First-principles investigation of azoxybenzene adsorption and N–O bond activation on Cu, Ag, and Au clusters

The adsorption and activation of azoxybenzene on Cu, Ag, and Au clusters were systematically investigated using density functional theory (DFT) calculations. Structural analyses reveal that azoxybenzene covers a large portion of the cluster surface and preferentially adsorbs at the boundary(111) site, where multi-point contacts maximize binding stability. Upon adsorption, the N-O bond undergoes significant elongation accompanied by contraction of adjacent C-N bonds and distortion of molecular planarity, with the oxygen atom tilting toward the surface metal. The extent of these changes depends strongly on the metal, following the order Cu > Ag > Au. Adsorption energies confirm that Cu binds azoxybenzene most strongly (-2.409 eV), compared to Ag (-1.406 eV) and Au (-1.573 eV). Electronic structure analyses demonstrate substantial charge transfer from the clusters to the oxygen atom, leading to population of N-O antibonding orbitals. Projected density of states (PDOS) and frontier orbital analysis further confirm strong O-p and metal d orbital hybridization, especially on Cu, which accounts for the pronounced N-O activation. While Au exhibits stronger dispersion-driven stabilization than Ag, its limited N-O elongation suggests weaker reactivity. The results establish copper as the most effective catalyst among the three for promoting N-O bond activation in azoxybenzene, offering mechanistic insight into selective catalytic reduction pathways.