TiO2-Coated Cenospheres as Catalysts for Photocatalytic

8908

Ind. Eng. Chem. Res. 2010, 49, 8908–8919

TiO2-Coated Cenospheres as Catalysts for Photocatalytic Degradation of Methylene Blue, p-Nitroaniline, n-Decane, and n-Tridecane under Solar Irradiation Praveen K. Surolia, Rajesh J. Tayade, and Raksh V. Jasra*,† Discipline of Inorganic Materials and Catalysis, Central Salt & Marine Chemicals Research Institute, Council of Scientific & Industrial Research (CSIR) G. B. Marg, BhaVnagar-364002, India

Fly ash cenosphere particles generated in coal-fired thermal power plants were coated with TiO2 using the sol-gel technique. Thus prepared photocatalysts were used for the degradation of methylene blue (MB), nitro-aromatic compound p-nitroaniline (PNA), and photocatalytic oxidation of n-decane and n-tridecane from aqueous solution under solar light irradiation. The morphology, composition, and crystallinity of both pristine and TiO2-coated cenospheres were studied. The UV-visible spectroscopic analysis was used to monitor the progress of the reaction for MB and PNA. The decrease in the amount of n-decane and n-tridecane was determined by gas chromatographic analysis. The decrease in the concentration for a 25 ppm initial concentration of MB and PNA was observed to be 99 and 26%, respectively, after 3 h of reaction time. This demonstrates that MB is more amenable toward degradation and mineralization as it acts as a sensitizer. Decreases up to 51 and 36% for n-decane and n-tridecane were observed with initial amounts of 3-12 mL, respectively, after 6 h. The coated cenosphere catalyst could be recovered by filtration and reused. The coating was found to be stable even after three uses of the catalyst in the photocatalytic degradation reaction of MB and PNA. The mineralization of dye and nitro-aromatic compound was confirmed by the chemical oxygen demand analysis. Cenospheres, because of their light weight, would make the catalyst particles float on the aqueous surface and hence could be directly activated by sunlight. 1. Introduction Photocatalysis has emerged as a potential technique for the degradation and mineralization of organic and inorganic pollutants1-6 for environmental detoxification. These applications, particularly for effluent treatment, are generally studied using catalysts in fine powdered form, which are cumbersome to recover and reuse. However, to make photocatalytic oxidation widely successful, the catalyst needs to be easily recoverable, reusable, and also active under solar light irradiation. The photocatalytic materials have a high density and do not float on the aqueous surface. The saturated hydrocarbons that have a density less than 1 g cc-1 get concentrated on the aqueous surface and take a longer time to decompose while using such catalysts. To overcome these problems, attempts have been made to prepare TiO2-coated buoyant materials such as polystyrene beads, sand, activated carbon, porcelain beads, and ceramic microspheres.7-14 These studies reported successful photooxidation but showed a lack of reusability due to leaching of photocatalytic material during use. Therefore, the strong adhesion of photocatalytic material on buoyant surface is needed so that the coating is stable even after multiple uses. Furthermore, the support’s surface must not get photooxidized, as this would also cause leaching of the TiO2 particles. In coal-fired power generation plants, the production of fly ash cenospheres is a major issue, as these are small particles (50 µm average particle size) and pose disposable and environmental problems. Research efforts are directed toward effective uses and value-added cenospheres by exploring their uses for the production of ceramics, zeolites, ceramic glass, and * To whom correspondence should be addressed. Tel: +91 265 6693935. Fax: +91 265 6693934. E-mail: [email protected] or [email protected]. † Present address: Reliance Technology Group, Vadodara-391346, Reliance Industries Limited, Gujarat, India.

other materials such as bricks, improved cements, and concretes. Fly ash cenospheres are stable, nontoxic, nonmetallic hollow particles that can float on the water surface, and there are few studies reported involving the cenosphere as a catalyst or catalytic support.7 The photooxidation of spilled petroleum constituents is the key factor in weathering processes in the marine environment15,16 in which hydrocarbons get transformed to water-soluble and small mobile compounds. Some studies deal with the photocatalytic transformation of hydrocarbons and possible application of photocatalysis in cleaning up marine oil spills.7,17,18 In this study, we report the synthesis of TiO2-coated cenosphere particles employing a sol-gel method to obtain stable TiO2 coatings. TiO2-coated cenosphere catalysts can float in aqueous solutions. The catalytic activity of these catalysts was investigated by the photocatalytic degradation of methylene blue (MB) and p-nitroaniline (PNA) as well as the photocatalytic oxidation of saturated hydrocarbons n-decane and n-tridecane under solar light irradiation. Although solar light consists of a small number of photons that have sufficient energy to promote the valence band electron of TiO2 to a conduction band, solar light-irradiated photocatalytic clean up can be quite fast, even with the residual photons in the wavelength range 300-390 nm. TiO2-coated cenosphere materials have potential for the photocatalytic oxidation of the saturated hydrocarbons, which makes a layer on the water surface during oil spillages and can be very perilous to the aquatic ecology. 2. Experimental Section 2.1. Chemicals and Materials. Titanium(IV) isopropoxide was purchased from Aldrich, USA. Fly ash cenospheres were obtained from a coal-fired power plant of the National Thermal Power Corporation (NTPC) (Shaktinagar, U.P. India). MB,

