JP2008279319A - Exhaust gas purification catalyst and method for producing acidic oxide-supported alumina used therefor - Google Patents

Abstract

[PROBLEMS] To suppress the emission of H ₂ S while maintaining the same three-way activity as before.
The catalyst includes an oxide-supported alumina in which an acidic oxide composed of a metal oxide having a higher acidity than alumina is selectively supported on the alumina.
The basic point of alumina in the acidic oxide-supported alumina is neutralized by the acidic oxide. As a result, adsorption of SO ₃ or SO ₄ is suppressed and accumulation on the catalyst is suppressed, so that generation of H ₂ S in a stoichiometric or rich atmosphere is suppressed, and emission of H ₂ S can be suppressed. .
[Selection] Figure 1

Description

The present invention relates to an exhaust gas purifying catalyst (three-way catalyst) for purifying exhaust gas discharged from an internal combustion engine that is combustion-controlled in a stoichiometric atmosphere, and more specifically, an exhaust gas purifying catalyst capable of suppressing the discharge of hydrogen sulfide (H ₂ S). About.

HC in exhaust gas of an automobile, as a catalyst for purifying CO and NO _x, the three-way catalyst is widely used. This three-way catalyst is a porous oxide carrier such as alumina, ceria, zirconia, ceria-zirconia, etc. supported on platinum group noble metals such as Pt and Rh, and oxidizes and purifies HC and CO. , to purify by reduction of NO _x. Since these reactions proceed most efficiently in an atmosphere in which an oxidizing component and a reducing component are present in approximately equivalent amounts, in a vehicle equipped with a three-way catalyst, the vicinity of the stoichiometric air-fuel ratio (Stoichi) (A / F = 14.6 ± The air-fuel ratio is controlled so that it is burned at about 0.2).

However, the three-way catalyst has a problem that when the exhaust gas atmosphere moves toward the reduction side, sulfur oxides in the exhaust gas are reduced and discharged as H ₂ S. For example, alumina is an essential component of a three-way catalyst, but in an automobile equipped with a three-way catalyst using alumina, H ₂ S is generated when the exhaust gas atmosphere is rich (reducing atmosphere) such as during acceleration. There was a problem. The mechanism generated by H ₂ S is explained as follows.

SO ₂ in the exhaust gas is oxidized by the catalyst in a lean atmosphere to become SO ₃ or SO ₄ . SO ₃ or SO ₄ is adsorbed on the base point of alumina, and the adsorbed SO ₃ or SO ₄ is gradually concentrated on the alumina. Then, it is considered that SO ₃ or SO ₄ is reduced and H ₂ S is generated in a stoichiometric or rich atmosphere. H ₂ S is perceived by human olfaction even in a trace amount and gives discomfort, so it is necessary to suppress its discharge.

Therefore, it is conceivable to further use an oxide of Ni or Cu as a component of the three-way catalyst. Ni or Cu oxide suppresses the generation of H ₂ S because SO _{2 is changed} to SO ₃ or SO ₄ in an oxidizing atmosphere, and sulfur components are stored as sulfides such as Ni ₃ S ₂ in a reducing atmosphere. Can do.

For example, Japanese Examined Patent Publication No. 08-015554 describes an exhaust gas purifying catalyst in which a noble metal is supported on a carrier made of nickel-barium composite oxide, alumina, and ceria. In this carrier, alumina and ceria capture sulfur oxides as sulfates in a lean atmosphere, and H ₂ S is captured by nickel barium complex oxide in a reducing atmosphere. Therefore, generation of H ₂ S can be suppressed.

Japanese Patent Publication No. 2000-515419 or Japanese Patent No. 02598817 describes that the formation of H ₂ S is suppressed by using a carrier in which NiO ₂ , Fe ₂ O _3, or the like is mixed. Japanese Patent Application Laid-Open No. 07-194978 describes that the formation of H ₂ S is suppressed by using a support on which Ni and Ca are supported.

  However, since Ni or Cu is an environmentally hazardous substance, there is a current situation that its use is being limited for automobile exhaust gas purification catalysts. Further, when barium or the like is added to the three-way catalyst, the original purification performance may be deteriorated.

Further, JP-A-09-075739 and JP-A-11-005035 describe a catalyst in which a platinum group noble metal is supported on a porous oxide containing zinc oxide, tin oxide and the like in addition to alumina and zirconia. Yes. However, these catalysts are all used in exhaust gas containing oxygen in excess of the theoretical reaction amount, and it is unlikely to be applied to a three-way catalyst.
No. 08-015554 Special table 2000-515419 Patent No. 02598817 JP 07-194978 JP 09-075739 JP 11-005035 A

The present invention has been made in view of the above circumstances, and suppresses the emission of H ₂ S without using Ni or Cu, which are environmentally hazardous substances, while maintaining the ternary activity equivalent to the conventional one. It is a problem to be solved.

A feature of the exhaust gas purifying catalyst of the present invention that solves the above problems is an exhaust gas purifying catalyst that includes a support made of a porous oxide containing at least alumina, and a noble metal supported on the support,
The support includes an acidic oxide-supported alumina in which an acidic oxide composed of a metal oxide having a higher acidity than alumina is selectively supported on the alumina.

  Typical examples of the acidic oxide include oxides of at least one metal selected from W, Ti, Si, P, and Mo. It is desirable that the acidic oxide and alumina form a composite oxide.

