Ind. Eng. Chem. Res. 2006, 45, 5231-5238
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Transition Metal Ion Impregnated Mesoporous TiO2 for Photocatalytic Degradation of Organic Contaminants in Water Rajesh J. Tayade,† Ramchandra G. Kulkarni,‡ and Raksh. V. Jasra*,† Silicates and Catalysis Discipline, Central Salt & Marine Chemicals Research Institute, G. B. Marg, BhaVnagar-364002, India, and Department of Physics, Saurashtra UniVersity, Rajkot-360005, India
Mesoporous nanocrystalline TiO2 was prepared by hydrolysis of titanium isopropoxide, and the band gap of the TiO2 was modified with transition metal ions Ag, Co, Cu, Fe, and Ni having different work functions by the wet impregnation method. X-ray diffraction (XRD), X-ray photoelectron spectrophotometer, diffuse reflectance spectrophotometer (DRS), scanning electron microscope (SEM), and BET techniques were used for the characterization of the catalysts. By using the DRS technique, the highest red shift of 11 nm and lowest of 1.5 nm were observed for Ni and Fe ion impregnated catalysts, respectively. The investigations were carried out to demonstrate the effect of ionic radius and work function of metal ions on photocatalytic activity of mesoporous nanocrystalline TiO2 for degradation of acetophenone and nitrobenzene in aqueous medium under ultraviolet light irradiation. 1. Introduction Wastewater effluents from industry, at times, contain toxic organic chemicals which need to be treated prior to effluent disposal. Semiconductor photocatalysis is emerging as a potent technique for treating such effluents. The main advantage of photocatalysis lies in the fact that organic contaminants are completely mineralized without requiring secondary treatment of concentrated wastes. Furthermore, photocatalysis has been reported to have potential to be an effective method for treating a wide range of pollutants both from water and air.1-7 TiO2 has emerged as a most viable semiconductor photocatalyst as it is stable in aqueous media and is tolerant to both acidic and alkaline solutions.8 It is recyclable and relatively simple to prepare. Furthermore, its band gap includes the redox potential for the H2O/•OH reaction (-2.8 eV), thus allowing degradation of many organic compounds. However, it has limitation due to its wide band gap (3.2 eV) that is activated by UV radiation. As a result, only 5-8% of sunlight photons have the requisite energy to activate TiO2. Semiconductor photocatalysis takes advantage of the valence/ conduction band gap specific to semiconductor molecules. Incoming photons with energies at or above the band gap will cause valence electrons to become excited and move to the conduction shell, leaving holes in the valence band. These excited charge carriers can then react with molecules adsorbed on the semiconductor surface, thus acting as catalytically active species. There are several competing effects, which might limit the effectiveness of the catalysts. Most of the activated charge carriers will undergo recombination before reaching the surface to interact with adsorbed molecules. In fact, up to 90% of the generated carriers are lost within a nanosecond of their generation, leading to low photoactivity. The efficiency of TiO2 as a catalyst can be enhanced by three methods, namely increasing its surface-to-volume ratio, sensitization using dye molecules,9-10 and doping of nonmetals such as nitrogen, carbon, and sulfur and addition of metal ions.11-14 * To whom correspondence should be addressed. Tel.: +91 278 2471793. Fax: +91 278 2567562. E-mail:
[email protected]. † Central Salt & Marine Chemicals Research Institute. ‡ Saurashtra University.
