- Email: [email protected]
spherical activated carbon
Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Preparation, characterization and photocatalytic activity evaluation of micro- and mesoporous TiO2/spherical activated carbon Mi-Hwa Baek a, Won-Chae Jung a, Ji-Won Yoon a, Ji-Sook Hong a, Young-Seak Lee b, Jeong-Kwon Suh a,* a b
Research Center for Environmental Resources and Processes, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, Republic of Korea Department of Fine Chemical Engineering and Applied Chemistry, BK21-E2 M, Chungnam National University, Daejeon 305-764, Republic of Korea
A R T I C L E I N F O
Article history: Received 10 May 2012 Accepted 28 August 2012 Available online 3 September 2012 Keywords: TiO2 Photocatalysis Spherical activated carbon Adsorption Humic acid
A B S T R A C T
The influences of heat-treatment temperature and activation time on the properties of TiO2 supported on spherical activated carbon (TiO2/SAC) were investigated. Nano-sized TiO2 was dispersed on the spherical activated carbon with the size of 10–30 nm. Some anatase phase of TiO2 was transformed to rutile phase of TiO2 with an increase of heat-treatment temperature. All of the TiO2/SAC photocatalysts had microporous structure, with the mesopore volume increasing over an activation time of 6 h. The TiO2/SAC photocatalysts obtained at activation times of 6 h and 9 h were observed synergistic effects between adsorption and photocatalysis in the removal of humic acid. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Humic acid as model compounds of natural organic matter (NOM) can react with chlorine during water disinfection processes to produce disinfection by-products (DBPs) such as trihalomethanes, a known carcinogen [1–3]. Thus, the removal of humic acid is required prior to chlorination of drinking water. Recently, numerous approaches have been studied for the elimination of HA from water [1–6]. The conventional treatments of humic acid such as biodegradation, coagulation, and activated carbon adsorption are relatively slow processes and the removal efficiency is comparatively low [4–6]. However, photocatalysis is a fast and effective method for elimination of humic acid from aqueous solution [3,6]. In water purification systems, titanium dioxide (TiO2) is becoming an irresistible photocatalyst due to its excellent oxidizing behavior. TiO2 is chemically inert, non-toxic, cheap, efficient and with long-term stability in reactions [7–12]. The powders such as P25 TiO2 have been commercially available for several years. Powder type TiO2 has high photocatalytic activity due to its large surface area and high degree of dispersion in reaction media. However, the powder type TiO2 leads to several problems such as: (1) the difficult process of filtration or
* Corresponding author. Tel.: +82 42 860 7334; fax: +82 42 860 7533. E-mail address: [email protected] (J.-K. Suh).
separation of the powder from the solution after a photocatalytic reaction, (2) the aggregation of particles in suspension, in the case of high loadings, and (3) its application to continuous flow systems is quite challenging [13–17]. Due to the aforementioned problems, photocatalysts (e.g. TiO2) have been immobilized on various supports such as silica gel, alumina, and activated carbon [13,14,16–18]. Especially, activated carbon has been extensively studied as a support for TiO2 photocatalyst [14]. Of all types of activated carbons, the spherical activated carbon has the advantages of a smoother surface, better fluidity, higher mechanical strength, and lower resistance to liquid diffusion over the powdered and the granular activated carbons [19–25]. Ion-exchange resin has been used as suitable precursor to prepared spherical activated carbon due to its high carbon yield and low ash content [21,26,27]. We have developed a spherical activated carbon containing photocatalysts (ZnS and ZnO) that has the characteristics of both activated carbon and photocatalyst [28–30]. It was reported that the spherical activated carbon containing ZnS or ZnO have good photocatalytic activity [28–30]. The aim of this study is to enhance the adsorption capacity and the photocatalytic activity of the TiO2/SAC by controlling activation time. In this work, influence of activation time on the TiO2/SAC photocatalysts properties was investigated by various physicochemical techniques. In addition, the photocatalytic activity of TiO2/SAC photocatalysts was evaluated for the degradation of humic acid under UV irradiation.
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.08.026
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
470
2. Experimental 2.1. Materials The strong acid ion-exchange resin (SAIR, Diaion SK1BH, Samyang Co. Ltd.), brown color with a diameter range of 400– 500 mm, was used as a support of the TiO2/SAC photocatalysts. Titanium trichloride (TiCl3, 20%, Kanto Chemical) as a Ti precursor was used without any further purification. Commercial TiO2 powder P25 used as a reference was obtained from Degussa Chemical, Germany. Humic acid was purchased from Aldrich Chemical Company, which has carbon, hydrogen, and nitrogen content of 39.44%, 4.74%, and 0.79%, respectively. Double-distilled water was used in all the experiments. 2.2. Preparation of photocatalysts The TiO2/SAC was prepared as follows [29]: 24.2 mL of titanium trichloride was added into 730 mL distilled water under the stirring at 150 rpm for 10 min, and 300 g SAIR was added intermittingly with continuous stirring for 1 h at room temperature. The sample was filtered and washed several times with distilled water until the pH values of the solution reached neutral pH, and then dried at 110 8C for 24 h in an oven. The dried sample was heat-treated for good stability as a support and also for effective conversion to carbonaceous porous material. A schematic diagram of the heat-treatment process is presented in Fig. 1. A quartz tube reactor of 10 cm height and 6 cm diameter was used in the heat-treatment process. The sample was stabilized at 300 8C for five hours in atmosphere at a heating rate of 1 8C/min using a tube furnace. Carbonization followed at 700 8C for 10 min under a nitrogen flow of 2 L/min with a heating rate of 3 8C/min. Further heating of the same sample under a nitrogen at a heating rate of 1 8C/min was carried out at 900 8C and then activated under nitrogen/steam at 900 8C for 0.5–9 h. The flow rate of nitrogen gas was 2 L/min and distilled water was infused into the quartz tube reactor by a pump at 15 mL/min. The TiO2/SAC photocatalysts obtained at 300 8C and 700 8C were labeled TiO2/SAC-300 and TiO2/ SAC-700. According to different activation times of 0.5, 1, 2, 6, and 9 h, the TiO2/SAC photocatalysts obtained at 900 8C were assigned TiO2/SAC-900-0.5, TiO2/SAC-900-1, TiO2/SAC-900-2, TiO2/SAC900-6, and TiO2/SAC-900-9, respectively.
crystalline phases of the TiO2/SAC photocatalysts, XRD analysis was performed using X-ray diffractometer (XRD, D/MAX-IIIB, Rigaku) with Cu-Ka radiation and operating at 2.0 kW with a scanning speed of 58/min over the 2u range of 20–808. Scanning electron microscopy (SEM, JSM-6700F, JEOL) and field emission transmission electron microscopy (FE-TEM, Tecnai F20, FEI Co.) were used to observe the surface morphology of the TiO2/SAC photocatalysts. Distribution of elements in the TiO2/SAC photocatalysts was analyzed using scanning transmission electron microscopy combined with energy dispersive X-ray spectrometer (STEM-EDS). Energy-dispersive X-ray spectroscopy (EDS, Bruker Quantax 200) was also used to confirm the titanium content in the TiO2/SAC. The specific surface area and pore structure of the TiO2/ SAC photocatalysts were measured by the Brunauer–Emmett– Teller (BET) method using a Micromeritics ASAP 2010 apparatus. In order to compare and evaluate the compressive strength of each sample, the single sphere strength of each sample was measured using compressive strength meter, as described in [29]. The titanium ion amount in the solution before and after ion-exchange reaction was analyzed with inductively coupled plasma (ICP, iCAP 6000 series, Thermo Scientific). 2.4. Preparation of humic acid solution Stock solution of humic acid was prepared by dissolving 1 g of humic acid in a liter of double-distilled water and was kept for one day. This solution was filtered through 0.45 mm filter paper to remove all suspended solid. As a result, the concentration of stock solution of humic acid is less than 1000 mg/L. The stock solution was stored in a refrigerator (<4 8C) and dark environment to prevent natural light degradation. The stock solution of humic acid was diluted fifty times to use in adsorption and photocatalytic experiments. The initial concentration of humic acid and total organic carbon (TOC) were 20 mg/L and 8 mg/L, respectively. 2.5. Adsorption and photocatalytic activity Adsorption equilibrium experiments were performed by adding 1.2 g TiO2/SAC photocatalysts with 200 mL humic acid solution. The solution was stirred using a shaker with 25 8C and 150 rpm and kept under dark conditions for 10 h. The amount of humic acid adsorbed by the TiO2/SAC qt (mg/g) at each time interval was calculated according to the expression:
2.3. Characterization qt ðmg=gÞ ¼ X-ray photoelectron spectroscopy (XPS, ESCALAB 210, VG Scientific Ltd.) was used to investigate the chemical composition and state of the photocatalysts. In order to elucidate the
Fig. 1. Schematic diagram of the heat-treatment process.
ðC 0 C t ÞV m
(1)
where C0 and Ct are the initial concentration of humic acid (mg/L) and the remaining concentration of humic acid at certain time (mg/ L), respectively. V is the volume of the humic acid solution (L), and m is the amount of the TiO2/SAC used (g). Degradation of humic acid under UV irradiation was performed to evaluate the photocatalytic activity of TiO2/SAC photocatalysts, as described in [29]. 16 W low-pressure mercury lamp (lmax = 254 nm, G18T5C, LIGHTTECH) was used as a UV light source. Photocatalysis experiments were carried out using a 2 L batch-type reactor (40 cm in height, 10 cm in diameter), and by adding 6 g of the TiO2/SAC photocatalysts to 1 L of humic acid aqueous solution, after which the solution was stirred in the dark for 10 h in order to reach the adsorption equilibrium prior to illumination. Air was supplied into the photocatalytic reactor at a rate of 1 L/min for smooth floating of photocatalysts. The pH of the solution was not controlled and all photocatalytic experiments were carried out at room temperature. At regular intervals, certain volume of the samples was withdrawn from the photoreactor. Reference experiment was conducted under the same conditions without the TiO2/SAC photocatalysts. In order to compare the
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
(A) O 1s
C 1s
Intensity (CPS)
prepared TiO2/SAC photocatalysts with commercially available TiO2, degradation of humic acid on P25 TiO2 was also achieved. In the photocatalytic experiment using P25 TiO2, collected samples were centrifuged at 13,000 rpm for 15 min to separate P25 TiO2 particles from humic acid solution before analysis. In order to examine change of humic acid concentration in solution according to reaction time, the absorbance of the collected solution was measured by UV–vis spectrophotometer (Qvis 4000, Cmac) at a wavelength of 254 nm. TOC of the solution was analyzed with a total organic carbon analyzer (TOC-5000A, Shimazu). In order to check the leaching of titanium during long-term water treatment, the amount of titanium in the solution before and after photocatalysis experiment was analyzed by ICP.
