WO2006119653A1

WO2006119653A1 - Multi-nozzle flame aerosol synthesis of nanoparticle-powders

Info

Publication number: WO2006119653A1
Application number: PCT/CH2005/000270
Authority: WO
Inventors: Reto Strobel; Lutz Mädler; Sotiris E. Pratsinis; Alfons Baiker
Original assignee: Eth Zurich
Priority date: 2005-05-13
Filing date: 2005-05-13
Publication date: 2006-11-16

Abstract

Disclosed is a flame spray method for the production of nanopowders with controlled mixedness at the submicro-level of individual nanoparticles as well as their use, in particular in heterogeneous catalysis. Said method comprises the steps of i) providing at least two spray nozzles suitable for performing flame spray pyrolysis, each spray nozzle being connected to at least one reservoir, each reservoir comprising a liquid precursor composition, each precursor composition comprising at least one dissolved or finely divided precursor compound, ii) positioning said at least two spray nozzles at an angle and in a distance suitable for getting the desired non-segregated submicron mixture of individual nanoparticles, iii) feeding said at least two liquid precursor compositions to their respective spray nozzle, iv) dispersing, igniting, combusting and mixing said at least two liquid precursor compositions, and v) collecting the nanopowder.

Description

MULTI-NOZZLE FLAMB AEROSOL SYNTHESIS OF NANOPARTICLE-POWDERS

Technical Field

The present invention concerns a method for the production of particulate material with controlled mixedness, in particular with controlled mixedness at the micro level .and/or the nano level as well as respectively producible materials .

Background Art

The production of primary particles of a size smaller than 100 run by flame spray pyrolysis from one precursor liquid is known (see e.g. [I]) . It is also already known from the patent to

Shanmugham to produce a coating on a substrate utilizing a two nozzle configuration where at least one nozzle is forming a flame that is used to fuse, cure or process in general a material deposited on a substrate [2] . Hitherto known methods start from one liquid precursor composition such that mixture at the atomic level can be obtained (see e.g. [3-5]) or the method is leading to segregation and embedding of the two components in one particle [6] .

Disclosure of the Invention

Hence, it is a general object of the inven- tion to provide a dry method and in particular a flame spray method for the production of nanopowders comprising a non-segregated submicron mixture of individual nanopar- tides, in particular sub-micron sized component mixtures of individual nanoparticles that have specific -features due to the fact that they are not related to the thermodynamics of the mixture within the reaction environment because the mixing is achieved after the formation of the individual components .

Now, in order to implement these and still further objects of the invention, which will ^'become more readily apparent as the description proceeds, the flame spray method for the production of nanopowders with controlled mixedness at the submicron-level, e.g. nanopowders comprising a non-segregated submicron mixture of individual nanoparticles, is manifested by the features that said method comprises i) providing at least two spray nozzles suitable for performing flame spray pyrolysis, each spray nozzle being connected to at least one reservoir, each reservoir comprising a liquid precursor composition, each precursor composition comprising at least one dissolved or finely divided precursor compound, with the proviso that at least one precursor compound is dissolved, ii) positioning said at least two spray nozzles at an angle and in a distance suitable for getting the desired non-segregated submicron mixture of individ- ual nanoparticles, iii) feeding said at least two liquid precursor compositions to their respective spray nozzle, iv) dispersing, igniting, combusting and mixing said at least two liquid precursor compositions, and v) collecting the nanopowder.

Another object of the present invention is the specific material, obtainable by the above-described method .

Still other objects of the present invention are specific uses of the inventive particulate material.

The term nanoparticles as used herein means individual primary particles of homogeneous or heteroge- neous composition having a particle size of smaller than 200 run, preferably smaller than 100 nm, especially smaller than 50 nm but in general larger than 0.5 nm. Said nanoparticles, in view of the high flame temperature may be agglomerated/sintered to particles of the same or other kind.

