SUSPENSION OF NANOPARTICLES OF A MIXED OXIDE

Abstract

The present invention relates to a suspension of nanoparticles of a mixed oxide based on cerium and zirconium. It also relates to the use of said suspension for the preparation of a catalysed gasoline particulate filter.

Claims

1. A suspension of particles of at least one mixed oxide of zirconium, of cerium, of optionally lanthanum, and optionally of at least one rare-earth element other than cerium and other than lanthanum (RE) in an aqueous liquid medium comprising an acid which is either a mineral acid or a carboxylic acid containing from 2 to 12 carbon atoms, the suspension having the following characteristics: a pH of the suspension is between 2.0 and 7.0, this latter value being excluded; a proportion of the mixed oxide is between 20.0 wt % and 50.0 wt %; a specific surface area of the suspension is higher than 15 m.sup.2/g if the mixed oxide does not comprise La nor RE and higher than 21 m.sup.2/g if the mixed oxide comprises La and/or RE, the specific surface area being determined after calcination in air at 1100° C. for 4 hours of the solid isolated from the suspension; wherein: the particles of the mixed oxide exhibit a D50 between 20.0 nm and 900 nm; the proportions of the elements Ce, Zr, La and RE, expressed by weight of oxide with respect to the mixed oxide, are the following: between 20.0 wt % and 55.0 wt % of cerium; up to 10.0 wt % of lanthanum; up to 15.0 wt % of the rare earth element(s) (RE(s)) other than cerium and other than lanthanum; the remainder as zirconium.

2. The suspension according to claim 1, which consists of: particles of at least one mixed oxide of zirconium, of cerium, of optionally lanthanum, and optionally of at least one rare-earth element other than cerium and other than lanthanum (RE); an aqueous liquid medium; and an acid which is either a mineral acid or a carboxylic acid containing from 2 to 12 carbon atoms.

3. (canceled)

4. The suspension according to claim 1 wherein the mixed oxide also comprises the element hafnium, the proportion of which being lower or equal to 2.5 wt %, this proportion being expressed by weight of oxide with respect to the total weight of the mixed oxide.

5. (canceled)

6. (canceled)

7. The suspension according to claim 1, wherein the proportion of cerium in the mixed oxide is: between 30.0 wt % and 55.0 wt %; or between 30.0 wt % and 45.0 wt %; or between 25.0 wt % and 35.0 wt %.

8. The suspension according to claim 1, wherein the proportion of lanthanum in the mixed oxide is between 1.0 wt % and 10.0 wt %.

9. (canceled)

10. The suspension according to claim 1, wherein the pH is between 3.0 and 6.0.

11. (canceled)

12. The suspension according to claim 1, wherein the acid is nitric acid or a carboxylic acid of formula R.sup.1—COOH wherein R.sup.1 is a linear or branched alkyl radical containing from 1 to 11 carbon atoms.

13. (canceled)

14. The suspension according to claim 12, wherein the acid is acetic acid.

15. The suspension according to claim 12, wherein the acid is a carboxylic acid containing at least one functional group other than COOH, which is selected from the group consisting of OH, C═O, anhydride and ester groups.

16. The suspension according to claim 1, wherein the suspension does not comprise any metal oxide other than the mixed oxide, the metal oxide being defined as an oxide of at least one metal.

17. (canceled)

18. The suspension according to claim 1, exhibiting a specific surface area (BET) of at least 35 m.sup.2/g, the specific surface area being determined after calcination in air at 1000° C. for 4 hours of the solid isolated from the suspension.

19. (canceled)

20. The suspension according to claim 1, wherein the suspension exhibits a total pore volume TPV greater than 0.50 mL/g, the TPV being determined by mercury porosimetry on a solid isolated by the method comprising the following steps: (i) the solid is isolated from the suspension; (ii) then, the solid is dried in air at 500° C. for 1 hour.

21. The suspension according to claim 1, wherein the suspension exhibits a pore volume determined by mercury porosimetry for the pores having a diameter below 300 nm (PV.sub.0-300 nm) which is greater than 0.15 mL/g, PV.sub.0-300 nm being determined by mercury porosimetry on a solid isolated by the method comprising the following steps: (i) the solid is isolated from the suspension; (ii) then, the solid is dried in air at 500° C. for 1 hour.

