Catalytic Decolorization of Rhodamine B, Congo Red, and Crystal Violet Dyes, with a Novel Niobium Oxide Anchored Molybdenum (Nb–O–Mo)

: In this work, a new metal-to-metal charge transfer (MMCT) heterogeneous catalyst (Nb–O–Mo) was synthesized by a chemical grafting method under an inert atmosphere. The activity of the covalently anchored oxo-bridged Nb–O–Mo catalyst was estimated for decolorization of Rh B, congo red, and crystal violet dyes in an aqueous solution under ﬂuorescent light. The catalyst was characterized via X-ray di ﬀ raction, scanning electron microscopy with energy-dispersive X-ray spectrometer, Fourier-transform infrared, and FT-Raman. The catalytic decolorization was evaluated from the UV spectra of dyes in aqueous solution by changing di ﬀ erent factors, including dye concentration, temperature, and catalyst loading. Decolorization percentages were 83%–89%, 86%–95%, 97%–99% for Rh B, Congo Red and Crystal Violet in 1 min at 298 K, showing the best performance among other catalysts. Decolorization e ﬃ ciency for 50 ppm of Rh B was improved from 92% to 98%, with a temperature increase to 318 K.


Introduction
Industrial activities bring in massive amounts of contaminants into aquatic environments, which may disturb the ecosystem. Dye organics in wastewater mainly come from dyeing, printing, textile, and food industries, resulting in negative effects [1,2]. Generally, dyes are classified into three categories, such as cationic, nonionic and anionic dyes. Rh B is a well-known xanthene dye widely used in printing, textile, and photographic industries, as well as in foodstuffs [3,4]. It is reported as harmful to human beings and animals because of its toxic nature, which causes skin irritation, eyes, and respiratory tract. Congo red is the most used dye in the textile industry, also called benzidine based anionic diazo dye [5]. This dye is harmful to the human body and causes carcinogens and mutagen. Crystal violet (Cv) is commonly used in ink industries to produce colored textiles [6]. This dye interacts easily with cell membrane surfaces in mammals and enters into cells [7], and excess inhalation causes vomiting, headache, diarrhea, dizziness, and damage to the gastrointestinal tract [8].
Several methods have been reported to purify dye contaminated water, such as biological oxidation, adsorption, and coagulation [9]. There are some problems in conventional techniques, respectively), a metal-to-metal charge transfer (MMCT) occurs, e.g.,  To the best of our knowledge, this is the first study to report a novel Nb-O-Mo MMCT catalyst for the catalytic decolorization of three dyes, including Rh B, Congo Red and Crystal Violet. The developed materials contain a purely inorganic redox center used for the catalytic decolorization of Rh B dye in aqueous solution. Decolorization performance was evaluated by monitoring the decrease of Rh B dye concentration in aqueous solutions under ambient conditions, using UV-vis spectroscopy. The advantage of prepared catalyst is the decolorization of Rh B by Nb-O-Mo occurred in aqueous medium under normal light (without external energy or light source) and the time required for the decolorization was reduced compared to previously reported studies. The catalyst was extensively characterized in terms of morphology, as well as structural, crystalline, and catalytic properties. The decolorization process was further confirmed by using NMR and monitoring dye concentrations, with scavengers which prevent the decolorization process.

Result and Discussion
There are several methods for the synthesis of catalysts such as precipitation, incipient wet impregnation, sol-gel method, physical/direct mixing, grafting, and ion exchange [40][41][42]. In the present study, the chemical grafting method was used for the synthesis of the Nb-O-Mo MMCT catalyst. During the synthesis, the deprotonated NbO-is formed on the surface of Nb 2 O 5 by the basicity of TMG (1,1,3,3-tetramethylguanidine), where Mo from MoCl 5 is linked to form Nb-O-Mo MMCT linkage, which helps the photocatalytic process under fluorescent light. The XRD pattern of Nb 2 O 5 was assigned via JCPDS PDF # 27-1003 [43]. The characteristic diffraction peaks of Nb 2 O 5 at 22.5 • , 28.3 • , 36.5 • , 46.1 • , and 50.9 • , ( Figure 1) were identical to the orthorhombic structure of Nb 2 O 5 reported in the literature [43]. New diffraction peaks appeared at 18.55 and 19.36 • when Mo is anchored on Nb 2 O 5 . Moradi et al. [44] found that the intensity of the Fe-doped TiO 2 peaks was decreased compared to those of undoped TiO 2 , indicating that the addition of Fe 3+ decreased the crystallinity of TiO 2 . In addition, doping of Mo in VO 2 (M) barely affected XRD patterns, while the doping of Nb in VO 2 (M) resulted in decreases in XRD peaks [45]. A similar trend was observed in the intensity of the XRD peaks for the Nb-O-Mo catalyst compared to pure Nb 2 O 5 , indicating slightly reduced crystallinity. The reduced peak intensities, with corresponding angles, are shown in Table 1. and the time required for the decolorization was reduced compared to previously reported studies. The catalyst was extensively characterized in terms of morphology, as well as structural, crystalline, and catalytic properties. The decolorization process was further confirmed by using NMR and monitoring dye concentrations, with scavengers which prevent the decolorization process.

