Molecular sieve-based catalyst with shell structure and preparation method and application thereof

Abstract

The application discloses a molecular sieve-based catalyst with a shell structure and a preparation method and application thereof, and belongs to the technical field of molecular sieve-based automobile exhaust treatment catalysts. The molecular sieve-based catalyst with the shell structure comprises a shell structure molecular sieve carrier and a metal component. The carrier is a core-shell material with cerium dioxide as a shell and SSZ-13 as a core. The metal component is Pd. The mass ratio of the carrier to the metal is 1:0.005-0.02. The molecular sieve-based catalyst with the shell structure is applied to a diesel vehicle PNA catalyst. By loading a Pd-based catalyst material on a carrier with SSZ-13 molecular sieve as a core and CeO2 as a shell, an excellent nitrogen oxide adsorption catalyst is expected to be obtained. The proper CeO2 shell layer in the core-shell structure material helps to improve the dispersion degree and stability of the metal Pd, significantly improves the adsorption activity of the catalyst on nitrogen oxide, and the catalyst has excellent stability and anti-poisoning property. The application provides a valuable reference for the design and synthesis of the PNA catalyst with the core-shell structure.

Description

Technical Field

[0001] The present invention relates to the technical field of molecular sieve-based catalysts, and in particular to a molecular sieve-based catalyst with a shell structure, a preparation method thereof, and an application thereof. Background Art

[0002] Vehicles are increasingly being used in people's daily lives, bringing great convenience to people's travel. In recent years, electric vehicles have seen strong development momentum, but it is generally expected that internal combustion engines will continue to maintain their dominant position in the market for at least the next few decades. However, diesel engines produce nitrogen oxides (NO x ), for total NO x Emissions have a large contribution. Due to the restrictions of higher emission regulations and taking into account NO x Harm to human body, design and preparation of diesel engine exhaust NO x The research on emission catalysts is urgent. Therefore, people have been committed to effectively controlling the emissions of diesel engines.

[0003] Currently, selective catalytic reduction (SCR) and NO x Storage reduction (NSR) is a method to reduce NO x There are two effective technologies for reducing NO emissions and they have been successfully commercialized. However, both technologies have the disadvantage of poor low-temperature activity and cannot effectively remove NO below 200°C (the temperature during vehicle cold start). x , there will be a large number of NO x Leakage into the air during cold start, therefore, current NSR and SCR catalysts alone no longer meet current regulatory requirements.

[0004] Therefore, it is necessary to provide a molecular sieve-based catalyst with a shell structure and a preparation method and application thereof, in order to solve the above problems. Summary of the Invention

[0005] In view of the deficiencies in the prior art, the purpose of the embodiments of the present invention is to provide a molecular sieve-based catalyst with a shell structure and a preparation method and application thereof, so as to solve the problems in the above-mentioned background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A molecular sieve-based catalyst with a shell structure, wherein the carrier of the molecular sieve-based catalyst is SSZ-13@CeO2 and the first active component is Pd.

[0008] A method for preparing a molecular sieve-based catalyst having a shell structure comprises the following steps:

[0009] Step S1: preparing SSZ-13 molecular sieve;

[0010] Step S2: preparing a core-shell structure material with ceria as the shell and SSZ-13 molecular sieve as the core;

[0011] Step S3: The core-shell structure material obtained in step S2 was dried in an oven at 80°C for 2 hours, and palladium nitrate was dissolved in water. The palladium nitrate solution was then added to the core-shell structure materials obtained in steps S1 and S2, respectively. The samples were dried in an oven at 80°C for 12 hours, and calcined in air at 500°C for 6 hours. The obtained samples were named Pd / SSZ-13 and Pd / (SSZ-13@CeO2-n), respectively.

