Cerium oxide particles and method for production thereof

Abstract

The present invention relates to cerium oxide particles that have excellent heat resistance and/or pore volume especially useful for catalysts, functional ceramics, solid electrolyte for fuel cells, polishing, ultraviolet absorbers and the like, and particularly suitable for use as a catalyst or cocatalyst material, for instance in catalysis for purifying vehicle exhaust gas. The present invention also relates to a method for preparing such cerium oxide particles, and a catalyst, such as for purifying exhaust gas, utilizing these cerium oxide particles.

Claims

1. Cerium oxide particles having the following properties: a specific surface area (SBET) ranging between 55 and 80 m.sup.2/g after calcination at 900° C. for 5 hours, under air; and a specific surface area (SBET) ranging between 75 and 90 m.sup.2/g after calcination at 700° C. for 4 hours, under a gaseous atmosphere containing 10% by volume of 02, 10% by volume of H.sub.2O and the balance of N.sub.2.

2. Cerium oxide particles according to claim 1 exhibiting a specific surface area (SBET) ranging between 30 and 60 m.sup.2/g after calcination at 900° C. for 24 hours, under air.

3. Cerium oxide particles according to claim 1 exhibiting a specific surface area (SBET) ranging between 22 and 40 m.sup.2/g after calcination at 1000° C. for 5 hours, under air.

4. Cerium oxide particles according to claim 1 exhibiting a total pore volume ranging between 0.8 and 4.0 ml/g after calcination at 400° C. for 10 hours, under air.

5. Cerium oxide particles according to claim 1 exhibiting a total pore volume ranging between 0.70 and 1.9 ml/g after calcination at 900° C. for 24 hours, under air.

6. Cerium oxide particles according to claim 1 exhibiting a pore volume of the pores having a pore size in the range of no more than 200 nm ranging between 0.27 and 0.9 ml/g after calcination at 400° C. for 10 hours, under air.

7. Cerium oxide particles according to claim 1 exhibiting a pore volume of the pores having a pore size in the range of no more than 200 nm ranging between 0.23 and 0.65 ml/g after calcination at 900° C. for 24 hours, under air.

8. Cerium oxide particles according to claim 1 exhibiting a total pore volume ranging between 1.5 and 4.0 ml/g after calcination at 400° C. for 10 hours, under air.

9. Cerium oxide particles according to claim 8 exhibiting a total pore volume ranging between 1.5 and 3.8 ml/g after calcination at 900° C. for 24 hours, under air.

10. Cerium oxide particles according to claim 8 exhibiting a pore volume of the pores having a pore size in the range of no more than 200 nm ranging between 0.3 and 1.5 ml/g after calcination at 400° C. for 10 hours, under air.

11. The cerium oxide particles according to claim 8 exhibiting a pore volume of the pores having a pore size in the range of no more than 200 nm ranging between 0.25 and 1.0 ml/g after calcination at 900° C. for 24 hours, under air.

12. Cerium oxide particles according to claim 8 exhibiting a specific surface area (SBET) of at least 40 m.sup.2/g after calcination at 900° C. for 24 hours, under air.

13. Cerium oxide particles according to claim 8 exhibiting a specific surface area (SBET) of at least 25 m.sup.2/g after calcination at 1000° C. for 5 hours, under air.

14. Cerium oxide particles according to claim 1, exhibiting an amount of NO.sub.x adsorbed higher than or equal to 22 μg NO.sub.x g CeO.sub.2 after calcination at 700° C. for 4 hours, under a gaseous atmosphere containing 10% by volume of O.sub.2, 10% by volume of H.sub.2O and the balance of N.sub.2.

15. A catalyst comprising cerium oxide particles as defined by claim 1.

16. The catalyst according to claim 15, wherein said catalyst is a co-catalyst comprising the cerium oxide particles, for purifying exhaust gas.

17. A composition comprising cerium oxide particles as defined by any one of claims 1 and 2-14.

18. A composition according to claim 17, wherein said composition includes at least one metal oxide other than cerium oxide.

19. A composition according to claim 18, wherein the metal of the at least one other metal oxide is an element selected from the group consisting of: (1) metal elements in Group 4A in the periodic table; (2) metal elements in Group 4B in the periodic table; (3) metal elements in Group 3A in the periodic table; (4) alkali metal elements; and (5) rare earth elements (REEs).

20. A composition according to claim 19, wherein (1) the metal elements in Group 4A of the periodic table are chosen from silicon and/or tin; (2) the metal elements in Group 4B of the periodic table are chosen from titanium and/or zirconium; (3) the metal element in Group 3A of the periodic table is chosen from aluminum; (4) the alkali metal elements are chosen from barium and/or strontium; and (5) the REEs are chosen from any of fifteen lanthanides, scandium, or yttrium.

21. A composition according to claim 18, wherein said at least one other metal oxide is a rare earth element oxide selected from the group consisting of: lanthanium oxide (La.sub.2O.sub.3), praseodymium oxide (Pr.sub.6O.sub.11), neodymium oxide (Nd.sub.2O.sub.3) and yttrium oxide (Y.sub.2O.sub.3).

Description

DETAILS OF THE INVENTION

(1) About the Method for Preparing the Cerium Oxide

(2) The method for preparing the cerium oxide of the invention involves the use of Ce.sup.IV and Ce.sup.III cations. According to the present method, first a cerium salt aqueous solution, wherein between 90 and 100 mol % of the cerium cations are tetravalent is provided in step (a). Thus, the aqueous solution of Ce.sup.IV and optionally Ce.sup.III cations is characterized by a molar ratio Ce.sup.IV/total cerium of at least 0.9. This ratio may be 1.0.

