NOx trap catalyst support material composition

Abstract

The present invention relates to a method of making a support material composition comprising an Mg/AI oxide, a cerium oxide and at least another rare earth element oxide, to a support material composition and to the use of the support material composition as a nitrogen oxide storage component within a catalyst for treating exhaust gases to reduce NOx content.

Claims

1. A method of preparing a support material composition, the composition comprising two phases: a first phase comprising a Mg/Al mixed oxide; and a second phase comprising a cerium based oxide, and rare-earth element(s) based oxide other than cerium oxide, wherein the second phase is a solid-solution; the method comprising the following steps: i) preparing an aqueous suspension of a Mg/Al mixed oxide precursor; ii) preparing an aqueous solution of a cerium salt; iii) preparing an aqueous solution of one or more rare-earth element salt(s) other than cerium salt; iv) combining, in any order, at least the aqueous suspension in step i), with the aqueous solution in step ii), and the aqueous solution of step iii) to form an aqueous mixture; v) dying the aqueous mixture to form a dried particulate material; and vi) calcining the dried particulate material; wherein the content of the one or more rare-earth element salt(s) other than cerium salt is between 5 and 50 wt. %, relative to the sum of the cerium salt from the aqueous solution of step ii) and the one or more rare earth element salt(s) other than cerium salt from the aqueous solution of step iii) and, wherein each of the salts are calculated as their oxides.

2. The method of claim 1, wherein the Mg/Al mixed oxide precursor is prepared by hydrolysis of a mixture of corresponding alkoxides of aluminium and magnesium that form a mixture of hydrotalcite, boehmite, and water.

3. The method of claim 1, wherein the cerium salt comprises one or more of cerium nitrate, ammonium cerium nitrate, cerium sulfate, cerium carbonate, and cerium acetate.

4. The method of claim 1, wherein the rare-earth element salt(s) other than cerium salt comprises salts of La, Pr, Nd, Y, Sm or mixtures thereof.

5. The method of claim 4, wherein the rare earth element salt(s) is an acetate of La, Pr, Nd, Y or mixtures thereof.

6. The method of claim 1, wherein the solutions of step ii) and step iii) are first combined, and then combined with the suspension of step i).

7. The method of claim 1, wherein the mixture of the suspension and the solutions is spray dried to form a particulate material.

8. The method of claim 1, wherein the particulate material is calcined at a temperature of between 500° C. and 1100° C., for a period of between 30 minutes and 5 hours to obtain the support material composition.

9. A support material composition prepared according to the method of claim 1, the support material composition comprising two phases: a first phase comprising a Mg/Al mixed oxide; and a second phase comprising a cerium based oxide, and rare-earth element(s) based oxide other than cerium oxide, wherein the second phase is a solid-solution, wherein the content of the rare-earth element(s) oxide other than cerium oxide in the second phase is between 10 and 35 wt. %, calculated as the rare earth element(s) oxide other than cerium oxide relative to the total second phase.

10. The support material composition of claim 9 comprising a BET surface area of above 50 m.sup.2/g.

11. The support material composition of claim 9 comprising a pore volume between 0.1 and 1.5 ml/g.

12. The support material composition of claim 9 comprising less than 500 ppm Na.sub.2O.

13. Use of the support material composition of claim 9 as a nitrogen oxide storage component within a catalyst for treating exhaust gases to reduce the NOx content.

14. A support material composition comprising two phases: i) a first phase comprising a Mg/Al mixed oxide; and ii) a second phase comprising a cerium based oxide, and rare-earth element(s) based oxide other than cerium oxide, wherein the second phase is a solid-solution; the content of the first phase being at least 50 wt. % of the total support material composition, wherein the amount of Mg in the first phase is between 1 and 40 wt. %, calculated as MgO based on the weight of the first phase, calculated as MgO and Al.sub.2O.sub.3; and wherein the content of the rare-earth element(s) oxide other than cerium oxide in the second phase is between 10 and 35 wt. %, calculated as the rare earth element(s) oxide other than cerium oxide relative to the total second phase.

15. The support material composition of claim 14 comprising a BET surface area of above 50 m.sup.2/g.

16. The support material composition of claim 14 comprising a pore volume between 0.1 and 1.5 ml/g.

17. The support material composition of claim 14 comprising less than 500 ppm Na.sub.2O.

Description

(1) The invention will now be described with reference to the following non-limiting FIGURE and examples in which:

(2) FIG. 1 represents the powder X-ray diffraction pattern of the support material composition of Example 1 together with the theoretical patterns of MgAl.sub.2O.sub.4 (dotted lines) and CeO.sub.2 (straight lines) showing the distinguishable first and second phases of the support material composition and the absence of any other constituents.

