- Email: [email protected]
Floating photocatalysts based on TiO2 supported on high surface area exfoliated vermiculite for water decontamination
Catalysis Communications 7 (2006) 538–541 www.elsevier.com/locate/catcom
Floating photocatalysts based on TiO2 supported on high surface area exfoliated vermiculite for water decontamination Luiz C.R. Machado a
a,b
, Charles B. Torchia a, Rochel M. Lago
a,*
Departamento de Quı´mica, Universidade Federal de Minas Gerais, UFMG, Belo Horizonte 31270901, MG, Brazil b UNIVALE, Governador Valadares, MG, Brazil Received 8 June 2005; received in revised form 18 October 2005; accepted 23 October 2005 Available online 9 March 2006
Abstract In this work, novel photocatalysts based on composites TiO2/high surface area vermiculite are described. These catalysts show the unique feature of floating on the water surface where optimum illumination and oxygenation occurs leading to a strong increase in their photocatalytic efficiency. The catalysts were prepared by impregnation of Ti(OCH(CH3)2)4 on exfoliated vermiculite (EV) followed by hydrolysis with HCl/H2O vapor and calcination. XRD, SEM, SBET, TG/DTA analyses suggested the formation of anatase highly dispersed on the EV surface. Photocatalytic tests with the probe molecule the dye drimaren red showed that in non-stirred reactions the floating photocatalysts, especially TiO2(20 wt%)/EV, are very active whereas the TiO2 P25 sinks to the bottom of the reactor and remains inactive due to the poor illumination and oxygenation. 2006 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide is the most promising photocatalyst for the oxidation of organic contaminants and it has been extensively studied over the last decade [1]. Several efforts have been directed towards the improvement of the photocatalytic activity of TiO2 in order to increase its efficiency for the treatment of wastewaters. Some of these approaches are: TiO2 doping with transition metals [2–4], modification of the TiO2 surface by noble metals (e.g. Pd, Au, Ag) [5,6] and coupling of TiO2 with different semiconductors [7]. Also different reactor designs, for example thin TiO2 films in rotating disk [8] and in solar cascade reactors [9], have been used to increase the photocatalytic activity by improving the illumination and oxygenation processes. In this work it is developed a new concept named ‘‘floating photocatalyst’’, which is the TiO2 photocatalyst synthesized on the surface of a floatable substrate (Fig. 1).
*
Corresponding author. Tel.: +55 31 34995777; fax: +55 31 34995700. E-mail address: [email protected] (R.M. Lago).
1566-7367/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.10.020
Some important features of a floatable photocatalyst are: (i) it maximizes the illumination/light utilization process, especially in a system using solar irradiation, (ii) it maximizes the oxygenation of the photocatalyst by the proximity with the air/water interface, especially for nonstirred reactions. The optimization of illumination and oxygenation should result in higher rates of radical formation and oxidation efficiencies. The floating photocatalysts can be applied for solar remediation in situ, i.e. directly in the contaminated wastewater reservoirs located in remote places without any special equipment or installation. Also, they can be used for the more efficient destruction of suspended insoluble organic contaminants, e.g. in oil-spill accidents, since the EV can absorb/adsorb and pre-concentrate the contaminant. For the floating substrate, we used vermiculite, a low cost and non-toxic clay mineral that shows very good chemical, mechanical and thermal stability. The vermiculite, a 2:1 phyllosilicate [10,11] is composed of tetrahedral SiO4 layers and octahedral Al(OH)3 or Mg(OH)2 layers. This clay mineral contains typically 5–20% of water located in the interlayer space [12]. An interesting property of
L.C.R. Machado et al. / Catalysis Communications 7 (2006) 538–541
Fig. 1. Schematic representation of a floatable photocatalyst and its operation.
