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Visible light photodegradation of organics over VYO composite catalyst
Journal of Hazardous Materials 169 (2009) 855–860
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Visible light photodegradation of organics over VYO composite catalyst Yiming He a , Ying Wu b,∗ , Hai Guo a , Tianlu Sheng c , Xintao Wu c a
College of Mathematics, Physics and Information Engineering, Zhejiang Normal University, Jinhua 321004, China Institute of Physical Chemistry, Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Zhejiang Normal University, Jinhua 321004, China c State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China b
a r t i c l e
i n f o
Article history: Received 3 March 2009 Received in revised form 5 April 2009 Accepted 7 April 2009 Available online 14 April 2009 Keywords: Photodegradation Photocatalyst V2 O5 YVO4 Acetone
a b s t r a c t This paper presents a visible-light driven photocatalyst, VYO composite, which was synthesized by doping YVO4 with V2 O5 . The catalytic testing results indicate that the photocatalyst shows high activity for photodegradation of acetone under both UV and visible light. The highest acetone conversion was obtained over V1.5 Y1 Ox composite catalyst. By doping a small amount of metal Pt the photocatalytic efficiency of the catalyst could be promoted further. It was also proved to be efficient for the photodegradation of methanol, ethanol, 2-propanol, and benzene. The physical and photophysical properties of the VYO composites were characterized by BET, XRD, FT-IR, Raman, UV–vis spectra, and photoluminescence spectra, respectively. On the basis of the investigation results, the high photocatalytic activity might be attributed to the coupling effect between YVO4 and V2 O5 . Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction Photodegradation of organic pollutants in the presence of TiO2 appears to be a viable decontamination process with widespread application, regardless of the state (gas or liquid) or chemical nature of the process target [1]. However, its technological applications seem to be limited by several factors, among which the most restrictive one is the need to use an ultraviolet (UV) excitation source. The efficient use of solar light may then appear to be an appealing challenge for developing the future generation of photocatalytic materials. Currently, most of the investigations in this area are focused on the improvements of doped TiO2 catalysts. Several approaches have been reported. These approaches include the incorporation of metal ions (such as Fe3+ , V5+ , La3+ ) or anions (such as N3− , S2− , F− ) into the matrix of TiO2 [2–6], the introduction of oxygen vacancies into the lattice of TiO2 [7,8], and the combination of two different semiconductors [9,10]. In these approaches, the third method attracted many attentions. The reason is that the combination of TiO2 and a narrow band gap semiconductor not only could promote the absorbing ability for the visible light, but also could retard the recombination of electron–hole pairs. This strategy has been effective for other semiconductors with large band gap. Up to now, a large variety of coupled semiconductor systems have been reported, such as CdS/ZnO, WS2 /WO3 , V2 O5 /MgF2 [9–17].
Tetragonal phased YVO4 has been extensively used as the red phosphor with several rare-earth metal ions as dopant in cathode ray tubes and color television in powder form [18,19]. In addition, many attentions have been paid on its thermal catalytic properties such as oxidative dehydrogenation of propane [20–22]. However, in the photocatalysis there have been few studies focused on the photocatalytic performance of yttrium orthovanadate. Recently, Xu et al. studied the photocatalytic properties of YVO4 [23]. The results showed that YVO4 could photodegrade methyl orange effectively under UV light. The high activity could be attributed to the wide band gap (3.5 eV) which makes the excited electron and hole have the high potential to oxidize the organics. However, the wide band gap of YVO4 also limits its photoadsorption to the ultraviolet region of the solar spectrum. In this paper, we present a new composite catalyst V2 O5 /YVO4 , which shows high photocatalytic activity on photodegradation of organic substrates under visible light. The semiconductor V2 O5 was chosen as sensitizer because of its small band gap (∼2.0 eV), which could make it active in a wide region of visible light wavelength (<600 nm). Lots of photocatalysts containing V2 O5 have been reported, and it has been proved that V2 O5 is a good semiconductor sensitizer [14–16]. 2. Experimental 2.1. Catalysts preparation
∗ Corresponding author. Tel.: +86 0579 82283920; fax: +86 0579 82283920. E-mail address: [email protected] (Y. Wu).
