Sunlight-Induced Photocatalytic Degradation of Organic Pollutants by

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Sunlight-Induced Photocatalytic Degradation of Organic Pollutants by Carbon-Modified Nanotitania with Vegetable Oil as Precursor Jeevitha R. Raji and Kandasamy Palanivelu* Centre for Environmental Studies, Anna University Chennai, Chennai 600 025, India ABSTRACT: An attempt has been made in this study to prepare a nanosized titanium dioxide catalyst by an ultrasonication assisted sol-gel process at low temperature. The as-synthesized titania was modified with carbon from coconut oil as a carbon precursor in two different proportions and then subjected to thermal and microwave treatment. The characterization of the pure and carbon modified titania catalysts was accomplished by XRD, SEM, HR-SEM/EDX, UV-vis absorption spectroscopy, and BET surface area analysis. The average size of the carbon modified TiO2 catalysts was found to be in the range of 90 nm. The solar photocatalytic activity of the catalysts was monitored by the degradation of a cationic dye, methylene blue, and an anionic dye, methyl orange. The effect of pH on the degradation of the dyes was also investigated. The study revealed that the degradation of the cationic dye was highly pronounced in the alkaline pH, whereas, the degradation of the anionic dye was effective in acidic pH. A solar photochemical study was also conducted with 2,4-dichlorophenol to ensure the degradation of visible light nonabsorbing organic compounds. The study revealed that carbon-modified catalysts with about 8.0 wt % of carbon was found to have the highest solar phoactivity, when compared to the pure titania and the carbon modified titania with 42% of carbon.

’ INTRODUCTION Photocatalysis is a rapidly emerging and promising technology coming up with clean, green, and sustainable innovation in environmental applications.1 Titanium dioxide is the most widely used photocatalyst, which creates powerful hydroxyl radicals and superoxide anions in the presence of light and degrades the harmful organic compounds in wastewater into harmless CO2 and water. Though TiO2 acts as an efficient photocatalyst, the wide band gap energy (3.2 eV) limits anatase TiO2 to using visible light as the light source. This has consequential implications for the use of titania materials as solar or room-light-activated catalysts, because the majority of sunlight consists of visible light (∼45%) and includes only 3-5% UV light.2,3 To enhance the photoactivity of TiO2 and broaden the absorption to solar spectrum, a number of attempts have been made to change the physical characteristics and chemical composition of TiO2 by metal/nonmetal doping as it extends the optical absorption of the catalyst to the visible spectral region.4 However, the metal doping of TiO2 materials has limitations such as thermal instability, low-quantum efficiency, and expensive facilities in ion implantation.5-8 Also, doping of heavy metals such as chromium is toxic in nature. All these limitations affect the practical application of metal-doped TiO2 in the degradation of refractory organics. To overcome these limitations, special efforts are being taken in doping TiO2 with nonmetals to achieve better band gap narrowing and visible light response. A variety of nonmetals such as carbon, nitrogen, fluorine, iodine, phosphorus, sulfur, and boron, etc. were found to be doped into TiO2 for the visible light active degradation of organic pollutants.9-16 Recent experimental studies have demonstrated that among the nonmetal dopants, carbon doping dramatically improves the photocatalytic activity of TiO2 in the visible-light region.17 In a study by Reddy and co-workers carbon-modified TiO2 nanoparticles were synthesized by sol-gel method using titanium chloride r 2011 American Chemical Society

