Method for Preparing Noble Metal Catalyst

Abstract

The present invention discloses a method for preparing a catalyst, comprising the following steps: (1) taking a noble metal salt solution A, adding a modified alumina support material, stirring until uniform and standing; (2) drying the material obtained in step (1) in a vacuum, and calcining at 500° C.-600° C. for 1-4 hours to obtain a powder material containing the noble metal; (3) mixing the noble metal powder material, an adhesive and other components to be added, and ball-milling to obtain a uniform slurry; (4) preparing a noble metal solution B and adjusting pH to 0.5-1; and (5) mixing the slurry of the step (3) with the noble metal solution B, coating the mixture on a support, drying, and calcining at 500° C.-600° C. for 1-2 hours to obtain the target product. The method for preparing the catalyst of the present invention is simple, the conditions of the preparation process are easy to control and the preparation method has strong practicality. The prepared catalyst has a good quality, a low ignition temperature and a high catalytic conversion rate for methane at a relatively low temperature.

Claims

1. A method for preparing a catalyst, comprising the following steps: (1) taking a noble metal salt solution A, adding a modified alumina support material, stirring until uniform and standing; (2) drying the material obtained in step (1) in a vacuum, and calcining at 500° C.-600° C. for 1-4 hours to obtain a powder material containing the noble metal; (3) mixing the powder material containing the noble metal prepared in step (2) with an adhesive and performing ball-milling, to obtain a uniform slurry; (4) preparing a noble metal solution B and adjusting pH to 0.5-1; and (5) mixing the slurry obtained in step (3) with the noble metal salt solution B, coating the mixture on a support, drying, and calcining at 500° C.-600° C. for 1-4 hours to obtain a target product; wherein the noble metal in the noble metal salt solution A and the noble metal salt solution B is one or more of palladium, platinum and rhodium.

2. The method for preparing a catalyst according to claim 1, characterized in that the noble metal salt solutions A and B are noble metal soluble salt solutions, the noble metal is palladium and/or platinum, and the soluble salt is nitrate, hydrochloride or ammonia salt.

3. The method for preparing a catalyst according to claim 1, characterized in that a concentration of the noble metal salt solution A is 0.1-2.0 wt %, wherein a weight ratio of Pt to Pd is Pd:Pt=1-0:0-1.

4. The method for preparing a catalyst according to claim 1, characterized in that a weight ratio of noble metal components contained in the noble metal solution A and in the noble metal solution B is 1:9-9:1.

5. The method for preparing a catalyst according to claim 1, characterized in that a concentration of the noble metal salt solution B is 0.1-2.0%, wherein a weight ratio of Pt to Pd is Pd:Pt=1-0:0-1.

6. The method for preparing a catalyst according to claim 1, characterized in that the modified alumina support material is La—Al.sub.2O.sub.3 or γ-Al.sub.2O.sub.3.

7. The method for preparing a catalyst according to claim 1, characterized in that the time for standing in step (1) is more than 0.5 hours.

8. The method for preparing a catalyst according to claim 1, characterized in that water content in the noble metal salt solution A in step (1) is 90% to 100% of the water absorption capacity of the added modified alumina.

9. The method for preparing a catalyst according to claim 1, characterized in that a temperature for drying in a vacuum in step (2) is 80° C. to 100° C.

10. The method for preparing a catalyst according to claim 1, characterized in that after the slurry obtained in step (3) is mixed with the noble metal salt solution B, the mixture stands for 0.5 to 2 hours.

11. The method for preparing a catalyst according to claim 2, characterized in that a concentration of the noble metal salt solution A is 0.1-2.0 wt %, wherein a weight ratio of Pt to Pd is Pd:Pt=1-0:0-1.

12. The method for preparing a catalyst according to claim 2, characterized in that a concentration of the noble metal salt solution B is 0.1-2.0%, wherein a weight ratio of Pt to Pd is Pd:Pt=1-0:0-1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 is an activity evaluation result of catalysts prepared in comparative examples 1-2 and example 1.

[0035] FIG. 2 is an activity evaluation result of catalysts prepared in examples 2-8.

DETAILED DESCRIPTION OF THE INVENTION

[0036] A catalyst for automobile exhaust treatment is provided, where the catalyst is made by supporting a part of noble metal A on La—Al.sub.2O.sub.3 or γ-Al.sub.2O.sub.3 which contains Al or modified alumina, a part of noble metal B is added during catalyst milling, where the noble metal is Pt and/or Pd, the contents in percentage by weight of the noble metals are as follows: the concentration of the noble metal A that is supported on the material is 0.1%-2.0%, where a weight ratio of Pt to Pd is Pd:Pt=1-0:0-1; the concentration of the noble metal B that is added during slurry milling is 0.1%-2.0%, where a weight ratio of Pt to Pd is Pd:Pt=1-0:0-1; and the pH value of the noble metal B solution that is added during slurry milling is 0.5-1.0.

