CN112675845B - Pd-Rh single-coating catalyst for purifying tail gas of natural gas vehicle and preparation method thereof - Google Patents
Pd-Rh single-coating catalyst for purifying tail gas of natural gas vehicle and preparation method thereof Download PDFInfo
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Abstract
The invention discloses a Pd-Rh single-coating catalyst for purifying tail gas of a natural gas vehicle and a preparation method thereof. The catalyst comprises a substrate and a single-layer catalyst coating which is uniformly coated on the substrate, wherein the single-layer catalyst coating is Rh/Ce (d)/Pd/A catalyst, and is used for purifying tail gas of natural gas vehicles and can simultaneously convert methane (CH)4) Nitrogen monoxide (NO) and carbon monoxide (CO). In the catalyst, Pd and Rh are loaded on the same carrier material, and the synergistic effect of Pd and Rh is effectively exerted. Meanwhile, a Ce layer is formed between Pd and Rh by a deposition precipitation method, so that the Pd and Rh can be effectively prevented from directly contacting to form an alloy at high temperature. The catalyst is a single-coating catalyst, and compared with the traditional double-coating catalyst, the preparation process is simpler, and the cost can be reduced to a certain extent.
Description
Technical Field
The invention belongs to the field of automobile exhaust purification catalysts, and particularly relates to a Pd-Rh single-coating catalyst and a preparation method thereof.
Background
Natural gas is a rich energy source and contains fewer impurities, and natural gas vehicles emit CO as compared to gasoline and diesel vehicles2、NOxAnd less PM. Natural gas is one of the most specific alternatives to traditional petroleum fuels. The main pollutant in the tail gas of natural gas vehicles is unburned methane (CH)4) Nitrogen Oxide (NO)x) And carbon monoxide (CO). The crane-type natural gas vehicle executed national six emission standards since 2019, 7 months and 1 day. The mainstream technical route of the six-stage heavy natural gas engine in China adopts equivalence ratio combustion,the post-treatment catalyst adopts a three-way catalyst. Three-way catalyst capable of purifying HC simultaneouslys、NOxAnd CO, and is widely applied to gasoline vehicles. The three-way catalyst used by the natural gas vehicle is different from the three-way catalyst used by the gasoline engine, mainly because of HC in tail gas of the natural gas vehiclesIs mainly CH4，CH4Is the hydrocarbon with the most stable chemical structure, and the purification difficulty of the hydrocarbon is more than that of HC in the traditional gasoline vehiclesTherefore, the content of the noble metal in the three-way catalyst for the natural gas vehicle is about three times that of the three-way catalyst for the gasoline vehicle.
Three-way catalysts are generally composed of a honeycomb ceramic or metal substrate and a catalyst coating attached to the honeycomb substrate. And the catalyst coating layer is generally composed of an inorganic oxide material having a large specific surface area, a cerium-based oxygen storage material, and an active component (generally, Platinum Group Metal (PGM)s) ) is prepared. At present, Pd-Rh bimetallic catalysts are mainly used, wherein Pd is applied to HCsAnd CO has high catalytic activity, while Rh is mainly used for NOxAnd (4) purifying. Different PGMsAnd PGMsThe interaction with the carrier has a great influence on the catalytic performance of the catalyst. Currently, the industry usually supports Pd and Rh on different carriers, and respectively coats them to form a double-coating catalyst. This double layer structure avoids the formation of Pd-Rh alloys at high temperatures. However, since Pd and Rh are distributed on different coatings, the coordination of the Pd and Rh cannot be reflected, and the catalyst cannot achieve the optimal effect. And the double-coating structure makes the preparation process of the catalyst more complicated. Therefore, it is necessary to find a simplified single-coating catalyst that can not only avoid the formation of Pd-Rh alloy at high temperature, but also exert the coordination between Pd and Rh.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a Pd-Rh single-coating catalyst for purifying tail gas of a natural gas vehicle and a preparation method thereof, and the single-coating catalyst which can not only avoid the generation of Pd-Rh alloy but also fully play the synergistic effect between Pd and Rh is obtained, so that the NO in the tail gas of the natural gas vehicle is improved and purifiedx、HCsAnd CO, greatly simplifying the preparation process of the Pd-Rh catalyst and reducing the cost.
