Sustainable magnetically recoverable Iridium-coated Fe3O4 nanoparticles for enhanced catalytic reduction of organic pollutants in water

The reduction of nitroarenes to aromatic amines is one of the potential pathways to remediate the hazardous impact of toxic nitroarenes on the aquatic environment. Aromatic amines obtained from the reduction of nitroaromatics are not only less toxic than nitroaromatics but also act as important intermediates in the synthesis of dyes, drugs, pigments, herbicides, and polymers. There is a huge demand for the development of cost-effective, and eco-friendly catalysts for the efficient reduction of nitroarenes. In the present study, Fe3O4@trp@Ir nanoparticles were explored as efficient catalysts for the reduction of nitroarenes. Fe3O4@trp@Ir magnetic nanoparticles were fabricated by surface coating of Fe3O4 with tryptophan and iridium by co-precipitation method. As-prepared Fe3O4@trp@Ir nanoparticles are environmentally benign efficient catalysts for reducing organic pollutants such as 4-nitrophenol (4-NP), 4-nitroaniline (4-NA), and 1-bromo-4-nitrobenzene (1-B-4-NB). The key parameters that affect the catalytic activity like temperature, catalyst loading, and the concentration of reducing agent NaBH4 were optimized. The obtained results proved that Fe3O4@trp@Ir is an efficient catalyst for reducing nitroaromatics at ambient temperature with a minimal catalyst loading of 0.0025%. The complete conversion of 4-nitrophenol to 4-aminophenol took only 20 s with a minimal catalyst loading of 0.0025% and a rate constant of 0.0522 s−1. The high catalytic activity factor (1.040 s−1 mg−1) and high turnover frequency (9 min−1) obtained for Fe3O4@trp@Ir nanocatalyst highlight the possible synergistic effect of the two metals (Fe and Ir). The visible-light photocatalytic degradation of 4-NP was also investigated in the presence of Fe3O4@trp@Ir. The photocatalytic degradation of 4-NP by Fe3O4@trp@Ir is completed in 20 min with 95.15% efficiency, and the rate of photodegradation of 4-NP (0.1507 min−1) is about twice the degradation rate of 4-NP in the dark (0.0755 min−1). The catalyst was recycled and reused for five cycles without significant reduction in the conversion efficiency of the catalyst.


Introduction
The presence of pollutants including industrial effluents, dyes, drugs, nuclear waste products, pesticides, and herbicides in concentrations greater than the permissible levels has greatly affected the quality of human life and disturbed the balance of the aquatic ecosystem (Gusain et al. 2019;Samanta et al. 2019). Various methods have been proposed to tackle the issue of water pollution such as physical separation, precipitation, and adsorption. The identification of the type of pollutants is the preliminary step in dealing with the problem of environmental pollution. The pollutants are broadly categorized into organic and inorganic pollutants based on the chemical constituents present. Inorganic pollutants generally include heavy metals, halides, radioactive waste, etc. whereas organic pollutants cover a wide range of pollutants such as dyes, polyaromatic hydrocarbons, and drugs. The byproducts produced by industries come under the category of persistent organic pollutants (POPs) (Samanta et al. 2019). These POPs can deposit in different environmental streams causing serious deleterious effects.
Nitroaromatics and their derivatives are used worldwide for the production of explosives, dyes, fungicides, and other industrial compounds (Ju and Parales 2010). Due to their widespread use and improper disposal into wastewater streams and the environment, the nitroaromatics contaminate the soil and groundwater. The presence of nitrophenols and their derivatives in the water streams produce harmful effects on aquatic organisms, and human beings (Santhiago et al. 2014). The electronic withdrawing ability of the nitro group in nitroaromatics prohibits oxidative reduction and biodegradation. Currently, more attention is centered on the transformation of these toxic organic pollutants into useful chemical products by employing catalysts. One of the amicable solutions is to convert the nitroaromatics into amines using metal catalysts (Scheme 1). Aromatic amines obtained from the reduction of nitroaromatics are not only less toxic than nitroaromatics but also act as important intermediates in the synthesis of dyes, drugs, pigments, herbicides, and polymers (Veisi et al. 2019;Rawat et al. 2021). The selective reduction of a nitro group into amines with cost-effective and environmentally benign catalysts is an attractive alternative that has been recently garnering massive attention.
The antibiotics and unused medicines disposed of unintentionally into the aquatic environment are also of serious concern to the aquatic organisms. The persistence of antibiotics such as tetracycline, sulfadiazine, and albendazole in the water environment has greatly impacted human health and aquatic life (Khan et al. 2021;Chen et al. 2022a). The persistent antibiotics present in the water lead to the disappearance of the key microbial groups contributing to the ecological imbalance (Bilal et al. 2019). Effective remediation of organic pollutants from water streams is a pressing global issue and the need of the hour is to develop novel strategies to remove organic contaminants from water streams using eco-friendly and cost-effective processes. Various methods have been developed for the removal of organic pollutants from wastewater such as adsorption, electrochemical processes, and photocatalysis (Mafa et al. 2021). Among various methods available, photocatalytic degradation is one of the most powerful methods for degrading organic pollutants due to its simplicity, environment-friendly nature, cost-effectiveness, and generation of non-toxic by-products (Sun et al. 2019;Chen et al. 2020Chen et al. , 2022aYu et al. 2022).
