4526
Ind. Eng. Chem. Res. 2000, 39, 4526-4530
Synthesis and Photocatalytic Properties of Fibrous Titania Prepared from Protonic Layered Tetratitanate Precursor in Supercritical Alcohols Shu Yin* and Tsugio Sato Institute for Chemical Reaction Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Fibrous titania samples consisting of monoclinic TiO2, anatase, and rutile were synthesized by heat treatment of H2Ti4O9‚nH2O, which was prepared by the ion-exchange reaction of potassium tetratitanate in HCl, in different supercritical media. The phase transformation temperature and microstructure of the products changed significantly depending on the heating environment. The critical temperature at which anatase appeared in liquid media was much lower than that at which it appeared in air, indicating that the phase transformation of the monoclinic TiO2 to anatase proceeded by the dissolution-precipitation mechanism. Fibrous titania consisting of nanosize crystals of TiO2 possessing high crystallinity could be obtained by treatment using supercritical ethanol or methanol as the reaction medium. The photocatalytic activities for hydrogen evolution from an aqueous methanol solution and for phenol photodegradation were determined for various titania samples. Amorphous TiO2, which possessed large amounts of crystal defects, showed no photocatalytic activity for either reaction. The titania that crystallized in supercritical ethanol or methanol consisted of a mixture of monoclinic TiO2 and anatase and showed excellent hydrogen evolution activity, and the hydrogen evolution activity of fibrous titania changed with heat treatment media in the following sequence: ethanol > methanol > water > 2-propanol > n-butanol > n-hexane > P-25 > air. On the other hand, the phenol degradation activity of titania decreased with decreasing specific surface area as follows: P-25 > water > ethanol > methanol > n-butanol > air. 1. Introduction Because of its excellent physical and chemical properties, titania has been used for wide applications such as white pigment, cosmetics, catalyst, catalyst carrier, etc.1-6 Usually, titania is prepared by heat treatment of amorphous titania gel produced by the hydrolysis of oxotitanium ion or alkoxides.7-9 The physicochemical properties of titania change significantly depending on the heat treatment conditions. In our previous papers,10,11 it was reported that fine crystals of anatase prepared by the heat treatment of amorphous gel in methanol exhibited much better sinterability and photochemical hydrogen evolution activity than those fabricated by hydrothermal reactions and/or by calcination in air. The excellent properties of titania may have been a result of their consistency as fine powders with good crystallinity. It was also reported that the photocatalytic behavior of titania greatly depended on the preparation methods.12 Various forms of titania, such as monoclinic titania, anatase, and rutile, can be prepared by the thermal decomposition of the H2Ti4O9‚nH2O.13,14 In a previous paper,15 we reported that monoclinic titania, anatase, and rutile with fibrous morphology could be prepared by solvothermal reactions in water, methanol, and ethanol at 200-325 °C using fibrous K2Ti4O9 precursor. It is to be expected that the photocatalytic properties of fibrous titania change depending on the heat treatment conditions. As an aspect of basic research in solarchemical energy conversion, the H2 evolution ability of * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: +81-22-217-5599.
a photocatalyst receives much of researchers’ attention. On the other hand, photodegradation of soluble toxic organic waste has become one of the most applicable techniques on photocatalysts. Phenol has the simplest structure of compounds containing a benzene ring and might be used as a representative of water-soluble pollution substances in photocatalytic research. In the present study, a series of tests was conducted to evaluate the hydrogen evolution and phenol degradation activity of fibrous titania prepared by the heat treatment of protonic tetratitanate in air, water, methanol, ethanol, and other organic solvents. 2. Experimental Sections 2.1. Starting Materials. Fibrous potassium tetratitanate, K2Ti4O9, prepared by the flux method (Otsuka Chemical Co. Ltd., Tokyo, Japan) was used as a starting material. After being washed with hot water to remove flux, the sample was crushed to -355 µm. H2Ti4O9‚ 0.25H2O was prepared by dispersing 10 g of K2Ti4O9 in 1000 cm3 of vigorously stirred 1 M HCl at 30 °C for 2 h. The solid product was separated by filtration and washed with distilled water three times before being vacuum-dried at 60 °C overnight. 2.2. Preparation of Fibrous Titania. After dispersing 1 g of H2Ti4O9‚0.25H2O in 75 cm3 of a reaction solution such as water, ethanol, or other organic solvents, the obtained slurry was placed into an SUS 314 stainless steel autoclave with an internal volume of 120 cm3 and heated with a heating rate of 10 °C/min to the desired temperature, which was maintained for 1 h to form fibrous titania. The product was filtered, washed with each special grade chemical solvent in air atmo-
