Reactivity and Spectroscopic Studies of Oxygen Reduction Reaction of Bio-Inspired Molecular Catalysts

Studies of oxygen reduction reaction (ORR) catalyzed by bio-inspired porphyrin catalysts is important for developing non-precious metal catalysts of fuel cell and for fundamental understanding of the reaction mechanism.  Molecular catalysts have advantage in study of ORR with more facile access to the reaction mechanism experimentally compared to biological and inhomogeneous systems.  We have previously shown that an axial ligand tethered iron porphyrin complex shows a crucial role in dioxygen activation [1].  As a next generation model a hanging hydrogen bond donor was incorporated at the distal site of heme that could stabilize heme bound intermediates and may induce proton-coupled electron transfer mechanism.  In ORR desorption of an OH ligand and formation of OOH intermediate are most energy demanding steps [2].  Our new model showed that the hydrogen bonding interaction can alter the thermodynamic properties of ORR facilitating efficient electron transfer to heme bound O₂.  Resonance Raman studies of reaction intermediates provide direct evidence of reaction mechanisms as well as probe of metal-ligand and ligand internal covalency: As expected the incorporated hydrogen bond donor was found to interact with heme bound oxygen ligand.  The thermochemical properties of ORR were examined with density functional theory calculations, demonstrating H-bonding and axial ligation of heme are crucial structural mechanism in activation of O₂.  The Sabatier principle states metal-ligand interaction is a descriptor of catalysts [2].  However in inhomogeneous system the reaction mechanism is not understood well due to difficulty of vibrational spectroscopic studies of the in situ reaction.  Thus our approach to study ORR provides deep insights into the molecular mechanism of electrocatalysts.  The structure/reactivity correlation gained by our reactivity and spectroscopic studies of bio-inspired molecular catalysts will be discussed.

[1] (a) J.-G. Liu, T. Ohta, S. Yamaguchi, T. Ogura, S. Sakamoto, Y. Maeda, Y. Naruta, Angew. Chem. Int. Ed., 48, 9262-9267 (2009).  (b) T. Ohta, J.-G. Liu, Y. Naruta, Coord. Chem. Rev., 257, 407-413 (2013).

[2] J. K. Nørskov et al., J. Phys. Chem. B, 108, 17886-17892 (2004)