Preparation method and application of chromium-based nitrogen carrier for chemical looping synthesis of ammonia

Abstract

The present application relates to the technical field of mild ammonia synthesis, and particularly relates to a preparation method of a chromium-based nitrogen carrier for chemical looping ammonia synthesis. The chromium-based nitrogen carrier is prepared by ammonia decomposition of a chromium chloride complex precursor, and a transition metal is loaded on the nitrogen carrier by an impregnation method to control the performance of the nitrogen carrier, so that the chemical looping ammonia synthesis rate of the chromium-based nitrogen carrier is increased by more than one order of magnitude. In the application of chemical looping ammonia synthesis, the loaded metal significantly improves the nitrogen conversion rate of the nitrogen carrier. The lattice nitrogen conversion rate of Co-loaded CrN is more than 50%, and in the chromium-based nitrogen carrier, the proportion of the lattice nitrogen loss converted into ammonia gas is more than 95%. The good nitrogen transfer characteristics of the chromium-based nitrogen carrier expand the application space of the chromium-based nitrogen carrier as a nitrogen carrier for chemical looping ammonia synthesis. In a continuous experiment of 12 chemical looping cycles, the chemical looping ammonia synthesis rate of the Co-loaded chromium-based nitrogen carrier is stable, and the average reaction rate is 466.1 micromoles per gram per second. ‑1 h ‑1 The prepared nitrogen carrier has the characteristics of low cost, stability and high efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of mild ammonia synthesis, and particularly relates to a preparation method of a chromium-based nitrogen carrier for chemical looping ammonia synthesis. BACKGROUND

[0002] The synthesis of ammonia has changed the development of human society. In recent years, in addition to being used as an industrial raw material, a cold storage refrigerant, and a nitrogen fertilizer such as urea and ammonium bicarbonate, ammonia has also attracted widespread attention from the scientific and industrial communities as a zero-carbon hydrogen energy carrier. Ammonia (NH3) has a high hydrogen storage capacity of 17.7% and a high energy density (23 MJ kg-1) similar to that of fossil fuels, and the decomposition products do not contain greenhouse gases and harmful gases, so ammonia is expected to become a clean and efficient energy carrier. However, the existing industrial ammonia synthesis process is based on the Haber-Bosch process, which requires the reaction of hydrogen and nitrogen to generate ammonia gas under high temperature and high pressure, consumes a large amount of energy and produces a large amount of carbon dioxide emissions during the process, which severely limits the application of ammonia as an energy carrier. -1

[0003] In order to realize the large-scale application of renewable energy, various new ammonia synthesis strategies based on renewable energy have been proposed. Among them, the chemical looping ammonia synthesis decouples the overall reaction into two separate reaction processes of nitrogen fixation and ammonia production, has the advantages of being able to operate under normal pressure conditions, simplifying the process flow, and being applicable to distributed and small-scale ammonia production systems. Since the catalyst is a stable nitride or oxide, the chemical looping ammonia synthesis method has the advantages of easy start and stop of facilities, easy coupling with renewable energy, optimization of reactants, temperature and pressure in each step such as nitrogen fixation and ammonia production, and avoidance of the problem of competitive adsorption of N2 and H2 or H2O.

[0004] ​Inorganic nitrogen-containing compounds are widely concerned in the field of ammonia synthesis due to their more reaction sites and significant kinetic characteristics, mainly including metal nitrides, oxynitrides, nitride hydrides, amide or imide compounds, etc. There are mainly three kinds of chemical looping ammonia synthesis: (1) Chemical looping ammonia synthesis of metal nitride-oxide pairs: such as Cr-Cr2N-Cr2O3 cycle, Li-based chemical looping process (Li-Li3N-LiOH), etc. This cycle requires high reaction temperature (>1000℃), low energy utilization rate, and further study is needed for the reduction method of metal oxides. (2) Chemical looping ammonia synthesis of lean-nitrogen-rich-nitrogen nitride pairs: For example, the previously reported Mn3N2 has very low reactivity, only 3.1% of the lattice nitrogen can react with hydrogen to generate ammonia, and the other 73% of the lattice nitrogen is lost in the form of N2. Fe-Mn-based composite nitrogen carriers solve the problems of nitrogenation difficulty of Fe-based nitrogen carriers and ammonia difficulty of Mn-based nitrogen carriers to a certain extent. This cycle still has problems such as low conversion rate of lattice nitrogen and difficulty in nitrogen fixation of lean-nitrogen-rich-nitrogen nitride, but due to its moderate reaction temperature (500-700℃), there is still a lot of room for improvement. (3) Chemical looping ammonia synthesis of hydride-imido compound pairs: Taking TM-LiH and Ni-BaH2 as examples, high-efficiency ammonia synthesis can be achieved at 300℃ and 1bar, however, the A-H in metal hydride is easily hydrolyzed and deactivated, and the catalyst will self-ignite and explode when it comes into contact with air, which seriously limits its scale application.