Nanoparticles with controlled mixedness means mixtures of particles that are distinguishable with respect to their chemical composition and/or their physical properties, but also compositions comprising nanoparticles of one or more than one kind, at least one kind of said nanoparticles being within themselves inhomogeneous . The term precursor as it is used in the scope of the present invention in general designates an organic or an inorganic salt or an organometallic compound that is combusted to form the nanoparticles. However, it may also be a carbonate or preferably an oxide powder that is processed .through a flame and in the flame mixed with a flame produced nanoparticulate material. Thus, at least one of the precursor compounds must be a combustible material, preferably a dissolved combustible material.

The term predominantly as used herein means more than 50%, preferably more than 80%, especially more than 90%, most preferred about 100%. According to the present invention, particulate material of controlled mixedness is formed in the gas phase by flame spraying precursor liquids, in particular different precursors containing liquids, from multiple individual spray nozzles. The precursors of the particulate components of the final product are separately sprayed from at least two nozzles. The nozzles are arranged to enable a mixing of the individual components in the aerosol phase (see Figure 1 for the exemplary case of a three nozzle system) . The individual components can be metals, metal oxides, mixed metal oxides, metal salts, i.e. nitrates, phosphates etc., carbon-containing materials, in particular carbonates, or any combination of those. At least one component of the nanopowder is formed by flame spray pyrolysis as described e.g. in [1] resulting in primary particles of size in general smaller than 100 nm. Other sprayed precursor containing fuels can be individually ignited before mixing with the other components in the spray or be ignited by one of the flames in the system at the point of mixing. One of the liquid precursor compositions can be a particulate suspension of at least one of the components in a suitable solvent. Such a liquid precursor composition may also comprise more than one particulate material and/or one or more than one dissolved precursor. The particulate material comprised in the liquid precursor composition may be a combustible salt or a non combustible material, such as an oxide. In general at least one precursor in at least one liquid precursor composition is present in dissolved form, preferably all precursors .

The one- or more solvents used in the liquid precursor compositions alone or as solvent mixture com- prise and preferably consist of combustible solvents. In general the solvent or solvent mixture is ignitable.

The solvents are selected in view of the combustion enthalpy and their ability to dissolve at least one of the precursors . The solvents or solvent mixtures are chosen such that^' the desired - in view of the often diverging necessities for solubility and flame temperature usually the optimized - solubility and flame temperature are obtained.

Examples for such solvents and solvent compo- sitions are alcohols, acids, aromatic and aliphatic hydrocarbons, ethers, ketones, aldehydes, glycoles etc., and mixtures thereof .

Preferred precursors are metal salts that preferably are used in dissolved form, such as salts with combustible anions, e.g. alkoxides, acetates and aceto- nates, but also nitrates and carbonates and mixed salts thereof. Insoluble precursors are oxides and/or carbonates in powder form.

The metals of the precursor can be any metal (or metal combination if a mixed precursor salt or a mixed organometallic compound is used) but in particular a metal selected from the group comprising silicon, titanium, strontium, cerium, zirconium, palladium, tantalum, zinc, silver, gold, ruthenium, and especially barium, aluminum, and platinum. The precursors can be divided into separate precursor solutions based on the product desired. For e.g. use as NO_x storage material in catalysts, a first liquid precursor composition comprising an aluminum oxide and/or carbonate precursor and a second liquid precursor composition comprising a barium oxide and/or carbonate precursor and a platinum precursor are prepared and separately sprayed (see below) . In this case it is preferred that both liquid precursor compositions are precursor solutions . The nozzles can be arranged in any configuration which enables the mixing in the aerosol phase leading to a product of controlled mixedness at the micro- and/or nano- and/or atomic levels. For example, a product of the invention can be well-mixed at the micro-level but segregated at the nano- and at the atomic-levels.

To ensure a system allowing a broad applicability or the production of a broad variety of products, respectively, it is much desired that at least two of the spray nozzles, preferably all spray nozzles are individu- ally adjustable and individually ignitable. This enables to ignite at least one of the liquid precursor compositions prior to its combination with the at least one further liquid precursor composition.

The distribution of the precursors to the in- dividual liquid precursor compositions and the position of the second (or further) nozzle (s) is dependent on the desired combustion temperature and the desired combustion time prior to combination of the individual spray streams, both influencing the degree of combustion and the particle size prior to combination as well as the growth/ sintering of the particles in the combined streams .

