Environ. Sci. Technol. 2004, 38, 2729-2736
Highly Efficient Liquid-Phase Photooxidation of an Azo Dye Methyl Orange over Novel Nanostructured Porous Titanate-Based Fiber of Self-Supported Radially Aligned H2Ti8O17‚1.5H2O Nanorods N I N G Z H O N G B A O , * ,†,‡ X I N F E N G , † ZHUHONG YANG,† LIMING SHEN,§ AND X I A O H U A L U * ,† College of Chemical Engineering and College of Material Science and Engineering, Nanjing University of Technology, Nanjing, 210009, Peoples’ Republic of China
Novel nanostructured porous fibers of self-supported, radially aligned H2Ti8O17‚1.5H2O nanorods were prepared from layered H2Ti4O9‚1.2H2O tetratitanate fibers by novel solvothermal reaction in glycerine at 150-250 °C. The H2Ti8O17‚1.5H2O fibers with diameters of 0.5-1.5 µm and lengths of 10-20 µm consist of multi-scale nanopores and nanostructures. They also are of high crystallinity, large surface area of 127 m2 g-1, and stable phase up to 350 °C. Photocatalytic activity of the H2Ti8O17‚1.5H2O fibers was evaluated in aqueous photooxidation of an azo dye methyl orange in the presence of UV irradiation and O2, using P-25 as the standard sample. Both the photocatalytic activity and the dispersity-agglomeration property of H2Ti8O17‚ 1.5H2O fibers are pH-controllable. Highly photooxidative activity, superior to that of P-25, occurs at pH 6.0-11.0 due to certain distinguishable material characteristics and to large amounts of adsorbed reactants of surface active OH• free radicals, surface hydroxyl OH, O2•-, O•OH, and methyl orange. The agglomeration of H2Ti8O17‚1.5H2O fibers becomes more serious from pH 2.0 to pH 5.0 and from pH 6.0 to pH 11.0. Well-dispersed H2Ti8O17‚1.5H2O fibers occur at pH 6.0. Both the total photodegradation of waste chemicals and the entire sedimentation of H2Ti8O17‚ 1.5H2O fibers can be timed to end simultaneously at suitable pH value. The photocatalyst-free reaction solution is then easily removed, and the fresh wastewater is added again. Standard unit operation processes of chemical engineering are used to design a continuous, low-cost, largescale, liquid-phase photocatalysis technique based on the H2Ti8O17‚1.5H2O fibers.
Introduction Liquid-phase photocatalysis by TiO2 is an attractive photon energy conversion approach for decomposing many chemi* Corresponding author e-mail:
[email protected] (N.B.) or
[email protected] (X.L). † College of Chemical Engineering. ‡ Present address: Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, 2-5-1 Akbono-cho, Kochi 780-8520, Japan. § College of Material Science and Engineering. 10.1021/es034388k CCC: $27.50 Published on Web 04/02/2004
2004 American Chemical Society
cals in wastewater (1-3). The photocatalytic efficiency is determined by both material characteristics (e.g., phase type, phase structure, crystallinity, surface area, surface hydroxyls, and particle size) (2-8) and photocatalytic conditions such as light intensity, initial concentration of chemicals, catalyst concentration, dispersity of TiO2, and pH value (9-15). Nanocrystalline anatase is generally recognized as the most active phase in titania photocatalysts (2-8). However, it has not been used in a practical industrial-based photocatalytic process due to high production cost and difficult catalyst recovery from solution (2, 3, 9-11, 16-20). Although nanocrystalline TiO2 was supported on a large carrier matrix for economic photocatalysis (2, 3, 21-23), the efficiency of the overall process decreases due to decreased surface area to volume ratio, partial loss of active surface sites, and difference for interfacial structures and mass-transfer limitations for photocatalysts (2, 3, 24-26). As a result, novel photocatalysts and photocatalytic techniques are required to attain the lowcost, large-scale photocatalytic application. Many TiO2-based materials with various morphologies, phase types, crystallinities, and chemical components have been prepared from layered tetratitanate by intercalation reaction; ion-exchange reaction; and subsequent low-cost, large-scale calcination (27-64). They have been the subject of intensive research as photocatalysts (27-34, 56-64). TiO2based fibers with large micrometer size can be separated easily from solution, which can overcome the problems of nanocrystalline anatase in photocatalysis. However, current bottlenecks lie in low photocatalytic activity and in corresponding photocatalytic technique of the fibrous photocatalyst. Layered tetratitanate nanocomposites with enhanced photocatalytic activity were prepared by the intercalation reaction (27-34, 56-64). Thermal dehydration reactions in air as well as calcinations or in supercritical alcohols (ethanol, methanol, and 1-butanol) at T > 250 °C as well as hydro/ solvothermal reactions were also used to prepare TiO2-based materials such as exfoliated H2Ti4O9 