(518f) Capturing Toxic Chemicals at Low Concentrations Using Lithiated SWNT: A Combined Experimental and Theoretical Study

Abstract

It is previously known that pristine single walled carbon nanotubes (SWNTs) interact weakly with many gas molecules and it is also known that these interactions can be enhanced via alkali metal doping. Thus, alkali metal doped SWNTs can play an important role in capturing toxic gas molecules. In this work, the reaction of chloromethane on Li-doped SWNTs has been studied using experiments and first principles density functional theory (DFT) calculations. Experiments using temperature programmed desorption (TPD) clearly show that CH₃Cl reacts with Li-doped SWNTs below room temperature, as can be seen from the dramatic decrease in the CH₃Cl TPD signal with the increase of Li concentration on the SWNTs. Moreover, we can infer from the experiments that CH₃ groups become irreversibly bound to the SWNTs as a result of this reaction. Our calculations show that CH₃Cl is not likely to react with either Li on pristine nanotubes or with a single Li atom 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. The reaction proceeds such that one Li atom produces LiCl and the second atom acts catalytically to lower the reaction energy required to break the C?Cl bond in CH₃Cl. The products of the reaction are LiCl and CH₃ that is chemically bound to defect sites on the nanotubes. The reactions are found to be highly exothermic and apparently irreversible, in agreement with experimental observations. Our DFT calculations indicate that binding of LiCl to pristine or defective nanotubes is relatively weak, with a binding energy of -0.50 to -0.91 eV. However, when we add an additional Li atom to the system we calculate a binding energy of about -1.78 eV for LiCl interacting with a Li atom at a defect site on the nanotube. This energy is in remarkable agreement with the activation energy (~ 1.80 eV) of LiCl evolution calculated using Redhead analysis of the TPD experiments. This study indicates the utility of using Li doped SWNTs and other carbons in producing highly dispersed reaction centers for the destruction of toxic materials containing carbon-chlorine bonds.

Acknowledgement: We thank the Defense Threat Reduction Agency (DTRA) for support of this work under DTRA contract no. HDTRA1-09-1-0008.