The present work aims to synthesize nanoscale well dispersed TiO2/SiO2 and TiO2/Al2O3 nanoparticle photocatalysts via an impregnation method for the removal of methyl orange, which was used as a model compound of organic pollutant in wastewater, from an aqueous medium. Also within this frame work, La and Ce metals were loaded onto the surfaces of TiO2/SiO2 and TiO2/Al2O3 by an impregnation method to enhance the photocatalytic activity of the nanoparticles; the activities and physicochemical properties of the photocatalysts were compared before and after loading of metallic La and Ce. The oxide system was characterized by different techniques, including XRD, UV-Vis spectroscopy, FT-IR spectroscopy, SEM, and EDX spectroscopy. Finally, the optimal conditions to complete the photocatalytic oxidation of methyl orange dye were studied. This work holds promise for the efficient photodegradation of pollutants by nanoparticle photocatalysts.
Methyl orange is widely used as an acid-base indicator, and it has also been employed in chemical, technological, and biomedical industries. Its chemical properties have been widely investigated in aqueous solutions and in water and organic mixed solvents [
All chemicals used in the preparation of TiO2/SiO2, TiO2/Al2O3, La-TiO2/Al2O3, and Ce-TiO2/Al2O3 nanoparticles were analytical grade reagents. The following high purity raw materials were used: Titanium(IV) oxide, anatase, 325 mesh, ≥99% metals basis, TiO2, Sigma-Aldrich Laboratories Supplies [Canada]. Titanium(IV) isopropoxide, 98%, C12H28O4Ti, ACROS Organics. Titanium tetrachloride, TiCl4, Fluka AG Laboratories Supplies. Aluminum oxide, Al2O3, Fluka Laboratories Supplies [Switzerland]. Silica precipitated, SiO2, BDH Laboratories Supplies [Poole, England]. Lanthanum(III) nitrate hexahydrate, La(NO3)3·6H2O, 99.999%, ACROS Organics Laboratories Supplies [New Jersey, USA]. Ammonium cerium(IV) nitrate, 99%, H8CeN8O18, ACROS Organics Laboratories Supplies [New Jersey, USA]. Methyl orange indicator, Pacegrove Limited Laboratories Supplies [UK].
The loading of titanium tetra chloride (TiCl4) or titanium isopropoxide (TIP) into silica (SiO2) or aluminum oxide (Al2O3) was performed by the impregnation method (Img).
Five grams of silica was mixed with a certain amount of titanium isopropoxide (TIP) or titanium tetra chloride (TiCl4). Then, a minimal amount of distilled water was added dropwise to the mixture, under vigorous stirring, to obtain 2.5, 5, 10, and 20 wt.% TiO2, as shown in Table
Compositions of different TiO2/SiO2 samples.
Concentration of TiO2 (wt.%) in TiO2/SiO2 | Source of titanium | Support type |
---|---|---|
2.5 | TiCl4 | SiO2 |
5 | ||
10 | ||
20 | ||
2.5 | TIP | |
5 | ||
10 | ||
20 |
Five grams of aluminum oxide (Al2O3) was mixed with a certain amount of titanium isopropoxide (TIP) or titanium tetra chloride (TiCl4). Then, a minimal amount of distilled water was added dropwise to the mixture, under vigorous stirring, to obtain 2.5, 5, 10, and 20 wt.% TiO2, as shown in Table
Compositions of different TiO2/Al2O3 samples.
Concentration of TiO2 (wt.%) in TiO2/Al2O3 | Source of titanium | Support type |
---|---|---|
2.5 | TiCl4 | Al2O3 |
5 | ||
10 | ||
20 | ||
2.5 | TIP | |
5 | ||
10 | ||
20 |
Lanthanum(III) nitrate hexahydrate, La(NO3)3·6H2O, was used as the source of the La(III) dopant. The detailed procedure for the preparation of 4 dopant concentrations (0.05, 0.1, 0.2, and 0.3 mole ratio of La to Ti) by the impregnation method is as follows. An appropriate amount of La(NO3)3·6H2O was dissolved in a small amount of distilled water. This solution was then added dropwise to 5 g of TiO2/Al2O3 (5 wt.% TiO2) with constant stirring, as shown in Table
Composition of different La-doped samples.
La : Ti mole ratio | wt.% loading |
---|---|
0.05 | 5 |
0.1 | |
0.2 | |
0.3 |
Ammonium cerium(IV) nitrate, (NH4)2Ce(NO3)6, was used as the source of the Ce(IV) dopant. The detailed procedure for the preparation of 4 dopant concentrations (0.05, 0.1, 0.2, and 0.3 mole ratio of Ce to Ti) by the impregnation method is as follows. The appropriate amount of (NH4)2Ce(NO3)6 was dissolved in a small amount of distilled water. This was then added dropwise to 5 g of TiO2/Al2O3 (5 wt.% TiO2) with constant stirring, as shown in Table
Composition of different Ce-doped samples.
