Influence of Catalysts on the Preparation of Carbon ... - ACS Publications

Mar 27, 2009 - Department of Safety Health and EnVironmental Engineering, Ming-Chi UniVersity of Technology, Taishan. Township, Taipei County 243, ...
1 downloads 0 Views 1MB Size
Download PDF
4202

Ind. Eng. Chem. Res. 2009, 48, 4202–4209

Influence of Catalysts on the Preparation of Carbon Nanotubes for Toluene Oxidation Kui H. Chuang,† Zhen S. Liu,‡ Chi Y. Lu,§ and Ming Y. Wey*,† Department of EnVironmental Engineering, National Chung Hsing UniVersity, Taichung 402, Taiwan, R.O.C., Department of Safety Health and EnVironmental Engineering, Ming-Chi UniVersity of Technology, Taishan Township, Taipei County 243, Taiwan, R.O.C., and Department of Public Health, Chung Shan Medical UniVersity, Taichung 402, Taiwan, R.O.C.

This study aims to investigate the effect of catalysts on the preparation of carbon nanotubes (CNTs) and the catalytic properties of Co/CNT for toluene oxidation between 175 and 250 °C. A different ratio of Co to Mo supported on Al2O3 was used in the chemical vapor deposition method to synthesize the carbon nanotubes. Moreover, the catalytic activities of Al2O3-supported catalysts for toluene oxidation were also measured, and the results were compared with the catalytic activities of Co/CNT catalysts. To confirm the characterization of CNT and Co/CNT, the composition and morphology of various catalysts were analyzed by using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Raman spectra, BET, and X-ray photoelectron spectroscopy (XPS). The results indicated that the catalytic activity of Co/ CNT catalysts was higher than that of the Al2O3-supported catalysts. Furthermore, the catalytic activity of Co/CNT prepared by using 2.4% Co-0.6% Mo/Al2O3 for toluene oxidation was 100% at 225 °C. The results demonstrated that the removal efficiency of toluene depends on the species of catalysts synthesized on the carbon nanotubes. 1. Introduction Volatile organic compounds (VOCs) are major air pollutants which are emitted from industrial processes and automobile exhaust. VOCs are extremely toxic to human health and are also harmful to the environment. Many different methods are developed to control VOCs, including adsorption,1 absorption,2 biofiltration,3 thermal incineration, and catalytic combustion.4 Among these methods, catalytic combustion is most appealing due to its low reacting temperature and high removal efficiency. The catalytic efficiency of VOCs in catalytic reactions depends mostly on the supports and nature of the active site. Therefore, the choice of supports and catalysts is especially important. Two groups of catalysts used widely for VOCs oxidation include noble metal catalysts (Pt, Pd, Rh, Au) and base metal catalysts (Mn, Co, Cu, Fe, Ni). Noble metal catalysts possess high activity for the oxidation of VOCs at a low temperature.5-7 However, these catalysts are uneconomical for wide application due to their high costs. As a result, the use of base metal catalysts for VOCs oxidation has been investigated widely as the potential alternative.8,9 Among base metals, cobalt-supported catalysts are promising for the catalytic combustion of VOCs. To increase the stability and dispersion of active metal, supports such as Al2O3, SiO2, TiO2, CeO2, MgO, and ZrO210-13 are generally employed. For the catalytic combustion of VOCs, Al2O3 is considered as the best support because of its high surface area and excellent mechanical properties. However, cobalt species that react with Al2O3 during preparation or catalysis will lead to the formation of mixed compounds.14,15 Therefore, the use of carbon materials as a catalytic support for VOCs oxidation to prevent these problems has been explored in our previous paper.16 Carbon nanotubes (CNTs) that possess high surface * To whom correspondence should be addressed. Tel.: +886-422852455. Fax: +886-4-22862587. E-mail address: mywey@ dragon.nchu.edu.tw. † National Chung Hsing University. ‡ Ming-Chi University of Technology. § Chung Shan Medical University.

