CHAPTER 3. CHARACTERIZATION AND PHOTOCATALYTIC ACTIVITY OF MESOPOROUS MIXED Fe 2 O 3 /TiO 2

Transcription

1 78 CHAPTER 3 CHARACTERIZATION AND PHOTOCATALYTIC ACTIVITY OF MESOPOROUS MIXED Fe 2 O 3 /TiO INTRODUCTION Titania (TiO 2 ) is one of the most commonly used photocatalysts for pollutant degradation (Choi 2006, Ghezzar et al 2009, Addamo et al 2008). As it is environmentally friendly, cheap, non-photo corrosive, stable, easy availability and capable of mineralizing pollutants completely, it has been largely exploited as the photocatalyst for water decontamination (Berger et al 2006). However, its application is limited due to absorbance in the near UV region. As a consequence it requires a UV source for bandgap excitation during photocatalysis. Hence TiO 2 can make use only 5% of the UV light from the solar beam that reaches the earth (Neppolian et al 2002, Anpo and Takeuchi 2003). In order to harvest maximum energy of the sunlight, excitation of TiO 2 by both UV and visible light is essential. To overcome this limitation attempts have been made to sensitize TiO 2 in order to absorb light in the visible region. Photosensitization was effected by doping of transition metals (Angelis et al 2007, Morikawa et al 2008, Snaith and Gratzel 2006, Sun et al 2012, Vijayan et al 2009) and doping with non-metals (Mitoraj and Kisch 2008, Vereba et al 2013). Photosensitization of TiO 2 using Bi 2 O 3 has been already reported in the literature (Bian et al 2008, Kanga et al 2005). The design of TiO 2 with well defined mesoporous structure is also a promising way to achieve high photocatalytic activity since mesopore channel can facilitate rapid intra particle molecular transfer (Li et al 2007, Yu et al

2 ). Since mesoporous TiO 2 alone was not efficient without a photosensitizer, it was impregnated with transition metal oxides. In such a composite, the photosensitizer demonstrated as a fine dispersion in TiO 2, thus led to enhance photocatalytic degradation. Synthesis of mesoporous TiO 2 has been reported in the literature (Yang et al 1998, Crepaldi et al 2003, Kumaresan et al 2011). Photosensitizer was added to it by impregnation in a separate step. In contrast to such a technique, in-situ synthesis of mesoporous Fe 2 O 3 /TiO 2 photocatalyst exhibits better features as this technique can also provide TiO 2 incorporated with iron or vice-versa. If Fe incorporated TiO 2 is formed, then light absorbance may be extended to visible region. Further, the usual photosensitization of Fe 2 O 3 can also persist. In view of the advantages, the present study attempted the synthesis of mesoporous Fe 2 O 3 /TiO 2 using P123 tri-block copolymer by sol-gel method and examined the photocatalytic degradation of 4-chloropheenol, which is one of the important classes of water pollutant (Keith and Telliard 1979). 4-Chlorophenol is potentially a carcinogenic and mutagenic to mammalian as well as aquatic life. It is listed among the priority water pollutant by U.S.EPA (Janda and Svecova 2000). It is generated as a by-product in plastic, paper making, insecticidal and petrochemical industries. It causes serious effects on human health and environment (Yue et al 2002). Hence, design of a suitable process for complete mineralization of 4-chlorophenol in aqueous medium is an important issue. The photocatalytic degradation of 4-chlorophenol was extensively studied under UV-light illumination using MgAl hydrotalcites (Mantilla et al 2011), mesoporous TiO 2 film (Rathousky et al 2011) and Fe (III) citrate complex (Abida et al 2012). These materials showed promising photocatalytic efficiency in the degradation of 4-chlorophenol under UV-light irradiation. Visible-light assisted photocatalytic degradation of 4-chlorophenol has been carried out using ZnFe 2 O 4 modified TiO 2 nanotube array electrode (Hou et al 2010) and mesoporous carbon nitride (Cui et al

