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Zn–Al layered double hydroxides: Synthesis, characterization and photocatalytic application
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Microporous and Mesoporous Materials 113 (2008) 296–304 www.elsevier.com/locate/micromeso
Zn–Al layered double hydroxides: Synthesis, characterization and photocatalytic application E.M. Seftel a,b,*, E. Popovici a, M. Mertens c, K. De Witte b, G. Van Tendeloo d, P. Cool b, E.F. Vansant b a
Department of Physical and Theoretical Chemistry and Materials Chemistry, ‘‘Al. I. Cuza” University of Iasi, Boulevard, Carol I, No 11, 700506, Romania b Laboratory of Adsorption and Catalysis, University of Antwerpen (CDE), Universiteitsplein 1, 2610 Wilrjik, Antwerpen, Belgium c VITO Flemish Institute for Technological Research, Boeretang 200, B-2400, Belgium d EMAT, University of Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium Received 10 September 2007; received in revised form 16 November 2007; accepted 21 November 2007 Available online 4 January 2008
Abstract Zn/Al-LDHs with cationic ratio of 1–4 were prepared by the co-precipitation method at constant pH. The XRD patterns showed that additional phase is present in all samples due to the lattice strains created when more Zn2+ is added and the longer periods of hydrothermal treatment. The as-synthesized samples were calcined at different temperatures and the phase transformations were fully investigated by XRD, IR, TG/DTG, UV–vis-DR, N2 adsorption/desorption, SEM and EDX methods. Infrared spectroscopy revealed that the characteristic layered double hydroxide structure is not fully destroyed. The EDX analysis showed that the increase of the calcination temperature leads to a diffusion of the Zn2+ cations to the surface of the particles. The photocatalytic activity was evaluated for the degradation of the methyl-orange dye. The band gap energy decreases as the calcination temperature increases indicating that less energy is needed for the photocatalytic process. The photocatalytic activity increases with the increase of the cationic ratio and the calcination temperature. 93% of the dye could be removed by the Zn/Al-LDH with the cationic ratio of 4 and calcined at 500 °C. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Zn2+ containing-LDHs; Co-precipitation; Cationic ratio; Phase transformation; Photodegradation
1. Introduction Methyl-orange is a well known acid–base indicator and is considered as a model of a series of common azo-dyes used in the industry. Photocatalytic techniques using metal semiconductors, such as ZnO, TiO2, SnO2 or CdS, have been widely applied for the degradation of the organic pollutants in aqueous solutions [1–5]. The layered double hydroxides (LDHs), also known as hydrotalcite-like materials or as anionic (more properly speaking, anion exchanging) clays, are a large group of * Corresponding author. Address: Laboratory of Adsorption and Catalysis, University of Antwerpen (CDE), Universiteitsplein 1, 2610 Wilrjik, Antwerpen, Belgium. Tel.: +32 3 820 23 68; fax: +32 3 820 23 74. E-mail address: [email protected] (E.M. Seftel).
1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.11.029
natural and synthetic materials readily produced when suitable mixtures of metal salts are exposed to base [6,7]. III These materials have the general formula ½MII 1x Mx xþ n II III ðOHÞ2 ðAx=n Þ mH2O, where M and M are the divalent and trivalent cations, respectively, and An the anions that maintain the electro-neutrality of the brucite-like sheets. The anions and the water molecules are hosted together in the interlayer gallery. The nature of the cations in the brucite-like sheets (which is not restricted to +2/+3 combinations) and the interlayer anions together with the coefficient x value may be varied in a broad range, giving rise to a large class of isostructural materials [7]. Co-precipitation is probably the best technique for the synthesis of LDHs, as it allows homogeneous precursors as starting materials. The thermal stability of these materials is strongly related with several factors, such as both the
E.M. Seftel et al. / Microporous and Mesoporous Materials 113 (2008) 296–304
nature of the cations and the cationic ratio in the brucitelike sheets, the nature of the compensating anions and the crystallinity degree of the LDH lattice [6,7]. In this study we used the co-precipitation at low supersaturation method for the synthesis of layered double hydroxides containing Zn2+ and Al3+ with the cationic ratio of 1–4 and CO2 3 as charge balancing anions. Phase transformations during calcination were fully investigated. Both the uncalcined and calcined products were tested for the photodegradation of the methyl-orange anionic dye in aqueous solution. 2. Experimental 2.1. Sample preparation The layered double hydroxides containing Zn/Al with different cationic ratio were prepared by the co-precipitation at low supersaturation method at constant pH [6,7]. The cationic ratio (Zn2+/Al3+) was ranging between 1 and 4. The synthesis was carried out by the slow addition of a mixed metal nitrates solution (1 M in total) (Zn(NO3)2 6H2O, Acros Organics, 98% and Al(NO3)3 9H2O, Acros Organics, 99+%) to a Na2CO3 (2 103 M) solution under magnetic stirring. During the synthesis the pH value was kept constant at 7.5 by adding suitable amounts of 2 M NaOH solution. The resulting slurry was aged about 1 h at room temperature and then placed for 24 h on oil bath at 80 °C under magnetic stirring and reflux. The final products were recuperated by filtration, washed several times with distilled water and dried at 80 °C overnight. These samples were denoted as ZnAl-r, where r stands for the cationic ratio. Part of these samples was calcined 4 h (1°/min) at 300 °C and 500 °C in a Lenton furnace. These samples were denoted as CZnAl-r-T, where C – calcined sample, r – the cationic ratio and T – the calcination temperature. The calcined samples were stored in a nitrogen box before use. A reference ZnO sample was prepared following the same method, but the starting solution contained only zinc nitrate hexahydrate (1 M) (Acros Organics, 98%) and the pH was kept constant at 9. The final product was recuperated by filtration, washed with distilled water, dried and calcined 4 h (1°/min) at 500 °C in a Lenton furnace. 2.2. Photocatalytic tests The photocatalytic activity of both the uncalcined and calcined samples was tested for the photodegradation of methyl-orange in aqueous solution. The ratio between the catalyst and the anionic dye was 0.1 g/L. 2.5 mg of solid was added to 25 mL methyl-orange solution (4 105 M) in a plastic flask and stirred for 30 min without UV-irradiation in order to establish the adsorption–desorption equilibrium between the dye and the surface of the catalyst. After 30 min, the suspension was irradiated with UV-light for 60 min. The UV-light source was a 100 W Hg lamp
