SnAg2O3-Coated Adhesive Tape as a Recyclable Catalyst for Efficient Reduction of Methyl Orange

Silver oxide-doped tin oxide (SnAg2O3) nanoparticles were synthesized and different spectroscopic techniques were used to structurally identify SnAg2O3 nanoparticles. The reduction of 4-nitrophenol (4-NP), congo red (CR), methylene blue (MB), and methyl orange (MO) was studied using SnAg2O3 as a catalyst. Only 1.0 min was required to reduce 95% MO; thus, SnAg2O3 was found to be effective with a rate constant of 3.0412 min−1. Being a powder, SnAg2O3 is difficult to recover and recycle multiple times. For this reason, SnAg2O3 was coated on adhesive tape (AT) to make it recyclable for large-scale usage. SnAg2O3@AT catalyst was assessed toward MO reduction under various conditions. The amount of SnAg2O3@AT, NaBH4, and MO was optimized for best possible reduction conditions. The catalyst had a positive effect since it speed up the reduction of MO by adding more SnAg2O3@AT and NaBH4 as well as lowering the MO concentration. SnAg2O3@AT totally reduced MO (98%) in 3.0 min with a rate constant of 1.3669 min−1. These findings confirmed that SnAg2O3@AT is an effective and useful catalyst for MO reduction that can even be utilized on a large scale for industrial purposes.


Introduction
Recently, much attention has been given to heterogeneous catalysis because of the enormously important role it plays in environmentally and industrially significant catalytic reactions.Catalysis has extensive applications, comprising the synthesis of crucial pharmaceutical scaffolds, the transformation of harmful chemicals into useful ones, the degradation and reduction of contaminants and toxins, etc.However, the effectiveness and durability of catalysts play a major part in catalysis [1][2][3].Therefore, the primary goal of catalysis is to prepare stable and effective catalysts, which have high catalytic activities on a large scale.The catalyst's ability to catalyze reactions is mostly determined by the catalyst's active sites, the molecules' ability to interact with the catalyst, and the mass transformation of the reacting molecules into the product [4][5][6].Therefore, it is essential to design effective catalysts with extraordinary efficiency for industrial and environmental purposes [7][8][9].
The most effective catalysts are metal oxides, which are frequently utilized in numerous industrial and environmental applications [10][11][12][13][14][15].Tin oxide (SnO 2 ) nanomaterials have attracted a lot of attention and have proven their efficiency as crucial heterogeneous catalysts.SnO 2 is an effective catalyst that has been used in a variety of environmental applications and synthetic reactions.SnO 2 has been employed in photo-catalysis, catalysis, Li-ion batteries, sensing, and other processes [16][17][18].However, doped metal oxides and metal oxide nanocomposites are substantially more effective, stable, and competent catalysts for several processes, therefore, it has gained considerable attention.Metal oxides can be doped with other metals to increase their surface area and reaction sites, which improves the catalyst's ability to catalyze various reactions.Because doping affects the characteristics of metal oxides and demonstrated outstanding catalytic activity in several catalytic reactions [19][20][21][22].Thus, in response to the growing demand for synthetic and environmental applications, silver oxide-tin oxide nanocomposite was prepared to improve catalytic efficiency.However, SnAg 2 O 3 is a powder, thus, one of the main issues with powder catalysts is that they tend to aggregate, which limits its ability to catalyze.The powder catalyst also cannot be recycled and used again by merely recovering it from the reaction medium.Therefore, to make the catalyst easier to reuse, polymeric or other supports can be employed since they play a significant role as hosts for metal oxides and other nanomaterials [23][24][25][26].Benali et al. developed a catalyst based on a Zn(II)-crosslinked alginate loaded with Ag NPs for dye reduction [27].Mallakpour et al. reported the catalytic reduction of nitrophenols, dyes, and metal ion pollutants based on mesoporous Ca-alginate/melamine-rich covalent organic polymer/cupric oxide microgel beads [28].Meena and Saini described the synthesis of a PANI/ZnO/MnO 2 ternary nanocomposite which was employed in the catalytic reduction of 4-nitrophenol as well as the adsorption of crystal violet dye from aqueous solutions [29].
Here, SnAg 2 O 3 nanoparticles were synthesized and structurally characterized via numerous techniques.After that, the catalytic performance of SnAg 2 O 3 was evaluated for the catalytic reduction of several contaminants.To examine the catalytic ability of SnAg 2 O 3 , the catalytic reduction of dyes and nitrophenols was used as a model electron exchange process.SnAg 2 O 3 was discovered to be an effective catalyst and subsequently used for catalytic reactions involving electron transfers in 4-NP, MO, CR, and MB reduction.SnAg 2 O 3 was found to be effective for MO reduction and it was revealed that the catalyst amount has a beneficial impact and greatly improves the reaction rate.Further, SnAg 2 O 3 was coated on AT to make the catalyst easier to be recycled.In the reduction of MO, BH 4 − ions diffuse into the surface of SnAg 2 O 3 and SnAg 2 O 3 @AT in the first stage, where borohydride injects electrons onto the catalyst.Then, the electrons are subsequently transferred to MO.These catalysts serve as an electron relay.Thus, SnAg 2 O 3 and SnAg 2 O 3 @AT are excellent nanocatalysts for the reduction of MO.The prepared nanocatalysts have the potential to compete and eventually substitute the well-known commercial catalysts because of their superior efficiency, cost-effectiveness, and simplicity in processing.The novelty of the developed efficient catalyst is that it overcomed the limitations like aggregation, regeneration, and non-recyclability.These limitations could be avoided by coating SnAg 2 O 3 on AT support which made the reuse of the SnAg 2 O 3 catalyst easier.AT was used as a support on which SnAg 2 O 3 was coated by just rubbing where the glue had bound the catalyst.The coating of a powder catalyst is an ideal method for the advancement of the catalyst because it is a challenge to implement powder nanomaterials as a catalyst for large scale application.The coating of powder nanomaterials on a flexible support was developed to address the problems of aggregation and recyclability.This allowed powder catalysts to be recyclable and widen the catalyst applicability.

