Engineering d−p Orbital Hybridization Sites in Coupled Pd Atom-Cluster with Bi-Doped Hydroxyapatite for Enhanced Semi-Hydrogenation of Alkynes

Developing chemoselective hydrogenation of alkynes into alkenes is one of the major strategies of the fine chemical industry; however, it presents significant challenges in achieving the semi-hydrogenation of carbon−carbon triple bond (C=C) to carbon−carbon double bond (C=C) without further hydrogenation. Herein, it is proposed that d−p orbital hybridization between the Pd single-atom-cluster sites and the bismuth species can serve as a descriptor for revealing the catalytic activity. Guided by DFT simulation, the well-dispersed Pd clusters and single-atom Pd−O4 sites on Bi-doped hydroxyapatite (PdSA+C/HAP-Bi) are fabricated for semi-hydrogenation of phenylacetylene, realizing the trade-off between activity and selectivity with a robust turnover frequency of 10,140 h−1, surpassing those of the commercial Pd/C catalyst and most of the reported catalysts. It also displays good stability for enduring six cycles with minimal declines in conversion and selectivity. Notably, this catalyst offers a general platform, showcasing broad substrate compatibility across at least 30 types of alkynes. Particularly, the introduction of well-dispersed Bi and PdC can induce prominent electron redistribution at the Pd−O4 sites and adjust the hybridization between the central Pd 3d and Bi 4f orbitals. Simultaneously, the strong electronic coupling in Pd−O−Bi pairs facilitated the d-electron domination near the Fermi level, boosted the activation of H2 and phenylacetylene at PdC sites, and improved the desorption capacity of styrene at Pd−O4 sites. This synergy ultimately lowers the energy barrier for the phenylacetylene semi-hydrogenation.