WO2003020643A1 - Highly reducible composition based on cerium oxide, zirconium oxide and another rare earth oxide, the preparation method thereof and the use of same as a catalyst - Google Patents

Highly reducible composition based on cerium oxide, zirconium oxide and another rare earth oxide, the preparation method thereof and the use of same as a catalyst Download PDF

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Publication number
WO2003020643A1
WO2003020643A1 PCT/FR2002/002991 FR0202991W WO03020643A1 WO 2003020643 A1 WO2003020643 A1 WO 2003020643A1 FR 0202991 W FR0202991 W FR 0202991W WO 03020643 A1 WO03020643 A1 WO 03020643A1
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rare earth
oxide
cerium
zirconium
additional rare
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PCT/FR2002/002991
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French (fr)
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Jean-Valéry MARTIN
Yvan Montardi
Emmanuel Rohart
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Rhodia Electronics And Catalysis
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9445—Simultaneously removing carbon monoxide, hydrocarbons or nitrogen oxides making use of three-way catalysts [TWC] or four-way-catalysts [FWC]
    • B01D53/945—Simultaneously removing carbon monoxide, hydrocarbons or nitrogen oxides making use of three-way catalysts [TWC] or four-way-catalysts [FWC] characterised by a specific catalyst
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/002—Mixed oxides other than spinels, e.g. perovskite
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/10—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of rare earths
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/03—Precipitation; Co-precipitation
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G25/00—Compounds of zirconium
    • C01G25/006—Compounds containing, besides zirconium, two or more other elements, with the exception of oxygen or hydrogen
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2523/00—Constitutive chemical elements of heterogeneous catalysts
    • B01J35/60—
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/82—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by IR- or Raman-data
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/12—Surface area
    • C01P2006/13—Surface area thermal stability thereof at high temperatures
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/10—Internal combustion engine [ICE] based vehicles
    • Y02T10/12—Improving ICE efficiencies

Definitions

  • the present invention relates to a composition with high reducibility and based on cerium oxide, zirconium oxide and an oxide of another rare earth, its preparation process and its use as a catalyst.
  • multifunctional catalysts are used for the treatment of exhaust gases from internal combustion engines (automotive post-combustion catalysis).
  • Multifunctional means the catalysts capable of carrying out not only the oxidation in particular of carbon monoxide and of the hydrocarbons present in the exhaust gases but also the reduction in particular of the nitrogen oxides also present in these gases (catalysts "three ways").
  • Zirconium oxide and cerium oxide appear today as two particularly important and interesting constituents for this type of catalyst. To be effective, these catalysts must have a large specific surface even at high temperature. Another quality required for these catalysts is reducibility.
  • reducibility is meant, here and for the rest of the description, the capacity of the catalyst to reduce in a reducing atmosphere and to reoxidize in an oxidizing atmosphere.
  • This reducibility can be measured by the capacity to capture hydrogen. It is due to cerium in the case of compositions of the type of those of the invention, the cerium having the property of reducing or oxidizing. This reducibility must be high. In addition, it is interesting that this reducibility does not vary too much when the catalyst is subjected to high temperatures.
  • the object of the invention is therefore to provide products with significant reducibility.
  • the composition of the invention is based on cerium oxide, zirconium oxide and at least one additional rare earth oxide other than yttrium and it is characterized in that it has a cubic crystal structure determined by X-ray analysis, in that it has a phase t 'detectable by Raman analysis, the additional rare earth being at least lanthanum.
  • the invention also relates to a process for the preparation of such a composition which is characterized in that it comprises the following stages: - A mixture is prepared in a liquid medium containing a zirconium compound and a cerium compound;
  • composition of the invention has a high reducibility but it can also exhibit a certain stability of its reducibility as well as of its specific surface when it is subjected to high temperatures, for example 1000 ° C.
  • the term "specific surface” means the specific surface B.E.T. determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established from the BRUNAUER - EMMETT-TELLER method described in the periodical "The Journal of the American Chemical Society, 60, 309 (1938)".
  • rare earth is meant the elements of the group consisting of the elements of the periodic classification with atomic number between 57 and 71 inclusive.
  • the composition of the invention is based on at least three compounds which are cerium oxide, zirconium oxide and one or more oxides of an additional rare earth other than cerium.
  • This other rare earth is a rare earth other than yttrium and it must be at least lanthanum.
  • the composition of the invention comprises at least oxide of lanthanum. It may include another additional rare earth in addition to lanthanum which may be more particularly neodymium or praseodymium.
  • composition of the invention also has structural characteristics. Its crystal structure, as determined by X-ray analysis, is of the cubic type, of the fluorite CaF 2 type. Preferably, this crystal structure or phase is unique, that is to say that there is no parasitic secondary phase, which indicates the existence of a pure solid solution.
  • composition has a phase t 'which can be detected by Raman analysis.
  • the composition of the invention has the following contents of cerium, zirconium and additional rare earth (TR):
  • TR can denote either lanthanum or several rare earths and in this case, at least one is lanthanum.
  • the Ce / Zr atomic ratio is preferably at least equal to 1.
  • the lanthanum content is generally at least 0.5% by mass, preferably at least 2% and more particularly at least 5%.
  • the additional rare earth content that is to say other than cerium, is preferably at most 10%, more particularly at most 5%, all these contents also being expressed as oxide mass relative to the whole. oxides.
  • compositions according to the invention have large specific surfaces even at high temperatures.
  • they can have a specific surface of at least 30m 2 / g, preferably at least 40m 2 / g.
  • the surface can still be at least 5m 2 / g, preferably at least 10m 2 / g.
  • the first step of the process consists in preparing a mixture in a liquid medium of a zirconium compound, a cerium compound and at least one additional rare earth (TR) compound which is at least lanthanum.
  • TR rare earth
  • the liquid medium is generally water.
  • the compounds are preferably soluble compounds. It can be in particular salts of zirconium, cerium and rare earth. It is also possible to use soils of these elements.
  • the mixture can be obtained either from compounds initially in the solid state which will then be introduced into a bottom of the water tank for example, or even directly from solutions of these compounds and then mixing, in any order, said solutions.
  • the zirconium compounds can be chosen from zirconium sulfate, zirconyl nitrate or zirconyl chloride. Zirconyl nitrate is most commonly used. It is also possible to use organic zirconium salts such as zirconium acetate.
  • ceric nitrate is used. It is advantageous to use salts with a purity of at least 99.5 and more particularly of at least 99.9%.
  • An aqueous solution of ceric nitrate can for example be obtained by reaction of nitric acid with a hydrated ceric 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.
  • the aqueous solutions of cerium salts and zirconyl salts may have a certain initial free acidity which can be adjusted by the addition of a base or an acid.
  • a basic compound can be, for example, an ammonia solution or else alkali hydroxides (sodium, potassium, etc.), but preferably an ammonia solution.
  • thermohydrolysis The temperature at which this heating or heat treatment is carried out, also called thermohydrolysis, is preferably at least 70 ° C. and even more particularly at least 100 ° C. It can thus be between 100 ° C and the critical temperature of the reaction medium, in particular between 100 and 350 ° C, preferably between 100 and 200 ° C.
