Mesoporous Al-MCM-41@Ag/TiO2 nanocomposites were synthesized successfully by combining the sol-gel method and hydrothermal treatment, using titanium isopropoxide (TTIP), AgNO3, and Vietnamese bentonite as precursors of Ti, Ag, and Si, respectively. The synthesized materials were well characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption-desorption isotherm measurements, energy dispersive X-ray spectroscopy (EDX), UV-visible diffuse reflectance spectroscopy (UV-Vis/DRS), and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity was evaluated by the photodegradation of dibenzothiophene (DBT) under both UV and visible light irradiation. MCM-41@Ag/TiO2 catalyst exhibited high catalytic activity for the oxidative desulfurization (ODS) of DBT reaching almost 100% conversions at 50°C after 2 h under UV and visible light irradiations. The significant enhanced degradation of DBT over Al-MCM-41@Ag/TiO2 might be due to the synergy effects of high surface area of MCM-41, well-distributed TiO2 anatase, and reduced electron-hole recombination rates due to the dispersion of Ag nanoparticles.
Dibenzothiophene presenting in diesel is one of the main sulfur-containing organic pollutants in fuel oils and is difficult to be removed [
The incorporation and doping of noble metal nanoparticles (e.g., Ag, Au, and Pt) into the crystal lattice of TiO2 (metal-TiO2) to absorb the abundant visible light due to their surface plasmon resonance (SPR) have been investigated to overcome this limitation in the TiO2 photocatalyst material [
Dibenzothiophene (DBT), triblock Pluronic F127 (EO106PO70EO106, 99 wt%,
Firstly, Si and Al precursors were obtained following the protocol described by Ali-dahmane et al. [
In a typical Al-MCM-41 synthesis, 1.2 g CTAB was added to 20 mL distilled water. Then, 42 mL of the supernatant was added to the above solution and stirred at room temperature for 6 h at pH of about 9-10, which was adjusted by acetic acid. The crystallization step was carried out at 100°C in a stainless steel autoclave for 24 hours. The white precipitate was then filtered and washed three times with distilled water and ethanol and dried overnight at 100°C. Finally, the template CTAB was removed by calcining at 550°C for 6 h in air.
Al-MCM-41@nAg/TiO2 nanocomposite microspheres were synthesized by a sol-gel method. 0.1 g of the synthesized Al-MCM-41 was dispersed in 100 mL ethanol via sonication for 1 h. Then, 0.2 g F127 and 2 mL distilled water were added to the above solution. The mixture was stirred at 50°C for 30 minutes.
A TiO2 precursor (solution A) was prepared by dissolving titanium(IV) isopropoxide (TTIP) in a mixed solvent of ethanol and nitric acid to form a final composition of 1 TTIP : C2H5OH : 2 H2O : 0.2 HNO3. Solution B was prepared by adding
Dibenzothiophene was dissolved in
Powder X-ray diffraction (XRD) patterns were recorded on a D8-Advance Bruker with Cu-K
Wide-angle XRD patterns of Al-MCM-41, Al-MCM-41@TiO2, and Al-MCM-41@Ag/TiO2 with different Ag concentrations are shown in Figure
XRD patterns of (a) Al-MCM-41@0.15Ag/TiO2, (b) Al-MCM-41@0.1Ag/TiO2, (c) Al-MCM-41@0.05Ag/TiO2, (d) Al-MCM-41@TiO2, and (e) Al-MCM-41. Inset is the small angle X-ray diffraction patterns.
Figure
SEM images of Al-MCM-41 and Al-MCM-41@0.1Ag/TiO2 are shown in Figure
SEM images of (a) Al-MCM-41 and (b) Al-MCM-41@0.1Ag/TiO2.
Figures
TEM images of Al-MCM-41@0.1Ag/TiO2.
The UV-Vis diffuse reflectance spectra of Al-MCM-41, Al-MCM-41@TiO2, and Al-MCM-41@nAg/TiO2 composites in the range of 250–800 nm are shown in Figure
UV-Vis diffuse reflectance spectra of Al-MCM-41@0.15Ag/TiO2, Al-MCM-41@0.1Ag/TiO2, Al-MCM-41@0.05Ag/TiO2, Al-MCM-41@TiO2, and Al-MCM-41.
The EDX of Al-MCM-41@0.1Ag/TiO2 (Figure
EDX spectrum of Al-MCM-41@0.1Ag/TiO2.
