WO2011012510A2

WO2011012510A2 - Redox loop method using a composition comprising supported cerium oxide or supported cerium, zirconium and/or rare earth oxide as a redox mass

Info

Publication number: WO2011012510A2
Application number: PCT/EP2010/060534
Authority: WO
Inventors: Virginie Harle; Michael Lallemand
Original assignee: Rhodia Operations
Priority date: 2009-07-31
Filing date: 2010-07-21
Publication date: 2011-02-03
Also published as: WO2011012510A3; FR2948748B1; FR2948748A1

Abstract

The invention relates to a redox loop method which, by way of a redox mass, uses a mass comprising a composition containing an oxide consisting of a cerium oxide or a mixture of a cerium oxide and at least an oxide of one other element selected from zirconium and rare earths other than cerium, said oxide being supported on a on a substrate containing silica, alumina, oxide of titanium or zirconium and taking the form of particles deposited on the substrate individually or as aggregates, with a maximum size of 500 nm. Following calcination at a temperature of at least 800°C for 6 hours, the composition has a reducibility value of at least 80%, measured between 50°C et 900°C.

Description

LOOP OXYDO-REDUCTION PROCESS USING THE SAME

OXYDO-REDUCTIVE MASS OXIDE-BASED COMPOSITION

SUPPORTED BY CERIUM OR CERIUM, ZIRCONIUM AND / OR

RARE EARTH

The present invention relates to a looped oxidation-reduction method using as mass redox a mass comprising a composition based on an oxide supported cerium or a mixture of cerium oxide and at least one oxide another element selected from zirconium and rare earths other than cerium.

In the fight against greenhouse gases, especially against CO2, the techniques and processes for capturing and storing greenhouse gases such as CO2 are of great interest.

The combustion processes of coal or natural gas, used for example to produce energy or electricity, emit significant amounts of CO2 and the capture of CO2 thus produced is now key to the development of such processes. We also seek to recover CO ₂ as pure as possible to allow liquefaction and this under the most favorable conditions economically and energetically.

A combustion technique that facilitates the recovery of CO ₂ consists in carrying out oxidation-reduction reactions of an active mass in order to decompose the combustion reaction into two successive reactions.

In a first reduction reactor, the previously oxidized active mass M _x O _y is reduced in contact with a reducing gas C _n H _2m , thus causing its combustion according to the following reaction:

(1) C _n H _2m + M _x O _y -> nCO ₂ + m H ₂ O + M _x O _y-2n- m

In a second oxidation reactor, the reduced active mass M _x O _y-2n -m is restored in its oxidized state by an oxidation reaction with air, possibly wet, according to the other reaction below:

(2) M _x O _y-2n-m + (n + m / 2) O ₂ -> M _x O _y

In the remainder of this description, the term "oxidation-reduction method in a loop" will be referred to as any process of this type using an active mass or redox mass which thus alternately passes from its oxidized form to its reduced form and vice versa and describes a oxidation-reduction cycle according to the two reactions of the type above. Such a method thus makes it possible to transmit fumes consisting mainly of CO2 and H ₂ O and depleted in N ₂ which facilitates the recovery of CO ₂ .

Active molecules are therefore sought with significant capacity to release oxygen, thus high capacities to oxidize and reduce cyclically. It is also possible to search for active masses having the capacity to release oxygen at the lowest possible temperatures, such as 700 ^° C., or even less.

The reactions of this process are also at high temperatures typically between 700 and 1100 ⁰ C and therefore require that the active mass is stable over time at these temperatures. The stability can result in a quantity of oxygen released during several oxidation-reducing cycles substantially stable as the cycles of these cycles.

Redox-reducing masses that can be used in such a process are known. They are generally based on a redox couple NiO / Ni type, CuO / Cu or CoO / Co associated with a binder such as alumina or yttrié zirconia. However, these active masses have several defects. Thus, their oxygen transfer capacity can be limited, either because of the very amount of transferable oxygen or because of the kinetics of oxygen transfer. In addition, performance losses are observed during oxidation-reduction cycles related to a modification of the active mass at high temperature.

