JPH07323228A - Phosphoric acid based composition and catalyst for reductive destructing nox using the same - Google Patents

Abstract

PURPOSE:To enable to stably purify an exhaust gas from a future diesel engine by providing high NOx reduction and destruction action in a high oxygen concn. atmosphere and improving heat resistance. CONSTITUTION:A phosphoric acid based composition is composed of at least copper(Cu), zirconium(Zr), phosphorus(P) and oxygen(O) as the structural components, satisfies 0.70<=y/z<=1.0 and 0.08<=x/(x+y+z)<=0.15 when the atomic ratio of copper(Cu), zirconium(Zr) and phosphorus(P) is x:y:z and is used for the catalyst for reductive destruction NOx.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel phosphoric acid composition, and more particularly to a composition suitable for a catalyst for NOx reduction decomposition and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, environmental pollution has become an important issue, and among them, there is an urgent need to develop a method for decomposing and removing harmful substances such as NOx and COx in exhaust gas of automobiles. Thus, as a conventional exhaust gas purifying catalyst for automobiles, a three-way catalyst that simultaneously oxidizes carbon monoxide (COx) and hydrocarbons (CxHy) and reduces nitrogen oxides (NOx) is widely used. Has been done. As such a catalyst, γ-Al ₂ O of a noble metal such as Pd, Pt, Rh is used.
A mixture with ₃ is used, and this catalyst is supported on a refractory carrier such as cordierite.

[0003]

However, such a catalyst can efficiently purify the exhaust gas only at an oxygen concentration of about 1%, and the oxygen concentration of the exhaust gas is higher than the above optimum value. There is a problem that it cannot work effectively when it gets higher. Therefore, it is usual to measure the oxygen concentration in the exhaust gas and control it so that it always has the optimum air-fuel ratio.However, it is almost impossible to purify exhaust gas with a higher oxygen concentration than this. Is.

On the other hand, in a gasoline engine, research and development of a lean burn system has been carried out in order to reduce fuel consumption. In this case, the oxygen concentration in the exhaust gas becomes several%,
There is a drawback that exhaust gas cannot be purified due to oxygen poisoning of precious metals. Even in the diesel engine, the exhaust gas is not purified at all under the current combustion system because the oxygen concentration in the exhaust gas is high.

Further, in a fixed combustion apparatus such as a factory, NOx in exhaust gas having a high oxygen concentration is reduced by mixing a reducing agent such as ammonia into the exhaust gas. There was a problem in terms of safety in mounting the vehicle on a mobile combustion device such as an automobile.

[0006] Therefore, the present applicant has previously proposed that Cu, Zr, P
It has been proposed that a phosphoric acid-based composition composed of (phosphorus) and O (oxygen) can reductively decompose NOx even under a relatively high oxygen concentration and can be used as a catalyst. It is considered to be the heat resistant temperature of the catalyst 10
It has been found that there is a problem that the catalytic activity is significantly reduced after being kept at around 00 ° C for a long time.

[0007]

[Means for Solving the Problems] The inventors of the present invention have studied the above phosphoric acid-based composition in detail in terms of NOx reductive decomposability in high-concentration oxygen.
In CuZr ₄ (PO ₄ ) ₆ composition, which is one of the phosphoric acid-based compositions, by making the Zr content more than the stoichiometric composition, it has a high NOx reducing property even at an oxygen concentration of about 5% and a high heat resistance. The present invention has been accomplished by finding that the deterioration of catalytic performance after treatment at high temperature is small.

That is, the phosphoric acid composition of the present invention is a phosphoric acid composition containing at least copper (Cu), zirconium (Zr), phosphorus (P) and oxygen (O) as constituent elements. When the atomic ratio of (Cu), zirconium (Zr) and phosphorus (P) is x: y: z, 0.70 ≦ y
/ Z ≦ 1, 0.08 ≦ x / (x + y + z) ≦ 0.16, wherein the composition further comprises CuZr ₄ (PO) at 700 to 1000 ° C.
₄ ) It is characterized in that the _6th phase is the main crystal phase, and according to the present invention, the phosphoric acid-based composition is used as a catalyst for NOx reduction decomposition.

The present invention will be described in detail below. The phosphoric acid-based composition according to the present invention contains Cu, Zr, P (phosphorus) and O (oxygen) as constituent elements, and the atomic ratio (y / z) of Zr to P is 0.70 ≦ y /. z ≦ 1, particularly 0.70 ≦ y / z ≦ 0.85, and Cu, Zr
And atomic ratio of Cu to all atoms of P (x / x + y +
z) is preferably 0.08 ≦ x / (x + y + z) ≦ 0.16, and more preferably 0.08 ≦ x / (x + y + z) ≦ 0.15.

In the composition of the present invention, the composition of each element is limited to the above range by the y / z ratio, x / x + y + z.
This is because in any composition where the ratio deviates from the above-mentioned composition range, a crystal phase other than the above-mentioned crystal phase is precipitated and the catalytic action is lowered.

When the phosphoric acid-based composition of the present invention is used as a catalyst, it is maintained at a temperature of 800 to 1000 ° C., but the composition of the present invention changes its crystal even in this temperature range. Without CuZr
₄ (PO ₄ ) ₆ crystal phase exists as a main phase, but in addition to this main crystal phase, a trace amount of zirconium phosphate or zirconium phosphate or an amorphous phase is included as long as the NOx conversion rate under a high oxygen concentration is not reduced. May be included.