The high surface-to-volume ratio inherent in nanoparticles is useful for photocatalysis so most of the studies were focused on the nanosized TiO2 with the purpose of improving the light absorption. Additionally, the small size of TiO2 crystals can make indirect band electron transition possible and increase the generation rate of electrons and holes. The principle of dye-sensitized TiO2 has been used for improving the TiO2 efficiency particularly in photovoltaics.15-17 Unfortunately, all of the known sensitizers used are toxic or unstable in aqueous medium, thus making them inappropriate for application in photocatalysis. Recently some investigation on the doping of nonmetals such as nitrogen, carbon, sulfur, and fluorine was done to have efficient photocatalytic activity.11-14 Addition of a low percentage of metal ion also improves the photocatalytic activity of the photocatalysts.18-19 The addition of metal can be achieved in different ways: doping, i.e., molecular combination of metal oxide in the lattice of TiO2; metallization, i.e., deposition of noble metal on TiO2 crystallite; impregnation of TiO2 with the salt of a metal followed by evaporation; addition of a low concentration of transition metal to the solution of substrate. It is proposed that the addition of certain transition metal ions offers a way to trap the charge carrier and extend the lifetime of one or both of the charge carriers improving the efficiency of the catalyst by decorating the particle surface with noble metals, which increases the surface charge transfer by stabilizing the electron-hole pairs once they reach the catalyst surface. The ions reported to be doped into the lattice of the TiO2 include Pt, Ag, Au, Cu, Ni, and Pd for prevention of electron and holes recombination.19-21 This is due to the reason that a metal with a work function higher than that of the semiconductor provides a Schottky barrier that facilitates the transfer of electrons from the semiconductor to the metal thus improving the catalytic efficiency. The transition metal-doped TiO2 is reported to be prepared by adding metal salt into the TiO2 colloid. Choi et al. have studied this in detail and have prepared 21 metal ion-doped colloids using this method and conducted systematic studies on them.22 This method allowed the metal ions to be located both in substitutional and interstitial positions of the TiO2 lattice. In addition, other methods such as sol-gel, mechanochemical doping, hydrothermal crystallization, metallorganic chemical
10.1021/ie051362o CCC: $33.50 © 2006 American Chemical Society Published on Web 06/14/2006
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vapor deposition, and advanced metal ion implantation23-27 have also been reported for preparing metal-doped TiO2. This study reports the photocatalytic characteristics of mesoporous nanocrystalline TiO2 including exploring the effect of impregnation of transition metal ions (∼0.5%) with different ionic radii and work functions on photocatalytic activity of TiO2 for degradation of acetophenone and nitrobenzene in aqueous medium using UV irradiation. The extent of degradation was followed by a UV spectrophotometer and COD measurements. Acetophenone degradation was used for measuring the comparative activities of the photocatalysts. Nitrobenzene was chosen as it is toxic in nature and is reported20-21,23-25 to be highly resistant to degradation or known to inhibit the biodegradation of other components of the waste in several biodegradation studies.26-33 The major source of nitrobenzene release into the atmosphere is during its manufacture and use as a chemical intermediate in the synthesis of aniline. Nitrobenzene is also found in wastewater from the organics and plastics industries. A small amount of nitrobenzene may cause mild irritation if it contacts the skin or eyes directly; however, repeated exposures to a high concentration of nitrobenzene can result in methemoglobinemia, a condition in which the blood’s ability to carry oxygen is reduced.24 This study demonstrated that the photocatalytic activity of the silver and nickel metal ion impregnated TiO2 photocatalysts has an increased initial rate of degradation as compared to that for synthesized bare TiO2 and standard P25 Degussa catalysts. 2. Experimental Section 2.1. Chemicals and Materials. Titanium tetraisopropoxide (97%), copper acetate, LR grade, and nickel acetate were procured from Aldrich, Milwaukee, WI. Silver nitrate, AR grade, was procured from Ranbaxy, Fine Chemicals Limited, Mumbai, India. Cobalt chloride, ferric chloride, and nitrobenzene, AR grade (99.0%), were procured from s. d. Fine Chem. Limited, Mumbai, India. Acetophenone, AR grade, and COD standard chemical reagents (solution A, 1.145 38; solution B, 1.14681, 1.14682) were purchased from E. Merck, Mumbai, India. 2.2. Catalysts Preparation. Bare mesoporous nanocrystalline TiO2 was prepared by hydrolysis of titanium isopropoxide. The mixture of absolute ethanol (100 mL) and titanium tetraisopropoxide (30 mL) was taken in a 250 mL round-bottom flask and continuously stirred for 30 min followed by 30 min of sonication (ULTRAsonik 28X). Distilled water (24 mL) was added at rate of 1.0 mL/min with continuous stirring (ca. 500 rpm). The thus obtained mixture was dried using a rotavapor (Buchi Rotavapor, R-205) under reduced pressure (350 mmHg) at 343 K. The powder was then kept in an oven at 398 K for 12 h. The dried sample was thoroughly grounded with an agate mortar and pestle and then was calcined at 773 ( 10 K temperature for 11 h in a tubular furnace under airflow (ramp rate ) 5 K min-1; flow rate ) 3 LPM). The catalyst thus obtained is termed as MT-10. Metal-impregnated mesoporous nanocrystalline TiO2 catalysts were prepared by the incipient wetness impregnation method.34 Typically, the prepared bare TiO2 catalyst (MT-10) was suspended in aqueous solutions of metal salts according to the metal used. The mixture was stirred for 48 h to get a loading of metal ion of 0.5% following which the slurry was dried in oven at 353 K for 12 h; the thus dried catalysts was thoroughly ground with an agate mortar and pestle and calcined at a 773 K temperature for 4 h. The impregnated mesoporous TiO2 catalysts with metal ion is denoted as Ti-M, where M indicates the impregnated metal ion.