471
Ti 2p
3. Results and discussion 600
Remaining concentration of titanium ion after ion-exchange reaction was analyzed through ICP to assess the efficiency of ionexchange. The result showed that efficiency of titanium ionexchange was 97%.
3.1.2. XRD analysis The phase structure change of the TiO2 was investigated using X-ray diffraction. The XRD patterns of the TiO2/SAC photocatalysts obtained at different heat-treatment temperatures and activation times are presented in Fig. 4. The SAC before TiO2 immobilization is an amorphous phase. At a stabilization temperature of 300 8C, a broad peak at 2u = 25.28, 37.68, 48.28, 55.18, and 62.98 corresponds to the anatase [37–40]. Increasing the carbonization temperature up to 700 8C, the peak intensities of anatase increased and became sharp, indicating the formation of greater TiO2 crystallites and enhancement of crystallization [40]. Small peak at 2u = 27.58, 36.18, 41.38, 44.18, and 56.78 corresponds to rutile which began to appear
400
300
200
Binding energy (eV)
TiO2 /SAC-300
Ti 2p3/2
TiO2 /SAC-700
3.1. Physicochemical characteristics
(B)
TiO2 /SAC-900
Intensity (CPS)
3.1.1. XPS study The XPS survey spectrum of the TiO2/SAC-300 is shown in Fig. 2(A). The characteristic energy spectrum of Ti, O, and C can be observed, and binding energies for Ti 2p, O 1s, and C 1s were 458.6, 531.3, and 284.5 eV, respectively. The Ti 2p and O 1s peaks was attributed to Ti4+ and O2, and the C ls corresponds to the SAC. Fig. 2(B) displays the XPS spectra of Ti 2p for the TiO2/SAC photocatalysts prepared at 300–900 8C. When the temperature of heat-treatment was 300 8C, the two peaks located at 458.9 eV and 464.5 eV can be assigned to the binding energies of Ti4+ 2p3/2 and Ti4+ 2p1/2, respectively [31–33]. The binding energies of Ti4+ 2p3/2 and Ti4+ 2p1/2 for the TiO2/SAC photocatalysts prepared between 700 8C and 900 8C were similar to that of 300 8C. It can be concluded that titanium element existed mainly as state of Ti4+ in the TiO2/SAC. XPS spectra of C 1s for the TiO2/SAC photocatalysts prepared at different heat-treatment temperatures are shown in Fig. 3. The C 1s spectra of SAIR show three peaks: 284.7 eV, graphitic carbon; 286 eV, carbon in phenolic, alcohol, ether; 290.6 eV, carbon in carbonate groups [34]. The TiO2/SAC photocatalysts prepared at 300 8C showed additional peak, located at 288 eV which can be assigned to Ti-O-C structure [35]. Also for the TiO2/SAC photocatalysts obtained at 700 8C and 900 8C, their binding energies at 285.3 eV may be assigned to C–O bond [36]. For all the TiO2/SAC, a main peak was observed at 284.7 eV due to C–C bond. This peak increased significantly as the heat-treatment temperature increased and this behavior may be traced to the sp2-bonded carbon content. From the results of XPS, it can be seen that titanium ion crystallized and the SAIR changed to carbonaceous materials through heat-treatment.
500
Ti 2p1/2
468
466
464
462
460
458
456
Binding energy (eV) Fig. 2. (A) XPS survey spectrum of the TiO2/SAC-300 and (B) XPS spectra of Ti 2p for the TiO2/SAC photocatalysts prepared at different heat-treatment temperatures.
when the activation temperature was increased to 900 8C [39–41]. This demonstrates that some of the anatase type was transformed into the rutile type during heat-treatment, which agrees with other literatures [38–40,42]. That is, heat-treatment temperature has great influence on phase transformation of TiO2. Also, reported by Ambrus et al. [43], anatase transforms to rutile at temperatures between 600 8C and 1100 8C depending on the primary particle size and the method of preparation. As shown in Fig. 4, the intensity of the anatase peak decreased, whereas the intensity of the rutile peak increased sharply with an increase in the activation time from 0.5 h to 6 h. At the TiO2/SAC-900-9, the main peak was rutile while anatase peak nearly disappeared. The crystallite size of TiO2 was determined by XRD results, as summarized in Table 1. The anatase crystallite size increased with increasing heat-treatment temperature, suggesting that high temperature led to TiO2 crystal growth. The anatase crystallite size gradually decreased whereas rutile crystallite size increased as activation time increase. 3.1.3. SEM image and strength of photocatalyst The morphologies of the SAIR and TiO2/SAC photocatalysts obtained at different heat-treatment temperatures and activation times were observed by SEM. Fig. 5(A) illustrates the SEM photograph of the SAIR, which has spherical shapes. As shown in Fig. 5(B)–(E), the TiO2/SAC-700, TiO2/SAC-900-0.5, TiO2/SAC-9002, and TiO2/SAC-900-6 maintained smooth surfaces and good
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
472
Fig. 3. XPS spectra of C 1s for the TiO2/SAC photocatalysts prepared at different heat-treatment temperatures.
Table 1 Characteristics of the TiO2/SAC photocatalysts prepared at different heat-treatment temperatures and activation times. Sample
TiO2/SAC-300 TiO2/SAC-700 TiO2/SAC-900-0.5 TiO2/SAC-900-1 TiO2/SAC-900-2 TiO2/SAC-900-6 TiO2/SAC-900-9
Temp. (8C)
300 700 900 900 900 900 900
Activation time (h)
0.5 1 2 6 9
Specific surface area (m2/g)
– – 391 478 604 1274 1427
spherical shapes with a diameter of 450–600 mm after the heattreatment and activation process. The SEM images of TiO2/SAC-300 and TiO2/SAC-900-1 were similar to Fig. 5(A)–(E). While for the TiO2/SAC-900-9, cracks can be seen in some spheres as shown in
Fig. 4. XRD patterns of the SAC, TiO2/SAC-300, TiO2/SAC-700, and the TiO2/SAC photocatalysts prepared at 900 8C for different activation times.
Total pore volume (cm3/g)
– – 0.153 0.195 0.260 0.863 1.217
Crystallite (nm)
size
Anatase
Rutile
7.9 17.9 28.1 14.9 14.1 11.8 11.6
– – 8 11.9 13.1 14.6 14.7
Ti content (wt.%)
– – 2.7 3.6 3.8 6.2 10
Fig. 5(F). The TiO2/SAC photocatalysts prepared by the ionexchange method and activation process, used in the present study, can immobilize titanium ion without a binder and can also maintain a smooth surface. This smooth surface can prevent problems of surface abrasion by inter-particle collision in floating photocatalytic application. Moreover, the TiO2/SAC photocatalysts possessing a good spherical shape with a diameter in the range of 450–600 mm can be separated from the solution without a special separation process after the photocatalytic reaction. The compressive strength of the TiO2/SAC photocatalysts obtained at activation time of 0.5–6 h was high and in the range of 7–10 kg/unit. It can enhance durability in floating photocatalysts systems. But, the TiO2/SAC-900-9 had relatively low compressive strength of 4 kg/unit; this is due to the cracking phenomenon, which equally agrees with the result of Fig. 5(F). More cracks and strength loss may occur when activation time goes beyond 9 h. 3.1.4. TEM image and EDS analysis Fig. 6(A) and (B) displays the FE-TEM image of TiO2/SAC-900-6. The results showed that the sizes of TiO2 measured directly from TEM photograph were approximately 10 to 30 nm. These values did not agree with crystallite size obtained from XRD results. It was
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
473
Fig. 5. SEM images of the TiO2/SAC: (A) SAIR, (B) TiO2/SAC-700, (C) TiO2/SAC-900-0.5, (D) TiO2/SAC-900-2, (E) TiO2/SAC-900-6, and (F) TiO2/SAC-900-9.
evident that TiO2 in the TiO2/SAC photocatalysts contains amorphous TiO2. To investigate the distribution of elements, an EDS mapping of the elements was performed. Fig. 6(C)–(F) exhibits STEM image and EDS elemental mapping of the TiO2/SAC-900-6.
The mapping images represent the presence of carbon, titanium and oxygen atoms in the TiO2/SAC. It also reveals the distribution of nano-sized Ti in the SAC matrix and the distribution of O is similar to that of Ti.
Fig. 6. FE-TEM image of the TiO2/SAC-900-6 (A), magnified image (B), STEM image (C), and EDS elemental mapping of the TiO2/SAC-900-6 (D–F).
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
474 800
(A)
TiO2 /SAC-900-1
700
3
Volume Adsorbed (cm /g) STP
as COx and SOx, which were progressively released with extending activation time.
TiO2 /SAC-900-0.5 TiO2 /SAC-900-2 TiO2 /SAC-900-6
600
TiO2 /SAC-900-9
500 400 300 200 100 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Relative Pressure (P/P0 ) 1.0
(B)
TiO2 /SAC-900-0.5 TiO2 /SAC-900-1 TiO2 /SAC-900-2 TiO2 /SAC-900-6
3
Pore volume (cm /g-nm )
0.8
TiO2 /SAC-900-9
0.6
0.4
0.2
0.0 10
20
30
40
50
60
70
80
90
100
Pore diameter (nm) Fig. 7. (A) Nitrogen adsorption–desorption isotherms and (B) pore-size distribution calculated adsorption branches of the isotherms for the TiO2/SAC photocatalysts prepared at different activation times.
