On the Coordination Environment of Single-Atom Catalysts.

ConspectusSingle-atom catalysis has become one of the most active frontiers in catalysis in the past decade. This concept not only gives birth to a new kind of heterogeneous catalysts featuring well-defined isolated active sites and strong covalent (or electronic) metal-support interaction, which deliver unique catalytic activity, selectivity, and stability distinct from their nanoparticulate counterparts, but also together with the principles and concepts in history, reshapes our understanding of heterogeneous catalysis and drives the catalysis research from the nanoscale and subnanoscale to the more precise atomic scale.Due to the extremely high free energy, the isolated gaseous metal atoms cannot survive alone but are stabilized on the support materials through strong chemical binding, forming metal-centered coordination moieties resembling organometallics in homogeneous catalysis. The coordination environment, including the inner shell and outer shell comprised of the support, reactants, and environmental molecules, determines the electronic and geometric properties of central atoms and, in turn, the catalytic performance of SACs. In some cases, the neighboring atoms on the support can be directly involved in the catalysis in a metal-support concerted catalysis manner. Therefore, the coordination environment not only serves as an effective descriptor of the structure-performance relationship but also provides great opportunities to fabricate better-performed SACs through modulation of the coordination atoms, number, promoter, and so on.In this Account, recent advances in our group in the identification and modulation of the coordination environment of SACs are highlighted. First, the heterogeneity of the coordination environment of SACs is discussed regarding the nonuniform structure and composition of the support. Then, various approaches to tune the inner-shell and outer-shell coordination environments are shown for the effective improvement of the catalytic performance of SACs. Finally, the structure evolution of SACs driven by external stimuli and reactant/products is discussed. The atomic understanding of the coordination environment of SACs will help to elucidate the nature of single-atom catalysis and enrich the theoretical aspects of heterogeneous catalysis.