
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

71016
10.1038/s41598-024-71016-y
Article
Improvement of photocatalytic ammonia production of cobalt ferrite nanoparticles utilizing microporous ZSM-5 type ferrisilicate zeolite
Khatamian Maasoumeh khatamian@tabrizu.ac.ir
mkhatamian@yahoo.com

Malekani Mohammad
Fazayeli Monireh
Yavari Azin
https://ror.org/01papkj44 grid.412831.d 0000 0001 1172 3536 Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
1 9 2024
1 9 2024
2024
14 2030122 5 2024
23 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
The development of decarbonized synthesis approaches is a critical step in the fabrication of ammonia, an indispensable chemical and a potential carbon–neutral energy carrier. In this regard, the photocatalytic production technology has gained ample attention as a sustainable alternative to energy-intensive and environmentally detrimental Haber–Bosch process. Here, we present cobalt ferrite nanoparticles supported on microporous ZSM-5 type ferrisilicate zeolite as a desirable novel photocatalyst for the ammonia generation. The zeolite introduced as a microporous support increasing the catalytically active sites. A straightforward one-pot sol–gel method was used to synthesize cobalt ferrite (CoFe2O4) and CoFe2O4/ferrisilicate (CF/FS) nanocomposites with various weight percentages (10, 25 and 50%) of CoFe2O4. The photocatalytic performances of the samples in the production of ammonia were investigated under visible light irradiation. The highest rate of NH4+ production (484.74 µmol L−1 h−1) was achieved using the CF50%/FS photocatalyst. The distribution of < 50 nm-sized CoFe2O4 nanoparticles on the surface of the zeolite, as demonstrated by TEM images, and extensive BET surface areas are presented as convincing evidences for the improved photocatalytic activity paticularly in CF50%/FS photocatalyst.

Keywords

Cobalt ferrite
Ferrisilicate zeolite
Photocatalytic ammonia production
Nanocomposite
N2 photoreduction
Subject terms

Chemistry
Materials science
Nanoscience and technology
http://dx.doi.org/10.13039/501100007831 University of Tabriz 624/44 issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

The use of ammonia is widespread in a variety of industries due to its wide range of applications. It is a leading feedstock in manufacturing of numerous chemicals especially nitrogenous fertilizers. Moreover, possessing high hydrogen storage capacity (17.6 wt%), high energy density (3 kWh kg−1) and facile liquefaction process makes ammonia to be in the limelight as a carbon-free energy source1,2.

Currently, ammonia is predominantly produced through the Haber–Bosch process, which utilizes H2 and N2 gases as feedstocks. This process not only requires high temperatures and pressures but also generates more than 450 million tons of CO2 that is about 1.2% of global CO2 emissions3,4. To address these issues, researchers have been seeking a sustainable mild alternative that would replace the Haber–Bosch process. In recent years, photocatalysis has emerged as a potential solution to global energy and environmental challenges5–9. One particularly promising aspect is photocatalytic N2 reduction in the presence of water, which has stimulated intense interest due to its ability to produce green NH3 utilizing sun light as a free and inexhaustible energy source10,11. Throughout this process, the light is absorbed by a semiconductor to generate electrons and holes, which can subsequently participate in the nitrogen reduction and water oxidation reactions, respectively. Indeed, the protons from water oxidation and the photoexcited electrons in the conduction band of the semiconductor activate the N≡N bond and convert it to NH3 through multistep reactions12,13.

Since the discovery of nitrogen photoreduction on iron-doped titanium dioxide by Schrauzer and Guth in 197714, numerous attempts have been dedicated to design efficacious photocatalysts. In this regard, semiconductors with narrow band gaps that can absorb the visible light (which makes up about 42% of sunlight) are preferred to the wide band gap UV-absorber materials owing to the fact that the UV component of the sunlight is negligible. Nevertheless, it is worth mentioning that in addition to the band gap, the energy levels of the valence band and conduction band edges play crucial role in the photocatalytic redox reactions. While the valence band should be more positive than the potential of water oxidation, the conduction band should be in more negative levels than the potential of nitrogen reduction15.

On the other hand, conversion of N2 molecules into ammonia requires an activation of the strong N≡N triple bond and its dissociation. Inspiring from the N2 fixation in nature with the nitrogenase enzymes16, which are made up of Fe- and FeMo- containing proteins, Fe-based catalysts have been captured copious interests. Hu and coworkers have synthesized Fe3+ doped g-C3N4, which showed remarkable N2 photofixation compared to pristine g-C3N417. From the DFT simulations and DOS results, they demonstrated that the Fe sites could both activate the triple bond and transfer electrons from the g-C3N4 to the N2 molecules. Li et al.18 highlighted the importance of the iron by comparing the performance of a Fe-based MOF (MIL-101(Fe)) and a Cr-based MOF (MIL-101(Cr)). While MIL-101(Cr) was inactive toward N2 photoreduction, MIL-101(Fe) showed 50.355 µmol L−1 h−1 NH4+ generation rate. Based on the in situ FTIR and quantum computation results, the observed performances was attributed to the higher electron density and lower reaction activation energy in Fe catalytic centers. Likewise, similar consequences was achieved by Shen et al.19 using Fe3+ doped flower-like BiOCl. The rate of NH4+ production was raised from 0 to 30 µmol L−1 h−1 through the doping of Fe3+ sites into the BiOCl. In fact, transition metal ions such as Fe3+, with the ability of adding d electrons into the π∗ antibonding orbitals of N2 molecules, act as π-donating centers. The formation of π-backbond, weakens the N≡N triple bond and ultimately leads to the activation of N2 molecules20.

The substantial role of iron in photocatalytic generation of NH3 arouses the curiosity about the performance of ferrites in this process. Spinel ferrites, as the most common widely used sort of ferrites, are represented by the general formula MFe2O4 where M signifies divalent metal cations such as Zn2+, Co2+, Fe2+, Ni2+, Cu2+ and Mn2+21–24. In addition to their magnetic properties, these compounds possess semiconducting features, which give them the capability to act as photocatalysts25. They benefit from the narrow band gap (around 2 eV) and mixed metal catalytic sites in the spinel crystal structure26. Cobalt ferrite (CoFe2O4) with outstanding characteristics including large cubic magneto crystalline anisotropy, high coercivity and high physical and chemical stability27 has been widely used in diverse applications such as magnetic resonance imaging (MRI)28, magnetic memories29, drug delivery30, and gas sensors31. Besides, as an n-type semiconductor, CoFe2O4 nanoparticles have demonstrated admirable photocatalytic activity in various processes like degradation of organic pollutants32 and hydrogen production33. However, to the best of our knowledge the evaluation of their performance in nitrogen photocatalytic fixation is limited to two works: By synthesizing terephthalic acid-doped cobalt ferrite, Shen and coworkers reached 216 µmol L−1 h−1 nitrogen fixation rate under simulated sunlight irradiation34; in another study, Zhang et al.35 found that the g-C3N4/CoFe2O4 exhibited nitrogen photofixation efficiency of 14.66 mmol g−1 h−1 within 120 min.

