions). Since the particle itself has the size of this crystallite, there is only one crystal per particle, therefore, the authors call this composition crystalline cerium dioxide to distinguish it from the prior art, where the nanoparticles were formed from agglomerates of crystallites of different sizes.

Example 2. The deposition of ~ 1.5 nm nanoparticles of CeO ₂ .

This deposition follows Example 1, except that a combination of 20:80 EDTA and lactic acid stabilizers and 76.4 gm of disodium EDTA disodium salt and 74.0 gm of 85% lactic acid were used instead of the MEEEA stabilizer. Fig. 3A is a high magnification TEM showing grain sizes substantially less than 5 nm and calculated 1.1 +/- 0.3 nm. 3B is an electron diffraction image of a typical precipitate sample. Fig. 3C contains table 1, in which the intensities of different diffraction rings {311}, {220}, {200} and {111} were analyzed in the structure: cubic CeO ₂ , cubic and hexagonal Ce ₂ O ₃ and Ce (OH) ₃ . It is clear that the percentage deviations of the analyzed ring intensity from the crystallization form are minimal for the cubic fluorite structure of CeO ₂ , thus proving the existence of this polymorphic modification up to this grain diameter.

Example 3. Obtaining CeO ₂ nanoparticles by isothermal double-jet deposition of CeO ₂

1117 grams of distilled water was added to a 3 liter round bottom stainless steel reaction vessel. The standard Rv 100 propeller was lowered into the reaction vessel, and the mixer head was positioned slightly above the bottom of the reaction vessel. The mixer was set at 700 rpm, and the reactor was brought to a temperature of approximately 70 ° C. Then 59.8 grams (98%) of methoxyacetic acid was added to the reactor. Two-jet deposition was carried out over a period of five minutes by pumping 250 ml of a solution containing 120.0 grams of Ce (NO ₃ ) ₃ · 6H ₂ O into the reactor simultaneously with a solution containing 69.5 grams (28-30%) of ammonium hydroxide. The supply of distilled water to the reactor cleaned the lines of reagents from residual materials. Then 10.2 grams of 50% unstabilized hydrogen peroxide was added to the reactor and its contents over a period of 40 seconds. The initial reaction mixture was an opaque dark orange-brownish liquid in the pH range from 6 to 7. The reaction mixture was heated for an additional 60 minutes, during which the pH was dropwise adjusted to 4.25 (state with the release of hydroxonium ions according to reactions (3a) and (3b)), and the mixture became a 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 led to the concentration of the dispersion of approximately 10 times, or nominally 1 M particles of CeO ₂ . A frequency analysis of particle size using transmission electron microscopy (FIG. 4) gave an average particle size of 2.2 nm with a frequency distribution of size having a coefficient of variation of COV (standard deviation divided by mean diameter) of 23%. The calculated yield was 62.9%.

Example 4. Copper-containing CeO ₂ nanoparticles Ce _0.9 Cu _0.1 O _1.95

The conditions of Example 3 were repeated except that the cerium nitrate solution contained 108.0 grams of cerium nitrate hexahydrate and 6.42 grams of Ce (NO ₃ ) ₃ · 2.5H ₂ O. The salts of these metals were dissolved separately and then combined to form 250 ml solution. The reaction proceeded as described in Example 3, except that hydrogen peroxide was added over a period of 40 seconds after cerium and ammonia were added. A frequency analysis of particle size using transmission electron microscopy (FIG. 5) gave an average particle size of 2.5 nm with a frequency distribution of size having a coefficient of variation of COV (standard deviation divided by mean diameter) of 25%. Note the absence of a bimodal distribution; a secondary peak would be an indication that Cu did not enter the CeO ₂ lattice, but instead exists as a separate population of Cu ₂ O ₃ .

Example 5. Iron-containing CeO ₂ nanoparticles Ce _0.9 Fe _0.1 O _1.95 (CeO-255)

The conditions of Example 4 were repeated except that the metal salt solution contained 108.0 grams of cerium nitrate hexahydrate and 11.16 grams of Fe (NO ₃ ) ₃ · 9H ₂ O. The salts of these metals were dissolved separately and then combined to form 250 ml of solution. The reaction proceeded as described in Example 4. TEM of the deposited particles (Fig. 6A) and frequency analysis of particle size using transmission electron microscopy (Fig. 6B) gave an average particle size of 2.2 +/- 0.7 nm with a frequency distribution of size having a coefficient of variation of COV (standard deviation divided by mean diameter) 32%. The calculated yield was 55.1%.

