JPH11169728A - Methane oxidation catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a methane oxidation catalyst highly durable and capable of efficiently oxidizing unburned methane even in the conditions that the temperature of a waste gas is low and steam coexists by improving the catalytic activity of a methane oxidation catalyst. SOLUTION: A ceramic material having 0.01 ml/g of voids of primary particles with not smaller than 100 Å and not larger than 1,000 Å, especially a zirconia containing a prescribed amount of yttria, is used as a catalyst support and a tetraamminepalladium hydroxide is used as a palladium source and palladium is highly densely deposited on the surface and in the voids of the primary particles of the catalyst support by an impregnat ion method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a methane oxidation catalyst for oxidizing unburned methane contained in exhaust gas of a lean-burn gas engine, and more particularly, to a lean-burn engine under an exhaust gas condition in which steam coexists. The present invention relates to a methane oxidation catalyst capable of efficiently oxidizing unburned methane discharged from a gas engine and maintaining high durability.

[0002]

2. Description of the Related Art A gas engine is one of engines used for cogeneration in which heat is generated by utilizing gas combustion and its exhaust heat is used for heat demand such as cooling, heating and hot water supply. . Various types of gas engines have been developed, and if classified according to exhaust gas treatment technology, they can be divided into a three-way catalyst method and a lean combustion method.

[0003] The three-way catalyst method is a method in which three components of NOx, CO and HC are simultaneously treated with the same catalyst in a narrow range where the excess air ratio is close to one. On the other hand, the lean burn method burns gas in excess air, lowers the flame temperature, and suppresses the amount of NOx generated. Unlike the three-way catalyst method, the lean burn method does not require stopping the engine for catalyst replacement, has advantages such as high engine thermal efficiency and low initial cost.

[0004] However, the lean burn method has a drawback that although it has low NOx, it has a large amount of unburned components such as methane (CH ₄ ) contained in exhaust gas. Unburned components such as methane (CH ₄ ) contained in exhaust gas are not regulated at present, but are expected to be regulated in the future because they have a greenhouse effect about 21 times that of carbon dioxide (CO ₂ ) It is desirable to suppress as much as possible from the viewpoint of the global environment.

Furthermore, if unburned components such as methane contained in exhaust gas can be oxidized, waste heat recovery efficiency can be improved due to an increase in the temperature of the exhaust gas, and there is also an economic merit. Therefore, various oxidation catalysts for oxidizing such unburned methane and the like have been developed. For example, a catalyst in which a cordierite honeycomb is wash-coated with alumina as a carrier and palladium is carried thereon by an impregnation method is already known.

[0006]

However, a catalyst in which palladium is supported on alumina by an impregnation method is usually one.
Despite having a high specific surface area of about 00 m ² / g, the exhaust gas temperature of a lean burn gas engine (about 400 ° C)
There is a disadvantage that the methane oxidation activity is low in a low temperature region corresponding to.

Further, under actual exhaust gas conditions including steam, the catalytic activity is reduced because the steam contained in the exhaust gas inhibits the oxidation reaction of methane, and the carried metal is affected by the steam contained in the exhaust gas. There is a drawback that sintering occurs, the activity is reduced, and the durability of the catalyst is low. Further, in order to obtain a practically high catalytic activity, the amount of supported palladium should be reduced to 1 in anticipation of a decrease in catalytic activity.
It is necessary to be about 0 g / l (estimated 6 wt%), and there is a disadvantage that the cost of the catalyst increases.

An object of the present invention is to improve the catalytic activity of a methane oxidation catalyst and to drastically improve the oxidation rate of unburned methane under exhaust gas conditions in which water vapor coexists. And to reduce the price of the methane oxidation catalyst, to prevent catalyst deterioration due to sintering of the catalyst metal, and to provide a methane oxidation catalyst capable of maintaining high catalytic activity for a long period of time.

[0009]

In order to solve the above-mentioned problems, a methane oxidation catalyst according to the present invention has a temperature of 100 ° C. or more.
The voids of primary particles having a diameter of not more than
The gist is that a ceramic material containing g or more is used as a catalyst carrier, and palladium is highly dispersed and supported on the surface of the catalyst carrier and in the voids of the primary particles.