(3) In step (a), the cerium salt solution, may preferably be a cerium nitrate solution, a cerium ammonium nitrate solution, a cerium sulfate solution and/or a cerium ammonium sulfate solution. Cerium salts may be ionic compounds usually resulting from the neutralization reaction of an acid and a base or dissolution of a cerium compound, such as cerium hydroxide, with an acid. They are composed of cerium cations and anions so that the product is electrically neutral.

(4) It is advantageous to use a salt of cerium with a purity of at least 99.5%, more particularly of at least 99.9%. The cerium salt solution may be an aqueous ceric nitrate solution. This solution is obtained by reaction of nitric acid with an hydrated cerix oxide prepared conventionally by reaction of a solution of a cerous salt and of an aqueous ammonia solution in the presence of aqueous hydrogen peroxide to convert Ce.sup.III cations into Ce.sup.IV cations. It is also particularly advantageous to use a ceric nitrate solution obtained according to the method of electrolytic oxidation of a cerous nitrate solution as disclosed in FR 2570087. A solution of ceric nitrate obtained according to the teaching of FR 2570087 may exhibit an acidity of around 0.6 N.

(5) The cerium salt solution of the present invention may have a cerium cation concentration comprised between 5 and 150 g/L expressed in terms of cerium oxide. For instance, a concentration of 225 g/L of cerium nitrate corresponds to 100 g/L of CeO.sub.2; a concentration of 318 g/L of cerium ammonium nitrate corresponds to 100 g/L of CeO.sub.2; a concentration of 193 g/L of cerium sulfate corresponds to 100 g/L of CeO.sub.2; a concentration of 270 g/L of cerium ammonium sulfate corresponds to 100 g/L of CeO.sub.2. The cerium salt concentration of the cerium solution may be adjusted between 10 and 120 g/L, more preferably between 15 and 100 g/L, in terms of cerium oxide, usually with water, preferably with deionized water.

(6) The cerium salt solution may be acidic. The acidity can be developed by dissolving the salt in water or an acid, such as HNO.sub.3 and H.sub.2SO.sub.4 can be added. The amount of H.sup.+ can be from 0.01 and 1 N.

(7) A typical cerium salt solution contains Ce.sup.IV, optionally Ce.sup.III, H.sup.+ and NO.sub.3.sup.−. It may be obtained by mixing the appropriate quantities of nitrate solutions of Ce.sup.IV and Ce.sup.III and by optionally adjusting the acidity. An example of an aqueous solution that may be used is disclosed in example 1.

(8) According to the present method, next the cerium salt solution prepared in step (a) is held between 60 and 220° C. under heating to cause reaction of the cerium solution in step (b), in order to obtain a suspension comprising a liquid medium and a precipitate. The precipitate may be in the form of cerium hydroxide. Any reaction vessel may be used in step (b) without critical limitation, and either a sealed vessel or an open vessel may be used. Specifically, an autoclave reactor may preferably be used.

(9) In step (b), the temperature is comprised between 60 and 220° C., preferably between 80 and 180° C., more preferably between 90 and 160° C. Duration of heat treatment is usually between 10 minutes and 48 hours, preferably between 30 minutes and 36 hours, more preferably between 1 hour and 24 hours. As detailed in the examples, the temperature can be between 90° C. and 140° C. The duration can be between 30 min to 1 hour. Without wishing to be bound by any particular theory, the function of this heating step is to improve the crystallinity of the precipitate and result in a better heat resistance of the cerium oxide of the invention.

(10) In the present invention, the steps (a) and (b) may be conducted in one step. In such instance, the combined step comprises heating said cerium salt solution comprising anions and cations, wherein between 90 and 100 mol % of the cerium cations are tetravalent cerium cations at a temperature comprised between 60 and 220° C. in order to obtain a suspension comprising a liquid medium and a precipitate.

(11) In step (c), water is added to decrease the concentration of the anions present in the liquid medium. The concentration of the anions from the cerium salt present in the liquid medium is decreased by between 10 and 90 mol %, preferably between 15 and 85 mol %, in comparison with anions comprised in the liquid medium in step (b). If the same amount of anions is present in step (a) and (b), the decrease of anions in the liquid medium in step (c) may be calculated in comparison with anions comprised in the liquid medium in step (a) or (b). Thus, [anions] in step (c)/[anions] in step (b) is between 10-90%, preferably 15-85%. When the method involves the typical aqueous solution disclosed above, [NO.sub.3.sup.−] in step (c)/[NO.sub.3.sup.−] in step (b) is between 10-90%, preferably 15-85%.

(12) Anions from the cerium salts may be for instance nitrate from cerium nitrate or sulfate from cerium sulfate.

(13) This decrease of anions concentration present in the liquid medium may be obtained by at least one of the following methods: addition of water, preferably deionized water, to the suspension obtained in step (b); and/or removing at least a part of the liquid medium from the suspension obtained in step (b) and then adding water, preferably deionized water, to the medium. Said medium is defined according to partial removal or complete removal of liquid medium as previously expressed; i.e. medium may be a precipitate in case of complete removal of liquid medium in step or rather a mixture of precipitate and liquid medium in case of partial removal of liquid medium.

(14) Separation of the liquid medium from the precipitate may be carried out, for example, by Nutsche filter method, centrifuging, filter pressing, or decantation.

