A facile method to synthesize water-soluble Pd₈ nanoclusters unraveling the catalytic mechanism of p-nitrophenol to p-aminophenol

Hydrogenation of p-nitrophenol (PNP) towards the conversion to p-aminophenol (PAP) by metal catalysis is known as a simple and eco-friendly technique for the production of corresponding industrial and pharmaceutical intermediates. While continuous efforts are paid for more sustainable and greener procedures by using transition metal catalysts, atomic-precise reaction mechanism for the PNP-to-PAP is still illusive to be fully understood. Utilizing a dry-wet combined strategy, here we have synthesized water-soluble Pd₈ nanoclusters (NCs) with mercaptosuccinic acid (H₂SMA) as the ligand, and the Pd₈ NCs found high catalytic performance for the conversion of PNP-to-PAP, as identified by the electrospray ionization mass spectrometer (ESI-MS) measurement. The gradual changes over time of ultraviolet-visible (UV-vis) spectra of PNP really display the catalytic reduction by NaBH₄ in presence of Pd₈ NCs. Further, in-depth charge transfer interactions between PNP and the Pd₈ clusters at the proton-rich conditions are investigated by natural bond orbital (NBO) analysis and electron density difference (EDD) analysis. The exothermic and kinetic-favorable reaction pathways are addressed, based on successive PNP hydrogenation and H₂O removal processes, clarifying the reaction mechanism of Pd catalysts. It is worth noting that this solid-state synthetic route for such Pd₈ clusters enables gram-scale quantity of production in likely practical use.