Single-Site Catalysts\nfor the Oxygen Reduction Reaction:\nWhy Iron Is Better than Platinum

This paper aims to elucidate the origin and the different\ncatalytic\nproperties toward the oxygen reduction reaction in the acidic electrolyte\nof Fe–octaethylporphyrin (FeOEt) and Pt–octaethylporphyrin\n(PtOEt) supported on Au(111) electrodes. The electrocatalytic process\nin the two systems is monitored by using in situ scanning tunneling\nmicroscopy, allowing observation of the different redox states of\nthe metal center and the different coordination of oxygen, which manifests\nas a net difference in topography. The coordination of oxygen at the\ntwo metal centers was confirmed by computational models, which observed\na much stronger interaction between Fe–O<sub>2</sub> (1.75\nÅ) than in Pt–O<sub>2</sub> (3 Å). Cyclic voltammetry\nat still and rotating ring disc electrodes evidenced that at FeOEP,\nthe ORR occurred according to redox-catalysis-like, <i>E</i><sub>onset</sub> (ORR) = 0.5 V vs reversible hydrogen electrode (RHE),\nwhere the variation of the metal center redox state mediates the reduction\nof the oxygen molecule, recovering its original oxidation state by\nreduction at the electrode. Conversely, PtOEP, which does not possess\na redox behavior, results in worse performances, <i>E</i><sub>onset</sub> (ORR) = 0.275 V vs RHE, but certain catalysis is\nstill observed. A tetraelectronic reduction process to H<sub>2</sub>O was observed at both metal centers, and the mechanism was fully\ninterpreted by computational analysis.