NH4 VO3 (>99%), Y(NO3 )3 ·6H2 O (>99.99%), and P25 (Degussa TiO2 ) as reference, were purchased commercially and used
0304-3894/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.04.018
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without further purification. V2 O5 was obtained by calcining NH4 VO3 at 500 ◦ C for 4 h. YVO4 was prepared by precipitation method: solutions of NH4 VO3 and Y(NO)3 with a V to Y molar ratio of 1.0 were mixed to obtain a deposit. The pH value of the solution was adjusted to 7–8 by a solution of NH3 . After aging at room temperature for 5 h, the deposit was filtered, washed three times by water, dried at 100 ◦ C for 12 h and calcined at 500 ◦ C for 4 h. The VYO composite catalysts were prepared by the following steps: solutions of NH4 VO3 and Y(NO3 )3 with different V/Y mole ratio were mixed and evaporated to give a solid precursor. After dried at 100 ◦ C for 12 h, the precursor solid was calcined at 500 ◦ C for 4 h and then cooled to room temperature to yield the catalyst. The 0.1 wt.% Pt/V1.5 Y1 Ox catalyst was prepared by impregnation–photoreduction method [15]: 4.5 g of V1.5 Y1 Ox catalyst were added into 3.0 mL of H2 PtCl6 ·6H2 O solution (Pt: 0.0015 g/mL). The sample was kept in the dark for 5 h, and then drying at 80 ◦ C for 10 h. After that, the dried sample was put on the xenon lamp irradiation for 2 h (the strong light could decompose the H2 PtCl6 to metal Pt) to obtain the 0.1 wt.% Pt/V1.5 Y1 Ox catalyst. 2.2. Catalytic tests The catalytic reaction under UV light was carried out in a quartz tube (ID 5.0 mm) reactor and two 500 W high pressure mercury lamps were used as UV light sources. When the reaction was carried out under visible light, two 400 W xenon lamps were used as visible light sources and a glass tube (ID 5.0 mm) reactor which could cut off most of the UV light was used. In each reaction, the bed length of catalyst (500–800 mg) is about 4.5 cm and the other part of the reactor was wrapped by aluminum paper to exclude the contribution of the blank reaction (Fig. 1). A thermocouple which clung to the reactor closely was used to detect the reaction temperature. The reactor tube was cooled by a fan. Because of the heat from the lamps, although we tried to cool down the reactor by the fan, the reaction temperature is still between 130 and 140 ◦ C. Pure oxygen was used as the oxidant. The organic reactants (acetone, methanol, 2-propanol, benzene) were fed into the reactor by bubbling gas (O2 ) through liquid organic at 0 ◦ C (cooled in a water-ice bath) to obtain the reactant mixture. The flow of the mixture was controlled at 8.0 mL/min. The concentration of organic substrates was analyzed by GC and the results are shown in Table 1. Before each catalytic testing, the photocatalyst was allowed to equilibrate
Fig. 1. The reactor system.
in the reaction gas for at least 60 min. The reaction products were analyzed on a GC (equipped with a GDX-203 column and a 5A carbon molecular sieve column, the samples were injected by hand, injection volume 100 L) with TCD. All the data were collected after 3 h of online reaction. In order to rule out the thermal reaction, both the V1.5 Y1 Ox and 0.1 wt.% Pt/V1.5 Y1 Ox catalysts were tested for acetone oxidation in the dark at the same reaction temperature (140 ◦ C). The dark reaction shows that acetone did not react with oxygen over V1.5 Y1 Ox or 0.1 wt.% Pt/V1.5 Y1 Ox catalysts at 140 ◦ C. 2.3. Characterizations The XRD characterization of catalysts was carried out on an Xray diffraction spectroscopy meter (RIGAKU DMAX2500) using Cu K␣ radiation (40 kV/40 mA). The specific surface areas of the catalysts were measured on Autosorb-1 (Quantachrome Instruments). The Raman spectra of the catalysts were collected on RM1000 spectrometer (Renishaw) with an Ar ion laser (514.5 nm) as excitation source. The FTIR spectra of the catalysts were recorded on PerkinElmer Magna 750 with a resolution of 4 cm−1 . The UV–vis spectra of catalysts were recorded on a UV–vis spectrometer (PerkinElmer Lambda900) equipped with an integrating sphere. The photoluminescence (PL) spectra of catalysts were collected on FLS-920 (Edinbergh Instrument). The light source was a Xe lamp (excitation at 280 nm). 3. Results and discussions 3.1. Photocatalytic performance of catalyst The photocatalytic performance of catalysts was tested in the photodegradation reaction of acetone under UV and visible light. Fig. 2 shows the photocatalytic activities of P25 (TiO2 , Degussa), V2 O5 , YVO4 and VYO composite catalysts. Under UV light, P25 has higher acetone degradation conversion than YVO4 . But under visible light the activity of P25 is lower than that of YVO4 . 27.9% acetone conversion was obtained over YVO4 and only 14.5% acetone conversion was obtained over P25. V2 O5 has low activity under both UV and visible light. For the VYO composite, as shown in Fig. 2, the acetone degradation conversion increases with the increase of V/Y ratio from 1.0 to 1.5 and decreases at even higher V/Y molar ratio under UV light. The highest acetone conversion was obtained on catalyst V1.5 Y1 Ox . The acetone conversion reached 98.4% under visible light. The result of Fig. 2 shows that the VYO composite catalysts are active as P25 under UV light. Under visible light, it shows much higher photoactivity than P25, YVO4 , or V2 O5 . Obviously, doping of V2 O5 to YVO4 formulated a series of active photodegradation catalysts. In the above sections, only acetone degradation reaction was discussed over catalysts. To be an efficient photodegradation catalyst, not only is high catalytic activity required, but also suitable product selectivity is needed. In photodegradation reactions, CO2 and H2 O are the preferred final products, and any other organic intermediate compound formed in the photodegradation reaction is still viewed as a pollutant. Although V1.5 Y1 Ox catalyst showed high activity in acetone photodegradation reaction, the catalyst was still not able to oxidize acetone completely. There is a significant amount of 1hydroxy-2-propanone (selectivity 0.8%) and CO (selectivity 51.8%) left as partial oxidation products (Table 1). In the photodegradation reactions of methanol, ethanol, 2-propanol, and benzene, CO and other harmful by-products are also formed in the reactions (Table 1). Loading a small amount of noble metal (such as Pt, Au, Ag, Ru, etc.) to photocatalyst is a practical way to promote the complete oxidation of organic pollutants [16,24,25]. The loaded metal
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Table 1 Photodegradation of organics over V1.5 Y1 Ox and 0.1 wt.% Pt/V1.5 Y1 Ox catalysts under visible light. Organics
Acetone
Methanol
X/%
10
Conversion/%
98.4 (99.8)
SCO2 /%
47.4 (100)
SCO /%
51.8 (0)
5
1.5
100 (100)
100 (100)
8.6 (100) 91.4 (0)
0.8 (0)a
Sother /%
Ethanol
0 (0)
2-Propanol 1
Benzene 2.2
99.8 (100)
82.5 (98.9)
36.9 (100)
42.3 (100)
69.5 (100)
63.1 (0)
57.6 (0)
30.5 (0)
0.1 (0)
0.1 (0)b
0 (0)
Note: The numbers in bracket represent the performance of 0.1 wt.% Pt/V1.5 Y1 Ox , X is the molar concentration of organic compound in oxygen. a Represents selectivity to 1-hydroxy-2-propanone. b Represents selectivity to acetone.