3 ). Though these materials exhibit good photocatalytic activity, fabrication process using these materials is tedious and not economical. Hence in the present study, it is focused to fabricate a simple method using earth abundant source. Mesoporous Fe 2 O 3 /TiO 2 with different wt% of Fe 2 O 3 were prepared using P123 triblock copolymer as structure directing agent with titanium tetraisopropoxide and iron (III) nitrate hexahydrate as titanium and iron precursors respectively. The characterization of these materials are discussed in the following sections. 3.2 PHYSICO-CHEMICAL CHARACTERIZATION OF MESOPOROUS Fe 2 O 3 /TiO X-ray Diffraction Patterns of Mesoporous Fe 2 O 3 /TiO 2 The XRD patterns of mesoporous TiO 2, mesoporous Fe 2 O 3 /TiO 2 (meso-10 wt% Fe 2 O 3 /TiO 2, meso-30 wt% Fe 2 O 3 /TiO 2, meso-50 wt% Fe 2 O 3 /TiO 2, meso-70 wt% Fe 2 O 3 /TiO 2 and meso-90 wt% Fe 2 O 3 /TiO 2 ) and mesoporous Fe 2 O 3 are shown in Figure 3.1. There was a shift in the angular positions of peaks for all mesoporous Fe 2 O 3 /TiO 2 in their XRD patterns. However, meso-90 wt% Fe 2 O 3 /TiO 2 showed reflections slightly different from that of meso-fe 2 O 3. Hence upto 70 wt% Fe 2 O 3 loading isomorphic substitution of Fe 3+ in TiO 2 was evident. In the case of meso-90 wt% Fe 2 O 3 /TiO 2, there were reflections similar to that of mesoporous Fe 2 O 3 without the characteristics of meso-tio 2. But a shift in the angular positions of the peaks was observed compared to meso-fe 2 O 3. Hence there might be framework incorporation of titanium in meso-fe 2 O 3. The XRD results thus confirmed the incorporation of Fe 3+ into the lattice of TiO 2, and Ti 4+ in the lattice of Fe 2 O 3. Isomorphic substitution of Ti 4+ by Fe 3+ in TiO 2 was also reported in the literature (Thimsen et al 2009). But they reported a thin film containing titanium, iron and oxygen.

4 81 The intensity of low angle XRD reflections of meso-tio 2 and meso-30 wt% Fe 2 O 3 /TiO 2 was also compared in Figure 3.2. The meso-30 wt% Fe 2 O 3 /TiO 2 calcined at 500 C revealed the absence of characteristic reflections illustrating the disappearance of ordered mesopores. The low angle XRD patterns of meso-10 wt% Fe 2 O 3 /TiO 2, meso-30 wt% Fe 2 O 3 /TiO 2, meso-50 wt% Fe 2 O 3 /TiO 2, meso-70 wt% Fe 2 O 3 /TiO 2 and meso-90 wt% Fe 2 O 3 /TiO 2 materials are presented in Figure 3.3. In each of the pattern, an intense reflection occurred between 0.5 and 1 (2 ). This supported the existence of mesoporous structure in all the materials. meso-fe 2 O 3 meso-90 wt% Fe 2 O 3 /TiO 2 Intensity (a.u.) meso-70 wt% Fe 2 O 3 /TiO 2 meso-50 wt% Fe 2 O 3 /TiO 2 meso-30 wt% Fe 2 O 3 /TiO 2 meso-10 wt% Fe 2 O 3 /TiO Theta degree meso-tio 2 Figure 3.1 XRD patterns of meso-tio 2, mesoporous Fe 2 O 3 /TiO 2 (10, 30, 50, 70 and 90 wt% Fe 2 O 3 ) and meso-fe 2 O 3

5 82 (i) meso TiO 2 calcined at 400 C Intensity (a.u.) (ii) meso-30 wt% Fe 2 O 3 /TiO 2 calcined at 400 C (iii) meso-30 wt% Fe 2 O 3 /TiO 2 calcined at 500 C Theta (Degree) Figure 3.2 Low angle XRD patterns of (i) meso-tio 2, (ii) meso- 30 wt% Fe 2 O 3 /TiO 2 calcined at 400 C and (iii) meso-30 wt% Fe 2 O 3 /TiO 2 calcined at 500 C Calcined at 400 C meso-10 wt% Fe 2 O 3 /TiO 2 Intensity (a.u.) meso-30 wt% Fe 2 O 3 /TiO 2 meso-50 wt% Fe 2 O 3 /TiO 2 meso-70 wt% Fe 2 O 3 /TiO 2 meso-90 wt% Fe 2 O 3 /TiO Theta (Degree) Figure 3.3 Low angle XRD patterns of mesoporous Fe 2 O 3 /TiO 2 (10, 30, 50, 70 and 90 wt% Fe 2 O 3 ) calcined at 400 C

