JP3932437B2 - Exhaust gas purification catalyst and exhaust gas purification method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification method for decomposing nitrogen oxides contained in exhaust gas emitted from combustion apparatuses such as turbines, boilers, internal combustion engines, etc. in an oxygen-excess atmosphere using methane as a reducing agent, and exhaust gas purification used in this method Relates to the catalyst.
[0002]
[Prior art]
A NOx selective catalytic reduction method (NH ₃ -SCR) using NH ₃ as a reducing agent is known as a technique for reducing nitrogen oxide (NOx) in an excess of oxygen atmosphere and converting it into harmless nitrogen. Although this technique can efficiently reduce and remove NOx with a simple device, it requires storage, maintenance and appropriate safety management of NH ₃ used as a reducing agent, which leads to an increase in denitration cost.
[0003]
A technique for decomposing NOx into nitrogen and oxygen without using a reducing agent is currently being researched and developed, but no method has yet emerged that can achieve practical results.
[0004]
A denitration method (HC-SCR) using unburned hydrocarbons contained in combustion exhaust gas as a reducing agent has been studied, but this has not yet been put into practical use.
[0005]
As one of the HC-SCR techniques, a method for decomposing NOx using methane as a reducing agent under an oxygen-excess atmosphere and a catalyst used therefor are disclosed in Japanese Patent Application Laid-Open No. 2000-61308.
[0006]
However, according to this publication, the NOx conversion rate, which was about 60% at the beginning of the reaction, is 52% after 30 hours, 45% after 50 hours, and 35% after 88 hours, gradually with time. Thus, a long-term stable NOx conversion rate is not obtained.
[0007]
[Problems to be solved by the invention]
In view of the above points, the present invention provides a method for purifying NOx in exhaust gas, which can stably decompose NOx for a long period of time using methane as a reducing agent in an oxygen-excess atmosphere, and an exhaust gas purifying catalyst used in this method. The main purpose is to provide. Another object of the present invention is to provide a plate-shaped catalyst or a honeycomb-shaped catalyst which is a practical usage form of this catalyst, and to provide an exhaust gas purification method using the catalyst.
[0008]
[Means for Solving the Problems]
As a result of investigating the cause of a decrease in activity of a catalyst in which palladium and platinum are supported on zirconia, as disclosed in JP-A-2000-61308, the inventors have conducted research to solve the above-mentioned problems. Further, the present inventors have found that a catalyst added with tungsten maintains high NOx decomposition activity stably for a long period of time when NOx is decomposed using methane as a reducing agent in an oxygen-excess atmosphere, and the present invention has been completed.
[0009]
It is considered that the selective reduction denitration reaction of NOx with methane proceeds by the following mechanism.
[0010]
Methane adsorbed on the catalytic active site is remarkably activated and reacts rapidly with the catalytic lattice oxygen without waiting for the arrival and activation of oxygen molecules from the gas phase.
[0011]
At the same time, the NOx adsorbed on the catalyst is desorbed as nitrogen gas by being deprived of oxygen by the catalyst in which oxygen defects are generated by the reaction with methane as described above.
[0012]
Here, the functions desired for the catalyst are as follows.
[0013]
(1) Adsorption and activation of methane (2) Provision of lattice oxygen that is easy to react with activated methane (3) Suppression of reaction between oxygen defects and gas phase oxygen (4) Adsorption of NOx and NO bond Cutting (5) Acceleration of reaction between oxygen ions or radicals generated by decomposition of NOx and oxygen defect points
When the activity of the catalyst was examined from the viewpoint of physical properties of the catalyst, the activity of the catalyst shown in JP-A-2000-61308 was that zirconia special crystals obtained by calcination of zirconium hydroxide and platinum and palladium supported thereon were obtained. It was found that the activity decreases as the surface of the unstable zirconia crystal changes.
