Bimetallic Fe, Co-Modified TiO₂ Derived from NH₂-MIL-125(Ti) as an Efficient Photocatalyst for N₂ Fixation

Abstract

The conversion of nitrogen (N₂) and water (H₂O) into NH₃ by photocatalysis under ambient conditions has been considered an environmentally friendly strategy. However, developing effective catalysts for N₂ fixation is still challenging. Herein, we report a bimetallic JH Fe, Co/TiO₂ derived from NH₂-MIL-125(Ti) by the fast Joule heating (FJH) method for visible–light–driven catalytic N₂ fixation. It was found that the photocatalytic N₂ reduction efficiency of bimetallic FC@TiO₂-JH was improved, enabling an NH₃ yield rate of 110.14 µmol g⁻¹ h⁻¹ without any sacrificial agents. Furthermore, the rate was higher than those of Fe@TiO₂-JH and Co@TiO₂-JH, suggesting that the synergistic effect between Fe and Co broke the electronic equilibrium and increased the center of its d-band, enhancing electronic feedback to the antibonding π* orbitals of N₂ while weakening the bonding energy of N≡N. Meanwhile, the rate was about 2.75 times higher than that of FC@TiO₂-TF, which was calcined in a tube furnace. It is assumed that FJH might lead to the formation of lattice defects, leading to localized charge deficiency, enhanced carrier separation, and transport. Thus, doping of Fe and Co synergistically interacted with the defects produced from FJH, facilitating the photocatalytic reduction process. As detected, it had a greater ability to separate hole–electron pairs and transferred electrons to adsorbed N₂ at faster rates. Our work demonstrates a prospective strategy for designing bimetallic catalysts derived from NH₂-MIL-125(Ti) for N₂ fixation.

1. Introduction

Ammonia plays an important role in chemical feedstock and is considered to be a clean energy carrier with a high hydrogen content [1,2]. At present, large-scale NH₃ production is almost dependent on the Haber–Bosch process [3]: $N_2+3H_2\to 2N{H}_3$ (350–550 °C, 15–35 MPa), which consumes a mass of energy fossil fuels and generates numerous greenhouse gases [4,5]. In recent years, photocatalysis—which only consumes energy-rich solar radiation—has attracted widespread attention [6,7]. However, most photocatalysts are difficult to absorb and activate N₂, which is due to the high barrier of N₂. Furthermore, the fast photogenerated electron–hole recombination also results in a low production rate [8,9]. Therefore, we urgently need to develop more efficient photocatalysts for the nitrogen reduction reaction (NRR).

Metal–organic frameworks (MOFs) have attracted attention due to their large surface areas, high porosities, and changeable organic linkers [10,11]. Furthermore, they offer a good precursor to derive highly efficient nanocomposites due to their unique properties, such as rationally designed structures, and various choices of morphologies [12,13]. TiO₂, the most traditional and classic photocatalyst, is derived from NH₂-MIL-125(Ti) and retains its properties [14,15]. This indicates that NH₂-MIL-125(Ti) is an highly-efficient photocatalyst. Compared to MIL-125(Ti), NH₂-MIL-125(Ti) can broaden the absorption wavelength and narrow the bandgap [8]. However, its photocatalytic activity stands as a limitation because of the excessive photo-carrier recombination and the wide bandgap [16,17]. To further enhance the NH₃ production rate, elemental doping is considered to be an effective strategy [4,18,19]. The efficiency and durability of single-atom doping are not sufficient for practical applications, whereas co-doping metal atoms allows for precisely modulating the electronic structure of the active center and thus controls the catalytic activity. Secondly, it will directly affect the binding energy with the co-active center and further improve the stability of the catalyst [20,21]. Fe and Co, as low-cost metals, are widely used in industrial and some catalytic applications. Fe and Co are considered to be great dopants for TiO_2, because they have similar radii to Ti⁴⁺, with 0.69 Å, 0.63 Å, and 0.75 Å, respectively [22]. Fe³⁺ has the potential to trap photogenerated electrons and holes because the energy levels of Fe²⁺/Fe³⁺ are similar to those of Ti³⁺/Ti⁴⁺. Fe³⁺ enhances the photocatalytic activity of TiO₂ by forming charge-trapping sites after Fe³⁺ replaces Ti⁴⁺. Moreover, the appropriate doping amount of Co²⁺ substitution of Ti⁴⁺ consumes the effective charge carriers in the TiO₂ lattice, inhibiting electron–hole recombination [4]. Thus, constructing bimetallic dopants provides more active sites and can inhibit the recombination of photogenerated carriers by the metal-to-metal charge transition. Recently, the fast Joule heating (FJH) method has attracted more interest [23]. The FJH method can avoid particle aggregation, reduce the particle size, and fabricate lattice defects that might produce more active sites for N₂ fixation of photocatalysis.

