5 research outputs found
Methyl Radical Reactivity on the Basal Plane of Graphite
The reaction of submonolayer Li atoms with CH<sub>3</sub>Cl at
100 K on a highly oriented pyrolytic graphite (HOPG) surface has been
studied under ultrahigh vacuum. We exploit the low defect density
of the high quality HOPG used here (∼10<sup>9</sup> defects
cm<sup>–2</sup>) to eliminate the effects of step edges and
defects on the graphite surface chemistry. Li causes C–Cl bond
scission in CH<sub>3</sub>Cl, liberating CH<sub>3</sub> radicals below
130 K. Ordinarily, two CH<sub>3</sub> species would couple to form
products such as C<sub>2</sub>H<sub>6</sub>, but in the presence of
graphite, CH<sub>3</sub> preferentially adsorbs on the flat basal
plane of Li-treated graphite. A C–CH<sub>3</sub> bond of 1.2
eV is formed, which is enhanced relative to CH<sub>3</sub> binding
to clean graphite (0.52 eV) due to donation of electrons from Li into
the graphite and back-donation from graphite to CH<sub>3</sub>. A
low yield of C<sub>1</sub>, C<sub>2</sub>, and C<sub>3</sub> hydrocarbon
products above 330 K is found along with a low yield of H<sub>2</sub>. The low yield of these products indicates that the majority of
the CH<sub>3</sub> groups are irreversibly bound to the basal plane
of graphite, and only a small fraction participate in the production
of C<sub>1</sub>–C<sub>3</sub> volatile products or in extensive
dehydrogenation. Spin-polarized density functional theory calculations
indicate that CH<sub>3</sub> binds to the Li-treated surface with
an activation energy of 0.3 eV to form a C–CH<sub>3</sub> adsorbed
surface species with sp<sup>3</sup> hybridization of the graphite,
and the methyl carbon atoms is involved in bond formation. Bound CH<sub>3</sub> radicals become mobile with 0.7 eV activation energy and
can participate in combination reactions for the production of small
yields of C<sub>1</sub>–C<sub>3</sub> hydrocarbon products.
We show that alkyl radical attachment to the graphite surface is kinetically
preferred over hydrocarbon product desorption