W.-S. Chang et al. / Applied Catalysis A: General 407 (2011) 224–230 225composites, the recyclability test of the SVO/SBA-15 composites were calculated using the Brunauer–Emmett–Teller (BET) methodwas investigated using DRIFTS. in the relative pressure range of 0.06–0.2. The pore size distri- butions were determined from the analysis of the adsorption isotherm using the Barret–Joyner–Halenda (BJH) algorithm. The2. Experimental procedure total pore volumes were estimated from the adsorbed N2 amount at P/P0 = 0.973.2.1. Preparation of photocatalyst 2.3. Photocatalytic activity evaluation with GC–MS and DRIFTS SBA-15 was synthesized with Pluronic P123 (EO20 PO70 EO20 ,Mav = 5800; Aldrich) and tetraethylorthosilicate (TEOS) according The photocatalytic oxidation of toluene was performed in situto a previous report . Brieﬂy, 4.0 g of P123 was dissolved in 30 g in an IR cell with ZnSe windows. An LED lamp, with a wave-of de-ionized water and 120.0 g of HCl solution (2 M) with stirring at length ranging from 430 to 620 nm with a photon intensity of313 K for 2 h. Then, 8.5 g of TEOS was added into the P123 solution, 4 mW/cm2 , was used as the visible-light source . A gaseouswhich was stirred for another 22 h until a white gel precipitated. toluene/N2 mixture was generated, corresponding to the targetThe gel was transferred to a Teﬂon bottle and heated at 403 K for toluene concentration of 200 ppmv. Prior to the experiments, the24 h. The precipitate was ﬁltered, washed several times with de- samples were pre-treated by heating and ﬂushing with N2 ﬂowionized water, dried overnight at 373 K, and then calcined at 773 K (20 ml/min) from room temperature to 523 K, held at 523 K for(heating rate of 1 K/min) for 4 h in air. 30 min, and then cooled to 303 K. The procedure for all PCO experi- The silver-vanadate-loaded SBA-15 (SVO/SBA-15) was prepared ments was as follows: (1) toluene/N2 ﬂow was introduced into thevia the incipient wetness impregnation procedure. In the prepara- photoreactor at a constant ﬂow rate of 20 ml/min; (2) when thetion process, 0.204 g AgNO3 was dissolved in urea aqueous solution photoreactor inlet and outlet toluene concentrations were approx-(120 g H2 O, 0.577 g urea) with stirring at room temperature for 0.5 h imately equal, the LED lamp was turned on and the toluene/N2to obtain solution A. Solution B was prepared by mixing 0.047 g ﬂow was stopped; (3) the photoreactor was ﬂushed with the N2NH4 VO3 with 0.3 g SBA-15 in de-ionized water at 343 K for 1 h ﬂow at 5 ml/min for 30 min, and then the N2 ﬂow was stopped.under an ultrasonic bath. A suspension formed when solution A was The gaseous products (intermediates) were analyzed on-line byadded dropwise to solution B under vigorous stirring for 1 h. The GC–MS. GC–MS analyses were carried out on a gas chromato-molar composition of the suspension AgNO3 /NH4 VO3 /CO(NH2 )2 graph (GC, Perkin-Elmer, Clarus 500) using a 60-m DB-624 capillarywas 3.0/1.0/12.0. The suspension was titrated to pH 7 using ammo- column, coupled with a quadrupole mass spectrometer (MS, SRS,nia solution, followed by additional stirring at room temperature QMS 300) at regular intervals. The spectra of the adsorbed speciesfor 24 h. Finally, the as-obtained suspension was transferred into on the catalyst surface were recorded under both darkness anda Teﬂon-lined autoclave with hydrothermal treatment (temper- illumination. The spectra of the catalyst and the reaction intermedi-ature: 413 K, time: 4 h). After the hydrothermal procedure, the ates during the reaction were expressed in units of Kubelka–Munkresulting precipitates were collected and washed with de-ionized (K–M).water three times, and then dried at 353 K for 12 h. These sampleswere denoted xSVO/SBA-15, where x represents the weight per- 2.4. DRIFT characterization of NH3 speciescentage of silver vanadate (wt%). The sample synthesized underidentical conditions without the addition of SBA-15 was denoted To determine the types of acid site present on the sam-SVO. ples, temperature-programmed desorption of ammonia (NH3 ) was carried out using DRIFT. Ammonia usually provides the probe2.2. Sample characterization molecules in spectroscopic