vermiculite is that upon a sudden heating at temperatures higher than 700 C, the water molecules evaporate abruptly from the inside of the clay structure producing layer separation. As a result of this process, the clay volume increases by 3–20 times [13] and its density strongly decreases to ca. 0.05–0.30 g cm3 and the vermiculite can float on water. This expanded or exfoliated vermiculite (EV) with highly developed porous structure formed by the interlamellar space has been used for different applications as adsorbent and absorbent [14]. 2. Experimental Vermiculite was obtained from Catala˜o (Brazil) and showed an approximate composition according to SEM/ EDS microprobe of: ðAl0:30 Ti0:04 Fe0:63 Mg2:00 ÞðSi3:21 Al0:79 ÞO10 ðOHÞ2 Mg0:13 Na0:02 K0:10 ðH2 OÞn The granulometric fraction used, ca. 0.2–0.5 mm, is considered a waste by the mining industry. The exfoliation was carried out by introducing the vermiculite in a quartz tube at 1000 C for 60 s. The composites TiO2/EV with titania content of 20, 40 and 50 wt% were prepared by impregnation of tetraisopropylorthotitanate, Ti(OCH(CH3)2)4 (from 0.3 to 2.1 g) (Aldrich 97%) in hexane on exfoliated vermiculite (EV) (2 g) followed by drying at 80 C in vacuum. The EV impregnated with Ti(OCH(CH3)2)4 was hydrolyzed by HCl/H2O vapor produced by a HCl solution (0.5 mol L1 at 90 C) carried by flowing air (30 mL min1). The hydrolyzed composites were dried at 80 C for 12 h and calcined in air at 400– 600 C for 1 h. The catalysts were characterized by BET surface area (22 cycles N2 adsorption/desorption in an Autosorb 1 Quantachrome) and by powder XRD (Rigaku ˚ , 4 min1). Geigerflex, Ni-filtered Cu Ka k = 1.5418 A Thermogravimetric analyses were carried out in a Shimadzu TGA/DTA 50 H in air flow and heating rate of 10 C min1. Scanning electron microscopy (SEM) was performed in a Jeol JKA 8900RL. The photocatalytic studies were carried out with the reactive textile dye drimaren
539
red (Color Index 18286) as a probe molecule. The reactions were performed with 200 mL dye solution at 50 mg L1 concentration and 200 mg of photocatalyst. The photocatalytic reactor with surface (ca. 120 cm2) illumination by an unfiltered low pressure Hg UV lamp (15 W, 254 nm) was kept at 10 cm distant from the solution. The reaction temperature was maintained at 28 ± 2 C. Before the reaction, the catalyst (200 mg) was kept in the dye solution (200 mL, 50 mg L1) in the dark for 48 h to reach adsorption equilibrium. No stirring was used in the reactions in order to avoid oxygenation of the solution and to simulate the use of a floating photocatalyst. The reaction was started by turning on the UV light and during 360 min aliquots of 1 mL were collected, centrifuged for 30 min at 5000 rpm. The color removal in the samples was determined by spectrophotometric analysis at k = 534 nm in an UV–Vis Analyser 800M spectrophotometer. 3. Results and discussion The floating photocatalysts were prepared with TiO2 contents of 20, 40 and 50 wt% by impregnation of Ti(OCH(CH3)2)4 on the surface of exfoliated vermiculite (EV). The higher TiO2 content resulted in non-floating catalysts due to composite densities greater than 1 g cm3. TGA/DTA analyses were carried out to investigate the thermal behavior of the hydrolyzed composites. The obtained results for the composite hydrolyzed and noncalcined TiO2(40%)/EV showed an endothermic peak in the range 50–130 C with weight loss of ca. 30% related to the evaporation of H2O and isopropyl alcohol. At 170– 250 C a weight loss of ca. 8%, probably related to the decomposition of organics and late condensation reactions is observed. Finally, a weight loss of ca. 10% occurs at 420– 520 C, which is likely related to dehydroxylation processes. Previous work on sol–gel synthesis of TiO2 using the same precursor suggests that in this temperature range oxidation of organic residues and crystallization of TiO2 to anatase also take place [15]. Powder XRD diffractograms of the exfoliated vermiculite (EV), the EV hydrolyzed with HCl/H2O vapor and the TiO2(40%)/EV composite are shown in Fig. 2. It can be observed that upon hydrolysis with HCl/H2O vapor the well defined XRD peaks of EV disappeared to produce only a very broad peak between 2h 15 and 50. These results suggest that the hydrolysis process strongly affects the EV crystalline structure. For the composite TiO2 (40%)/EV calcined at 500 C clear peaks for the anatase phase TiO2 were observed (JCPDS-ICCD 1996, 21-1272). Similar result was observed for the composite TiO2(50%)/ EV whereas no diffraction peaks could be detected for the photocatalyst TiO2(20%)/EV suggesting that the TiO2 is highly dispersed on the EV surface. Nitrogen adsorption analysis of EV showed BET surface area of 17 m2 g1 which strongly increased to 84 m2 g1 after treatment with HCl/H2O vapor (Table 1).