According to the methane oxidation catalyst having the above-described structure, when palladium is supported on a catalyst carrier containing a predetermined amount or more of primary particle voids having a predetermined size, a part of the palladium is supported on the surface of the carrier. However, the remaining palladium will be carried in the primary particle voids. When a gas containing a small amount of methane is passed through such a catalyst, the inside of the primary particle voids becomes a reaction field for methane oxidation, and the methane oxidation reaction proceeds more actively through the palladium supported therein. Will be

Further, sintering hardly occurs in the palladium supported in the voids of the primary particles even in the presence of steam, and high activity is maintained for a long time. This makes it possible to obtain a methane oxidation catalyst having high oxidation activity and durability even when the amount of supported catalyst metal is small.

Here, it is necessary that the catalyst carrier made of the ceramic material contains primary particle voids having a diameter of 100 ° or more and 1000 ° or less. The primary particle voids present in the catalyst carrier serve as a reaction field for methane oxidation, and the size of the reaction field has an appropriate range for efficiently performing the methane oxidation reaction. This is because the efficiency of the oxidation reaction is reduced whether the size is too large or too small. Preferably, it is not less than 300 ° and not more than 700 °. The “primary particle void” is a space inside the carrier surrounded by ceramic fine particles made of a single crystal, and is a space communicating with the surface.

It is necessary that the catalyst support contains at least 0.01 ml / g of primary particle voids having a diameter within the above range. As described above, the primary particle voids serve as a reaction field for methane oxidation. If the content is less than 0.01 ml / g, the number of reaction fields is reduced and the catalyst supported on the surface of the catalyst carrier is reduced. This is because sintering of metal cannot be suppressed, and the oxidation efficiency of unburned methane decreases. Preferably, it is 0.05 ml / g or more. The volume of the primary particle voids is preferably as large as possible as long as a catalyst carrier can be produced.

The ceramic material used as the catalyst carrier is not particularly limited as long as it contains primary particle voids having a diameter in a predetermined range in a predetermined amount or more. For example, alumina, zirconia, cordierite,
Various ceramic materials such as oxide ceramics such as mullite, carbide ceramics such as silicon carbide and boron carbide, and nitride ceramics such as silicon nitride and boron nitride can be used.

However, the ceramic material is
Desirably, it is a zirconia-based material. The zirconia-based material has a good sinterability, it is easy to control the internal structure of the carrier as described above, and it has an action of suppressing a decrease in catalytic activity in the presence of steam. .

Particularly, as the zirconia-based material, 1
The partially stabilized zirconia containing 0 mol% or less of yttria is excellent, and can improve the durability as compared with a zirconia-based material containing no yttria. Incidentally, if the yttria content is too large, the catalytic activity is rather lowered, so the yttria content is 10 mol
% Is desirable.

Palladium has a function as a catalyst for efficiently oxidizing methane having a high ignition point.
A methane oxidation catalyst having high methane oxidation activity can be obtained by highly dispersing and supporting the catalyst on the surface of the zirconia catalyst having the above configuration or in the primary particle voids.

Here, it is sufficient that the palladium is highly dispersed and supported on the surface of the catalyst carrier and in the voids of the primary particles, and the palladium source used and the method of supporting the palladium are not particularly limited. However, it is preferable that the palladium be one supported on the catalyst carrier by an impregnation method using tetraammine palladium hydroxide as a palladium source.

The tetraammine palladium hydrochloride does not contain harmful elements that may cause catalyst poisoning,
Hydration is high, and it is in a liquid state just before water disappears.By supporting palladium by an impregnation method using tetraammine palladium hydroxide as a palladium source, it is easily formed on the surface of the catalyst support and the primary particle voids. It becomes possible to uniformly disperse palladium inside.

Further, it is desirable that the carried amount of the palladium is 0.5 wt% or more and 5.0 wt% or less. If the amount of supported palladium is less than 0.5% by weight, sufficient methane oxidation activity cannot be obtained.
This is because the methane oxidation activity is not greatly improved even if it is supported in excess of wt%, but the cost is rather high. Preferably, it is 1.0 wt% or more and 3.0 wt% or less.

