Synergetic Effects of Thermal and Photo-Catalysis in Purification of

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J. Phys. Chem. C 2009, 113, 4970–4975

Synergetic Effects of Thermal and Photo-Catalysis in Purification of Dye Water over SrTi1-xMnxO3 Solid Solutions Xiaoyan Sun and Jun Lin* Department of Chemistry, Renmin UniVersity of China, Beijing 100872, People’s Republic of China ReceiVed: NoVember 21, 2008; ReVised Manuscript ReceiVed: January 29, 2009

The solid solutions SrTi1-xMnxO3 (x ) 0.025 and 0.05) were synthesized by a modified sol-gel method. The substitution of Mn cations, mainly present as Mn4+ and Mn3+, for octahedrally coordinated Ti4+ in the host SrTiO3 exhibited an absorption extending up to the visible region in optical absorption spectrum, and established oxo-bridged bimetallic linkages TiIV-O-MnIV (or MnIII) in two neighboring octahedrons of the solid solutions. Similar to a thermal catalyst MnO2, the solid solutions were examined to be catalytic active for RhB degradation in dark due to the presence of Mn4+ species with a high oxidation potential. The RhB degradation over the solid solutions was demonstrated to be significantly enhanced under visible-light irradiation. Difference diffuse reflectance spectra and ESR measurements revealed that the visible-light induced MMCT of TiIV-O-MnIII f TiIII-O-MnIV over the solid solutions provides an additional channel for the fast regeneration of Mn4+ species, synergically promoting the catalytic process of RhB degradation. Introduction To eliminate environmentally unacceptable compounds, the scientific and engineering interest in the development and application of environmental catalysis has grown exponentially over the past several decades. Manganese dioxide thermal catalysis and semiconductor photocatalysis are two important areas of investigation in environmental catalysis due to their excellent performances and potential applications in organic pollutant transformation and organic matter decomposition.1-6 Manganese dioxide is well-known to be present as different structural forms, R-, β-, γ-, and δ-MnO2, when the basic structural unit, [MnO6] octahedron, is linked in different ways.7-9 MnO2 in various forms was demonstrated to be a catalyst, to different extents, for thermally catalyzed degradation of organic compounds, and the oxidation of carbon monoxide.10-12 The catalytic reactivity of MnO2 is, likely, associated with the capacity of both ready conversions between Mn4+ and Mn3+ or Mn2+ in the oxide lattice.13,14 It was revealed that the Mn-O bond strength of MnO2 and transformation of the intermediate Mn2O3 or Mn3O4 into MnO2 play a crucial role in the catalytic behaviors of MnO2.15 The weak strength of the Mn-O bond in MnO2 and easy regeneration of MnO2 from Mn2O3 or Mn3O4 would generate an enhanced catalytic performance.16-18 Semiconductor photocatalysis with a primary focus on several semiconductors such as TiO2, SrTiO3, and ZnO as efficient photocatalysts has been already used for remediation of contaminated water and air since the later 1970s.5,6 For a practical application, the design and development of photocatalytic systems working under visible light became indispensable and an extensively studied topic. Recent work about N-doped TiO2 by Asahi et al. is a typical representative of a few successful visible-light photocatalyst examples.19 More recently, a new version of a visible-light photocatalytic system was developed by Frei et al. and Hashimoto et al. for the use of oxo-bridged heterobimetallic assembly as a visible-light sensitive photoinduced redox center.20-22 In the work by Hashimoto, the * Corresponding author. E-mail: [email protected]. Phone: (+8610)62516222. Fax: (+8610)-62516444.

