Electrochemical nitrate reduction to high-value ammonia on two-dimensional molybdenum carbide nanosheets for nitrate-containing wastewater upcycling

Donglin Zhu; Guoguang Li; Xu Yan; Chunxia Geng; Li Gao

doi:10.1016/j.scitotenv.2023.163145

Electrochemical nitrate reduction to high-value ammonia on two-dimensional molybdenum carbide nanosheets for nitrate-containing wastewater upcycling

Sci Total Environ. 2023 Jun 20:878:163145. doi: 10.1016/j.scitotenv.2023.163145. Epub 2023 Mar 29.

Authors

Donglin Zhu¹, Guoguang Li¹, Xu Yan², Chunxia Geng³, Li Gao⁴

Affiliations

¹ School of the Environment, Nanjing University, Nanjing 210023, PR China.
² Huizhou Innovation Research Institute of Next Generation Industrial Internet, Huizhou 516006, PR China.
³ Beijing Water Planning Institute, Beijing 100089, PR China.
⁴ Institute for Sustainable Industries and Liveable Cities, Victoria University, PO Box 14428, Melbourne, Victoria 8001, Australia. Electronic address: Li.gao@vu.edu.au.

PMID: 37001674
DOI: 10.1016/j.scitotenv.2023.163145

Abstract

Electrochemical conversion of nitrate wastewater into high-value ammonia fertilizer has attracted extensive attention in wastewater treatment and resource recovery, but presents great challenges due to complicated reaction pathways and competing side reactions. Herein, we report a feasible method for the successful fabrication of Mo₂C nanosheets (Mo₂C NSs) as electrocatalyst for the electroreduction of nitrate to ammonia. Compared to Mo₂C nanoparticles, the Mo₂C NSs exhibited superior activity and selectivity in NH₃ electrosynthesis with an NH₃ yield rate of 25.2 mg·h^-1·mg^-1_cat. at -0.4 V and a Faradaic efficiency of 81.4 % at -0.3 V versus reversible hydrogen electrode. The X-ray diffraction and transmission electron microscopy characterization verifted the controllable conversion of 2D MoO₂ NSs into 2D Mo₂C NSs. In situ spectroscopic studies and on-line differential electrochemical mass spectrometry revealed the proposed reaction pathway of NO₃^- to NH₃ conversion, *NO₃^- → *NO₂^- → *NO→*NOH → *NH₂OH → *NH₃. Density functional theory calculations further verified the effective N-end NOH pathway with the conversion of *NH₂OH to *NH₂ as the rate-determining step requiring a low energy barrier of 0.58 eV. Importantly, the key hydrogenation of *NO to form *NOH species underwent a lower energy barrier of 0.39 eV compared with the formation of *ONH species (1.06 eV).

Keywords: Ammonia electrosynthesis; Electrocatalysis; Electrochemical nitrate reduction; Molybdenum carbide; Nitrate wastewater upcycling.