A low-rhodium three-way catalytic catalyst for efficient conversion of NOx and its preparation method and application

Abstract

The present invention discloses a low-rhodium three-way catalytic catalyst for efficiently converting NOx, and a preparation method and application thereof, wherein the catalyst contains a transition metal modified molecular sieve layer; the transition metal used in the transition metal modified molecular sieve layer is one or more of Fe, Co, Cu, and Mn, and the molecular sieve used for modification is one of MFI type, beta type, SSZ type, and AEI type molecular sieves; the transition metal modified molecular sieve layer is coated under a noble metal Rh coating and a noble metal Pt and Pd coating, and is used to improve the catalyst's NO conversion efficiency and reduce the amount of the noble metal Rh. The catalyst of the present invention applies the transition metal modified molecular sieve to the three-way catalytic catalyst, and as an independent coating, improves the catalyst's NO conversion efficiency when the O2 content in the exhaust gas is greater than the theoretical air-fuel ratio, reduces the amount of the noble metal rhodium in the catalyst, and achieves the purpose of increasing efficiency and reducing costs, and is particularly suitable for the field of motorcycle exhaust treatment, and has good economic value and promotion prospects.

Description

Technical Field

[0001] The invention belongs to the field of catalysts, and in particular relates to a low-rhodium three-way catalytic catalyst for efficiently converting NOx, and a preparation method and application thereof. Background Art

[0002] Motorcycle exhaust, like automobile exhaust, contains pollutants such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx, mainly NO). In order to prevent motorcycle exhaust from polluting the environment, various countries and regions have formulated relevant emission regulations. At present, domestic motorcycle exhaust emissions need to meet the National IV emission standard. Compared with the previous National III emission standard, the National IV emission standard further reduces the limit values ​​of pollutants. Among them, the emission limit value of THC (total hydrocarbons) is reduced by 50% compared with the emission limit value of the National III emission standard, the emission limit value of CO is reduced by 43% compared with the emission limit value of the National III emission standard, and the emission limit value of NOx is reduced the most, which is 53% lower than the emission limit value of the National III emission standard.

[0003] At present, the most effective technology for treating motorcycle exhaust is three-way catalytic catalyst, which converts HC, CO and NOx in motorcycle exhaust into harmless gases through the coordinated action of precious metals such as rhodium (Rh), platinum (Pt), palladium (Pd) and rare earth composite oxides. Its working principle is as follows: Figure 1 As shown in the figure, when the oxygen (O2) content in the exhaust gas is lower than the theoretical air-fuel ratio, the catalyst reduces NOx to nitrogen (N2) and O2 through the action of the precious metal Rh; when the O2 content in the exhaust gas is higher than the theoretical air-fuel ratio, the catalyst further oxidizes CO to carbon dioxide (CO2) and oxidizes HC compounds to water (H2O) and CO2 through the action of the precious metals Pt and Pd. Therefore, Rh is an indispensable element in the three-way catalytic catalyst, and the three-way catalytic catalyst can only efficiently eliminate the three pollutants under the premise that the oxygen storage characteristics of the vehicle and the catalyst itself match. However, Rh is expensive, which makes the catalyst cost unable to come down. At the same time, due to users' pursuit of fuel consumption, manufacturers have repeatedly improved the oxygen storage capacity of motorcycles and catalysts. Although excessive O2 helps to fully burn the fuel and reduce fuel consumption, it also makes the O2 content in the exhaust gas of the motorcycle greater than the theoretical air-fuel ratio, resulting in a decrease in the elimination efficiency of the three-way catalyst for NOx.

[0004] In order to improve the performance of the catalyst, in the absence of other reaction paths, more precious metals must be used, especially the precious metal Rh for NOx reduction, which further leads to the high cost of the catalyst. Therefore, the development of low-rhodium motorcycle three-way catalytic catalyst is an important way to reduce the cost of the catalyst. CN117899925A discloses a low-rhodium motorcycle three-way catalytic catalyst and a preparation method thereof, which uses the relatively low-priced precious metal ruthenium (Ru) to replace part of the expensive Rh, thereby reducing the cost of the three-way catalytic catalyst. However, the catalytic effect of Ru is weaker than that of Rh, and its dosage needs to be increased accordingly to ensure the exhaust gas purification effect; at the same time, the patented technical solution is to mix the precious metals Pt and Pd with rare earth oxides and the like and then coat them on the inner layer, while the precious metals Rh and Ru are coated on the outer layer. When the O2 content in the exhaust gas is greater than the theoretical air-fuel ratio, the dosage of the precious metals Rh and Ru still needs to be further increased. However, the metal honeycomb carrier used for carrying the three-way catalyst in motorcycles is small in size and has limited carrying capacity, which limits the amount of precious metal Rh and Ru coatings used, and also limits the catalytic effect and the amount of precious metal Rh that is replaced. Moreover, Ru is still a precious metal, and as the amount of Ru metal added increases, the cost is too high, which goes against the original intention of cost reduction. Excessive addition of Ru will also affect the dispersion effect of the coating, resulting in uneven coating distribution of the effective ingredients in the coating, which in turn affects the catalytic effect.

