Environ. Sci. Technol. 2010, 44, 9123–9127
Catalyzed Degradation of Azo Dyes under Ambient Conditions JIN-MING WU* AND WEI WEN State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, P. R. China
Received August 10, 2010. Revised manuscript received October 9, 2010. Accepted October 12, 2010.
Phase-pure layered perovskite La4Ni3O10 powders were synthesized by a solution combustion approach. It is found that, in the presence of the La4Ni3O10 powders, aqueous azo dyes can be degraded catalytically and efficiently under ambient conditions. Neither light nor additional reagents are needed in the catalytic reaction. The dye degradation procedure can be accelerated markedly by magnetic stirring. A systemic series of chemical and electrochemical experiments suggested that the dye degradation proceeds through electron transfers from the dye molecules to the catalyst and then to electron acceptors such as dissolved oxygen. The present catalytic degradation requires no additional reagents or external energy input, which hence provides a potentially low-cost alternative for the remediation of azo-dye effluents.
Introduction Dye effluents have caused serious environmental pollution worldwide (1). Azo dyes constitute ca. 50% of all dyes produced (2); therefore, degrading azo dyes efficiently is an emerging yet challenging task. To date, many methods have been developed to treat dye effluents, including adsorption tactics (3), biological degradations (4), Fenton-like reactions (5), reductive degradations utilizing zero-valent iron (2, 6, 7), and photocatalysis (8). However, the absorption tactic is actually a transfer of pollutants, and the traditional biological approach achieves poor efficiency. The OH• radicals produced by the Fenton-like reactions are effective in degrading azo dyes; yet consumption of H2O2 is significant, and the subsequent treatment of the ferrous slurry is a nuisance. Photocatalysis is believed to degrade azo dyes thoroughly; unfortunately, the most utilized photocatalyst of TiO2 is active only in the ultraviolet light range due to its wide band gap (3.2 eV for anatase). Many reports have appeared on modifications of TiO2 (9) and development of other visiblelight-active photocatalysts (10); however, the quantum yield is still not satisfying, thus limiting its practical applications. It is therefore of great interest to achieve dye degradation that demands neither light illumination (as is the situation in the photocatalytic degradations) nor additional reagents (as is the situation in Fenton-like reactions). Over the past two decades, the layered perovskite oxide La4Ni3O10, which is a stacking of three LaNiO3 perovskite layers separated by LaO rock salt layers along the c axis (11), has attracted much attention because of its unique electronic (11) and magnetic (12) properties, and it has found applications as cathodes for solid-oxide fuel cells (13, 14). Several techniques have been developed to synthesize this compound, including solid-state routes (15), Pechini routes * Corresponding author e-mail:
[email protected]. 10.1021/es1027234
2010 American Chemical Society
Published on Web 11/04/2010
(13, 14), citrate routes (15), and continuous hydrothermal flow synthesis methods (16). However, it is still not easy to achieve phase-pure La4Ni3O10 because of the easy coexistence of several other phases encountered in the La-Ni-O system, such as LaNiO3, La2NiO4, and La3Ni2O7. Meanwhile, most of the techniques utilized to date are energy- and timeconsuming and require complicated steps and/or instruments. In the current investigation, we utilized a solution combustion approach, which is a versatile and energyefficient method for preparing inorganic oxide (17, 18), to prepare single-phase La4Ni3O10 powders. Interestingly, we discovered that the achieved La4Ni3O10 powders can efficiently achieve the catalytic degradation of azo dyes under ambient conditions, requiring neither light nor additional reagents.
