AU639425B2

AU639425B2 - Ceric oxide and a process for its production

Info

Publication number: AU639425B2
Application number: AU47104/89A
Authority: AU
Inventors: Claire David; Jean-Luc Le Loarer; Francoise Picard
Original assignee: Rhone Poulenc Chimie SA
Current assignee: Rhodia Chimie SAS
Priority date: 1988-12-23
Filing date: 1989-12-20
Publication date: 1993-07-29
Anticipated expiration: 2009-12-20
Also published as: FI97374C; NO895150D0; EP0388567B1; FI97374B; FR2640954B1; CA2006563A1; JPH0832555B2; ZA899764B; EP0388567A1; NO895150L; FR2640954A1; ATE120436T1; KR920007596B1; KR900009443A; DE68921977T2; BR8906721A; NO178854C; ES2072913T3; FI896265A0; AU4710489A

Abstract

A ceric oxide with new morphological characteristics. The invention also applies to one of the processes for obtaining the said oxide. The process of the invention is characterised in that a ceric hydroxide prepared by hydrolysis of a solution of a cerium(IV) salt in a basic medium is subjected to a thermal treatment in solvent before the calcining operation.

Description

COMMONWEALTH OF AUSTRALIA

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6 3 9 4 2 FORM 10 PATENTS ACT 1952 COMPLETE SPECIFICATION FOR OFFICE USE: Class Int.Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority: Related Art: ""Name of Applicant: ***.Iddress of Applicant: Actual Inventor: RHONE-POULENC CHIMIE 25, Quai Paul Doumer, Cedex, France 92408, Courbevoie, Jean-Luc Le Loarer, Francoise Picard and Claire David.

Address for Service: SHELSTON WATERS, 55 Clarence Street, Sydney Complete Specification for the Invention entitled: Complete Specification for the Invention entitled: "CERIC OXIDE AND A PROCESS FOR ITS PRODUCTION" 0 6 C The following statement is a full description of this invention, including the best method of performing it known to us:- *6 -1 la CERIC OXIDE AND A PROCESS FOR ITS PRODUCTION The present invention pertains to a ceric oxide with improved morphological characteristics. The invention also pertains to one of the processes for the production of the said oxide.

In the specification of the invention that follows is meant by specific surface area the specific surface area determined by nitrogen adsorption in accordance with the o0 standard ASTM D 3663-78, based on the method BRUNAUER EMMETT TELLER described in the periodical "The Journal of American Society, 60, 309 (1938)".

It is known that ceric oxide can be used as catalyst or as a substrate for catalyst. One may mention, for instance, the work of Paul MERIAUDAU and colleagues in relation to the synthesis of methanol, starting from CO H 2 on platinum catalysts deposited onto ceric 15 oxide Acad. Sc. Paris, Vol. 297 Series 11- 471 1983).

It is also well known that the effectiveness of a catalyst usually increases as the surface area of contact between the catalyst and the reagents becomes larger. To this end, it is necessary that the catalyst remains in as divided a state as possible, i.e. that the solid particles which comprise it be as small and distinct as possible. The fundamental role of the 20 substrate is therefore to keep the catalyst particles or crystallites in contact with the reagents, in as divided a state as possible.

During prolonged use of a catalyst substrate, a reduction of the specific surface area occurs as a result of the coalescence of the very fine micropores. During the course of this coalescence, a fraction of the catalyst is engulfed within the mass of the substrate, and is 25 therefore no longer in contact with the reagents.

Up to now, most of the ceric oxides prepared exhibit a specific surface area which decreases rapidly at working temperatures higher than 5000C. Thus, R. ALVERO and colleagues Chem. Soc. Dalton Trans. 1984, 87) obtained, starting from ammonium cerinitrate, a ceric oxide exhibiting, following calcination at a temperature of 6000C, a specific surface area of 29 m 2 /g.

Furthermore, a ceric oxide exhibiting a specific surface area of at least 85 5 m 2 /g following calcination between 350 and 450°C and, preferably, between 100 and 130 m 2 /g following calcination between 400 and 4500C, was described in FR-A 2 559 754. The said oxide is prepared by hydrolysis of an aqueous solution of ceric nitrate in acid nitric medium, then by separation of the precipitate obtained, washing with an organic solvent, possibly drying, then calcination. The ceric oxide obtained exhibits an interesting specific surface area when it is used within a calcination temperature range of 300 to 6000C. However, following calcination at a higher temperature, a drop of the specific surface area is seen, the specific surface area being 10 m 2 /g following calcination at 8000C.

