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A sustainable approach for spent V2O5-WO3/TiO2 catalysts management: Selective recovery of heavy metal vanadium and production of value-added WO3-TiO2 photocatalysts Qijun Zhang, Yufeng Wu, Lili Li, and Tieyong Zuo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b03192 • Publication Date (Web): 28 Jul 2018 Downloaded from http://pubs.acs.org on July 28, 2018

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ACS Sustainable Chemistry & Engineering

A sustainable approach for spent V2O5-WO3/TiO2 catalysts management: Selective recovery of heavy metal vanadium and production of value-added WO3-TiO2 photocatalysts Qijun Zhang 1, Yufeng Wu 1, *, Lili Li 1, and Tieyong Zuo 1

1

Institute of Circular Economy, Beijing University of Technology, No. 100,

Pingleyuan Street, Chaoyang District, Beijing, 100124, P.R. China.

*

Corresponding Author: Fax: +86-10-67396234; Tel: +86-10-67396234; E-mail:

[email protected].

1 ACS Paragon Plus Environment

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Abstract

In order to control nitrogen oxides emissions, V2O5-WO3/TiO2 catalysts are widely applied in coal-fired power plants. Consequently, a large number of V2O5-WO3/TiO2 catalysts are spent annually because of their short operating life. Although these spent catalysts contain amounts of heavy metals, they have also been regarded as a potential secondary resource for the recovery of valuable elements titanium, tungsten, and vanadium. Therefore, this study developed an efficient method for selective leaching of heavy metal vanadium with an “H2SO4 + Na2SO3” acid reduction system. The use of this leaching solution achieved nearly 100% efficiency in vanadium removal. And the effects of the leaching parameters on the vanadium leaching efficiencies were investigated. Subsequently, the titanium-enriched residue obtained from the leaching process was used to produce high-performance WO3-TiO2 photocatalysts with dominant {001} facets via a hydrothermal treatment. The influence of the amount of hydrogen fluoride on the morphology and percentage exposure of the {001} facets of the photocatalysts was studied systematically. The method proposed in this study constitutes a novel and sustainable approach for the disposal of spent V2O5-WO3/TiO2 catalysts.

Keywords: spent V2O5-WO3/TiO2 catalysts, recovery, photocatalysts, {001} facets.


vanadium, WO3-TiO2

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Introduction Coal burning in the coal-fired power plants produces large amounts of nitrogen oxides (NOx). These NOx emissions are major air pollutants that have severe impact on local air quality; however, they also constitute a regional environmental risk. Thus, before they are discharged into the atmosphere, they must be transformed into environmentally friendly substances. In response to the environmental threats posed by large amounts of NOx emissions, the Chinese government has established the national standard “Emission Standard of Air Pollutants for Thermal Power Plants” to control NOx emissions from coal-fired power plants.1 This standard stipulates that the concentration limits of NOx emissions must be reduced to 100 mg/Nm3 from thermal power boilers and gas turbines in key areas. To achieve this target, V2O5-WO3/TiO2 catalysts are largely used in coal-fired power plants.2 However, commercial V2O5-WO3/TiO2 catalysts have a short operating life because they are degraded by the volatility of the active components, high-temperature sintering, and fly ash contaminants.3 Consequently, since 2014, China has had to process increasingly large amounts of V2O5-WO3/TiO2 catalyst waste as the catalysts reach the end of their design life. The production of spent V2O5-WO3/TiO2 catalysts is projected to stabilize at 200,000-250,000 m3 year-1 after 2020.1 The spent V2O5-WO3/TiO2 catalysts are categorized as hazardous waste because they contain amounts of heavy metal vanadium. However, spent V2O5-WO3/TiO2 catalysts have also been regarded as a potential secondary resource for the recovery of valuable elements such as titanium, tungsten, and vanadium, which could protect the environment and alleviate pressure on domestic resources. Conventional methods for the recovery of valuable elements from spent V2O5-WO3/TiO2

catalysts

include

pyrometallurgical

and

hydrometallurgical

technologies such as soda roasting and concentrated alkaline solution leaching.4-8 For example, Kim et al. investigated the leaching of vanadium and tungsten from spent V2O5-WO3/TiO2 catalysts via a pressure leaching process that used a NaOH solution as the leaching reagent.4 They subsequently separated V and W from the as-obtained 3 ACS Paragon Plus Environment


