When many small particles of metal oxides and hydroxides are suspended in a liquid they tend to agglomerate. This is disadvantageous if one is trying to obtain ultra-fine particles. Added to this it is well known to coat such particles in order to try and stabilize them but agglomeration prevents an efficient coating to take place.

The agglomeration leads to internal surfaces of the agglomerated particle not being completely exposed and therefore available to the coating agent. It is, of course, possible to break down the agglomerates but typically, if a suspension of the particles in a hydrocarbon solvent, for instance, is broken down the exposed uncoated surfaces do not properly wet the solvent and have a tendency to re-agglomerate and separate from the solvent. There is, therefore, a need to provide a process which enables one to obtain ultra-fine particles which are fully coated.

According to the present invention there is provided a process for preparing coated particles of a metal or metalloid oxide or hydroxide which comprises comminuting the oxide or hydroxide in an organic solvent in the presence of a coating agent which is an organic acid, anhydride or ester or a Lewis base. It has been found that, in this way which involves coating in situ, it is possible to significantly improve the coating of the oxide or hydroxide. Further, the resulting product can, in many instances, be used directly without any intermediate step. Thus in some coating procedures it is necessary to dry the coated particles before dispersing them in a hydrocarbon solvent. In accordance with the present invention this is not necessary although it is possible if desired.

As used herein, a metal or metalloid oxide or hydroxide may be an individual metal or metalloid oxide or hydroxide or a mixture of two or more metal and/or metalloid oxides and/or hydroxides, and may include further metal atoms as dopant.

The process of the present invention is applicable to a wide range of oxides

and hydroxides but it has particular application to cerium oxide.

Cerium oxide acts as a catalyst in automotive exhaust systems. Cerium oxide releases oxygen and is therefore capable of regulating the oxygen partial pressure in the exhaust system. With the engine working under lean conditions, cerium oxide removes excess oxygen from the exhaust gas and catalysed by, for example, platinum, NO, is reduced to nitrogen. During rich cycles, cerium oxide releases oxygen to oxidise carbon monoxide to carbon dioxide. There is, therefore, a need to make a cerium oxide readily available for catalyst systems and this can be achieved most easily by incorporating the cerium oxide into the fuel, i. e. as a fuel additive.

For this purpose, the cerium oxide needs to be dispersible or soluble in the fuel. In accordance with the process of the present invention, one can obtain coated cerium oxide particles dispersed or soluble in the fuel or another hydrocarbon compatible with the fuel.

Apart from cerium oxide, the present invention is applicable to other rare earth oxides and hydroxides and indeed other metal oxides and hydroxides including metals of group II of the Periodic table such as magnesium, calcium, strontium and barium, aluminium, zirconium e. g. ZrO2, titanium e. g. TiO2, nickel e. g. NiO, and iron as Fe203 and Fe304 and other transition metals as well as actinide metal oxides as well as metalloids such as silicon. Particles of mixed oxides and hydroxides such as a mixture of cerium oxide and zirconium oxide can also be coated in accordance with the present invention. Additional examples of oxides which may be used in the present invention include complex materials of formula ZZX. OY : M or ZZOY : M where Z and X are metals or metalloids and M is one or more dopant metals, each added at a level of, for example, 0 to 5 mole%. Examples of such complex materials include Zn2Si04 : Mn, YV04 : Dy, Y203 : Eu, Gd203 : Tb and ZnO: Zn.

The oxides and hydroxides and mixed oxides and hydroxides used in the present invention may be produced by a variety of different routes including plasma vapour synthesis, combustion synthesis, flame pyrolysis and colloid chemistry.

The present invention is particularly applicable to nanometre sized particles.

The particles which are subjected to the process should have as large a surface area as

possible and preferably the particles have a surface area, before coating, of at least 10 m2/g and preferably a surface area of at least 50 or 75 m2/g, for example 80-150 m2/g.

It is believed that the surface chemistry of the particles also has an effect on the ability to coat the particles. Thus the presence of hydroxide groups on the surface is believed to assist the coating process. The resulting particles generally have a size not exceeding 1 micron and especially not exceeding 250 nm, for example 100 to 150 nm, such as 120-130 nm. Specific cerium oxide particles which can be coated include those obtainable from Nanophase, typically with a primary particle size of about 180 nm (although agglomerates are also generally present such that, typically, about 75% of the particles are below 1 micron), Rhodia and Meldform.

The coating agent is an organic acid, anhydride or ester or a Lewis base. It will normally be soluble in the organic solvent employed. Of course the coating agent should not react chemically with the particles although there may be some binding at the interface with the particle surface. The coating agent is preferably an organic carboxylic acid or an anhydride, typically one possessing at least 8 carbon atoms, for example 10 to 25 carbon atoms, especially 12 to 18 carbon atoms such as stearic acid. It will be appreciated that the carbon chain can be saturated or unsaturated, for example ethylenically unsaturated as in oleic acid. Similar comments apply to the anhydrides which can be used. A preferred anhydride is dodecylsuccinic anhydride (DDSA). Other organic acids, anhydrides and esters which can be used in the process of the present invention include those derived from phosphoric acid and sulphonic acid. The esters are typically aliphatic esters, for example alkyl esters where both the acid and ester parts have 4 to 18 carbon atoms.

