Highly active Ni-Mg-Al Ni–Mg–Al catalyst effect on carbon nanotube production from waste biodegradable plastic catalytic pyrolysis

The catalyst composition effect on the biodegradable plastic (Poly Lactic (PolyLactic Acid, PLA) catalytic pyrolysis in generating hydrogen and high-value carbon nanotubes (CNTs) was examined using Ni-Mg-Al Ni–Mg–Al oxygen carrier catalysts. Experiments were carried out in a two-stage fixed bed reactor, with the plastic pyrolyzed in the first stage and the resulting volatiles activated over the catalyst in the second stage. This research investigates the different catalytic pyrolysis temperatures (500, 600, and 700 °C) that could improve the hydrogen production and CNTs yield formed on the catalyst. The results reveal that the catalyst composition and pyrolytic temperature significantly affected the development of CNTs and hydrogen production. The Ni/MgAl₂O₄ produced the highest hydrogen (73 vol.%) and CNTs yield (374 mg g⁻¹_plastic) among the monometallic catalysts. Adding Ni to the Mg-based catalyst increased metal particle dispersion and metal-support interactions, resulting in better catalytic activity in proportion to filamentous carbon production. Increasing the Mg-Al Mg–Al concentration could promote CNTs graphitization. Therefore, the 1:2 ratio of Ni: Mg catalysts exhibited the most significant catalytic activity in the quantity and quality of generated CNTs. The formation mechanism of bamboo-like CNTs from pyrolysis and catalytic decomposition of PLA was also discussed.