[0005] Therefore, how to seek new reaction pathways, improve the conversion efficiency of NO, reduce the amount of precious metal rhodium in the catalyst while taking into account fuel consumption, and further reduce the catalyst cost, is still an urgent problem to be solved in motorcycle exhaust treatment technology. Summary of the invention

[0006] In view of the above-mentioned problems of the existing low-rhodium three-way catalytic catalyst, such as the load and the conversion efficiency limit of NO, the present invention provides a low-rhodium three-way catalytic catalyst for efficiently converting NOx, and its preparation method and application. By applying transition metal modified molecular sieves to the three-way catalytic catalyst, the reduction conversion reaction path of NO is increased, so as to achieve the purpose of improving the conversion efficiency of the catalyst to NO, reducing the amount of precious metal rhodium in the catalyst, and further reducing the cost of the catalyst. The specific technical scheme is as follows:

[0007] Firstly, the present invention provides a low-rhodium three-way catalytic catalyst for efficiently converting NOx. The catalyst is characterized in that the catalyst contains a transition metal modified molecular sieve layer; the transition metal used in the transition metal modified molecular sieve layer is one or more of Fe, Co, Cu, and Mn, and the molecular sieve used for modification is one of MFI type, beta type, SSZ type, and AEI type molecular sieves; and the transition metal modified molecular sieve layer is coated under a precious metal Rh coating and a precious metal Pt and Pd coating, so as to improve the catalyst's NO conversion efficiency and reduce the amount of precious metal Rh.

[0008] The aforementioned low-rhodium three-way catalytic catalyst for efficient conversion of NOx, the transition metal modified molecular sieve, wherein the silicon aluminum atomic molar ratio of the molecular sieve is 20-200:1, and the mass of the transition metal element is 1-12wt% of the total weight of the transition metal modified molecular sieve.

[0009] The aforementioned low-rhodium three-way catalytic catalyst for efficient NOx conversion comprises a pre-treatment section and a post-treatment section, and there is a vacuum gap between the pre-treatment section and the post-treatment section; the coatings of the pre-treatment section and the post-treatment section are the same, which are, from top to bottom: a precious metal Rh coating, a precious metal Pt-Pd coating and a transition metal modified molecular sieve coating.

[0010] Preferably, in the aforementioned low-rhodium three-way catalytic catalyst for efficient NOx conversion, the dry material components of the precious metal Rh coating include, by mass percentage, Rh 0.01%~0.5%, CeZrO2 25%~59%, ZrO2 2%~4%, and the balance is Al2O3; the dry material components of the precious metal Pt-Pd coating include, by mass percentage, Pt 0.01%~1%, Pd 0.01%~1%, CeZrO2 25%~50%, and the balance is Al2O3; the dry material components of the transition metal modified molecular sieve coating include, by mass percentage, transition metal 5~10%, molecular sieve 85~90%, and the balance is Al2O3 (as a binder).

[0011] Further preferably, in the aforementioned low-rhodium three-way catalytic catalyst for efficient NOx conversion, the precious metal Rh in the precious metal Rh coating, the precious metals Pt and Pd in ​​the precious metal Pt-Pd coating, and the transition metal in the transition metal-modified molecular sieve coating are all nitrates.

[0012] The aforementioned low-rhodium three-way catalytic catalyst for efficient NOx conversion, each coating of the catalyst also contains a humectant and a thickener; the humectant is polyethylene glycol, and its amount is 0.5-1wt% of the total weight of the dry material components of each coating; the thickener is hydroxyethyl cellulose, and its amount is 0.1-0.5wt% of the total weight of the dry material components of each coating.

[0013] Secondly, the present invention provides a method for preparing the aforementioned low-rhodium three-way catalytic catalyst for efficient conversion of NOx, comprising the following steps:

[0014] 1) Making slurry:

[0015] The dry material components, moisturizing agent and thickener of each coating are weighed according to the corresponding weight percentage, and then prepared into precious metal Rh coating slurry, precious metal Pt-Pd coating slurry and transition metal modified molecular sieve coating slurry for standby use; the solid content of each coating slurry is: transition metal modified molecular sieve layer, 18wt% to 30wt%, precious metal Pt-Pd layer, 22wt% to 35wt%, precious metal Rh layer, 24wt% to 35wt%;

[0016] 2) Slurry coating:

[0017] The coating machine is used to sequentially coat each coating slurry on the catalyst substrate in the order of transition metal modified molecular sieve layer → noble metal Pt-Pd layer → noble metal Rh layer, and each coating slurry is dried and calcined at high temperature by a dryer before coating the next layer. After the three layers of coating are coated, the low rhodium three-way catalytic catalyst for efficient NOx conversion is obtained.

[0018] Specifically, the slurry preparation process is:

[0019] 1-1) Preparation of transition metal modified molecular sieve layer slurry:

[0020] Weigh the dry material components, moisturizing agent and thickener of the transition metal modified molecular sieve layer according to the corresponding weight percentage; add deionized water to the container, add the molecular sieve to be modified into the container and stir for more than 30 minutes, add transition metal nitrate, stir for more than 1 hour, then add Al2O3 and stir for more than 30 minutes; adjust pH to 3-6; then add moisturizing agent and stir for more than 30 minutes, then add thickener and stir for more than 4 hours to obtain transition metal modified molecular sieve layer slurry;

[0021] 1-2) Preparation of precious metal Pt-Pd layer slurry

[0022] Weigh the dry material components of the precious metal Pt-Pd layer, the moisturizing agent and the thickener according to the corresponding weight percentage; add deionized water to the container, then put Al2O3 into the container and stir for more than 30 minutes, add platinum nitrate and palladium nitrate and stir for more than 1 hour, then add CeZrO2 and stir for more than 1 hour; adjust the pH to 3-6; then add the moisturizing agent and stir for more than 30 minutes, then add the thickener and stir for more than 4 hours to obtain the precious metal Pt-Pd layer slurry;

[0023] 1-3) Preparation of precious metal Rh layer slurry

[0024] Weigh the dry material components of the precious metal Rh layer, the moisturizing agent and the thickener according to the corresponding weight percentage; add deionized water to the container and stir, then put Al2O3 into the container and stir for more than 30 minutes, add ZrO2 and stir for more than 30 minutes, then add rhodium nitrate and stir for more than 1 hour, then add CeZrO2 and stir for more than 1 hour; adjust the pH to 3-6; then add the moisturizing agent and stir for more than 30 minutes, then add the thickener and stir for more than 4 hours to obtain the precious metal Rh layer slurry;