[0012] As a further embodiment of the present invention, the preparation method of SSZ-13 in step S1 comprises the following steps:

[0013] Step a1: dissolving NaOH and Al(OH)3 particles in deionized water in sequence under magnetic stirring at 300-800 rpm at room temperature and stirring until clear to obtain a solution;

[0014] Step a2: adding a silicon source to the solution obtained in step a1, and continuously stirring at room temperature for 3 hours to obtain a uniformly dispersed precursor gel;

[0015] Step a3: Transfer the uniformly dispersed precursor gel to a polytetrafluoroethylene liner and assemble it into a stainless steel reactor;

[0016] Step a4: crystallization at 160° C. in a homogeneous reactor;

[0017] Step a5: centrifuging the product obtained in step a4, and then washing and centrifuging the product 3-4 times until it is neutral;

[0018] Step a6: Dry the product of step a5 in a constant temperature oven overnight, and calcine the obtained white solid powder at 650° C. for 6 h in air to remove the template. The obtained product is SSZ-13 molecular sieve.

[0019] As a further solution of the present invention, the method for preparing the core-shell structure material in step S2 comprises the following steps:

[0020] Step b1: 0.1-5 g of SSZ-13 was dispersed in 10-200 ml of a 1 / 1 ethanol / water mixed solution, the mixed solution was first ultrasonically treated for 0.5-2 h, and then the mixed solution was stirred at room temperature and 500-1000 rpm for 30-120 min;

[0021] Step b2: Add 0.5-2 g of white solid powder polyvinylpyrrolidone to the ethanol / water solution in step b1 and stir for 1.5-5 hours until it is completely dissolved;

[0022] Step b3: Add 0.1-1 g of Ce(NO3)3·6H2O and 0.1-1 g of hexamethylenetetramine to the solution obtained in step b2, and stir the mixture under continuous magnetic stirring at 500-100 rpm for 2-5 h;

[0023] Step b4: removing the liquid obtained in step b3 by centrifugation to obtain a light yellow solid product;

[0024] Step b5: The product was washed repeatedly 3-4 times, dried in a constant temperature oven overnight, and calcined at 650°C for 6 h under static conditions. The final yellow solid material obtained was SSZ-13@CeO2-n, where n represents the mass of Ce(NO3)3·6H2O added during the preparation of the core-shell material, and n=0.2, 0.3, 0.5 and 0.6.

[0025] As a further embodiment of the present invention, the method for preparing the catalyst containing 0.1-2.0 wt% of metal Pd in ​​step S3 comprises the following steps:

[0026] Step c1: Prepare a Pd(NO3)2·2H2O solution and apply an impregnation method to load 0.1-2.0 wt% Pd onto SSZ-13 and SSZ-13@CeO2-n;

[0027] Step c2: The white solid powder obtained in step c1 was calcined at 400-550° C. in static air for 6 h. The obtained samples were named Pd / SSZ-13 and Pd / SSZ-13@CeO2-n, respectively, where n=0.2, 0.3, 0.5 and 0.6.

[0028] As a further solution of the present invention, the molar composition ratio of the synthetic material feed of the SSZ-13 is 1500-5000H2O:8-30TMAdaOH:50-300SiO2:7-20NaOH:1-10Al2O3.

[0029] As a further solution of the present invention, the silicon source in step a2 is AS-40 silica sol.

[0030] As a further solution of the present invention, the oven temperature in step a6 and the oven temperature in step b5 are both 80°C.

[0031] As a further solution of the present invention, the crystallization time in step a4 is 96 hours.

[0032] As a further solution of the present invention, the molecular sieve-based catalyst with a shell structure is used in a PNA catalyst for diesel vehicles.

[0033] In summary, the embodiments of the present invention have the following beneficial effects compared with the prior art:

[0034] The present invention uses a Pd-based core-shell structure material with SSZ-13 molecular sieve as the core and CeO2 as the shell, which is expected to become an excellent PNA catalyst. The appropriate amount of CeO2 shell in the core-shell structure material helps to significantly improve NO x The adsorption activity of CeO2 shell structure provides a valuable reference for the design and synthesis of PNA catalysts with high catalytic efficiency and significant improvement of NO x of the adsorption activity and the effect of improving the reference.

[0035] In order to more clearly illustrate the structural features and effects of the present invention, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 NO of the embodiment of the invention x Adsorption performance test experimental scheme (a) and peak analysis in different temperature ranges (b).

