Cationic Polystyrene Resin Bound Silver Nanocomposites Assisted Fourier Transform Infrared Spectroscopy for Enhanced Catalytic Reduction of 4-Nitrophenol in Aqueous Medium

Abstract

The present work reported a novel strategy to construct supported cationic-polystyrene-resin-bound silver nanocomposites for enhanced catalytic reduction of 4-nitrophenol in an aqueous medium. The Fourier transform infrared spectroscopy (FTIR) was used as a model instrument for the study of catalytic reduction of 4-nitrophenol using cationic-polystyrene-resin-bound silver nanocomposite materials. The mechanism is based on the reduction of 4-nitrophenol to 4-aminophenol due to the electron transfer process that occurred between donor borohydride (BH₄⁻) and acceptor 4-nitrophenol. The polystyrene resin provides support and surface area to increase the catalytic activity of silver nanoparticles. The diffused reflectance-Fourier transform infrared spectroscopy revealed the binding of silver particles onto the surface of cationic polystyrene resin beads. Furthermore, the catalyst was easily separated by the filtration and drying process and was able to reuse. A quantitative analysis of this work has also been performed. The linearity range, the limit of detection, and the limit of quantification obtained for the present method were 0.1 × 10⁻⁴ to 1.0 M, 0.6 M, and 2.1 M, respectively. Moreover, a good catalytic efficiency was found to be 96.8%. The advantages of the current method are its simplicity, sensitivity, rapidity, low cost, ease of preparation, and excellent catalytic efficiency to reduce 4-nitrophenol from an aqueous solution.

1. Introduction

The nitro-aromatic compounds (NAC) are important primary resources for fabricating different types of industrial chemicals, dyes, insecticides, fungicides, pharmaceuticals, and volatile products. The NAC mainly includes nitrophenol and nitrobenzene constituents, which have toxic and perilous properties [1]. The exposure of nitrophenol compounds to environmental parts may cause different types of health problems. According to the US Environmental Protection Agency, nitrophenol has been considered one priority pollutant [2,3]. It causes eye irritation, nausea, headaches, tiredness, and cyanosis in humans [3,4]. Many techniques, such as degradation, adsorption, electrochemical cure, etc., have been proposed regarding its removal from aquatic resources [4,5]. On the other hand, the shortcomings of these conventional managements include high cost, strict operating conditions, slow degradation rate, and efficiency can remarkably slow down their applications on a large scale [4,5]. The nitrophenol conversion or reduction product aminophenol has an essential role in manufacturing palliatives, antipyretics, and different cosmetic products [3,6]. Therefore, the current reduction methods of hydrogenating reduction may be eye-catching for researchers in several fields because it is necessary to convert the pollutants to renewable and reliable resources.

In the past few years, nanotechnology has received enormous attention due to its several potential applications [7]. Further, recent studies of composite materials fabrication and their impending applications in catalysis, electronics, photonics, and magnetic have much attention and play an important role in materials research [8,9,10,11]. The nanocomposite materials formation from the combination of ion exchange resins with metal nanoparticles plays an essential role in material research. Nano-material means contrived or incidental material containing particles as an aggregate or agglomerate or in an unbound state [12,13]. The use of nanocomposite materials has been proven to be an important field to study the importance of storage and stabilization of nanoparticles on a polymer or solid support [13,14]. On the other hand, the reactivity of the planner surface of metal nanoparticles has been investigated, and many researchers have proven its important applications [15]. Silver has drawn great scientific attention among many metal nanoparticles (i.e., Gold, Silver, copper nanoparticles, etc.) because of its broad applications in catalysis. The presence of several less coordinated atoms on the surface of the nanoparticles matrix shows one distinct property [13,16]. The meticulous synthesis of silver immobilized cationic polystyrene resin composites has much interest due to its special chemical and physical properties, such as high surface area, fine shape and size, high reactivity, sensitivity, Fermi potential, regenerability, etc., as compared to particular nanoparticles [17,18], since different types of noble metal nanoparticles were synthesized on the solid support and used as efficient catalysts for several oxidation and reduction reactions [19,20,21]. Pal et al. reported the citrate-capped gold nanoparticles (AuNPs) immobilized resin materials as an efficient catalyst for reducing 4-nitrophenol to 4-aminophenol [22].

