Copper Oxide-Antimony Oxide Entrapped Alginate Hydrogel as Efficient Catalyst for Selective Reduction of 2-Nitrophenol

Copper oxide-antimony oxide (Cu2O-Sb2O3) was prepared and entrapped inside Na-alginate hydrogel (Alg@Cu2O-Sb2O3). The developed Alg@Cu2O-Sb2O3 was used as catalytic reactor for the reduction of 4-nitrophenol (4-NP), 2-nitrophenol (2-NP), 2,6-dinitrophenol (2,6-DNP), methyl orange (MO), congo red (CR), acridine orange (AO), methylene blue (MB) and potassium ferricyanide (K3[Fe(CN)6]). Alg@Cu2O-Sb2O3 was found to be selective and more efficient for the reduction of 2-NP among all the pollutants. Therefore, 2-NP was selected for a detailed study to optimize various parameters, e.g., the catalyst amount, reducing agent concentration, 2-NP concentration and recyclability. Alg@Cu2O-Sb2O3 was found to be very stable and easily recyclable for the reduction of 2-NP. The Alg@Cu2O-Sb2O3 nanocatalyst reduced 2-NP in 1.0 min, having a rate constant of 3.8187 min−1.


Introduction
Environmental pollution achieved worrying levels due to the continuous release and disposal of numerous pollutants into the environment and wastewater. Many of these pollutants are dyes and nitrophenols and are unsustainable in nature, which is of particularly great concern due to environmental regulations. These pollutants have high toxicity, hazardous nature, carcinogenicity, mutagenic nature, low biodegradability, and the fact that they are harmful and dangerous to living organisms [1,2]. Currently, dyes as well as phenols are believed to be tenacious, lethal, and dangerous contaminants coming from industrial wastewater [3,4]. These pollutants exist in the wastes of agrochemical, pharmaceutical, textile, and chemical industries because several industries are accountable for throwing out various pollutants into the ecosystem through wastewater streams. Consequently, the processes related to the disposal of the above-mentioned pollutants have foremost limitations, having a very slow removal proficiency, a poor elimination rate as well as being very costly and complex, which limits its applications [5,6]. Fortunately, researchers have focused a lot on detecting significant pollutants for different water bodies, but less work has been done on the removal or conversion of toxic pollutants [1,2]. Therefore, the efficient reduction and fast degradation of these pollutants is a central concern, considering its environmental and health hazards. One way to manage these pollutants from wastewater is by performing their reduction so as to produce less toxic or useful compounds.
Various techniques are utilized regarding the removal of lethal pollutants from contaminated water [7]. However, these processes are believed to not be very useful for the complete removal of dangerous pollutants, or they require a long time. Second, the high cost also reduces their importance. Thus, easy, uncomplicated, and cheap methods are required for managing polluted wastewater [3,8]. Catalytic reduction is considered an inexpensive and good technique as it is easy, fast, and proficient. Catalytic reduction involves a very low quantity of solvent, and the time required for removal is also very short [9][10][11]. In recent times, many efforts have been made to prepare new catalysts and enhance their catalytic performance and stability as well as their selectivity regarding water pollutants [12][13][14].
Nanomaterials have eccentric and numerous applications in relation to human health, which they confer significantly to medicine and the environment. Nanomaterials demonstrate a good ability as catalysts, which is hardly possible when considering conventional systems. Nanomaterials retain a distinct shape, small size, and remarkable surface activity [15][16][17][18][19]. Tin oxide possesses a good ability and many applications, but still, it requires reformation to enhance its catalytic properties [20][21][22]. When considering many techniques, doping is the one that is used to reveal excellent properties in various areas. Dopants works to increase the surface area and alter the structure, modifying the morphology, which leads to enhanced properties of nanomaterials [23,24].
Doped metal oxides are efficient catalysts, but their utilization in powder form has several drawbacks such as aggregation, regeneration of the catalyst, recyclability and leaching. Therefore, to solve these problems, a Cu 2 O-Sb 2 O 3 nanomaterial was wrapped inside Na-alginate hydrogel to prevent this agglomeration and to make the recycling of metal oxide nanocatalysts easy, which is the novelty of the current study. For the current study, Cu 2 O-Sb 2 O 3 nanomaterial was prepared and checked by FESEM, EDS, XRD and FTIR. Further Cu 2 O-Sb 2 O 3 was entrapped inside Na-alginate hydrogel. Alg@Cu 2 O-Sb 2 O 3 was initially tested for MO, CR, ArO, MB and K 3 [Fe(CN) 6 ], 4-NP, 2-NP and 2,6-DNP, and we explored the efficiency of the Alg@Cu 2 O-Sb 2 O 3 nanocatalyst for the reduction of these pollutants. The Alg@Cu 2 O-Sb 2 O 3 nanocatalyst achieved the greatest selectivity toward 2-NP, which underwent a reduction in 1.0 min, having a superior rate constant.