(TiCl4) and tetra n-butyl ammonium hydroxide as precursors.9 The particle size of the resulted carbon-modified nanoparticles was found to be in the range of 12-15 nm. In another study by Wu et al. (2007), carbon-modified TiO2 spheres were formed on Ti substrates and carbon-modified TiO2 nanotubes were developed within nanochannels of alumina template by chemical vapor deposition. In the synthesis procedure, titanium tetraisopropoxide was used as a precursor of both titanium and carbon. The diameter of the formed carbon-modified TiO2 spheres varied from 100 nm to several micrometers, and the diameter of the TiO2 nanotubes was about 100 nm with a wall thickness of around 15 nm. The estimated optical band gap for the carbonmodified TiO2 microspheres was 2.78 eV and that of the carbonmodified TiO2 nanotubes was 2.72 eV.18 Ren et al. prepared a visible-light-active carbon-doped TiO2 photocatalyst by a hydrothermal method using glucose as carbon source. It was found that the resulting carbon-doped TiO2 exhibited significantly higher photocatalytic activity than the undoped TiO2 and Degussa P25 in the degradation of rhodamine B under visible light irradiation (λ > 420 nm).10 In the present study, an attempt has been made to synthesize nanosized titanium dioxide by an ultrasonication-assisted solgel process followed by calcination at low temperature. The assynthesized titania was then modified with coconut oil, the carbon precursor, in two different proportions. The carbon-modified catalysts were then subjected to thermal treatment with/without subsequent microwave irradiation. The pure and carbon treated titania samples were characterized by XRD, SEM, HR-SEM/ EDX, UV-vis absorption spectroscopy, and BET surface area Received: June 10, 2010 Accepted: January 11, 2011 Revised: November 25, 2010 Published: February 07, 2011 3130

dx.doi.org/10.1021/ie101259p | Ind. Eng. Chem. Res. 2011, 50, 3130–3138

Industrial & Engineering Chemistry Research analysis. The solar photoactivity of the carbon-treated catalysts was then monitored by the decolorization of a cationic dye, methlylene blue, and an anionic dye, methyl orange, in the presence of natural sunlight. Since dyes have the tendency of absorbing visible light, a solar photochemical study was also conducted with a visible light nonabsorbing pollutant, 2,4-dichlorophenol. The study demonstrates that the treatment of TiO2, with a minor proportion of coconut oil as a carbon source effectively enhances the solar photoactivity of the catalyst due to enhanced visible light response.

’ EXPERIMENTAL SECTION Materials. All chemicals used in the present study were of analytical grade. Titanium tetraisopropoxide (Spectrochem), isopropyl alcohol (SRL Chemicals), coconut oil (Parachute, India), methylene blue (Merck), methyl orange (Merck), and 2,4-dichlorophenol (Fischer) were used as received from the suppliers without further purification. All reagents and solutions were prepared using double distilled water. Preparation of TiO2 and C-Modified TiO2. Undoped TiO2 was prepared by an ultrasonication-assisted sol-gel process with titanium isopropoxide as the titania precursor. In the synthesis procedure, titanium tetraisoproxide was dispersed in isopropyl alcohol in a 1:1 w/w ratio under stirring using a magnetic stirrer for 2-3 min and then subjected to ultrasonication using a bathtype ultrasonicator (model: Sonorex RK 52 30 kV capacity). During the ultrasonication process, 30 mL of water was added dropwise to the mixture, and the ultrasonication was continued for 3.5 h for the formation of nanosized titania particles. The sol was then aged overnight and the resulting gel was initially dried in a water bath for the evaporation of solvent, ground to a fine powder, and then calcined at low temperature in the range of 80150 °C for 12 h in a hot air oven. The low temperature heating of TiO2 was preferred to provide an economical way of calcination, which also prevents the agglomeration of TiO2 particles through sintering and also avoids the formation of rutile phase. The assynthesized undoped titania was then designated as UD-TiO2 and a portion of it was then taken for carbon modification as follows. In the carbon modification process, the precursor used as carbon source was coconut oil. On the basis of the proportion of coconut oil, two different methods were followed in the preparation of carbon modified titania. In the first method, a thick paste of the titania was made with coconut oil and kept undisturbed for 24 h and the resulting material was designated as C-TiO2-1. In the second method, coconut oil was first dissolved in acetone and the solution was then uniformly mixed with the pure titania. While mixing, the pure TiO2, coconut oil, and acetone was added in the w/w ratio of 1:0.1:0.9. The acetone solvent was then evaporated by heating the mixture on a hot plate for 5 min. It was then kept undisturbed for 24 h, and the resulting material was designated as C-TiO2-2. After the mixture preparation, both the materials were subjected to two types of treatment, namely, (a) simple thermal treatment and (b) thermal treatment with subsequent microwave irradiation. In the simple thermal treatment process, the mixtures C-TiO2-1 and C-TiO2-2 were spread individually as a thin layer onto the surface of a hindalium metal plate and closed with a metal closure in such a way that the catalyst layer was not exposed to air. It was then subjected to thermal treatment at a temperature of 250 °C until there were no fumes. The resulting thermally treated catalysts were designated as C-TiO2-1a and