FR-A 2 559 755 may also be mentioned. It pertains to a ceric oxide exhibiting a specific surface area of at least 85 5 m 2 /g following calcination between 350 and 500°C and, preferably, included between 150 and 180 m 2 /g following calcination between 400 and 450 0 C. This oxide is obtained according to a process which consists of precipitating a basic ceric sulphate by reacting an aqueous solution of ceric nitrate with an aqueous solution containing sulphate ions, of separating the precipitate obtained, washing it by means of a solution of ammonia, possibly drying it then calcinating it at a temperature varying between 300 and 5000C. The eerie oxide thus prepared exhibits a large specific surface area, but when subjected to calcination at 8000C, its specific surface area decreases dramatically, and is close to 10 m 2 /g.

In the European patent application no 88 401593.4, the Applicant described a process to increase and stabilise, at high temperature, the specific surface area of a ceric oxide.

This process consists of subjecting ceric hydroxide, precursor of ceric oxide, to a solvothermal treatment prior to the operation of calcination.

15 More particularly, the process described in the said application consists of: suspending the ceric hydroxide in a liquid medium, heating it within a sealed vessel to a temperature and a pressure below the critical temperature and critical pressure of the said medium, respectively, coo!!ng the reaction medium and bringing i' back to atmospheric pressure, 20 separating the ceric hydroxide so treated, then calcinating it.

By ceric hydroxide is meant hydrated ceric oxide CeO 2 ,2H 2 0 or else a ceric hydroxide possibly containing residual amounts of bound or adsorbed anions such as, for instance, chlorides, sulphates, nitrates, acetates, formiates, etc 25 A preferred operative mode of the process described in European patent application i n° 88 401593.4 consists of using a solution of base as rutoclaving liquid medium.

Such a process permits not only the increase of the specific surface area of the ceric oxide obtained, but also the conservation of a high specific surface area at temperatures up to 9000C.

By subjecting a ceric hydroxide prepared by reacting a solution of cerium salt and a base possibly in the presence of an oxidising agent, under pH conditions above 7, to autoclaving in basic medium, the Applicant proposes according to European patent application no 88 401594.2, a ceric oxide presenting a specific surface area, at 800-900C, never reached by the products described within the state of the art.

The ceric oxide thus obtained has a specific surface area of at least 15 m 2 /g measured following calcination at a temperature between 800 and 9000C and, preferably, included between 20 and 60 m2/g measured following calcination at a temperature of 8000C.

It has a specific surface area included between 160 and 15 m 2 /g measured following calcination at a temperature included between 350 and 9000C.

Thus it can exhibit a specific surface area varying between 70 and 160 m 2 /g and, preferably, between 100 and 160 m 2 /g following calcination between 350 and 4500C.

In the present application, the specific surface areas that are indicated are measured on a product having undergone a calcination of at least 2 hours at the given temperature.

Pursuing her research work, the Applicant has found that by applying the process described in the European patent application no 88 401593.5, but selecting the starting ceric hydroxide and the dispersion liquid medium, a ceric oxide displaying morphological characteristics yet improved was unexpectedly obtained.

In relation to earlier products, the Applicant proposes a ceric oxide exhibiting a high 10 specific surface area following calcination at low temperature as well as at high temperature.

The characteristic of the ceric oxide of the invention is to have a specific surface area °of at least 190 m 2 preferably of at least 200 m 2 following calcination at a temperature

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between 350 and 450°C. More precisely, the specific surface area that is given is measured on a ceric oxide having undergone calcination at the temperature indicated.

The preferred ceric oxide of the invention has a specific surface area included between 220 and 280 m 2 /g following calcination at a temperature between 350 and 4500C.

Another characteristic of the ceric oxide of the invention is to present a large specific surface area following calcination at high temperature.

20 Thus, it has a specific surface area of at least 15 m 2 /g measured at a temperature between 800°C and 9000C and, preferably, between 20 and 100 m2/g following calcination at a temperature of 8000C.

The ceric oxide of the invention has a large specific surface area of a value between 190 and 280 m 2 /g following calcination between 3500C and 4500C. When subjected to a 25 higher temperature possibly reaching 9000C, at the time of its use, especially in the field of catalysis, it has the characteristic of retaining a high specific surface area.

The latter is included between 20 and 90 m 2 /g for a treatment temperature of 8000C.