10.1021/ie100388m  2010 American Chemical Society Published on Web 08/27/2010

Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010

8909

Scheme 1. TiO2 Coating Process on the Cenosphere Surfaces: (a) Overall Coating Process, (b) Hydrolysis of Titanium Tetraisopropoxide, and (c) Condensation Reaction between Si-OH/Al-OH and Ti-OH Groups at Calcination Temperature

Figure 1. Particle size distribution of cenosphere particles.

PNA, n-decane, and n-tridecane AR grade were purchased from s.d. fine-chem Ltd. (Mumbai, India). Deionized, distilled water was used to make up the reaction mixture. 2.2. Synthesis of the Catalysts. Titanium tetraisopropoxide was chosen as a titanium source for the TiO2 coating on fly ash cenospheres. The coating procedure followed is described in Scheme 1. The 0.1 M solution of titanium tetraisopropoxide was prepared in 100 mL of dry ethanol, and this solution was stirred for 30 min to make the solution homogeneous. Five grams of cenospheres was added to this solution and allowed to stir for 12 h. The used cenosphere particles were not of the same size. The average particle size of used cenosphere was 47 µm with a range up to 120 µm size (Figure 1). This slurry was hydrolyzed with controlled addition of distilled water (rate of 1 mL/min) followed by solvent evaporation using a rotavapor. The sample was dried in an oven at 393 K and calcined at 723 K for 5 h under the air. 2.3. Photocatalytic Irradiation Experiments. The photocatalytic activity of the prepared catalysts was determined by the oxidation of MB, PNA, n-decane, and n-tridecane under solar light irradiation. The experiments were carried out in a Petri dish with an inner diameter of 90 mm and an outer diameter of 93 mm. This Petri dish was covered with another Petri dish having an inner diameter of 96 mm and an outer diameter of 99 mm to check the water vapor loss due to evaporation. The amount of the resultant solution obtained was 49 mL as compared to the 50 mL initial reaction sample amount with a loss of 1 mL. For MB and PNA, 50 mL of reactant solution was taken in the Petri dish, and 1 g of catalyst was spread over it. The initial concentrations of the MB and PNA taken for the study were 10 and 25 ppm. The progress of reaction was monitored by measuring the concentration decrease of MB and PNA at different irradiation time intervals. The decrease in the concen-

Figure 2. XRD pattern of (a) pristine cenosphere and (b) TiO2-coated cenosphere catalyst. The asterisk shows the peak for TiO2 at 2θ ) 25.3.

tration due to adsorption on the catalyst was determined by keeping one reaction sample in the dark for the time equal to the highest solar light illumination time used for the degradation, which was 1.5 h for 10 ppm MB and 3 h for the remaining

8910


Table 1. Conditions of the Day the Reaction Was Performed (a) Conditions of the Day the Reaction Was Performed with 10 ppm MB solar light intensity (Lux) reaction cycle a

time 11:00 11:30 12:00 12:30

a.m. a.m. noon p.m.

1

2

3

blank

March 01, 2008 (Saturday)

March 04, 2008 (Tuesday)

March 05, 2008 (Wednesday)

March 01, 2008 (Saturday)

60500 67800 68300 70100

61300 67200 68500 71000

60800 67500 71000 71200

60500 67800 68300 70100

(b) Conditions of the Day the Reaction Was Performed with 25 ppm MB solar light intensity (Lux) reaction cycle a

time 11:00 12:00 13:00 14:00

a.m. noon p.m. p.m.

1

2

3

blank

February 26, 2008 (Tuesday)

March 02, 2008 (Sunday)

March 03, 2008 (Monday)

February 29, 2008 (Friday)

61500 68800 71500 72100

61400 68200 72000 72400

61100 70300 72000 72500

62000 70500 72000 72700

(c) Conditions of the Day the Reaction Was Performed with 10 ppm NA solar light intensity (Lux) reaction cycle timea 11:00 12:00 13:00 14:00

a.m. noon p.m. p.m.

1

2

3

blank

April 14, 2008 (Monday)

April 16, 2008 (Wednesday)

April 18, 2008 (Friday)

April 14, 2008 (Monday)

61000 68300 72300 72500

61500 67000 72500 72500

61000 67500 72700 72500

61100 68000 72500 72600

(d) Conditions of the Day the Reaction Was Performed with 25 ppm PNA solar light intensity (Lux) reaction cycle timea 11:00 12:00 13:00 14:00

a.m. noon p.m. p.m.

