WO1998024726A1

WO1998024726A1 - Composition with base of cerium oxide or cerium and zirconium oxides, in extruded form, method of preparation and use as catalyst

Info

Publication number: WO1998024726A1
Application number: PCT/FR1997/002190
Authority: WO
Inventors: Gilbert Blanchard; Eric Quemere
Original assignee: Rhodia Chimie
Priority date: 1996-12-06
Filing date: 1997-12-03
Publication date: 1998-06-11
Also published as: KR20000057428A; JP2000505771A; NO992741L; CN1244177A; FR2756819B1; NO992741D0; CA2274013C; TW426542B; CA2274013A1; EP0950022A1; FR2756819A1

Abstract

The invention concerns a composition with base of cerium oxide or cerium and zirconium oxide, in extruded form, its method of preparation and its use as catalyst. The method of preparation of this composition is characterised in that it consists in extruding a hydroxide or an oxygen hydroxide of cerium or hydroxides or oxygen hydroxides of cerium and zirconium. The composition can be used as catalyst or catalyst support, particularly in the treatment of exhaust gases of internal combustion engines, in the process of dehydrogenation of ethyl benzene into styrene, in methanation catalysis, in the treatment of a solution or suspension of organic compounds by wet oxidation.

Description

COMPOSITION BASED ON CERIUM OXIDE OR CERIUM OXIDES AND

ZIRCONIUM. IN EXTRUDED FORM. ITS PREPARATION PROCESS AND

ITS USE AS A CATALYST

RHONE-POULENC CHEMISTRY

The present invention relates to a composition based on cerium oxide or on cerium and zirconium oxides, in extruded form, its preparation process and its use as a catalyst.

Compositions based on cerium oxide or on mixtures of cerium oxide and zirconium oxide are well known. They are used in particular as catalyst or catalyst support, in particular for the afterburning catalysis of motor vehicles. These compositions are generally used in a coating technique, that is to say by mixing them with a binder oxide such as alumina or silica and by depositing the mixture obtained, in the form of a layer, on a support. However, the binder oxide can cause either rapid deactivation of the compositions or a loss of their selectivity. Given this drawback, it would be advantageous to be able to obtain these compositions directly in extruded form. However, to the knowledge of the Applicant, these compositions have never been obtained so far in this form.

The object of the invention is therefore to provide these compositions in the form of extrudates.

The Applicant has discovered that the use of a specific starting product makes it possible to resolve this problem.

The invention therefore relates to a process for the preparation of a composition based on cerium oxide or cerium and zirconium oxides which is characterized in that a product which is based on a hydroxide or d is extruded '' a cerium oxyhydroxide or cenum and zirconium hydroxide or oxyhydroxides.

The invention also covers a composition based on cerium oxide or on cenum and zirconium oxides, characterized in that it is in extruded form.

Finally, the invention relates to the use of a composition of the above type as catalyst or catalyst support, in particular in the treatment of exhaust gases from internal combustion engines, in the process of dehydrogenation of ethylbenzene to styrene , in the methanation catalysis. Other characteristics, details and advantages of the invention will appear even more completely on reading the description which follows, as well as various concrete but nonlimiting examples intended to illustrate it. The product of the invention will first be described. The essential characteristic of the compositions of the invention is their form.

They are in fact in the form of extrudates. The term “extrude” means any object obtained by ejection of a paste under pressure, through nozzles or dies of selected shapes. The objects thus obtained can have various shapes, they can, for example, have cylindrical or semi-cylindrical, square, polygonal sections or even sections in the shape of lobes, like trilobes. Objects can be full or hollow. They can have the shape of a monolith, a honeycomb, a cylinder for example.

The compositions of the invention are based on cerium oxide or on cenum and zirconium oxides. It is meant by that the cenum oxide or cerium oxide in combination with the zirconium oxide represent at least 50 % by weight of the entire composition. They can consist essentially or only of cenum and zirconium oxides with, in addition, where appropriate, one or more additives of the type which will be described below.

In the case of compositions based on cerium and zirconium oxides, the respective proportions of cenum and zirconium can vary within wide limits. More particularly, this proportion, expressed by the atomic ratio Zr / Ce, can vary between 1/20 and 20/1, more particularly between 1/9 and 9/1.

