English

• 0.   Introduction

  With rapid industrialization and urbanization in China, air pollution has become increasingly serious, restricting sustainable social, and economic development. Toluene is widely used in the chemical industry as an excellent organic solvent. Its high volatility and chemical stability make it easy to diffuse into the air and difficult to degrade naturally, and its accumulation over a long time can lead to photochemical smog and the generation of O₃. This is a persistent polluter of the environment^[1-3]. People exposed to such environments for long periods can develop a range of acute and chronic diseases that threaten human health^[4-5].

  The first way to control toluene is recycling, however, the low concentration of toluene is not easy to recycle, generally using adsorption and concentration of combustion, biotechnology, plasma technology, or photocatalytic oxidation technology, such as purification to meet the standards and then discharged. Photocatalytic degradation has been emphasized due to its high efficiency, greenness, and other advantages. Vacuum ultraviolet (VUV) lamps not only emit 185-256 nm ultraviolet light for direct photolysis of toluene but also enable O₂ and H₂O to produce O₃ and hydroxyl radicals (·OH) with strong oxidizing properties to accelerate the degradation of toluene^[3-4]. It was found that the electron affinity of O₃ was higher than that of O₂, and it was easier for O₃ to capture electrons from the catalyst's conduction band and generate ·OH in photocatalysis, which effectively suppressed the photogenerated electron-hole complexation, and improved the degradation and mineralization rates of toluene^[6-10].

  TiO₂ semiconductor materials are widely used for environmental purification due to their chemical stability and high photocatalytic activity^[10-11]. However, the large energy band gap of anatase TiO₂ and the high complexation rate of photogenerated electron-hole pairs greatly limit the use of TiO₂. In recent years, the photocatalytic performance of TiO₂ has been improved by morphology modulation, metal/non-metal doping, noble metal deposition, and preparation of semiconductor heterojunctions^[12-14]. Co₃O₄ has excellent antioxidant ability as a p-type semiconductor catalyst, but its photocatalytic activity is low. Fortunately, its photocatalytic activity can be improved by combining it with TiO₂^[15-17]. The Co₃O₄/TiO₂ composite catalyst can promote the separation of photogenerated electron-hole pairs and enhance the efficiency of photocatalytic degradation of toluene^{[16, 18]}. In particular, the synergistic effect of VUV and Co₃O₄/TiO₂ can effectively improve the degradation of toluene^[19].

  Based on the above studies, we synthesized Co₃O₄/TiO₂ catalysts by a hydrothermal method and investigated the degradation of toluene by VUV and Co₃O₄/TiO₂ under nitrogen and air as carrier gas.

  1.   Experimental

  1.1   Materials

  The anatase phase TiO₂, sodium hydroxide, hydrochloric acid, cobalt nitrate hexahydrate, urea, and anhydrous ethanol were analytically pure and purchased from National Pharmaceutical Reagent Co. Toluene standard gas was purchased from Jining Xieli Special Gas Co.

  1.2   Synthesis of Co₃O₄/TiO₂

  Preparation of TiO₂ nanoribbons: first, anatase phase TiO₂ (0.8 g) was added into 60.0 mL of NaOH solution (10.0 mol·L^-1), stirred uniformly and then ultrasonicated for 30 min, which was transferred into a 100 mL polytetrafluoroethylene reactor, and then reacted at 180 ℃ for 24 h before being cooled to room temperature; Next, 0.1 mol·L^-1 HCl solution was added to the obtained slurry and stirred to pH=2.0, centrifuged and washed three times with deionized water and ethanol, respectively, the resulting solid was dried at 60 ℃ for 12 h; Finally, the obtained samples were heated up to 500 ℃ at 4 ℃·min^-1 and kept for 4 h, the TiO₂ nanoribbons were obtained by cooling to room temperature.