The ceric Ixide of the invention exhibits a porous volume varying between 0,15 cm 3 /g and approximately 0,30 cm 3 following calcination at a temperature between 3500C and 4500C.

The porous volume corresponding to pores of a diameter smaller than 60 nm (600 A) is measured with a mercury porosimeter according to the standard ASTM D4284-83, or following the method of nitrogen adsorption isotherms, the aforementioned B.E.T. method.

Following calcination at a temperature of 800°C, it has a porous volume varying between 0,15 and approximately 0,30 cm3/g.

The ceric oxide of the invention obtained following calcination between 350°C and 4500C has a pore mean diameter (d 5 0 varying between 2 nm (20 A) and 10 nm (100 A).

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C. CC ew C CC C w The mean diameter is defined as being a diameter such that all pores under this diameter constitute 50 of the total porous volume (Vp) of the pores of diameter below nm (600 A).

Following calcination at a temperature of 800 0 C, it has a mean pore diameter varying between approximately 10 nm (100 A) and approximately 20 nm (200 A).

X ray diffraction analysis shows that the ceric oxide of the invention has a crystalline phase of CeO 2 type, with a mesh size varying from 0, 542 nm (5,42 A) to 0,544 nm (5,44 A and a high crystallisation rate, greater than 80 for a ceric oxide obtained following calcination at a temperature between 3500C and 450°C, and greater than 90 for a ceric 0L oxide obtained following calcination at a temperature of 8000C.

The process for the production of a ceric oxide exhibiting a specific surface area of at least 190 m 2 /g following calcination at a temperature between 350°C and 450°C is characterised by the fact that it consists of suspending in an aqueous solution of a decomposable base, a ceric hydroxide complying with general formula Ce (M)x (OH)y (NO 3 z (I) where: M represents an alkali metal or a quaternary ammonium radical, x lies between 0,01 and 0,2, y is such that y 4 z x, z lies between 0,4 and 0,7.

heating the said suspension in a sealed vessel to a temperature and pressure below the critical temperature and pressure, respectively, of the said medium (also called autoclaving), cooling the reaction medium and bringing it back to atmospheric pressure, separating the ceric hydroxide thus treated, then calcinating it.

The Applicant has discovered that a ceric oxide with a large specific surface area at low temperature as well as at high temperature could be obtained by subjecting a ceric hydroxide or hydrated ceric oxide, obtained by hydrolysis of a salt of cerium IV in basic medium, to an autoclaving treatment performed in basic medium.

A ceric hydroxide complying with formula thus intervenes in the invention process.

It is the subject of patent application EP-A 0 208 580.

More precisely, it consists of an hydroxinitrate of cerium IV which has the property of breaking up, i.e. of giving a sol by mere dispersion in water.

Thermal differential analysis of the said product reveals, during its calcination in air, the existence of an exothermal peak from 2500°C to 3000°C for a temperature increase of 3000°C per hour.

It has a crystalline phase of CeO 2 type, with a mesh size varying from 0, 542 nm (5,42 A) to 0,544 nm (5,44 A) and a crystallisation rate varying from 30 to 70 and, preferably, from to 60 The ceric oxide complying with formula with the aforementioned characteristics that is used according to the invention is prepared according to a process described in EP-A 0 208 580, which consists of preparing a colloidal dispersion of a compound of cerium IV by 10 reacting a cerium IV salt aqueous solution with a base so that a rate of neutralisation lower e than or equal to 3,0 is obtained of subjecting the said dispersion to a thermal treatment than of separating the precipitate formed.

In the first step of the process described in EP-A 0 208 580, a colloidal dispersion of a compound of cerium IV is prepared. In the latter, cerium is both in the form of ion and in the form of colloids, meaning that there is presence of particles of colloidal dimensions. This ioncolloid mixture is designated as "colloidal dispersion".

The preparation of The said colloidal dispersion is achieved by reacting an aqueous solution of a salt of cerium IV with a base under the conditions described below.

The cerium IV salt solution can be an aqueous solution of ceric nitrate or an aqueous 20 solution of ceri-ammoniacal nitrate. The said solution may contain cerium in the cerous state without disadvantage, but is is desirable that it contains at least 85 of cerium IV.

The concentration of the cerium salt solution is not critical and may vary between 0,1 and 2 moles/liter, preferably between 1 and 2 moles/liter the said concentration being expressed as cerium IV.

