A high-temperature reverse water-gas shift reaction MoOx / Mo2N catalyst and its preparation method

Abstract

The application discloses a high-temperature reverse water-gas shift reaction MoO x / Mo2N catalyst and a preparation method thereof, and comprises the following steps: synthesizing MoO3 powder by using a hydrothermal method; grinding the MoO3 powder, and then performing nitriding reaction on the MoO3 powder with pure NH3 at 600-700 DEG C for 3-5 h to obtain Mo2N; cooling the Mo2N, and then performing passivation on the Mo2N by using a mixed gas of O2 and Ar to obtain a molybdenum nitride catalyst MoO3 / Mo2N with a surface MoO3 passivation layer; treating the MoO3 / Mo2N in a reducing atmosphere to obtain MoO x / Mo2N, and x<3. The MoO x / Mo2N catalyst exhibits high CO2 conversion rate (about 48%), complete CO selectivity and persistent cycle stability.

Description

Technical Field

[0001] This invention belongs to the field of catalyst preparation technology, specifically relating to a high-temperature reverse water-gas shift (RWGS) reaction of MoO2. x / Mo2N catalyst and its preparation method. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] In the entire CO2 hydrogenation reaction process, the catalyst surface sites play a crucial role in adsorbing reactants, coupling or dissociating adsorbed molecules, and desorbing products, making them key sites driving the catalytic reaction. Therefore, the development of efficient catalysts is closely related to the design of effective active surface structures. Supported active metal catalysts, due to the formation of an active interface between the active metal and the support, exhibit active catalytic properties and have thus become the main catalytic materials for RWGS reactions. However, the catalytic properties of the active interface are influenced by both the active metal and the support, meaning that the aggregation of the active metal and the sintering of the support can lead to rapid deactivation of the catalytic interface. Especially for high-temperature RWGS reactions, harsh reaction conditions (high temperature, strong reducing atmosphere) are more likely to damage the active interface structure. Furthermore, supported active metal catalysts require the participation of an active metal, making the catalyst preparation process more complex and costly. Therefore, constructing efficient catalysts without active metal support is more practically valuable, but also more challenging.

[0004] Due to the unique catalytic properties of oxygen vacancies, defect engineering based on reducible oxides has been widely applied to CO2-related catalytic reactions. On the one hand, the presence of oxygen vacancies can modulate the geometry and electronic structure of active metal sites, thereby promoting CO2 activation and the formation of active intermediates at the metal-oxygen vacancy interface. On the other hand, as a typical coordinating unsaturated site, oxygen vacancies themselves can adsorb CO2 molecules to participate in the catalytic reaction. However, for traditional reducible oxides, with the formation of oxygen vacancies, the exposed unsaturated coordinating metal sites cannot provide sufficient electrons to the antibonding orbitals of CO2 molecules, limiting the dissociation of the C=O double bond and directly preventing oxygen vacancies from acting as independent catalytic sites for the entire RWGS reaction. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a high-temperature reverse water-gas shift reaction MoO x / Mo2N catalyst and its preparation method.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0007] In a first aspect, the present invention provides a high-temperature reverse water-gas shift reaction MoO x The preparation method of / Mo2N catalyst includes the following steps:

[0008] MoO3 powder was synthesized using a hydrothermal method.

[0009] MoO3 powder was ground and then reacted with pure NH3 at 600℃-700℃ for 3-5 hours to obtain Mo2N.

[0010] After cooling Mo2N, it was passivated with a mixture of O2 and Ar gas to obtain a molybdenum nitride catalyst MoO3 / Mo2N with a surface MoO3 passivation layer.

[0011] MoO3 / Mo2N was treated in a reducing atmosphere to obtain MoO x / Mo2N, x<3.

