DE10032400A1 - Immobilization of silver nanoparticles on a support, useful as a catalyst for the oxidation of alkanes, comprises addition of a compound to a silver salt solution to form a poorly soluble silver salt - Google Patents

Abstract

A process for the immobilization of silver nanoparticles (I) having a diameter of less than 10 nm on a support material by deposition of silver onto the dispersed support material comprises addition of a compound (II) that forms a poorly soluble salt with silver to a silver salt solution. An Independent claim is included for a catalyst (III) containing silver nanoparticles (I) immobilized on a support material.

Description

For gold nanoparticles immobilized on metal oxides are special Properties based on the presence of gold particles smaller than 10 nm be returned. These properties are particularly in one catalytic activity corresponding to the size of the gold particles, d. H. the selectivity and activity in a reaction depends on a. on the particle size from.

For example, catalyze immobilized on titanium dioxide Gold particles approximately 4 nm in diameter in one Hydrogen / oxygen mixture the epoxidation of propene, while at one Diameter of about 2 nm the hydrogenation of propene under otherwise the same conditions is catalyzed. [M. Haruta; Catal. Today 36 (1997) 153- 166]. The author expressly emphasizes that incipient-wetness- Impregnation made gold catalysts no such specific catalytic activity can be achieved, but only for by precipitation Gold Catalysts Manufactured [M. Haruta; Catalysis surveys Japan 1 (1997) 61- 73].

For gold nanoparticles immobilized on titanium dioxide by evaporation a gold-titanium mixture by means of an electrical plasma, subsequent rapid quenching of the metal vapor in a gas stream (DACS), and ultimately collecting the gas stream containing the colloids Introducing into a slurry containing the carrier was also produced Activity found for propene epoxidation [E. E. Stangland, K. B. Stavens, R.P. Andres, W.N. Delgass; J. Catal. 191 (2000) 332-347].

For silver immobilized on titanium dioxide by incipient wetness impregnation No such activity was found in particles [T. Hayashi, K. Tanaka, M. Haruta; J. Catal. 178 (1998) 566-575].  

The method for producing gold immobilized on metal oxides Nanoparticles are known [US 4,839,327], as is their usability as Oxidation, reduction and electrode catalysts as well as sensor elements.

For gold nanoparticles immobilized on titanium dioxide, the use as Catalysts for the selective production of oxygenates (in particular Alcohols, ketones and epoxides) from hydrocarbons [US 5,623,090] patented. Immobilized gold nanoparticles are also known Silicon dioxide carrier on which titanium dioxide is dispersed and their use as catalysts for the selective production of epoxides from olefins [US 5,965,754]. These catalysts are used to oxidize the respective Hydrocarbons in an oxygen / hydrogen atmosphere at low Temperatures possible.

These patents on the selective catalytic oxidation of hydrocarbons in Presence of hydrogen with the help of immobilized gold nanoparticles prove the wide applicability and do not mention any alternatives to gold Active components. The gold loading of these catalysts is typical high, which makes the production of the catalysts at considerable cost is connected, which affects the profitability of possible processes.

Inexpensive silver catalysts with small silver particles are known to be active and selective for crotonaldehyde hydrogenation to crotyl alcohol. The production of silver nanoparticles immobilized on TiO ₂ by incipient wetness impregnation results in a hydrogenation catalyst which selectively reduces crotonaldehyde to crotyl alcohol. Depending on the reduction temperature, this catalyst has very small silver particles: 2.8 nm at 200 ° C, 1.4 nm at 400 ° C. [P. Claus, H. Hofmeister; J. Phys. Chem. B 103 (1999) 2766-2775]. Silver can also be suitable as a catalyst in oxidation reactions. B. for the epoxidation of ethene or the oxidation of alcohols to aldehydes [G. Franz, RA Sheldon; Oxidation in Ullmann's Encyclopedia of Industrial Chemistry, Ed. 5, VCH Publishers, New York 1991, Vol. A 18, pp. 262-311]. However, catalytic properties similar to those described for the gold catalysts discovered by Haruta have so far been excluded for silver catalysts.