area and uniform porosity have been widely used as catalytic supports for heterogeneous catalysis reaction.17-19 The chemical vapor deposition (CVD) method is a CNT synthesis technique with relatively high yield and low cost. The structure and morphology of CNTs depends on the synthesis catalyst, carbon source, and synthesis temperature. The CNTs were synthesized by using Fe, Co, Ni, Mo, or mixed metal.20-24 Many investigations have discussed the catalytic activity of CNT with different treatments. For instance, Hsu et al. reported25 the PtRu catalyst as supported on CNT for methanol electro-oxidation. In our previous paper,26 the CNT-supported Co composite showed excellent catalytic activity for the oxidation of CO. However, the catalytic activity of CNT as synthesized by different catalysts for the oxidation of VOCs has not yet been examined. Therefore, this study aims to determine the oxidative conversion of toluene by using Co supported on the different types of CNTs synthesized by different catalysts. Different catalysts (Co and Co-Mo) that were supported on Al2O3 were used in the chemical vapor deposition method to synthesize the CNT supports. The catalytic activities of the Al2O3-supported catalysts for toluene oxidation were also estimated, and the results were compared with the catalytic activities of the Co/CNT catalysts. To confirm the characterization of the catalysts, the composition and morphology of the Al2O3-supported catalysts and CNTsupported catalysts were analyzed by using field emission Table 1. Code and Compositions of Various Catalysts catalyst

support

Co content (wt %)

3.0% Co/Al2O3 2.4% Co-0.6% Mo/Al2O3 1.5% Co-1.5% Mo/Al2O3 Co/CNT-A Co/CNT-B Co/CNT-C

Al2O3 Al2O3 Al2O3 CNT-Aa CNT-Ba CNT-Ca

3.0 2.4 1.5 3.0 3.0 3.0

Mo content (wt %) 0.6 1.5

a CNT-A, CNT-B, and CNT-C were prepared by using 3.0% Co/ Al2O3, 2.4% Co-0.6% Mo/Al2O3, and 1.5% Co-1.5% Mo/Al2O3 as catalysts, respectively.

10.1021/ie801692g CCC: $40.75  2009 American Chemical Society Published on Web 03/27/2009

Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4203

Figure 1. FESEM images of the CNTs prepared by using different catalysts: (a) 3.0% Co/Al2O3; (b) 2.4% Co-0.6% Mo/Al2O3; (c) 1.5% Co-1.5% Mo/ Al2O3.

Figure 2. TEM images of Al2O3-supported catalysts: (a) 3.0% Co/Al2O3; (b) 2.4% Co-0.6% Mo/Al2O3; (c) 1.5% Co-1.5% Mo/Al2O3.

scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Raman spectra, BET, and X-ray photoelectron spectroscopy (XPS). 2. Experimental Section 2.1. Preparation of Al2O3-Supported Catalyst. Co and Mo that were supported on Al2O3 were prepared by the excesssolution impregnation method. The total metal content was kept at 3 wt %. The precursors of Co(NO3)2 · 6H2O and (NH4)6Mo7O24 · 4H2O were dissolved in distilled water and the support of Al2O3 was added into the solution. Then the samples were dried in an oven at 80 °C and calcined in hydrogen at a temperature of 500 °C. The nomenclature and compositions of the catalysts are listed in Table 1.

2.2. Synthesis of CNTs and Preparation of Co/CNTs. The synthesis of CNTs was carried out at atmospheric pressure via catalytic decomposition of acetylene in a horizontal quartz tubular reactor (34 mm i.d.). For the synthesis of the CNTs, 3 g of the Al2O3-supported catalysts were placed in the middle part of the reactor, heated in H2 up to 550 °C and then in argon at 700 °C. Acetylene was introduced at this temperature at a flow rate of 30 cm3 min-1 for 1 h. Finally, the sample was cooled down in the flow of pure argon. The yield of carbon nanotubes over 3% Co/Al2O3, 2.4% Co-0.6% Mo/Al2O3, and 1.5% Co-1.5% Mo/Al2O3 is 0.31, 0.38, and 0.36 g CNT g catalyst-1, respectively. The produced CNT was purified by first dissolving it in hydrochloric acid (37%) for 3 days to remove the impurities, followed by filtering the solution. The remaining solids were

4204 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009

Figure 3. TEM images of the CNTs prepared by using different catalysts: (a) 3.0% Co/Al2O3; (b) 2.4% Co-0.6% Mo/Al2O3; (c) 1.5% Co-1.5% Mo/Al2O3.

