Introduction

Sulfur elimination from fluidized catalytic cracker naphtha to the desirable restriction is a crucial issue in the refining industry. One of the major components in gasoline pool is The FCC naphtha. Thiophenes are considered the preeminent sulfur-incorporating compounds in FCC naphtha. Thiophenes are inert relative to other sulfur-containing compounds since they are aromatic. Hydrodesulfurization process is one of the most important methods that have been utilized in removal of thiophenes (Teng-fei et al. 2015; Boukoberine and Hamada 2016; Kabe et al. 1992; Gates and Topsoe 1997; Ma et al. 1994, 1995; Olguin-Orozco et al. 1997; Kilanowski et al. 1978). The drawbacks of this method are its high cost besides, affecting the octane number of gasoline. Many alternative efficient and economical methods have been examined, for thiophene elimination, to overcome these drawbacks such as pervaporation method (Lin et al. 2009; Bettermann and Staudt 2009; Qi et al. 2006; Jain et al. 2015, 2016). The superiority of pervaporation method is the relatively lower demands of temperature and pressure compared to those in hydrodesulfurization method and it was concluded that thiophene could be removed successfully from FCC gasoline up to any desirable limit via the pervaporation method. Other successful alternative techniques were investigated too for thiophene elimination. For instance, reactive adsorption using solid adsorbents and H2 (Song 2003), selective adsorption in absence of H2 at ambient temperature (Ma et al. 2001, 2002a, b; Velu et al. 2002; 2003a, b; Qi et al. 2015), hydrodesulfurization accompanied with distillation (Rock 2002; Rock and Shorey 2003), absorption using ionic liquids (Bösmann et al. 2001; Zhang and Zhang 2002; Mafi et al. 2016). Although there are broad scale applications of these processes they are very costly. Recently, different polymeric materials such as polyhedral oligomeric silsesquioxane (POSS), polyvinylidene fluoride (PVDF), polyimide (PI), polyethylene glycol (PEG), etc. were handled as membranes for thiophene withdrawal from gasoline (Yu et al. 2015; Konietzny et al. 2014; Amaral et al. 2014; Qu et al. 2010; Chen et al. 2008; Zhao et al. 2008; Jain et al. 2017; Yang et al. 2014; Liu et al. 2014a, b; Lin et al. 2012, 2014; Yang et al. 2013). In modern trends, extraction desulfurization has been devoted to be the most effective and suitable method as a result of its mild operational conditions of temperature and pressure without any hydrogen consumption (Mokhtarani et al. 2014). Recently, there has been an extensive significance in heterogeneous photocatalysis adopting semiconductors for the pollutant removal. The main advantage of photocatalysis is that it provides a more environmentally sustainable solution for pollutant removal without any need for further treatment. Thiophene destruction over polyaniline/mesoporous Cu2O nanocomposites was investigated (Mohamed and Aazam 2014). It was found that the semiconductor/conductive polymer composite had large photocatalytic activity under visible light. TiO2 was doped with Ag and the resulted photocatalyst was supported on MWCNTs, and then Ag–TiO2/MWCNT photocatalysts were used to degrade thiophene by photocatalysis under visible light irradiation in an aqueous solution and it was found that 0.02:1.0:0.05 was the optimum mass ratio of MWCNT:TiO2:Ag, which lead to about 100% photocatalyst’s experimental performance for thiophene oxidation in a 600 mg/l solution within 30 min (Aazam 2014). Other researchers studied the photooxidation of thiophene using different materials as, RuO2/SO2–TiO2 (Lina et al. 2016), NiO/AgInS2 nanoparticles (Baeissa 2014), Pd/ZrO2–chitosan nanocomposite (Abdelaala and Mohamed 2014), Ag–BiVO4 (Gao et al. 2013), TiO2/Cr-MCM-41 (Marques et al. 2008), TiO2 (Dedual et al. 2014), TiO2/Ni-ZSM-5 (Wang et al. 2013). Pt/PbS nanoparticles (Mohamed and Aazam 2014), MoO3/γ-Al2O3 (Xue et al. 2017), titania/MWCNT composite (Barmala et al. 2015). The notable and wonderful characteristics involving dielectric, electrical and optical properties of Barium stannate (Zhang et al. 2007; Mizoguchi et al. 2004), makes it one of the most significant materials to be used as photocatalyst, catalyst support, solar cell and capacitor, etc. (Wang et al. 2014; Shin et al. 2013; Cerda et al. 2002). A new simple coprecipitation method was applied to prepare nanocrystalline barium stannate (Moshtaghi et al. 2016). Many other attempts had been made to prepare barium stannate (Upadhyay et al. 1997; Reddy et al. 2001; Upadhyay 2013; Ihlefeld et al. 2008). Doped barium stannate could be prepared via different methods (Ansaree and Upadhyay 2015; Bévillon et al. 2008; Kumar et al. 2007; Singh et al. 2005; Wang et al. 2007). Indium is elected to be the dopant for different metal oxide nanostructures; this selection is attributed to its ability to magnify the photocatalytic activity properties of these metal oxides. Different morphologies of indium-doped ZnO nanostructures have been prepared via solvothermal method (Rezapourn and Talebian 2014). Indium-doped titania particles in a nano-scale were prepared via a sol–gel method (Tahir and Amin 2015). In addition, indium doping of different materials were prepared by various methods (Singh et al. 2010; Chava and Kang 2017; Feng et al. 2016; Kumar et al. 1999; Nishio et al. 2006; Saquib et al. 2008; Yang et al. 2018). In this project, we address the preparation of a novel In/BaSnO3 nanocomposite through sol–gel method and we apply this material for thiophene degradation.