anchored Ce(III)-O-Ti(IV) bimetallic assemblies on MCM41 sieves showed the metal-to-metal charge transfer (MMCT) of Ce(III)-O-Ti(IV) f Ce(IV)-O-Ti(III) under visible-light irradiation, and such MMCT was demonstrated to have the ability to drive the photocatalytic oxidation of 2-propanal via the oxidizing species of O2-• by photogenerated Ti(III). Subsequently, we reported the enhanced photocatalytic decomposition of organic dye driven by visible-light induced MMCT of Fe(II)-O-Ti(IV) linkages formed in Fe-doped SrTiO3.23 A particular feature of our system is to reveal that the visiblelight induced MMCT could also occur in the solid solution with more than two metals differing in oxidation state and cause a photochemical reaction. In the current article, we reported the synergetic effects of the thermal and visible-light photocatalysis of the SrTi1-xMnxO3 solid solutions on the purification of dye water. The degradation of rhodamine B (RhB), an organic dye, in aqueous suspension was used as a probe reaction to evaluate the catalytic performance of the SrTi1-xMnxO3 solid solutions. Similar to Mn4+ of MnO6 octahedron in MnO2 lattice, Mn4+ substituting for Ti4+ of TiO6 octahedron in the crystal structure of SrTi1-xMnxO3 was examined to be catalytic active for the degradation of RhB in aqueous solution in dark. The catalytic degradation of RhB over SrTi1-xMnxO3 was significantly improved in the presence of visible-light irradiation. The visible-light induced MMCT of bimetallic linkage Mn-O-Ti(IV) formed in the SrTi1-xMnxO3 solid solutions was found to play a crucial role in this improved catalytic activity. Finally, the synergetic relation between the thermal and visible-light photocatalysis was also discussed in more detail. Experimental Section Reagents. Rhodamine B (RhB), titanium isopropoxide, Sr(NO3)2 and Mn(NO3)2, all of analytical grade, were purchased from Aldrich. Other chemicals were of analytical grade and purchased from Beijing Chemical Co., China. All chemicals were used as received without further purification. Deionized water was used throughout the experiments. For a clear identification, the structure of rhodamine B (RhB) is given below

10.1021/jp810227y CCC: $40.75  2009 American Chemical Society Published on Web 03/04/2009

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Synthesis of SrTi1-xMnxO3 Solid Solutions. The SrTi1-xMnxO3 solid solutions (with x ) 0.025 and 0.05, respectively) employed in the present study were prepared by a modified sol-gel method similar to that reported by Subramanian et al.24 In a typical synthesis of the modified sol-gel method, a solution of titanium isopropoxide precursor in acidic acid was added into an aqueous solution of Sr(NO3)2, Mn(NO3)2, and citric acid to make the molar ratio of total metals to citric acid at 1. The molar ratio of Sr to (Ti + Mn) was controlled stoichiometrically at 1:1 to facilitate the incorporation of Mn ions at the Ti site of the SrTiO3 host lattice. The amount of Mn ion added was 2.5% or 5 mol % of the strontium. The resulting mixture was allowed to mix for several hours until the solution became transparent and clear, followed by the addition of ethylene glycol and glycerol. Subsequently, the precursor solution was stirred for another 12 h to remove water at room temperature before heated first at 50 °C over 1 day to evaporate most of the water and again reheated at 120 °C. Finally a green sticky paste and solid was obtained upon heating the clear and transparent sol solution. At last this green solid was moved into a ceramic crucible and was calcined for 6 h at 900 °C in a stove. The obtained solids were ground into fine powder prior to characterization and catalytic activity examination. Pure SrTiO3 was also prepared in the same manner. Characterization. Polycrystalline X-ray diffraction (XRD) patterns of as-prepared samples were recorded with Rigaku D/max-2500 rotating anode X-ray diffractometer using graphitemonochromated Cu KR radiation (λ ) 0.154 nm) under 40 kV and 200 mA in the range of 2θ ) 20∼80°. A step of 2θ ) 0.02° and 0.5 s/step were used. X-ray photoelectron spectra (XPS) of these samples were measured on an ESCALab220iXL X-ray photoelectron spectrometer using 300 W Al KR radiation. The base pressure was about 3 × 10-9 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. The relative composition of each sample was calculated from the relative XPS peak areas. The measurements on UV-vis diffuse reflectance spectra of the samples were carried out on a Hitachi U-3310 spectrophotometer equipped with a diffuse reflectance accessory and absorption spectra were referenced to BaSO4. ESR experiments were carried out at room temperature by using a Bruker ESR 300E spectrometer. The settings for ESR spectrometer were centerfield ) 3490 G, microwave frequency ) 9.775 GHz, and power ) 12.71 mW. The powder samples in the presence of oxygen with and without visible-light (λ > 420nm) irradiation were used in ESR measurements, respectively. Catalytic Activity Evaluation. The catalytic reaction of the pure SrTiO3 and SrTi1-xMnxO3 solid solutions for the degradation of RhB was conducted in a Pyrex reactor, in which 100 mg of the catalyst was suspended in 100 mL of RhB aqueous solution (∼10-5 M). O2 was continuously bubbled throughout the reaction. Above the Pyrex reactor, a 300 W halogen lamp (Institute of Electric Light Source, Beijing) with a special cutoff filter as a light source was positioned inside a cylindrical Pyrex vessel surrounded by a recirculating cooling water jacket (Pyrex). The special cutoff filter was placed outside the Pyrex jacket to cut off the wavelengths shorter than 400 nm and wavelengths between 500 and 700 nm. This is to ensure that

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Figure 1. Polycrystalline X-ray diffraction patterns of pure SrTiO3 and SrTi1-xMnxO3.

