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TiO2–CeO2 catalysts
Desalination 268 (2011) 55–59
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Catalytic wet hydrogen peroxide oxidation of H-acid in aqueous solution with TiO2–CeO2 and Fe/TiO2–CeO2 catalysts Binxia Zhao ⁎, Binchu Shi, Xiaoli Zhang, Xin Cao, Yaozhong Zhang College of Chemical Engineering, Northwest University, Xi'an, Shaanxi 710069, China
a r t i c l e
i n f o
Article history: Received 18 February 2010 Received in revised form 26 September 2010 Accepted 27 September 2010 Available online 18 October 2010 Keywords: Catalytic wet hydrogen peroxide oxidation (CWPO) Metal oxide CeO2 TiO2 H-acid
a b s t r a c t TiO2–CeO2 and Fe/TiO2–CeO2 catalysts were prepared by the methods of co-precipitation and impregnation, respectively, and evaluated through the catalytic wet peroxide oxidation treatment of H-acid solution in the high concentration aqueous medium under mild experimental conditions. Furthermore, the catalysts were characterized by BET, XRD, SEM-EDX and TEM. The results showed that iron-containing samples were comparatively more active, Fe/TiO2–CeO2 (Ti/Ce 9/1, 2 wt.% Fe) was a very efficient catalyst to oxidize the pollutants of dye industry such as H-acid into biodegradable species and doping cerium into TiO2 obviously restrained the growth of crystal, greatly enhancing the surface areas of the catalysts. At the reaction temperature of 100 °C, initial pH of 5.0, the atmospheric pressure and the theoretical dosage of peroxide, 98.1% color removal, 89.6% COD and 65.4% TOC reduction with Fe/TiO2–CeO2 catalyst were obtained. © 2010 Elsevier B.V. All rights reserved.
1. Introduction H-acid (1-amino-8-naphthol-3, 6-disulfonic acid) is an important dye intermediate which is widely used in chemical industry for the synthesis of direct, acidic, reactive and azoic dye, as well as in the pharmaceutical industry [1,2]. Since the production process of H-acid is complicated and the utilization ratio of raw materials is low, the wastewater from the manufacturing processes is rich in various substituted derivatives of naphthalene compound and is of dark color and strong acidity. Organic substances in dye intermediate wastewater are often aromatic compounds substituted by some groups, such as amino (−NH2), nitro (−NO2), etc, that are extremely toxic to organisms. The biological processes can ineffectively degrade these substances and decolorize the H-acid wastewater. As aromatic ring with sulfonic (−SO3H) is easily dissolved in water, the general chemical and physical methods are very inefficient [3]. Wet air oxidation (WAO) is a method of oxidizing dissolvable or suspended organic compounds as well as reducible inorganic compounds with oxygen or air at high temperature and high pressure conditions. The application of traditional WAO is limited because of its severe operation conditions and rather costly investment [4–6]. Catalytic wet hydrogen peroxide oxidation (CWPO) has been developed in recent years, which can decompose high concentration effluents, as well as poisonous, detrimental and hardly degradable wastewater [7,8]. By adding catalyst and oxidant, CWPO process can
⁎ Corresponding author. Tel.: + 86 29 88302632; fax: + 86 29 88373052. E-mail address: [email protected] (B. Zhao). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.09.050
work well under mild conditions without too much energy consumption, because the •OH radicals generated in the reaction are highly oxidative, non-selective, and able to decompose many organic compounds including dyes and pesticides. In recent years, many investigators have been trying to improve the catalytic activity and stability of heterogeneous oxidation catalysts to enhance the efficiency of CWPO. Transition metals (mainly Fe, but also Cr, Mn, Co, Ni, and Cu) are supported over different materials: AC [9], pillared clays [10], ZSM5 [11], CeO2 [12], zeolite [13], SiO2 [14] and γ-Al2O3 [15]. However, TiO2–CeO2-based CWPO systems have not yet been investigated. Cerium oxide and CeO2-containing materials have been studied as a good alternative for the oxidation catalysts and supports. It has been shown that, when associated with transition metal oxides and noble metals, cerium oxide promotes oxygen storage and release to enhance oxygen mobility, and forms surface and bulk vacancies to improve the catalyst redox properties of the system [16–18]. In the CWAO, TiO2 with the good stability do not display the activity, and was often used as the support of metals [19–21]. No previous CWPO of H-acid studies dealing with Fe/TiO2–CeO2 catalysts have been reported. Yang et al. [22] presented an investigation of catalytic wet air oxidation (CWAO) of phenol over CeO2–TiO2 catalysts. They observed an increase in the mineralization efficiency due to a promoting effect of the ceria in the structural and redox properties of titanium dioxide. They found that the catalytic activity was influenced by Ce/Ti mol ratio seriously. In the paper, TiO2–CeO2 and Fe/TiO2–CeO2 catalysts are prepared by co-precipitation and impregnation, respectively, and characterized by BET nitrogen adsorption method, scanning electron microscope (SEMEDX), transmission electron microscope (TEM) and X-ray diffraction
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(XRD). The purpose is to study the performance of TiO2–CeO2 and Fe/ TiO2–CeO2 for the catalytic oxidation of H-acid with hydrogen peroxide. 2. Materials and methods
UV–VIS spectrophotometer to follow the progress of the decolorization during wet peroxide oxidation. An induced coupled plasma (ICP Model: IRIS Advantage) was used for determination of dissolved metal in solution. Residual hydrogen peroxide was determined by a colorimetric method.
