DE102011005489A1 - Enveloped iron oxide particles - Google Patents

Abstract

Process for the production of particles with an amorphous silicon dioxide shell and a core which contains hematite and at least one further iron oxide selected from magnetite and maghemite, in which magnetite particles and / or maghemite particles, an oxygen-containing gas or a hydrogen-containing gas, water vapor and causes a silicon compound to react at high temperature, subsequently optionally cooling and the fent.

Description

The invention relates to processes for the preparation of silica-coated iron oxide particles.

The inductive heating of composites containing magnetic particles is a promising way for a rapid and gentle curing of adhesive bonds or for the crosslinking of polymers. It has proven to be advantageous to use coated magnetic particles. The shell is attributed the function, on the one hand to improve the incorporation into the composites and on the other to prevent unwanted growth of the magnetic phases.

In US4280918 discloses a process for producing magnetic particles surrounded by a silica shell in which a dispersion containing colloidal silica particles having a negative zeta potential is present in a dispersion containing magnetic particles having a positive zeta potential. However, the particles thus produced, for example under shear conditions, do not have sufficient bonding of the magnetic core and the silica shell.

In WO03 / 58649 discloses a process for producing silica-coated iron oxide containing less than 20% by weight of silicon, in which silicate deposition from water glass or silica sol on the iron oxide particles occurs.

In DE-A-102006054173 discloses a process for producing porous magnetic silica particles. In this case, a pH-value modifier and an organic pore-forming agent are added to a liquid reaction mixture which contains magnetic particles and at least one compound serving as the silica source, and the resulting reaction mixture is subsequently spray-dried.

In WO02 / 09125 discloses an oil dispersion crosslinking process for preparing spherical pore size magnetic silica particles comprising silica hydrosol formed by acid-catalytic hydrolysis of an aqueous silane dispersion and admixed with a magnetic colloid or ferrofluid or magnetic particles; dispersed in a water-immiscible organic phase and crosslinked during the dispersion process by adding a base.

In EP-A-961653 there is disclosed a method of making low core temperature core-shell particles wherein the shell is silicon dioxide and the core is superparamagnetic iron oxides, wherein a silica precursor is added to a dispersion of the core material, followed by adding acid to the pH Value is lowered to 6 to 9.

In WO2010 / 063557 discloses iron-silicon oxide particles having a core-shell structure which has a BET surface area of 10 to 80 m ² / g, a shell thickness of 2 to 30 nm and an iron oxide content of 60 to 90% by weight. wherein the core is crystalline and the iron oxides comprise hematite, magnetite and maghemite, the shell is made of amorphous silica and at least partially present between shell and core are one or more compounds consisting of the elements silicon, iron and oxygen. It is obtained by a flame oxidation / flame hydrolysis process in which a mixture of an aerosol of an iron oxide precursor and a vaporous silicon compound burns under defined conditions in a flame.

The majority of the processes mentioned in the prior art are those which take place in liquid, generally aqueous, phase. At the in WO2010 / 063557 The disclosed process is a gas phase process. The material thus produced is ideally suited for inductive heating.

The technical object of the present invention is to provide further methods resulting in coated magnetic particles. Here, the core, as in the case of in WO2010 / 063557 In addition to magnetic iron oxides, the disclosed gas phase method also contains the non-magnetic hematite, since obviously the combination of magnetic iron oxides and hematite is a particularly advantageous composition for inductive heating.

• a) Magnetitpartikel und/oder Maghemitpartikel
• b) ein Sauerstoff enthaltendes Gas oder ein Wasserstoff enthaltendes Gas
• c) Wasserdampf und
• d) eine Siliciumverbindung

zur Reaktion bringt, nachfolgend gegebenenfalls kühlt und den Feststoff von gas- oder dampfförmigen Stoffen abtrennt.A first object of the invention is a process for the production of particles with an envelope of amorphous silica and a core containing hematite and at least one further iron oxide selected from magnetite and maghemite, wherein at a temperature of 800 to 1200 ° C, preferably from 900 to 1100 ° C, comprising a stream

• a) magnetite particles and / or maghemite particles
• b) an oxygen-containing gas or a hydrogen-containing gas
• c) water vapor and
• d) a silicon compound

is reacted, then optionally cooled and the solid separated from gaseous or vaporous substances.

