JPS581627B2 - Catalyst for exhaust gas purification - Google Patents

Description

Detailed Description of the Invention The present invention can highly efficiently purify nitrogen oxides, carbon monoxide, and hydrocarbons, which are harmful components in exhaust gas emitted from internal combustion engines, etc., and has high temperature strength and This invention relates to an exhaust gas purification catalyst with excellent high-temperature durability.

Currently, various catalysts have been proposed for purifying harmful components in exhaust gas as mentioned above, among which catalyst components supported on alumina carriers include platinum, palladium,
It is said that those using rhodium have relatively excellent purification activity.

However, these catalysts still satisfy the requirements for purifying automobile exhaust gas, such as highly efficient purification of the three harmful components mentioned above, and superior strength and durability at high temperatures. I can't.

The present invention has been made with the aim of overcoming such problems.

That is, the present invention provides alumina-magnesia spinel (M
gA1204), alumina (A1203), magnesia (MgO) and ceria (CeO2), alumina and magnesia are each 2% of the above spinel.
A catalyst for exhaust gas purification is characterized in that the carrier is a porous material present at a weight ratio of 5 or less, and one or both of platinum (Pt) and palladium (Pd) is supported on the carrier.

According to the present invention, nitrogen oxides (NO
x) A catalyst capable of purifying carbon monoxide (Co) and hydrocarbons (HC) with high efficiency can be obtained.

Furthermore, this catalyst exhibits particularly high activity in purifying NO.

The reason for this effect is thought to be that since the porous body contains ceria, oxygen can be easily taken in and out of the surface of the porous body, activating the reaction on the catalyst surface.

The reason why oxygen can be taken in and out easily in this way is that ceria exists between MgAl2O4 spinel, which is the matrix of the porous material, and cerium (Ce), which is a constituent element of ceria, and oxygen (
This is thought to be due to the fact that CeO2 or Ce203 is relatively easily combined or separated from CeO2 and Ce203.

In addition, the catalyst particularly has an air-fuel ratio (weight ratio of air to gasoline fed to the internal combustion engine) of 13.5 to 15.5.
It exhibits an excellent effect in simultaneously purifying the above-mentioned harmful components of exhaust gas from an internal combustion engine operated within the above range.

Further, the catalyst according to the present invention has high mechanical strength at high temperatures because the porous body as a carrier contains magnesia alumina (MgA120,) spinel.

In addition, even when used at high temperatures, the crystal structure of alumina does not change as in the case of general alumina supports, and there is no decrease in surface area or strength due to such changes, and the durability of catalyst activity at high temperatures is maintained. Demonstrates excellent effects.

In the porous body, as described above, alumina and magnesia in the components constituting the porous body must each be present in an amount of 25% by weight or less based on the spinel.

If more alumina or magnesia is present than this amount, the effects of the presence of spinel, as described above, cannot be achieved.

Note that the above effects can be achieved even when almost no alumina and magnesia are present.

In addition, in the above, the amount of ceria with respect to the total amount of spinel, alumina, and magnesia is 0.05 to 20
Preferably, it is % by weight.

If the amount is less than 0.05φ, it is difficult to obtain the above effect due to the presence of ceria, and if the amount is more than 20%, this effect becomes greater, but the bond between spinels becomes weaker and the strength of the catalyst decreases. This is because it is difficult to obtain the above effect.

Further, the average pore diameter of the porous material as the carrier is 0.01
Preferably, the thickness is between 2μ and 2μ.

If the average pore diameter is outside this range, it is difficult to exhibit excellent activity as an exhaust gas purifying catalyst.

When supporting platinum, palladium, or both of the catalyst components on the porous body, it is carried out in the same manner as in the case of supporting ordinary catalyst components, such as platinum nitrate, chloroplatinic acid, palladium nitrate, palladium chloride. Immersing the porous body in a solution of raw materials for forming a catalyst component such as
Dry and bake.

Further, in this case, it is preferable that the total amount of catalyst components supported on the porous body is 0.01 to 5% by weight.

If it is less than 0.01%, the purification activity becomes low, and if it is less than 5%
Above this, even if it is supported more than that, the activity will not be improved to a commensurate extent.