Preparation of Cu 2 O-Sb 2 O 3 and Alg@Cu 2 O-Sb 2 O 3
0.1 M solutions of antimony chloride and copper nitrate were initially prepared and then mixed. The mixed aqueous solution was stirred continuously, and then 0.1 M NH 4 OH was added and the pH was increased to 10. The mixed solution was heated at 150.0 • C for 10 h in an oven using an autoclave. The product was then filtered. After filtration, the product was dried after washing it thoroughly and was calcined at 400.0 • C for 5 h. The preparation process of Alg@Cu 2 O-Sb 2 O 3 and its utilization has been shown in Scheme 1. Alg@Cu 2 O-Sb 2 O 3 composite hydrogel was readily prepared by dispersing 3, 5 and 10 mg of Cu 2 O-Sb 2 O 3 in 1 mL of Alg individually. After that, HCl solution was added to the mixed solution. Alg was turned into hydrogel by H + after adding HCl solution because of H + crosslinking Alg. Alg@Cu 2 O-Sb 2 O 3 was finally removed and washed by utilizing distilled water for the purpose of removing the unreacted H + present on the top surface of the hydrogel. The samples were examined by a field emission scanning electron microscope (FESEM, INSTRUMENT JSM-7600F, Japan), energy dispersive X-ray spectrometry (EDS, Oxford instruments Inc system), powder X-ray diffraction (XRD, ARL X'tra Thermo Scientific diffractometer), and a Fourier Transform Infrared spectrometer. The catalytic reduction was examined by a UV-Vis spectrometer (Evolution 300 by Thermo Scientific). spectrometry (EDS, Oxford instruments Inc system), powder X-ray diffraction (XRD, ARL X'tra Thermo Scientific diffractometer), and a Fourier Transform Infrared spectrometer. The catalytic reduction was examined by a UV-Vis spectrometer (Evolution 300 by Thermo Scientific). Scheme 1. Scheme of Alg@Cu2O-Sb2O3 preparation and catalytic reduction of 2-NP.

Catalytic Reduction
At first, the Alg@Cu 2 O-Sb 2 O 3 hydrogel nanocatalyst was assessed with different dyes (CR (0.07 mM), MO (0.07 mM), ArO (0.07 mM), MB (0.07 mM)), K 3 [Fe(CN) 6 ] (2.0 mM) and nitrophenols (4-NP (0.13 mM), 2-NP (0.13 mM) and 2,6-DNP (0.13 mM)). The catalytic reaction of the selected dyes, K 3 [Fe(CN) 6 ] and nitrophenols was performed by Alg@Cu 2 O-Sb 2 O 3 catalyst in the presence of NaBH 4 . 2.5 mL of pollutant solution was poured in a cuvette, and its UV spectrum was checked. 0.5 mL of NaBH 4 (0.1 M) was poured into it, followed by the addition of the Alg@Cu 2 O-Sb 2 O 3 hydrogel nanocatalyst to the reaction system, and then the UV-vis spectrum was measured with an interval of 1.0 min. Consequently, the catalytic reactions of CR, MO, ArO, MB, K 3 [Fe(CN) 6 ], 4-NP, 2-NP and 2,6-DNP were analyzed by measuring the absorbance spectra at 490 nm, 460 nm, 465 nm, 658 nm, 415 nm, 400 nm, 413 nm and 428 nm, respectively, and the percent reduction (%R) was measured using Equation (1): C i = initial and C e = final concentrations of pollutants. doped metal oxides could be an excellent selection for the reduction reaction of pollutants. Consequently, Cu 2 O-Sb 2 O 3 and Alg@Cu 2 O-Sb 2 O 3 were prepared and tested in a reduction reaction of pollutants and discovered to be good catalysts because the Alg@Cu 2 O-Sb 2 O 3 nanocatalyst showed a superb catalytic activity in relation to the catalytic reduction reaction of pollutants. However, it was more efficient for the reactions involving the 2-NP reduction, and it was, therefore, suggested that the Alg@Cu 2 O-Sb 2 O 3 nanocatalyst was a better choice for the reduction of 2-NP. Additionally, the composition, chemical structure, and morphology of Cu 2 O-Sb 2 O 3 was evaluated by FESEM, EDS, XRD and ATR-FTIR. The wrapping of Cu 2 O-Sb 2 O 3 inside Alg was confirmed by XRD and ATR-FTIR.

