NOx trap catalyst support material with improved stability against BaAl2O4 formation

Abstract

The present invention relates to a method for the production of a support material for a nitrogen oxide storage component that is applicable in catalysts for treating exhaust gases from lean-burn engines and a support material made according to said process that is stable against the reaction with a Barium compound to form BaAl.sub.2O.sub.4.

Claims

1. A method of making a support material comprising the following steps: i) providing a first suspension comprising a homogeneous Mg/Al mixed oxide precursor; ii) drying the first suspension; iii) calcining the Mg/Al mixed oxide precursor to obtain a calcined Mg/Al mixed oxide; iv) re-suspending the calcined Mg/Al mixed oxide to obtain a second suspension comprising the Mg/Al mixed oxide; v) doping the re-suspended calcined Mg/Al mixed oxide with a precursor comprising a manganese oxide precursor to form a Mg/Al mixed oxide doped with at least manganese; vi) drying the second suspension; and vii) calcining the Mg/Al mixed oxide doped at least with manganese to form a doped Mg/Al mixed oxide.

2. The method of claim 1 comprising the further step of bringing together the homogeneous mixed Mg/Al mixed oxide precursor in the first suspension with a cerium based oxide precursor.

3. The method of claim 2 wherein the cerium based oxide precursor and/or the rare earth (other than cerium) oxide precursor is selected from one or more members of the group comprising acetate salts, nitrate salts, hydrated oxides, hydroxides, oxyhydrates and carbonates.

4. The method of claim 2 comprising the further step of bringing together the first suspension comprising the homogeneous mixed Mg/Al mixed oxide precursor with a cerium based oxide precursor and a rare earth (other than cerium) oxide precursor.

5. The method of claim 1 comprising the further step of bringing together the first suspension comprising the homogeneous mixed Mg/Al mixed oxide precursor with a cerium based oxide precursor and a rare earth (other than cerium) oxide precursor.

6. The method of claim 5 wherein the rare earth oxide precursor comprises lanthanum oxide, praseodymium oxide, neodymium oxide, yttrium oxide, or mixtures thereof.

7. The method of claim 6 wherein the cerium based oxide precursor and/or the rare earth (other than cerium) oxide precursor is selected from one or more members of the group comprising acetate salts, nitrate salts, hydrated oxides, hydroxides, oxyhydrates and carbonates.

8. The method of claim 5 wherein the cerium based oxide precursor and/or the rare earth (other than cerium) oxide precursor is selected from one or more members of the group comprising acetate salts, nitrate salts, hydrated oxides, hydroxides, oxyhydrates and carbonates.

9. The method of claim 1 wherein the amount of magnesium oxide, calculated as MgO, within the homogeneous Mg/Al mixed oxide precursor is in the range of 1 to 40 wt. %, relative to the doped Mg/Al mixed oxide.

10. The method of claim 1 wherein the suspension medium of the first suspension and the second suspension is water.

11. The method of claim 1 wherein the precursor comprising a manganese oxide precursor of step v) exclusively consists of a manganese oxide precursor.

12. The method of claim 11 wherein the amount of manganese oxide precursor added to the re-suspended Mg/Al mixed oxide is between 5 to 20% wt. calculated as MnO.sub.2, relative to the doped Mg/Al mixed oxide.

13. The method of claim 1 wherein the amount of manganese oxide precursor added to the re-suspended Mg/Al mixed oxide is between 5 to 20% wt. calculated as MnO.sub.2, relative to the doped Mg/Al mixed oxide.

14. The method of claim 1 wherein the precursor of step v) comprising a manganese oxide precursor comprises a mixture of a manganese oxide precursor and a cerium oxide precursor.

15. The method of claim 14 wherein a rare-earth (other than cerium) oxide precursor is added to the second suspension together with the manganese oxide precursor and the cerium oxide precursor.

16. The method of claim 1 wherein a rare-earth (other than cerium) oxide precursor is added to the second suspension together with the manganese oxide precursor and the cerium oxide precursor.

