Degradation of Cationic Red GTL by Catalytic Wet Air Oxidation over

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Degradation of Cationic Red GTL by Catalytic Wet Air Oxidation over Mo−Zn−Al−O Catalyst under Room Temperature and Atmospheric pressure Yin Xu,† Xiaoyi Li,† Xiang Cheng,† Dezhi Sun,†,* and Xueye Wang‡ †

Beijing Key Laboratory for Source Control Technology of Water Pollution, Beijing Forestry University, Beijing 100083, P. R. China Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan, Hunan 411105, P. R. China

‡

S Supporting Information *

ABSTRACT: To overcome the drawback of catalytic wet air oxidation (CWAO) with high temperature and high pressure, the catalytic activity of Mo−Zn−Al−O catalyst for degradation of cationic red GTL under room temperature and atmospheric pressure was investigated. Mo−Zn−Al−O catalyst was prepared by coprecipitation and impregnation. XRD, TGDTG, and XPS were used to characterize the resulting sample. Central composition design using response surface methodology was employed to optimize correlation of factors on the decolorization of cationic red GTL. The results show that the optimal conditions of pH value, initial concentration of dye and catalyst dosage were found to be 4.0, 85 mg/L and 2.72 g/L, respectively, for maximum decolorization of 80.1% and TOC removal of 50.9%. Furthermore, the reaction on the Mo−Zn−Al− O catalyst and degradation mechanism of cationic red GTL was studied by Electron spin resonance (ESR) and GC-MS technique. The possible reaction mechanism was that the Mo−Zn−Al−O catalyst can efficiently react with adsorbed oxygen/ H2O to produce ·OH and 1O2 and finally induce the degradation of cationic red GTL. GC-MS analysis of the degradation products indicates that cationic red GTL was initiated by the cleavage of NN and the intermediates were further oxidized by ·OH or 1O2.

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activity under mild conditions. The catalysts such as Fe2O3− CeO 2 −TiO 2 /γ-Al 2 O 3 , La 4 Ni 3 O 10 , CuO-MoO 3 −P 2 O 5 , MoO3:Ce, and Zn1.5PMo12O40 have been proved to get a good catalytic performance for treatment of dye wastewater under mild conditions.12−16 In these catalysts, molybdenum oxide as environmentally friendly catalyst has attracted much attention because of formidable structural variety and specific properties including nanosized dimension, change and surface activity. Besides of currently available catalyst, searching for a more active heterogeneous stable catalyst is a never-ending task. Even though such kinds of molybdenum oxide have been used in CWAO process to degrade the dye wastewater, information related to the detailed mechanisms of CWAO process is limited.17,18 In this study, cationic red GTL used in textile industry, especially for terylene, nitrilon, and other polyester fibers was selected as a cationic azo dye model. The primary aim is to understand the structure of Mo−Zn−Al−O catalyst and its catalytic activity for the degradation of cationic red GTL under

INTRODUCTION Organic dyes are used in a wide range of industrial application, which means that they are frequently found in the wastewater. Recently, the production of dye mainly made in China registers a growth of about 3.5% at 29 500 tonnes, and 1−20% of the total world production of dyes is discharged to the aquatic environment.1 Some dyes, especially cationic azo dyes, are known to bring very serious threats to the living species (http://www.food-info.net/uk/colour/azo.htm). Through their absorption and reflection of sunlight entering in the water, they inhibit the growth of aquatic organisms.2 As international environmental standards are becoming more stringent, various technologies for the removal of dye wastewater have been recently developed.3−6 In recent years, advanced oxidation processes (AOPs) for dye wastewater have received an increasing concern because of the formation of very active oxidative species such as singlet oxygen (1O2), superoxide radical anion (O2·−) and hydroxyl radical (·OH).7,8 Among AOPs technology, catalytic wet air oxidation (CWAO) has been proved to be one of the most effective methods to degrade dye wastewater.9−11 Unfortunately, high temperature and high pressure are needed, which has limited the practical application. To overcome this drawback, some attempts have been made to develop the heterogeneous catalyst with high © 2012 American Chemical Society

Received: Revised: Accepted: Published: 2856

October 5, 2011 February 9, 2012 February 14, 2012 February 27, 2012 dx.doi.org/10.1021/es203531q | Environ. Sci. Technol. 2012, 46, 2856−2863

Environmental Science & Technology

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Figure 1. XRD patterns of Zn−Al LDHs precursor and the Mo−Zn−Al−O catalyst (a: before calcination, b: after calcination).