Figure 4. TEM images of the different CNT-supported catalysts: (a) Co/CNT-A; (b) Co/CNT-B; (c) Co/CNT-C.

extensively washed with water and were collected after drying. The sample is used as a support for Co catalyst preparation. The Co/CNT catalysts were prepared by the excess-solution impregnation method. The precursor was Co(NO3)2 · 6H2O, and the total metal content was kept at 3 wt %. The samples were

dried in an oven at 80 °C and then calcined at 500 °C for 3 h in the presence of hydrogen to obtain the Co/CNT catalysts. 2.3. Characterization of Catalysts. Field emission scanning electron microscopy (FESEM) was used for the morphology analysis of the samples. FESEM images were obtained by a

Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4205

with VOC during the VOC catalytic oxidation, and it produced CO2 and H2O. In the toluene catalytic oxidation process, the main reaction is the following: C6H5CH3 + 9O2 f 7CO2 + 4H2O

(1)

The conversion of toluene selectivity of CO2 was calculated using the following equations: conversion of toluene )

selectivity of CO2 )

(toluenein - tolueneout) × 100% toluenein (2) COout 2

7(toluenein - tolueneout)

× 100% (3)

3. Result and Discussion

Figure 5. Raman of the different CNT-supported catalysts (a) Co/CNT-A; (b) Co/CNT-B; (c) Co/CNT-C.

JEOL JSM-6700F electron microscope using an accelerating voltage of 5 kV. In order to enhance their conductivity, samples were placed in aluminum stubs and coated with a platinum layer by sputtering. Transmission electron microscopy (TEM) observations were made with a JEOL JEM-1200CX Π microscope that operated at 120 keV to observe the dispersion of active site on support surface. The samples were crushed in the mortar and suspended in ethanol. After ultrasonic dispersion, a droplet was deposited on a copper grid supporting a perforated carbon film. Raman analysis of deposited carbon was carried out with Raman spectroscopy (Renishaw system 1000) under ambient conditions. The Raman spectra were recorded by using a 514.5 nm argon ion laser that equipped with a charge-coupled device (CCD) detector. X-ray photoelectron spectroscopic (XPS) spectra of the fresh samples were collected in a PHI 5000 Multifunctional electron spectrometer under monochromatic A1 KR radiation. The anode was operated at 12 kV and 15 mA. BET (Brunauer-Emmett-Teller method) surface area and pore size were measured at 77 K on a PMI’s BET sorptometer (BET-201-AEL). 2.4. Activity Test. Catalytic oxidation of toluene was determined at atmospheric pressure in a fixed-bed flow reactor. The toluene vapor was carried by nitrogen from a saturator filled with liquid toluene. The concentration of toluene was 275 ( 10 ppm. The material and inner diameter of the reactor were quartz and 16 mm, respectively. The catalysts of 0.5 g were placed on a quartz filter board. A thermocouple was placed in the center of the catalyst bed to record the reaction temperature and was also used to control the furnace. The reactions were performed in the temperature range of 175-250 °C. The space velocity (SV) of the reactant flows was 35 000 h-1 under atmospheric pressure. The concentrations of CO2 and toluene at the inlet and outlet of the reactor were monitored by an online flue gas analyzer (Horiba, PG-250) and online GC (Agilent, GC-6890N), respectively. In the present work, most experimental conditions produced only toluene; CO, CO2, and other byproduct were not be detected. Before the experiments were carried out, the system was tested for its stability by repeating it twice to improve the accuracy of the measured data. From the respect of catalytic oxidation, the oxygen may be reacted

