Catalytic activity of using tungsten oxide with hydrogen peroxide for methyl orange degradation

In this work, a tungsten oxide nanoparticle was developed with a simple method, using tungsten powder. The orthorhombic, hexagonal and monoclinic crystalline structures obtained were characterized by X-ray diffraction, scanning electron microscopy, as well as Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The coupling of these tungsten oxide nano particles with hydrogen peroxide is carried out on methyl orange removal from wastewater. The use of Hydrated orthorhombic particles showed the highest removal rate up to 89.7% comparatively to hexagonal and monoclinic crystalline structures respectively. The reusability of these particles showed a good stability with monoclinic crystalline structure after four cycles.


INTRODUCTION
Azo dyes are present in wastewaters from the textile, paper, plastics, leather, food, cosmetics and some other industries. Unfortunately, they constitute one of the major sources of environmental degradation of water (Pavithra et al., 2019). Azo dyes are not only dangerous to aquatic life, but in many cases carcinogenic to humans and animals (Fatima et al., 2017). Researchers are thinking about creating an effective technique for the removal of pestilent dyes from wastewaters. Several methods like adsorption (Ayati et al., 2016), hydrogenation (Fu et al., 2019), photocatalysis (Yang et al., 2018Chen et al., 2019;Zeng et al., 2019;Feng et al., 2020), degradation by biological (Rajasulochana and Preethy, 2016) and chemical oxidation (Collivignarelli et al., 2019;Katheresan et al., 2018) have been applied for industrial wastewaters remediation before their disposal to the environment. The use of hydrogen peroxide as an oxidizing agent was also proposed for a long time but proved to be ineffective when the load of dye is high (Wang et al., 2016). So, research groups have been focused on advanced oxidation processes, some environmentally friendly methods allow treatment of dyes in wastewater. These technologies have already shown their potential in treating toxic and biologically recalcitrant organic pollutants. The advanced oxidation processes include O 3 (Biard et al., 2018), UV (Seo et al., 2019), *Corresponding author. E-mail: aime.ello@univ-fhb.edu.ci.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License UV/H 2 O 2 , (Li et al., 2020), O 3 /UV (Wardenier et al., 2019), and Fenton (Bello et al., 2019;Zhu et al., 2019;Xiao et al., 2020). Among all advanced oxidation processes, Fenton reaction using soluble iron salts in Fe2 + or Fe 3+ form and H 2 O 2 can produce powerfully oxidizing radical species such HO • (Fe 2+ + H 2 O 2 → Fe3 + + HO • + OH − (1) Fe 3+ + H 2 O 2 → Fe 2+ + HO 2 • + H + (2)) for decomposition of dyes. However, the implementation of homogeneous Fenton oxidation based on ferrous or ferric salts has several limitations, including the conversion of Fe(OH) 3 into sludge at a pH above 4 (Zhu et al., 2019) which requires an additional treatment (Cetinkaya et al., 2018) and a cost for their elimination. To improve the oxidation efficiency heterogeneous Fenton-like reaction has been developed. Heterogeneous Fenton-like processes can be established by replacing Fe 2+ in the Fenton reagent with a solid catalyst including metal oxide. According to literature (Thornburg et al., 2016;Ziolek et al., 2015;Ziolek and Sobezak, 2017) reactive oxygen species can be generated in situ on the surface of selected metal oxides (such as Nb 2 O 5 , V 2 O 5 , TiO 2 , Fe 2 O 3 ) by the treatment with hydrogen peroxide, and then successfully used as heterogeneous catalysts for degradation of organic pollutants (Bello et al., 2019;Wiedmer et al., 2016;Zhu et al., 2019). The advantages of using metal oxide as solid catalyst in heterogeneous Fenton-like process comparatively to homogeneous Fenton process are: (a) The reactions shows performance in a wide range of pH (Zhu et al., 2019) (b) metal oxides can be recovered easily; (c) metal oxides decompose hydrogen peroxide H 2 O 2 to hydroxyl radical HO • like in classical Fenton reaction, and prevent precipitating of iron hydroxide (sludge) which reduce the efficiency of global oxidation process (Zhu et al., 2019). Among the heterogeneous Fenton-like processes, Photo-Fenton has been developed as a promising method using catalysts such Fe 2 O 3 (Domacena et al., 2020), Fe 3 O 4 , (Arshad et al., 2017) FeOOH (Liu et al., 2017), Fe 2 O 3 /Al 2 O 3 (Di Lucia et al., 2018) to overcome the classic homogeneous Fenton drawbacks mentioned above. However, UV and Visible light-based photo Fenton oxidation also has practical limitations. At neutral pH, a photo Fenton process showed low efficiency and needed the addition of iron complexing agents (Ahile et al., 2019). Consequently, significant efforts to develop various new heterogeneous Fenton catalysts have been made including; CuO (Deka et al., 2016), CeO 3 (Wei et al., 2019) and Mn 3 O 4 (Osgouei et al., 2020). The aim of this work is to propose a new alternative metal oxide such as tungsten oxide coupled with H 2 O 2 for the treatment of methyl orange. WO 3 can be a remarkable candidate as solid Fenton catalyst because in nano size, transition metal oxides has a great interest for oxidation due to is large surface area, which exhibits many advantages in the chemical catalysis field, such as a low diffusion resistance, easy accessibility to reactants, and large number of active sites (Gadipelly and Mannepalli, 2019). The use of WOx/H 2 O 2 has not been reported in the literature as heterogeneous Fentonlike process. Thus, to evaluate the outstanding efficiency using the WO 3 catalyst, a comparative study with other metal oxides for removal methyl orange from aqueous solution was presented.