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leached solution using CaCl2 and NH4OH precipitation processes, respectively. Chen et al. developed a Na2CO3 roasting method to treat spent V2O5-WO3/TiO2 catalysts at 700 °C for 3.5 h. 6 However, these methods are considered unsustainable because of the high temperatures involved in the recycling process, long process flow and toxic waste emissions. Thus, with consideration of both the need for resource recycling and the environmental burden, it is urgently desirable to develop an efficient and green way in which to dispose of spent V2O5-WO3/TiO2 catalysts. Nanostructured TiO2 photocatalysts have received considerable attention in recent decades because they can be used in energy and environmental fields.9-11 However, some weaknesses remain in the practical application of nanostructured TiO2 photocatalysts, e.g., (1) low usage of solar energy and low quantum yield and (2) high cost because the cost of expensive chemical reagents usually used in the laboratory for preparing TiO2.12-15 To overcome such problems, this work proposed using spent V2O5-WO3/TiO2 catalysts as a secondary raw material. The mass fraction of the TiO2 component is > 80% in the spent V2O5-WO3/TiO2 catalysts. Thus, reuse of thses spent catalysts could protect Ti resources and reduce the cost of TiO2 photocatalysts. Importantly, WO3 present in the spent V2O5-WO3/TiO2 catalysts would couple with the TiO2 to improve the solar energy utilization and to enhance the photogenerated electron-hole pair separation, which has been proven effective in improving photocatalyst efficiency.16-19 This finding implies that production of WO3-TiO2 photocatalysts from spent V2O5-WO3/TiO2 catalysts is a sustainable option. The activity of a TiO2 photocatalysts is associated closely with its exposed facets. For example, the theoretical surface energy of the {001} facets is 0.98 J m-2, while the theoretical surface energies of the {100} and {101} facets are only 0.63 and 0.45 J m-2, respectively.20 Therefore, TiO2 with exposed {001} facets shows higher photocatalytic activity

than

those

with

other

exposed

facets.

Li

et

al.

synthesized

W18O49-photosensitized TiO2 with exposed {001} facets. Their as-synthesized W18O49-photosensitized TiO2 exhibited excellent photocurrent response and photocatalytic activity for decomposition organic contaminants under visible light, 4 ACS Paragon Plus Environment

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full-spectrum, and NIR irradiation.21 However, to the best of our knowledge, there have been no reports on the synthesis of WO3-TiO2 photocatalysts with exposed {001} facets from spent V2O5-WO3/TiO2 catalysts. In this study, we developed an efficient and sustainable process for the recovery of vanadium and for the production of WO3-TiO2 photocatalytic materials with exposed {001} facets from spent V2O5-WO3/TiO2 catalysts. First, the heavy metal vanadium in the spent V2O5-WO3/TiO2 catalysts was selectively leached using an “H2SO4 + Na2SO3” acid reduction system. Then, WO3-TiO2 photocatalysts with exposed {001} facets were prepared using a hydrothermal treatment on the derived titanium-enriched residue. This work not only demonstrates an effective method with which to lower the cost of production of TiO2 photocatalysts, but is also highlights a new pathway for the full recovery and high-value reuse of spent V2O5-WO3/TiO2 catalysts. Experimental Materials All chemical reagents were purchased from the Chemical Reagent Company of Beijing. The spent V2O5-WO3/TiO2 catalysts were supplied by the Zhejiang Tuna Environmental Science & Technology Co., Ltd. The spent V2O5-WO3/TiO2 catalysts were crushed, ground, and dried at 105 °C until a constant weight was obtained. Before use, the spent catalysts were dry milled using a planetary mill (48 h; 400 rpm) and passed through a 0.074 mm sieve using dry sieve method. Materials characterization The chemical composition of samples was analysed using X-ray fluorescence (XRF, PW2403, PANalytical, The Netherlands). The crystalline phases of samples were investigated using X-ray diffraction (XRD) analysis (Bruker AXS D8, Bruker, Germany). The Fourier transform infrared spectroscopy (FTIR) spectra of the catalysts were recorded on a Bruker FTIR (Bruker TENSOR II, Bruker Corporation, USA). The morphology of the samples was examined by field-emission scanning electron microscopy (SEM) (SU-8020, Hitachi, Japan) and transmission electron 5 ACS Paragon Plus Environment


