Microwave-Specific Effects in Various TiO2 Specimens. Dielectric

Mar 18, 2009 - Chiba-ken 278-8510, Japan, Department of Materials and Life Sciences, ... Organica, UniVersita di PaVia, Via Taramelli 10, PaVia 27100,...
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J. Phys. Chem. C 2009, 113, 5649–5657

5649

Microwave-Specific Effects in Various TiO2 Specimens. Dielectric Properties and Degradation of 4-Chlorophenol Satoshi Horikoshi,*,†,‡ Futoshi Sakai,‡ Masatsugu Kajitani,‡ Masahiko Abe,† Alexei V. Emeline,§ and Nick Serpone*,| Research Institute for Science and Technology, Tokyo UniVersity of Science, 2641 Yamazaki, Noda-shi, Chiba-ken 278-8510, Japan, Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia UniVersity, 7-1; Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan, V.A. Fock Research Institute of Physics, St. Petersburg State UniVersity, UlyanoVskaya St. 1, St. Petersburg 198504, Russia, and Gruppo Fotochimico, Dipartimento di Chimica Organica, UniVersita di PaVia, Via Taramelli 10, PaVia 27100, Italia ReceiVed: NoVember 13, 2008; ReVised Manuscript ReceiVed: February 11, 2009

Microwave specific effects on various potential TiO2 photocatalyst specimens were examined for the photodecomposition of 4-chlorophenol (4-CP) with regard to physical and chemical properties and by the characterization of dielectric properties such as the dielectric loss factor (ε′′), the dielectric constant (ε′), and the dielectric loss tangent (tan δ ) ε′′/ ε′). Dielectric properties were measured in both dispersed TiO2 particles in a 4-CP aqueous solution and in TiO2 pellets prepared from the specimens’ powders. The irradiation efficiency of the microwave for the TiO2 particles in aqueous media is discussed on the basis of the changes in the dielectric loss factor and penetration depth from the increase in the temperature of water. Changes in crystalline shape and the estimated band gap energies were analyzed before and after irradiation by the microwaves. Of particular significance, the band gap energies were estimated by an in situ observation method under UV and microwave irradiation conditions. 1. Introduction Research to enhance catalytic activity in heterogeneous media has been carried out extensively in the field of organic synthesis.1 Early fundamental research into the use of microwaves in the sintering of ceramics was reported by Shimomura and coworkers2 in the early 1970s on the microwave heating of alumina. Several research studies were reported in the late 1980s.3,4 Microwave sintering of ceramics may be a reason as to why microwaves are diverted principally to the catalyst. During these two decades, application of the microwave technology to solid materials concerned mostly the field of heterogeneous catalysis. Microwave irradiation is an attractive dielectric heating method that has led to some remarkable improvements in reaction dynamics in various organic reactions. Microwave heating effects in such reactions have been delineated from conventional heating by considerations that a catalyzed reaction may be influenced by temperature gradients or by hot-spots created within the catalyst beds.5-7 However, other studies have argued for some special (i.e., nonthermal) microwave effects in the enhancement of reaction dynamics. Thermal and specific nonthermal effects of the microwave radiation are expected to affect a variety of reactions. In this regard, we reported recently that not all heterogeneous Ni-based catalysts were particularly made more active by microwave dielectric heating compared to conventional heating under otherwise identical temperature conditions.8 This notwithstanding, however, some environmental remediation studies have shown that the TiO2-assisted degrada* To whom correspondence should be addressed. E-mail: horikosi@ rs.noda.tus.ac.jp (S.H.), [email protected] (N.S.). † Tokyo University of Science. ‡ Sophia University. § St. Petersburg State University. | Universita di Pavia.

tion of contaminants can be accelerated by simultaneous irradiation of the metal-oxide dispersions with both microwaves and UV light.9 Although most degradations are not enhanced by conventional heating or at identical temperature conditions with the use of microwave radiation, degradation processes have been shown to be accelerated further in a microwave radiation field if the processes were carried out at ambient temperatures (silicone oil bath coolant maintained at -20 °C).10 The microwave-assisted photoassisted degradation of 4-chlorophenol (4-CP) in aqueous media has been reported and a degradation pathway proposed from a comparison between the dynamics of the microwave-assisted protocol and the analogous method in which the heat was supplied by conventional methods.11 Other microwave-assisted photodegradation studies of various model compounds such as bisphenol A,12 of substances containing various functional groups,13 as well as a dye pollutant14 examined potential mechanistic details. The present article examines the TiO2-photoassisted degradations of the otherwise extensively studied 4-chlorophenol in the presence of various TiO2 specimens of different origins so as to investigate microwave thermal and nonthermal effects using the advanced oxidation process (AOP) of the test pollutant as induced further by microwave irradiation. The results are correlated with the chemical and physical properties of the TiO2 samples. 2. Experimental Section 2.1. Chemical Reagents. High-purity grade 4-chlorophenol was purchased from Tokyo Kasei Kogyo Co. Ltd. The P-25 titanium dioxide specimen was supplied by Degussa, whereas the Hombikat UV-100 TiO2 was obtained from Sachtleben Chemie (Germany). The TiO2/Al2O3 (1.5 wt %) powder was a gift from a cosmetic company that uses the material as a sunscreen active agent. Other commercial titanium dioxide

10.1021/jp810002z CCC: $40.75  2009 American Chemical Society Published on Web 03/18/2009

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Figure 2. Temperature-time profiles on heating aqueous TiO2 dispersions containing a 4-CP solution when using the TiO2/UV/MW, TiO2/UV/CH, and TiO2/UV protocols.