[0012] This invention reveals the unique redox properties of sub-nanometer MoO3 passivated surfaces on bulk Mo2N and constructs a dynamically changing catalytic surface, achieving direct CO2 dissociation activation. Under a reducing atmosphere, accompanied by the reduction of Mo-O species, sub-nanometer MoO3 with a high density of oxygen vacancies forms on the bulk Mo2N structure. x (x<3) Surface. This highly disordered sub-nanometer surface possesses a large number of coordinate-unsaturated Mo sites, providing a favorable geometric and electronic microenvironment. Electron transfer mediated by the exposed Mo sites directly cleaves the stable C=O double bond in CO2 (CO2→CO+O). surface Oxygen vacancies occupied by oxygen atoms in CO2 are easily reduced and regenerated by H2 (H2 + O). surface →H2O). The in-situ recycling and generation of oxygen vacancies accelerates the transfer of oxygen atoms from CO2 to the catalyst surface and then to the final product CO, thus effectively catalyzing the hydrogenation of CO2 to CO (i.e., reverse water-gas shift reaction, RWGS reaction).

[0013] In some embodiments, the method for preparing MoO3 powder includes the following steps:

[0014] P123 was uniformly dispersed in water, and Na2MoO4 was added. Under vigorous stirring, concentrated hydrochloric acid was added, and stirring was continued for 20-40 minutes. Then, a hydrothermal reaction was carried out in a high-pressure reactor. The precipitate was washed, dried, and calcined to obtain MoO3 powder.

[0015] During the preparation process, hydrochloric acid was added to reduce the Na content in the system. + / H +The proportion of P123 induced the growth of MoO3 nanoribbons. In this process, P123 acts as a structure guide for MoO3 production. P123 molecules aggregate into micelles and adsorb onto the growing MoO3 core, thus ensuring that MoO3 grows only in a specific direction, generating MoO3 nanoribbons.

[0016] Preferably, the hydrothermal reaction temperature is 100-150℃ and the hydrothermal reaction time is 5-15h.

[0017] Preferably, the washing is performed using water and anhydrous ethanol.

[0018] Preferably, the calcination temperature is 350-450℃ and the calcination time is 3-5h.

[0019] In some embodiments, the volume ratio of O2 to Ar in the O2 and Ar mixture is 1:90-100.

[0020] In some embodiments, the passivation temperature is 20-40°C and the passivation time is 1-3 hours.

[0021] In some embodiments, the method for treating MoO3 / Mo2N in a reducing atmosphere is as follows: the MoO3 / Mo2N sample is placed in a quartz tube, a H2 / Ar mixed gas is introduced, the temperature is raised from room temperature to 550-650°C, and maintained for 0.5-1.5 hours.

[0022] Specifically as follows:

[0023] The MoO3 / Mo2N sample was placed in a quartz tube, and a 5% H2 / Ar mixed gas was introduced (30 mL / min). The temperature was increased from room temperature to 600℃ (10℃ / min) and maintained at 600℃ for 1 h.

[0024] Secondly, this invention provides a high-temperature RWGS reaction of MoO x The / Mo2N catalyst is prepared by the method described above.

[0025] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows:

[0026] Under harsh reaction conditions (600℃, WHSV = 200,000 mL / g) cat / h) under the condition of no load metal, MoO x The / Mo2N catalyst exhibited high CO2 conversion (~48%), complete CO selectivity, and long-lasting cycle stability (900h), surpassing most supported metal catalysts and demonstrating great potential for industrial applications. Attached Figure Description

[0027] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0028] Figure 1 In the figure, (a) XRD results of samples before and after the reaction; (b) Raman results of samples before and after the reaction; (c) In-situ Raman spectra of samples synthesized under air, Ar and H2 atmospheres with continuous switching; (d) MoO x H2-TPR results of / Mo2N; (e) MoO under continuous switching of Ar, CO2, Ar, H2 x TPSR results for / Mo2N; (f) In-situ Raman spectra of H2, Ar and CO2 atmospheres at 500℃.

