Transition-Metal Single-Atom Alloy Electrocatalysts for Nitrate Reduction

Electrochemical nitrate reduction (E-NRR) powered by renewable energy would enable the elimination of toxic nitrate (NO3–) via conversion to benign N2 or upgrading to valuable NH3. Single-atom alloys (SAAs) based on Pd and Ru substitutions in Cu have shown high E-NRR activity and selectivity to N2 and NH3, respectively. Here, we systematically investigate the E-NRR pathway of 8 neat and 88 SAA surfaces constructed from 11 elements across the transition metal block to understand how d-band properties of the resulting SAAs control E-NRR activity. We find that d-band center (εd), and to a lesser extent, position of the Fermi level (εF) and work function are controlling factors in the E-NRR free energy landscape and adsorbate site preferences; substitutions which raise the local εd will increase the binding strength of E-NRR intermediates and exothermicity of N–H combination events, thus favoring the formation of NH3─the converse is true of substitutions which lower the local εd. Combinations of coinage metal hosts (Cu, Ag, Au) with early transition metals (Y, Ti, Mo, W) thermodynamically favor production of NH3 while suppressing the hydrogen evolution reaction (HER), but substitution by a platinum group metal (PGM) will favor N2 production due to weakly localized E-NRR intermediates. On the SAA surfaces investigated, we find that overall activity and (NO3–) selectivity behavior such as surface poisoning and preference for hydrogen evolution are dependent on the host metal, whereas the nitrate reduction product is dictated by a combination of host and single-atom substitution identity.