6 X-ray Photoelectron Spectra of Mesoporous Fe 2 O 3 /TiO 2 The XPS spectra were recorded to establish the oxidation state and electronic environment of the elements present in the photocatalyst. The XPS survey spectrum of meso-30 wt% Fe 2 O 3 /TiO 2 is illustrated in Figure 3.4a and the core level spectra of characteristic elements are represented in Figures 3.4b, 3.5a and 3.5b. The peak at ev was due to O 1s bonded to Ti 4+ and Fe 3+. The presence of OH group was ascribed to the peak at ev. Similar results were also reported earlier by Pradhan et al (2012). The peaks due to Fe 2p 3/2 of Fe 2 O 3 occurred at ev and that of Fe 2p 1/2 at ev. The same values were also reported in the literature (Wagner et al 1979, Yamashita and Hayes 2008, Bhargaba et al 2007). The peaks at ev and ev were due to Ti 2p 3/2 and Ti 2p 1/2 (Bian et al 2008). Hence, the oxidation state of Fe in meso-30 wt% Fe 2 O 3 /TiO 2 was established to be +3. Since Ti is tetravalent and Fe is trivalent, the lattice might carry an extra negative charge. This was compensated by the acidic protons present on the surface. The shoulder at ev was due to oxygen of the defective - OH groups. Thus XPS spectra confirmed the presence of Ti, Fe, O and defective OH groups in the material.

7 84 O 1s (a) Ti 3p O 2s Ti 3s C 1s Intensity (a.u.) Ti 2p Ti 2s Fe 2p O KLL Binding Energy (ev) Fe 2p (b) 2P 3/2 2P 1/2 Intensity (a.u.) Binding Energy (ev) Figure 3.4 XPS spectra of meso-30 wt% Fe 2 O 3 /TiO 2 spectrum and (b) Fe 2p spectrum (a) survey

8 85 Ti 2P (a) Ti 2p 3/2 Intensity (a.u.) Ti 2p 1/ Binding Energy (ev) O 1s O 2- (b) Intensity (a.u.) OH Binding Energy (ev) Figure 3.5 XPS spectra of meso-30 wt% Fe 2 O 3 /TiO 2 (a) Ti 2p spectrum and (b) O 1s spectrum

9 Diffuse Reflectance Ultraviolet - Visible Spectra of Mesoporous Fe 2 O 3 /TiO 2 The DRS-UV-Vis spectra of mesoporous TiO 2, mesoporous Fe 2 O 3 /TiO 2 and mesoporous Fe 2 O 3 are shown in Figure 3.6. Mesoporous TiO 2 exhibited an absorbance just below 400 nm. It was due to bandgap excitation, and the bandgap value was equal to 3.23 ev. The spectrum of mesoporous Fe 2 O 3 showed a high energy absorbance peak at 550 nm. In addition, an absorbance was also occurred close to 700 nm. All other spectra showed a shift in absorbance. There was also an absorbance close to 500 nm for meso-10 wt% Fe 2 O 3 /TiO 2, which was attributed to bandgap excitation of Fe 2 O 3. meso-fe 2 O 3 meso-90 wt% Fe 2 O 3 /TiO 2 meso-70 wt% Fe 2 O 3 /TiO 2 Absorbance (a.u.) meso-50 wt% Fe 2 O 3 /TiO 2 meso-30 wt% Fe 2 O 3 /TiO 2 meso-10 wt% Fe 2 O 3 /TiO 2 meso-tio Wavelength (nm) Figure 3.6 DRS-UV-Vis spectra of meso-tio 2, mesoporous Fe 2 O 3 /TiO 2 (10, 30, 50, 70 and 90 wt% Fe 2 O 3 ) and meso-fe 2 O 3.

10 87 The shoulder appeared close to 440 nm assigned to bandgap excitation of TiO 2. The shift of bandgap excitation to longer wavelength in mesoporous TiO 2 confirmed the framework incorporation of Fe 3+. The same features were also displayed by meso-30 wt% Fe 2 O 3 /TiO 2 and meso-50 wt% Fe 2 O 3 /TiO 2. The bandgap due to TiO 2 was not evident in the spectra of meso- 70 wt% Fe 2 O 3 /TiO 2, meso-90 wt% Fe 2 O 3 /TiO 2 and mesoporous Fe 2 O 3. Hence Ti 4+ might be incorporated in the framework of Fe 2 O 3. This was also confirmed by the slight shift in the bandgap excitation to longer wavelength. Similarly, the absorbance in longer wavelength close to 630 nm was assigned to charge transfer transition from oxygen to iron (Pradhan et al 2012, Pradhan and Parida 2011). Thus the DRS-UV-Vis spectra confirmed the framework incorporation of Fe 3+ in TiO 2 and Ti 4+ in Fe 2 O 3. These results are also in accordance with the results of XRD N 2 Sorption Studies of Mesoporous Fe 2 O 3 /TiO 2 Nitrogen adsorption-desorption isotherms of the catalysts are depicted in Figure 3.7. All the isotherms showed similar features with a hysteresis loop, evidencing the presence of mesopores. The specific surface area, pore volume and pore diameter are presented in Table 3.1. Mesoporous TiO 2 showed higher surface area than all other catalysts. Since all catalysts were prepared by the same method, the decrease in surface area was ascribed to increase in the wall thickness. It was also supported by the large pore diameter for Fe 3+ containing materials compared to mesoporous TiO 2. Mesoporous TiO 2 showed high surface area but low pore diameter. Hence its wall thickness might be lower than the others. In other words, it was presumed that meso-tio 2 carried more number of pores per gram than others. It was also partially evident from TEM images. The pore volume of mesoporous TiO 2 was also higher than others. Generally mesoporous materials like MCM-41 showed surface area close to 1000 m 2 /g