The temperatures of 800 to 1200 ° C required in this process according to the invention can be provided for example in the form of an external heating. Preferably, they are formed by a flame resulting from the ignition of a mixture containing at least one hydrogen-containing fuel gas and an oxygen-containing gas. Suitable hydrogen-containing fuel gases are hydrogen, methane, ethane and / or propane, with hydrogen being used with particular preference. The molar ratio of O ₂ in the oxygen-containing gas / hydrogen-containing fuel gas is preferably from 0.8 to 1.5 and particularly preferably 1.05 to 1.3.

This can be an internal or external flame. In the case of an internal flame, the reaction of the starting materials, iron oxide or iron oxide and silicon dioxide precursor, takes place in the presence of the flame or its reaction products, such as water in the case of a hydrogen / oxygen flame. In the case of an external flame, this merely provides the temperature for reacting the starting materials, while the reaction products are conducted separately from the reaction or are only subsequently combined with the products of the reaction.

Water vapor is formed by the reaction of the hydrogen-containing fuel gas and the oxygen-containing gas. This may represent part or all of the water vapor required in the process according to the invention.

The average residence time in the region of the reactor in which the temperature of 800 to 1200 ° C is preferably 10 ms to 1 s.

• a) Magnetitpartikel und/oder Maghemitpartikel
• b) ein Sauerstoff enthaltendes Gas oder ein Wasserstoff enthaltendes Gas und
• c) Wasserdampf

zur Reaktion bringt, und in einem zweiten, dem ersten nachfolgenden, Bereich des Reaktors bei einer Temperatur von 400 bis 800°C, bevorzugt 600 bis 650°C, wobei die Temperatur im zweiten Bereich geringer ist als im ersten Bereich, einen Strom
umfassend

• d) das den ersten Bereich verlassende Reaktionsgemisch und eine Siliciumverbindung und
• e) Wasserdampf

zur Reaktion bringt und das den zweiten Bereich verlassende Reaktionsgemisch gegebenenfalls kühlt, bevorzugt durch Einspeisung von Wasser, und nachfolgend den Feststoff von gas- oder dampfförmigen Stoffen abtrennt.A second aspect of the invention is a process for producing particles comprising an amorphous silica shell and a core containing hematite and at least one further iron oxide selected from magnetite and maghemite, in which
in a first region of a reactor at a temperature of 650 to 1000 ° C, preferably 700 to 850 ° C, comprising a stream

• a) magnetite particles and / or maghemite particles
• b) an oxygen-containing gas or a hydrogen-containing gas and
• c) water vapor

brings to a reaction, and in a second, the first subsequent, region of the reactor at a temperature of 400 to 800 ° C, preferably 600 to 650 ° C, wherein the temperature in the second region is lower than in the first region, a stream
full

• d) the reaction mixture leaving the first region and a silicon compound and
• e) water vapor

brings to reaction and optionally the reaction mixture leaving the second region, preferably by feeding water, and subsequently separating the solid from gaseous or vaporous substances.

