SYNTHESIS OF Zn-Fe BIMETALLIC ZIFs AND STUDY OF THE PHOTOCATALYTIC NITROGEN FIXATION PERFORMANCE

Abstract

In recent years, in-depth studies of zeolitic imidazolate framework-8 (ZIF-8) materials have revealed that they have many unique properties and uses. Zn-Fe bimetallic ZIFs with bimetallic active centers can be designed and fabricated based on ZIF-8 with good water resistance. The structure, composition, and photoelectrochemical properties of the Zn-Fe bimetallic ZIFs were systematically analyzed by field emission scanning electron microscopy (FESEM), mapping, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT IR), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), and photoluminescence (PL). The bimetallic Zn-Fe ZIFs exhibited strong light absorption properties in the visible region at 390 nm, and the forbidden bandwidths were greatly reduced compared to those of ZIF-8. The photoelectrochemical properties of bimetallic Zn-Fe ZIFs and the separation and migration efficiency of photogenerated charge carriers can be optimized by adjusting the amount of metal introduced. The highest average photocatalytic rate of 1363.53 μmol L^-1 g^-1 h^-1 was observed for bimetallic Zn-Fe ZIFs when Zn:Fe = 1:0.3. Finally, the photocatalytic nitrogen fixation performance of Zn-Fe bimetallic ZIFs is discussed.

Keywords: Zn-Fe bimetallic ZIFs; structural characterization; photoelectrochemical properties; photocatalytic nitrogen fixation.

INTRODUCTION

Ammonia, the second most consumed basic chemical after sulfuric acid, is widely used in agriculture, industry and household chemicals.¹-⁴ Eighty percent of ammonia produced worldwide is used in the production of nitrogen fertilizers such as urea, ammonium nitrate, ammonium sulfate, and ammonium hydrogen phosphate.² In addition, with its high hydrogen content (hydrogen density of 17.6 wt.%) and energy density (3 kWh kg^-1), the ability to store hydrogen at room temperature and the fact that no CO₂ is produced when hydrogen is released,⁵-⁸ ammonia has been recognized as a potential substitute for fossil fuels in recent years. The traditional production process for ammonia is the Haber-Bosch (H-B) process (350-550 °C, 15-35 MPa) using iron-based catalysts. Currently, the synthesis capacity of ammonia based on this process is 220 million tons per year (Mt y^-1), and the production capacity is 170 Mt y^-1 worldwide,⁹ but the high temperature and pressure in the operating environment and the low conversion rate have prompted researchers to look for a green production process.

Metal-organic frameworks (MOFs) are novel crystalline porous materials composed of metal nodes and organic connectors.¹⁰ Due to their large specific surface area, structural diversity and tunable nanopore shape and size, MOF materials have attracted great research interest in photocatalysis.¹¹-¹³ However, MOFs also have drawbacks such as poor structural stability and low electrical conductivity, making them difficult to apply in a wider range of applications. In recent years, researchers have synthesized numerous MOF derivatives that inherit the advantages of MOFs and consider the conductivity and stability of the materials,¹⁴-¹⁶ which has led to further applications within photocatalysis, which include the decomposition of water for hydrogen production,¹⁷,¹⁸ photocatalytic CO₂ reduction,¹⁹,²⁰ the photocatalytic degradation of organic pollutants²¹,²² and photocatalytic organic synthesis.²³,²⁴ MOFs also have obvious advantages in photocatalytic nitrogen reduction reactions. The large specific surface area and porous structure of MOFs are conducive to the adsorption of N₂ molecules, and at the same time, MOFs polarize the adsorbed N₂ molecules through their unique ligand-electron transfer process, which results in the activation of N₂ molecules; ultimately, they can achieve highly efficient photocatalytic nitrogen fixation performance.²⁵ Among MOFs, there is a class of ZIF (zeolitic imidazolate framework) materials with aqueous stability and tunable macroporous structures, which is favorable for photocatalytic applications, so bimetallic MOFs containing two different valence states or different kinds of metals and organic ligands coordinated to generate bimetallic MOFs are receiving increasing attention in photocatalytic nitrogen fixation. Zhao et al. ²⁶ designed and synthesized MIL-53 (Fe^II/Fe), a photocatalyst with a metal-organic skeleton and a photocatalytic nitrogen fixation rate of up to 306 μmol g^-1 h^-1, which is more than ten times that of the same type of photocatalyst. An et al. ²⁷ were inspired by the bimetallic active center in MoFe proteins in nitrogen fixation enzymes and designed and synthesized bimetallic Zr-Hf MOFs based on Uio-66, with an average photocatalytic nitrogen fixation performance of up to 116 μmol g^-1 h^-1, which is a substantial improvement over that of Uio-66. ZIF-8, composed of Zn²⁺ and 2-methylimidazole, is a more typical material for ZIFs and has been repeatedly reported in photocatalysis. However, ZIF-8 itself has a high forbidden bandwidth, which allows it to be excited only by high-energy light, severely limiting its application as an efficient nitrogen fixation photocatalyst.²⁸,²⁹ Therefore, in this work, bimetallic ZIFs of Zn-Fe were prepared by introducing the transition metal element Fe into the coordination synthesis of Zn²⁺ and 2-methylimidazole through simple temperature and time modulation to reduce the forbidden bandwidth of the ZIFs, increase the light absorption and utilization of the ZIFs, improve the photogenerated charge separation of the ZIF transport rate, and finally enhance the photocatalytic nitrogen fixation performance of the ZIFs.