2.1. Preparation of catalysts The TiO2–CeO2 catalysts were prepared by co-precipitation method. The hydrolysis of TiCl4 was performed at 0 °C to get Ti aqueous solution. The mixture solution of the aqueous Ti and Ce (NO3)3 with different molecular ratios of Ti and Ce was added dropwise to excess ammonia solution at room temperature under stirring, and then stirred for 2 h and aged in the 80 °C for 3 h. The precipitate was washed with distilled water to remove Cl−, and dried at 110 °C for 12 h. After that, the precursor was calcined in the air at 350 °C for 3 h to obtain TiO2–CeO2 powder catalyst (pure CeO2 or TiO2 catalysts were prepared with co-precipitation by adding Ce(NO3)3 or Ti aqueous solution to excess ammonia solution).These catalysts were referred to as Ti/Ce 10/0 (pure titanium oxide), Ti/Ce 9/1, Ti/Ce 8/2, Ti/ Ce 7/3, Ti/Ce 6/4, Ti/Ce 5/5, Ti/Ce 4/6, Ti/Ce 3/7, Ti/Ce 2/8, Ti/Ce 1/9 and Ti/Ce 0/10 (pure cerium oxide). The TiO2–CeO2 (Ti/Ce 9/1) was chosen from CWPO tests as support, and 0.5, 1, 2, and 3 wt.% Fe (the weight ratio of Fe to carrier)/ TiO2–CeO2 catalysts were prepared by impregnating it in a solution with different concentrations of Fe(NO3)3 under room condition for 12 h, then evaporated at 80 °C and dried at 110 °C for overnight. The obtained catalysts were calcined in a furnace at 350 °C for 3 h. After calcinations, the catalysts were stored in a dessicator. 2.2. Characterization of samples The surface areas of the samples were measured at 77 K using the BET method performed on Autosorb(MT)-1 Series-Surface Area and Pore Size Analyzers. Powder X-ray diffraction (XRD) patterns of the catalysts were obtained with a D/max-3C powder diffractometer by using nickel-filtered Cu Kα radiation at a scanning range of 20–70° and under a speed of 4°/min. X-ray tube voltage was 35 kV and the electric current was 40 mA. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) techniques were used to determine the catalyst granule morphology and elemental distribution of the catalyst particles using JSM-5800 Oxford ISIS-200EDX scanning electron microscope. Transmission electron microscopy (TEM) measurements were performed on a Hitachi H-600 transmission electron microscopy. 2.3. Determination of catalytic activity
3. Results and discussion 3.1. XRD and BET analysis of TiO2–CeO2 XRD was used to investigate the phase structure and the phase composition of TiO2-CeO2 catalysts. Fig. 1 shows the power XRD patterns of the 11 Ti/Ce catalyst samples. According to the main features of the patterns, the samples can be divided into two groups: Ti-rich catalysts (with titanium content of 80% and above) and Ce-rich catalysts (with cerium content 50% and above). For pure TiO2, only the peaks of anatase titania (2θ = 25.20°, 37.76°, 47.92°, and 53.78°) were detected. It means that TiO2 catalyst calcined at 350 °C exists as anatase structure. In Ti-rich samples, the peaks of anatase titania became much weaker and the wider with the increase of adding Ce content into TiO2, while no peaks of cerium oxides were observed in the spectra of XRD. This means that the crystal size of TiO2–CeO2 particles decreased with the increase of Ce content in Ti-rich samples. When the Ti/Ce mol ratios were between 7/3 and 6/4, the peaks became very faint scattering, indicating that TiO2–CeO2 existed as amorphous phase. The Ti/Ce 7/3 exhibited the most asymmetric diffraction peaks. This can be associated with large lattice distortion resulting from the introduction of dopant/vacancy. Accordingly, this explains that the Ti/Ce 7/3 sample exhibited the largest BET surface area (as in Fig. 2). For pure CeO2, the strong peaks were attributed to cubic CeO2 (2θ = 28.57°, 33.09°, 47.49°, and 56.26°). In Ce-rich samples, the dominant diffraction peaks are the characteristic of cerianite CeO2. No titanium oxide phases were detected by XRD. This may be due either to the formation of Ti–Ce oxide ‘solid solutions’ with cerianite structure or to the occurrence of amorphous titanium oxide. Besides, in Ti-rich catalysts, with the increasing amount of Ce, some peaks of anatase titania moved left. This shift indicates that part of cerianite species enters into the titanium lattice and provokes the contraction of its unit cell and shaping Ti–Ce oxide ‘solid solutions’ with anatase titania structure. It is also noticeable that in Ce-rich catalysts, a progressive shift of the diffraction peaks to higher Bragg angles was observed, which was due to the insertion of Ti ions into the lattice of CeO2, also shaping Ce–Ti oxide ‘solid solutions’ with cerianite structure. The effect of ratio of Ti and Ce on BET surface area is depicted in Fig. 2. The surface area of pure TiO2 (68.25 m2/g) was bigger than that of pure CeO2 (55.93 m2/g). The BET surface area increased
CWPO process was carried out in the 0.5 L autoclave equipped with a condenser, stirrer and heating device that keeps the constant temperature. The 1.0 g of solid catalyst and 17.6 mL of hydrogen peroxide (30% w/w, corresponding to the theoretical stoichiometric amount of H2O2 for complete oxidation of H-acid up to CO2 and H2O) were introduced into 250 mL of aqueous H-acid solution (10 g/L). The reaction was conducted at atmospheric pressure and the temperature of 100 °C. When the reaction temperature reached the setting value, the reaction started and the time was zero, the solution was analyzed to confirm the absence of adsorption by the catalyst. For all runs the reaction time was 90 min.
Ti:Ce=0:10 Ti:Ce=1:9 Ti:Ce=2:8 Ti:Ce=3:7 Ti:Ce=4:6 Ti:Ce=5:5 Ti:Ce=6:4 Ti:Ce=7:3 Ti:Ce=8:2 Ti:Ce=9:1 Ti:Ce=10:0
2.4. Analyses Total organic carbon was determined using a Vario model TOC analyzer. The analysis of COD was conducted in accordance with standard method. The pH was measured by means of a PHS-3B pHmeter. The visible light absorbance at the characteristic wavelength of the sample, i.e. 528 nm, was measured using a UV-2550 Shimadzu
20
25
30
35
40
45
50
55
60
65
Diffraction angle (2.Theta) Fig. 1. XRD patterns of the different TiO2–CeO2 catalysts.
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B. Zhao et al. / Desalination 268 (2011) 55–59 Table 1 The BET surface area and the particle size of the catalysts.
160
BET Surface area/(m2/g)
57
120
80
40
Samples
The surface area (m2/g)
The size of the particles (nm)
TiO2–CeO2 0.5% Fe/TiO2–CeO2 1% Fe/TiO2–CeO2 2% Fe/TiO2–CeO2 3% Fe/TiO2–CeO2
104.2 95.0 92.7 91.9 93.3
11.7 12.7 12.2 13.1 12.4
decreased monotonically with increasing contents of iron, up to 91.9 m2/g. However, when the Fe content rose to 3%, the surface area rose slightly to 93.28 m2/g.