Next, as a method for manufacturing the above porous body,
There is a method in which magnesia powder, alumina powder, and ceria or a powder of a cerium compound that becomes ceria upon heating are mixed, and the mixture is heated and fired.

In the above, the molded body of the mixed powder is heated to 1000°C to 1°C.
Heat and bake at 600°C.

At temperatures lower than 1000° C., sintering is not sufficient, the amount of MgAl20 and spinel produced is small, and the strength of the carrier is weak.

Moreover, when the temperature is higher than 1600° C., the MgA1204 spinel particles grow too much and the pore volume decreases.

In this case, when firing at 1200 to 1600° C., magnesia and alumina react and 75% or more of them become MgAl2O4 spinel, making it possible to obtain a carrier with better heat resistance and strength.

Since the above-mentioned spinel is produced by reacting equimolar amounts of magnesia and alumina, unreacted magnesia or alumina remains as is in the carrier.

Note that the equimolar amount of magnesia and alumina corresponds to a case where the amount of alumina (alumina/magnesia) to magnesia is 2.6 times by weight in terms of weight ratio.

Further, in order to obtain a porous body with an average pore size of 0.01 to 2μ, an alumina powder having an average particle size of 0.01 to 2μ is used.

The term "particle size" used herein means the weight average particle size.

In addition to the above alumina, α (alpha)-alumina,
Aluminas such as γ (gamma)-alumina can also be used.

Magnesia powder mixed with alumina powder, etc. can be said to be the optimal binder for alumina powder in the sintered porous body, and its particle size is not particularly limited, but it can be mixed almost evenly with alumina powder. In order to form alumina and spinel together and to make the obtained carrier substantially uniform in pore size, it is preferable to use particles with a particle size of 0.1 to 500 μm.

The mixing ratio of alumina powder to magnesia powder (alumina/magnesia) is preferably 1.5 to 3.5 times by weight.

If it is less than 1.5 times the weight, the amount of magnesia is too large, and the heating during firing causes magnesia to crystallize, producing large crystals of only magnesia, and reducing the number of pores in the porous body. I end up.

Furthermore, the alumina is wrapped in a large amount of magnesia, making it difficult to obtain the desired porous body.

The ceria or cerium compound is mixed in an amount of 0.05 to 20% by weight based on the total amount of magnesia and alumina powder in terms of cerium oxide (ceria) due to the reason for the presence of ceria in the porous body as described above. .

Examples of the cerium compound that becomes ceria upon heating include cerium acetate, cerium carbonate, and cerium nitrate.

These cerium compounds decompose into ceria when the mixed powder is heated and fired.

Further, the particle size of the powder of ceria or the above-mentioned cerium compound is not particularly limited, but preferably has an average particle size of 0.01 to 2μ.

Next, when firing the mixed powder, a small amount of organic glue such as dextrin is added and mixed with the mixed powder, and the mixture is molded into a desired size using a tablet molding machine or the like.
This is fired in an electric furnace or the like.

The mixed powder is shaped into a desired shape such as granules, columns, or honeycombs.

In addition, the above-mentioned porous body containing ceria can be obtained by impregnating the surface of a porous body made only of alumina with a magnesia compound that becomes magnesia by heating, such as magnesium nitrate or magnesium chloride, and heating it at 1000 to 1600°C. ,
It can also be produced by once forming spinel on its surface, then impregnating it with a ceria compound, heating it, and supporting ceria.

Example 1 66% (weight ratio) of α-alumina powder with an average particle size of 0.37 μm, 24% of magnesia powder with an average particle size of 1 μm, and 10% ceria powder with an average particle size of 1 μm were mixed, and a small amount was added to this. of water was added, thoroughly mixed, and molded into spherical pellets having a diameter of about 3 mm using a marmerizer (tablet molding machine).

Next, the above pellets were heated and sintered at 1350°C for 10 hours in an electric furnace to form a porous body (carrier A) as a carrier.
l) was created.

The carrier has a composition of MgAl20483%, alumina 7%, ceria (Ce02) 10%, and has a pore volume of 0.35c.
m3/g, average pore diameter 0.3μ, and surface area 6m3/g.

