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Preparation of multi-walled carbon nanotube supported TiO2 and its photocatalytic activity in the reduction of CO2 with H2O
Carbon 45 (2007) 717–721 www.elsevier.com/locate/carbon
Preparation of multi-walled carbon nanotube supported TiO2 and its photocatalytic activity in the reduction of CO2 with H2O Xiao-Hong Xia a
a,*
, Zhi-Jie Jia a, Ying Yu a, Ying Liang a, Zhuo Wang b, Li-Li Ma
a
Institute of Nano-Science and Technology, Department of Physics, Central China Normal University, Wuhan 430079, China b Department of Physics Science and Technology, Wuhan University, Wuhan 430072, China Received 7 March 2006; accepted 30 November 2006 Available online 11 January 2007
Abstract Multi-walled carbon nanotube (MWCNT) supported TiO2 composite catalysts were prepared by sol–gel and hydrothermal methods. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy and N2-adsorption analysis were carried out to characterize the composite catalysts. In using the sol–gel method, the MWCNTs were coated with anatase TiO2 nanoparticles, and by the hydrothermal method, rutile TiO2 nanorods were uniformly deposited on the MWCNTs. The photocatalytic activities of the composite catalysts were evaluated by the reduction of CO2 with H2O. The results indicate that the addition of an appropriate amount of MWCNTs as supports for TiO2 could remarkably improve the efficiency of the photocatalytic reaction. The composite catalysts prepared by the sol–gel method lead to the main formation of C2H5OH, while HCOOH is found to be the major product on the sample prepared by the hydrothermal method. Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction The reduction of carbon dioxide has recently been regarded as an important research area in chemistry and materials, not only for solving many problems resulting from environmental pollution, but also for finding ways to maintain the carbon resources which are being depleted by the burning of fossil fuels [1]. The photocatalytic reduction of CO2 with H2O by semiconductors is of vital interest especially for the utilization of solar energy [2]. Titania is one of the most widely used semiconductors for this purpose due to its strong oxidizing power, nontoxicity, and long-term photostability [3]. However, the activity of pure titania in the photocatalytic reduction of CO2 with H2O is not high enough for practical use. Therefore, many methods have been explored to improve the photocatalytic activity of TiO2. For example, Yamashita [1] and Anpo [4] have dispersed TiO2 within mesoporous zeolites and
*
Corresponding author. Tel./fax: +86 276 7861 185. E-mail address: [email protected] (X.-H. Xia).
0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.11.028
molecular sieves. However, the materials are difficult to synthesize and the photocatalytic activities of them are still very low [5]. Thus, we need to develop some other methods for modifying TiO2 in order to improve the efficiency of the photocatalytic reduction of CO2 with H2O. Multi-walled carbon nanotubes (MWCNTs) have attracted considerable attention since their discovery [6]. With the development of the synthesis technology, the price of MWCNTs has reduced significantly and it is possible for them to be used on a large scale [7]. Taking advantage of the unique electronic properties of the MWCNTs, we expect that the combination of MWCNTs with titania may induce interesting charge transfer and thus enhance the photocatalytic activity of titania. Previous studies have demonstrated that the addition of MWCNTs as supports for TiO2 could improve the efficiency of the photocatalytic degradation of organics [8]. In this work, MWCNTs were used as supports for TiO2 in the photocatalytic reduction of CO2 with H2O. To the best of our knowledge, this work may be the first report about utilizing MWCNT supported TiO2 composite catalysts in the photocatalytic reduction of CO2 with H2O.