The Pd-Rh single-coating catalyst for purifying tail gas of natural gas vehicles comprises a substrate and a single-layer catalyst coating uniformly coated on the substrate, wherein the single-layer catalyst coating is Rh/Ce (d)/Pd/A catalyst, A is a carrier material, the content of Pd is 1.5-2 wt%, the content of Ce is 3-10 wt%, and the content of Rh is 0.1-0.5 wt%.
The Pd-Rh single-coating catalyst is characterized in that the carrier material A is a commercial alumina carrier containing 4wt% of La; the substrate is a honeycomb-shaped ceramic or metal substrate.
The total loading amount of metal Pd and Rh on a substrate of the Pd-Rh single-coating catalyst is 100-150 g/ft3。
The invention also provides a Rh/Ce (d)/Pd/A catalyst, wherein A is a carrier material, the content of Pd is 1.5-2 wt%, the content of Ce is 3-10 wt%, and the content of Rh is 0.1-0.5 wt%.
The catalyst can be used for purifying natural gas vehicle tail gas, slurry is prepared during use and is coated on a substrate to prepare an integral catalyst, only a single layer needs to be coated, Pd and Rh are loaded on the same carrier and coated on the same layer, the coordination effect between Pd and Rh is exerted, and meanwhile, a Ce layer between Pd and Rh can avoid the generation of Pd-Rh alloy at high temperature, so that CH in the natural gas vehicle tail gas can be effectively purified4NO and CO.
The invention provides a preparation method of a Pd-Rh single-coating catalyst for purifying tail gas of a natural gas vehicle, which comprises the following steps:
(1) respectively weighing a certain amount of palladium nitrate solution and a certain amount of catalyst carrier material A, adding the catalyst carrier material A into the palladium nitrate solution by adopting an equal water pore volume impregnation method, uniformly stirring, standing, drying, and calcining at 500-600 ℃ for 2-5 hours to obtain Pd/A powder;
(2) adding Ce (NH)4)2(NO3)6Dissolving in deionized water to obtain cerium ammonium nitrate solution; adding the Pd/A powder obtained in the step (1) into a ceric ammonium nitrate solutionAfter being stirred uniformly, the alkali liquor precipitator is added dropwise under stirring to lead Ce to be4+Depositing on Pd/A powder; to Ce4+After the deposition is completed, continuously stirring for 1-3 h, filtering, drying, and calcining at 500-600 ℃ for 2-5 h to obtain Ce (d)/Pd/A powder;
or, adding Ce (NH)4)2(NO3)6Dissolving in deionized water to obtain cerium ammonium nitrate solution; dipping the Pd/A powder obtained in the step (1) by using a ceric ammonium nitrate solution by adopting an equal water pore volume dipping method, drying, and then calcining for 2-5 h at 500-600 ℃ to obtain Ce (i)/Pd/A powder;
(3) weighing a certain amount of rhodium nitrate solution, adding the Ce (d)/Pd/A powder obtained in the step (2) into the rhodium nitrate solution by adopting an equal water pore volume impregnation method, uniformly stirring, standing, drying, and calcining at 500-600 ℃ for 2-5 h to obtain Rh/Ce (d)/Pd/A catalyst powder;
in the steps (1) - (3), the palladium nitrate, the ceric ammonium nitrate and the rhodium nitrate are used in amounts such that the obtained Rh/Ce (d)/Pd/A catalyst powder contains 1.5-2 wt% of Pd, 3-10 wt% of Ce and 0.1-0.5 wt% of Rh;
(4) mixing and ball-milling the Rh/Ce (d)/Pd/A catalyst powder obtained in the step (3), deionized water and an adhesive to prepare uniform slurry, and controlling the solid content of the slurry to be 40-50 wt%; and uniformly coating the obtained slurry on a honeycomb ceramic or metal matrix, drying, and calcining at 500-600 ℃ for 2-5 h to obtain the Pd-Rh single-coating catalyst.
In the above method, the mass concentration of the cerium ammonium nitrate in the step (2) is further 3 to 10%.
In the above method, further, the precipitant in the step (2) is used in an amount of 2 times the volume of the cerium ammonium nitrate solution.
In the method, further, in the step (1) and the step (3), the standing time is 1-5 h; in the steps (1) - (4), drying is carried out for 8-10 h under the condition of 60-120 ℃.
