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  Deep desulfurization of diesel has become an important research topic in petroleum refining industry due to the heightened interest for cleaner air and increased stringent environmental regulations for fuel sulfur concentration worldwide.

  Traditional hydrodesulfurization (HDS) is indeed efficient in removing thiols, sulfides, and disulfides from crude oils, however, it is difficult to reduce refractory sulfur-containing compounds such as dibenzothiophene (DBT) and its alkylated derivatives. In order to achieve the goal of ultra-deep desulfurization by the existing hydrotreating technology, the catalytic process need to be operated at elevated Temperatures ( > 300 ℃) and high H₂ pressures (2-10 MPa), which becomes more expensive and less efficient as it woulc require a much larger catalyst bed, higher temperature and pressure, and more H₂ consumption^[1].

  To these problems, the development of alternative non-hydrotreating approaches for more effective ultra-deep desulfurization has become essential. Some new approaches such as selective adsorption, bio-desulfurization, oxidative desulfurization and ionic liquid extraction have attracted much interests^[2-4]. Among them, photocatalytic oxidative desulfurization (PODS) is considered to be one of the most promising desulfurization methods because of its mild reaction conditions and high efficiency. In PODS, refractory sulfur compounds are oxidized to their corresponding polar compounds (sulfoxides or sulfones), which can be easily removed from the fuel by solvent extraction or adsorption.

  Recently, many researchers have investigated the desulfurization of aromatic sulfur compounds by photocatalytic technology^[5].Wang et al^[6] prepared TiO₂ in ionic liquid via microwave radiation for PODS and achieved 98.2% of DBT removal from model oil. Zhang^[7] reported that TiO₂ doped with Cu led to significant absorption of visible light and showed better photo-activity compared to undoped TiO₂. Wang et al^[8] prepared the TiO₂/g-C₃N₄ composites and applied in photocatalytic oxidative desulfurization. TiO₂, as a great photocatalyst, has been widely used in the field of photocatalytic degradation of organic pollutants. It should be mentioned that part of photocatalytic efficiency are limited by nature of TiO₂, for example, the relatively low surface area and easy recombination of the electron-hole pair. More recently, researchers focus their efforts on the development of some new non-TiO₂ photocatalyst which have a low band gaps. For example, BiVO₄^[9], Bi₂WO₆^[10] and BiOBr^[11] have been reported to be applied in the photo-degradation of organic pollutants under visible light irradiation.

  Among them, BiOBr with a band gap of 2.75 eV that allows direct photo-activation under visible light is attractive because of its low toxicity, low cost and high stability. Different doping chemicals have been reported for example, C₃N₄, Bi₂WO₆ and AgBr, in order to further improve the photocatalytic activity.

  Graphene, a two-dimensional (2D) nanocarbon has proved to be an exceptionally promising and versatile building block for the design of useful devises and materials due to its excellent electronic, mechanical and thermal properties, high specific surface areas. In addition, enhanced photocatalytic activities have also been observed using graphene-based composite materials^[12].

  Therefore, recently a BiOBr-graphene combined photocatalysts has been prepared and the photocatalytic performance was shown^[13]. The wrapping of semiconductor materials with graphene allows for an increased contact between graphene and the photocatalyst, thus improve the charge transport characteristic^[14-17]. However, to date the application of a BiOBr-graphene combined photocatalysts for oxidative removal of DBT has not been reported. The present work focused on the structural and catalytic characterization of BiOBr-graphene photocatalysts for the photo-catalytic oxidation of sulfur species in model oil. DBT was used as model aromatic sulfur compounds. The photocatalysts were characterized by XRD, SEM, TEM, Raman, PL and UV-vis DRS. The optimal conditions were studied by varying the reaction temperature, the amount of hydrogen peroxide and graphene loading.

  1   Experimental

  1.1   Reagents and synthesis method

  All chemicals were of analytical grade and were used as received without further purification. The model oil was prepared by dissolving (10^-4-4×10^-4) of dibenzothiophene (DBT, 98%) in cyclohexane (99%). The RGO was first prepared with 0.6 g/L GO following a modified Hummers' methods^[18]. Different amounts of BiOBr are deposited on RGO via a hydrothermal method. In a typical synthesis, 1 mmol of Bi (NO₃)₃·5H₂O was dissolved in 30 mL of aqueous solution with 5 mL of acetic-acid under stirring for 0.5 h. Then 1.2 mmol of KBr and an appropriate amount of GO were added to the above solution and under stirring for 30 min. After that, the resulting precursor solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated to 160 ℃ for 12 h. The obtained precipitated was washed three times with deionized water and dried in oven at 120 ℃ for 12 h. Pure BiOBr was prepared following the same procedure as mentioned above, however, without the addition of GO. The obtained BiOBr-graphene samples were denoted as BRG1, BRG2, and BRG3, where 1, 2, 3 represents the different weight ratios of RGO.

