Engineered Spatial Confinement of Cu Single-Atoms with Diagonal N─Cu─N Motifs for High-Rate CO₂ Methanation

The renewable-electricity-powered carbon dioxide reduction (eCO₂R) to value-added fuels and feedstocks like methane (CH₄) holds the sustainable and economically viable carbon cycle at meaningful scales. However, this kinetically challenging eight-electron multistep deep-reduction encounters insufficient catalyst design principles to steer complex CO₂ reduction pathways. Utilizing atomic copper (Cu) structures with unitary active sites can boost eCO₂R-to-CH₄ selectivity due to the efficient suppression of unwanted C─C coupling. Herein, we report a sequential ion exchange strategy to fabricate periodic Cu single-atom catalysts within a polymeric carbon nitride (PCN) matrix, where the uniformly dispersed, diagonally coordinated N─Cu─N configuration hosts low-valent Cu^δ+ centers. Leveraging the periodic N-anchoring sites with delocalized π-electron conjugation in the PCN matrix, the isolated Cu sites are obtained with an interatomic distance of ∼4.2 Å under high metal-loading conditions. This engineered spatial configuration effectively inhibits C─C coupling to avoid subsequent multicarbon product formation. The optimized Cu₁/PCN demonstrates exceptional eCO₂R-to-CH₄ performance, achieving 71.1% CH₄ Faradaic efficiency with a high partial current density of 426.6 mA cm^-2 at -1.50 V versus reversible hydrogen electrode, outpacing the state-of-the-art catalysts. This work delves into effective concepts for steering desirable reaction pathways via precisely modulating active site structures at the atomic level to create favorable microenvironments.