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microscopy (TEM) (JEM-2100F, JEOL, Japan). The concentrations of elements in each solution were analyzed using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Optima 8000, PerkinElmer, Shelton, CT 06484, USA). The photoluminescence (PL) (F7000, Hitachi, Japan) spectra were recorded with a fluorescence spectrophotometer using a 300 nm line from a xenon lamp. The elements composition in samples and their corresponding valence states in samples were determined by X-ray photoelectron spectroscopy (XPS). The measurements were acquired by an ESCALab220i-XL electron spectrometer (Thermo Fisher Scientific Company, USA) using 300 WAl KR radiation in a base pressure of 3 × 10-9 mbar. Proposed Process An efficient and sustainable process for the selective recovery of vanadium and the production of WO3-TiO2 photocatalysts with exposed {001} facets from spent V2O5-WO3/TiO2 catalysts was proposed, as shown in Fig. 1. Leaching of vanadium Vanadium in the spent V2O5-WO3/TiO2 catalysts was selectively leached using the H2SO4 + Na2SO3 acid reduction system. First, in a typical acidolysis process, 500 mL 5 wt. % H2SO4 was added into a 1-L beaker. Then, 50 g of spent V2O5-WO3/TiO2 catalyst and 1 g Na2SO3 were added into the acidic solution under mechanical stirring at 95 °C. Importantly, Na2SO3 has low solubility under acidic conditions; therefore, we had to add 1 g Na2SO3 into the mixture solution every 0.5 h. After 2 h leaching, the resulting solution was filtered to produce a vanadium sulphate solution and a titanium-enriched residue. The unreacted titanium-enriched residue was then leached by HF solution. The concentration of vanadium in the obtained vanadium sulphate solution and in titanium-enriched residue were analysed using ICP-OES. The leaching efficiency of vanadium (x) was calculated by the following formula: x% =

Vs × 100% Vs + Vr

(1)

where Vs and Vr are the concentrations of vanadium in the solution and in the titanium-enriched residue, respectively.


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Production of WO3-TiO2 photocatalysts The WO3-TiO2 photocatalysts with exposed {001} facets were regenerated using a hydrothermal method. In a typical process, the dried titanium-enriched residue obtained from the H2SO4 leaching process was dissolved in a hydrogen fluoride (HF) solution to produce the precursor solution. Then, 6 mL of precursor solution was mixed with 3 mL H2O2 and 21 mL H2O under mechanical stirring, following which the

mixture

was

transferred

to

a

stainless

steel

autoclave

lined

with

polytetrafluoroethylene (Teflon®) (50 mL). The hydrothermal synthesis was conducted at 180 °C for 12 h in an electric oven. After synthesis, the autoclave was removed from the oven and allowed to cool naturally to room temperature. The prepared WO3-TiO2 photocatalysts were obtained by centrifugation. Then, rinsed with dilute hydrochloric acid, deionized water and dried fully at 60 °C.

Photocatalytic activities test The photocatalytic activity of the samples was evaluated for the photocatalytic removal of Methyl orange (MO) in aqueous solutions at ambient temperature. Before the photocatalytic experiments, all samples were cleaned with 0.1 M NaOH aqueous solution. Then rinsed extensively with deionized water and dried at 60 °C. All samples were loaded onto slides (4.0 × 4.0 cm) using a screen printing method.22 Finally, all prepared films were sintered at 500 °C for 30 min. A 30-W UV lamp (365 nm) was used as the illumination source. The films were immersed in 50 mL of MO solution (10 ppm) in a quartz cell for the photodegradation experiments. Prior to irradiation, all films were dipped in a 20-ppm MO solution for the saturated adsorption process. After a given period of irradiation, the concentration of MO in the resulting solution was analysed using a UV-vis spectrophotometer (U-3900H, Hitachi, Japan). The Lambert-Beer rule was applied using the characteristic absorbance band of the dye at 464 nm to determine the changes in concentration.