  Further, in producing the exhaust gas purifying catalyst of the present invention, as a method for producing the acidic oxide-supported alumina in advance, a step of preparing a mixed solution in which the metal acid salt and the soluble aluminum salt constituting the acidic oxide are dissolved; A step of dropping the mixed solution into an alkaline aqueous solution capable of neutralizing the total amount of acid ions in the mixed solution to form a precipitate composed of an oxide precursor, and then a step of firing the precipitate. Is desirable.

According to the exhaust gas purifying catalyst of the present invention, the basic point of alumina in the acidic oxide-supported alumina is in a neutralized state by the acidic oxide. Therefore, it is considered that adsorption of SO ₃ or SO ₄ is suppressed and accumulation on the catalyst is suppressed. Accordingly, the generation of H ₂ S in a stoichiometric or rich atmosphere is suppressed, and the emission of H ₂ S can be suppressed. In addition, since it does not contain environmentally hazardous substances, there are no environmental problems.

And according to the acidic oxide carrying | support alumina manufactured by the manufacturing method of the acidic oxide carrying | support alumina of this invention, the particle size of acidic oxide is very small, and a surface area is large. Accordingly, H ₂ S emission can be effectively suppressed. In addition, since the specific surface area of alumina itself can be maintained high, the purification performance of HC, CO, and NO _x can be maintained high.

The exhaust gas purifying catalyst of the present invention includes a support made of a porous oxide containing at least alumina and a noble metal supported on the support, and the support contains at least acidic oxide-supported alumina. As alumina, γ-Al ₂ O ₃ , θ-Al ₂ O _{3 and the} like can be used. In addition to alumina, the support may contain other porous oxides such as ceria, zirconia, ceria-zirconia, and titania. In particular, it is desirable to contain ceria, ceria-zirconia and the like. Ceria or a complex oxide containing ceria has an oxygen absorption / release capability and can adjust atmospheric fluctuations, so that purification performance is improved. It is also possible to form the carrier only from the acidic oxide-supported alumina.

As the noble metal, one that oxidizes HC and CO by its catalytic action and one that reduces NO _x are used, and Pt, Rh, Pd and the like are typically exemplified. It is preferable to use together Pt excellent in oxidation activity and Rh excellent in reduction activity. The amount of noble metal supported may be the same as that of the conventional three-way catalyst, and varies depending on the metal species, but is preferably in the range of 0.1 to 10 g per liter of the catalyst.

  The greatest feature of the present invention lies in that the support includes acidic oxide-supported alumina in which an acidic oxide composed of a metal oxide having a higher acidity than alumina is selectively supported on alumina.

  Typical examples of the metal element constituting the metal oxide having higher acidity than alumina include W, Ti, Si, P, and Mo, and the acid oxide is selected from W, Ti, Si, P, and Mo. At least one metal oxide is preferred. Among these, W and Ti are excellent in the action of forming a composite oxide such as aluminum tungstate and aluminum titanate together with Al and eliminating the base point of alumina. Si also has an excellent effect of eliminating the basic point of alumina. Therefore, it is particularly preferable to use an oxide of at least one metal selected from W, Si and Ti.

The CO ₂ desorption amount described later can also be used as an index for defining the acidic oxide. The amount of CO ₂ desorbed refers to a catalyst in which 0.1 mol / L of metal oxide is supported on alumina and 1 g / L of Pt, and nitrogen gas containing 0.5% CO ₂ is circulated at 90 ° C for 10 minutes. Te of CO ₂ adsorbed is then amount of CO ₂ desorbed when heating at 40 ° C. / min up to 810 ° C. from 90 ° C. in a nitrogen stream, the acidic oxide, the CO ₂ desorption The amount can be defined as 1 mmol / L or less.

The amount of acidic oxide supported on the acidic oxide-supported alumina is preferably in the range of 0.01 to 0.3 mol per liter of catalyst. When the amount of acidic oxide supported is less than 0.01 mol / L, it becomes difficult to achieve the effect and H ₂ S is likely to be generated. On the other hand, if it exceeds 0.3 mol / L, the specific surface area of alumina may decrease and the purification performance of HC, CO, and NO _x may decrease.

  In order to produce acidic oxide-supported alumina, it can be produced by impregnating alumina with an aqueous solution of an acid salt of a metal element constituting the acidic oxide, and baking it to obtain an acidic oxide. Alternatively, a suspension is prepared by mixing alumina in an aqueous solution of an acid salt of a metal element constituting an acidic oxide, and while stirring it, an aqueous alkaline solution such as aqueous ammonia is added to convert the oxide precursor into an alumina particle. It can also be deposited on the surface and fired to carry an acidic oxide.

However, acidic oxides supported by these methods have a small surface area due to their relatively large particle size. Therefore, unless the amount of acidic oxide supported is relatively large, the H ₂ S emission suppressing effect is small. In addition, the acidic oxide may block the pores of the alumina, resulting in a problem that the specific surface area of the alumina is reduced.

  Therefore, in the method for producing acidic oxide-supported alumina according to the present invention, a mixed solution in which a metal acid salt and a soluble aluminum salt constituting the acidic oxide are dissolved can be prepared, and the total amount of acid ions in the mixed solution can be neutralized. A mixed solution is dropped into an alkaline aqueous solution to form a precipitate made of an oxide precursor, and then the precipitate is fired. According to this manufacturing method, an alumina precursor and an acidic oxide precursor are generated almost simultaneously, and are fired to form a composite oxide in which acidic oxide and alumina are mixed at a molecular level. In the composite oxide, since the number of metal atoms constituting the acidic oxide is generally smaller than the number of Al atoms, there is no problem that the effect of the high specific surface area by alumina is reduced.