An EDS analysis was performed to determine the content (wt.%) of titanium in the TiO2/SAC. The titanium in the TiO2/SAC photocatalysts was clearly identified from EDS and the titanium content increased with increasing activation time from 0.5 h to 9 h (Table 1). Especially, the TiO2/SAC-900-9 had the highest titanium content of 10 wt.%. This may be attributed to volatile matters such
3.1.5. BET analysis As shown in Table 1, total pore volume slightly increased with increasing activation time from 0.5 h to 2 h. However, total pore volume greatly increased as activation time increase from 6 h to 9 h, indicating the formation of mesoporous structure. As a result, the steam activation time has an effect on the specific surface area and could enhance porosity [44,45]. Fig. 7(A) shows the nitrogen adsorption–desorption isotherms for the TiO2/SAC photocatalysts prepared at different activation times. The isotherms of the TiO2/SAC-900-0.5, TiO2/SAC-900-1, and TiO2/SAC-900-2 were observed as type I according to IUPAC classification, indicating dominant microporous character and occurrence of pore filling at low relative pressures [46–48]. In contrast, the TiO2/SAC-900-6 and TiO2/SAC-900-9 exhibited a type IV with hysteresis loops at high relative pressure, revealing the presence of mesopores [22,49–51]. The observed hysteresis loops at relative pressure in the range of 0.4–0.95 closes at P/P0 = 0.42. This can be explained by the tensile strength effect (TSE) [52]. The effect means that the TiO2/SAC-900-6 and TiO2/SAC-900-9 contain ink bottle pores having narrow entrance and wide bodies [53–56]. The volume of nitrogen adsorbed in the relative pressure range of 0.5–1.0 increased with increasing activation time (6–9 h), demonstrating a remarkable increase in mesopores. This may have occurred due to pore enlargement, which resulted from the gasification of volatile matter such as COx and SOx [25,44]. The pore-size distributions of the TiO2/SAC photocatalysts prepared at different activation time are presented in Fig. 7(B). Pore diameter in the range of about 5–30 nm remarkably increased as activation time increase from 6 h to 9 h, which indicates that activation time is helpful to the formation of more pores and porewidening. From above results, the proposed mechanism for the formation of the micro- and mesoporous TiO2/SAC is illustrated in Fig. 8. 3.2. Adsorption In order to evaluate the adsorption capacities of the TiO2/SAC photocatalysts prepared at different activation times, adsorption kinetics was performed for 10 h in the dark conditions (Fig. 9). It was found that the TiO2/SAC photocatalysts obtained at activation times of 6 h and 9 h showed adsorption of humic acid. The reason may be supported by the increase in mesopores (peak pore: ca. 5– 30 nm) of the TiO2/SAC-900-6 and TiO2/SAC-900-9, as shown in
Fig. 8. Schematic procedure for the formation of the micro- and mesoporous TiO2/SAC.
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
0.5
TiO2/SAC-900-6 remarkably enhanced photocatalytic activity (73% for 12 h), but degradation of humic acid by TiO2/SAC-900-9 was nearly same with TiO2/SAC-900-6. The kinetics of humic acid degradation by TiO2/SAC photocatalysts was analyzed using the first-order model expressed in Eq. (2) [33,57]:
TiO2 /SAC-900-0.5 TiO2 /SAC-900-1 TiO2 /SAC-900-2 TiO2 /SAC-900-6
0.4
475
TiO2 /SAC-900-9
qt (mg/g )
0.3
0.2
dC t ¼ k1 C t dt
(2)
After integrating with the initial conditions, the equation is expressed as follows: Ct ln (3) ¼ k1 t Co
0.1
0.0 0
2
4
6
8
10
Time (h) Fig. 9. Adsorption kinetics of humic acid on the TiO2/SAC photocatalysts prepared at different activation times (initial concentration of humic acid = 20 mg/L and photocatalyst amount = 6 g/L).
Fig. 7(B). Generally, the presence of mesopores in the activated carbon can significantly enhance adsorption capacities, especially for large adsorbates [25]. To eliminate the influence of adsorption, photocatalytic experiment was carried out after adsorption reaction of 10 h. 3.3. Photocatalytic activity The photocatalytic activity of TiO2/SAC photocatalysts prepared at different temperatures and activation times was evaluated under UV irradiation for 12 h. Under UV illumination, the SAC, TiO2/SAC-300, and TiO2/SAC-700 did not exhibit photocatalytic activity because Ti does not exist in SAC, and also TiO2/SAC photocatalyst prepared at 300 8C and 700 8C were not activated. The formation of anatase TiO2 crystals in the TiO2/SAC-300 and TiO2/SAC-700 confirmed by XRD, while their crystallinity is substantially low. Therefore, anatase TiO2 supported on uncarbonized resins did not show photocatalytically active for the degradation of humic acid. As shown in Fig. 10, TOC removal during the photocatalytic degradation of humic acid gradually increased with increasing activation time from 0.5 h to 2 h. The
where k1 is the rate constant of the first-order reaction (h1), Co is the equilibrium concentration of TOC after adsorption in the dark, and Ct is the remaining concentration of TOC at certain irradiation time. The relationship observed is approximately linear, and the slope of this linear relation represents the rate constant (k1) for the photocatalysis of humic acid in the solution (figure not shown). The k1 values are presented in Fig. 11. With increasing activation time between 0.5 h and 2 h, the rate constant gradually increased owing to increase in specific surface area, micro/mesopore volume, and titanium content. Increase in the specific surface area can offer more active adsorption site and photocatalytic reaction [58,59]. The rate constant of degradation of the humic acid for the TiO2/SAC-900-6 remarkably increased, implying that mesoporous structure can enhance photocatalytic activity [60]. On the other hand, k1 values of the TiO2/SAC-900-9 seldom changed compared to that of the TiO2/ SAC-900-6, despite its highest specific surface area (1427 m2/g), total pore volume (1.2 cm3/g), and high titanium content (10 wt.%). This result suggests that TiO2 photocatalytic activity may be related to the crystalline phases. The main active crystal phases of TiO2 are anatase and rutile. Among the two phases, anatase is typically more active [60]. As shown in Fig. 4, rutile phase of TiO2 was quite dominant at 9 h. Therefore, the photocatalytic activity of the TiO2/ SAC-900-9 is offset by the conversion of anatase to the less photocatalytically active rutile phase. According to Ambrus et al. [43], photocatalyst containing anatase phase is more efficient than rutile-only catalyst. This was also supported by Yu et al. [61] who reported that rutile phase showed no photocatalytic activity. From the entire results obtained in this study, it can be said that the activation time of 6 h presents the optimum condition for use in floating photocataytic reactor. Thus, in comparison experiment
1.0 0.15
0.10
0.6 -1
k (h )
TOC t /TOC 0
0.8
0.4
0.05 UV only SAC TiO2 /SAC-300
0.2
TiO2 /SAC-700
TiO2 /SAC-900-0.5 TiO2 /SAC-900-1 TiO2 /SAC-900-2 TiO2 /SAC-900-6 TiO2 /SAC-900-9
0.0 0
2
4
6
8
10
12
Irradiation time (h) Fig. 10. Variation of TOC removal during the photocatalytic degradation of humic acid by the TiO2/SAC photocatalysts prepared at different activation times (initial concentration of humic acid = 20 mg/L and photocatalyst amount = 6 g/L).
0.00
0.5
1
2 Activation time (h)
6
9
Fig. 11. Rate constant for degradation of humic acid by the TiO2/SAC photocatalysts prepared at different activation times (initial concentration of humic acid = 20 mg/L and photocatalyst amount = 6 g/L).
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
476
surface area of the TiO2/SAC photocatalyts increased from 391 m2/ g to 1427 m2/g with increase in activation time, implying that more pores were formed by increasing activation time. The micropore volume increased with the increase in activation time, especially, the volume of micro- and mesopore greatly increased over an activation time of 6 h. There was adsorption of humic acid on the TiO2/SAC photocatalyts prepared at activation times of 6 h and 9 h as a result of developed mesopores. When heat-treatment temperature was 900 8C, degradation rate of humic acid increased with increasing activation time from 0.5 h to 6 h, but for activation time of 9 h there was really no increase due to conversion of anatase to rutile. From the results obtained in this study, it can be concluded that micro- and mesoporous TiO2/SAC-900-6 has adsorption capacity and highest photocatalytic activity.
80
TOC removal (%)
60
40
20
Acknowledgements
0 1
2
3
Number of runs Fig. 12. TOC removal during photocatalytic degradation of humic acid by repeated use of the TiO2/SAC-900-6 (initial concentration of humic acid = 20 mg/L, photocatalyst amount = 6 g/L, and irradiation time = 12 h).