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3. Results and discussion The XRD patterns of samples A–E are given in Fig. 1. The two peaks of sample A correspond to the (0 0 2) and (1 0 0) reflections of the MWCNTs, respectively [10]. The peaks in all the diffraction patterns of samples B, C and D correspond to the anatase TiO2 (JCPDS no. 21-1272). The (0 0 2) reflection due to the MWCNTs overlaps the anatase TiO2 (1 0 1) reflection in samples C and D. It is worth noticing that the intensity of the anatase diffraction peaks decreases from sample B to E and the width at the half height of the peaks increases. The crystallite sizes estimated from the line broadening of the anatase TiO2 (2 0 0) reflection plane (2h = 48.1°) of samples B, C and D, where there is hardly any interference from the MWCNTs, are 18.5, 8.5 and 6.3 nm, respectively. This result is consistent with the increase of the MWCNTs from 0 g to 0.01 g and 0.1 g, which favors the decrease of the crystallized TiO2 on the surfaces of the MWCNTs and thus prevents the TiO2 particles from agglomerating [11]. The peaks present
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The MWCNTs were synthesized by the chemical vapor deposition of propylene in a fixed-bed flow reactor at 740 °C using Al2O3 supported Fe catalyst [9]. For the purification, the raw MWCNTs were boiled in concentrated nitric acid (65% HNO3) for 30 min. Then the suspension was centrifuged and washed with distilled water to remove the residual nitric acid. After that, the purified MWCNTs were dried at 80 °C for 24 h and labeled as sample A. Titanium tetrachloride (98% TiCl4), hydrochloric acid (38% HCl) and distilled water with a molar ratio of TiCl4:HCl:H2O = 0.1:1:200 were homogenized under magnetic stirring for 30 min. 250 ml of the obtained transparent solution was then used as the raw material to prepare samples B–E. Sample B was prepared by adding ammonia into the raw material under magnetic stirring until the solution reached neutral. Sample C and sample D were prepared through dispersing 0.01 g and 0.1 g of the purified MWCNTs into two raw material solutions respectively via sonication, followed by adding ammonia slowly into the obtained suspensions with vigorous stirring until the pH values of the solutions reached 7. Hydrothermal treatment of another suspension prepared through the same way as that of sample C was performed in a Teflon-steel autoclave at 140 °C for 12 h to get sample E. Samples B–E were then washed with distilled water and dried at 80 °C for 24 h. The samples were characterized by transmission electron microscopy (TEM, JEM-2010), scanning electron microscopy (SEM, JEOL-6700 F), X-ray diffraction (XRD, D/max-rB, Cu Ka radiation) and Brunauer-Emmett-Teller N2-adsorption analysis (Geminni 2360 V 5.00). Commercial photocatalyst Degussa P25 (75% anatase/25% rutile with a BET surface area of 50 m2g1, Nippon Aerosil Co. Ltd.), with the same quantity as that of sample B, was used for comparison and labeled as sample F. Samples A–F were calcined at 450 °C for 2 h under the protection of argon before being used as the photocatalyst. The photocatalytic activities of samples A–F were evaluated by the reduction of CO2 with H2O. Taking sample A for example, firstly, the sample was laid over a piece of transparent glass and then put into a home-made stainless steel reactor. Secondly, the reactor was washed with argon for several times and vacuumed. Thirdly, H2O and CO2 with a mole ratio of 5:1 were introduced into the reactor, which was then kept in the dark for 2 h to establish an adsorption-desorption balance. Fourthly, a 15 W UV lamp with the wavelength of 365 nm was turned on and the photocatalytic reaction was conducted for 5 h at room temperature. Finally, the gas phase product was analyzed by gas chromatography after the photocatalytic reaction was completed. Samples B–F were then tested in the same way.
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Fig. 1. XRD patterns of sample A (a), sample B (b), sample C (c), sample D (d) and sample E (e).
in the diffraction pattern of sample E correspond to the rutile TiO2 (JCPDS no. 77-0441). The high intensity and the small width at the half height of the diffraction peaks indicate the big crystallized portion of the TiO2 particles on the surfaces of the nanotubes. Moreover, the crystalline extent of the MWCNTs is much lower than that of the TiO2 in sample E, leading to the shielding of the peaks of MWCNTs by those of TiO2 [12]. SEM images of samples A and B are shown in the supplementary data. The purified carbon nanotubes in sample A have very smooth surfaces and they are about 30 nm in diameter and several micrometers in length. The sol–gel synthesized pure TiO2 in sample B are agglomerate particles with individual diameter of about 20 nm. TEM images of the MWCNT supported TiO2 composites are presented in Fig. 2. The image of sample C prepared by the sol–gel method with 0.01 g of MWCNTs (Fig. 2(a)) shows a homogeneous sample with individual nanotubes covered with a thick layer of TiO2 particles, which is consistent with the SEM observation in the supplementary data. For sample D prepared by the sol–gel method with 0.1 g of MWCNTs (Fig. 2(b)), the nanotubes are coated with dispersed TiO2 nanoparticles and no TiO2 aggregates are observed. For sample E prepared by the hydrothermal process (Fig. 2(c)), the TiO2 covered on the surfaces of the MWCNTs are rodlike. The diameters of the rods are about 20–60 nm and the lengths of them are in the range of 50–150 nm. The coat is not so uniform in comparison with the samples obtained by the sol–gel method. The gas chromatographic graphs of the photocatalytic products on samples A–F are shown in the supplementary data. UV irradiation of the pure TiO2 and the MWCNT supported TiO2 composite catalysts in the presence of a mixture of CO2 and H2O led to the evolution of CH4, HCOOH and C2H5OH. The formation of these products was neither detected in the dark condition nor without photocatalyst. These results clearly show that the presence
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Fig. 2. TEM images of sample C (a), sample D (b) and sample E (c).
