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Carbon-Chlorine Bond Scission in Li-Doped Single-Walled Carbon Nanotubes: Reaction of CH3Cl and Lithium† Lynn Mandeltort,‡ Michael Bu¨ttner,‡ John T. Yates, Jr.,*,‡ Pabitra Choudhury,§ Li Xiao,§ and J. Karl Johnson§,| Department of Chemistry, UniVersity of Virginia, Virginia 22904, Department of Chemical and Petroleum Engineering, UniVersity of Pittsburgh, PennsylVania 15261, and National Energy Technology Laboratory, Pittsburgh, PennsylVania 15236 ReceiVed: April 30, 2010; ReVised Manuscript ReceiVed: July 12, 2010
The doping of single-walled carbon nanotubes (SWNTs) under ultrahigh vacuum by Li atoms has been explored experimentally and theoretically. The chemical effect of Li in breaking the C-Cl bond in chloromethane has been observed. Temperature programmed desorption (TPD) experiments show that at low coverage CH3Cl is physisorbed to the undoped SWNT sample, exhibiting a desorption process near 178 K. The CH3Cl desorption peak shifts to about 240 K for lithiated SWNTs, indicating an increase in binding energy of about 0.16 eV. More importantly, the integrated intensity of the CH3Cl desorption peak is dramatically reduced in the lithiated SWNT case, and CH3, C2H6, or related species are not observed in significant quantities in the TPD experiments up to a temperature of 500 K. This strongly indicates that CH3Cl reacts on lithiated SWNTs to produce an irreversibly bound species. Products LiCl and Li2Cl2 are observed to desorb near 700 K. Density functional theory calculations present possible reaction mechanisms and clues to the fate of the reacted CH3Cl. Our calculations show that at least two Li atoms are required to dissociate CH3Cl through a low-energy pathway. The products of the reaction are LiCl and CH3. The CH3 is chemically bound to defect sites on the nanotubes, and CH3 + CH3 radical recombination is suppressed. The second Li atom acts catalytically to lower the reaction energy required to break the C-Cl bond in CH3Cl. Furthermore, the CH3 bound to SWNT defect sites is observed to dehydrogenate at high temperatures in molecular dynamics simulations. This study indicates the potential for Li doping of SWNTs and other high surface area carbons to produce highly dispersed reaction centers for the destruction of toxic materials containing carbon-chlorine bonds. I. Introduction Single-walled carbon nanotubes (SWNTs) are potential candidates for advanced sorbent materials due to their large surface area, strong binding sites, and robust nature.1 A number of recent studies have investigated the adsorption of molecules on SWNTs using temperature programmed desorption (TPD) to discriminate the various binding sites present. These studies have shown that: (1) different SWNT binding sites can be selectively occupied by changing the conditions of adsorption;2,3 (2) SWNTs can shield molecules bound on the inside from reactive species on the outside to affect the reaction efficiency;4 (3) SWNTs can coadsorb molecules with similar binding energies, and the stronger binding energy molecule will displace the second molecule to a weaker adsorption site;5 and (4) the SWNT surface can be modified to enhance a molecule’s binding energy.6 Molecular adsorption in undoped, nondefective SWNTs is essentially due to van der Waals interactions.1 It is known that molecules adsorbed inside SWNTs can be easily displaced into the gas phase by molecules that have a stronger binding affinity for the SWNT surface.5 Such a process could lead to the release of toxic molecules into the air when SWNTs are used as sorbent materials for toxic clean up or for air filters. Therefore, modified †
Part of the “D. Wayne Goodman Festschrift”. University of Virginia. § University of Pittsburgh. | National Energy Technology Laboratory. ‡
SWNTs that can bind adsorbed molecules more strongly or even destroy such molecules are of interest. Single-walled nanotubes are model surfaces for adsorption on more complex carbon adsorbents. Because of their structure, only a few types of adsorption sites are available on SWNTs compared to the broad range of sites and pore sizes on technological carbon adsorbents. Adsorption studies on SWNTs therefore can probe fundamental issues more clearly than studies on complex carbon adsorbents. It has recently been shown that doping SWNTs with alkali metal vapor enhances the binding energy of nonpolar adsorbates.6 Intercalated Li atoms ionize inside the nanotube and the Li+ ions create a local electrostatic field that induces a dipole in neighboring polarizable molecules, thus enhancing the adsorbate interaction with the SWNT surface and causing an increase in the temperature of desorption in TPD experiments.6 Compared to a nonpolar molecule, a polar molecule, such as CH3Cl, is expected to have a stronger binding enhancement near Li+ ions on SWNTs. CH3Cl is an appropriate model for other toxic molecules, which often contain carbon-halogen bonds and dipole moments of similar magnitudes.7,8 In this work, the adsorption of chloromethane on Li-doped SWNTs and subsequent chemical processes have been studied using ultrahigh vacuum (UHV) techniques including TPD and Auger electron spectroscopy (AES) along with complementary first principles density functional theory (DFT) calculations. Both experiments and theory indicate that the intercalation of Li into SWNT bundles leads to C-Cl bond scission in adsorbed
10.1021/jp103942n 2010 American Chemical Society Published on Web 08/02/2010
C-Cl Bond Scission in Li-Doped SWNTs
Figure 1. STM Image of a SWNT Bundle. Derivative image showing chiral and achiral SWNTs (Egap ) -1.3 eV; Itunneling ) 0.5 nA).