The products obtainable by the method of the invention are particulate powders that are collected on and recovered from a powder collection system.

The material of the present invention is dis- tinct from the materials obtained by conventional flame spraying methods (e.g. according to [3-6]) by a non- segregated sub-micron mixture of individual nanoparti- cles. By using more than one metal/metal oxide precursor in one liquid precursor composition in the method of the present invention, nanoparticles with a mixture of the two compounds on a molecular level can be obtained. Either such nanoparticles being a mixture of two or more compounds or nanoparticles consisting of one compound can be used as one stream that according to the invention can be combined with a second or more streams of the before- mentioned kind. In the present case the sub-micron sized component mixtures of individual nanoparticles are not related to the thermodynamics of the mixture within the reaction environment because the mixing is achieved after the formation of the individual components.

Due to their specific structure, the nanopow- ders of the present invention are better suitable for many applications than powders that e.g. have the same composition on a macro level but different distribution on a nano level.

Applications of the described nano- particulate materials with separate phases or nanoparticles, which are homogeneously distributed on a submicron level, comprise heterogeneous catalysts (specific appli- cation see below), polymer fillers, biomaterials including bone replacement and dental fillers (matching the refractive index of the polymer with appropriate mixtures of nanoparticles to obtain high transparency besides improved mechanical properties of such composites) , pigmentary applications (homogeneously mixing individual color and effect centers, in this case nanoparticles, within a length scale much smaller than the visible light) , and microelectronics .

Brief Description of the Drawings

The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes refer- ence to the annexed drawings, wherein the figures show:

Figure 1: Schematic of the flame spray py- rolysis setup using three separate nozzles.

Figure 2: Geometric parameters for two nozzle flame synthesis. Figure 3: XRD pattern of the as-prepared

Pt/BaZAl₂O₃. a: prepared using a single nozzle; b: precursor system A using two nozzles (di =2.5cm, d₂=6.5cm); c-e: precursor system B using two nozzles (di=2.5cm, d₂=2.5 , 4.5 and 6.5 cm for c, d and e, respectively). Figure 4: TPD profiles for Pt/Ba/Al₂O₃ made with precursor B by two nozzle flame spray pyrolysis as function of nozzle 2 to center distance (d₂) .

Figure 5: Relative amount of Ba in the form of crystalline BaCO₃ as estimated from XRD analysis and in the form of BaCO₃ as measured by the total amount of CO₂ desorbed during TPD as a function of nozzle 2 to center distance (d₂) for powders consisting of Pt/BaZAl₂O₃ (1:20:100) prepared with precursor B (α = 120°, di = 2.5 cm) . ^• Figure 6: TEM image and corresponding EDX analysis of Pt/Ba/Al₂θ₃ made by two nozzle flame spray synthesis . Figure 7 : STEM images and corresponding EDX spot analyses (arrow) of Pt/Ba/Al₂θ₃ made by two nozzle flame spray synthesis. Well dispersed BaCO₃ species are discernible as bright, rough shaped particles together with very fine Al₂O₃ particles.

Figure 8: Typical TPD-NO profile for Pt/Ba/Al₂O₃ after saturation with NO.

Figure 9 : NO storage capacity as a percentage of the theoretical maximum (all Ba converted into Ba (NO₃) ₂) of Pt/Ba/Al₂O₃ made with precursor B (α = 120°, di = 2..5 cm) as a function of the distance of nozzle 2 from the center (d2) .

Figure 10 shows different kinds of nanoparti- cles that may be comprised in a nanoparticulate powder with controlled mixedness of the present invention or which may constitute such nanoparticulate powder.