Result and Discussion
There are several methods for the synthesis of catalysts such as precipitation, incipient wet impregnation, sol-gel method, physical/direct mixing, grafting, and ion exchange [40][41][42]. In the present study, the chemical grafting method was used for the synthesis of the Nb-O-Mo MMCT catalyst. During the synthesis, the deprotonated NbO-is formed on the surface of Nb2O5 by the basicity of TMG (1,1,3,3-tetramethylguanidine), where Mo from MoCl5 is linked to form Nb-O-Mo MMCT linkage, which helps the photocatalytic process under fluorescent light. The XRD pattern of Nb2 O5 was assigned via JCPDS PDF # 27-1003 [43]. The characteristic diffraction peaks of Nb2O5 at 22.5°, 28.3°, 36.5°, 46.1°, and 50.9°, (Figure 1) were identical to the orthorhombic structure of Nb2O5 reported in the literature [43]. New diffraction peaks appeared at 18.55 and 19.36° when Mo is anchored on Nb2O5. Moradi et al. [44] found that the intensity of the Fe-doped TiO2 peaks was decreased compared to those of undoped TiO2, indicating that the addition of Fe 3+ decreased the crystallinity of TiO2. In addition, doping of Mo in VO2(M) barely affected XRD patterns, while the doping of Nb in VO2(M) resulted in decreases in XRD peaks [45]. A similar trend was observed in the intensity of the XRD peaks for the Nb-O-Mo catalyst compared to pure Nb2O5, indicating slightly reduced crystallinity. The reduced peak intensities, with corresponding angles, are shown in Table 1.   The surface morphologies of Nb 2 O 5 and Nb-O-Mo catalyst samples were identified based on SEM analysis ( Figure 2). The average particle sizes, measured using ImageJ [46], were decreased from 46.2 to 29.5 µm when Mo was anchored on Nb 2 O 5 , while the uniformity of morphologies of Nb 2 O 5 was reduced. Particle size was reduced possibly due to a physical stirring while doping MO into Nb 2 O 5 . A similar phenomenon was reported for Co-doped TiO 2 lattice. The particle size of TiO 2 was decreased from 15-20 nm to 5-7 nm, after Co doping with a stirring [47]. The SEM-EDS images ( Figure 3) and EDS spectra ( Figure S1 in Supporting Information) showed the uniform dispersion of Nb, O, and Mo elements on the sample with atomic ratios (Table S1 in supporting information) of 17.6%, 76.6%, and 5.8% for Nb, O, and Mo with a ratio of 3 to 1 for Nb and Mo.  The surface morphologies of Nb2O5 and Nb-O-Mo catalyst samples were identified based on SEM analysis ( Figure 2). The average particle sizes, measured using ImageJ [46], were decreased from 46.2 to 29.5 µm when Mo was anchored on Nb2O5, while the uniformity of morphologies of Nb2O5 was reduced. Particle size was reduced possibly due to a physical stirring while doping MO into Nb2O5. A similar phenomenon was reported for Co-doped TiO2 lattice. The particle size of TiO2 was decreased from 15-20 nm to 5-7 nm, after Co doping with a stirring [47]. The SEM-EDS images ( Figure  3) and EDS spectra ( Figure S1 in Supporting Information) showed the uniform dispersion of Nb, O, and Mo elements on the sample with atomic ratios (Table S1 in supporting information) of 17.6%, 76.6%, and 5.8% for Nb, O, and Mo with a ratio of 3 to 1 for Nb and Mo.  N 2 adsorption-desorption isotherms at 77 K were measured for the analysis of porous structures. Before measurements, the samples were degassed for 3 h at 383 K under vacuum. The samples exhibited typical type IV curves and hysteresis loops according to IUPAC classification [40,41]. This behavior specifies the presence of mesopores. When Mo was anchored on Nb 2 O 5 , the BET surface area and pore volume for Nb-O-Mo were slightly decreased compared to those of Nb 2 O 5 (surface area from 6.01 to 5.30 m 2 /g; pore volume from 0.0119 to 0.0084 cm 3 /g in Table S2 of Supporting Information). The pore size distribution was derived from the non-local density functional theory (NLDFT) method, based on N 2 adsorption-desorption isotherms at 77 K ( Figure S2 in Supporting Information). Pore size distribution analysis for Nb 2 O 5 and Nb-O-Mo exhibits that pore distribution of Nb-O-Mo was shifted to larger pore width, because the molybdenum anchored on niobium oxide resulted in [48,49].