  1.2   Characterizations

  SEM were collected with HITACHI S-4800. TEM were performed on JEM-2100. XRD analysis was performed using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation operated at 30 mA and 40 kV at a scanning range of 10°-80° following Joint Committee on Powder Diffraction Standards (JCPDS).Raman spectrum was acquired with a Dilor XY microspectrometer using a 532 nm excitation wavelength. The UV-vis diffusion reflectance spectra of the samples were obtained on a UV-vis spectrophotometer (UV-3600, Shimadzu, Japan). PL spectrum was measured using a Hitachi F-7000 fluorescence spectrophotometer at room temperature.

  1.3   Photocatalytic oxidative desulfurization (PODS) test

  The catalytic measurements were carried out in a self-build PODS reactor equiped with a 150 W Xenon lamp as the visible light source. Typically 1 g photocatalyst was mixed with 100 mL model fuel (sulfur content of 500 μg/g) in the PODS reactor. The mixture was subsequently kept in the dark for 30 min and with constant stirring to establish an adsorption-desorption equilibrium. The treated model oil was sampled every 20 min. The oxidized sample fuels was first centrifuged to remove the solid catalyst and then extracted with N-methyl pyrrolidone (NMP). Subsequently, the total sulfur concentration of product was analyzed by a WK-2D microcoulometric detector, and the sulfur removal efficiency (η) of model oil was calculated by Eq (1):

  Where C₀ and C are the initial and final sulfur concentration of model oil, respectively.

  2   Results and discussion

  2.1   Sample characterization

  2.2   Desulfurization performance

  2.3   Pathways of PODS

  As illustrated in Figure 9, the electrons of BiOBr can migrate from the valence band to the conduction band after being excited, leaving holes in the valence band of BiOBr (Eq.(2)). The graphene was beneficial to the separation of electrons and holes. These electrons can quickly move and react with adsorbed H₂O₂ to produce ·OH radicals (Eq.(3)), and the holes on the surface can react with adsorbed H₂O (Eq.(4)), forming ·OH radicals. With the existence of ·OH, DBT was easily oxidized to corresponding sulfone in the boundary of water and oil phase (Eq.(5)), from the literature^[38], the main products are expected to be DBTO and DBTO2.

  Figure 9.  Illustration of the photocatalytic oxidation of DBT with oxidants and catalyzed by BRG2

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  2.1.4   PL

  In photocatalytic reaction, the activity is largely affected by the recombination of the photo induced electrons and holes, the recombination of photo-induced electrons and holes in the semiconductor could be verified by PL^{[26, 27]}, the higher the PL intensity is, the less efficient carriers participate in the photocatalytic procedure, the spectra of BRG1, BRG2, BRG3 sample and pure BiOBr are shown in Figure 4. The PL spectra intensity of pure BiOBr is found to be much stronger than those of BRG1, BRG2 and BRG3 materials. The intensity of this emission peak decreases with the increased RGO loading, especially, the BRG2 composite exhibits the lowest PL peak intensity, indicating that the well-matched band structure could result in the low recombination rate of photo-induced electrons and holes in BRG2 samples. This result further demonstrates the photo-induced electrons flow from BiOBr to RGO, and the loading amount is favorable for suppressing the recombination of photo-induced hole-electron pairs^[28]. An increasing RGO loading (3%) results in a higher PL intensity than that of BRG1. This could attribute to the fact that the excessive RGO would act as a recombination center and which promotes the combination of electron-hole pairs in RGO, thus lead to the increased PL intensity.

  Figure 4.  PL spectra of obtained samples

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  2.1.5   UV-vis DRS

  The optical absorption of obtained samples was investigated by using a UV-vis spectrometer. As shown in Figure 5, all the samples show significant visible light absorption. The BRG1, BRG2, BRG3 exhibit redshift of band edge in comparison with pure BiOBr, which favors the absorption of visible light, respectively. Such differences may be attributed to the changes of crystallite phase and the size of coupled oxides, defects, and so on^[29].

  Figure 5.  UV-vis DRS spectra of obtained samples

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  In this work, the shift may ascribe to the size effect. In addition, the increase of absorption in the visible light region is due to the introduction of black body properties typical of graphite-like materials, and these results are also consistent with the result obtained by XRD and SEM. Optical absorption property of a photocatalyst is often closely associated with its optical energy gap. The energy band structure feature of a semiconductor is considered as a key factor to determinate its photocatalytic activity^[30]. The band gap energy is calculated by the equation E_g=1 240/λ(eV).The band gap energies are estimated to be 2.85, 280, 2.75 and 2.72 eV for BiOBr, BRG1, BRG2, and BRG3 powders.

