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Carbon Nanotubes as a Superior Sorbent for Nitrogen Oxides Richard Q. Long and Ralph T. Yang* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136
Carbon nanotubes were prepared and investigated as sorbents for NO, SO2, and CO2 in the presence of O2. The preliminary results indicate that they are a good and reversible sorbent for NO removal at room temperature. An uptake amount of 78 mg/g of NOx was obtained by TGA when the carbon nanotubes were exposed to 1000 ppm NO + 5% O2/He for 120 min. The equilibrium amount was near 90 mg/g after 12 h. TPD profiles show that NO2 and NO desorb from the NOx-saturated carbon nanotubes at well below 300 °C. FTIR spectra indicate that the adsorbed species are nitrates and trace amounts of NO2 and (NO)2 dimers. By comparison, only 29 mg/g of SO2 (at 500 ppm) and 2 mg/g of CO2 (at 10%) are adsorbed onto the carbon nanotubes in 120 min. Introduction Carbon nanotubes are derivatives of C60 buckyballs. They are formed from graphite (or graphene) sheets rolled up into tubes, generally in the range of 1-10 nm in diameter and 200-500 nm in length. Since their recent discovery, these materials have attracted intense interest because of their potential applications in a variety of nanotechnologies. Significant efforts have been devoted to improving their syntheses, determining their structures, measuring their properties, and developing applications.1-6 Carbon nanotubes have electrons located among the carbon atoms because of the sp2 hybridization. Their unique electronic properties and structure have led to interest in their potential applications as quantum nanowires, electron field emitters, catalyst supports, chemical sensors,4 and sorbents for hydrogen storage.5-8 It is of particular interest to explore carbon nanotubes as sorbents for other gases. There has been a major effort in developing systems to remove NOx (NO + NO2) emissions from the combustion of fossil fuels because of the environmental importance. NO adsorption is a promising alternative approach for NOx removal. NOx adsorption can occur at low or high temperatures. After the sorbent is saturated with NOx, the NOx can be desorbed by either increasing the temperature or decreasing the pressure. The resulting desorption stream concentrated in NOx can be recycled to the combustion zone for NO decomposition into N2, or the NOx can be reduced to N2 through the injection of reducing gases such as H2, CO, or hydrocarbons. The sorbents for NOx removal at low temperatures include ion-exchanged zeolites, activated carbon, FeOOH dispersed on activated carbon fibers, and mixed oxides, as reviewed recently.9,10 It is known that zeolite is not a good sorbent for NO at room temperature. By comparison, NO can be effectively adsorbed onto activated carbon because of the assistance of surface functional groups, although the adsorption amount is still not large.11,12 However, dispersion of FeOOH on activated carbon fibers increased the amount of NO adsorbed significantly.11 * Author to whom correspondence should be addressed. Tel.: (734) 936-0771. Fax: (734) 763-0459. E-mail: yang@ umich.edu.
In this work, we first report NO adsorption on carbon nanotubes at room temperature in the presence of oxygen. In addition, because SO2 and CO2 are present in exhaust gases along with NO, the adsorption of SO2 and CO2 on the carbon nanotubes is also studied. The initial results indicate that carbon nanotubes are a very good and reversible sorbent for NO removal. The surface-adsorbed species is also studied by FTIR spectroscopy. Experimental Section Preparation of Carbon Nanotubes. Carbon nanotubes were obtained by catalytic decomposition of acetylene on 2.5 wt % Co/Y catalyst, as described in detail elsewhere.13 The catalyst was prepared by doping 2.17 g of Co(CH3COO)2‚4H2O (99%, Aldrich) on 20 g of Na-Y zeolite (Al/Si ) 1/2.4, Zeolyst) and then calcining in air at 500 °C for 6 h. In the synthesis of the carbon nanotubes, 1.4 g of 2.5 wt % Co/Y catalyst was loaded in a fixed-bed quartz reactor, and 15% C2H2/N2 (130 mL/ min) was passed over the catalyst at 650 °C for 1 h. Subsequently, the obtained sample was mixed with a 48% HF solution at room temperature and stirred for 24 h. The catalyst particles were dissolved in the solution during this process. The mixture was then filtered and washed with deionized water. Finally, the obtained carbon