Photodegradation of organic pollutants is based on the generation of highly reactive species such as free radicals for the mineralization of organic compounds (Chen et al. 2022b). Photocatalysts employing semiconductors have emerged as one of the most effective methods for the degradation of organic pollutants such as dyes, antibiotics, and heavy metal ions. The use of nanostructured metal oxides such as TiO 2 , ZnO, CeO 2 , and Fe 3 O 4 has arrived as one of the most productive approaches for the degradation of organic pollutants using visible light photocatalysis (Padhi et al. 2017). Various materials are known in the literature which possesses excellent photocatalytic activity but their practical use in environmental remediation is limited due to the problem associated with their separation, recovery, and reuse of catalysts after photochemical reaction (Mansingh et al. 2019). These issues can be addressed by the design and development of magnetically separable novel photocatalysts. Magnetite (Fe 3 O 4 ) has emerged as one of the most effective material among all other potential candidates such as TiO 2 , ZnO, and CeO 2 as it is easy to prepare, it possesses excellent conductivity and magnetic property, it is non-toxic, and it has high light-responsive nature making it suitable for photocatalysis.
Fe 3 O 4 can generate excitons by harvesting visible light owing to its narrow band gap (Mohanta and Ahmaruzzaman 2021). The rapid agglomeration and high charge carrier recombination rate of bare Fe 3 O 4 nanoparticles limit the use of magnetite nanoparticles for practical applications. This problem can be overcome by the functionalization of Fe 3 O 4 with various materials such as proteins, amino acids, chitosan, and graphene oxide (Atacan et al. 2016). Padhi et al. have introduced graphene on the surface of Fe 3 O 4 which not only reduces the agglomeration of magnetic nanoparticles but also suppresses the rate of electron-hole recombination. Fe 3 O 4 nanocomposites possess a narrow bandgap, superior magnetism, and the ability to effectively transport photogenerated electrons making them ideal candidates for visible light photocatalysis (Dinari and Dadkhah 2021). Over the past decade, the Fe 3 O 4 nanocomposites decorated with different noble metals are employed in the photodegradation of nitroaromatics (Seoudi and Al-Marhaby 2016;Samuel et al. 2020). The photocatalytic efficiency of the synthesized nanocatalyst can also be improved by incorporating noble metals in the nanocomposite. The incorporation of noble metals enhances the light absorption ability of the photocatalyst due to the surface plasmon resonance effect of noble metals (Pham et al. 2021). In the present study, we have exploited this strategy further by immobilization of iridium metal on the surface of tryptophan (AA)-grafted Fe 3 O 4 nanoparticles Scheme 1 Reduction of nitroarenes to aromatic amines to develop eco-friendly, and cost-effective catalysts for the reduction of nitroarenes under ambient conditions.
Recently, Thekkathu and co-workers reported a magnetically recoverable iridium nanocatalyst supported on a silica shell for nitrophenol reduction (Thekkathu et al. 2020). Although these materials possess improved stability and enhanced recyclability, the procedures for synthesis are complicated and involve multiple steps. The surface coating with silica shell not only enhances the diameter of the synthesized nanoparticles but also causes a reduction in the magnetic property. A feasible option involves the direct functionalization of magnetic nanoparticles with biocompatible molecules such as amino acids (AA) (Tie et al. 2006). AA not only possesses active groups for interaction with different biological molecules but also provides colloidal stability to the MNPs (Pav et al. 2020). The carboxylic acid functional group of AA can be efficiently grafted on Fe 3 O 4 and nitrogen donor atoms of AA can participate in the coordination to the noble metal centers (Ir, Pt, Ag, etc.) to produce stable metal complexes for application in catalysis. For example, Jain et al. recently reported the immobilization of Ag NPs on the Fe 3 O 4 @glutathione super magnetic nanoparticles (Kumari et al. 2019) for the reduction of nitroarenes. The Fe 3 O 4 -Glu-Ag NPs were efficient in the reduction of 4-NP even in the gram scale employing water as a solvent, and NaBH 4 as a reducing agent providing the pathway for the development of green catalysts for application in the industry (Kumari et al. 2019).
Despite the excellent catalytic activity and corrosion resistivity of Ir-based nanocatalysts, their use in the hydrogenation of nitroarenes has not been explored rigorously. Specifically, the photocatalytic reduction of nitroaromatics using iridium nanoparticles or homogeneous Ir catalysts has been scarcely reported. Moreover, large-scale industrial use of Ir NPs is limited due to their high cost, difficulty in separation, limited supply, and their tendency to undergo aggregation. Reducing the amounts of the catalyst used and improving the catalytic efficiency is among the top priorities for their large-scale practical applications in industries. To address these challenges, iridium metal has been immobilized on the surface of magnetic Fe 3 O 4 @trp nanoparticles to enhance the recyclability and catalytic efficiency in the reduction of nitroarenes. Fe 3 O 4 @trp@ Ir is an efficient catalyst for reducing nitroaromatics at ambient temperature with a minimal catalyst loading of 0.0025%, and with nominal iridium metal content (0.17% by atom concentration). The use of a minimal amount of iridium metal source, cheap magnetically separable Fe 3 O 4 catalyst support, and bio-compatible tryptophan ligand for complexation of iridium are the key highlights of the present work that provide the suitable platform for the development of cost-effective and environmentally benign catalyst of nitroaromatics reduction into aromatic amines.
