Preparation of diesel oxidation catalyst via deposition of colloidal nanoparticles

Abstract

The present invention relates to a process for preparing a catalyst, at least comprising the steps of adding a protecting agent to an aqueous solution of a metal precursor to give a mixture (M1), adding a reducing agent to mixture (M1) to give a mixture (M2), adding a support material to mixture (M2) to give a mixture (M3), adjusting the pH of mixture (M3), and separating the solid and liquid phase of mixture (M3). Furthermore, the present invention relates to the catalyst as such and its use as diesel oxidation catalyst.

Claims

1. A catalyst composite comprising a catalyst on a substrate for purification of an exhaust gas of a combustion engine, the catalyst comprising: a precious metal component; and a support material for the precious metal component; wherein the precious metal component comprises colloidally-delivered nanoparticles on the support material that are dispersed upon aging; wherein the catalyst is effective as a diesel oxidation catalyst; and the substrate comprises a flow through substrate or a wall flow substrate.

2. The catalyst composite of claim 1, wherein the precious metal component comprises: platinum, palladium, rhodium, gold, silver, or mixtures thereof, and the support material comprises particles of aluminum oxide, silicon oxide, cerium oxide, zirconium oxide, titanium oxide, magnesium oxide alone or as mixtures thereof or solid solutions thereof.

3. The catalyst composite of claim 1, wherein the precious metal component comprises colloidally- and protective agent-delivered nanoparticles.

4. The catalyst composite of claim 3, wherein the colloidally- and protective agent-delivered nanoparticles are deposited onto the support material in an aqueous mixture by reduction and pH adjustment of a mixture of a protecting agent and a precursor of the metal component.

5. The catalyst composite of claim 3, wherein a protecting agent is selected from a group consisting of soluble homo- and co-polymers having one or more amino, amido, carboxylic, aldehydic, or hydroxyl groups, and organic molecules having one or more amino, amido, carboxylic, aldehydic, or hydroxyl groups and mixtures thereof.

6. The catalyst composite of claim 1 comprising platinum and palladium as the precious metal component, wherein after treatment of the catalyst at 800 C. for 12 h in an oxidizing atmosphere (10% H.sub.2O in air), no less than 36% of particles of the precious metal component have an average diameter below 22 nm.

7. The catalyst composite of claim 1, the precious metal component comprising platinum and palladium, wherein after treatment of the catalyst at 800 C. for 12 h in an oxidizing atmosphere (10% H.sub.2O in air), no less than 90% of precious metal particles are constituted by both Pt and Pd.

8. The catalyst composite of claim 1, the precious metal component comprising platinum and palladium in a substantially equal molar ratio, wherein after treatment of the catalyst at 800 C. for 12 h in an oxidizing atmosphere (10% H.sub.2O in air), no less than 78% of precious metal particles are constituted by a molar ratio of Pt:Pd in the range of 0.8 to 1.2.

9. The catalyst composite of claim 1, the precious metal component comprising platinum and palladium in a substantially equal molar ratio, wherein after treatment of the catalyst at 800 C. for 12 h in an oxidizing atmosphere (10% H.sub.2O in air), no less than 63% of precious metal particles are constituted by a molar ratio of Pt:Pd in the range of 0.9 to 1.1.

10. The catalyst composite of claim 1, prepared by a process comprising the steps of: (1) adding a protecting agent to an aqueous solution of a metal precursor to give a mixture (M1), (2) adding a reducing agent to mixture (M1) to give a mixture (M2), the reducing agent being selected from the group consisting of alkali metal borohydrides and alkali metal citrates, (3) adding a support material to mixture (M2) to give a mixture (M3) where the support material is suspended in a liquid phase, (4) adjusting the pH of mixture (M3) to form the catalyst comprising metal particles on the support material, the catalyst being suspended in the liquid phase, (5) separating the catalyst and the liquid phase of mixture (M3), (6) depositing the catalyst on a substrate; wherein the catalyst is effective as a diesel oxidation catalyst.

11. The catalyst composite of claim 10, wherein the protecting agent is selected from a group consisting of soluble homo- and co-polymers having one or more amino, amido, carboxylic, aldehydic, or hydroxyl groups, and organic molecules having one or more amino, amido, carboxylic, aldehydic, or hydroxyl groups, and mixtures thereof.

12. The catalyst composite of claim 10, wherein the metal precursor is selected from a group consisting of metal salt of platinum, palladium, rhodium, gold, silver, and mixtures thereof.

13. The catalyst composite of claim 10, wherein the support material is selected from a group consisting of aluminum oxide, silicon oxide, cerium oxide, zirconium oxide, titanium oxide, magnesium oxide alone or as mixtures thereof, and solid solutions thereof.

14. The catalyst composite of claim 10, wherein in step (5) the catalyst and liquid phase of mixture (M3) are separated by filtration or evaporation of the solvent.

15. The catalyst composite of claim 10 comprising platinum and palladium as metals, wherein after treatment of the catalyst at 800 C. for 12 h in an oxidizing atmosphere (10% H.sub.2O in air), no less than 36% of the metal particles have an average diameter below 22 nm.

