Next Article in Journal
Photocatalytic Hydrogen Evolution
Next Article in Special Issue
Iron-Loaded Catalytic Silicate Adsorbents: Synthesis, Characterization, Electroregeneration and Application for Continuous Removal of 1-Butylpyridinium Chloride
Previous Article in Journal
Special Issue on Environmental Biocatalysis
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 10
Issue 5
10.3390/catal10050491
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

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

by
Rahul L. Kadam
1,†,
Yeonhee Kim
2,†,
Sanjit Gaikwad
1,
Mincheol Chang
3,*,
Naresh H. Tarte
4,* and
Sangil Han
1,*
1
Department of Chemical Engineering, Changwon National University, Changwon 51140, Korea
2
School of Energy and Chemical Engineering, UNIST, Ulsan 44919, Korea
3
School of Polymer Science and Engineering, Chonnam National University, Gwangju 61186, Korea
4
Department of Chemistry and Biology, Korea Science Academy of KAIST, Baegyanggwanmun-ro 105-47, Busanjin-gu, Busan, Korea
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this study.
Catalysts 2020, 10(5), 491; https://doi.org/10.3390/catal10050491
Submission received: 24 March 2020 / Revised: 27 April 2020 / Accepted: 28 April 2020 / Published: 1 May 2020
(This article belongs to the Special Issue Regeneration of Adsorbent by Catalytic Process)
Browse Figures
Versions Notes

Abstract

:
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 diffraction, 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 different 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 efficiency for 50 ppm of Rh B was improved from 92% to 98%, with a temperature increase to 318 K.

1. 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, such as secondary pollutants, slow process, regeneration cost, high sludge production, etc. [10] To avoid such problems, photocatalyst, where a photon absorption with enough energy leads to charge separation of the photocatalyst from the valence state to the conduction state [11], has been introduced for the degradation of dyes in wastewater in the presence of external energy source [12]. This technique is more popular due to advantages such as dye degradation without producing secondary pollutants and degraded products are eco-friendly with biological systems. Zhang et al. [13] previously studied photocatalytic activities for degrading Rh B in an aqueous solution of anatase, rutile, and brookite TiO2, and Yin et al. [14] reported the degradation of Rh B under visible light irradiation with BiVO4/TiO2 showing 79% degradation (9 wt % BiVO4/TiO2), 1.14 times higher than that of pure TiO2. Kostic et al. [15] reported on solar light-induced Rh B degradation assisted by TiO2-Zn-Al layered double hydroxide (LDH)-based photocatalysts. Shi et al. [16] studied Ag/AgBr/ZnO photocatalyst for Rh B degradation under visible light. Yang et al. [17] reported different cocatalyst by metal deposition method; among them, maximum degradation activities were observed for the Au/ZnGa2O4 catalyst with 96.7% RhB degradation in 180 min under ultraviolet light. Tian et al. [18] reported TiO2/ZrO2 composite using chitosan (CMCS) as a template, 5% TiO2/ZrO2 composite showed 90.5% Rh B degradation at Ph = 10.3 in 4.5 h under ultraviolet light. Pica et al. [19] reported ZrP/AgBr composites for RhB degradation with a faster degradation (90% in 3 min), compared to the parent AgBr (90% in 30 min).
Advanced oxidation processes (AOPs) have recently been widely investigated as efficient routes for the destruction of organic dyes. In particular, semiconductor-based photocatalysts including iron oxide [20,21], Nb-based oxides [22,23], and titanium dioxides [24,25] are of great interest for hazardous wastewater removal to solve environmental problems. The main drawbacks of these photocatalysts lie in their limited activity and UV irradiation requirements due to their wide bandgaps. To overcome the limited activity, the doping of anions such as N, S, and C into semiconductor photocatalysts has been proposed to extend their absorption and conversion capacities in the visible light region [26,27,28]. In addition, various transition metals, including Ag+, Pb2+, and Cr3+ were doped onto TiO2 or SrTiO3 semiconductors with wide band gaps, extending the photocatalytic activity to the visible region [29,30]. Among the semiconductors, TiO2 is a well-known and promising candidate for the degradation of contaminants. TiO2 has been made into thin layers and films based on TiO2 nanotubes. Krbal et al. [31] reported a comparative study between sulfur treated anodic TiO2 nanotube and parent blank TiO2 nanotube. Sulfur treated TiO2 nanotube showed that the photocatalytic degradation rate was 2.3 times higher than the reference TiO2. Motola et al. [32] developed TiO2 nanotube layers using titanium mesh and Ti sputtered on silicon substrates for the antibiotic, photocatalytic and antimicrobial activities.
Nb2O5 exhibits a similar semiconductivity to TiO2, in addition to good chemical stability non-toxicity, and recyclability [33]. Niobium oxides are promising materials for various reactions, such as hydration, esterification, condensation, and heterogeneous photocatalysis in AOPs, due to their high Brunauer–Emmett–Teller (BET) surface areas, high selectivities, and acidic sites [34,35]. Niobium catalysts are efficient for the oxidation of organic compounds in aqueous media for the photodegradation of pollutants under UV radiation [36]. Furthermore, niobium oxides have been combined with other metals, such as Pt, Ru, and Rh, to improve their performances in many catalytic processes [37].
When two metal ions are present in different oxidation states in a complex with an oxo-bridged linkage, such as in M1m-O-M2n (where m and n represent the oxidation states of metals M1 and M2, respectively), a metal-to-metal charge transfer (MMCT) occurs, e.g., M1m-O-M2n → M1m-1-O-M2n+1, upon absorption of visible light. Using this strategy, Frei et al. [38] developed a novel class of MMCT visible light-absorbing chromophores and mixed-valence complexes with oxygen linkages (Ti(IV)–O–Cu(I) and Ti(IV)–O–Sn(II)). Hashimoto et al. [39] also successfully developed Ce(III)–O–Ti (IV) bimetallic assemblies on MCM-41 molecular sieves that demonstrate MMCT activity under visible-light irradiation, showing the photocatalytic oxidation of 2-propanol under visible light.
Herein, based on the MMCT mechanism, a niobium oxide anchored molybdenum (Nb–O–Mo) unit using Nb2O5 and MoCl5 as optimum precursors in a 1:2 ratio was successfully synthesized. 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.

