JPH08131833A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE: To provide a catalyst having NOx removal activity in a high temp. region by carrying rhodium on tungsten oxide. CONSTITUTION: Rhodium is deposited on tungsten oxide to obtain the objective catalyst. This tungsten oxide is known as a hydrogenation catalyst and it is known that olefin HC in exhaust gas has relatively high activity and paraffin HC has low activity. When NOx in HC-contg. exhaust gas is removed, the tungsten oxide acts like a hydrogenation catalyst to HC at the time of oxidation combustion of HC with the rhodium and converts high activity olefin HC into low activity paraffin HC or prevents sudden oxidation of HC. As a result, HC becomes a high activity reducing agent effective in decomposing NOx at a higher temp. and the catalyst has NOx removal activity in a higher temp. region.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst, and more particularly to an exhaust gas purifying catalyst effective for removing NOx (nitrogen oxide) in exhaust gas.

[0002]

2. Description of the Related Art In a conventional mobile internal combustion engine, a three-way catalyst is installed in an exhaust passage to remove NOx contained in exhaust gas into CO and HC.
It is usually purified (decomposed) together with (hydrocarbon). As the three-way catalyst, a catalyst in which a precious metal such as Pt is supported on a carrier such as alumina is often used. However, this conventional three-way catalyst has a high exhaust gas purification rate when the internal combustion engine is operated at the stoichiometric air-fuel ratio, but has a low NOx purification rate when it is operated at the lean air-fuel ratio.

On the other hand, Pt, Rh and W are added to the above carrier.
Also, there is a proposal to increase the window width of the three-way catalyst (the air-fuel ratio range where a purification rate of 80% or more can be obtained) by supporting an alkali metal chloride (JP-B-58-
No. 45288). Further, by using zeolite as a carrier and supporting a noble metal catalyst thereon, NOx can be purified even at a lean air-fuel ratio.
Ox catalysts are also known.

[0004]

However, there is a limit to the expansion of the window width of the three-way catalyst because it is necessary to purify all of HC, CO and NOx, and NOx is reduced at a lean air-fuel ratio of A / F = 16 or more. It is difficult to fully purify.

In the case of a NOx catalyst in which a noble metal catalyst is supported on zeolite, for example, Pt is used as the noble metal catalyst.
With the adoption of NOx, NOx is relatively high even with a lean air-fuel ratio.
Purification rate can be obtained. However, the peak of the activity is, for example, in the case where the NOx catalyst is coated on the monolith carrier, the catalyst inlet temperature is around 220 to 230 ° C., and when the temperature is higher than that, for example, around 300 ° C., N
There is a problem that the Ox purification rate is greatly reduced.

That is, in the case of an internal combustion engine of an automobile, the temperature of exhaust gas is relatively high, and the temperature at the catalyst inlet is often around 300 ° C. although it slightly varies depending on the arrangement of the catalyst.
The low activation temperature of the NOx catalyst means NOx
Will be disadvantageous in purifying effectively. Therefore, the present invention provides an exhaust gas purifying catalyst having a high activation temperature range in NOx purification.

[0007]

Means for Solving the Problem and Its Action As a result of various experiments and studies on the problem, the present inventor has found that Rh as a catalyst metal and tungsten oxide as a carrier are used. The inventors have found that the combination can achieve the intended purpose, and have completed the present invention. Hereinafter, the invention according to each claim will be specifically described.

<Invention of Claim 1> The present invention is an exhaust gas purifying catalyst comprising rhodium supported on an oxide of tungsten.

That is, it is considered that purification of NOx by the catalyst proceeds by using HC in the exhaust gas as a reducing agent, and HC in this case functions as a reducing agent of NOx in the process of being oxidized and decomposed by the catalyst metal. On the other hand, P
Although t and Pd are so effective as catalytic metals for NOx, they have too strong an oxidation catalyst function to decompose HC in exhaust gas even when the exhaust gas temperature is relatively low.
This lowers the temperature at which the activity develops. Therefore, in the present invention, Rh (rhodium), which has a relatively mild activity as an oxidation catalyst, is selected as an active species of the catalyst among the catalytic metals effective for purification of NOx.

Zeolite is first considered as a carrier of Rh, but since it adsorbs HC well and has a high function as a cocatalyst, it is advantageous in purifying NOx by the HC. It is not effective from the viewpoint of raising the activation temperature range of the catalyst. From this point of view, on the contrary, the inert carrier is advantageous because NOx is not purified unless the temperature becomes high. However, when the present inventor conducted experiments to use other materials such as NiO, ZnO, CuO, and CoO as a carrier in place of the above zeolite,
With the NiO carrier, the effect of shifting to the higher temperature side of the above-mentioned activation temperature range was not so great, with the ZnO carrier, the effect of shifting was small, and with the CuO carrier and the CoO carrier, the NOx purification rate itself was low.

