JP5164821B2 - Nitrogen oxide selective catalytic reduction catalyst - Google Patents

Description

The present invention relates to a catalyst for reducing nitrogen oxides NO _x (mainly NO and NO ₂ ) contained in exhaust gas to harmless nitrogen N ₂ using ammonia as a reducing agent. This catalyst is particularly useful for purifying exhaust gas from automobile engines, but is also useful for purifying flue gas from factory plants.

One of the technologies for removing nitrogen oxides (NO _x ) contained in automobile exhaust gas and flue gas from factory plants (denitration technology) is selective catalytic reduction (SCR). The method of ammonia was added as a reducing agent in a gas containing NO _x, is a method of reducing the NO _x to N ₂ with a catalyst. Many types of SCR catalysts have been known so far and some have been put into practical use, but the required important performance is high NO _{x in} both low temperature range (<350 ° C.) and high temperature range (> 350 ° C.). It was difficult to satisfy all of the demands that show the conversion rate, the NO _x conversion rate does not decrease even in the presence of water vapor, and that the generation of nitrous oxide N ₂ O is low at high temperatures.

For example, Patent Document 1 discloses an SCR catalyst made of synthetic zeolite ion-exchanged with Fe. It is said that the production of N ₂ O is suppressed by ion exchange of Fe. However, a decrease in catalyst performance due to the presence of water vapor is inevitable. Patent Document 2 discloses an SCR catalyst made of synthetic zeolite ion-exchanged with Fe and rare earth metal. In addition to Fe, ion exchange with rare earth metals is said to suppress a decrease in activity when exposed to a high-temperature atmosphere containing water vapor. However, the conversion rate in the low temperature range is lower than the corresponding catalyst ion-exchanged with Fe alone.

One type of SCR catalyst in practical use is a titanium / tungsten composite oxide catalyst using V ₂ O ₅ as an active species. This catalyst is not suitable for automobile exhaust gas purification because V ₂ O ₅ is a substance that prohibits diffusion in the atmosphere.

Non-Patent Document 1 discloses an SCR catalyst comprising a Mn—Ce binary composite oxide having a high activity in a low temperature range of 150 ° C. or lower and a ternary composite oxide obtained by adding a third metal oxide thereto. ing. However, the activity of these catalysts is remarkably lowered by water vapor, and N ₂ O is generated at a high temperature of 300 ° C. or higher, and the activity is also lowered because of the oxidation of NH ₃ .

JP 2006-136776 A JP 2006-305423 A G. Qi et al. , Applied Catalysis B: Environmental 51 (2004) 93-106.

The object of the present invention is to at least partially remedy the drawbacks of the previously known SCR catalysts. These catalysts are satisfactory in any performance of (a) high activity at low and high temperatures, (b) activity that does not decrease in the presence of water vapor, and (c) low N ₂ O production, especially at high temperatures. It wasn't.

  The present invention provides an SCR catalyst that is improved in any of these performances compared to previously known catalysts.

As described above, MnO x _/ CeO ₂ binary composite oxide catalyst significantly reduced activity by water vapor, but also includes generating a large amount of N _{2 O} at 300 ° C. or higher, even active for oxidation also occurs ammonia descend. According to the present invention, to the two-component composite oxide catalyst, further mixing the tungsten oxide by the MnO x _{/ CeO} 2 / WO ₃ ternary composite oxide, surprisingly a two-component composite oxide It has been found that the defects of objects can be improved significantly.

  Therefore, the present invention provides an SCR catalyst for removing nitrogen oxides mainly composed of cerium oxide and containing manganese oxide and tungsten oxide.

SCR catalyst of the present invention is to produce a _CeO 2 / MnO _x complex oxide and _CeO 2 / WO ₃ composite oxide by a coprecipitation method, mixing the two, mainly the _{CeO 2, CeO 2> MnO x} > WO _To obtain a mixture having a weight ratio of ₃ .

