A transition metal doped modified spinel oxygen carrier and a preparation method thereof

Abstract

The application discloses a transition metal doped modified spinel oxygen carrier and a preparation method thereof. x Ni 1‑x Fe2O4; wherein M is one of Co, Cu or Mn; and x is 0.1-0.9. The preparation method of the spinel oxygen carrier comprises the following steps: (1) weighing nickel salt, iron salt and transition metal salt according to stoichiometric ratio, dissolving the nickel salt, iron salt and transition metal salt in deionized water, adding a complexing agent and adjusting the pH of the system, stirring and heating to make metal ions undergo complexing and gelation reactions, and obtaining a precursor sol; (2) adding a binder and a dispersant to the precursor sol, and then adding deionized water to adjust the solid content, to obtain a sprayable precursor slurry; (3) spray granulating the sprayable precursor slurry to obtain precursor particles; and (4) drying the precursor particles and then calcining in an air atmosphere to obtain the transition metal doped modified spinel oxygen carrier M x Ni 1‑ x Fe 2 O 4. The spinel oxygen carrier has good reactivity and structural stability.

Description

Technical Field

[0001] This invention relates to a spinel oxygen carrier modified by transition metal doping, and also to a method for preparing the above-mentioned spinel oxygen carrier. Background Technology

[0002] Oxygen carriers are core functional materials in chemical looping combustion, and their reactivity, cycle stability, and structural integrity directly affect oxygen transfer efficiency, energy utilization efficiency, and material cycle life. Among existing oxygen carrier systems, spinel-type oxygen carriers have attracted widespread attention due to their structural stability and strong resistance to sintering. NiFe2O4 (nickel-iron spinel), as a representative spinel oxygen carrier, has a relatively stable Fe-O octahedral framework, and the introduction of Ni is beneficial to improving local electronic structure and redox performance. However, NiFe2O4 suffers from insufficient reactivity, insufficient lattice oxygen migration capacity, and insufficient structure-activity synergy optimization in actual cycling processes. The main reason is that methane oxidation in chemical looping combustion usually follows a Mars-van Krevelen type lattice oxygen participation mechanism. The fuel conversion rate depends not only on surface active sites but also on the migration rate of bulk lattice oxygen to the surface and the rate of oxygen vacancy replenishment. Unmodified NiFe2O4 has a limited effective oxygen vacancy concentration and lattice oxygen migration channels, making it difficult to replenish surface lattice oxygen in a timely manner after consumption, thus limiting the deep oxidation of methane and CO2 selectivity. Meanwhile, in high-temperature repeated redox cycles, excessive oxygen vacancies, cation migration and phase separation can easily lead to structural degradation. Therefore, it is necessary to simultaneously consider the lattice oxygen migration capability and maintain the stability of the spinel framework. Summary of the Invention

[0003] Purpose of the invention: The purpose of this invention is to provide a spinel oxygen carrier with good reactivity and cycling stability (maintaining structural integrity after multiple cycles of use). Another purpose of this invention is to provide a method for preparing the above-mentioned spinel oxygen carrier.

[0004] Technical solution: The transition metal-doped modified spinel oxygen carrier of the present invention has the chemical formula M x Ni 1- x Fe2O4; wherein M is one of Co, Cu or Mn; x is 0.1 to 0.9, preferably 0.3 to 0.7, more preferably 0.5.

[0005] The above-mentioned method for preparing spinel oxygen carriers employs a sol-gel combined spray granulation-calcination method, specifically including the following steps:

[0006] (1) Weigh nickel salt, iron salt and transition metal salt according to stoichiometric ratio, dissolve the nickel salt, iron salt and transition metal salt in deionized water, add citric acid as a complexing agent and ethylene glycol as a gelling aid, adjust the pH of the system to 6-8 (pH is adjusted by ammonia water, dilute nitric acid or a combination of ammonia water and dilute nitric acid), stir and heat at 60-90°C for 1-3 hours to allow the metal ions to complex and gradually form a uniform precursor sol or precursor gel; preferably, the water bath heating temperature is 80°C, the heating time is 2 hours, and the total molar amount of metal ions and the molar ratio of citric acid to ethylene glycol are 1:1:1;