Fig. 3. Life-time testing of V1.5 Y1 Ox and 0.1 wt.% Pt/V1.5 Y1 Ox catalyst samples under visible light. Fig. 2. Acetone photodegradation over VYO catalysts with different V/Y molar ratio.
could change the distribution of the electrons in the photocatalyst and form the Schottky barriers at each metal-semiconductor contact region, which leads to charge separation. Among the noble metals, it is reported that Pt is the best dopant [16]. Hence, we doped the V1.5 Y1 Ox catalyst with a small amount of Pt (0.1 wt.%). Table 1 shows the catalytic performance of the catalysts for organic compounds degradation reaction. The 0.1 wt.% Pt/V1.5 Y1 Ox catalyst shows higher activity than catalyst V1.5 Y1 Ox , and only CO2 and H2 O were detected in the degradation products. Besides the selectivity of products, the stability of the catalytic performance is another important evaluation criterion for a catalyst. Fig. 3 shows the life-testing results of V1.5 Y1 Ox and 0.1 wt.% Pt/V1.5 Y1 Ox catalyst under visible light. As shown in Fig. 3, both of them were stable within 24 h of continuous reaction. An average acetone conversion of 99.0% was obtained over 0.1 wt.% Pt/V1.5 Y1 Ox catalyst. The results of Fig. 3 indicate that the V1.5 Y1 Ox catalyst has potential application perspectives because of its high activity and stability.
much larger specific surface area. With the increase of V to Y molar ratio the BET surface area of catalysts increased first and then decreased. V1.2 Y1 Ox catalyst has the highest specific surface area (73 m2 /g). Fig. 4 shows the XRD patterns of VYO composite catalysts. Pure YVO4 (tetragonal) shows several strong diffraction peaks at 2 = 18.7◦ , 24.9◦ , 33.5◦ , 49.8◦ (JCPDS 17-0341). When vanadium was doped into YVO4 phase, several new peaks at 2 = 15.3◦ , 20.3◦ , 21.7◦ , 26.1◦ appeared, which could be assigned to V2 O5 (JCPDS 41-1426). With the increase of V/Y atomic ratio the peak intensity of V2 O5
3.2. Characterizations 3.2.1. BET and XRD analysis of catalysts Table 2 shows the specific surface area of catalysts. The BET surface area of YVO4 is 11 m2 /g. It is larger than that of V2 O5 (6 m2 /g). In comparison with YVO4 or V2 O5 , VYO composite catalysts show Table 2 Specific surface area of catalysts. Catalysts
YVO4
V1 Y1 Ox
V1.2 Y1 Ox
V1.5 Y1 Ox
V2.0 Y1 Ox
V3.2 Y1 Ox
V2 O5
S/m2 g−1
11
64
73
50
33
20
6
Fig. 4. XRD patterns of YVO4 (a), V2 O5 (h) and VYO composite catalysts, (b) V1 Y1 Ox ; (c) V1.2 Y1 Ox ; (d) V1.5 Y1 Ox ; (e) V2.0 Y1 Ox ; (f) V3.2 Y1 Ox ; (g) used V1.5 Y1 Ox .
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Fig. 6. Raman spectra of catalysts.
Fig. 5. FT-IR spectra of YVO4 (a), V2 O5 (h) and VYO composite catalysts, (b) V1 Y1 Ox ; (c) V1.2 Y1 Ox ; (d) V1.5 Y1 Ox ; (e) V2.0 Y1 Ox ; (f) V3.2 Y1 Ox ; (g) used V1.5 Y1 Ox .
