An alkaline earth metal oxide-supported Mo-based nitrogen carrier, its preparation method and application

Abstract

This application discloses an alkaline earth metal oxide-supported Mo-based nitrogen carrier, its preparation method, and its application. The method includes: preparing a Mo-HMT precursor; preparing an aqueous solution of an alkaline earth metal salt; wherein the alkaline earth metal salt is selected from at least one of soluble Mg salt, Ca salt, Sr salt, or Ba salt; adding the Mo-HMT precursor to the aqueous solution of the alkaline earth metal salt and mixing, then drying to obtain an alkaline earth metal-supported Mo-HMT precursor; pyrolyzing the alkaline earth metal-supported Mo-HMT precursor at 700-800℃ under N2, then reducing it under H2 to obtain the alkaline earth metal oxide-supported Mo-based nitrogen carrier. The alkaline earth metal oxide surface has coordinatingly unsaturated metal cations or Lewis acidic sites composed of surface oxygen vacancies, which can adsorb intermediates in the NH3 formation process, reducing the formation energy barrier of NH3 intermediates and greatly improving the performance of chemical chain ammonia synthesis.

Description

Technical Field

[0001] This application belongs to the field of nitrogen carrier preparation technology, and relates to an alkaline earth metal oxide supported Mo-based nitrogen carrier, its preparation method and application, specifically an alkaline earth metal oxide supported Mo-based nitrogen carrier that can fix nitrogen under N2 and its application in the chemical chain synthesis of ammonia. Background Technology

[0002] Ammonia, as a carbon-free energy carrier, is a potential alternative to current energy sources and possesses enormous research potential. As a major raw material for nitrogen fertilizers and a basic raw material for most nitrogen-containing chemicals, ammonia is irreplaceable in maintaining global food security and the modern chemical industry system. The Haber Bosch (HB) process is the main method for industrial ammonia synthesis; however, its operating conditions are harsh (350-550℃, 10-30 MPa). Furthermore, high greenhouse gas emissions are also widely recognized as a major drawback of HB, emitting up to 420 million tons of CO2 annually. Therefore, there is an urgent need to find a green and mild ammonia production method. Chemical chain synthesis of ammonia achieves ammonia synthesis under mild conditions by breaking down the reaction into relatively independent "nitrogen fixation" and "hydrogenation" reactions.

[0003] Transition metal nitrides (TMNs) possess variable metal valence states and hold great promise for applications in CLAS (Clean-Acting Synthesis). However, similar to conventional catalytic ammonia synthesis, the relationship between lattice nitrogen fixation and hydrogen release from lattice nitrogen in TMNs is severely constrained by the Brønsted-Evans-Polanyi (BEP) restriction relationship. An amorphous Mo-based nitrogen support prepared by a one-step pyrolysis method exhibits a good initial ammonia production rate of 4500 μmol·g⁻¹. -1 ·h -1 However, it is difficult to fix nitrogen under pure nitrogen gas, and the cycle rate is only 100 μmol·g. -1 ·h -1 . Summary of the Invention

[0004] One effective method to improve the activity of nitrogen carriers is to introduce transition metals as hydrogenation active sites, such as Fe, Co, Ni, and Pd. In addition, introducing a support is also an effective means to improve the activity of ammonia synthesis. However, the addition of transition metals and supports cannot change the intrinsic characteristics of the nitrogen fixation reaction of Mo-based nitrogen carriers. Specifically, the fixation of N2 into lattice nitrogen by elemental molybdenum (Mo) requires the assistance of H2.

[0005] Alkaline earth metal oxides have good applications in the catalytic synthesis of ammonia. As good electron donors and with Lewis acid sites, they lower the nitrogen fixation energy barrier in the synthesis of ammonia.