[0037] Figure 2 SEM images of all samples prepared in the inventive examples: Pd / SSZ-13 (a), Pd / (SSZ-13@CeO2-0.2) (b), Pd / (SSZ-13@CeO2-0.3) (c) (d), Pd / (SSZ-13@CeO2-0.5) (e) (f) and Pd / (SSZ-13@CeO2-0.6) (g) (h).

[0038] Figure 3 These are the N2 adsorption-desorption isotherms of the five catalytic materials prepared in the inventive examples.

[0039] Figure 4 The XPSPd3d spectra of the five samples prepared in the embodiment of the invention are: Pd / SSZ-13 (a), Pd / (SSZ-13@CeO2-0.2) (b), Pd / (SSZ-13@CeO2-0.3) (c), Pd / (SSZ-13@CeO2-0.5) (d) and Pd / (SSZ-13@CeO2-0.6) (e), as well as the Pd 2+ Histogram of content.

[0040] Figure 5UV-Vis spectra of samples in the embodiments of the invention: Pd / SSZ-13 (a), Pd / (SSZ-13@CeO2-0.2) (b), Pd / (SSZ-13@CeO2-0.3) (c), Pd / (SSZ-13@CeO2-0.5) (d) and Pd / (SSZ-13@CeO2-0.6) (e) as well as the Pd(H2O) of each sample obtained by analysis 2+ / PdO x Histogram of the values ​​(f).

[0041] Figure 6 is the NO of the sample in the embodiment of the invention x Variation of NO concentration (a) and NO / NO2 concentration (b) with time and temperature. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0043] The specific implementation of the present invention is described in detail below with reference to specific embodiments.

[0044] In one embodiment of the present invention, a molecular sieve-based catalyst having a shell structure is provided. The carrier of the molecular sieve-based catalyst is SSZ-13@CeO2, and the first active component is Pd.

[0045] In one embodiment of the present invention, a method for preparing a molecular sieve-based catalyst having a shell structure comprises the following steps:

[0046] Step S1: preparing SSZ-13 molecular sieve;

[0047] Step S2: preparing a core-shell structure material;

[0048] Step S3: The core-shell structure material obtained in step S2 was dried in an oven at 80°C for 2 h, and Pd(NO3)2·2H2O was dissolved in water. Then, the carrier SSZ-13 or SSZ-13@CeO2-n was added to a mortar together with the palladium nitrate solution and ground for 30 min. The obtained samples were named Pd / SSZ-13 and Pd / (SSZ-13@CeO2-n), respectively.

[0049] The preparation method of SSZ-13 in step S1 comprises the following steps:

[0050] Step a1: dissolving NaOH and Al(OH)3 particles in deionized water in sequence under magnetic stirring at 300-800 rpm at room temperature and stirring until clear to obtain a solution;

[0051] Step a2: adding a silicon source to the solution obtained in step a1, and continuously stirring at room temperature for 3 hours to obtain a uniformly dispersed precursor gel;

[0052] Step a3: Transfer the uniformly dispersed precursor gel to a polytetrafluoroethylene liner and assemble it into a stainless steel reactor;

[0053] Step a4: crystallization at 160° C. in a homogeneous reactor;

[0054] Step a5: centrifuging the product obtained in step a4, and then washing and centrifuging the product 3-4 times until it is neutral;

[0055] Step a6: Dry the product of step a5 in a constant temperature oven overnight, and calcine the obtained white solid powder at 650° C. for 6 h in air to remove the template. The obtained product is SSZ-13.

[0056] The method for preparing the core-shell structure material in step S2 comprises the following steps:

[0057] Step b1: 0.1-5 g of SSZ-13 was dispersed in 10-200 ml of a 1 / 1 ethanol / water mixed solution, the mixed solution was first ultrasonically treated for 0.5-2 h, and then the mixed solution was stirred at room temperature and 500-1000 rpm for 30-120 min;

[0058] Step b2: Add 0.5-2 g of white solid powder PVP to the ethanol / water solution in step b1 and stir for 1.5-5 hours until it is completely dissolved;