Ce : Ti mole ratio | wt.% loading |
---|---|
0.05 | 5 |
0.1 | |
0.2 | |
0.3 |
The synthesized samples were evaluated and characterized by determining photocatalytic activity, particle size, and phase using XRD, UV-Vis, FT-IR, SEM, and EDX. Experimental details are as follows.
The crystalline phase and crystallite size of all catalyst nanoparticle samples were analyzed by X-ray powder diffraction (XRD) using a Rigaku X-ray diffractometer system equipped with a RINT 2000 wide angle Goniometer and Cu K
The concentration of the dye was followed up using UV-Vis spectrophotometer (Thermo Fisher Scientific Evolution 300).
To study the surface chemical structures of all catalyst nanoparticle samples, Fourier transform infrared (FT-IR) spectra were recorded on a PerkinElmer FT-IR instrument 100 series.
Surface morphologies and shapes of all catalyst nanoparticle samples were examined via a field-emission scanning electron microscope (SEM), which was obtained using JEOL JSM-7600F. This system was combined with energy dispersive X-ray spectroscopy for composition and elemental analysis.
The photocatalytic activity of each prepared sample was evaluated by degradation of a pollutant in water. Methyl orange (MO), a common textile dye, was used as the model pollutant. The activity of the photocatalyst was tested by analyzing the decomposition of MO under UV irradiation. The MO solutions varied in concentration from 1 to 20 ppm and were prepared by dissolving MO powder in distilled water: 250 mL of 1 ppm MO solution was added to a Pyrex beaker (600 mL) and 0.5 g of TiO2/SiO2, TiO2/Al2O3, La-TiO2/Al2O3, or Ce-TiO2/Al2O3 was added to the solution. The suspension was magnetically stirred in the dark for various time intervals. An 8 W Philip’s lamp was used as the UV source at a fixed distance of 25 cm from the top of the magnetic stir bar. All components of the experimental set-up were located inside a box, as shown in Figure
Experimental set-up for the photocatalytic reaction.
The concentration of MO solutions was determined by measuring the absorbance at approximately 465 nm using UV-Vis spectrophotometer (Thermo Fisher Scientific Evolution 300, USA).
For catalyst characterization, knowing the catalyst’s texture and phase, as well as its chemical composition, represents an essential minimum. XRD, FT-IR, SEM, EDX, and UV-Vis are basic methods that provide the required information.
The XRD patterns of 5 wt.% loaded samples on Al2O3 only showed the crystalline of corundum phase of the support; there was no indication of any crystalline phase of TiO2. This could be explained by the low detection limit of XRD towards TiO2 or by TiO2 being well dispersed over Al2O3 support (Figure
XRD pattern of 5 wt.% TiO2/Al2O3 (using TIP as a source of TiO2) nominated with
The FT-IR spectra of different doped and undoped samples are shown in Figures
FT-IR spectra of 5 wt.% TiO2/Al2O3 (a) before reaction and (b) after reaction (using TIP as a source of TiO2).
FT-IR spectra of 0.05, 0.1, 0.2, and 0.3 mole ratio La-TiO2/Al2O3 (a) before reaction and (b) after reaction.
FT-IR spectra of 0.05, 0.1, 0.2, and 0.3 mole ratio Ce-TiO2/Al2O3 (a) before reaction and (b) after reaction.
FT-IR spectra of La-doped and undoped samples.
Doping with La or Ce did not change the Ti–O and Al–O bands; however, after the reaction of most samples, a slight change in the positions and the shapes of these peaks was observed. This could be explained by a physical change of the support after reacting.
In Figure
Figures
SEM image of 0.05 mole ratio La-loaded on TiO2/Al2O3.
SEM image of 0.3 mole ratio La-loaded on TiO2/Al2O3.
SEM image of 0.05 mole ratio Ce-loaded on TiO2/Al2O3.
SEM image of 0.3 mole ratio Ce-loaded on TiO2/Al2O3.
SEM image of 5 wt.% TiO2/Al2O3 (using TIP as a source of TiO2).
Table
EDX analysis of selected samples.
wt.% | Al2O3% | TiO2% | La2O3% | CeO2% |
---|---|---|---|---|
La/TIP (0.05) | 99.21061 | 0.789391 | 0 | 0 |
La/TIP (0.3) | 98.41669 | 1.583311 | 0 | 0 |
Ce/TIP (0.05) | 98.98106 | 1.018944 | 0 | 0 |
Ce/TIP (0.3) | 98.18298 | 0 | 0 | 1.817018 |
Ce/TIP (0.3) | 97.56178 | 0 | 0 | 2.438221 |
Ce/TIP (0.3) | 99.25471 | 0.745288 | 0 | 0 |
Ce/TIP (0.3) | 87.76509 | 0 | 0 | 12.23491 |
TIP/Al2O3 (5%) | 97.57552 | 2.424481 | 0 | 0 |
Tables
Theoretical wt.% of loading of different samples.