3.1. Catalyst Characterization. 3.1.1. FESEM Analysis. The FESEM photographs of the produced CNTs prepared by different catalysts supported on Al2O3 are shown in Figure 1. A high abundance of produced CNTs is evident by chemical vapor deposition over three catalysts (3.0% Co/Al2O3, 2.4% Co-0.6% Mo/Al2O3, and 1.5% Co-1.5% Mo/Al2O3). It seems clear that the Co catalyst and Co-Mo catalyst were successfully used to produce CNTs. Additionally, the surfaces of the produced CNTs synthesized by 3.0% Co/Al2O3 and 1.5% Co-1.5% Mo/Al2O3 are smooth (see magnified images in Figure 1). However, the surfaces of CNT synthesized by 2.4% Co-0.6% Mo/Al2O3 show defects. 3.1.2. TEM Analysis. Figure 2a-c shows the TEM images of 3.0% Co/Al2O3, 2.4% Co-0.6% Mo/Al2O3, and 1.5% Co-1.5% Mo/Al2O3, respectively. A comparison of the three TEM images shows that the 2.4% Co-0.6% Mo/Al2O3 catalyst had finely dispersed catalyst particles on Al2O3. The results show that the addition of less Mo improves the catalysts dispersion on Al2O3. Similar results have been report in the case of Ni-Mo/ γ-Al2O3 system with different Mo contents.27 Figure 3a-c shows the TEM images of the produced CNTs synthesis from 3.0% Co/Al2O3, 2.4% Co-0.6% Mo/Al2O3, and 1.5% Co-1.5% Mo/Al2O3, respectively. The nanotubes are hollow with outer-diameters of the CNTs synthesis from 3.0% Co/Al2O3, 2.4% Co-0.6% Mo/Al2O3, and 1.5% Co-1.5% Mo/ Al2O3 of about 10-20, 10-15, and 5-25 nm (as shown in Figure 3), respectively. The results demonstrated that the diameter of the CNT is correlated with the Co/Mo atomic ratio, and the more uniform diameter of the CNT is synthesized by using 2.4% Co-0.6% Mo/Al2O3. Figure 4a-c shows the TEM images of Co/CNT-A, Co/CNTB, and Co/CNT-C, respectively. A comparison of the three TEM images show, that the Co/CNT-B catalyst had finely dispersed cobalt species on CNT-B. The average sizes of the Co particles follow the order: CNT-B (4 nm) < CNT-A (5 nm) < CNT-C (30 nm), and the particle sizes are correlated with the kinds of CNT support. This is attributed to the presence of graphite defects and edges on Co/CNT-B as observed from the FESEM image (Figure 1b). The TEM image of Co/CNT-A catalyst almost dispersed cobalt particles on CNT-A surface and a partial of cobalt particles were aggregated. As shown in Figure 4c, the Co particles were aggregated over the Co/CNT-C catalyst. 3.1.3. Raman Analysis. Figure 5 shows the Raman spectra of the Co/CNT catalysts. The Raman peaks appearing in the 1500-1605 and 1250-1450 cm-1 regions correspond to the stretching mode of crystal graphite (graphitic band, G-band) and the poor crystallization of graphite (disorder band, D-band),

4206 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009

Figure 6. Co 2p spectra of the different catalysts: (a) 3% Co/Al2O3; (b) 2.4% Co-0.6% Mo/Al2O3; (c) 1.5% Co-1.5% Mo/Al2O3; (d) Co/CNT-A; (e) Co/CNT-B; (f) Co/CNT-C. Table 2. Surface Concentration of Co and Phase Content by XPS catalyst 3% Co/Al2O3 2.4% Co-0.6% Mo/Al2O3 1.5% Co-1.5% Mo/Al2O3 Co/CNT-A Co/CNT-B Co/CNT-C

Co0 (%)

Co3O4 (%)

Co2+ (%)

CoAl2O4 (%)

3.2 5.9 9.8

18.4

41.5 60.3 44.2 44.6 34.9

36.9 33.8 46.0

12.5

55.4 100 52.6

Table 3. BET Surface Area and Structural Characteristics of CNT-Supported Catalysts catalyst

BET surface area (m2 g-1)

pore volume (cm3 g-1)

average pore diameter (Å)

Co/CNT-A Co/CNT-B Co/CNT-C

76.8 86.3 70.7

0.23 0.21 0.21

119.0 95.2 119.6

respectively.28,29 The ratio (ID/IG) of D-band intensity (ID) to G-band intensity (IG) is an index to determine the CNT structure. When the value of ID/IG was more than 1, the graphite of sample was more poor crystallization. The ID/IG results for all samples are shown in Figure 5. The value of ID/IG of the catalysts follows the order Co/CNT-B > Co/CNT-A > Co/CNT-C, which indicates the more presence of graphite defects and edges in the Co/CNT-B catalyst. In our experiment, the produced CNTs were purified by first dissolving it in hydrochloric acid (37%) for 3 days to remove the impurities and Al2O3-supported catalysts. Thus, the D-band may be contributed mainly from defects of the samples. The defects of CNT-supported catalyst can be occurred in two ways. One is the graphite defect of a CNT. In a CNT, carbon atoms are arranged in regular honeycomb lattice (hexagon). However, pentagons and heptagons will exist on curve carbon nanotubes. The graphite defect means pentagons and heptagons on carbon nanotubes. CNTs have defects in the form of structural deficiencies in hexagon. The other is the edge

Figure 7. Toluene conversion as a function of reaction temperature using various catalysts that were supported on Al2O3: (9) 3.0% Co/Al2O3, (•) 2.4% Co-0.6% Mo/Al2O3, (2) 1.5% Co-1.5% Mo/Al2O3.