[0005] Compared with the ultra-high energy consumption of metal oxide reduction and the difficulty in preparation and storage of metal hydride, metal nitride materials have many advantages such as easy regeneration, low price, and stable properties. In order to solve the problems of low effective conversion rate of lattice nitrogen and difficulty in nitrogen fixation of nitrogen carriers after denitrification in the above-mentioned lean-nitrogen-rich-nitrogen nitride pair ammonia synthesis process, the present application determines that chromium-based nitrogen carriers have good nitrogen fixation capacity by combining the thermodynamic parameters of different metal nitride nitrogen fixation reactions, and innovatively applies chromium-based nitrogen carriers to chemical looping ammonia synthesis production by combining their low price and rich intermediate valence characteristics. It is found that the composite nitrogen carrier material prepared by impregnation method can significantly improve the conversion rate of lattice nitrogen in chromium-based nitrogen carrier, and the consumed lattice nitrogen can be used to synthesize NH3, and the synergistic effect of the two active sites significantly enhances the reaction rate of chemical looping ammonia synthesis. SUMMARY

[0006] The purpose of this invention is to address the problems existing in the background technology by proposing a method for preparing a chromium-based nitrogen support for chemical chain synthesis of ammonia. This method is characterized by low cost, stability, and high efficiency. The performance of the nitrogen support has been regulated, and its feasibility and application properties for ammonia synthesis via chemical chain method have been verified and enhanced.

[0007] The technical solution of this invention is a method for preparing a chromium-based nitrogen support for chemical chain synthesis of ammonia, comprising:

[0008] (1) Dissolve chromium chloride hexahydrate in anhydrous ethanol to obtain a concentrated solution;

[0009] (2) Add the concentrated solution from step (1) dropwise to the saturated urea / ethanol solution and stir in an oil bath until a large amount of green precursor (Cr-urea coordination compound) precipitates out.

[0010] (3) Filter the precipitate from step (2) to obtain a blocky green Cr-urea precursor, and then vacuum dry it.

[0011] (4) Grind the complex precursor in step (3) into powder and directly ammonolycide it in a tube furnace to prepare a chromium-based nitrogen-supported catalyst.

[0012] (5) Dissolve the corresponding stoichiometric amount of transition metal nitrate (0.12-0.60 g) in 3-5 ml of anhydrous ethanol;

[0013] (6) Add the nitrate solutions from step (5) to the precursor powder from step (4), stir continuously until the impregnation is uniform, and then vacuum dry.

[0014] (7) Grind the precursor after impregnation in step (6) and ammonolycide it in a tube furnace to prepare chromium-based nitrogen carriers loaded with different metals.

[0015] The raw material for preparing the chromium-based nitrogen carrier in step (1) is chromium chloride hexahydrate (CrCl3·6H2O), and the amount used is 0.04-0.05 mol.

[0016] In step (2), the oil bath temperature is controlled at 75-80℃, and the total time for adding the concentrated solution after step (1) is about 1 hour.

[0017] In step (3), the temperature of the vacuum drying oven is set to 60-80℃. The drying time is 8-10 hours.

[0018] In step (4), the gas introduced into the tubular furnace is a 50% NH3 / N2 mixture, and the gas flow rate is 80 ml / min. -1 Before heating, purge the air from the furnace by introducing a mixed gas for 0.5-1.0 hours.

[0019] In steps (4) and (7), the temperature during ammonolysis of the complexing precursor in the tubular furnace is 750℃, and the heating rate is 5-10℃ / min. -1 The calcination time is 6 hours.

[0020] In step (5), the impregnated and loaded transition metals include cobalt, nickel, and iron. The calculated metal loadings in the catalyst are 5% wt, 10% wt, and 15% wt, while the actual measured loadings range from 2.0% to 7.6% wt (SEM-EDS). The loaded metals are derived from their corresponding nitrates.

