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

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 NHx 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 (UL) 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 UL 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.