Selective Nitrate-to-Nitrite Electroreduction on Ag/Au Core-Shell Nanocubes

Abstract Electrochemical nitrate reduction (NO3RR) offers a promising route for the production of valuable fertilizers and nitrites (common food preservatives), yet the relevant active sites remain poorly defined. Here, we combine density functional theory (DFT) with operando electrochemical scanning tunneling microscopy (n-EC-STM) to analyze tunneling current noise and establish structure–activity relationships in Ag/Au(111) and Ag/Au(100) model systems. DFT calculations show that Ag overlayers strengthen nitrate adsorption relative to bare Au and facilitate the initial NO3* → NO2* step. The adsorption energetics depend on both the structure of the surface facet and adsorption geometry, with tridentate nitrate adsorption generally favored on Au(100)-based surfaces and more variable behavior on Au(111)-based surfaces. Experimentally, Ag-coated Au(111) and Au(100) exhibit strong coverage-dependent NO3RR activity, with maximum activity near completion of an Ag monolayer on both facets. AgML/Au(100) consistently outperforms AgML/Au(111). n-EC-STM identifies Ag(100)-like monolayer terraces as the dominant active sites for NO3RR, whereas adjacent Au sites remain inactive. The same design principle is reproduced in Au{100} nanocubes, where Ag5nm@Au43nm/C with a thinner Ag shell outperforms Ag11nm@Au43nm/C and Au43nm/C. Together, these results identify terrace-confined Ag monolayers on Au as the key active sites for the electrochemical nitrate reduction reaction and highlight the roles of strain, ligand presence, and electronic effects at the Ag/Au–electrolyte interface.