2. 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 Fe3+ 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 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.
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 m2/g; pore volume from 0.0119 to 0.0084 cm3/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 Nb–O–Mo sample. The several peaks at 1000–1490 cm−1 and 1609 correspond to characteristic peaks of Mo-O linkage [51], whereas the peak at 873 cm−1 was assigned to Mo–O stretching. The bands between 500 and 950 cm−1 correspond to the angular vibrations of Nb–O–Nb and Mo–O–Mo in the Nb–O–Mo sample after anchoring Mo on the Nb2O5 sample [36].
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 UV-vis spectra in Figure 6 present the concentration decrease of Rh B in aqueous solution withdrawn from the batch reactor under the catalytic activity of Nb–O–Mo. After the addition of Nb–O–Mo catalyst to the 10 ppm Rh B aqueous solution, the absorption peak at 554 nm ascribed to a significant decrease in Rh B concentration by 80% within 1 min, indicating the decreased concentration of Rh B. The color of the Rh B dye solution almost disappeared within 10 min of the addition of the catalyst (by approximately 90%). Even with the increased concentration of Rh B, the decolorization percentages were similar to 10 ppm solution, showing 93% and 90% for 20 and 50 ppm in 10 min (Figure 7). When there is no doping of Mo, a decrease in dye concentration is negligible for Nb2O5 because of a lack of MMCT mechanism (Figure S3 in supporting information), showing only a 9% decrease in RhB concentration in 10 min without doping.
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/MoO3/SiO2 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/C0 as a function of reaction time (Figure 7), where C0 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 WO3/MoCl5 [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/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 performance of the developed Nb–O–Mo catalyst outperforms that of the reported catalytic materials. Wang et al. [57] prepared homogeneously decorated Pt quantum dots on TiO2 nanotube arrays, using a modified photo irradiation-reduction method for Rh B decolorization. They found that 73.47% of the Rh B was degraded after 180 min of UV irradiation. Abdel-Messih et al. [58] used TiSn10 and pure titania as catalysts, achieving approximately 92% and 72% decolorization of Rh B in 3 h under UV irradiation. Kuo et al. [59] achieved photocatalytic decolorization of Rh B (~94%) in 300 min, by illuminating the surface of ZnO nanowires on silicon wafers.
To investigate the reaction mechanism of Rh B decolorization with Nb–O–Mo, different scavengers, such as benzoquinone (BQ, O2−• scavenger) of 0.108 g [60], NaHCO3 (h+ scavenger) of 0.168 g [60], and isopropyl alcohol of 0.060 g (IPA, •OH scavenger) [61] were separately added to reaction mixtures containing Rh B dye solution and catalyst, where the scavengers interact with reactive radicals. Superoxide anion radical (O2−•), electron-hole (h+), and •OH are the primary active radicals in decolorization processes [62]. Each addition of the scavengers into Rh B solution showed decreases in concentrations (Figure 9), compared to that without scavengers, indicating that O2−•, h+ and •OH participate in the decolorization process. Moreover, when BQ was added into the Rh B solution with Nb–O–Mo, the decolorization of Rh B was more depressed, revealing that O2−• is likely to be the dominant active species in the decolorization process. The anchoring of Mo5+ with Nb2O5 broadens its visible light absorption and more electron-hole pairs are formed. The formed oxo-bridged linkage between Mo and Nb2O5 can accelerate the e−/h+ separation and inhibit the recombination of the generated electron-hole pairs; as a result, more generated electrons can react 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 1H 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.
Based on the previously reported MMCT mechanism [65], the schematic illustration of Rh B dye decolorization by Nb–O–Mo linkages is provided in Scheme 1. The oxo-bridged bimetallic linkage of Nb(V)–O–Mo(V) is supposed to be the function of the catalytic redox center. There is a possibility of MMCT transition of Nb(V) –O–Mo(V) to Nb(IV)–O–Mo(VI), 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).

3. Experimental

3.1. Materials and Chemicals

Niobium oxide (Nb2O5, 99.99%), 1,1,3,3-tetramethylguanidine (99%), molybdenum pentachloride (MoCl5, 95%), and deionized water were purchased from Sigma Aldrich (St. Louis, MO, USA). Anhydrous toluene (≥99.8 %) was obtained from Honeywell Chemicals (Harvey St., Muskegon, MI, USA). All chemicals and solvents were of analytical grade and used without further purification.

