Recyclable Hydrazine-Passivated NiB_x/Ni Heterostructured Catalyst for Enhanced Hydrogenation of Polystyrene Pyrolysis Oil

Nickel boride (Ni₂B) is known to be an excellent catalyst for hydrogenation of olefin structures under mild reaction conditions. Its susceptibility to oxidation in air environments, however, limits its practical applications. Here, a hydrazine-passivated nickel boride/nickel (NiB_x/Ni) heterostructured catalyst is developed to address the oxidation problem while improving the conversion efficiency of styrene to ethylbenzene in a real polystyrene (PS) pyrolysis oil. The calculated turnover frequency for styrene to ethylbenzene conversion is 24.9 mmol/h·g, with 99.9% selectivity, representing a 38.3% improvement compared with the control Ni₂B catalyst (18.0 mmol/h·g). This outstanding performance is attributed to the increased charge density on both Ni and B, induced by a strong hydrazine reductant, which surpasses other amine-structured ligands, such as ethanolamine and ethylamine. Additionally, the enhanced magnetic properties of NiB_x/Ni, resulting from the Ni(111) nanocrystalline structure along the easy axis of magnetization, enable facile recovery after a neodymium magnet. The catalyst demonstrated excellent stability, retaining catalytic efficacy over five consecutive cycles with only a 4% loss in activity. This study highlights the potential of the NiB_x/Ni catalyst for low-cost and efficient hydrogenation in industrial applications.