1

2

3

blank

April 08, 2008 (Tuesday)

April 11, 2008 (Friday)

April 12, 2008 (Saturday)

April 10, 2008 (Thursday)

60500 68000 72500 72500

61000 67000 72000 72000

61000 67500 72200 72300

60700 67700 71500 72000

(e) Conditions of the Day the Reaction Was Performed with n-Decane solar light intensity (Lux) timea 10:00 11:00 12:00 13:00 14:00 15:00 16:00

a.m. a.m. noon p.m. p.m. p.m. p.m.

3 mL

8 mL

12 mL

March 21, 2008 (Friday)

March 25, 2008 (Tuesday)

April 02, 2008 (Wednesday)

60000 65000 71000 71000 62500 57500 56800

57500 60000 63500 70500 62500 58500 57200

56500 60500 65500 71000 71000 64000 59400

(f) Conditions of the Day the Reaction Was Performed with n-Tridecane solar light intensity (Lux) timea 10:00 11:00 12:00 13:00 14:00 15:00 16:00 a

a.m. a.m. noon p.m. p.m. p.m. p.m.

Time ) Indian standard time.

3 mL

8 mL

12 mL

May 02, 2008 (Friday)

May 12, 2008 (Monday)

May 22, 2008 (Thursday)

64500 68700 70100 72000 67500 62300 58700

62400 67800 69500 71900 65400 59300 57700

63800 68400 69900 72200 69300 64000 60200


8911

Figure 3. SEM images of cenospheres: (a and b) pristine cenospheres, (c and d) TiO2-coated cenospheres, and (e and f) TiO2-coated cenospheres after the reaction was performed.

reaction mixtures of MB and PNA. The possible degradation due to solar light alone was studied by running one blank run without the catalyst for 1.5 h for 10 ppm MB and 3 h for the remaining reaction mixtures of MB and PNA. The samples were filtered to separate the catalyst, and the solution was analyzed by an UV-visible spectrophotometer. The standard solutions of MB and PNA of 5, 10, 20, 30, 40, and 50 ppm were prepared, and the calibration curves were plotted for solutions by measuring absorption of these samples. The slopes of the curves were calculated and used to determine the concentration employing the Beer-Lambert’s law from the absorption data. The mineralization of MB and PNA in aqueous solution was confirmed by chemical oxygen demand (COD) analysis of the samples taken at different reaction time intervals. The COD was measured by using a HACH DR 2800 Photometer. The reagents for COD analysis and 3 mL of sample taken at different times were mixed together in glass cells and digested in a HACH DRB 200 Thermodigester for 2 h at 421 K. After digestion, the mixture was cooled down to room temperature, and the COD was measured using the photometer. The COD was measured for the original solution, and the filtered sample was taken out at different time intervals.

For n-decane and n-tridecane, the total 50 mL of reactant solution with both water and hydrocarbon was taken in a Petri dish, and 1 g of catalyst was spread over it. The initial amounts of n-decane and n-tridecane were 3, 8, and 12 mL in the aqueous phase. The progress of the reaction was monitored by considering the decrease in the initial amount of the hydrocarbon as analyzed by gas chromatography (GC). The calibration curves were plotted for both of the hydrocarbons, and for the purpose, the standard mixtures of the hydrocarbons with the toluene were prepared. The amounts of hydrocarbons were taken as 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 3.0, and 4.0 mL with a constant amount of 10 mL of toluene, and these mixtures were analyzed with GC. The calibration curves were plotted between the peak area ratio of hydrocarbon with toluene and the amount of hydrocarbon taken in the samples. The slopes of the curves were calculated, which were used for the calculation of the hydrocarbon amount. The decrease in the hydrocarbon concentration due to solar light irradiation that triggered evaporation was determined by running one blank run without the catalyst for the time equal to the maximum solar light illumination time for the degradation (6 h for each sample).

