A method for preparing high specific surface area pure-phase α-type molybdenum carbide by hydrogen intercalation modification of MoO3 and its application

Abstract

This invention belongs to the field of catalytic material preparation technology, specifically relating to a method for preparing high specific surface area pure phase α-type molybdenum carbide by hydrogen intercalation modification of MoO3 and its application, including the following steps: (1) adding MoO3 powder to hydrochloric acid and dispersing it to prepare a mixture of MoO3 and hydrochloric acid; (2) adding metal powder to the mixture in step (1) and performing pre-reduction treatment by in-situ hydrogenation of the metal and hydrochloric acid to prepare H x MoO3 precursor; (3) H prepared in step (2) x The MoO3 precursor is carbonized under a carbon source gas to obtain a pure α-type molybdenum carbide with a high specific surface area. This method is simple and effective, avoids the use of corrosive NH3 gas during carbonization, and does not require a metal as a medium, thus solving the current problem of not being able to prepare metal-free modified pure α-MoC in one step.

Description

Technical Field

[0001] This invention belongs to the field of catalytic material preparation technology, specifically relating to a method for preparing high specific surface area pure phase α-type molybdenum carbide by hydrogen intercalation modification of MoO3 and its application. Background Technology

[0002] Hydrogen, as a new energy source, boasts advantages such as being pollution-free and having a high calorific value, and is hailed as the most promising clean energy carrier in future energy systems. Currently, the "hydrogen economy" has attracted widespread research interest from scientists. Water-gas shift reaction (WGS), a crucial process in traditional hydrogen production, converts CO and H2O into H2 and CO2, releasing a certain amount of heat. To achieve a balance between high equilibrium conversion rate and high reactivity, researchers have focused on developing low-temperature shift catalysts, among which oxide-supported noble metal-based catalysts have been extensively studied. Generally, noble metal-based oxide catalysts suffer from two main problems: reducible oxide supports are prone to irreversible over-reduction in a hydrogen atmosphere, leading to deactivation; while non-reducible oxide supports can avoid this problem, their reactivity still needs further improvement due to inherent differences in support properties. Therefore, finding suitable supports to disperse and anchor active metals is the core of catalyst design and preparation.

[0003] Previous research by the inventors' group revealed that α-MoC (face-centered cubic structure, FCC type) has a larger specific surface area and more active sites compared to β-Mo2C (hexagonal close-packed structure, HCP type). Furthermore, α-MoC exhibits excellent low-temperature H2O dissociation capabilities. Therefore, we loaded active metals Au and Pt onto α-MoC, utilizing the strong interaction between α-MoC and the active metals to anchor and disperse the metals, obtaining a series of metal / α-MoC catalysts with excellent low-temperature vapor shift activity. However, traditional α-MoC nanoparticles have insufficient specific surface area and a limited number of active sites, which restricts further improvement in low-temperature vapor shift activity and also hinders further metal dispersion. Therefore, designing and preparing α-MoC with a high specific surface area is crucial. This not only facilitates anchoring the active metal and improving its dispersion but also creates more active sites, thus enhancing reaction activity.

[0004] Generally, the preparation of pure-phase α-MoC from MoO3 carbonization involves a nitriding followed by carbonization method. First, MoO3 is treated with NH3 at high temperature to obtain γ-Mo2N, and then γ-Mo2N is carbonized in a methane and hydrogen atmosphere to transform it into α-MoC. This synthesis method involves a topological phase transition, meaning that during carbonization, the crystal forms of molybdenum carbide and molybdenum nitride remain consistent, both exhibiting a face-centered cubic structure. Although this method has become a widely used synthetic method for α-MoC, it still has several drawbacks: ① The nitriding process requires the use of NH3, a gas with a strong pungent odor and corrosive properties, placing high demands on equipment and posing a safety hazard; ② During carbonization, the exchange of nitrogen and carbon atoms in the molybdenum nitride lattice requires high temperatures (above 700℃), which intensifies the polymerization of free carbon on the particle surface, forming carbon deposits and significantly reducing the specific surface area of ​​molybdenum carbide and the exposure of surface catalytic active sites; ③ The preparation process is complex, requiring frequent changes in the reaction atmosphere, resulting in a long preparation cycle. To address the aforementioned issues, the inventors' research group conducted a series of studies on the synthesis of pure α-MoC catalysts via a one-step carbonization method (i.e., carbonization without nitriding), and achieved certain research results. For example, the inventors' research group has been granted a patent (CN104923274A) entitled "A Pure α-Phase Molybdenum Carbide Supported Noble Metal Catalyst and Its Preparation Method and Application," which uses a non-equilibrium plasma treatment process to replace the traditional precursor calcination process, and can directly obtain noble metal-modified pure α-type molybdenum carbide through a one-step carbonization method; in addition, the inventors' research group has published a patent application entitled "An In-situ Induced Generation of MoO2." x H y The method for preparing non-noble metal modified pure phase α-type molybdenum carbide by one-step carbonization (patent publication number: CN112387293A) also generates a molybdenum oxide precursor (MoO3) by in-situ pre-reduction treatment of metal / MoO3. x H y After that, a non-precious metal modified pure phase α-type molybdenum carbide was obtained by a carbonization step.

