Adsorption of Polycyclic Aromatic Hydrocarbons and C₆₀ onto Forsterite: C–H Bond Activation by the Schottky Vacancy

Abstract

Understanding how to catalytically break the C–H bond of aromatic molecules, such as polycyclic aromatic hydrocarbons (PAHs), is currently a big challenge and a subject of study in catalysis, astrochemistry, and planetary science. In the latter, the study of the breakdown reaction of PAHs on mineral surfaces is important to understand if PAHs are linked to prebiotic molecules in regions of star and planet formation. In this work, we employed a periodic density functional theory along with Grimme’s D4 (DFT-D4) approach for studying the adsorption of a sample of PAHs (naphthalene, anthracene, fluoranthene, pyrene, coronene, and benzocoronene) and fullerene on the [010] forsterite surface and its defective surfaces (Fe-doped and Ni-doped surfaces and a MgO-Schottky vacancy) for their implications in catalysis and astrochemistry. On the basis of structural and binding energy analysis, large PAHs and fullerene present stronger adsorption on the pristine, Fe-doped, and Ni-doped forsterite surfaces than small PAHs. On a MgO-Schottky vacancy, parallel adsorption of the PAH leads to the chemisorption process (C–Si and/or C–O bonds), whereas perpendicular orientation of the PAH leads to the catalytic breaking of the aromatic C–H bond via a barrierless reaction. Spin density and charge analysis show that C–H dissociation is promoted by electron donation from the vacancy to the PAH. As a result of the undercoordinated Si and O atoms, the vacancy acts as a Frustrated Lewis Pair (FLP) catalyst. Therefore, a MgO-Schottky vacancy [010] forsterite surface proved to have potential catalytic activity for the activation of C–H bond in aromatic molecules.

Laboratory and field analogs of the Martian (sub)surface

“…If the PAHs are not strongly bound to the surface, they may not be affected by the electron transfer reaction and oxidizing radicals produced by the Fenton mechanism (Reactions 2.8-2.11). In general the binding energy increases with the PAH size/surface area, a trend attributed to the increasing number of contact points (though pyrene is an exception to this, having higher binding energies than fluoranthene because of its more compact geometry) (Campisi et al, 2022). Conversely, triphenylene in our experiments may have a lower binding energy because of its less compact geometry, facilitating its preservation.…”

“…Interestingly, only pyrene, fluoranthene, and perylene show degradation in their IR spectra and seem to be significantly affected by this mechanism. Though probably experiencing only weak bonding or long range interactions with the nontronite surface (Campisi et al, 2021(Campisi et al, , 2022, in our experiments pyrene, fluoranthene, and perylene were subject to photodegradation under a dry, hypoxic atmosphere. A summary of the results can be seen in Table 2.6.…”

“…Environmental pollution studies have highlighted the photocatalytic effects of metal oxides and clays that facilitate the degradation of PAHs (Dong et al, 2010a,b). However, PAHs bond to mineral grains via π-cloud interactions (Campisi et al, 2021(Campisi et al, , 2022. Experiencing only weak bonds or long range interactions with their mineral substrates, the extent of photocatalytic effects on their degradation in a dry, hypoxic environment without abundant (oxy)hydroxide radical production is questioned.…”

“…In order for PAHs to be broken down by photocatalysis on TiO 2 , they must be adsorbed to the surfaces of the mineral grains. Generally PAHs bond to mineral grains via π-cloud interactions, characterized by long-range interactions and/or weak bonding between the PAH and the mineral surface (Campisi et al, 2021(Campisi et al, , 2022. The binding and adsorption energies of the PAH-mineral system will dic-tate the efficacy of the metal-catalyzed degradation mechanisms that the PAH is subjected to.…”