JP2002113367A - Catalyst for saturated hydrocarbon oxidation - Google Patents

Abstract

[PROBLEMS] To further improve the activity of oxidizing saturated hydrocarbons in a low temperature range and to improve the durability at high temperatures. SOLUTION: At least one noble metal selected from Pd, Pt, Rh, Au and Ir, and Fe, Co, Ni, Sn, Mo, W, Zn,
At least one transition metal selected from V, Nb, Ta and Cr was supported on an oxide carrier. By carrying the composite of the noble metal and the transition metal, the intervening particles suppress the grain growth of each other, and it is considered that a part of the noble metal and the transition metal form a complex, and the thermal stability of the noble metal is improved. .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a catalyst for oxidizing and decomposing a saturated hydrocarbon. INDUSTRIAL APPLICABILITY The catalyst of the present invention is useful for purification of automobile exhaust gas, purification of factory exhaust gas, and the like, and can also be used for oxidative purification of methane, which is considered difficult to purify by oxidation.

[0002]

2. Description of the Related Art Three-way catalysts have been widely used as exhaust gas purifying catalysts for purifying automobile exhaust gas.
This three-way catalyst uses platinum (P) on a porous carrier such as alumina.
t) and other precious metals.
O, it is possible to efficiently purify HC and NO _x.

By the way, the noble metal supported on the three-way catalyst does not cause a catalytic reaction in a temperature range lower than its activation temperature. Therefore, the three-way catalyst does not function sufficiently in exhaust gas in a low temperature range such as when the engine is started, and there is a problem that a large amount of HC is emitted. At the time of cold start, the air-fuel ratio is often set to a fuel-rich atmosphere, and the HC in the exhaust gas
The large amount also contributes to the above problem.

[0004] Therefore, improvement of the low-temperature activity of the three-way catalyst has become an issue. For example, the catalyst has been arranged immediately below the engine. In this case, since the heat of the exhaust gas is quickly transmitted to the catalyst, the temperature can be quickly raised to the activation temperature. Precious metals are also supported at a high concentration at the upstream end of the exhaust gas stream of the catalyst. The upstream end rises to the activation temperature earlier than the downstream end, and the reaction is active at the upstream end where the activation temperature is raised. As a result, the activity of the catalyst as a whole in a low temperature range is improved.

[0005] Among HCs, olefinic hydrocarbons are relatively easy to purify, but saturated hydrocarbons are more difficult to purify than olefinic hydrocarbons, and methane is a saturated hydrocarbon that is particularly difficult to purify by oxidation. Therefore, the development of a catalyst capable of purifying methane has been promoted, and palladium (P
d) has been found to be effective. For example, JP-A-11-137998 discloses that Pd, Ru,
A catalyst that supports at least one selected from Ir and Cu and exhibits methane purification ability is disclosed. In addition, JP-A-7-05
Japanese Patent No. 3976 discloses a methane oxidation catalyst using a coprecipitate of Pd and Co as a catalyst metal.

[0006]

However, even if the above-mentioned means are used, purification of HC in exhaust gas in a low-temperature region is still insufficient, and further improvement in low-temperature activation is required.

For example, the catalyst disclosed in Japanese Patent Application Laid-Open No. 11-137998 has a problem that when a high-temperature durability test at 700 ° C. or more is performed, Pd grows in a large grain size and the activity is significantly deteriorated.

Further, the catalyst disclosed in Japanese Patent Application Laid-Open No. 7-053976 has a drawback that the particles in the coprecipitate of Pd and Co become coarse because no carrier is used, and the activity is particularly greatly reduced after the durability test. The publication also discloses that a binder may be added to the coprecipitate to form a slurry, which may be used after being applied to a honeycomb-shaped refractory substrate such as alumina, magnesia, or cordierite. However, also in this case, the co-precipitate of Pd and Co is on the surface of the substrate such as alumina, and is not supported, so that the interaction between the substrate and the co-precipitate is weak and coarse. The activity is significantly reduced after the durability test due to the formation of the resin.

The present invention has been made in view of such circumstances, and an object of the present invention is to further improve the activity of oxidizing saturated hydrocarbons in a low temperature range and improve the durability at high temperatures.