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C-TiO2-2a, respectively. In the second process, that is, the thermal treatment with subsequent microwave irradiation, a portion of C-TiO2-1a and C-TiO2-2a was taken and subjected to microwave irradiation for 1 h in medium-high mode using a domestic microwave oven (model: IFB 17PM-MEPI), to augment the crystallinity of the catalyst particles19 and the resulting catalysts were designated as C-TiO2-1b and C-TiO2-2b,respectively. In the synthesis procedure, plain microwave irradiation was not chosen, since the microwave irradiation alone was insufficient to remove the entire oil content of the mixture (in both the preparation methods). Hence, microwave irradiation was employed after the thermal treatment of the corresponding material. Catalyst Characterization. The synthesized undoped and carbon modified titania catalysts were characterized by high angle powder X-ray diffraction (XRD) (ET 816 X-ray diffractometer using Cu KR radiation λ = 1.5405 Å with scintillation counter as detector; 2θ = 10.000-70.000) and scanning electron microscopy (SEM) (JSM 6360 with carbon coating technique) to observe the surface morphology, high resolution scanning electron microscopy combined with energy dispersive X-ray spectroscopy (HR-SEM/EDX from Hitachi) (with gold coating technique) to obtain the particle size, and elemental composition, respectively, and UV-vis spectrophotometry (Cary 5E UV-vis spectrophotometer in the spectral range of 200-800 nm) to investigate the visible light response of the catalysts. The specific surface area of the samples was determined by BET surface area analysis (Micrometrics ASAP2020) with N2 physisorption at liquid nitrogen temperature. Sunlight-Induced Photocatalytic Experiments. The photochemical experiments were carried out in glass beakers with 200 mL of aqueous solution containing 0.1 g of catalyst and 10 mg/L of the dye (methylene blue/methyl orange). The pH adjustments of the dye solutions were carried out by the addition of diluted NaOH/ H2SO4, and the pH of all the solutions was measured using a calibrated pH meter (model: Elico L1 120). The dye suspensions were stirred in the dark for 1 h to attain the adsorption equilibrium and then subjected to daylight solar irradiation at noon for 3 h when the intensity of sunlight was ∼90 000 LUX, measured using a LUX meter (model: TES 1332). For every 30 min, an aliquot of 5 mL was taken and centrifuged to eliminate the catalyst, and the absorbance of the supernatant was then measured at the corresponding wavelength of the dye using a spectrophotometer (model: Systronics Visiscan 167). In the case of dyes, the decolorization may also be due to adsorption onto the catalyst to a large extent. Also, dyes have the tendency of absorbing visible light and undergoing partial degradation under solar light. Hence, a solar photodegradation experiment was also conducted with a visible light nonabsorbing pollutant, 2,4-dichlorophenol (DCP) to evaluate the degradation of visible light nonabsorbing organic compounds. In the experimental procedure, 200 mL of aqueous solution containing 100 mg/L of DCP was taken, and 2 drops of 1 M NaOH solution was added for better dissolution of chlorophenol in water. Subsequently, 0.1 g of catalysts was added, stirred in the dark for 1 h and subjected to solar irradiation as mentioned above. During the adsorption and irradiation period, aliquots of samples were collected at each stage, and the liberation of chloride ion into the DCP solution was measured using an ion chromatograph (model: Dionex DX120) equipped with a conductivity detector (Dionex, Wommelgem, Belgium) and a 4-mm anionic exchanger column (IonPac AS14). The sample injection volume was 25 μL and the mobile phase was a 3 and 1.5 mmol of sodium carbonate and sodium 3131

dx.doi.org/10.1021/ie101259p |Ind. Eng. Chem. Res. 2011, 50, 3130–3138

Industrial & Engineering Chemistry Research

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Figure 1. XRD patterns of (a) UD-TiO2, (b) C-TiO2-1a, (c) C-TiO2-1b, (d) C-TiO2-2a, (e) C-TiO2-2b.