[0037] Further, the contents in percentage by weight of the noble metal A supported on the material and the noble metal B added during slurry milling are as follows: the concentration of A is 1.6%, where the weight ratio of Pt to Pd is Pd:Pt=7:1; and the concentration of B is 0.4%, wherein the weight ratio of Pt and Pd is Pd:Pt=2:1.

[0038] A method for preparing a catalyst, including the following steps: (1) weighing corresponding La—Al.sub.2O.sub.3 or γ-Al.sub.2O.sub.3 containing Al or modified alumina powders according to the above mentioned catalyst composition; (2) weighing solution A according to the above mentioned catalyst composition, adding an appropriate amount of water with a water addition amount of 0.9 to 1.0 times than a water absorption amount of the material in step (1), and mixing until uniform; (3) adding the powder material obtained in step (1) to the solution in step (2), stirring rapidly until uniform, standing for larger than or equal to 1 hour, drying in a vacuum at 80° C. for 8 hours, and calcining at 550° C. for 2 hours to obtain a powder material containing the noble metal; (4) adding an adhesive to a mixture of the noble metal powder material obtained in step (3) and other components required to be added, and ball-milling until uniform; and (5) adjusting pH of the solution B to 0.5-1.0, adding to (4) to prepare a slurry, coating it on a catalyst support after standing for 1 hour, drying at 120° C. for 2 hours, and calcining at 550° C. for 2 hours to obtain a catalyst.

COMPARATIVE EXAMPLE 1

Preparation of a Catalyst to Which Noble Metals Are All Added When They are Firstly Supported

[0039] A platinum nitrate solution and a palladium nitrate solution (containing 3.333g of Pt and 16.667g of Pd) are respectively weighed and added to 800 g of deionized water, and stirred until uniform. 980 g of La—Al.sub.2O.sub.3 powder is added to a mixed solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/La—Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, and the slurry is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

COMPARATIVE EXAMPLE 2

Preparation of a Catalyst to Which Noble Metals Are All Added During Slurry Milling

[0040] An adhesive is added to 980 g of La—Al.sub.2O.sub.3 powder to prepare a slurry, and a platinum salt solution and a palladium salt solution (containing 3.333g of Pt and 16.667 g of Pd) are added, and ball-milled until uniform. The slurry is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 1

Preparation of a Catalyst With a Ratio of an Amount of Noble Metal Firstly Supported (Pt:Pd=1:5) to An Amount of Noble Metal During Slurry Milling (Pt:Pd=1:5) of 2:8

[0041] A platinum salt solution and a palladium salt solution (containing 0.6666 g of Pt and 3.3334 g of Pd) are respectively weighed and added to 800 g of deionized water, and stirred until uniform. 980 g of La—Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/La—Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 2.6664 g of Pt and 13.3336 g of Pd) are taken, pH of the noble metal solution is adjusted to 0.5, the solutions having adjusted their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2; and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

[0042] The above various materials, nitrates and the like are all commercially available.

[0043] Detection Results:

[0044] The catalysts in the above examples are subject to an activity evaluation test. The testing conditions are as follows:

[0045] the composition in volume of gases by simulating exhaust gases of lean-burn engines: methane: 1000 ppm, carbon monoxide: 3000 ppm, carbon dioxide: 10%, nitric oxide: 1000 ppm oxygen: 6.5%, nitrogen: a balance gas; space velocity: 40000 h.sup.−1.

[0046] The results are shown in FIG. 1.

[0047] FIG. 1 Methane Conversion Curves in Different Comparative Examples and Example 1

TABLE-US-00001 TABLE 1 Activity data of catalyst CH.sub.4 in different comparative examples and Example 1 No. T.sub.50 (° C.) T.sub.90 (° C.) Comparative 363 444 example 1 Comparative 377 530 example 2 Example 1 346 405

[0048] It can be learned from Table 1 that:

[0049] (1) The performance of the catalyst in Example 1 for CH.sub.4 is better than that of comparative example 2, which illustrates that the conversion performance of the catalyst for methane which is prepared by impregnating the powder and ball-milling a slurry after calcining is superior to that of the catalyst to which the noble metals are all added during slurry milling.