Modes for Carrying out the Invention

As already mentioned above, the term "nano- particles with controlled mixedness" means mixtures of particles that are distinguishable with respect to their chemical composition and/or their physical properties, but also compositions comprising nanoparticles of one or more than one kind, at least one kind of said nanoparticles being within themselves inhomogeneous . Possible nanoparticles or powder compositions covered by said definition are represented in Figure 10. The composition designated A shows a composition composed of two kinds of nanoparticles wherefrom each kind may be homogeneous or homogneously mixed on a nano level or atomic level. Composition B shows a nanoparticle of one kind on the surface of which nanoparticles of a second kind are depos- ited or to the surface of which nanoparticles of a second kind are to some extent sintered. C shows one nanoparticle which is a fusion of two nanoparticles of different kind and D shows a nanoparticle which is an inclusion body as it might result from strong sintering or if the "matrix" is formed after the formation of the "included" particles. It is also within the scope of the present in- vention to e.g. produce mixtures comprising nanoparticles of more than one of the different kinds A to D shown in Figure 10.

A presently preferred nanoparticulate powder of the present invention is Pt/Ba/Al₂O₃. Although the product described herein was shown to be in general Pt/BaCO₃/Al₂Os, and since it is generally accepted to name such products irrespective of the actually present Ba compound Pt/Ba/Al₂O_3/ this generally accepted nomenclature was adopted in the scope of the present specification. Pt/Ba/Al2θ3 can well be synthesized by using two separate flame spray nozzles. Each spray nozzle can be fed with at least one and preferably one liquid precursor composition, much preferred precursor solutions. One of said precursor solutions suitably comprises the aluminum salt precursor, the second precursor solution the barium salt . The platinum precursor can be added to the first and/or second precursor solution. If the barium comprising precursor solution is sprayed into an aluminum comprising combustion flame, a mixed particulate powder can be obtained wherein the barium forms crystalline BaCO₃ nanoparticles, discernible from TEM/EDX and XRD analysis, whereas with the single nozzle process only amorphous Ba species could be identified. The BaCO₃ and Al₂O₃ particles were well mixed on a submicron scale. The platinum was detected to be present as platinum metal and/or platinum oxide. The as-prepared materials showed high NO_x-Storage capacity compared to conventionally, wet-phase made catalysts. The geometry of the two nozzles resulted in a mixing of the two flames at different tem- peratures and states of particle formation and it could be shown that this geometry plays an important role in the final product properties, i.e. BaCO₃ con- tent/crystallinity and NO_x storage capacity. NO_x storage/reduction (NSR) catalysts based on Pt/Ba/Al are well known. However, the generally used methods lead to Ba/Al mixed oxides that are at least predominately amorphous . By the present invention, nanoparticles of pure AI2O3 are obtained that have the advantage to enhance the thermal stability. The BaO (or rather BaCU3) and the platinum form separate particles . The platinum condenses as metal and/or metal oxide after the barium carbonate particles have been formed.

It was found that by the inventive method a large amount of crystalline BaCθ3 is formed which is considered as having improved NO_x storage capacity. Surprisingly, the analysis of the barium particles revealed that only BaCθ3 was formed from which CO2 can be released below 900°. The BaO/BaCO3 particles are mixed with the AI2O3 particles in a relatively cold part of the flame to prevent fusion of the particles.

Since according to the invention individual particles are formed that need no further processing, the original pore size is not reduced as it is in the case of conventional impregnation methods .

The ratio of Pt: Ba : AI2O3 in general is about 0 to 5 : 5 to 100 : 100, in particular 1 : 10 to 40 : 100.

Materials of this composition are applicable as NO_x-storage-reduction catalysts for the treatment of exhaust gases emerging from engines operated under lean conditions . Other materials produced according to the present invention can e.g. be tooth fillers visible in x- ray analysis, but also polymer fillers with e.g. one particle type enhancing the mechanical stability, and a second particle type providing UV-stability . Multi nozzle flame synthesis may be applied for various materials consisting of at least two components in the form of individual nanoparticles that are well mixed on a submicron scale, and it can lead to materials with favorable properties .

Examples : Materials and apparatus

A nozzle suitable for flame spray pyrolysis (FSP) consists of a two phase spray nozzle as described earlier [V] . Two of these spray nozzles were used for the synthesis of submicron mixed nanoparticles . Two liquid precursor compositions were fed through the two nozzles, where they were separately dispersed, ignited and combusted, resulting in two spray flames. The two nozzles were aligned in such a way that the two flames mixed at a well defined height. Various metal compounds can be used as precursors for the conversion into metal oxides and/or metals in the FSP process, i.e. alkoxides, acetates, nitrates, carbonates, acetonates, and caboxylates.