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(Sylvania Par 38, 365 nm). During the irradiation, at intervals of 10 min, samples (5 mL) of the suspension were collected, centrifuged and analyzed at 463 nm using water as reference with a Thermo-electron evolution 500 UV–vis spectrometer. 2.3. Characterization techniques The final composition of the as-synthesized samples was determined by electron probe micro analysis measurements (EPMA) and energy dispersed X-ray spectroscopy (EDX). The structure and the textural properties of the final solids were investigated by X-ray diffraction, FT-IR and UV–vis diffuse reflectance spectroscopy (UV–vis-DR), TGA, N2 adsorption/desorption and scanning electron microscopy methods. X-ray diffractions were recorded on a PANalytical X’Pert PRO MPD diffractometer with filtered Cu Ka radiation; measurements were done in the 2h mode using a bracket sample holder with a scanning speed of 0.04°/4 s continuous mode. Diffuse reflectance infrared Fourier transform spectra (DRIFT) were measured on a Nicolet 20 DXB FTIR Spectrometer, equipped with a spectra-tech diffuse reflectance accessory. About 200 scans were taken with a 4 cm1 resolution. UV–vis-DR spectra were obtained at room temperature on a Nicolet Evolution 500 UV–vis spectrometer, with a diffuse reflectance accessory using KBr standard white as reflectance. TGA measurements were performed on a Mettler TG50 thermobalance, equipped with a M3 microbalance and connected to a TC10A processor. Samples were heated at a heating rate of 5 °C/min under O2 flow. Porosity and surface area studies were performed on a Quantachrome Autosorb 1-MPautomated gas adsorption system using nitrogen as the absorbate at liquid nitrogen temperature (196 °C). All the samples were outgassed under vacuum for 16 h at 25 °C before adsorption measurements. The surface area was calculated using the BET method in the range of relative pressure 0.05–0.35. The SEM images were obtained using a JSM 5510 microscope, operating at an accelerating voltage of 15 kV. 3. Results and discussion Fig. 1 illustrates the XRD patterns for the as-synthesized ZnAl-r samples with the Zn2+/Al3+ cationic ratio of 4–1 in the synthesis mixture. For all the cationic ratios the layered structure is formed. The XRD patterns exhibit the characteristic reflections of layered double hydroxides with the basal peaks for (0 0 3) and (0 0 6) planes at low 2h angle and the nonbasal peaks for (1 0 1), (0 1 5), (0 1 8), (1 1 0) and (1 1 3) planes at high 2h angle. Additional ZnO phase is present in all the samples as indicated by the XRD patterns. The unit cell parameters and the crystallite size for the as-synthesized samples were calculated and the results are listed in Table 1.
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Fig. 1. The XRD patterns of (a) ZnAl-4, (b) ZnAl-3, (c) ZnAl-2 and (d) ZnAl-1, . ZnO phase.
Table 1 The calculated unit cell parameters and the textural features for the assynthesized samples Sample ˚) a (A ˚) c (A ˚) c’ (A ˚) The interlayer thicknessa (A ˚) D110b (A ˚) D003b (A SBET (m2/g) Vp (cc/g)
ZnAl-4
ZnAl-3
ZnAl-2
ZnAl-1
3.06 23.27 7.75 2.96 136.2 84.6 49 0.14
3.03 23.61 7.87 3.07 163.0 41.05 35 0.15
3.01 23.89 7.96 3.12 160.5 51.5 32 0.14
2.98 24.45 8.15 3.35 212.3 36.9 25 0.14
a = 2d110, c = 3d003 and c = 3c0 . a ˚ (the thickness of the brucite-like The difference between c’ and 4.8 A sheet) [1]. b Calculated using the Scherrer equation [8,9].
In general, the number, the size, the orientation and the strength of the bonds between the anions and the hydroxyl groups of the brucite-like layers determine the thickness of the interlayer. In this case the unit thickness of the interlayer increase with the decrease of the cationic ratio (Table 1). This observation can be attributed to an increase of the charge density on the brucite-like sheets when the cationic ratio decrease, leading to a higher amount of carbonate
anions required to maintain the electro-neutrality of the final material. The peak centered at 2h = 34.5 in all the XRD patterns can be associated with the ZnO phase formed on the surface of the brucite-like sheets. The intensity of the reflection increases with the increase of the cationic ratio, therefore with the Zn2+ content, as demonstrated by Fig. 1. Moreover, low values of x lead to a high density of Zn octahedra in the brucite-like sheet, acting as nuclei for the formation of Zn(OH)2. Therefore, larger quantities of ZnO phase are present in the sample with the cationic ratio of 4 as indicated by the presence of the peaks at 2h = 31.9°, 34.5°, 36.3°, 47.6°, 56.8°, 62.9°, 68.2° corresponding to the reflections from (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes, respectively [10]. The ZnO particles have a preferential orientation along the (0 0 2) plane, as this is the peak with the highest intensity. When the samples are heated at 500 °C, the structure collapses and only the reflection characteristic for the ZnO are observed (Fig. 2). The lattice parameters a and c for the ZnO phase were calculated from the peak positions using standard relations (Table 2) [10,11]. The value of the c/a ratio varies between 1.60 and 1.604. These values are smaller than 1.633 which
Fig. 2. The XRD patterns of the (a) ZnO and (b) CZnAl-4–500 °C.
E.M. Seftel et al. / Microporous and Mesoporous Materials 113 (2008) 296–304 Table 2 The textural features and the calculated unit cell parameters for the ZnO reference sample and for the ZnO phase in the ZnAl-4 and CZnAl-4-T
˚) a (A ˚) c (A c/a ˚) D(002)a (A SBET (m2/g) Vp (cc/g)
ZnAl-4
CZnAl-4– 300 °C
CZnAl-4– 500 °C
ZnO reference
2.803 5.195 1.604 172.7 49 0.14
3.25 5.21 1.603 197.3 84 0.15
3.26 5.21 1.60 211.4 91 0.16
3.26 5.21 1.60 272.4 9 0.015
a = 2 30.5 d100 and c = 2d002. a Calculated using the Scherrer equation [8,9].