The results are shown in FIG. 1, in which the symbol Rh / NiO is a catalyst in which rhodium is supported on NiO as a carrier, and Rh / ZnO is rhodium supported on ZnO as a carrier. Catalyst, Rh / Cu
O means a catalyst in which rhodium is supported on CuO as a carrier, and Rh / CoO means a catalyst in which rhodium is supported on CoO as a carrier.

[0012] Therefore, the present inventor has made further studies from the viewpoint of a carrier that has an effect of shifting the active temperature range to the high temperature side by exerting some action on HC, and the above tungsten oxide is used as a carrier of Rh. As a result, the activation temperature range can be shifted to the high temperature side without causing a decrease in the NOx purification rate. The reason why the active temperature range can be shifted to the high temperature side by using this tungsten oxide as the carrier is presumed to be as follows.

That is, it is known that the oxide of tungsten is known as a hydrogenation catalyst, and that the HC in the exhaust gas has a relatively high activity for olefins and a low activity for paraffins. ing. In the purification of NOx of exhaust gas containing such HC, tungsten oxide acts as a hydrogenation catalyst on the HC during the oxidative combustion of HC by Rh, so that a highly active olefin-based catalyst is used. It reforms HC to paraffinic HC with low activity or prevents abrupt oxidation of HC. Therefore, the temperature at which the HC becomes a highly active reducing agent effective for NOx decomposition (an intermediate produced during oxidative combustion) becomes high,
As a result, the active temperature range for NOx purification of the catalyst becomes higher.

Even if such a tungsten oxide is used as a carrier, the NOx purification rate itself does not significantly decrease because the tungsten oxide does not impair the catalytic function itself of Rh, and the tungsten oxide does not interfere with the exhaust gas. It is considered to have a co-catalyst function of trapping HC in the interior or capturing NOx.

The tungsten oxide may be tungsten oxide or tungstic acid. Further, as a method of supporting Rh on such a tungsten oxide, an evaporation dry method or a spray dry method, or a method of impregnating Rh in a coating layer of the tungsten oxide after coating the monolith carrier with the tungsten oxide is carried. Various methods such as a method of performing can be adopted.

<Invention of Claim 2> The present invention is the exhaust gas purifying catalyst according to claim 1, wherein the rhodium content is 0.175 parts by weight or more relative to 100 parts by weight of the tungsten oxide. An exhaust gas purifying catalyst, which is carried.

By setting the ratio of Rh to tungsten oxide as described above, it is possible to secure a relatively high NOx purification rate while shifting the NOx purification activation temperature range of the catalyst to the high temperature side.

<Invention of Claim 3> The present invention relates to the exhaust gas purifying catalyst according to claim 1, wherein the rhodium content of 0.35 to 1. The exhaust gas purifying catalyst is characterized in that it carries 4 parts by weight.

When the ratio of Rh to tungsten oxide is set as described above, it becomes more advantageous to secure a relatively high NOx purification rate while shifting the NOx purification active temperature range of the catalyst to the high temperature side. Here, the Rh amount is set to 1.4 parts by weight or less because a large improvement in the NOx purification rate cannot be expected even if the Rh amount becomes larger than this.

<Invention of Claim 4> This invention is provided in an exhaust passage of an internal combustion engine operated at a lean air-fuel ratio, and is described in any one of claims 1 to 3. Exhaust gas purification catalyst.

As described above, the purpose of using each of the exhaust gas purifying catalysts of claims 1 to 3 is to purify the exhaust gas of an internal combustion engine operated at a lean air-fuel ratio, because the exhaust gas has a high oxygen concentration. However, the catalyst effectively works to purify NOx in the exhaust gas.

[0022]

According to the first aspect of the present invention, since it is an exhaust gas purifying catalyst in which rhodium is supported on an oxide of tungsten, the active temperature range of NOx purification in an oxygen excess atmosphere becomes high, and the exhaust gas is exhausted. The desired NOx purification can be performed when the gas temperature is relatively high.

According to the second aspect of the present invention, the rhodium is added to 100 parts by weight of the tungsten oxide.
Since 175 parts by weight or more is supported, it is advantageous in increasing the NOx purification rate when the exhaust gas temperature is high.

According to the third aspect of the present invention, the rhodium is added to 100 parts by weight of the tungsten oxide.
Since 35 to 1.4 parts by weight are supported, it is more advantageous in increasing the NOx purification rate when the exhaust gas temperature is high.

