A catalyst for reducing the amount of N2O generated during ammonia conversion and a preparation method thereof

Abstract

The present invention belongs to the technical field of catalyst preparation, and particularly relates to a catalyst for reducing the generation amount of N₂O during the ammonia conversion process and a preparation method thereof. The catalyst of the present invention comprises a carrier, on which an ammonia adsorption and conversion functional coating and a NO reduction functional coating are sequentially coated; the NO reduction functional coating comprises a first molecular sieve and Cu or Fe, and further comprises a pore-forming template agent, and Cu or Fe is loaded on the first molecular sieve; the ammonia adsorption and conversion functional coating comprises a second molecular sieve and an active metal. The inner ammonia adsorption and conversion functional coating of the catalyst of the present invention can adsorb the excessive ammonia injected upstream at low temperature. As the temperature rises, the ammonia conversion ability increases, converting ammonia into NO. The formed NO rapidly diffuses to the outer NO reduction functional coating and reacts with the ammonia adsorbed by the outer NO reduction functional coating to generate N₂ and H₂O, effectively reducing the over-oxidation of ammonia and reducing the generation amount of N₂O.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalyst preparation, and particularly relates to a catalyst for reducing the generation amount of N2O in the ammonia conversion process and a preparation method thereof. Background Art

[0002] The NH3-SCR technology catalyst for post-treatment of lean-burn engines can effectively solve the problem of nitrogen oxide (NO x ) emissions in the exhaust gas and achieve the engineering target value. However, in practical applications, because the catalyst under low-temperature conditions (<250 °C) does not reach the optimal working region, the conversion efficiency of NO x is relatively low (<90%), as reported by Kwak J H et al. [1] . In order to achieve the nitrogen oxide emission target, the system will inject ammonia in excess, resulting in ammonia leakage problems. The role of the ammonia conversion catalyst in the post-treatment system is to convert the excessively injected ammonia into N2. Currently, the active metal that oxidizes ammonia in this catalyst is Pt. Because of its relatively high activity, a large amount of N2O will inevitably be generated during the catalytic process due to the over-oxidation of ammonia. With the increasingly strict future regulations, especially the limit requirements for N2O, the existing catalyst solutions face huge challenges. In addition, with the cost reduction requirements of upstream vehicles, catalyst companies are also facing huge cost reduction pressures. Since the precious metals in the catalyst are expensive, they have become the first choice for cost reduction by each manufacturer. However, while realizing the functions and efficiencies of the catalyst, how to reduce the use of precious metals is also a challenge faced by post-treatment companies. To address the above challenges, how to optimize the ammonia conversion catalyst unit module to achieve low N2O emissions and cost reduction requirements is an urgent problem that current post-treatment catalyst manufacturers need to solve.

[0003] References: [1] Kwak J H, Tran D, Burton S D, et al. Effects of hydrothermal aging on NH3-SCR reaction over Cu / zeolites[J]. Journal of Catalysis, 2012, 287(none):203-209. DOI:10.1016 / j.jcat.2011.12.025. Summary of the Invention

[0004] The object of the present invention is to overcome the deficiencies existing in the prior art and provide a catalyst for reducing the generation amount of N2O during the ammonia conversion process and a preparation method thereof. The catalyst of the present invention includes a carrier and an inner layer ammonia adsorption and conversion functional coating and an outer layer NO reduction functional coating sequentially coated on the carrier. The inner layer ammonia adsorption and conversion functional coating can adsorb the excessive ammonia injected upstream at low temperature. As the temperature rises, the ammonia conversion ability increases, converting ammonia into NO. The formed NO quickly diffuses to the outer layer NO reduction functional coating and reacts with the ammonia adsorbed by the outer layer NO reduction functional coating to generate N2 and H2O, effectively reducing the over-oxidation of ammonia and reducing the generation amount of N2O.