2.3. Catalyst Characterization. The synthesized bare mesoporous TiO2 as well as transition metal impregnated catalysts were characterized by a powder X-ray diffractometer (XRD), and diffraction patterns were recorded at 295 K with a Phillips X’pert MPD system using Cu KR1 radiation (λ ) 0.154 05 nm). The diffraction pattern measured in 2θ ranged from 5 to 60° at a scan speed of 0.1° s-1. The XRD peaks of crystal plane 101 for anatase appeared at 25.3° (2θ), and that for crystal plane 110 for rutile at 27.4° (2θ). These peaks were selected to determine the percentage of anatase and rutile phases35 in the TiO2 sample. The percentage of anatase, A (%), was determined using the equation
A (%) ) 100/(1 + 1.265IR/IA)
(1)
where IR is the intensity of the rutile peak at 2θ ) 27.4° and IA is the intensity of the anatase peak at 2θ ) 25.3°. The crystallite size of TiO2 was determined from the characteristic peak of 2θ ) 25.3° (101) for anatase and 2θ ) 27.4° (110) for rutile using the Scherrer formula, with a shape factor36 (K) of 0.9:
crystallite size ) Kλ/W cos θ
(2)
Here W ) Wb - Ws, Wb is the broadened profile width of the experimental sample, Ws is the standard profile width of the reference silicon sample, and λ is the wavelength of X-ray radiation (Cu KR1 ) 0.154 05 nm). To determine the composition and the binding energy of the metal ion impregnated catalysts, analysis was carried out using a X-ray photoelectron spectrophotometer (PHI 1257, PerkinElmer, Eden Prarie, MN). The band gap energy of the catalysts was determined using diffuse reflectance spectroscopy (DRS). The spectrophotometer (Shimadzu UV-3101PC) was equipped with an integrating sphere, and BaSO4 was used as a reference.37 The spectra were recorded at room temperature in the wavelength range of 250-600 nm. The band gap energies of catalysts were calculated according to the equation
band gap (EG) ) hc/λ
(3)
where EG is the band gap energy (eV), h Planck’s constant, c the light velocity (m/s), and λ the wavelength (nm). An Oxford Instruments scanning electron microscope (Leo series 1430 VP) equipped with INCA, an energy dispersive system (EDX), was used to confirm the presence of impregnated of metal on mesoporous TiO2 as well as to determine the morphology of catalysts. The sample powder was supported on aluminum stubs using silver paint and then coated with gold by plasma prior to measurement. An inductively coupled plasma-optical emission spectrophotometer (Optima2000 DV, Perkin-Elmer, Eden Prarie, MN) was used to determine the percentage of the metal ion present in the degraded solution after performing photocatalytic experiments. Specific surface area, pore volume, and pore size distributions of catalysts were determined from N2 adsorption-desorption isotherms at 77 K by using a volumetric adsorption setup (ASAP 2010, Micromeritics, Norcross, GA). The catalysts were degassed under vacuum (10-2 Torr) at 573 K for 4 h, prior to measurement. Surface area and pore size distribution were determined using the BET equation and BJH method, respectively.37 2.4. Adsorption Studies in the Dark. Adsorption studies in the dark were performed separately using an aqueous 50 ppm solution of acetophenone and nitrobenzene with bare mesoporous TiO2 and with metal-impregnated TiO2 for 8 h at 25 °C in
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Figure 1. Spectral distribution of the UV source.