Figure 2. Diffraction peak positions of (110) plane of various samples.

irradiation was achieved by the visible light wavelengths which excluded the light range for RhB excitation. During the reaction, at the given irradiation time intervals, ca. 3 mL of reaction suspension was taken out and separated by centrifugation before the analysis on a Hitachi U-3310 spectrophotometer. The concentration of RhB was determined by monitoring the change in the absorbance at 554 nm. The catalytic activities of all samples for the degradation of RhB with and without the visiblelight irradiation were measured, respectively. Results and Discussion Characterizations of Pure SrTiO3 and SrTi1-xMnxO3 (x ) 0.025 and 0.05). 1. XRD Studies. The polycrystalline X-ray diffraction profiles of pure SrTiO3 and SrTi1-xMnxO3 (x ) 0.025 and 0.05) solid solutions are presented in Figure 1. From Figure 1, it can be found that all samples exhibit a cubic crystallographic structure of typical perovskite-type SrTiO3 without any second phase for all Mn concentration range (x up to 5 atom %) according to JCPDS file (35-734) and literature results.25,26 Figure 2 shows the (110) diffraction peaks of three samples. We can see that the (110) diffraction peak position exhibits an obvious shift to a higher 2θ value with the increase in the amount of the doping cation Mn (mostly present as +4 oxidation state based on XPS measurements below) in the SrTiO3 host lattice, indicating a decrease in the lattice parameters of two solid solutions. Obviously, the decrease is caused by the substitution of the doped Mn4+ with small ionic radius (0.53 Å) for the host cation Ti4+ with large ionic radius (0.605 Å) or Sr2+ with larger ionic radius (1.44 Å).27 On the basis of ionic size and charge compensation consideration, it could be sup-

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Sun and Lin

TABLE 1: Lattice Parameters and Crystalline Sizes of Pure SrTiO3 and SrTi1-xMnxO3 sample

lattice parameter (Å)

crystallite size (nm)

pure SrTiO3 SrTi0.975Mn0.025O3 SrTi0.95Mn0.05O3

3.9073(1) 3.896(1) 3.887(2)