To our knowledge, there are no reports in the literature where the iridium-tryptophan complex immobilized on the surface of Fe 3 O 4 was employed in the catalytic reduction of nitroarenes at ambient conditions. The catalyst can be recovered after each catalytic cycle by magnetic separation and reused for the next catalytic cycle. Due to the recyclability of the catalyst, the cost associated with the use of Ir metal is a more economical paying way for a costeffective approach. Moreover, catalysis reactions are performed in an aqueous environment without compromising the product yields, thus it has potential application in the industry as a sustainable catalysis process for the reduction of nitroarenes into aromatic amines.
Here in this report, it has been postulated that the tryptophan AA anchored on the surface of Fe 3 O 4 will enhance the stability of magnetic nanoparticles towards surface oxidation, and efficiently reduce the agglomeration of iridium nanoparticles (Dhanalakshmi et al. 2020;Alaghmandfard and Madaah Hosseini 2021;Belachew et al. 2021). It has also been hypothesized that the presence of iridium on the surface of Fe 3 O 4 @trp@Ir will definitely aid in enhanced photocatalytic reduction of organic pollutants, specifically nitroaromatics. A simple and effective approach for the fabrication of Fe 3 O 4 @trp@Ir nanoparticles is reported in this work. The primary aim of the present work is to develop a robust environmentally benign catalyst (Fe 3 O 4 @ trp@Ir) for the rapid reduction of nitroarenes under mild reaction conditions. The generated nanocatalyst's catalytic efficiency was demonstrated in the elimination of different organic pollutants by NaBH 4 in an aqueous medium. The catalyst morphology and microscopic structural features were characterized using SEM-EDX, X-ray diffraction (XRD), and TEM analysis. The catalytic activity in dark vs light was investigated and compared with the existing catalysts. The photoelectrochemical studies and EPR spectral analysis were performed to obtain insights into the potential pathways for the photochemical degradation of nitrophenols.

Characterization of catalysts and instrumentation
The UV-Visible absorption spectra were recorded using Systronics Double beam Spectrophotometer 2203. The asprepared Fe 3 O 4 @trp and Fe 3 O 4 @trp@Ir nanocatalysts were characterized by Fourier transform infrared spectroscopy (FTIR, Perkin Elmer Spectrum Two spectrometer) and XRD (Bruker Model D8-Advance, Cu anode, 40 kV, 40 mA). The particle size and surface morphologies were obtained using transmission electron microscopy (TEM, JEOL JEM-F200) and field emission scanning electron microscopy (FESEM, Model JSM6100 Jeol). The elemental composition was determined by X-ray photoelectron spectroscopy (XPS, Physical Electronics, and Model PHI 5000 VersaProbe III). The specific surface areas of Fe 3 O 4 @trp@Ir nanocatalyst were determined by Brunaeur-Emmett-Teller (BET, Quantachrome). A vibrating sample magnetometer (VSM, Lake Shore Model-7410 Series) was used to determine the magnetism of synthesized nanocatalysts. The zeta potential of nanoparticles was measured by Zeta potential analyzer (Malvern Zetasizer Nano ZS). EPR spectra were acquired using a Bruker EMX MicroX EPR spectrometer with the following settings: center field 3400G, sweep width, 800G; microwave frequency, 9.395 GHz; microwave power, 0.708 mW; modulation frequency 100 kHz; modulation amplitude, 5.00G; conversion time, 75 ms; time constant, 5.12 ms; receiver gain, 1000; data points 1024; the number of X-Scans, 5.

Photoelectrochemical and electrochemical measurements
The electrochemical impedance spectra (EIS) analysis and transient photocurrent measurements were carried out to study the photoelectrochemical performance of the synthesized nanocatalysts. The EIS measurements were carried out in the electrolyte solution containing 5 mM [Fe(CN) 6 ] 3−/4− and 0.1 M KCl with a frequency range of 100 kHz to 0.1 Hz. The surface of the glassy carbon electrode (GCE) was polished with alumina slurry and washed with distilled water before the suspension of drop on the surface of GCE. Five microliters of Fe 3 O 4 (5 mg/5 ml), Fe 3 O 4 @ trp, and Fe 3 O 4 @trp@Ir were drop casted on the surface of GCE and left to dry in air for 5-6 h. The EIS analysis was done using FRA2 software.
The photoelectrochemical (PEC) studies were done in a three-electrode system on a potentiostat (Autolab, Metrohm). A solar simulator of Peccell design (PEC-L01) having a Xenon lamp and AM 1.5 G filter of intensity = 100 mW/cm 2 was utilized to carry out linear sweep voltammetry (LSV) and transient photocurrent response studies under dark and light conditions. The samples were spray-coated onto ITO glass and were used as working electrodes whereas Pt-wire and Ag/AgCl were utilized as the counter and reference electrodes respectively. For working electrode fabrication samples were dissolved in D.I (2 mg/ml) and spray coated 10 times. Along with that 0.1 M, sodium sulfate was used as a supporting electrolyte for all PEC studies.

Preparation of Fe 3 O 4 @trp
Direct co-precipitation method was adopted for the preparation of Fe 3 O 4 @trp with slight modification (Moeini et al. 2018). It was synthesized by mixing Fe 2+ (2 mmol) and Fe 3+ (4 mmol) salts in 50mL distilled water and agitated continuously for 30 min under nitrogen atmosphere before the addition of 8 mmol tryptophan. Stirring was continued for another 30 min followed by the addition of 5 mL of NH 3 solution. The reaction mixture was stirred for another 1.5 h, and Fe 3 O 4 @trp precipitates were collected using a super magnet, washed with methanol (20 mL × 1), and deionized water (20 mL × 3), and dried in an oven at 80 °C for 24 h.