The current approach has been made for further meaningful achievements, employing Fourier transform infrared (FTIR) spectroscopy method. The FTIR spectroscopy combined with nanocomposite materials, i.e., Ag⁺ ions coated cationic resin beads, could be a new approach. Through this, the chemical analysis of the reduction reaction of nitrophenol to the corresponding aminophenol in the presence of a nanocomposite catalyst is possible in a more precise manner. Commonly, FTIR spectroscopy is used for the characterization, quantification, and qualitative analysis of biological samples and organic and inorganic compounds from the environment in different compositions and origins [23,24,25,26]. Consequently, such a method is more eco-friendly, has greater sensitivity, has easy operation conditions, and is organic solvent-free. These key points are made more customarily.

This manuscript reports a facial approach for synthesizing cationic polystyrene resin-bound silver nanocomposites (CR-AgNCs) and the reduction of the 4-nitrophenol compound to amine derivative by using CR-AgNCs as a catalyst through the FTIR method. The interaction step for silver nanoparticles with resin beads involved an electrostatic force of attraction between silver complex and polystyrene beads. The prepared CR-AgNCs catalyst was characterized by SEM, EDX, XPS, UV-Visible spectroscopy, and FTIR studies.

2. Materials and Method

2.1. Reagents and Solutions

All Chemicals used in this work were Analytical grade. Ultrapure water was used throughout the experiment. Dowex^® 50WX2, cation exchange resin (hydrogen form, 50–100 mesh), and 4-Nitrophenol were purchased from Sigma-Aldrich, St. Louis, MO, USA, and used without further sanitization. Silver Nitrate (AgNO₃) and sodium borohydride (NaBH₄) were used as received from Merck (Rahway, NJ, USA).

2.2. Instruments

The absorption spectra were recorded in a Carry 60 UV-Visible spectrophotometer (Aligent technology, range: 200–800 nm), taking the solution in a 1 cm quartz cuvette. Further, the absorption spectra of solid materials were recorded in a Thermo Fisher Scientific EVOLUTION 220 double-beam (integrated) UV-Visible spectrophotometer. All spectral scans using FTIR were made in the region 4000–400 cm⁻¹ using Nicolet iS10, Thermo Fisher Scientific, Madison, WI, USA. For solid sample analysis, ATR, and for liquid sample analysis, DRS-FTIR, was used. Where minimum resolution of 4 cm⁻¹ with spectral measurements, a potassium bromide (KBr) beam splitter was used. Omnic 9 and TQ analysis software packages (Nicolate iS10) were used to gain spectra. An average of 32 scans were acquired for each spectrum of DRS-FTIR. A magnetic heating stirrer (5MLH), Mfd. By Remi Equipment Pvt. Ltd., Mumbai, India, was used to prepare catalyst and catalytic reactions. A Sartorius electronic balance with a precision of 10 mg (CP225D, AG Gottingen, Gottingen, Germany) was used as weight measurements. The aqueous solutions were prepared with ultrapure water. Thermo Fisher Scientific Barnstead Smart2pure water system (conductivity 18.2 Ω) was used to obtain ultrapure water. The particle morphology was examined using a Scanning electron microscope (SEM) with an energy-dispersive X-ray (EDX) machine (JEOL JSM-6701F FE-SEM) attached to the instrument.

2.3. Preparation of Cationic Polystyrene Resin Bound Silver Nanocomposites (CR-AgNCs)

An ion-exchange process prepared the CR-AgNCs, in-betweens the cationic beads of Dowex 50, and the solution of different concentrations of silver nitrate, which was chemically reduced by NaBH₄ [13]. The typical procedure followed for synthesizing CR-AgNCs is shown in Figure 1, where 1 g of Dowex 50 was mixed with 10 mL silver nitrate solution (i.e., from 1 mM to 1 M). The solution was magnetically stirred for 1 h at ~25 °C in the dark for ion exchange. Then after complete occurrence of the ion exchange process, at room temperature, 10 mL of NaBH₄ (ranging from 2 mM to 2 M) was added drop wise to the reaction mixture under continuous stirring. The spectra of absorption were measured by UV-Vis spectrophotometer with a maximum of 430 nm.

2.4. Catalytic Reduction Study Employing DRS-FTIR

In a catalytic reaction process, as described by Jana et al. [8], in a quartz cuvette, 0.3 mL of 0.1 × 10⁻³ M aqueous solution of 4-nitrophenol was taken, and the volume of the solution was made up to 3 mL. In the next step, about 0.3 mL of 0.1 M aqueous NaBH₄ was added to the reaction mixture. After that, 40 mg of catalyst particles were added to the solution mixture. The appearance of characteristic absorption bands (before and after addition of catalyst) was recorded using DRS-FTIR and UV-Vis spectrophotometer.