The aqueous solution of cerium IV salt usually has a certain initial acidity and may have a normality varying between 0,1 and 4 N and, preferably, between 0,1 N and 1 N.

The solution of ceric nitrate obtained according to the process of electrolytic oxidation of a solution of cerous nitrate, which is described in FR-A 2 570 087 constitutes a raw material of choice.

With respect to the basic solution used, one may call upon an aqueous solution of ammonia, of sodium or potassium hydroxide. It is also possible to use gaseous ammonia. A solution of ammonia is preferred.

The normality of the basic solution is not critica! and may vary between 0,1 N and 11 N, preferably between 0,1 N and 5 N.

The proportion of basic solution to cerium IV salt solution must be such that the rate of neutralisation is greater than or equal to 0,01 and lower than or equal to The neutralisation rate r is defined by the following equation 3 n 2 r ni where: n, represents the number of moles of Ce IV present in the final colloidal dispersion n 2 represents the number of moles of OH' required to neutralise the acidity brought by the aqueous solution of salt of cerium IV n represents the total number of moles of OH- brought by the addition of base.

The neutralisation rate reflects the colloidal state of the cerium IV when r 4, the cerium IV precipitates in gelatinous form S- when r 0, the cerium IV is in ionic form o when 0 r 4, the cerium IV is in ionic and/or colloidal form.

15 For a final concentration of cerium IV lower than 0,5 mole/liter, a neutralisation rate greater than or equal to 0,01 and lower than or equal to 2.5 is selected, and for a concentration greater than 0,5 mole/liter, it is chosen, preferabiy, greater than or equal to 0,01 and lower than or equal to In practise, to obtain a desired neutralisation rate r, chosen within the aforementioned 20 bracket for a given concentration of Ce IV in the final colloidal dispersion, the concentration of the basic solution is adjusted so that it satisfies the following equation (ni r n 2 [Ce IV]f [Ce IV] 25 n 1 ([Ce IV] [Ce IV]f) [OH] represents the concentration, in moles/liter, of the basic solution [Ce IV]f represents the concentration in Ce IV, in moles/liter, of the final colloidal dispersion [Ce IV] i represents the concentration in Ce IV, in moles/liter, of the aqueous solution of cerium IV salt n and n 2 are determined by conventional titration of the aqueous solution of cerium IV salt: n 1 by potentiometric titration by means of a solution of ferrous salt n 2 by acido-basic titration following chelation of cerium by means of oxalate ions.

The reaction between the aqueous solution of cerium IV salt and the base used in the previously defined quantities is performed at a temperature which may vary between 0°C and but preferably at room temperature i.e. most often 15 to 25 0

C.

The aforementioned reagents may be mixed according to several variants. For example, one may proceed to the simultaneous mixing of the aqueous solution of the cerium IV salt and of the basic solution, under agitation, or else add, continuously or all in one, the base into the aqueous solution of cerium IV salt, or vice versa.

Mixing time may vary between 0,1 second and 30 hours and is selected, preferably, between 2 hours and 6 hours.

In accordance with the process described in EP-A 0 208 580, a colloidal dispersion of a compound of cerium IV in aqueous medium is obtained.

Its cerium IV concentration can vary between 0,1 and 2,0 moles/liter, preferably from es..

*10 0,3 and 0,8 mole/liter. The proportion of cerium IV in colloidal form is usually between 10 and of the cerium IV used.

••The mean hydrodynamic diameter of the colloids, determined by light quasi elastic scattering according to the method described by Michael. L. Mc CONNELL in Analytical Chemistry, 53, no 8, 1007A (1981), can vary between 4 and 30 nm.

15 In accordance with the process described in EP-A 0 208 580, the dispersion thus obtained is subjected to thermal treatment at a temperature between 800C and 3000C, preferably between 900C and 1100C, and even more preferentially, at the reflux temperature :eof the reaction medium.

The conditions of the thermal treatment are not critical it may be conducted under 20 atmospheric pressure or under a pressure such as, for example, the saturating steam S. pressure corresponding to the temperature of the thermal treatment. The treatment is conducted either in air or in an atmosphere of inert gas, preferably nitrogen.

•The duration of the treatment may vary, within large limits, between 2 and 48 hours, preferably between 2 and 24 hours.

S 25 At the end of the operation, a solid precipitate is recovered, which is separated according to conventional separation techniques filtration, decantation, blotting and centrifugation.

