Composite catalyst, preparation method and application thereof

Abstract

The present application relates to the technical field of doped nanocrystal catalyst preparation, and particularly relates to a composite catalyst, a preparation method and application thereof.The present application comprises the following steps: slowly adding a NaOH solution into a mixed solution of Fe(NO3)3.9H2O and Ce(NO3)3.6H2O, stirring at room temperature to obtain a mixture; the molar ratio of Fe(NO3)3.9H2O and Ce(NO3)3.6H2O is 1:20; transferring the mixture into a polytetrafluoroethylene hydrothermal reactor to perform a hydrothermal reaction, and the reaction time is 24 hours to obtain a solid; separating, washing and drying the solid to obtain a nanomaterial.The present application is applied to a catalytic CO oxidation reaction, and two kinds of Fe x Ce 1‑ x O 2‑δ solid solutions are successfully prepared by means of an improved hydrothermal method, a specific morphology Fe x Ce 1‑x O 2‑δ composite catalyst is realized, high-yield synthesis and uniform doping are realized, and the activation pretreatment atmosphere and the activity sequence of different exposed crystal surfaces on the CO catalytic oxidation reaction are studied.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of doped nanocrystal catalyst preparation, and particularly relates to a composite catalyst, a preparation method and application thereof. BACKGROUND

[0002] In recent years, cerium-based nanomaterials have attracted extensive attention in the field of catalysis due to their excellent redox ability and high oxygen storage capacity. However, the catalytic performance of cerium-based nanomaterials still needs to be further improved in practical applications. At present, there are mainly two strategies to optimize the catalytic performance of cerium oxide, which are crystal face regulation strategy and doping strategy. The morphology and size of nanomaterials have a great influence on their physical and chemical properties, and in structure-sensitive catalytic reactions, the catalytic activity often depends on the specific crystal faces exposed. Therefore, synthesizing nanocatalysts with different morphologies can not only be used for theoretical research on the structure-activity relationship, but also can enhance or regulate their inherent physical and chemical properties according to actual needs, such as regulating their catalytic activity by changing different exposed crystal faces.

[0003] At present, researchers have successfully synthesized cerium oxide nanoparticles with various morphologies, including rods (mainly exposing {110} crystal faces), cubes (mainly exposing {100} crystal faces), octahedrons (mainly exposing {111} crystal faces), etc. In the catalytic CO oxidation reaction, the rod-shaped catalyst has the highest activity, followed by the cubic catalyst, because the oxygen vacancy formation energy of the {110} crystal face is the lowest, which can produce more catalytic reaction sites. According to theoretical calculations, the surface energy of each crystal face of cerium oxide is in the order of {111} < {110} < {100}, so the {100} crystal face should be the most active and has great potential for regulation in practical applications. Therefore, the crystal face regulation strategy has important research significance for improving the catalytic performance of cerium oxide nanomaterials.

[0004] In addition, the doping strategy is also a main way to improve the performance of cerium-based catalysts. By introducing other metal elements to modify cerium oxide, new active sites can be created through synergistic effect. Since iron element has similar ionic radius and lower redox potential than cerium ions, it can successfully enter the cerium oxide lattice to form a uniform iron-cerium solid solution, so iron is considered to be the most suitable doping material for cerium-based catalysts. 3+ / Fe 2+ The redox potential of Fe 4 + / Ce 3+ (1.61V), so Fe 3+ and Ce 4+The electronic interaction between them can occur, thereby improving the redox performance of the cerium-based catalyst, and further leading to the activity improvement of the cerium-based composite catalyst.

[0005] At present, there are relevant literatures studying the influence of the crystal face effect of pure-phase cerium oxide nanomaterials on the catalytic activity thereof, however, when another metal element is doped into the cerium oxide with the morphology, the influence of the crystal face effect of the modified composite catalyst on the catalytic activity thereof is still a pending problem. Therefore, synthesizing the cerium-based composite nanomaterials with a dopant and exposing specific crystal faces has important significance for designing high-efficiency cerium-based catalysts with more excellent catalytic performance in the future. SUMMARY

[0006] In view of the above problems in the prior art, the application provides a composite catalyst, a preparation method and application thereof, and specifically provides a preparation method of efficiently and controllably synthesizing different morphology cerium-based composite catalysts doped with iron elements, which realizes the high-yield synthesis and uniform doping of the composite catalyst, and studies the activity sequence of the activation pretreatment atmosphere and different exposed crystal faces on the CO catalytic oxidation reaction. x Ce 1-x O 2-δ The high-yield synthesis and uniform doping of the composite catalyst are realized, and the activity sequence of the activation pretreatment atmosphere and different exposed crystal faces on the CO catalytic oxidation reaction is studied.