This work is supported by Korea Ministry of Environment as ‘‘Converging technology project’’ (no. 2009-192-091-010). Authors greatly appreciate this support. References
with commercial P25 TiO2, the amount of P25 TiO2 was controlled by titanium content in the TiO2/SAC-900-6. TOC removal during photocatalytic degradation of humic acid by P25 TiO2 for 3 h UV irradiation was 71% (figure not shown). The photocatalytic degradation of humic acid by the TiO2/SAC photocatalysts was not as fast as that of P25 TiO2 due to mass transfer limitation of immobilized systems [62]. However, powder type as P25 TiO2 requires an additional process step for separation of photocatalyst from treated water after photocatalysis. On the other hand, the TiO2/SAC photocatalysts used in this study is free from a separation process. Because leaching of titanium from TiO2/SAC photocatalyst in the solution may reduce photocatalytic activity, the titanium ion existed in the solution was analyzed by ICP. From the ICP analysis, there was no leaching of titanium during photocatalysis experiments for the TiO2/SAC photocatalysts prepared at activation time between 0.5 h and 6 h, but the TiO2/SAC-900-9 leached titanium of 0.1 mg/L for 12 h irradiation, but this value is negligible. The reuse of photocatalyst is significantly important for its practical application in water treatment. The stability of photocatalyst was investigated through recycling of the TiO2/ SAC-900-6, which possesses optimal condition in this study. As shown in Fig. 12, the TOC removal efficiency during photocatalytic degradation of humic acid by TiO2/SAC-900-6 decreased slightly as recycling times increase. At the second cycles, 87% of the initial TOC was removed after photocatalytic reaction of 12 h. The efficiency of TOC removal in the third cycles was similar to that of the second cycles. This result indicates that the TiO2/SAC-900-6 is relatively stable photocatalyst for the photocatalytic degradation of humic acid. 4. Conclusion TiO2 supported on spherical activated carbon was prepared through heat-treatment after ion-exchange reaction of SAIR and titanium ion in aqueous solution. The effects of heat-treatment temperature and activation time on the TiO2/SAC photocatalyts properties were studied. Heat-treatment temperature and activation time had great influence on the phase structure; that is some anatase type of TiO2 was transformed into rutile type. The TiO2/SAC photocatalyts prepared at 900 8C for 0.5–6 h has good spherical shape and smooth surface without cracks, whereas the morphology of the TiO2/SAC-900-9 exhibited some cracking. The specific
[1] G. Xue, H. Liu, Q. Chen, C. Hills, M. Tyrer, F. Innocent, Journal of Hazardous Materials 186 (2011) 765. [2] W.S. Wan Ngah, M.A.K.M. Hanafiah, S.S. Yong, Colloids and Surfaces B: Biointerfaces 65 (2008) 18. [3] S. Liu, M. Lim, R. Fabris, C. Chow, K. Chiang, M. Drikas, R. Amal, Chemosphere 72 (2008) 263. [4] A. Bansal, S. Madhavi, T.T.Y. Tan, T.M. Lim, Catalysis Today 131 (2008) 250. [5] G.S. Wang, C.H. Liao, F.J. Wu, Chemosphere 42 (2001) 379. [6] J. Wiszniowski, D. Robert, J. Surmacz-Gorska, K. Miksch, J.V. Weber, Journal of Photochemistry and Photobiology A: Chemistry 152 (2002) 267. [7] Y. Xu, J. Jia, D. Zhong, Y. Wang, Chemical Engineering Journal 150 (2009) 302. [8] Z.M. El-Bahy, A.A. Ismail, R.M. Mohamed, Journal of Hazardous Materials 166 (2009) 138. [9] W. Wang, J. Zhang, F. Chen, D. He, M. Anpo, Journal of Colloid and Interface Science 323 (2008) 182. [10] F. Peng, L. Cai, L. Huang, H. Yu, H. Wang, Journal of Physics and Chemistry of Solids 69 (2008) 1657. [11] X. Wang, T.T. Lim, Applied Catalysis B: Environmental 100 (2010) 355. [12] V. Puddu, H. Choi, D.D. Dionysiou, G.L. Puma, Applied Catalysis B: Environmental 94 (2010) 211. [13] N.F. Zainudin, A.Z. Abdullah, A.R. Mohamed, Journal of Hazardous Materials 174 (2010) 299. [14] R. Leary, A. Westwood, Carbon 49 (2011) 741. [15] I. Sopyan, M. Watanabe, S. Murasawa, K. Hashimoto, A. Fujishima, Journal of Photochemistry and Photobiology A: Chemistry 98 (1996) 79. [16] X. Zhang, M. Zhou, L. Lei, Applied Catalysis A: General 282 (2005) 285. [17] G. Jiang, Z. Lin, C. Chen, L. Zhu, Q. Chang, N. Wang, W. Wei, H. Tang, Carbon 49 (2011) 2693. [18] X. Zhang, L. Lei, Journal of Hazardous Materials 153 (2008) 827. [19] A.J. Romero-Anaya, M.A. Lillo-Ro´denas, A. Linares-Solano, Carbon 48 (2010) 2625. [20] Q. Wang, X. Liang, W. Qiao, C. Liu, X. Liu, L. Zhan, L. Ling, Fuel Processing Technology 90 (2009) 381. [21] Q. Wang, X. Liang, W. Qiao, C. Liu, X. Liu, R. Zhang, L. Ling, Applied Surface Science 255 (2009) 3499. [22] Q. Wang, X.Y. Liang, R. Zhang, C.J. Liu, X.J. Liu, W.M. Qiao, L. Zhan, L.C. Ling, New Carbon Materials 24 (2009) 55. [23] Z. Liu, L. Ling, W. Qiao, L. Liu, Carbon 37 (1999) 663. [24] Z. Liu, L. Ling, W. Qiao, L. Liu, Carbon 37 (1999) 2063. [25] Z. Zhu, A. Li, L. Yan, F. Liu, Q. Zhang, Journal of Colloid and Interface Science 316 (2007) 628. [26] C. Long, J.D. Lu, A. Li, D. Hu, F. Liu, Q. Zhang, Journal of Hazardous Materials 150 (2008) 656. [27] W. Bratek, K. Bratek, M. Kułaz˙yn´ski, Fuel Processing Technology 81 (2003) 87. [28] J.J. Lee, J.K. Suh, J.S. Hong, J.M. Lee, J.W. Park, Materials Science Forum 569 (2008) 37. [29] J.J. Lee, J.K. Suh, J.S. Hong, J.M. Lee, Y.S. Lee, J.W. Park, Carbon 46 (2008) 1648. [30] J.J. Lee, J.K. Suh, J.S. Hong, J.M. Lee, J.W. Park, Research on Chemical Intermediates 35 (2009) 337. [31] Y. Fu, H. Du, S. Zhang, W. Huang, Materials Science and Engineering A 403 (2005) 25. [32] H. Liu, W. Yang, Y. Ma, J. Yao, Applied Catalysis A: General 299 (2006) 218. [33] X. Zhang, M. Zhou, L. Lei, Carbon 44 (2006) 325. [34] J.H. Zhou, Z.J. Sui, J. Zhu, P. Li, D. Chen, Y.C. Dai, W.K. Yuan, Carbon 45 (2007) 785. [35] Q. Xiao, J. Zhang, C. Xiao, Z. Si, X. Tan, Solar Energy 82 (2008) 706. [36] A. Jitianu, T. Cacciaguerra, R. Benoit, S. Delpeux, F. Be´guin, S. Bonnamy, Carbon 42 (2004) 1147.
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477 [37] J.R. Raji, K. Palanivelu, Industrial and Engineering Chemistry Research 50 (2011) 3130. [38] P. Go´rska, A. Zaleska, E. Kowalska, T. Klimczuk, J.W. Sobczak, E. Skwarek, W. Janusz, J. Hupka, Applied Catalysis B: Environmental 84 (2008) 440. [39] K. Lv, Q. Xiang, J. Yu, Applied Catalysis B: Environmental 104 (2011) 275. [40] J. Yu, Y. Su, B. Cheng, Advanced Functional Materials 17 (2007) 1984. [41] Y. Chen, D.D. Dionysiou, Applied Catalysis B: Environmental 62 (2006) 255. [42] J.C. Yu, J. Yu, L. Zhang, W. Ho, Journal of Photochemistry and Photobiology A: Chemistry 148 (2002) 263. [43] Z. Ambrus, K. Mogyoro´si, A. Szalai, T. Alapi, K. Demeter, A. Dombi, P. Sipos, Applied Catalysis A: General 340 (2008) 153. [44] Q. Cai, Z.H. Huang, F. Kang, J.B. Yang, Carbon 42 (2004) 775. [45] J. Sun, X. Wang, C. Wang, Q. Wang, Journal of Applied Polymer Science 99 (2006) 2565. [46] X. Wu, N.C. Gallego, C.I. Contescu, H. Tekinalp, V.V. Bhat, F.S. Baker, M.C. Thies, Carbon 46 (2008) 54. [47] C.H. Yun, Y.H. Park, C.R. Park, Carbon 39 (2001) 559. [48] S. Barrientos-Ramı´rez, G. Montes de Oca-Ramı´rez, A. Sepu´lveda-Escribano, M.M. Pastor-Blas, A. Gonza´lez-Montiel, F. Rodrı´guez-Reinoso, Catalysis Today 150 (2010) 42. [49] S.J. Yang, J.H. Im, T. Kim, K. Lee, C.R. Park, Journal of Hazardous Materials 186 (2011) 376.