  Preparation of xCo₃O₄/TiO₂ (x=1.5, 3.0, 6.0, 12.0, x mL is the volume of Co(NO₃)₂ solution) composite catalysts: 0.40 g of prepared TiO₂ nanoribbons, 0.9 g of urea, 30.0 mL of ethanol, 30.0 mL of deionized water, and a certain volume (1.5, 3.0, 6.0, and 12.0 mL based on the mass fractions of 3.0%, 6.0%, 12.0%, 24.0% of Co₃O₄ in xCo₃O₄/TiO₂ respectively) of 0.1 mol·L^-1 Co(NO₃)₂ solution were mixed. After stirring for 30 min and ultrasonication for 30 min, the mixture was transferred to a 100 mL polytetrafluoroethylene reactor and reacted for 10 h at 180 ℃. It was cooled to room temperature, centrifuged, and washed three times with deionized water and ethanol, and dried for 12 h at 60 ℃. Finally, the xCo₃O₄/TiO₂ catalysts with different Co₃O₄ loadings were obtained by elevating the temperature to 500 ℃ at 4 ℃·min^-1 for 4 h calcination.

  1.3   Characterization

  X-ray powder diffraction (XRD) spectroscopy was performed using an X′Pert3Power model X diffractometer (working voltage: 40 kV, working current: 100 mA, diffraction light source: Cu Kα (λ=0.154 nm), scanning speed: 8 (°)·min^-1, step size: 0.02°, scanning angle 2θ: 10°-80°) from PANalytical, Netherlands; Transmission electron microscopy (TEM) was performed using a JEOL JEM-2100F transmission electron microscope (accelerating voltage: 200 kV, point resolution: 0.19 nm, lattice resolution: 0.1 nm, tilt angle: ±25°; X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250Xi X-ray photoelectron spectrometer from Thermo Electron Corporation (USA) and an XmaxN80T IE250 spectrometer from Oxford Corporation (UK). A fluorescence spectrometer (JASCO FP-6500) was used to obtain the catalyst′s photoluminescence (PL) spectra with a standard excitation wavelength of 200.0-750.0 nm and resolution of 1.0 nm.

  1.4   Catalyst performance evaluation

  The fluidized bed unit used for the experiments is shown in Fig. 1, and the reactor was equipped with two VUV lamps (6 W, Heraeus, Germany) and a quartz glass tube with catalyst, and the gas flow rate was all controlled by a mass flow meter. The humidity of the gas was regulated by passing N₂ into the water. The gas humidity at the inlet was monitored by a hygrometer after each gas was mixed in the mixer. Ozone concentrations were measured using an ozone analyzer (2B Technologies, M-106L, USA). In this experiment, the total gas flow rate was 1.0 L·min^-1, the space velocity was 1.05×10⁴ h^-1, and the volume concentration of toluene was 25.0 μL·L^-1. After the gas flow was stabilized, the gas was passed into a quartz reactor with an effective volume of 1.8 L for the photocatalytic reaction.

  Figure 1

  

  Figure 1.  Diagram of catalyst performance evaluation device

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  The degradation rate and mineralization rate of toluene were used to measure the catalyst performance and were calculated from the following equations 1 and 2:

  $ Q=\frac{x_{\mathrm{i}, \mathrm{C}_7 \mathrm{H}_8}-x_{\mathrm{o}, \mathrm{C}_7 \mathrm{H}_8}}{x_{\mathrm{i}, \mathrm{C}_7 \mathrm{H}_8}} \times 100 \% $

  (1)

  $ \mathrm{MR}=\frac{x_{\mathrm{o}, \mathrm{CO}}+x_{\mathrm{o}, \mathrm{CO}_2}}{7 x_{\mathrm{i}, \mathrm{C}_7 \mathrm{H}_8} Q} \times 100 \% $

  (2)

  where Q is the degradation rate of toluene, x_{i, C7H8} and x_{o, C7H8} are the toluene volume concentrations at the inlet and outlet; MR is the toluene mineralization rate, x_{o, CO2} and x_{o, CO} are the concentration of CO₂ and CO at the outlet.