0 0
0.2
0.4
0.6
0.8
1
Ti/(Ti+Ce) Fig. 2. Effect of Ti/(Ti + Ce) composition on BET surface area.
monotonically with increasing contents of cerium, up to 173.6 m2/g for the Ti/Ce 7/3 sample, and then gradually dropped to 55.93 m2/g, the BET surface area of pure cerium oxide. For doped-CeO2 and dopedTiO2 catalysts, the surface areas of TiO2–CeO2 catalysts are higher than that of pure CeO2 or TiO2 catalyst. As can be seen, the BET surface area of all the composite oxide samples far outweighed that predicted for mere mechanical mixtures of the two metal oxides. This suggests that with the current catalysts, there was a strong intimate interaction between titanium and cerium oxides. 3.2. XRD and BET analysis of Fe/TiO2–CeO2 The XRD patterns of all the compositions (0, 0.5, 1, 2, and 3%) Fe/ TiO2–CeO2 (Ti/Ce 9/1) calcined at 350 °C for 3 h is shown in Fig. 3. The XRD patterns of Fe-doped TiO2–CeO2 samples almost coincide with that of bare TiO2–CeO2 showing no crystalline phase attributed to iron oxide. Anatase type structure is kept almost same in all Fe-doped TiO2–CeO2 catalysts, only the peaks of anatase titania became much stronger than bare TiO2–CeO2. There are two reasons responsible for this result. One possible reason is that the Fe3+ content in the Fe/TiO2– CeO2 samples is below the detection limit of this technique. Another is that all Fe3+ ions might substitute Ti4+ ions and insert into the crystal lattice of TiO2–CeO2 because the radii of Fe3+ (0.69 A) is similar to that of Ti4+ (0.745 A), so Fe3+ can be easily incorporated into the crystal lattice of TiO2, forming an iron–titanium oxide solid solution [23–25]. Table 1 shows the BET surface areas and the particle sizes of the Fe/ TiO2–CeO2 (Ti/Ce 9/1) catalysts. The BET surface areas of Fe/TiO2– CeO2 were smaller than that of bare TiO2–CeO2, moreover, they
3% Fe/TiO 2 -CeO 2 2% Fe/TiO 2 -CeO 2 1% Fe/TiO 2-CeO 2 0.5% Fe/TiO 2 -CeO 2 TiO 2 -CeO 2 20
25
30
35
40
45
50
55
60
65
Diffraction angle(2.Theta) Fig. 3. XRD patterns of the different Fe/TiO2–CeO2 catalysts.
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3.3. SEM and TEM analysis Fig. 4 presents three representative TEM images of the following samples: pure TiO2, TiO2–CeO2 (Ti/Ce 9/1) and 2% Fe/TiO2–CeO2 (Ti/Ce 9/1). In all cases, the small and irregular particles of the catalysts were observed. Moreover, it can be seen that from Fig. 4(a) slight aggregates of TiO2 particles are observed, but the better dispersion can be achieved by doping of CeO2 as shown in Fig. 4(b). It is because of the insertion of Ce into the lattice of TiO2 and replacement of Ti ion proved by the fact that no peaks of cerium were detected in the XRD pattern, leading to great enhancement of the dispersion. It is also noticeable that since the Fe–Ce ions inserted into the lattice of TiO2, the structure of Fe/TiO2–CeO2 catalyst was better dispersion and more irregular which could partially explain the reason why the surface area of Fe/TiO2–CeO2 catalyst was higher than that of TiO2. The chemical composition of the catalyst on the surface of 2% Fe/ TiO2–CeO2 (Ti/Ce 9/1) catalyst was determined by SEM-EDX. The results obtained from SEM-EDX suggest that there was a little Fe (0.65%) which was much less than loading content (2%) and no Ce was observed in the surface of the catalyst since most Fe and all Ce were inserted into the lattice of TiO2. It was confirmed that no Fe was detected by XRD because of its low content in the surface. 3.4. Effect of the ratio of Ti and Ce Fig. 5 shows the activity of the different catalysts in the CWPO of Hacid under the reaction temperature of 100 °C, atmospheric pressure, a catalyst dosage of 1.0 g, H2O2 amount of 17.6 mL and reaction time of 90 min. It was possible to achieve 40.1% color removal, 35.1% COD reduction, and 15.1% TOC reduction without catalyst, and 32.5.2% and 15.3% TOC conversion were obtained in CWPO of H-acid over pure TiO2 and CeO2 catalysts, indicating that pure TiO2 was more active than pure CeO2 which had little activity. The ratio of Ti and Ce affected the activity of the TiO2–CeO2 catalyst greatly. When the ratio of Ti and Ce was higher than 9/1, doping a little CeO2 into TiO2 could obviously improve the catalytic activity. However, when the ratio of Ti and Ce was smaller than 9/1, the catalytic activity was decreased with the increase of Ce content. Using the Ti/Ce 9/1 catalyst, which is the most active catalyst, 89.1% color removal, 69.6% COD, and 35.8% reductions were obtained in CWPO of H-acid. It is noticed that the activities of TiO2–CeO2 catalysts were not in agreement with that of the surface areas of the TiO2–CeO2 catalysts (in Fig. 2). This finding can be explained by considering that the concentration of the chemisorbed oxygen decreases on the surface of the catalysts with the higher surface areas, because the chemisorbed oxygen is the most active oxygen specie, and plays an important role in the CWPO of organic compounds [22]. 3.5. Effect of Fe content in Fe/TiO2–CeO2 Iron oxide appeared as a promising alternative in the CWPO of refractory contaminant such as phenol and dye [12,15]. The different
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100
a Percentage removal (%)
Color 80
COD TOC
60 40 20 0
10/0 9/1
8/2
7/3
6/4
5/5
4/6
3/7
2/8
1/9 0/10 None
Ti/Ce
b
Fig. 5. COD removal of H-acid solution with the different TiO2–CeO2 catalysts (pH=5.0, catalyst amount [TiO2–CeO2]=1.0 g, H2O2 dosage=17.6 mL, reaction temperature=100 °C, and reaction time=90 min).