  2.1.3   Raman

  The Raman scattering has been used to affirm the presence of graphene and the interaction between BiOBr and graphene.The Raman spectrum of GO (Figure 3) indicates the characteristic D and G bands of graphene at 1 350 and 1 600 cm^-1, respectively^[23]. In addition, no diffraction peak corresponding to the restacking of graphene to form graphite can be observed, suggesting that the agglomeration of graphene sheets during the reduction process can be effectively prevented. In general, the value of I_d/I_g ratio reflects the surface defects density of graphene. The I_d/I_g integral intensity ratio corresponding to BRG1, BRG2 and BRG3 is higher than that of GO^{[24, 25]}, and the results could confirm that the GO was reduced greatly after hydrothermal treatment.

  Figure 3.  Raman spectra of obtained samples

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  2.1.1   XRD

  To identify the change of phase structures before and after hybridization, the XRD patterns of BRG1, BRG2, BRG3 photocatalyst referred to that of pure BiOBr are shown in Figure 1, all the diffraction peaks can be readily indexed to the tetragonal phase BiOBr of (JCPDS card no. 09-0393) for pure BiOBr. No other diffraction peaks are observed, which indicates that RGO does not alter the crystal structure of BiOBr. Compared to pure BiOBr the RGO doped BRG1, BRG2 and BRG3 samples showed a weakened featured peaks of tetragonal phase BiOBr, suggesting the crystallinity of BiOBr decreased after the insertation of RGO. The typical diffraction peak of RGO could not be observed which could be attributed to the low loading amount and relatively low diffraction intensity of the RGO^{[11, 19, 20]}. And it can also be inferred that RGO dispersed uniformly on the surface of BiOBr, which exhibits a short-range order of RGO arrangement.

  Figure 1.  XRD patterns of obtained samples

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  2.1.2   SEM and TEM

  The morphology of the hybrid material and pure BiOBr is studied by scanning electron microscopy. As shown in Figure 2(a), the pure BiOBr sample displays a flower-like nanostructure and the particle size is around 5 μm. These micro-flowers is structured with some nanosheets with the thickness around 20 nm. After the introduction of RGO, it is found the micro-flowers structure is transformed into some plate nanosheets. From Figure 2(b)-(d) we can see the increased RGO content shows negligible effect on the plate nanosheets structure. The average size of RGO doped BiOBr nanosheets aggregations is about 50 nm, which is bigger than pure BiOBr sample, and these observations are consistent with the result obtained from XRD. It provides the evidence that the introduction of RGO would change the crystallization of BiOBr, as a result the original micro-flowers nano-structure of BiOBr was destroyed^{[21, 22]}.

  Figure 2.  SEM images of obtained samples

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  TEM images of the BRG2 sample in Figure 2(e) clearly show the combination of BiOBr microspheres with graphene sheets. These results suggest that the GO sheets can be selectively loaded on BiOBr without free dispersion in solution in the synthesis process.

  2.2.1   Selection of catalyst

  BiOBr with different amount of RGO were investigated to optimize photocatalyst. It is obvious that the activity of catalyst is associated with the loading of RGO. The lower catalytic activity is obtained by small loading amount. However, increasing the loading amount too much, the catalytic activity is always declined because of accumulation of the active center. Figure 6 shows the photocatalytic oxidation of DBT under visible light. It is found that BRG1, BRG2, and BRG3 show better photocatalytic activities than that of pure BiOBr, indicating that doping RGO played an important role in the enhancement of photocatalytic activity. This can be explained that RGO suppressed the recombination of photogenerated electron-hole pairs and provided more active sites for the degradation reaction of DBT^[31], especially, the BRG2 has the highest photocatalytic activity^[32], which is confirmed by the 98.5% of desulfurization rates of DBT by BRG2, indicating 2% RGO weight content is enough to provide active species for the PODS process of DBT. Therefore, BRG2 was selected for the following discussion of the catalytic conditions^[33].

  Figure 6.  Effect of different catalysts on desulfurization performance

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  2.2.2   Influence of reaction temperature on desulfurization performance

  The reaction temperature is an important factor for the removal of sulfur compounds in the process of PODS. The effect of temperature on the catalytic activity of BRG2 was shown in Table 1, it is found that the DBT removal at 40 ℃ was 78.8% after 2 h. where as an almost complete removal of DBT could be achieved by increasing the reaction temperature to 60 ℃. It is likely that the sulfur removal efficiency decreased with temperature increasing from 60 ℃ to 70 ℃^[34-36]. Therefore, the PODS process is further investigated at the reaction temperature 60 ℃.