Next, the above carrier was immersed in an aqueous solution of platinum nitrate or an aqueous solution of palladium nitrate, dried, and then heated at 600°C in air for 3 hours.
Platinum (Pt) catalyst (NoA1) shown in Table 1
), a palladium (Pd) catalyst (NoA2) was prepared.

In addition, a platinum-palladium (Pt-Pd) catalyst (NoA3) was prepared by once supporting platinum on the carrier in the same manner as above and then supporting palladium again.

Next, in order to evaluate the purification activity of these catalysts, the catalysts were filled in a quartz tube, heated and maintained at 500°C, and exhaust gas from an automobile internal combustion engine was poured into the tube at a space velocity of 3000°C.
It was introduced at 0/hour.

The above exhaust gas operates at a stoichiometric air-fuel ratio (A/F=1
455), and the air-fuel ratio was changed at intervals of 0.8 above and below it.

In addition, the average concentration of harmful components in the exhaust gas in the above operation is approximately 0.1 in terms of volume ratio of nitrogen oxides (NOx).
%, carbon monoxide (CO) 0.62%, hydrocarbon (HC)
0.05 hiro carbon dioxide (Co2) 12%, hydrogen (H2)
0.2%, oxygen (O2) 0.54%, water (H20) 13
%, balance nitrogen (N2).

The above-mentioned purification activity was evaluated by the purification rate of the above-mentioned harmful components.

The results are also listed in Table 1. Table 1 also shows, as a comparative example, using a conventional γ-alumina carrier for a catalyst carrier.
A carrier (NoC1) was prepared in which ceria was supported on this catalyst, and the purification rate of a catalyst (ASI, 82) in which platinum or palladium was supported on the carrier in the same manner as above is also shown.

This AC1 carrier was created by impregnating the above γ-alumina carrier with a 10% aqueous solution of cerium nitrate, drying it, and then heating it in air at 600°C for 3 hours, and ceria was present on its surface. .

In addition, the properties of this ACl carrier are that the pore volume is 0.5 cm3/
g, average pore diameter 0.02μ, and surface area 98m3/g.

Note that the γ-alumina carrier has a pore volume of 0.5 cm3.
/g, average pore diameter 0.02μ, surface area 100m2/g
Met.

As can be seen from Table 1, the catalyst according to the present invention has high purification activity.

In addition, a carrier (ACI) in which ceria is supported on a conventional carrier
It can be seen that the catalysts using (S1, S2) have considerably lower activity than the catalysts according to the invention.

Example 2 A mixed powder consisting of 74% alumina and 26% magnesia was sintered in the same manner as in Example 1 to obtain a porous body consisting of 92% alumina/magnesia spinel and 8% alumina. A carrier (No. 2) on which ceria was supported was prepared, and a catalyst was prepared in which platinum was supported on the carrier.

This carrier is a porous material on which spinel is formed, and ceria is further supported on the surface of the porous material, so that the spinel, alumina, and ceria are present on the surface of the carrier.

This carrier was created by immersing the porous body in a 10% aqueous solution of cerium nitrate, drying it, and then baking it in air at 600° C. for 3 hours.

The above-mentioned cerium nitrate turned into ceria by the above-mentioned heating and was supported on the surface of the carrier.

Further, this ceria was present in a proportion of about 2% by weight in the total amount of the carrier.

The properties of the above carrier are shown in Table 2.

Next, a platinum catalyst (catalyst NoA4) using this carrier was prepared, and the exhaust purification activity was measured in the same manner as in Example 1. The results are shown in Table 3.

As is known from the above, catalysts using a carrier in which ceria exists in the surface layer together with spinel and alumina also
It is found that it has excellent purifying activity.

Example 3 A catalyst was prepared using a carrier in which MgAl2O4 spinel was formed on the surface of a γ-alumina porous material and ceria was further supported on the spinel.

That is, first, the commercially available γ-alumina carrier for catalyst carrier shown in the comparative example of Example 1 was immersed in a 30% magnesium nitrate aqueous solution, dried, and then heated at 1000°C in air for 4 hours to coat the surface with MgAl2O4 spinel. A porous body was created.

