Surface pH Effects on Catalytic Behavior of Pyridinic Nitrogen on Nitrogen-doped Carbon Nanotube in CO₂ Electrochemical Reduction

Nitrogen-doped carbon nanotubes (NCNTs) have been considered a promising catalyst for the electrochemical reduction of CO2 (CO2ER) to generate CO. Although pyridinic N sites have been suggested to be the active center of NCNTs, their behavior in the reaction remains unclear because of the lack of experimental evidence. Herein we focused on the pH dependence of CO2ER activity of NCNT and investigated the effects of local pH at the electrode surface to estimate the catalytic role of the pyridinic N. The results of the in situ local pH measurements using surface-enhanced Raman spectroscopy (SERS) revealed that CO2ER activity disappears in an acidic environment at pH below 4. SERS detected no CO species at the surface during the reaction in the acidic electrolyte, and ex situ X-ray photoelectron spectroscopy indicated the protonation of the pyridinic N. These results suggest the protonation of pyridinic N, the active site of NCNT, inhibits the CO2 adsorption and the following reduction to define the catalytic activity.