35.59 36.12 35.12

ported, to a great extent, that the doping cation Mn4+ substitutes for octahedrally coordinated Ti4+ in the SrTiO3 host lattice to form SrTi1-xMnxO3 solid solution. Thus, in the SrTi1-xMnxO3 structure, the TiO6 octahedron together with MnO6 octahedron links through corners to form a three-dimensional cubic lattice, and Sr2+ cation is in the center of the cube. The interaction between the host ion Ti and foreign ion Mn in the solid solution can be described to be the oxo-bridged bimetallic linkage Mn-O-Ti in two corner-sharing octahedrons. The lattice parameters of pure SrTiO3 and SrTi1-xMnxO3 (x ) 0.025 and 0.05) were refined by a least-squares procedure. Crystallite sizes (D) of these samples were estimated using the diffraction peak of the sample (110) crystalline plane with Scherrer equation. Both results are represented in Table 1. As obtained from Table 1, the lattice parameter of SrTi1-xMnxO3 decreases linearly with the increase of Mn content from x ) 0 to 0.05 (nominal value) with an average slope da/dx ≈ 0.4 Å. This average slope is higher than that reported for the SrTi1-xMnxO3 prepared by a conventional mixed oxide method,28 indicating that our synthesis method favors the incorporation of more doped-Mn at Ti site and causes a larger distortion of the host lattice. The crystallite sizes of pure SrTiO3 and SrTi1-xMnxO3 (x ) 0.025 and 0.05) solid solutions are 35.59, 36.12, and 35.16 nm, respectively. This result suggests that doping Mn cation up to 5 atom % almost has no obvious influence on the growth of the host structure. 2. XPS Analysis. Ti 2p3/2 peak located at 458.5 eV and Sr 3d peak at 133.0 eV show that Ti cation is still in +4 oxidation state, and Sr cation in +2 oxidation state in both SrTi1-xMnxO3 (x ) 0.025 and 0.05) solid solutions.29 No significant variations in the binding energies of Ti2p and Sr3d are observed upon doping Mn cation in the host SrTiO3. Figure 3 shows the high resolution XPS spectra of Mn 2p and O 1s in the sample SrTi1-xMnxO3 (x ) 0.05) before and after used for RhB degradation in dark. As observed in Figure 3A, it can be found that the peak of binding energy for Mn2p in two samples consists of two signals at the binding energies of 642.4 and 641.6 eV, which are assigned to Mn4+ and Mn3+, respectively.29,30 Most of the doping cations are present as Mn4+. The presence of Mn3+ cation would result in the formation of oxygen vacancy in the solid solution for charge compensation. The formation of oxygen vacancy should be also responsible for the high decreasing slope da/dx of the lattice parameter mentioned above. From the relative XPS area, RhB degradation over the solid solution SrTi1-xMnxO3 (x ) 0.05) in dark gives rise to the atomic percentage of Mn3+ species at the Ti site in the solid solution (from 1.27% to 1.65%), indicating that the partial Mn4+ is reduced into Mn3+ during RhB degradation in dark. Tomita et al. reported that the β-MnO2 was partially converted into Mn2O3 after being used for thermally catalyzed oxidation of phenol.31 Liang et al. also found that the crystallinity of MnO2 in various forms was significantly decreased after these manganese oxides were utilized for CO catalytic oxidation.10 All these suggest that crystalline MnO2 as a thermal catalyst could not be easily regenerated after catalytic reaction. Suib et al. used amorphous MnO2 as a catalyst for the oxidation of 2-propanol, and found that with the help of light irradiation, the amorphous MnO2 is

Figure 3. High resolution Mn 2p and O 1s XPS spectra of the SrTi1-xMnxO3 (x ) 0.05) before and after used for RhB degradation in dark.

more readily regenerable than crystalline MnO2.32 Two surface oxygen species could be obviously observed in the O1s XPS spectra (Figure 3B). The binding energy at the range of 529∼530 eV is characteristic of the lattice oxygen (Oa), and the binding energy of 531∼532 could be attributed to defect oxide or the surface oxygen ions with low coordination situation (Ob).33 On the basis of the relative XPS areas, the atomic percentages of Ob at O site in the solid solution SrTi1-xMnxO3 (x ) 0.05) before and after used for RhB degradation in dark are 31.0% and 35.5%, respectively, which shows the RhB degradation over the solid solution in dark also increases the amount of the surface oxygen ions with low coordination situation. This result is well consistent with the increase of Mn3+ observed in the solid solution after used for RhB degradation in dark since the reduction of Mn4+ into Mn3+ in an octahedron MnO6 would cause the cleaving of the partial Mn-O bonds. In addition, the Mn4+ and Mn3+ species are also found to be present in the solid solution SrTi1-xMnxO3 (x ) 0.025) based on XPS measurements. Since the doping cation Mn is detected by XPS to be present as Mn4+ and Mn3+ species in the solid solutions, the oxo-bridged bimetallic linkage Ti-O-Mn in two corner-sharing octahedrons of the solid solutions should be in the forms of TiIV-O-MnIV and TiIV-O-MnIII. 3. UV-Vis Diffuse Reflectance Spectra. Figure 4 shows the UV-vis diffuse reflectance spectra of pure SrTiO3 and SrTi1-xMnxO3 solid solution (x ) 0.025 and 0.05). As seen in Figure 4A, the substitution of Mn4+ (and Mn3+) cation for Ti4+ generates a significant effect on the absorption characteristics of the host SrTiO3. The SrTi1-xMnxO3 solid solution exhibits an optical absorption tail extending to about 800 nm in addition to the band gap absorption of the host SrTiO3. The increase in the Mn cation content leads to an increase in the absorption band in the visible range. The shape of the diffuse reflectance

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Figure 5. ESR spectra at room temperature of the solid solution SrTi1-xMnxO3 (x ) 0.05) in the presence of oxygen (a) before and (b) after visible-light (λ > 420 nm) irradiation for 1.5 h.

