Rapid oxidative degradation of methylene blue by various metal oxides doped with vanadium

Abstract

Nanoparticles of ZrO₂, CeO₂ and TiO₂ doped with vanadium were synthesized and the physicochemical properties of the compounds were characterized by X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The catalytic degradation of methylene blue (MB), an organic dye, in the presence of these materials and hydrogen peroxide (H₂O₂) as a green oxidant was studied at room temperature. The effects of solution pH, catalyst composition and radical scavenging agents on the degree of degradation of MB were also studied. Finally, the recoverability and reusability of the V-doped ZrO₂ catalyst were analysed.

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Introduction

Textile industries use a large amount of dyestuff, but only part of the dyestuff is actually used. The rest of the dye compounds dissolve in wastewater effluent, entering bodies of water such as rivers and lakes. During a chemical or biological reaction pathway, these dye compounds not only deplete the dissolved oxygen in bodies of water and present significant environmental problems, but also release toxic compounds which endanger aquatic life. Therefore, the chemical, biochemical and photochemical degradation of organic dyes has atracted much attention.^1–6

Metal oxide nanomaterials represent highly customisable and robust multifunctional platforms in many industries, especially given their heightened chemical, physical, and electronic properties compared with their bulk counterparts.^7–9 Metal oxide nanomaterials are versatile materials that can be used for applications such as environmental remediation, medical technology, energy, water treatment, and personal care products, with their use projected to increase.^10–14 Hence, many chemicals such as solvents, raw materials, reagents, and template materials have been utilized to produce nanomaterials.^15,16

Methylene blue (MB), a deep blue coloured pigment, is one of the most commonly used substances for dyeing cotton, wood and silk.¹⁷ It is potentially harmful to the environment due to its non-biodegradability and high toxicity to aquatic creatures and carcinogenic effects in humans, therefore the degradation of MB dye has attracted much attention.^18–20 To protect the aquatic environment, many methods such as adsorption, photo-degradation, biological treatment, coagulation, liquid–liquid extraction and ultrasonic decomposition have been used to remove MB from wastewater.^21–24 Oxidative degradation of MB is one of the most common degradation pathways but perhaps the most complex one.^25–27 Among the various oxidants, hydrogen peroxide (H₂O₂) is one of the most commonly used owing to its eco-friendly nature.^28,29 Hence, in recent decades, activation of H₂O₂ by transition-metal ions for organic dye degradation has been actively explored.^30,31

Vanadium ions in a structure can activate H₂O₂ and generate ¹O₂, O₂˙⁻ and ˙OH radicals which can unselectively attack and decompose organic contaminants. This process is heterogeneous and can operate at basic and near neutral pH, which is of considerable interest since it could offer some advantages, such as easy operation, lack of sludge generation and the possibility of recycling the promoter.⁷ Thus, metal oxide nanoparticles doped with various metal ions, particularly with vanadium, hold the potential to be very efficient catalytic materials.^32–34 Herein we report the first preparation of nano-vanadium supported on ZrO₂, CeO₂ and TiO₂ as an efficient catalyst for the green decomposition of MB using H₂O₂ as the oxidant.

Experimental

Materials

Chemicals and solvents were purchased from Merck and Fluka and were used without further purification. V/TiO₂ nanoparticles were synthesized according to our previously published procedure.⁷ For the synthesis of V/ZrO₂, an aqueous solution (10 cm³) of ZrOCl₂ (1.0 g) was basified with a KOH solution (1.8 M), and a white precipitate of ZrO(OH)₂ was obtained. The solid ZrO(OH)₂ was washed with water several times and dried at 50 °C. Then, the required amount of NH₄VO₃ and a 1 : 1 molar ratio of urea (as a fuel) and metal ions (Zr + V) were mixed and pulverized with ZrO(OH)₂. After addition of methanol (20 mL) to the mixture of NH₄VO₃ and ZrO(OH)₂, the mixture was mechanically stirred to ensure the homogeneous distribution of materials. Finally, CH₃OH was evaporated under vacuum using a rotary evaporator. After the evaporation process, the residual solid was ground and calcinated at 500 °C for 4 h. For the synthesis of V/CeO₂, an aqueous solution (10 cm³) of Ce(NO₃)₃·6H₂O (1.0 g) was basified with a NaOH solution (1.0 M), and a white precipitate of Ce(OH)₄ was obtained. The solid Ce(OH)₄ was washed with water several times and dried at 50 °C. Then, the required amount of NH₄VO₃ and a 1 : 1 molar ratio of urea (as a fuel) and metal ions (Ce + V) were mixed and pulverized with Ce(OH)₄. Methanol (20 mL) was added to the mixture of NH₄VO₃ and Ce(OH)₄. This mixture was mechanically stirred to ensure the homogeneous distribution of materials. Then, CH₃OH was evaporated under vacuum using a rotary evaporator and the residual solid was ground and calcinated at 400 °C for 5 h.