The ratio of palladium carried in the voids of the primary particles is preferably as large as possible. The catalyst metal present on the surface of the catalyst carrier acts as a catalyst for accelerating the oxidation reaction of methane, but the catalyst metal supported in the primary particle voids has a condition in which water vapor coexists. This is because the high dispersion degree of the catalyst metal is maintained even below, so that the activity is high and the durability is excellent.

[0022]

Embodiments of the present invention will be described below in detail. The methane oxidation catalyst according to the present invention can be produced by various methods. For example, in order to produce a catalyst carrier made of zirconia, using zirconia fine powder as a raw material, adding an appropriate binder, kneading, extruding or the like to form a honeycomb shaped body, and firing this,
A honeycomb-shaped carrier made of zirconia is obtained.

After firing the granules obtained by binding the zirconia fine powder with a binder, the granules are pressed and pulverized to 16 to 6 particles.
It may be a granular carrier having a size of 0 mesh.
Further, a honeycomb substrate may be prepared by using cordierite or the like as a raw material, and a slurry containing fine zirconia powder may be wash-coated thereon to form a carrier.

To support the catalyst metal on the catalyst carrier thus obtained, the carrier is immersed in an aqueous solution containing a palladium salt, and the aqueous solution is poured into the primary particle voids formed inside the carrier. After the impregnation, moisture may be removed under heating or reduced pressure and dried, and after reduction, firing may be performed in an oxygen stream.

Example 1 1 wt% of palladium was supported on a catalyst carrier made of zirconia containing no stabilizer using various palladium sources, and its initial activity was examined. Zirconia, which is a raw material of the catalyst carrier, has a specific surface area of 1
A zirconia fine powder (RC-100, manufactured by Daiichi Kagaku Kagaku Kogyo Co., Ltd.) containing a stabilizer of 00 m ² / g was used. This zirconia fine powder is heated at 1000 ° C. for 6 hours in the air.
Firing conditions.

Next, the calcined zirconia powder was added to aqueous solutions containing various palladium salts, and the mixture was heated and stirred at 80 ° C. for 12 hours, and then heated to 80 to 100 ° C. and evaporated to dryness. Then, at 120 ° C x
After drying for 12 hours, the product was scraped out, reduced in a stream of hydrogen at 300 ° C. for 3 hours, and then 600 ° C. for 6 hours in an oxygen stream.
It baked on condition of. After solidifying the obtained powdery catalyst with a CIP molding machine, it is pulverized to obtain 1 wt% of palladium.
% Of the catalyst was obtained.

As the palladium salt, tetraammine palladium hydroxide (Pd (NH ₃ ) ₄ (OH) ₂ ),
Tetraamminedichloropalladium (Pd (NH ₃ ) ₄ C
l ₂ ), palladium chloride (PdCl ₂ ), palladium nitrate (Pd (NO ₃ ) ₂ ) and palladium acetate (Pd (OC
Five types of OCH ₃ ) ₂ ) were used.

Each of the catalysts obtained as described above was subjected to a reaction test for examining the initial activity for methane oxidation. The procedure is as follows. That is, 1
0.1g of granular catalyst and 0.2g of quartz sized to 6-60 mesh were packed in a reaction tube, which was installed in a normal-pressure fixed-bed flow-type reaction test apparatus to blow off moisture and impurities attached to the catalyst. As a pretreatment, firing was performed in air at 600 ° C. for 2 hours.

Next, after the temperature of the reaction tube was lowered to 250 ° C., the reaction gas containing a small amount of methane was supplied at a flow rate of 5000 m while the temperature in the reaction tube was raised to 250 ° C. to 600 ° C.
It flowed into the reaction tube at 1 / hr. The methane oxidation rate was determined by analyzing the product gas after the reaction test using a gas chromatograph (GC) with a flame ionization detector (FID). Note that, in Example 1, 10 g
% Water vapor (CH ₄ = 4000 ppm, CO
= 500 ppm, CO ₂ = 5%, H ₂ O = 10%, balance N
₂ ) was used.

FIG. 1 is a diagram showing the relationship between the reaction temperature and the methane oxidation rate. From FIG. 1, it can be seen that the methane oxidation activity differs depending on the palladium source used, even though the catalyst carrier and the supported amount of palladium are the same.

