Monometallic and Bimetallic Ni, Fe, and Co Catalysts to Produce Carbon Nanotubes from Pyrolysis-Catalysis of Waste Polypropylene

Abstract The development of advanced chemical recycling routes that generate high-value products such as carbon nanotubes (CNTs) from waste plastics is of current interest. The specific role of monometallic versus bimetallic nickel, iron, and cobalt catalysts in controlling the formation of CNTs remains insufficiently resolved. In this study, we investigate the application of mono-metallic and bimetallic catalysts under identical two-stage pyrolysis–catalysis conditions to correlate CNTs morphology and crystallinity with catalyst composition and alloy formation. The results showed that monometallic nickel/alumina and iron/alumina catalysts produced high yields of carbon nanotubes (~ 285 mg g⁻ 1 plastic), whereas the synergistic interaction within the bimetallic nickel–iron/alumina catalyst generated significantly higher yields (420.7 mg g⁻ 1 plastic). Conversely, the cobalt/alumina catalyst produced low yields of carbon nanotubes, attributed to strong metal–support interactions and limited carbon solubility. Parallel to the solid carbon formation, the bimetallic nickel–iron catalyst significantly enhanced the dehydrogenation of the plastic-derived hydrocarbons, achieving a maximum hydrogen yield of (45.3 mmol g⁻ 1 plastic). High-resolution microscopy and diffraction analysis confirmed that the formation of multi-walled carbon nanotubes with graphitic interlayer spacings was directly influenced by alloy formation and metal–support interactions. The results clarify how catalyst composition and alloy formation govern hydrocarbon decomposition, carbon dissolution–precipitation behaviour, and nanotube growth, providing mechanistic guidance for optimising simultaneous hydrogen and carbon nanotube production from waste polypropylene. Graphical Abstract