[0050] (2) Comparative example 1 and comparative example 2 are inferior in conversion performance to CH.sub.4 in Example 1, which illustrates that the method that the noble metals are calcined by partially impregnating the powder material and partially added during the slurry milling is superior to the way that the noble metals are impregnated the powder material and calcined or that the noble metals are all added during ball-milling to prepare a slurry.

[0051] The particular examples by changing the ratio of the noble metal A supported on the material and the noble metal B added during slurry milling and the material types are as follows.

EXAMPLE 2

Preparation of a Catalyst Firstly Supported With a Noble Metal Pd, and Supplemented With a Noble Metal Pt During Slurry Milling

[0052] A palladium salt solution (containing 16.667 g of Pb) is weighed and added to 800 g of deionized water, and stirred until uniform. 980 g of La—Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/La—Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution (containing 3.333 g of Pt) is taken, pH of the noble metal solution is adjusted to 0.5, the solution with its pH properly adjusted is added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 3

Preparation of a Catalyst Firstly Supported With a Noble Metal Pt, and Supplemented With a Noble Metal Pd During Slurry Milling

[0053] A platinum salt solution (containing 3.333 g of Pt) is weighed and added to 800 g of deionized water, and stirred until uniform. 980 g of La—Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/La—Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A palladium salt solution (containing 16.667 g of Pd) is taken, pH of the noble metal solution is adjusted to 0.5, the solution with its pH properly adjusted is added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 4

Preparation of a Catalyst With a Ratio of An Amount of Noble Metal Firstly Supported (Pt:Pd=1:5) to An Amount of Noble Metal During Slurry Milling (Pt:Pd=1:5) of 5:5

[0054] A platinum salt solution and a palladium salt solution (containing 1.6665 g of Pt and 8.3335 g of Pd) are respectively weighed and added to 800 g of deionized water, and stirred until uniform. 980 g of La—Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/La—Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 1.6665 g of Pt and 8.3335 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 5

Preparation of a Catalyst With a Ratio of An Amount of Noble Metal Firstly Supported (Pt:Pd=1:7) to An Amount of Noble Metal During Slurry Milling (Pt:Pd=1:2) of 4:1

[0055] A platinum salt solution and a palladium salt solution (containing 2 g of Pt and 14 g of Pb) are respectively weighed and added to 800 g of deionized water, and stirred until uniform. 980 g of La—Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/La—Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 1.333 g of Pt and 2.667 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 6

Preparation of a Catalyst With a Ratio of An Amount of Noble Metal Firstly Supported (Pt:Pd=1:4.2) to An Amount of Noble Metal Pd During Slurry Milling of 6.5:1

[0056] A platinum salt solution and a palladium salt solution (containing 3.333 g of Pt and 14 g of Pd) are respectively weighed and added to 800 g of deionized water, and stirred until uniform. 980 g of La—Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/La—Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution (containing 2.667 g of Pd) is taken, pH of the noble metal solution is adjusted to 0.5, the solution with its pH properly adjusted is added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support with a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst with a noble metal content of 80 g/ft.sup.3.

EXAMPLE 7

Preparation of a Catalyst With a Ratio of An Amount of Noble Metal Firstly Supported (Pt:Pd=1:7) to An Amount of Noble Metal During Slurry Milling (Pt:Pd=1:2) of 4:1 When a γ-Al.SUB.2.O.SUB.3 .Material is Used

[0057] A platinum salt solution and a palladium salt solution (containing 2 g of Pt and 14 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of γ-Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/La—Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 1.333 g of Pt and 2.667 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 8

Preparation of a Catalyst With a Ratio of An Amount of Noble Metal Firstly Supported (Pt:Pd=1:7) to An Amount of Noble Metal During Slurry Milling (Pt:Pd=1:2) of 4:1, When a Zr—Y/Al.SUB.2.O.SUB.3 .Material is Used

[0058] A platinum salt solution and a palladium salt solution (containing 2 g of Pt and 14 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of Zr—Y/Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/Zr—Y/Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 1.333 g of Pt and 2.667 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

[0059] The results of activity evaluation are shown in FIG. 2, which respectively represents the data of activity evaluation in Example 2, Example 3, Example 4, Example 5, Example 6, Example 7 and Example 8.

[0060] The results are shown in FIG. 2.

[0061] FIG. 2 Methane Conversion Curves in Different Examples

TABLE-US-00002 TABLE 2 Activity data of catalyst CH.sub.4 of different examples NO. T.sub.50 (° C.) T.sub.90 (° C.) Example 2 358 416 Example 3 366 438 Example 4 328 380 Example 5 316 374 Example 6 319 378 Example 7 318 386 Example 8 312 370

[0062] It can be learned from Table 2 that:

[0063] (1) The catalyst performance of Example 2 for CH.sub.4 is superior to that of Example 3, which illustrates that the effect of Pd being firstly supported is better than that of Pt being firstly supported.