Reagents and Chemicals
Sodium borohydride (NaBH 4 , 97%) was purchased from BDH chemicals, England.Silver chloride, tin chloride, 4-nitrophenol (4-NP), congo red (CR), methyl orange (MO), and methylene blue (MB) were purchased from Sigma-Aldrich.To prepare all solutions, distilled water was utilized.Adhesive tape, which is normally used for packing purposes, was bought from the local market and used as a supporting glue-based tape for the catalyst.

Preparation of SnAg 2 O 3 and SnAg 2 O 3 @AT
Initially, the solution was made by placing the salts of AgNO 3 and SnCl 2 .2H 2 O in 100 mL of water, and stirring the mixture (1:1 molar ratio) for 30 min.Then, the pH of the solution was adjusted to 10 by dropping NaOH solution, and the solution was stirred continuously at 60.0 • C overnight while being kept on a hot plate stirrer [17,18].As soon as the reaction was stopped, SnAg 2 O 3 settled down as a precipitate.Then, SnAg 2 O 3 was washed, dried, and then further calcined for five hours at 500 • C. SnAg 2 O 3 @AT was prepared by coating SnAg 2 O 3 on AT, as shown in Scheme 1.
Materials 2023, 16, x FOR PEER REVIEW 5 of 5 solution was adjusted to 10 by dropping NaOH solution, and the solution was stirred continuously at 60.0 °C overnight while being kept on a hot plate stirrer [17,18].As soon as the reaction was stopped, SnAg2O3 settled down as a precipitate.Then, SnAg2O3 was washed, dried, and then further calcined for five hours at 500 °C.SnAg2O3@AT was prepared by coating SnAg2O3 on AT, as shown in Scheme 1.
Scheme 1. Graphic illustration of SnAg2O3@AT nanocatalyst preparation and its application in MO reduction.

Characterization of SnAg2O3
The morphology of SnAg2O3 was examined using a JEOL Scanning Electron Microscope (JSM-7600F, Akishima-shi, Japan).SnAg2O3 elemental composition was determined using the Oxford-EDS technique for Energy-Dispersive X-Ray Spectrometry (EDS).Thermo Scientific's (Waltham, MA, USA) X-ray diffractometer was used to study the crystal structure of SnAg2O3.Thermo Fisher Scientific's NicoletTM iS50 FTIR Spectrometer was used to identify the functional groups of SnAg2O3.Using a Thermo Scientific Evolution 300 UV-vis spectrometer, spectral data for all chemicals were recorded over the wavelength range of 200-800 nm.