17. The method of claim 1 wherein in step v) a cerium based oxide precursor and/or a rare earth (other than cerium) oxide precursor are added simultaneously with the manganese oxide precursor.

18. The method of claim 1 wherein the drying of the first suspension and/or the second suspension comprises spray drying.

19. A support material produced according to a method of claim 1.

20. The support material of claim 19 comprising no BaAl.sub.2O.sub.4 after a thermal aging treatment at 850 C. for 4 h.

Description

BRIEF DESCRIPTION OF THE FIGURES

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

(2) FIG. 1 represents a portion of the X-ray diffraction patterns of the materials of Examples 1 to 3 and Comparative Examples 1 and 2 after a thermal treatment at 850 C. for 4 h, showing the effect on the support stability against BaAl.sub.2O.sub.4 formation by the addition of manganese oxide to a cerium oxide doped Mg/Al mixed oxide; and

(3) FIG. 2 represents a portion of the X-ray diffraction patterns of the materials of Examples 1 and Comparative Example 2 together with Comparative Examples 3 and 4 after a thermal treatment at 850 C. for 4 h, showing the influence of different preparation processes on the stability against BaAl.sub.2O.sub.4 formation.

(4) FIG. 3 represents a portion of the X-ray diffraction patterns of the material of Examples 5 after a thermal treatment at 850 C. for 4 h, showing the stability against BaAl.sub.2O.sub.4 formation by the absence of the reflection at 40 2Theta.

EXPERIMENTAL SECTION

(5) Preparation of Support Materials

Example 1

(6) An aqueous suspension of a 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.

(7) The powder was re-suspended in water, ball milled and mixed with a solution of manganese acetate. The mixture was spray dried and the resulting powder calcined at 550 C. for 3 h. The composition of the support material is given in Table I.

Example 2

(8) The support material was prepared as in Example 1 but with a lower amount of manganese oxide, as shown in Table I.

Example 3

(9) The support material was prepared as in Example 2 but with a lower amount of manganese oxide, as shown in Table I.

Comparative Example 1

(10) The support material was prepared without the addition of manganese oxide.

(11) 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.

Comparative Example 2

(12) The support material having the same composition as in Example 1 was prepared including a different sequential addition of manganese oxide, i.e. by using a different doping process that is very well known in the art.

(13) 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. Then, the powder was treated by an incipient wetness impregnation with an aqueous solution of manganese acetate, dried at 120 C. and finally calcined at 550 C. for 3 h.

(14) To be noted is that in this comparative example the calcined Mg/Al mixed oxide is not re-suspended prior to the manganese doping step as per the present invention.

Example 4

(15) The support material was prepared as in Example 1 but adding a mixed solution of cerium acetate and manganese acetate to the suspended cerium oxide doped homogenous Mg/Al mixed oxide. The composition of the support material is given in Table 1.

Example 5

(16) An aqueous suspension of mixed Mg/Al oxide precursor (Pural MG20, MgO content of 20 wt. %) was mixed with a cerium and a neodymium 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. The powder was re-suspended in water, ball milled and mixed with a solution of manganese acetate. The mixture was spray dried and the resulting powder calcined at 550 C. for 3 h. The composition of the support material is given in Table I.

(17) TABLE-US-00001 TABLE I Composition of support materials Mg/Al-oxide CeO.sub.2 MnO.sub.2 Example (wt. %) (wt. %) (wt. %) Example 1 76 14 10 Example 2 78 15 7 Example 3 80 15 5 Comp. Example 1 85 15 0 Comp. Example 2 76 14 10 Mg/Al-oxide CeO.sub.2 MnO.sub.2 Nd.sub.2O.sub.3 (wt. %) (wt. %) (wt. %) (wt. %) Example 5 81 7.2 10 1.8

(18) Testing of the Support Materials Stability Against BaAl.sub.2O.sub.4 Formation (Examples 1 to 5 and Comparative Examples 1 and 2)

(19) The support materials obtained in Examples 1-4 and Comparative Examples 1 and 2, respectively, were suspended in water and ball milled until a d50 of 3 m was obtained. After adding an aqueous solution of barium acetate to get a concentration of 16% of BaO, a powder was obtained by spray drying. The resulting powder was fired first at 550 C. for 3 h followed by a thermal treatment at 850 C. for 4 h. This latter treatment has been found to induce BaAl.sub.2O.sub.4 formation when state of the art materials are used as support materials for the Ba-compound. These samples were then investigated by X-ray diffraction, in particular the occurrence of the reflection of BaAl.sub.2O.sub.4 at around 40 2theta was evaluated. Table II summarizes the results.