[0007] (2) Polyvinyl alcohol and sodium polyacrylate are added to the precursor sol, and then deionized water is added to adjust the solid content to obtain a sprayable precursor slurry (the solid content of the sprayable precursor slurry is 10~30wt%, preferably 15~25wt%, more preferably 20wt%); wherein, polyvinyl alcohol is used to improve droplet formation and particle mechanical integrity, and sodium polyacrylate is used to improve slurry dispersibility and improve particle forming quality;

[0008] (3) Spray the sprayable precursor slurry through a dual-fluid nozzle for granulation. The atomization pressure is 0.1~0.4MPa, the inlet air temperature is 150~230℃, and the outlet air temperature is 80~120℃, so that the atomized droplets are quickly dehydrated to form precursor particles.

[0009] (4) The collected precursor particles were dried at 90~130℃ for 6~18h, and then kept at 700~950℃ in air for 2~8h to obtain transition metal doped spinel oxygen carrier M. x Ni 1-x Fe2O4; the preferred drying temperature is 110℃, the calcination temperature is 850℃, and the holding time is 4h.

[0010] After introducing Co, Cu or Mn into NiFe2O4, the doped products all retain the spinel main phase and no obvious impurity phase appears. This indicates that the method of the present invention can achieve uniform doping of transition metals while maintaining the main framework, and can obtain better spinel phase formation and the obtained particles have good uniformity.

[0011] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: The present invention introduces differences in valence state and ionic radius by partially replacing Ni sites in NiFe2O4 with Co, Cu or Mn, which causes moderate distortion and charge compensation effect in the spinel lattice, promotes the formation of oxygen vacancies and improves the oxygen migration ability of the bulk lattice, thereby obtaining a spinel oxygen carrier with both good reactivity and cycle stability. Attached Figure Description

[0012] Figure 1The NiFe2O4 prepared in Comparative Example 1 and the Mn prepared in Examples 1-3 0.5 Ni 0.5 Fe2O4, Co 0.5 Ni 0.5 Fe2O4 and Cu 0.5 Ni 0.5 XRD pattern of Fe2O4;

[0013] Figure 2 The NiFe2O4 prepared in Comparative Example 1 and the Mn prepared in Examples 1-3 0.5 Ni 0.5 Fe2O4, Co 0.5 Ni 0.5 Fe2O4 and Cu 0.5 Ni 0.5 EPR diagram of Fe2O4;

[0014] Figure 3 The NiFe2O4 prepared in Comparative Example 1 and the Mn prepared in Examples 1-3 0.5 Ni 0.5 Fe2O4, Co 0.5 Ni 0.5 Fe2O4 and Cu 0.5 Ni 0.5 SEM images of Fe2O4; (c) NiFe2O4; (d) Mn 0.5 Ni 0.5 Fe2O4; (e) is Co 0.5 Ni 0.5 Fe2O4; (f) is Cu 0.5 Ni 0.5 Fe2O4;

[0015] Figure 4 Cu obtained in Example 3 0.5 Ni 0.5 TEM / HR-TEM image of Fe2O4;

[0016] Figure 5 Cu obtained in Example 3 0.5 Ni 0.5 Elemental distribution diagram of Fe2O4;

[0017] Figure 6 The NiFe2O4 prepared in Comparative Example 1 and the Mn prepared in Examples 1-3 0.5 Ni 0.5 Fe2O4, Co 0.5 Ni 0.5 Fe2O4 and Cu 0.5 Ni 0.5 O 1s XPS spectrum of Fe2O4;

[0018] Figure 7 The NiFe2O4 prepared in Comparative Example 1 and the Mn prepared in Examples 1-3 0.5 Ni 0.5 Fe2O4, Co 0.5 Ni 0.5 Fe2O4 and Cu 0.5 Ni 0.5 Ni 2p XPS spectrum of Fe2O4;