3.2. 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 Nb2O5 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 Nb2O5 was vacuum dried overnight at 363 K. Similarly, 1 g of MoCl5 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 Nb2O5 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 MoCl5 solution from flask B was transferred to a flask containing Nb2O5 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.

3.3. 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.

3.4. 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, N2 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 D2O 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.

4. 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 Nb2O5 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], TiO2/RCN heterojunction photocatalyst of 0.5 g/L (98.6% decolorization in 240 min) [71], and MnO2 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.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/10/5/491/s1, Figure S1: EDS spectrum of Nb–O–Mo, Figure S2: (a) N2 adsorption-desorption isotherms and (b) pore size distribution of Nb2O5 and Nb–O–Mo at 77 K, Figure S3: Catalytic decolourization of Rh B (10 ppm) by Nb2O5 and Nb–O–Mo catalyst at 298K, Figure S4: UV-vis spectral changes of Rh B 50 ppm dye solution upon addition of Nb–O–Mo catalyst at 318 K at various time intervals, Figure S5: Control experiment of dyes without catalyst at 298K, Figure S6: Fluorescence spectra for TAOH formation with respect to reaction time, Figure S7: 1H NMR spectra of samples with different decolourization percentages (Rh B(Gray), Blue(1 min), Green(5 min) and Red(10 min) decolourization) under a catalytic decolourization by Nb–O–Mo, Figure S8: Calibration curves for Mo(V) using ICP-MS, Figure S9: Calibration curves for a) Rh B, b) Cango Red and c) Crystal violet, Table S1: EDS elemental analysis of Nb–O–Mo, Table S2: Surface area and pore volume of Nb2O5 and Nb–O–Mo.

Author Contributions

Conceptualization, R.L.K. and N.H.T.; methodology, S.G.; software, S.G.; validation, Y.K. and M.C.; formal analysis, Y.K.; investigation, S.H.; resources, S.G.; data curation, R.L.K.; writing—original draft preparation, R.L.K.; writing—review and editing, S.H.; visualization, Y.K.; supervision, S.H.; project administration, S.H.; funding acquisition, S.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science, ICT and Future Planning of Korea (2020R1H1A1003814 and #NRF-2017R1C1B1002851), the Cooperative R&D between Industry, Academy, and Research Institute funded Korea Ministry of SMEs and Startups in 2019 (S2652595), and the Korea Science Academy of KAIST with funds from the Ministry of Science and ICT.