because the iron (active phase) reacts with the hydrogen peroxide constituting a modified Fenton system capable of generating hydroxyl (•OH) and perhydroxil (HO•2) radicals to carry out the oxidation of the organic molecules [26,27]: 3+
−Cat + H2 O2 →Fe
2+
−Cat + H2 O2 →Fe
Fe
Fe
•
þ
2+
−Cat + HO2 + H
3+
−Cat + HO + HO
•
ð1Þ −
ð2Þ
•
ð3Þ
RH + OH→H2 O + Rd→further oxidation •
3+
HO2 + Fe
c
þ
2+
−Cat→H + O2 + Fe
•
−Catd
ð4Þ
•
ð5Þ
•
ð6Þ
HO2 + H2 O2 →HO + H2 O + O2 •
H2 O2 + OH→HO2 + H2 O:
The hydroxyl radicals generated on the inner surface of the microporous material can diffuse to the external surface to break the large molecule into smaller fragments which can then diffuse inside the microporous material. The small number of sites on the external surface of the microporous material crystals may be sufficient to break the large molecule into smaller fragments [28]. The degradation efficient is strongly related to the consumption of H2O2 which will be decomposed into hydroxyl [29]. H2O2 decomposition after 90 min was complete for our experiment with Fe/TiO2– CeO2. The decomposition of H2O2 might give two hydroxyl radicals which react with H-acid in water. Fig. 4. TEM photographs of catalyst samples; (a) TiO2; (b) TiO2–CeO2 (TiCe 9/1); (c) 2% Fe/TiO2–CeO2 (Ti/Ce 9/1).
Percentage removal (%)
iron loading content in Fe/TiO2–CeO2 catalysts were prepared in order to improve the degradation efficiency of H-acid. Fig. 6 shows the degradation efficiency, obtained for the different Fe/TiO2–CeO2 catalysts at 100 °C (after 90 min of reaction). It was evident that Fe loading could obviously improve the activity of the catalyst, and that greater catalytic activity did not always correspond to the maximum Fe content (catalyst 3% Fe/TiO2–CeO2). It might rather be related with a better iron dispersion in TiO2–CeO2 catalysts. When Fe content was 2%, the degradation efficiency was the highest, arrived 98.1% color removal, 89.6% COD and 65.4% TOC reduction, and it can be concluded from the results of CWPO that the TOC reduction can be improved by about 30% compared to using TiO2–CeO2 catalyst under the same reaction condition. Nevertheless, a general tendency to increase the catalytic activity as the iron content increases was observed, probably
100 80 60 Color
40
COD TOC
20 0
0
0.5
1
2
3
Fe loading (%) Fig. 6. COD removal of H-acid solution with different Fe/TiO2–CeO2 catalysts (pH = 5.0, catalyst amount [2% Fe/TiO 2 –CeO 2 ] = 1.0 g, H 2 O 2 dosage = 17.6 mL, reaction temperature = 100 °C, and reaction time = 90 min).
B. Zhao et al. / Desalination 268 (2011) 55–59
3.6. Stability of Fe/TiO2–CeO2 As well known, the stability of the CWPO systems is one of the important factors for practical application. For this reason, recycling experiment was carried out using 2% Fe/TiO2–CeO2 catalyst in order to determine its stability. In particular, the 2% Fe/TiO2–CeO2 catalyst was recovered by filtration from the solution after treatment, washed with ultra-pure water, dried at 110 °C overnight and then re-used under the same reaction conditions: pH 5.0, 1.0 g catalyst, 17.6 mL H2O2, t = 100 °C, and 90 min reaction time. 98.1%, 83.5%, and 63.5% color removal of H-acid solution were obtained by three-run consecutive experiments, respectively. 35.3% of the iron was leached after threerun consecutive experiments. The results clearly show the presence of Fe ions in the solution coming from the catalyst and the stability of 2% Fe/TiO2–CeO2 decreases fast in successive runs. During the catalytic wet peroxide oxidation the active components leach out from the catalysts, which could cause additional pollution. To investigate the stability of 2% Fe/TiO2–CeO2 with respect to metal leaching, the concentrations of dissolved Fe, Ti, and Ce in the solution after catalytic wet peroxide oxidation for 90 min were analyzed using ICP, and 4.93 and 0.45 mg/L for Fe and Ti, respectively. No detectable amount of dissolved Ce could be measured. This result indicates that the loss of color removal mainly was due to the leaching of Fe. Much effort should be given to the improvement of active components leaching from the catalysts in the next future. 4. Conclusions TiO2–CeO2 and Fe/TiO2–CeO2 (Ti/Ce 9/1) catalysts prepared with co-precipitation and impregnation had good activity in CWPO of Hacid. The activity of TiO2–CeO2 catalysts was strongly affected by catalyst composition. The most active catalyst was Ti/Ce 9/1. Fe loading could obviously improve the activity of TiO2–CeO2 catalyst, and the greater catalytic activity did not always correspond to the maximum Fe content. The optimum Fe loading content was 2%. XRD, BET and SEM techniques showed the textural and structural properties. The interaction of metallic ions Ti, Ce and Fe causes the formation of Ti–Ce and Fe–Ti–Ce oxide ‘solid solutions’. For TiO2–CeO2 catalysts, adding a little of Ce to TiO2, the surface area of TiO2–CeO2 increases, and in our Ti/Ce catalytic systems, surface area alone cannot account for the trend of activity change with composition. 35.8% and 65.4% TOC removals were obtained with TiO2–CeO2 (Ti/Ce 9/1) and the Fe/TiO2–CeO2 (Ti/Ce 9/1, 2% Fe) in CWPO of H-acid under the reaction temperature of 100 °C, atmospheric pressure, a catalyst dosage of 1.0 g, H2O2 amount of 17.6 mL and reaction time of 90 min, respectively. It is necessary to improve the stability of Fe/TiO2–CeO2 further. References [1] W. Zhu, Z. Yang, L. Wang, Application of ferrous-hydrogen peroxide for the treatment of H-acid manufacturing process wastewater, Water Res. 30 (1996) 2949–2954. [2] M. Noorjaha, M.P. Reddy, V.D. Kumari, B. Lavédrine, P. Boule, M. Subrahmanyam, Photocatalytic degradation of H-acid over a novel TiO2 thin film fixed bed reactor and in aqueous suspensions, J. Photoch. Photobio. A Chem. 156 (2003) 179–187.