The compositions of the invention can comprise, in addition to cerium and zirconium. additives. These additives will be chosen from those known to improve the catalytic properties of the cenum or zirconium. Additives can thus be used to stabilize the specific surface of these compositions or those known to increase their oxygen storage capacity.

As additives, mention may be made of those belonging to the group consisting of aluminum, silicon, thorium, titanium, niobium, tantalum and rare earths. By rare earth is meant the elements of the group constituted by yttrium and the elements of the periodic classification with atomic number included inclusively between 57 and 71. Among the rare earths, there may be mentioned more particularly yttrium, lanthanum, neodymium and praseodymium.

Mention may also be made, as additives, of those belonging to the group consisting of magnesium, scandium, hafniu, gallium and boron.

Finally, mention may be made of the additives belonging to the group consisting of iron, bismuth, nickel, manganese, tin and chromium. It goes without saying that all the additives mentioned here can be present in the compositions of the invention alone or in combination regardless of the group to which they belong. In addition, these additives are generally present in the compositions in the form of oxides. The amounts of additives can vary within wide limits. The maximum amount is at most 50% expressed by weight of additive oxide relative to the weight of the entire composition. The minimum quantity is that necessary to obtain the desired effect. Generally, this amount is at least 0.1%. More particularly, the amount of additive can be between 1 and 20% and even more particularly between 1 and 10%.

The compositions of the invention can have large specific surfaces, even after calcination at high temperature. This surface depends on the nature of the constituents of the composition. The highest surfaces will be obtained for the compositions in which zirconium is the majority. More specifically, the compositions of the invention in which cerium is present with zirconium and where the cenum is predominant, that is to say that the Ce / Zr atomic ratio is greater than 1, may have specific surfaces of at least minus 20m2 / g, more particularly at least 30m2 / g, after calcination at 900 ° C for 6 hours. With one or more additives, in particular such as scandium and rare earths and in particular lanthanum, praseodymium or neodymium, these surfaces can be at least 35m2 / g, or even at least 40m2 / g and even more particularly at least 45m2 / g after calcination under the same conditions. These compositions with additives can also have a surface of at least 20 m 2 / g and even more particularly of at least 30 m 2 / g after calcination at 1000 ° C. for 6 hours.

Furthermore, the compositions of ^the invention in which the cerium is present with zirconium or zirconium but a majority, that is to say that the atomic ratio Ce / Zr is less than 1, may have surface areas of at minus 30m2 / g, more particularly at least 40m2 / g, after calcination at 900 ° C for 6 hours. With one or more additives, in particular rare earths and in particular lanthanum, praseodymium or neodymium, these surfaces can be at least δOm ^ / g, or even at least 55rr) 2 / g and even more particularly at least δOm ^ / g after calcination under the same conditions. These compositions with additives can also have a surface of at least 30 m 2 / g and even more particularly of at least 40 m 2 / g after calcination at 1000 ° C. for 6 hours.

By specific surface is meant the BET specific surface determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established from BRUNAUER - EM ETT- TELLER method described in the periodical "The Journal of the American Society, fiQ, 309 (1938)"

The process for preparing the extruded compositions of the invention will now be described. The main characteristic of the process of the invention is to start from a specific product. This product can be defined in two ways.

We can first of all define the product which undergoes the extrusion as being a product based on a hydroxide or an oxyhydroxide of cenum or hydroxides or oxyhydroxides of cenum and zirconium. Such a hydroxide can generally be represented by the formula (1) M (OH) _x (X) _v , nH2θ, in which M represents cerium or zirconium, X an anion, x + y being at most equal to 4 and x being different from 0, y and n being able to be zero The anion X is the anion of the compound of cenum or zirconium, in particular a salt, which is generally used in the preparation of hydroxide as will be described below It is also possible to start from a product based on an oxyhydroxide of formula (2) MO _z (OH) _x (X) y, nH2θ, in which M and X have the same meaning as above, or x + y + z is at most equal to 4, x and z being different from 0, y and n possibly being zero Such an oxyhydroxide can be obtained by drying a hydroxide of formula (1).