Preparation of Fe 3 O 4 @trp@Ir
Fe 3 O 4 @trp (300 mg) was dispersed in distilled water (80 mL) by ultrasonication. Solution of IrCl 3 ·3H 2 O (10 mg) in water (20 mL) was added to the above solution and stirring was continued for 12 h. Freshly prepared NaBH 4 solution was added to reduce Ir(III) ions to Ir(0). Stirring was further continued for 3 h and precipitates were separated magnetically, washed with deionized water (20 mL × 3) and ethanol (20 mL × 3), and dried in an oven at 80 °C to obtain Fe 3 O 4 @trp@Ir nanoparticles.

Catalytic reduction of 4-NP, 4-NA, and 1-B-4-NB using Fe 3 O 4 @trp@Ir nanocatalyst
The catalytic activity of the formed nanocatalyst for the reduction of nitroarenes was investigated using 4-nitrophenol as a model substrate (Dutta et al. 2014). A stock solution of 10 −2 M 4-nitrophenol was prepared and kept in dark. 0.1 M NaBH 4 solution was freshly prepared before the experiment. Thirty microliters of 4-NP (10 −2 M) and 0.3 mL of NaBH 4 solution (0.1 M) were diluted to 3 mL followed by the addition of 0.1 mg/mL of the nanocatalyst where the final concentration of 4-NP and NaBH 4 and the final nanocatalyst became 7.4 × 10 −5 M, 0.0074 M, and 0.025 mg/mL respectively. The progress of the reaction was monitored with time by recording UV-Visible spectra. After the reaction, the catalyst was recovered using an external magnet and washed several times with ethanol and distilled water before utilizing it for the next catalytic cycle.

Photocatalytic reduction of 4-NP using Fe 3 O 4 @trp@ Ir nanocatalyst
A comparative study was performed to test the photocatalytic activity of the synthesized catalyst in the reduction of 4-NP at room temperature using 200 watts LED flood lamp ( λ > 400 nm) as a source of visible light (Paul and Dhar 2020). The catalytic reduction process was carried out by the addition of 0.005 g of Fe 3 O 4 @trp@Ir nanocatalyst to the solution containing 50 ml of 4 NP (0.01 mmol L −1 ) and 5 ml of NaBH 4 (0.05 M) in a beaker and stirring was continued. The kinetics of the photocatalytic reduction of 4-NP was studied by recording UV-Vis spectra of a small aliquot of the sample at regular time intervals. The catalyst was retrieved using a magnet after the completion of the reaction and used for 5 successive cycles after washing with water and ethanol. The percentage degradation of 4-NP is given: where Ao is the absorbance of the sample before irradiation in visible light, and A is the absorbance of the sample after the light irradiation process (Shabib et al. 2022).

Results and discussion
In this work, Fe 3 O 4 @trp nanoparticles were synthesized by conventional co-precipitation method from FeSO 4 ·7H 2 O and FeCl 3 ·6H 2 O, and tryptophan in (2:4:8) ratio followed by immobilization of Iridium on the surface of Fe 3 O 4 @ trp (Scheme 2). To gain insights into the morphology and microscopic structural features of the as-prepared nanoparticles, a combination of analytical techniques such as XRD, FTIR, FE-SEM, TEM, XPS, TGA, CV, and VSM, From the XRD analysis of Fe 3 O 4 @trp@Ir nanoparticles, no characteristic peak of iridium in the final catalyst was obtained (Fig. 1a). This may be due to its low loading and small peak size (Boruah et al. 2015). The average crystallite size of Fe 3 O 4 @trp@Ir nanocatalyst estimated using the Debye-Scherrer equation was 9.3 nm. The FTIR spectra of magnetic Fe 3 O 4 nanoparticles showed strong absorption peaks at 633 and 578 cm −1 which were associated with Fe-O stretching vibration modes (Mihaela et al. 2022). A broad peak at 3415 cm −1 was assigned to stretching vibrations of adsorbed water molecules on the surface of Fe 3 O 4 nanoparticles (Fig. 1b). Two more distinctive bands were observed in the case of Fe 3 O 4 @ trp, at 1622 and 1400 cm −1 , which correspond to asymmetric COO − stretching vibration and symmetric COO − stretching vibration, respectively (Theerdhala et al. 2010). The retention of these peaks in the final catalyst proved that tryptophan remains attached even after binding with Ir nanoparticles. Thermal stability and decomposition behavior of as-prepared nanoparticles were studied by TGA (Fig. 1c). The weight loss below 200 °C was due to adsorbed water molecules and hydroxyl groups on the surface of magnetic Fe 3 O 4 nanoparticles. The decomposition of well-grafted organic groups on the surface of Fe 3 O 4 nanoparticles caused weight loss in the 200-600 °C range. After immobilization of tryptophan and iridium on the surface of Fe 3 O 4 nanoparticles, the stability of Fe 3 O 4 @trp@Ir nanocatalyst was enhanced effectively. The energy-dispersive X-ray spectrum (EDS) analysis confirmed the existence of Fe, O, C, N, and Ir in the formed nanocatalyst (Fig. 1d). The surface morphology of the formed nanocatalyst and its physical properties was determined by FE-SEM (Fig. 2a, b). The nanoparticles appeared to have a spherical shape and were nearly Scheme 2 Schematic illustration for the preparation of Fe 3 O 4 @trp and Fe 3 O 4 @trp@Ir At room temperature, the magnetic behavior of bare and modified Fe 3 O 4 nanoparticles was studied using a vibrating sample magnetometer (VSM) in the field range of − 10,000 to + 10,000 Oe (Fig. 3a). The magnetization saturation values (Ms) of Fe 3 O 4 , Fe 3 O 4 @trp, and Fe 3 O 4 @trp@Ir were found to be 71.11, 52.97, and 50.37 emu/g respectively. The superparamagnetic behavior of the final catalyst was clearly shown from the VSM graph with negligible remanence and coercivity. The lower Ms values obtained for Fe 3 O 4 @trp and Fe 3 O 4 @trp@Ir were caused by non-magnetic materials becoming immobilized on the surface of magnetic Fe 3 O 4 nanoparticles. The final catalyst showed enough saturation magnetization so that it could easily be separated using a super magnet (Sharif et al. 2019).