EXPERIMENTAL

Synthesis of bimetallic Zn-Fe ZIFs

Bimetallic Zn-Fe ZIFs were synthesized by a one-step solvothermal method. The individual steps were as follows: 70 mL of methanol was added to a 250 mL beaker, and 3.30 g of 2-methylimidazole was subsequently weighed and dissolved with stirring, followed by the addition of 0.60 g of ferric nitrate hydrate (Fe(NO₃)₃.9H₂O) under ultrasonic dispersion conditions, 0.50 g of polyvinylpyrrolidone (PVP), and 1.10 g of zinc acetate dihydrate (Zn(CH₃COO)₂.2H₂O). After being sonicated for 10 min, the mixture was transferred to a 100 mL polytetrafluoroethylene lined autoclave and reacted for 3 h at 180 °C in an electrically heated constant-temperature drying oven with a programmed temperature increase rate of 5 °C min^-1. After cooling at the end of the reaction, the samples were washed three times by centrifugation with methanol at a speed of 3000 r min^-1 for 3 min and finally immersed in methanol solution. This sample was designated Zn-Fe-ZIF (1:0.3), the content of ferric nitrate (Fe(NO₃)₃.9H₂O) hydrate was adjusted to 0.20, 0.40 and 0.80 g, and the other conditions remained unchanged. The resulting samples were designated Zn Fe ZIF (1:0.1), Zn Fe ZIF (1:0.2) and Zn-Fe-ZIF (1:0.4).

Characterization method

The morphology of the samples was characterized using a field emission scanning electron microscope (SU8010, Hitachi). During the test, conductive adhesive was spread on a copper electroscope plate, and then, the prepared sample was spread, sprayed with gold and then tested at a voltage of 5 kV.

The types and distribution of elements in the samples were analyzed by elemental distribution (mapping) performed on an energy spectrometer with a field emission scanning electron microscope (SU8010, Hitachi) at a test voltage of 15 kV.

The structure and crystal type of the samples were determined using an X-ray powder diffractometer (Bruker D8Advance) with Cu Kα radiation (λ = 1.5406 Å) and a power of 3 kW. The tests were carried out with the samples placed flat in the grooves of the XRD test plate at a test voltage of 45 kV, a current of 40 mA, 2θ = 5-80° and a duration of 4 min.

The structure of the samples and the type of chemical bonding were analyzed using a Fourier transform infrared spectrometer (FTS 40, Bio-Rad). The samples and dried KBr were thoroughly ground under tungsten light and tested with a tablet press after pressing the samples.

Beam photoelectron spectroscopy (Escalab 250Xi, Thermo Scientific) was used to analyze the elemental distribution and valence states on the sample surface. During the test, the conductive adhesive was glued to the center of aluminum foil, and then, the sample was placed on the conductive adhesive and completely encapsulated by folding the aluminum foil several times.

The light absorption of the samples was determined using a UV Vis diffuse reflectance spectrometer (Lambda 950, PerkinElmer). The light absorption properties of the samples were tested by thoroughly grinding 20 mg of the sample and 200 mg of barium sulfate and using barium sulfate as a reference.

