Selective reduction of NO3− to N2 using a highly stable Cu0/Cu2O-PANI-CNTs as a photocatalyst at neutral pH

Abstract

The selective photoreduction of nitrate (NO3−) to nitrogen (N2) has emerged as an energy-efficient and environmentally friendly NO3− removal route under mild conditions. However, NO3− photoreduction often suffers low NO3− removal efficiency in the absence of hole scavengers at neutral pH, the generation of undesired side products (nitrite (NO2−), ammonium (NH4+), nitrogen oxides (NOx), etc.) and poor photocatalyst stability. In this study, a novel copper/copper(I) oxide-polyaniline-carbon nanotubes (Cu0/Cu2O-PANI-CNTs) photocatalyst was prepared through a combination of in situ polymerization and liquid phase reduction processes and used to reduce NO3− to N2. In the Cu0/Cu2O-PANI-CNTs/hv system, NO3− was reduced to NO2− by photogenerated electrons (ecb−) on the surface of Cu0/Cu2O, and the generated NO2− was further reduced to N2 by PANI, resulting in high N2 selectivity. –NH– in PANI acted as photogenerated hole (hvb+) capture centres, leading to NO3− photoreduction without additional hole scavengers. Given the interaction between Cu0, ecb− and PANI, Cu0/Cu2O-PANI-CNTs show excellent repeatability. A 100% NO3− removal efficiency and 100 % N2 selectivity could be achieved by the Cu0/Cu2O-PANI-CNTs/hv system at neutral pH. With river water, secondary effluent and the fourth reuse of the catalyst, the NO3− removal efficiencies were as high as 93.0%, 91.2% and 84.4%, respectively, with 100 % N2 selectivity.