Method of preparing catalyst for low-temperature synergistic catalytic purification of NO.SUB.x .and HCN in flue gas, and use thereof

Abstract

The present invention discloses a method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in a flue gas, and the use thereof. Citric acid is dissolved in ethanol to obtain a citric acid/ethanol solution; tetrabutyl titanate is added, mixed uniformly to obtain a tetrabutyl titanate-citric acid/ethanol solution; glacial acetic acid is added dropwise to react for 30-40 min to obtain a solution A; the metal salt solution was added dropwise into the solution A, mixed uniformly and added with nitric acid, ammonium hydroxide is added dropwise to adjust the pH value, and the temperature is raised at a constant speed to obtain a gel B; dried and then then baked at a temperature of 300-500° C. for 3-4 h, cooled in the furnace, pulverized, tableted and sieved to obtain the catalyst for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas.

Claims

1. A method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas, comprising the following specific steps: (1) dissolving an active metal salt in deionized water to obtain a metal salt solution, wherein the active metal salt is two or more of a lanthanum salt, a cobalt salt, an aluminum salt, a copper salt, an iron salt, a manganese salt, a cerium salt, a nickel salt and a niobium salt; (2) dissolving citric acid in ethanol under stirring to obtain a citric acid/ethanol solution; then adding tetrabutyl titanate into the citric acid/ethanol solution, mixing uniformly, and reacting for 20-30 min to obtain a tetrabutyl titanate-citric acid/ethanol solution; (3) adding glacial acetic acid dropwise into the tetrabutyl titanate-citric acid/ethanol solution of the step (2) under stirring to react for 30-40 min to obtain a solution A; (4) adding the metal salt solution of the step (1) dropwise into the solution A of the step (3), mixing uniformly and adding nitric acid, then adding ammonium hydroxide dropwise to adjust the pH value of the system to 3-5 or 9-11, and rising the temperature at a constant speed until the temperature is 70-80° C. to obtain a gel B; and (5) subjecting the gel B of the step (4) to a constant-temperature treatment under a condition of a temperature of 90-110° C. for 2-3 days, then baking under a condition of a temperature of 300-500° C. for 3-4 h in a furnace, cooling in the furnace, pulverizing, tableting, and sieving to obtain a catalyst M-N/TiO.sub.2 for low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas, wherein M-N is two or more metal oxides of active metals lanthanum, cobalt, aluminum, copper, iron, manganese, cerium, nickel and niobium.

2. The method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas according to claim 1, wherein the mass percentage of the active metal oxide M-N in the catalyst M-N/TiO.sub.2 is 5%-15%.

3. The method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas according to claim 1, wherein the molar ratio of citric acid to tetrabutyl titanate in the step (2) is (1-1.2):1, and the concentration of citric acid in the citric acid/ethanol solution is 0.16-0.26 g/mL.

4. The method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas according to claim 1, wherein the volume ratio of the citric acid/ethanol solution to tetrabutyl titanate in the step (2) is (2.4-4.1):1.

5. The method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas according to claim 1, wherein the volume ratio of glacial acetic acid to tetrabutyl titanate in the step (3) is (0.6-1.4):1.

6. The method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas according to claim 1, wherein the volume ratio of nitric acid to tetrabutyl titanate in the step (4) is (0.6-1.4):1, and the constant rate of temperature rising is 2.5-3.5° C./h.

7. A catalyst prepared by the method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas according to claim 1.

8. A catalyst prepared by the method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas according to claim 2.

9. A catalyst prepared by the method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas according to claim 3.

10. A catalyst prepared by the method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas according to claim 4.

11. A catalyst prepared by the method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas according to claim 5.

12. A catalyst prepared by the method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas according to claim 6.

13. Use of the catalyst according to claim 7 in low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas.

14. Use of the catalyst according to claim 8 in low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas.

15. Use of the catalyst according to claim 9 in low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas.

16. Use of the catalyst according to claim 10 in low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas.

17. Use of the catalyst according to claim 11 in low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas.

18. Use of the catalyst according to claim 12 in low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows nitrogen adsorption-desorption isotherms of the catalyst La.sub.1Cu.sub.9/TiO.sub.2 of Example 1 and Comparative Example 1;