Results and Discussion
FESEM was utilized to check the morphology of Cu 2 O-Sb 2 O 3, and the FESEM images are displayed in Figure 1. The surface morphology of Cu 2 O-Sb 2 O 3 was explicated by comparing both the low and high magnification images. The Cu 2 O-Sb 2 O 3 images indicate that it is grown like sheets, which combine to construct a flower-shaped structure. Thus, Cu 2 O-Sb 2 O 3 is grown in nanosheets, for which the sheet diameter was observed to bẽ 10 nm. The images clearly show that Cu 2 O-Sb 2 O 3 has a flower-shaped morphology, where the flower is made of aggregated nanosheets, collectively giving a flower structure.

Selection and Structure Interpretation of Cu2O-Sb2O3 and Alg@Cu2O-Sb2O3
In search of an efficient, stable, and economical catalyst, a huge number of materials have been investigated. Among different materials, nanomaterials containing metal oxides play a very important part as a catalyst. As a catalyst, metal oxides have displayed good catalytic activities when considering different reactions. Thus, it was thought that doped metal oxides could be an excellent selection for the reduction reaction of pollutants. Consequently, Cu2O-Sb2O3 and Alg@Cu2O-Sb2O3 were prepared and tested in a reduction reaction of pollutants and discovered to be good catalysts because the Alg@Cu2O-Sb2O3 nanocatalyst showed a superb catalytic activity in relation to the catalytic reduction reaction of pollutants. However, it was more efficient for the reactions involving the 2-NP reduction, and it was, therefore, suggested that the Alg@Cu2O-Sb2O3 nanocatalyst was a better choice for the reduction of 2-NP. Additionally, the composition, chemical structure, and morphology of Cu2O-Sb2O3 was evaluated by FESEM, EDS, XRD and ATR-FTIR. The wrapping of Cu2O-Sb2O3 inside Alg was confirmed by XRD and ATR-FTIR.
FESEM was utilized to check the morphology of Cu2O-Sb2O3, and the FESEM images are displayed in Figure 1. The surface morphology of Cu2O-Sb2O3 was explicated by comparing both the low and high magnification images. The Cu2O-Sb2O3 images indicate that it is grown like sheets, which combine to construct a flower-shaped structure. Thus, Cu2O-Sb2O3 is grown in nanosheets, for which the sheet diameter was observed to be ~10 nm. The images clearly show that Cu2O-Sb2O3 has a flower-shaped morphology, where the flower is made of aggregated nanosheets, collectively giving a flower structure. By using EDS, the compositional analysis of nanomaterial was examined, as shown in Figure 2. EDS displayed peaks related to antimony (Sb), copper (Cu) and oxygen (O), showing the existence of the corresponding elements and suggesting their existence in the prepared nanomaterial. EDS revealed 71 wt% of Sb and 3 wt% of Cu, indicating the growth of Cu2O-doped Sb2O3. The lack of any extra peak in EDS suggested that the synthesized Cu2O-Sb2O3 nanomaterial contained no impurity. The XRD patterns of the prepared material clearly reveal peaks that correspond to the crystalline phase of Cu 2 O and Sb 2 O 3 ( Figure 3a) The XRD patterns of the prepared material clearly reveal peaks that correspond to the crystalline phase of Cu2O and Sb2O3 ( Figure 3a). Hence, the XRD patterns imply that the prepared material is comprised of Cu2O and Sb2O3 phases. Cu2O-Sb2O3 revealed diffraction peaks at 2θ = 20.0, 26 (200), were assigned to Cu2O [25,26]. All other peaks are well matched with the literature and could be assigned to the Sb2O3 phase. The intensities of the Cu2O peaks are low in the XRD pattern, which might be due to the low quantity of Cu2O phase present in Cu2O-Sb2O3 when compared to the Sb2O3 phase. Therefore, the Sb2O3 diffraction peaks have significantly high intensities when compared to the Cu2O peaks. The XRD pattern suggests the possible formation of a two-phase material composed of Cu2O and Sb2O3 phases [24,[27][28][29]. The Alg@Cu2O-Sb2O3 hydrogel XRD only demonstrated the pattern of the amorphous material. This peak can be attributed to the amorphous phase of Alg. The Alg@Cu2O-Sb2O3 pattern did not show any peak associated with the Cu2O-Sb2O3 phase, the reason for this being either the presence of a lower quantity of Cu2O-Sb2O3 in Alg@Cu2O-Sb2O3 or Cu2O-Sb2O3 being entirely wrapped up inside the Alg hydrogel. Consequently, the XRD pattern of Alg@Cu2O-Sb2O3 only displayed peaks associated with the Alg phase. Thus, the XRD pattern suggested that Cu2O-Sb2O3 was ensnared inside the Alg hydrogel.  The pseudo-first-order equation was utilized to evaluate the reaction kinetics [32].
where r stands for the reactant rate; C is the reactant concentration; t stands for the time of reaction; k represents the reaction rate constant; C t stands for the dyes concentration at time t; and C 0 stands for the dyes concentration at time 0. The current study proves that the dyes reduction obeys pseudo-first-order kinetics. The rate constants were assessed as being 1.1929 min −1 , 1.4045 min −1 , 0.1131 min −1 and 0.9571 min −1 for CR, MO, ArO and MB, respectively. These high-rate constants suggest that there is a quick diffusion of BH 4 and dye molecules into Alg@Cu 2 O-Sb 2 O 3 , where electron transfers take place from NaBH 4 to dye molecules and quickly diffuse out the product.