Figure 4. (A) UV-vis diffuse reflectance spectra of pure SrTiO3 and SrTi1-xMnxO3 solid solution. (B) Difference UV-vis spectrum before and after exposure to 420 nm irradiation for 2 h in the presence of O2 for the SrTi1-xMnxO3 (x ) 0.05).

spectra of the solid solution is similar to those reported by Kudo et al. and Matsumura et al.34,35 This shape also indicates that there are different isolated energy levels formed by the Mn cation substitution in the forbidden band of the host SrTiO3. The existence of two absorption bands of a shoulder at approximately 510 nm and a broadband around 650 nm reveals there are two oxidation states for Mn cations in the forbidden band, which is in agreement with the XPS measurements. Two absorption bands should be mainly attributed to 3d-electron excitation from Mn4+ and Mn3+ to the conduction band of the host SrTiO3, respectively. The absorption tail at λ > 650 nm is probably due to the d-d band transition of Mn cation.36 Based on the XRD and XPS results, there are oxo-bridged bimetallic linkages Ti4+-O-Mn4+ (or Mn3+) formed in the solid solutions. Generally, oxo-bridged metal-to-metal charge-transfer (MMCT) moieties of mixed metal oxides are known to absorb visible light and even near-IR light.37 Frei et al. and Hashimoto et al. reported the visible-light induced MMCT for different oxobridged bimetallic systems such as TiIV-O-CuI, TiIV-O-SnII, and TiIV-O-CeIII, respectively.20-22 Our recent work also revealed the visible-light induced MMCT of TiIV-O-FeII linkages formed in Fe-doped SrTiO3.23 According to the oxidation states of Mn cations in the bimetallic linkages, thus, the visible-light induced MMCT of TiIV-O-MnIII f TiIII-O-MnIV is supposed to occur in the solid solutions SrTi1-xMnxO3, which also contributes to the absorption extending to visible range observed in Figure 4A. The optical difference spectroscopy following an exposure of the SrTi1-xMnxO3 (x ) 0.05) solid solution to 420 nm irradiation for 2 h in the presence of O2 (in Figure 4B) clearly demonstrates the oxo-bridged MMCT occurring in our solid solution system.

As shown in Figure 4B, a slight rise of the Mn(IV) d-d band at 700 nm36 and the an obvious absorption loss in the 300∼520 nm are observed in the difference spectra. Since the isolated energy levels including the oxygen vacancy created by the substitution of cations Mn4+ and Mn3+ for Ti4+ in the host SrTiO3 acting as e-/h+ recombination do not allow the photogenerated electron diffuse to the surface for O2 trapping,19,38-41 the absorption loss observed in Figure 4B is only assigned to the MMCT transition TiIV-O-MnIII f TiIII-O-MnIV. 4. ESR Measurements. ESR spectra of the solid solution SrTi1-xMnxO3 (x ) 0.05) in the presence of oxygen before and after visible-light (λ > 420 nm) irradiation for 1.5 h at room temperature are shown in Figure 5. Both spectra exhibit an asymmetric and incomplete sextet of hyperfine lines (centered at g ) 1.996) together with a large signal among these hyperfine lines in the detection range (275∼425 mT), which looks similar to that Azzoni et al. reported on Mn-substituted strontium titanate.42 According to the description by Azzoni et al. and other investigators,42-44 the observed sextet of hyperfine lines could be assigned to Mn4+ cations substituting for Ti4+ in regular octahedral sites, whereas the large signal is expected for Mn4+ in octahedral sites in sample regions with a high Mn concentration. The ESR signals from Mn4+ at the regular sites are superimposed by the large signal, resulting in the formation of the asymmetric and incomplete sextet of hyperfine lines observed in Figure 5. It is to be noted that the visible-light irradiation significantly enhances the signals of various Mn4+ species, confirming that the visible-light induced MMCT of TiIV-O-MnIII f TiIII-O-MnIV occurs in the system. The ESR results also indicate that the doping cations Mn4+ and Mn3+ might be concentrated somewhere in the solid solutions, probably at or near the lattice surface.23 Catalytic Performance. 1. RhB Degradation oWer Pure SrTiO3 and SrTi1-xMnxO3 Solid Solutions (x ) 0.025 and 0.05). The catalytic activities of various samples for the degradation of RhB in aqueous solution in dark and upon visible-light irradiation are shown in panels A and B of Figure 6, respectively. To evaluate the RhB adsorbed on various samples during the reaction, total RhB including aqueous and adsorbed RhB was measured by extracting with ethanol (1:1 V/V). We found that the total RhB is only slightly higher than aqueous RhB. Thus, the variation of aqueous RhB concentration can be directly used for the evaluation of the RhB degradation by the catalysts in dark and under visible-light irradiation. As seen in Figure 6A, obviously, the degradation of RhB is quite different over these samples in the dark. Doping Mn cation into the host SrTiO3 produces the catalytic activity for RhB degradation. Approximately 50% and 30% RhB degradation are reached

