Coal gangue-supported catalyst for the efficient conversion of waste plastics into hydrogen and carbon nanotubes

From an economic and environmental perspective, pyrolysis-catalysis is a promising method for converting waste plastics into high-value products. Low-cost catalysts are crucial for the large-scale processing of waste plastics. This study prepared a nickel-based catalyst supported on solid waste coal gangue (CG) using a simple impregnation method and investigated the effect of catalyst composition on the catalytic pyrolysis of polyethylene to produce hydrogen and high-value carbon nanotubes (CNTs). Experiments were conducted in a two-stage fixed bed reactor, where the plastic was pyrolyzed in the first stage (500 °C) and the resulting volatiles were activated on the catalyst in the second stage. The effects of catalytic temperature (700–900 °C) and Ni loading (5%–20%) on the products were studied. The results showed that the 15% Ni/CG catalyst exhibited optimal activity at a catalytic temperature of 750 °C, with maximum hydrogen concentration and yield reaching 66.48 vol% and 31.48 mmol/g, respectively, and CNTs yield reaching 79.71 mg g⁻¹. At this temperature, the resulting carbon had a high degree of graphitization and fewer defects, with an ID/IG value of 0.72. Additionally, the gas-phase oxidation method effectively regenerates the coal gangue-based catalyst. After five cycles of pyrolysis-catalysis experiments, the hydrogen concentration and yield remained high, at approximately 63.0 vol% and 27 mmol g⁻¹. Low-temperature oxidation removed most amorphous carbon, enriching the catalyst surface with up to 73.9 wt% CNTs of high quality (ID/IG ratio of 0.67). The mechanism of CNTs formation from polyethylene pyrolysis and catalytic decomposition was also discussed.