Efficient Photocatalytic Degradation of Volatile Organic Compounds by

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Environ. Sci. Technol. 2010, 44, 1380–1385

Efficient Photocatalytic Degradation of Volatile Organic Compounds by Porous Indium Hydroxide Nanocrystals TINGJIANG YAN,† JINLIN LONG,† XICHENG SHI,‡ DONGHUI WANG,‡ Z H A O H U I L I , † A N D X U X U W A N G * ,† State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350002, People’s Republic of China and Research Institute of Chemical Defence, Beijing 100191, People’s Republic of China

Received September 11, 2009. Revised manuscript received November 30, 2009. Accepted December 23, 2009.

Nanosized porous In(OH)3 photocatalysts with high surface areas (as much as 110 m2 · g-1) were successfully synthesized by peptization of colloidal precipitates under ultrasound radiation. The resulting catalysts were characterized by X-ray powder diffraction (XRD), thermogravimetric analysis, nitrogen adsorption, transition electron microscopy, and UV-vis diffuse reflection spectroscopy. The photocatalytic activities of the samples were evaluated by the gas-phase decomposition of several volatile organic pollutants (acetone, benzene, and toluene) under UV light illumination and were compared with that of the commercial titania (Degussa P25). Results revealed that the as-synthesized In(OH)3 exhibited much higher photocatalytic activity and durability than both In2O3 and TiO2. One, therefore, can conclude that nanosized In(OH)3 has potential application in environmental treatment, especially in the removal of benzenecontaining exhaust emissions from shoemaking plants in China. The excellent photocatalytic performance of In(OH)3 can be attributed to its strong oxidation capability, abundant surface hydroxyl groups, and high BET surface area as well as the porous texture.

Introduction Volatile organic compounds (VOCs), which are emitted largely from shoemaking, semiconducting, and chemical industries, such as automobile, house paints, and so forth are the most baleful air pollutants. Many VOCs are toxic and some, such as benzene and its derivatives, are considered to be carcinogenic, mutagenic, or teratogenic (1, 2). Numerous VOC removal technologies including reuse or recycle, incineration, absorption, adsorption, and biological treatment such as biofiltering and bioscrubbing have been developed (3-6). However, these cleanup technologies are not ideal due to secondary pollutants or hazardous byproducts. Photocatalytic oxidation has been established as a potential alternative to air treatment strategies currently in use, due to its advantageous activity under ambient conditions (7-9). But * Corresponding author phone/fax: +86-591-83779251; e-mail: [email protected]. † State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou University. ‡ Research Institute of Chemical Defence. 1380

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it has been documented that the widely used TiO2 photocatalyst suffers from rapid deactivation and low photocatalytic conversion for degradation of VOCs, especially the aromatic hydrocarbons (10-13). Great effort has been made in the last two decades to improve the photocatalytic performance of TiO2. It was reported that addition of a small quantity of water vapor to the system could slightly meliorate the catalyst durability (14, 15), but the method has no significant application because of the low inherent activity of TiO2 for the reaction. Loading noble metals (e.g., Pt, Pd, Au, or Rh) on TiO2 particles was also shown to be favorable for enhancing photooxidation efficiency of TiO2 (16-18), but there is concern about its durability during the treatment of stable aromatic compounds such as benzene at high concentrations because of the oxidation of noble metal nanoparticles on the surface of the TiO2 (19). Therefore, development of novel photocatalysts degrading VOCs with high stability is indispensable for practical application of photocatalysis for air treatment, yet it presently remains a great challenge. In(OH)3 is a wide-gap semiconductor with a direct band gap energy of 5.15 eV (20-22). However, the majority of past studies were with the purpose of preparation of In(OH)3 with the good crystalline or morphologies, little attention has been paid to its practical application as a semiconductor. Until recently, In(OH)3 was shown to be a sensitive material to ammonia atmosphere and has promising application in gas detectors (23). Besides, only a few studies dealt also with the modified In(OH)3 as photocatalyst (24). In previous works, we first reported that In(OH)3 nanocrystal was an active photocatalyst for benzene oxidation (25) and performed preparative research on the material (26, 27). However, it was found that the well-crystallized In(OH)3 cubes had low photocatalytic activity both for benzene degradation and for producing hydrogen from water under UV light irradiation (26). The objective of this work is to systematically study the practicability of In(OH)3 photocatalyst applied to treat exhaust gases. Acetone, benzene, and toluene, several main components of emission exhaust gases from shoemaking plants, are used as simulated targets of photocatalysis in this work. The results indicated that the nanocrystal In(OH)3 with mesopore prepared by peptization of a colloidal precipitate shows very excellent photocatalytic activity and durability for the degradation of the several VOCs and has significant superiority over commercial TiO2 photocatalyst. One, therefore, can conclude that nanosized In(OH)3 has potential application in environmental treatment, especially in removal of benzene-containing exhaust emissions from shoemaking plants in China. Furthermore, the factors that contributed to the better photocatalytic performance of In(OH)3 have also been discussed in detail.

