Electrochemical Hydrogenation of Nitrobenzene: From Electrocatalysis to Redox Mediator Catalysis

Introducing polyoxometalates (POMs) as redox mediators can decouple direct electrochemical reactions into surface-homogeneous conversion steps. And the formed redox catalysis is beneficial to achieve more efficient hydrogenation of nitrobenzene (Ph-NO2) to aniline (Ph-NH2) at low overpotentials. Notably, the redox potentials of POMs can adjust the energy barrier of the hydrogenation reaction, thus improving conversion efficiency. In particular, by using phosphotungstic acid ({PW12}) as the redox mediator, the potential of the hydrogenation of Ph-NO2 to Ph-NH2 was improved to 0.04 V vs RHE with a very high kapp of 0.0339 min–1. This indicates its superior kinetic performance over that of most previously reported electrocatalysts. In addition, through comparative mechanistic studies of electrocatalysis and redox mediator catalysis, EC-SERS revealed the preferential adsorption of mediator molecules and their direct interactions with intermediates. This work is significant to a deep understanding of the mechanistic behaviors and potential tuning effects of redox mediators, which will help to develop more efficient mediators for the hydrogenation of Ph-NO2.