Reetu
H2 is one of the most important molecules in the interstellar medium. It plays a pivotal role in the interstellar chemistry through reaction with ions and radicals. Furthermore, the energetics of the H2 formation reaction directly affect the thermal balance of the interstellar medium. It is widely accepted that the dominant mechanism for the formation of H2 in the interstellar clouds is through surface-catalysed reaction on dust-grains. One of the possible mechanism for the gas-surface reaction is the Eley-Rideal reaction mechanism in which an atom from the gas-phase reacts directly with an atom adsorbed on the surface. The key step in this reaction is the adsorption of the first H atom on dust grains, which in our studies are taken to be graphitic in origin.
In the literature, it has been shown that for adsorption of H atoms on graphitic surfaces, calculations using either the full graphene sheet (via periodic boundary conditions) or using poly-aromatic hydrocarbons (PAHs) give similar results [1,2]. However for adsorption of other abundant elements such as C, O and N these comparisons have not been made yet, although the interactions with PAHs have been studied[3-5]. Moreover, there is still a question over the influence of dispersion interactions on the energetic of the absorption of atoms in these systems.
To understand the related reactive process and to study the interactions (and reactions) of PAHs and graphite/grapheme with C, O and N atoms present in the interstellar medium. We have performed Density functional theory (DFT) calculations. In this poster we will compare PAHs to graphitic surfaces and the influence of graphitic systems. Moreover, we will also show the influence of the inclusion of dispersion interactions in these systems.
References:
[1] Morella Sanchez and Fernando Ruette, Chem. Phys. Lett., (2015), 640, 11-15.
[2] L. Jeloaica, V. Sidis, Chem. Phys. Lett., (1999), 300, 157-162.
[3] F. Dulieu, L. Amiaud, E. Congiu, J.-H. Fillion, E.Matar, A.Momeni, V. Pirronello and J.L.Lemaire, A&A, (2010), 512, A30.
[4] T. Fromherz, C. Mendoza and F. Ruette, Mon. Not. R. Astron. Soc., (1993), 263, 851-860.
[5] H. Bergeron, N. Rougeau, V. Sidis, M. Sizun, D. Teillet-Billy and F. Aguillon, J. Phys. Chem. A, (2008), 112, 11921-11930.
Computational methods:
All the calculations are performed using density functional theory (DFT). For adsorption of atoms on coronene surface, we have used Gaussian09 software with density functional B3LYP. We have also study the importance of dispersion forces by adding empirical dispersion with density functional B3LYP-GD3BJ. The basis set we have used is 6-311G(d,p).
For graphene study we have used VASP v.5.4.4 with density functional PBE and plane wave basis set.
During these calculation we have given complete relaxation to the coronene ring. On the other hand in case of graphene we have relaxed only four carbon atom.
Results:
In Figure 2, panel A, B, C, and D represents the potential energy surface of atomic gases on coronene and graphene surface in the form of binding energy with variation of distance of atoms from the surface. These shows that at a large distance there is no interaction of these atoms with the surface. However with closer approach C and O show barrierless chemisorptions. Whereas H show chemisorptions through an energy barrier, which is in good agreement with the previous studies. On the other hand N does not show any interaction with the surface at large distance and shows metastable stable adsorption with closer approach through an energy barrier.
In addition to this carbon atom show a double well adsorption on the graphene surface. In the first well carbon form chemical bond with the surface carbon atom and then it start to repel the surface carbon atom. This increase the surface energy and form a complex geometry. Then finally replace the surface carbon atom and reduce the surface energy and we get a second well.
By adding the empirical dispersion, in addition to chemisorption we get physisorption well at a large distance in case of H and N. Moreover, this also reduces the energy barrier and provides a higher adsorption energy value.
Future work:
Study of interactions of H, C, O, N, and other small atoms and molecules on Bi-layer graphene surface. Study of reactions of atoms on pre-adsorbed surface.
Comparisons of results of graphene and PAH interactions.
References:
M. Sachez and F. Ruette, CPL, (2015), 640. 11-15
L. Jeloaica, V. Sidis, CPL, (1999), 300, 157-162.
F. Dulieu and et al., A&A, (2010), 512, A30.