bicarbonate solution with a flow rate of 1 mL/min. The extent of liberation of chloride ion into the DCP solution was then taken as an indication of the extent of degradation of the pollutant.

’ RESULTS AND DISCUSSION Crystal Characterization. To determine the crystal phase and crystalline nature of the undoped and carbon-modified titania catalysts, powder X-ray diffraction patterns of the catalysts were taken. Figure 1 panels a, b, c, d, and e represent the X-ray diffraction patterns of UD-TiO2, C-TiO2-1a, C-TiO2-1b, C-TiO2-2a and C-TiO2-2b titania catalysts, respectively. For all the catalysts, the peaks related to the anatase phase were predominantly obtained at the corresponding 2θ values of 25°, 37°, 48°, 55°, and 62°.13 Thus, the X-ray diffraction patterns reveal the predominant presence of anatase phase in both undoped and carbon-modified samples which may be attributed to the low temperature calcination of TiO2 (145 to 150 °C) during the synthesis rather than calcination at higher temperatures. The low temperature calcination prevented the formation of less active rutile phase which is evident from the absence of rutile phase peaks at 2θ values20 of 28°, 36°, 42°, and 57°. Also, the thermal treatment of the titania catalyst during the pyrolytic

carbon modification and microwave irradiation were not found to affect the crystal phase of the catalysts. Thus, the results reveal the predominant presence of the most active anatase titania phase in all the synthesized catalysts. In a study by Ren et al., similar results were obtained in which the low temperature hydrothermal treatment at 160 °C and carbon modification favored the formation of anatase phase.10 The XRD patterns also show that when compared to the undoped titania (UD-TiO2) and thermally treated C-TiO2 catalysts (C-TiO21a and C-TiO2-2a), the crystallinity of the microwave irradiated C-TiO2 catalysts (C-TiO2-1b and C-TiO2-2b) was much higher which is evident from the presence of more intense peaks in Figure 1c,e. This is because microwave irradiation produces heating without breaking or weakening the chemical bonds within the molecules.21 In a study conducted by Liu et al., also, much intense XRD patterns were obtained for MW irradiated TiO2 crystallites than the titania without MW irradiation.19 Morphological Characterization. The surface morphology of the undoped and carbon-modified titania catalysts was chracterized by scanning electron microscopy. Low- and high-magnification SEM images of the undoped and microwave irradiated carbonmodified titania samples (C-TiO2-1b and C-TiO2-2b) are presented in Figure 2. It could be seen from the low-magnification 3132



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Figure 2. Low- and high-magnification SEM micrographs of UD-TiO2 (a and b); C-TiO2-1b (c and d); and C-TiO2-2b (e and f).

SEM images that all the samples acquire a large number of petite identical spherical particles. Their corresponding high-magnification SEM images reveal that the morphology of both the undoped and carbon-treated titania particles was similar, which shows that thermal treatment of the catalyst during carbon modification did not affect the surface morphology of catalyst particles. It could also be seen from Figure 2 that most of the catalyst particles present in the SEM micrographs of undoped titania are agglomerated, whereas the particles shown in the SEM micrographs of microwave-treated C-TiO2 samples are present as tiny individual particles which facilitate the effective degradation of organic contaminants on each catalyst particle. This beneficial nature of the microwave treated C-TiO2 particles was due to the rapid and homogeneous heating produced by the microwaves, which induces better crystallinity.21,22 The high-magnification SEM images of MW irradiated C-TiO2 samples illustrate that the diameters of the spherical particles were below 1 μm. Similar kind of results were obtained in the study conducted by Wu et al., in which carbon doping at different temperatures yielded uniform microspheres with diameters within 2 μm.18 The particle size of the microwave treated C-TiO2 samples in this study was further confirmed with the aid of HR-SEM.