According to the invention of claim 4, the exhaust gas purifying catalyst according to any one of claims 1 to 3 is provided in an exhaust passage of an internal combustion engine operated at a lean air-fuel ratio. Thus, NOx can be efficiently purified even when the oxygen concentration in the exhaust gas is high.

[0026]

Embodiments of the present invention will be described below.

-Preparation of catalyst-A powder of tungstic acid (H ₂ WO ₄ ) was prepared, and hydrated alumina was added as a binder to 20 wt% of the tungstic acid. An appropriate amount of water was added to this mixture to obtain a slurry. 400 cells / in this slurry
The inch ² honeycomb carrier was dipped and pulled up, excess slurry was blown off by air blow, dried, and then fired at 500 ° C. for 2 hours in the atmosphere. The amount of the above mixture supported was 30 wt% of the above honeycomb carrier.

Next, an aqueous rhodium nitrate solution having a predetermined concentration was prepared, and the above-mentioned rhodium nitrate aqueous solution was impregnated into the honeycomb carrier carrying the above mixture and dried sufficiently at 200 ° C., and then 500 ° C. × 2 in air. By performing calcination for an hour, an exhaust gas purifying catalyst having an amount of supported Rh of 0.5 g per liter of honeycomb carrier volume was obtained. In this case, Rh
Tungstic acid 1 when the supported amount is 1 g / liter
Since 0.7 part by weight of Rh is supported with respect to 00 parts by weight, the amount of supported Rh of the catalyst is 0.35.
It means parts by weight.

-Evaluation of Catalyst- The above exhaust gas purifying catalyst was incorporated into a gas reactor of a fixed pressure fixed bed flow type, and a model gas (A / F =) having the composition shown in Table 1 was used.
22) was circulated at a space velocity of 55000 h ⁻¹ , and the NOx purification rate at that time was measured. In this case, the model gas is preheated to 400 ° C. by a heater, and the temperature is lowered at a rate of 20 ° C./min while the NO gas is simultaneously discharged.
The method of analyzing the concentration of x was adopted. The results are shown in FIG. 1 by the symbol Rh / WO.

[0030]

[Table 1]

According to FIG. 1, the peak of the NOx purification rate appears near the catalyst inlet temperature of 300 ° C., which is clear from the comparison with the NOx purification characteristics of the catalyst using the carrier other than tungsten oxide shown in FIG. Thus, it can be seen that when tungsten oxide is used as the carrier, the NOx purification active temperature range shifts to the high temperature side. The conditions for adjusting each catalyst by the other carriers in FIG. 1 are the same as those in Rh / WO described above, and a honeycomb catalyst is similarly used.

-Influence of Rh loading amount- Several kinds of aqueous rhodium nitrate solutions having different concentrations were prepared, and each of the above-mentioned honeycomb carriers carrying the mixture was impregnated with these aqueous rhodium nitrate solutions and dried sufficiently at 200 ° C. After that, several kinds of samples (exhaust gas purifying catalysts) having different Rh loadings were obtained by firing at 500 ° C. for 2 hours in the atmosphere. The amount of Rh supported on each sample was adjusted within the range of 0.1 to 2.0 g per liter of honeycomb carrier volume. Therefore, Rh is supported in the range of 0.07 to 1.4 parts by weight with respect to 100 parts by weight of tungstic acid.

Each of the above samples was incorporated into a gas reactor of atmospheric pressure fixed bed flow type, and the HC concentration was set to 4000.
A model gas having the same composition as shown in Table 1 except for ppm was circulated at a space velocity of 55000 h ⁻¹ , and NO at that time was used.
The x purification rate was measured by the same method as in the previous example. The results are shown in Figure 2 and Table 2.

[0034]

[Table 2]

According to FIG. 2, the NOx purification rate increases as the amount of Rh carried increases, but the peak temperature of the NOx purification rate deviates to the low temperature side. However, as shown in Table 2, the NOx purification rate at the catalyst inlet temperature of 300 ° C. does not decrease even when the Rh loading amount increases. Also, Rh
When the loading amount becomes 0.25 g / liter (0.175 parts by weight), the peak value of the NOx purification rate at 300 ° C. becomes considerably high, and when it becomes 0.5 g / liter (0.35 parts by weight), the NOx purification rate increases. Although the rate becomes higher, the NOx purification rate at 300 ° C hardly changes even if the rate is exceeded.