Experimental Section Synthesis of Samples. All of the reagents were analytical grade and used without further purification. The indium hydroxide sol was prepared by peptization of colloidal precipitates under ultrasound radiation. In a typical synthesis, 30 mL of diluted ammonia solution (12 wt %) was dropped slowly into 500 mL of indium nitrate aqueous solution (0.05 mol · L-1) with magnetically stirring at room temperature until pH value of the solutions reached ∼8. The resulting white precipitate was recovered and washed thoroughly with distilled water until the total concentration of ionic species was lower than 10 ppm and then dispersed into 420 mL of distilled water under ultrasound radiation (100 W/cm2, 20 10.1021/es902702v

 2010 American Chemical Society

Published on Web 01/19/2010

kHz) for 3 h to form a transparent sol. After the sol was dried at 60 °C in air, the formed gel was divided into several portions and respectively calcined at temperature of 120, 160, 200, 250, and 300 °C (ramp: 2 °C/min) in air for 3 h to obtain different In(OH)3 and In2O3 samples. Characterization. X-ray diffraction (XRD) patterns of the sample were collected on a Bruker D8 Advance X-ray diffractometer (Cu KR irradiation, λ ) 1.5406 Å). The thermogravimetric (TG) analysis curve was recorded using a Perkin-Elmer TGA7 thermogravimetric analyzer with a ramp rate of 5 °C min-1 and an air purging gas. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained by a JEOL model JEM 2010 EX instrument at an acceleration voltage of 200 kV. Nitrogen adsorption-desorption isotherm was collected at 77 K using OMNISORP100CX equipment. The sample was degassed at 110 °C and 10-6 Torr prior to the measurement. The UV-vis diffuse reflection spectra of the samples over a range of 200-800 nm were determined on a Varian Cary 500 Scan UV/vis spectrophotometer. ESR signals of the radical spin-trapped by DMPO were examined with a Bruker ESP 300E spectrometer. The concentration of DMPO was 0.2 mol L-1, and the relevant irradiation source was a 200 W xenon lamp with a cutoff filter (λ ) 254 nm). The settings for the ESR spectrometer were: center field, 3510.00 G; microwave frequency, 9.79 GHz; power, 5.05 mW. Measurements of Photocatalytic Activity. Photocatalytic degradation of gaseous acetone was carried out in a 2.4 L glass container. The container was connected to a quartzglass reactor with diameter of 4 mm and interfaced to a gas chromatograph (GC, Hewlett-Packard 4890). A closed circulation reaction system, including the container, the reactor, and the GC, was established with the aid of a circulation pump. The catalyst (0.1 g, 50-40 mesh) was loaded in the reactor that was surrounded by four UV lamps with a wavelength centered at 254 nm (Philips, TUV 4W/G4 T5). A given amount of liquid acetone was introduced to the reaction system with a microsyringe by a septum. Prior to the photodegradation experiment, the circulation pump was turned on and allowed to reach a steady adsorption of acetone on catalyst in the dark. The circulation rate of the system was 20 mL min-1. In the condition, the equilibrium concentration of acetone was determined as 420 ppm. During photocatalytic reaction, the reaction temperature was controlled at 30 ( 1 °C by an air-cooling system. The concentrations of residual acetone and the produced CO2 were measured at an interval of 30 min by gas chromatography equipped with a Porapak R column, a flame ionization detector, and a thermal conductivity detector. The benzene and toluene photo-oxidation experiments were conducted in a single-pass mode, same as the reported procedure (26). The catalyst (0.3 g, 50-70 mesh) was loaded in the 11-cm long 2.4-mm diameter reactor surrounded by four 4 W UV lamps (the same as above). In a typical run, benzene or toluene vapor (at 0 °C) saturated O2 gas was introduced into the reactor along with a flow rate of 20 mL min-1. After the gas-solid adsorption, equilibrium of benzene or toluene was achieved in the reactor under dark conditions, photo irradiation of the reactor was started. The concentrations of benzene or toluene and the amount of produced CO2 were analyzed by an online gas chromatograph. The original concentration of benzene, toluene, and CO2 were 920, 1220, and 0 ppm, respectively. The reaction temperature of the photocatalytic system was controlled at 25 °C with an air-cooling system. Percent conversion and mineralization of reactants were calculated using eqs 1 and 2, % conversion ) [(C0 - C/C0)] × 100