That is, comparing the methane oxidation activities at 450 ° C., palladium acetate (Pd (OCOCH ₃ ) ₂ ) and palladium nitrate (Pd (N
The catalysts using O ₃ ) ₂ ), palladium chloride (PdCl ₂ ) and tetraammine dichloropalladium (Pd (NH ₃ ) ₄ Cl ₂ ) have 59%, 62%, 68% and 79%, respectively. , A catalyst using tetraammine palladium hydroxide (Pd (NH ₃ ) ₄ (OH) ₂ )
Even in the presence of steam, the methane oxidation rate was 100%.

At 400 ° C. corresponding to the exhaust gas temperature of a lean burn gas engine, palladium acetate (Pd (OC
OCH ₃ ) ₂ ), palladium nitrate (Pd (NO ₃ ) ₂ ), palladium chloride (PdCl ₂ ), and catalysts using tetraamminedichloropalladium (Pd (NH ₃ ) ₄ Cl ₂ ) all have a methane oxidation rate of 10 % Or less, whereas tetraammine palladium hydroxide (Pd (NH ₃ ) ₄
The catalyst using (OH) ₂ ) showed a high methane oxidation rate of 85%.

As described above, the catalyst using tetraammine palladium hydroxide as a palladium source exhibits high activity because it does not contain a harmful element which may be a catalyst poison, and in addition, tetraammine palladium hydroxide. It is considered that palladium has a high hydration property and is in a liquid state until immediately before water disappears in the evaporating and drying step, so that palladium is more likely to be highly dispersed and supported than other palladium sources which are solid at room temperature. From the above results, it was found that tetraammine palladium hydroxide was the most excellent palladium source for the methane oxidation catalyst.

Example 2 Using zirconia containing no stabilizer as a catalyst carrier, the effects of the specific surface area of the carrier and the pore size of the primary particles on the methane oxidation rate were examined. The catalyst carrier was set at a sintering temperature of zirconia powder of 600 ° C.
Except for four types of 0 ° C, 1000 ° C and 1200 ° C,
It was prepared under the same conditions as in Example 1. Also, 1 wt% of palladium was added to the obtained zirconia support by the same procedure as in Example 1 using tetraammine palladium hydroxide (Pd (NH ₃ ) ₄ (OH) ₂ ) as a palladium source.
% As a catalyst.

The specific surface area of the obtained granular methane oxidation catalyst was determined by the BET method, the degree of Pd dispersion was determined by the CO pulse method, the zirconia particle diameter was determined by the X-ray diffraction method, and the pore distribution was determined by the mercury intrusion method. I asked for each. Further, for each catalyst, a reaction test was performed in the same procedure as in Example 1. The reaction gas used in the reaction test contained 10% water vapor (CH ₄ = 4000 ppm, CO = 500
ppm, CO ₂ = 5%, H ₂ O = 10%, balance N ₂ )
Those not containing water vapor (CH ₄ = 4000 ppm, CO
= 500 ppm, CO ₂ = 5%, balance N ₂ ).

FIG. 2 shows the relationship between the reaction temperature and the methane oxidation rate for various zirconia catalysts containing 1 wt% palladium. Under the condition in which steam does not coexist, as shown by the dotted line in FIG. 2, the methane oxidation rate increased with the firing temperature until the firing temperature of the zirconia powder was up to 1000 ° C., but the firing temperature was 1200 ° C.
, The rate of methane oxidation fell in reverse.

That is, the firing temperature is 600 ° C. (Pd /
ZrO ₂ (600)), 800 ° C. (Pd / ZrO ₂ (80
0)) and 1000 ° C. (Pd / ZrO ₂ (1000))
The methane oxidation rate at a reaction temperature of 400 ° C. increased to 70%, 97% and 98% as the temperature rose, but when the firing temperature reached 1200 ° C. (Pd / ZrO ₂ (1200)), 46% Down to. Similarly, if the firing temperature is 6
As the temperature rises to 00 ° C., 800 ° C., and 1000 ° C., the reaction temperature at which the methane oxidation rate becomes 50% is also 381 ° C., 33 ° C.
The temperature decreased to 2 ° C. and 322, but increased to 405 ° C. when the firing temperature reached 1200 ° C.