In a particular example this apparatus and such precursors were used for the flame spray synthesis of Pt/Ba/Al₂θ₃- This material is specifically interesting as a NO_x-storage/reduction catalyst in car tail exhaust treatment of lean-fuel engines [8, 9], where a high NO_x storage capacity is essential. Two different precursor systems were investigated for the production of this catalyst system. Each precursor system consisted of two solutions, one containing the Al precursor and the other containing the Ba and Pt precursor.

Precursor system A Precursor 1: Aluminium tri-sec-butoxide was dissolved in a 1:1 mixture of diethylene glycol monobutyl ether and acetic anhydride resulting in a molar Al concentration of 0.5 M.

Precursor 2 : Barium 2-ethylhexanoate and platinum acetylacetonate were dissolved in a sec- butanol/2-ethylhexanoic acid (10:1) mixture. Precursor System B

Precursor 1: Aluminium tri-sec-butoxide was dissolved in sec-butanol resulting in a molar Al concentration of 0.5 M. Precursor 2: Barium 2-ethylhexanoate and platinum acetylacetonate were dissolved in a sec- butanol/2-ethylhexanoic acid (10:1) mixture.

Example 1: Production of nanoparticulate Pt/Ba/Al₂O₃ powder

Each liquid precursor composition was dispersed simultaneously by an individual flame spray nozzle and separately ignited resulting in two flames as shown in Figure 2. The angle between the two nozzles ((X) was kept constant at 120°. The center of nozzle 1 (precursor 1) was placed at 2.5 cm (di) , whereas the distance of nozzle 2 (d.2) was varied between 2.5 and 6.5 cm from the center (see Figure 2) . The feed rate of precursor 1 was 5 ml/min which was dispersed by oxygen (5 L/min) . The feed rate of precursor 2 was 3 ml/min and the dispersion gas flow rate was 5 L/min. The as-prepared material was collected on a glass-fiber filter with the aid of a vacuum pump. The nominal weight ratio Pt: Ba -.A^O₃ of all samples was 1:20:100. For comparison a powder with the same composition was made with a conventional, one nozzle system [1] . The liquid precursor composition corresponded to precursor system B, as described above. However, all three components (Al, Ba, and Pt) were dissolved together in sec-butanol resulting in one liquid precursor composition.

Example 2 : Investigation of the materials produced according to Example 1 Figure 3 shows X-ray diffraction (XRD) patterns of Pt/Ba/Al2θ₃ powders prepared using one or two nozzles. The material produced with one nozzle (a) showed only crystalline peaks attributed to the gamma-Al₂θ₃ phase (ICSD 28260) and no crystalline Ba-containing phases were detectable. However, the samples prepared with the two nozzle system exhibited crystalline BaCO₃ in the form of monoclinic BaCO₃ (ICSD 63257) or witherite (ICSD 15196) in the case of precursor A (b) or B (c-e) , respectively. The XRD patterns were used to obtain the mass fraction of crystalline BaCO₃ based on the fundamental parameter approach and the Rietveld method (Topas) . Ti02 (Degussa, P25) was used as internal standard in the XRD analysis. Increasing the distance of nozzle 2 from the center (d₂, Figure 2) resulted in a larger amount of crystalline BaCO₃ formed (Figure 5) .