Other coating or capping agents which can be used include Lewis bases which possess an aliphatic chain of at least 8 carbon atoms including mercapto compounds, phosphines, phosphine oxides and amines as well as long chain ethers, diols, esters and aldehydes. Polymeric materials including dendrimers can also be used provided that they possess a hydrophobic chain of at least 8 carbon atoms and one or more Lewis base groups, as well as mixtures of two or more such acids and/or Lewis bases.

Typical polar Lewis bases include tdalkylphosphine oxides P (RI) 30, especially trioctylphosphine oxide (TOPO), trialkylphosphines, P (R3) 3, amines N (R3) 2, thiocompounds S (R3) 2 and carboxylic acids or esters R3COOR4 and mixtures thereof, wherein each R3, which may be identical or different, is selected from Cl 24 alkyl groups, 2-24 alkenyl groups, alkoxy groups of formula-O (Cl 24 alkyl), aryl groups and heterocyclic groups, with the proviso that at least one group R3 in each molecule is other than hydrogen ; and wherein W is selected from hydrogen and Cl 24 alkyl groups, preferably hydrogen and C1-4 alkyl groups. Typical examples of C1-24 and C1-4 alkyl groups, C2-24 alkenyl groups, aryl groups and heterocyclic groups are described below.

It is also possible to use as the polar Lewis base a polymer, including dendrimers, containing an electron rich group such as a polymer containing one or more of the moieties P (R3)3O, P (R3)3, N (R3) 2, S (R3) 2 or R3COOR4 wherein R3 and R4 are as defined above ; or a mixture of Lewis bases such as a mixture of two or more of the compounds or polymers mentioned above.

As used herein, a C1-4 alkyl group is an alkyl group as defined above which contains from 1 to 4 carbon atoms. Cl 4 alkyl groups include methyl, ethyl, i-propyl, n-propyl, n-butyl and tert-butyl.

As used herein, a 2. 24 alkenyl group is a linear or branched alkenyl group which may be unsubstituted or substituted at any position and which may contain heteroatoms selected from P, N, O and S. Typically, it is unsubstituted or carries one or two substituent. Suitable substituent include halogen, hydroxyl, cyano,-NR2, nitro, oxo,-CO2R,-SOR and-SO2R wherein each R may be identical or different and is selected from hydrogen or C1-4 alkyl.

As used herein, a C2 4 alkenyl group is an alkenyl group as defined above which contains from 2 to 4 carbon atoms. C2-4 alkenyl groups include ethenyl, propenyl and butenyl.

As used herein, an aryl group is typically a C6 l0 aryl group such as phenyl or naphthyl, preferably phenyl. An aryl group may be unsubstituted or substituted at any position, with one or more substituent. Typically, it is unsubstituted or carries

one or two substituent. Suitable substituent include Cl 4 alkyl, C, 4 alkenyl, each of which may be substituted by one or more halogens, halogen, hydroxyl, cyano,-NR2, nitro, oxo,-CO2R,-SOR and-SO2R wherein each R may be identical or different and is selected from hydrogen and Cl alkyl.

As used herein, a heterocyclic group is a 5-to 10-membered ring containing one or more heteroatoms selected from N, O and S. Typical examples include pyridyl, pyrazinyl, pyrimidinyl, pyridazinyl, furanyl, thienyl, pyrazolidinyl, pyrrolyl and pyrazolyl groups. A heterocyclic group may be substituted or unsubstituted at any position, with one or more substituent. Typically, a heterocyclic group is unsubstituted or substituted by one or two substituent. Suitable substituent include C, 4 alkyl, CI-4 alkenyl, each of which may be substituted by one or more halogens, halogen, hydroxyl, cyano,-NR2, nitro, oxo,-CO2R,-SOR and-SO2R wherein each R may be identical or different and is selected from hydrogen and Cl 4 alkyl.

As used herein, halogen is fluorine, chlorine, bromine or iodine, preferably fluorine, chlorine or bromine.

The process of the present invention is carried out in an organic solvent.

Preferably, the solvent is non-polar and is also preferably non-hydrophilic. It can be an aliphatic or an aromatic solvent. Typical examples include toluene, xylene, petrol, diesel fuel as well as heavier fuel oils. Naturally, the organic solvent used should be selected so that it is compatible with the intended end use of the coated particles.

The presence of water should be avoided; the use of an anhydride as coating agent helps to eliminate any water present.