[0019] Figure 8 The NiFe2O4 prepared in Comparative Example 1 and the Mn prepared in Examples 1-3 0.5 Ni 0.5 Fe2O4, Co 0.5 Ni 0.5 Fe2O4 and Cu 0.5 Ni 0.5 Reactivity and 20-cycle performance of Fe2O4 in methane chemical looping combustion process;

[0020] Figure 9 The NiFe2O4 prepared in Comparative Example 1 and the Mn prepared in Examples 1-3 0.5 Ni 0.5 Fe2O4, Co 0.5 Ni 0.5 Fe2O4 and Cu 0.5 Ni 0.5 XRD patterns of Fe2O4 during 20 redox cycles (20 cycles of the methane chemical chain combustion process); (a) NiFe2O4; (b) Mn 0.5 Ni 0.5 Fe2O4; (c) is Co 0.5 Ni 0.5 Fe2O4; (d) is Cu 0.5 Ni 0.5 Fe2O4;

[0021] Figure 10 The NiFe2O4 prepared in Comparative Example 1 and the Mn prepared in Examples 1-3 0.5 Ni 0.5 Fe2O4, Co 0.5 Ni 0.5 Fe2O4 and Cu 0.5 Ni 0.5 Figure 1 shows the results of the CH4 pulse experiment on Fe2O4;

[0022] Figure 11 The NiFe2O4 prepared in Comparative Example 1 and the Mn prepared in Examples 1-3 0.5 Ni 0.5Fe2O4, Co 0.5 Ni 0.5 Fe2O4 and Cu 0.5 Ni 0.5 Schematic diagram of oxygen vacancy formation energy, lattice oxygen diffusion barrier and lattice oxygen migration in Fe2O4.

[0023] Figure 12 The NiFe2O4 prepared in Comparative Example 1 and the Mn prepared in Examples 1-3 0.5 Ni 0.5 Fe2O4, Co 0.5 Ni 0.5 Fe2O4 and Cu 0.5 Ni 0.5 H2-TPR spectra of Fe2O4 sample and sample after 20 redox cycles. Detailed Implementation

[0024] This invention relates to a transition metal-doped modified spinel oxygen carrier Mn 0.5 Ni 0.5 The preparation method of Fe2O4, taking 0.01 mol of the target product as an example, specifically includes the following steps:

[0025] (1) Weigh 1.255g Mn(NO3)2·4H2O, 1.454g Ni(NO3)2·6H2O and 8.080g Fe(NO3)3·9H2O according to the molar ratio of Mn:Ni:Fe=0.5:0.5:2, add them to 50mL of deionized water, stir at room temperature for 1h to form a homogeneous precursor solution;

[0026] (2) Add 6.30g of citric acid monohydrate and 1.86g of ethylene glycol to the above precursor solution, adjust the pH of the system to about 7.0 with ammonia, and then continue to stir and heat at 80°C for 2 hours to allow the metal ions to fully complex and form a fluid precursor sol.

[0027] (3) Add 0.20g of polyvinyl alcohol and 0.02g of sodium polyacrylate to the obtained precursor sol, and then add deionized water to adjust it into a sprayable precursor slurry (the solid content of the sprayable precursor slurry is 20wt%, which refers to the total mass fraction of all non-aqueous components in the precursor slurry). After stirring for 1 hour, spray granulation is performed using a dual-fluid nozzle with an atomization pressure of 0.2MPa, a spray drying inlet air temperature of 200℃, and an outlet air temperature of 100℃ to obtain precursor particles.

[0028] (4) The obtained precursor particles were dried at 110°C for 12 h, and then placed in a muffle furnace and held at 850°C for 4 h in air atmosphere to obtain spinel oxygen carrier Mn. 0.5 Ni0.5 Fe2O4.

[0029] The obtained samples were characterized by XRD, EPR, and XPS. Mn 0.5 Ni 0.5 Fe2O4 retains the spinel main phase and exhibits a more obvious peak shift and stronger oxygen vacancy correlation signal compared to undoped NiFe2O4; the proportion of surface adsorbed oxygen / defect-related oxygen components in the O 1s spectrum can reach 44.2%, indicating that Mn doping is more likely to induce lattice distortion and oxygen vacancy formation, thereby promoting lattice oxygen migration.

