MgF2

Nov 18, 2009 - College of Biotechnology and Pharmaceutical Engineering, Nanjing University of Technology, Nanjing 210009, P. R. China, and Microvast I...
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21106

J. Phys. Chem. C 2009, 113, 21106–21113

Characterization of Photodegradation Catalyst 9.1% VOx/MgF2 Fei Chen,† He Huang,† and Xiao P. Zhou*,‡ College of Biotechnology and Pharmaceutical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, P. R. China, and MicroVast Inc., 12603 Southwest Freeway, Suite 210, Stafford, Texas 77477 ReceiVed: June 25, 2009; ReVised Manuscript ReceiVed: October 20, 2009

Catalysts with the composition 9.1% VOx/MgF2 were prepared at different calcination temperatures and tested for the acetone photodegradation reaction under visible light. The catalysts calcined below 773 K were found to contain V2O5 and MgF2 phases and to show high activities for acetone photodegradation reaction under visible light. When the catalysts were calcined above 773 K, the V2O5 phase reacted with the MgF2 phase to form MgO, Mg2V2O7, and unknown new phase(s) that are not active for acetone photodegradation reaction. XRD, SEM, laser Raman, FT-IR, and UV-vis characterizations of the 9.1% VOx/MgF2 catalysts suggest that the V2O5 domains dispersed in MgF2 matrix are the active sites. 1. Introduction As an effective approach for organic pollutant degradation, the catalytic photodegradation of organic pollutants has been investigated extensively.1-5 TiO2 has been proven to be an excellent photodegadation catalyst for almost all organic compounds.5-9 However, TiO2 is active only under ultraviolet light. To carry out chemical reactions over TiO2 catalysts, a special ultraviolet light source is needed. Solar light is the cheapest light source, but most solar light is visible light, which could not drive chemical reactions over TiO2 catalysts. Hence, to make use of solar light, it is highly desired to develop highefficiency visible-light-active catalysts. It is known that transition-metal oxides, such as V2O5, Cr2O3, Mn2O3, MnO2, Fe2O3, CoO, Co2O3, NiO, and CuO, absorb visible light. However, these metal oxides are not active photodegradation catalysts. This suggests that the absorption of visible light is not accompanied by the generation of separated electron-hole pairs that are able to lead to reactions. Very likely, the Mn+ centers, although acting as absorption centers of light, also favor the recombination of carriers, thus suppressing the photoactivities of these metal oxides. For the purpose of making this type of metal oxide active for photodegradation reactions of organic pollutants, Zhou et al.10,11 dispersed V2O5 into MgF2 or LaF3 matrixes to prepare catalysts and demonstrated the visible-light activities of these catalysts for the photodegradation of acetone, methanol, and benzene. Their investigations also showed that doping Pt, Pd, or Rh into VOx/MgF2 improved the activities of the catalysts. As pointed out by Zhou et al., metal fluorides, such as MgF2 and LaF3, might be the ideal matrixes to stabilize the electron-hole pairs. The investigation of catalyst V2O5/LaF3 revealed that the V2O5 domains in LaF3 matrix11 are the photoexciting sites, whereas other phases, such as LaVO4, are not active. As reported by Zhou et al., V2O5/MgF2 is more active than V2O5/LaF3 for acetone degradation under visible light.11,12 However, the correlations between the phase structure and the activity of V2O5/MgF2 catalysts are still not fully understood. Hence, we believe that it is necessary to conduct detailed investigations into this aspect. * Corresponding author. E-mail: [email protected]. Phone: 8673188821017. Fax: 8673188821017. † Nanjing University of Technology. ‡ Microvast Inc.

In a previous investigation, we found that VOx/MgF2 catalyst with a vanadium loading of 9.1% (calculated based on V and Mg atomic mole numbers) has the highest activity.12 Hence, 9.1% VOx/MgF2 catalyst was selected as the “model catalyst” for a detailed investigation in this work. A wide range of techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), laser Raman spectroscopy, FT-IR spectroscopy, and UV-vis spectroscopy were applied in the characterization of the catalysts. 2. Experimental Section 2.1. Catalyst Preparation. Catalysts composed of 9.1% VOx/ MgF2 (where the percentage is in mole percent calculated from the V and Mg mole numbers) were prepared by the following method: Mg(NO3) · 6H2O (41.360 g) was dissolved in H2O (100 mL) to obtain solution A. NH4VO3 (1.890 g) was dissolved in 100 mL of H2O at 353 K to obtain solution B (which is stable even at room temperature). NH4F (11.940 g) was dissolved in H2O (100 mL) to obtain solution C. Solution C was added to solution A under stirring to obtain a gel mixture, denoted D. Solution B was added to mixture D under vigorous stirring to obtain a mixture. The mixture was aged at room temperature for 12 h, dried at 293 K in an oven for 10 h, and then calcined in air at 723 K for 5 h to obtain 9.1% VOx/MgF2 catalyst. Except for the calcination temperatures, other 9.1% VOx/MgF2 catalysts calcined at different temperatures (523, 573, 623, 673, 773, 823, 873, and 1173 K) were prepared by the same method. The catalyst 9.1% VOx/MgF2-m was prepared by ball-milling V2O5 (bought from Adrich) powder and MgF2 (bought from Adrich) powder at room temperature for 24 h. The 9.1% VOx/CaF2 catalyst was prepared by the same method as used for 9.1% VOx/MgF2 with Ca(NO3)2 · 4H2O, NH4VO3, and NH4F as precursors. The 9.1% VOx/SrF2 catalysts were also prepared by the same method as used for 9.1% VOx/MgF2 with Sr(NO3)2, NH4VO3, and NH4F as precursors. The calcination temperatures of 9.1% VOx/CaF2 and 9.1% VOx/SrF2 catalysts are provided in the corresponding sections. 2.2. Catalyst Testing. Acetone photodegradation reactions were carried out in a Pyrex glass (passing band wavelength λ > 320 nm) tube reactor (i.d., 9.0 mm; o.d., 14.0 mm; length, 60.0 cm). Two 500-W high-pressure mercury lamps were used as light sources. Between the reactor tube and the mercury

