Plasma radicals control synthesis of platinum supported on nitrogen-doped graphitic carbon for ambient-pressure ammonia synthesis

Integrating isolated metal atoms and subnanometer clusters within highly doped, structurally ordered carbon frameworks remains challenging because thermal synthesis promotes dopant loss, metal migration, and structural disorder. We report a biphasic liquid–liquid solution plasma strategy that produces nitrogen-doped graphitic carbons with a record-high nitrogen content of 22.4 atomic% while stabilizing atomically dispersed platinum sites and subnanometer clusters. Analysis of 53 cyclic organic precursors by optical emission spectroscopy and machine learning identifies CN and C2 radicals as positive descriptors for nitrogen incorporation and graphitic ordering, whereas Hβ emission correlates with hydrogen termination and disorder. In ambient-pressure nonthermal plasma-assisted ammonia synthesis, this material used as a catalyst achieves 3.97 gNH3 kWh‒1 and sustained operation for 85 h without degradation. Infrared spectroscopy, isotope-switching experiments, and density functional theory are consistent with a proposed cooperative catalytic pathway involving nitrogen activation at isolated platinum sites, hydrogenation at platinum clusters, and a Mars–van Krevelen-like contribution from Pt–Nx–C motifs. Highly doped, structurally ordered carbon frameworks that simultaneously host isolated metal atoms and sub-nanometer clusters are difficult to synthesize. Here, the authors develop a biphasic liquid–liquid solution plasma strategy that yields nitrogen-doped graphitic carbons with a record-high nitrogen content of 22.4 at.% while co-stabilizing platinum as atomically dispersed sites and sub-nanometer clusters.