(1)

% mineralization ) [CO2]produced /{x[CxHyOz]converted} × 100 (2) where C0 is the initial concentration of the reactant and C is the concentration of the reactants after photocatalytic reaction.

Results and Discussion Changes of the as-synthesized samples with calcination temperatures were characterized by XRD. As shown in Figure 1, a notable transformation of crystal phase takes place at about 200 °C. Below the temperature (Figure 1a-c), the sample is mainly In(OH)3 phase, as evidenced by the strong diffraction peaks at 2θ ) 22.29° and 45.32° that are indexed respectively to the (200) and (400) planes of cubic In(OH)3 (JCPDS Card No. 76-1463). Both diffraction peaks show a little attenuation of intensity with increasing temperature between 60 and 160 °C, but the diffraction pattern is typical of the crystal. The slight decrease in the diffraction intensity can result from the condensation between structural hydroxyl groups on In(OH)3 surface. Additionally, for three In(OH)3 samples, several reflection peaks at 2θ ) 24.72°, 28.79°, and 33.41°, which are indefinable on the basis of available literatures, are observed and can belong to a small quantity of unknown phase of indium species. The diffraction plot of the present In(OH)3 sample is very similar to that depicted by Matijevic et al. for plateletlike indium hydroxide (20). As increasing temperature to 200 °C, the diffraction peaks of In(OH)3 severely decline along with emergence of a new phase indexed to cubic In2O3 (JCPDS Card No. 71-2194). The highly crystallized In2O3 is rapidly formed (Figure 1e,f) at the temperature range of 250-300 °C. The average crystallite sample sizes, which were determined from the broadening of the corresponding X-ray spectral peak by the Scherrer formula, are listed in Table S1 (see the Supporting Information). Obviously, the thermal treatment results in the shrinkage of nanoparticles and the average diameters of crystal particles decrease from ∼20 nm (250 °C). This shrinking is caused by transformation of In(OH)3 to In2O3 via dehydration reconstitution. The thermal transformation of In(OH)3 to In2O3 can be clearly observed by TG/DTG analysis, which was determined in air in the temperature range of 30-550 °C. As shown in Figure 2, the TG curve can be divided into three weight loss steps. The first step occurs at 30-80 °C and accounts for about 3.5% of weight loss, which is considered as the physical water evaporation from the surface of In(OH)3. The second step, taking place at a temperature range of 130-270 °C and accounting for ∼14.5% of the weight loss, is a result of

FIGURE 1. X-ray diffraction patterns of the as-synthesized samples treated at different temperatures: (a) 60 °C, (b) 120 °C, (c) 160 °C, (d) 200 °C, (e) 250 °C, and (f) 300 °C. VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. TG/DTG curves of In(OH)3 xerogels tested in air at temperature raising rate of 5 °C/min.