Results and Discussion Waste characterization Table 1 shows the XRF results of the element analysis of the spent 7 ACS Paragon Plus Environment


V2O5-WO3/TiO2 catalysts in this study. The main chemical components of the spent V2O5-WO3/TiO2 catalysts are TiO2, SiO2, WO3, CaO, VOx, SO3 and Al2O3, and their corresponding mass fractions are 85.1%, 6.63%, 4.39%, 1.24%, 0.841%, 0.795% and 0.682% , respectively. It is notable that vanadium species are highly toxic. Therefore, leaching and recovery of vanadium from spent V2O5-WO3/TiO2 catalysts is an essential issue in environmental protection and conservation. In addition, the high contents of TiO2 and WO3 in the spent V2O5-WO3/TiO2 catalysts make them suitable for the recovery of the valuable elements titanium and tungsten. The surface morphology and dispersion of the spent V2O5-WO3/TiO2 catalysts were observed by SEM, as shown in Fig. 2. The spent V2O5-WO3/TiO2 catalysts are obviously aggregated with diameters of 0.05-3.0 µm, which might be a result of high-temperature sintering for the long-term denitrification catalytic process (Fig. 2a).

The crystal structures of the spent V2O5-WO3/TiO2 catalysts are shown in Fig. 2c. The major peaks at 2θ = 25.3, 37.9, 48.1, 54.0, 55.1, 62.8, 68.8, 70.4, and 75.1° are consistent with those obtained for the standard anatase TiO2 phase (JCPDS No. 21-1272).

Fig. 3a shows the FTIR spectra of the spent V2O5-WO3/TiO2 catalysts. The bands in the range 800-400 cm-1 represent the stretching vibration of Ti-O and Ti-O-Ti.23 The bands observed at 1634 and 3364 cm-1 can be attributed to strong interaction between -OH groups and Ti4+.23 The band at 1050 cm-1 is related to W=O stretches.24 The weak intensity vibration bands at 2321 and 2358 cm-1 are assigned to adsorbed carbon dioxide.25 The vibration bands at 1127 and 1221 cm-1 are attributed to sulphates.25 The bands at 1042 and 1433 cm-1 can be attributed to surface VOx species.26 XPS is applied to investigate the elements composition in the spent V2O5-WO3/TiO2 catalysts, and also to confirm their corresponding valence states (Fig.

4). It can be observed that the primary elements of O, V, Ti, and W elements along with other minor elements were detected in the spent V2O5-WO3/TiO2 catalysts (Fig.

4a). The V 2p3/2 spectrum exhibits three peaks at 517.4, 516.6, and 515.6 eV, and their 8 ACS Paragon Plus Environment

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corresponding area ratios are 35.6, 49.3, and 15.1%, respectively (Fig. 4b). The peak at 517.4 eV can be assigned to V5+.27 The peaks at 516.6 and 515.6 eV can be assigned to V4+ and V3+, respectively.3 As shown in Fig. 4c, it can be obviously observed that two peaks located at 464.8 and 459.1 eV, which can be ascribed to Ti 2p1/2 and Ti 2p3/2, respectively, consistent with the typical values of TiO2.16 The W 4f5/2 and W 4f7/2 peaks at approximately 37.8 and 35.9 eV are also found, respectively (Fig. 4d).28, 29

Completely and selective leaching of V To determine the optimal concentration of H2SO4 for maximum recovery of vanadium, leaching was conducted at 1, 2, 5, and 10 wt. % H2SO4 with leaching time of 2h and temperature of 95 °C. The results shown in Fig. 5a indicate that the leaching efficiency of vanaium can be affected substantially by the concentration of H2SO4. The leaching efficiency of vanadium increased considerably from 23.1% to 100.0% when the H2SO4 concentration was increased from 1 to 5 wt. %. However, higher concentrations of H2SO4 can dissolve the titanium component, resulting in wastage lots of titanium and tungsten. Consequently, 5 wt. % was considered the optimal concentration of H2SO4. The results shown in Fig. 5a also indicate that the leaching efficiency of vanadium can be significantly affected by the addition of Na2SO3. Without Na2SO3, only 61.2% of vanadium was leached for 2 h. However, with the additive of Na2SO3, nearly 100.0% of vanadium was leached under the same leaching parameters. These results demonstrate that Na2SO3 with H2SO4 show a synergistic enhancement effect on vanadium leaching. This might be attributed to the V2O5 and V2O3 were converted to VOSO4 through the reactions with Na2SO3 and H2SO4, according to the following mechanism:

V2 O5 + Na 2SO3 + 2H 2SO 4 → 2VOSO 4 + Na 2SO 4 + 2H 2 O V2 O5 + V2 O3 + 4H 2SO 4 → 4VOSO 4 + 4H 2 O

(2) (3)

The effects of leaching temperature on vanadium recovery yield were investigated in the range of 30-95 °C with a H2SO4 concentration of 5 wt. % and the 9 ACS Paragon Plus Environment


addition of Na2SO3. The results shown in Fig. 5b indicate that the leaching efficiency of vanadium increased with the increasing of leaching temperature. For example, after leaching for 2 h, the vanadium recovery yield increased from 38.5% to 100.0% when the leaching temperature increased from 30 to 95 °C, respectively. For a complete vanadium recovery, the suggested leaching temperature and time will be 95 °C and 2 h, respectively. Under these optimized leaching parameters, nearly 100.0% of vanadium was leached. However, leaching efficiencies of titanium and tungsten were only 0.84% and 1.65% under these optimized leaching parameters, respectively. The titanium-enriched residue obtained from the “H2SO4 + Na2SO3” leaching process was also analysed by XRF, as shown in Table 1, which confirmed that the VOx species were dissolved completely in the “H2SO4 + Na2SO3” solution. In the FTIR spectra of the titanium-enriched residue (Fig. 3b), the peaks at 1042 and 1433 cm-1 belonging to the VOx species cannot be observed, which further confirms that the VOx species were removed completely.

Fig. 2b shows an SEM image of the titanium-enriched residue. It reveals that the morphologies and particles size of the titanium-enriched residue remained essentially unchanged. Fig. 2d shows the crystal structure of the titanium-enriched residue. The XRD patterns of the titanium-enriched residue exhibit anatase TiO2 (JCPDS No. 21-1272).

Production of WO3-TiO2 photocatalysts The second objective of this work was to synthesize low-cost WO3-TiO2 photocatalysts with exposed {001} facets using the titanium-enriched residue obtained from the “H2SO4 + Na2SO3” leaching process as a raw material. First, we dissolved the titanium-enriched residue in an HF solution to produce a precursor solution, which was then processed using a hydrothermal treatment.

Fig. 6a shows an overview SEM image of the regenerated sample, which reveals that the resulting sample consists of uniform well-defined sheet-like structures with square outlines. As shown in Fig. 6b, the side length and thickness of an individual 10 ACS Paragon Plus Environment

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nanosheet were approximately 1.7 and 0.64 µm, respectively. A well-defined rectangle is clearly observed in the TEM image in Fig. 6c with an average size of approximately 1.8 µm. Fig. 6d displays a high-resolution TEM image of a regenerated sample. The spacing of the lattice fringes is approximately 0.235 nm, which can be assigned to the {001} facets of anatase TiO2.30 Commonly, anatase TiO2 nanocrystals are primarily dominated by eight low-energy {101} facets, according to Wulff construction. However, the as-regenerated sample has eight equivalent low-energy {101} facets and two high-energy {001} facets as shown in Fig. 6f. The described series of characterizations confirm that the {001} facets of the as-regenerated TiO2 can be obtained with the presence of F- ions. This is because F- ions can be strongly adsorbed on the {001} facets, which can reduce the surface energy of high-energy {001} facets and delay crystal growth along the [001] axis.31 According to the formula in Fig. 6f, the percentage of {001} crystal facets is calculated as 61.7%. The primary elements of O, Ti, F, and W were detected with mass ratios of 46.96, 42.69, 4.87, and 5.48 wt. %, respectively, as shown in the EDS spectrum of the as-regenerated sample (Fig. 6e). The XRF results in Table S1 confirm that the main components of the as-obtained products were TiO2 (93.21 wt. %) and WO3 (5.62 wt. %). The full XPS spectra of the regenerated WO3-TiO2 products, shown in Fig.

