Development of Ni- and Fe- based catalysts with different metal particle sizes for the production of carbon nanotubes and hydrogen from thermo-chemical conversion of waste plastics

Co-production of valuable hydrogen and carbon nanotubes (CNTs) has obtained growing interest for the management of waste plastics through thermo-chemical conversion technology. Catalyst development is one of the key factors for this process to improve hydrogen production and the quality of CNTs. In this work, Ni/SiO2 and Fe/SiO2 catalysts with different metal particle sizes were investigated in relation to their performance on the production of hydrogen and CNTs from catalytic gasification of waste polypropylene, using a two-stage fixed-bed reaction system. The influences of the type of metals and the crystal size of metal particles on product yields and the production of CNTs in terms of morphology have been studied using a range of techniques; gas chromatography (GC); X-ray diffraction (XRD); temperature programme oxidation (TPO); scanning electron microscopy (SEM); transmission electron microscopy (TEM) etc. The results show that the Fe-based catalysts, in particular with large particle size (∼80 nm), produced the highest yield of hydrogen (∼25.60 mmol H2 g−1 plastic) and the highest yield of carbons (29 wt.%), as well as the largest fraction of graphite carbons (as obtained from TPO analysis of the reacted catalyst). Both Fe- and Ni-based catalysts with larger metal particles produced higher yield of hydrogen compared with the catalysts with smaller metal particles, respectively. Furthermore, the CNTs formed using the Ni/SiO2-S catalyst (with the smallest metal particles around 8 nm) produced large amount of amorphous carbons, which are undesirable for the process of CNTs production.