16. The catalyst composite of claim 10 comprising platinum and palladium as metals, wherein after treatment of the catalyst at 800 C. for 12 h in an oxidizing atmosphere (10% H.sub.2O in air), no less than 90% of the metal particles are constituted by both Pt and Pd.

17. A process for oxidizing diesel exhaust wherein the diesel exhaust is brought into contact with a catalyst composite according to claim 10.

18. A catalyst for purification of an exhaust gas of a combustion engine, the catalyst comprising: a precious metal component comprising: platinum, palladium, or a mixture thereof; a support material for the precious metal component comprising particles of aluminum oxide; wherein after treatment of the catalyst at 800 C. for 12 h in an oxidizing atmosphere (10% H.sub.2O in air), no less than 36% of particles of the precious metal component have an average diameter below 22 nm; and wherein the catalyst is effective as a diesel oxidation catalyst.

19. The catalyst of claim 18, wherein the precious metal component comprises colloidally- and protective agent-delivered nanoparticles that are deposited onto the support material in an aqueous mixture by reduction and pH adjustment of a mixture of a protecting agent and a precursor of the metal component, wherein a reducing agent comprises an alkali metal borohydride, and the protective agent comprises poly(vinylalcohol), poly(vinylpyrrolidone), poly(ethyleneimine), poly(acrylic acid), carbohydrates or alkali metal citrates.

20. The catalyst of claim 18, wherein platinum and palladium are in a substantially equal molar ratio and wherein after treatment of the catalyst at 800 C. for 12 h in an oxidizing atmosphere (10% H.sub.2O in air), no less than 78% of precious metal particles are constituted by a molar ratio of Pt:Pd in the range of 0.8 to 1.2.

Description

DETAILED DESCRIPTION OF THE FIGURES

(1) FIG. 1. shows the transmission electron microscope of a Pt/Pd sample on an alumina support prepared following the procedure according to Example 3 below and detailing the particle size composition. The x-axis of the diagram shows the number of particles (#), the y-axis the ratio of Pt/Pd (in mol/mol).

(2) FIG. 2. shows the XRD spectrum of 1% Pt on aluminium oxide prepared according to the process of the invention. The x-axis shows the 2 Theta scale (in ), the y-axis the intensity (in lincounts; I/LC).

(3) FIG. 3. shows the XRD spectrum of 1% Pt on aluminium oxide prepared according to a process according to the state of the art. The x-axis shows the 2 Theta scale (in ), the y-axis the intensity (in lincounts; I/LC).

(4) FIG. 4. shows the XRD spectrum of 0.67% and Pt 0.33% Pd on aluminium oxide prepared according to the process of the invention. The x-axis shows the 2 Theta scale (in ), the y-axis the intensity (in lincounts; I/LC).

(5) FIG. 5. shows the XRD spectrum of 0.67% Pt and 0.33% Pd on aluminium oxide prepared according to a process according to the state of the art. The x-axis shows the 2 Theta scale (in ), the y-axis the intensity (in lincounts; 1/LC).

(6) FIG. 6. shows a diagram comparing the gas activity of catalysts prepared according to the process of the invention with that of prior art catalysts. A detailed description of FIG. 6 is to be found in the context of Example 11 herein under.

(7) The present invention is further illustrated by way of the following examples.

EXAMPLES

Example 1

(8) 10.2 g of an H.sub.2PtCl.sub.6 solution containing 5.1*10.sup.2 moles of Pt per liter of solution were diluted in 400 ml of water and an opportune amount of a PVP solution containing 10 mg of PVP per ml of solution was added in order to achieve a Pt/PVP weight ratio equal to 1. After letting the solution stir at room temperature in air for 1 hour, NaBH.sub.4 was added to the solution at room temperature. The amount of NaBH.sub.4 was chosen in order to have a Pt/NaBH.sub.4 weight ratio of . Following stirring for 1 hour in air of the obtained mixture, an appropriate amount of alumina powder was added to the solution in order to achieve a total metal loading of 1% wt/wt and the pH adjusted to a value of 2.4 with an HCl solution containing 15% HCl in weight. After 30 minutes of stirring the solution was filtered and the solid powder recovered.

Example 2

(9) The same process and quantities of reagents were used as in Example 1 with the exception of the PVP addition. Here an opportune amount of PVP solution containing 10 mg of PVP per mg of solution was added in order to achieve a Pt/PVP weight ratio equal to 2.

Example 3

(10) The same process and quantities of reagents were used as in Example 2 with the exception that 6.6 g of an H.sub.2PtCl.sub.6 solution containing 5.1*10.sup.2 moles of Pt per liter of solution were diluted in 400 ml of water together with 110 mg of K.sub.2PdCl.sub.4.

(11) As one can see from FIG. 1, the precious metal nanoparticles comprise both platinum and palladium and the composition is the same as one would expect from the relative ratio of platinum and palladium when calculated based on the mol of precious metal.