It has now surprisingly been found that silver catalysts precipitated on a support can be produced which have a comparable catalytic activity to gold catalysts of the Haruta type. For this purpose, silver nanoparticles are deposited on the carrier material dispersed in the solution from a silver salt solution with the addition of a compound which forms a poorly soluble salt with silver. The precipitate can optionally be tempered after drying. Metal oxides are suitable as carrier material, e.g. B. MnO ₂ , Fe ₂ O ₃ , Co ₃ O ₄ , NiO, CuO, CuMnO ₂ , MgO, Al ₂ O ₃ , SiO ₂ , V ₂ O ₅ , MoO ₃ , WO ₃ or mixed oxides of these metal oxides, preferably one titanium-containing carrier used, for. B. titanium dioxide, or a carrier coated with titanium dioxide. As a readily soluble silver salt z. B. silver nitrate, acetate, chlorate, perchlorate, bromate, fluoride, lactate or propionate can be used. To precipitate such a solution, a compound is added, which with z. B. forms a salt of the ions OH ^- _, O ^2- _, CO ₃ ^2- or C ₂ O ₄ ^2- . Catalysts produced in this way can, for. B. for the epoxidation of alkenes and for the selective oxidation of alkanes at low temperatures by means of a hydrogen / oxygen mixture, for the total oxidation of volatile organic hydrocarbons, for the hydrogenation of alkenes and oxygenates, as electrode catalysts, as gas sensors or for low-temperature oxidation of carbon monoxide become.

Surprisingly, these catalysts behave similarly to Catalysts based on gold nanoparticles, i. H. the epoxidation of Propene to propene oxide, for example, is characterized by a high selectivity (<90%), the only by-product being easily removable carbon dioxide is. These catalysts also show deactivation over several  Days passed, and moderate activity. Another common feature is that the Reaction takes place only in the presence of hydrogen and the catalyst is regenerable. TEM images of these catalysts show that silver Titanium dioxide is evenly dispersed on the carrier and the particles are smaller than 10 nm, mostly less than 5 nm.

Incipient wetness impregnation shows immobilized silver particles special catalytic activity is not, although even after this procedure produced catalysts, depending on the chosen synthesis parameters, Can have particle sizes smaller than 10 nm.

An equal relationship between the chosen synthetic route, i. H. precipitation or incipient wetness impregnation, and specific activity is also for Gold is known and is immobilized on a gold nanoparticle different or disadvantageous size distribution of the particles.

From the particle sizes we found for incipient wetness Impregnation on the one hand and silver nanoparticles produced by precipitation on the other hand, no size dependency can be derived. This leads to the Assumption that a specific silver-carrier interaction, that of chosen synthetic route depends, as the cause of the specific catalytic Activity. It has thus surprisingly been found that Silver catalysts have similar catalytic properties to those after Haruta synthesized gold catalysts when used appropriately getting produced. This enabled an inexpensive catalyst alternative for a number of uses are provided.  

example 1 catalyst preparation Preparation of a silver catalyst immobilized on titanium dioxide precipitation

In a solution of 1 g of sodium carbonate in 200 ml of chloride-free dist. water 1.5 g of titanium dioxide (Hombifine N, Sachtleben Chemie) were stirred dispersed and stirred for half an hour. The pH of the solution was 11. Zu this mixture was a solution of 55 mg silver nitrate in 100 ml chloride-free dist. Water within a minute with stirring with a Magnetic stirring core (600 revolutions / minute) evenly added. To After stirring for one hour, the titanium dioxide, together with the precipitate Silver compound from the supernatant solution whose pH was 11, filtered off, twice with 50 ml of chloride-free dist. Water washed and 6 Dried in air at 90 ° C for hours. Then the dried The filtrate is calcined, being heated to 250 ° C. at one degree per minute and this temperature was held for 10 hours. A silver titanium dioxide Obtained catalyst with 2.3% by weight of silver.