10.1021/jp905963b  2009 American Chemical Society Published on Web 11/18/2009

VOx/MgF2 Catalyst Characterization lamps, there were optical filters (λ > 380 nm) to cut off UV light when visible light was needed. The configuration of the reactor system was described elsewhere.10-12 The reactor tube was cooled by a fan. In the center of the catalyst bed, there was a thermal probe to monitor the reaction temperature. Because of the heat from the mercury lamps, even though the reactor tube was cooled by the fan, the temperature inside the catalyst bed was still between 383 and 403 K. The flow of oxygen was 5.0 mL/min, which was controlled by a mass flow controller. The reactant mixture of acetone/oxygen was prepared by passing oxygen through a glass bubbler containing liquid acetone. The glass bubbler was placed in a water/ice bath to maintain the temperature at 273 K. The mole concentration of acetone was analyzed to be 10.0% in oxygen. In each reaction, 5.00 g of catalyst was loaded. The bed length of the 9.1% VOx/ MgF2 catalyst was about 11.5 cm. The blank testing was carried out in the dark at the same temperature in the same reactor with an acetone/oxygen gas mixture (containing 10.0% of acetone) over 9.1% VOx/MgF2, which was heated to 403 K. The dark reaction did not show acetone degradation. The reaction products were analyzed on a gas chromatography (GC; Agilent 6890N) instrument with a thermal conductivity detector (TCD) and by gas chromatography/mass spectroscopy (GC/MS; Agilent 6890N/ 5973N). 2.3. Catalyst Charaterization. The specific surface areas of the catalysts were measured by nitrogen adsorption on a Quantansorb SI instrument (made by Quantachrome Instruments). Before measurements, the samples were degassed at 523 K. Thermogravimetric analysis (TGA) was performed on a Netzschsta 449C apparatus between 300 and 875 K. X-ray diffraction (XRD) characterizations of catalysts were carried out on an X-ray diffraction spectroscopy meter (PW3040/ 60 made by Philips). Cu KR was used as the radiation (40 kV/ 40 mA) source. Laser Raman spectra of catalysts were collected on an RM1000 spectrometer (Renishaw) with an Ar ion laser (514.5 nm) as the excitation source. Scanning electron microscopy (SEM) images were recorded on a field-emission scanning electron microscope (LEO-1530). Fourier transform infrared (FT-IR) spectra of catalysts were recorded on an FT-IR spectroscopy meter (Nicolet Nexus 670). The UV-vis spectra of the catalysts were recorded on a UV-vis spectrometer (Nicolet Evolution 500) equipped with an integrating sphere. 3. Results and Discussion 3.1. Catalyst Testing. For the purposes of comparison, V2O5, MgF2, and 9.1% VOx/MgF2-m were tested for the acetone photodegradation reaction. The ball-milled catalyst 9.1% VOx/ MgF2-m has a particle size below 120 nm (analyzed on Mastersizer 2000E made by Malvern Instruments Ltd.) and contains only V2O5 and MgF2 (see Supporting Information Figures 1s and 2s for the XRD spectra and particle size distribution, respectively). The catalytic reactions were carried out under light with λ > 380 nm. At the given reaction conditions, V2O5 gave 5.4% acetone conversion and MgF2 gave 1.5% acetone conversion. At the same reaction conditions, 9.1% VOx/MgF2-m gave 31.4% acetone conversion with CO2, CO, acetaldehyde, and acetol selectivities of 71.2%, 11.4%, 15.1%, and 2.3%, respectively. A trace of acetic acid was also detected in the products by GC/MS. The results indicate that even V2O5 particles dispersed in MgF2 particles are much more active photodegradation catalyst than pure V2O5 or MgF2.