Experimental techniques

Synthesis of BaSnO3

Chemicals in this study were used without further purification as they are of analytical grade. 1 mmol of tin(IV) isopropoxide solution and 1 mmol of barium nitrate were dissolved in a mixture containing 30 ml of ethanol, 20 ml of de-ionized water and 0.05 mmol of HNO3. The resultant mixture was stirred for 90 min, then added to glass vessel and kept in a microwave at 180 °C for 20 min. After that, the product was gathered via centrifugation and then washed many times using absolute ethanol and de-ionized water. The delivered sample was left to dry at about 80 °C overnight and air-heated at 400 °C for 1 h. The produced specimen was titled BaSnO3.

Synthesis of In/BaSnO3

The synthesized BaSnO3 nanorods were added to 20 ml of distilled water containing suitable amount of indium(III) nitrate hydrate. The suspension was stirred and irradiated overnight by strong UV lamp. The resultant material was gathered and heated at 140 °C for 1 h in air. In was permitted to be doped within BaSnO3 structure with the ratios of 0.1, 0.2, 0.3, and 0.4% wt, and the products were named 0.1 wt% In/BaSnO3, 0.2 wt% In/BaSnO3, 0.3 wt% In/BaSnO3 and 0.4 wt% In/BaSnO3, respectively.

Identification techniques

X-ray diffracto-grams of both BaSnO3 and In–BaSnO3 nanocomposites were measured using X-ray diffraction (XRD) analysis via Cu-Kα radiation (λ = 1.540 Å). A Nova-2000 instrument was adopted for specific surface area determination of the synthesized BaSnO3 and In–BaSnO3 nanocomposites through N2-adsorption at 77 K. Before each measurement, specimens were heated at about 250 °C for 4 h to remove gases from these specimens. Band gap energies corresponding to the synthesized BaSnO3 as well as In–BaSnO3 nanocomposites were measured applying a spectro-photometer (V-570, JASCO, Japan) via determination of UV–Visible diffuse reflectance spectra (UV–Vis-DRS) in air at ambient temperature within the wavelength range of 200 up to 800 nm. Morphology and microstructure of the prepared nanocomposites were investigated using scanning electron microscopy (JEOL-JEM-5410). The elemental analysis of the synthesized specimens was attained using X-ray photoelectron spectroscope (XPS) of Thermo Scientific K-ALPHA type, England.