Catalytic Studies of SnAg2O3 and SnAg2O3@AT
SnAg2O3 was initially used for the selectivity study of a reduction reaction of 4-NP, CR, MB, and MO.Each compound's reduction was evaluated after 2.5 mL of 4-NP (0.1 mM), CR (0.07 mM), MB (0.07 mM), and MO (0.07 mM) were individually mixed with 0.5 mL of NaBH4 (0.1 M) and verified each compound's decrease.The reduction was then observed by continuously measuring the UV-vis spectra of CR, MB, and MO at maximum wavelengths of 488 nm, 660 nm, and 460 nm, respectively.However, after adding NaBH4, 4-NP was tested at 400 nm due to the production of nitrophenolate ions.Next, SnAg2O3 (20 mg) was added to each compound.Because SnAg2O3 was found to be more efficient and selective for MO reduction, SnAg2O3@AT was evaluated for further reduction studies using 0.02 mM, 0.05 mM, and 0.07 mM of MO, with 0.3 mL, 0.5 mL, and 1.0 mL of NaBH4, using 5.5, 8.5 and 9.5 mg of SnAg2O3@AT, and different reduction parameters were optimized.
The following Equation (1) was used to estimate the percent reduction of each pollutant [30][31][32]: where Co = Ao = initial concentration and Ct = At = final concentration of each compound.
Scheme 1. Graphic illustration of SnAg 2 O 3 @AT nanocatalyst preparation and its application in MO reduction.

Characterization of SnAg 2 O 3
The morphology of SnAg 2 O 3 was examined using a JEOL Scanning Electron Microscope (JSM-7600F, Akishima-shi, Japan).SnAg 2 O 3 elemental composition was determined using the Oxford-EDS technique for Energy-Dispersive X-Ray Spectrometry (EDS).Thermo Scientific's (Waltham, MA, USA) X-ray diffractometer was used to study the crystal structure of SnAg 2 O 3 .Thermo Fisher Scientific's NicoletTM iS50 FTIR Spectrometer was used to identify the functional groups of SnAg 2 O 3 .Using a Thermo Scientific Evolution 300 UV-vis spectrometer, spectral data for all chemicals were recorded over the wavelength range of 200-800 nm.
2.4.Catalytic Studies of SnAg 2 O 3 and SnAg 2 O 3 @AT SnAg 2 O 3 was initially used for the selectivity study of a reduction reaction of 4-NP, CR, MB, and MO.Each compound's reduction was evaluated after 2.5 mL of 4-NP (0.1 mM), CR (0.07 mM), MB (0.07 mM), and MO (0.07 mM) were individually mixed with 0.5 mL of NaBH 4 (0.1 M) and verified each compound's decrease.The reduction was then observed by continuously measuring the UV-vis spectra of CR, MB, and MO at maximum wavelengths of 488 nm, 660 nm, and 460 nm, respectively.However, after adding NaBH 4 , 4-NP was tested at 400 nm due to the production of nitrophenolate ions.Next, SnAg 2 O 3 (20 mg) was added to each compound.Because SnAg 2 O 3 was found to be more efficient and selective for MO reduction, SnAg 2 O 3 @AT was evaluated for further reduction studies using 0.02 mM, 0.05 mM, and 0.07 mM of MO, with 0.3 mL, 0.5 mL, and 1.0 mL of NaBH 4 , using 5.5, 8.5 and 9.5 mg of SnAg 2 O 3 @AT, and different reduction parameters were optimized.
The following Equation (1) was used to estimate the percent reduction of each pollutant [30][31][32]: where C o = A o = initial concentration and C t = A t = final concentration of each compound.

Catalyst Selection
The efficiency, stability, and reusability of the catalyst are three crucial considerations that should be taken into account in preparing an effective catalyst.Several catalysts are effective; however, they have issues with stability and reusability.In order to minimize the recyclability difficulties and provide a reliable catalyst for industrial and commercial large-scale use, an effective catalyst was prepared based on SnAg 2 O 3 and further used to coat an AT.SnAg 2 O 3 was made as a powder, however, powder catalysts typically suffer from aggregation, separation, and recyclability issues.Several spectroscopy approaches were used to structurally study the prepared SnAg 2 O 3 and SnAg 2 O 3 @AT.

Physiochemical Characterization of SnAg 2 O 3
SEM images of the SnAg 2 O 3 nanocatalyst, shown in Figure 1, were used to determine its size and surface morphology.According to the SEM pictures at various magnifications, SnAg 2 O 3 had a morphology like well-scattered particles.The particles were found to have grown in significant numbers while maintaining their spherical form.

Catalyst Selection
The efficiency, stability, and reusability of the catalyst are three crucial considerations that should be taken into account in preparing an effective catalyst.Several catalysts are effective; however, they have issues with stability and reusability.In order to minimize the recyclability difficulties and provide a reliable catalyst for industrial and commercial large-scale use, an effective catalyst was prepared based on SnAg2O3 and further used to coat an AT.SnAg2O3 was made as a powder, however, powder catalysts typically suffer from aggregation, separation, and recyclability issues.Several spectroscopy approaches were used to structurally study the prepared SnAg2O3 and SnAg2O3@AT.