• a) Magnetitpartikel und/oder Maghemitpartikel und einen Teil der Gesamtmenge einer Siliciumverbindung und
• b) ein Sauerstoff enthaltendes Gas oder ein Wasserstoff enthaltendes Gas und
• c) Wasserdampf

zur Reaktion bringt und gegebenenfalls in einem zweiten, dem ersten nachfolgenden, Bereich des Reaktors bei einer Temperatur von 400 bis 800°C, bevorzugt 600 bis 650°C, wobei die Temperatur im zweiten Bereich geringer ist als im ersten Bereich, einen Strom umfassend

• d) das den ersten Bereich verlassende Reaktionsgemisch und den verbleibenden Teil der Gesamtmenge der Siliciumverbindung und
• e) Wasserdampf

zur Reaktion bringt und das den zweiten Bereich verlassende Reaktionsgemisch gegebenenfalls kühlt, bevorzugt durch Einspeisung von Wasser, und nachfolgend den Feststoff von gas- oder dampfförmigen Stoffen abtrennt.A third object of the invention is a process for the production of particles with an envelope of amorphous silica and a core containing hematite and at least one further iron oxide selected from magnetite and maghemite, in which
in a first region of a reactor at a temperature of 650 to 1000 ° C, preferably 700 to 850 ° C, comprising a stream

• a) Magnetitpartikel and / or Maghemitpartikel and a part of the total amount of a silicon compound and
• b) an oxygen-containing gas or a hydrogen-containing gas and
• c) water vapor

brings to reaction and optionally in a second, the first subsequent, region of the reactor at a temperature of 400 to 800 ° C, preferably 600 to 650 ° C, wherein the temperature in the second region is lower than in the first region comprising a stream

• d) the reaction mixture leaving the first region and the remaining part of the total amount of the silicon compound and
• e) water vapor

brings to reaction and optionally the reaction mixture leaving the second region, preferably by feeding water, and subsequently separating the solid from gaseous or vaporous substances.

The temperatures of 650 to 1000 ° C required in the process according to the invention can be provided for example in the form of an external heating. Preferably, they are formed by a flame resulting from the ignition of a mixture containing at least one hydrogen-containing fuel gas and an oxygen-containing gas. Suitable hydrogen-containing fuel gases are hydrogen, methane, ethane and / or propane, with hydrogen being used with particular preference. The molar ratio of O ₂ in the oxygen-containing gas / hydrogen-containing fuel gas is preferably 0.8 to 1.5, and more preferably 1.05 to 1.3.

This can be an internal or external flame. In the case of an internal flame, the reaction of the starting materials, iron oxide or iron oxide and silicon dioxide precursor, takes place in the presence of the flame or its reaction products, such as water in the case of a hydrogen / oxygen flame. In the case of an external flame, this merely provides the temperature for reacting the starting materials, while the reaction products are conducted separately from the reaction or are only subsequently combined with the products of the reaction. In 1 is schematically a method with an internal flame, in 2 a method with an external flame shown. In this case, A = maghemite and / or magnetite powder, B = air, C = hydrogen, D = silicon compound and E = cooling.

Water vapor is formed by the reaction of the hydrogen-containing fuel gas and the oxygen-containing gas. This may represent part or all of the water vapor required in the process according to the invention.

The average residence time in the region of the reactor in which the temperature is from 650 to 1000 ° C. is preferably 10 ms to 2 s. The mean residence time in the region of the reactor in which the temperature is from 400 to 800 ° C. is preferably from 0.1 to 10 s.

The embodiment of the invention in which the silicon compound is introduced both in the temperature range from 650 to 1000 ° C and in the temperature range from 400 to 800 ° C, is preferably carried out so that more than 50 mol% of the silicon compound, particularly preferably 60 to 90 mol%, are introduced in the temperature range of 400 to 800 ° C.

In the inventive method, the magnetite and / or hematite particles can be used either in powder form or as a dispersion. They are preferably introduced into the reactor as aerosol, either as solid / gaseous aerosol when using pulverulent particles or as solid / liquid / gaseous aerosol when using dispersions. For this purpose, the known to those skilled units, such as multi-component nozzles can be used. The carrier gas used is generally air or nitrogen. If a dispersion of maghemite and / or magnetite particles is used, their content in the dispersion is generally from 0.5 to 25% by weight, preferably from 3 to 10% by weight. The dispersion may contain dispersing additives such as polyacrylic acid and salts thereof. However, the simpler process variant consists in using the maghemite and / or magnetite particles as powder.