The photoluminescence (PL) spectra of the samples were obtained using a stationary/transient fluorescence spectrometer (FLS980, Shimadzu). The samples were placed on a quartz sample holder and tested using a 300 W xenon lamp as the excitation light source and an appropriate slit width.

An amount of 20 mg of the sample and 10 μL of the Nafion solution were placed in 500 μL of anhydrous ethanol and dispersed ultrasonically for 1 h. Then, the sample was uniformly coated on conductive glass (FTO), and after natural drying, the photocurrent transient curve, Mott-Schottky curve and electrochemical impedance spectra were measured using a standard three-electrode system on an electrochemical workstation (Shanghai Chenhua CHI660E). The working electrode was the prepared ZIF material, the reference electrode was an Ag/AgCl electrode, the counter electrode was a Pt metal foil, and the electrolyte was a solution of 0.5 mol L^-1 Na₂SO₄.

Quantification of ammonia nitrogen

In this experiment, the salicylic acid spectrophotometric method was used to determine the photocatalytic production of ammonia as follows:

(i) Preparation of the color developer

Preparation of the salicylic acid-potassium sodium tartrate solution: 5 g of salicylic acid was weighed, 10 mL of water was added, 16 mL of 2 mol L^-1 NaOH solution was added, and the mixture was stirred so that it dissolved completely. Then, 5 g of potassium sodium tartrate dissolved in an appropriate amount of water was weighed, and the mixture was heated in a 100 °C water bath for 30 min. The mixture was cooled and mixed with the above solution and transferred to a 100 mL volumetric flask after the capacity of the bottle was adjusted, after which the mixture was placed in an amber glass bottle for storage.

Preparation of the sodium nitrosoferricyanide solution: first, 0.1 g of sodium nitrosoferricyanide was weighed and dissolved in 10 mL of water.

Preparation of the sodium hypochlorite solution: first, the effective chlorine content and free alkali content in the original sodium hypochlorite solution were calibrated, the calibrated sodium hypochlorite solution was added, an appropriate amount of water and 2 mol L^-1 NaOH solution was added, the solution was diluted to a sodium hypochlorite solution with an effective chlorine concentration of 3.5 g L^-1 and a free alkali concentration of 0.75 mol L^-1. Then, the solution was placed in an amber glass bottle for storage.

(ii) Preparation of the ammonia-nitrogen standard solution

Preparation of the ammonia-nitrogen standard storage solution: a total of 3.819 g of NH₄Cl was weighed, dried at 100 °C for 2 h, and dissolved in water. The solution was transferred to a 1000 mL volumetric flask and then stored in a fixed volume.

Preparation of the ammonia-nitrogen standard intermediate solution: first, 10.00 mL of the ammonia-nitrogen standard stock solution was pipetted into a 100 mL volumetric flask and stored after calibration.

Preparation of the ammonia-nitrogen standard solution: first, 1.00 mL of the ammonia-nitrogen standard intermediate solution was pipetted into a 100 mL volumetric flask and maintained at a constant volume.

Preparation of the ammonia-nitrogen standard concentration solution: a certain amount of the ammonia-nitrogen standard solution was pipetted into a 10 mL colorimetric tube at a time, and ammonia-nitrogen standard solutions of 0.025, 0.05, 0.10, 0.15, and 0.20 μg mL^-1 were prepared.

(iii) Measurement of the ammonia-nitrogen standard curve

A total of 4 mL of the prepared ammonia-nitrogen standard solution was added to a sealed centrifuge tube, 500 μL of sodium hypochlorite-potassium tartrate solution and 50 μL of sodium nitrosoferricyanide solution were added, the mixture was shaken well, 45-50 μL of sodium hypochlorite solution was added, and the absorbance was measured at 697 nm after 1 h of color development.

Figure 1a shows the UV-Vis absorbance of different concentrations of the ammonia-nitrogen standard solution. The ammonia-nitrogen standard solution has a maximum absorbance peak at approximately 697 nm after color development, and the intensity of the absorbance peak and the concentration of the ammonia-nitrogen standard solution are linearly related. The standard curve for ammonia-nitrogen was plotted according to the relationship between the absorbance and concentration of the ammonia-nitrogen standard solution, as shown in Figure 1b, and the equation of the standard curve was Y = 0.97293X + 0.00079, where R² = 0.99947.