The  N2 adsorption-desorption isotherms at 77 K were measured for the analysis of porous structures. Before measurements, the samples were degassed for 3 h at 383 K under vacuum. The samples exhibited typical type ⅠV curves and hysteresis loops according to IUPAC classification [40,41]. This behavior specifies the presence of mesopores. When Mo was anchored on Nb2O5, the BET surface area and pore volume for Nb-O-Mo were slightly decreased compared to those of Nb2O5 (surface area from 6.01 to 5.30 m 2 /g; pore volume from 0.0119 to 0.0084 cm 3 /g in Table S2 of Supporting Information). The pore size distribution was derived from the non-local density functional theory (NLDFT) method, based on N2 adsorption-desorption isotherms at 77 K ( Figure S2 in Supporting Information). Pore size distribution analysis for Nb2O5 and Nb-O-Mo exhibits that pore distribution of Nb-O-Mo was shifted to larger pore width, because the molybdenum anchored on niobium oxide resulted in [48,49].
The FTIR spectra of the Nb2O5 and Nb-O-Mo catalyst samples are shown in Figure 4. The peaks that appeared at 3400 cm −1 correspond to the OH stretching of Nb-OH in Nb2O5 [50] and   To further investigate the formation of Mo on Nb2O5, Raman spectra of Nb2O5 and Nb-O-Mo were measured. The weak band at 990 cm −1 ( Figure 5) has resulted from the vibration modes (ν1) of NbO6 octahedra in Nb2O5 [52]. The Raman band at approximately 700 cm −1 was assigned to orthorhombic Nb2O5. The strongest symmetric Nb-O stretching mode of the niobium oxide polyhedra was related to the bond order of the niobia polyhedra and structure order [53]. After Mo doping, almost all bands disappeared, except at 700 cm −1 ; similar behavior has been reported after the doping of metal [54].   [52]. The Raman band at approximately 700 cm −1 was assigned to orthorhombic Nb 2 O 5 . The strongest symmetric Nb-O stretching mode of the niobium oxide polyhedra was related to the bond order of the niobia polyhedra and structure order [53]. After Mo doping, almost all bands disappeared, except at 700 cm −1 ; similar behavior has been reported after the doping of metal [54].
To further investigate the formation of Mo on Nb2O5, Raman spectra of Nb2O5 and Nb-O-Mo were measured. The weak band at 990 cm −1 ( Figure 5) has resulted from the vibration modes (ν1) of NbO6 octahedra in Nb2O5 [52]. The Raman band at approximately 700 cm −1 was assigned to orthorhombic Nb2O5. The strongest symmetric Nb-O stretching mode of the niobium oxide polyhedra was related to the bond order of the niobia polyhedra and structure order [53]. After Mo doping, almost all bands disappeared, except at 700 cm −1 ; similar behavior has been reported after the doping of metal [54].  The effect of temperature on Rh B decolorization at a fixed catalyst loading showed that at high temperature (318 K), the characteristic absorption peak of Rh B at 554 nm diminished by 98% in 10 min after addition of the Nb-O-Mo catalyst to the 50 ppm Rh B solution ( Figure S4 in Supporting Information). The dye molecules were more mobile at increased temperatures and reached the  The catalytic performance of Nb-O-Mo was also examined for the other two dyes of Congo Red and Crystal Violet, with 20 and 50 ppm (Figure 8). The concentrations of Congo Red and Crystal Violet were significantly decreased by 86%−95% and 97%−99%, within 1 min, respectively. The kinetics followed a biphasic trend with an initial very fast phase followed by a slow phase. For simplicity and practical reasons, kinetics have been determined using first-order model, taking into account the experimental points of the fast degradation phase (0-1 min). The rate constant was determined using the equation, -ln(C/C0) = kt, [56], where C0 and C are dye concentrations at time zero and t; k is a rate constant. The rate constant (Table 2) was increased by twice for Crystal Violet when the concentration was increased from 20 to 50 ppm. The effect of temperature on Rh B decolorization at a fixed catalyst loading showed that at high temperature (318 K), the characteristic absorption peak of Rh B at 554 nm diminished by 98% in 10 min after addition of the Nb-O-Mo catalyst to the 50 ppm Rh B solution ( Figure S4 in Supporting Information). The dye molecules were more mobile at increased temperatures and reached the catalyst surface more rapidly for transformation, due to the increased kinetic energy of dye molecules. Yuan et al. [55] reported the effect of temperature for the oxidation of Safranin T over microporous ZnO/MoO 3 /SiO 2 hybrid catalysts at 293-308 K. The decolorization efficiency reached 98% with an increase in temperature of up to 308 K.