(2) FIG. 2 shows the pore diameter distribution of the catalyst La.sub.1Cu.sub.9/TiO.sub.2 of Example 1 and Comparative Example 1;

(3) FIG. 3 is an H.sub.2-TPR diagram of the catalyst La.sub.2O.sub.3—CuO/TiO.sub.2 of Example 1 and Comparative Example 1;

(4) FIG. 4 is a NH.sub.3-TPD diagram of the catalyst La.sub.2O.sub.3—CuO/TiO.sub.2 of Example 1 and Comparative Example 1; and

(5) FIG. 5 is an O1sXPS diagram of the catalyst La.sub.2O.sub.3—CuO/TiO.sub.2 of Example 1 and Comparative Example 1.

DESCRIPTION OF THE EMBODIMENTS

(6) The present invention will be further described in detail hereafter with reference to specific embodiments, but the claimed scope of the present invention is not limited to the disclosure.

(7) Example 1: in this example, the carrier of the catalyst for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas was TiO.sub.2, the active components were La.sub.2O.sub.3 and CuO, and the mass percentage of the active components (La.sub.2O.sub.3 and CuO) was 10%, where La.sub.2O.sub.3 was 1% and CuO was 9%, and this was recorded as La.sub.1Cu.sub.9/TiO.sub.2.

(8) A method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas, included the following specific steps:

(9) (1) the active metal salt La(NO.sub.3).sub.3.6H.sub.2O and Cu(NO.sub.3).sub.2.6H.sub.2O was dissolved in deionized water to obtain a metal salt solution, where the concentration of the metal cation in the metal salt solution was 0.475 mol/L;

(10) (2) citric acid was dissolved in ethanol under stirring to obtain a citric acid/ethanol solution, where the molar ratio of citric acid to tetrabutyl titanate was 1:1, and the concentration of citric acid in the citric acid/ethanol solution was 0.19 g/mL; then tetrabutyl titanate was added into the citric acid/ethanol solution, mixed uniformly, and reacted for 25 min to obtain a tetrabutyl titanate-citric acid/ethanol solution, where the volume ratio of the citric acid/ethanol solution to tetrabutyl titanate was 3.2:1;

(11) (3) glacial acetic acid was added dropwise into the tetrabutyl titanate-citric acid/ethanol solution of step (2) under stirring for 30 min to obtain a solution A; where the volume ratio of glacial acetic acid to tetrabutyl titanate was 0.6:1;

(12) (4) the metal salt solution of the step (1) was added dropwise into the solution A of the step (3), mixed uniformly, and added with nitric acid, where the volume ratio of nitric acid to tetrabutyl titanate was 0.6:1; then ammonium hydroxide was added dropwise to adjust the pH value of the system to 10, and the temperature was raised at a constant speed until the temperature was 70° C. to obtain a gel B; where the constant rate of temperature rising was 2.5° C./h; and

(13) (5) the gel B of the step (4) was subjected to a constant-temperature treatment under a condition of a temperature of 90° C. for 3 days, and then baked at a temperature of 350° C. for 4 h, cooled in the furnace, pulverized, tableted and sieved to obtain the catalyst La.sub.1Cu.sub.9/TiO.sub.2 for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas.

(14) The method of applying this example in the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the coke oven flue gas, included the following specific steps:

(15) (1) placing the catalyst La.sub.1Cu.sub.9/TiO.sub.2 for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas of this example in a fixed bed quartz tube reactor;

(16) (2) the temperature of the fixed bed reactor was set as 150° C., and the simulated flue gas was mixed uniformly and introduced into the reactor for a catalytic reaction, where the simulated flue gas contained 100 ppm of NO.sub.x, 100 ppm of HCN, 50 ppm of NH.sub.3, had a relative humidity of 10%, a O.sub.2 volume fraction of 5%, and the balance of N.sub.2; the total gas flow rate was 600 mL/min, and the reactor air speed was 50,000 h.sup.−1; and

(17) (3) the concentrations of NO.sub.x, HCN, NH.sub.3, CO, and CO.sub.2 in the simulated flue gas at the outlet of the fixed reactor in the step (2) was detected.

