J. Phys. Chem. C 2008, 112, 9723–9729
9723
Photocatalytic Degradation of Acetone over a V2O5/LaF3 Catalyst under Visible Light Jie Wang,† Fei Chen,†,‡ and Xiao P. Zhou*,†,‡ Institute of Physical Chemistry, Zhejiang Normal UniVersity, Jinhua, 321004, China, and MicroVast, Incorporated, 12603 Southwest Freeway, Suite 210, Stafford, Texas 77477 ReceiVed: February 17, 2008; ReVised Manuscript ReceiVed: April 9, 2008
Visible light active photodegradation catalysts were prepared by doping V2O5 into a LaF3 matrix. In the photodegradation of acetone, the highest conversion was obtained over a catalyst containing 16.0 atom % vanadium. When VOx/LaF3 catalysts were calcined below 673 K, a LaF3 phase was observed in XRD characterization. At even higher catalyst calcination temperatures (higher than 673 K), LaF3 and LaVO4 phases were observed in VOx/LaF3 catalysts. The laser Raman and FTIR characterizations indicated that there are V2O5 species (possibly amorphous) and LaVO4 phases formed in the VOx/LaF3 catalysts. The photoluminescence characterization showed that the photoelectron-hole pair recombination rate over the VOx/LaF3 catalyst was slower than that over pure V2O5. The UV-vis spectrum of VOx/LaF3 indicates that this material is sensitive under visible light with wavelengths of 320 (%) X>380 (%)
99.5 9.6
5.2 0
9.6 5.4
a X>320 is acetone conversion when λ > 320 nm. X>380 is acetone conversion when λ > 380 nm.
Figure 1. Photocatalytic reactor.
12 h and then calcined at 673 K for 4 h to obtain LaF3. V2O5 was obtained by calcining ammonium metavanadate at 673 K for 4 h. The V2O5 doped LaF3 catalysts were prepared according to the following procedure. A total of 1.1374 g of ammonium metavanadate NH4VO3 (99%) was dissolved in 120 mL of H2O to obtain solution C. A total of 5.9083 g of ammonium fluoride (96%) was dissolved in 40.0 mL of H2O to obtain solution D. Solution C was added to solution D with stirring to obtain mixture E. A total of 18.8988 g of La(NO3)3 · xH2O (44% La2O3) was dissolved in 50.0 mL of H2O to obtain solution F. Solution F was added to mixture E with vigorous stirring to obtain a suspension. The suspension was dried at 393 K in an oven for 12 h and then calcined in air at 673 K for 4 h to obtain catalyst 16.0 atom % VOx/LaF3. Other catalysts with different vanadium (atomic molar) concentrations and catalysts calcined at different temperatures were prepared by a similar method, only the vanadium concentration or calcination temperature was changed. 2.2. Catalyst Testing. Catalytic reactions were carried out in a Pyrex glass (passing band wavelength λ > 320 nm) tube reactor (i.d. 9.0 mm and o.d. 14 mm). Two 500 W high pressure mercury lamps were used as light sources, and in front of the mercury lamps, there were optical filters (λ > 380 nm) to eliminate UV light when visible light was needed (Figure 1). The reactor tube was cooled by a fan. In the center of the catalyst bed, there was a thermal probe that was used to monitor the reaction temperature. Because of the heat from the mercury lamps, even when we tried to cool the reactor with a fan, the temperature inside the catalyst bed was still between 120 and 130 °C. 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 that contained liquid acetone. The glass bubbler