11 88 (Umamaheswari et al 2002), but the surface area of the prepared materials in this study was between 50 and 150 m 2 /g. Hence all the particles might not be used in the construction of mesopores. It was also partially evident from the XRD patterns where the intensity of intense peak was not high but broad. The intensity of the intense peak was less than 500 counts/s for mesoporous TiO 2. The Figure 3.8 indicated pore size distribution of meso-10 wt% Fe 2 O 3 /TiO 2, meso-30 wt% Fe 2 O 3 /TiO 2, meso-50 wt% Fe 2 O 3 /TiO 2, meso-70 wt% Fe 2 O 3 /TiO 2 and meso-90 wt% Fe 2 O 3 /TiO 2. Volume of N 2 adsorbed (cm 3 /g) meso-90 wt% Fe 2 O 3 /TiO 2 meso-70 wt% Fe 2 O 3 /TiO 2 meso-50 wt% Fe 2 O 3 /TiO 2 meso-30 wt% Fe 2 O 3 /TiO 2 meso-10 wt% Fe 2 O 3 /TiO Relative pressure (p/p o ) Figure 3.7 N 2 sorption isotherms of meso-tio 2 and mesoporous Fe 2 O 3 /TiO 2 (10, 30, 50, 70 and 90 wt% Fe 2 O 3 ).

12 89 Table 3.1 Textural parameters of mesoporous TiO 2, mesoporous Fe 2 O 3 /TiO 2 and mesoporous Fe 2 O 3 Catalyst Specific surface area (m 2 g -1 ) Pore volume (cm 3 g 1 ) Pore diameter (nm) meso TiO meso-10 wt% Fe 2 O 3 /TiO meso-30 wt% Fe 2 O 3 /TiO Reused catalyst meso-30 wt% Fe 2 O 3 /TiO meso-50 wt% Fe 2 O 3 /TiO meso-70 wt% Fe 2 O 3 /TiO meso-90 wt% Fe 2 O 3 /TiO meso Fe 2 O meso-90 wt% Fe 2 O 3 /TiO 2 meso-70 wt% Fe 2 O 3 /TiO 2 meso-50 wt% Fe 2 O 3 /TiO 2 meso-30 wt% Fe 2 O 3 /TiO 2 Pore volume (cm 3 /g) meso-10 wt% Fe 2 O 3 /TiO Pore diameter (Å) Figure 3.8 Pore size distribution of mesoporous Fe 2 O 3 /TiO 2 (10, 30, 50, 70 and 90 wt% Fe 2 O 3 )

13 Inductively Coupled Plasma-Optical Emission Spectroscopic Studies of Mesoporous Fe 2 O 3 /TiO 2 The Fe content in Fe 2 O 3 /TiO 2 was estimated by ICP OES analysis before and after photocatalytic reaction and the results are presented in Table 3.2. The results revealed that there is no change in Fe content before and after the photocatalytic reaction suggesting that the material is quite stable during photocatalytic reaction. Moreover, in order to check the leaching of Fe in the photocatalytic suspension, Fe 2 O 3 /TiO 2 was collected from the suspension by filtration after photocatalytic reaction. The filtrate was analyzed by ICP-OES and it is found that the filtrate did not contain either Fe or Ti, thus confirmed the absence of leaching of either Fe or Ti into the aqueous solution. Table 3.2 Weight % of Fe 2 O 3 in fresh and reused catalyst analyzed by ICP-OES Fresh Catalyst Reused catalyst Catalyst Weight % of Fe 2 O 3 Weight % of Fe 2 O 3 meso-10 wt% Fe 2 O 3 /TiO (10) 8.78 meso-30 wt% Fe 2 O 3 /TiO (30) meso-50 wt% Fe 2 O 3 /TiO (50) meso-70 wt% Fe 2 O 3 /TiO (70) meso-90 wt% Fe 2 O 3 /TiO (90) Scanning Electron Microscopic (SEM) Images of Mesoporous Fe 2 O 3 /TiO 2 The SEM images of mesoporous TiO 2, mesoporous Fe 2 O 3 /TiO 2 and mesoporous Fe 2 O 3 are shown in Figure 3.9. The SEM image of mesoporous TiO 2 is depicted in Figure 3.9a (inset). There were large aggregates of tiny particles, and the aggregates did not possess any definite shape. The inset