In addition to the XRD measurements, tempera- ture programmed desorption / decomposition (TPD) was used to quantify the amount of barium in the form of BaCO₃. Figure 4 depicts BaCO₃ decomposition profiles [10] , measured by mass spectroscopy on M/Z = 44, for a powder prepared with two nozzles with nozzle 2 placed at three dif- ferent distances (Precursor B). Bellow 300 ⁰C physically adsorbed CO₂ desorbs from the surface. The main peak between 600 - 650 ⁰C can be attributed to the thermal decomposition of BaCO₃ in the form of low LT-BaCO₃ [10] . Bulk BaCO₃ or HT-BaCO₃ would decompose at temperatures above 900 ⁰C, which was not observed for any flame-made materials [10] . Figure 5 depicts the percentage of Ba in the form of BaCO₃ derived from the total amount of CO₂ desorbed during TPD for different flame-made materials. To account for physically adsorbed CO2 15% of the total amount of CO2 desorbed -was subtracted. Increasing the distance of nozzle 2 from the center (d₂) , or in other words the distance between the two flames, resulted in a higher amount of barium in the form of BaCO₃, corroborating the results of the XRD analysis (Fig. 3), where an increase in the distance between the two nozzles resulted in a higher fraction of crystalline BaCO₃ formed (Fig. 5) . This indicates that the temperature and the state of newly formed particles at the point of mixing of the two flames have a crucial influence on the final material properties. For each individual spray-flame the flame temperature decreases with height above nozzle. An in- crease of the distance of the two nozzles results in the mixing of the two flames further downstream, where burning and reaction of the precursors had mostly proceeded and cooling of the mixing jets by air entrainment becomes important. Therefore, the particle mixing takes place at lower temperatures. It may be assumed, that in the case of barium and alumina the lower flame temperatures in the mixing zone favor the formation of crystalline BaCO₃ species/particles .

The homogeneous distribution of the BaCU₃ particles was confirmed by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) analysis. Figure 6 shows a TEM-image of a Pt/Ba/Al₂θ₃ powder prepared by using two nozzles with the corresponding energy-dispersive X-ray spectroscopy (EDX) analysis. EDX analysis of a region with diameter of 2 μin revealed well dispersed BaCO₃ particles on the AI₂O₃ support or in other words homogenously mixed nanoparticles on a submicron scale. The STEM images as shown in Figure 7 revealed small BaCO₃ particles (ca. 20nm) that are well dispersed over the AI₂O3 support. EDX spot analysis of a small area (circle) further corroborates the formation of BaCO₃ and Al₂O₃ particles .

Example 3: Catalytic application of the as- derived nano-powders

The NO_x storage capacity of the as-prepared powders was measured by saturating the catalyst with pulses of NO (0.35 ml) in a flow of 5% O₂ in He at 300 ⁰C. The NO_x storage capacity was then determined by meas- uring the amount of NO (M/Z = 30) desorbed during temperature programmed decomposition (TPD) in flowing helium between 300 and 900⁰C. Figure 8 shows a typical TPD-NO evolution profile after saturation with NO for a flame- made material. The amount of desorbed NO was quantified by injecting a calibrated pulse of NO after each measurement. Figure 9 depicts the NO_x storage capacity for different powders prepared with the two nozzle system (precursor B) . These catalysts, and especially the ones made with higher internozzle distances, exhibited a high NO_x storage capacity of up to 69% compared to materials prepared by conventional wet-phase preparation techniques, where usually NO_x storage capacities bellow 40% are observed [8] . Increasing the nozzle distance, lowered the temperature in the mixing zone that resulted in higher NO_x storage capacities, indicating that small, crystalline BaCO₃ particles are more active than amorphous Ba-species in the NO_x storage process (compare Figure 5) . Compared to the high storage capacity of the materials prepared with two nozzles, the single-nozzle-made catalysts showed only a very low NO_x storage capacity in the order of no Ba containing Pt/Al₂O₃. This small amount can be attributed to the adsorption of NO_x on the AI₂O₃ surface, and indicates that the amorphous Ba contaning powders made with one nozzle (Fig. 3) are rather inactive for NO_x storage. While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims. References

1. L. Madler, H. K. Kammler, R. Mύller, and S. E. Prat- sinis, J. Aerosol. Sci . , 33(2), 369-389 (2002).

2. S. Shanmugham, A. T. Hunt, G. Deshpande, T.J. Hwang, E. Moore, and Y. Jiang, WO0202246 (January 2002) and US20030047617 (March 2003 ),

3. W.J. Stark, L. Madler, M. Maciejewski, S. E. Prat- sinis, and A. Baiker, Chem. Commun. , (5), 588-589 (2003) . 4. W.J. Stark, L. Madler, and S. E. Pratsinis, WO

2004005184 Al and EP 1378489 Al, (January 2004) .

5. P. Burtscher, L. Madler, S. E. Pratsinis, and N. Moszner, DE 10 2004 006 564.0, (pending 08.02.2004).

6. T. Tani, L. Madler, and S. E. Pratsinis, J. Mater. Sci., 37 (21) , 4627-4632 (2002).

7. L. Madler, W.J. Stark, and S. E. Pratsinis, J. Mater. Res., 17(6), 1356-1362 (2002).

8. W. S. Epling, L. E. Campbell, A. Yezerets, N.W. Currier, and J. E. Parks II, Catalysis Reviews, 46(2), 163-245 (2004) .