[0007] To achieve the above purpose, in one aspect, the application provides a preparation method of a composite catalyst, comprising:

[0008] Step one: slowly adding a NaOH solution into a mixed solution of Fe(NO3)3·9H2O and Ce(NO3)3·6H2O, stirring at room temperature to obtain a mixture; the molar ratio of the Fe(NO3)3·9H2O to the Ce(NO3)3·6H2O is 1:20;

[0009] Step two: transferring the mixture obtained in the step one into a polytetrafluoroethylene hydrothermal reaction kettle to perform hydrothermal reaction, the reaction time is 24 h, to obtain a solid;

[0010] Step three: separating, washing and drying the solid obtained in the step two to obtain a nanomaterial;

[0011] When the hydrothermal temperature of the hydrothermal reaction in the step two is 100℃, the step three obtains a rod-shaped Fe x Ce 1-x O 2-δ nanomaterial; when the hydrothermal temperature of the hydrothermal reaction in the step two is 180℃, the step three obtains a cubic block-shaped Fe x Ce 1-x O 2-δ nanomaterial.

[0012] In some embodiments, the composite catalyst is prepared by activating the nanomaterials, which includes nitrogen activation and / or oxygen activation.

[0013] In some embodiments, the nitrogen activation includes roasting at 400℃ for 2h under N2 atmosphere.

[0014] In some embodiments, the oxygen activation includes roasting at 400℃ for 2h under O2 atmosphere.

[0015] In some embodiments, the step one includes stirring at room temperature for 30min.

[0016] Another aspect of the present application provides a composite catalyst prepared by the method of any of the above technical solutions.

[0017] In some embodiments, the rod-shaped Fe x Ce 1-x O 2-δ The nanomaterial has a length of any value in the range of 50-100nm, a width of any value in the range of 5-10nm, and a main exposed crystal face of {110} crystal face.

[0018] In some embodiments, the cubic block-shaped Fe x Ce 1-x O 2-δ The nanomaterial has an average edge length of 10nm and a main exposed crystal face of {100} crystal face.

[0019] The present application also provides a use of the composite catalyst of any of the above technical solutions in catalyzing CO oxidation reaction, which includes: mixing the composite catalyst and quartz sand uniformly, placing the mixture on quartz wool, placing the quartz wool in a reaction tube of a normal pressure fixed bed reaction device, and detecting the gas composition by an infrared analyzer after the reaction gas passes through the reaction tube.

[0020] In some embodiments, the reaction gas includes 1% CO, 10% O2 and 89% N2, and the gas flow rate is 60mL / min.

[0021] Compared with the prior art, the present application has the following advantages:

[0022] The present application provides a method for preparing a composite catalyst, which successfully prepares two kinds of Fe x Ce 1-x O 2-δSolid solution: nanorods with {110} exposed crystal surface and nanocubes with {100} exposed crystal surface; the obtained samples were activated and pretreated in N2 or O2 atmosphere respectively to obtain composite catalysts, providing a simple and efficient preparation method of specific morphology doped cerium-based composite catalysts;

[0023] The above composite catalysts were applied to the CO+O2 model reaction, wherein the cubic block Fe x Ce 1-x O 2-δ The composite catalyst, after oxygen activation, the catalytic activity was reversed to the rod-shaped Fe x Ce 1-x O 2-δ The composite catalyst, for the theoretical research of the regulation of different morphologies of cerium oxide on the catalytic reaction activity, provides a material basis, and has important significance for further research on doped cerium-based composite catalysts and corresponding structure-activity relationship. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0025] Figure 1 TEM, HRTEM and STEM-EDS images of Fe x Ce 1-x O 2-δ Nanorods prepared in Example 1, (a, b) are TEM, HRTEM and STEM-EDS images of Fe x Ce 1-x O 2-δ Nanorods prepared in Example 2, (c, d) are TEM, HRTEM and STEM-EDS images of Fe x Ce 1-x O 2-δ Nanocubes, TEM, HRTEM and STEM-EDS images;

[0026] Figure 2 Catalytic oxidation of CO conversion rate and temperature relationship diagram of Fe x Ce 1-x O 2-δ Nanorods of Example 1 and Fe x Ce 1-x O 2-δ Nanocubes of Example 2;

[0027] Figure 3HRTEM images of the activated pre-treated nanomaterials obtained in Example 4, where (a, b) are the FFe obtained in Example 4 pre-treated with nitrogen x Ce 1-x O 2-δ HRTEM images of nanorods, (c, d) are the Fe obtained in Example 4 pre-treated with oxygen x Ce 1-x O 2-δ HRTEM images of nanorods, (e, f) are the Fe obtained in Example 4 pre-treated with nitrogen x Ce 1-x O 2-δ HRTEM images of nanocubes, (g, h) are the Fe obtained in Example 4 pre-treated with oxygen x Ce 1-x O 2-δ HRTEM images of nanocubes;