Figure 1. Experimental setup used in the photoassisted decomposition of 4-CP in aqueous powdered dispersions using an integrated UV-vis source and a source of MW radiation, (a) a real photograph and (b) a sketch of the setup.

specimens (Anatase and Rutile) were from Wako Pure Chemicals Co. Ltd. The commercial anatase TiO2 powder and rutile TiO2 powder were mixed in a ratio of 8:2 (MAR) so as to mimic the mixture ratio inherent in the P-25 TiO2 specimen. 2.2. Experimental Setup for the Influence of Microwave Irradiation on the TiO2 Specimens. Continuous microwave irradiation was achieved using a 2.45 GHz microwave generator (maximal power, 800 W), a power monitor, and an isolator (air cooling device) fabricated by the Hitachi Kyowa Engineering Co. Ltd. (Figure 1). The 30 W continuous microwaves emitted from the magnetron of a microwave source were measured using a power monitor. An aqueous 4-CP solution (30 mL, 0.025 mM) containing the metal-oxide particles (loading, 60 mg) was introduced into a closed high-pressure 150 mL Pyrex glass cylindrical reactor (Taiatsu Techno Co.; size, 160 mm (H) × 37 mm (i.d.); maximal pressure, 1 MPa) from the top side and subsequently irradiated with a Toshiba 75 W high-pressure Hg lamp (irradiance ca. 0.1 mW cm-2) through a fiber optic. The solution temperature was measured with a K-type thermocouple. The reactor was sealed with two Byton O-rings and a stainless steel cap. A pressure gauge and a release bulb were connected to the cover of the reactor. The reaction mixture was continually stirred magnetically under batch and reflux conditions. We also confirmed that the increase in the rate of temperature rise for pure water under microwave irradiation was not influenced by the presence of the magnetic stirring bar. 2.3. Experimental Degradation Methodologies. Four different methodologies were examined to achieve the decomposition of the 4-chlorophenol sample. The first was the photoassisted degradation under UV light and microwave irradiation (TiO2/UV/MW). The second method entailed the photodegradation of 4-CP by UV irradiation alone (TiO2/UV), whereas the third method involved the thermally assisted photodegradation of the dispersions with UV light and externally applied

conventional heat (TiO2/UV/CH). For the latter method, the external heat was supplied by coating one part of the cylindrical photoreactor with a thin metallic film on one side and at the bottom of the reactor. The uncoated side was used to permit UV irradiation. The rate of increase of temperature (error e (1 °C) and the pressure in the TiO2/UV/CH method were maintained at levels otherwise identical to those used for the TiO2/UV/MW method. The fourth and last method involved irradiation with microwaves only (MW). The temperature profiles of the aqueous dispersions subjected to the TiO2/UV/ MW, TiO2/UV/CH, and TiO2/UV methods are reported in Figure 2. No differences were observed in the temperature profiles using either microwave or conventional heating. 2.4. Analytical Methods. The time profiles of the disappearance of 4-CP during the degradation process were obtained by monitoring the concentration changes using a JASCO liquid chromatograph (HPLC) equipped with a JASCO UV-2070 UV-vis diode array, a multiwavelength detector, and a JASCO Crestpak C-18S column. The eluent consisted of a mixed solution of methanol and H2O (1:2, v/v ratio). The number of • OH radicals photogenerated in UV- and microwave-irradiated TiO2 aqueous suspensions were determined relative to a Mn2+ standard by ESR spectroscopy using the DMPO (5,5-dimethyl1-pyrrolidine-N-oxide) spin-trap by a procedure reported earlier.15 The UV-vis diffuse reflectance spectra of the TiO2 (P-25) powder were observed by an in situ observation technique (Figure 3). The quartz cell containing the P-25 powder was located at the window of an integrating sphere unit (JASCO ISN-470 unit with both direct and diffuse light system) attached to a JASCO V-570 UV-vis spectrophotometer. The microwave power was measured with a power meter connected between the N-type connector and the microwave coaxial cable from the microwave generator. The actual useful power of the microwave irradiation was 12 W. The samples were irradiated with microwaves from a monopole antenna. Both the antenna and the quartz cell were setup in a metallic box so as to prevent leakage of the microwave radiation. The temperature of the P-25 powder increased to 60 °C under microwave irradiation for about 5 min. 2.5. Physical and Chemical Properties of the TiO2 Specimens. The physical and chemical properties of the various TiO2 specimens are listed in Table 1. The anatase to rutile ratio of the crystalline samples, the particle sizes, the surface area, and the estimated band gap energies were analyzed with a Rigaku X-ray Rint-2100 diffraction apparatus, a Hitachi H-7650 transmission electron microscope (TEM), a Yuasa Ionics AUTOSORB-1-KR BET method, and a JASCO diffraction UV-vis spectroscopic method, respectively. The influence of the microwave effects on the various metal-oxide specimens was examined with regard to their respective physical properties.

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Figure 3. Sketch (a) and actual photograph (b) of the apparatus used for the in situ observation of the UV-vis diffuse reflectance spectra under irradiation by a microwave source.

TABLE 1: Physical Properties of Various TiO2 Specimens

TiO2

% anatase

particle size (nm)

surface area (m2g-1)

estimated band-gap energy (eV)

P-25 anatase rutile MAR UV-100 TiO2/Al2O3

82 100 0 80 100 98

33 369 263 mixture 10 43

52 10 16 mixture 323 55

3.10 3.20 2.97 3.16 3.14 3.30

3. Results and Discussion 3.1. Photoassisted Degradation of 4-CP. The temporal disappearance of 4-CP was monitored at 279 nm by HPLC techniques (aqueous solutions of 4-CP display absorption bands at 194, 223, and 279 nm). Figure 4 summarizes the temporal behavior of the various photodegradation processes that involved

Figure 4. Temporal decrease of 4-chlorophenol concentration by (1) microwave irradiation (TiO2/MW), (2) photoassisted oxidation (TiO2/ UV), (3) thermally assisted photooxidation (TiO2/UV/CH), and by an (4) integrated microwave-assisted photodegradation (TiO2/UV/MW) in aqueous TiO2 dispersions using UV absorption loss at 279 min in HPLC chromatograms.