[0021] In step (6), the mass of the precursor is 4.0-4.2g, and the equal volume method is used to ensure uniform impregnation (corresponding to the measurement of 3-5ml of anhydrous ethanol in step (5)).

[0022] Furthermore, the aforementioned chromium-based nitrogen support was applied to a chemical chain synthesis experiment to verify the activity and stability of the nitrogen support in this process. H2 and N2 were alternately introduced into the constructed experimental setup to test the reactivity of the chromium-based nitrogen support at different temperatures.

[0023] The fixed-bed reactor operates at atmospheric pressure with a total gas flow rate of 60 ml / min. -1 (WHSV = 60000 mlg) -1 h -1 The product is passed into a collection device containing a dilute sulfuric acid solution, and the reaction rate is detected online by conductivity method.

[0024] The aforementioned chromium-based nitrogen support was prepared, its performance was regulated, and it was applied to a chemical chain ammonia synthesis experiment. The results showed that the unloaded chromium-based nitrogen support could achieve sustained ammonia production only at temperatures above 600 °C. Loading with transition metals such as cobalt, nickel, and iron significantly reduced the onset temperature of the ammonia production from the nitrogen support's reaction with H₂. At 700 °C, the promoting effect of the loaded metal on the chromium-based nitrogen support was cobalt > nickel > iron, with ammonia production of 368.2 μmol, 158.9 μmol, and 127.1 μmol within 6 h, respectively (at ambient pressure, 60 mg catalyst).

[0025] Compared with the prior art, the present invention has the following beneficial technical effects:

[0026] This invention prepares a high-purity, stable chromium-based nitrogen support via ammonolysis precursors. Significant performance regulation of the nitrogen support is achieved by loading transition metals using an impregnation method. In the ammonia production step with H₂, transition metal loading significantly improves the nitrogen conversion rate of the nitrogen support; specifically, the lattice nitrogen conversion rate of Co-loaded CrN increases from 4.3% to 50.7%. The lattice nitrogen conversion rate of Ni-CrN increases to 22.3%, and that of Fe-CrN increases to 18.5%. Simultaneously, only trace amounts of N are lost as N₂ from the lattice nitrogen consumed by the nitrogen support, and the proportion of consumed lattice nitrogen converted to ammonia gas consistently exceeds 95%.

[0027] The excellent nitrogen transfer properties of chromium-based nitrogen carriers expand their application scope in chemical chain synthesis of ammonia. In a 12-cycle continuous chemical chain experiment, the ammonia production rate of Co-supported chromium-based nitrogen carriers remained stable at 380-500 μmol g. -1 h -1 The average rate of ammonia production in the chemical chain was 466.1 μmol g. -1 h -1 The reaction rate was higher than that of the unsupported catalyst (46.2 μmol g). -1 h -1 This represents an improvement of more than an order of magnitude (700℃, atmospheric pressure). The disclosed nitrogen-carrying material can be stored in air at room temperature, enabling efficient energy thermal conversion in the field of novel mild ammonia synthesis. Attached Figure Description

[0028] Figure 1 This is a flowchart illustrating the preparation process of the chromium-based nitrogen carrier used in this invention;

[0029] Figure 2 This is a diagram of the experimental apparatus used in this invention;

[0030] Figure 3 These are the crystal phase and morphology characterization diagrams of the cobalt-supported chromium-based nitrogen-supported body prepared in this invention;

[0031] Figure 4 This is a diagram showing the ammonia production characteristics of the unsupported chromium-based nitrogen support prepared according to the present invention;

[0032] Figure 5 This is a diagram showing the ammonia production characteristics of chromium-based nitrogen carriers with different metal loads prepared according to the present invention;

[0033] Figure 6 This is a diagram showing the nitrogen conversion ratio of the chromium-based nitrogen carrier lattice prepared according to the present invention;

[0034] Figure 7 This is a graph showing the chemical chain synthesis rate of ammonia using different chromium-based nitrogen carriers prepared in this invention;

[0035] Figure 8This is a multi-cycle stability test diagram of the cobalt-supported chromium-based nitrogen carrier prepared in this invention. Detailed Implementation

[0036] Example 1

[0037] A method for preparing and controlling the performance of a high-efficiency chromium-based nitrogen support and its characterization

[0038] like Figure 1 As shown, taking a cobalt-supported chromium-based nitrogen carrier as an example, the process includes the following steps:

[0039] (1) Weigh 0.04 mol CrCl3·6H2O into a beaker, add 15 ml of anhydrous ethanol, and stir vigorously at room temperature to obtain solution A; add 0.24 mol urea and 20 ml of anhydrous ethanol into the beaker, and stir vigorously at room temperature to obtain solution B.