It is possible to subject the product obtained to a drying operation, which can be conducted at a temperature between 150C and 1200C, preferably at room temperature, and in air or under reduced pressure, for example, between 133,322 Pa and 13332,2 Pa.

The hydroxinitrate of cerium IV of formula obtained directly following separation, without drying it, is preferred for the invention process.

In accordance with the invention process, the said product prepared by hydrolysis of a nitrate salt of cerium IV is subjected to an autoclaving treatment, prior to proceeding to the operation of calcination.

To this end, one uses the ceric hydroxide in the form of a suspension in an aqueous solution of a decomposable base, which constitutes the liquid medium of autoclaving.

By decomposable base is meant a compound with a pka lower than 7 and likely to decompose under the conditions of calcination of the invention.

By way of illustration of these bases, one may mention ammonia, urea, ammonium acetate, ammonium hydrogenocarbonate, ammonium carbonate, or a primary, secondary or tertiary amine such as, for instance, methylamine, ethylamine, propylamine, n-butylamine, sec-butylamine, n-pentylamine, amino-2 pentane, amino-2 methyl-2 butane, amino-1 methyl-3 butane, diamino-1,2 ethane, diamino-1,2 propane, diamino-1,3 propane, diamino-1,4 butane, pentane, diamino-1,6 hexane, dimethylamine, diethylamine, trimethylamine, triethylamine or a quaternary amine such as, for example, tetraalkylammnonium hydroxide preferably containing alkyl radicals with 1 to 4 carbon atoms, and one more particularly calls upon tetramethylammonium hydroxide or tetraethylammonium hydroxide.

10 It is also possible to use a mixture of bases.

A solution of ammonia, of tetraalkylammonium hydroxide or their mixtures is S preferred.

The concentration of the base in the autoclaving liquid medium is not critical S••according to the invention. It may vary within large limits, for example, between 0,1 N and 11 15 N, but a solution with a concentration varying between 1 and 10 N is preferred.

In the liquid medium, the concentration of ceric hydroxide, expressed as CeO 2 preferably varies between 0,1 and 3,0 moles/liter, and, even more preferably, between 0,2 and 1,0 mole/liter.

The autoclaving operation is performed at a temperature between the reflux 20 temperature and the critical temperature of the reaction mixture. A temperature between 100°C and 3500C is preferred, and even more so between 1500C and 3500C.

S* The speed at which the temperature is raised is not critical. The reaction temperature is reached by heating between 30 minutes and 4 hours, for instance.

se'* The invention process may be carried out by introducing the ceric hydroxide in suspension in the liquid medium into a sealed vessel, the pressure therefore only results from Sheating the reaction medium.

Under the temperature conditions given above, and in aqueous medium, it can be specified, by way of indication, that the pressure varies between 1 (105 Pa) and 165 (165.105 Pa), and preferably between 5 (5.105 Pa) and 165 Bars (165.105 Pa).

It is also possibly to apply an external pressure which would then add to that resulting from heating.

The duration of the autoclaving operation is not critical. It may vary between minutes and 6 hours.

At the end of the latter, the system is left to cool under its inertia, and brought back to atmospheric pressure.

The product in suspension in the liquid medium is separated according to conventional solid-liquid separation techniques such as decantation, blotting, filtration and/or centrifugation.

The product recovered may eventually be washed, preferably with water, and/or dried under conditions such as previously described.

According to a last step of the Invention process, the product obtained is calcinated at a temperature between 3000°C and 10000°C and, preferably, selected between 3500°C and 8000C.

Calcination time may vary within large limits, between 30 minutes and 10 hours, and preferably between 2 and 6 hours.

According to a variant of the invention process, the ceric hydroxide in suspension in a liquid medium is subjected to a first thermal treatment within a sealed vessel to a temperature *":010 and pressure below, respectively, the critical temperature and pressure of the liquid medium, in other words, to a first autoclaving.

Avantageously, the liquid medium is water.

S* The autoclaved suspension that is obtained is then subjected to a second autoclaving such as described above, i.e. with a liquid medium containing a decomposable base.

15 The suspension resulting from the first autoclaving may be concentrated or diluted, or the ceric hydroxide so autoclaved may be separated and recovered, for example by filtration, prior to being redispersed in the second autoclaving medium. The precipitate thus recovered may be washed and dried prior to its redispersion.

The conditions of this first autoclaving are similar to those described for autoclaving in 20 basic medium. However, the ceric hydroxide concentration, expressed as CeO 2 may be higher and for example included between 0,.1 and 4 moles/liter.

