JPH08206498A - Catalyst for purifying exhaust gas and manufacture thereof - Google Patents

Abstract

PURPOSE: To exhibit a high NOX purification ratio even in an oxygen excessive atmosphere and to enhance the heat resistance by making a secondary aggregate particle comprising an aggregate of powder of alumina crystal carry noble metals as catalyst metals. CONSTITUTION: In a secondary aggregate particle 1 of powders 2 of alumina crystal, there are recesses or pockets, where crystal particles are combined, forming a mesopore 3. Surface electric potential at the mesopore is high so that a noble metal 4 can be easily carried by the mesopore. And, the mesopore 3 is an area, into which NOX in exhaust gas or HC, which serves as a reducing agent when NOX is decomposed, can be easily taken. Therefore, oxidation decomposition of HC in exhaust gas due to catalytic action of the noble metal 4 and following reducing decomposition of NOX can be effected efficiently. And, the powders 2 are thermally stable in comparison with zeolite, so that even if they are exposed to exhaust gas at a high temperature for a long period of time, NOX purifying performance is not deteriorated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst.

[0002]

2. Description of the Related Art As a catalyst for purifying exhaust gas from an internal combustion engine, particularly an internal combustion engine of an automobile, a three-way catalyst in which a noble metal (Pt, Rh, Pd, etc.) is supported on alumina is known. It is also known that a catalyst carrier is coated with this three-way catalyst, and a mixture of CeO ₂ and alumina, which has been heat-treated at 800 to 1000 ° C. for 1 to 5 hours, is coated on the catalyst layer. (JP-A-62-7
1541). The CeO ₂ has an O ₂ storage effect and is used for improving the catalytic performance.
Further, the CeO ₂ causes sintering and deteriorates when heated to a high temperature, so that the heat treatment is performed in advance.

[0003]

However, the above three-way catalyst efficiently purifies HC (hydrocarbon), CO (carbon monoxide) and NOx (nitrogen oxide) in the exhaust gas near the stoichiometric air-fuel ratio. However, the NOx purification rate significantly decreases in an oxygen excess atmosphere (lean). On the contrary,
A NOx catalyst has been developed in which a noble metal or other transition metal is supported on zeolite. With this catalyst, NOx can be purified even in the above oxygen excess atmosphere. However, zeolite is generally more expensive than the above-mentioned alumina, has low heat resistance, and suddenly causes structural destruction at high temperatures to fail to exhibit its intended function.

[0004]

Means and Actions for Solving the Problems The present inventor has conducted various experiments and studies to utilize the above-mentioned alumina for purification of NOx in an oxygen-excess atmosphere, and found that the alumina was subjected to a predetermined heat treatment. It has been found that a high NOx purification rate is exhibited, and the present invention has been completed. Hereinafter, the invention according to each claim will be specifically described.

<Invention of Claim 1> The present invention is an exhaust gas purifying catalyst, characterized in that a noble metal is supported as a catalytic metal on secondary agglomerated particles formed by agglomeration of alumina crystal powder.

In the present invention, compared with the case where the noble metal is supported on the alumina crystal powder, N in the oxygen excess atmosphere is increased.
Ox purification rate increases. The reason is considered as follows.

That is, in the secondary agglomerated particles of the alumina crystal powder, the portions where the crystal particles are bonded to each other form recesses or pockets to form mesopores. The surface potential of this mesopore site is high, so
The noble metal is easily supported on the portion. In addition, the mesopores are also sites where NOx in the exhaust gas and HC that serves as a reducing agent when decomposing the NOx are easily taken in. Since the noble metal is supported on such mesopores, it is considered that the catalytic action of the noble metal efficiently promotes the oxidative decomposition of HC in the exhaust gas and the accompanying reduction decomposition of NOx.

As a result, the above-mentioned alumina crystal particles are more thermally stable than Zeoseit, and therefore, even if they are exposed to high-temperature exhaust gas for a long time, the NOx purification performance does not deteriorate.

The specific surface area of the secondary agglomerated particles of alumina is 1
It is preferably 00 m ² / g or more (for example, 100 to 350 m ² / g). This is because a larger specific surface area is more advantageous for supporting precious metals and adsorbing gas. Further, as the above-mentioned noble metal, Pt, Rh, Pd, Ir or any other suitable substance may be adopted, and one kind of noble metal may be carried, or two or more kinds of noble metals may be carried in combination.

