Methyl Radical Reactivity on the Basal Plane of Graphite

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