Physiochemical Characterization of SnAg2O3
SEM images of the SnAg2O3 nanocatalyst, shown in Figure 1, were used to determine its size and surface morphology.According to the SEM pictures at various magnifications, SnAg2O3 had a morphology like well-scattered particles.The particles were found to have grown in significant numbers while maintaining their spherical form.The elemental composition of SnAg2O3 was verified via EDS.According to the EDS analysis, Sn, Ag, and O peaks were observed at around 1.5, 2.6 and 3.0 kV with respective weight percentages of 14.51%, 32.79%, and 20.27%, respectively, as shown in Figure 2. The spectrum showed that SnAg2O3 comprised Sn, Ag, and O, in which Ag had the highest mass percentage, indicating that it was the primary component of SnAg2O3.The EDS spectrum indicated that the preparation of SnAg2O3 was successful.X-ray diffraction confirmed the crystal structure for SnAg2O3 (Figure 3a).The SnAg2O3 XRD pattern showed many intense peaks at 26.6°, 27.8°, 32.2°, 33.5°, 38.0°, 46.1°, 52.2°, 54.7°, 57.4°, and 64.4°.The typical diffraction peaks of low intensities at 26.6°, 33.5°, 52.2°, and 64.4°, which were indexed to (110), ( 101), (211), and (301) planes fitting with tetragonal SnO2.The characteristic hexagonal Ag2O peaks of (110), ( 111), ( 200), ( 211), (220), and (221) planes were shown at 27.8°, 32.2°, 38.0°, 46.1°, 54.4°, and 57.4°, respectively.The XRD pattern perfectly matched the hexagonal Ag2O and tetragonal SnO2 crystal structures [33,34].This indicated that the material includes both Ag2O and SnO2 according to the XRD spectrum of SnAg2O3.ATR-FTIR analysis was used to evaluate the SnAg2O3 nanocatalyst chemical composition.The SnAg2O3 ATR-FTIR spectrum showed a strong band at 519 cm −1 (Figure 3b) that was responsible for the M-O bond (M = Sn, Ag).Additionally, the OH group broad bands were observed at 3200 cm −1 and 1637 cm −1 .It also showed two bands at 901 cm −1 and 1404 cm −1 , which were responsible for the nitrate group, suggesting that SnAg2O3 may contain the nitrate of the precursor.The FTIR spectrum of the prepared nanocatalyst supports the growth of SnAg2O3.X-ray diffraction confirmed the crystal structure for SnAg2O3 (Figure 3a).The SnAg2O3 XRD pattern showed many intense peaks at 26.6°, 27.8°, 32.2°, 33.5°, 38.0°, 46.1°, 52.2°, 54.7°, 57.4°, and 64.4°.The typical diffraction peaks of low intensities at 26.6°, 33.5°, 52.2°, and 64.4°, which were indexed to (110), ( 101), (211), and (301) planes fitting with tetragonal SnO2.The characteristic hexagonal Ag2O peaks of (110), ( 111), ( 200), ( 211), (220), and (221) planes were shown at 27.8°, 32.2°, 38.0°, 46.1°, 54.4°, and 57.4°, respectively.The XRD pattern perfectly matched the hexagonal Ag2O and tetragonal SnO2 crystal structures [33,34].This indicated that the material includes both Ag2O and SnO2 according to the XRD spectrum of SnAg2O3.ATR-FTIR analysis was used to evaluate the SnAg2O3 nanocatalyst chemical composition.The SnAg2O3 ATR-FTIR spectrum showed a strong band at 519 cm −1 (Figure 3b) that was responsible for the M-O bond (M = Sn, Ag).Additionally, the OH group broad bands were observed at 3200 cm −1 and 1637 cm −1 .It also showed two bands at 901 cm −1 and 1404 cm −1 , which were responsible for the nitrate group, suggesting that SnAg2O3 may contain the nitrate of the precursor.The FTIR spectrum of the prepared nanocatalyst supports the growth of SnAg2O3.ATR-FTIR analysis was used to evaluate the SnAg 2 O 3 nanocatalyst chemical composition.The SnAg 2 O 3 ATR-FTIR spectrum showed a strong band at 519 cm −1 (Figure 3b) that was responsible for the M-O bond (M = Sn, Ag).Additionally, the OH group broad bands were observed at 3200 cm −1 and 1637 cm −1 .It also showed two bands at 901 cm −1 and 1404 cm −1 , which were responsible for the nitrate group, suggesting that SnAg 2 O 3 may contain the nitrate of the precursor.The FTIR spectrum of the prepared nanocatalyst supports the growth of SnAg 2 O 3 .