<Invention of Claim 2> In the exhaust gas-purifying catalyst according to Claim 1, the present invention has a mesopore diameter having the largest pore volume in the pore distribution of the secondary agglomerated particles. The exhaust gas purifying catalyst is characterized by having a thickness of 3 to 13 nm.

The mesopores of the secondary agglomerated particles are sites that play an important role in supporting the precious metal and taking in NOx or HC as described above, and the mesopore diameter is 3 nm.
When it is less than the above range, it is disadvantageous for the uptake of NOx or HC, and when it is larger than 13 nm, the surface potential is not so high and it is disadvantageous for the selective uptake of NOx or HC and is also disadvantageous for supporting the noble metal. .

<Invention of Claim 3> In the exhaust gas purifying catalyst according to claim 1, the present invention has a mesopore diameter having the largest pore volume in the pore distribution of the secondary aggregated particles. The exhaust gas purifying catalyst is characterized by having a thickness of 6 to 10 nm.

In the present invention, the reason why the mesopore diameter of the secondary agglomerated particles is 6 to 10 nm is because it is more advantageous for supporting the precious metal and taking in NOx or HC.

<Invention of Claim 4> The present invention is a method for producing the exhaust gas purifying catalyst of each of the inventions of claims 1 to 3, wherein the alumina crystal powder is subjected to heat treatment. The alumina crystal powder is agglomerated to produce 2
The secondary agglomerated particles are formed, and the secondary agglomerated particles are loaded with a noble metal as a catalytic metal.

In the present invention, since the noble metal is supported on the secondary aggregated particles of the alumina crystal powder, the noble metal is easily supported on the mesopore portion of the secondary aggregated particles.

A heating means is used for the agglomeration (sintering) of the alumina crystal powder. This heating may be carried out under atmospheric pressure, under reduced pressure, or under pressure. The lower the atmospheric pressure, the higher the heating temperature needs to be. Further, the above heating can be performed in an inert atmosphere to avoid mixing of impurities, but aluminum nitride is easily formed in a nitrogen gas atmosphere,
For example, it is suitable to use other gas such as argon gas.

In addition, for agglomeration of the above-mentioned alumina crystal powder,
The process of evaporation-condensation of alumina contributes, but Mg,
The inclusion of a very small amount of Y, Fe, Ti, etc. lowers the melting point of alumina, which is advantageous for the formation of secondary agglomerated particles.

<Invention of Claim 5> The present invention is the method for producing an exhaust gas purifying catalyst as set forth in claim 4, wherein the heat treatment is carried out by heating the alumina crystal powder to 700 to 1000 ° C. at atmospheric pressure. The method for producing an exhaust gas purifying catalyst is characterized in that the above temperature is maintained for 1 hour or more.

Performing the above heat treatment under atmospheric pressure is advantageous in simplifying the equipment for the heat treatment, but if the heating temperature in that case is less than 700 ° C., the heating time is shortened. Agglomeration of alumina crystal powder does not easily proceed even if the length is increased, and the mesopore diameter is, for example, the above-mentioned claim 2 to claim 3.
It becomes difficult to grow the secondary agglomerated particles until the target size is reached as in each of the above inventions. On the other hand, when the heating temperature is higher than 1000 ° C., the agglomeration of the alumina crystal powder tends to proceed excessively, and it becomes difficult to obtain the desired mesopores. The preferred temperature is 800 to 1000 ° C.

If the heating time is too short, the agglomeration of the alumina crystal powder does not proceed sufficiently, so the heating time is 1
It is desirable that the heating time be longer than that, but the heating for a long time does not matter so much when the heating temperature is high.

[0021]

According to the invention of claim 1, since the noble metal is supported as the catalytic metal on the secondary agglomerated particles formed by agglomeration of the alumina crystal powder, the heat resistance of the catalyst is increased,
It is possible to increase the NOx purification rate in an oxygen-excess atmosphere, which is advantageous in terms of cost.

According to the invention of claim 2, the mesopore diameter having the largest pore volume in the pore distribution of the secondary agglomerated particles is set to 3 to 13 nm.
It is advantageous for the uptake of x or HC, and the NOx purification rate can be increased.

According to the third aspect of the invention, the mesopore diameter having the largest pore volume in the pore distribution of the secondary agglomerated particles is set to 6 to 10 nm, which is further advantageous in improving the NOx purification rate.

According to the invention of claim 4, heat treatment is applied to the alumina crystal powder to agglomerate the alumina crystal powder to form secondary agglomerated particles, and the noble metal is used as a catalyst metal in the secondary agglomerated particles. Since the noble metal is supported, the noble metal can be supported on the mesopore portion of the secondary agglomerated particles, and the exhaust gas purifying catalyst according to each of the first to third aspects of the invention can be manufactured.