  • the heating operation can be carried out by introducing the liquid medium containing the abovementioned species into a closed enclosure (closed reactor of the autoclave type), the necessary pressure then resulting only from the heating of the reaction medium (autogenous pressure).
  • autogenous pressure the necessary pressure then resulting only from the heating of the reaction medium
  • the pressure in the closed reactor can vary between a value greater than 1 Bar (10 5 Pa) and 165 Bar (1 , 65. 10 7 Pa), preferably between 5 Bar (5 ⁇ 10 5 Pa) and 165 Bar (1, 65. 10 7 Pa). It is of course also possible to exert an external pressure which is then added to that following the heating.
  • Heating can also be carried out in an open reactor for temperatures close to 100 ° C.
  • the heating can be carried out either in air 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.
  • the temperature rise takes place at a speed which is not critical, and the fixed reaction temperature can thus be reached by heating the medium, for example between 30 minutes and 4 hours, these values being given entirely indicative fact.
  • reaction medium thus obtained is brought to a basic pH.
  • This operation is carried out by adding a base to the medium such as, for example, an ammonia solution.
  • basic pH is meant a pH value greater than 7 and preferably greater than 8.
  • this embodiment is not preferred, it is possible to introduce into the reaction mixture obtained after heating, in particular at the time of the addition of the base, the rare earth compound (s) additional in particular in the form which has been described above, if this or these have not been introduced during the first 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.
  • the product as recovered can then be subjected to washing.
  • the process can optionally include a ripening which makes it possible to improve the crystallinity and the specific surface of the product obtained.
  • This ripening can be carried out directly on the reaction medium obtained after addition of a base in order to be at basic pH.
  • a base has the effect of cooling the reaction medium, the ripening takes place by heating it again.
  • the temperature to which the medium is heated is at least 40 ° C, more particularly at least 60 ° C and even more particularly at least 100 ° C.
  • the medium is thus maintained at a constant temperature for a period which is usually at least 30 minutes and more particularly at least 1 hour.
  • the ripening can be done at atmospheric pressure or possibly at a higher pressure.
  • the ripening can also be carried out on a suspension obtained after putting the precipitate back into water.
  • the pH of this suspension can be adjusted to a value greater than 7 and preferably greater than 8.
  • the precipitate recovered after optionally maturing, washing and / or drying, can then be calcined.
  • This calcination makes it possible to develop the crystallinity of the product obtained, and it can also be adjusted and / or chosen as a function of the subsequent temperature of use reserved for the composition according to the invention, and this taking into account the fact that the surface
  • the specific product is lower the higher the calcination temperature implemented.
  • Such calcination is generally carried out in air, but calcination carried out for example under inert gas or under a controlled atmosphere (oxidizing or reducing) is obviously not excluded.
  • the calcination temperature is generally limited to a range of values between 300 and 1000 ° C.
  • compositions of the invention as described above or as obtained by the process mentioned above are in the form of powders but they can optionally be shaped to be in the form of granules, balls, cylinders or nests. bee of variable dimensions.
  • These compositions can be applied to any support usually 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, phosphates crystalline aluminum.
  • compositions can also be used in catalytic systems.
  • These catalytic systems can comprise a coating (wash coat) with catalytic properties and based on these compositions, on a substrate of the type, for example metallic or ceramic monolith.
  • the coating may also include a support of the type of those mentioned above. This coating is obtained by mixing the composition with the support so as to form a suspension which can then be deposited on the substrate.
  • catalytic systems and more particularly the compositions of the invention can find numerous applications. They are thus particularly well suited to, and therefore usable in, the catalysis of various reactions such as, for example, dehydration, hydrosulfurization, hydrodenitrification, desuif u ration, hydrodesulfurization, dehydrohalogenation, reforming, steam reforming, cracking, hydrocracking, hydrogenation, dehydrogenation, isomerization, disproportionation, oxychlorination, dehydrocyclization of hydrocarbons or other organic compounds, oxidation and / or reduction, Claus reaction, exhaust gas treatment of internal combustion engines, demetallation, methanation, shift conversion.
  • various reactions such as, for example, dehydration, hydrosulfurization, hydrodenitrification, desuif u ration, hydrodesulfurization, dehydrohalogenation, reforming, steam reforming, cracking, hydrocracking, hydrogenation, dehydrogenation, isomerization, disproportionation, oxychlorination, de
  • the compositions of the invention are used in combination with precious metals.
  • the nature of these metals and the techniques for incorporating them into these compositions are well known to those skilled in the art.
  • the metals can be platinum, rhodium, palladium or iridium, they can in particular be incorporated into the compositions by impregnation.
  • the treatment of exhaust gases from internal combustion engines constitutes a particularly interesting application.
  • the invention also relates to the use of a composition or of a catalytic system as described above in the manufacture of a catalyst for automotive post combustion.
  • the reducibility of the compositions is determined by measuring their capacity for capturing hydrogen. This measurement is made by programmed reduction in temperature as follows. We use an AMI-1 Altamira device with a silica reactor and a 100 mg sample. The gas is hydrogen at 10% by volume in argon and with a flow rate of 30ml / min. The temperature rise is from ambient to 900 ° C at a rate of 10 ° C / min. The signal is detected with a thermal conductivity detector at 70mA. Hydrogen uptake is calculated from the missing area of the hydrogen signal from the baseline at room temperature to the baseline at 900 ° C.
  • the Raman analysis was carried out using a Raman spectrometer with dispersive optics and with single-channel Jobin Yvon Ramanor HG2S ® detection, the laser being of the ionized argon type (green line at 514.5 nm).
  • the analyzed sample was slightly ground in a mortar and then compacted on a sample holder.
  • This example relates to the synthesis of a compound of formula CeO 2 ZrO 2 / La2 ⁇ 3 in the following proportions by mass 48/47/5%.
  • the mixture is then heated to 100 ° C. under atmospheric pressure, then is maintained at this temperature for 4 hours. We then observe the formation of a solid which sediments when the stirring is stopped. The suspension is then cooled in air.
  • the X-ray analysis shows that the oxide obtained does indeed have a cubic crystal structure.
  • Raman analysis highlights a phase t '.
  • This example relates to the synthesis of a compound of formula Ce ⁇ 2 / ZrO 2 / La 2 O 3 / Nd 2 O 3 in the following proportions by mass 48/47/ 2 / 3 %.
  • the area indicated is that measured on the products obtained in the examples and after additional calcination at the indicated temperature for 4 hours.
  • FIG. 1 gives two Raman spectra (intensity as a function of the wave number).
  • Spectra 1 and 2 correspond to the compounds of Examples 1 and 2 respectively.

Abstract

The invention relates to a composition based on cerium oxide, zirconium oxide and at least one additional rare earth oxide other than yttrium. The inventive composition is characterised in that it has a cubic crystalline structure which is determined by X-ray analysis and a t' phase which can be detected by Raman analysis, the additional rare earth being at least lanthanum. Said composition is obtained using a method consisting in: preparing a mixture in liquid medium containing a zirconium compound and a cerium compound; heating said mixture; bringing the reaction medium obtained following the heating step to a basic pH; and calcining the precipitate thus obtained, the additional rare earth (RE) compound being added either to the original liquid mixture or to the reaction mixture obtained at the end of the heating step. The quantities of cerium, zirconium and additional rare earth compounds used verify the following relationships which are expressed by weight of oxides of the elements: 47 % ≤ Ce +TR ≤ 58 % and 42 % ≤ Zr ≤ 53 %. The inventive composition can be used as a catalyst.