  2.   Results and discussion

  2.1   XRD

  Fig. 2 shows the XRD patterns of Co₃O₄/TiO₂ catalysts with different Co₃O₄ loadings. The characteristic diffraction peaks with 2θ of 25.3°, 37.8°, 48.0°, 53.8°, 55.1°, 62.7°, and 75.0° correspond to the (101), (004), (200), (105), (211), (204), and (215) crystal planes of anatase TiO₂(A), respectively (PDF No.21-1272). The characteristic diffraction peaks at 2θ of 28.6°, 29.7°, 33.3°, 43.5°, 44.5°, and 48.5° correspond to the (002), (401), (311), (003), (601), and (020) crystallographic planes of monoclinic TiO₂(B), respectively (PDF No.46-1238). Diffraction peaks of TiO₂(A) and TiO₂(B) were observed in all samples, indicating that the prepared material has a mixed phase of TiO₂(A) and TiO₂(B)^[20]. The characteristic diffraction peaks with 2θ of 31.3°, 36.8°, 59.4°, and 65.2° in the XRD plots correspond to the (220), (311), (511), and (440) crystal planes of Co₃O₄ (PDF No.43-1003), respectively^[21]. Moreover, as the amount of Co₃O₄ increased, the intensity of the characteristic diffraction peaks of the anatase and TiO₂ phases decreased, and the intensity of the characteristic diffraction peaks of Co₃O₄ increased, indicating that Co₃O₄ was loaded onto the TiO₂ nanoribbons.

  Figure 2

  

  Figure 2.  XRD patterns of Co₃O₄/TiO₂ composites with different Co₃O₄ loadings

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  2.2   SEM and TEM

  Fig. 3a and 3d show the TiO₂ and 3.0Co₃O₄/TiO₂ SEM characterization results. It can be seen that both TiO₂ and 3.0Co₃O₄/TiO₂ exhibit nanoribbon structures with widths of 100-200 nm and lengths of 3 μm, and the loading of Co₃O₄ did not change the main structure of TiO₂.

  Figure 3

  

  Figure 3.  (a) SEM images and (b, c) TEM images of TiO₂; (d) SEM images, (e, f) TEM images of 3.0Co₃O₄/TiO₂; (g, h, l) Elements distribution mappings of 3.0Co₃O₄/TiO₂

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  Fig. 3b and 3e are the TEM images of TiO₂ and 3.0Co₃O₄/TiO₂, respectively. Both materials have a banded structure and a comparison between Fig. 3b and 3e shows that the surface of TiO₂ is grown with 10-20 nm Co₃O₄ nanoparticles. The HRTEM images in Fig. 3c and 3f show that TiO₂ and Co₃O₄ have high crystallinity and display clear lattice stripes on the nanocrystal surface. The crystal plane spacing of 0.357, 0.352, and 0.202 nm corresponds to the (110) crystal plane of TiO₂(B), the (101) crystal plane of TiO₂(A), and the (400) crystal plane of Co₃O₄. The distribution of the elements shown in Fig. 3g, 3h, and 3i confirmed the simultaneous presence of Co, Ti, and O elements in the 3.0Co₃O₄/TiO₂ catalyst. The distribution of Co₃O₄ on TiO₂ nanoribbons is conducive to improving the contact between toluene and catalyst, which can shorten the diffusion path of carriers and effectively improve photocatalytic performance.

  2.3   XPS

  To investigate the surface properties of the catalysts, XPS was used to study the surface chemical states of TiO₂ and 3.0Co₃O₄/TiO₂. Fig. 4a shows the XPS spectra of TiO₂ and 3.0Co₃O₄/TiO₂, illustrating that Co₃O₄ has been loaded onto TiO₂. In the XPS spectra of Ti2p of TiO₂ and 3.0Co₃O₄/TiO₂ shown in Fig. 4b, the characteristic double peaks of Ti⁴⁺ centered at 464.3 and 458.6 eV correspond to Ti2p_1/2 and Ti2p_3/2^[22]. Fig. 4c shows the XPS spectra of catalyst O1s with three peaks at 529.7, 530.5, and 532.1 eV corresponding to lattice oxygen (O_β), surface adsorbed oxygen (O_α), and chemisorbed water (O_γ)^[23-24]. The number of adsorbed oxygen atoms on the surface of 3.0Co₃O₄/TiO₂ composites was significantly increased, and the adsorbed oxygen atoms can capture the electrons in the photo-excited electron-hole, separate the photogenerated electron-hole pairs, increase the quantum yield, and improve the photocatalytic performance^[25-26]. The Co2p spectrum of the 3.0Co₃O₄/TiO₂ composite is shown in Fig. 4d, and the two peaks at 780.0 and 795.0 eV in the Co2p spectrum correspond to the surface phase of Co₃O₄^[27]. The peaks at 779.9 and 794.9 eV correspond to Co³⁺. The peaks at 781.9 and 797.1 eV correspond to Co²⁺. The results indicate the successful preparation of Co₃O₄/TiO₂ composites.