of both the photocatalyst and the UV irradiation are indispensable for the photocatalytic reduction of CO2 with H2O. The yields of the CH4, HCOOH and C2H5OH on samples A–F as well as the surface areas of samples A–F are listed in Table 1. It can be seen from the table that only trace amounts of CH4 and HCOOH were produced when sample A was used as the photocatalyst. Among the sol–gel synthesized samples, the yields of the photocatalytic products on sample D are lower than those on sample B. The photocatalytic activity of sample C is higher than that of sample B and than the literature reports [1,4]. It reveals that the addition of 0.1 g of MWCNTs in sample D as supports for TiO2 suppressed the photocatalytic reduction of CO2 with H2O while the addition of 0.01 g of MWCNTs in sample C could remarkably improve the activity of TiO2. It can also be seen from the table that sample C has larger surface area than that of sample B. This may be one reason why sample C exhibits superior photocatalytic activity to sample B. However, the photocatalytic activity of sample D is worse than that of sample B though it has larger surface area. Therefore, surface area may not be the only factor affecting the photocatalytic activity of TiO2. There are two important species involved in the photoreduction of CO2 with H2O, HÆ (hydrogen atom) and Table 1 The yields of the photocatalytic products on samples A–I and the surface areas (SBET) of samples A–I Sample
A B C D E F G H I
SBET (m2/g)
292 107 168 263 150 50 953 638 729
Photocatalytic products (lmol/g) CH4
HCOOH
C2H5OH
Total carbonaceous (lmol/g)
4.93 18.67 58.7 13.5 63.3 73.33 3.37 21.57 11.2
13.29 29.87 93.35 29.7 125.1 92.94 11.52 32.43 26.4
0 115.75 149.36 0 34.6 5.16 0 119.16 0
18.22 280.04 450.77 43.2 257.6 176.59 14.89 292.32 37.6
CO 2 (carbon dioxide anion radical) which are produced by the electron transfer from the conduction band of TiO2 as follows [5]: TiO2 þ h m ! e þ hþ ; H þ þ e ! H ;
2H2 O þ 4hþ ! O2 þ 4Hþ ; CO2 þ e ! CO 2;
þ Methane formation: CO 2 þ 8H þ h ! CH4 þ 2H2 O, Formic acid formation: CO2 þ 2H þ hþ ! HCOOH, þ Ethanol formation: 2 CO 2 þ 12H þ 2h ! C2 H5 OHþ 3H2 O.
The electron–hole pairs generated by the UV irradiation are the key factors for the photocatalytic reaction. However, they are inclined to recombine. Thus the recombination rate of the electron–hole pairs has to be reduced in order to enhance the photocatalytic activity of TiO2. The MWCNTs in samples C and D function as the dispersing agents to prevent the TiO2 particles from agglomerating, which decrease the recombination rate of the e/h+ pairs thus the photocatalytic efficiency of sample C increased. However, the excessive MWCNTs in sample D shield the TiO2 from absorbing the UV light [12]. This is the reason why sample D has lower photocatalytic activity than samples B and C though the TiO2 particles in sample D dispersed better on the surfaces of the MWCNTs. Therefore, only the addition of an appropriate amount of MWCNTs can greatly improve the photocatalytic activity of TiO2. It can be concluded from Table 1 that the selective formation of HCOOH or C2H5OH strongly depends on the type of the catalyst. Samples B and C prepared by the sol–gel method lead to the formation of a considerable amount of C2H5OH, while the formation of HCOOH is found to be the major reaction on sample E prepared by the hydrothermal method and on sample F. From the XRD diffraction patterns of the samples we could see that samples B and C are anatase TiO2 while sample E is rutile TiO2, and sample F also contains 25% of rutile TiO2. The
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Photocatalytic products(μmol/g)
difference in the crystal phase may be the reason for the selective formation of C2H5OH and HCOOH. Recently, activated carbons (AC) have been reported to enhance the activity of TiO2 in the degradation of organics [13]. In this work, AC (Shanxi Taiyuan Activated Carbon Factory, China) was used in the photocatalytic reduction of CO2 with H2O for comparison and labeled as sample G. Two AC/TiO2 composites labeled as samples H and I were prepared via replacing the MWCNTs in samples C and D by the same amount of AC, respectively. The yields of the photocatalytic products on samples G, H and I are also shown in Table 1. The yields of the products on sample H are a little higher than those on sample B but lower than those on sample C. And the photocatalytic activity of sample I is lower than that of sample D. It suggests that the addition of AC as supports for TiO2 could slightly improve the efficiency of the photocatalytic reduction of CO2 with H2O, and the AC has worse performance than the MWCNTs. Compared with activated carbons, carbon nanotubes have the unique hollow and layered structure. The electron–hole pairs could transport freely along the cylindrical nanostructure, which thus suppresses the
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recombination rate of the photo-generated e/h+ pairs [14]. This seems to be the main reason in our study for the enhancement of the photocatalytic activity of TiO2 by the addition of the MWCNTs. In order to study the feasibility to reuse the photocatalysts, samples B and C were heated at 124 °C in a vacuum oven for 12 h to degas the surfaces of the catalysts. After that, the treated catalysts were reused in the photocatalytic reduction of CO2 with H2O. The yields of the photocatalytic products on the two samples both decrease stepwise in the three reusing cycles as shown in Fig. 3. And the photocatalytic efficiency of sample B decreased faster than that of sample C. After the three reusing cycles, the yields of the products on sample B decreased to about 40% while those on sample C remained about 80% of the original test. It demonstrates that the MWCNT supported TiO2 exhibited higher photocatalytic activity than the pure TiO2 in the reusing cycles.
