Catalytic activity toward oxygen reduction of transition metal porphyrins covalently linked to single-walled carbon nanotubes: A density functional study

The interaction of molecular oxygen with the metal center of transition metal porphyrins (MP, with M = Mn, Fe, Co, and Ni) covalently linked to single-walled carbon nanotubes (CNTs) are addressed by density functional theory calculations. Two geometries for the CNT sidewall functionalization with porphyrin radicals are proposed, considering sp(2) and sp(3) chemical bonds. By computing the stability and electronic properties of the CNT-MP complexes, and the interaction of the O-2 molecule with the metal center, we investigate their catalytic activity toward the oxygen reduction reaction (ORR). According to our results, CNT-MnP, CNT-CoP, and CNT-FeP linked by sp(2) covalent bonds are highly stable, preserving the CNT metallic character. We also find a significant O-O bond weakening after the oxygen adsorption on the porphyrin metal center, showing favorable conditions toward ORR. These results support experimental evidences of ORR activity in CNT-based transition metal-N-4 centers.