[0015]
Desirable crystals can be obtained when sulfate groups coexist during the formation of zirconia crystals. However, sulfate groups are detached during a long-time reaction, and the crystal surface changes accordingly.
[0016]
Considering the activity of the catalyst from the reaction side, the violent activation of methane decreases as the crystal surface changes, and the adsorbed methane reacts with gas-phase oxygen according to the general oxidation reaction mechanism. That is, it is considered that methane reaching the catalyst surface is not consumed effectively for the reaction with NOx, and the denitration performance is lowered.
[0017]
In other words, it is considered that the added sulfate radical suppresses the oxidation of methane by gas phase oxygen and improves the selectivity of the reaction between methane and NOx. The drawback of the catalyst described in JP-A-2000-61308 is a decrease in the denitration performance due to the elimination of the sulfate radical.
[0018]
Based on the above research results, the present inventors have searched for a substance that exhibits the same action on the zirconia crystal type as the sulfate radical and does not easily desorb, and the addition of tungsten oxide is effective. I found out.
[0019]
The first exhaust gas purifying catalyst according to the present invention is obtained by supporting palladium and platinum on a zirconia carrier and further adding tungsten.
[0020]
The exhaust gas second purification catalyst according to the present invention is formed by supporting tungsten on a zirconia support and then supporting palladium and platinum.
[0021]
In the first catalyst, a preferable zirconia support contains a sulfate group, and this catalyst is obtained by supporting palladium and platinum on the sulfate group-containing zirconia support and further adding tungsten.
[0022]
The first catalyst includes, for example, palladium and platinum supported on a sulfate radical-supporting zirconia support described in JP-A-2000-61308, and then the catalyst is immersed in an aqueous solution of ammonium tungstate and dried. It can be obtained by baking at 450 to 650 ° C for 1 to 24 hours, preferably 2 to 12 hours.
[0023]
Te second catalyst odor, supported tungsten to di zirconia carrier was immersed a commercially available zirconium hydroxide to ammonium tungstate aqueous solution, dried in air from 550 to 1,100 ° C., a temperature of preferably five hundred and fifty to eight hundred fifty ° C. For 1 to 24 hours, preferably 2 to 12 hours, for example about 3.5 hours.
[0024]
In the first and second catalysts, the amount of tungsten is preferably 2 to 20% by weight relative to the support. The supported amount of palladium is preferably 0.01 to 0.5% by weight based on the carrier . The amount of data tungsten is preferable to use palladium is from 10 to 200 wt%.
[0025]
The first and second catalysts may be in the form of a plate as described below, in addition to granular and powdery forms. That is, a fine catalyst powder is dispersed and held between fibers of a preform plate (sheet, mat, paper, cloth, etc.) made of ceramic fibers to obtain a plate catalyst. More specifically, the catalyst is ground to 200 mesh or less, and the pulverized product is added to an aqueous nitric acid solution having a pH of 2 to 3 to obtain a paste slurry having a solid content of 40 to 60% by weight. A plate-like material is obtained by press-fitting between the fibers and drying.
[0026]
The form of the first and second catalysts may also be the following honeycomb catalyst. That is, this is a honeycomb structure formed by laminating a folded product of a preform plate-like body made of ceramic fibers, or between fibers of a honeycomb structure formed by alternately laminating a folded product and a flat plate, This is a honeycomb catalyst in which fine particles of catalyst are dispersed and held. A folded product of the preform plate is obtained by pressing and drying the preform in a wet state with the slurry against the template. In the preparation of the plate catalyst and the honeycomb catalyst, silica sol can be added to the slurry as a binder as necessary. Moreover, after preparing a honeycomb-shaped catalyst without adding a binder, it can be immersed in a binder colloid solution or slurry such as silica sol, dried, and fired to improve the mechanical strength of the honeycomb structure.
[0027]
The exhaust gas purification method using the exhaust gas purification catalyst according to the present invention is a method in which the first or second catalyst is brought into contact with the exhaust gas in an oxygen-excess atmosphere to decompose NOx in the exhaust gas using methane as a reducing agent.