[0059] Step b3: Add 0.1-1 g of Ce(NO3)3·6H2O and 0.1-1 g of hexamethylenetetramine to the solution obtained in step b2, and stir the mixture under continuous magnetic stirring at 500-100 rpm for 2-5 h;

[0060] Step b4: removing the liquid obtained in step b3 by centrifugation to obtain a light yellow solid product;

[0061] Step b5: The product was washed repeatedly 3-4 times, dried in a constant temperature oven overnight, and calcined at 650°C for 6 h under static conditions. The final yellow solid material obtained was SSZ-13@CeO2-n, where n represents the mass of Ce(NO3)3·6H2O added during the preparation of the core-shell material, and n=0.2, 0.3, 0.5 and 0.6.

[0062] The preparation method of the catalyst containing 0.1-2.0 wt% of metal Pd in ​​step S3 comprises the following steps:

[0063] Step c1: Prepare a Pd(NO3)2·2H2O solution and apply an impregnation method to load 0.1-2.0 wt% Pd onto SSZ-13 and SSZ-13@CeO2-n;

[0064] Step c2: The white solid powder obtained in step c1 was calcined at 400-550° C. in static air for 6 h. The obtained samples were named Pd / SSZ-13 and Pd / SSZ-13@CeO2-n, respectively, where n=0.2, 0.3, 0.5 and 0.6.

[0065] The molecular sieve-based catalyst with a shell structure can be applied to PNA catalysts for diesel vehicles.

[0066] In this embodiment, the molar composition ratio of the synthetic material feed amount of the SSZ-13 is 1500-5000H2O: 8-30TMAdaOH: 50-300SiO2: 7-20NaOH: 1-10Al2O3, the silicon source in the step a2 is AS-40 silica sol, the oven temperature in the step a6 and the oven temperature in the step b5 are both 80°C, and the crystallization time in the step a4 is 96h. The present invention is expected to become an excellent PNA catalyst by using a Pd-based core-shell structure material with SSZ-13 molecular sieve as the core and CeO2 as the shell. The appropriate amount of CeO2 shell in the core-shell structure material helps to significantly improve NO x This study provides a valuable reference for the design and synthesis of CeO2 shell structured PNA catalysts, which have high catalytic efficiency and can significantly improve the adsorption of NO. x of the adsorption activity and the effect of improving the reference.

[0067] 1. Catalytic activity test

[0068] A plug flow reactor was used to conduct constant temperature NO x Adsorption and temperature-programmed desorption experiments. The outlet gas concentration was recorded by an MKS Multigas 2030 FTIR online gas analyzer. Figure 1 As shown in a, in the adsorption of NO x Before, 100 mg of sample (40-60 mesh) was first pretreated in a 10% O2 flowing gas balanced with nitrogen at 500°C for 30 min. Then, the temperature was lowered to 100°C and a 200 ppm NO x and 3.5% H2O with a total flow rate of 200 ml / min to carry out NO xStorage and release process. During the entire adsorption-desorption process, the gas concentration was first stabilized in the bypass pipe of the reactor for 10 minutes, and then NO was adsorbed at 100 °C. x After maintaining the temperature for 10 min, the temperature was raised to 600 °C at a heating rate of 10 °C / min.

[0069] NO x The amount stored and released was calculated from the integration of the adsorption–desorption curves, e.g. Figure 1 As shown in b, NO is obtained from the negative peak below about 200 ppm. x The adsorption amount of NO can be calculated from the positive peak (about greater than 200 ppm) generated during the temperature programming process. x The desorption amount of NO was calculated according to the following equation x Adsorption capacity (Qads / Qdes):

[0070]

[0071] where Q NOx (μmol·g -1 ) means NO x Adsorption or desorption amount, F (mL / min) represents the gas flow rate, Fin (ppm) is the initial NO x concentration, and Fout (ppm) is the outlet NO x concentration, m(g) is the mass of the sample, and t(min) represents the duration of the adsorption or desorption process.