Under the condition where 10% steam coexists, as shown by the solid line in FIG. 2, the methane oxidation rate of all the catalysts decreased as compared with the case without steam.
There was no change in the rank of methane oxidation activity. That is, the firing temperature of the zirconia powder is 600 ° C., 800 ° C., and 10 ° C.
As the temperature rises to 00 ° C., the methane oxidation rates at a reaction temperature of 400 ° C. increase to 23%, 73% and 86%, respectively.
It decreased to 5%.

Similarly, the reaction temperature at which the methane oxidation rate becomes 50% is as follows: the firing temperature of the zirconia powder is 600 ° C .;
℃ and 1000 ℃, respectively, 424
° C, 378 ° C and 363 ° C, and the firing temperature is 12
When the temperature reached 00 ° C, the temperature rose to 451 ° C.

FIG. 3 shows the pore distribution of various methane oxidation catalysts having different firing temperatures for zirconia powder. Catalyst with firing temperature of 600 ° C (Pd / ZrO
₂ (600)), peaks were observed at two positions of 170 ° and 3100 °. The former corresponds to primary particle voids, and the latter corresponds to secondary particle voids.

Similarly, a catalyst having a calcination temperature of 800 ° C.
(Pd / ZrO ₂ (800)), 320 ° and 40 °
A peak was observed at the position of 00 °, and the firing temperature was 1000
° C (Pd / ZrO ₂ (1000))
Peaks were observed at 30 ° and 4200 °. On the other hand, a catalyst (Pd / ZrO ₂
In (1200)), the peak at the position corresponding to the primary particle void disappeared, and only the peak at 4200 ° was observed.

Table 1 shows the Pd dispersity (%) of various methane oxidation catalysts having different zirconia firing temperatures.
It shows specific surface area (m ² / g), ZrO ₂ particle size (Å), and primary particle void size (Å).

[0043]

[Table 1]

From Table 1, it can be seen that the firing temperature of zirconia is 600
C, 800 ° C., 1000 ° C. and 1200 ° C., the degree of dispersion of Pd (%) monotonously decreased to 7 ° C., respectively.
0%, 31%, 18% and 13%. Similarly, the specific surface area (m ² / g) of the catalyst also decreases monotonically,
37.6m was ^{2 /g,20.4m 2 /g,9.7m 2} / g and 3.0 m ² / g.

On the other hand, the ZrO ₂ particle diameter (Å) is determined when the firing temperature of zirconia is 600 ° C., 800 ° C., 1000 ° C., and 1 ° C.
As the temperature rises to 200 ° C, it increases monotonically,
210 °, 360 °, 960 ° and 4500 °. Similarly, the primary particle void diameter monotonically increases as the firing temperature increases, and the primary particle voids disappear at the firing temperature of 1200 ° C., as described above.

The following can be said from the above results. In other words, the methane oxidation activity monotonously increases with the firing temperature from 600 ° C. to 1000 ° C., even though the primary particle void diameter monotonically increases. This does not mean that the smaller the size of the primary particle void is, the higher the methane oxidation activity becomes.
This is considered to indicate that there is an appropriate range for providing a highly active methane oxidation catalyst. According to Table 1, in order to obtain a high methane oxidation activity, the primary particle void diameter is 1
It is found that the angle is required to be not less than 00 ° and not more than 1000 °, and preferably not less than 300 ° and not more than 700 °.

A catalyst calcined at 1000 ° C.
When compared with the catalyst calcined at 00 ° C., a large difference is observed in the methane oxidation activity, although no large difference is observed in the degree of dispersion of Pd. Since the primary particle voids disappear in the catalyst calcined at 1200 ° C. in which the methane oxidation activity is significantly reduced, it is considered that the primary particle voids are a main reaction field for methane oxidation.

Therefore, in order to exhibit a high methane oxidation activity, the amount of the primary particle voids having a diameter in the above range must be:
It is considered that the larger the amount, the better. However, as shown in FIG. 3, in the catalyst calcined at 1200 ° C., the content is at least 0.01 ml / g or more, and preferably 0.05 ml / g or more.

The specific surface area of the catalyst support corresponds to the sum of the total volume of the primary particle voids and the total volume of the secondary particle voids, and is not necessarily the total volume of the primary particle voids contained in the catalyst support. Is not displayed. However, the above-described microstructure of the catalyst support which is a feature of the present invention is usually
It is obtained by baking under predetermined conditions such that the ultrafine powder having a specific surface area of about 100 m ² / g is used as a starting material and the diameter and content of the primary particle voids are within predetermined ranges.

