A monolithic catalyst for low-temperature deep oxidation of methanol and a preparation method thereof

Abstract

The present application relates to a kind of monolithic catalyst for low-temperature deep oxidation of methanol and its preparation method, which is composed of Pd nanoparticles, cerium lanthanum solid solution and cordierite honeycomb ceramic.Pd nanoparticles are highly dispersed on the surface of cerium lanthanum solid solution, and the cerium lanthanum solid solution loaded with Pd nanoparticles is coated on the cordierite honeycomb ceramic substrate after ball milling to form a Pd / CeLaO monolithic catalyst.The catalyst is placed in the center of the steel pipe of the continuous fixed bed reactor, and then N2 is introduced as the loading gas (also the reaction gas) to remove the air in the pipe for 5 min.After that, low-concentration methanol is loaded into the mixer through a low-temperature methanol saturator, mixed with another N2 as dilution gas, and then enters the reaction pipe.The reaction system gradually undergoes low-temperature deep oxidation of methanol under the action of the catalyst as the temperature rises.The present application provides a low-cost catalyst and its preparation method, and the prepared Pd / CeLaO monolithic catalyst pretreated by methanol can achieve methanol ignition at 40℃ and complete conversion at 80℃.

Description

Technical Field

[0001] This invention relates to a monolithic catalyst for the low-temperature deep oxidation of methanol and its preparation method, belonging to the field of catalyst preparation technology. Background Technology

[0002] Methanol fuel, as a high-performance liquid fuel, has been proven to have the potential to improve engine performance and efficiency due to its excellent combustion properties, such as high anti-knock properties, high heat of vaporization, smokeless combustion, high oxygen content, low C / H ratio, high octane number, and high latent heat of vaporization. As an ideal alternative fuel to traditional automotive fuels, methanol can alleviate dependence on fossil fuels and significantly reduce CO and NO emissions from vehicle exhaust. x This is in contrast to the emissions of other conventional hydrocarbon pollutants. Currently, methanol is widely used as a fuel in ships, automobiles, and other fields. However, methanol-fueled vehicles have low exhaust temperatures during idling and cold starts, which may lead to incomplete combustion and the release of harmful pollutants such as formaldehyde and unburned methanol vapor. The toxic effects of methanol on the human body are caused by methanol and its metabolites formaldehyde and formic acid, characterized primarily by damage to the central nervous system, eye damage, and metabolic acidosis. Methanol can cause toxic reactions when ingested through the digestive tract, respiratory tract, or skin. Therefore, there is an urgent need to develop efficient methanol exhaust purification technologies for methanol-fueled vehicles.

[0003] Since the purification targets of methanol-fueled vehicle exhaust are mainly unburned methanol and some oxidation products of methanol, and the operating conditions of methanol-fueled vehicle exhaust emissions are often high-space-velocity, low-temperature, or even oxygen-deficient environments, achieving efficient methanol purification under these harsh conditions is increasingly difficult. Catalytic oxidation can completely combust methanol, directly converting it into less harmful products such as carbon dioxide and water. Furthermore, catalytic oxidation has advantages such as simple process, cost-effectiveness, and high efficiency, and can achieve efficient purification of methanol-fueled vehicle exhaust through low-temperature deep oxidation of methanol. Noble metal catalysts (platinum, gold, palladium, etc.) are currently the best-performing catalysts in the field of low-temperature catalytic oxidation of methanol. Among them, supported noble metal palladium catalysts have shown very high activity in low-temperature methanol oxidation. However, the high price of noble metal Pd, and how to ensure the low-temperature catalytic oxidation performance of the catalyst while reducing the amount of noble metal used, is a major obstacle to the industrialization of this catalyst. In addition, most industrial catalysts at present are powder catalysts, which are prone to generating huge bed pressure drops and air resistance during use, which significantly affects the catalytic effect. Therefore, it is urgent to study a catalyst with low noble metal usage, simple preparation method, and the ability to meet the requirements of low-temperature deep oxidation of methanol under high space velocity conditions. Summary of the Invention

