WO2014027308A1

WO2014027308A1 - Ceria-zirconia-mixed oxide particles and process for their production by pyrolysis

Info

Publication number: WO2014027308A1
Application number: PCT/IB2013/056617
Authority: WO
Inventors: Stefan Hannemann; Dirk Grossschmidt; Mirko Arnold; Gerd Grubert; Olga Gerlach; Andreas Sundermann; Stefan Kotrel
Original assignee: Basf Se; Basf (China) Company Limited; Basf Schweiz Ag
Priority date: 2012-08-14
Filing date: 2013-08-13
Publication date: 2014-02-20
Also published as: EP2885245A1; EP2885245A4; KR20150043436A; CN104718155A; JP2015526377A

Abstract

Ceria-Zirconia-Mixed oxide particles and process for their production by pyrolysis, wherein the process comprising: (1) providing a mixture comprising a solvent, one or more precursor compounds of ceria, one or more precursor compounds of zirconia, and one or more precursor compounds of one or more rare earth oxides other than ceria and/or one or more precursor compounds of yttria; (2) forming an aerosol of the mixture provided in step (1); and (3) pyrolyzing the aerosol of step (2) to obtain mixed oxide particles; wherein the content of the rare earth oxides other than ceria and/or of yttria in the mixed oxide particles formed in step (3) is comprised in the range of from 0.1 to 4.9 wt.-% based on the total weight of the rare earth oxides, yttria, and zirconia contained in the mixed oxide particles, as well as to mixed oxide particles obtainable from flame spray pyrolysis and to their use as an oxygen storage component, a catalyst and/or as a catalyst support.

Description

Ceria-Zirconia-Mixed Oxide Particles and Process for their Production by Pyrolysis

The present invention relates to a process for the production of mixed oxide particles compris- ing ceria and zirconia as well as to mixed oxide particles obtainable from the inventive process and in particular from flame spray pyrolysis. Furthermore, the present invention relates to the use of mixed oxide particles obtainable according to the inventive process.

INTRODUCTION

In the field of exhaust gas treatment and in particular in methods for the combustive or oxidative treatment thereof employing oxygen storage components (OSC), cerium and zirconium containing mixed oxides have found use therein, in particular as OSC component in automotive cata- lysts. As regards the methods for their production, a variety of processes have been employed such as solid state synthesis (e.g. ceramic method and mechanical grinding), liquid to solid synthesis (e.g. precursor method), various precipitation methods, hydrothermal and solvothermal synthesis, sol-gel methods, emulsion and microemulsion methods, impregnation methods, as well as gas to solid synthesis (e.g. chemical vapor deposition).

In practice, co-precipitation methods have found wide use. In these cases, from a solution of salts of the desired products the oxides are precipitated with the aid of precipitation or flocculating agents. In these methods, the solubility of the individual compounds plays an important role. In particular, the solubility of the individual compounds must be very similar for avoiding a sepa- ration of phases. As a result of this, the use of said method with mixtures consisting of more than two cations becomes difficult. A further disadvantage in these methods is the formation of large amounts of salt containing waste materials during the production of such mixed oxide particles. In addition to this, co-precipitation methods necessitate a considerable number of workup steps including washing, filtration, drying, and calcination.

Various studies have shown that flame spray synthesis (FSS) and in particular flame spray pyrolysis (FSP) are suitable for producing oxygen storage materials displaying an improved thermal stability. Thus, Stark et al. in Chem. Comm. 2003, pp. 588-589 were able to produce ceria- zirconia mixed oxides with high surface areas having satisfactory oxygen storage characteristics using such spray flame synthetic methods. In Stark et al. in Chem. Mater. 2005, vol. 17, pp. 3352-3358 ceria-zirconia mixed oxides were produced via flame synthesis for attempting to obtain materials having an improved oxygen storage capacity at lower temperatures. Furthermore, Stark ef al. in Chem. Mater. 2005, vol. 17, pp. 3352-3358 concerns the flame synthesis of ceria- zirconia mixed oxides further including platinum which may be obtained using a single step of flame synthesis. In analogy to the co-precipitation methods, flame spray synthesis has also been used for producing ceria-zirconia mixed oxides containing further additives such as e.g. silica and alumina. Thus, in Schuiz et al. in J. Mater. Chem. 2003, vol. 13, pp. 2979-2984 it was found that smaller amounts of silica were able to improve the oxygen storage capacity whereas alumina apparent- ly had no effect thereon. Larger amounts of additives were found to be disadvantageous since these led to the formation of layers which would inhibit the oxygen exchange in the particles.

Jossen et al. Chem. Vap. Deposition 2006, vol. 12, pp. 614-619 investigated the thermal stability of ceria-zirconia mixed oxides using flame spray synthesis. In particular, it was found that ceria-zirconia mixed oxides having a cerium content of 35 wt.-% allowed for a production of particles with a high surface area which show an increased resistance to thermal aging. Furthermore, it was found that the addition of aluminum oxide and lanthanum oxide was able to improve the thermal stability. In this respect, the optimum as regards the stabilization effect was found for a mixed oxide consisting of 10 wt.-% lanthanum, 25 wt.-% cerium, and 65 wt.-% zirco- nium based on the total weight of the rare earth oxides and zirconium oxides.

Wang et al. in Journal of Molecular Catalysis A: Chemical 2011 , vol. 339, pp. 52-60 relates to co-precipitated ceria-zirconia mixed oxides of the formula Ceo.2Zro.8O2 containing 5 wt.-% of rare earth elements such as lanthanum, neodymium, praseodymium, samarium, and yttrium. Wang ef al. in Environmental Science and Technology 2010, vol. 44, pp. 3870-3875 concerns ceria- zirconia mixed oxides of the formula Ceo.2Zro.8O2 modified with rare earth elements and in particular with lanthanum, neodymium, praseodymium, samarium, and yttrium as well as their use in a three way catalyst for the treatment of automotive exhaust gases, wherein the rare earth containing ceria-zirconia mixed oxide is obtained by a co-precipitation method. In particular, it was respectively found in Wang et al. that the addition of 5 wt.-% of rare earth oxides to Ceo.2Zro.8O2 had the effect of improving the thermostability as well as the oxygen storage capacity of the resulting material. In particular, the addition of lanthanum, neodymium, and praseodymium showed a clear improvement in comparison with the pure ceria-zirconia mixed oxides. These materials, however, were not produced using flame spray synthesis but rather using a co-precipitation method. Li ef al. in Journal of Rare Earths 2011 , vol. 29, no. 6, pp. 544-549, relate to a ceria-zirconia mixed oxide Ceo.8Zro.2O2 containing 5 wt.-% lanthana and which has been produced by a co-precipitation method. Cao ef al. in Materials Letters 2008, vol. 62, pp. 2667-2669 on the other hand concerns an oxide ceramic material of the formula La2(Ceo.7Zro.3)207 which is obtained from solid state synthesis.

US 201 1/0281 1 12 A1 relates to a method of producing ceria using flame spray pyrolysis.

Stark ef al. in Chemical Communications 2003, pp. 588-589, WO 2004/103900 A1 , and WO 2004/005184 A1 respectively relate to the production of ceria-zirconia mixed oxides using flame spray pyrolysis methods. EP 1 378 489 A1 concerns a method for the production of mixed metal oxides from flame synthesis and in particular to ceria-zirconia mixed oxides having high zirconium levels. US 7,220,398 B2 concerns ceria-zirconia mixed oxide including alumina which are formed via flame spray pyrolysis, wherein the particles consist of gamma-alumina onto which a solid solution of ceria and zirconia segregate.

Although improvements have been made relative to the methods of obtaining mixed oxide materials containing various additives in addition to the main components and in particular ceria and zirconia, there remains an ongoing need for high performance oxygen storage components which may be produced in a highly efficient and thus cost effective manner for providing cost effective materials. This applies in particular with respect to oxygen storage component materials employed in automotive catalysts which to a large extent is motivated by the costs of the precursor materials and in particular of additives employed in ceria-zirconia mixed oxides for improving their properties. Thus, although improvements have been achieved in view of the flame spray pyrolysis methods which may be performed in a single step, there remains the problem that such methods nevertheless involve the use of larger amounts of precursor materials and in particular of ceria and other rare earth compounds as additive materials for providing the desired performance in oxygen storage capacity in the resulting materials.

DETAILED DESCRIPTION

It is therefore the object of the present invention to provide an improved process for the production of ceria-zirconia mixed oxides. In particular, it is the aim of the present invention to provide a ceria-zirconia mixed oxide material having an excellent oxygen storage capacity and aging resistance in particular relative to the amount of the costly precursor compounds ceria and further additives including rare earth oxides other than ceria for providing the desired features and performance of the oxygen storage materials.

Thus, it has quite surprisingly been found that ceria-zirconia mixed oxide particles containing a very low amount of one or more further rare earth metals other than ceria and/or containing a very low amount of yttria may be obtained from flame synthesis which offer an unexpectedly high performance based on the amount of said rare earth elements other than ceria and/or of yttria contained therein. More specifically, it has quite unexpectedly been found that relative to the oxygen storage capacity of such materials based on the amounts of rare earth metal oxide other than ceria and/or of yttria contained therein considerably less of said materials are necessary for achieving a comparable of even an improved performance compared to ceria-zirconia based mixed oxide materials known in the art.

Therefore, the present invention relates to a process for the production of mixed oxide particles comprising: (1 ) providing a mixture comprising a solvent, one or more precursor compounds of ceria, one or more precursor compounds of zirconia, and one or more precursor compounds of one or more rare earth oxides other than ceria and/or one or more precursor compounds of yttria;

(2) forming an aerosol of the mixture provided in step (1 ); and

(3) pyrolyzing the aerosol of step (2), preferably in an atmosphere containing oxygen, to obtain mixed oxide particles; wherein the content of the rare earth oxides other than ceria and/or of yttria in the mixed oxide particles formed in step (3) is in the range of from 0.1 to 4.9 wt.-% based on the total weight of the rare earth oxides, yttria, and zirconia contained in the mixed oxide particles, preferably of from 0.3 to 4.5 wt.-%, more preferably of from 0.5 to 4 wt.-%, more preferably of from 0.7 to 3.5 wt.-%, more preferably of from 0.8 to 3 wt.-%, more preferably of from 0.9 to 2.5 wt.-%, and even more preferably of from 1 to 2 wt.-%.

As regards the provision of the mixture in step (1 ) of the inventive process, there is no particular restriction as to the means which are employed for forming a mixture provided that a homoge- nous mixture may be provided. Thus, according to the present invention, in instances wherein one or more of the precursor compounds is at least in part insoluble in the solvent provided for forming the mixture, it is preferred that means of homogenizing the mixture are employed for achieving a high dispersion of said one or more precursor compounds therein. Thus, by way of example, in such cases the homogenous mixture may be provided by appropriate means of agitation such as by stirring, shaking, rotating, and sonication, wherein preferably the mixture is provided by appropriate stirring of the one or more precursors in the solution for providing a high dispersion thereof. According to the present invention, however, it is preferred that the one or more precursor compounds provided in step (1 ) are respectively soluble in the solvent which is provided such that a homogenous mixture is provided by dissolution of all components in the solvent.