According to the invention of claim 5, the alumina crystal powder is heated to a temperature of 700 to 1000 ° C.
Since the heating and holding is carried out for more than a time, it becomes possible to obtain the alumina secondary agglomerated particles having the desired mesopore diameter with a simple heating facility.
It is advantageous in producing the exhaust gas purifying catalyst of each of the inventions.

[0026]

Embodiments of the present invention will be described below.

<Preparation of Catalyst> -Production of Alumina Crystal Powder-A sol-gel method was adopted for this production. That is, a predetermined amount of aluminum alkoxide Al (OR) ₃ is dissolved in dimethylpropanol (or other alcohol such as ethanol), heated to a predetermined temperature of 80 to 150 ° C., and then a predetermined amount of ion-exchanged water is added thereto. To produce aluminum hydroxide, which is
Alumina crystal powder was obtained by firing at 0 ° C.
The reaction formula is as follows.

Al (OR) ₃ + H ₂ O → AlO (OH) + 3ROH AlO (OH) + H ₂ O → Al (OH) ₃ 2Al (OH) ₃ → Al ₂ O ₃ + 3H ₂ O

Note that Al (O
H) ₃ may be synthesized and then fired.

-Agglomeration of Alumina Crystal Powder-Alumina secondary agglomerated particles were obtained by heating the alumina crystal powder at 900 ° C. for 50 hours. The specific surface area of the secondary agglomerated particles is 100 m ² / g or more, and when the pore distribution was examined, the mesopore diameter was 5 to 5.
The pore volume of 10 nm was the largest.

-Supporting Noble Metals-The above-mentioned secondary aggregated particles of alumina are wash-coated on a honeycomb-shaped monolithic carrier (400 cells / inch ² ) made of cordierite, dried and calcined. The desired honeycomb catalyst was obtained by immersing in a mixed aqueous solution of rhodium chloride, pulling it up, and drying and firing. The total amount of Pt and Rh supported is 4.5 g per liter of the carrier.
The weight ratio of the supported amount of was 1: 1.

The secondary agglomerated particles of the obtained catalyst are shown as a model in FIG. That is, in the figure, 1 is an alumina secondary agglomerated particle, several alumina crystal particles 2 are agglomerated and bonded, and noble metal 4 is added to the mesopore 3 site.
Is carried.

<Evaluation of Catalyst> The above catalyst was evaluated by using the honeycomb catalyst of the example prepared by the above catalyst preparation method.
It was carried out by comparing the NOx purification performance with the honeycomb catalyst of the comparative example using the alumina crystal powder which was not subjected to the above-mentioned agglomeration treatment (heat treatment). With respect to the catalyst of the comparative example, the supporting treatment of the above-mentioned noble metal was performed under the same conditions and methods as those of the examples.

In the NOx purification performance comparison test, the catalysts of the above-mentioned examples and comparative examples were installed in a simulated gas flow apparatus, and the simulated gas was flown through the catalyst, and the gas temperature was changed from room temperature to 350.
After raising the temperature to 0 ° C., the temperature is lowered from that temperature to room temperature, and the NOx purification rate at the time of the temperature reduction is measured at each temperature. The composition and space velocity of the simulated gas are as follows.

-Composition of simulated gas-HC: 4000 ppm, NOx; 250 ppm, CO;
0.15%, O ₂ ; 7.5%, H ₂ ; 650 ppm, N
₂ Balance-Space velocity-SV = 55000h ^-1

The results are shown in FIG. According to the figure, the catalyst of the example has a higher NOx purification rate than the catalyst of the comparative example, and it is effective to agglomerate the alumina crystal powder and then carry the noble metal on the NOx purification rate. I understand.

FIG. 3 shows the results of investigating the NOx purification performance under the same conditions and methods as in the above case after aging treatment (850 ° C. × 50 hours) for the catalyst of the above-mentioned example and the catalyst of the comparative example. Is.

According to the figure, even when the catalyst is aged, it is effective to increase the NOx purification rate by aggregating the alumina crystal powder and then supporting the noble metal as in the example. Recognize.

<Relationship between Mesopore Diameter and NOx Purification Rate> In order to determine the optimum range for the mesopore diameter of the secondary alumina agglomerated particles, the alumina crystal powder is heat-treated at various temperatures, Several types of secondary alumina agglomerated particles having different mesopore diameters showing the largest pore volume in the pore distribution were manufactured. Then, using these, a honeycomb catalyst supporting a noble metal by the above-described noble metal supporting treatment was prepared. The total amount of Pt and Rh supported on these honeycomb catalysts was 4.5 g / L,
The weight ratio of the amount of Pt carried to the amount of Rh carried was 75: 1.