The silver-titanium dioxide catalyst obtained in this way showed on TEM images Silver particles with a maximum diameter of 5 nm. Through EDX analysis with a 10 nm measuring window could evenly distribute the silver be detected.

Example 2 Oxidation of propene with hydrogen and oxygen

In a tubular reactor with an inner diameter of 6 mm and a thermocouple inserted into the catalyst bed, 0.4 g of the silver-titanium dioxide catalyst (125-250 μm) synthesized according to Example 1 was introduced. The tubular reactor was heated so that the temperature measured in the catalyst bed was a constant 50 ° C. A gas stream of propene, oxygen, hydrogen and nitrogen (volume ratio 10/10/10/70) was then passed through the catalyst bed at a volume flow rate of 800 ml / h (weight-related space velocity: 2000 ml g (cat.) ^-1 h ^-1 ) directed. The product gas stream from the catalyst bed was analyzed with a gas chromatograph to determine the reaction conversions. The reaction results are shown in Table 1.

Table 1

Example 3 Oxidation of propene with hydrogen and oxygen

In a parallel reactor with 16 catalyst receptacles with an inner diameter of 5 mm, 100 mg each of silver-titanium dioxide catalysts synthesized according to Example 1 with different silver weight contents and calcination temperatures were introduced. A gas flow of propene, oxygen, hydrogen and nitrogen (volume ratio 10/10/10/70) was through the 16 catalyst recordings with a volume flow rate of 6400 ml / h (weight-related space velocity: 4000 ml g (cat.) ^-1 h ^-1 ) directed. The reactor temperature was 50 ° C. The product gas stream from the respectively measured catalyst uptake was analyzed with a gas chromatograph in order to determine the reaction conversions. The propene oxide yields determined for various catalysts are shown in Table 2. In the same way, an already deactivated catalyst with a silver weight content of 2%, which was regenerated at 400 ° C. in an air atmosphere, was tested.

Table 2

Example 4 Oxidation of carbon monoxide

In a parallel reactor with 16 catalyst receptacles with an inner diameter of 5 mm, 100 mg each of silver-titanium dioxide catalysts synthesized according to Example 1 with different silver weight contents and calcination temperatures were introduced. The parallel reactor was heated to different temperatures and a gas stream of carbon monoxide and air (volume ratio 1/99) through the 16 catalyst holders with a volume flow rate of 16000 ml / h (weight-related space velocity: 10000 ml g (cat.) ^-1 h ^-1 ) directed. The carbon dioxide content of the product gas stream from the measured catalyst uptake was determined using a non-dispersive IR detector in order to determine the respective reaction conversion. Carbon monoxide conversions of selected catalysts determined at different temperatures are shown in Table 3.

Table 3

Example 5 Oxidation of propane with hydrogen and oxygen

0.13 g of the silver-titanium dioxide catalyst (125-250 μm) synthesized according to Example 1 was introduced into a tubular reactor with an inner diameter of 6 mm and a thermocouple introduced into the catalyst bed. The tubular reactor was heated in such a way that the temperature measured in the catalyst bed was a constant 100 ° C. A gas stream of propane, oxygen, hydrogen and nitrogen (volume ratio 10/10/10/70) was then passed through the catalyst bed at a volume flow rate of 520 ml / h (weight-related space velocity: 4000 ml g (cat.) ^-1 h ^-1 ) directed. The product gas stream from the catalyst bed was analyzed with a gas chromatograph to determine the reaction conversions. Traces of acetone (approx. 0.007% yield) and CO ₂ in the exhaust gas stream could be determined.

Example 6 Catalyst preparation using the incipient-wetness process Comparative example

0.825 g of titanium dioxide (Hombifine N; Sachtleben Chemie) was added concurrently mortar a solution of 30.2 mg silver nitrate in 0.6 ml chloride-free dist. Water added in small portions so that none recognizable liquid phase was next to the moistened powder. The received Powder was dried at 90 ° C for 6 hours. Then that was  dried powder calcined, with one degree per minute to 250 ° C. heated and this temperature was held for 10 hours by a 2.3% to get weight percent silver titanium dioxide catalyst.