CV was used to record the electrochemical behavior of Fe 3 O 4 @trp and Fe 3 O 4 @trp@Ir in 0.1 M KOH solution using the ink drop coating method at a scan rate of 0.1 V/s (Tipsawat et al. 2018). Strong oxidation and reduction peaks were obtained at 1.17 V and − 1.16 V respectively in Fe 3 O 4 @trp which were attributed to irreversible redox reactions between Fe 2+ and Fe 3+ (Fig. 3b).
Zeta potential was used to determine the surface charge of Fe 3 O 4 @trp and Fe 3 O 4 @trp@Ir at neutral pH. The reported zeta potential of Fe 3 O 4 nanoparticles was − 35.6 mV (Hung et al. 2016). It can be seen from Fig. 3 c that as-prepared Fe 3 O 4 @trp is negatively charged with a zeta potential value of − 24.8 mV. The decrease in negative charge upon functionalization of Fe 3 O 4 nanoparticles with tryptophan amino acid is due to the protonation of free NH 2 groups on the surface of amino acids (Pav et al. 2020). The synthesized Fe 3 O 4 @trp@Ir nanocatalyst is positively charged with a zeta potential of + 3.02 mV (Fig. 3d). As NaBH 4 is present in excess in the reduction of the 4-NP, the 4-nitrophenolate ion formed is an anion species and is likely to be adsorbed on the positively charged nanocatalyst's surface (Aditya et al. 2017). The zeta potential value of positively charged Fe 3 O 4 @trp@Ir nanocatalyst also indicates the rapid reaction of negatively charged molecules on the surface of the nanocatalyst. The absorption of light by the material and the movement of photon-induced holes and electrons are some of the key factors which are involved in controlling a photocatalytic reaction. These key factors are related to the electronic structure of the material (Chishti et al. 2021). The UV-Vis diffuse reflectance spectrum of Fe 3 O 4 @trp and Fe 3 O 4 @trp@Ir is shown in Fig. 4 a. The UV-Vis spectra of as-prepared nanocatalyst shows a strong absorption peak at 295 nm (Fig. 4b).
The bandgap energy (Eg) of Fe 3 O 4 @trp and Fe 3 O 4 @trp@Ir as calculated from the Tauc plot was found to be 2.51 eV and 2.32 eV respectively (Fig. 4c). The decrease in the bandgap of Fe 3 O 4 @trp@Ir compared to the precursors indicates the favorable conditions for enhanced light absorption, therefore contributing to the enhanced photochemical catalytic activity compared to the bare Fe 3 O 4 magnetic nanoparticles. The bandgap energy is calculated using Tauc's relation: where Eg is the bandgap energy (eV), hν is the energy of the photon (eV), B is a constant, n is dependent on the type of electronic transition, and α is the material's absorption coefficient (cm −1 ) (Bagbi et al. 2017;Shabib et al. 2022).
The electron-hole recombination was determined using the PL spectra (Fig. 4d). The lower PL intensity of Fe 3 O 4 @ trp@Ir nanocatalyst indicates a lower electron-hole recombination rate in the presence of light (Zhao et al. 2018;Mancuso et al. 2021). Thus, the synthesized nanocatalyst increased the lifetime of electron/hole pairs and causes enhanced photocatalytic activity.
The elemental composition and oxidation states of the metallic species in the Fe 3 O 4 @trp@Ir nanocatalyst were investigated with the help of XPS spectral analysis. The survey scan of the formed Fe 3 O 4 @trp@Ir nanocatalyst shows distinctive peaks of O1s (526.81 eV), and C1s (282.24 eV), Fe2p (708.82 eV), N1s (394.62 eV), and Ir4f (61.14 eV) (Fig. 5a). XPS spectra of Ir4f in the final nanocatalyst are marked by the presence of doublet in the region 63.2 and 62.4 eV arising from the j-j coupling or spin-orbit coupling of ground electronic states (Ir4f 5/2 and Ir4f 7/2 ) as shown in Fig. 5 b. The two binding energies of Ir4f in XPS spectra of Fe 3 O 4 @trp@Ir nanocatalyst are due to the presence of metallic iridium (Ir 0 ) (Kundu and Liang 2011). The XPS analysis suggests the successful incorporation of tryptophan ligand and iridium metal on the surface of Fe 3 O 4 . The relative atomic concentration (%) of iridium in the synthesized Fe 3 O 4 @trp@Ir nanocatalyst is 0.34% as determined by XPS. XPS spectra of Fe2p, C1s, N1s, and O1s in the final nanocatalyst are shown in S3.