It will be noted that in formulas (1) and (2), the respective values of x, y, z and of π can vary depending in particular on the preparation methods used to obtain the hydroxides or oxyhydroxides. Thus, by way of example, n can vary between approximately 0 and 20, y can be at most 0.5, X being in particular a nitrate anion. It is emphasized here that these values are not limiting, the essential characteristic of the invention being the use for the extrusion operation of a product comprising the hydroxide anion.

We can also define the product which undergoes extrusion by its preparation processes.

For the preparation of such a product, reference may be made in particular to patent application EP-A-300852, in the name of the Applicant and the teaching of which is incorporated here, which describes the preparation of a ceric hydroxide by making react a solution of cerium salt and a base, optionally in the presence of an oxidizing agent, the proportion of base being such that the pH of the reaction medium is greater than 7. The cenum hydroxide thus obtained can then undergo a solvothermal treatment in which it is resuspended in water or in a decomposable base and it is heated in a closed enclosure to a temperature and a pressure lower respectively than the temperature and the critical pressure of the reaction medium. Cenum IV hydroxides can be obtained by hydrolysis of an aqueous solution of cenum IV in an acid medium (patent application FR-A-2596380 in the name of the Applicant)

With regard to the preparation of cenum and zirconium hydroxides or oxyhydroxides or of cenum and zirconium products suitable for extrusion in the context of the present invention, reference may be made to patent applications FR -A-2699524 and FR-A-2714370 in the name of the Applicant and the teaching of which is incorporated here and which describe in particular methods which make it possible to obtain, at the end of some of their stages, products of this type.

Thus, a first method of this type which can be described comprises the following steps. A liquid medium is formed comprising compounds of cerium and zirconium, and, where appropriate, at least one compound of an additive; the medium obtained is heated, the precipitate formed is recovered, said precipitate is optionally dried. The product thus obtained at the end of these stages is suitable for extrusion for the present invention. The first step of this process therefore consists in preparing a liquid medium, generally an aqueous medium, containing compounds of the cenum and zirconium, and, if necessary, at least one compound of an additive. These compounds are generally salts of the aforementioned elements and, preferably, soluble salts. The liquid medium can be obtained either from compounds initially in the solid state which are subsequently introduced into a base of the water tank for example, or even directly from solutions of these compounds and then mixing, in any order, said solutions

By way of compounds of the cenum, mention may in particular be made of cerium salts such as cenum IV salts such as nitrates or ceπ-ammoniacal nitrates for example, which are particularly suitable here. Preferably, cenic nitrate is used. The solution of _salts, CENUM IV may contain CENUM in the cerous state but it is preferred that it contain at least 85% of CENUM IV. An aqueous solution of cene nitrate can for example be obtained by reaction of nitric acid with a hydrated cene oxide prepared in a conventional manner by reaction of a solution of a cerous salt, for example cerous nitrate, and an ammonia solution in the presence of hydrogen peroxide. It is also possible to use a ceric nitrate solution obtained according to the electrolytic oxidation process of a cerous nitrate solution as described in document FR-A-2 570 087, and which can constitute an advantageous raw material. The zirconium compounds can be chosen from zirconium sulfate, zirconyl nitrate or zirconyl chloride. The zirconyl nitrate is particularly suitable can be mentioned more particularly the use of a zirconyl nitrate from ^nitric attack on a zirconium carbonate. The compound zirconium can also be a salt of an organic acid such as acetic acid or citric acid.