Catalytic Properties of SnAg 2 O 3
In order to assess the SnAg 2 O 3 catalytic impact, it was first evaluated as a nanocatalyst for the reduction of 4-NP, CR, MB, and MO via a reducing agent.Initially, only a modest amount (20 mg) of SnAg 2 O 3 was scrutinized in the reduction process of each individual pollutant.Figure 4 clearly shows that the absorbance of 4-NP, CR, MB, and MO gradually decreased, indicating that SnAg 2 O 3 had a positive impact on their reduction.These findings confirm that the SnAg 2 O 3 nanocatalyst reduced MO and CR within 1.0 min and 8.0 min, respectively, while it took 4.0 min to reduce both MB and 4-NP.Using Equation (1), the percentage reduction was further computed, and it was found that the SnAg 2 O 3 nanocatalyst reduced 95.77% of 4-NP, 93.12% of CR, 95.23% of MB, and 95.22% of MO (Figure 5a).decreased, indicating that SnAg2O3 had a positive impact on their reduction.These findings confirm that the SnAg2O3 nanocatalyst reduced MO and CR within 1.0 min and 8.0 min, respectively, while it took 4.0 min to reduce both MB and 4-NP.Using Equation ( 1), the percentage reduction was further computed, and it was found that the SnAg2O3 nanocatalyst reduced 95.77% of 4-NP, 93.12% of CR, 95.23% of MB, and 95.22% of MO (Figure 5a).Further, to examine the rate constants for all pollutants that are reduced in the presence of SnAg2O3, the pseudo-first-order kinetic presented in Equation ( 2) was used [35][36][37]: where At = Ct is the absorbance or concentration of reduced pollutant at various reaction times, Ao = Co is the absorbance or reducing chemical concentration prior to the reaction, and K (min −1 ) is the slope-based rate constant calculated from the graph of ln Ct/Co vs. SnAg2O3 reduction time (Figure 5b).The rate constant values for 4-NP, CR, MB, and MO reduction reactions usin SnAg2O3 were 0.8573 min −1 , 0.3186 min −1 , 0.7859 min −1 , and 3.0412 min −1 , respectively.Thi suggested that SnAg2O3 is selective for MO reduction.Therefore, SnAg2O3 was more effi cient in reducing MO based on the rate constants that reflect its catalytic activity.
Since MO reduction was completed using SnAg2O3 in 1.0 min, it was selected for th comprehensive analysis and optimization of reaction conditions using the SnAg2O3@A nanocatalyst.Only SnAg2O3@AT was used for further MO reduction investigations in or der to minimize recycling concerns and provide a superior catalyst for large-scale appl cations since powder catalysts have problems with aggregation, separation, and recycla bility.

Catalytic Activity of SnAg2O3@AT toward MO
SnAg2O3@AT was used for MO reduction to test the catalytic activity.The reductio of 2.5 mL of MO (0.07 mM) was first investigated in the presence of only NaBH4.Withou SnAg2O3 or SnAg2O3@AT, a negligible reduction took place, and the concentration of MO was slightly changed.This demonstrates unequivocally that a catalyst is necessary for MO reduction.To reduce MO effectively, SnAg2O3@AT was added and the reaction was mon itored by measuring UV-vis absorption.The NaBH4 + MO reaction was initially invest gated with 8.5 mg of SnAg2O3@AT.
Since there was a steady loss of the MO color as the reaction progressed along wit the drop in absorbance, the reduction of MO was visually detected.This shift in color an drop in absorbance indicated MO reduction, which occurred in 1.0 min and 3.0 min b employing 20 mg of SnAg2O3 and 8.5 mg of SnAg2O3@AT, as shown in Figure 6.Further, to examine the rate constants for all pollutants that are reduced in the presence of SnAg 2 O 3 , the pseudo-first-order kinetic presented in Equation ( 2) was used [35][36][37]: where A t = C t is the absorbance or concentration of reduced pollutant at various reaction times, A o = C o is the absorbance or reducing chemical concentration prior to the reaction, and K (min −1 ) is the slope-based rate constant calculated from the graph of ln C t /C o vs. SnAg 2 O 3 reduction time (Figure 5b).The rate constant values for 4-NP, CR, MB, and MO reduction reactions using SnAg 2 O 3 were 0.8573 min −1 , 0.3186 min −1 , 0.7859 min −1 , and 3.0412 min −1 , respectively.This suggested that SnAg 2 O 3 is selective for MO reduction.Therefore, SnAg 2 O 3 was more efficient in reducing MO based on the rate constants that reflect its catalytic activity.
Since MO reduction was completed using SnAg 2 O 3 in 1.0 min, it was selected for the comprehensive analysis and optimization of reaction conditions using the SnAg 2 O 3 @AT nanocatalyst.Only SnAg 2 O 3 @AT was used for further MO reduction investigations in order to minimize recycling concerns and provide a superior catalyst for large-scale applications since powder catalysts have problems with aggregation, separation, and recyclability.