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Catalytic wet hydrogen peroxide oxidation of H-acid in aqueous solution with TiO2–CeO2 and Fe/TiO2–CeO2 catalysts Binxia Zhao ⁎, Binchu Shi, Xiaoli Zhang, Xin Cao, Yaozhong Zhang College of Chemical Engineering, Northwest University, Xi'an, Shaanxi 710069, China
a r t i c l e
i n f o
Article history: Received 18 February 2010 Received in revised form 26 September 2010 Accepted 27 September 2010 Available online 18 October 2010 Keywords: Catalytic wet hydrogen peroxide oxidation (CWPO) Metal oxide CeO2 TiO2 H-acid
a b s t r a c t TiO2–CeO2 and Fe/TiO2–CeO2 catalysts were prepared by the methods of co-precipitation and impregnation, respectively, and evaluated through the catalytic wet peroxide oxidation treatment of H-acid solution in the high concentration aqueous medium under mild experimental conditions. Furthermore, the catalysts were characterized by BET, XRD, SEM-EDX and TEM. The results showed that iron-containing samples were comparatively more active, Fe/TiO2–CeO2 (Ti/Ce 9/1, 2 wt.% Fe) was a very efficient catalyst to oxidize the pollutants of dye industry such as H-acid into biodegradable species and doping cerium into TiO2 obviously restrained the growth of crystal, greatly enhancing the surface areas of the catalysts. At the reaction temperature of 100 °C, initial pH of 5.0, the atmospheric pressure and the theoretical dosage of peroxide, 98.1% color removal, 89.6% COD and 65.4% TOC reduction with Fe/TiO2–CeO2 catalyst were obtained. © 2010 Elsevier B.V. All rights reserved.
1. Introduction H-acid (1-amino-8-naphthol-3, 6-disulfonic acid) is an important dye intermediate which is widely used in chemical industry for the synthesis of direct, acidic, reactive and azoic dye, as well as in the pharmaceutical industry [1,2]. Since the production process of H-acid is complicated and the utilization ratio of raw materials is low, the wastewater from the manufacturing processes is rich in various substituted derivatives of naphthalene compound and is of dark color and strong acidity. Organic substances in dye intermediate wastewater are often aromatic compounds substituted by some groups, such as amino (−NH2), nitro (−NO2), etc, that are extremely toxic to organisms. The biological processes can ineffectively degrade these substances and decolorize the H-acid wastewater. As aromatic ring with sulfonic (−SO3H) is easily dissolved in water, the general chemical and physical methods are very inefficient [3]. Wet air oxidation (WAO) is a method of oxidizing dissolvable or suspended organic compounds as well as reducible inorganic compounds with oxygen or air at high temperature and high pressure conditions. The application of traditional WAO is limited because of its severe operation conditions and rather costly investment [4–6]. Catalytic wet hydrogen peroxide oxidation (CWPO) has been developed in recent years, which can decompose high concentration effluents, as well as poisonous, detrimental and hardly degradable wastewater [7,8]. By adding catalyst and oxidant, CWPO process can
⁎ Corresponding author. Tel.: + 86 29 88302632; fax: + 86 29 88373052. E-mail address: [email protected] (B. Zhao). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.09.050
work well under mild conditions without too much energy consumption, because the •OH radicals generated in the reaction are highly oxidative, non-selective, and able to decompose many organic compounds including dyes and pesticides. In recent years, many investigators have been trying to improve the catalytic activity and stability of heterogeneous oxidation catalysts to enhance the efficiency of CWPO. Transition metals (mainly Fe, but also Cr, Mn, Co, Ni, and Cu) are supported over different materials: AC [9], pillared clays [10], ZSM5 [11], CeO2 [12], zeolite [13], SiO2 [14] and γ-Al2O3 [15]. However, TiO2–CeO2-based CWPO systems have not yet been investigated. Cerium oxide and CeO2-containing materials have been studied as a good alternative for the oxidation catalysts and supports. It has been shown that, when associated with transition metal oxides and noble metals, cerium oxide promotes oxygen storage and release to enhance oxygen mobility, and forms surface and bulk vacancies to improve the catalyst redox properties of the system [16–18]. In the CWAO, TiO2 with the good stability do not display the activity, and was often used as the support of metals [19–21]. No previous CWPO of H-acid studies dealing with Fe/TiO2–CeO2 catalysts have been reported. Yang et al. [22] presented an investigation of catalytic wet air oxidation (CWAO) of phenol over CeO2–TiO2 catalysts. They observed an increase in the mineralization efficiency due to a promoting effect of the ceria in the structural and redox properties of titanium dioxide. They found that the catalytic activity was influenced by Ce/Ti mol ratio seriously. In the paper, TiO2–CeO2 and Fe/TiO2–CeO2 catalysts are prepared by co-precipitation and impregnation, respectively, and characterized by BET nitrogen adsorption method, scanning electron microscope (SEMEDX), transmission electron microscope (TEM) and X-ray diffraction
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(XRD). The purpose is to study the performance of TiO2–CeO2 and Fe/ TiO2–CeO2 for the catalytic oxidation of H-acid with hydrogen peroxide. 2. Materials and methods
UV–VIS spectrophotometer to follow the progress of the decolorization during wet peroxide oxidation. An induced coupled plasma (ICP Model: IRIS Advantage) was used for determination of dissolved metal in solution. Residual hydrogen peroxide was determined by a colorimetric method.