The magnetite and / or maghemite particles used in the process according to the invention have, in each case independently of one another, an average diameter of preferably 10 nm to 1 μm and particularly preferably 100 to 500 nm.

The silicon compounds used in the process according to the invention are hydrolyzable and / or oxidizable compounds. These are preferably selected from the group consisting of SiCl ₄ , CH ₃ SiCl ₃ , (CH ₃ ) ₂ SiCl ₂ , (CH ₃ ) ₃ SiCl, HSiCl ₃ , (CH ₃ ) ₂ HSiCl and CH ₃ C ₂ H ₅ SiCl ₂ , H ₄ Si, Si (OC ₂ H ₅ ) ₄ , Si (OCH ₃ ) ₄ or mixtures of the abovementioned. SiCl ₄ , and Si (OC ₂ H ₅ ) ₄ can be used with particular preference. SiCl ₄ can be very particularly preferred in the process steps that in the higher Temperature are carried out, namely from 800 to 1200 ° C or 650 to 1000 ° C are used. Si (OC ₂ H ₅ ) ₄ can very particularly preferably be used in the process steps which are carried out at the lower temperature, namely from 400 to 800 ° C.

In the process according to the invention, the weight ratio of maghemite particles and / or magnetite particles to the silicon compound, calculated as SiO ₂ , may be preferably 10:90 to 95: 5. In general, high induction temperatures are desired, so that a weight ratio of 60:40 to 95: 5 is to be regarded as particularly preferred.

The embodiment of the invention in which the silicon compound is introduced both in the temperature range from 650 to 1000 ° C and in the temperature range from 400 to 800 ° C, is preferably carried out so that more than 50 mol% of the silicon compound, particularly preferably 60 to 90 mol%, are introduced in the temperature range of 400 to 800 ° C.

The particles produced according to the methods of the invention consist of a dense envelope of amorphous silica and a core containing hematite and at least one further iron oxide selected from magnetite and maghemite. The processes according to the invention are preferably carried out such that the core contains magnetite, maghemite and hematite. Particular preference is given to a weight distribution of 30 to 50% maghemite, 40 to 60% hematite and 5 to 20% magnetite, the sum of the proportions adding up to 100%. The BET surface area of the particles is preferably 5 to 80 m ² / g. The thickness of the shell is preferably from 3 to 40 nm. The crystallite sizes of the core constituents determined by means of Debye-Scherrer are preferably less than 100 nm. The crystallite size thus determined of hematite is 40 to 70 nm, magnetite 30 to 90 nm and maghemite 20 are particularly preferred up to 40 nm.

In contrast to the silica-coated iron oxide particles obtainable in the prior art, the processes according to the invention allow the production of particles having particularly high heating rates. During the production by the novel processes, complex conversions of the iron oxides take place at high temperatures, which apparently lead to a particularly favorable arrangement of the core constituents, which are bound to the amorphous silicon dioxide shell via chemical bonds. Magnetite and maghemite ensure the electromagnetic coupling of energy into the core, while hematite provides for high thermal conductivity.

Examples

analytics

The content of iron oxide is determined by digestion with NaOH, dissolution in dilute H ₂ SO ₄ and subsequent iodometric titration. The BET surface area is determined by DIN 66131 ,

The quantitative determination of the core components is carried out by X-ray diffractometry (reflection, θ / θ diffractometer, Co-K _α , U = 40 kV, I = 35 mA, scintillation counter, simulated graphite monochromator, angular range (2Θ) / step size / measuring time: 10-100 ° / 0.04 ° / 6 s (4 h)). The hematite is clearly identifiable because of the freestanding reflexes. The maghemite is significantly detectable by the reflexes (110) and (211) in the front angle range. Using the Rietveld method, a quantitative phase analysis is performed (error about 10% relative). Quantitative phase analysis is performed using set 60 of the ICDD database PDF4 + (2010). The quantitative phase analysis and determination of the crystallite done with the Rietveld program SiroQuant ^® version 3.0 (2005).

