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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...

We show with angle-resolved photoemission spectroscopy that a new energy band appears in the electronic structure of electron doped hydrogenated monolayer graphene (H-graphene). Its occupation can be controlled with the hydrogen amount and allows for tuning of graphene's doping level. Our calculations of the electronic structure of H-graphene suggest that this state is largely composed from hydrogen 1s orbitals and remains extended for low H coverages despite the random chemisorption of H. Further evidence for the existence of a hydrogen state is provided by X-ray absorption studies of undoped H-graphene which are clearly showing the emergence of an additional state in the vicinity of the π * -resonance.

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