That is, in the obtained acidic oxide-supported alumina, the acidic oxide is dispersed at the atomic level in the alumina, and the particle size of the acidic oxide is extremely small and the surface area is large. Therefore, adsorption of SO _x onto alumina is suppressed, and H ₂ S emission can be effectively suppressed. Moreover, since the specific surface area of alumina can be maintained high, the purification performance of HC, CO, and NO _x can be maintained high.

  In addition, since the particle size of the acidic oxide is extremely small, when the acidic oxide-supported alumina coexists with ceria, ceria-zirconia, etc., mutual reaction is unlikely to occur and the oxygen storage / release capability is prevented from being impaired. be able to.

  The exhaust gas purifying catalyst of the present invention is used as a pellet shape, a honeycomb shape, a foam shape or the like. For example, in the case of a honeycomb-shaped catalyst, a slurry containing a porous oxide such as alumina or ceria-zirconia and an acidic oxide-supported alumina is washed on a honeycomb substrate made of cordierite or metal, and then fired. Then, a coating layer may be formed, and a noble metal may be supported on the coating layer by an adsorption supporting method or a water absorbing supporting method.

  Moreover, the catalyst of this invention can also be prepared by preparing the catalyst powder which carry | supported the noble metal beforehand to the porous oxide, mixing acidic oxide carrying | support alumina powder in it, and forming a coating layer from it.

When the support contains a porous oxide having oxygen storage ability such as ceria or ceria-based composite oxide, it is desirable that the porous oxide having oxygen storage ability does not carry an acidic oxide. For example ceria acidic oxide was supported tend to its capability of adsorbing and releasing oxygen is reduced, HC and atmosphere change mitigation action is reduced, CO, purifying performance of the NO _x is lowered.

  Hereinafter, the present invention will be specifically described with reference to test examples, examples and comparative examples.

(Test example)
First, 100 parts by weight of θ-Al ₂ O ₃ powder was prepared, impregnated with a predetermined amount of ammonium tungstate aqueous solution, and baked at 500 ° C. for 2 hours to support W / Al ₂ O ₃ (acidic oxide). (Supported alumina) powder was prepared. The supported amount of W is 0.1 mol with respect to 100 parts by weight of θ-Al ₂ O ₃ powder.

Next, a total amount of W / Al ₂ O ₃ powder, alumina hydrate as a binder (3 parts by weight as alumina), and a predetermined amount of pure water were mixed and milled to prepare a slurry.

  A honeycomb substrate having a straight flow structure (35 cc, diameter 30 mm, length 50 mm) was prepared, the slurry was washcoated, dried at 120 ° C., and fired at 450 ° C. for 2 hours to form a coat layer. The coating layer was formed in an amount of 100 g per liter of catalyst.

  Further, a honeycomb substrate having a coating layer was impregnated with a predetermined amount of a dinitrodiammine platinum solution having a predetermined concentration and supported for water absorption, dried at 120 ° C., and fired at 450 ° C. for 2 hours to support Pt. The amount of Pt supported is 1 g per liter of catalyst.

  Alumina powder carrying Ti, P, Mo, Fe, Mg and Ba oxides in the same manner as described above using water-soluble salts of Ti, P, Mo, Fe, Mg and Ba instead of ammonium tungstate The catalyst was prepared in the same manner.

Each catalyst is placed in an evaluation device, and nitrogen gas containing 0.5% CO ₂ is allowed to flow at 90 ° C for 10 minutes to adsorb CO ₂ and then in a nitrogen stream from 90 ° C to 810 ° C at 40 ° C / min. The amount of CO ₂ desorbed when the temperature was increased at a rate was measured. The results are shown in FIG.

FIG. 5 shows that the catalyst supporting W, Ti, P, and Mo oxides has significantly less CO ₂ desorption than the catalyst supporting Fe, Mg, and Ba oxides, respectively. That is, the oxides of W, Ti, P, and Mo are more likely to desorb CO ₂ than the oxides of Fe, Mg, and Ba, that is, the acidity is high. The threshold value is the amount of CO ₂ desorption. Is 1 mmol / L or less.

Example 1
FIG. 1 shows the three-way catalyst of this example. This catalyst comprises a honeycomb substrate 1 having a straight flow structure, a lower coat layer 2 formed on the cell wall surface of the honeycomb substrate 1, and an upper coat layer 3 formed on the surface of the lower coat layer 2. The lower coat layer 2 includes ceria-zirconia solid solution powder 20 and acidic oxide-supported alumina 21 in which tungsten oxide (×) is supported on θ-Al ₂ O ₃ powder, and Pt (●) as a whole. Are uniformly supported. Further, the upper coat layer 3 uses acidic oxide-supported alumina 21 in which tungsten oxide (x) is supported on θ-Al ₂ O ₃ powder as a carrier, and Rh (（) is uniformly supported on the whole.

First, θ-Al ₂ O ₃ powder is prepared, impregnated with a predetermined amount of ammonium tungstate aqueous solution, and calcined at 500 ° C. for 2 hours to support tungsten oxide and W / Al ₂ O ₃ (acidic oxide supporting alumina) A powder was prepared.

60 parts by weight of ceria-zirconia solid solution powder having a specific surface area of 3 m ² / g, 37 parts by weight of W / Al ₂ O ₃ powder, and alumina hydrate (3 parts by weight as alumina) as a binder in a predetermined amount It was mixed with pure water and milled to prepare a slurry.