Figure 3 panels a and b show the HR-SEM images of C-TiO2-1b and C-TiO2-2b, which depict that spherical particles with diameters in the range of 90 nm were present in both the samples. The formation of such nanoparticles was attributed to the ultrasonication process during the sol-gel synthesis of the catalysts. Compositional Analysis. To determine the wt % of carbon in the carbon modified titania samples, EDS analysis was performed. Figure 4 panels a and b show the EDS spectra of C-TiO21b and C-TiO2-2b. Figure 4a shows a very high composition of 41.71% of carbon in C-TiO2-1b due to the addition of high amount of coconut oil in the synthesis procedure. In the case of C-TiO2-2b samples, the % of coconut oil added in the synthesis procedure was 10 wt % and the percentage of carbon shown in the corresponding EDS spectrum was 8.27 wt %. Remaining composition was contributed chiefly by titanium, then by oxygen. Optical Light Characterization. To inspect the visible light absorption characteristics of the undoped and carbon-modified titania samples, UV-vis absorption spectra of the samples were taken and is presented in Figure 5. From the UV-vis absorption spectra, it could be seen that the absorption edge of pure undoped titania (UD-TiO2) was at λ = 390 nm which corresponds with a 3133



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Figure 3. HR-SEM images of (a) C-TiO2-1b and (b) C-TiO2-2b.

Figure 4. EDS spectra of (a) C-TiO2-1b and (b) C-TiO2-2b.

Figure 5. UV-vis spectra of pure and carbon-modified titania catalysts.

band gap of 3.18 eV, being in good agreement with the reported bandgap of anatase titania (3.2 eV).23 For all the carbon-treated titania catalysts, a slight red shift of the absorbance spectrum toward visible light region was observed. This red shift clearly shows the modification of the TiO2 crystal lattice with carbon. A similar kind of justification was given in the study conducted by Wu et al., in which a control experiment was carried out with a coating of carbon on the surface of P-25 titania film and the UV spectrum was obtained. The spectrum was similar to the pure P-25 titania film. It was reported that there is no obvious absorption edge shift when the carbon is simply coated on the surface.17 In another study conducted by Sun et al., also, TiO2 catalysts were found to exhibit visible light response due to doping with carbon and sulfur.24

Surface Area Analysis. The specific surface areas of the samples were found to be 180 m2/g for UD-TiO2 and 135 m2/g for C-TiO22b. The surface area of the carbon modified titania sample seemed to be low when compared with pure titania, but, it is to be noted that the specific surface areas of both the titania samples before and after carbon modification were very high when compared with the commercially renowned Degussa P25 catalyst which is reported to be around 50 m2/g.25,26 Solar Photoactivity of Carbon-Modified Titania. To evaluate the solar-light-induced photoactivity of carbon-modified titania catalysts in the degradation of refractory organic compounds, a study was conducted to monitor the decolourization of methylene blue and methyl orange as model organic pollutants. 3134



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Table 1. Decolorization Profile of Methylene Blue Dye Using Carbon-Modified TiO2 at pH 5.8 (Initial Concentration of the Dye = 10.0 ppm) concentration of methylene blue (ppm) after corresponding irradiation duration concn of methylene blue after s. no.

sample code

adsorption (ppm)

30 min

60 min

90 min

120 min

150 min

180 min

1

UD-TiO2

9.33

6.57

3.76

2.21

1.47

0.88

0.37

2

C-TiO2-1a

6.35

4.48

3.10

2.28

1.52

1.16

0.56

3

C-TiO2-1b

6.12

4.36

3.32

2.14

1.40

0.66

0.51

4 5

C-TiO2-2a C-TiO2-2b

8.93 8.99

6.27 6.13

2.14 2.36

0.66 0.44

0.44 0.15

BDL BDL

BDL BDL

Table 2. Decolorization Profile of Methylene Blue Dye Using Carbon-Modified TiO2 at pH 9 (Initial Concentration of the Dye = 10.0 ppm) concentration of methylene blue (ppm) after corresponding irradiation duration concn of methylene blue s. no.