8912


The reactions were carried out in Bhavnagar city in the Gujarat state of India. This city is situated at latitude 21° 26′ North and longitude 72° 09′ East. The sample time and day that the reaction was performed and conditions of the reactions are given in the Table 1. The time given in the table is as per Indian standard time. After the irradiation, the samples were taken to the dark. MB and PNA samples were filtered to recover the catalyst and analyzed. The unconverted amount of the hydrocarbon in the samples was extracted with CH2Cl2. The CH2Cl2 was removed by solvent evaporation using rotavapor. The obtained hydrocarbon was mixed with a constant 10 mL amount of toluene and analyzed by GC (GC-1048, Shimadzu) with a packed column (chromopack column, length 25 m, and carrier gas nitrogen). The injector and detector temperature was kept at 200 °C, and the column temperature was programmed from 70 to 140 °C. 2.4. Catalyst Characterization. The X-ray diffraction data of the catalyst samples were collected on a Phillips X’pert MPD system using Cu KR1 (λ ) 0.15405 nm) radiation at 295 K. Diffraction patterns were taken over a 2θ range of 10-70° at a scan speed of 0.1° s-1. The BET surface area of the catalysts was determined from nitrogen adsorption data measured at 77.4 K using a Micromeritics ASAP 2010 volumetric adsorption apparatus. The band gap energy of the TiO2 network coated on the cenosphere surface was determined using diffuse reflectance spectroscopy (DRS). The spectrophotometer (Shimadzu UV3101PC) equipped with an integrating sphere and BaSO4 was used as a reference. The spectra were recorded at room temperature in the wavelength range of 225-800 nm. The band gap energy was calculated according to the equation band gap (EG) ) hc/λ where EG is the band gap energy (eV), h is Planck’s constant, c is the light velocity (m/s), and λ is the wavelength (nm). Fourier transform infrared (FT-IR) spectroscopic measurements were carried out using a Perkin-Elmer GX spectrophotometer. The spectra were recorded in the range 400-4000 cm-1 with a resolution of 4 cm-1 as KBr pellets. The morphology of the synthesized catalysts was determined using scanning electron microscopy (SEM) (Leo Series VP1430) equipped with INCA, energy dispersive system (EDX), and an Oxford instrument. This was also used to confirm the elements present in the cenosphere and the increased Ti elemental concentration at the cenosphere surface. The sample powder was supported on aluminum stubs prior to measurement. Phase identification of the coating was also conducted using transmission electron microscopy (JEOL JEM 2100 model 794). The optical micrographs were viewed under a light microscope (Carl Zeiss Axion imager). 3. Results and Discussion 3.1. Structural, Textural, and Electronic Properties. The X-ray diffraction measurement patterns of pristine and coated cenospheres are presented in Figure 2. All sharp peaks are identified to be due to either mullite M (3Al2O3 · 2SiO3) or quartz Q (SiO2), showing that the cenosphere particles are mainly composed of a mixture of mullite and quartz.19 The TiO2-coated samples exhibit a peak at 2θ ) 25.3° corresponding to the anatase phase of the titania. The anatase phase is responsible for the photocatalytic activity of the catalyst. The broad diffraction peak maxima centered at around 2θ ) 24° indicates

Figure 4. EDAX results of (a) pristine cenospheres and (b) TiO2-coated cenospheres. Table 2. Elemental Values of the Uncoated and Uncoated Cenospheres Particles by EDAX Analysis bare cenospheres

TiO2-coated cenosphere

element

weight %

atomic %

weight %

atomic %

Si Al Fe K Ca Ti O

22.86 13.91 3.00 2.23 0.31 1.00 56.70

16.24 10.29 1.07 1.14 0.15 0.42 70.70

13.98 9.07 1.19 1.42

10.17 6.86 0.43 0.74

15.47 58.88

6.60 75.19

the presence of the amorphous (glassy) aluminosilicate phase in the fly ash sample.20 The crystallinity of the pristine and coated cenosphere was found to be nearly the same from XRD data. SEM images of the uncoated and coated fly ash cenosphere particles show that the particles are spherical in shape, and TiO2 particles have formed a reasonably uniform layer on it (Figure 3). The cenospheres are formed as spherical in shape due to the surface tension forces acting on the melt to minimize the surface free energy during cooling of the fly ash. This coating is due to the development of a TiO2 network on the cenosphere particle surfaces as shown in Scheme 1. SEM images of the used TiO2-coated cenosphere catalysts also show the presence of TiO2 on the cenosphere surface, which confirms the stability of the coating. Some TiO2 clusters can also be seen to be deposited on the surface. EDAX analysis was carried out to find out the chemical composition of these particles (Figure 4). X-ray analysis confirms that these particles are mainly composed of a mixture of oxides of Si and Al, and the main components are mullite and quartz, while other oxides of trace elements such as K, Fe, Ca, Ti, and Mg are also present. The peaks corresponding to “Ti” and “O” were found to be increased after coating. The Si:Al ratio was observed to be the same before and after coating.


8913

Figure 5. Transmission electron micrographs (a-d) and selected area electron diffraction patterns (e and f) of TiO2-coated cenospheres.

The weight and atomic percentage values are given in Table 2. SEM and EDAX analysis suggest successful TiO2 coating on the surface of fly ash cenosphere particles by the sol-gel process. The coating and presence of the TiO2 layer on the cenosphere particles was also studied using TEM analysis, and typical micrographs and selected area electron diffraction patterns of the synthesized catalysts are shown in Figure 5a-d and e,f, respectively. The selected area electron diffraction patterns were taken at positions A and B shown in the circles in Figure 5d. TEM images show the hexagonal morphology of the TiO2 particles. Figure 5e shows the selected area electron diffraction pattern of TiO2 particles on the center of cenosphere surface, while Figure 5f shows the TiO2 particles from the side of the cenosphere particles. This selected area electron diffraction

pattern shows the planes 101, 004, 200, and 105 indexed to the anatase crystalline phase of TiO2. The optical micrographs were taken to confirm shapes of the particles and the stability of the coating toward the mechanical forces. The ultrasonication was applied as a mechanical perturbation force. The micrographs of the prepared catalyst are shown in Figure 6. The coated TiO2 particles were found at the cenospheres surface even after applying the ultrasonication force as shown in Figure 6c, confirming the stability of the coating toward the mechanical perturbations. FT-IR spectra of coated and uncoated fly ash cenospheres are shown in Figure 7. The uncoated fly ash cenosphere showed FT-IR bands at 1095 and 451 cm-1 with shoulders at 1189 and 552 cm-1. The bands at 1095 and 1189 cm-1 are attributed to the symmetric stretching vibration of Si-O-Si and asymmetric

