Methyl Radical Reactivity on the Basal Plane of Graphite

Abstract

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.

Rapid Atomic Li Surface Diffusion and Intercalation on Graphite: A Surface Science Study

“…The sample was inserted into the ultrahigh vacuum chamber on a manipulator that allowed the HOPG sample to be electrically heated and cooled via tungsten wires attached to an oxygen-free high conductivity (OFHC) copper support assembly connected to a liquid nitrogen coldfinger. The temperature agreement between n -heptane thermal desorption kinetics from graphite measured here (not shown, see ref ) and in another laboratory indicates that our temperature scale is correct to within 5 K at 208 K. The HOPG surface was cleaned at ∼1100 K for several hours under ultrahigh vacuum. Auger electron spectroscopy was used to monitor the elemental composition of the surface (1 keV beam; 10 eV/s scan rate unless otherwise noted; 2 V peak-to-peak modulation; I e / A = 10 –4 A cm –2 ).…”

“…TPD with a heating rate of either 0.5 or 2 Ks –1 was performed, and desorbing species were monitored with the closely coupled quadrupole mass spectrometer. Further details about TPD measurements in this vacuum system can be found elsewhere …”

Hyperaccumulation Route to Ca-Rich Hard Carbon Materials with Cation Self-Incorporation and Interlayer Spacing Optimization for High-Performance Sodium-Ion Batteries

“…Therefore, it suggests that the Ca ion occupies between interlayers. Here, we indeed observe the significantly reduced d -interlayer spacing from ∼0.40 to ∼0.38 nm (Figure d,f) by removing metal cations (HC-1200H). − Besides, the corresponding selected area electron diffraction (SAED) pattern inset suggests the typical polycrystalline characteristics of HC materials due to the presence of local turbostratic structures. ,,, The SAED patterns become sharper by increasing the temperature, which indicates the trend of the lattice graphitization. In order to further examine the residue Ca ions in lattice, the EDS elemental mappings were conducted and shown in Figure .…”

“…In terms of C 1s spectra, three typical peaks can be deconvoluted with the HC-1200, including the 284. 35,36,53 Also, the C−Cl bond can reside in the interlayer spacing or edges of carbon layers to balance the charges of the excess cation, which is favorable for the stability of residue Ca ions in lattice. 49,54 In this regard, the sp 2 carbon atoms are arranged in conjugated honeycomb lattices, relating to the graphitized carbon, while the sp 3 carbon atoms refer to the defects on graphite layers.…”

“…It is worth noting that an additional peak at 287.0 eV can be identified with HC-1200 corresponding to the C–Cl bond. Both the KCl and CaCl 2 are reported to bound to the defects of HC materials through the C–Cl bond. ,, Also, the C–Cl bond can reside in the interlayer spacing or edges of carbon layers to balance the charges of the excess cation, which is favorable for the stability of residue Ca ions in lattice. , In this regard, the sp 2 carbon atoms are arranged in conjugated honeycomb lattices, relating to the graphitized carbon, while the sp 3 carbon atoms refer to the defects on graphite layers. , By contrast, the C 1s spectra shows higher content of sp 3 carbon and O–CO bonds after acid-washing, whereas the sp 2 carbon atoms greatly decrease. It stands to the reason that more defects on graphite structures have been created due to the removal of Ca ions in lattice. , In addition, the content of Ca ions is indeed reduced by the comparison of Ca 1s spectra before and after the acid-washing.…”