CeO2

Ind. Eng. Chem. Res. 2010, 49, 1625–1630

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Low-Pressure Catalytic Wet Air Oxidation of Aniline Over Co3O4/CeO21 Gu¨lin Erso¨z* and Su¨heyda Atalay† Chemical Engineering Department, Faculty of Engineering, Ege UniVersity, 35100 BornoVa, I˙zmir, Turkey

Low -pressure catalytic wet air oxidation of aniline was investigated in a bubble reactor over Co3O4 (10% wt)/ CeO2. The catalyst was prepared by sol-gel technology and characterized by using scanning electron microscope, X-ray diffraction, nitrogen adsorption, and thermogravimetric analysis techniques. The aim was to search for the conditions to destroy the aniline content by avoiding production of byproducts such as ammonium, nitrate, and nitrite ions. The reaction was optimized at 0.5 g/L catalyst loading at 150 °C with a pressure of 4 atm, in 2 h with an air flow rate of 1.36 L/min. A 35.15% amount of aniline was removed, and 14% of the input nitrogen was converted into N2 gas. To evaluate the stability of the catalyst, two consecutive runs were performed by reusing the catalyst recovered. The highest removal with the reused catalyst was found as 34.94%, showing that Co3O4/CeO2 is a stable catalyst. 1. Introduction Aromatic amines such as aniline are known to be toxic and hazardous water pollutants. They are present in several types of wastewaters including those from textile, petroleum, paper, and the coal industry and are widely used in the manufacturing of rubbers and plastics, azo dyes, agrochemicals, pharmaceuticals, and pesticides. They are known to be toxic water pollutants, and their presence in wastewater even in very low concentrations is harmful to aquatic life.1 Aromatic amines therefore must be treated in order to meet the specifications for discharge. The treatment of these kinds of organic wastes, by biological processes, is often unsuitable due to their inherent toxicity to microorganisms. The use of traditional noncatalytic chemical processes or incineration may be too costly and energy intensive.2 Recent literature shows that, among the wastewater treatment techniques, catalytic wet air oxidation (CWAO) of organic wastes in water seems to be effective and promising.2-6 The wet air oxidation (WAO) process has well-known capabilities for breaking down biologically refractory compounds to simpler, easily treated materials, before they are released into the environment. In general, this aqueous-phase flameless combustion process takes place at high reaction temperatures (473-593 K) and pressures (20-200 bar) by means of an active oxygen species.7 The use of catalysts in WAO not only makes it possible to reduce the reaction temperature and pressure and to increase oxidation rates but also avoids the formation of harmful products by complete oxidation of organic contaminants to harmless carbon dioxide, water, and nitrogen.8,9 Indeed, in the past decades, numerous authors have demonstrated CWAO efficiency with a large range of compounds such as carboxylic acids, aromatics, polymers, N- and O-containing organic compounds, and treatment of toxic nitrogen-containing compounds as one of the major applications of the CWAO processes.10

The different steps of the CWAO of nitrogen-containing compounds, involved in the process, can be summarized in Scheme 1.11 Consequently, to evaluate the total process efficiency, it is essential to know the production of byproducts such as ammonia, nitrogen, nitrates, and nitrites. The desired product is molecular nitrogen. In literature there are valuable studies performed on CWAO of various industrial wastes.10-13 Nevertheless, the reaction conditions of CWAO are still somewhat severe, and the search for an active and durable catalyst has become the focus of investigations. Though some inspiring results have been reported, it is still necessary to develop more active, stable, and environmentally friendly catalysts. In this research the main aim was to study the effectiveness of the WAO of aqueous aniline over the prepared nanostructured catalyst to make the stream ecofriendly by minimizing the organic content and also minimizing byproducts such as ammonium, nitrate, and nitrite ions by selectively converting the N atom in aniline into N2 gas by searching for the suitable conditions. 2. Experimental Section 2.1. Catalyst Preparation. Co3O4/CeO2 particles were prepared by the sol-gel method as reported in literature.14-16 The procedure is mainly based on two steps. First, CeO2 support was prepared by sol-gel technology, and then it was impregnated with Co3O4 by the incipient wetness impregnation method. 2.2. Catalyst Characterization. The surface morphology of the catalyst prepared was investigated using a scanning electron microscope (SEM) analyzer (Phillips XL_30S FEG). Their phases were analyzed with powder X-ray diffraction (Phillips X’Pert Pro X-ray diffraction). In addition to these the characterization was enriched by nitrogen adsorption (Quantachrome Scheme 1. CWAO of Nitrogen-Containing Compounds

1

* To whom correspondence should be addressed. Tel.: 00 90 232 3884000/3061. Fax: 00 90 232 3887776. E-mail: gulin.aytimur@ ege.edu.tr. † Tel.: 00 90 2323887600. Fax: 00 90 2323887776. E-mail: suheyda.atalay@ ege.edu.tr. 10.1021/ie901383e  2010 American Chemical Society Published on Web 01/15/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010

Figure 1. Schematic diagram of the experimental setup.

