Interaction of hydrogen molecules with polycyclic aromatic hydrocarbons modified with Zr atoms: different modifications yield distinct properties

Abstract

Ability of polycyclic aromatic hydrocarbons (PAH), namely coronene (C₂₄H₁₂), circumcoronene (C₅₄H₁₈) and circumpyrene (C₄₂H₁₆) modified with Zr atoms to bind H₂ molecules is investigated using density functional theory. It is shown that PAHs doped with Zr atoms show different H₂ binding properties than the ones decorated with Zr atoms. The studied Zr-doped PAHs are able to bind up to seven H₂ molecules per one Zr atom via weak chemisorption regardless of their size (number of C atoms). Calculated gravimetric H₂ densities of Zr-doped PAHs increase in the following order: Zr-circumcoronene (1.9 wt%) < Zr-circumpyrene (2.3 wt%) < Zr-coronene (3.7 wt%). In the case of Zr-decorated PAHs, the first H₂ molecule undergoes dissociation upon interaction with Zr atom, with an energy outcome of approximately −160 kJ mol⁻¹. Additional five H₂ molecules can be bound to Zr-decorated PAH either by Kubas interaction or physisorption. The H₂ binding capacity of these systems can be increased by decoration with Zr atoms from both sides of the PAH backbone. In particular, coronene decorated with two Zr atoms is able to bind up to twelve H₂ molecules, corresponding to gravimetric density of 5.0 wt%.