In this study, we fabricated a novel bimetallic dopant photocatalyst using Fe and Co by the FJH method for N₂ fixation. Given the current problems of nitrogen fixation, such as difficult nitrogen activation and a high photogenerated carrier complexation rate, the active sites of the catalysts were regulated by adjusting parameters such as the doping ratios of the bimetals. The creation of lattice defects and Co, Fe doping enhanced light harvesting (almost the full spectrum), reduced the bandgap, and suppressed the recombination of photogenerated carriers. As a result, the photocatalytic activity of FC@TiO₂-JH (0.75:0.75) in the NRR was enhanced, reaching 110.14 µmol g⁻¹ h⁻¹. It is expected that this study will provide a reference for bimetallic-doped photocatalytic N₂ fixation.

2. Experimental Section

2.1. Preparation of NH₂-MIL-125(Ti)

NH₂-MIL-125 (Ti) was prepared by using a simple hydrothermal method [24]. Typically, 3.53 g of 2-aminoterephthalic (AAPD, 98%, Macklin, Shanghai, China) and 2.1 mL of titanium butoxide (TTIP, purity > 99.0%, Aladdin Ltd., Shanghai, China) were dissolved in 6.0 mL of methanol and 54.0 mL of DMF under magnetic stirring for 180 min. The well-dispersed solution was then sealed in a 100 mL stainless steel autoclave lined with PTFE and heated in an oven at 150 °C for 24 h. After natural cooling to ambient temperature, the sample was soaked in methanol for 12 h to exchange DMF. Then, the samples were washed sequentially by centrifugation using methanol and deionized water, and finally dried in a vacuum drying oven at 80 °C overnight.

2.2. Preparation of FC@TiO₂

Next, 0.40 g of the obtained NH₂-MIL-125(Ti) was dispersed in 40 mL of deionized water. Fe(NO₃)₃·9H₂O and Co(NO₃)₃·6H₂O (Macklin, 99.99%) were added in different proportions (0.75:0, 0:0.75, 0.5:1, 1:0.5, 1.0:1.0, 0.75:0.75 wt%) and stirred at a rate of 450 r/min, keeping a temperature of 60 °C for 6 h. The obtained products were washed 3–4 times by centrifugation with deionized water and then dried in an oven at 80 °C overnight to obtain FC@NH₂-MIL-125(Ti). FC@ NH₂-MIL-125(Ti) was calcined as a precursor by the FJH (In-situ High-tech, CIS-JH3.2) method under different atmospheres at 600 °C to finally obtain FC@TiO₂-JH. FC@TiO₂-TF as a control sample was prepared by calcination in a tube furnace (OTF–1200X–S, Hefei Kejing Material Technology Co., Ltd., Hefei, China) at 600 °C. Fe, M@TiO₂-JH (M = Ni, Cu) was prepared in the same way, using NiNO₃·6H₂O and Cu(NO₃)₂·xH₂O (99.99%, Macklin).

2.3. Characterizations

X-ray diffraction (XRD) patterns were obtained by German-Bruker-D8 Advance to test the physical phase of the samples. Scanning electron microscopy (FE-SEM SU-8010, HITACHI, Tokyo, Japan) and transmission electron microscopy (TEM, JEM-2100PLUS Electron Microscope, Japan Electronics Co., Ltd, Amagasaki, Japan) were used to test the morphology and microstructure of the samples. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) was used to test the elemental species and valence information of the samples. UV–Vis diffuse reflectance spectra (DRS) data were measured by a TU-1901 spectrophotometer (Pu xi Ltd. of Beijing, China).