540
L.C.R. Machado et al. / Catalysis Communications 7 (2006) 538–541
Relative intensity/ a.u.
A A
A
A
A TiO2/VE
EV hydrolized HCl vapor
EV (intensity/5) 10
20
30
40
50
60
70
80
2 / degree Fig. 2. Powder XRD of the EV, the EV after hydrolysis with HCl/H2O vapor and the composite TiO2(40wt%)/EV obtained by hydrolysis with HCl/H2O and calcination at 500 C (A = anatase).
Table 1 BET surface areas of EV, EV after hydrolysis and the composites TiO2/EV (hydrolysis HCl/H2O vapor, calcination at 500 C) ´ rea (m2 g1) Materials A Pure EV Pure EV exposed to HCl/H2O vapor TiO2(20%)/EV TiO2(40%)/EV TiO2(50%)/EV
17 84 120 76 89
This increase in SBET is likely related to the reaction of HCl with the clay mineral structure leaching Al and Si and creating porosity in the material [16,17]. This process also leads to a loss of crystallinity of the clay which was observed by powder XRD. It is interesting to observe that the presence of titanium dioxide at 20% on the EV produced a significant increase on the surface area from 84 to 120 m2 g1 probably due to the formation of a highly dispersed porous TiO2 phase. For higher TiO2 content, i.e. 40 and 50%, it is observed a decrease on the surface area to 76 and 89 m2 g1, respectively. SEM images of the EV and the TiO2/EV composites are shown in Fig. 3. Fig. 3a shows the exfoliated lamellae of the pure vermiculite. Figs. 3b and c for TiO2(20%)/EV show agglomerated particles spread all over the EV surface. For the TiO2(40%)/EV (Figs. 3d and e) and TiO2(50%)/EV (Fig. 3f) quasi spherical particles of TiO2 located in the interlamellar space and on the flat surface of the vermiculite sheets can be observed. The photocatalytic experiments were carried out using the reactive textile dye drimaren red as a probe molecule and the reaction was monitored by the discoloration rate. A complete adsorption study with the composites was carried out showing that 23 mgdye g1 composite is adsorbed in a very slow process which needs 45 h to reach equilibrium. Therefore, before the photocatalytic tests the composites were exposed in the dark to a solution of 50 mg L1 of the dye for 48 h. After this equilibrium period the system
Fig. 3. SEM of (a) EV, (b and c) TiO2(20%)/EV and (d and e) TiO2(40 wt%) and (f) TiO2(50%)/EV.
was exposed to a UV light in a special reactor designed to produce a preferential surface irradiation and no stirring to avoid artificial oxygenation of the reaction medium (Fig. 4). This reactor was designed to simulate environments where floating photocatalysts can be applied. The preliminary catalytic results obtained are shown in Fig. 5.
Hg UV Lamp
TiO2 thermocouple EV Floating photocatalyst
Dye solution
Covered reaction vessel (to expose only the water surface to irradiation)
Fig. 4. Schematic representation of the photocatalytic reactor used for the oxidation of the textile dye drimaren red.