Figure 1
(a) UV-Vis absorption of the ammonia-nitrogen standard solution, (b) standard curve

Photocatalytic nitrogen fixation performance test

Due to the low solubility of nitrogen in water, the catalyst in a conventional photocatalytic reactor can only adsorb and activate a small amount of N₂ molecules dissolved in the liquid phase, which undoubtedly leads to a lower activation efficiency of nitrogen adsorption and a lower catalytic rate. To solve this problem, we studied the industrial absorber tower model in detail and adopted the design of the industrial absorber tower, in which the gas and liquid phases are efficiently brought into contact, to develop a novel photocatalytic nitrogen fixation reactor. As shown in Figure 2, this reactor is a three-way three-phase reaction device (gas-liquid-solid). During photocatalysis, ultrapure water saturated with N₂ circulates from the upper vessel to the lower vessel, nitrogen circulates from the lower vessel to the upper vessel, and the catalyst is located in the middle part of the reactor, which is the center of catalytic activity. The new photocatalytic nitrogen fixation reactor that we developed solves the problem of the low solubility of nitrogen molecules in water during photocatalysis.

Figure 2
Schematic diagram of the photocatalytic nitrogen fixation device

During the photocatalytic performance test, 20 mg of catalyst was typically added to the center of the reactor while nitrogen was passed through the transverse port on one side of the catalyst. An amount of 100 mL of N₂-saturated ultrapure water was added to the top vessel and passed through the catalytic activity center in the middle of the reactor at a flow rate of 0.083 mL min^-1. The NH₃ produced by the photocatalytic reaction dissolved rapidly in water to form NH₄ ⁺, which was then transferred to the bottom vessel. A total of 4 mL was removed from the lower vessel every hour. The active middle part of the reactor was irradiated with a 300 W xenon lamp, and the ammonia content of the samples was determined by salicylic acid spectrophotometry.

RESULTS AND DISCUSSION

Characterization of the physicochemical properties of Zn-Fe-ZIF

In this experiment, the morphology and size of the fabricated bimetallic Zn-Fe ZIFs were analyzed by field emission scanning electron microscopy (FESEM) (see Figure 3), which revealed that there was a great difference in the morphologies of the bimetallic Zn-Fe ZIFs when the Fe contents were different. As shown in Figures 3a-3d, when the Fe content is low, the prepared bimetallic Zn-Fe ZIFs exhibit the morphology of several hexagonal prismatic octahedral agglomerates, in which the diameter of the Zn Fe ZIF sample (1:0.1) is relatively smaller and the agglomerates are closer together. In the Zn-Fe-ZIF (1:0.2) sample, relatively larger crystalline particles appear, and the regularity of the crystals is relatively better. As shown in Figures 3c-3f, the prepared Zn-Fe bimetallic ZIFs exhibit a relatively dispersed hexagonal prismatic octahedral morphology with a relatively uniform dispersion and a relatively small variation in the diameter of the samples when the Fe content is high. Furthermore, analyzing and comparing the degree of dispersion and the size of the four sample groups shows that, when the Fe content was relatively low, the dispersion of the bimetallic Zn-Fe-ZIF was relatively poor, and its diameter was relatively small. As the Fe content increased, the dispersion of the samples improved, and the diameter tended to increase. At Zn:Fe = 1:0.3, the resulting crystals were almost completely separated, and the size became more homogeneous.

Figure 3
FESEM images of the synthesized bimetallic Zn-Fe-ZIF samples: (a,b) Zn-Fe-ZIF (1:0.1); (c,d) Zn-Fe-ZIF (1:0.2); (e,f) Zn-Fe-ZIF (1:0.3); (g,h) Zn-Fe-ZIF (1:0.4)

To study and analyze the elemental species and distribution within the bimetallic Zn-Fe-ZIF, an elemental distribution test was carried out. Figure 4 shows the elemental distribution map of the prepared Zn-Fe-ZIF (1:0.2) sample, from which it can be determined that the sample contains C, O, N, Zn, and Fe. The concentration of C and O is high due to the presence of C and O in the conductive adhesive, and the N and Zn dispersed in the sample surface are more uniform because the Fe is less common and may be dispersed in the interior of the sample. Thus, the mapping diagram of the sample surface shows that Fe is less abundant but can still be seen in the distribution.

