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.

Figure 1

Stable geometries of O${}_2$ adsorbed on the metal center of porphyrins anchored to a carbon nanotube (CNT-MP). (a) O${}_2$ in the sideon geometry and CNT-MP in the $s{p}^2$ bonding structure. (b) O${}_2$ in the endon geometry and CNT-MP in the $s{p}^3$ bonding structure.

Figure 2

Spin-resolved band structure of carbon nanotube-metal porphyrin complexes: (a)–(d) in the $s{p}^2$ structure for CNT-MnP, CNT-FeP, CNT-CoP, and CNT-NiP, (e)–(h) in the $s{p}^3$ structure for CNT-MnP, CNT-FeP, CNT-CoP, and CNT-NiP. Blue (red) curves represent majority (minority) spin subbands.

Figure 3

Spin-density isosurfaces for O${}_2$ adsorbed on the porphyrin metal centers of (a) CNT-MnP, (b) CNT-FeP, (c) CNT-CoP, and (d) CNT-NiP. The isosurfaces correspond to an electronic charge of 0.02 $e$/Å${}^3$, and different colors indicate antiparallel spin-density configurations.