The catalytic decolorization of Rh B was examined at different concentrations of 10, 20, and 50 ppm, to understand the influence of concentration on decolorization efficiency from the graphs of C/C 0 as a function of reaction time (Figure 7), where C 0 is the initial concentration of Rh B and C is the concentration at a certain time. It should be noted that there is no significant decrease in decolorization performance in 10 min, even with an increase in the concentration of Rh B from 10 to 50 ppm, while decolorization performances in 1 min were increased for all concentrations. Such a significant decolorization in a short time was reported for catalysts (80%~97% decolorization in 1 min for WO 3 /MoCl 5 [56] with a rate constant of 104.4/min). When the temperature was increased in 50 ppm solution from 298 to 318 K, the performance was improved after 1 min. When the catalyst amount decreased from 0.8 and 0.4 g/L for 20 ppm Rh B solution (Figure 7), the decolorization performance was decreased by 53% for 1 min and 19% for 10 min. The control experiments without catalysts under a fluorescent light were carried out for all three dyes with 20 ppm concentration at 298 K ( Figure S5 in supporting information), showing no significant decreases of dye concentration, due to the absence of catalysts.
The catalytic performance of Nb-O-Mo was also examined for the other two dyes of Congo Red and Crystal Violet, with 20 and 50 ppm (Figure 8). The concentrations of Congo Red and Crystal Violet were significantly decreased by 86%−95% and 97%−99%, within 1 min, respectively. The kinetics followed a biphasic trend with an initial very fast phase followed by a slow phase. For simplicity and practical reasons, kinetics have been determined using first-order model, taking into account the experimental points of the fast degradation phase (0-1 min). The rate constant was determined using the equation, -ln(C/C 0 ) = kt, [56], where C 0 and C are dye concentrations at time zero and t; k is a rate constant. The rate constant ( Table 2) was increased by twice for Crystal Violet when the concentration was increased from 20 to 50 ppm.     [63]. In addition, generated • OH radicals in the reaction were monitored based on the oxidation of terephthalic acid (TA) to hydroxyterephthalic acid (TAOH) by • OH, where TA reacts with • OH to produce a fluorescent TAOH compound. The fluorescence intensity of TAOH increased with respect to time (Figure 10), representing the presence of • OH radicals generated in the catalytic reaction with Nb-O-Mo (see Figure S6 in Supporting Information for spectra changes over time). FTIR spectroscopy was taken to identify the disappearance of Rh B due to the decolorization process (Figure 11), by comparing FTIR spectra before and after Rh B decolorization. The absence of major peaks at 2961, 2920, 2855 cm −1 of Rh B [64] in Nb-O-Mo after Rh B decolorization indicates that the Rh B was degraded by the Nb-O-Mo catalyst. Additionally, the structural changes of Rh B by decolorization were evaluated, by using proton NMR analysis. Four samples as one Rh B solution and three reaction mixture samples with different times of 1, 5 and 10 min were evaluated using 1 H NMR ( Figure S7 in Supporting Information). The disappearance of characteristic peaks at δ 1.05, 1.99, 2.1 and 2.21 and appearance of new peaks at δ 2.07, 2.14, 2.67 and 2.92 ppm (in red color) indicates the breakdown of the Rh B structure.
