Chemical origin of effective functionalization of single atom-MXene catalysts

The chemical and atomic structures of Cu-, Ni-, or CuNi-embedded MXene (Ti₃C₂T_x, T = O or OH) nanosheet catalysts are examined by using various characterization methods to demonstrate the chemical origin of their composition-dependent evolution. The results of combined X-ray spectroscopy studies and the electrochemical test reveal that Cu ions in (Cu or CuNi):MXene remain active having a +1 valence and form metallic Cu-Cu bonds to enhance the catalytic activity for nitrate reduction. By contrast, Ni ions in (Ni or CuNi):MXene tend to remain bound to O as in Ni²⁺O_x staying inactive, and, furthermore, hinder the catalytic activity of Cu when co-doped on MXene. It is also demonstrated that chemistry of MXene itself varies by donating electrons from Ti^2+/3+ to Cu²⁺ to stabilize the active Cu⁺ ions. These findings support a combinational mechanism in which both the abundant metallic bonds and the cooperative chemical reconstruction that happened via MXene-to-Cu charge transfer facilitate the single atom-aided functionalization of MXene catalysts.