MgF2

ARTICLE pubs.acs.org/IECR

Photocatalytic Degradation of Acetone over Sulfated MoOx/MgF2 Composite: Effect of Molybdenum Concentration and Calcination Temperature Yiming He,*,† Leihong Zhao,‡ Yongjiao Wang,‡ Tingting Li,‡ Tinghua Wu,‡ Xintao Wu,§ and Ying Wu*,‡ †

Department of Materials Physics, Zhejiang Normal University, Jinhua, 321004, People’s Republic of China Institute of Physical Chemistry, Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Zhejiang Normal University, Jinhua, 321004, People’s Republic of China § State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, People’s Republic of China ‡

bS Supporting Information ABSTRACT: This paper presents a novel visible-light-driven catalyst, a SO42
/MoOx/MgF2 composite, which was synthesized by a simple solution method. Multiple techniques, including Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance spectroscopy (DRS) were applied to investigate the physical and photophysical properties of the catalysts. The photocatalytic activities were evaluated in the degradation of acetone in gas phase. In the photodegradation of acetone, the highest conversion was obtained over a catalyst containing 5 mol % molybdenum. The XRD and Raman characterizations indicate that the molybdenum oxide was highly dispersed in the MgF2 matrix. These MoOx species might be the active sites of the catalysts, which is the reason for the visible-light response of the composite catalyst. The MgF2 matrix acts to isolate the MoOx species and retard the electron
hole pair recombination. When the molybdenum concentration is >5 mol %, crystalline MoO3 phase was observed. The large MoO3 particle would decrease the separation efficiency. Thus, the photocatalytic activity was reduced. Besides the molybdenum concentration, the calcination temperature also shows a great effect on the activity. A sulfated 5 mol % MoOx/MgF2 catalyst that was calcined at 350 °C showed the highest photocatalytic activity. Based on the results of the characaterization, the origin of the high activity was discussed. The light absorption ability and the MoOx size effect are considered as the key factors.

1. INTRODUCTION Since 1972, photocatalysis has received considerable attention as one of the most promisingly practical technologies, especially for the photocatalytic decomposition of organic contaminants.1
4 To date, many organics have been found to be decomposed on TiO2. However, the photocatalytic activities of TiO2 are limited to irradiation wavelengths in the ultraviolet (UV) region, because it has a wide band gap and can only absorb UV light instead of moreabundant visible light.2
4 In order to utilize the solar irradiation efficiently, the development of a photocatalyst that is sensitive to visible light is desired. In recent years, there has been considerable interest in the coupling of two types of semiconductor particles with different redox energy levels of their corresponding conduction and valence bands. Such systems have better characteristics in photoexcited charge separation and photoabsorption for visible light. Therefore, the use of two semiconductors in contact can actually be considered to be one of the most important promising methods to synthesize the visible-light-driven photocatalyst. Until now, a large variety of coupled semiconductor systems have been reported, such as CdS/TiO2,5 V2O5/TiO2,6 MoO3/ TiO2,7 WO3/TiO2,7
9 MoS2/TiO2,10 V2O5/YVO4,11 Bi2O3/ SrTiO3,12 CuFeO2/SnO2,13 CdS/KNbO3,14 WO3/SrNb2O6,15 r 2011 American Chemical Society

CuO/ZnO,16 CdS/ZnO,17 Bi2O3/BiOCl,18 and Fe2O3/SnO2 couples.19 For these coupled systems, the semiconductor with a small band gap plays the role of sensitizer, while another semiconductor acts as a retarder for the recombination of electron
hole pairs. In order to obtain high photoquantum efficiency, the successful charge transfer from the sensitizer to the retarder phase is important. Besides, the retarder semiconductor should have some characteristics in charge separation and they are usually good photocatalysts under UV light (TiO2, ZnO, SrTiO3, etc.). However, Zhou et al.20
23 found, for the first time, that a metal fluoride, which is an insulator, can also play the role of retarder. Both the VOx/LaF3 and VOx/MgF2 couples can photodegrade acetone efficiently under visible-light irradiation.20,21 In general, the recombination of electron
hole pairs in metal oxides is fast. One important reason is that a hole, such as •OH, could grab electrons from the vicinity O2
anions (since all of the anions are O2
anions) to create new holes (•OH) and, therefore, lead to holes moving from place to place, which leaves a high possibility Received: November 28, 2010 Accepted: May 5, 2011 Revised: May 1, 2011 Published: May 06, 2011 7109

dx.doi.org/10.1021/ie102389q | Ind. Eng. Chem. Res. 2011, 50, 7109–7119

Industrial & Engineering Chemistry Research for the recombination of electron
hole pairs.22,23 But in metal fluoride, this process could be avoided. This is the reason why a metal fluoride can be a good matrix for separation of the photoelectrons and holes. Based on the work of Zhou, MgF2 shows a higher performance than LaF3.20,21 Hence, coupling MgF2 with a small band gap semiconductor (such as MnO2, MoO3, MoS2, and Fe2O3) might be a new possible route to prepare a high-efficiency photocatalyst. However, except the work of Zhou, no corresponding research has been reported until now. Herein, we present a novel MgF2-based catalyst, a SO42
/ MoOx/MgF2 composite, which shows high photocataltyic performance in photodegradation of acetone under both UV and visible-light irradiation. Actually, this catalyst is a surprising result, because our primary design is a MoS2/MgF2 couple. A detail characterization about the catalyst was carried out to elucidate the surprising result. The high photocatalytic activity of the SO42
/MoOx/MgF2 composite can be attributed to the synergetic effect of SO42
, MoOx, and MgF2 species. The role of SO42
has been studied in our previous work.24 In this paper, we investigated the effect of molybdenum concentration and calcination temperature on the photocatalytic activity of SO42
/ MoOx/MgF2 catalyst in acetone photodegradation.