In the above method, further, the alkali liquor precipitant in step (2) is a mixed solution of ammonia water and ammonium carbonate solution, wherein NH is3:(NH4)2CO3The molar concentration ratio of (2-1) to (1-2).
In the above method, further, the substrate in the step (4) is a cordierite ceramic honeycomb substrate.
The invention also provides a preparation method of the Rh/Ce (d)/Pd/A catalyst, which is the steps (1) to (3) of the preparation method of the Pd-Rh single-coating catalyst.
Compared with the prior art, the invention has the beneficial effects that:
1. the active components Pd and Rh of the catalyst are loaded on the same carrier material and are in the same single-layer coating, so that the coordination effect between Pd and Rh can be fully exerted, and meanwhile, the Ce layer deposited between Pd and Rh can be used as a physical barrier to effectively prevent Pd and Rh from forming Pd-Rh alloy at high temperature, and the catalytic performance of the catalyst is improved. The catalyst is used as a three-way catalyst for natural gas vehicles and is used for treating Hydrocarbon (HC)s) Carbon monoxide (CO) and Nitrogen Oxides (NO)x) Has good transformation effect.
2. The catalyst is a Pd-Rh single-coating catalyst, and compared with the traditional double-coating catalyst, the catalyst has simpler preparation process and can greatly save the preparation cost of the catalyst.
Drawings
Figure 1 is the XRD diffractogram of three catalysts (example 1, example 2, comparative example 1).
FIG. 2 shows the three catalysts (example 1, example 2, comparative example 1) vs. CH4Activity diagram (reaction gas composition 1000ppm CH)4，4600ppm CO，950ppm NO，3118ppmO2，10vol.％H2O，10vol.％CO，N2The space velocity is 50000h for balancing gas-1)。
FIG. 3 is a graph of the activity of three catalysts (example 1, example 2, comparative example 1) on NO (reaction gas composition 1000ppm CH)4，4600ppm CO，950ppm NO，3118ppmO2，10vol.％H2O，10vol.％CO，N2The space velocity is 50000h for balancing gas-1)。
FIG. 4 shows three catalysts (example 1, example 2, p)Ratio 1) activity diagram on CO (reaction gas composition 1000ppm CH)4，4600ppm CO，950ppm NO，3118ppmO2，10vol.％H2O，10vol.％CO，N2The space velocity is 50000h for balancing gas-1)。
Detailed Description
The invention is further illustrated by the following examples. It should be noted that the following examples are only for illustrating the present invention and should not be construed as limiting the scope of the present invention, and those skilled in the art can make certain insubstantial modifications and adaptations of the present invention based on the above disclosure and still fall within the scope of the present invention.
Example 1
Preparation of Rh/Ce (d)/Pd/A catalyst powder
(1) Weighing 1.2329g of palladium nitrate solution (with the concentration of 14.60wt percent), adding a proper amount of deionized water for dilution, and dipping 9.3gLa-Al by adopting an equal water pore volume dipping method 203And drying the powder at 60-120 ℃ for 8-10 h, and calcining at 550 ℃ for 3h to obtain Pd/A powder.
(2) 28.8g (NH) are weighed4)2CO3Fully dissolving in proper amount of deionized water, and adding 22.4ml of NH3·H2And O (the concentration is 25 wt%), finally adding a proper amount of deionized water to the constant volume of 100mL, and uniformly mixing to obtain a precipitator for later use.
(3) 1.5926g of Ce (NH) were weighed out4)2(NO3)6Fully dissolving the mixture in 32mL of deionized water, adding the Pd/A powder obtained in the step (1), uniformly mixing, slowly dropwise adding the precipitator obtained in the step (2) while stirring to ensure that Ce is added4+Depositing on Pd/A powder; to Ce4+After the deposition is completed, stirring is continuously carried out for 2h, then filtering is carried out, drying is carried out for 8-10 h at the temperature of 60-120 ℃, and then calcining is carried out for 3h at the temperature of 550 ℃, so as to obtain Ce (d)/Pd/A powder (d represents that Ce is deposited by a deposition precipitation method).
