Adsorption of O2 on Cobalt-(n)Pyrrole Molecules from First-Principles Calculations

In order to clarify the adsorption mechanism of the O-2 molecule on Co-polypyrrole composite metallo-organic catalyst, we have investigated the interaction between the molecule and Co-(n)pyrrole model clusters (n = 4, 6) using the density functional theory. The stable adsorption site of the O-2 molecule on Co-(4)pyrrole is found to be at the O-O center of mass located on top of the Co atom in side-on configuration, while for the case of Co-(6)pyrrole cluster, the O-2 molecule is slightly deviated from the side-on configuration. The O-O bonds of the O-2/Co-(4)pyrrole and the O-2/Co-(6)pyrrole systems have elongated by 10.84 and 9.86%, respectively. The elongation mechanism of O-2 on Co-(n)pyrrole is induced by the interaction between the cobalt d-orbitals and the O-2 anti-bonding pi* orbital, which results in a charge transfer from the cobalt atom toward the O-2 molecule. This effect seems important in the adsorption of the O-2 molecule on Co-(n)pyrrole. It is likely that the extra charge in the O-2 molecule would fill its anti-bonding orbital and consequently weaken the O-O bond. In Co-(4)pyrrole, the elongation of the O-2 bond is larger than that of Co-(6)pyrrole since a complete side-on configuration has more symmetric overlapping between the cobalt d-orbitals and the O-2 anti-bonding orbital.