Langmuir 2008, 24, 9907-9915
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Effect of Calcination Temperature on the Structure of a Pt/TiO2 (B) Nanofiber and Its Photocatalytic Activity in Generating H2 Chiu-Hsun Lin,*,† Jiunn-Hsing Chao,‡ Chun-Hsuan Liu,† Jui-Chun Chang,† and Feng-Chieh Wang† Department of Chemistry, National Changhua UniVersity of Education, Changhua 500, Taiwan, and Nuclear Science and Technology DeVelopment Center, National Tsing Hua UniVersity, Hsinchu 300, Taiwan ReceiVed February 22, 2008. ReVised Manuscript ReceiVed April 23, 2008 Hydrogen trititanate (H2Ti3O7) nanofibers were prepared by a hydrothermal method in 10 M NaOH at 403 K, followed by acidic rinsing and drying at 383 K. Calcining H2Ti3O7 nanofibers at 573 K led to the formation of TiO2 (B) nanofibers. Calcination at 673 K improved the crystallinity of the TiO2 (B) nanofibers and did not cause any change in the morphology and dimensions of the nanofibers. TiO2 (B) and H2Ti3O7 nanofibers are 10-20 nm in diameter and several micrometers long, but FE-SEM reveals that several of these nanofibers tend to bind tightly to each other, forming a fiber bundle. Calcination at 773 K transformed TiO2 (B) nanofibers into a TiO2 (B)/anatase bicrystalline mixture with their fibrous morphology remaining intact. Upon increasing the calcination temperature to 873 K, most of the TiO2 (B) nanofibers were converted into anatase nanofibers and small anatase particles with smoother surfaces. In the photocatalytic dehydrogenation of neat ethanol, 1% Pt/TiO2 (B) nanofiber calcined at 673 K was the most active catalyst and generated about the same amount of H2 as did 1% Pt/P-25. TPR indicated that the calcination of 1% Pt/TiO2 (B) nanofiber at 573 K produced a poor Pt dispersion and poor activity. Calcination at a temperature higher than 773 K (in ambient air) resulted in an SMSI effect similar to that observed over TiO2 in the reductive atmosphere. As suggested by XPS, such an SMSI effect decreased the surface concentration of Pt metal and created Ptδs sites, preventing Pt particles from functioning as a Schottky barrier and leading to a lower activity. Because of the synergetic effect between TiO2 (B) and anatase phases, the bicrystalline mixture, produced by calcining at 773 K, was able to counter negative effects such as the reduction in surface area and the SMSI effect and maintained its photocatalytic activity.
1. Introduction Because global oil reserves are expected to be depleted in a few decades, alternative energy sources must be found to replace the current petroleum-based energy system. Hydrogen gas will play a vital role in the future energy system because it is a renewable, clean-burning energy source. Accordingly, many researchers have reported on the photocatalytic production of H2 from neat or aqueous aliphatic alcohol solution over platinized titanium oxide. Under deaerated conditions, both primary and secondary alcohols can be effectively dehydrogenated to aldehydes or ketones with the evolution of an equimolar amount of H2 over a semiconductor photocatalyst suspended in the alcohol solution.1–5 The most frequently investigated semiconductor photocatalyst of this reaction is TiO2,1–5 but other nontitaniumbased photocatalysts such as CdS,6 ZnS,7 and TiSe28 have been used. The purpose of loading TiO2 with Pt is to generate at the interface between Pt and TiO2 a Schottky barrier, which effectively * Corresponding author. E-mail:
[email protected]. Tel: 886-47232105 ext. 3541. Fax: 886-4-7292361. † National Changhua University of Education. ‡ National Tsing Hua University. (1) Zou, J.-J.; Liu, C.-J.; Yu, K.-L.; Cheng, D.-G.; Zhang, Y.-P.; He, F.; Du, H.-Y.; Gui, L. Chem. Phys. Lett. 2004, 400, 520. (2) Enea, O.; Ali, A.; Duprez, D. Int. J. Hydrogen Energy 1988, 13, 569. (3) Sakata, T.; Kawai, T. Chem. Phys. Lett. 1981, 80, 341. (4) Bamwanda, G. R.; Tsubota, S.; Nakamaru, T.; Haruta, M. J. Photochem. Photobiol. 1995, 89, 177. (5) Teratani, S.; Nakamichi, J.; Taya, K.; Tanaka, K. Bull. Chem. Soc. Jpn. 1982, 55, 1688. (6) Jin, Z.; Li, Q.; Zheng, X.; Xi, C.; Wang, C.; Zhang, H.; Feng, L.; Wang, H.; Chen, Z; Jiang, Z. J. Photochem. Photobiol., A 1993, 71, 85. (7) Chen, L.; Zhu, X.; Wang, F.; Gu, W. J. Photochem. Photobiol., A 1993, 73, 217. (8) Iseda, K.; Osaki, T.; Taoda, H.; Yamakita, H. Bull. Chem. Soc. Jpn. 1993, 66, 1038.
