CN114832853B - Cu-based mixed crystal molecular sieve NH 3 SCR catalyst and method for producing same - Google Patents
Cu-based mixed crystal molecular sieve NH 3 SCR catalyst and method for producing same Download PDFInfo
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Abstract
The invention discloses aCu-based mixed crystal molecular sieve NH 3 -SCR catalyst and method of preparing the same. The Cu-based mixed crystal molecular sieve NH 3 The SCR catalyst is a Cu/CHA+AEI supported molecular sieve catalyst, wherein Cu is supported on a CHA+AEI mixed crystal molecular sieve, the CHA+AEI mixed crystal molecular sieve consists of the CHA molecular sieve and the AEI molecular sieve, and crystal grains of the CHA+AEI mixed crystal molecular sieve are CHA and AEI mixed crystal grains. The preparation method comprises using CHA as main crystal phase, adding AEI molecular sieves with different contents into precursor solution of CHA as seed crystal to construct Cu/CHA+AEI mixed crystal NH with high hydrothermal stability 3 -an SCR catalyst. A series of desired molecular sieve powders can be synthesized during the preparation process by varying the AEI seed content. The invention can further improve the NH3-SCR activity of the Cu/CHA catalyst and the low-temperature and high-temperature hydrothermal stability of the Cu/CHA catalyst, and prolong the service life of the catalyst.
Description
Technical Field
The invention belongs to NH 3 The field of SCR catalysts, in particular to a seed crystal pair copper molecular sieve based NH 3 Optimization and application studies of SCR catalysts.
Background
Nitrogen Oxides (NO) of diesel vehicle exhaust emissions x ) As one of urban atmospheric pollutants, the problems of environmental pollution such as acid rain, haze, photochemical smog and the like are caused, diseases such as cancers are induced, and the health of human beings is seriously threatened. Selective Catalytic Reduction (SCR) by Engelhard corporation through urea injection NH 3 Then selectively catalyzing and reducing NO by using catalyst x 。NH 3 Particulate trap (DPF) periodic high temperature (T) in SCR catalyst susceptible diesel vehicle aftertreatment systems>600 ℃ C.) and high content (10 vol.%) of H due to the lean burn characteristics of the diesel engine 2 O is present in the tail gas of the diesel vehicle to enable NH 3 The hydrothermal stability of SCR catalysts has been of great concern.
Cu-based molecular sieve catalyst in NH 3 The SCR field has great performance advantages, but to meet the increasingly stringent emission regulation limits, the hydrothermal stability of such catalysts still needs to be optimized. In the high-temperature hydrothermal aging process, the molecular sieves with different microenvironments have different potential energies required for structural damage, so that the molecular sieve structures of different types showDifferences in stability are noted. The synthesis of a stable molecular sieve structure is one of the important means for solving the performance degradation of Cu/molecular sieve catalysts after hydrothermal treatment. The molecular sieve support structure that is currently very representative is a small pore molecular sieve having an 8-membered ring structure. The small pore structure well limits migration and agglomeration of framework atoms of the molecular sieve in the bond breaking process, so that the removed framework atoms can enter the framework structure of the molecular sieve again at a slightly low temperature, and the complete structure of the molecular sieve is restored. Existing 11 kinds of small pore molecular sieves can be used for NH 3 The SCR field, in which SAPO-34 and SSZ-13 represented by CHA structure have been most widely studied for their advantages of performance and synthesis.
Although single molecular sieve supports exhibit good NH 3 SCR performance, but its catalytic result is still not very satisfactory for the later more stringent emission regulations. The molecular sieve carrier with the composite crystal structure can be combined with the structural advantages of various molecular sieves, and has better denitration performance. The composite molecular sieve catalyst solves the problems of poor high-temperature hydrothermal stability of Cu/SSZ-13 and poor low-temperature hydrothermal stability of Cu/SAPO-34 by taking the CHA of the same type as a seed crystal for assisting in constructing the Cu/(SSZ-13+SAPO-34) catalyst. In addition, the crystallization temperature and conditions can be regulated in the SAPO-34 synthesis process to obtain the AFI with the composition similar to that of the SAPO-34 element and the structure completely different, so that the CHA+AFI mixed crystal molecular sieve can be formed, the problems of high temperature and low temperature hydrothermal stability of Cu/SAPO-34 can be slightly relieved, but the stability of the two composite molecular sieve catalysts still needs to be further improved due to the limitations of the property of the CHA molecular sieve and the sensitivity of the SAPO-5 to synthesis conditions and the like, so that the requirements of the standard of quasi-zero emission of the tail gas of the future diesel vehicle can be met.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provide a Cu-based mixed crystal molecular sieve NH 3 SCR catalyst and method for its preparation to further increase NH of Cu/CHA catalyst 3 SCR activity and its hydrothermal stability, prolonging the service life of the catalyst.
