Methyl Radical Reactivity on the Basal Plane of Graphite

Abstract

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