Then, when the maximum NOx purification rates of the respective honeycomb catalysts having different mesopore diameters were examined, the results shown in FIG. 4 were obtained. The simulated gas used is as follows and the space velocity is 55000 h ^-1 .

HC: 4000 ppm C, NOx: 250
ppm, O ₂ ; 7.5%

According to FIG. 4, the mesopore diameter is 3 to 13 nm.
It can be seen that the NOx purification rate becomes high when, and the mesopore diameter is preferably 5 to 11 nm, particularly preferably 6 to 10 nm.

<Regarding Relation between Aggregation Treatment Temperature of Alumina Crystal Powder and Pore Volume> The relation between the heating temperature of the alumina crystal powder and the pore volume was examined. The heating was performed in the atmosphere, and the heating time was 50 hours. As a result, the total pore volume of the alumina is shown in FIG.
The pore volume of 10 nm is shown in FIG. 6, and the pore volume of 5 nm or less is shown in FIG.

According to FIG. 5, the total pore volume increases up to 900 ° C. as the heating temperature rises, and conversely decreases at higher heating temperatures. The increase in the total pore volume is due to the formation of mesopores due to the aggregation of the alumina crystal powder, and the decrease in the total pore volume is considered to be the result of excessive aggregation of the alumina crystal powder.

According to FIG. 6, the pore volume having a pore diameter of 5 to 10 nm is larger than that of fresh (without heat treatment) at a heating temperature of 700 ° C., and 110
At 0 ° C, it is smaller than that of untreated one.
Further, according to FIG. 7, as the heating temperature increases, 5n
The pores of m or less are dented, which is a result of the aggregation of the alumina crystal powder.

From the above results shown in FIGS. 5 to 7, in order to produce secondary alumina agglomerated particles having a large amount of mesopores having a diameter of 5 to 10 nm, the heating temperature of the alumina crystal powder is set to 700 to 1000 ° C. It can be seen that the preferable heating temperature is 800 to 1000 ° C.

[Brief description of drawings]

FIG. 1 is a diagram showing secondary agglomerated particles of alumina.

FIG. 2 is a graph diagram comparing NOx purification characteristics of catalysts of Examples and Comparative Examples.

FIG. 3 is a graph chart comparing the NOx purification characteristics after aging for the catalysts of the example and the comparative example.

FIG. 4 Mesopore diameter and maximum NO of secondary agglomerated particles of alumina
x Graph showing the relationship with the purification rate

FIG. 5 is a graph showing the relationship between the heating temperature of alumina crystal powder and the total pore volume.

FIG. 6: Heating temperature of alumina crystal powder and diameter 5-10 nm
Graph showing the relationship between the pore volume and

FIG. 7 is a graph showing the relationship between the heating temperature of alumina crystal powder and the volume of pores having a diameter of 5 nm or less.

[Explanation of symbols]

 1 Alumina Secondary Aggregated Particle 2 Alumina Crystal Particle 3 Mesopore 4 Noble Metal

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01D 53/94 B01J 21/04 ZAB A 23/42 A // B01J 23/38 ZAB A

Claims (5)

[Claims] 1. An exhaust gas purifying catalyst, wherein a noble metal is supported as a catalytic metal on secondary agglomerated particles formed by agglomeration of alumina crystal powder. 2. The exhaust gas purifying catalyst according to claim 1, wherein the mesopore diameter having the largest pore volume in the pore distribution of the secondary agglomerated particles is 3 to 13 nm. Gas purification catalyst. 3. The exhaust gas purifying catalyst according to claim 1, wherein the mesopore diameter having the largest pore volume in the pore distribution of the secondary agglomerated particles is 6 to 10 nm. Gas purification catalyst. 4. Exhaust gas, characterized in that the alumina crystal powder is subjected to a heat treatment to aggregate the alumina crystal powder to form secondary aggregate particles, and the secondary aggregate particles carry a noble metal as a catalyst metal. Method for producing gas purification catalyst. 5. The method for producing an exhaust gas purifying catalyst according to claim 4, wherein the heat treatment is performed by applying alumina crystal powder to an atmospheric pressure of 7
A method for producing an exhaust gas purifying catalyst, characterized by holding at a temperature of 00 to 1000 ° C. for 1 hour or more.