[0029] Figure 2 Medium, (a)MoO x CO2 conversion rate of / Mo2N in 6-cycle start-up cooling cycle (experimental conditions: 0.1MPa, 300℃-600℃, WHSV=200,000mL / g) cat / h;(b)MoO x (c) CO selectivity of / Mo2N in 6-cycle start-up cooling; (d) MoO under harsh reaction conditions of 600 °C and 200,000 mL / g / h space velocity. x Results of three rounds of stability testing for / Mo2N (with a time interval of 55 days between two rounds of testing).

[0030] Figure 3 Medium, (a)MoO x / Mo2N CO yield at different space velocities; (b)MoO x Stability comparison of / Mo2N and other reference samples.

[0031] Figure 4 In the reaction (a,b), MoO2 is generated. x HAADF-STEM results of the Mo2N catalyst; (c) MoO after reaction x STEM and elemental distribution results of the / Mo2N catalyst. Detailed Implementation

[0032] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0033] The present invention will be further described below with reference to the embodiments.

[0034] Example 1

[0035] Preparation of high-temperature RWGS reaction catalyst

[0036] MoO3 Precursor: The MoO3 precursor was synthesized via a hydrothermal method. First, 1 g of P123 (a triblock copolymer, officially named: ethylene oxide-propylene oxide-ethylene oxide triblock copolymer) was dissolved in a polytetrafluoroethylene (PTFE) liner containing 40 mL of deionized water. Next, 0.9 g of Na₂MoO₄·H₂O and 5 mL of deionized water were added to the suspension. Subsequently, 3 mL of concentrated hydrochloric acid (37%) was added to the solution under vigorous stirring. After stirring for 30 min, the PTFE liner was sealed in a stainless steel high-pressure reactor and heated in an oven at 100 °C for 12 h. After the hydrothermal reactor cooled naturally to room temperature, the precipitate was collected by centrifugation and washed with deionized water and anhydrous ethanol. The resulting solid product was then dried in an oven at 70 °C for 10 h and calcined in a tube furnace at 400 °C for 4 h to obtain the MoO3 precursor.

[0037] MoO3 / Mo2N and MoO x Preparation of the / Mo2N heterostructure catalyst: The MoO3 powder prepared above was ground and transferred to a quartz tube, and nitrided with pure NH3 (40 mL / min) at 650 °C for 4 h. After cooling to room temperature, the prepared Mo2N was passivated with 1% O2 / Ar mixed gas (O2 to Ar volume ratio of 1:99) for 2 h. This yielded a molybdenum nitride catalyst with a surface MoO3 passivation layer. The catalyst exposed to air was labeled MoO3 / Mo2N, while the catalyst obtained after treatment in a reducing atmosphere (i.e., reduction at 600 °C for 1 h with 5% H2 / Ar) was labeled MoO2N. x / Mo2N(x<3).

[0038] Catalyst performance testing

[0039] The catalytic performance of the catalyst was tested in a fixed-bed reactor at atmospheric pressure. 15 mg of sieved sample (20-40 mesh) was mixed with 90 mg of inert SiO2 and packed into a quartz tube. First, the sample was pretreated at 600 °C with 5% H2 / Ar (30 mL / min) for 1 h. The catalyst temperature was then lowered to room temperature, and the gas flow was switched to an RWGS reaction gas flow (23% CO2, 69% H2, 8% N2) at a flow rate of 50 mL / min. The reaction was carried out for at least 1 h at each test temperature. The effluent product was analyzed by an online gas chromatograph equipped with a thermal conductivity detector (TCD). The gas flow rate was determined using the internal standard method, with N2 used as the internal standard. The CO2 conversion and CO selectivity were calculated using the following formulas.

[0040]

[0041] in, It is the CO2 concentration in the reactant gas. It is the CO2 concentration in the outlet gas. and These refer to the chromatographic peak areas of CO2 and N2 in the inlet gas, respectively. and Indicate the chromatographic peak areas of CO2 and N2 in the exit gas, respectively. The peak area of ​​each component is directly proportional to the concentration of that component.

