The effect of heteroatoms in carbonaceous surfaces: computational analysis of H chemisorption on to a PANH and Si-doped PAH

Abstract

ABSTRACT In this work, we studied the effect of a heteroatom (nitrogen and silicon) inside the main skeleton of the carbonaceous surface in the H chemisorption reaction. The process taking place on to an N-doped polycyclic aromatic hydrocarbon (PAH), known as PANHs, shows differences in the energetic parameters only when the process is carried out on to the N atom. When N is located in an external site of the surface, the process is barrierless, whereas if N is in an internal position of the surface the activation energy drastically increases. The aromaticity of these N-doped systems does not change much concerning pristine coronene. In a Si-doped PAHs, the chemisorption on to the Si atom takes place in the absence of activation energy, regardless the position of Si on the surface. Moreover, the adsorption on to their neighbour carbon atoms is carried out with lower activation energies than those found in the reaction on to pristine PAH, indicating that the presence of silicon atoms in the surface favours H chemisorption. This might be due to a loss of aromaticity on the surface. In both cases, the reactions become significantly more exoenergetic. Finally, the presence of heteroatoms favours kinetically the reaction, where the rate coefficient of H2 formation process, calculated considering all of the sites of every PAH studied in this work, reaches a close value to the reported for diffuse interstellar medium and photodissociation regions ($R_{_{\mathrm{ H}\mathrm{ }_2}} = 1 \times 10^{-17}$ cm3 s−1 at 40 K).