[0004] The purpose of this invention is to provide a highly efficient catalyst for the low-temperature deep oxidation of methanol with low noble metal loading and its preparation method. First, noble metal Pd nanoparticles were prepared using a liquid-phase reduction method, which allows for the controllable synthesis of Pd nanoparticles with specific sizes. A cerium-lanthanum solid solution was then prepared using a hydrothermal synthesis method, a simple and easy-to-operate procedure that facilitates comparative experiments with different lanthanum ion doping ratios. Next, an assembly method was used to achieve high dispersion of Pd nanoparticles of specific sizes on the surface of the cerium-lanthanum solid solution, resulting in Pd / CeLaO catalyst powder. Anhydrous ethanol was used as the solvent during assembly, and high-speed stirring not only achieved high dispersion of Pd nanoparticles on the cerium-lanthanum solid solution surface, improving the dispersibility and utilization rate of noble metal Pd, but also rapidly evaporated the anhydrous ethanol solvent, reducing subsequent drying time and temperature, thus lowering both time and economic costs. Finally, a monolithic Pd / CeLaO catalyst was prepared using a ball milling coating method. Preparing the catalyst powder into a monolithic catalyst can resist the problems of high bed pressure drop and high air resistance under high space velocity reaction conditions, reducing the impact of high bed pressure drop and high air resistance on the practicality of the catalyst and improving its lifespan.

[0005] The technical solution adopted by the present invention to achieve the above objectives is as follows: a monolithic catalyst for low-temperature deep oxidation of methanol and its preparation method, characterized in that: lanthanum oxide is doped into the lattice of cerium dioxide to form a cerium-lanthanum solid solution support, Pd nanoparticles are highly dispersed on the surface of the cerium-lanthanum solid solution support, and the cerium-lanthanum solid solution loaded with Pd nanoparticles is ball-milled into a slurry and then coated onto a cordierite honeycomb ceramic substrate to form a Pd / CeLaO monolithic catalyst.

[0006] A method for preparing a monolithic catalyst for low-temperature deep oxidation of methanol, comprising the following steps: Weigh out a certain mass of polyvinylpyrrolidone and L-ascorbic acid as the protective agent and reducing agent, respectively, and dissolve them completely in 16 mL of deionized water. Then, add Pd... 2+ The precursor solution was mixed with the above solution and stirred and reduced in a water bath at 80~100 ℃. After the reaction was completed, the mixture was washed with acetone and centrifuged three times to obtain Pd nanoparticles with a particle size of 2~3 nm.

[0007] Weigh out Ce(NO3)6·6H2O, La(NO3)3·6H2O, and NaOH according to the molar ratio, dissolve them completely in 50 mL of deionized water, and then add the Ce-containing solution. 4+ La 3+The solution was mixed with NaOH solution and magnetically stirred at room temperature until the solution was uniformly mixed. The liquid was then transferred to a reaction vessel and heated to 80-120 °C for hydrothermal reaction for 16-24 h. After the reaction was completed, the reaction product was filtered, washed, collected, dried, and finally calcined in a muffle furnace to obtain CeLaO solid solution support.

[0008] According to the loading requirements, take an appropriate amount of Pd nanoparticles and CeLaO solid solution, disperse them in 100 mL of anhydrous ethanol, and stir magnetically overnight at room temperature. Then, continue stirring under water bath heating at 50-80 °C until most of the solvent evaporates. Then, add 10-50 wt% of polyethylene glycol solution to adjust the slurry, spray dry, and control the average particle size of the powder to be 5-10 micrometers. Finally, vacuum dry the catalyst powder at 50-80 °C overnight, and then transfer it to a muffle furnace at 300-500 °C for calcination for 3-6 h to obtain the formed powdered Pd / CeLaO catalyst.

[0009] Powdered Pd / CeLaO catalyst, viscous additive, glacial acetic acid, and deionized water were ball-milled into a slurry in a specified ratio. Then, a cordierite honeycomb ceramic substrate was immersed in the slurry, and excess catalyst slurry was blown off with compressed air. Finally, the cordierite honeycomb ceramic substrate coated with the slurry was dried at 60~100 ℃ for 2~4 h, and then heat-treated at 200~500 ℃ in air, hydrogen, methanol vapor, and reaction atmosphere for 2~6 h respectively to obtain Pd / CeLaO monolithic catalyst pretreated in the corresponding atmosphere.

[0010] A method for low-temperature deep oxidation of methanol using a Pd / CeLaO monolithic catalyst is as follows: The monolithic catalyst is placed in a steel tube of a fixed-bed reactor. First, N2 is passed through for 5 minutes to purge the air from the reaction tube. Then, O2 is introduced as a carrier gas (which also serves as the reaction gas). Low-concentration methanol is loaded into a mixer via a low-temperature methanol saturator and mixed evenly with the N2 used as a dilution gas before entering the reaction tube. The reaction system is heated, and under the action of the catalyst, the methanol undergoes a slow low-temperature deep oxidation reaction as the temperature rises. The reaction space velocity after passing through the catalyst is controlled at 30,000 h⁻¹. -1 The mixed gas is subjected to gas chromatography to achieve online detection of the reaction product components and the content of each component.