It should be noted here that the aqueous solutions of cenum IV salts and zirconyl salts may have a certain initial free acidity. According to the present invention, it is as much possible to use an initial solution of cenum IV and zirconium salts effectively exhibiting a certain free acidity as mentioned above, than solutions which will have been more or less neutralized beforehand. . This neutralization can be done by adding a basic compound to the above mixture so as to limit this acidity. This basic compound can be, for example, an ammonia solution or else alkali hydroxides (sodium, potassium, etc.), but preferably an ammonia solution. A neutralization rate (r) of the initial solution of cenum and zirconium can then be defined in practical terms by the following equation: r = n3 - n2 m

wherein ni represents the total number of moles of Ce IV and zirconium present in the solution after neutralization; n2 represents the number of moles of OH ^' ions effectively necessary to neutralize the initial free acidity provided by the aqueous solutions of cenum IV salt and zirconium; and n3 represents the total number of moles of OH ions ^" supplied by the addition of the base. When the" neutralization "variant is used, in all cases an amount of base is used which must be imperatively less than the quantity of base which would be necessary to obtain the total precipitation of the cenum zirconium hydroxide species, this quantity depending on the composition synthesized, in practice this is limited to neutralization rates not exceeding 2.

According to a particular embodiment, a zirconium solution which has the following characteristic is used as the zirconium compound. The quantity of base necessary to reach the equivalent point during an acid-base assay of this solution must verify the condition of the molar ratio OH7Zr <1.65. More particularly, this ratio can be at most 1.5, and even more particularly at most 1.3.

The acid-base assay is carried out in a known manner. To carry it out under optimal conditions, it is possible to dose a solution which has been brought to a concentration of approximately 3.10 ^" ^ mole per liter expressed in zirconium element. A 1N sodium hydroxide solution is added thereto with stirring. , the determination of the equivalent point (change of the pH of the solution) is made in a clear manner, this equivalent point is expressed by the molar ratio OH7Zr. As compounds of the additives which can be used in the process of the invention, mention may, for example, be made of the salts of inorganic or organic acids, for example of the sulfate, nitrate, chloride or acetate type. Note that nitrate is particularly suitable. These compounds can also be provided in the form of soils. These soils can be obtained for example by neutralization with a base of a salt of these compounds.

The amounts of cenum, zirconium and optionally additives present in the mixture must correspond to the stoichiometric proportions required for obtaining the desired final composition.

The initial liquid medium being thus obtained, one then proceeds, in accordance with the second step of the process, to its heating.

The temperature at which this heat treatment, also called thermohydrolysis, is carried out can be between 80 ° C. and the critical temperature of the reaction medium, in particular between 80 and 350 ° C., preferably between 90 and 200 ° C.

This treatment can be carried out, depending on the temperature conditions adopted, either under normal atmospheric pressure, or under pressure such as for example the saturated vapor pressure corresponding to the temperature of the heat treatment.

When the treatment temperature is chosen to be higher than the reflux temperature of the reaction mixture (that is to say generally greater than 100 ° C.), for example chosen between 150 and 350 ° C., the operation is then carried out by introducing the mixture aqueous containing the aforementioned species in a closed enclosure (closed reactor more commonly called autoclave), the necessary pressure resulting then only from the heating of the reaction medium (autogenous pressure). Under the temperature conditions given above, and in aqueous media, it can thus be specified, by way of illustration, that the pressure in the closed reactor varies between a value greater than 1 Bar (10 ⁵ Pa) and 165 Bar (165. 10 ⁵ Pa), preferably between 5 Bar (5. 10 ⁵ Pa) and 165 Bar

(165. 1 0 ⁵ Pa). It is of course also possible to exert an external pressure which is then added to that following the heating.

Heating can be carried out either in an air atmosphere or in an inert gas atmosphere, preferably nitrogen. The duration of the treatment is not critical, and can thus vary within wide limits, for example between 1 and 48 hours, preferably between 2 and 24 hours.

At the end of the heating step, a solid precipitate is recovered which can be separated from its medium by any conventional technique of solid-liquid separation such as for example filtration, decantation, spinning or centrifugation. It may be advantageous at the end of this second step to bring the reaction medium thus obtained to a basic pH. This operation is carried out by adding a base to the medium such as, for example, an ammonia solution. By basic pH is meant a pH value greater than 7 and preferably greater than 8.

It is also possible to add hydrogen peroxide in the same way, after the heating step. Note that it is of course possible to repeat one or more times, identical or not, after recovery of the product and possible addition of the base or hydrogen peroxide, a heating step as described above , then putting the product back in a liquid medium, especially in water, and by carrying out, for example, heat treatment cycles. The product as recovered can then be subjected to washes with water and / or ammonia, at a temperature between room temperature and the boiling temperature. To remove the residual water, the washed product can finally, optionally, be dried, for example in air, and this at a temperature which can vary between 80 and 300 ° C., preferably between 100 and 150 ° C., the drying being continued until a constant weight is obtained.

