Abstract
The role of the effect of the support on the reactivity of heterogeneous catalysts cannot be over-emphasized. Therefore, the study documented in this article investigated the effect of different metal oxide supports (MgO, CaO and TiO2) and mixed oxide supports (CaTiO3) on the performance of a bimetallic NiMo catalyst prepared via the sol–gel method during the catalytic growth of carbon nanotubes (CNTs) from waste polypropylene (PP). Waste PP was pyrolyzed at 700 °C in a single-stage chemical vapor deposition reactor and off-gas was utilized in-situ as a cheap carbon feedstock for the growth of CNTs under similar conditions for all the prepared NiMo catalysts (supported and unsupported). The structures of the prepared catalysts and deposited carbon were extensively characterized using X-ray diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), etc. The catalytic performance of NiMo supported and unsupported catalysts was evaluated in terms of the yield, purity, and morphology of synthesized CNTs. The results revealed that the stabilizing role of supports is fundamental in preventing nanoparticle agglomeration and aggregation, thereby resulting in improved yield and quality of CNTs. Supported NiMo catalysts produced better aligned graphitic and high-quality CNTs. The NiMo/CaTiO3 catalyst produced the highest carbon of 40.0%, while unsupported NiMo produced low-quality CNTs with the lowest carbon yield of 18.4%. Therefore, the type of catalyst support and overall stability of catalytic materials play significant roles in the yield and quality of CNTs produced from waste PP.
Highlights
Polypropylene represents the second largest polymer market worldwide, accounting for more than 25% of the global polymer demand, on an estimated annual average growth rate of around 5.2% [1,2,3]
Recycling technologies for waste PP materials and products are usually low-grade since waste PP is limited to a small number of reprocessing and reuse cycles as these products lose some of their essential properties, such as strength and stability, after a number of recycling cycles; the carbon content remains intact [4]
Since carbon nanotubes were first discovered by Ijima, a great deal of research has been undertaken in this field on several potential applications of CNTs which are very plausible given their tremendous chemical, electrical, optical, thermal, and mechanical properties [9]
Summary
Polypropylene represents the second largest polymer market worldwide, accounting for more than 25% of the global polymer demand, on an estimated annual average growth rate of around 5.2% [1,2,3]. From economic and environmental points of view, low-value waste PP has been widely studied as an inexpensive feedstock for high-value-added carbon nanotubes [5,6,7]. Investigated and compared CNT production over an Fe/Al2 O3 catalyst during the catalytic pyrolysis of different types of plastic wastes comprising polypropylene, polyethylene (PE), i.e., low-density polyethylene (LDPE) and high-density polyethylene (HDPE), and polystyrene (PE), i.e., (high-impact and general-purpose). Since carbon nanotubes were first discovered by Ijima, a great deal of research has been undertaken in this field on several potential applications of CNTs which are very plausible given their tremendous chemical, electrical, optical, thermal, and mechanical properties [9]
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