S1a, confirm that Ti, O, and W are the primary constituents of the as-obtained products. The peaks at around 35.7 and 37.8 eV can be assigned to W 4f7/2 and W 4f5/2 for WO3, respectively (Fig. S1b). The structural properties and facets of the regenerated sample were examined by XRD. As depicted in Fig. 7a, the as-regenerated sample was present in the anatase TiO2 phase, which corresponds to 2θ values of 25.1, 36.7, 37.7, 47.7, 53.9, 54.6, 61.2, 62.4, 68.5, 69.9, and 74.7°, indicating (101), (103), (004), (200), (105), (211), (213), (204), (116), (220), and (215) diffractions (JCPDS No. 21-1272), respectively. The PL spectrum is an important tool for the analysis of the separation efficiency of electron-hole pairs in TiO2 photocatalysts.32 Fig. 7b demonstrates that both the as-regenerated WO3-TiO2 and the P25 photocatalysts exhibit PL peaks in the range of 11 ACS Paragon Plus Environment


350-550 nm with an excitation at 325 nm. By comparing the curves of as-regenerated WO3-TiO2 and P25 photocatalysts, we observe that the intensity of the peaks decreases in the order P25 > as-regenerated WO3-TiO2 photocatalysts. The decrease of the PL intensity corresponds to greater efficiency in the transfer and separation of photogenerated electron-hole pairs; therefore, the PL results are consistent with the photocatalytic activities.33 Thus, we can conclude that the photocatalytic activities of the samples should be in the order as-regenerated WO3-TiO2 > P25 photocatalysts. To investigate the effects of HF on the morphology and the percentage exposure of the {001} facets of the as-regenerated WO3-TiO2 photocatalysts, the amounts of precursor solution were tested with 3 mL H2O2 and 21 mL H2O at 180 °C for 12 h, as shown in Fig. 8. When 2 mL of precursor solution was used, an amount of irregular agglomerates and a notably trace amount of octahedral structures were obtained (Fig.

8a). Thus, the exposure percentage of the {001} facets could be negligible. Limited Fions in the precursor solution are unavailable to reduce the surface energy of the {001} facets to a level below that of {101} facets; therefore, the as-regenerated WO3-TiO2 nanoparticles possess no (or notably few) exposed {001} facets.34 When the amount of precursor solution was increased to 4 mL, a certain amount of octahedral structures with an average thickness of about 2 µm was produced (Fig. 8b). A further increase in the amount of precursor solution (to 6 mL), which corresponded to a decrease in the thickness of the octahedral structures to approximately 0.64 µm, resulted in an increased exposure percentage of the {001} facets (Fig. 8c). When the amount of precursor solution was increased to 8 mL, the nanosheets were seriously corroded and fractured, leading to a decreased percentage of exposed {001} facets (Fig. 8d). The observed phenomena of corrosion and fracture might be the result of HF concentration in the precursor solution on the etching or dissolution effects on TiO2 {001} facets.35 These results indicate the amount of HF has enormous influence on the as-regenerated products. The photocatalytic activity of the as-regenerated WO3-TiO2 photocatalysts with exposed {001} facets, which were prepared in 6 mL precursor solution, 3 mL H2O2, 12 ACS Paragon Plus Environment

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and 21 mL H2O at 180 °C for 12 h, was tested using the photocatalytic degradation of the MO under UV irradiation, as shown in Fig. 9. For comparison, a WO3-TiO2 (WTO) photocatalyst with 5.62 wt. % WO3 was prepared using a sol-gel method.36 The photocatalytic experiments with the P25 and WTO were conducted under identical conditions (Figs. 9 and S2). Without the photocatalyst particles (denoted as blank), the MO solution showed negligible reduction in concentration under UV irradiation. The photocatalytic degradation efficiencies of the as-regenerated WO3-TiO2, P25, and WTO were 56.2%, 41.7%, and 40.9%, respectively (Figs. 9a and