Description

COMPOSITION A REDUCTIBILITE ELEVEE ET A BASE DOXYDE DE HIGH REDUCTIBILITY COMPOSITION BASED ON DOXIDE
CERIUM, DOXYDE DE ZIRCONIUM ET DON OXYDE D'UNE AUTRECERIUM, ZIRCONIUM DOXIDE AND DON OXIDE FROM ANOTHER
TERRE RARE, SON PROCEDE DE PREPARATION ET SON UTILISATIONRARE EARTH, ITS PREPARATION METHOD AND ITS USE
COMME CATALYSEURAS A CATALYST
La présente invention concerne une composition à réductibilité élevée et à base d'oxyde de cérium, d'oxyde de zirconium et d'un oxyde d'une autre terre rare, son procédé de préparation et son utilisation comme catalyseur.The present invention relates to a composition with high reducibility and based on cerium oxide, zirconium oxide and an oxide of another rare earth, its preparation process and its use as a catalyst.
On utilise à l'heure actuelle pour le traitement des gaz d'échappement des moteurs à combustion interne (catalyse postcombustion automobile) des catalyseurs dits multifonctionnels. Par multifonctionnels, on entend les catalyseurs capables d'opérer non seulement l'oxydation en particulier du monoxyde de carbone et des hydrocarbures présents dans les gaz d'échappement mais également la réduction en particulier des oxydes d'azote également présents dans ces gaz (catalyseurs "trois voies"). L'oxyde de zirconium et l'oxyde de cérium apparaissent aujourd'hui comme deux constituants particulièrement importants et intéressants pour ce type de catalyseurs. Pour être efficaces, ces catalyseurs doivent présenter une surface spécifique importante même à température élevée. Une autre qualité requise pour ces catalyseurs est la réductibilité. On entend par réductibilité, ici et pour le reste de la description, la capacité du catalyseur à se réduire en atmosphère réductrice et à se réoxyder en atmosphère oxydante. Cette réductibilité peut se mesurer par la capacité à capter l'hydrogène. Elle est due au cérium dans le cas des compositions du type de celles de l'invention, le cérium ayant la propriété de se réduire ou de s'oxyder. Cette réductibilité doit être élevée. En outre, il est intéressant que cette réductibilité ne varie pas de manière trop importante quand le catalyseur est soumis à des températures élevées.At the present time, so-called multifunctional catalysts are used for the treatment of exhaust gases from internal combustion engines (automotive post-combustion catalysis). Multifunctional means the catalysts capable of carrying out not only the oxidation in particular of carbon monoxide and of the hydrocarbons present in the exhaust gases but also the reduction in particular of the nitrogen oxides also present in these gases (catalysts "three ways"). Zirconium oxide and cerium oxide appear today as two particularly important and interesting constituents for this type of catalyst. To be effective, these catalysts must have a large specific surface even at high temperature. Another quality required for these catalysts is reducibility. By reducibility is meant, here and for the rest of the description, the capacity of the catalyst to reduce in a reducing atmosphere and to reoxidize in an oxidizing atmosphere. This reducibility can be measured by the capacity to capture hydrogen. It is due to cerium in the case of compositions of the type of those of the invention, the cerium having the property of reducing or oxidizing. This reducibility must be high. In addition, it is interesting that this reducibility does not vary too much when the catalyst is subjected to high temperatures.
L'objet de l'invention est donc de fournir des produits à réductibilité importante.The object of the invention is therefore to provide products with significant reducibility.
Dans ce but, la composition de l'invention est à base d'oxyde de cérium, d'oxyde de zirconium et d'au moins un oxyde d'une terre rare supplémentaire autre que l'yttrium et elle est caractérisée en ce qu'elle possède une structure cristalline cubique déterminée par analyse aux rayons X, en ce qu'elle présente une phase t' détectable par analyse Raman, la terre rare supplémentaire étant au moins le lanthane.For this purpose, the composition of the invention is based on cerium oxide, zirconium oxide and at least one additional rare earth oxide other than yttrium and it is characterized in that it has a cubic crystal structure determined by X-ray analysis, in that it has a phase t 'detectable by Raman analysis, the additional rare earth being at least lanthanum.
L'invention concerne aussi un procédé de préparation d'une telle composition qui est caractérisé en ce qu'il comprend les étapes suivantes : - on prépare un mélange en milieu liquide contenant un composé du zirconium et un composé du cérium;The invention also relates to a process for the preparation of such a composition which is characterized in that it comprises the following stages: - A mixture is prepared in a liquid medium containing a zirconium compound and a cerium compound;
- on chauffe ledit mélange;- Heating said mixture;
- on amène le milieu réactionnel obtenu à l'issue du chauffage à un pH basique;- the reaction medium obtained after the heating is brought to a basic pH;
- on récupère le précipité ainsi obtenu;- The precipitate thus obtained is recovered;
- on calcine ledit précipité; un composé de la terre rare supplémentaire (TR) étant ajouté soit a'u mélange en milieu liquide de départ soit au mélange réactionnel obtenu à l'issue du chauffage, les quantités de composés de cérium, de zirconium et de la terre rare supplémentaire utilisées vérifiant les relations suivantes exprimées en masse d'oxydes des éléments :- Said precipitate is calcined; an additional rare earth compound (TR) being added either to the mixture in the starting liquid medium or to the reaction mixture obtained at the end of the heating, the amounts of cerium, zirconium and additional rare earth compounds used verifying the following relationships expressed in mass of oxides of the elements:
47% < Ce+TR < 58% 42% < Zr < 53% La composition de l'invention possède une réductibilité élevée mais elle peut présenter en outre une certaine stabilité de sa réductibilité ainsi que de sa surface spécifique lorsqu'elle est soumise à des températures élevées, par exemple de 1000°C.47% <Ce + TR <58% 42% <Zr <53% The composition of the invention has a high reducibility but it can also exhibit a certain stability of its reducibility as well as of its specific surface when it is subjected to high temperatures, for example 1000 ° C.
D'autres caractéristiques, détails et avantages de l'invention apparaîtront encore plus complètement à la lecture de la description qui va suivre, des exemples concrets mais non limitatifs destinés à l'illustrer et du dessin annexé dans lequel :Other characteristics, details and advantages of the invention will appear even more completely on reading the description which follows, concrete but non-limiting examples intended to illustrate it and the attached drawing in which:
- la figure unique représente deux spectres Raman de compositions selon l'invention. Pour la suite de la description, on entend par surface spécifique, la surface spécifique B.E.T. déterminée par adsorption d'azote conformément à la norme ASTM D 3663-78 établie à partir de la méthode BRUNAUER - EMMETT- TELLER décrite dans le périodique "The Journal of the American Chemical Society, 60, 309 (1938)". Par terre rare on entend les éléments du groupe constitué par les éléments de la classification périodique de numéro atomique compris inclusivement entre 57 et 71.- The single figure represents two Raman spectra of compositions according to the invention. For the remainder of the description, the term "specific surface" means the specific surface B.E.T. determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established from the BRUNAUER - EMMETT-TELLER method described in the periodical "The Journal of the American Chemical Society, 60, 309 (1938)". By rare earth is meant the elements of the group consisting of the elements of the periodic classification with atomic number between 57 and 71 inclusive.