The ceric oxide of the invention has a large specific surface area following calcination at low and high temperature so that it is perfectly well suited to the field of catalysis, as catalyst or as a substrate for cataiyst.

25 It is particularly suitable for use as catalytic substrate in the reactions of treatment of exhaust gases in internal combustion engines.

The following Examples illustrate the novel ceric oxide of invention and its production process. They are given by way of illustration, without any limitative character.

EXAMPLES 1 1. Synthesis of the ceric hydroxide In a triple neck flask of 6 liters equipped with a thermometer, a stirrer, a device to introduce the reagents (measuring pump), the following are introduced at room temperature 1000 cm 3 of a solution of ceric nitrate containing 1,24 moles/liter of cerium IV and with a free acidity of 0,332 N 2555 cm 3 of a 0,3726 N solution of ammonia.

The addition of the solution of ammonia into the solution of ceric nitrate, at room temperature, is performed at 1664 cm3/hour.

An aqueous colloidal dispersion of a compound of cerium IV with a concentration, expressed as CeO 2 equal to 60 g/I and a neutralisation rate r 0,5, is c t ained.

In a second step, the dispersion obtained is subjected to thermal treatment, in a dc uble wall reactor equipped with a cooling system and a stirrer, at 1000C for 4 hours.

Following filtration of the precipitate, 287 g of a yellow product are recovered.

Chemical analysis of the product obtained reveals the following chemical e g.

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15 composition: loss on ignition CeO 2 molar ratio NOg3/CeIV molar ratio NH 4 +/CeV 2. Autoclavina of the ceric hv6yoxide 20 0,49 0,025

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*o 25 C C In an autoclave of 1 liter of effective volume are introduced successively 400 cm 3 of a 2 N solution of ammonia and 24 o of the ceric hydroxide prepared as described in 1.

Following homogenisation of the latter in its medium, the ensemble is brought to 1800C, i.e. appriximately 12 bars (12.105 Pa) for 1 hour, by means of adequate heating.

At the end of this treatment, the precipitate is filtered on Buchner.

Two fractions of this moist product are then subjected to calcination under the following conditions 6 hours at 350°C and 6 hours at 8000C.

The specific surface area of the ceric oxide obtained, as well as its porous volume, are then determined according to the methods defined in the specification.

The size of the crystallites perpendicular to directions 110 and 120 and the rate of crystallisation are determined by X ray diffraction. The results obtained are summarised in Table I: TABLE I Examole Calcination Specific Porous Crystallite Crystallisation temperature surface volume size rate area OC m2/g cm3/g nm 1 350 250 0,17 4 1 800 50 0,15 20 It can be noticed that the ceric oxide of the invention has a high specific surface area following calcination at low temperature (3500C) and at high temperature (8000C).

11 EXAMPLES 2 AND 3 1. Synthesis of the ceric hydroxide In these tests, a ceric hydroxide prepared according to Example 1 is used, except that the amount of base added is adjusted so that the neutralisation rate is equal to 1,5 in Example 2 and to 3,0 in Example 3.

2. Autoclavina of the ceric hydroxide According to the operative protocol of Example 1, 24 g of ceric hydroxide as 10 prepared in 1 are suspended in 400 cm 3 of a 2 N aqueous solution of ammonia and is °autoclI ;ed at a temperature of 1800C, for 1 hour.

At the end of this thermal treatment, the precipitate is separated by filtration and subjected to an operation of calcination under the following conditions a fraction is calcinated for 6 hours at 3500C and two other fractions are calcinated for 6 hours at 6000C and 8000C.

The results obtained are summarised in Table 11 TABLE II

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0* 0 0 0*0 0 0* Example Calcination temperature Specific surface area oC m2/g 2 350 260 2 600 150 2 800 3 350 260 3 600 140 3 800 EXAMPLE 4 1. Synthesis of the ceric hydroxide In this test is used a ceric hydroxide prepared according to Example 1.

2. AutoclEvina of the ceric hydroxide a) The ceric hydroxide is suspended in water at a concentration, expressed as CeO 2 of 400 g/l. This suspension is brought to 180°C within a sealed vessel for 1 hour.

The autoclaving pressure is approximately 12 bars (12.105 Pa).

b) after cooling of the suspension, 2 N ammonia is added to obtain a 12 suspension at 60 g/ of CeO 2 This suspension is then treated as in Example 2.

