Abstract
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 (∼80nm), produced the highest yield of hydrogen (∼25.60mmol H2g−1 plastic) and the highest yield of carbons (29wt.%), 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 8nm) produced large amount of amorphous carbons, which are undesirable for the process of CNTs production.
Highlights
Plastics are one of the most widely-used and multi-purpose materials
Compared to the Fe/SiO2-S, sharp diffraction peaks were observed for the Fe/SiO2-L catalyst, supporting that large metal particles were formed in the Fe/SiO2-L catalyst
The results are consistent with the X-ray diffraction (XRD) analysis and temperature programmed reduction (TPR) analysis (Fig. 3), where much higher temperature was required
Summary
Plastics are one of the most widely-used and multi-purpose materials. Due to increasing demand, global plastics production has continuously grown to 322 million tonnes in 2015, indicating a nearly 60% increase compared to the level in 1990 [1]. Nanioka et al [7] used Ru based catalysts to enhance hydrogen production from steam gasification of polystyrene using a fixed-bed reactor. A two-stage continuous reactor was used to optimize process conditions including reaction temperature and weight space velocity for gasification of polypropylene using Ru based catalysts [8]. Elordi et al [9] used HZSM-zeolite with different ratios of SiO2/Al2O3 as catalyst to investigate coke formation during hydrogen production from gasification of mixed plastics waste. Ni catalysts supported on different metal oxides including Al2O3, ZrO2, TiO2, MgO and CemO2 and Cu/Mg/Al have been investigated with the aim to reduce the formation of coke on the surface of the reacted catalyst [13,14,15]
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