[0030] Example 2

[0031] This invention relates to a transition metal-doped modified spinel oxygen carrier, Co. 0.5 Ni 0.5 The preparation method of Fe2O4, taking 0.01 mol of the target product as an example, specifically includes the following steps:

[0032] (1) Weigh out 1.455g Co(NO3)2·6H2O, 1.454g Ni(NO3)2·6H2O and 8.080g Fe(NO3)3·9H2O according to the molar ratio of Co:Ni:Fe=0.5:0.5:2, add them to 50mL of deionized water, stir at room temperature for 1h to form a homogeneous precursor solution;

[0033] (2) Add 6.30g of citric acid monohydrate and 1.86g of ethylene glycol to the above precursor solution, adjust the pH of the system to about 7.0 with ammonia, and then continue to stir and heat at 80°C for 2 hours to allow the metal ions to fully complex and form a fluid precursor sol.

[0034] (3) Add 0.20g of polyvinyl alcohol and 0.02g of sodium polyacrylate to the obtained precursor sol, and then add deionized water to adjust it into a sprayable precursor slurry (the solid content of the sprayable precursor slurry is 20wt%). After stirring for 1 hour, spray granulation is performed using a dual-fluid nozzle. The atomization pressure is 0.2MPa, the spray drying inlet air temperature is 200℃, and the outlet air temperature is 100℃ to obtain precursor particles.

[0035] (4) The obtained precursor particles were dried at 110°C for 12 h, and then placed in a muffle furnace and held at 850°C for 4 h in air atmosphere to obtain spinel oxygen carrier Co. 0.5 Ni 0.5 Fe2O4.

[0036] The obtained samples were characterized by XRD, EPR, and XPS. 0.5 Ni 0.5Fe2O4 retains the spinel main phase, and the oxygen vacancy signal is enhanced compared with undoped NiFe2O4, indicating that Co doping does not significantly destroy the main framework of NiFe2O4 and can improve defect structure and cycle stability.

[0037] Example 3

[0038] This invention relates to a transition metal-doped modified spinel oxygen carrier Cu. 0.5 Ni 0.5 The preparation method of Fe2O4, taking 0.01 mol of the target product as an example, specifically includes the following steps:

[0039] (1) Weigh 1.208g Cu(NO3)2·3H2O, 1.454g Ni(NO3)2·6H2O and 8.080g Fe(NO3)3·9H2O according to the molar ratio of Cu:Ni:Fe=0.5:0.5:2, add them to 50mL of deionized water, stir at room temperature for 1h to form a homogeneous precursor solution;

[0040] (2) Add 6.30g of citric acid monohydrate and 1.86g of ethylene glycol to the above precursor solution, adjust the pH of the system to about 7.0 with ammonia, and then continue to stir and heat at 80°C for 2 hours to allow the metal ions to fully complex and form a fluid precursor sol.

[0041] (3) Add 0.20g of polyvinyl alcohol and 0.02g of sodium polyacrylate to the obtained precursor sol, and then add deionized water to adjust it into a sprayable precursor slurry (the solid content of the sprayable precursor slurry is 20wt%). After stirring for 1 hour, spray granulation is performed using a dual-fluid nozzle. The atomization pressure is 0.2MPa, the spray drying inlet air temperature is 200℃, and the outlet air temperature is 100℃ to obtain precursor particles.

[0042] (4) The obtained precursor particles were dried at 110°C for 12 h, and then placed in a muffle furnace and held at 850°C for 4 h in air atmosphere to obtain spinel oxygen carrier Cu. 0.5 Ni 0.5 Fe2O4.