La composition de l'invention est à base d'au moins trois composés qui sont l'oxyde de cérium, l'oxyde de zirconium et d'un ou plusieurs oxydes d'une terre rare supplémentaire autre que le cérium. Cette autre terre rare est une terre rare autre que l'yttrium et ce doit être au moins du lanthane. En d'autres termes, la composition de l'invention comprend au moins de l'oxyde de lanthane. Elle peut comprendre une autre terre rare supplémentaire en plus du lanthane qui peut être plus particulièrement le néodyme ou le praséodyme.The composition of the invention is based on at least three compounds which are cerium oxide, zirconium oxide and one or more oxides of an additional rare earth other than cerium. This other rare earth is a rare earth other than yttrium and it must be at least lanthanum. In other words, the composition of the invention comprises at least oxide of lanthanum. It may include another additional rare earth in addition to lanthanum which may be more particularly neodymium or praseodymium.
La composition de l'invention présente par ailleurs des caractéristiques de structure. Sa structure cristalline, telle que déterminée par analyse aux rayons X est de type cubique, type fluorine CaF2. De préférence, cette structure ou phase cristalline est unique, c'est à dire qu'il n'y a pas de phase secondaire parasite, ce qui traduit l'existence d'une solution solide pure.The composition of the invention also has structural characteristics. Its crystal structure, as determined by X-ray analysis, is of the cubic type, of the fluorite CaF 2 type. Preferably, this crystal structure or phase is unique, that is to say that there is no parasitic secondary phase, which indicates the existence of a pure solid solution.
En outre, la composition présente une phase t' que l'on peut détecter par analyse Raman.In addition, the composition has a phase t 'which can be detected by Raman analysis.
La mise en évidence de la phase t' est faite en interprétant les spectres Raman selon la méthode de M. Yashima et Coll. J. Am. Soc. 77(4), 1067, 1994 et en se basant sur un élargissement de la raie phonon Ce-O vers 470cm"1 et la présence d'une raie à 300cm"1. Généralement, la composition de l'invention présente les teneurs suivantes en cérium, zirconium et terre rare supplémentaire (TR) :The demonstration of the phase t 'is done by interpreting the Raman spectra according to the method of M. Yashima and Coll. J. Am. Soc. 77 (4), 1067, 1994 and based on a widening of the Ce-O phonon line around 470cm "1 and the presence of a line at 300cm " 1 . Generally, the composition of the invention has the following contents of cerium, zirconium and additional rare earth (TR):
47% < Ce+TR < 58% 42% < Zr < 53% Les teneurs données ci-dessus sont exprimées en masse d'oxyde de ces éléments et en pourcentage par rapport à l'ensemble de ces oxydes. Comme cela a été indiqué plus haut TR peut désigner soit le lanthane soit plusieurs terres rares et dans ce cas, au moins une est le lanthane.47% <Ce + TR <58% 42% <Zr <53% The contents given above are expressed in mass of oxide of these elements and in percentage compared to all of these oxides. As indicated above TR can denote either lanthanum or several rare earths and in this case, at least one is lanthanum.
Le rapport atomique Ce/Zr est de préférence au moins égal à 1.The Ce / Zr atomic ratio is preferably at least equal to 1.
La teneur en lanthane est généralement d'au moins 0,5% en masse, de préférence d'au moins 2% et plus particulièrement d'au moins 5%. La teneur en terre rare supplémentaire, c'est à dire autre que le cérium est de préférence d'au plus 10%, plus particulièrement d'au plus 5%, toutes ces teneurs étant aussi exprimées en masse oxyde par rapport à l'ensemble des oxydes.The lanthanum content is generally at least 0.5% by mass, preferably at least 2% and more particularly at least 5%. The additional rare earth content, that is to say other than cerium, is preferably at most 10%, more particularly at most 5%, all these contents also being expressed as oxide mass relative to the whole. oxides.
Les compositions selon l'invention présentent des surfaces spécifiques importantes même à température élevées. Ainsi, elles peuvent présenter après calcination 4 heures à 1000°C une surface spécifique d'au moins 30m2/g, de préférence d'au moins 40m2/g. Après calcination 4 heures à 1100°C, la surface peut être encore d'au moins 5m2/g, de préférence d'au moins 10m2/g.The compositions according to the invention have large specific surfaces even at high temperatures. Thus, after calcination for 4 hours at 1000 ° C., they can have a specific surface of at least 30m 2 / g, preferably at least 40m 2 / g. After 4 hours calcination at 1100 ° C, the surface can still be at least 5m 2 / g, preferably at least 10m 2 / g.
Le procédé de préparation des compositions selon l'invention va maintenant être décrit.The process for preparing the compositions according to the invention will now be described.
La première étape du procédé consiste à préparer un mélange en milieu liquide d'un composé du zirconium, d'un composé du cérium et d'au moins un composé de la terre rare supplémentaire (TR) qui est au moins le lanthane. Les différents composés du mélange sont présents dans des quantités telles que soient vérifiées les relations suivantes exprimées en masse d'oxydes des éléments :The first step of the process consists in preparing a mixture in a liquid medium of a zirconium compound, a cerium compound and at least one additional rare earth (TR) compound which is at least lanthanum. The various compounds of the mixture are present in quantities such that the following relationships, expressed in mass of oxides of the elements, are verified:
47% < Ce+TR < 58% 42% < Zr < 53%47% <Ce + TR <58% 42% <Zr <53%
Le milieu liquide est généralement l'eau.The liquid medium is generally water.
Les composés sont de préférence des composés solubles. Ce peut être notamment des sels de zirconium, de cérium et de terre rare. Il est aussi possible d'utiliser des sols de ces éléments. Le mélange peut être indifféremment obtenu soit à partir de composés initialement à l'état solide que l'on introduira par la suite dans un pied de cuve d'eau par exemple, soit encore directement à partir de solutions de ces composés puis mélange, dans un ordre quelconque, desdites solutions.The compounds are preferably soluble compounds. It can be in particular salts of zirconium, cerium and rare earth. It is also possible to use soils of these elements. The mixture can be obtained either from compounds initially in the solid state which will then be introduced into a bottom of the water tank for example, or even directly from solutions of these compounds and then mixing, in any order, said solutions.
Les composés du zirconium peuvent être choisis parmi le sulfate de zirconium, le nitrate de zirconyle ou le chlorure de zirconyle. Le nitrate de zirconyle est utilisé le plus généralement. On peut aussi utiliser des sels organiques du zirconium comme l'acétate de zirconium.The zirconium compounds can be chosen from zirconium sulfate, zirconyl nitrate or zirconyl chloride. Zirconyl nitrate is most commonly used. It is also possible to use organic zirconium salts such as zirconium acetate.