Example 6. Zirconium-containing CeO ₂ nanoparticles Ce _0.9 Zr _0.15 O ₂ (CeO-257)

The conditions of Example 4 were repeated except that the metal salt solution contained 101.89 grams of cerium nitrate hexahydrate and 9.57 grams of ZrO (NO ₃ ) ₂ · 6H ₂ O. The salts of these metals were dissolved separately and then combined to form 250 ml of solution. The reaction proceeded as described in example 4 except that the reaction temperature was maintained at 85 ° C. Frequency analysis of particle size using transmission electron microscopy (Fig. 7A) gave an average particle size of 2.4 +/- 0.7 nm with a frequency distribution of size having a coefficient of variation of COV (standard deviation divided by mean diameter) of 29%. Inductively coupled plasma atomic emission spectroscopy gave a stoichiometry of Ce _0.82 Zr _0.18 O _1.91 , which meant the relative insolubility of ZrO ₂ in CeO ₂ , taking into account the increased Zr content (18% instead of 15%).

Example 7a Zirconium and iron-containing CeO ₂ nanoparticles Ce _0.9 Zr _0.15 Fe _0.1 O _1.95 (CeO-270)

The conditions of Example 4 were repeated except that the metal salt solution contained 84.0 grams of cerium nitrate hexahydrate, 11.16 grams of Fe (NO ₃ ) ₃ · 9H ₂ O and 12.76 grams of ZrO (NO ₃ ) ₂ · 6H ₂ O The salts of these metals were dissolved separately and then combined to form 250 ml of solution. The reaction proceeded as described in example 4, except that the reaction temperature was maintained at 85 ° C and the hydrogen peroxide solution (50%) was raised to 20.4 gm and added over a period of ten minutes. Particle TEM (Fig. 8A) and particle size analysis by transmission electron microscopy (Fig. 8B) gave an average particle size of 2.2 +/- 0.6 nm with a frequency distribution of size having a coefficient of variation of COV (standard deviation divided by on average diameter) 27%. Once again, the monodisperse unimodal distribution supported the idea of joint introduction, in contrast to individual populations of the nucleated grains of ZrO ₂ and Fe ₂ O ₃ . The calculated yield was 78%. Atomic emission spectroscopy of inductively coupled plasma gave a stoichiometry of Ce _0.69 Fe _0.14 Zr _0.17 O _1.915 . Again, the relatively more concentrated Fe and Zr relative to the nominal amounts reflect the greater insolubility of their hydroxide precursors compared to the cerium hydroxide precursor. Also shown in FIG. 8C is an X-ray diffraction pattern of this sample (upper curve) compared to a transition metal free CeO ₂ . The absence of a peak (indicated by an arrow) at 32 degrees two theta means that there is no free ZrO ₂ , i.e. everything is embedded in the cerium lattice. Also, the absence of peaks of 50 and 52 degrees two theta indicates the absence of a separate population of Fe ₂ O ₃ (i.e., the incorporation of Fe into the cerium lattice). We note a shift with an increase of 2 theta at a large angle of two theta of scattering, which indicates distortion or compression of the lattice (nλ / 2d = sinθ), which corresponds to smaller ionic radii of Fe ³⁺ (0.78 Å) and Zr ⁴⁺ (0.84 Å) ) relative to Ce ⁴⁺ (0.97 Å), which is replaced. Thus, the authors concluded that the transition metals were embedded in the CeO ₂ lattice and did not represent a separate population of explicit ZrO ₂ and Fe ₂ O ₃ nanoparticles. The unimodal frequency distribution of size also confirms this conclusion.

Examples 7b-f. Zirconium and iron-containing CeO ₂ nanoparticles with a systematically changing amount of iron (15%, 20%, 25%, 30%) at 15% zirconium and 20% iron at 20% zirconium.

