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Surface effect of natural zeolite (clinoptilolite) on the photocatalytic activity of TiO2
Applied Surface Science 252 (2005) 1410–1416 www.elsevier.com/locate/apsusc
Surface effect of natural zeolite (clinoptilolite) on the photocatalytic activity of TiO2 Fangfei Li, Yinshan Jiang *, Lixin Yu, Zhengwen Yang, Tianyi Hou, Shenmei Sun Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, and Department of Materials Science and Engineering, Jilin University, Changchun 130026, China Received 18 January 2005; received in revised form 18 February 2005; accepted 19 February 2005 Available online 26 April 2005
Abstract The surface interaction between TiO2 and natural zeolite, clinoptilolite, has been investigated by means of transmission electron microscope (TEM), atom force microscope (AFM), X-ray diffractometer (XRD), diffuse reflectance infrared Fourier transform (DRIFT) and far Fourier transform infrared ray (FTIR) spectroscopy. And the photocatalytic degradation (PCD) rate of methyl orange (MO), a model of recalcitrant azo dye, in aqueous system has been measured to compare the photocatalytic activities of different photocatalysts. A model has been carried out to explain the incorporation between TiO2 particles and natural zeolite. The results show that the TiO2 particles loaded on zeolite are 50 nm or so, smaller than the pure one, and combine with zeolite via chemical force. Since the reserved adsorption ability and the existence of electron trapper, the TiO2–zeolite performed more efficient at low initial concentration and in the later period of PCD process, as compared with pure TiO2 nanopowders. # 2005 Elsevier B.V. All rights reserved. Keywords: TiO2; Natural zeolite; Clinoptilolite; TiO2–zeolite; PCD
1. Introduction The photocatalytic degradation (PCD) of organic pollutants on TiO2 photocatalysts, nanoscaled or modified, has attracted considerable attentions in waste treatment due to the high PCD rate for various * Corresponding author. Present address: College of Material Science and Engineering, Jilin University, 6, West-Minzhu Avenue, Changchun 130026, China. E-mail address: [email protected] (Y. Jiang).
organs [1–6]. But the difficulties in recycling and preconcentration restricted the utilization of finer TiO2 particles. Thus, the TiO2 nanoparticles have been supposed to be fixed on the inert supports for practical applications [7–11]. Among various supports, zeolites have attracted more interests due to their unique uniform pores and straight channels [11–13]. Previous works have mainly emphasized on the synthetic zeolites, for example, the Y zeolite, as TiO2 support [11–13]. However, the high cost of the support limited the extensive application of such hosts of
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.02.111
F. Li et al. / Applied Surface Science 252 (2005) 1410–1416
photocatalysts in usual industry. Comparing with the synthetic zeolites, the natural zeolites are much cheaper, more abundant and more easily available. But reports on the loading effect, even the PCD activity, of TiO2 onto such natural zeolite are very rare. In this article, natural clinoptilolite was used as the support for TiO2 loading. Considering the relatively small pore size of clinoptilolite (0.4–0.7 nm), most of the PCD reaction must take place at the surface of the compound photocatalyst. Thus, it is worthwhile to investigate the surface or interface effect between the natural zeolite and TiO2. To reveal the above interaction, the morphology changes and the bond vibrations have been investigated systematically before and after TiO2 loading. In addition, the kinetic processes of PCD reaction of methyl orange (MO) have been measured in order to examine the photoactivity and utility of different photocatalysts.
2. Experimental Clinoptilolite (Jiutai, Jilin) was employed as the support of TiO2. Some of its properties are listed in Table 1 [14]. For the sake of recovery, the purified raw zeolite was screened to obtain homogeneous granules. It should be noted that there still existed some photocatalytic inert impurities in the purified raw zeolite to some extent, for instance, the feldspar and quartz. The transparent TiO2 solution was prepared by slowly instilling the Ti(OC4H9)4 and ethanol mixture into diluted HNO3 aqueous solution. In typical synthesis of TiO2–zeolite, 1 g of natural zeolite was Table 1 Properties of clinoptilolite [14] Molecular formula Crystal form ˚) Lattice constant (A Dimension Direction ˚) Channel size (A Pore volume (%) Density of frame (g/cm3)
Na8[Al8Si40O96]32H2O Oblique a = 7.410, b = 17.89, c = 15.85, b = 918290 II //a //c 4.0 5.5 4.4 7.2, 4.1 4.7 34 1.71
The ‘‘direction’’ and ‘‘channel size’’ of C, which were not shown in reference directly, were extracted from the data of heulandite which has been considered to have similar structure to C.