For DRS-FTIR determination of reduction reaction, 25 µL of the above solution mixture was withdrawn with the help of a micro-syringe (Model 1710 Hamilton, Bonaduz, Switzerland) and added to granular KBr matrix. Then the matrix was dried at a temperature of around 60–85 °C in a hot air oven. After drying, it was appropriately ground using a mortar pestle and subsequently filled into the sample holder for FTIR spectral analysis. This process is also known as the dry-state method because of converting the analyte state from wet to dry. Khalkho et al., 2021 also reported the dry-state SEIRS analysis method for glutathione detection in blood and serum samples [27]. The schematic procedure for reducing 4-nitrophenol using CR-AgNCs catalyst in an aqueous solution and its analysis through DRS-FTIR is depicted in Figure 2. Furthermore, the background spectrum was collected before the FTIR analysis to remove the interference from carbon dioxide (CO₂) and water (H₂O) vapours. In order to eliminate possible interferences, the instrument was purged with nitrogen gas (analytical grade, >99.99%, iS10, iZ10 model, Thermo Fisher Scientific) for 25–30 min. The characteristic absorption band of product aminophenol was investigated at ~1600.00 cm⁻¹ for bending vibration of the -NH₂ group. The % reduction reaction rate was calculated using the change in peak intensity, i.e., the FTIR band obtained for reactant and product with change in time, during the presence of catalyst particles Equation (1). Similarly, the catalytic efficiency was calculated using the following Equation (2).

$$\%\text{ Reduction reaction rate }=\frac{\Delta \mathrm{PI}}{\Delta \mathrm{t}}$$

(1)

$${\mathrm{Q}}_{\mathrm{e}}=({\mathrm{PI}}_{\mathrm{o}}-{\mathrm{PI}}_{\mathrm{e}})\times \frac{\mathrm{W}}{\mathrm{V}}$$

(2)

where ΔPI and Δt represent the change in peak intensity and time (i.e., for conversion of reactant to the product in the presence of a catalyst), respectively. Q_e represents the catalytic efficiency of catalyst CR-AgNCs, PI₀ and PI_e are the peak intensity of initial concentration of 4-nitrophenoland equilibrium concentration of 4-nitrophenolsolution with NaBH₄ and catalyst, respectively.

3. Result and Discussion

3.1. Selection and Characterization of CR-AgNCs

The cationic resin Dowex-50 has H⁺ ions when reacted with the AgNO₃ solution in the presence of NaBH_4; the product CR-AgNCs was used as a reactive catalyst because of its astonishing properties, for example, elemental composition, uniform size distribution, crystal structure, control of aggregation, reactivity, purity, stabilization, and reproducibility. Other properties such as having higher stability, feasibility, high surface area to volume ratio, and ease of chemical or physical modification made the CR-AgNCs more preferable catalyst than other nanocomposites [28]. The catalytic performance of CR-AgNCs towards 4-nitrophenol conversion, functional groups such as –SO₃, –NO₃, –H, etc., are normally initiated onto the surface of CR-AgNCs due to the chemical modifications [29]. The high signal intensity of CR-AgNCs material was observed when an evanescent wave of reflectance electromagnetic radiation fell on the surface of nanocomposite materials in DRS-FTIR spectroscopy. Therefore, CR-AgNCs were used as an efficient catalyst for switching the nitrophenol to aminophenol and studied with DRS-FTIR spectroscopy in the mid-IR region.

The characterizations of the prepared catalyst (CR-AgNCs) employ UV-Vis spectroscopy, SEM, EDX, XPS, and FTIR spectroscopy. CR-AgNCs have optical properties that are sensitive to size, shape, concentration, agglomeration state, and refractive index near the metallic surface in nano range scale, which makes UV-Vis spectroscopy an important tool to identify, characterize and investigate these materials, and evaluate the stability of CR-AgNCs colloidal solutions. UV-Vis spectrophotometry experiments were performed for swift detection of silver nanoparticle (AgNPs) formation with a polymeric resin. The coalescence of silver atoms to larger particles in the chemical synthesis of colloidal silver gives rise to the localized surface plasmon resonance (LSPR) band. A strong absorption peak at near 400 nm in the UV-Vis spectrum indicates the formation of silver colloids [13,30]. The full width at a half maximum (FWHM) can be used to determine particle dispersion [31]. Figure 3A shows the LSPR bands for successive AgNPs formation with the resin matrix. As shown in Figure 3A, (a) and (b) did not exhibit any absorbance band specifying silver colloids, as they were formed from a low concentration of silver precursor with Dowex resin. The absence of the LSPR band could be attributed to the formation of a small amount of AgNPs, and thus it can be stated that the resin beads hindered the absorption. Figure 3A (c) displayed the characteristic sharp absorption band at 395 nm, confirming the formation of silver colloidal particles. Similarly, a red shift of the maximum absorbance was obtained at nearly 420 nm (Figure 3B (d)), a sign of a broadening of the absorption band that signifies an increase in the particle size of the AgNPs. The polystyrene resin Dowex^® 50WX2 was also characterized by Evolution 220 double beam UV-Vis spectroscopy (i.e., for solid sample analysis). Figure 3B shows the UV-Vis spectra of Dowex resin beads.