It is also possible to carry out spray drying of the precipitate obtained. In this case, it is not then necessary to separate the precipitate from the reaction medium in which it was obtained. The reaction medium can be allowed to settle, then the supernatant is drawn off and finally spray drying is carried out. A second process for preparing the hydroxides or oxyhydroxides of cerium and zirconium can also be implemented in which precipitation is carried out. According to this second method, a liquid medium is formed comprising a compound of the cenum or compounds of the cenum and of zirconium, and, if necessary, at least one compound of an additive, a base is added to the medium obtained; the precipitate formed is recovered; said precipitate is optionally dried. The product thus obtained at the end of these stages is suitable for extrusion for the present invention.

What was described above on the compounds of cerium, zirconium and additives also applies here.

In the second step of this second process, a base is added to the liquid medium formed previously. Hydroxide type products can be used as a base. Mention may be made of alkali or alkaline-earth hydroxides. It is also possible to use secondary, tertiary or quaternary amines. However, amines and ammonia may be preferred insofar as they reduce the risks of pollution by alkaline or alkaline earth cations. Mention may also be made of urea. The order of introduction of the reactants can be arbitrary, the base being able to be introduced into the mixture or vice versa or the reagents being able to be introduced simultaneously into the reactor. The addition can be carried out at once, gradually or continuously, and it is preferably carried out with stirring. This operation can be carried out at a temperature between room temperature (18-25 ° C) and the reflux temperature of the reaction medium, the latter possibly reaching 120 ° C for example. It is preferably carried out at room temperature.

At the end of the addition of the base, the reaction medium can optionally be kept stirred for some time, in order to perfect the precipitation.

When the base is added continuously, the pH of the reaction medium is preferably maintained between approximately 7 and approximately 11, more particularly between 7.5 and 9.5.

A variant of this second method can be recalled here, this variant corresponding to the teaching of FR-A-2714370 which was mentioned above. In this variant, the reaction with the base takes place in the presence of a carbonate or bicarbonate. The term carbonate should be understood as possibly also comprising a hydroxycarbonate. By way of example, mention may be made of ammonium carbonate or bicarbonate. Furthermore, the reaction is carried out under conditions such that the pH of the reaction medium remains neutral or basic. The pH value of the reaction medium is generally at least 7 and is between 7 and 7.5 in the case of a neutral medium and more particularly at least 8 in the case of a basic medium. More precisely, this value can be between 7.5 and 14, in particular between 8 and 11 and more particularly between 8 and 9.

According to a particular embodiment of this variant, the liquid medium comprising the compounds of cenum and zirconium and, optionally of the additive, with the carbonate or bicarbonate is introduced into a basic solution. It is therefore possible, for example, to form a base stock with the basic solution into which the liquid medium is introduced.

It is also possible in this variant to proceed continuously. In this case, the liquid medium comprising the cerium and zirconium compounds and, optionally the additive, the base and the carbonate or the bicarbonate is introduced simultaneously into a reactor, ensuring an excess of base to fulfill the pH condition.

At the end of the precipitation step of the second process, a mass of a solid precipitate is recovered which can be separated from its medium and optionally dried as described above for the first process. The hydroxide or oxyhydroxides of cenum and zirconium as well as the product or products which have been obtained by the methods described above will then be shaped by extrusion. They can be extruded directly or as a mixture with an acid solution. The presence of an acid solution facilitates shaping. As the acid, it is possible to use, for example, nitric acid, stearic acid, oxalic acid or acetic acid. The amount of acid used is generally between approximately 0.1 and 5% expressed in moles of acid relative to the sum of the moles of cerium and zirconium. The hydroxide or oxyhydroxides of cenum and zirconium as well as the product or products which have been obtained by the processes described above can also be extruded in admixture with known shaping additives such as cellulose, carboxymethyl cellulose, carboxyethyl cellulose, xanthan gums and succinoglycans, surfactants, flocculating agents such as polyacrylamides, carbon black, starches, polyacrylic alcohol, polyvinyl alcohol, glucose, polyethylene glycol.