Catalytic Activity of SnAg 2 O 3 @AT toward MO
SnAg 2 O 3 @AT was used for MO reduction to test the catalytic activity.The reduction of 2.5 mL of MO (0.07 mM) was first investigated in the presence of only NaBH 4 .Without SnAg 2 O 3 or SnAg 2 O 3 @AT, a negligible reduction took place, and the concentration of MO was slightly changed.This demonstrates unequivocally that a catalyst is necessary for MO reduction.To reduce MO effectively, SnAg 2 O 3 @AT was added and the reaction was monitored by measuring UV-vis absorption.The NaBH 4 + MO reaction was initially investigated with 8.5 mg of SnAg 2 O 3 @AT.
Since there was a steady loss of the MO color as the reaction progressed along with the drop in absorbance, the reduction of MO was visually detected.This shift in color and drop in absorbance indicated MO reduction, which occurred in 1.0 min and 3.0 min by employing 20 mg of SnAg 2 O 3 and 8.5 mg of SnAg 2 O 3 @AT, as shown in Figure 6.Equation ( 2) was also used to determine the kinetic reduction of MO SnAg2O3@AT.As a result, K was found to be 1.3669 min −1 (Figure 7b).As indicated in 1, the rate constants of SnAg2O3 and SnAg2O3@AT were also compared with those of reported catalysts.Equation ( 2) was also used to determine the kinetic reduction of MO via SnAg 2 O 3 @AT.As a result, K was found to be 1.3669 min −1 (Figure 7b).As indicated in Table 1, the rate constants of SnAg 2 O 3 and SnAg 2 O 3 @AT were also compared with those of other reported catalysts.Equation ( 2) was also used to determine the kinetic reduction of MO via SnAg2O3@AT.As a result, K was found to be 1.3669 min −1 (Figure 7b).As indicated in Table 1, the rate constants of SnAg2O3 and SnAg2O3@AT were also compared with those of other reported catalysts.To explore the dose effect, 2.5 mL of MO was reduced by 0.5 mL of NaBH 4 (0.1 M) in the presence of 5.5 mg, 8.5 mg, and 9.5 mg of SnAg 2 O 3 @AT.With a high dose of SnAg 2 O 3 @AT, the reduction process of MO occurred more rapidly.MO reduction with 8.5 mg and 9.5 mg of SnAg 2 O 3 @AT took only 3.0 min, which was faster than the lower dose (took 4.0 min).Equations ( 1) and ( 2) were used to determine the catalytic efficiency of SnAg 2 O 3 @AT in the reduction of MO, as illustrated in Figure 7, which showed that a superior reduction was quickly achieved by utilizing 8.5 mg of SnAg 2 O 3 @AT (98.11%) in just 3.0 min with a rate constant of 1.3669 min −1 .By using 9.5 mg, SnAg 2 O 3 @AT reduced 90.56% of MO (K = 0.7751 min −1 ).On the other hand, 5.5 mg reduced 95.36% of MO in 4.0 min (K = 0.7936 min −1 ).These findings demonstrated that the amount of the catalyst has a beneficial effect on MO reduction because it is directly related to the percent reduction.