2.1. Preparation of catalysts The TiO2–CeO2 catalysts were prepared by co-precipitation method. The hydrolysis of TiCl4 was performed at 0 °C to get Ti aqueous solution. The mixture solution of the aqueous Ti and Ce (NO3)3 with different molecular ratios of Ti and Ce was added dropwise to excess ammonia solution at room temperature under stirring, and then stirred for 2 h and aged in the 80 °C for 3 h. The precipitate was washed with distilled water to remove Cl−, and dried at 110 °C for 12 h. After that, the precursor was calcined in the air at 350 °C for 3 h to obtain TiO2–CeO2 powder catalyst (pure CeO2 or TiO2 catalysts were prepared with co-precipitation by adding Ce(NO3)3 or Ti aqueous solution to excess ammonia solution).These catalysts were referred to as Ti/Ce 10/0 (pure titanium oxide), Ti/Ce 9/1, Ti/Ce 8/2, Ti/ Ce 7/3, Ti/Ce 6/4, Ti/Ce 5/5, Ti/Ce 4/6, Ti/Ce 3/7, Ti/Ce 2/8, Ti/Ce 1/9 and Ti/Ce 0/10 (pure cerium oxide). The TiO2–CeO2 (Ti/Ce 9/1) was chosen from CWPO tests as support, and 0.5, 1, 2, and 3 wt.% Fe (the weight ratio of Fe to carrier)/ TiO2–CeO2 catalysts were prepared by impregnating it in a solution with different concentrations of Fe(NO3)3 under room condition for 12 h, then evaporated at 80 °C and dried at 110 °C for overnight. The obtained catalysts were calcined in a furnace at 350 °C for 3 h. After calcinations, the catalysts were stored in a dessicator. 2.2. Characterization of samples The surface areas of the samples were measured at 77 K using the BET method performed on Autosorb(MT)-1 Series-Surface Area and Pore Size Analyzers. Powder X-ray diffraction (XRD) patterns of the catalysts were obtained with a D/max-3C powder diffractometer by using nickel-filtered Cu Kα radiation at a scanning range of 20–70° and under a speed of 4°/min. X-ray tube voltage was 35 kV and the electric current was 40 mA. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) techniques were used to determine the catalyst granule morphology and elemental distribution of the catalyst particles using JSM-5800 Oxford ISIS-200EDX scanning electron microscope. Transmission electron microscopy (TEM) measurements were performed on a Hitachi H-600 transmission electron microscopy. 2.3. Determination of catalytic activity
3. Results and discussion 3.1. XRD and BET analysis of TiO2–CeO2 XRD was used to investigate the phase structure and the phase composition of TiO2-CeO2 catalysts. Fig. 1 shows the power XRD patterns of the 11 Ti/Ce catalyst samples. According to the main features of the patterns, the samples can be divided into two groups: Ti-rich catalysts (with titanium content of 80% and above) and Ce-rich catalysts (with cerium content 50% and above). For pure TiO2, only the peaks of anatase titania (2θ = 25.20°, 37.76°, 47.92°, and 53.78°) were detected. It means that TiO2 catalyst calcined at 350 °C exists as anatase structure. In Ti-rich samples, the peaks of anatase titania became much weaker and the wider with the increase of adding Ce content into TiO2, while no peaks of cerium oxides were observed in the spectra of XRD. This means that the crystal size of TiO2–CeO2 particles decreased with the increase of Ce content in Ti-rich samples. When the Ti/Ce mol ratios were between 7/3 and 6/4, the peaks became very faint scattering, indicating that TiO2–CeO2 existed as amorphous phase. The Ti/Ce 7/3 exhibited the most asymmetric diffraction peaks. This can be associated with large lattice distortion resulting from the introduction of dopant/vacancy. Accordingly, this explains that the Ti/Ce 7/3 sample exhibited the largest BET surface area (as in Fig. 2). For pure CeO2, the strong peaks were attributed to cubic CeO2 (2θ = 28.57°, 33.09°, 47.49°, and 56.26°). In Ce-rich samples, the dominant diffraction peaks are the characteristic of cerianite CeO2. No titanium oxide phases were detected by XRD. This may be due either to the formation of Ti–Ce oxide ‘solid solutions’ with cerianite structure or to the occurrence of amorphous titanium oxide. Besides, in Ti-rich catalysts, with the increasing amount of Ce, some peaks of anatase titania moved left. This shift indicates that part of cerianite species enters into the titanium lattice and provokes the contraction of its unit cell and shaping Ti–Ce oxide ‘solid solutions’ with anatase titania structure. It is also noticeable that in Ce-rich catalysts, a progressive shift of the diffraction peaks to higher Bragg angles was observed, which was due to the insertion of Ti ions into the lattice of CeO2, also shaping Ce–Ti oxide ‘solid solutions’ with cerianite structure. The effect of ratio of Ti and Ce on BET surface area is depicted in Fig. 2. The surface area of pure TiO2 (68.25 m2/g) was bigger than that of pure CeO2 (55.93 m2/g). The BET surface area increased
CWPO process was carried out in the 0.5 L autoclave equipped with a condenser, stirrer and heating device that keeps the constant temperature. The 1.0 g of solid catalyst and 17.6 mL of hydrogen peroxide (30% w/w, corresponding to the theoretical stoichiometric amount of H2O2 for complete oxidation of H-acid up to CO2 and H2O) were introduced into 250 mL of aqueous H-acid solution (10 g/L). The reaction was conducted at atmospheric pressure and the temperature of 100 °C. When the reaction temperature reached the setting value, the reaction started and the time was zero, the solution was analyzed to confirm the absence of adsorption by the catalyst. For all runs the reaction time was 90 min.
Ti:Ce=0:10 Ti:Ce=1:9 Ti:Ce=2:8 Ti:Ce=3:7 Ti:Ce=4:6 Ti:Ce=5:5 Ti:Ce=6:4 Ti:Ce=7:3 Ti:Ce=8:2 Ti:Ce=9:1 Ti:Ce=10:0
2.4. Analyses Total organic carbon was determined using a Vario model TOC analyzer. The analysis of COD was conducted in accordance with standard method. The pH was measured by means of a PHS-3B pHmeter. The visible light absorbance at the characteristic wavelength of the sample, i.e. 528 nm, was measured using a UV-2550 Shimadzu
20
25
30
35
40
45
50
55
60
65
Diffraction angle (2.Theta) Fig. 1. XRD patterns of the different TiO2–CeO2 catalysts.
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B. Zhao et al. / Desalination 268 (2011) 55–59 Table 1 The BET surface area and the particle size of the catalysts.
160
BET Surface area/(m2/g)
57
120
80
40
Samples
The surface area (m2/g)
The size of the particles (nm)
TiO2–CeO2 0.5% Fe/TiO2–CeO2 1% Fe/TiO2–CeO2 2% Fe/TiO2–CeO2 3% Fe/TiO2–CeO2
104.2 95.0 92.7 91.9 93.3
11.7 12.7 12.2 13.1 12.4
decreased monotonically with increasing contents of iron, up to 91.9 m2/g. However, when the Fe content rose to 3%, the surface area rose slightly to 93.28 m2/g.