The thickness of the outer shell is determined by transmission electron microscopy (TEM). In addition, it is checked whether the shell is tight. By this is meant that upon contact of the particles with hydrochloric acid less than 50 ppm of iron are detectable. At room temperature, 0.33 g of the particles are brought into contact with 20 ml of 1 N hydrochloric acid solution for 15 minutes. Part of the solution is subsequently analyzed for iron by means of suitable analysis techniques, for example ICP (inductively coupled plasma spectroscopy).

The heating time up to a temperature of 100 ° C is determined in a silicone composition. The silicone composition is obtained by mixing 33 g ELASTOSIL ^® E50, Fa. Momentive Performance Materials, 13 g of silicone oil type M 1000, Fa. Momentive Performance Materials, 4 g of Aerosil ^® 150, Fa. Evonik Degussa and 2.5 g, equivalent to 4 , 76 wt .-% silica coated iron oxide mixed by means of a SpeedMixers 2 × 30 sec. And 2 × 45 sec. At 3000 rpm. Subsequently, the silicone composition in a thickness of about 1 mm on a Glass slide applied. The energy input is made by induction in the alternating magnetic field of a frequency of 670 MHz with an electrical power of 2.5 kW.

feedstocks

• Maghemite: Auer-Remy, mean particle size 20 to 50 nm, specific surface area> 30 m ² / g, approximately spherical.
• Magnetite: Auer-Remy, average particle size 20 to 30 nm, specific surface area> 60 m ² / g, spherical.

Examples 1 to 3 are carried out by means of indirect flame, examples 4 to 6 by means of direct flame.

EXAMPLE 1 In a first region of a flow-through reactor, a mixture of 1250 g / h of pulverulent maghemite and 4.5 Nm ³ / h of air is subjected to a temperature-controlled treatment in a high-temperature zone. The high-temperature zone is formed by an external hydrogen / oxygen flame, which results from ignition of 6.5 Nm ³ / h of hydrogen and 30 Nm ³ / h of air. The mean residence time of the reaction mixture in this first region is about 0.1 s. The temperature 50 cm below the burner mouth is 825 ° C.

In the second region of the flow-through reactor, 0.6 kg / h of vaporous Si (OC ₂ H ₅ ) ₄ together with 3 Nm ³ / h of nitrogen are added to the stream of the hot reaction mixture from the first region. The mean residence time of the reaction mixture in the second range is 0.9 s. The temperature 20 cm above the point of addition is about 640 ° C.

Subsequently, the reaction mixture is cooled and the resulting solid is deposited on a filter of the gaseous substances.

The solid has an iron oxide content, calculated as Fe ₂ O ₃ , of 89% by weight. Its BET surface area is 30 m ² / g. The proportions by weight of maghemite, magnetite and hematite are 54, 9 and 37%. The heating temperature after 60 s is 312 ° C. After treatment with hydrochloric acid, 15 ppm iron is detectable.

Example 2 is carried out analogously to Example 1, but the atomization of the magnetite takes place in a reducing atmosphere. Furthermore, the flame parameters are selected differently than in Example 1, so that the temperature in the first region is lower.

Example 3 is carried out analogously to Example 1, but a 1: 1 mixture, each 0.325 kg / h, of magnetite and maghemite is used.

Example 4: In a first region of a flow reactor, a mixture of 800 g / h of pulverulent maghemite and 4.5 Nm ³ / h of air are reacted in a hydrogen / oxygen flame. The flame results from the ignition of a mixture of 8 Nm ³ / h hydrogen and 21 Nm ³ / h air results. The mean residence time of the reaction mixture in this first range is about 1.2 s. The temperature 50 cm below the burner mouth is 1061 ° C.

