Efficient electrocatalytic nitrate reduction on molecular catalyst with electron-deficient single-atom Cuδ+ sites

Abstract

Electrocatalytic nitrate (NO3−-N) reduction to ammonia (NH3-N) (ENRR) coupled with NH3-N separation represents a sustainable process for nitrate pollution mitigation. The −(Cuδ+-N4)- moiety-bearing carbon materials (SCMs) are robust ENRR catalysts but suffer from energy-intensive synthetic process. Furthermore, the complexity in altering the coordination environment of Cuδ+ raises the difficulty in optimizing its electron-deficiency for an enhanced ENRR. Herein the copper phthalocyanine (CuPc) and copper perfluorocyanine (CuF16Pc) molecular catalysts, which share the same −(Cuδ+-N4)- moiety with SCMs but are commercially available, are attempted for ENRR. The CuF16Pc, which differs from CuPc by the substitution of the H in Pc with F, shows alleviated intermolecular agglomeration and increased electron-deficiency of.the Cuδ+ center, owing to the electron-withdrawing effect of F. The alleviated intermolecular agglomeration exposes more Cuδ+, which makes CuF16Pc outperform CuPc (2254.9 vs. 1979.3 mg-NH3 h−1 gCu−1) and most of the reported catalysts in the mass activity of ENRR. However, the Cuδ+ with a larger electron-deficiency in CuF16Pc is less active than that in CuPc for ENRR, owing to the higher energy barrier for NO* hydrogenation to NOH* on it. It reminds us that the CuPc is more ideal for ENRR if its agglomeration issue can be addressed. Finally, an integrated continuous-flow system composed of the CuF16Pc-driven ENRR and PVDF fibers-mediated NH3 separation is customized, which enables a steady NO3−-N removal efficiency of 96.4 % and NH3-N recovery efficiency of 97.2 % from simulated nitrate-contaminated water.