L.C.R. Machado et al. / Catalysis Communications 7 (2006) 538–541
4. Conclusion
no catalyst
TiO2 P25
Absorbance /a.u.
1.0
TiO2(40%)/EV
0.9
TiO2(20%)/EV 0.8
TiO2(50%)/EV 0.7 0
100
200
541
300
400
Time /min
The preliminary results presented in this work have shown that exfoliated vermiculite can be used to support TiO2 to prepare active floating photocatalysts. The clay mineral vermiculite is a non-toxic low cost natural material and as a catalyst support offers good chemical, thermal and mechanical resistance. The Ti(OCH(CH3)2)4 can be used as precursor to prepare highly dispersed TiO2 on the EV surface. The hydrolysis process carried out with HCl/H2O vapor showed the best results producing a strong increase of the EV surface area, a good TiO2 dispersion and the best catalytic performance. This floating photocatalyst can be used to remediate contaminated waters and can be recovered and recycled after the treatment is finished.
Fig. 5. Discoloration of the reactive textile dye drimaren red (C0 = 50 mg L1, catalyst 200 mg at 28 ± 2 C).
Acknowledgments Although TiO2 P25 Degussa is a very active photocatalyst, under the reaction conditions employed it showed nearly no activity for the oxidation of the dye with discoloration rates comparable to the control experiment with irradiation and no catalyst. This is likely related to the fact that TiO2 P25 with density of ca. 1.8 g cm3 sinks and at the bottom of the reactor no photooxidation takes place due to the poor illumination and oxygenation. On the other hand, the floating photocatalyst showed much higher discoloration activity. The approximate linear behavior in Fig. 5 suggests a discoloration with pseudozero order rates of 2.1 · 101, 1.7 · 101 and 1.1 · 101 min1 for the composites with 20%, 50% and 40% of TiO2/EV, respectively. This pseudo-zero order behavior is likely related to an important mass transfer limitation in a non-stirred reactor. As the photocatalytic reaction, surprisingly, the TiO2(20%)/EV showed the highest photocatalytic activity which might be related to the higher surface area and better dispersion of the TiO2 over the clay surface. The floating photocatalysts were also prepared by hydrolysis using HCl aqueous solution instead HCl/H2O vapor. However, the composites obtained by HCl solution hydrolysis showed much lower surface areas (ca. 20–30 m2 g1) and were nearly inactive for the photocatalytic oxidations. Also, the highest photocatalytic activities were obtained for the catalysts calcined at temperatures near to 500 C. Probably, upon calcination at 400 C, the active phase TiO2 was not completely formed as suggested by XRD measurements and at higher temperature, especially 700 and 800 C, the formation of rutile less active phase and/or sintering might be responsible for the decrease in activity observed [15].
The authors are grateful to CNPq, FAPEMIG, CAPES for financial support and for the Lab. Microscopia Eletroˆnica UFMG. References [1] K. Kabra, R. Chaudhary, R.L. Sawhney, Ind. Eng. Chem. Res. 43 (2004) 7683. [2] P. Cheng, M.Y. Gu, Y.P. Jin, Prog. Chem. 17 (2005) 8. [3] J.P. Yan, Z.L. Tang, S.H. Luo, Z.T. Zhang, Rare Metal Mater. Eng. 32 (2003) 582. [4] S. Klosek, D. Raftery, J. Phys. Chem. B 105 (2001) 2815. [5] S. Sakthivel, M.V. Shankar, M. Palanichamy, B. Arabindoo, D.W. Bahnemann, V. Murugesan, Water Res. 38 (2004) 3001. [6] I.M. Arabatzis, T. Stergiopoulos, D. Andreeva, S. Kitova, S.G. Neophytides, P. Falaras, J. Catal. 