Concerning the formation of an aerosol in step (2), on the other hand, there is again no particular restriction according to the present invention as to the means which may be employed for forming such an aerosol provided that it may be pyrolysed in step (3) of the inventive process. Thus, by way of example, the aerosol may be formed by any appropriate means for dispersing the mixture provided in step (1 ) in a gaseous medium such as by spraying the mixture provided in step (1 ) into said medium. According to a preferred embodiment of the present invention, the mixture provided in step (1 ) is sprayed into a gas stream for obtaining a stream of said aerosol which may then be conducted into a pyrolysing zone for achieving step (3) of the inventive pro- cess. As regards the step of pyrolysing of the aerosol provided in step (2) of the inventive process, there is again no particular restriction as to the method which is employed for achieving said pyrolysis, provided that at least a portion of the aerosol is converted to mixed oxide particles as a result of said thermal treatment. Thus, be way of example, the pyrolysis in step (3) may be achieved with the aid of any suitable heat source of which the temperature is sufficient for pyrolysing at least a portion of the aerosol provided in step (2). According to the present invention, the process for the production of mixed oxide particles is conducted in a continuous mode, wherein the aerosol according to particular and preferred embodiments of the present invention is provided as a gas stream which is allowed to pass a pyrolysing zone for obtaining mixed ox- ide particles from at least a portion of said aerosol in the gas stream exiting the pyrolysing zone. According to said preferred embodiments of the present invention wherein the pyrolysis is conducted in a continuous mode, there is no particular restriction as to the weight hourly space velocity of the aerosol gas stream which is conducted to the pyroylsing zone, nor is there any restriction as to the extent of the pyrolysing zone provided that the weight hourly space velocity is chosen such depending on the extent of the pyrolysing zone at least a portion of the aerosol may be pyrolysed in step (3) for obtaining mixed oxide particles.

As regards the gas in which the aerosol is formed in step (2) of the inventive process, there is again no particular restriction regarding its composition such that it may contain one type of gas or several different types of gases. Accordingly, the gas employed for providing the aerosol in step (2) may consist of one or more inert gases, wherein according to the present invention said one or more inert gases do not react under the conditions of pyrolysis in step (3) of the inventive process. According to the present invention, it is however preferred that at least a portion of the gas employed for forming an aerosol in step (2) is a gas which reacts with at least a portion of the mixture provided in step (1 ), wherein it is further preferred that said gas has an oxidizing effect on the mixture provided in step (1 ), in particular during pyrolysis of the mixture in step (3). According to said embodiments wherein at least a portion of the gas contained in the aerosol formed in step (2) acts as an oxidizing agent towards the mixture in step (1 ), there is no particular restriction as to the type of gas which may be used to this effect provided that it may oxidize at least a portion of the mixture provided in step (1 ). According to said preferred embodiments of the present invention it is further preferred that the portion of the gas contained in the aerosol provided in step (2) which has an oxidizing effect on the mixture provided in step (1 ) reacts with at least a portion of the mixture during pyrolysis in step (3), wherein said reaction is exothermic for providing at least a portion of the heat source required in step (3) for the pyrolysis of the mix- ture provided in step (1 ). As regards the type of gas which may be used according to said particularly preferred embodiments, there is again no particular restriction provided that it may react with at least a portion of the mixture provided in step (1 ) in an exothermic fashion for providing at least part of the heat necessary for the pyrolysis in step (3). According to particularly preferred embodiments of the present invention, the oxidizing gas comprised in the aerosol in step (2) comprises oxygen, wherein more preferably the oxidizing gas contained in the aerosol of step (2) is oxygen. As regards the one or more precursor compounds of zirconia provided in step (1 ) it is noted that within the meaning of the present invention, the term "zirconia" designates zirconia, hafnia, and mixtures thereof. Concerning the mixture provided in step (1 ) of the inventive process, there is no particular restriction as to the amounts of the solvent, nor with respect to the amount of the one or more precursor compounds of ceria, zirconia, or of the one or more rare earth oxides other than ceria and/or of yttria, provided that depending on the specific parameters and conditions which are employed in the steps of providing the mixture in step (1 ), of forming the aerosol from said mix- ture in step (2), and of pyrolysing the aerosol in step (3), at least a portion of the mixed oxide particles formed in step (3) requires rare earth oxides other than ceria and/or yttria in an amount in the range of from 0.1 to 4.9 wt.-% based on the total weight of the rare earth oxides, yttria, and zirconia contained in the mixed oxide particles. According to a particular meaning of the present invention, at least a portion of the mixed oxide particles formed in step (3) contain the rare earth oxides other than ceria and/or yttria in an amount comprised in the range of from 0.1 to 4.9 wt.-% based on the total weight of the rare earth oxides, yttria, and zirconia contained in the mixed oxide particles when at least part of the individual mixed oxide particles formed in step (3) fulfill this definition, wherein preferably at least 50 % of the individual mixed oxide particles formed in step (3) contain the one or more rare earth oxides other than ceria and/or yttria in an amount comprised in the range of from 0.1 to 4.9 wt.-% based on the total weight of the rare earth oxides, yttria, and zirconia contained in the mixed particles, more preferably 60 wt.-% or more of the mixed oxide particles formed in step (3), more preferably 70 wt.-% or more, more preferably 80 wt.-% or more, more preferably 90 wt.-% or more, more preferably 95 wt.-% or more, more preferably 98 wt.-% or more, more preferably 99 wt.-% or more, and more prefera- bly 99.5 wt.-% or more. According to particularly preferred embodiments of the present invention, 99.9 wt.-% or more of the mixed oxide particles formed in step (3) contain one or more of the rare earth oxides other than ceria and/or yttria in an amount comprised in the range of from 0.1 to 4,9 wt.-% based on the total weight of the rare earth oxides and zirconia contained in the mixed oxide particles, wherein said content of rare earth oxides other than ceria refers to the content in the individual particles of the mixed oxide. Same applies accordingly relative to the further preferred embodiments of the present invention wherein the content of the rare earth oxides other than ceria in the mixed oxide particles formed in step (3) is comprised in the range of from 0.3 to 4.5 wt.-%, more preferably of from 0.5 to 4 wt.-%, more preferably of from 0.7 to 3.5 wt.-%, more preferably of from 0.8 to 3 wt.-%, more preferably of from 0.9 to 2.5 wt.-%, and even more preferably of from 1 to 2 wt.-%.

As regards the one or more rare earth oxides other than ceria provided in step (1 ) of the inventive process, there is no particular restriction according to the present invention neither with respect to the type nor with respect to the number of the one or more precursor compounds of one or more rare earth oxides other than ceria which may be provided. According to the present invention it is however preferred that said one or more rare earth oxides other than ceria comprise one or more of lanthana, praseodymia, and neodymia, including mixtures of two or three thereof, wherein it is further preferred that the one or more rare earth oxides other than ceria comprise lanthana and/or neodymia. According to particularly preferred embodiments of the present invention, the one or more rare earth oxides other than ceria provided in step (1 ) of the inventive process include lanthana, wherein even more preferably the rare earth oxide other than ceria is lanthana. According to the present invention, unless otherwise specified, the designation of the rare earth oxides does not refer to a particular type thereof, in particular relative to the oxidation state of the rare earth metal, such that in principle any one or more rare earth oxides may be designated. Thus, by way of example, unless otherwise specified, the term "ceria" principally refers to the compounds Ce02, Ce203, and any mixtures of the aforementioned compounds. According to a preferred meaning of the present invention, however, the term "ceria" designates the compound Ce02. Same applies accordingly relative to the term "praseo- dymia" such that in general said term designates any one of the compounds Pr203, PreOn, Pr02, and any mixtures of two or more thereof. According to a preferred meaning of the present invention, the term "praseodymia" designates the compound Pr203.

Therefore, according to preferred embodiments of the inventive process, the one or more rare earth oxides other than ceria is selected from the group consisting of lanthana, praseodymia, neodymia, and mixtures of two or three thereof, wherein the one or more rare earth oxides preferably comprises lanthana and/or neodymia, preferably lanthana, wherein even more preferably the rare earth oxide other than ceria is lanthana.

Concerning the concentration of the one or more precursor compounds of the one or more rare earth oxides other than ceria, there is again no particular restriction according to the present invention as to the amounts in which said compounds may be provided in step (1 ) of the in- ventive process provided that depending on the specific means for its execution and the parameters chosen therein the content of the rare earth oxides other than ceria and/or of yttria in the mixed oxide particles formed in step (3) is comprised in the range of from 0.1 to 4.9 wt.-% based on the total weight of the rare earth oxides, yttria, and zirconia contained in the mixed oxide particles. Same applies accordingly with respect to the one or more precursor compounds of yttria according to embodiments containing the same. Thus by way of example, the concentration of the one or more precursor compounds of the one or more rare earth oxides other than ceria and/or of the one or more precursor compounds of yttria calculated as their respective oxides contained in the mixture provided in step (1 ) may be comprised in the range of anywhere from 0.01 to 5 wt.-% based on the total weight of the mixture provided in step (1 ), wherein pref- erably the concentration thereof is comprised in the range of from 0.05 to 2 wt.-%, more preferably of from 0.1 to 1 .5 wt.-%, more preferably of from 0.3 to 1 .2 wt.-%, more preferably of from 0.5 to 1 wt.-%, and more preferably of from 0.7 to 0.9 wt.-%. According to particularly preferred embodiments of the present invention, the concentration of the one or more precursor compounds of the one or more rare earth oxides other than ceria and/or of the one or more precur- sor compounds of yttria calculated as their respective oxides is comprised in the range of from 0.75 to 0.85 wt.-% based on the total weight of the mixture provided in step (1 ). As concerns the solvent provided in step (1 ) of the inventive process, there is again no particular restriction neither with respect to the composition nor with respect to the amount of said solvent provided that the content of the rare earth oxides other than ceria and/or of yttria in the mixed oxide particles formed in step (3) is comprised in the range of from 0.1 to 4.9 wt.-%. Thus, by way of example, the solvent provided in step (1 ) may comprise one or more compounds such as those selected from the group consisting of aliphatic and aromatic hydrocarbons, alcohols, heterocyclic compounds, carboxylic acids, water, and mixtures of any two or more thereof. According to preferred embodiments, the solvent provided in step (1 ) of the inventive process comprises one or more selected from the group consisting of aromatic hydro- carbons, N-containing heterocycles, tetrahydrofurane, (C5-Cio)hydrocarbons, (Ci-C5)alcohols, (Ci-C8)carboxylic acids, water, and combinations of two or more thereof, more preferably from the group consisting of (C6-C12) aromatic hydrocarbons, pyrrolidine, pyrrole, piperidine, pyridine, azepane, azepine, tetrahydrofurane, (C5-C7)hydrocarbons, (Ci-C3)alcohols, (C2-C8)carboxylic acids, water, and combinations of two or more thereof, more preferably from the group consist- ing of Cs aromatic hydrocarbons, pyrrolidine, pyrrole, piperidine, pyridine, tetrahydrofurane, pen- tane, hexane, ethanol, methanol, propanol, (C6-C8)carboxylic acids, acetic acid, propionic acid, water, and combinations of two or more thereof, more preferably from the group consisting of toluene, ethylbenzene, xylene, hexane, propanol, acetic acid, Cs-carboxylic acids, propionic acid, water, and combinations of two or more thereof, and even more preferably from the group consisting of xylene, hexane, n-propanol, acetic acid, 2-ethylhexanoic acid, water, and combinations of two or more thereof.