9. N. Miyoshi, T. Tanziawa, K. Kasahara, and S. Tateishi, ΞP0669157, (August 1995).

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Claims

1. A flame spray method for the production of nanopowders with controlled mixedness at the submicro- level of individual nanoparticles, said method comprising i) providing at least two spray nozzles suitable for performing flame spray pyrolysis, each spray nozzle being connected to at least one reservoir, each reservoir comprising a liquid precursor composition, each precursor composition comprising at least one dissolved or finely divided precursor compound, with the proviso that at least one precursor compound is dissolved, ii) positioning said at least two spray nozzles at an angle and in a distance suitable for getting the desired non-segregated submicron mixture of individual nanoparticles, iii) feeding said at least two liquid precursor compositions to their respective spray nozzle, iv) dispersing, igniting, combusting and mix- ing said at least two liquid precursor compositions, and v) collecting the nanopowder.

2. The method of claim 1, wherein in step iv) only one of the dispersed liquid precursor compositions is ignited prior to the mixing of said at least two liq- uid precursor compositions .

3. The method of claim 1, wherein in step iv) all of the dispersed liquid precursor compositions are ignited prior to the mixing of said at least two liquid precursor compositions.

4. The method of anyone of the preceding claims, wherein two liquid precursor compositions and two nozzles are used.

5. The method of anyone of the preceding claims, wherein at least one liquid precursor composition is a precursor solution.

6. The method of anyone of the preceding claims, wherein all liquid precursor compositions are precursor solutions .

7. The method of anyone of the preceding claims, wherein the solvent or solvent mixture comprised in the liquid precursor composition is ignitable.

8. The method of claim 7, wherein the combustible organic solvent or the mixture of combustible organic solvents is selected from the group consisting of alcohols, acids, aromatic and aliphatic hydrocarbons, ethers, ketones, aldehydes, glycoles, and mixtures thereof .

9. The method of anyone of the preceding claims, wherein the precursors are dissolved or finely divided metal salts with combustible organic anions e.g. anions selected from alkoxides , acetates, acetonates, carboxylates as well as nitrates, carbonates and mixed salts of any of the above mentioned anions, or an oxide and/or a carbonate powder.

10. The method of anyone of the preceding claims, wherein the precursors are dissolved or finely divided organometallic compounds .

11. The method of anyone of the preceding claims, wherein the metals comprised in the precursors are selected from silicon, titanium, strontium, cerium, zirconium, palladium, tantalum, zinc, silver, gold, ruthenium, and in particular from barium, aluminum and platinum, and mixtures thereof.

12. The method of anyone of the preceding claims, wherein both liquid precursor compositions are precursor solutions and wherein the first precursor solution comprises an aluminum salt as precursor, and wherein the second precursor solution comprises a barium salt as precursor and wherein either the first or the second pre- cursor solution comprises a platinum salt as precursors, preferably the second solution.

13. The method of anyone of the preceding claims, wherein the spray nozzles are individually adjustable .

14. A nanopowder producable according to the method of anyone of the preceding claims, in particular a nanopowder comprising a non-segregated submicron mixture of individual nanoparticles .

15. The nanopowder of claim 14 which is a Pt/Ba/Al2θ3 powder comprising individual nanoparticles containing Ba predominately in the form of BaCC>3 and Al predominately in the form of AI2O3.

16. The nanopowder of claim 15, wherein the BaCC>3 releases CO2 at a temperature below 900⁰C.

17. The nanopowder of claim 15 or 16, wherein at least about 50% of the BaCC>3 is crystalline.