22. The suspension according to claim 1, wherein D50 is between 20.0 nm and 800.0 nm.

23. (canceled)

24. (canceled)

25. (canceled)

26. (canceled)

27. (canceled)

28. (canceled)

29. The suspension according to claim 1, wherein the suspension exhibits a viscosity V which is lower than 1000 cP, V being measured at 20° C. with a rheometer using a cylindrical couette geometry according to the following method: the sample is first subjected to a shear rate increasing from 1 s.sup.−1 to 100 s.sup.−1 over a ramp time of 5 minutes, then to a shear rate decreasing from 100 s.sup.−1 to 1 s.sup.−1 also over 5 minutes and the viscosity value (V) corresponds to the value measured at a shear rate of 10 s.sup.−1 during the decrease of the shear rate.

30. (canceled)

31. (canceled)

32. The suspension according to claim 22, with a D50 between 20 nm and 100 nm and a D90 between 50 nm and 120 nm.

33. (canceled)

34. The suspension according to claim 1, wherein the weight ratio ZrO.sub.2/CeO.sub.2 is higher than 1.0, or even higher than 1.5.

35. A method of preparation of a suspension according to claim 1 wherein a dispersion comprising a mixed oxide M dispersed in an aqueous liquid medium comprising the acid undergoes a mechanical treatment so as to reduce the size of the particles of the mixed oxide.

36. (canceled)

37. The method according to claim 35 wherein the mechanical treatment is based on an attrition of the mixed oxide with beads in contact with the solid and put into motion.

38. (canceled)

39. A method of preparation of a catalytic composition wherein a suspension according to claim 1 is brought into contact with at least one mineral material and optionally at least one PGM.

40. (canceled)

41. (canceled)

Description

EXAMPLES

[0136] Specific Surface (BET)

[0137] The surface area was determined by BET Flow method (multi point) with N.sub.2 adsorption at liquid N.sub.2 temperature (77 K) on a Micromeritics TRISTAR 3020 analyzer. The specific surface area was calculated by the well-known Brunauer-Emmett-Teller (BET) method. Prior to the measurements, the samples were pre-treated in a vacuum oven at 300° C. for 15 min to remove any residual moisture and adsorbed species.

[0138] As discussed above, the specific surface area (BET) of the suspension is determined on the solid isolated from the dispersion.

[0139] Hg Porosimetry

[0140] The pore volumes were determined with a Micromeritics Autopore IV 9500 Automatic Mercury Porosimeter following the guidelines of the constructor. A value of 130° was used for the contact angle (8) and the surface tension of mercury was fixed at 480 dyne/cm. Vacuum is made on samples up to 50 μm Hg. The mercury intrusion curves were collected in the pressure range from 0.98 Psi up to 30 Psi for low pressure slot and from 30 Psi up to 60000 Psi for high pressure slot; this enables the analysis of pores in the a large range, typically 3 nm-200 μm in diameter.

[0141] Particle Size Analysis

[0142] A laser particle size analyzer was used. A relative refractive index of 1.6 was used. The suspension (approx. 10 g) was diluted in an aqueous solution (60 mL) with a pH identical to the pH of the suspension. After sonication of the diluted suspension (5 min; 45 kHz) in an external ultrasound bath, the diluted suspension was introduced into the measuring cell. It is preferable that the measurement is made with a PIDS (polarization intensity differential scattering) in the range 45%-55% (45%<PIDS<55%).

[0143] Viscosity of the Suspension

[0144] A Malvern Kinexus Pro+ stress-controlled rheometer with a cylindrical couette geometry was used. The rheology of the suspension was measured at 20° C. The sample was first subjected to a shear rate increasing from 1 s.sup.−1 to 100 s.sup.−1 over a ramp time of 5 minutes, then to a shear rate decreasing from 100 s.sup.−1 to 1 s.sup.−1 also over 5 minutes. The viscosity value (V) which is retained is the value measured at a shear rate of 10 s.sup.−1 during the decrease of the shear rate from 100 s.sup.−1 to 1 s.sup.−1.

[0145] XRD

[0146] X-ray powder diffraction patterns were acquired on an X'pertPro MPD powder diffractometer (PANAlytical Company) equipped with a Cu Kα (1.5406 Angstrom) radiation source and a linear detector X Celerator Detector. The scattered intensity data were collected from 28 values of 19-85° by scanning at 0.017° steps with a counting time of 28 s at each step. Crystalline phases were identified by matching with the International Centre for Diffraction Data Powder Diffraction File (ICDD-PDF). The average crystallite size (D.sub.SRD) of the samples was determined with the help of Scherrer equation from line broadening with taking into account the instrumental width and the lattice parameters were estimated by a standard cubic indexation method using the intensity of the most prominent peak (1 1 1).

