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Electron-Transfer Reaction of Oxygen Species on TiO2 Nanoparticles Induced by Sub-band-gap Illumination Kenji Komaguchi,* Takanori Maruoka, Haruka Nakano, Ichiro Imae, Yousuke Ooyama, and Yutaka Harima* Department of Applied Chemistry, Graduate School of Engineering, Hiroshima UniVersity, Higashi-Hiroshima 739-8527, Japan ReceiVed: October 9, 2009; ReVised Manuscript ReceiVed: NoVember 27, 2009
Molecular oxygen species formed on the surface of partially reduced TiO2 (rutile) nanoparticles have been studied by in situ electron spin resonance (ESR) and diffuse-reflectance spectroscopies. ESR signals due to O2- (gz ) 2.020) and Ti3+ appeared upon visible-light illumination at 77 K and vanished by raising the temperature in the dark. The numbers of O2- and Ti3+ radicals formed by sub-band-gap illumination were equal, suggesting a reversible electron transfer between peroxo O22- species and the adjacent Ti4+ ion at an oxygen vacancy site on the TiO2 surface: Ti4+ · · · O22- · · · Ti4+ f Ti3+ + O2- · · · Ti4+ (forward reaction). The ESR intensity was saturated by a prolonged illumination and a surface coverage of O2 molecules adsorbed at the oxygen vacancy site was evaluated as 1.3 × 1013 sites cm-2. The spectral response for the generation rate of O2- exhibited a broad peak at around 480 nm, in agreement with the absorption band observed by the diffuse-reflectance measurements. It was concluded that F-type color centers generated in subsurface layers of TiO2 absorb the visible light to induce indirectly the electron-transfer reaction from O22- to Ti4+ at the surface oxygen vacancy site. 1. Introduction Since TiO2 is a metal-oxide semiconductor having unique and usable features of low cost, nontoxicity, and high stability against UV irradiation, a great deal of effort has been devoted to the study on TiO2 applicable to photocatalysts, dye-sensitized solar cell, and so on.1-7 The TiO2 photocatalysts can decompose organic pollutants in water or air by the band gap excitation of TiO2 with sun light. At present, the TiO2 photocatalysts have been already put to a practical use for air purification and selfcleaning of restroom tiles and exterior walls of housing.3,8,9 One of the current interests in the TiO2 photocatalyst is to design and fabricate highly efficient photocatalysts operative under visible-light illumination.10-13 The photocatalytic reactions on TiO2 are initiated by electron-hole pairs generated by the band gap irradiation, followed by multistep redox processes involving a variety of reactive intermediates on the TiO2 surface. The photogenerated electrons and holes which escaped from the geminate recombination are separately trapped on/in TiO2; otherwise they are captured by adsorbed species and participate in the subsequent reactions. Oxygen species such as O2-, O3-, O22-, and O- are well-known to be formed by capturing the photogenerated electrons and holes and to play a crucial role in TiO2 photocatalytic processes.14-24 Among these oxygen species, the O2species has been investigated most intensively, because it is a key reduced product of molecular oxygen on the TiO2 surface and participates in both the reductive and oxidative processes in the TiO2 photocatalytic systems.3,11,21,25 In addition, because of its paramagnetic nature, O2- has been used in the electron spin resonance (ESR) study as a molecular probe to explore the active sites including its environment on the TiO2 surface.17,19,21-23,25-27 The ESR technique is useful to characterize * Corresponding authors. E-mail:
[email protected] (K.K.);
[email protected] (Y.H.). Fax: +81-824245494.