[0010]

The feature of the catalyst for oxidizing saturated hydrocarbons of the present invention that solves the above-mentioned problems is that an oxide carrier,
At least one noble metal selected from Pd, Pt, Rh, Au and Ir supported on an oxide carrier;
e, at least one transition metal selected from Co, Ni, Sn, Mo, W, Zn, V, Nb, Ta and Cr.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the catalyst of the present invention, a noble metal and a transition metal are supported on an oxide carrier. By supporting the noble metal and the transition metal in a composite manner, the activity of each other is improved and the high-temperature durability is improved because the growth of each other is suppressed. Further, it is considered that a part of the noble metal and the transition metal form a composite, and the thermal stability of the noble metal is improved.

Further, the transition metal has higher oxide stability than the noble metal, and releases the lattice oxygen in the transition metal oxide in a high temperature range where the noble metal oxide is decomposed. By virtue of the presence, active oxygen can be supplied onto the noble metal, the noble metal oxide as an active species can be stabilized, and the oxidizing activity of the hydrocarbon can be maintained.

As the oxide carrier, Al ₂ O ₃ , SiO ₂ , Zr
It can be selected from O ₂ , TiO ₂ , CeO ₂ , MgO, or a combination of a plurality of these oxides, but γ-Al ₂ O ₃ having a large specific surface area and excellent heat resistance is desirable. The specific surface area of the oxide carrier is desirably 40 m ² / g or more. If the specific surface area is smaller than this, the activity becomes low and oxidation of methane or the like becomes difficult.

Although a solid oxide powder may be used as the oxide carrier, it is preferable to use a hollow oxide powder as disclosed in JP-A-11-314035. When the hollow oxide powder is used as a carrier, gas diffusion into the carrier layer is improved, and the inside of the hollow layer can be used as a reaction field, so that the contact probability between the supported noble metal and transition metal and the gas is reduced. The activity is further improved.

The noble metal is at least one selected from Pd, Pt, Rh, Au and Ir, and one or more of them can be supported. Pd, which has a particularly high oxidation activity for saturated hydrocarbons, is most desirable. The transition metals are Fe, Co,
It is at least one selected from the group consisting of Ni, Sn, Mo, W, Zn, V, Nb, Ta, and Cr. Among them, one type or a plurality of types can be supported. Among them, at least one selected from Fe, Co, Ni and Sn has particularly high activity.

Although the activity of the noble metal is recognized if the amount of the noble metal supported is small, the noble metal is supported in an amount of 0.5 to 15%.
It is desirable to be in the range of weight%. If the amount of the noble metal carried is less than this range, the oxidation activity of the saturated hydrocarbon is too low to be practical, and if the amount is more than this range, the activity is saturated and the cost is disadvantageous.

The amount of the transition metal carried is desirably in the range of 0.1 to 20 times the atomic ratio of the noble metal. If the atomic ratio is less than 0.1 times the noble metal, the effect of supporting is not obtained and the activity is low, and if the atomic ratio is more than 20 times the noble metal, the active species will be coated with the noble metal and the activity will decrease. is there.

The loading of the noble metal and the transition metal on the oxide carrier can be carried out by using a solution of the noble metal compound and a solution of the transition metal compound in the same manner as the conventional method of loading a noble metal. For example, a co-impregnation loading method in which a mixture of both solutions is impregnated into an oxide carrier powder and supported, or a method in which one of a noble metal compound solution or a transition metal compound solution is impregnated in the oxide carrier powder, and then the other solution is impregnated. An impregnation carrying method or the like can be used. In some cases, it is also possible to use an adsorption-carrying method. Among them, it is desirable to use the co-impregnation carrying method. This is considered to increase the probability that the noble metal and the transition metal are supported in contact with each other, and a higher activity is exhibited as compared with the sequential impregnation-supporting method.