A titre de composés solubles dans l'eau du cérium, on peut citer notamment les sels de cérium IV tels que nitrates ou nitrates céri-ammoniacal par exemple, qui conviennent ici particulièrement bien. De préférence, on utilise du nitrate cérique. Il est avantageux d'utiliser des sels de pureté d'au moins 99,5 et plus particulièrement d'au moins 99,9%. Une solution aqueuse de nitrate cérique peut par exemple être obtenue par réaction de l'acide nitrique sur un oxyde cérique hydraté préparé d'une manière classique par réaction d'une solution d'un sel céreux, par exemple le nitrate céreux, et d'une solution d'ammoniaque en présence d'eau oxygénée. On peut également, de préférence, utiliser une solution de nitrate cérique obtenue selon le procédé d'oxydation électrolytique d'une solution de nitrate céreux tel que décrit dans le document FR-A- 2 570 087, et qui constitue ici une matière première intéressante.Mention may in particular be made, as water-soluble compounds of cerium, of cerium IV salts such as nitrates or cerium-ammoniacal nitrates, which are particularly suitable here. Preferably, ceric nitrate is used. It is advantageous to use salts with a purity of at least 99.5 and more particularly of at least 99.9%. An aqueous solution of ceric nitrate can for example be obtained by reaction of nitric acid with a hydrated ceric 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, preferably, to use a ceric nitrate solution obtained according to the electrolytic oxidation process of a cerous nitrate solution as described in the document FR-A-2 570 087, which here constitutes an advantageous raw material .
On notera ici que les solutions aqueuses de sels de cérium et de sels de zirconyle peuvent présenter une certaine acidité libre initiale qui peut être ajustée par l'addition d'une base ou d'un acide. Selon la présente invention, il est autant possible de mettre en œuvre une solution initiale de sels de cérium et de zirconium présentant effectivement une certaine acidité libre comme mentionné ci-dessus, que des solutions qui auront été préalablement neutralisées de façon plus ou moins poussée. Cette neutralisation peut se faire par addition d'un composé basique au mélange précité de manière à limiter cette acidité. Ce composé basique peut être par exemple une solution d'ammoniaque ou encore d'hydroxydes d'alcalins (sodium, potassium,...), mais de préférence une solution d'ammoniaque.It will be noted here that the aqueous solutions of cerium salts and zirconyl salts may have a certain initial free acidity which can be adjusted by the addition of a base or an acid. According to the present invention, it is as much possible to use an initial solution of cerium and zirconium salts effectively exhibiting a certain free acidity as mentioned above, than solutions which have been neutralized in more or less depth 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.
Le mélange initial étant ainsi obtenu, on procède ensuite, conformément à la deuxième étape du procédé selon l'invention, à son chauffage.The initial mixture being thus obtained, one then proceeds, in accordance with the second step of the process according to the invention, to its heating.
La température à laquelle est mené ce chauffage ou traitement thermique, aussi appelé thermohydrolyse, est de préférence d'au moins 70°C et encore plus particulièrement d'au moins 100°C. Elle peut ainsi être comprise entre 100°C et la température critique du milieu réactionnel, en particulier entre 100 et 350°C, de préférence entre 100 et 200°C.The temperature at which this heating or heat treatment is carried out, also called thermohydrolysis, is preferably at least 70 ° C. and even more particularly at least 100 ° C. It can thus be between 100 ° C and the critical temperature of the reaction medium, in particular between 100 and 350 ° C, preferably between 100 and 200 ° C.
L'opération de chauffage peut être conduite en introduisant le milieu liquide contenant les espèces précitées dans une enceinte close (réacteur fermé du type autoclave), la pression nécessaire ne résultant alors que du seul chauffage du milieu réactionnel (pression autogène). Dans les conditions de températures données ci-dessus, et en milieux aqueux, on peut ainsi préciser, à titre illustratif, que la pression dans le réacteur fermé peut varier entre une valeur supérieure à 1 Bar (105 Pa) et 165 Bar (1 ,65. 107 Pa), de préférence entre 5 Bar (5. 105 Pa) et 165 Bar (1 ,65. 107 Pa). Il est bien entendu également possible d'exercer une pression extérieure qui s'ajoute alors à celle consécutive au chauffage.The heating operation can be carried out by introducing the liquid medium containing the abovementioned species into a closed enclosure (closed reactor of the autoclave type), the necessary pressure then resulting 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 can vary between a value greater than 1 Bar (10 5 Pa) and 165 Bar (1 , 65. 10 7 Pa), preferably between 5 Bar (5 × 10 5 Pa) and 165 Bar (1, 65. 10 7 Pa). It is of course also possible to exert an external pressure which is then added to that following the heating.
On peut aussi effectuer le chauffage dans un réacteur ouvert pour les températures voisines de 100°C.Heating can also be carried out in an open reactor for temperatures close to 100 ° C.
Le chauffage peut être conduit soit sous air, soit sous atmosphère de gaz inerte, de préférence l'azote. La durée du traitement n'est pas critique, et peut ainsi varier dans de larges limites, par exemple entre 1 et 48 heures, de préférence entre 2 et 24 heures. De même, la montée en température s'effectue à une vitesse qui n'est pas critique, et on peut ainsi atteindre la température réactionnelle fixée en chauffant le milieu par exemple entre 30 minutes et 4 heures, ces valeurs étant données à titre tout à fait indicatif.The heating can be carried out either in air 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. Likewise, the temperature rise takes place at a speed which is not critical, and the fixed reaction temperature can thus be reached by heating the medium, for example between 30 minutes and 4 hours, these values being given entirely indicative fact.
A l'issue de cette deuxième étape, on amène le milieu réactionnel ainsi obtenu à un pH basique. Cette opération est effectuée en ajoutant au milieu une base telle que par exemple une solution d'ammoniaque.At the end of this second step, the reaction medium thus obtained is brought to a basic pH. This operation is carried out by adding a base to the medium such as, for example, an ammonia solution.
Par pH basique on entend une valeur du pH supérieure à 7 et de préférence supérieure à 8.By basic pH is meant a pH value greater than 7 and preferably greater than 8.
Bien que ce mode de réalisation ne soit pas préféré, il est possible d'introduire au mélange réactionnel obtenu à l'issue du chauffage, notamment au moment de l'addition de la base, le ou les composés de la terre rare supplémentaire notamment sous la forme qui a été décrite plus haut, si celui-ci ou ceux-ci n'ont pas été introduits lors de la première étape.Although this embodiment is not preferred, it is possible to introduce into the reaction mixture obtained after heating, in particular at the time of the addition of the base, the rare earth compound (s) additional in particular in the form which has been described above, if this or these have not been introduced during the first step.
A l'issue de l'étape de chauffage, on récupère un précipité solide qui peut être séparé de son milieu par toute technique classique de séparation solide- liquide telle que par exemple filtration, décantation, essorage ou centrifugation.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.
Le produit tel que récupéré peut ensuite être soumis à des lavages.The product as recovered can then be subjected to washing.
Le procédé peut comprendre éventuellement un mûrissement qui permet d'améliorer la cristallinité et la surface spécifique du produit obtenu.The process can optionally include a ripening which makes it possible to improve the crystallinity and the specific surface of the product obtained.