An important advantage of the invention is that the product to be extruded can be extruded in the absence of the binders that are usually used in this type of technique. Of course, it would not be departing from the scope of the present invention to use a binder which can be chosen from silica, alumina, clays, silicates, titanium sulphate, ceramic fibers, in particular in the proportions generally used. , i.e. up to about 30% by weight.

The hydroxide or oxyhydroxides of cerium and zirconium as well as the product or products which have been obtained by the processes described above, either alone or in admixture with the acid solution or the abovementioned shaping additives or the aforementioned binders , preferably have, before extrusion, a loss on ignition of between 25 and 75%, more particularly between 40 and 65%. The loss on ignition (PAF) is measured as the weight loss corresponding to the ratio: PAF in% = (PQ-P - |) 'PO in which

- PQ is the initial weight of the raw material

- P- ₎ is the weight of this raw material after calcination for 2 hours at 1000 ° C. and cooling to ambient temperature in an anhydrous enclosure.

Prior to extrusion, the hydroxide or oxyhydroxides of cerium and zirconium as well as the product or products which have been obtained by the processes described above are kneaded. The duration of the mixing can vary within wide limits, for example between 1 minute and three hours.

The extrusion is done in any suitable device.

The extruded product is optionally dried and then calcined. The calcination is carried out at a temperature generally between 200 and 1200 ° C, preferably between 300 and 900 ° C and even more particularly between

500 ° C and 900 ° C This calcination temperature must be sufficient to transform the precursors into oxides, and it is also chosen according to the temperature subsequent use of the catalytic composition and taking into account that the specific surface area of the product is lower the higher the calcination temperature used is higher The duration of the calcination can vary over wide ranges limits, for example between 1 and 24 hours, preferably between 4 and 10 hours. Calcination is generally carried out in air, but calcination carried out for example under inert gas is obviously not excluded.

The extrudates thus obtained can be used in combination with catalytically active metals of the precious metal type. The nature of these metals and the techniques for incorporating them into these compositions are well known to those skilled in the art. For example, the metals can be platinum, rhodium, palladium, ruthenium or iridium, they can in particular be incorporated into the compositions by impregnation.

The extrudes of the invention can be used very particularly in the treatment of exhaust gases from internal combustion engines, in the process of dehydrogenation of ethylbenzene to styrene, in the methanation catalysis.

More generally, they can be used in the catalysis of various reactions such as, for example, dehydration, hydrosulfurization, hydrodenitrification, desulfurization, hydrodesulfurization, dehydrohalogenation, reforming, steam reforming, cracking , hydrocracking, hydrogenation, dehydrogenation, isomenation, disproportionation, oxychlorination, dehydrocyclization of hydrocarbons or other organic compounds, oxidation and / or reduction reactions, Claus reaction , demetallation, shift conversion or treatment of a solution or suspension of organic compounds by wet oxidation.

This latter treatment of aqueous solution or suspension of organic compounds is carried out at an elevated temperature and pressure by oxidation of the organic compounds with an oxygen-containing gas in the presence of an oxidation catalyst to reduce the chemical oxygen demand of said solution or suspension at a predetermined level II is characterized in that the catalyst comprises a catalytically active phase present on a support constituted by a composition based on a cenum oxide and a zirconium oxide in an atomic proportion ceπum / zirconium of at least 1, having a specific surface after calcination for 6 hours at 900 ° C of at least 35m2 / g and an oxygen storage capacity at 400 ° C of at least 1.5 ml of 02. This support can be in the form of an extrude. The catalytically active phase consists of ruthenium or iridium in metal form or in oxide form.

The oxidation reaction is carried out using an oxygen-containing gas, such as, for example, pure oxygen, air, oxygen-enriched air, waste gases containing oxygen, as the oxidizing gas. oxygen. The quantity of gas supplied is determined from the chemical oxygen demand (COD) of the solution to be treated. Generally, the oxygen-containing gas is used in an amount equal to 1 to 1.5 times the theoretical amount of oxygen.