[0042] The calculation process for CO selectivity is as follows:

[0043]

[0044] in, and Indicate the concentrations of CO and CH4 in the outlet gas, respectively. and These are the relative correction factors for CO to N2 and CH4 to N2, respectively, which are calibrated using standard gases. and It represents the peak areas of CO and CH4 detected by TCD in the outlet gas.

[0045] Figure 1 As shown in Figure a, for the catalyst before and after the reaction, only the diffraction peaks of γ-Mo2N were detected in the XRD results, indicating that the bulk structure of the catalyst before and after the reaction is stable γ-Mo2N. However, the Raman spectroscopy results only showed the signal of MoO3, indicating that a MoO3 passivation layer exists on the surface of the bulk γ-Mo2N structure before and after the reaction. Figure 1 b).

[0046] Subsequently, in-situ Raman spectroscopy was performed to investigate the in-situ evolution of the surface MoO3 structure under different atmospheres. Figure 1 As shown in c, after switching the synthesis air to Ar at room temperature, the MoO3 signal transforms into MoO. x (x < 3) signal. This result indicates that even at room temperature and under an inert atmosphere, the MoO3 passivation layer on the Mo2N surface can undergo reduction to generate oxygen-vacancy-rich MoO. x Structure. MoO₂ can be detected at relatively low test temperatures when H₂ is introduced. x Signal. However, when the test temperature gradually increased to 500℃, MoO x The Raman signal disappeared, indicating that MoO2 at high temperatures... x The structure underwent deep reduction, and the increased disorder on the catalyst surface made it impossible to detect the Raman signal.

[0047] The redox properties of the catalyst were further investigated using H2-TPR. Figure 1 As shown in Figure d, H2 begins to be consumed as the test temperature increases. The H2 consumption peak in H2-TPR can be attributed to the reduction of Mo-O species on the Mo2N surface. Even at a high temperature of 950℃, H2 consumption can still be detected, indicating that some oxygen atoms are tightly coordinated with Mo atoms and are difficult to be reduced by H2.

[0048] Generally, oxygen vacancies, as unique coordinating unsaturated sites, can adsorb and activate oxygen-containing reactant molecules to participate in catalytic reactions. CO2 is a typical oxygen-containing molecule, and its stable C=O bond makes its catalytic conversion very difficult. Here, we used a continuous switching experiment of Ar→CO2→Ar→H2 to investigate the highly reduced MoO2. x The adsorption and activation behavior of CO2 molecules on the surface. Before TPSR testing, the MoO3 / Mo2N heterostructure catalyst was pretreated with 5% H2 / Ar at 600℃ for 1 h, thereby constructing oxygen-rich defective MoO3 with vacancies in situ. x surface.

[0049] like Figure 1 As shown in Figure e, for the sample pretreated with H2, the CO signal significantly increased after the introduction of CO2, indicating that CO2 was directly converted into gaseous CO. The gradual decrease in the CO signal indicates that the number of sites converting CO2 into CO is gradually decreasing. With the introduction of H2, the H2O signal was clearly observed, indicating that CO2 is converted into gaseous CO in highly reduced MoO2. x On the surface, a direct C=O bond cleavage occurs, and the oxygen atoms generated by the dissociation of H2 and CO2 react to produce H2O.

[0050] In-situ Raman results with continuous switching of H2, Ar and CO2 ( Figure 1 f) shows that under H2 atmosphere at 500℃, the MoO3 surface is deeply reduced, leading to increased surface disorder, and no Raman signal of Mo-O species was detected. Subsequently, MoO3 was further reduced under CO2 atmosphere. x The generation of Raman signals (x < 3) indicates that CO2 can partially oxidize the highly reduced catalyst surface structure.