S2a). The degradation efficiency of MO was not the desired value, possibly because the amount of photocatalyst in our photocatalytic experiments was too small. However, under identical conditions, we can conclude the as-regenerated WO3-TiO2 with exposed {001} facets produced considerably better photocatalytic performance than either P25 or WTO. The initial concentration of MO (10 ppm) was small; therefore, the apparent rate constant (κapp) for the photocatalytic oxidation of MO with different samples was evaluated using a pseudo-first-order model. The apparent rate constant κapp values are listed in Table 2. Clearly, the as-regenerated WO3-TiO2 photocatalyst exhibited superior photocatalytic performance (Figs. 9b and S2a). For the kinetics of degrading MO, the slope of the as-regenerated WO3-TiO2 is 1.35 and 2.36 times that of P25 and WTO, respectively, indicating that the as-regenerated WO3-TiO2 photocatalysts is most efficient in the degradation of MO. Figs. S2b, 9c, and 9d show the absorption spectra of an MO solution in the presence of WTO, P25 TiO2, These results show in

Figs. S2b, 9c, and 9d indicate that MO was completely photocatalytically degraded into the end products of CO2 and H2O. To demonstrate the reusability of as-regenerated WO3-TiO2 photocatalysts, the stability of the as-regenerated WO3-TiO2 photocatalysts was tested. The catalyst films were soaked in 10% HNO3 solution for 12 h after the photocatalytic reactions. Then, washed with deionized water and ethanol for several times, and then dried at 60 °C. After cleaning, the used WO3-TiO2 photocatalysts were tested in fresh MO solutions 13 ACS Paragon Plus Environment


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under the described sample experimental conditions. After three cycles of use, we found that the loss of photocatalytic activity of the as-regenerated WO3-TiO2 photocatalysts was negligible, which indicates its outstanding reusability. Based on the photocatalytic experiments and results of the above series of characterizations,

the

enhanced

photocatalytic

activities

of

the

WO3-TiO2

photocatalysts can be ascribed primarily to the synergistic effects of WO3 coupled with TiO2 and the exposed high-energy {001} facets. On the one hand, WO3 coupled with TiO2 yields excellent electron-scavenging capacity and it increases the separation efficiency of the photogenerated electron-hole pairs, according to the PL results. Thus, under light irradiation, the photogenerated holes are transferred from the valence band (VB) of WO3 to TiO2 because the VB of WO3 is higher than that of TiO2, according to previous reports.37-39 While the photogenerated electrons move in opposite directions to the photogenerated holes (Fig. 10). Electrons can be scavenged by the adsorbed O2 to form superoxide radical (O2·-). The holes can also react with H2O or OH- species to produce •OH radicals.40, 41 On the other hand, the high instability and reactivity of O atoms on the {001} facets can produce more oxygen vacancies.42 Thus, these oxygen vacancies can react with H2O or OH- species to form more •OH radicals. Finally, dyes can be strongly decomposed by these superoxide species to produce CO2 and H2O.

Conclusions This work addressed the development of a novel and sustainable approach to the selective recovery of vanadium and the production of valuable WO3-TiO2 photocatalysts from spent V2O5-WO3/TiO2 catalysts. The VOx species in the spent V2O5-WO3/TiO2 catalysts were selectively leached using an “H2SO4 + Na2SO3” acid reduction system. Then, the titanium-enriched residue was used to produce valuable WO3-TiO2 photocatalysts with exposed {001} facets via a hydrothermal treatment. The essential results are listed below: (i) An efficient "H2SO4 + Na2SO3" acid reduction leaching process was proposed to selective leach vanadium and proven feasible in this work. It is revealed that H2SO4 14 ACS Paragon Plus Environment