8914


Figure 6. Optical micrograph of (a) pristine cenospheres, (b) TiO2-coated cenospheres, and (c) TiO2-coated cenospheres after ultrasonication treatment.

vibration of Al-O-Si, respectively, while the band at 451 cm-1 is attributed to the bending vibration of Si-O-Si. An additional band is observed at 552 cm-1 in the case of tectosilicate such as quartz, crystobalite, etc. The peaks at 800 and 720 cm-1 are due to the asymmetric stretching vibration and symmetric vibration of Si-O-Si and Al-O-Si, respectively. The peaks at 3441 and 1692 cm-1 are due to the OH-stretching and bending vibrations of the adsorbed water or surface hydroxyl groups.

The silicate bands are broad and diffuse because of the overlapping of different types of silicate molecular vibrations resulting from various silicate minerals. Hence, only those vibrations that are unique and do not interfere with other silicate minerals are observed distinctly in the spectrum. After coating of TiO2 on the cenosphere surface, the peaks due to the Si-O-Si and Al-O-Si are observed to decrease. This is due to the formation of Si-O-Ti and Al-O-Ti bonds at the surface of cenosphere. However, the bands of Si-O-Ti and Al-O-Ti near at 450 cm-1 are not identified individually. The surface water molecule and hydroxyl groups remain almost the same after the coating. The FT-IR spectra were also recorded after the use of a TiO2-coated cenosphere for the first and second reaction cycles. These spectra bear a resemblance to fresh TiO2coated cenospheres, which confirms the stability of the coating. This stability results due to the formation of strong Si-O-Ti and Al-O-Ti covalent bonds. The diffuse reflectance spectra of uncoated and TiO2-coated cenosphere are shown in Figure 8. Figure 8b shows the differential diffuse reflectance spectra. In the differential spectra of TiO2-coated cenospheres, a minimum was observed at 367 nm, and the calculated band gap is 3.3 eV. This spectrum confirms the presence of TiO2 at the cenosphere surface, and the formed TiO2 network exhibits the semiconductor nature of the material. The minimum was not observed in the differential spectra of bare cenosphere materials. The surface area of uncoated fly ash cenosphere was observed only at 0.72 m2 g-1, which confirms the nonporous nature of the material. After TiO2 coating, the BET surface area was found to increase to 45 m2 g-1, which is due to the porosity of TiO2 network developed on the surface of the cenosphere. The surface area was observed to decrease up to 26 m2 g-1 after the calcination, which could be due to the shrinkage of the TiO2 network after the calcination. 3.2. Photocatalytic Activity. 3.2.1. Photocatalytic Oxidation of MB and PNA. The two parameters used to compare the photocatalytic activity of the prepared catalyst material toward the degradation of MB and PNA were the final degradation percentage and the mineralization after 3 h of irradiation. Nearly complete degradation was observed for the 10 ppm MB within 1.5 h only. The reusability of the catalyst was studied for three consecutive cycles. For the recycling, the catalyst was filtered and dried under the solar light and reused for the reaction. The decrease in the concentration due to adsorption and blank study (under solar light in absence of catalyst) was also considered. Figures 9-11 show the concentration vs solar light irradiation curves, COD decrease, and percentage degradation, respectively, for MB and PNA. Each data point in these plots corresponds to an independent experiment that was stopped at the required time and analyzed to determine the MB and PNA content of the solution. 3.2.1.1. Removal of MB. Degradation was observed 99% after the 3 h for the 25 ppm initial concentration of MB (Figure 11). However, the reaction was stopped after 1.5 h only for the 10 ppm initial concentration of MB, as 98% degradation was observed within this time period. The concentration of MB decreased to 58 and 45% after the same reaction time 3 and 1.5 h as in the blank study for 25 and 10 ppm initial concentration of MB. The decrease in the concentration due to adsorption was 29 and 36% for 25 and 10 ppm initial concentration. However, these values are far less as compared to respective degradation values of 99 and 98% observed in the presence of the catalyst. The reusability of the catalyst was observed, and second and third runs were also carried out under


8915

Figure 7. FT-IR spectra of (a) pristine cenospheres, (b) TiO2-coated cenospheres, (c) TiO2-coated cenospheres after the first cycle, and (d) TiO2-coated cenospheres after the second cycle.

