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Microwave-Assisted Preparation of TiO2/Activated Carbon Composite Photocatalyst for Removal of Methanol in Humid Air Streams Yong Tao, Chang-Yu Wu,* and David W. Mazyck Department of EnVironmental Engineering Sciences, UniVersity of Florida, 406 Black Hall, P.O. Box 116450, GainesVille, Florida 32611
TiO2/activated carbon composite photocatalyst was prepared by a microwave-assisted impregnation method and was employed for the removal of methanol from humid air streams. A commercial microwave oven (800 W) was used as the microwave source. Under 2450 MHz microwave irradiation, titanium tetra-isopropoxide (TTIP) was quickly hydrolyzed and anatase TiO2 was formed in a short time (17.9 MΩ/ cm), and the suspension was stirred overnight to reach equilibrium. The sample was then filtered, and the pH of the filtrate was measured by an Accumet AP71 pH meter.26 The results are listed in Table 2 and showed that the pH of the carbon surface increased after the microwave process. This may result from the decomposition of oxygenated surface groups. Because of the instability of F400 AC under a high power level, this level was not used in subsequent studies and only power levels equal to or less than medium were used. Table 2 also lists BET surface area and total pore volume of virgin F400 AC and the F400 AC after 20 min of medium level microwave irradiation. As shown, the microwave process did not significantly affect the specific surface area or total pore volume of F400 AC. This also proved that the F400 AC was stable under medium power level irradiation. Because carbon and 2-propanol solvent are flammable, it may not be safe to run the microwave process in air, if the vapor pressure is high. In each experiment, 1.00 g of carbon and 1 mL of TTIP solution were used because these conditions were deemed safe when applying under a medium or lower power level. As far as the
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Figure 4. SEM images of TiO2/AC samples.
Figure 3. Weight loss curves of TiO2/AC samples (based on 1 g of AC).
flammability is concerned, it can be controlled by using inert protective gas. Figure 3 shows the weight loss in the preparation of samples 1-4. The weight of each sample was normalized by 1.00 g of AC. The results showed that as the irradiation intensity increased (i.e., higher microwave power), weight loss occurred faster. Considering that heat was dissipated to the environment through the surface continuously, the bulk temperature and heating rate of the sample under different microwave power levels should be different. On the basis of energy balance, the temperature change can be described as
FCp
dT ) Qabs - Qloss dt
(2)
where Qabs is the total adsorbed microwave power in the material and Qloss is the total heat loss including the heat loss of evaporation and the heat loss of thermal convection, conduction, and irradiation.27 Qabs is dependent on qabs, the microwave power density absorbed by the material, which is related to the electric field by the following equation:
qabs ) $eff′′|E|2
(3)
where ω is the angular frequency, |E| is the root-mean-square magnitude of the electric field in the material, and eff′′ is the effective loss factor.27 Information on these physical parameters is not available in the literature to quantitatively predict the bulk temperature. However, the equation can be used to qualitatively predict the trend. Although the bulk temperature was not measured because of facility limitation, it can be conjectured that the bulk temperature and heating rate increased with the power level. Assuming that all the TTIP was converted into TiO2, 0.13 g of TiO2 would be generated. The weights of samples 1-4 were 1.13, 1.13, 1.32, and 1.10 g, respectively. Clearly, the low power level was not enough to vaporize the chemicals, and there was still volatile material (solvent, byproducts, and/or unhydrolyzed TTIP) adsorbed on sample 3. Hence, low power level was not considered in subsequent experiments. Considering the weight loss of virgin AC under microwave irradiation (0.03 g), the expected weight was 1.10. Therefore, the TTIP conversions of samples 1 and 2 perhaps were also incomplete. Another important parameter is the water content. When the microwave power level is the same (samples 1 and 4), the weight loss depends on the water content. Because the water content added
Figure 5. Cross section of sample 1.
