Environ. Sci. Technol. 2008, 42, 7387–7391
Degradation of Benzene over a Zinc Germanate Photocatalyst under Ambient Conditions JIANHUI HUANG, XINCHEN WANG,* YIDONG HOU, XIUFANG CHEN, LING WU, AND XIANZHI FU* Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou 350002, People’s Republic of China
Received March 14, 2008. Revised manuscript received July 2, 2008. Accepted July 21, 2008.
A rod-shaped Zn2GeO4 photocatalyst has been successfully prepared by a surfactant-assisted hydrothermal method. The photocatalyst was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, UV/vis, N2 adsorption-desorption, and FTIR techniques. The photocatalytic activity of the sample was evaluated by the decomposition of benzene in the gas phase under UV light illumination and was compared with that of bulk Zn2GeO4, commercial titania (Degussa P25), and Pt/P25. The results revealed that the Zn2GeO4 nanorods had the best photocatalytic activity for mineralizing benzene to CO2 among the catalysts examined. No obvious deactivation of Zn2GeO4 nanorods was observed during the prolonged operation of 140 h. It was found that the Zn2GeO4 was also more active and stable than TiO2-based catalysts toward photocatalytic decomposition of other volatile aromatic pollutants (e.g., toluene and ethylbenzene).
Introduction Benzene is one of the most abundant aromatic hydrocarbons found in polluted urban atmospheres. The main sources are chemical manufactories, evaporative emissions from paints, plastics, rubber, and gasoline, auto exhaust, and cigarette smoke. Benzene has been shown to produce severe health effects in humans, even causing cancers such as leukemia (1-3). A recent study showed that even low doses (140 h.
Experimental Section Preparation of Catalysts. Zn2GeO4 nanorods were synthesized by a surfactant-assisted hydrothermal method. In a typical synthesis, 0.10 g of cetyltrimethylammonium bromide (CTAB; Sinopharm Chemical Reagent, >99.0%), 0.52 g of GeO2 (Sinopharm Chemical Reagent, 99.999%), and 1.10 g of Zn(CH3COO)2 · 2H2O (Sinopharm Chemical Reagent, >99.0%) were added to 15 mL of hot deionized water. The pH value of the resulting mixture was adjusted to pH 8 by adding NaOH solution (30 wt %). The mixture was stirred for 1 h and then transferred to a stainless Teflon-lined autoclave of 20 mL inner volume. The autoclave was maintained at 140 °C for 24 h, followed by cooling naturally to room temperature. The product was centrifuged, filtered out, and rinsed with alcohol and deionized water several times. Finally, the product was dried at 80 °C overnight. A white Zn2GeO4 powder was obtained with a high yield of ∼90%. Bulk Zn2GeO4 was prepared by a conventional solid-state reaction according to the literature (21). In a typical synthesis, an equimolar ratio of ZnO (Sinopharm Chemical Reagent, >99.0%) and GeO2 (Sinopharm Chemical Reagent, 99.999%) was mixed sufficiently in an agate. The resulting mixture was calcined at 1200 °C (ramp rate 5 °C min-1) for 16 h in air to obtain bulk Zn2GeO4. A Pt/TiO2 sample was prepared by impregnating TiO2 (Degussa P25) with an aqueous solution of H2PtCl6. The initial ratio of Pt to TiO2 was fixed at 1 wt %. The impregnated sample was dried at 110 °C and then calcined at 350 °C for 3 h. The resulting solid was subsequently reduced with a NaBH4 solution (0.1 M) to produce Pt/TiO2 (17). Characterization. X-ray diffraction (XRD) patterns were collected in θ-θ mode using a Bruker D8 Advance X-ray diffractometor (Cu KR1 irradiation, λ ) 1.5406 Å). The morphology of the sample was investigated by field emission scanning electron microscopy (SEM) (JSM-6700F). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded on an FEI Tecnai 20 microscope. Nitrogen sorption experiments were carried out VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. XRD pattern of Zn2GeO4 nanorods and bulk Zn2GeO4 particles. at 77 K by using Micromeritics ASAP 2020 equipment. A Varian Cary 500 scan UV/vis system equipped with a Labsphere diffuse reflectance accessory was used to obtain the reflectance spectra of the catalysts. FTIR spectra were recorded on a Nicolet Nexus 670 FTIR spectrometer at a resolution of 4 cm-1. The thermogravimetric analysis (TGA) curve was recorded using a Perkin-Elmer TGA7 thermogravimetric analyzer with a ramp rate of 5 °C min-1 and a N2 purging gas. A Bruker model ESP 300E spectrometer equipped with a quanta-Ray Nd:YAG laser (λ ) 266 nm) was used for measurements of the electron paramagnetic resonance (EPR) signals of radicals spin-trapped by DMPO. The settings were the following: center field 3480.00 G, microwave frequency 9.79 GHz, and power 5.05 mW. Photocatalytic Activity Measurements. Photocatalytic experiments were conducted with a fixed bed tubular quartz reactor operated in a single-pass mode. The catalyst (0.3 g) was loaded in the reactor surrounded by four 4 W UV lamps with a wavelength centered at 254 nm (Philips, TUV 4W/G4 T5). Benzene diluted in a pure oxygen stream was used to afford a reactant stream. The initial concentrations of benzene and carbon dioxide in the stream were determined to be 300 and 0 ppm, respectively. The flow rate of the reactant stream was kept at 20 mL min-1. Simultaneous determination of benzene and CO2 concentrations was performed with an online gas chromatograph (HP6890) equipped with a flame ionization detector, a thermal conductivity detector, and a Porapak R column. The reaction temperature was controlled at 29 ( 1 °C by an air-cooling system. In the case of photocatalytic degradation of toluene and ethylbenzene, the experimental conditions were the same as those of benzene photo-oxidation reaction. The aromatic pollutants (benzene, toluene, and ethylbenzene) were found to be stable in the catalyst-loaded reactor without illumination, and no degradations were observed when they were illuminated in the absence of catalysts.
Results and Discussion Figure 1 shows the XRD pattern of the as-synthesized Zn2GeO4 nanorods. The XRD pattern of bulk Zn2GeO4 crystals is also shown for comparison. All of the diffraction peaks for both nanorods and bulk Zn2GeO4 samples can be assigned to the rhombohedral phase of Zn2GeO4 (JCPDS11-0687), with lattice constants of a ) 14.231 and c ) 9.53 Å. There is no trace of an impurity phase under the instrument’s resolution. Note that the XRD peaks of the Zn2GeO4 nanorods are broader than the corresponding peaks of the bulk Zn2GeO4 crystals, reflecting the nanostructural nature of the former. In addition, the intensity of the (113) reflection of the Zn2GeO4 nanorods is higher than expected, indicating a preferential crystal growth of the nanorods. 7388
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FIGURE 2. Structural characterization and general morphology of Zn2GeO4 nanorods: (a) SEM image; (b) TEM image; (c) TEM image of a Zn2GeO4 nanorod; (d) HRTEM image of area e1 in (c); (e) enlarged image of area f1 in (d); (f) SAED pattern recorded along the zone axis [11j0] of the Zn2GeO4 nanorods.
