Space-Confined Carbon-Doped Pd Nanoparticles as a Highly Efficient Catalyst for Selective Phenol Hydrogenation

Pd-based catalysts have been widely applied in hydrogenation reactions for production of both bulk and fine chemicals; however, selective and efficient hydrogenation of substrates with multiple functional groups is challenging. Here, we synthesize the C-doped Pd NPs confined in the hollow-structured carbons with a closed shell (C-Pd@HCS), a single hole in the shell (C-Pd@HCH), and a bowl-like shell (C-Pd@HCB). Based on the three comparative samples, void-confinement effects in phenol hydrogenation are investigated. It is found that the reaction rate of phenol hydrogenation is always decelerated gradually in the order of C-Pd@HCS > C-Pd@HCH > C-Pd@HCB, regardless of what reaction conditions are applied. The highest rate over C-Pd@HCS should be related to the void-confinement effect induced by its closed space. Moreover, the selectivity toward cyclohexanone over C-Pd@HCS/HCH/HCB is always higher than 95% due to the C-doped Pd NPs (hindering the Pd β-hydride formation and disfavoring the cyclohexanone adsorption as revealed by experimental and theoretical results).