(15) According to the invention, partial removal or complete removal of liquid medium is understood to mean that the liquid medium is partially, or completely removed from the precipitate. For example between 10 and 100% by weight, preferably between 10 and 90% by weight, more preferably between 15 and 95% by weight, especially between 20 and 90% by weight, of the liquid medium present in step (b) may be removed in step (c).

(16) The decrease of the concentration can be determined through mass balance and calculations of the amount of anions in the liquid media of step (b) and step (c). In the case of cerium nitrate, decrease of concentration of anions present in the liquid medium in step (c) in comparison with anions comprised in the liquid medium in step (b), may be calculated as follows in case of cerium nitrate:

(17) Materials at the start of step (a) are Ce(IV)(NO.sub.3).sub.4 and Ce(III)(NO.sub.3).sub.3 and optionally HNO.sub.3.

(18) 1. Calculation of total number of NO.sub.3.sup.− ions (mol)
NO.sub.3.sup.−(mol)=A/172.12×[B/100×4+(100−B)/100×3]+C=D
wherein: A is quantity of cerium cations in terms of CeO.sub.2 (gram), in step (a) B is percentage of tetravalent cerium cations per total cerium cations, at the start of step (b) C is quantity of HNO.sub.3 (mol) if any, in step (a)
2. Calculation of NO.sub.3.sup.− concentration in step (b)
[NO.sub.3.sup.−](mol/l)=D/E
wherein E is volume (liter) of reaction medium in step (b). B may be measured directory such as using measuring cylinder, or gauge.
3. Calculation of NO.sub.3.sup.− concentration in step (c)
[NO.sub.3.sup.−](mol/l)=F/G F is quantity of NO.sub.3.sup.− ions (mol). F=D if the liquid medium is not removed.
F=D×removal ratio of liquid medium if the liquid medium is removed. G is volume (liter) after adding of water.
4. Decrease ratio of NO.sub.3.sup.− concentration decrease ratio of [NO.sub.3.sup.−](%)=[NO.sub.3.sup.−] in step (c)/[NO.sub.3.sup.−] in step (b)×100=(F/G)/(D/E)×100.

(19) Control of the anion concentration may be conducted by a direct measurement of the anion concentration of step (b) and (c). NO.sub.3.sup.− concentration can be analyzed by ion chromatography or adsorptiometer, both apparatus being commonly used to analyze NO.sub.3.sup.− concentration in the liquid medium. A part of the liquid medium is put in the analyzer to automatically measure the NO.sub.3.sup.− concentration. It is then possible to compare the both concentrations to calculate the decrease ratio of NO.sub.3.sup.− concentration.

(20) In step (d), the suspension is heated at a temperature comprised between 100 and 300° C., preferably comprised between 110 and 150° C. Any reaction vessel may be used without critical limitation, and either a sealed vessel or an open vessel may be used. Specifically, an autoclave reactor may preferably be used. The duration of heat treatment is usually between 10 minutes and 48 hours, preferably between 30 minutes and 36 hours.

(21) Following step (d), the heated suspension may be cooled in an optional step (e). The suspension may usually be cooled under stirring. Means for cooling are not critical, and it may be cooling in an atmosphere or forced cooling with cooling tube. The temperature of the suspension after cooling may be comprised between 20 and 90° C.

(22) According to step (f), a basic compound is then added to the suspension, or the suspension having been cooled. This basic compound may be for example sodium hydroxide, potassium hydroxide, an aqueous ammonia solution, ammonia gas, or mixtures thereof, with an aqueous ammonia solution being preferred. The basic compound may be added by first preparing an aqueous solution of the basic compound at a suitable concentration and adding the solution to the cooled suspension prepared in step (e) under stirring, or when ammonia gas is used, by blowing the ammonia gas into the reaction vessel under stirring. The amount of the basic compound may easily be decided by tracing the pH change of the solution. Usually, a sufficient amount is such that the pH of the solution is not lower than 7, and a preferred amount is such that the pH is between 7 and 9. Basic compounds are especially useful to precipitate Ce.sup.3+ ions which are dispersed in the suspension at the end of step (d) or (e) to form then Ce(OH).sub.3 precipitates.

(23) In the method according to the present invention, the suspension prepared in accordance with the step (f) is held between 60 and 300° C. in step (g). Without wishing to be bound by particular theory, this step is believed to modify crystallinity of cerium oxide precursors present in the solution obtained by step (f) so to increase the crystal size thereof.

(24) Any reaction vessel may be used in step (b) without critical limitation, and either a sealed vessel or an open vessel may be used. Specifically, an autoclave reactor may preferably be used.

(25) In step (g), the temperature is comprised between 60 and 220° C., preferably between 90 and 200° C., more preferably between 90 and 160° C. Duration of heat treatment is usually between 10 minutes and 48 hours, preferably between 30 minutes and 36 hours, more preferably between 1 hour and 24 hours. In the present invention, the temperature in the step (g) may be between 90 and 120° C. In the present invention, the duration of heat treatment in the step (g) may be from 1 hour to 2 hours.

(26) In step (h), separation of the liquid medium from the precipitate, may be carried out, for example, by Nutsche filter method, centrifuging, filter pressing, or decantation. The precipitate may optionally be washed with water, preferably with water at basic pH, for example aqueous ammonia solution. Further, the precipitate may optionally be dried.