[0005] While the above methods successfully prepared pure-phase α-MoC, avoiding the defects of the traditional two-stage nitriding-carburizing method, all of them utilized a metal as a medium to induce MoO. x C y Intermediate species are generated, ultimately leading to the synthesis of α-MoC. To date, no method has been reported for the direct carbonization of molybdenum trioxide (without metal support) to synthesize pure α-MoC catalysts with high specific surface area using a one-step carbonization process. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing high specific surface area pure phase α-molybdenum carbide by hydrogen intercalation modification of MoO3 and its application. This method is simple and effective, avoids the use of corrosive gas NH3 in the carbonization process, and does not require metal as a medium, thus solving the current problem of not being able to prepare metal-free modified pure phase α-MoC in one step.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] This invention provides a method for preparing high specific surface area pure-phase α-type molybdenum carbide by hydrogen intercalation modification of MoO3, comprising the following steps:

[0009] (1) Add MoO3 powder to hydrochloric acid and disperse to prepare a mixture of MoO3 and hydrochloric acid;

[0010] (2) Add metal powder to the mixture in step (1) and perform pre-reduction treatment by in-situ hydrogenation of the metal with hydrochloric acid to prepare H x MoO3 precursor;

[0011] (3) Take the H prepared in step (2) x The MoO3 precursor was carbonized under a carbon source gas to finally obtain a pure phase α-type molybdenum carbide with a high specific surface area.

[0012] Furthermore, in step (1), the concentration of the hydrochloric acid is 1-6 mol / L, and the dispersion method is ultrasonic dispersion.

[0013] Furthermore, in step (2), the metal powder includes one or more of Al, Zn, and Fe.

[0014] Furthermore, in step (2), the pre-reduction process takes 10-12 hours.

[0015] Furthermore, in step (3), the carbon source gas is a mixture of methane and hydrogen, wherein the volume percentage of hydrogen in the mixture is 75%-85%.

[0016] Furthermore, in step (3), the specific steps of carbonization are as follows: heating to 300°C at a heating rate of 5°C / min, then heating to 590-750°C at a heating rate of 1°C / min, and holding at that temperature for 2 hours, followed by cooling to room temperature and passivating in air for 6-10 hours.

[0017] The present invention also provides a high specific surface area pure phase α-type molybdenum carbide prepared by the above preparation method.

[0018] The present invention also provides an application of the high specific surface area pure phase α-type molybdenum carbide described above in a low-temperature water-vapor shift reaction.

[0019] The beneficial effects of this invention are as follows:

[0020] 1) This invention develops a method for directly obtaining pure-phase α-MoC catalyst by in-situ hydrogenation of MoO3 followed by one-step carbonization. This method is simple and effective, avoids the use of corrosive gas NH3 during carbonization, and eliminates the need for a metal as a medium, thus solving the current problem of not being able to prepare metal-free modified pure-phase α-MoC in one-step carbonization.

[0021] 2) This invention avoids the impact of the formation of a large amount of surface carbon in the traditional preparation process on the specific surface area of ​​the catalyst and the exposure of active sites. The prepared pure-phase α-MoC has a surface area as high as 217m². 2 The specific surface area is / g, which is much higher than that of α-MoC catalysts prepared by the traditional two-stage method of nitriding followed by carbonization.

[0022] 3) The α-MoC catalyst prepared in this invention exhibits excellent low-temperature conversion activity in the water-gas shift reaction, with a CO conversion rate of 53.36% at 150℃. This catalytic activity is more than twice that of α-MoC prepared by conventional methods. Furthermore, the catalyst also demonstrates superior stability. This indicates that it possesses excellent water-gas shift performance combined with good stability. Attached Figure Description

[0023] Figure 1 The XRD patterns of the α-MoC catalysts prepared in Examples 1-3 and the α-MoC catalyst obtained in Comparative Example 1 are shown.