10.1021/ie000165g CCC: $19.00 © 2000 American Chemical Society Published on Web 10/27/2000
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4527
sphere three times, and vacuum-dried at 60 °C overnight. The heat treatment of H2Ti4O9‚0.25H2O in air was also carried out. The purity of the fibrous TiO2 obtained was more than 99% (main impurities: K2O, 0.68 wt %; Nb2O5, 0.23 wt %). 2.3. Analysis. The phase constitution of the products was determined by X-ray diffraction analysis (XRD, Shimadzu XD-D1) with continuous scanning mode at a rate of 2°/min using graphite-monochromised Cu KR radiation. The specific surface areas were determined by the amount of nitrogen adsorption at 77 K (BET, Sibata, SA-1000). Microstructures were observed by scanning electron micrograph (SEM, Hitachi S-900) at 15 kV and transmission electron micrograph (TEM, JEOL JEM-2010) at 200 kV. The photocatalytic reaction was carried out in a Pyrex reactor of 500 cm3 capacity attached to an inner radiation type 100 W high-pressure mercury lamp. The inner cell had thermostatic water flowing through a jacket between the mercury lamp and the reaction chamber and was constructed of Pyrex glass, which served to filter out the UV emission of the mercury arc below 290 nm. The hydrogen evolution activity of the catalyst was determined by measuring the volume of H2 evolved with a gas buret during the irradiation of the suspension of 0.25 g of catalyst in 500 cm3 of a 10:90 (vol %) methanol/water mixed solution at 60 °C. Evolved H2 gas was identified by gas chromatography (Shimadzu, GC-8A). The photodegradation of phenol was determined by measuring the concentration of phenol during the irradiation of the suspension of catalyst at 60 °C with air bubbling at a flow rate of 250 mL/min in 500 cm3 of phenol solution. The initial concentration of phenol and the initial pH were 0.5 mM and 3.0, respectively. The concentration of phenol was determined colorimetrically by a UV-visible spectrophotometer (Hitachi, U-1000). For comparison, the photocatalytic activities of the commercial titania powder (Degussa P-25) and fine powders of anatase with high specific surface area prepared by crystallizing amorphous gel from titanium tetraisopropoxide precursor were also determined. 3. Results and Discussions 3.1. Phase Constitution and Microstructure. Titania was prepared by the heat treatment of H2Ti4O9‚ 0.25H2O in air and various solvents such as water, methanol, ethanol, etc. In a previous paper,15 it was reported that H2Ti4O9‚0.25H2O transformed in successive steps to H2Ti8O17, monoclinic TiO2, anatase, and rutile with increasing calcination temperature of 200, 300, 650, and 1100 °C, respectively. The phase transformation was accelerated by the existence of liquid media. The XRD patterns of the samples prepared in hightemperature media are shown in Figure 1. The H2Ti4O9‚ 0.25H2O transformed to monoclinic titania in air even at temperatures as high as 550 °C. However, it transformed to a mixture of monoclinic titania and anatase in supercritical ethanol or methanol at 325 °C and a pressure of 28 or 32 MPa. Single-phase anatase was obtained in subcritical water at a temperature as low as 250 °C and a pressure of 4 MPa. The phase transformation from H2Ti4O9‚0.25H2O to monoclinic titania and anatase was promoted by the presence of liquid media, especially water, indicating that crystallization and phase transformation in liquid media proceeded via the dissolution-reprecipitation mechanism. The XRD
Figure 1. XRD patterns of (a) H2Ti4O9‚0.25H2O and the products prepared by heat treatments of H2Ti4O9‚0.25H2O in (b) air at 550 °C, (c) methanol at 325 °C, (d) ethanol at 325 °C, and (e) water at 250 °C for 1 h. (3, anatase TiO2; 1, monoclinic TiO2; b, H2Ti4O9‚ 0.25H2O)