Experimental
Tungsten oxide NPs were prepared by a simple dispersion of metallic tungsten powder (1.0 g) (APS 1-5 μm, purity 99,9%, Alfa Aesar Co.) in deionized water (10 ml), glacial acetic acid (1 ml , ACS reagent, purity ≥99.7%) followed by addition of H 2 O 2 (10 ml, aqueous solution; 30%, Fisher Scientific Co.) dropwise. The mixture was kept at 0°C in an ice bath. The oxidative dissolution process was very exothermic; therefore it was realized carefully under a well-ventilated fume hood. The solution was put in the glass reactor equipped with a condenser and heated at 85 °C for 3h. A precipitate is formed and centrifuged at 8000 rpm. Hydrated tungsten oxide WO 3 -H 2 O material was obtained without any surfactant by washing the precipitate with water and ethano,l then dried at 60°C overnight (16 h). Hydrated tungsten oxide WO 3 -H 2 O material followed two treatments: it was annealed at 200 and 600°C for 4 h and obtained h-WO 3 and WO 3 respectively.

Characterization
Powder X-ray diffraction (XRD) patterns of the samples were obtained on a Bruker D8 Advance X-ray diffractometer equipped with a Cu Kα radiation source (λ = 1.5418 Å). Fourier transform infrared (FTIR) absorption spectra were measured with an FTS 45 infrared spectrophotometer using KBr pellet technique. Scanning electron microscopy (SEM) images were taken on a JEOL 6360 instrument at an accelerating voltage of 3 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG Escalab 220i-XL using a polychromatic Al source (hν = 1486.6 eV) operating at 15 kV and 20 mA. The signal was filtered with a hemispherical analyser (pass energy = 20 eV) and the detection was performed with a multi-channel detector. The base pressure inside the spectrometer during analysis was 3 × 10 -10 Torr.