There is therefore a need for masses whose performance is improved, especially as regards stability.

For this purpose, the loop oxidation-reduction method of the invention is characterized in that a mass comprising a composition based on an oxide which is a cerium oxide or a mixture is used as the redox mass. cerium oxide and at least one oxide of another element selected from zirconium and rare earths other than cerium, said oxide being supported on a support based on silica, alumina, titanium or zirconia and this oxide being in the form of particles which are deposited on said support, in individualized form or in the form of aggregates, with a size of at most 500 nm, said composition having after calcination for 6 hours at a temperature of at least 800 ° C, a reducibility measured between 50 ⁰ C and 900 ° C of at least 80%.

Other features, details and advantages of the invention will appear even more fully on reading the description which follows, as well as various concrete but non-limiting examples intended to illustrate it. For the purposes of this description, the term "rare earth" refers to the elements of the group consisting of the ytthum and the elements of the periodic classification of atomic number inclusive of between 57 and 71.

By specific surface is also meant the specific surface area B. AND. determined by nitrogen adsorption according to ASTM D

3663-78 established from the method BRUNAUER - EMMETT-TELLER described in the journal "The Journal of the American Chemical Society, 60,

309 (1938) ".

In addition, the calcinations at which the surface values are given are calcinations under air. Moreover, the specific surface values which are indicated for a given temperature and duration correspond, unless otherwise indicated, to calcinations at a temperature step over the indicated time.

The contents are given in oxides unless otherwise indicated. The cerium oxide is in the form of ceric oxide, the oxides of the other rare earths in Ln ₂ θ3 form, except otherwise indicated and with the exception of praseodymium expressed in the form Pr ₆ On, Ln denoting the rare earth.

It is also specified for the remainder of the description that, unless otherwise indicated, in all ranges or limits of values that are given, the values at the terminals are included, the ranges or limits of values thus defined thus covering any value at least equal to and greater than the lower bound and / or at most equal to or less than the upper bound.

The definition of the loop oxidation-reduction method also applies to a hydrogen production process implementing the oxidation-reduction cycle mentioned above.

The process of the invention is characterized by the use, as active mass or redox mass, of a specific composition which will be described more precisely below.

This composition comprises an oxide supported in the form of particles of nanometric dimensions, these particles being deposited on a support.

This supported oxide may first be a single oxide cerium oxide, usually in the form of ceric oxide CeO ₂ .

It can also be a mixture of a cerium oxide and at least one oxide of another element which is selected from zirconium and rare earths other than cerium.

The rare earth may be more particularly ytthum, neodymium, lanthanum or praseodymium. According to a particular embodiment, lanthanum and praseodymium are present in combination. The content of rare earth oxide is generally at most 25%, preferably when the rare earth is lanthanum, more particularly at most 20% and preferably at most 15% by weight. The minimum content is not critical but generally it is at least 1%, more preferably at least 2% and preferably at least 5% by weight. This content is expressed as the rare earth oxide relative to the mass of all the cerium oxide and the rare earth oxide or rare earth oxides when several rare earths are present in combination.

The supported oxide of the composition of the active mass may also be based on a mixture of cerium oxide and zirconium oxide. This oxide can meet the formula Cei _ _x Zr _x θ2. In this formula, x, which defines an atomic proportion of the elements Ce and Zr, is less than 1 and may be more particularly between 0.8 and 0.2, and more particularly between 0.50 and 0.2.

The supported oxide of the composition of the active mass may also be based on a mixture of cerium oxide, zirconium oxide and at least one oxide of a rare earth other than cerium. This oxide may correspond to the formula Ce _x ZryM _z θ2 in which M represents at least one element selected from the group consisting of rare earths with the exception of cerium and where x, y and z give the atomic proportions of elements Ce, Zr and M of the composition.

In this formula z is different from Oet and preferably has a value of at most 0.3 and which may be more particularly between 0.02 and 0.2 and the ratio x / y may be between 1 and 19, plus particularly between 1 and 9 and even more particularly between 1, 5 and 4, and x, y and z being linked by the relation x + y + z = 1.