(27) It is also possible to add at any point between after step (c) and before step (i) of the process a metal compound other than cerium compound, for example a metal element (except for cerium) in the form of nitrate, chloride, oxide, hydroxide, carbonate, halide, oxyhalide, oxynitrate, and/or sulfate. Examples of the metal element (except for cerium) include (1) metallic elements in Group 4A in the periodic table, such as silicon (Si) and tin (Sn), (2) metal elements in Group 4B in the periodic table, such as titanium and zirconium, (3) metal elements in Group 3A in the periodic table, such as aluminum, (4) alkali metal elements, such as barium and strontium, and (5) rare earth element (REE) or rare earth metal being one of a set of seventeen chemical elements in the periodic table, meaning the fifteen lanthanides plus scandium and yttrium. Preferably, the rare earth element oxides are chosen in the group consisting of: lanthanium oxide (La.sub.2O.sub.3), praseodymium oxide (Pr.sub.6O.sub.11), neodymium oxide (Nd.sub.2O.sub.3) and yttrium oxide (Y.sub.2O.sub.3).

(28) Cerium oxide particles obtained by the process of the invention may then comprise at least one metal oxide, other than cerium oxide, for instance in a proportion comprised between 1 and 40% by weight of oxide, preferably in a proportion comprised between 1 and 20% by weight of oxide. Oxide refers there to final mixed oxide defined as integration of cerium oxide and the other metal oxide.

(29) In step (i), an organic texturing agent is added to the suspension obtained in the preceding step (g) or the precipitate obtained in step (h) once separated from the liquid medium. According to one embodiment of the present invention, the method of the present invention is conducted without the step (i). According to another embodiment of the present invention, the method of the present invention is conducted including the step (i).

(30) An organic texturing agent usually refers to an organic compound, such as a surfactant, able to control or modify the mesoporous structure of the cerium oxide. “Mesoporous structure” basically describes a structure which specifically comprises pores with an average diameter comprised between 2 and 50 nm, described by the term “mesopores”. Typically, these structures are amorphous or crystalline compounds in which the pores are generally distributed in random fashion, with a very wide pore-size distribution.

(31) The organic texturing agent may be added directly or indirectly. It can be added directly to the suspension or precipitate resulting from the preceding step. It can also be first added in a composition, for instance comprising a solvent of the organic texturing agent, and said composition being then added to the suspension or precipitate as previously obtained.

(32) The amount of organic texturing agent used, expressed as percentage by weight of additive relative to the weight of the cerium in terms of CeO.sub.2, is generally between 5 and 100%, more particularly between 15 and 60%, preferably between 20 to 30%.

(33) The organic texturing agent may be adsorbed on the surface of secondary particles and primary particles of the precipitates. For instance, the organic texturing agent adsorbed on the primary particles will lead to increase the size of mesopores and pore volume of the precipitate.

(34) Organic texturing agents are preferably chosen in the group consisting of: anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids and their salts, and surfactants of the carboxymethylated fatty alcohol ethoxylate type. With regard to this additive, reference may be made to the teaching of application WO-98/45212 and the surfactants described in this document may be used.

(35) As surfactants of anionic type, mention may be made of ethoxycarboxylates, ethoxylated fatty acids, sarcosinates, phosphate esters, sulfates such as alcohol sulfates, alcohol ether sulfates and sulfated alkanolamide ethoxylates, and sulfonates such as sulfosuccinates, and alkylbenzene or alkylnapthalene sulfonates.

(36) As nonionic surfactants, mention may be made of acetylenic surfactants, alcohol ethoxylates, alkanolamides, amine oxides, ethoxylated alkanolamides, long-chain ethoxylated amines, copolymers of ethylene oxide/propylene oxide, sorbitan derivatives, ethylene glycol, propylene glycol, glycerol, polyglyceryl esters and ethoxylated derivatives thereof, alkylamines, alkylimidazolines, ethoxylated oils and alkylphenol ethoxylates. Mention may in particular be made of the products sold under the brands Igepal®, Dowanol®, Rhodamox® and Alkamide®.

(37) With regard to the carboxylic acids, it is in particular possible to use aliphatic monocarboxylic or dicarboxylic acids and, among these, more particularly saturated acids. Fatty acids and more particularly saturated fatty acids may also be used. Mention may thus in particular be made of formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, caproic acid, caprylic acid, capric acid, lauric acid, myristic acid and palmitic acid. As dicarboxylic acids, mention may be made of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid and sebacic acid.

(38) Salts of the carboxylic acids may also be used, in particular the ammonium.

(39) By way of example, mention may be made more particularly of lauric acid and ammonium laurate.

(40) Finally, it is possible to use a surfactant which is selected from those of the carboxymethylated fatty alcohol ethoxylate type.

(41) The expression “product of the carboxymethylated fatty alcohol ethoxylate type” is intended to mean products consisting of ethoxylated or propoxylated fatty alcohols comprising a —CH.sub.2—COOH group at the end of the chain.

(42) These products may correspond to the formula:
R.sub.1—O—(CR.sub.2R.sub.3—CR.sub.4R.sub.5—O).sub.n—CH.sub.2—COOH
in which R.sub.1 denotes a saturated or unsaturated carbon-based chain of which the length is generally at most 22 carbon atoms, preferably at least 12 carbon atoms; R.sub.2, R.sub.3, R.sub.4 and R.sub.5 may be identical and may represent hydrogen or else R.sub.2 may represent an alkyl group such as a CH.sub.3 group and R.sub.3, R.sub.4 and R.sub.5 represent hydrogen; n is a non-zero integer that may be up to 50 and more particularly between 5 and 15, these values being included. It will be noted that a surfactant may consist of a mixture of products of the formula above for which R.sub.1 may be saturated or unsaturated, respectively, or alternatively products comprising both —CH.sub.2—CH.sub.2—O— and —C(CH.sub.3)—CH.sub.2—O— groups.