the electronic structure and dynamics of paramagnetic species. In 1978, Iwamoto et al. with the ESR technique have found that O2- species formed at three different Ti4+ sites on an evacuated TiO2 (anatase) surface exhibit different thermal stabilities.17 Recently, Carter et al. have reported that there are at least three different trapping sites for the O2- formation on thermally treated TiO2 (P-25) and that the O2- species have different thermal, chemical, and photochemical (UV-irradiation) reactivities depending on the sorts of trapping sites.23 In previous studies on photocatalytic systems based on a single-phase TiO2, very little attention has been paid to the effect of the visible-light illumination. This is because the photocatalytic reactions on TiO2 start in principle with band gap illumination. We have recently demonstrated that an electron transfer from the anatase phase to the rutile phase can be induced by the sub-band-gap illumination of partially reduced mixedphase TiO2 nanoparticles (P-25).28 This finding suggests the involvement of the visible light in the photocatalytic processes on the surface of TiO2 (P-25) where trapped electrons (or holes) were produced by thermal treatment or band gap excitation. In addition, it has been reported that intrinsic local structural defects formed on/in TiO2 by thermo- and photoactivated TiO2 work as absorption centers of visible light.29 To the best of our knowledge, however, little is known about whether the visible light can cause the photoresponse of oxygen species or not and how the photoresponse depends on the sort of trapping sites. In a recent Letter, we reported a photoresponse of the ESR spectra of O2- and Ti3+ by the visible-light illumination.30 The ESR results demonstrated the photoinduced electron transfer from O22- to Ti3+ to generate O2- and Ti3+ at the oxygen vacancy sites. In this paper, the photoresponse of O2- species was further examined by more complete and quantitative analysis of new experimental data obtained by sophisticated ESR technique combined with the optical measurements. The reaction
10.1021/jp909678e 2010 American Chemical Society Published on Web 12/16/2009
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mechanism proposed earlier is confirmed, and the active center that absorbs visible light could be assigned to F-type color centers. 2. Experimental Methods The TiO2 powders were kindly donated by Sakai Chemical Industry (rutile: STR-60C and STR-60C-LP (STR-60C-LP is basically the same as STR-60C, but its surface is modified with organopolysiloxane), i.d. ) 20 nm; STR-100C, i.d. ) 10 nm), Tayca Corp. (anatase: AMT-100, i.d. ) 6 nm), and Japan Aerosil (Degussa P-25). The TiO2 powder (10 mg) was placed in a Suprasil quartz glass tube of 5 mm in diameter, unless otherwise stated. The sample was annealed in air for 1 h at 573 K and then evacuated at a pressure of 0.15 Pa. The sample was heated at 773 K, exposed to H2 (27 kPa) for 1 h, cooled to room temperature, and then evacuated again. Subsequently, O2 was admitted at a pressure of 4.0 kPa for 5 min and then the residual gas was removed by evacuation. Finally, the sample tube was sealed with He gas (27 kPa) at room temperature and subjected to the ESR measurements. The He gas was used to reduce a possible temperature rise of the sample by illumination. In fact, the temperature rise was as high as 20 K in the absence of He, but its increase was less than 1 K under the same condition except in the presence of He.31 ESR spectra were recorded at 77 K on a JEOL JES-RE1X and Bruker ESP300E spectrometers (X-band) at a microwave power of 1 mW, which was small enough to avoid saturation of the signals. The g-values were determined for the spectra taken on ESP300E with a gauss meter (ER 035M, Bruker,) combined with a microwave frequency counter (5350B, Hewlett-Packard). The number of spins was determined on JES RE-1X by comparison with the Mn2+/MgO signal area (mI ) 3/2) as the secondary standard, which was calibrated with a DPPH/benzene (10-4 mol/L) solution. A 500 W xenon lamp (UXL-500SX, Ushio) with a set of cutoff glass filters combined with ND filters, a water filter, and a cold filter was used as a light source for visible-light illumination. In measuring a spectral response of the ESR signal intensity, a series of interference filters (Asahi Spectra Co.) with a full width at half-maximum (fwhm) of 10 nm in the visible range were used. UV-vis-near-IR diffuse-reflectance spectra were measured in the wavelength range of 200-2600 nm on a spectrophotometer (SHIMADZU, UV-3150) with an integrating sphere assembly (ISR-3100). The TiO2 samples for the optical measurements were prepared in the same way as those for ESR measurements, but in a quartz cell with an inner thickness of 1 mm and a width of 10 mm. 3. Results The ESR spectra of the rutile TiO2 sample (STR-60C) at different treatment stages are depicted in Figure 1. The spectrum of the sample thermally treated under H2 atmosphere consists of a broad singlet with a line width of 3 mT at g ≈ 1.96 (Figure 1a). The broad ESR signal is attributable to the Ti3+ cations (d1 electronic configuration) formed on the TiO2 surface, as reported previously by us28 and other groups.23,24,32 Upon exposing the sample to O2 at room temperature, the ESR spectrum changed drastically (Figure 1b): the intensity of the Ti3+ signal decreased by 1/20 times, and several ESR peaks appeared in the low magnetic field relative to Ti3+. The reduction of the signal intensity of Ti3+ by O2 admission is due, most likely, to the stoichiometric restoration of the TiO2 surface.33 On the other hand, the fully anisotropic signals are characteristic of the static structure of O2- species adsorbed on the TiO2 surface,27 where the degeneracy of the antibonding 2pπg* orbitals is lifted by
Figure 1. ESR spectra observed at 77 K for rutile TiO2 nanoparticles treated in the following sequence: (a) preheated at 573 K in air followed by thermal treatment at 773 K under H2 atmosphere (27 kPa), (b) exposed to O2 (4.0 kPa) for 5 min at room temperature, and (c) illuminated with visible light for 10 min. The line shapes around the gz-component in spectra b and c are expanded for clarification. Asterisks indicate signals due to a standard marker, Mn2+/MgO.