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added to 3 mL of the resultant TiO2 solution. Thus, the content of TiO2 in zeolite was estimated to be 5 wt%. After drying, the loaded catalyst was calcined at 200 8C. The detailed preparation process of the TiO2– zeolite material has been described previously [15,16]. As comparison, pure TiO2 nanopowders were also prepared using the same procedure except for the addition of natural zeolite. During the PCD experiments of MO, 10 mL of MO aqueous solution with known initial concentration (C0 = 4, 8, 12, 16, 30, 40 mg/L, respectively) was mixed with 0.04 g of TiO2–zeolite (or 0.002 g pure TiO2 nanopowders) into a cell of 25 mL. Then the obtained suspension was illuminated under a 250 W high-Hg lamp with a 254–365 nm wavelength at room temperature. No proceeding molecular oxygen was added into the system in order to check the real photoactivity of catalysts in the usual industrial conditions. The light intensity was conserved to be same for all experiments. And the kinetic process of PCD of MO (color removal) was investigated using a UV-754 spectrophotometer at 463 nm light [17]. The structures of the raw zeolite and TiO2–zeolite were examined with an X-ray diffractometer (XRD; Shimadzu, D/max-rA with Cu Ka radiation). The morphology of the resultant pure and loaded TiO2 was obtained with a transmission electron microscope (TEM; JEM-2000FX, Japan) and atom force microscope (AFM; Dimension 3100, USA). And the bond vibration was analyzed by IR spectrometer (Nexus 6700, USA). The diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was carried out by KBr powder and collected 32 scans at the range of 4000–400 cm1. And the far Fourier transform infrared ray (FTIR) spectroscopy was carried out by polyethylene film and collected 70 scans at the range of 600–50 cm1.
3. Results and discussion 3.1. Loading effect of TiO2 on the morphology of natural zeolite clinoptilolite Fig. 1 shows the AFM images of the clinoptilolite before and after TiO2 loading. It is obvious that the surface morphology has changed greatly after the raw zeolite loaded with TiO2. The surface of raw zeolite is
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Fig. 1. The stereoscopic phase images of AFM on the surfaces changes between raw zeolite (a) and TiO2–zeolite (b). The scan size is 600.0 nm.
relatively smooth (Fig. 1a). And after loading, the roughness of the surface is remarkably increased and some protrusions can be observed (Fig. 1b). The average size of these protrusions is about 50 nm. As known, the pore size of clinoptilolite is 0.4–0.7 nm. Thus, TiO2 particles are not able to enter into the pores. Therefore, we suggest that the protrusions are TiO2 and nearly all of the TiO2 particles have been loaded on the surface of zeolite, instead of into the pores and cavities. It means that the PCD of MO mostly takes place at the zeolite surface, but not in the pores. In addition, as shown in Fig. 1b, the small TiO2 particles does not distribute compactly but reserves lacunes on the zeolite surface, which enable the support to show its adsorption ability. Fig. 2 shows the TEM images of pure TiO2 nanoparticles. As seen, the particle size of pure TiO2 is 80 nm or so, which is much larger than that of the loaded ones. It indicates that the existence of microporous zeolite dose benefit for restraining the particle growth of TiO2 during the synthesis. As compared with the pure TiO2 system, it is much easier for the TiO2–zeolite compound system to obtain smaller sized TiO2 particles. To reveal the interaction between the TiO2 and the zeolite, the crystal structures of the raw zeolite and the TiO2–zeolite calcined at different temperature were measured, as shown in Fig. 3. It is clear that the XRD patterns of TiO2–zeolite consist with the raw zeolite
very well as calcined below 500 8C, and no diffraction peaks corresponded to typical TiO2, including anatase and rutile, can be observed. And similar results have also been reported by other researchers [13,18]. It implies that the frame structure of zeolite after TiO2 loading has not been destructed and less amount of TiO2 has loaded on zeolites. When the annealing temperature goes further higher, the crystalline of TiO2–zeolite starts to destruct: one of the main diffusion peaks (2u = 8.928) becomes very weaker. While, in the case of raw zeolite, the diffusion peak at 8.928 heated at high temperature (600 and 700 8C) shows rare difference from the curves of XRD heated at low temperature (300, 400 and 500 8C), indicating
Fig. 2. TEM image of resultant pure TiO2 nanopowders heated at 200 8C.
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Scheme 1. Proposed incorporation of TiO2 on natural zeolite. –R represents the –OH or the –OC4H9.
