Density functional theory study of CO formation through reactions of polycyclic aromatic hydrocarbons with atomic oxygen (O(3P))

Abstract

To provide a qualitative analysis on how the reaction energies and mechanisms of initial steps of char combustion depend on a) the size of polycyclic aromatic hydrocarbons (PAHs) and b) the reaction sites, we investigate the initial steps of the individual reactions of thirteen PAHs with atomic oxygen with density functional theory. Specifically, we have studied three elementary steps: i) O(3P) addition, ii) subsequent oxyradical formation through H abstraction, and iii) CO elimination from the oxyradicals. We find that the CO elimination from the oxyradicals follows either a three, two, or single step mechanism, depending on the geometrical strain at the reaction site. In accordance, we observe that as the geometrical strain at the reaction site increases, the activation energy of the first step of the CO elimination from the oxyradicals increases. Important findings that are relevant for a multiscale modeling of the oxidation of large PAHs, e.g. during char burnout, are that the activation energies for these reactions can be related to the reaction energies through Bell-Evans-Polanyi (BEP) fits, and that for the O addition and the last step of the three or two step CO elimination mechanisms, i.e. the CO elimination from carbonyl radicals, the reaction energies are correlated with the stabilization energies calculated from the Hückel molecular orbital theory.