2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. (NH4)6Mo7O24 3 4H2O (>99%), Mg(NO3)2 3 6H2O (>99%), NH4F (>96%), (NH4)2S (20% sulfur content), Ti(SO4)2 (>99%), and SiO2 (235 m2/g), with P25 (Degussa TiO2) as a reference, were purchased commercially and used without further purification. MoO3 was prepared by calcination of (NH4)6Mo7O24 3 4H2O in air at 500 °C for 4 h. MgF2 was prepared by directly mixing Mg(NO3)2 3 6H2O aqueous solution and NH4F aqueous solution with a Mg2þ:F
molar ratio of 1:2, dried at 90 °C for 12 h, and then calcined in air at 400 °C for 2 h. Nitrogen-doped TiO2 (NTiO2) was prepared according to the literature.25 In a typical procedure, 10.0 g Ti(SO4)2 was dissolved in 40 mL of deionized water. Then, a 35 wt % solution of ammonia was added dropwise with vigorous stirring until pH 8.0. After aging in the mother liquid for 12 h, the precipitate obtained was filtered and washed with distilled water until it was free of sulfate ions. The N-TiO2 was finally obtained after the as-prepared filter residue was dried at 80 °C for 12 h and calcined in air at 400 °C for 2 h. The sulfated MoOx/MgF2 catalysts were prepared according to the following procedure. A total of 10.00 g of Mg(NO3)2 3 6H2O was dissolved into 20 mL of H2O to obtain solution A. A total of 2.880 g of NH4F was dissolved in 10 mL of H2O to obtain solution B. A total of 0.353 g of (NH4)6Mo7O24 3 4H2O was dissolved in 10 mL of H2O and 3.3 mL of (NH4)2S solution (20% sulfur content) to obtain solution C (nS/nMo= 4). Solution A was mixed with solution B and solution C under stirring to obtain a mixture. The water in the mixture was removed by a rotary evaporator to obtain a precursor solid. After being dried at 90 °C for 12 h, the precursor solid was calcined in N2 at 400 °C for 2 h and then cooled to room temperature to obtain the sulfated 5 mol % MoOx/MgF2 catalyst. Other sulfated MoOx/MgF2 catalysts were prepared by a similar method, only the Mo/Mg ratio or the calcination temperature was changed. For the sake of clarity, the synthesized SO42
/MoOx/MgF2 composite is denoted as SMMC-T. Herein, C represents the Mo/Mg molar ratio and T represents the calcination temperature. For example, the sulfated 5 mol % MoOx/MgF2 catalyst calcined at 350 °C can be denoted

ARTICLE

Figure 1. Schematic diagram of the photoreaction apparatus.

as SMM-5-350. For comparative purpose, sulfated 5 mol % MoOx/SiO2 (denoted as SMS-5-350) was also prepared by the similar method. Solutions of (NH4)6Mo7O24 3 4H2O and (NH4)2S was mixed to obtain a solution of (NH4)2MoS4. Then, SiO2 powder was added. After an impregnating process for 5 h, the mixture was dried and calcined in air at 350 °C for 2 h. 2.2. Characterization. The X-ray diffraction (XRD) characterization of the catalysts was carried out on a Rigaku Model DMAX2500 X-ray diffractometer, using Cu KR radiation (40 kV/40 mA) with λ = 0.15406 nm. The specific surface areas (BET) of the catalysts were measured by nitrogen adsorption on Autosorb-1 (Quantachrome Instruments). The scanning electron microscopy (SEM) pictures were taken using a fieldemission scanning electron microscope (Model LEO-1530). The Raman spectra of the catalysts were collected on a Model RM1000 spectrometer (Renishaw) with an Ar ion laser (514.5 nm) as excitation source. The Fourier transform infrared spectroscopy (FT-IR) spectra of the catalysts were recorded on a Perkin
Elmer Model Magna 750 with a resolution of 4 cm
1. The X-ray photoelectron spectroscopy (XPS) measurements were performed with a Quantum 2000 Scanning ESCA Microprobe instrument using Al KR radiation. The C 1s signal was set to a position of 284.6 eV. The UV-vis diffuse reflectance spectroscopy (DRS) spectra of the catalysts were recorded on a Perkin
Elmer Model Lambda900 system that was equipped with an integrating sphere, using BaSO4 as the background. The photoluminescence (PL) spectra of catalysts were collected on a Model FLS-920 device (Edinbergh Instrument). The light source was a xenon lamp (excitation at 274 nm). 2.3. Photocatalyic Reaction. The catalytic reaction under UV lights was carried out in a quartz tube reactor (inner diameter (ID) of 5.0 mm) and two 500-W high-pressure mercury lamps were used as UV light sources. When the reaction was carried out under light with a wavelength of λ > 380 nm and λ > 420 nm, two 400-W xenon lamps were used as visible-light sources and two optical filters (λ > 380 nm and λ > 420 nm) were placed in front of the xenon lamps to eliminate UV light (Figure 1). After the light had passed through the optical filters, the power densities of the light at the position of catalysts were 489 mW/cm2 (Hg lamps), 411 mW/cm2 (>380 nm), and 302 mW/cm2 (>420 nm). In each reaction, the volume of catalyst was controlled to be ∼1.0 mL. A thermocouple was placed in the center of the catalyst bed, to detect the reaction temperature. The reactor tube was cooled by a powerful fan. Although the researchers attempted to cool the reactor using the fan, the reaction temperature, however, 7110