was placed in a water-ice bath to maintain a temperature of 0 °C. The mol concentration of acetone was analyzed as 10.0%. In each reaction, 5.00 g of catalyst was used. The bed length of the VOx/LaF3 catalyst was ∼11.5 cm. To rule out thermal reactions, the 16.0 atom % VOx/LaF3 catalyst was tested for acetone oxidation in dark at the same reaction temperature of 130 °C. The dark reaction did not show acetone degradation. The reaction products were analyzed on a GC (Agilent 6890N) instrument with TCD and GC/MS (Thermo GC/MS DSQ). All data were collected after 5 h of online reaction. 2.3. Catalyst Characterizations. The XRD characterizations of catalysts were carried out on an X-ray diffraction spectroscopy meter (Philips PW3040/60) using Cu KR radiation (40 kV/40 mA). The specific surface areas (SBET) of catalysts were measured by nitrogen adsorption on Autosorb-1 (made by Quantachrome Instruments). The Raman spectra of catalysts were collected on a RM1000 spectrometer (Renishaw) with an Ar ion laser (514.5 nm) as the excitation source. The FTIR spectra of catalysts were recorded on a FTIR spectroscopy meter (Nicolet Nexus 670) with a resolution of 4 cm-1. The UV-vis spectra of catalysts were recorded on a UV-vis spectrometer (Nicolet Evolution 500) equipped with an integrating sphere. The photoluminescence (PL) spectra of catalysts were collected on Horiba Jobin Yvon (FL3-P-TCSPC) instrument. The light source was a Xe lamp (excitation at 280 nm). 3. Results and Discussion 3.1. Catalyst Testing. The catalytic reactions were carried out in a reaction system as shown in Figure 1. P25 (TiO2 Degussa), LaF3, and V2O5 were tested in the photodegradation reaction of acetone. Results are listed in Table 1. Under light with λ > 320 nm, P25 had a high activity for acetone degradation. An acetone conversion of 99.5% was obtained. LaF3 and V2O5 had low activities for acetone degradation. LaF3 is not active under visible light (λ > 380 nm), but P25 and V2O5 showed low activities. Figure 2 shows the photodegradation conversions of acetone over VOx/LaF3 with different vanadium concentrations under both UV (λ > 320 nm) and visible lights (λ > 380 nm). The acetone degradation conversion increased with an increase in vanadium concentration from 0 to 16.0 atom % and decreased at even higher vanadium concentrations. The highest acetone conversion was obtained on catalyst 16.0 atom % VOx/LaF3. The acetone conversion reached 75.9% under UV light and 58.4% under visible light. The results of Table 1 and Figure 2 show that under UV light, P25 is more active than VOx/LaF3 catalysts. However, under visible light, the VOx/LaF3 catalysts are more active than P25. The VOx/LaF3 catalysts are more active for acetone photodegradation than both LaF3 and V2O5 under both UV and visible light. Clearly, the doping of V2O5 in LaF3 formulated a series of active photodegradation catalysts. In the photodegradation of acetone, the typical products were CO2, CO, acetol (CH3COCH2OH), and H2O. As an example, for the photodegradation of acetone over 16.0 atom % VOx/ LaF3 under visible light, the selectivities of CO2, CO, and acetol were 62.7, 29.4, and 7.9%, respectively.
V2O5 Doped LaF3 Photodegradation Catalyst
J. Phys. Chem. C, Vol. 112, No. 26, 2008 9725 TABLE 2: Specific Surface Areas of Catalysts Calcined at 673 Ka catalyst
S (m2/g)
LaF3 V2O5 10.0 atom % VOx/LaF3 12.0 atom % VOx/LaF3
5.3 9.3 26. 4 26.5
a
S (m2/g)
catalyst 14.0 16.0 18.0 20.0
atom atom atom atom
% % % %
VOx/LaF3 VOx/LaF3 VOx/LaF3 VOx/LaF3
27.0 25.2 20.0 19.4
S is the specific surface area of catalyst.
TABLE 3: Specific Surface Area of 16.0 Atom % VOx/LaF3 Catalyst Calcined at Different Temperaturesa T (K) 573 623 673 a
S (m2/g)
T (K)
S (m2/g)
31.7 29.5 25.2
723 773
21.4 9.7
T is catalyst calcination temperature. S is specific surface area.
Figure 2. Acetone conversions over VOx/LaF3 catalysts (calcined at 673 K) with different vanadium concentrations under light with wavelengths λ > 320 and 380 nm.
Figure 4. XRD spectra of catalysts (calcined at 673 K) with different vanadium concentrations.
Figure 3. Acetone conversions over 16.0 atom % VOx/LaF3 catalyst calcined at different temperatures under light with wavelengths λ > 320 and 380 nm.