FIGURE 3. Diffuse absorption coefficient F(R) of Zn2GeO4. The inset is the optical band gap energy Eg of Zn2GeO4. Figure 2a,b and Figure S1 (Supporting Information) show typical SEM and TEM images of the Zn2GeO4 nanorods, respectively. A large quantity of nanorods 20-50 nm in width and 150-600 nm in length can be observed. Parts d and e of Figure 2 are HRTEM images of a Zn2GeO4 nanorod (Figure 2c). Well-resolved lattice fringes are clearly visible across the entire nanorod (Figure 2e), with an interplanar distance of 0.29 nm corresponding to the (113) d spacing of the rhombohedral Zn2GeO4 structure, and most of the Zn2GeO4 nanorods have the same clear lattice fringes as shown in Figure S2 (Supporting Information). The selected area electron diffraction (SAED) pattern (Figure 2f) of the nanorod recorded along the [11j 0] zone axis can be indexed as a rhombohedral Zn2GeO4 phase. Energy-dispersive X-ray (EDX) analysis (Figure S3, Supporting Information) shows that the chemical composition of the sample was Zn2GeO4 and there was no detectable signal for carbon, nitrogen, and bromide. TGA data (Figure S4, Supporting Information) show only ∼3% mass loss after the sample is heated to 800 °C. Both EDX and TGA results show that almost all the surfactant had been removed during the preparation. The diffuse reflectance spectrum (Figure 3) shows that the Zn2GeO4 had a band gap of 4.7 eV, corresponding to an optical absorption edge of 266 nm. Conversions of C6H6 + O2 in a signal-pass flow-type reactor at atmospheric pressure and room temperature were determined by gas chromatographic analysis of the products,
FIGURE 4. (a) Photocatalytic conversion of benzene and (b) concentration of produced CO2 in the stream over the Zn2GeO4 nanorods against the reaction time, with TiO2 (Degussa P25) as a reference catalyst. using 4W 254 nm UV lamps for illumination. Figure 4 shows the conversion of benzene and the concentration of the produced CO2 in the stream over the Zn2GeO4 nanorods against the reaction time, together with TiO2 data as a comparison. It shows that benzene conversion on TiO2 was ∼5% in the initial 1 h, and after 24 h of reaction it decreased to ∼1.3% with only ∼12 ppm CO2 concentration on the stream. Finally after 90 h of reaction the white TiO2 turned black due to the deposition of reaction intermediates, leading to the deactivation of TiO2 (18). For the Zn2GeO4 nanorods, however, both the benzene conversion and CO2 concentration were kept steady at ∼21% and ∼280 ppm, respectively, on the stream regardless of the reaction time, with a high mineralization ratio of ∼75%. Trace amounts of formaldehyde, acetaldehyde, and acetone adsorbed on the catalyst surface are detected by the temperature-programmed desorption-mass spectrometry (TPD-MS) analysis, and infrared gas analysis also revealed the formation of a trace amount of carbon monoxide in the off gas, which explains the carbon mass imbalance in the reaction stream (Figure S7, Supporting Information). The photocatalytic activity was stable in 142 h of reaction, producing 2.10 mmol of CO2, exceeding the amount of catalyst used (1.12 mmol). The conversion of benzene stopped when the light was turned off and was resumed immediately at the same rate when the light was turned on again. Control experiments showed that no reaction took place when benzene was illuminated in the absence of catalysts and that benzene was stable in the presence of catalysts without illumination. It is noted that GeO2 and ZnO catalysts cannot convert benzene molecules under the same experimental conditions. Considering the negligible conversion of excited-state benzene on dielectric oxides (19), these results indicate that the reaction proceeded photocatalytically on the Zn2GeO4 nanorods. XRD and FTIR
FIGURE 5. Photocatalytic performance of the Zn2GeO4 nanorods, bulk Zn2GeO4 particles, TiO2, and Pt/TiO2 for decomposing (A) benzene, (B) toluene, and (C) ethylbenzene in the gas phase. The initial concentration of benzene in the stream is 300 ppm, and the initial concentration of toluene and ethylbenzene is 270 ppm. analyses (Figures S5 and S6, Supporting Information) show that there was no observable structural difference between the sample before and after reaction. Figure 5 displays the photocatalytic conversion and the concentration of produced CO2 over the Zn2GeO4 catalyst toward degradation of benzene (Figure 5A), toluene (Figure 5B), and ethylbenzene (Figure 5C) in the gas phase. The data of TiO2 (P25), Pt/P25, and bulk Zn2GeO4 are also shown for comparison. The specific activities of Zn2GeO4 and TiO2 catalysts at the steady state of the reactions are listed in Table 1. The results reveal that the Zn2GeO4 nanorods exhibited high VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Specific Catalytic Activity of Different Catalysts for Benzene, Toluene, and Ethylbenzene Photo-oxidation Reactions CRb a
catalyst c
Zn2GeO4 TiO2d Pte/TiO2 Zn2GeO4f
SBET (m2/g)
benzene
toluene
ethylbenzene
36 51 51