Subsequently, the reaction mixture is cooled and the resulting solid is deposited on a filter of the gaseous substances.

The solid has an iron oxide content, calculated as Fe ₂ O ₃ , of 82% by weight. Its BET surface area is 30 m ² / g. The proportions by weight of maghemite, magnetite and hematite are 20, 62 and 18%. The heating temperature after 60 s is 352 ° C. After treatment with hydrochloric acid, 25 ppm iron is detectable.

Example 5 is carried out analogously to Example 4, but magnetite is used instead of maghemite.

Example 6 is carried out analogously to Example 4, but a 1: 1 mixture of magnetite and maghemite is used and the atomization takes place in a reducing atmosphere.

The starting materials and reaction conditions as well as the physicochemical properties of the powders obtained are summarized in Table 1.

The examples show that it is possible by means of the processes according to the invention to produce silica-coated iron oxide particles. The short-term high-temperature treatment makes it possible to modify the iron oxide particles used so that the core of the particles produced by the process according to the invention contains three iron oxide modifications, namely, irrespective of the iron oxide, magnetite, maghemite and hematite used. The composition of the core can be varied as shown in the examples and leads to high temperatures during inductive heating. Table 1: Starting materials and reaction conditions; physicochemical data example 1 2 3 4 5 6 Iron oxide [kg / h] Maghemite 1.25 Magnetite 1.25 Maghemite: magnetite ^e) 0.65 Maghemite 1.25 Magnetite 1.25 Maghemite: magnetite ^e) 1,25 SiO ₂ precursor [kg / h] TEOS ^f) 0.60 TEOS 0.60 TEOS 0.17 SiCl ₄ 0.80 SiCl ₄ 0.80 SiCl ₄ 0.60 Flame Arrangement H ₂ [Nm ³ / h] Air [Nm ³ / h] lambda indirectly 6.5 30 1.94 indirectly 5.5 30 2.29 indirectly 5.0 30 2.52 directly 8.0 21 1,10 directly 8.0 20 1.05 directly 8.0 19 1.00 Atomization air [Nm ³ / h] N ₂ [Nm ³ / h] H ₂ / N ₂ ^a) [Nm ³ / h] 4,5 - - - - 4,5 2,0 - - 4,5 - - - 4,5 - - - 4,5 Temperature [° C] 1st area ^b) 2nd area ^c) 825 640 748,623 740 610 1061 - 1053 - 1057 - Avg. Dwell time [s] 1st area 2nd area 0.10 0.9 0.11 1.0 0.34 1.1 1,2 - 1,3 - 1,4 - Particle BET [m ² / g] hematite [%] magnetite [%] maghemite [%] t _heating ^d) [S] Fe in HCl [ppm] 30 54 9 37 6.5 15 26 46 11 43 6,0 12 61 52 12 36 6.3 14 71 20 62 18 4,1 25 75 35 30 35 5.9 32 72 28 42 30 5.5 33 a) 90:10 H ₂ / N _2; b) 50 cm below burner mouth; c) second area 20 cm above the addition point); d) heating time up to 100 ° C; 670 MHz; in silicone matrix; e) 1: 1 parts by weight; f) TEOS = Si (OC ₂ H ₅ ) ₄

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 4280918 [0003]
• WO 03/58649 [0004]
• DE 102006054173 A [0005]
• WO 02/09125 [0006]
• EP 961653A [0007]
• WO 2010/063557 [0008, 0009, 0010]

Cited non-patent literature

• DIN 66131 [0030]

Claims (15)