[0006] Objective: To overcome the shortcomings of existing technologies and address the aforementioned defects or improvement needs of existing molybdenum-based nitrogen carriers, this application provides an alkaline earth metal oxide-supported Mo-based nitrogen carrier capable of nitrogen fixation under N2, along with its preparation method and applications. The preparation process is simple and easy to operate. In this application, the alkaline earth metal oxide acts as an electron accelerant, assisting the Mo-based nitrogen carrier in injecting d-orbital electrons into the N2 antibonding orbitals and providing Lewis acidic sites, thus overcoming the intrinsic nitrogen fixation defects of elemental molybdenum (Mo). This design concept can be applied to various transition metal nitrogen carrier materials where nitrogen fixation is difficult.

[0007] Technical solution: The technical solution adopted in this application is as follows:

[0008] According to a first aspect of this application, a method for preparing an alkaline earth metal oxide-supported Mo-based nitrogen carrier is provided, comprising:

[0009] Step S1: Prepare Mo-HMT precursor;

[0010] Step S2: Prepare an aqueous solution of an alkaline earth metal salt; wherein the alkaline earth metal salt is selected from at least one of soluble Mg salt, Ca salt, Sr salt or Ba salt;

[0011] Step S3: Add Mo-HMT precursor to an alkaline earth metal salt aqueous solution and mix, then dry to obtain alkaline earth metal-supported Mo-HMT precursor;

[0012] Step S4: The alkaline earth metal-supported Mo-HMT precursor is first pyrolyzed under N2 at 700~800℃, and then reduced under H2 to obtain an alkaline earth metal oxide-supported Mo-based nitrogen carrier.

[0013] In some embodiments, step S1, preparing the Mo-HMT precursor, includes:

[0014] The molybdenum source and hexamethylenetetramine were dissolved in water, mixed and reacted to precipitate, and the product was washed and dried to obtain the Mo-HMT precursor.

[0015] Furthermore, the molar ratio of hexamethylenetetramine to molybdenum source, calculated as Mo element, is (1~2):1, preferably (1.4~1.6):1.

[0016] In some embodiments, the molybdenum source is selected from at least one of ammonium heptamolybdate or its hydrate, ammonium tetramolybdate or its hydrate, ammonium molybdate or its hydrate, and ammonium octamolybdate or its hydrate.

[0017] In some embodiments, the reaction temperature is room temperature (10~30℃), the drying temperature is 60~100℃, preferably 80℃, and the drying time is 10-18 h.

[0018] In some embodiments, in the alkaline earth metal oxide-supported Mo-based nitrogen carrier, the alkaline earth metal element exists in the form of alkaline earth metal oxide or alkaline earth metal molybdate; the molar ratio of alkaline earth metal element to Mo element is 0.1~0.8, preferably 0.2~0.6.

[0019] In some embodiments, in step S4, pyrolysis under N2 involves heating from room temperature to 700-800°C at a heating rate of 1-20°C / min, followed by constant-temperature calcination at 700-800°C for at least 0.5 hours; reduction under H2 involves constant-temperature reduction at 700-800°C for at least 0.5 hours.

[0020] Preferably, the pyrolysis under N2 involves heating from room temperature to 750°C at a heating rate of 10°C / min, followed by constant-temperature calcination at 750°C for 1 hour; the reduction under H2 involves constant-temperature reduction at 750°C for 1 hour.

[0021] According to a second aspect of this application, an alkaline earth metal oxide-supported Mo-based nitrogen carrier is provided, which is prepared by the aforementioned preparation method.

[0022] It should be noted that the alkaline earth metal oxide-supported Mo-based nitrogen carrier, due to the loading of alkaline earth metal oxides on the surface, increases the electron cloud density of the Mo surface, promoting the injection of d-orbital electrons from electron-rich Mo atoms into the N2 antibonding orbitals. The presence of Lewis acidic sites composed of coordinated unsaturated metal cations or surface oxygen vacancies on the alkaline earth metal oxide surface can adsorb intermediates in the NH3 formation process, lowering the formation energy barrier of these intermediates. Simultaneously, the loading of alkaline earth metal oxides leads to a reduction in the particle size of the nitrogen carrier material and an increase in the dispersion of its surface active sites, improving the migration efficiency of lattice nitrogen. This nitrogen carrier can be applied to the chemical chain synthesis of ammonia.