The peak temperature of the NOx purification rate deviates to the low temperature side as the amount of Rh carried increases.
The amount of tungstic acid decreases relative to the amount,
It is recognized that the effect of suppressing the HC combustion becomes insufficient with respect to the Rh amount. From the above results, it is preferable that the amount of Rh supported is 0.25 g / liter (0.175 parts by weight) or more, and 0.5 g / liter (0.35 parts by weight) or more. It is understood that it is more preferable, and that the amount of Rh supported is more than 2.0 g / liter (1.4 parts by weight) is not necessary from the viewpoint of increasing the NOx purification rate at high temperature.

-Influence of Pt- In the step of impregnating rhodium nitrate in the above-mentioned sample preparation, a predetermined amount of an aqueous solution of tetraammineplatinum chloride was added as an additive to carry out the impregnation. The amount of Rh supported was adjusted to 0.5 g / liter (0.35 parts by weight), and the amount of Pt was adjusted in the range of 0 to 200% of the amount of Rh. Then, the NOx purification rate of each of the obtained samples was measured under the same conditions as the above-influence of the amount of Rh carried-. The results are shown in Figure 3 and Table 3.

[0038]

[Table 3]

According to FIG. 3 and Table 3, the NOx purification rate on the high temperature side decreases as the Pt amount increases, and the peak temperature of the NOx purification rate shifts to the low temperature side. The addition is not suitable for increasing the NOx purification rate on the high temperature side, and the Pt amount is 10% of the Rh amount.
It can be seen that the content is preferably not more than%.

-Combination with Low Temperature NOx Catalyst- As described above, the catalyst Rh / WO of the present invention can be said to be a high temperature NOx catalyst exhibiting a high NOx purification rate when the exhaust gas temperature is high. It can be seen that when used for exhaust gas purification of an engine, it is suitable to combine with a low temperature type NOx catalyst showing a high NOx purification rate when the exhaust gas temperature is low (for example, 200 to 250 ° C.).

FIG. 4 shows such an example. In FIG. 4, a high temperature type catalyst (R
h / WO) 2 is arranged, and the low temperature type catalyst (Pt
/ Z) 3 is arranged. The low temperature type catalyst (Pt / Z) is
A honeycomb carrier is wash-coated with a catalyst powder obtained by supporting a noble metal on a crystalline metal-containing silicate in which a metal typified by zeolite constitutes a part of a crystal lattice. As the catalyst powder, for example, zeolite (ZSM-5) with Pt and Rh of 75: 1
It is preferable that the carrier is supported by a spray dry method or the like at a weight ratio of. According to this, the peak temperature of the NOx purification rate appears near the catalyst inlet gas temperature of 220 to 230 ° C.

According to such a combination of the low temperature type and high temperature type NOx catalysts 2 and 3, NOx is purified by the low temperature type NOx catalyst 3 on the downstream side when the exhaust gas temperature is low, and the exhaust gas temperature is increased. When it becomes higher, NOx is purified by the high temperature type NOx catalyst 2 on the upstream side.

Regarding the arrangement of both NOx catalysts, it is not preferable to arrange the low temperature type NOx catalyst on the upstream side and the high temperature type NOx catalyst on the downstream side. This is because when the exhaust gas temperature is low, NOx is purified by the low temperature NOx catalyst on the upstream side, but when the exhaust gas temperature is high, only HC is decomposed by the low temperature NOx catalyst on the upstream side. That is, since the HC required for NOx decomposition by the high temperature NOx catalyst on the downstream side is decomposed and consumed by the low temperature NOx catalyst on the upstream side, the high temperature NOx catalyst is consumed.
The catalyst will not function effectively.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of different carriers on the NOx purification rate.

FIG. 2 is a graph showing the effect of the amount of Rh carried on the NOx purification rate.

FIG. 3 is a graph showing the effect of the amount of Pt added on the Ox purification rate.

FIG. 4 is a block diagram showing an example of using the catalyst of the present invention.

[Explanation of symbols]

 1 Internal Combustion Engine 2 High Temperature NOx Catalyst (Catalyst of the Present Invention) 3 Low Temperature NOx Catalyst

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01J 23/64 ZAB A B01D 53/36 102 H 104 A

Claims (4)

[Claims] 1. A catalyst for purifying exhaust gas, wherein rhodium is supported on an oxide of tungsten. 2. The exhaust gas purifying catalyst according to claim 1, wherein 0.175 parts by weight or more of the rhodium is supported on 100 parts by weight of the tungsten oxide. Purification catalyst. 3. The exhaust gas purifying catalyst according to claim 1, wherein 0.35 to 1.4 parts by weight of the rhodium is supported on 100 parts by weight of the tungsten oxide. Exhaust gas purification catalyst. 4. The exhaust gas purifying catalyst according to claim 1, which is provided in an exhaust passage of an internal combustion engine operated at a lean air-fuel ratio.