2.4. Electrochemical Measurement

Electrochemical measurements were performed on an electrochemical workstation (CHI 660E, CH Instruments Inc., Bee Cave, TX, USA). Electrochemical impedance spectroscopy (EIS) and transient photocurrents were tested to investigate the photogenerated carrier separation and transfer capabilities. The standard three-electrode system consisted of a working electrode, a counter electrode (graphite carbon rod), and a reference electrode (saturated calomel electrode). Typically, the working electrode was prepared by dispersing 1 mg of photocatalyst and 5 μL of Nafion solution in 40 μL of deionized water, and then dropping 40 μL of the solution onto an L-shaped glassy carbon electrode with a rubber-tipped dropper, and finally drying under an infrared lamp. The transient photocurrent and electrochemical impedance spectra (EIS) were tested by using 0.5 M Na₂SO₄ aqueous solution as an electrolyte.

2.5. Photocatalytic Reaction

Firstly, 30 mg of photocatalyst and 60 mL of pure water were added to the beaker, and that was placed an ultrasonic cleaner so the photocatalyst would dissolve. We sealed the solution after placing it in the photoreactor, and N₂ (>99.999%) was bubbled for 0.5 h to remove the other gases. Then, the solution spent one hour under a 300 W Xe lamp. After the reaction, we removed 10 mL solution, and it was filtered using a filter membrane (0.22 μm). Then, Nessler’s reagent was added to the solution. The color renderer promoted the N₂ change to NH₄⁺, and the NH₄⁺ concentration was quantified by spectrophotometry and the indophenol blue method. In particular, excellent selectivity for N₂ photo-fixation was demonstrated with no N₂H₄ or NO₃⁻ detected as byproducts. AQE was calculated according to Equation (1) as follows:

$$AQE={\displaystyle \frac{N_{\mathrm{e}}}{N_{\mathrm{P}}}}\times 100\%={\displaystyle \frac{3\times n_{AM}\times N_A}{\frac{W\times A\times t}{h\times \nu }}}\times 100\%$$

(1)

where N_e and N_P are the total number of reactive electrons transferred and the number of incident photons, respectively. n_AM represents the number of moles of ammonia (mol), W represents the irradiation intensity (measured as 491 mW cm⁻²), A represents the irradiation area (cm²), t represents the reaction time (s), ν represents the incident light frequency, and h represents the Planck constant.

3. Results and Discussion

3.1. Morphology and Structural Properties

Figure 1a gives a schematic diagram of the FC@TiO₂-JH synthesis procedure. The bimetallic-based catalysts of FC@TiO₂-JH were synthesized firstly by the impregnation method in a 50 mL test tube, which realized Fe and Co adsorption on NH₂-MIL-125(Ti). Then, FC@TiO₂-JH was derived from FC@NH₂-MIL-125(Ti) by fast Joule heating in an Ar atmosphere at 600 °C. The morphologies of NH₂-MIL-125(Ti) and FC@TiO₂-JH were observed by SEM and TEM. The NH₂-MIL-125(Ti) sample was similar to round slices, with different sizes (250 nm–650 nm) (Figure 1b). However, the original round slice structure was retained after rapid heating and cooling by the FJH treatment, producing the smaller-sized round slices of FC@TiO₂-JH (Figure 1c,d). The elemental mapping of FC@TiO₂-JH in Figure 1h revealed that the Ti, O, Fe, and Co elements were uniformly distributed in the sample. As displayed in Figure 1e, TEM further confirmed the tablet-like structure of the nanocomposite and surface roughness. Moreover, the HRTEM image (Figure 1f) further revealed that protrusions were TiO₂. The lattice fringe was 0.35 nm, assigning to (1 0 1) [25] dominant facets of the anatase TiO₂. Additionally, Figure 1g shows lattice defects (red circles) in FC@TiO₂-JH due to the FJH method, leading to localized charge deficiency, which might be potential active sites. EDS mapping (Figure 1h–k) further confirmed the uniform distribution of Ti, O, Fe, and Co elements in FC@TiO₂-JH. In summary, the successful preparation of FC@TiO₂-JH was demonstrated.

