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.

Figure 1

Optimized slab models of [010] forsterite at two different orientations: (a) [010]-fo, (b) Fe-[010]-fo, (c) Ni-[010]-fo, and (d) V_MgO-[010]-fo (the location of the vacancy is indicated by the Si and O labels reported on the corresponding undercoordinated atoms). The vacuum region is located along the z-axis, and the atomic labels of the corresponding atoms (Si, Mg, O, Ni, and Fe) are reported.

Figure 2

Side-view figure that shows the labels of the atomic positions of the [010]-fo forsterite. M shows the location of an Mg, Fe, and Ni atoms or MgO vacancy. The table reports distances (d), in Angstrom (Å), and angles, in degrees (°), of the atoms found in the top figure.

Figure 3

The skeleton structure of PAHs and the location of carbon–transition-metal interactions less than 3 Å (see the Supporting Information), on the PAH structure, when they adsorb on the Fe and Ni-[010]-fo. The legend shows the symbol related to C–Fe (circles) and C–Ni (squares) interactions.

Figure 4

Side views using two different perspectives (second perspective in thumbnail) of the optimized geometries of benzocoronene adsorbed on (a) [010]-fo, (b) V_MgO-[010]-fo, (c) Fe-[010]-fo, and (d) Ni-[010]-fo. Atomic labels are reported on the corresponding atoms.

Figure 5

Side views using two different perspectives (second perspective in thumbnail) of the optimized geometries of fullerene adsorbed on (a) [010]-fo, (b) Fe-[010]-fo, (c) Ni-[010]-fo, and (d) V_MgO-[010]-fo. Atomic labels are reported on the corresponding atoms.

Figure 6

For the PAHs adsorbed on the V_MgO-[010]-fo surface, we report a schematic representation of the atomic labels of the hexagonal rings chemisorbed with the undercoordinated Si and O for (a) naphthalene, anthracene, fluoranthene, benzocoronene, and fullerene, (b) pyrene, and (c) coronene. A table reporting the average distance and distance (d) in Angstrom (Å), angles and average dihedral angles , in degrees (deg), coordination number (C.N.) for the specified atom, and the number (n) of atoms (C and Mg) interacting with each other ([nC–nMg ]) for the vacancy surface, PAHs, and C₆₀. Missing values indicate no interaction between the atoms. The average values are calculated from the geometrical parameters reported in the Supporting Information.

Figure 7

Vacancy reconstruction caused by the adsorption of coronene on V_MgO-[010]-fo. In the thumbnail is reported the close-up perspective of the reconstruction. The atomic labels on the corresponding atoms indicate the reconstructed atoms.

Figure 8

Binding energies of the adsorption of PAHs on [010] forsterite surfaces modeled in this study. The surfaces follow the order shown in the legend for all PAHs. Numerical values are reported in the Supporting Information. The asterisks (*) on the bars show data reported from a prior work.

Figure 9

A schematic representation of the (a) parallel and (b) perpendicular adsorption of naphthalene on V_MgO-[010]-fo. Atomic labels depict the bond formation due to the adsorption.

Figure 10

Spin-density isosurface (isovalue 0.007 e/A³) of V_MgO-[010]-fo (a) and the dissociation of the C−H bond of naphthalene on the V_MgO-[010]-fo (b). Yellow density indicates the spin-up population and red density the spin-down one.