NOx reduction catalyst for exhaust gas and method for producing same

Abstract

To provide a catalyst having excellent performance and durability by improving a NOx reduction ratio at 350 C. or higher without deteriorating excellent durability of a TiVMoP catalyst in view of problems of conventional art. A NOx reduction catalyst for exhaust gas, which is composed of a catalyst composition that comprises titanium (Ti), an oxide of phosphorous, molybdenum (Mo) and/or tungsten (W), oxide of vanadium (V), and high-silica zeolite that has an SiO.sub.2/Al.sub.2O.sub.3 ratio of not less than 20 is obtained by kneading in the presence of water, drying and calcining (1) titanium oxide, and phosphoric acid or an ammonium salt of phosphoric acid in an amount of more than 1% by weight and not more than 15% by weight relative to the titanium oxide in terms of H.sub.3PO.sub.4, (2) an oxo acid or oxo acid salt of molybdenum (Mo) and/or tungsten (W) and an oxo acid salt of vanadium (V) or vanadyl salt respectively in an amount of more than 0% by atom and not more than 8% by atom relative to the titanium oxide and (3) high-silica zeolite in an amount of more than 0% by weight and not more than 20% by weight relative to the titanium oxide.

Claims

1. A NOx reduction catalyst for exhaust gas, in which the catalyst has adsorption sites for NH.sub.3 and the catalyst comprises a calcination product comprising titanium (Ti), an oxide of phosphorous (P), molybdenum (Mo) and/or tungsten (W), an oxide of vanadium (V), and a high-silica zeolite having a SiO.sub.2/Al.sub.2O.sub.3 ratio of not less than 20 and not more than 31, in which the high-silica zeolite is mordenite or pentasil type zeolite.

2. The NOx reduction catalyst according to claim 1, in which the titanium is derived from titanium oxide, the oxide of phosphorous is derived from phosphoric acid or an ammonium salt of phosphoric acid in the amount of more than 1% by weight and not more than 15% by weight relative to the titanium oxide in terms of H3PO4, the molybdenum (Mo) and/or tungsten (W) are/is derived from an oxo acid or oxo acid salt of molybdenum (Mo) and/or tungsten (W) in the amount of more than 0% by atom and not more than 8% by atom relative to the titanium oxide, the oxide of vanadium (V) is derived form an oxo acid salt of vanadium (V) or vanadyl salt in the amount of more than 0% by atom and not more than 8% by atom relative to the titanium oxide, and the amount of the high-silica zeolite is more than 0% by weight and not more than 20% by weight relative to the titanium oxide.

3. The NOx reduction catalyst according to claim 1, in which the calcination product consists of titanium (Ti), the oxide of phosphorous (P), molybdenum (Mo) and/or tungsten (W), the oxide of vanadium (V), and the high-silica zeolite.

4. The NOx reduction catalyst according to claim 1, in which the high-silica zeolite has an SiO.sub.2/Al.sub.2O.sub.3 ratio of not less than 20 and not more than 30.

5. A method for producing a NOx reduction catalyst for exhaust gas according to claim 1, the method comprising the steps of bringing titanium oxide into contact with phosphoric acid or ammonium salt of phosphoric acid in the presence of water to adsorb phosphoric acid ion on the surface of the titanium oxide, adding an oxo acid or an oxo acid salt of molybdenum (Mo) and/or tungsten (W), an oxo acid salt of vanadium (V) or vanadyl salt, and high-silica zeolite to the titanium oxide adsorbed with phosphoric acid ion to obtain a mixture, wherein the high-silica zeolite has a SiO.sub.2/Al.sub.2O.sub.3 ratio of not less than 20 and not more than 31, in which the high-silica zeolite is mordenite or pentasil type zeolite, kneading the mixture in the presence of water, drying the kneaded mixture, and calcining the dried mixture.

6. The method according to claim 5, in which the amount of the phosphoric acid or the ammonium salt of phosphoric acid is more than 1% by weight and not more than 15% by weight relative to the titanium oxide in terms of H3PO4, the amount of the oxo acid or the oxo acid salt of molybdenum (Mo) and/or tungsten (W) is more than 0% by atom and not more than 8% by atom relative to the titanium oxide, the amount of the oxo acid salt of vanadium (V) or the vanadyl salt is more than 0% by atom and not more than 8% by atom relative to the titanium oxide, and the amount of the high-silica zeolite is more than 0% by weight and not more than 20% by weight relative to the titanium oxide.

7. The method according to claim 5, in which the high-silica zeolite has an SiO.sub.2/Al.sub.2O.sub.3 ratio of not less than 20 and not more than 30.

8. A method for producing a NOx reduction catalyst for exhaust gas according to claim 1, the method comprising the steps of kneading titanium oxide, phosphoric acid or an ammonium salt of phosphoric acid, an oxo acid or oxo acid salt of molybdenum (Mo) and/or tungsten (W), an oxo acid salt of vanadium (V) or vanadyl salt, and high-silica zeolite in the presence of water to obtain a kneaded mixture, wherein the high-silica zeolite has a SiO.sub.2/Al.sub.2O.sub.3 ratio of not less than 20 and not more than 31, in which the high-silica zeolite is mordenite or pentasil type zeolite, drying the kneaded mixture, and calcining the dried mixture.