FIGURE 3. UV-vis spectra of the samples treated at different temperatures: (a) 60 °C, (b) 120 °C, (c) 160 °C, (d) 200 °C, (e) 250 °C, and (f) 300 °C. dehydration between structural hydroxyls of In(OH)3. The third step, between 270 and 320 °C, might originate from the translation of amorphous phase to crystalline of In2O3 (28) along with ∼1.1% of water loss. The total weight loss in the latter two steps is about 15.6%, very close to the theory loss (16.3%) calculated by the reaction 2 In(OH)3 f In2O3 + 3H2Ov. It is concluded from these results that In(OH)3 undergoes the dehydration of structural hydroxyl groups between 130-320 °C to transform into the high crystalline In2O3 phase and is in accordance with the XRD results. Figure 3 shows the light absorption properties of the samples calcined at different temperatures. The photoabsorption of the samples is remarkably red-shifted with increasing temperatures. The wavelength at the absorption edge, λ, is determined as the intercept on the wavelength axis for a tangent line drawn on the absorption spectra. For the In(OH)3 treated at 60 and 120 °C, the absorption edge is 263 and 275 nm and corresponds to band gap of 4.71 and 4.51 eV, respectively. The photoabsorption red-shift of ∼0.44-0.64 eV for our In(OH)3 samples with respect to the normal value (5.15 eV) (22) can be due to the change in the chemical state of surface, imperfections, and defects of crystal structure. After calcined at higher temperatures (160 and 200 °C), the sample becomes a mixture of In(OH)3 and In2O3 due to decomposition of In(OH)3 into In2O3 (Figure 2). An obvious absorption edge located at 343 and 422 nm corresponding to a band gap of 3.62 and 2.94 is observed, respectively. The absorption edge of the In2O3 samples calcined at 250 and 300 °C is ∼441 nm, corresponding to a 1382

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band gap of 2.81 eV, which is also red-shifted compared with that of bulk In2O3 (3.75 eV) (29). The photocatalytic activities of samples were evaluated first by photocatalytic oxidation of acetone in air under 254 nm UV light irradiation and compared with that of P25 TiO2, as shown in Figure 4. All of the samples are active for photocatalytic oxidization of acetone; however, the catalytic efficiency decreases with the phase transformation from In(OH)3 to In2O3, and only the In(OH)3 obtained at 120 °C shows the superior photocatalytic performance to TiO2. It is well-known that a good catalyst has not only high activity but also high durability of activity for practical application. For this purpose, cyclic experiments of the photooxidation of acetone over In(OH)3 treated at 120 °C and TiO2 photocatalysts were further carried on under the same conditions. The results are shown in Figure 5. It can be seen that the In(OH)3 sample shows no obvious change in the activity after 10 repeat reactions within total 50 h while TiO2 displays good photocatalytic activity only for the first run, followed by a quick deactivation. Moreover, it is also observed that the In(OH)3 catalyst almost keeps its primitive color after the long time of reaction. In contrast, the color of TiO2 sample was changed from white to brown during the reaction. The inactivation of TiO2 in the oxidation of acetone was also reported previously in literature (30). The inactivation originates from the blockage of stable intermediates on the active sites of surface (31). The activity stability of In(OH)3 can be due to its stronger oxidation capacity compared to TiO2, which will be discussed later. Photocatalytic activity of the In(OH)3 and In2O3 samples was further evaluated with the photodecomposition of benzene in air under UV light irradiation, and compared with that of P25 TiO2. As shown for TiO2 in Figure 6, the initial conversion of benzene is only 5% and only a small amount of CO2 (30 ppm) is produced in the process. Furthermore, the benzene conversion rapidly decreases with increasing reaction time until zero along with change in color from white to black after 13 h of reaction. The low activity and severe deactivation of TiO2 during photocatalytic degradation of benzene were reported in the literatures (10, 32), and the deactivated TiO2 was showed to be not regenerable even calcined under oxygen at 200 °C for 5 h. In contrast, the In(OH)3 sample presents 6 times higher initial photodegradation activity than the TiO2 catalyst under the same conditions. Although In(OH)3 suffers also from a visible decrease of activity at initial stage, the reaction can reach a steady state after 10 h, hereafter the conversion of benzene and the amount of produced CO2 keep at a stable level of ∼15% and 340 ppm (corresponding to mineralization rate of 40%), respectively. Only a faint change in color of the In(OH)3 photocatalyst is observed after more than 30 h of reaction. This indicates that In(OH)3 possesses higher activity and better activity durability than TiO2 for photocatalytic oxidation of benzene. With In2O3 as photocatalyst, the benzene conversion and the amount of produced CO2 keep approximately at a level of 5% and 40 ppm, respectively, which are higher than those on TiO2 but greatly lower than those on In(OH)3. In(OH)3 also exhibits superior photocatalytic performance for toluene degradation to TiO2. As shown in Figure 7, the toluene conversion over In(OH)3 is 25%, 5× higher than that (5%) over TiO2. The amount of CO2 produced with In(OH)3 (250 ppm) is also much higher than that with TiO2 (20 ppm). Interestingly, for toluene, the In(OH)3 sample keeps its ready activity throughout more than 60 h of reaction. The results above show that In(OH)3 is more active than both TiO2 and In2O3 for photocatalytic degradation of different VOCs under UV light irradiation. The excellent behavior of In(OH)3 can be first explained in terms of its electronic structures. The top of the valence band of In(OH)3 is made