(18) The test results were as follows: when at 150° C., the catalyst La.sub.1Cu.sub.9/TiO.sub.2 (pH=10) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN of this example was used, the purification efficiencies of NO.sub.x and HCN reached 95.50% and 100%, respectively; and the catalyst performed well under the condition of the complex gas composition: the catalyst had a long service life, a high synergistic catalytic activity, and a stable performance.

(19) Example 2: the method for preparing the catalyst of this example was basically the same as that of Example 1, except that in the step (4) ammonium hydroxide was added dropwise to adjust the pH value of the system to 11.

(20) The application of the catalyst of this example was the same as that of the method of Example 1, and the test results were as follows: when at the temperature of 150° C., the catalyst La.sub.1Cu.sub.9/TiO.sub.2 (pH=11) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN was used, the purification efficiencies of NO.sub.x and HCN were 94.28% and 98.65%, respectively.

(21) Example 3: the method for preparing the catalyst of this example was basically the same as that of Example 1, except that in the step (4) ammonium hydroxide was added dropwise to adjust the pH value of the system to 9.

(22) The application of the catalyst of this example was the same as that of the method of Example 1, and the test results were as follows: when at the temperature of 150° C., the catalyst La.sub.1Cu.sub.9/TiO.sub.2 (pH=9) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN was used, the purification efficiencies of NO.sub.x and HCN were 92.15% and 96.69%, respectively.

(23) Comparative Example 1: the method for preparing the catalyst of this comparative example was basically the same as that of Example 1, except that in the step (4) ammonium hydroxide was added dropwise to adjust the pH value of the system to 8;

(24) The application of the catalyst of this comparative example was the same as that of Example 1, and the test results were as follows: when at the temperature of 150° C., the catalyst La.sub.1Cu.sub.9/TiO.sub.2 (pH=8) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN was used, the purification efficiencies of NO.sub.x and HCN were 49.67% and 58.65%, respectively.

(25) The nitrogen adsorption-desorption isotherm diagram of the catalyst La.sub.1Cu.sub.9/TiO.sub.2 of Example 1 and this Comparative Example was as shown in FIG. 1. As could be seen from FIG. 1, the two adsorption-desorption isotherms, in each of which a hysteresis loop occurred and for the catalyst La.sub.1Cu.sub.9/TiO.sub.2 at pHs of 8 and 10 respectively, both belonged to the isotherm of type IV; no adsorption saturation platform appeared in a region had a relatively higher pressure, demonstrating that the pore distribution was uneven, and both the surface and the interior of the catalyst contained micropores and mesopores; and the hysteresis loop of the isotherm at the pH of 8 was narrower than that at the pH of 10, indicating that the sample having the pH of 8 had a narrower pore diameter distribution.

(26) The pore diameter distribution diagram of the catalyst La.sub.1Cu.sub.9/TiO.sub.2 of Example 1 and this comparative example was as shown in FIG. 2. As could be seen from FIG. 2, the pores of the sample having the pH of 10 were distributed as having two sharp peaks between 3-7 nm while the pores of the sample having the pH of 8 had a plurality of small peaks distributed within 1-10 nm and had a relatively smaller pore diameter; and Table 1 showed the surface area, the pore volume and the pore diameter of the catalyst La.sub.1Cu.sub.9/TiO.sub.2. In connection with the pore diameter distribution diagram of FIG. 2 and Table 1, the catalyst having the pH of 10 had a specific surface area of 64.369 m.sup.2/g, which was much greater than the specific surface area of 13.505 m.sup.2/g of the catalyst having the pH of 8.

(27) TABLE-US-00002 TABLE 1 The surface areas, pore volumes and pore diameters of the catalyst La.sub.1Cu.sub.9/TiO.sub.2 of Example 1 and this comparative example BET surface area Pore volume Pore Diameter Samples (m.sup.2/g) (cm.sup.3/g) (nm) 8 13.505 0.047 1.486 10 64.369 0.112 3.935

(28) In connection with the pore diameter distribution diagram of FIG. 2 and Table 1, the catalyst having the pH of 10 had a specific surface area of 64.369 m.sup.2/g, which was much greater than the specific surface area of 13.505 m.sup.2/g of the catalyst having the pH of 8.