The standard Brunaeur-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods were used to determine the specific surface area and porosity properties respectively (Bagbi et al. 2017). The specific surface area of Fe 3 O 4 nanoparticles and Fe 3 O 4 @trp@Ir nanocatalyst calculated by plotting N 2 adsorption-desorption isotherm was 64.49 and 77.145 m 2 /g respectively ( Fig. 5c and d). From Fig. 5 d, it can be seen that the N 2 adsorption-desorption curve of as-prepared nanocatalyst is a type IV adsorption isotherm possessing mesoporous features. The presence of mesoporous structures was confirmed by the average pore diameter (10.63 nm). The total pore volume of the as-synthesized nanocatalyst is 0.405 cm 3 /g. The presence of a large specific surface area of synthesized nanocatalyst (77.145 m 2 /g) significantly enhances the photocatalytic activity by providing an increased no. of active sites for photodegradation (Chishti et al. 2021).
The surface morphology of the synthesized Fe 3 O 4 @ trp@Ir nanocatalyst was also studied by TEM (Fig. 6). Figure 6 a, b, and c reveal the quasi-spherical morphology of the synthesized nanoparticles with an average diameter of 16.5 ± 1.2 nm. The particles possess narrow size distribution with sizes ranging from 8 to 25 nm and no visible change in the morphology. The surface coating of Fe 3 O 4 nanoparticles with tryptophan and iridium can be seen from the TEM images of the Fe 3 O 4 @trp@Ir nanocatalyst. Further, EDS, FE-SEM, and XPS also support the successful coating of Fe 3 O 4 nanoparticles with tryptophan and iridium.
To understand the nature of charge transfer kinetics during the photochemical degradation of nitrophenol by Fe 3 O 4 @ trp@Ir nanocatalyst, the EIS analysis was performed. The EIS analysis suggests enhanced photoelectric currents obtained with the Fe 3 O 4 @trp@Ir nanocatalyst in comparison to that obtained in the case of Fe 3 O 4 and Fe 3 O 4 @trp. Nyquist plots of Fe 3 O 4 , Fe 3 O 4 @trp, and Fe 3 O 4 @trp@Ir obtained through EIS analysis are illustrated in Fig. 7 a  electrons leading to increase in the rate of the reaction and lowers the charge carrier recombination rate. These results support our hypothesis that the incorporation of tryptophan and Irridium on the surface of the Fe 3 O 4 results in enhanced photocurrent intensity, further contributing to the more efficient photodegradation of nitrophenols in the presence of visible light. Nyquist plot signifies that Fe 3 O 4 @trp@Ir is an excellent photocatalyst which is also supported by the lower PL intensity of the final nanocatalyst.
The PEC studies of samples with 10 coating layers were investigated for their photo response. LSV studies showed there is an increase in current density in light conditions consequently revealing photo-responsive behavior (Fig. 8a). The transient photocurrent response of Fe 3 O 4 , Fe 3 O 4 @trp, and Fe 3 O 4 @trp@ Ir was done at 1 V vs Ag/AgCl potential under the chopped light condition with 50-s time interval. It confirmed the better photoresponse in the case of Fe 3 O 4 @trp@Ir due to the fast switching of photocurrent in dark and light (Fig. 8b).
It can be concluded from the PEC study that the photoelectrode fabricated by using sample Fe 3 O 4 @trp@Ir showed better photoresponse as compared to other samples. These results are in good agreement with the experimental results obtained in the catalytic degradation of nitrophenols employing Fe 3 O 4 @trp@Ir as a photocatalyst in comparison to the bare magnetic nanoparticles Fe 3 O 4 . As evidenced by EIS analysis and PEC measurement studies, and other complimentary characterization studies, the Fe 3 O 4 @trp@Ir nanoparticles possess suitable physio-chemical properties for the photochemical degradation of organic pollutants. We have explored the potential application of Fe 3 O 4 @trp@Ir nanoparticles for the reduction of nitrophenols both in the dark and in the light and compared the catalytic activity.

Catalytic activity
The catalytic activity of the formed Fe 3 O 4 @trp@Ir nanocatalyst was investigated by choosing 4-NP as a model substrate. The peak for 4-NP absorption at 317 nm was shifted to 400 nm upon the addition of an aqueous solution of NaBH 4 (Fig. 9a). This was due to the formation of a highly stable 4-nitrophenolate anion. In time-dependent UV-Visible absorption spectra, the addition of 0.025 mg/ mL of Fe 3 O 4 @trp@Ir nanocatalyst resulted in a decrease in peak intensity at 400 nm, and the appearance of a new peak at 295 nm attributed to 4-AP was observed. The complete reduction of 4-NP to 4-AP took 6 min and completion of the reaction was indicated with the disappearance of the peak at 400 nm (Fig. 9b). With the addition of 0.05 mg/mL of nanocatalyst, the conversion of 4-NP to 4-AP takes only 20 s with apparent rate constants (K app ) as 0.0522 s −1 (Fig. 10a). Magnetic separation of catalyst is shown in Fig. 9d. In the absence of a catalyst, the absorbance of the peak at 400 nm decreased slightly, but no new absorption peak corresponding to 4-AP emerged at 295 nm. The catalytic activity of as-prepared Fe 3 O 4 @trp was also investigated for the reduction of 4-NP. The results obtained indicated that even after 2 h, only 5% conversion of 4NP to 4-AP took place. The catalytic reduction of 4-NP was also carried out in the presence of bare Fe 3 O 4 nanoparticles under both dark and light conditions. The UV-Visible results obtained signify that no significant reduction of 4-NP to 4-AP took place in the presence of Fe 3 O 4 nanoparticles even after 24 h. Ir-doped Fe 3 O 4 was also employed as a catalyst for the reduction of 4-NP. It was comparatively less effective in reducing 4-NP as compared to the Fe 3 O 4 @trp@Ir nanocatalyst. Even after 1 h under visible light, there was only a 50% conversion to the product under similar experimental conditions.
