Revealing the pH-dependent mechanism of nitrate electrochemical reduction to ammonia on single-atom catalysts

Nitrate electrochemical reduction to ammonia (NO₃RR) catalyzed by single-atom catalysts (SACs) is an attractive and efficient way for solving the problem of nitrate pollution in water and obtaining valuable product ammonia through low temperature synthesis. It is well known that the pH conditions can be regulated to tune the performance of NO₃RR, however, there have been few studies aimed at gaining theoretical insight into the origin of pH-dependent catalytic performance among SACs. Herein, taking 3d-transition metal (Fe, Co, Ni and Mn) single-atoms supported on diverse anchor sites of MoS₂ as an example (SA-MoS₂), we explore the activity and selectivity for NO₃RR towards ammonia (NH₃ and NH₄⁺) under different pH conditions by density functional theory calculations. It is found that priority reaction pathways, the potential determining step and limiting potentials of SA-MoS₂ exhibit pH-dependent characteristics, which can be described by a contour map of catalytic reactivity, spanned by adsorption free energies (G_NO* and G_NH2*), and further determined by local coordination environment and electronic states of active sites. Our three-step screening method reveals that the Co single-atom adsorbed MoS₂ edge catalyst is the most promising catalyst among the studied SA-MoS₂ because of its low limiting potential (-0.3-0.4 V, RHE), excellent selectivity in the competition with the hydrogen evolution reaction (HER), as well as stability against aggregation and electrochemical dissolution across the full pH range. This work demonstrates a theoretical insight into the pH-dependent mechanism of supported SA catalyzed NO₃RR, which proposes a screening strategy for finding new SACs, and provides motivation for further experimental exploration.

A highly active copper-nanoparticle-based nitrate reduction electrocatalyst prepared by in situ electrodeposition and annealing

In recent years, copper-based electrodes have attracted intense attention for the electrochemical reduction of nitrate (NO₃^-), the so-called ECRN. However, these electrodes suffer from low activity and selectivity. Herein, we report a novel Cu-based electrode (IE-Cu-400) for the ECRN fabricated by loading Cu-based nanoparticles onto graphite felt using in situ electrodeposition followed by annealing. Compared with traditional Cu-based electrodes, the IE-Cu-400 is comprised of smaller particles and the copper is present in a high oxidation state (Cu²⁺ in CuO). During operation, the CuO is converted to Cu, which is the active ECRN species. In addition, an increased surface area and high density of grain boundaries resulting from the reduction of CuO were observed for IE-Cu-400. This resulted in a 3.38-fold increase in the NO₃^- removal rate and a 1.36-fold increase in NH₄⁺ selectivity. Further analyses revealed that the enhanced ECRN performance of IE-Cu-400 is linked to its increased number of active sites, as well as its improved adsorption and reduction ability for NO₂^-. Moreover, IE-Cu-400 displays high stability for the ECRN. Finally, the produced NH₄⁺ was effectively oxidised to N₂ with approximately 100% selectivity via chlorination. Hence, the two-stage treatment strategy (i.e. ECRN by IE-Cu-400 + chlorination treatment) presented here shows great potential for the complete electrocatalytic denitrification of water. Further, this work highlights the beneficial effect of decreasing the particle size and controlling the surface oxidation of Cu-based catalysts simultaneously for enhancing the ECRN and offers new suggestions for the design of high-performance electrode materials for the ECRN.