flat bottom flask with continuous stirring by a magnetic stirrer. Then, the 5 mL solutions were taken out after every 10 min for the first 1 h and then after every 1 h. The catalyst was separated from this solution by using a centrifuge, and the concentration of organic compounds was measured by a UV-vis-NIR spectrophotometer (Cary 500 Scan). 2.5. UV Irradiation Experiment Procedure. Photocatalytic degradation of organic compounds was carried out using a reactor consisting of two parts. The first part is an inner quartz double-wall jacket with inlet and outlet for the water circulation to maintain the temperature of the reaction mixture. This jacket has an empty chamber at the center for immersion of a mercury vapor lamp. The high-pressure mercury vapor lamp is used to carry out the UV irradiation with 125 W (Crompton Greaves Ltd., Mumbai, India). The spectral response of the subject UV source is as shown in Figure 1. The second part is the outer borosilicate glass container (volume 250 mL after insertion of the inner part) in which the reaction takes place. The magnetic stirrer was kept below the reactor for continuous stirring. The reaction mixture was withdrawn from the port by syringe. The photocatalytic activity of impregnated and bare mesoporous TiO2 catalyst was evaluated by measuring the decrease in concentration of acetophenone or nitrobenzene (pH ) 5) from the reaction solution. Prior to commencing illumination, a suspension containing 50 mg of the catalyst and 250 mL of aqueous solution of ca. 50 ppm of substrate was stirred continuously for 30 min in the dark. Following this, the sample was withdrawn by syringe from the irradiated suspension at each interval of 10 min for the first 1 h and every 1 h afterward. For analysis the catalyst was separated by centrifuge from the aqueous solution prior to analysis. The concentrations of organic compounds in the solution were determined by a UV-visible spectrophotometer (Cary 500). The oxygen equivalent of the organic matter of a sample, i.e., chemical oxygen demand (COD), was measured by using a Spectroquant Nova 60 photometer. The reagents for COD analysis and 3 mL of a sample taken at different times were mixed together in glass cells and digested in a Spectroquant TR 320 Thermodigester for 2 h at 421 K. After digestion, the mixture was cooled to room temperature and the COD was measured using the photometer. The COD was measured for the original solution and the centrifuged sample taken out at different time intervals.
Figure 2. XRD pattern of metal ion impregnated and bare TiO2 catalysts.
3. Results and Discussion 3.1. Structural Properties. Figure 2 shows the X-ray diffraction pattern of the bare TiO2 catalyst and metal ion impregnated TiO2 catalysts. The major crystalline phase detected in prepared mesoporous nanocrystalline TiO2 was anatase (91%), with rutile being observed as 8%. The XRD pattern also shows that the bare catalyst is highly crystalline in nature with a crystallite size of 38 nm. The XRD pattern (shown in Figure 2) of impregnated catalysts shows that the crystallinity of metalimpregnated catalysts was nearly same after impregnation of metal ions and without any detectable impregnated metal ions peak. This could be due to the fact that the impregnated metal ion goes to the substitutional sites on the TiO2 lattice or octahedral interstitial sites or may be because of the small amount of impregnation of the metal. Due to the smaller ionic radii of Fe and Ni as compared to Ti, impregnated metal ions can easily substitute into the TiO2 lattice, while due to the larger ionic radii silver could take the interstitial position into the TiO2 lattice.38 The crystallite size of TiO2 depends on the calcination temperature, and it is reported that, with an increase in calcination temperature, crystallite size also increases.39 During our synthesis, we have kept fixed the calcination temperature at 773 ( 10K. For transition metal impregnated samples, an increase of 6-12 nm in the crystallite size as compared to the bare catalyst was observed. A change in color of TiO2 (Table 1) was observed on transition metal impregnation. The percentage of the anatase and rutile phases and the crystallite sizes of all the catalysts are given in Table 1. The percentage of anatase to rutile in all the catalysts after impregnation of the metal ion is nearly the same. 3.2. Textural Properties. The surface area plays a major role in the photocatalytic reactions. The BET surface areas of various catalysts given in Table 1 vary from 26 to 38 m2/g. Figure 3 shows the pore size distribution curve and the corresponding nitrogen adsorption-desorption isotherms (inset) of MT-10. All the samples are observed to possess mesopores with pore radii in the range 56-70 Å. The adsorption isotherm was found to be of type IV, with a hysteresis that is typical for mesoporous materials. The isotherms for all metal-impregnated catalysts were of a similar nature. There was no change observed in the shape of the isotherm plot after metal impregnation on the catalyst.