According to the present invention, it is not necessary that the solvent provided in step (1 ) be in the liquid state at room temperature. Within the meaning of the present invention, the term "room temperature" refers to a temperature of 25°C. Thus, according to particular embodiments of the present invention, the solvent provided in step (1 ) is not in a liquid state but rather in a solid or semi-solid state at room temperature and the mixture provided in step (1 ) is employed in the inventive process at a temperature greater than room temperature for forming an aerosol in step (2). According to said alternatively preferred embodiments, the solvent provided in step (1 ) therefore comprises one or more compounds having a melting point above room temperature, wherein said one or more compounds may accordingly be selected from the group consisting of higher molecular weight aliphatic and aromatic hydrocarbons, alcohols, heterocyclic compounds, carboxylic acids, and mixtures of two or more thereof, wherein said compounds have melting points above room temperature, respectively. According to the present invention, said one or more higher molecular weight compounds having melting points above room temperature may be comprised in the solvent provided in step (1 ) together with one or more compounds having a melting point at and/or below room temperature. According to particular embodiments of the present invention, however, the solvent provided in step (1 ) substantially consists of one or more compounds having a melting point above room temperature, wherein according to pre- ferredn said one or more compounds are preferably selected from the group consisting of aliphatic and aromatic hydrocarbons, alcohols, heterocyclic compounds, carboxylic acids, and mixtures of two or more thereof, and more preferably from the group consisting of aliphatic hydrocarbons, alcohols, carboxylic acids, and mixtures of two or more thereof.

According to particularly preferred embodiments of the present invention, the solvent provided in step (1 ) comprises xylene. According to an alternative embodiment of the present invention which is particularly preferred, the solvent provided in step (1 ) comprises a mixture of acetic acid and water.

As regards the aromatic hydrocarbons preferably comprised in the solvent in step (1 ) of the in- ventive process, there is again no particular restriction relative to the particular type or types of aromatic hydrocarbons which may be employed in the inventive process provided that the content of the rare earth oxides other than ceria and/or of yttria in the mixed oxide particles formed in step (3) is comprised in the range of 0.1 to 4.9 wt.-% based on the total weight of the rare earth oxides, yttria and zirconia contained in the mixed oxide particles. Thus, by way of exam- pie, the aromatic hydrocarbons may be selected from the group consisting of (C6-Ci2)hydrocarbons, including any mixtures of two or more thereof, wherein it is however preferred that the aromatic hydrocarbons comprise one or more (C7-Cn)hydrocarbons, more preferably of (C8-Cio)hydrocarbons, more preferably of (Cs-C^hydrocarbons, and even more preferably of Ce-hydrocarbons, wherein even more preferably the solvent comprises one or more ar- omatic hydrocarbons selected from the group consisting of toluene, ethylbenzene, xylene, me- sitylene, durene, and mixtures of two or more thereof, more preferably from the group consisting of toluene, ethylbenzene, xylene, and mixtures of two or more thereof, wherein even more preferably the solvent comprises toluene and/or xylene, preferably xylene. Same applies accordingly relative to the aliphatic hydrocarbons preferably comprised in the solvent provided in step (1 ) of the inventive process. Thus, as regards the aliphatic hydrocarbons which are preferably comprised in the mixture provided in step (1 ) of the inventive process, these may in principle be any one or more branched or unbranched aliphatic hydrocarbons or any conceivable mixture of branched and/or unbranched hydrocarbons, wherein the aliphatic hydrocarbons are preferably unbranched. According to said preferred embodiments, it is further preferred that the aliphatic hydrocarbons comprise one or more hydrocarbons selected from the group consisting of unbranched (C4-Ci2)hydrocarbons, preferably of (C5-Cio)hydrocarbons, more preferably of (C6-C8)hydrocarbons, more preferably of (C6-C7)hydrocarbons, and even more preferably of branched and/or unbranched, preferably unbranched C6-hydrocarbons, wherein even more preferably the solvent comprises one or more aliphatic hydrocarbons selected from the group consisting of pentane, hexane, heptane, octane, and mixtures of two or more thereof, wherein even more preferably the aliphatic hydrocarbons comprise pentane and/or hexane, preferably hexane. As for the hydrocarbons which are preferably comprised in the mixture provided in step (1 ) of the inventive process, there is also no particular restriction which would apply relative to the carboxylic acids comprised in the mixture provided in step (1 ) according to preferred embodi- ments of the inventive process, provided again that the content of the rare earth oxides other than ceria and/or of yttria in the mixed oxide particles formed in step (3) as comprised in the range of from 0.1 to 4.9 wt.-% based on the total weight of the rare earth oxides, yttria and zir- conia contained in the mixed oxide particles. Thus, by way of example, the one or more carbox- ylic acids may be selected from the group consisting of (Ci-C8)carboxylic acids, wherein preferably the one or more carboxylic acids are selected from the group consisting of (Ci- C6)carboxylic acids, more preferably from the group consisting of (C1-C5) carboxylic acids, more preferably from the group consisting of formic acid, acetic acid, propionic acid, butyric acid, and mixtures of two or more thereof, more preferably from the group consisting of acetic acid, propi- onic acid, butyric acid, 2-ethylhxanoic acid and mixtures of two or more thereof, wherein more preferably the carboxylic acid comprises acetic acid and/or propionic acid, preferably acetic acid.

As regards the one or more precursor compounds of ceria comprised in the mixture provided in step (1 ) of the inventive process, there is no particular restriction neither with respect to the particular type or number of precursor compounds which may be employed nor with respect to the amount in which they may be provided in the mixture provided that depending on the further components provided in the mixture and the specific means of executing steps (1 ), (2), and (3) of the inventive process affords mixed oxide particles in step (3) of which the content of rare earth oxides other than ceria and/or of yttria is comprised in the range of from 0.1 -4.9 wt.-% based on the total weight of the rare earth oxides, yttria and zirconia contained in the mixed oxide particles. The same applies accordingly with respect to the one or more precursor compounds of zirconia as well as with respect to the one or more precursor compounds of the one or more rare earth oxides other than ceria and with respect to the one or more precursor com- pounds of yttria. Thus, as regards the one or more precursor compounds of ceria and/or of the rare earth oxides other than ceria and/or of yttria comprised in the mixture provided in step (1 ), any one or more of said precursor compounds may be provided in any suitable form provided that their interaction with the solvent and/or with the further components of the mixture in step (1 ) as well as the specific methods for forming an aerosol employed in step (2) and the pyrolysis of the aerosol in step (3) allows for the formation of mixed oxide particles in step (3) of which the content of the rare earth oxides other than ceria and/or of yttria is comprised in the range of from 0.1 to 4.9 wt.-% based on the total weight of the rare earth oxides, yttria, and zirconia contained in the mixed oxide particles. Thus, by way of example, the one or more precursor compounds of ceria and/or of the rare earth oxides other than ceria and/or of yttria may be any suit- able compound of the rare earth metals or yttrium, wherein it is preferred that one or more salts of the rare earth metals and/or of yttrium be provided in step (1 ) of the inventive process. As regards the preferred salts of the rare earth metals and/or of yttrium, any conceivable salts may again be employed, wherein it is preferred that the one or more salts may completely dissolve in the solvent provided in step (1 ), wherein the type of salt chosen may accordingly depend on the type and amount of salts chosen for the other precursor compounds provided in step (1 ) and in particular on the type of solvent and the amount thereof provided in the mixture. Thus, according to the present invention, it is particularly preferred that the one or more precursor com- pounds of ceria and/or of the rare earth oxides other than ceria and/or of yttria comprise one or more salts selected from the group consisting of carboxylates, nitrates, carbonates, alcoholates, and chelating ligand containing complexes, wherein the carboxylates are preferably selected from (C4-Ci2)carboxylates, more preferably from (C5-Cn)carboxylates, more preferably from (Ce- Cio)carboxylates, more preferably from (C7-Cg)carboxylates, more preferably from C&- carboxylates, more preferably from branched Cs-carboxylate, wherein more preferably the one or more precursor compounds comprise a 2-ethylhexanoate salt, and wherein even more preferably the one or more precursor compounds of ceria and/or of the rare earth oxides other than ceria, and preferably of all of the precursor compounds of the rare earth oxides provided in step (1 ) are 2-ethylhexanoate salts.

According to the present invention, it is further preferred that according to the preferred embodiments, wherein the one or more precursor compounds comprise one or more salts, that said salts do not lower the solubility of the one or more further precursor compounds as a result of the specific type of salt which is used. Furthermore, it is preferred that the salts which are preferably used as the one or more precursor compounds do not have a negative impact on the apparatus which is used and in particular does not generate reactive side products which may damage said apparatus, e.g., by corrosion thereof. Accordingly, it is preferred according to the present invention that the mixture provided in step (1 ) does not contain any halides and in par- ticular does not contain any fluorides, chlorides, and/or bromides and even more preferably does not contain any fluorides and/or chlorides. Within the meaning of the present invention, the mixture provided in step (1 ) does not contain any halides when no substantial amount of a hal- ide-containing salt is present in the mixture provided in step (1 ), wherein the term "substantial" as employed for example in the terms "substantially not", or "not any substantial amount of" within the meaning of the present invention respectively refer to there practically being not any amount of said component in the mixture provided in step (1 ) and/or in the aerosol formed in step (2) of the inventive process, wherein preferably 0.1 wt.-% or less of said one or more components is contained therein based on the total weight of the mixture and/or of the liquids and/or solids contained in the aerosol, preferably an amount of 0.05 wt.-% or less, more preferably of 0.001 wt.-% or less, more preferably of 0.0005 wt.-% or less, and even more preferably of 0.0001 wt.-% or less.