(29) The H2-TPR diagram of the catalyst La.sub.2O.sub.3—CuO/TiO.sub.2 of Example 1 and this comparative example was as shown in FIG. 3. As could be seen from FIG. 3, for the catalyst having the pH of 10: the reduction peak near 110° C. belonged to the reduction of highly dispersed CuO; the reduction peak near 147° C. corresponded to reducing Cu.sup.2+ into elemental Cu; and the shoulder peak near 200° C. belonged to the reduction of the bulk CuO; for the catalyst having the pH of 8: there was a main peak near 155° C., which corresponded to reducing Cu.sup.2+ into elemental Cu; and the reduction peak temperature of the catalyst having the pH of 10 was lower than that of the catalyst having the pH of 8, indicating that the catalyst having the pH of 10 had a higher reductivity at a lower temperature, which was also the reason why this catalyst had a higher conversion rate of HCN and a higher catalytic conversion efficiency of NO.sub.x at low temperature.

(30) The NH.sub.3-TPD diagram of the catalyst La.sub.2O.sub.3—CuO/TiO.sub.2 of Example 1 and this comparative example was as shown in FIG. 4. As could be seen from FIG. 4, three major NH.sub.3 desorption peaks occurred at one time from low temperature to high temperature, which corresponded to a weakly acidic adsorption site, a moderately-to-highly acidic site and a highly acidic adsorption site respectively; the catalyst having the pH of 10 had a stronger desorption peak at high temperature, indicating that the catalyst had more highly acidic sites than the catalyst having the pH of 8; and as there were more highly acidic adsorption sites, more of NH.sub.3 generated by hydrolysis of HCN in the flue gas and the NH.sub.3 added into the system could be adsorbed onto the surface of the catalyst to carry out the SCR denitration reaction.

(31) The O1s XPS diagram of the catalyst La.sub.2O.sub.3—CuO/TiO.sub.2 of Example 1 and this comparative example was as shown in FIG. 5. As could be seen from FIG. 5, the figure showed two O 1 s peaks, where the peak near 529.8 eV belonged to lattice oxygen, and the peak near 531.7 eV belonged to chemisorbed oxygen on the surface of the catalyst; and the catalyst having the pH of 10 had a binding energy of chemisorbed oxygen higher than that of the catalyst having the pH of 8, indicating that the interaction between the chemisorbed oxygen and the catalyst was higher in the catalyst having the This shows that the interaction between chemisorbed oxygen and the catalyst was stronger in the catalyst having the pH of 10.

(32) Comparative Example 2: the method for preparing the catalyst of this comparative example was basically the same as that of Example 1, except that in the step (4) ammonium hydroxide was added dropwise to adjust the pH value of the system to 12.

(33) The application of the catalyst of this comparative example was the same as that of Example 1, and the test results were as follows: when at the temperature of 150° C., the catalyst La.sub.1Cu.sub.9/TiO.sub.2 (pH=12) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN was used, the purification efficiencies of NO.sub.x and HCN were 50.32% and 62.73%, respectively.

(34) Example 4: in this example, the carrier of the catalyst for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas was TiO.sub.2, the active components were CeO.sub.2 and CuO, and the mass percentage of the active components (CeO.sub.2 and CuO) was 15%, where CeO.sub.2 was 7% and CuO was 8%, and this was recorded as Ce.sub.7Cu.sub.8/TiO.sub.2.

