Analysis
of Porphyrines as Catalysts for Electrochemical
Reduction of O<sub>2</sub> and Oxidation of H<sub>2</sub>O
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Abstract
Bioinspired
structures are promising as improved catalysts for
various redox reactions. One example is metal hangman-porphyrines
(MHP), which recently have been suggested for oxygen reduction/evolution
reaction (ORR/OER). The unique properties of the MHPs are attributed
to both the hangman scaffold and the C<sub>6</sub>F<sub>5</sub> side
groups. Herein, the OER/ORR over various transition metal MHPs is
investigated by density functional theory calculations within an electrochemical
framework. A comparison of the reaction landscape for MHP, metal porphyrine
(MP) and metaltetrafluorophenyloporphyrines (MTFPP), allow for a disentanglement
of the different roles of the hangman motif and the side groups. In
agreement with experimental studies, it is found that Fe and Co are
the best MHP metal centers to catalyze these reactions. We find that
the addition of the three-dimensional moiety in the form of hangman
scaffold does not break the apparently universal energy relation between
*OH and *OOH intermediates. However, the hangman motif is found to
stabilize the oxygen intermediate, whereas addition of C<sub>6</sub>F<sub>5</sub> groups reduces the binding energy of all reaction intermediates.
Our results indicate that the combination of these two effects allow
new design possibilities for macromolecular systems with enhanced
catalytic OER/ORR activity