Structurally Defined Low-Coordination Single-Atom Strategy for CO2 Photoconversion to Formic Acid.

A central challenge in single-atom catalysis lies in the precise construction of structurally well-defined local coordination environments at isolated metal sites while retaining low coordination numbers. Here, we present an edge-bonding strategy that confines isolated metal atoms to well-defined edge sites of covalent triazine frameworks, enabling deterministic construction of structurally defined low-coordination environments. With Ni as a model, this approach yields single-atom sites with a well-defined Ni-N1-C6 coordination motif and allows precise control over their spatial distribution. The strategy is readily extendable to other metals, affording single-atom catalysts with structurally defined and highly accessible low-coordination environments. Such geometrically low-coordination sites optimize photogenerated carrier separation and transport while selectively stabilizing the key *OCHO intermediate in the CO2 reduction pathway, thereby directing the reaction toward HCOOH with 98.5% selectivity. This work establishes a principle for achieving low-coordination microenvironments at single-atom sites via macroscopic regulation of support structures, providing a rational strategy for single-atom catalyst design.