(4) 0.2501g of rhodium nitrate solution (with the concentration of 7.996 wt%) is weighed, added with a proper amount of deionized water for dilution, the Ce (d)/Pd/A powder obtained in the step (3) is impregnated by adopting an equal water pore volume impregnation method, then the powder is dried for 8-10 h at the temperature of 60-120 ℃, and then the powder is calcined for 3h at the temperature of 550 ℃ to obtain Rh/Ce (d)/Pd/A catalyst powder.
Preparation of monolithic catalyst:
mixing the obtained Rh/Ce (d)/Pd/A catalyst powder with deionized water and a bonding agent, uniformly ball-milling to prepare slurry, controlling the solid content of the slurry to be 45%, uniformly coating the slurry on a phi 11mm 25mm/400cpsi cordierite ceramic honeycomb substrate, and controlling the total loading capacity of noble metals Pd and Rh to be 120g/ft3And drying the catalyst for 8 to 10 hours at the temperature of between 60 and 120 ℃, and calcining the catalyst for 3 hours at the temperature of between 550 ℃ to obtain the Pd-Rh single-coating catalyst.
Example 2
Rh/Ce (i)/Pd/A (i stands for Ce deposited using impregnation) catalyst powder preparation:
(1) weighing 1.2329g of palladium nitrate solution (14.60 wt%), adding a proper amount of deionized water for dilution, and soaking 9.3gLa-Al by adopting an equal water pore volume soaking method 203And drying the powder at the temperature of 60-120 ℃ for 8-10 h, and calcining at the temperature of 550 ℃ for 3h to obtain Pd/A powder.
(2) 1.5926g of Ce (NH) were weighed out4)2(NO3)6Fully dissolving the raw materials in a proper amount of deionized water to obtain a solution, dipping the Pd/A powder obtained in the step (1) by adopting an equal water pore volume dipping method, drying the Pd/A powder at the temperature of between 60 and 120 ℃ for 8 to 10 hours, and calcining the Pd/A powder at the temperature of between 550 ℃ for 3 hours to obtain Ce (i)/Pd/A powder.
(3) 0.2501g of rhodium nitrate solution (7.996 wt%) is weighed, added with a proper amount of deionized water for dilution, and then the Ce (i)/Pd/A powder obtained in the step (2) is impregnated by adopting an equal water pore volume impregnation method, dried for 8-10 h at the temperature of 60-120 ℃, and calcined for 3h at the temperature of 550 ℃ to obtain Rh/Ce (i)/Pd/A catalyst powder.
Preparation of monolithic catalyst:
mixing the obtained Rh/Ce (i)/Pd/A catalyst powder with deionized water and a bonding agent, uniformly ball-milling to prepare slurry, controlling the solid content of the slurry to be 45-50%, uniformly coating the slurry on a phi 11mm 25mm/400cpsi cordierite ceramic honeycomb substrate, and controlling the total loading capacity of noble metals Pd and Rh to be 120g/ft3And drying the catalyst for 8 to 10 hours at the temperature of between 60 and 120 ℃, and calcining the catalyst for 3 hours at the temperature of between 550 ℃ to obtain the Pd-Rh single-coating catalyst.
Comparative example 1
Preparation of (Rh + Ce + Pd)/A catalyst powder:
1.5926g of Ce (NH) were weighed out4)2(NO3)6Fully dissolving the raw materials in a proper amount of deionized water, adding 1.2329g of palladium nitrate solution (14.60 wt%) and 0.2501g of rhodium nitrate solution (7.996 wt%), uniformly mixing, and dipping 9.3gLa-Al by an equal water pore volume dipping method 203Drying the powder at 60-120 ℃ for 8-10 h, and calcining at 550 ℃ for 3h to obtain (Rh + Ce + Pd)/A catalyst powder.
Preparation of monolithic catalyst:
mixing the obtained (Rh + Ce + Pd)/A catalyst powder with deionized water and an adhesive, uniformly ball-milling to prepare slurry, controlling the solid content of the slurry to be 45-50%, uniformly coating the slurry on a phi 11mm 25mm/400cpsi cordierite ceramic honeycomb substrate, and controlling the total loading capacity of noble metals Pd and Rh to be 120g/ft3And drying the catalyst for 8 to 10 hours at the temperature of between 60 and 120 ℃, and calcining the catalyst for 3 hours at the temperature of between 550 ℃ to obtain the Pd-Rh single-coating catalyst.