14 91 showed tiny particles of different sizes, and each particle was an aggregate. The SEM image of meso-10 wt% Fe 2 O 3 /TiO 2, as shown in Figure 3.9a, also revealed similar features as that of the inset. The particles did not possess any definite shape. Hence the growth of TiO 2 even in the presence of Fe did not form different morphology. The XRD patterns showed incorporation of Fe in the lattice of mesoporous TiO 2 but even after the entry of Fe, the crystal growth characteristics were remained the same. In other words the tendency of agglomeration of individual particles was not affected even in the presence of Fe in the TiO 2 crystal lattice. Similar features were also observed for meso-30 wt% Fe 2 O 3 /TiO 2 (Figure 3.9b). The image of meso-50 wt% Fe 2 O 3 /TiO 2, shown in Figure 3.9c, showed large aggregates in addition to small particles dispersed on them. Such small particles may be attributed to the presence of Fe 2 O 3 which is in support of XRD pattern that revealed the formation of Fe 2 O 3 phase. The SEM images of meso-70 wt% Fe 2 O 3 /TiO 2 and meso-90 wt% Fe 2 O 3 /TiO 2 are shown in Figures 3.9d and 3.9e respectively. They also revealed tiny crystallites on large agglomerates. The tiny crystallites as well as large aggregates were assigned to Fe 2 O 3. The TiO 2 phase was insignificant in these two composites as revealed in the XRD pattern. The SEM image of mesoporous Fe 2 O 3 is shown in Figure 3.9f for comparison. The morphology was almost similar to the earlier one. Hence, both mesoporous TiO 2 and mesoporous Fe 2 O 3 /TiO 2 exhibited similar crystallization property even in meso-90 wt% Fe 2 O 3 /TiO 2. Hence the formation of nanoparticles and their tendency for agglomeration might be followed similarly in all the cases.

15 92 a b c d e f Figure 3.9 SEM images of (a) meso-10 wt% Fe 2 O 3 /TiO 2 (inset meso-tio 2 ), (b) meso-30 wt% Fe 2 O 3 /TiO 2, (c) meso-50 wt% Fe 2 O 3 /TiO 2, (d) meso-70 wt% Fe 2 O 3 /TiO 2, (e) meso-90 wt% Fe 2 O 3 /TiO 2 and (f) meso-fe 2 O 3

16 Transmission Electron Microscopic (TEM) Images of Mesoporous Fe 2 O 3 /TiO 2 The TEM image of meso-30 wt% Fe 2 O 3 /TiO 2 is shown in Figure 3.10a. The particles were roughly spherical in shape, and the particle size was varying in the range from 20 to 30 nm. The other materials also exhibited similar spherical shape. The crystalline nature of the particles was confirmed by lattice fringes. The aggregation of particles was evident from HRTEM image (Figure 3.10b). a b Figure 3.10 (a) TEM image of meso-30 wt% Fe 2 O 3 /TiO 2 and (b) magnified image of meso-30 wt% Fe 2 O 3 /TiO Temperature Programmed Desorption Studies of Mesoporous Fe 2 O 3 /TiO 2 The TPD results of meso-tio 2, meso-fe 2 O 3 and meso-30 wt% Fe 2 O 3 /TiO 2 are depicted in Figure The TPD analysis established the

17 94 presence of surface protons that counter balanced the negative charge in the crystal lattice of TiO 2, originated due to substitution of Ti 4+ by Fe 3+. As expected meso-30 wt% Fe 2 O 3 /TiO 2 showed more density of acid sites than meso-tio 2 or meso-fe 2 O 3. Almost all the three types of acid sites were present. The desorption of ammonia in the temperature from 100 to 200 C was due to weak acid sites, from 200 to 300 C due to medium acid sites and from 300 to 450 C due to strong acid sites. meso-30 wt% Fe 2 O 3 /TiO 2 TCD signal (a.u.) meso-fe 2 O 3 meso-tio Temperature ( C) Figure 3.11 TPD of meso-tio 2, meso-fe 2 O 3 and meso-30 wt% Fe 2 O 3 /TiO Fourier Transform - Infrared (FT-IR) Spectra of Mesoporous Fe 2 O 3 /TiO 2 The FT-IR spectra of as-synthesized meso-30 wt% Fe 2 O 3 /TiO 2 and its calcined form are shown in Figure The spectrum of as-synthesized material showed a broad peak at 3250 cm -1 due to O-H stretching vibration of water. The CH 2 stretching vibration of template showed a peak just below 3000 cm -1. The bending vibration of water appeared at 1620 cm -1. The CH 2