the different titanium dioxide specimens; the relevant zero-order kinetic data for the photodegradation of 4-CP under various conditions are reported in Table 2. The time profiles of the degradation of 4-CP carried out for each specimen with the four protocols do not correlate with the crystalline phase of the samples reported in Table 1, nor is there any evident correlation with the particle size and surface area of the TiO2 specimens, with the latter two properties varying in the order UV-100 (10 nm) < P-25 (33 nm) < anatase (369 nm), and UV-100 (323 m2 g-1) > P-25 (52 m2 g-1) > anatase (10 m2 g-1), respectively. Clearly, none of the properties appear to be associated with any microwave thermal effects and/or with any microwave specific nonthermal effects. Microwave irradiation alone (MW) of 4-CP solutions in the presence of various TiO2 specimens was inconsequential in the degradation of 4-CP (parts a-f of Figure 4). Examination of the data indicates that for the degradation of 4-CP with the P-25 TiO2 sample by the TiO2/UV/MW method (part a of Figure 4) a microwave specific effect seems to operate because the analogous method but with conventional heating (TiO2/UV/CH) the rate of degradation is smaller (k ) 2.5 × 10-4 mM min-1 versus 1.7 × 10-4 mM min-1). For anatase TiO2 particles (part b of Figure 4), there are but negligible differences in the thermal component between conventional heating and microwave dielectric heating effects when using the TiO2/UV, TiO2/UV/MW, and TiO2/UV/CH methods as the kinetics are essentially identical within experimental error (k ) 1.5-1.8 × 10-4 mM min-1). Evidently, microwave irradiation of the anatase TiO2 specimen displays no particular advantage with regard to either the thermal or specific microwave effects. With rutile TiO2 (part c of Figure 4), the dynamics of photodegradation of the 4-CP system were 1 to 2 orders of magnitude slower than for the anatase and the P-25 TiO2 specimens. Clearly microwave irradiation has no identifiable effect on the rutile phase of titanium dioxide. For the mixed anatase/rutile TiO2 mixture (MAR, part d of Figure 4) the degradation tendency for each of the methods was somewhat less than for the anatase and P-25 specimens, except for the TiO2/UV/MW protocol for which the rate was 2-fold smaller (Table 2). We thus infer that much of the degradation of 4-CP was caused by the anatase component in the MAR mixed sample. The photoactivity of the UV-100 TiO2 sample (part e of Figure 4) was enhanced 2-fold under microwave irradiation (k ) 0.6 × 10-4 mM min-1 versus 1.37 × 10-4 mM min-1). However, the degradation dynamics were nearly the same within experimental error for the microwave dielectrically heated versus

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TABLE 2: Number of DMPO-•OH Spin-Adducts (Relative to a Mn2+ Standard) Produced in an Aqueous Microwave-Irradiated TiO2 Dispersion (TiO2/MW), UV-Irradiated (TiO2/UV), and a TiO2 Dispersion Simultaneously Subjected to both 2.45 GHz Microwaves and UV Radiation (TiO2/UV/MW) for Various TiO2 Specimens; the Zero-Order Rates for the Degradation of 0.025 mM 4-CP Using the Several Protocols Are also Reported relative number of DMPO-•OH spin-adducts in TiO2dispersions

samples P-25 Anatase Rutile MAR UV-100 a

MW 25 11 5 2 4

UV 182 110 110 21 45

UV/MW 259 92 76 35 51

kdega (TiO2/MW) ∼0.07 ∼0.08 ∼0.02 ∼0.05 0.21 ( 0.04

kdega (TiO2/UV)

kdega (TiO2/UV/MW)

kdega (TiO2/UV/CH)

2.5 ( 0.2 1.7 ( 0.2 0.05 ( 0.01 1.3 ( 0.04 1.37 ( 0.01

1.7 ( 0.1 1.8 ( 0.2 0.17 ( 0.02 1.25 ( 0.04 1.54 ( 0.24

1.0 ( 0.1 1.5 ( 0.1 ∼0.02 1.1 ( 0.1 0.6 ( 0.1

Zero-order rates of degradation of 4-CP given as 10-4 mM min-1. No •OH radicals were produced in H2O under any conditions.

the conventionally heated systems. Clearly, the thermal effect appears to be a significant factor in the photoactivity of the Hombicat UV-100 specimen, and (specific) nonthermal effects of the microwave radiation had no influence on this metal-oxide specimen. Some differences were observed in the disposition of the 4-CP pollutant with the alumina-coated TiO2/Al2O3 system. After 120 min of irradiation, the order of the degradation dynamics of 4-CP was TiO2/UV/CH (k ) 0.37 × 10-4 mM min-1) > TiO2/UV (k ) 0.17 × 10-4 mM min-1) > TiO2/UV/ MW (k ) 0.03 × 10-4 mM min-1). Such variations likely originate from differences in the adsorption capacity of this particular alumina-coated TiO2 specimen when subjected to the various protocols. In summary, no microwave specific effects were evident in many of the photoassisted reactions. That is, any specific microwave effect(s) attributable to some nonthermal factor appears to be limited in metal-oxide heterogeneous dispersion systems. To get a better handle on the degradation behaviors observed in Figure 4, we also determined the number of DMPO-•OH spin-adducts (Table 2) relative to a Mn2+ ion standard that were produced in microwave-irradiated (TiO2/MW) and UV-irradiated (TiO2/UV) aqueous dispersions, and for TiO2 dispersions simultaneously irradiated with both UV light and microwave radiation (TiO2/UV/MW) at the 2.45 GHz frequency for the various TiO2 specimens (note that the dispersions contained no 4-CP). Under microwave irradiation alone, the number of •OH radicals formed on the TiO2 particles was considerably less than for the other two systems by as much as a factor of 10. For both TiO2/UV and TiO2/UV/MW the largest number of •OH radicals occurred for the P-25 TiO2 and the smallest number for the mixed anatase/rutile MAR particulates, even though the ratio of anatase to rutile was identical to the anatase/rutile ratio in P-25 TiO2. Evidently, P-25 TiO2 is more than a simple mechanical mixture of anatase and rutile. A better anatase-rutile interfacial contact is likely formed during the high-temperature preparation of P-25 TiO2. The overall order of decreasing number of •OH radicals for both systems is (Table 2): P-25 . anatase ≈ rutile > UV-100 > MAR. This order correlates neither with the TiO2/UV method nor with the TiO2/UV/MW protocol in the rates of the photoassisted degradation of the 4-chlorophenol. For the latter method, the rates decrease as P-25 > anatase > UV-100 ≈ MAR . rutile. Most remarkable about the data of Table 2 is that even though the number of •OH radicals formed for the rutile system is 1-3 times smaller than for P-25 and anatase TiO2, the rates of degradation of 4-CP are nearly 50-fold and 34-fold smaller, respectively; a similar trend was observed for the TiO2/UV protocol. Another curious but no less interesting observation is that the sum of •OH radicals produced for the TiO2/MW and TiO2/UV systems is smaller (207) than the number of •OH

