Letter pubs.acs.org/JPCL
Reaction of the Basal Plane of Graphite with the Methyl Radical 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 Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States § National Energy Technology Laboratory, Pittsburgh, Pennsylvania 15236, United States ‡
ABSTRACT: The reaction of methyl radicals with the basal plane of graphite has been observed to occur with an activation energy of less than 0.3 eV. This reaction is initiated by Li-induced CH3Cl dissociation to produce CH3 radicals on the graphite surface. It is found that ∼3/4 of the methyl radicals remain on the graphite surface up to 700 K at puckered sp3 carbon sites, while 1/4 of the CH3 radicals participate in CH4 formation and small amounts of C2 and C3 hydrocarbon formation. CH3 radicals become mobile over an activation energy barrier of ∼0.7 eV.
SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis e report the first experimental observation of facile reactivity of the atomically clean basal plane of highly oriented pyrolytic graphite (HOPG) with methyl radicals at low temperature. While a reaction with surface carbon atoms is expected at edge sites where dangling bonds are present, it is unexpected at planar, coordinatively saturated sp2 sites of graphite.1,2 Although basal plane functionalization has been realized using electrochemical and wet chemical methods, these studies utilized high temperatures or applied potentials.3,4 Using surface science experiments under conditions where surface defects are not involved, it was found that methyl radicals (generated by Li atom-induced breaking of the C−Cl bond in CH3Cl) efficiently attack the basal plane of graphite at temperatures near 100 K. The CH3 radicals produced are strongly chemisorbed on the graphite, and as the temperature is raised, about 3/4 of the CH3 species remain attached to graphite, and about 1/4 react to produce CH4 and traces of C2 and C3 hydrocarbons. We have used the Vienna ab initio simulation package (VASP)5−7 to perform van der Waals corrected8 spin-polarized periodic density functional theory (DFT) calculations to compute geometries, binding energies, and reaction barriers for reactions on model graphite surfaces as a complement to the experiments. From the calculations, we find that the activation energy required for producing the C− CH3 bond on graphene is only ∼0.28 eV and that the C−CH3 bond has a strength of ∼0.52 eV. This bonding converts the graphene sp2 hybridization to sp3 hybridization. Temperature-programmed desorption (TPD) performed on HOPG exposed to CH3 radicals produced from CH3Cl and Li (see Experimental section) yields small quantities of CH4 and C2−C3 hydrocarbons desorbing below 1000 K. Figure 1 shows the relative molar amounts of CH3Cl converted to carbon-
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© 2012 American Chemical Society
Figure 1. Carbon mole fraction of CH3Cl consumed and C1 − C3 hydrocarbons produced by CH3 radical production and reaction below 700 K using excess metallic Li on graphite. The mass spectral cracking patterns of the small yield of desorbing products have been converted to the molar quantity of carbon contained in each molecule. Quantification was made by integrating the TPD peak areas and using the known sensitivity of the spectrometer to the individual chemical species. It should be noted that if CH3 desorbs directly, it would make only a small contribution to the C1 products because of efficient CH3 removal by the walls of the mass spectrometer.
containing products. It may be seen that less than ∼1/4 of the initially deposited CH3Cl is evolved as hydrocarbons at temperatures in the range of 330 to ∼700 K. Thus, about 3/ 4 of the methyl radicals produced from CH3Cl are retained on the graphite surface to at least 330 K or above, leading to the postulate of extensive CH3 adsorption on the graphite surface. This conclusion is verified by the observation of a very low Received: May 7, 2012 Accepted: June 5, 2012 Published: June 5, 2012 1680
dx.doi.org/10.1021/jz300578x | J. Phys. Chem. Lett. 2012, 3, 1680−1683
The Journal of Physical Chemistry Letters
Letter
yield of hydrogen in the TPD measurements, comparable to the CH4 yield. In addition, LiCl and Li2Cl2 are observed to desorb at about 650 K. No additional products were observed up to 1000 K. The lack of CH3Li products in the TPD indicates that this adduct is not formed in appreciable amounts. Figure 2 shows the decreasing yield of CH3Cl as increasing coverages of Li are employed to react with a constant coverage
Figure 2. Fraction of CH3Cl reacted as a function of Li coverage as measured by Auger spectroscopy. The exposure of CH3Cl is 2.3 × 1014 cm−2 (∼0.3 ML) in all cases. (Inset) TPD spectra of CH3Cl for increasing precoverage of Li.
Figure 3. Snapshots (top) and energy profile of the reaction of CH3 with the graphene surface. The left panel shows the initial state (IS) with CH3 in a weakly bound preadsorbed state. The middle panel shows the transition state (TS), where both the CH3 radical and the C atom in graphene to which it is binding begin to show sp3 character (bonds shown in white). The right panel gives the structure of the final state (FS), highlighting the sp3 character (bonds shown in white) of the C atom in graphene to which the CH3 group is bonded. The zero of the energy profile is arbitrarily taken as CH3 in the weakly bound preadsorbed state.
