JPH0699072A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE:To provide a catalyst for purification of exhaust gas capable of efficient removal of NOx in a relatively high temp. region in an excess oxygen- contg. atmosphere such as exhaust gas from a lean combustion engine. CONSTITUTION:This catalyst for purification of exhaust gas is made of a bimetallosilicate with at least one kind of noble metal selected among Pt, Rh, Pd, Ir and Ru and Al and/or Ga introduced into the crystal lattices. A transition metal such as Cu or Co may further be carried on the bimetallosilicate by ion exchange. This catalyst is used for removal of NOx from exhaust gas contg. oxygen in excess.

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 for purifying nitrogen oxides (NOx) in exhaust gas discharged from, for example, an internal combustion engine of an automobile, and more particularly to exhaust gas from a lean burn engine. The present invention relates to an exhaust gas purifying catalyst capable of efficiently purifying NOx in a relatively high temperature range in an excess oxygen atmosphere such as gas.

[0002]

2. Description of the Related Art Nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC), which are harmful substances in exhaust gas discharged from an internal combustion engine, have platinum, rhodium, palladium, etc. on an alumina carrier. It is known to remove by a three-way catalyst supported on. This is because engine control is performed so that the exhaust gas has a stoichiometric air-fuel ratio, and nitrogen oxides could not be removed when the exhaust gas had an oxidizing atmosphere, such as in a diesel engine.

In recent gasoline engines,
Lean combustion has become necessary for the purpose of reducing fuel consumption and reducing carbon dioxide emissions, but since the exhaust gas of this lean burning gasoline engine is an oxygen excess atmosphere, the conventional three-way catalyst described above It wasn't valid.

Under such circumstances, various catalysts for purifying exhaust gas of automobiles have been proposed which simultaneously oxidize carbon monoxide (CD) and hydrocarbons (HC) and reduce nitrogen oxides (NOx). . As such a catalyst, for example, a noble metal ion-exchanged zeolite catalyst (see, for example, JP-A-1-135541) is known as a NOx purification catalyst in an oxygen excess atmosphere. This is Pt, R for ZSM-5 and other zeolites.
It is prepared by ion-exchange of noble metals such as h, Pd, Ir, and Ru.
It has the feature of high NOx removal capacity.

[0005]

However, the above-mentioned noble metal ion-exchanged zeolite catalyst has a problem that its NOx purification action temperature is on a considerably low temperature side. This is because the noble metal supported on the zeolite is present near the catalyst surface, and its oxidation activity is too high, and hydrocarbons (HC) are oxidized and removed at about 200 ° C. This is because NOx cannot be removed at this temperature.

In order to solve such a problem, the present inventors previously evacuated an oxygen-rich exhaust gas composed of a noble metal silicate in which a noble metal such as platinum, rhodium, palladium, iridium and ruthenium was incorporated in the crystal lattice of the zeolite. We have developed an exhaust gas purification catalyst that removes nitrogen oxides from gas in a relatively high temperature range (see Japanese Patent Application No. 4-214334, filed on August 11, 1992). However, although this catalyst has purifying activity in a relatively high temperature range, it does not adsorb hydrocarbons due to the absence of solid acid sites, and therefore the hydrocarbons present in the exhaust gas are not utilized effectively. , NO
x The purification rate did not necessarily improve to a practically satisfactory value.

Therefore, the present invention provides a catalyst for purifying exhaust gas which can purify NOx more efficiently in a relatively high temperature range in an oxygen excess atmosphere such as exhaust gas from a lean burn engine. To aim.

[0008]

According to the present invention, platinum,
Removal of nitrogen oxides from oxygen-excess exhaust gas consisting of bimetallosilicate in which at least one noble metal selected from rhodium, palladium, iridium and ruthenium and aluminum and / or gallium are incorporated in the crystal lattice. An exhaust gas purification catalyst is provided.

According to the present invention, nitrogen oxide is contained in exhaust gas in excess of oxygen, which is obtained by further carrying out ion exchange loading of a transition metal on a bimetallosilicate in which the noble metal and aluminum and / or gallium are incorporated in a crystal lattice. There is provided an exhaust gas purifying catalyst for removing.

As described above, the exhaust gas purifying catalyst according to the present invention has platinum (Pt), rhodium (Ph), palladium (Pd), iridium (Ir) or ruthenium (Ru) in the silicate crystal lattice. Precious metals and aluminum (A
l) and / or gallium (Ga). The noble metal content in this noble metal-containing bimetallosilicate is Si
The molar ratio of / noble metal is preferably 50 to 600, more preferably 70 to 200. If this ratio is less than 50, the amount of the noble metal is too large to synthesize the bimetallosilicate, and the noble metal is present on the surface, so that the above problems cannot be solved. Also, the above
If the Si / noble metal molar ratio exceeds 600, the amount of noble metal is too small to obtain sufficient catalytic activity.