TABLE 3: Dielectric Loss Factor (ε′′), Dielectric Constant (ε′) and Dielectric Loss Tangent (tan δ) of 0.025 mM Aqueous 4-CP Solution/TiO2 Dispersions (200 mL) with Various TiO2 Specimens at a 2.45 GHz Microwave Frequency

samples

dielectric loss factor (ε′′)

dielectric constant (ε′)

dielectric loss tangent (tan δ ) ε′′/ ε′)

4-CP solution P-25 anatase rutile MAR UV-100

10.28 9.982 10.08 9.522 9.915 9.844

78.74 77.72 77.49 77.45 77.46 77.51

0.131 0.128 0.130 0.123 0.128 0.127

radicals (259) produced by the TiO2/UV/MW system. This suggests some sort of synergistic effect when both UV and MW radiations are brought to bear on the metal-oxide particulates. Finally, factors other than the number of •OH radicals formed impact negatively on the photodegradations with rutile TiO2. One such factor may be the differences in the extent of adsorption of the organic substrate on the rutile particle surface (also see below). 3.2. Relationship(s) between Photoactivity and Dielectric Properties of Various TiO2 Samples. The dielectric constant (ε′) and the dielectric loss tangent (tan δ ) ε′′/ε′; also referred to as the dissipation factor) for the aqueous 4-CP/TiO2 dispersions were analyzed using an Agilent Technologies HP-85070B Network Analyzer. The dielectric loss factors (ε′′) were calculated from the values of the latter two experimentally determined dielectric properties. Prior to the determination of the dielectric properties of the TiO2 samples, the methodology was tested using pure water; the result accorded with the literature. Note that all of the experiments in the determination of the dielectric properties were carried out in triplicate for all of the specimens under otherwise identical conditions the same day. The results reported in Table 3 thus represent the average of the triplicate experiments; variations in the values for any one sample were insignificant. The dielectric properties (ε′′ and ε′) for the 4-CP solution in the presence of TiO2 systems showed a slight decrease relative to those of the TiO2-free 4-CP solution, and no particularly large differences in the dielectric properties were evident for the various TiO2 dispersions. Evidently, the efficiency of absorption of the microwave radiation for these dispersions does not depend on the nature of the TiO2 samples. When microwave irradiation is carried out under identical applied microwave power conditions, the thermal factor for the bulk solution is constant. Therefore, any difference in the rate of decomposition of 4-CP between the TiO2/UV/MW and TiO2/UV/CH protocols under identical temperature conditions must originate from microwave effects other than thermal.

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TABLE 4: Dielectric Properties at 2.45 GHz for the P-25, Anatase, Rutile, and UV-100 TiO2 in Pellet Forma

specimen

dielectric loss factor (ε′′)

dielectric constant (ε′)

dielectric loss tangent (tan δ)

P-25 anatase rutile UV-100

0.181 0.142 0.317 0.150

5.626 6.525 9.550 6.728

0.0322 0.0218 0.0332 0.0223

a Values of the dielectric properties are those of the TiO2 specimens in pellet form including possibly trapped air so that the values reported here may be different from those of pure TiO2. Important are the trends observed.

Figure 6. Temperature profiles of (a) the changes in the dielectric properties ε′ and ε′′ of water, (b) the dielectric loss tangent (tan δ),and (c) the penetration depth (cm) of the microwaves into the aqueous medium (pure water). Figure 5. Temperature-time profiles of the absorption of microwave radiation by the powdered TiO2 systems packed in a tube.

Absorption of microwave radiation by TiO2 particles in the aqueous dispersions should depend on the dielectric properties of all of the systems’ components. However, the selective measurement of the permittivity of the TiO2 particle in solution is a rather formidable challenge, if not impossible. On the other hand, measurements of the dielectric loss tangent and the dielectric constant on pristine TiO2 particles become possible as such measured values represent the sum total of the TiO2 particles and air. Accordingly, the absorption efficiency of the microwave radiation by each of the TiO2 specimens was examined from two different approaches. In the first experimental attempt, we assessed the dielectric loss factor of pristine TiO2 pellets prepared using a press operated under high pressures (ca. 1000 kg cm-2). Note that the crystalline structure of the specimens in pellet form was not modified under the high pressures. The dielectric loss factors (ε′′) of the naked TiO2 pellets estimated from the measured dielectric loss tangents (tan δ) and dielectric constants (ε′) are listed in Table 4. Perusal of the data shows that the dielectric loss factor, which reflects the heating efficiency of the TiO2 pellets by the microwave radiation varied in the order: rutile > P-25 > UV-100 ≈ anatase. This order correlates with the extent of anatase/rutile (i.e., crystalline phase) content in the various specimens. Strangely, however, the dielectric constant (ε′) of the P-25 TiO2 sample, which contains ca. 18% rutile, is smaller than that of anatase, whereas the dielectric loss tangent (tan δ) of P-25 is nearly identical to that of rutile. A high loss tangent (tan δ) means that there is a large internal attenuation of the microwave radiation in the P-25 TiO2 particles. That is, the microwaves are easily confined in the bulk of the P-25 TiO2 particles. In the second experimental approach, the level of microwave absorption was monitored by the temperature increase (Figure 5) of each naked TiO2 powder (300 mg) placed in a packed tube measured at the center of the tube using an appropriate thermocouple. The temperature measurement was performed three times; variations between readings were negligible. The rates of temperature increase were P-25 (92.5 °C min-1) > rutile (58.9 °C min-1) > TiO2/Al2O3 (28.6 °C min-1) > UV-100 (6.3 °C min-1) ≈ MAR (4.7 °C min-1) ≈ anatase (2.2 °C min-1).