with O2 to form O2 radicals. Zhang et al. demonstrated that the photoinduced Zr(III) via the MMCT excitation of Zr(IV)-O-Ce(III) to Zr(III)-O-Ce(IV) reduced O2 to O2 −• , which was liable for the oxidation of Rh B [63]. In addition, generated • OH radicals in the reaction were monitored based on the oxidation of terephthalic acid (TA) to hydroxyterephthalic acid (TAOH) by • OH, where TA reacts with • OH to produce a fluorescent TAOH compound. The fluorescence intensity of TAOH increased with respect to time (Figure 10), representing the presence of • OH radicals generated in the catalytic reaction with Nb-O-Mo (see Figure S6 in Supporting Information for spectra changes over time).  FTIR spectroscopy was taken to identify the disappearance of Rh B due to the decolorization process (Figure 11), by comparing FTIR spectra before and after Rh B decolorization. The absence of major peaks at 2961, 2920, 2855 cm −1 of Rh B [64] in Nb-O-Mo after Rh B decolorization indicates that the Rh B was degraded by the Nb-O-Mo catalyst. Additionally, the structural changes of Rh B by decolorization were evaluated, by using proton NMR analysis. Four samples as one Rh B solution and three reaction mixture samples with different times of 1, 5 and 10 min were evaluated using 1 H NMR ( Figure S7    , which can serve as active sites for dye decolorization. The generated Nb(IV) can be trapped by the O 2 molecules to return to Nb(V), producing the superoxide anion radicals as a strong oxidizing agent for the decolorization of organic compounds; meanwhile, the Mo(VI) is reduced to Mo(V) by receiving an electron from water molecule in dye solution, leaving H + and • OH radicals, which contribute to dye decolorization.
FTIR spectroscopy was taken to identify the disappearance of Rh B due to the decolorization process (Figure 11), by comparing FTIR spectra before and after Rh B decolorization. The absence of major peaks at 2961, 2920, 2855 cm −1 of Rh B [64] in Nb-O-Mo after Rh B decolorization indicates that the Rh B was degraded by the Nb-O-Mo catalyst. Additionally, the structural changes of Rh B by decolorization were evaluated, by using proton NMR analysis. Four samples as one Rh B solution and three reaction mixture samples with different times of 1, 5 and 10 min were evaluated using 1 H NMR ( Figure S7 in Supporting Information). The disappearance of characteristic peaks at δ 1.05, 1.99, 2.1 and 2.21 and appearance of new peaks at δ 2.07, 2.14, 2.67 and 2.92 ppm (in red color) indicates the breakdown of the Rh B structure.  , which can serve as active sites for dye decolorization. The generated Nb(IV) can be trapped by the O2 molecules to return to Nb(V), producing the superoxide anion radicals as a strong oxidizing agent for the decolorization of organic compounds; meanwhile, the Mo(VI) is reduced to Mo(V) by receiving an electron from water molecule in dye solution, leaving H + and • OH radicals, which contribute to dye decolorization. The reusability of a catalyst was examined by performing recycling experiments, because it is another important aspect as a catalyst in water treatment, due to which it can be used as an industrial catalyst and avoiding secondary pollution. After each reaction in Rh B solution, Nb-O-Mo was collected from the solution by a centrifuge and washed with deionized water, dried and reused for the next experiment. The decolorization activities of Nb-O-Mo ( Figure 12) were effectively maintained during three cycles, with a 7% decrease in efficiency, which is similar to the reported catalysts, including CoxFe3−xO4 nanocatalyst [66], Ag-Fe3O4 composite [67], and Ag2CO3 catalyst [68].
Inductively coupled plasma-mass spectroscopy (ICP-MS) analysis was performed to investigate the metal leaching. The Rh B (10 ppm) solution with 0.8 g/L catalyst loading was used for 10 min. The formed reaction solution after the decolorization was centrifuged at 1000 rpm and finally passed through a mixed cellulose ester syringe filter. For the ICP-MS analysis, 2 ml of the above solution was diluted to 4 ml, by using deionized water and comparing with the standard solution of Mo with 1, 10 and 100 ppm for a calibration curve ( Figure S8 in supporting information), showing that the leaching of Mo in reaction aliquot was 7.8 ppm, less than 1% of total catalyst loading (mass of Mo leaching = 0.0078 g/L, total catalyst loading = 0.8 g/L). The reusability of a catalyst was examined by performing recycling experiments, because it is another important aspect as a catalyst in water treatment, due to which it can be used as an industrial catalyst and avoiding secondary pollution. After each reaction in Rh B solution, Nb-O-Mo was collected from the solution by a centrifuge and washed with deionized water, dried and reused for the next experiment. The decolorization activities of Nb-O-Mo ( Figure 12) were effectively maintained during three cycles, with a 7% decrease in efficiency, which is similar to the reported catalysts, including Co x Fe 3−x O 4 nanocatalyst [66], Ag-Fe 3 O 4 composite [67], and Ag 2 CO 3 catalyst [68].