Reduction of K 3 [Fe(CN) 6 ]
Alg@Cu 2 O-Sb 2 O 3 was tested in the reduction reaction of K 3 6 ] bearing a yellow color faded immediately within few seconds and that within 3.0 min the absorbance band at 415 nm declined and disappeared, as demonstrated in Figure 5a. This indicates the reduction of Fe +3 to Fe +2 and hence the formation of [Fe(CN) 6 ] −4 [33]. The conversion (%) was 97.98% for K 3 [Fe(CN) 6 ] within 3.0 min (Figure 5b), and thus the current study indicated that a good reduction was attained by Alg@Cu 2 O-Sb 2 O 3 ; additionally, the results suggested a high catalytic property of Alg@Cu 2 O-Sb 2 O 3 toward the K 3 [Fe(CN) 6 ] reduction. The kinetics of the K 3 [Fe(CN) 6 ] reduction were analyzed by applying Equation (2), and the rate constant was assessed to be 1.3180 min −1 .
olymers 2022, 14, x FOR PEER REVIEW 7 of 15 electron transfers take place from NaBH4 to dye molecules and quickly diffuse out the product.

Reduction of K 3 [Fe(CN) 6 ]
Alg@Cu2O-Sb2O3 was tested in the reduction reaction of K 3 [Fe(CN) 6 ] and NaBH4. At first, the K 3 [Fe(CN) 6] reduction was only performed by NaBH4 without Alg@Cu2O-Sb2O3. The K 3 [Fe(CN) 6] reduction was very slow without Alg@Cu2O-Sb2O3. Then, 10 mg of Alg@Cu2O-Sb2O3 were placed into the mixture of NaBH4 and K 3 [Fe(CN) 6 ]. Using visual observation, it was noticed that K 3 [Fe(CN) 6] bearing a yellow color faded immediately within few seconds and that within 3.0 min the absorbance band at 415 nm declined and disappeared, as demonstrated in Figure 5a. This indicates the reduction of Fe +3 to Fe +2 and

