Coordination Engineering Directs d-Band Center Optimization for Efficient CO2 Hydrogenation to Formic Acid on Ir–Nx Single-Atom Catalysts

The in-depth exploration of electronic structure serves as a cornerstone for rational catalyst design, which is essential to unlock the precise structure-activity relationships governing heterogeneous CO 2 hydrogenation to formic acid. Herein, we report a coordination engineering strategy to tailor the electronic structure of iridium single-atom sites by synthesizing a series of Ir-N x catalysts with well-defined N-coordination numbers ( x =4, 3, 2). Significantly, the optimized Ir-N 2 catalyst exhibits exceptional activity, achieving a high turnover number (TON) of 19400, significantly surpassing most reported heterogeneous catalysts. Combined X-ray absorption fine structure (XAFS) analysis and density functional theory (DFT) calculations reveal that the reduced coordination in Ir-N 2 sites optimizes the local electron density, resulting in enhanced d-band charge occupancy and an upshifted d-band center. This favorable electronic structure facilitates reactant activation and stabilizes key intermediates, thereby lowering the overall energy barrier of the reaction. This work establishes a direct correlation between coordination geometry and catalytic performance through electronic modulation, offering a generalizable design principle for high-performance single-atom catalysts in CO 2 conversion.