According to a particular embodiment, the supported oxide is in crystallized form.

According to another particular embodiment, the mixture of oxides is in the form of a solid solution (mixed oxide) of the rare earth element and / or zirconium, in cerium or in the form of a solid solution of cerium and, optionally, rare earth, in zirconium, this depending on the respective quantities of cerium and zirconium. In the first case, the supported oxide has a single phase which corresponds in fact to a crystalline structure of the fluorine type, just like the ceric oxide CeO 2 crystallized, and whose mesh parameters are more or less staggered with respect to a pure ceric oxide. , thus reflecting the incorporation of the other elements, rare earth and, possibly, zirconium, into the crystalline lattice of cerium oxide. In the second case, the supported oxide has a single phase corresponding to that of a zirconium oxide crystallized in the tetragonal system in particular, thus reflecting the incorporation of cerium and the other rare earth element into the crystal lattice of the zirconium oxide.

The supported oxide is present in the composition of the redox mass in the form of particles of nanometric dimensions. By this is meant that the particles have a size of at most 500 nm. It should be noted here that these particles can be either individualized or in the form of aggregates. The value of 500 nm applies either to the size of the particles when they are individualized or to the size of the aggregate when there are aggregated particles. Moreover, this size may be more particularly at most 50 nm and even more particularly at most 10 nm. The highest size values given above correspond to the cases where the particles are most often in an aggregated form while the lowest values correspond to the cases where the particles are most often in an individualized form. More particularly, the particles are in individualized form with a size of approximately at most 50 nm, even more particularly at most 10 nm, or else between 10 nm and 5 nm, and in the case where the oxide is crystallized. they correspond to crystallites of this oxide. According to a particular embodiment, the particles are in individualized form with a size of at most 5 nm, this embodiment corresponding to the case where the supported oxide content of the composition is relatively low, this low content being further specified below. The size values given here and in the present description are determined either by transmission electron microscopy (TEM) or X-ray diffraction (XRD), particularly when the oxide is crystallized.

These particle size values of the supported oxide are given for a composition which has been calcined for 6 hours at a temperature of at least 500 ^° C., more particularly at least 800 ^° C.

As indicated above, the particles are deposited on a support.

By this is meant that the particles are predominantly present on the surface of this support, it being understood that the particles may be present inside the pores of the support but remaining on the surface of these pores.

The support is based on silica, alumina, titanium oxide or zirconia. Preferably, this support must have a high and stable surface area, that is to say which remains at a sufficient value even after exposure to a high temperature. The constituent oxides of the support which have just been mentioned are well known and used in the field of catalysis. The description below concerning these supports is therefore given for purely illustrative purposes and is not limiting.

In the case of a silica-based support, it may be a precipitated silica or a pyrogenation silica. The silica may optionally be stabilized by a stabilizing element such as aluminum.

In the case of an alumina-based support, any type of alumina capable of having a specific surface area sufficient for an application in catalysis can be used here. Mention may be made of aluminas resulting from the rapid dehydration of at least one aluminum hydroxide, such as bayerite, hydrargillite or gibbsite, nordstrandite, and / or at least one aluminum oxyhydroxide such as boehmite. , pseudoboehmite and diaspore.

According to a particular embodiment of the invention, a composition in which the support is a stabilized alumina is used as the redox mass. As a stabilizing element, mention may be made of rare earths, barium, strontium, silicon and zirconium. As a rare earth, cerium, lanthanum or the lanthanum-neodymium mixture can be particularly mentioned. These elements can be used alone or in combination. Advantageous combinations include La-Ba, Ba-Pr, La-Pr-Ba and La-Pr.

The stabilized alumina is prepared in a manner known per se, in particular by impregnation of the alumina with salt solutions, such as nitrates, with the aforementioned stabilizing elements or by co-drying an alumina precursor and salts of these elements then calcination.