(35) A method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas, included the following specific steps:

(36) (1) the active metal salt (Ce(NO.sub.3).sub.3.6H.sub.2O and Cu(NO.sub.3).sub.2.6H.sub.2O) was dissolved in deionized water to obtain a metal salt solution; and the concentration of the metal cation in the solution was 0.562 mol/L;

(37) (2) citric acid was dissolved in ethanol under stirring to obtain a citric acid/ethanol solution, where the molar ratio of citric acid to tetrabutyl titanate was 1.05:1, and the concentration of citric acid in the citric acid/ethanol solution was 0.23 g/mL; then tetrabutyl titanate was added into the citric acid/ethanol solution, mixed uniformly, and reacted for 30 min to obtain a tetrabutyl titanate-citric acid/ethanol solution, where the volume ratio of the citric acid/ethanol solution to tetrabutyl titanate was 4.1:1;

(38) (3) glacial acetic acid was added dropwise into the tetrabutyl titanate-citric acid/ethanol solution of step (2) under stirring for 35 min to obtain a solution A; where the volume ratio of glacial acetic acid to tetrabutyl titanate was 1.4:1;

(39) (4) the metal salt solution of the step (1) was added dropwise into the solution A of the step (3), mixed uniformly, and added with nitric acid, where the volume ratio of nitric acid to tetrabutyl titanate was 1.4:1; then ammonium hydroxide was added dropwise to adjust the pH value of the system to 4, and the temperature was raised at a constant speed until the temperature was 75° C. to obtain a gel B; where the constant rate of temperature rising was 3.5° C./h; and

(40) (5) the gel B of the step (4) was subjected to a treatment under a condition of a temperature of 100° C. for 2.5 days, and then baked at a temperature of 450° C. for 4 h, cooled in the furnace, pulverized, tableted and sieved to obtain the catalyst Ce.sub.7Cu.sub.8/TiO.sub.2 for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas.

(41) The method of applying this example in the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the coke oven flue gas, included the following specific steps:

(42) (1) placing the catalyst Ce.sub.7Cu.sub.8/TiO.sub.2 for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas of this example in a fixed bed quartz tube reactor;

(43) (2) the temperature of the fixed bed reactor was set as 150° C., and the simulated flue gas was mixed uniformly and introduced into the reactor for a catalytic reaction, where the simulated flue gas contained 200 ppm of NO.sub.x, 100 ppm of HCN, 150 ppm of NH.sub.3, had a relative humidity of 10%, a O.sub.2 volume fraction of 5%, and the balance of N.sub.2; the total gas flow rate was 600 mL/min, and the reactor air speed was 50,000 h.sup.−1; and

(44) (3) the concentrations of NO.sub.x, HCN, NH.sub.3, CO, and CO.sub.2 in the simulated flue gas at the outlet of the fixed reactor in the step (2) was detected.

(45) The test results were as follows: when at the temperature of 150° C., the catalyst Ce.sub.7Cu.sub.8/TiO.sub.2 (pH=4) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN was used, the purification efficiencies of NO.sub.x and HCN reached 91.13% and 98.67%, respectively; and the catalyst performed well under the condition of the complex gas composition: it had a high catalytic reduction property, a high resistance to loss, and a stable performance.

(46) Example 5: the method for preparing the catalyst of this example was basically the same as that of Example 4, except that in the step (4) ammonium hydroxide was added dropwise to adjust the pH value of the system to 3;

(47) The application of the catalyst of this example was the same as that of the method of Example 4, and the test results were as follows: when at the temperature of 150° C., the catalyst Ce.sub.7Cu.sub.8/TiO.sub.2 (pH=3) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN was used, the purification efficiencies of NO.sub.x and HCN were 89.16% and 95.63%, respectively.

(48) Example 6: the method for preparing the catalyst of this example was basically the same as that of Example 4, except that in the step (4) ammonium hydroxide was added dropwise to adjust the pH value of the system to 5;

(49) The application of the catalyst of this example was the same as that of the method of Example 4, and the test results were as follows: when at the temperature of 150° C., the catalyst Ce.sub.7Cu.sub.8/TiO.sub.2 (pH=5) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN was used, the purification efficiencies of NO.sub.x and HCN were 88.27% and 91.49%, respectively. Comparative Example 3: the method for preparing the catalyst of this comparative example was basically the same as that of Example 4, except that in the step (4) ammonium hydroxide was added dropwise to adjust the pH value of the system to 2.

(50) The application of the catalyst of this comparative example was the same as that of the method of Example 4, and the test results were as follows: when at the temperature of 150° C., the catalyst Ce.sub.7Cu.sub.8/TiO.sub.2 (pH=2) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN was used, the purification efficiencies of NO.sub.x and HCN were 53.27% and 57.31%, respectively.