[0005] To achieve the above technical objectives, the technical solution adopted in the embodiments of the present invention is as follows:

[0006] In the first aspect, an embodiment of the present invention provides a catalyst for reducing the generation amount of N2O during the ammonia conversion process, including a carrier, and an ammonia adsorption and conversion functional coating and a NO reduction functional coating are sequentially coated on the carrier;

[0007] The NO reduction functional coating includes a first molecular sieve and Cu or Fe, and also includes a pore-forming template agent, and Cu or Fe is loaded on the first molecular sieve;

[0008] The ammonia adsorption and conversion functional coating includes a second molecular sieve and an active metal.

[0009] Further, the first molecular sieve loaded with Cu or Fe is one or more of SSZ-13, SSZ-16, SSZ-39, SSZ-52, SSZ-98, ZK-4, ZK-5, ZK-20, ZK-22, ZSM-5, ZSM-35, ZSM-58, MOR, and BETA.

[0010] Further, the second molecular sieve is one or more of SSZ-13, SSZ-16, SSZ-39, SSZ-52, SSZ-98, ZK-4, ZK-5, ZK-20, ZK-22, ZSM-5, ZSM-35, ZSM-58, MOR, and BETA.

[0011] Further, the active metal is one or more of Cu, Fe, Mn, Ce, Zr, La, Al, Y, Nd, and Hf.

[0012] Further, in the ammonia adsorption and conversion functional coating, the active metal is physically mixed with the second molecular sieve in the form of an oxide, or the active metal is loaded on the second molecular sieve in an ionic state.

[0013] Further, the loading amount of the NO reduction functional coating on the carrier is 80 - 200 g / L, and the loading amount of the ammonia adsorption and conversion functional coating on the carrier is 40 - 100 g / L.

[0014] Further, the weight percentage of the second molecular sieve in the ammonia adsorption and conversion functional coating is 10% - 50%.

[0015] Further, in the NO reduction functional coating, the weight percentage of Cu or Fe is 1% - 5%.

[0016] Further, the pore-forming template agent includes one or more of starch, polydextrose, carboxymethyl cellulose, hydroxyethyl cellulose, polyethylene, polypropylene, polyimide, and polyvinyl alcohol fiber, and the addition amount is 0.5 wt% - 1 wt% of the weight of the NO reduction functional coating.

[0017] In a second aspect, an embodiment of the present invention provides a preparation method of the catalyst described in the first aspect, including the following steps:

[0018] (1) Coating of the inner layer ammonia adsorption and conversion functional coating: Coating the prepared ammonia adsorption and conversion functional coating slurry on the carrier, drying at 120 - 150 °C for 1 - 2 h, and calcining at 500 - 550 °C for 1 - 2 h to obtain a semi-finished catalyst A for use;

[0019] (2) Coating of the outer layer NO reduction functional coating: Making the first molecular sieve loaded with Cu or Fe into a suspension, adding aluminum sol and stirring evenly, then adding the pore-forming template agent to the suspension, and then coating it on the semi-finished catalyst A prepared in step (1) according to the loading amount requirements, drying at 120 - 150 °C for 1 - 2 h, and calcining at 500 - 550 °C for 1 - 2 h to obtain a finished catalyst;

[0020] The ammonia adsorption and conversion functional coating slurry is prepared by one of the following methods:

[0021] a. Physical mixing of the active metal in the form of an oxide with the molecular sieve: Adding the active metal in the form of an oxide to the second molecular sieve suspension, and after mixing, adding aluminum sol and stirring evenly;

[0022] b. Loading the active metal in an ionic state into the molecular sieve: Calcining the second molecular sieve at 300 - 350 °C for 0.5 - 1 h, placing it in a dryer and cooling to room temperature, adding the active metal in the form of an ionic salt solution to the uniformly stirred second molecular sieve powder, standing for 12 - 24 h and then drying at 120 - 150 °C by microwave for 0.5 - 1 h, and then preparing the above dried powder into a suspension and adding aluminum sol and stirring evenly.