The conception of the invention is as follows: AEI molecular sieves are structurally similar to CHA molecular sieves, but because of the adjacent doubletsThe difference in spatial distribution of the six-membered ring structure results in a different cage structure between the two, the former being capable of stabilizing more active copper species and thus exhibiting excellent hydrothermal stability. The invention takes CHA as a main crystal phase, AEI molecular sieves with different contents are added into a precursor solution of the CHA to serve as seed crystals, and a Cu-based molecular sieve catalyst with high stability is constructed in an auxiliary way for NH 3 -SCR reaction, modification of Cu/CHA molecular sieve catalyst by seed crystals.
The invention provides a CHA+AEI mixed crystal molecular sieve, which consists of a CHA molecular sieve and an AEI molecular sieve, wherein crystal grains are CHA and AEI mixed crystal grains.
The topological structure code of the novel mixed crystal molecular sieve framework structure is CHA and AEI molecular sieves proposed by the International Zeolite Association, wherein the AEI molecular sieve is used as seed crystal, and the silicon-aluminum ratio of the AEI molecular sieve is 5-50.
The invention provides a Cu-based mixed crystal molecular sieve NH3-SCR catalyst based on the CHA+AEI mixed crystal molecular sieve, wherein the catalyst is a Cu/CHA+AEI supported molecular sieve catalyst, cu is supported on the CHA+AEI mixed crystal molecular sieve, the CHA+AEI mixed crystal molecular sieve consists of the CHA molecular sieve and the AEI molecular sieve, and crystal grains of the CHA+AEI mixed crystal molecular sieve are CHA and AEI mixed crystal grains.
The Cu/CHA+AEI supported molecular sieve catalyst in the specification is Cu/CHA+AEI mixed crystal form NH 3 -an SCR catalyst.
The invention provides a monolithic catalyst, which comprises a cordierite matrix and Cu/CHA+AEI supported molecular sieve catalyst powder, wherein the Cu/CHA+AEI supported molecular sieve catalyst powder is supported on the cordierite matrix.
The invention provides a preparation method of the CHA+AEI mixed crystal molecular sieve, which adopts a hydrothermal synthesis method, and AEI seed crystal is added into a precursor, and the AEI seed crystal is utilized to construct a high-stability Cu/CHA+AEI mixed crystal NH 3 -an SCR catalyst.
The precursor used for synthesis is prepared by the following steps of molecular sieve, aluminum: phosphorus: silicon: structure directing agent (templating agent): the molar ratio of water is (0.06-0.10): (0.06-0.10): (0.040 to 0.050): (0.12-0.35): (0.5-6) feeding, wherein the dosage of the AEI seed crystal is precursor SiO 2 0 to 30wt% of the mass, and the amount of the AEI seed crystal is not 0.
Further, the method comprises the steps of:
(1) Uniformly mixing pseudo-boehmite, phosphoric acid and a part of water in the total water consumption, stirring and ageing at normal temperature to prepare P/Al oxide mixed gel, adding a Si source, a structure directing agent, the rest water and AEI seed crystals into the mixed gel, and standing and ageing at normal temperature to obtain a mixed solution;
(2) The mixed solution is kept at 120-160 ℃ for 2-20 h, then heated to 190-210 ℃ for 5-30 h, naturally cooled to room temperature, then solid is separated, and then baked at 500-600 ℃ for 5-8 h to burn out the structure directing agent, thus obtaining CHA+AEI mixed crystal molecular sieve powder.
Further, the step (1) is stirred for 1 to 24 hours when preparing the P/Al oxide mixed gel, and the mixture is aged for 1 to 24 hours at normal temperature.
Further, the temperature rise rate of the temperature in the step (2) is 3 ℃ min -1 。
Further, the structure directing agent is triethylamine TEA or tetraethylammonium hydroxide TEAOH.
A series of desired molecular sieve powders were synthesized during the preparation by varying the AEI seed content.