increased. There is no new peak observed in the XRD pattern of the VYO composite catalyst, indicating that no new phase was generated. The XRD patterns of V1.5 Y1 Ox before reaction (Fig. 4 curve d) and after reaction (Fig. 4 curve g) were recorded. The XRD patterns of the two samples show that, even when the catalyst was subjected to more than 24 h of online reaction, the phase structure of the catalyst did not change. 3.2.2. FT-IR analysis of catalysts Fig. 5 shows the FT-IR spectra of VYO composite catalysts. YVO4 has a strong broad peak between 750 and 950 cm−1 . On VYO composite catalysts a new peak at 1020 cm−1 was observed, which could be assigned to the V O stretching vibration in the V2 O5 phase [26]. With the increase of the V/Y molar ratio, the peak of the V2 O5 phase became stronger. The FT-IR spectrum of V1.5 Y1 Ox catalyst after reaction was also recorded. No changes were observed in the FT-IR spectra of the catalyst V1.5 Y1 Ox before and after reaction. The FT-IR characterizations are consistent to those of XRD shown in Fig. 4. 3.2.3. Raman analysis of catalysts Both the XRD and FT-IR analysis of the VYO composite catalysts showed that only V2 O5 and YVO4 phases were formed in catalysts, but does not give any information about surface structure. Fig. 6 shows the Raman spectra of the catalysts. YVO4 has several strong peaks at 258, 378, 814, 838, and 889 cm−1 , while V2 O5 shows several strong peaks at 280, 403, 526, 690, 990 cm−1 . The Raman spectrum of V1.5 Y1 Ox shows that there is some V2 O5 phase besides the YVO4 phase. There is no new peak (belonging to a new phase) observed over the surface of the catalyst. The data in Figs. 4–6 indicate that the V2 O5 domains are well distributed in the photocatalyst. 3.2.4. UV–vis analysis of catalysts The UV–vis spectra of P25 (TiO2 Degussa), YVO4 , V2 O5 and VYO catalysts are shown in Fig. 7. P25 could only absorb the UV light. YVO4 also has a strong absorption in the UV region. The band gap absorption edge is determined to be 367 nm, corresponding to the band gap energy 3.38 eV. Xu et al. reported that the band gap of nanosized YVO4 particles is about 3.5 eV [23]. Our result is a little smaller. It could due to the quantum-sized effect of the nano-system. Besides, the UV–vis spectrum of YVO4 shows it has some photoadsorption performance in the visible region. The oxygen vacancy in the YVO4 might be the possible reason [27]. V2 O5 has a small band gap and could absorb most of the visible light. So, doping V2 O5 into YVO4 greatly promoted the catalyst’s pho-
toadsorption performance. With the increase of vanadium content, the peak intensity in the visible region (400–600 nm) increases. But when the molar ratio of V to Y is greater than 1.5, only a little difference is observed. 3.3. Discussions The primary factors that influence a photocatalytic reaction are mainly: (1) the adsorption ability of the reactant on the catalyst surface, (2) the ability of the catalyst to absorb the incident radiation, and (3) the efficient separation and transport of light-induced electrons and holes in the catalyst. In the current case, the VYO composites have larger surface area than YVO4 or V2 O5 . So, they could adsorb more organic compounds to the catalyst surface and show higher photocatalytic activity. However, the consistency of the surface area and activity was not observed in the VYO composite with different V/Y molar ratio. The V1.2 Y1 Ox (73 m2 /g) and V3.2 Y1 Ox (20 m2 /g) catalysts show lower acetone conversion than V1.5 Y1 Ox (50 m2 /g) catalyst under both UV and visible light. It means the specific area is not the only factor to influence the catalytic activity. The most important factors to influence the activity might be radiation absorption and charge separation, which are often related to the phase composition of the catalysts.
Fig. 7. UV–vis spectra of YVO4 (a), V2 O5 (g), P25 (h) and VYO composite catalysts, (b) V1 Y1 Ox ; (c) V1.2 Y1 Ox ; (d) V1.5 Y1 Ox ; (e) V2.0 Y1 Ox ; (f) V3.2 Y1 Ox .
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4. Conclusions It is concluded that the VYO composite catalyst exhibits high photocatalytic activity for the photodegradation of acetone under both UV and visible light irradiation. The optimized V to Y molar ratio is 1.5. By doping a small amount of Pt the catalytic performance of V1.5 Y1 Ox catalyst could be promoted further. 99% of acetone was completely photodegradated to CO2 and H2 O under visible light. In order to elucidate the origin of the photocatalyst, we characterized the VYO composite by different techniques. The results indicated that the high performance of these catalysts might be attributed to the enhanced charge separation by the electrons and/or holes transfer between V2 O5 and YVO4 semiconductors. Acknowledgements
Fig. 8. PL spectra of YVO4 and V1.5 Y1 Ox catalysts.
The characterizations revealed that only the V2 O5 and YVO4 phases exist in the VYO composite catalysts. V2 O5 is inactive because of the rapid recombination of electron–hole pairs. YVO4 also shows low activity under visible light because of the limited photoadsorption ability. However, when the two semiconductors were combined, a high photocatalytic activity under visible light was obtained. It shows the typical phenomenon of the coupled photocatalyst. So, it is plausible that the stronger photoactivity of the VYO composite catalysts results from the coupling effect between V2 O5 and YVO4 . Many researchers have noted the particularity of the photoactivity of composite systems consisting of two semiconductors in contact [9–17], and attributed the improvement of activity to the enhanced charge separation, which is due to the electron or hole transfer between the coupled semiconductors. In our case, we think this mechanism also works. That means that in the VYO composite the electrons or holes transfer might occur from one semiconductor to another. This simultaneous charge transfer could increase the adsorption ability for visible light, promote the space charge separation, limit electron–hole recombination and thus promote the photo-oxidation efficiency [28]. The photoluminescence spectra of the photocatalysts are useful to disclose the migration, transfer, and recombination processes of the photogenerated electron–hole pairs in the semiconductor. Fig. 8 shows the PL spectra of YVO4 and V1.5 Y1 Ox catalysts. The PL spectrum of YVO4 shows a strong emission, which indicates that the electrons and holes recombine rapidly [29,30]. However, over V1.5 Y1 Ox catalyst, the peaks intensity is greatly decreased. It suggests that the doping with V2 O5 decreased the electron–hole pair recombination. The V2 O5 could also be the centers of recombination reactions. Hence, there exists an optimum dosage for the V2 O5 phase (V1.5 Y1 Ox catalyst). When the content of V2 O5 is higher than its optimal level, V2 O5 would retard the electron–hole recombination. However, when the concentration of V2 O5 is higher than its optimal level, V2 O5 would promote the recombination reaction and decreased the photocatalytic activity. The Pt doped V1.5 Y1 Ox catalyst was also studied in this paper. However, such a small amount of Pt (0.1 wt.%) did not give rise to any detectable changes in the specific surface area and structure except the PL performance. As shown in Fig. 8, nearly no PL peak was observed over 0.1 wt.% Pt/catalyst. It means that the electrons and holes recombine very slowly, and that the doped Pt retards the recombination of electron–hole pairs further. It is in good agreed with the photocatalytic testing results (Table 1).