The size distribution, degree of aggregation, surface charge, and surface area were studied using SEM analysis. The SEM image obtained for Dowex resin shows microspheres with a smooth surface (Figure 4). Further, the SEM analysis of 1 M silver loaded on the surface of the resin beads revealed with concordant increment in the particle size of AgNPs being formed. This result is depicted in Figure 5. Significantly, the Dowex resin beads’ size and shape remained the same after the chemical reduction and ion exchange process with Ag ions. These results suggested that the cationic polystyrene beads are appropriate for use as a secure matrix/template for forming metal-polymer nanocomposite material, i.e., CR-AgNCs. As shown in Figure 5b, the large silver precipitates (AgPs) were found scattered uniformly, overlapping smaller AgNPs covering the surface of the polymer beads. The cation exchange resin has SO₃ negatively charged surface that confines the uptake of the Ag⁺ ions. The glut amount of silver precursor used to form CR-AgNCs seemed to overwhelm the surface area available on the resin beads, consequentially appearing as a covered surface of resin beads with a layer of metallic Ag⁺. Moreover, the glut Ag⁺ ions available in the solution were chemically reduced without an anchoring matrix, resulting in free AgPs.

The elemental analysis was conducted using the EDX to determine the amount of silver loaded onto the surface of cation exchange resin beads throughout the formation of CR-AgNCs. The EDX spectra of the free Dowex resin matrix and CR-AgNCs are shown in Figure 6a,b. EDX detected the emission of elements of carbon (C), oxygen (O), nitrogen (N), sodium (Na), sulfur (S),Silver (Ag), etc. The weight percentage (wt%) of all elements were investigated for Dowex resin beads as: O (average 47.06%), C (18.43%), S (21.62%), Na (3.70%), Ag (0.60%), and Pt (0.53%). Similarly, the wt% of elements present in CR-AgNCscatalyst particles was investigated as: O (43.43%), Ag (24.12%), S (21.93%), C (10.41%), N (2.21%), and Pb (0.08).

Next, the direct evidence for the deposition of metallic silver on the surface of cationic resin beads was investigated by X-ray photoelectron spectroscopy. The prepared nanocomposite catalyst exhibits two specific peaks with binding energies of 366 eV and 374 eV due to the Ag 3d_5/2 and Ag 3d_3/2 electrons of metallic silver (Ag⁰), respectively. As shown in Figure 7, the obtained spectral results suggest the chemical deposition of metallic silver on the surface of resin beads.

The further characterization of nanocomposite was performed employing FTIR. FTIR determined the presence of functional groups in the Dowex-50WX2 resin and CR-AgNCs, and the results are shown in Figure 8a,b. The FTIR spectrum of Dowex-50WX2, a characteristic peak at 3363.6 cm⁻¹ and 1635.2 cm⁻¹, corresponds to the O-H and S-O stretching vibration of SO₃H moiety, respectively. The absorption band of 1169.3 cm⁻¹ (S-O), 1125.2cm⁻¹, 1034.2 cm⁻¹, and 1006.9cm⁻¹ {symmetric stretching vibration of S=O (SO₃⁻group)} of the resin beads as shown in Figure 8a. This peak position was somehow shifted in the spectrum of silver precursor immobilized resin beads, demonstrating the presence of interaction between the cationic resin beads and the Ag⁺ ions. Figure 8b shows the IR peaks of prepared CR-AgNCs material. The intense peak at 3423.9 cm⁻¹ represents the stretching vibration of alcohol (O-H group), 2923.3 cm⁻¹ assigned to the symmetrical stretching vibration of the C-H group, and 1638.3 cm⁻¹ to 1000 cm⁻¹ assigned to the stretching vibration of S=O.

3.2. DRS-FTIR Spectral Assignment of Catalytic Reduction of 4-Nitrophenol Employing CR-AgNCs as a Catalyst

The catalytic activity of the prepared CR-AgNCs catalyst was confirmed through the reduction of 4-nitrophenol in the presence of NaBH₄ as a reductant. The NAC (i.e., nitrophenol) is one of the water pollutants with high toxicity and immense environmental distress [32,33]. The electron-withdrawing nitro group in nitrophenol is resistant to biological, chemical oxidation, and hydrolysis. Several reductions and separation techniques have been developed for the 4-nitrophenol compound [32,34].