  Next, a cordierite-made honeycomb substrate (volume: 0.9 L, cell density: 400 cpsi) was prepared, and the slurry was wash-coated, dried at 120 ° C. and fired at 450 ° C. for 2 hours to form an undercoat layer. The lower coat layer was formed in an amount of 100 g per liter of the honeycomb substrate.

  A honeycomb base material having the above lower coat layer was impregnated with a predetermined amount of a dinitrodiammine platinum solution having a predetermined concentration and supported for water absorption, dried at 120 ° C., and fired at 450 ° C. for 2 hours to support Pt.

Subsequently, 57 parts by weight of Rh / W / Al ₂ O ₃ catalyst powder in which 0.35% by weight of Rh was previously supported on the W / Al ₂ O ₃ powder prepared in the same manner as described above, and alumina hydrate ( 3 parts by weight as alumina) was mixed with a predetermined amount of pure water and milled to prepare a slurry. Then, the surface of the lower coat layer carrying Pt was wash coated, dried at 120 ° C., and then baked at 450 ° C. for 2 hours to form an upper coat layer. The upper coat layer was formed in an amount of 60 g per liter of the honeycomb substrate.

  The supported amount per liter of the catalyst is 0.078 mol of W, 1.0 g of Pt, and 0.2 g of Rh.

(Comparative Example 1)
A catalyst having upper and lower coat layers was prepared in the same manner as in Example 1, except that θ-Al ₂ O ₃ powder not supporting tungsten oxide was used instead of W / Al ₂ O ₃ powder.

(Example 2)
A catalyst having upper and lower coat layers was prepared in the same manner as in Example 1 except that W / Al ₂ O ₃ powder having a different loading of tungsten oxide was used. The supported amount per liter of the catalyst is 0.039 mol of W, 1.0 g of Pt, and 0.2 g of Rh.

(Example 3)
A catalyst having upper and lower coat layers was prepared in the same manner as in Example 1 except that W / Al ₂ O ₃ powder having a different loading of tungsten oxide was used. The supported amount per liter of the catalyst is 0.019 mol of W, 1.0 g of Pt, and 0.2 g of Rh.

Example 4
Prepare the same ceria-zirconia solid solution powder as in Example 1, impregnate a predetermined amount of ammonium tungstate aqueous solution, and calcinate at 500 ° C. for 2 hours to prepare W / CeO ₂ —ZrO ₂ powder carrying tungsten oxide. did. A catalyst having upper and lower coat layers was prepared in the same manner as in Example 1 except that W / CeO ₂ —ZrO ₂ powder was used instead of ceria-zirconia solid solution powder. The supported amount per liter of catalyst is 0.086 mol of W, 1.0 g of Pt, and 0.2 g of Rh.

(Example 5)
Prepare the same ceria-zirconia solid solution powder as in Example 1, impregnate with a predetermined amount of ammonium titanate aqueous solution and calcinate for 2 hours at 500 ° C. to prepare Ti / CeO ₂ —ZrO ₂ powder supporting titanium oxide did. A catalyst having upper and lower coat layers was prepared in the same manner as in Example 1 except that Ti / CeO ₂ —ZrO ₂ powder was used instead of ceria-zirconia solid solution powder. The supported amount per liter of the catalyst is 0.078 mol of W, 0.023 mol of Ti, 1.0 g of Pt, and 0.2 g of Rh.

(Example 6)
A catalyst having upper and lower coat layers was prepared in the same manner as in Example 1 except that the same ceria-zirconia solid solution powder as in Example 1 was used except that the specific surface area was 0.8 m ² / g. The supported amount per liter of the catalyst is 0.078 mol of W, 1.0 g of Pt, and 0.2 g of Rh.

(Example 7)
95 parts by weight of γ-Al ₂ O ₃ powder and 100 parts by weight of Al ₂ O ₃ —CeO ₂ —ZrO ₂ composite oxide powder (weight ratio Al ₂ O ₃ : CeO ₂ : ZrO ₂ = 26 / 39.5 / 34.5) Then, alumina hydrate as a binder (3 parts by weight as alumina) was mixed with a predetermined amount of pure water and milled to prepare a slurry.

  Next, a honeycomb base material similar to that in Example 1 was prepared, this slurry was wash coated, dried at 120 ° C. and then fired at 450 ° C. for 2 hours to form a coat layer. The coating layer was formed in an amount of 198 g per liter of honeycomb substrate volume.

  Next, using a dinitrodiammine platinum solution and an aqueous rhodium nitrate solution, Pt and Rh were supported on the coating layer, respectively, and an tungsten tungstate aqueous solution was further used to support tungsten oxide. The supported amount per liter of the catalyst is 0.18 mol of W, 1.0 g of Pt, and 0.2 g of Rh.

(Example 8)
A predetermined amount of γ-Al ₂ O ₃ powder was impregnated with a predetermined amount of an aqueous solution of ammonium tungstate and fired at 500 ° C. for 2 hours to prepare W / Al ₂ O ₃ powder supporting tungsten oxide. 95 parts by weight of this W / Al ₂ O ₃ powder, 100 parts by weight of the same Al ₂ O ₃ —CeO ₂ —ZrO ₂ composite oxide powder as in Example 7, and alumina hydrate (3 wt. Part) was mixed with a predetermined amount of pure water and milled to prepare a slurry.