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Visible light photodegradation of organics over VYO composite catalyst Yiming He a , Ying Wu b,∗ , Hai Guo a , Tianlu Sheng c , Xintao Wu c a
College of Mathematics, Physics and Information Engineering, Zhejiang Normal University, Jinhua 321004, China Institute of Physical Chemistry, Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Zhejiang Normal University, Jinhua 321004, China c State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China b
a r t i c l e
i n f o
Article history: Received 3 March 2009 Received in revised form 5 April 2009 Accepted 7 April 2009 Available online 14 April 2009 Keywords: Photodegradation Photocatalyst V2 O5 YVO4 Acetone
a b s t r a c t This paper presents a visible-light driven photocatalyst, VYO composite, which was synthesized by doping YVO4 with V2 O5 . The catalytic testing results indicate that the photocatalyst shows high activity for photodegradation of acetone under both UV and visible light. The highest acetone conversion was obtained over V1.5 Y1 Ox composite catalyst. By doping a small amount of metal Pt the photocatalytic efficiency of the catalyst could be promoted further. It was also proved to be efficient for the photodegradation of methanol, ethanol, 2-propanol, and benzene. The physical and photophysical properties of the VYO composites were characterized by BET, XRD, FT-IR, Raman, UV–vis spectra, and photoluminescence spectra, respectively. On the basis of the investigation results, the high photocatalytic activity might be attributed to the coupling effect between YVO4 and V2 O5 . Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction Photodegradation of organic pollutants in the presence of TiO2 appears to be a viable decontamination process with widespread application, regardless of the state (gas or liquid) or chemical nature of the process target [1]. However, its technological applications seem to be limited by several factors, among which the most restrictive one is the need to use an ultraviolet (UV) excitation source. The efficient use of solar light may then appear to be an appealing challenge for developing the future generation of photocatalytic materials. Currently, most of the investigations in this area are focused on the improvements of doped TiO2 catalysts. Several approaches have been reported. These approaches include the incorporation of metal ions (such as Fe3+ , V5+ , La3+ ) or anions (such as N3− , S2− , F− ) into the matrix of TiO2 [2–6], the introduction of oxygen vacancies into the lattice of TiO2 [7,8], and the combination of two different semiconductors [9,10]. In these approaches, the third method attracted many attentions. The reason is that the combination of TiO2 and a narrow band gap semiconductor not only could promote the absorbing ability for the visible light, but also could retard the recombination of electron–hole pairs. This strategy has been effective for other semiconductors with large band gap. Up to now, a large variety of coupled semiconductor systems have been reported, such as CdS/ZnO, WS2 /WO3 , V2 O5 /MgF2 [9–17].
Tetragonal phased YVO4 has been extensively used as the red phosphor with several rare-earth metal ions as dopant in cathode ray tubes and color television in powder form [18,19]. In addition, many attentions have been paid on its thermal catalytic properties such as oxidative dehydrogenation of propane [20–22]. However, in the photocatalysis there have been few studies focused on the photocatalytic performance of yttrium orthovanadate. Recently, Xu et al. studied the photocatalytic properties of YVO4 [23]. The results showed that YVO4 could photodegrade methyl orange effectively under UV light. The high activity could be attributed to the wide band gap (3.5 eV) which makes the excited electron and hole have the high potential to oxidize the organics. However, the wide band gap of YVO4 also limits its photoadsorption to the ultraviolet region of the solar spectrum. In this paper, we present a new composite catalyst V2 O5 /YVO4 , which shows high photocatalytic activity on photodegradation of organic substrates under visible light. The semiconductor V2 O5 was chosen as sensitizer because of its small band gap (∼2.0 eV), which could make it active in a wide region of visible light wavelength (<600 nm). Lots of photocatalysts containing V2 O5 have been reported, and it has been proved that V2 O5 is a good semiconductor sensitizer [14–16]. 2. Experimental 2.1. Catalysts preparation
∗ Corresponding author. Tel.: +86 0579 82283920; fax: +86 0579 82283920. E-mail address: [email protected] (Y. Wu).
NH4 VO3 (>99%), Y(NO3 )3 ·6H2 O (>99.99%), and P25 (Degussa TiO2 ) as reference, were purchased commercially and used
0304-3894/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.04.018
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without further purification. V2 O5 was obtained by calcining NH4 VO3 at 500 ◦ C for 4 h. YVO4 was prepared by precipitation method: solutions of NH4 VO3 and Y(NO)3 with a V to Y molar ratio of 1.0 were mixed to obtain a deposit. The pH value of the solution was adjusted to 7–8 by a solution of NH3 . After aging at room temperature for 5 h, the deposit was filtered, washed three times by water, dried at 100 ◦ C for 12 h and calcined at 500 ◦ C for 4 h. The VYO composite catalysts were prepared by the following steps: solutions of NH4 VO3 and Y(NO3 )3 with different V/Y mole ratio were mixed and evaporated to give a solid precursor. After dried at 100 ◦ C for 12 h, the precursor solid was calcined at 500 ◦ C for 4 h and then cooled to room temperature to yield the catalyst. The 0.1 wt.% Pt/V1.5 Y1 Ox catalyst was prepared by impregnation–photoreduction method [15]: 4.5 g of V1.5 Y1 Ox catalyst were added into 3.0 mL of H2 PtCl6 ·6H2 O solution (Pt: 0.0015 g/mL). The sample was kept in the dark for 5 h, and then drying at 80 ◦ C for 10 h. After that, the dried sample was put on the xenon lamp irradiation for 2 h (the strong light could decompose the H2 PtCl6 to metal Pt) to obtain the 0.1 wt.% Pt/V1.5 Y1 Ox catalyst. 2.2. Catalytic tests The catalytic reaction under UV light was carried out in a quartz tube (ID 5.0 mm) reactor and two 500 W high pressure mercury lamps were used as UV light sources. When the reaction was carried out under visible light, two 400 W xenon lamps were used as visible light sources and a glass tube (ID 5.0 mm) reactor which could cut off most of the UV light was used. In each reaction, the bed length of catalyst (500–800 mg) is about 4.5 cm and the other part of the reactor was wrapped by aluminum paper to exclude the contribution of the blank reaction (Fig. 1). A thermocouple which clung to the reactor closely was used to detect the reaction temperature. The reactor tube was cooled by a fan. Because of the heat from the lamps, although we tried to cool down the reactor by the fan, the reaction temperature is still between 130 and 140 ◦ C. Pure oxygen was used as the oxidant. The organic reactants (acetone, methanol, 2-propanol, benzene) were fed into the reactor by bubbling gas (O2 ) through liquid organic at 0 ◦ C (cooled in a water-ice bath) to obtain the reactant mixture. The flow of the mixture was controlled at 8.0 mL/min. The concentration of organic substrates was analyzed by GC and the results are shown in Table 1. Before each catalytic testing, the photocatalyst was allowed to equilibrate
Fig. 1. The reactor system.