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Sun and Lin SCHEME 1: Schematic Illustration of Thermal and Photocatalytic Synergetic Effects over the Solid Solution SrTi1-xMnxO3

Figure 6. Catalytic degradation of RhB in aqueous solution over pure SrTiO3 and SrTi1-xMnxO3 (A) in dark and (B) under visible-light irradiation.

over the two solid solution samples, respectively, after 4 h of reaction, whereas almost no RhB degradation is observed over pure SrTiO3 sample within the same reaction time. Furthermore, the visible-light irradiation significantly enhances RhB degradation over two solid solution samples, as shown in Figure 6B. Upon visible-light irradiation, the removal of RhB is increased by 80% and 40% over two solid solution samples, respectively, after 4 h of reaction. Pure SrTiO3 still does not exhibit any reactivity under the same conditions. These results indicate that both the substitution of Mn cation for Ti4+ in the host SrTiO3 and visible-light irradiation would be responsible for RhB degradation, and the catalytic activity of the solid solution also strongly depends on Mn content in the host SrTiO3. 2. Thermal Catalytic Effects of SrTi1-xMnxO3 Solid Solutions on RhB Degradation. MnO2 with different structure forms such as R-, β-, γ-, and δ-MnO2 is widely investigated as an important thermal catalyst in facilitating organic pollutant oxidation because of its high redox potential.7-12 The Mn4+ cation in various oxide forms is 6-fold coordinated with oxygen atoms as MnO6. Generally, it is partly converted into Mn3+, leaving oxygen anion vacancy in the lattice, or forming Mn2O3 or Mn3O4 during a thermal catalysis.15,45 On the basis of the XRD and XPS results above, the doped Mn cation, mainly present as Mn4+, substitutes for octahedrally coordinated Ti4+ site in the host structure to form SrTi1-xMnxO3 solid solution. As estimated from Table 1, the average Mn-O bond length of SrTi1-xMnxO3 solid solution is not shorter than 1.94 Å, which is longer than that of β- or γ-MnO2 (1.88 and 1.91 Å).46,47 This indicates that Mn-O bond strength of the solid solutions is weaker and more capable of favoring the formation of other oxidation states (Mn3+ or Mn2+) as compared to that of β- or γ-MnO2. Therefore, it is also possible for the SrTi1-xMnxO3 solid solution to drive a catalytic oxidation process like MnO2 with respect to the structural environment and oxidation state of Mn cation. The possibility is well evidenced by the increases in the

atomic percentages of Mn3+ and Ob species in the sample SrTi1-xMnxO3 (x ) 0.05) after used for RhB degradation in dark, as observed in Figure 3. The increases in the atomic percentages of Mn3+ and Ob species suggests that partial Mn4+ in the solid solution is reduced into Mn3+ during the reaction. Thus, the RhB degradation in the dark shown in Figure 6A originates from the thermal catalytic oxidation over the solid solutions like MnO2. The increase in the Mn content in the host lattice offers more active sites (Mn4+) for RhB degradation, resulting in a higher catalytic activity over SrTi1-xMnxO3 (x ) 0.05) than over SrTi1-xMnxO3 (x ) 0.025). 3. Thermal and Photocatalytic Synergetic Effects in RhB Degradation. As observed in Figure 6B, the visible-light irradiation significantly enhances degradation of RhB over the SrTi1-xMnxO3 solid solutions. It was reported that organic dyes could be degraded over some wide band gap semiconductors such as TiO2 and SrTiO3 under visible-light irradiation via a self-photosensitized oxidative transformation.48,49 However, a glass filter in front of the light source is used to cut off the wavelengths exciting RhB in our photoreaction experiments. Also a negligible decrease in RhB concentration over the host compound SrTiO3 under visible-light irradiation observed in Figure 6B indicates that the use of the cutoff filter effectively inhibits the RhB degradation through a photosensitization process. Thus, it could be excluded that the enhanced RhB degradation over the solid solutions observed in Figure 6B is caused by a photosensitization process. As discussed above, it is the Mn4+ of octahedron MnO6 in the solid solution via the reduction of Mn4+ into Mn3+ (causing the cleaving of Mn-O bond) to catalytically degrade RhB in dark like MnO2 thermal catalysis. The main contribution to a thermal catalytic oxidation over manganese oxide also includes the reoxidation of Mn4+ in addition to its reducibility.13,14 It was reported that a fast and easy regeneration of the lattice Mn4+, with a high oxidation potential, from the Mn3+ or Mn2+ is consistent to a high catalytic activity for MnO2 catalytic oxidation process.16-18 Generally, the catalyst can return to its initial state (MnO2) during reaction via the process the adsorbed oxygen molecule obtains electron from the reduced manganese and fills oxygen anion vacancies in the lattice. However, the regeneration process of manganese oxide after a catalytic reaction seems to be quite slow based on our results and other investigations mentioned above. Our optical difference spectroscopy (shown in Figure 4B) reveals there is a visible-light induced MMCT of TiIV-O-MnIII f TiIII-O-MnIV over the solid solution, as confirmed by ESR measurement. This visible-light induced MMCT promotes the formation of Mn4+ from Mn3+ in the solid solution. Certainly,