The invention also provides a method for preparing the Cu-based mixed crystal molecular sieve NH by taking the mixed crystal molecular sieve as a carrier 3 The method of the SCR catalyst comprises the steps of mixing and stirring a CHA+AEI mixed crystal molecular sieve and a copper salt solution until the mixture is sticky by using a copper salt compound as a precursor, so that copper is uniformly dispersed in the molecular sieve; and then standing the mixture, drying in a water bath, and roasting in a muffle furnace at 300-600 ℃ for 3-6 h to obtain Cu/CHA+AEI supported molecular sieve catalyst powder.
Further, the copper salt is selected from Cu (NO 3 ) 2 、Cu(Ac) 2 、Cu(COOH) 2 At least one of them.
Preferably, the mass of Cu in the copper salt solution accounts for 3% of the mass of the Cu/CHA+AEI mixed crystal type molecular sieve catalyst.
Further, the standing time is 1-15 h, the water bath drying temperature is 50-80 ℃ and the drying time is 1-7 h.
The invention also providesIntegrated Cu-based mixed crystal molecular sieve NH 3 A process for the preparation of SCR catalysts, i.e. a process for preparing the Cu/cha+aei supported catalysts described above into monolithic catalysts: preparing Cu/CHA+AEI supported catalyst powder into slurry and coating the slurry on a cordierite substrate, wherein the loading capacity of the catalyst powder on the cordierite is 120-130 g.L -1 The method comprises the steps of carrying out a first treatment on the surface of the And then drying the coated substrate at 70-150 ℃ and roasting the substrate at 300-600 ℃ for 3-6 hours.
Further, the cordierite substrate is selected from the group consisting of Corning, 400 mesh, 2.5cm, U.S 3 。
Compared with the prior art, the invention has the following beneficial effects:
by adding AEI seed crystal in the synthesis process, the novel mixed crystal molecular sieve with CHA+AEI structure is synthesized, thereby improving NH of Cu/CHA catalyst 3 SCR activity and possesses better high-temperature and low-temperature hydrothermal stability than Cu supported pure carrier CHA molecular sieves (see examples and comparative examples).
Drawings
FIG. 1 shows the results of comparison of the activities of the catalysts described in comparative example 1, comparative example 2 and examples 1 to 3.
FIG. 2 shows the results of comparison of the activities of the catalysts described in example 2 and example 4.
Fig. 3 shows XRD results of the catalysts described in comparative example 1, comparative example 2, and example 4.
FIG. 4 shows the results of the activity of the catalysts described in comparative example 3 and example 5.
FIG. 5 is a comparison of the activities of the catalysts described in example 5 and example 6.
FIG. 6 shows the results of the activity of the catalysts described in comparative example 4 and example 7.
FIG. 7 shows the results of the activity of the catalysts described in comparative example 5 and example 8.
FIG. 8 shows the electron paramagnetic resonance results of the catalysts described in comparative example 1, comparative example 3, example 2, and example 5 and the change of the active copper species content after hydrothermal aging at 800 ℃.
Detailed Description
The invention is further described in connection with the following detailed description. The method according to the invention is further described in the following by means of specific embodiments. The following embodiments are merely examples of one embodiment of the present invention, and not all examples. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Comparative example 1 preparation of pure CHA molecular sieve powder
Mixing 11g of pseudo-boehmite, 18g of phosphoric acid and 18g of water uniformly, stirring for 12 hours, and aging for 12 hours at normal temperature to prepare the P/Al oxide mixed gel. 8g of 44 nm-sized Si sol supplied by Shanghai Nalce Co., ltd.) was added as Si source, 28g of structure directing agent TEA was added, and the remaining 54g of distilled water was added to be uniformly mixed and stirred for 24 hours, and the mixture was aged at normal temperature for 12 hours to obtain a mixed solution. Transferring the mixed solution into a hydrothermal kettle, and placing the hydrothermal kettle into an oven at 3 degrees min -1 The temperature is raised to 140 ℃ for 6 hours, and the mixture is heated to 200 ℃ for 18 hours. Naturally cooling to room temperature, and centrifugally washing for 4 times. Finally, roasting at 550 ℃ for 6 hours to burn out the structure directing agent to obtain CHA molecular sieve powder.