the same reaction conditions. The results given in Figures 9-11 show that the efficiency of the catalyst remains the same even after the three cycles. The decrease in the COD value was 30% for the blank experiment, while it was observed to be around 85% in the presence of the catalyst for all three runs for the 25 ppm concentration after 3 h. The decrease in the COD value for 10 ppm MB was observed as only 12% in the blank experiment, while a nearly 69% decrease in the COD value was observed in the presence of the catalyst in all of the three runs after 1.5 h of reaction time. The results demonstrate the contribution because of the catalyst for the degradation and mineralization of MB. 3.2.1.2. Removal of PNA. Figures 9c,d, 10c,d, and 11c,d show the photocatalytic results for the PNA degradation after the 3 h reaction time. The PNA degradation was observed 26 and 63% with 25 and 10 ppm of initial concentration after 3 h of reaction. Blank control experiments showed that the PNA concentration remains constant for 25 ppm initial concentration and was found to decrease to 10% with 10 ppm initial concentration when the irradiation was carried out under identical conditions but in the absence of any photocatalyst. This shows that PNA is more resistant toward photocatalytic degradation as compared to MB. The decrease in the PNA concentration due to adsorption was negligible in both cases. The reusability results confirm nearly the same efficiency of

the catalyst even after the third cycle in the case of PNA degradation as in MB degradation. The decrease in COD value was 1 and 3% only with the blank experiments for the 25 and 10 ppm concentration, respectively. These values were observed 17 and 51% in the presence of the catalyst for all of the three cycles. The results of MB and PNA degradation demonstrate that MB is more amenable towards degradation and mineralization as it acts as a photosensitizer and can absorb the photons more promptly in the solar light region (λmax for MB ) 644 nm and for PNA ) 380 nm). This photon energy then can get transferred to the catalyst for the e-/h+ generation in the catalyst and help for the degradation of the MB. Dyes have the visible light sensitivity and redox property, which can be used for photocatalytic systems.21,22 When the dyes are illuminated by visible light, these excite from S0 to S1 states by photon absorption. In the excited state, these can eject electrons to the conduction band of semiconductors to initiate the catalytic reactions, typically on an ultrafast time scale, leaving behind positively charged dye molecules. hν

dyefdye*

(1)

8916

Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010 Table 3. Percent Decrease in the Hydrocarbon Amount after the 6 h Reaction under the Solar Light n-decane

n-tridecane

amount of hydrocarbon (mL)

solar light/ catalyst

only solar light

solar light/ catalyst

only solar light

3 8 12

51 47 29

16 13 12

36 23 19

5 3 2

The e-/h+ pair generation process occurs in TiO2 particles attached at the cenospheres surface. These charge carriers migrate rapidly to the surface of catalyst particles where these are ultimately trapped and poised to undergo redox chemistry with suitable substrates. This process is supposed to occur at the aqueous surface as the coated cenosphere particles float at the aqueous surface. The presence of water facilitates the oxidation process by creating active hydroxyl radicals, which are well-known to take part in the photocatalytic oxidation process.23 The buoyant property of the prepared catalyst makes it effective for the photocatalytic oxidation of the saturated hydrocarbons, which forms a slick at the water surface. The photocatalytic oxidation process of hydrocarbons can occur in following steps: TiO2 + hV f TiO2(ecb- + hvb+)

(1)

O2 + TiO2(ecb-) f TiO2 + O2•-

(2)

O2•- + H2O f •OOH + OH-

(3)

OH- + TiO2(hvb+) f TiO2 + •OH

(4)

TiO2(hvb+) + H2O f TiO2 + •OH + H+

(5)

OOH + H+ + TiO2(ecb-) f TiO2 + •OH + H-

(6)

Figure 8. DRS spectra (a) and differential DRS spectra (b) of pristine and TiO2-coated cenosphere. TiO2

dye* f dye+ + e-

(2)

dye* + e- f dye

(3) •

Thus, the improved photocatalyst activity of TiO2 by dye is due to sensitization, inhibiting recombination by increasing the charge separation and increasing the spectrum response range, that is, excitation of wide band gap semiconductors by visible light. 3.2.2. Photocatalytic Oxidation of n-Decane and n-Tridecane. The progress of the reaction was monitored by considering the decrease in the initial amount of the hydrocarbon as analyzed by GC. The decrease in the amount of hydrocarbons under solar light in absence of catalyst was also considered. Figures 12 and 13 show the amount of hydrocarbon vs solar light irradiation time curves for n-decane and n-tridecane, respectively. Each data point in these plots corresponds to an independent experiment that was stopped at the required time and analyzed to determine the n-decane and n-tridecane contents of the solution. The decreases in the amount of the n-decane were observed 51, 47, and 29% for 3, 8, and 12 mL of initial amounts after 6 h of reaction under the solar light. The blank study showed the decrease in the amount of n-decane to be 16, 13, and 12% for 3, 8, and 12 mL of the initial amount, respectively, for the same period of 6 h as given in Table 3. The percentage decrease for the n-tridecane was observed as 36, 23, and 19% for 3, 8, and 12 mL of the initial amount of n-tridecane. The blank study showed the 5, 3, and 2% decreases for similar initial amounts after the 6 h solar irradiation time. The change in the amount of hydrocarbons was found to decrease with the increase in the initial amount taken.