Inductively coupled plasma-mass spectroscopy (ICP-MS) analysis was performed to investigate the metal leaching. The Rh B (10 ppm) solution with 0.8 g/L catalyst loading was used for 10 min. The formed reaction solution after the decolorization was centrifuged at 1000 rpm and finally passed through a mixed cellulose ester syringe filter. For the ICP-MS analysis, 2 ml of the above solution was diluted to 4 ml, by using deionized water and comparing with the standard solution of Mo with 1, 10 and 100 ppm for a calibration curve ( Figure S8 in supporting information), showing that the leaching of Mo in reaction aliquot was 7.8 ppm, less than 1% of total catalyst loading (mass of Mo leaching = 0.0078 g/L, total catalyst loading = 0.8 g/L). the next experiment. The decolorization activities of Nb-O-Mo ( Figure 12) were effectively maintained during three cycles, with a 7% decrease in efficiency, which is similar to the reported catalysts, including CoxFe3−xO4 nanocatalyst [66], Ag-Fe3O4 composite [67], and Ag2CO3 catalyst [68].
Inductively coupled plasma-mass spectroscopy (ICP-MS) analysis was performed to investigate the metal leaching. The Rh B (10 ppm) solution with 0.8 g/L catalyst loading was used for 10 min. The formed reaction solution after the decolorization was centrifuged at 1000 rpm and finally passed through a mixed cellulose ester syringe filter. For the ICP-MS analysis, 2 ml of the above solution was diluted to 4 ml, by using deionized water and comparing with the standard solution of Mo with 1, 10 and 100 ppm for a calibration curve ( Figure S8 in supporting information), showing that the leaching of Mo in reaction aliquot was 7.8 ppm, less than 1% of total catalyst loading (mass of Mo leaching = 0.0078 g/L, total catalyst loading = 0.8 g/L).

Preparation of the Catalyst
The niobium oxide anchored molybdenum (Nb-O-Mo) catalysts were prepared using a Schlenk line under a nitrogen atmosphere. In a typical synthesis, 0.5 g of Nb 2 O 5 was added to a 100 mL single-necked Schlenk flask (A), capped with rubber septa. Nitrogen gas was purged and evacuated twice to remove air and the Nb 2 O 5 was vacuum dried overnight at 363 K. Similarly, 1 g of MoCl 5 was placed in another single necked Schlenk flask (B) and maintained under a nitrogen atmosphere. Anhydrous toluene (25 mL) was transferred to both flasks (A and B) using a cannula tube. Further, 0.5 mL of tetramethylguanidine was added to flask A containing the Nb 2 O 5 solution under a nitrogen atmosphere using a glass syringe. Both flask solutions were stirred for 1 h under a nitrogen atmosphere and room temperature. After 1 h, the MoCl 5 solution from flask B was transferred to a flask containing Nb 2 O 5 solution using a cannula tube. Finally, the reaction mixture was stirred for 2 h maintaining the nitrogen atmosphere. The resulting product was filtered and washed with toluene a couple of times under inert vacuum conditions. The final product was dried under vacuum overnight at 388 K.

Experimental Procedure for Oxidative Decolorization of Rh B, Congo Red, and Crystal Violet
Dye solutions of 10, 20, and 50 ppm were prepared for dyes. In a batch reactor, 100 mL of dye solution at the desired concentration was mixed with 80 mg of the catalyst (0.8 g/L) and then stirred for 10 min. At certain time intervals from 0 to 10 min, an approximately 4 ml aliquot of the reaction mixture was withdrawn for concentration analysis by using a UV spectrometer. The absorbances of Rh B, Congo Red, and Crystal Violet were recorded at 554, 498, and 590 nm respectively ( Figure S9 for calibration curves in supporting information). The catalytic activity was performed under normal light conditions and at room temperature (298 K), where there is no highly concentrated light source, such as a UV lamp, directly irradiating the reactor. The only light source was a fluorescent light 480 nm to 570 nm with a frequency of 120 Hz located at the ceiling to brighten the laboratory, providing small energy of light.