  Next, a honeycomb base material similar to that in Example 1 was prepared, this slurry was wash coated, dried at 120 ° C. and then fired at 450 ° C. for 2 hours to form a coat layer. The coating layer was formed in an amount of 160 g per liter of the honeycomb substrate.

  Next, using a dinitrodiammine platinum solution and an aqueous rhodium nitrate solution, Pt and Rh were supported on the coating layer, respectively. The supported amount per liter of the catalyst is 0.18 mol of W, 1.0 g of Pt, and 0.2 g of Rh.

Example 9
FIG. 2 shows a method for producing the acidic oxide-carrying alumina of this example. 95 g of aluminum nitrate corresponding to γ-Al ₂ O ₃ was dissolved in pure water, 0.18 mol of ammonium tungstate was further added, and the pH was adjusted to prepare a mixed aqueous solution. While stirring aqueous ammonia capable of neutralizing the total amount of acid ions in the mixed aqueous solution, the mixed aqueous solution was added dropwise to produce a precipitate. Most of the precipitate is considered to be hydroxides of W and Al. The precipitate was filtered and water was removed under a reduced pressure and constant temperature environment, followed by firing at 500 ° C. for 5 hours to prepare acidic oxide-supported alumina powder.

Next, 100 g of the same Al ₂ O ₃ —CeO ₂ —ZrO ₂ composite oxide powder as in Example 7 and alumina hydrate (3 g as alumina) as the binder were used with respect to the total amount of the acidic oxide-supported alumina powder obtained. ) And pure water were added to prepare a slurry, and a coating layer was formed in the same manner as in Example 7. Next, using a dinitrodiammine platinum solution and an aqueous rhodium nitrate solution, Pt and Rh were supported on the coating layer, respectively. The supported amount per liter of the catalyst is 0.18 mol of W, 1.0 g of Pt, and 0.2 g of Rh.

(Comparative Example 2)
Example 7 is the same as Example 7 except that no tungsten oxide was supported.

<Test and evaluation>
Table 1 summarizes the catalyst compositions of the Examples and Comparative Examples.

(H ₂ S inhibition test)
LA # 4 mode (Bagt1-3) using gasoline fuel with a sulfur concentration of 300ppm on the underfloor catalyst of a vehicle equipped with an inline 4-cylinder 2.4L engine and each of the examples and comparative examples. Ran. The accumulated amount of H ₂ S discharged during purging after running was evaluated. The ratio of the catalyst of Comparative Example 2 to the accumulated amount of H ₂ S was calculated, and the results are shown in FIG.

From FIG. 3, it can be seen that the catalyst of each Example has a smaller amount of H ₂ S desorption than the catalysts of Comparative Examples 1 and ₂ . Further, it can be seen from the comparison of Examples 1 to 3 that the amount of desorbed H ₂ S decreases as the amount of acidic oxide-supported alumina increases. That is, it is clear that the emission of H ₂ S can be greatly suppressed by including acidic oxide-supported alumina.

The catalyst of Example 9 has a smaller amount of H ₂ S desorption than the catalysts of Examples 7 and 8 although the amount of acidic oxide-supported alumina is the same as that of Examples 7 and 8. That is, the catalyst using the acidic oxide-supported alumina produced by the production method of the present invention has a H ₂ S desorption amount as compared with the catalyst using the acidic oxide-supported alumina produced by a general impregnation support method. It is clear that can be further reduced.

(Oxygen storage capacity test)
The catalyst of Example 7 and Comparative Example 2 were respectively mounted on the exhaust system of a V-type 8-cylinder 4.3L engine, and the inlet gas temperature was 850 ° C, and A / F = 15 and A / F = 14 were oscillated at 1 Hz A 50-hour durability test was conducted.

The catalyst of Example 7 after the endurance test and the catalyst of Comparative Example 2 were mounted on the rear stage of the inline 4-cylinder 2.4L engine, and burned at the stoichiometric air-fuel ratio until the temperature of the gas with catalyst reached 500 ° C. Thereafter, the A / F was switched at the inversion timing of the sub O ₂ sensor so as to sweep several cycles from 14.1 to 15.1 and then from 15.1 to 14.1. Based on the following equation, the catalyst inflow O ₂ amount was calculated, and the oxygen storage amount was measured from the difference from the oxygen amount discharged from the catalyst. The results are shown in FIG.

Catalyst inflow O ₂ amount = O ₂ mass ratio × ΔA / F × fuel injection amount From FIG. 4, the catalyst of Example 7 has a smaller oxygen storage amount than the catalyst of Comparative Example 2. This is apparent that in Al ₂ O ₃ -CeO ₂ -ZrO ₂ composite oxide powder due to the fact that the carrying tungsten oxide. Therefore, it is desirable that the porous oxide having oxygen storage capacity does not carry an acidic oxide.

  In addition, the ternary activity of the catalyst of each Example and each comparative example was almost equivalent, and had sufficient activity.

(Example 10)
First, pure water is heated to 70 to 75 ° C., citric acid is added, and the mixture is stirred at 70 to 75 ° C. A 1/4 molar amount of titanium isopropoxide is slowly added dropwise to citric acid, and the mixture is stirred and held at 80 ° C. for 6 hours or more. This is cooled to room temperature to obtain a solution of Ti citrate complex.

Next, a predetermined amount of γ-Al ₂ O ₃ powder commensurate with the Ti concentration in the Ti citrate complex solution is mixed, dried after stirring for 1 hour, and fired at 500 ° C. for 1 hour to support Ti / Al carrying Ti. ₂ O ₃ powder was prepared.