the crystal field due to the connected Ti4+ ion on the surface. When both the molecular axis (z) and the 2pπg*(x) orbital containing the unpaired electron of the O2- molecule are parallel to the TiO2 surface, the g-tensor components for the O2- ion can be expressed by the following equations, if the relation λ < δ ,E holds:18,34,35
gx ) ge
(1)
gy ) ge + (2λ/E)
(2)
gz ) ge + (2λ/δ)
(3)
where ge denotes the free-spin value of 2.00231, δ and E are the energy separations between the two antibonding orbitals in O2-, 2pπg*(x) and 2pπg*(y), and between 2σg and 2πg*(x) orbitals, respectively, and λ is the spin-orbit coupling constant of the O- ion (0.014-0.017 eV).18,34 From the above equations, one can see gx < gy < gz and find that the gz value is very sensitive to δ. On this basis, the g-tensor components of O2- were determined as gx ) 2.003, gy ) 2.010, and gz ) 2.023. In the next stage when the TiO2 sample treated as above was illuminated with a visible light (500-800 nm), the ESR signal of Ti3+ increased its intensity and the signal intensities of O2at gx and gy were also enhanced, accompanying the appearance of a new peak denoted by gz′ in Figure 1c. We note here that the ESR peak intensity at the gz position was unchanged by illumination. Two or more sorts of O2- species have been reported on the TiO2 surfaces,17,19,23,25,36 reflecting the presence of different sorts of trapping sites. In view of this, we presume
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Figure 3. Time profiles for the spin numbers of (O) O2-(B) and (b) Ti3+ under visible-light illumination (500-800 nm) and after the termination of illumination at 77 K. The inset shows a relationship between the spin numbers of O2-(B) and Ti3+. The dashed line with a unity slope is a guide for the eye.
Figure 2. ESR spectra observed at 77 K for TiO2 powders of (a) STR100C (rutile), (b) STR-60C-LP (rutile), (c) AMT-100 (anatase), and (d) P-25 (mixed phase). The samples were preheated at 573 K in air followed by thermal treatment at 773 K under H2 atmosphere (27 kPa) and exposed to O2 (4.0 kPa) for 5 min at room temperature. The dotted line shows signals due to glass damages in the liquid N2 Dewar.
TABLE 1: g-Tensor Components Evaluated for O2Formed on the Surface of a Variety of TiO2 Samples gx
gy
gz
STR-60C
2.003
2.010
STR-100C
2.003
2.010
STR-60C-LP AMT-100
2.004 2.003
2.010 2.010
P-25
2.003
2.010
2.023 2.020 2.023 2.020 2.021 2.025 2.021 2.029 2.024 2.021
that another sort of O2- having a different gz value and the same gx and gy values as those of O2- presented initially is generated by illumination of the TiO2 sample. Indeed, one or more gz peaks were observed with other TiO2 samples such as STR-100C (rutile), STR-60C-LP (rutile), AMT-100 (anatase), and P-25 (mixed phase). The ESR spectra of these TiO2 samples illuminated with a dim light are shown in Figure 2, and g-tensor values for STR-60C and other TiO2 samples are summarized in Table 1. In the present study, the O2- species with gz ) 2.023 and 2.020 for STR-60C are tentatively represented as O2-(A) and O2-(B), respectively. It is O2-(B) that is generated by the visible-light illumination, and eq 3 implies that O2-(B) is subject to a stronger crystal field than O2-(A). It is worth mentioning that once the illuminated sample was warmed to room temperature for a moment, the ESR spectrum of Figure 1c was completely restored to the original one shown in Figure 1b, demonstrating that this photoinduced change of the ESR spectrum is reversible. Figure 3 shows time profiles of the numbers of spins for O23+ during and after illumination of STR-60C. The (B) and Ti absolute spin numbers were evaluated for a series of spectra of the identical sample by a line shape simulation based on a
Figure 4. Time profiles for the spin number of O2-(B) observed at 77 K with the flat-type cell (see main text) under visible-light illumination (500-800 nm) at different intensities and after the termination of illumination. The inset depicts the saturated spin number for O2-(B) plotted against the intensity of the visible illuminated light.
