Majid Vafaeezadeh, Werner R. Thiel

• Atomically-dispersed heterogeneous metal catalysts can be considered as an “upgraded” version of classical solid metal catalysts. Unlike traditional heterogeneous metallic catalysts that contain nanoparticles or bulk micro-sized transition metal species, the active sites of these novel types of catalysts are atomically distributed on the support's surface. This provides several advantages compared to classical catalysts such as higher activity and need for just very small amounts of metal precursors for the catalyst preparation. The latter issue is a key point in green chemical processes and of importance to achieve low-cost pathways for industrial-scale synthesis. The atomically dispersed metal sites permit a maximum of metal dispersion and additionally allow to achieve more reproducible heterogeneous catalysts. This review summarizes an overview on breakthrough findings in synthesis, applications and characterization techniques developed in the area of single-atom, dual-atom, single-atom-layer, single-site as well as single-atom nanozyme/enzyme catalysis. The characteristics and properties of each system provide an appropriate understanding for designing a nanomaterial that is optimized for a specific requirement.