Quantum dynamics study of the Langmuir–Hinshelwood H+H recombination mechanism and H2 formation on a graphene model surface

Exploration of Graphene Defect Reactivity toward a Hydrogen Radical Utilizing a Preactivated Circumcoronene Model

A preexisting chemisorbed defect is well-known to increase the reactivity of graphene which is normally chemically inert. Specifically, the presence of chemisorbed hydrogen atoms forming an sp³-hybridized C-H bond is known to increase the reactivity of neighboring carbon atoms toward additional hydrogenation with wide-ranging applications from materials science to astrochemistry. In this work, static DFT and DFT-based direct dynamics simulations are used to characterize the reactivity of a graphene sheet around an existing C-H bond defect. The spin density landscape shows how to guide subsequent H atom additions, always bonding most strongly to the carbon atom with greatest spin density. Molecular dynamics of an impinging H atom under thermal conditions with defect graphene was used to determine the statistics of probable reactions. The most frequent outcome is inelastic scattering (48%) and then Eley-Rideal (ER) abstraction of the chemisorbed H atom as vibrationally hot H₂ (40%), while the least likely, but probably most interesting, result is formation of a novel C-H bond (12%). The C-H bonds always form in the β sublattice. The carbon atom in the para position shows to be most reactive toward the incoming H atom, followed by the ortho carbon, in agreement with the spin density computed in the static calculations. Globally, the graphene energy surface is repulsive, but the defects create local channels into this energy surface through which reactants can move locally through and react with the activated surface without a barrier.

First-Principles Study of the Reaction Mechanism of CHO + H on Graphene Surface

Many organic molecules observed in the interstellar medium are considered to be formed on dust grains and populated into the gas phase. We analyzed the reaction of HCO + H on a graphene surface using ab initio molecular dynamics simulations as a case study of the formation and desorption of organic molecules on interstellar dust particles. During the reactions of chemisorbed CHO (chemisorbed at the C atom) with free H, CO was generated and efficiently desorbed from the surface. These results suggest that the reactions, of which the reactant forms a covalent bond with the surface while the product does not, cause efficient desorption of the product upon reaction. In such reactions a repulsive force between the product and the surface would be generated and accelerate translation of the product in a specific direction. In addition, it was also shown that the branching ratio of the reactions between radical species on the surface would be affected by the form of the adsorption on the surface, e.g., when a free H reacted with the CHO chemisorbed at the C atom, CH₂O was not generated.