captures the photogenerated electrons and reduces the rate of electron-hole recombination9 and promotes the formation of H2 gas. Numerous transition metals had been investigated as the promoter for this photocatalytic dehydrogenation reaction, and 1 wt % Pt was the most effective one.5,10
C2H5OH f CH3CHO + H2 Anatase, rutile, and brookite are the three well-known crystallographic forms of TiO2 in photocatalysis. In contrast, very few works have addressed the photocatalytic properties of TiO2 (B).11–13 TiO2 (B) has a monoclinic unit cell with a ) 1.21787 nm, b ) 0.37412 nm, c ) 0.65249 nm, and β ) 107.054° and a crystal structure that consists of two edge-sharing octahedra that are linked to the neighboring doublet octahedra unit at their corners.14,15 We and others12,13,16 have reported the preparation of a TiO2 (B) nanotube utilizing a hydrothermal method reported by Kasuga.17 This TiO2 (B) nanotube was an excellent photocatalyst, producing 20% more H2 than P-25 in the photocatalytic dehydrogenation of neat ethanol.12 However, calcining a TiO2 (9) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr Chem. ReV. 1995, 95, 735. (10) Ohtani, B.; Iwai, K.; Nishimoto, S.-I.; Sato, S. J. Phys. Chem. B 1997, 101, 3349. (11) Zhu, J.; Zhang, J.; Chen, F.; Anpo, M. Mater. Lett. 2005, 59, 3378. (12) Kuo, H.-L.; Kuo, C.-Y.; Liu, C.-H.; Chao, J.-H.; Lin, C.-H. Catal. Lett. 2007, 113, 7. (13) Lin, C.-H.; Lee, C.-H.; Chao, J.-H.; Kuo, C.-Y.; Cheng, Y.-C.; Huang, W.-N.; Huang, Y.-M.; Shih, M.-K. Catal. Lett. 2004, 98, 61. (14) Marchand, R.; Brohan, L.; Tournoux, M. Mater. Res. Bull. 1980, 15, 1129. (15) Feist, T. P.; Mocarski, S. J.; Davies, P. K.; Jacobson, A. J.; Lawandowski, A. J. Solid State Ionics 1988, 28-30, 1338. (16) Armstrong, G.; Armstrong, A. R.; Canales, J.; Bruce, P. G. Chem. Commun. 2005, 2454. (17) Kasuga, T.; Hiramatsu, M.; Hoson, A. Langmuir 1998, 14, 3160.
10.1021/la800572g CCC: $40.75 2008 American Chemical Society Published on Web 08/09/2008
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(B) nanotube at 673 K altered its morphology and phase composition, which in turn affected the photocatalytic activity of the nanotube. A solid fiber should be more resistant to thermal calcination than a hollow nanotube and provided a broader temperature range for investigation. The TiO2 (B) nanofiber can be prepared using the same hydrothermal method to prepare the TiO2 (B) nanotube, except that a higher hydrothermal temperature should be used.18,19 Therefore, in this study, we prepared a platinized TiO2 (B) nanofiber and examined, in more detail, the effect of calcination temperature on the Pt promoter as well as on the pore size and pore volume, morphology, crystalline phase transformation, and band gap energy of the nanofiber.20,21 We focused on the influence of the changes in physical and chemical properties of the Pt/nanofiber caused by calcination on its ability to produce H2 from neat ethanol.