(51) Comparative Example 4: the method for preparing the catalyst of this comparative example was basically the same as that of Example 4, except that in the step (4) ammonium hydroxide was added dropwise to adjust the pH value of the system to 6.

(52) The application of the catalyst of this comparative example was the same as that of the method of Example 4, and the test results were as follows: when at the temperature of 150° C., the catalyst Ce.sub.7Cu.sub.8/TiO.sub.6 (pH=6) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN was used, the purification efficiencies of NO.sub.x and HCN were 45.46% and 48.19%, respectively.

(53) Example 7: in this example, the carrier of the catalyst for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas was TiO.sub.2, the active components were CeO.sub.2 and MnO.sub.2, and the mass percentage of the active components (CeO.sub.2 and MnO.sub.2) was 5%, where CeO.sub.2 was 3% and MnO.sub.2 was 2%, and this was recorded as Ce.sub.3Mn.sub.2/TiO.sub.2.

(54) A method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas, included the following specific steps:

(55) (1) the active metal salt Ce(NO.sub.3).sub.3.6H.sub.2O was dissolved in deionized water, then added with a Mn(NO.sub.3).sub.2 solution and mixed uniformly to obtain a metal salt solution; where the concentration of the metal cation in the solution was 0.162 mol/L;

(56) (2) citric acid was dissolved in ethanol under stirring to obtain a citric acid/ethanol solution, where the molar ratio of citric acid to tetrabutyl titanate was 1.1:1, and the concentration of citric acid in the citric acid/ethanol solution was 0.26 g/mL; then tetrabutyl titanate was added into the citric acid/ethanol solution, mixed uniformly, and reacted for 20 min to obtain a tetrabutyl titanate-citric acid/ethanol solution, where the volume ratio of the citric acid/ethanol solution to tetrabutyl titanate was 2.4:1;

(57) (3) glacial acetic acid was added dropwise into the tetrabutyl titanate-citric acid/ethanol solution of step (2) under stirring for 40 min to obtain a solution A; where the volume ratio of glacial acetic acid to tetrabutyl titanate was 0.86:1;

(58) (4) the metal salt solution of the step (1) was added dropwise into the solution A of the step (3), mixed uniformly, and added with nitric acid, where the volume ratio of nitric acid to tetrabutyl titanate was 0.86:1; then ammonium hydroxide was added dropwise to adjust the pH value of the system to 5, and the temperature was raised at a constant speed until the temperature was 80° C. to obtain a gel B; where the constant rate of temperature rising was 3.5° C./h; and

(59) (5) the gel B of the step (4) was subjected to a treatment under a condition of a temperature of 110° C. for 2 days, and then baked at a temperature of 450° C. for 4 h, cooled in the furnace, pulverized, tableted and sieved to obtain the catalyst Ce.sub.3Mn.sub.2/TiO.sub.2 for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas.

(60) The method of applying this example in the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the coke oven flue gas, included the following specific steps:

(61) (1) placing the catalyst Ce.sub.3Mn.sub.2/TiO.sub.2 for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas of this example in a fixed bed quartz tube reactor;

(62) (2) the temperature of the fixed bed reactor was set as 150° C., and the simulated flue gas was mixed uniformly and introduced into the reactor for a catalytic reaction, where the simulated flue gas contained 300 ppm of NO.sub.x, 100 ppm of HCN, 250 ppm of NH.sub.3, had a relative humidity of 10%, a O.sub.2 volume fraction of 5%, and the balance of N.sub.2; the total gas flow rate was 600 mL/min, and the reactor air speed was 50,000 h.sup.−1; and

(63) (3) the concentrations of NO.sub.x, HCN, NH.sub.3, CO, and CO.sub.2 in the simulated flue gas at the outlet of the fixed reactor in the step (2) was detected.

