JP4224780B2 - Exhaust gas purification catalyst - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a low temperature active catalyst capable of efficiently purifying harmful components such as CO or HC in automobile exhaust gas from a low temperature range.
[0002]
[Prior art]
Conventionally, as a catalyst for exhaust gas purification of automobiles, a three-way catalyst that purifies by performing CO and HC oxidation and NO _x reduction simultaneously in exhaust gas at a stoichiometric air-fuel ratio (stoichiometric) has been used. As such a three-way catalyst, for example, a coat layer is formed from a porous oxide such as γ-alumina on a heat-resistant substrate made of cordierite, and platinum (Pt), rhodium (Rh) is formed on the coat layer. Those carrying precious metals such as are widely known.
[0003]
In recent years, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem from the viewpoint of protecting the global environment. As a solution, so-called lean burn that causes lean combustion in a lean atmosphere with excess oxygen is proposed. Promising. In this lean burn, CO ₂ emissions can be reduced by improving fuel efficiency.
[0004]
If the three-way catalyst is disposed in the exhaust gas from the lean burn engine, the three-way catalyst mainly functions as an oxidation catalyst, and can oxidize and purify CO and HC in the exhaust gas. The atmosphere lean - stoichiometric - be used in case of change and rich acts mainly as an oxidation catalyst in a lean atmosphere, the stoichiometric-rich atmosphere also function to reduce NO _x by CO and HC in the exhaust gas as a reducing catalyst Can be purified.
[0005]
By the way, it is the supported noble metal that exhibits the catalytic action in the three-way catalyst, but the temperature range in which the noble metal can purify HC, for example, is in a relatively high temperature range of about 300 ° C. or higher, which is harmful in the low temperature range. There is a problem that it is difficult to purify the components. For this reason, there is a problem that the purification activity is low when the exhaust gas is in a low temperature range such as in winter or at the start. Also, at the time of cold start, the air-fuel ratio is often a fuel-rich atmosphere, and the amount of HC in the exhaust gas is large, which is one of the above-mentioned problems.
[0006]
Thus, improvement of the low-temperature activity of the three-way catalyst has become a problem, and for example, the catalyst is disposed directly under the engine. In this way, the heat of the exhaust gas is quickly transmitted to the catalyst, so that it is possible to quickly raise the temperature to the activation temperature. In addition, a precious metal is supported at a high concentration on the upstream end of the exhaust gas flow of the catalyst. The upstream end rises to the activation temperature earlier than the downstream side, and the reaction takes place actively at the upstream end that has risen to the activation temperature. As a result, the activity of the catalyst in the low temperature range is improved.
[0007]
Japanese Patent Laid-Open No. 09-000928 discloses a catalyst containing a noble metal and a NO ₂ absorbent as a catalyst for efficiently purifying HC in exhaust gas at the cold start. According to this catalyst, it is possible to high NO ₂ absorption oxidation activity to NO ₂ absorbent reacts with HC in the low temperature range, to increase the temperature of the catalyst by the reaction heat. In addition, since the HC oxidation reaction proceeds with the ignition of the reaction as a trigger, HC in the exhaust gas at the cold start can be efficiently purified.
[0008]
Furthermore, Japanese Patent Application Laid-Open No. 2003-024747 describes a method for purifying ethylene oxide or the like by contacting exhaust gas with a silver catalyst before contacting exhaust gas with a platinum catalyst. Since the silver catalyst exhibits activity at a lower temperature than the platinum catalyst, the exhaust gas temperature can be increased before contacting the platinum catalyst, and the activity of the platinum catalyst can be expressed early.
[0009]
[Patent Document 1]
JP 09-000928 [Patent Document 2]
Japanese Patent Laid-Open No. 2003-024747
[Problems to be solved by the invention]
As the active species of the three-way catalyst, Pt having a high oxidation activity is generally used. However, in the stoichiometric to rich atmosphere, CO in the exhaust gas is adsorbed by Pt, so that there is a problem that the activity of Pt decreases. For this reason, even if the above-described means is used, purification of HC in the exhaust gas in the low temperature region is still insufficient, and further improvement in low temperature activation is required.
[0011]
This invention is made | formed in view of such a situation, and it aims at further improving the purification activity in a low temperature range.