Therefore, in this case, it is considered that the size of the specific surface area of the catalyst carrier indirectly indicates the diameter and the content of the primary particle voids. Can be specified by the specific surface area of the entire catalyst carrier. From Table 1, it can be seen that the specific surface area of the catalyst carrier should be 5 m ² / g or more and 50 m ² / g or less in order to express high methane oxidation activity. Preferably, it is 5 m ² / g or more and 30 m ² / g or less.

Further, the firing temperature is from 600 ° C. to 1000 ° C.
Up to ° C, the methane oxidation activity monotonically increases with increasing firing temperature, despite the fact that the degree of dispersion of Pd monotonously decreases. This does not mean that the greater the degree of dispersion of Pd, the higher the oxidizing activity of methane becomes, but that the degree of dispersion of Pd has an appropriate range for efficiently performing methane oxidation. Conceivable.
Table 1 shows that in order to obtain high methane oxidation activity, the Pd dispersity needs to be at least 15% or more and 80% or less, and the range of 15% or more and 40% or less is preferable.

When the primary particle void diameter is compared with the Pd dispersion degree, the smaller the primary particle void diameter is, the larger the Pd dispersion degree is, and a correlation is recognized. It is believed that a significant proportion is carried in the primary particle voids. Therefore, it is considered that the high methane oxidation activity was exhibited by the present invention because the primary particle voids act as a reaction field for methane oxidation, and at the same time, give Pd clusters having a size suitable for methane oxidation.

Example 3 Alumina having a specific surface area of 160 m ² / g (catalyst JRC-ALO-1) was used as a raw material of the catalyst carrier, and the alumina powder was calcined at 1000 ° C. for 6 hours at 1000 ° C. x24hr,
Palladium was reduced to 1 according to the same procedure as in Example 2 except that the temperature was changed to 1000 ° C. × 48 hr and 1000 ° C. × 72 hr.
A methane oxidation catalyst supporting wt% was prepared. For each of the obtained catalysts, the measurement of the specific surface area and the reaction test were performed in the same manner as in Example 2.

When the specific surface area of the obtained catalyst was measured, the specific surface area monotonously decreased with the increase of the calcination time of the alumina powder, and as the calcination time increased to 6, 24, 48 and 72 hours. , 70m ² /
g, 42 m ² / g, 20 m ² / g and 14 m ² / g.

When a reaction test was carried out, as shown by the solid line in FIG. 4, in the presence of water vapor, the methane oxidation rate of the catalyst calcined under the conditions of 1000 ° C. for 48 hours with a specific surface area of 20 m ² / g was found. Highest, reaction temperature 400 ° C
Methane oxidation rate is 79% and methane oxidation rate is 50%
% Was 370 ° C. On the other hand, even if the specific surface area was further reduced (72 hr) or increased (6 hr, 24 hr), the methane oxidation rate decreased.

Further, 1000 ° C. x having the highest catalytic activity
When the methane oxidation rate of the catalyst calcined under the condition of 48 hours in the absence of steam was measured, the methane oxidation rate at a reaction temperature of 400 ° C. was 100% and the methane oxidation rate was 50%, as shown by the dotted line in FIG. Reaction temperature is
320 ° C.

Compared with the catalyst using zirconia fired at 1000 ° C. for 6 hours as shown in FIG. 2, alumina has a slightly higher catalytic activity in the absence of steam, whereas zirconia in the presence of steam has a slightly higher catalytic activity. Conversely, the catalyst activity is higher. Although details are unknown, it is considered to be due to chemical factors.

From the above results, even when alumina is used as the catalyst carrier, the ultrafine powder having a high specific surface area is used as a starting material, and by optimizing the calcination conditions, the specific surface area of the catalyst carrier becomes appropriate. It was found that the methane oxidation activity could be remarkably improved if it was within the range. In addition, it was found that zirconia has less reaction inhibition due to water vapor than alumina.