experiments to determine the type of acid site in heterogeneous catalysis: Brønsted sites or Lewis sites. The X-ray diffraction (XRD) patterns of the powders were mea- Prior to the experiments, the samples were heated in situ fromsured using an X-ray diffractometer (PANalytical X’Pert PRO) with room temperature to 523 K at 10 K/min in N2 ﬂow (30 ml/min),Cu radiation ( = 0.15418 nm) in the 2Â range of 20–60◦ . High- held at 523 K for 30 min, and then cooled down to 303 K. The sam-resolution transmission electron microscopy (HRTEM) images of ples were saturated at 303 K with a gas mixture of 5% NH3 in N2the samples were observed on a Philips Tecnai G2 F20 micro- (30 ml/min) for 30 min. At the end of the saturation process, thescope equipped with energy-dispersive X-ray spectroscopy (EDX) samples were ﬂushed with N2 ﬂow (30 ml/min). Then, the sam-operated at an accelerating voltage of 200 kV. Photoluminescence ples were heated again at a heating rate of 10 K/min from 30 K to(PL) spectra were recorded by a ﬂuorescence spectrophotome- 523 K and held at 523 K for 60 min. The DRIFT spectra with a reso-ter (Dongwoo, Optron) under excitation at 325 nm. In situ lution of 4 cm−1 were collected in the interval of 1200–1700 cm−1DRIFTS measurements were performed using a FTIR spectrome- for determining surface acidity.ter (PerkinElmer, Spectrum GX) and a diffuse reﬂectance accessory(Harrick Scientiﬁc, DRP-PE 9) with a temperature- and atmosphere- 3. Results and discussioncontrolled high-temperature low-pressure reaction cell (HarrickScientiﬁc, HVC-DRP 3). Prior to the IR measurements, the sam- 3.1. X-ray diffraction analysisples were dehydrated under vacuum from room temperatureto 523 K at 10 K/min in N2 ﬂow (30 ml/min), held at 523 K for The XRD patterns of SVO and SVO-SBA-15 are shown in Fig. 1.30 min, and then cooled to 303 K. DRIFT spectra with a resolu- Fig. 1a shows the low-angle XRD patterns of SBA-15 and SVO/SBA-tion of 4 cm−1 were collected in the interval of 2800–4000 cm−1 15 samples. The XRD pattern of SBA-15 shows three well resolvedfor surface hydroxyl functional groups . The surface area and peaks: a sharp peak at 1.0◦ and two weak peaks near 1.5◦ andpore volume of the as-prepared samples were determined using 2.0◦ , respectively, which can be indexed to (1 0 0), (1 1 0), anda volumetric sorption analyzer (Micromeritics, ASAP 2020). The (2 0 0) reﬂections of an ordered hexagonal P6mm space group,samples were degassed at 473 K under vacuum conditions for a respectively. For SVO/SBA-15 samples, the XRD peaks decreased inperiod of at least 4 h prior to measurements. The nitrogen adsorp- intensity, which was probably caused by the decreasing scatter con-tion/desorption isotherms were measured over a relative pressure trast between pore walls and the pore space with the introduction(P/P0 ) range of approximately 10−3 to 0.995. The surface areas of SVO. For the samples with high SVO content, the intensities of the
226 W.-S. Chang et al. / Applied Catalysis A: General 407 (2011) 224–230 Fig. 2. N2 adsorption–desorption isotherms of SBA-15 and SVO/SBA-15 samples. silver vanadates were increased to 60 and 70%, all peaks attributed to ␣-Ag3 VO4 disappeared and pure Ag4 V2 O7 was observed. 3.2. Porosity and surface area characterization Fig. 2 shows the nitrogen adsorption-desorption isotherms of SBA-15 and SVO/SBA-15 samples. The N2 adsorption–desorption isotherms of SBA-15 show type IV adsorption with a H1 hysteresis loop, indicating a mesoporous characteristic. For the SVO/SBA-15 samples, a sharp inﬂection in the relative pressure (P/P0 ) range of 0.65–0.95, corresponding to capillary condensation within uni- form mesopores, was observed. Compared to SBA-15, the capillary condensation of SVO/SBA-15 shifted to higher relative pressure, indicating an increase of the pore diameter when SVO was loaded into/onto SBA-15. The textural properties of SBA-15 support and SVO/SBA-15 samples are listed in Table 1. It can be seen that the speciﬁc surface area and pore volumes of SVO/SBA-15 sam- ples signiﬁcantly decrease with increasing silver vanadate loading, whereas the pore diameter increases. When the SVO/SBA-15 samples were synthesized using a post-synthesis method, the silver and vanadate species reacted on the surface of SBA-15; thus, the BET surface and pore volumes of SVO/SBA-15 com- posites decreased with increasing amount of SVO. The average pore diameter increased for SVO/SBA-15, which might be due to the small pores of SBA-15 being obstructed by silver vanadate nanoparticles. 