correspond to a closed-packed hexagonal (wurtzite) structure indicating some strains in the crystals. Thermoanalytical measurements were recorded to investigate the phase transformations during the calcination treatment (Fig. 3). The first weight loss at lower temperatures (40–180 °C) corresponds to the water loss from internal gallery surfaces and the external non-gallery surfaces. The second one, at higher temperatures (210–260 °C) is due to the dehydroxylation of the brucite-like sheets as well as the decomposition of the carbonate anions (partial overlap) [12–14]. A small endothermic peak is observed in the
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range between 300 and 500 °C which probably corresponds to the conversion of the double layer into ZnO lattice. Accordingly, the XRD pattern of the CZnAl-4–500 °C refers to the ZnO phase (Fig. 2). The formation of the zinc oxide phase was confirmed also with the UV–vis-DR measurements. The reference sample shows the absorption edge 386 nm attributed to the ZnO nanoparticles [10,15–17] (Fig. 4). For our samples, the absorption edges at around 380 nm are attributed to the ZnO nanoparticles formed on the surface of the layered double hydroxides, which corresponds well with the XRD patterns. The blue shift of the absorption edge can be assigned to the quantum size effect if very small particle size is taken into consideration. The UV-DR measurements revealed that the intensity of the absorption bands in this region increases with the cationic ratio indicating that more ZnO phase was formed. On the other hand, there is a shift of the absorption edge as the calcination temperature increases (Fig. 5, the inset part). This is induced by a decrease of the band gap energy that can be associated with the increase of the size of the ZnO crystals [18] (i.e. 3.23 eV (kg = 384 nm) for the ZnAl-4 sample, 3.22 eV (kg = 385 nm) for the CZnAl-4–300 °C and 3.21 eV (kg = 386 nm) for the CZnAl-4–500 °C sample,
Fig. 3. Thermoanalytical curves (TG/DTG) of the ZnAl-4 sample.
Fig. 4. UV–vis-DR spectra of the ZnO reference sample.
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Fig. 5. UV–vis-DR spectra of the (a) CZnAl-1-, (b) CZnAl-2-, (c) CZnAl-3- and (d) CZnAl-4- sample calcined at 300 °C (left) and 500 °C (right). The inset picture shows the enlarged spectra in the region k/nm 360–430.
respectively), which is in good agreement with the XRD results (see Table 2). Then the decrease of the band gap energy suggests that less energy is needed for the photocatalytic degradation of the methyl-orange dye. Upon calcination, a small amount of hydrotalcite-like phase remains in all the samples, although the main compound is ZnO. The infrared data are in agreement with this supposition. The IR spectra recorded for the uncalcined samples show the presence of an intense band at 1384 cm1 attributed to the carbonate anions from the interlayer gallery [19–21]. Upon calcination, usually this band disappears as hydrotalcite decomposes. In our case, these bands are always present indicating that the layered structure is not fully destroyed (Figs. 6 and 7) [22,23]. Only for the CZnAl-1–500 °C sample, this band is significantly diminished. To determine the composition of the small particles observed in the SEM micrographs (Fig. 8), the local elemental analysis was determined by EDX. Taking into account the weight and size of these two cations, the mobility of Al is higher than Zn [24].
The EPMA results showed that in the as-synthesized samples, the amounts of Zn and Al are in agreement with the ones in the starting mixed aqueous solutions, within the experimental errors. The chemical formulas could not be calculated since the products obtained are a mixture of LDH and ZnO phase. The EDX results showed that the Zn/Al ratios decrease upon calcination (CZnAl-1-T samples) indicating the formation of particles with a Zn enriched core (Table 3). On the other hand, if more Zn is added, aluminium inhibits its diffusion, the lattice strain is such that zinc forms zinc oxide (the case of the ZnAl-4 sample). This observation is in good agreement with the XRD data (Fig. 1a). Upon calcination, the particles change to a different structure, where the core is Al enriched (probably small regions of LDH structure) and the particle surface is constituted by a ZnO form (Table 3) represented by the brighter spots in the SEM pictures (Fig. 8). These results could be in disagreement with the XRD results. But, to observe a compound by X-ray diffraction it has to be present in more than 3%, the crystallite size has to be larger
Fig. 6. The infrared spectra of the (a) ZnAl-4, (b) CZnAl-4–300 °C and (c) CZnAl-4–500 °C sample.
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Fig. 7. The infrared spectra of (a) ZnAl-1, (b) CZnAl-1–300 °C and (c) CZnAl-1–500 °C sample.
Fig. 8. The SEM micrographs of the (a) ZnAl-4, (b) CZnAl-4–300 °C and (c) CZnAl-4–500 °C sample. The arrows in the left figures indicate where the enlargements (in the right figures) were taken. The marked spots (in the right figures) indicate the part taken for the EDX measurements.
˚ and on the surface [22]. In our case, the LDH than ca. 25 A structure is sheltered with large amounts of ZnO phase and could not be detectable with X-ray diffraction. In infrared spectroscopy, a band appears below these constrains and the EDX results show the presence of both Zn and Al in the calcined samples. Moreover, the heat treatment at 500 °C is not sufficient for the formation of Al2O3 crystals
and the aluminium is present presumably in the AlO(OH) form [25]. Therefore, we might conclude that upon calcination, a small part of the layered structure remains in the core of the particles and the shell is constituted by ZnO form. These considerations are also very important for the photocatalytic application of these materials. The photo-
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Table 3 Atomic ratios determined by EPMA for the as-synthesized samples and by EDX for the CZnAl-1-T and CZnAl-4-T samples Sample
ZnAl-4 ZnAl-3 ZnAl-2 ZnAl-1 a
Zn/Al (EPMA)
3.85 2.83 1.96 0.96
xa
Zn/Al (EDX)
Starting
Final
300 °C
500 °C
0.2 0.25 0.33 0.5
0.21 0.26 0.33 0.51
3.90 nd nd 0.722
4.60 nd nd 0.721
x = MIII/(MII + MIII); nd – not determined.
catalytic activity is much higher for the ZnAl-4 sample than for the ZnAl-1 sample due to the presence of more ZnO crystals on the surface of the brucite-like layers (Fig. 9). The adsorption–desorption equilibrium between the anionic dye and the catalyst surface is established in the first 30 min without UV-illumination. The increased adsorption of the dye on the catalyst surface can be attributed to the increased surface area as the cationic ratio increases (Table 1). The adsorption and desorption isotherms of nitrogen show an uptake at intermediate relative pressures, typical for mesoporous materials (Fig. 10). The mesoporous structure of LDHs arises from the interparticle space, as it was reported previously [26]. The shape of the hysteresis loops can be explained by the presence of agglomerates defined as rigidly joined particles [27]. It is well known that calcination of the LDHs give rise not only to changes in their structure, but also to changes in their surface area and pore development [6,28]. The same kind of isotherm shape is recorded after the thermal treatments as for the as-synthesized samples (Fig. 11). The increase of the adsorbed dye in the first 30 min can be
Fig. 10. The N2 adsorption/desorption isotherms of the ZnAl-r samples.
associated with the increased surface area of the calcined samples (Table 2). The photocatalytic activity is highly improved when samples are calcined at 500 °C due to the large amounts of ZnO phase formed. Maximal degradation is observed for the CZnAl-4–500 °C sample (Fig. 12 and Table 4). Our results showed that even at this temperature the LDH-metal oxide conversion is incomplete as the LDH structure partially remains intact in the core of the particles. The above described results allow us to conclude that high amounts of Zn2+ cations and an elevate calcination temperature provides a powerful photo-oxidative catalyst.