[0051] Generally, in the harsh conditions of RWGS reactions, supported catalysts with active metals may exhibit high initial activity, but they are highly susceptible to deactivation due to the sintering of the active metal. Because MoO x / Mo2N catalysts possess unique redox properties, abundant highly active oxygen vacancies, and no supported active metals, making them promising for achieving a balance between high activity and high stability in high-temperature RWGS reactions.

[0052] like Figure 2 As shown in a, at 200,000 mL / g cat At a high reactive space velocity (WHSV) of / h, MoO x The / Mo2N catalyst exhibits extremely high CO2 conversion. At 600℃, the CO2 conversion approaches the thermodynamic equilibrium limit. The catalytic activity remained well maintained throughout six rounds of heating and cooling tests, indicating good temperature stability. Furthermore, the catalyst showed 100% CO selectivity, and no CH4 was detected throughout the activity evaluation, demonstrating its effective catalysis of the RWGS reaction rather than methanation. Figure 2 b). To evaluate MoO x The durability of the / Mo2N catalyst in the high-temperature RWGS reaction was assessed through three rounds of stability testing at 600℃. Each test lasted 300 hours, with a 55-day interval between rounds. Figure 2 As shown in Figure c, the catalyst maintained stable CO2 conversion and CO selectivity over 900 h, with CO2 conversion exceeding 45% and CO selectivity at 100%. Importantly, even after the post-reaction sample was exposed to air for 55 days, the active sites in the catalyst did not become deactivated due to air corrosion, indicating that the catalyst has high recycling value.

[0053] It is worth noting that as the external diffusion restriction weakens, MoO x / Mo2N at 3,800,000 mL / g cat At extremely high space velocities of up to 159.6 × 10⁶ h, its CO formation rate can reach 159.6 × 10⁶ h. -5 mol / g cat / s( Figure 3 a) This surpasses almost all catalysts reported in previous literature. Furthermore, compared to other reported supported metal catalysts, this MoO2 catalyst, which does not support any active metal, exhibits superior performance. x The / Mo2N catalyst effectively avoids deactivation caused by the sintering of active metals. Furthermore, since it does not support any active metal, this catalyst effectively saves on the preparation costs associated with loading active metals, demonstrating its significant practical application value. Figure 3 b).

[0054] For the samples after the high-temperature RWGS reaction, apart from the structural information of Mo2N, no MoO3 lattice fringes could be observed in the HAADF-STEM test results. However, a highly disordered layered structure with a thickness of less than 1 nm could be clearly observed on the Mo2N surface. Figure 4 (a, b) It can be inferred that this structure can form oxygen-vacancy-rich sub-nanometer MoO₂ during CO₂ hydrogenation. x Thin layers were formed, enabling CO2 dissociation and activation. Simultaneously, elemental distribution results showed that the O signal was highly dispersed around the Mo element, confirming the presence of Mo-O species on the surface. Figure 4 c).

[0055] Example 2

[0056] Preparation of high-temperature RWGS reaction catalyst

[0057] The preparation method of the MoO3 precursor is the same as in Example 1.

[0058] MoO3 / Mo2N and MoO x Preparation of the / Mo2N heterostructure catalyst: The MoO3 powder prepared above was ground and transferred to a quartz tube, and nitrided with pure NH3 (40 mL / min) at 600 °C for 5 h. After cooling to room temperature, the prepared Mo2N was passivated with 5% O2 / Ar mixed gas (O2 to Ar volume ratio of 5:95) for 1 h. This yielded a molybdenum nitride catalyst with a surface MoO3 passivation layer. The catalyst exposed to air was labeled MoO3 / Mo2N, while the catalyst obtained after treatment in a reducing atmosphere (i.e., reduction with 5% H2 / Ar at 550 °C for 1.5 h, H2 to Ar volume ratio of 5:95) was labeled MoO2 / Mo2N. x / Mo2N(3<x).