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Figure 3. Isotherm plot (insert) and pore size distribution of mesoporous TiO2 MT-10. Table 1. Properties of the Catalysts catalyst property anatase phase content (%) rutile phase content (%) crystallite size (nm) BET surface area (m2 g-1) pore diameter (Å) ionic radius of impregnated metal (Å) work function of metal band edge (nm) red shift (nm) band gap (eV) color of catalyst
MT-10 92 8 38 38 112 384 3.229 white
Ti-Ag
Ti-Co
Ti-Cu
Ti-Fe
Ti-Ni
91 9 44 32 132 1.26 4.73 393 09.0 3.155 white
90 10 49 26 132 0.74 5.00 392 10.0 3.163 light green
90 10 50 27 137 0.72 4.70 389.5 05.0 3.183 light yellow
90 10 45 31 128 0.64 4.50 385.5 01.5 3.216 light yellow
90 10 48 30 139 0.72 5.15 395 11.0 3.139 light pink
The data for the N2 sorption study in Table 1 show that the impregnation of metal on bare catalyst has reduced the surface area by about 6-12 m2 g-1 only. This decrease in the surface area is indicative of the impregnation of metal on the bare catalyst. It was observed that the morphology of all the metalimpregnated catalysts is nearly the same (Figure 4); all the catalysts are of spherical shape with wide distributions of spherical particles present in range of 2-4 µm. The results of the energy dispersive system (EDX), with analysis given in Table 2, show the presence of metal ions in the synthesized
Figure 4. SEM images of catalysts.
Table 2. Energy Dispersive System Results of Catalysts catal
Ti wt %
O wt %
M wt %
MT-10 Ti-Ag Ti-Co Ti-Cu Ti-Fe Ti-Ni
63.63 52.75 54.65 67.77 59.87 69.13
36.37 46.74 44.71 31.79 39.86 30.35
0.51 0.64 0.44 0.47 0.52
catalysts. The amount of impregnated material was nearly the same as was taken for impregnation. 3.3. Electronic Properties. Comparison of the diffuse reflectance spectra of bare and impregnated catalysts showed a small change in the band gap of bare and metal-impregnated catalysts (Figure 5). The catalysts prepared by this method may form a doped semiconductor structure. The background observed in the DRS spectra could be due to the presence of anions retained on titania during the impregnation of the metal salt. It is observed from the data in Table 1 that the diffuse reflectance
Figure 5. DRS spectra of catalysts.
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Figure 6. Differential spectra of DRS.
Figure 8. Degradation of organic compounds using metal ion impregnated catalysts (A) acetophenone and (B) nitrobenzene.
Figure 7. XPS analysis of metal-impregnated catalysts of the Ti 2p region.
spectra of all the impregnated TiO2 catalysts have extended a red shift. The observed red shift was 1.5 nm in the case of Fe and 11 nm in the case if Ni showing that the nature of impregnating ion influences the band gap value. The observed red shift could be attributed to a charge-transfer transition between the metal ion and the TiO2 conduction or valance band.21,45-46 To get a more precise band edge for metal ion impregnated catalysts and the bare catalyst, a differential calculation was done and the resulting curves are shown in Figure 6 with values given in Table 1. The shifts in the binding energies of titanium in the various catalysts determined using XPS are shown in Figure 7. The peaks for metallic titanium are expected at 453.8 (Ti 2p3/2) and 459 (2p1/2) eV.43 Our results showed that the peaks get shifted to 460.5 and 466.3 eV for silver, 462.3 and 467.7 eV for cobalt, 463.0 and 468.9 eV for copper, and 461.9 and 467.6 eV for nickel. As the percentage of impregnated metal ion is small, a peak related to the impregnated metal ion could not be observed. The observed shift in the binding energy of titanium in the catalyst show the presence of metal ion in the catalysts which is also supported by the shift in band gap of the catalysts measured by DRS as well as surface analysis using EDX.