[0012]
[Means for Solving the Problems]
Features of the exhaust gas purifying catalyst of the present invention for solving the above-mentioned problems, CeO ₂ - and a carrier comprising a ZrO ₂ composite oxide, at least CeO ₂ - and Pt supported on ZrO ₂ composite oxide, at least CeO ₂ - ZrO It consists of a supported Ag ₂ composite oxide, and the Pt and Ag in that it is carried as a composite metal colloid.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
In the exhaust gas purifying catalyst of the present invention, CeO ₂ - Pt and Ag to ZrO ₂ composite oxide is supported adjacent to one another as a composite metal colloid. Here, Ag has a characteristic that it can easily release adsorbed oxygen compared to Pt . Therefore, when exhaust gas of lean atmosphere are in contact, but the Pt least oxygen is adsorbed, the oxygen adsorbed on Pt is easy to move through the adjacent Ag to a carrier, CeO ₂ - together are inserted in the ZrO ₂ composite oxide It easily reacts with CO or HC near the interface between Ag and support. The NO _x adsorbed on Pt also tends to move through the Ag to the carrier, likely to be reduced in the vicinity of the interface between the Ag and the carrier.
[0014]
On the other hand, when the exhaust gas in the stoichiometric to rich atmosphere comes into contact, CO in the exhaust gas is adsorbed on Pt, but the adsorbed CO easily moves to the carrier through the adjacent Ag . Accordingly adsorption decrease in activity due to the CO can be suppressed, moreover CO is CeO ₂ in the vicinity of the interface between the Ag and the carrier - is purified is oxidized by oxygen released from the ZrO ₂ composite oxide. Further, CeO ₂ - oxygen released from ZrO ₂ composite oxide is moved on the Pt through the Ag, purifying by oxidizing CO and HC on Pt.
[0015]
That is, Ag functions as a diffusion promoting phase that promotes at least oxygen diffusion . It is not clear in what state the gas component diffuses through Ag , but it may be diffused in the state of molecules, atoms, ions or radicals.
[0016]
And the movement and reaction of oxygen through Ag occurs even in the low temperature range of less than 300 ° C, so that harmful components can be purified from the low temperature range.
[0019]
CeO ₂ - ZrO ₂ composite oxide, preferably at least a part of ZrO ₂ is be used solid solution dissolved in CeO _2, the atomic ratio Ce: Zr is 40: 60-95: the use of the range of 5 Is preferred. If the Ce: Zr is less than 60, the absolute amount of oxygen absorbed and released is insufficient and the activity decreases. If the Ce is greater than 95: 5, the oxygen absorption / release of CeO ₂ due to the solid solution of ZrO ₂ Since the performance improving effect becomes insufficient, the absolute amount of absorbed and released oxygen is insufficient and the activity is lowered.
[0020]
Support at least the CeO ₂ - may be included ZrO ₂ composite oxide, CeO ₂ - may be constituted only with ZrO ₂ composite oxide, CeO ₂ - Al ₂ O ₃ in ZrO ₂ composite oxide, ZrO ₂ , TiO ₂ , SiO ₂ and other oxides may be mixed to form a carrier. However, the mixing ratio of other oxides is desirably 50 mol% or less. When other oxides are mixed in an amount exceeding 50 mol%, the absolute amount of oxygen absorbed and released is insufficient and the activity is lowered.
[0021]
The amount of Pt supported is in the range of 0.1 to 10% by weight with respect to the support as in the case of a three-way catalyst. If the amount of Pt supported is less than this range, the purification activity is low, and even if it is supported more than this range, the purification activity is saturated and there is a problem in terms of cost. For supporting Pt , a conventionally known supporting method such as an adsorption supporting method or a water absorbing supporting method can be used.
[0023]
The supported amount of Ag is preferably in the range of 5 to 200 mol% as a metal with respect to Pt . If the loading amount is less than this range, it is difficult to express the above-mentioned effects and the low-temperature activity is lowered. Even if the loading is more than this range, the effect is saturated and the Pt activity is inhibited. Therefore, it is preferable that the amount of Ag supported is smaller than the amount of Pt supported.
[0024]
Pt and Ag are supported adjacent to each other. This smoothly moves at least oxygen through Ag and improves low-temperature activity. The Ag is, CeO ₂ in a state adjacent to Pt - is supported on ZrO ₂ composite oxide. FIG. 1 shows an ideal view of the carrying state. Thus Ag contacts the entire circumference of Pt, but yet the state where Pt is also exposed is ideal, but not limited thereto.