Figure 3 shows acetone conversions under both UV and visible light over catalyst 16.0 atom % VOx/LaF3, which was calcined at different temperatures. The acetone conversion increased with the increase of calcination temperature from 573 to 673 K and reached maximum conversion at 673 K. When the catalyst was calcined at even higher temperatures than 673 K, the acetone conversion decreased. The change of acetone conversion with catalyst calcination temperature might be caused by changes in the catalyst phase structure and specific surface area. Table 2 lists the specific surface areas of catalysts calcined at 673 K but having different vanadium concentrations. The VOx/LaF3 catalysts have higher specific surface areas than pure V2O5 or LaF3. With the increase of vanadium concentration from 10.0 to 16.0 atom %, the specific surface area of the catalyst changes, but the change is very small (25.2-27.0 m2). Such a small change in specific surface area does not match the large change in catalytic activities of the catalysts (Figure 2). Hence, for catalysts having different vanadium concentrations, the influence of specific surface area on catalytic activity is not the
most important factor. The most important factor that influences the catalytic activity is the composition of the catalyst. Catalysts having the same chemical composition, but calcined at different temperatures, show large differences in catalytic activities. Figure 3 shows that catalyst 16.0 atom % VOx/LaF3 calcined at 673 K has the highest activity. The data in Table 3 show that with the increase of the catalyst calcination temperature, the specific surface area decreased. However, there is no direct correspondence between the catalytic activity and the specific surface area. The catalyst calcined at 673 K (25.2 m2) is much more active (see Figure 3) than that calcined at 573 K (31.7 m2) and 723 K (21.4 m2) under visible light. Hence, the phase structure of catalysts might be the major factor that influences catalytic activity. There might be significant differences in catalyst structure when the calcination temperature changed from 673 to 773 K. 3.2. Characterization of Catalysts. Figure 4 shows the XRD patterns of the VOx/LaF3 catalysts with different vanadium concentrations after calcination at 673 K for 4 h. The pure LaF3 is in the hexagonal phase (consistent with the JCPDC 00-0080416). With the increase of vanadium loadings from 16.0 to 20.0 atom %, only the hexagonal LaF3 phase was observed. Figure 5 shows the XRD patterns of the catalyst 16.0 atom % VOx/LaF3 calcined at different temperatures. When the catalyst was calcined at temperature below 723 K, there was
9726 J. Phys. Chem. C, Vol. 112, No. 26, 2008
Figure 5. XRD spectra of catalyst 16.0 atom % VOx/LaF3 calcined at different temperatures.
only hexagonal LaF3 phase. When the catalyst was calcined at even higher temperatures, a new phase of LaVO4 (monoclinic, JCPDC 00-023-0324), which has peaks at 26.2, 28.9, 30.1, 32.8, 39.6, 40.3, and 41.2°, appeared. The XRD analysis indicated that at higher calcination temperatures, vanadium component(s) reacted with LaF3 to form LaVO4. From the XRD characterization results of Figures 4 and 5, when the VOx/LaF3 catalysts were calcined at temperatures below 723 K, there was no new phase formation, and the vanadium component(s) existed as amorphous phase(s), which do not show XRD peaks. At higher calcination temperatures (above 723 K), the vanadium component(s) reacted with LaF3 to form LaVO4, and in this case, the catalysts lost activity. To obtain more information on the vanadium component(s), we conducted calculations of lattice constants based on XRD data in Figures 4 and 5 (by using MDI Jade 5.0). The results are listed in Table 4. The lattice constants changed randomly with the increase in vanadium concentration. There is no clear lattice constant increase or decrease trend with the increase in vanadium concentration. The maximum length variation in a or b is 0.13% and in c is 0.28%. If there is up to 20.0% vanadium component(s) doped in the lattice of LaF3, there should be large changes in the lattice constants of LaF3. On the basis of these results, we believe that there might be a certain amount of vanadium component(s) doped into the lattice of LaF3, but some of the vanadium component(s) might still stay out of the LaF3 lattice. The results in Table 5 also show