Proceedings of the 53rd DAE Solid State Physics Symposium (2008)
Photodegradation of aqueous methylene blue by TiO2 @ Mo-MCM-41 H. Shankar1, R. Saravanan1, V. Narayanan2 and A. Stephen1* 1
Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025 2 Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai 600 025 *
Corresponding author:
[email protected] Abstract
The activity of TiO2-loaded Mo-incorporated MCM-41 (TiO2@Mo-MCM-41) was studied as a catalyst for photodegradation of aqueous Methylene blue. A series of Mo-MCM-41 mesoporous molecular sieves of varying Mo content (Si/Mo =25,50,75) were synthesized by hydrothermal method and loaded with 25 wt.% TiO2 utilizing sol–gel method. X-ray diffraction (XRD), UV–vis diffuse reflectance spectra (UV–vis), Fourier transform infrared spectroscopy (FTIR) were used to investigate the effects of the incorporated elements on the structure of MCM-41. It was found that the photodegradation ability of TiO2@Mo-MCM-41 was highly related to the amount of Mo present in the catalyst INTRODUCTION In recent years, a great deal of interests has been devoted to the photocatalytic degradation of organic water pollutants by semiconductor particles. Among the semiconductors employed, the TiO2 of anatase phase is the most preferable material for the photocatalytic process due to its high photosensitivity, non-toxic nature, large bandgap and stability [1,2]. In this work, we report the synthesis of a new photocatalyst TiO2@Mo-MCM-41 and its application for the photodegradation of methylene blue for the first time.
to higher angle with increasing Molybdenum content, although the hexagonal structure still remained intact. The peak intensity of Mo-MCM-41 decreases with increase in the Mo concentration. These results suggest that the regularity of the mesoporous structure decreased with introduction of Molybdenum-ion content. No XRD peak (in the range 2θ < 5°) appears for the samples with Si/Mo ratio higher than 25, which proves that the MCM-41 structure cannot be formed at high Mo concentrations.
EXPERIMENTAL Synthesis of Mo-MCM-41 nanoporous molecular sieve was carried out in laboratory using the method mentioned in reference [3] with some modifications. The reagents used in the synthesis were 40% Ludox HS-40 colloidal silica, 25% tetramethyl ammonium hydroxide, cetyltrimethyl ammonium bromide (CTAB), Ammonium Molybdate and distilled water. A weighed quantity of 0.5 gm of Mo-MCM-41 catalyst was dispersed in 50 ml of (99.8%) isopropyl alcohol and 0.64 ml of titanium (IV) isopropoxide (97%) was added to achieve 25% loading of Titania. The system was dried while stirring at ambient temperature. It was then placed in the oven to dry at 1000 C for 1 hour and calcined at 4500 C for 3 hours. RESULT AND DISCUSSION The small angle XRD patterns of Mo-MCM-41and TiO2 @Mo-MCM-41 are shown in the Fig.1. All samples exhibit well defined 100,110 and 200 reflections indexed on the hexagonal lattice in their XRD patterns [4], suggesting the formation of the long range ordered MCM-41 nanopores. The peaks were broadened and shifted slightly
Fig.1 XRD pattern of Mo-MCM-41 and TiO2 @Mo-MCM-41
The XRD reflections of TiO2 @Mo-MCM-41 samples are determined to be almost at the same location as those of unloaded Mo-MCM-41 indexed on the hexagonal lattice, suggesting the formation of the long range ordered MCM41 nano pores even after the loading of TiO2. Moreover, the (100) peak of TiO2@Mo-MCM-41 demonstrates an obvious decrease in intensity, which should be attributed to the filling of TiO2 nanoparticles. . In the FTIR spectra of as-synthesized Mo-MCM41(Si/Mo=75), peaks at 2851 and 2920 cm–1 can be attributed to the C–H symmetric and antisymmetric stretching vibration of CTAB, while these absorption bands are absent for calcined Mo-MCM-41. This confirms the
augmentation of Mo atom in the catalyst, the probability of rapid recombination of excited electrons/holes during photoreaction decreases. This can lead to the large generation of OH* free radicals and photocatalytic degradation rate will increase greatly. The resulting OH* radicals, being a very strong oxidizing agent (Oxidation potential +2.80 V) can oxidize most of methylene blue.
Fig. 2 FTIR Spectra of TiO2@Mo-MCM-41 (Si/Mo=75)
perfect removal of surfactant from the sample. The peak at 963 cm−1 can be attributed to the vibration of hydroxyl groups shared by framework Si and Mo atoms [5, 6]. This is the evidence for the incorporatMo into the framework of silica. It is observed that peaks at 1086, 963, 800 cm−1 in Mo-MCM-41 are narrow and more intense due to free vibration of (Si-O-Si) and (Si-O-Mo) bond as compared to Titania loaded Mo-MCM-41 sample. This supports that, the TiO2 nanoparticles are bonded with silica by Si-O-Ti bond. The UV-vis spectra of Mo-MCM-41(Si/Mo=25) and TiO2@Mo-MCM-41 (Si/Mo=25,50 and 75) are examined in the range 200 – 800 nm in order to assess the light absorption characteristics of the catalysts is shown in fig.3
Fig.4 UV-vis Spectra of aqueous methylene blue sample collected at various time intervals CONCLUSION Mo incorporated MCM-41 was synthesized at various Si/Mo ratios (25, 50, and 75) then they were loaded with 25% of TiO2 via sol-gel method. Characterization of MoMCM-41 and TiO2@Mo-MCM-41 materials by XRD, UVvis and FTIR revealed that the Molybdenum ions are well incorporated inside the framework Mo-MCM-41.It was found that the photodegradation ability of TiO2@MoMCM-41 was highly related to the amount of Mo content present in the sample. The 25% TiO2@Mo-MCM-41 (Si/Mo=50) is an active catalyst for the degradation of methylene blue when compared with other catalysts. ACKNOWLEDGEMENT The authors are grateful to UGC for the financial assistance for this work.
Fig .3 UV-vis Spectra of Mo-MCM-41and TiO2@W-MCM-41
One can clearly observe that the absorption of UV light is very weak in the case of Mo-MCM-41, whereas the absorption of UV light is very strong for TiO2@Mo-MCM41 (Si/Mo=25) followed by TiO2@Mo-MCM-41 (Si/Mo=50 and 75). The absorption ability of the prepared catalyst is directly related to the amount of Mo atom in the sample Fig.4 shows the UV-vis spectra of aqueous methylene blue samples collected at various time intervals during photodegradation. The high photocatalytic activity of 25% TiO2@Mo-MCM-41 (Si/Mo=50) can be attributed to the high surface area of the material and also to the presence of optimum amount of Mo atom in the sample. With the
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