nanotubes were calcined at 300 °C in He for 3 h. Characterization of Carbon Nanotubes. TEM (transmission electron microscope) images were taken with a 2000FX instrument (JEOL). A Micromeritics ASAP 2010 micropore size analyzer was used to measure the N2 adsorption isotherm at liquid N2 temperature (-196 °C). The specific surface area was determined from the linear portion of the BET plots (P/P0 ) 0.05-0.20). The pore size distribution was calculated from the desorption branch of the N2 adsorption isotherm using the Barrett-Joyner-Halenda (BJH) formula, because the desorption branch can provide more information about the degree of blocking than the adsorption branch can. Prior to the measurements, the sample was dehydrated at 300 °C for 6 h. Adsorption of NO, SO2, and CO2. A thermogravimetric analyzer (TGA, Cahn 2000 System 113), equipped with a programmed temperature-control unit, was used
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to study the sorption of NO, SO2, and CO2 on the carbon nanotubes. The sample (10 mg) was held in a quartz bucket suspended in the heated zone of the quartz tube. A thermocouple was placed as close to the bucket as possible to indicate the temperature of the sample. The gas mixtures were obtained by blending gas streams of controlled flow rates. The feed gas composition was as follows: 1000 ppm NO (when used), 500 ppm SO2 (when used), 10% CO2 (when used), 5% O2, and the balance He. The total flow rate was 250 mL/min. The sample was pretreated at 500 °C for 30 min in He and was then cooled to room temperature for adsorption. Calibration for changes in gas composition was done to accurately account for differences due to buoyancy and friction losses. TPD of NOx. Temperature-programmed desorption (TPD) was used to investigate the behavior of NO desorption and the reversibility of NO adsorption on the carbon nanotubes. The experiment was carried out in a fixed-bed quartz reactor. The sample (50 mg) was first treated at 300 °C for 30 min and was then cooled to room temperature in He. Subsequently, 1000 ppm NO + 5% O2/He (100 mL/min) was passed through the sample for 180 min, and then the mixed gas was switched to He for 30 min. Finally, TPD was performed in He (100 mL/ min) by ramping the temperature at a rate of 10 °C/ min to 400 °C. The desorbed NO and NO2 were continually monitored by a chemiluminescent NO/NOx analyzer (Thermo Environmental Instruments Inc., model 42C). Also, a magnetic deflection-type mass spectrometer (AERO VAC, Vacuum Technology Inc.) was used for continuous detection of the effluent gas, which contained N2 + CO (m/e ) 28), NO + NO2 (m/e ) 30), O2 (m/e ) 32), and N2O + CO2 (m/e ) 44). FTIR Studies. Fourier transform infrared (FTIR) spectra were recorded with a Nicolet Impact 400 FTIR spectrometer with a TGS detector. The carbon nanotubes were first mixed with KBr in a ratio of 1/20 by weight. A self-supporting wafer was prepared by pressing 30 mg of the mixture, and it was then loaded into an IR cell with BaF2 windows. The background spectrum was recorded in flowing He at room temperature and was subtracted from the sample spectrum during the experiment. Subsequently, the He gas was switched to 1000 ppm NO + 5% O2/He (100 mL/min), and IR spectra were recorded at different times by accumulating 100 scans at a spectral resolution of 1 cm-1. The technique using KBr pellets has recently been discussed in detail.14 Results and Discussion The TEM image of the multiwall carbon nanotubes is shown in Figure 1. The quality of the carbon nanotubes was high. The material was mainly composed of carbon nanotubes, in addition to a trace amount of amorphous carbon (Figure 1). The catalyst particles had been removed by the HF solution. From the TEM image, it can be seen that the outer diameter of the carbon nanotubes was in the range of 5-10 nm and that the inner diameter was between 2 and 4 nm. The pore size distribution was also measured by the N2 adsorption isotherm at -196 °C, as shown in Figure 2. The carbon nanotubes had micropore and mesopore size distributions with two maxima at 2.0 and 3.9 nm. This is quite consistent with the TEM results. A broad peak appeared at around 45.7 nm, which likely can be attributed to internanotube spaces. The BET surface area and pore volume were 462 m2/g and 0.41 cm3/g, respectively.
Figure 1. TEM image of multiwall carbon nanotubes.
Figure 2. Pore size distribution of carbon nanotubes.