Fig. 3. XRD comparison between raw zeolite (a) and TiO2–zeolite (b) during heating.
that the TiO2–zeolite shows higher destruction degree than the raw zeolite at relatively high temperature. It means that the interaction between TiO2 and zeolite decreased the structural stability of microporous frame of zeolite during the heat treatment. 3.2. Proposed incorporation of TiO2 on natural zeolite On the basis of the above analysis, a rough model for TiO2 loading on zeolite is carried out for further study (shown in Scheme 1). Before loading (Scheme 1a), the purified raw zeolite surface is covered with oxide thin layer [19]. And small cations, such as H+, Na+ and K+, are attracted close to aluminum by the static force due to the replacement of Si with Al. In this
work, the cations on the natural zeolite are all H+ because the TiO2 solution is acidic. It is notable that the O at the surface, which is combined with Al, is much more active than that combined with Si, due to the different electronic capability of Al and Si. Since the TiO2 solution is the hydrolysate of Ti(OC4H9)4 which has gone through a contracting process, the structure of TiO2 in the solution is more like a random thread with –O–Ti– as the backbone and –OH or –O–C4H9 covering the surface, as described in Scheme 1b. When the two materials are mixed together, there are three probable methods, we think, for TiO2 colloid particles to combine with the zeolite. The detailed processes are described as follows (Scheme 1c): 1. –H from the TiO2 colloid particle (Scheme 1b) reacts with the surface –OH, which has been combined with Al from zeolite, to create H2O. And the left BBTi–O– combines with –AlBB to create the new Ti–O–Al bond. 2. The TiO2 colloid provides –OH to react with the surface H– from zeolite to produce H2O. Thus, the left BBTi– is near to the O–AlBB. Then because of the static stress, the new bond Ti–O–Al comes into being.
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3. When the surface Si is deposed at the Y crossing, the special Si atom is combined with the other three Si–O tetrahydron, which makes the bond between the special Si and the surface O much weaker. Since the static capability of Si is lower than Al, it could not act as Al in the second probable method. Thus, the reaction process may follow like this: the acid TiO2 solution provides H+ and –H to react with the surface O of special Si to create H2O. And then the left BBTi–O– combines with special –SiBB to create Ti–O–Si new bond. To prove and further realize the above model, we studied ion-exchange capacity of zeolite and the IR spectra. On one hand, before synthesis the adsorption capacity for NH4+ of raw zeolite by ion-exchange is 157.8 mmol/100 g. While after the TiO2 loading the value drops sharply to 51.9 mmol/100 g. The result indicates that the defects, which are caused by the replacement of Si with Al, are mostly eliminated by the combination of Ti and Al. On the other hand, the DRIFT spectra were measured, shown in Fig. 4. The stretching vibration of O–H (3610 cm1), mostly corresponding to the surface –OH, shows a little weaker after TiO2 loading, indicating that a certain amount of –OH has been demolished by the load of TiO2. In addition, a new absorption band is found in the DRIFT spectra of TiO2–zeolite comparing with the raw one. The band covers a range from 945 to 905 cm1 corresponding to
Fig. 5. Far FTIR spectra of pure TiO2, raw zeolite and TiO2–zeolite.
the stretching vibration of Ti–O–Si and Ti–O–Al, which is similar to the results from Liu et al. [20]. The far FTIR spectra are shown in Fig. 5. As compared with raw zeolite, the TiO2–zeolite shows a new broad absorption band at ca. 219 cm1 with the shape similar to pure TiO2 (238 cm1). The red shift of ca. 20 cm1, we think, is mainly caused by the surface loading effect. As known, the infrared ray ranging from 400 to 200 cm1 is mostly reflecting the bond torsions. When the TiO2 are combined with zeolite via Ti–O–Si or Ti–O–Al, the torsions of Ti–O– are restricted by the solid surface bonding. Thus, the energy of the torsion vibration of such bonds becomes weaker, leading to the red shift of the loaded TiO2 in far FTIR spectra. Based on the results of ion-exchange capacity test and infrared spectra analysis, we conclude that during the synthesis process a certain amount of TiO2 has reacted with the surface –OH of raw zeolite, and created the new bonds like Ti–O–Si and Ti–O–Al. The bond stress is strong enough to keep the TiO2 caught on the zeolite during the PCD process and even the recycling process. 3.3. The photocatlytic activity during the PCD process of MO
Fig. 4. DRIFT spectra of raw zeolite and TiO2–zeolite.