(64) The test results were as follows: when at the temperature of 150° C., the catalyst Ce.sub.3Mn.sub.2/TiO.sub.2 (pH=5) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN was used, the purification efficiencies of NO.sub.x and HCN reached 89.28% and 96.67%, respectively; and the catalyst performed well under the condition of the complex gas composition: it had a high anti-poisoning performance, high catalytic efficiency, a stable performance, and a long service life.

(65) Comparative Example 5: the method for preparing the catalyst of this comparative example was basically the same as that of Example 7, except that in the step (4) ammonium hydroxide was added dropwise to adjust the pH value of the system to 7.

(66) The application of the catalyst of this comparative example was the same as that of the method of Example 7, and the test results were as follows: when at the temperature of 150° C., the catalyst Ce.sub.3Mn.sub.2/TiO.sub.2 (pH=7) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN was used, the purification efficiencies of NO.sub.x and HCN were 35.77% and 47.09%, respectively.

(67) Example 8: in this example, the carrier of the catalyst for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas was TiO.sub.2, the active components were La.sub.2O.sub.3 and CoO, and the mass percentage of the active components (La.sub.2O.sub.3 and CoO) was 8%, where La.sub.2O.sub.3 was 2% and CoO was 6%, and this was recorded as La.sub.2Co.sub.6/TiO.sub.2.

(68) A method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas, included the following specific steps:

(69) (1) the active metal salt (La(NO.sub.3).sub.3.6H.sub.2O and Co(NO.sub.3).sub.2.6H.sub.2O) was dissolved in deionized water to obtain a metal salt solution; and the concentration of the metal cation in the solution was 0.37 mol/L;

(70) (2) citric acid was dissolved in ethanol under stirring to obtain a citric acid/ethanol solution, where the molar ratio of citric acid to tetrabutyl titanate was 1.2:1, and the concentration of citric acid in the citric acid/ethanol solution was 0.22 g/ml; then tetrabutyl titanate was added into the citric acid/ethanol solution, mixed uniformly, and reacted for 23 min to obtain a tetrabutyl titanate-citric acid/ethanol solution, where the volume ratio of the citric acid/ethanol solution to tetrabutyl titanate was 2.7:1;

(71) (3) glacial acetic acid was added dropwise into the tetrabutyl titanate-citric acid/ethanol solution of step (2) under stirring for 37 min to obtain a solution A; where the volume ratio of glacial acetic acid to tetrabutyl titanate was 0.8:1;

(72) (4) the metal salt solution of the step (1) was added dropwise into the solution A of the step (3), mixed uniformly, and added with nitric acid, where the volume ratio of nitric acid to tetrabutyl titanate was 0.8:1; then ammonium hydroxide was added dropwise to adjust the pH value of the system to 3, and the temperature was raised at a constant speed until the temperature was 75° C. to obtain a gel B; where the constant rate of temperature rising was 2.7.0° C./h; and

(73) (5) the gel B of the step (4) was subjected to a treatment under a condition of a temperature of 110° C. for 2 days, and then baked at a temperature of 500° C. for 3 h, cooled in the furnace, pulverized, tableted and sieved to obtain the catalyst La.sub.2Co.sub.6/TiO.sub.2 for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas.

(74) The method of applying this example in the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the coke oven flue gas, included the following specific steps:

(75) (1) placing the catalyst La.sub.2Co.sub.6/TiO.sub.2 for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas of this example in a fixed bed quartz tube reactor;

(76) (2) the temperature of the fixed bed reactor was set as 150° C., and the simulated flue gas was mixed uniformly and introduced into the reactor for a catalytic reaction, where the simulated flue gas contained 300 ppm of NO.sub.x, 100 ppm of HCN, 250 ppm of NH.sub.3, had a relative humidity of 10%, a O.sub.2 volume fraction of 5%, and the balance of N.sub.2; the total gas flow rate was 600 mL/min, and the reactor air speed was 50,000 h.sup.−1; and

(77) (3) the concentrations of NO.sub.x, HCN, NH.sub.3, CO, and CO.sub.2 in the simulated flue gas at the outlet of the fixed reactor in the step (2) was detected.