•

•

RCH2CH2CH2R′ + OH f RCHCH2CH2R′ + H2O

•

•

RCOO- + OH + H+ f R + CO2 + H2O •

(7)

(11)

•

R + O2 f ROO f f f CO2 + H2O

(12)

Semiconductor photocatalyzed oxidation of organic compounds in aqueous system is explained in terms of band gap model of semiconductors.24 An electron promotes from the valence band to the conduction band by absorption of a photon with an energy equal or higher than the band gap energy (Eg), leaving an electron deficiency or hole in the valence band as shown in eq 1. Thus, the generated


8917

Figure 9. Concentration vs solar irradiation time for (a) 10 ppm MB, (b) 25 ppm MB, (c) 10 ppm PNA, and (d) 25 ppm PNA.

Figure 10. COD decrease with solar irradiation time for (a) 10 ppm MB, (b) 25 ppm MB, (c) 10 ppm PNA, and (d) 25 ppm PNA.

charged species e- and h+ pair reacts with a surface hydroxyl group or water25,26 and dissolved oxygen to produce hydroxyl, peroxide, and superoxide radical anions as shown from reaction steps 2-5. However, some studies report that adsorbed hydroxyl groups and water molecules cannot be photooxidized with VB holes.27 The structural •OH radicals can be photogenerated via electroreduction of •OOH with conduction band electrons as shown in eq 6. These hydroxyl radicals generated further react with saturated hydrocarbons n-decane and n-tridecane to initiate their oxidation to water-soluble organic products such as aldehydes, ketones, and carboxylates, which finally convert to

carbon dioxide and water as demonstrated in steps 7-12. It is a catalyzed free radical thermodynamically favorable spontaneous process and proceeds at ambient temperature. 4. Conclusions A photocatalyst with buoyant ability was synthesized by coating the cenosphere particles with TiO2 nanoparticles. The catalyst was successfully used for the photocatalytic oxidation of MB, PNA, and saturated hydrocarbons n-decane and ntridecane under solar light irradiation. The coating was found

8918


Figure 11. Percent degradation after the completion of reaction for (a) 10 ppm MB, (b) 25 ppm MB, (c) 10 ppm PNA, and (d) 25 ppm PNA.

Figure 12. Amount of n-decane with solar irradiation time for (a) 3, (b) 8, and (c) 12 mL.

to be stable even after three times of use of the catalyst in the photocatalytic degradation reaction of MB and PNA. The catalyst was efficient for the mineralization also, which was confirmed by the COD analysis. PNA showed resistance for the degradation as compared to MB. The blank and adsorption study showed that the presence of the catalyst and solar light was essential for the degradation as well as mineralization. The results demonstrated that the catalyst can be used effectively

Figure 13. Amount of n-tridecane with solar irradiation time for (a) 3, (b) 8, and (c) 12 mL.

for the aquatic organic pollutants, which float at the water surface under natural solar light.

Acknowledgment We thank the Council of Scientific and Industrial Research, New Delhi, for the financial assistance and analytical science discipline of the institute for analytical support.