J. Phys. Chem. C, Vol. 113, No. 50, 2009 21107 TABLE 1: Performance Characteristicsa of 9.1% VOx/MgF2 Catalysts Calcined at Different Temperatures for Acetone Photodegradation Reaction under Visible Light (λ > 380 nm) T (K)

conversion (%)

SCO2 (%)

SCO (%)

Sacetol (%)

523 573 623 673 723 773 823 873 1173

77.2 93.8 94.5 95.8 95.4 34.7 4.8 1.2 0

74.2 69.7 67.5 68.6 70.3 71.5 89.8 90.5

20.7 24.4 24.9 25.6 25.2 16.8 10.2 9.4

5.0 5.9 7.6 5.9 4.5 11.6 0 0

a T is the calcination temperature of catalyst. S is the selectivity of product.

TABLE 2: Acetone Degradation Conversions (X) over V2O5, CaF2, SrF2, 9.1% VOx/CaF2, and 9.1% VOx/SrF2 catalyst

X (%)

CaF2 SrF2 9.1% V2O5/CaF2 9.1% V2O5/SrF2 V2O5

1.1 0.9 5.0 7.5 5.4

The reaction results obtained over 9.1% VOx/MgF2 catalysts are listed in Table 1. The catalysts calcined at relatively lower temperatures (below 773 K) show high acetone photodegradation activities. The typical oxidation products of acetone were CO2, CO, acetol (CH3COCH2OH), and H2O. As an example, the selectivities of CO2, CO, and acetol were 68.5%, 25.6, and 5.9%, respectively, over the catalyst calcined at 673 K. The acetone conversion increased from 77.2% to 93.8% when the catalyst calcination temperature was raised from 523 to 573 K. The low activity of the catalyst calcined at 523 K might be caused by the partial decomposition of NH4VO3 at low calcination temperature. This is discussed further in section 3.2, in the discussion of TGA characterization. The acetone degradation conversion increased slightly from 93.8% to 95.8% when the calcination temperature was raised from 573 to 673 K and reached a maximum value at 673 K. When the catalyst 9.1% VOx/MgF2 was calcined at temperatures higher than 723 K, the acetone conversion decreased. There was an abrupt activity decrease when the catalyst was calcined between 723 and 823 K. The variation in acetone conversion with increasing catalyst calcination temperature might be caused by changes in the catalyst phase structure and specific surface area. Catalysts 9.1% VOx/CaF2 and 9.1% VOx/SrF2 (prepared at 723 K) were also tested for the photodegradation reaction of acetone. The acetone conversions are reported in Table 2. For the purpose of comparison, the photodegradation conversions of acetone over V2O5, CaF2, and SrF2 are also included in Table 2. The activities of CaF2, SrF2, and V2O5 for acetone photodegradation reaction are low. Catalysts 9.1% VOx/CaF2 and 9.1% VOx/SrF2 show only very low activities for acetone photodegradation reaction. The acetone conversions over 9.1% VOx/CaF2 and 9.1% VOx/SrF2 were 5.0% and 7.5%, respectively. The low catalytic activities of catalysts 9.1% VOx/CaF2 and 9.1% VOx/SrF2 might be determined by their phase structures. 3.2. Characterization of Catalysts. Table 3 shows the specific surface areas of 9.1% VOx/MgF2 catalysts calcined at different temperatures. With an increase of the calcination temperature from 523 to 723 K, the specific surface area of the catalyst decreases, but the magnitude of the decrease is small

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TABLE 3: Specific Surface Area of 9.1% VOx/MgF2 Calcined at Different Temperatures Ta (K)

Sb (m2/g)

523 573 623 673 723 773 823 1173

126.8 104.4 112.7 99.2 82.4 64.9 7.1 0.2

a Catalyst calcination temperature. catalyst.

b

Specific surface area of the

Figure 1. TGA spectrum of NH4VO3.

(from 126.8 to 82.4 m2/g). In this range of calcination temperatures, the change in specific surface area of the catalyst does not cause a significant change in catalytic activity. Hence, the specific surface area is not the major factor influencing the catalytic activity. However, between 773 and 823 K, the specific surface area of the catalyst decreases significantly with increasing calcination temperature. The decrease in catalytic activity is in good agreement with the decrease in specific surface area between 773 and 823 K. However, in the calcination temperature range of 723-773 K, the specific surface area of catalyst changed only from 82.4 to 64.9 m2/g, which was not a large change, but the acetone conversion decreased from 95.4% to 34.7%, which corresponds to an unusually large decrease in catalytic activity. Such a significant decrease in catalytic activity might not be caused solely by the decrease in specific surface area. There might be other reasons that lead to such a significant decrease in catalytic activity. The SEM images (see Supporting Information Figure 3s) of catalyst 9.1% VOx/MgF2 calcined at different temperatures show that, below a calcination temperature of 723 K, the catalyst had an average particle size of less than 20 nm. With increasing calcination temperature, the crystals grew larger. Upon calcination of the catalyst at 823 K, the average particle size of the catalyst reached about 100 nm. When the catalyst was calcined above 873 K, it finally fused. The SEM images of catalysts calcined at different temperatures show that the crystals grow increasingly large with increasing calcination temperature, which is consistent with the change of the specific surface area versus calcination temperature. Figure 1 shows the TGA results for NH4VO3. The TGA results show that, if NH4VO3 were completely decomposed to V2O5, the compound would have to have been heated to 638 K