[0147] For all examples 1-6 and comparative examples 1-2, use was made as a starting material of a mixed oxide M with the following composition (% expressed by weight of oxides): Zr/Ce/La 58.5%/36.0%/5.5% which is characterized by the following specific surface area: 29 m.sup.2/g after calcination in air at 1100° C. for 4 hours. This mixed oxide was prepared according to the teaching of EP 1527018 B1. The powder of the mixed oxide M used in the examples exhibits initially a D50 of 4.1 μm and a D90 of 11.5 μm. The crystallite size as determined by XRD was approximately 8 nm. It was observed that the crystallite size of the mixed oxide after the mechanical treatment remained substantially unchanged.

Example 1

[0148] A solution with 120 mL deionized water was prepared with 0.1 g acetic acid so as to reach a pH of 3.5. Then, the dispersion of mixed oxide was prepared by adding 80 g of M powder into this 120 mL solution. The pH of the dispersion was 6.2. The pH was further adjusted by adding drop by drop acetic acid (2.5 g acetic acid at 96 wt %) until a pH of 4.0 was obtained (final weight of the dispersion=202.6 g). The dispersion was homogenized for some hours under high stirring with a magnetic stirrer.

[0149] Then, for the wet milling, 116 mL of the homogeneous dispersion (167.8 g dispersion) were placed in a bowl of lab beads mill (container of 500 ml capacity, bowl diameter 10 cm) containing zirconia beads (605.5 g; average size 350 μm). The dispersion was milled in the bowl with beads agitator at 1500 rpm for 20 minutes (this corresponds to a duration of 300 minutes per kg oxide).

Example 2

[0150] Example 1 was reproduced under the same conditions as in example 1.

Example 3

[0151] Example 3 was reproduced under the conditions of example 1 except that the proportion of the mixed oxide M is lower: 25 wt % instead of 39 wt %. Characteristics of the suspension after milling: D50=70 nm and D90=250 nm. As can be seen in Table I, a decrease of D50 and D90 was observed with a more diluted suspension. The pH during milling also significantly increased from initial pH of 4.0 to pH 4.6. Measured viscosity of suspension is low at 3 cP for any shearing rate between 1 s.sup.−1 to 1000 s.sup.−1.

Example 4

[0152] This example was performed with a milling time of 40 min instead of 20 min for examples 1 and 2. The dispersion was prepared as in example 1. The slurry was milled in the bowl with beads stirring at 1500 rpm for 40 minutes. The increase of milling time leads to a reduction of particles size: D50=140 nm and D90=590 nm. It must be noted that, due to some losses of material on the surface of the bowl and on the beads, the proportion of the mixed oxide in the recovered suspension was found to be 36.6%.

Example 5

[0153] This example was performed with a pH of the dispersion higher than for example 1. The dispersion was prepared as in example 1, the proportion of the mixed oxide being 36.6% (after milling) and pH before milling being 6 instead of pH 4. The slurry is milled in the bowl with beads agitator at 1500 rpm for 40 minutes. Compared to example 4, the higher pH slightly affect D50 and D90 which tends to increase: D50=250 nm and D90=650 nm. The pH during milling also significantly increased from initial pH of 6.0 to pH 6.6.

Example 6

[0154] The acid used for the preparation of the dispersion was nitric acid instead of acetic acid. A dispersion of mixed oxide was prepared by adding 80 g of M powder into solution of 120 mL of deionized water plus some drops of nitric acid. The pH of the dispersion was 5.9. The pH was then adjusted by adding more nitric acid to get a final pH of 4.0 (in total 1.3 g of nitric acid at 69 wt % used). The final weight of suspension prepared was 201.3 g. The slurry was then milled in the bowl with the beads agitator at 1500 rpm for 40 minutes. Characteristics of the particles after milling: D50=180 nm and D90=590 nm.

Example 7

[0155] Example 7 was reproduced under the conditions of example 4 except that the mixed oxide M had a different composition (expressed by weight of oxides): Zr/Ce/La/Y 60.0%/30.0%/5.0%/5.0%. This mixed oxide was prepared by following the recipe of the example of EP 1527018 B1. The proportion of mixed oxide in the suspension was 36.9 wt %. The final pH was 4.7. The obtained suspension was as fluid as the suspension of example 4.

Example 8

[0156] Example 8 was reproduced under the conditions of examples 4 and 7 except that the mixed oxide M has a different composition (expressed by weight of oxides): Zr/Ce/LaN/Nd 45.0%/40.0%/2.0%/8.0%/5.0%. This mixed oxide was prepared by following the recipe of the example of EP 1527018 B1. The proportion of mixed oxide in the suspension is 37.8 wt %. The obtained suspension was as fluid as the suspension of example 7.