(43) Optionally in step (j) the precipitate is separated off from the liquid medium, for example, by Nutsche filter method, centrifuging, or filter pressing. The precipitate may optionally be washed with an aqueous solution, preferably with water at basic pH, for example aqueous ammonia solution. Further, the precipitate may optionally be dried to a suitable extent for improving the efficiency in the following step.

(44) In step (k), the precipitate obtained in the preceding step is calcined to obtain the cerium oxide particles which are the object of the invention. Calcination is preferably conducted in air.

(45) Process of the invention then comprises either a step (h) of separation of the liquid medium from the precipitate, either a step (j) separating off the precipitate from the liquid medium, or both step (h) and step (j); in order to proceed with calcination of the precipitate obtained at the end of step (g) or obtained at step (h), (i) or (j) to obtain cerium oxide particles.

(46) In step (k), the calcination temperature may suitably be selected from the range of usually between 250 and 900° C. The selection of the temperature may be made as desired, depending on the required values of the specific surface area and bulk density. From a practical point of view to prepare a catalyst or a co-catalyst material wherein the specific surface area is important, the calcination temperature in step (k) may preferably be between 250 and 800° C., more preferably between 250 and 700° C., most preferably between 280 and 450° C. The duration of calcination may suitably be determined depending on the temperature, and may preferably be between 1 and 20 hours.

(47) After step (k), the cerium oxide particles obtained may usually be pulverized. The pulverization may sufficiently be performed in an ordinary pulverizer, such as a hammer mill, to obtain a powder of a desired particle size. The cerium oxide obtained by the present method may be given a desired particle size through the above mentioned pulverization. For use as a co-catalyst in a catalyst for purifying exhaust gas, for example, a preferred average particle size D50 of the cerium oxide is between 0.05 and 50 μm, D50 being measured by laser diffraction with a distribution in volume. The measurement can be performed on a suspension of the cerium oxide particles in water. The distribution may be obtained for instance with a LA-920 particle size analyzer commercialized by Horiba, Ltd.

(48) In the method according to the present invention, trivalent Ce.sup.III cations must be present in one of the steps before the precipitation step (f). Ce.sup.III cations may for instance be present in the cerium salt solution of step (a). According to a preferred embodiment, Ce.sup.III cations are present after step (c). The presence of Ce.sup.III cations may be obtained by addition of a Ce.sup.III salt, such as nitrate, chloride, hydroxide, carbonate, and sulfate of Ce.sup.III. It may also be obtained by the addition of a reducing agent to reduce a part of Ce.sup.IV cations already present, for instance by adding hydrogen peroxide or oxalic acid or both. The molar ratio of Ce.sup.III cations relative to the amount of cerium (III) cations and cerium (IV) cations in or after the step (c) and before the step (f) may be from 5 to 30. The molar ratio of Ce.sup.III cations is preferably at least 7. The molar ratio of Ce.sup.III cations is preferably controlled to no more than 20, more preferably no more than 15.

(49) The conditions of examples 1-3 may be used to prepare the cerium oxide particles of the invention. The cerium oxide particles of the present invention may preferably be prepared by the method according to the present invention.

(50) About the Cerium Oxide Particles

(51) Cerium oxide particles having surprisingly excellent heat resistance, especially having high heat resistance under “humid” conditions (700° C.; gaseous atmosphere containing 10% by volume of O.sub.2, 10% by volume of H.sub.2O and the balance of N.sub.2), can be obtained via the method of the present invention. Thus, the invention relates to cerium oxide particles as defined in claim 1.

(52) The cerium oxide particles preferably have a specific surface area (SBET) comprised between 75 and 90 m.sup.2/g, more particularly between 78 and 90 m.sup.2/g after calcination at 700° C. for 4 hours, under a gaseous atmosphere containing 10% of O.sub.2, 10% of H.sub.2O and the balance of N.sub.2. The specific conditions of the “humid” conditions may also be found in the examples.

(53) The cerium oxide particles preferably have a specific surface area (SBET) comprised between 60 and 80 m.sup.2/g after calcination at 900° C. for 5 hours, under air.

(54) The cerium oxide particles may be further characterized by a total pore volume comprised between 0.8 and 4.0 ml/g, more particularly between 0.8 and 2.5 ml/g, even more particularly between 0.8 and 2.0 ml/g after calcination at 400° C. for 10 hours, under air.

(55) The cerium oxide particles may be further characterized by a total pore volume comprised between 0.70 and 1.9 ml/g after calcination at 900° C. for 24 hours, under air.

(56) The cerium oxide particles may be further characterized by a pore volume of the pores having a pore size in the range of no more than 200 nm comprised between 0.27 and 0.9 ml/g, more particularly between 0.27 and 0.7 ml/g after calcination at 400° C. for 10 hours, under air.

(57) The cerium oxide particles may be further characterized by a pore volume of the pores having a pore size in the range of no more than 200 nm comprised between 0.23 and 0.65 ml/g after calcination at 900° C. for 24 hours, under air.

(58) The cerium oxide particles may be obtained from the method of the present invention, in particular the method in which the addition of an organic texting agent in step (i) is not conducted, yet the present invention is not limited thereto.