The efficiency of absorption of microwave radiation by the P-25 and rutile powders was greater than those of the other TiO2 powdered specimens. The rapid increase of temperature for the rutile TiO2 is reasonable because the dielectric loss factor (ε′′) is relatively high; in fact it is the highest (Table 3) compared to the loss factor of the anatase TiO2 particles. However, the dielectric loss factor (ε′′) of P-25 is much smaller than that of rutile. Clearly, the efficiency of heat generated by the absorption of microwave radiation by TiO2 must involve factors other than the crystalline structures. On the other hand, there was no difference between the dielectric loss factor and the temperature rate of increase between the UV-100 and anatase specimens as the former is also 100% anatase. Consequently, the behavior of P-25 must be seen as abnormal. The rate of microwave absorption by the bulk solutions in the presence of TiO2 particles showed no dependence on the nature of the TiO2 samples, at least at ambient temperature. Accordingly, we examined the changes of the dielectric parameters (dielectric constant, ε′, and dielectric loss factor ε′′) as a function of temperature changes of the aqueous medium (pure water) as illustrated in part a of Figure 6. The data indicate that the efficiency of absorption of microwaves by the medium decreased with microwave irradiation time because the dielectric loss factor (ε′′) decreased with the rise in temperature. The dielectric loss tangent also decreases with rise in temperature (part b of Figure 6). Consequently, because of this temperature increase the efficiency of absorption of microwaves by TiO2 particles in the dispersions should improve with an increase in the temperature of the dispersion. For low loss dielectrics (i.e., for ε′′/ε′ , 1), the depth (Dp, in cm) to which the microwave radiation penetrates the microwave absorbing media can be estimated from eq 1,16 where λo is the wavelength of the irradiation (λo(2.45 GHz) ) 0.122 m). It denotes the depth at which the power density of the microwaves is reduced to 1/e of its initial value. Part c of Figure 6 shows that the increase of temperature increases the depth of penetration of the microwaves into the bulk solution. This penetration depth increases 3.7-fold by the increase in temperature from ambient to 90 °C owing to a decrease of microwave absorption by the medium.

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Dp )

( )( ) λo √ε′ 2π ε′′

Horikoshi et al.

(1)

The incident and reflected microwave power levels were monitored by the power monitor against irradiation time. The incident power (30 W) was independent of changes in the irradiation time and temperature. By contrast, some of the reflected power did change with variations in temperature. Initially, the reflected power was 5 W, and subsequent to microwave irradiation of the aqueous TiO2 dispersion containing 4-CP the temperature increased to over 95 °C at which time the reflected power increased to ca. 6-7 W for the P-25 TiO2 system. However, this increase of 1-2 W was smaller than the change in the dielectric loss factor and penetration depth (Figure 6). Thus, the reflected microwave power increased but slightly, although microwave absorption by the dispersion decreased. Hence, we infer that the microwaves were absorbed by the TiO2 particles. Changes in the crystalline phase in an anatase TiO2 tablet (10 mm (dia.) × 8 mm (height)) under microwave irradiation were reported by Ikuma and Shigemura.17 In that study, the bulk of the TiO2 tablet changed from anatase to rutile, but no phase changes occurred at the surface of the tablet, which suggests that microwave internal heating may have caused the phase changes. Moreover, because the heat at the TiO2 surface is radiated out to the particle surroundings, temperature variations likely occurred between the bulk of the tablet and the surface. The proposed thermal conditions of a dispersed TiO2 particle in aqueous media are sketched in Scheme 1. In the initial stage, when the TiO2 particle in solution is heated by the microwave radiation, the microwaves are absorbed by the medium (part a of Scheme 1) such that the penetration of the microwaves decreases from the glass-water interface to the bulk of the medium (part b of Scheme 1), whereas the rate of microwave absorption by the TiO2 particles in the medium increases. Note that the microwaves can infiltrate the core of the TiO2 particles, and to the extent that the microwave-generated heat in the TiO2 radiates away from the surface of the particles it will be reabsorbed immediately by the aqueous medium. Accordingly, TiO2 particles having a high surface area cannot retain the thermal energy and are suitable for radiating heat to the surroundings. For some yet unknown reason, however, P-25 TiO2 particles seem to have some structural features that confine the microwaves and retain the heat. We infer that this may be one of the factors that may lead to microwave specific effects in such a heterogeneous system. For the P-25 case (part a of Figure 4), no difference was observed in the extent of decomposition of 4-CP between the TiO2/UV/MW and TiO2/UV/CH methods for a 30 min microwave irradiation period. However, differences are notable after 60 min. Such differences are significantly smaller for the other TiO2 specimens. Note that the temperature of the microwaveirradiated dispersions reached 100 °C after only 17 min (Figure 2). 3.3. Effect of Microwave Absorption by TiO2 and Possible Formation of Color Centers (P-25). A sample of the P-25 TiO2 powder was introduced into a closed quartz reactor, after which the reactor was purged with hydrogen gas and illuminated with UV light for 30 min to yield an orangish-yellow TiO2 powder. In this regard, formation of surface peroxo-TiIV complexes on the TiO2 particles also yields yellow-colored TiO2 particulates.18 However, because oxygen was removed by the purging process it is unlikely that such surface complexes formed in the present instance. Microwave irradiation of the H2-treated P-25 powder