The uptake rates of NOx, SO2, and CO2 on the carbon nanotubes at room temperature are shown in Figure 3. When the 1000 ppm NO + 5% O2 mixture was introduced into the carbon nanotubes, the weight gain was significant. The NOx uptake amount reached 78 mg/g of carbon nanotubes in 120 min. After the mixed gas was switched to He, 4 mg of NOx per gram was desorbed in 60 min. By comparison, when a mixture of 500 ppm SO2 + 5% O2 was introduced into the carbon nanotubes, the adsorption rate for SO2 was lower than that for NOx. An uptake amount of 29 mg/g was obtained in 120 min. When the mixed gas was switched to He, 8.6 mg of SO2 per gram was desorbed in 60 min. The CO2 adsorption on carbon nanotubes was unexpectedly low. An uptake amount of less than 2 mg/g was observed in 120 min when a 10% CO2 + 5% O2 mixture was used, and the adsorbed CO2 was desorbed completely in He (Figure 3). The desorption behavior of NO was studied by the TPD experiment. The TPD profile of NOx on the carbon
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Figure 3. Adsorption/desorption of NOx, SO2, and CO2 on carbon nanotubes at 25 °C. Conditions: for adsorption, 1000 ppm NO (when used), 500 ppm SO2 (when used), 10% CO2 (when used), 5% O2, and balance of He and 250 mL/min of total flow rate; for desorption, in He (250 mL/min).
Figure 4. TPD profiles of carbon nanotubes that were pretreated with 1000 ppm NO + 5% O2/He at 25 °C.
nanotubes is shown in Figure 4. As the temperature was increased from 20 to 400 °C, NOx desorption was observed. The peak for NOx desorption was at 130 °C, suggesting that the bond between NOx and carbon nanotubes is not strong. NO2 was the major product at low temperatures, whereas NO became dominant at higher temperatures. Essentially all of the NOx desorbed at temperatures below 300 °C. This indicates that NOx adsorption on the carbon nanotubes is reversible. The carbon nanotubes can be regenerated by heating at temperatures below 300 °C. During the desorption of NOx, a small amount of CO2 and/or N2O (m/e ) 44) was also formed at high temperatures, but O2 (m/e ) 32) and N2 or CO (m/e ) 28) were not detected. The nature of the adsorbed NOx species was investigated using a carbon nanotube/KBr pellet with FTIR spectroscopy. As shown in Figure 5, after the 1000 ppm NO + 5% O2/He mixture was introduced to the sample
Figure 5. FTIR spectra taken at room temperature upon passing a 1000 ppm NO + 5% O2/He mixture over carbon nanotubes for (a) 5, (b) 15, (c) 30, (d) 60, and (e) 180 min and then (f) purging with He for 15 min.
cell for 5-30 min, a strong band at 1351 cm-1 and three weak bands at 1766, 1555, and 832 cm-1 were observed. Because these IR bands were not observed on NOxtreated KBr, they can be attributed only to the NOx species that were adsorbed on the carbon nanotubes. The bands at 1351 and 832 cm-1 can be assigned to the nitrate species, whereas the band at 1555 cm-1 is likely due to the adsorbed NO2 species.15,16 The weak band at 1766 cm-1 is very close to the asymmetric N-O stretching band of trans-(NO)2 dimers (which is known to be at 1764 cm-1);15,16 hence, it is most likely due to NO dimers formed in the carbon nanotubes. The intensities of the above bands increased with time. After the sample was exposed to NO + O2 for 180 min and then purged with He for 15 min, the bands due to nitrate, NO2, and NO dimers were still clearly detectable (Figure 5). The above results indicate that carbon nanotubes are a good sorbent for NO removal. An uptake amount of 78 mg/g NOx was obtained in 120 min when a mixture of 1000 ppm NO + 5% O2 was introduced at room temperature. A separate experiment showed that the equilibrium amount was near 90 mg/g after 12 h. This amount is much higher than the amounts for all other known sorbents, including zeolites and activated carbon.12 As reported by Kaneko,12 only 15-17 mg of NO per gram was adsorbed on activated carbons at 30 °C when 13 kPa (i.e., 12.6%) NO was used. The best sorbent that has been reported in the literature was FeOOHdoped activated carbon fibers.9-12 The maximum NO uptakes at a high NO partial pressure (80%) were 320 and 235 mg/g at 1 and 30 °C, respectively. However, at very low partial pressures, e.g., 1000 ppm, less than 2 mg of NO uptake per gram was obtained on the FeOOHdoped activated carbon fibers, according to the reported isotherms.11,12 This comparison shows that the uptake amount of NOx on carbon nanotubes is again much higher than that for the FeOOH-doped activated carbon fibers at low NO partial pressures. The NOx adsorption