[0025] The coating process of the slurry is:

[0026] 2-1) Transition metal modified molecular sieve layer slurry coating:

[0027] Use a coating machine to coat the transition metal modified molecular sieve layer slurry on one end of the catalyst substrate, the fan frequency is 15Hz~50Hz, and the coating height is 50%±5% of the total height of the catalyst substrate; after coating, use a dryer to dry, the dryer temperature is 110℃±10℃;

[0028] Then turn the catalyst substrate over, and use a coating machine to apply the transition metal modified molecular sieve layer slurry to the other end of the catalyst substrate, the fan frequency is 15Hz~50Hz, and the coating height is 50%±5% of the total height of the catalyst substrate; after coating, use a dryer to dry, the dryer temperature is 110℃±10℃;

[0029] Then high-temperature roasting is carried out, and the roasting furnace parameters are set as follows: the first stage temperature is 150±15℃, the second stage temperature is 310±15℃, the third stage temperature is 470±20℃, the fourth stage temperature is 550~600℃, and the total roasting time is 90~120 min;

[0030] 2-2) Precious metal Pt-Pd layer slurry coating

[0031] Use a coating machine to coat the precious metal Pt-Pd layer slurry on one end of the catalyst substrate, the fan frequency is 15Hz~50Hz, and the coating height is 50%±5% of the total height of the catalyst substrate; after coating, use a dryer to dry, the dryer temperature is 110℃±10℃;

[0032] Then turn the catalyst over, and use a coating machine to apply the precious metal Pt-Pd layer slurry to the other end of the catalyst substrate, with a fan frequency of 15Hz~50Hz and a coating height of 50%±5%; after coating, use a dryer to dry it, and the dryer temperature is 110℃±10℃;

[0033] Then high-temperature roasting is carried out, and the roasting furnace parameters are set as follows: the first stage temperature is 150±15℃, the second stage temperature is 310±15℃, the third stage temperature is 470±20℃, the fourth stage temperature is 550~600℃, and the total roasting time is 90~120 min;

[0034] 2-3) Precious metal Rh layer slurry coating

[0035] Use a coating machine to coat the precious metal Rh layer slurry on one end of the catalyst substrate, the fan frequency is 15Hz~50Hz, and the coating height is 50%±5% of the total height of the catalyst substrate; after coating, use a dryer to dry, the dryer temperature is 110℃±10℃;

[0036] Then turn the catalyst over, and use a coating machine to apply the precious metal Rh layer slurry to the other end of the catalyst substrate, with a fan frequency of 15Hz~50Hz and a coating height of 50%±5%; after coating, use a dryer to dry, and the dryer temperature is 110℃±10℃;

[0037] Then high-temperature roasting is carried out, and the roasting furnace parameters are set as follows: first-stage temperature 150±15°C, second-stage temperature 310±15°C, third-stage temperature 470±20°C, fourth-stage temperature 550-600°C, and total roasting time 90-120 min.

[0038] As a preferred technical solution, the preparation method of the aforementioned low-rhodium three-way catalytic catalyst for efficient NOx conversion, the loading amount of each coating layer is calculated based on the unit carrier loading weight: transition metal modified molecular sieve layer, the loading amount is 40 to 60 g / L; precious metal Pt-Pd layer, the loading amount is 60 g / L to 110 g / L; precious metal Rh layer, the loading amount is 70 to 120 g / L.

[0039] In addition, the present invention provides an application of the aforementioned low-rhodium three-way catalytic catalyst for highly efficient NOx conversion, specifically, an application in an exhaust gas treatment device for a fuel vehicle or motorcycle.

[0040] Beneficial effects of the present invention:

[0041] 1) The catalyst of the present invention applies transition metal modified molecular sieves to three-way catalytic catalysts, and utilizes the HC-SCR performance of the molecular sieves modified by transition metal elements, thereby increasing the reaction path of traditional motorcycle catalysts, broadening the catalyst working window, and increasing the NO conversion efficiency of the catalyst when the O2 content in the exhaust gas is greater than the theoretical air-fuel ratio, thereby reducing the use of expensive precious metal rhodium in the catalyst and achieving the purpose of reducing costs.

[0042] 2) The present invention uses the transition metal modified molecular sieve as a separate coating, which is convenient for coating dispersant coating; and the coating is located at the bottom layer, and the remaining unconverted NO after catalysis by the precious metal rhodium coating is screened and converted again, which not only improves the catalyst's conversion rate of NOx, but also plays a gatekeeping role; at the same time, this layer, as the final purification barrier, can synergize with the precious metal Pt and Pd coatings to further reduce the emission of THC in the tail gas and ensure the tail gas purification effect. The catalyst purification principle of the present invention is: first, the NOx in the tail gas is partially reduced by the outermost precious metal Rh coating, and the HC and CO in the tail gas are partially purified by the middle Pt-Pd coating. The remaining HC, CO and NOx all reach the transition metal modified molecular sieve coating. Under the action of the transition metal modified molecular sieve, the remaining HC and CO are treated with the Pt-Pd coating as a reducing agent, and the remaining NOx converted by the Rh coating is further converted and reduced to generate N2, and the remaining unconverted HC after catalysis by the Pt-Pd coating is oxidized again to generate pollution-free CO2 and H2O:

[0043] HC+O2+NO→CO2+H2O+N2;

[0044] According to the National IV emission standards, the emission limit for HC is 380mg / km, while the emission limit for NO is 68mg / km. Therefore, in the exhaust gas reaching the transition metal modified molecular sieve coating, there are enough HC and oxygen to support the redox reaction of NOx and HC, ensuring the catalyst's ability to reduce NO and further eliminate NO and HC in the exhaust gas, making the final exhaust gas purification level higher and meeting the National IV emission standards.