[0023] According to a third aspect of this application, the use of the aforementioned alkaline earth metal oxide-supported Mo-based nitrogen carrier in chemically chained ammonia synthesis is provided.

[0024] In some embodiments, the reaction temperature for chemically synthesized ammonia is 600℃~700℃. Further, the reaction pressure is 0.1 MPa, and the flow rates of N2 and H2 are 80~100 mL·min, respectively. -1 .

[0025] Compared with existing technologies, this application has the following advantages: This application provides an alkaline earth metal oxide-supported Mo-based nitrogen support capable of nitrogen fixation under N2, its preparation method, and its application. The preparation method is simple and easy to operate. By adding an appropriate amount of alkaline earth metal oxide to the nitrogen support, the introduction of the alkaline earth metal oxide as an electronic aid assists the Mo-based nitrogen support in injecting d-orbital electrons into the N2 antibonding orbitals and provides Lewis acidic sites, thus overcoming the intrinsic nitrogen fixation defect of elemental molybdenum (Mo). The prepared alkaline earth metal oxide-supported Mo-based nitrogen support significantly improves the performance of chemically chained ammonia synthesis as a catalyst. Attached Figure Description

[0026] Figure 1 This is a schematic diagram showing the performance of the alkaline earth metal oxide-supported Mo-based nitrogen carrier prepared in Examples 1-3 of this application for chemical chain ammonia synthesis.

[0027] Figure 2 This is a schematic diagram showing the performance results of the alkaline earth metal oxide-supported Mo-based nitrogen carrier prepared in Examples 3-6 of this application for chemical chain ammonia synthesis.

[0028] Figure 3 The images show the XRD patterns of the Mo-based nitrogen-supported alkaline earth metal oxides prepared in Examples 3-7 of this application.

[0029] Figure 4 The image shows the XRD pattern of the Mo-based nitrogen carrier supported on alkaline earth metal oxide prepared in Example 3 of this application and after nitrogen fixation for 1 hour.

[0030] Figure 5 This is a SEM image of the Mo-based nitrogen carrier prepared in Example 7 of this application.

[0031] Figure 6 This is a SEM image of the alkaline earth metal oxide-supported Mo-based nitrogen carrier prepared in Example 3 of this application.

[0032] Figure 7 This is a SEM image of the Mo-based nitrogen carrier supported on alkaline earth metal oxides prepared in Example 4 of this application.

[0033] Figure 8 This is a SEM image of the alkaline earth metal oxide-supported Mo-based nitrogen carrier prepared in Example 5 of this application.

[0034] Figure 9 This is a SEM image of the alkaline earth metal oxide-supported Mo-based nitrogen carrier prepared in Example 6 of this application. Detailed Implementation

[0035] The present application will be further described below with reference to the accompanying drawings and embodiments.

[0036] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0037] For the purposes of this specification and the appended claims, unless otherwise stated, all expressions, percentages, or proportions, and other numerical values ​​used in this specification and the appended claims, are to be understood to be modified by the term "about" in all cases. Furthermore, all scopes disclosed herein include their endpoints and can be combined independently.

[0038] In this embodiment of the application, the method for preparing the Mo-HMT precursor includes:

[0039] S11: Weigh out ammonium heptamolybdate tetrahydrate (NH4)2Mo7O 24 7g of 4H2O (molecular weight 1235.86) and 8g of hexamethylenetetramine (molecular weight 140.19) were prepared into clear solutions with appropriate amounts of deionized water.

[0040] S12: Mix the two solutions obtained in step S11 and continue stirring to obtain a white suspension. Then wash the product with deionized water, centrifuge, and dry at 80°C for 15 h to obtain the Mo-HMT precursor for later use.