3.3. Morphology Fig. 1. (a) Low-angle and (b) wide-angle XRD patterns of SVO and SVO/SBA-15. To better understand the formation of SVO nanocrystals on the mesopores and explore the effect of SVO on the pore structure of SVO/SBA-15 photocatalyst, HRTEM images were recorded. HRTEMpeaks are much weaker but can still be discerned, demonstratingthat the uniform porous structure of SBA-15 is retained. The unit- Table 1cell parameter (a0 ) of SVO/SBA-15 catalysts (ca. 13.0–14.0 nm) is Speciﬁc surface areas and pore properties of P25, SVO, SBA-15, and SVO/SBA-15.larger than that of SVO (11.8 nm). This may be ascribed to the partial Sample SBET (m2 g−1 ) Pore volume Pore sizeincorporation of SVO into the framework of SBA-15. Fig. 1b shows (cm3 g−1 ) (nm)wide-angle XRD patterns of SVO/SBA-15 samples and SVO. As canbe seen, the SVO and SVO-SBA-15 samples have three kinds of P25 56 0.25 17.52 SVO 2 0.002 7.30XRD patterns, assigned to the pure Ag4 V2 O7 , to the pure ␣-Ag3 VO4 , SBA-15 787 1.09 7.61and to the mixed phases of Ag4 V2 O7 and ␣-Ag3 VO4 , respectively. 17SVO/SBA-15 193 0.70 14.90The pure SVO and 51SVO-SBA-15 samples had mixed phases of 34SVO/SBA-15 141 0.54 16.14␣-Ag3 VO4 and Ag4 V2 O7 . When the weight percentages of silver 51SVO/SBA-15 99 0.35 15.32 60SVO/SBA-15 96 0.34 13.61vanadates were lower than 51%, pure ␣-Ag3 VO4 was obtained for 70SVO/SBA-15 85 0.33 15.8617 and 34SVO/SBA-15 samples. When the weight percentages of
W.-S. Chang et al. / Applied Catalysis A: General 407 (2011) 224–230 227 Fig. 3. HRTEM images of (a) SBA-15 and (b) 51SVO/SBA-15.images of SBA-15 and 51SVO/SBA-15 samples are shown in Fig. 3. 3.4. Adsorption and photocatalytic oxidation of tolueneThe highly ordered mesoporous channel structure of pure SBA-15can be observed in Fig. 3a, which shows a pore diameter of around The adsorption and photodegradation curves of gaseous toluene7–8 nm, in agreement with N2 adsorption–desorption results. Due over samples were shown in Fig. 5. Prior to photocatalytic exper-to the electronic density contrast of TEM, clear and gray stripes can iments, the concentration of gaseous toluene rapidly decreasedbe observed between empty channels and silica walls. When the during the initial 10 min, which was due to the adsorption ofSVO loading was increased to 51 wt% (Fig. 3b), the ordered array of toluene on catalyst surface. After 30 min, the concentration ofmesopores remained intact. Silica walls remained white whereas gaseous toluene is increased to the initial state, indicating that thethe SVO particles, with a larger electronic density, turned black. The adsorption of toluene reaches equilibrium. In the absence of irradi-morphology results indicate that nanocrystalline SVO was inserted ation, the as-prepared SVO/SBA-15 composites except 17SVO/SBA-into the mesoporous channels of SBA-15 when urea was used as 15 exhibit a much higher adsorption capability of toluene thanthe chelating agent, which indicates a strong interaction between those of SVO and P-25. The toluene adsorption capacity is thusurea and metal ions that prevents the precipitation of silver and affected by the silver vanadate content of SVO/SBA-15 composites.vanadium ions before the formation of silver vanadate–silica com- It is well known that photocatalytic oxidation of organic pollu-posites. EDX elemental analysis of a selected point of SBA-15 and tants in the initial stage follows Langmuir–Hinshelwood kinetics;the SBA-15 supported catalyst (Fig. 3) conﬁrms the presence of the L–H model can be simpliﬁed to a pseudo-ﬁrst-order expres-SVO particles in the center of a SBA-15 channel. Fig. 4 shows sion: ln(Ce /C) = kt (where Ce and C are the equilibrium concentrationSEM images for SBA-15 support and SVO/SBA-15 composites. SEM of adsorption and the concentration of VOC at the exposureimage (Fig. 