Methods

TEM measurements were carried out on pristine samples irradiated at 200 kV using an FEI Tecnai F20 G² TEM. Scanning electron microscopy (SEM) was performed on a Nano-SEM machine (Nova 200, FEI, The Netherlands), with an image resolution of 1.0 nm. The morphologies and sizes of the products were further observed by scanning electron microscopy (SEM, FEI XL-30 FEG, resolution < 2 nm) with energy-dispersive X-ray spectroscopy (EDX) for percentage composition. XANES measurements at O K-edges along with the V L_3,2-edge were carried out at the 10D (XAS-KIST) beam line of the Pohang Light Source, South Korea and BL20A1 of NSRRC, Taiwan. The NEXAFS data of these edges were collected in the total fluorescence yield (TFY) mode by detecting the florescence signal using a multi-channel plate detector. The resolution of both beamlines was 600 meV at O K-edge energy. The spectra were normalized by pre-edge background subtraction, incident beam intensity I₀ and by keeping the area under the spectra in the energy range between 500 and 580 eV for the O K-edge/V L_3,2-edge fixed. A base pressure of 5 × 10⁻⁹ Torr was used during measurements and the temperature was kept at 300 K. The raw data were energy-calibrated (O K-edge energy of the standard oxide sample, first inflection point), smoothed, background corrected, and normalized using Athena.³⁵ The degradation rates of MB solutions were scanned periodically using a CARY 100 Bio VARIAN UV-vis spectrophotometer and the maximum absorption wavelength of the MB solution was identified as 664 nm. The FT-IR spectrum was obtained with a Unicam Matson 1000 FT-IR spectrophotometer using KBr disks at room temperature.

Methylene blue degradation

The catalytic activity of the catalyst was demonstrated by degrading MB in aqueous solution. A round-bottom flask was charged with 50 mL of aqueous MB (10 mg L⁻¹) and 10 mg of catalyst was added. H₂O₂ (1 mL, 10 mmol) was added and the reaction mixture was stirred under ambient conditions. At a given time interval, a small quantity of the mixture was pipetted into a quartz cell, and its absorption spectrum was measured using an UV-visible spectrophotometer.

Results and discussion

Catalyst characterization

The structure and phase formation of V doped on ZrO₂ and CeO₂ supports were investigated using the XRD technique. Fig. 1 shows the XRD patterns of the Zr_1−xV_xO₂ nanoparticles (x = 0, 0.07, 0.15 and 0.17) synthesized in the present work. It is clear that all the samples with V-content of up to 15% can be indexed to the cubic phase of ZrO₂.³⁶ No vanadium oxide peak is observed even at 17% vanadium loading. This observation hints that vanadium ions are likely to occupy the lattice position of ZrO₂.³⁷

Fig. 1 XRD pattern of Zr_1−xV_xO₂ (x = 0, 0.07, 0.15 and 0.17).

It is well known that the EDX technique can be used to determine the composition of individual components in the outermost surface layers. Investigation of V/ZrO₂ by EDX showed that the relative atomic abundance of vanadium present on the surface of the ZrO₂ was 0%, 7%, 15% and 17%, respectively (Fig. 2).

Fig. 2 EDX investigation of Zr_1−xV_xO₂: (a) x = 0; (b) 0.07; (c) 0.15; (d) 0.17.

Fig. 3 shows the XRD patterns of Ce_1−xV_xO₂ catalysts with different loadings (x = 0, 0.08 and 0.16). All the samples exhibited the characteristic peaks of CeO₂, which were very close to those for cubic fluorite-structured CeO₂ crystal in JCPDS.³⁸ Additionally, the characteristic pattern of CeVO₄ (JCPDS card 12-0757) appeared for Ce_1−xV_xO₂ nanoparticles containing 8 and 16 wt% vanadium.³⁹ Knözinger et al. suggested that VO_x and CeO₂ began to react to form CeVO₄ on the surface at 573 K.⁴⁰ The characteristic peaks of CeVO₄ became more intense and sharper with increased vanadium loading, illustrating the formation of more CeVO₄ and the growth of particle size.