Reduction of Nitrophenols
Alg@Cu 2 O-Sb 2 O 3 was also tested in the reduction reaction of 4-NP, 2-NP and 2,6-DNP with NaBH 4 . Figure 6a reveals that 4-NP displayed an absorption peak at 317 nm. The addition of NaBH 4 caused a shift in absorption from 317 to 400 nm, signifying the formation of 4-nitrophenolate ions [24,34]. After adding Alg@Cu 2 O-Sb 2 O 3 , the 4-nitrophenolate band at 400 nm gradually declined with time, which indicated that the -NO 2 group present in 4-NP got reduced to the -NH 2 group. The decline in the 4-nitrophenolate ion peak was faster for Alg@Cu 2 O-Sb 2 O 3 , and the reaction was completed within 3.0 min. Similarly, Alg@Cu 2 O-Sb 2 O 3 was further analyzed in the reduction reaction of 2-NP and 2,6-DNP with NaBH 4 using UV-Vis spectrometry. Figure 6b,c illustrates that 2-NP and 2,6-DNP gave absorption peaks at 347 nm and 428 nm. Upon adding NaBH 4 solution, a change of color was noticed along with a shift in the absorption band from 347 nm to 413 nm for 2-NP, signifying the existence of nitrophenolate ions [24,34]. Furthermore, upon adding Alg@Cu 2 O-Sb 2 O 3 , the absorption peaks recorded at 413 nm and 428 nm declined, while, on the other hand, another absorption peak at 280 nm could be seen with an increasing trend in intensity. The results verify the conversion of nitrophenols to aminophenol by reducing NO 2 present in 2-NP and 2,6-DNP (Figure 6b,c). The 2-NP and 2,6-DNP reduction reactions were faster in the presence of Alg@Cu 2 O-Sb 2 O 3 , and both reactions were completed within 1.0 min and 3.0 min, respectively. Alg@Cu 2 O-Sb 2 O 3 reduced more than 97% of 4-NP, 2-NP and 2,6-DNP in 3.0 min, 1.0 min and 3.0 min, respectively. Further, rate constants were obtained by applying Equation (2), and they were 1.3279 min −1 , 3.8187 min −1 and 1.2426 min −1 for 4-NP, 2-NP and 2,6-DNP, respectively. These results verify that the Alg@Cu 2 O-Sb 2 O 3 nanocatalyst is an effective catalyst, displaying more selectivity toward 2-NP.  [33]. The conversion (%) was 97.98% for K 3 [Fe(CN) 6 ] within 3.0 min (Figure 5b), and thus the current study indicated that a good reduction was attained by Alg@Cu2O-Sb2O3; additionally, the results suggested a high catalytic property of Alg@Cu2O-Sb2O3 toward the K 3 [Fe(CN) 6 ] reduction. The kinetics of the K 3 [Fe(CN) 6 ] reduction were analyzed by applying Equation (2), and the rate constant was assessed to be 1.3180 min −1 .  6 ] in the presence of NaBH4 using 10 mg Alg@Cu2O-Sb2O3 nanocatalyst.

Reduction of Nitrophenols
Alg@Cu2O-Sb2O3 was also tested in the reduction reaction of 4-NP, 2-NP and 2,6-DNP with NaBH4. Figure 6a reveals that 4-NP displayed an absorption peak at 317 nm. The addition of NaBH4 caused a shift in absorption from 317 to 400 nm, signifying the formation of 4-nitrophenolate ions [24,34]. After adding Alg@Cu2O-Sb2O3, the 4-nitrophenolate band at 400 nm gradually declined with time, which indicated that the -NO2 group present in 4-NP got reduced to the -NH2 group. The decline in the 4-nitrophenolate ion peak was faster for Alg@Cu2O-Sb2O3, and the reaction was completed within 3.0 min. Similarly, Alg@Cu2O-Sb2O3 was further analyzed in the reduction reaction of 2-NP and 2,6-DNP with NaBH4 using UV-Vis spectrometry. Figure 6b,c illustrates that 2-NP and 2,6-DNP gave absorption peaks at 347 nm and 428 nm. Upon adding NaBH4 solution, a change of color was noticed along with a shift in the absorption band from 347 nm to 413 nm for 2-NP, signifying the existence of nitrophenolate ions [24,34]. Furthermore, upon adding Alg@Cu2O-Sb2O3, the absorption peaks recorded at 413 nm and 428 nm declined, while, on the other hand, another absorption peak at 280 nm could be seen with an increasing trend in intensity. The results verify the conversion of nitrophenols to aminophenol by reducing NO2 present in 2-NP and 2,6-DNP (Figure 6b,c). The 2-NP and 2,6-DNP reduction reactions were faster in the presence of Alg@Cu2O-Sb2O3, and both reactions were completed within 1.0 min and 3.0 min, respectively. Alg@Cu2O-Sb2O3 reduced more than 97% of 4-NP, 2-NP and 2,6-DNP in 3.0 min, 1.0 min and 3.0 min, respectively. Further, rate constants were obtained by applying Equation (2), and they were 1.3279 min −1 , 3.8187 The powder catalyst has the drawback that it cannot be collected and separated from reaction media in order to recycle it. Hence, a requirement for a good catalyst would be to remove or reduce these concerns. Consequently, Cu 2 O-Sb 2 O 3 was packed in hydrogel for the purpose of handling and surmounting all the above-mentioned concerns.