[0064] (2) When the same support material is used, the performance of Example 5 is superior to that of Example 1, Example 2, Example 3, Example 4, and Example 6.

[0065] (3) The catalyst performance of Example 8 for CH.sub.4 is superior to that of Example 5 and Example 7, which illustrates that under the condition of a same preparation method, the materials had an effect on the catalyst, and the performance of the catalysts prepared with rare earth modified alumina are superior to that of the catalysts without modification.

EXAMPLE 9

Preparation of a Catalyst Firstly Supported With Noble Metal Pt:Pd=1:1 When a γ-Al.SUB.2.O.SUB.3 .Material is Used

[0066] A platinum salt solution and a palladium salt solution (containing 1 g of Pt and 1 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of γ-Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/γ-Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 2.3333 g of Pt and 15.6667 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 10

Preparation of a Catalyst Firstly Supported With Noble Metal Pt:Pd=2:1 when a γ-Al.SUB.2.O.SUB.3 .material is used

[0067] A platinum salt solution and a palladium salt solution (containing 2 g of Pt and 1 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of γ-Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/y-A1203 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 1 3333g of Pt and 15.6667g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 11

Preparation of a Catalyst Firstly Supported With Noble Metal Pt:Pd=1:2 When a γ-Al.SUB.2.O.SUB.3 .Material is Used

[0068] A platinum salt solution and a palladium salt solution (containing 1 g of Pt and 2 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of γ-Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/γ-Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 2.3333 g of Pt and 14.6667 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 12

Preparation of a Catalyst Firstly Supported With Noble Metal Pt:Pd=3:1 When a γ-Al.SUB.2.O.SUB.3 .Material is Used

[0069] A platinum salt solution and a palladium salt solution (containing 3 g of Pt and 1 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of γ-Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/γ-Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 0.3333 g of Pt and 15.6667 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 13

Preparation of a Catalyst Firstly Supported With Noble Metal Pt:Pd=1:4 When a γ-Al.SUB.2.O.SUB.3 .Material is Used

[0070] A platinum salt solution and a palladium salt solution (containing 1 g of Pt and 4 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of γ-Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/γ-Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 2.3333 g of Pt and 12.6667 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 14

Preparation of a Catalyst Firstly Supported With Noble Metal Pt:Pd=1:10 When a γ-Al.SUB.2.O.SUB.3 .Material is Used

[0071] A platinum salt solution and a palladium salt solution (containing 1 g of Pt and 10 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of γ-Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/γ-Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 2.3333 g of Pt and 6.6667 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

[0072] The results of activity evaluation are shown in FIG. 3, which respectively represents the data of activity evaluation in Example 9, Example 10, Example 11, Example 12, Example 13 and Example 14

[0073] The results are shown in FIG. 3.

[0074] FIG. 3 Methane Conversion Curves in Different Examples

TABLE-US-00003 TABLE 3 Activity data of catalyst CH.sub.4 in different examples No. T.sub.50 (° C.) T.sub.90 (° C.) Example 9 324 393 Example 10 319 376 Example 11 325 380 Example 12 322 373 Example 13 319 377 Example 14 325 386

EXAMPLE 15

Preparation of a Catalyst With Noble Metal Pt:Pd=1:1 During Slurry Milling When a γ-Al.SUB.2.O.SUB.3 .Material is Used

[0075] A platinum salt solution and a palladium salt solution (containing 2.3333 g of Pt and 15.6667 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of γ-Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/γ-Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 1 g of Pt and 1 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 16

Preparation of a Catalyst With Noble Metal Pt:Pd=10: 1 During Slurry Milling When a γ-Al.SUB.2.O.SUB.3 .Material is Used

[0076] A platinum salt solution and a palladium salt solution (containing 0.3333 g of Pt and 16.3667 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of γ-Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/γ-Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 3 g of Pt and 0.3 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 17

Preparation of a Catalyst With Noble Metal Pt:Pd=1:10 During Slurry Milling When a γ-Al.SUB.2.O.SUB.3 .Material is Used

[0077] A platinum salt solution and a palladium salt solution (containing 2.3333 g of Pt and 6.6667 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of γ-Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/γ-Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A.