Nova Win2) and thermogravimetric analysis, TGA (PerkinElmer Diamond TG/DTA), studies. 2.3. Experimental Setup and Procedure. The WAO reactions were performed as a semibatch process in a bubble column reactor of 500 mL volume (h ) 500 mm; D ) 35 mm), which is made of a stainless steel material. A schematic diagram of the experimental setup is illustrated in Figure 1. For a typical run, 300 mL of aniline solution (1 g/L) is charged and heated to the desired temperature by electrical wires wrapped around the reactor and the reaction temperature controlled by a PID controller (1/32DIN temperature controller). To minimize the heat loss, the reactor is insulated using glasswool. There is a condenser at the top of the reactor, which is used to condense the vapors formed during the reaction and send the condensate back to the reactor. Air is fed continuously and distributed in the reactor via a sparger placed at the bottom of the reactor. A rotameter (Sho-Rate) is used to adjust the flow rate of the air stream. In every run, at the end of experiment, samples were taken for determination of the concentrations of aniline, NH4+, NO3-, NO2-, and CO2. In addition to these, in some experiments, the detection of ammonia in the gas phase was performed. 2.4. Experiments. After characterization of the catalysts, the oxidation reactions were performed to find out the optimum conditions. For the removal of aniline and selective conversion of the nitrogen atom in aniline into N2 gas, the parameters tested

were as follows: Catalyst loading, temperature, reaction time, air flow rate, and pressure. The range of the parameters tested was chosen according to a literature survey. Table 1 represents the experimental sets performed in this study. 2.5. Product Analysis. From the literature survey it can be concluded that it is generally expected to have aniline, phenol, low molecular weight acids, NO3-, NO2-, and NH4+ in the product of the CWAO of aniline.11 Consequently, to investigate the reaction mechanism both the analyses of organic and inorganic forms of nitrogen were performed. However N2 could not be analyzed; it was calculated from the nitrogen balance after experimental determination of aniline, ammonia, nitrates, and nitrites. In literature, a possible relevance of the deposit of nitrogen organic compound on the catalyst was also reported.11 However, this analysis was not performed and hence not included in the balance. The organic form of nitrogen (aniline) was analyzed with an HP gas chromatograph 5890 Series II with a column of HP-5 30 m, 0.32 mm, and 0.25 µm. The inorganic forms of nitrogen (NH4+, NO3-, and NO2-) were measured using a Spectroquant NOVA 400 Merck UVspectrometer. In addition, to see the aniline conversion for the formation of CO2, the measurement of CO2 gas was also performed. CO2 gas measurement was performed on analysis involving a titration. The amount of C02 generated during an experiment was determined by absorption in a NaOH solution at known concentration. It is known that CO2 gas readily reacts with aqueous hydroxides to make carbonates and water. Hence the reactor gas outlet stream was continuously bubbled into the NaOH solution in a gas washing bottle. At the end of each experiment, the solution was titrated with a HCl solution with known concentration and the CO2 gas generated was determined. It was observed that the pH of the aqueous solution after the reaction remained constant, which indicated that the formation of low molecular weight acids, such as acetic acid, were not detected. Reactor off-gases were also absorbed in a series of absorbers containing dilute H2SO4 for the detection of ammonia in the gas phase of the reactor at the end of the reaction. The remaining acid was titrated to find the ammonia. However, no ammonia in the gas phase of the reactor was detected. 3. Results and Discussion 3.1. Catalyst Characterization. In Figure 2, the SEM images of CeO2 support (a), fresh Co3O4/CeO2 (b), and used Co3O4/CeO2 (c), at ×50000 magnification are given. The SEM image of the catalyst support depicts large agglomerations of particles (Figure 2a). The particles have a porous structure. SEM analysis (Figure 2b) shows that, after