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on carbon nanotubes was also reversible. The NOxadsorbed carbon nanotubes could be regenerated by heating to 300 °C in He. The good capacity for NOx might be related to the unique structure, electronic properties, and surface functional groups (e.g., CdO on graphitic edges and defects) of carbon nanotubes. When NO + O2 was passed over the carbon nanotubes, NO was possibly oxidized to NO2 and then adsorbed on the surface as a nitrate species. Mochida et al. have reported a high catalytic activity of activated carbon fibers for the oxidation of NO to NO2 at room temperature.17 Also, a trace amount of NO dimers was also formed in the nanotubes through micropore filling at room temperature. By comparison, SO2 can also be adsorbed on the carbon nanotubes, but the adsorption rate and amount are less than those for NO. CO2 was hardly adsorbed on the carbon nanotubes. Further studies on the adsorption of NO and SO2 on carbon nanotubes are in progress. Acknowledgment Support by the NSF (Grant CTS-9819008) and by NGK Insulators, LTD, Nagoya, Japan, is acknowledged. Discussions with Tomonori Takahashi of NGK are appreciated. Note Added after ASAP Posting This article was released ASAP on 5/19/01 with an error in the Abstract. The correct version was posted on 8/1/01. Literature Cited (1) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, CA, 1995. (2) Ajayan, P. M. Carbon Nanotubes: Novel Architecture in Nanometer Space. Prog. Cryst. Growth Charact. Mater. 1997, 34, 37. (3) Subramoney, S. Novel NanocarbonssStructure, Properties, and Potential Applications. Adv. Mater. 1998, 10, 1157. (4) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Nanotube Molecular Wires as Chemical Sensors. Science 2000, 287, 622.
(5) Chambers, A.; Park, C.; Baker, R. T. K.; Rodriguez, N. M. Hydrogen Storage in Graphite Nanofibers. J. Phys. Chem. B 1998, 102, 4253. (6) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Hydrogen Storage in Single-Walled Carbon Nanotubes at Room Temperature. Science 1999, 286, 1127. (7) Chen, P.; Wu, X.; Lin, J.; Tan, K. L. High H2 Uptake by Alkali-Doped Carbon Nanotubes under Ambient Pressure and Moderate Temperatures. Science 1999, 285, 91. (8) Yang, R. T. Hydrogen Storage by Alkali-Doped Carbon NanotubessRevisited. Carbon 2000, 38, 623. (9) Arai, H.; Machida, M. Removal of NOx through sorptiondesorption cycles over metal oxides and zeolites. Catal. Today 1994, 22, 97. (10) Long, R.; Yang, R. T. Selective and Reversible Adsorbents for Nitric Oxide from Hot Combustion Gases. Stud. Surf. Sci. Catal. 1999, 120, 435. (11) Kaneko, K. Anomalous Micropore Filling of NO on R-FeOOH-Dispersed Activated Carbon Fibers. Langmuir 1987, 3, 357. (12) Kaneko, K. Control of Supercritical Gases with Solid NanospacesEnvironmental Aspects. Stud. Surf. Sci. Catal. 1999, 120, 635. (13) Colomer, J.-F.; Piedigrosso, P.; Willems, I.; Journet, C.; Bernier, P.; van Tendeloo, G.; Fonseca, A.; Nagy, J. B. Purification of Catalytically Produced Multi-Wall Nanotubes. J. Chem. Soc., Faraday Trans. 1998, 94, 3753. (14) Mawhinney, D. B.; Rossin, J. A.; Gerhart, K.; Yates, J. T., Jr. Adsorption Studies by Transmission IR Spectroscopy: A New Method for Opaque Materials. Langmuir 1999, 15, 4617. (15) Laane, J.; Ohlsen, J. R. Characterization of Nitrogen Oxides by Vibrational Spectroscopy. Prog. Inorg. Chem. 1980, 27, 465. (16) Hadjiivanov, K. I. Identification of Neutral and Charged NxOy Surface Species by IR Spectroscopy. Catal. Rev.-Sci. Eng. 2000, 42, 71. (17) Mochida, I.; Kawabuchi, Y.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. High Catalytic Activity of Pitch-Based Activated Carbon Fibers of Moderate Surface Area for Oxidation of NO to NO2 at Room Temperature. Fuel 1997, 76, 543.
Received for review November 20, 2000 Revised manuscript received April 16, 2001 Accepted April 18, 2001 IE000976K