In view of the analytical techniques, FTIR is a highly responsive technique widely used in analyzing chemical species through the adsorption or absorption of analyte onto the selected substrate or probe. For a few years, our research group exploited the importance of the FTIR spectroscopic technique for different quantitative analyses of amino acids, dyes, surfactants, and different pollutants in real environmental samples based on the measurement of selected characteristic IR absorption band of the analyte [23,35,36,37]. Eid et al. (2020) also report the IR method as a green chemistry approach for quantitative analysis of ternary mixtures using citrate functionalized silver nanoparticles [38]. In these assays, the DRS-FTIR approach is employed to analyze the reduction reaction of 4-nitrophenol using R-Ag NCs as a catalyst.

The initial analysis was performed using 4-nitrophenol to determine the presence of IR spectral bands of functional groups. Secondly, to observe the characteristic absorption peaks of formation of 4-aminophenol in the reduction process of nitro derivative, analysis with and without adding the amount of catalyst, i.e., R-Ag NCs to the sample solution with optimized conditions, was performed DRS-FTIR approach. For this, 10 µL of prepared reaction solution of 4-nitrophenol was taken and dropped into the finely ground KBr powder and dried for 5 min in a hot air oven for FTIR spectral analysis. Next, the addition of an appropriate quantity of NaBH₄ and R-AgNCs catalyst to the solution of 4-nitrophenol was similarly taken with varying times, and the reduction of nitrophenol into aminophenol was confirmed by analysis of the appearance of characteristic IR spectral band of an amino group at 1596.16 cm⁻¹ and disappearance of a characteristic band of the nitro group at 1509.78 cm⁻¹. The results are shown in Figure 9. In FTIR spectra of pure 4-nitrophenol, as shown in Figure 9a, a strong band at 1509.78 cm⁻¹ and 1342.00 cm⁻¹ corresponding to N=O asymmetrical and symmetrical stretching confirm the presence of the nitro group in the compound. Further, the absorption band at 3319.25 cm⁻¹ corresponds to O-H valency superimposed by C˗H valence of arene and 3083.13 cm⁻¹ for C˗H bond stretching, respectively. A prominent peak is shown as medium–weak, and multiple bands were obtained between 1400–1600 cm⁻¹, which was assigned to C=C symmetry stretching. The appearance of stretching vibrational bands is due to the aromatic ring of NAC. Similarly, Figure 9b shows the FTIR spectral bands for 4-aminophenol, where the instance sharp peak at 1596.16 cm⁻¹ assigned to N-H bending vibration (for –NH₂ group), 1341.10 cm⁻¹ for N-O stretching (i.e., for nitro-phenolate ion), 1289.34 cm⁻¹ and 1109.94 cm⁻¹ for C-N bond stretching. A broad band obtained between 3000 to 3600 cm⁻¹ was assigned to O-H symmetry stretching.

UV-Vis Study of 4-Nitrophenol Reduction

As mentioned above, 4-aminophenol has been synthesized by catalytic reduction of 4-nitrophenol over CR-AgNCs, and the progress of the catalytic reduction was also studied employing a UV-Vis spectrophotometer. For these in a quartz cuvette, 0.3 mL of 0.1 × 10⁻³ M aqueous solution of 4-nitrophenol was taken, and the volume of the solution was made up to 3 mL. About 0.3 mL of 0.1 M aqueous NaBH₄ was added to the reaction mixture, and afterward, 50 mg of catalyst particles were added to the solution mixture. Finally, the absorbance was taken using a UV-Vis. Figure 10 shows typical UV-Vis absorption spectra for successive reduction of 4-nitrophenol by the catalyst particles. An aqueous solution of 4-nitrophenol shows a distinct spectral contour with an absorption maximum at ~317 nm, as shown in Figure 10 (a). Upon the immediate addition of an ice-cold aqueous solution of freshly prepared NaBH₄ to this solution mixture, there was a shift of λ_max value of 4-nitrophenol to higher wavelength (red shift), i.e., from 317 nm to 400 nm (Figure 10 (b)) because of deprotonation of 4-nitrophenol [39,40]. Additionally, the color of the reaction mixture turned from light yellow to vivid yellow. The obtained peak indicates the formation of 4-nitrophenolate ions in an alkaline medium. The reducing agent (NaBH₄) alone is incapable of reducing nitrophenol to the corresponding amino derivative. Thus the peak at 400 nm remains constant with time and suggests that the reduction reaction does not proceed without any catalyst. With the subsequent addition of catalyst particles into the solution mixture of 4-nitrophenolate ions, the absorption spectrum at 400 nm decreases with time. At this stage, the peak for 4-nitrophenolate ions decreases and shows an increase in the peak at λ_max 297 nm, which suggests the formation of reduction product 4-aminophenol (as shown in Figure 10 (c)) [39]. Figure 10 (d) shows the single spectra at ~297 nm corresponding to 4-aminophenol.

