Owning to unsaturated coordination environment, quantum size effect and metal-support interaction, single- or dual-atom metal
sites, such as Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Rh, Pd, Ag, Sn, Ir, Pt, Au, Bi, and Er coordinated with nonmetallic elements such as
O, N, P, and S, exhibit different electronic configurations, which endow them with high catalytic performances in multiple redox
reactions and versatile applications in organic synthesis, environmental remediation, energy conversion, and biomedicine. Despite
intense research, the relation of structure-activity for single-atom catalysts (SACs) still bedazzles researchers, since diversified
configurations of active sites would bring about difficulty in structural identification and theoretical simulations. Here, recent results
on the applications of SACs are reviewed with an emphasis on identifying the active sites and discussing the relation between
structure and property.