According to preferred embodiments of the present invention, wherein one or more chelating ligand-containing complexes are comprised as the one or more precursor compounds of ceria and/or of the rare earth oxides other than ceria and/or of yttria in the mixture provided in step (1 ), there is in principle no particular restriction as to the type or number of chelating ligand- containing complexes which may be comprised in said mixture. Thus, there is no particular restriction according to the present invention as to the type of the one or more chelating ligands such that said ligands may for example be selected from the group consisting of bi-, tri-, tetra-, penta-, and hexadentate ligands. According to preferred embodiments of the present invention, the chelating ligand-containing complexes comprise one or more chelating ligands selected from the group consisting of oxalate, ethylenediamine, 2,2'-bipyridine, 1 ,10-phenanthroline, acetylacetonate, 2,2,2-crypt, diethylenetriamine, dimethylglyoximate, EDTA, ethylenediaminetri- acetate, glycinate, triethylenetetramine, tris(2-aminoethyl)amine, and combinations of two or more thereof, more preferably from the group consisting of oxalate, ethylenediamine, acetylacetonate, diethylenetriamine, dimethylglyoximate, EDTA, ethylenediaminetriacetate, glycinate, triethylenetetramine, tris(2-aminoethyl)amine, and combinations of two or more thereof, more preferably from the group consisting of oxalate, ethylenediamine, acetylacetonate, diethylenetriamine, EDTA, ethylenediaminetriacetate, triethylenetetramine, and combinations of two or more thereof, wherein even more preferably the chelating ligand containing complexes comprise acetylacetonate.

As regards the concentration of the one or more precursor compounds of ceria which may be contained in the mixture provided in step (1 ), as for the one or more precursor compounds of the one or more rare earth oxides other than ceria and/or of the one or more precursor compounds of yttria, there is again no particular restriction in this respect provided that depending on the type and amount of the other components provided in the mixture of step (1 ) and the specific steps and parameters chosen in steps (2) and (3) of the inventive process allow for the generation of mixed oxide particles of which the content of the rare earth oxides other than ceria and/or of yttria is comprised in the range of from 0.1 to 4.9 wt.-% based on the total weight of the rare earth oxides, yttria, and zirconia contained in the mixed oxide particles. Thus, by way of example, the concentration of the one or more precursor compounds of ceria calculated as Ce02 contained in the mixture contained in step (1 ) may be comprised anywhere in the range of from 0.1 to 15 wt.-% based on the total weight of the mixture provided in step (1 ), wherein preferably the concentration of the one or more precursor compounds of ceria is comprised in the range of from 0.5 to 10 wt.-%, more preferably of from 1 to 7 wt.-%, more preferably of from 2 to 5 wt.-%, more preferably of from 2.5 to 4 wt.-%, and more preferably of from 2.7 to 3.5 wt.-%. According to particularly preferred embodiments of the present invention, the concentration of the one or more precursor compounds of ceria calculated as Ce02 contained in the mixture provided in step (1 ) is comprised in the range of from 3 to 3.2 wt.-%. As regards the one or more precursor compounds of zirconia provided in step (1 ) of the inventive process, as for the one or more precursor compounds of ceria or of the other precursor compounds of the one or more rare earth oxides other than ceria and/or of the one or more precursor compounds of yttria, again no particular restrictions apply in this respect for the same reasons as mentioned in the foregoing relative to the other components of the mixture in step (1 ). Thus, by way of example, the concentration of the one or more precursor compounds of zirconia calculated as Zr02 contained in the mixture provided in step (1 ) may be comprised in the range of anywhere from 0.1 to 15 wt.-% based on the total weight of the mixture provided in step (1 ), wherein preferably the concentration of the one or more precursor compounds of zirconia is comprised in the range of from 0.5 to 10 wt.-%, more preferably of from 1 to 7 wt.-%, more preferably of from 2 to 5 wt.-%, more preferably of from 2.5 to 4 wt.-%, and more preferably of from 2.7 to 3.5 wt.-%. According to particularly preferred embodiments of the present in- vention, the concentration of the one or more precursor compounds of zirconia contained in the mixture provided in step (1 ) is comprised in the range of from 3 to 3.2 wt.-%.

Regarding the specific type of precursor compounds which may be employed as the one or more precursor compounds of zirconia, as for the one or more precursor compounds of ceria and of the one or more rare earth oxides other than ceria and/or of yttria, in principle any conceivable precursor compound or compounds of zirconium may be employed, wherein again one or more salts are preferably employed as the one or more precursor compounds for the same reasons as given above with respect to the further one or more precursor compounds contained in the mixture according to step (1 ) of the inventive process. Thus, by way of example, the one or more salts of zirconium preferably comprised in the mixture of step (1 ) preferably comprise one or more salts selected from the group consisting of halides, carboxylates, nitrates, carbonates, alcoholates, and chelating ligand containing complexes, preferably diketone ligand containing complexes, and more preferably acetylacetonate complexes, wherein the alco- holates are preferably selected from (C2-C5) alcoholates, more preferably from (C3-C4) alcoholates, and more preferably from C3-alcoholat.es, wherein more preferably the one or more precursor compounds comprise zirconium(IV)-propoxide, and wherein even more preferably the one or more precursor compounds of zirconia is zirconium(IV)-propoxide. Concerning the zirconium salts which may be employed in the inventive process which do not form a complex with the counter-ion, it is preferred according to the present invention that said zirconium salts contain the zirconyl cation, wherein according to particularly preferred embodiments the one or more precursor compounds of zirconia comprise one or more zirconyl salts, preferably one or more ziconyl halides, more preferably zirconyl bromide and/or zirconyl halide, and more preferably zirconyl chloride.

Concerning the mixture provided in step (1 ) of the inventive process, there is also no particular restriction which would apply relative to any further compounds which may be contained therein, provided that mixed oxide particles according to any of the particular or preferred embodiments of the present invention may be formed in step (3). Thus, any suitable auxiliary agent may fur- ther be comprised in the mixture of step (1 ) and/or any further compound or compounds may be provided therein for incorporation into the mixed oxide particles formed in step (3) of the inventive process. In this respect, it is particularly preferred that one or more transition metal- containing compounds be provided in step (1 ) as precursor compounds for the incorporation of said one or more transition metals into the mixed oxide particles generated in step (3) of the inventive process. According to particularly preferred embodiments, one or more platinum group metals are included in the mixture of step (1 ) for incorporation thereof in the metal oxide particles resulting from the inventive process. According to the present invention, it is further preferred that the one or more platinum group metals are preferably selected from the group consisting of Ru, Rh, Pd, Os, Ir, Pt, and mixtures of two or more thereof, more preferably from the group consisting of Rh, Pd, Pt, and mixtures of two or more thereof, wherein more preferably the platinum group metal is Pd and/or Pt, preferably Pd. Regarding the preferred embodiments of the inventive process, wherein one or more transition metals and in particular one or more platinum group metals is further added to the mixture provided in step (1 ) of the inventive process, there is in principle no particular restriction as to the amounts in which said one or more metals may be added thereto, provided that mixed oxide particles according to particular and/or preferred embodiments of the present invention may be formed in step (3) of the inventive process, in particular with respect to the content of the rare earth oxides other than ceria and/or of yttria in the mixed oxide particles formed in step (3). Thus, by way of example, the one or more transition metals and in particular the one or more platinum group metals may be included in the mixture provided in step (1 ) in an amount ranging anywhere from 0.01 to 15 wt.-% calculated as the metal based on the total weight of the mixture provided in step (1 ), wherein preferably the amount thereof is comprised in the range of from 0.05 to 14 wt.-%, more preferably of from 0.1 to 13 wt.-%, more preferably of from 0.5 to 12 wt.-%, more preferably of from 2 to 10 wt.-%, more preferably of from 3 to 9 wt.-%, and more preferably of from 4 to 8 wt.-%. According to particularly preferred embodiments of the present invention, the mixture provided in step (1 ) comprises the one or more transition metals and in particular the one or more platinum group metals in an amount ranging from 5 to 7 wt.-%. According to alternatively preferred embodiments, on the other hand, the preferred amount of the one or more transition metals and in particular the one or more platinum group metals comprised in the mixture provided in step (1 ) comprised in the range of from 0.01 to 6 wt.-% calcu- lated as the metal based on the total weight of the mixture provided in step (1 ), and preferably in the range of from 0.05 to 4 wt.-%, more preferably of from 0.08 to 3 wt.-%, more preferably of from 0.09 to 2.5 wt.-%, and even more preferably of from 0.1 to 2 wt.-%.

As regards the pyrolysis performed in step (3) of the inventive process, there is no particular restriction as to the temperature at which said step is performed, provided that mixed oxide particles according to the particular and preferred embodiments of the present invention are produced therein, in particular with respect to the content of the rare earth oxides other than ceria and/or of yttria contained therein. Thus, by way of example, the temperature at which the pyrolysis is performed may be comprised in the range of anywhere from 800 to 2,200°C, wherein preferably the temperature in step (3) is comprised in the range of from 900 to 1 ,800°C, more preferably of from 950 to 1 ,500°C, and more preferably of from 1 ,000 to 1 ,300°C. According to particularly preferred embodiments of the present invention, pyrolysis in step (3) is performed at a temperature comprised in the range of from 1 ,050 to 1 ,150°C. In addition to providing a process for the production of mixed oxide particles, the present invention further relates to the mixed oxide particles per se which are obtained according to the inventive process as well as to mixed oxide particles which are obtainable according to any of the particular or preferred embodiments of the inventive process irrespective of the actual method according to which the mixed oxide particles are actually produced. Therefore, the present invention also relates to mixed oxide particles obtainable and/or obtained, preferably obtained according to any of the particular and preferred embodiments of the inventive process. Furthermore, the present invention also relates to mixed oxide particles obtainable from flame- spray pyrolysis, wherein the particles comprise ceria, zirconia, and one or more oxides of one or more rare earth elements other than Ce, and/or yttria, wherein the content of the rare earth oxides other than ceria, and/or of yttria in the mixed oxide calculated as their respective oxides is comprised in the range of from 0.1 to 4.9 wt.-% based on the total weight of the one or more rare earth oxides, zirconia, and optional yttria contained in the mixed oxide particles, preferably of from 0.3 to 4.5 wt.-%, more preferably of from 0.5 to 4 wt.-%, more preferably of from 0.7 to 3.5 wt.-%, more preferably of from 0.8 to 3 wt.-%, more preferably of from 0.9 to 2.5 wt.-%, and even more preferably of from 1 to 2 wt.-%. According to particularly preferred embodiments of the present invention, the mixed oxide particles are obtainable from flame-spray pyrolysis according to the preferred embodiments of the inventive process, wherein said specific pyrolysis method is at least partly applied in step (3) for obtaining mixed oxide particles according to any of the particular or preferred embodiments of the present invention.

Regarding the one or more oxides of one or more rare earth elements other than ceria which may be comprised in the mixed oxide particles, there is no particular restriction according to the present invention neither with respect to the type nor with respect to the number of the one or more rare earth oxides other than ceria which may be comprised therein. According to the pre- sent invention it is however preferred that said one or more rare earth oxides other than ceria comprise one or more of lanthana, praseodymia, and neodymia, including mixtures of two or three thereof, wherein it is further preferred that the one or more rare earth oxides other than ceria comprise lanthana and/or neodymia. According to particularly preferred embodiments of the present invention, the one or more rare earth oxides other than ceria include lanthana, wherein even more preferably the rare earth oxide other than ceria is lanthana.

Therefore, according to preferred embodiments of the mixed oxide particles according to the present invention, the one or more rare earth oxides other than ceria are selected from the group consisting of lanthana, praseodymia, neodymia, and combinations of two or three thereof, wherein the one or more rare earth oxides preferably comprise lanthana and/or neodymia, preferably lanthana, wherein even more preferably the rare earth oxide other than ceria is lanthana.

