CERIUM-TIN-BASED COMPOSITE OXIDE CATALYST FOR CATALYZING PURIFICATION OF NITROGEN OXIDE, PREPARATION METHOD AND APPLICATION THEREOF

Abstract

The present application relates to a cerium-tin-based composite oxide catalyst for catalyzing purification of a nitrogen oxide, a preparation method and an application thereof. The catalyst has the following chemical composition: a cerium-tin oxide and an M oxide, wherein the M is selected from any one of or a combination of at least two of P, Ti, Zr, V, Mn, Fe, Cu, Al, Si, Ni, Hf, Nb, Ta, Cr, Mo, W, or Re. According to the present application, a cerium-tin-based composite oxide catalyst having the characteristics such as high catalytic activity, high hydrothermal stability, excellent N.sub.2 generation selectivity, a wide operation temperature window, and adaptation to high space velocity reaction conditions is prepared by means of a non-toxic and harmless raw material and a simple method, and the present application is applicable to a device for catalyzing purification of a mobile source nitrogen oxide represented by diesel vehicle exhaust gas and a fixed source nitrogen oxide represented by flue gas from a coal-fired power plant.

Claims

1. A cerium-tin-based composite oxide catalyst for catalytic purification of nitrogen oxide, comprising the following chemical composition: cerium oxide, tin oxide and amorphous MO.sub.x, wherein the M element is selected from any one or a combination of at least two of P, Ti, Zr, V, Mn, Fe, Cu, Al, Si, Ni, Hf, Nb, Ta, Cr, Mo, W or Re; MO.sub.x is dispersed on the surfaces of cerium oxide and tin oxide in a form of unimer, oligomer or amorphous cluster, wherein x is a number of O required to satisfy the valence balance of the elements; a molar ratio of cerium element, tin element and M element is 1:(0.05-5.0):(0.05-3.5), and a molar ratio of M element and Sn element is (0.01-3):1.

2. The catalyst according to claim 1, wherein, in the cerium-tin-based composite oxide catalyst, a particle size of CeO.sub.2 is 5 nm to 35 nm, a particle size of SnO.sub.2 is 1 nm to 10 nm, and the particle size of SnO.sub.2 is smaller than the particle size of CeO.sub.2; optionally, the oligomer comprises dimer and/or trimer; optionally, the molar ratio of M element and Sn element is (0.2-0.4):1.

3. The catalyst according to claim 1, wherein the M element is selected from any one or a combination of at least two of W, Mo or Nb, and optionally selected from any one of Nb or W, or a combination of W and Nb of a molar ratio (0.1-10):1; optionally, the M element is W element, and a molar ratio of cerium element, tin element and W element is 1:(0.1-4.5):(0.08-3.0), and the molar ratio of M element and Sn element is (0.03-3):1.

4. A preparation method of the catalyst according to claim 1, comprising a co-precipitation method, a sol-gel method, a citric acid complex method, a hydrothermal synthesis method or an impregnation method.

5. The preparation method according to claim 4, wherein the co-precipitation method comprises the following steps: (1) subjecting a cerium salt, a tin salt and a M salt to the preparation of a mixed solution; (2) with excess precipitant, carrying out the reaction by stirring the system at a temperature of 20° C. to 95° C. for 0.5 h to 48 h; (3) performing suction filtration and washing to obtain a filter cake; and (4) drying the filter cake, and calcining it in air at 500° C. to 1000° C. to obtain the cerium-tin-based composite oxide catalyst; in the mixed solution, based on a molar ratio of metal elements, the molar ratio of cerium element, tin element and M element is 1:(0.05-5.0):(0.05-3.5), and the molar ratio of M element and Sn element is (0.01-3):1; optionally, the precipitant comprises any one or a combination of at least two of urea, aqueous ammonia, ammonium carbonate, ammonium bicarbonate, sodium carbonate, ammonium bicarbonate, potassium carbonate or potassium bicarbonate, and is optionally selected from aqueous ammonia.

6. The preparation method according to claim 4, wherein the sol-gel method comprises the following steps: (1) subjecting a cerium salt, a tin salt and a M salt to the preparation of a mixed solution; (2) stirring the mixed solution at room temperature for 0.5 h to 72 h to obtain a sol; (3) allowing the obtained sol to stand at normal temperature and pressure for 0.5 h to 12 d to obtain a gel; and (4) drying the gel, and calcining it in air at 500° C. to 1000° C. to obtain the cerium-tin-based composite oxide catalyst; in the mixed solution, based on a molar ratio of metal elements, the molar ratio of cerium element, tin element and M element is 1:(0.05-5.0):(0.05-3.5), and the molar ratio of M element and Sn element is (0.01-3):1.