to sample 4 was less, the bulk temperature and heating rate of sample 4 should be higher than that of sample 1. Consequently, the weight loss of sample 4 was faster to reach the final stage than that of sample 1. The TiO2 loading, BET surface area, and total pore volume of the TiO2/AC sample are also listed in Table 1. The ash content of F400 AC after microwave process was used to calculate the TiO2 loading. The various preparation conditions listed in Table 1 did not exhibit significant influences on the TiO2 loading, specific surface area, and total pore volume of samples 4-6. Compared with samples 4-6, these properties of samples 1 and 2 were lower. This further supports that the TTIP conversion of samples 1 and 2 was not complete. Compared with virgin F400 AC, however, the specific surface area and total pore volume of samples 4-6 were lower which resulted from the TiO2 deposited on the carbon surface that blocked the pores. Product Characterization. Figure 4 shows the SEM images of TiO2/AC prepared by the described method and the virgin carbon. It demonstrates that TiO2 particles were formed on the carbon surface. The preparation conditions did not significantly affect the TiO2 morphology because the actual power output rate at each condition was the same; just the irradiation time was changed. Figure 5 shows the SEM image of a cross section of sample 1 sliced by a blade in the middle. Because the particle was irregular, part of the outer surface is also visible. Figure 6 shows the SEM images, EDS spectra, and EDS mapping of the Ti element on section 1 (external surface) and section 2 (internal surface) in Figure 5. Obviously, the formed TiO2 mainly deposited on the external surface of carbon. The deposition of TiO2 on the external surface is preferred because UV light cannot penetrate into small pores. Because water was added later, thermal reaction of TTIP inside the pores could yield TiO2 on the internal surface. Under microwave irradiation, however, part of TTIP inside the pores could also be desorbed and/or evaporated out before decomposition. These TTIP molecules reacted with water and then deposited on the outer surface. Figure 7 shows the XRD patterns of different samples. A fast scanning speed, 0.05°/s, was used initially. If any clear peaks were detected, a slow scanning speed, 0.005°/s, was used to
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Figure 6. Regions 1 and 2 in Figure 5.
Figure 7. XRD patterns of different samples (scanning speed, 0.05 °/s for samples 1 and 2; 0.005°/s for samples 4-6).
verify the result. No significant peak was detected on samples 1 and 2. However, peaks were detected on samples 4-6.
Therefore, a slow scanning speed was used to rerun samples 4-6. The results revealed that the anatase phase was formed
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Figure 8. Methanol effluent concentration profiles: inlet methanol concentration was 22.4 ppm, inlet water vapor concentration was 19 mg L-1, and EBCT was 0.35 s.
Figure 9. Average methanol removal efficiencies: inlet methanol concentration was 38.8 ppm, inlet water concentration was 19 mg L-1, and EBCT was 0.35 s.
on samples 4 and 6 and the rutile phase was formed on samples 5 and 6 that resulted from the high bulk temperature. The formation of anatase and rutile TiO2 is important because of its photocatalytic performance, which will be explained in the next paragraph. Methanol Removal Testing. Evaluation of the original F400 AC and TiO2/AC composites for the methanol removal was carried out with and without UV light. The inlet methanol concentration was 22.4 ppm. The normalized effluent methanol concentration profiles for the virgin AC and TiO2/AC (sample 4) are shown in Figure 8. It is apparent from Figure 8 that the effluent methanol concentration increased quickly when treated by the virgin AC with and without UV light. When treated by TiO2/AC without UV light irradiation, a similar adsorption profile was observed, and the methanol adsorption capacity for the sample 4 composite was actually lower than that of the virgin carbon because of the lower surface area. However, with UV light irradiation, the methanol concentration did not reach saturation for the duration of the experiment. Another thing that should be mentioned was that acetone and 2-propanol were detected by GC in the impinger samples of sample 1 and sample 2 (with UV light). The curves of samples 1 and 2 were not shown in Figure 8. Acetone is the reaction product between TTIP and the carbonyl groups on the AC surface,28 while 2-propanol is likely the residual solvent left over. Because acetone and 2-propanol were not the products of methanol degradation, their presence further proved that the TTIP conversion of samples 1 and 2 was not complete. Regarding sample 4 with UV light irradiation, the effluent methanol concentration increased during the first 2 h and then was maintained at about 53% removal. Therefore, the average of the last four data points was used to calculate the average methanol removal efficiency for subsequent analysis. Figure 9 shows the average methanol removal efficiencies of samples 4-6 with UV irradiation. No acetone or 2-propanol were detected by GC in the impinger samples of these samples, and their average methanol removal efficiencies were similar. This revealed that increasing the irradiation time or changing the water/TTIP ratio cannot further increase the photocatalytic activity once the TTIP conversion was completed.