[0024] Figure 2 XRD patterns of the α-MoC catalysts prepared in Examples 1 and 4-5;

[0025] Figure 3 Nitrogen adsorption-desorption curves of the α-MoC catalysts prepared in Examples 1-3 and the α-MoC catalyst obtained in Comparative Example 1;

[0026] Figure 4 Nitrogen adsorption-desorption curves of the α-MoC catalysts prepared in Examples 1 and 4-5;

[0027] Figure 5 SEM images of the α-MoC catalysts obtained in Examples 1-5;

[0028] Figure 6 Comparison of the activities of the α-MoC catalysts prepared in Examples 1-3 and Comparative Example 1, and the catalyst obtained in Comparative Example 2, in the low-temperature water-gas shift reaction;

[0029] Figure 7 The results show the stability evaluation of the α-MoC catalysts prepared in Example 1 and Comparative Example 1 in the water-gas shift reaction. Detailed Implementation

[0030] The detailed preparation method of the catalyst provided by this invention is as follows:

[0031] 1) Preparation of MoO3 and hydrochloric acid mixture: First, a certain amount of ammonium molybdate was placed in a crucible and then calcined in a muffle furnace at 500°C for 4 hours to obtain MoO3 powder. Then, a certain amount of MoO3 powder was added to hydrochloric acid with a concentration of 1-6 mol / L and ultrasonically dispersed to obtain a hydrochloric acid suspension containing MoO3.

[0032] 2) In-situ hydrogenation preparation of H x MoO3 precursor: Add a certain amount of Al, Zn, or Fe powder to the above mixture, stir at room temperature for 10-12 hours, and hydrogenate in situ to generate H. x MoO3 precursor.

[0033] 3) Preparation of high specific surface area α-MoC catalyst: The H generated by the above in-situ hydrogenation is used to prepare the catalyst. x MoO3 precursor (40-60 mesh) was placed in a fixed-bed reactor, and 15-25% CH4 / H2 mixed gas was introduced. The temperature was raised to 300℃ at a rate of 5℃ / min, and then raised to 590-750℃ at a rate of 1℃ / min and held for 2 hours. Subsequently, the temperature was lowered to room temperature and passivated in air for 6-10 hours to obtain an α-MoC catalyst with a high specific surface area.

[0034] The α-MoC catalyst with high specific surface area described above undergoes pretreatment before the water-gas shift reaction, as follows: A 15% CH4 / H2 mixture is introduced into a reaction tube containing the catalyst, and the temperature is increased to 590°C at a rate of 10°C / min and maintained for 2 hours. Subsequently, the temperature is lowered to the target temperature under a pure Ar atmosphere, and then switched to the reaction gas for water-gas shift performance evaluation. Preferably, the conditions for the water-gas shift reaction are: a reaction space velocity of 30000-90000 mL / g / h; a reaction temperature of 120-300°C; and a reaction atmosphere of 5-11% CO, 10-26% H2O, 10-26% H2, 2-7% CO2, and a balance gas of Ar.

[0035] The following experimental examples and embodiments are used to further illustrate the present invention, but are not limited to the present invention.

[0036] Example 1

[0037] Catalyst preparation

[0038] 1) Preparation of the MoO3 and hydrochloric acid mixture: First, 15g of ammonium molybdate was placed in a crucible and then calcined in a muffle furnace at 500℃ for 4 hours to obtain MoO3 powder. Then, 2g of MoO3 powder was added to 45mL of 4mol / L hydrochloric acid and ultrasonically dispersed to obtain a hydrochloric acid suspension containing MoO3.

[0039] 2) In-situ hydrogenation preparation of H x MoO3 precursor: Add 2.328g of iron powder to the above mixture and stir at room temperature for 11 hours to generate H2O through in-situ hydrogenation. x MoO3 precursor.

[0040] 3) Preparation of high specific surface area α-MoC catalyst: The H generated by the above in-situ hydrogenation is used to prepare the catalyst. x MoO3 precursor (40-60 mesh) was placed in a fixed-bed reactor, and a 20% CH4 / H2 mixed gas was introduced. The temperature was raised to 300℃ at a rate of 5℃ / min, then raised to 700℃ at a rate of 1℃ / min and held for 2 hours. Subsequently, the temperature was lowered to room temperature and passivated in air for 8 hours to obtain an α-MoC catalyst with a high specific surface area (denoted as α-MoC-Fe:Mo = 3-700℃).