in the reaction gas for at least 60 min. The reaction products were analyzed on a GC (equipped with a GDX-203 column and a 5A carbon molecular sieve column, the samples were injected by hand, injection volume 100 L) with TCD. All the data were collected after 3 h of online reaction. In order to rule out the thermal reaction, both the V1.5 Y1 Ox and 0.1 wt.% Pt/V1.5 Y1 Ox catalysts were tested for acetone oxidation in the dark at the same reaction temperature (140 ◦ C). The dark reaction shows that acetone did not react with oxygen over V1.5 Y1 Ox or 0.1 wt.% Pt/V1.5 Y1 Ox catalysts at 140 ◦ C. 2.3. Characterizations The XRD characterization of catalysts was carried out on an Xray diffraction spectroscopy meter (RIGAKU DMAX2500) using Cu K␣ radiation (40 kV/40 mA). The specific surface areas of the catalysts were measured on Autosorb-1 (Quantachrome Instruments). The Raman spectra of the catalysts were collected on RM1000 spectrometer (Renishaw) with an Ar ion laser (514.5 nm) as excitation source. The FTIR spectra of the catalysts were recorded on PerkinElmer Magna 750 with a resolution of 4 cm−1 . The UV–vis spectra of catalysts were recorded on a UV–vis spectrometer (PerkinElmer Lambda900) equipped with an integrating sphere. The photoluminescence (PL) spectra of catalysts were collected on FLS-920 (Edinbergh Instrument). The light source was a Xe lamp (excitation at 280 nm). 3. Results and discussions 3.1. Photocatalytic performance of catalyst The photocatalytic performance of catalysts was tested in the photodegradation reaction of acetone under UV and visible light. Fig. 2 shows the photocatalytic activities of P25 (TiO2 , Degussa), V2 O5 , YVO4 and VYO composite catalysts. Under UV light, P25 has higher acetone degradation conversion than YVO4 . But under visible light the activity of P25 is lower than that of YVO4 . 27.9% acetone conversion was obtained over YVO4 and only 14.5% acetone conversion was obtained over P25. V2 O5 has low activity under both UV and visible light. For the VYO composite, as shown in Fig. 2, the acetone degradation conversion increases with the increase of V/Y ratio from 1.0 to 1.5 and decreases at even higher V/Y molar ratio under UV light. The highest acetone conversion was obtained on catalyst V1.5 Y1 Ox . The acetone conversion reached 98.4% under visible light. The result of Fig. 2 shows that the VYO composite catalysts are active as P25 under UV light. Under visible light, it shows much higher photoactivity than P25, YVO4 , or V2 O5 . Obviously, doping of V2 O5 to YVO4 formulated a series of active photodegradation catalysts. In the above sections, only acetone degradation reaction was discussed over catalysts. To be an efficient photodegradation catalyst, not only is high catalytic activity required, but also suitable product selectivity is needed. In photodegradation reactions, CO2 and H2 O are the preferred final products, and any other organic intermediate compound formed in the photodegradation reaction is still viewed as a pollutant. Although V1.5 Y1 Ox catalyst showed high activity in acetone photodegradation reaction, the catalyst was still not able to oxidize acetone completely. There is a significant amount of 1hydroxy-2-propanone (selectivity 0.8%) and CO (selectivity 51.8%) left as partial oxidation products (Table 1). In the photodegradation reactions of methanol, ethanol, 2-propanol, and benzene, CO and other harmful by-products are also formed in the reactions (Table 1). Loading a small amount of noble metal (such as Pt, Au, Ag, Ru, etc.) to photocatalyst is a practical way to promote the complete oxidation of organic pollutants [16,24,25]. The loaded metal
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Table 1 Photodegradation of organics over V1.5 Y1 Ox and 0.1 wt.% Pt/V1.5 Y1 Ox catalysts under visible light. Organics
Acetone
Methanol
X/%
10
Conversion/%
98.4 (99.8)
SCO2 /%
47.4 (100)
SCO /%
51.8 (0)
5
1.5
100 (100)
100 (100)
8.6 (100) 91.4 (0)
0.8 (0)a
Sother /%
Ethanol
0 (0)
2-Propanol 1
Benzene 2.2
99.8 (100)
82.5 (98.9)
36.9 (100)
42.3 (100)
69.5 (100)
63.1 (0)
57.6 (0)
30.5 (0)
0.1 (0)
0.1 (0)b
0 (0)
Note: The numbers in bracket represent the performance of 0.1 wt.% Pt/V1.5 Y1 Ox , X is the molar concentration of organic compound in oxygen. a Represents selectivity to 1-hydroxy-2-propanone. b Represents selectivity to acetone.