Table 1. Experimental Sets operating conditions set I

II III IV V

aim to optimize the catalyst loading and temp value

aniline initial concn, g/L

temp, °C

1

to optimize reacn time to optimize flow rate

1

to optimize pressure to evaluate the stability of the catalyst

1

1

1

pressure, atm 4

150 (optimized in the first set) 150 (optimized in the first set) 150 (optimized in the first set) 150 (optimized in the first set)

4

air flow rate, L/min 1.36 for 300 mL of wastewater

1.36 for 300 mL of wastewater

4

5 (optimized in the fourth set)

catalyst loading, g/L

1.36 (optimized in the third set) 1.36 (optimized in the third set)

reacn time, min

parameters

120

catalyst loading: no catalyst, 0.5 g/L, 2 g/L

0.5 (optimized in the first set) 0.5 (optimized in the first set)

120 (optimized in the second set)

0.5 (optimized in the first set) 0.5 (optimized in the first set)

120 (optimized in the second set) 120 (optimized in the second set)

temp: 100, 125, and 150 °C reacn time:30, 60, 90, 120, and 150 min flow rate: 0.23, 0.61, 1.36, and 1.72 L/min for 300 mL of wastewater. pressure:3, 4, and 5 atm -


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Figure 2. SEM images of CeO2 support (a), fresh Co3O4/CeO2 (b), used Co3O4/CeO2 (c), at ×50000 magnification. Figure 4. XRD patterns of fresh Co3O4/CeO2 catalyst.

Table 2. Results of Nitrogen Adsorption Studies for given catalyst

2

BET surface area (m /g) Langmuir surface area (m2/g) t-plot external surface area (m2/g) micropore area (m2/g) total pore vol (cm3/g) micropore vol (cm3/g) av pore radius (nm)

CeO2

Co3O4/CeO2

57.93 96.10 52.52 5.41 0.18 2.25 × 10-3 6.23

38.30 71.11 37.40 0.85 0.12 2.15 × 10-4 6.48

impregnation, the particles seem to have more intense density. This finding can be strengthened with the nitrogen adsorption study results as shown in Table 2. It is clear that the micropore area decreased after the impregnation. The micropore area of the support is 5.41 m2/g, and after impregnation the area reduces to 0.85 m2/g and also the catalyst support has a higher specific surface area than its impregnated form. For example, the BET surface area of the support is 57.93 m2/g, whereas after impregnation the area is 38.30 m2/g. In Figure 3, the N2 adsorption isotherm of Co3O4/CeO2 is shown. As seen from the figure, the isotherm is of type IV, indicating the presence of mesoporosity. In the XRD analysis of Co3O4/CeO2 catalyst shown in Figure 4, two phases which are Co3O4 and CeO2 are observed. The peaks at diffraction angles 2θ ) 28.6, 33.5, 47.5, 56.4, and 59° characterize the CeO2 crystals, and the others represent the Co3O4 crystals. Additionally, for further understanding of the activity of Co3O4/CeO2 catalyst, the catalyst particles used in the reaction were recovered, washed with toluene, calcinated at 500 °C, and investigated. The images taken are shown in Figure 2c. As seen in Figure 2c, the particles were swollen after the reaction and the structure changed a little. They had a honeycomb shape, and because of the swelling, macropores became more insignificant while micropores in the particles became more significant. Before the reaction, peaks seemed to be sharper, indicating that after being used in the reaction the crystallite structure was corrupted. Characterization of the CeO2/Co3O4 catalyst was carried out to measure the weight loss as a result of the increase in sample

Figure 3. Nitrogen adsorption isotherm of Co3O4/CeO2 catalyst.

Figure 5. The effect of temperature on aniline degradation (no catalyst; aniline initial concentration, 1 g/L; pressure, 4 atm; reaction time, 120 min; air flow rate, 1.36 L/min for 300 mL of wastewater).

temperature. The result of thermogravimetric analysis indicates an overall weight loss of 18% which is the removal of the physisorbed water. 3.2. Determination of Optimum Operating Parameters. The experiment efficiency was evaluated according to the aniline % removal and the selectivities. They are expressed as aniline % removal ) initial aniline concn - final aniline concn × 100 initial aniline concn The selectivities are calculated as follows: S1 ) S2 ) S3 )

produced CO2 removed aniline

produced nitrogen removed aniline

produced NO3- + NO2removed aniline

3.2.1. Influence of Reaction Temperature and Catalyst Loading on Removal Efficiency. The influence of reaction temperature on aniline oxidation and selectivity was investigated between 100 and 150 °C at 0.5 and 2 g/L catalyst loading in the conditions as shown in Table 1. In addition to this, a reaction set without using a catalyst was performed. The progressions of the oxidation of aniline at various temperatures and catalyst loadings are shown in Figures 5-7. The reactions performed without any catalyst seem not to be very promising either, taking into account the parameter aniline removal as well as N2 formation. In this case the highest removal achieved was only 13% at the highest temperature value (150

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Figure 6. Effect of temperature on aniline degradation (0.5 g/L catalyst; aniline initial concentration, 1 g/L; pressure, 4 atm; reaction time, 120 min; air flow rate, 1.36 L/min for 300 mL of wastewater).