combination of the two spectral components for O2- and Ti3+ with their relative ratio as a variable parameter. Prior to the simulations, the lineshapes were best fitted for O2-(B) and Ti3+ separately and their spin numbers were calibrated by using Mn2+/MgO. It is seen from Figure 3 that the spin numbers of O2-(B) and Ti3+ increase with the illumination time and decrease very slowly after the termination of illumination. A salient feature to be noticed is that the spin numbers of O2-(B) and Ti3+ are equal during and after illumination, as shown more clearly by the inset of Figure 3. Another feature of Figure 3 is a slow increase of spin numbers during illumination, showing no signal saturation even 30 min after the start of illumination. By considering that inhomogeneous illumination of the TiO2 powder in the thick ESR tube could be responsible for the slow photoresponse, a flat-type cell of 0.5 and 5 mm in inner thickness and width consisting of a Suprasil quartz glass was used in subsequent ESR experiments. Details will be reported in our future publication.31 The time profile of the number of O2-(B) species measured with the flat-type cell is shown in Figure 4, where the signal height of the first-derivative peak at g ) 2.020 was monitored by fixing the magnetic field to its position, and the spin numbers were calculated in the same way as in Figure 3. The signal intensity increased much more rapidly than when the quartz tube of 5 mm in diameter was employed and then was saturated during illumination. Not only the initial growth rate but also
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Figure 5. Spectral response for the photoinduced evolution of O2-(B) at 77 K. The ESR spectra were measured at 50 s after illumination to avoid a saturation of the signal, and their intensities were normalized by intensities of the respective monochromatic lights.
Figure 6. Diffuse-reflectance UV-vis-near-IR spectra of the TiO2 sample treated in the following sequence: (a) annealed at 773 K in vacuum, (b) exposed to H2 (27 kPa) for 1 h at 773 K, and (c) exposed to O2 (4.0 kPa) for 5 min at room temperature. The spectrum for the preheated sample (573 K in air) was used as a control.
the saturated intensity depended on the light intensity. It is likely that the variation in the saturated intensity is caused by the backward reaction which may not be negligible in the experiments with lower intensities of light. There is no doubt about the dependence of the saturated intensity, which was obtained at low light levels by carefully checked experimental procedures, though the contradictory statements had been given at the preliminary experimental stage.30 The inset in the figure depicts a relationship between the saturated ESR intensity and the light intensity. The plot tends to level off at higher light intensities, and the saturation value for the total amount of the O2-(B) species can be evaluated as ca. 3 × 1016 spins for 10 mg of the TiO2 sample (STR-60C). This value corresponds to one O2-(B) species per 18 oxygen vacancy sites present on the TiO2 surface, assuming a (110) surface with the specific surface area of 74 m2 g-1. The spectral response for the generation rate of O2-(B) was studied in order to figure out the origin for the visible-light absorption leading to the simultaneous generation of O2-(B) and Ti3+. The experiments were made by measuring the ESR intensity of O2-(B) at 50 s after illumination with weak monochromatic lights ranging from 420 to 700 nm. The observed ESR intensities were divided by the corresponding light intensities, and the result is depicted in Figure 5 as a plot of normalized ESR intensity vs wavelength. Diffuse-reflectance spectra for the TiO2 sample treated with the same procedures as for the ESR measurements were measured at room temperature (Figure 6). As reported earlier, the broad absorption band extending to the near-IR region is attributable to Ti3+ ions.10,29,37 This absorption band was enhanced by the H2 treatment (27 kPa) at 773 K and then decreased by exposure to O2 (4.0 kPa) at room temperature, in accord with the change in ESR spectra of the Ti3+ ions. Noted here is an absorption peak at around 460 nm which appears by thermal treatment of TiO2 at 773 K under vacuum. The absorption peak remained unchanged even after the thermal treatment of TiO2 in H2 and subsequent exposure to O2 at room temperature. The peak wavelength is in good agreement with that in the spectral response of Figure 5 for the generation rate of O2-(B).