(Example 4) A durability test was carried out at the exhaust gas temperature (400 ° C) on the catalyst prepared using zirconia as a carrier prepared in Example 2 and the catalyst prepared using alumina as a support prepared in Example 3. The endurance test was carried out using a granular catalyst of 0.1
g and a quartz content of 0.2 g were packed in a reaction tube, which was set in a normal-pressure fixed-bed flow-type reaction test apparatus, calcined in air at 600 ° C. for 2 hours, and then cooled to 400 ° C.
The reaction was carried out by flowing a reaction gas containing a small amount of methane into the reaction tube at a flow rate of 5000 ml / hr while maintaining the temperature at ° C. The reaction gas is a reaction gas containing water vapor (CH ₄ = 40
00 ppm, CO = 500 ppm, CO ₂ = 5%, H ₂ O
= 10%, balance N ₂ ). FIG. 5 shows the results.

800 ° C. for 6 hours and 1000 ° C. for 6 hours
The calcined zirconia catalyst conditions, specific surface area, respectively, is a 20 m ² / g and 10 m ² / g, and maintained for endurance test, both the initial methane oxidation rate (60-80%) No change with time in the methane oxidation rate was observed.

On the other hand, the specific surface area of the alumina-based catalyst calcined at 1000 ° C. for 48 hours was 20 m ² / g, which was the same as that of the zirconia-based catalyst calcined at 800 ° C. for 6 hr. In the early stage of the durability test, the methane oxidation rate was almost the same as that of the zirconia-based catalyst, but after 100 hours, it decreased to 50%, and a change with time was observed.

From the above results, the catalyst using alumina as the carrier has almost the same oxidation activity in the early stage of the reaction as the catalyst using zirconia as the carrier, but the activity decreases with time as compared with the zirconia-based catalyst. It turned out to be big.

Example 5 As a raw material of a catalyst carrier, zirconia containing no stabilizer and 3 mol% of yttria (Y) were calcined at 1,000 ° C. for 6 hours in the air.
Partially stabilized zirconia (3YSZ) containing ₂ O ₃ ) and 8
mol% yttria and partially stabilized zirconia (8
Except for using YSZ), a catalyst supporting 1 wt% of palladium was prepared in the same procedure as in Example 2.
For each of the obtained catalysts, the measurement of the specific surface area and the reaction test were performed in the same manner as in Example 2.

The specific surface area of the catalyst using zirconia containing no stabilizer (Pd / ZrO ₂ ), the catalyst using 3YSZ (Pd / 3YSZ) and the catalyst using 8YSZ (Pd
d / 8YSZ), about 10 m ² / g and 11 m ² / g, respectively.
m ² / g and 13 m ² / g, and there was no significant difference between the catalysts.

The initial activity of methane oxidation in the presence of water vapor was examined. As shown in FIG. 6, the methane oxidation rate at a reaction temperature of 400 ° C. was 86% for a catalyst using zirconia containing no stabilizer. Whereas 3
The catalyst using YSZ was 90%, indicating that the use of 3YSZ slightly improved the methane oxidation rate as compared to the catalyst using zirconia containing no stabilizer.
On the other hand, in the catalyst using 8YSZ, the methane oxidation rate at a reaction temperature of 400 ° C. was almost the same as the catalyst using zirconia containing no stabilizer, but the reaction temperature was 400
The catalyst activity in the low temperature range below ℃ was lower than that of other catalysts.

A durability test was performed on a catalyst using zirconia as a carrier and a catalyst using 3YSZ and 8YSZ without a stabilizer in the same manner as in Example 4. FIG. 7 shows the results. Immediately after the durability test, catalysts using 3YSZ and 8YSZ as carriers show higher catalytic activity,
No change in the catalyst activity with time was observed, and it was found that the catalyst exhibited a high methane oxidation rate exceeding 80% even after 100 hours.

From the above results, 3
When YSZ and 8YSZ were used, the initial activity at a reaction temperature of 400 ° C. was only slightly improved as compared with zirconia containing no stabilizer, but it was found that the durability in the presence of steam was significantly improved. .

It should be noted that the present invention is not limited to the above-described embodiment at all, and various modifications can be made without departing from the gist of the present invention. For example, in the above embodiment,
Although the amount of supported palladium is 1 wt%, the present invention is not limited to this. If the amount of supported palladium is increased, the initial activity and durability can be further improved. The optimum amount of the supported catalyst may be appropriately selected.