[0043] pass Figure 1 It can be seen that Cu 0.5 Ni 0.5 Fe2O4 retains the spinel main phase; through Figure 4 It can be seen that the TEM-displayed particles are mainly composed of irregular nanoparticles of about 75~150 nm, and the interplanar spacings of about 0.21 nm and 0.26 nm in the HR-TEM can be attributed to the (400) and (311) crystal planes of the spinel structure, respectively; through Figure 5Elemental distribution results show that Ni, Cu, and Fe are uniformly distributed within the particles, indicating that Cu doping did not cause significant elemental segregation. Figure 3 It can be seen that the oxygen carrier particles prepared by the method of the present invention are uniformly dispersed and the doping elements do not undergo obvious segregation, indicating that the preparation process does not lead to severe phase separation or structural collapse.

[0044] Comparative Example 1

[0045] NiFe2O4 without transition metal doping was prepared by the same sol-gel combined spray granulation method as in Example 1, specifically: (1) 2.908g Ni(NO3)2·6H2O and 8.080g Fe(NO3)3·9H2O were weighed according to the molar ratio of Ni:Fe=1:2 and added to 50mL of deionized water. The mixture was stirred at room temperature for 1h to form a uniformly mixed precursor solution.

[0046] (2) Add 6.30g of citric acid monohydrate and 1.86g of ethylene glycol to the above precursor solution, adjust the pH of the system to about 7.0 with ammonia, and then continue to stir and heat at 80°C for 2 hours to allow the metal ions to fully complex and form a fluid precursor sol.

[0047] (3) Add 0.20g of polyvinyl alcohol and 0.02g of sodium polyacrylate to the obtained precursor sol, and then add deionized water to adjust it into a sprayable precursor slurry (the solid content of the sprayable precursor slurry is 20wt%). After stirring for 1 hour, spray granulation is performed using a dual-fluid nozzle. The atomization pressure is 0.2MPa, the spray drying inlet air temperature is 200℃, and the outlet air temperature is 100℃ to obtain precursor particles.

[0048] (4) The obtained precursor particles were dried at 110°C for 12 hours and then placed in a muffle furnace and kept at 850°C for 4 hours in an air atmosphere to obtain the oxygen carrier NiFe2O4.

[0049] The oxygen carriers prepared in Examples 1-3 and Comparative Example 1 were used to evaluate the performance of methane chemical looping combustion. A fixed-bed reactor was used in the experiment, with an oxygen carrier loading of 300 mg. The reducing gas was a CH4 / N2 mixture containing 5% vol CH4, and the reducing gas flow rate was 100 mL / min. After heating to the target temperature (850 °C) under a pure N2 atmosphere, the reducing gas was introduced for a reduction reaction for 15 min. Subsequently, air was introduced as the oxidant at a flow rate of 500 mL / min for oxidation and regeneration for 10 min. The cycle performance was tested at 850 °C, and 20 consecutive redox cycles were performed. The outlet gases CH4, H2, CO, and CO2 were analyzed online using gas chromatography and mass spectrometry.

[0050] The methane conversion rate XCH4 and CO2 selectivity SCO2 are calculated using the following formulas: XCH4=(mCH4,in-mCH4,out) / mCH4,in; SCO2=mCO2 / (mCO2+mCO), where mCH4,in and mCH4,out are the inlet and outlet molar flow rates of methane, respectively, and mCO2 and mCO are the outlet molar flow rates of CO2 and CO, respectively.

[0051] Table 1 shows the reactivity of the oxygen carriers prepared in Examples 1-3 and Comparative Example 1 in the chemical looping combustion of methane at 850°C.

[0052]

[0053] Comparative Example 2

[0054] The preparation method of Comparative Example 2 is the same as that of Example 3, except that in step (4), the calcination temperature is 700℃. Specifically, the obtained precursor particles are dried at 110℃ for 12 hours, and then placed in a muffle furnace and kept at 700℃ for 4 hours in an air atmosphere to obtain the oxygen carrier Cu. 0.5 Ni 0.5 Fe2O4.

[0055] At this temperature, the precursor decomposition and crystallization were insufficient, and the spinel characteristic peak intensity decreased and the peak shape broadened in the XRD, indicating that the crystal phase development was insufficient. Since the lattice oxygen migration channel had not been fully formed, the reactivity of the obtained oxygen carrier in the chemical chaining combustion of methane was much lower than that in Example 3.