Next, a sol-gel method will be described as an example for producing the phosphoric acid composition. First, a water-soluble compound containing Cu and a water-soluble compound containing Zr are prepared as raw materials, respectively, and a mixed solution of a Cu-containing aqueous solution and a Zr-containing aqueous solution is prepared first. on the other hand,
A water-soluble P (phosphorus) -containing compound solution is prepared, and this is added dropwise to the above aqueous solution containing Cu and Zr. At this time, Cu, Zr, and P are prepared so as to have the above composition ratio.

Next, the pH of the mixed solution obtained as described above is adjusted to a range of 6.0 ± 0.5. Ammonia water or urea is used for adjustment. By adjusting the pH in this way, the mixed solution becomes a sol. After filtering this, the residue is 650 to 1050 ° C,
In particular, it can be obtained by heat treatment at a temperature of 700 to 1000 ° C. for 10 hours or more.

This is because in the above production method, if the pH value deviates from the above range, it becomes difficult to control the Zr / Cu / P molar ratio molar ratio accurately. The heat treatment temperature is 6
If the temperature is lower than 50 ° C., the crystallization is not sufficiently performed, so that the crystal phase having the X-ray diffraction peak does not precipitate, and 1050
This is because when the temperature exceeds ° C, other crystal phases are precipitated and the catalytic properties are deteriorated.

NO using the composition obtained according to the invention
x When a catalyst for reductive decomposition is produced, the synthetic powder obtained as described above is appropriately crushed and then used as a catalyst, but the powder that is calcinated again at 1200 to 1600 ° C. and crushed is used. May be. The specific surface area of the powder as a catalyst is preferably controlled so that the BET value is 10 m ² / g or more.

[0016]

According to the present invention, in the phosphoric acid compound consisting of Cu, Zr, P (phosphorus) and O (oxygen), CuZ
By making Zr excessively present in the stoichiometric system represented by r ₄ (PO ₄ ) ₆ , the NOx conversion rate can be improved and at the same time, the heat resistance can be improved and stable characteristics at high temperature can be obtained. It can be maintained. It is considered that this is because the increase of Zr amount slows down the progress of crystallization and phase transition.

[0017]

【Example】

Example As starting materials, Cu (NO ₃ ) ₂ .3H ₂ O and ZrO
Using (NO ₃ ) ₂ · 2H ₂ O and NH ₄ H ₂ PO ₄ ,
Atomic ratio of Cu, Zr and P, y / x, x / x + y + z
Were weighed so that the ratios shown in Table 1 were obtained. First, Cu (NO ₃ ) ₂ .3H ₂ O and ZrO (NO ₃ ) ₂
・ Dissolve 2H ₂ O in pure water and stir it sufficiently. afterwards,
An NH ₄ H ₂ PO ₄ aqueous solution is gradually added dropwise to this solution while stirring, and then the pH is adjusted to 6 with aqueous ammonia and the mixture is stirred for 1 hour. Then, after suction filtration, washing with water, freeze-drying and hot-air drying, the mixture was baked at 700 ° C. for 80 hours in the air.

Next, the obtained sample was 3 ton / cm ^2.
After cold isostatic pressing, the powder was sized to powder of 40 mesh pass and 80 mesh on. Then, using this sample, N
200 ° C. under the conditions of O = C ₂ H ₄ = 1000 ppm, O ₂ = 5%, He = the rest, SV (space velocity) = 4800 / h
The reductive decomposition activity of NO was measured in the range from 1 to 500 ° C. using a gas chromatograph. The results are shown in Table 1.

Further, the obtained sample was heat-treated in air at 1000 ° C. for 16 hours, and the NOx conversion rate after the treatment was also measured. Further, X-ray diffraction measurement is performed on each sample to identify the crystal phase to be detected, and the sample is subjected to 1
After the heat treatment for 6 hours, the same measurement was performed to examine the change of the crystal phase.

[0020]

[Table 1]

According to Table 1, samples Nos. 1, 6, 7, in which the ratio of y / z and x / x + y + z deviates from the scope of the present invention.
Nos. 8, 9, 10, and 14 had low NOx conversion rates and large changes in characteristics after heat treatment. On the other hand, all the products of the present invention showed a high NOx conversion rate, and the deterioration of the characteristics after the heat treatment was small.

[0022]

As described in detail above, the phosphoric acid-based composition of the present invention has a high NOx reduction decomposition action in an atmosphere of high oxygen concentration and is excellent in heat resistance, and therefore, it is suitable for future diesel engines and the like. Exhaust gas can be stably purified.

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Claims (3)

[Claims] 1. At least copper (Cu), zirconium (Z
r), phosphorus (P) and oxygen (O) as constituent elements, wherein the atomic ratio of copper (Cu), zirconium (Zr) and phosphorus (P) is x: y: z. , 0.70 ≦ y / z ≦ 1, 0.08 ≦ x / (x + y
+ Z) ≦ 0.16 is satisfied, the phosphoric acid composition. 2. The main crystal phase at 700 to 1000 ° C. is C
The phosphoric acid composition according to claim 1, which has a uZr ₄ (PO ₄ ) ₆ phase. 3. At least copper (Cu), zirconium (Z
r), phosphorus (P) and oxygen (O) as constituent elements, wherein the atomic ratio of copper (Cu), zirconium (Zr) and phosphorus (P) is x: y: z. , 0.70 ≦ y / z ≦ 1, 0.08 ≦ x / (x + y
+ Z) ≦ 0.16, which is a phosphoric acid-based composition, and is a catalyst for NOx reduction decomposition.