Figure 4
Elemental characteristics of the Zn-Fe-ZIF (1:0.2) sample (red: C; blue: O; green: N; violet: Zn; yellow: Fe)

The XRD pattern of the bimetallic Zn-Fe-ZIF is shown in Figure 5. The analysis shows that the XRD peaks of the bimetallic Zn-Fe ZIFs are almost the same as those of ZIF-8, indicating that the structures and crystal types of the bimetallic Zn-Fe ZIFs with different Fe contents are similar to those of ZIF-8; the diffraction peaks at 2θ = 7.3, 10.4, 12.7, 14.6, 16.4, 18.1, 22.0, 24.5, 26.6, and 29.6° for the bimetallic Zn-Fe ZIFs are stronger and correspond to the (110), (002), (112), (022), (013), (222), (114), (233), (015), and (044) crystal faces, respectively.³⁰ Notably, the intensity of the Zn Fe ZIF (1:0.4) diffraction peak is lower compared to that of the other samples, indicating that the crystallinity of the bimetallic Zn-Fe-ZIF decreases when the content of Fe is increased too much.

Figure 5
XRD patterns of bimetallic Zn-Fe ZIFs

To further analyze the structure and type of chemical bonding of different bimetallic Zn-Fe ZIFs, we performed Fourier transform infrared (FTIR) spectroscopy. As shown in Figure 6, the absorption bands of bimetallic Zn-Fe ZIFs are almost the same as those of ZIF 8, indicating that the introduction of different amounts of Fe hardly changes the structure of the ZIFs. In the infrared spectra of bimetallic Zn-Fe ZIFs, the absorption bands at 3131 and 2929 cm^-1 are from the asymmetric telescopic vibration of the C‒H bond, the absorption peak at 1582 cm^-1 is caused by the stretching vibration of C=C, the band at 1419 cm^-1 is attributed to the stretching vibration of the C‒O bond, the peak at 1310 cm^-1 is attributed to the stretching vibration of C‒H, the peak at 1145 cm^-1 is from the stretching vibration of C‒N, the peak at 1031 cm^-1 is attributed to the in-plane bending vibration of C‒H, the peak at 995 cm^-1 is caused by the bending vibration of C‒N, the peak at 760 cm^-1 is attributed to the bending mode of the C‒H bond, the peak at 693 cm^-1 is attributed to the out-of-plane bending vibration of the 2-methylimidazole ring, and the absorption peak at 419 cm^-1 is attributed to the chemical bonding of Zn²⁺, Fe³⁺ and 2-methylimidazole. Combined with the XRD pattern shown in Figure 5, these results demonstrate that the bimetallic Zn-Fe ZIFs synthesized by the solvothermal method have a stable structure similar to that of ZIF-8.

Figure 6
FTIR spectra of bimetallic Zn-Fe ZIFs

Figure 7a shows the UV-Vis diffuse reflectance spectra of different bimetallic Zn-Fe ZIFs, which show that the Zn Fe ZIFs have strong absorption peaks near 210 and 390 nm. The stronger absorption peak near 210 nm is caused by the migration of photogenerated carriers of 2-methylimidazole in the sample, and the fringe of the absorption peak at 390 nm can reach up to 675 nm, which suggests that all the bimetallic Zn-Fe ZIFs have strong visible light absorption. Figure 7b shows the Kubelka-Munk transformation function of Zn Fe ZIFs versus photon energy, from which the forbidden bandwidths of Zn Fe-ZIF (1:0.1), Zn Fe ZIF (1:0.2), Zn-Fe-ZIF (1:0.3), and Zn Fe ZIF (1:0.4) are 2.67, 2.50, 2.42, and 2.54 eV, respectively. Compared with the other samples, Zn Fe ZIF (1:0.3) has a relatively higher light absorption capacity and a lower forbidden bandwidth (only 2.42 eV).³¹

Figure 7
(a) UV-Vis diffuse reflectance spectra, (b) Kubelka-Munk transformation function versus photon energy for bimetallic Zn-Fe ZIFs