[0056] Comparative Example 3

[0057] The preparation method of Comparative Example 3 is the same as that of Example 3, except that in step (4), the calcination temperature is 700℃. Specifically, the obtained precursor particles are dried at 110℃ for 12 hours, and then placed in a muffle furnace and kept at 950℃ for 4 hours in an air atmosphere to obtain the oxygen carrier Cu. 0.5 Ni 0.5 Fe2O4.

[0058] Although the oxygen at this temperature can form a spinel phase, high-temperature treatment can easily lead to grain growth, particle sintering, and a decrease in specific surface area. At the same time, it will weaken the surface defect and oxygen vacancy regulation effect, resulting in a decrease in methane conversion rate and CO2 selectivity compared to Example 3. Furthermore, the structure is prone to deterioration during the methane chemical loop combustion reaction cycle, thus reducing cycle stability.

[0059] Comparative Example 4

[0060] The preparation method of Comparative Example 4 is the same as that of Example 3, except that in step (2), the water bath heating temperature is 60°C. Specifically, 6.30g of citric acid monohydrate and 1.86g of ethylene glycol are added to the above precursor solution, the pH of the system is adjusted to about 7.0 with ammonia water, and then the mixture is stirred and heated at 60°C for 2 hours to allow the metal ions to fully complex and form a fluid precursor sol.

[0061] At this temperature, the complexation of metal ions such as Ni, Fe, and Cu by citric acid and the gelation process involving ethylene glycol proceed slowly, resulting in insufficient viscosity growth in the system, an incomplete precursor sol network, and decreased uniformity of multi-metal components in subsequent sprayed particles. This is not conducive to the formation of the spinel phase and weakens the regulation effect of oxygen vacancies.

[0062] Comparative Example 5

[0063] The preparation method of Comparative Example 5 is the same as that of Example 3, except that in step (2), the water bath heating temperature is 95°C. Specifically, 6.30g of citric acid monohydrate and 1.86g of ethylene glycol are added to the above precursor solution, the pH of the system is adjusted to about 7.0 using ammonia water, and then the mixture is stirred and heated at 95°C for 2 hours to allow the metal ions to fully complex and form a fluid precursor sol.

[0064] At this temperature, the solvent evaporates and concentrates too quickly, which can easily lead to local gelation or a rapid increase in viscosity. This results in poor dispersibility and decreased atomization stability of the spray slurry. The resulting particle size distribution becomes wider and local agglomeration may occur. Consequently, the structure of the oxygen carrier is prone to deterioration during the methane chemical loop combustion reaction cycle, thus reducing cycle stability.

[0065] Comparative Example 6

[0066] The preparation method of Comparative Example 6 is the same as that of Example 3, except that in step (2), the amount of citric acid monohydrate added is 3.15g and the amount of ethylene glycol added is 0.93g, that is, the total molar amount of metal ions: citric acid: ethylene glycol is 1:0.5:0.5 (molar ratio); specifically, 3.15g of citric acid monohydrate and 0.93g of ethylene glycol are added to the above precursor solution, the pH of the system is adjusted to about 7.0 with ammonia water, and then the mixture is stirred and heated at 80°C for 2h to allow the metal ions to fully complex and form a fluid precursor sol.

[0067] Comparative Example 7

[0068] The preparation method of Comparative Example 7 is the same as that of Example 3, except that in step (2), the amount of citric acid monohydrate added is 12.6g and the amount of ethylene glycol added is 3.72g, that is, the total molar amount of metal ions: citric acid: ethylene glycol is 1:2:2 (molar ratio); specifically, 12.6g of citric acid monohydrate and 3.72g of ethylene glycol are added to the above precursor solution, the pH of the system is adjusted to about 7.0 with ammonia water, and then the mixture is stirred and heated at 80°C for 2h to allow the metal ions to fully complex and form a fluid precursor sol.