[0078] A platinum salt solution and a palladium salt solution (containing 1 g of Pt and 10 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 18

Preparation of a Catalyst With Noble Metal Pt:Pd=1:5 During Slurry Milling When a γ-Al.SUB.2.O.SUB.3 .Material is Used

[0079] A platinum salt solution and a palladium salt solution (containing 2.3333 g of Pt and 11.6667 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of γ-Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/γ-Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 1 g of Pt and 5 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2 and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

EXAMPLE 19

Preparation of a Catalyst With Noble Metal Pt:Pd=1:3 During Slurry Milling When a γ-Al.SUB.2.O.SUB.3 .Material is Used

[0080] A platinum salt solution and a palladium salt solution (containing 2.3333 g of Pt and 13.6667 g of Pd) are respectively weighed and added to 1000 g of deionized water, and stirred until uniform. 980 g of γ-Al.sub.2O.sub.3 powder is added to a palladium nitrate solution, which is rapidly stirred until uniform and then stands for 1 hour. Then the solution is dried in a vacuum at 80° C. for 8 hours and calcined at 550° C. for 2 hours, to obtain Pd—Pt/γ-Al.sub.2O.sub.3 powder (which is yellowish brown powder). An adhesive is added to the yellowish brown powder to prepare a slurry, which is labeled as slurry A. A platinum salt solution and a palladium salt solution (containing 1 g of Pt and 3 g of Pd) are taken, pH of the noble metal solutions is adjusted to 0.5, the solutions with their pH properly adjusted are added to the slurry A and then ball-milled and mixed until uniform, and the mixture which already stands for 1 hour is coated on a cordierite ceramic honeycomb support having a mesh number of 400 cell/in.sup.2and a volume of 2.5 ml, and is then dried and calcined, to obtain a platinum-palladium catalyst having a noble metal content of 80 g/ft.sup.3.

[0081] The results of activity evaluation are shown in FIG. 4, which respectively represents the data of activity evaluation in Example 15, Example 16, Example 17, Example 18 and Example 19.

[0082] The results are shown in FIG. 4.

[0083] FIG. 4 Methane Conversion Curves in Different Examples

TABLE-US-00004 TABLE 4 Activity data of catalyst CH.sub.4 in different examples No. T.sub.50 (° C.) T.sub.90 (° C.) Example 15 327 396 Example 16 321 385 Example 17 324 377 Example 18 324 380 Example 19 316 368

[0084] The bench test of vehicles is as follows:

[0085] Scheme 1: after being prepared in accordance with the method in Example 5, the slurry is coated on a support having a mesh number of 400 cell/in.sup.2 and an external dimension of Φ150 mm (diameter)×100 (length). A finished catalyst (with Pt:Pd:Rh=1:5:0, 80 g/ft.sup.3) is prepared.

[0086] Scheme 2: after being prepared in accordance with the method in Comparative example 1, the slurry is coated on a support having a mesh number of 400 cell/in.sup.2 and an external dimension of Φ150 mm (diameter)×100 (length). A finished catalyst (with Pt:Pd:Rh=1:5:0, 80 g/ft.sup.3) is prepared.

[0087] Scheme 3: after being prepared in accordance with the method in Comparative example 2, the slurry is coated on a support having a mesh number of 400 cell/in.sup.2 and an external dimension of Φ150 mm (diameter)×100 (length). A finished catalyst (with Pt:Pd:Rh=1:5:0, 80 g/ft.sup.3) is prepared. The abovementioned catalyst is installed on a bus with an engine model YC6L manufactured by certain company, and the bench test is performed in accordance with ETC test procedures as prescribed by National V Emission Standard in China The results are as follows:

[0088] ETC Test Results of YC6L Engine, Table 4

TABLE-US-00005 Standard limited Comparative Comparative Example Example Example Test items values example 1 example 2 5 13 19 CO g/(kW .Math. h) 4.0 0.341 0.359 0.586 0.362 0.586 NOx g/(kW .Math. h) 2.0 1.88 1.82 1.86 1.86 1.85 CH.sub.4 g/(kW .Math. h) 1.1 1.32 1.07 0.668 0.575 0.713 NMHC g/(kW .Math. h) 0.55 0.51 0.463 0.259 0.302 0.276

[0089] It can be seen from the ETC test results of YC6L engine that after being installed with the catalysts of Examples 5, 13 and 19, the engine produces the minimal amount of emission pollutants, indicating that Examples 5, 13 and 19 are significantly better than Comparative examples 1 and 2 in CH.sub.4 purification performance.

[0090] In this example, the catalysts with excellent CH.sub.4 conversion performance prepared by the preparation methods of Examples 5, 13 and 19 have the optimal result when used for treatment of exhaust gases from lean-burn natural gas vehicles, and satisfy a wide variety of relevant standards and requirements in National V Emission Standard