dx.doi.org/10.1021/ie102389q |Ind. Eng. Chem. Res. 2011, 50, 7109–7119

Industrial & Engineering Chemistry Research

ARTICLE

Figure 2. Scanning electron microscopy (SEM) micrographs of (a) MoO3, (b) MgF2, (c) SMM-5-400, and (d) SMM-5-350.

Table 1. Specific Surface Area of MgF2, MoO3, P25, and SMM Catalysts specific surface area,

specific surface area,

S (m2 g
1)

catalyst

S (m2 g
1)

MgF2

40

SMM-9-400

64

MoO3

19

SMM-11-400

51

P25 SMM-1-400

52 44

SMM-5-250 SMM-5-300

192 188

catalyst

SMM-3-400

91

SMM-5-350

146

SMM-5-400

112

SMM-5-450

70

SMM-7-400

70

remained between 120 °C and 130 °C, because of the heat from the lamps. Pure oxygen was used as the oxidant and carrier gas. The organic substrate acetone was fed into the reactor by bubbling gas (O2) through liquid acetone at 0 °C (cooled in a water-ice bath) to obtain the reactant mixture. The flow of mixture was controlled at 8.0 mL/min. The concentration of acetone was analyzed to be 10 mol % via gas chromatography (GC). The reaction products were analyzed on a GC system (Model GC-950, equipped with a GDX-203 column and a 5A carbon molecular sieve column) with a thermal conductor detector. The catalyst activity and the molar concentration of acetone were calculated by the area normalization method. All the data were collected after 3 h of online reaction. In order to rule out the thermal reaction, the SMM-5-350 catalyst was tested for acetone oxidation in darkness at different temperatures. The dark reaction did not show acetone degradation until the temperature increased to 250 °C. It indicated that the contribution of thermal reaction can be ignored in the reaction system (see Table S1 in the Supporting Information). The blank reaction was also tested. Under UV light irradiation, 7.4% of the acetone was degraded. Under light

with λ > 380 nm and λ > 420 nm, no acetone photolysis reaction was observed.

3. RESULTS AND DISCUSSION 3.1. BET and SEM Analysis. The morphology of the prepared samples was investigated via SEM experimental analysis. Figure 2 shows SEM pictures of MoO3, MgF2, and two representative catalysts (SMM-5-400 and SMM-5-350). As shown in Figure 2, MoO3 particle presents an average size of 0.3
0.8 μm. For pure MgF2, the sample particles are greatly agglomerated. The agglomerated particle exhibits a very large size of 3
4 μm. The SMM catalysts exhibit a different morphology than MgF2 or MoO3. Highly dispersed small particles (380 nm) (%)

14.5

48.6

6.0

23.8

1.2

XVis(λ>420 nm) (%) a

N-TiO2

3.6

Note: X represents the conversion of acetone.

the well-hybridized absorption of MoOx and MgF2. This phenomenon was also observed on VOx/MgF2 and In2O3/ CaIn2O4 coupled composites.20,22,38 3.7. Photocatalytic Activity. Acetone degradation was used as the probe reaction to evaluate the activity of the catalyst. Figure 9 shows the photocatalytic activities of SMM catalysts under both UV and visible light (λ > 380 nm and λ > 420 nm). As shown in Figure 9a, the acetone degradation conversion increases as the molybdenum concentration increases from 1 mol % to 5 mol % and decreases at even higher molybdenum concentrations. The highest acetone conversion was obtained on the SMM-5-400 catalyst. The acetone conversion reaches 73.5% under UV light, 57.5% under light with a wavelength of λ > 380 nm, and 32.3% under light with a wavelength of λ > 420 nm. For comparative purposes, the photocatalytic activities of the P25, N-TiO2, MgF2, and MoO3 catalysts were also investigated. The results are listed in Table 3. The investigation shows that catalyst P25 has a high activity in UV light, but it is not active under light with λ > 380 nm and λ > 420 nm. The MgF2 and 7115

dx.doi.org/10.1021/ie102389q |Ind. Eng. Chem. Res. 2011, 50, 7109–7119

Industrial & Engineering Chemistry Research

ARTICLE

the photodegradation of acetone. It can be formed via the following process:39,40 CH3 COCH3 þ O
f CH3 COCH2 • þ OH


