Selective electrosynthesis of hydroxylamine from aqueous nitrate/nitrite by suppressing further reduction

Abstract

The electrocatalytic reduction of nitrogenous waste offers a sustainable approach to producing nitrogen-containing chemicals. The selective synthesis of high-value hydroxylamine (NH₂OH) is challenging due to the instability of NH₂OH as an intermediate. Here, we present a rational electrocatalyst design strategy for promoting NH₂OH electrosynthesis by suppressing the competing pathways of further reduction. We screen zinc phthalocyanines (ZnPc) with a high energy barrier for NH₂OH reduction by regulating their intrinsic activity. Additionally, we discover that carbon nanotube substrates exhibit significant NH₃-producing activity, which can be effectively inhibited by the high coverage of ZnPc molecules. In-situ characterizations reveal that NH₂OH and HNO are generated as intermediates in nitrate reduction to NH₃, and NH₂OH can be enriched in the ZnPc electrode. In the H-cell, the optimized ZnPc catalyst demonstrates a Faradaic efficiency (FE) of 53 ± 1.7% for NH₂OH with a partial current density exceeding 270 mA cm^-2 and a turnover frequency of 7.5 ± 0.2 s^-1. It also enables the rapid electrosynthesis of cyclohexanone oxime from nitrite with a FE of 64 ± 1.0%.