Testing of X-Fe₂O₄/Al₂O₃ (X = NiZn, NiMg, MgZn) Composite Catalysts for Microwave-Driven Conversion of HDPE Plastic into Carbon Nanotubes and Hydrogen

This study examined a series of X–Fe₂O₄/Al₂O₃ (X = NiZn, NiMg, MgZn) composite catalysts for microwave-assisted catalytic breakdown of high-density polyethylene (HDPE) waste into hydrogen fuel and carbon nanotubes (CNTs). A single-stage catalytic pyrolysis was conducted at 450 °C using Al₂O₃–NiZnFe₂O₄, Al₂O₃–NiMgFe₂O₄, and Al₂O₃–MgZnFe₂O₄) composite catalysts. The catalysts were characterized before pyrolysis using XRD, UV-Vis spectroscopy, FTIR, and SEM, and their physicochemical properties were correlated with their catalytic performance. Among the tested systems, Al₂O₃–NiZnFe₂O₄ showed the highest efficiency, producing 47.03 mmol/g_plastic of hydrogen and 287 mg/g_plastic of CNTs. This enhanced activity was attributed to its smaller particle size, greater dispersion, and more effective microwave absorption, which facilitated the creation of localized hot spots and promoted the cracking of polymer chains. The presence of Al₂O₃ in the composites further supported hydrogen release and CNT growth by converting intermediate hydrocarbons formed during thermal cracking. A proposed reaction mechanism includes polymer chain scission, catalytic dehydrogenation, and carbon deposition on ferrite nanoparticles. These results demonstrate the potential of microwave-responsive ferrite–Al₂O₃ catalysts for the eco-friendly recycling of plastics into clean hydrogen and valuable carbon nanomaterials.