The properties of the product obtained are as follows: following calcination at 350°C for 6 hours: specific surface area: 187 m 2 /g mean size of the crystallites 5,5 6 nm crystallisation 97 10 following calcination at 800°C for 6 hours: specific surface area: 80 m 2 /g •mean size of the crystallites 15 nm crystallisation 98 The product exhibits a loss on ignition of the order of

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Claims

1. Ceric oxide characterised by the fact that it has a specific surface area of at least 190 m 2 /g following calcination at a temperature between 350°C and 4500C.

2. Ceric oxide according to claim 1 characterised by the fact that it has a specific surface area included between 220 and 280 m 2 /g following calcination at a temperature between 3500C and 4500C. 10 3. Ceric oxide according to one of claims 1 and 2 characterised by the fact that it has a s.e* specific surface area of at least 15 m2/g following calcination at a temperature between 8000C and 9000C. C

4. Ceric oxide according to claim 3 characterised by the fact that it has a specific 15 surface area included between 20 and 100 m 2 /g following calcination at a temperature of 8000C.

5. Ceric oxide according to one of claims 1 to 4 characterised by the fact that it has a specific surface area of at least 190 m 2 /g following calcination at a temperature 20 between 3500C and 4500C and that it retains a specific surface area of at least m 2 /g when subjected to a temperature between 8000C and 9000C.

6. Ceric oxide according to claim 5 characterised by the fact that it has a specific surface area included between 220 and 280 m 2 /g following calcination at a 25 temperature between 350°C and 4500C and that it retains a specific surface area included between 20 and 90 m 2 /g when subjected to a temperature of 8000C.

7. Ceric oxide according to one of claims 1 and 2 charactorised by the fact that it has a porous volume included between 0,15 cm3/g and 0,30 cm3/g following calcination at a temperature between 350°C and 4500C.

8. Ceric oxide according to claim 7 characterised by the fact that it has a pore mean diameter varying between 2 nm and 10 nm.

9. Ceric oxide according to one of claims 3 and 4 characterised by the fact that it has a porous volume included between 0,15 cm 3 /g and 0,30 cm 3 /g following calcination at a temperature of 8000C. 14 Ceric oxide according to claim 9 characterised by the fact that it has a pore mean diameter varying between and

11. Ceric oxide according to one of claims 1 and 2 characterised by the fact that it has a crystallisation rate greater than

12. Ceric oxide according to one of claims 3 and 4 characterised by the fact that it has a crystallisation rate greater than

13. Process for the preparation of the ceric oxide described in one of claims 1 to 12 characterised by the fact that it consists of: suspending in an aqueous solution of a decomposable base, as hereinbefore described, a ceric hydroxide complying with the general formula Ce(M) (OH) y(NO 3 )z So where S- M represents an alkali Tretal or a quaternary S"ammonium radical, x lies between 0,01 and 0,2, y is such that y 4 z x, *I z lies between 0,4 and 0,7. heating the said suspension within a sealed vessel to a temperature and pressure lower than the critical temperature and pressure, respectively, of the said suspension, cooling the reaction suspension and bringing it back to atmospheric pressure, separating the ceric hydroxide thus treated, S- then calcinating it.

14. Process for the preparation of the ceric oxide described in one of claims 1 to 12 characterised by the fact that it consists of: suspending in an aqueous solution of a decomposable base, a ceric hydroxide obtained according to a process which consists of preparing 15 a colloidal dispersion of a compound of cerium IV by reacting an aqueous solution of a salt of cerium IV with a base so that a rate of neutralisation, as hereinbefore described, lower than or equal to is obtained; of subjecting the said dispersion to thermal treatment; then of separating the precipitate formed, heating the said suspension within a sealed vessel to a temperature and pressure lower than the critical temperature and pressure, respectively, of the said suspension. cooling the reaction suspension and bringing it back to atmospheric pressure, separating the ceric hydroxide thus treated, then calcinating it. Process according to claim 13 or 14, characterised in that prior to suspending a ceric hydroxide in an aqueous solution of a decomposable base, this hydroxide is suspended in water, the said aqueous suspension being heated within a sealed vessel to a temperature and to a pressure lower than, respectively, the critical temperature and pressure of the suspension.

16. Procedure according to claim 15 characterised in 0*S0 that the decomposable base is added to the previously thermally treated aqueous suspension of ceric hydroxide.

17. Process according to claim 15 characterised in that S.the ceric hydroxide contained within the previously thermally treated aqueous suspension is separated and recovered prior to bein resuspended in an aqueous solution of a decomposable base.