[0023] The technical solutions in the embodiments of the present invention have the following beneficial effects:

[0024] By optimizing the catalyst composition, the present invention adds a second molecular sieve to the inner layer ammonia adsorption and conversion functional coating to increase the ammonia adsorption capacity at low temperatures, effectively alleviating the problem of too low NO conversion efficiency in the NH3-SCR process at low temperatures and the leakage problem caused by excessive ammonia injection in the SCR system. At the same time, a templating agent is added to the outer layer NO reduction functional coating to increase the porosity of the outer layer coating, which not only facilitates the inward transmission of ammonia, but also facilitates the rapid transmission of NO formed by the conversion of ammonia in the inner layer to the outer layer and its reaction with the ammonia adsorbed in the outer layer. At the same time, the amount of N2O generated during the ammonia conversion process is reduced. And the present invention does not use precious metals including Pt, Pd, and Rh, which can reduce the cost of the catalyst. x By optimizing the catalyst composition, the present invention adds a second molecular sieve to the inner layer ammonia adsorption and conversion functional coating to increase the ammonia adsorption capacity at low temperatures, effectively alleviating the problem of too low NO conversion efficiency in the NH3-SCR process at low temperatures and the leakage problem caused by excessive ammonia injection in the SCR system. At the same time, a templating agent is added to the outer layer NO reduction functional coating to increase the porosity of the outer layer coating, which not only facilitates the inward transmission of ammonia, but also facilitates the rapid transmission of NO formed by the conversion of ammonia in the inner layer to the outer layer and its reaction with the ammonia adsorbed in the outer layer. At the same time, the amount of N2O generated during the ammonia conversion process is reduced. And the present invention does not use precious metals including Pt, Pd, and Rh, which can reduce the cost of the catalyst. x By optimizing the catalyst composition, the present invention adds a second molecular sieve to the inner layer ammonia adsorption and conversion functional coating to increase the ammonia adsorption capacity at low temperatures, effectively alleviating the problem of too low NO conversion efficiency in the NH3-SCR process at low temperatures and the leakage problem caused by excessive ammonia injection in the SCR system. At the same time, a templating agent is added to the outer layer NO reduction functional coating to increase the porosity of the outer layer coating, which not only facilitates the inward transmission of ammonia, but also facilitates the rapid transmission of NO formed by the conversion of ammonia in the inner layer to the outer layer and its reaction with the ammonia adsorbed in the outer layer. At the same time, the amount of N2O generated during the ammonia conversion process is reduced. And the present invention does not use precious metals including Pt, Pd, and Rh, which can reduce the cost of the catalyst. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a graph showing the change in ammonia adsorption capacity of the catalyst in Example 1 and Comparative Example 1.

[0026] Figure 2 It is a curve showing the change in ammonia concentration in the exhaust gas during the ammonia conversion performance evaluation in Example 1 and Comparative Example 1.

[0027] Figure 3 It is a graph showing the change in the amount of N2O generated during the ammonia conversion in Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] In order to make the objectives, technical solutions and advantages of the present invention clearer, 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 used to limit the present invention.

[0029] The carriers used in preparing the catalysts in Examples 1-3 and Comparative Example 1 are all ceramic honeycomb carriers, with a size of 143.8 mm × 101.6 mm and a mesh number of 400 meshes.

[0030] Example 1

[0031] A preparation method of a catalyst for reducing the amount of N2O generated during ammonia conversion, comprising the following steps:

[0032] (1)Coating of the inner layer ammonia adsorption and conversion functional coating: MnO2, CeO2, and ZrO2 were added to the SSZ-13 molecular sieve suspension according to the molar ratio of Mn:Ce:Zr of 0.5:1:4 (where the SSZ-13 molecular sieve accounted for 10% of the total weight of the entire coating), and after mixing, 1 wt% aluminum sol (calculated based on the mass of alumina) was added and stirred for 1 h. The above suspension was coated on the honeycomb carrier at a loading of 40 g / L, dried at 120 °C for 2 h, and calcined at 500 °C for 1 h to obtain the semi-finished catalyst A for use;

[0033] (2)Coating of the outer layer NO reduction functional coating: The SSZ-13 molecular sieve material loaded with 3 wt% Cu was made into a suspension, 1 wt% aluminum sol (calculated based on the mass of alumina) was added and stirred for 1 h, then 1% of the coating weight of polydextrose was added to the suspension, and then it was coated on the semi-finished catalyst A at a loading of 120 g / L, dried at 120 °C for 2 h, and calcined at 500 °C for 1 h to obtain the finished catalyst.