Fig. 1 is a result of characterization of XRD of the catalysts prepared in example 1, example 2 and comparative example 1, and it can be seen that Pd and Rh are not alloyed.
Activity evaluation experiment:
the activity evaluation of the catalyst is carried out in a multi-path fixed continuous flow microreactor, and the simulated natural gas automobile exhaust comprises the following components: 1000ppm CH4，4600ppm CO，950ppm NO，3118ppmO2，10vol.％H2O，10vol.％CO，N2The space velocity is 50000h for balancing gas-1. All catalyst samples were pretreated for 3h at 550 ℃ in a reaction atmosphere before reaction. Then testing at a heating rate of 5 ℃/min, wherein the temperature range of the activity test is 300-550 ℃, and recording the activity every 10 ℃. CH (CH)4The concentrations of NO and CO were measured using a fourier infrared analyzer. The final results of the activity evaluation are shown in FIGS. 2, 3 and 4.
As can be seen from the activity results of FIG. 2, for the conversion of methane, at the light-off temperature (T)50) Front-410 deg.C (about 410 deg.C), three catalyst pairs CH4Has no obvious difference in transformation at T50The catalyst of example 1 then shows a gradual advantage, in particular the presence of CH on the catalyst4Increases with increasing temperature, after-460 ℃ CH4The conversion of (a) was almost stabilized at 100%. However, on the catalysts in example 2 and comparative example 1, CH after 460 ℃ is4The conversion of (a) decreased with increasing temperature and did not reach full conversion throughout the entire temperature range tested, especially CH on the catalyst in comparative example 14The conversion of (c) was always less than 90%.
As can be seen from the activity results of fig. 3, there was NO significant difference in the conversion of NO on the catalysts of example 1 and example 2, whereas the conversion of NO on the catalyst of comparative example 1 was significantly inferior to the other two catalysts.
As can be seen from the activity results of fig. 4, there was no significant difference in the conversion of CO for the three catalysts, which were above 95% CO conversion over the entire temperature range tested.
Comparing several activity diagrams, it can be known that, compared with the step impregnation method (example 2) and the co-impregnation method (comparative example 1), the Ce layer formed between Pd and Rh by the deposition precipitation method can effectively separate Pd and Rh so that the interaction between Pd and Rh is not strong but not insufficient, and the obtained Pd-Rh bimetallic catalyst has higher efficiency for purifying tail gas of natural gas vehicles.
At present, the three-way catalyst of the natural gas vehicle mainly uses a Pd-Rh double-coating catalyst, the coordination effect between Pd and Rh cannot be effectively exerted, the preparation process is complex and tedious, and the cost is increased by coating for many times. The invention has the advantages that Pd and Rh are loaded on the same carrier, a Ce layer is formed between Pd and Rh by adopting a deposition precipitation method, and the Pd-Rh single-coating catalyst is obtained by coating, so that the generation of Pd-Rh alloy at high temperature is avoided, the coordination effect between Pd and Rh is exerted, and the CH in the tail gas of the natural gas vehicle can be more effectively purified4NO and CO. In addition, the single-layer coating simplifies the preparation process of the catalyst and saves the cost to a certain extent.
Claims (10)
1. The Pd-Rh single-coating catalyst for purifying tail gas of a natural gas vehicle is characterized by comprising a substrate and a single-layer catalyst coating uniformly coated on the substrate, wherein the single-layer catalyst coating is a Rh/Ce (d)/Pd/A catalyst, A is a carrier material, d represents Ce deposited by a deposition precipitation method, the content of Pd is 1.5-2 wt%, the content of Ce is 3-10 wt%, and the content of Rh is 0.1-0.5 wt%; in the Rh/Ce (d)/Pd/A catalyst, Pd, Ce and Rh are sequentially loaded on the same carrier material A, and a Ce layer is deposited between the Pd and the Rh to serve as a physical barrier.
2. The Pd-Rh single-coating catalyst according to claim 1, characterized in that the support material a is a commercial alumina support containing 4wt% La; the substrate is a honeycomb-shaped ceramic or metal substrate.