peak intensity of the powders prepared by heat treatment in water was almost the same as that of samples prepared in supercritical alcohols and higher than that of samples prepared in air, indicating that the crystallinity of the powders are in the sequence water = alcohol . air. The scanning electron micrographs and transmission electron micrographs of fibrous titania prepared by the calcination and solvothermal reactions are shown in Figure 2. All samples exhibited fibrous morphology similar to that of the potassium tetratitanate used as the starting material; however, the microstructure of the fiber changed significantly depending on the reaction conditions. The fibrous titania prepared by calcination consisted of strongly agglomerated ultrafine particles less than 20 nm in diameter. In contrast, the fibrous titania prepared by the solvothermal reaction in supercritical ethanol at 325 °C consisted of rodlike fine crystals of ca. 30-50 nm in width, and that prepared in water at 250 °C consisted of comparatively larger crystals of 50-100 nm in size. The crystal growth in water seems greater than that in other solvents because water exhibits a higher dielectric constant. Consequently, well-crystallized fine particles of titania might be formed by crystallization in supercritical ethanol and methanol, which have the appropriate dielectric constants. These results also suggested that the reaction proceeds according to the dissolutionreprecipitation mechanism in liquid media. 3.2. Photocatalytic Activity for Hydrogen Evolution and Phenol Photodegradation. The photocatalytic activities of various titania samples for hydrogen evolution and phenol photodegradation were determined. Fibrous samples were prepared by heat treatment of H2Ti4O9‚0.25H2O at 325 °C in supercritical ethanol (28 MPa), methanol (32 MPa), 2-propanol (25 MPa), n-butanol (15 MPa), and n-hexane (18 MPa) and
4528
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000
Figure 2. Scanning and transmission electron micrographs of titania prepared by heat treatments of H2Ti4O9‚0.25H2O in (a) air at 550 °C, (b) ethanol at 325 °C, and (c) water at 250 °C for 1 h.
Figure 3. Cumulative amounts of hydrogen evolved from 0.25 g samples dispersed in 500 cm3 solutions at 60 °C using 100-W highpressure mercury arc radiation. Samples were prepared by heat treatment of H2Ti4O9‚0.25H2O for 1 h in (2) ethanol at 325 °C, (4) methanol at 325 °C, (O) water at 250 °C, (9) 2-propanol at 325 °C, (0) n-butanol at 325 °C, (3) n-hexane at 325 °C, and (1) air at 550 °C; ×, amorphous gel from Ti(i-C3H7O)4 precursor; [, heat treatment of amorphous TiO2 gel in 2-propanol at 250 °C; and b, Degussa P-25 TiO2.
at 250 °C in subcritical water (4 MPa) for 1 h. For comparison, the activity of the commercial titania powder, Degussa P-25, and fine powders of amorphous titania and anatase prepared from Ti(i-C3H7O)4 precursor were also determined. The cumulative amounts of hydrogen evolved and phenol decomposed with typical samples are shown in Figures 3 and 4. The amount of hydrogen gas evolved increased linearly with time for all samples, but the slope of the straight line changed significantly depending on the heat treatment conditions. All fibrous samples prepared in liquid media showed much higher hydrogen
Figure 4. Phenol photodegradation from 0.25 g samples dispersed in 500 cm3 solutions at 60 °C using 100-W high-pressure mercury arc radiation. Samples were prepared by heat treatment of H2Ti4O9‚0.25H2O for 1 h in (O) water at 250 °C, (2) ethanol at 325 °C, (4) methanol at 325 °C, (0) n-butanol at 325 °C, and (1) air at 550 °C; ×, amorphous gel from Ti(i-C3H7O)4 precursor; [, heat treatment of amorphous TiO2 gel in 2-propanol at 250 °C; and b, Degussa P-25 TiO2.
evolution activity than P-25, whereas the hydrogen evolution activity of the sample formed by calcination in air was smaller than that of P-25. The hydrogen evolution activities of fibrous titania prepared by heat treatment of H2Ti4O9‚0.25H2O under different conditions changed in the following sequence: ethanol > methanol > water > 2-propanol > n-butanol > n-hexane > P-25 > air. On the other hand, although the degree of phenol decomposition also increased linearly with time for all samples up to 90%, the phenol degradation activity of fibrous titania showed a different sequence than that of hydrogen evolution as follows: P-25 > water > ethanol > methanol > n-butanol > air. The
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4529
Figure 6. Schematic illustration of the energy correlation and the redox mechanism on titania catalyst surface (a) with and (b) without the presence of oxygen.