[0059] Example 3

[0060] Preparation of high-temperature RWGS reaction catalyst

[0061] The preparation of the MoO3 precursor is the same as in Example 1.

[0062] MoO3 / Mo2N and MoO xPreparation of the / Mo2N heterostructure catalyst: The MoO3 powder prepared above was ground and transferred to a quartz tube, and nitrided with pure NH3 (40 mL / min) at 700 °C for 3 h. After cooling to room temperature, the prepared Mo2N was passivated with 1% O2 / Ar mixed gas (O2 to Ar volume ratio of 1:99) for 3 h. This yielded a molybdenum nitride catalyst with a surface MoO3 passivation layer. The catalyst exposed to air was labeled MoO3 / Mo2N, while the catalyst obtained after treatment in a reducing atmosphere (i.e., reduction with 5% H2 / Ar at 650 °C for 0.5 h) was labeled MoO2N. x / Mo2N(x<3).

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A high-temperature reverse water-gas shift reaction MoO x The method for preparing / Mo2N catalyst is characterized by: Includes the following steps: MoO3 powder was synthesized using a hydrothermal method. MoO3 powder was ground and then reacted with pure NH3 at 600℃-700℃ for 3-5 hours to obtain Mo2N; After cooling Mo2N, it was passivated with a mixture of O2 and Ar gas to obtain a molybdenum nitride catalyst MoO3 / Mo2N with a surface MoO3 passivation layer. MoO3 / Mo2N was treated in a reducing atmosphere to obtain MoO x / Mo2N, x 3; The method for treating MoO3 / Mo2N in a reducing atmosphere is as follows: the MoO3 / Mo2N sample is placed in a quartz tube, H2 / Ar mixed gas is introduced, the temperature is raised from room temperature to 550-650 ℃, and maintained for 0.5-1.5 h.

2. The high-temperature reverse water-gas shift reaction MoO as described in claim 1 x The method for preparing / Mo2N catalyst is characterized by: The method for preparing MoO3 powder includes the following steps: P123 is uniformly dispersed in water, Na2MoO4 is added to it, concentrated hydrochloric acid is added under vigorous stirring, and stirring is continued for 20-40 minutes; then a hydrothermal reaction is carried out in a high-pressure reactor, and the precipitated product is washed, dried, and calcined to obtain MoO3 powder.

3. The high-temperature reverse water-gas shift reaction MoO as described in claim 2 x The method for preparing / Mo2N catalyst is characterized by: The hydrothermal reaction temperature is 100-150℃, and the hydrothermal reaction time is 5-15h.

4. The high-temperature reverse water-gas shift reaction MoO as described in claim 2 x The method for preparing / Mo2N catalyst is characterized by: The washing process involves using water and anhydrous ethanol.

5. The high-temperature reverse water-gas shift reaction MoO as described in claim 2 x The method for preparing / Mo2N catalyst is characterized by: The calcination temperature is 350-450℃, and the calcination time is 3-5h.

6. The high-temperature reverse water-gas shift reaction MoO as described in claim 1 x The method for preparing / Mo2N catalyst is characterized by: In a mixture of O2 and Ar gases, the volume ratio of O2 to Ar is 1:90-100.

7. The high-temperature reverse water-gas shift reaction MoO as described in claim 1 x The method for preparing / Mo2N catalyst is characterized by: The passivation temperature is 20-40℃, and the passivation time is 1-3 hours.

8. The high-temperature reverse water-gas shift reaction MoO as described in claim 1 x The method for preparing / Mo2N catalyst is characterized by: The heating rate is 5-15℃ / min.

9. The high-temperature reverse water-gas shift reaction MoO as described in claim 8 x The method for preparing / Mo2N catalyst is characterized by: The heating rate is 10℃ / min.

10. A high-temperature reverse water-gas shift reaction MoO x / Mo2N catalyst, characterized in that: It is prepared by any one of the preparation methods described in claims 1-9.

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