9. The method according to claim 8, in which the amount of the phosphoric acid or the ammonium salt of phosphoric acid is more than 1% by weight and not more than 15% by weight relative to the titanium oxide in terms of H.sub.3PO.sub.4, the amount of the oxo acid or the oxo acid salt of molybdenum (Mo) and/or tungsten (W) is more than 0% by atom and not more than 8% by atom relative to the titanium oxide, the amount of the oxo acid salt of vanadium (V) or the vanadyl salt is more than 0% by atom and not more than 8% by atom relative to the titanium oxide, and the amount of the high-silica zeolite is more than 0% by weight and not more than 20% by weight relative to the titanium oxide.

10. The method according to claim 8, in which the high-silica zeolite has an SiO.sub.2/Al.sub.2O.sub.3 ratio of not less than 20 and not more than 30.

11. The NOx reduction catalyst for exhaust gas according to claim 1, wherein the catalyst further comprises a metal substrate processed into a lath or a ceramic fiber formed into a net-like shape.

12. The NOx reduction catalyst for exhaust gas according to claim 1, wherein the catalyst further comprises binders, a silica sol, or inorganic fibers.

Description

EXAMPLES

(1) The present invention is described below in detail by way of examples.

Example 1

(2) 900 g of titanium oxide (Ishihara Sangyo Kaisha, Ltd., specific surface area of 290 m.sup.2/g), 84.5 g of 85% phosphoric acid, 219 g of silica sol (product name: OS sol, from Nissan Chemical Industries, Ltd.), and 5568 g of water were placed in a kneader and kneaded for 45 minutes to let the phosphoric acid adsorb on the surface of TiO.sub.2. To this, 113 g of ammonium molybdate, 105 g of ammonium metavanadate, and 90 g of H type mordenite (TSZ-650 from Tosoh Corporation, SiO.sub.2/Al.sub.2O.sub.3 ratio=23) were added and the mixture was further kneaded for 1 hour, so that Mo and V compounds were supported on the surface of TiO.sub.2 adsorbed with phosphoric acid. Subsequently, 151 g of silica alumina based ceramic fiber (Toshiba Fine Flex K.K.) was slowly added while kneading for 30 minutes to obtain a uniform paste. The paste thus obtained was then placed on a metal lath substrate having a thickness of 0.7 mm made from SUS430 steel plate having a thickness of 0.2 mm. After sandwiching between two polyethylene sheets, the substrate was passed through a pair of pressuring rollers, coating and filling the openings of the metal lath substrate with the paste. After air drying, it was calcined at 500 C. for 2 hours to obtain the catalyst. The catalyst had the compositional atomic ratio of Ti/MoN=88/5/7, and the additive amounts of H.sub.3PO.sub.4 and zeolite were 8% by weight and 10% by weight, respectively, relative to TiO.sub.2.

Examples 2 and 3

(3) The catalysts were prepared in the same way as in Example 1 except that the additive amount of phosphoric acid was changed to 10.6 g and 42.4 g, respectively.

Example 4

(4) The catalyst was prepared in the same way as in Example 1 except that the additive amount of phosphoric acid was changed to 159 g, the additive amount of ammonium metavanadate was changed to 121 g, and 180 g of H type mordenite, TSZ-640 (product name) from Tosoh Chemicals (SiO.sub.2/Al.sub.2O.sub.3=22) was used. The catalyst had the compositional atomic ratio of Ti/Mo/V=88/5/8, and the additive amounts of H.sub.3PO.sub.4 and zeolite were 15% by weight and 20% by weight, respectively, relative to TiO.sub.2.

Examples 5 and 6

(5) The catalysts were prepared in the same way as in Example 1 except that the titanium oxide used in Example 1 was changed to a titanium oxide having a specific surface area of 90 m.sup.2/g, the additive amount of phosphoric acid to the catalyst was changed to 4% by weight relative to TiO.sub.2, and the amounts of ammonium metavanadate and ammonium molybdate were each changed to 6.8 g and 61.8 g, and 27.7 g and 62.7 g, respectively, and 27 g of H type mordenite, TSZ-660 (product name) from Tosoh Chemicals (SiO.sub.2/Al.sub.2O.sub.3=31) was used. The catalysts had the compositional atomic ratio of Ti/Mo/V=96.5/3/0.5, and 95/3/2, and the additive amounts of H.sub.3PO.sub.4 and zeolite were 4% by weight and 3% by weight, respectively, relative to TiO.sub.2.

