JP2001137706A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the heat resistance of an NOx occluding and reducing type catalyst for cleaning exhaust gas containing TiO2 in its carrier and to still more enhance the NOx cleaning efficiency thereof after high temperation endurance. SOLUTION: In an NOx occluding and reducing type catalyst for cleaning exhaust gas consisting of a carrier comprising porous oxide containing at least TiO2 and the noble metal and NOx occluding material carried on the carrier, vanadium oxide (V2O5) is further supported on the carrier. Since the particle growth of both of TiO2 and the noble metal is suppressed by the presence of V2O5 though a reason is obscure, the NOx cleaning efficiency of the catalyst after endurance is enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to a catalyst for purifying exhaust gas for purifying exhaust gas of an automobile, and more particularly to a catalyst for purifying an exhaust gas of the NO _x storage reduction.

[0002]

2. Description of the Related Art In recent years, global warming due to carbon dioxide has become a problem, and reducing carbon dioxide emissions has become an issue. In automobiles, reduction of the amount of carbon dioxide in exhaust gas has become an issue, and lean burn engines have been developed in which fuel is burned lean in an oxygen-excess atmosphere. According to this lean burn engine, the amount of fuel used is reduced, so that the amount of carbon dioxide emission can be suppressed.

[0003] When purifying harmful components in the exhaust gas from lean-burn engines, the reduction purification is an oxygen-rich atmosphere because NO _x becomes difficult. Therefore,
No. 317652, etc., include alkali metals with precious metals,
A NO _x storage reduction catalyst supporting a NO _x storage material selected from alkaline earth metals and rare earth elements is disclosed. Using this NO _x storage-and-reduction type catalyst, by controlling the mixture composition as a stoichiometric-rich atmosphere in a pulsed manner during the lean atmosphere, to proceed efficiently and reduction of HC and CO oxidation and NO _x And high purification performance can be obtained.

[0004] That NO _x becomes oxidized is NO in the exhaust gas in a lean atmosphere, because it is occluded in _the NO _x storage material NO _x
Emission is suppressed. When the the stoichiometric-rich atmosphere, NO _x from _the NO _x storage material is released, it is to be reduced by reaction with reducing components such as HC present in the exhaust gas, NO _x emissions can be suppressed. Therefore it is possible to suppress the emission of the NO _x in all atmosphere rich-lean.

[0005] However, the exhaust gas contains SO _x generated by burning sulfur (S) contained in the fuel, and this is oxidized by a catalytic metal in an oxygen-excess atmosphere to form SO ₃ . And it will be readily sulfate by water vapor also contained in the exhaust gas, they are to generate sulfites and sulfates react with _the NO _x storage material, _the NO _x storage material is found to be degraded poisoned Was. The porous carrier such as alumina since the property that easily absorbs SO _x, there is a problem that the sulfur poisoning is facilitated.

[0006] When thus _the NO _x storage material becomes sulfites and sulfates, it becomes difficult to occlude NO _x longer, the result above catalyst, that the purification performance of the NO _x after the durability is lowered There was a defect.

[0007] Further, an experiment was conducted in which the use of a titania carrier was recalled. As a result, SO _x flowed downstream as it was without being adsorbed by titania, and came in direct contact with precious metals
Since only SO ₂ was oxidized, it was revealed that the degree of sulfur poisoning was small. However, it has also been found that the titania carrier has a problem that the initial activity is low and the NO _x purification performance after durability remains low.

Therefore, Japanese Patent Application Laid-Open No. Hei 8-099034 discloses that TiO ₂ -A
It has been proposed to use a composite carrier consisting of l ₂ O ₃ .
In addition, the applicant of the present application has proposed in Japanese Patent Application No. 9-359690 that a carrier obtained by adding rutile-type titania to alumina is used.

[0009] may be maintained in this way by using the composite support was alumina and titania and mixtures or complex oxides, high initial of the NO _x purification rate by virtue of the alumina. The titania difficult to adsorb SO _x in comparison with the alumina, and so easily desorbed at a lower temperature than in the case SO _x adsorbed on the titania adsorbed on alumina, NO
_The probability of contact between the _x storage material and SO _x is reduced. Therefore, when the composite carrier is used, a high NO _x purification rate is secured even at the initial stage, and sulfur poisoning is suppressed, so that the NO _x purification rate after durability is improved.

