Unveiling the Role of Ru–N ₄ on Ru–N–C Single-Atom Catalyst in C–O/C–C Bonds’ Oxidative Cleavage in Lignin

M–N–C single-atom catalysts (SACs) represent promising candidates owing to their atomically dispersed active sites and tunable catalytic properties and have shown broad potential in various catalysis reactions. However, the mechanisms and true active sites involved in lignin conversion, particularly oxidative depolymerization, remain unclear. Herein, a Ru–N–C SAC with a well-defined configuration, including coordination environment and coordination number, was synthesized via a straightforward ball-milling method for lignin oxidation. The Ru–N–C SAC prepared with 12 h of ball milling demonstrated high catalytic performance in the oxidative depolymerization of various β-O-4 model compounds and diverse lignin feedstocks. Structural analysis via X-ray absorption spectroscopy demonstrated that the Ru–N4 motif constitutes the predominant coordination environment in Ru–N–C, which is regarded as the primary active site in activating O2 into superoxide radicals, as confirmed by free-radical quenching experiments and electron paramagnetic resonance analysis; meanwhile, it also served as a basic site in polarizing Cβ–H bonds in β-O-4 that favored C–O/C–C bond cleavage, which was disclosed by CO2 temperature-programmed desorption and electron localization function analysis. The critical role of Ru–N4 in the activation of O2 and C–O/C–C bond cleavage was further confirmed by density functional theory calculation, which indicated that the Ru–N4 center exhibits strong adsorption toward both the O2 and β-O-4 linkages. This work provides a deep understanding on the active sites within Ru–N–C SACs for lignin oxidative cleavage and offers great potential on the rational design of next-generation SACs in biomass valorization.