Single Atom Alloy Clusters Agn−1X− (X = Cu, Au; n = 7−20) Reacting with O2: Symmetry-adapted Orbital Model

Single atom alloy AgCu catalyst has attracted great attention, since doping the single Cu atom introduces narrow free-atom-like Cu 3 d 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 catalyst 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 effect. 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.