[0010]

However, the above-mentioned Japanese Patent Application Laid-Open
In an exhaust gas purifying catalyst containing titania in a carrier such as disclosed in 8-099034 Publication, titania does not have sufficient heat resistance, and when exposed to a high temperature for a long time, grain growth occurs in titania, There was a problem that the noble metal supported on titania grew. In particular, Pt easily grows in a high-temperature lean atmosphere. Therefore redox activity by a decrease in active sites due to the decrease in surface area is reduced, NO _x activity to reduce sulfate storage material also lower _the NO _x purification ratio after durability to decrease, the effect of suppressing the sulfur poisoning Is not sufficiently expressed.

[0011] The present invention has been made in view of such circumstances, carriers improves the heat resistance of the NO _x storage reduction catalyst for purifying an exhaust gas containing a titania, after the high-temperature durability NO _x
The purpose is to further improve the purification rate.

[0012]

Features of the exhaust gas purifying catalyst of the present invention to solve the above problems SUMMARY OF THE INVENTION may, at least a carrier comprising a porous oxide including TiO _2, precious metal and NO _x storage material that is supported on a carrier When a more becomes NO _x storage-and-reduction type catalyst for purifying an exhaust gas, in that it contains vanadium oxide (V ₂ O ₅₎ in the carrier.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying catalyst of the present invention,
The carrier contains vanadium oxide (V ₂ O ₅ ). The existence of V ₂ O ₅ suppresses the grain growth of TiO _{2 and} the grain growth of the noble metal, although the reason is unknown, and therefore the NO _x purification rate after durability is improved.

V ₂ O ₅ has a function of accelerating the redox reaction on the catalyst surface.
The effect of promoting the NO _x storage reduction reaction is also exhibited.

In the exhaust gas purifying catalyst of the present invention, the carrier only needs to contain at least TiO ₂ and may be composed of only TiO _2. However, since TiO ₂ alone has low activity from the beginning, alumina, silica, zirconia, etc. It is desirable to use it in combination with another oxide. It is also preferable to use a composite oxide such as Al ₂ O ₃ —TiO ₂ . It is preferable that the carrier further contains CeO ₂ . Purification performance is further improved by the oxygen storage / release capability of CeO ₂ . If CeO ₂ stabilized with ZrO ₂ (CeO ₂ -ZrO ₂ composite oxide) is used, the durability is further improved.

The content of V ₂ O ₅ contained in the carrier is preferably in the range of 0.005 to 0.1 mol per liter of the carrier. V ₂ O ₅
If the content of TiO ₂ is less than this range, it is difficult to exhibit the effect, and TiO ₂ will grow at high temperatures. Further, when the content of V ₂ O ₅ exceeds this range, aggregation of V ₂ O ₅ itself occurs, and the effect of suppressing the grain growth of TiO ₂ decreases.

For example, a porous oxide powder containing TiO ₂ and V ₂
A mixture with O ₅ powder can be used as a carrier. And it is possible to coat the mixed powder to a carrier base material of a honeycomb shape formed from such cordierite, it manufacturing the exhaust gas purifying catalyst of the present invention by supporting a noble metal and NO _x storage material. Further, a coat layer made of a porous oxide powder containing TiO ₂ may be formed on a honeycomb-shaped carrier substrate, and V ₂ O ₅ may be supported on the coat layer. To carry V ₂ O ₅ , the carrier is made to absorb water using an aqueous solution of vanadyl oxalate or vanadium complex, and then calcined.
This can be achieved by setting V ₂ O ₅ .

As the noble metal supported on the carrier, Pt, R
One or more of h, Pd, Ir and Ru can be used. As the NO _x storage material, at least one selected from alkali metals, alkaline earth metals, and rare earth elements can be used. Above all, high alkalinity and NO _x
It is preferable to use at least one of an alkali metal and an alkaline earth metal having a high storage capacity.