There may also be mentioned another preparation of stabilized alumina in which the alumina powder resulting from the rapid dehydration of an aluminum hydroxide or oxyhydroxide is subjected to a ripening operation in the presence of an agent. stabilizer consisting of a lanthanum compound and, optionally, a neodymium compound, this compound may be more particularly a salt. The ripening can be done by suspending the alumina in water and then heating to a temperature of, for example, between 70 and 110 ^° C. After maturing, the alumina is subjected to a heat treatment.

Another preparation consists of a similar type of treatment but with barium or strontium. The stabilizer content expressed as weight of stabilizer oxide relative to stabilized alumina is generally between 1.5% and 35%, or between 1.5% and 25%, especially between 1.5% and 15%. %. This content may be more particularly between 2.5% and 20%, more particularly between 2.5 and 11% or between 5% and 20%.

According to a particular embodiment, the stabilizer is barium and / or strontium in a content expressed by weight of stabilizer oxide relative to stabilized alumina of less than 10%.

According to two other particular embodiments, the support of the composition of the redox mass is based on alumina stabilized by a stabilizing element of the group consisting of barium, strontium and lanthanum and the oxide supported is in the first case a mixture of cerium oxide, zirconium oxide and oxide of a rare earth other than cerium which may be more particularly praseodymium or lanthanum. In the second case, the supported oxide is a mixture of cerium oxide, praseodymium oxide and rare earth oxide other than cerium and praseodymium, which may be more particularly lanthanum. For these two embodiments, the stabilizing element content, expressed as oxide, may be at least 10% by weight relative to the stabilized alumina and may be more particularly between 10 and 30%, especially between 10 and 30% by weight. and 25%.

According to other particular embodiments, the support consists essentially of alumina (first mode) or consists essentially of alumina stabilized by at least one element chosen from rare earths, silicon and zirconium (second mode ). By "essentially consists of" is meant in the case of the first mode that the carrier contains only alumina and no other element capable of stabilizing it, the alumina may possibly contain the usual impurities. In the case of the second mode, it is understood that the support contains only alumina and, as stabilizer, one or more elements in combination chosen only in the group consisting of rare earths, silicon and zirconium with, again, the possible presence of impurities usually bonded to the alumina and the aforementioned element.

In the case of a support based on alumina stabilized by a stabilizing element selected from rare earths, barium and strontium or consisting essentially of stabilized alumina with the same stabilizing element, it may be noted here that the preparation stabilized alumina can be achieved during the actual preparation of the composition of the redox mass according to the invention.

The support may also be titanium oxide, more particularly titanium oxide in anatase form. Such an oxide can be prepared for example by the methods described in EP-A-319365 or in EP-A-351270 and it can also comprise a rare earth or alkaline earth stabilizer.

The support may finally be a zirconia, tetragonal or monoclinic, optionally stabilized for example by a rare earth.

It will also be noted that the support may consist of a combination of the abovementioned oxides, either in the form of mixtures of these oxides or in the form of mixed oxides (solid solutions), such as mixed silica / alumina, titanium / silica or zirconia / silica oxides.

It will be noted that it is also possible to introduce the stabilizing elements of the various supports which have been mentioned after having prepared the composition (support supported oxide without stabilizer) for example by an impregnation technique.

The supported oxide content, cerium oxide for example, of the composition of the redox mass of the invention is generally at most 75% by weight of the entire composition. It may especially be at most 60%, more particularly at most 50% and even more particularly at most 30%. In the particular case of a zirconia support, the supported oxide content may correspond to the values that have just been given and it may also be more particularly at most 55%.

The minimum content of supported oxide is set according to the desired performance of the composition. By way of example only, this minimum content is generally at least 3%, more particularly at least 4% by weight.

As indicated above, the particles of the supported oxide may be in individualized form with a size of at most 5 nm. This embodiment preferably corresponds to compositions in which the supported oxide content is at most 20% by weight, more particularly at most

15% and even more particularly at most 10%.

In the case of the particular embodiments described above for which the support is based on barium-stabilized alumina and the supported oxides, mixtures of cerium oxide, zirconium oxide and oxide of earth rare other than cerium or cerium oxide, praseodymium oxide and rare earth oxide other than praseodymium, the content of supported oxides may be more particularly equal to or greater than 15%, more particularly between 15 and 30%.