The N2 adsorption-desorption isotherms of Al-MCM-41 and Al-MCM-41@0.1Ag/TiO2 were typical for type IV, with a hysteresis loop characteristic of mesoporous materials (Figure
N2 adsorption-desorption isotherms of (a) Al-MCM-41 and (b) Al-MCM-41@0.1Ag/TiO2.
XPS analysis was carried out to analyze the surface composition and chemical states of Al-MCM-41@0.1Ag/TiO2 (Figure
XPS spectra of (a) Al-MCM-41@0.1Ag/TiO2, (b) Ag3d, and (c) Ti2p.
The photocatalytic performance of Al-MCM-41@Ag/TiO2 with different Ag loadings was evaluated by the oxidative desulfurization of DBT in the model fuel under the visible light source in 30 minutes at 70°C. The photocatalyst with low Ag loading (0.5wt% Ag/TiO2) exhibited a weak performance (conversion of DBT∼48%) due to the low concentration of active catalytic sites, whereas the highest efficiency was obtained for Al-MCM-41@0.1Ag/TiO2 (78% DBT conversion). Due to more silver decorated on the surface of TiO2 for Al-MCM-41@0.15Ag/TiO2 compared with Al-MCM-41@0.1Ag/TiO2, although they have almost the same band gaps, the overlapping of the plasmonic field region makes the photocatalytic activity of Al-MCM-41@0.15Ag/TiO2 mesoporous structure decline (conversion of DBT is 65%). However, overloading of Ag will reduce the amount of available active sites due to the spatial charge repulsion, thereby affecting the photoactivity. Hence, Al-MCM-41@0.1Ag/TiO2 as a photocatalyst has the best photocatalytic activity due to its suitable plasma resonance band, narrow band gap, and available active sites, which is also consistent with the literature [
For comparison, the degradation of DBT over Al-MCM-41@TiO2 by the irradiation of UV and visible light was carried out. At 70°C and after 2 hours, the Al-MCM-41@TiO2 photocatalyst degraded only about 40% of DBT under UV light (Figure
Conversion of DBT over Al-MCM-41@TiO2 and Al-MCM-41@0.1Ag/TiO2 under UV/visible light irradiation. Experimental conditions:
The effect of temperature on the oxidative desulfurization of DBT during UV and visible irradiations are shown in Figures
Conversion of DBT under UV at various reaction temperatures. Experimental conditions:
Conversion of DBT under visible irradiation at various reaction temperatures. Experimental conditions:
After 2 h at the reaction temperature slightly above room temperature (30°C), the deep desulfurization could be achieved with 89% and 81% conversions under UV and visible light irradiation, respectively. This was attributed to the formation of conduction band (CB) electrons (e−) and valence band (VB) holes (h+) under irradiation. This indicates that the visible light absorption of TiO2 samples was considerably improved by adding Ag and Al-MCM-41 to TiO2.
Al-MCM-41 has a uniform pore structure and high surface area, which facilitates the high adsorption of DBT. The Ag nanoparticles were photoexcited to enable the generation of electron and Ag+ (h+) due to the surface plasmon resonance effect, and the photoexcited electrons can be further introduced into the conduction band of TiO2 (Equation (
Species •OH and O2•− obtained in the presence of the photocatalyst and H2O2 as the oxidant under irradiation could effectively oxidize DBT to its corresponding sulfone [
In summary, the Al-MCM-41@Ag/TiO2 nanocomposites have been successfully synthesized using the Vietnamese bentonite as Si and Al sources and well characterized by various analytical techniques. Al-MCM-41@Ag/TiO2 composites exhibited much higher photocatalytic activities for degrading DBT under visible light irradiation than Al-MCM-41@TiO2, and Al-MCM-41@0.1Ag/TiO2 was found to have the best photocatalytic performance. Incorporating Ag and TiO2 into Al-MCM-41 substrate had positive effects on the photocatalytic activity of the TiO2, under both visible light and UV irradiations. At a relatively mild condition of 30°C, DBT degraded 90% under UV light irradiation and 81% under visible light after 2h. At higher temperature (70°C), the DBT photooxidative desulfurization efficiency of 100% could be achieved after 2 hours under visible light. The Ag nanoparticle dispersed on Al-MCM-41@TiO2 nanocomposites has shown its superiority and the potential as a promising material for the removal of toxic organic pollutants either in the UV or visible light region.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED)(grant number 105.99-2015.21).
The results of the HPLC analysis of the conversion of DBT were investigated over time at various reaction temperatures under UV and visible irradiation conditions using Al-MCM41@0.1Ag/TiO2 and Al-MCM-41@TiO2 catalysts.