[0025]
In order to support Pt and Ag adjacent to each other , they are supported as a composite metal colloid as described in, for example, JP-A-2000-279824 . If Pt and Ag are supported as a composite metal colloid, Pt and Ag are supported in a state of being adjacent to each other at the micro level or the nano level, and the above-described action is effectively exhibited. Furthermore, since Pt and Ag serve as a barrier to each other, grain growth in a high temperature atmosphere is suppressed and durability is improved. Also, since Pt is supported as colloidal particles with 10 to several thousand atoms gathered, PtO ₂ detachment / diffusion in a lean atmosphere containing oxygen at a high temperature is suppressed compared with the case where it is supported in an atomic form, and grain growth Is suppressed.
[0026]
The particle size of the composite metal colloid is preferably in the range of 1 to 10 nm. Theoretically, if the particle size is 1 to 10 nm, the number of constituent atoms is 10 to 5000. If the particle size is smaller than 1 nm, it becomes close to an atomic state and grain growth is likely to occur. On the other hand, when the particle size is larger than 10 nm, the surface area of the composite metal colloid is reduced, so that the initial purification rate is lowered. By setting the thickness within the range of 1 to 10 nm, the grain growth can be effectively suppressed, so that the durability is improved and the high surface area can ensure a high initial purification activity.
[0027]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
[0028]
(Example 1)
PVP (Polyvinylpyrrolidene “K-90” manufactured by Wako Pure Chemical Industries , Ltd.) 6.9g dissolved in 1000ml of 1: 1 mixed solution of ethanol and pure water, 1g of H ₂ PtCl ₆ in terms of Pt, AgClO ₄ Ag 0.25 g was dissolved in terms of conversion, and refluxed at 85 ° C. for 3 hours in the air to prepare a colloidal solution.
[0029]
100 g of CeO ₂ —ZrO ₂ complex oxide powder (atomic ratio Ce: Zr = 50: 50) was added to the resulting colloidal solution, sufficiently impregnated, and then the solvent was removed. The organic component was removed by baking for a period of time. The obtained catalyst powder was compacted, pulverized, and sized into pellets having a particle size of 0.5 to 1 mm to obtain the catalyst of Example 1.
[0030]
The composition of this catalyst is such that 1 g of Pt is supported and 0.25 g of Ag is supported on 100 g of CeO ₂ —ZrO ₂ . The average particle size of the supported composite metal colloid is 3 nm.
[0031]
(Example 2)
A colloid solution was prepared in the same manner as in Example 1 except that 0.5 g of AgClO ₄ in terms of Ag was used, and a catalyst was prepared in the same manner. The composition of this catalyst is such that 1 g of Pt is supported and 0.5 g of Ag is supported on 100 g of CeO ₂ —ZrO ₂ . The average particle size of the supported composite metal colloid is 3 nm.
[0032]
(Example 3)
A colloidal solution was prepared in the same manner as in Example 1 except that 1 g of AgClO ₄ in terms of Ag was used, and a catalyst was prepared in the same manner. As for the composition of this catalyst, 1 g of Pt is supported and 1 g of Ag is supported on 100 g of CeO ₂ —ZrO ₂ . The average particle size of the supported composite metal colloid is 3 nm.
[0033]
(Example 4)
A colloidal solution was prepared in the same manner as in Example 1 except that 2 g of AgClO ₄ in terms of Ag was used, and a catalyst was prepared in the same manner. The composition of this catalyst is such that 1 g of Pt is supported and 2 g of Ag is supported per 100 g of CeO ₂ —ZrO ₂ . The average particle size of the supported composite metal colloid is 3 nm.
[0034]
(Comparative Example 1)
A predetermined amount of a CeO ₂ —ZrO ₂ composite oxide powder similar to that in Example 1 is mixed with a predetermined amount of an aqueous solution in which H ₂ PtCl ₆ is dissolved at a predetermined concentration, and the mixture is stirred and evaporated. Baked for 3 hours. The obtained catalyst powder was compacted, pulverized and sized into pellets having a particle size of 0.5 to 1 mm to obtain a catalyst of Comparative Example 1. As for the composition of this catalyst, 1 g of Pt is supported on 100 g of CeO ₂ —ZrO ₂ .