SCHEME 1: Schematic of the Microwave Irradiation of a TiO2 Particle by a Temperature Change of the Bulk Medium: (a) Shallow Penetration of the Microwaves, (b) Deep Penetration of the Microwaves

and the untreated white powder led to an increase of temperature of both particulate systems (Figure 7) as measured with the thermocouple at the center of the reactor. The two systems clearly show different behaviors with the increase of temperature. Accordingly, microwave dielectric heating must (somehow) have a different effect on the electronic characteristics of the H2-treated and untreated TiO2 particles. One possibility is that such temperature variations may be caused by the occurrence of oxygen vacancies that cause changes to the electronic properties of the TiO2 specimens and thus to the heating efficiency of the microwave irradiation. A microwave-specific effect may be involved in TiO2-photoassisted reactions as a result of electronic features inherent to the TiO2 structure. An alternative view would suggest that the photoinduced coloration of the P-25 specimen is caused by the photoadsorption of the hydrogen molecule as demonstrated for ZrO2 by Emeline and co-workers.19 The first step in such photoadsorption implicates reaction of H2 with a surface-oxidized oxide ion (O-, eq 3) produced by the trapping of a valence band hole.

O-+ H2 f OH-+ H•

(3)

Stabilization of the hole state resulting from this hole trapping and decrease of recombination with free electrons through this hole state increases the number of electron color centers such as F and Ti3+ centers (see below). The F color centers are produced by electron trapping into anion vacancies.19 According to our earlier interpretation of the absorption spectra of several colored TiO2 systems,20 electron centers absorb the violet-blue light thereby imparting the yellow color of the samples. Moreover, some of these centers can be thermally ionized by absorption of microwave radiation leading to a greater concentration of free electrons. The remaining hydrogen atom from

Figure 7. Temperature-time profiles of untreated P-25 TiO2 and H2treated P-25 TiO2 powder subjected to microwave irradiation.

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Figure 8. (a) In situ observation of the diffuse reflectance spectra of P-25 TiO2 under microwave irradiation for 0 (dashed blue line) and 10 min (red solid line) in air, (b) absorption spectrum as 102∆R () R10 R0) of the P-25 TiO2 after 10 min of MW irradiation (error ca. ( 0.001 in ∆R).

reaction 3 subsequently interacts with surface oxygens (reaction 4) to yield additional free electrons and thus additional electron color centers on trapping the electron(s) by the anion vacancies or by the Ti4+ species.21

H•+ O2- f OH-+ e-

(4)

3.4. Microwave Radiation Effects on the P-25 TiO2 Structure. Changes in the physical/chemical properties of the P-25 TiO2 specimen that may take place from the continuous microwave irradiation of a tube packed with naked P-25 particles for 0, 10, and 60 min were examined by X-ray diffraction (XRD) and by in situ UV-vis diffuse reflectance spectroscopy. No changes in the ratio of anatase/rutile phases were observed by XRD at all irradiation times following microwave dielectric heating in a narrow range limited to hot spots. In addition, no changes in the band gap of the TiO2 specimen were evident when monitoring the UV-vis spectra at 1 min intervals from 0 to 10 min (part a of Figure 8), consistent with the XRD results. Consequently, any difference in properties of the specimens occurring from temperature variations must originate from changes in the optoelectronic properties. Part b of Figure 8 illustrates the absorption spectrum of the P-25 TiO2 specimen after 10 min of microwave irradiation calculated from the data of part a of Figure 8. Two bands are clearly discernible, one at ca. 400 nm (3.09 eV) and the other at ca. 365 nm (3.39 eV) together with broad absorption beyond 420 nm (hV < 2.94 eV). The bands at 400 and 365 nm are reminiscent of the spectral variations of the quantum yield (Φ) of the photostimulated adsorption of O2 by P-25 TiO2 within the spectral range corresponding to the fundamental absorption, which displayed the typical double-band-like structure at ca. 385 nm (3.20 eV) and ∼340 nm (ca. 3.65 eV)22 in different heterogeneous systems (liquid-solid and gas-solid) involving pristine (undoped) TiO2 samples.22-24 Possible origins of such variations have been discussed elsewhere.20,25 Nonetheless, germane to this discussion, a theoretical examination by Lu and co-workers26 of the spectral features at 2.9 and 1.7 eV in the absorption spectrum of a reduced rutile TiO2 single crystal inferred that these features involve the defect center (Ti3+-VO) with the oxygen vacancy VO located nearest the site of the central Ti3+ ion. Along these lines, Sekiya et al.27 reported various color changes when a single TiO2 anatase crystal was heat-treated in the presence of H2 and O2, with the as-grown crystal changing color from pale blue f dark blue f dark green f yellow f colorless on annealing the TiO2 first in a H2 atmosphere and subsequently in the presence of oxygen. The polarized crystal spectra reported by Sekiya et al.27,28 revealed a band at ca. 2.9 eV and a very broad band around 1.8 eV that disappeared upon annealing in O2. Related EPR spectra of these

multicolored TiO2 crystalline specimens (pale blue f dark blue/ dark green) displayed only a single peak attributed to the trapped electron in Ti3+ (pale-blue crystal) and to the presence of both Ti3+ and (Ti3+-VO) centers in the dark-blue/dark-green specimens. The diamagnetic yellow TiO2 crystal contained F centers.27 The studies of Sekiya and co-workers27 contrasted other EPR studies29 on powdered TiO2 nanoparticles, which displayed signals assigned to electrons trapped in oxygen (anion) vacancies (i.e., to an F+ center). A close examination of several other studies20,30 in which absorption band assignments correlated with EPR results suggested that the bands in the ranges 2.9-3.0 and 2.4-2.6 eV are likely due to Jahn-Teller split 2T2 f 2E transitions of Ti3+ centers in doped TiO2, whereas the band in the range 1.7-2.1 eV presumably implicates F+ centers. Although assignments of optical UV-vis absorption bands in TiO2 specimens remain a daunting task, they do provide a starting point with which the microwave-induced changes in the P-25 TiO2 specimen could (in principle) be described. To the extent that microwave dielectric heating of a specimen occurs inside-out (Scheme 1), that is from the lattice bulk to the surface in contrast to conventional heating, which heats up the specimen from the surface to the bulk, it is not unreasonable to suppose that additional defect sites such as anion and cation vacancies (Va and Vc, respectively) are produced in the lattice and subsequently at the surface. Absorption of microwave energy can also lead to the formation31 of electron/hole pairs so that eqs 5-9 are also likely to occur in air.22