[0028] Figure 4 Conversion of CO catalytic oxidation versus temperature for the nanomaterials pre-treated with nitrogen and pre-treated with oxygen, where (a) is the Fe pre-treated with nitrogen x Ce 1-x O 2-δ nanorods and Fe pre-treated with oxygen x Ce 1- x O 2-δ Conversion of CO catalytic oxidation versus temperature for the nanorods, (b) is the Fe pre-treated with nitrogen x Ce 1-x O 2-δ nanocubes and Fe pre-treated with oxygen x Ce 1-x O 2-δ Conversion of CO catalytic oxidation versus temperature for the nanocubes. DETAILED DESCRIPTION

[0029] The technical solutions of the present application will be described in detail below with reference to the accompanying drawings and specific examples, so as to further understand the purposes, solutions and effects of the present application, but not as a limitation of the protection scope of the appended claims of the present application.

[0030] Example 1

[0031] A 35 mL NaOH solution (7.45 mol / L) was slowly added to a mixed solution of 5 mL Fe(NO3)3·9H2O (0.025 mol / L) and Ce(NO3)3·6H2O (0.5 mol / L), and after stirring at room temperature for 30 min, it was transferred to a 50 mL hydrothermal reactor, and reacted in an oven at 100°C for 24 h. The obtained precipitate was washed with deionized water and anhydrous ethanol three times respectively, and placed in a 60°C oven to dry overnight to obtain Fe x Ce 1-x O 2-δ nanorods. The TEM, HRTEM and STEM-EDS images of the Fe x Ce 1-x O 2-δ nanorods of Example 1 of the present application are shown in Figs. Figure 1 (a, b), and the results prove the successful doping of iron elements. The length of the obtained rod-shaped composite catalyst is 50-100 nm, the width is 5-10 nm, and the main exposed crystal face is {110} crystal face.

[0032] Example 2

[0033] A 35 mL NaOH solution (7.45 mol / L) was slowly added to a mixed solution of 5 mL Fe(NO3)3·9H2O (0.025 mol / L) and Ce(NO3)3·6H2O (0.5 mol / L), and after stirring at room temperature for 30 min, it was transferred to a 50 mL hydrothermal reactor, and reacted in an oven at 180°C for 24 h. The obtained precipitate was washed with deionized water and anhydrous ethanol three times respectively, and placed in a 60°C oven to dry overnight to obtain Fe x Ce 1-x O 2-δ nanocubes. The TEM, HRTEM and STEM-EDS images of the Fe x Ce 1-x O 2-δ nanocubes of Example 2 of the present application are shown in Figs. Figure 1 (c, d), and the results prove the successful doping of iron elements. The average edge length of the obtained cube-shaped composite catalyst is 10 nm, and the main exposed crystal face is {100} crystal face.

[0034] Example 3

[0035] The two kinds of Fe x Ce 1-x O 2-δ composite catalysts obtained in Example 1 and Example 2 were subjected to catalytic performance testing, and a CO+O2 reaction model was selected. The Fe x Ce 1-x O 2-δThe composite catalyst and quartz sand were mixed uniformly and placed on quartz wool. The quartz wool was placed in a reaction tube of a normal pressure fixed bed reaction device. After the reaction gas (1% CO, 10% O2 and 89% N2) passed through the quartz reaction tube at a flow rate of 60 mL / min, the gas composition was detected by an infrared analyzer. As shown in Figure 2 Fe x Ce 1-x O 2-δ The CO conversion rate of the nanorod reached 90% at 276°C, while the CO conversion rate of the Fe x Ce 1-x O 2-δ Nanocube could reach nearly 90% at 315°C.

[0036] Example 4

[0037] In this example, the Fe x Ce 1-x O 2-δ Composite catalysts of two morphologies were activated and pretreated. Specifically, the Fe x Ce 1-x O 2-δ Nanorod was calcined at 400°C for 2h under N2 atmosphere to obtain the activated rod-shaped catalyst Rod-N, and calcined at 400°C for 2h under O2 atmosphere to obtain the activated rod-shaped catalyst Rod-O. The Fe x Ce 1-x O 2-δ Nanocube was calcined at 400°C for 2h under N2 atmosphere to obtain the activated cube-shaped catalyst Cube-N, and calcined at 400°C for 2h under O2 atmosphere to obtain the activated cube-shaped catalyst Cube-O.