sample code

after adsorption (ppm)

30 min

60 min

90 min

120 min

150 min

180 min

1 2

UD-TiO2 C-TiO2-1a

6.33 7.65

5.37 5.9

2.86 3.7

1.83 2.56

1.23 2.00

0.66 1.71

BDL 0.60

3

C-TiO2-1b

7.02

5.52

3.41

2.32

1.42

0.84

0.40

4

C-TiO2-2a

6.10

2.60

0.54

BDL

BDL

BDL

BDL

5

C-TiO2-2b

6.03

2.56

BDL

BDL

BDL

BDL

BDL

Table 3. Decolorization Profile of Methyl Orange Dye Using Carbon-Modified TiO2 at pH 5.45 (Initial Concentration of the Dye = 10.0 ppm) concentration of methyl orange (ppm) after corresponding irradiation duration concn of methyl orange s. no.

sample code


30 min

60 min

90 min

120 min

150 min

180 min

1

UD-TiO2

9.2

8.09

7.08

6.21

5.33

3.72

2.1

2

C-TiO2-1a

7.88

7.01

6.12

5.11

4.78

3.82

3.01

3 4

C-TiO2-1b C-TiO2-2a

7.60 9.10

6.71 6.00

6.00 4.20

5.00 2.81

4.10 1.11

3.60 0.41

2.90 BDL

5

C-TiO2-2b

9.11

5.80

3.71

2.01

0.81

BDL

BDL

Table 4. Decolorization Profile of Methyl Orange Dye Using Carbon-Modified TiO2 at pH 3 (Initial Concentration of the Dye = 10.0 ppm) concentration of methyl orange (ppm) after corresponding irradiation duration concn of methyl orange s. no.

sample code


30 min

60 min

90 min

120 min

150 min

180 min

1

UD-TiO2

9.52

7.14

5.65

4.65

2.41

1.90

1.42

2

C-TiO2-1a

8.33

7.26

6.19

5.01

4.65

3.22

2.38

3 4

C-TiO2-1b C-TiO2-2a

8.57 9.05

7.03 3.18

5.96 2.02

5.19 1.19

4.49 0.61

3.11 BDL

2.14 BDL

5

C-TiO2-2b

9.52

2.38

0.476

BDL

BDL

BDL

BDL

The decolourization profile of methylene blue with undoped and carbon modified titania catalysts is given in Table 1. The results in Table 1 show that at the actual pH (pH 5.8) of methylene blue 93.9% of decolourization of the dye was observed in the case of the pure titania at the end of 3 h of sunlight irradiation. But, complete decolourization of the dye was observed after 2.5 h of irradiation in the presence of the carbon modified titania catalysts, “C-TiO2-2a and C-TiO2-2b” in which the percentage of carbon accounts to ∼8 wt %. This enhanced efficiency in the decolourization of the dye of the carbon modified catalysts was

due to the enhanced visible light absorption from the sunlight due to the red shift after modification with carbon. Whereas, in the case of C-TiO2-1a and C-TiO2-1b catalysts, only around 85% of the dye was found to be decolourized at the end of 3 h of sunlight irradiation. This reduced efficiency of the catalysts when compared to the efficiency of UD-TiO2, C-TiO22a and C-TiO2-2b was due to the coating of the TiO2 surface and blocking the interaction of TiO2 active sites with the pollutant, due to the large amount of carbon (∼42 wt %) present in the catalysts. This justification could be supported by the results 3135