It will be appreciated that the particles which are subjected to comminution may desirably be at least partially coated with a coating agent e. g. an organic acid such as stearic acid to assist transfer from an aqueous phase in which they are generally produced to the organic phase in which the comminution is conducted.

Thus the particles may be at least partially dispersed before comminution. The addition of a dispersant such as a polymeric dispersant can be beneficial.

The process of the present invention involves comminuting the particles so as to prevent any agglomerates from forming. The technique employed should be

chosen so that the particles are adequately wetted by the agent and a degree of pressure or shear is desirable. Techniques which can be used for this purpose include high-speed stirring, mixing or tumbling, the use of a colloid mill, ultrasonics or ball milling. Ultrasonics, high speed mixing and ball milling are preferred. Typically, ball milling can be carried out in a pot where the larger the pot the larger the balls.

By way of example, ceramic balls of 7 to 10 mm diameter are suitable when the milling takes place in a 1.25 litre pot. The time required will of course, be dependent on the nature of the particles but, generally, at least 4 hours is required. Good results can generally be obtained after 24 hours so that the typical time is 12 to 36 hours.

Ultrasonic treatment can be carried out with a conventional ultrasonicator for, say, 10 minutes to 2 hours, typically 30 minutes to 1 hour. High speed mixing is typically carried out with a rotor/stator mixer at speeds typically from 1000 to 4000, generally 1500 to 2500, rpm.

The effectiveness of the process of the present invention can be assessed by studying the stability of the resulting suspension. A turbidity procedure can be used to assess the extent to which the particles remain suspended and therefore un- agglomerated. The agglomerated particles will, of course fall out of suspension and therefore reduce the turbidity of the suspension. By way of example, it has been found that the addition of a suspension of cerium oxide particles obtained by the process of the present invention is sufficient to act as a fuel catalyst when present in a concentration of about 4 ppm. This compares with a concentration of in excess of 40 ppm for an existing coated cerium oxide product.

If desired the resulting particles can be dried and re-dispersed in another organic solvent or in a polymer. Examples of suitable polymers include homo-and co-polymers of ethylene, propylene or styrene, and hydrocarbon-based elastomers such as those containing propylene, butadiene or isoprene.

The following Examples further illustrate the present invention.

Example 1

A porcelain ball mill pot (1.25 1 capacity) with the corresponding ball charge was charged with 50 g of cerium oxide, Nanotech material from Nanophase (primary particle size about 180 nm with about 76% of the particles being below 1 micron), 200 ml of low sulphur diesel and 5 g of dodecylsuccinic anhydride. The mill was operated for 24 hours A smooth stable slurry was obtained with substantially all the particles being of the primary size (about 180 nm). It is clear, therefore, that the particles have been coated by the anhydride.

Example 2 10 grams of a 4% w/w suspension of cerium oxide in a non-polar organic solvent (diesel fuel) previously prepared from an aqueous suspension of the oxide with the use of stearic acid was placed in a 30 ml. wide-mouth glass bottle. 0.08 grams of DDSA, representing 20% of the mass of the cerium oxide, was dissolved separately in 5 ml. of similar solvent, diesel, and added to the suspension of the oxide. The bottle was placed in a dish of water to act as a cooling bath and an ultrasonic probe dipped into the suspension. The suspension was subjected to ultrasonic treatment for 30 minutes. The instrument used was a Sonics & Materials Inc. VibroCell controller and a Model 1VA transducer fitted with a probe-head 18mm. diameter and 80mm in length.

Example 3 Example 2 was repeated on a larger scale. 175.6 grams of a cerium oxide slurry (4.1% w/w) was placed in a 500ml wide-mouth bottle and a solution of 1.44 grams of DDSA in Shellsol D70 (a white spirit comprising Cll hydrocarbons with paraffins and naphthenic) added. During the ultrasonic treatment the contents of the glass bottle were stirred magnetically. The suspension was treated for 60 minutes in this case.

Example 4 A test was conducted using an EEL Absorptiometer with 100mm pathlength cells to determine the transmittance of white light through a sample of diesel containing 5ppm of cerium oxide which had been pretreated in one of two ways-A and B as given below. Transmittance was determined as a function of time. The results are shown in the accompanying Figure. It can be seen that the inclusion of DDSA increased the clarity of the diesel (B) over the sample containing no DDSA (A). The increase in transmittance as a function of time results from settling of the cerium oxide. It should be noted that at time zero there was no evidence of settling in either case.

A-cerium oxide dispersed with the addition of stearic acid and Solsperse 3000 (a polymeric dispersant) B-cerium oxide dispersed with the addition of stearic acid followed by the addition of DDSA as described in Example 2.

Example 5 25.8g of a slurry of cerium oxide (9.15% solids) in Shellsol D70 was mixed with a solution of 0. 47g DDSA dissolved in 15ml of the same solvent in a 60 ml wide- mouth glass bottle and subjected to a high speed mixing for 15 minutes using a small Silverson mixer. A satisfactory dispersion was obtained.