Fig. 9. Kinetic curves of the methyl-orange photo-oxidation in the presence of the ZnAl-r samples.
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Fig. 11. The N2 adsorption/desorption isotherms of the ZnAl-4 sample and the calcined form.
Fig. 12. Kinetic curves of the methyl-orange photo-oxidation in the presence of the CZnAl-r–500 °C samples.
Table 4 The catalytic performances of the calcined Zn/Al-samples Sample
a
b
4. Conclusions c
Adsorbed dye (%)
Degraded dye (%)
Removed dye (%)
300 °C CZnAl-1–300 °C CZnAl-2–300 °C CZnAl-3–300 °C CZnAl-4–300 °C
23 51 55 48
27 41 52 63
44 71 78 81
500 °C CZnAl-1–500 °C CZnAl-2–500 °C CZnAl-3–500 °C CZnAl-4–500 °C
19 23 30 55
39 52 56 85
51 63 69 93
a b c
Determined with UV–vis after 30 min of stirring in dark. Determined with UV–vis after 60 min of irradiation with UV-light. The total amount of dye removed after 90 min of reaction.
Zn/Al-LDHs with different cationic ratios were prepared by the conventional co-precipitation method at constant pH. Phase transformations were investigated by thermoanalytical measurements. The ZnAl-4 dried at 80 °C already provided a photo-oxidative effect. The larger Zn2+ cations generate such a lattice strain that part of it forms ZnO detected by X-ray diffraction and UV–vis-DR spectroscopy. Further heat treatment up to 500 °C showed a significant improvement in the photocatalytic activity due to the formation of large amounts of ZnO phase. Infrared spectroscopy and local elemental analysis (EDX) reveal that the hydrotalcite-type structure is never fully destroyed. In a methyl-orange photo-oxidation experiment 93% of the
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dye could be removed by a Zn/Al-LDH with the cationic ratio of 4 and calcined at 500 °C. Acknowledgments This study was supported by the NoE project ‘‘Inside Pores” and the GOA project funded by the Special Fund for Research of the University of Antwerp. This work is also part of the CEEX No. 1/S1/2005 project. References [1] R. Comparelli, E. Fanizza, M.L. Curri, P.D. Cozzoli, G. Mascolo, A. Agostiano, Appl. Cat. B: Environ. 60 (2005) 1. [2] G. Colon, J.M. Sa´nchez-Espan˜a, M.C. Hidalgo, J.A. Navı´o, J. Photochem. Photobiol. A: Chem. 179 (2006) 20. [3] S. Liao, H. Donggen, D. Yu, Y. Su, G. Yuan, J. Photochem. Photobiol. A: Chem. 168 (2004) 7. [4] S.K. Kansal, M. Singh, D. Sud, J. Hazard. Mater. 141 (2007) 581. [5] L. Wu, J.C. Yu, X. Fu, J. Mol. Catal. A: Chem. 244 (2006) 25. [6] P.S. Braterman, Z.P. Xu, F. Yarberry, in: S.M. Auerbache, K.A. Carrado, P.K. Dutta (Eds.), Handbook of Layered Materials, Marcel Dekker Inc., New York, 2004, p. 373. [7] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 201. [8] Z. Chang, D.G. Evans, X. Duan, C. Vial, J. Ghabaja, V. Prevot, M. De Roy, C. Forano, J. Solid State Chem. 178 (2005) 2766. [9] Y. Feng, D. Li, Y. Wang, D.G. Evans, X. Duan, Polym. Degrad. Stabil. 91 (2006) 789. [10] R.E. Marotti, D.N. Guerra, C. Bello, G. Machado, E.A. Dalchiele, Sol. Energy Mater. Sol. Cells 82 (2004) 85.
[11] J.P. Enrı´quez, X. Mathew, Sol. Energy Mater. Sol. Cells 76 (2003) 313. [12] R.L. Frost, W. Martens, Z. Ding, J.T. Kloprogge, J. Therm. Anal. Calorim. 71 (2003) 429. [13] M. Tu, J. Shen, Y. Chen, J. Therm. Anal. Calorim. 58 (1999) 441. [14] R.L. Frost, W.N. Martens, K.L. Erickson, J. Therm. Anal. Calorim. 82 (2005) 603. [15] R.E. Marotti, P. Giorgi, G. Machado, E.A. Dalchiele, Sol. Energy Mater. Sol. Cells 90 (2006) 2356. [16] H. Zhang, Y. Cui, Y. Men, X. Liu, J. Lumin. 121 (2006) 601. [17] S.E. Dapurkar, S.K. Badamali, P. Selvam, Catal. Today 68 (2001) 63. [18] S. Wang, G. Xia, J. Shao, Z. Fan, J. Alloys Compd. 424 (2006) 304. [19] J.T. Kloprogge, R.L. Frost, J. Solid State Chem. 146 (1999) 506. [20] J.T. Kloprogge, L. Hickey, R.L. Frost, J. Raman Spectrosc. 35 (2004) 967. [21] J.T. Kloprogge, D. Wharton, L. Hickey, R.L. Frost, Am. Mineral. 87 (2002) 623. [22] J.A. Rivera, G. Fetter, Y. Jime´nez, M.M. Xochipa, P. Bosch, Appl. Catal. A: Gen. 316 (2007) 207. [23] A.J. Marchi, C.R. Apesteguı´a, Appl. Clay Sci. 13 (1998) 35. [24] J.A. Rivera, G. Fetter, P. Bosch, Micropor. Mesopor. Mater. 89 (2006) 306. ´ . Patzko´, R. Kun, V. Hornok, I. De´ka´ny, T. Engelhardt, N. Schall, [25] A Colloids Surf. A: Physicochem. Eng. Asp. 265 (2005) 64. [26] P. Benito, F.M. Labajos, V. Rives, Micropor. Mesopor. Mater. 94 (2006) 148. [27] F. Rouquerol, J. Rouquerol, K.S.W. Sing, Adsorption by Powders and Porous Solids, Principles, Methodology and Applications, Academic Press, London, 1991. [28] R. Bechara, A. D’Huysser, M. Fournier, L. Forni, G. Fornasari, F. Trifiro`, A. Vaccari, Catal. Lett. 82 (2002) 59.