No particular restriction applies according to the present invention as to the content of ceria and in particular of Ce02 which may be contained in the mixed oxide particles such that the amount of ceria contained therein may for example range anywhere from 1 to 95 wt.-% based on the total weight of the one or more rare earth oxides, zirconia, and optional yttria contained in the mixed oxide particles. According to preferred embodiments of the present invention, the content of ceria and in particular of Ce02 in the mixed oxide particles is comprised in the range of from 5 to 80 wt.-%, more preferably of from 10 to 70 wt.-%, more preferably of from 30 to 60 wt.-%, more preferably of from 40 to 55 wt.-%, and more preferably of from 45 to 52 wt.-%. According to particularly preferred embodiments of the present invention, the content of ceria and in par- ticular of Ce02 in the mixed oxide particles is comprised in the range of from 47.5 to 50.5 wt.-% based on the total weight of the one or more rare earth oxides, zirconia, and optional yttria contained in the mixed oxide particles. As regards the content of ceria in the mixed oxide particles, said content may in principle relate to any form of ceria and in particular to Ce02, Ce203, and any mixture of said cerium oxides, wherein the content of ceria in the mixed oxide particles of the present invention preferably refer to the cerium (IV) oxide Ce02.

According to alternatively preferred embodiments of the present invention, and in particular relative to embodiments of the mixed oxide particles for use in oxidative applications, in particular in the field of automotive exhaust gas treatment, and even more particularly as oxidation catalyst and preferably for use in diesel oxidation catalysts (DOC), the content of ceria and in particular of CeC"2 in the mixed oxide particles is comprised in the range of from 5 to 99 wt.-% based on the total weight of the one or more rare earth oxides, zirconia and optional yttria contained in the mixed oxide particles, preferably from 15 to 98 wt.-%, more preferably from 30 to 95 wt.-%, more preferably from 40 to 90 wt.-%, and more preferably from 45 to 87 wt.-%. According to said alternatively preferred embodiments of the present invention, it is particularly preferred that the content of ceria in the mixed oxide particles and in particular of CeC"2 is comprised in the range of from 50 to 80 wt.-%.

According to a further alternative embodiment of the present invention which is also preferred, and in particular to embodiments of the mixed oxide particles for use as oxygen storage components in the field of automotive exhaust gas treatment and in particular for use as an oxygen storage component in three-way catalysts (TWC), the content of ceria and in particular of CeC"2 in the mixed oxide particles is comprised in the range of from 1 to 80 wt.-% based on the total weight of the one or more rare earth oxides, zirconia, and optional yttria contained in the mixed oxide particles, more preferably from 5 to 70 wt.-%, more preferably from 10 to 60 wt.-%, more preferably from 15 to 55 wt.-%, and more preferably from 18 to 50 wt.-%. According to particularly preferred embodiments of said alternative embodiments, the content of ceria and in particular of CeC"2 in the mixed oxide particles is comprised in the range of from 20 to 45 wt.-%. As regards the content of zirconia in the mixed oxide particles, as for ceria, there is no particular restriction in this respect provided that a mixed oxide particle according to any of the particular or preferred embodiments of the present invention is provided. Thus, by way of example, the content of zirconia in the mixed oxide particles is comprised in the range of anywhere from 1 to 95 wt.-% based on the total weight of the one or more rare earth oxides, zirconia, and optional yttria contained in the mixed oxide particles, wherein within the meaning of the present invention, the term "zirconia" generally refers to zirconia, hafnia, and mixtures thereof, wherein according to a preferred definition said term designates the chemical compound ZrC"2. According to the present invention, it is, however, preferred that zirconia is contained in the mixed oxide particles in an amount comprised in the range of from 5 to 80 wt.-%, and more preferably of from 10 to 70 wt.-%, more preferably of from 30 to 60 wt.-%, more preferably of from 40 to 55 wt.-%, and more preferably of from 43 to 52 wt.-%. According to particularly preferred embodi- ments of the present invention, the content of zirconia in the mixed oxide particles is comprised in the range of from 45 to 51 .5 wt.-%. According to alternatively preferred embodiments of the present invention, and in particular relative to embodiments of the mixed oxide particles for use in oxidative applications, in particular in the field of automotive exhaust gas treatment, and even more particularly as oxidation catalyst and preferably for use in diesel oxidation catalysts (DOC), the content of zirconia in the mixed oxide particles is comprised in the range of from 0.5 to 80 wt.-% based on the total weight of the one or more rare earth oxides, zirconia, and optional yttria contained in the mixed oxide particles, wherein more preferably the content of zirconia in the mixed oxide particles is comprised in the range of from 1 to 70 wt.-%, more preferably of from 5 to 60 wt.-%, more preferably of from 10 to 55 wt.-%, and more preferably of from 13 to 50 wt.-%. According to particularly preferred embodiments of said alternative embodiments, the content of zirconia in the mixed oxide particles is comprised in the range of from 15 to 45 wt.-%.

According to further alternative embodiments of the present invention which are particularly preferred, and in particular to embodiments of the mixed oxide particles for use as oxygen storage components in the field of automotive exhaust gas treatment and in particular for use as an oxygen storage component in three-way catalysts (TWC), the content of zirconia in the mixed oxide particles is comprised in the range of from 5 to 95 wt.-% based on the total weight of the one or more rare earth oxides, zirconia, and optional yttria contained in the mixed oxide particles, wherein more preferably the content of zirconia is comprised in the range of from 15 to 90 wt.-% and more preferably of from 30 to 85 wt.-%, more preferably of from 40 to 80 wt.-% and more preferably of from 45 to 77 wt.-%. According to said alternatively preferred embodiments, it is particularly preferred that the content of zirconia in the mixed oxide particles is comprised in the range of from 50 to 75 wt.-%. As regards the surface area of the mixed oxide particles according to the present invention, there is no particular restriction as to the surface area which the mixed oxide particles may display such that surface areas and in particular surface areas determined according to the BET method, may be comprised in the range of anywhere from 2 to 200 m²/g, wherein preferably surface areas and in particular BET surface areas ranging from 5 to 150 m²/g are preferred, and more preferably from 10 to 1 10 m²/g, more preferably of from 20 to 95 m²/g, and more preferably of from 50 to 90 m²/g. According to particularly preferred embodiments of the present invention, the surface area and in particular the BET surface area of the mixed oxide particles is comprised in the range of from 80 to 87 m²/g. Regarding the preferred surface areas of the mixed oxide particles according to the present invention, it is noted that said preferred and par- ticularly preferred values relate in particular to embodiments of the mixed oxide particles for use as an oxidation catalyst in the treatment of automotive exhaust gases and in particular for use in diesel oxidation catalysts. According to alternatively preferred embodiments of the present in- vention, the mixed oxide particles display a surface area and in particular a BET surface area comprised in the range of from 20 to 100 m²/g, and more preferably of from 30 to 90 m²/g, more preferably of from 40 to 85 m²/g, and more preferably of from 45 to 80 m²/g. According to said alternatively preferred embodiments, it is particularly preferred that the surface area and in par- ticular the BET surface area of the mixed oxide particles is comprised in the range of from 50 to 75 m²/g. As regards said alternatively preferred embodiments of the mixed oxide particles according to the present invention, it is noted that said embodiments are particularly adapted for use as oxygen storage components in exhaust gas treatment applications and in particular for use as an oxygen storage component in three-way catalysts. As regards the BET surface area as defined in the present invention, it is noted that this refers in particular to a BET surface area determined according to DIN 66135.

Concerning the dimensions of the mixed oxide particles according to the present invention, in principle these may adopt any conceivable values. According to the present invention, it is, however, preferred that the mixed oxide particles are microcrystalline, wherein it is preferred that the average particle size of the mixed oxide particles is comprised in the range of from 5 to 100 nm, and preferably of from 6 to 50 nm, more preferably of from 7 to 30 nm, more preferably of from 8 to 20 nm, more preferably of from 9 to 50 nm, and more preferably of from 10 to 30 nm. According to particularly preferred embodiments of the present invention, the mixed oxide particles display an average particle size comprised in the range of from 1 1 to 12.5 nm. As regards the values of the average particle size as defined in the present application, these refer in particular to the average particle size of the mixed oxide particles as obtained using the Scher- rer formula as follows:

KX

' β€ΟΕ θ wherein K is the shape factor, lambda (λ) is the X-ray wave length, beta (β) is the line broadening at half the maximum intensity (FWHM) in radians, and theta (Θ) is the Bragg angle. As regards tao (τ), this stands for the mean size of the ordered (crystalline) domains, which may be smaller or equal to the grain size. The dimensionless shape factor has a typical value of about 0.9, and may be adapted to the actual shape of the crystallite if necessary.

Concerning the state of ceria, zirconia, and of the one or more oxides of one or more rare earth elements other than cerium and/or of yttria, said components are at least in part contained in the mixed oxide particles as a mixed oxide and in particular as a solid solution. This applies in particular to the ceria and zirconia in the mixed oxide particles and preferably to ceria, zirconia, the one or more oxides of one or more rare earth elements other than cerium, and/or yttria, all of which form a mixed oxide in the form of a solid solution. In this quality, the crystalline phases formed in the mixed oxide particles are accordingly crystalline phases formed by the mixed ox- ides and in particular by the ceria-zirconia mixed oxides. As a result, the crystalline phase of the mixed oxide particles made a tetragonal and/or a cubic structure in particular as determined by X-ray diffraction, wherein typically at least part of the crystalline mixed oxide is in the cubic phase. In fact, it has quite surprisingly been found that as opposed to ceria-zirconia mixed oxides containing one or more oxides of one or more rare earth elements other than cerium, and/or yttria, wherein the content of the latter exceeds the inventive range of 0.1 to 4.9 wt.-% according to the present application, such mixed oxide particles are mainly in the cubic phase, and in particular are entirely converted to the cubic phase upon ageing. According to the present invention, it has however been found that mixed oxide particles wherein the content of the rare earth oxides other than ceria, and/or of yttria is comprised in the range of from 0.1 to 4.9 wt.-% based on the total weight of the one or more rare earth oxides, zirconia, and optional yt- tria contained in the mixed oxide particles display an exceptionally low content of the cubic phase. Furthermore, it has quite unexpectedly been found that upon ageing of the inventive materials, the content of the cubic phase only gradually increases such that even after ageing a considerable proportion of the crystalline phase in the mixed oxide is not in a cubic state and, in particular, is still in a tetragonal state. Thus, without being bound to theory, it would appear that the surprising technical effects achieved by the very low content of the one or more oxides of one of more rare earth elements other than cerium, and/or of yttria in the inventive materials may very well be due to the exceptionally low proportion of the cubic phase in the crystalline material which is quite unexpectedly stabilized by said low contents of the one or more oxides of one or more rare earth elements other than cerium, and/or of yttria even after ageing.