[0041] Example 1: A method for preparing a Mo-based nitrogen support supported on an alkaline earth metal oxide, comprising:

[0042] Weigh 0.22 g of Mg(NO3)2·6H2O and dissolve it in 3 mL of deionized water by stirring for half an hour. Then, add 1 g of Mo-HMT precursor and mix to obtain a precursor solution, which is dried in an oven at 80 °C for 10 h to obtain the Mg-Mo-HMT precursor. Subsequently, [the solution is then processed at 100 mL / min]. -1 Under a flow rate of N2, the nitrogen-carrying sample 1 was obtained by constant calcination at 750℃ for 1 h and constant reduction at H2 for 1 h in a fixed bed at a heating rate of 10℃ / min, followed by cooling and grinding.

[0043] Example 2: A method for preparing a Mo-based nitrogen support supported on an alkaline earth metal oxide, comprising:

[0044] Weigh 0.67 g of Mg(NO3)2·6H2O and dissolve it in 3 mL of deionized water by stirring for half an hour. Then, add 1 g of Mo-HMT precursor and mix to obtain a precursor solution, which is dried in an oven at 80 °C for 10 h to obtain the Mg-Mo-HMT precursor. Subsequently, [the solution is then processed at 100 mL / min].-1 At a flow rate of N2, the sample was calcined at 750℃ for 1 h in a fixed bed at a heating rate of 10℃ / min, followed by H2 reduction for 1 h. After cooling, the sample was ground to obtain nitrogen-supported sample 2.

[0045] Example 3: A method for preparing a Mo-based nitrogen support supported on an alkaline earth metal oxide, comprising:

[0046] Weigh 0.45 g of Mg(NO3)2·6H2O and dissolve it in 3 mL of deionized water by stirring for half an hour. Then, add 1 g of Mo-HMT precursor and mix to obtain a precursor solution, which is dried in an oven at 80 °C for 10 h to obtain the Mg-Mo-HMT precursor. Subsequently, [the solution is then processed at 100 mL / min]. -1 Under a flow rate of N2, the nitrogen-carrying sample 3 was obtained by constant calcination at 750℃ for 1 h and constant reduction at H2 for 1 h in a fixed bed at a heating rate of 10℃ / min, followed by cooling and grinding.

[0047] Example 4: A method for preparing a Mo-based nitrogen support supported on an alkaline earth metal oxide, comprising:

[0048] Weigh 0.41 g of Ca(NO3)2·4H2O and dissolve it in 3 mL of deionized water by stirring for half an hour. Then, add 1 g of Mo-HMT precursor and mix to obtain a precursor solution, which is dried in an oven at 80 °C for 10 h to obtain the Ca-Mo-HMT precursor. Subsequently, [the solution is then processed at 100 mL / min]. -1 Under a flow rate of N2, the nitrogen-carrying sample 4 was obtained by constant calcination at 750℃ for 1 h and constant reduction at H2 for 1 h in a fixed bed at a heating rate of 10℃ / min, followed by cooling and grinding.

[0049] Example 5: A method for preparing a Mo-based nitrogen support supported on an alkaline earth metal oxide, comprising:

[0050] Weigh 0.14 g of Sr(NO3)2 and dissolve it in 3 mL of deionized water by stirring for half an hour. Then, add 1 g of Mo-HMT precursor and mix to obtain a precursor solution, which is dried in an oven at 80 °C for 10 h to obtain the Sr-Mo-HMT precursor. Subsequently, [the solution is then processed at 100 mL / min]. -1 At a flow rate of N2, the sample was calcined at 750℃ for 1 h in a fixed bed at a heating rate of 10℃ / min, followed by constant-temperature reduction with H2 for 1 h. After cooling, the sample was ground to obtain nitrogen-supported sample 5.