Chen et al. and the sample would have had a mass loss of 22.2%. This is in good agreement with the change in catalytic activity as a function of catalyst calcination temperature. The data in Table 1 show that the catalyst calcined at 673 K has the highest acetone degradation conversion. The results indicate that the catalyst could reach the highest activity only if the ammonium in the catalyst was fully driven out. At 523 K, the mass loss of NH4VO3 reaches only 15.5%. Therefore, part of the NH4VO3 must still be left in the catalyst and must not act in the role of active sites. The low activity of the catalyst calcined at 523 K might be caused by the presence of this small amount of NH4VO3 in the catalyst. The TGA results indicate that the catalyst could reach the highest activity upon calcination at or above 638 K. The data in Table 1 also show that the catalyst calcination temperature should be controlled below 773 K. Hence, to prepare the most active catalyst, the calcination temperature should be selected in the range of 638-723 K. Figure 2 shows the XRD patterns of 9.1% VOx/MgF2 catalysts calcined at different temperatures. The catalysts calcined between 523 and 723 K contain only tetragonal MgF2 and orthorhombic V2O5 phases (Figure 2A). The average crystal size of MgF2 in the catalyst calcined at 723 K was calculated to be 17.5 nm from the XRD data. The XRD peaks of V2O5 were too weak for the average crystal size of V2O5 to be calculated. From the SEM image, we know that V2O5 and MgF2 have similar average crystal sizes in the catalyst calcined at 723 K. Hence, the average crystal size of V2O5 should be close to 17.5 nm. As mentioned with respect to TGA, catalysts calcined below 638 K might still contain some NH4VO3, but we did not find NH4VO3 in catalysts calcined at 523 and 573 K by XRD. Our TGA results show that almost 70% of the NH4VO3 decomposed at 500 K. Because the mole concentration of V is only 9.1%, the concentration of the undecomposed NH4VO3 at 523 K should be lower than 2.4%. The XRD analysis might not be able to detect 2.4% of NH4VO3 in catalyst. The SEM images of catalysts calcined below 723 K show that the catalysts have uniform particle sizes. It is not possible to distinguish V2O5 particles from MgF2 particles (based on morphology) observed by SEM. Hence, the V2O5 domains (the V2O5 domains are not V2O5 molecular units but rather could be composed of many V2O5 units of nanometer-level size) are uniformly dispersed in the MgF2 matrix. The catalyst calcined at 773 K (Figure 2B) contains unknown phase(s). At even higher calcination temperatures (Figure 2C), new phases are developed. Some of the diffraction peaks could be assigned to MgO and Mg2V2O7 phases. The XRD analysis indicates that V2O5 reacted with MgF2 at high calcination temperatures to form the new phases MgO and Mg2V2O7, as well as unknown phases that are not active for acetone photodegradation reaction. The catalyst calcined below 773 K has relatively higher activity for acetone photodegradation, and in this case, the catalyst contains MgF2 and V2O5 phases as the major components. At 773 K, V2O5 starts to react with MgF2 to form new phase(s), and the catalyst loses activity. At 823 K, almost all of the V2O5 reacts with MgF2 to form Mg2V2O7, MgO, and other unknown phase(s), and the resulting catalyst is not active for acetone photodegradation reaction. Figure 3 shows the XRD patterns of 9.1% VOx/CaF2 catalysts calcined at different temperatures. In 9.1% VOx/CaF2 calcined at 723 K, strong peaks of fluorite structure CaF2 phase and weak peaks of CaV2O6 were observed. No V2O5 phase was observed. Upon calcination of 9.1% VOx/CaF2 at 673 K, the three phases CaF2, CaV2O6, and V2O5 were observed. At even lower calcination temperatures, such as 623 and 573 K, CaF2 and V2O5

VOx/MgF2 Catalyst Characterization

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Figure 3. XRD spectra of CaF2 and 9.1% VOx/CaF2 catalyst calcined at 723 K.

Figure 4. XRD spectra of SrF2 and 9.1% VOx/SrF2 catalyst calcined at 723 K.

Figure 2. XRD spectra of 9.1% VOx/MgF2 catalysts calcined at different temperatures.

phases were observed in the catalyst. The XRD characterization results show that CaF2 reacts much more easily with V2O5 than does MgF2. As mentioned in a previous section, 9.1% VOx/ CaF2 calcined at 723 K is not active for the acetone photodegradation reaction. Figure 4 shows the XRD patterns of 9.1% VOx/SrF2 catalysts calcined at 673 and 723 K. The catalyst calcined at 673 K contains SrF2, Sr5V3O12F, and Sr(NO3)2. No V2O5 phase was found. The XRD results show that the catalyst calcination temperature should not be lower than 673 K; otherwise,