Figure 7. Effect of temperature on aniline degradation (2 g/L catalyst; aniline initial concentration, 1 g/L; pressure, 4 atm; reaction time, 120 min; air flow rate, 1.36 L/min for 300 mL of wastewater).

°C) studied. The selectivity of CO2 does not change with temperature, while the N2 and (NO3- and NO2) change significantly. As seen in Table 3, the selectivity toward CO2 formation was not affected seriously by the temperature increase, whereas the selectivity toward NO3- and NO2 formation increased a little when the temperature was increased to 125 °C. Further increase in temperature did not cause a significant effect. Even though increasing the temperature increased the N2 amount formed, the selectivity to its formation was affected unexpectedly especially at 125 °C. The reason for this behavior can be explained because the reaction chose the route of NH4+ formation (higher NH4+ amounts formed at this temperature value). As seen in Figure 5, 4.96% of the input nitrogen was in the form of NH4+, and as the temperature increased, this percentage decreased to 3.1%. Introduction of a Co3O4/CeO2 catalyst considerably increased the process efficiency (Figure 6). This result is consistent with the results reported in literature.17,11 With this gradual increase in the catalyst loading (0.5 g/L), the removal of aniline and the amount of N2 formed increased considerably. Both of these indicate that the catalyst has good activity in the CWAO of aniline. In this case the temperature effect was more considerable. Especially from 100 to 125 °C, both the removal of aniline and N2 formation increased. For example at 100 °C the removal was 5.48%, whereas at 125 °C the removal increased to 29.73%. Even the NO3- and NO2 formed seemed to be constant; as the temperature was increased, the selectivity toward the formation of them decreased (Table 3). Thus it may be concluded that the temperature of 150 °C was the optimum temperature value.

It is well-known that high catalyst concentration can influence the reaction and the selectivity of the product formed during aniline oxidation. In Figure 7, the results gained are plotted. For example at 100 °C, an increase in catalyst loading (from 0.5 to 2 g/L) increased both the conversion of aniline (from 5.48 to 10.15%) and N2 formation. But the effect was not as much as expected, especially at higher temperatures. The relationship between the conversion and the catalyst concentration did not appear to be proportional. For example, increasing the loading from 0.5 to 2 g/L, at 125 °C, just increased the removal from 29.73 to 34.44%. At a lower catalyst concentration (0.5 g/L) NH4+ seemed to be one of the major products formed. As seen in Figure 7 and Table 3 with an increase in the catalyst concentration NH4+ disappeared a little, whereas the selectivity to N2 increased, considerably. As Reddy and Mahajani reported that at lower catalyst loading a higher percentage of nitrogen was in the form of NH4+, but at higher catalyst loading, the degradation of the NH2 group in aniline resulted in more N2 gas.17 For the following step it was decided to continue with 0.5 g/L catalyst loading and 150 °C. 3.2.2. Influence of Reaction Time. At the decided optimum temperature and catalyst loading value, the effect of reaction time was investigated. As seen in Figure 8, the rate of removal of aniline was the highest in the first 30 min. The removal continued to increase until 120 min but not as fast as in the first 30 min of the reaction. The amount of NH4+ formed increased in the first 90 min and then started to decrease in the following 30 min. It seems that nitrogen was converted to NH4+ in the first 90 min and then converted into N2 gas, because, as it is seen in Figure 8, after 90 min, the N2 amount increased. In Table 3, it can be seen that, in the first 60 min, selectivity toward CO2 formation increased, and after this time it started to decrease until 120 min. For the following studies, it was decided to perform experiments at reaction duration of 120 min. 3.2.3. Influence of Air Flow Rate. In Figure 9, the results obtained at various air flow rates are given. As the air flow rate was increased, there was a considerable difference in aniline removal. This trend was followed until the flow rate was increased to 1.36 L/min. Further increase in the flow rate was not very effective. Increasing the air flow rate also had a positive effect on the N2 gas formation. If Table 3 is investigated, it can be seen that the air flow rate affected the selectivity to CO2 formation. As the flow rate was increased, the N2 gas formed was higher. For the next step, the air flow rate was suggested as 1.36 L/min. 3.2.4. Influence of Pressure. As the last parameter, the effect of pressure was tested. In the Figure 10, the results collected are plotted. From the results, it can be concluded that pressure was not so effective on the aniline removal. Increasing it from 3 to 4 atm had a positive but unrecordable effect in both removal of aniline and N2 gas formation. But further increases, although increasing the nitrogen formation, in general had no considerable effect especially on the aniline removal. Reddy and Mahajani also reported similar results on the effect of oxygen pressure on the formation of N2 gas. So it was decided that P ) 4 atm was the optimum operating pressure. 3.2.5. Reusability and Leaching of the Catalyst. To evaluate the stability of the Co3O4/CeO2 catalyst, two consecutive runs were performed by reusing the catalyst particles recovered by filtration, washed with toluene and calcinated at 500 °C. At the end it was found that there is no recordable loss of performance in the two consecutive runs. The highest removals