Example 1
Mixing 11g of pseudo-boehmite, 18g of phosphoric acid and 18g of water uniformly, stirring for 12 hours, and aging for 12 hours at normal temperature to prepare the P/Al oxide mixed gel. 6g of 44 nm-sized Si sol supplied by Shanghai Nalce Co., ltd.) was added as Si source, 20g of structure directing agent TEA was added, the remaining 54g of distilled water was added, 5wt.% of AEI (Si/Al ratio: 30) powder was added, and the mixture was uniformly mixed and stirred for 24 hours, and aged at normal temperature for 12 hours to obtain a mixed solution. Transferring the mixed solution into a hydrothermal kettle, and placing the hydrothermal kettle into an oven at 3 degrees min -1 The temperature is raised to 140 ℃ for 6 hours, and the mixture is heated to 200 ℃ for 18 hours. Naturally cooling to room temperature, and centrifugally washing for 4 times. Finally, roasting at 600 ℃ for 6 hours to burn out the structure directing agent to obtain CHA+AEI molecular sieve powder.
Example 2
Mixing 11g of pseudo-boehmite, 18g of phosphoric acid and 30g of water uniformly, stirring for 12 hours, and aging for 12 hours at normal temperature to prepare the P/Al oxide mixed gel. 6g of 44nm Si sol supplied by Shanghai Narce Co., ltd. Was added as Si source, and 20g of structure was addedThe guiding agent TEA, the rest 42g of distilled water and 10wt.% of AEI (Si/Al ratio of 30) powder are added, and the mixture is uniformly mixed and stirred for 24 hours, and the mixture is stood and aged for 12 hours at normal temperature to obtain a mixed solution. Transferring the mixed solution into a hydrothermal kettle, and placing the hydrothermal kettle into an oven at 3 degrees min -1 The temperature is raised to 140 ℃ for 6 hours, and the mixture is heated to 200 ℃ for 18 hours. Naturally cooling to room temperature, and centrifugally washing for 4 times. Finally, roasting at 600 ℃ for 6 hours to burn out the structure directing agent to obtain CHA+AEI molecular sieve powder.
Example 3
Mixing 11g of pseudo-boehmite, 18g of phosphoric acid and 30g of water uniformly, stirring for 12 hours, and aging for 12 hours at normal temperature to prepare the P/Al oxide mixed gel. 6g of 44 nm-sized Si sol supplied by Shanghai Nalce Co., ltd.) was added as Si source, 20g of structure directing agent TEA was added, the remaining 42g of distilled water was added, 15wt.% of AEI (Si/Al ratio: 30) powder was added, and the mixture was uniformly mixed and stirred for 24 hours, and aged at normal temperature for 12 hours to obtain a mixed solution. Transferring the mixed solution into a hydrothermal kettle, and placing the hydrothermal kettle into an oven at 3 degrees min -1 The temperature is raised to 140 ℃ for 6 hours, and the mixture is heated to 200 ℃ for 18 hours. Naturally cooling to room temperature, and centrifugally washing for 4 times. Finally, roasting at 550 ℃ for 6 hours to burn out the structure directing agent to obtain CHA+AEI molecular sieve powder.
Example 4
CHA molecular sieve powder was prepared according to the procedure described in comparative example 1, and this powder was physically mixed with AEI molecular sieve (Si/Al ratio: 30) powder by ball milling in the same ratio as in example 2 to give a physically mixed CHA+AEI molecular sieve powder.
The molecular sieves prepared in examples 1 to 4 and comparative example 1 were prepared as supported catalysts according to the following methods, and further made into monolithic catalysts:
the copper active component is loaded by an isowater pore volume impregnation method. The method comprises the steps of using a copper salt compound (e.g. Cu (NO) 3 ) 2 、Cu(Ac) 2 And Cu (COOH) 2 ) Preparing Cu-supported CHA+AEI mixed crystal molecular sieve catalyst for precursor, dissolving copper salt compound in deionized water to prepare a metal salt solution with a certain concentration, respectively weighing 10g of molecular sieve powder prepared in examples 1-4 and comparative example 1, pouring into the copper salt solution, and connectingStirring is continued until the mixture is viscous to ensure uniform dispersion of the copper species in the molecular sieve. The mixture is kept stand for 10 hours, transferred to a 70 ℃ water bath for drying for 6 hours, and then baked in a muffle furnace at 500 ℃ for 5 hours, so as to obtain Cu/CHA and Cu/CHA+AEI supported catalyst powder with different AEI contents.