FIGURE 4. The photocatalytic activities for oxidizing acetone over the samples treated at different temperatures: (a) 60 °C, (b) 120 °C, (c) 160 °C, (d) 200 °C, and (e) 250 °C. (f) P25 TiO2 was used as a reference.

FIGURE 5. Cyclic runs for the photooxidation of acetone over In(OH)3 (treated at 120 °C) and P25-TiO2 photocatalysts.

FIGURE 6. The photocatalytic conversion of benzene and the yield of CO2 over In(OH)3, In2O3, and TiO2 as a function of reaction time. In(OH)3 and In2O3 were obtained at 120 and 300 °C thermal treatment, respectively.

FIGURE 7. The photocatalytic conversion of toluene and the amount of produced CO2 over In(OH)3 treated at 120 °C and TiO2 as a function of reaction time. The initial concentration of toluene in the stream was 1220 ppm. up of O2p levels, while the bottom of its conduction band is made up of the hybridized orbital of In5s-In5p (24). Such a dispersive conduction band structure can promote the mobility of the photoinduced electron, and therefore decrease the recombination of the electron-hole. Besides this character, the valence band position of In(OH)3 estimated from the equation EVB ) X - E e + 0.5Eg is 4.24 V (vs NHE) (33),

more positive than that of TiO2 (3.00 V), In2O3 (2.17 V) and E° (OH•/OH-) (2.38 V vs NHE) (34). Thermodynamically, the photogenerated holes of In(OH)3 are more favorable to react with adsorbed H2O to produce more active OH• radicals and maintain a clean surface of catalyst (25). This speculation is supported by determination and comparison of the active radicals by a spin-trapping technique. Figure 8 gives the spinVOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. ESR signals of the DMPO-OH• adducts in an aqueous dispersion of (a) In(OH)3, (b) TiO2, and (c) In2O3 under 254 nm light irradiation. trapping ESR spectra of In(OH)3, TiO2, and In2O3. There is no signal in dark. However, when the system is irradiated with UV light, the characteristic 1:2:2:1 quadruple peaks of the DMPO-OH• adduct are observed on all samples, and it is found that the intensity is much stronger in In(OH)3 than in TiO2 or In2O3. This result is a strong indication that the photogenerated charge carriers in In(OH)3 have strong oxidation ability and are long-lived enough to react with surface hydroxyl groups and H2O adsorbed on the catalyst surface. This might be the principal reason for activity durability of In(OH)3. Another important reason contributing to the high photocatalytic performance of In(OH)3 can be related to its abundant hydroxyl groups (OH-), especially of the surface OH-. Since a unit cell of In(OH)3 including 8 indium atoms and 24 OH(35), it goes without saying that abundant OH- are presented on the surface of In(OH)3. Many investigations have evidenced that the surface OH- can capture the photoinduced holes to form hydroxyl radicals (OH•) as well as to inhibit electron-hole recombination and then enhance the photocatalytic activity (36). In addition, the large surface areas and porous structure of In(OH)3 would also favor the photocatalysis (37, 38). It is generally known that the photocatalysts with higher specific surface areas and larger pore volumes are beneficial to the enhancement of photocatalytic performance due to more surface active sites for the adsorption of reactant molecules, ease transportation of reactant molecules and products through the interconnected porous networks, and enhanced harvesting of exciting light by multiple scattering within the porous framework (39). Therefore, it is not surprising that the present porous In(OH)3 nanoparticles exhibit much higher photocatalytic