Example 9

[0157] Example 9 was reproduced under the conditions of examples 4 and 7 except that the mixed oxide M had a different composition (expressed by weight of oxides): Zr/Ce/La/Nd 63.0%/30.0%/1.75%/5.25%. The proportion of mixed oxide in the suspension was 34.1 wt %. The final pH of the suspension was 4.7. The obtained suspension was as fluid as the suspension of example 7.

Example 10

[0158] Example 10 was reproduced under the conditions of examples 4 and 7 except that the mixed oxide M had a different composition (expressed by weight of oxides): Zr/Ce/La/Y 50.0%/40.0%/5.0%/5.0%. The proportion of mixed oxide in the suspension was 23.0 wt %. The obtained suspension was as fluid as the suspension of example 7.

Example 11

[0159] Example 11 was reproduced under the conditions of example 10 except that the proportion of the mixed oxide M in the suspension was 19.0 wt %. The obtained suspension was as fluid as the suspension of example 7.

Example 12

[0160] Example 12 was reproduced under the conditions of examples 4 and 7 except that the mixed oxide M has a different composition (expressed by weight of oxides): Zr/Ce/La 59.0%/35.5%/5.5%. The proportion of mixed oxide in the suspension was 29.0 wt %. The final pH was 4.2. The pH during milling also increased from initial pH of 3.8 to pH 4.2. The obtained suspension was as fluid as suspension of example 4.

Comparative Example 1

[0161] The pH of the suspension was different than in the previous examples. The dispersion was prepared as in example 1 but at a pH of 7.0. It was observed that at this pH, the viscosity of the dispersion at around 39 wt % was much higher than for the suspension of example 1, so that the dispersion had to be diluted in to 34.4 wt % order to perform the milling for 30 min at 1500 rpm. After 30 minutes, the viscosity of the suspension became too high and milling was stopped. It was not possible to decrease D90 below 1 μm (D90=7.15 μm).

Comparative Example 2

[0162] A suspension at 27.8 wt % was prepared as in example 1 but without any acid. The pH of the suspension was 8.3. Milling was performed in the bowl with beads agitator at 1500 rpm for 30 minutes. After 30 minutes, the viscosity of suspension became too high and milling was stopped. It was observed that it was not possible to decrease D90 below 1 μm (D90=10.7 μm).

[0163] Table I provides the characteristics of the suspensions. D50 and D90 were obtained with a laser particle size analyzer LS13320 of Beckman-Coulter.

TABLE-US-00001 TABLE 1 SPECIFIC MERCURY XRD BEADS MILLING STEP LASER SURFACE POROSIMETRY Primary Milling Solid GRANULOMETRY SSA TPV particles time content D50 D90 (1100° C./4 h) TPV (0-300 nm) V size min Stabilizer (% w/w) pH (nm) (nm) (m.sup.2/g) (mL/g) (mL/g) (cP) (nm) REFERENCE* 0 Acetic acid 39.5 4.0 4100 11500 27 1.11 0.58 7.8 EXAMPLE 1 20 Acetic acid 39 4.1 320 700 EXAMPLE 2 20 Acetic acid 39 4.1 420 790 EXAMPLE 3 20 Acetic acid 25 4.6 70 250 21 0.62 0.29 3 EXAMPLE 4 40 Acetic acid 36.6 4.2 140 590 24 0.62 0.29 8 7.9 EXAMPLE 5 40 Acetic acid 36.6 6.6 250 650 25 0.73 0.38 50 7.9 EXAMPLE 6 40 Nitric acid 36.3 5.0 180 590 21 0.63 0.32 EXAMPLE 7 45 Acetic acid 36.9 4.7 66 88 21 0.5 0.45 EXAMPLE 8 45 Acetic acid 37.8 4.1 60 80 23 0.52 0.25 EXAMPLE 9 50 Acetic acid 34.1 4.7 65 89 19 0.57 0.28 EXAMPLE 10 60 Acetic acid 23 4.0 70 90 21 0.50 0.24 EXAMPLE 11 45 Acetic acid 19 4.0 60 80 22 0.56 0.24 EXAMPLE 12 45 Acetic acid 29 4.2 70 90 23 0.51 0.25 COUNTER-EXAMPLE 1 30 Acetic acid 34.4 7.4 550 7150 27 0.86 0.29 400 COUNTER-EXAMPLE 2 30 None 27.8 8.3 3520 10700 29 1.23 0.63 *Reference slurry at pH 4: unmilled. ** V: viscosity measured at shear rate of 10 s−1