JPH08117603A - Aldehyde decomposition catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE: To provide an aldehyde decomposition catalyst for exhaust gas capable of decomposing aldehyde at a low temp. discharged from a combustion engine using an oxygen containing fuel such as methanol. CONSTITUTION: The catalyst decomposes aldehyde in unburned components discharged from the combustion engine using the oxygen containing fuel such as methanol and is obtained by incorporating a conjugate oxide expressed by a chemical formula, Ce1-x Gdx Nd1-y Agy O3-z [wherein, (x) is 0.1 or 0.2; (y) is 0.05 or 0.1; (z) is <=1.5] with palladium of a catalytic active component.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an exhaust gas purifying aldehyde decomposition catalyst, and more particularly to an exhaust gas purifying aldehyde decomposition catalyst capable of decomposing aldehydes discharged from a combustion engine using an oxygen-containing fuel such as methanol at a low temperature. .

[0002]

2. Description of the Related Art As a conventional aldehyde decomposition catalyst, an aldehyde decomposition for exhaust gas purification showing a high oxidation activity for aldehydes by arranging Cs at the A site of a perovskite type composite oxide containing silver and combining it with palladium. A catalyst has been proposed (JP-A-3-288549).

The catalyst described in the above-mentioned publication makes silver having excellent low temperature reactivity into a solid solution in the crystal structure of a perovskite type complex oxide excellent in high temperature durability and maintains the low temperature reactivity of silver. However, after imparting high-temperature stability, palladium is supported to improve reaction stability in a wide temperature range and atmosphere, and high oxidative activity is exhibited for aldehydes.

[0004]

However, although such a conventional aldehyde decomposition catalyst is excellent in low-temperature reactivity, it is possible to form a complex oxide crystal at high temperature in the presence of Co contained in combustion exhaust gas. It has been found that Cs incorporated in the structure is poisoned by Co and destroys the thermally stable perovskite crystal structure, resulting in silver crystal growth. Therefore, there is a problem that the activity is lowered when exposed to an atmosphere containing Co for a long time.

Therefore, the present invention has been made by paying attention to such a conventional problem, and silver excellent in low temperature activity is mixed with an inorganic composite oxide excellent in sulfur poisoning resistance and high temperature durability (eg, perovskite). Gd that does not cause Co poisoning when incorporated into the crystal of (type complex oxide) is added to the A site to impart sulfur poisoning resistance and high temperature stability while maintaining the low temperature reactivity of silver. The purpose is to solve the above problems.

[0006]

SUMMARY OF THE INVENTION The above objects of the present invention are as follows.
Among unburned components discharged from a combustion engine using an oxygen-containing fuel such as methanol, the catalyst is a catalyst for decomposing aldehyde at a low temperature and has a chemical formula: Ce _1-X Gd _X Nd _1-y Ag _y O _3-z (In the formula, x is 0.1 or 0.2, y is 0.05 or 0.1, and z is 1.5 or less). It was achieved by an aldehyde decomposition catalyst for exhaust gas purification, which is characterized by being combined with.

The present invention will be described in more detail below. First, explaining the configuration of the present invention, the aldehyde decomposition catalyst for exhaust gas purification of the present invention, on the surface of the monolith carrier,
A base metal composite oxide containing silver and a high specific surface area ceria and cerium-containing activated alumina having an O ₂ storage capacity are added to P.
The Pd-containing alumina carrying d is applied.

The silver-containing composite oxide used herein has the chemical formula: Ce _1-x Gd _x Nd _1-y Ag _y O _3-z (where x is 0.1 or 0.2 and y is 0). 0.05 or 0.1 and z is 1.5 or less).
This is a so-called perovskite complex oxide.

The above complex oxide is prepared by mixing hydrochlorides of respective metals in a predetermined stoichiometric ratio and adding them into a sodium octylate solution to synthesize an octylate compound, followed by drying at a low temperature and air flow. It is obtained by firing in.

Cerium oxide is known as an oxide having an O ₂ storage ability. Since the O ₂ storage capacity is proportional to the specific surface area of the oxide, an active oxide having a high specific surface area is used. As the alumina supporting Pd used at the same time, cerium-supporting activated alumina obtained by mixing and supporting a cerium salt solution on boehmite alumina (aluminum oxide monohydrate) and firing the mixture is used.