impregnation that 20 g Zn−Al LDH was impregnated in 100 mL aqueous solution containing 2 mol/L of Mo concentration. The mixture was maintained at 55 °C for 12 h before it was filtered and washed thoroughly by deionized water several times to remove suspended materials. After that the resulting product was dried at 80 °C for 10 h and calcined at 400 °C for 1 h. After cooling to room temperature, the resulting solid was denoted as the Mo−Zn−Al−O catalyst.21 Characterization of the Catalyst. X-ray diffraction (XRD) patterns were recorded on a D/MAX-RB X-ray diffractometer (D/MAX-RB, Japan) using nickel-filter Cu Kα radiation (λ = 0.15418 nm) with a graphite monchrometer at 40 kV and 120 mA. Thermogravimetric-differential thermogravimetric (TG-DTG) analyses were carried out under a nitrogen atmosphere as a reference at a heating rate of 10 °C/ min (Labsys TM, France). XPS spectra of the catalyst were recorded using a PHI-5300ESCA analyzer. Al/Mg was used as an X-ray source operating at 400 W and 15 kV. The kinetic energy of photoelectrons was measured using a hemispherical electrostatic analyzer working in a constant pass energy mode. The C1s peak from the adventitious carbon-based contaminant with bind energy of 284.8 eV was used as the reference for calibration.

room temperature and atmospheric pressure. The operating conditions such as pH value, initial concentration of cationic red GTL, and catalyst dosage using response surface methodology (RSM) were evaluated. Briefly, RSM is one of the relevant multivariate technique with minimum number of experiments, and understanding combined interaction of all the factors to optimize the operating conditions.19,20 The secondary aim is to explore the possible CWAO mechanism under room temperature and atmospheric pressure through several methods below: (1) to assess the singlet oxygen by DMPO−1O2 and active radical species by DMPO-·OH using ESR and (2) to detect degradation products using GC-MS.

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EXPERIMENTAL SECTION Materials and Reagents. Cationic red GTL was purchased from Shanghai Luojing Dyeing Chemical Co., Ltd. (China). Its structure is given in Supporting Information (SI) (Figure S1. The other reagents were of AR grade. Preparation of the Mo−Zn−Al−O Catalyst. The Mo− Zn−Al−O catalyst was prepared by coprecipitation and impregnation. First, the Zn−Al LDH was prepared by coprecipitation with Zn(NO3)2·6H2O and Al(NO3)3·9H2O at pH 9.5−10.0. The Mo−Zn−Al−O catalyst was prepared by 2857

dx.doi.org/10.1021/es203531q | Environ. Sci. Technol. 2012, 46, 2856−2863


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Response Surface Methodology. The oxidation of cationic red GTL dye wastewater was performed in 400 mL beaker equipped with an air-flow controller. The Mo−Zn−Al− O catalyst was added into 100 mL cationic red GTL dye wastewater. The reaction was conducted at room temperature and atmospheric pressure, and the flow rate of air was kept at 400 mL/min for 60 min. After the oxidation reaction, the suspension was centrifuged to get the used catalysts. During the oxidation reaction, the decolorization of cationic red GTL was measured based on the absorbency of dye solution at 488 nm by UV−vis spectrophotometere. A typical procedure was carried out as follows: 2.72 g/L Mo−Zn−Al−O catalyst and Zn−Al LDHs after calcination was introduced into 100 mL of 85 mg/L cationic red GTL. The flow rate of air were kept at 400 mL/min. Central composite design was carried out considering the minimum and maximum levels for pH, initial concentration of dye and catalyst dosage (see SI Table 1S). The decolorization of cationic red GTL wastewater over Mo−Zn− Al-O catalyst was investigated. The experiments and relationship between codified and real values were summarized. Five levels were donated by (−1.68), (−1.00), (0.00), (1.00), and (1.68) respectively (see SI Table S2). Sample Analysis. Electron spin resonance (ESR) signal of radicals was obtained on a Bruker ESR 300 E spectrometer with an irradiation source of this instrument of Quanta-Ray Nd: YAG pulsed laser system. The regents for spin-trapping ·OH and 1O2 were T5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and 2,2,6,6-Tetramethylp (TEMP) respectively. For all samples, the same quartz capillary tube was used to minimize experimental errors. For GC-MS analysis, an Agilent 7890 gas chromatography with a HP-5MS capillary column (30 m × 0.25 mm × 0.25 mm) coupled with an Agilent 5975 mass spectrometer (Agilent Technologies, Palo Alto, CA) was used. The gas chromatographer conditions were as follows: initial column temperature was held constant at 40 °C for 3 min, ramped at 10 °C/min to 150 °C for 1 min, and then ramped further at 10 °C/min to 250 °C for 1 min. The other experimental conditions were EI impact ionization 70 eV, helium as the carrier gas, injection temperature 250 °C, source temperature 150 °C.