95 parts by weight of this Ti / Al ₂ O ₃ powder and Al ₂ O ₃ —CeO ₂ —ZrO ₂ composite oxide powder (weight ratio Al ₂ O ₃ : CeO ₂ : ZrO ₂ = 26 / 39.5 / 34.5) 100 parts by weight Alumina hydrate (3 parts by weight as alumina) as a binder was mixed with a predetermined amount of pure water and milled to prepare a slurry.

  A honeycomb substrate similar to that in Example 1 was prepared, and this slurry was wash-coated, dried at 120 ° C. and then fired at 450 ° C. for 2 hours to form a coat layer. The coating layer was formed in an amount of 198 g per liter of honeycomb substrate volume. Next, using a dinitrodiammine platinum solution and an aqueous rhodium nitrate solution, Pt and Rh were supported on the coating layer, respectively.

  The supported amount per liter of catalyst is 0.18 mol of Ti, 1.0 g of Pt, and 0.2 g of Rh.

(Comparative Example 3)
Example 10 is the same as Example 10 except that γ-Al ₂ O ₃ powder is used instead of Ti / Al ₂ O ₃ powder.

(Example 11)
First, θ-Al ₂ O ₃ powder was prepared, and Ti was supported in the same manner as in Example 10 to prepare Ti / Al ₂ O ₃ powder.

60 parts by weight of ceria-zirconia solid solution powder (weight ratio CeO ₂ : ZrO ₂ = 53/47) having a specific surface area of 3 m ² / g, 94 parts by weight of Ti / Al ₂ O ₃ powder, and alumina hydrate as a binder (3 parts by weight as alumina) was mixed with a predetermined amount of pure water and milled to prepare a slurry.

  Next, a cordierite-made honeycomb substrate (volume: 0.9 L, cell density: 400 cpsi) was prepared, and the slurry was wash-coated, dried at 120 ° C. and fired at 450 ° C. for 2 hours to form an undercoat layer. The lower coat layer was formed in an amount of 100 g per liter of the honeycomb substrate.

  A honeycomb base material having the above lower coat layer was impregnated with a predetermined amount of a dinitrodiammine platinum solution having a predetermined concentration and supported for water absorption, dried at 120 ° C., and fired at 450 ° C. for 2 hours to support Pt.

Subsequently, 57 parts by weight of Rh / Ti / Al ₂ O ₃ catalyst powder in which 0.35% by weight of Rh was previously supported on Ti / Al ₂ O ₃ powder prepared in the same manner as above, and alumina hydrate as a binder ( 3 parts by weight as alumina) was mixed with a predetermined amount of pure water and milled to prepare a slurry. Then, the surface of the lower coat layer carrying Pt was wash coated, dried at 120 ° C., and then baked at 450 ° C. for 2 hours to form an upper coat layer. The upper coat layer was formed in an amount of 60 g per liter of the honeycomb substrate.

  The supported amount per liter of catalyst is 0.078 mol of Ti, 1.0 g of Pt, and 0.2 g of Rh.

(Comparative Example 4)
A catalyst having upper and lower coat layers was prepared in the same manner as in Example 11 except that θ-Al ₂ O ₃ powder not supporting Ti was used instead of Ti / Al ₂ O ₃ powder.

(Example 12)
First, Al ₂ O ₃ powder was fired at 1200 ° C. for 5 hours to prepare a low specific surface area alumina powder having a specific surface area reduced from 100 m ² / g to 40 m ² / g. A Ti / Al ₂ O ₃ powder supporting Ti was prepared in the same manner as in Example 10 using this low specific surface area alumina powder instead of the γ-Al ₂ O ₃ powder. A catalyst having upper and lower coat layers was prepared in the same manner as in Example 11 except that this Ti / Al ₂ O ₃ powder was used.

(Comparative Example 5)
A catalyst having upper and lower coat layers was prepared in the same manner as in Example 12 except that low specific surface area alumina powder was used instead of Ti / Al ₂ O ₃ powder.

(Example 13)
Using the same low specific surface area alumina powder as in Example 12, Ti / Al ₂ O ₃ powder supporting Ti was prepared in the same manner as in Example 10. 94 parts by weight of this Ti / Al ₂ O ₃ powder, 60 parts by weight of ceria-zirconia solid solution powder (weight ratio CeO ₂ : ZrO ₂ = 53/47) having a specific surface area of 3 m ² / g, and hydration of alumina as a binder A product (3 parts by weight as alumina) was mixed with a predetermined amount of pure water and milled to prepare a slurry.

  Next, prepare a cordierite honeycomb substrate (volume: 0.9 L, cell density: 400 cpsi), wash coat the slurry on the upstream half, dry at 120 ° C., and fire at 450 ° C. for 2 hours to upstream coat layer Formed. The upstream coat layer was formed in an amount of 100 g per liter of honeycomb substrate volume. The upstream coat layer was impregnated with a predetermined amount of a dinitrodiammine platinum solution having a predetermined concentration and supported for water absorption, dried at 120 ° C., and baked at 450 ° C. for 2 hours to support Pt.

Subsequently, 57 parts by weight of Rh / Ti / Al ₂ O ₃ catalyst powder in which 0.35% by weight of Rh was previously supported on Ti / Al ₂ O ₃ powder prepared in the same manner as above, and alumina hydrate as a binder ( 3 parts by weight as alumina) was mixed with a predetermined amount of pure water and milled to prepare a slurry. Then, the downstream half of the upstream coat layer was wash-coated, dried at 120 ° C., and then fired at 450 ° C. for 2 hours to form a downstream coat layer. The downstream coat layer was formed in an amount of 60 g per liter of honeycomb substrate volume.

