From double‐atom catalysts to single‐cluster catalysts: A new frontier in heterogeneous catalysis

In recent years, atomically dispersed metal catalysts with well‐defined structures have attracted great interest in heterogeneous catalysis due to their high atomic utilization efficiency, activity, stability, and selectivity. The rapid development of single‐atom catalysts (SACs) has simultaneously stimulated the emergence of diatomic catalysts (DACs) and single cluster catalysts (SCCs). Compared with SACs, DACs, and SCCs possess higher metal loading and more structurally flexible active sites, which provide great potential for achieving higher catalytic performance. DACs and SCCs have become a new field of heterogeneous catalysis. In this review, we first focus on the latest developments of DACs/SCCs, including synthesis methods and characterization approaches including experimental and theoretical tools. In addition, the relationship between structure and catalytic performance of DACs/SCCs are well discussed, including the effect of supports, synergistic metal atoms, and coordination environment. At last, the similarities and differences between SACs and DACs/SCCs are systematically summarized and analyzed. Double‐atom catalysts (DACs) and single‐cluster catalysts (SCCs) anchor more metal atoms at an adsorption site to possess higher metal loading, which provide adjacent unsaturated coordination sites for adsorption of multiple reactants at the same time. The synergistic effect between the metal atoms changes the adsorption mode of key intermediates and optimizes reaction pathways, resulting in significantly improved catalytic activity and selectivity.