Influence of Carbon Nanotube Support on Electrochemical Nitrate Reduction Catalyzed by Cobalt Phthalocyanine Molecules

The electrocatalytic conversion of waste nitrate (NO3–) into value-added ammonia (NH3) is a promising water treatment approach to remedy environmental pollution. However, developing catalyst design and optimization strategies to control the reaction selectivity remains a challenge. We report on an underexplored approach to overcome this challenge by tuning the multiwalled carbon nanotube (CNT) support for the cobalt phthalocyanine (CoPc) molecular catalyst. With pristine CNTs as the support, the CoPc/CNT hybrid catalyst is selective for NO3– reduction to NH3 with a maximum Faradaic efficiency of 70%. In sharp contrast, CoPc supported on oxidized CNTs (OCNTs) generates mostly hydrogen (H2) under the same conditions. On the basis of kinetic measurements which reveal that the rate-determining step of NO3– reduction is limited by the first electron transfer without involving a proton, we propose that the oxygen functional groups on the OCNT support help deliver protons and steer the supported CoPc molecules from catalyzing NO3– reduction to performing H2 evolution. This work highlights the importance of tailoring the catalyst support to advance reactivity in environmental catalysis.