[0034] Example 2

[0035] A preparation method of a catalyst for reducing the generation amount of N2O during the ammonia conversion process, comprising the following steps:

[0036] (1)Coating of the inner layer ammonia adsorption and conversion functional coating: The SSZ-13 molecular sieve loaded with 5 wt% Cu was calcined at 300 °C for 1 h, placed in a dryer and cooled to room temperature. Manganese nitrate, cerium nitrate, and zirconium nitrate were added to the uniformly stirred Cu-SSZ-13 molecular sieve powder in the form of ionic salt solutions according to the molar ratio of Mn:Ce:Zr of 0.5:1:4 (where the SSZ-13 molecular sieve accounted for 30% of the total weight of the entire coating), allowed to stand for 12 h, then dried at 150 °C by microwave for 0.5 h. Then the above dried powder was made into a suspension and 1 wt% aluminum sol (calculated based on the mass of alumina) was added and stirred for 1 h. The suspension was coated on the honeycomb carrier at a loading of 100 g / L, dried at 120 °C for 2 h, and calcined at 500 °C for 1 h to obtain the semi-finished catalyst A for use;

[0037] (2)Coating of the outer layer NO reduction functional coating: The BETA molecular sieve material loaded with 1 wt% Fe was made into a suspension, 1 wt% aluminum sol (calculated based on the mass of alumina) was added and stirred for 1 h, then 1% of the coating weight of carboxymethyl cellulose was added to the suspension, and then it was coated on the semi-finished catalyst A at a loading of 80 g / L, dried at 120 °C for 2 h, and calcined at 500 °C for 1 h to obtain the finished catalyst.

[0038] Example 3

[0039] A preparation method of a catalyst for reducing the generation amount of N2O during ammonia conversion, comprising the following steps:

[0040] (1) Coating of the inner layer ammonia adsorption and conversion functional coating: Calcinate ZSM-5 molecular sieve loaded with 10 wt% Cu at 300 °C for 1 h, place it in a dryer and cool it to room temperature. Add aluminum nitrate, cerium nitrate and zirconium nitrate in the form of an ionic salt solution according to the molar ratio of Al:Ce:Zr of 2:1:2 to the uniformly stirred Cu-ZSM-5 molecular sieve powder (where the ZSM-5 molecular sieve accounts for 50% of the total weight of the entire coating). After standing for 12 h, dry it at 150 °C by microwave for 0.5 h. Then prepare the above dried powder into a suspension and add 1 wt% aluminum sol (calculated by the mass of alumina) and stir for 1 h. Coat the suspension on the honeycomb carrier at a loading amount of 80 g / L, dry it at 120 °C for 2 h, and calcine it at 500 °C for 1 h to obtain semi-finished catalyst A for later use;

[0041] (2) Coating of the outer layer NO reduction functional coating: Prepare a suspension of BETA molecular sieve material loaded with 5 wt% Cu, add 1 wt% aluminum sol (calculated by the mass of alumina) and stir for 1 h. Then add 1% of hydroxyethyl cellulose based on the weight of the coating to the suspension, and then coat it on semi-finished catalyst A at a loading amount of 200 g / L, dry it at 120 °C for 2 h, and calcine it at 500 °C for 1 h to obtain the finished catalyst.