Examples of the alkali metal include Li, Na, K and Cs. The alkaline earth metal refers to an element in Group 2A of the periodic table, and examples include Ba, Be, Mg, Ca, and Sr. Examples of the rare earth element include Sc, Y, La, Ce, Pr, Nd, Dy, and Yb.

The amount of the noble metal carried on the carrier is preferably 0.1 to 10 g, particularly preferably 0.5 to 10 g, in the case of Pt and Pd per liter of the carrier substrate. Also 0 for Rh.
It is preferably from 01 to 10 g, particularly preferably from 0.05 to 5 g. Also
The loading amount of the NO _x storage material is 0.
A range of 05 to 1.0 mole is desirable. NO _x
When the amount is less than 0.05 mol / L, the NO _x storage ability is reduced, and when the amount is more than 1.0 mol / L, grain growth of the noble metal is promoted.

In the exhaust gas purifying catalyst of the present invention, the air-fuel ratio
(A / F) is an oxygen-excess atmosphere burned at, for example, 18 or more
NO in exhaust gas is oxidized by contact with
NO _xBecome NO_xIt is stored in the storage material. And air fuel
By periodically varying the ratio from stoichiometric to overfueled.
I mean, NO_xNO from occlusion material_xIs released and it becomes precious metal
Therefore, it reacts with HC and CO in the exhaust gas to be reduced and purified.

In the carrier, the presence of TiO ₂
Sulfur poisoning of the NO _x storage material is suppressed, Ti due to the presence of V ₂ O ₅
O ₂ grain growth is suppressed. Thus also the grain growth inhibition of the noble metal it is possible to sufficiently suppress the sulfur poisoning after a high temperature durability is improved _the NO _x purification performance.

[0023]

The present invention will be specifically described below with reference to examples and comparative examples.

Example 1 A Rh / ZrO ₂ powder was prepared by impregnating a ZrO ₂ powder with an aqueous solution of rhodium nitrate containing a predetermined amount of Rh, and evaporating it to dryness, followed by baking to carry Rh. Rh
The amount of Rh supported in the / ZrO ₂ powder is 0.42% by weight.

This Rh / ZrO ₂ powder, TiO ₂ powder, γ-Al ₂
The mixed powder was prepared by mixing with O ₃ powder. TiO ₂ and γ-Al ₂
O ₃ was mixed at a weight ratio of 1 to 1, and Rh / ZrO ₂ powder mixed 1/10 of the total weight of TiO ₂ and γ-Al ₂ O ₃ . Furthermore, CeO ₂ −
The ZrO ₂ composite oxide powder is mixed with TiO ₂ and γ-Al ₂ O ₃ at 1/1 of the total weight.
The mixture was mixed so as to be 10 and mixed well to prepare a mixed powder.

This mixed powder was slurried by a predetermined method, wet-coated on the surface of a 1.3 L ceramic honeycomb substrate, and dried at 250 ° C. for 15 minutes to form a coat layer. The coat layer is formed in an amount of 270 g per liter of the honeycomb substrate, and the carried amount of Rh is 0.1 g per liter of the honeycomb substrate.

The honeycomb substrate having the coating layer is impregnated with a predetermined amount of a vanadyl oxalate (VOC ₂ O ₄ ) aqueous solution having a predetermined concentration, dried at 250 ° C. for 15 minutes, and baked at 500 ° C. for 30 minutes to obtain V ₂ O. ₅ was carried. The loading amount of V ₂ O ₅ is 0.
01 mole.

Next, a honeycomb substrate having a coating layer supporting V ₂ O ₅ is impregnated with a predetermined amount of a barium acetate aqueous solution having a predetermined concentration, dried at 250 ° C. for 15 minutes, and baked at 500 ° C. for 30 minutes to obtain a Ba substrate. Was carried. The supported amount of Ba is 0.2 mol per 1 L of the honeycomb substrate. This was immersed in an aqueous solution of ammonium bicarbonate having a concentration of 15 g / L for 15 minutes, and then dried at 250 ° C. for 15 minutes to convert Ba into a carbonate.

Further, the substrate was immersed in a nitric acid aqueous solution of a dinitrodiamineplatinum complex of a predetermined concentration, pulled up and blown off excess droplets, and then dried and calcined at 300 ° C. for 15 minutes to carry Pt. Pt
Is 2.0 g per liter of the honeycomb substrate.