[0045] 3) The transition metal of the transition metal modified molecular sieve of the present invention is common metals such as Fe, Co, Cu, and Mn, which can not only increase the activity of NOx, so that it can be reduced when the O2 content in the tail gas is greater than the theoretical air-fuel ratio, but also its price is very low, which further reduces the cost of the catalyst. Among them, when the transition metal Co modified MFI molecular sieve is used as the bottom coating component, the multi-channel of the MFI molecular sieve can provide a better confinement effect. After loading cobalt, it can limit the mutual agglomeration between cobalt atoms under high temperature conditions, and play a dispersing effect while inhibiting the active deactivation of the cobalt element after high temperature, ensuring the selective catalytic reduction ability of the coating, and achieving a better catalytic effect. Experiments have shown that when the transition metal Co modified MFI molecular sieve is used as the bottom coating component, the reduction conversion rate of NO in the catalyst is 2 to 3 times that of the NO reduction conversion rate when other transition metal modified molecular sieves are the bottom coating components; and under the same replacement amount of precious metal rhodium, when the same conversion efficiency is achieved, the catalytic effect of the catalyst of the present invention far exceeds the substitution effect of the precious metal Ru.

[0046] 4) There is a vacuum gap between the pre-treatment part and the post-treatment part of the catalyst of the present invention. After the tail gas is purified by the pre-treatment part, it is mixed in the vacuum chamber in the middle, and then enters the post-treatment part for secondary purification, so that the pollutants in the tail gas are fully in contact with the catalyst, and double purification. In addition, the coatings of the pre-treatment part and the post-treatment part of the catalyst of the present invention are the same, both with transition metal modified molecular sieve coatings, so that the tail gas is repeatedly screened by the transition metal modified molecular sieve coating on the same purification path, so that the three pollutants of HC, CO, and NOx in the tail gas are fully converted, ensuring the emission volume meets the National IV emission limit standard.

[0047] 5) The catalyst of the present invention increases the HC-SCR reaction path through the molecular sieve modified by the transition metal element, and increases the reaction window on the basis of the original two-layer purification coating, so as to achieve the purpose of taking into account the fuel consumption while reducing the amount of precious metal rhodium in the catalyst, thereby improving the catalyst durability mileage, and its catalytic effect is excellent, and the problems of uneven coating mixing and limited loading capacity when using precious metals such as Ru to replace rhodium can be avoided. It can be used in vehicle purifiers such as fuel vehicles and motorcycles, and is particularly suitable for smaller metal honeycomb carriers for motorcycles, and has good practical value and important economic value. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is a schematic diagram of the working principle of three-way catalytic catalyst;

[0049] Figure 2 The schematic diagram of the coating structure of the low-rhodium three-way catalytic catalyst for efficient NOx conversion of the present invention (in the figure: M is a transition metal, and Zeolite refers to a molecular sieve);

[0050] Figure 3 It is a schematic structural diagram of a low-rhodium three-way catalytic catalyst for efficiently converting NOx according to the present invention;

[0051] Figure 4 This is a physical picture of the catalyst of the present invention;

[0052] Figure 5 It is a curve diagram of the effect of the catalyst of the present invention in converting NO. DETAILED DESCRIPTION

[0053] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described clearly and completely in combination with the embodiments below. The drugs involved in the embodiments are all purchased from the market, and those without purity are all industrial or reagent grade.

[0054] Example 1

[0055] This embodiment is to prepare a low-rhodium three-way catalytic catalyst for efficient conversion of NOx. The catalyst contains a transition metal modified molecular sieve; the transition metal used in the transition metal modified molecular sieve layer is Co, and the molecular sieve used for modification is an MFI type molecular sieve. The transition metal modified molecular sieve is used alone as the main effective component of the coating, and is coated under the precious metal Rh coating and the precious metal Pt and Pd coatings to improve the catalyst's conversion efficiency to NO and reduce the amount of precious metal Rh. The specific preparation process is as follows:

[0056] First, make each coating slurry:

[0057] Weigh 298g of cobalt nitrate hexahydrate (AR) (about 60g of cobalt), 860g of MFI molecular sieve ZSM-5, 80g of Al2O3, 8g of PEG, and 3g of Natrosol. Add 3950mL of deionized water to a container, add ZSM-5 molecular sieve to the container and stir for 35min, add cobalt nitrate hexahydrate, stir for 80min, then add Al2O3 and stir for 40min; adjust pH to 5; then add PEG and stir for 35min, then add Natrosol and stir for 5h to obtain transition metal modified molecular sieve layer slurry, and its solid content is about 28wt% for standby use.

[0058] Weigh 10.5g of platinum nitrate (purity 99%) (5g of platinum), 10.8g of palladium nitrate (GR) (5g of palladium), 450g of CeZrO2, 540g of Al2O3, 8g of PEG, and 3g of Natrosol. Add 3150mL of deionized water to a container, add Al2O3 to the container and stir for 40min, add platinum nitrate and palladium nitrate, stir for 90min, then add CeZrO2 and stir for 45min; adjust pH to 4; then add PEG and stir for 40min, then add Natrosol and stir for 6h to obtain a precious metal Pt-Pd layer slurry with a solid content of 32.5wt% for standby use.

[0059] Weigh 2.8g of rhodium nitrate (industrial grade purity 99.5%) (rhodium 1g), 500g of CeZrO2, 30g of ZrO2, 469g of Al2O3, 8g of PEG, and 3g of Natrosol. Add 3050mL of deionized water to a container, add Al2O3 to the container and stir for 45min, add rhodium nitrate, stir for 100min, then add CeZrO2 and stir for 50min; adjust pH to 3; then add PEG and stir for 40min, then add Natrosol and stir for 6h to obtain a precious metal Pt-Pd layer slurry with a solid content of 33.2% for standby use.