7. The preparation method according to claim 4, wherein the citric acid complex method comprises the following steps: (1) subjecting a cerium salt, a tin salt and a M salt to the preparation of a mixed solution; (2) adding a certain amount of citric acid to the mixed solution, and a molar ratio of the total amount of metal ions and citric acid is 0.5-5.0; (3) stirring the mixed solution at a temperature of 20° C. to 95° C. for 0.5 h to 48 h; (4) allowing the mixed solution to stand at normal temperature and pressure for 0.5 h to 5 d; and (5) drying the obtained product, and calcining it in air at 500° C. to 1000° C. to obtain the cerium-tin-based composite oxide catalyst; in the mixed solution, based on a molar ratio of metal elements, the molar ratio of cerium element, tin element and M element is 1:(0.05-5.0):(0.05-3.5), and the molar ratio of M element and Sn element is (0.01-3):1.

8. The preparation method according to claim 4, wherein the hydrothermal synthesis method comprises the following steps: (1) subjecting a cerium salt, a tin salt and a M salt to the preparation of a mixed solution; (2) stirring the mixed solution at room temperature for 0.5 h to 2 h, and then transferring the solution into a stainless steel reactor with polytetrafluoroethylene liner; (3) allowing the reactor to stand at 80° C. to 200° C. for 3 h to 12 d; and (4) centrifugally washing and drying the obtained product, and calcining it in air at 500° C. to 1000° C. to obtain the cerium-tin-based composite oxide catalyst; optionally, in the mixed solution, based on a molar ratio of metal elements, the molar ratio of cerium element, tin element and M element is 1:(0.05-5.0):(0.05-3.5), and the molar ratio of M element and Sn element is (0.01-3):1.

9. The preparation method according to claim 4, wherein the impregnation method comprises the following steps: (1) subjecting a cerium salt and a tin salt to the preparation of a mixed solution; (2) with excess precipitant, stirring the mixed solution at a temperature of 20° C. to 95° C. for 0.5 h to 48 h; (3) performing suction filtration and washing to obtain a filter cake; (4) drying the filter cake, and calcining it in air at 500° C. to 1000° C. to obtain a cerium-tin composite oxide catalyst; (5) preparing an M salt solution, and adding an appropriate amount of cerium-tin composite oxide catalyst into the M salt solution; (6) stirring the mixed solution at a temperature of 40° C. to 95° C. for 0.5 h to 48 h until the solution is evaporated to dryness completely, or subjecting the mixed solution to vacuum rotary evaporation at a temperature of 40° C. to 70° C. for 2 h to 6 h until the solution is evaporated to dryness completely; and (7) calcining the residue in air at 500° C. to 1000° C. to obtain the cerium-tin-based composite oxide catalyst; based on a molar ratio of metal elements, in the mixed solution of step (1), a molar ratio of tin and cerium is 0.05-5.0, and the used amount of M salt solution and cerium-tin composite oxide catalyst described in step (5) satisfies: a molar ratio of M and cerium is 0.05-3.5, and the molar ratio of M and Sn is (0.01-3):1.

10. The preparation method according to claim 4, wherein the cerium salt comprises any one or a combination of at least two of cerium chloride, cerium nitrate, cerium ammonium nitrate or cerium sulfate; optionally, the tin salt comprises any one or a combination of at least two of tin chloride, stannous oxalate, tin nitrate or tin sulfate; optionally, the M salt comprises M salts and/or acid salts; optionally, the M element in the M salt is selected from any one or a combination of at least two of W, Mo or Nb, and optionally selected from any one of Nb or W, or a combination of W and Nb with a molar ratio of (0.1-10):1; optionally, the calcination has a temperature of 750° C. to 850° C.

11. A method for catalytic purification of nitrogen oxide in a gas, wherein the cerium-tin-based composite oxide catalyst according to claim 1 is utilized in the method.

12. The method according to claim 11, wherein the method comprises: loading the cerium-tin-based composite oxide catalyst to a carrier, preparing and forming to obtain catalyst, for use in catalytic purification of nitrogen oxide in a gas.