Fig. 3. Life-time testing of V1.5 Y1 Ox and 0.1 wt.% Pt/V1.5 Y1 Ox catalyst samples under visible light. Fig. 2. Acetone photodegradation over VYO catalysts with different V/Y molar ratio.
could change the distribution of the electrons in the photocatalyst and form the Schottky barriers at each metal-semiconductor contact region, which leads to charge separation. Among the noble metals, it is reported that Pt is the best dopant [16]. Hence, we doped the V1.5 Y1 Ox catalyst with a small amount of Pt (0.1 wt.%). Table 1 shows the catalytic performance of the catalysts for organic compounds degradation reaction. The 0.1 wt.% Pt/V1.5 Y1 Ox catalyst shows higher activity than catalyst V1.5 Y1 Ox , and only CO2 and H2 O were detected in the degradation products. Besides the selectivity of products, the stability of the catalytic performance is another important evaluation criterion for a catalyst. Fig. 3 shows the life-testing results of V1.5 Y1 Ox and 0.1 wt.% Pt/V1.5 Y1 Ox catalyst under visible light. As shown in Fig. 3, both of them were stable within 24 h of continuous reaction. An average acetone conversion of 99.0% was obtained over 0.1 wt.% Pt/V1.5 Y1 Ox catalyst. The results of Fig. 3 indicate that the V1.5 Y1 Ox catalyst has potential application perspectives because of its high activity and stability.
much larger specific surface area. With the increase of V to Y molar ratio the BET surface area of catalysts increased first and then decreased. V1.2 Y1 Ox catalyst has the highest specific surface area (73 m2 /g). Fig. 4 shows the XRD patterns of VYO composite catalysts. Pure YVO4 (tetragonal) shows several strong diffraction peaks at 2 = 18.7◦ , 24.9◦ , 33.5◦ , 49.8◦ (JCPDS 17-0341). When vanadium was doped into YVO4 phase, several new peaks at 2 = 15.3◦ , 20.3◦ , 21.7◦ , 26.1◦ appeared, which could be assigned to V2 O5 (JCPDS 41-1426). With the increase of V/Y atomic ratio the peak intensity of V2 O5
3.2. Characterizations 3.2.1. BET and XRD analysis of catalysts Table 2 shows the specific surface area of catalysts. The BET surface area of YVO4 is 11 m2 /g. It is larger than that of V2 O5 (6 m2 /g). In comparison with YVO4 or V2 O5 , VYO composite catalysts show Table 2 Specific surface area of catalysts. Catalysts
YVO4
V1 Y1 Ox
V1.2 Y1 Ox
V1.5 Y1 Ox
V2.0 Y1 Ox
V3.2 Y1 Ox
V2 O5
S/m2 g−1
11
64
73
50
33
20
6
Fig. 4. XRD patterns of YVO4 (a), V2 O5 (h) and VYO composite catalysts, (b) V1 Y1 Ox ; (c) V1.2 Y1 Ox ; (d) V1.5 Y1 Ox ; (e) V2.0 Y1 Ox ; (f) V3.2 Y1 Ox ; (g) used V1.5 Y1 Ox .
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Fig. 6. Raman spectra of catalysts.
Fig. 5. FT-IR spectra of YVO4 (a), V2 O5 (h) and VYO composite catalysts, (b) V1 Y1 Ox ; (c) V1.2 Y1 Ox ; (d) V1.5 Y1 Ox ; (e) V2.0 Y1 Ox ; (f) V3.2 Y1 Ox ; (g) used V1.5 Y1 Ox .