of CH3Cl, achieved by quantitative dosing of CH3Cl at 105 K to a coverage of 2.3 × 1014 CH3Cl cm−2 (∼0.3 ML). Li predeposited on HOPG at low temperature exhibits no 2-D structure and is known to form clusters on graphene at room temperature.9,10 Although Li donates most of its outer 2s electron to the surface, the electron remains localized for chemical activity.11 As seen from the inset, essentially all CH3Cl has reacted when the Li/CH3Cl stoichiometry reaches ∼2:1. This ratio is in agreement with calculations shown below, which indicate that two Li atoms are needed for every one CH3Cl, one Li atom to break the C−Cl bond and a second to act catalytically in the C−Cl bond scission, lowering the activation energy barrier. The reaction pathway for a CH3 radical chemically binding to the pristine graphene surface from a physically preadsorbed state was computed. We have verified that graphene is a quantitatively reasonable model for graphite when large supercells are used; the difference in binding energies of CH3 to mono- and bilayer graphene was only 0.05 eV when a supercell containing 160 carbon atoms per layer was used. In contrast, using a supercell with 72 carbon atoms per layer gives a difference in CH3 binding energy of 0.6 eV going from monoto bilayer graphene, in agreement with previous calculations that suggested the dependence of the binding energy on the number of graphene layers used.12,13 Our calculations indicate that the observed layer thickness dependence is actually an artifact of using a model with too few atoms per layer in the periodic cell. We used the climbing-image nudged elastic band method,14−16 as implemented within VASP,5−7 to compute the barrier height. The CH3 in the preadsorbed state is weakly bound to the graphene surface by about 0.13 eV due mainly to van der Waals interactions. The geometry of the CH3 radical is planar, as can be seen in Figure 3 for the initial state (IS), and located 3.2 Å above the graphene surface at the atop site. The energy of the transition state (TS) is 0.28 eV above the preadsorbed state and shows the transition of CH3 from a
planar sp2 configuration to a bent sp3-like geometry (TS in Figure 3). Note also that the TS involves the initial puckering of the graphene carbon atom, which we highlight by coloring the bonds white in the center panel of Figure 3. Similar puckering of the graphene layer was also found during modeling of H chemisorption.12,17 The final state (FS) of CH3−graphene is about 0.4 eV lower in energy than the preadsorbed state. The local sp3 character of the C atom in graphene to which the CH3 moiety is bound is clearly seen (bonds shown in white). We have also computed the reaction pathway for dissociation of CH3Cl and subsequent binding of CH3 on the lithiated graphene surface in order to closely model the experiments. As with our previous work on lithiated carbon nanotubes, we find that two Li atoms are required for facile reaction, with the second Li atom acting as a catalyst for the reaction.18,19 Figure 4 shows three stages of the reaction, the IS just prior to CH3Cl dissociation, a local minimum (LM) intermediate state where the CH3 radical is stabilized by the Li atom, and the FS where the CH3 radical is chemically bonded to a carbon atom in the graphene surface. The chemisorption of CH3 occurs from the LM over a barrier of ∼0.3 eV, as seen in Figure 4. One notes from the structural analysis in the right-hand panel that the expected puckering (sp3 hybridization) of the graphene sheet occurs at the reacted carbon site where CH3 is bonded. It has been suggested that the proximal presence of Li radical species would result in the formation of Li-containing compounds in preference to chemisorption of the CH3 radical to the graphene 1681
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The Journal of Physical Chemistry Letters
Letter
breaking reaction. Diffusion, on the other hand, is a kinetic event that does depend on the barrier shown in Figure 3. While at present the details of the mechanism of the reactions to produce these minor hydrocarbon products are not known, it is likely that the known propensity for mobile CH3 radicals to abstract H atoms from neighboring CH3 radicals by an activated process of ∼0.5 eV23 is involved in CH4 production. We calculated the barrier to H abstraction by CH3 to be 0.56 eV in the gas phase. The reported activation energy for CH3 diffusion is consistent with CH4 desorption at and above ∼330 K. A more detailed account of these findings will be published elsewhere.24 In summary, the reaction of CH3Cl + Li on a graphite surface has been used to generate CH3 radicals. These radicals mainly react with the graphite basal plane with less than ∼1/4 of the CH 3 radicals reacting to form CH 4 and C 2 and C 3 hydrocarbons. The involvement of surface defects in this chemistry is unlikely. CH3 radicals are bonded to graphite carbon atoms to form a 0.52 eV chemical bond (sp3) and become mobile at temperatures above ∼330 K. The bonding of CH3 to the basal plane of graphite is reminiscent of the recent discovery that activated atomic H, with a kinetic energy of ∼0.4 eV, will also form covalent C−H bonds with the basal plane of graphite, whereas less energetic H atoms are unreactive.25,26
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Figure 4. Snapshots (top) and energy profile for dissociation of CH3Cl on graphene and subsequent CH3 bonding to the surface containing 2Li/CH3Cl. The left panel shows the IS before the C−Cl bond cleavage. The middle panel shows the LM intermediate state, with one Li atom stabilizing the CH3 radical. The right panel gives the structure of the FS, highlighting the sp3 character (bonds shown in white) of the C atom in graphene to which the CH3 group is bonded. The two transition states, TS1 and TS2, exhibit barriers of about 0.1 and 0.3 eV, respectively.
EXPERIMENTAL METHODS Experiments were carried out in ultrahigh vacuum using highquality HOPG (SPI-1 grade, ∼109 edge sites per cm2 or
