Metasequoia‐like Nanocrystal of Iron‐Doped Copper for Efficient Electrocatalytic Nitrate Reduction into Ammonia in Neutral Media

Abstract

It is of significance to design catalysts for achieving high‐performance electrochemical nitrate reduction to ammonia (NRA) in mild neutral media. However, the faradaic efficiency and selectivity are still far from satisfactory. Here, the fabrication of an efficient catalyst was achieved by rationally doping Fe to Cu into a metasequoia‐like nanocrystal of CuFe for NRA in neutral media. Fe doping was found to deepen energy level of the Cu 3d band, favorably tuning adsorption energies of reaction intermediates to promote the NRA. At an applied potential of −0.7 V vs. the reversible hydrogen electrode, the CuFe with approximately 2 % Fe doping content delivered a catalytic current density of 55.6 mA cm−2, which was 2.1 times that of the Cu material. The CuFe also exhibited a faradaic efficiency up to 94.5 %, and a good selectivity of 86.8 %.

Theoretical Insights into Superior Nitrate Reduction to Ammonia Performance of Copper Catalysts

“…At a potential between 0.4 and −0.4 V vs RHE, the hydrogenation of *NO is the RDS, while the adsorption of nitrate becomes the RDS at a potential below −0.43 V. When the potential goes up to 0.42 V, *NO 2 → *NO becomes the RDS, and Δ G also increases. This trend is in excellent agreement with our experimentally observed volcanic shape Faradaic efficiency–potential curve . Besides, the negatively charged potential can reduce the activation barriers, as shown in Figure b.…”

“…Then, the influence of the applied potential on the NRA performance was investigated. The NRA experiments were reported to work at 0.3 to −0.3, −0.1 to −0.8, and 0.4 to −0.6 V vs reverse hydrogen electrode (RHE). Figure presents the potential-dependent RDS and activation barriers of key steps in the NRA3 pathway on Cu(111) at pH = 14.…”

“…Then, the influence of the applied potential on the NRA performance was investigated. The NRA experiments were reported to work at 0.3 to −0.3, 9 −0.1 to −0.8, 16 and 0.4 to The Gibbs free-energy changes of RDS decrease first when the electrode potential shifts negatively. However, the too negative potential can lead to difficulty in adsorption of nitrate (as shown in Figure 6a, ΔG of NO 3 − → *NO 3 increases when U is lower than −0.43 V).…”

“…Among the many pure metals that have been developed to catalyze the nitrate reduction process, Cu is considered to be one of the most promising electrocatalysts available either alone or as a promoter for NRA due to its low cost and relatively high activity. ,− Earlier nitrate reduction experiments on copper indicated that the nitrogenous byproduct was potential-, surface-, and electrolyte-dependent. However, the detailed understanding of these fundamental questions is hindered by the susceptibility to oxidation of Cu surfaces, gradually changing pH values (hydroxyl ions are continuously produced) during the electrochemical process.…”

Progress on electrocatalytic reduction of nitrate on copper-based catalysts

“…酸盐污染防治已成为备受瞩目的全球性问题。 目前为止，去除污水中硝酸盐的方法可分为 3 类：生物法 [14] 、物理法 [15] 和化学还 原法 [16,17] 。生物法主要是通过微生物体内的酶对其进行还原，该方法有效、经济，关 键在于找到合适的菌种 [18] 。然而，生物法面临的问题是工业污水成分不稳定，严重影 响微生物的还原活性；其产生的污泥，后处理成本较高，且耗时长 [19] 。物理法去除水 中的硝酸盐主要有离子交换法、反渗透法和电渗析法 [20] 。虽然利用物理法去除水中的 硝酸盐具有操作简单、效率高、便于自动化等优点 [21] ，但是离子交换树脂和渗透膜价 格较昂贵，使用寿命有限，运行成本过高 [22,23] 。化学法是利用化学催化剂将硝酸盐直 接还原为氮气(N 2 )或具有经济价值的氨(NH 3 ) [13] 。其主要方法包括光催化还原法、 液相还原法和电催化还原法等。光催化还原法的原理是利用高能量紫外光照射反应溶 液，产生高还原能力的自由基，进行还原反应。此方法具有合成简单、毒性低及成本 低等优势，但是存在紫外光吸收范围窄、光能利用率低、溶液透光度差等问题 [24] [29,30] 。因此，发展 电催化还原法去除硝酸盐成为未来解决硝酸盐污染以及资源化的一个有效途径。 目前，电催化硝酸盐还原技术的发展和应用重点在于阴极催化剂材料的研究。良 好的阴极催化剂材料必须具备稳定性好，抗腐蚀性强，导电率高，寿命长，选择性好 等优势 [31] 。先前的研究表明铅(Pb) 、镍(Ni) 、锡(Sn) 、锌(Zn) 、铑(Rh) 、钌 (Ru) 、铱(Ir) 、钯(Pd) 、铜(Cu) 、银(Ag)和金(Au)等催化材料都具有一定的 硝酸盐还原活性 [32~34] 。其中，铜由于导电性好、析氢能力弱、催化性能稳定等优势， 引起研究者的广泛关注。例如 Fu 等人 [3] 报道了 Cu(111)纳米片电催化硝酸盐还原 合成 NH 3 的速率是普通 Cu 箔的 400 倍，是 Cu 纳米块的 3.1 倍，是 Cu 纳米颗粒 的 1.7 倍，大大提升了电催化硝酸盐还原的性能。同时，Wang 等人 [35] 基于 Cu 电子结 构优化得到 CuFe 纳米枝晶，其电催化硝酸盐还原性能得到了很大的提升。目前，研究 者在铜基催化剂开展了系列研究 [36,37] ammonia production at different applied potentials. [3 ] (Copyright © 2020 Tsinghua University Press and Springer-Verlag GmbH Germany) [49] .…”