4. Conclusions MWCNT supported TiO2 composite catalysts synthesized by the sol–gel and hydrothermal methods were used as the photocatalysts in the reduction of CO2 with H2O. The presence of the MWCNTs in the composite catalysts can mitigate the agglomeration of the TiO2 particles and transport the electron–hole pairs generated by the UV irradiation along the tubes, so as to decrease the recombination rate of the e/h+ pairs and thus improve the photocatalytic activity of TiO2. The composite catalysts prepared by the sol–gel method lead to the main formation of C2H5OH, while HCOOH is found to be the major product on the sample prepared by the hydrothermal method. Compared with activated carbons, MWCNTs have better performance in the photocatalytic reactions as supports for TiO2. The MWCNT supported TiO2 composite catalyst prepared by the sol–gel method exhibits higher activity than the pure TiO2 in the reusing cycles. Taking into account the semiconducting properties of TiO2, these MWCNT supported TiO2 nanocomposites can be applied for sensors, high-density electronic devices, lithium ion batteries, as well as the photocatalytic decomposition of pollutants and so on. Moreover, the photocatalytic products CH4, HCOOH, C2H5OH can be reused as energy sources.
3
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20 0
CH4
HCOOH
C2 H5 OH
Fig. 3. Photocatalytic products on reused sample B and sample C. Zero denotes the original test; 1, 2 and 3 represents the first, the second and the third reusing cycle, respectively.
This work was supported by the National Natural Science Foundation of China (No. 20207002, 90510012) and the Important Item Nano-Specialized Foundation of Wuhan (No. 20041003068). Discussions with Professor Yiwen Tang, Ming Tan and Jialin Li are gratefully acknowledged.
X.-H. Xia et al. / Carbon 45 (2007) 717–721
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon. 2006.11.028. References [1] Yamashita H, Fujii Y, Ichihashi Y, Zhang SG, Ikeue K, Park DR, et al. Selective formation of CH3OH in the photocatalytic reduction of CO2 with H2O on titanium oxides highly dispersed within zeolites and mesoporous molecular sieves. Catal Today 1998;45:221–7. [2] Yamashita H, Shiga A, Kawasaki S, Ichihashi Y, Ehara S, Anpo M. Photocatalytic synthesis of CH4 and CH3OH from CO2 and H2O on highly dispersed active titanium oxide catalysts. Energy Convers Mgmt 1995;36:617–20. [3] Anpo M, Yamashita H, Ichihashi Y, Ehara S. Photocatalytic reduction of CO2 with H2O on various titanium oxide catalysts. J Electroanalyt Chem 1995;396:21–6. [4] Anpo M, Yamashita H, Ikeue K, Fuji Y, Zhang SG, Ichihashi Y, et al. Photocatalytic reduction of CO2 with H2O on Ti-MCM-41 and Ti-MCM-48 mesoporous zeolite catalysts. Catal Today 1998;44:327–32. [5] Usubharatana P, McMartin D, Veawab A, Tontiwachwuthikul P. Photocatalytic process for CO2 emission reduction from industrial flue gas streams. Ind Eng Chem Res 2006;45:2558–68.