A process for the production of particles with an envelope of amorphous silica and a core containing hematite and at least one further iron oxide selected from magnetite and maghemite, characterized in that at a temperature of 800 to 1200 ° C, a stream comprising a) magnetite particles and or Maghemitpartikel b) an oxygen-containing gas or a hydrogen-containing gas c) water vapor and d) brings a silicon compound to the reaction, then optionally cooled and separates the solid from gaseous or vaporous materials. A method according to claim 1, characterized in that the temperature of 800 to 1200 ° C is formed by a flame resulting from the ignition of a mixture containing at least one hydrogen-containing fuel gas and an oxygen-containing gas. A method according to claim 2, characterized in that the molar ratio of O ₂ in the oxygen-containing gas / hydrogen-containing fuel gas is 0.8 to 1.5. Process according to claims 1 to 3, characterized in that in the region of the reactor in which the temperature of 800 to 1200 ° C, the average residence time is 10 ms to 1 s. A method of producing particles having an amorphous silica shell and a core containing hematite and at least one further iron oxide selected from magnetite and maghemite, characterized in that in a first region of a reactor at a temperature of 650 to 1000 ° C comprising a stream a) magnetite particles and / or maghemite particles b) an oxygen-containing gas or a hydrogen-containing gas and c) water vapor reacting, and in a second, the first subsequent, region of the reactor at a temperature of 400 to 800 ° C, wherein the temperature in the second region is lower than in the first region comprising a stream d) the reaction mixture leaving the first region and a silicon compound and e) water vapor brings to a reaction and the reaction mixture leaving the second region is optionally cooled and subsequently separates the solid from gaseous or vaporous substances. A method of producing particles having an amorphous silica shell and a core containing hematite and at least one further iron oxide selected from magnetite and maghemite, characterized in that one in a first region of a reactor at a temperature of 650 to 1000 ° C comprising a stream a) Magnetitpartikel and / or Maghemitpartikel and a part of the total amount of a silicon compound and b) an oxygen-containing gas or a hydrogen-containing gas and c) water vapor brings to reaction and in a second, the first subsequent, region of the reactor at a temperature of 400 to 800 ° C, wherein the temperature in the second region is lower than in the first region comprising a stream d) the reaction mixture leaving the first region and the remaining part of the total amount of the silicon compound and e) water vapor brings to a reaction and the reaction mixture leaving the second region is optionally cooled and subsequently separates the solid from gaseous or vaporous substances. A method according to claims 5 or 6, characterized in that the temperature of 650 to 1000 ° C is formed by a flame resulting from the ignition of a mixture containing at least one hydrogen-containing fuel gas and an oxygen-containing gas. A method according to claim 7, characterized in that the molar ratio of O ₂ in the oxygen-containing gas / hydrogen-containing fuel gas is 0.8 to 1.5. Process according to claims 5 to 8, characterized in that in the region of the reactor in which the temperature of 650 to 1000 ° C, the average residence time is 10 ms to 2 s. A method according to claim 5 to 9, characterized in that in the region of the reactor in which the temperature of 400 to 800 ° C, the average residence time is 0.1 to 10 s. Process according to claims 1 to 10, characterized in that the maghemite particles and / or magnetite particles are introduced in the form of an aerosol. Process according to claims 1 to 11, characterized in that the maghemite particles and / or magnetite particles have an average diameter of 10 nm to 1 μm. Process according to claims 1 to 12, characterized in that the silicon compound is selected from the group consisting of SiCl ₄ , CH ₃ SiCl ₃ , (CH ₃ ) ₂ SiCl ₂ , (CH ₃ ) ₃ SiCl, HSiCl ₃ , (CH ₃ ) ₂ HSiCl and CH ₃ C ₂ H ₅ SiCl ₂ , H ₄ Si, Si (OC ₂ H ₅ ) ₄ and / or Si (OCH ₃ ) ₄ . Process according to claims 1 to 13, characterized in that the weight ratio of maghemite particles and / or magnetite particles to the silicon compound, calculated as SiO ₂ , is 10: 90-95: 5. Process according to Claims 5 to 14, characterized in that more than 50 mol% of the silicon compound is introduced in the region of the reactor in which the temperature is from 400 to 800 ° C.

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