CH3Cl as well as to enhancement of the binding energy of the unreacted CH3Cl. II. Methodology A. Experimental Section. Experiments were performed in a stainless steel ultrahigh vacuum chamber, described in detail elsewhere.9 The chamber is pumped with a turbomolecular pump (150 L/s), an ion pump (260 L/s), and a titanium sublimation pump to maintain a base pressure of 1.0 eV) for the single Li bound to a defective nanotube (Figure 11). Adding an additional Li atom to the system does stabilize the intermediate state, as we had supposed. By comparing Figures 11 and 12, we see that the intermediate state is about 0.7-0.8 eV lower in energy than the initial state when two Li atoms are in the system, making a difference in the relative energy of the intermediate state of 2 eV or more compared to the system with a single Li atom. Moreover, the reaction is found to be exothermic by 2.2 and 3.19 eV on the inside and outside nanotubes, respectively (Figure 12), which can be considered essentially irreversible. The additional Li atom in the 2Li + CH3Cl system acts to catalyze the dissociation of CH3Cl by stabilizing the CH3 radical, as can be seen by comparing the intermediate geometries shown in Figure 13a-d. The Li-Cl distance is about the same in Figure 13 for both one and two Li atoms in the system. However, when two Li atoms are present, the distance between the C in CH3 and the second Li is about 2.2 Å for inside and outside reactions, as shown in Figure 13c,d. This distance is small, enough to allow electron exchange between Li and CH3, resulting in a more stable intermediate state (i.e., lowering the energy landscape). Hence, the presence of the second Li atom on both the inside and
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Figure 14. Reaction product states for CH3Cl reacting with two Li atoms. Product state for (a) inside and (b) outside the SWNT with one defect site. These states correspond to the final states from Figure 11.
Figure 13. Reaction intermediate states involving one or two Li atoms. CH3Cl reacting with (a) single Li inside, (b) single Li outside, (c) 2Li inside, and (d) 2Li outside SWNT with a SWNT having a single defect site. These states correspond to the intermediate states from Figures 10 and 11.