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[6] Iijima S. Helical microtubeles of graphitic carbon. Nature 1991;354:56–8. [7] Yu Y, Yu JC, Chan CY, Che YK, Zhao JC, Ding L, et al. Enhancement of adsorption and photocatalytic activity of TiO2 by using carbon nanotubes for the treatment of azo dye. Appl Catal B: Environ 2005;61:1–11. [8] Ebbesen TW, Lezee HJ, Hiura H, Neentt JW, Ghaemi HF, Thio T. Electrical conductivity of individual carbon nanotubes. Nature 1996;382:54–6. [9] Yu Y, Ma LL, Huang WY, Du FP, Yu JC, Yu JG, et al. Sonication assisted deposition of Cu2O nanoparticles on multiwall carbon nanotubes with polyol process. Carbon 2005;43:670–3. [10] Lee S, Sigmund WM. Formation of anatase TiO2 nanoparticles on carbon nanotubes. Chem Commum 2003:780–2. [11] Wang WD, Serp P, Kalck P, Faria JL. Photocatalytic degradation of phenol on MWNT and titania composite catalysts prepared by a modified sol–gel method. Appl Catal B: Environ 2005;56:305–12. [12] Yu Y, Yu JC, Yu JG, Kwok YC, Che YK, Zhao JC, et al. Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes. Appl Catal A: General 2005;289:186–96. [13] Matos J, Laine J, Herrmann JM. Effect of the type of activated carbons on the photocatalytic degradation of aqueous organic pollutants by UV-irradiated titania. J Catalysis 2001;200:10–20. [14] Charlier JC. Defects in carbon nanotubes. Acc Chem Res 2002;35:1063–9.
Preparation of multi-walled carbon nanotube supported TiO2 and its photocatalytic activity in the reduction of CO2 with H2O Xiao-Hong Xia a
a,*
, Zhi-Jie Jia a, Ying Yu a, Ying Liang a, Zhuo Wang b, Li-Li Ma
a
Institute of Nano-Science and Technology, Department of Physics, Central China Normal University, Wuhan 430079, China b Department of Physics Science and Technology, Wuhan University, Wuhan 430072, China Received 7 March 2006; accepted 30 November 2006 Available online 11 January 2007
Abstract Multi-walled carbon nanotube (MWCNT) supported TiO2 composite catalysts were prepared by sol–gel and hydrothermal methods. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy and N2-adsorption analysis were carried out to characterize the composite catalysts. In using the sol–gel method, the MWCNTs were coated with anatase TiO2 nanoparticles, and by the hydrothermal method, rutile TiO2 nanorods were uniformly deposited on the MWCNTs. The photocatalytic activities of the composite catalysts were evaluated by the reduction of CO2 with H2O. The results indicate that the addition of an appropriate amount of MWCNTs as supports for TiO2 could remarkably improve the efficiency of the photocatalytic reaction. The composite catalysts prepared by the sol–gel method lead to the main formation of C2H5OH, while HCOOH is found to be the major product on the sample prepared by the hydrothermal method. Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction The reduction of carbon dioxide has recently been regarded as an important research area in chemistry and materials, not only for solving many problems resulting from environmental pollution, but also for finding ways to maintain the carbon resources which are being depleted by the burning of fossil fuels [1]. The photocatalytic reduction of CO2 with H2O by semiconductors is of vital interest especially for the utilization of solar energy [2]. Titania is one of the most widely used semiconductors for this purpose due to its strong oxidizing power, nontoxicity, and long-term photostability [3]. However, the activity of pure titania in the photocatalytic reduction of CO2 with H2O is not high enough for practical use. Therefore, many methods have been explored to improve the photocatalytic activity of TiO2. For example, Yamashita [1] and Anpo [4] have dispersed TiO2 within mesoporous zeolites and
*
Corresponding author. Tel./fax: +86 276 7861 185. E-mail address: [email protected] (X.-H. Xia).
0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.11.028
molecular sieves. However, the materials are difficult to synthesize and the photocatalytic activities of them are still very low [5]. Thus, we need to develop some other methods for modifying TiO2 in order to improve the efficiency of the photocatalytic reduction of CO2 with H2O. Multi-walled carbon nanotubes (MWCNTs) have attracted considerable attention since their discovery [6]. With the development of the synthesis technology, the price of MWCNTs has reduced significantly and it is possible for them to be used on a large scale [7]. Taking advantage of the unique electronic properties of the MWCNTs, we expect that the combination of MWCNTs with titania may induce interesting charge transfer and thus enhance the photocatalytic activity of titania. Previous studies have demonstrated that the addition of MWCNTs as supports for TiO2 could improve the efficiency of the photocatalytic degradation of organics [8]. In this work, MWCNTs were used as supports for TiO2 in the photocatalytic reduction of CO2 with H2O. To the best of our knowledge, this work may be the first report about utilizing MWCNT supported TiO2 composite catalysts in the photocatalytic reduction of CO2 with H2O.
