In situ uncovering the catalytic cycle of electrochemical and chemical oxygen reduction mediated by an iron porphyrin†

Abstract

As one of the critical reactions in biotransformation and energy conversion processes, the oxygen reduction reaction (ORR) catalyzed by iron porphyrins has been widely explored by electrochemical, spectroscopic, and theoretical methods. However, experimental identification of all proposed intermediates of iron porphyrins in one catalytic cycle is rather challenging in the mechanistic studies of the ORR driven by electrochemical or chemical methods. Herein, we report the application of electrochemical mass spectrometry (EC-MS) and chemical reaction mass spectrometry (CR-MS) to in situ uncover the catalytic cycle of electrochemical and chemical ORRs mediated by an iron porphyrin molecular catalyst. Five crucial iron–oxygen intermediates detected by both EC-MS and CR-MS help to build the whole catalytic cycle and indicate the details of the 4e⁻/4H⁺ pathway to produce H₂O in the electrochemical and chemical ORRs. By combining in situ MS methods with electrochemical and spectroscopic methods to characterize the intermediates and study the selectivities, this work provides a mechanistic comparison of the electrochemical and chemical ORRs catalyzed by one model iron porphyrin.