In order to compare the photoactivity of different catalysts, the kinetic processes of PCD of MO, one typical azo dye, in aqueous system have been studied. The curve of C versus t of the raw zeolite is similar to
F. Li et al. / Applied Surface Science 252 (2005) 1410–1416
the curve of simple MO aqueous solution at the same C0 (not shown in this article), indicating that the zeolite support is photo-inert in the PCD process of MO. Therefore, the degradation curve of raw zeolite was subtracted from the apparent degradation curves of TiO2–zeolite, in order to achieve the adjusted reaction rate. That means the degradation of MO on raw zeolites was treated as the background of the PCD process on TiO2–zeolite. As the effect of pollutant concentration is of importance in any process of water treatment, it is necessary to investigate its dependence. The effects of C0 on the evolution of the PCD of MO by different photocatalysts are shown in Fig. 6. Generally, the PCD of the organs always tends to follow pseudo-first-order kinetics, which has been widely used in both aqueous system and gas system. Therefore, the rate of pseudo-first-order (k1) reaction was calculated by a linear plot of ln (C0/C) versus t. And the values of k1 at various degradation degree of MO (85, 90, 95%) are summarized in Table 2 as a function of initial concentration C0. Seen from Table 2, the TiO2–zeolite displays higher photocatalytic activity than pure TiO2 nanopowders especially at low concentration of MO. That may be caused by the super adsorption capability of the zeolite support. In the case of low initial concentration, the bottleneck of the whole PCD process is the supply of organic substance onto the photoactive TiO2 particles. From the AFM analysis we know that some of the zeolite pores and cavities are still open to the surroundings even after the load of TiO2, which enable the TiO2–zeolite keeps the adsorption ability to some extent. Thus, the TiO2– zeolite shows more efficient than pure TiO2 nanopowders due to its remarkable ability in gathering the organic substance near to the TiO2 particles.
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Fig. 6. Evolution of the MO concentration vs. t for pure TiO2 and TiO2–zeolite at different C0.
Comparing the value of k1 at different degradation degree of the same initial concentration, we find that the TiO2–zeolite shows higher efficiency in the later period of PCD process. For example, when C0 = 16 mg/L, the TiO2–zeolite needs 46 min to achieve 85% degradation of MO which is 6 min later than the pure TiO2 nanopowders. When it comes to the
Table 2 The fitted pseudo-first-order rate constant (k1) of different catalysts at various initial concentration of MO (C0) C0 (mg/L)
TiO2–zeolite
Pure TiO2 nanopowders
85%
90%
95%
85%
90%
95%
4 8 12 16 30 40
0.16441 0.10207 0.06491 0.04005 0.02772 0.01826
0.12781 0.09939 0.06312 0.04113 0.02797 0.01892
0.10906 0.08529 0.06190 0.04309 0.02974 0.01668
0.04551 0.04177 0.05828 0.04953 0.03925 0.03427
0.03840 0.03906 0.05624 0.04084 0.03296 0.02714
0.03620 0.03849 0.04120 0.03144 0.02670 0.02147
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90% degradation, the TiO2–zeolite only needs 54 min, which is 6 min ahead of the pure TiO2. And when in view of 95% degradation, the TiO2–zeolite shows 24 min quicker than the pure TiO2 nanopowders (94 min). The possible origins of the above interesting phenomena are mainly in two ways. On one hand, since the adsorption ability of zeolite is much high than that of pure TiO2 nanopowders, surrounding substances, for example, the OH (hydroxyl radical), on the surface of TiO2 are easily transferred onto the surface of zeolite. That means the organic pollutants, which have already been adsorbed on the non-photoactive zeolite, have chances to be degraded due to the appearance of OH, resulting in the enhancement of photo-degradation performance of TiO2–zeolite. Moreover, the adsorption ability for organs of TiO2–zeolite enhances the supply and delivery of the PCD process of MO. On the other hand, the existence of Ti–O–Al and Ti–O–Si, observed from the DRIFT and far FTIR spectra, may generate positive ion defects, considered to be the electron trapper at the interface the zeolite and TiO2, due to the combination of high positive ion, TiIV, with the surface O of zeolite. Thus, the recombination of the photogenerated electron–hole pair can be avoided to some extant, resulting in the increase of the photoactivity of TiO2–zeolite in the anti-passivation.
4. Conclusions The present study has focused on the interface effect between the TiO2 and natural zeolite on the photocatalytic activity of TiO2–zeolite. As a result, the growth of TiO2 particle is restrained by the zeolite surface. And the infrared analysis confirms that the TiO2 colloid particles combine with the active sites of natural zeolite, clinoptitolite, by Ti–O–Al and Ti–O– Si which make the load of TiO2 more durable than simplice physical combination. The PCD results show that, comparing with pure TiO2, the TiO2–zeolite displays higher photocatalytic efficiency at low initial
concentration and tends to benefit for avoiding the passivation of TiO2 in the later period of PCD process.
