Co-Production of Hydrogen and Carbon Nanotubes From Waste Plastic Over Ni-Fe/Hydrotalcite Catalysts: Process Optimization and Mechanistic Pathways.

To tackle the urgent energy depletion issue, this research presents a catalytic upcycling approach for plastic waste through thermochemical conversion mediated by Ni-Fe nanoparticles anchored on layered double hydroxide substrates. The effects of Ni/Fe loading molar ratios, catalytic temperatures, and steam flow rates were explored to optimize catalyst performance and operational parameters for hydrogen (H2) production. Attributed to the formation of a spinel phase, the Ni-Fe1.5/hydrotalcite catalyst demonstrated the highest catalytic activity during the catalytic reforming process, yielding 51.41 mmol/gPP of H2 (where PP denotes polypropylene) without steam and 121.97 mmol/gPP with steam (8 mL/h). Additionally, carbon nanomaterials produced at a catalytic temperature of 800°C showed the highest carbon yield. Furthermore, the catalyst exhibited robust cyclic stability, retaining approximately 90.3% of its initial H2 yield after five consecutive reaction-regeneration cycles, with its mild deactivation primarily driven by bimetallic sintering and amorphous carbon encapsulation. Possible reaction pathway mechanisms of PP and methane (CH4) proposed based on Gaussian simulations demonstrate consistency with the experimental results. Finally, the mechanism of catalytic temperature on the production of carbon nanotubes (CNTs) was proposed. These findings provide valuable insights for applications of H2 and CNTs production through waste plastic pyrolysis.