220 (2003) 127. [7] A.L. Stroyuk, V.V. Shvalagin, S.Y. Kuchmii, J. Photochem. Photobiol. A 173 (2005) 185. [8] D.D. Dionysiou, M.T. Suidan, I. Baudin, J.M. Laine, Appl. Catal. B 50 (2004) 259. [9] A.H.C. Chan, C.K. Chan, J.P. Barford, J.F. Porter, Water Res. 37 (2003) 1125. [10] R.E. Grim, Clay Mineralogy, McGraw-Hill, New York, 1968, p. 596. [11] G.W. Brindley, G. Brown, Crystal Structure of Clay Minerals and Their X-ray Identification, Mineralogical Society, London, 1980, p. 495. [12] C. De la Calle, H. Suquet, in: S.W. Bailey (Ed.), Reviews in Mineralogy: Hydrous Phyllosilicates, vol. 19, Mineralogical Society of America, Washington, 1988, p. 725 (Chapter 12). [13] P.S. Santos, R. Navajas, Rev. Bras.Technol. 7 (1976) 415. [14] L.C.R. Machado, F.W.J. Lima, R. Paniago, J.D. Ardisson, K. Sapag, R.M. Lago, Appl. Clay Sci. (2006), in press. [15] S. Mahanty, S. Roy, S. Sen, J. Cryst. Growth 261 (2004) 77. [16] G. Jozefaciuk, G. Bowanko, Clays Clay Min. 50 (2002) 771. [17] H. Suquet, S. Chevalier, C. Marcilly, D. Barthomeuf, Clay Min. 26 (1991) 49.
Floating photocatalysts based on TiO2 supported on high surface area exfoliated vermiculite for water decontamination Luiz C.R. Machado a
a,b
, Charles B. Torchia a, Rochel M. Lago
a,*
Departamento de Quı´mica, Universidade Federal de Minas Gerais, UFMG, Belo Horizonte 31270901, MG, Brazil b UNIVALE, Governador Valadares, MG, Brazil Received 8 June 2005; received in revised form 18 October 2005; accepted 23 October 2005 Available online 9 March 2006
Abstract In this work, novel photocatalysts based on composites TiO2/high surface area vermiculite are described. These catalysts show the unique feature of floating on the water surface where optimum illumination and oxygenation occurs leading to a strong increase in their photocatalytic efficiency. The catalysts were prepared by impregnation of Ti(OCH(CH3)2)4 on exfoliated vermiculite (EV) followed by hydrolysis with HCl/H2O vapor and calcination. XRD, SEM, SBET, TG/DTA analyses suggested the formation of anatase highly dispersed on the EV surface. Photocatalytic tests with the probe molecule the dye drimaren red showed that in non-stirred reactions the floating photocatalysts, especially TiO2(20 wt%)/EV, are very active whereas the TiO2 P25 sinks to the bottom of the reactor and remains inactive due to the poor illumination and oxygenation. 2006 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide is the most promising photocatalyst for the oxidation of organic contaminants and it has been extensively studied over the last decade [1]. Several efforts have been directed towards the improvement of the photocatalytic activity of TiO2 in order to increase its efficiency for the treatment of wastewaters. Some of these approaches are: TiO2 doping with transition metals [2–4], modification of the TiO2 surface by noble metals (e.g. Pd, Au, Ag) [5,6] and coupling of TiO2 with different semiconductors [7]. Also different reactor designs, for example thin TiO2 films in rotating disk [8] and in solar cascade reactors [9], have been used to increase the photocatalytic activity by improving the illumination and oxygenation processes. In this work it is developed a new concept named ‘‘floating photocatalyst’’, which is the TiO2 photocatalyst synthesized on the surface of a floatable substrate (Fig. 1).
*
Corresponding author. Tel.: +55 31 34995777; fax: +55 31 34995700. E-mail address: [email protected] (R.M. Lago).