In the present work, we report the synthesis of cobalt ferrite/ferrisilicate zeolite nanocomposites and their performance in the photocatalytic reduction of nitrogen to ammonia for the first time. Using the microporous ZSM-5 type ferrisilicate zeolite as a substrate, we aimed to prevent the agglomeration of the cobalt ferrite nanoparticles and enhance the photocatalytically active sites in the resultant composites. Besides, considering the importance of iron in the cleavage of triple bond of nitrogen, the iron-containing substrate seemed to be a rational option.

Methods

Synthesis of CoFe2O4 nanoparticles

CoFe2O4 nanoparticles were synthesized with the sol–gel method. Briefly, the grinded mixture of Co(NO3)2.6H2O (Merck), Fe(NO3)3.9H2O (Merck) and citric acid with 1:2:5 molar ratio was dissolved in deionized water and stirred at 85 °C for 1.5 h till a dark red gel formed. The resulting gel was dried at 110 °C overnight to remove the remaining water. After crushing the obtained brown porous mass, it was transferred to a furnace and heated to 850 °C at a rate of 2 °C min−1. The calcination was performed at this temperature for 6 h.

Synthesis of ferrisilicate zeolite

The ZSM-5 type ferrisilicate was prepared according to our previous work36. First, iron (ΙΙΙ) nitrate (Merck) was dissolved in water and the pH was adjusted to 0.5 by adding sulfuric acid. Subsequently, the solution of silicic acid and sodium carbonate in distilled water was slowly added to the first solution. After being stirred for an hour at pH ≈ 9, TPABr (tetrapropylammonium bromide, Merck) was added to the prepared mixture. The obtained mixture was hydrothermally treated at 170 °C for 5 days in order for the crystallization of the zeolite. The obtained product was washed with distilled water, dried at 120 °C overnight and calcined at 550 °C for 5 h.

Synthesis of CoFe2O4 /ferrisilicate

Cobalt ferrite/ferrisilicate composites were synthesized similarly to the synthesis of the bare cobalt ferrite. According to the considered percentages of cobalt ferrite in the composites (10%, 25% and 50%), 1.4 g, 0.42 g, and 0.14 g of ferrisilicate were added to the precursor solution of CoFe2O4, respectively. The mixtures were kept stirring for an hour before heating at 85 °C. The subsequent steps were followed likewise. The samples were designated as CF10%/FS, CF25%/FS, and CF50%/FS.

Photocatalytic ammonia production

The photocatalytic ammonia production experiments were conducted in a homemade photoreactor at room temperature and atmospheric pressure. The visible illumination was provided by a 50 Watt LED lamp. For each test, a particular amount of the photocatalyst (50 mg) was distributed in 50 mL of distilled water in the reactor, and 0.789 mg L−1 of methanol was utilized as a sacrificial agent. During the process, the input nitrogen gas flow rate was maintained at 100 mL min−1. Prior to light irradiation, the reaction suspension was stirred in nitrogen gas in the dark for 30 min and then the sampling was performed every 30 min for 2.5 h. To determine the amount of ammonia, a few drops of Nessler's reagent were added to the centrifuged solutions, and the absorption was measured at the wavelengths of 425 nm. The photocatalytic experiment was carried out three times for each photocatalyst under the same conditions, and the average results were presented as the final data.

In order to elucidate the mechanism by which methanol functions, an examination was conducted to determine the existence of hydroxyl radicals during the reaction. In this regard, the reaction employing the most effective catalyst (CF50%/FS) was carried out by: (1) omitting methanol, and (2) substituting it with terephthalic acid (TA) as hydroxyl radical scavenger.

Characterization apparatus

X-ray diffraction (XRD) method was used to determine the crystal structure of the samples. The patterns were collected by a Siemens D500 diffractometer with Cu Kα radiation (λ = 1.5418 Å, 2θ = 4–70°) at room temperature. The step sizes were chosen to be 0.02°. The FT-IR spectra were acquired using KBr pellets on a Bruker TENSOR 27 spectrometer in the range of 400–4000 cm−1 wavenumbers. The morphology of the products and their chemical compositions were studied using a Tescan MIRA3 field emission-scanning electron microscope equipped with the energy dispersive X-ray spectrometer (EDS) and a Philips EM 208S Transmission electron microscope. By monitoring the N2 adsorption/desorption isotherms, the surface area and pore size distribution of the resulting composites were calculated applying the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) models. Thermogravimetric analysis (TGA) was carried out from room temperature to 800 °C (with a rate of 10 °C/min) in the air atmosphere using Mettler Toledo instrument. By diffuse reflectance spectroscopy technique, the optical absorption behavior of samples was studied at the wavelength range of 380–900 nm using a Scinco 4100 spectrophotometer. A vibrating sample magnetometer (VSM) operating at room temperature was used to elaborate the magnetic characteristics of the samples.

Results and discussion

Characterization

The XRD patterns of the synthesized CoFe2O4 nanoparticles, ferrisilicate and CoFe2O4 /ferrisilicate nanocomposites are depicted in Fig. 1. The pattern of CoFe2O4 nanoparticles (Fig. 1a) represents diffraction peaks at 18.55°, 30.55°, 35.97°, 37.62°, 43.77°, 54.37°, 57.79°, and 63.44°, which are consistent with the standard pattern of CoFe2O4 with cubic crystal structure (JCPDS Card No. 22–1086). The peaks are attributed to the reflections from planes with the Miller indices of (111), (220), (311), (222), (400), (422), (511) and (440), respectively. Based on the FWHM (Full-width at half maxima) values of three main peaks ((220), (311) and (400)), the mean size of CoFe2O4 crystallites was calculated to be 47nm by using the Scherrer’s equation (D =KλβCosθ).Fig. 1 XRD patterns of (a) CoFe2O4, (b) ferrisilicate zeolite, (c) CF10%/FS, (d) CF25%/FS and (e) CF50%/FS.

The formation of the ZSM-5 type ferrisilicate zeolite is proven by the presence of peaks at 7.97°, 8.84°, 23.08°, and 23.9°, which are ascribed to the (011), (020), (051) and (033) planes, respectively. According to the patterns of the CoFe2O4 /ferrisilicate nanocomposites, the peaks related to CoFe2O4 have appeared along with the peaks of ferrisilicate and intensified by increasing the percentage of the CoFe2O4 in the composites.