(59) The total pore volume or pore volume of the pores having a pore size in the range of no more than 200 nm may be measured by ordinary mercury porosimeter, which is further detailed in the following examples.

(60) Further, the inventors of the present invention have surprisingly found cerium oxide particles having a certain porosity, an outstanding pore volume and heat stability associated therewith. Thus, according to an embodiment, the present invention as to product also relates to cerium oxide particles having a specific surface area (SBET) comprised between 37 and 60 m.sup.2/g, more particularly between 40 and 60 m.sup.2/g, even more particularly between 45 and 60 m.sup.2/g after calcination at 900° C. for 24 hours, under air.

(61) The cerium oxide particles according to the embodiment may have a specific surface area (SBET) comprised between 26 and 40 m.sup.2/g after calcination at 1000° C. for 5 hours, under air.

(62) The cerium oxide particles according to the embodiment may have a total pore volume comprised between 1.5 and 4.0 ml/g, more particularly between 1.7 and 4.0 ml/g, after calcination at 400° C. for 10 hours, under air.

(63) The cerium oxide particles according to the embodiment may have a total pore volume comprised between 1.5 and 3.8 ml/g, more particularly between 1.7 and 3.8 ml/g after calcination at 900° C. for 24 hours, under air.

(64) The cerium oxide particles according to the embodiment may have a pore volume of the pores having a pore size in the range of no more than 200 nm comprised between 0.3 and 1.5 ml/g, more particularly between 0.4 and 1.5 ml/g, after calcination at 400° C. for 10 hours, under air.

(65) The cerium oxide particles according to the embodiment may have a pore volume of the pores having a pore size in the range of no more than 200 nm comprised between 0.25 and 1.0 ml/g, more particularly between 0.4 and 1.0 ml/g, after calcination at 900° C. for 24 hours, under air.

(66) The cerium oxide particles according to the embodiment preferably have a specific surface area (SBET) of at least 40 m.sup.2/g, more preferably at least 43 m.sup.2/g, still more preferably 45 m.sup.2/g, after calcination at 900° C. for 24 hours, under air.

(67) The cerium oxide particles according to the embodiment have a specific surface area (SBET) of at least 25 m.sup.2/g, more particularly of at least 28 m.sup.2/g, more preferably at least 30 m.sup.2/g, still more preferably 32 m.sup.2/g, after calcination at 1000° C. for 5 hours, under air.

(68) The cerium oxide particles according to the embodiment preferably have a total pore volume of at least 1.6, more preferably at least 1.7, still more preferably at least 1.8, yet more preferably at least 1.9, further more preferably at least 2.0, far more preferably at least 2.1, still far more preferably at least 2.2, yet far more preferably at least 2.3, especially preferably at least 2.4, particularly preferably at least 2.5, most preferably at least 2.6 ml/g, after calcination at 400° C. for 10 hours under air.

(69) The cerium oxide particles according to the embodiment preferably have a total pore volume of at least 1.6, more preferably at least 1.7, still more preferably at least 1.8, yet more preferably at least 1.9, further more preferably at least 2.0, far more preferably at least 2.1, still far more preferably at least 2.2, yet far more preferably at least 2.3, especially preferably at least 2.4, particularly preferably at least 2.5 ml/g, after calcination at 900° C. for 24 hours under air.

(70) The cerium oxide particles according to the embodiment preferably have a pore volume of the pores having a pore size in the range of no more than 200 nm, of at least 0.35, more preferably at least 0.40, still more preferably at least 0.45, yet more preferably at least 0.50, further more preferably at least 0.55, far more preferably at least 0.60, still far more preferably at least 0.65, yet far more preferably at least 0.70 ml/g, after calcination at 400° C. for 10 hours under air.

(71) The cerium oxide particles according to the embodiment preferably have a pore volume of the pores having a pore size in the range of no more than 200 nm of at least 0.30, more preferably at least 0.35, still more preferably at least 0.40, yet more preferably at least 0.45 ml/g, after calcination at 900° C. for 24 hours under air.

(72) The cerium oxide particles according to the embodiment may be obtained from the method of the present invention, in particular the method in which the addition of an organic texturing agent in step (i) is included in the method, yet the present invention is not limited thereto.

(73) The cerium oxide particles of the present invention may also comprise at least one metal oxide, other than cerium oxide, for instance in a proportion comprised between 1 and 40% by weight of oxide, preferably in a proportion comprised between 1 and 20% by weight of oxide. Oxide refers there to final mixed oxide defined as integration of cerium oxide and metal oxide.

(74) Cerium oxide particles as described above or as obtained by means of the method of preparation previously described may be in the form of powders, but they can optionally be formed so as to be in the form of granules, pellets, foams, beads, cylinders or honeycombs of variable dimensions.

(75) The present invention also concerns a catalyst comprising at least cerium oxide particles as previously defined, such as a catalyst for purifying exhaust gas. The cerium oxide particles of the invention may be applied as such or within a composition to any support commonly used in the field of catalysis, that is to say in particular thermally inert supports. This support can be chosen from alumina, titanium oxide, cerium oxide, zirconium oxide, silica, spinels, zeolites, silicates, crystalline silicoaluminum phosphates or crystalline aluminum phosphates.

(76) The catalyst for purifying exhaust gas according to the present invention may be of any type, as long as it has a co-catalyst containing the cerium oxide of the present invention. The catalyst may be produced, for example, by a commonly known method and with commonly known other materials.