[0060] Then, each coating slurry is sequentially coated on a catalyst substrate, the specification of which is 45 (43) * 120 (50.8 + 50.8) -300 cpsi, as follows:

[0061] 1) Coating transition metal modified molecular sieve layer

[0062] Use a coating machine to coat the transition metal modified molecular sieve layer slurry on one end of the catalyst substrate, the fan frequency is 45Hz, the coating height is 55% of the total height of the catalyst substrate, and the loading amount is 50g / L; after coating, use a dryer to dry, and the dryer temperature is 100℃; then turn the catalyst substrate over, and use a coating machine to coat the transition metal modified molecular sieve layer slurry on the other end of the catalyst substrate, the fan frequency is 45Hz, the coating height is 55% of the total height of the catalyst substrate, and the loading amount is 50g / L; after coating, use a dryer to dry, and the dryer temperature is 100℃; finally, high-temperature calcination is carried out, and the calcination furnace parameters are set as: first stage temperature 135℃, second stage temperature 295℃, third stage temperature 450℃, fourth stage temperature 560℃, and calcination for 100min.

[0063] 2) Coating of precious metal Pt-Pd layer

[0064] A coating machine is used to coat the precious metal Pt-Pd layer slurry on one end of the catalyst substrate, with a fan frequency of 35 Hz, a coating height of 55% of the total height of the catalyst substrate, and a loading amount of 80 g / L. After coating, a dryer is used for drying, and the dryer temperature is 120°C. The catalyst is then turned over, and a coating machine is used to coat the precious metal Pt-Pd layer slurry on the other end of the catalyst substrate, with a fan frequency of 35 Hz, a coating height of 55%, and a loading amount of 80 g / L. After coating, a dryer is used for drying, and the dryer temperature is 120°C. Finally, high-temperature calcination is carried out, and the calcination furnace parameters are set as follows: first stage temperature 165°C, second stage temperature 325°C, third stage temperature 490°C, fourth stage temperature 570°C, and calcination for 100 minutes.

[0065] 2-3) Applying the precious metal Rh layer

[0066] Use a coating machine to coat the precious metal Rh layer slurry on one end of the catalyst substrate, the fan frequency is 25Hz, the coating height is 55% of the total height of the catalyst substrate, and the loading amount is 90g / L; after coating, use a dryer to dry, and the dryer temperature is 120℃; then turn the catalyst over, turn the catalyst over, and use a coating machine to coat the precious metal Rh layer slurry on the other end of the catalyst substrate, the fan frequency is 25Hz, the coating height is 55%, and the loading amount is 90g / L; after coating, use a dryer to dry, and the dryer temperature is 120℃; finally, high-temperature roasting is carried out, and the roasting furnace parameters are set as: first stage temperature 165℃, second stage temperature 325℃, third stage temperature 490℃, fourth stage temperature 580℃, and roasting 110min.

[0067] After the three coatings are applied, a low-rhodium three-way catalytic catalyst 1 (hereinafter referred to as product 1) with high efficiency in converting NOx is obtained. Figures 2 to 4 shown.

[0068] Example 2

[0069] This example is also for preparing a low-rhodium three-way catalytic catalyst for efficient conversion of NOx. The transition metal in the transition metal modified molecular sieve layer of the catalyst is iron nitrate (Fe 60g) and the modified molecular sieve is ZSM-5 molecular sieve. The slurry preparation and coating process and parameters of each coating layer of the catalyst are the same as those in Example 1, and a low-rhodium three-way catalytic catalyst 2 (product 2) for efficient conversion of NOx is obtained.

[0070] Example 3

[0071] This example is also for preparing a low-rhodium three-way catalytic catalyst for efficient conversion of NOx. The transition metal in the transition metal modified molecular sieve layer of the catalyst is copper nitrate (Cu 60g), and the molecular sieve used for modification is ZSM-5 molecular sieve. The slurry preparation and coating process and parameters of each coating layer of the catalyst are the same as those in Example 1, and a low-rhodium three-way catalytic catalyst 3 (product 3) for efficient conversion of NOx is obtained.

[0072] Example 4

[0073] This example is also for preparing a low-rhodium three-way catalytic catalyst for efficient conversion of NOx. The transition metal in the transition metal modified molecular sieve layer of the catalyst is manganese nitrate (Mn 60g), and the molecular sieve used for modification is ZSM-5 molecular sieve. The slurry preparation and coating process and parameters of each coating layer of the catalyst are the same as those in Example 1, and a low-rhodium three-way catalytic catalyst 4 (product 4) for efficient conversion of NOx is obtained.

[0074] Example 5

[0075] This embodiment is also for preparing a low-rhodium three-way catalytic catalyst for efficient conversion of NOx. The transition metal in the transition metal modified molecular sieve layer of the catalyst is cobalt nitrate, and the molecular sieve used for modification is beta molecular sieve. The slurry preparation and coating process and parameters of each coating layer of the catalyst are the same as those in Example 1, and a low-rhodium three-way catalytic catalyst 5 (product 5) for efficient conversion of NOx is obtained.

[0076] Example 6

[0077] This embodiment is also for preparing a low-rhodium three-way catalytic catalyst for efficient conversion of NOx. The transition metal in the transition metal modified molecular sieve layer of the catalyst is cobalt nitrate, and the molecular sieve used for modification is SSZ molecular sieve. The slurry preparation and coating process and parameters of each coating layer of the catalyst are the same as those in Example 1, and a low-rhodium three-way catalytic catalyst 6 (product 6) for efficient conversion of NOx is obtained.