increased. There is no new peak observed in the XRD pattern of the VYO composite catalyst, indicating that no new phase was generated. The XRD patterns of V1.5 Y1 Ox before reaction (Fig. 4 curve d) and after reaction (Fig. 4 curve g) were recorded. The XRD patterns of the two samples show that, even when the catalyst was subjected to more than 24 h of online reaction, the phase structure of the catalyst did not change. 3.2.2. FT-IR analysis of catalysts Fig. 5 shows the FT-IR spectra of VYO composite catalysts. YVO4 has a strong broad peak between 750 and 950 cm−1 . On VYO composite catalysts a new peak at 1020 cm−1 was observed, which could be assigned to the V O stretching vibration in the V2 O5 phase [26]. With the increase of the V/Y molar ratio, the peak of the V2 O5 phase became stronger. The FT-IR spectrum of V1.5 Y1 Ox catalyst after reaction was also recorded. No changes were observed in the FT-IR spectra of the catalyst V1.5 Y1 Ox before and after reaction. The FT-IR characterizations are consistent to those of XRD shown in Fig. 4. 3.2.3. Raman analysis of catalysts Both the XRD and FT-IR analysis of the VYO composite catalysts showed that only V2 O5 and YVO4 phases were formed in catalysts, but does not give any information about surface structure. Fig. 6 shows the Raman spectra of the catalysts. YVO4 has several strong peaks at 258, 378, 814, 838, and 889 cm−1 , while V2 O5 shows several strong peaks at 280, 403, 526, 690, 990 cm−1 . The Raman spectrum of V1.5 Y1 Ox shows that there is some V2 O5 phase besides the YVO4 phase. There is no new peak (belonging to a new phase) observed over the surface of the catalyst. The data in Figs. 4–6 indicate that the V2 O5 domains are well distributed in the photocatalyst. 3.2.4. UV–vis analysis of catalysts The UV–vis spectra of P25 (TiO2 Degussa), YVO4 , V2 O5 and VYO catalysts are shown in Fig. 7. P25 could only absorb the UV light. YVO4 also has a strong absorption in the UV region. The band gap absorption edge is determined to be 367 nm, corresponding to the band gap energy 3.38 eV. Xu et al. reported that the band gap of nanosized YVO4 particles is about 3.5 eV [23]. Our result is a little smaller. It could due to the quantum-sized effect of the nano-system. Besides, the UV–vis spectrum of YVO4 shows it has some photoadsorption performance in the visible region. The oxygen vacancy in the YVO4 might be the possible reason [27]. V2 O5 has a small band gap and could absorb most of the visible light. So, doping V2 O5 into YVO4 greatly promoted the catalyst’s pho-
toadsorption performance. With the increase of vanadium content, the peak intensity in the visible region (400–600 nm) increases. But when the molar ratio of V to Y is greater than 1.5, only a little difference is observed. 3.3. Discussions The primary factors that influence a photocatalytic reaction are mainly: (1) the adsorption ability of the reactant on the catalyst surface, (2) the ability of the catalyst to absorb the incident radiation, and (3) the efficient separation and transport of light-induced electrons and holes in the catalyst. In the current case, the VYO composites have larger surface area than YVO4 or V2 O5 . So, they could adsorb more organic compounds to the catalyst surface and show higher photocatalytic activity. However, the consistency of the surface area and activity was not observed in the VYO composite with different V/Y molar ratio. The V1.2 Y1 Ox (73 m2 /g) and V3.2 Y1 Ox (20 m2 /g) catalysts show lower acetone conversion than V1.5 Y1 Ox (50 m2 /g) catalyst under both UV and visible light. It means the specific area is not the only factor to influence the catalytic activity. The most important factors to influence the activity might be radiation absorption and charge separation, which are often related to the phase composition of the catalysts.
Fig. 7. UV–vis spectra of YVO4 (a), V2 O5 (g), P25 (h) and VYO composite catalysts, (b) V1 Y1 Ox ; (c) V1.2 Y1 Ox ; (d) V1.5 Y1 Ox ; (e) V2.0 Y1 Ox ; (f) V3.2 Y1 Ox .
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4. Conclusions It is concluded that the VYO composite catalyst exhibits high photocatalytic activity for the photodegradation of acetone under both UV and visible light irradiation. The optimized V to Y molar ratio is 1.5. By doping a small amount of Pt the catalytic performance of V1.5 Y1 Ox catalyst could be promoted further. 99% of acetone was completely photodegradated to CO2 and H2 O under visible light. In order to elucidate the origin of the photocatalyst, we characterized the VYO composite by different techniques. The results indicated that the high performance of these catalysts might be attributed to the enhanced charge separation by the electrons and/or holes transfer between V2 O5 and YVO4 semiconductors. Acknowledgements
Fig. 8. PL spectra of YVO4 and V1.5 Y1 Ox catalysts.
The characterizations revealed that only the V2 O5 and YVO4 phases exist in the VYO composite catalysts. V2 O5 is inactive because of the rapid recombination of electron–hole pairs. YVO4 also shows low activity under visible light because of the limited photoadsorption ability. However, when the two semiconductors were combined, a high photocatalytic activity under visible light was obtained. It shows the typical phenomenon of the coupled photocatalyst. So, it is plausible that the stronger photoactivity of the VYO composite catalysts results from the coupling effect between V2 O5 and YVO4 . Many researchers have noted the particularity of the photoactivity of composite systems consisting of two semiconductors in contact [9–17], and attributed the improvement of activity to the enhanced charge separation, which is due to the electron or hole transfer between the coupled semiconductors. In our case, we think this mechanism also works. That means that in the VYO composite the electrons or holes transfer might occur from one semiconductor to another. This simultaneous charge transfer could increase the adsorption ability for visible light, promote the space charge separation, limit electron–hole recombination and thus promote the photo-oxidation efficiency [28]. The photoluminescence spectra of the photocatalysts are useful to disclose the migration, transfer, and recombination processes of the photogenerated electron–hole pairs in the semiconductor. Fig. 8 shows the PL spectra of YVO4 and V1.5 Y1 Ox catalysts. The PL spectrum of YVO4 shows a strong emission, which indicates that the electrons and holes recombine rapidly [29,30]. However, over V1.5 Y1 Ox catalyst, the peaks intensity is greatly decreased. It suggests that the doping with V2 O5 decreased the electron–hole pair recombination. The V2 O5 could also be the centers of recombination reactions. Hence, there exists an optimum dosage for the V2 O5 phase (V1.5 Y1 Ox catalyst). When the content of V2 O5 is higher than its optimal level, V2 O5 would retard the electron–hole recombination. However, when the concentration of V2 O5 is higher than its optimal level, V2 O5 would promote the recombination reaction and decreased the photocatalytic activity. The Pt doped V1.5 Y1 Ox catalyst was also studied in this paper. However, such a small amount of Pt (0.1 wt.%) did not give rise to any detectable changes in the specific surface area and structure except the PL performance. As shown in Fig. 8, nearly no PL peak was observed over 0.1 wt.% Pt/catalyst. It means that the electrons and holes recombine very slowly, and that the doped Pt retards the recombination of electron–hole pairs further. It is in good agreed with the photocatalytic testing results (Table 1).
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