Further, a predetermined amount of an aqueous solution containing predetermined concentrations of potassium nitrate and lithium nitrate was impregnated, dried at 250 ° C., and calcined at 500 ° C. for 30 minutes to carry K and Li. K and Li
Was supported in an amount of 0.1 mol per 1 L of the honeycomb substrate.

Example 2 An exhaust gas purifying catalyst of Example 2 was prepared in the same manner as in Example 1 except that the carried amount of V ₂ O ₅ was 0.02 mol per liter of the honeycomb substrate.

Example 3 An exhaust gas purifying catalyst of Example 3 was prepared in the same manner as in Example 1 except that the amount of V ₂ O ₅ carried was 0.05 mol per liter of the honeycomb substrate.

Comparative Example An exhaust gas purifying catalyst of a comparative example was prepared in the same manner as in Example 1 except that V ₂ O ₅ was not supported.

(Test / Evaluation) The honeycomb catalysts of Examples 1 to 3 and Comparative Example were respectively mounted on the exhaust system of a 1.8 L lean burn engine, and the gas was switched from a steady state of rich gas to a steady state of lean gas on an engine bench. _{Then, the} amount of NO _x stored by each catalyst (NO _x saturated storage amount) until the NO _x concentration of the exhaust gas became steady was measured. Table 1 shows the results.

Further, when the sulfur additive is used, the sulfur concentration in the fuel is 500 p
A 50-hour accelerated endurance test was performed in a pattern simulating urban driving, using fuel added to pm. Thereafter, the NO _x saturation storage amount was measured in the same manner as above, and the results are shown in Table 1. Further, the particle size of Pt on each catalyst after the durability test was measured by X-ray diffraction, and the results are shown in Table 1.

[0036]

[Table 1]

From Table 1, it can be seen that the particle size of Pt after durability was reduced by carrying V ₂ O ₅ , and the NO _x storage ability after durability was improved. The negative correlation was observed between the particle diameter and _the NO _x storage capacity of Pt after the durability, the results grain growth of Pt by carrying the V ₂ O ₅ is suppressed, is _the NO _x storage ability after durability It is clear that it has improved. It is considered that the reason why the growth of Pt was suppressed by the loading of V ₂ O ₅ was that the growth of TiO ₂ was suppressed.

It is also recognized that the initial NO _x storage capacity is improved by the loading of V ₂ O ₅ . Furthermore, it was also found that when the amount of V ₂ O ₅ supported increased, the effect of suppressing the grain growth of Pt was reduced, and it is considered that a supported amount of about 0.01 to 0.02 mol is optimal.

[0039]

According to the exhaust gas purifying catalyst of the present invention, the growth of TiO ₂ at high temperature is suppressed by the presence of V ₂ O ₅ , so that the growth of noble metal is also suppressed. NO _x storage reduction ability is improved.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (reference) F01N 3/28 301 B01D 53/36 102B B01J 23/64 102A F-term (reference) 3G091 AB06 GB02Y GB03Y GB04Y GB05W GB06W GB07W GB10X 4D048 AA06 AB02 BA03X BA07X BA08X BA14X BA15X BA19X BA23X BA30X BA33X BA41X BB02 BC05 CA01 EA04 4G069 AA01 AA04 AA08 AA09 BA01B BA01C BA04A BA04B BA04C BA05B BA05C BA13B BA13C BB04A BB04B BB04C BB06A BB06B BB06C BC03B BC03C BC04B BC04C BC13B BC13C BC32A BC33A BC43B BC43C BC54A BC54B BC54C BC69A BC69B BC69C BC71B BC71C BC75B BC75C CA03 CA08 CA13 DA06 EA18 ED06 ED07 FA02 FA06 FB13

Claims (1)

[Claims] [1 claim: a carrier comprising a porous oxide including at least titania (TiO _2), a more becomes NO _x storage-and-reduction type exhaust gas purifying catalyst is supported noble metal and NO _x storage material to the carrier An exhaust gas purifying catalyst, wherein the carrier further contains vanadium oxide (V ₂ O ₅ ).