Ti4++ e- f Ti3+

(5)

Va+ e- f F+

(6)

Ti3+ + 1/2O2 f Ti4++ Oads

(7)

F+ + 1/2O2 f Va+ Oads

(8)

Vc+ h+ f Vs(Os)

(9)

where Vs is a hole color center at the surface and Oads- denote surface-trapped holes. One of the factors that may determine the ability of metal-oxide solids to absorb microwave radiation is the presence of free charge carriers: electrons in the case of the n-type TiO2 semiconductor. As a result of the specific coexistence of anatase and rutile phases in P-25 TiO2, photogenerated free electrons on the anatase component migrate to the rutile domain because of differences in the positions of the lowest energy levels of their respective conduction bands. Accordingly, we expect to see an accumulation of free electrons in the rutile component of the P-25 particles. In other words, in rutile we expect to see additional free electrons that were transferred from anatase in addition to those produced from UV excitation of the rutile phase per se. Therefore, the local concentration of free electrons in the rutile component of P-25 will be much greater than that in either of the separate rutile or anatase particles of other TiO2 specimens. This may be one of the reasons for the greater heating efficiency of P-25 under the microwave radiation (Figure 5) and for the substantively greater number of •OH radicals produced on P-25 relative to the other samples (Table 2) as a result of a process reminiscent of the interparticle electron transfer process.32

5656 J. Phys. Chem. C, Vol. 113, No. 14, 2009 Concluding Remarks The P-25 TiO2 consisting of well interwoven anatase (ca.80%) and rutile (ca. 20%) crystallite forms is a well-known potential photocatalyst that has manifested significant photoactivity in a multitude of photoreductive and photooxidative processes. Using various approaches, the present study has examined the effect(s) that microwave radiation may have on the P-25 material in comparison to other potential TiO2 photocatalyst systems. Germane to the present discussion, Ohno and co-workers33 examined the structural features of P-25 particles by TEM and electron diffraction methods, and concluded that P-25 TiO2 consists of a 3:1 anatase/rutile ratio, confirming XRD results reported earlier by several workers. However, the most interesting result of the study33 was that P-25 TiO2 particles consist of agglomerates of anatase and rutile elementary particles with average agglomerate sizes of ca. 85 and 25 nm, respectively. Under practical operational conditions of photoassisted reactions, the agglomerates apparently disintegrate with the anatase and rutile particles remaining in contact, however. This mixed anatase/rutile structure must be the key to the high photoactivity of the P-25 powder due to a synergistic effect between the anatase and rutile elementary particles. In this regard, Hurum and co-workers34 probed the charge separation characteristics of the highly active mixed-phase Degussa P-25 titania by EPR spectroscopy and deduced that the charges produced on the rutile phase by Visible light irradiation are stabilized through electron transfer to lower energy lattice trapping sites in the anatase phase. However, this may be muted by the concomitant formation of electron-hole pairs generated in both anatase and rutile when irradiated with photons commensurate with their bandgap energies of ca. 3.2 eV and ca. 3.0 eV, respectively. The relative photochemical inactivity of the pure-phase rutile, as seen in Figure 4, is likely due, at least in part, to a rapid charge carrier recombination in the semiconductor.34 The EPR results34 confirmed the affirmations of Ohno et al.33 in that within the mixed-phase P-25 titania there is a morphology of nanoclusters that contain atypically small rutile crystallites interwoven with anatase crystallites. The EPR studies also inferred that the rutile crystallites act as antenna extending the photoactivity of P-25 to longer wavelengths and that the anatase/rutile crystallite structural arrangement in P-25 titania creates potential catalytically reactive hot spots at the anatase/rutile interface.34 In this regard, it is relevant to note that the mixed anatase/rutile MAR specimen with a composition similar to that of P-25 TiO2 is less photoactive than the latter. The anomaly of P-25 TiO2 is then likely due to a better anatase/rutile interfacial construct that facilitates the charge carrier transfer event. The combination of microwave irradiation and UV light irradiation might allow for the choice of an optimal TiO2 product to carry out efficient microwave-assisted/photoassisted reactions. The particle size and the surface area of the metal oxide specimens were not principal factors for the enhancement by the microwave effect(s). In this regard, the absorption of microwaves by the TiO2 particles is improved by an increase in the temperature of the dispersions, with the P-25 specimen being the most efficient. In conclusion, therefore, the different electronic and structural characteristics of the P-25 specimen may be the key(s) to elucidate the elusive microwave specific effects. Future studies will examine the degradation of another halogenated phenol along with phenol to ascertain whether the 4-chlorophenol examined herein degraded solely by a photoreductive process upon electron attachment of photogenerated free electrons onto the phenyl ring of the 4-CP substrate.

