Breaking the Scaling Relationship Limit: From Single-Atom to Dual-Atom Catalysts

ConspectusRecent decades have witnessed the rapid development of catalytic science, especially after Taylor and Armstrong proposed the notion of the "active site" in 1925. By optimizing reaction paths and reducing the activation energies of reactions, catalysts appear in more than 90% of chemical production reactions, involving homogeneous catalysis, heterogeneous catalysis, and enzyme catalysis. Because of the 100% efficiency of active atom utilization and the adjustable microenvironment of metal centers, single-atom catalysts (SACs) shine in various catalytic fields for enhancing the rate, conversion, and selectivity of chemical reactions. Nevertheless, a solo active site determines a fixed adsorption mode, and the adsorption energies of intermediates from multistep reactions linking with a solo metal site are related to each other. For a specific multistep reaction, it is almost impossible to optimally adjust the adsorption of every intermediate on the solo site simultaneously. This phenomenon is termed the scaling relationship limit (SRL) and is an unavoidable obstacle in the development of pure SACs.Dual-atom catalysts (DACs), perfectly inheriting the advantages of SACs, can exhibit better catalytic performance than simple SACs and thus have gradually gained researchers' attention. Depending on the dual-metal structure, dual-metal sites (DMSs) in DACs can be divided into two separated heterometal sites, two linked homometal sites, and two linked heterometal sites. Two separated heterometal sites prescribe a distance limit between two metal sites for electron interaction. Currently, the active origins of DACs can be summarized in the following three points: (1) electronic effect, in which only one metal center serves as active site and the other plays an electronic regulatory role; (2) synergistic effect, in which two metal centers separately catalyze different core steps to improve catalytic performance together; (3) adsorption effect, in which offering additional sites changes the adsorption structures to break the SRL based on SACs. Among the three active origins, optimizing the adsorption structure of intermediates upon DMSs is one of the most effective technologies to boost the catalytic property of DACs on the basis of SACs. To date, few contributions have focused on the development of DACs in various heterogeneous catalysis environments, including O2 reduction reaction, O2 evolution reaction, H2 evolution reaction, CO2 reduction reaction, N2 reduction reaction, and other conversion reactions.In this Account, a summary of recent progress regarding DACs in heterogeneous catalysis will be presented. First, the evolution of DACs from an unpopular discovery to research hot spot is illustrated through a timeline. In the next section, the DACs are divided into three categories, and the potential active origins of DACs are revealed by comparison with SACs. In addition, the techniques for constructing DACs are systematically summarized, including preparation of carbonous, pyrolysis-free, noncarbon-supported, and complex-type DACs. Furthermore, the underlying active origins of DACs in specific energy- and environment-related reactions are introduced in detail with assistance of theoretical calculations. Finally, we affirm the contribution of DACs to catalysis, particularly heterogeneous electrocatalysis, and provide an outlook regarding the development direction for DACs by discussing the major challenges. It is anticipated that this Account can inspire researchers to propel the advance of DACs.