ð1Þ

CH3 COCH2 • þ O2 f CH3 COCH2 OO•

ð2Þ

CH3 COCH2 OO• þ O
f CH3 CHO þ CO2 þ OH
ð3Þ The acetaldehyde can be further photodegraded by a sequential mechanism in which acetic acid is generated from acetaldehyde and successively transformed into formaldehyde, formic acid, and finally CO2.41 CO is generated because of the incomplete oxidation and self-decomposition of formaldehyde, as explained in eqs 4 and 5:42 Figure 10. Effect of reaction time on the photoactivity of P25, N-TiO2, and the SMM-5-350 composite.

MoO3 did not show much activity under either UV or visible light. For the N-TiO2 catalyst, acetone conversions of 85.0%, 48.6%, and 23.6% were reached under UV light, and light with λ > 380 nm and 420 nm, respectively. The results in Figure 9a and Table 3 show that, under UV light, both P25 and N-TiO2 are more active than sulfated MoOx/MgF2 catalysts. However, under visible light (λ > 380 nm and λ > 420 nm), the SMM catalysts are more active than P25 or N-TiO2. Clearly, the SMM composite is a visible-light-active photocatalyst and exhibits high activity for acetone photodegradation. Contrary to the SMM photocatalyst, an SMS composite presents a very poor photocatalytic activity. Figure 9b presents the photocatalytic activity of the sulfated 5 mol % MoOx/MgF2 catalyst calcined at different temperatures. As shown in Figure 9b, the acetone conversion increases as the calcination temperature increases from 250 °C to 350 °C, and then reaches maximum conversion at 350 °C. The acetone conversion reaches 99.0% under UV light, 97.9% under light with λ > 380 nm, and 70.3% under light with λ > 420 nm. When the catalyst was calcined at temperatures even higher than 350 °C, the acetone conversion decreased. The stability of SMM composite catalyst was also evaluated by continuous reaction. Figure 10 shows the life-testing result of the SMM-5-350 composite. For the sake of clarity, the activity of SMM-5-350 and the N-TiO2 catalyst under UV light is not presented. As shown in Figure 10, the SMM-5-350 catalyst is stable with 27 h of continuous reaction and shows much higher photocatalytic activity than N-TiO2 and P25 under visible-light irradiation (λ > 420 nm). The result in Figure 10 indicates that the SMM composite is promising for practical application for air purification, because of its high activity and stability. In the photodegradation of acetone, the typical degradation products are CO2, CO, acetaldehyde, acetol (CH3COCH2OH), and H2O (see Table S2 in the Supporting Information). For example, if the photodegradation of acetone under light with λ > 380 nm is performed over the SMM-5-350 catalyst, the selectivities to CO2, CO, acetaldehyde, and acetol are 70.1%, 27.4%, 1.9%, and 0.6%, respectively. This indicates that most of the acetone was completely degraded. However, there are still some amounts of CO, acetaldehyde, and acetol left as partial oxidation products. Acetaldehyde is a common intermediate in

HCHO þ hv ð þ O2 Þ f 2HO2 þ CO

ð4Þ

HCHO þ hv f H2 þ CO

ð5Þ

Compared to the other intermediates, CO shows high stability, which makes it become the main byproduct. However, the acetol is unusual. Although Zhou detected the product in the photodegradation of acetone, he did not explain the formation mechanism of the acetol.22 Based on the result of Heller et al.,43 a simple guess is presented below. O 2 þ e
f O 2 •


ð6Þ

O2 •
þ Hþ f •OOH

ð7Þ

CH3 COCH2 OO• þ •OOH f CH3 COCH2 OOOOH ð8Þ CH3 COCH2 OOOOH þ CH3 COOH f CH3 COCH2 OH þ CH2 O þ O2 þ CO2 ð9Þ In addition, the coupling of HO• and CH3COCH2• species might be another possible route for the formation of acetol. CH3 COCH2 • þ HO• f CH3 COCH2 OH

ð10Þ

Of course, the guesses still need to be proven. Besides, it is very strange that some intermediates (such as CH3COOH, which are also a common byproduct in the photodegradation of acetone) are not observed. The low sensitivity of TCD might be a possible reason. In our opinion, however, the formation mechanism of acetol did not deserve much attention since only trace acetol was observed and CO2 and H2O are the preferred products. A further modification (such as the use of noble metal) is preferred for the SMM catalyst to promote the complete oxidation ability. This work is ongoing. 3.8. Discussion. It is well-known that the catalytic activity of a photocatalyst is mainly dependent on whether the electron
hole pairs can be separated effectively. In the current case, SMM composite shows high activity for acetone photodegradation under both UV and visible light, indicating that the recombination of electron
hole pairs can be retarded effectively. The structure characterization indicates that the SMM composite consists of MoO3, MgF2, and SO42
. Generally, the SO42
species would not contribute the separation of electron
hole pairs, with the exception case of SO42
/TiO2.44 Therefore, the key composition of the SMM might be MoOx/MgF2, which was the origin of the high photocatalytic activity. Actually, Zhou et al. have reported a MgF2-based composite photocatalyst (VOx/MgF2), 7116

dx.doi.org/10.1021/ie102389q |Ind. Eng. Chem. Res. 2011, 50, 7109–7119

Industrial & Engineering Chemistry Research

Figure 11. PL spectra of the MoO3, SMS-5-350, and SMM-5-350 catalysts.