[0040] (2) Solution A was slowly added dropwise to solution B, and B was simultaneously mixed in an oil bath at 80°C and stirred until a large amount of green precipitate was formed; then, the Cr-urea complex precursor was prepared by filtration, vacuum drying, grinding and other processes.

[0041] (3) Add 0.2933g Co(NO3)2·6H2O and 4ml anhydrous ethanol to a beaker and stir evenly at room temperature to obtain solution C; weigh 4.2g Cr-urea precursor powder, add it to solution C, and stir at room temperature for 15-30min to complete the loading of Co nitrate.

[0042] (4) The obtained sample was placed in a tube furnace and ammonia-nitrogen mixed gas was introduced for 1 hour to remove the air in the furnace; then the temperature was increased to 750°C at a rate of 5°C / min, held for 6 hours and then cooled naturally in the same gas flow.

[0043] (5) Remove the sample from the tube furnace and grind it to obtain a cobalt-loaded chromium-based nitrogen carrier. The loading of other metals is similar to that of cobalt, and the corresponding mass of nitrate needs to be weighed and impregnated into the Cr-urea precursor.

[0044] The crystal structure and surface morphology of the above-mentioned Co-CrN materials are as follows: Figure 3 As shown, the material exhibits distinct peaks corresponding to elemental Co and CrN, and its surface is composed of tiny spherical nanoparticles. Furthermore, surface EDS scanning revealed the distribution of Co, Cr, and N, indicating the successful uniform loading of trace amounts of Co onto the CrN surface. This facilitates the construction of a dual-active-site catalyst, enabling synergistic catalytic action between components and thus significantly improving the rate of ammonia synthesis via chemical chaining.

[0045] Example 2

[0046] The performance of a high-efficiency chromium-based nitrogen carrier in converting lattice nitrogen to ammonia

[0047] The apparatus for chemical chain synthesis of ammonia in this invention is as follows: Figure 2 As shown, nitrogen and hydrogen are supplied from corresponding gas cylinders. After the gas flow rate is adjusted by a mass flow meter, the gas is alternately introduced into the fixed-bed reactor under valve control. The product is introduced into a dilute sulfuric acid solution, and the ammonia yield is measured by a conductivity meter.

[0048] like Figure 4 As shown, the reaction between CrN without transition metal loading and H2 in this invention is difficult to proceed, and the ammonia production within 6 hours is 46.2 μmol g. -1 h -1 (700℃, atmospheric pressure). Furthermore, the ammonia production reaction can only proceed continuously at temperatures above 600℃. In contrast, the loading of transition metals significantly promotes the reactivity of lattice nitrogen with H2, such as... Figure 5 As shown, under the same reaction conditions (700℃, atmospheric pressure), the ammonia production of CrN supported on Co, Ni, and Fe within 6 h was 368.2 μmol, 158.9 μmol, and 127.1 μmol, respectively. The promoting effect of the supported metal on the chromium-based nitrogen support was Co>Ni>Fe, and the starting temperature for sustained ammonia production also decreased to 450-500℃.

[0049] like Figure 6 The nitrogen (N) content of different materials before and after the reaction was characterized by elemental analysis, and N balance analysis was performed accordingly. During the ammonia production process with H₂, Co loading increased the lattice nitrogen conversion rate of the nitrogen carrier from 4.3% to 50.7%; the lattice nitrogen conversion rate of Ni-CrN increased to 22.3%; and the lattice nitrogen conversion rate of Fe-CrN increased to 18.5%. Simultaneously, only trace amounts of N were lost as N₂ from the lattice nitrogen consumed by the nitrogen carrier, and the proportion of consumed lattice nitrogen converted to ammonia gas was over 95% in all cases. Based on the above test results, it can be seen that the ammonia production rate of the chromium-based nitrogen carrier after performance regulation in this application is much higher than that of other nitrogen carrier materials currently reported. The excellent lattice nitrogen transfer characteristics demonstrate its potential for industrial chemical chain ammonia synthesis.