Effect of NaBH 4 Concentration
To examine the influence of NaBH 4 concentration, 0.3 mL, 0.5 mL, and 1.0 mL of NaBH 4 (1.0 M) were studied on the reduction of 2.5 mL of MO utilizing 8.5 mg of SnAg 2 O 3 @AT.The findings showed that NaBH 4 had a negligible impact on SnAg 2 O 3 @AT ability to reduce MO.It was discovered that 8.5 mg of SnAg 2 O 3 @AT reduced 94.94% and 98.11% of MO in 5.0 min and 3.0 min, respectively, using 0.3 mL and 0.5 mL of NaBH 4 .
While applying 1.0 mL of NaBH 4 , 8.5 mg of SnAg 2 O 3 @AT reduced 85.34% of MO in 1.0 min.As a result, it was discovered that by adding more NaBH 4 (0.3 mL~1.0 mL), the MO reduction reaction rate was increased using SnAg 2 O 3 @AT, in which the rate constants were 0.6301 min −1 , 1.3669 min −1 , and 1.9199 min −1 for 0.3 mL, 0.5 mL, and 1.0 mL, respectively, as illustrated in Figure 8. Specifically, SnAg 2 O 3 @AT transfers electrons from BH 4 − to MO.However, if the concentration of electron-rich moieties is in a large quantity, there is still competition for electrons to reach MO, which may reduce the % reduction of MO as a result of competition and steric hindrance.Overall, it can be concluded that the amount of NaBH 4 has a good impact on MO reduction when it is used in appropriate amounts.

Effect of MO Concentration
To assess the catalytic behavior of SnAg2O3@AT, the reduction process was tested using various concentrations of MO.A total of 8.5 mg of SnAg2O3@AT was utilized for the reduction of 2.5 mL of MO + 0.5 mL of NaBH4, in which the concentrations of MO were 0.02 mM, 0.05 mM, and 0.07 mM.As illustrated in Figure 9, it was discovered that SnAg2O3@AT reduced 0.02 mM MO much faster (2.0 min) than 0.05 mM and 0.07 mM (3.0 min).This means that higher concentrations of MO require more time to be reduced than lower concentrations.Accordingly, there is a direct proportion between the concentration of MO and its reduction time.

Effect of MO Concentration
To assess the catalytic behavior of SnAg 2 O 3 @AT, the reduction process was tested using various concentrations of MO.A total of 8.5 mg of SnAg 2 O 3 @AT was utilized for the reduction of 2.5 mL of MO + 0.5 mL of NaBH 4 , in which the concentrations of MO were 0.02 mM, 0.05 mM, and 0.07 mM.As illustrated in Figure 9, it was discovered that SnAg 2 O 3 @AT reduced 0.02 mM MO much faster (2.0 min) than 0.05 mM and 0.07 mM (3.0 min).This means that higher concentrations of MO require more time to be reduced than lower concentrations.Accordingly, there is a direct proportion between the concentration of MO and its reduction time.

Effect of MO Concentration
To assess the catalytic behavior of SnAg2O3@AT, the reduction process was tested using various concentrations of MO.A total of 8.5 mg of SnAg2O3@AT was utilized for the reduction of 2.5 mL of MO + 0.5 mL of NaBH4, in which the concentrations of MO were 0.02 mM, 0.05 mM, and 0.07 mM.As illustrated in Figure 9, it was discovered that SnAg2O3@AT reduced 0.02 mM MO much faster (2.0 min) than 0.05 mM and 0.07 mM (3.0 min).This means that higher concentrations of MO require more time to be reduced than lower concentrations.Accordingly, there is a direct proportion between the concentration of MO and its reduction time.The stability and reusability of a catalyst are important factors from an economic and environmental perspective [45][46][47].Thus, SnAg2O3@AT was investigated for recyclability.The recyclability of SnAg2O3@AT was evaluated by performing a reduction of 2.5 mL MO

Recyclability
The stability and reusability of a catalyst are important factors from an economic and environmental perspective [45][46][47].Thus, SnAg 2 O 3 @AT was investigated for recyclability.The recyclability of SnAg 2 O 3 @AT was evaluated by performing a reduction of 2.5 mL MO (0.07 mM).Three further reactions were analyzed using 8.5 mg of SnAg 2 O 3 @AT.It was simple to remove SnAg 2 O 3 @AT from the reaction medium after each use.Then, SnAg 2 O 3 @AT was cleaned with distilled water before being used in the following process.In the first, second, and third cycles, the data showed that SnAg 2 O 3 @AT completely (>94%) reduced MO in 3.0, 5.0, and 8.0 min, respectively.Figure 10 shows the catalytic activity of SnAg 2 O 3 @AT in each cycle.The results showed that SnAg2O3@AT was able to effectively reduce MO in 8.0 min up to the third cycle, which indicated the stability and recyclability of the material based on the catalytic activity in each cycle.These findings imply that SnAg2O3@AT can be used repeatedly without significant losing of its superior catalytic activity.