(78) The test results were as follows: when at the temperature of 150° C., the catalyst La.sub.2Co.sub.6/TiO.sub.2 (pH=3) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN was used, the purification efficiencies of NO.sub.x and HCN reached 88.18% and 89.49%, respectively; and the catalyst performed well under the condition of the complex gas composition: it had a high anti-poisoning performance, high catalytic efficiency, a stable performance, and a long service life.

(79) Example 9: in this example, the carrier of the catalyst for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas was TiO.sub.2, the active components were La.sub.2O.sub.3 and MnO, and the mass percentage of the active components (La.sub.2O.sub.3 and MnO) was 12%, where La.sub.2O.sub.3 was 3% and MnO was 9%, and this was recorded as La.sub.3Mn.sub.9/TiO.sub.2.

(80) A method of preparing a catalyst for low-temperature synergistic catalytic purification of NO.sub.x and HCN in flue gas, included the following specific steps:

(81) (1) the active metal salt La(NO.sub.3).sub.3.6H.sub.2O was dissolved in deionized water, then added with a Mn(NO.sub.3).sub.2 solution and mixed uniformly to obtain a metal salt solution; where the concentration of the metal cation in the solution was 0.488 mol/L;

(82) (2) citric acid was dissolved in ethanol under stirring to obtain a citric acid/ethanol solution, where the molar ratio of citric acid to tetrabutyl titanate was 1.15:1, and the concentration of citric acid in the citric acid/ethanol solution was 0.24 g/mL; then tetrabutyl titanate was added into the citric acid/ethanol solution, mixed uniformly, and reacted for 27 min to obtain a tetrabutyl titanate-citric acid/ethanol solution, where the volume ratio of the citric acid/ethanol solution to tetrabutyl titanate was 4.2:1;

(83) (3) glacial acetic acid was added dropwise into the tetrabutyl titanate-citric acid/ethanol solution of step (2) under stirring for 40 min to obtain a solution A; where the volume ratio of glacial acetic acid to tetrabutyl titanate was 1.2:1;

(84) (4) the metal salt solution of the step (1) was added dropwise into the solution A of the step (3), mixed uniformly, and added with nitric acid, where the volume ratio of nitric acid to tetrabutyl titanate was 1.2:1; then ammonium hydroxide was added dropwise to adjust the pH value of the system to 9, and the temperature was raised at a constant speed until the temperature was 75° C. to obtain a gel B; where the constant rate of temperature rising was 2.8° C./h; and

(85) (5) the gel B of the step (4) was subjected to a treatment under a condition of a temperature of 100° C. for 2.8 days, and then baked at a temperature of 300° C. for 3.5 h, cooled in the furnace, pulverized, tableted and sieved to obtain the catalyst La.sub.3Mn.sub.9/TiO.sub.2 for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas.

(86) The method of applying this example in the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the coke oven flue gas, included the following specific steps:

(87) (1) placing the catalyst La.sub.3Mn.sub.9/TiO.sub.2 for the low-temperature synergistic catalytic purification of NO.sub.x and HCN in the flue gas of this example in a fixed bed quartz tube reactor;

(88) (2) the temperature of the fixed bed reactor was set as 150° C., and the simulated flue gas was mixed uniformly and introduced into the reactor for a catalytic reaction, where the simulated flue gas contained 300 ppm of NO.sub.x, 100 ppm of HCN, 250 ppm of NH.sub.3, had a relative humidity of 10%, a O.sub.2 volume fraction of 5%, and the balance of N.sub.2; the total gas flow rate was 600 mL/min, and the reactor air speed was 50,000 h.sup.−1; and

(89) (3) the concentrations of NO.sub.x, HCN, NH.sub.3, CO, and CO.sub.2 in the simulated flue gas at the outlet of the fixed reactor in the step (2) was detected.

(90) The test results were as follows: when at the temperature of 150° C., the catalyst La.sub.3Mn.sub.9/TiO.sub.2 (pH=9) for the low-temperature synergistic catalytic purification of NO.sub.x and HCN of this example was used, the purification efficiencies of NO.sub.x and HCN reached 90.28% and 94.67%, respectively; and the catalyst performed well under the condition of the complex gas composition: it had a high anti-poisoning performance, high catalytic efficiency, a stable performance, and a long service life.