which showed high performance in the photodegradation of organic compounds.20,22,23 The V2O5 domains dispersed in the MgF2 matrix are the considered active sites. The high activity of the catalyst was attributed to the isolation effect of MgF2. The SMM catalyst presents a similar phase composition with VOx/MgF2. Therefore, it is very possible that both catalysts could be explained by the same mechanism. Similar to V2O5, pure MoO3 shows poor photocatalytic activity, because of the fast recombination of electron
hole pairs. In the SMM catalyst, however, the isolation effect of MgF2 makes the MoO3 present a small particle size. The decreased particle size means that many more electrons or holes can migrate from their formation site to the catalyst surface and react with the adsorbed acetone. The high photocatalytic activity was thus obtained. However, there still some doubt about the role of MgF2. Although it has been proven that SiO2 can also isolate the MoO3 phase effectively,27 SO42
/ 5 mol % MoOx/SiO2 catalyst presents a poor activity. It indicates that the isolation effect of MgF2 cannot explain the high photocatalytic activity solely. The MgF2 matrix might prolong the lifetime of electrons and holes in another way. Generally, MgF2 is considered to be an insulator, because of its huge band gap. Herein, however, the results of XRD, XPS, and DRS indicate that nitrogen was doped into the lattice of MgF2, which made the synthesized MgF2 present some semiconductor properties. It has a relative small band gap (almost as same as that of ZrO2) and exhibits a certain photocatalytic activity under UV light. Therefore, we think that a coupling effect might exist between the MoO3 and the N-doped MgF2. Since a chemically bonded contact is assumed to bridge between the MoO3 and MgF2 in the composite system, the photoexcited electrons in the conduction band of MoO3 get injected efficiently into the MgF2, while the photoexcited holes remain in the valence band of MoO3. This process can retard the recombination of photoexicted electrons and holes, prolong the life of charges, and, consequently, enhance the photocatalytic activity. A powerful proof for the mechanism is the result of the PL experiment. The PL technique is useful in disclosing the migration, transfer, and recombination processes of the photogenerated electron
hole pairs in the semiconductor.45 As shown in Figure 11, the PL spectrum of MoO3 shows a strong peak at ∼410 nm, which agreed well with a previous report.46 In addition, the strong emission indicates that the electrons and holes recombine rapidly,47,48 which might be the origin of the low photocatalytic activity of MoO3. The PL peak of the SMS-5-350 sample is still strong, although the MoO3 concentration is very low. It is reported that the PL intensity of the semiconductor usually decreases as the particle size increases, which is ascribed to the decrease in the surface oxygen vacancy and defect content with increasing particle size.45

ARTICLE

Therefore, the strong PL peak of the SMS-5-350 catalyst might be attributed to the decreased MoO3 particle size. The MgF2 matrix also can isolate MoO3 like SiO2. However, different from the SMS-5-350 sample, the PL peak of the SMM-5-350 catalyst is weakened significantly, indicating that the recombination of the electron
hole pairs is very slow. Evidently, the N-doped MgF2 matrix hindered the rapid recombination of photoinduced electron
hole pairs. This is consistent with the analysis as discussed above. In the SMM composites, only MoOx can absorb the visible light and can be considered as the active phase for the photocatalysis reaction. With increasing molybdenum concentration, more active sites are available and the activity of catalyst is thus promoted. But when the Mo/Mg molar ratio is >5%, the photocatalytic activity decreased, based on the result of Figure 8a. This phenomenon can be explained by a synergistic effect of its MoOx domain size effect, BET surface area, and surface charge capture. Both the XRD and Raman experiments indicate that, in the region of low molybdenum concentration, Mo oxide is greatly dispersed in the MgF2 matrix. The MoOx domains and MgF2 exhibit small particle size, because of the strong interaction between them. The small particle size of MoOx indicates that the volume recombination of the electron
hole pairs, which is the dominant process in large MoO3 particles, would be suppressed. Compared with large MoO3 particles, the decreased particle size of MoOx can also lead to an increase in the interfacial charge-carrier transfer rates. Thus, the isolator MgF2 matrix can retard the recombination of the electrons and holes more effectively. For MgF2, the small particle size means that MgF2 has a large specific surface area, which can promote the adsorption of reactant. Both of the above two factors are beneficial for the increase of the photocatalytic activity. When the molybdenum concentration is higher than 5 mol %, the MoOx domains congregated and formed the large crystalline MoO3 particle (see Figures 3a and 4a). The BET surface area of catalysts also decreased. As a result of the change, the recombination of electron
hole pairs is accelerated, the adsorption of reactant is retarded, and the activity of catalyst is thus decreased. The surface molybdenum oxide species might also play an important role in the photodegradation of acetone. The XPS characterization indicates that there exist some Mo5þ besides Mo6þ in the catalyst with low molybdenum concentration (5 mol %, Mo4þ was observed and its content increased, along with the molybdenum concentration. Generally, the Mo5þ ion is considered to be the low trap of hole, while the Mo4þ ion is considered to be the deep trap of hole. The Mo5þ species is preferred for retarding the recombination of electron
hole pairs.49 The above analysis is also suitable to explain the optimal calcination temperature (350 °C). However, note that the consistency between surface area and photoactivity was not observed in the catalysts calcined at different temperatures. The SMM-5-250 (192 m2 g
1) and SMM-5-300 (188 m2 g
1) samples show lower acetone conversion than the SMM-5-350 sample (146 m2 g
1) under both UV and visible light. This indicates that the specific surface area is not the major factor that influences catalytic activity; the key factor might be the light absorption and the separation efficiency of electron
hole pairs. For the SMM catalyst calcined at different temperatures, the dispersal of molybdenum oxides is found to be related to the calcination temperature. The small MoOx particle was obtained 7117

