Heterogeneous metal trimer catalysts on Mo₂TiC₂O₂ MXene for highly active N₂ conversion to NH₃

Activating the inert basal plane of functional-group-terminated MXenes is highly desirable for non-noble metal MXenes applied for the N₂ reduction reaction (NRR) to value-added ammonia (NH₃), but remains a significant challenge. Herein, we report the coupled interactions and catalytic mechanism of the supported trimeric metal M₃ single-cluster catalysts (SCCs) on oxygen-vacancy-rich Mo₂TiC₂O₂ (V_O−Mo₂TiC₂O₂) MXene materials for catalyzing this difficult reaction via spin-polarized density functional theory (DFT) calculations. Energy and electronic property analysis showed that the robust structural stability of the M₃/V_O−Mo₂TiC₂O₂ catalysts stemmed from the vacancy defects, electronegativity difference, and significant charge redistribution. Based on the unique bidirectional electron transfer activation mechanism, the N₂ adsorption strength can be well represented by a simple descriptor of the valence electrons in the d orbitals of M₃ trimers. Remarkably, these confined Rh₃, Ru₃, Ni₃ and Co₃ SCCs on V_O−Mo₂TiC₂O₂ were capable of providing superior electrocatalytic activity and selectivity toward the NRR with ultra-low limiting potentials of −0.23, −0.32, −0.42 and −0.44 V, respectively. Finally, a mechanistic investigation illustrated that the valence electrons, charge transfer, and d-band center of the M₃ active sites could collectively act as good descriptors to correlate the structure and NRR activity.