2. Experimental Section 2.1. Photocatalyst Preparations. Anatase TiO2 powder (2.0 g, Aldrich) was mixed with 600 mL of 10 M NaOH in a 1.0 L perfluoroalkoxy container, and the mixture was kept at 403 K for 7 days. The hydrothermal temperature used here is 20 K higher than that used in the preparation of the nanotube.12 The resulting slurry was filtered to dryness by vacuum filtration to yield a paste. The paste was then washed a few times in deionized water and then in 0.10 M HCl. The washing process was very thorough: 1.0 g of paste was dispersed in 1000 mL of deionized water or 0.10 M HCl, and the suspension solution was stirred at 300 rpm for 1.0 h using a mechanical stirrer. Between washings, the suspension was always filtered to dryness before the next washing. After it had been washed with HCl, the paste was again washed in deionized water as described above until no Cl- was present in the filtrate. The final paste was then dried at 383 K for 24 h to produce the hydrogen trititanate nanofiber. The titanate nanofiber was calcined between 573 and 873 K at a heating rate of 1 K min-1 for 3 h to yield the final TiO2 (B) nanofiber. An impregnation method was adopted to deposit Pt onto the TiO2 (B) nanofiber. The nanofiber was first mixed with 60 mL of deionized water in a flask to generate a suspension solution. The calculated volume of 1.0 mM H2PtCl6 (Showa) was added slowly to the suspension and stirred for 2 h at room temperature. A rotary evaporator with a water bath at 323 K was utilized to remove water from the suspension solution. The resulting paste was dried at 383 K for 12 h to yield a yellow powder, which was calcined between 573 and 873 K in air for 3 h to produce the catalyst precursors. The catalyst precursor was then reduced in the flowing hydrogen at a rate of 30 mL min-1 at 423 K for 3 h to yield a dark-gray Pt metal-loaded nanofiber. The reduction temperature of Pt was chosen on the basis of the results of the temperature-programmed reduction experiments (section 2.2). A physical mixture of Pt-loaded TiO2 (B) nanofiber and the anatase phase was prepared to test the bicrystalline synergetic effect. The nanofiber that was calcined at 873 K (pure anatase form) was mixed thoroughly with that calcined at 673 K (pure TiO2 (B) form) at different mole % values, and then Pt metal was deposited onto this bicrystalline mixture following the same impregnation procedure described above. 2.2. Photocatalyst Characterizations. The BET surface areas of the photocatalysts were measured with Micromeritics ASAP 2010 using N2 gas at liquid-nitrogen temperature, and their pore size distributions were determined by the BJH method. X-ray powder diffraction patterns of calcined nanofibers were obtained using a Shimadzu Laboratory-X XRD-6000 spectrometer with Fe KR irradiation (λ ) 1.93604 Å). A JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM) was employed to observe (18) Armstrong, A. R.; Armstrong, G.; Canales, J.; Bruce, P. G. Angew. Chem., Int. Ed. 2004, 43, 2286. (19) Pavasupree, S.; Suzuki, Y.; Yoshikawa, S.; Kawahata, R. J. Solid State Chem. 2005, 178, 3110. (20) Yu, H.; Yu, J.; Cheng, B. Chemosphere 2007, 66, 2050. (21) Yu, J.; Yu, H.; Cheng, B.; Trapalis, C. J. Mol. Catal. A 2006, 249, 135.
Lin et al. the morphology of the photocatalyst at various calcination temperatures. A Phillips Tecani 20 transmission electron microscope coupled with an EDX detector (energy-dispersive X-ray) was adopted to observe the fine structure of the prepared photocatalysts at an accelerating voltage of 200 kV and to analyze their elemental compositions. Raman spectra were obtained using a 3D nanometer-scale Raman PL microspectrometer (Tokyo Instruments, Inc.) to monitor the formation of the TiO2 (B) phase during calcination. The diffusereflectance UV-vis spectra of these photocatalysts were recorded using a Shimazru 3560 UV-vis spectrometer that was equipped with an integrating sphere to estimate the band gap energies of the photocatalysts. The sodium ion content of these photocatalysts was determined by neutron activation analysis using the neutron source from the THOR nuclear reactor locating at Hsinchu, Taiwan, and Na2CO3 was the calibration standard. Temperature-programmed reduction experiments were performed using a Micromeritics Autochem 2910. The instrument has a cryocooler device that can lower the reactor temperature to 203 K using cold N2 gas that is generated from liquid N2 and is used to conduct subambient temperature TPR experiments (SAT-TPR). The SAT-TPR procedure is as follows: After 100 mg of catalyst was placed in a U-shaped quartz reactor, the instrument was purged with Ar (99.999%) at a flow rate of 30 mL min-1 for 30 min at room temperature. The reactor was then cooled to 223 K in the flowing Ar. Once the baseline was stabilized, the SAT-TPR experiments were initiated by switching the gas to 10% H2/Ar and simultaneously ramping the temperature from 223 to 673 K at a heating rate of 5 K min-1. 2.3. Photocatalytic Activity Tests. An 80 mL quartz tube that was sealed with a rubber septum was used as the photoreactor to study the production of H2 gas from neat ethanol. The ethanol was dried over molecular sieves 5A before use. Ten milligrams of photocatalyst and 15 mL of neat ethanol were placed in the photoreactor in an ice bath and purged with Ar for 20 min to remove O2 that was dissolved in the ethanol. In these experiments, the photoreactor was carefully placed at a fixed distance in front of the UV lamps. The suspension solution was stirred with a magnetic stirrer and irradiated with two 15 W UV lamps (λmax ) 352 nm, Sankyo Denki) for various periods of time at ambient temperature. Ten microliters of the gaseous products was sampled using a gastight syringe and analyzed using a Varian 3300 GC that was equipped with a TCD detector and a CP-Carbon PLOT P7 capillary column with Ar as the carrier gas. The liquid products (0.4 µL) were analyzed using an HP 6890 GC that was equipped with an FID detector and an HP-5 5% phenyl methyl siloxane capillary column with N2 as the carrier gas.
