Article pubs.acs.org/JPCC
Methyl Radical Reactivity on the Basal Plane of Graphite Lynn Mandeltort,† Pabitra Choudhury,‡ J. Karl Johnson,‡,§ and John T. Yates, Jr.*,† †
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States § National Energy Technology Laboratory, Pittsburgh, Pennsylvania 15236, United States ‡
S Supporting Information *
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.
I. INTRODUCTION The basal plane of graphite is widely considered to possess little or no chemical reactivity due to its stability originating from extended delocalization of electrons in the hexagonally linked sp2 carbon sheet. In this work we expand upon the recent finding that chemically active CH3 radical species are strongly bound to the basal plane of highly oriented pyrolytic graphite (HOPG) at temperatures below ∼300 K1 and that a small fraction will react with each other as the temperature is raised, evolving C1, C2, and C3 hydrocarbons. Such processes would be expected to occur at defect (edge plane) sites, as on defective carbon nanotubes and defective HOPG,2,3 but the experiments presented here eliminate this possibility by working with graphite surfaces with a very low fraction (∼10−6) of defect sites. In recent work, the basal plane of graphite has been found to be reactive with atomic H when H atoms are incident with a kinetic energy of ∼0.4 eV,4−6 suggesting that other radical species should also be reactive with this surface. Other reactions with the basal plane of graphite have been reported, each involving high temperatures, aggressive reagents such as diazonium salts, and applied voltages.7,8 Carbon surface addition reactions are of interest to the astrochemistry community studying radical species interacting with carbonaceous grain surfaces in the interstellar medium.4,9 Such reactions may also be of interest to researchers studying the formation of the solid electrolyte interphase (SEI) in Li-ion © 2012 American Chemical Society
batteries with graphite electrodes. This passivating layer, formed on the surface of a lithiated carbon electrode during the first operating cycle of the battery, plays a large role in battery lifetime, performance, and safety.10 While much effort has been directed toward characterizing the SEI in Li ion batteries, there appears to be a lack of molecular information about its initial formation. Components of the SEI have been characterized for a variety of conditions, and it has been shown that SEI formation on the basal plane of graphite is dominated by organic polymeric species, while the cross-sectional edge sites contain an abundance of inorganic species.11 The modification of the basal plane by CH3 radicals presented in our work suggests that SEI formation could involve the covalent attachment of organic free radical species to the graphite basal plane. Working under ultrahigh vacuum, we employ submonolayer Li atom coverages deposited onto HOPG at ∼100 K as an active reagent. CH3Cl molecules, deposited in submonolayer quantities following the predeposition of Li, experience C−Cl bond scission. We find that the experimental activation energy for Li-induced CH3Cl dissociation is