[0078] Example 7

[0079] This embodiment is also for preparing a low-rhodium three-way catalytic catalyst for efficient conversion of NOx. The transition metal in the transition metal modified molecular sieve layer of the catalyst is cobalt nitrate, and the molecular sieve used for modification is AEI molecular sieve. The slurry preparation and coating process and parameters of each coating layer of the catalyst are the same as those in Example 1, and a low-rhodium three-way catalytic catalyst 7 (product 7) for efficient conversion of NOx is obtained.

[0080] Comparative Example 1

[0081] This example is to prepare a low-rhodium three-way catalytic catalyst. Except that the catalyst does not contain a transition metal modified molecular sieve layer, the slurry preparation and coating process and parameters of the other coatings are the same as those of Example 1. A low-rhodium three-way catalytic catalyst comparative product 1 (referred to as comparative product 1) is obtained.

[0082] Effect Example 1

[0083] This embodiment is to test the removal rate of the three pollutants HC, CO and NOx in the exhaust gas by the rhodium-type three-way catalytic catalyst products prepared in Examples 1 to 7 and Comparative Example 1, and at the same time test the durability mileage of each catalyst product. Specifically, the rhodium-type three-way catalytic catalyst products prepared in Examples 1 to 7 and Comparative Example 1 are respectively used as accessories of the exhaust gas treatment device of a motorcycle (model HJ125T-23, displacement 125mL, the overall calibration of this model is lean, that is, the oxygen content in the exhaust gas under most working conditions is higher than the theoretical stoichiometric ratio), and installed on the motorcycle for experimental testing to test the purification effect and durability mileage. The detection method refers to the provisions of the national standards GB 14622-2016 "Emission Limits and Measurement Methods of Pollutants for Motorcycles (China Phase IV)" and GB 18176-2016 "Emission Limits and Measurement Methods of Pollutants for Light Motorcycles (China Phase IV)". Among them,

[0084] 1) The removal rate test method of the three pollutants HC, CO and NOx is as follows: Under the same working conditions, an analyzer sampling port is set at the tail end of the catalyst. The analyzer can simultaneously analyze the concentrations of HC, CO and NO, and convert them into units of mg / km, which are compared with the regulatory limits; at the same time, a blank carrier without a catalyst coating is used, and the original exhaust emissions of the engine are analyzed by the analyzer to compare and calculate the conversion efficiency of different pollutants HC / CO / NO. The test results are shown in Table 1.

[0085] Table 1. Test results of the removal rates of THC, CO and NOx pollutants in motorcycle exhaust by various catalyst products

[0086]

[0087] It can be seen from the test results in Table 1 that, firstly, compared with the blank carrier, the catalyst of the present invention can effectively remove the three pollutants of HC, CO and NOx in the exhaust gas; secondly, compared with the removal effect of the comparative product 1, the transition metal modified molecular sieve layer in the catalyst of the present invention has a further removal effect on the three pollutants of HC, CO and NOx, among which the effect on the removal of NOx is the most significant; in addition, the removal effect of the three pollutants of HC, CO and NOx by product 1 is better than that of products 2 to 7, among which the removal effect on NOx is 2 to 3 times that of products 2 to 7. This is because when the transition metal Co modified MFI molecular sieve is used as the bottom coating component, the multi-channel of the MFI molecular sieve can provide a better confinement effect. After loading cobalt, the mutual agglomeration of cobalt atoms under high temperature conditions can be limited, which can play a dispersing effect while inhibiting the inactivation of the cobalt element after high temperature, ensuring the selective catalytic reduction ability of the coating, and achieving a better catalytic effect.

[0088] 2) The catalyst durability mileage test method is: each catalyst product is operated in the vehicle, and runs 10,000 km according to the regulatory conditions (after multiplying by the degradation number according to the regulations, it is equivalent to the 20,000 km mileage required by the regulations), and then the removal rate of the three pollutants HC, CO and NOx is tested, and the test method is the same as above. The test results are shown in Table 2.

[0089] Table 2. Test results of the removal rate of HC, CO and NOx in exhaust gas after each catalyst product worked on the vehicle for 10,000 km

[0090]

[0091] It can be seen from the test results in Table 2 that, relative to the comparative product 1 which does not contain transition metal modified molecular sieve coating, the catalysts of the present invention can continuously and well remove the three pollutants, and the effect of removing NOx is still within the limit of National IV emission standards, which is fully in line with the new regulatory restrictions.

[0092] Effect Example 2

[0093] This example is to investigate the removal effect of different coating amounts of transition metal modified molecular sieve coating on three pollutants, HC, CO and NOx in exhaust gas. This example uses the low rhodium type three-way catalytic catalyst prepared in Example 1 as the basis for investigation. The coating amount of each product involved in the investigation is shown in Table 3. Each catalyst product works on the vehicle and runs 10,000 km according to the regulatory working conditions. The detection method is the same as in Effect Example 1, and the detection results are shown in Table 4.

[0094] Table 3. Coating loading of each catalyst product (g / L)

[0095]

[0096] Table 4. Effect of different coating amounts of transition metal modified molecular sieve layer on the removal rate of pollutants in motorcycle exhaust

[0097]

[0098] It can be seen from the test results in Table 4 that if the coating amount of the transition metal modified molecular sieve coating is too little, the conversion rate of the remaining NOx converted by the rhodium coating is insufficient, which will result in the emission volume failing to meet the National IV emission standard limit for pollutants; while when the coating amount of the rhodium coating and the platinum palladium coating remains unchanged, with the increase of the coating amount of the transition metal modified molecular sieve coating, the removal effect of the three pollutants, HC, CO and NOx, is enhanced, and when the coating amount of the transition metal modified molecular sieve coating increases too much, the removal effect converges. Therefore, under the premise of ensuring the ideal purification effect, Xu reasonably controls the coating amount of the transition metal modified molecular sieve coating to achieve the best comprehensive effect.