Next, a method for producing the aldehyde decomposition catalyst for purifying exhaust gas will be described. First, a predetermined amount is supported by immersing an aqueous solution of cerium nitrate or the like in boehmite alumina powder, dried and then calcined in an air stream at 600 ° C. for 2 hours to obtain cerium-supported activated alumina. Then nitric acid P
A d solution is used to impregnate and support a predetermined amount of Pd, which is dried and then calcined in an air stream at 400 ° C. for 2 hours to obtain Pd-supporting activated alumina.

Next, cerium, gadolinium, neodymium and silver hydrochloride were dispersed and mixed in pure water, and then added to a sodium octylate solution with stirring, and after sufficient reaction, dried at low temperature, Silver-containing composite oxide (Ce _1-x Gd _x Nd _1-y Ag) was obtained by firing at 800 ° C for 5 hours in an air stream.
_y O _3-z ) is obtained.

A slurry obtained by mixing the above silver-containing composite oxide, Pd-supporting cerium-containing activated alumina and high specific surface area ceria with nitric acid acidic alumina sol is applied to a monolith carrier and then dried, and the mixture is dried in a combustion gas atmosphere at 400 The catalyst is obtained by calcining at ℃. When octylic acid is used in synthesizing the perovskite composite oxide, it is possible to efficiently obtain a composite oxide having a large specific surface area even at a relatively low temperature of 800 ° C. 1100 to 1300 in the normal method
It can be synthesized only in the range of ° C. At such a high temperature, the specific surface area of the resulting complex oxide becomes small, and the purification performance becomes poor when used as a catalyst.

[0014]

[Function] Among the exhaust gas discharged from the combustion engine using the oxygen-containing fuel, the existing oxidization catalyst mainly composed of platinum, palladium, rhodium, etc. is utilized for the decomposition and removal of a large amount of aldehyde generated particularly at low temperature. There is.

However, the undecomposed component of the oxygen-containing fuel may cause a decomposition reaction at a temperature of 200 ° C. or lower under an oxidation catalyst such as platinum, and may rather form an aldehyde, so that the object can be sufficiently achieved. There is a problem that says no.
Up to now, there is a cost problem because it has to be dealt with by devising the combustion system side.

Therefore, the present invention makes it possible to use a silver catalyst, which is highly effective in decomposing aldehydes at low temperatures, in an exhaust gas atmosphere of an internal combustion engine such as an automobile, so that silver is a composite oxide having excellent high temperature stability. The purpose of the present invention is to provide an exhaust gas-purifying catalyst having high activity and high durability by incorporating it into the crystal structure of.

The silver-containing composite oxide has a so-called perovskite type structure having a basic structure of ABO ₃ and is therefore extremely stable crystallographically. B in this crystal structure
By forming a solid solution of silver in the site to form a stable silver oxide, it is possible to provide sulfur poisoning resistance without losing the low temperature activity of the silver catalyst.

In particular, for the purpose of the silver-catalyst interaction effect of the A-site ion, in addition to its own resistance to Co poisoning, gadolinium is dissolved in cerium to form an abnormal valence state together with neodymium of the B-site. As a result, oxygen defects are introduced, and as a result, a high degree of oxygen adsorption / desorption ability is provided, and the activity of the silver catalyst is stabilized.

Generally, as a result of the introduction of the oxygen defects, the sorbed oxygen, which is important for the oxidation activity, is increased. The sorbed oxygen is a) alpha oxygen: desorbed in a wide temperature range of 800 ° C. or lower and sorbed in oxygen vacancies generated by partial substitution of A site ions. b) Beta oxygen: 820
It is known that there are two kinds which are desorbed in the shape of a sharp peak in the vicinity of ° C and correspond to the reduction of the B site metal to a low valence. The presence of this type 2 N sorbed oxygen stabilizes silver.

A catalyst in which the surface of a monolith carrier is coated with the silver-containing composite oxide obtained as described above and the high specific surface area ceria-Pd-supporting cerium-containing activated alumina which itself has a strong O ₂ storage capacity. Thus, a catalyst having high activity and high durability for decomposing and purifying aldehyde in combustion exhaust gas can be obtained.

[0021]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto.