[0051] Example 6: A method for preparing a Mo-based nitrogen support supported on an alkaline earth metal oxide, comprising:

[0052] 0.12 g of Ba(NO3)2 was weighed and dissolved in 3 mL of deionized water by stirring for half an hour. Then, 1 g of Mo-HMT precursor was added and mixed to obtain a precursor solution, which was dried in an oven at 80 °C for 10 h to obtain the Ba-Mo-HMT precursor. Subsequently, it was calcined at 750 °C for 1 h in a fixed bed at a flow rate of 10 °C / min under N2 at a flow rate of 100 mL, followed by reduction under H2 at a constant temperature for 1 h. After cooling, it was ground to obtain nitrogen-supported sample 6.

[0053] Example 7: A method for preparing a Mo-based nitrogen support supported on an alkaline earth metal oxide, comprising:

[0054] Take 0.2 g of Mo-HMT precursor and saline it in 100 mL·min -1 Under a flow rate of N2, the sample was calcined at 750℃ for 1 h in a fixed bed at a heating rate of 10℃ / min, then reduced at 1 h in a fixed bed with H2, and finally ground after cooling to obtain Mo-based nitrogen support sample 7 without alkaline earth metal oxide loading.

[0055] This application uses a fixed-bed reactor to evaluate the nitrogen carriers prepared in Examples 1-6. The specific operating steps are as follows: all samples were tested at 80 mL / min. -1 The H2 gas stream was pre-reduced at 650 °C for 2 h. Afterwards, it was pretreated at the same temperature, with the gas flow rate of N2 and H2 switched twice for 15 min each time. Then, the flow rate of N2 and H2 was continuously switched for 1 h each time for the reaction. The reaction tail gas was passed into a 0.1 mmol / L dilute sulfuric acid solution at 25 °C, and the change in conductivity was measured. The amount of NH3 generated was calculated using a standard curve.

[0056] The test results of the samples in Examples 1-3 are as follows: Figure 1 As shown, all samples exhibit chemical loop ammonia production performance, with Example 3 showing the best loop ammonia production rate, i.e., the mass ratio of MgO to Mo2N is approximately 1:5, and each 1 mol of Mo corresponds to approximately 0.515 mol of Mg.

[0057] The test results of the samples in Examples 3-6 are as follows: Figure 2 Similarly, Example 3 exhibits the best chemical loop ammonia production performance. This demonstrates that the addition of alkaline earth metals can promote nitrogen fixation of Mo under N2, and the addition of Mg has the greatest effect on improving the chemical loop ammonia production performance of Mo.

[0058] XRD analysis of nitrogen-carrying samples in Examples 3-7 is as follows: Figure 3As shown, in Example 2, Mg exists in the form of MgO; in Example 3, Ca exists in the forms of CaO and CaMoO4; in Example 4, Sr exists in the form of SrMoO4; and in Example 5, Ba exists in the form of BaMoO4. Furthermore, in both Examples 4 and 5, samples with the same reduction time showed some incomplete reduction of Mo2N. This indicates that Mo2N has a strong electron transfer effect on Mo, leading to increased Mo-N bond strength or excessive coating, making lattice nitrogen migration difficult and hindering ammonia production.

[0059] The XRD patterns of the nitrogen-carrying sample in Example 3 before and after nitrogen fixation are as follows: Figure 4 This demonstrates that the nitrogen carrier in this application enables the regeneration of Mo2N under pure nitrogen gas.

[0060] SEM and particle size of the nitrogen carrier in Example 7 are shown below. Figure 5 , Figure 5 The image shows the SEM and particle size of the pure Mo nitrogen-supported material in Example 7 at a particle size of 10 μm, with a particle size distribution of approximately 11.5 μm.