[0099] Effect Example 3

[0100] This example is to investigate the effect of the transition metal modified molecular sieve layer on the substitution of precious metal rhodium in the catalyst product. This example also uses the low rhodium type three-way catalytic catalyst prepared in Example 1 as the basis for investigation, and the products involved in the investigation include:

[0101] Sample 1: low rhodium three-way catalytic catalyst prepared in Example 1;

[0102] Sample 2: This catalyst does not contain a transition metal modified molecular sieve layer, but the content of rhodium in the noble metal rhodium coating is twice that in the rhodium coating in Example 1, and the rest of the preparation method is the same as that in Example 1;

[0103] Sample 3: This catalyst does not contain a transition metal modified molecular sieve layer, but a precious metal ruthenium with a content of 10 times the rhodium is added to the precious metal rhodium coating, and the rest of the preparation method is the same as that of Example 1;

[0104] Sample 4: product prepared in Comparative Example 1.

[0105] The amount of precious metal rhodium in the coating of each sample is shown in Table 5.

[0106] Table 5. Amount of precious metals used in each coating (g)

[0107]

[0108] The coating method and parameters of Sample 2 and Sample 3 are consistent with the coating method of the noble metal Pt-Pd layer and the noble metal Rh layer in the embodiment. The inspection standard is: inspect the temperature corresponding to the NOx conversion efficiency. The lower the temperature, the better the catalytic conversion effect. The test results are shown in Table 6 and Figure 5 shown.

[0109] Table 6. Temperature values ​​corresponding to 60% NOx conversion efficiency of each sample (℃)

[0110]

[0111] The test results show that the NOx conversion efficiency of sample 1 (low rhodium three-way catalytic catalyst prepared by the present invention) is higher than that of the other three samples. That is, under the condition of replacing the same amount of precious metal rhodium, when the same conversion efficiency is achieved, the catalytic effect of the catalyst of the present invention exceeds the effect of replacing 10 times the precious metal Ru, and the effect is significant.

[0112] In general, the catalyst of the present invention increases the HC-SCR reaction path through the molecular sieve modified by the transition metal element, and increases the reaction window on the basis of the original two-layer purification coating, so as to achieve the purpose of taking into account the fuel consumption while reducing the amount of precious metal rhodium in the catalyst, thereby improving the catalyst durability mileage, and its catalytic effect is excellent, and the problems of uneven coating mixing and limited loading capacity when using precious metals such as Ru to replace rhodium can be avoided. It can be used in vehicle purifiers such as fuel vehicles and motorcycles, and is particularly suitable for smaller metal honeycomb carriers for motorcycles, and has good practical value and important economic value.

[0113] It is obvious to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, the embodiments should be regarded as exemplary and non-restrictive from any point of view. In addition, it should be understood that although this specification is described in accordance with the implementation methods, it does not contain only one technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in the embodiments can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A low-rhodium three-way catalytic catalyst for efficient NOx conversion, characterized by: The catalyst comprises a front-stage processing part and a rear-stage processing part, and there is a vacuum gap between the front-stage processing part and the rear-stage processing part; The coatings of the front-stage treatment part and the back-stage treatment part are the same, which are: precious metal Rh coating, precious metal Pt-Pd coating and transition metal modified molecular sieve coating from top to bottom. The transition metal used in the transition metal modified molecular sieve layer is one or more of Fe, Co, Cu, and Mn, and the molecular sieve used for modification is one of MFI type, beta type, SSZ type, and AEI type molecular sieve; The transition metal modified molecular sieve layer is coated under the noble metal Rh coating and the noble metal Pt and Pd coatings to improve the conversion efficiency of the catalyst to NO and reduce the amount of noble metal Rh; The dry material components of the noble metal Rh coating include, by mass percentage: Rh 0.01%~0.5%, CeZrO2 25%~59%, ZrO2 2%~4%, The balance is Al2O3; The dry material components of the noble metal Pt-Pd coating include, by mass percentage: Pt 0.01%~1%, Pd 0.01%~1%, CeZrO2 25%~50%, The balance is Al2O3; The dry material components of the transition metal modified molecular sieve coating include, by mass percentage: Transition metals 5-10%, Molecular sieve 85-90%, The balance is Al2O3.

2. The low-rhodium three-way catalytic catalyst for efficient NOx conversion according to claim 1, characterized in that: The transition metal modified molecular sieve, wherein the silicon to aluminum atomic molar ratio of the molecular sieve is 20 to 200:1, and the mass of the transition metal element is 1 to 12 wt % of the total weight of the transition metal modified molecular sieve.

3. The low-rhodium three-way catalytic catalyst for efficient NOx conversion according to claim 1, characterized in that: Each coating layer of the catalyst also contains a moisturizer and a thickener; The moisturizing agent is polyethylene glycol, and its usage is 0.5-1wt% of the total weight of the dry material components of each coating; The thickener is hydroxyethyl cellulose, and its usage is 0.1-0.5wt% of the total weight of the dry ingredients of each coating.

4. A method for preparing a low-rhodium three-way catalytic catalyst for efficient conversion of NOx according to any one of claims 1 to 3, characterized in that: The steps include: 1) Making slurry: Weigh the dry material components, moisturizer and thickener of each coating according to the corresponding weight percentage, and then prepare them into precious metal Rh coating slurry, precious metal Pt-Pd coating slurry and transition metal modified molecular sieve coating slurry for standby use; 2) Slurry coating: A coating machine is used to coat each coating slurry on the catalyst substrate in the order of transition metal modified molecular sieve layer → precious metal Pt-Pd layer → precious metal Rh layer, and each layer of slurry is dried and calcined at high temperature in a dryer before coating the next layer; after the three layers of coating are applied, the low-rhodium three-way catalytic catalyst for efficient NOx conversion is obtained.