3.3. Mechanism Involved in the Preparation of CR-AgNCs and Their Catalytic Performance in DRS-FTIR Spectroscopic Study

The reaction mechanism for the formation of CR-AgNCs catalyst and the selective detection of catalytic performance is demonstrated by performing different sets of experiments in DRS-FTIR spectroscopy. The catalyst was formed using precursor AgNO₃, Dowex 50 resin beads, and NaBH₄.The chemical synthesis of the CR-AgNCs catalyst was carried out in two simple steps, as shown in Figure 11. In step I, the cation exchange resin Dowex-50, having a protonated (H⁺) surface, was mixed and stirred with AgNO₃ solution to form an intermediate, i.e., silver-resin bead (Ag⁺-R). The cationic resin beads are composed of copolymerization of styrene and divinylbenzene, by which multiple chains of polystyrene are bound together. The sulphonic acid groups (-SO₃^-) containing a negative charge were disseminated across the benzene rings of the styrene-divinylbenzene polymer beads, while the positively charged H⁺ mobile ions are ionically bound with the negatively charged -SO₃⁻ sites [41,42]. Nevertheless, it is reported that the -SO₃^- sites have a superior affinity towards larger cations such as Ag⁺ [43]. For the sulphonic acid group containing ion exchange resins, the cation exchange affinity series depends on the charge and size of the cation to be exchanged. In addition, when the charges present in the cations are similar, the exchange capacity increases with the atomic number of the cation [42,43]. The interaction involved in the equilibrium process of immobilization of silver precursor onto the surface of cationic polystyrene resin beads is ambitious by non-covalent electrostatic interaction [14]. Here, the ion exchange procedure proceeds to generate Ag⁺-R until the Dowex resins ion exchange capacity in close proximity to exhaustion. This typical reaction step is shown in the following Equation (3).

In step II, the Ag⁺-R was chemically reduced by using NaBH₄ to form metallic Ag.

$$\mathrm{R}-{\mathrm{H}}^{+}+{\mathrm{AgNO}}_3\to \mathrm{R}-{\mathrm{Ag}}^{+}$$

(3)

$$\mathrm{R}-{\mathrm{Ag}}^{+}+{\mathrm{NaBH}}_4\to \mathrm{R}+\mathrm{Ag}+½{\mathrm{H}}_2+½{\mathrm{B}}_2{\mathrm{H}}_6+{\mathrm{Na}}^{+}$$

(4)

For the chemical reduction involving ion exchange resin in the formation of silver nanoparticles, Alonso et al. and Yee et al. [13,44] have proposed an alternative pathway. Where after the reduction process, ion exchange resin is regenerated to its original form. Even as the metallic silver is produced and adsorbed on the surface of resin beads, the stepwise reaction is given in Equations (5) and (6).

$$\mathrm{R}–{\mathrm{SO}}_3\mathrm{H}+{\mathrm{AgNO}}_3\to \mathrm{R}–{\mathrm{SO}}_3\mathrm{Ag}+{\mathrm{HNO}}_3$$

(5)

$$\mathrm{R}–{\mathrm{SO}}_3\mathrm{Ag}+{\mathrm{HNO}}_3+{\mathrm{NaBH}}_4+{\mathrm{H}}_{2}O\to \mathrm{R}–{\mathrm{SO}}_3\mathrm{H}+½{\mathrm{H}}_2+\raisebox{1ex}{$7$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{B}\left(\mathrm{OH}\right)}_3+{\mathrm{NaNO}}_3+{\mathrm{Ag}}^0$$

(6)

Subsequently, the reduction reaction occurs in a stepwise process, as shown in Figure 12. Basically, the electron transfer phenomenon occurred between donor borohydride (BH₄⁻) and acceptor nitrophenol. At first, one free hydrogen atom will move close and bind to the oxygen atom of nitrophenol, which constitutes an intermediate having a hydroxyl-like structure. In addition, it was seen as the change in color of the solution mixture to darken yellow. Then another hydrogen atom will approach and combine with the intermediate first as similar to the first one (making a dihydroxyl-like structure). After that, one H₂O molecule is released owing to the hydroxyl dehydration and making up a nitrosophenol molecule. Afterward, another two hydrogen atoms approached the oxygen and nitrogen atom of nitrosophenol molecule and then released the second H₂O molecule. In the end, two new hydrogen atoms will draw near the last residual nitrogen atom of the cluster, forming an amino group. All the possible listed formed intermediates (In 1–8) are shown in Figure 12b.

