Metal Porphyrins as Single Site Catalyst Models Explored by Electrochemical Scanning Tunneling Microscopy: A New Perspective in Electrocatalysis

M-N-C materials featuring M-N-x single site catalysts, where for example M = Co, Mg, or Fe coordinating x atoms of nitrogen, have been intensely investigated as promising candidates to replace Pt group metals in a variety of electrocatalytic reactions, for example in the O-2 reduction reactions (ORR). However, despite the recent emerging of atomic resolution techniques, there is still a lack of understanding of the precise configuration of the M-N-x single site. For example, metal porphyrins are known to act as catalysts for O-2 and they are indeed good model systems for mimicking M-N-4 sites. In particular, it would be of interest to gain more information on the precise dynamic of the catalytic process at atomic scale, especially in an electrochemical environment, i.e., at electrode/electrolyte interphase. An approach that is becoming more and more consolidated is to combine the information coming from electrochemistry and scanning tunneling microscopy techniques in electrochemical scanning tunneling microscopy (EC-STM) and the result is that a precise dynamic of the catalytic site at atomic scale can be investigated. This review aims to emphasize the main results and advances of EC-STM in investigating small molecule electrocatalysis and specifically O-2 reduction at metal porphyrins M-N-4 model system.