of a CNT. The “edge” means the damaging sidewall of carbon nanotubes and the open-end of carbon nanotubes (as shown in Figure 1b). 3.1.4. X-ray photoelectron spectroscopy (XPS). The XPS technique, through the detection of Co 2p photoelectrons, determined the states of the fresh catalyst before toluene oxidation reaction. Figure 6 presents the XPS spectra of 3% Co/Al2O3, 2.4% Co-0.6% Mo/Al2O3, 1.5% Co-1.5% Mo/ Al2O3, Co/CNT-A, Co/CNT-B, and Co/CNT-C. The binding energy of the spin-orbit components Co 2p3/2 and Co 2p1/2 peaks were 780.3 and 796.2 eV.30 The percent intensities of different cobalt phases Co0, Co3O4, Co2+, and CoAl2O4 present over different catalysts are listed in Table 2. In Al2O3-supported catalysts, the major cobalt species of 3% Co/Al2O3, 2.4%

Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4207

Figure 8. Toluene conversion as a function of reaction temperature using different CNTs as support: (9) Co/CNT-A, (•) Co/CNT-B, (2) Co/ CNT-C.

Figure 9. Toluene conversion on various catalysts that were supported on Al2O3 and CNT at 250 °C.

Co-0.6% Mo/Al2O3, and 1.5% Co-1.5% Mo/Al2O3 is Co2+, Co2+, and CoAl2O4, respectively. In Table 2, one can see that the addition of less Mo may decrease the formation of CoAl2O4, and the percentages of the CoAl2O4 follow the order: 2.4% Co-0.6% Mo/Al2O3 (33.8%) < 3% Co/Al2O3 (36.9%) < 1.5% Co-1.5% Mo/Al2O3 (46.0%). The Co 2p binding energies of the 3% Co/Al2O3 and 2.4% Co-0.6% Mo/Al2O3 catalyst are assigned to Co2+ for the formation of CoO or CoMoO4.31 In

CNT-supported catalysts, the fraction of Co3O4 on CNT-A, CNT-B, and CNT-C is 55.4%, 100%, and 52.6%, respectively. It can be found that the fraction of Co3O4 follow the order CNT-B > CNT-A > CNT-C. 3.1.5. BET Surface Area Measurement. The BET specific surface area, pore volume, and average pore size of CNTsupported catalysts have been summarized in Table 3. The specific surface area were 76.8, 86.3, and 70.7 m2 g-1 for Co/ CNT-A, Co/CNT-B, and Co/CNT-C. When comparing Table 3 with Figure 4, it can be found that the higher BET surface analysis result of Co/CNT-B was caused by the Co particles high dispersion on CNT-B than with other CNT-supported catalysts. Trepanier et al.18 show that the specific surface area, pore volume, and average pore size of cobalt supported on carbon nanotubes is about 145-160 m2 g-1, 0.41-0.50 cm3 g-1, and 50-57 Å, respectively. In our study, the lower surface area was caused by the smaller pore volume and bigger average pore diameter than in Trepanier’s work. 3.2. Catalytic Activities of Various Catalysts for Toluene Oxidation. The blank experiment results show that the toluene conversions over three kinds of as-prepared CNTs were low and less than 20% at a reaction temperature of 250 °C (not show). Thus, the affect of as-prepared CNTs on the toluene conversion was be neglected in our experiment. Figure 7 shows toluene conversion as function of the reaction temperature using various catalysts that were supported on Al2O3. The inlet feed stream comprised of 275 ( 10 ppm toluene and 7% O2 at a space velocity of 35 000 h-1. The results show that the catalytic activities of various catalysts supported on Al2O3 increase as reaction temperature increases. Figure 7 reveals that the activities of various catalysts supported on Al2O3 follow the order 2.4% Co-0.6% Mo/Al2O3 > 3.0% Co/Al2O3 > 1.5% Co-1.5% Mo/Al2O3. In Al2O3-supported catalysts, the XPS spectra indicated that the percentage of the CoAl2O4 follow the order: 2.4% Co-0.6% Mo/Al2O3 (33.8%) < 3% Co/Al2O3 (36.9%) < 1.5% Co-1.5% Mo/Al2O3 (46.0%). Comparing metal chemical state with catalytic activity, Gao et al.32 found that the CoAl2O4 showed lower activity than Co2+. This result agrees with our experiment. The catalytic activity of 2.4% Co-0.6% Mo/Al2O3 and 3% Co/Al2O3 is higher than that of 1.5% Co-1.5% Mo/Al2O3 (Figure 7). A comparison of the TEM images of Al2O3-supported catalysts shows that the 2.4% Co-0.6% Mo/Al2O3 catalyst had finely dispersed catalyst particles on Al2O3. Hence, the results demonstrated that the catalytic activity of 2.4% Co-0.6% Mo/Al2O3 was high performance. The results show that the modification of Co/Al2O3 by adding a lesser amount of Mo can improve the conversion of toluene. Toluene conversion as a function of reaction temperature that uses CNT as support, which was prepared by different catalysts,

Figure 10. CO2 selectivity with respect to the conversion of toluene as a function of reaction temperature over Co/CNT-A and Co/CNT-B catalysts: (a) Co/CNT-A and (b) Co/CNT-B.