(77) The invention also concerns a composition, preferably a liquid composition, comprising at least cerium oxide particles as previously obtained and defined. More preferably said composition is a suspension comprising at least a liquid medium and cerium oxide particles as previously obtained and defined.

(78) According to an embodiment of the invention, the invention also relates to the use of cerium oxide particles as defined and/or as obtained in the above identified process for the polishing application. For instance, a composition, such as a suspension, for polishing comprising at least the cerium oxide particles of the invention may be obtained. This composition can be used for polishing glass, for example in the crystal-making or mirror industry, flat glass, television screens or spectacles, or else for polishing ceramics or other materials of vitreous type. This composition can also be used most particularly for CMP-type polishing in the electronics industry and therefore for polishing metal substrates which go to make up microprocessors, but also for polishing insulating layers or Interlayer Dielectric (ILD) layers of these same microprocessors, the suspension of the invention being particularly suitable for the polishing of said layers. Chemical mechanical planarization (CMP) is a key process enabling Shallow Trench Isolation (STI), which is used in current integrated circuit manufacturing processes to achieve device isolation. These layers are generally made of silica, such as doped silica or porous silica. This suspension may also be used for metal CMP for wiring and barrier in integrated circuit, polishing a photomask substrate, especially made of a synthetic quartz glass. In the present invention, the term “polishing” is understood to include the meaning of both conventional polishing, such as glass polishing, and polishing of surface of semiconductive materials, such as silicon wafer in semiconductor application, often called as “planarization.”

(79) In general, such compositions comprise, in addition to the compound with abrasive property, such as the oxide particles, additives such as a dispersing agent and/or an oxidant.

(80) The present invention also concerns a method of removing a portion of a substrate, for instance in a CMP operation, comprising: providing at least a composition, for instance a suspension, comprising cerium oxide particles of the invention, contacting at least the composition and the substrate to be polished, and performing the polishing on the substrate.

(81) The following examples are included to illustrate embodiments of the invention. Needless to say, the invention is not limited to described examples.

(82) Experimental Part

(83) Specific Surface Areas

(84) The specific surface areas (BET) by adsorption of N.sub.2 were determined automatically on a Flowsorb II 2300 or a Macsorb analyzer model 1-1220 (Mountech Co., LTD.). Prior to any measurement, the samples are carefully degassed to desorb any adsorbed species. To do so, the samples may be heated at 200° C. for 2 hours in a stove, then at 300° C. for 15 min in the cell.

(85) Hg Porosity

(86) The porosity measurements were conducted on a Micromeritics Autopore IV 9500 equipped with penetrometer adapted for powders following the guidelines of Micromeritics. Similarly, prior to any measurement, the samples are carefully degassed to desorb any adsorbed species. The following parameters may be used: penetrometer 3.2 ml; capillary volume: 0.412 ml; head pressure: 4.68 psi; contact angle: 130°; mercury superficial tension: 485 dynes/cm; mercury density: 13.5335 g/ml. A vacuum is first applied on the sample (50 mm Hg; 5 min). The duration of the equilibrium is: low pressures (1.3-30 psi): 20 s; (30-60 000 psi): 30 s. Before the measurement, the samples are heated at 400° C. for 10 hours. Total pore volume is determined directly on the porosity curves.

(87) “Humid” Conditions

(88) The cerium oxide particles are evaluated under “humid” conditions (700° C.; gaseous atmosphere containing 10% by volume of O.sub.2, 10% by volume of H.sub.2O and the balance of N.sub.2). The samples are prepared in the following way. the solid is compacted in the form of a cylindrical pellet of 32 mm diameter by applying the powder to a pressure of 30 tons for 2 min; the pellet so obtained is then deagglomerated in a mortar to give a powder which was sieved so as to retain only the fraction of the powder which passed through a sieve of 250 μm and retained by a sieve of 125 μm; the powder (2.2 g) thus obtained is hydrothermally aged at 700° C. for 4 h under a synthetic gas containing 10% by volume O.sub.2, 10% by volume H.sub.2O and balance N.sub.2 (24 L/h flow rate (20° C.; 1 atm) over the powder); the SBET is then measured in accordance with the BET measurement method explained in the above.

(89) The same conditions could be applied on the cerium oxide particles with platinum which can be prepared as follows: 9.9 grams of the cerium oxide in the form of a powder is impregnated with amine solution of platinum hydroxide (containing 0.1 gram of Pt). The product is dried at around 120° C. and further calcined under air at 500° C. for 4 h; the whole solid is then compacted in the form of a cylindrical pellet of 32 mm diameter by applying the powder to a pressure of 30 tons for 2 min; the pellet is then deagglomerated in a mortar to give a powder which was sieved so as to retain only the fraction of the powder which passed through a sieve of 250 μm and retained by a sieve of 125 μm; the cerium oxide with Pt (2.2 g) thus obtained is hydrothermally aged at 700° C. for 4 h under a synthetic gas containing 10% by volume O.sub.2, 10% by volume H.sub.2O and balance N.sub.2 (24 L/h flow rate (20° C.; 1 atm) over the powder); the SBET is then measured in accordance with the BET measurement method explained in the above.

Example 1

(90) 50 g of a ceric nitrate solution in terms of CeO.sub.2 containing not less than 90 mol % tetravalent cerium cations was measured out, and adjusted to a total amount of 1 L with deionized water. The obtained solution was heated to 100° C., maintained at this temperature for 30 minutes, and allowed to cool down to 25° C., to thereby obtain a cerium suspension.