Advantageously, the oxygen pressure is between 1 and 50 bars, the total pressure of the gases being high enough to maintain the solution or suspension in the liquid state at the reaction temperature.

This reaction temperature is advantageously between 100 ° C and 400 ° C, preferably between 120 ° C and 200 ° C. This temperature depends in particular on the nature of the organic compounds present in the effluents to be treated. The aqueous solutions or suspensions which can be treated by this process are waters which preferably contain oxidizable organic substances such as aqueous effluents having a moderately concentrated chemical oxygen demand, advantageously less than 200 g / l.

Examples of waste water are, for example, waste water from industrial installations such as the chemical or petroleum industries, municipal effluents, waste water containing oils, waste water from gas purification process or activated sludge process. Advantageously, to avoid fouling of the installations and of the catalyst, these waters can be filtered before being treated. Examples will now be given.

EXAMPLE 1

This example illustrates the preparation of an oxide support in the form of an extrusion of formula Ceo _ι62 Zr _0ι 38θ2 In the stoichiometric proportions required for obtaining the above mixed oxide, a solution of ceric nitrate and a solution are mixed. zirconyl nitrate. The latter was obtained by attacking a carbonate using concentrated nitric acid. The solution responds, in the sense defined above, to the condition molar ratio OH7Zr = 0.94. The concentration of this mixture (expressed as the oxide of the various elements) is adjusted to 80 g / l. This mixture is then brought to 150 ° C for 4 hours.

An ammonia solution is then added to the reaction medium so that the pH is greater than 8.5. The reaction medium thus obtained is brought to boiling for 2 hours. After decantation then racking, the solid product is resuspended and the medium thus obtained is treated for 1 hour at 100 ° C. The product is then filtered. The filter cake thus obtained has a loss on ignition at 1000 ° C. of 60.6%. The cerium-zirconium hydroxide or oxyhydroxide is kneaded for 15 minutes before being shaped by extrusion through a 3.2 mm die. The material used is a single screw extruder sold by the company LHOMARGIE. The extrusions obtained have the shape of a cylinder with a diameter of 1.2 mm. They are then dried at 100 ° C for 1 hour before being calcined at 600 ° C in air.

The oxide formed has a specific surface after treatment at 900 ° C in air for 6 hours at 39m2 / g _and at 1000 ° C 6 hours at 17m2 / g.

EXAMPLE 2

This example illustrates the preparation of an oxide support in the form of an extrusion of the formula Ceo 7Zro _ι 83θ2- To a solution of cenum nitrate IV, a solution of zirconyl nitrate is added (obtained by attacking a Zr carbonate by nitric acid) in the respective proportions by weight of oxide of 20/80 and such that the ratio r as defined above is 0.5. The concentration is adjusted to 80 g / l, then the solution is brought to 150 ° C. for 6 hours. After cooling, the pH of the reaction medium is brought to a value of 8.5 using an ammoniacal solution. The temperature is then brought to 100 ° C. After cooling, is removed by decanting the mother liquor and an equivalent amount is added ^water. The reaction medium is again brought to 100 ° C. After decantation, the supernatant is removed, and the product is spray-dried. The powder obtained is kneaded with an aqueous solution of nitric acid of concentration such as the support HNθ3 / Zr + Ce is equal to 0.025 and the loss on ignition of the dough at 1000 ° C of 45%. ^The obtained cerium-zirconium hydroxide is then shaped as in Example 1 and then dried at 100 ° C 1 hour before being calcined at 500 ° C under air.

The specific surface area of the oxide thus obtained is 45m2 / g after calcination for 6 hours at 900 ° C.

The X-ray diffraction analysis shows that the oxide obtained is in the form of a pure solid solution phase.

EXAMPLE 3 This example illustrates the preparation of an oxide support in the form of an extrudate of formula

We start from the following two solutions.

