Microwave-assisted pyrolysis of plastics with iron-based catalysts for hydrogen and carbon nanotubes production

This study presents a microwave-assisted pyrolysis (MAP) for plastics using iron-based catalysts with the aim to convert high-density polyethylene into hydrogen and carbon nanotubes (CNTs). A multistep mechanism was proposed based on the microwave-induced ‘micro-hot spots’ theory, indicating that the yield and composition of pyrolytic products are mainly determined by the microwave absorbing property and the activity of the catalysts. To validate the mechanism, iron-based catalysts with different microwave-absorbing properties were prepared using different catalyst supports including activated carbon, silicon carbide, and silicon dioxide. The experimental results showed that the catalysts with high dielectric loss promoted the gas yield and the growth of CNTs. This was due to the local high-temperature domains generated over the catalyst surface under microwave irradiation. In addition, the role of catalytic activity was explored by designing FeAlOx catalysts with different iron loadings. An increase in iron content from 7% to 22% improved the gas yield from 86.3% to 93.7% but has insignificant effect on the morphology of CNTs. Conclusions drawn from this study can guide the future design and development of microwave-responsive catalysts for the microwave-assisted pyrolysis of plastic wastes towards circular economy.