18 95 bending vibration of template showed peaks between 1500 and 1250 cm -1. The intense sharp peak at about 1050 cm -1 was due to C-O stretching vibration of alcohol used in the synthesis. The broad peak at 510 cm -1 was due to M-O stretching vibration. The FT-IR spectrum of calcined meso-30 wt% Fe 2 O 3 /TiO 2 is also shown in the same figure. The -OH stretching vibration of water, alcohol and template and the CH 2 bending vibrations were completely absent. The peaks at 480 and 540 cm -1 were due to Fe-O vibrations (Pala et al 1999). The peak at 800 cm -1 was due to Ti-O vibration (Zhu et al 2000). The other wt% of Fe 2 O 3 /TiO 2 materials also showed similar spectra. However, the intensity of Fe-O vibration at 480 and 540 cm -1 increased with increase in the wt% of Fe 2 O 3. calcined meso-30 wt% Fe 2 O 3 /TiO 2 as-synthesized meso-30 wt% Fe 2 O 3 /TiO 2 Transmittance (%) Wavelength (cm -1 ) Figure 3.12 FT-IR spectra of as-synthesized and calcined meso-30 wt% Fe 2 O 3 /TiO 2

19 Thermogravimetric Analysis (TGA) of Mesoporous Fe 2 O 3 /TiO 2 TGA results of as-synthesized meso-30 wt% Fe 2 O 3 /TiO 2 and its calcined form are shown in Figure The initial weight loss upto 200 C in the thermogram of as-synthesized material was assigned to desorption of water and alcohol, the presence of which was also confirmed by FT-IR analysis. The weight loss that continued from 200 to 350 C was due to degradation of template. A total weight loss of about 35% was observed upto 600 C, of which about 8% was due to loss of water. The thermogram of calcined material showed gradual increase in weight loss, but the total weight loss was only about 6%. All other as-synthesized and calcined materials also showed nearly similar results calcined meso-30 wt% Fe 2 O 3 /TiO 2 90 weight loss (%) as-synthesized meso-30 wt% Fe 2 O 3 /TiO Temperature ( C) Figure 3.13 TGA of as-synthesized and calcined meso-30 wt% Fe 2 O 3 /TiO 2

20 PHOTOCATALYTIC ACTIVITY OF Fe 2 O 3 /TiO 2 The photocatalytic reaction was carried out for 180 minutes with each of the catalysts, and the progress of the reaction was monitored by determining the total organic carbon (TOC) at different time intervals to examine the photocatalytic activity. The results of degradation of 4 chlorophenol are illustrated in Figure The decrease of TOC in the reaction over meso-tio 2 and meso-fe 2 O 3 with increase in time was low compared to mesoporous Fe 2 O 3 /TiO 2. Thus meso-tio 2 or meso-fe 2 O 3 was found to be less active than mesoporous Fe 2 O 3 /TiO 2 catalysts. The lesser activity of mesoporous TiO 2 than others was due to low level of UV light (5%) in sunlight. But mesoporous Fe 2 O 3 showed slightly higher activity than mesoporous TiO 2, as its bandgap is equal to 2.1 ev (TiO 2 bandgap is 3.23 ev) and hence it can absorb light in the visible region. Although Fe 2 O 3 can absorb light in the visible region, it exhibited lesser activity than mesoporous Fe 2 O 3 /TiO 2. This may be attributed to its low bandgap (2.1 ev), which promoted rapid electron hole recombination as the conduction band edge of Fe 3+ incorporated TiO 2 is not enough to reduce oxygen by one electron reduction. Meso-30 wt% Fe 2 O 3 /TiO 2 showed much rapid decrease of TOC than others. Although DRS-UV-Vis spectrum of it was nearly similar to meso-10 wt% Fe 2 O 3 /TiO 2, it showed higher activity. Hence Fe 3+ incorporation in meso-30 wt% Fe 2 O 3 /TiO 2 was found to be higher than meso-10 wt% Fe 2 O 3 /TiO 2. This was also evident from DRS-UV-Vis analysis. In meso-30 wt% Fe 2 O 3 /TiO 2, there might be more framework incorporation of Fe 3+ in TiO 2 compared to all others. As a consequence of framework incorporation of Fe 3+, shift of bandgap excitation to longer wavelength was evident from DRS-UV-Vis spectrum. This confirmed higher solar light absorbance of mesoporous Fe 2 O 3 /TiO 2 than mesoporous TiO 2. Once the

