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 k_app 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.