On the PES for the interaction of an H atom with an H chemisorbate on a graphenic platelet.

Motivated by the problem of H(2) formation in diffuse clouds of the interstellar medium (ISM), we study the effect of including van der Waals-type corrections in DFT calculations on the entrance PES of the Eley-Rideal reaction H(b) + H(a)-GR → H(b)-H(a) + GR for a graphenic surface GR. The present calculations make use of the PBE-D3 dispersion corrected functional of Grimme et al. (2010) and are carried out on cluster models of graphenic surfaces: C(24)H(12) and C(54)H(18). To assess the soundness of the chosen functional we start by revisiting the H-GR adsorption potential. We find a satisfactory on top physisorption well (43-48 meV) correctly located at an H-GR distance of 3 Å. We then revisit the H(b)-H(a)-GR system using both the PW91 and PBE functionals. Our calculations do not reproduce the tiny potential barrier reported earlier for large H(b)distances from the surface. The barrier in the calculations of Sidis et al. (2000) and Morisset et al. (2003, 2004) has been traced to their previous use of an LSDA + POSTSCF PW91 procedure rather than the genuine PW91 one. The new PBE-D3 PES for the H(b)-H(a)-GR system is reported as a function of the H(b) distance to the surface and its impact parameter relative to the H(a) chemisorbate for the so-called "fixed puckered" ("diabatic" or "sudden") approach. The results are discussed in relation to recent experimental and theoretical work.

Effects of a nonrigid graphene surface on the LH associative desorption of H atoms and on the deexcitation of nascent H2 molecules colliding with model walls of carbonaceous porous material.

A planar slab of 200 C atoms bound by the Brenner potential is used to study the Langmuir-Hinshelwood (LH) recombination of two physisorbed H atoms on a graphene sheet and to simulate afterward successive collisions of the nascent H(2) molecule with pore walls of a carbonaceous dust grain of the interstellar medium. The study is based on successive propagations of classical trajectories for the 200 C + 2 H atoms. The characteristics of H(2) molecules formed by the LH reaction on the flexible surface are found to differ but negligibly from those formed on a rigid one. Collisions of those H(2) molecules with graphitic pore walls are studied next. Reflection from and "trapping" onto the surface is observed and discussed. The most important energy transfer is from the molecule vibration to its rotation. This conversion mediates the transfer of the molecule internal energy to its translation or to surface heating. It is found that a single H(2)-surface impact has little effect on the internal energy of the molecules. The grain absorbs on the average but a very weak energy. Several impacts are required to appreciably cool the molecule. The molecule cooling is accompanied by a significant increase of its translational energy. The swifter the molecules are or get, the larger the number of their impacts on the surface they undergo per unit time and the more efficiently cooled they get.

OH formation from O and H atoms physisorbed on a graphitic surface through the Langmuir-Hinshelwood mechanism: a quasi-classical approach.

We study the quasi-classical dynamics of OH formation on a graphitic surface through the Langmuir-Hinshelwood (LH) mechanism when both O and H ground-state atoms are initially physisorbed on the surface. The model proceeds from previous theoretical work on the LH formation of the H 2 molecule on graphite [Morisset, S.; Aguillon, F.; Sizun, M.; Sidis, V. J. Chem. Phys. 2004, 121, 6493; ibid 2005, 122, 194704]. The H-graphite system is first revisited with a view to get a tractable DFT-GGA computational prescription for the determination of atom physisorption onto graphitic surfaces. The DZP-RPBE combination is found to perform well; it is thereafter used along with MP2 calculations to determine the physisorption characteristics of atomic oxygen on graphitic surfaces. We also deal with chemisorption. In accordance with previous work, we find that O chemisorbs on graphite in a singlet spin state epoxy-like conformation. In the triplet state we find only "metastable" chemisorption with an activation barrier of 0.2 eV. The physisorption results are then used in the LH dynamics calculation. We show that in the [0.15 meV, 12 meV] relative collision energy range of the reacting O and H atoms on the surface, the OH molecule is produced with a large amount of internal energy ( approximately = 4eV) and a significant translation energy (>or=100 meV) relative to the surface.

Unrestricted study of the Eley-Rideal formation of H(2) on graphene using a new multidimensional graphene-H-H potential: role of the substrate.

The Brenner potential is adapted to handle chemical interactions and reactions of H atoms at a graphene surface. The adapted potential reproduces several important features of DFT computed data and reveals an extended puckering of the surface upon its adsorption of an H atom. This potential is used to investigate in a much more realistic way than has been done before, the Eley-Rideal abstraction reaction producing H(2) in H + H-graphene collisions at energies E(col)< or = 0.2 eV. The graphene surface is represented by a slab of 200 carbon atoms and the study is carried out using classical molecular dynamics for vertical incidences in a cylinder centered about the chemisorption axis. A highlight of the present study is that upon the arrival of the gas phase H atom, the adsorbent C atom is attracted and pulls out its surrounding surface atoms. The hillock thus formed remains puckered until the newly formed molecule is released. The range of impact parameters leading to reaction depends on the collision energy and is governed by the shape of the entrance channel potential; the reaction probability in this range is 100%. On average, in the studied E(col) range, the available energy (3.92 eV + E(col)) is shared as: 69-52% for the internal energy, 11-23% for the translation energy and 20-25% for the energy imparted to the surface. Also, the average vibration and rotation energy levels of the nascent H(2) molecule are, respectively, v = 5-4 and j = 2-4.

Wave-packet study of H2 formation on a graphite surface through the Langmuir-Hinshelwood mechanism.

We have studied the formation of the H2 molecule on a graphite surface, when both H atoms are initially physisorbed. The graphite surface is assumed to be planar. The interaction potential is modeled to reproduce the experimental properties of H physisorption on graphite. Extending our previous work [S. Morisset, F. Aguillon, M. Sizun, and V. Sidis, J. Chem. Phys. 121, 6493 (2004)], full-dimensionality quantum calculations are presented for collision energies ranging from 4 to 50 meV. It is shown that the reaction occurs with a large cross section and produces the H2 molecule with a considerable amount of vibrational energy. The mechanism is either direct or involves the formation of an intermediate complex.

Quantum dynamics of H2 formation on a graphite surface through the Langmuir Hinshelwood mechanism.

We have studied the formation of the H2 molecule on a graphite surface, when both H atoms are initially physisorbed. The graphite surface is assumed to be planar, and a model potential is obtained in a semiempirical way to reproduce the experimental properties of H physisorption on graphite. The reaction probability has been computed in the case when the angular momentum of the relative H-H motion lies parallel to the surface plane. Three-dimensional wave packet calculations have been performed for collision energies ranging from 2 to 50 meV. It is shown that the reaction occurs with a significant probability and produces the H2 molecule with a considerable amount of vibrationnal energy. A simple mechanical model is presented, where desorption of the nascent H2 molecule results from two successive binary elastic collisions.