Cu/CHA and modified Cu/CHA+AEI molecular sieve based catalysts having different AEI contents were coated on cordierite carriers (Corning Co., U.S. 400 mesh, 2.5 cm) by the following coating methods, respectively 3 ) The catalyst is prepared into a monolithic catalyst, and the specific method is as follows:
a) Uniformly mixing the obtained Cu/CHA and modified Cu/CHA+AEI molecular sieve-based catalyst powder with different AEI contents with zirconium acetate and acetic acid respectively to prepare coating slurry, controlling the solid content of the slurry to be 45%, controlling the zirconium acetate content of the slurry to be 3% and controlling the acetic acid content of the slurry to be 3%;
b) Coating the slurry on a cordierite honeycomb ceramic matrix, and controlling the loading of the catalyst to be 120 g.L -1 ；
c) The coated substrate is gradually heated to 100 ℃ from room temperature for drying, and then baked for 5 hours at 550 ℃ to obtain the monolithic Cu/CHA and modified Cu/CHA+AEI molecular sieve-based catalysts with different AEI contents.
Comparative example 2
The monolithic Cu/AEI molecular sieve-based catalyst was prepared as described above using an AEI molecular sieve (Si/Al ratio of 30) as a support.
The structure of the molecular sieves prepared in example 2, example 4 and comparative examples 1 and 2 was analyzed by XRD, and the results are shown in fig. 3. The results of fig. 3 show that example 2 with the modified structure has CHA as the predominant crystal structure, but the catalyst exhibits a diffraction peak of AEI at 2θ=24°, which indicates that the AEI crystal structure is present in example 2, and therefore the cha+aei mixed crystal structure is present in example 2.
Catalytic Activity test
NH in laboratory self-assembled multichannel microreactors 3 SCR conversion test, simulated diesel vehicle tail gas composition and experimental conditions are shown in table 1. Before testing, the integral catalyst is placed in reaction gas (simulated diesel vehicle tail gas) at 550 ℃ for pretreatmentAnd (5) managing. And then adopting a cooling test method, and carrying out activity test after each temperature point is balanced. Unconverted NO, NO 2 And N 2 O was tested using a Fourier infrared gas analyzer (Thermol Fisher Scientific) and the simulated tail gas composition is shown in Table 1.
Table 1: simulating tail gas conditions
Fresh catalyst NH 3 The SCR activity results are shown in figure 1. Example 2 can achieve 82% NO at 150℃ x Conversion was significantly better than 72% on comparative example 1 and 66% on comparative example 2; at the same time, example 2 can also be carried out at T>High temperature range of 350 ℃ realizes NO similar to comparative example 2 x And (3) transformation. However, when the AEI addition is further increased, the catalytic performance of the modification system in the high temperature range is not changed, but the low temperature performance is slightly reduced, and only 74% of NO is realized at 150 DEG C x And (3) transformation. This demonstrates that the Cu/CHA+AEI molecular sieve based catalyst is effective in improving low temperature performance over Cu/AEI molecular sieve based catalysts and in enhancing NH over Cu/CHA molecular sieve based catalysts at the appropriate AEI addition levels 3 SCR performance, better catalytic activity can be obtained in both the low temperature and high temperature sections. In addition, catalyst-generated N 2 The O content is lower than 7ppm.
To explore the difference between seed modification and pure physical mixing of the prepared cha+aei mixed crystal form molecular sieve catalyst, we prepared example 4 by thoroughly physical mixing the synthesized single CHA molecular sieve powder with the single AEI molecular sieve powder in a quartz mortar. The ratio of the two molecular sieves in example 4 was the same as in example 2. The crystal structure information of the mixed powder was examined by XRD, and as a result, as shown in the inset of fig. 3, the mixed powder also detected AEI characteristic diffraction peaks at the same diffraction angles. The catalyst prepared by supporting copper in example 4 was applied to cordierite for NH 3 SCR activity test, the results are shown in figure 2. EXAMPLE 2 NO throughout the reaction zone x The conversion efficiency was significantly higher than in example 4, indicating that the seed modification had higher than physical mixingIs used for the catalytic performance of the catalyst.
Example 5
To examine the effect of CHA+AEI mixed crystal structure on the 800 ℃ hydrothermal stability of the monolithic Cu/CHA+AEI catalyst. Example 2 was run at 10vol.% H 2 In the flowing air of O, the activity test is carried out by hydrothermal aging at 800 ℃ for 12 hours.
