Single atom alloy clusters Agn−1X− (X = Cu, Au; n = 7–20) reacting with O2: Symmetry-adapted orbital model.

Single atom alloy AgCu catalysts have attracted great attention, since doping the single Cu atom introduces narrow free-atom-like Cu 3d states in the electronic structure. These peculiar electronic states can reduce the activation energies in some reactions and offer valuable guidelines for improving catalytic performance. However, the geometric tuning effect of single Cu atoms in Ag catalysts and the structure–activity relationship of AgCu catalysts remain unclear. Here, we prepared well-resolved pristine Ag_n⁻ as well as single atom alloy Ag_n−1Cu⁻ and Ag_n−1Au⁻ (n = 7–20) clusters and investigated their reactivity with O₂. We found that replacing an Ag atom in Ag_n⁻ (n = 15–18) with a Cu atom significantly increases the reactivity with O₂, while replacement of an Ag with an Au atom has negligible effects. The adsorption of O₂ on Ag_n⁻ or Ag_n−1Cu⁻ clusters follows the single electron transfer mechanism, in which the cluster activity is dependent on two descriptors, the energy level of α-HOMO (strong correlation) and the α-HOMO–LUMO gap (weak correlation). Our calculation demonstrated that the cluster arrangements caused by single Cu atom alloying would affect the above activity descriptors and, therefore, regulates clusters' chemical activity. In addition, the observed reactivity of clusters in the representative sizes with n = 17–19 can also be interpreted using the symmetry-adapted orbital model. Our work provides meaningful information to understand the chemical activities of related single-atom-alloy catalysts. [ABSTRACT FROM AUTHOR]