[0038] The activated Fe x Ce 1-x O 2-δ Composite catalysts of different morphologies were characterized by HRTEM, as shown in Figure 3 Regardless of the pretreatment atmosphere (nitrogen or oxygen), the original exposed crystal face of the iron-cerium composite catalyst was not changed. The Fe x Ce 1-x O 2-δ Nanorod still exposed the {110} crystal face, and the Fe x Ce 1-x O 2-δ Nanocube still exposed the {100} crystal face.

[0039] Example 5

[0040] The two morphologies of Fe x Ce 1-xO 2-δ The catalytic performance of the composite catalyst was tested, and the CO+O2 reaction model was selected, with Fe... x Ce 1-x O 2-δ The composite catalyst and quartz sand were mixed evenly and placed on quartz wool. The quartz wool was then placed in the reaction tube of a fixed-bed reactor at atmospheric pressure. Reaction gases (1% CO, 10% O2, and 89% N2) passed through the quartz reaction tube at a flow rate of 60 mL / min and were then analyzed by an infrared analyzer. Figure 4 As shown in (a), for Fe x Ce 1-x O 2-δ For nanorods, the two pretreatment methods yielded almost identical results, with T100 (temperature at which CO is completely converted) of 290℃ and 285℃ for Rod-N and Rod-O, respectively. However, for Fe... x Ce 1-x O 2-δ For nanocubes, two different activation atmospheres resulted in completely different catalytic effects in the bulk catalyst, such as... Figure 4 As shown in (b), the catalytic performance of Cube-N after nitrogen activation is similar to that of the unactivated block catalyst, with a final CO conversion temperature as high as 335°C. However, after pretreatment under oxygen conditions, the initial CO conversion temperature of Cube-O is less than 100°C, and the final conversion temperature is only 220°C.

[0041] The improved catalytic performance of Cube-O is mainly attributed to the migration of some lattice iron atoms to the more reactive {100} crystal plane during oxygen activation, which increases the availability of oxygen vacancies and Fe atoms on the catalyst surface. 3+ The increased number of [something] improves the chemical properties and defect structure of the catalyst surface, which is beneficial for oxygen adsorption and activation, and enhances catalytic activity.

[0042] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any simple modifications, equivalent changes, and modifications 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 protection scope of the present invention.

Claims

1. Use of a composite catalyst in catalyzing a CO oxidation reaction, characterized in that, The application relates to a composite catalyst and a preparation method thereof. The composite catalyst is mixed with quartz sand, and the mixture is placed on quartz wool, and the quartz wool is placed in a reaction tube of a normal-pressure fixed-bed reaction device; a reaction gas passes through the reaction tube and enters an infrared analyzer to detect the gas composition; The preparation method of the composite catalyst comprises the following steps: In step one, a NaOH solution is slowly added into a mixed solution of Fe(NO3)3*9H2O and Ce(NO3)3*6H2O, and stirring is conducted at room temperature to obtain a mixture; the molar ratio of the Fe(NO3)3*9H2O to the Ce(NO3)3*6H2O is 1:20; In step two, the mixture obtained in step one is transferred into a polytetrafluoroethylene hydrothermal reaction kettle to conduct hydrothermal reaction, the reaction lasts for 24 hours, and a solid is obtained; In step three, the solid obtained in step two is separated, washed and dried to obtain a nanomaterial; The nanomaterial is further subjected to activation pretreatment to obtain the composite catalyst, and the activation pretreatment comprises oxygen activation pretreatment. when the hydrothermal temperature of the hydrothermal reaction of step two is 100 °C, step three results in Fe x Ce 1-x O 2-δ nanomaterials; when the hydrothermal temperature of the hydrothermal reaction of step two is 180 °C, step three results in Fe x Ce 1-x O 2-δ nanomaterials; The rod-like morphology of Fe x Ce 1-x O 2-δ The nanomaterial has a length of any value ranging from 50 to 100 nm and a width of any value ranging from 5 to 10 nm, and a main exposed crystal face is a {110} crystal face. The cubic block-shaped morphology Fe x Ce 1-x O 2-δ The average side length of the nanomaterial is 10 nm, and the main exposed crystal face is a {100} crystal face.

2. Use according to claim 1, characterized in that, The oxygen activation pretreatment comprises: 400 DEG C calcination under an O2 atmosphere for 2 hours.

3. Use according to claim 1, characterized in that, In step one, the stirring is conducted at room temperature for 30 minutes.

4. Use according to claim 1, characterized in that, The reaction gas comprises: 1% CO, 10% O2 and 89% N2, and the gas flow rate is 60 mL / min.

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