Abstract
In this research different composites of impregnated TiO2 with LTA or FAU zeolites were used as different weight% ratio for photodegradation of organic dye. Normal laboratory UV-lamps were used as a source of UV irradiation. In addition a setup of system of mirrors was used to collect real Jeddah sunlight. A comparison of UV and real sunlight photodegradation activity showed that the real sunlight enhances new centers of active sites exhibiting higher catalytic activity than that of UV irradiated samples.
1. Introduction
Decomposition of organic pollutants photocatalytically in the presence of TiO2 appears as a sustainable decontamination process of common application, as gas or liquid [1, 2]. The ultraviolet (UV) energy/excitation source becomes the most practical applications due to the fact that the absorption edge of anatase TiO2 is about 380 nm. The most solar radiation intensity reaching the earth surface is in the visible range. Systems with visible-light as an active radiation have become a priority for developing the upcoming generation of photocatalytic materials.
The composite of TiO2 fine particles with other supports such as active carbon has been proved to exhibit efficient photocatalytic reactivity [3–5]. However, black active carbon inhibits the incident UV-vis radiation to excite semiconductor photocatalysts. In comparison, TiO2 supported on transparent constituents in UV-vis regions, such as porous materials [6–10] or clays [11], has also been conveyed to show greater photocatalytic reactivity than sole TiO2. Moreover, TiO2 particles loaded on ZSM-5 zeolite [12] as well as TiO2 powders mechanically grinded with MOR zeolite [13] were proved for efficient removal of highly volatile acetaldehyde.
In addition, TiO2/Y-zeolite hybrid photocatalysts [14] were used efficiently for removal of toluene or benzene molecules.
The above behavior could be explained by the fact that the zeolitic material adsorbs the organic compounds and easily supplies them onto the TiO2 photocatalyst surface. Another advantage of zeolite-TiO2 composite is that the adsorption capacity of the pure TiO2 sample rapidly deteriorated, while the TiO2/zeolite hybrid system maintained a high adsorption efficiency to remove such aromatic compounds for a long period [14]. In this research, we will study the effect of different composites of TiO2-zeolite, namely, LTA and FAU materials, on the photocatalytic decomposing of organic dye using UV irradiation. In addition a developing system using direct sunlight was also used as a future system for photocatalytic decomposition of such systems.
2. Experimental
2.1. Materials Used
Materials used were titanium isopropoxide (Merck), LTA, FAU zeolites (synthesized from kaolin by hydrothermal treatment with NaOH), LTA zeolite chemical structure being Na12Al12Si12O48·37H2O with pore size of 4.5 Å, FAU chemical structure being Na86Al86Si94.6O361·228·5H2O with pore size diameter of 7.5 Å, and methyl orange (Merck).
2.2. Synthesis
Titanium isopropoxide was loaded by different wt% composition on LTA or FAU, namely, 20, 50, and 70 wt%, after which calcination at 550°C was performed.
2.3. Characterization Techniques
2.3.1. Photocatalytic Decomposition
A 100 mL of 500 ppm solution of methyl orange was mixed with 0.5 g of composite using a horizontal cylinder annular batch reactor. A xenon lamp (300 W), covered by a UV filter, was used for irradiation of the photocatalyst. The photocatalytic reaction was carried out at room temperature.
The direct sunlight reactor shown in schematic diagram 1 was used using the same previous concentration of dye and same amount of the catalyst (Scheme 1).
2.3.2. XRD (X-Ray Diffraction Analysis)
X-ray diffractograms of various solids were collected using a Bruker D8 advance instrument with CuK1 target with secondly monochromator 40 kV, 40 mA. Crystal lattice and space group analysis were performed using PhilpsX’Pert Plus V. 1.0 23. 04. 1999.
2.3.3. SEM and EDX Analysis
SEM images combined with EDX analysis were taken using instrument and EDX analysis was performed using instrument “JXA-840 A electron probe microanalyzer, Japan”.
2.3.4. UV Visible Spectrophotometer
The decomposition of organic dye followed using UV spectrophotometer.
2.3.5. HPLC
Some selected samples were reanalyzed by HPLC system and nearly the same results of UV were obtained. Visible spectrophotometer was attained.