When a noble metal is supported, it is also preferable to use a method in which a solution of a compound containing a noble metal is brought into contact with hydrazine to precipitate the noble metal by a reduction reaction and to support the noble metal. According to this method, the noble metal can be supported as finer particles, and the high-temperature durability is further improved. When this method is used, an oxide carrier is mixed in a noble metal compound solution, hydrazine is added thereto, and the mixture is stirred, whereby the noble metal can be supported on the oxide carrier by a reduction reaction. The transition metal may be supported thereafter. Alternatively, an oxide carrier preliminarily supporting a transition metal may be mixed in a noble metal compound solution, and hydrazine may be added thereto to support the noble metal. Hydrazine may be added after the oxide carrier is added to the mixed solution of the noble metal compound and the transition metal compound.

The reducing agent is not limited to hydrazine, and in addition to hydrazine, sodium borohydride, lithium aluminum hydrate, aldehydes such as formaldehyde and acetaldehyde, reducing sugars (aldose) such as fructose and maltose, and ethanol. Can be used.

The noble metal supported as described above has a thickness of 10 nm.
Supported on the oxide carrier in a fine state of less than. Since the transition metal is supported in close proximity, the grain growth of each other is suppressed. For example, even after a heat resistance test in which heating is performed at 800 ° C. for 5 hours, the thickness is only about 20 nm, and high activity is exhibited even after high-temperature durability.

[0022]

The present invention will be specifically described below with reference to examples and comparative examples.

(Example 1) Pd (NO ₃ ) ₃ aqueous solution and Fe (NO ₃ ) _3.
An aqueous solution was prepared by mixing with 9H ₂ O aqueous solution, and γ-Al ₂
After O ₃ powder was added and sufficiently stirred, the mixture was evaporated to dryness and calcined at 300 ° C. for 2 hours in the atmosphere to prepare a catalyst powder.
In this catalyst powder, Pd is 5 g for 120 g of alumina.
0.24 mol of Fe is supported.

The obtained catalyst powder was compacted and pulverized to prepare a pellet catalyst having a particle size of 0.5 to 1 mm.

Example 2 In the same manner as in Example 1 except that an aqueous solution of Co (NO ₃ ) ₃ .6H ₂ O was used in place of the aqueous solution of Fe (NO ₃ ) ₃ .9H ₂ O, alumina 120 g was used. On the other hand, Pd is 5g and Co is 0.
A 24 mole supported pellet catalyst was prepared.

[0026] (Example ₃₎ Fe (NO 3) in place of ₃ · 9H ₂ O aqueous Ni (NO ₃₎ except for the use of ₃ · 6H ₂ O aqueous solution in the same manner as in Example 1, the alumina 120g On the other hand, Pd is 5g and Ni is
A pellet catalyst supported by 0.001 mol was prepared.

[0027] (Example 4) Fe (NO ₃₎ in place of ₃ · 9H ₂ O aqueous Ni (NO ₃₎ except for the use of ₃ · 6H ₂ O aqueous solution in the same manner as in Example 1, the alumina 120g On the other hand, Pd is 5g and Ni is
A pellet catalyst loaded with 0.005 mol was prepared.

[0028] (Example 5) Fe (NO ₃₎ in place of ₃ · 9H ₂ O aqueous Ni (NO ₃₎ except for the use of ₃ · 6H ₂ O aqueous solution in the same manner as in Example 1, the alumina 120g On the other hand, Pd is 5g and Ni is 0.
A 01 mol supported pellet catalyst was prepared.

[0029] Except (Example _{6) Fe (NO 3) 3} · 9H 2 in place of the O solution Ni (NO ₃₎ that was used ₃ · 6H ₂ O aqueous solution in the same manner as in Example 1, the alumina 120g On the other hand, Pd is 5g and Ni is 0.
A 05 mol supported pellet catalyst was prepared.

[0030] (Example 7) Fe (NO ₃₎ in place of ₃ · 9H ₂ O aqueous Ni (NO ₃₎ except for the use of ₃ · 6H ₂ O aqueous solution in the same manner as in Example 1, the alumina 120g On the other hand, Pd is 5g and Ni is 0.
A 24 mole supported pellet catalyst was prepared.