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (2020R1H1A1003814 and #NRF-2017R1C1B1002851), the Cooperative R&D between Industry, Academy, and Research Institute funded Korea Ministry of SMEs and Startups in 2019 (Grants No. S2652595), and the Korea Science Academy of KAIST with funds from the Ministry of Science and ICT. Rahul L. Kadam is thankful to Jin Ho Oh, Department of Chemistry, KSA of KAIST for his support during this research activity.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Banazadeh, A.; Salimi, H.; Khaleghi, M.; Shafiei-Haghighi, S. Highly efficient decolorization of hazardous dyes in aqueous phase by supported palladium nanocatalyst—A green approach. J. Environ. Chem. Eng. 2016, 4, 2178–2186. [Google Scholar] [CrossRef]
  2. Ameen, S.; Park, D.-R.; Shin, H.S. Silicon nanowires arrays for visible light driven photocatalytic decolorization of rose bengal dye. J. Mater. Sci. Mater. Electron. 2016, 27, 10460–10467. [Google Scholar] [CrossRef]
  3. Guo, C.; Xu, J.; He, Y.; Zhang, Y.; Wang, Y. Photodecolorization of rhodamine B and methyl orange over one-dimensional TiO2 catalysts under simulated solar irradiation. Appl. Surf. Sci. 2011, 257, 3798–3803. [Google Scholar] [CrossRef]
  4. Richardson, S.D.; Willson, C.S.; Rusch, K.A. Use of rhodamine water tracer in the marshland upwelling system. Groundwater 2004, 42, 678–688. [Google Scholar] [CrossRef]
  5. Liu, X.; Li, W.; Chen, N.; Xing, X.; Dong, C.; Wang, Y. Ag–ZnO heterostructure nanoparticles with plasmon-enhanced catalytic decolorization for Congo red under visible light. RSC Adv. 2015, 5, 34456–34465. [Google Scholar] [CrossRef]
  6. Singh, K.P.; Gupta, S.; Singh, A.K.; Sinha, S. Optimizing adsorption of crystal violet dye from water by magnetic nanocomposite using response surface modeling approach. J. Hazard. Mater. 2011, 186, 1462–1473. [Google Scholar] [CrossRef]
  7. Li, S. Removal of crystal violet from aqueous solution by sorption into semi-interpenetrated networks hydrogels constituted of poly (acrylic acid-acrylamide-methacrylate) and amylose. Bioresour. Technol. 2010, 101, 2197–2202. [Google Scholar] [CrossRef]
  8. Ghosh, D.; Bhattacharyya, K.G. Adsorption of methylene blue on kaolinite. Appl. Clay Sci. 2002, 20, 295–300. [Google Scholar] [CrossRef]
  9. Zaied, M.; Chutet, E.; Peulon, S.; Bellakhal, N.; Desmazières, B.; Dachraoui, M.; Chaussé, A. Spontaneous oxidative decolorization of indigo carmine by thin films of birnessite electrodeposited onto SnO2. Appl. Cataly. B Environmen. 2011, 107, 42–51. [Google Scholar] [CrossRef]
  10. Sharma, S.K. Green Chemistry for Dyes Removal from Waste Water: Research Trends and Applications; John Wiley & Sons: Hoboken, NJ, USA, 2015. [Google Scholar]
  11. Akpan, U.G.; Hameed, B.H. Parameters affecting the photocatalytic decolorization of dyes using TiO2-based photocatalysts: A review. J. Hazard. Mater. 2009, 170, 520–529. [Google Scholar] [CrossRef]
  12. Guo, J.; Yuan, S.; Jiang, W.; Yue, H.; Cui, Z.; Liang, B. Adsorption and photocatalytic decolorization behaviors of rhodamine dyes on surface-fluorinated TiO2 under visible irradiation. RSC Adv. 2016, 6, 4090–4100. [Google Scholar] [CrossRef]
  13. Zhang, J.; Song, Y.; Lu, F.; Fei, W.; Mengqiong, Y.; Genxiang, L.; Qian, X.; Xiang, W.; Can, L. Photocatalytic decolorization of rhodamine B on anatase, rutile, and brookite TiO2. Chin. J. Catal. 2011, 32, 983–991. [Google Scholar] [CrossRef]
  14. Hu, Y.; Li, D.; Wang, H.; Zeng, G.; Li, X.; Shao, Y. Role of active oxygen species in the liquid-phase photocatalytic decolorization of RhB using BiVO4/TiO2 heterostructure under visible light irradiation. J. Mol. Catal. A Chem. 2015, 408, 172–178. [Google Scholar] [CrossRef]
  15. Hadnadjev-Kostic, M.; Vulic, T.; Marinkovic-Neducin, R. Solar light induced rhodamine B decolorization assisted by TiO2–Zn–Al LDH based photocatalysts. Adv. Powder Technol. 2014, 25, 1624–1633. [Google Scholar] [CrossRef]
  16. Shi, L.; Liang, L.; Ma, J.; Meng, Y.; Zhong, S.; Wang, F.; Sun, J. Highly efficient visible light-driven Ag/AgBr/ZnO composite photocatalyst for degrading Rhodamine B. Ceram. Int. 2014, 40, 3495–3502. [Google Scholar] [CrossRef]
  17. Yang, J.; Sun, X.; Yang, W.; Zhu, M.; Shi, J. The Improvement of Coralline-Like ZnGa2O4 by Cocatalysts for the Photocatalytic Decolorization of Rhodamine B. Catalysts 2020, 10, 221. [Google Scholar] [CrossRef] [Green Version]
  18. Tian, J.; Shao, Q.; Zhao, J.; Pan, D.; Dong, M.; Jia, C.; Ding, T.; Wu, T.; Guo, Z. Microwave solvothermal carboxymethyl chitosan templated synthesis of TiO2/ZrO2 composites toward enhanced photocatalytic decolorization of Rhodamine B. J. Colloid Interface Sci. 2019, 541, 18–29. [Google Scholar] [CrossRef]
  19. Pica, M.; Calzuola, S.; Donnadio, A.; Gentili, P.L.; Nocchetti, M.; Casciola, M. De-ethylation and cleavage of rhodamine B by a zirconium phosphate/silver bromide composite photocatalyst. Catalysts 2019, 9, 3. [Google Scholar] [CrossRef] [Green Version]
  20. Hou, X.; Shen, W.; Huang, X.; Ai, Z.; Zhang, L. Ascorbic acid enhanced activation of oxygen by ferrous iron: A case of aerobic decolorization of rhodamine B. J. Hazard. Mater. 2016, 308, 67–74. [Google Scholar] [CrossRef] [Green Version]
  21. Prihod’ko, R.; Stolyarova, I.; Gündüz, G.; Taran, O.; Yashnik, S.; Parmon, V.; Goncharuk, V. Fe-exchanged zeolites as materials for catalytic wet peroxide oxidation. Decolorization of Rodamine G dye. Appl. Catal. B Environ. 2011, 104, 201–210. [Google Scholar] [CrossRef]