Characterization
The X-ray diffraction (XRD) patterns of the samples were recorded using an X-ray diffractometer (XPert Pro-MPD, PANanlytical, Netherlands) with Cu anodes from 10 to 70 • under 40 kV and 40 mA. Fourier-transform infrared (FTIR) spectra were recorded using an FT/IR 6300 spectrometer (JASCO, Japan) from 400 to 4000 cm −1 . KBr pellets were prepared to obtain FTIR spectra by mixing the Nb-O-Mo catalyst with KBr in a weight ratio of 1:99. The scanning electron microscopy (SEM) images and atomic percentages of Nb, O, and Mo were determined using a scanning electron microscope (Merlin compact, Oberkochen, Germany), equipped with an energy-dispersive X-ray spectrometer (EDS) to observe the specific microstructures of the prepared samples. The UV-vis spectra of the various liquid samples were recorded using a Cary 5000 UV-Vis-NIR spectrophotometer (Agilent, Santa Clara, CA, USA), from 350 to 800 nm. To measure the specific surface area of the samples, N 2 adsorption-desorption isotherms were measured at 77 K using liquid nitrogen, and the specific surface areas of the samples were calculated using the BET method with BELSORP-Mini-II BET equipment (Microtrac BEL, Japan). Then, 1H nuclear magnetic resonance (1H NMR) spectra of the dye solution were achieved using Bruker NMR spectrometer (San Jose, CA, USA) operating at 600 MHz at 300 K. To prepare for the samples for NMR analysis, an appropriate amount of the catalyst was added to 2 mL of Rh B solution in D 2 O and vortexed for a desired time to achieve catalytic decolorization. After the reaction, the mixture was centrifuged to remove Nb-O-Mo catalyst particles and the residue was used for NMR analysis.
To investigate the presence of •OH radicals in the reaction, the oxidation of terephthalic acid (TA) was performed according to the following method: Nb-O-Mo catalyst (40 mg) was added to TA (6 × 10 −3 M) aqueous solution of 50 ml with 0.02 M NaOH, and the reaction was carried out for 60 min. During the reaction, sample aliquots were collected and filtered at every 10 min and the formation of hydroxyterephthalic acid (TAOH) was identified by using a fluorescence spectrophotometer (HITACHI F-7000) at 350−500 nm.

Conclusions
Niobium oxide anchored with a molybdenum catalyst was successfully synthesized using a Schlenk line. The as-synthesized catalyst was analyzed using various characterization techniques, including XRD, SEM with EDS, FTIR, and FT-Raman spectroscopy to explore the structure, morphology, and linkage between elements. XRD studies showed the orthorhombic structure of Nb 2 O 5 and EDS mapping elucidated the presence and relative proportion of each element in the catalyst. The catalytic activity was explored for the oxidation of Rh B, Congo Red, and Crystal Violet and was examined using UV-vis spectra in at 554 nm. Decolorization intensity was retained over time, achieving 91%, 93%, and 92% decolorization of Rh B at initial concentrations of 10, 20 and 50 ppm, respectively, within 10 min at 298 K. Large decolorization was also observed 86%−95% and 97%−99% in 1 min for Congo Red and Crystal Violet respectively. The obtained results indicate that the decolorization efficiency of niobium oxide anchored molybdenum was not affected by even high initial dye concentration (50 ppm). The decolorization efficiency was enhanced to 98% for 50 ppm at 318 K, with a catalyst loading of 0.8 g/L, which was the best performance among other materials, such as ZnO nanoparticles of 1 g/L (96% decolorization in 180 min) [69], CdS/AgBr-rGO nanocomposite of 0.6 g/L (96% decolorization in 60 min) [70], TiO 2 /RCN heterojunction photocatalyst of 0.5 g/L (98.6% decolorization in 240 min) [71], and MnO 2 of 2.5 g/L (90% in 25 min) [72]. The decolorization of Rh B was confirmed by FTIR and NMR analysis, as well as performing scavenger experiments. In conclusion, the results presented herein indicate that the prepared catalyst is a potential candidate for the catalytic oxidation of hazardous cationic organic compounds in water to environmentally innocuous products.