Comparison of various metal catalysts in the microwave-assisted pyrolysis of polyethylene revealed bimetallic Co₁Fe₉O_x to be the most effective and efficient catalyst, with a H₂ yield of approximately 90 % and abundant carbon nanotubes with tube diameters of 20–30 nm.

Abstract

Environmental issues caused by waste polyethylene are becoming increasingly severe. Among potential treatment processes, microwave-assisted catalytic pyrolysis is promising for converting waste plastics into valuable products owing to its energy efficiency and environmental sustainability. Herein, a modified citric acid combustion method was used to prepare a series of metal oxide catalysts with loose porous structures. The prepared Fe-based catalysts doped with Co, Ni, or Cu were employed in the microwave-assisted catalytic pyrolysis of polyethylene. The bimetallic Co₁Fe₁O_x catalyst exhibited the best performance, yielding hydrogen at a rate of 60.7 mmol/g_plastic. Further variation in the Co : Fe ratio revealed that the Co₁Fe₉O_x catalyst achieved the highest hydrogen production efficiency (63.64 mmol/g_plastic). Similar oil-phase products were obtained over the various catalysts, as revealed by infrared spectroscopy and proton nuclear magnetic resonance spectroscopy. Furthermore, scanning electron microscopy (SEM) identified carbon nanotubes as the major solid product of pyrolysis, which were attached to the catalyst surface. Finally, a combination of thermogravimetric analysis, SEM, and energy-dispersive X-ray spectroscopy indicated that the reduction in catalytic activity following recycling was caused by the accumulation of carbonaceous products on the catalyst surface. Overall, Co₁Fe₉O_x catalysts were favorable for obtaining H₂ and carbon nanotubes by the microwave-assisted pyrolysis of polyethylene.