sheets, H2Ti8O17, TiO2 (B), monoclinic titania, anatase, and rutile, from the layered tetratitanate (27-46, 52-64). However, disadvantages remain because these TiO2-based materials and layered tetratitanate nanocomposites still have poor thermal stability, low surface area, poor crystallinity, low photooxidative activity, and uncontrollable structure transformations. Moreover, the intercalation reaction is difficult to perform due to the low swelling ability of the layered tetratitanate. Hydro/solvothermal reactions have been used to prepare advanced inorganic materials with fabricated or tailed novel properties, which are hardly achieved by the traditional routes (27-40, 65-68). The nonporous octatitanate hydrate (H2Ti8O17‚1.5H2O) with small surface area can be prepared from the layered tetratitanate. It has very low photocatalytic activity due to the following material characteristics: impurity, low surface area, low crystallinity, and unstable structure. In this study, we prepared novel nanostructured porous titanatebased fibers of self-supported radially aligned H2Ti8O17‚ 1.5H2O nanorods from layered tetratitanate by novel atmospheric solvothermal reaction in glycerine at 150-250 °C. This method has a low production cost when combined with the low-cost calcination synthesis of K2Ti4O9 and controllable ion-exchange synthesis of H2Ti4O9‚1.2H2O (41-46). The nanostructured porous H2Ti8O17‚1.5H2O fibers prepared in this work are of highly photooxidative activity, stable phase up to 350 °C, large surface area of 127 m2 g-1, and surface modification with glycerine. They also have large size in micrometers, pH-controllable suspension-sedimentation VOL. 38, NO. 9, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Chemical Component and Material Characteristics of Samples samples
component
K2Ti4O9 A B C1 C2 D E1 E2 P-25
K2Ti4O9 H2Ti4O9‚1.2H2O H2Ti4O9‚1.2H2O H2Ti8O17‚1.5H2O H2Ti8O17‚1.5H2O H2Ti8O17‚1.5H2O anatase anatase anatase(70%), rutile(30%)
synthesis condition media T (°C) air glycerine air glycerine glycerine
60 60 150 150 250
glycerine
150
BET surface area (m2 g-1)
microporea ( B (0.0108) > C1 (0.0085) > A (0.0054) The k value sequence of P-25 (0.0940) > E1 (0.0308) > C1 (0.0085) > A (0.0054) indicates that the photooxidative activities of all samples are directly related to their basic titania matrixes as well as to tunnel-like arrangements of [Ti4O9]2- sheets of H2Ti8O17‚1.5H2O, layered [Ti4O9]2- sheets of H2Ti4O9‚1.2H2O, anatase, and a mixture of anatase and rutile of P-25. Usually, P-25 has relatively high photooxidation activity as a standard photocatalyst. The photooxidative activity of the basic titania matrix of H2Ti8O17‚1.5H2O with tunnel crystal structure is higher than that of H2Ti4O9‚1.2H2O with layered crystal structure but lower than that of anatase. The k value sequence of E1 (0.0308) > E2 (0.0169) indicates that photocatalytic activity of anatase cannot be improved by thermal treatment in glycerine although the k value sequence of D (0.3211) > C2 (0.0945) > C1 (0.0085) • P-25 (0.0940) > B (0.0108) > A (0.0054) indicates that photocatalytic activities of both H2Ti4O9‚1.2H2O and H2Ti8O17‚1.5H2O can be improved by the solvothermal reactions in glycerine. After the solvothermal reactions in glycerine at T > 150 °C, the nanostructured porous H2Ti8O17‚1.5H2O fibers show highly photooxidative activity equal to that of P-25 (for sample C2, prepared at 150 °C) or superior to that of P-25 (for sample D, prepared at 250 °C). This is due to the increased crystallinity of H2Ti8O17‚1.5H2O. As a result, the solvothermal reaction in glycerine is quite beneficial for improving the photooxidative activity of materials consisting of basic titania matrix of [Ti4O9]2- sheets, similar to those in H2Ti8O17‚1.5H2O and H2Ti4O9‚1.2H2O. The material characteristic plays a key role in the highly efficient photooxidation of methyl orange over the nanostructured porous H2Ti8O17‚1.5H2O fibers of sample D (see Supporting Information). The sequence of D (0.3211) > C2 (0.0945) • P-25 (0.0940) > C1 (0.0085) also indicates that the solvothermal reaction in glycerine is suitable to improve the photooxidative activity of H2Ti8O17‚1.5H2O. Although samples C1 and C2 share nearly identical material characteristics in their crystal structure, surface area, pore structure, and pore size distribution, samples C1 and C2 differ in surface adsorption components. As a result, the surface modification during the solvothermal reaction in glycerine also plays a key role in the highly efficient photooxidation of methyl orange over the nanostructured porous H2Ti8O17‚1.5H2O fibers of sample D. Influence of pH Values on both Photooxidative Activity and Dispersion-Agglomeration Behavior of H2Ti8O17‚1.5H2O Fibers in Solution. When compared with nanocrystalline TiO2, novel nanostructured