[0072] However, NO x The storage efficiency and NO x Desorption efficiency is also a factor worthy of attention in PNA research. x Storage efficiency (NSE) refers to the NO adsorbed and stored during the adsorption period. x Accounting for NO delivered to the reactor x Percentage of NO x The desorption efficiency (NDE) is defined as the amount of stored NO desorbed during the TPD period. x The percentages are calculated using the following formulas:

[0073]

[0074]

[0075] Where t is NO x Storage time, Fin is NO x Inlet NO during storage x concentration, Fout is the outlet NO x concentration, t(T0) is the NO concentration after heating x-TPD process starts time, t(T) is the NO x The time corresponding to the end of desorption.

[0076] 2. Experimental Results

[0077] 1. Structural Characterization of Prepared Samples

[0078] The morphology of the five samples prepared and the effect of different CeO2 amounts on the sample morphology were studied by SEM. Figure 2 As shown in the figure, all samples have a regular cubic morphology, which is the morphology of the main carrier SSZ-13 molecular sieve. This shows that after loading the active Pd species and continuing high-temperature calcination, the morphology of SSZ-13 remains a complete cube without significant changes. In the process of preparing the core-shell material, the other reagents added did not seriously damage the morphology of SSZ-13. Secondly, Figure 2 a and b are SEM images of Pd / SSZ-13 and Pd / (SSZ-13@CeO2-0.2) respectively. The surfaces of these two samples are smooth. This is because the latter contains less CeO2 and no obvious difference can be seen. Figure 2 cf, with Figure 2 The difference between a and b is that small particles can be seen on the outer surface of the SSZ-13 cubes, indicating that the prepared CeO2 particles are very small. The CeO2 particles accumulate and coat the outer surface of SSZ-13, forming an SSZ-13@CeO2-n core-shell structure. The SEM results are consistent with the XRD results.

[0079] N2 physical adsorption was used to further investigate the fine structural characteristics of the prepared samples. The N2 adsorption-desorption isotherms of all the prepared samples were shown in Figure 3 All catalyst materials showed type I isotherms, indicating that the prepared samples were all microporous materials; as shown in Table 1, the micropore specific surface area (Smic) of Pd / SSZ-13, Pd / (SSZ-13@CeO2-0.2), Pd / (SSZ-13@CeO2-0.3), Pd / (SSZ-13@CeO2-0.5) and Pd / (SSZ-13@CeO2-0.6) were 523m 2 g -1 、517m 2 g -1 、468m 2 g -1 、463m 2 g -1 and 461m 2 g -1It can be seen that after adding the CeO2 shell, the micropore specific surface area of ​​the sample decreases, which shows that the CeO2 shell can reduce the micropore specific surface area; at the same time, with the increase of CeO2, it can be observed that the micropore volume of the sample decreases slightly, indicating that the coated CeO2 shell will cover part of the micropores of the SSZ-13 molecular sieve.

[0080] ICP-OES analysis of the Si / Al ratio, Pd, and Ce content of all samples was performed. As shown in Table 1, the Si / Al ratio for all samples was approximately 13, and the Pd content was approximately 1.0 wt%, consistent with the target loading. The Ce contents of the catalyst samples Pd / (SSZ-13@CeO2-0.2), Pd / (SSZ-13@CeO2-0.3), Pd / (SSZ-13@CeO2-0.5), and Pd / (SSZ-13@CeO2-0.6) were 2.78 wt%, 4.42 wt%, 5.26 wt%, and 7.17 wt%, respectively. The Ce content increased with increasing Ce(NO3)3·6H2O loading during the preparation process.

[0081] Table 1 Element content and structural characteristics of prepared catalyst samples

[0082]

[0083] a The silicon-aluminum ratio (Si / Al), the content of Pd and Ce elements in the prepared samples were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). b. The micropore specific surface area (S mic ) and its micropore volume (V mic ).