Accordingly, as regards the proportion of the cubic phase in the mixed oxide particles of the present invention, it is preferred that said proportion as determined according to the Rietveld method is comprised in the range of from 0.1 to 29 percent, and more preferably in the range of from 0.5 to 25 percent, more preferably of from 1 to 20 percent, more preferably of from 2 to 15 percent, more preferably of from 3 to 10 percent, and more preferably of from 4 to 8 percent. According to particularly preferred embodiments according to the present invention, the proportion of the cubic phase in the mixed oxide particles as determined according to the Rietveld method is comprised in the range of from 5 to 6 percent. Furthermore, as regards the determination of the proportion of the cubic phase according to the Rietveld method, it is preferred that the composition of the mixed oxide particle is employed as a constraint when determining said proportion of the cubic phase.

For determining the proportion of the cubic phase in the mixed oxide particles according to the Rietveld method, it is preferred according to the present invention that for determining the crys- talline phases and their microstructure via X-ray diffraction, the Rietveld-Software TOPAS v4.1 (Bruker AXS) is employed. In particular, it is preferred to use a model consisting of the cubic Ce02 structure wherein the lattice parameters and the size of the crystallites are freely refined. More specifically, the refinement reduces the weighted error square between the measured and simulated data points. The alignment of the calculated curve to the measured data is examined with respect to deviations, in particular in the region of 43° 2Theta. It is at this diffraction angle that the only free reflex of the tetragonal modification Ceo.5Zro.5O2 may be found. In the event that the difference curve indicates the presence of this phase, it is then introduced into the model and a quantification of both phases is carried out. In the quantification, the errors preferably lie around 1 wt.-% and the relative errors in the calculation of the size of the crystallites preferably around 15%. Furthermore, as regards the proportion of the cubic phase after ageing of the mixed oxide particles, it is preferred according to the present invention that said proportion is comprised in the range of from 30 to 100 percent, and more preferably of from 35 to 99 percent, more preferably of from 38 to 90 percent, and more preferably of from 40 to 60 percent. Within the meaning of the present invention, the term "mixed oxide" preferably refers to a single solid solution of differ- ent oxides. According to particularly preferred embodiments of the present invention, the proportion of the cubic phase as determined according to the Rietveld method after ageing of the mixed oxide particles is comprised in the range of from 42 to 45 percent. According to the present invention, it is preferred that the values defined for the proportion of the cubic phase of the mixed oxide particles after ageing specifically refer to those values obtained after ageing of the mixed oxide particles having a proportion of the cubic phase as determined according to the Rietveld method in the fresh state prior to ageing comprised in any of the particular or preferred ranges according to the present invention, wherein the specific ageing treatment performed on said mixed oxide particles in the fresh state involves the exposure thereof at 1 ,100°C to air containing 10 percent H2O and preferably 10 volume percent H2O for a period of 40 hours.

Regarding the use of the mixed oxide particles according to the present invention, there is no restriction whatsoever as to the applications or methods in which the inventive materials may be used. According to preferred embodiments of the present invention, however, the mixed oxide particles are used as a catalyst and/or as a catalyst support. Alternatively, the mixed oxide par- tides are preferably used as an oxygen storage component involving the reversible uptake of oxygen, wherein preferably the application as an oxygen storage component relates to a particular use of the inventive materials in catalytic applications either as a catalyst and/or as a catalyst support. Thus, according to particularly preferred embodiments of the present invention, the mixed oxide particles according to any of the particular or preferred embodiments of the present invention are employed as an oxygen storage component and/or as a catalyst or catalyst component. Regarding said preferred uses, there is again principally no restriction whatsoever as to the specific applications and/or methods in which the inventive materials may act as an oxygen storage component and/or as a catalyst or catalyst component, wherein it is preferred that the inventive materials are used as such in catalysts for the treatment of exhaust gas and, in partic- ular, in the treatment of automotive exhaust gas. According to said preferred embodiments, it is yet further preferred that the inventive materials are used as an oxygen storage component and/or as a catalyst or catalyst component in a three way catalyst and/or in a diesel oxidation catalyst. Therefore, the present invention further relates to the use of mixed oxide particles according to any of the particular or preferred embodiments as defined in the present application as an oxygen storage component, a catalyst and/or as a catalyst support, preferably as an oxygen stor- age component and/or as a catalyst or catalyst component in a three way catalyst and/or diesel oxidation catalyst for the treatment of exhaust gas, preferably of automotive exhaust gas.

The present invention includes the following embodiments, wherein these include the specific combinations of embodiments as indicated by the respective interdependencies defined therein:

1 . A process for the production of mixed oxide particles comprising:

(1 ) providing a mixture comprising a solvent, one or more precursor compounds of ce- ria, one or more precursor compounds of zirconia, and one or more precursor compounds of one or more rare earth oxides other than ceria and/or one or more precursor compounds of yttria;

(2) forming an aerosol of the mixture provided in step (1 ); and

(3) pyrolyzing the aerosol of step (2) to obtain mixed oxide particles;

wherein the content of the rare earth oxides other than ceria and/or of yttria in the mixed oxide particles formed in step (3) is in the range of from 0.1 to 4.9 wt.-% based on the total weight of the rare earth oxides, yttria, and zirconia contained in the mixed oxide particles, preferably of from 0.3 to 4.5 wt.-%, more preferably of from 0.5 to 4 wt.-%, more preferably of from 0.7 to 3.5 wt.-%, more preferably of from 0.8 to 3 wt.-%, more preferably of from 0.9 to 2.5 wt.-%, and more preferably of from 1 to 2 wt.-%. 2. The process of embodiment 1 , wherein the one or more rare earth oxides other than ceria is selected from the group consisting of lanthana, praseodymia, neodymia, and mixtures of two or three thereof, wherein the one or more rare earth oxides preferably comprises lanthana and/or neodymia, preferably lanthana, wherein even more preferably the rare earth oxide other than ceria is lanthana. 3. The process of embodiment 1 or 2, wherein the concentration of the one or more precursor compounds of the one or more rare earth oxides other than ceria and/or of the one or more precursor compounds of yttria calculated as the respective oxides contained in the mixture provided in step (1 ) is comprised in the range of from to 0.01 to 5 wt.-% based on the total weight of the mixture provided in step (1 ), preferably of from 0.05 to 2 wt.-%, more preferably of from 0.1 to 1 .5 wt.-%, more preferably of from 0.3 to 1.2 wt.-%, more preferably of from 0.5 to 1 wt.-%, more preferably of from 0.7 to 0.9 wt.-%, and even more preferably of from 0.75 to 0.85 wt.-%.

4. The process of any of embodiments 1 to 3, wherein the solvent comprises one or more selected from the group consisting of aliphatic and aromatic hydrocarbons, alcohols, het- erocyclic compounds, carboxylic acids, water, and mixtures of two or more thereof, preferably from the group consisting of aromatic hydrocarbons, N-containing heterocycles, tet- rahydrofurane, (C5-Cio)hydrocarbons, (Ci-C5)alcohols, (Ci-C8)carboxylic acids, water, and combinations of two or more thereof, more preferably from the group consisting of (C6-C12) aromatic hydrocarbons, pyrrolidine, pyrrole, piperidine, pyridine, azepane, azepine, tetra- hydrofurane, (C5-C7)hydrocarbons, (Ci-C3)alcohols, (C2-C8)carboxylic acids, water, and combinations of two or more thereof, more preferably from the group consisting of Cs aromatic hydrocarbons, pyrrolidine, pyrrole, piperidine, pyridine, tetrahydrofurane, pentane, hexane, ethanol, methanol, propanol, (C6-C8)carboxylic acids, acetic acid, propionic acid, water, and combinations of two or more thereof, more preferably from the group consisting of toluene, ethylbenzene, xylene, hexane, propanol, acetic acid, Cs-carboxylic acids, propionic acid, water, and combinations of two or more thereof, and even more preferably from the group consisting of xylene, hexane, n-propanol, acetic acid, 2-ethylhexanoic acid, water, and combinations of two or more thereof.

The process of embodiment 4, wherein the aromatic hydrocarbons comprise one or more aromatic hydrocarbons selected from the group consisting of (C6-Ci2)hydrocarbons, preferably of (C7-Ci i)hydrocarbons, more preferably of (C8-Cio)hydrocarbons, more preferably of (C8-Cg)hydrocarbons, and even more preferably of Cs-hydrocarbons, wherein even more preferably the solvent comprises one or more aromatic hydrocarbons selected from the group consisting of toluene, ethylbenzene, xylene, mesitylene, durene, and mixtures of two or more thereof, more preferably from the group consisting of toluene, ethylbenzene, xylene, and mixtures of two or more thereof, wherein even more preferably the solvent comprises toluene and/or xylene, preferably xylene.

The process of embodiment 4 or 5, wherein the aliphatic hydrocarbons comprise one or more hydrocarbons selected from the group consisting of branched and/or unbranched, preferably unbranched (C4-Ci2)hydrocarbons, preferably of (C5-Cio)hydrocarbons, more preferably of (C6-C8)hydrocarbons, more preferably of (C6-C7)hydrocarbons, and even more preferably of branched and/or unbranched, preferably unbranched C6-hydrocarbons, wherein even more preferably the solvent comprises one or more aliphatic hydrocarbons selected from the group consisting of pentane, hexane, heptane, octane, and mixtures of two or more thereof, wherein even more preferably the aliphatic hydrocarbons comprise pentane and/or hexane, preferably hexane.

The process of any of embodiments 4 to 6, wherein the carboxylic acid is selected from the group consisting of (Ci-Cs) carboxylic acids, preferably from the group consisting of (C-I-C6) carboxylic acids, more preferably from the group consisting of (C1-C5) carboxylic acids, more preferably from the group consisting of formic acid, acetic acid, propionic acid, butyric acid, and mixtures of two or more thereof, more preferably from the group consisting of acetic acid, propionic acid, butyric acid, 2-ethylhxanoic acid and mixtures of two or more thereof, wherein more preferably the carboxylic acid comprises acetic acid and/or propionic acid, preferably acetic acid. The process of any of embodiments 1 to 7, wherein the concentration of the one or more precursor compounds of ceria calculated as Ce02 contained in the mixture provided in step (1 ) is comprised in the range of from 0.1 to 15 wt.-% based on the total weight of the mixture provided in step (1 ), preferably of from 0.5 to 10 wt.-%, more preferably of from 1 to 7 wt.-%, more preferably of from 2 to 5 wt.-%, more preferably of from 2.5 to 4 wt.-%, more preferably of from 2.7 to 3.5 wt.-%, and even more preferably of from 3 to 3.2 wt.-%.

The process of any of embodiments 1 to 8, wherein the concentration of the one or more precursor compounds of zirconia calculated as Zr02 contained in the mixture provided in step (1 ) is comprised in the range of from 0.1 to 15 wt.-% based on the total weight of the mixture provided in step (1 ), preferably of from 0.5 to 10 wt.-%, more preferably of from 1 to 7 wt.-%, more preferably of from 2 to 5 wt.-%, more preferably of from 2.5 to 4 wt.-%, more preferably of from 2.7 to 3.5 wt.-%, and even more preferably of from 3 to 3.2 wt.-%.

