Mechanistic understanding of the effect of alloying Au with Ni on N2 electroreduction into NH3: theoretical considerations

Abstract

Bimetallic AuNi alloy electrocatalysts were theoretically designed to determine the effect of the alloying of Au with Ni on N₂-electroreduction activity and mechanism. Our calculated results indicate that increasing the Ni surface composition could facilely activate N₂ molecules, which could be attributed to increasing the overpotential at electrochemical interfaces. Notably, we found that the Au₁Ni₁(111) alloy with medium Ni surface composition had the lowest N₂H-formation barrier and the highest barrier for HER, thus leading to the optimal N₂-electroreduction activity. This phenomenon can be explained by the commonly accepted Sabatier principle. The present studies indicated that the Au₁Ni₁ alloy could remarkably decrease the barrier for initial N₂ electroreduction into N₂H species as the rate-determining step at the pure Au(111)/H₂O interface and suppressed HER. Thus, AuNi alloy with the atomic ratio of 1 : 1 was employed to study the effect of the alloying of Au with Ni on the N₂-electroreduction mechanisms. The calculated results suggest that only the associative distal mechanism could occur at Au₁Ni₁(111)/H₂O interfaces. By contrast, the associative alternating and distal mechanisms could parallelly occur at pure Au(111)/H₂O interfaces. Thus, we can conclude that the alloying of Au with Ni not only facilitates N₂ electroreduction but also changes the N₂-electroreduction pathways. The origin of the enhanced N₂-electroreduction activity was attributed to the greater electron transfer from the Au₁Ni₁(111)/H₂O interface to the empty π orbital of the adsorbed N₂ molecule. Moreover, we also observed that Au atoms exhibited a negative ion property on the bimetallic AuNi alloy surface due to electron transfer from the transition metal Ni to Au. Our present study can offer a theoretical guideline for rationally designing and preparing highly active bimetallic Au-based alloy electrocatalysts for N₂ electroreduction.