activity than the In(OH)3 cubes previously reported (25). As for the acetone degradation over different In(OH)3 samples (treated at 60, 120, and 160 °C), it is noted that the 120 °C calcined sample has the highest photocatalytic activity (as shown in Figure 4). There are several possible reasons associated with its good photocatalytic activity. The first important one is the surface area. Numerous studies have verified that the surface area has a great influence on the photocatalytic activity of semiconductors in gas-solid photocatalysis (40). Since the sample treated at 120 °C has an especially higher specific surface area (121.4 m2 · g-1) than the other two samples (see Table S1 of the Supporting Information), it can be expected that more reactant can be adsorbed on the surface of catalyst, leading to a higher photocatalytic efficiency. The order of photocatalytic activities was 120 °C sample >60 °C sample >160 °C sample, which 1384

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is in accordance with the corresponding specific surface areas. Therefore, larger surface area could be responsible for the higher photocatalytic activity of 120 °C treated sample. In addition to the surface area, the porous property of 120 °C treated sample could also contribute to its higher activity (see Figure S1 of the Supporting Information). This is because the pores existing in the nanoparticles can promote the rapid diffusion of reactants during the photocatalytic reaction and enhance the rate of the photocatalytic reaction (41). The third reason that we have to consider is the crystal phase. Note that thermal treating of In(OH)3 at a temperature of above 130 °C caused the phase transformation from In(OH)3 to In2O3, which means that the 160 °C treated sample is indeed a mixture of In(OH)3 and amorphous In2O3 that was not observed by the TEM characterization (see Figure S2 of the Supporting Information). The decreased content of In(OH)3 in the sample gives rise to poor photocatalytic activity. Furthermore, an enormous decrease in photocatalytic activity can be found in the case of further increasing the calcination temperature to 250 °C. In summary, compared to commercial TiO2 photocatalyst, nanosized porous In(OH)3 shows higher activity and longerterm durability for photocatalytic decomposition of acetone, benzene, and toluene in gas phase under UV light irradiation, and therefore has potential application in the removal of benzene-containing exhaust emissions from shoemaking plants in China. The unique electronic structures and favorable surface properties endow In(OH)3 with strong oxidation ability, abundant surface hydroxyl groups, and high surface area as well as accessible porous configuration, and thus better photocatalytic performance for decomposing VOCs.

Acknowledgments Financial support of this work by the National Natural Science Foundation of China (Grant Nos. 20673020 and 20873022), National Basic Research Program of China (973 Program, No. 2007CB613306), and National High Technology Research and Development Program of China (863 Program, No. 2008AA06Z326) is gratefully acknowledged.

Supporting Information Available Nitrogen-sorption isotherms and pore size distributions of the samples treated at different temperatures (Figure S1), Texture parameters of the samples obtained at different temperatures (Table S1), TEM and HRTEM images of In(OH)3 samples calcined at 60, 120, and 160 °C (Figure S2). This material is available free of charge via the Internet at http:// pubs.acs.org/.

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