4a) of parent SBA-15 sample presents well-deﬁned time, t, respectively, and k is the apparent rate constant). Thesausage-like particles of approximately 1.5 m in length. These calculated kapp of toluene decreased in the order: 51SVO/SBA-sausage-like particles cluster together to form rope-like aggregates, 15 (0.032 min−1 ) > 34SVO/SBA-15 (0.024 min−1 ) > 60SVO/SBA-15which are parallel straight mesochannel arraying along the long (0.020 min−1 ) > 70SVO/SBA-15 (0.019 min−1 ) > SVO (0.018 min−1 )axis. The SEM images in Fig. 4b–d demonstrate that the ordered > 17SVO/SBA-15 (0.016 min−1 ) > P25 (0.015 min−1 ).rope-like domains are preserved after silver-vanadate impregna- The evolution of gaseous benzaldehyde as a function of irra-tion for 17, 34, and 51SVO/SBA-15 samples. On the other hand, 60 diation time was plotted in Fig. 6. During the ﬁrst 10 min ofand 70SVO/SBA-15 samples show the presence of an overgrowth toluene photodegradation, the amounts of benzaldehyde pro-as the irregular ragged particles when high amounts of silver- duced for all SVO/SBA-15 samples were higher than those ofvanadate nanoparticles are deposited (Fig. 4e and f). However, the SVO and P25. However, at longer reaction times, the amountstacked nanoparticles do not lead to any signiﬁcant change in the of benzaldehyde generated for 17SVO/SBA-15 is lower thanmorphology of silica. that of SVO. The amount of benzaldehyde decreased in the
228 W.-S. Chang et al. / Applied Catalysis A: General 407 (2011) 224–230 Fig. 4. SEM images of various SBA-15 samples: (a) parent SBA-15, (b) 17SVO/SBA-15, (c) 34SVO/SBA-15, (d) 51SVO/SBA-15, (e) 60SVO/SBA-15, and (f) 70SVO/SBA-15.following sequence: 51SVO/SBA-15 > 34SVO/SBA-15 > 60SVO/SBA- It is well known that the adsorption capacity is mainly deter-15 > 70SVO/SBA-15 ∼ SVO > 17SVO/SBA-15 > P25. A big degree of mined by the speciﬁc surface area and nature of the surface ofphotodegradation of toluene takes place in case of 51SVO/SBA- the photocatalyst. Studies have shown that the much higher spe-15. Generally speaking, photocatalytic activity is related to the ciﬁc surface area and pore volume of TiO2 -containg mesoporouscrystalline phase. Konta et al.  reported that ␣-Ag3 VO4 showed silica composites compared to those of pure titania are beneﬁ-stronger photocatalytic activity than those of ␤-AgVO3 and cial for the adsorption of organic pollutants [13–15]. As shownAg4 V2 O7 for O2 evolution from an aqueous silver nitrate solu- in Fig. 5, 51SVO/SBA-15 exhibited the highest adsorption capac-tion under visible-light irradiation. For the SVO/SBA-15 series, it ity of toluene instead of 17SVO/SBA-15. However, 17SVO/SBA-15was observed that the 51SVO/SBA-15 sample, with mixed phases had much higher speciﬁc surface area and pore volume than thoseof ␣-Ag3 VO4 and Ag4 V2 O7 , exhibits the highest photocatalytic of 51SVO/SBA-15; therefore, the toluene adsorption capacity isactivity, whereas 34SVO/SBA-15 shows lower activity than that of thus greatly inﬂuenced by the silver vanadate content, rather51SVO/SBA-15 even though 34SVO/SBA-15 has a high crystallinity than the speciﬁc surface area. To further characterize the sur-of ␣-Ag3 VO4 . An increasing number of studies have indicated that face attributes that affect toluene adsorption, the surface acidity,a bicrystalline framework of anatase and rutile show much better the existence of Brønsted and Lewis acid sites, of the samplesphotocatalytic activity than that of pure anatase TiO2 [10–12]. was examined using DRIFTS to detect ammonia adsorption on
W.-S. Chang et al. / Applied Catalysis A: General 407 (2011) 224–230 229Fig. 5. Time-dependent concentrations of gaseous toluene in the dark and under Fig. 8. Normalized PL intensity of the samples measured at 300 K.visible light. vanadate surfaces is supported by the presence of a band at 1425 cm−1 due to the asymmetric deformation mode [16–18]. The band at 1604 cm−1 is assigned to NH3 coordinately bonded to Lewis acid sites [19–21]. It can be concluded that both Brønsted and Lewis acid sites exist on the surfaces of SVO and SVO/SBA-15. The intensities of Brønsted and Lewis acid sites, detected at 1425 and 1604 cm−1 , respectively, follow the sequence: 51SVO/SBA- 15 > 34SVO/SBA-15 > 17SVO/SBA-15 ∼ SVO. The trend of Brønsted and Lewis acid sites is in agreement with that of the adsorp- tion capacity. That is, higher intensities of Brønsted and Lewis acids of silver vanadate indicate a larger adsorption capacity for toluene. 