  The catalyst thus obtained can be used for denitration by coating and fixing on the surface of a support having a honeycomb structure.

The SCR catalyst of the present invention can be obtained by mixing a CeO ₂ / MnO _x composite oxide produced by the coprecipitation method and a CeO ₂ / WO ₃ composite oxide produced by the coprecipitation method. A composite oxide of CeO ₂ and MnO _x is formed by neutralizing an aqueous solution containing a water-soluble cerium salt such as cerium nitrate and a water-soluble manganese salt such as manganese nitrate with an alkali such as aqueous ammonia. The cake obtained by filtering can be produced by baking at a high temperature, for example, 550 ° C. after drying. A composite oxide of CeO ₂ and WO ₃ is prepared by neutralizing an aqueous solution of a water-soluble cerium salt such as cerium nitrate with aqueous ammonia, filtering the formed precipitate, and treating the cake with a water-soluble tungsten compound such as ammonium metatungstate. It can be obtained by pouring it into an aqueous solution, evaporating the moisture and drying it, followed by firing at 550 ° C.

A two-component composite oxide containing MnO _x, the catalyst of the present invention obtained by mixing a two-component composite oxide containing WO _{3 is} that the composition ratio of CeO 2> _MnO x> WO ₃ is maintained In the above conditions, it is appropriate that manganese oxide is in the range of 5 to 30% by weight and tungsten oxide is in the range of 2 to 10% based on cerium oxide. Outside this range, as the ratio of manganese oxide to cerium oxide increases, it approaches the CeO ₂ / MnO _x binary composite oxide, and tends to decrease activity due to water vapor and increase the production of nitrous oxide at high temperatures. On the contrary, the ratio of manganese oxide to cerium oxide decreases, and the overall SCR catalytic activity of NO _x tends to decrease as the ratio of tungsten oxide increases relatively. A catalyst having a weight ratio of CeO ₂ > WO ₃ > MnO _x is also low in overall activity.

The catalyst of the present invention must be fixed to a support in order to use it for a gas phase reaction. In this case, it is general to use a catalyst coating layer fixed on the surface of a heat-resistant support such as a metal such as stainless steel or a ceramic such as cordierite by, for example, a wash coating method. In order to ensure a contact area with the exhaust gas, a honeycomb support having a cell density of 200 to 600 cells / in ² is generally used. The SCR catalyst of the present invention can also be used by being fixed to the honeycomb structure support. In this case, the catalyst of the present invention is preferably supported in an amount of 30 to 300 g / L based on the volume of the honeycomb support.

  In addition, ammonia has been shown as the reducing agent used in the SCR reaction so far, but urea can also be used as the reducing agent.

  The invention is explained in more detail below by means of non-limiting examples.

Example 1
1) Preparation of catalyst a) 120 g of cerium nitrate hexahydrate and 195 g of manganese nitrate hexahydrate were dissolved in 3 L of water. To this solution, 28% ammonia water was slowly added dropwise with stirring, and neutralized to pH 8.5. The formed precipitate was filtered, and the cake was dried at 120 ° C. for 18 hours and then calcined at 550 ° C. for 6 hours to obtain a quantitative yield of manganese oxide / cerium oxide composite oxide powder (weight ratio of oxide based on 50/50). Got in.

b) Separately, 126 g of cerium nitrate hexahydrate was dissolved in 3 L of water, and 28% aqueous ammonia was slowly added dropwise to this solution while stirring to neutralize the solution to pH 8.5. The produced precipitate was filtered, and the cake was put into 1 L of an aqueous solution containing 10 g / L of ammonium metatungstate and stirred. This slurry was directly heated at 120 ° C. for 18 hours to evaporate to dryness, calcined at 550 ° C. for 6 hours, and tungsten oxide / cerium oxide composite oxide powder (oxide-based tungsten / cerium weight ratio = 9.1 / 90). .9) was obtained.

c) Take the powder obtained in step a) and the powder obtained in step b) into a mortar so that the weight ratio is 1: 2, thoroughly grind and mix until uniform, cerium oxide / oxidation Manganese / tungsten oxide composite oxide (oxide-based cerium / manganese / tungsten weight ratio = 77.3 / 16.7 / 6) was obtained. The contents of manganese oxide and tungsten oxide with respect to cerium oxide of this catalyst are 26.6% and 7.8%, respectively, on a weight basis.