[0069] Under these conditions, an excess of organic complex network leads to strong heat release and gas release during the decomposition of organic matter during calcination, which can easily cause particle cracking, uneven pore structure, or secondary agglomeration. At the same time, excessive organic matter may cause local reducing atmosphere and excessive formation of defects, resulting in decreased structural stability.

[0070] The oxygen carriers prepared in Comparative Examples 2-7 were used to evaluate the performance of methane chemical looping combustion (the experimental procedure was the same as in Examples 1-3). A fixed-bed reactor was used in the experiments, with an oxygen carrier loading of 300 mg; the reducing gas was a CH4 / N2 mixture containing 5% vol CH4, and the reducing gas flow rate was 100 mL / min; after heating to the target temperature (850 °C) under a pure N2 atmosphere, the reducing gas was introduced for a reduction reaction for 15 min; the outlet gases CH4, H2, CO, and CO2 were analyzed online using gas chromatography and mass spectrometry.

[0071] Table 2 shows the reactivity of the oxygen carriers prepared in Comparative Examples 2-7 during methane chemical looping combustion at 850℃.

[0072]

[0073] Figure 1 The XRD patterns showed that NiFe2O4 and Co, Cu, and Mn-doped samples all exhibited spinel phase characteristic diffraction peaks, including the (220), (311), (400), (422), (511), and (440) crystal planes, and no obvious impurity phases were detected, indicating that Co, Cu, or Mn can enter the NiFe2O4 lattice and maintain the spinel main structure. After doping, some diffraction peaks shifted to lower angles, indicating that the lattice expanded or distorted, which is related to the difference in the radius of the dopant ions and the formation of oxygen vacancies. Figure 2 The EPR spectrum shows an oxygen vacancy-related signal at g≈2.003, and the signal intensity follows the Mn... 0.5 Ni 0.5 Fe2O4> Cu 0.5 Ni 0.5 Fe2O4>Co 0.5 Ni 0.5 The increasing sequence of Fe2O4 indicates that transition metal doping can increase the oxygen vacancy concentration. Figure 6In the O 1s XPS spectrum, lattice oxygen (OI) is located at approximately 529.7 eV, surface adsorbed oxygen / defect-related oxygen (OII) is located at approximately 531.5 eV, and surface hydroxyl or adsorbed species (OIII) is located at approximately 532.9 eV; the proportion of OII is increased in the doped sample, among which Mn 0.5 Ni 0.5 The OII ratio of Fe2O4 can reach 44.2%, further proving that doping induces the formation of oxygen vacancies. Figure 7 In the Ni 2p XPS spectrum, Ni 2+ and Ni 3+ The peaks are located at approximately 856.0 eV and 861.3 eV, respectively, after Ni doping. 3+ The increased proportion indicates that charge compensation caused by doping promotes Ni 2+ / Ni 3+ It transforms and enhances electron transfer capabilities.

[0074] Figure 10 CH4 pulse experiments showed that, under conditions of no external oxygen replenishment, the Mn-doped sample exhibited the most significant recovery after extending the pulse interval, indicating that it possessed the strongest ability to replenish bulk lattice oxygen to the surface; the overall order of lattice oxygen migration ability was Mn 0.5 Ni 0.5 Fe2O4> Cu 0.5 Ni 0.5 Fe2O4>Co 0.5 Ni 0.5 Fe₂O₄ > NiFe₂O₄. DFT calculations show that the oxygen vacancy formation energy and the lattice oxygen diffusion barrier both exhibit a trend of undoped > Co-doped > Cu-doped > Mn-doped, with the Mn-doped sample showing a reduction in oxygen vacancy formation energy and oxygen diffusion barrier to 2.13 eV and 1.42 eV, respectively. These results indicate that doping can lower the energy barriers for oxygen vacancy formation and lattice oxygen migration, thereby improving the reactivity of methane chemical looping combustion.