Literature Cited (1) Fox, M. A.; Doan, K. E.; Dulay, M. T. The Effect of the “Inert” Support on Relative Photocatalytic Activity in the Oxidative Decomposition of Alcohols On Irradiated Titanium Dioxide Composites. Res. Chem. Intermed. 1994, 20, 711. (2) Kim, Y.; Yoon, M. TiO2/Y-Zeolite encapsulating intramolecular charge transfer molecules: a new photocatalyst for photoreduction of methyl orange in aqueous medium. J. Mol. Catal. A: Chem. 2001, 168, 257. (3) Lee, S.-K.; Mills, A. Detoxification of water by semiconductor Photocatalysis. J. Ind. Eng. Chem. 2004, 2, 173. (4) Mills, A.; Lee, S.-K. A web-based overview of semiconductor photochemistry-based current commercial applications. J. Photochem. Photobiol., A 2002, 152, 233. (5) Augustynski, J. The role of the surface intermediates in the photoelectrochemical behaviour of anatase and rutile TiO2. Electrochim. Acta 1993, 38, 43. (6) Al-Ekabi, H.; Seone, N. Kinetics studies in heterogeneous photocatalysis. I. Photocatalytic degradation of chlorinated phenols in aerated aqueous solutions over titania supported on a glass matrix. J. Phys. Chem. 1988, 92, 5726. (7) Nair, M.; Luo, Z.; Heller, A. Rates of Photocatalytic Oxidation of Crude Oil on Salt Water on Buoyant Cenosphere-Attached Titanium Dioxide. Ind. Eng. Chem. Res. 1993, 32, 2318. (8) Fabiyi, M. E.; Skelton, R. L. Photocatalytic mineralization of methylene blue using buoyant TiO2-coated polystyrene beads. J. Photochem. Photobiol., A 2000, 132, 121. (9) Matos, J.; Laine, J.; Hermann, J. M. Synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon. Appl. Catal., B 1998, 18, 281. (10) Piperata, G.; Meitchtry, J. M.; Litter, M. I. Photocatalytic reaction over TiO2 supported on porcelain spheres. Prog. Colloid Polym. Sci. 2004, 128, 303. (11) Lohse, S.; Rosentreter, J. J. Photooxidation of aqueous trichloroethylene using a buoyant photocatalyst with reaction progress monitored via micro-headspace GC/MS. Microchem. J. 2006, 82, 66. (12) Huo, P.; Yan, Y.; Li, S.; Huang, W. Preparation and characterization of Cobalt Sulfophthalocyanine/TiO2/fly-ash Cenospheres photocatalyst and study on degradation activity under visible light. Appl. Surf. Sci. 2009, 255, 6914. (13) Koopman, M.; Chawla, K. K.; Ricci, W.; Carlisle, K.; Gladsyz, G. M.; Lalor, M.; Jones, M. L.; Kerr, K.; George, M. P.; Gouadec, G.; Tournie, A. Titania-coated glass microballoons and cenospheres for environmental applications. J. Mater. Sci. 2009, 44, 1435.

8919

(14) Wang, S. Application of Solid Ash Based Catalysts in Heterogeneous Catalysis. EnViron. Sci. Technol. 2008, 42, 7055. (15) Payne, J. R.; Phillips, C. R. Photochemistry of petroleum in water. EnViron. Sci. Technol. 1985, 19, 569. (16) Bongiovanni, R.; Borgareello, E.; Pelizzetti, E. Oil spills in the aquatic environment: The chemistry and photochemistry at water/oil interface. Chim. Ind. (Milan) 1989, 71, 12. (17) Hashimoto, K.; Kawai, T.; Sakata, T. Photocatalytic reactions of hydrocarbons and fossil fuels with water. Hydrogen production and oxidation. J. Phys. Chem. 1984, 88, 4083. (18) Pelizzetti, E.; Minero, C.; Maurino, V.; Hidaka, H.; Serpone, N.; Terzian, R. Photocatalytic degradation of dodecane and of some dodecyl derivatives. Ann. Chim. (Rome) 1990, 80, 81. (19) Aixiang, Z.; Weihao, X.; Jian, X. Electroless Ni-P coating of Cenospheres using silver nitrate activator. Surf. Coat. Technol. 2005, 197, 142. (20) Matsunaga, T.; Kim, J. K.; Hardcastle, S.; Rohatgi, P. K. Crystallinity and selected properties of fly ash particles. Mine. Sci. Eng. A 2002, 325, 333. (21) Gurunathan, K.; Maruthamuthu, P.; Sastri, M. V. C. Int. J. Hydrogen Energy 1997, 22, 57. (22) Dhanalakshmi, K. B.; Latha, S.; Anandan, S.; Maruthamuthu, P. Int. J. Hydrogen Energy 2001, 26, 669. (23) Surolia, P. K.; Tayade, R. J.; Jasra, R. V. Effect of Anions on the Photocatalytic Activity of Fe(III) Salts Impregnated TiO2. Ind. Eng. Chem. Res. 2007, 46, 6196. (24) Yang, Y.; Ma, J.; Qin, Q.; Zhai, X. Degradation of nitrobenzene by nano-TiO2 catalyzed ozonation. J. Mol. Catal. A 2007, 267, 41. (25) Surolia, P. K.; Lazar, M. A.; Tayade, R. J.; Jasra, R. V. Photocatalytic degradation of 3,3′-dimethylbiphenyl-4,4′-diamine (ortho-tolidine) over nanocrystalline TiO2 synthesized by sol-gel, solution combustion and hydrothermal methods. Ind. Eng. Chem. Res. 2008, 47, 5847. (26) Poulios, I.; Makri, D.; Prohaska, X. Photocatalytis treatment of olive milling waste water: oxidation of protocatachuic acid. Global Nest Int. J. 1999, 1, 55. (27) Salvador, P. On the Nature of Photogenerated Radical Species Active in the Oxidative Degradation of Dissolved Pollutants with TiO2 Aqueous Suspensions: A Revision in the Light of the Electronic Structure of Adsorbed Water. J. Phys. Chem. C 2007, 111, 17038.

ReceiVed for reView February 20, 2010 ReVised manuscript receiVed July 17, 2010 Accepted July 27, 2010 IE100388M

TiO2-Coated Cenospheres as Catalysts for Photocatalytic

Recommend Documents