- Solution 1 1 16.7 g of Ce III nitrate (29.5% Ceθ2)

24.7 g zirconyl nitrate (19.9% Zrθ2) l 24 g H ₂ 0

- Solution 2. 47.4 g ammonium bicarbonate

35.2 g NH4OH (29% NH3) 240.4 g H2O Solution 1 is preheated to 80 ° C. It is added in 5 minutes to solution 2 originally maintained at a temperature of 25 ° C. Filtered through a diameter of 15cm Buchner and washed with 500ml of ^water. The product is then dried at 125 ° C in a ventilated oven.

The powder obtained is shaped as in Example 1 with the addition of an aqueous solution of acetic acid of concentration such as the support CH3Cθ2H / Zr + Ce is equal to 0.030 and the loss on ignition of the dough to 1000 ° C of 51%.

The ceπum-zirconium hydroxide obtained is dried at 100 ° C for 1 hour before being calcined at 700 ° C in air

The oxide formed has a specific surface after treatment at 900 ° C in air for 6 hours at 30m2 / g.

Claims

1 - Composition based on cenum oxide or on cenum and zirconium oxides, characterized in that it is in extruded form.

2- Composition according to claim 1, characterized in that it is based on cenum and zirconium oxides in an atomic ratio Zr / Ce of between 1/20 and 20/1, more particularly between 1/9 and 9 / 1

3- Composition according to claim 1 or 2, characterized in that it further comprises at least one additive chosen from the group consisting of aluminum, silicon, thorium, titanium, niobium, tantalum and earths rare.

4- Composition according to I one of the preceding claims, characterized in that it further comprises at least one additive chosen from the group consisting of magnesium, scandium, hafmum, gallium and boron.

5- Composition according to one of the preceding claims, characterized in that it further comprises at least one additive chosen from the group consisting of iron, bismuth, nickel, manganese, etam and chromium.

6- Composition according to one of the preceding claims, characterized in that it has a specific surface of at least 20rτ) 2 / g, more particularly at least 30m2 / g, after calcination at 900 ° C for 6 hours .

7- Process for the preparation of a composition according to one of the preceding claims, characterized in that a product which is based on a hydroxide or an oxyhydroxide of cenum or hydroxides or oxyhydroxides is extruded cerium and zirconium.

8- A method of preparing a composition according to one of claims 1 to 6, characterized in that a product obtained is extruded by a process in which a liquid medium is formed comprising a compound of the cenum or compounds of cerium and zirconium, and, where appropriate, at least one compound of an additive; the medium obtained is heated, the precipitate formed is recovered, said precipitate is optionally dried.

9- A method for preparing a composition according ^to one of claims 1 to 6, characterized in that extrudes a product obtained by a process which comprises forming a liquid medium comprising a compound of the cenum or compounds of cerium and zirconium, and, where appropriate, at least one compound of an additive; a base is added to the medium obtained, the precipitate formed is recovered; said precipitate is optionally dried.

10- A method for ^preparing a composition according to one of claims 1 to 6, characterized in ^that one extrudes a product obtained by a process which comprises reacting a CENUM salt solution and a base, optionally in the presence of an oxidizing agent, the proportion of base being such that the pH of the reaction medium is greater than 7

1 1 - A process for preparing ^a composition according to one of claims 1 to 6, characterized in that one extrudes a product obtained by a process wherein an aqueous solution is hydrolyzed ae cerium IV in acid medium.

12- Method according ^to one of claims 7-1 1, characterized in that extrudes a mixture of ^the one part from the or hydroxides or oxyhydroxides above or at least one of the products obtained by the above methods and, on the other hand, an acid solution.

13- Method according ^to one of claims 7 to 12, characterized in that the extruded or one or hydroxides or oxyhydroxyαes above or at least one of the products obtained by the above methods, or a mixture obtained from one or hydroxides or oxyhydroxides above ^or at least one of the products obtained by the above processes, which exhibit a loss on ignition between 25 and 75%, more particularly between 40 and 65%.

14- Use ^of a comQos is according ^to one of claims 1 to 6 as a catalyst or catalyst support, in particular in the treatment of exhaust gases from internal engines comoustion, in the dehydrogenation process of ethylbenzene to styrene , in methanation catalysis, the treatment of a solution or suspension of organic compounds by wet oxidation.