18. Process according to one of claims 15 to 17 characterised in that the ceric hydroxide concentration of the aqueous suspension having to be thermally treated i beforehand, expressed as CeO 2 lies between 0,1 mole and 4 moles/litre.

19. Process according to claim 14 characterised by the fact that the colloidal dispersion of a compound of the SIcerium IV is prepared by reacting a aqueous solution of a 15a salt of cerium IV with a base so that a neutralisation rate greater than or equal to 0,01 and lower than or equal to 3,0 is obtained. Process according to claim 19 characterised by the fact that the solution of salt of cerium IV is an aqueous solution of ceric nitrate or an aqueous solution of ceriammoniacal nitrate.

21. Process according to claim 20 characterised by the fact that the aqueous solution of salt of cerium IV is a solution originating from the electrochemical oxidation of a solution of cerous nitrate.

22. Process according to one of claim 19 to 21 characterised by the fact that the concentration of the solution of cerium salt, expressed as cerium IV, varies between 0,1 and 2 moles per litre. *0 S** S S 99 eo S S

23. Process accordng to claim 19 charactarised by the fact that the base is an aqueous solution of ammonia.

24. Process according to one of claims 19 and 23 characterised by the fact that the normality of the basic solution is between 0,1 and 5 N. Process according to claim 20 characterised by the fact that the temperature of the reaction between the aqueous solution of salt of cerium IV and the base is between OOC and 600C.

26. Process according to claim 19 characterised by the fact that one proceeds to the simultaneous mixing of the aqueous solution of salt of cerium IV and of the basic S solution or that one adds the basic solution into the sction of cerium IV, or vice versa.

27. Process according to claim 14 characterised by the fact that the colloidal dispersion of the compound of cerium IV is thermally treated at a temperature between and 3000C. 20 28. Process according to claim 27 characterised by the fact that the temperature of the thermal treatment is included between 90°C and 1100C. S*

29. Process according to one of claims 27 and 28 characterised by the fact that the duration of the thermal treatment varies between 2 hours and 24 hours.

30. Process according to one of c.aims 13 to 18 characterised by the fact that the decomposable base is ammonia, urea, ammonium hydrogenocarbonate, ammonium carbonate, a primary, secondary, tertiary or quaternary amine or their mixtures.

31. Process according to claim 30 characterised by the fact that the decomposable base is ammonia, a tetraalkylammonium hydroxide or their mixtures.

32. Process according to one of claims 13 and 14 characterised by the fact that the concentration of the basic solution varies between 1 and 10 N.

33. Process according to one of claims 13 and 14 characterised by the fact that the ceric hydroxide concentration within the aqueous suspension of a decomposable base, expressed as CeO 2 varies between 0,1 and 3,0 moles/liter. 17

34. Process according to claim 33 characterised by the fact that the said concentration is between 0,2 and mole/litre. Process according to one of claims 13 to characterised by the fact that the temperature at which said suspension within said vessel is heated in aqueous medium or in medium containing a decomposable base varies between 100 and 350 0 C.

36. Process according to claim 35 characterised by the fact that the said temperature is between 150 and 350 0 C.

37. Process according to one of claims 13 to characterised by the fact that the pressure in aqueous suspension or in suspension containing a decomposable base varies between 1(10 Pa) and 165 bars (165.10 Pa).

38. Process according to claim 37 characterised by the fact that the said pressure is between 5(5.105 Pa) and 165 bars (165.105 Pa).

39. Process according to one of claims 13 and 14 characterised by the fact that the duration of the S: heating of the aqueous suspension or the suspension containing the decomposable bases within the sealed a S vessel in aqueous suspension or in suspension containing 0 a decomposable base varies between 30 minutes and 6 hours.

40. Process according to one of claims 13 to characterised by the fact that the calcination temperature is included between 300 and 1000 0 C.

41. Process according to claim 40 characterised by the fact that the said temperature lies between 350 and 800 0 C.

42. Process according to claims 13 to 15 characterised by the fact that the calcination time varies between 2 and 6 hours.

43. Ceric oxide substantially as herein described with reference to the examples.

44. Process for the preparation of ceric oxide I. :substantially as herein described with reference to the examples. DATED this 19th Day of May 1993 RHONE-POULENC CHIMIE Attorney: IAN T. ERNST SFellow Institute of Patent Attorneys of Australia of SHELSTON WATERS