Activate N₂-Selective Pathways for Electrochemical Ammonia Oxidation—Dual Active Site Tandem Catalysis in 2D Layered Nanospace

Ammonia oxidation reaction (AOR) plays a vital role in various applications including hydrogen production, direct ammonia fuel cell, and removal of ammonia-containing wastewater. During ideal AOR in an aqueous electrolyte, NH₃ undergoes dehydrogenation to form NH<i>_x</i> (<i>x</i> = 1 or 2), followed by the catalytical dimerization and electrochemical dehydrogenation to produce N₂.¹ However, most of the reported AOR catalysts, such as Pt²and NiCu alloys³, suffer from its low N₂ selectivity (undesired nitrogen oxides (NO<i>_x</i>) formation) and/or low long-term stability (catalyst poisoning from N_ad species).<br/>In this study, we introduce 2D layered MnO₂ composites co-intercalated with a dual active site, that is, hydrated Ni^II and Cu^II ions, for N₂-selective and stable AOR catalyst. Co-intercalated hydrated Ni^II and Cu^II ions are expected to accelerate the key (electro)chemical process, dehydrogenation and dimerization process, respectively, to achieve high N₂ selectivity.<br/>Various spectroscopic methods such as X-ray diffraction, X-ray photoelectron spectroscopy, and high-energy X-ray total scattering were performed to clarify the electron and structural states of Ni^II and Cu^II within the layers. The result confirms that co-intercalated Ni^II and Cu^II coexist closely with a single ion state with negligible interaction, which is ideal for dual active site catalysis. Linear sweep voltammogram and gas chromatography analysis confirm the significant improvement in N₂-selectivity for Ni^II-Cu^II co-intercalated catalyst compared with single active site (Ni^II and Cu^II) counterparts. Furthermore, the role of Ni^II and Cu^II active sites for N₂-selective AOR was revealed by <i>operando </i>surface-enhanced infrared absorption spectroscopy. The result confirms that NH₃ was dehydrogenated in the vicinity of Ni^II site, and subsequent oxidation of Ni^II triggers the hand-over process of NH<i>_x</i> to Cu^IIsite, followed by the dimerization/dehydrogenation process in the vicinity of Cu^II to selectively produce N₂. This work revealed that the unique catalytic activity can be evoked by introducing multiple isolated metal complexes into 2D nano-spaces, providing a novel strategy for designing electrocatalysts with ideal efficiency and selectivity.<br/><br/>1 Gerischer, H. <i>et al., J. Electroanal. Chem.</i> <b>1970</b>, 25, 421–433.<br/>2 Matsui, T. <i>et al., Langmuir</i> <b>2015</b>, 31, 11717–11723.<br/>3 Adli, N. M. <i>et al., J. Electrochem. Soc.</i> <b>2018</b>, 165, J3130.