4208 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009

is shown in Figure 8. The results show that the catalytic activities of cobalt catalysts supported on different CNTs increase when the reaction temperature increases. The catalytic activity sequence of toluene oxidation with respect to CNT-supported cobalt catalysts was Co/CNT-B > Co/CNT-A > Co/CNT-C. The surfaces of the Co/CNT-B catalyst are not smooth as verified from FESEM analysis. The Raman analysis results show that the more presence of graphite defects in the Co/CNT-B catalyst. The defects on carbon nanotubes surface would improve the dispersion of catalyst on carbon nanotubes in the catalyst preparation. Chai et al.33 point out CNTs with open tips have more advantages than the enclosed-tip CNTs because the former provides the hollow parts which can be readily utilized for filling and adsorption purposes. Choi et al.34 reported the selective deposition of silver nanoparticles on the defect of single-walled carbon nanotubes. Our study agrees with those researches. The TEM image proved that the cobalt particles are well dispersed on the CNT-B. The higher BET surface analysis result of Co/ CNT-B was caused by the Co particles high dispersion on CNT-B than with other CNT-supported catalysts. In addition, the XPS results indicated that the major cobalt species of the CNT-supported catalysts is Co3O4. The fraction of Co3O4 follow the order CNT-B > CNT-A > CNT-C. Therefore, the conclusions may be concluded that the Co/CNT-B catalyst has high fraction of Co3O4 phases and good dispersion of cobalt which will contribute to high catalytic activity for conversion of toluene at 225 °C (as shown in Figure 8). Toluene conversion on various catalysts at 250 °C is shown in Figure 9. The results indicate that the oxidative efficiency of toluene over CNT-supported catalysts is higher than that over Al2O3-supported catalysts. The major cobalt species of CNTsupported catalysts are Co3O4 as determined by the XPS results. These results showed that the toluene conversion of the CNTsupported catalysts was better than that of the Al2O3-supported catalysts. This means that the Co3O4 species are more efficient for toluene oxidation. Several researchers have reported that Co3O4 is an effective catalyst for VOCs oxidation.11,35,36 The activities of Co/CNT-A and Co/CNT-B for the removal of toluene at 250 °C are similar (approximately 80%), so the selectivity of CO2 will further be discussed to estimate toluene conversion into CO2. The CO2 selectivity with respect to the conversion of toluene as a function of reaction temperature over Co/CNT-A and Co/CNT-B catalysts is shown in Figure 10. The results show that the selectivity of CO2 of Co/CNT-A is lower than that of Co/CNT-B when the reaction temperature is higher than 200 °C. Therefore, the Co/CNT-B catalyst can effectively catalyze toluene and transform it into CO2 when the reaction temperature is higher than 200 °C. 4. Conclusion The influence of catalysts on the preparation of carbon nanotubes and the catalytic properties of Co/CNT for toluene oxidation between 175 and 250 °C were investigated. The inlet feed stream comprised of 275 ( 10 ppm toluene and 7% O2 in N2 at a space velocity of 35 000 h-1. A different ratio of Co to Mo that was supported on Al2O3 was used in the chemical vapor deposition method to synthesize the carbon nanotubes. Moreover, the catalytic activities of the Al2O3-supported catalysts for toluene oxidation were also estimated, and the results were compared with the catalytic activities of Co/CNT catalysts. The findings showed that the toluene conversion of the CNTsupported catalysts were higher than that of the Al2O3-supported catalysts because the former that provided Co3O4 species were more efficient for toluene oxidation. They also indicate that the