Catalytic tests
Catalytic tests were carried out in a reactor capacity of 250 ml attached with a condenser. 100 ml of aqueous solution of methyl orange was introduced and 50 mg of various tungsten oxides obtained following 2.5 ml of H 2 O 2 . The reactor was put in the oil bath previously heated and maintained at 65°C. During the reaction, the mixture was continuously stirred using magnetic stirrer (agitation rate ~ 400 rpm) for 3 h. The residual concentration of dye in the aqueous solution was determined each 20 min via UV-vis spectrophotometer at 464 nm. At the end of the reaction time, the tungsten metal oxide particles were separated from aqueous solution by using centrifuge at 8000 rpm during 10 min. Tungsten metal oxide particles were recovered then washed with ethanol and water several times and dried at 60°C overnight for reusability.
In order to investigate the surface morphology of these tungsten oxide particles, scanning electron microscopy was carried out for the various samples. Figure 2 shows morphology of the tungsten oxides particles. Figure 2A shows the nanoplates obtained before calcination. The shape evolved with the temperature of calcination, giving nanorod-like structure ( Figure 2B) at 200°C and pallets with large thickness at 600°C ( Figure  2C). The particles are not uniform in micro or nano size but regular without addition of surfactant. The X-ray photoelectron spectroscopy (XPS) is a useful tool to investigate the composition and oxidation states of elements presented over the surface of the tungsten oxide particles. Figure 3A shows a wide survey scan of the samples, clearly indicating the presence of W and O elements. The deconvolution of W4f spectrum in Figure  3B clearly shows two peaks at 35.6 and 37. 7 eV which were assigned to W 4f 7/2 and W4f 5/2 respectively for WO 3 -H 2 O. The binding energy at 35.7 and 37.8 eV were assigned to W4f 7/2 and W 4f 5/2 for h-WO 3 , while the peaks located at 35.4 and 37.5 eV were attributed to W 4f 7/2 and W 4f 5/2 of monoclinic WO 3 .
XPS analysis is one of the fundamental tool and the Figure  3B depicted  ions or +6 oxidation state (Kolhe et al., 2020). Figure 3C shows O1s spectra. The deconvolution of these peaks showed various weak peaks at different binding energy only for h-WO 3 and WO 3 -H 2 O. The monoclinic tungsten oxide structure showed a peak at 530 eV and both h-WO 3 and WO 3 -H 2 O samples at 530,2 eV which are attributed to the lattice oxygen bonded with W (Parthibavarman et al., 2018). WO 3 -H 2 O showed a high peak of adsorbed H 2 O at 532.5 eV which confirms XDR results. The peak of adsorbed H 2 O shifted to a weak at 532.4 eV due to a very small amount of water containing h-WO 3 . The peak at 531.4 eV is due to OH groups. XPS study strongly supported the formation of WO 3 and confirmed XRD patterns observed for tungsten oxide structures. The same results were observed by several authors during the synthesis of tungsten oxide (Thornburg et al., 2016;Kolhe et al., 2020;Parthibavarman et al., 2018). The small characteristic peak of the carbon element is observed at 284 eV in Figure 3A and is attributed to the carbon dioxide of the air or to the impurities of the reagents or to the washing solvent. These same results were demonstrated by Geng et al. (2017). Figure 4 shows the FT-IR spectra of the various tungsten oxide synthesized.
Identification of molecular structures reveals normal vibration modes by stretching (ν), plane bending (δ). The vibration bands are mainly the fundamental vibrations of the W=O, W-O and W-O-W chromophores and the stretching of O-W-O bonds (Ziolek et al., 2015). Below 1000 cm -1 , the FT-IR spectra showed significantly various vibration peaks at 942 and 671 cm -1 assigned as the stretching of W=O and O-W bonds in the hydrated tungsten oxide WO 3 -H 2 O crystalline structure. These stretching bonds exist also in both hexagonal and monoclinic structure. The increase of the calcination temperature up to 600°C created a characteristic peak of O-W-O stretching vibrations in the WO 3 monoclinic crystal lattice.
The absence of carbonyl peak around 1730 cm -1 indicated that no carbon contained all the samples, which confirmed the impurities shown in XPS spectra ( Figure  3A). For hydrated tungsten oxide, the symmetric stretching vibrations related to WO 3 were observed and the intercalated water molecules (W-OH ... was obtained at 85°C without calcination, that did not allow removal of water molecules; thus creating their inter reaction with tungsten oxide structure. The peak at 3397 cm -1 is weak and not significant for the h-WO 3 comparatively to WO 3 -H 2 O structure. The bending vibration is observable at approximately δ = 1613 cm -1 . The h-WO 3 shows that water molecules are removed but there is a small amount intercalated water molecules containing on the plane. The water intercalated was shown by XPS ( Figure 3C). Increasing the calcination temperature up to 600°C removed water and led to obtain non-hydrated crystal structure WO 3 because there are no vibration peaks in the range of 3000-3700 cm -1 corresponding to the presence of water. These results confirmed structures proposed by XRD and XPS.