Example 1 A cerium nitrate aqueous solution was dipped in boehmite alumina powder having an alumina purity of 60% by weight or more to support 8.0% by weight of cerium metal on alumina.
After supporting, it was dried at 150 ° C. for 3 hours and then calcined in an air stream at 600 ° C. for 2 hours to obtain a cerium-supported activated alumina carrier. A Pd nitrate solution was used as the obtained activated alumina carrier, and 18.0% by weight as Pd metal based on alumina.
Carried. Then, it was dried at 150 ° C. for 3 hours and calcined in an air stream at 400 ° C. for 2 hours to obtain Pd-supporting cerium-containing activated alumina.

Next, 172.6 g of cerium chloride, 25.0 g of gadolinium chloride, 206.6 g of neodymium chloride and 4.65 g of silver chloride were dispersed and mixed in 1 L of pure water, and this mixture was prepared to a concentration of 3 mol / L. It was put into 3 L of sodium octylate [Na (C ₇ H ₁₅ COO)] solution with stirring. When the solution became colorless and transparent, the solution and the precipitate were separated by filtration, and the precipitate was dried at a low temperature (50 ° C. or lower) for about 15 hours to obtain 2 mol of oxide (about 660 g).

After drying, 800 ° C in an air stream
The silver-containing composite oxide was obtained by firing for 5 hours. Obtained silver included
When the complex oxide was measured by XRD, it was found that Ce_0.9
Gd _0.1Nd_0.95Ag_0.05O₃Perovskite type structure
It was showing structure. Also, when the lattice constant was calculated, it was 1
It was 0.979. This is Ce_0.5Nd_0.5O_1.75
Well agrees with the lattice constant 10.990 of the perovskite type
Therefore, it has an oxygen defect structure as originally designed.
confirmed.

As described above, 650 g of Pd-supported cerium-containing activated alumina and 588 g of silver-containing composite oxide powder,
617 g of high specific surface area ceria and 2135 g of nitric acid boehmite sol (sol obtained by adding 10% by weight nitric acid to 10% by weight suspension of boehmite alumina) were put into a magnetic ball mill pot and mixed and ground to obtain a slurry. It was coated on a carrier substrate (1.7 L, 400 cells). After coating, dry at 130 ℃ × 1 hour,
Catalyst 1 was obtained by calcining in a combustion gas stream at 400 ° C. for 2 hours. The catalyst coating amount (catalyst layer amount) obtained here was 2
It was set to 00 g / L.

Comparative Example 1 The silver-containing composite oxide added to the catalyst layer was neodymium and silver.
Made from NdAgO ₃(Nd_0.95Ag_0.05O₃)
A catalyst 2 was obtained in exactly the same manner as in Example 1 except that the above was used.

Example 2 The composition of the silver-containing composite oxide was Ce _0.8 Gd _0.2 Nd _0.95.
A catalyst 3 was obtained in exactly the same manner as in Example 1 except that Ag _0.05 O ₃ was used.

Example 3 The composition of the silver-containing composite oxide was Ce _0.9 Gd _0.1 Nd _0.9.
A catalyst 4 was obtained in the same manner as in Example 1 except that Ag _0.1 O ₃ was used.

Example 4 The composition of the silver-containing composite oxide was Ce _0.8 Gd _0.2 Nd _0.9 A.
A catalyst 5 was obtained in exactly the same manner as in Example 1 _{except that} g _0.1 O ₃ was used.

Comparative Example 2 According to the method described in JP-A-3-288549, 8.0 wt% of cerium metal was supported on an activated alumina carrier by immersing an aqueous cerium nitrate solution in the carrier, and
It was dried at 150 ° C for 3 hours and calcined in an air stream at 600 ° C for 2 hours to obtain a cerium-supported activated alumina carrier. The resulting alumina carrier was loaded with dinitrodiammine Pd nitric acid solution so that the Pd content would be 1.91% by weight.
After drying at 50 ° C. for 3 hours, it was calcined in an air stream at 400 ° C. for 2 hours to obtain Pd-supported activated alumina powder.

Next, a mixed powder of lanthanum carbonate, cesium carbonate, neodymium carbonate and silver carbonate is dispersed in pure water, and then the slurry obtained by heating and pressurizing is dried at a low temperature, and then in an air stream at 850 ° C. It was baked for 2 hours or more to obtain a silver-containing composite oxide.

The Pd-supported activated alumina 542 obtained above.
g, silver-containing composite oxide powder 588 g, high specific surface area ceria 735 g, and nitric acid boehmite sol 2135 g were charged into a magnetic ball mill pot, and mixed and ground to obtain a slurry, which was coated on a monolith carrier substrate. 130 ° C
After being dried for 1 hour at 400 ° C. for 2 hours in a combustion gas stream, a catalyst 6 was obtained. The coating amount of the catalyst obtained here was set to 270 g / piece.