With the addition of catalyst beads (i.e., CR-AgNCs) into the reaction mixture, the product 4-aminophenol was formed quickly with reaction time. After adding the catalyst particles, which increases the likeliness of the reaction, the reduction was completed immediately, and the color of the reaction solution became lighter. It indicates the complete occurrence of the electron transfer process between donor and acceptor molecules. The reduction of 4-nitrophenol into 4-aminophenol was confirmed by the DRS-FTIR spectroscopic study of characteristic spectral bands of specific groups. For these, the characteristic peak for the nitro group (4-nitrophenol) was obtained at 1509.78 cm⁻¹. After the reduction, the shift of the spectral band annoying formation of the product, the characteristic peak for the amino group was obtained at 1596.16 cm⁻¹. Furthermore, the peak regarding nitro compound disappeared from the product FTIR spectrum (Figure 9).

Further, the recently developed method was authenticated by determining the rate of the reduction reaction, catalytic efficiency, linearity range, limit of detection (LOD), Limit of quantification (LOQ), and correlation coefficient (R²). The linearity range is obtained by constructing the calibration curve using the product peak area at 1599.16 cm⁻¹ (N-H bending) versus different concentrations of 4-nitrophenol, as shown in Figure 13. The linearity range showed by the calibration curve was 0.1 × 10⁻⁴ M to 1.0 M with a good correlation coefficient value (R²) of 0.989. Furthermore, the relative standard deviation percentage (RSD%) of the present method was calculated by four replicate measurements under the optimized conditions for DRS-FTIR analysis. The RSD% was calculated at 4.5% for the catalytic reduction reaction of nitrophenol in a water solution. The value of LOD is analyzed by using a peak area three times the standard deviation (SD) of four replicate analyses (LOD = 3SD/b). The value of ‘b’ indicates the slope value. Therefore, the LOD of the current method was calculated to be 0.6 M. Similarly, the LOQ is the smallest concentration of analyte, which can be analyzed and quantified at the stated confidence limit. It is calculated by using the formula: LOQ = 10SD/b. The LOQ obtained for the present method was 2.1 M. The rate of reduction reaction of 4-nitrophenol with time in DRS-FTIR analysis was calculated using Equation (1), as mentioned in Section 2.4. The rate of reduction and catalytic efficiency was obtained at about 8.0×10⁻³ min⁻¹ and 96.8%, respectively, for DRS-FTIR spectroscopic study. The obtained values for statistical parameters are shown in

Table 1. DRS-FTIR spectroscopic approach for 4-nitrophenol reduction catalyzed by CR-AgNCs in aqueous solution.

S. No.	Statistical Parameters	Values Obtained
1.	Linear range (M)	0.1 × 10−4 to 1.0
2.	LOD (M)	0.6
3.	LOQ (M)	2.1
4.	RSD (%)	4.5
5.	Correlation coefficient (R2)	0.989
6.	Catalytic efficiency (%)	96.8
7.	Reduction reaction rate (min−1)	8.0 × 10−3

. Since the reduction depends only on the concentration of one reactant molecule, in order that as a kinetic viewpoint, it is stated as a first-order reaction.

3.4. Factors Affecting the Catalysis Process

3.4.1. Effect of Catalyst Dosage

The different amount of catalyst was varied to determine/verify the effect of reaction catalysis. Figure 14a shows the effect of catalyst dose upon reduction reaction, i.e., a plot between signal intensity vs. the amount of catalyst. As a result, by increasing the amount of catalyst, an enhanced peak intensity of the product aminophenol was obtained with a low reaction time.

3.4.2. Effect of Time

To probe the effect of time ranging from 2 min to 30 min (i.e., 2, 5, 10, 15, 20, 25, 30 min) upon reduction reaction was performed with 0.1 × 10⁻³ M of 4-nitrophenol, 0.1 M of NaBH₄ and 40 mg of catalyst dosage. As observed, the signal intensity at 1596.16 cm⁻¹ increases with time, but after 15 min, the signal intensity remains the same. This indicates that the reaction was completed. The graphic is shown in Figure 14b.