dx.doi.org/10.1021/ie102389q |Ind. Eng. Chem. Res. 2011, 50, 7109–7119

Industrial & Engineering Chemistry Research only over the sample that was calcined at suitable temperature (350 °C) (see Figure 4b). The volume recombination of electron
hole pairs is thus retarded and the photocatalytic activity is promoted. Besides the size effect of MoOx, another important reason for the optimal calcination temperature lies on the photoabsorption ability of catalyst. The UV
vis experiment shows that the calcination temperature greatly affected the photoabsorption ability. The SMM-5-350 composite shows the highest photoabsorption performance, indicating that the catalyst can absorb much more visible light and generate more electrons and holes than the other samples. The highest photocatalytic efficiency is thus obtained.

4. CONCLUSIONS In summary, a surprising composite photocatalyst, SO42
/ MoOx/MgF2, was synthesized from the aqueous solutions of (NH4)6Mo7O24, (NH4)2S, Mg(NO3)2, and NH4F. It shows high activity in the photodegradation of acetone under both UV and visible-light irradiation. The optimal molybdenum concentration in MgF2 was found to be 5 mol %. Higher molybdenum concentrations reduced the activity of the catalyst. In the highly active catalysts, the characterization indicates that there are highly dispersed MoOx species that are considered to be the active phase. The dispersion of the MoOx species shows a great effect on the separation of electron
hole pairs. In order to obtain high activity, the sulfated 5 mol % MoOx/MgF2 catalyst should be calcined at 350 °C. ’ ASSOCIATED CONTENT

bS

Supporting Information. Photocatalytic activity of SMM3-400 catalyst at different temperatures (Table S1) and performance of sulfated 5 mol % MoOx/MgF2 catalyst calcined at different temperatures under UV light, light with a wavelength of λ > 380 nm, and light with a wavelength of λ > 420 nm (Table S2). (PDF) This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel./Fax: þ86-0579-82283920 (Y.H.), þ86-0579-82283920 (Y.W.). E-mail: [email protected] (Y.H.), [email protected] (Y.W.).

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21003109), the Science Foundation of Zhejiang Education Department (No. Y200909374), and the Doctor Startup Fund of Zhejiang Normal University (No. ZC304008169). ’ REFERENCES (1) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37–38. (2) Hoffmann, M. R.; Martin, S. T.; Chio, W.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69–96. (3) Fujishima, A.; Rao, T. N.; Tryk, D. A. Titanium dioxide photocatalysis. J. Photochem. Photobiol. C: Photochem. Rev. 2000, 1, 1–21. (4) Malato, S.; Fern
andez-Ib
a~nez, P.; Maldonado, M. I.; Blanco, J.; Gernjak, W. Decontamination and disinfection of water by solar