Example 7

(6) The catalyst was prepared in the same way as in Example 1 except that 113 g of ammonium molybdate used in Example 1 was changed to 162 g of ammonium metatungstate, and the additive amount of H type mordenite was changed to 9 g. The catalyst had compositional atomic ratio of Ti/W/V=88/5/7, and the additive amounts of H.sub.3PO.sub.4 and zeolite were 8% by weight and 1% by weight, respectively, relative to TiO.sub.2.

Comparative Examples 1 to 7

(7) The catalysts were prepared in the same way as in Examples 1 to 7 except that zeolite addition was not performed.

Comparative Examples 8 to 11

(8) The catalysts were prepared in the same way as in Example 1, and 5 to 7 except that phosphoric acid addition and adsorption steps were not carried out.

Test Example 1

(9) The catalysts of Examples 1 to 7 and Comparative Examples 1 to 7 were cut out into pieces of 20 mm wide100 mm long, and the NOx reduction performance of respective catalysts were measured under the condition listed in Table 1. The results are shown in Table 2.

(10) As can be seen in Table 2, when the performance of catalysts of Examples and Comparative Examples of the present invention are compared, the difference among them are small at 350 C., however, at 400 C. the performance of the Examples are significantly higher, suggesting that they are superior catalysts with improved NOx reduction ability at temperatures of 350 C. or higher.

Test Examples 2 and 3

(11) In order to clarify the advantage of the catalysts of the present invention, the catalysts of Examples 1 and 5 to 7, and Comparative Examples 8 to 17 were cut out into pieces of 20 mm wide100 mm long, and impregnated with aqueous solution of potassium carbonate so that its additive amount would be 0.5% by weight relative to the catalyst component in terms of K.sub.2O, Subsequently, they were dried at 150 C., and subjected to a test simulating the deterioration by the potassium compounds contained in the biomass combustion ash.

(12) Independently of this, the catalysts of Examples 1 and 5 to 7 and Comparative Examples 8 to 17, were impregnated with aqueous solution of arsenious acid so that the amount of As.sub.2O.sub.3 would be 2% by weight relative to the catalyst component. Subsequently, they were calcined at 350 C. for 1 hour and a test simulating the deterioration by high S coal exhaust gas was performed.

(13) In regard to the catalysts subjected to the two tests described above and the catalysts not subjected to the tests, the NOx reduction performance were measured using the conditions shown in Table 3, and the resistance to catalyst poisoning was evaluated for each of the catalysts. The results are summarized in Table 4.

(14) As can be seen in Table 4, when the performance of each of the catalysts are compared, the catalysts according to the Examples of the present invention exhibit significantly less deterioration by the potassium and arsenic compounds, and are superior in durability.

(15) From this result and results of test examples, it is clear that the catalyst of the present invention is superior in terms of its characteristics at a high temperature 350 C. or higher, as well as its resistance against deterioration by the catalyst poisons such as K and As.

(16) TABLE-US-00001 TABLE 1 Category Value 1. gas compositional ratio NO.sub.x 200 ppm NH.sub.3 240 ppm SO.sub.2 500 ppm O.sub.2 3% CO.sub.2 12% H.sub.2O 12% 2. gas flow rate 3.7 liter/min 3. temperature 350 deg C./ 400 deg C. 4. catalyst load amount 20 mm width 100 mm (length) 1 piece

(17) TABLE-US-00002 TABLE 2 350 deg C. NO.sub.x 400 deg C. NO.sub.x catalyst reduction ratio [%] reduction ratio [%] Ex. 1 75.5 76.4 Ex. 2 81.7 85.3 Ex. 3 79.5 82.1 Ex. 4 74.2 75.3 Ex. 5 64.9 69.2 Ex. 6 70.9 72.2 Ex. 7 76.1 78.1 Comp. Ex. 1 74.8 73.0 Comp. Ex. 2 81.5 81.5 Comp. Ex. 3 78.9 79.0 Comp. Ex. 4 72.0 71.7 Comp. Ex. 5 62.2 64.0 Comp. Ex. 6 70.6 71.3 Comp. Ex. 7 76.9 74.5

(18) TABLE-US-00003 TABLE 3 Category Value 1. gas compositional ratio NO.sub.x 200 ppm NH.sub.3 240 ppm SO.sub.2 500 ppm O.sub.2 3% CO.sub.2 12% H.sub.2O 12% 2. gas flow rate 3.7 liter/min 3. temperature 350 deg C. 4. catalyst load amount 20 mm width 100 mm (length) 3 pieces

(19) TABLE-US-00004 TABLE 4 Initial NO.sub.x NO.sub.x reduction ratio NO.sub.x reduction ratio reduction [%] after K [%] after As catalyst ratio [%] deterioration test deterioration test Ex. 1 99.1 98.1 94.8 Ex. 5 95.3 83.9 89.8 Ex. 6 97.4 87.7 93.0 Ex. 7 99.1 98.1 96.1 Comp. Ex. 8 99.2 78.4 85.8 Comp. Ex. 9 96.4 62.7 72.1 Comp. Ex. 10 98.2 69.9 77.5 Comp. Ex. 11 99.5 72.5 80.9