Figure 2 demonstrates the FT-TR spectra of the synthesized samples. The fundamental vibrations of the ions in the crystal lattice of metal ferrites occur in the range of 300–700 cm−137.The appearance of two main absorption bands in this region is a common feature of these compounds37. Here, for CoFe2O4 nanoparticles, two peaks at 420 cm−1 and 582 cm−1 are assigned to the stretching vibrations of metal–oxygen bonds with octahedral and tetrahedral coordination, respectively38. Moreover, in the ferrisilicate sample, the peaks attributed to the bending vibrations in TO4 tetrahedra, external T–O–T symmetric and internal O–T–O asymmetric stretching vibrations can be observed at 450 cm−1, 800 cm−1 and 1095 cm−1, respectively36. In addition, the characteristic peaks of ZSM-5 type zeolite at 549 cm−1 (double ring vibrations) and 1222 cm−1 (external T–O–T asymmetric stretching vibration) confirm the zeolitic structure of the ferrisilicate39. The mentioned peaks are present in the spectra of the CoFe2O4/ferrisilicate composites, as well. A more detailed look on these spectra indicates a broadening of the peak at 549 cm−1 occurs and the absorption in this region increases from CF10%/FS sample to CF50%/FS. It can be imputed to the presence of a strong absorption band of CoFe2O4 at 582 cm−1 and its overlap with the peak of zeolite at 549 cm−1. Therefore, the relative intensity of the peaks of CoFe2O4 to zeolite increases by increasing the weight percentage of the cobalt ferrite in the composites.Fig. 2 FTIR spectra of (a) CoFe2O4, (b) ferrisilicate zeolite, (c) CF10%/FS, (d) CF25%/FS and (e) CF50%/FS.

The morphology and elemental composition of the samples were investigated using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). The bare CoFe2O4 particles are agglomerated owing to their magnetic characteristics, as seen in Fig. 3a, b. The presence of Fe and Co in this sample is proven from the EDS spectrum. On the other hand, Fig. 3c demonstrates the ferrisilicate zeolite comprised microspheres with rugged surfaces. These spheres are composed of merged cuboids containing smaller nanoparticles (Fig. 3d).Fig. 3 FE-SEM images of (a, b) CoFe2O4 and (c, d) ferrisilicate zeolite.

Through the synthesis of CoFe2O4/ferrisilicate composites, the surfaces of the zeolite particles are found to be covered by the cobalt ferrite nanoparticles with the sizes lower than 50 nm (Fig. 4). Hence, the particle size of ferrite can be significantly reduced by the synthesis of CoFe2O4 on ferrisilicate. Here again, the magnetic feature of the CoFe2O4 leads to the aggregation of the zeolite particles. The EDS spectra confirm the incremental loading amount of the CoFe2O4 on the ferrisilicate by increasing its weight percentage in the samples. The enhancement of relative intensities of Co and Fe peaks are observable from the spectra (Fig. 5). Additionally, the loaded nanoparticles with the sizes below 50 nm on the ferrisilicate microspheres are clearly depicted in TEM images of the CF50%/FS sample (Fig. 6).Fig. 4 FE-SEM images of (a–c) CF10%/FS, (d–f) CF25%/FS and (g–i) CF50%/FS.

Fig. 5 EDS spectra of (a) CoFe2O4, (b) ferrisilicate zeolite, (c) CF10%/FS, (d) CF25%/FS and (e) CF50%/FS.

Fig. 6 TEM images of CF50%/FS nanocomposite.

Thermogravimetric analysis was performed on the uncalcined cobalt ferrite nanoparticles. The TGA–DTG curves of the sample are exhibited in Fig. 7. As is evident, a weight loss occurs in the region of 20–140 °C, which is arising from the egress of moisture and water molecules from the structure. In addition, a substantial step of the weight reduction process has appeared in the range of 180–320 °C. An intense weight loss with a sharp peak at ~ 200 °C in the DTG curve can be associated with the decomposition and oxidation of citrate. Moreover, two shoulders at 240 °C and 280 °C can imply the combustion of nitrate ions as well as hydroxyl and carboxylate groups coordinated the metal centers. The absence of considerable weight loss at the temperatures higher than 320 °C reveals the completely removal of excess compounds except the metal oxide phase. The total weight loss of the sample up to 800 °C is approximately 79.15%.Fig. 7 TGA and DTG curves of the CoFe2O4 before calcination.

Diffuse reflectance spectroscopy was used to evaluate the optical properties of the photocatalysts and determine their band gaps. Figure 8a demonstrates the absorption spectra of CoFe2O4 and CF25%/FS samples. Both have wide absorption in the visible region up to wavelength of 800 nm. Therefor they can be used as photocatalysts under visible light irradiation. To calculate the band gap energies, Kubelka–Munk function (FR=(1-R)2/2R , where R is reflectance) was used. From the Tauc plots presented in Fig. 8b, both samples have the band gaps of 1.57 eV.Fig. 8 (a) UV–vis absorption spectra and (b) Tauc plots of CoFe2O4 and CF25%/FS.

Nitrogen adsorption/desorption isotherms of CoFe2O4, CF10%/FS, CF25%/FS, and CF50%/FS were provided for the exploration of the surface areas and pore size distribution. As illustrated in Fig. 9a–d, the isotherms of CF10%/FS, CF25%/FS, and CF50%/FS represent a combination of type I and IV isotherms, while the recorded profile for ferrite is consistent with the type III isotherm. For all samples, no plateau was observed at high pressure, corroborating the presence of abundant mesopores (pore size between 2 and 50 nm) and macropores (pore size larger than 50 nm) in the resultant CF10%/FS, CF25%/FS, and CF50%/FS nanocomposites. Based on the BET model, the surface areas for CoFe2O4, CF10%/FS, CF25%/FS, and CF50%/FS are estimated to be 2.4, 301.7, 241.8, and 139.7 m2 g−1, respectively. As can be seen, by increasing the content of ferrite nanoparticles in composites, the BET surface area decreases due to the blocking of zeolite pores. Additionally, the BJH curves (Fig. 9e-g) were utilized to observe the pores size distribution in the samples. As illustrated, two types of pore (mesopores and micropores (2˃ pore size)) are illustrated in the BJH profiles relevant to the nanocomposites. On the other hand, loading higher amounts of nanoparticles leads to the larger pores, so that CF50%/FS is predominantly mesoporous.Fig. 9 Nitrogen adsorption/desorption isotherms of (a) CoFe2O4, (b) CF10%/FS, (c) CF25%/FS and (d) CF50%/FS. The BJH curves for (e) CF10%/FS, (f) CF25%/FS and (g) CF50%/FS.

Vibrating sample magnetometer (VSM) analysis was employed to assess the magnetic characteristics of the samples. In Fig. 10, the M–H curves of CoFe2O4, CF25%/FS and CF50%/FS demonstrate hysteresis loops that imply the ferromagnetic behavior40. The three key parameters including saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) are presented in Table 1. It is likely that the low Ms value in CF25%/FS is due to the low proportion of cobalt ferrite in the sample. The squareness ratio (Rs = Mr/Ms) for the samples was evaluated and the results are presented in Table 1. The obtained values, which are lower than 0.5, indicate that all the samples possess multi-domain structures. In addition, the small coercivity in all samples suggests their soft magnetic character40. Nevertheless, the coercivity of the composites is greater than that of the bare CoFe2O4, which can be attributed to the reduced particle size of cobalt ferrite within the composites. It was reported that in the multi-domain particles, the coercivity rises as the particle size decreases, reaching a maximum level at the critical diameter associated with the transition from multi to single-domain state41.Fig. 10 M–H curves of the (a) CoFe2O4, (b) CF25%/FS and (c) CF50%/FS.

