Kinetic Isotope Effect in the Hydrogenation and Deuteration of Graphene

Abstract

Summary We have observed a strong kinetic isotope effect (KIE) for the hydrogenation/deuteration reaction on quasi-free-standing monolayer graphene leading to substantially higher saturation coverage of D as compared to H. The unique geometry of the experiment and the relatively low flux of H/D atoms allowed us to study the surface chemistry of graphene during hydrogenation/deuteration monitoring the whole reaction in situ and in real time by photoemission spectroscopy. The experimental results for hydrogenation/deuteration are well described using a phenomenological kinetic model with terms for chemisorption, associative desorption and reflection of H/D atoms. The observed significant difference in the reaction involving H and D with graphene is relevant for isotope specific chemical reactivity in functional carbon materials and is important for the intensive research regarding atomic control of chemical reactivity at the nanoscale. The deuterium-doped graphene develops a band-gap similar to that observed for hydrogen [Haberer et al, Nano Letters 6, (2010), 3360] and consequently that the electronic isotope effect is weak. Electronic isotope effect MotivationKinetic isotope effects (KIEs), i.e. a different reaction constant for isotopes, are important phenomena in physical chemistry. They have been studied for a long time in their relation to the activation and rate of chemical reactions. A significant contribution to KIE are vibrational zero-point energy (ZPE) effects that upshift the energy of a chemical bond by half the phonon frequency of its constituents. Hence, KIEs are easy to observe in the hydrogen isotopes (deuterium and tritium) due to their large relative mass difference and were studied, even at the single-molecule level for hydrogen transfer in organic reactions including acid and base catalysis, enzyme reactions and catalytic decomposition. Most of these reactions were carried out with H and D in a molecular configuration and it was found that the reaction constant for the D compounds is lower than for the corresponding H compounds. These results are explained by the larger ZPE for H bonds, which results in a lower potential energy barrier to be overcome for Hbonds breaking as compared to D bonds. As a result, the bond-breaking reaction proceeds faster for the H compound than for the D compound. There have been very few reports on experiments carried out with one reactant being atomic H or D such as the H-D exchange reactions or reactions of H(D) with oxygen, where it was found that the oxygen-H(D) reaction proceeds equally fast for D and H. This was attributed to the absence of vibrational zero-point energy effects for the atomic species. Much less is known about the kinetics of these reactions in functional carbon materials based on hydrogenated graphene, such as novel 2D polymers.Here we present a novel inverse kinetic isotope effect involving the reaction of H/D radicals with the carbon atoms of epitaxial graphene. The advantages of graphene include a flat, well ordered large area surfa...

Growth of carbon nanotubes via twisted graphene nanoribbons

“…we speculate that the flexible ribbons twist to adopt a versatile orientation222324 under such moderate heating condition. The hydrogen atoms along the side edges can be randomly removed as H 2 via associative desorption2526 during this stage, before the subsequent tubes formation. These helical intermediates start to exhibit a salient tubular form from 1,000 °C, as indicated by the characteristic inner tube peaks emerged at 300–400 cm −1 in the radial breathing modes (RBM) region.…”

Cooperative Interplay of van der Waals Forces and Quantum Nuclear Effects on Adsorption: H at Graphene and at Coronene

“…Recently, in an elegant set of experiments on quasi free-standing graphene it was shown that D adsorption was more facile than H adsorption. 37 The H and D atoms were obtained with the standard approach of thermally cracking molecular hydrogen at very high temperatures (3000 K). As a result the H/D atoms impinging on the surface were translationally hot and at a much higher temperature than the graphene substrate; consequently in these measurements Our results suggest that at low temperatures the chemisorption of a first H at carbonaceous materials is easier than previously thought.…”

Quantum dynamics of hydrogen atoms on graphene. II. Sticking

“…Though experimental studies have shown adsorption-induced metal-insulator transitions, 7,8 reversible opening of a band-gap 9,10 and even ferromagnetic hysteresis, 11 the necessary precise control on the hydrogenation process has yet to be achieved. Hydrogenation of graphite has been studied in a number of experimental works, 3,[12][13][14][15][16][17][18][19][20][21][22][23][24] with a variety of surface-science techniques including thermal desorption, high-resolution electron-energy-loss spectroscopy, scanning tunneling microscopy, low-energy electron diffraction, angle a) Electronic mail: matteo.bonfanti@unimi.it b) Electronic mail: rocco.martinazzo@unimi.it resolved photo-emission spectroscopy, and X-ray photoemission spectroscopy. It has now been well established that sticking is an activated process, with a barrier related to the surface reconstruction accompanying the sp 2 → sp 3 re-hybridization of the carbon atom involved in the bond formation process.…”

“…Furthermore, a marked isotope effect has been found when hydrogenating epitaxial graphene grown on Au/Ni and it has been argued that it directly relates to the sticking cross sections. 23 More recently, the role of the specific graphenic substrate employed has been addressed, and substantial differences in hydrogen saturated structures have been reported between quasi-free-standing graphene and metal-bound graphene. 24 At present, a complete and thorough description of this vast phenomenology is lacking, and experimental results, although pointing towards strong dynamical effects, are affected by such a large variety of almost uncontrollable factors -the hydrogen coverage, the quality of the substrate, and the nature and energy distribution of the incident hydrogen atom beam -that they are all quite inconclusive for the very first adsorption events.…”