  The supported amount per liter of catalyst is 0.078 mol of Ti, 1.0 g of Pt, and 0.2 g of Rh.

(Comparative Example 6)
A catalyst having an upstream coat layer and a downstream coat layer was prepared in the same manner as in Example 13 except that a low specific surface area alumina powder was used instead of the Ti / Al ₂ O ₃ powder.

<Test and evaluation>
LA # 4 mode (Bagt1-3) using gasoline fuel with a sulfur concentration of 300ppm on the underfloor catalyst of a vehicle equipped with an inline 4-cylinder 2.4L engine and each of the examples and comparative examples. Ran. The accumulated amount of H ₂ S discharged during purging after running was evaluated. The ratio of the catalyst of Comparative Example 3 to the accumulated amount of H ₂ S was calculated, and the results are shown in FIG.

From FIG. 6, it can be seen that the catalyst of each example has a smaller amount of H ₂ S desorption than the catalyst of the comparative example. That is, it is clear that the emission of H ₂ S can be greatly suppressed by including Ti-supported alumina.

(Example 14)
Example 11 is the same as Example 11 except that a Ti / Al ₂ O ₃ powder prepared using an ethanol solution of titanium (IV) tetrabutoxy monomer is used.

(Example 15)
Example 11 is the same as Example 11 except that Ti / Al ₂ O ₃ powder prepared using an aqueous titanium chloride solution is used.

<Test and evaluation>
Each catalyst of Example 11, Example 14, Example 15, Comparative Example 3 and Comparative Example 4 was cut to a test piece size, and model gas containing SO ₂ was circulated to cause sulfur poisoning. Thereafter, the accumulated amount of H ₂ S discharged when the model gas shown in Table 2 was circulated at 30 ° C. and a flow rate of 25 L / min was evaluated. The ratio of the catalyst of Comparative Example 3 to the accumulated amount of H ₂ S was calculated, and the results are shown in FIG.

Also from this test, it can be seen that the catalyst of each example has a smaller amount of H ₂ S desorption than the catalyst of the comparative example. That is, it is clear that the emission of H ₂ S can be greatly suppressed by including Ti-supported alumina. It can also be seen that the amount of H ₂ S desorption varies depending on the Ti loading method.

(Example 16)
An alcohol solution of tetramethoxysilane was added to γ-Al ₂ O ₃ powder and baked at 500 ° C. for 1 hour to prepare Si / Al ₂ O ₃ powder supporting Si.

95 parts by weight of this Si / Al ₂ O ₃ powder and Al ₂ O ₃ —CeO ₂ —ZrO ₂ composite oxide powder (weight ratio Al ₂ O ₃ : CeO ₂ : ZrO ₂ = 26 / 39.5 / 34.5) 100 parts by weight Alumina hydrate (3 parts by weight as alumina) as a binder was mixed with a predetermined amount of pure water and milled to prepare a slurry.

  A honeycomb substrate similar to that in Example 1 was prepared, and this slurry was wash-coated, dried at 120 ° C. and then fired at 450 ° C. for 2 hours to form a coat layer. The coating layer was formed in an amount of 198 g per liter of honeycomb substrate volume. Next, using a dinitrodiammine platinum solution and an aqueous rhodium nitrate solution, Pt and Rh were supported on the coating layer, respectively.

  The supported amount per liter of the catalyst is 0.1 mol of Si, 1.0 g of Pt, and 0.2 g of Rh.

(Comparative Example 7)
Example 16 is the same as Example 16 except that γ-Al ₂ O ₃ powder was used instead of the Si / Al ₂ O ₃ powder. This catalyst is the same as the catalyst of Comparative Example 3.

(Example 17)
First, θ-Al ₂ O ₃ powder was prepared, and Si was supported in the same manner as in Example 16 to prepare Si / Al ₂ O ₃ powder.

60 parts by weight of ceria-zirconia solid solution powder (weight ratio CeO ₂ : ZrO ₂ = 53/47) having a specific surface area of 3 m ² / g, 94 parts by weight of Si / Al ₂ O ₃ powder, and alumina hydrate as a binder (3 parts by weight as alumina) was mixed with a predetermined amount of pure water and milled to prepare a slurry.

  Next, a cordierite-made honeycomb substrate (volume: 0.9 L, cell density: 400 cpsi) was prepared, and the slurry was wash-coated, dried at 120 ° C. and fired at 450 ° C. for 2 hours to form an undercoat layer. The lower coat layer was formed in an amount of 100 g per liter of the honeycomb substrate.

  A honeycomb base material having the above lower coat layer was impregnated with a predetermined amount of a dinitrodiammine platinum solution having a predetermined concentration and supported for water absorption, dried at 120 ° C., and fired at 450 ° C. for 2 hours to support Pt.

Subsequently, 57 parts by weight of Rh / Si / Al ₂ O ₃ catalyst powder in which 0.35% by weight of Rh was previously supported on the Si / Al ₂ O ₃ powder prepared in the same manner as described above, and alumina hydrate as a binder ( 3 parts by weight as alumina) was mixed with a predetermined amount of pure water and milled to prepare a slurry. Then, the surface of the lower coat layer carrying Pt was wash coated, dried at 120 ° C., and then baked at 450 ° C. for 2 hours to form an upper coat layer. The upper coat layer was formed in an amount of 60 g per liter of the honeycomb substrate.