Fig. 3 XRD pattern of Ce_1−xV_xO₂ (x = 0, 0.08 and 0.16).

The presence of vanadium on the CeO₂ surface is clearly indicated in the EDX spectrum (Fig. 4) for Ce_1−xV_xO₂ nanoparticles containing 8 and 16 wt% V. The EDX spectrum also shows other elements which are present in the support.

Fig. 4 EDX investigation of Ce_1−xV_xO₂: (a) x = 0; (b) 0.08; (c) 0.16.

X-ray absorption near edge structure (XANES) spectroscopy is a powerful local probe of electronic structure as well as the atomic structure of materials.^41–44 It probes the empty energy bands by measuring transitions from core levels, and does not require a crystalline sample. Fig. 5 shows room-temperature XANES data of vanadium-doped ZrO₂ and CeO₂ at the V L_3,2- and O K-edge regions as a function of vanadium doping.

Fig. 5 Room-temperature XANES data of (a) V-doped ZrO₂ and (b) V-doped CeO₂ at the V L_3,2- and O K-edge regions as a function of vanadium doping.

The V L_3,2-edge region shows a well-resolved pre-peak at about 517.5 eV and the main V L₃- and L₂-edges at about 519.8 and 526.7 eV, respectively. The L₃- and L₂-edges are due to electron transitions from 2p_3/2 and 2p_1/2 energy levels to V 3d–O 2p hybridized bands. Due to the strong interaction between the 2p core hole and the 3d electrons in the final state, the apparent spin–orbit splitting of the V 2p level in the absorption spectra (∼6.9 eV) shows a reduction in comparison with the splitting observed by XPS (∼7.7 eV).^45,46 These interactions are of the same order of magnitude as the V 2p spin–orbit splitting, which causes a large redistribution of intensity throughout the entire spectra.⁴³ The peak position of the L₃-edge suggests that V is mainly in the +5 oxidation state in the ZrO₂ and CeO₂.⁴⁷ The pre-peak at about 517.5 eV is a signature of the electron transition from V 2p to the unoccupied V 4s states.⁴⁸ Since the 4s states are occupied in elemental vanadium, the presence of the pre-peak implies that V is not in the elemental form and that there is no metallic clustering of V-atoms in the ZrO₂ and CeO₂. There is no significant shift in the peak positions as a function of V doping. This observation suggests that the oxidation states of V in the ZrO₂ and CeO₂ remain the same. As expected, the peak intensities increase with V doping. Also, the O K-edge XANES patterns of vanadium-doped ZrO₂ and CeO₂ show clear peaks of the cubic phases of ZrO₂ and CeO₂. The difference between the XANES of Ce_1−xV_xO₂ (x = 0.16) and Ce_1−xV_xO₂ (x = 0.08) reflects the different chemical environments of the cerium in these materials. As the VO_x coverage in the ceria increases, the product distribution approaches that of CeVO₄. Because XANES is sensitive to oxidation states, it is clear that the cerium in CeO₂ and CeVO₄ is in the +4 and +3 oxidation states, respectively.⁴⁹

The particle morphology and textural properties of the catalysts were also studied carefully by SEM and TEM. Representative SEM and TEM images recorded for the materials are shown in Fig. 6 and 7, respectively.

Fig. 6 SEM images of (a) Zr_1−xV_xO₂ (x = 0); (b) Zr_1−xV_xO₂ (x = 0.15); (c) Ce_1−xV_xO₂ (x = 0); and (d) Ce_1−xV_xO₂ (x = 0.16).

Fig. 7 TEM images of (a) Zr_1−xV_xO₂ (x = 0.15) and (b) Ce_1−xV_xO₂ (x = 0.16).

The TEM images of the catalysts show that rod-like particles of Zr_1−xV_xO₂ (x = 0.15) and sphere-like particles of Ce_1−xV_xO₂ (x = 0.16) are formed. The length of the rod-like particles of Zr_1−xV_xO₂ (x = 0.15) ranges from 0.1 to 0.4 μm, with a diameter from 20 to 40 nm. The sphere-like particles of Ce_1−xV_xO₂ (x = 0.16) have an average particle size of between 10 and 50 nm.