Table 1 Ms, Hc, Mr and Rs parameters of CoFe2O4, CF25%/FS and CF50%/FS.

	Ms (emu/g)	Hc (Oe)	Mr (emu/g)	Rs	
CoFe2O4	38.73	135.11	13.79	0.36	
CF25%/FS	5.59	260.42	0.87	0.15	
CF50%/FS	39.45	495.40	10.71	0.27	

Photocatalytic production of ammonia

The performances of the synthesized samples in the photocatalytic reduction of N2 to ammonia were evaluated under visible light irradiation. As previously mentioned, the concentration of the ammonia was quantified via UV–vis spectroscopy following the addition of Nessler’s reagent. As a result of the reaction between ammonia and Nessler’s solution, OHg2NH2I is formed according to the reaction (1), which leads to the yellowing of the solution42:1 2HgI2.2KI+NH4OH+3NaOH→OHg2NH2I+3H2O+4KI+3NaI

The obtained results for the synthesized samples are presented in Fig. 11. As is apparent from the figure, in the case of bare CoFe2O4, the concentration of the ammonia increases over time and reaches the maximum point (755.53 µmol L−1) after 120 min light irradiation. A decline in the concentration is observable with further irradiation time, which can be attributed to the photodegradation of ammonia43,44. To scrutinize the mechanism of the nitrogen photoreduction over CoFe2O4 nanoparticles, the band gap energy and band edge positions should be assessed.Fig. 11 The performances of the synthesized samples in photocatalytic ammonia production.

The synthesized CoFe2O4 with the band gap of 1.57 eV can absorb the light in the visible region and generate electrons and holes. The energy level of the CB in CoFe2O4 was determined to be − 0.4 V in our previous work33. By using the formula of EVB = ECB + Eg, the energy of the VB is calculated as 1.17 V. The energy of the VB (1.17 V vs NHE, pH = 7) is more positive than the water oxidation potential (O2/ H2O, 0.81 V vs NHE)13 and the CB level (− 0.4 V vs NHE) is more negative than the nitrogen reduction potential (N2/NH3, − 0.278 V vs NHE)44. Therefore, by providing electrons and protons the conversion of nitrogen to ammonia can take place as belows13:2 H2O+2h+→2H++12O2

3 N2+6H++6e-→2NH3

In addition to the proper potential energies, the free charge carriers are required for proceeding a photocatalytic process. In this respect, one of the methods to prevent e−-h+ recombination is utilizing scavengers45. Here, methanol as a hole-scavenger has boosted the ammonia production. In terms of the mechanism of methanol action, there are different scenarios that can be considered46: (1) According to the (Eq. 4) methanol can directly consume the holes in VB and prevent their recombination with the electrons in CB or (2) by following the reaction (5) it can react with the ºOH which can oxidize the produced NH3 and decrease its concentration.4 CH3OH+h+→∘CH2OH+H+

5 CH3OH+∘OH→∘CH2OH+H2O

To carry out the second reaction, the possibility of the presence of hydroxyl radicals in the solution should be investigated. These radicals can be produced through the reactions below47:6 H2O→OH-+H+

7 OH-+h+→∘OH

Theoritically, the redox potential of OH−/ºOH (1.99 V vs NHE)44 is more positive than the potential of the VB of CoFe2O4, thereby no ºOH can be generated. For a practical approach to investigating the presence of ºOH in the reaction system, terephthalic acid was introduced as ºOH scavenger agent. Referring to Fig. 12, it can be noticed that TA addition did not influence the ammonia concentration, suggesting that ºOH may not exist in the photocatalytic system and the second mechanism (Eq. 5) is improbable. On the other hand, the substituting TA with methanol increases the ammonia production. Based on these observations, it can be inferred that methanol impedes the electron–hole recombination by prompt grabbing of holes from VB and thus promote the ammonia production through the first mechanism (Eq. 4). It should be noted that by comparing the energy level of VB and redox potential of NO3-/NH3 (+ 0.363 vs NHE)44, the oxidation of ammonia as a side reaction can thermodynamically happen over the synthesized photocatalyst and decrease the concentration of NH3.Fig. 12 Trapping test of ºOH and h+ in photocatalytic reaction using CF50%/FS sample.

By loading various amounts of CoFe2O4 (10%, 25% and 50%) on the ferrisilicate zeolite in CoFe2O4/ferrisilicate composites, the maximum ammonia concentration is changed to 599.11, 896.18 and 969.48 µmol L−1, respectively. It is clear that comparing to the bare CoFe2O4, the ammonia production is decreased for CF10%/FS owing to the lowest amount of cobalt ferrite photocatalyst in this composite. On the other hand, for the two other composites, the ammonia concentration is elevated and it reaches to the highest amount in the CF50%/FS sample. Large BET surface areas of 301.7, 241.8, and 139.7 m2 g−1 in the synthesized composites are advantageous for N2 adsorption, as indicated by the nitrogen sorption isotherms discussed in the previous section. These large surface areas originate from the presence of zeolite with porous structure in nanocomposites. Therefore as a support for the cobalt ferrite nanoparticles, the ferrisilicate zeolite can provide more nitrogen molecules for the photocatalytic process. Moreover, as established by SEM and TEM images, the distribution of the cobalt ferrite particles on the surface of the zeolite effectively decreases the particle size of ferrite. This leads to the increment of photocatalytically active sites and boosts the surface area of CoFe2O4, which can promote the ammonia production. In addition, Co2+ and Fe3+ as transition metals with electrons in the d orbitals (ed) can activate the triple bond in N≡N through π backdonation (ed (Co2+, Fe3+)→ π* (N2))48 and thereby facilitate the photoreduction of N2. A comparative assessment of ammonia production rates for the optimal synthesized sample, photocatalysts with ferrite nanoparticles, and those containing Co and Fe sites can be found in Table 2. The data reveals that the CF50%/FS sample holds substantial potential in N2 photofixation among the literature. Table 2 Comparison of photocatalytic ammonia production in the present study and previously reported photocatalysts.