porous H2Ti8O17‚1.5H2O fibers of sample D with large surface area can be prepared from the layered tetratitanate fiber in large scale at low cost, using a multi-step process involving calcination, ion exchange, and VOL. 38, NO. 9, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Influence of pH value on the photodegradation kinetic rate constant of methyl orange over nanostructured porous H2Ti8O17‚ 1.5H2O fibers of sample D. the present solvothermal reaction in glycerine. They are large in micrometer scale and can be separated easily from solution. Photooxidation of methyl orange in solution over sample D is mainly determined by both pH value and the dispersity of H2Ti8O17‚1.5H2O fibers, in addition to the photooxidative activity of photocatalyst itself. This is due to pH value influence on the surface electrochemical potential and the dispersity of H2Ti8O17‚1.5H2O fibers; the dispersity of H2Ti8O17‚1.5H2O fibers also influences surface photooxidative kinetic conditions of catalysts in solution. Figure 2 shows the influence of pH value on the kinetic rate constants of methyl orange photooxidation over nanostructured porous H2Ti8O17‚1.5H2O fibers of sample D. Photooxidative activity of sample D increases slowly from pH 2.0 to pH 4.0, increases rapidly from pH 4.0 to pH 8.0, and then decreases again from pH 9.0 to pH 11.0. Sample D maintains the highest photooxidative activity at pH 8.0-9.0 and relatively high photooxidation activity at pH 6.0-8.0 and pH 9.0-11.0. The pH values affect photocatalytic rates by shifting the electrochemical potential of the surface of nanostructured porous H2Ti8O17‚1.5H2O fibers. This is similar to the electrochemical potential at the TiO2 surface (84). The potential of a semiconductor in contact with aqueous media (i.e., the positions of the band edges) is shifted by -59 mV per pH unit at 298 K over a wide pH range. This Nernstian behavior has been observed for TiO2 electrodes and colloidal particles. Thus, the redox potential of conduction band electrons becomes more negative at higher pH, increasing the driving force for O2 reduction with conduction band electrons to form O2•- as well as increasing the rate of this reduction reaction at the TiO2-solution interfaces, thus shortening the lifetimes of conduction band electrons. Most experiments are carried out at pH 7 or lower, with the lowest step of O2 reduction being incidental to conditions employed (4-8, 1216, 75-77) rather than being a condition intrinsic to TiO2 (85, 86). Here, the photooxidative activity of H2Ti8O17‚1.5H2O fibers at pH 6.0 is not the highest, as shown in Figure 2. However, the photooxidative activity of H2Ti8O17‚1.5H2O fibers at pH 6.0 still is relatively high. Further, H2Ti8O17‚1.5H2O fibers have good self-dispersion behavior at pH 6.0, as explained in the following discussion. Figure 3 shows optical microscope images of heterogeneous reaction solutions containing 0.5 g L-1 H2Ti8O17‚1.5H2O fibers of sample D at pH 2.0-11.0, demonstrating that the dispersion-agglomeration behavior of H2Ti8O17‚1.5H2O fibers depends on pH values of the solution at room temperature. The agglomeration of H2Ti8O17‚1.5H2O fibers becomes more serious as pH values increase from 2.0 to 5.0, and at pH 6.0 the H2Ti8O17‚1.5H2O fibers become well dispersed in the 2732
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FIGURE 3. Optical microscope images showing the dispersion and agglomeration behavior of fibers in solution at pH values between 2.0 and 11.0 at room temperature. The fiber content in solution was 0.5 g L-1, and the pH values were adjusted by adding concentrated HNO3 or NaOH solutions. solution. Similarly, the agglomeration of H2Ti8O17‚1.5H2O fibers becomes more serious as pH values increase from 7.0 to 11.0. The weak agglomeration of H2Ti8O17‚1.5H2O fibers occurs at pH from 2.0 to 3.0 and from 7.0 to 9.0, with a serious agglomeration of H2Ti8O17‚1.5H2O fibers occurring at pH 5.0, 10.0, and 11.0. The monodispersed H2Ti8O17‚1.5H2O fiber aqueous solution occurs at pH 6.0. Figure 4 shows the sedimentation curves of H2Ti8O17‚1.5H2O fibers of sample D in aqueous solution at pH values of 2.0-11.0, also indicating a dispersion and agglomeration behavior of H2Ti8O17‚1.5H2O fibers that depends on pH values of solution at room temperature. The sequence of sedimentation time for H2-
FIGURE 5. Concentration percent of photodegraded methyl orange [with (C0 - C)/C0, %; C0 and C as the methyl orange initial and residual concentrations, respectively] in solution over time of photodegradation of sample D for three times. FIGURE 4. Sedimentation curves of multi-scale, nanostructured porous H2Ti8O17‚1.5H2O fibers in aqueous solution at pH values from 2.0 to 11.0. The fiber content in solution was 0.5 g L-1, and pH values were adjusted by adding concentrated HNO3 or NaOH solutions.