Figure 5. (a) Hydrogen evolution and (b) phenol photodegradation activity of titania samples prepared under different conditions as a function of the specific surface area. Samples were prepared by heat treatment of H2Ti4O9‚0.25H2O for 1 h in (2) ethanol at 325 °C, (4) methanol at 325 °C, (O) water at 250 °C, (0) n-butanol at 325 °C, and (1) air at 550 °C; ×, amorphous gel from Ti(i-C3H7O)4 precursor; [, heat treatment of amorphous TiO2 gel in 2-propanol at 250 °C; and b, Degussa P-25 TiO2.
difference in the sequence might be caused by the different reaction mechanisms between hydrogen evolution and phenol degradation. The fine powder of anatase prepared from Ti(i-C3H7O)4 showed excellent phenol degradation activity but moderate hydrogen evolution activity. On the other hand, the photocatalytic activity of amorphous TiO2 was quite small for both reactions. It was difficult to compare the photocatalytic activity of the monoclinic titania with that of anatase or rutile titania under the present experimental conditions, as it was almost impossible to prepare the powders of monoclinic, anatase, or rutile titania as a single phase consisting of the same particle size, microstructure, and specific surface area in different media. 3.3. Possible Mechanism. Figure 5 shows the relationship between photocatalytic activities for (a) hydrogen evolution and (b) phenol decomposition and the specific surface area of the samples. The specific surface area was in the order amorphous titania (138 m2/g) > anatase fine powder (77.0 m2/g) > P-25 (44.7 m2/g) > fibrous titania (13-22 m2/g). Although the amorphous TiO2 gel had a very high specific surface area, no noticeable hydrogen evolution or phenol degradation activity was observed. These results suggested that photoinduced electrons and holes recombined quickly in amorphous titania because the crystal defects acted as a recombination center. Fibrous titania prepared in supercritical ethanol and methanol showed higher hydrogen evolution activities than those of P-25 and anatase fine powder, and no obvious relationship between specific surface area and hydrogen evolution activity was observed in the present study. On the other hand, the phenol degradation activity strongly related to the specific surface area of crystallized titania. These results are thought to be due to the difference in the reaction mechanisms.
It is accepted that TiO2 particles absorb UV light of energy greater than the band gap of ca. 3.2 eV to generate electron/hole pairs (eq 1). The electrons are photoinduced to the conduction band (e-BC), and the holes in the valence band (h+VB) are subsequently trapped by H2O to yield H+ and •OH radicals (eq 2). hν
TiO2 798TiO2 (e-CB + h+VB)
(1)
Figure 6 illustrates the energy diagram and photoinduced charge-transfer process in a TiO2 particle. Although the band gap of titania is larger than the potential energy for the electrochemical decomposition of water (1.23 eV), pure H2O cannot be photodecomposed on a clean titania surface because of the rapidly occurring backward reaction of H2 and O2. In the absence of oxygen and the presence of sacrificial species such as methanol, the holes generated by the light would oxidize methanol to HCHO, etc., while electrons in the conduction band of the particle would simultaneously reduce water and/or protons in the solution to form gaseous H2 as shown by eqs 2-5.16,17 These reactions proceed competitively with the recombination of the photoinduced electrons and holes. Because the recombination rate of electrons and holes decreases with increasing crystallinity of titania powders, i.e., decreasing crystal defects, the hydrogen evolution activity strongly depends on the crystallinity but depends only slightly on the specific surface area of titania.
h+VB + H2O f •OH + H+
(2)
CH3OH + •OH f •CH2OH + H2O
(3)
•CH2OH f HCHO + H+ + e-CB
(4)
2H2O + 2e-CB f H2v+ 2OH-
(5)
On the other hand, in the presence of oxygen, the electrons in the conduction band can be trapped rapidly by the molecular oxygen, adsorbed on the TiO2 particle surface, to form •O2-, which can then generate highly active •OOH radicals 17 (eqs 6-7). The phenol reacts
4530
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000
with these reactive oxygen radicals, and/or with molecular oxygen, to produce intermediates or complete decomposition products, CO2 and H2O18,19 (eq 8).
h+VB + H2O f •OH + H+
(2)
e-CB + O2 f •O2-
(6)
•O2- + H+ f •OOH
(7)
C6H5OH +{•OH, •OOH, and/or O2} f intermediates f CO2 + H2O (8) In this case, the recombination between electrons and holes is difficult because the oxygen molecules quickly consume the photoinduced electrons. The reaction between holes and phenol molecules becomes the controlling factor of the phenol degradation. The number of reaction centers usually increases with increasing specific surface area of the catalyst. Therefore, the holes in the valence band could be used effectively for the oxidation reaction with increasing specific surface area. Consequently, phenol degradation activity is related strongly to the specific surface area. 4. Conclusions (1) The photocatalytic activity of fibrous titania prepared from protonic layered tetratitanate in supercritical alcohols was much better than that of titania samples prepared by hydrothermal reaction and calcination. (2) Fibrous titania prepared in supercritical ethanol or methanol at 325 °C consisted of well-crystallized fine particles of a mixture of monoclinic titania and anatase. (3) The hydrogen evolution activity increased with increasing crystallinity of titania and showed no obvious relation to the specific surface area, whereas the phenol degradation activity increased with increasing crystallinity and specific surface area. Literature Cited (1) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; John Wiley & Sons, Inc.: New York, 1988; pp 654-655. (2) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (3) Kawai. T.; Sakata. T. Conversion of Carbohydrate into Hydrogen Fuel by a Photocatalytic Process. Nature 1980, 286, 474-476.