Comparative Example 3 A silver-containing composite oxide was mixed with Ce _0.7 Gd _0.3 Nd _0.95 Ag.
_A catalyst 7 was prepared in the same manner as in Example 1 _{except that 0.05} O ₃ was used.
I got When the crystal structure was confirmed by XRD, Gd ₂ O
_Three single oxides were confirmed.

Comparative Example 4 A silver-containing composite oxide was mixed with Ce _0.9 Gd _0.1 Nd _0.85 Ag.
_A catalyst 8 was prepared in the same manner as in Example 1 _{except that 0.15} O ₃ was used.
I got When the crystal structure was confirmed by XRD, a single oxide of AgO was confirmed.

Comparative Example 5 A silver-containing composite oxide was mixed with Ce _0.95 Gd _0.05 Nd _0.95 Ag.
_A catalyst 9 was prepared in the same manner as in Example 1 _{except that 0.05} O ₃ was used.
I got When the crystal structure was confirmed, a single oxide of CeO ₂ was confirmed.

Comparative Example 6 A silver-containing composite oxide was mixed with Ce _0.9 Gd _0.1 Nd _0.97 Ag.
Catalyst 1 was prepared in the same manner as in Example 1 _{except that 0.03} O ₃ was used.
I got 0.

Comparative Example 7 25% ammonia water was added to a mixed aqueous solution of cerium chloride, gadolinium chloride, neodymium chloride and silver chloride to adjust the pH.
Adjusted to 7.8. After stirring this solution well, the precipitate was filtered off and the obtained precipitate was dried at 120 ° C. for 6 hours and then calcined at 850 ° C. for 5 hours to obtain a silver-containing composite oxide. The crystal structure of the silver-containing composite oxide obtained here is XR
As confirmed by D, oxides of each metal and part A
It was confirmed to be a mixture of BO ₃ type complex oxides.
A catalyst 11 was obtained in exactly the same manner as in Example 1 except that this composite oxide powder was used.

Test Example The catalysts obtained in Examples 1 to 4 and Comparative Examples 1 to 7 were subjected to durability (laboratory heat, sulfur, Co atmosphere durability) under the following conditions, and the formaldehyde decomposition purification rate was measured by model gas evaluation. The results are shown in FIG. 1 as the aldehyde purification rate temperature characteristic results.

[0039]

Performance Evaluation Conditions Device: Atmospheric pressure flow reactor Evaluation gas composition: Formaldehyde 100 ppm C ₂ H ₄ 300 ppm CO 1.0% Air balance SV: 30,000 Hr ⁻¹ Evaluation temperature: 25 to 250 ° C. 10 ° C./min Temperature rising

[0041]

Industrial Applicability As described above, the aldehyde decomposition catalyst for purifying exhaust gas of the present invention has excellent low-temperature oxidation activity, and has high heat resistance, sulfur poisoning resistance and CO poisoning resistance. Since the silver-containing composite oxide and the specific surface area ceria having a high storage capacity of Pd and O ₂ supported on activated alumina to which cerium is supported to impart heat resistance are allowed to coexist, in an atmosphere containing sulfur and CO However, the durability is sufficiently maintained, and the aldehyde contained in the exhaust gas can be efficiently purified at a low temperature. Therefore, according to the aldehyde decomposition catalyst for exhaust gas purification of the present invention, it becomes possible to adapt to a vehicle system or the like using an oxygen-containing fuel,
It can be expected that the air pollution prevention effect can be improved by the early commercialization of the system.

[Brief description of drawings]

FIG. 1 is a graph showing the results of aldehyde purification rate temperature characteristics.

Claims (2)

[Claims] 1. A catalyst for decomposing an aldehyde among unburned components discharged from a combustion engine using an oxygen-containing fuel such as methanol at a low temperature, represented by a chemical formula: Ce _1-X Gd _X Nd _1-y Ag. a composite oxide represented by _y O _3-z (wherein x is 0.1 or 0.2, y is 0.05 or 0.1, and z is 1.5 or less) An aldehyde decomposition catalyst for purifying exhaust gas, which is characterized by being combined with palladium which is a catalytically active component. 2. The aldehyde decomposition catalyst for exhaust gas purification according to claim 1, wherein octylic acid is used in the production of the composite oxide.