The process of any of embodiments 1 to 9, wherein the one or more precursor compounds of ceria and/or of the rare earth oxides other than ceria and/or of yttria comprise one or more salts, preferably one or more salts selected from the group consisting of car- boxylates, nitrates, carbonates, alcoholates, and chelating ligand containing complexes, wherein the carboxylates are preferably selected from (C4-Ci2)carboxylates, more preferably from (C5-Cn)carboxylates, more preferably from (C6-Cio)carboxylates, more preferably from (C7-Cg)carboxylates, more preferably from Cs-carboxylates, more preferably from branched Cs-carboxylate, wherein more preferably the one or more precursor compounds comprise a 2-ethylhexanoate salt, and wherein even more preferably the one or more precursor compounds of ceria and/or of the rare earth oxides other than ceria, and preferably of all of the precursor compounds of the rare earth oxides provided in step (1 ) are 2- ethylhexanoate salts.

The process of embodiment 10, wherein the chelating ligand containing complexes comprise one or more chelating ligands selected from the group consisting of bi-, tri-, tetra-, penta-, and hexadentate ligands, more preferably from the group consisting of oxalate, ethylenediamine, 2,2'-bipyridine, 1 ,10-phenanthroline, acetylacetonate, 2,2,2-crypt, diethy- lenetriamine, dimethylglyoximate, EDTA, ethylenediaminetnacetate, glycinate, triethylenetetramine, tris(2-aminoethyl)amine, and combinations of two or more thereof, more preferably from the group consisting of oxalate, ethylenediamine, acetylacetonate, diethylenetri- amine, dimethylglyoximate, EDTA, ethylenediaminetnacetate, glycinate, triethylenetetramine, tris(2-aminoethyl)amine, and combinations of two or more thereof, more preferably from the group consisting of oxalate, ethylenediamine, acetylacetonate, diethylenetri- amine, EDTA, ethylenediaminetnacetate, triethylenetetramine, and combinations of two or more thereof, wherein even more preferably the chelating ligand containing complexes comprise acetylacetonate. 12. The process of any of embodiments 1 to 1 1 , wherein the one or more precursor compounds of zirconia comprise one or more salts, preferably one or more salts selected from the group consisting of carboxylates, nitrates, carbonates, alcoholates, and chelating lig- and containing complexes, preferably diketone ligand containing complexes, and more preferably acetylacetonate complexes, wherein the alcoholates are preferably selected from (C2-C5) alcoholates, more preferably from (C3-C4) alcoholates, and more preferably from C3-alcoholat.es, wherein more preferably the one or more precursor compounds comprise zirconium(IV)-propoxide, and wherein even more preferably the one or more precursor compounds of zirconia is zirconium(IV)-propoxide. 13. The process of any of embodiments 1 to 12, wherein the mixture provided in step (1 ) further comprises one or more platinum group metals, preferably one or more platinum group metals selected from the group consisting of Ru, Rh, Pd, Os, Ir, Pt, and mixtures of two or more thereof, more preferably from the group consisting of Rh, Pd, Pt, and mixtures of two or more thereof, wherein more preferably the platinum group metal is Pd and/or Pt, pref- erably Pd.

14. The process of embodiment 13, wherein the mixture provided in step (1 ) comprises the one or more platinum group metals in an amount ranging from 0.01 to 15 wt.-% calculated as the metal based on the total weight of the mixture provided in step (1 ), preferably from 0.05 to 14 wt.-%, more preferably from 0.1 to 13 wt.-%, more preferably from 0.5 to 12 wt- %, more preferably from 2 to 10 wt.-%, more preferably from 3 to 9 wt.-%, more preferably from 4 to 8 wt.-%, and even more preferably from 5 to 7 wt.-%.

15. The process of any of embodiments 1 to 14, wherein pyrolysis in step (3) is performed in an atmosphere containing oxygen.

16. The process of any of embodiments 1 to 15, wherein pyrolysis in step (3) is performed at a temperature comprised in the range of from 800 to 2,200°C, preferably of from 900 to

1 ,800°C, preferably of from 950 to 1 ,500°C, more preferably of from 1 ,000 to 1 ,300°C, and even more preferably of from 1 ,050 to 1 ,150°C.

17. Mixed oxide particles obtainable and/or obtained, preferably obtained by a process according to any of embodiments 1 to 16. 18. Mixed oxide particles obtainable from flame spray pyrolysis, preferably according to any of embodiments 1 to 16, wherein the particles comprise ceria, zirconia, and one or more oxides of one or more rare earth elements other than Ce, and/or yttria, wherein the content of the rare earth oxides other than ceria, and/or of yttria in the mixed oxide calculated as their respective oxides is comprised in the range of from 0.1 to 4.9 wt.-% based on the to- tal weight of the one or more rare earth oxides, zirconia, and optional yttria contained in the mixed oxide particles, preferably of from 0.3 to 4.5 wt.-%, more preferably of from 0.5 to 4 wt.-%, more preferably of from 0.7 to 3.5 wt.-%, more preferably of from 0.8 to 3 wt- %, more preferably of from 0.9 to 2.5 wt.-%, and even more preferably of from 1 to 2 wt- %.

The mixed oxide particles of embodiment 17 or 18, wherein the one or more rare earth oxides other than ceria are selected from the group consisting of lanthana, praseodymia, neodymia, and combinations of two or three thereof, wherein the one or more rare earth oxides preferably comprise lanthana and/or neodymia, preferably lanthana, wherein even more preferably the rare earth oxide other than ceria is lanthana.

The mixed oxide particles of any of embodiments 17 to 19, wherein the content of ceria in the mixed oxide particles is comprised in the range of from 1 to 95 wt.-% based on the total weight of the one or more rare earth oxides, zirconia, and optional yttria contained in the mixed oxide particles, preferably from 5 to 80 wt.-%, more preferably from 10 to 70 wt.-%, more preferably from 30 to 60 wt.-%, more preferably from 40 to 55 wt.-%, more preferably from 45 to 52 wt.-%, more preferably from 47.5 to 50.5 wt.-%.

The mixed oxide particles of any of embodiments 17 to 20, wherein the content of Zr02 in the mixed oxide particles is comprised in the range of from 1 to 95 wt.-% based on the total weight of the one or more rare earth oxides, zirconia, and optional yttria contained in the mixed oxide particles, preferably from 5 to 80 wt.-%, more preferably from 10 to 70 wt.-%, more preferably from 30 to 60 wt.-%, more preferably from 40 to 55 wt.-%, more preferably from 43 to 52 wt.-%, more preferably from 45 to 51.5 wt.-%.

The mixed oxide particles of any of embodiments 17 to 21 , wherein the BET surface area of the mixed oxide particles is comprised in the range of from 2 to 200 m²/g, preferably of from 5 to 150 m²/g, more preferably of from 10 to 1 10 m²/g, more preferably of from 20 to 95 m²/g, more preferably of from 50 to 90 m²/g, and even more preferably of from 80 to 87 m²/g.

The mixed oxide particles of any of embodiments 17 to 22, wherein the average particle size of the mixed oxide particles is comprised in the range of from 5 to 100 nm, preferably of from 6 to 50 nm, more preferably of from 7 to 30 nm, more preferably of from 8 to 20 nm, more preferably of from 9 to 15 nm, more preferably of from 10 to 13 nm, and even more preferably of from 1 1 to 12.5 nm, wherein preferably the average particle size is obtained using the Scherrer formula. The mixed oxide particles of any of embodiments 17 to 23, wherein the proportion of the cubic phase as determined according to the Rietveld method is comprised in the range of from 0.1 to 29%, preferably of from 0.5 to 25%, more preferably of from 1 to 20%, more preferably of from 2 to 15%, more preferably of from 3 to 10%, more preferably of from 4 to 8%, and even more preferably of from 5 to 6%.

The mixed oxide particles of embodiment 24, wherein the proportion of the cubic phase as determined according to the Rietveld method after aging of the mixed oxide particles is comprised in the range of from 30 to 100%, preferably of from 35 to 99%, more preferably of from 38 to 90%, more preferably of from 40 to 60%, and even more preferably of from 42 to 45%, wherein aging is preferably performed by heating the mixed oxide particles at 1 ,100°C in air with 10% H₂O for 40 h.

Use of mixed oxide particles according to any of embodiments 17 to 25 as an oxygen storage component, a catalyst and/or as a catalyst support, preferably as an oxygen storage component and/or as a catalyst or catalyst component in a three way catalyst and/or diesel oxidation catalyst for the treatment of exhaust gas, preferably of automotive exhaust gas.

DESCRPTION OF THE FIGURES

Figure 1 shows the oxygen storage capacity of ceria-zirconia mixed oxide materials obtained from flame spray pyrolysis according to Examples 1 to 4 and Comparative Examples 2 to 5 compared to a sample according to the prior art obtained from a co- precipitation method as reference depending on the amount of the lanthana additive contained in the ceria-zirconia mixed oxide materials. In the figure, the content of lanthana in weight along the abscissa and the oxygen storage capacity relative to the reference material is plotted against the ordinate. Regarding the tested samples, the oxygen storage capacity of the as- synthesized materials in their fresh state as indicated by the symbol "■" whereas the oxygen storage capacity of the aged materials is indicated by the symbol

Figure 2 shows the BET-surface area of Examples 1 to 4 and Comparative Examples 2 to 6 after aging relative to the amount of lanthana contained in the ceria-zirconia mixed oxides as additive. In the figure, the BET-surface area in m²/g is plotted along the ordinate whereas the content of lanthana in the ceria-zirconia mixed oxide materials is shown along the abscissa.

Figure 3 displays the burner configuration (top and side view) in an apparatus which may be employed for flame spray pyrolysis. EXAMPLES For the synthesis of the mixed oxide particles in the experimental section, a flame spray pyroly- sis apparatus as depicted in Figure 3 was employed. More specifically, the nozzles for the flame spray pyrolysis are located in the upper portion of a burning chamber. As indicated in Figure 3 (top view), the main nozzle for the vaporization of the precursor is located in the middle of the nozzle arrangement. Two lines are connected to said main nozzle wherein the first line is con- nected to a piston pump for pumping the precursor solution into the chamber. The second line provides air wherein the oxygen content thereof may be varied by the injection of oxygen or nitrogen therein. Both lines converge in the main nozzle. Further nozzles are arranged around said main nozzle, wherein in the first half of said nozzles a gas mixture of air/oxygen/nitrogen is introduced and in the second half ethylene, respectively. The further nozzles provide auxiliary flames for providing a constant temperature and a uniform combustion in the burning chamber. In the lower portion of the burning chamber, a quench portion is provided for rapid cooling of the particles generated in the upper portion of the burning chamber.