[0031] (Example _{8) Ni (NO 3) 3} · 6H 2 O-aqueous gamma-A
After l ₂ O ₃ powder was added and sufficiently stirred, the mixture was evaporated to dryness and calcined in air at 300 ° C. for 2 hours. Next, the obtained powder was mixed with an aqueous solution of Pd (NO ₃ ) ₃ and sufficiently stirred, evaporated to dryness, and calcined in the air at 300 ° C. for 2 hours to prepare a catalyst powder. Thereafter, pelletization was performed in the same manner as in Example 1 to prepare a pellet catalyst in which 5 g of Pd and 0.24 mol of Ni were supported per 120 g of alumina.

Example 9 Commercially available aluminum nitrate nonahydrate was dissolved in deionized water to prepare 0.1 to 2 mol / mol.
L of aluminum nitrate aqueous solution and commercially available lanthanum nitrate 6
An aqueous lanthanum nitrate solution of 0.1 to 2 mol / L prepared by dissolving a hydrate in deionized water is mixed with a water phase by a predetermined amount so that the molar ratio of lanthanum to aluminum is La / Al = 0.05. did.

On the other hand, commercially available kerosene is used as an organic solvent, and a dispersant (“Sunsoft No. 818H” manufactured by Taiyo Kagaku Co., Ltd.) is used.
5) to 10% by weight with respect to kerosene to obtain an oil phase.

The water phase and the oil phase are defined as water phase / oil phase = 40-70 / 60-
The mixture was mixed so as to have a volume ratio of 30 (% by volume), and the mixture was stirred with a homogenizer at 1,000 to 20,000 rpm for 5 to 30 minutes to obtain a W / O emulsion. From the result of observation with an optical microscope, the diameter of the water droplet in the emulsion was about 1 to 2 μm.

The W / O emulsion prepared above was
Spray combustion was performed using the apparatus disclosed in Japanese Patent Application Laid-Open No. 11-314035 to burn the oil phase and to form oxide powder.
This synthesis was carried out with the spray flow rate of the emulsion and the amount of air (the amount of oxygen) controlled so that the sprayed emulsion completely burned and the flame temperature became a constant temperature of 650 to 750 ° C.

The obtained alumina powder has a hollow shape having a large number of internal spaces, and the thickness of the hollow shell is 10 nm. The primary particle size of this alumina powder was 500 nm, and the specific surface area was 55 m ² / g. The primary particle diameter was determined from the average value of the particle diameters of 50 particles measured from the powder SEM image.
The specific surface area was measured by the BET method.

[0037] The gamma-Al ₂ O ₃ powder instead be for the use of hollow alumina powder mentioned above, and _{Fe (NO 3) 3 · 9H} 2 in place of the O solution _{Ni (NO 3) 3 · 6H} 2 O Except that an aqueous solution was used, the same procedure as in Example 1 was repeated except that Pd was 5
A pellet catalyst supporting 0.24 mol of g and Ni was prepared.

Example 10 An aqueous solution of Pd (NO ₃ ) ₃ and Ni (NO ₃ ) _3.
6H ₂ O aqueous solution was mixed to prepare γ-Al ₂
O ₃ powder and a predetermined amount of an aqueous hydrazine solution were added, sufficiently stirred, evaporated to dryness, and calcined at 300 ° C. for 2 hours in the atmosphere to prepare a catalyst powder. In this catalyst powder, 5 g of Pd and 0.24 of Ni were added to 120 g of alumina as in Example 3.
Molar supported.

The obtained catalyst powder was compacted and pulverized to prepare a pellet catalyst having a particle size of 0.5 to 1 mm.

Comparative Example 1 The same procedure as in Example 1 was repeated except that an aqueous solution of Pd (NO ₃ ) ₃ was used without using an aqueous solution of Fe (NO ₃ ) ₃ .9H ₂ O. A pellet catalyst supporting 5 g of Pd was prepared.

<Test / Evaluation> The catalysts of Examples and Comparative Example 1 were respectively placed in an evaluation device, and a Lean steady model gas shown in Table 1 was allowed to flow from room temperature while flowing at a space velocity SV = 200,000 h ^-1. At a heating rate of 12 ° C / min to 550 ° C,
Meanwhile, the purification rate of methane was measured continuously. Then, the 50% purification temperature of methane was calculated, and the results are shown in Table 3.