As indicated in Figure 2a, the NH₂-MIL-125(Ti) XRD pattern was consistent with the standard cards, indicating that NH₂-MIL-125(Ti) was successfully prepared [26]. After rapid calcination with the FJH method, FC@TiO₂-TF and FC@TiO₂-JH showed anatase TiO₂ (PDF#21-1272) and the × represented the standard card of TiO₂. characteristic diffraction peaks. Additionally, FC@TiO₂-JH and FC@TiO₂-TF both showed poor crystallinity from the derivatives of MOF. The C 1s peaks of TiO₂-JH and FC@TiO₂-JH appeared at 284.8, 285.8, and 288.2 eV, attributed to C=C, C-O, and O-C=O bonds, respectively (Figure S1). The peaks at 529.7 and 531.7 eV in the FC@TiO₂-JH O 1 s spectrum (Figure 2b) were attributed to the Ti–O bond and the OH groups on the carbon surface, respectively. Deconvoluted N 1s peaks of TiO₂-JH and FC@TiO₂-JH appeared at 400.6 and 398.7 eV. These could be attributed to the pyrrolic N species at 400.6 and assigned to the pyridine-like N atoms at 398.7 eV present in the porous carbon matrix, respectively (Figure 2c). Furthermore, there was no peak between 396 and 397 eV, which would have been assigned to Ti-N bonds, indicating that N atoms were not doped into TiO₂ [27]. As displayed in Figure 2d, compared to TiO₂-JH, the FC@TiO₂-JH peak at 458.4 eV shifted 0.29 eV to lower binding energies, indicating Fe, Co was successfully doped into TiO₂. Furthermore, Fe 2p peaks of FC@TiO₂-JH are displayed in Figure 2e, with peaks at 709.2 and 724.1 eV, which were attributed to Fe²⁺. The peaks of Fe³⁺ were at 710.9 eV and 725.2 eV. Furthermore, the peak at 714.8 eV was identified as a satellite peak assigned to Fe²⁺ (Fe 2p_3/2). Meanwhile, the satellite peak belonging to Fe³⁺ was at 718.2 eV (Fe 2p_1/2) [23]. In the Co 2p XPS spectra for FC@TiO₂-JH (Figure 2f), the peaks of Co²⁺ were at 779.8 (Co 2p_3/2) and 794.8 eV (Co 2p_1/2). The peaks at 785.1 eV (Co 2p_3/2) and 801.9 eV (Co 2p_1/2) were Co²⁺ satellites [28]. In conclusion, Fe and Co replaced Ti⁴⁺ and were doped into TiO₂ crystal lattices, and N was present as pyrrolic N species and pyridine-like N atoms.

3.2. Photocatalytic Nitrogen Fixation Test

As depicted in Figure 3a, the NH₃ production rate was 10.57 μmol g⁻¹ h⁻¹ for TiO₂-TF, which efficaciously established the foundation of photocatalytic N₂ reduction activity. TiO₂-JH (17.14 μmol g⁻¹ h⁻¹) was higher than TiO₂-TF, indicating that the FJH method produced more defects as active sites, leading to localized charge deficiency, enhanced carrier separation, and transport [29]. As was foreseeable, the yield rate of NH₃ was greatly increased by the doped Co and Fe. Compared to FC@TiO₂-TF, FC@TiO₂-JH exhibited the highest NH₃ yield rate (110.14 µmol g⁻¹ h⁻¹). In addition, as the Fe/Co weight ratio (wt%) increased, the photocatalytic NH₃ yield rate of FC@TiO₂-JH increased firstly and then decreased (Figure 3b). Of all the samples prepared, the wt% of Fe/Co (0.75:0.75) exhibited the highest NH₃ production rate. As shown in Figure 3c, other metals (M=Ni, Cu) with the ratio of Fe/M 0.75:0.75 by the FJH method were also investigated in this work, but the NH₃ yield rate of those catalysts was lower than that of FC@TiO₂-JH. This might have been due to the synergistic effect between Fe and Co breaking the electronic equilibrium and increasing the center of its d-band, which enhanced electronic feedback to the antibonding π* orbitals of N₂ and weakened the bonding energy of N≡N [20]. Figure 3d shows the NH₃ yield rates of FC@TiO₂-JH in various atmospheres. The activity was notably higher in an Ar atmosphere than in an air or H₂/Ar atmosphere, indicating that the inert atmosphere produced more active sites absorbing N₂. After four cycle tests, its photocatalytic performance slightly decreased, reflecting that FC@TiO₂-JH possessed a relatively stable photocatalytic performance (Figure 3e). The AQE results of the NH₃ product over FC@TiO₂-JH were determined as 0.69, 0.42, 0.25, and 0.15% at 385, 400, 500, and 600 nm, respectively, which agreed well with photoresponsivity as detected in the DRS test (Figure 3f). Furthermore, the NH₃ production in our work was higher than the majority of those for reported photocatalysts (Figure 3g) [23,29,30,31,32,33,34].