Next, ceria was further supported on the surface of this porous body in the same manner as in Example 2 to produce a carrier (43) having the spinel and ceria on the surface.

Table 4 shows the properties of the carrier A3 above.

Next, using this carrier, platinum-supported catalysts shown in Table 5 were prepared in the same manner as in Example 1.

The exhaust purification activity of this catalyst was measured in the same manner as in Example 1, and the results are shown in the table.

As can be seen from Table 5, the catalyst in which a mixed layer of MgA12O4 spinel and ceria was formed on the surface also had high purification activity.

Example 4 The catalyst according to the present invention shown in Example 1 (A1, A2, A
Regarding 3), the CO and HC purification activities were measured in oxygen-rich conditions.

The purification activity measurement in this example is to test the performance as a so-called oxidation catalyst in an exhaust gas purification method.

This is because the measurements in Examples 1 to 3 are NOx
, CO, and HC simultaneously, which tests the performance of a so-called "three-way catalyst."

Therefore, in this example, the exhaust gas sent to the catalyst layer is the exhaust gas used in Examples 1 to 3, to which air is further added to contain a large amount of oxygen.

The composition of this gas is 0.9% CO, HCO. 045%
, CO2 12%, NOx 0. 0 9%, H20.
18%, O22%, H2O13%, balance nitrogen.

The preparation of the carrier, preparation of the catalyst, and measurement of purification activity were carried out in the same manner as in Example 1 except for the above.

As a result, for both catalysts, CO was 10
Even for 0% HC, the purification rate was almost 100%.

Comparative Experimental Example Apart from the comparative example shown in the above example, a carrier containing alumina/magnesia spinel but not ceria was prepared,
A catalyst was prepared in which the catalyst component was supported on the carrier, and its activity was measured.

"Part 1" α-alumina powder with an average particle size of 0.1μ and an average particle size of 0.
3μ of magnesia powder in the proportions shown in Table 6, and in the same manner as shown in Example 1, the composition shown in Table 6, the pore volume, average pore diameter, and A porous material having a surface area was prepared as a carrier.

Next, platinum, palladium, or both were supported on the above carrier in the same manner as shown in Example 1 to prepare comparative catalysts shown in Table 8.

Next, the purification activities of these catalysts were evaluated in the same manner as in Example 1, and the results are shown in Table 9.

As is known from Table 9 above and Table 1 of Example 1 above, the catalysts according to the present invention (NoA1 to A3: all have higher NOx purification activity than the comparative catalysts (NoS to S7). It turns out that it is an excellent catalyst.

"Part 2" A comparative catalyst was prepared using a porous body in which MgA12O4 spinel was formed on the surface of a γ-alumina carrier.

That is, the commercially available γ-alumina particles for a catalyst carrier (support NoC1) shown in Example 1 were immersed in a 30% magnesium nitrate aqueous solution, dried, and then heated at 1000°C in air for 4 hours to coat the surface with MgAl2O4 spinel. (AC 5 ) was prepared.

Table 10 shows the properties of the NoC5 carrier described above.

Next, using this carrier, platinum-supported catalysts shown in Table 11 were prepared in the same manner as in Example 1.

The exhaust purification activity of this catalyst (NoS8) was measured in the same manner as in Example 1, and the results are shown in the table.

As is known from Table 11 above and Table 5 of Example 3, the catalyst according to the present invention (NoA5) is different from the comparative catalyst (N
It can be seen that it has higher purifying activity for all NOx, CO, and HC than oS8).

Claims (1)

[Scope of Claims] 1 A porous body consisting of alumina-magnesia spinel, alumina, magnesia and ceria, in which alumina and magnesia each exist in a weight ratio of 25 or less with respect to the spinel, is used as a carrier, and platinum is added to the carrier. 1. An exhaust gas purifying catalyst for simultaneously purifying nitrogen oxides, carbon monoxide, and hydrocarbons in exhaust gas, characterized by supporting one or both of palladium and palladium. 2. The exhaust gas purifying catalyst according to claim 1, wherein the porous body has an average pore diameter of 0.01 to 2μ.