[0041] Nitrogen physical adsorption results showed that the catalyst had a high adsorption capacity of up to 205 m. 2 Specific surface area per g.

[0042] Catalyst activity evaluation

[0043] The water-gas shift reaction was carried out in a fixed-bed reactor with an inner diameter of 4 mm in a quartz tube. The flow rates of each gas were adjusted and controlled by a mass flow meter, and the mixture was homogeneous before flowing into the reactor. 100 mg of α-MoC-Fe:Mo (3-700℃) was weighed into the quartz tube and treated at 590℃ for 2 hours under a 15% CH4 / H2 mixed gas. Then, the temperature was lowered to the target temperature, and activity testing was conducted under the following conditions: reaction atmosphere 10.5% CO / 21% H2O / N2; reaction temperature 150℃; reaction space velocity 36000 mL / g / h; at 150℃, the CO conversion rate of this catalyst reached 53.36%. Figure 6 As shown.

[0044] Example 2

[0045] The steps and process conditions in this embodiment are the same as those in Example 1, with the only differences being the following two points: ① 2g of MoO3 powder was weighed and added to 30mL of hydrochloric acid with a concentration of 4mol / L. After ultrasonic dispersion, a hydrochloric acid suspension containing MoO3 was prepared; ② During in-situ hydrogenation, 1.552g of iron powder was added to the mixture. The catalyst obtained after carbonization was denoted as α-MoC-Fe:Mo = 2-700℃.

[0046] Example 3

[0047] The steps and process conditions in this embodiment are the same as those in Example 1, with the only differences being the following two points: ① 2g of MoO3 powder was weighed and added to 15mL of hydrochloric acid with a concentration of 4mol / L. After ultrasonic dispersion, a hydrochloric acid suspension containing MoO3 was prepared; ② During in-situ hydrogenation, 0.776g of iron powder was added to the mixture. The catalyst obtained after carbonization was denoted as α-MoC-Fe:Mo=1-700℃.

[0048] Example 4

[0049] The steps and process conditions in this embodiment are the same as in Example 1, except for the following: During carbonization, a 20% CH4 / H2 mixed gas is introduced, and the temperature is raised to 300°C at a rate of 5°C / min, then raised to 650°C at a rate of 1°C / min, and held at that temperature for 2 hours. Subsequently, the temperature is lowered to room temperature and passivated in air for 6-10 hours to obtain an α-MoC catalyst with a high specific surface area (denoted as α-MoC-Fe:Mo = 3-650°C).

[0050] Example 5

[0051] The steps and process conditions in this embodiment are the same as in Example 1, except for the following: during carbonization, a 20% CH4 / H2 mixed gas is introduced, and the temperature is raised to 300°C at a rate of 5°C / min, then raised to 600°C at a rate of 1°C / min, and held at that temperature for 2 hours. Subsequently, the temperature is lowered to room temperature and passivated in air for 6-10 hours to obtain an α-MoC catalyst with a high specific surface area (denoted as α-MoC-Fe:Mo = 3-600°C).

[0052] Comparative Example 1

[0053] Catalyst preparation

[0054] 1) Preparation of MoO3 precursor: 15g of ammonium molybdate was placed in a crucible and then heated to 500℃ in a muffle furnace at a heating rate of 10℃ / min. The mixture was then calcined at 500℃ for 4 hours to obtain the MoO3 precursor.

[0055] 2) Preparation of pure α-MoC catalyst: The above-mentioned MoO3 precursor (40-60 mesh) was placed in a fixed-bed reactor. Pure NH3 was first introduced, and the temperature was raised to 700°C at a heating rate of 5°C / min and held for 2 hours. Then, the temperature was rapidly reduced to room temperature, the atmosphere was switched to 20% CH4 / H2, and the temperature was raised to 700°C again at a heating rate of 5°C / min. After holding for 2 hours, the temperature was cooled to room temperature and passivated in air for 6-10 hours to obtain conventional α-MoC.

[0056] Catalyst activity evaluation

[0057] The catalyst activity was evaluated under the reaction conditions described in Example 1. At 150°C, the CO conversion rate of the catalyst was 25.1%.