Example 4

(12) 6.6 g of an H.sub.2PtCl.sub.6 solution containing 5.1*10.sup.2 moles of Pt per liter of solution were diluted in 400 ml of water and an opportune amount of a PVP solution containing 10 mg of PVP per ml of solution was added in order to achieve a Pt/PVP weight ratio equal to 1. After letting the solution stir at room temperature in air for 1 hour, NaBH.sub.4 was added to the solution at room temperature. The amount of NaBH.sub.4 was chosen in order to have a Pt/NaBH.sub.4 weight ratio of . The resulting solution was stirred for 30 minutes and then 110 mg of K.sub.2PdCl.sub.4 were added to the solution. After 30 minutes, NaBH.sub.4 was added to the solution at room temperature. The amount of NaBH.sub.4 was chosen in order to have a Pd/NaBH.sub.4 weight ratio of . Following stirring for 1 hour in air of the obtained mixture, an appropriate amount of alumina powder was added to the solution in order to achieve a total metal loading of 1% wt/wt and the pH adjusted to a value of 2.4 with an HCl solution containing 15% HCl in weight. After 30 minutes of stirring the solution was filtered and the solid powder recovered.

Example 5

(13) The same process and quantities of reagents were used as in Example 4 with the exception that the order of addition of H.sub.2PtCl.sub.6 and of K.sub.2PdCl.sub.4 has been inverted.

Example 6

(14) The same process and quantities of reagents were used as in Example 1 with the exception of the PVP addition. Here an opportune amount of PVP solution containing 10 mg of PVP per mg of solution was added in order to achieve a Pt/PVP weight ratio equal to 4.

Example 7

(15) The same process and quantities of reagents were used as in Example 1 with the exception of the H.sub.2PtCl.sub.6 and support quantities which were chosen to obtain a catalyst having 2% wt/wt of precious metal with respect to the support.

Example 8

(16) The same process and quantities of reagents were used as in Example 3 with the exception of the H.sub.2PtCl.sub.6, K.sub.2PdCl.sub.4 and alumina quantities which were chosen to obtain a catalyst having 4% wt/wt of precious metal with respect to the support.

Example 9

Comparative Example

(17) Referring to FIG. 2, there is shown an XRD spectrum of a sample comprising 1% Pt wt/wt with respect to the support material deposited on an alumina support, prepared following the same process of Example 1, which was thermally aged for 12 hours at 800 C.

(18) FIG. 3 shows an XRD spectrum of a sample comprising 1% Pt wt/wt with respect to the support material deposited on an alumina support, prepared from the same precious metal precursor according to state of the art incipient wetness impregnation methods, which was thermally aged for 12 hours at 800 C.

(19) As one can see, in FIG. 2 the Pt diffraction peak is broader and less intense than in the case of the sample prepared according to state of the art incipient wetness impregnation, thus indicating a smaller average particle size.

Example 10

Comparative Example

(20) Referring to FIG. 4, there is shown an XRD spectrum of a sample comprising 0.67% Pt wt/wt and 0.33% Pd wt/wt with respect to the support material deposited on an alumina support, prepared following the same process of Example 3, which was thermally aged for 12 hours at 800 C.

(21) FIG. 5 shows an XRD spectrum of a sample comprising 0.67% Pt wt/wt and 0.33% Pd wt/wt with respect to the support material deposited on an alumina support, prepared from the same precious metal precursors according state of the art incipient wetness impregnation methods, which was thermally aged for 12 hours at 800 C.

(22) As one can see, in FIG. 4 the Pt/Pd diffraction peak is broader and less intense than in the case of the sample prepared according to state of the art incipient wetness impregnation, thus indicating a smaller average particle size.

Example 11

Comparison of Examples and State of the ArtExamples

(23) FIG. 6 shows the gas activity of the sample tested in a laboratory reactor simulating the exhaust emissions of a conventional diesel engine. The reaction conditions used were a fixed bed tube reactor where 40 mg of powder were diluted with 100 mg of cordierite material and the mixture was crushed and sieved in the range of 250-500 micrometer. The total gas flow rate was 200 mL/min and the resulting space velocity was equivalent to 15,000-20,000 per hour that would be experienced by a monolith sample. The gas composition used in the powder reactor testing comprised CO 2000 ppm, NO 100 ppm, C.sub.3H.sub.8 300 ppm, C.sub.3H.sub.8 300 ppm, toluene 350 ppm, O.sub.2 12%, H.sub.2O 5%. Unless otherwise specified, hydrocarbon (HC) concentrations are reported on a C1 basis.

(24) At the beginning of the light-off test, the powder sample was equilibrated in the gas mixture for 20 minutes at 50 C. The temperature at which 50% conversion was observed is denoted as T50 and was used as the measure of catalyst activity: the lower the 150, the better the catalyst performance. The activity after thermal aging for 12 h at 800 C. of the samples prepared according to the process of the invention as outlined in Example 2, Example 3, Example 7 and Example 8, was compared to that of samples prepared according to state of the art impregnation incipient wetness methods (IW) from the same precious metal precursors, deposited on the same support material and having the same precious metal content as in Example 2, Example 3, Example 7 and Example 8.

(25) As one can see, the catalytic activity of the samples prepared according to the process of invention is higher than that of the samples prepared according to state of the art impregnation methods as indicated by the lower T50 value of CO in the feed stream used for the evaluation.