Another characteristic of the compositions of the redox masses of the invention is their reducibility.

The reducibility of the compositions is determined by measuring their hydrogen consumption measured between 50 ^° C. and 700 ^° C. or 900 ° C. This measurement is made by programmed temperature reduction using diluted hydrogen in argon. A signal is detected with a thermal conductivity detector. The consumption of hydrogen is calculated from the missing surface of the baseline hydrogen signal at 50 ^° C at baseline at 700 ° C or 900 ° C. The degree of reducibility represents the percentage of cerium reduced, it being understood that 1/2 mol of H ₂ consumed and measured by the method described above corresponds to 1 mole of reduced Ce IV.

For the purposes of the present invention, the compositions of the active mass may contain in addition to cerium another reducible element of the rare earth type such as praseodymium, the overall reducibility of cerium and of this element, praseodymium for example, is calculated. The overall reducibility of cerium and this element is calculated by relating the experimental hydrogen consumption of the composition to the sum of the theoretical hydrogen consumptions corresponding to the complete reduction of the cerium and to the complete reduction of the other element such as praseodymium. The experimental hydrogen consumption is calculated from the missing area of the hydrogen signal to baseline at 50 ⁰ C to baseline at 900 ° C when measured reducibility between 50 ° C and 900 ⁰ C. The reducibility measured between 50 ° C and 700 ° C corresponds to the same measurement but made in the temperature range of 50 ⁰ C to 700 ° C.

The theoretical hydrogen consumption for the complete reduction of cerium is calculated as above considering that all the cerium is initially at the degree + IV and that 1/2 mole of H ₂ is consumed to reduce 1 mole of Ce IV. The theoretical hydrogen consumption for the complete reduction of praseodymium, for example, is calculated analogously considering that 2/3 of the praseodymium is present at the oxidation state + IV in Pr ₆ On and that 1/2 mole of H ₂ is consumed to reduce 1 mole of Pr IV.

On the other hand, and for the whole of the description, the values which are given for the reducibility result from measurements made on compositions which have previously been calcined for 6 hours at a temperature of at least 800 ° C. in air. As indicated above, the compositions reducing oxido masses of the invention exhibit reducibility measured between 50 ⁰ C and

900 ⁰ C of at least 80%. This reducibility thus measured can be more particularly at least 85% and even more particularly at least 90%.

A more particular embodiment of the invention relates to compositions whose reducibility is important even in a low temperature range, i.e. at most 700 ° C. This embodiment corresponds to the case where the supported oxide is a mixture of a cerium oxide and a zirconium oxide, with an oxide of a rare earth other than cerium, and with a zirconium content such that the atomic ratio Ce / Zr is at least 1. In this case, these compositions may have a reducibility measured between 50 ^° C. and 700 ° C. (after also calcining for 6 hours at a temperature of at least 800 ° C.) at least 35%, more particularly at least 40%. Of course, these compositions also have the same values of reducibility measured between 50 ⁰ C and 900 ° C as those given above.

Finally, the compositions of the redox compositions of the invention may have a high BET surface area which may, after calcination for 6 hours at 800 ° C., be at least 95 m ² / g, more particularly at least 110 m ² / g, this surface depending on the nature of the support used.

The compositions which have been described above are known compositions. Reference can be made in particular to the entire description of the patent application WO 2008/025753 in which the characteristics of these compositions and their preparation process are described. It will be noted that the measurements of reducibility between 30 ⁰ C and 900 ° C described in the application WO 2008/025753 give the same results as those made between 50 ° C and 900 ⁰ C as in the present application.

The redox mass may further comprise, in addition to a composition of the type described above, at least one reducible oxide chosen from the group comprising the oxides of the elements nickel, copper, cobalt, iron and manganese, iron being preferred.

This reducible oxide may be present in an amount preferably of at most 40% by weight relative to the whole of the redox mass. This amount may be more particularly between 10% and 30%.

According to a particular embodiment and in the case where the support of the oxide of the redox mass is alumina, this reducible oxide can be present in the alumina itself and therefore it can be brought by the support to the composition of the mass. In this case, this reducible oxide may be present in a quantity by weight of between 5 and 30% relative to the alumina.