[0084] 2. Distribution of Pd species in the catalyst

[0085] Table 2 Surface Pd obtained by XPS spectral analysis 2+ Species content

[0086]

[0087] XPS was used to analyze the presence of Pd metal species within the catalyst surface depth of 10nm. The XPS results of Pd3d of the prepared sample are shown in Figure 4 In. Pd 2+ Corresponding Pd3d 5 / 2 The binding energies of Pd3d3 / 2 and PdO3d3 / 2 are around 337.1-337.6eV and 342.3-342.9eV respectively. x Pd3d 5 / 2 and Pd3d 3 / 2The binding energies of the two are around 338.8-338.9eV and 344.1-344.2eV respectively. Figure 4 From Table 2, we can see that in all catalyst samples, Pd 2+ The main Pd metal species accounted for more than 70%, indicating that the active Pd metal species in all the prepared catalyst samples were relatively dispersed. Compared with Pd / SSZ-13, the Pd in ​​the catalyst sample Pd / (SSZ-13@CeO2-0.3) 2+ The content increased by 15%.

[0088] The relative contents of Pd and Ce species in different samples were analyzed by UV-Vis spectroscopy, and the characteristic absorption peaks of different metal species were fitted. The fitting peak area ratios of different species of the same metal were calculated. The analysis results are shown in Table 2 and Figure 5 In. Figure 5 As shown in Figure 3, the sample containing a cerium oxide shell is fitted with five characteristic absorption peaks, among which the ligand-metal charge transfer (LMCT) between the molecular sieve framework oxygen to Pd ions and Ce ions shows a characteristic absorption band centered at 209 nm, and the absorption band at 250 nm is attributed to the presence of Ce in the prepared sample. 3+ The 4f-5d transition of the species, and the band at 300 nm corresponds to the O 2- To Ce 4+ The charge transfer characteristics of the molecular sieve are shown in Figure 2. The characteristic absorption peak at 410 nm is attributed to Pd(H2O)4 2+ The characteristic peak greater than 410 nm is attributed to the weak interaction between PdO and molecular sieve. x The absorption peak of the cluster. Sample Pd / SSZ-13( Figure 5 a) Fitting into four absorption peaks, namely the ligand-metal charge transfer from molecular sieve framework oxygen to Pd ion at 209nm and 266nm, and the Pd(H2O)4 at around 410nm and 480nm respectively. 2+ complex and PdO x The dd transition of Pd in ​​the cluster. Figure 5 For Ce species, it can be clearly observed that as the Ce content increases, Ce 4+ The characteristic absorption peaks of Pd / (SSZ-13@CeO2-0.2), Pd / (SSZ-13@CeO2-0.2), and Pd / (SSZ-13@CeO2-0.2) listed in Table 2 are significantly enhanced, indicating that there are more and more CeO2 nanocrystals outside the framework, which is consistent with the XRD results obtained. 2 -0.3)、Pd / (SSZ-13@CeO 2 -0.5) and Pd / (SSZ-13@CeO2-0.6) 3+ / Ce 4+ The fitted peak area ratios are 0.58, 0.49, 0.33 and 0.27, respectively, which also proves that more CeO2 species are generated. Figure 5 The histogram f shows the relative content of Pd species in the sample. It can be found that with the increase of Ce content, the Pd(H2O)4 2+ / PdO x The ratio shows a trend of increasing first and then decreasing, among which Pd / (SSZ-13@CeO2-0.3) has the highest Pd(H2O)4 2+ / PdO x value, indicating that this sample has more Pd(H2O)4 2+ , which is composed of Pd 2+ species hydrated, so the results show that in all prepared samples, Pd in ​​Pd / (SSZ-13@CeO2-0.3) 2+ or Pd(OH) 2+ is the richest.

[0089] 3. PNA activity of catalyst samples

[0090] During the heating process from 100 to 200℃, at about 143℃ ( Figure 6 a) Another relatively small negative adsorption peak appeared. It can be clearly seen from the figure that the second negative adsorption peaks of the four samples containing cerium oxide shells are significantly larger than those of Pd / SSZ-13 samples, indicating that the coated CeO2 shell helps to regulate the Pd 2+ or the distribution of Pd(OH)2+, thereby increasing NO x Secondary adsorption. Figure 6 In b, NO and NO2 show a negative adsorption peak, while NO2 shows a positive desorption peak. The adsorption / desorption temperatures of the two are the same, but the peak sizes are different. Figure 6 The second NO of a x The storage peak indicates that NO and NO2 are not simply stored / released in a 1:1 ratio. According to the literature, this part of the adsorption peak has two sources: (1) H2O from the active Pd center (Pd 2+ or Pd(OH) 2+ ) desorption, followed by NO storage; (2) NO is oxidized to NO2 by active sites, leading to NO adsorption. Therefore, the results indicate that CeO2 facilitates the generation of active Pd centers.