For determining the proportion of the cubic phase in the mixed oxide particles according to the Rietveld method, the crystalline phases of the samples and their microstructure were examined via X-ray diffraction. To this effect, the samples were analyzed in a D8 Advance (Bruker AXS) Bragg-Brentano diffractometer with a resolution of about 0.05° 2Theta. The powder samples were filled into a sample container and smoothed out using a glass plate. The reflection data was then collected in the range from 5 to 80° 2Theta and the data evaluated with the Rietveld- Software TOPAS v4.1 (Bruker AXS). In particular, a model was used consisting of the cubic Ce02 structure wherein the lattice parameters and the size of the crystallites are freely refined. More specifically, the weighted error square between the measured and simulated data points was reduced by refinement of the model. The alignment of the calculated curve to the measured data was examined with respect to deviations, in particular in the region of 43° 2Theta. It is at this diffraction angle that the only free reflex of the tetragonal modification Ceo.5Zro.5O2 may be found. In the event that the difference curve indicated the presence of this phase, it was then introduced into the model and a quantification of both phases was carried out. In the quantification, the errors lay around 1 wt.-% and the relative errors in the calculation of the size of the crystallites lay around 15%.

Flame Spray Pyrolysis

In the synthesis of the examples and comparative examples, the flow of the precursor solution was set to 320 ml/h and the flow of air and ethylene to the main and auxiliary nozzles regulated such that an average temperature of 1 100 °C was sustained in the burning chamber for the pyrolysis of the precursor solution. After having obtained the mixed oxide products from flame spray pyrolysis, the powders were analyzed in their fresh state after which they were subject to hydrothermal aging by exposure to air with 10 vol.-% of H2O at a temperature of 1 100 °C for 40 h. The composition of the precursor solutions as well as the characteristics of the fresh and aged products is shown in Table 1 below, wherein in particular the BET-surface area of the fresh and aged products as well as the proportion of the cubic crystalline phase of the mixed oxide materials prior to and after aging is indicated in addition to the XRD diameter of the as- synthesized materials.

Table 1

The oxygen storage capacity of the mixed oxide materials according to the examples and comparative examples in the fresh and aged states were respectively determined. The results of said testing is indicated in Figure 1 , wherein the oxygen storage capacity is indicated relative to a sample according to the prior art obtained from a co-precipitation method as the reference material, wherein said reference sample contained 40 wt.-% ceria, 45 wt.-% zirconia, 2 wt.-% lanthana, 5 wt.-% neodymia, and 8 wt.-% yttria. Thus, as may be taken from Figure 1 , it has quite surprisingly been found that materials according to the present invention having a particularly low amount of additives in addition to ceria and zirconia not only display an oxygen storage capacity comparable to materials having a greater amount of additives in the fresh state. Quite more unexpectedly, it has been found that in the aged materials particularly low levels of an additive in the inventive materials affords an oxygen storage capacity which is clearly superior to materials produced in the same fashion yet containing higher levels of said additive, an effect which is most pronounced at additive levels of around 1 to 2 wt.-%

Consequently, as a result of said highly unexpected finding, it is actually possible to provide an improved oxygen storage component which may not only be used in a highly cost efficient manner, in particular with respect to the additive components. Quite surprisingly it is actually even possible to provide a considerably improved oxygen storage component relative to the aging stability compared to oxygen storage component materials produced with a higher amount of the additives and which would normally be expected not only to provide a greater oxygen storage capacity but in particular to allow for an improved hydrothermal stability of the resulting materials compared to such materials having little or no additive components. Furthermore, as may be taken from the values for the BET-surface area after aging as indicated in Figure 2, the BET-surface area of the inventive materials, an increased surface area of the aged materials compared to a sample devoid of additive is observed which may clearly compete with surface areas of aged materials containing clearly larger amounts thereof. Consequently, it is also observed relative to the hydrothermal stability of the inventive materials that quite unex- pectedly the inventive materials may compete in their quality with materials having a multiple of the amount of additives and being therefore clearly less cost efficient in their production than the inventive materials.

Finally, it has quite unexpectedly been found that upon ageing of selected examples of the in- ventive materials (see in particular results for Examples 1 and 2 in Table 1 ), the content of the cubic phase only gradually increases such that even after ageing a considerable proportion of the crystalline phase in the mixed oxide is not in a cubic state and, in particular, is still in a tetragonal state. Thus, without being bound to theory, it would appear that part of the surprising technical effects achieved by the very low content of the one or more oxides of one of more rare earth elements other than cerium, and/or of yttria in the inventive materials may very well be due to exceptionally low proportions of the cubic phase in the crystalline material which is quite unexpectedly stabilized by low contents of the one or more oxides of one or more rare earth elements other than cerium, and/or of yttria even after ageing. In particular, again without being bound to theory, said highly unexpected finding would appear to correlate with the exceptional hydrothermal stability of the inventive materials, in particular as displayed for Examples 1 and 2 in Fig. 1 , respectively. Accordingly, as demonstrated above, a highly improved oxygen storage component may be obtained according to the present invention which is furthermore highly cost efficient compared to other materials in view of the very low amount of additive materials in addition to ceria and zirconia employed therein.

Cited Prior Art Documents

- Stark et al. in Chem. Comm. 2003, pp. 588-589

- Stark et al. in Chem. Mater. 2005, vol. 17, pp. 3352-3358

- Schulz et al. in J. Mater. Chem. 2003, vol. 13, pp. 2979-2984

- Jossen et al. Chem. Vap. Deposition 2006, vol. 12, pp. 614-619

- Wang ef al. in Journal of Molecular Catalysis A: Chemical 2011 , vol. 339, pp. 52-60

- Wang ef al. in Environmental Science and Technology 2010, vol. 44, pp. 3870-3875 - Li ef al. in Journal of Rare Earths 2011 , vol. 29, no. 6, pp. 544-549

- Cao ef al. in Materials Letters 2008, vol. 62, pp. 2667-2669

- US 201 1/0281 1 12 A1

- Stark ef al. in Chemical Communications pp. 588-589

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Claims

A process for the production of mixed oxide particles comprising:

(2) forming an aerosol of the mixture provided in step (1 ); and

(3) pyrolyzing the aerosol of step (2) to obtain mixed oxide particles;

wherein the content of the rare earth oxides other than ceria and/or of yttria in the mixed oxide particles formed in step (3) is in the range of from 0.1 to 4.9 wt.-% based on the total weight of the rare earth oxides, yttria, and zirconia contained in the mixed oxide particles.

The process of claim 1 , wherein the one or more rare earth oxides other than ceria is selected from the group consisting of lanthana, praseodymia, neodymia, and mixtures of two or three thereof.

3. The process of claim 1 or 2, wherein the concentration of the one or more precursor compounds of the one or more rare earth oxides other than ceria and/or of the one or more precursor compounds of yttria calculated as the respective oxides contained in the mixture provided in step (1 ) is comprised in the range of from to 0.01 to 5 wt.-% based on the total weight of the mixture provided in step (1 ).

4. The process of any of claims 1 to 3, wherein the solvent comprises one or more selected from the group consisting of aliphatic and aromatic hydrocarbons, alcohols, heterocyclic compounds, carboxylic acids, water, and mixtures of two or more thereof. 5. The process of claim 4, wherein the aromatic hydrocarbons comprise one or more aromatic hydrocarbons selected from the group consisting of (C6-Ci2)hydrocarbons.

6. The process of claim 4 or 5, wherein the aliphatic hydrocarbons comprise one or more hydrocarbons selected from the group consisting of branched and/or unbranched (C₄- Ci2)hydrocarbons. 7. The process of any of claims 4 to 6, wherein the carboxylic acid is selected from the group consisting of (Ci-Cs) carboxylic acids. The process of any of claims 1 to 7, wherein the concentration of the one or more precursor compounds of ceria calculated as Ce02 contained in the mixture provided in step (1 ) is comprised in the range of from 0.1 to 15 wt.-% based on the total weight of the mixture provided in step (1 ).

The process of any of claims 1 to 8, wherein the concentration of the one or more precursor compounds of zirconia calculated as Zr02 contained in the mixture provided in step (1 ) is comprised in the range of from 0.1 to 15 wt.-% based on the total weight of the mixture provided in step (1 ).

The process of any of claims 1 to 9, wherein the one or more precursor compounds of ceria and/or of the rare earth oxides other than ceria and/or of yttria comprise one or more salts.

1 1 . The process of claim 10, wherein the chelating ligand containing complexes comprise one or more chelating ligands selected from the group consisting of bi-, tri-, tetra-, penta-, and hexadentate ligands. 12. The process of any of claims 1 to 1 1 , wherein the one or more precursor compounds of zirconia comprise one or more salts.

13. The process of any of claims 1 to 12, wherein the mixture provided in step (1 ) further comprises one or more platinum group metals.

14. The process of claim 13, wherein the mixture provided in step (1 ) comprises the one or more platinum group metals in an amount ranging from 0.01 to 15 wt.-% calculated as the metal based on the total weight of the mixture provided in step (1 ).

15. The process of any of claims 1 to 14, wherein pyrolysis in step (3) is performed in an atmosphere containing oxygen.

The process of any of claims 1 to 15, wherein pyrolysis in step (3) is performed at a temperature comprised in the range of from 800 to 2,200°C.

17. Mixed oxide particles obtainable and/or obtained by a process according to any of claims 1 to 16.

18. Mixed oxide particles obtainable from flame spray pyrolysis, wherein the particles corr prise ceria, zirconia, and one or more oxides of one or more rare earth elements other than Ce, and/or yttria, wherein the content of the rare earth oxides other than ceria, ar of yttria in the mixed oxide calculated as their respective oxides is comprised in the rai of from 0.1 to 4.9 wt.-% based on the total weight of the one or more rare earth oxides zirconia, and optional yttria contained in the mixed oxide particles.

19. The mixed oxide particles of claim 17 or 18, wherein the one or more rare earth oxide; other than ceria are selected from the group consisting of lanthana, praseodymia, neo dymia, and combinations of two or three thereof.

The mixed oxide particles of any of claims 17 to 19, wherein the content of ceria in the mixed oxide particles is comprised in the range of from 1 to 95 wt.-% based on the tot; weight of the one or more rare earth oxides, zirconia, and optional yttria contained in t mixed oxide particles.

21 . The mixed oxide particles of any of claims 17 to 20, wherein the content of ZrC"2 in the mixed oxide particles is comprised in the range of from 1 to 95 wt.-% based on the tot; weight of the one or more rare earth oxides, zirconia, and optional yttria contained in t mixed oxide particles.

22. The mixed oxide particles of any of claims 17 to 21 , wherein the BET surface area of t mixed oxide particles is comprised in the range of from 2 to 200 m²/g. 23. The mixed oxide particles of any of claims 17 to 22, wherein the average particle size the mixed oxide particles is comprised in the range of from 5 to 100 nm.

24. The mixed oxide particles of any of claims 17 to 23, wherein the proportion of the cubi phase as determined according to the Rietveld method is comprised in the range of fr< 0.1 to 29%. 25. The mixed oxide particles of claim 24, wherein the proportion of the cubic phase as de termined according to the Rietveld method after aging of the mixed oxide particles is c prised in the range of from 30 to 100%.

26. Use of mixed oxide particles according to any of claims 17 to 25 as an oxygen storage component, a catalyst and/or as a catalyst support.