Do we achieve “1 + 1 > 2” in dual-atom or dual-single-atom catalysts?

In recent years, the study of high utilization rates of metals in multiphase catalysis has sparked interest in various atomically dispersed catalysts. Compared to single-atom catalysts, diatomic catalysts not only retain advantages such as high active sites, unsaturated coordination environments, and unique electronic structures, but also incorporate multiple interactive mechanisms. This integration enables them to surpass the limits of single-atom catalysts in catalytic performance, demonstrating a unique ability of "1 + 1 > 2". Consequently, diatomic catalysts turn out to be a hot topic in the field of catalytic reactions driven by coupling processes. Currently, catalysts that are loaded on material surfaces with two different single-atom-level metal coordination environments are commonly referred to as diatomic catalysts. Reviews reported to date mainly categorize and study the differences and advantages of homonuclear and heteronuclear bimetallic active centers in diatomic catalysts. However, this conceptual distinction has obscured the impact of spatial configuration differences on catalytic mechanisms. For diatomic catalysts, the differences in coordination environments between the two atoms lead to changes in spatial configuration, which in turn produce specific synergistic effects, greatly influencing the activity, selectivity, and stability of the catalytic reactions. With this in mind, this paper, for the first time, clarifies the definition of diatomic catalysts based on different spatial configurations, resolving long-standing confusion. Catalysts with two types of atoms dispersed in isolation without any connection are defined as dual-single-atom catalysts; those with two atoms connected either through a heteroatom bridge or direct bonding are defined as dual-atom catalysts. Additionally, this paper summarizes the latest research progress in the synthesis methods and technical characterization of these two types of catalysts. Furthermore, it explores the mechanisms of dual-core synergy and primary-secondary synergy in dual-single-atom catalysts, as well as the electron effects caused by distance effects and unique functional environments in dual-atom catalysts that contribute to enhancing the catalytic performance of dual-atom catalysts. These are key to achieving the "1 + 1 > 2" catalytic performance in various catalytic fields for both dual-single-atom catalysts and dual-atom catalysts. Further, in the field of electrocatalysis, this paper reviews the structure–activity relationships of diatomic catalysts for various reactions and focuses on how in situ characterization techniques and first-principles calculations can reveal these relationships in depth. Finally, it explores the challenges faced by dual-atom catalysts and dual-single-atom catalysts in practical applications, offering new perspectives for the rational design and thorough research of efficient dual-atom catalysts and dual-single-atom catalysts in heterogeneous catalysis.