The silver-titanium dioxide catalyst obtained in this way showed on TEM images Silver particles with a maximum diameter of 5 nm. Through EDX analysis with a 10 nm measuring window could evenly distribute the silver be detected.

Example 7 Oxidation of propene with hydrogen and oxygen

0.2 g of the silver-titanium dioxide catalyst (125-250 μm) synthesized according to Example 6 was introduced into a tubular reactor with an inner diameter of 6 mm and a thermocouple introduced into the catalyst bed. The tubular reactor was heated so that the temperature measured in the catalyst bed was a constant 50 ° C. A gas stream of propene, oxygen, hydrogen and nitrogen (volume ratio 10/10/10/70) was then passed through the catalyst bed at a volume flow rate of 400 ml / h (weight-related space velocity: 2000 ml g (cat.) ^-1 h ^-1 ) directed. The product gas stream from the catalyst bed was analyzed with a gas chromatograph to determine the reaction conversions. No reaction turnover could be determined.

Claims (21)

1. A method for immobilizing silver nanoparticles with a diameter <10 nm on a carrier material, characterized in that silver is deposited on the dispersed carrier material from a silver salt solution with the addition of a compound which forms a sparingly soluble salt with silver. 2. The method of claim 1, wherein the diameter of the silver nanoparticles is less than 5 nm. 3. The method according to claims 1-2, wherein a metal oxide as the carrier material is used. 4. The method according to claims 1-3, wherein the metal oxide is MnO ₂ , Fe ₂ O ₃ , Co ₃ O ₄ , NiO, CuO, CuMnO ₂ , MgO, Al ₂ O ₃ , SiO ₂ , V ₂ O ₅ , MoO ₃ , WO ₃ or mixed oxides of these metal oxides can be used. 5. The method according to claim 3, wherein titanium dioxide is used as the metal oxide becomes. 6. The method according to claim 1-2, wherein the carrier material with titanium dioxide is coated. 7. The method according to claim 1-2, wherein a titanium-containing as the carrier material Carrier is used. 8. The method according to claims 1-7, wherein as silver salt silver nitrate, acetate, -chlorate, -perchlorate, -bromate, -fluoride, -lactate or -propionate is used.   9. The method according to claims 1-8, wherein a compound is added, which forms a salt of silver with the ions OH ^- , O ^2- _, CO ₃ ^2- or C ₂ O ₄ ^2- . 10. Catalyst containing silver nanoparticles with a diameter <10 nm, which are immobilized on a carrier material, characterized in that a method according to claims 1-9 is used for its production. 11. The catalyst of claim 10, wherein the diameter of the silver Nanoparticles is <5 nm. 12. Use of the catalyst according to claim 10 or 11 for implementation of oxidation reactions. 13. Use according to claim 12, wherein the catalyst with a to oxidizing compound, hydrogen and oxygen or an oxygen releasing compound containing gas stream or reaction medium is contacted. 14. Use according to claim 13, wherein the gas stream or reaction medium Hydrogen peroxide is added instead of hydrogen and the addition of Oxygen or an oxygen-releasing compound is not mandatory is required. 15. Use according to claims 12-14 for the selective oxidation of alkanes, Alkenes or carbon monoxide. 16. Use according to claims 12-15 for the production of propene oxide Propene.   17. Use according to claims 12-14 for total oxidation of volatile organic compounds. 18. Use of a catalyst according to claims 10-11 for selective Hydrogenation of an alkene, the catalyst with an alkene and Hydrogen-containing gas stream or reaction medium is contacted. 19. Use of a catalyst according to claims 10-11 for selective Hydrogenation of an oxygenate, wherein the catalyst with an oxygenate and Hydrogen-containing gas stream or reaction medium is contacted. 20. Use of a catalyst according to claims 10-11 with a Electrode material as a support for electrocatalysis. 21. Use of a catalyst according to claims 10-11 with a Electrode material as a carrier as a gas sensor for organic molecules.