Mechanism of MO reduction
The process of the catalytic reduction of MO depends on catalyst-mediated electron transmittance.NaBH4 dissociates initially into BH4 − and gets adsorbed on SnAg2O3@AT, where it donates electrons to the surface of SnAg2O3@AT which then attack MO molecules.Azo bonds are activated by electrons existing on the surface of SnAg2O3@AT when MO molecules get adsorbed onto it.As a result, the breakage of azo bonds takes place, thus, the formation of aromatic amines occurs during the reduction process resulting in the disappearance of MO color which confirms the completion of its catalytic reduction.

Conclusion
This study demonstrated the straightforward manufacturing of SnAg2O3 nanomaterial, which was successfully produced using the co-precipitation method.With the help of SEM, EDS, XRD, and FTIR, the physiochemical characterization of SnAg2O3 was examined.The reduction of different compounds was investigated (4-NP, CR, MB, and MO) in the presence of SnAg2O3.Using SnAg2O3 and SnAg2O3@AT, MO was reduced within 1.0 min and 3.0 min with NaBH4 at reduction rates of 3.0412 min −1 and 1.3669 min −1 , respectively.Thus, based on the catalytic activities, SnAg2O3 and SnAg2O3@AT were specified as efficient catalysts for the MO reduction.MO reduction was therefore selected for further analysis and reaction condition optimization.The results suggested that The results showed that SnAg 2 O 3 @AT was able to effectively reduce MO in 8.0 min up to the third cycle, which indicated the stability and recyclability of the material based on the catalytic activity in each cycle.These findings imply that SnAg 2 O 3 @AT can be used repeatedly without significant losing of its superior catalytic activity.

Mechanism of MO Reduction
The process of the catalytic reduction of MO depends on catalyst-mediated electron transmittance.NaBH 4 dissociates initially into BH 4 − and gets adsorbed on SnAg 2 O 3 @AT, where it donates electrons to the surface of SnAg 2 O 3 @AT which then attack MO molecules.Azo bonds are activated by electrons existing on the surface of SnAg 2 O 3 @AT when MO molecules get adsorbed onto it.As a result, the breakage of azo bonds takes place, thus, the formation of aromatic amines occurs during the reduction process resulting in the disappearance of MO color which confirms the completion of its catalytic reduction.

Figure 1 .
Figure 1.SEM images of the SnAg2O3 nanocatalyst at (a) low and (b) high magnifications.

Figure 5 .
Figure 5. (a) % Reduction and (b) kinetics using 20 mg of SnAg2O3 and 0.5 mL of NaBH4 for redu tion of different pollutants.

Figure 5 .
Figure 5. (a) % Reduction and (b) kinetics using 20 mg of SnAg 2 O 3 and 0.5 mL of NaBH 4 for reduction of different pollutants.

Materials 2023 ,Figure 6 .
Figure 6.Time-dependent UV-vis spectra for MO reduction using (a) 20 mg of SnAg2O3 and ( mg of SnAg2O3@AT and 0.5 mL of NaBH4.(c) Comparison of nanocatalysts' catalytic perform toward MO reduction.

Figure 6 .
Figure 6.Time-dependent UV-vis spectra for MO reduction using (a) 20 mg of SnAg 2 O 3 and (b) 8.5 mg of SnAg 2 O 3 @AT and 0.5 mL of NaBH 4 .(c) Comparison of nanocatalysts' catalytic performance toward MO reduction.

Figure 6 .
Figure 6.Time-dependent UV-vis spectra for MO reduction using (a) 20 mg of SnAg2O3 and (b) 8.5 mg of SnAg2O3@AT and 0.5 mL of NaBH4.(c) Comparison of nanocatalysts' catalytic performance toward MO reduction.

Figure 10 .
Figure 10.Time-dependent UV-vis spectra for reduction of 0.07 mM MO; (a) 1st cycle, (b) 2nd cycle, and (c) 3rd cycle using 8.5 mg of SnAg2O3@AT and 0.5 mL of NaBH4.(d) % Reduction of MO using 8.5 mg of SnAg2O3@AT for three cycles.(e) Recyclability performance of SnAg2O3@AT nanocatalyst in MO reduction.

Figure 10 .
Figure 10.Time-dependent UV-vis spectra for reduction of 0.07 mM MO; (a) 1st cycle, (b) 2nd cycle, and (c) 3rd cycle using 8.5 mg of SnAg 2 O 3 @AT and 0.5 mL of NaBH 4 .(d) % Reduction of MO using 8.5 mg of SnAg 2 O 3 @AT for three cycles.(e) Recyclability performance of SnAg 2 O 3 @AT nanocatalyst in MO reduction.

Table 1 .
Rate constant comparison of SnAg 2 O 3 and SnAg 2 O 3 @AT with literature.