In addition to palladium, Pt, Ce,
Other catalyst noble metals such as La may be supported, and other stabilizers such as scandia (Sc ₂ O ₃ ) and ceria (CeO ₂ ) may be used in addition to yttria (Y ₂ O ₃ ). Thereby, catalyst activity and durability can be improved.
Furthermore, the present invention is used not only as a catalyst for methane oxidation, but also as a catalyst for purifying CO, NOx, HC, and the like contained in exhaust gas of a gasoline engine or a diesel engine in which the reaction of the coexisting water vapor is concerned. can do.

[0070]

According to the present invention, a ceramic material containing a predetermined amount or more of primary particle voids having a diameter in a predetermined range is used as a catalyst carrier, and palladium as a catalytic metal is supported on the surface of the carrier and in the primary particle voids. As a result, a reaction field for methane oxidation is secured, and sintering of the catalyst metal supported in the primary particle voids is suppressed, so that the catalytic activity of methane oxidation is significantly improved, and exhaust gas conditions in which steam coexists are used. Also, there is an effect that the oxidation rate of unburned methane can be dramatically improved.

When a zirconia-based material, particularly a partially stabilized zirconia containing a predetermined amount of yttria, is used as a material for the catalyst carrier, a decrease in the initial activity of the catalyst due to steam can be suppressed, and the catalyst can be used in the presence of steam. A highly durable methane oxidation catalyst capable of maintaining high methane oxidation activity for a long time can be obtained.

Further, when tetraammine palladium hydroxide is used as a palladium source and palladium is supported by the impregnation method, the catalyst can be supported on the catalyst carrier with a high degree of dispersion.
There is an effect that the methane oxidation rate at a low temperature can be improved.

As described above, according to the methane oxidation catalyst of the present invention, the methane oxidation rate can be significantly improved while suppressing the initial activity and durability of methane oxidation caused by water vapor. As a result, the amount of catalytic metal consumption can be significantly reduced, the price of the methane oxidation catalyst can be reduced, and if this is used as an exhaust gas treatment catalyst for lean burn gas engines, etc., it will contribute to the reduction of greenhouse gases. This is an invention having an extremely large industrial effect.

[Brief description of the drawings]

FIG. 1 is a view showing the initial activity of a Pd / ZrO ₂ -based catalyst using various palladium sources.

FIG. 2 is a view showing the initial activity of a Pd / ZrO ₂ -based catalyst calcined at various temperatures.

FIG. 3 is a view showing a pore distribution of each catalyst shown in FIG. 2;

FIG. 4 is a diagram showing the initial activity of a Pd / Al ₂ O ₃ based catalyst calcined at various temperatures.

FIG. 5 is a diagram showing the results of a durability test of a Pd / ZrO ₂ catalyst and a Pd / Al ₂ O ₃ catalyst.

FIG. 6 is a diagram showing the initial activities of a Pd / ZrO ₂ catalyst, a Pd / 3YSZ catalyst, and a Pd / 8YSZ catalyst.

FIG. 7 is a diagram showing the results of endurance tests of a Pd / ZrO ₂ catalyst, a Pd / 3YSZ catalyst, and a Pd / 8YSZ catalyst.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI F01N 3/10 F01N 3/20 D 3/20 3/28 Q 3/28 301P 301 B01J 23/56 301Z

Claims (5)

[Claims] A ceramic material containing at least 0.01 ml / g of primary particle voids having a diameter of 100 ° or more and 1000 ° or less is used as a catalyst carrier, and palladium is highly dispersed and supported on the surface of the catalyst carrier and in the primary particle voids. A methane oxidation catalyst. 2. The methane oxidation catalyst according to claim 1, wherein the ceramic material is a zirconia-based material. 3. The method according to claim 1, wherein the zirconia-based material is 10 mol%.
The methane oxidation catalyst according to claim 2, which is partially stabilized zirconia containing the following yttria. 4. The methane according to claim 1, wherein the palladium is supported on the catalyst carrier by an impregnation method using tetraammine palladium hydroxide as a palladium source. Oxidation catalyst. 5. The palladium has a supported amount of 0.5.
The methane oxidation catalyst according to any one of claims 1, 2, 3 and 4, wherein the content of the methane oxidation catalyst is not less than wt% and not more than 5.0 wt%.