  Table 1.  Effect of reaction temperature on desulfurization performance

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  Reaction temperature t/℃	Sulfur removal efficiency η/%
  40	78.8
  50	90.4
  60	98.5
  70	95.2
  V (model oil)=20 mL, m (BRG2)=0.2 g, O/S (mol ratio)=6, t=2 h

  Table 1.  Effect of reaction temperature on desulfurization performance

  2.2.4   Influence of the feature of the substrates on desulfurization performance

  Although DBT was a representative sulfur compound in the study of desulfurization, BT and 4, 6-DMDBT are also mainly sulfur compounds in diesel, so we choose BT, DBT and 4, 6-DMDBT as model compounds.

  For the typical PODS reaction, catalytic performances of BG2 catalyst in photocatalytic oxidative removal of BT, DBT and 4, 6-DMDBT are shown in Figure 8, the removal trend of sulfur compounds in decreasing order of DBT 4, 6-DMDBT BT, the total S removal achieved is 85.5%, 92.2% and 98.5% of BT, 4, 6-DMDBT and DBT after 2 h. These results can be attributed to two factors: the electron densities on the sulfur atoms and steric strain in the reaction products. The electron densities on the sulfur atoms are 5.739 for BT, 5.758 for DBT, 5.760 for 4, 6-DMDBT^[39]. BT exhibits the lowest reactivity for its lowest electron density, for DBT and 4, 6-DMDBT, the difference in the electron density is very small, so the steric hindrance of the methyl groups determines the reactivity, and 4, 6-DMDBT has a lower reactivity for the two extra methyl groups, which make it difficult for 4, 6-DMDBT to be contact with catalyst.

  Figure 8.  Effect of different S-compounds on desulfurization performance

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  2.2.3   Influence of the amount of H₂O₂ on desulfurization performance

  The concentration of H₂O₂ is an important factor in the DBT oxidation process. As can be seen from Figure 7, when the O/S mol ratio rose from 5 to 6, the desulfurization rate increased. When the O/S mol ratio further increased, the sulfur removal efficiency decreases^[37].

  Figure 7.  Effect of different O/S mol ratioon desulfurization performance

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  This process is mainly about that hydrogen peroxide gets an electron and produces OH· in the initial stage of photocatalytic reaction, and OH· oxides the sulfur compound. However, the excessive amount of hydrogen peroxide may hinder the absorption of the reactants on the photocatalyst surface. The O/S mol ratio of 6 was selected for further studies. These findings demonstrate that not all hydrogen peroxide introduced is utilized and the remaining portion is decomposed with the evolution of oxygen gas^[38].

  3   Conclusions

  In summary, BiOBr-graphene has been successfully prepared by hydrothermal treatment. The catalytic activity of BiOBr-graphene for the oxidative desulfurization of DBT using H₂O₂ as oxidant was investigated. The photocatalytic activity of the composite strongly depends on the amount of RGO and the composite prepared with 2% RGO showed the highest photocatalytic activity. The enhanced photocatalytic activity can be attributed to the increased light absorption and more effective charge transportation and separation arisen from the strong chemical bonding between BiOBr and RGO.

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  Figure 1  XRD patterns of obtained samples

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  Figure 2  SEM images of obtained samples

  (a): BiOBr; (b): BRG1; (c): BRG2; (d): BRG3; (e): TEM image of BRG2

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  Figure 3  Raman spectra of obtained samples

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  Figure 4  PL spectra of obtained samples

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  Figure 5  UV-vis DRS spectra of obtained samples

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  Figure 6  Effect of different catalysts on desulfurization performance

  V (model oil)=20 mL, m (catalyst)=0.2 g, O/S (mol ratio)=6, t=60 ℃

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  Figure 7  Effect of different O/S mol ratioon desulfurization performance

  V (model oil)=20 mL, m (BRG2)=0.2 g, t=30 ℃, ■: O/S (mol ratio)=5:1, ●: O/S (mol ratio)=6:1, ▲: O/S (mol ratio)=7:1

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  Figure 8  Effect of different S-compounds on desulfurization performance

  V (model oil)=20 mL, m (BRG2)=0.2 g, O/S (mol ratio)=6, t=60 ℃, t=2 h

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  Figure 9  Illustration of the photocatalytic oxidation of DBT with oxidants and catalyzed by BRG2

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  Table 1.  Effect of reaction temperature on desulfurization performance

  Reaction temperature t/℃	Sulfur removal efficiency η/%
  40	78.8
  50	90.4
  60	98.5
  70	95.2
  V (model oil)=20 mL, m (BRG2)=0.2 g, O/S (mol ratio)=6, t=2 h

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