Microporous and Mesoporous Materials 113 (2008) 296–304 www.elsevier.com/locate/micromeso
Zn–Al layered double hydroxides: Synthesis, characterization and photocatalytic application E.M. Seftel a,b,*, E. Popovici a, M. Mertens c, K. De Witte b, G. Van Tendeloo d, P. Cool b, E.F. Vansant b a
Department of Physical and Theoretical Chemistry and Materials Chemistry, ‘‘Al. I. Cuza” University of Iasi, Boulevard, Carol I, No 11, 700506, Romania b Laboratory of Adsorption and Catalysis, University of Antwerpen (CDE), Universiteitsplein 1, 2610 Wilrjik, Antwerpen, Belgium c VITO Flemish Institute for Technological Research, Boeretang 200, B-2400, Belgium d EMAT, University of Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium Received 10 September 2007; received in revised form 16 November 2007; accepted 21 November 2007 Available online 4 January 2008
Abstract Zn/Al-LDHs with cationic ratio of 1–4 were prepared by the co-precipitation method at constant pH. The XRD patterns showed that additional phase is present in all samples due to the lattice strains created when more Zn2+ is added and the longer periods of hydrothermal treatment. The as-synthesized samples were calcined at different temperatures and the phase transformations were fully investigated by XRD, IR, TG/DTG, UV–vis-DR, N2 adsorption/desorption, SEM and EDX methods. Infrared spectroscopy revealed that the characteristic layered double hydroxide structure is not fully destroyed. The EDX analysis showed that the increase of the calcination temperature leads to a diffusion of the Zn2+ cations to the surface of the particles. The photocatalytic activity was evaluated for the degradation of the methyl-orange dye. The band gap energy decreases as the calcination temperature increases indicating that less energy is needed for the photocatalytic process. The photocatalytic activity increases with the increase of the cationic ratio and the calcination temperature. 93% of the dye could be removed by the Zn/Al-LDH with the cationic ratio of 4 and calcined at 500 °C. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Zn2+ containing-LDHs; Co-precipitation; Cationic ratio; Phase transformation; Photodegradation
1. Introduction Methyl-orange is a well known acid–base indicator and is considered as a model of a series of common azo-dyes used in the industry. Photocatalytic techniques using metal semiconductors, such as ZnO, TiO2, SnO2 or CdS, have been widely applied for the degradation of the organic pollutants in aqueous solutions [1–5]. The layered double hydroxides (LDHs), also known as hydrotalcite-like materials or as anionic (more properly speaking, anion exchanging) clays, are a large group of * Corresponding author. Address: Laboratory of Adsorption and Catalysis, University of Antwerpen (CDE), Universiteitsplein 1, 2610 Wilrjik, Antwerpen, Belgium. Tel.: +32 3 820 23 68; fax: +32 3 820 23 74. E-mail address: [email protected] (E.M. Seftel).
1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.11.029
natural and synthetic materials readily produced when suitable mixtures of metal salts are exposed to base [6,7]. III These materials have the general formula ½MII 1x Mx xþ n II III ðOHÞ2 ðAx=n Þ mH2O, where M and M are the divalent and trivalent cations, respectively, and An the anions that maintain the electro-neutrality of the brucite-like sheets. The anions and the water molecules are hosted together in the interlayer gallery. The nature of the cations in the brucite-like sheets (which is not restricted to +2/+3 combinations) and the interlayer anions together with the coefficient x value may be varied in a broad range, giving rise to a large class of isostructural materials [7]. Co-precipitation is probably the best technique for the synthesis of LDHs, as it allows homogeneous precursors as starting materials. The thermal stability of these materials is strongly related with several factors, such as both the
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nature of the cations and the cationic ratio in the brucitelike sheets, the nature of the compensating anions and the crystallinity degree of the LDH lattice [6,7]. In this study we used the co-precipitation at low supersaturation method for the synthesis of layered double hydroxides containing Zn2+ and Al3+ with the cationic ratio of 1–4 and CO2 3 as charge balancing anions. Phase transformations during calcination were fully investigated. Both the uncalcined and calcined products were tested for the photodegradation of the methyl-orange anionic dye in aqueous solution. 2. Experimental 2.1. Sample preparation The layered double hydroxides containing Zn/Al with different cationic ratio were prepared by the co-precipitation at low supersaturation method at constant pH [6,7]. The cationic ratio (Zn2+/Al3+) was ranging between 1 and 4. The synthesis was carried out by the slow addition of a mixed metal nitrates solution (1 M in total) (Zn(NO3)2 6H2O, Acros Organics, 98% and Al(NO3)3 9H2O, Acros Organics, 99+%) to a Na2CO3 (2 103 M) solution under magnetic stirring. During the synthesis the pH value was kept constant at 7.5 by adding suitable amounts of 2 M NaOH solution. The resulting slurry was aged about 1 h at room temperature and then placed for 24 h on oil bath at 80 °C under magnetic stirring and reflux. The final products were recuperated by filtration, washed several times with distilled water and dried at 80 °C overnight. These samples were denoted as ZnAl-r, where r stands for the cationic ratio. Part of these samples was calcined 4 h (1°/min) at 300 °C and 500 °C in a Lenton furnace. These samples were denoted as CZnAl-r-T, where C – calcined sample, r – the cationic ratio and T – the calcination temperature. The calcined samples were stored in a nitrogen box before use. A reference ZnO sample was prepared following the same method, but the starting solution contained only zinc nitrate hexahydrate (1 M) (Acros Organics, 98%) and the pH was kept constant at 9. The final product was recuperated by filtration, washed with distilled water, dried and calcined 4 h (1°/min) at 500 °C in a Lenton furnace. 2.2. Photocatalytic tests The photocatalytic activity of both the uncalcined and calcined samples was tested for the photodegradation of methyl-orange in aqueous solution. The ratio between the catalyst and the anionic dye was 0.1 g/L. 2.5 mg of solid was added to 25 mL methyl-orange solution (4 105 M) in a plastic flask and stirred for 30 min without UV-irradiation in order to establish the adsorption–desorption equilibrium between the dye and the surface of the catalyst. After 30 min, the suspension was irradiated with UV-light for 60 min. The UV-light source was a 100 W Hg lamp