Photocatalyst	Light source	Sacrificant agent	NH3 production rate (μmol L−1 g−1 h−1)	References	
MIL-101(Fe)	300 W Xenon lamp	None	1007	18	
TPA@CoFe2O4	500 W Solar Simulator	Na2SO3	7200	34	
g-C3N4/ CoFe2O4	250 W Xenon lamp	CH3OH	146,600	35	
TiO2/ZnFe2O4	250 W Xenon lamp	CH3OH	1776	49	
NH2-MIL-101(Fe,Co)	300 W Xenon lamp	None	3357	50	
MIL-53 (FeII/FeIII)	300 W Xenon lamp	K2SO3	3060	51	
PANI@ ZnIn2S4	300 W Xenon lamp	CH3OH	5800	52	
ZnO/CuCo2O4	500 W Xenon lamp	Ethanol	3460	53	
Co3O4/GQDs	50 W LED lamp	CH3OH	30,408	54	
CoFe2O4 50%/FS	50 W LED lamp	CH3OH	9694	This work	

To investigate the recyclability of the catalysts, five runs were carried out under the identical reaction conditions. For this purpose, two of the best performed samples (CF25%/FS and CF50%/FS) were selected. After each cycle, the photocatalyst was separated from the solution, washed and dried to reuse. As can be seen in Fig. 13, the photocatalytic performances are decreased to 690.54 and 675.21 µmol L−1 after the fifth cycle for CF25%/FS and CF50%/FS, respectively.Fig. 13 Recyclability of CF25%/FS and CF50%/FS photocatalysts.

Conclusion

To sum up, cobalt ferrite and its nanocomposites with ferrisilicate zeolite were synthesized using sol–gel method. The narrow band gap of the cobalt ferrite (1.57 eV) and the proper positions of the VB and CB made it to be a candidate for the photoreduction of nitrogen to ammonia under visible light irradiation. On the other hand, as a microporous substrate with a large specific surface area, ferrisilicate zeolite inhibited the agglomeration of the cobalt ferrite particles and boosted their surface area. As a result, CoFe2O4 (50%)/ferrisilicate introduced as the best photocatalyst which produced 969.48 µmol L−1 ammonia after 120 min light irradiation. However, the performance of the bare ferrite was 755.53 µmol L−1 in this time length. Therefore, it seems that utilizing cobalt ferrites as photocatalyst and zeolites as substrate can be promising in photocatalytic production of ammonia.

Acknowledgements

The authors acknowledge the University of Tabriz for the financial support of this research under Grant 624/44.

Author contributions

M. Kh.: Supervision and editing. M. M.: Conducting the experiments, interpretation of data, and writing the original draft. M. F.: Investigation and interpretation of data and writing the original draft. A. Y.: Interpretation of data and editing.

Data availability

All data generated or analyzed during this study are included in this published article.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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References