3.5. Photoluminescence spectraFig. 6. Time-dependent concentrations of gaseous benzaldehyde during toluene PL spectra have been widely used to disclose the migra-photocatalytic degradation with P25, SVO, and SVO/SBA-15. tion, transfer, and recombination processes of the photogenerated electron-hole pairs in the semiconductor particles . Fig. 8 shows the PL spectra with 325 nm excitation wavelength in thethe photocatalysts surface. A literature survey indicates that IR range of 400–600 nm for P25, SVO, and SVO loaded compos-spectroscopic studies of ammonia adsorbed on solid surfaces ites at room temperature. The PL intensity of these sampleshave made it possible to distinguish between Brønsted and Lewis decreases in the order of P25 > SVO > 17SVO/SBA-15 > 60SVO/SBA-acid sites of a catalyst [16–21]. Before the DRIFT measurements, 15 > 70SVO/SBA-15 > 34SVO/SBA-15 > 51SVO/SBA-15. The PL ofthe samples were saturated with NH3 /N2 , ﬂushed with N2 ﬂow SVO/SBA-15 samples show obvious decrease in the intensity of PLto remove physically adsorbed ammonia, and then heated from spectra as compared to SVO, indicating the recombination of pho-303 K to 523 K. The intensity of chemisorption was determined toelectrons and holes is efﬁciently suppressed in the compositebased on the irreversible adsorption of ammonia. Fig. 7 shows semiconductors. The intensity of photoluminescence spectra cor-the IR spectra of ammonia adsorbed onto SVO and SVO/SBA-15 responds to the recombination rates of the holes formed in the O2psamples which were heated at 523 K for 30 min. The existence band and the electron in the V3d band. The slower recombinationof NH4 + ions adsorbed onto Brønsted acid sites of the silver process of photogenerated charges (the less the PL intensity) can facilitate the enhancement of photocatalytic activity of SVO/SBA-15 composite. 3.6. DRIFTS study for recyclability of photocatalyst During the photocatalytic reaction, organic pollutants are supposed to be adsorbed or concentrated in situ and then photo- degraded in vitro under factory process; therefore, it is important to evaluate the catalyst recyclability in situ. Since 51SVO/SBA- 15 had the highest apparent rate constant, it was chosen for the recyclability test. A complete cycle was composed of adsorption under darkness for 30 min and then an illumination reaction for 60 min. After the 1st cycle, the catalyst was regenerated in situ by further irradiating the LED light for 1 h. Then, the catalyst was reused in situ without any washing or thermal treatment for the 2nd cycle. The reliability test was stopped when the catalyst was deactivated as ascertained from DRIFTS and color analyses. The cat- alyst reliability results were expressed as IR spectra. The resultsFig. 7. IR spectra of NH3 adsorbed onto SVO and SVO/SBA-15 samples at 523 K. of P25 and 51SVO/SBA-15 are shown in Fig. 9. After exposing P25
230 W.-S. Chang et al. / Applied Catalysis A: General 407 (2011) 224–230 4. Conclusion Environmentally friendly SVO/SBA-15 photocatalysts were synthesized using hydrothermal synthesis without a high- temperature calcination process. XRD and HRTEM results indicate that SVO/SBA-15 composites had three kinds of phase and that SVO dispersed well in the channels or on the surface of SBA-15 without affecting the SBA-15 mesoporous structure, respectively. The mass spectra indicate that the main oxidation product of toluene is ben- zaldehyde, which is strongly adsorbed on the surface of the catalyst. The SVO/SBA-15 photocatalyst has a much higher photodegrada- tion ability for toluene than does P25. Based on the recyclability test, the accumulation of benzaldehyde on the P25 surface appears to be responsible for its deactivation; no deactivation was observed for 51SVO/SBA-15, which was still stable after 2nd cycle as evidenced by DRIFTS analysis. 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