(91) After the mother liquor was removed from the cerium suspension thus obtained, the total volume was adjusted to 1 L with deionized water; concentration of anions is hence decreased by 44%, in comparison with anions comprised in the liquid medium after heating. After the removal of the mother liquor, a solution of trivalent Ce.sup.III cations in a form of nitrate (Ce(NO.sub.3).sub.3) was added so as to control the amount of Ce.sup.III cations based on the total amount of cerium cations to 6 mol %.

(92) Then the cerium suspension was maintained at 120° C. for 2 hours, allowed to cool, and neutralized to pH 8.5 with aqueous ammonia.

(93) The obtained mixture was heated to 120° C., maintained at this temperature for 1 hour, and allowed to cool down to 25° C., thereby obtaining a slurry. The obtained slurry was subjected to solid-liquid separation through a Nutsche filter to obtain a filter cake. The cake was calcined in the air at 400° C. for 10 hours to obtain cerium oxide powder.

(94) The obtained composite oxide powder was measured of the specific surface area by the BET method after calcination at 900° C. for 5 hours under air, 700° C. for 4 hours under the humid conditions as already described above, total pore volume (before and after the calcination at 900° C. for 24 hours under air), and pore volume of pores having no more than 200 nm size (before and after the calcination at 900° C. for 24 hours under air).

Example 2

(95) A cerium oxide powder was prepared in the same way as in Example 1 except that after the thermal aging at the temperature of 120° C. for 1 hour, the obtained slurry was allowed to cool down to 40° C., and then, 12.5 g of lauric acid was added to the slurry.

(96) The obtained composite oxide powder was measured of the specific surface area by the BET method after calcination at 900° C. for 5 hours under air, 900° C. for 24 hours under air, 1000° C. for 5 hours under air, total pore volume (before and after the calcination at 900° C. for 24 hours under air), and pore volume of pores having no more than 200 nm size (before and after the calcination at 900° C. for 24 hours under air).

Example 3

(97) A cerium oxide powder was prepared in the same way as in Example 2 except that the amount of Ce.sup.III cations based on the total amount of cerium cations was controlled to 8 mol %, instead of 6 mol %.

(98) The properties of the oxide powder thus obtained were evaluated in the same way as in Example 2, except for SBET 1000° C./5 h under air.

Comparative Example 1

(99) A cerium oxide powder was prepared in accordance with the method disclosed in Patent Publication U.S. Pat. No. 7,361,322 B2. Thus, 20 g of a ceric nitrate solution in terms of CeO.sub.2 containing not less than 90 mol % tetravalent cerium cations was measured out, and adjusted to a total amount of 1 L with deionized water. The obtained solution was heated to 100° C., maintained at this temperature for 24 hours, and allowed to cool down to the room temperature. Then aqueous ammonia was added to neutralize to pH 8 to obtain cerium oxide hydrate in the form of the slurry.

(100) The slurry was then subjected to solid-liquid separation with a Nutsche filter to obtain a filter cake. The cake was calcined in the air at 300° C. for 10 hours to obtain cerium oxide powder.

(101) Those properties measured in Examples 1 and 2 of the oxide powder thus obtained were evaluated in the same way. Properties of the cerium oxides prepared in the above defined examples are mentioned in Table 1.

(102) TABLE-US-00001 TABLE 1 Examples Ex. 1 Ex. 2 Ex. 3 Comp. 1 SBET 900° C./5 h 56 63 61 46 SBET 900° C./24 h — 49 48 34 SBET 1000° C./5 h — 32 — 25 SBET 700° C./4 h 80 — — 75 under “humid” conditions (700° C.; gaseous atmosphere containing 10% by volume of O.sub.2, 10% by volume of H.sub.2O and the balance of N.sub.2) total pore volume 400° C./10 h 0.83 1.79 2.07 0.75 total pore volume 900° C./24 h 0.79 1.72 1.71 0.65 pore volume <200 nm 0.31 0.45 0.75 0.25 400° C./10 h pore volume <200 nm 0.25 0.44 0.45 0.20 900° C./24 h SBET in m2/g; pore volume in ml/g

Example 4: Low Temperature NO.SUB.x .Storage Capacity Testing

(103) The cerium oxides of Example 1 and Comparative example 1 are calcined under a synthetic gas containing 10% by volume of O.sub.2, 10% by volume of H.sub.2O and balance being N.sub.2 (24 L/h flow rate over 2.2 g catalyst). The NO.sub.x storage capacity is then measured in the following way: a synthetic gas mixture (30 L/h), representative of the catalytic process with the composition A given below is flushed during 90 min at 120° C. through 150 mg of cerium oxide placed in a fixed bed reactor. The amount of NO.sub.x stored is monitored on line in function of the time, owing to an Antaris IGS FTIR Spectrometer.

(104) TABLE-US-00002 Composition A (vol %) NO 0.014 NO.sub.2 0.018 H.sub.2O 5 CO.sub.2 5 O.sub.2 10 N.sub.2 balance

(105) NO.sub.x adsorption of both cerium oxides of Example 1 and Comparative example 1 at 90 mins is shown in Table 2:

(106) TABLE-US-00003 TABLE 2 NO.sub.xadsorbed (μg NOx/g Ce oxide) Ex. 1 22.6 Comp. 1 19.2

(107) It appears then that the cerium oxide of the present invention has a higher NO.sub.x capture performance than conventional cerium oxide after hydro-thermal aging. NSC (NO.sub.x storage capacity) is an indicator to evaluate the NO.sub.x emission performance.