The redox mass may further comprise a binder, especially in order to increase its mechanical strength. As an example of a binder, mention may be made of alumina, aluminate spinels, titanium dioxide, silica, zirconia, kaolin and yttria.

The proportion of binder in the redox mass can vary from 10% to 95% by weight, preferably from 20% to 80% by weight, and even more particularly from 30% to 70% by weight.

The compound comprising the cerium oxide composition, the reducible oxide and the binder optionally may be obtained by simple physical mixing of these components.

The redox mass may optionally be shaped to be in the form of powder, granules, balls, cylinders or honeycombs of varying sizes. The size and shape of the redox mass are chosen according to the technological constraints of the process. In the case of a process employing a fluidized bed, it will advantageously be chosen to form in the form of micron-sized grains of between 10 and 100 μm in size. The shaping can be carried out according to any technique known to those skilled in the art.

The method of the invention may in particular be implemented using the redox mass in a circulating bed or fluidized bed or in a rotary reactor. This process can be used in the general field of energy production, (production of electricity, steam or heat) for example in gas turbines, boilers and furnaces, in the oil, chemical, glass industry and cement plant. This process can also be used to produce hydrogen with simultaneous capture of CO2 for example by combustion of methane on the reducing mass forming a gas CO, CO2, H ₂ and H ₂ O. The latter is then enriched in H ₂ , CO ₂ by a water gas shift reaction between CO and H ₂ O.

Examples will now be given.

EXAMPLE 1

This example relates to the preparation of an oxide-reductive mass based on a cerium oxide on an alumina support. A mixture containing 80% by weight of lanthanum stabilized Al 2 O 3 and 20% by weight of CeO 2 is produced by mixing in a beaker with stirring, 80 g of MI-386 alumina powder marketed by Rhodia dispersed in 500 ml of H ₂ O at pH 0.5 with a colloidal solution of CeO ₂ (pH 0.5) containing 20 g of CeO ₂ (CeO ₂ 50 g / l).

The mixture is then spray-dried on a Bϋchi 190 mini Spray Drier type apparatus, with an inlet temperature of 230 ^° C. and an outlet temperature of 115 ° C.

The powder obtained is calcined under air at 600 ^° C. for 2 hours.

The specific surface of the redox mass CeO ₂ -Al ₂ O ₃ is

161 m ² / g.

EXAMPLE 2

This example concerns the evaluation of the ability of a material to undergo oxidation-reduction cycles.

For this purpose, 200 mg of powder are heated in a ramp of 20 ° C./min up to 900 ^° C. under a 10% hydrogen reducing flux in Ar (25 ml / min).

The consumption of hydrogen in the flow is measured via a katharometer type detector.

The degree of reducibility of the capture mass is calculated by the percentage of cerium reduced under these conditions by considering the following reaction:

2 CeO ₂ + H ₂ -> Ce ₂ O ₃ + H ₂ O

This degree of reducibility or volume of hydrogen consumed reflects the ability of the material to give oxygen to a reducing agent, here hydrogen.

By way of comparison, the redox mass of Example 1 is compared with pure cerine marketed by Rhodia under the name HSA5.

This oxide was first calcined in air at 600 ° C. for 2 hours and thus developed 159 m ² / g.

The following Table 1 indicates that under these conditions, the cerine of the ceria-alumina mass according to the invention is 100% reducible at 900 ^° C. whereas the pure cerine is only reducible at 24%. . Table 1

EXAMPLE 3

This example concerns the evaluation of the ability of a material to undergo oxidation-reduction cycles and to maintain this capacity during a cycle.

For this, the same type of test is carried out as in Example 2 except that the following steps are followed:

Reduction N ° 1: we realize the first reduction under H ₂ in Ar between

50 and 700 ^° C.: the volume of hydrogen is measured and the reduction ratio (25 ml / min) is calculated.