[0011] Compared with existing technologies, the advantages of this invention are as follows: It achieves the controllable synthesis of Pd nanoparticles with specific sizes using a liquid-phase reduction method. Anhydrous ethanol is used as the solvent during the assembly of Pd nanoparticles and the cerium-lanthanum solid solution. High-speed stirring ensures high dispersion of Pd nanoparticles on the surface of the cerium-lanthanum solid solution, improving the dispersibility and utilization rate of the precious metal Pd. Simultaneously, the anhydrous ethanol solvent is rapidly evaporated, reducing subsequent drying time and temperature, thus lowering both time and economic costs. The use of ball milling coating to prepare the catalyst powder into a monolithic catalyst resists the problems of high bed pressure drop and high air resistance under high space velocity reaction conditions, reducing the impact of high bed pressure drop and high air resistance on the practicality of the catalyst and improving its service life. Attached Figure Description

[0012] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort. Figure 1 The XRD diffraction patterns of the Pd / CeLaO-A, Pd / CeLaO-H, Pd / CeLaO-M and Pd / CeLaO-R powder catalysts obtained in Examples 1-4 of this invention are shown below. Figure 2 These are TEM images of the Pd / CeLaO-A, Pd / CeLaO-H, Pd / CeLaO-M, and Pd / CeLaO-R powder catalysts obtained in Examples 1-4 of this invention. Figure 3 The images show the activity curves of the monolithic catalysts Pd / CeLaO-A, Pd / CeLaO-H, Pd / CeLaO-M and Pd / CeLaO-R obtained in Examples 1-4 of this invention for the low-temperature deep oxidation of methanol. Detailed Implementation

[0013] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for explanation and illustration only and are not intended to limit the present invention. Some advantages and features of the present invention will be apparent from the following description and will be understood by those skilled in the art through study and practice of the present invention.

[0014] Example 1: The preparation steps of the air-pretreated Pd / CeLaO monolithic catalyst are as follows: First, Pd nanoparticles with a particle size of approximately 3 nm were prepared by PVP protection and liquid-phase reduction. The specific preparation process was as follows: 0.1110 g of PVP and 0.0432 g of L-ascorbic acid were dissolved in 16 mL of deionized water, and the solution was heated to 80 °C and maintained for 15 min under high-speed stirring. Then, a pre-prepared Na₂PdCl₄ solution containing 0.24 mmol of Na₂PdCl₄ was rapidly added to the above solution, and the mixture was stirred continuously for 2–4 h (preferably 3 h). After the reaction, Pd nanoparticles with a particle size of approximately 3 nm were obtained. The obtained nanoparticles were centrifuged, washed three times with 200 mL of acetone, and then dispersed in anhydrous ethanol for storage. Next, cerium-lanthanum solid solutions with different lanthanum ion doping amounts were prepared using a hydrothermal synthesis method. The specific preparation process is as follows: 5.21 g of Ce(NO3)3·6H2O and 0.58 g of La(NO3)3·6H2O were weighed out according to a molar ratio of 0.90:0.10, and an appropriate amount of deionized water was added and stirred thoroughly to obtain a cerium-lanthanum nitrate precursor mixture. 16 g of NaOH was weighed out and dissolved in an appropriate amount of deionized water. The total mass of deionized water used in the two dissolution processes was 50 mL. After the NaOH solution cooled to room temperature, it was transferred to the cerium-lanthanum nitrate precursor mixture and magnetically stirred at room temperature for more than 30 min to form a suspension. After stirring, the mixed suspension was transferred to a 100 mL polytetrafluoroethylene liner and sealed in a stainless steel reactor. The reactor was placed in a forced-air drying oven and heated to 80~120 °C (preferably 100 °C) for hydrothermal reaction for 16~24 h (preferably 20 h). After the hydrothermal reaction was completed and cooled to room temperature, the reaction vessel was removed and the reaction precipitate was washed several times with a large amount of deionized water until the pH was about 7. Then, an appropriate amount of anhydrous ethanol was added for washing, the mixture was filtered, the filter cake was collected, dried at 80 °C overnight, and calcined at 400 °C for 4 h to obtain CeLaO solid solution support. Then, the Pd nanoparticles and CeLaO support were assembled using an assembly method to form a Pd / CeLaO powder catalyst. The specific assembly process is as follows: 2g of CeLaO support was weighed and dispersed in an appropriate amount of anhydrous ethanol. After sonication for 30 min, Pd nanoparticles with a pre-sonicated ethanol solution of about 3 nm were added according to a Pd loading of 1wt%. The mixture was magnetically stirred for more than 12 h to disperse the Pd nanoparticles on the CeLaO support as much as possible. Then, the mixed suspension was placed in a 60 °C water bath and stirred to evaporate most of the solvent. Then, a polyethylene glycol solution containing 0.40g of polyethylene glycol was added to make a slurry. The mixture was spray-dried to control the average particle size of the powder to be 5 μm. The obtained powder was vacuum dried at 60 °C for more than 12 h, and then calcined in a muffle furnace at 400 °C for 4 h to obtain the formed air-pretreated powdered Pd / CeLaO catalyst. Take 4g of the Pd / CeLaO catalyst powder prepared above, 0.32mL of 25wt% glacial acetic acid and an appropriate amount of deionized water, and ball mill them to prepare a slurry. Then, a cordierite honeycomb ceramic matrix (64 cell cm) is formed. -2 The coated catalyst (Φ: 10 mm, L: 25 mm) is immersed in slurry, and then the excess slurry is blown out with compressed air. The coated catalyst is dried at 80 °C for 3 h, and then calcined in air at 400 °C for 4 h to obtain the air-pretreated Pd / CeLaO monolithic catalyst, named Pd / CeLaO-A, denoted as Cat1.