[0042] Comparative Example 1

[0043] (1) Inner layer coating: Add 3 g / ft³ of Pt to the alumina suspension, then add 1 wt% aluminum sol (calculated by the mass of alumina) and stir for 1 h. Coat the above suspension on the honeycomb carrier at a loading amount of 40 g / L, dry it at 120 °C for 2 h, and calcine it at 500 °C for 1 h to obtain semi-finished catalyst A for later use;

[0044] (2) Outer layer coating: Prepare a suspension of SSZ-13 molecular sieve material loaded with 3 wt% Cu, add 1 wt% aluminum sol (calculated by the mass of alumina) and stir for 1 h. Then add 1% of dextrin based on the weight of the coating to the suspension, and then coat it on semi-finished catalyst A at a loading amount of 120 g / L, dry it at 120 °C for 2 h, and calcine it at 500 °C for 1 h to obtain the traditional noble metal-based finished catalyst.

[0045] Evaluation of the NOx conversion performance of the catalyst: The experiment was carried out by loading samples in a fixed-bed micro-reactor and detecting the tail gas components with an MKS2030 Fourier transform infrared analyzer. The experimental conditions were: space velocity 60000 h -1 , NO 500 ppm, NH3 500 ppm, 14% O2, 5% H2O, and N2 as the balance gas.

[0046] Evaluation of the ammonia conversion performance of the catalyst: The experiment was carried out by loading samples in a fixed-bed micro-reactor, and the tail gas components were detected by an MKS2030 Fourier transform infrared analyzer. The experimental conditions were: space velocity 60000 h -1 , 150 ppm of NO, 450 ppm of NH3, 14% O2, 5% H2O, and N2 as the balance gas.

[0047] Ammonia adsorption performance of the catalyst coating: The experiment was carried out by loading samples in a fixed-bed micro-reactor, and the tail gas components were detected by an MKS2030 Fourier transform infrared analyzer. The experimental conditions were: space velocity 60000 h -1 , 500 ppm of NH3, 5% H2O, and N2 as the balance gas.

[0048] Figure 1 is the curve of the ammonia adsorption storage capacity of the catalyst coating. Since the outer coatings of the catalysts in Example 1 and Comparative Example 1 are the same, the difference in the ammonia adsorption storage capacity is caused by the difference in the inner coatings. From Figure 1 it can be seen that the inner coating of Comparative Example 1 has almost no ammonia adsorption storage capacity, while the catalyst in Example 1 has a higher ammonia adsorption amount, and the ammonia adsorption amount decreases with the increase of temperature.

[0049] Figure 2 is the curve of the change in ammonia concentration in the tail gas during the ammonia conversion performance evaluation experiment. It can be seen from the figure that the ammonia concentration in the tail gas of both Example 1 and Comparative Example 1 decreases continuously with the increase of temperature. However, in the range of 150 - 265 °C, the ammonia concentration in Example 1 is significantly less than that in Comparative Example 1. The decrease in ammonia concentration in Example 1 is mainly because the ammonia adsorption and conversion function of the inner coating works at low temperatures. Since the inner coating of the catalyst in Comparative Example 1 has almost no ammonia adsorption storage capacity, the decrease in ammonia concentration in Comparative Example 1 is mainly due to the adsorption of the outer layer and the conversion reaction of the inner layer. When the temperature is greater than 265 °C, the ammonia concentration in Example 1 is slightly greater than that in Comparative Example 1, mainly because the inner coating of the catalyst in Comparative Example 1 uses noble metal Pt, showing excellent ammonia conversion ability.

[0050] Figure 3 is the curve of the change in N2O concentration in the tail gas during the ammonia conversion evaluation experiment. It can be seen from the figure that both catalysts produce different amounts of N2O. The N2O production amount in Example 1 reaches the peak at about 300 °C, and the N2O production amount in Comparative Example 1 reaches the peak at about 250 °C. Moreover, the peak concentration of Example 1 is about 40 ppm lower than that of Comparative Example 1, and the total amount of N2O produced in the whole temperature window is about 13.5% lower.