21 98 electron is bandgap excited, it might be immediately picked up by Fe 3+ and reduced it to Fe 2+. Since Fe 2+ is not stable, it is rapidly oxidized to Fe 3+ by dissolved oxygen. Hence the electron-hole recombination was suppressed, and the rate of degradation of 4 chlorophenol was increased by the hole. Although the surface area of meso-30 wt% Fe 2 O 3 /TiO 2 was lower than meso-10 wt% Fe 2 O 3 /TiO 2, the degradation rate of former was higher than that of the later. Hence surface area of the catalyst alone is not the prime factor to account for the rate of degradation. As mentioned above, the level of Fe 3+ incorporation in the lattice of TiO 2 is important for light absorption in the visible region. However, further increase of Fe 2 O 3 content beyond 30 wt %, the rate of degradation decreased. The rate of decrease of TOC was lower in meso-50 wt% Fe 2 O 3 /TiO 2, meso-70 wt% Fe 2 O 3 /TiO 2 and meso-90 wt% Fe 2 O 3 /TiO 2 than that of meso-10 wt% Fe 2 O 3 /TiO 2 and meso-30 wt% Fe 2 O 3 /TiO 2. This is attributed to the presence of high percentage of free Fe 2 O 3 rather than incorporation of Fe 3+ in the lattice of TiO TOC (mg/l) meso-tio 2 meso- Fe 2 O 3 meso-90 wt% Fe 2 O 3 /TiO 2 meso-70 wt% Fe 2 O 3 /TiO 2 meso-50 wt% Fe 2 O 3 /TiO 2 meso-10 wt% Fe 2 O 3 /TiO 2 meso-30 wt% Fe 2 O 3 /TiO Irradiation time (min) Figure 3.14 Photocatalytic degradation of 4-chlorophenol using meso- TiO 2, mesoporous Fe 2 O 3 /TiO 2 (10, 30, 50, 70 and 90 wt% Fe 2 O 3 ) and meso-fe 2 O 3 catalysts

22 99 During degradation chloride ion concentration was also monitored with respect to time, and the results are depicted in Figure The chloride ion concentration increased with increase in time, and the total amount of chloride removal was found to be 25 ppm, out of the total theoretically calculated amount of chlorine (28 ppm) in 4-chlorophenol. Hence it is concluded that 4 chlorophenol was completely mineralized, and chlorine in 4-chlorophenol was released as chloride ion. The same reaction was also studied without the catalyst, with meso-30 wt% Fe 2 O 3 /TiO 2, meso-tio 2 and meos-fe 2 O 3 in dark and with meso-30 wt% Fe 2 O 3 /TiO 2 in sunlight. The results of decrease in TOC with irradiation time of the above reactions are illustrated in Figure Since the decrease in TOC was insignificant in the absence of catalyst, the degradation was not photolytic. Though decrease in TOC was observed with meso-30 wt% Fe 2 O 3 /TiO 2, meso-tio 2 and meos-fe 2 O 3 in dark, such a decrease was less than 10 percent which might be due to adsorption. In contrast, a very rapid decrease in TOC was observed in the presence of meso-30 wt% Fe 2 O 3 /TiO 2 and sunlight, and the total TOC at the end of 180 minutes was found to be zero. Hence the degradation was confirmed to be photocatalytic.

23 Chloride ion concentration (ppm) Time (h) Figure 3.15 Formation of chloride ion during degradation of 4-chlorophenol using meso-30 wt% Fe 2 O 3 /TiO Without catalyst TOC (mg/l) with Fe 2 O 3 in dark with meso-30 wt% Fe 2 O 3 /TiO 2 in dark with TiO 2 in dark with meso-30 wt% Fe 2 O 3 /TiO 2 under sunlight Irradiation time (min) Figure 3.16 Photocatalytic degradation of 4-chlorophenol with meso-30 wt% Fe 2 O 3 /TiO 2 in sunlight and dark, meso-tio 2, meos-fe 2 O 3 in dark and without the catalyst