[0015] Example 2: The preparation process of the hydrogen-pretreated Pd / CeLaO monolithic catalyst is roughly the same as that of the catalyst in Example 1. The difference is that after air calcination, the catalyst needs to be calcined at 400 °C in a hydrogen atmosphere for 4 h to obtain the hydrogen-pretreated Pd / CeLaO monolithic catalyst, named Pd / CeLaO-H, and denoted as Cat2.

[0016] Example 3: The preparation process of the methanol-pretreated Pd / CeLaO monolithic catalyst is roughly the same as that of the catalyst in Example 1. The difference is that after air calcination, the catalyst needs to be calcined at 400 °C in a methanol vapor atmosphere of 200 ppm for 4 h to obtain the methanol-pretreated Pd / CeLaO monolithic catalyst, named Pd / CeLaO-M, and denoted as Cat3.

[0017] Example 4: The preparation process of the pretreated Pd / CeLaO monolithic catalyst is roughly the same as that of the catalyst in Example 1. The difference is that after air calcination, the catalyst needs to be calcined for 4 h in a mixed atmosphere of 400 °C, 200 ppm methanol, and 2 vol% oxygen to obtain the methanol pretreated Pd / CeLaO monolithic catalyst, named Pd / CeLaO-R, and denoted as Cat4.

[0018] The specific method for the low-temperature deep oxidation of methanol using the Pd / CeLaO monolithic catalyst is as follows: The Pd / CeLaO monolithic catalysts prepared in Examples 1-4 and pretreated under different atmospheres were respectively loaded into the steel tubes of a fixed-bed reactor. First, N2 was passed through for 5 minutes to purge the air from the reaction tubes. Then, another stream of O2 was introduced as the carrier gas (which also serves as the reactant gas). Low-concentration methanol was fed into a mixer through a 0 °C methanol saturator and mixed thoroughly with the N2 used as a dilution gas before entering the reaction tubes (CH3OH = 200 ppm, O2 = 2 vol%, balance N2). The space velocity (SHV) of the catalyst was controlled at 30,000 h⁻¹. -1The reaction system was heated, and under the action of a catalyst, a slow, low-temperature deep oxidation reaction of methanol occurred as the temperature increased. The composition of the products was analyzed and detected online using an Agilent GC7890B gas chromatograph. The test results are shown in Table 1 (where: T 50 This represents the ignition temperature of methanol. T 90 Indicates the temperature at which methanol is completely converted. ΔT = T 90 - T 50 For the methanol oxidation reaction, the lower the temperature at which a specific methanol conversion rate is achieved, the higher the catalytic activity of the catalyst. T 50 The lower the value, the better the low-temperature activity of the catalyst.

[0019] Table 1. Comparison of the activities of the monolithic catalysts prepared in Examples 1-4: As shown in Table 1, the low-temperature methanol oxidation activity of the Pd / CeLaO monolithic catalyst after heat treatment with hydrogen, methanol, and reaction gas is significantly improved compared with that of the Pd / CeLaO-A monolithic catalyst after air heat treatment. Among them, the Pd / CeLaO-M monolithic catalyst after methanol preheating treatment has the best catalytic activity, which can basically achieve methanol low-temperature ignition at 40 ℃ and complete conversion at 80 ℃, which can basically meet the requirements of methanol fuel vehicle exhaust purification for methanol low-temperature ignition and conversion. Cat 1 118 155 37 Cat 2 51 142 91 Cat 3 40 80 40 Cat 4 50 80 30 .