Experimental Section La4Ni3O10 powders were synthesized by the solution combustion method. All reagents were of analytical grade and were used without further purification. In a typical combustion synthesis, 18 mmol of La(NO3)3, 13.5 mmol of Ni(NO3)2, and 22.5 mmol of citric acid were dissolved in 20 mL of triply distilled water in a crucible, which was then transferred to a preheated furnace maintained at 500 °C. Within a few minutes, the solution boiled and was ignited to produce a self-propagating combustion, yielding a fluffy black product. The product obtained was ground in a mortar and then annealed in air at 1000 °C for 3 h. X-ray diffraction (XRD) measurements were conducted on a Rigaku D/max-3B diffractometer with Cu KR radiation, operated at 40 kV and 100 mA (λ ) 0.15406 nm). The powder morphology was observed using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800, Tokyo, Japan), equipped with an energy-dispersive X-ray analysis (EDS) system. Transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) observations were conducted employing a JEM-2010 microscope (JEOL, TokyoJapan) working at 200 kV. The BrunauerEmmett-Teller (BET) approach using adsorption data over the relative pressure range of 0.05-0.16 was utilized to determine the specific surface area. The sample was degassed at 150 °C to remove physisorbed gases prior to the measurement. Catalytic activities of the sample were evaluated by decomposition of methyl orange (MO) in aqueous solution. For each run, 0.010-0.150 g (corresponding to a catalyst loading of 0.2-3.0 g/L) of the catalyst was added to 50 mL of MO solution with an initial concentration of 5-15 mg/L in a vessel (with an inside diameter of 80 mm) under magnetic stirring at specified speeds of 0-1000 rpm. At given time intervals, ca. 3.5 mL of the suspension was sampled and centrifuged to remove the catalyst particles, and the residual MO concentration was determined using a 752 UV-visible spectrophotometer at a wavelength of 464 nm. In some cases, pure N2 at a flow rate of 300 sccm was purged into the solution for 1 h prior to the reaction and throughout the reaction process to remove the dissolved oxygen. Other dyes including methyl red (MR), orange G (OG), Congo red (CR), rhodamine B (RB), and methylene blue (MB) were also used to evaluate the activity of the sample. For the electrochemical characterizations, the La4Ni3O10 film electrode was prepared as follows: Titanium plates of dimensions 1 × 1 × 0.01 cm3 were etched at ambient temperature for 1 min in a 1:3:6 (by volume) mixture of aqueous solutions of HF (55 wt %), HNO3 (63 wt %), and VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Results and Discussion
FIGURE 1. XRD patterns of the La4Ni3O10 powders: (a) asprepared and (b) after the MO degradation experiment. distilled water and then cleaned with water in an ultrasonic bath. The La4Ni3O10 particles were added to ethanol to give a viscous suspension. Then, the suspension was coated onto the etched metallic Ti substrate and dried in air at 80 °C for 12 h, to give a film weight of 0.01 g. All electrochemical experiments were performed with an electrochemical working station (CHI-660C, Shanghai Chenhua Instruments) in a standard three-electrode configuration using a platinum plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. A 0.25 M Na2SO4 solution was used as the supporting electrolyte, which was exposed to air. Electrochemical impedance spectra (EIS) were obtained at the open-circuit potential of the specimen, with an amplitude of 10 mV. The frequency span was from 100 kHz to 0.01 Hz. Current-potential curves and electrochemical impedance spectra were also measured by adding MO (5 mg/L) to the supporting electrolyte under stirring with a given speed.
Well-crystallized pure La4Ni3O10 black powders were synthesized through a solution combustion approach followed by a subsequent calcination at 1000 °C for 3 h. Figure 1a shows the XRD pattern of the product. All of the peaks correspond well to the orthorhombic La4Ni3O10 structure according to JCPDS standard card no. 50-0243. Figure 2a,b shows that the average particle size is 100-200 nm. The HRTEM image shown in Figure 2c clearly indicates a structure typical of layered perovskites. The fringes with a neighboring distance of 1.32 nm match those of the (002) crystallographic plane of La4Ni3O10. The EDS results (Figure S1 in the Supporting Information) demonstrate that the powder contains only La, Ni, and O and that the atomic ratio is close to the stoichiometric value. The BET specific surface area of the powder is ca. 2.8 m2/g, which is larger than the value reported in the literature (16). Interestingly, methyl orange (MO), a typical azo dye, is gradually degraded in the presence of La4Ni3O10 powders under magnetic stirring (Figure 3). The newly developed peak at 244 nm in the UV-vis absorption spectra (see Figure 3b) is attributed to the aromatic intermediates (6, 7), which suggests that MO is indeed degraded, not just adsorbed by La4Ni3O10. The maximum located at 464 nm in the UV-vis absorption spectrum disappeared after ca. 3.5 h under stirring at a speed of 650 rpm. When the duration was prolonged to ca. 8.5 h, the peak at 244 nm almost disappeared, which means that further degradation of the intermediates is also possible. Measurements of total organic carbon (TOC) revealed a TOC removal of 21% for the MO dye after degradation for 8.5 h. It is worth noting that MO can also be degraded by La4Ni3O10 in the dark without stirring (see Figure S3, Supporting Information), providing that the duration is long enough, that is, ca. 3 days. Increasing the stirring speed promotes the degradation efficiency. Among the other investigated dyes, namely, MR, OG, CR, RB, and MB, the first
FIGURE 2. (a, b) SEM and TEM images and (c) HRTEM images of the La4Ni3O10 powder.