Sr(NO3)2 would not be fully decomposed. At 723 K, only SrF2 and Sr5V3O12F phases were found in the 9.1% VOx/SrF2 catalyst. Again, no V2O5 phase was observed. Raman spectra of 9.1% VOx/MgF2 catalysts calcined at different temperature are shown in Figure 5A-C. MgF2 does not exhibit Raman bands between 100 and 1100 cm-1.12 The Raman bands centered at 103, 145, 195, 282, 303, 403, 483, 529, 700, and 995 cm-1 are assigned to V2O5 species.13-15 The 996 cm-1 band is assigned to the symmetrical stretching vibration of the short vanadium-oxygen bond VdO.16 The band at 700 cm-1 is assigned to the stretching vibration of the VsO bond.14 All of the Raman bands of catalyst 9.1% VOx/MgF2 match the Raman bands of bulk V2O5. The so-called monomeric band (1040 cm-1) was not observed over 9.1% VOx/MgF2 catalyst.17,18 Hence, no monomeric V2O5 exists in the catalyst. The Raman spectra indicate that the catalysts calcined at temperatures from 523 to 673 K have strong peaks of V2O5 species (Figure 5A). Upon calcination of the catalyst at 723 and 773 K, the peaks of V2O5 species become weaker (Figure 5B). When the catalysts are calcined at 823, 873, and 1173 K, the peaks of V2O5 species disappear and new peaks, as shown in Figure 5C, are developed. These new peaks belong to unidentified phases (and could be vibrations from MgO and Mg2V2O7). Referring to the data in Table 1, the Raman characterizations of catalysts also show that the V2O5 phase is

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Chen et al.

Figure 6. Laser Raman spectra of V2O5, CaF2, and 9.1% VOx/CaF2 calcined at 723 K.

Figure 7. Laser Raman spectra of V2O5, SrF2, and 9.1% VOx/SrF2 calcined at 723 K.

Figure 5. Raman spectra of 9.1% VOx/MgF2 catalysts calcined at different temperatures.

the active phase. The catalysts calcined at/or below 773 K contain V2O5 species, and they are active for acetone photodegradation. However, when the catalysts are calcined at/or above 823 K, the V2O5 phase is transformed into other phases that are not active for acetone photodegradation. The Raman characterization results are consistent with the XRD characterization results. Figure 6 shows the Raman spectra of V2O5 (given as a reference), CaF2, and 9.1% VOx/CaF2 calcined at 723 K. The bands at 164, 241, 256, 339, 432, 553, 721, 885, and 959 cm-1 were observed for 9.1% VOx/CaF2. There is no band matching

that observed for bulk V2O5. Hence, there is no V2O5 species in 9.1% VOx/CaF2 calcined at 723 K. We also did not observe isolated VO43- species with a band at 830 cm-1 or a twodimensional polymeric network of distorted octahedra species with a band at 970 cm-1 for 9.1% VOx/CaF2.19,20 In the XRD characterization earlier in this section, CaV2O6 was found to be the vanadium-containing species. Hence, the bands at 241, 339, 432, 553, 721, and 959 cm-1 might be absorption bands of CaV2O6 species. This assignment requires more investigation to be confirmed. The band at 885 cm-1 over 9.1% VOx/CaF2 was also observed on CaF2. It might be caused by certain vibrational mode of the CaF2 lattice, which we cannot assign in detail yet based on the current investigation. It can also be found that weak bands between 930 and 1100 cm-1 observed over CaF2 did not appear over 9.1% VOx/CaF2. This result reveals that, even though 9.1% VOx/CaF2 catalyst contains CaF2 as the major component, the surface structure of 9.1% VOx/ CaF2 is different from that of CaF2. For example, CaV2O6 species might be enriched on the surface of the catalyst. Figure 7 shows Raman spectra of V2O5 (given as a reference), 9.1% VOx/SrF2 (calcined at 723 K), and SrF2. Raman bands at 284, 345, 400, 484, 529, 700, 816, 842, 855, 995, and 1072 cm-1 were observed for 9.1% VOx/SrF2. The bands at 284, 400, 484, 529, 700, and 995 cm-1 are assigned to V2O5 species. The bands at 345, 816, 842, and 855 cm-1 are assigned to SrF2 species. The band at 1072 cm-1 is a new band that could not

VOx/MgF2 Catalyst Characterization

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Figure 9. UV-vis spectra of MgF2, TiO2, and V2O5.

