Electrocatalytic reduction of nitrate at the stabilizedelectron-deficient Cu<sup><italic>δ</italic>+</sup> active sites

Abstract

<p indent="0mm">With the extensive use of nitrogen fertilizers, sewage discharges and industrial processes that consume nitric acid, the exposure level of nitrate (NO₃⁻-N) in environmental water bodies has significantly increased in the past decades. The NO₃⁻ anion has a stable chemical structure, and is hardly removed by natrual self-cleaning. It has adverse effect, and is primarily responsible for the eutrophication in lakes as well as some physiological diseases in neighboring mammal inhabitants, such as the methemoglobinemia. Strict regulations on the maximum contaminant level of <sc>10 mg/L</sc> for NO₃⁻-N in the drinking water has then been set by the World Health Organization, the United States Environmental Protection Agency and the China Environmental Protection Administration. To minimize the adverse effects of NO₃⁻-N, various technologies, including ion exchange, electrolysis, biological denitrification, and catalytic reduction by H₂ or Fe⁰, have then been developed to eliminate the NO₃⁻-N from water. Among them, electrocatalytic reduction of nitrate (ENRR), driven by the renewable electric energy, is receiving increasing research interests, due to its substantial nitrate removal capacity, mild reaction conditions and minimal daily maintenances. Copper-based materials are favored as the ENRR catalysts by researchers because of their high activity, chemical stability and low cost. The electron-deficient Cu^{<italic>δ</italic>+} is the main active site at the catalyst surface, but it is readily to be reduced to Cu⁰ at the very negative ENRR potentials, leading to the performance deterioration during the long-term reaction process. In this paper, we reported a facile approach to construct the Cu^{<italic>δ</italic>+}-1,4-naphthalene dicarboxylic acid (1,4-NDC) catalytic active sites through the surface decoration of the Cu(OH)₂ particles with the 1,4-NDC molecule at room temperature, followed by an <italic>in-situ</italic> electrochemical activation process under a negative potential of <sc>−0.40 V</sc> vs. RHE (reversible hydrogen electrode, the same below) (the product is denoted as <sc>Cu(OH)₂/1,4-NDC-AT).</sc> In this structure, the Cu^{<italic>δ</italic>+} is confirmed by the X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) to survive under the negative working potential during the long-term ENRR, possibly due to the high electronegativity and chemical stability of the oxygen atoms in carboxyl group (the carboxyl group is chelated with the Cu^{<italic>δ</italic>+} site). The batch ENRR activity test results show that the Cu(OH)₂/1,4-NDC-AT delivers a NH₃ selectivity of &gt;90% in product, a specific activity of <sc>1034.7 mg N/(h m²)</sc> and a mass activity of <sc>89.1 mg N/(h</sc> g_Cu) in treating <sc>22.5 mg/L</sc> NO₃⁻-N solution at <sc>−0.40 V.</sc> The specific and mass activities are nearly 2.3 and 5.1 times those achieved by the ligand-free Cu(OH)-AT, respectively, and also higher than that of most reported catalysts. Optimizing the 1,4-NDC dosage and working potential can further improve the ENRR performance, and the mass activity increases to 147.1 and <sc>104.6 mg N/(h</sc> g_Cu) at the <inline-formula id="INLINE13"><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub other="0"><mml:mi other="0">n</mml:mi><mml:mrow other="1"><mml:mtext other="1">1</mml:mtext><mml:mo other="1">,</mml:mo><mml:mtext other="1">4-NDC</mml:mtext></mml:mrow></mml:msub><mml:mo>/</mml:mo><mml:msub other="0"><mml:mi other="0">n</mml:mi><mml:mrow other="1"><mml:mtext other="1">Cu</mml:mtext><mml:msub other="1"><mml:mrow other="1"><mml:mrow other="1"><mml:mo other="1">(</mml:mo><mml:mrow other="1"><mml:mtext other="1">OH</mml:mtext></mml:mrow><mml:mo other="1">)</mml:mo></mml:mrow></mml:mrow><mml:mtext other="2">2</mml:mtext></mml:msub></mml:mrow></mml:msub></mml:mrow></mml:math></inline-formula> of 2.16 and the potential of <sc>−0.50 V,</sc> respectively. Finally, this paper proposed the appratus for the treatment of nitrate wastewater in different concentrations. Specifically, an integrated “ENRR+NH₃-N adsorption” technology is proposed to deal with the dilute nitrate wastewater, which enables the elimination of the nitrogen-containing species in water. For the concentrated nitrate wastewater, an integrated “ENRR+NH₃-N recovery” technology is proposed, which can recover 98.0% of the produced NH₃-N via absorption by H₂SO₄ aqueous solution. This work provides new strategies and robust materials specific for NO₃⁻-N removal, which should advance the development of ENRR technology in water pollution remediation.