FIGURE 3. (a) Changes in the normalized MO concentration as a function of the reaction time in the presence of La4Ni3O10 under stirring at various speeds, (b) time-dependent UV-vis spectra of MO in the presence of La4Ni3O10 under stirring at a speed of 650 rpm. Initial MO concentration: 5 mg/L. 9124
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SCHEME 1. Proposed Mechanism for Catalytic Degradation of MO in the Presence of La4Ni3O10
FIGURE 4. MO degradation curves demonstrating the effects of electron acceptors, at a stirring speed of 650 rpm. three, which are also anionic azo dyes, can be degraded by La4Ni3O10 (Figure S4, Supporting Information). Therefore, La4Ni3O10 might be effective only for the degradation of anionic azo dyes. The degradation of CR was the most striking, achieving a thorough decoloration for the dye with an initial concentration of 10 mg/L after only 1 h under stirring. The degradation rate of MO increased when the catalyst loading was increased from 0.2 to 1.0 g/L and then remained almost unchanged with a further increase in the loading to 3.0 g/L (Figure S5, Supporting Information). On the other hand, increasing the initial MO concentration from 5 to 10 and then 15 mg/L resulted in a markedly reduced degradation rate (Figure S6, Supporting Information). A further increase in the degradation efficiency is demanded for practical application. The crystal structure of La4Ni3O10 remains unchanged after the degradation reaction (see Figure 1b for the XRD pattern), which provides rough evidence that the present MO degradation is a catalytic process. To rule out the possibility that the observed degradation of azo dyes is caused by the leaching of any ions from the powder, a slurry with the La4Ni3O10 powder (1.5 g/L) was adjusted with dilute HCl to pH 6.1, which is the same as that of an MO solution with a concentration of 5 mg/L, and incubated for 2 h. The powders were then removed by centrifugation to obtain the leaching solution. No MO degradation was observed when MO molecules were added to the leaching solution under stirring for 3.5 h at a speed of 650 rpm; on the contrary, the MO concentration as determined by the relative absorption maximum approached nearly zero in the presence of the La4Ni3O10 powders (see Figure S7, Supporting Information). Thus, the degradation of azo dyes can be attributed to a reaction catalyzed by the La4Ni3O10 catalyst, rather than the leaching of ions. The additive of Ag+ in the reaction system, which serves as an electron acceptor, markedly enhanced the MO degradation rate, as illustrated in Figure 4. This suggests that MO molecules were oxidized, not reduced as is the case in the degradation tactic utilizing zero-valent iron (6, 7). The MO degradation was totally inhibited in the absence of dissolved O2, which was achieved by thoroughly purging the solution with N2. Purging the solution containing Ag+ ions with N2 achieved MO degradation, but at a rate lower than in the system open to air. This can be explained by the fact that Ag+ ions play the same role as dissolved O2 in the solution, that is, electron acceptors. The control experiments revealed that neither O2 nor Ag+ can oxide the MO (see Figure S8 in the Supporting Information); therefore, La4Ni3O10 serves as a “bridge” for electron transfer during the reaction. Based on these results, we suggest here a possible mechanism for the catalytic degradation. As illustrated in Scheme 1, electrons
from MO molecules transfer to La4Ni3O10 and further react with either dissolved O2 or Ag+ ions absorbed on the powder surface, resulting in MO degradation. When the system is open to air, further deep degradation of the intermediates, by the reactive oxygen species produced, such as O2- groups, as illustrated in the scheme, is also possible (Figure 3b). If no suitable electron acceptors exist to receive the electrons, they would recombine with the MO molecule, which inhibits the degradation procedure. The electron transfer between MO and La4Ni3O10 can find some hints from the research conducted by Seo et al. (19), who verified that ligand-tometal charge transfer (LMCT) occurs between polyaromatic hydrocarbons and TiO2 not only through covalent bonding but also through physical adsorption. Photodecomposition of H2O2 on TiO2 surfaces under visible light irradiation has been attributed to the formation of surface complexes of H2O2/TiO2 that