Photocatalytic efficiency

A pyrex reaction cell was handled for thiophene oxidation using the photocatalyst through O2, the oxidant, bubbling in a steady-state flow. 1 g/l photocatalyst was spread in thiophene-containing acetonitrile solution (initial content of sulfur = 6 × 102 ppm). After that, the suspension was agitated in the absence of light for 0.5 h, to attain equilibrium, before being irradiated by a 125-W mercury lamp with a UV cut filter. The reaction solution temperature was then kept at low temperature (12 °C) through cooling water flow. At the end of the reaction and after catalyst separation, the major and minor products were analyzed by GC-FPD (Agilent 7890, FFAP column) and GC–MS.

Results and discussion

Phase composition, morphology and microstructure

XRD diffractograms of both BaSnO3 and In/BaSnO3 nanocomposites are illustrated in Fig. 1. The patterns of Fig. 1 indicate that BaSnO3 and In/BaSnO3 nanocomposites are primarily composed of BaSnO3 phase (JCPDS Card: 15-0780), this result reveals that the BaSnO3 skeleton will persist after indium doping. It is clear from the diffraction pattern of In/BaSnO3 sample that the peak characteristic to indium are absent. The absence of the characteristic peaks of indium in the patterns of In/BaSnO3 sample may be ascribed to the low indium-dopant content. Evidently, the data illustrate that indium is well dispersed within the BaSnO3 lattice. In fact, indium played a prominent aspect in the process of crystallization since the characteristic diffraction peaks of BaSnO3 phase became broader and the diffraction peaks’ intensities became lesser by increasing indium loading.

Fig. 1
figure 1

XRD patterns of BaSnO3 and In/BaSnO3 nanocomposites

Full size image

XPS spectra of In3d for the 0.3 wt% In/BaSnO3 nanocomposite are displayed in Fig. 2. The existence of the peaks committed to the indium–indium at about 443.8 and 451.3 eV for In3d5/2 and In3d3/2, respectively, confirms the formation of indium metal in a nano-sized scale.

Fig. 2
figure 2

XPS spectra of In3d for the 0.3 wt% In/BaSnO3 nanocomposite

Full size image

The SEM micrographs of BaSnO3 and In/BaSnO3 nanocomposites are presented in Fig. 3. The results reveal that as weight percent of indium metal increases, the dispersion on the surface of BaSnO3 nanorods increases and this finding is valid up to 0.3 wt% of indium dopant (Fig. 3a–d). On the contrary, indium is doped as aggregate by increasing weight percent of indium over 0.3 wt% as shown in Fig. 3e.

Fig. 3
figure 3

SEM images of BaSnO3 and In/BaSnO3 nanocomposites, where wt% of In is 0.0 (a); 0.1 (b); 0.2 (c); 0.3 (d); and 0.4 (e)

Full size image

Surface area measurement

Specific surface areas (SBET) of both BaSnO3 and In/BaSnO3 nanocomposites were determined. The surface area of the parent BaSnO3 and In/BaSnO3 nanocomposites are given in Table 1. The SBET values are found to be 45, 43, 41, 39 and 34 m2/g for BaSnO3, 0.1 wt% In/BaSnO3, 0.2 wt% In/BaSnO3, 0.3 wt% In/BaSnO3 and 0.4 wt% In/BaSnO3, respectively. In fact, the bigger characters of the specific surface area of BaSnO3 in comparison to those of the In/BaSnO3 samples reveal that indium doping causes some pores to be blocked.