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(Sylvania Par 38, 365 nm). During the irradiation, at intervals of 10 min, samples (5 mL) of the suspension were collected, centrifuged and analyzed at 463 nm using water as reference with a Thermo-electron evolution 500 UV–vis spectrometer. 2.3. Characterization techniques The final composition of the as-synthesized samples was determined by electron probe micro analysis measurements (EPMA) and energy dispersed X-ray spectroscopy (EDX). The structure and the textural properties of the final solids were investigated by X-ray diffraction, FT-IR and UV–vis diffuse reflectance spectroscopy (UV–vis-DR), TGA, N2 adsorption/desorption and scanning electron microscopy methods. X-ray diffractions were recorded on a PANalytical X’Pert PRO MPD diffractometer with filtered Cu Ka radiation; measurements were done in the 2h mode using a bracket sample holder with a scanning speed of 0.04°/4 s continuous mode. Diffuse reflectance infrared Fourier transform spectra (DRIFT) were measured on a Nicolet 20 DXB FTIR Spectrometer, equipped with a spectra-tech diffuse reflectance accessory. About 200 scans were taken with a 4 cm1 resolution. UV–vis-DR spectra were obtained at room temperature on a Nicolet Evolution 500 UV–vis spectrometer, with a diffuse reflectance accessory using KBr standard white as reflectance. TGA measurements were performed on a Mettler TG50 thermobalance, equipped with a M3 microbalance and connected to a TC10A processor. Samples were heated at a heating rate of 5 °C/min under O2 flow. Porosity and surface area studies were performed on a Quantachrome Autosorb 1-MPautomated gas adsorption system using nitrogen as the absorbate at liquid nitrogen temperature (196 °C). All the samples were outgassed under vacuum for 16 h at 25 °C before adsorption measurements. The surface area was calculated using the BET method in the range of relative pressure 0.05–0.35. The SEM images were obtained using a JSM 5510 microscope, operating at an accelerating voltage of 15 kV. 3. Results and discussion Fig. 1 illustrates the XRD patterns for the as-synthesized ZnAl-r samples with the Zn2+/Al3+ cationic ratio of 4–1 in the synthesis mixture. For all the cationic ratios the layered structure is formed. The XRD patterns exhibit the characteristic reflections of layered double hydroxides with the basal peaks for (0 0 3) and (0 0 6) planes at low 2h angle and the nonbasal peaks for (1 0 1), (0 1 5), (0 1 8), (1 1 0) and (1 1 3) planes at high 2h angle. Additional ZnO phase is present in all the samples as indicated by the XRD patterns. The unit cell parameters and the crystallite size for the as-synthesized samples were calculated and the results are listed in Table 1.
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Fig. 1. The XRD patterns of (a) ZnAl-4, (b) ZnAl-3, (c) ZnAl-2 and (d) ZnAl-1, . ZnO phase.
Table 1 The calculated unit cell parameters and the textural features for the assynthesized samples Sample ˚) a (A ˚) c (A ˚) c’ (A ˚) The interlayer thicknessa (A ˚) D110b (A ˚) D003b (A SBET (m2/g) Vp (cc/g)
ZnAl-4
ZnAl-3
ZnAl-2
ZnAl-1
3.06 23.27 7.75 2.96 136.2 84.6 49 0.14
3.03 23.61 7.87 3.07 163.0 41.05 35 0.15
3.01 23.89 7.96 3.12 160.5 51.5 32 0.14
2.98 24.45 8.15 3.35 212.3 36.9 25 0.14
a = 2d110, c = 3d003 and c = 3c0 . a ˚ (the thickness of the brucite-like The difference between c’ and 4.8 A sheet) [1]. b Calculated using the Scherrer equation [8,9].
In general, the number, the size, the orientation and the strength of the bonds between the anions and the hydroxyl groups of the brucite-like layers determine the thickness of the interlayer. In this case the unit thickness of the interlayer increase with the decrease of the cationic ratio (Table 1). This observation can be attributed to an increase of the charge density on the brucite-like sheets when the cationic ratio decrease, leading to a higher amount of carbonate
anions required to maintain the electro-neutrality of the final material. The peak centered at 2h = 34.5 in all the XRD patterns can be associated with the ZnO phase formed on the surface of the brucite-like sheets. The intensity of the reflection increases with the increase of the cationic ratio, therefore with the Zn2+ content, as demonstrated by Fig. 1. Moreover, low values of x lead to a high density of Zn octahedra in the brucite-like sheet, acting as nuclei for the formation of Zn(OH)2. Therefore, larger quantities of ZnO phase are present in the sample with the cationic ratio of 4 as indicated by the presence of the peaks at 2h = 31.9°, 34.5°, 36.3°, 47.6°, 56.8°, 62.9°, 68.2° corresponding to the reflections from (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes, respectively [10]. The ZnO particles have a preferential orientation along the (0 0 2) plane, as this is the peak with the highest intensity. When the samples are heated at 500 °C, the structure collapses and only the reflection characteristic for the ZnO are observed (Fig. 2). The lattice parameters a and c for the ZnO phase were calculated from the peak positions using standard relations (Table 2) [10,11]. The value of the c/a ratio varies between 1.60 and 1.604. These values are smaller than 1.633 which
Fig. 2. The XRD patterns of the (a) ZnO and (b) CZnAl-4–500 °C.
E.M. Seftel et al. / Microporous and Mesoporous Materials 113 (2008) 296–304 Table 2 The textural features and the calculated unit cell parameters for the ZnO reference sample and for the ZnO phase in the ZnAl-4 and CZnAl-4-T
˚) a (A ˚) c (A c/a ˚) D(002)a (A SBET (m2/g) Vp (cc/g)
ZnAl-4
CZnAl-4– 300 °C
CZnAl-4– 500 °C
ZnO reference
2.803 5.195 1.604 172.7 49 0.14
3.25 5.21 1.603 197.3 84 0.15
3.26 5.21 1.60 211.4 91 0.16
3.26 5.21 1.60 272.4 9 0.015
a = 2 30.5 d100 and c = 2d002. a Calculated using the Scherrer equation [8,9].