[0028]
Methane is preferably unburned methane from the combustion device.
[0029]
When the catalyst according to the invention is used as a granular catalyst, the space velocity (GHSV) is preferably from 2,000 to 200,000 h ⁻¹ . Further, when the catalyst according to the present invention is used as a plate-like catalyst and a honeycomb-like catalyst, the area (GHAV) is preferably 5 to 30 m / h. The catalyst layer temperature is preferably 350 to 500 ° C.
[0030]
The NOx concentration is not limited, and when sufficient methane coexists, the reaction rate is almost proportional to the NOx concentration.
[0031]
The methane concentration can vary depending on the required denitration rate and other conditions, but in order to obtain a high denitration rate, it is preferably 1 or more with respect to the NOx concentration. When the concentration of methane contained in the exhaust gas is higher than the amount necessary for decomposing NOx, it is not necessary to newly add methane to the exhaust gas, and when the amount necessary for decomposing NOx does not exist, An appropriate amount may be newly added.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be specifically described based on examples.
[0033]
Example 1
Preparation of W / 0.5% Pd-0.5% Pt / sulfate radical zirconia catalyst 150 g of zirconium hydroxide was immersed in 150 ml of an aqueous solution in which 15 g of ammonium sulfate was dissolved for 10 hours. This immersion body was dried and then calcined at 550 ° C. for 3.5 hours to obtain sulfate zirconia.
[0034]
1.25 g of palladium nitrate solution containing 10 wt% as Pd and 8.56 g of tetraammineplatinum nitrate aqueous solution containing 1.47 wt% as Pt are mixed and stirred, and the resulting mixture is diluted to 20 ml by adding pure water did.
[0035]
25 g of the above sulfate radical zirconia was immersed in the solution thus prepared in advance for 10 hours. The soaked sulfate zirconia was dried and then calcined at 500 ° C. for 9 hours to obtain a 0.5% Pd-0.5% Pt / sulfate zirconia catalyst.
[0036]
Next, 2 g of an ammonium tungstate aqueous solution containing 50 wt% as WO ₃ was diluted to 20 ml by adding pure water, and 10 g of the above 0.5% Pd-0.5% Pt / 25 g of zirconia sulfate was added to the resulting solution. Soaked for hours. The dried 0.5% Pd-0.5% Pt / sulfate zirconia was dried and then calcined at 500 ° C. for 9 hours to obtain a W / 0.5% Pd-0.5% Pt / sulfate zirconia catalyst. Got.
[0037]
Example 2
Preparation of 0.5% Pd-0.5% Pt / W supported zirconia catalyst 5 g of an ammonium tungstate aqueous solution containing 50 wt% as WO ₃ was diluted to 25 ml with pure water, and 25 g of zirconium hydroxide was added to the resulting solution. Soaked for 10 hours. After drying this immersion body, it baked at 650 degreeC for 3.5 hours, and W carrying | support zirconia was obtained.
[0038]
1.25 g of a palladium nitrate solution containing 10% by weight as Pd and 8.56 g of a tetraammineplatinum nitrate aqueous solution containing 1.47% by weight as Pt are mixed and stirred, and the resulting mixture is made 20 ml by adding pure water. Diluted.
[0039]
25 g of the above W-supported zirconia was immersed in the solution thus prepared for 10 hours. The W-supported zirconia after immersion was dried and then calcined at 500 ° C. for 9 hours to obtain 0.5% Pd-0.5% Pt / W-supported zirconia.
[0040]
Comparative Example 1
Preparation of 0.5% Pd-0.5% Pt / sulfuric acid radical zirconia catalyst 150 g of zirconium hydroxide was immersed in 150 ml of an aqueous solution in which 15 g of ammonium sulfate was dissolved for 10 hours. This immersion body was dried and then calcined at 550 ° C. for 3.5 hours to obtain sulfate zirconia.