1566-7367/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.10.020
Some important features of a floatable photocatalyst are: (i) it maximizes the illumination/light utilization process, especially in a system using solar irradiation, (ii) it maximizes the oxygenation of the photocatalyst by the proximity with the air/water interface, especially for nonstirred reactions. The optimization of illumination and oxygenation should result in higher rates of radical formation and oxidation efficiencies. The floating photocatalysts can be applied for solar remediation in situ, i.e. directly in the contaminated wastewater reservoirs located in remote places without any special equipment or installation. Also, they can be used for the more efficient destruction of suspended insoluble organic contaminants, e.g. in oil-spill accidents, since the EV can absorb/adsorb and pre-concentrate the contaminant. For the floating substrate, we used vermiculite, a low cost and non-toxic clay mineral that shows very good chemical, mechanical and thermal stability. The vermiculite, a 2:1 phyllosilicate [10,11] is composed of tetrahedral SiO4 layers and octahedral Al(OH)3 or Mg(OH)2 layers. This clay mineral contains typically 5–20% of water located in the interlayer space [12]. An interesting property of
L.C.R. Machado et al. / Catalysis Communications 7 (2006) 538–541
Fig. 1. Schematic representation of a floatable photocatalyst and its operation.
vermiculite is that upon a sudden heating at temperatures higher than 700 C, the water molecules evaporate abruptly from the inside of the clay structure producing layer separation. As a result of this process, the clay volume increases by 3–20 times [13] and its density strongly decreases to ca. 0.05–0.30 g cm3 and the vermiculite can float on water. This expanded or exfoliated vermiculite (EV) with highly developed porous structure formed by the interlamellar space has been used for different applications as adsorbent and absorbent [14]. 2. Experimental Vermiculite was obtained from Catala˜o (Brazil) and showed an approximate composition according to SEM/ EDS microprobe of: ðAl0:30 Ti0:04 Fe0:63 Mg2:00 ÞðSi3:21 Al0:79 ÞO10 ðOHÞ2 Mg0:13 Na0:02 K0:10 ðH2 OÞn The granulometric fraction used, ca. 0.2–0.5 mm, is considered a waste by the mining industry. The exfoliation was carried out by introducing the vermiculite in a quartz tube at 1000 C for 60 s. The composites TiO2/EV with titania content of 20, 40 and 50 wt% were prepared by impregnation of tetraisopropylorthotitanate, Ti(OCH(CH3)2)4 (from 0.3 to 2.1 g) (Aldrich 97%) in hexane on exfoliated vermiculite (EV) (2 g) followed by drying at 80 C in vacuum. The EV impregnated with Ti(OCH(CH3)2)4 was hydrolyzed by HCl/H2O vapor produced by a HCl solution (0.5 mol L1 at 90 C) carried by flowing air (30 mL min1). The hydrolyzed composites were dried at 80 C for 12 h and calcined in air at 400– 600 C for 1 h. The catalysts were characterized by BET surface area (22 cycles N2 adsorption/desorption in an Autosorb 1 Quantachrome) and by powder XRD (Rigaku ˚ , 4 min1). Geigerflex, Ni-filtered Cu Ka k = 1.5418 A Thermogravimetric analyses were carried out in a Shimadzu TGA/DTA 50 H in air flow and heating rate of 10 C min1. Scanning electron microscopy (SEM) was performed in a Jeol JKA 8900RL. The photocatalytic studies were carried out with the reactive textile dye drimaren
539
red (Color Index 18286) as a probe molecule. The reactions were performed with 200 mL dye solution at 50 mg L1 concentration and 200 mg of photocatalyst. The photocatalytic reactor with surface (ca. 120 cm2) illumination by an unfiltered low pressure Hg UV lamp (15 W, 254 nm) was kept at 10 cm distant from the solution. The reaction temperature was maintained at 28 ± 2 C. Before the reaction, the catalyst (200 mg) was kept in the dye solution (200 mL, 50 mg L1) in the dark for 48 h to reach adsorption equilibrium. No stirring was used in the reactions in order to avoid oxygenation of the solution and to simulate the use of a floating photocatalyst. The reaction was started by turning on the UV light and during 360 min aliquots of 1 mL were collected, centrifuged for 30 min at 5000 rpm. The color removal in the samples was determined by spectrophotometric analysis at k = 534 nm in an UV–Vis Analyser 800M spectrophotometer. 