13. The method according to claim 11, wherein, during use, the catalyst is placed in a pipeline of the gas to be treated, a reducing agent is injected upstream of the catalyst to mix with the gas to be treated, the reducing agent is ammonia and/or urea, and a molar amount of reducing agent is 0.8 times to 1.2 times as large as that of nitrogen oxide in the gas to be treated; optionally, the gas to be treated comprises a mobile source nitrogen oxide-containing gas or a fixed source nitrogen oxide-containing gas, which optionally is selected from diesel vehicle exhaust.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0084] FIG. 1 is a diagram that shows the relationship that the nitrogen conversion rates of the catalysts described in Examples 1-5 and Comparative Example 1 change along with the temperature;

[0085] FIG. 2 is a diagram that shows the relationship that the nitrogen conversion rates of the catalysts described in Examples 3, 6 and 7 change along with the temperature;

[0086] FIG. 3a-3d show the FE-SEM (field emission scanning electron microscope) image of the catalyst provided in Example 3, and the Sn, W and Ce element distribution results in sequence;

[0087] FIG. 4 shows the XRD (X-Ray Diffraction) comparison among the catalysts described in Examples 3, 6 and 7.

DETAILED DESCRIPTION

[0088] The technical solution of the present application will be further described below with reference to the accompanying drawing and through specific embodiments.

[0089] To facilitate the understanding of the present application, the present application lists the following examples. Those skilled in the art should understand that the examples described herein are merely used for a better understanding of the present application and should not be construed as specific limitations to the present application.

EXAMPLE 1

[0090] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0091] ammonium metatungstate, cerium nitrate and tin chloride were dissolved in deionized water in sequence, and a solution having Ce:Sn:W with a molar ratio of 1:0.8:0.6 (the molar ratio of W and Sn was 0.75:1) was prepared and shaken uniformly; 30 mL of aqueous ammonia was added to the solution; the mixture was stirred continuously at 25° C. for 12 h, and then subjected to suction filtration and washing; the filter cake was placed in an oven and dried at 100° C. overnight, and finally calcined through a muffle furnace in air at 800° C. for 3 h to prepare the target catalyst.

[0092] The prepared catalyst was ground and sieved, so as to obtain the catalyst with particle size of 40 mesh to 60 mesh, and such catalyst was called Catalyst A.

EXAMPLE 2

[0093] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0094] other conditions remained unchanged as in Example 1, except the precipitant of aqueous ammonia was replaced with 40 g of urea, and the precipitation temperature was set to 90° C., so as to prepare Catalyst B.

EXAMPLE 3

[0095] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0096] other conditions remained unchanged as in Example 1, except the molar ratio of Ce:Sn:W was changed to 1:2:0.5 (the molar ratio of W and Sn was 0.25:1), so as to prepare Catalyst C1.

EXAMPLE 4

[0097] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0098] other conditions remained unchanged as in Example 1, and the molar ratio of Ce:Sn:W was changed to 1:1.5:0.5 (the molar ratio of W and Sn was 0.33:1), so as to prepare Catalyst C2.

EXAMPLE 5

[0099] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0100] other conditions remained unchanged as in Example 1, and the molar ratio of Ce:Sn:W was changed to 1:1:0.1 (the molar ratio of W and Sn was 0.01:1), so as to prepare Catalyst C3.

EXAMPLE 6

[0101] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0102] Catalyst C1 was aged at 800° C. in air containing 10% water for 16 h, so as to prepare Catalyst D.

EXAMPLE 7

[0103] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0104] Catalyst C1 was aged at 900° C. in air containing 10% water for 16 h, so as to prepare Catalyst E.

EXAMPLE 8

[0105] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0106] other conditions remained unchanged as in Example 3, except ammonium tungstate was replaced with ammonium molybdate, so as to prepare Catalyst F.

EXAMPLE 9

[0107] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0108] niobium oxalate, cerium nitrate and tin chloride were dissolved in deionized water in sequence, and a solution having Ce:Sn:Nb with a molar ratio of 1:2:1 (the molar ratio of Nb and Sn was 0.5:1) was prepared and shaken uniformly; 40 mL of aqueous ammonia was added to the solution; the mixture was stirred continuously at 25° C. for 12 h, and then subjected to suction filtration and washing; the filter cake was placed in an oven and dried at 90° C. overnight, and finally calcined through a muffle furnace in air at 800° C. for 5 h to prepare the catalyst as powder.

[0109] The prepared catalyst was compressed into tablets, ground and sieved, and the particle of 40 mesh to 60 mesh was taken for later use, which was called Catalyst G.