[0075] Figure 12 The H2-TPR results showed that the main reduction peak of the freshly doped sample shifted towards lower temperatures compared to NiFe2O4, indicating that doping improved the reducibility of the oxygen carrier. After 20 cycles, the reduction peaks of all samples shifted towards higher temperatures, indicating that the reduction difficulty increased during the cycling process. The peak shift was most significant in the Mn-doped sample, indicating that its high oxygen vacancy concentration brought strong initial activity but also accelerated structural degradation. The peak shift was smaller in the Cu-doped sample, indicating that it had better redox reversibility and structural stability. Figure 9 The cyclic XRD results further show that after 20 cycles, the relative spinel phase retention rates of the undoped, Co-doped, Cu-doped, and Mn-doped samples are approximately 72.3%, 68.9%, 64.1%, and 53.3%, respectively.

[0076] pass Figure 11 It is known that transition metal doping can lower the oxygen vacancy formation energy and the lattice oxygen diffusion barrier in NiFe2O4. NiFe2O4, Co 0.5 Ni 0.5 Fe2O4, Cu 0.5 Ni 0.5 Fe2O4, Mn 0.5 Ni 0.5 The oxygen vacancy formation energy of Fe2O4 decreases sequentially, and the bulk oxygen diffusion barrier also shows the same trend. This indicates that after Co, Cu and Mn partially replace Ni sites, they can weaken some metal-oxygen bonds by adjusting the local electronic structure and lattice distortion, thereby promoting the formation of surface oxygen vacancies and the migration of bulk lattice oxygen to the surface.

Claims

1. A transition metal-doped modified spinel oxygen carrier, characterized in that: Its chemical formula is M x Ni 1-x Fe2O4; where M is one of Co, Cu or Mn; x takes the value of 0.1 to 0.

9.

2. The transition metal-doped modified spinel oxygen carrier according to claim 1, characterized in that: x takes values ​​from 0.3 to 0.

7.

3. The method for preparing the spinel oxygen carrier according to claim 1, characterized in that, Includes the following steps: (1) Weigh nickel salt, iron salt and transition metal salt according to stoichiometric ratio, dissolve the nickel salt, iron salt and transition metal salt in deionized water, add complexing agent and adjust the pH of the system, stir and heat to make the metal ions undergo complexation and gelation reaction to obtain precursor sol; (2) Add binder and dispersant to the precursor sol, and then add deionized water to adjust the solid content to obtain a sprayable precursor slurry; (3) Spray granulation is performed on the sprayable precursor slurry to obtain precursor particles; (4) After drying the precursor particles, they are calcined in an air atmosphere to obtain a transition metal-doped modified spinel oxygen carrier M. x Ni 1-x Fe2O4.

4. The preparation method according to claim 3, characterized in that: In step (1), the nickel salt is nickel nitrate, nickel acetate, or nickel chloride; the iron salt is ferric nitrate, ferric chloride, or ferric acetate; and the transition metal salt is one of cobalt nitrate, copper nitrate, or manganese nitrate.

5. The preparation method according to claim 3, characterized in that: In step (1), the complexing agent is citric acid, and ethylene glycol is also added as a gelling aid in step (1); wherein, the molar ratio of the total molar amount of metal ions to citric acid is 1:0.5~2, and the molar ratio of ethylene glycol to citric acid is 0.5~2:

1.

6. The preparation method according to claim 3, characterized in that: In step (1), the heating and stirring are carried out by water bath heating, with a water bath heating temperature of 60~90℃ and a heating time of 1~3h.

7. The preparation method according to claim 3, characterized in that: In step (1), the pH of the system is 6-8.

8. The preparation method according to claim 3, characterized in that: In step (2), the binder is polyvinyl alcohol, the dispersant is sodium polyacrylate, and the solid content of the sprayable precursor slurry is 10~30wt%.

9. The preparation method according to claim 3, characterized in that: In step (3), the spray granulation uses a dual-fluid nozzle with an atomization pressure of 0.1~0.4MPa, an inlet air temperature of 150~230℃, and an outlet air temperature of 80~120℃.

10. The preparation method according to claim 3, characterized in that: In step (4), the drying temperature is 90~130℃ and the drying time is 6~18h; the calcination temperature is 700~950℃ and the calcination time is 2~8h.

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