diffraction peaks were identified to the compound of ZnMoO4 (JCPDS 35-0765), MoO2 (JCPDS 32-0671), and ZnO structure. For ZnMoO4 phase, the main characteristic diffraction peaks of 2θ value were 24°, 26°, and 27°. The main diffraction patterns of 2θ at 24°, 27°, 37° and 54° were the characteristic diffraction peaks of MoO2 phase. However, the matching score of ZnMoO4, MoO2 and ZnO was 92%, 80%, and 79%, which indicates that the catalyst has a special structure. It has been noted that when Zn−Al LDH doped with Mo, Mo6+ ion diffuse into the crystal lattice and influence the phase, then form new crystal that ZnMoO4 molecular moieties were proposed to graft and isolate at the zinc or alumina surface. The structure of Zn−Al LDHs before calcination was different from Zn−Al LDHs after calcination. Also the similar result was shown in the XRD results of Mo−Zn−Al−O catalyst before and after calcination. This fact indicates that the textural properties of the samples are different due to the thermal treatment. However, the characteristic diffraction peaks of aluminum oxide exited were not found in the XRD patterns, the possible reason is that aluminum oxide could not be crystal in the catalyst. TG-DTG Results of the Mo−Zn−Al−O Catalyst. The catalyst was decomposed in the calcination process and became the compound with catalytic activity. Thermal behavior of the catalyst during heating in the air, revealed the transformation of precursors into mixed oxides. TG-DTG curve of Mo−Zn−Al− O catalyst is shown in Figure 2. It had three endothermic peaks

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RESULTS AND DISUSSION XRD Patterns of Mo−Zn−Al−O Catalyst. To identify the structure change of the catalyst in the preparation, the carrier (Zn−Al LDH precursor) before and after calcination was characterized by XRD and the results are shown in Figure 1(A). Typical diffraction peaks of LDH (JCPDS 38−0486) and ZnO (JCPDS 65−3411) were shown in precursor Zn−Al LDH before calcination. The Zn−Al LDH sample exhibited patterns characteristic of the well-crystallized layered structure of hydrotalcite-like type material. The diffraction intensities of basal reflections on the (003) and (006) faces were strong and sharp compared with other reflections, owing to the layer structure. The 2θ value for ZnO typical peak were 36°, 37°, and 63°. The all diffraction peaks of Zn−Al LDH precursor after calcination are corresponding well to the ZnO structure. There was no the characteristic diffraction peaks of aluminum oxide in the XRD results of the Zn−Al LDH carrier. XRD patterns of Mo−Zn−Al−O catalyst before and after calcination were also obtained and they are shown in Figure 1(B). In the Mo−Zn−Al−O catalyst before calcination, the main peaks were attributed to LDH and Zn5Mo2O11·5H2O (JCPDS 30-1486). After calcination, the characteristic

Figure 2. TG-DTG results of the Mo−Zn−Al−O catalyst before it was calcined.

at 87 °C, 223 and 305 °C, which were corresponding to the three processes of weight loss on the DTG curve respectively. First, the endothermic peak with weight loss of 2% at 87 °C was due to the loss of physically adsorbed and interlayer water molecules. Then, the peaks at 223 and 305 °C were attributed to the loss of the dehydroxylation of structural water (hydroxyl group in the brucite-like layer) and the elimination of anions (CO32‑ in the layer of LDH) respectively, according to the XRD result of Mo−Zn−Al−O catalyst. As the temperature increased to 400 °C, the total weight loss of the catalyst reached 13.4% in the TG curve. The weight in the three steps became constant when the temperature exceeded 400 °C. XPS Results of the Mo−Zn−Al−O Catalyst. Because that aluminum was not found in the XRD patterns, further 2858



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Figure 3. XPS results of the Mo−Zn−Al−O catalyst (a: before reaction, b: after reaction).