Purification of Dye Water the Mn3+ cations could include the original and the reduced ones during the catalytic reaction. Thus, the visible-light induced MMCT in the solid solution could play a synergetic role in the regeneration of the catalyst for the thermally catalyzed oxidation of RhB. According to the work reported by Hashimoto et al.,22 the photogenerated Ti3+ via the visible-light induced MMCT of TiIV-O-MnIII f TiIII-O-MnIV could reduce O2 to produce O2-•. The superoxide anion radical O2-• is also a powerful oxidizing species and can degrade organic compounds.50 A possible thermal and photocatalytic synergetic process for RhB degradation upon visible-light irradiation is proposed in Scheme 1 based on the characterization results and discussion above. From Scheme 1, with the visible-light irradiation, it clearly appears that the solid solution catalyst becomes more readily regenerable via providing the visible-light induced MMCT of TiIV-O-MnIII f TiIII-O-MnIV, an additional channel for the regeneration of Mn4+. This, we believe, is the main cause of the increased RhB degradation over the solid solutions under visible light irradiation observed in Figure 6B. Meanwhile, the production of the superoxide anion radical O2-• from photogenerated Ti3+ may also contribute to the RhB degradation. Acknowledgment. The work described in this paper was supported by grants from the National Natural Science Foundation of China (20673145) and National Basic Research Program of China (973 Program, No. 2007CB613302). References and Notes (1) Kobayashi, M.; Matsumoto, H.; Kobayashi, H. J. Catal. 1971, 21, 48. (2) Jiang, S. P.; Ashton, W. R.; Tseung, A. C. C. J. Catal. 1991, 131, 88. (3) Zhang, H.; Huang, C. EnViron. Sci. Technol. 2005, 39, 4471. (4) Petrie, R. A.; Grossl, P. R.; Sims, R. C. EnViron. Sci. Technol. 2002, 36, 3744. (5) Hoffmann, M. R.; Matin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (6) Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993. (7) Wang, X.; Li, Y. D. J. Am. Chem. Soc. 2002, 124, 2880. (8) Subramanian, V.; Zhu, H. W.; Vajtai, R.; Ajayan, P. M.; Wei, B. Q. J. Phys. Chem. B 2005, 109, 20207. (9) Wang, X.; Li, Y. D. Chem. Eur. J. 2003, 9, 300. (10) Liang, S.; Teng, F.; Bulgan, G.; Zong, R.; Zhu, Y. F. J. Phys. Chem. C 2008, 112, 5307. (11) Nico, P. S.; Zasoski, R. J. EnViron. Sci. Technol. 2001, 35, 3338. (12) Xu, R.; Wang, X.; Wang, D. S.; Zhou, K. B.; Li, Y. D. J. Catal. 2005, 229, 206. (13) Craciun, R.; Dulamita, N. Ind. Eng. Chem. Res. 1999, 38, 1357. (14) Ramesh, K.; Chen, L.; Chen, F.; Liu, Y.; Wang, Z.; Han, Y.-F. Catal. Today 2008, 131, 477. (15) Tian, Z. R.; Xia, G. G.; Luo, J.; Suib, S. L.; Navrotsky, A. J. Phys. Chem. B 2000, 104, 5035.

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