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3. Results and discussion The XRD patterns of samples A–E are given in Fig. 1. The two peaks of sample A correspond to the (0 0 2) and (1 0 0) reflections of the MWCNTs, respectively [10]. The peaks in all the diffraction patterns of samples B, C and D correspond to the anatase TiO2 (JCPDS no. 21-1272). The (0 0 2) reflection due to the MWCNTs overlaps the anatase TiO2 (1 0 1) reflection in samples C and D. It is worth noticing that the intensity of the anatase diffraction peaks decreases from sample B to E and the width at the half height of the peaks increases. The crystallite sizes estimated from the line broadening of the anatase TiO2 (2 0 0) reflection plane (2h = 48.1°) of samples B, C and D, where there is hardly any interference from the MWCNTs, are 18.5, 8.5 and 6.3 nm, respectively. This result is consistent with the increase of the MWCNTs from 0 g to 0.01 g and 0.1 g, which favors the decrease of the crystallized TiO2 on the surfaces of the MWCNTs and thus prevents the TiO2 particles from agglomerating [11]. The peaks present
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The MWCNTs were synthesized by the chemical vapor deposition of propylene in a fixed-bed flow reactor at 740 °C using Al2O3 supported Fe catalyst [9]. For the purification, the raw MWCNTs were boiled in concentrated nitric acid (65% HNO3) for 30 min. Then the suspension was centrifuged and washed with distilled water to remove the residual nitric acid. After that, the purified MWCNTs were dried at 80 °C for 24 h and labeled as sample A. Titanium tetrachloride (98% TiCl4), hydrochloric acid (38% HCl) and distilled water with a molar ratio of TiCl4:HCl:H2O = 0.1:1:200 were homogenized under magnetic stirring for 30 min. 250 ml of the obtained transparent solution was then used as the raw material to prepare samples B–E. Sample B was prepared by adding ammonia into the raw material under magnetic stirring until the solution reached neutral. Sample C and sample D were prepared through dispersing 0.01 g and 0.1 g of the purified MWCNTs into two raw material solutions respectively via sonication, followed by adding ammonia slowly into the obtained suspensions with vigorous stirring until the pH values of the solutions reached 7. Hydrothermal treatment of another suspension prepared through the same way as that of sample C was performed in a Teflon-steel autoclave at 140 °C for 12 h to get sample E. Samples B–E were then washed with distilled water and dried at 80 °C for 24 h. The samples were characterized by transmission electron microscopy (TEM, JEM-2010), scanning electron microscopy (SEM, JEOL-6700 F), X-ray diffraction (XRD, D/max-rB, Cu Ka radiation) and Brunauer-Emmett-Teller N2-adsorption analysis (Geminni 2360 V 5.00). Commercial photocatalyst Degussa P25 (75% anatase/25% rutile with a BET surface area of 50 m2g1, Nippon Aerosil Co. Ltd.), with the same quantity as that of sample B, was used for comparison and labeled as sample F. Samples A–F were calcined at 450 °C for 2 h under the protection of argon before being used as the photocatalyst. The photocatalytic activities of samples A–F were evaluated by the reduction of CO2 with H2O. Taking sample A for example, firstly, the sample was laid over a piece of transparent glass and then put into a home-made stainless steel reactor. Secondly, the reactor was washed with argon for several times and vacuumed. Thirdly, H2O and CO2 with a mole ratio of 5:1 were introduced into the reactor, which was then kept in the dark for 2 h to establish an adsorption-desorption balance. Fourthly, a 15 W UV lamp with the wavelength of 365 nm was turned on and the photocatalytic reaction was conducted for 5 h at room temperature. Finally, the gas phase product was analyzed by gas chromatography after the photocatalytic reaction was completed. Samples B–F were then tested in the same way.