can absorb visible light (20). Kim et al. (21) also showed that visible-light-induced photocatalytic degradation of 4-chlorophenol on pure TiO2 occurs through LMCT with the transfer of electrons from 4-chlorophenol to the conduction band of TiO2. Interestingly, in the current investigation, the electron transfer from MO molecules to La4Ni3O10 can occur in the dark (see Figure S3 in the Supporting Information). It is argued that the anionic azo dyes might be adsorbed on the La4Ni3O10 surface to form a special complex, whose electrons can directly transfer to La4Ni3O10 and further react with dissolved O2 to form reactive oxygen species, resulting in further oxidation of organic compounds. The exact complex is not specified at the present time, and the charge-transfer procedure is also not clear; however, the La4Ni3O10 catalyst surely plays a key role because the control experiment revealed that neither La2O3 nor NiO, Ni2O3, or their mixtures induces the catalytic degradation of MO. It is also possible for the electrostatic interactions between the catalyst and the dyes to contribute to the formation of the complex and hence the degradation reaction, when one considers that, among the six dyes studied in the current investigation, the four degradable azo dyes of MO, MR, OG, and CR are anionic whereas RB and MB, which are resistant to degradation by La4Ni3O10, are cationic. The efficiency of La4Ni3O10 in assisting MO degradation increased with increasing stirring speed, which can be attributed to the enhanced electron-transfer process and also to the increasing amounts of dissolved O2 in the solution. To further support the above mechanism, we measured the current-voltage curve and EIS spectra of the La4Ni3O10 film electrode. Figure 5a demonstrates the enhanced positive current for the La4Ni3O10 film electrode in the electrolyte containing MO, which can be ascribed to the electron transfer from MO to the La4Ni3O10 film electrode (22, 23). The current increased with increasing stirring speed, which is in accordance with the fact that the degradation efficiency increased with increasing stirring speed (Figure 3a). Furthermore, the semicircular radii on the EIS Nynquist plot of the La4Ni3O10 film electrode (Figure 5b) decreased with increasing stirring speed, which suggests a faster charge transfer on the interface between MO and the La4Ni3O10 electrode (24, 25). The control experiments revealed that the VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. (a) Current-voltage characteristics of the La4Ni3O10 film electrode under various conditions; (b) EIS Nynquist plots of the La4Ni3O10 film electrode under a bias potential of 0.5 V vs SCE with corresponding enlargement of the first semicircle (inset). current detected for the Ti electrode remains unchanged with either the addition of MO in the electrolyte or an increase of the stirring speed (see Figure S9, Supporting Information). Therefore, it is safe to conclude that electrons can transfer from MO molecules to La4Ni3O10 to initiate the catalytic degradation. Increasing the stirring speed significantly promotes the catalytic degradation process. The catalytic activity can be maintained for at least five runs (Figure S10, Supporting Information), which is of great importance for potential practical applications. The XRD (Figure 1) results further confirm the good stability of the La4Ni3O10 catalyst. The catalytic degradation process reported herein requires no additional reagents or external energy input, which we believe is of potential importance for the low-cost treatment of azo-dye effluents. The mechanism behind this process is also of interest for the design of novel catalysts and the related theory. Of course, further study is demanded to clarify those factors affecting the interfacial charge transfer, with the intention of further improving the efficiency and exploring other similar catalysts.
Acknowledgments
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This work was supported by Zhejiang Provincial Natural Science Foundation (Grant Y4080001).
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Supporting Information Available
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Ten figures showing additional details of our analysis. This material is available free of charge via the Internet at http:// pubs.acs.org.
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