Table 1 BET surface area of BaSnO3 and In/BaSnO3 nanocomposites
Full size table

Optical characterization

The spectra corresponding to UV–Vis diffuse reflectance of BaSnO3 as well as In/BaSnO3 nano-materials are demonstrated in Fig. 4. The results of Fig. 4 illustrates that the introduction of indium metal into the BaSnO3 lattice causes a shift of spectra towards higher wavelengths from 526 to 653 nm (red shift) by the various percentages of indium metal, comparing to BaSnO3wavelength at nearly 400 nm. The band gaps for both BaSnO3 and In/BaSnO3 nanocomposites were determined from their own spectra of reflection found in the form proposed by Kumar et al. (1999), the band gap characters of the both synthesized nanocomposites are given in Table 2. Evidently, it is obvious from the data of Table 2 that the band gap energy decreases with increasing the weight percentages of the dopant indium. The values of band gap were found to be 3.1, 2.36, 2.16, 1.92 and 1.90 eV for the parent BaSnO3, 0.1 wt% In/BaSnO3, 0.2 wt% In/BaSnO3, 0.3 wt% In/BaSnO3 and 0.4 wt% In/BaSnO3, respectively. This finding illustrates that indium doping enhances the photocatalytic activity of the catalyst via its band gap narrowing.

Fig. 4
figure 4

UV–Vis absorption spectra of BaSnO3 and In/BaSnO3 nanocomposites

Full size image
Table 2 Band gap of BaSnO3 and In/BaSnO3 nanocomposites
Full size table

The deportation of holes and photogenerated electrons was studied via investigating photoluminescence (Pl) emission spectra. The PI emission spectra for the different investigated samples are shown in Fig. 5. It is obvious from the illustrations of Fig. 5 that the intensity of Pl is largely decreased with increasing the indium metal percentage. Moreover, separation of the photogenerated electron–hole couples occurs. This finding might be accredited to the capturing of photogenerated electrons from the CB by indium metal which acts as a trapping center. It is generally acknowledged that an enhancement in light retention of the catalysts in the wave length range of the visible region may occur as a result of the rare metal nanoparticles’ embodiment into catalysts made of semiconductors. And so a deflection of the absorption threshold towards higher values of wavelengths occurs pointing out a reduction in the band gap energy. Consequently, extra photogenerated electrons along with holes will cooperate in the photocatalytic reaction. In the current study, indium implies to vary the interface of BaSnO3 in such a manner that develops the system in which photo-originated charge carriers experience reconsolidation. And so, it will strengthen BaSnO3 to be highly stimulated in the visible region. On the other hand, the displacement in the location of emission could be correlated to the conduction band (CB) of BaSnO3 as a semiconductor and the charge transfer between the indium-generated bands.

Fig. 5
figure 5

Pl spectra of BaSnO3 and In/BaSnO3 nanocomposites

Full size image

Photocatalytic efficiency

Effect of catalyst kind

Figure 6 shows the photocatalytic degradation of thiophene compound over both BaSnO3 and In/BaSnO3 nanocomposites in the wavelength of the visible light. The examination was accomplished using the subsequent settings: 500 mL of thiophene solution having the concentration 600 ppm and 0.4 g/l catalyst. The results confirm the lower activity of BaSnO3 photocatalyst beneath visible light. Moreover, the photocatalytic efficiency of In/BaSnO3 doped with various loadings of In is increased in the following order: 0.1 wt% In/BaSnO3 < 0.2 wt% In/BaSnO3 < 0.3 wt% In/BaSnO3 ≤ 0.4 wt% In/BaSnO3, this finding is in agreement with those found in SEM, XRD, and band gap investigations.