Acknowledgement The author gratefully thanks the financially supports of National Natural Science Foundation of China (Grant No. 40172020).
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Surface effect of natural zeolite (clinoptilolite) on the photocatalytic activity of TiO2 Fangfei Li, Yinshan Jiang *, Lixin Yu, Zhengwen Yang, Tianyi Hou, Shenmei Sun Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, and Department of Materials Science and Engineering, Jilin University, Changchun 130026, China Received 18 January 2005; received in revised form 18 February 2005; accepted 19 February 2005 Available online 26 April 2005
Abstract The surface interaction between TiO2 and natural zeolite, clinoptilolite, has been investigated by means of transmission electron microscope (TEM), atom force microscope (AFM), X-ray diffractometer (XRD), diffuse reflectance infrared Fourier transform (DRIFT) and far Fourier transform infrared ray (FTIR) spectroscopy. And the photocatalytic degradation (PCD) rate of methyl orange (MO), a model of recalcitrant azo dye, in aqueous system has been measured to compare the photocatalytic activities of different photocatalysts. A model has been carried out to explain the incorporation between TiO2 particles and natural zeolite. The results show that the TiO2 particles loaded on zeolite are 50 nm or so, smaller than the pure one, and combine with zeolite via chemical force. Since the reserved adsorption ability and the existence of electron trapper, the TiO2–zeolite performed more efficient at low initial concentration and in the later period of PCD process, as compared with pure TiO2 nanopowders. # 2005 Elsevier B.V. All rights reserved. Keywords: TiO2; Natural zeolite; Clinoptilolite; TiO2–zeolite; PCD
1. Introduction The photocatalytic degradation (PCD) of organic pollutants on TiO2 photocatalysts, nanoscaled or modified, has attracted considerable attentions in waste treatment due to the high PCD rate for various * Corresponding author. Present address: College of Material Science and Engineering, Jilin University, 6, West-Minzhu Avenue, Changchun 130026, China. E-mail address: [email protected] (Y. Jiang).
organs [1–6]. But the difficulties in recycling and preconcentration restricted the utilization of finer TiO2 particles. Thus, the TiO2 nanoparticles have been supposed to be fixed on the inert supports for practical applications [7–11]. Among various supports, zeolites have attracted more interests due to their unique uniform pores and straight channels [11–13]. Previous works have mainly emphasized on the synthetic zeolites, for example, the Y zeolite, as TiO2 support [11–13]. However, the high cost of the support limited the extensive application of such hosts of
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.02.111
F. Li et al. / Applied Surface Science 252 (2005) 1410–1416
photocatalysts in usual industry. Comparing with the synthetic zeolites, the natural zeolites are much cheaper, more abundant and more easily available. But reports on the loading effect, even the PCD activity, of TiO2 onto such natural zeolite are very rare. In this article, natural clinoptilolite was used as the support for TiO2 loading. Considering the relatively small pore size of clinoptilolite (0.4–0.7 nm), most of the PCD reaction must take place at the surface of the compound photocatalyst. Thus, it is worthwhile to investigate the surface or interface effect between the natural zeolite and TiO2. To reveal the above interaction, the morphology changes and the bond vibrations have been investigated systematically before and after TiO2 loading. In addition, the kinetic processes of PCD reaction of methyl orange (MO) have been measured in order to examine the photoactivity and utility of different photocatalysts.
2. Experimental Clinoptilolite (Jiutai, Jilin) was employed as the support of TiO2. Some of its properties are listed in Table 1 [14]. For the sake of recovery, the purified raw zeolite was screened to obtain homogeneous granules. It should be noted that there still existed some photocatalytic inert impurities in the purified raw zeolite to some extent, for instance, the feldspar and quartz. The transparent TiO2 solution was prepared by slowly instilling the Ti(OC4H9)4 and ethanol mixture into diluted HNO3 aqueous solution. In typical synthesis of TiO2–zeolite, 1 g of natural zeolite was Table 1 Properties of clinoptilolite [14] Molecular formula Crystal form ˚) Lattice constant (A Dimension Direction ˚) Channel size (A Pore volume (%) Density of frame (g/cm3)
Na8[Al8Si40O96]32H2O Oblique a = 7.410, b = 17.89, c = 15.85, b = 918290 II //a //c 4.0 5.5 4.4 7.2, 4.1 4.7 34 1.71
The ‘‘direction’’ and ‘‘channel size’’ of C, which were not shown in reference directly, were extracted from the data of heulandite which has been considered to have similar structure to C.