Ce mûrissement peut s'effectuer directement sur le milieu réactionnel obtenu après addition d'une base pour se trouver en pH basique. Comme l'addition d'une base a pour effet de refroidir le milieu réactionnel, le mûrissement se fait en chauffant de nouveau celui-ci. La température à laquelle est chauffé le milieu est d'au moins 40°C, plus particulièrement d'au moins 60°C et encore plus particulièrement d'au moins 100°C. Le milieu est maintenu ainsi à une température constante pendant une durée qui est habituellement d'au moins 30 minutes et plus particulièrement d'au moins 1 heure. Le mûrissement peut se faire à la pression atmosphérique ou éventuellement à une pression plus élevée.This ripening can be carried out directly on the reaction medium obtained after addition of a base in order to be at basic pH. As the addition of a base has the effect of cooling the reaction medium, the ripening takes place by heating it again. The temperature to which the medium is heated is at least 40 ° C, more particularly at least 60 ° C and even more particularly at least 100 ° C. The medium is thus maintained at a constant temperature for a period which is usually at least 30 minutes and more particularly at least 1 hour. The ripening can be done at atmospheric pressure or possibly at a higher pressure.
Le mûrissement peut aussi s'effectuer sur une suspension obtenue après remise dans l'eau du précipité. On peut ajuster le pH de cette suspension à une valeur supérieure à 7 et de préférence supérieure à 8.The ripening can also be carried out on a suspension obtained after putting the precipitate back into water. The pH of this suspension can be adjusted to a value greater than 7 and preferably greater than 8.
Il est possible de faire plusieurs mûrissements. Ainsi, on peut remettre en suspension dans l'eau, le précipité obtenu après l'étape de mûrissement et éventuellement un lavage puis effectuer un autre mûrissement du milieu ainsi obtenu. Cet autre mûrissement se fait dans les mêmes conditions que celles qui ont été décrites pour le premier. Bien entendu, cette opération peut être répétée plusieurs fois.It is possible to make several ripenings. Thus, it is possible to resuspend in water, the precipitate obtained after the ripening step and optionally a washing then carry out another ripening of the medium thus obtained. This other ripening takes place under the same conditions as those which have been described for the first. Of course, this operation can be repeated several times.
Dans une dernière étape du procédé selon l'invention, le précipité récupéré, après éventuellement mûrissement, lavage et/ou séchage, peut ensuite être calciné. Cette calcination permet de développer la cristallinité du produit obtenu, et elle peut être également ajustée et/ou choisie en fonction de la température d'utilisation ultérieure réservée, à la composition selon l'invention, et ceci en tenant compte du fait que la surface spécifique du produit est d'autant plus faible que la température de calcination mise en œuvre est plus élevée. Une telle calcination est généralement opérée sous air, mais une calcination menée par exemple sous gaz inerte ou sous atmosphère contrôlée (oxydante ou réductrice) n'est bien évidemment pas exclue. En pratique, on limite généralement la température de calcination à un intervalle de valeurs comprises entre 300 et 1000°C.In a last stage of the process according to the invention, the precipitate recovered, after optionally maturing, washing and / or drying, can then be calcined. This calcination makes it possible to develop the crystallinity of the product obtained, and it can also be adjusted and / or chosen as a function of the subsequent temperature of use reserved for the composition according to the invention, and this taking into account the fact that the surface The specific product is lower the higher the calcination temperature implemented. Such calcination is generally carried out in air, but calcination carried out for example under inert gas or under a controlled atmosphere (oxidizing or reducing) is obviously not excluded. In practice, the calcination temperature is generally limited to a range of values between 300 and 1000 ° C.
Les compositions de l'invention telles que décrites plus haut ou telles qu'obtenues par le procédé mentionné précédemment se présentent sous forme de poudres mais elles peuvent éventuellement être mises en forme pour se présenter sous forme de granulés, billes, cylindres ou nids d'abeille de dimensions variables. Ces compositions peuvent être appliquées sur tout support utilisé habituellement dans le domaine de la catalyse, c'est à dire notamment des supports inertes thermiquement. Ce support peut être choisi parmi l'alumine, l'oxyde de titane, l'oxyde de cérium, l'oxyde de zirconium, la silice, les spinelles, les zéolites, les silicates, les phosphates de silicoaluminium cristallins, les phosphates d'aluminium cristallins.The compositions of the invention as described above or as obtained by the process mentioned above are in the form of powders but they can optionally be shaped to be in the form of granules, balls, cylinders or nests. bee of variable dimensions. These compositions can be applied to any support usually 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, phosphates crystalline aluminum.
Les compositions peuvent aussi être utilisées dans des systèmes catalytiques. Ces systèmes catalytiques peuvent comprendre un revêtement (wash coat) à propriétés catalytiques et à base de ces compositions, sur un substrat du type par exemple monolithe métallique ou en céramique. Le revêtement peut comporter lui aussi un support du type de ceux mentionnés plus haut. Ce revêtement est obtenu par mélange de la composition avec le support de manière à -former une suspension qui peut être ensuite déposée sur le substrat.The compositions can also be used in catalytic systems. These catalytic systems can comprise a coating (wash coat) with catalytic properties and based on these compositions, on a substrate of the type, for example metallic or ceramic monolith. The coating may also include a support of the type of those mentioned above. This coating is obtained by mixing the composition with the support so as to form a suspension which can then be deposited on the substrate.
Ces systèmes catalytiques et plus particulièrement les compositions de l'invention peuvent trouver de très nombreuses applications. Ils sont ainsi particulièrement bien adaptés à, et donc utilisables, dans la catalyse de diverses réactions telles que, par exemple, la déshydratation, l'hydrosulfuration, l'hydrodénitrification, la désuif u ration, l'hydrodésulfuration, la déshydrohalogénation, le reformage, le reformage à la vapeur, le craquage, l'hydrocraquage, l'hydrogénation, la déshydrogénation, l'isomérisation, la dismutation, l'oxychloration, la déshydrocyclisation d'hydrocarbures ou autres composés organiques, les réactions d'oxydation et/ou de réduction, la réaction de Claus, le traitement des gaz d'échappement des moteurs à combustion interne, la démétallation, la méthanation, la shift conversion.These catalytic systems and more particularly the compositions of the invention can find numerous applications. They are thus particularly well suited to, and therefore usable in, the catalysis of various reactions such as, for example, dehydration, hydrosulfurization, hydrodenitrification, desuif u ration, hydrodesulfurization, dehydrohalogenation, reforming, steam reforming, cracking, hydrocracking, hydrogenation, dehydrogenation, isomerization, disproportionation, oxychlorination, dehydrocyclization of hydrocarbons or other organic compounds, oxidation and / or reduction, Claus reaction, exhaust gas treatment of internal combustion engines, demetallation, methanation, shift conversion.
Dans le cas de ces utilisations en catalyse, les compositions de l'invention sont employées en combinaison avec des métaux précieux. La nature de ces métaux et les techniques d'incorporation de ceux-ci dans ces compositions sont bien connues de l'homme du métier. Par exemple, les métaux peuvent être le platine, le rhodium, le palladium ou l'iridium, ils peuvent notamment être incorporés aux compositions par imprégnation. Parmi les utilisations citées, le traitement des gaz d'échappement des moteurs à combustion interne (catalyse post combustion automobile) constitue une application particulièrement intéressante.In the case of these uses in catalysis, the compositions of the invention are used in combination with precious metals. 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 or iridium, they can in particular be incorporated into the compositions by impregnation. Among the uses cited, the treatment of exhaust gases from internal combustion engines (post-combustion catalysis for cars) constitutes a particularly interesting application.