Theoretical (wt.%) | Al2O3% | TiO2% | La2O3% | CeO2% |
---|---|---|---|---|
La/TIP (0.05) | 95 | 4.537242 | 0.462758 | 0 |
La/TIP (0.3) | 95 | 3.101841 | 1.898159 | 0 |
Ce/TIP (0.05) | 95 | 4.513598 | 0 | 0.486402 |
Ce/TIP (0.3) | 95 | 4.513598 | 0 | 0.486402 |
Ce/TIP (0.3) | 95 | 4.513598 | 0 | 0.486402 |
Ce/TIP (0.3) | 95 | 4.513598 | 0 | 0.486402 |
Ce/TIP (0.3) | 95 | 4.513598 | 0 | 0.486402 |
TIP/Al2O3 (5 |
95 | 5 | 0 | 0 |
Excess surface concentrations of investigated samples.
wt.% | Surface excess concentration | |||
---|---|---|---|---|
Al2O3% | TiO2% | La2O3% | CeO2% | |
La/TIP (0.05) | 4.43222 | −82.602 | −100 | 0 |
La/TIP (0.3) | 3.596515 | −48.9558 | −100 | 0 |
Ce/TIP (0.05) | 4.190585 | −77.425 | −100 | −100 |
Ce/TIP (0.3) | 3.350508 | −100 | −100 | 273.5633 |
Ce/TIP (0.3) | 2.696609 | −100 | −100 | 401.2774 |
Ce/TIP (0.3) | 4.478644 | −83.4879 | −100 | −100 |
Ce/TIP (0.3) | −7.61569 | −100 | −100 | 2415.392 |
TIP/Al2O3 (5%) | 2.711073 | −51.5104 | 0 | 0 |
From these tables it could be observed that the presence of La2O3, as a dopant, facilitated the diffusion of TiO2 into the alumina support, as reflected by the remarkably small amount of TiO2 when compared to values for the theoretical load and those observed for undoped samples. The facile diffusion of TiO2 into alumina appeared to exist even in undoped samples. It is suggested that approximately half of TiO2 diffused into undoped samples when compared to the theoretical loading values. The introduction of La2O3 as a dopant with a 0.05 Ti : La mole ratio resulted in a dramatic increase (30%) of TiO2 diffusion. From the data below, we can conclude that there are two types of active sites: one is predominantly a surface site associated with TiO2, and the other is bulk or diffused TiO2. From Figure
This possible interaction was not observed with XRD, due to the detection limit; however, it is well observed from FT-IR spectra in Figure
In addition, we can discuss the observation of M–O bands in the FT-IR spectra of doped samples (Figure
Returning to the CeO2-doped samples, the EDX analysis showed nearly the same enhancement of TiO2 diffusion at the low doping concentration (0.05 mole ratio), which was not reflected in its catalytic activity. Moreover, by increasing the dopant level (0.3 mole ratio) and through a different spot analysis, CeO2 seemed to aggregate on the surface. This could be explained as the affinity of CeO2 towards alumina being higher than its affinity for TiO2, which is in contrast to La2O3, which first interacts with TiO2.
Figures
Curve of dye concentration (MO) divided by % TiO2/Al2O3 (using TIP as a source of TiO2) versus time during the photocatalytic degradation experiment.
Curve of dye concentration (MO) divided by % TiO2/SiO2 (using TIP as a source of TiO2) versus time during the photocatalytic degradation experiment.
Curve of dye concentration (MO) divided by % TiO2/Al2O3 (using TiCl4 as a source of TiO2) versus time during the photocatalytic degradation experiment.
Curve of dye concentration (MO) divided by % TiO2/SiO2 (using TiCl4 as a source of TiO2) versus time during the photocatalytic degradation experiment.
The effect of different doping levels with La 0.05, 0.1, 0.2, and 0.3 mole ratio compared to 5 wt.% TiO2/Al2O3 (using TIP as a source of TiO2).
For Al2O3 support it could be concluded that the 5 wt.% loading was the optimum loading with respect to catalytic activity and the decomposition rate of the organic dye.
Figure
Moreover, Figure
The effect of different doping levels with Ce 0.05, 0.1, 0.2, and 0.3 mole ratio compared to 5 wt.% TiO2/Al2O3 (using TIP as a source of TiO2).
From the above research, the following conclusions were drawn: Titania supported on silica or alumina could be used as a good photocatalyst for the degradation of organic dyes. Titanium isopropoxide (TIP) was found to be more effective than titanium tetrachloride (TiCl4). The optimum loading value was found to be 5 wt.% TiO2 which insures its presence in nanoscale. The alumina support (corundum phase) was found to be more effective than that of amorphous silica. In trying to increase catalytic activity by dopant inclusion of either La2O3 or CeO2, we determined that La2O3 enhances the catalytic activity more than CeO2. The interactions between La2O3, TiO2, and alumina were correlated with catalytic activity. The 0.05 mole ratio for the dopant was found to be the optimum value, above which the catalytic activity decreased.
The authors declare that they have no competing interests.
This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under Grant no. 387-247-1436G. The authors, therefore, acknowledge with thanks DSR technical and financial support.