The aluminum (Al) and / or gallium (Ga) metal incorporated in the silicate crystal lattice of the exhaust gas purifying catalyst according to the present invention has a Si / metal molar ratio of 15 to 200.
Is preferred, and 20-100 is more preferred. If this ratio is less than 15, the synthesis of bimetallosilicate becomes difficult and
If it exceeds, the expression of solid acid points is reduced and the desired effect cannot be sufficiently obtained. In addition, Si / (noble metal + Al and / or Ga) cannot be successfully synthesized unless it is 10 or more.

According to another aspect of the present invention, an exhaust gas purifying catalyst for removing nitrogen oxides from an oxygen-excessive exhaust gas is provided with a noble metal and Al and / or Ga incorporated in a silicate crystal lattice. Copper (Cu) in metallosilicate,
It is formed by ion-exchange supporting transition metals such as cobalt (Co), iron (Fe), and nickel (Ni). The ion exchange rate of the transition metal is preferably 40 to 160%, more preferably 80 to 120%. For example, in the case of copper, Cu / Al ₂ = 1.0 and 100% is preferable. By this ion exchange of transition metals, NOx decomposing activity at a high temperature of 400 ° C or less is remarkably improved.
Compared to Cu ion exchange supported zeolite (ZSM-5) catalyst 2
It also improves NOx decomposing activity at around 50-350 ℃. If the ion exchange rate is too low, these effects cannot be obtained sufficiently, and conversely, if too high, the performance deteriorates.

A purification method using an exhaust gas purifying catalyst according to the present invention is a catalyst and exhaust gas containing NOx, CO and HC, particularly oxygen excess exhaust gas (that is, air exhausted from an internal combustion engine such as an automobile). This can be performed by bringing the exhaust gas in a state of a large fuel ratio (so-called lean region) into contact with each other by a usual method. In the purification method according to the present invention, the space velocity (SV) of introducing the exhaust gas into the catalyst layer is not particularly limited, but a range of 30,000 to 400,000 / hour is desirable for maintaining the activity, for example.

[0014]

According to the present invention, in order to suppress the oxidation activity of the noble metal-supported zeolite catalyst, which is a problem of the prior art, a noble metal and Al and / or Ga are preliminarily mixed at the time of synthesis, By using the noble metal-containing bimetallosilicate incorporated in the silicate crystal lattice, the oxidation activity of the noble metal is suppressed, and the solid acid points are expressed by the presence of Al and / or Ga, so that the hydrocarbons in the exhaust gas are effective. It was possible to achieve a NOx purification rate that is practically well tolerated in a relatively high temperature range by solving the above-mentioned problems. According to another aspect of the present invention, a noble metal-containing bimetallosilicate in which the above noble metal and Al and / or Ga are incorporated into a crystal lattice is further provided with Cu, Co.
By making the transition metal such as ion-exchange supported,
The NOx decomposing activity at 400 ℃ or higher is improved, and the activity at 250 ℃ to 350 ℃ is also improved compared with the conventional Cu ion exchange supported zeolite (ZSM-5) catalyst.

[0015]

EXAMPLES The present invention will be further described below with reference to examples, but it goes without saying that the scope of the present invention is not limited to these examples.

Example 1 Synthetic noble metal salts of metallosilicates include H ₂ PtCl ₄ , Pt (NH ₃ ) ₄ Cl ₂ , Rh (NO ₃ ) ₃ ,
Pd (NO ₃ ) _{2 and the} like were used, and Al (NO ₃ ) ₃ and Ga (NO ₃ ) ₃ were used as the aluminum and gallium salts. As a typical example, Pt, Al- bimetallosilicate (Si / Al molar ratio = 2
0, Si / Pt molar ratio = 600) and various bimetallosilicates were synthesized by changing the kind and amount of noble metal salt.

Table 1 (Si / Al molar ratio = 20, Si / Pt molar ratio = 600) ───────────────────────────── ─ Liquid reagent Reagent for gel Supernatant ───────────────────────────── A H ₂ O 60ml 60ml H ₂ PtCl ₄ 3.062ml 3.062 ml NaAlO ₂ 1.656g 1.656g H ₂ SO ₄ 6.2g 6.2g Al (NO ₃ ) ₃ 3.2g 3.2g TPAB ^{* 1} 5.75g 7.53g NaCl 11.95g − ─────────────── ──────────────── B H ₂ O 45ml 45ml Water glass 69g 69g ──────────────────────── ────── CH ₂ O 208ml 104ml NaCl 40.59g 26.27g H ₂ SO ₄ 2.85g − TPAB ^{* 1} 2.16g − NaOH 2.39g − ───────────────── ────────────── * 1: Tetrapropylammonium bromide

