High-yield hydrogen from mixed plastic waste via response surface optimized coke-resistant ni-Fe/CPA-Al2O3 catalysis

Thermochemical upcycling of mixed plastic waste to hydrogen offers a combined route for plastic-waste valorisation and energy recovery, although catalyst deactivation by carbon deposition and heavy intermediates remains a major limitation. In this study, a bimetallic Ni-Fe catalyst supported on cassava peel ash containing 20 wt% Al 2 O 3 (Ni-Fe/CPA-20 %Al 2 O 3 ) was developed and evaluated in a two-stage fixed-bed pyrolysis-steam-reforming system. Mixed-plastic feeds comprised polyethylene terephthalate (PET), polypropylene (PP), and high-density polyethylene (HDPE), with PET systematically varied from 25 to 75 wt% to evaluate the influence of an oxygenated aromatic polymer relative to polyolefin-rich feedstocks on hydrogen production and gas-phase carbon distribution. A four-factor Box-Behnken response-surface design investigated PET fraction (25–75 wt%), reforming temperature (700–900 °C), steam-to-plastic ratio (S/P = 1.0–3.0), and catalyst loading (1.0–2.0 g). The corrected quadratic models showed strong agreement with the experimental responses ((R 2 = 0.9978) for H 2 and 0.9914 for CO 2 ), with non-significant lack of fit. Catalyst resistance to carbon accumulation was supported experimentally by thermogravimetric analysis of the spent Ni-Fe/CPA-20 %Al 2 O 3 catalyst, which showed only ∼ 1.54 wt% mass loss attributable to deposited/retained carbonaceous species, compared with ∼ 2.53–3.11 wt% for the corresponding Ni-Fe/Al 2 O 3 catalyst, together with a comparatively weak carbon-oxidation feature. Reusability testing over five cycles further showed sustained H 2 production with a reported coke yield of 0.044 wt%, supporting improved resistance to carbon deposition. The behaviour is consistent with the combined effects of Ni-Fe active sites and the K-, Ca-, and Mg-containing CPA matrix in promoting steam-assisted carbon removal, although these roles are interpreted mechanistically rather than directly measured in situ. Temperature and S/P exerted the strongest effects on the gas responses, while feed composition significantly influenced product distribution. Numerical optimization originally identified 857.9 °C, S/P = 2.80, 1.94 g catalyst, and 52.13 wt% PET. Overall, the results demonstrate the potential of waste-derived CPA-Al 2 O 3 -supported Ni-Fe catalysts for hydrogen-rich syngas production from compositionally heterogeneous plastic feeds while limiting catalyst carbon accumulation.