3. The Pd-Rh single-coating catalyst as recited in claim 1, wherein the total loading of the metals Pd and Rh on the substrate is in the range of 100 to 150g/ft3。
4. An Rh/Ce (d)/Pd/A catalyst is characterized in that A is a carrier material, d represents Ce deposited by a deposition precipitation method, the content of Pd is 1.5-2 wt%, the content of Ce is 3-10 wt%, and the content of Rh is 0.1-0.5 wt%; in the Rh/Ce (d)/Pd/A catalyst, Pd, Ce and Rh are sequentially loaded on the same carrier material A, and a Ce layer is deposited between the Pd and the Rh to serve as a physical barrier.
5. The preparation method of the Pd-Rh single-coating catalyst for purifying tail gas of the natural gas vehicle is characterized by comprising the following steps of:
(1) respectively weighing a certain amount of palladium nitrate solution and a certain amount of catalyst carrier material A, adding the catalyst carrier material A into the palladium nitrate solution by adopting an equal water pore volume impregnation method, uniformly stirring, standing, drying, and calcining at 500-600 ℃ for 2-5 hours to obtain Pd/A powder;
(2) adding Ce (NH)4)2(NO3)6Dissolving in deionized water to obtain cerous nitrateAn ammonium solution; adding the Pd/A powder obtained in the step (1) into a ceric ammonium nitrate solution, uniformly stirring, and then dropwise adding an alkali liquor precipitator while stirring to obtain Ce4+Depositing on Pd/A powder; to Ce4+After the deposition is completed, continuously stirring for 1-3 h, filtering, drying, and calcining at 500-600 ℃ for 2-5 h to obtain Ce (d)/Pd/A powder, wherein d represents depositing Ce by using a deposition precipitation method;
or, adding Ce (NH)4)2(NO3)6Dissolving in deionized water to obtain cerium ammonium nitrate solution; dipping the Pd/A powder obtained in the step (1) by using a ceric ammonium nitrate solution by adopting an equal water pore volume dipping method, and drying; then calcining for 2-5 h at 500-600 ℃ to obtain Ce (i)/Pd/A powder, wherein i represents Ce deposited by using an immersion method;
(3) weighing a certain amount of rhodium nitrate solution, adding the Ce (d)/Pd/A powder or the Ce (i)/Pd/A powder obtained in the step (2) into the rhodium nitrate solution by adopting an equal water pore volume impregnation method, uniformly stirring, standing, drying, and then calcining at 500-600 ℃ for 2-5 h to obtain Rh/Ce (d)/Pd/A or Rh/Ce (i)/Pd/A catalyst powder;
in the steps (1) to (3), the palladium nitrate, the ammonium ceric nitrate and the rhodium nitrate are used in amounts such that the obtained Rh/Ce (d)/Pd/A or Rh/Ce (i)/Pd/A catalyst powder contains 1.5 to 2wt% of Pd, 3 to 10wt% of Ce and 0.1 to 0.5wt% of Rh;
(4) mixing and ball-milling the Rh/Ce (d)/Pd/A or Rh/Ce (i)/Pd/A catalyst powder obtained in the step (3) with deionized water and an adhesive to prepare uniform slurry, wherein the solid content of the slurry is controlled to be 40-50 wt%; and uniformly coating the obtained slurry on a honeycomb ceramic or metal matrix, drying, and calcining at 500-600 ℃ for 2-5 h to obtain the Pd-Rh single-coating catalyst.
6. The method according to claim 5, wherein the mass concentration of the cerium ammonium nitrate in the step (2) is 3 to 10%.
7. The method of claim 5, wherein the precipitant in step (2) is used in an amount of 2 times the volume of the cerium ammonium nitrate solution.
8. The method according to claim 5, wherein in the step (1) and the step (3), the standing time is 1-5 h; the drying conditions in the steps (1) to (4) are 60 to 120 ℃ for 8 to 10 hours.
9. The method as claimed in claim 5, wherein the lye precipitating agent in step (2) is a mixed solution of ammonia and ammonium carbonate solution, wherein NH3:(NH4)2CO3The molar concentration ratio of (2-1) to (1-2).
A method for preparing Rh/Ce (d)/Pd/A catalyst, which is characterized in that the method comprises the steps (1) to (3) of the method for preparing the Pd-Rh single-coating catalyst according to claim 5.
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