0 0
0.2
0.4
0.6
0.8
1
Ti/(Ti+Ce) Fig. 2. Effect of Ti/(Ti + Ce) composition on BET surface area.
monotonically with increasing contents of cerium, up to 173.6 m2/g for the Ti/Ce 7/3 sample, and then gradually dropped to 55.93 m2/g, the BET surface area of pure cerium oxide. For doped-CeO2 and dopedTiO2 catalysts, the surface areas of TiO2–CeO2 catalysts are higher than that of pure CeO2 or TiO2 catalyst. As can be seen, the BET surface area of all the composite oxide samples far outweighed that predicted for mere mechanical mixtures of the two metal oxides. This suggests that with the current catalysts, there was a strong intimate interaction between titanium and cerium oxides. 3.2. XRD and BET analysis of Fe/TiO2–CeO2 The XRD patterns of all the compositions (0, 0.5, 1, 2, and 3%) Fe/ TiO2–CeO2 (Ti/Ce 9/1) calcined at 350 °C for 3 h is shown in Fig. 3. The XRD patterns of Fe-doped TiO2–CeO2 samples almost coincide with that of bare TiO2–CeO2 showing no crystalline phase attributed to iron oxide. Anatase type structure is kept almost same in all Fe-doped TiO2–CeO2 catalysts, only the peaks of anatase titania became much stronger than bare TiO2–CeO2. There are two reasons responsible for this result. One possible reason is that the Fe3+ content in the Fe/TiO2– CeO2 samples is below the detection limit of this technique. Another is that all Fe3+ ions might substitute Ti4+ ions and insert into the crystal lattice of TiO2–CeO2 because the radii of Fe3+ (0.69 A) is similar to that of Ti4+ (0.745 A), so Fe3+ can be easily incorporated into the crystal lattice of TiO2, forming an iron–titanium oxide solid solution [23–25]. Table 1 shows the BET surface areas and the particle sizes of the Fe/ TiO2–CeO2 (Ti/Ce 9/1) catalysts. The BET surface areas of Fe/TiO2– CeO2 were smaller than that of bare TiO2–CeO2, moreover, they
3% Fe/TiO 2 -CeO 2 2% Fe/TiO 2 -CeO 2 1% Fe/TiO 2-CeO 2 0.5% Fe/TiO 2 -CeO 2 TiO 2 -CeO 2 20
25
30
35
40
45
50
55
60
65
Diffraction angle(2.Theta) Fig. 3. XRD patterns of the different Fe/TiO2–CeO2 catalysts.
70
3.3. SEM and TEM analysis Fig. 4 presents three representative TEM images of the following samples: pure TiO2, TiO2–CeO2 (Ti/Ce 9/1) and 2% Fe/TiO2–CeO2 (Ti/Ce 9/1). In all cases, the small and irregular particles of the catalysts were observed. Moreover, it can be seen that from Fig. 4(a) slight aggregates of TiO2 particles are observed, but the better dispersion can be achieved by doping of CeO2 as shown in Fig. 4(b). It is because of the insertion of Ce into the lattice of TiO2 and replacement of Ti ion proved by the fact that no peaks of cerium were detected in the XRD pattern, leading to great enhancement of the dispersion. It is also noticeable that since the Fe–Ce ions inserted into the lattice of TiO2, the structure of Fe/TiO2–CeO2 catalyst was better dispersion and more irregular which could partially explain the reason why the surface area of Fe/TiO2–CeO2 catalyst was higher than that of TiO2. The chemical composition of the catalyst on the surface of 2% Fe/ TiO2–CeO2 (Ti/Ce 9/1) catalyst was determined by SEM-EDX. The results obtained from SEM-EDX suggest that there was a little Fe (0.65%) which was much less than loading content (2%) and no Ce was observed in the surface of the catalyst since most Fe and all Ce were inserted into the lattice of TiO2. It was confirmed that no Fe was detected by XRD because of its low content in the surface. 3.4. Effect of the ratio of Ti and Ce Fig. 5 shows the activity of the different catalysts in the CWPO of Hacid under the reaction temperature of 100 °C, atmospheric pressure, a catalyst dosage of 1.0 g, H2O2 amount of 17.6 mL and reaction time of 90 min. It was possible to achieve 40.1% color removal, 35.1% COD reduction, and 15.1% TOC reduction without catalyst, and 32.5.2% and 15.3% TOC conversion were obtained in CWPO of H-acid over pure TiO2 and CeO2 catalysts, indicating that pure TiO2 was more active than pure CeO2 which had little activity. The ratio of Ti and Ce affected the activity of the TiO2–CeO2 catalyst greatly. When the ratio of Ti and Ce was higher than 9/1, doping a little CeO2 into TiO2 could obviously improve the catalytic activity. However, when the ratio of Ti and Ce was smaller than 9/1, the catalytic activity was decreased with the increase of Ce content. Using the Ti/Ce 9/1 catalyst, which is the most active catalyst, 89.1% color removal, 69.6% COD, and 35.8% reductions were obtained in CWPO of H-acid. It is noticed that the activities of TiO2–CeO2 catalysts were not in agreement with that of the surface areas of the TiO2–CeO2 catalysts (in Fig. 2). This finding can be explained by considering that the concentration of the chemisorbed oxygen decreases on the surface of the catalysts with the higher surface areas, because the chemisorbed oxygen is the most active oxygen specie, and plays an important role in the CWPO of organic compounds [22]. 3.5. Effect of Fe content in Fe/TiO2–CeO2 Iron oxide appeared as a promising alternative in the CWPO of refractory contaminant such as phenol and dye [12,15]. The different
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100
a Percentage removal (%)
Color 80
COD TOC
60 40 20 0
10/0 9/1
8/2
7/3
6/4
5/5
4/6
3/7
2/8
1/9 0/10 None
Ti/Ce
b
Fig. 5. COD removal of H-acid solution with the different TiO2–CeO2 catalysts (pH=5.0, catalyst amount [TiO2–CeO2]=1.0 g, H2O2 dosage=17.6 mL, reaction temperature=100 °C, and reaction time=90 min).