[0091] Table 3 Calculated NO of 5 samples x Adsorption amount and NO x / Pd value

[0092]

[0093] When the temperature rises to 200-500℃, the curve shows a positive peak greater than 200ppm, which is NO x Release process. Figure 6 a, Main NO of sample Pd / SSZ-13 x The release temperature is 443℃, and the main NO x The release temperature is 327℃ and NO x The desorption is relatively concentrated, indicating that the core-shell structure carrier composed of molecular sieve and cerium oxide can make the active center distribution more uniform and reduce NO x The desorption temperature of Figure 6 b It can be found that when the temperature is close to 500℃, the NO concentration curve decreases and the NO2 concentration increases with the increase of temperature, but the overall performance is Figure 6 The plateau curve around 200 ppm in a is attributed to the catalytic oxidation of NO.

[0094] NO was calculated by integrating the negative peak below 200 ppm. x Storage capacity, NO x The release amount can be obtained by integrating the positive peaks above 200 ppm in the desorption stage. x The / Pd value can indicate the Pd utilization efficiency of the catalyst. Table 3 shows that the calculated adsorption amounts of Pd / SSZ-13, Pd / (SSZ-13@CeO2-0.2), Pd / (SSZ-13@CeO2-0.3), Pd / (SSZ-13@CeO2-0.5) and Pd / (SSZ-13@CeO2-0.6) are 70.5, 72.7, 80.7, 73.6 and 71.4 μmol / g, respectively, and NO x The / Pd values ​​are 0.72, 0.79, 0.87, 0.82 and 0.78 respectively. It can be seen that the adsorption amount of Pd / (SSZ-13@CeO2-0.3) and NO x The / Pd value is the highest among all samples, indicating that Pd / (SSZ-13@CeO2-0.3) has excellent NO x Adsorption activity and Pd utilization efficiency.

[0095] Compared with Pd / SSZ-13, the low-temperature NOx adsorption activity of Pd / (SSZ-13@CeO2-0.3) with a Ce content of 4.42wt% is significantly improved, with an adsorption capacity of 80.7μmol / g, and the desorption temperature (327℃) is also within the temperature range of 200-350℃ required by PNA. This is because the shell layer formed by the stacking of CeO2 particles in the sample Pd / (SSZ-13@CeO2-0.3) is evenly distributed on the outer surface of SSZ-13, which can promote the dispersion of active Pd metal species and form more ionic Pd active centers (Pd 2+ or Pd(OH) 2+ In addition, compared with Pd / SSZ-13, the Pd / (SSZ-13@CeO2-0.3) sample with CeO2 shell has a more obvious NO x Therefore, this study shows that the Pd-based core-shell structure material with SSZ-13 molecular sieve as the core and CeO2 as the shell is expected to become an excellent PNA catalyst. The appropriate amount of CeO2 shell in the core-shell structure material helps to significantly improve the NO x The results show that the adsorption activity of CeO2 shell-structured PNA catalysts can be improved, and provide a valuable reference for the design and synthesis of CeO2 shell-structured PNA catalysts.

[0096] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a diesel engine PNA molecular sieve-based catalyst having a shell structure, characterized in that: The carrier of the molecular sieve-based catalyst is SSZ-13@CeO2, and the first active component is Pd; The preparation method of the molecular sieve-based catalyst comprises the following steps: Step S1: preparing SSZ-13 molecular sieve; Step S2: preparing a core-shell structure material with ceria as the shell and SSZ-13 molecular sieve as the core; Step S3: The core-shell structure material obtained in step S2 is dried in an oven at 80°C for 12 hours, and palladium nitrate is dissolved in water. The palladium nitrate solution is then added to the core-shell structure material obtained in step S2, and the mixture is dried in an oven at 80°C for 12 hours. After calcination in air at 400-550°C for 6 hours, the obtained sample is named Pd / (SSZ-13@CeO2-n), where n represents the mass of Ce(NO3)3·6H2O added during the preparation of the core-shell structure material, and n=0.2, 0.3, 0.5 and 0.

