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 $s{p}^{2}$ and $s{p}^{3}$ 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 $s{p}^{2}$ 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.