Beyond the Lindlar catalyst: highly-oxidized Pd single atoms as promoters for alkyne semi-hydrogenation

The semi-hydrogenation of alkynes is crucial for the synthesis of steroid hormone drugs, yet conventional approaches relying on Pd surface poisoning additives sacrifice activity and sustainability for selectivity. Herein, we present a "grafting-then-coordination" strategy to construct a Pd/C-NH₂ catalyst featuring coexisting tetravalent Pd single atoms (Pd_IV SAs) and Pd nanoparticles (NPs), which achieves inhibitor-free and highly efficient hydrogenation of steroidal alkynes. The Pd/C-NH₂ catalyst, functionalized with 3-aminopropyltriethoxysilane (APTES), exhibits 99% conversion with 97% selectivity in the selective hydrogenation of mifepristone under 0.1 MPa at 25 °C, with a remarkable turnover frequency (TOF) of 3675 h^-1, representing a 17-fold enhancement over the conventional Lindlar catalyst. Mechanistic studies reveal that the Pd_IV SAs are stabilized through Pd-N/O coordination by leveraging oxygen-containing groups of the support and amino groups of the ligand. The electron-deficient Pd_IV SAs adsorb mifepristone, mitigating substrate self-poisoning on Pd NPs, while Pd NPs activate H₂ and promote hydrogen spillover to Pd_IV SAs, enabling hydrogenation via a dual-site cooperative mechanism. The stable Pd_IV SAs transform conventional poisoning sites into productive active centers, offering valuable insights for the rational design of advanced selective hydrogenation catalysts.