correspond to a closed-packed hexagonal (wurtzite) structure indicating some strains in the crystals. Thermoanalytical measurements were recorded to investigate the phase transformations during the calcination treatment (Fig. 3). The first weight loss at lower temperatures (40–180 °C) corresponds to the water loss from internal gallery surfaces and the external non-gallery surfaces. The second one, at higher temperatures (210–260 °C) is due to the dehydroxylation of the brucite-like sheets as well as the decomposition of the carbonate anions (partial overlap) [12–14]. A small endothermic peak is observed in the
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range between 300 and 500 °C which probably corresponds to the conversion of the double layer into ZnO lattice. Accordingly, the XRD pattern of the CZnAl-4–500 °C refers to the ZnO phase (Fig. 2). The formation of the zinc oxide phase was confirmed also with the UV–vis-DR measurements. The reference sample shows the absorption edge 386 nm attributed to the ZnO nanoparticles [10,15–17] (Fig. 4). For our samples, the absorption edges at around 380 nm are attributed to the ZnO nanoparticles formed on the surface of the layered double hydroxides, which corresponds well with the XRD patterns. The blue shift of the absorption edge can be assigned to the quantum size effect if very small particle size is taken into consideration. The UV-DR measurements revealed that the intensity of the absorption bands in this region increases with the cationic ratio indicating that more ZnO phase was formed. On the other hand, there is a shift of the absorption edge as the calcination temperature increases (Fig. 5, the inset part). This is induced by a decrease of the band gap energy that can be associated with the increase of the size of the ZnO crystals [18] (i.e. 3.23 eV (kg = 384 nm) for the ZnAl-4 sample, 3.22 eV (kg = 385 nm) for the CZnAl-4–300 °C and 3.21 eV (kg = 386 nm) for the CZnAl-4–500 °C sample,
Fig. 3. Thermoanalytical curves (TG/DTG) of the ZnAl-4 sample.
Fig. 4. UV–vis-DR spectra of the ZnO reference sample.
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Fig. 5. UV–vis-DR spectra of the (a) CZnAl-1-, (b) CZnAl-2-, (c) CZnAl-3- and (d) CZnAl-4- sample calcined at 300 °C (left) and 500 °C (right). The inset picture shows the enlarged spectra in the region k/nm 360–430.
respectively), which is in good agreement with the XRD results (see Table 2). Then the decrease of the band gap energy suggests that less energy is needed for the photocatalytic degradation of the methyl-orange dye. Upon calcination, a small amount of hydrotalcite-like phase remains in all the samples, although the main compound is ZnO. The infrared data are in agreement with this supposition. The IR spectra recorded for the uncalcined samples show the presence of an intense band at 1384 cm1 attributed to the carbonate anions from the interlayer gallery [19–21]. Upon calcination, usually this band disappears as hydrotalcite decomposes. In our case, these bands are always present indicating that the layered structure is not fully destroyed (Figs. 6 and 7) [22,23]. Only for the CZnAl-1–500 °C sample, this band is significantly diminished. To determine the composition of the small particles observed in the SEM micrographs (Fig. 8), the local elemental analysis was determined by EDX. Taking into account the weight and size of these two cations, the mobility of Al is higher than Zn [24].
The EPMA results showed that in the as-synthesized samples, the amounts of Zn and Al are in agreement with the ones in the starting mixed aqueous solutions, within the experimental errors. The chemical formulas could not be calculated since the products obtained are a mixture of LDH and ZnO phase. The EDX results showed that the Zn/Al ratios decrease upon calcination (CZnAl-1-T samples) indicating the formation of particles with a Zn enriched core (Table 3). On the other hand, if more Zn is added, aluminium inhibits its diffusion, the lattice strain is such that zinc forms zinc oxide (the case of the ZnAl-4 sample). This observation is in good agreement with the XRD data (Fig. 1a). Upon calcination, the particles change to a different structure, where the core is Al enriched (probably small regions of LDH structure) and the particle surface is constituted by a ZnO form (Table 3) represented by the brighter spots in the SEM pictures (Fig. 8). These results could be in disagreement with the XRD results. But, to observe a compound by X-ray diffraction it has to be present in more than 3%, the crystallite size has to be larger
Fig. 6. The infrared spectra of the (a) ZnAl-4, (b) CZnAl-4–300 °C and (c) CZnAl-4–500 °C sample.
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Fig. 7. The infrared spectra of (a) ZnAl-1, (b) CZnAl-1–300 °C and (c) CZnAl-1–500 °C sample.
Fig. 8. The SEM micrographs of the (a) ZnAl-4, (b) CZnAl-4–300 °C and (c) CZnAl-4–500 °C sample. The arrows in the left figures indicate where the enlargements (in the right figures) were taken. The marked spots (in the right figures) indicate the part taken for the EDX measurements.
˚ and on the surface [22]. In our case, the LDH than ca. 25 A structure is sheltered with large amounts of ZnO phase and could not be detectable with X-ray diffraction. In infrared spectroscopy, a band appears below these constrains and the EDX results show the presence of both Zn and Al in the calcined samples. Moreover, the heat treatment at 500 °C is not sufficient for the formation of Al2O3 crystals
and the aluminium is present presumably in the AlO(OH) form [25]. Therefore, we might conclude that upon calcination, a small part of the layered structure remains in the core of the particles and the shell is constituted by ZnO form. These considerations are also very important for the photocatalytic application of these materials. The photo-
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Table 3 Atomic ratios determined by EPMA for the as-synthesized samples and by EDX for the CZnAl-1-T and CZnAl-4-T samples Sample
ZnAl-4 ZnAl-3 ZnAl-2 ZnAl-1 a
Zn/Al (EPMA)
3.85 2.83 1.96 0.96
xa
Zn/Al (EDX)
Starting
Final
300 °C
500 °C
0.2 0.25 0.33 0.5
0.21 0.26 0.33 0.51
3.90 nd nd 0.722
4.60 nd nd 0.721
x = MIII/(MII + MIII); nd – not determined.
catalytic activity is much higher for the ZnAl-4 sample than for the ZnAl-1 sample due to the presence of more ZnO crystals on the surface of the brucite-like layers (Fig. 9). The adsorption–desorption equilibrium between the anionic dye and the catalyst surface is established in the first 30 min without UV-illumination. The increased adsorption of the dye on the catalyst surface can be attributed to the increased surface area as the cationic ratio increases (Table 1). The adsorption and desorption isotherms of nitrogen show an uptake at intermediate relative pressures, typical for mesoporous materials (Fig. 10). The mesoporous structure of LDHs arises from the interparticle space, as it was reported previously [26]. The shape of the hysteresis loops can be explained by the presence of agglomerates defined as rigidly joined particles [27]. It is well known that calcination of the LDHs give rise not only to changes in their structure, but also to changes in their surface area and pore development [6,28]. The same kind of isotherm shape is recorded after the thermal treatments as for the as-synthesized samples (Fig. 11). The increase of the adsorbed dye in the first 30 min can be
Fig. 10. The N2 adsorption/desorption isotherms of the ZnAl-r samples.
associated with the increased surface area of the calcined samples (Table 2). The photocatalytic activity is highly improved when samples are calcined at 500 °C due to the large amounts of ZnO phase formed. Maximal degradation is observed for the CZnAl-4–500 °C sample (Fig. 12 and Table 4). Our results showed that even at this temperature the LDH-metal oxide conversion is incomplete as the LDH structure partially remains intact in the core of the particles. The above described results allow us to conclude that high amounts of Zn2+ cations and an elevate calcination temperature provides a powerful photo-oxidative catalyst.