Industrial & Engineering Chemistry Research obtained in the phenol photodecomposition study conducted by Tryba et al., in which the degradation efficiency of the carbonmodified samples with percentage of carbon exceeding the value of 8 wt % decreased. This was reported to be due to the slow process of phenol diffusion to the TiO2 surface through a thick layer of carbon.27 The decolourization profile of methylene blue at pH 9 is represented in Table 2. The results in the table reveal that a complete decolourization of methylene blue dye was observed at the end of 60 min in the case of C-TiO2-2b. Thus the rate of decolourization of methylene blue was high in the presence of the catalyst at pH 9 when compared to the decolourization rate at the actual pH of the dye where complete decolourization was observed only after 150 min (Table 1). This increase in the rate of decolourization of the dye at pH 9 could be explained as follows. The measured value of the pH of the zero point of charge (pHzpc) for TiO2 is around 5.7, evincing that the particle surface is predominantly positively charged below pH 5 and negatively charged in neutral and alkaline solutions. Since, methylene blue is a cationic dye, it is expected to be predominantly in its cationic form in alkaline pH, and hence, it prefers to electrostatically interact with the negatively charged catalyst surface and undergoes better degradation. Thus, due to the dissociation nature of the dye, the decolourization was maximum and faster in the alkaline pH when compared to that in the actual pH. Similar results were obtained in the study by Yun et al., in which the decolouization of methylene blue was favored28 at pH 9. Also, when observing the results from Table 2, the majority of methylene blue was found to be adsorbed after the dark adsorption period, which proves the maximum attraction of the cationic dye toward the catalyst in alkaline pH. It could also be observed from Table 2 that the MW-irradiated carbon modified catalysts possess a little superior degradation efficiency than the corresponding non-MW-irradiated carbonmodified catalysts. This may be due the improved crystallinity in the catalysts induced by the microwave radiation.

Figure 6. Chloride liberation into DCP after solar irradiation.

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In the case of methyl orange, the decolourization profile observed was similar to that in the case of methylene blue decolourization. As observed in Tables 1 and 2, the results represented in Tables 3 and 4 also show that the carbon-modified catalysts with 8 wt % of carbon was found to perform better than all the other catalysts. The catalysts with carbon content exceeding 8 wt % were found to be poorer performers than the undoped and the catalysts with 8 wt % carbon. As observed in the case of methylene blue, the decolourization of methyl orange was maximized in the pH at which the dissociation of the dye is maximum. Thus, decolourization was favored at pH 3 than at the actual pH (pH 5.45), since, methyl orange is an anionic dye. When a comparison is made of the rate of decolourization of the two dyes, methylene blue was found to be decolourized faster and better than methyl orange. This is due to the obvious reason that methyl orange is an azo dye and hence its degradation is complicated when compared to the degradation of nonazo dyes like methylene blue.29 When the results from Tables 1-4 are observed, it is clear that the carbon modified catalysts with 42 wt % of carbon majorly adsorbs the dyes, but, leastly degrades them. The degradation profile of the visible-light-nonabsorbing pollutant, 2,4-dichlorophenol (DCP) is given in Figure 6. 2,4Dichlorophenol (DCP) is a toxic derivative of phenol, which has no absorption in the visible region. The general photodegradation reaction of DCP could be expressed as in eq 1. TiO2 , hv, H2 O 2C6 H4 OCl2 þ 12O2 sf 12CO2 þ 2H2 O þ 4HCl ð1Þ Theoretically, for 100 ppm of 2,4-dichlorophenol, 43.5 ppm of chloride ion should be released into the solution in case of its complete degradation. The amount of chloride released into DCP after the adsorption period was negligible which showed that during the adsorption period the degradation of the pollutant was negligible (figure not shown). From Figure 6, it is clear that after 180 min of solar irradiation, the amount of chloride released into the DCP solution was significantly higher. The percent degradation of DCP by UDTiO2, C-TiO2-1a, C-TiO2-1b, C-TiO2-2a and C-TiO2-2b after 180 min of solar irradiation was calculated to be 31.5%, 32%, 35%, 48.6%, and 54%, respectively. When comparing the degradation profile in the case of the dyes and DCP, the results were as expected, that the degradation of the dyes was better than that of DCP, since dyes themselves play a role in absorbing visible light and undergo degradation. All the results represented in Tables 1,-4 and in Figure 6 depict that when compared to the undoped titania catalysts, the carbon-modified catalysts with ∼8 wt % of carbon performed better in the degradation of the organic pollutants, and the C-TiO2-2b catalyst was a little superior. This shows that the microwave irradiation of catalysts during carbon modification could yield better results when compared to the simple thermal treatment.