3. Results and discussion The floating photocatalysts were prepared with TiO2 contents of 20, 40 and 50 wt% by impregnation of Ti(OCH(CH3)2)4 on the surface of exfoliated vermiculite (EV). The higher TiO2 content resulted in non-floating catalysts due to composite densities greater than 1 g cm3. TGA/DTA analyses were carried out to investigate the thermal behavior of the hydrolyzed composites. The obtained results for the composite hydrolyzed and noncalcined TiO2(40%)/EV showed an endothermic peak in the range 50–130 C with weight loss of ca. 30% related to the evaporation of H2O and isopropyl alcohol. At 170– 250 C a weight loss of ca. 8%, probably related to the decomposition of organics and late condensation reactions is observed. Finally, a weight loss of ca. 10% occurs at 420– 520 C, which is likely related to dehydroxylation processes. Previous work on sol–gel synthesis of TiO2 using the same precursor suggests that in this temperature range oxidation of organic residues and crystallization of TiO2 to anatase also take place [15]. Powder XRD diffractograms of the exfoliated vermiculite (EV), the EV hydrolyzed with HCl/H2O vapor and the TiO2(40%)/EV composite are shown in Fig. 2. It can be observed that upon hydrolysis with HCl/H2O vapor the well defined XRD peaks of EV disappeared to produce only a very broad peak between 2h 15 and 50. These results suggest that the hydrolysis process strongly affects the EV crystalline structure. For the composite TiO2 (40%)/EV calcined at 500 C clear peaks for the anatase phase TiO2 were observed (JCPDS-ICCD 1996, 21-1272). Similar result was observed for the composite TiO2(50%)/ EV whereas no diffraction peaks could be detected for the photocatalyst TiO2(20%)/EV suggesting that the TiO2 is highly dispersed on the EV surface. Nitrogen adsorption analysis of EV showed BET surface area of 17 m2 g1 which strongly increased to 84 m2 g1 after treatment with HCl/H2O vapor (Table 1).
540
L.C.R. Machado et al. / Catalysis Communications 7 (2006) 538–541
Relative intensity/ a.u.
A A
A
A
A TiO2/VE
EV hydrolized HCl vapor
EV (intensity/5) 10
20
30
40
50
60
70
80
2 / degree Fig. 2. Powder XRD of the EV, the EV after hydrolysis with HCl/H2O vapor and the composite TiO2(40wt%)/EV obtained by hydrolysis with HCl/H2O and calcination at 500 C (A = anatase).
Table 1 BET surface areas of EV, EV after hydrolysis and the composites TiO2/EV (hydrolysis HCl/H2O vapor, calcination at 500 C) ´ rea (m2 g1) Materials A Pure EV Pure EV exposed to HCl/H2O vapor TiO2(20%)/EV TiO2(40%)/EV TiO2(50%)/EV
17 84 120 76 89
This increase in SBET is likely related to the reaction of HCl with the clay mineral structure leaching Al and Si and creating porosity in the material [16,17]. This process also leads to a loss of crystallinity of the clay which was observed by powder XRD. It is interesting to observe that the presence of titanium dioxide at 20% on the EV produced a significant increase on the surface area from 84 to 120 m2 g1 probably due to the formation of a highly dispersed porous TiO2 phase. For higher TiO2 content, i.e. 40 and 50%, it is observed a decrease on the surface area to 76 and 89 m2 g1, respectively. SEM images of the EV and the TiO2/EV composites are shown in Fig. 3. Fig. 3a shows the exfoliated lamellae of the pure vermiculite. Figs. 3b and c for TiO2(20%)/EV show agglomerated particles spread all over the EV surface. For the TiO2(40%)/EV (Figs. 3d and e) and TiO2(50%)/EV (Fig. 3f) quasi spherical particles of TiO2 located in the interlamellar space and on the flat surface of the vermiculite sheets can be observed. The photocatalytic experiments were carried out using the reactive textile dye drimaren red as a probe molecule and the reaction was monitored by the discoloration rate. A complete adsorption study with the composites was carried out showing that 23 mgdye g1 composite is adsorbed in a very slow process which needs 45 h to reach equilibrium. Therefore, before the photocatalytic tests the composites were exposed in the dark to a solution of 50 mg L1 of the dye for 48 h. After this equilibrium period the system
Fig. 3. SEM of (a) EV, (b and c) TiO2(20%)/EV and (d and e) TiO2(40 wt%) and (f) TiO2(50%)/EV.
