(700f) Chemical Reaction of •CH3 with the Basal Plane of Graphite

Authors: 
Choudhury, P., University of Pittsburgh
Johnson, J. K., University of Pittsburgh
Mandeltort, L., University of Virginia


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.

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