Single-Site Catalysts for the Oxygen Reduction Reaction: Why Iron Is Better than Platinum

This paper aims to elucidate the origin and the different catalytic properties toward the oxygen reduction reaction in the acidic electrolyte of Fe-octaethylporphyrin (FeOEt) and Pt-octaethylporphyrin (PtOEt) supported on Au(111) electrodes. The electrocatalytic process in the two systems is monitored by using in situ scanning tunneling microscopy, allowing observation of the different redox states of the metal center and the different coordination of oxygen, which manifests as a net difference in topography. The coordination of oxygen at the two metal centers was confirmed by computational models, which observed a much stronger interaction between Fe-O2 (1.75 Å) than in Pt-O2 (3 Å). Cyclic voltammetry at still and rotating ring disc electrodes evidenced that at FeOEP, the ORR occurred according to redox-catalysis-like, Eonset (ORR) = 0.5 V vs reversible hydrogen electrode (RHE), where the variation of the metal center redox state mediates the reduction of the oxygen molecule, recovering its original oxidation state by reduction at the electrode. Conversely, PtOEP, which does not possess a redox behavior, results in worse performances, Eonset (ORR) = 0.275 V vs RHE, but certain catalysis is still observed. A tetraelectronic reduction process to H2O was observed at both metal centers, and the mechanism was fully interpreted by computational analysis.