Fig. 6
figure 6

Effect of catalyst type on photocatalytic conversion of thiophene

Full size image

Concerning the investigation of the photoproducts, the gas from the products’ outlet is introduced to 0.2 M NaOH aqueous solution. When 0.2 M Ba(NO3)2 aqueous solution was added into the latter NaOH aqueous solution, a precipitate of white color was produced (designated as precipitate 1). The XRD pattern of precipitate 1 is illustrated in Fig. 7a. The XRD pattern proves the presence of BaCO3, which is in acceptable convenience with the standard card of ICDD-PDF no. 05-0378. This finding ensures that thiophene can be oxidized to CO2 in the presence of photocatalyst and captured in the NaOH aqueous solution. Meanwhile, if HNO3 solution is added to precipitate 1, part of the white precipitate will still remain without dissolving in HNO3 solution, designated as precipitate 2. The XRD diffractogram of precipitate 2 is displayed in Fig. 7b. The data of Fig. 7b indicate the formation of BaSO4, which agrees with the standard card of ICDD-PDF no. 24-1035. This illustrates that the sulfur atom in thiophene can be oxidized to SO3 in the presence of the photocatalyst. In conclusion, thiophene could be readily photocatalytically oxidized to both CO2 and SO3. And so, the photocatalytic degradation of thiophene will be as follows:

$${\text{Thiophene}} + {\text{photo-catalyst}}\, \longrightarrow {\text{CO}}_{ 2} + {\text{SO}}_{ 3} + {\text{H}}_{ 2} {\text{O}}.$$
Fig. 7
figure 7

a XRD pattern of the precipitate 1, b XRD pattern of the precipitate 2

Full size image

Effect of photocatalyst loading

The photocatalyst loading is considered another crucial factor that governs photocatalytic destruction of thiophene solution under Vis light irradiation. In this investigation, 0.3 wt% In/BaSnO3 having loadings ranging from 0.2 up to 1.4 g/l in 600 mg/l thiophene solutions, were operated. The data of Fig. 8 illustrates that the time needed for thiophene oxidation decreases from 150 to 60 min by increasing the catalyst dose from 0.2 up to 0.8 g/l, respectively. On the contrary, further increase of the photocatalyst dose, above 0.8 g/l, leads to a repeated expansion in the reaction time up to 150 min. Actually, the increase of the photocatalyst dose will develop the total number of active centers on the photocatalyst (Nishio et al. 2006). And so, the number of the absorbed photons and thiophene molecules increases. However, at photocatalyst loadings above 0.8 g/l, the time needed to oxidize thiophene is increased due to the rendering of light entrance by the extra load of photocatalyst (Saquib et al. 2008).

Fig. 8
figure 8

Effect of loading of 0.3 wt% In/BaSnO3 on photocatalytic conversion of thiophene

Full size image

Photocatalyst recovery

From the economic point of view, handling the photocatalyst several times is a serious subject. Photocatalytic activity of 0.3 wt% In/BaSnO3 photocatalyst after recycling five times is shown in Fig. 9. The data confirm that the photocatalytic activity remains without change after recycling up to about five times. Hence, recycling and separation of 0.3 wt% In/BaSnO3 photocatalyst could be preceded easily.

Fig. 9
figure 9

Recycling and reusing of 0.3 wt% In/BaSnO3 photocatalysts for photocatalytic conversion of thiophene

Full size image

Conclusion

On the basis of our study, the subsequent conclusions could be stated

  1. 1.

    In/BaSnO3, photocatalyst was profitably synthesized and verified to be a talented catalyst due to its great oxidation capability of pollutants in the wavelength range of visible light region.

  2. 2.

    Weight percentage of doped indium in BaSnO3 controls the red shift phenomenon.

  3. 3.

    In/BaSnO3 with a 0.3 wt% of In performed the greatest catalytic efficiency.

  4. 4.

    The synthesized photocatalyst is considered to be an efficient photocatalytic catalyst towards water disinfection.

  5. 5.

    Optimum conditions in our study were found to be; 0.3 wt% In/BaSnO3, 0.8 g/l photocatalyst, 600 mg/l thiophene solution and these conditions yielded 100% oxidation of thiophene solution after 60 min of irradiation of visible light.

  6. 6.

    It was found that the photocatalyst under investigation remains impressive after about five cycles, which illustrates the talented recovery of the In/BaSnO3 photocatalyst.