  Figure 4

  

  Figure 4.  (a) XPS survey spectra and (b) Ti2p, (c) O1s high-resolution XPS spectra of TiO₂ and 3.0Co₃O₄/TiO₂; (d) Co2p high-resolution XPS spectrum of 3.0Co₃O₄/TiO₂

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  2.4   PL

  To analyze the photogenerated electron and hole separation abilities of the catalysts, PL analysis was performed on TiO₂ and 3.0Co₃O₄/TiO₂. As shown in Fig. 5, the PL spectrum of TiO₂ was more intense than that of 3.0Co₃O₄/TiO₂. It indicates that the loading of Co₃O₄ onto TiO₂ nanoribbons to form Co₃O₄/TiO₂ catalysts is more conducive to the improvement of the separation efficiency of the photogenerated electron-hole pairs and the reduction of the complexation of photogenerated electron-hole pairs. Thus, 3.0Co₃O₄/TiO₂ exhibits better catalytic performance.

  Figure 5

  

  Figure 5.  PL spectra of TiO₂ nanobelt and 3.0Co₃O₄/TiO₂ catalyst

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  2.5   Photocatalytic performance test

  The prepared xCo₃O₄/TiO₂ catalysts were used for the VUV degradation of toluene. In the same degradation time, the degradation rate of toluene by VUV and different catalysts with N₂ as the carrier gas is shown in Fig. 6a. The graph indicates that toluene was degraded, the degradation rate was less than 35.0%. Under VUV, high-energy photons generated by VUV lamps act directly on toluene, breaking the covalent bonds in toluene, and water vapor can combine with electrons in photogenerated electron holes to produce the active substance ·OH, which promotes the degradation of toluene^[28]. However, the degradation rate of toluene by the most effective 3.0Co₃O₄/TiO₂ catalyst was only about 4.4% higher than that of VUV, indicating that the degradation efficiency of toluene by the Co₃O₄/TiO₂ catalyst was insignificant. The synergistic effect of the Co₃O₄/TiO₂ catalyst and VUV was not obvious, and the degradation of toluene mainly depended on the photolysis of VUV.

  Figure 6

  

  Figure 6.  Degradation performance graph of toluene using (a) N₂ and (b) air as carrier gases under VUV; (c) Mineralization rate map of toluene using air as the carrier gas of the samples

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  Fig. 6b and 6c show the degradation and mineralization rates of toluene, respectively, when air was used as the carrier gas. The toluene degradation rate under VUV irradiation was stable at 71.4% and the mineralization rate was 58.3%. The degradation rate of toluene was 82.1% and the mineralization rate was 70.4% after the addition of TiO₂ catalyst, which increased the degradation rate of toluene by 10.7% and the mineralization rate by 12.1%. After adding xCo₃O₄/TiO₂, the degradation rates of toluene were all above 85.0%, with 91.7% toluene degradation and 74.6% mineralization with the addition of the 3.0Co₃O₄/TiO₂ catalysts, which increased the toluene degradation rate by 20.3% and the toluene mineralization rate by 16.3% compared with that of VUV. Under the action of VUV, O₂ can generate O₃, which has an electron affinity energy of 2.1 eV for better capture of photogenerated electrons, and O₃ can react with water vapor to produce more ·OH for toluene degradation and mineralization^[29-30]. In addition, 3.0Co₃O₄/TiO₂ has a higher photogenerated electron-hole separation capacity than TiO₂, which promotes the degradation and mineralization of toluene over the catalysts. Therefore, the synergistic effect of VUV and Co₃O₄/TiO₂ greatly enhanc the degradation and mineralization rates of toluene under simulated air conditions. The synergistic effect of 3.0Co₃O₄/TiO₂ and VUV showed the best degradation and mineralization of toluene, with 91.7% toluene degradation and 74.6% mineralization.