Figure 4. Concentration and TOC of cationic red GTL in CWAO process over Mo−Zn−Al−O catalyst.

calcination with the optimum value of each factor was investigated. As shown in Figure 4, without catalyst, but with air bubbling, decolorization efficiency and TOC removal was zero in 60 min. It indicated that the oxidation ability of air is very limited under a room condition. For MoO2 and Zn−Al LDH with air condition, the decolorization efficiency was only 7.0% and 11.6%, respectively; whereas over the Mo−Zn−Al−O catalyst it was 80.1%. The TOC removal efficiency over Mo− Zn−Al−O catalyst was 50.9% under air condition. The fact that the decolorization efficiency of cationic red GTL by the Mo− Zn−Al−O catalyst with air bubbling was higher than that without air bubbling indicates that Mo−Zn−Al−O catalyst has an excellent catalytic activity. Decolorization efficiency by Mo− Zn−Al−O catalyst was higher than that by Zn−Al LDH. This means that intrinsic structure of Mo has the main influence for these reaction systems. Meanwhile, the leaching tests were carried out in the CWAO process. The leaching concentrations of Zn, Al, and Mo from Mo−Zn−Al−O catalyst were increased with the reaction time and they can reach 0.22%, 0.02%, and 0.36% after 60 min. The amount of Mo leaching was far lower than the literature which was reported that the amount of Mo leaching was 2.5% in treating 10 mg/L Safranin T by air and ZnO/MoO3/SiO2 (1 g/L) under room conditions.15 Obviously, the application of Mo−Zn−Al−O catalyst was economic than ZnO/MoO3/SiO2. Meanwhile, besides analysis of solution for metals after catalyst testing and separation, to

investigation on the component masses and XPS of the catalyst was done. XPS spectrum for the Mo−Zn−Al−O catalyst is shown in Figure 3 (curve a). The presence of Mo 3d, Mo 3p, Zn 2p, Zn 3S, Zn 3p, Zn 3d, Al 2s, Al 2p, O 1s, and C 1s was detected. These results are not surprising as the composed catalyst was prepared with Zn−Al LDH doped with Mo, and each of the peaks could come from the interaction between any two elements. The XPS analysis mainly reflects the composition and chemical elementary state of the surface and the inferior surface of catalyst. A typical peak at 116.3 eV for Al 2s can be ascribed to AlOx, which originates from aluminum nitrate. Distinctive peaks of Mo 3d at binding energies of 232.9 and 236.0 eV are identified to be a typical chemical shift of Mo (VI) compounds and Mo (IV) compounds.13,15,22,23 In XPS spectrum for Mo 3p, the spin−orbit components of the peaks were well-defined by peaks at approximately 397.9 and 414.1 eV, respectively. This demonstrates that molybdenum is transformed from molybdate ammonium to Mo compounds in the course of doping and calcining treatment. Two typical peaks at 1042.5 and 1023.8 eV for Zn 2p suggest that Zn is possibly combined with oxygen.24 Catalytic Activity on the Degradation of Cationic Red GTL. To confirm the Mo−Zn−Al-O catalyst had a higher catalytic activity and Mo was the active component, decolorization efficiency and TOC removal of cationic red GTL by Zn−Al LDH and Mo−Zn−Al−O catalyst after 2859



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Figure 5. Response surface showing decolorization of cationic red GTL as a function of: (A) pH and initial concentration of dye, (B) pH and catalyst dosage and (C) initial concentration of dye and catalyst dosage.

interaction of pH value and initial dye concentration. With the same catalyst dosage, both parameters seemed to affect positively the final performance. Changes in the initial pH value could influence the adsorption of cationic red GTL on the catalyst surface. The decolorization efficiency increased with a decrease of initial pH value at low initial dye concentrations and then decreased with a decrease of initial pH value at high initial dye concentrations. This means that there exists an optimal initial pH and initial concentration of dye for the best decolorization efficiency. Figure 5B shows that decolorization was increased with the elevated catalyst dosage. The possible reason is that the increased catalyst dosage leads to growth in the number of active sites on the catalyst surface, which in turn enhanced the degradation efficiency of cationic red GTL. Obviously, initial pH value affected the decolorization at a low catalyst dosage, while the removal efficiency was over 60% for a high catalyst dosage, as is shown in Figure 5B. A similar result was observed when changing the initial dye concentration and catalyst dosage (Figure 5C). Based on the experimental design methodology, the optimal conditions were obtained: pH value was 4.0, initial concentration was 85 mg/L and catalyst dosage was 2.72 g/L. At these optimum conditions, the predicted maximal decolorization was 80.1%. After verifying by a further experiment, the result indicates that 79.5% of decolorization was obtained when the value of each factors were set at the