ARTICLE

photocatalysis: Recent overview and trends. Catal. Today 2009, 147, 1–59. (5) Serpone, N.; Borgarello, E.; Gratzel, M. Visible light induced generation of hydrogen from H2S in mixed semiconductor dispersion: Improved efficiency through inter-particle electron transfer. J. Chem. Soc., Chem. Commun. 1984, 342–344. (6) Martin, S. T.; Morrison, C. L.; Hoffmann, M. R. Photochemical mechanism of size-quantized vanadium-doped TiO2 particles. J. Phys. Chem. 1994, 98, 13695–13704. (7) Papp, J.; Soled, S.; Dwight, K.; Wold, A. Surface acidity and photocatalytic activity of TiO2, WO3/TiO2, and MoO3/TiO2 photocatalysts. Chem. Mater. 1994, 6, 496–500. (8) Keller, V.; Bernhardt, P.; Garin, F. Photocatalytic oxidation of butyl acetate in vapor phase on TiO2, Pt/TiO2 and WO3/TiO2 catalysts. J. Catal. 2003, 215, 129–138. (9) Li, X. Z.; Li, F. B.; Yang, C. L.; Ge, W. K. Photocatalytic activity of WOx
TiO2 under visible light irradiation. J. Photochem. Photobiol. A: Chem. 2001, 141, 209–217. (10) Pourabbas, B.; Jamshidi, B. Preparation of MoS2 nanoparticles by a modified hydrothermal method and the photo-catalytic activity of MoS2/TiO2 hybrids in photo-oxidation of phenol. Chem. Eng. J. 2008, 138, 55–62. (11) He, Y. M.; Wu, Y.; Guo, H.; Tian, L. S.; Wu, X. T. Visible light photodegradation of organics over VYO composite catalyst. J. Hazard. Mater. 2009, 169, 855–860. (12) Zhang, H. T.; Ouyang, S. X.; Li, Z. S.; Liu, L. F.; Yu, T.; Ye, J. H.; Zou, Z. G. Preparation, characterization and photocatalytic activity of polycrystalline Bi2O3/SrTiO3 composite powders. J. Phys. Chem. Solid 2006, 67, 2501–2505. (13) Derbal, A.; Omeiri, S.; Bouguelia, A.; Trari, M. Characterization of new heterosystem CuFeO2/SnO2 application to visible-light induced hydrogen evolution. Int. J. Hydrogen Energy 2008, 33, 4274–4282. (14) Ryu, S. Y.; Choi, J.; Balcerski, W.; Lee, T. K.; Hoffmann, M. R. Photocatalytic production of H2 on nanocomposite catalysts. Ind. Eng. Chem. Res. 2007, 46, 7476–7488. (15) Huang, T.; Lin, X. P.; Xing, J. C.; Wang, W. D.; Shan, Z. C.; Huang, F. Q. Photocatalytic activities of hetero-junction semiconductors WO3/SrNb2O6. Mater. Sci. Eng., B 2007, 141, 49–54. (16) Liu, Z. L.; Deng, J. C.; Deng, J. J.; Li, F. F. Fabrication and photocatalysis of CuO/ZnO nano-composites via a new method. Mater. Sci. Eng., B 2008, 150, 99–104. (17) Hotchandani, S.; Kamat, P. V. Charge-transfer processes in coupled semiconductor systems: Photochemistry and photoelectrochemistry of the colloidal CdS
ZnO system. J. Phys. Chem. 1992, 96, 6834–6839. (18) Chai, S. Y.; Kim, Y. J.; Jung, M. H.; Chakraborty, A. K.; Jung, D.; Lee, W. I. Heterojunctioned BiOCl/Bi2O3, a new visible light photocatalyst. J. Catal. 2009, 262, 144–149. (19) Niu, M. T.; Huang, F.; Cui, L. F.; Huang, P.; Yu, Y. L.; Wang, Y. S. Hydrothermal synthesis, structural characteristics, and enhanced photocatalysis of SnO2/R-Fe2O3 semiconductor nanoheterostructures. ACS Nano 2010, 4, 681–688. (20) Chen, F.; Wu, T. H.; Zhou, X. P. The photodegradation of acetone over VOx/MgF2 catalysts. Catal. Commun. 2008, 9, 1698–1703. (21) Wang, J.; Chen, F.; Zhou, X. P. Photocatalytic degradation of acetone over a V2O5/LaF3 catalyst under visible light. J. Phys. Chem. C 2008, 112, 9723–9729. (22) Chen, F.; Wang, J.; Xu, J. Q.; Zhou, X. P. Visible light photodegradation of organic compounds over V2O5/MgF2 catalyst. Appl. Catal., A 2008, 348, 54–59. (23) Chen, F.; Huang, H.; Zhou, X. P. Characterization of photodegradation catalyst 9.1% VOx/MgF2. J. Phys. Chem. C 2009, 113, 21106–21113. (24) He, Y. M.; Sheng, T. L.; Wu, Y.; Chen, J. S.; Fu, R. B.; Hu, S. M.; Wu, X. T. Visible light-induced degradation of acetone over SO42
/ MoOx/MgF2 catalysts. J. Hazard. Mater. 2009, 168, 551–554. (25) Ma, Y. F.; Zhang, J. L.; Tian, B. Z.; Chen, F.; Wang, L. Z. Synthesis and characterization of thermally stable Sm,N co-doped TiO2 with highly visible light activity. J. Hazard. Mater. 2010, 182, 386–393. 7118