Figure 8. FT-IR spectra of 9.1% VOx/MgF2 catalyst calcined at different temperatures.

be assigned to either V2O5 or SrF2 species. This band might be caused by Sr5V3O12F species. In the XRD characterization of 9.1% VOx/SrF2, no V2O5 phase was found. The reason for this could be that Raman spectroscopy is a more sensitive surface analysis tool than XRD, which can detect even a monolayer of V2O5 polymer on the catalyst surface.13 The weak band of V2O5 indicates that there is only a very small amount of V2O5 left on the surface of the catalyst. Most of the V2O5 was converted into the new compound Sr5V3O12F. Figure 8A shows FT-IR spectra of 9.1% VOx/MgF2 catalysts calcined between 523 and 723 K. Two bands are observed for the catalysts. The band at 1036 cm-1 does not change with increasing calcination temperature. The other band shifts position from 941, 954, 956, and 945 cm-1 to 933 cm-1 when the calcination temperature is raised from 523, 573, 623, and 673 K, respectively, to 723 K. The band at 1036 cm-1 is assigned to the VdO stretching vibration mode of crystalline V2O5.21-25 Nakagawa et al.26 studied monolayer V2O5 species in V2O5/ TiO2 catalysts by FT-IR spectroscopy and found a shift from 1020 cm-1 (pure V2O5) to 980 cm-1 for the VdO stretching frequency. At low vanadium content (below 2.0 wt % V2O5), there was a band at 983 cm-1 due to the amorphous VOx.26 At low vanadium content, VOx supported on Al2O3 and ZrO2 showed only one band at about 970 cm-1.26 Because our 9.1% VOx/MgF2 catalysts were prepared by mixing a MgF2 gel solution with NH4VO3 solution, highly dispersed NH4VO3 species might be packed in the MgF2 gel matrix. After calcination, these MgF2-packed NH4VO3 species might decompose to V2O5 clusters containing a few vanadium atoms that are similar to amorphous VOx. The amorphous VOx species have

vibration bands in the range of 941-956 cm-1. In an investigation of low-V2O5-loading catalysts, Frederickson et al. assigned the FT-IR bands between 990 and 910 cm-1 to the lattice vibrations of V2O4 species over supported catalyst.27,28 The vibration bands between 941 and 956 cm-1 are also located in this wavenumber range. Hence, it is difficult to assign the bands at 941, 954, 956, 945, and 933 cm-1 to VdO stretching vibrations of MgF2-packed amorphous V2O5 species or to lattice vibrations of low-coordinated V2O4 species. The FT-IR characterization indicates that crystalline V2O5 (1036 cm-1) and possibly highly dispersed amorphous V2O5 species are present in 9.1% VOx/MgF2 catalysts calcined between 523 and 723 K. Figure 8B presents FT-IR spectra of 9.1% VOx/MgF2 catalysts calcined between 723 and 1173 K. With increasing calcination temperature, many new IR bands develop. It was found that, as the calcination temperature was increased from 723 to 1173 K, the 933 cm-1 band shifted in the high-wavenumber direction whereas the 1030 cm-1 band shifted in the low-wavenumber direction and, finally, the two bands combined into one band at 971 cm-1 at 1173 K. The position shifting and intensity reducing process of the 1036 cm-1 band with increasing calcination temperature indicates that the typical VdO bond in V2O5 was converted into other chemical bond(s). This is consistent with the XRD characterization results (V2O5 reacted with MgF2). Figure 9 shows the UV-vis spectra of MgF2, TiO2, and V2O5. MgF2 does not absorb light in the wavelength range of 250-700 nm. The TiO2 absorbs ultraviolet light (250-400 nm). V2O5 absorbs light in the wavelength range of 250-600 nm. All three compounds are not active under visible light for acetone photodegradation reaction. The band gap of V2O5 was calculated to be 2.0 eV (based on the UV-vis spectrum). Figure 10 shows UV-vis spectra of 9.1% VOx/MgF2 catalysts calcined at different temperatures. The catalysts calcined at temperatures from 523 to 723 K have an absorption threshold at 600 nm (band gap 2.0 eV), which is almost the same as that of V2O5. The absorption edge of the catalyst calcined at 773 K shifts to shorter wavelength, and the catalyst has low activity under visible light. The catalysts calcined at temperatures from 823 to 1173 K do not absorb much visible light and are not active under visible light. The results of the UV-vis characterization of 9.1% VOx/MgF2 catalysts calcined at different temperature are in good agreement with the catalytic activity testing results in Table 1. Figure 11 shows UV-vis spectra of V2O5 (included as a reference), CaF2, and 9.1% VOx/CaF2 calcined at 723 K. CaF2 does not absorb visible light. Catalyst 9.1% VOx/CaF2 absorbs

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Figure 10. UV-vis spectra of 9.1% VOx/MgF2 catalysts calcined at different temperatures.

Figure 11. UV-vis spectra of V2O5, CaF2, and 9.1% VOx/CaF2 calcined at 723 K.