477
[50] D. Long, R. Zhang, W. Qiao, L. Zhang, X. Liang, L. Ling, Journal of Colloid and Interface Science 331 (2009) 40. [51] V. Zelenˇa´k, V. Hornebecq, P. Llewellyn, Microporous and Mesoporous Materials 83 (2005) 125. [52] J.P. Thielemann, F. Girgsdies, R. Schlo¨gl, C. Hess, J. Beilstein, Beilstein Journal of Nanotechnology 2 (2011) 110. [53] S. Hu, J. Xiang, L. Sun, M. Xu, J. Qiu, P. Fu, Fuel Processing Technology 89 (2008) 1096. [54] G. Liu, M. Jia, Z. Zhou, L. Wang, W. Zhang, D. Jiang, Journal of Colloid and Interface Science 302 (2006) 278. [55] N. Banic´, B. Abramovic´, J. Krstic´, D. Sˇojic´, D. Loncˇarevic´, Z. Cherkezova-Zheleva, V. Guzsva´ny, Applied Catalysis B: Environmental 107 (2011) 363. [56] J. Zhou, S. Yang, J. Yu, Z. Shu, Journal of Hazardous Materials 192 (2011) 1114. [57] W.T. Tsai, M.K. Lee, T.Y. Su, Y.M. Chang, Journal of Hazardous Materials 168 (2009) 269. [58] X.F. Chen, X.C. Wang, X.Z. Fu, Energy & Environmental Science 2 (2009) 872. [59] J. Yu, L. Zhang, B. Cheng, Y. Su, Journal of Physical Chemistry C 111 (2007) 10582. [60] S. Kim, S.H. Ehrman, Journal of Colloid and Interface Science 338 (2009) 304. [61] J.C. Yu, J. Yu, W. Ho, J. Zhao, Journal of Photochemistry and Photobiology A: Chemistry 148 (2002) 331. [62] H.M. Coleman, V. Vimonses, G. Leslie, R. Amal, Journal of Hazardous Materials 146 (2007) 496.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Preparation, characterization and photocatalytic activity evaluation of micro- and mesoporous TiO2/spherical activated carbon Mi-Hwa Baek a, Won-Chae Jung a, Ji-Won Yoon a, Ji-Sook Hong a, Young-Seak Lee b, Jeong-Kwon Suh a,* a b
Research Center for Environmental Resources and Processes, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, Republic of Korea Department of Fine Chemical Engineering and Applied Chemistry, BK21-E2 M, Chungnam National University, Daejeon 305-764, Republic of Korea
A R T I C L E I N F O
Article history: Received 10 May 2012 Accepted 28 August 2012 Available online 3 September 2012 Keywords: TiO2 Photocatalysis Spherical activated carbon Adsorption Humic acid
A B S T R A C T
The influences of heat-treatment temperature and activation time on the properties of TiO2 supported on spherical activated carbon (TiO2/SAC) were investigated. Nano-sized TiO2 was dispersed on the spherical activated carbon with the size of 10–30 nm. Some anatase phase of TiO2 was transformed to rutile phase of TiO2 with an increase of heat-treatment temperature. All of the TiO2/SAC photocatalysts had microporous structure, with the mesopore volume increasing over an activation time of 6 h. The TiO2/SAC photocatalysts obtained at activation times of 6 h and 9 h were observed synergistic effects between adsorption and photocatalysis in the removal of humic acid. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Humic acid as model compounds of natural organic matter (NOM) can react with chlorine during water disinfection processes to produce disinfection by-products (DBPs) such as trihalomethanes, a known carcinogen [1–3]. Thus, the removal of humic acid is required prior to chlorination of drinking water. Recently, numerous approaches have been studied for the elimination of HA from water [1–6]. The conventional treatments of humic acid such as biodegradation, coagulation, and activated carbon adsorption are relatively slow processes and the removal efficiency is comparatively low [4–6]. However, photocatalysis is a fast and effective method for elimination of humic acid from aqueous solution [3,6]. In water purification systems, titanium dioxide (TiO2) is becoming an irresistible photocatalyst due to its excellent oxidizing behavior. TiO2 is chemically inert, non-toxic, cheap, efficient and with long-term stability in reactions [7–12]. The powders such as P25 TiO2 have been commercially available for several years. Powder type TiO2 has high photocatalytic activity due to its large surface area and high degree of dispersion in reaction media. However, the powder type TiO2 leads to several problems such as: (1) the difficult process of filtration or
* Corresponding author. Tel.: +82 42 860 7334; fax: +82 42 860 7533. E-mail address: [email protected] (J.-K. Suh).
separation of the powder from the solution after a photocatalytic reaction, (2) the aggregation of particles in suspension, in the case of high loadings, and (3) its application to continuous flow systems is quite challenging [13–17]. Due to the aforementioned problems, photocatalysts (e.g. TiO2) have been immobilized on various supports such as silica gel, alumina, and activated carbon [13,14,16–18]. Especially, activated carbon has been extensively studied as a support for TiO2 photocatalyst [14]. Of all types of activated carbons, the spherical activated carbon has the advantages of a smoother surface, better fluidity, higher mechanical strength, and lower resistance to liquid diffusion over the powdered and the granular activated carbons [19–25]. Ion-exchange resin has been used as suitable precursor to prepared spherical activated carbon due to its high carbon yield and low ash content [21,26,27]. We have developed a spherical activated carbon containing photocatalysts (ZnS and ZnO) that has the characteristics of both activated carbon and photocatalyst [28–30]. It was reported that the spherical activated carbon containing ZnS or ZnO have good photocatalytic activity [28–30]. The aim of this study is to enhance the adsorption capacity and the photocatalytic activity of the TiO2/SAC by controlling activation time. In this work, influence of activation time on the TiO2/SAC photocatalysts properties was investigated by various physicochemical techniques. In addition, the photocatalytic activity of TiO2/SAC photocatalysts was evaluated for the degradation of humic acid under UV irradiation.
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.08.026
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
470
2. Experimental 2.1. Materials The strong acid ion-exchange resin (SAIR, Diaion SK1BH, Samyang Co. Ltd.), brown color with a diameter range of 400– 500 mm, was used as a support of the TiO2/SAC photocatalysts. Titanium trichloride (TiCl3, 20%, Kanto Chemical) as a Ti precursor was used without any further purification. Commercial TiO2 powder P25 used as a reference was obtained from Degussa Chemical, Germany. Humic acid was purchased from Aldrich Chemical Company, which has carbon, hydrogen, and nitrogen content of 39.44%, 4.74%, and 0.79%, respectively. Double-distilled water was used in all the experiments. 2.2. Preparation of photocatalysts The TiO2/SAC was prepared as follows [29]: 24.2 mL of titanium trichloride was added into 730 mL distilled water under the stirring at 150 rpm for 10 min, and 300 g SAIR was added intermittingly with continuous stirring for 1 h at room temperature. The sample was filtered and washed several times with distilled water until the pH values of the solution reached neutral pH, and then dried at 110 8C for 24 h in an oven. The dried sample was heat-treated for good stability as a support and also for effective conversion to carbonaceous porous material. A schematic diagram of the heat-treatment process is presented in Fig. 1. A quartz tube reactor of 10 cm height and 6 cm diameter was used in the heat-treatment process. The sample was stabilized at 300 8C for five hours in atmosphere at a heating rate of 1 8C/min using a tube furnace. Carbonization followed at 700 8C for 10 min under a nitrogen flow of 2 L/min with a heating rate of 3 8C/min. Further heating of the same sample under a nitrogen at a heating rate of 1 8C/min was carried out at 900 8C and then activated under nitrogen/steam at 900 8C for 0.5–9 h. The flow rate of nitrogen gas was 2 L/min and distilled water was infused into the quartz tube reactor by a pump at 15 mL/min. The TiO2/SAC photocatalysts obtained at 300 8C and 700 8C were labeled TiO2/SAC-300 and TiO2/ SAC-700. According to different activation times of 0.5, 1, 2, 6, and 9 h, the TiO2/SAC photocatalysts obtained at 900 8C were assigned TiO2/SAC-900-0.5, TiO2/SAC-900-1, TiO2/SAC-900-2, TiO2/SAC900-6, and TiO2/SAC-900-9, respectively.
crystalline phases of the TiO2/SAC photocatalysts, XRD analysis was performed using X-ray diffractometer (XRD, D/MAX-IIIB, Rigaku) with Cu-Ka radiation and operating at 2.0 kW with a scanning speed of 58/min over the 2u range of 20–808. Scanning electron microscopy (SEM, JSM-6700F, JEOL) and field emission transmission electron microscopy (FE-TEM, Tecnai F20, FEI Co.) were used to observe the surface morphology of the TiO2/SAC photocatalysts. Distribution of elements in the TiO2/SAC photocatalysts was analyzed using scanning transmission electron microscopy combined with energy dispersive X-ray spectrometer (STEM-EDS). Energy-dispersive X-ray spectroscopy (EDS, Bruker Quantax 200) was also used to confirm the titanium content in the TiO2/SAC. The specific surface area and pore structure of the TiO2/ SAC photocatalysts were measured by the Brunauer–Emmett– Teller (BET) method using a Micromeritics ASAP 2010 apparatus. In order to compare and evaluate the compressive strength of each sample, the single sphere strength of each sample was measured using compressive strength meter, as described in [29]. The titanium ion amount in the solution before and after ion-exchange reaction was analyzed with inductively coupled plasma (ICP, iCAP 6000 series, Thermo Scientific). 2.4. Preparation of humic acid solution Stock solution of humic acid was prepared by dissolving 1 g of humic acid in a liter of double-distilled water and was kept for one day. This solution was filtered through 0.45 mm filter paper to remove all suspended solid. As a result, the concentration of stock solution of humic acid is less than 1000 mg/L. The stock solution was stored in a refrigerator (<4 8C) and dark environment to prevent natural light degradation. The stock solution of humic acid was diluted fifty times to use in adsorption and photocatalytic experiments. The initial concentration of humic acid and total organic carbon (TOC) were 20 mg/L and 8 mg/L, respectively. 2.5. Adsorption and photocatalytic activity Adsorption equilibrium experiments were performed by adding 1.2 g TiO2/SAC photocatalysts with 200 mL humic acid solution. The solution was stirred using a shaker with 25 8C and 150 rpm and kept under dark conditions for 10 h. The amount of humic acid adsorbed by the TiO2/SAC qt (mg/g) at each time interval was calculated according to the expression:
2.3. Characterization qt ðmg=gÞ ¼ X-ray photoelectron spectroscopy (XPS, ESCALAB 210, VG Scientific Ltd.) was used to investigate the chemical composition and state of the photocatalysts. In order to elucidate the
Fig. 1. Schematic diagram of the heat-treatment process.
ðC 0 C t ÞV m
(1)
where C0 and Ct are the initial concentration of humic acid (mg/L) and the remaining concentration of humic acid at certain time (mg/ L), respectively. V is the volume of the humic acid solution (L), and m is the amount of the TiO2/SAC used (g). Degradation of humic acid under UV irradiation was performed to evaluate the photocatalytic activity of TiO2/SAC photocatalysts, as described in [29]. 16 W low-pressure mercury lamp (lmax = 254 nm, G18T5C, LIGHTTECH) was used as a UV light source. Photocatalysis experiments were carried out using a 2 L batch-type reactor (40 cm in height, 10 cm in diameter), and by adding 6 g of the TiO2/SAC photocatalysts to 1 L of humic acid aqueous solution, after which the solution was stirred in the dark for 10 h in order to reach the adsorption equilibrium prior to illumination. Air was supplied into the photocatalytic reactor at a rate of 1 L/min for smooth floating of photocatalysts. The pH of the solution was not controlled and all photocatalytic experiments were carried out at room temperature. At regular intervals, certain volume of the samples was withdrawn from the photoreactor. Reference experiment was conducted under the same conditions without the TiO2/SAC photocatalysts. In order to compare the
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
(A) O 1s
C 1s
Intensity (CPS)
prepared TiO2/SAC photocatalysts with commercially available TiO2, degradation of humic acid on P25 TiO2 was also achieved. In the photocatalytic experiment using P25 TiO2, collected samples were centrifuged at 13,000 rpm for 15 min to separate P25 TiO2 particles from humic acid solution before analysis. In order to examine change of humic acid concentration in solution according to reaction time, the absorbance of the collected solution was measured by UV–vis spectrophotometer (Qvis 4000, Cmac) at a wavelength of 254 nm. TOC of the solution was analyzed with a total organic carbon analyzer (TOC-5000A, Shimazu). In order to check the leaching of titanium during long-term water treatment, the amount of titanium in the solution before and after photocatalysis experiment was analyzed by ICP.