that when the 16.0 atom % VOx/LaF3 catalyst was calcined at high temperatures (higher than 723 K), even the vanadium component(s) reacted with LaF3 to form LaVO4, but there was still no large change found in the LaF3 lattice constants. Although the lattice constant data do not provide strong support for our viewpoint, we believe that some of the vanadium component(s) exist outside of the LaF3 lattice. They may be amorphous V2O5 species. Figure 6 shows the Raman spectra of LaF3, V2O5, and catalysts with different vanadium concentrations. LaF3 does not show Raman bands between 100 and 1100 cm-1. As reported in the literature,29–31 the Raman bands centered at 103, 145, 195, 282, 303, 403, 483, 529, 700, and 995 cm-1 are assigned to V2O5, while the peaks at 860 and 375 cm-1 are assigned to LaVO4.32,33 As shown in Figure 6, all of the VOx/LaF3 catalysts have peaks that could be assigned to V2O5 and LaVO4. However, when the vanadium concentration is higher than 12.0%, the bands of V2O5 are much stronger than those of LaVO4. At relatively lower vanadium concentrations (lower than 14.0%), the Raman bands of LaVO4 are stronger than those of V2O5. The results indicate that there were V2O5 species and
Wang et al. LaVO4 phases on the surface of VOx/LaF3 when catalysts were calcined at 673 K. The Raman spectra of catalyst 16.0 atom % VOx/LaF3 calcined at different temperatures are shown in Figure 7. It was found that catalysts calcined at 573 and 623 K only have peaks ascribed to V2O5. When the catalyst was calcined at temperatures higher than 673 K, Raman bands of LaVO4 appeared. At even higher calcination temperatures of 723 and 773 K, the Raman bands of V2O5 disappeared, and only the peaks of LaVO4 were left. The photocatalytic reaction results indicate that the catalyst calcined at higher temperatures (equal to or higher than 723 K) shows a very low photodegradation activity for the acetone degradation reaction. Hence, we believe that possibly only the highly dispersed V2O5 species are active for the acetone degradation reaction, while the LaVO4 phases are not active. Figure 8 shows the FTIR spectra of the VOx/LaF3 catalysts with different vanadium concentrations. The absorption shoulder at 1020 cm-1 is assigned to the VdO stretching vibration in V2O5.29,34 Generally, the IR band of VdO in crystalline V2O5 shows at 1020-1025 cm-1 and the Raman band at 995 cm-1.35This is consistent with our Raman characterizations. The band at 821 cm-1 is attributed to the coupled vibration between VdO and V-O-V.34,36 The bands at 910 and 837 cm-1 are ascribed to the symmetric and antisymmetric stretching vibrations of the orthovanadate VO43- group, respectively.37,38 Hence, the broad band in the 730-980 cm-1 region could be interpreted as an overlap of the characteristic bands of V2O5 and LaVO4. FTIR characterization results are consistent with those of the Raman characterizations. As shown in Figure 9, the catalyst 16.0 atom % VOx/LaF3 calcined at different temperatures also was characterized by FTIR. The VdO stretching vibration band at 1020 cm-1 and the vibration bands below 900 cm-1, which are assigned to V2O5, were observed in 16.0 atom % VOx/LaF3 calcined at 573 and 623 K.36,39 However, the VdO stretching vibration band at 1020 cm-1 disappeared, and the well-resolved bands at 783, 802, 821, 852, and 910 cm-1 appeared in the spectra of the catalyst that was calcined at 773 K. The bands at 783, 802, 821, 852, and 910 cm-1 are assigned to LaVO4 with a low monoclinic symmetry.37,40 The FTIR characterization results are also consistent to those of Raman results shown in Figure 7. In the XRD characterizations, the V2O5 phase was not found in the atomic concentration region of 10.0-20.0% (Figure 4). The laser Raman (Figure 6) and FTIR (Figure 8) characterizations show that there are V2O5 species in the VOx/LaF3 catalysts. The reason could be that either the size of the V2O5 species is too small to be detectable by XRD or it is amorphous V2O5. However, the laser Raman and FTIR spectra proved the existence of the V2O5 species. The V2O5 species could be transferred to the LaVO4 phase (Figures 5, 7, and 9), when the catalysts were calcined at temperatures higher than 673 K and the catalysts calcined at high temperatures lost activity. Hence, we believe that only highly dispersed V2O5 species are photoactive in VOx/LaF3 catalysts. Pure LaF3 absorbs UV light (λ