short time, at atmospheric pressure. F400 AC was stable under this power level, and the formed submicrometer TiO2 particles were rich on the external surface of carbon. When the TTIP conversion was completed, the irradiation time and water/TTIP ratio would no longer pose any significant impact on the final product. The prepared TiO2/AC composite photocatalyst showed lower adsorption capacity for methanol than virgin carbon as a result of pore blockage by the newly formed TiO2 particles. Photocatalytic oxidation of methanol from humid air was successfully accomplished by the composite, and the material did not reach saturation for the duration of the experiment.
Conclusions Under a medium level of 800 W microwave irradiation, anatase TiO2 was quickly formed from the TTIP precursor in a
Acknowledgment This project is supported by the Department of Energy, Award No. DE-FC36-03ID14437. The authors greatly appreciate the Major Analytical Instrument Center at UF for the use of SEM and XRD instruments, Mr. Anadi Misra for his work with the XRD analysis, and Ms. Ameena Y. Khan for her training of NOVA 2200. Literature Cited (1) Springer, A. M. Industrial EnVironmental Control: Pulp and Paper Industry; TAPPI Press: Atlanta, GA, 2000. (2) Varma, V. K. Experience with the collection, transport, and burning off Kraft mill high Volume low concentration gases; NCASI Special Report No. 03-03; NCASI: Research Triangle Park, NC, 2003. (3) Liu, J.; Crittenden, J. C.; Hand, D. W.; Perram, D. L. Regeneration of adsorbents using heterogeneous photocatalytic oxidation. J. EnViron. Eng. 1996, 8, 707-713. (4) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. ReV. 1995, 95, 69-96. (5) Alberici, R. M.; Jardim, W. F. Photocatalytic destruction of VOCs in the gas-phase using titanium dioxide. Appl. Catal., B 1997, 14, 55-68. (6) Pitoniak, E.; Wu, C.-Y.; Londeree, D.; Mazyck, D.; Bonzongo, J.C.; Powers, K.; Sigmund, W. Nanostructured Silica-Gel Doped with TiO2 for Mercury Vapor Control. J. Nanopart. Res. 2003, 5, 281-292. (7) Crittenden, J. C.; Suri, R. P. S.; Perram, D. L.; Hand, D. W. Decontamination of water using adsorption and photocatalysis. Water Res. 1997, 31, 411-418. (8) Chang, C.; Chen, J.; Lu, M.; Yang, H. Photocatalytic oxidation of gaseous DMF using thin film TiO2 photocatalyst. Chemosphere 2005, 58, 1071-1078. (9) El-Sheikh, A. H.; Newman, A. P.; Al-paffaee, H.; Phall, S.; Cresswell, N.; York, S. Deposition of anatase on the surface of activated carbon. Surf. Coat. Technol. 2004, 187, 284-292. (10) Khan, A. M. Titanium Dioxide Coated Activated Carbon: A Regenerative Technology for Water Recovery. Master’s Thesis, University of Florida, Gainesville, FL, 2003.
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ReceiVed for reView January 9, 2006 ReVised manuscript receiVed May 1, 2006 Accepted May 8, 2006 IE0600341