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added to 3 mL of the resultant TiO2 solution. Thus, the content of TiO2 in zeolite was estimated to be 5 wt%. After drying, the loaded catalyst was calcined at 200 8C. The detailed preparation process of the TiO2– zeolite material has been described previously [15,16]. As comparison, pure TiO2 nanopowders were also prepared using the same procedure except for the addition of natural zeolite. During the PCD experiments of MO, 10 mL of MO aqueous solution with known initial concentration (C0 = 4, 8, 12, 16, 30, 40 mg/L, respectively) was mixed with 0.04 g of TiO2–zeolite (or 0.002 g pure TiO2 nanopowders) into a cell of 25 mL. Then the obtained suspension was illuminated under a 250 W high-Hg lamp with a 254–365 nm wavelength at room temperature. No proceeding molecular oxygen was added into the system in order to check the real photoactivity of catalysts in the usual industrial conditions. The light intensity was conserved to be same for all experiments. And the kinetic process of PCD of MO (color removal) was investigated using a UV-754 spectrophotometer at 463 nm light [17]. The structures of the raw zeolite and TiO2–zeolite were examined with an X-ray diffractometer (XRD; Shimadzu, D/max-rA with Cu Ka radiation). The morphology of the resultant pure and loaded TiO2 was obtained with a transmission electron microscope (TEM; JEM-2000FX, Japan) and atom force microscope (AFM; Dimension 3100, USA). And the bond vibration was analyzed by IR spectrometer (Nexus 6700, USA). The diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was carried out by KBr powder and collected 32 scans at the range of 4000–400 cm1. And the far Fourier transform infrared ray (FTIR) spectroscopy was carried out by polyethylene film and collected 70 scans at the range of 600–50 cm1.
3. Results and discussion 3.1. Loading effect of TiO2 on the morphology of natural zeolite clinoptilolite Fig. 1 shows the AFM images of the clinoptilolite before and after TiO2 loading. It is obvious that the surface morphology has changed greatly after the raw zeolite loaded with TiO2. The surface of raw zeolite is
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Fig. 1. The stereoscopic phase images of AFM on the surfaces changes between raw zeolite (a) and TiO2–zeolite (b). The scan size is 600.0 nm.
relatively smooth (Fig. 1a). And after loading, the roughness of the surface is remarkably increased and some protrusions can be observed (Fig. 1b). The average size of these protrusions is about 50 nm. As known, the pore size of clinoptilolite is 0.4–0.7 nm. Thus, TiO2 particles are not able to enter into the pores. Therefore, we suggest that the protrusions are TiO2 and nearly all of the TiO2 particles have been loaded on the surface of zeolite, instead of into the pores and cavities. It means that the PCD of MO mostly takes place at the zeolite surface, but not in the pores. In addition, as shown in Fig. 1b, the small TiO2 particles does not distribute compactly but reserves lacunes on the zeolite surface, which enable the support to show its adsorption ability. Fig. 2 shows the TEM images of pure TiO2 nanoparticles. As seen, the particle size of pure TiO2 is 80 nm or so, which is much larger than that of the loaded ones. It indicates that the existence of microporous zeolite dose benefit for restraining the particle growth of TiO2 during the synthesis. As compared with the pure TiO2 system, it is much easier for the TiO2–zeolite compound system to obtain smaller sized TiO2 particles. To reveal the interaction between the TiO2 and the zeolite, the crystal structures of the raw zeolite and the TiO2–zeolite calcined at different temperature were measured, as shown in Fig. 3. It is clear that the XRD patterns of TiO2–zeolite consist with the raw zeolite
very well as calcined below 500 8C, and no diffraction peaks corresponded to typical TiO2, including anatase and rutile, can be observed. And similar results have also been reported by other researchers [13,18]. It implies that the frame structure of zeolite after TiO2 loading has not been destructed and less amount of TiO2 has loaded on zeolites. When the annealing temperature goes further higher, the crystalline of TiO2–zeolite starts to destruct: one of the main diffusion peaks (2u = 8.928) becomes very weaker. While, in the case of raw zeolite, the diffusion peak at 8.928 heated at high temperature (600 and 700 8C) shows rare difference from the curves of XRD heated at low temperature (300, 400 and 500 8C), indicating
Fig. 2. TEM image of resultant pure TiO2 nanopowders heated at 200 8C.
F. Li et al. / Applied Surface Science 252 (2005) 1410–1416
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Scheme 1. Proposed incorporation of TiO2 on natural zeolite. –R represents the –OH or the –OC4H9.