1. Chang F Gao W Guo J Chen P Emerging materials and methods toward ammonia-based energy storage and conversion Adv. Mater. 2021 33 2005721 10.1002/adma.202005721
Chang, F., Gao, W., Guo, J. & Chen, P. Emerging materials and methods toward ammonia-based energy storage and conversion. Adv. Mater. 33, 2005721 (2021).10.1002/adma.202005721
2. Wang C-Y Mesenchymal stem cell-conditioned medium facilitates angiogenesis and fracture healing in diabetic rats J. Tissue Eng. Regen. Med. 2012 6 559 569 10.1002/term.461 21916015
Wang, C.-Y. et al. Mesenchymal stem cell-conditioned medium facilitates angiogenesis and fracture healing in diabetic rats. J. Tissue Eng. Regen. Med. 6, 559–569 (2012).21916015 10.1002/term.461
3. Liu X Elgowainy A Wang M Life cycle energy use and greenhouse gas emissions of ammonia production from renewable resources and industrial by-products Green Chem. 2020 22 5751 5761 10.1039/D0GC02301A
Liu, X., Elgowainy, A. & Wang, M. Life cycle energy use and greenhouse gas emissions of ammonia production from renewable resources and industrial by-products. Green Chem. 22, 5751–5761 (2020).10.1039/D0GC02301A
4. Smith C Hill AK Torrente-Murciano L Current and future role of Haber–Bosch ammonia in a carbon-free energy landscape Energy Environ. Sci. 2020 13 331 344 10.1039/C9EE02873K
Smith, C., Hill, A. K. & Torrente-Murciano, L. Current and future role of Haber–Bosch ammonia in a carbon-free energy landscape. Energy Environ. Sci. 13, 331–344 (2020).10.1039/C9EE02873K
5. Li S Cai M Wang C Liu Y Ta3N5/CdS core–shell S-scheme heterojunction nanofibers for efficient photocatalytic removal of antibiotic tetracycline and Cr(VI): Performance and mechanism insights Adv. Fiber Mater. 2023 5 994 1007 10.1007/s42765-022-00253-5
Li, S., Cai, M., Wang, C. & Liu, Y. Ta3N5/CdS core–shell S-scheme heterojunction nanofibers for efficient photocatalytic removal of antibiotic tetracycline and Cr(VI): Performance and mechanism insights. Adv. Fiber Mater. 5, 994–1007 (2023).10.1007/s42765-022-00253-5
6. Wang Q Fang Z Zhang W Zhang D High-efficiency g-C3N4 based photocatalysts for CO2 reduction: Modification methods Adv. Fiber Mater. 2022 4 342 360 10.1007/s42765-021-00122-7
Wang, Q., Fang, Z., Zhang, W. & Zhang, D. High-efficiency g-C3N4 based photocatalysts for CO2 reduction: Modification methods. Adv. Fiber Mater. 4, 342–360 (2022).10.1007/s42765-021-00122-7
7. Li X Growth of BiOBr/ZIF-67 nanocomposites on carbon fiber cloth as filter-membrane-shaped photocatalyst for degrading pollutants in flowing wastewater Adv. Fiber Mater. 2022 4 1620 1631 10.1007/s42765-022-00189-w
Li, X. et al. Growth of BiOBr/ZIF-67 nanocomposites on carbon fiber cloth as filter-membrane-shaped photocatalyst for degrading pollutants in flowing wastewater. Adv. Fiber Mater. 4, 1620–1631 (2022).10.1007/s42765-022-00189-w
8. Zhou D Efficient photocatalytic degradation of the persistent PET fiber-based microplastics over Pt nanoparticles decorated N-doped TiO2 nanoflowers Adv. Fiber Mater. 2022 4 1094 1107 10.1007/s42765-022-00149-4
Zhou, D. et al. Efficient photocatalytic degradation of the persistent PET fiber-based microplastics over Pt nanoparticles decorated N-doped TiO2 nanoflowers. Adv. Fiber Mater. 4, 1094–1107 (2022).10.1007/s42765-022-00149-4
9. Huang Y Fiber-optic microfiber: Tracking activity enhancement and suppression of heterogeneous photocatalysts Adv. Fiber Mater. 2023 5 527 542 10.1007/s42765-022-00245-5
Huang, Y. et al. Fiber-optic microfiber: Tracking activity enhancement and suppression of heterogeneous photocatalysts. Adv. Fiber Mater. 5, 527–542 (2023).10.1007/s42765-022-00245-5
10. Nguyen DLT Recent development of high-performance photocatalysts for N2 fixation: A review J. Environ. Chem. Eng. 2021 9 104997 10.1016/j.jece.2020.104997
Nguyen, D. L. T. et al. Recent development of high-performance photocatalysts for N2 fixation: A review. J. Environ. Chem. Eng. 9, 104997 (2021).10.1016/j.jece.2020.104997
11. Han Q Jiao H Xiong L Tang J Progress and challenges in photocatalytic ammonia synthesis Mater. Adv. 2021 2 564 581 10.1039/D0MA00590H
Han, Q., Jiao, H., Xiong, L. & Tang, J. Progress and challenges in photocatalytic ammonia synthesis. Mater. Adv. 2, 564–581 (2021).10.1039/D0MA00590H
12. Ithisuphalap K Photocatalysis and photoelectrocatalysis methods of nitrogen reduction for sustainable ammonia synthesis Small Methods 2019 3 1800352 10.1002/smtd.201800352
Ithisuphalap, K. et al. Photocatalysis and photoelectrocatalysis methods of nitrogen reduction for sustainable ammonia synthesis. Small Methods 3, 1800352 (2019).10.1002/smtd.201800352
13. Chen X Li N Kong Z Ong W-J Zhao X Photocatalytic fixation of nitrogen to ammonia: State-of-the-art advancements and future prospects Mater. Horiz. 2018 5 9 27 10.1039/C7MH00557A
Chen, X., Li, N., Kong, Z., Ong, W.-J. & Zhao, X. Photocatalytic fixation of nitrogen to ammonia: State-of-the-art advancements and future prospects. Mater. Horiz. 5, 9–27 (2018).10.1039/C7MH00557A
14. Schrauzer G Guth T Photolysis of water and photoreduction of nitrogen on titanium dioxide J. Am. Chem. Soc. 2002 99 7189 7193 10.1021/ja00464a015
Schrauzer, G. & Guth, T. Photolysis of water and photoreduction of nitrogen on titanium dioxide. J. Am. Chem. Soc. 99, 7189–7193 (2002).10.1021/ja00464a015
15. Choe S Rational design of photocatalysts for ammonia production from water and nitrogen gas Nano Converg. 2021 8 1 12 10.1186/s40580-021-00273-8 33403521
Choe, S. et al. Rational design of photocatalysts for ammonia production from water and nitrogen gas. Nano Converg. 8, 1–12 (2021).33403521 10.1186/s40580-021-00273-8
16. Hoffman BM Lukoyanov D Yang Z-Y Dean DR Seefeldt LC Mechanism of nitrogen fixation by nitrogenase: The next stage Chem. Rev. 2014 114 4041 4062 10.1021/cr400641x 24467365
Hoffman, B. M., Lukoyanov, D., Yang, Z.-Y., Dean, D. R. & Seefeldt, L. C. Mechanism of nitrogen fixation by nitrogenase: The next stage. Chem. Rev. 114, 4041–4062 (2014).24467365 10.1021/cr400641x
17. Hu S Fe3+ doping promoted N2 photofixation ability of honeycombed graphitic carbon nitride: The experimental and density functional theory simulation analysis Appl. Catal. B 2017 201 58 69 10.1016/j.apcatb.2016.08.002
Hu, S. et al. Fe3+ doping promoted N2 photofixation ability of honeycombed graphitic carbon nitride: The experimental and density functional theory simulation analysis. Appl. Catal. B 201, 58–69 (2017).10.1016/j.apcatb.2016.08.002
18. Li G Li F Liu J Fan C Fe-based MOFs for photocatalytic N2 reduction: Key role of transition metal iron in nitrogen activation J. Solid State Chem. 2020 285 121245 10.1016/j.jssc.2020.121245
Li, G., Li, F., Liu, J. & Fan, C. Fe-based MOFs for photocatalytic N2 reduction: Key role of transition metal iron in nitrogen activation. J. Solid State Chem. 285, 121245 (2020).10.1016/j.jssc.2020.121245