Fig. 9. Kinetic curves of the methyl-orange photo-oxidation in the presence of the ZnAl-r samples.
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Fig. 11. The N2 adsorption/desorption isotherms of the ZnAl-4 sample and the calcined form.
Fig. 12. Kinetic curves of the methyl-orange photo-oxidation in the presence of the CZnAl-r–500 °C samples.
Table 4 The catalytic performances of the calcined Zn/Al-samples Sample
a
b
4. Conclusions c
Adsorbed dye (%)
Degraded dye (%)
Removed dye (%)
300 °C CZnAl-1–300 °C CZnAl-2–300 °C CZnAl-3–300 °C CZnAl-4–300 °C
23 51 55 48
27 41 52 63
44 71 78 81
500 °C CZnAl-1–500 °C CZnAl-2–500 °C CZnAl-3–500 °C CZnAl-4–500 °C
19 23 30 55
39 52 56 85
51 63 69 93
a b c
Determined with UV–vis after 30 min of stirring in dark. Determined with UV–vis after 60 min of irradiation with UV-light. The total amount of dye removed after 90 min of reaction.
Zn/Al-LDHs with different cationic ratios were prepared by the conventional co-precipitation method at constant pH. Phase transformations were investigated by thermoanalytical measurements. The ZnAl-4 dried at 80 °C already provided a photo-oxidative effect. The larger Zn2+ cations generate such a lattice strain that part of it forms ZnO detected by X-ray diffraction and UV–vis-DR spectroscopy. Further heat treatment up to 500 °C showed a significant improvement in the photocatalytic activity due to the formation of large amounts of ZnO phase. Infrared spectroscopy and local elemental analysis (EDX) reveal that the hydrotalcite-type structure is never fully destroyed. In a methyl-orange photo-oxidation experiment 93% of the
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dye could be removed by a Zn/Al-LDH with the cationic ratio of 4 and calcined at 500 °C. Acknowledgments This study was supported by the NoE project ‘‘Inside Pores” and the GOA project funded by the Special Fund for Research of the University of Antwerp. This work is also part of the CEEX No. 1/S1/2005 project. References [1] R. Comparelli, E. Fanizza, M.L. Curri, P.D. Cozzoli, G. Mascolo, A. Agostiano, Appl. Cat. B: Environ. 60 (2005) 1. [2] G. Colon, J.M. Sa´nchez-Espan˜a, M.C. Hidalgo, J.A. Navı´o, J. Photochem. Photobiol. A: Chem. 179 (2006) 20. [3] S. Liao, H. Donggen, D. Yu, Y. Su, G. Yuan, J. Photochem. Photobiol. A: Chem. 168 (2004) 7. [4] S.K. Kansal, M. Singh, D. Sud, J. Hazard. Mater. 141 (2007) 581. [5] L. Wu, J.C. Yu, X. Fu, J. Mol. Catal. A: Chem. 244 (2006) 25. [6] P.S. Braterman, Z.P. Xu, F. Yarberry, in: S.M. Auerbache, K.A. Carrado, P.K. Dutta (Eds.), Handbook of Layered Materials, Marcel Dekker Inc., New York, 2004, p. 373. [7] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 201. [8] Z. Chang, D.G. Evans, X. Duan, C. Vial, J. Ghabaja, V. Prevot, M. De Roy, C. Forano, J. Solid State Chem. 178 (2005) 2766. [9] Y. Feng, D. Li, Y. Wang, D.G. Evans, X. Duan, Polym. Degrad. Stabil. 91 (2006) 789. [10] R.E. Marotti, D.N. Guerra, C. Bello, G. Machado, E.A. Dalchiele, Sol. Energy Mater. Sol. Cells 82 (2004) 85.
[11] J.P. Enrı´quez, X. Mathew, Sol. Energy Mater. Sol. Cells 76 (2003) 313. [12] R.L. Frost, W. Martens, Z. Ding, J.T. Kloprogge, J. Therm. Anal. Calorim. 71 (2003) 429. [13] M. Tu, J. Shen, Y. Chen, J. Therm. Anal. Calorim. 58 (1999) 441. [14] R.L. Frost, W.N. Martens, K.L. Erickson, J. Therm. Anal. Calorim. 82 (2005) 603. [15] R.E. Marotti, P. Giorgi, G. Machado, E.A. Dalchiele, Sol. Energy Mater. Sol. Cells 90 (2006) 2356. [16] H. Zhang, Y. Cui, Y. Men, X. Liu, J. Lumin. 121 (2006) 601. [17] S.E. Dapurkar, S.K. Badamali, P. Selvam, Catal. Today 68 (2001) 63. [18] S. Wang, G. Xia, J. Shao, Z. Fan, J. Alloys Compd. 424 (2006) 304. [19] J.T. Kloprogge, R.L. Frost, J. Solid State Chem. 146 (1999) 506. [20] J.T. Kloprogge, L. Hickey, R.L. Frost, J. Raman Spectrosc. 35 (2004) 967. [21] J.T. Kloprogge, D. Wharton, L. Hickey, R.L. Frost, Am. Mineral. 87 (2002) 623. [22] J.A. Rivera, G. Fetter, Y. Jime´nez, M.M. Xochipa, P. Bosch, Appl. Catal. A: Gen. 316 (2007) 207. [23] A.J. Marchi, C.R. Apesteguı´a, Appl. Clay Sci. 13 (1998) 35. [24] J.A. Rivera, G. Fetter, P. Bosch, Micropor. Mesopor. Mater. 89 (2006) 306. ´ . Patzko´, R. Kun, V. Hornok, I. De´ka´ny, T. Engelhardt, N. Schall, [25] A Colloids Surf. A: Physicochem. Eng. Asp. 265 (2005) 64. [26] P. Benito, F.M. Labajos, V. Rives, Micropor. Mesopor. Mater. 94 (2006) 148. [27] F. Rouquerol, J. Rouquerol, K.S.W. Sing, Adsorption by Powders and Porous Solids, Principles, Methodology and Applications, Academic Press, London, 1991. [28] R. Bechara, A. D’Huysser, M. Fournier, L. Forni, G. Fornasari, F. Trifiro`, A. Vaccari, Catal. Lett. 82 (2002) 59.