[0051] In summary, the inner layer ammonia adsorption and conversion functional coating of the catalyst of the present invention can adsorb the excessive ammonia injected upstream at low temperature. As the temperature rises, the ammonia conversion ability increases, converting ammonia into NO. The formed NO quickly diffuses to the outer layer NO reduction functional coating and reacts with the ammonia adsorbed by the outer layer NO reduction functional coating to generate N2 and H2O, effectively reducing the over-oxidation of ammonia, capable of reducing the generation amount of N2O. Moreover, precious metals such as Pt are not used in the catalyst of the present invention, which can greatly reduce the production cost of the catalyst.

[0052] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Although the present invention has been described in detail with reference to the examples, those skilled in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A catalyst for reducing the production amount of N2O during the ammonia conversion process, characterized in that, It includes a carrier, on which an ammonia adsorption and conversion functional coating and a NO reduction functional coating are sequentially coated; The NO reduction functional coating includes a first molecular sieve and Cu or Fe, and also includes a pore-forming template agent, and Cu or Fe is loaded on the first molecular sieve; The ammonia adsorption and conversion functional coating includes a second molecular sieve and an active metal; The loading amount of the NO reduction functional coating on the carrier is 80 - 200 g / L, and the loading amount of the ammonia adsorption and conversion functional coating on the carrier is 40 - 80 g / L; The weight proportion of the second molecular sieve in the ammonia adsorption and conversion functional coating is 10% - 50%; In the NO reduction functional coating, the weight proportion of Cu or Fe is 1% - 5%; The pore-forming template agent includes one or several of starch, polydextrose, carboxymethyl cellulose, hydroxyethyl cellulose, polyethylene, polypropylene, polyimide, and polyvinyl alcohol fiber, and the addition amount is 0.5 wt% - 1 wt% of the weight of the NO reduction functional coating; The first molecular sieve loaded with Cu or Fe is one or more of SSZ-13, SSZ-16, SSZ-39, SSZ-52, SSZ-98, ZK-4, ZK-5, ZK-20, ZK-22, ZSM-5, ZSM-35, and ZSM-58; The second molecular sieve is one or more of SSZ-13, SSZ-16, SSZ-39, SSZ-52, SSZ-98, ZK-4, ZK-5, ZK-20, ZK-22, ZSM-5, ZSM-35, and ZSM-58; The active metal is one or more of Cu, Fe, Mn, Ce, Zr, La, Al, Y, Nd, and Hf; In the ammonia adsorption and conversion functional coating, the active metal is physically mixed with the second molecular sieve in the form of an oxide, or the active metal is loaded in the second molecular sieve in an ionic state.

2. The preparation method of the catalyst according to claim 1, characterized in that, It includes the following steps: (1) Coating of the inner ammonia adsorption and conversion functional coating: Coat the prepared ammonia adsorption and conversion functional coating slurry on the carrier, dry it at 120 - 150 °C for 1 - 2 h, and calcine it at 500 - 550 °C for 1 - 2 h to obtain a semi-finished catalyst A for standby; (2) Coating of the outer NO reduction functional coating: Make the first molecular sieve loaded with Cu or Fe into a suspension, add aluminum sol and stir evenly, then add the pore-forming template agent to the suspension, and then coat it on the semi-finished catalyst A prepared in step (1) according to the loading amount requirement, dry it at 120 - 150 °C for 1 - 2 h, and calcine it at 500 - 550 °C for 1 - 2 h to obtain a finished catalyst; The ammonia adsorption and conversion functional coating slurry is prepared by one of the following methods: a. Physically mixing the active metal with the molecular sieve in the form of an oxide: Add the active metal in the form of an oxide to the second molecular sieve suspension, and after mixing, add aluminum sol and stir evenly; b. The active metal is loaded in the molecular sieve in an ionic state: The second molecular sieve is calcined at 300 - 350 °C for 0.5 - 1 h, placed in a dryer and cooled to room temperature. The active metal is added to the uniformly stirred second molecular sieve powder in the form of an ionic salt solution. After standing for 12 - 24 h, it is dried at 120 - 150 °C by microwave for 0.5 - 1 h. Then, the dried powder is prepared into a suspension and aluminum sol is added and stirred evenly.

Images