[0020] Figure 1 and Figure 2 The XRD patterns and TEM images of the Pd / CeLaO-A, Pd / CeLaO-H, Pd / CeLaO-M, and Pd / CeLaO-R powder catalysts prepared in Examples 1-4 of this invention are shown respectively. Figure 1 It can be concluded that La2O3 was successfully doped into the CeO2 lattice to form a CeLaO solid solution. Due to the low loading of the noble metal Pd and its high dispersion on the surface of the CeLaO solid solution, no obvious diffraction peaks of Pd species were observed in the XRD. Figure 2 As can be seen, small Pd nanoparticles can be uniformly dispersed on CeLaO nanorod supports on several Pd / CeLaO powder catalysts that have undergone heat treatment in different atmospheres; Figure 3The activities of the monolithic Pd / CeLaO catalysts prepared in Examples 1-4 of this invention and obtained by heat treatment in different atmospheres are compared. It can be seen that the catalytic activity of the monolithic Pd / CeLaO-M catalyst pretreated with methanol is significantly higher than that of the monolithic Pd / CeLaO catalysts pretreated in other atmospheres.

[0021] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. The application of a monolithic catalyst in the low-temperature deep oxidation reaction of methanol, characterized in that, The preparation steps of the monolithic catalyst are as follows: S1. Weigh out a certain mass of polyvinylpyrrolidone and L-ascorbic acid as the protective agent and reducing agent, respectively, and dissolve them completely in 16 mL of deionized water. Then, add Pd... 2+ The precursor solution was mixed with the above solution and reduced by stirring in a water bath at 80~100 ℃. After the reaction was completed, the mixture was washed with acetone and centrifuged three times to obtain Pd nanoparticles with a particle size of 2~3 nm. S2. Weigh a certain mass of Ce(NO3)3·6H2O, La(NO3)3·6H2O, and NaOH according to the molar ratio, dissolve them completely in 50 mL of deionized water, and then add the Ce-containing solution. 3+ La 3+ The solution was mixed with NaOH solution and magnetically stirred at room temperature until the solution was uniformly mixed. The liquid was then transferred to a reaction vessel for hydrothermal reaction. After the reaction was completed, the reaction product was filtered, washed, collected, dried, and finally calcined in a muffle furnace to obtain CeLaO solid solution support. S3. Take a certain amount of Pd nanoparticles synthesized in step S1 and CeLaO solid solution support synthesized in step S2, disperse them in 100 mL of anhydrous ethanol, and magnetically stir to make the Pd nanoparticles uniformly dispersed on the surface of CeLaO solid solution. Then, under magnetic stirring, heat the mixture in a water bath to evaporate most of the solvent. Then, add 10-50 wt% of polyethylene glycol solution to adjust the slurry, spray dry, and control the average particle size of the powder to be 5-10 micrometers. Finally, vacuum dry the catalyst powder overnight, and then transfer it to a muffle furnace for calcination to obtain the formed powdered Pd / CeLaO catalyst. S4. The powdered Pd / CeLaO catalyst prepared in step S3, the viscous additive, glacial acetic acid and deionized water are ball-milled into a slurry in a certain proportion. Then, the cordierite honeycomb ceramic matrix is ​​immersed in the slurry, and the excess catalyst slurry is blown off with compressed air. Finally, the cordierite honeycomb ceramic matrix coated with slurry is dried and calcined in air at 400 °C for 4 h. Then, the catalyst is calcined at 400 °C and 200 ppm methanol vapor atmosphere for 4 h to obtain the methanol pretreated monolithic Pd / CeLaO catalyst.

2. The application as described in claim 1, characterized in that, The hydrothermal reaction temperature in step S2 is 80~120 ℃, the hydrothermal reaction time is 16~24h, the muffle furnace calcination temperature is 300~500 ℃, and the calcination time is 3~6h.

3. The application as described in claim 1, characterized in that, In step S3, the water bath heating temperature and the vacuum drying temperature are both 50~80 ℃, the muffle furnace calcination temperature is 300~500 ℃, and the calcination time is 3~6h.

4. The application as described in claim 1, characterized in that, In step S4, the drying temperature is 60–100 °C and the drying time is 2–4 h.

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