between 400 and 600 nm. Because no V2O5 phase was found in catalyst 9.1% VOx/CaF2 calcined at 723 K in the XRD and Raman characterizations and the CaV2O6 phase was found to be a new component, the absorption of 9.1% VOx/CaF2 catalyst in visible light (400 and 600 nm) might be caused by CaV2O6. Figure 12 shows UV-vis spectra of SrF2 and 9.1% VOx/ SrF2 calcined at 723 K. The spectra indicate that neither SrF2 nor 9.1% VOx/SrF2 absorb visible light. However, in the Raman characterization of 9.1% VOx/SrF2 catalyst, weak bands of V2O5 species were observed. The reason for this difference might be that Raman spectroscopy is a more sensitive surface analysis tool than XRD and that, even though there is only very small amount of V2O5 on the surface of the catalyst, it gives a Raman signal, but this amount of V2O5 does not absorb much visible light. The spectroscopy characterizations of 9.1% VOx/MgF2 catalysts calcined at different temperatures show that the active sites of the catalysts are MgF2-matrix-isolated V2O5 domains, which could be nanometer-level V2O5 particles or crystals. Even the ball-milled V2O5 and MgF2 powder (particle size below 120 nm) shows high acetone photodegradation activity (31.4% acetone conversion was reached). The 9.1% VOx/MgF2 catalyst calcined at 723 K has a much higher activity for acetone photodegradation than ball-milled V2O5 and MgF2 powder. More than 95% acetone conversion was confirmed for our specific

Chen et al.

Figure 12. UV-vis spectra of SrF2 and 9.1% VOx/SrF2 calcined at 723 K.

reaction conditions. The SEM and XRD investigations showed that 9.1% VOx/MgF2 calcined below 723 K contains MgF2 and V2O5 crystals with an average size of 17.5 nm. When the catalysts were calcined above 773 K, the Raman and FT-IR investigations showed that V2O5 reacts with MgF2 to form new compounds, and the XRD characterization confirmed that the newly formed compounds are MgO, Mg2V2O7, and unknown phases. With the consumption of V2O5 in reaction with MgF2 at high calcination temperatures, the catalyst loses activity and the UV-vis absorption threshold of 9.1% VOx/MgF2 catalyst shifts to the ultraviolet light range. All of these phenomena indicate that nanometer-level V2O5 domains (from a few nanometers to hundreds of nanometers) are the active sites. The role of the MgF2 matrix is to isolate the individual V2O5 domains to retard the recombination of the photoexcited electron-hole pairs, which is similar to the mechanism of LaF3-isolated V2O5 domains in catalyst VOx/LaF3.11 This also explains why pure V2O5 is not active, because there is nothing to retard electron-hole pair recombination over pure V2O5. The viewpoint of MgF2-isolated V2O5 domain active sites is also supported by our investigations of 9.1% VOx/CaF2 and 9.1% VOx/SrF2 catalysts. Our investigations showed that the reaction temperature of V2O5 with CaF2 or SrF2 is much lower than the reaction temperature of V2O5 with MgF2. Even at 723 K, the XRD and Raman investigations confirmed that almost all of the V2O5 was converted into new compounds CaV2O6 and Sr5V3O12F, respectively. In this case, no V2O5 domains are present in 9.1% VOx/SrF2 or 9.1% VOx/CaF2. The 9.1% VOx/ SrF2 material does not absorb visible light and shows very low activity for the acetone photodegradation reaction. In contrast, 9.1% VOx/CaF2 absorbs visible light (400-600 nm), but is not active. The reason could be that the electron-hole pairs recombine too fast to allow the chemical reaction to occur. The reaction between V2O5 and CaF2 could lead to a large number of lattice defects. Although a suitable lattice defect concentration might favor the formation of electron-hole pairs, a high lattice defect concentration would enhance electron-hole pair recombination.29 The UV-vis absorption investigations of the catalysts also offer some support for so-called MgF2-matrix-isolated V2O5 domain active sites. The band gap of pure V2O5 is 2.0 eV (calculated from the UV-vis spectrum of V2O5). This band gap is the energy difference between the valence band and the conduction band of V2O5. The valence band is made up of O

VOx/MgF2 Catalyst Characterization 2p orbitals and V 3d orbitals. The conduction band is made up of V 3d orbitals and O 2p orbitals. In the valence band, the bottom part is mainly composed of the O 2p and V 3d orbitals, and the top part is mainly composed of the O 2p orbitals. In the conduction band, the bottom part is mainly composed of V 3d orbitals, and the top part is mainly composed of V 3d and O 2p orbitals. The band gap is actually close to the difference between the O 2p orbital and the V 3d orbital.30,31 For 9.1% VOx/MgF2 catalyst, we also calculated the band gap from the UV-vis spectrum. The 9.1% VOx/MgF2 catalyst calcined at 723 K has a band gap of 2.0 eV, which is the same as that of pure V2O5. The results indicate that the energy gap between the valence band and the conduction band does not change after V2O5 domains are dispersed into the MgF2 matrix. This suggests that the orbitals of Mg and F atoms do not take part in the formation of the valence and conduction bands of the catalyst, which does not influence the width of the energy gap. In other words, the photoexciting sites are still V2O5 crystals. 4. Conclusions SEM, XRD, laser Raman, and FT-IR characterizations show that 9.1% VOx/MgF2 calcined below 773 K contains V2O5 domains that are dispersed in a MgF2 matrix. These V2O5 domains, having the same UV-vis absorption threshold as bulk V2O5, act as the active sites for the acetone photodegradation reaction. When the 9.1% VOx/MgF2 catalyst is calcined at or above 773 K, the V2O5 phase reacts with MgF2 to form MgO, Mg2V2O7, and other unknown phases that are not active for the photodegradation reaction. Hence, in 9.1% VOx/MgF2 catalysts, the V2O5 domains are the necessary sites that control the generation of electron-hole pairs under the irradiation of light, and the inert MgF2 matrix acts in the role of isolating these V2O5 domains to retard the recombination of electron-hole pairs. The narrow band gap of V2O5 guarantees the visiblelight activity of these catalysts to generate electron-hole pairs and the isolating effect of the MgF2 matrix with respect to the V2O5 domains guarantees a long enough lifetime of the electron-hole pairs to allow chemical reactions occur. Acknowledgment. This work was supported by the State Key Laboratory of Materials Oriented Chemical Engineering of Nanjing University of Technologies (PJ 38901122). Supporting Information Available: XRD spectra of analyzed materials, particle size distribution of 9.1% VOx/MgF2-