Effect of Catalyst Amount
For a comprehensive study, we examined the impact of Alg@Cu2O-Sb2O3 hydrogel and powder Cu2O-Sb2O3 amounts on 2-NP reduction using NaBH4 reducing agent. Hence, 3, 5 and 10 mg each of Cu2O-Sb2O3 and Alg@Cu2O-Sb2O3 nanocatalysts were added for the reduction of 2-NP to ascertain the impact of catalyst quantity (Figure 7). Thus, the role of Cu2O-Sb2O3 and Alg@Cu2O-Sb2O3 was evidently noticed, and hence, the Cu2O-Sb2O3 and Alg@Cu2O-Sb2O3 quantity plays a substantial role in the rate of 2-NP reduction. Therefore, Cu2O-Sb2O3 is very effective, but powder Cu2O-Sb2O3 has limitations, i.e., it is difficult to separate from the reaction in comparison with the present hydrogel. The powder catalyst has the drawback that it cannot be collected and separated from reaction media in order to recycle it. Hence, a requirement for a good catalyst would be to remove or reduce these concerns. Consequently, Cu2O-Sb2O3 was packed in hydrogel for the purpose of handling and surmounting all the above-mentioned concerns.

Effect of NaBH4 Concentration
The impact of the NaBH4 amount was assessed in the 2-NP reduction using the Alg@Cu2O-Sb2O3 nanocatalyst. To scrutinize various quantities of NaBH4, 0.3, 0.5 and 0.7 mL of 0.1 M NaBH4 were utilized with 5 mg of Alg@Cu2O-Sb2O3. Figure 8d shows the % reduction against the time plot. 2-NP got reduced up to 95.40%, 96.36% and 96.43% using 0.3, 0.5 and 0.7 mL of 0.1 M NaBH4 solution. Further, the reaction rates calculated in the current study were 0.4649 min −1 , 0.7937 min −1 and 1.0599 min −1 .   Recyclability of Alg@Cu2O-Sb2O3 Catalyst consistency and recovery are important from an environmental and costbased point of view [13,35]. Therefore, we checked the recyclability property of the Alg@Cu2O-Sb2O3 nanocatalyst ( Figure 10). We have analyzed the Alg@Cu2O-Sb2O3 nanocatalyst four times for an easy removal of the product by means of just removing the hydrogel. Figure 10d specifies the time utilized for the reduction reaction of 2-NP in every cycle with the same Alg@Cu2O-Sb2O3 nanocatalyst, which suggests the stability as well as the recyclability property of Alg@Cu2O-Sb2O3. Alg@Cu2O-Sb2O3 efficiently reduced 2-NP in 4.0 min until the 3 rd cycle, which suggests that Alg@Cu2O-Sb2O3 can be used numerous times with no loss in catalytic activity. The results were compared with the literature [36][37][38][39][40], and it was fount that Alg@Cu2O-Sb2O3 is an efficient catalyst that can be used several Catalyst consistency and recovery are important from an environmental and cost-based point of view [13,35]. Therefore, we checked the recyclability property of the Alg@Cu 2 O-Sb 2 O 3 nanocatalyst ( Figure 10). We have analyzed the Alg@Cu 2 O-Sb 2 O 3 nanocatalyst four times for an easy removal of the product by means of just removing the hydrogel. Figure 10d specifies the time utilized for the reduction reaction of 2-NP in every cycle with the same Alg@Cu 2 O-Sb 2 O 3 nanocatalyst, which suggests the stability as well as the recyclability property of Alg@Cu 2 O-Sb 2 O 3 . Alg@Cu 2 O-Sb 2 O 3 efficiently reduced 2-NP in 4.0 min until the 3 rd cycle, which suggests that Alg@Cu 2 O-Sb 2 O 3 can be used numerous times with no loss in catalytic activity. The results were compared with the literature [36][37][38][39][40], and it was fount that Alg@Cu 2 O-Sb 2 O 3 is an efficient catalyst that can be used several times. This indicate that nanocomposite can play important role as a catalyst since nanocomposites have shown an enormous range of applications in different sectors [41][42][43].

Conclusions
Cu2O-Sb2O3 nanoparticles were prepared and placed inside the Alg hydrogel. Both Cu2O-Sb2O3 and Alg@Cu2O-Sb2O3 nanocatalysts were excellent catalysts for the selective reduction reaction of 2-NP. The influence of various parameters on the 2-NP reduction using Alg@Cu2O-Sb2O3 showed that increasing the catalyst and NaBH4 amounts and decreasing the 2-NP concentration preceded a greater catalytic efficiency. The rate constant when utilizing 10 mg Alg@Cu2O-Sb2O3 was calculated as being 3.8187 min −1 for the 2-NP reduction. Lastly, Alg@Cu2O-Sb2O3 maintained a greater activity after recyclability assessments.