Figure 8 shows the steady-state photoluminescence (SSPL) spectra of bimetallic Zn-Fe ZIFs with different Fe contents. All the prepared bimetallic Zn-Fe ZIFs have excitation wavelengths of approximately 460 nm and emission wavelengths of approximately 575 nm. The figure shows that Zn-Fe-ZIF (1:0.3) has the lowest fluorescence intensity, indicating that it has the lowest fluorescence performance. An analysis and comparison of the fluorescence intensities of the other samples indicated that the photogenerated carriers of Zn-Fe-ZIF (1:0.3) had the highest migration rate and the lowest complexation rate, and the photocatalytic nitrogen fixation performance of this sample was likely the best.

Figure 8
Photoluminescence spectra of bimetallic Zn-Fe ZIFs

To further analyze the separation rate of photogenerated electron-hole pairs in bimetallic Zn-Fe ZIFs with different Fe contents, transient photocurrent response curves were obtained. Figure 9 shows that under light conditions, compared with similar types of materials in the literature,³²,³³ all bimetallic Zn-Fe ZIFs have higher current densities, indicating that they can all produce a larger number of photogenerated carriers; among them, the current density of Zn-Fe-ZIF (1:0.3) is the highest, 0.63 μA cm^-2, which, in combination with the UV-Vis diffuse reflectance spectra and steady-state photoluminescence spectra of this sample, demonstrated that it can produce a larger number of photogenerated carriers, and the carriers have the highest migration rate and the lowest compounding rate. In addition, the electrochemical impedance spectra of the different bimetallic Zn-Fe ZIFs show that the conductivity strength of the samples almost matches the photocurrent response strength, and Zn-Fe-ZIF (1:0.3) has a smaller arc radius, indicating that it is more favorable for carrier transport.

Figure 9
(a) Photocurrent response curve, (b) electrochemical impedance spectra of bimetallic Zn-Fe ZIFs

To study the flat band potential and reduction potential of bimetallic Zn-Fe ZIFs, we performed Mott-Schottky curve tests. As shown in Figure 10, the 1 C^-2 versus potential curves of the different bimetallic Zn-Fe ZIFs have positive slopes, indicating that all they have n-type semiconductor properties. The linear part of the curve is extrapolated to the horizontal coordinate, and its intersection point is the conduction band potential (Ecb). With increasing Fe content, the flat band potentials (Ecb) of the bimetallic Zn-Fe ZIFs are -0.53, -0.57, -0.60, and -0.55 eV (vs. Ag/AgCl). It can be concluded that all the prepared bimetallic Zn-Fe ZIFs with different Fe contents have suitable N₂ reduction conduction band potentials. In addition, the Ecb value of Zn-Fe-ZIF (1:0.3) is -0.60 eV, which is higher than that all of the other samples, indicating that, compared with the other samples, it has a higher photogenerated electron energy and a relatively higher N₂ reduction capacity.

Figure 10
Mott-Schottky curves of bimetallic Zn-Fe ZIFs: (a) Zn-Fe-ZIF (1:0.1), (b) Zn-Fe-ZIF (1:0.2), (c) Zn-Fe-ZIF (1:0.3), (d) Zn-Fe-ZIF (1:0.4)

As shown in Figure 11, the energy band structures of different bimetallic Zn-Fe ZIFs are arranged. The forbidden bandwidth (Eg) of the bimetallic Zn-Fe ZIFs is derived from the plot of the Kubelka-Munk transformation function versus the photon energy, the conduction band potential (Ecb) is obtained according to the Mott-Schottky curves, and the corresponding valence band potential (Evb) is derived according to the empirical formula Evb = Eg + Ecb, which can lead to the electronic energy band structure of different samples. The electronic energy band structure of different samples can be obtained. As shown in Figure 11, the conduction band potentials (Ecb) of all the different bimetallic Zn Fe ZIFs were greater than the reduction potentials (vs. NHE) of N₂/NH₃, indicating that they possessed energy band structures suitable for nitrogen fixation. Among all the samples, Zn Fe ZIF (1:0.3) has the smallest forbidden bandwidth and the most negative conduction band potential, so it is the most favorable for photocatalytic nitrogen fixation.