471
Ti 2p
3. Results and discussion 600
Remaining concentration of titanium ion after ion-exchange reaction was analyzed through ICP to assess the efficiency of ionexchange. The result showed that efficiency of titanium ionexchange was 97%.
3.1.2. XRD analysis The phase structure change of the TiO2 was investigated using X-ray diffraction. The XRD patterns of the TiO2/SAC photocatalysts obtained at different heat-treatment temperatures and activation times are presented in Fig. 4. The SAC before TiO2 immobilization is an amorphous phase. At a stabilization temperature of 300 8C, a broad peak at 2u = 25.28, 37.68, 48.28, 55.18, and 62.98 corresponds to the anatase [37–40]. Increasing the carbonization temperature up to 700 8C, the peak intensities of anatase increased and became sharp, indicating the formation of greater TiO2 crystallites and enhancement of crystallization [40]. Small peak at 2u = 27.58, 36.18, 41.38, 44.18, and 56.78 corresponds to rutile which began to appear
400
300
200
Binding energy (eV)
TiO2 /SAC-300
Ti 2p3/2
TiO2 /SAC-700
3.1. Physicochemical characteristics
(B)
TiO2 /SAC-900
Intensity (CPS)
3.1.1. XPS study The XPS survey spectrum of the TiO2/SAC-300 is shown in Fig. 2(A). The characteristic energy spectrum of Ti, O, and C can be observed, and binding energies for Ti 2p, O 1s, and C 1s were 458.6, 531.3, and 284.5 eV, respectively. The Ti 2p and O 1s peaks was attributed to Ti4+ and O2, and the C ls corresponds to the SAC. Fig. 2(B) displays the XPS spectra of Ti 2p for the TiO2/SAC photocatalysts prepared at 300–900 8C. When the temperature of heat-treatment was 300 8C, the two peaks located at 458.9 eV and 464.5 eV can be assigned to the binding energies of Ti4+ 2p3/2 and Ti4+ 2p1/2, respectively [31–33]. The binding energies of Ti4+ 2p3/2 and Ti4+ 2p1/2 for the TiO2/SAC photocatalysts prepared between 700 8C and 900 8C were similar to that of 300 8C. It can be concluded that titanium element existed mainly as state of Ti4+ in the TiO2/SAC. XPS spectra of C 1s for the TiO2/SAC photocatalysts prepared at different heat-treatment temperatures are shown in Fig. 3. The C 1s spectra of SAIR show three peaks: 284.7 eV, graphitic carbon; 286 eV, carbon in phenolic, alcohol, ether; 290.6 eV, carbon in carbonate groups [34]. The TiO2/SAC photocatalysts prepared at 300 8C showed additional peak, located at 288 eV which can be assigned to Ti-O-C structure [35]. Also for the TiO2/SAC photocatalysts obtained at 700 8C and 900 8C, their binding energies at 285.3 eV may be assigned to C–O bond [36]. For all the TiO2/SAC, a main peak was observed at 284.7 eV due to C–C bond. This peak increased significantly as the heat-treatment temperature increased and this behavior may be traced to the sp2-bonded carbon content. From the results of XPS, it can be seen that titanium ion crystallized and the SAIR changed to carbonaceous materials through heat-treatment.
500
Ti 2p1/2
468
466
464
462
460
458
456
Binding energy (eV) Fig. 2. (A) XPS survey spectrum of the TiO2/SAC-300 and (B) XPS spectra of Ti 2p for the TiO2/SAC photocatalysts prepared at different heat-treatment temperatures.
when the activation temperature was increased to 900 8C [39–41]. This demonstrates that some of the anatase type was transformed into the rutile type during heat-treatment, which agrees with other literatures [38–40,42]. That is, heat-treatment temperature has great influence on phase transformation of TiO2. Also, reported by Ambrus et al. [43], anatase transforms to rutile at temperatures between 600 8C and 1100 8C depending on the primary particle size and the method of preparation. As shown in Fig. 4, the intensity of the anatase peak decreased, whereas the intensity of the rutile peak increased sharply with an increase in the activation time from 0.5 h to 6 h. At the TiO2/SAC-900-9, the main peak was rutile while anatase peak nearly disappeared. The crystallite size of TiO2 was determined by XRD results, as summarized in Table 1. The anatase crystallite size increased with increasing heat-treatment temperature, suggesting that high temperature led to TiO2 crystal growth. The anatase crystallite size gradually decreased whereas rutile crystallite size increased as activation time increase. 3.1.3. SEM image and strength of photocatalyst The morphologies of the SAIR and TiO2/SAC photocatalysts obtained at different heat-treatment temperatures and activation times were observed by SEM. Fig. 5(A) illustrates the SEM photograph of the SAIR, which has spherical shapes. As shown in Fig. 5(B)–(E), the TiO2/SAC-700, TiO2/SAC-900-0.5, TiO2/SAC-9002, and TiO2/SAC-900-6 maintained smooth surfaces and good
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
472
Fig. 3. XPS spectra of C 1s for the TiO2/SAC photocatalysts prepared at different heat-treatment temperatures.
Table 1 Characteristics of the TiO2/SAC photocatalysts prepared at different heat-treatment temperatures and activation times. Sample
TiO2/SAC-300 TiO2/SAC-700 TiO2/SAC-900-0.5 TiO2/SAC-900-1 TiO2/SAC-900-2 TiO2/SAC-900-6 TiO2/SAC-900-9
Temp. (8C)
300 700 900 900 900 900 900
Activation time (h)
0.5 1 2 6 9
Specific surface area (m2/g)
– – 391 478 604 1274 1427
spherical shapes with a diameter of 450–600 mm after the heattreatment and activation process. The SEM images of TiO2/SAC-300 and TiO2/SAC-900-1 were similar to Fig. 5(A)–(E). While for the TiO2/SAC-900-9, cracks can be seen in some spheres as shown in
Fig. 4. XRD patterns of the SAC, TiO2/SAC-300, TiO2/SAC-700, and the TiO2/SAC photocatalysts prepared at 900 8C for different activation times.
Total pore volume (cm3/g)
– – 0.153 0.195 0.260 0.863 1.217
Crystallite (nm)
size
Anatase
Rutile
7.9 17.9 28.1 14.9 14.1 11.8 11.6
– – 8 11.9 13.1 14.6 14.7
Ti content (wt.%)
– – 2.7 3.6 3.8 6.2 10
Fig. 5(F). The TiO2/SAC photocatalysts prepared by the ionexchange method and activation process, used in the present study, can immobilize titanium ion without a binder and can also maintain a smooth surface. This smooth surface can prevent problems of surface abrasion by inter-particle collision in floating photocatalytic application. Moreover, the TiO2/SAC photocatalysts possessing a good spherical shape with a diameter in the range of 450–600 mm can be separated from the solution without a special separation process after the photocatalytic reaction. The compressive strength of the TiO2/SAC photocatalysts obtained at activation time of 0.5–6 h was high and in the range of 7–10 kg/unit. It can enhance durability in floating photocatalysts systems. But, the TiO2/SAC-900-9 had relatively low compressive strength of 4 kg/unit; this is due to the cracking phenomenon, which equally agrees with the result of Fig. 5(F). More cracks and strength loss may occur when activation time goes beyond 9 h. 3.1.4. TEM image and EDS analysis Fig. 6(A) and (B) displays the FE-TEM image of TiO2/SAC-900-6. The results showed that the sizes of TiO2 measured directly from TEM photograph were approximately 10 to 30 nm. These values did not agree with crystallite size obtained from XRD results. It was
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
473
Fig. 5. SEM images of the TiO2/SAC: (A) SAIR, (B) TiO2/SAC-700, (C) TiO2/SAC-900-0.5, (D) TiO2/SAC-900-2, (E) TiO2/SAC-900-6, and (F) TiO2/SAC-900-9.
evident that TiO2 in the TiO2/SAC photocatalysts contains amorphous TiO2. To investigate the distribution of elements, an EDS mapping of the elements was performed. Fig. 6(C)–(F) exhibits STEM image and EDS elemental mapping of the TiO2/SAC-900-6.
The mapping images represent the presence of carbon, titanium and oxygen atoms in the TiO2/SAC. It also reveals the distribution of nano-sized Ti in the SAC matrix and the distribution of O is similar to that of Ti.
Fig. 6. FE-TEM image of the TiO2/SAC-900-6 (A), magnified image (B), STEM image (C), and EDS elemental mapping of the TiO2/SAC-900-6 (D–F).
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
474 800
(A)
TiO2 /SAC-900-1
700
3
Volume Adsorbed (cm /g) STP
as COx and SOx, which were progressively released with extending activation time.
TiO2 /SAC-900-0.5 TiO2 /SAC-900-2 TiO2 /SAC-900-6
600
TiO2 /SAC-900-9
500 400 300 200 100 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Relative Pressure (P/P0 ) 1.0
(B)
TiO2 /SAC-900-0.5 TiO2 /SAC-900-1 TiO2 /SAC-900-2 TiO2 /SAC-900-6
3
Pore volume (cm /g-nm )
0.8
TiO2 /SAC-900-9
0.6
0.4
0.2
0.0 10
20
30
40
50
60
70
80
90
100
Pore diameter (nm) Fig. 7. (A) Nitrogen adsorption–desorption isotherms and (B) pore-size distribution calculated adsorption branches of the isotherms for the TiO2/SAC photocatalysts prepared at different activation times.