EXAMPLE 10

[0110] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0111] ammonium metatungstate, niobium oxalate, cerium nitrate and tin chloride were dissolved in deionized water in sequence, and a solution having Ce:Sn:W:Nb with a molar ratio of 1:2:0.24:0.76 (the molar ratio of W+Nb and Sn was 0.5:1) was prepared and shaken uniformly; citric acid was added to the mixed solution; a molar ratio of the total amount of metal ions and citric acid was 1; the mixture was stirred continuously at 30° C. for 24 h, and allowed to stand for 3 d at normal temperature and pressure; the obtained product was dried and then calcined through a muffle furnace in air at 500° C. for 3h to prepare the catalyst as powder. The prepared catalyst was compressed into tablets, ground and sieved, and the particle of 40 mesh to 60 mesh was taken for later use, so as to obtain Catalyst H.

EXAMPLE 11

[0112] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0113] cerium nitrate and tin chloride were dissolved in deionized water in sequence, and a solution having Ce:Sn with a molar ratio of 1:2 was prepared and shaken uniformly; with 30 mL of aqueous ammonia as precipitant, the mixture was stirred at 30° C. for 12 h, and then subjected to suction filtration and washing to obtain a filter cake; the filter cake was dried and then calcined in air at 600° C. to obtained a cerium-tin composite oxide carrier; 0.6 g of (NH.sub.4).sub.3PO.sub.4 was dissolved in deionized water to prepare a solution; 2 g of the cerium-tin composite oxide carrier was added to the above solution; the mixture was stirred at 80° C. until the aqueous solution was evaporated to dryness completely; the residue was calcined at 600° C. in air to obtain the 15% wt.PO.sub.x/Ce.sub.1Sn.sub.2O.sub.x catalyst (x was a number of O required to satisfy the valence balance of the elements), so as to obtain Catalyst I.

EXAMPLE 12

[0114] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0115] other conditions remained unchanged as in Example 3, and the calcination temperature was replaced with 500° C., so as to prepare Catalyst K.

EXAMPLE 13

[0116] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0117] other conditions remained unchanged as in Example 3, and the calcination temperature was replaced with 900° C., so as to prepare Catalyst L.

EXAMPLE 14

[0118] This example provides a cerium-tin-based composite oxide catalyst, of which the preparation method is as follows:

[0119] other conditions remained unchanged as in Example 3, and ammonium metatungstate was replaced by a mixture of ammonium metatungstate and niobium oxalate in a molar ratio of 4:1, so as to prepare Catalyst M.

COMPARATIVE EXAMPLE 1

[0120] This comparative example is different from Example 5 in that ammonium tungstate was not added, so as to prepare Catalyst J.

APPLICATION EXAMPLE

[0121] The reaction activity for NH.sub.3 selective catalytic reduction of NO.sub.x was investigated with applying the cerium-tin-based composite oxide catalysts prepared in Examples 1-14 and Comparative Example 1 to a fixed-bed reactor.

[0122] The use amount of catalyst was 0.5 g, and the composition of the reaction gas mixture included: [NO]═[NH.sub.3]=500 ppm, [O.sub.2]=5 vol. %, 5 vol. % H.sub.2O and N.sub.2 was used as an equilibrium gas; the total gas flow was 500 mL/min, the space velocity was 120,000 h.sup.−1, and the reaction temperature was 150° C. to 500° C. NO, NH.sub.3 and by-product N.sub.2O and NO.sub.2 were all detected using an infrared gas cell. The reaction results are shown in Table 1.

[0123] FIG. 1 is a diagram that shows the relationship that the nitrogen conversion rates of the catalysts described in Examples 1-5 and Comparative Example 1 change along with the temperature.

[0124] FIG. 2 is a diagram that shows the relationship that the nitrogen conversion rates of the catalysts described in Examples 3, 6 and 7 change along with the temperature.

[0125] FIG. 3a-3d show the FE-SEM image of the catalyst described in Example 3 and the respective images of element distribution results, from which it can be found that Sn element agglomerated together and Ce element agglomerated together, while W element was uniformly distributed on the catalyst surface.

[0126] FIG. 4 shows the XRD comparison among the catalysts described in Examples 3, 6 and 7, from which it can be found that only diffraction spectra of cubic phase CeO.sub.2 and tetragonal phase SnO.sub.2 were obtained by detecting the three catalysts, and no tungsten phase was observed, and with the elemental analysis results in FIG. 3, it is proved that M existed in an amorphous state.