6.

2. The method for preparing a diesel engine PNA molecular sieve-based catalyst having a shell structure according to claim 1, characterized in that: The preparation method of SSZ-13 in step S1 comprises the following steps: Step a1: dissolving NaOH and Al(OH)3 particles in deionized water in sequence under magnetic stirring at 300-800 rpm at room temperature and stirring until clear to obtain a solution; Step a2: adding a silicon source to the solution obtained in step a1, and continuously stirring at room temperature for 3 hours to obtain a uniformly dispersed precursor gel; Step a3: Transfer the uniformly dispersed precursor gel to a polytetrafluoroethylene liner and assemble it into a stainless steel reactor; Step a4: crystallization at 160° C. in a homogeneous reactor; Step a5: centrifuging the product obtained in step a4, and then washing and centrifuging the product 3-4 times until it is neutral; Step a6: Dry the product of step a5 in a constant temperature oven overnight, and calcine the obtained white solid powder at 650° C. for 6 h in air to remove the template. The obtained product is SSZ-13 molecular sieve.

3. The method for preparing a diesel engine PNA molecular sieve-based catalyst having a shell structure according to claim 2, characterized in that: The method for preparing the core-shell structure material in step S2 comprises the following steps: Step b1: 0.1-5 g of SSZ-13 molecular sieve is dispersed in 10-200 ml of a 1 / 1 ethanol / water mixed solution, the mixed solution is first ultrasonically treated for 0.5-2 h, and then the mixed solution is stirred at room temperature and 500-1000 rpm for 30-120 min; Step b2: Add 0.5-2 g of white solid powder polyvinylpyrrolidone to the ethanol / water solution in step b1 and stir for 1.5-5 hours until it is completely dissolved; Step b3: adding 0.2 g, 0.3 g, 0.5 g or 0.6 g of Ce(NO3)3·6H2O and 0.1-1 g of hexamethylenetetramine to the solution obtained in step b2, respectively, and continuously stirring with magnetic force at 500-100 rpm for 2-5 h; Step b4: removing the liquid obtained in step b3 by centrifugation to obtain a light yellow solid product; Step b5: The product was washed repeatedly for 3-4 times, dried in a constant temperature oven overnight, and calcined at 650°C for 6 hours under static conditions. The final yellow solid material obtained was SSZ-13@CeO2-n.

4. The method for preparing a diesel engine PNA molecular sieve-based catalyst having a shell structure according to claim 3, characterized in that: The step S3 comprises the following steps: Step c1: Prepare Pd(NO3)2·2H2O solution and apply the impregnation method to load 0.1-2.0wt% Pd onto SSZ-13@CeO2-n; Step c2: calcining the white solid powder obtained in step c1 at 400-550° C. in static air for 6 h.

5. The method for preparing a diesel engine PNA molecular sieve-based catalyst having a shell structure according to claim 2, characterized in that: The molar composition ratio of the synthetic material feed of the SSZ-13 molecular sieve is 1500-5000H2O:8-30TMAdaOH:50-300SiO2:7-20NaOH:1-10Al2O3.

6. The method for preparing a diesel engine PNA molecular sieve-based catalyst having a shell structure according to claim 2, characterized in that: The silicon source in step a2 is AS-40 silica sol.

7. The method for preparing a diesel engine PNA molecular sieve-based catalyst having a shell structure according to claim 3, characterized in that: The oven temperature in step a6 and the oven temperature in step b5 are both 80°C.

8. The method for preparing a diesel engine PNA molecular sieve-based catalyst having a shell structure according to claim 2, characterized in that: The crystallization time in step a4 is 96 hours.

9. Use of the catalyst prepared by the method for preparing a diesel engine PNA molecular sieve-based catalyst with a shell structure according to any one of claims 1 to 8 in a diesel vehicle PNA catalyst.

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