Catalytic effects

The extent of dye decomposition was monitored using UV-vis spectroscopic techniques. The activity of the catalysts is expressed as the percentage decrease in dye concentration during the reaction, as measured by absorption intensity. In order to understand the effect of catalyst supports on the catalytic activity, first, a catalyst screening was carried out for the reaction. Fig. 8 shows the degradation of MB using the catalysts synthesized in this work: Zr_1−xV_xO₂ (x = 0.15), Ce_1−xV_xO₂ (x = 0.16) and Ti_1−xV_xO₂ (x = 0.15). As can be seen, almost no degradation was observed over 20 min when H₂O₂ alone was added to the MB solution. When the V/ZrO₂ catalyst was added to the MB solution containing H₂O₂, the removal of MB was complete after 3 min, showing that V/ZrO₂ facilitated the oxidation of MB by H₂O₂. The catalytic activity of the system decreased dramatically when V/ZrO₂ was replaced by V/CeO₂ and V/TiO₂, as shown in Fig. 7. It can be seen that V/CeO₂ did not significantly decolourise MB over the duration of the experiment. When V/TiO₂ was used as the catalyst, removal of MB took more than 50 min. Control experiments confirmed that when only V/ZrO₂ catalyst was added to the solution, within 60 min, minimal removal of MB (about 5%) occurred, indicating that the effect of adsorption on MB removal is not obvious (Fig. 8e). Additionally, the reaction carried out in the dark did not show a difference in the spectrum when compared with the control experiments.

Fig. 8 Changes in the UV-vis absorbance spectrum of MB dye using various conditions: (a) H₂O₂ without catalyst; (b) V/ZrO₂ with H₂O₂; (c) V/CeO₂ with H₂O₂; (d) V/TiO₂ with H₂O₂; and (e) V/ZrO₂ without H₂O₂.

The effect of varying the initial H₂O₂ concentration from 0 to 1 mmol in the presence of Zr_1−xV_xO₂ (x = 0.15) was investigated, and the effect of the amount of H₂O₂ on the percentage decomposition of MB is shown in Fig. 9. When the amount of H₂O₂ was varied from 0 to 0.1 mmol, the percentage decomposition of MB increased from 0 to ca. 62% over 3 min. With a further increase of H₂O₂ to 0.8 mmol, complete decomposition of MB occurred within 3 min. The results demonstrated that the percentage removal of MB has a direct relationship with the amount of H₂O₂.

Fig. 9 Effect of the amount of H₂O₂ on the percentage removal of MB in the presence of Zr_1−xV_xO₂ (x = 0.15).

It has been reported that the catalytic removal of MB in the presence of H₂O₂ follows a radical oxidation pathway.³⁰ This pathway may be taken due to hydrogen peroxide being a source of ˙OH and ¹O₂ radicals when decomposed in the presence of catalyst. To further verify the existence and roles of ¹O₂ and ˙OH radicals in the V/ZrO₂–H₂O₂ suspension, the effects of various radical scavengers on the rate of MB decolorization were investigated. DMSO and t-butanol were selected as ˙OH radical scavengers, and sodium azide was selected as a ¹O₂ scavenger. As shown in Fig. 10, the decolorization of MB was almost suppressed in the presence of sodium azide, and the removal of MB over the course of 15 min was decreased from 100% to 43%. Further, there was no significant difference in MB decolorization with or without either DMSO or t-butanol. These results confirmed that ¹O₂ was the main ROS in the decolorization of MB in the V/ZrO₂–H₂O₂ suspension, while ˙OH played a limited role.

Fig. 10 Effect of NaN₃ on the percentage removal of MB in the presence of Zr_1−xV_xO₂ (x = 0.15). Conditions: 1 mg catalyst, 5 mL MB (10 mg L⁻¹), 0.1 mL H₂O₂ (30%), 0.4 mmol NaN₃.

Therefore, the degradation of MB using V/ZrO₂ corresponds to the ¹O₂ mechanism. H₂O₂ is activated by the catalyst surface to form radical ¹O₂:

The oxidative degradation of organic compounds in the presence of H₂O₂ is typically described as a second-order reaction:⁵⁰

where C and [¹O₂] are the concentrations of MB and ¹O₂ radical, respectively, k is the second-order rate constant, and t is the reaction time. By assuming that the instantaneous ¹O₂ concentration is constant, the kinetics of MB degradation can be described according to the pseudo-first-order equation given below:

k_app = k[¹O₂] (4)

where C₀ is the initial concentration of MB and k_app is the apparent pseudo-first-order rate constant (min⁻¹). The k_app constant was obtained from the slopes of the straight lines by plotting −ln(C_t/C₀) as a function of time t, through regression (Fig. 11). The apparent pseudo-first-order rate constants (k_app) for V/ZrO₂, V/CeO₂ and V/TiO₂ are 1.67 min⁻¹, 0.016 min⁻¹ and 0.096 min⁻¹, respectively. Good correlation coefficients (r² > 0.93) were obtained for our systems.