[0035]
(Comparative Example 2)
A predetermined amount of a CeO ₂ —ZrO ₂ composite oxide powder similar to that in Example 1 is mixed with a predetermined amount of an aqueous solution in which AgClO ₄ is dissolved at a predetermined concentration, and the mixture is stirred and evaporated. Baked. The obtained catalyst powder was compacted, pulverized and sized into pellets having a particle size of 0.5 to 1 mm to obtain a catalyst of Comparative Example 2. The composition of this catalyst is 0.5 g of Ag supported on 100 g of CeO ₂ —ZrO ₂ .
[0036]
(Comparative Example 3)
50 g of CeO ₂ -ZrO ₂ composite oxide powder similar to that in Example 1 was mixed with an aqueous solution in which 1 g of H ₂ PtCl ₆ was dissolved in terms of Pt, stirred, evaporated to dryness, and then calcined at 400 ° C in the atmosphere for 3 hours. did. In addition, 50 g of CeO ₂ —ZrO ₂ composite oxide powder similar to that in Example 1 was mixed with an aqueous solution in which 0.5 g of AgClO ₄ was dissolved in terms of Ag, stirred, evaporated to dryness, and then fired at 400 ° C. in the atmosphere for 3 hours. did. Both catalyst powders thus prepared were mixed in a mortar, compacted and pulverized, and sized into pellets having a particle size of 0.5 to 1 mm to obtain a catalyst of Comparative Example 3. The composition of this catalyst is such that 1 g of Pt and 0.5 g of Ag are supported on 100 g of CeO ₂ —ZrO ₂ .
[0037]
<Test and evaluation>
[0038]
[Table 1]
[0039]
1g of each pellet catalyst of Example and Comparative Example is filled in the evaluation device, and the steady model gas shown in Table 1 is flowed at a flow rate of 7L / min. The temperature was raised, and C ₃ H ₆ , CO, and NO purification rates were measured continuously during that time. The 50% purification temperature of C ₃ H _{6 with} the clearest order was calculated, and the results are shown in Table 2.
[0040]
[Table 2]
[0041]
From Table 2, the catalyst of Example 2 has a higher HC purification activity in the low temperature range than the catalysts of the comparative examples. This is because Pt and Ag are adjacent to each other by supporting Pt and Ag as a composite metal colloid. This is the effect of being carried.
[0042]
The comparison between each example and Comparative Example 1 suggests that there is an optimum value for the amount of Ag supported.
[0043]
And it is thought that it is based on the following mechanisms that the catalyst of each Example is excellent in low temperature activity.
[0044]
That is, in the catalyst of each example, Ag is supported in a state extremely adjacent to Pt. Therefore, for example, oxygen radicals generated by NO or oxygen molecules adsorbed on Pt move to Ag, which is easier to oxidize than Pt, and then are stored in a CeO ₂ -ZrO ₂ carrier that has oxygen absorption / release capacity. Is done. Therefore, oxygen does not remain on the Pt surface, and the Pt surface is always cleaned, and as a result, the low temperature activity is improved.
[0045]
In addition, when a reducing agent such as C ₃ H ₆ is adsorbed on the Pt surface, oxygen in the CeO ₂ -ZrO ₂ carrier rapidly moves onto Pt via Ag, and the reducing agent is oxidized by the oxygen. Is done. Further, when CO is adsorbed on Pt, it rapidly moves to the CeO ₂ -ZrO ₂ carrier via Ag, so that poisoning of the Pt surface by CO is alleviated.
[0046]
【The invention's effect】
That is , according to the exhaust gas purifying catalyst of the present invention, the activity in the low temperature range is remarkably improved as compared with the conventional catalyst, and harmful components such as CO or HC in the automobile exhaust gas are efficiently removed even in the low temperature range such as at the start. Can be purified.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing an ideal structure of an exhaust gas purifying catalyst of the present invention.

Claims (1)

CeO ₂ - and a carrier comprising a ZrO ₂ composite oxide, at least the CeO ₂ - and Pt supported on ZrO ₂ composite oxide, at least the CeO ₂ - consists of a Ag supported on ZrO ₂ composite oxide, Pt And Ag are supported as a composite metal colloid, and an exhaust gas purifying catalyst .