  2.6   Mechanistic analysis of photocatalytic performance

  Based on the results of the above experiments and related literature^{[29, 31-32]}. The possible processes of VUV synergistic Co₃O₄/TiO₂-catalyzed oxidation of toluene are as follows: firstly, in the photolysis process, under VUV light irradiation, water is decomposed to produce ·OH; O₂ is decomposed to produce triplet oxygen (³O₂) and singlet oxygen (¹O₂), which in turn generate O₃ and ·OH with O₂ and water. The corresponding process is shown in equation 3-6.

  $ \mathrm{H}_2 \mathrm{O}+h \nu \rightarrow \mathrm{H}+\cdot \mathrm{OH} $

  (3)

  $ \mathrm{O}_2+h \nu \rightarrow{ }^3 \mathrm{O}_2+{ }^1 \mathrm{O}_2 $

  (4)

  $ { }^3 \mathrm{O}_2+\mathrm{O}_2 \rightarrow \mathrm{O}_3 $

  (5)

  $ { }^1 \mathrm{O}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \cdot \mathrm{OH} $

  (6)

  Next is the photocatalytic oxidation process (equation 7-10). Photogenerated electron-hole pairs are generated on TiO₂ semiconductors under VUV irradiation, the electrons from TiO₂ are transferred to the conduction band of Co₃O₄, O₃ accepts the electrons to produce ·O₃^- groups that effectively inhibit electron-hole recombination, and the ·O₃^- groups combine with water vapor to produce ·OH^[33-34]. The combination of cavities in TiO₂ and water vapor produces ·OH^[35-36]. Therefore, the toluene degradation and mineralization rates are effectively enhanced under the strong synergistic influence of VUV and Co₃O₄/TiO₂.

  $ \mathrm{TiO}_2+h \nu \rightarrow \mathrm{e}^{-}+\mathrm{h}^{+} $

  (7)

  $ \mathrm{h}^{+}+\mathrm{H}_2 \mathrm{O} \rightarrow \cdot \mathrm{OH}+\mathrm{H}^{+} $

  (8)

  $ \mathrm{O}_3+\mathrm{e}^{-} \rightarrow \cdot \mathrm{O}_3^{-} $

  (9)

  $ \mathrm{O}_3^{-}+\mathrm{H}_2 \mathrm{O} \rightarrow \cdot \mathrm{OH}^{+}+\mathrm{OH}^{-}+\mathrm{O}_2 $

  (10)

  3.   Conclusions

  Co₃O₄/TiO₂ nanoribbon catalysts with different Co₃O₄ loadings were prepared by the hydrothermal method. Toluene was used as a pollutant to explore the synergistic effect of VUV and Co₃O₄/TiO₂. It was shown that the Co₃O₄/TiO₂ catalyst with 6.0% Co₃O₄ loading had a high synergistic effect with VUV, the degradation rate of toluene was increased from 71.4% to 91.7%, and the mineralization rate was increased from 58.3% to 74.6%, which has a better prospect of application for gaseous pollutant treatment.

  Disclosure statement: No potential conflict of interest was reported by the authors.

  
  
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  Figure 1  Diagram of catalyst performance evaluation device

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  Figure 2  XRD patterns of Co₃O₄/TiO₂ composites with different Co₃O₄ loadings

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  Figure 3  (a) SEM images and (b, c) TEM images of TiO₂; (d) SEM images, (e, f) TEM images of 3.0Co₃O₄/TiO₂; (g, h, l) Elements distribution mappings of 3.0Co₃O₄/TiO₂

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  Figure 4  (a) XPS survey spectra and (b) Ti2p, (c) O1s high-resolution XPS spectra of TiO₂ and 3.0Co₃O₄/TiO₂; (d) Co2p high-resolution XPS spectrum of 3.0Co₃O₄/TiO₂

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  Figure 5  PL spectra of TiO₂ nanobelt and 3.0Co₃O₄/TiO₂ catalyst

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  Figure 6  Degradation performance graph of toluene using (a) N₂ and (b) air as carrier gases under VUV; (c) Mineralization rate map of toluene using air as the carrier gas of the samples

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