Fig. 11 Plot of ln(C_t/C₀) as a function of time; fitted by pseudo-first-order rate law.

It is known that the adsorption capacity of the catalyst for the dye is a key factor, i.e., the more target molecules are adsorbed on the catalyst, the faster or more completely these molecules decompose. Therefore, adsorption experiments were carried out to study the adsorption of dye on Zr_1−xV_xO₂ (x = 0.15) nanoparticles at various pH values. The effect of pH on the degradation efficiency of MB was examined in the range of 3–11. The initial pH was adjusted by addition of 1.0 M NaOH or HCl. As shown in Fig. 12, V/ZrO₂ is an efficient catalyst over a wide pH range from 3 to 11, which is the pH range found in most natural and polluted waters. The activity follows the order pH 7 > 5 > 9 > 11 > 3. In general, the catalyst exhibits higher activity at neutral pH.

Fig. 12 Effect of pH on the degradation of MB in the presence of Zr_1−xV_xO₂ (x = 0.15).

The typical IR spectra of MB in the region 4000–500 cm⁻¹ before and after treatment also demonstrated the total degeneration of the dye (Fig. 13). The IR spectra of MB exhibit peaks corresponding to the stretching vibrations of C=N and C–N in the heterocycle of MB at 1604 and 1399 cm⁻¹, respectively, and the C–N bond connected with the benzene ring and N–CH₃ at 1356 and 1332 cm⁻¹, respectively.²⁸ After exposing the dye in air for about 60 min, these peaks decrease in intensity and disappear, indicating the destruction of the aromatic ring of the dye molecule in the presence of Zr_1−xV_xO₂ (x = 0.15) catalyst and H₂O₂. The degradation products, H₂SO₄ and HCl, were identified gravimetrically by precipitating them as BaSO₄ and AgCl, respectively, using BaCl₂ and AgNO₃ as precipitants.

Fig. 13 FT-IR spectrum of MB during degradation; (a) before treatment and (b) after treatment.

The lifespan of a catalyst is an important parameter in its evaluation. Finally, the effect of repeated uses of the catalyst on its catalytic activity was examined. After the reaction was completed, the suspensions were centrifuged, and Zr_1−xV_xO₂ nanoparticles (x = 0.15) were decanted to the bottom of the reactor, which could be easily separated and reused. The catalyst was thoroughly washed with water to remove the dye compound attached to the catalyst surface before reusing the catalyst in the next experiment. The results shown in Fig. 14 demonstrate that V/ZrO₂ maintained similar activity after four runs of the catalyst.

Fig. 14 Recycling studies of the Zr_1−xV_xO₂ (x = 0.15) catalyst in the degradation of MB.

Conclusions

Nano-vanadium supported on ZrO₂, CeO₂ and TiO₂ was successfully synthesized and characterized by XRD, XANES, EDX, SEM, and TEM. The new materials were found to be effective catalysts for the degradation of MB, an important industrial dye and problematic pollutant. The catalytic activity of the system decreased dramatically when V/ZrO₂ was replaced by V/CeO₂ and V/TiO₂. The effects of the amount of oxidant and the solution pH on the degree of decomposition of MB dye in the presence of V/ZrO₂ catalyst were also studied. To study the mechanism of the degradation of MB in the V/ZrO₂ oxidation system, the effects of DMSO and t-butanol as ˙OH radical scavenging agents and NaN₃ as a ¹O₂ scavenger were examined. The results confirmed that singlet molecular oxygen (¹O₂) is generated in the presence of catalytic amounts of V/ZrO₂. By assuming that the instantaneous ¹O₂ concentration was constant, the kinetics of MB degradation could be described according to the pseudo-first-order equation. Changes in the FT-IR spectrum of MB during degradation demonstrated the total degeneration of the dye. Finally, a further set of experiments was carried out to check the reusability of the V/ZrO₂ catalyst for the degradation of MB at room temperature.

Acknowledgements

M. Amini thanks the Research Council of the University of Maragheh for financial support of this work. S. G. is thankful to Prof. J. M. Chen (BL20A1, NSRRC) for experimental support and also acknowledges the travel subsidy given by Panjab University for Synchrotron experiments at NSRRC, Hsinchu, Taiwan.

Notes and references

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