  22. Song, Y.; Zhao, S.; Tchounwou, P.; Liu, Y.-M. A nanoparticle-based solid-phase extraction method for liquid chromatography–electrospray ionization-tandem mass spectrometric analysis. J. Chromatogr. A 2007, 1166, 79–84. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  23. Merka, O.; Yarovyi, V.; Bahnemann, D.W.; Wark, M. pH-control of the photocatalytic decolorization mechanism of rhodamine B over Pb3Nb4O13. J. Phys. Chem. C 2011, 115, 8014–8023. [Google Scholar] [CrossRef]
  24. Lin, C.; Lin, K.-S. Photocatalytic oxidation of toxic organohalides with TiO2/UV: The effects of humic substances and organic mixtures. Chemosphere 2007, 66, 1872–1877. [Google Scholar] [CrossRef] [PubMed]
  25. Rasalingam, S.; Wu, C.-M.; Koodali, R.T. Modulation of pore sizes of titanium dioxide photocatalysts by a facile template free hydrothermal synthesis method: Implications for photocatalytic decolorization of rhodamine B. Appl. Mater. Interfaces 2015, 7, 4368–4380. [Google Scholar] [CrossRef] [PubMed]
  26. Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001, 293, 269–271. [Google Scholar] [CrossRef]
  27. Sakthivel, S.; Kisch, H. Daylight photocatalysis by carbon-modified titanium dioxide. Angew. Chem. Int. Ed. 2003, 42, 4908–4911. [Google Scholar] [CrossRef] [PubMed]
  28. Yu, J.C.; Ho, W.; Yu, J.; Yip, H.; Wong, P.K.; Zhao, J. Efficient visible-light-induced photocatalytic disinfection on sulfur-doped nanocrystalline titania. Environ. Sci. Technol. 2005, 39, 1175–1179. [Google Scholar] [CrossRef]
  29. Irie, H.; Maruyama, Y.; Hashimoto, K. Ag+-and Pb2+-doped SrTiO3 photocatalysts. A correlation between band structure and photocatalytic activity. J. Phys. Chem. C 2007, 111, 1847–1852. [Google Scholar] [CrossRef]
  30. Shim, W.G.; Lee, J.W.; Moon, H. Adsorption equilibrium and column dynamics of VOCs on MCM-48 depending on pelletizing pressure. Microporous Mesoporous Mater. 2006, 88, 112–125. [Google Scholar] [CrossRef]
  31. Krbal, M.; Ng, S.; Motola, M.; Hromadko, L.; Dvorak, F.; Prokop, V.; Sopha, H.; Macak, J.M. Sulfur treated 1D anodic TiO2 nanotube layers for significant photo-and electroactivity enhancement. Appl. Mater. Today 2019, 17, 104–111. [Google Scholar] [CrossRef]
  32. Motola, M.; Dworniczek, E.; Satrapinskyy, L.; Chodaczek, G.; Grzesiak, J.; Gregor, M.; Plecenik, T.; Nowicka, J.; Plesch, G. UV light-induced photocatalytic, antimicrobial, and antibiofilm performance of anodic TiO2 nanotube layers prepared on titanium mesh and Ti sputtered on silicon. Chem. Pap. 2019, 73, 1163–1172. [Google Scholar] [CrossRef]
  33. Zhao, T.; Hou, C.; Zhang, H.; Zhu, R.; She, S.; Wang, J.; Li, T.; Liu, Z.; Wei, B. Electromagnetic wave absorbing properties of amorphous carbon nanotubes. Sci. Rep. 2014, 4, 5619. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  34. Lopes, O.F.; Paris, E.C.; Ribeiro, C. Synthesis of Nb2O5 nanoparticles through the oxidant peroxide method applied to organic pollutant photodecolorization: A mechanistic study. Appl. Catal. B Environ. 2014, 144, 800–808. [Google Scholar] [CrossRef]
  35. Zarei-Chaleshtori, M.; Hosseini, M.; Edalatpour, R.; Masud, S.S.; Chianelli, R.R. Photocatalytic decontamination of wastewater with porous material HNb3O8. Microchem. J. 2013, 110, 361–368. [Google Scholar] [CrossRef]
  36. Oliveira, L.; Gonçalves, M.; Guerreiro, M.; Ramalho, T.; Fabris, J.; Pereira, M.; Sapag, K. A new catalyst material based on niobia/iron oxide composite on the oxidation of organic contaminants in water via heterogeneous Fenton mechanisms. Appl. Catal. A Gen. 2007, 316, 117–124. [Google Scholar] [CrossRef]
  37. Rooke, J.C.; Barakat, T.; Brunet, J.; Li, Y.; Finol, M.F.; Lamonier, J.-F.; Giraudon, J.-M.; Cousin, R.; Siffert, S.; Su, B.L. Hierarchically nanostructured porous group Vb metal oxides from alkoxide precursors and their role in the catalytic remediation of VOCs. Appl. Catal. B Environ. 2015, 162, 300–309. [Google Scholar] [CrossRef]
  38. Lin, W.; Frei, H. Anchored metal-to-metal charge-transfer chromophores in a mesoporous silicate sieve for visible-light activation of titanium centers. J. Phy. Chem. B 2005, 109, 4929–4935. [Google Scholar] [CrossRef]
  39. Nakamura, R.; Okamoto, A.; Osawa, H.; Irie, H.; Hashimoto, K. Design of all-inorganic molecular-based photocatalysts sensitive to visible light: Ti (IV)− O− Ce (III) bimetallic assemblies on mesoporous silica. J. Am. Chem. Soc. 2007, 129, 9596–9597. [Google Scholar] [CrossRef]
  40. Bock, C.; Halvorsen, H.; MacDougall, B. Catalyst synthesis techniques. In PEM Fuel Cell Electrocatalysts and Catalyst Layers; Springer: Berlin/Heidelberg, Germany, 2008; pp. 447–485. [Google Scholar]
  41. Munnik, P.; de Jongh, P.E.; de Jong, K.P. Recent developments in the synthesis of supported catalysts. Chem. Rev. 2015, 115, 6687–6718. [Google Scholar] [CrossRef]
  42. Kim, W.; Yuan, G.; McClure, B.A.; Frei, H. Light induced carbon dioxide reduction by water at binuclear ZrOCoII unit coupled to Ir oxide nanocluster catalyst. J. Am. Chem. Soc. 2014, 136, 11034–11042. [Google Scholar] [CrossRef]
  43. Wu, J.; Li, J.; Liu, J.; Bai, J.; Yang, L. A novel Nb2O5/Bi2WO6 heterojunction photocatalytic oxidative desulfurization catalyst with high visible light-induced photocatalytic activity. RSC Adv. 2017, 7, 51046–51054. [Google Scholar] [CrossRef] [Green Version]