24 101 The reaction was also followed using HPLC. The chromatograms of 4-chlorophenol and the formation of intermediates in 60 min irradiation are shown in Figures 3.17a and 3.17b respectively. A single peak was observed for initial 4-chlorophenol solution. The chromatogram observed for the reaction mixture after 60 minutes, as depicted in Figure 3.17b, showed several peaks at lower retention time than that of 4-chlorophenol. These peaks were attributed to the formation of intermediates in the degradation of 4-chlorophenol. In addition, the intensity of 4-chlorophenol peak was also reduced significantly after 60 minutes. So the degradation process was established to be through formation of several intermediates. The intermediates were also analyzed by GC-MS technique. The results revealed that 4-chlorophenol was mainly transformed into hydroxylated aromatic intermediates such as benzoquinone, hydroquinone, 4-chlorocatechol and hydroxyhydroquinone. Based on the intermediates, the plausible mechanism and the role of Fe 2 O 3 are shown in Scheme 3.1. After complete degradation of 4-chlorophenol, the catalyst was separated by filtration and washed repeatedly with water for several time to remove the adsorbed molecules. The separated catalyst was dried at 200 C for 2 h and reused. The photocatalytic activity of the recovered meso-30 wt% Fe 2 O 3 /TiO 2 catalyst was tested thrice under similar conditions and the results are depicted in Figures 3.18 and The results revealed that the catalytic activity of the used catalyst did not show any significant loss of activity upto 4 cycles. The comparison of compositional analysis of fresh and used meso-30 wt% Fe 2 O 3 /TiO 2 catalysts are shown in BET, ICP-OES and XPS studies. The results revealed that the catalytic activity and the composition of the spent catalyst were retained as that of parent catalyst, suggesting the absence of catalyst degradation or deactivation. The XRD patterns of the spent catalyst were almost finger print of the fresh catalyst.

25 102 The characterization of used meso-30 wt% Fe 2 O 3 /TiO 2 is presented in the respective figures. (a) (b) Figure 3.17 HPLC chromatograms of (a) 4-chlorophenol initially and (b) degradation intermediates of 4-chlorophenol

26 103 OH Cl +. OH -. Cl OH OH + 2 h H e H + O O... CO 2 + H 2 O +. OH -. H OH Cl OH +. OH -. Cl OH OH OH... CO 2 + H 2 O Scheme 3.1 Plausible reaction pathway for the degradation of 4-chlorophenol TOC (mg/l) st use 2nd use 3rd use 4th use Irradiation time (min) Figure 3.18 TOC data of reused meso-30 wt% Fe 2 O 3 /TiO 2

27 % of degradation (180 min) Cycles Figure 3.19 Percentage recyclability of meso-30 wt% Fe 2 O 3 /TiO Photocatalytic Mechanism The band structures of bare TiO 2 and Fe 2 O 3 /TiO 2 are shown in Figure The O 2p orbitals contribute to make up a valence band (VB) while Ti 3d contribute for a conduction band (CB). Since the distance between VB (O 2p) and CB (Ti 3d) of pure TiO 2 is large (3.23 ev) (Figure 3.20a), absorption of visible-light is negligible and hence TiO 2 showed low photocatalytic activity in the degradation of 4-chlorophenol. On the other hand, the new absorption band appeared below the conduction band edge of TiO 2 in Fe 2 O 3 /TiO 2 (Figure 3.20b) due to incorporation of Fe 3+ which is responsible for visible light absorption in Fe 2 O 3 /TiO 2. When visible-light was illuminated on Fe 2 O 3 /TiO 2, the electrons excited from the valence band to forbidden band leaving holes in the valence band of TiO 2. The excited electrons in the forbidden band is transferred to Fe 3+ resulting the reduction of Fe 3+ to Fe 2+. Hence the holes generated in the valence band of Fe 2 O 3 /TiO 2 caused oxidation of 4-chlorophenol to produce carbon

28 105 dioxide and water, while the transferred electrons in Fe 3+ caused efficient reduction as shown in Figure 3.20b. Thus, the holes with strong oxidation power generated in the VB of Fe 2 O 3 /TiO 2 and the electrons in the forbidden band are effectively separated upon visible-light irradiation. Figure 3.20 Plausible mechanism for the degradation of 4-chlorophenol in TiO 2 and Fe 2 O 3 /TiO CONCLUSION Sol-gel synthesis of mesoporous Fe 2 O 3 /TiO 2 with different loading of Fe 2 O 3 yielded mesoporous Fe 3+ incorporated TiO 2 and mesoporous Ti 4+ incorporated Fe 2 O 3. Fe 3+ incorporated TiO 2 absorbed light in the visible region and it is proved to be better catalyst than TiO 2 for visible light photocatalytic degradation 4-chlorophenol. The presence of free Fe 2 O 3 also aided photosensitization of TiO 2 and hence both are important for solar light assisted degradation of water pollutants. Thus the synergistic effects of visible light absorption and photosensitization of Fe 2 O 3 /TiO 2 are demonstrated in the degradation of 4-chlorophenol in the presence solar light. This catalyst can

29 106 also be used to mineralize other organic pollutants in wastewater. Since the catalyst surface carried protons, amine based pollutants such as dyes can be well adsorbed and decomposed rapidly.