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Table 3. Selectivities to CO2 (S1), N2 (S2), and NO3 + NO2 (S3) for Each Set selectivities (%) set I

II

III

IV

catalyst loading (g/L)

pressure (atm)

temp (°C)

time (min)

air flow rate (L/min)

S1

S2

S3

0 0 0 0.5 0.5 0.5 2 2 2 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 3 4 5

100 125 150 100 125 150 100 125 150 150 150 150 150 150 150 150 150 150 150 150 150

120 120 120 120 120 120 120 120 120 30 60 90 120 150 120 120 120 120 120 120 120

1.36 1.36 1.36 1.36 1.36 1.36 1.36 1.36 1.36 1.36 1.36 1.36 1.36 1.36 0.23 0.62 1.36 1.72 1.36 1.36 1.36

5.51 9.89 4.82 6.86 14.75 10.69 6.17 7.27 4.85 3.77 20.30 19.02 10.69 11.16 25.50 18.68 10.69 16.29 14.28 10.69 17.92

57.18 20.64 53.31 21.71 38.49 41.11 49.79 76.74 78.22 42.79 32.86 29.43 41.11 52.16 29.11 40.76 41.11 45.19 44.30 41.11 46.98

12.40 27.15 22.84 51.14 7.34 7.38 28.22 1.66 1.19 5.75 5.97 5.72 7.38 0.85 11.28 3.75 7.38 4.81 5.58 7.38 5.03

with fresh catalyst and the reused catalyst were 35.15 and 34.94%, respectively. The metal leaching of catalyst might be important, since continuous leaching of metal ions is one of the direct causes of catalyst deactivation.18 To understand this, the concentrations of dissolved Ce and Co in the solution after oxidation were

Figure 8. The effect of reaction time on aniline degradation (aniline initial concentration, 1 g/L; temperature, 150 °C; pressure, 4 atm; catalyst loading, 0.5 g/L; air flow rate, 1.36 L/min for 300 mL of wastewater).

Figure 9. The effect of air flow rate on aniline degradation (aniline initial concentration, 1 g/L; temperature, 150 °C; pressure, 4 atm; catalyst loading, 0.5 g/L; reaction time, 120 min).

analyzed by atomic adsorption spectroscopy (Varian 10 Plus). The catalyst showed good chemical stability with no leaching ions. 4. Conclusions In this research the main aim was to study the efficiency of the CWAO of aqueous aniline to make the stream ecofriendly while minimizing the organic content and also byproducts such as ammonium, nitrate, and nitrite ions by selectively converting the N atom in aniline into N2 gas. Results show the following: From the characterization analysis it was found that, after impegnation, the surface area and the micropore area decreased. The N2 adsorption isotherm of the catalyst is of type IV, indicating the presence of mesoporosity. The following are found with the catalyst Co3O4 (10% wt)/ CeO2: In the following conditions the highest removal with the fresh catalyst was found to be 35.15%: temperature (°C), 150; pressure (atm), 4; catalyst loading (g/L), 0.5; reaction time (min), 120; air flow rate (L/min), 1.36. When the removal obtained is compared with the results in literature, it is seen that, in literature, even though the conversions obtained are high, the operating conditions are

Figure 10. Effect of pressure on aniline degradation (aniline initial concentration, 1 g/L; temperature, 150 °C; catalyst loading, 0.5 g/L; air flow rate, 1.36 L/min; reaction time, 120 min).

