Dual Pd2+ and Pd0 Sites on CeO2 for Selective Oxidative Cleavage C-C Bond of β-O-4 Linkage in Lignin

Selective C-C bond cleavage in lignin is a pivotal step toward its efficient utilization, enabling the production of diverse and valuable chemicals from this abundant and renewable resource. Among the catalysts reported for oxidative cleavage of C-C bond in lignin β-O-4 ketones, CeO₂-supported Pd catalysts have demonstrated outstanding activity. However, the exact nature of the active Pd sites responsible for C-C bond cleavage remains poorly understood, limiting mechanistic insights into this transformation. In this study, CeO₂ supports with four distinct morphologies including rods (R), spheres (S), cubes (C), and polyhedrons (P) were synthesized to investigate their influence on the catalytic activity of Pd/CeO₂ catalysts in the oxidative cleavage of β-O-4 ketones. The catalytic performance was found to depend strongly on the morphology of CeO₂, with Pd/CeO₂-P exhibiting the highest activity for C-C bond cleavage. This catalyst possessed the lowest Pd⁴⁺ content and the highest Pd⁰/Pd²⁺ ratio among the series. By systematically tuning this ratio, a volcano-type relationship between Pd⁰/Pd²⁺ ratio and catalytic activity was established, indicating a dual-site mechanism in which Pd⁰ facilitates the formation of reactive oxygen species, while Pd²⁺ serves as the active center for C-C bond cleavage.