TABLE 2. Relation of Weight Percent of H2Ti8O17‚1.5H2O Fibers Remaining in Solution and pH Values at Fixed Sedimentation Time sedimentation time (min) 20 40 80
pH values corresponding to different wt % of H2Ti8O17‚1.5H2O fibers remaining in solution >70%
70-65% 65-60% 60-55% 55-40%
2, 3, 6, 7 2, 3, 6 2, 3
7 6
tpH 6 > tpH 7 > (tpH 4 = tpH 8 = tpH 9) > tpH 5 = tpH 10 > tpH 11 Table 2 shows the relationship of weight percent for H2Ti8O17‚1.5H2O fibers remaining in solution to pH values at fixed sedimentation time. Over 70% of the total H2Ti8O17‚ 1.5H2O fibers remain in solution at pH 2.0-3.0 and 6.0-7.0 within 80 min. Furthermore, the photodegradation of methyl orange can be completed within 30 min (see Figure 1), and H2Ti8O17‚1.5H2O fibers maintain highly photocatalytic activity at pH 6.0-11.0 (see Figure 2). As a result, H2Ti8O17‚1.5H2O fibers are suspended in solution at pH 6.0-7.0 stably by themselves before the total photodegradation of methyl orange. If the pH value extends to between 8.0 and 9.0, then over 40% of the total H2Ti8O17‚1.5H2O fibers remain in solution within 80 min, also ensuring the total photodegradation of methyl orange before the sedimentation of entire H2Ti8O17‚ 1.5H2O fibers. Around 45% and 30% of the total H2Ti8O17‚ 1.5H2O fibers can remain in solution at pH 5.0 and 10.0 within 20 and 40 min, respectively. Because the photodegradation kinetic rate of methyl orange over sample D at pH 10.0 is 1.5 times faster than that at pH 6.0, all methyl orange can be nearly photodegraded at pH 10.0 within 20 min before the sedimentation of the entire H2Ti8O17‚1.5H2O fibers. Photodegradation of methyl orange over H2Ti8O17‚1.5H2O fibers of sample D is completed within 20 min at wide pH values ranging from 6.0 to 10.0, and most of the H2Ti8O17‚1.5H2O fibers are suspended in solution at the same time. At pH
11.0, both the fast sedimentation and the highly efficient photooxidation of methyl orange over nanostructured porous H2Ti8O17‚1.5H2O fibers of sample D occur. Prospect of Low-Cost Large-Scale Controllable Photocatalysis Technique Based on H2Ti8O17‚1.5H2O Fibers. For practical photocatalysis applications, in addition to the requirement of maintaining a low production cost for photocatalysts, the operating process of photocatalytic reaction also should be easy, low cost, and cyclic, and the photocatalysts should be reused. On the basis of the cases in which the porous H2Ti8O17‚1.5H2O fibers of sample D are of both highly photocatalytic activity and controllable dispersion-agglomeration property, depending on pH values of the solution, we propose the following design idea of the liquid-phase photocatalytic technique of the nanostructured porous H2Ti8O17‚1.5H2O fibers. For the wasterwater with pH values ranging from 6.0 to 10.0, photocatalysis reactions over H2Ti8O17‚1.5H2O fibers are operated under conditions in which H2Ti8O17‚1.5H2O fibers disperse well in solution or waste chemicals totally decompose before H2Ti8O17‚1.5H2O fibers entirely deposit on the bottom. If a certain amount of H2Ti8O17‚1.5H2O fibers still remains in solution after the total photocatalytic degradation of waste chemicals is completed, the pH value of the solution could be adjusted to the closest pH value at which the remaining H2Ti8O17‚1.5H2O fibers in solution are deposited quickly on the bottom. For wastewater with pH value of 11.0, waste chemicals are photodegraded quickly and H2Ti8O17‚1.5H2O fibers are deposited on the reactor bottom at the same time. For the wasterwater with pH value of >11.0 or