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severe. Especially the pressures are so high. With consideration of these severe conditions and from the economic point of view, at these moderate conditions, the removal obtained can be considered as reasonable. Even though Co3O4/CeO2 had low surface area (38.30 m2/ g), rather high conversions were obtained, indicating that cobalt is an effective metal for CWAO of aniline. And it is found to be promising in the conversion of the -NH2 group in aniline into N2 gas while minimizing the production of byproduct. At these conditions the highest removal with the reused catalyst was 34.94%, showing that Co3O4 is a stable catalyst. Literature Cited ¨ zler, M.; Ayar, A. Removal of aniline from (1) Gu¨rten, A.; Uc¸an, S.; O aqueous solution by PVC-CDAE ligand-exchanger. J. Hazard. Mater. 2005, 120, 381. (2) Masende, Z. Catalytic wet oxidation of organic wastes using platinum catalysts, Thesis, Technische Universiteit Eindhoven, Eindhoven, The Netherlands, 2004. (3) Aytimur, G.; Atalay, S. Treatment of Afyon alkaloid factory wastewater by biological oxidation and/or chemical oxidation. Energy Sources 2004, 26, 661. (4) Bıc¸aksız, Z.; Aytimur, G.; Atalay, S. Low pressure catalytic wet air oxidation of a high strength industrial wastewater using Fenton’s reagent. Water EnViron. Res. 2008, 540. (5) Luck, F. Wet air oxidation: past, present and future. Catal. Today 1999, 53, 81. (6) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet air oxidation. Ind. Eng. Chem. Res. 1995, 34, 2. (7) Levec, J.; Pintar, A. Catalytic wet-air oxidation processes: A review. Catal. Today 2007, 124, 172.

(8) Batygina, M. V.; Dobrynkin, N. M.; Noskov, A. S. Oxidation of organic substances in aqueous solutions over Ru catalysts by oxygen. AdV. EnViron. Res. 2000, 4, 123. (9) Donlagi, J.; Levec, J. Does the catalytic wet oxidation yield products more amenable to biodegradation. Appl. Catal., B 1998, 17, L1. (10) Barbier, J.; Oliviero, L.; Renard, B.; Duprez, D. Catalytic wet air oxidation of ammonia over M/CeO2 catalysts in the treatment of nitrogencontaining pollutants. Catal. Today 2002, 75, 29. (11) Oliviero, J.; Barbier, J.; Duprez, D. Wet air oxidation of nitrogencontaining organic compounds and ammonia in aqueous media. Appl. Catal., B 2003, 40, 163. (12) Dobrynkin, N. M.; Batygina, M. V.; Noskov, A. S. Solid catalysts for wet oxidation of nitrogen-containing organic compounds. Catal. Today 1998, 45, 257. (13) Garcia, J.; Gomes, H. T.; Serp, P.; Kalck, P.; Figueiredo, J. L.; Faria, J. L. Platinum catalysts supported on MWNT for catalytic wet air oxidation of nitrogen containing compounds. Catal. Today 2005, 101. (14) Chen, C.; Weng, H. Nanosized CeO2-supported metal oxide catalysts for catalytic reduction of SO2 with CO as a reducing agent. Appl. Catal., B 2005, 55, 115. (15) Cheng, X.; Wang, Z.; Wang, S.; Zhang, S.; Huang, W.; Wu, S. Preparation, characterization and catalytic properties of CuO/CeO2 system. Mater. Sci. Eng. 2005, C- 25, 516. (16) Kuang, W.; Fan, Y.; Chen, Y. Preparation, characterization, and catalytic properties of ultrafine mixed Fe-Mo oxide particles. J. Colloid Interface Sci. 1999, 215, 364. (17) Reddy, G. R.; Mahajani, V. V. Insight into wet oxidation of aqueous aniline over a Ru/SiO2 catalyst. Ind. Eng. Chem. Res. 2005, 44, 7320. (18) Yang, S.; Zhu, W.; Jiang, Z.; Chen, Z.; Wang, J. The surface properties and the activities in catalytic wet air oxidation over CeO2-TiO2 catalyst. Appl. Surf. Sci. 2006, 252, 8499.

ReceiVed for reView September 3, 2009 ReVised manuscript receiVed December 4, 2009 Accepted December 22, 2009 IE901383E

CeO2

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