[0058] Comparative Example 2

[0059] The commercial Cu / ZnO / Al₂O₃ catalyst (HiFUEL™ W220) was purchased from Alfa Aesar. The catalyst activity testing procedures and process parameters were consistent with those described in the examples, except that the commercial Cu / ZnO / Al₂O₃ catalyst was first treated with 20% H₂ / Ar (100 mL / min) at a temperature increased to 250 °C for 2 hours at 5 °C / min before activity evaluation. The catalyst achieved a CO conversion rate of 14.87% at 150 °C.

[0060] The final comparison results of the specific surface area of ​​the α-MoC catalysts prepared in Examples 1-5 and Comparative Example 1 are shown in Table 1.

[0061] Table 1. Comparison of specific surface area of ​​α-MoC catalysts prepared in Examples 1-5 and Comparative Example 1.

[0062] catalyst <![CDATA[Specific surface area (m 2 / g)]]> α-MoC-Fe:Mo = 3-700℃ 205 α-MoC-Fe:Mo = 2-700℃ 190 α-MoC-Fe:Mo = 1-700℃ 177 α-MoC-Fe:Mo = 3-650℃ 217 α-MoC-Fe:Mo = 3-600℃ 169 α-MoC 105

[0063] The XRD patterns of the α-MoC catalysts prepared in Examples 1-5 and Comparative Examples 1-2 were obtained by combining the results from Examples 1-3 and Comparative Example 1, as shown below. Figure 1 As shown; the XRD patterns of the α-MoC catalysts prepared in Examples 1 and 4-5 are as follows. Figure 2 As shown; the nitrogen adsorption-desorption curves of the α-MoC catalysts prepared in Examples 1-3 and the α-MoC catalyst obtained in Comparative Example 1 are shown in the figure. Figure 3 As shown; the nitrogen adsorption-desorption curves of the α-MoC catalysts prepared in Examples 1 and 4-5 are shown in the figure. Figure 4 As shown; SEM images of the α-MoC catalysts obtained in Examples 1-5 are shown below. Figure 5 As shown in the figure; a comparison of the activities of the α-MoC catalysts prepared in Examples 1-3 and Comparative Example 1, and the catalyst obtained in Comparative Example 2 in the low-temperature water-gas shift reaction is shown in the figure. Figure 6 As shown; the stability evaluation results of the α-MoC catalysts prepared in Example 1 and Comparative Example 1 in the water-gas shift reaction are as follows. Figure 7 As shown in the figure. Based on the above results, it can be seen that the prepared pure-phase α-MoC has a high 217m... 2The specific surface area of ​​this catalyst is significantly higher than that of α-MoC catalysts prepared by the traditional two-stage method of nitridation followed by carbonization. Furthermore, the α-MoC catalyst prepared in this invention exhibits excellent low-temperature shift activity in the water-gas shift reaction, achieving a CO conversion rate of 53.36% at 150℃, which is more than twice that of α-MoC prepared by the conventional method. Simultaneously, the catalyst also demonstrates superior stability.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing high specific surface area pure phase alpha molybdenum carbide by modifying MoO3 by hydrogen insertion, characterized in that, The method comprises the following steps: (1) adding MoO3 powder into hydrochloric acid to prepare a mixture of MoO3 and hydrochloric acid by dispersion; (2) adding metal powder to the mixed solution in step (1) and performing a pre-reduction treatment using in-situ hydrogenation of the metal and hydrochloric acid to prepare H x MoO3precursor; (3) H x MoO3 precursor is carbonized under a carbon source gas, and finally a high specific surface area pure phase α-type molybdenum carbide is obtained. In step (1), the concentration of the hydrochloric acid is 1-6 mol / L, and the dispersion is ultrasonic dispersion; In step (2), the metal powder comprises one or more of Al, Zn and Fe; In step (2), the time of the pre-reduction treatment is 10-12 hours; In step (3), the carbon source gas is a mixed gas of methane and hydrogen, and the volume percentage of hydrogen in the mixed gas is 75%-85%.

2. The method for preparing high specific surface area pure phase alpha type molybdenum carbide by modifying MoO3 through hydrogen spillover according to claim 1, characterized in that, In step (3), the specific steps of the carbonization are as follows: increasing the temperature to 300℃ at a temperature increasing rate of 5℃ / min, then increasing the temperature to 590-750℃ at a temperature increasing rate of 1℃ / min, and keeping the temperature for 2 hours, then decreasing the temperature to room temperature and passivating in air for 6-10 hours.

3. High specific surface area pure phase α-type molybdenum carbide prepared by the preparation method of any one of claims 1-2.

4. Application of the high specific surface area pure phase α-type molybdenum carbide of claim 3 in low-temperature water vapor shift reaction.

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