19. Shen Z Enhanced N2 photofixation activity of flower-like BiOCl by in situ Fe(III) doped as an activation center J. Colloid Interface Sci. 2021 584 174 181 10.1016/j.jcis.2020.09.111 33069017
Shen, Z. et al. Enhanced N2 photofixation activity of flower-like BiOCl by in situ Fe(III) doped as an activation center. J. Colloid Interface Sci. 584, 174–181 (2021).33069017 10.1016/j.jcis.2020.09.111
20. Zhang S Zhao Y Shi R Waterhouse GI Zhang T Photocatalytic ammonia synthesis: Recent progress and future EnergyChem 2019 1 100013 10.1016/j.enchem.2019.100013
Zhang, S., Zhao, Y., Shi, R., Waterhouse, G. I. & Zhang, T. Photocatalytic ammonia synthesis: Recent progress and future. EnergyChem 1, 100013 (2019).10.1016/j.enchem.2019.100013
21. Kumar V Sol-gel assisted synthesis and tuning of structural, photoluminescence, magnetic and multiferroic properties by annealing temperature in nanostructured zinc ferrite Mater. Today Proc. 2021 47 6242 6248 10.1016/j.matpr.2021.05.215
Kumar, V. et al. Sol-gel assisted synthesis and tuning of structural, photoluminescence, magnetic and multiferroic properties by annealing temperature in nanostructured zinc ferrite. Mater. Today Proc. 47, 6242–6248 (2021).10.1016/j.matpr.2021.05.215
22. Chamani S Khatamian M Peighambardoust NS Aydemir U Microwave-assisted auto-combustion synthesis of binary/ternary Cox Ni1− x ferrite for electrochemical hydrogen and oxygen evolution ACS Omega 2021 6 33024 33032 10.1021/acsomega.1c05183 34901654
Chamani, S., Khatamian, M., Peighambardoust, N. S. & Aydemir, U. Microwave-assisted auto-combustion synthesis of binary/ternary Cox Ni1− x ferrite for electrochemical hydrogen and oxygen evolution. ACS Omega 6, 33024–33032 (2021).34901654 10.1021/acsomega.1c05183
23. Eivazzadeh-Keihan R Magnetic copper ferrite nanoparticles functionalized by aromatic polyamide chains for hyperthermia applications Langmuir 2021 37 8847 8854 10.1021/acs.langmuir.1c01251 34259525
Eivazzadeh-Keihan, R. et al. Magnetic copper ferrite nanoparticles functionalized by aromatic polyamide chains for hyperthermia applications. Langmuir 37, 8847–8854 (2021).34259525 10.1021/acs.langmuir.1c01251
24. Akhlaghi N Najafpour-Darzi G Manganese ferrite (MnFe2O4) nanoparticles: From synthesis to application—A review J. Ind. Eng. Chem. 2021 103 292 304 10.1016/j.jiec.2021.07.043
Akhlaghi, N. & Najafpour-Darzi, G. Manganese ferrite (MnFe2O4) nanoparticles: From synthesis to application—A review. J. Ind. Eng. Chem. 103, 292–304 (2021).10.1016/j.jiec.2021.07.043
25. Suresh R Rajendran S Kumar PS Vo D-VN Cornejo-Ponce L Recent advancements of spinel ferrite based binary nanocomposite photocatalysts in wastewater treatment Chemosphere 2021 274 129734 10.1016/j.chemosphere.2021.129734 33548641
Suresh, R., Rajendran, S., Kumar, P. S., Vo, D.-V.N. & Cornejo-Ponce, L. Recent advancements of spinel ferrite based binary nanocomposite photocatalysts in wastewater treatment. Chemosphere 274, 129734 (2021).33548641 10.1016/j.chemosphere.2021.129734
26. Casbeer E Sharma VK Li X-Z Synthesis and photocatalytic activity of ferrites under visible light: A review Sep. Purif. Technol. 2012 87 1 14 10.1016/j.seppur.2011.11.034
Casbeer, E., Sharma, V. K. & Li, X.-Z. Synthesis and photocatalytic activity of ferrites under visible light: A review. Sep. Purif. Technol. 87, 1–14 (2012).10.1016/j.seppur.2011.11.034
27. López-Ortega A Lottini E Fernandez CdJ Sangregorio C Exploring the magnetic properties of cobalt-ferrite nanoparticles for the development of a rare-earth-free permanent magnet Chem. Mater. 2015 27 4048 4056 10.1021/acs.chemmater.5b01034
López-Ortega, A., Lottini, E., Fernandez, Cd. J. & Sangregorio, C. Exploring the magnetic properties of cobalt-ferrite nanoparticles for the development of a rare-earth-free permanent magnet. Chem. Mater. 27, 4048–4056 (2015).10.1021/acs.chemmater.5b01034
28. Alazmi A Cobalt ferrite supported on reduced graphene oxide as a T2 contrast agent for magnetic resonance imaging RSC Adv. 2019 9 6299 6309 10.1039/C8RA09476D 35517263
Alazmi, A. et al. Cobalt ferrite supported on reduced graphene oxide as a T2 contrast agent for magnetic resonance imaging. RSC Adv. 9, 6299–6309 (2019).35517263 10.1039/C8RA09476D
29. Dai Q Self-assembled ferrimagnet−polymer composites for magnetic recording media Nano Lett. 2010 10 3216 3221 10.1021/nl1022749 20698640
Dai, Q. et al. Self-assembled ferrimagnet−polymer composites for magnetic recording media. Nano Lett. 10, 3216–3221 (2010).20698640 10.1021/nl1022749
30. De D Studies on cancer cell death through delivery of dopamine as anti-cancer drug by a newly functionalized cobalt ferrite nano-carrier Colloids Surf. A 2021 627 127202 10.1016/j.colsurfa.2021.127202
De, D. et al. Studies on cancer cell death through delivery of dopamine as anti-cancer drug by a newly functionalized cobalt ferrite nano-carrier. Colloids Surf. A 627, 127202 (2021).10.1016/j.colsurfa.2021.127202
31. Prasad PD Hemalatha J Enhanced magnetic properties of highly crystalline cobalt ferrite fibers and their application as gas sensors J. Magn. Magn. Mater. 2019 484 225 233 10.1016/j.jmmm.2019.04.026
Prasad, P. D. & Hemalatha, J. Enhanced magnetic properties of highly crystalline cobalt ferrite fibers and their application as gas sensors. J. Magn. Magn. Mater. 484, 225–233 (2019).10.1016/j.jmmm.2019.04.026
32. Dutta V Review on augmentation in photocatalytic activity of CoFe2O4 via heterojunction formation for photocatalysis of organic pollutants in water J. Saudi Chem. Soc. 2019 23 1119 1136 10.1016/j.jscs.2019.07.003
Dutta, V. et al. Review on augmentation in photocatalytic activity of CoFe2O4 via heterojunction formation for photocatalysis of organic pollutants in water. J. Saudi Chem. Soc. 23, 1119–1136 (2019).10.1016/j.jscs.2019.07.003
33. Chamani S Photocatalytic hydrogen evolution performance of metal ferrites/polypyrrole nanocomposites Int. J. Hydrog. Energy 2022 47 32940 32954 10.1016/j.ijhydene.2022.07.193
Chamani, S. et al. Photocatalytic hydrogen evolution performance of metal ferrites/polypyrrole nanocomposites. Int. J. Hydrog. Energy 47, 32940–32954 (2022).10.1016/j.ijhydene.2022.07.193
34. Shen Y Nitrogen fixation from air at normal temperature and pressure via Cobalt-iron photocatalyst day and night Mol. Catal. 2022 518 112091 10.1016/j.mcat.2021.112091
Shen, Y. et al. Nitrogen fixation from air at normal temperature and pressure via Cobalt-iron photocatalyst day and night. Mol. Catal. 518, 112091 (2022).10.1016/j.mcat.2021.112091
35. Zhang S Construct vacancy nitrogen controllable Z-scheme 3D porous g-C3N4/CoFe2O4 composite material for high-efficient photofixation nitrogen Diam. Relat. Mater. 2023 138 110167 10.1016/j.diamond.2023.110167