The kinetics studies indicate that the reaction catalyzed by Fe 3 O 4 @trp@Ir was pseudo-first-order with a linear relationship obtained when ln (Co/Ct) versus time profiles were plotted (Fig. 9c).
where Ct is the absorbance of 4-NP at time t, Co is the initial absorbance of 4-NP, and k is the rate constant (Priya and Asharani 2018) The effect of temperature on the degradation of 4-NP was studied at 40 °C, 50 °C, and 60 °C (Table 1). At 40 °C, the reaction was completed in 5 min with an apparent rate constant of 0.456 min −1 , and on increasing the temperature to 50 °C and 60 °C, the reaction was completed in 4 min with Kapp as 0.4741 min −1 and in 3 min with Kapp as 0.7262 min −1 respectively. 4-NA and 1B-4-NB are two other toxic pollutants that were degraded under the same experimental conditions as those of 4-NP. In the case of 1-bromo-4-nitrobenzene (1B-4-NB), the reduction of 1B-4-NB to 4-bromoaniline was completed in 4 min (Fig. 9g). The absorption peak at 282 nm completely disappeared and a new absorption peak at 239 nm confirms the formation of 4-bromoaniline. Similarly, the reduction of 4-nitroaniline (4-NA) to 1, 4-phenylenediamine took place in 4 min (Fig. 9e). The absorption The rate equation is given as ln (Ct∕Co) = −kt Fig. 6 TEM images of Fe 3 O 4 @ trp@Ir nanocatalyst (a, b, c) and particle size distribution (d) peak at 375 nm completely disappeared upon the addition of catalyst and the product obtained on reduction showed a peak at 305 nm which confirms the formation of 1, 4-phenylenediamine. Both reactions follow pseudo-first-order kinetics (Fig. 9f, h). Upon increasing the synthesized nanocatalyst concentration to 0.05 mg/mL, the reduction of 4-NA and 1B-4-NB was completed in 30 s and 60 s respectively (Fig. 10b, c). The catalytic efficiency can be defined in terms of the catalytic activity factor (k a ) as shown in Table 2. The catalytic activity factor is defined as the ratio of rate constant (k) in s −1 to the amount of catalyst used in milligrams (Aditya et al. 2017). The catalytic activity factor (k a ) for the reduction of 4-NP using Fe 3 O 4 @trp@Ir nanocatalyst was found to be 1.040 s −1 mg −1 . The observed catalytic activity factor was higher as compared to the other nanocatalysts in the literature (Table 2) suggesting the higher catalytic efficiency of synthesized Fe 3 O 4 @trp@Ir nanocatalyst. The efficiency of the synthesized nanocatalyst was also determined by calculating the turnover frequency (TOF). The TOF was calculated by dividing the no. of moles of 4-NP consumed by the no. of moles of catalyst used in the reaction per unit time (Nariya et al. 2020). The high TOF obtained for the reduction of 4-NP by Fe 3 O 4 @trp@Ir nanocatalyst (9 min −1 ) indicates that the catalyst's surface has a large no. of active sites which provides effective contact with 4-NP and efficient transfer of electrons from the catalyst surface to 4-NP.  The superior TOF and activity factor for the as-prepared Fe 3 O 4 @trp@Ir NPs is also attributed to the cooperative effect of two metals (Fe, Ir). The Fe metal in the Fe 3 O 4 magnetic particles plays a major role in the enhanced thermal and photostability of Fe 3 O 4 @trp@Ir NPs, whereas the noble metal Ir effectively participates in the reduction of nitroaromatics. As evidenced by positive Zeta potential values for Fe 3 O 4 @trp@Ir NPs, the immobilized Ir metal transfers electrons to Fe 3 O 4 support to create a positively charged surface. The reduction of nitrophenol is a surface phenomenon; therefore, accumulation of positive charges on the surface provides a suitable environment for the facile Scheme 3 A plausible mechanism for the 4-NP reduction reduction of nitrophenol. At present, we do not have solid evidence for the synergistic effect of the two metals, but further investigations on understanding the contribution of each metal in enhancing the catalytic activity of the Fe 3 O 4 @trp@ Ir NPs are currently in progress. The proposed mechanism for the 4-NP reduction using Fe 3 O 4 @trp@Ir nanocatalyst is illustrated in Scheme 3. In the first step, BH 4 − ions are adsorbed on the nanocatalyst's surface followed by the transfer of hydrogen species on the surface of iridium (Thekkathu et al. 2020). In the second step, the adsorption of 4-nitrophenolate ions took place on the surface of the catalyst. The adsorbed hydrogen species reacts with the adsorbed 4-nitrophenoate ions to form the desired 4-AP.