  44. Moradi, H.; Eshaghi, A.; Hosseini, S.R.; Ghani, K. Fabrication of Fe-doped TiO2 nanoparticles and investigation of photocatalytic decolorization of reactive red 198 under visible light irradiation. Ultrason. Sonochem. 2016, 32, 314–319. [Google Scholar] [CrossRef] [PubMed]
  45. Kim, J.-M.; Jung, Y.-H.; Kwak, J.-Y.; Kim, Y.-I. Synthesis and Characterization of Nb, Mo-doped and Nb/Mo-codoped Monoclinic VO2 Nanoparticles and Their Thin Films by Hydrothermal/Post-Thermal Transformation and Wet-Coating Method. J. Korean Chem. Soc. 2019, 63, 94–101. [Google Scholar]
  46. Gaikwad, S.; Han, S. A microwave method for the rapid crystallization of UTSA-16 with improved performance for CO2 capture. Chem. Eng. J. 2019, 371, 813–820. [Google Scholar] [CrossRef]
  47. Meshesha, D.S.; Matangi, R.C.; Tirukkovalluri, S.R.; Bojja, S. Synthesis, characterization and visible light photocatalytic activity of Mg2+ and Zr4+ co-doped TiO2 nanomaterial for decolorization of methylene blue. J. Asian Ceram. Soc. 2017, 5, 136–143. [Google Scholar] [CrossRef] [Green Version]
  48. Morais, L.A.; Adán, C.; Araujo, A.S.; Guedes, A.P.; Marugán, J. Photocatalytic Activity of Suspended and Immobilized Niobium Oxide for Methanol Oxidation and Escherichia coli Inactivation. J. Adv. Oxid. Technol. 2016, 19, 256–265. [Google Scholar] [CrossRef]
  49. Zhang, H.; Lin, Q.; Ning, S.; Zhou, Y.; Lin, H.; Long, J.; Zhang, Z.; Wang, X. One-step synthesis of mesoporous Pt–Nb2O5 nanocomposites with enhanced photocatalytic hydrogen production activity. RSC Adv. 2016, 6, 96809–96815. [Google Scholar] [CrossRef]
  50. Da Conceição, L.R.V.; Carneiro, L.M.; Rivaldi, J.D.; de Castro, H.F. Solid acid as catalyst for biodiesel production via simultaneous esterification and transesterification of macaw palm oil. Ind. Crops Prod. 2016, 89, 416–424. [Google Scholar] [CrossRef]
  51. Chiang, T.H.; Yeh, H.C. The synthesis of α-MoO3 by ethylene glycol. Materials 2013, 6, 4609–4625. [Google Scholar] [CrossRef] [Green Version]
  52. Varghese, B.; Haur, S.C.; Lim, C.-T. Nb2O5 nanowires as efficient electron field emitters. J. Phy. Chem. C 2008, 112, 10008–10012. [Google Scholar] [CrossRef]
  53. Brayner, R.; Bozon-Verduraz, F. Niobium pentoxide prepared by soft chemical routes: Morphology, structure, defects and quantum size effect. Phys. Chem. Chem. Phys. 2003, 5, 1457–1466. [Google Scholar] [CrossRef]
  54. Nakamura, R.; Frei, H. Visible light-driven water oxidation by Ir oxide clusters coupled to single Cr centers in mesoporous silica. J. Am. Chem. Soc. 2006, 128, 10668–10669. [Google Scholar] [CrossRef] [PubMed]
  55. Yuan, M.; Wang, S.; Wang, X.; Zhao, L.; Hao, T. Removal of organic dye by air and macroporous ZnO/MoO3/SiO2 hybrid under room conditions. Appl. Surf. Sci. 2011, 257, 7913–7919. [Google Scholar] [CrossRef]
  56. Kim, D.; Kim, G.; Bae, H.; Kim, E.; Moon, B.; Cheon, D.; Tarte, N.H. An External Energy Independent WO3/MoCl5 Nano-Sized Catalyst for the Superior Decolorization of Crystal Violet and Rhodamine B Dye. Catalysts 2019, 9, 642. [Google Scholar] [CrossRef] [Green Version]
  57. Wang, Y.; Zhu, Y.; Zhao, X.; Yang, X.; Li, X.; Chen, Z.; Yang, L.; Zhu, L.; Gao, T.; Sha, Z. Improving photocatalytic Rhodamine B degrading activity with Pt quantum dots on TiO2 nanotube arrays. Surf. Coat. Technol. 2015, 281, 89–97. [Google Scholar] [CrossRef]
  58. Abdel-Messih, M.; Ahmed, M.; El-Sayed, A.S. Photocatalytic decolorization of Rhodamine B dye using novel mesoporous SnO2–TiO2 nano mixed oxides prepared by sol–gel method. J. Photochem. Photobiol. A Chem. 2013, 260, 1–8. [Google Scholar] [CrossRef]
  59. Kuo, T.-J.; Lin, C.-N.; Kuo, C.-L.; Huang, M.H. Growth of ultralong ZnO nanowires on silicon substrates by vapor transport and their use as recyclable photocatalysts. Chem. Mater. 2007, 19, 5143–5147. [Google Scholar] [CrossRef]
  60. Li, J.; Wen, X.; Zhang, Q.; Ren, S. Adsorption and visible-light photodecolorization of organic dyes with TiO2/conjugated microporous polymer composites. RSC Adv. 2018, 8, 34560–34565. [Google Scholar] [CrossRef] [Green Version]
  61. Hu, C.; Peng, T.; Hu, X.; Nie, Y.; Zhou, X.; Qu, J.; He, H. Plasmon-induced photodecolorization of toxic pollutants with Ag− AgI/Al2O3 under visible-light irradiation. J. Am. Chem. Soc. 2010, 132, 857–862. [Google Scholar] [CrossRef]
  62. Ji, P.; Zhang, J.; Chen, F.; Anpo, M. Study of adsorption and decolorization of acid orange 7 on the surface of CeO2 under visible light irradiation. Appl. Catal. B Environ. 2009, 85, 148–154. [Google Scholar] [CrossRef]
  63. Zhang, C.; Lin, J. Visible-light induced oxo-bridged Zr IV− O− Ce III redox centre in tetragonal ZrO2–CeO2 solid solution for decolorization of organic pollutants. Phys. Chem. Chem. Phys. 2011, 13, 3896–3905. [Google Scholar] [CrossRef] [PubMed]
  64. Lamdab, U.; Wetchakun, K.; Kangwansupamonkon, W.; Wetchakun, N. Effect of a pH-controlled co-precipitation process on rhodamine B adsorption of MnFe2O4 nanoparticles. RSC Adv. 2018, 8, 6709–6718. [Google Scholar] [CrossRef] [Green Version]
  65. Xie, T.-H.; Sun, X.; Lin, J. Enhanced photocatalytic decolorization of RhB driven by visible light-induced MMCT of Ti (IV)− O− Fe (II) formed in Fe-doped SrTiO3. J. Phys. Chem. C 2008, 112, 9753–9759. [Google Scholar] [CrossRef]