Zhang, S. et al. Construct vacancy nitrogen controllable Z-scheme 3D porous g-C3N4/CoFe2O4 composite material for high-efficient photofixation nitrogen. Diam. Relat. Mater. 138, 110167 (2023).10.1016/j.diamond.2023.110167
36. Khatamian M Khandar A Haghighi M Ghadiri M Darbandi M Synthesis, characterization and acidic properties of nanopowder ZSM-5 type ferrisilicates in the Na+/K+ alkali system Powder Technol. 2010 203 503 509 10.1016/j.powtec.2010.06.012
Khatamian, M., Khandar, A., Haghighi, M., Ghadiri, M. & Darbandi, M. Synthesis, characterization and acidic properties of nanopowder ZSM-5 type ferrisilicates in the Na+/K+ alkali system. Powder Technol. 203, 503–509 (2010).10.1016/j.powtec.2010.06.012
37. Waldron R Infrared spectra of ferrites Phys. Rev. 1955 99 1727 10.1103/PhysRev.99.1727
Waldron, R. Infrared spectra of ferrites. Phys. Rev. 99, 1727 (1955).10.1103/PhysRev.99.1727
38. Gabal M Al Angari Y Effect of chromium ion substitution on the electromagnetic properties of nickel ferrite Mater. Chem. Phys. 2009 118 153 160 10.1016/j.matchemphys.2009.07.025
Gabal, M. & Al Angari, Y. Effect of chromium ion substitution on the electromagnetic properties of nickel ferrite. Mater. Chem. Phys. 118, 153–160 (2009).10.1016/j.matchemphys.2009.07.025
39. Shukla DB Pandya VP Estimation of crystalline phase in ZSM-5 zeolites by infrared spectroscopy J. Chem. Technol. Biotechnol. 1989 44 147 154 10.1002/jctb.280440206
Shukla, D. B. & Pandya, V. P. Estimation of crystalline phase in ZSM-5 zeolites by infrared spectroscopy. J. Chem. Technol. Biotechnol. 44, 147–154 (1989).10.1002/jctb.280440206
40. Prabhakaran T Hemalatha J Combustion synthesis and characterization of cobalt ferrite nanoparticles Ceram. Int. 2016 42 14113 14120 10.1016/j.ceramint.2016.06.025
Prabhakaran, T. & Hemalatha, J. Combustion synthesis and characterization of cobalt ferrite nanoparticles. Ceram. Int. 42, 14113–14120 (2016).10.1016/j.ceramint.2016.06.025
41. Majumder D Karan S Ceramic nanocomposite 2013 Elsevier 51 91
Majumder, D. & Karan, S. Ceramic nanocomposite 51–91 (2013).
42. Leonard RH Quantitative range of Nessler's reaction with ammonia Clin. Chem. 1963 9 417 422 10.1093/clinchem/9.4.417
Leonard, R. H. Quantitative range of Nessler’s reaction with ammonia. Clin. Chem. 9, 417–422 (1963).10.1093/clinchem/9.4.417
43. Rusina O Linnik O Eremenko A Kisch H Nitrogen photofixation on nanostructured iron titanate films Chem. A Eur. J. 2003 9 561 565 10.1002/chem.200390059
Rusina, O., Linnik, O., Eremenko, A. & Kisch, H. Nitrogen photofixation on nanostructured iron titanate films. Chem. A Eur. J. 9, 561–565 (2003).10.1002/chem.200390059
44. Ye L Ni2P loading on Cd0.5Zn0.5S solid solution for exceptional photocatalytic nitrogen fixation under visible light Chem. Eng. J. 2017 307 311 318 10.1016/j.cej.2016.08.102
Ye, L. et al. Ni2P loading on Cd0.5Zn0.5S solid solution for exceptional photocatalytic nitrogen fixation under visible light. Chem. Eng. J. 307, 311–318 (2017).10.1016/j.cej.2016.08.102
45. Zhao Y Ammonia detection methods in photocatalytic and electrocatalytic experiments: How to improve the reliability of NH3 production rates? Adv. Sci. 2019 6 1802109 10.1002/advs.201802109
Zhao, Y. et al. Ammonia detection methods in photocatalytic and electrocatalytic experiments: How to improve the reliability of NH3 production rates. Adv. Sci. 6, 1802109 (2019).10.1002/advs.201802109
46. Guzman F Chuang SS Yang C Role of methanol sacrificing reagent in the photocatalytic evolution of hydrogen Ind. Eng. Chem. Res. 2013 52 61 65 10.1021/ie301177s
Guzman, F., Chuang, S. S. & Yang, C. Role of methanol sacrificing reagent in the photocatalytic evolution of hydrogen. Ind. Eng. Chem. Res. 52, 61–65 (2013).10.1021/ie301177s
47. Puangpetch T Sreethawong T Yoshikawa S Chavadej S Hydrogen production from photocatalytic water splitting over mesoporous-assembled SrTiO3 nanocrystal-based photocatalysts J. Mol. Catal. A: Chem. 2009 312 97 106 10.1016/j.molcata.2009.07.012
Puangpetch, T., Sreethawong, T., Yoshikawa, S. & Chavadej, S. Hydrogen production from photocatalytic water splitting over mesoporous-assembled SrTiO3 nanocrystal-based photocatalysts. J. Mol. Catal. A: Chem. 312, 97–106 (2009).10.1016/j.molcata.2009.07.012
48. Mao C Anion (O, N, C, and S) vacancies promoted photocatalytic nitrogen fixation Green Chem. 2019 21 2852 2867 10.1039/C9GC01010F
Mao, C. et al. Anion (O, N, C, and S) vacancies promoted photocatalytic nitrogen fixation. Green Chem. 21, 2852–2867 (2019).10.1039/C9GC01010F
49. Rong X An all-solid-state Z-scheme TiO2/ZnFe2O4 photocatalytic system for the N2 photofixation enhancement Chem. Eng. J. 2019 371 286 293 10.1016/j.cej.2019.04.052
Rong, X. et al. An all-solid-state Z-scheme TiO2/ZnFe2O4 photocatalytic system for the N2 photofixation enhancement. Chem. Eng. J. 371, 286–293 (2019).10.1016/j.cej.2019.04.052
50. Feng H Xu Q Lv T Liu H Bimetallic NH2-MIL-101 (Fe, Co) as highly efficient photocatalyst for nitrogen fixation Appl. Catal. B: Environ. Energy 2024 351 123949 10.1016/j.apcatb.2024.123949
Feng, H., Xu, Q., Lv, T. & Liu, H. Bimetallic NH2-MIL-101 (Fe, Co) as highly efficient photocatalyst for nitrogen fixation. Appl. Catal. B: Environ. Energy 351, 123949 (2024).10.1016/j.apcatb.2024.123949
51. Zhao Z Nitrogenase-inspired mixed-valence MIL-53 (FeII/FeIII) for photocatalytic nitrogen fixation Chem. Eng. J. 2020 400 125929 10.1016/j.cej.2020.125929
Zhao, Z. et al. Nitrogenase-inspired mixed-valence MIL-53 (FeII/FeIII) for photocatalytic nitrogen fixation. Chem. Eng. J. 400, 125929 (2020).10.1016/j.cej.2020.125929
52. Chen S Zhao X Xie F Tang Z Wang X Efficient charge separation between ZnIn2S4 nanoparticles and polyaniline nanorods for nitrogen photofixation New J. Chem. 2020 44 7350 7356 10.1039/D0NJ01102A
Chen, S., Zhao, X., Xie, F., Tang, Z. & Wang, X. Efficient charge separation between ZnIn2S4 nanoparticles and polyaniline nanorods for nitrogen photofixation. New J. Chem. 44, 7350–7356 (2020).10.1039/D0NJ01102A
53. Almojil SF Constructing a ZnO/CuCo2O4 pn heterojunction photocatalyst for efficiently hexavalent chromium–phenol detoxification and nitrogen fixation J. Phys. Chem. Solids 2023 172 111057 10.1016/j.jpcs.2022.111057
Almojil, S. F. et al. Constructing a ZnO/CuCo2O4 pn heterojunction photocatalyst for efficiently hexavalent chromium–phenol detoxification and nitrogen fixation. J. Phys. Chem. Solids 172, 111057 (2023).10.1016/j.jpcs.2022.111057
54. Mokhtari A Khatamian M Tuning the photocatalytic activity of graphene quantum dots via decorating the X%(Co3O4) as modern photocatalysts to produce ammonia J. Mol. Liq. 2023 391 123223 10.1016/j.molliq.2023.123223
Mokhtari, A. & Khatamian, M. Tuning the photocatalytic activity of graphene quantum dots via decorating the X%(Co3O4) as modern photocatalysts to produce ammonia. J. Mol. Liq. 391, 123223 (2023).10.1016/j.molliq.2023.123223