2) Immobilization of catalyst 30 g of the catalyst of the present invention obtained in the above step c), 4 g of alumina sol (Nissan Chemical's alumina sol, 20 wt% as Al ₂ O ₃ ) and silica sol (Nissan Chemical's Snowtech O, SiO ₂₎ 8 g) and an appropriate amount of water were mixed, 50 g of 1.5 mm glass beads were added as a grinding medium, and pulverized for 10 minutes with a paint conditioner to obtain a washcoat slurry. This is applied to a cordierite honeycomb substrate of 400 cells per in ^{2 and} fired at 550 ° C. for 6 hours to produce a honeycomb catalyst structure carrying the catalyst component on the surface at a rate of 200 g / L (honeycomb volume). did.

Example 2
1) Preparation of catalyst a) 195 g of cerium nitrate hexahydrate and 86 g of manganese nitrate hexahydrate were dissolved in 3 L of water. To this solution, 28% ammonia water was slowly added dropwise with stirring, and neutralized to pH 8.5. The formed precipitate was filtered, and the cake was dried at 120 ° C. for 18 hours and then calcined at 550 ° C. for 6 hours to obtain a manganese oxide / cerium oxide composite oxide powder (oxide reference Mn / Ce weight ratio = 25/75 ). Got.
b) Separately, Step b) of Example 1 was repeated to obtain a tungsten oxide / cerium oxide composite oxide powder (oxide-based tungsten / cerium weight ratio = 9.1 / 90.9).
c) Take the powder obtained in step a) and the powder obtained in step b) into a mortar so that the weight ratio is 1: 2, thoroughly grind and mix until uniform, cerium oxide / oxidation Manganese / tungsten oxide composite oxide was obtained.
The contents of manganese oxide and tungsten oxide with respect to cerium oxide of this catalyst are 9.7% and 7.0%, respectively, on a weight basis.
2) Fixation of catalyst As in Example 1, the catalyst obtained above was fixed as a honeycomb catalyst structure.

Example 3
1) Preparation of catalyst a) Step a) of Example 2 was repeated to obtain manganese oxide / cerium oxide composite oxide powder (oxide reference Mn / Ce weight ratio = 25/75 ).
b) Separately, 126 g of cerium nitrate hexahydrate was dissolved in 3 L of water, and 28% aqueous ammonia was slowly added dropwise to this solution while stirring to neutralize it to pH 8.5. The produced precipitate was filtered, and the cake was put into 1 L of an aqueous solution containing 5 g / L of ammonium metatungstate and stirred. This slurry was directly heated at 120 ° C. for 18 hours to evaporate to dryness, and calcined at 550 ° C. for 6 hours. 95.3) was obtained.
c) Take the powder obtained in step a) and the powder obtained in step b) into a mortar so that the weight ratio is 1: 2, thoroughly grind and mix until uniform, cerium oxide / oxidation Manganese / tungsten oxide composite oxide was obtained. The contents of manganese oxide and tungsten oxide with respect to cerium oxide of this catalyst are 9.4% and 3.5%, respectively, on a weight basis.
2) Fixation of catalyst As in Example 1, the catalyst obtained above was fixed as a honeycomb catalyst structure.

Comparative Example 1
Instead of the cerium oxide / manganese oxide / tungsten oxide composite oxide obtained in Step c) of Example 1, the cerium oxide / manganese oxide composite oxide obtained in Step a) of Example 1 was used. The operation was repeated to produce a honeycomb catalyst structure of Comparative Example 1.