dx.doi.org/10.1021/ie102389q |Ind. Eng. Chem. Res. 2011, 50, 7109–7119

Industrial & Engineering Chemistry Research (26) Jeziorowski, H.; Kn€ozinger, H. Raman and ultraviolet spectroscopic characterization of molybdena on alumina catalysts. J. Phys. Chem. 1979, 83, 1166–1173. (27) Willianms, C. C.; Ekerdt, J. G.; Jehng, J. M.; Hardcastle, F. D.; Turek, A. M.; Wachs, I. E. A Raman and ultraviolet diffuse reflectance spectroscopic investigation of silica-supported molybdenum oxide. J. Phys. Chem. 1991, 95, 8781–8791. (28) Haber, J.; Wojciechowska, M. Surface structure and catalytic properties of the MoO3
MgF2 system. J. Catal. 1988, 110, 23–26. (29) Henker, M.; Wendlandt, K. P. Structure of MoO3/Al2O3
SiO2 catalysts. Appl. Catal. 1991, 69, 205–220. (30) Prinetto, F.; Cerrato, G.; Ghiotti, G.; Chiorino, A.; Campa, M. C.; Gazzoli, D.; Indovina, V. Formation of the MoVI surface phase on MoOx/ZrO2 catalysts. J. Phys. Chem. 1995, 99, 5556–5567. (31) Barraud, E.; Bosc, F.; Edwards, D.; Keller, N.; Keller, V. Gas phase photocatalytic removal of toluene effluents on sulfated titania. J. Catal. 2005, 235, 318–326. (32) Oertelz, R. P.; Plane, R. A. Raman and infrared study of nitrate complexes of bismuth(III). Inorg. Chem. 1968, 7, 1192–1196. (33) Chen, D. M.; Jiang, Z. Y.; Geng, J. Q.; Wang, Q.; Yang, D. Carbon and nitrogen co-doped TiO2 with enhanced visible-light photocatalytic activity. Ind. Eng. Chem. Res. 2007, 46, 2741–2746. (34) Yufit, V.; Golodnitsky, D.; Burstein, L.; Nathan, M.; Peled, E. X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy studies of electrodeposited molybdenum oxysulfide cathodes for lithium and lithium-ion microbatteries. J. Solid State Electrochem. 2008, 12, 273–285. (35) Asakura, K.; Nakatani, K.; Kubota, T.; Iwasawa, Y. Characterization and kinetic studies on the highly active ammoxidation catalyst MoVNbTeOx. J. Catal. 2000, 194, 309–317. (36) Simanovskii, D. M.; Schwettman, H. A. Midinfrared optical breakdown in transparent dielectrics. Phys. Rev. Lett. 2003, 91 (10), 107601–107604. (37) Ashi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visiblelight photocatalysis in nitrogen-doped titanium oxides. Science 2001, 293, 269–271. (38) Ge, L. Preparation of novel visible-light-driven In2O3
CaIn2O4 composite photocatalyst by sol
gel method. J. Sol
Gel Sci. Technol. 2007, 44, 263–268. (39) Hern
andez-Alonso, M. D.; Tejedor-Tejedor, I.; Coronado, J. M.; Anderson, M. A.; Soria, J. Operando FTIR study of the photocatalytic oxidation of acetone in air over TiO2
ZrO2 thin films. Catal. Today 2009, 143, 364–373. (40) Attwood, A. L.; Edwards, J. L.; Rowlands, C. C.; Murphy, D. M. Identification of a surface alkylperoxy radical in the photocatalytic oxidation of acetone/O2 over TiO2. J. Phys. Chem. A 2003, 107, 1779–1782. (41) Nimlos, M. R.; Wolfrum, E. J.; Brewer, M. L.; Fennell, J. A.; Bintner, G. Gas-phase heterogeneous photocatalytic oxidation of ethanol: Pathways and kinetic modeling. Environ. Sci. Technol. 1996, 30, 3102–3110. (42) Liu, H. M.; Lian, Z. W.; Ye, X. J.; Shangguan, W. F. Kinetic analysis of photocatalytic oxidation of gas-phase formaldehyde over titanium dioxide. Chemosphere 2005, 60, 630–635. (43) Schwitzgebel, J.; Ekerdt, J. G.; Gerischer, H.; Heller, A. Role of the oxygen molecule and of the photogenerated electron in TiO2-photocatalyzed air oxidation reactions. J. Phys. Chem. 1995, 99, 5633–5638. (44) Barraud, E.; Bosc, F.; Edwards, D.; Keller, N.; Keller, V. Gas phase photocatalytic removal of toluene effluents on sulfated titania. J. Catal. 2005, 235, 318–326. (45) Jing, L. Q.; Qu, Y. C.; Wang, B. Q.; Li, S. D.; Jiang, B. J.; Yang, L. B.; Fu, W.; Fu, H. G.; Sun, J. Z. Review of photoluminescence performance of nano-sized semiconductor materials and its relationships with photocatalytic activity. Sol. Energy Mater. Sol. Cells 2006, 90, 1773–1787. (46) Chen, X. H.; Lei, W. W.; Liu, D.; Hao, J.; Cui, Q. L.; Zou, G. T. Synthesis and characterization of hexagonal and truncated hexagonal shaped MoO3 nanoplates. J. Phys. Chem. C 2009, 113, 21582–21585.

ARTICLE

(47) Tang, J. W.; Zou, Z. G.; Ye, J. H. Photophysical and photocatalytic properties of AgInW2O8. J. Phys. Chem. B 2003, 107, 14265–14269. (48) Li, F. B.; Li, X. Z. Photocatalytic properties of gold/gold ionmodified titanium dioxide for wastewater treatment. Appl. Catal., A 2002, 228, 15–27. (49) Choi, W.; Termin, A.; Hoffmann, M. R. The role of metal ion dopants in quantum-sized TiO2: correlation between photoreactivity and charge carrier recombination dynamics. J. Phys. Chem. 1994, 98, 13669–13679.

7119

dx.doi.org/10.1021/ie102389q |Ind. Eng. Chem. Res. 2011, 50, 7109–7119