Turning metal-organic frameworks into efficient single-atom catalysts via pyrolysis with a focus on oxygen reduction reaction catalysts

Single-atom catalysts (SACs) have attracted extensive attention because of their maximal atom utilization, unique electronic structure and high activity. Metal-organic frameworks (MOFs) could be used as perfect self-sacrificed precursors/templates for preparing SACs due to their uniformly distributed and spatially separated metal nodes and organic linkers as well as designable pore structures. Recently, numerous studies have been devoted to utilizing MOFs to prepare SACs through pyrolysis. Herein, this review summarizes the most recent strategies of turning selected MOFs into SACs, focusing on oxygen reduction reaction (ORR) catalysts. First, the inherent metal sites in MOFs are directly turned into single-atom sites via the high-temperature treatment with/without acid etching. Second, additional metal precursors are introduced into MOFs by various methods to further supplement active sites in the obtained SACs. Third, nonmetal heteroatom-rich (i.e., N, P and S) precursors are combined with MOFs to provide more coordination sites to anchor metal atoms. Finally, perspectives on future opportunities for selecting and designing MOFs as SAC precursors are also proposed.