3.4. Photocatalytic Activity. The photocatalytic activities for the degradation of AP and NB for the bare catalyst and impregnated catalysts are compared in Figure 8 with an error of ((5-10) × 10-5 M. It is observed that the extents of adsorption of NB from aqueous solution on catalysts MT-10, Ti-Ag, Ti-Co, Ti-Cu, Ti-Fe, and Ti-Ni were 16, 22, 8, 8, 10, and 20%, respectively, and in case of AP it was 9, 26, 16, 16, 14, and 18%, respectively. From these data, it appears that silver- and nickel-impregnated samples show higher adsorption for both AP and NB. As higher degradation is observed for silver- and nickel-impregnated TiO2, compared to other metal ions, it seems that adsorption could be helping in more efficient degradation. During the photocatalytic experiments the leaching of metal ion was observed. The leaching of silver, cobalt, copper, nickel, and Fe metal ion was observed to be 4.25, 4.6, 2.2, 6.7, and 18.0%, respectively, after performing the photocatalytic experiments. As the leaching of the transition metal ions is low, the metal ion impregnated TiO2 (except for Fe2+/3+) could be used as a photocatalyst. The results show that the photocatalytic activity for the degradation of AP and NB gets enhanced in ion-impregnated catalysts. In particular, for Ag- and Niimpregnated catalyst, the degradation of AP was observed to increase to 100% from 89% with the bare catalysts while it was 91, 97, and 98% in the case of Co, Fe, and Cu metal ion impregnated catalysts in a 4 h time duration. Similarly the degradation of NB was found to increase from 57% with the bare catalyst to 89, 88, 84, 83, and 72% for Ag-, Co-, Cu-, Fe-, and Ni-impregnated catalysts, respectively. These data show that the photocatalytic activity of TiO2 increases with transition metal ion impregnation in case of AP while in case of NB only silver-
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Figure 9. Initial rates of degradation of AP and NB using catalyst (A) MT-10, (B) Ti-Ag, (C) Ti-Co, (D) Ti-Cu, (E) Ti-Fe, and (F) Ti-Ni under UV light.
and nickel-impregnated catalysts showed higher photocatalytic activity maybe due to better charge separation. In our earlier studies we reported the highest initial rate of NB degradation 8.45 × 10-6 M/min using the synthesized nanocrystalline TiO2 catalysts.47 The catalysts prepared by this method showed the higher initial rate of degradation in the case of silver and nickel metal ion impregnated catalysts to 13.6 × 10-6 and 8.6 × 10-6 M/min. The initial rates of degradation of AP and NB were determined and were observed to be in the order of Ti-Ag > Ti-Ni > Ti-Cu > Ti-Co > Ti-Fe > MT-10 and Ti-Ag > Ti-Ni > Ti-Cu > Ti-Co > MT-10 > Ti-Fe (Figure 9). Figure 9 shows the initial rate of photocatalytic degradation of AP using all metal ion impregnated catalyst was higher as compared to standard photocatalyst Degussa-P25. However, for AP and NB photocatalytic activity was observed to be higher for silver and nickel metal ion impregnated catalyst as compared to Degussa-P25. It is also observed that the catalysts Ti-Co showed less initial rate of degradation as compared to MT-10 in the case of AP and NB degradation, whereas Ti-Cu showed less initial rate of degradation for NB while it was higher in the case of AP. The different photocatalytic activities of the metal ions could be explained in terms of their location in TiO2 and variation in adsorption of NB or AP in metal-impregnated TiO2. The combination of these factors could result in the observed findings. The decreases in COD values for both organic compounds are tabulated in Table 3. The decrease in COD values confirms the degradation of the organic compounds. Both organic compounds were found to show the highest decrease in COD values using the Ag-impregnated catalyst. The decrease in COD values for other metal ions is not significantly different for both acetophenone and nitrobenzene as seen in Table 3.