was exposed to a UV light in a special reactor designed to produce a preferential surface irradiation and no stirring to avoid artificial oxygenation of the reaction medium (Fig. 4). This reactor was designed to simulate environments where floating photocatalysts can be applied. The preliminary catalytic results obtained are shown in Fig. 5.
Hg UV Lamp
TiO2 thermocouple EV Floating photocatalyst
Dye solution
Covered reaction vessel (to expose only the water surface to irradiation)
Fig. 4. Schematic representation of the photocatalytic reactor used for the oxidation of the textile dye drimaren red.
L.C.R. Machado et al. / Catalysis Communications 7 (2006) 538–541
4. Conclusion
no catalyst
TiO2 P25
Absorbance /a.u.
1.0
TiO2(40%)/EV
0.9
TiO2(20%)/EV 0.8
TiO2(50%)/EV 0.7 0
100
200
541
300
400
Time /min
The preliminary results presented in this work have shown that exfoliated vermiculite can be used to support TiO2 to prepare active floating photocatalysts. The clay mineral vermiculite is a non-toxic low cost natural material and as a catalyst support offers good chemical, thermal and mechanical resistance. The Ti(OCH(CH3)2)4 can be used as precursor to prepare highly dispersed TiO2 on the EV surface. The hydrolysis process carried out with HCl/H2O vapor showed the best results producing a strong increase of the EV surface area, a good TiO2 dispersion and the best catalytic performance. This floating photocatalyst can be used to remediate contaminated waters and can be recovered and recycled after the treatment is finished.
Fig. 5. Discoloration of the reactive textile dye drimaren red (C0 = 50 mg L1, catalyst 200 mg at 28 ± 2 C).
Acknowledgments Although TiO2 P25 Degussa is a very active photocatalyst, under the reaction conditions employed it showed nearly no activity for the oxidation of the dye with discoloration rates comparable to the control experiment with irradiation and no catalyst. This is likely related to the fact that TiO2 P25 with density of ca. 1.8 g cm3 sinks and at the bottom of the reactor no photooxidation takes place due to the poor illumination and oxygenation. On the other hand, the floating photocatalyst showed much higher discoloration activity. The approximate linear behavior in Fig. 5 suggests a discoloration with pseudozero order rates of 2.1 · 101, 1.7 · 101 and 1.1 · 101 min1 for the composites with 20%, 50% and 40% of TiO2/EV, respectively. This pseudo-zero order behavior is likely related to an important mass transfer limitation in a non-stirred reactor. As the photocatalytic reaction, surprisingly, the TiO2(20%)/EV showed the highest photocatalytic activity which might be related to the higher surface area and better dispersion of the TiO2 over the clay surface. The floating photocatalysts were also prepared by hydrolysis using HCl aqueous solution instead HCl/H2O vapor. However, the composites obtained by HCl solution hydrolysis showed much lower surface areas (ca. 20–30 m2 g1) and were nearly inactive for the photocatalytic oxidations. Also, the highest photocatalytic activities were obtained for the catalysts calcined at temperatures near to 500 C. Probably, upon calcination at 400 C, the active phase TiO2 was not completely formed as suggested by XRD measurements and at higher temperature, especially 700 and 800 C, the formation of rutile less active phase and/or sintering might be responsible for the decrease in activity observed [15].
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