Dual-metal porphyrin-graphene hybrids as oxygen catalysts: Comparative DFT insights into O2 adsorption and activation

The efficient activation of molecular oxygen (O2) underpins electrochemical energy conversion; however, the design strategies for non-precious catalysts for the oxygen reduction reaction remain incomplete. Transition metal porphyrins supported on conductive substrates offer a versatile platform, but the mechanism by which different metal centers cooperate to control O2 activation is not well understood. In this work, we used density functional theory to explore heterometallic (Fe, Mn) porphyrin-graphene hybrids and reveal the decisive role of axial-core metal synergy. Across FeTPyP-Fe/Gr, FeTPyP-Mn/Gr, and MnTPyP-Fe/Gr, axial (bridging) sites consistently promote stronger O2 binding, greater charge transfer, and more pronounced weakening of the O-O bond than their core counterparts. Electronic-structure analysis showed that this effect arises from enhanced orbital overlap and π* occupation at the axial position, while Mn incorporation tunes the ligand field to further optimize O2 activation. The most effective configuration combines axial Fe binding with Mn-mediated electronic modulation, demonstrating that the complementary roles of distinct metals can be harnessed in a single catalytic architecture. These findings provide mechanistic insights into oxygen reduction and establish clear design principles for the engineering of earth-abundant porphyrin catalysts. More broadly, they highlight heterometallic coordination as a powerful strategy for tailoring molecular electrocatalysts for sustainable energy conversion.