SCHEME 1: Illustrations for Adsorption of O2 Molecules on the Partially Reduced TiO2 Surface and Electron-Transfer Reaction Induced by Visible-Light Illumination
4. Discussion Assignment of O2-(A) and O2-(B). Two sorts of O2- species were detected on TiO2 nanoparticles (STR-60C). O2-(A) (gz ) 2.023) was observed after exposure of partially reduced TiO2 nanoparticles to O2 in the dark, while O2-(B) (gz ) 2.020) appeared only after illumination of the O2-exposed TiO2 sample
with visible light. The ESR intensity of O2-(A) showed no photoresponse, whereas the increase of ESR intensity of O2-(B) by visible-light illumination was accompanied by a simultaneous increase of the surface Ti3+ ions at an equimolar ratio. It has been reported so far that there are at least two specific sites for O2 adsorption on the reduced TiO2 (110) surface that is the most thermodynamically stable: a five-coordinate Ti3+ site and an oxygen vacancy site. Adsorption of O2 at the five-coordinate site may generate O2- by one-electron transfer from Ti3+ to the adsorbed O2 molecule. On the other hand, the oxygen vacancy site consists of two Ti3+ ions adjacent to each other and an O2 molecule that attached to this site will form a diamagnetic peroxide ion O22- by coordination with the two Ti3+ ions. The formation of O22- on the TiO2 surface has been experimentally evidenced by a number of studies.38-43 When an electron is transferred from O22- to one of the two Ti4+ ions in Ti4+ · · · O22- · · · Ti4+ and thus the corresponding O-Ti coordination is broken, a pair of O2- and Ti3+ species would be concomitantly formed. On these bases, one can reasonably assign O2-(A) and O2-(B), respectively, to O2 molecules at a fivecoordinate Ti3+ site and an oxygen vacancy site. The adsorption of O2 molecules on the two different sites and the photoinduced reaction are schematically shown in Scheme 1. This assignment is consistent with the g-value of O2-(B) greater than that of O2-(A), because the oxygen vacancy site should have a crystal field stronger than the five-coordinate site.17,23 Our ESR observation shows further that the O2-(B) signal which appeared by illumination is more intense than the O2-(A) signal. This implies that the adsorption of molecular oxygen at the oxygen vacancy site is predominant as compared with the five-coordinate site, in
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agreement with the previous reports (Figure 1).20,40 From Figure 4, the maximal surface coverage of O2 molecules at the oxygen vacancy site was determined to be 1.3 × 1013 sites cm-2, being slightly smaller than 4 × 1013 sites cm-2 reported by Henderson et al. who annealed the TiO2 (110) surface at 850 K in ultrahigh vacuum for 10 min and evaluated the coverage by the H2O temperature programmed desorption.20 The difference in the surface coverage may arise from the fact that the present reductive condition is rather mild as compared with the condition of Henderson et al. Photoactive Center Responsible for Visible-Light Absorption. O22- is quite unlikely to be responsible for visible-light absorption, because it absorbs light in the UV region. Likewise, Ti3+ which exhibits a broad absorption band in the vis-nearIR can be ruled out as a photoactive center such as generating O2-(B) and Ti3+ most effectively upon illumination of 460 nm light. A number of experimental and theoretical studies have been made on the appearance of the near-edge absorptions in TiO2 nanoparticles.44-47 Kuznetsov and Krutitskaya have reported an optical absorption band at 2.8 eV (450 nm) for slightly reduced TiO2 (P-25), and this band was assigned to oxygen vacancies in the surface and/or subsurface layers.45 Sekiya et al. who used a polarized absorption spectroscopy have concluded that an absorption band that was observed at ca. 3 eV for a slightly reduced TiO2 (anatase) on oxygen annealing originates from an oxygen vacancy with two electrons.46 The peak wavelengths reported so far for the oxygen vacancies are close to 460 nm at which the photoinduced electron transfer takes place.47,48 The coincidence leads to a reasonable assignment of the 460 nm band to the oxygen vacancies (color centers) in subsurface layers of TiO2 nanoparticles. The oxygen