The HPLC system used consisted of an Alliance Waters separations module 2695, waters 2996 photodiode array detector, and Waters 2475 multi fluorescence detector (Milford, MA, USA). HPLC system control and data processing were performed by Empower software (Build 1154, Waters). Screw capped V-shaped vials, 300 L, with PTFE liners were used (Alltech, GmbH, Unterhaching, Germany). Heating oven (Heraeus, Kendro, Hanau, Germany) was adjusted at 60°C. Calibrated digital microtransfer pipettes, 5–250 L, Brand, Wertheim, Germany, were used.
3. Results and Discussion
3.1. XRD
The XRD diffraction patterns showed that at high loading of TiO2 (70%) only anatase phase is showed with no trace of zeolite matrix for both LTA and FAU zeolite. For low loading, in case of FAU, also only anatase phase is observed with very low trace of FAU zeolite. In contrast at low loading (20%) over LTA only zeolite matrix was observed with no trace of anatase phase.
The crystal lattice analysis (Table 1) showed that in case of 20% TiO2/LTA the lattice parameters slightly decreased meaning that the disappearance of anatase phase is accompanied by dissolving the TiO2 into the LTA zeolite substituting the framework [15]. In other words the change in lattice parameters of TiO2 loaded sample compared with pure LTA zeolite, (pure LTA) to (20% TiO2/LTA), gives indication that the TiO2 was incorporated into the framework of zeolite.
3.2. SEM and EDX
Figures 1 and 2 showed the SEM images of different loaded samples. The images showed that in case of high loaded samples only TiO2 agglomerate matrix was observed while for low loaded samples small agglomerate of TiO2 is dispersed over an observable zeolite matrix. The EDX analysis was manipulated in Tables 2 and 3. From this table it is clearly observed that at 20% TiO2/LTA zeolite the surface TiO2 observed by EDX analysis is much lower than those corresponding to 20% TiO2/FAU which supports the previous XRD analysis data that the TiO2 is incorporated inside the framework of LTA zeolite.
3.3. Photocatalytic Activity
The photocatalytic activity kinetic curves of all samples (UV irradiated and sunlight irradiated) are presented in Figure 3. From this figure it could be observed that the two zeolites LTA and FAU showed two different behaviors. Thus, for UV irradiation sample loaded on FAU, the catalytic decomposition activity of 20, 50% TiO2 loaded samples showed nearly no catalytic activity while only 70% showed about 80% decomposition after nearly 50 min. This could be explained that at this critical composite ratio the adsorbed dye is easily submitted to the surface of TiO2 while other ratios of loading the adsorbed dye are not transferred easily, which may be due to high adsorption power of FAU.
With sunlight irradiation, a dramatic change occurs that all catalytic activity increased meaning that there are new active sites activated only by sunlight (visible range); these active sites may be due to either some impurities in zeolite matrix or due to partial substitution of Ti into the framework of zeolite.
In contrast to FAU zeolite, LTA loaded with TiO2 showed another amazing behavior. Thus, regarding the UV irradiation, a dramatic increase of 20% TiO2/LTA up to 80% decomposition comparing with only 45% decomposition for both 50 and 70% TiO2/LTA could be explained in the light of previous XRD and SEM and EDX data that explained for this composite ratio most of TiO2 incorporated into the matrix of zeolite generating new active site to whole matrix accompanied by absence of any anatase phase of TiO2. It appeared that the catalytic activity of these new active sites is much higher than that of even high loaded TiO2 with anatase phase detected. Studying the same system under sunlight showed that catalytic activity of 70% TiO2/LTA decreased than those values obtained by UV irradiation while 50% TiO2/LTA increased dramatically under sun irradiation to be nearly the same with 20% TiO2/LTA to be 75% decomposition. This could be explained by that the incorporated Ti into zeolite active sites is active in both sunlight and UV irradiation while at 50% loading both anatase phases with the new incorporated active site seemed to make a synergetic effect increasing dramatically the catalytic activity. However, pure anatase loaded on LTA (70%) is less active in sunlight.
4. Conclusions
The following conclusions could be observed from this research.(1)Microporous materials such as LTA and FAU could be effectively used as a good support for TiO2 photocatalyst.(2)Loading TiO2 on these supports leads to either amorphous dispersed TiO2 or pure anatase phase of TiO2.(3)In case of low loading of LTA (20%) some XRD evidences for incorporation of some of titanium into the framework sites of zeolite were found.(4)More than one evidence exists of more than one photoactive site for both UV and visible irradiations.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgment
This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah (under Grant no. 225/247/1432). The authors, therefore, acknowledge with thanks DSR for technical and financial support.