Example 6
To examine the influence of the CHA+AEI mixed crystal structure on the 800 ℃ hydrothermal stability of the integral Cu/CHA+AEI catalyst prepared by a physical ball milling method. The Cu/CHA+AEI monolithic catalyst prepared in example 4 was prepared at a level of 10vol.% H 2 O in flowing air, carrying out activity test after being subjected to hydrothermal aging at 800 ℃ for 12 hours.
Example 7
To examine the effect of the CHA+AEI mixed crystal structure on the hydrothermal stability of the monolithic Cu/CHA+AEI catalyst at 850 ℃. The Cu/CHA+AEI catalyst prepared in example 2 was reacted in the presence of 10vol.% H 2 O in flowing air, carrying out activity test after hydrothermal aging at 850 ℃ for 12 hours.
Example 8
To examine the effect of the CHA+AEI mixed crystal structure on the 70℃low-temperature hydrothermal stability of the integrated Cu/CHA+AEI catalyst. The Cu/CHA+AEI catalyst prepared in example 2 was prepared at 30vol.% H 2 O in flowing air, carrying out activity test after hydrothermal aging at 70 ℃ for 12 hours.
Comparative example 3
10g of the CHA molecular sieve prepared in comparative example 1 was taken and used to prepare a Cu/CHA catalyst according to the same isowater pore volume impregnation method described above. The resulting Cu/CHA catalyst powder was coated onto a cordierite honeycomb ceramic substrate according to the coating method described above. Then at 10vol.% H 2 O in flowing air, carrying out activity test after being subjected to hydrothermal aging at 800 ℃ for 12 hours.
Comparative example 4
10g of the CHA molecular sieve prepared in comparative example 1 was taken and used to prepare a Cu/CHA catalyst according to the same isovolumetric impregnation method described above. The resulting Cu/CHA catalyst powder was coated onto a cordierite honeycomb ceramic substrate according to the corresponding coating method. Then at 10vol.% H 2 O in flowing air, after hydrothermal aging at 850 ℃ for 12 hoursActivity testing was performed.
Comparative example 5
10g of the molecular sieve with CHA prepared in comparative example 1 was taken to prepare a Cu/CHA catalyst according to the same isovolumetric impregnation method as described above. The resulting Cu/CHA catalyst powder was coated onto a cordierite honeycomb ceramic substrate according to the corresponding coating method. Then at 30vol.% H 2 O in flowing air, carrying out activity test after hydrothermal aging at 70 ℃ for 12 hours.
Hydrothermal aging at 800 ℃ for 12 hours, and NH of the catalyst 3 SCR performance results are shown in figures 5-8. EXAMPLE 5 NO in the reaction zone of 175-550℃ x The conversion is significantly higher than in comparative example 3, the conversion of the former reaches a maximum of about 98% at 200℃and the highest NO of the latter x The conversion was only 80%. In addition, N 2 O formation results indicate that the N of the aged catalyst 2 The O production amount was less than 7ppm.
Evaluation of NH after hydrothermal aging of a physically Mixed catalyst 3 SCR performance, the results are shown in figure 6. Example 5 still shows a higher NO than example 6 x Conversion, which indicates that example 5 of the seed modification preparation has higher hydrothermal stability.
The performance test results of the aging treatment of the Cu/CHA+AEI catalyst and the Cu/CHA catalyst under more severe hydrothermal conditions of high temperature (850 ℃,12 h) and low temperature (70 ℃,12 h) were examined deeply for the applicability of the modification scheme are shown in FIGS. 7 and 8. Comparative example 4, after hydrothermal treatment at 850 ℃, still shows the worst catalytic performance, achieving only 72% of maximum NO at 250 °c x Conversion, whereas modified example 7 achieves 97% maximum NO x And (3) transformation. Example 8 may still have a higher NH than comparative example 5 after hydrothermal treatment at 70℃ 3 -SCR performance. The former can achieve 97% maximum NO at 250 DEG C x Conversion was higher than 89% conversion of comparative example 5. Moreover, example 8 also makes it possible to achieve a NO of more than 90% in the reaction zone of 180-350 DEG C x And (3) transformation.
From the electron paramagnetic resonance results shown in fig. 8, it is found that the content of the active copper species in comparative example 3 (after aging) was reduced by 52.7% from that in comparative example 1 (before aging) and the content of the active copper species in example 5 (after aging) was increased by 35.7% from that in example 2 (before aging) after hydrothermal aging, which indicates that the active copper species content of the cha+aei molecular sieve-supported catalyst prepared by the seed modification was not reduced during hydrothermal aging, but was able to stabilize more active copper species, thereby exhibiting higher hydrothermal stability after aging.
