There is a dire need for selective catalysts that allow us to consistently achieve a desired outcome in a chemical reaction. It is this consistency that allows for more efficient, energy-saving ways of producing fuel. A team of researchers from Tohoku University, the Indian Institute of Technology Indore, and Dalhousie University have revealed that a Cu₁₄ nanocluster (NC) with just a single exposed Cu site exhibits remarkably high ammonia (NH₃) selectivity (~80%) and production rate for the electrochemical nitrate ion reduction reaction. They highlight methods for controlling catalyst selectivity, which are expected to greatly contribute to the creation of various metal NC catalysts in the future. Their findings support a more efficient way to create ammonia – a highly versatile source of green energy. The findings were published in the ACS Catalysis on December 9, 2025.

Ammonia (NH₃) has garnered significant attention as a next-generation, clean fuel. While it has the major benefit of not emitting carbon dioxide upon its combustion, the method for producing ammonia in the first place is not as kind to the environment. The dominant production method necessitates high temperatures and pressures, imposing a substantial environmental burden.

To overcome this, NH₃ synthesis via electrochemical nitrate ion (NO₃^-) reduction reaction (eNO₃^-RR) powered by electricity derived from renewable energy sources has emerged as a promising alternative to conventional chemical synthesis. This technology allows the direct conversion of NO₃^- into NH3 under ambient temperature and pressure conditions, so production no longer damages the environment. Accordingly, the development of efficient catalysts for eNO₃^-RR is gaining momentum.

In recent years, it has become increasingly evident that atomically precise metal nanoclusters (NCs) hold significant potential as highly active catalysts for a wide range of reactions. They hold the possibility for selective promotion of a desired reaction while suppressing unwanted side reactions. However, for many metal NCs, organic ligand coverage hinders the exposure of surface metal atoms that serve as active sites, often resulting in relatively low activity and selectivity.

"Since organic ligands were hiding what would otherwise be useful active sites, we sought a way to unveil them by manipulating the ligand interactions in a certain way," explains Tokuhisa Kawawaki (Institute of Multidisciplinary Research for Advanced Materials).

They found that the judicious selection of thiolate or SR ligands enabled the controlled preparation of Cu₁₄ NCs exhibiting either the presence or absence of coordinatively unsaturated, exposed active Cu sites. They reasoned that targeting SR ligands lacking Ph groups limits the structural distortion of Cu NCs. As a result, this study details the successful synthesis of Cu₁₄ NCs co-protected by a combination of SR and PPh₃ ligands.

Electrochemical evaluation revealed a markedly superior performance for E-Cu₁₄/CB featuring exposed active Cu sites, as evidenced by an exceptionally high Faradaic Efficiency (FE) for NH₃ production (FE_NH3 = 78%). Furthermore, the rates of NH₃ synthesis over the E-Cu₁₄/CB catalyst were 2.72-fold and 5.69-fold greater than those observed for their counterpart lacking exposed Cu sites (UE-Cu₁₄/CB) and a conventionally prepared Cu NP/CB catalyst, respectively. This underscores a significantly enhanced efficiency in NH₃ production and a relatively sustained operational stability of the eNO₃^-RR process.

"This achievement not only demonstrates the influence of overall geometric structure on catalytic activity in metal nanoclusters but also directly shows that exposing just a single active site can dramatically alter catalytic activity," remarks Yuichi Negishi (Institute of Multidisciplinary Research for Advanced Materials). "We are excited to mark a new milestone in metal nanocluster research."