Comparative Example 2
In place of the cerium oxide / manganese oxide / tungsten oxide composite oxide obtained in step c) of Example 1, the cerium oxide / tungsten oxide composite oxide obtained in step b) of Example 1 was used. The operation was repeated to produce a honeycomb catalyst structure of Comparative Example 2.

Comparative Example 3
1) Preparation of catalyst a) Step a) of Example 2 was repeated to obtain manganese oxide / cerium oxide composite oxide powder (oxide reference Mn / Ce weight ratio = 25/75 ).
b) In step b) of Example 1, an aqueous solution containing 15 g / L of metatungstic acid was used in place of the aqueous solution containing 10 g / L of metatungstic acid, and a tungsten oxide / cerium oxide composite oxide powder (oxide) (Conversion tungsten / cerium weight ratio = 87.0 / 13.0).
c) Take the powder obtained in step a) and the powder obtained in step b) into a mortar so that the weight ratio is 1: 2, thoroughly grind and mix until uniform, cerium oxide / oxidation Manganese / tungsten oxide composite oxide was obtained. The contents of manganese oxide and tungsten oxide with respect to cerium oxide of this catalyst are 10.2% and 10.6%, respectively.
2) Fixation of catalyst As in Example 1, the catalyst obtained above was fixed as a honeycomb catalyst structure.

Comparative Example 4
1) Preparation of catalyst carrier 151.7 g of titanyl sulfate crystal (TM crystal manufactured by Teika Co., Ltd., TiO ₂ content 33%) was dissolved in 1 L of water, and this solution and 28% aqueous ammonia were slowly added dropwise with stirring to pH 6 The mixture was neutralized to 5 and stirred for 10 minutes for aging. Thereafter, ammonia water was dropped again to adjust the pH to 6.5, followed by filtration and washing with water, and then repulping into water to remove water-soluble impurities, followed by pH adjustment, filtration and washing with water. To the slurry obtained by dispersing the obtained filter cake in 500 mL of water, 4.27 g of ammonium paratungstate (manufactured by Nippon Inorganic Chemical Industry Co., Ltd.) was added, stirred for 10 minutes, and then directly filtered without filtration. Heat at 24 ° C. for 24 hours and evaporate to dryness. This was calcined at 600 ° C. for 3 hours and used as a support for V ₂ O ₅ titania / tungsten oxide composite oxide powder (oxide standard TiO ₂ / WO ₃ weight ratio = 90/10, specific surface area 85 m ² / g) was obtained.

2) Manufacture of honeycomb catalyst structure in which V ₂ O ₅ / WO ₃ / TiO ₂ catalyst is fixed To titania / tungsten oxide composite oxide 30 g obtained in step 1), titania sol (TKS-202, manufactured by Taca Co., Ltd.) After mixing 12 g and an appropriate amount of water, 50 g of 15 mmφ glass beads were added as a grinding medium, and pulverized for 10 minutes with a paint conditioner to obtain a washcoat slurry. This was applied to the same honeycomb substrate as used in the Examples, and fired at 500 ° C. for 1 hour to obtain a honeycomb structure having the catalyst carrier fixed on the surface at a rate of 200 g / L (honeycomb volume). The obtained honeycomb was impregnated in an ammonium metavanadate aqueous solution prepared in advance so that the impregnation amount was 10 g / L (honeycomb volume) as V ₂ O ₅ , dried, and fired again at 500 ° C. for 2 hours to obtain V ₂ O. A honeycomb catalyst structure having a ₅ / WO ₃ / TiO ₂ composite oxide catalyst fixed thereto was produced.

〔performance test〕
Each honeycomb catalyst structure (volume 2 mL) of the example and the comparative example is installed in an atmospheric pressure fixed bed, the reaction gas of condition 1 or 2 in Table 1 is used at a space velocity of 50,000 h ⁻¹ , a reaction temperature of 150 ° C. to 400 ° C. The NO _x concentration of the apparatus inlet gas and the outlet gas was measured using a chemiluminescence NO _x meter (Yanamoto Seisakusho ECL-77 type), and the NO _x conversion rate was calculated according to the following formula.

