Beyond Monosite Catalysis: Multi‐Tiered Site Engineering for Advanced CO 2 Valorization

Abstract Single‐atom catalysts (SACs) have redefined the landscape of heterogeneous catalysis by offering atom‐level precision and remarkable activity in small‐molecule transformations. Yet, their inherently isolated active centers limit the ability to mediate complex multi‐step reactions such as cascade, tandem, and multi‐intermediate conversions. To address this challenge, multi‐tiered site engineering (MTSE), a conceptual framework in which spatially and electronically coupled active sites cooperate to transcend the constraints of conventional mono‐site catalysis is introduced. MTSE advances SAC design along four key dimensions: dual‐atom ensembles, single‐atom/nanostructure hybrids, functional group cooperativity, and support‐mediated electronic modulation. By orchestrating synergistic adsorption and stepwise intermediate conversion, these strategies unlock efficient routes for CO 2 conversion into fuels and high‐value chemicals. Progress across thermal, electrocatalytic, and photocatalytic systems, highlighting how MTSE enhances activity, selectivity, and durability through deliberate multi‐site interplay is critically assessed. Cutting‐edge in situ and operando spectroscopies are emphasized as indispensable for unraveling dynamic intermediates and structure‐function correlations. Looking forward, pressing challenges, including atomically precise synthesis, mechanistic deconvolution of cooperative pathways, and integration into cascade process design are identified. This review positions MTSE as a transformative paradigm for next‐generation SACs and a pathway toward sustainable CO 2 valorization.