Claims (7)
1. Cu-based mixed crystal molecular sieve NH based on CHA+AEI mixed crystal molecular sieve 3 The catalyst is a Cu/CHA+AEI supported molecular sieve catalyst, wherein Cu is supported on a CHA+AEI mixed crystal molecular sieve, crystal grains of the CHA+AEI mixed crystal molecular sieve are CHA and AEI mixed crystal grains, the CHA+AEI mixed crystal molecular sieve adopts a hydrothermal synthesis method, AEI seed crystals are added into a precursor, and the AEI seed crystals are utilized to construct a high-stability Cu/CHA+AEI mixed crystal NH 3 -an SCR catalyst; the precursor used for synthesis is prepared by the following steps of molecular sieve, aluminum: phosphorus: silicon: structure directing agent: the molar ratio of water is (0.06-0.10): (0.06-0.10): (0.040 to 0.050): (0.12-0.35): (0.5-6) feeding, wherein the dosage of the AEI seed crystal is precursor SiO 2 0 to 30 percent of wt percent of the mass, and the using amount of the AEI seed crystal is not 0, and the method specifically comprises the following steps:
(1) Uniformly mixing pseudo-boehmite, phosphoric acid and a part of water in the total water consumption, stirring and ageing at normal temperature to prepare P/Al oxide mixed gel, adding a Si source, a structure directing agent, the rest water and AEI seed crystals into the mixed gel, and standing and ageing at normal temperature to obtain a mixed solution;
(2) The mixed solution is kept at the temperature of 120-160 ℃ for 2-20 h, then heated to the temperature of 190-210 ℃ for 5-30 h, naturally cooled to room temperature, then separated out solid, and then baked at the temperature of 500-600 ℃ for 5-8 h to burn out the structure directing agent, thus obtaining CHA+AEI mixed crystal molecular sieve powder.
2. The cha+aei mixed crystal molecular sieve based Cu-based mixed crystal molecular sieve NH of claim 1 3 The SCR catalyst is characterized in that the mixed gel of the P/Al oxide is stirred for 1 to 24 percent h when the mixed gel is prepared in the step (1), and the mixed gel is aged at normal temperature1 to 24 g h; in the step (2), the temperature rising rate of the mixed solution in the process of preserving the heat of 2 to 20 and h at the temperature of 120 to 160 ℃ is 3 ℃ min -1 The method comprises the steps of carrying out a first treatment on the surface of the The structure directing agent is triethylamine TEA or tetraethylammonium hydroxide TEAOH.
3. The Cu-based mixed crystal molecular sieve NH of claim 1 3 -a process for the preparation of an SCR catalyst, characterized in that a cha+aei mixed crystal molecular sieve according to claim 1 is mixed with a copper salt solution with a copper salt compound as a precursor and stirred until the mixture is viscous, allowing copper to be dispersed homogeneously in the molecular sieve; and then standing the mixture, drying in a water bath, and roasting in a muffle furnace at 300-600 ℃ for 3-6 h to obtain Cu/CHA+AEI supported molecular sieve catalyst powder.
4. A method according to claim 3, characterized in that the copper salt is selected from Cu (NO 3 ) 2 、Cu(Ac) 2 、Cu(COOH) 2 At least one of them.
5. A method according to claim 3, wherein the standing time is 1-15-h, the water bath drying temperature is 50-80 ℃, and the drying time is 1-7-h.
6. The Cu-based mixed crystal molecular sieve NH of claim 1 3 -a monolithic catalyst of an SCR catalyst, characterized in that it comprises a cordierite substrate and a Cu/cha+aei-supported molecular sieve catalyst powder as claimed in claim 5, wherein the Cu/cha+aei-supported molecular sieve catalyst powder is supported on the cordierite substrate.
7. The Cu-based mixed crystal molecular sieve NH of claim 6 3 A process for preparing monolithic catalyst of SCR catalyst, characterized in that the Cu/CHA+AEI supported catalyst powder according to claim 5 is applied to a cordierite substrate as a slurry, the catalyst powder having a loading of 120-130 g L on cordierite -1 The method comprises the steps of carrying out a first treatment on the surface of the Then the coated substrate is dried at 70-150 ℃ and then baked at 300-600 ℃ for 3-6 h.
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