Methyl Radical Reactivity on the Basal Plane of Graphite

The reaction of submonolayer Li atoms with CH3Cl 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 (∼109 defects cm–2) to eliminate the effects of step edges and defects on the graphite surface chemistry. Li causes C–Cl bond scission in CH3Cl, liberating CH3 radicals below 130 K. Ordinarily, two CH3 species would couple to form products such as C2H6, but in the presence of graphite, CH3 preferentially adsorbs on the flat basal plane of Li-treated graphite. A C–CH3 bond of 1.2 eV is formed, which is enhanced relative to CH3 binding to clean graphite (0.52 eV) due to donation of electrons from Li into the graphite and back-donation from graphite to CH3. A low yield of C1, C2, and C3 hydrocarbon products above 330 K is found along with a low yield of H2. The low yield of these products indicates that the majority of the CH3 groups are irreversibly bound to the basal plane of graphite, and only a small fraction participate in the production of C1–C3 volatile products or in extensive dehydrogenation. Spin-polarized density functional theory calculations indicate that CH3 binds to the Li-treated surface with an activation energy of 0.3 eV to form a C–CH3 adsorbed surface species with sp3 hybridization of the graphite, and the methyl carbon atoms is involved in bond formation. Bound CH3 radicals become mobile with 0.7 eV activation energy and can participate in combination reactions for the production of small yields of C1–C3 hydrocarbon products. We show that alkyl radical attachment to the graphite surface is kinetically preferred over hydrocarbon product desorption.