[0041]
1.25 g of a palladium nitrate solution containing 10% by weight as Pd and 8.56 g of a tetraammineplatinum nitrate aqueous solution containing 1.47% by weight as Pt were mixed and stirred, and the resulting mixture was diluted to 20 ml by adding pure water. .
[0042]
25 g of the above sulfate radical zirconia was immersed in the solution thus prepared in advance for 10 hours. After the immersion, the sulfate zirconia was dried and then calcined at 500 ° C. for 9 hours to obtain 0.5% Pd−0.5% Pt / sulfate zirconia.
[0043]
Catalyst Activity Tests The catalysts obtained in Examples 1 and 2 and Comparative Example 1 were formed into a disk shape, which was then pulverized and sized into meshes 22 to 42. The obtained catalyst is filled in a reaction tube, and a simulated exhaust gas in which 150 ppm of nitrogen monoxide and 2000 ppm of methane are mixed with a gas obtained by diluting compressed air with nitrogen so that the oxygen concentration becomes 10%, is obtained at a space velocity of 30000 h ^−1. The catalyst activity (denitration rate) was measured over time through a reaction tube under a reaction temperature of 450 ° C. The NOx concentration at the inlet and outlet of the catalyst layer was measured with a chemiluminescent NOx analyzer.
[0044]
The actual exhaust gas contains about 10% water vapor, but it was separately confirmed that this does not affect the activity under the reaction temperature of 450 ° C.
[0045]
The obtained catalytic activity test results are shown in FIG.
[0046]
【The invention's effect】
The catalyst according to the present invention stably has high NOx decomposition performance over a long period of time when NOx is decomposed using methane as a reducing agent.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between time and denitration rate.

Claims (11)

Palladium and platinum supported on a zirconia support, further ing the addition of tungsten, oxygen-rich atmosphere methane reducing agent to the exhaust gas purifying catalyst used in the reaction for decomposing the nitrogen oxides under. Tungsten supported on a zirconia support, then palladium and platinum ing carrying the oxygen-rich atmosphere methane reducing agent to the exhaust gas purifying catalyst used in the reaction for decomposing the nitrogen oxides under. Zirconia carrier includes a sulfate group, a palladium and platinum supported on a sulfate ion-containing zirconia support, further comprising adding a tungsten claim 1, wherein the exhaust gas purifying catalyst. Loading of tungsten to di zirconia carrier is impregnated with a aqueous solution of ammonium metatungstate hydroxide zirconia powder, then according to claim 2, wherein the same powder is made by baking for 1 to 24 hours at from 550 to 1,100 ° C. in air Exhaust gas purification catalyst.   The exhaust gas purifying catalyst according to any one of claims 1 to 4, wherein the amount of tungsten is 2 to 20% by weight based on the carrier.   The exhaust gas purifying catalyst according to any one of claims 1 to 5, wherein the supported amount of palladium is 0.01 to 0.5 wt% with respect to the support. The exhaust gas purification catalyst according to any one of claims 1 to 6 , wherein the amount of tungsten is 10 to 200 wt% with respect to palladium. An exhaust gas purification plate catalyst, wherein the catalyst fine powder according to any one of claims 1 to 7 is dispersed and held in a preform plate body made of ceramic fibers. A honeycomb structure obtained by laminating a folded product of a preform plate-like body made of ceramic fibers, or a folded product and a flat plate are alternately laminated, the catalyst fine powder according to any one of claims 1 to 7. An exhaust gas purifying honeycomb catalyst dispersed and held in a honeycomb structure. An exhaust gas purification method in which the catalyst according to any one of claims 1 to 9 is brought into contact with exhaust gas in an oxygen-excess atmosphere to decompose nitrogen oxides in the exhaust gas using methane as a reducing agent . The exhaust gas purification method according to claim 10, wherein the methane is unburned methane from a combustion device.

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