  66. Su, S.; Guo, W.; Leng, Y.; Yi, C.; Ma, Z. Heterogeneous activation of Oxone by CoxFe3− xO4 nanocatalysts for decolorization of rhodamine B. J. Hazard. Mater. 2013, 244, 736–742. [Google Scholar] [CrossRef]
  67. Ai, L.; Zeng, C.; Wang, Q. One-step solvothermal synthesis of Ag-Fe3O4 composite as a magnetically recyclable catalyst for reduction of Rhodamine B. Catal. Commun. 2011, 14, 68–73. [Google Scholar] [CrossRef]
  68. Yang, X.; Li, R.; Wang, Y.; Wu, K.; Chang, S.; Tang, H. Solvent-induced controllable synthesis of recyclable Ag2CO3 catalysts with enhanced visible light photocatalytic activity. Ceram. Int. 2016, 42, 13411–13420. [Google Scholar] [CrossRef]
  69. Duraimurugan, J.; Shanavas, S.; Ramesh, R.; Acevedo, R.; Anbarasan, P.; Maadeswaran, P. Hydrothermal assisted phytofabrication of zinc oxide nanoparticles with different nanoscale characteristics for the photocatlytic decolorization of Rhodamine B. Optik 2020, 202, 163607. [Google Scholar]
  70. Zhang, J.; Zhang, Z.; Zhu, W.; Meng, X. Boosted photocatalytic decolorization of Rhodamine B pollutants with Z-scheme CdS/AgBr-rGO nanocomposite. Appl. Surf. Sci. 2020, 502, 144275. [Google Scholar] [CrossRef]
  71. Huang, H.; He, M.; Yang, X.; Tian, Z.; Hu, J.; Wen, B. One-pot hydrothermal synthesis of TiO2/RCN heterojunction photocatalyst for production of hydrogen and rhodamine B decolorization. Appl. Surf. Sci. 2019, 493, 202–211. [Google Scholar] [CrossRef]
  72. Das, M.; Bhattacharyya, K.G. Oxidation of Rhodamine B in aqueous medium in ambient conditions with raw and acid-activated MnO2, NiO, ZnO as catalysts. J. Mol. Catal. A Chem. 2014, 391, 121–129. [Google Scholar] [CrossRef]
Catalysts 10 00491 g001 550
Figure 1. X-ray diffraction pattern of pure Nb2O5 and Nb–O–Mo samples.
Figure 1. X-ray diffraction pattern of pure Nb2O5 and Nb–O–Mo samples.
Catalysts 10 00491 g001
Catalysts 10 00491 g002 550
Figure 2. SEM images of (a) Nb2O5 and (b) Nb–O–Mo.
Figure 2. SEM images of (a) Nb2O5 and (b) Nb–O–Mo.
Catalysts 10 00491 g002
Catalysts 10 00491 g003 550
Figure 3. (a) SEM image of Nb–O–Mo and corresponding EDS element mapping of (b) Nb, (c) O, and (d) Mo.
Figure 3. (a) SEM image of Nb–O–Mo and corresponding EDS element mapping of (b) Nb, (c) O, and (d) Mo.
Catalysts 10 00491 g003
Catalysts 10 00491 g004 550
Figure 4. FTIR of the Nb2O5 and Nb–O–Mo samples.
Figure 4. FTIR of the Nb2O5 and Nb–O–Mo samples.
Catalysts 10 00491 g004
Catalysts 10 00491 g005 550
Figure 5. Raman spectra of the Nb2O5 and Nb–O–Mo samples.
Figure 5. Raman spectra of the Nb2O5 and Nb–O–Mo samples.
Catalysts 10 00491 g005
Catalysts 10 00491 g006 550
Figure 6. UV-vis spectra of Rh B (10 ppm) solution by Nb–O–Mo catalyst at various times.
Figure 6. UV-vis spectra of Rh B (10 ppm) solution by Nb–O–Mo catalyst at various times.
Catalysts 10 00491 g006
Catalysts 10 00491 g007 550
Figure 7. Catalytic decolorization of Rh B at various dye concentrations, with different loadings of Nb–O–Mo catalyst and temperature. Error bars represent standard deviations (n = 3).
Figure 7. Catalytic decolorization of Rh B at various dye concentrations, with different loadings of Nb–O–Mo catalyst and temperature. Error bars represent standard deviations (n = 3).
Catalysts 10 00491 g007
Catalysts 10 00491 g008 550
Figure 8. Catalytic decolorization of (a)Congo Red and (b) Crystal Violet at various dye concentrations at 298 K, with a fixed loading of Nb–O–Mo catalyst. Error bars represent standard deviations (n = 3).
Figure 8. Catalytic decolorization of (a)Congo Red and (b) Crystal Violet at various dye concentrations at 298 K, with a fixed loading of Nb–O–Mo catalyst. Error bars represent standard deviations (n = 3).
Catalysts 10 00491 g008
Catalysts 10 00491 g009 550
Figure 9. Catalytic decolorization of Rh B (50 ppm) by Nb–O–Mo with scavengers of BQ, NaHCO3, and IPA at 318 K.
Figure 9. Catalytic decolorization of Rh B (50 ppm) by Nb–O–Mo with scavengers of BQ, NaHCO3, and IPA at 318 K.
Catalysts 10 00491 g009
Catalysts 10 00491 g010 550
Figure 10. TAOH formation by •OH radicals generated in the heterogeneous system.
Figure 10. TAOH formation by •OH radicals generated in the heterogeneous system.
Catalysts 10 00491 g010
Catalysts 10 00491 g011 550
Figure 11. FTIR spectra of Nb–O–Mo before decolorization and Nb–O–Mo after decolorization.
Figure 11. FTIR spectra of Nb–O–Mo before decolorization and Nb–O–Mo after decolorization.
Catalysts 10 00491 g011
Catalysts 10 00491 sch001 550
Scheme 1. Schematic illustration of Rh B decolorization by Nb–O–Mo MMCT catalyst.
Scheme 1. Schematic illustration of Rh B decolorization by Nb–O–Mo MMCT catalyst.
Catalysts 10 00491 sch001
Catalysts 10 00491 g012 550
Figure 12. Rh B decolorization efficiency of Nb–O–Mo for three cycles.
Figure 12. Rh B decolorization efficiency of Nb–O–Mo for three cycles.
Catalysts 10 00491 g012
Table
Table 1. Comparison of XRD peak intensities for Nb2O5 and Nb–O–Mo.
Table 1. Comparison of XRD peak intensities for Nb2O5 and Nb–O–Mo.
Angle (2 Ɵ) Intensity
Nb2O5Nb–O–Mo
22.5916,4529156
28.3611,1086451
36.5844702577
46.1431071682
49.901468965
50.9819491222
55.4320451258
Table
Table 2. The rate constant for dye decolorization.
Table 2. The rate constant for dye decolorization.
Dye ConcentrationR2k [min−1]
Rh B 20 ppm 298 K0.99481.944
Rh B 50 ppm 298 K0.99952.220
Rh B 50 ppm 318 K0.99932.226
Congo Red 20 ppm 298 K0.99312.940
Congo Red 50 ppm 298 K0.99881.986
Crystal Violet 20 ppm 298 K0.98963.486
Crystal Violet 50 ppm 298 K0.98347.116