  The supported amount per liter of the catalyst is 0.1 mol of Si, 1.0 g of Pt, and 0.2 g of Rh.

(Comparative Example 8)
A catalyst having upper and lower coat layers was prepared in the same manner as in Example 17 except that θ-Al ₂ O ₃ powder not supporting Si was used instead of Si / Al ₂ O ₃ powder.

<Test and evaluation>
LA # 4 mode (Bagt1-3) using gasoline fuel with a sulfur concentration of 300ppm on the underfloor catalyst of a vehicle equipped with an inline 4-cylinder 2.4L engine and each of the examples and comparative examples. Ran. The accumulated amount of H ₂ S discharged during purging after running was evaluated. The ratio of the catalyst of Comparative Example 7 to the accumulated amount of H ₂ S was calculated, and the result is shown in FIG.

FIG. 8 shows that the catalyst of each Example has a smaller amount of H ₂ S desorption than the catalyst of the comparative example. That is, it is clear that the discharge of H ₂ S can be greatly suppressed by including Si-supported alumina.

(Example 18)
Example 16 is the same as Example 16 except that Si / Al ₂ O ₃ powders prepared using alcohol solutions having different tetramethoxysilane concentrations are used. The supported amount per liter of catalyst is 0.02 mol of Si, 1.0 g of Pt, and 0.2 g of Rh.

(Example 19)
Example 16 is the same as Example 16 except that Si / Al ₂ O ₃ powders prepared using alcohol solutions having different tetramethoxysilane concentrations are used. The supported amount per liter of catalyst is 0.5 mol of Si, 1.0 g of Pt, and 0.2 g of Rh.

<Test and evaluation>
Each catalyst of Example 16, Example 18, Example 19, and Comparative Example 7 was cut out to a test piece size, and sulfur gas was poisoned by circulating a model gas containing SO ₂ . Thereafter, the accumulated amount of H ₂ S discharged when the model gas shown in Table 2 was circulated at 30 ° C. and a flow rate of 25 L / min was evaluated. The ratio of the catalyst of Comparative Example 7 to the cumulative amount of H ₂ S was calculated, and the results are shown in FIG.

Also from this test, it can be seen that the catalyst of each example has a smaller amount of H ₂ S desorption than the catalyst of the comparative example. That is, it is clear that the discharge of H ₂ S can be greatly suppressed by including Si-supported alumina. It can also be seen that the greater the amount of Si supported, the greater the amount of H ₂ S desorption.

  The exhaust gas-purifying catalyst of the present invention is useful as a three-way catalyst for automobiles, but is not limited to automobiles as long as it is an internal combustion engine that is combustion controlled in the vicinity of a stoichiometric atmosphere.

It is typical explanatory drawing which shows the structure of the catalyst for exhaust gas purification of this invention. It is explanatory drawing of the manufacturing method of the acidic oxide carrying | support alumina of this invention. The H ₂ H ₂ S desorption amount of Comparative Example 2 the desorption amount of S is a graph showing by the relative ratio of 100. It is a graph which shows oxygen storage amount. 3 is a graph showing the amount of CO ₂ desorbed by a catalyst using supported alumina supporting each metal oxide as a support. The H ₂ H ₂ S desorption amount of Comparative Example 3 the desorption amount of S is a graph showing by the relative ratio of 100. The H ₂ H ₂ S desorption amount of Comparative Example 3 the desorption amount of S is a graph showing by the relative ratio of 100. The H ₂ S desorption amount in Comparative Example 7 the amount of desorbed H ₂ S is a graph showing by the relative ratio of 100. The H ₂ S desorption amount in Comparative Example 7 the amount of desorbed H ₂ S is a graph showing by the relative ratio of 100.

Explanation of sign

1: Honeycomb base material 2: Lower coat layer 3: Upper coat layer
20: Ceria-zirconia solid solution powder
21: Acid oxide-supported alumina powder

Claims (6)

An exhaust gas purifying catalyst comprising a support made of a porous oxide containing at least alumina, and a noble metal supported on the support,
An exhaust gas purifying catalyst characterized in that the carrier contains acidic oxide-supported alumina in which an acidic oxide made of a metal oxide having a higher acidity than alumina is selectively supported on alumina.   The exhaust gas-purifying catalyst according to claim 1, wherein the acidic oxide is an oxide of at least one metal selected from W, Ti, Si, P, and Mo.   The exhaust gas purifying catalyst according to claim 1 or 2, wherein the acidic oxide and alumina form a composite oxide.   The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the amount of the acidic oxide supported is 0.01 to 0.3 mol per liter of the catalyst.   2. The exhaust gas purifying catalyst according to claim 1, wherein, when the carrier includes a porous oxide having an oxygen storage capacity, the acidic oxide is not supported on the porous oxide having an oxygen storage capacity. In producing the exhaust gas purifying catalyst according to any one of claims 1 to 5, a method of producing the acidic oxide-supported alumina in advance,
Preparing a mixed solution in which a metal acid salt and a soluble aluminum salt constituting the acidic oxide are dissolved;
Dropping the mixed solution into an aqueous alkaline solution capable of neutralizing the total amount of acid ions in the mixed solution to produce a precipitate composed of an oxide precursor;
And then baking the precipitate. The method for producing acidic oxide-supported alumina.

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