The solutions A, B and C for gel in Table 1 were mixed with stirring, and after the respective solutions were dissolved,
While stirring Solution C with a homogenizer, H ₂ SO ₄ or NaOH
Was added dropwise to adjust the pH of the solution to 10.0 ± 0.5. Then this C
Solution A and solution B were simultaneously added dropwise to the solution while maintaining pH = 10 ± 0.5, and after completion of all dropping and mixing, a gel-like product was obtained. This was placed in a centrifuge (3,500 rpm / min) for 15 minutes, and the supernatant was discarded. This operation is repeated 3 times, and the obtained gel is designated as gel D.

On the other hand, regarding the supernatant solution in Table 1,
In exactly the same manner as above, mixing the solutions A, B and C under the same pH conditions, and the resulting gel product was centrifuged.
(3,500 rpm / min), and the solution portion is designated as supernatant E.

Next, the gel D obtained above was lump-cast for 15 minutes in an alumina mortar to remove water. This operation is repeated 3 times, the supernatant E is added, and this is designated as solution F. This F
The solution was put into an autoclave and hydrothermal synthesis was performed with stirring. The temperature pattern is 1 ℃ / min up to 160 ℃,
The sound was raised to 210 ° C at 0.16 ° C / min.

The product synthesized as above was washed with water, dried at a temperature of 100 ° C., and calcined at 540 ° C. for 3.5 hours,
The template is removed, and then ion exchange is performed using a 1N-NH ₄ NO ₃ aqueous solution, followed by firing at 540 ° C. for 3 hours to obtain an H type.

Noble metal silicate catalysts shown in Table 2 were synthesized in the same manner as above. Furthermore, as a comparative catalyst, H-
ZSM-5 (Si / Al = 20) zeolite was also prepared as a catalyst with a noble metal supported by ion exchange.

Table 2 ────────────────────────────────── Catalyst metal Si / metal (molar ratio) Noble metal content (Wt%) Noble metal / Al or Ga Noble metal / Al or Ga ─────────────────────────────────── 1 Pt / Al 40/20 7.5 2 Pt / Al 100/20 3.0 3 Pt / Al 600/20 0.5 4 Pt / Al 1000/20 0.3 5 Pt / Al 100/10 3.0 6 Pt / Al 100/40 3.0 7 Pt / Al 100/100 3.0 8 Pt / Al 100/400 3.0 9 Pt / Ga 100/40 3.0 10 Pd / Al 100/40 1.58 11 Rh / Al 100/40 1.64 12 Pd / Ga 100/40 1.58 13 Rh / Ga 100 / 40 1.64 ────────────────────────────────── A-1 − / Al − / 20 Pt (1.67) A -2 − / Al − / 20 Rh (1.67) A-3 − / Al − / 20 Pd (1.67) A-4 Pt / − − / 100 3.0 ──────────────── ─────────────────── B-1 Pt / − Al ₂ O ₃ 1.67 ──────────────────────────────────

The NOx purification characteristics of the above catalyst were evaluated using a model gas simulating automobile exhaust gas. The composition of the gas used was NOx (1,000 ppm), C ₃ H ₆ (1,000 ppm), O ₂ (4.0%) and N ₂ (the balance) at a space velocity (SV) of 150,000 / hr. The results are shown in FIGS.

First, as shown in FIG. 1, when Pt-containing bimetallosilicate is compared with Pt / ZSM-5,
The Ox purification activity is high, and the NOx purification activity at low temperatures is slightly improved compared to Pt-silicate. Also, from the results of FIGS. 2 and 3, it can be seen that even in the case of the Rh- and Pd-containing bimetallosilicate, it exhibits an activity close to that of the ion-exchange ZSM-5 at low temperatures and higher activity at 300 ° C. or higher. .
Furthermore, from the results of FIG. 4, the Pt content is Si / Pt = 40 (catalyst
NO.1) shows high activity equivalent to ion-exchange ZSM-5, and Si / Pt = 1000 (catalyst NO.4) shows low activity in each temperature range when Pt content is low. It turns out to be low.

Further, from the results of FIGS. 1 to 5, as for the Al content, Si / Al = 400 (catalyst NO.8) having a small Al content causes almost no solid acid point to appear, so that Pt-silicate is obtained. Almost the same. On the other hand, the area with much Al (Si / Al =
In 10), according to the identification by XRD, it does not have a silicate structure, so it is not mentioned here.

Example 2 After hydrothermal synthesis, a catalyst in which Cu or Co was ion-exchanged was prepared by using a noble metal-containing bimetallosilicate in H form with a copper acetate or cobalt acetate solution.