catalytic activity of Co/CNT-B prepared by using 2.4% Co-0.6% Mo/Al2O3 for toluene oxidation was 100% at 225 °C. The results can be explained by the Co particles being well dispersed in the Co/CNT-B catalyst, and the Co/CNT had structural defects that can be verified by FESEM image and Raman spectra. The XPS spectra indicated that the cobalt oxide of the CNTsupported catalysts contains Co3O4. Hence, the results demonstrated that the removal efficiency of toluene depends on the species of catalysts synthesized on the carbon nanotubes. Literature Cited (1) Das, D.; Gaur, V.; Verma, N. Removal of volatile organic compound by activated carbon fiber. Carbon 2004, 42, 2949. (2) Lalanne, F.; Malhautier, L.; Roux, J.-C.; Fanlo, J.-L. Absorption of a mixture of volatile organic compounds (VOCs) in aqueous solutions of soluble cutting oil. Bioresour. Technol. 2008, 99, 1699. (3) Pagans, E.; Font, X.; Sanchez, A. Coupling composting and biofiltration for ammonia and volatile organic compound removal. Biosyst. Eng. 2007, 97, 491. (4) Centeno, M. A.; Paulis, M.; Montes, M.; Odriozola, J. A. Catalytic combustion of volatile organic compounds on Au/CeO2/Al2O3 and Au/Al2O3 catalysts. Appl. Catal. A: Gen. 2002, 234, 65. (5) Gao, D.; Zhang, C.; Wang, S.; Yuan, Z.; Wang, S. Catalytic activity of Pd/Al2O3 toward the combustion of methane. Catal. Commun. 2008, 9, 2583. (6) Demoulin, O.; Le Clef, B.; Navez, M.; Ruiz, P. Combustion of methane, ethane and propane and of mixtures of methane with ethane or propane on Pd/γ-Al2O3 catalysts. Appl. Catal. A: Gen. 2008, 344, 1. (7) Kaisare, N. S.; Deshmukh, S. R.; Vlachos, D. G. Stability and performance of catalytic microreactors: Simulations of propane catalytic combustion on Pt. Chem. Eng. Sci. 2008, 63, 1098. (8) Scir, S.; Minic, S.; Crisafulli, C.; Galvagno, S. Catalytic combustion of volatile organic compounds over group IB metal catalysts on Fe2O3. Catal. Commun. 2001, 2, 229. (9) Iamarino, M.; Chirone, R.; Lisi, L.; Pirone, R.; Salatino, P.; Russo, G. Cu/γ-Al2O3 catalyst for the combustion of methane in a fluidized bed reactor. Catal. Today 2002, 75, 317. (10) Kim, S. C. The catalytic oxidation of aromatic hydrocarbons over supported metal oxide. J. Hazard. Mater. 2002, 91, 285. (11) Wyrwalski, F.; Lamonier, J. F.; Siffert, S.; Gengembre, L.; Aboukais, A. Modified Co3O4/ZrO2 catalysts for VOC emissions abatement. Catal. Today 2007, 119, 332. (12) Liotta, L. F.; Di Carlo, G.; Pantaleo, G.; Deganello, G. Catalytic performance of Co3O4/CeO2 and Co3O4/CeO2-ZrO2 composite oxides for methane combustion: Influence of catalyst pretreatment temperature and oxygen concentration in the reaction mixture. Appl. Catal. B: EnViron. 2007, 70, 314. (13) Ulla, M. A.; Spretz, R.; Lombardo, E.; Daniell, W.; Knozinger, H. Catalytic combustion of methane on Co/MgO: characterisation of active cobalt sites. Appl. Catal. B: EnViron. 2001, 29, 217. (14) El-Shobaky, G. A.; Ghozza, A. M.; El Warraky, A. A.; Mohamed, G. M. Surface and catalytic investigations of Co3O4-MoO3/Al2O3 system. Colloid Surf. A: Physicochem. Eng. Asp. 2003, 219, 97. (15) Solsona, B.; Davies, T. E.; Garcia, T.; Va´zquez, I.; Dejoz, A.; Taylor, S. H. Total oxidation of propane using nanocrystalline cobalt oxide and supported cobalt oxide catalysts. Appl. Catal. B: EnViron. 2008, 84, 176. (16) Lu, C.-Y.; Wey, M.-Y.; Chen, L.-I. Application of polyol process to prepare AC-supported nanocatalyst for VOC oxidation. Appl. Catal. A: Gen. 2007, 325, 163. (17) Guczi, L.; Stefler, G.; Geszti, O.; Koppany, Z.; Konya, Z.; Molnar, M.; Kiricsi, I. CO hydrogenation over cobalt and iron catalysts supported over multiwall carbon nanotubes: Effect of preparation. J. Catal. 2006, 244, 24. (18) Tavasoli, A.; Abbaslou, R. M. M.; Trepanier, M.; Dalai, A. K. Fischer-Tropsch synthesis over cobalt catalyst supported on carbon nanotubes in a slurry reactor. Appl. Catal. A: Gen. 2008, 345, 134. (19) Shen, J.; Hu, Y.; Li, C.; Qin, C.; Ye, M. Pt-Co supported on singlewalled carbon nanotubes as an anode catalyst for direct methanol fuel cells. Electrochim. Acta 2008, 53, 7276. (20) Emmenegger, C.; Bonard, J. M.; Mauron, P.; Sudan, P.; Lepora, A.; Grobety, B.; Zuttel, A.; Schlapbach, L. Synthesis of carbon nanotubes over Fe catalyst on aluminium and suggested growth mechanism. Carbon 2003, 41, 539.

Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4209 (21) Tran, K. Y.; Heinrichs, B.; Colomer, J.-F.; Pirard, J.-P.; Lambert, S. Carbon nanotubes synthesis by the ethylene chemical catalytic vapour deposition (CCVD) process on Fe, Co, and Fe-Co/Al2O3 sol-gel catalysts. Appl. Catal. A: Gen. 2007, 318, 63. (22) Lamouroux, E.; Serp, P.; Kihn, Y.; Kalck, P. Identification of key parameters for the selective growth of single or double wall carbon nanotubes on FeMo/Al2O3 CVD catalysts. Appl. Catal. A: Gen. 2007, 323, 162. (23) Niu, Z.; Fang, Y. Effect of temperature for synthesizing singlewalled carbon nanotubes by catalytic chemical vapor deposition over MoCo-MgO catalyst. Mater. Res. Bull. 2008, 43, 1393. (24) Alvarez, W. E.; Kitiyanan, B.; Borgna, A.; Resasco, D. E. Synergism of Co and Mo in the catalytic production of single-wall carbon nanotubes by decomposition of CO. Carbon 2001, 39, 547. (25) Hsu, N.-Y.; Chien, C.-C.; Jeng, K.-T. Characterization and enhancement of carbon nanotube-supported PtRu electrocatalyst for direct methanol fuel cell applications. Appl. Catal. B: EnViron. 2008, 84, 196. (26) Lu, C.-Y.; Wey, M.-Y. The performance of CNT as catalyst support on CO oxidation at low temperature. Fuel 2007, 86, 1153. (27) Youn, M. H.; Seo, J. G.; Kim, P.; Song, I. K. Role and effect of molybdenum on the performance of Ni-Mo/γ-Al2O3 catalysts in the hydrogen production by auto-thermal reforming of ethanol. J. Mol. Catal. A: Chem. 2007, 261, 276. (28) Tuinstra, F.; Koenig, J. L. Raman Spectrum of Graphite. J. Chem. Phys. 1970, 53, 1126. (29) Ferrari, A. C.; Robertson, J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. ReV. B 2000, 61, 14095.

(30) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Mullenberg, G. E. Handbook of X-Ray Photoelectron Spectroscopy; PerkinElmer: Minnesota, 1978. (31) Okamoto, Y.; Imanaka, T.; Teranishi, S. Surface structure of CoOMoO3/Al2O3 catalysts studied by X-ray photoelectron spectroscopy. J. Catal. 1980, 65, 448. (32) Gao, X. X.; Huang, C. J.; Zhang, N. W.; Li, J. H.; Weng, W. Z.; Wan, H. L. Partial oxidation of methane to synthesis gas over Co/Ca/Al2O3 catalysts. Catal. Today 2008, 131, 211. (33) Chai, S.-P.; Zein, S. H. S.; Mohamed, A. R. The effect of reduction temperature on Co-Mo/Al2O3 catalysts for carbon nanotubes formation. Appl. Catal. A: Gen. 2007, 326, 173. (34) Choi, S. K.; Chun, K.-Y.; Lee, S.-B. Selective decoration of silver nanoparticles on the defect sites of single-walled carbon nanotubes. Diam. Relat. Mat.,in press. (35) Yuranov, I.; Dunand, N.; Kiwi-Minsker, L.; Renken, A. Metal grids with high-porous surface as structured catalysts: preparation, characterization and activity in propane total oxidation. Appl. Catal. B: EnViron. 2002, 36, 183. (36) Liotta, L. F.; Di Carlo, G.; Pantaleo, G.; Deganello, G. Co3O4/ CeO2 and Co3O4/CeO2-ZrO2 composite catalysts for methane combustion: Correlation between morphology reduction properties and catalytic activity. Catal. Commun. 2005, 6, 329.

ReceiVed for reView November 6, 2008 ReVised manuscript receiVed February 11, 2009 Accepted March 10, 2009 IE801692G