Table 5. Comparison between C-TiO2-2b and Carbon-Modified Degussa P25 in MB Decolourization at pH 9 (Initial Concentration of the Dye = 10.0 ppm) concentration of methylene blue (ppm) after corresponding irradiation duration concn of methylene blue s. no.

sample code


30 min

60 min

90 min

1

C-TiO2-2b

5.86

2.64

0.009

BDL

2

carbon-modified Degussa P25

6.17

2.95

0.600

BDL

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Industrial & Engineering Chemistry Research To compare the efficiency of the best-acting microwavetreated C-TiO2-2b catalyst with the commercial Degussa P25, a comparative study was also conducted with the Degussa P25 before and after carbon modification (same as done in the case of C-TiO2-2b) and C-TiO2-2b catalyst to decolourize methylene blue. The observed results are tabulated and reported in Table 5. The observed results show that the majority of methylene blue degradation is possible with both the carbon modified assynthesized titania (C-TiO2-2b) and Degussa P25 samples in 60 min, and the results are well comparable.

’ CONCLUSIONS C-modified TiO2 nanoparticles were prepared by thermal decomposition of naturally occurring coconut oil. The crystal phase characterization of the catalysts by XRD reveals the predominant presence of highly active anatase phase in both the undoped and C-modified catalysts. The characterization thus shows that low-temperature calcinations of the catalyst from 140-150 °C prevented the formation of less photoactive rutile phase. SEM charcterization of the as-synthesized C-modified catalysts reveal that the thermal treatment and microwave irradiation of the catalyst during carbon modification reduced the agglomeration of titania particles and yielded petite, individual particles when compared to the undoped catalyst. HR-SEM images of the microwave-treated C-TiO2 shows the presence of particles in the range of 90 nm. EDS spectra of the carbon modified samples showed that the mass% of carbon in C-TiO2-1a sample was around 42% and in C-TiO2-1b sample was around 8%. UV-vis spectra of the catalysts reveal the red shift toward visible region during carbon modification. The solar degradation of methylene blue, methyl orange, and 2,4-dichlorophenol reveals that the carbon modified catalysts with ∼8 wt % of carbon can decolourize dyes with a greater efficiency than the undoped catalyst. The performance of the best acting C-TiO2-2b was found to be comparable with the same carbon-modified Degussa P25 in the decolourization of methylene blue. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: þ91- 44 - 22359014. Fax: þ91 - 44 - 22354717.

’ ACKNOWLEDGMENT We thank the University Grant Commission (UGC), New Delhi, for fellowship to Jeevitha R. Raji. ’ REFERENCES (1) Kabra, K.; Chaudhary, R.; Sawhney, L. Treatment of hazardous organic and inorganic compounds through aqueous-phase photocatalysis: A review. Ind. Eng. Chem. Res. 2004, 43, 7683. (2) Seery, M. K.; George, R.; Floris, P.; Pillai, S. C. Silver doped titanium dioxide nanomaterials for enhanced visible light photocatalysis. J. Photochem. Photobiol., A 2007, 189, 258. (3) Zhang, X.; Udagawa, K.; Liu, Z.; Nishimoto, S.; Xu, C.; Liu, Y.; Sakai, H.; Murakami, T. Fujishma. Photocatalytic and photoelectrochemical studies on N-doped TiO2 photocatalyst. J. Photochem. Photobio., A 2009, 202, 30. (4) Xiao, J.; Peng, T.; Li, T. R.; Peng, Z.; Yan, C. Preparation, phase transformation and photocatalytic activities of cerium doped mesoporous titania nanoparticles. J. Sol. State Chem. 2006, 179, 1161.

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