Followed the conditions of example 7a; however, the amount of iron or zirconium was varied to obtain the indicated nominal stoichiometries using an appropriate metal salt solution, while cerium nitrate hexahydrate was reduced to accommodate the increased concentration of iron or zirconium.

Fig.9 is a TEM field emission image of a particle lattice obtained 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. Water sols of various materials were heated for 30 minutes in a muffle furnace at 1000 ° C. These carefully dried samples were measured for OSC and the kinetics with which they reached their OSC maximum using thermogravimetric technology, as described by Sarkas et al., "Nanocrystalline Mixed Metal Oxides-Nonel Oxygen Storage Materials', Mat. Res. Soc. Symp.Proc Vol.788, L4.8.1 (2004) A very fast initial reduction rate in gaseous nitrogen containing 5% hydrogen was usually observed, followed by a second, slower rate.

The accompanying table 2 contains oxygen storage capacities (1 sigma reproducibility in brackets) and fast (k1) and slow (k2) speed constants (1 standard deviation in brackets) for the reduction of various cerium oxide nanoparticles with a given lattice (all 2 nm except Sigma Aldrich control) in gaseous nitrogen containing 5% H ₂ at 700 ° C. These values were rechecked on a second TGA instrument (average difference of 2.6%), with respect to differences in gas flow (average deviation of 1%) and repeated sampling at 1000 ° C for 30 minutes (average deviation of 1.54%). From the data in Table 2, the authors see that the OSC of cerium dioxide particles is independent of size in the range of about 2 nm to 20 μm. This may be due to sintering into large particles. Note that the OSC increases by approximately 50% with the addition of zirconium and is accompanied by a 10 x increase in speed. In addition, the addition of iron to the Zr-containing material gives an approximately three-fold increase in OSC at a 10-fold speed compared to cerium dioxide particles not containing transition metal ions. These values are more than three times the values in the cited reference. The predominant effect of citric acid on the rate constant of recovery, apparently, confirms that the stabilizer can affect the surface area of particles or morphology even after its pyrolysis.

table 2 Comparison of OSC for cerium dioxide nanoparticle variations Sample OSC (μmol / g) (st. Off. Mcmol / g) Recovery rate constant k1 × 10 ^ 3 (/ min) (st. Off) Recovery rate constant k2 × 10 ^ 3 (/ min) (st. Off) Sigma Aldrich CeO ₂ (20 μm) 296 (1/65) CeO ₂ (2 nm) 349 CeFe _0.10 O ₂ 470 (1% surface) CeZr _0.15 O ₂ 592 (3) CeZr _0.15 Fe _0.10 O ₂ 1122 (3) 3.1 (0.4) 0.9 (0.15) CeZr _0.15 Fe _0.15 O ₂ 1359 (33) 5.9 (0.04) 2.0 (0.2) CeZr _0.15 Fe _0.20 O ₂ 1653 (6) 3.4 (0.4) 1.1 (0.3) CeZr _0.15 Fe _0.25 O ₂ 2013 (1) 3.1 (0.4) 1.1 (0.2) CeZr _0.15 Fe _0.30 O ₂ 2370 (4) 2.6 (0.1) 1.0 (0.1) CeZr _0.20 Fe _0.20 O ₂ 1661 (7) 4.9 (1.3) 1.2 (0.2) CeZr _0.20 Fe _0.20 O ₂ citric acid 1636 (1) 9.5 (0.6) 3.9 (0.2)

Although the present invention has been described with reference to various specific embodiments, it should be understood that numerous changes can be made within the spirit and scope of the described proposed concepts. Accordingly, it is intended that the invention is not limited to the described embodiments, but will have the full scope defined by the language of the following claims.