3.4.3. Effect of Amount of NaBH₄

After seeing the consequence of catalyst amount and time on the rate of the reduction reaction, the action was performed by varying amounts of reducing agent (NaBH₄) to probe the effect on the reaction. The graph plotted between signal intensity vs. the amount of NaBH₄ is shown in Figure 14c. As observed, the signal intensity at 1596.16 cm⁻¹ increases with an increase in the amount of NaBH₄. Therefore, a faster rate of reduction takes place with the addition of reducing agents and follows a linear relationship initially. However, after a certain amount of NaBH₄, it saturates, and the reaction rate is almost constant. The reason behind it is the number of electrons required by 4-nitrophenol remains the same. Thus, after certain addition of NaBH_4, there is no effect on the reaction rate or signal intensity.

3.5. Comparison of Present CR-AgNCs/FTIR Method with Other Reported Methods

In order to show the advantages of the proposed CR-AgNCs/FTIR method, the obtained results for reduction catalysis were compared with several reported methods from the viewpoint of dissimilar experimental conditions [32,33,39,40]. In comparison with other reported works, the as-synthesized CR-AgNCs materials in DRS-FTIR showed a significant reduction reaction rate of 4-nitrophenol to 4-aminophenol.

Table 2. Comparison of present method with other reported methods.

S. No.	Catalyst	Catalysis System	Concentration of 4-Nitrophenol (M)	Catalytic Dose (mg)	Catalyst Efficiency (%)/ Reaction Rate (min−1)	Reaction Time (min)	Reference
1.	a AgNPs	UV-Vis	0.1 × 10−3	3.527 × 10−5 g L−1	1.024	8.33	[40]
2.	b Au-Ag BNPs@LDH	UV-Vis	0.1 × 10−3	20 µL/1 mg	c 3.75 × 10−2 and d 2.85 × 10−2	30	[39]
3.	e Au@MSNs	UV-Vis	0.5 × 10−2	~2	0.18	25	[33]
4.	f Au/SiO2	UV-Vis and HPLC	0.5 × 10−2	5	98.0	20	[32]
5.	CR-AgNCs	DRS-FTIR	0.1 × 10−3	40	96.8/8.0 × 10−3	10	Present method

^a Gold nanoparticles; ^b Au-Ag bimetallic nanoparticles supported on layer double hydroxide; ^c homogeneous; ^d heterogeneous; ^e gold at mesoporous silica nanospheres; ^f gold nanoparticles dispersed uniformly on mesoporous silica.

shows the comparison results. In the present method employing DRS-FTIR, % catalytic efficiency was also calculated using signal intensity and found to be ~97%. In this work, the FTIR technique shows a many-fold enhancement in the signal intensity of product aminophenol using the prepared CR-AgNCs catalyst as compared to those of AgNPs, Au-Ag BNPs@LDH, Au@MSNs, and Au/SiO₂in UV-Visible spectroscopy (UV-Vis) and high-performance liquid chromatography (HPLC) [32,33,39,40]. Therefore, it could be considered that as-synthesized CR-AgNCs could be employed as an alternative catalyst for the conversion of 4-nitrophenol to 4-aminophenol at reaction conditions.

4. Conclusions

In the present work, we efficiently and fruitfully synthesized the CR-AgNCs using the ion exchange and reduction process. The nanocomposite materials were used as a sensitive and selective catalyst for the reduction of 4-nitrophenol to aminophenol. The rapid synthesis of CR-AgNCs formation was shown using an ion exchange process between silver precursor and Dowex-50 cationic resin beads. The Dowex-50 resin microbeads were used as a supporting matrix as well as a template for the immobilization of Ag⁺ ions to form nanocomposites. The synthesized nanocomposite materials were extremely stable and remained months together and can be used as deliverables. The characterization of CR-AgNCs was performed employing FTIR, UV-Vis, XPS, SEM, and EDX analysis. The value of % catalytic efficiency and rate of reduction reaction obtained for the present method was 96.8% and 8.0 × 10⁻³ min⁻¹, respectively. The catalytic efficiency of CR-AgNCs for reduction of an aqueous solution of 4-nitrophenol was obtained when 0.1 M aqueous solution of NaBH₄ and 40 mg of catalyst beads were employed in 5 mL of 0.1 × 10⁻³ M nitrophenol solution within 15 min. Furthermore, the catalyst was easily separated by the filtration and drying process and was able to reuse. The advantages of the present method are simple, easy to handle, rapid, cost-effective, eco-friendly, and do not require any toxic chemicals and sophisticated instrumentations.