For the catalysts of Examples and Comparative Example 1, the model gas shown in Table 2 was changed to Lean gas 2 minutes / Rich gas 2
An endurance test was performed in which the sample was held at 800 ° C. for 5 hours while alternately flowing under the conditions of space velocity SV = 10,000 h ⁻¹ . For each catalyst after the durability test, the methane purification rate was measured in the same manner as above, and the 50% purification temperature of methane was calculated. The results are shown in Table 3.

Further, for each of the catalysts after the durability test, the particle size of the supported Pd was measured by a CO pulse adsorption method. The results are shown in Table 3. The particle size of Pd before the durability test was 6 nm or less for each of the catalysts.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

From Table 3, it can be seen that the catalysts of the Examples had higher methane purifying activity both in the initial stage and after the endurance than Comparative Example 1, and the Pd particle size was less than 20 nm even after the endurance test. It is clear that this is the effect of complex loading of Pd and transition metal.

Further, comparison of the results of Examples 3 to 7 suggests that there is an optimum value for the amount of the transition metal supported, and the amount of the supported transition metal is 0.001 mol (Pd of Pd) as in the catalyst of Example 3. 0.02 times the number of moles) is not sufficient,
It can be seen that it is desirable to support 0.005 mol or more of Example 4 (0.1 times the number of mols of Pd).

Further, from the comparison of Examples 7 to 10, it can be seen that the co-impregnation loading method is more preferable than the sequential impregnation loading method, and the reduction loading method using hydrazine is particularly preferred. It is also apparent that the activity is improved by using hollow alumina as compared with the case of using γ-Al ₂ O ₃ .

[0050]

According to the catalyst for oxidizing saturated hydrocarbons of the present invention, the oxidizing activity of saturated hydrocarbons in a low temperature range is extremely high, and methane which has conventionally been difficult to oxidatively decompose can be decomposed and removed. Since the noble metal grain growth is suppressed even after the high temperature durability test, the durability is excellent.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Takao Tani 41-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Laboratory Co., Ltd. (72) Inventor Tadashi Suzuki, Nagakute-cho, Aichi-gun, Aichi Prefecture No. 41, Yokomichi 1 Toyota Central Research Laboratory Co., Ltd. F-term (reference) BA01B BC22A BC33A BC35A BC38A BC42B BC54A BC56A BC58A BC59A BC60A BC66A BC66B BC67A BC67B BC68A BC68B BC71A BC72A BC72B BC74A BC75A CA03 CA10 CA15 DA06 EA02Y EC02X EC03X EC04X EC05X

Claims (9)

[Claims] 1. An oxide carrier and an oxide carrier
At least one noble metal selected from Pd, Pt, Rh, Au and Ir, and Fe, Co, Ni, Sn, Mo,
A catalyst for oxidizing saturated hydrocarbons, comprising: at least one transition metal selected from W, Zn, V, Nb, Ta and Cr. 2. The method according to claim 1, wherein the transition metal is at least one selected from Fe, Co, Ni, and Sn.
The catalyst for oxidizing a saturated hydrocarbon according to the above. 3. The catalyst for oxidizing saturated hydrocarbons according to claim 1, wherein the transition metal is supported in an atomic ratio of 0.1 to 20 times the noble metal. 4. The catalyst according to claim 1, wherein the noble metal is 0.5 to 15% by weight based on the whole catalyst.
The catalyst for oxidizing a saturated hydrocarbon according to claim 1, wherein the catalyst is supported. 5. The catalyst for oxidizing a saturated hydrocarbon according to claim 1, wherein the noble metal and the transition metal are simultaneously supported using a solution containing the noble metal and the transition metal. 6. The catalyst for oxidizing a saturated hydrocarbon according to claim 1, wherein the noble metal is supported by bringing a solution of the compound containing the noble metal into contact with hydrazine. 7. The catalyst for oxidizing a saturated hydrocarbon according to claim 1, wherein the oxide carrier has a specific surface area of 40 m ² / g or more. 8. The catalyst for oxidizing a saturated hydrocarbon according to claim 1, wherein the oxide carrier has a hollow shape. 9. The catalyst for oxidizing saturated hydrocarbons according to claim 1, which is mainly used for methane oxidation.