3.3. Optical and Electronic Properties

The optical properties of different catalysts were evaluated by UV–Vis DRS. As presented in Figure 4a, MIL-125(Ti) had an optical response in the UV region only from 200 nm to 380 nm. NH₂-MIL-125(Ti) could enhance the optical response from 200 nm to 500 nm, indicating that -NH₂ effectively increased light absorption. Moreover, TiO₂-JH and FC@TiO₂-JH had a wide range of response in the full spectrum from 200 to 800 nm. Furthermore, the light absorption intensity was enhanced, which might have been due to the doping of Fe, Co, and the creation of defects by the FJH method. As depicted in Figure 4b, the E_g values of MIL-125(Ti), NH₂-MIL-125(Ti), TiO₂-JH, and FC@TiO₂-JH were about 3.22, 2.52, 2.12, and 1.98 eV, respectively. Fe, Co doping and generation of defects have been demonstrated to narrow the bandgap. Furthermore, the XPS valence band (VB) spectrum showed that the VB of TiO₂-JH and FC@TiO₂-JH were 1.91 and 1.21 eV, respectively (Figure 4c). Based on the equation E_g = E_VB − E_CB, the conduction band (CB) of FC@TiO₂-JH was calculated as −0.77 eV (Figure 4d). The CB potential became more negative after Fe, Co doping and played a role in the photoreduction thermodynamically. Therefore, FC@TiO₂-JH showed the best NH₃ yield rate.

Photogenerated carrier separation and transfer capabilities were investigated through photocurrent responses and EIS tests. When compared with TiO₂ and FC@TiO₂-TF, Figure 4e,f shows that FC@TiO₂-JH had a higher current density and smaller Nyquist circle diameter. The results demonstrate that the doping of Fe, Co could productively improve the migration and separation of photogenerated carriers, which was favorable to increase NH₃ production. Furthermore, the defects that occurred during the rapid heating and cooling processes of the FJH method trapped charged electrons, preventing the direct complexation of light-generated electron holes.

4. Conclusions

In conclusion, bimetallic FC@TiO₂ photocatalysts with different Fe: Co ratios and different calcined methods were used for N₂ fixation. FC@TiO₂-JH catalytic activity was achieved at 110.14 μmol g⁻¹ when the ratio of Fe: Co was 0.75:0.75 without any sacrificial agents. The activity was 6.4 times higher than that of TiO₂-JH, and 2.75 times higher than that of FC@TiO₂-TF. Additionally, the FC@TiO₂-JH photocatalyst had good stability, and the AQE in 365 nm reached 0.69%. Furthermore, it had a greater ability to separate hole–electron pairs and transferred electrons to adsorb N₂ at faster rates due to the doped Fe and Co. Thus, doping Fe and Co synergistically interacted with the defects produced from the FJH treatment, facilitating the photocatalytic reduction process. The reasonable design of Fe, Co co-dopant transition metal oxides by the FJH method opens up a new approach for highly efficient photocatalytic N₂ fixation.