Fig. 3. XRD comparison between raw zeolite (a) and TiO2–zeolite (b) during heating.
that the TiO2–zeolite shows higher destruction degree than the raw zeolite at relatively high temperature. It means that the interaction between TiO2 and zeolite decreased the structural stability of microporous frame of zeolite during the heat treatment. 3.2. Proposed incorporation of TiO2 on natural zeolite On the basis of the above analysis, a rough model for TiO2 loading on zeolite is carried out for further study (shown in Scheme 1). Before loading (Scheme 1a), the purified raw zeolite surface is covered with oxide thin layer [19]. And small cations, such as H+, Na+ and K+, are attracted close to aluminum by the static force due to the replacement of Si with Al. In this
work, the cations on the natural zeolite are all H+ because the TiO2 solution is acidic. It is notable that the O at the surface, which is combined with Al, is much more active than that combined with Si, due to the different electronic capability of Al and Si. Since the TiO2 solution is the hydrolysate of Ti(OC4H9)4 which has gone through a contracting process, the structure of TiO2 in the solution is more like a random thread with –O–Ti– as the backbone and –OH or –O–C4H9 covering the surface, as described in Scheme 1b. When the two materials are mixed together, there are three probable methods, we think, for TiO2 colloid particles to combine with the zeolite. The detailed processes are described as follows (Scheme 1c): 1. –H from the TiO2 colloid particle (Scheme 1b) reacts with the surface –OH, which has been combined with Al from zeolite, to create H2O. And the left BBTi–O– combines with –AlBB to create the new Ti–O–Al bond. 2. The TiO2 colloid provides –OH to react with the surface H– from zeolite to produce H2O. Thus, the left BBTi– is near to the O–AlBB. Then because of the static stress, the new bond Ti–O–Al comes into being.
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3. When the surface Si is deposed at the Y crossing, the special Si atom is combined with the other three Si–O tetrahydron, which makes the bond between the special Si and the surface O much weaker. Since the static capability of Si is lower than Al, it could not act as Al in the second probable method. Thus, the reaction process may follow like this: the acid TiO2 solution provides H+ and –H to react with the surface O of special Si to create H2O. And then the left BBTi–O– combines with special –SiBB to create Ti–O–Si new bond. To prove and further realize the above model, we studied ion-exchange capacity of zeolite and the IR spectra. On one hand, before synthesis the adsorption capacity for NH4+ of raw zeolite by ion-exchange is 157.8 mmol/100 g. While after the TiO2 loading the value drops sharply to 51.9 mmol/100 g. The result indicates that the defects, which are caused by the replacement of Si with Al, are mostly eliminated by the combination of Ti and Al. On the other hand, the DRIFT spectra were measured, shown in Fig. 4. The stretching vibration of O–H (3610 cm1), mostly corresponding to the surface –OH, shows a little weaker after TiO2 loading, indicating that a certain amount of –OH has been demolished by the load of TiO2. In addition, a new absorption band is found in the DRIFT spectra of TiO2–zeolite comparing with the raw one. The band covers a range from 945 to 905 cm1 corresponding to
Fig. 5. Far FTIR spectra of pure TiO2, raw zeolite and TiO2–zeolite.
the stretching vibration of Ti–O–Si and Ti–O–Al, which is similar to the results from Liu et al. [20]. The far FTIR spectra are shown in Fig. 5. As compared with raw zeolite, the TiO2–zeolite shows a new broad absorption band at ca. 219 cm1 with the shape similar to pure TiO2 (238 cm1). The red shift of ca. 20 cm1, we think, is mainly caused by the surface loading effect. As known, the infrared ray ranging from 400 to 200 cm1 is mostly reflecting the bond torsions. When the TiO2 are combined with zeolite via Ti–O–Si or Ti–O–Al, the torsions of Ti–O– are restricted by the solid surface bonding. Thus, the energy of the torsion vibration of such bonds becomes weaker, leading to the red shift of the loaded TiO2 in far FTIR spectra. Based on the results of ion-exchange capacity test and infrared spectra analysis, we conclude that during the synthesis process a certain amount of TiO2 has reacted with the surface –OH of raw zeolite, and created the new bonds like Ti–O–Si and Ti–O–Al. The bond stress is strong enough to keep the TiO2 caught on the zeolite during the PCD process and even the recycling process. 3.3. The photocatlytic activity during the PCD process of MO
Fig. 4. DRIFT spectra of raw zeolite and TiO2–zeolite.