J. Phys. Chem. C, Vol. 113, No. 50, 2009 21113 m, and SEM images of 9.1% VOx/MgF2 calcined at different temperatures. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tang, J.; Zou, Z.; Ye, J. Chem. Mater. 2004, 16, 1644. (2) Hoffmann, M. R.; Martin, S. T.; Chio, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (3) Mills, A.; Hunte, S. L. J. Photochem. Photobiol. A 1997, 108, 1. (4) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C 2002, 1, 1. (5) Chatterjee, D.; Dasgupta, S. J. Photochem. Photobiol. C 2005, 6, 186. (6) Linsebigler, A. L.; Lu, G.; Yates, J. T. Chem. ReV. 1995, 95, 735. (7) Hu, Y.; Yuan, C. W. J. Cryst. Growth 2005, 274, 563. (8) Muggli, D. S.; Ding, L.; Odland, M. J. Catal. Lett. 2002, 78, 23. (9) Ma, W.; Li, J.; Tao, X.; He, J.; Xu, Y.; Zhao, J. C. Angew. Chem., Int. Ed. 2003, 42, 1029. (10) Chen, F.; Wu, T. H.; Zhou, X. P. Catal. Commun. 2008, 9, 1698. (11) Wang, J.; Chen, F.; Zhou, X. P. J. Phys. Chem. C 2008, 112, 9723. (12) Chen, F.; Wang, J.; Xu, J. Q.; Zhou, X. P. Appl. Catal. A: Gen. 2008, 348, 54. (13) Bond, G. C.; Tahir, S. F. Appl. Catal. 1991, 71, 1. (14) Wang, X. J.; Li, H. D.; Fei, Y. J.; Wang, X.; Xiong, Y. Y.; Nie, Y. X.; Feng, K. A. Appl. Surf. Sci. 2001, 177, 8. (15) Jayalakshimi, M.; Rao, M. M.; Venugopal, N.; Kim, K. B. J. Power Sources 2007, 166, 578. (16) Dines, T. J.; Rochester, C. H.; Ward, A. M. J. Chem. Soc., Faraday Trans. 1991, 87, 653. (17) Ismail, A. A.; Ibrahim, I. A.; Mohamed, R. M. Appl. Catal. B: EnViron. 2003, 45, 161. (18) Das, N.; Eckert, H.; Hangchun, H.; Wachs, I. E.; Walzer, J. F.; Feher, F. J. J. Phys. Chem. 1993, 97, 8240. (19) Roozeboom, F.; Mittelmeijer-Hazeleger, M. C.; Moulijn, J. A.; Medema, J.; Beer, V. H. J.; Gellings, P. J. J. Phys. Chem. 1980, 84, 2783. (20) Roozeboom, F.; Medema, J.; Gellings, P. J. Z. Phys. Chem. N.F. 1978, 111, 215. (21) Sohn, J. R.; Doh, I. J.; Pae, Y. I. Langmuir 2002, 18, 6280. (22) Sam, D. S. H.; Soenen, V.; Volta, J. C. J. Catal. 1990, 123, 417. (23) Sohn, J. R.; Cho, S. G.; Pae, Y. I.; Hayashi, S. J. Catal. 1996, 159, 170. (24) Inomata, M.; Miyamoto, A.; Murakami, Y. J. Catal. 1980, 62, 140. (25) Tarama, K.; Yoshida, S.; Ishida, S.; Kakioka, H. Bull. Chem. Soc. Jpn. 1969, 41, 2840. (26) Nakagawa, Y.; Ono, K.; Miyata, H.; Kubokawa, Y. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2929. (27) Frederickson, L. D.; Hausen, D. M. Anal. Chem. 1963, 35, 818. (28) Mori, K.; Miyamoto, A.; Murakami, Y. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3303. (29) Gracia, F.; Holgado, J. P.; Caballero, A.; Gonzalez-Elipe, A. R. J. Phys. Chem. B 2004, 108, 17466. (30) Zhang, L.; Fu, H.; Zhang, C.; Zhu, Y. J. Solid State Chem. 2006, 179, 804. (31) Clauws, P.; Vennik, J. Phys. Status Solidi (b) 1975, 69, 491.

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