(4) O’Regan, B.; Gratzel, M. A Low-cost, High-efficiency Solar Cell Based on Dye-sensitized Colloidal TiO2 Films. Nature 1991, 353, 737-739. (5) Barbe, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Gratzel, M. Nanocrystalline Titanium Oxide Electrodes for Photovoltaic Applications. J. Am. Ceram. Soc. 1997, 80, 3157-3171. (6) Mills, A.; Le Hunte, S. An Overview of Semiconductor Photocatalysis. J. Photochem. Photobiol. A. 1997, 108, 1-35. (7) Kominami, H.; Matsuura, T.; Iwai, K.; Ohtani, B.; Nishimoto, S.; Kera, Y. Ultra-highly Active Titanium(IV) Oxide Photocatalyst Prepared by Hydrothermal Crystallization from Titanium(IV) Alkoxide in Organic Solvents. Chem. Lett. 1995, 8, 693-694. (8) Oguri, Y.; Riman, R. E.; Bowen, K. H. Processing of Anatase Prepared from Hydrothermally Treated Alkoxy-derived Hydrous Titania. J. Mater. Sci. 1998, 23, 2897-2904. (9) Slunecko, J.; Kosec, M.; Holc, J.; Drazic, G. Morphology and Crystallization Behavior of Sol-gel-derived Titania. J. Am. Ceram. Soc. 1998, 81, 1121-1124. (10) Yin, S.; Inoue, Y.; Uchida, S.; Fujishiro, Y.; Sato, T. Crystallization of Titania in Methanol, n-Hexane and Water at High Temperatures. Rev. High-Pressure Sci. Technol. 1998, 7, 1438-1440. (11) Yin, S.; Inoue, Y.; Uchida, S.; Fujishiro, Y.; Sato, T. Characterization of Titania in Liquid Media and Photochemical Properties of Crystallized Titania. J. Mater. Res. 1998, 13, 844847. (12) Sclafani, A.; Palmisano. L.; Schiavello, M. Influence of the Preparation Methods of TiO2 on the Photocatalytic Degradation of Phenol in Aqueous Dispersion. J. Phys. Chem. 1990, 94, 829832. (13) Izawa, H.; Kikkawa, S.; Koizumi, M. Ion Exchange and Dehydration of Layered Titanates, Na2Ti3O7 and K2Ti4O9. J. Phys. Chem. 1982, 86, 5023-25026. (14) Sasaki, T.; Watanabe, M.; Komatsu, Y.; Fujiki, Y. Layered Hydrous Titanium Dioxide: Potassium Ion Exchange and Structural Characterization. Inorg. Chem. 1985, 24, 2265-2271. (15) Yin, S.; Uchida, S.; Fujishiro, Y.; Aki, M.; Sato, T. Phase Transformation of the Protonic Layered Tetratitanate under Solvothermal Conditions. J. Mater. Chem. 1999, 9, 1191-1195. (16) Kawai, T.; Sakata, T. Photocatalytic Hydrogen Production from Liquid Methanol and Water. J. Chem. Soc., Chem. Commun. 1980, 694-695. (17) Gerischer, H.; Heller, A. The Role of Oxygen in Photooxidation of Organic Molecules on Semiconductor Particles. J. Phys. Chem. 1991, 95, 5261-5267. (18) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Itaya, A. Heterogeneous Photocatalytic Decomposition of Phenol over TiO2 Powder. Bull. Chem. Soc. Jpn. 1991, 58, 2015-2022. (19) Auguliaro, V.; Davi, E.; Palmisona, L.; Schiavello, M.; Sclafani, A. Influence of Hydrogen Peroxide on the Kinetics of Phenol Photodegradation in Aqueous Titanium Dioxide Dispersion. Appl. Catal. 1990, 65, 101-106.
Received for review February 3, 2000 Revised manuscript received June 26, 2000 Accepted June 26, 2000 IE000165G