In this study, the synthesized catalysts have nearly the same anatase and rutile phases and nearly the same amount of metal ion impregnation with a slight change in the surface area. However, there was a significant change in the band gap of some of the metal ion impregnated catalysts which could be responsible for different photocatalytic activities of the catalysts. The photocatalytic activities of all the catalysts were observed as different for both the substrates which further shows that the photocatalytic activities of the catalysts depend on the types of substrates. The initial rate of degradation was found different for both substrates. It is reported19-21,39,45 that the addition or impregnation of a transition metal on the nanocrystalline TiO2 photocatalyst surface can enhance the photocatalytic degradation activity due to the charge trapping. The process of charge trapping is as follows:38
Ti4+ + ecb- f Ti3+ Mn+ + ecb- f M(n-1)+ Mn+ + hvb+ f M(n-1)+ OH- + hvb+ f OH• Here Mn+ is the impregnated metal ion. The energy level of Mn+/M(n-1)+ lies below the conduction band edge. Thus, the energy level of transition metal ions affects the trapping efficiency. The trapping electrons make it easy for holes to transfer onto the surface of TiO2 and react with OH- in the organic compound solution and form active OH•, hydroxyl radicals which participate in the degradation of organic compounds. For effective degradation reaction, the lifetime of electron and holes is critical. The lifetime of the holes can be
Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006 5237 Table 3. COD of Organic Substrates Using Bare and Metal-Impregnated Catalysts COD of reaction mixture of AP and NB with respect to irradiation time (mg/L) acetophenone (initial COD ) 120 mg/L)
nitrobenzene (initial COD ) 90 mg/L)
catal
1h
2h
3h
4h
1h
2h
3h
4h
MT-10 Ti-Ag Ti-Co Ti-Cu Ti-Fe Ti-Ni
50 27 32 35 32 28
35 21 22 26 29 18
27 14 16 18 17 13
20 08 18 10 14 09
35 16 37 39 41 48
28 14 25 28 31 37
22 10 19 21 24 26
19 06 10 12 16 13
enhanced by trapping electrons, thereby reducing the recombination of rate and allowing holes to diffuse to the particle surface and participate in oxidation reaction. If the energy level of dopant ions moves toward the conduction band edge, the efficiency of trapping becomes higher. In that case the traps have a larger tendency to act as shallow traps so that the holes generated by following photons cannot recombine with the already trapped electrons. Consequently, the lifetime of free holes can be extended. Our results demonstrated that the photocatalytic activity depends on the substrates and impregnated metal ion. These results show that the different metal ions have different effects on impeding the transportation of electrons and holes from the interface to the surface, thereby having different effects on catalyst efficiency. The impregnated metal ions at the interstitial positions are more helpful than the substitutional positions to enhance the photocatalytic activity of the photocatalyst. Table 1 shows the ionic radii of different metal ions; it is clear that except for the silver metal ion having a substantially higher ionic radius (1.26 Å) than Ti4+ (0.68 Å), all other metal ion having radii in the range 0.64-0.74 Å can have the Ti4+ substitutional position. The highest photocatalytic activity was obtained for silver-impregnated catalyst; this may be due to the maximum charge trapping because of the silver metal ion. The order of initial rate shows that the highest photocatalytic activity obtained for silver-impregnated catalysts is due to the interstitial position of impregnated silver metal ion in the TiO2 lattice which causes better charge separation and, therefore, less recombination. If the work function of the metal is higher than that of TiO2, then electrons are removed from TiO2 particles in the vicinity of each metal particle. This results in the formation of Schottky barriers at each metal-semiconductor contact region and results in charge separation. Similar enhancement in the photocatalytic activity of modified TiO2 surface by silver metal ion using various methods was reported for the degradation of organic compounds, dyes, and dye intermediates.48,49 One of the reasons is that the modified surface facilitates charge trapping.19-21,45,46 Tran et al.50 has reported enhanced photooxidation of carbohydrates and carboxylic acids with Ag/TiO2 that has been attributed to attack by photogenerated holes. It is further explained that silver deposits generate a greater number of holes by acting as electron sinks and assisting their transfer to solution which reduces the possibility of their recombination. Other metal ions such as copper, cobalt, iron, and nickel have smaller ionic radii (Table 1), which causes the substitutional position of metal ions in the TiO2 lattice. In case of metal ions at substitutional position catalysts, the results show that the metal ions which are having a higher work function showed the higher initial rate of degradation. However, Fe-impregnated catalyst showed an initial rate of degradation of AP slightly higher than the bare catalyst and it