An EDS analysis was performed to determine the content (wt.%) of titanium in the TiO2/SAC. The titanium in the TiO2/SAC photocatalysts was clearly identified from EDS and the titanium content increased with increasing activation time from 0.5 h to 9 h (Table 1). Especially, the TiO2/SAC-900-9 had the highest titanium content of 10 wt.%. This may be attributed to volatile matters such
3.1.5. BET analysis As shown in Table 1, total pore volume slightly increased with increasing activation time from 0.5 h to 2 h. However, total pore volume greatly increased as activation time increase from 6 h to 9 h, indicating the formation of mesoporous structure. As a result, the steam activation time has an effect on the specific surface area and could enhance porosity [44,45]. Fig. 7(A) shows the nitrogen adsorption–desorption isotherms for the TiO2/SAC photocatalysts prepared at different activation times. The isotherms of the TiO2/SAC-900-0.5, TiO2/SAC-900-1, and TiO2/SAC-900-2 were observed as type I according to IUPAC classification, indicating dominant microporous character and occurrence of pore filling at low relative pressures [46–48]. In contrast, the TiO2/SAC-900-6 and TiO2/SAC-900-9 exhibited a type IV with hysteresis loops at high relative pressure, revealing the presence of mesopores [22,49–51]. The observed hysteresis loops at relative pressure in the range of 0.4–0.95 closes at P/P0 = 0.42. This can be explained by the tensile strength effect (TSE) [52]. The effect means that the TiO2/SAC-900-6 and TiO2/SAC-900-9 contain ink bottle pores having narrow entrance and wide bodies [53–56]. The volume of nitrogen adsorbed in the relative pressure range of 0.5–1.0 increased with increasing activation time (6–9 h), demonstrating a remarkable increase in mesopores. This may have occurred due to pore enlargement, which resulted from the gasification of volatile matter such as COx and SOx [25,44]. The pore-size distributions of the TiO2/SAC photocatalysts prepared at different activation time are presented in Fig. 7(B). Pore diameter in the range of about 5–30 nm remarkably increased as activation time increase from 6 h to 9 h, which indicates that activation time is helpful to the formation of more pores and porewidening. From above results, the proposed mechanism for the formation of the micro- and mesoporous TiO2/SAC is illustrated in Fig. 8. 3.2. Adsorption In order to evaluate the adsorption capacities of the TiO2/SAC photocatalysts prepared at different activation times, adsorption kinetics was performed for 10 h in the dark conditions (Fig. 9). It was found that the TiO2/SAC photocatalysts obtained at activation times of 6 h and 9 h showed adsorption of humic acid. The reason may be supported by the increase in mesopores (peak pore: ca. 5– 30 nm) of the TiO2/SAC-900-6 and TiO2/SAC-900-9, as shown in
Fig. 8. Schematic procedure for the formation of the micro- and mesoporous TiO2/SAC.
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
0.5
TiO2/SAC-900-6 remarkably enhanced photocatalytic activity (73% for 12 h), but degradation of humic acid by TiO2/SAC-900-9 was nearly same with TiO2/SAC-900-6. The kinetics of humic acid degradation by TiO2/SAC photocatalysts was analyzed using the first-order model expressed in Eq. (2) [33,57]:
TiO2 /SAC-900-0.5 TiO2 /SAC-900-1 TiO2 /SAC-900-2 TiO2 /SAC-900-6
0.4
475
TiO2 /SAC-900-9
qt (mg/g )
0.3
0.2
dC t ¼ k1 C t dt
(2)
After integrating with the initial conditions, the equation is expressed as follows: Ct ln (3) ¼ k1 t Co
0.1
0.0 0
2
4
6
8
10
Time (h) Fig. 9. Adsorption kinetics of humic acid on the TiO2/SAC photocatalysts prepared at different activation times (initial concentration of humic acid = 20 mg/L and photocatalyst amount = 6 g/L).
Fig. 7(B). Generally, the presence of mesopores in the activated carbon can significantly enhance adsorption capacities, especially for large adsorbates [25]. To eliminate the influence of adsorption, photocatalytic experiment was carried out after adsorption reaction of 10 h. 3.3. Photocatalytic activity The photocatalytic activity of TiO2/SAC photocatalysts prepared at different temperatures and activation times was evaluated under UV irradiation for 12 h. Under UV illumination, the SAC, TiO2/SAC-300, and TiO2/SAC-700 did not exhibit photocatalytic activity because Ti does not exist in SAC, and also TiO2/SAC photocatalyst prepared at 300 8C and 700 8C were not activated. The formation of anatase TiO2 crystals in the TiO2/SAC-300 and TiO2/SAC-700 confirmed by XRD, while their crystallinity is substantially low. Therefore, anatase TiO2 supported on uncarbonized resins did not show photocatalytically active for the degradation of humic acid. As shown in Fig. 10, TOC removal during the photocatalytic degradation of humic acid gradually increased with increasing activation time from 0.5 h to 2 h. The
where k1 is the rate constant of the first-order reaction (h1), Co is the equilibrium concentration of TOC after adsorption in the dark, and Ct is the remaining concentration of TOC at certain irradiation time. The relationship observed is approximately linear, and the slope of this linear relation represents the rate constant (k1) for the photocatalysis of humic acid in the solution (figure not shown). The k1 values are presented in Fig. 11. With increasing activation time between 0.5 h and 2 h, the rate constant gradually increased owing to increase in specific surface area, micro/mesopore volume, and titanium content. Increase in the specific surface area can offer more active adsorption site and photocatalytic reaction [58,59]. The rate constant of degradation of the humic acid for the TiO2/SAC-900-6 remarkably increased, implying that mesoporous structure can enhance photocatalytic activity [60]. On the other hand, k1 values of the TiO2/SAC-900-9 seldom changed compared to that of the TiO2/ SAC-900-6, despite its highest specific surface area (1427 m2/g), total pore volume (1.2 cm3/g), and high titanium content (10 wt.%). This result suggests that TiO2 photocatalytic activity may be related to the crystalline phases. The main active crystal phases of TiO2 are anatase and rutile. Among the two phases, anatase is typically more active [60]. As shown in Fig. 4, rutile phase of TiO2 was quite dominant at 9 h. Therefore, the photocatalytic activity of the TiO2/ SAC-900-9 is offset by the conversion of anatase to the less photocatalytically active rutile phase. According to Ambrus et al. [43], photocatalyst containing anatase phase is more efficient than rutile-only catalyst. This was also supported by Yu et al. [61] who reported that rutile phase showed no photocatalytic activity. From the entire results obtained in this study, it can be said that the activation time of 6 h presents the optimum condition for use in floating photocataytic reactor. Thus, in comparison experiment
1.0 0.15
0.10
0.6 -1
k (h )
TOC t /TOC 0
0.8
0.4
0.05 UV only SAC TiO2 /SAC-300
0.2
TiO2 /SAC-700
TiO2 /SAC-900-0.5 TiO2 /SAC-900-1 TiO2 /SAC-900-2 TiO2 /SAC-900-6 TiO2 /SAC-900-9
0.0 0
2
4
6
8
10
12
Irradiation time (h) Fig. 10. Variation of TOC removal during the photocatalytic degradation of humic acid by the TiO2/SAC photocatalysts prepared at different activation times (initial concentration of humic acid = 20 mg/L and photocatalyst amount = 6 g/L).
0.00
0.5
1
2 Activation time (h)
6
9
Fig. 11. Rate constant for degradation of humic acid by the TiO2/SAC photocatalysts prepared at different activation times (initial concentration of humic acid = 20 mg/L and photocatalyst amount = 6 g/L).
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477
476
surface area of the TiO2/SAC photocatalyts increased from 391 m2/ g to 1427 m2/g with increase in activation time, implying that more pores were formed by increasing activation time. The micropore volume increased with the increase in activation time, especially, the volume of micro- and mesopore greatly increased over an activation time of 6 h. There was adsorption of humic acid on the TiO2/SAC photocatalyts prepared at activation times of 6 h and 9 h as a result of developed mesopores. When heat-treatment temperature was 900 8C, degradation rate of humic acid increased with increasing activation time from 0.5 h to 6 h, but for activation time of 9 h there was really no increase due to conversion of anatase to rutile. From the results obtained in this study, it can be concluded that micro- and mesoporous TiO2/SAC-900-6 has adsorption capacity and highest photocatalytic activity.
80
TOC removal (%)
60
40
20
Acknowledgements
0 1
2
3
Number of runs Fig. 12. TOC removal during photocatalytic degradation of humic acid by repeated use of the TiO2/SAC-900-6 (initial concentration of humic acid = 20 mg/L, photocatalyst amount = 6 g/L, and irradiation time = 12 h).