De ce fait, l'invention concerne aussi l'utilisation d'une composition ou d'un système catalytique tels que décrits plus haut à la fabrication de catalyseur pour post combustion automobile.As a result, the invention also relates to the use of a composition or of a catalytic system as described above in the manufacture of a catalyst for automotive post combustion.
Des exemples concrets mais non limitatifs vont maintenant être donnés.Concrete but non-limiting examples will now be given.
Dans ces exemples, la réductibilité des composition est déterminée par la mesure de leur capacité de captage de l'hydrogène. Cette mesure est faite par réduction programmée en température de la manière suivante. On utilise un appareil AMI-1 Altamira avec un réacteur en silice et un échantillon de 100mg. Le gaz est l'hydrogène à 10% en volume dans l'argon et avec un débit de 30ml/mn. La montée en température se fait de l'ambiante à 900°C à raison de 10°C/mn. La détection du signal se fait avec un détecteur de conductivité thermique à 70mA. Le captage de l'hydrogène est calculé à partir de la surface manquante du signal d'hydrogène de la ligne de base à la température ambiante à la ligne de base à 900°C.In these examples, the reducibility of the compositions is determined by measuring their capacity for capturing hydrogen. This measurement is made by programmed reduction in temperature as follows. We use an AMI-1 Altamira device with a silica reactor and a 100 mg sample. The gas is hydrogen at 10% by volume in argon and with a flow rate of 30ml / min. The temperature rise is from ambient to 900 ° C at a rate of 10 ° C / min. The signal is detected with a thermal conductivity detector at 70mA. Hydrogen uptake is calculated from the missing area of the hydrogen signal from the baseline at room temperature to the baseline at 900 ° C.
L'analyse Raman a été faite en utilisant un spectromètre Raman à optique dispersive et à détection monocanale Jobin Yvon Ramanor HG2S®, le laser étant du type argon ionisé (raie verte à 514,5nm). L'échantillon analysé a été légèrement broyé au mortier puis compacté sur un porte échantillon.The Raman analysis was carried out using a Raman spectrometer with dispersive optics and with single-channel Jobin Yvon Ramanor HG2S ® detection, the laser being of the ionized argon type (green line at 514.5 nm). The analyzed sample was slightly ground in a mortar and then compacted on a sample holder.
EXEMPLE 1EXAMPLE 1
Cet exemple concerne la synthèse d'un composé de formule CeO2 ZrO2/La2θ3 dans les proportions suivantes en masse 48/47/5 %.This example relates to the synthesis of a compound of formula CeO 2 ZrO 2 / La2θ3 in the following proportions by mass 48/47/5%.
Pour préparer 100 g de composé, on introduit dans un réacteur agité :To prepare 100 g of compound, the following are introduced into a stirred reactor:
- 192 ml d'une solution de nitrate de Ce(IV) à 250 g/L (exprimée en CeO2)- 192 ml of a Ce (IV) nitrate solution at 250 g / L (expressed as CeO 2 )
- 180 ml d'une solution de nitrate de Zr(IV) à 260 g/L (exprimée en ZrO2) - 11 ml d'une solution de nitrate de La(lll) à 450 g/L (exprimée en La2O3)- 180 ml of a Zr (IV) nitrate solution at 260 g / L (expressed in ZrO 2 ) - 11 ml of a La nitrate solution (III) at 450 g / L (expressed in La 2 O 3 )
- 617 ml d'eau déminéralisée.- 617 ml of demineralized water.
Le mélange est ensuite chauffé à 100°C sous pression atmosphérique , puis est maintenu à cette température durant 4h. On observe alors la formation d'un solide qui sédimente lorsqu'on arrête l'agitation. La suspension est ensuite refroidie à l'air.The mixture is then heated to 100 ° C. under atmospheric pressure, then is maintained at this temperature for 4 hours. We then observe the formation of a solid which sediments when the stirring is stopped. The suspension is then cooled in air.
On ajoute ensuite de l'ammoniaque à 6 mol.L"1 sous agitation jusqu'à atteindre un pH égal à 9. Puis l'agitation est stoppée, et le solide sédimente lentement. Après 1 h de sédimentation, on élimine 500 ml de jus clair que l'on remplace par 500 ml d'eau déminéralisée. Cette suspension est ensuite chauffée sous agitation à 100°C et sous pression atmosphérique durant 2h. Une fois la suspension refroidie à la température ambiante, on sépare le solide du liquide au moyen d'un filtre du type Buchner. Puis le produit est ensuite calciné dans un four électrique à une température de 800°C durant 2h. L'oxyde mixte formé est ensuite désaggloméré à l'aide d'un broyeur à broches.Then added ammonia at 6 mol.L "1 with stirring until a pH equal to 9 is reached. Then the stirring is stopped, and the solid settles slowly. After 1 h of settling, 500 ml of clear juice that one replaces with 500 ml of demineralized water. This suspension is then heated, with stirring, to 100 ° C. and at atmospheric pressure for 2 hours. Once the suspension has cooled to room temperature, the solid is separated from the liquid by means of a Buchner type filter. Then the product is then calcined in an electric oven at a temperature of 800 ° C for 2 hours. The mixed oxide formed is then deagglomerated using a pin mill.
L'analyse RX montre que l'oxyde obtenu présente bien une structure cristalline cubique. L'analyse Raman met en évidence une phase t'.The X-ray analysis shows that the oxide obtained does indeed have a cubic crystal structure. Raman analysis highlights a phase t '.
Les autres caractéristiques de l'oxyde sont données dans le tableau ci- dessous.The other characteristics of the oxide are given in the table below.
EXEMPLE 2EXAMPLE 2
Cet exemple concerne la synthèse d'un composé de formule Ceθ2/ZrO2/La2O3/Nd2O3 dans les proportions suivantes en masse 48/47/2/3 %.This example relates to the synthesis of a compound of formula Ceθ 2 / ZrO 2 / La 2 O 3 / Nd 2 O 3 in the following proportions by mass 48/47/ 2 / 3 %.
Pour préparer 100 g d'oxyde mixte, on introduit dans un réacteur agité :To prepare 100 g of mixed oxide, the following are introduced into a stirred reactor:
- 192 ml d'une solution de nitrate de Ce(IV) à 250 g/L (exprimée en CeO2)- 192 ml of a Ce (IV) nitrate solution at 250 g / L (expressed as CeO 2 )
- 180 ml d'une solution de nitrate de Zr(IV) à 260 g/L (exprimée en ZrO2) - 4.4 ml d'une solution de nitrate de La(lll) à 450 g/L (exprimée en La2Os)- 180 ml of a Zr (IV) nitrate solution at 260 g / L (expressed in ZrO 2 ) - 4.4 ml of a La nitrate solution (III) at 450 g / L (expressed in La 2 Os )
- 6 ml d'une solution de nitrate de Nd(lll) à 500 g/L (exprimée en Nd2O3)- 6 ml of a Nd (lll) nitrate solution at 500 g / L (expressed as Nd 2 O 3 )
- 617.6 ml d'eau déminéralisée.- 617.6 ml of demineralized water.