outside of the defective nanotube catalyzes the C-Cl bond scission. Detailed analysis of the intermediate and transition states will be part of a future publication. Going from the intermediate to the final state requires that the CH3 radical bind to the defect site by pushing LiCl away from the defect site. This is easily accomplished because the CH3 binding energy (-1.78 and -3.60 eV for inside and outside, respectively) is much more favorable than that of LiCl (-0.76 and -0.91 eV for inside and outside, respectively). The binding energy for LiCl to the inside of nondefective nanotubes is only 0.26 eV less than that of defect site, which indicates that the migration of the LiCl group from the defect site is facile. The displacement of LiCl away from the defect site can be seen from the end state geometries shown in Figure 14a,b. C. High-Temperature Chemistry of SWNT-Bound CH3 Groups. Our DFT calculations indicate that the binding of CH3 to the defect sites is highly exothermic and hence virtually irreversible at the temperatures covered in the TPD experiments. To gain more insight into the possible fate of CH3 bound to defect sites upon heating, we have performed DFT-MD simulations at high temperatures for systems containing CH3 groups
bound to different defect sites. Three different calculations were carried out: (1) CH3 bound to the end of a finite length tube with two adjacent carbon vacancies; (2) CH3 bound to the end of a finite length tube with a single carbon vacancy; (3) CH3 bound to a defect side on the outside of a periodic nanotube (Figure 5). The MD simulations were carried out for up to 5 ps. The temperature ranged from 1500 to 2500 K over the different simulations, with temperature fluctuations on the order of 500 K in a single simulation. In every case we observed the breaking of a C-H bond within the first few hundred femtoseconds of the simulations. At no time did we observe the scission of the C-CHn bond, implying that the CH3 group will lose hydrogens that then bind to carbons on the nanotube at high temperatures but that the CHn groups will not readily desorb. We are mindful of the dangers of extrapolating the behavior of the system over a few picoseconds to laboratory time scales of minutes. However, the fact that no dissociation was seen at the highest temperature of 2500 K is highly consistent with experimental observations that very little CH4 (12-16 amu) and C2 hydrocarbons are observed to desorb in the TPD experiments. The starting and ending configurations from the MD simulation on the periodic nanotube are shown in Figure 15. A movie of this simulation can be found in the online version of the Supporting Information. Note that a hydrogen atom from the CH3 group is bound to a neighboring unsaturated C atom in the nanotube leaving the CH2 group bound at its site. One might expect at high enough temperatures that the H atoms would desorb, leaving the C atom from the CH3 group behind to heal the defect in the SWNT. It has been shown that methyl radicals impinging on a hot (1273 K) defective HOPG surface readily form graphene sheets extending from individual defect sites.47 Such a reaction suggests the facile incorporation of carbon atoms into an sp2 C network, such as in SWNTs, at elevated temperatures. V. Conclusion We have found that CH3Cl undergoes a chemical reaction on lithiated carbon nanotubes that consumes essentially all the CH3Cl. There is no evidence of chemical reactions on nonlithiated nanotubes. The reaction of CH3Cl with the lithiated nanotubes appears to be irreversible up to the temperatures
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Mandeltort et al. References and Notes
Figure 15. Fate of CH3 bound to a defect site. The (a) initial and (b) final configurations from a DFT-MD simulation on a tube with a CH3 group bound to a single carbon atom vacancy defect. The total length of the run was 5 ps, and the temperature ranged from about 1500 to 2000 K in this simulation.
covered in our TPD experiments. Our density functional theory calculations show that CH3Cl is not likely to react either with Li on nondefective nanotubes or with a single Li on defective nanotubes because the barriers for forming the CH3 radical intermediate are too high. Conversely, our calculations have found that CH3Cl can react with two Li atoms to bind with defect sites on both the inside and outside of SWNTs. One Li atom produces LiCl and the second atom acts catalytically to lower the energy of the CH3 intermediate. The reactions are highly exothermic and apparently irreversible, as seen from short-time high-temperature first principles MD simulations. CH3 groups bound to defect sites lose one hydrogen atom within a few hundred femtoseconds in the simulation. This H atom binds to a neighboring unsaturated C atom in the nanotube and the resulting structure is stable at temperatures as high as 2500 K for several ps. Our combined experimental and theoretical work clearly demonstrates that Li-doped SWNTs can act as chemically reactive highly dispersed sorbents that irreversibly break carbon-chlorine bonds in chloroalkanes anchoring the alkyl group on the carbon surface at defect sites. This finding could prove useful for capturing and destroying toxic chemicals such as chemical warfare agents or toxic industrial chemicals at low concentrations, on both nanotube sorbents as well as on higharea lithiated carbon sorbents. Acknowledgment. We thank A. J. Kennedy for artistic help and the Defense Threat Reduction Agency (DTRA) for support of this work under DTRA contract no. HDTRA1-09-1-0008. Supporting Information Available: Auger electron spectroscopy of sample and reference materials, additional TPD spectra, binding site geometries for LiCl, CH3, CH3Cl, tables of binding energies computed from DFT, and a molecular dynamics movie of CH3 bound to the defective SWNT wall. This material is available free of charge via the Internet at http:// pubs.acs.org.
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