The adsorption energy is defined by E ad = E total -E COF -E m , where E total , E COF and E m are the energies of the whole system, COF and one small molecule, respectively. All computations were performed within the framework of density functional theory (DFT) as implemented in the Vienna Ab-initio Simulation Package (VASP) code by using the projector augmented wave method with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. The influence of vdW interactions is considered by using a modified version of vdW-DF, referred to as "optB86b-vdW". The projector augmented wave potentials were used with a kinetic energy cutoff of 500 eV. Brillouin zone integration was sampled by a 1 × 1 × 1 Gamma centered k-mesh. The system was simulated with a