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(b)
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Fig. 1. XRD patterns of sample A (a), sample B (b), sample C (c), sample D (d) and sample E (e).
in the diffraction pattern of sample E correspond to the rutile TiO2 (JCPDS no. 77-0441). The high intensity and the small width at the half height of the diffraction peaks indicate the big crystallized portion of the TiO2 particles on the surfaces of the nanotubes. Moreover, the crystalline extent of the MWCNTs is much lower than that of the TiO2 in sample E, leading to the shielding of the peaks of MWCNTs by those of TiO2 [12]. SEM images of samples A and B are shown in the supplementary data. The purified carbon nanotubes in sample A have very smooth surfaces and they are about 30 nm in diameter and several micrometers in length. The sol–gel synthesized pure TiO2 in sample B are agglomerate particles with individual diameter of about 20 nm. TEM images of the MWCNT supported TiO2 composites are presented in Fig. 2. The image of sample C prepared by the sol–gel method with 0.01 g of MWCNTs (Fig. 2(a)) shows a homogeneous sample with individual nanotubes covered with a thick layer of TiO2 particles, which is consistent with the SEM observation in the supplementary data. For sample D prepared by the sol–gel method with 0.1 g of MWCNTs (Fig. 2(b)), the nanotubes are coated with dispersed TiO2 nanoparticles and no TiO2 aggregates are observed. For sample E prepared by the hydrothermal process (Fig. 2(c)), the TiO2 covered on the surfaces of the MWCNTs are rodlike. The diameters of the rods are about 20–60 nm and the lengths of them are in the range of 50–150 nm. The coat is not so uniform in comparison with the samples obtained by the sol–gel method. The gas chromatographic graphs of the photocatalytic products on samples A–F are shown in the supplementary data. UV irradiation of the pure TiO2 and the MWCNT supported TiO2 composite catalysts in the presence of a mixture of CO2 and H2O led to the evolution of CH4, HCOOH and C2H5OH. The formation of these products was neither detected in the dark condition nor without photocatalyst. These results clearly show that the presence
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Fig. 2. TEM images of sample C (a), sample D (b) and sample E (c).
of both the photocatalyst and the UV irradiation are indispensable for the photocatalytic reduction of CO2 with H2O. The yields of the CH4, HCOOH and C2H5OH on samples A–F as well as the surface areas of samples A–F are listed in Table 1. It can be seen from the table that only trace amounts of CH4 and HCOOH were produced when sample A was used as the photocatalyst. Among the sol–gel synthesized samples, the yields of the photocatalytic products on sample D are lower than those on sample B. The photocatalytic activity of sample C is higher than that of sample B and than the literature reports [1,4]. It reveals that the addition of 0.1 g of MWCNTs in sample D as supports for TiO2 suppressed the photocatalytic reduction of CO2 with H2O while the addition of 0.01 g of MWCNTs in sample C could remarkably improve the activity of TiO2. It can also be seen from the table that sample C has larger surface area than that of sample B. This may be one reason why sample C exhibits superior photocatalytic activity to sample B. However, the photocatalytic activity of sample D is worse than that of sample B though it has larger surface area. Therefore, surface area may not be the only factor affecting the photocatalytic activity of TiO2. There are two important species involved in the photoreduction of CO2 with H2O, HÆ (hydrogen atom) and Table 1 The yields of the photocatalytic products on samples A–I and the surface areas (SBET) of samples A–I Sample
A B C D E F G H I
SBET (m2/g)
292 107 168 263 150 50 953 638 729
Photocatalytic products (lmol/g) CH4
HCOOH
C2H5OH
Total carbonaceous (lmol/g)
4.93 18.67 58.7 13.5 63.3 73.33 3.37 21.57 11.2
13.29 29.87 93.35 29.7 125.1 92.94 11.52 32.43 26.4
0 115.75 149.36 0 34.6 5.16 0 119.16 0
18.22 280.04 450.77 43.2 257.6 176.59 14.89 292.32 37.6
CO 2 (carbon dioxide anion radical) which are produced by the electron transfer from the conduction band of TiO2 as follows [5]: TiO2 þ h m ! e þ hþ ; H þ þ e ! H ;
2H2 O þ 4hþ ! O2 þ 4Hþ ; CO2 þ e ! CO 2;
þ Methane formation: CO 2 þ 8H þ h ! CH4 þ 2H2 O, Formic acid formation: CO2 þ 2H þ hþ ! HCOOH, þ Ethanol formation: 2 CO 2 þ 12H þ 2h ! C2 H5 OHþ 3H2 O.