3. Results and Discussion 3.1. Characterization of Morphology and Microstructure. Figure 1a displays the FE-SEM micrograph of the anatase TiO2 powder that was used to prepare the nanofiber. It indicates that the starting material comprises particles of sizes 50-150 nm. Figure 1b is the FE-SEM micrograph of the hydrothermal product after washing in deionized water and HCl and drying at 383 K. The micrograph in Figure 1b demonstrates that the hydrothermal product consists of many fiberlike materials with diameters of 20-200 nm and lengths of several micrometers. The highermagnification FE-SEM micrograph in the inset of Figure 1b reveals that these large-diameter fibers are not single fibers but fiber bundles. The TEM micrograph in Figure 2a confirms that the fiber bundle is actually composed of many smaller fibers with outer diameters of 10-20 nm that are tightly bound to each other. The lack of bright and dark contrast in the fibrous material in the micrograph in Figure 2a verifies that it is a solid fiber rather than a hollow tube. The HRTEM micrograph in Figure 2b shows that the HCl-washed nanofibers that are dried at 383 K have a lattice fringe of 0.90 nm; this value is close to the
Effect of Temperature on Pt/TiO2 (B) Structure
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Figure 1. FE-SEM micrograph of (a) the anatase particles (the starting material) and (b) the nanofibers after washing with 0.10 M HCl and drying at 383 K. The inset in b is a higher-magnification micrograph showing the fiber bundle.
Figure 2. TEM micrograph of (a) the fiber bundle after acidic washing and drying at 383 K, (b) the HRTEM micrograph showing the lattice fringe of the nanofiber, and (c) the EDX spectrum of the nanofiber.
interplanar distance of the (001) crystal plane of hydrogen trititanate (H2Ti3O7).22 The EDX spectrum in Figure 2c reveals that the material has only Ti and O elements and no Na ions are present in the material. The results of neutron activation analysis
are consistent with EDX analysis: 70%), the effect of low anatase surface area dominated the bicrystalline synergetic effect, resulting in a continuous decrease in photocatalytic activity.
4. Conclusions Calcination temperature has a profound effect on the morphology, surface area, pore volume, and crystalline phase
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composition of the prepared nanofiber, and these factors in turn affect its photocatalytic activity. The H2Ti3O7 nanofiber was prepared by a hydrothermal method in 10 M NaOH solution, followed by acidic washing and drying at 383 K. Calcining the H2Ti3O7 fiber at 673 K produced a crystalline TiO2 (B) nanofiber, which, after calcination at 773 K, transformed into a bicrystalline mixture containing TiO2(B) and anatase phases with fibrous morphology. Upon increasing the calcination temperature to 873 K, most of the TiO2 (B) nanofibers were converted into anatase nanofibers and small anatase particles. Nanofibers indeed have a higher temperature for phase transformation and morphology deformation than do nanotubes. Therefore, they are more stable against thermal calcination than nanotubes. In the photocatalytic dehydrogenation of ethanol, a TiO2 (B) nanofiber impregnated with 1% Pt and calcined at 673 K generated more H2 than those calcined at other temperatures. Its activity with respect to the catalytic dehydrogenation of ethanol is equal to that of 1% Pt/ P-25. TPR indicated that calcination at 573 K provided a poor Pt dispersion on the TiO2 (B) nanofiber and a lower activity. However, calcination at 773 and 873 K produced an SMSI effect that decreased the surface concentration of Pt metal and created Ptδs sites as suggested by XPS, preventing Pt particles from functioning like a Schottky barrier. Because of a synergetic effect between TiO2 (B) and the anatase phase, the bicrystalline mixture produced by calcining at 773 K was able to counter negative effects such as the reduction in surface area and the SMSI effect and maintained its photocatalytic activity. Acknowledgment. C-H.L. is grateful for a grant from the National Science Council of Taiwan (NSC-93-2113-M-018-006) in support of this research. Supporting Information Available: Sample preparation procedures for electron microscopy, Raman spectra of H2Ti3O7 nanofibers, and XRD spectra of Pt/TiO2 (B) nanofibers at different calcination temperatures. This material is available free of charge via the Internet at http://pubs.acs.org. LA800572G