Catalytic activity
The catalytic activities of tungsten oxides nanoparticles synthesized were carried out on the capacity of methyl orange remove from an aqueous solution. Efficiency of the removal process was monitored by UV-visible spectroscopy. Figure 5 shows the methyl orange removal rate, using tungsten oxide nanoparticles with hydrogen peroxide.
Hydrated tungsten oxide WO 3 -H 2 O alone eliminated around 3% of methyl orange after 3 h. The amount of methyl orange removed, increased up to 10% when H 2 O 2 was also used only, slightly high comparatively to hydrated tungsten oxide WO 3 -H 2 O. According to the literature, it is possible to oxidize many organic compounds with hydrogen peroxide . Coupling H 2 O 2 and the tungsten oxides nano particles increased the removal efficiency of methyl orange. The values reached 20, 30 and 47% for the coupling of hydrogen peroxide with WO 3 , h-WO 3 and WO 3 -H 2 O catalysts respectively. The use of hydrogen peroxide with tungsten oxide nanoparticles had a benefit effect for methyl orange removal. Many authors had shown that the catalytic activity of metal oxides in dye degradation is enhanced when metal oxides are combined with hydrogen peroxide in acid pH medium (Ziolek and Sobezak, 2017;Osgouei et al., 2020;Deka et al., 2016). The reaction of these tungsten oxide nanoparticles with H 2 O 2 led to generation of strongly reactive oxidative species such as O 2 •and O 2 2-, HO • (Ziolek et al., 2015). All the catalytic tests were carried out at pH = 6.9 that is very difficult in classical Fenton process. The coupling of WO 3 -H 2 O with hydrogen peroxide displayed a high performance comparatively to h-WO 3 and WO 3 respectively. Therefore, WO 3 -H 2 O led to formation of more reactive oxygen species than h-WO 3 and WO 3 . The presence of water in crystal lattice increased the ability of WO 3 -H 2 O to generate more oxidative species. Thus, the high production of hydroxyl radicals in the presence of water by the hydrated orthorhombic structure of tungsten oxide caused significant degradation of methyl orange comparatively to the hexagonal and monoclinic structures respectively. The reaction of H 2 O 2 with the metal oxides is very complex, but the mechanism for production of hydroxyl radicals is known and consists of two steps: adsorption of H 2 O 2 on metal oxide followed by its decomposition or cleavage to form hydroxyl radicals (Lousada et al., 2015). These two steps are also governed by the state of metal oxide surface and the kinetics of decomposition respectively (Lousada et al., 2015). The hydrated structures of tungsten oxide promoted the kinetics of hydrogen peroxide cleavage for the production of hydroxyl radicals. It is known that the kinetics of decomposition is strongly influenced by the pH of the reaction medium (Lousada et al., 2015;Wolski et al., 2019). To increase the performance of hydrated tungsten oxide, the catalytic reaction tests were done at a pH range as shown in Figure 6. The methyl orange removal rate increased when the pH medium decreased and maximum of methyl orange removal reached up 51% when the pH medium is 4. The results confirmed that shown in precious literature (Lousada et al., 2015;Wolski et al., 2019). The mass effect was also realized. Figure 7 displayed that the high rate of methyl orange removal was observed when the mass is 300 mg for WO 3 -H 2 O and WO 3 . At this mass value, the methyl orange removal reached up to 89.7 and 74.4% for WO 3 -H 2 O and WO 3 respectively. Thus, increasing the mass of tungsten oxide led to considerable removal of methyl orange from water. The UV-vis absorption spectra of methyl orange were recorded and shown in Figure 8. The spectrum of initial methyl orange shows two peaks located at 464 nm from the azo bond and at 298 nm attributed to benzene nucleus in the methyl orange molecule before treatment. After the end of reaction time of degradation, the decrease in these peaks was observed when H 2 O 2 is used, indicating that the azo bond was destroyed. The treatment of methyl orange with the coupling WO 3 -H 2 O/H 2 O 2 showed that the absorption peak from the azo bond decreased considerably, while the absorption at 298 nm from the benzene nucleus increase drastically and shifted to 288 nm, indicating that degradation intermediates containing the benzene ring were generated. These results are consistent with the work reported in previous literature (Chen et al., 2015;Omri et al., 2020).
The efficiently of monoclinic WO 3 is also increased. It was essential to compare the efficiency of the heterogeneous Fenton-like system using tungsten oxide for the removal of methyl orange with another heterogeneous Fenton-like system using other metal oxides. The Table 1 showed the performance of the Fenton-like process obtained with different metal oxides for the removal of methyl orange only. As showed in     process using tungsten metal oxide nanoparticle requires no UV input and showed high performance at pH= 4 in comparison to these other metal oxides, which need more strong acid pH medium. Reusability of the tungsten oxide nanoparticles was investigated. Unfortunately, it was difficult to do more than 2 cycles of methyl orange removal with WO 3 -H 2 O because of the loss of hydrated tungsten oxide particles as well as the leaching. As shown in SEM analysis, WO 3 -H 2 O showed the small size particles comparatively to monoclinic WO 3. It was then, not so easy to recover these particles. However, monoclinic WO 3 particles were recovered easily and reused in 4 consecutive runs. As shown in Figure 9, the removal efficiencies of WO 3 found are 74.4; 73. 2; 71.3 and 69.6% in first cycle, second, third and fourth cycle respectively. A slight loss was observed in efficiency of monoclinic tungsten metal oxide nanoparticles. Monoclinic WO 3 showed excellent stability even after four continuous cycles of activity, whereas the removal efficiency of monoclinic WO 3 is relatively

Conclusion
This study demonstrated that the combination of tungsten oxide nanoparticles with hydrogen peroxide as a Fentonlike system improved the removal of methyl orange from water. This process did not need pH = 3 condition to show high performances. The tungsten oxide particles in nano size have been well identified and characterized by various technical. Coupling the tungsten oxide nanoparticles with H 2 O 2 showed high methyl orange removal rate comparatively to hydrogen peroxide or tungsten oxide used alone. At pH = 4, the increase of the mass of tungsten oxides nanoparticles produced better conditions with H 2 O 2 to form more reactive oxygen species attributed to orthorhombic tungsten oxide WO 3 -H 2 O comparatively to h-WO 3 and WO 3 . The reusability of the monoclinic WO 3 was done and showed good stability. The Fenton-like process using WO 3 /H 2 O 2 showed higher capacity to remove methyl orange than many other metal oxides associated with hydrogen peroxide.