This work is supported by Korea Ministry of Environment as ‘‘Converging technology project’’ (no. 2009-192-091-010). Authors greatly appreciate this support. References
with commercial P25 TiO2, the amount of P25 TiO2 was controlled by titanium content in the TiO2/SAC-900-6. TOC removal during photocatalytic degradation of humic acid by P25 TiO2 for 3 h UV irradiation was 71% (figure not shown). The photocatalytic degradation of humic acid by the TiO2/SAC photocatalysts was not as fast as that of P25 TiO2 due to mass transfer limitation of immobilized systems [62]. However, powder type as P25 TiO2 requires an additional process step for separation of photocatalyst from treated water after photocatalysis. On the other hand, the TiO2/SAC photocatalysts used in this study is free from a separation process. Because leaching of titanium from TiO2/SAC photocatalyst in the solution may reduce photocatalytic activity, the titanium ion existed in the solution was analyzed by ICP. From the ICP analysis, there was no leaching of titanium during photocatalysis experiments for the TiO2/SAC photocatalysts prepared at activation time between 0.5 h and 6 h, but the TiO2/SAC-900-9 leached titanium of 0.1 mg/L for 12 h irradiation, but this value is negligible. The reuse of photocatalyst is significantly important for its practical application in water treatment. The stability of photocatalyst was investigated through recycling of the TiO2/ SAC-900-6, which possesses optimal condition in this study. As shown in Fig. 12, the TOC removal efficiency during photocatalytic degradation of humic acid by TiO2/SAC-900-6 decreased slightly as recycling times increase. At the second cycles, 87% of the initial TOC was removed after photocatalytic reaction of 12 h. The efficiency of TOC removal in the third cycles was similar to that of the second cycles. This result indicates that the TiO2/SAC-900-6 is relatively stable photocatalyst for the photocatalytic degradation of humic acid. 4. Conclusion TiO2 supported on spherical activated carbon was prepared through heat-treatment after ion-exchange reaction of SAIR and titanium ion in aqueous solution. The effects of heat-treatment temperature and activation time on the TiO2/SAC photocatalyts properties were studied. Heat-treatment temperature and activation time had great influence on the phase structure; that is some anatase type of TiO2 was transformed into rutile type. The TiO2/SAC photocatalyts prepared at 900 8C for 0.5–6 h has good spherical shape and smooth surface without cracks, whereas the morphology of the TiO2/SAC-900-9 exhibited some cracking. The specific
[1] G. Xue, H. Liu, Q. Chen, C. Hills, M. Tyrer, F. Innocent, Journal of Hazardous Materials 186 (2011) 765. [2] W.S. Wan Ngah, M.A.K.M. Hanafiah, S.S. Yong, Colloids and Surfaces B: Biointerfaces 65 (2008) 18. [3] S. Liu, M. Lim, R. Fabris, C. Chow, K. Chiang, M. Drikas, R. Amal, Chemosphere 72 (2008) 263. [4] A. Bansal, S. Madhavi, T.T.Y. Tan, T.M. Lim, Catalysis Today 131 (2008) 250. [5] G.S. Wang, C.H. Liao, F.J. Wu, Chemosphere 42 (2001) 379. [6] J. Wiszniowski, D. Robert, J. Surmacz-Gorska, K. Miksch, J.V. Weber, Journal of Photochemistry and Photobiology A: Chemistry 152 (2002) 267. [7] Y. Xu, J. Jia, D. Zhong, Y. Wang, Chemical Engineering Journal 150 (2009) 302. [8] Z.M. El-Bahy, A.A. Ismail, R.M. Mohamed, Journal of Hazardous Materials 166 (2009) 138. [9] W. Wang, J. Zhang, F. Chen, D. He, M. Anpo, Journal of Colloid and Interface Science 323 (2008) 182. [10] F. Peng, L. Cai, L. Huang, H. Yu, H. Wang, Journal of Physics and Chemistry of Solids 69 (2008) 1657. [11] X. Wang, T.T. Lim, Applied Catalysis B: Environmental 100 (2010) 355. [12] V. Puddu, H. Choi, D.D. Dionysiou, G.L. Puma, Applied Catalysis B: Environmental 94 (2010) 211. [13] N.F. Zainudin, A.Z. Abdullah, A.R. Mohamed, Journal of Hazardous Materials 174 (2010) 299. [14] R. Leary, A. Westwood, Carbon 49 (2011) 741. [15] I. Sopyan, M. Watanabe, S. Murasawa, K. Hashimoto, A. Fujishima, Journal of Photochemistry and Photobiology A: Chemistry 98 (1996) 79. [16] X. Zhang, M. Zhou, L. Lei, Applied Catalysis A: General 282 (2005) 285. [17] G. Jiang, Z. Lin, C. Chen, L. Zhu, Q. Chang, N. Wang, W. Wei, H. Tang, Carbon 49 (2011) 2693. [18] X. Zhang, L. Lei, Journal of Hazardous Materials 153 (2008) 827. [19] A.J. Romero-Anaya, M.A. Lillo-Ro´denas, A. Linares-Solano, Carbon 48 (2010) 2625. [20] Q. Wang, X. Liang, W. Qiao, C. Liu, X. Liu, L. Zhan, L. Ling, Fuel Processing Technology 90 (2009) 381. [21] Q. Wang, X. Liang, W. Qiao, C. Liu, X. Liu, R. Zhang, L. Ling, Applied Surface Science 255 (2009) 3499. [22] Q. Wang, X.Y. Liang, R. Zhang, C.J. Liu, X.J. Liu, W.M. Qiao, L. Zhan, L.C. Ling, New Carbon Materials 24 (2009) 55. [23] Z. Liu, L. Ling, W. Qiao, L. Liu, Carbon 37 (1999) 663. [24] Z. Liu, L. Ling, W. Qiao, L. Liu, Carbon 37 (1999) 2063. [25] Z. Zhu, A. Li, L. Yan, F. Liu, Q. Zhang, Journal of Colloid and Interface Science 316 (2007) 628. [26] C. Long, J.D. Lu, A. Li, D. Hu, F. Liu, Q. Zhang, Journal of Hazardous Materials 150 (2008) 656. [27] W. Bratek, K. Bratek, M. Kułaz˙yn´ski, Fuel Processing Technology 81 (2003) 87. [28] J.J. Lee, J.K. Suh, J.S. Hong, J.M. Lee, J.W. Park, Materials Science Forum 569 (2008) 37. [29] J.J. Lee, J.K. Suh, J.S. Hong, J.M. Lee, Y.S. Lee, J.W. Park, Carbon 46 (2008) 1648. [30] J.J. Lee, J.K. Suh, J.S. Hong, J.M. Lee, J.W. Park, Research on Chemical Intermediates 35 (2009) 337. [31] Y. Fu, H. Du, S. Zhang, W. Huang, Materials Science and Engineering A 403 (2005) 25. [32] H. Liu, W. Yang, Y. Ma, J. Yao, Applied Catalysis A: General 299 (2006) 218. [33] X. Zhang, M. Zhou, L. Lei, Carbon 44 (2006) 325. [34] J.H. Zhou, Z.J. Sui, J. Zhu, P. Li, D. Chen, Y.C. Dai, W.K. Yuan, Carbon 45 (2007) 785. [35] Q. Xiao, J. Zhang, C. Xiao, Z. Si, X. Tan, Solar Energy 82 (2008) 706. [36] A. Jitianu, T. Cacciaguerra, R. Benoit, S. Delpeux, F. Be´guin, S. Bonnamy, Carbon 42 (2004) 1147.
M.-H. Baek et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 469–477 [37] J.R. Raji, K. Palanivelu, Industrial and Engineering Chemistry Research 50 (2011) 3130. [38] P. Go´rska, A. Zaleska, E. Kowalska, T. Klimczuk, J.W. Sobczak, E. Skwarek, W. Janusz, J. Hupka, Applied Catalysis B: Environmental 84 (2008) 440. [39] K. Lv, Q. Xiang, J. Yu, Applied Catalysis B: Environmental 104 (2011) 275. [40] J. Yu, Y. Su, B. Cheng, Advanced Functional Materials 17 (2007) 1984. [41] Y. Chen, D.D. Dionysiou, Applied Catalysis B: Environmental 62 (2006) 255. [42] J.C. Yu, J. Yu, L. Zhang, W. Ho, Journal of Photochemistry and Photobiology A: Chemistry 148 (2002) 263. [43] Z. Ambrus, K. Mogyoro´si, A. Szalai, T. Alapi, K. Demeter, A. Dombi, P. Sipos, Applied Catalysis A: General 340 (2008) 153. [44] Q. Cai, Z.H. Huang, F. Kang, J.B. Yang, Carbon 42 (2004) 775. [45] J. Sun, X. Wang, C. Wang, Q. Wang, Journal of Applied Polymer Science 99 (2006) 2565. [46] X. Wu, N.C. Gallego, C.I. Contescu, H. Tekinalp, V.V. Bhat, F.S. Baker, M.C. Thies, Carbon 46 (2008) 54. [47] C.H. Yun, Y.H. Park, C.R. Park, Carbon 39 (2001) 559. [48] S. Barrientos-Ramı´rez, G. Montes de Oca-Ramı´rez, A. Sepu´lveda-Escribano, M.M. Pastor-Blas, A. Gonza´lez-Montiel, F. Rodrı´guez-Reinoso, Catalysis Today 150 (2010) 42. [49] S.J. Yang, J.H. Im, T. Kim, K. Lee, C.R. Park, Journal of Hazardous Materials 186 (2011) 376.
477
[50] D. Long, R. Zhang, W. Qiao, L. Zhang, X. Liang, L. Ling, Journal of Colloid and Interface Science 331 (2009) 40. [51] V. Zelenˇa´k, V. Hornebecq, P. Llewellyn, Microporous and Mesoporous Materials 83 (2005) 125. [52] J.P. Thielemann, F. Girgsdies, R. Schlo¨gl, C. Hess, J. Beilstein, Beilstein Journal of Nanotechnology 2 (2011) 110. [53] S. Hu, J. Xiang, L. Sun, M. Xu, J. Qiu, P. Fu, Fuel Processing Technology 89 (2008) 1096. [54] G. Liu, M. Jia, Z. Zhou, L. Wang, W. Zhang, D. Jiang, Journal of Colloid and Interface Science 302 (2006) 278. [55] N. Banic´, B. Abramovic´, J. Krstic´, D. Sˇojic´, D. Loncˇarevic´, Z. Cherkezova-Zheleva, V. Guzsva´ny, Applied Catalysis B: Environmental 107 (2011) 363. [56] J. Zhou, S. Yang, J. Yu, Z. Shu, Journal of Hazardous Materials 192 (2011) 1114. [57] W.T. Tsai, M.K. Lee, T.Y. Su, Y.M. Chang, Journal of Hazardous Materials 168 (2009) 269. [58] X.F. Chen, X.C. Wang, X.Z. Fu, Energy & Environmental Science 2 (2009) 872. [59] J. Yu, L. Zhang, B. Cheng, Y. Su, Journal of Physical Chemistry C 111 (2007) 10582. [60] S. Kim, S.H. Ehrman, Journal of Colloid and Interface Science 338 (2009) 304. [61] J.C. Yu, J. Yu, W. Ho, J. Zhao, Journal of Photochemistry and Photobiology A: Chemistry 148 (2002) 331. [62] H.M. Coleman, V. Vimonses, G. Leslie, R. Amal, Journal of Hazardous Materials 146 (2007) 496.