5. The method for preparing a low-rhodium three-way catalytic catalyst for efficient NOx conversion according to claim 4, characterized in that: In step 1), the slurry is prepared in the following specific process: 1-1) Preparation of transition metal modified molecular sieve layer slurry: Weigh the dry material components, moisturizing agent and thickener of the transition metal modified molecular sieve layer according to the corresponding weight percentage; add deionized water to the container, add the molecular sieve to be modified into the container and stir for more than 30 minutes, add transition metal nitrate, stir for more than 1 hour, then add Al2O3 and stir for more than 30 minutes; adjust pH to 3-6; then add moisturizing agent and stir for more than 30 minutes, then add thickener and stir for more than 4 hours to obtain transition metal modified molecular sieve layer slurry; 1-2) Preparation of precious metal Pt-Pd layer slurry Weigh the dry material components of the precious metal Pt-Pd layer, the moisturizing agent and the thickener according to the corresponding weight percentage; add deionized water to the container, then put Al2O3 into the container and stir for more than 30 minutes, add platinum nitrate and palladium nitrate and stir for more than 1 hour, then add CeZrO2 and stir for more than 1 hour; adjust the pH to 3-6; then add the moisturizing agent and stir for more than 30 minutes, then add the thickener and stir for more than 4 hours to obtain the precious metal Pt-Pd layer slurry; 1-3) Preparation of precious metal Rh layer slurry Weigh the dry material components of the precious metal Rh layer, the moisturizing agent and the thickener according to the corresponding weight percentage; add deionized water to the container and stir, then put Al2O3 into the container and stir for more than 30 minutes, add ZrO2 and stir for more than 30 minutes, then add rhodium nitrate and stir for more than 1 hour, then add CeZrO2 and stir for more than 1 hour; adjust the pH to 3-6; then add the moisturizing agent and stir for more than 30 minutes, then add the thickener and stir for more than 4 hours to obtain the precious metal Rh layer slurry; In step 2), the slurry is applied, and the specific process is as follows: 2-1) Transition metal modified molecular sieve layer slurry coating: Use a coating machine to coat the transition metal modified molecular sieve layer slurry on one end of the catalyst substrate, the fan frequency is 15Hz~50Hz, and the coating height is 50%±5% of the total height of the catalyst substrate; after coating, use a dryer to dry, the dryer temperature is 110℃±10℃; Then turn the catalyst substrate over, and use a coating machine to apply the transition metal modified molecular sieve layer slurry to the other end of the catalyst substrate, the fan frequency is 15Hz~50Hz, and the coating height is 50%±5% of the total height of the catalyst substrate; after coating, use a dryer to dry, the dryer temperature is 110℃±10℃; Then high-temperature roasting is carried out, and the roasting furnace parameters are set as follows: the first stage temperature is 150±15℃, the second stage temperature is 310±15℃, the third stage temperature is 470±20℃, the fourth stage temperature is 550~600℃, and the total roasting time is 90~120 min; 2-2) Precious metal Pt-Pd layer slurry coating Use a coating machine to coat the precious metal Pt-Pd layer slurry on one end of the catalyst substrate, the fan frequency is 15Hz~50Hz, and the coating height is 50%±5% of the total height of the catalyst substrate; after coating, use a dryer to dry, the dryer temperature is 110℃±10℃; Then turn the catalyst over, and use a coating machine to apply the precious metal Pt-Pd layer slurry to the other end of the catalyst substrate, with a fan frequency of 15Hz~50Hz and a coating height of 50%±5%; after coating, use a dryer to dry it, and the dryer temperature is 110℃±10℃; Then high-temperature roasting is carried out, and the roasting furnace parameters are set as follows: the first stage temperature is 150±15℃, the second stage temperature is 310±15℃, the third stage temperature is 470±20℃, the fourth stage temperature is 550~600℃, and the total roasting time is 90~120 min; 2-3) Precious metal Rh layer slurry coating Use a coating machine to coat the precious metal Rh layer slurry on one end of the catalyst substrate, the fan frequency is 15Hz~50Hz, and the coating height is 50%±5% of the total height of the catalyst substrate; after coating, use a dryer to dry, the dryer temperature is 110℃±10℃; Then turn the catalyst over, and use a coating machine to apply the precious metal Rh layer slurry to the other end of the catalyst substrate, with a fan frequency of 15Hz~50Hz and a coating height of 50%±5%; after coating, use a dryer to dry, and the dryer temperature is 110℃±10℃; Then high-temperature roasting is carried out, and the roasting furnace parameters are set as follows: first-stage temperature 150±15°C, second-stage temperature 310±15°C, third-stage temperature 470±20°C, fourth-stage temperature 550-600°C, and total roasting time 90-120 min.

6. The method for preparing a low-rhodium three-way catalytic catalyst for efficient NOx conversion according to claim 5, characterized in that: The solid contents of the prepared coating slurries are: Transition metal modified molecular sieve layer, 18wt%~30wt%, Precious metal Pt-Pd layer, 22wt%~35wt%, Precious metal Rh layer, 24wt%~35wt%.

7. The method for preparing a low-rhodium three-way catalytic catalyst for efficient NOx conversion according to claim 5, characterized in that: The loading amount of each coating layer is calculated based on the unit carrier loading weight: Transition metal modified molecular sieve layer, loading capacity 40~60g / L, Precious metal Pt-Pd layer, loading 60 g / L~110 g / L, Precious metal Rh layer, loading capacity 70~120g / L.

8. An application of a low-rhodium three-way catalytic catalyst for efficient NOx conversion according to any one of claims 1 to 3, characterized in that: Application in exhaust gas treatment devices of fuel vehicles or motorcycles.

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