Reoxidation: the powder is then cooled to 50 ° C., still under a flux of H ₂ 10% in Ar (25 ml / min) and then heated in a ramp of 20 ° C./min up to 700 ^° C. under a compound oxidizing stream 2.5% O ₂ in Ar (25 ml / min). The powder is again cooled to 50 ° C. still under 2.5% O ₂ flux in (25 ml / min).

Reduction No. 2: the powder is heated again at a ramp of 20 ° C./min up to 700 ^° C. under a flow of H ₂ in Ar and the consumption of hydrogen in the flow is measured again via a detector type catharometer.

The reduction rates of the first and second reductions are compared.

Table 2 below indicates that at 700 ° C. the reduction of the ceria-alumina active mass according to the invention is greater than that of pure cerine, both in terms of degree of cerium reduction and of volume. of hydrogen consumed (thus oxygen transferred). Moreover, the capacity to transfer oxygen is identical between the first and second reduction at

700 ⁰ C indicating a good stability of the capacities of the redox mass during oxidation-reduction cycling. Under the same conditions, the capacity of pure cerine fell by 22%, from 9% reduction of cerium to 7%. This example thus illustrates the better capacity of the oxide-reducing masses proposed, especially at low temperature, as well as their increased stability.

Table 2

Claims

1 - Redox oxidation-reduction process, characterized in that the oxide-reducing mass used comprises a composition based on an oxide which is a cerium oxide or a mixture of cerium oxide and at least one oxide of another element selected from zirconium and rare earths other than cerium, said oxide being supported on a support based on silica, alumina, titanium oxide or zirconia and this oxide being in the form of particles which are deposited on said support, in individualized form or in the form of aggregates, with a size of at most 500 nm, said composition having after calcination for 6 hours at a temperature of at least 800 ^° C. a reducibility measured between 50 ⁰ C and 900 ° C of at least 80%.

2. Process according to claim 1, characterized in that an oxido-reducing mass comprising the abovementioned composition in which the individualized particles or in the form of aggregates of the supported oxide have a size of at most 50 nm is used.

3- A method according to claim 1 or 2, characterized in that a redox mass comprising the aforementioned composition wherein the individualized particles or aggregates of the supported oxide have a size of at most 10 nm, more particularly at most 5 nm.

4. Method according to one of the preceding claims, characterized in that a redox mass is used in which the aforementioned composition has a reducibility of at least 85%, more particularly at least 90%.

5. Method according to one of the preceding claims, characterized in that a redox mass is used in which the supported oxide of the aforementioned composition is a mixture of cerium oxide, zirconium oxide and at least one oxide of a rare earth other than cerium, with a zirconium content such that the Ce / Zr atomic ratio is at least 1, and in that said composition has after calcination for 6 hours at a temperature of at least 800 ^° C., a reducibility measured between 50 ° C. and 700 ° C. of at least 35%, more particularly at least 40%. 6. Process according to one of the preceding claims, characterized in that a redox mass is used in which the support of the oxide of the aforementioned composition consists essentially of alumina or alumina stabilized with least one element selected from rare earths, silicon and zirconium.

7- Method according to one of the preceding claims, characterized in that a redox mass is used in which the supported oxide of the aforementioned composition is a mixture of a cerium oxide, a zirconium oxide and an oxide of a rare earth other than cerium, and in that the support of the oxide of said composition is based on alumina stabilized by a member of the group consisting of barium, strontium and lanthanum. 8- Method according to one of claims 1 to 6, characterized in that a redox mass is used in which the supported oxide of the aforementioned composition is a mixture of a cerium oxide, an oxide of praseodymium and an oxide of a rare earth other than cerium and praseodymium, and in that the support of the oxide of said composition is based on alumina stabilized by an element of the group consisting of barium, strontium and lanthanum.

9- Method according to one of the preceding claims, characterized in that a redox mass is used in which the aforementioned composition has a supported oxide proportion of at most 75% by weight, more particularly at most 50 % and even more particularly at most 30%.

10- Method according to one of the preceding claims, characterized in that the redox mass used comprises in addition at least one reducible oxide selected from the group comprising the oxides of the elements nickel, copper, cobalt, iron and manganese.