NO _x conversion rate (%) = (inlet NO _x concentration−outlet NO _x concentration) / (inlet NO _x concentration) × 100

The results are shown in Tables 2 and 3 and the graphs of FIGS. Further, using a gas chromatography meter (manufactured by Shimadzu Corporation), the concentration (ppm) of nitrous oxide N ₂ O in the outlet gas was measured, and the results shown in Table 4 and the graph of FIG. 3 were obtained.

As shown in the graph of Table 2 and FIG. 1 , the catalysts of Examples 1 to 3 are 300 ° C. higher than the catalyst of Comparative Example 1 that does not contain tungsten oxide and the catalyst of Comparative Example 2 that does not contain manganese oxide. A high NO _x conversion is shown at the above temperatures. The catalyst of Comparative Example 3 having a weight ratio of CeO ₂ > WO ₃ > MnO _x is similar to the catalyst of Comparative Example 2 in NO _x conversion. Further, when compared with the catalyst of Comparative Example 4 of the V ₂ O ₅ / WO ₃ / TiO ₂ system that has already been actually used, the catalysts of Examples 1 to 3 have a high NO _x conversion rate in a low temperature range up to 250 ° C. Show.

Referring to the graph of Table 3 and Figure 2, the catalyst of Example 1 and Comparative Example 1, tend to exhibit high NO _x conversion in the condition 2 H _{2 O} is not present than condition 1 H 2 _O is present However, the catalyst of Example 1 shows that the difference in the NO _x conversion rate under conditions 1 and 2 decreases as the reaction temperature increases. It also shows that the catalyst of Example 1 has high catalytic activity at temperatures up to 350 ° C., compared with the catalyst of Comparative Example 4 that has already been put into practical use.

Referring to Table 4 showing the production concentration of nitrous oxide N ₂ O and the graph of FIG. 3, the ternary composite oxide catalyst containing tungsten oxide of Examples 1 to 3 is that of Comparative Example 1 containing no tungsten oxide. Like a cerium oxide / manganese oxide binary composite oxide, it is less likely to produce a high concentration of N ₂ O at high temperatures.

Is a graph showing the conversion of the NO _x in the different temperatures of the catalysts of Examples and Comparative Examples. The catalysts of Examples and Comparative Examples, a graph showing the NO _x conversion at different temperatures in water vapor presence and absence. It is a graph which shows the nitrous oxide production amount in the high temperature of an Example and a comparative example.

Claims (6)

A catalyst for selectively catalytically reducing nitrogen oxide in a nitrogen oxide-containing gas to nitrogen using ammonia or urea as a reducing agent, comprising cerium oxide, manganese oxide, and tungsten oxide, CeO ₂ > MnO _x > Catalyst comprising a ternary composite oxide containing WO _{3 in} a weight ratio .
  The catalyst according to claim 1, wherein the content of manganese oxide in the catalyst is 5 to 30% by weight with respect to cerium oxide.   The catalyst according to claim 1 or 2, wherein the content of tungsten oxide in the catalyst is 2 to 10% by weight based on cerium oxide.   A honeycomb structure catalyst bed comprising the catalyst according to any one of claims 1 to 3 fixed on a surface of a honeycomb structure carrier.   The honeycomb structure catalyst bed according to claim 4, wherein the fixed amount of the catalyst is 30 to 300 g / L (honeycomb volume) with respect to the honeycomb structure carrier. The weight ratio of the binary composite oxide of (a) cerium oxide and manganese oxide and the binary composite oxide of (b) cerium oxide and tungsten oxide is such that CeO ₂ > MnO _x > WO _3. The method for producing a catalyst according to claim 1, wherein the catalyst is mixed.