vacancies are grouped in three types of color center, F2+ (without electron), F+ (with one electron), and F (with two electrons), depending on the number of the captured electrons.49 No trace of ESR signals due to the structural defect was observed during and after illumination, ruling out the possibility of the paramagnetic F+-type center as the origin of the visible-light absorption. The reduced samples employed in the study may facilitate the generation of the F-type color center rather than the F2+-type center. The relatively broad spectrum of Figure 5 may suggest coexistence of several sorts of F-type color centers that are located at different energy levels within the band gap. A reaction diagram is depicted in Figure 7 to illustrate the photoinduced electron-transfer processes involving O22-, Ti4+, and F-type color center with their relative energy levels located in the energy gap between the valence and conduction bands.50 By illuminating TiO2 with visible light, the color center absorbs the light energy and the electron is excited to the conduction band (a). The excess electron in the conduction band will be readily captured by Ti4+ at the surface oxygen vacancy site where the two Ti4+ ions are bridged by O22-, and at the same time O22- liberates an electron to the F-type color center (b). The successive electron-transfer processes account for the concomitant formation of O2-(B) and Ti3+ at an equimolar ratio (c). These steps are sufficiently fast relative to the time scale of X-band ESR measurements, since any new ESR signals were never detected during illumination. When the temperature is raised, an electron transfer from Ti3+ to O2-(B) takes place and the TiO2 surfaces will be restored (d). 5. Conclusions An electron transfer from O22- at an oxygen vacancy site to the adjacent Ti4+ is induced by sub-band-gap illumination of the partially reduced TiO2, and the backward reaction occurs
Komaguchi et al.
Figure 7. Schematic diagram illustrating the photoinduced electrontransfer reaction including relative energy levels of O22-, Ti3+, and F-type color center. The dotted arrows denote the direction of an electron flow.
in the dark at elevated temperatures. The electron transfer is initiated by absorption of visible light by F-type color centers. The excited electron in the TiO2 conduction band is captured by Ti4+ at the surface oxygen vacancy site to form Ti3+, and concurrently O22- passes an electron to restore the F-type color center. As a whole, the sub-band-gap illumination of the TiO2 leads to the concomitant formation of O2- and Ti3+ radicals. Acknowledgment. This work was supported in part by Grants-in-Aid for Scientific Research (Grant Nos. 19350094 and 21550190) from the Japan Society for the Promotion of Science. References and Notes (1) Fujishima, A.; Honda, K. Bull. Chem. Soc. Jpn. 1971, 44, 1148. (2) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735. (3) Heller, A. Acc. Chem. Res. 1995, 28, 503. (4) Anpo, M., Application of Titanium Oxide Photocatalysts To Improve Our Environment. In Green Chemistry Challenging PerspectiVes; Tundo, P., Anastas, P., Eds.; Oxford University Press: New York, 2000. (5) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (6) Horiuchi, T.; Fujisawa, J.; Uchida, S.; Gra¨tzel, M., Eds. Data Book on Dye-Sensitized Solar Cells; CMC: Tokyo, 2009. (7) Ooyama, Y.; Harima, Y. Eur. J. Org. Chem. 2009, 2009, 2903. (8) Nakajima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 7044. (9) Inoue, M.; Shimada, Y. Bosei Kanri 2005, 49, 75. (10) Thompson, T. L.; Yates, J. T., Jr. Chem. ReV. 2006, 106, 4428. (11) Anpo, M. Catal. SurV. Jpn. 1997, 1, 169. (12) Livraghi, S.; Paganini, M. C.; Giamello, E.; Selloni, A.; Valentin, C. D.; Pacchioni, G. J. Am. Chem. Soc. 2006, 128, 15666. (13) Yang, J.; Chen, C.; Ji, H.; Ma, W.; Zhao, J. J. Phys. Chem. B 2005, 109, 21900. (14) Naccache, C.; Meriaudeau, P.; Che, M. Trans. Faraday Soc. 1962, 67, 506. (15) Iyenger, R. D. AdV. Colloid Interface Sci. 1972, 3, 365. (16) Lansford, J. H. Catal. ReV. 1973, 8, 135–157. (17) Iwamoto, M.; Yoda, Y.; Yamazoe, N.; Seiyama, T. J. Phys. Chem. 1978, 82, 2564. (18) Che, M.; Tench, A. J. AdV. Catal. 1983, 32, 1.
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