On procède ensuite de la même manière que dans l'exemple 1. L'analyse RX montre que l'oxyde obtenu présente bien une structure cristalline cubique. L'analyse Raman met en évidence une phase t'.The procedure is then carried out in the same way as in Example 1. The X-ray analysis shows that the oxide obtained has indeed a cubic crystal structure. Raman analysis highlights a phase t '.
Les autres caractéristiques de l'oxyde sont données dans le tableau ci- dessous.The other characteristics of the oxide are given in the table below.
Figure imgf000011_0001
Figure imgf000011_0001
(1 ) la surface indiquée est celle mesuré sur les produits obtenus dans les exemples et après calcination complémentaire à la température indiquée pendant 4 heures.(1) the area indicated is that measured on the products obtained in the examples and after additional calcination at the indicated temperature for 4 hours.
(2) on donne le % de cérium réduit étant entendu qu'1/2 mole d'H2 consommée et mesurée par la méthode décrite plus haut correspond à 1 mole de CelV réduit. La mesure a été faite sur les produits obtenus dans les exemples et après calcination complémentaire de ceux-ci à la température indiquée pendant 4 heures. On note une variation faible du % de cérium réduit avec la température de calcination.(2) the% of reduced cerium is given, it being understood that 1/2 mole of H 2 consumed and measured by the method described above corresponds to 1 mole of reduced CelV. The measurement was made on the products obtained in the examples and after further calcination thereof at the temperature indicated for 4 hours. There is a slight variation in the% of cerium reduced with the calcination temperature.
La figure 1 annexée donne deux spectres Raman (intensité en fonction du nombre d'onde). Les spectres 1 et 2 correspondent aux composés des exemples 1 et 2 respectivement. The appended FIG. 1 gives two Raman spectra (intensity as a function of the wave number). Spectra 1 and 2 correspond to the compounds of Examples 1 and 2 respectively.

Claims

REVENDICATIONS
1- Composition à base d'oxyde de cérium, d'oxyde de zirconium et d'au moins un oxyde d'une terre rare supplémentaire autre que l'yttrium, caractérisée en ce qu'elle possède une structure cristalline cubique déterminée par analyse aux rayons X, en ce qu'elle présente une phase t' détectable par analyse Raman, la terre rare supplémentaire étant au moins le lanthane.1- Composition based on cerium oxide, zirconium oxide and at least one additional rare earth oxide other than yttrium, characterized in that it has a cubic crystal structure determined by analysis with X-rays, in that it has a phase t 'detectable by Raman analysis, the additional rare earth being at least lanthanum.
2- Composition selon la revendication 1 , caractérisée en ce qu'elle présente les teneurs suivantes en cérium, zirconium et terre rare supplémentaire (TR), exprimées en masse d'oxyde de ces éléments :2- Composition according to claim 1, characterized in that it has the following contents of cerium, zirconium and additional rare earth (TR), expressed in mass of oxide of these elements:
47% < Ce+TR < 58% 42% < Zr < 53%47% <Ce + TR <58% 42% <Zr <53%
3- Composition selon la revendication 1 ou 2, caractérisée en ce qu'elle présente une teneur en lanthane d'au moins0,5%, plus particulièrement d'au moins 2%.3- Composition according to claim 1 or 2, characterized in that it has a lanthanum content of at least 0.5%, more particularly at least 2%.
4- Composition selon l'une des revendications précédentes, caractérisée en ce qu'elle comprend une seconde terre rare supplémentaire choisie parmi le néodyme et le praséodyme.4- Composition according to one of the preceding claims, characterized in that it comprises a second additional rare earth chosen from neodymium and praseodymium.
5- Composition selon l'une des revendications précédentes, caractérisée en ce qu'elle présente après calcination 4 heures à 1000°C une surface spécifique d'au moins 30m2/g.5- Composition according to one of the preceding claims, characterized in that after calcination for 4 hours at 1000 ° C a specific surface of at least 30m 2 / g.
6- Procédé de préparation d'une composition selon l'une des revendications précédentes, caractérisé en ce qu'il comprend les étapes suivantes : - on prépare un mélange en milieu liquide contenant un composé du zirconium et un composé du cérium;6- A method of preparing a composition according to one of the preceding claims, characterized in that it comprises the following steps: - a mixture is prepared in a liquid medium containing a zirconium compound and a cerium compound;
- on chauffe ledit mélange;- Heating said mixture;
- on amène le milieu réactionnel obtenu à l'issue du chauffage à un pH basique; - on récupère le précipité ainsi obtenu;- the reaction medium obtained after the heating is brought to a basic pH; - The precipitate thus obtained is recovered;
- on calcine ledit précipité; un composé de la terre rare supplémentaire (TR) étant ajouté soit au mélange en milieu liquide de départ soit au mélange réactionnel obtenu à l'issue du chauffage, les quantités de composés de cérium, de zirconium et de la terre rare supplémentaire utilisées vérifiant les relations suivantes exprimées en masse d'oxydes des éléments :- Said precipitate is calcined; an additional rare earth compound (TR) being added either to the mixture in the starting liquid medium or to the reaction mixture obtained at the end of the heating, the quantities of cerium, zirconium and additional rare earth compounds used verifying the following relationships expressed in mass of oxides of the elements:
47% < Ce+TR < 58% 42% < Zr < 53%47% <Ce + TR <58% 42% <Zr <53%
7- Procédé selon la revendication 6, caractérisé en ce qu'on effectue un mûrissement du milieu réactionnel obtenu après chauffage et retour au pH basique.7- Method according to claim 6, characterized in that one carries out a ripening of the reaction medium obtained after heating and return to basic pH.
8- Procédé selon la revendication 6, caractérisé en ce qu'on effectue un mûrissement de la suspension obtenue après remise dans l'eau du précipité.8- A method according to claim 6, characterized in that the suspension obtained is cured after putting the precipitate back into water.
9- Procédé selon la revendication 7 ou 8, caractérisé en ce qu'on effectue le mûrissement précité à une température d'au moins 60°C.9- A method according to claim 7 or 8, characterized in that the aforementioned ripening is carried out at a temperature of at least 60 ° C.
10- Procédé selon l'une des revendications 6 à 9, caractérisé en ce qu'on utilise comme composés du zirconium et du cérium le nitrate de zirconyle, l'acétate de zirconium -ou le chlorure de zirconyle et le nitrate de cérium ou le nitrate céri-ammoniacal.10- Method according to one of claims 6 to 9, characterized in that as zirconium and cerium compounds are used zirconyl nitrate, zirconium acetate -or zirconyl chloride and cerium nitrate or ceri-ammoniacal nitrate.
11- Système catalytique caractérisé en ce qu'il comprend une composition selon l'une des revendications 1 à 5.11- Catalytic system characterized in that it comprises a composition according to one of claims 1 to 5.
12- Utilisation d'une composition selon l'une des revendications 1 à 5 ou d'un système catalytique selon la revendication 11 au traitement de gaz d'échappement des moteurs à combustion interne. 12- Use of a composition according to one of claims 1 to 5 or of a catalytic system according to claim 11 for the treatment of exhaust gas from internal combustion engines.
PCT/FR2002/002991 2001-09-03 2002-09-02 Highly reducible composition based on cerium oxide, zirconium oxide and another rare earth oxide, the preparation method thereof and the use of same as a catalyst WO2003020643A1 (en)

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