[0061] Figure 6 The image shows the SEM image of the alkaline earth metal Mg oxide-supported Mo-based nitrogen carrier prepared in Example 3 of this application, with a particle size distribution of about 75 nm. Figure 7 The image shows the SEM image of the alkaline earth metal Ca oxide-supported Mo-based nitrogen carrier prepared in Example 4 of this application, with a particle size distribution of about 110 nm. Figure 8 This is a SEM image of the alkaline earth metal Sr oxide-supported Mo-based nitrogen carrier prepared in Example 5 of this application, with a particle size distribution of approximately 68 nm. Figure 9 This is a SEM image of the alkaline earth metal Ba oxide-supported Mo-based nitrogen carrier prepared in Example 6 of this application. The particle size distribution is approximately 230 nm. Figures 5 to 9 It can be seen that the particle size of the Mo-based nitrogen carrier supported by alkaline earth metal oxides is reduced to the nanoscale. Therefore, the loading of alkaline earth metals reduces the particle size of the Mo-based nitrogen carrier and increases the exposed area of ​​active sites.

[0062] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A method for preparing a Mo-based nitrogen support loaded with an alkaline earth metal oxide, characterized in that, include: Step S1: Prepare Mo-HMT precursor; Step S2: Prepare an aqueous solution of an alkaline earth metal salt; wherein the alkaline earth metal salt is selected from at least one of soluble Mg salt, Ca salt, Sr salt or Ba salt; Step S3: Add Mo-HMT precursor to an alkaline earth metal salt aqueous solution and mix, then dry to obtain alkaline earth metal-supported Mo-HMT precursor; Step S4: The alkaline earth metal-supported Mo-HMT precursor is first pyrolyzed under N2 at 700~800℃, and then reduced under H2 to obtain an alkaline earth metal oxide-supported Mo-based nitrogen carrier.

2. The preparation method according to claim 1, characterized in that, Step S1, preparing the Mo-HMT precursor, including: The molybdenum source and hexamethylenetetramine were dissolved in water, mixed and reacted to precipitate, and the product was washed and dried to obtain the Mo-HMT precursor.

3. The preparation method according to claim 2, characterized in that, The molybdenum source, calculated as Mo, has a molar ratio of hexamethylenetetramine to molybdenum source of (1~2):1, preferably (1.4~1.6):

1.

4. The preparation method according to claim 2, characterized in that, The molybdenum source is selected from at least one of ammonium heptamolybdate or its hydrate, ammonium tetramolybdate or its hydrate, ammonium molybdate or its hydrate, and ammonium octamolybdate or its hydrate.

5. The preparation method according to claim 2, characterized in that, During the preparation of Mo-HMT precursors, any one of the following conditions must be met: The reaction temperature was room temperature; The drying temperature is 60~100℃; The drying temperature is 80℃ and the drying time is 10-18 hours.

6. The preparation method according to claim 1, characterized in that, In the Mo-based nitrogen support supported by alkaline earth metal oxides, the alkaline earth metal element exists in the form of alkaline earth metal oxides or alkaline earth metal molybdates; the molar ratio of alkaline earth metal element to Mo element is 0.1~0.8, preferably 0.2~0.

6.

7. The preparation method according to claim 1, characterized in that, In step S4, pyrolysis under N2 involves heating from room temperature to 700-800℃ at a heating rate of 1-20℃ / min, followed by constant-temperature calcination at 700-800℃ for at least 0.5 hours; reduction under H2 involves constant-temperature reduction at 700-800℃ for at least 0.5 hours. Preferably, the pyrolysis under N2 involves heating from room temperature to 750°C at a heating rate of 10°C / min, followed by constant-temperature calcination at 750°C for 1 hour; the reduction under H2 involves constant-temperature reduction at 750°C for 1 hour.

8. An alkaline earth metal oxide-supported Mo-based nitrogen carrier, characterized in that, It is prepared by the preparation method according to any one of claims 1-7.

9. The application of the alkaline earth metal oxide-supported Mo-based nitrogen carrier as described in claim 8 in the chemical chain synthesis of ammonia.

10. The application according to claim 9, characterized in that, In the chemical chain synthesis of ammonia, the reaction temperature is 600℃~700℃.

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