TABLE-US-00001 TABLE 1 Activity evaluation result of cerium-tin-based composite oxide catalyst NO.sub.x conversion rate at different temperature (%) No. 150° C. 200° C. 250° C. 300° C. 350° C. 400° C. 500° C. Example 1  A 5.5 8.7 28.3 61.2 80.6 93.6 93.3 Example 2  B 3.3 10.8 23.8 57.4 79.9 88.4 89.0 Example 3  C1 8.2 47.9 97.3 100 100 100 91.9 Example 4  C2 9.9 46.7 97.1 100 100 100 94.0 Example 5  C3 8.2 52.7 96.9 100 99.9 100 89.9 Example 6  D 8.1 39.7 90.8 99.1 99.6 98.9 91.4 Example 7  E 7.4 28.4 78.4 97.2 99.4 99.7 93.8 Example 8  F 5.2 31.2 87.4 98.7 99.4 99.0 77.0 Example 9  G 8.0 47.7 97.4 100 100 100 94.0 Example 10 H 8.0 46.8 96.9 100 100 100 90 Example 11 I 6.5 45.8 95.4 100 100 100 98.8 Example 12 K 14.1 70.4 96.7 100 99.9 97.4 52.0 Example 13 L 8.2 38.7 91.2 99.8 100 100 96.6 Example 14 M 8.2 52.7 100 100 99.9 100 95 Comparative Example 1 J 1.1 3.1 11.3 36.9 74.8 90.3 75.7

[0127] It can be seen from Table 1 that the performance of catalysts C1 and C2 in NH.sub.3—SCR was significantly better than that of catalysts A, B and C3, indicating that the molar ratio of Ce, Sn and W has a preferred range; the catalyst would have a better effect when elements cerium, tin and M had a molar ratio of 1:(0.05-5.0):(0.05-3.5) and elements M and Sn had a molar ratio of (0.2-0.4):1; with a space velocity of 120,000 h.sup.−1 and a temperature range of 250° C. to 500° C., both catalyst C1 and catalyst C2 could achieve the NO.sub.x conversion rate of more than or equal to 90%, and all the N.sub.2 selectivity was greater than 98%.

[0128] Catalyst D obtained after hydrothermal aging at 800° C. for 16 h was still able to achieve the NO.sub.x conversion rate of more than 90% with a space velocity of 120,000 h.sup.−1 and a temperature range of 250° C. to 500° C., and all the N.sub.2 generation selectivity was greater than 99%; Catalyst E obtained after hydrothermal aging at 900° C. for 16 h was still able to achieve the NO.sub.x conversion rate of more than 90% with a space velocity of 120,000 h.sup.−1 and a temperature range of 300° C. to 500° C., and all the N.sub.2 generation selectivity was greater than 98%, indicating that such catalyst has excellent resistance to high temperature hydrothermal aging.

[0129] Through comparing Example 1 and Example 2, it is found that the performance of the catalyst prepared by using the aqueous ammonia precipitant is better than that of the catalyst prepared by using the urea precipitant.

[0130] Through comparing Example 3 and Example 12, it is found that the calcination treatment with relatively low temperature (500° C.) significantly improves the activity of catalyst for low temperature NH.sub.3—SCR, which is mainly resulted from that the catalyst obtained by the low temperature calcination has a larger specific surface area, thereby facilitating the coupling and dispersion of more acidic sites and redox sites.

[0131] Through comparing Example 3 and Example 13, it is found that the calcination treatment with relatively high temperature (900° C.) significantly improves the activity of catalyst for high temperature NH.sub.3—SCR, which is mainly resulted from that the calcination treatment with relatively high temperature inhibits the catalyst from nonselective oxidation of ammonia gas in the high temperature section, thereby facilitating more NH.sub.3 to participate in the NH.sub.3—SCR reaction.

[0132] Through comparing Example 3 and Example 14, the use of W and Nb in a specific molar ratio can have both excellent low-temperature activity and high-temperature stability, which is mainly due to the synergistic effect of W and Nb.

[0133] The applicant has stated that although the specific process equipment and process flow in the present application are described through the above embodiments, the present application is not limited to the above specific process equipment and process flow, which means that the present application does not necessarily depend on the above specific process equipment and process flow to be implemented.

[0134] The applicant has stated that although the detailed method in the present application is described through the above embodiments, the present application is not limited to the above detailed method, which means that the present application does not necessarily depend on the above detailed method to be implemented.