Figure 11
Band structure arrangement of bimetallic Zn-Fe ZIFs

Photocatalytic nitrogen fixation performance and mechanism analysis of Zn-Fe-ZIF

Figures 12a and 12b show the graphs of the photocatalytic nitrogen fixation rates and compares the average rates of bimetallic Zn-Fe ZIFs, from which it can be determined that the photocatalytic nitrogen fixation performance of the bimetallic Zn-Fe ZIFs obviously changes with increasing Fe content, the overall tendency is to increase first and then decrease, and the decrease in the photocatalytic rate when there is too much Fe may be due to the limited coordination binding capacity of Fe³⁺ and 2-methylimidazole, and the introduction of too much Fe may decrease the crystallinity and stability of the samples and change the photoelectrochemical properties. The photocatalytic rate of the samples could be significantly enhanced by modulating the ratio of Fe. Zn-Fe-ZIF (1:0.3) had the greatest effect, and the ammonia content in the system reached 6817.64 μmol L^-1 g^-1 after 5 h of photocatalysis, with an average ammonia production rate of 1363.53 μmol L^-1 g^-1 h^-1.

Figure 12
(a) Photocatalytic nitrogen fixation rate diagram, (b) comparison of the average rates of bimetallic Zn-Fe ZIFs

To analyze the stability of bimetallic Zn-Fe ZIFs during photocatalysis, the samples after photocatalysis were structurally and morphologically characterized in this experiment. Figures 13a and 13b show the XRD patterns and FESEM images of Zn-Fe-ZIF (1:0.3) after photocatalysis, respectively. The positions and intensities of the XRD diffraction peaks of the samples after photocatalysis were almost the same as those of the original samples, indicating that they could maintain a stable structure during the catalytic process; moreover, the morphology of the samples after photocatalysis was still a hexagonal prismatic octahedron with improved dispersion, which indicated that the morphology could be maintained in a stable manner during catalysis. The above analysis demonstrates that Zn-Fe-ZIF (1:0.3) has good stability during photocatalysis.³⁴

Figure 13
(a) XRD patterns, (b) FESEM images of the Zn-Fe-ZIF (1:0.3) photocatalysts

CONCLUSIONS

The transition metal element Fe was selected for introduction into ZIF-8, bimetallic Zn-Fe sites were successfully constructed in ZIF-8 by a one-step solvothermal method, and the differences in morphology, structure, composition, photoelectrochemical properties, and photocatalytic nitrogen fixation performance of the bimetallic Zn Fe ZIFs were investigated with changes in the Fe content. Bimetallic Zn-Fe ZIFs with different Fe contents were prepared using a simple time and temperature modulation strategy, and the samples exhibited multiple hexagonal prismatic octahedral agglomerates when the Fe content was low. The dispersion of the samples improved, and the crystal size increased when the Fe content increased. All the bimetallic Zn-Fe ZIFs had stable structures and crystal types similar to those of ZIF-8. The bimetallic Zn-Fe ZIFs had good light-absorbing ability in the visible region near 390 nm, the number of photogenerated carriers and the rates of segregation and migration in the samples varied with the Fe content, and the corresponding photoelectrochemical properties could be optimized by adjusting the Fe content. By analyzing the photocatalytic nitrogen fixation performance of the samples, it can be concluded that Zn Fe ZIF (1:0.3) has the highest rate of photocatalytic nitrogen fixation, while the sample has good stability. This is attributed to the fact that Zn-Sn-ZIF (1:0.3) has a stronger light-absorbing ability, is a more suitable energy band structure for producing a larger number of photogenerated carriers, and produces photogenerated carriers with the highest detachment migration rate and the lowest complexation rate.³⁵,³⁶

ACKNOWLEDGMENTS

We are grateful to Yunxuan Liang, Institute of Natural Sciences, Chinese Academy of Sciences for providing experimental suggestions.

SUPPLEMENTARY MATERIAL

Complementary material for this work is available at http://quimicanova.sbq.org.br/, as a PDF file, with free access.

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Edited by

• Associate Editor handled this article:
  Eduardo H. S. Sousa

Publication Dates

• Publication in this collection
  20 Dec 2024
• Date of issue
  2025

History

• Received
  07 Apr 2024
• Accepted
  09 Sept 2024
• Published
  11 Nov 2024