The electron–hole pairs generated by the UV irradiation are the key factors for the photocatalytic reaction. However, they are inclined to recombine. Thus the recombination rate of the electron–hole pairs has to be reduced in order to enhance the photocatalytic activity of TiO2. The MWCNTs in samples C and D function as the dispersing agents to prevent the TiO2 particles from agglomerating, which decrease the recombination rate of the e/h+ pairs thus the photocatalytic efficiency of sample C increased. However, the excessive MWCNTs in sample D shield the TiO2 from absorbing the UV light [12]. This is the reason why sample D has lower photocatalytic activity than samples B and C though the TiO2 particles in sample D dispersed better on the surfaces of the MWCNTs. Therefore, only the addition of an appropriate amount of MWCNTs can greatly improve the photocatalytic activity of TiO2. It can be concluded from Table 1 that the selective formation of HCOOH or C2H5OH strongly depends on the type of the catalyst. Samples B and C prepared by the sol–gel method lead to the formation of a considerable amount of C2H5OH, while the formation of HCOOH is found to be the major reaction on sample E prepared by the hydrothermal method and on sample F. From the XRD diffraction patterns of the samples we could see that samples B and C are anatase TiO2 while sample E is rutile TiO2, and sample F also contains 25% of rutile TiO2. The
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Photocatalytic products(μmol/g)
difference in the crystal phase may be the reason for the selective formation of C2H5OH and HCOOH. Recently, activated carbons (AC) have been reported to enhance the activity of TiO2 in the degradation of organics [13]. In this work, AC (Shanxi Taiyuan Activated Carbon Factory, China) was used in the photocatalytic reduction of CO2 with H2O for comparison and labeled as sample G. Two AC/TiO2 composites labeled as samples H and I were prepared via replacing the MWCNTs in samples C and D by the same amount of AC, respectively. The yields of the photocatalytic products on samples G, H and I are also shown in Table 1. The yields of the products on sample H are a little higher than those on sample B but lower than those on sample C. And the photocatalytic activity of sample I is lower than that of sample D. It suggests that the addition of AC as supports for TiO2 could slightly improve the efficiency of the photocatalytic reduction of CO2 with H2O, and the AC has worse performance than the MWCNTs. Compared with activated carbons, carbon nanotubes have the unique hollow and layered structure. The electron–hole pairs could transport freely along the cylindrical nanostructure, which thus suppresses the
120
0
Sample B
1
100
2 80 60
3
40 20
0 0
1
2
1 2
3
3
0
HCOOH
CH4
C2H5OH
Photocatalytic products(μmol/g)
160
0
Sample C
140
1
2
3
120
0
100
2
80 60
1
0
1
2
3
recombination rate of the photo-generated e/h+ pairs [14]. This seems to be the main reason in our study for the enhancement of the photocatalytic activity of TiO2 by the addition of the MWCNTs. In order to study the feasibility to reuse the photocatalysts, samples B and C were heated at 124 °C in a vacuum oven for 12 h to degas the surfaces of the catalysts. After that, the treated catalysts were reused in the photocatalytic reduction of CO2 with H2O. The yields of the photocatalytic products on the two samples both decrease stepwise in the three reusing cycles as shown in Fig. 3. And the photocatalytic efficiency of sample B decreased faster than that of sample C. After the three reusing cycles, the yields of the products on sample B decreased to about 40% while those on sample C remained about 80% of the original test. It demonstrates that the MWCNT supported TiO2 exhibited higher photocatalytic activity than the pure TiO2 in the reusing cycles.
4. Conclusions MWCNT supported TiO2 composite catalysts synthesized by the sol–gel and hydrothermal methods were used as the photocatalysts in the reduction of CO2 with H2O. The presence of the MWCNTs in the composite catalysts can mitigate the agglomeration of the TiO2 particles and transport the electron–hole pairs generated by the UV irradiation along the tubes, so as to decrease the recombination rate of the e/h+ pairs and thus improve the photocatalytic activity of TiO2. The composite catalysts prepared by the sol–gel method lead to the main formation of C2H5OH, while HCOOH is found to be the major product on the sample prepared by the hydrothermal method. Compared with activated carbons, MWCNTs have better performance in the photocatalytic reactions as supports for TiO2. The MWCNT supported TiO2 composite catalyst prepared by the sol–gel method exhibits higher activity than the pure TiO2 in the reusing cycles. Taking into account the semiconducting properties of TiO2, these MWCNT supported TiO2 nanocomposites can be applied for sensors, high-density electronic devices, lithium ion batteries, as well as the photocatalytic decomposition of pollutants and so on. Moreover, the photocatalytic products CH4, HCOOH, C2H5OH can be reused as energy sources.
3
40
Acknowledgements
20 0
CH4
HCOOH
C2 H5 OH
Fig. 3. Photocatalytic products on reused sample B and sample C. Zero denotes the original test; 1, 2 and 3 represents the first, the second and the third reusing cycle, respectively.
This work was supported by the National Natural Science Foundation of China (No. 20207002, 90510012) and the Important Item Nano-Specialized Foundation of Wuhan (No. 20041003068). Discussions with Professor Yiwen Tang, Ming Tan and Jialin Li are gratefully acknowledged.
X.-H. Xia et al. / Carbon 45 (2007) 717–721
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