In order to compare the photoactivity of different catalysts, the kinetic processes of PCD of MO, one typical azo dye, in aqueous system have been studied. The curve of C versus t of the raw zeolite is similar to
F. Li et al. / Applied Surface Science 252 (2005) 1410–1416
the curve of simple MO aqueous solution at the same C0 (not shown in this article), indicating that the zeolite support is photo-inert in the PCD process of MO. Therefore, the degradation curve of raw zeolite was subtracted from the apparent degradation curves of TiO2–zeolite, in order to achieve the adjusted reaction rate. That means the degradation of MO on raw zeolites was treated as the background of the PCD process on TiO2–zeolite. As the effect of pollutant concentration is of importance in any process of water treatment, it is necessary to investigate its dependence. The effects of C0 on the evolution of the PCD of MO by different photocatalysts are shown in Fig. 6. Generally, the PCD of the organs always tends to follow pseudo-first-order kinetics, which has been widely used in both aqueous system and gas system. Therefore, the rate of pseudo-first-order (k1) reaction was calculated by a linear plot of ln (C0/C) versus t. And the values of k1 at various degradation degree of MO (85, 90, 95%) are summarized in Table 2 as a function of initial concentration C0. Seen from Table 2, the TiO2–zeolite displays higher photocatalytic activity than pure TiO2 nanopowders especially at low concentration of MO. That may be caused by the super adsorption capability of the zeolite support. In the case of low initial concentration, the bottleneck of the whole PCD process is the supply of organic substance onto the photoactive TiO2 particles. From the AFM analysis we know that some of the zeolite pores and cavities are still open to the surroundings even after the load of TiO2, which enable the TiO2–zeolite keeps the adsorption ability to some extent. Thus, the TiO2– zeolite shows more efficient than pure TiO2 nanopowders due to its remarkable ability in gathering the organic substance near to the TiO2 particles.
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Fig. 6. Evolution of the MO concentration vs. t for pure TiO2 and TiO2–zeolite at different C0.
Comparing the value of k1 at different degradation degree of the same initial concentration, we find that the TiO2–zeolite shows higher efficiency in the later period of PCD process. For example, when C0 = 16 mg/L, the TiO2–zeolite needs 46 min to achieve 85% degradation of MO which is 6 min later than the pure TiO2 nanopowders. When it comes to the
Table 2 The fitted pseudo-first-order rate constant (k1) of different catalysts at various initial concentration of MO (C0) C0 (mg/L)
TiO2–zeolite
Pure TiO2 nanopowders
85%
90%
95%
85%
90%
95%
4 8 12 16 30 40
0.16441 0.10207 0.06491 0.04005 0.02772 0.01826
0.12781 0.09939 0.06312 0.04113 0.02797 0.01892
0.10906 0.08529 0.06190 0.04309 0.02974 0.01668
0.04551 0.04177 0.05828 0.04953 0.03925 0.03427
0.03840 0.03906 0.05624 0.04084 0.03296 0.02714
0.03620 0.03849 0.04120 0.03144 0.02670 0.02147
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90% degradation, the TiO2–zeolite only needs 54 min, which is 6 min ahead of the pure TiO2. And when in view of 95% degradation, the TiO2–zeolite shows 24 min quicker than the pure TiO2 nanopowders (94 min). The possible origins of the above interesting phenomena are mainly in two ways. On one hand, since the adsorption ability of zeolite is much high than that of pure TiO2 nanopowders, surrounding substances, for example, the OH (hydroxyl radical), on the surface of TiO2 are easily transferred onto the surface of zeolite. That means the organic pollutants, which have already been adsorbed on the non-photoactive zeolite, have chances to be degraded due to the appearance of OH, resulting in the enhancement of photo-degradation performance of TiO2–zeolite. Moreover, the adsorption ability for organs of TiO2–zeolite enhances the supply and delivery of the PCD process of MO. On the other hand, the existence of Ti–O–Al and Ti–O–Si, observed from the DRIFT and far FTIR spectra, may generate positive ion defects, considered to be the electron trapper at the interface the zeolite and TiO2, due to the combination of high positive ion, TiIV, with the surface O of zeolite. Thus, the recombination of the photogenerated electron–hole pair can be avoided to some extant, resulting in the increase of the photoactivity of TiO2–zeolite in the anti-passivation.
4. Conclusions The present study has focused on the interface effect between the TiO2 and natural zeolite on the photocatalytic activity of TiO2–zeolite. As a result, the growth of TiO2 particle is restrained by the zeolite surface. And the infrared analysis confirms that the TiO2 colloid particles combine with the active sites of natural zeolite, clinoptitolite, by Ti–O–Al and Ti–O– Si which make the load of TiO2 more durable than simplice physical combination. The PCD results show that, comparing with pure TiO2, the TiO2–zeolite displays higher photocatalytic efficiency at low initial
concentration and tends to benefit for avoiding the passivation of TiO2 in the later period of PCD process.
Acknowledgement The author gratefully thanks the financially supports of National Natural Science Foundation of China (Grant No. 40172020).
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