Claims (44)

1. The method of producing cerium dioxide nanoparticles with a given lattice containing at least one transition metal (M), in which:
(a) an aqueous reaction mixture is obtained containing a source of cerium ions Ce ³⁺ , a source of ions of one or more transition metals (M), a source of hydroxide ions, at least one stabilizer of nanoparticles and an oxidizing agent leading to the oxidation of Ce ³⁺ ion to Ce ⁴⁺ ion at an initial temperature in the range of from about 20 ° C to about 95 ° C;
(b) carry out a mechanical shift of this mixture and its transmission through a perforated sieve, forming a homogeneously distributed suspension of cerium hydroxide nanoparticles; and
(c) create temperature conditions effective to oxidize the Ce ³⁺ ion to the Ce ⁴⁺ ion to form a product stream containing cerium dioxide nanoparticles containing a transition metal, Ce _1-x M _x O ₂ , where x has a value of from about 0 , 3 to about 0.8, said nanoparticles have a cubic fluorite structure, an average hydrodynamic diameter in the range of from about 1 nm to about 10 nm, and a geometric diameter of from about 1 nm to about 4 nm, the nanoparticle stabilizer being water It has a value of Log K _BC in the range from 1 to 14, where K _VS is the binding constant of the nanoparticle stabilizer with cerium ion in water, and the mentioned temperature conditions effective for oxidizing the Ce ³⁺ ion to Ce ⁴⁺ ion include a temperature from approximately 50 ° C to approximately 100 ° C. 2. The method according to claim 1, where the aforementioned mechanical shear is carried out in a colloidal mill. 3. The method according to claim 1, where the aforementioned temperature conditions effective to effect the oxidation of Ce ³⁺ ion to Ce ⁴⁺ ion include a temperature of from about 60 ° C to about 90 ° C. 4. The method according to claim 1, where said ion sources are introduced into said reaction mixture simultaneously or sequentially during said mechanical shift. 5. The method according to claim 1, where the said cerium dioxide nanoparticles have an average hydrodynamic diameter of approximately 6 nm. 6. The method according to claim 1, where the said cerium dioxide nanoparticles have a geometric mean (TEM) diameter of less than about 4 nm. 7. The method according to claim 1, wherein said cerium dioxide nanoparticles have a substantially monomodal size distribution and a substantially monodisperse frequency distribution of size. 8. The method according to claim 1, where said cerium dioxide nanoparticles are crystalline and the crystallite size is in the range from about 1.1 nm to about 2.5 nm ± 0.5 nm. 9. The method according to claim 1, where said source of Ce ³⁺ ions comprises cerium (III) nitrate. 10. The method according to claim 1, where said transition metal M in said cerium dioxide nanoparticles containing a transition metal, Ce _1-x M _x O ₂ , is selected from the group consisting of Fe, Mn, Cr, Ni, W, Co, V, Cu, Zr, Y, Mo, and combinations thereof. 11. The method according to claim 10, where x has a value from about 0.40 to about 0.60. 12. The method of claim 10, wherein said transition metal M comprises Zr or possibly Y. 13. The method of claim 12, wherein said transition metal M comprises Fe and Zr or possibly Y. 14. The method according to claim 1, where the aforementioned source of hydroxide ions includes ammonium hydroxide, and said hydroxide ions are present in a molar stoichiometric ratio to Ce ³⁺ ions from about 2: 1 OH: Ce to about 5: 1 OH: Ce. 15. The method according to claim 1, where the said oxidizing agent includes hydrogen peroxide or molecular oxygen. 16. The method according to claim 1, where the nanoparticle stabilizer is selected from the group consisting of alkoxy substituted carboxylic acid, α-hydroxycarboxylic acid, α-ketocarboxylic acid, and combinations thereof. 17. The method according to claim 1, where the stabilizer of the nanoparticles is selected from the group consisting of di-, tri- and tetracarboxylic acids and combinations thereof. 18. The method according to claim 1, where the nanoparticle stabilizer is selected from the group consisting of 2- [2- (2-methoxyethoxy) ethoxy] acetic acid, methoxyacetic acid, and combinations thereof. 19. The method according to claim 1, where the stabilizer of the nanoparticles is selected from the group consisting of ethylenediaminetetraacetic acid, gluconic acid, lactic acid, tartaric acid, pyruvic acid, citric acid, and combinations thereof. 20. The method according to claim 19, where the stabilizer of the nanoparticles is citric acid. 21. The method according to claim 7, where the aforementioned essentially monomodal size distribution and essentially monodisperse frequency distribution of size is controlled using a variable selected from the group consisting of nucleation temperature, choice of a nanoparticle stabilizer, concentration of a nanoparticle stabilizer, and combinations thereof . 22. The method according to claim 1, where said method is a continuous method or a batch method. 23. The method according to claim 1, where additionally:
(d) concentrating said product stream into a suspension having a density not exceeding about 35% (wt.% solids / wt. suspension) and a conductivity not exceeding about 5 mS / cm. 24. The method of claim 23, wherein said concentration of said product stream is carried out using at least one semi-permeable membrane. 25. A method of obtaining a homogeneous dispersion containing stabilized crystalline cerium dioxide nanoparticles with a given lattice, containing a transition metal, Ce _1-x M _x O ₂ , where M represents at least one transition metal and x has a value of from about 0.3 to approximately 0.8:
(a) an aqueous mixture is obtained which comprises stabilized cerium dioxide nanoparticles containing a transition metal, Ce _1-x M _x O ₂ having a cubic fluorite structure, said nanoparticles having an average hydrodynamic diameter in the range of from about 1 nm to about 10 nm, and geometric diameter less than about 4 nm;
(b) concentrating said aqueous mixture, which comprises said stabilized cerium dioxide nanoparticles containing a transition metal to form an aqueous concentrate;
(c) substantially all of the water is removed from said aqueous concentrate to form a substantially water-free concentrate of said stabilized cerium dioxide nanoparticles containing a transition metal;
(d) adding an organic diluent to said substantially water-free concentrate, forming an organic concentrate of said stabilized cerium dioxide nanoparticles containing a transition metal; and
(f) combining said organic concentrate with a surfactant in the presence of a non-polar medium to form said homogeneous dispersion containing stabilized crystalline cerium dioxide nanoparticles containing a transition metal, Ce _1-x M _x O ₂ , where M and x are as defined above . 26. The method according A.25, where the aforementioned transition metal M in said cerium dioxide nanoparticles containing a transition metal, Ce _1-x M _x O ₂ selected from the group consisting of Fe, Mn, Cr, Ni, W, Co, V, Cu, Zr, Y, Mo, and combinations thereof. 27. The method according A.25, where x has a value from about 0.40 to about 0.60. 28. The method of claim 26, wherein said transition metal M comprises Zr or Y. 29. The method according A.25, where the aforementioned transition metal M includes Fe and Zr or, possibly, Y. 30. The method of claim 25, wherein said organic diluent comprises glycol ether. 31. The method according A.25, where the aforementioned organic diluent further comprises alcohol. 32. The method of claim 25, wherein said organic diluent comprises diethylene glycol monomethyl ether and 1-methoxy-2-propanol. 33. The method according A.25, where the aforementioned surfactant further comprises a joint surfactant. 34. The method according A.25, where the aforementioned surfactant contains oleic acid. 35. The method according to clause 33, where the aforementioned joint surfactant contains 1-hexanol. 36. The method according A.25, where the aforementioned surfactant is free from sulfur. 37. The method according A.25, where the aforementioned homogeneous dispersion contains the aforementioned nanoparticles in a concentration of at least about 2 wt.% And water in a maximum concentration of about 0.5 wt.%. 38. The method according to clause 37, where the said homogeneous dispersion contains the aforementioned nanoparticles in a concentration of at least about 9 wt.%. 39. The method according to § 38, where the said homogeneous dispersion contains the said nanoparticles in a concentration of approximately 35 wt.%. 40. The method according A.25, where the aforementioned non-polar medium contains a hydrocarbon containing from about 6 to about 20 carbon atoms. 41. The method according A.25, where the aforementioned non-polar medium is selected from the group consisting of octane, decane, toluene, kerosene, naphtha, ultra-low sulfur diesel fuel, biodiesel and mixtures thereof. 42. The method according A.25, where additionally:
(f) diluting one part of said homogeneous dispersion of at least about 100 parts of hydrocarbon fuel. 43. The coating for the catalytic converter of the exhaust system of an internal combustion engine, where said coating is obtained using a homogeneous dispersion containing stabilized crystalline cerium dioxide nanoparticles containing a transition metal formed according to item 27. 44. A catalyst effective for a reduction reaction or an oxidation reaction, wherein said catalyst is prepared using a homogeneous dispersion containing stabilized crystalline cerium dioxide nanoparticles containing a transition metal formed according to claim 27.

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