(20) TABLE-US-00002 TABLE II Support materials stability test Composition storage material Support material Support material BaO BaAl.sub.2O.sub.4 used (%) (%) formation Example 1 84 16 No Example 2 84 16 No Example 3 84 16 Minor Comp. Example 1 84 16 Yes Comp. Example 2 84 16 Yes Example 4 84 16 No Example 5 85 15 No

(21) FIG. 1 shows a portion of the X-ray diffraction patterns of Examples 1 to 3 and Comparative Example 1. It is obvious that the characteristic (020) and (112) reflections of the BaAl.sub.2O.sub.4 phase at around 40 2theta are absent when the inventive support material of Example 1 is used. With decreasing amount of MnO.sub.2 within the support material the BaAl.sub.2O.sub.4 phase can be observed in minor amounts. When a cerium oxide doped Mg/Al mixed oxide state of the art material is utilized without the addition of manganese oxide as per Comparative Example 1, a significant amount of BaAl.sub.2O.sub.4 is observed.

Comparative Example 3

(22) The preparation process was the same as disclosed by Le Phuc. The Ce, Mn and Ba were added simultaneously to a homogenous Mg/Al mixed oxide to obtain the same composition as in Example 1.

(23) Firstly, a Mg/Al mixed oxide precursor (Pural MG20, MgO content of 20 wt. %) was calcined at 950 C. for 3 h. The resulting Mg/Al mixed oxide was suspended at 60 C. and pH 10 in water. The nitrate salts of cerium, manganese and barium were added simultaneously under stirring while the pH was maintained by the addition of ammonia. After 30 min, the solution was evaporated at 80 C. under air and the resulting powder was dried at 120 C. The dry powder was calcined at 550 C. for 3 h followed by a thermal treatment at 850 C. for 4 h.

Comparative Example 4

(24) The preparation process was the same as disclosed by Le Phuc. The Mn and Ba were added simultaneously to a cerium oxide doped homogenous Mg/Al mixed oxide to obtain the same composition as in Example 1.

(25) 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. The resulting Mg/Al mixed oxide was suspended at 60 C. and pH 10 in water. The nitrate salts of manganese and barium were added simultaneously under stirring while the pH was maintained by the addition of ammonia. After 30 min, the solution was evaporated at 80 C. under air and the resulting powder was dried at 120 C. The dry powder was calcined at 550 C. for 3 h followed by a thermal treatment at 850 C. for 4 h.

(26) TABLE-US-00003 TABLE III Mg/Al- oxide CeO.sub.2 MnO.sub.2 BaO BaAl.sub.2O.sub.4 Example (wt. %) (wt. %) (wt. %) (wt. %) formation Comp. Example 3 62 14 10 15 Yes Comp. Example 4 67 11 9 13 Yes Comp Example 2 63 12 9 16 Yes Example 1 64 12 8 16 No

(27) FIG. 2 shows a portion of the X-ray diffraction patterns of Example 1 (novel and inventive support material) together with Comparative Example 2 and Comparative Examples 3 and 4. All materials have comparable compositions as summarized in Table III but differ in their preparation processes. Comparative Example 2 not including the re-dispersing step and Comparative Example 3 including simultaneous addition of the Ce, Mn and Ba. This demonstrates that a support material that is stable against the formation of BaAl.sub.2O.sub.4 can only be obtained by the method disclosed by the present invention, as only the material made in Example 1 to 4 is characterized by the absence of the characteristic reflections of BaAl.sub.2O.sub.4 in the X-ray diffraction pattern.