Share and Cite

MDPI and ACS Style

Kadam, R.L.; Kim, Y.; Gaikwad, S.; Chang, M.; Tarte, N.H.; Han, S. Catalytic Decolorization of Rhodamine B, Congo Red, and Crystal Violet Dyes, with a Novel Niobium Oxide Anchored Molybdenum (Nb–O–Mo). Catalysts 2020, 10, 491. https://doi.org/10.3390/catal10050491

AMA Style

Kadam RL, Kim Y, Gaikwad S, Chang M, Tarte NH, Han S. Catalytic Decolorization of Rhodamine B, Congo Red, and Crystal Violet Dyes, with a Novel Niobium Oxide Anchored Molybdenum (Nb–O–Mo). Catalysts. 2020; 10(5):491. https://doi.org/10.3390/catal10050491

Chicago/Turabian Style

Kadam, Rahul L., Yeonhee Kim, Sanjit Gaikwad, Mincheol Chang, Naresh H. Tarte, and Sangil Han. 2020. "Catalytic Decolorization of Rhodamine B, Congo Red, and Crystal Violet Dyes, with a Novel Niobium Oxide Anchored Molybdenum (Nb–O–Mo)" Catalysts 10, no. 5: 491. https://doi.org/10.3390/catal10050491

APA Style

Kadam, R. L., Kim, Y., Gaikwad, S., Chang, M., Tarte, N. H., & Han, S. (2020). Catalytic Decolorization of Rhodamine B, Congo Red, and Crystal Violet Dyes, with a Novel Niobium Oxide Anchored Molybdenum (Nb–O–Mo). Catalysts, 10(5), 491. https://doi.org/10.3390/catal10050491

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop