Potential-Dependent Electrocatalytic Nitrogen Reduction Catalysis on Ni-Anchored γ-Al2O3(110) Surface

Abstract

The transition metal oxides (TMOs) as supports for metal clusters or nanoparticles with surface oxygen vacancies enhance charge transfer from the metal-oxide surface to the metal cluster. Experimental reports have shown that 5 wt % Ni loading on γ-Al₂O₃ enables N₂ activation with the presence of NH_x intermediates on the Ni surface. In this study, we investigated the electrocatalytic nitrogen reduction reaction mechanism (eNRR) on Ni₁ and Ni₄ clusters on γ-Al₂O₃(110) at room temperature by using the computational hydrogen electrode (CHE) model. Nickel atoms preferentially bind via oxygen atoms on the γ-Al₂O₃(110) surface. Moreover, Ni₄/γ-Al₂O₃(110) follows the enzymatic mechanism with a limiting potential (U_L) of −0.576 V, where **N₂ to *N*NH is the potential determining step (PDS). In contrast, Ni₁/γ-Al₂O₃(110) follows a mixed mechanism through an enzymatic route with a U_L of −1.272 V, also with **N₂ to *N*NH as the PDS. The electronic properties, such as the projected density of states (PDOS) and integrated crystal orbital Hamilton population (ICOHP), indicate effective overlap between the metal d-orbital and N₂ antibonding 2π* orbitals near the Fermi level in Ni₄/γ-Al₂O₃(110), whereas a weak interaction is observed in Ni₁/γ-Al₂O₃(110). The evaluation of NRR selectivity shows that Ni₁/Ni₄ sites are freely available for NRR rather than HER. Overall, the mechanistic investigation shows that Ni₄/γ-Al₂O₃(110) is a better electrocatalyst for NRR because of the strong electronic orbital overlaps between Ni metal and surface oxygen. This work highlights how the loading percentage of metal clusters on metal oxides affects catalysis and can be tuned to design better electrocatalysts.