[0050] Example 3

[0051] A highly efficient chromium-based nitrogen-supported catalyst for improving the average ammonia production rate and cycle stability in chemically looped ammonia synthesis.

[0052] The chromium-based nitrogen support disclosed in this invention has a high average ammonia production rate, such as... Figure 7 As shown, under the same reaction conditions (700℃, atmospheric pressure), the ammonia production rates of CrN supported on Co, Ni, and Fe were significantly higher than those of pure CrN. Co-supported CrN exhibited the highest chemical chain ammonia production rate (466.1 μmol g).-1 h -1 The ammonia production rate of Ni-CrN was increased by more than an order of magnitude compared to CrN, and was significantly higher than the rate of thermocatalytic ammonia production (H2 and N2 environment) under the same conditions (3-4 times). The average chemical chain ammonia production rates of Ni-CrN and Fe-CrN were 279.5 μmol g. -1 h -1 246.4 μmol g -1 h -1 The comparison between chemical chaining and thermocatalytic ammonia production rates demonstrates that the chromium-based nitrogen support, after performance regulation, breaks the competitive adsorption relationship between nitrogen-containing intermediates and H2.

[0053] The stability of multi-cycle internal cycling in Co-CrN-based chemical chain ammonia synthesis is as follows: Figure 8 As shown. In a 12-cycle chemical looping experiment, the ammonia production rate of the cobalt-supported chromium-based nitrogen carrier remained stable at 380–500 μmol g. -1 h -1 After the relatively high rates in the first two cycles, the rate of ammonia synthesis via chemical looping stabilizes within a high range without further decline. This indicates that cobalt loading not only promotes the hydrogenation of lattice nitrogen to ammonia but also synergizes with the activation of N2 on the surface of the reduced nitrogen carrier and the regeneration of lattice nitrogen. These results demonstrate the significant potential of chromium-based nitrogen carriers in future renewable energy conversion applications.

Claims

1. A method for preparing a chromium-based nitrogen support for chemical chain synthesis of ammonia, characterized in that, Includes the following steps: S1. Dissolve 0.04-0.05 mol of chromium chloride hexahydrate CrCl3·6H2O in anhydrous ethanol to obtain a concentrated solution, and stir magnetically for 6-8 h. S2. Add the concentrated solution after treatment in S1 dropwise to the saturated urea / ethanol solution and stir in an oil bath until a large amount of green precipitate is formed. S3. Filter the precipitate from S2 to obtain a green precursor of Cr-urea coordination complex, and then vacuum dry it. S4. Grind the complex precursor in S3 into powder; S5. Weigh the corresponding stoichiometric amounts of transition metal nitrates and dissolve them in 3-5 ml of anhydrous ethanol. S6. Immerse the green precursor powder in S4 into the solutions in S5 respectively, and stir to make the impregnation uniform; the impregnation-loaded transition metal is provided by the corresponding transition metal nitrate; wherein the loaded transition metal includes cobalt, nickel or iron. S7. The precursor impregnated in S6 is vacuum dried, ground, and then ammonolyzed in a tube furnace to obtain a chromium-based nitrogen-supported catalyst. The gas introduced into the tube furnace is a 50% NH3 / N2 mixture at a flow rate of 80 ml / min. -1 The temperature during ammonolysis is 750 ℃, and the heating rate is 5-10 ℃ min. -1 The ammonolysis time is 6 hours.

2. The method for preparing a chromium-based nitrogen support for chemical chain synthesis of ammonia according to claim 1, characterized in that, The oil bath temperature in S2 is 75-80 ℃.

3. The method for preparing a chromium-based nitrogen support for chemical chain synthesis of ammonia according to claim 1, characterized in that, The total time for adding the concentrated solution after the dropwise treatment in S2 is 0.8-1.0 h.

4. The method for preparing a chromium-based nitrogen support for chemical chain ammonia synthesis according to claim 1, characterized in that, The set temperature for vacuum drying in S3 is 60-80 ℃.

5. The method for preparing a chromium-based nitrogen support for chemical chain synthesis of ammonia according to claim 1, characterized in that, The precursor in S6 has a mass of 4.0-4.2 g and is uniformly impregnated using the equal volume method.

6. The application of a chromium-based nitrogen support in the chemical chain synthesis of ammonia, characterized in that, The chromium-based nitrogen support prepared by any one of the preparation methods described in claims 1-5 is used as a catalyst for the catalytic synthesis of ammonia.

Images