this solution was added dye and tested in air flow. There was no dye transformation in absence of solid and no obvious change of three metals in the solution. It means that the catalytic reaction happened in heterogeneous system. Response Surface Optimization. It has been previously shown that pH value (A), initial dye concentration (B) and catalyst dosage (C) can have dramatic effects on the decolorization of dye.19 Central composition design using response surface methodology with three variables was employed in this study to optimize correlation between these variables on the decolorization of cationic red GTL by CWAO under room temperature and atmospheric pressure. The quadratic regression model was suggested as the result of sequential model fitting for dye degradation (see SI Table S3). The ANOVA of quadratic model proved the validity of the model (see SI Table S4). The fact that F-value was 16.15 and Prob > F value was 0.0001 demonstrates that the model was highly significant. Not all the effects of parameters were significant. If value of Prob > F was less than 0.05, the model terms were significant.20 In this case, A, B, C and the interaction terms AC, BC, and C2 were significant, whereas the others were insignificant. The response surface modeling in a three-dimensional representation to put into evidence of the interaction terms AB, BC, and AC are illustrated in Figure 5. Figure 5A shows the 2860



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optimum value. It demonstrates that the RSM method was successful to optimize the CWAO conditions.19 Stability Test of the Mo−Zn−Al−O Catalyst. The regeneration of Mo−Zn−Al−O composite catalyst is of practical and economic importance.12−15 After the reaction, the suspensions were centrifuged and the Mo−Zn−Al−O catalyst was decanted. The catalyst was washed with deionized water in order to remove the dye compounds attached on the catalyst surface and it was dried at 110 °C. The results of the recovered Mo−Zn−Al−O composite after five recycling runs in the decolorization of cationic red GTL are depicted (see SI Figure S2). It can be clearly seen that the catalytic activity of Mo−Zn−Al−O catalyst on the degradation of cationic red GTL remained efficient after five experimental cycles. The XPS result of Mo−Zn−Al−O after reaction is shown in Figure 3 (curve b) and the results showed that the structure of Mo− Zn−Al−O catalyst did not change and the bonding energies of Mo did not shift after reaction, compared with a fresh catalyst. This means that Mo−Zn−Al−O catalyst after five experimental cycles still have high catalytic activity, just as the fresh catalyst. Verification of the Proposed Mechanism Using ESR. To determine the main active species responsible in CWAO system, a comparative study was undertaken for the scavengerloaded condition. The DMPO-·OH and TEMP−1O2 adducts under air condition and N2 condition are probed by ESR. The result shows that no characteristic peaks DMPO-·OH was observed in the suspension of Mo−Zn−Al−-O catalyst/ cationic red GTL (not shown). It is possibly due to the intensive and fast degradation of cationic red GTL, which acts as a quencher of ·OH.25 Cationic red GTL should be more active than DMPO for ·OH. TEMP−1O2 signal was detected in the suspension of Mo−Zn−Al−O catalyst/cationic red GTL. This means that cationic red GTL can react with 1O2 directly hardly comparing to TEMP. If it is believe that cationic red GTL and DMPO is in a competition reaction with oxygen active species, then the signals of DMPO-·OH and TEMP−1O2 adducts for the Mo−Zn−Al−O catalyst suspension in H2O would be detected. As depicted in Figure 6(A), four characteristic peaks of DMPO-·OH were obviously observed under air condition. It is noted that no such signal detected under N2 condition, indicating that air is essential to the generation of ·OH on the surface of the catalyst. Also shown is some date for TEMP−1O2 species under air condition (Figure 6(B)). TEMP−1O2 signal was detected in the Mo−Zn−Al−O catalyst under air condition and very weak such signal detected in the Mo−Zn−Al−O catalyst suspension under N2 condition. For a good understanding of the results with the heterogeneous catalyst, the reaction mechanism shown in Scheme 1 is proposed. In the initial stage, the Mo(IV)Zn2AlO8H5 catalyst reacted with oxygen and water to form Mo(VI)Zn2AlO9H5 intermediate and H2O2. The catalyst can react with O2 by adsorption or activation of adsorbed O2 leading to the transfer of electrons between the metal atoms and O2. Oxygen is typically reduced to H2O2 and Mo(IV) are correspondingly oxidized to Mo(VI). A free radical chain auto oxidation process was performed to generate ·OH radical. This Mo(VI)Zn 2 AlO 9 H 5 intermediate was unstable and easily was decomposed The statement about Mo (VI IV) redox transformations is confirmed by the H2-TPR results which show that two H2-TPR peaks correspond to the reduction of Mo6+ to Mo4+.21 The two peaks correspond to the reduction of Mo6+ to Mo4+ in the Mo−Zn−Al−O catalyst which confirm the Mo (VI, V, IV) redox transformations. As was proven,

Figure 6. ESR spectra of DMPO-·OH (A) and DMPO−1O2 (B) adducts under air and N2 bubbled condition for the Mo−Zn−Al−O catalyst dispersion in H2O.