Horikoshi et al. Acknowledgment. Financial support to S.H. from the Steel Industry Foundation for the Advancement of Environmental Protection is most gratefully appreciated. One of us (N.S.) thanks Prof. Albini of the University of Pavia for the constant gracious hospitality during the winter semesters in his laboratory (since 2002); N.S. is also particularly grateful to Prof. Abe and his group for a Visiting Professorship during July through August, 2008, and for their warm welcome to the Tokyo University of Science. We also greatly appreciate the technical assistance by the personnel of the Hitachi Kyowa Engineering Co. Ltd. References and Notes (1) Ha´jek, M. MicrowaVes in Organic Synthesis; Loupy, A., Ed.; WileyVCH Verlag: Weinheim, Germany, 2002; Chapter 14, pp 345-378. (2) Shimomura, K.; Miyazaki, T.; Tutiya, A. Annual Report, 4811 RIKEN: Japan, 1972. (3) (a) Sutton, W. H. Microwave Processing of Ceramics s An Overview. Proceedings of the Materials Research Society Symposium, Microwave Processing of Materials III, San Francisco, CA, April 1992, R. L. Beatty, Ed.; 1992; Vol. 269, pp 3-20. (4) Sutton, H. Am. Ceram. Soc. Bull. 1989, 68, 376. (5) Zhang, X.; Hayward, D. O. Inorg. Chim. Acta 2006, 359, 3421. (6) Stuerga, D.; Gaillard, P. Tetrahedron 1996, 52, 5505. (7) Cecilia, R.; Kunz, U.; Turek, T. Chem. Eng. Process. 2007, 46, 870. (8) Horikoshi, S.; Tsuzuki, J.; Sakai, F.; Kajitani, M.; Serpone, N. Chem. Commun. 2008, 4501. (9) See, e.g. (a; Horikoshi, S.; Hidaka, H.; Serpone, N. EnViron. Sci. Technol. 2002, 36, 1357. (b) Horikoshi, S.; Kajitani, M.; Hidaka, H.; Serpone, N. J. Photochem. Photobiol., A 2008, 196, 159. (c) Horikoshi, S.; Kajitani, M.; Horikoshi, N.; Dillert, R.; Bahnemann, D. W. J. Photochem. Photobiol., A 2008, 193, 284. (10) Horikoshi, S.; Kajitani, M.; Serpone, N. J. Photochem. Photobiol., A 2007, 188, 1. (11) Horikoshi, S.; Tokunaga, A.; Watanabe, N.; Hidaka, H.; Serpone, N. J. Photochem. Photobiol., A 2006, 177, 129. (12) Horikoshi, S.; Tokunaga, A.; Hidaka, H.; Serpone, N. J. Photochem. Photobiol., A 2004, 162, 33. (13) Horikoshi, S.; Hojo, F.; Hidaka, H.; Serpone, N. EnViron. Sci. Technol. 2004, 38, 2198. (14) Horikoshi, S.; Saitou, A.; Hidaka, H.; Serpone, N. EnViron. Sci. Technol. 2003, 37, 5813. (15) Horikoshi, S.; Hidaka, H.; Serpone, N. Chem. Phys. Lett. 2003, 376, 475. (16) (a) Bogdal, D. Microwave-Assisted Organic Synthesis-One hundred reaction procedures, Tetrahedron Organic Chemistry Series; Elsevier: New York, 2005; Vol. 25, pp 9-11. (b) Horikoshi, S.; Sakai, F.; Kajitani, M.; Abe, M.; Serpone, N. Chem. Phys. Lett. 2009, 470, 304. (17) Ikuma, Y.; Shigemura, T. J. Ceram. Soc. Jpn. 1993, 101, 900. (18) Wu, T.; Liu, G.; Zhao, J.; Hidaka, H.; Serpone, N. New J. Chem. 2000, 24, 93. (19) (a) Emeline, A. V.; Kuzmin, G. N.; Basov, L. L.; Serpone, N. J. Photochem. Photobiol., A 2005, 174, 214. (b) Emeline, A. V.; Panasuk, A. V.; Sheremetyeva, N.; Serpone, N. J. Phys. Chem. B 2005, 109, 2785. (c) Emeline, A. V.; Kataeva, G. V.; Litke, A. S.; Rudakova, A. V.; Ryabchuk, V. K.; Serpone, N. Langmuir 1998, 14, 5011. (20) Serpone, N. J. Phys. Chem. B 2006, 110, 24287. (21) Andreev, N. S.; Emeline, A. V.; Khudnev, V. A.; Polikhova, S. A.; Ryabchuk, V. K.; Serpone, N. Chem. Phys. Lett. 2000, 325, 288. (22) Emeline, A. V.; Smirnova, L. G.; Kuzmin, G. N.; Basov, L. L.; Serpone, N. J. Photochem. Photobiol., A 2002, 148, 99. (23) Emeline, A. V.; Kuzmin, G. N.; Purevdorj, D.; Ryabchuk, V. K.; Serpone, N. J. Phys. Chem. B 2000, 104, 2989. (24) Emeline, A. V.; Salinaro, A.; Serpone, N. J. Phys. Chem. B 2000, 104, 11202. (25) Kuznetsov, V. N.; Serpone, N. J. Phys. Chem. B 2006, 110, 25203. (26) Lu, T.-C.; Wu, S.-Y.; Lin, L.-B.; Zheng, W.-C. Physica B 2001, 304, 147. (27) Sekiya, T.; Yagisawa, T.; Kamiya, N.; Mulmi, D. D.; Kurita, S.; Murakami, Y.; Kodaira, T. J. Phys. Soc. Jpn. 2004, 73, 703. (28) Sekiya, T.; Tasaki, M.; Wakabayashi, K.; Kurita, S. J. Lumin. 2004, 108, 69. (29) (a) Sun, Y.; Egawa, T.; Shao, C.; Zhang, L.; Yao, X. J. Cryst. Growth 2004, 268, 118. (b) Sun, Y.; Egawa, T.; Shao, C.; Zhang, L.; Yao, X. J. Phys. Chem. Solids 2004, 75, 1793. (c) Nakamura, I.; Negishi, N.; Kutsuma, S.; Ihara, T.; Sugihara, S.; Takeuchi, K. J. Mol. Catal. A: Chem. 2000, 161, 205. (d) Ihara, T.; Miyoshi, M.; Ando, M.; Sugihara, S.; Iriyama, Y. J. Mater. Sci. 2001, 36, 4201. (e) Serwicka, E.; Schlierkamp, M. W.; Schindler, R. N. Z. Naturforsch. A 1981, 36, 226. (f) Serwicka, E. Colloids

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