EXAMPLES

(3) NOx Storage Capacity Measurements

(4) Samples were treated at 950° C. for 3 h in an air atmosphere before determining the NOx storage capacity.

(5) The NOx storage/release tests were performed in a fixed-bed reactor. The gas flows were controlled by mass flow controllers. The gas composition was continuously monitored by specific NDIR-UV gas analyzers for NO, NO.sub.2, CO, CO.sub.2, and O.sub.2, and the measurement data are recorded every 10 seconds. 100 mg of sample mixed with 300 mg SiC were used for every experiment. Upstream, 80 mg of commercial Pt catalyst were placed in order to adjust the NO.sub.2/NO ratio to the realistic value according to the measurement temperature. The sample was heated under nitrogen from room temperature to the adsorption temperature of 150° C. or 200° C. respectively at 10° C./min.

(6) Then the nitrogen gas flow was replaced by 500 ppm NO+5% O.sub.2/N.sub.2 at 500 ml/min until the outlet NOx concentration reaches a value close to that of the calibration. NOx desorption was carried out by changing to nitrogen gas flow and heating at 5° C./min from 150° C. or 200° C. respectively to 700° C. The amount of released NOx is determined and defined to be the NOx storage capacity.

Comparative Example 1

(7) An aqueous suspension of mixed Mg/Al oxide precursor (Pural MG20, MgO content of 20 wt. %) was mixed with a cerium acetate solution. After spray drying, the resulting powder was calcined at 950° C. for 3 h to obtain the cerium oxide doped homogenous Mg/Al mixed oxide.

Example 1

(8) An aqueous suspension of mixed Mg/Al oxide precursor (Pural MG20, MgO content of 20 wt. %) was mixed with a pre-mixed solution of cerium acetate and lanthanum acetate. After spray drying, the resulting powder was calcined at 950° C. for 3 h to obtain the cerium oxide/lanthanum doped homogenous Mg/Al mixed oxide.

(9) The powder X-ray diffraction together with the theoretical patterns of CeO.sub.2 (straight lines) and MgAl.sub.2O.sub.4 (dotted lines) shown in FIG. 1, reveals two separate phases, a first phase comprising a Mg/Al mixed oxide, and a second phase comprising cerium oxide and lanthanum oxide.

Example 2

(10) The same procedure as Example 1 was used but Nd acetate was pre-mixed with the cerium acetate as opposed to lanthanum acetate.

Example 3

(11) The same procedure as Example 1 was used but Y acetate was pre-mixed with the cerium acetate as opposed to lanthanum acetate.

(12) NOx storage was tested at 200° C. The results are included in Table 1:

(13) TABLE-US-00001 TABLE 1 Wt. % rare- NOx Wt. % earth oxide storage Second in Second Crystal d-value capacity Phase Phase size (nm) (Å) (mg NOx/g) Comparative 10 — 11.1 3.1273 4.14 Example 1 Example 1 12 33% La.sub.2O.sub.3 8.3 3.1406 9.15 Example 2 10 30% Nd.sub.2O.sub.3 6.8 3.1383 9.6 Example 3 9.6 25% Y.sub.2O.sub.3 6.5 3.1310 7.08

(14) The Examples dearly show that the support material composition of the present invention have a higher NOx storage capacity than the Comparative Example at 200° C.

Comparative Example 2

(15) An aqueous suspension of mixed Mg/Al oxide precursor (Pural MG20, MgO content of 20 wt. %) was mixed with a cerium acetate solution. After spray drying, the resulting powder was calcined at 950° C. for 3 h.

Example 4

(16) An aqueous suspension of mixed Mg/Al oxide precursor (Pural MG20, MgO content of 20 wt. %) was mixed with a mixed solution of cerium acetate and lanthanum acetate. After spray drying, the resulting powder was calcined at 950° C. for 3 h.

Example 5

(17) The same procedure as Example 4 was used but Pr acetate was pre-mixed with the cerium acetate as opposed to lanthanum acetate.

Example 6

(18) The same procedure as Example 4 was used but Pr acetate was pre-mixed with the cerium acetate as opposed to lanthanum acetate.

(19) NOx storage was tested at 150° C. The results are included in Table 2.

(20) TABLE-US-00002 TABLE 2 Wt. % rare- NOx Wt. % earth oxide CeO.sub.2 storage Second in Second Crystal d-value capacity (mg Phase Phase size (nm) (Å) NOx/g) Comparative 20 0 10 3.1308 4.95 Example 2 Example 4 20 25% La.sub.2O.sub.3 7 3.1435 5.61 Example 5 20 25% Pr.sub.6O.sub.11 8 3.1341 5.19 Example 6 20 10% Pr.sub.6O.sub.11 8.4 3.1322 6.42

(21) Again, the Examples clearly show that the support material composition of the present invention have a higher NOx storage capacity than the Comparative Example at 150° C.