because the iron (active phase) reacts with the hydrogen peroxide constituting a modified Fenton system capable of generating hydroxyl (•OH) and perhydroxil (HO•2) radicals to carry out the oxidation of the organic molecules [26,27]: 3+
−Cat + H2 O2 →Fe
2+
−Cat + H2 O2 →Fe
Fe
Fe
•
þ
2+
−Cat + HO2 + H
3+
−Cat + HO + HO
•
ð1Þ −
ð2Þ
•
ð3Þ
RH + OH→H2 O + Rd→further oxidation •
3+
HO2 + Fe
c
þ
2+
−Cat→H + O2 + Fe
•
−Catd
ð4Þ
•
ð5Þ
•
ð6Þ
HO2 + H2 O2 →HO + H2 O + O2 •
H2 O2 + OH→HO2 + H2 O:
The hydroxyl radicals generated on the inner surface of the microporous material can diffuse to the external surface to break the large molecule into smaller fragments which can then diffuse inside the microporous material. The small number of sites on the external surface of the microporous material crystals may be sufficient to break the large molecule into smaller fragments [28]. The degradation efficient is strongly related to the consumption of H2O2 which will be decomposed into hydroxyl [29]. H2O2 decomposition after 90 min was complete for our experiment with Fe/TiO2– CeO2. The decomposition of H2O2 might give two hydroxyl radicals which react with H-acid in water. Fig. 4. TEM photographs of catalyst samples; (a) TiO2; (b) TiO2–CeO2 (TiCe 9/1); (c) 2% Fe/TiO2–CeO2 (Ti/Ce 9/1).
Percentage removal (%)
iron loading content in Fe/TiO2–CeO2 catalysts were prepared in order to improve the degradation efficiency of H-acid. Fig. 6 shows the degradation efficiency, obtained for the different Fe/TiO2–CeO2 catalysts at 100 °C (after 90 min of reaction). It was evident that Fe loading could obviously improve the activity of the catalyst, and that greater catalytic activity did not always correspond to the maximum Fe content (catalyst 3% Fe/TiO2–CeO2). It might rather be related with a better iron dispersion in TiO2–CeO2 catalysts. When Fe content was 2%, the degradation efficiency was the highest, arrived 98.1% color removal, 89.6% COD and 65.4% TOC reduction, and it can be concluded from the results of CWPO that the TOC reduction can be improved by about 30% compared to using TiO2–CeO2 catalyst under the same reaction condition. Nevertheless, a general tendency to increase the catalytic activity as the iron content increases was observed, probably
100 80 60 Color
40
COD TOC
20 0
0
0.5
1
2
3
Fe loading (%) Fig. 6. COD removal of H-acid solution with different Fe/TiO2–CeO2 catalysts (pH = 5.0, catalyst amount [2% Fe/TiO 2 –CeO 2 ] = 1.0 g, H 2 O 2 dosage = 17.6 mL, reaction temperature = 100 °C, and reaction time = 90 min).
B. Zhao et al. / Desalination 268 (2011) 55–59
3.6. Stability of Fe/TiO2–CeO2 As well known, the stability of the CWPO systems is one of the important factors for practical application. For this reason, recycling experiment was carried out using 2% Fe/TiO2–CeO2 catalyst in order to determine its stability. In particular, the 2% Fe/TiO2–CeO2 catalyst was recovered by filtration from the solution after treatment, washed with ultra-pure water, dried at 110 °C overnight and then re-used under the same reaction conditions: pH 5.0, 1.0 g catalyst, 17.6 mL H2O2, t = 100 °C, and 90 min reaction time. 98.1%, 83.5%, and 63.5% color removal of H-acid solution were obtained by three-run consecutive experiments, respectively. 35.3% of the iron was leached after threerun consecutive experiments. The results clearly show the presence of Fe ions in the solution coming from the catalyst and the stability of 2% Fe/TiO2–CeO2 decreases fast in successive runs. During the catalytic wet peroxide oxidation the active components leach out from the catalysts, which could cause additional pollution. To investigate the stability of 2% Fe/TiO2–CeO2 with respect to metal leaching, the concentrations of dissolved Fe, Ti, and Ce in the solution after catalytic wet peroxide oxidation for 90 min were analyzed using ICP, and 4.93 and 0.45 mg/L for Fe and Ti, respectively. No detectable amount of dissolved Ce could be measured. This result indicates that the loss of color removal mainly was due to the leaching of Fe. Much effort should be given to the improvement of active components leaching from the catalysts in the next future. 4. Conclusions TiO2–CeO2 and Fe/TiO2–CeO2 (Ti/Ce 9/1) catalysts prepared with co-precipitation and impregnation had good activity in CWPO of Hacid. The activity of TiO2–CeO2 catalysts was strongly affected by catalyst composition. The most active catalyst was Ti/Ce 9/1. Fe loading could obviously improve the activity of TiO2–CeO2 catalyst, and the greater catalytic activity did not always correspond to the maximum Fe content. The optimum Fe loading content was 2%. XRD, BET and SEM techniques showed the textural and structural properties. The interaction of metallic ions Ti, Ce and Fe causes the formation of Ti–Ce and Fe–Ti–Ce oxide ‘solid solutions’. For TiO2–CeO2 catalysts, adding a little of Ce to TiO2, the surface area of TiO2–CeO2 increases, and in our Ti/Ce catalytic systems, surface area alone cannot account for the trend of activity change with composition. 35.8% and 65.4% TOC removals were obtained with TiO2–CeO2 (Ti/Ce 9/1) and the Fe/TiO2–CeO2 (Ti/Ce 9/1, 2% Fe) in CWPO of H-acid under the reaction temperature of 100 °C, atmospheric pressure, a catalyst dosage of 1.0 g, H2O2 amount of 17.6 mL and reaction time of 90 min, respectively. It is necessary to improve the stability of Fe/TiO2–CeO2 further. References [1] W. Zhu, Z. Yang, L. Wang, Application of ferrous-hydrogen peroxide for the treatment of H-acid manufacturing process wastewater, Water Res. 30 (1996) 2949–2954. [2] M. Noorjaha, M.P. Reddy, V.D. Kumari, B. Lavédrine, P. Boule, M. Subrahmanyam, Photocatalytic degradation of H-acid over a novel TiO2 thin film fixed bed reactor and in aqueous suspensions, J. Photoch. Photobio. A Chem. 156 (2003) 179–187.
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