was lower in the case of NB. The lack of enhancement by Fe loading was reported for the degradation of vinyl chloride and 2-chlorophenol26,51 also. The lower photocatalytic activity of Fe ion still not clear, but it could be related to the nonoptimal impregnation of Fe and the ionic radii of Fe. It also may be the source material used for the impregnation contained chloride anions which inhibit the photocatalytic activity of the catalysts. 4. Conclusions The photocatalytic activity of transition metal ion impregnated TiO2 was found to increase the degradation of acetophenone and nitrobenzene present in aqueous solution. The XRD results demonstrated that the there were no changes in the structure and crystallinity of the bare catalyst after impregnation of the metal ions (∼0.5%). However, there was a slight increase in the crystallite size of the metal-impregnated catalysts of 6-12 nm. The results of the N2 sorption study show that the BET surface area of the bare TiO2 reduced by 6-12 m2 g-1 and there was an increase in pore size by 1.62-2.65 nm in metalimpregnated catalysts. The highest red shift of 11 nm and lowest of 1.5 nm were obtained for Ni and Fe, respectively, in metalimpregnated catalysts. The initial rate of the photocatalytic degradation of AP and NB varies due to the change in band gap of the catalyst, work function, ionic radii, and the position of the impregnated metal ion on the TiO2 lattice. The silverimpregnated catalysts showed the highest initial rate of photocatalytic degradation for both compounds due to the interstitial position of impregnated silver metal ion in the TiO2 lattice. Acknowledgment We are thankful to the Council of Scientific and Industrial Research, New Delhi, and Dr. P. K. Ghosh, Director, CSMCRI, for the financial assistance and support. We are also thankful to Dr. Jince Sebastian, Mr. Shobhit Singh Chauhan, Mr. C. K. Chandrakanth, Dr. Pragnya Bhatt, Dr. Amajd Hussain for analytical support, and Dr. K. H. Modi for the COD measurement facility. Literature Cited (1) Lee S.-K; Mills A. Detoxification of water by semiconductor photocatalysis. J. Ind. Eng. Chem. 2004, 2, 173-187. (2) Mills A.; Lee S.-K. A web-based overview of semiconductor photochemistry-based current commercial applications. J. Photochem. Photobiol., A: Chem. 2002, 152, 233-247. (3) Ollis, D.; Pelizzetti, E.; Serpone, N. Photocatalyzed destruction of water contaminants. EnViron. Sci. Technol. 1991, 25, 1522-1529. (4) Hoffman, M. R.; Martin, S.; Choi, W.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. ReV. 1995, 95, 69-96. (5) Peral, J.; Domenech, X.; Ollis, D. F. Heterogeneous photocatalysis for purification, decontamination and deodorization of air. J. Chem. Technol. Biotechnol. 1997, 70, 117-140. (6) Zhao, J.; Yang, X., Photocatalytic oxidation for indoor air purification: a literature review. Build. EnViron. 2003, 38, 645-654. (7) Choi, H.; Stathatos, E.; Dionysiou, D. D. Sol-gel preparation of mesoporous photocatalytic TiO2 films and TiO2/Al2O3 composite membranes for environmental applications. Appl. Catal., B: EnViron. 2006, 63, 60-67. (8) Mills A.; Hunte S. L. An overview of semiconductor photocatalysis. J. Photochem. Photobiol., A: Chem. 1997, 108, 1-14. (9) Li, B.; Wang, X.; Yan, M.; Li, L. Preparation and characterization of nano-TiO2 powder. Mater. Chem. Phys. 2002, 78, 184-188. (10) Nagaveni K.; Sivalingam, G.; Hegde, M. S.; Madras, G., Solar photocatalytic degradation of dyes: high activity of combustion synthesized nano TiO2. Appl. Catal., B: EnViron. 2004, 48, 83-93. (11) Irie, H.; Watanabe, Y.; Hashimoto, K. Carbon-doped anatase TiO2 powders as a visible-light sensitive photocatalyst. Chem. Lett. 2003, 32, 772-773.
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ReceiVed for reView December 7, 2005 ReVised manuscript receiVed May 2, 2006 Accepted May 15, 2006 IE051362O