For example, a method for preparing a catalyst in which copper is ion-exchanged for H-Pt (600) -Al (20) -silicate will be described below. First, a 0.2 mol / l copper acetate aqueous solution is prepared, and aqueous ammonia is added dropwise thereto to adjust the pH of the solution to 11.
H-Pt (600) -Al (20) -silicate powder was mixed there, and 1
The mixture was stirred for 5 hours and washed and filtered 2-3 times. 110 this
Dry in air at 5 ° C for 5 hours, then at 500 ° C in air,
Bake for 2 hours. At this time, the ion exchange rate can be adjusted according to the charged amount.

Catalysts having the compositions shown in Table 3 were prepared in the same manner, and the NOx purification characteristics of these catalysts were evaluated in the same manner as in Example 1. The results are shown in FIGS. 6 and 7.

Table 3 ─────────────────────────────────── Catalyst metal Si / metal (molar ratio) Noble metal Content Ion exchange metal NO. Noble metal / Al or Ga Noble metal / Al or Ga (wt%) and exchange rate ──────────────────────────── ──────── 14 Pt / Al 100/20 3.0 Cu (94) 15 Pt / Al 3.0 Co (92) 16 Pt / Al 600/20 0.5 Cu (104) 17 Pt / Al 600/20 0.5 Co (100) 18 Pt / Ga 100/40 3.0 Cu (105) 19 Pt / Al 600/20 0.5 Cu (35) 20 Pt / Al 600/20 0.5 Cu (64) 21 Pt / Al 600/20 0.5 Cu (82 ) 22 Pt / Al 600/20 0.5 Cu (125) 23 Pt / Al 600/20 0.5 Cu (166) ────────────────────────── ──────────

As is clear from the results shown in FIGS. 6 and 7, C
350 to 50 by ion-exchange supporting u or Co
Compared to the catalyst that does not support Cu or Co at high temperature of 0 ℃
The NOx purification rate is significantly improved.

[0032]

As described above, when the catalyst of the present invention is used, in an excess oxygen atmosphere such as exhaust gas from a lean burn engine, the temperature is 250 ° C. or higher, particularly 250 to 400 ° C.
NOx can be efficiently purified in this temperature range. This is due to the inclusion of precious metals in the silicate lattice,
By intentionally reducing the oxidizing power, suppressing the oxidation of hydrocarbons and effectively using it for NOx purification.In addition to the effect found in this noble metal silicate, Al or Ga is mixed at the time of synthesis to simultaneously crystallize. By incorporating it into the catalyst, solid acid sites are expressed in the catalyst, the amount of hydrocarbon adsorbed is increased, and the decomposition efficiency of NOx is further enhanced.

Further, as in another catalyst of the present invention, the noble metal-containing bimetallosilicate is ion-exchanged with a transition metal such as Cu or Co to carry out high temperature (400 ° C.)
In the above), the NOx decomposing activity is remarkably improved, and in comparison with the conventional Cu / ZSM-5 catalyst, by incorporating the noble metal in the lattice, the activity at 250 to 350 ° C is further improved.

[Brief description of drawings]

FIG. 1 is a graph showing the evaluation results showing the temperature characteristics of NOx purification activity of the Pt-containing catalysts of the present invention and comparison.

FIG. 2 is a graph showing the evaluation results showing the temperature characteristics of NOx purification activity of the Rh-containing catalyst according to the present invention.

FIG. 3 is a graph showing the evaluation results showing the temperature characteristics of NOx purification activity of the Pd-containing catalysts of the present invention and comparison.

FIG. 4 is a graph showing the evaluation results showing the influence of Pt content and temperature on the NOx purification activity of the Pt-containing catalyst according to the present invention.

FIG. 5 is a graph showing the evaluation results showing the influence of Al content and temperature on the NOx purification activity of the Pt-containing catalyst according to the present invention.

FIG. 6 is a graph showing the evaluation results showing the temperature characteristics of NOx purification activity of the Co or Cu ion exchange catalyst according to the present invention.

FIG. 7 is a graph showing the evaluation results showing the influence of the Cu ion exchange rate on the NOx purification activity of the Cu ion exchange catalyst according to the present invention.

Claims (2)

[Claims] 1. An oxygen-excess exhaust gas comprising a bimetallosilicate in which at least one noble metal selected from platinum, rhodium, palladium, iridium and ruthenium and aluminum and / or gallium are incorporated in a crystal lattice. An exhaust gas purifying catalyst that removes nitrogen oxides from exhaust gas. 2. The exhaust gas purifying catalyst according to claim 1, wherein a transition metal is carried on the bimetallosilicate by ion exchange.