To elucidate the active species involved in 4-NP reduction, an EPR experiment using DMPO as a spin-trapping agent is carried out at low temperature (Nguyen et al. 2018;Jia et al. 2021). The EPR spectra indicate the generation Fig. 10 a Reduction of 4-NP using 0.05 mg/mL Fe 3 O 4 @trp@Ir catalyst, b reduction of 4-NA using 0.05 mg/mL nanocatalyst, c reduction of 1B-4-NB using 0.05 mg/mL nanocatalyst Fig. 11 EPR spectra of DMPO-H radical adduct in presence of a 4-NP, b 4-NP + NaBH 4 , c 4-NP + NaBH 4 + Fe 3 O 4 @trp@Ir, d 4-NP + NaBH 4 + Fe 3 O 4 @trp@Ir after 5 min of H* radical adducts as shown in (Fig. 11b). The signal in the EPR spectra can only be produced in the presence of NaBH 4 and DMPO which indicated that the H* radical adducts were mainly obtained from NaBH 4 . The control experiments revealed that no signal was obtained for 4-NP in the presence of DMPO (Fig. 11a). The intensity of the EPR signal increased upon the addition of Fe 3 O 4 @trp@Ir nanocatalyst which implies that the synthesized nanocatalyst promotes the generation of H* radical adducts resulting in 4-NP reduction (Fig. 11c). The intensity of signal diminished as the reaction proceeded as shown in Fig. 11d due to the consumpfigtion of H* radicals with time and finally the completion of 4-NP reduction on the surface of synthesized nanocatalyst.

Photocatalytic study
The effect of visible light on the catalytic reduction of 4-NP was also investigated (Fig. 12). In the presence of light, photocatalysis and noble metal catalysis plays an important role. However, in dark, the effect of photocatalysis is suppressed (Paquin et al. 2015). The catalytic reduction of 4-NP under visible light illumination takes only 20 min with a conversion efficiency of 95.15% (Fig. 12b). The conversion of 4-NP to 4-AP took 40 min in the absence of visible light (Fig. 12a). The value of the rate constant under visible light illumination is 0.1507 min −1 which is about twice the value of the rate constant obtained in the dark (0.0755 min −1 ) The localized surface plasmon resonance effect of iridium, enhancement of visible light absorption by Fe 3 O 4 nanoparticles, and visible light responsive behavior of the Fe 3 O 4 @trp@Ir nanocatalyst promote the catalytic process.
In addition to the activity of the catalyst, cyclic stability is another important parameter for the determination of the service life of the catalyst. The applicability of Fe 3 O 4 @trp@ Ir nanocatalyst was explored in practical applications by testing the reuse ability of the synthesized nanocatalyst. The potential recyclability and the ability to separate the catalyst magnetically are the two main advantages of the synthesized Fe 3 O 4 @trp@Ir nanocatalyst. The catalyst was magnetically retrieved, washed with DI water, and ethanol, and dried in an oven before using it for the next catalytic cycle. Fe 3 O 4 @ trp@Ir nanocatalyst possesses remarkable stability with a conversion efficiency of 4-NP as high as 89% even after the fifth catalytic cycle as shown in Fig. 13.
The reusability of the nanocatalyst was further studied by measuring the reaction rate constant after each catalytic cycle as shown in Fig. 14. The gradual reduction in the rate constant of each catalytic cycle was observed due to slight loss in the mass of the catalyst during the recycling process. The reasons for the excellent stability of synthesized  Fe 3 O 4 @trp@Ir nanocatalyst were further investigated by characterizing the structure of the nanocatalyst after five catalytic cycles. Figure 15 showed the FTIR spectra of as-synthesized nanocatalyst before and after the catalytic reduction of 4-NP. It was found that the peaks remain intact in the FTIR spectra of the nanocatalyst even after the catalytic reduction of 4-NP which indicated the stability of the catalyst in the catalytic reduction process. Figure 16 shows the XRD pattern of the recovered Fe 3 O 4 @trp@Ir nanocatalyst after 5 catalytic cycles. The six characteristic diffraction peaks remain intact even after 5 catalytic cycles which confirms the high cyclic stability of the synthesized nanocatalyst.
The reusability of the catalyst over several cycles and use of minimal catalytic loading without the compromise in the product yields in aqueous medium are some of the beneficial factors that can advance the use of this catalyst for practical industrial applications in the future.

Conclusion
A simple but effective approach for the preparation of robust and environmentally benign Fe 3 O 4 @trp@Ir nanocatalyst by co-precipitation method was investigated which could not only catalytically reduce 4-NP but also cause its photocatalytic degradation. The catalyst exhibits remarkable performance in the 4-nitrophenol degradation under visible light illumination. The prepared nanocatalyst possesses important features such as efficient magnetic separation, high stability, and recyclability. The TOF and catalytic activity factor of the Fe 3 O 4 @trp@ Ir nanocatalyst in the reduction of 4-NP are superior to some of the known bimetallic catalysts supported on the Fe 3 O 4 . The prepared nanocatalyst's remarkable catalytic activity in the degradation of 4-NP, 4-NA, and 1-B-4-NB is due to the existence of Ir/IrO 2 interfaces. The excellent photocatalytic performance of the novel catalyst makes it a suitable candidate for the environmental remediation process based on photo technology.