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concentration, temperature and time had a significant influence on leaching vanadium. The optimal leaching conditions for complete dissolution of the VOx species in the spent V2O5-WO3/TiO2 catalysts were: temperature 95 ºC, 5 wt. % of H2SO4, and leaching 2 h. And the addition of Na2SO3 has a significant effect on the leaching efficiencies of vanadium. The XRF and FTIR results of the titanium-enriched residue obtained from the leaching process indicate that the VOx species in the spent catalysts is dissolved completely in the solution. (ii) We successfully produced WO3-TiO2 photocatalysts with exposed {001} facets using the titanium-enriched residue obtained from the leaching process as a raw material. TEM and SEM observations confirmed the WO3-TiO2 photocatalysts exhibited a well-defined octahedral structure with dominant {001} facets, and the percentage of these {001} facets was 61.7%. The presence of Ti, O, and W in the products was verified by EDS and XPS. Remarkably, the WO3-TiO2 photocatalysts exhibited higher photocatalytic efficiency than P25 for the photo-oxidation of MO.

Acknowledgments. This research was financially supported by Beijing Natural Science Foundation (2174067, 2182009), and the Fundamental Research Fund Project of Beijing University of Technology (033000546317501).

Supporting Information. The XRF and XPS results of the regenerated WO3-TiO2 photocatalysts. And photocatalytic results of the WO3-TiO2 which prepared via sol-gel method.

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Figure Captions:

Fig. 1. Flowchart of V recycling and production of valuable WO3-TiO2 photocatalysts with exposed {001} facets from spent V2O5-WO3/TiO2 catalysts.


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Fig. 2. SEM images of (a) spent V2O5-WO3/TiO2 catalysts and (b) titanium-enriched residue; XRD patterns of (c) spent V2O5-WO3/TiO2 catalysts and (d) titanium-enriched residue.



Fig. 3. FTIR spectra of (a) spent V2O5-WO3/TiO2 catalysts and (b) titanium-enriched residue.


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Fig. 4. (a) XPS survey spectra, (b) V 2p, (c) Ti 2p, and (d) W 4f spectrum of spent V2O5-WO3/TiO2 catalysts.



Fig. 5. (a) Effect of H2SO4 concentration on leaching of V, and (b) effect of temperature on leaching of V.


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Fig. 6. (a) and (b) SEM images of the regenerated WO3-TiO2 photocatalysts with exposed {001} facets; (c) TEM image of the regenerated WO3-TiO2 photocatalysts; (d) high-resolution TEM image of the regenerated WO3-TiO2 photocatalysts; (e) EDS spectrum of the regenerated WO3-TiO2 photocatalysts; (f) three-dimensional model of the WO3-TiO2 nanosheet (Side lengths labelled (A) and (B), were used to calculate the percentage of exposed {001} facets).



Fig. 7. (a) XRD patterns of as-regenerated WO3-TiO2 photocatalysts, and (b) PL spectra of P25 and as-regenerated WO3-TiO2 excited at 325 nm.


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Fig. 8. SEM images of samples with different precursor solution content: (a) 2 mL, (b) 4 mL, (c) 6 mL, and (d) 8 mL.



Fig. 9. (a) Photodegradation of MO with all samples under UV irradiation, (b) photocatalytic reaction kinetics of MO as a function of reaction time, and UV-vis spectral changes of MO in the presence (c) P25 and (d) as-regenerated WO3-TiO2.


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Fig. 10. Schematic of the band energy levels and charge transfers for the as-regenerated WO3-TiO2 photocatalysts.



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Table Captions:

Table 1. Chemical compositions (wt. %) of samples. (XRF analysis). composition

TiO2

SiO2

WO3

CaO

V2O5

SO3

Al2O3

Others

85.10

6.63

4.39

1.24

0.841

0.795

0.682

0.412

87.34

6.68

4.59

0.120

0

0.410

0.302

0.558

Spent V2O5-WO3/TiO2 catalysts Titanium-enriched residue

Table 2. Reaction rate constant κapp (min-1) for the degradation of MO using various samples. Degradation of MO Samples

κapp (min-1)

R2

As-regenerated WO3-TiO2

3.721×10-2

0.99426

P25

2.758×10-2

0.99312

WTO

1.573×10-2

0.9974


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“For Table of Contents Use Only.”

We introduce a green and economical process for selective recovering heavy metal vanadium and regenerating low-cost and excellent performance WO3-TiO2 photocatalysts with exposed {001} facets.


TiO2 catalysts

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