Chemical Reaction of •CH3 with the Basal Plane of Graphite

Pristine graphite is known to be resistant to chemical attack. In this work, we present both experimental and theoretical work showing facile reaction between methyl radical species and the basal plane of graphite. We have used van der Waals corrected spin-polarized periodic density functional theory (DFT) to identify the reaction mechanism and energy barriers for catalytic dissociation of CH₃Cl on Li-doped graphite, followed by reaction of CH₃ radicals on the graphite surface. The energy barrier for CH₃ binding to graphite is found to be less than ~0.3 eV. Experiments have been performed on high quality HOPG (highly oriented pyrolytic graphite) with low defect density (~10⁹ cm^-2) to mitigate the effects of step edges and defects on the graphite surface chemistry. Theoretical calculations show that CH₃ radicals become mobile over an energy barrier of ~ 0.7 eV, consistent with experimental observations of CH₄ desorption at and above 330 K.  The calculations show that mobile CH₃ radicals abstract hydrogen from neighboring CH₃ radicals with an energy barrier of 0.56 eV in CH₄ production. Experimental observations indicate that ~ ¾ of the methyl radicals remain on the graphite surface up to 700 K at puckered sp³ carbon sites, while ¼ of the CH₃ radicals participate in CH₄ formation with small amounts of C₂ and C₃ hydrocarbons formation through the onset of surface mobility of CH₃ radicals bonded to the graphite surface.

 

Acknowledgement: We gratefully acknowledge the support by DTRA under Contract No. HDTRA1-09-1-0008. We also gratefully acknowledge NSF XSEDE (TeraGrid) resources under allocation numbers TG-DMR100097, TG-DMR110091 and TG-SEE090006. We thank the Center for Simulation and Modeling at the University of Pittsburgh for providing computational support.