Scheme 1. Proposed Reaction Pathway Over Mo−Zn−Al−O Catalyst in Aquatic Environment

aqueous H2O2 is converted at the surface of these catalysts into excited, singlet oxygen (1O2). The singlet oxygen diffuses away from the 1O2 generating centers and can perform selective oxygenations.26 Finally, these active oxygen radicals induce the decomposition of dye. Possible Degradation Pathway of Cationic Red GTL. The bonds length and angle of the main structure calculated by density functional theory (DFT) method at the B3LYP/6311G level with the GAUSSIAN-03 package, version E02, and are presented (SI Table S5). The bond lengths of N(1)−C(2), N(7)−C(8), N(7)−C(10), C(13)−N(16), and N(17)−C(18) 2861



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initiated by the cleavage of NN, with the hydrazo derivative, namely 2-((4-aminophenyl)(ethyl)amino)-N,N,Ntrimethylethanaminium and 2-chloro-4-nitroaniline as possible intermediates. After azo bond were attacked, the ·OH continued to react with the intermediates. Related to 2-[4amino(ethyl)anilino]-1-ethanol and 4-(diethylamino)phenol detected in CWAO process, these further proved that the attack of N(1)−C(2) was easier than others for dye degradation. The 2-chloro-4-nitroaniline was oxidized to 1chloro-3-nitrobenzene as shown in Scheme 2. With the continuous oxygen and the reaction time, these intermediates can be oxidized to acid, aldehyde, alcohol, and hydrocarbons. Unfortunately, such byproducts can not be detected due to the limitations associated with the analytical technique, or they are not accumulated in the solution. This fact of TOC removal proves that part of these intermediate would convert to the final oxidation products such as CO2 and H2O. In summary, the efficient Mo−Zn−Al−O catalyst for degradation of cationic red GTL by CWAO process under room temperature and atmospheric pressure had been prepared successfully and Mo was the active component in the CWAO process in present work. RSM method was successful to optimize the variables, namely pH value, initial concentration of dye, and catalyst dosage, and the optimal conditions were obtained. Moreover, on the basis of the obtained results, a possible reaction pathway on the degradation of dye under room temperature and atmospheric pressure over the Mo− Zn−Al−O catalyst was proposed.

are 1.55 A°, 1.48 A°, 1.41 A°, 1.41 A°, and 1.42 A° respectively. Comparing with the length and angles of N−C bonds, N(1)− C(2) is the most easily potential broken bond as for stability. The broken of N(7)−C(8) bond was easier than that of N(7)− C(10) bond. Degradation products of the cationic red GTL were identified by GC-MS method (Table 1) and the possible Table 1. Main Degradation Products in CWAO Process

Scheme 2. Possible Degradation Pathway of Cationic Red GTL in CWAO Process

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1−S3 show the structure of cationic red GTL, recycling runs and H2-TPR results of the catalyst and the calculations in response surface methodology and cationic red GTL were provided in Table S1−S5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+86)010-62336596; fax: (+86)010-62336596; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from Research Foundation of National Natural Science Foundation of China (No. 50978029). Wethank Mr. Peng Li from Chinese academy of Sciences for his help in the XRD and TG-DTG characterization. We also thank Jiangmei Wu for improving the English writing.

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degradation pathway was proposed (Scheme 2). Dye pollutants are adsorbed on the surface of catalysts by a hydrogen bonding interaction between hydroxyl groups and O atoms. As for the color formation theory, the color of dye is formed by the cooperation of the chromophore and the auxochrome.1,27,28 It has been researched that the azo bond of cationic red GTL is relatively easier to be attacked by heat energy and free radicals in CWAO process. Once the azo bond is broken, the color of dye is removed. Decomposition of cationic red GTL was first

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Degradation of Cationic Red GTL by Catalytic Wet Air Oxidation over

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